Science, Faculty of
Resources, Environment and Sustainability (IRES), Institute for
DSpace
UBCV
Bonfil-Sanders, Ramon
Doctor of Philosophy - PhD
University of British Columbia
An overview of the situation of elasmobranch fisheries around the world and problems for their assessment and management are presented. Four different studies are carried out, each attacking a particular problem under this general topic. The first, is an in-depth review of recent trends in elasmobranch exploitation and management on a worldwide basis aimed at closing the gap in baseline information about these fisheries on a global scale. In the second study, a deterministic age-structured simulation model is developed to analyse density-dependent changes in fecundity as a response to increased fishing mortality in a hypothetical shark population. The use of the model as an aid in management decision-making is exemplified with a case from a tropical shark fishery. Monte Carlo analysis is used in the third study, to evaluate the Schaefer and Fox surplus production models and the delay-difference model of Deriso-Schnute for the estimation of assessment and management parameters of elasmobranch fisheries. The fishery models are evaluated by comparing their estimates of stock assessment a n d management parameters against the known values of a full age-structured stochastic simulation model of a shark population. Different scenarios of stock recruitment relationship, fishable stock size, spatial behaviour of the sharks, and data quality are used for testing robustness. None of the fishery models performs satisfactorily under situations of density-dependent catchability. When catchability remains constant, the Deriso-Schnute model outperforms the Schaefer or Fox models, both for biomass and management parameter estimation. In the final study, the multispecies shark fishery of Yucatan, Mexico, is used as an example of the problems for elasmobranch stock assessment in the real world. The fishery is analysed by fitting the Schaefer model to catch and cpue data. The results highlight severe deficiencies in the data available for assessment which are characterised by a lack o f contrast in the cpue data. Some alternative management recommendations aimed at improving the data for assessment are given.
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Elasmobranch Fisheries: Status, Assessment and Management by Ramon Bonfil-Sanders B.Sc. (Hons), Universidad Autonoma de Baja California (Mexico), 1983 M.Sc, University College of North Wales (UK), 1991 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Resource Management and Environmental Studies) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA February 1996 © Ramon Bonfil-Sanders 1996 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of 2&t/£&M^MMr/hW£m£0MtV7?rl S7l»/£S The University of British Columbia Vancouver, Canada Date ^B3-X9-DE-6 (2/88) II A b s t r a c t . A n o v e r v i e w of the s i tuat ion of e l a s m o b r a n c h f i sher ies a r o u n d the w o r l d a n d p r o b l e m s for thei r a s s e s s m e n t a n d m a n a g e m e n t a re p r e s e n t e d . F o u r different s tud ie s a r e ca r r i ed out, e a c h a t t ack ing a par t icu lar p r o b l e m u n d e r this g e n e r a l top ic . T h e first, is a n in-depth r ev i ew of recent t r ends in e l a s m o b r a n c h explo i ta t ion a n d m a n a g e m e n t o n a w o r l d w i d e b a s i s a i m e d at c l o s i n g the g a p in b a s e l i n e informat ion abou t t h e s e f i she r i es o n a g l o b a l s c a l e . In the s e c o n d s tudy, a de te rmin i s t i c age - s t ruc tu red s imula t ion m o d e l is d e v e l o p e d to a n a l y s e d e n s i t y - d e p e n d e n t c h a n g e s in fecundi ty a s a r e s p o n s e to i n c r e a s e d f i sh ing mortal i ty in a hypo the t i ca l s h a r k popu l a t i on . T h e u s e of the m o d e l a s a n a i d in m a n a g e m e n t d e c i s i o n -m a k i n g is e x e m p l i f i e d with a c a s e from a t rop ica l s h a r k f i shery . M o n t e C a r l o a n a l y s i s is u s e d in the third s tudy, to eva lua t e the S c h a e f e r a n d F o x s u r p l u s p r o d u c t i o n m o d e l s a n d the de lay-d i f fe rence m o d e l o f D e r i s o - S c h n u t e for the e s t ima t ion o f a s s e s s m e n t a n d m a n a g e m e n t p a r a m e t e r s o f e l a s m o b r a n c h f i sher ies . T h e f i shery m o d e l s a re e v a l u a t e d by c o m p a r i n g their e s t ima te s o f s tock a s s e s s m e n t a n d m a n a g e m e n t p a r a m e t e r s aga ins t the k n o w n v a l u e s o f a full age - s t ruc tu red s tochas t i c s imu la t ion m o d e l o f a s h a r k popu la t i on . Different s c e n a r i o s o f s tock recrui tment re la t ionsh ip , f i shab le s tock s i z e , spa t i a l b e h a v i o u r o f the s h a r k s , a n d da t a qual i ty a re u s e d for tes t ing r o b u s t n e s s . N o n e o f the f i shery m o d e l s per forms sat isfactor i ly u n d e r s i tua t ions o f d e n s i t y - d e p e n d e n t ca tchab i l i ty . W h e n ca tchabi l i ty r e m a i n s cons tan t , the D e r i s o - S c h n u t e m o d e l ou tper forms the S c h a e f e r o r F o x m o d e l s , both for b i o m a s s a n d m a n a g e m e n t p a r a m e t e r e s t ima t ion . In the f inal s tudy , the m u l t i s p e c i e s s h a r k f i shery o f Y u c a t a n , M e x i c o , is u s e d a s a n e x a m p l e o f the p r o b l e m s for e l a s m o b r a n c h s tock a s s e s s m e n t in the rea l w o r l d . T h e f i shery is a n a l y s e d by fitting the S c h a e f e r m o d e l to c a t c h a n d c p u e da ta . T h e resul ts highl ight s e v e r e d e f i c i e n c i e s in the da t a a v a i l a b l e for a s s e s s m e n t w h i c h a re c h a r a c t e r i s e d by a l ack o f cont ras t in the c p u e da ta . S o m e al ternat ive m a n a g e m e n t r e c o m m e n d a t i o n s a i m e d at i m p r o v i n g the da t a for a s s e s s m e n t are g i v e n . iii Table of Contents Abstract ii Table of Contents iii List of tables viii List of figures xi Acknowledgements xx Preface ., xxiii Chapter 1. Elasmobranchs: a Specific Problem of Fisheries Assessment and Management 1 1.1 Introduction 1 1.2 A note on taxonomy 2 1.3 Problems for the Assessment and Management of Elasmobranch Fisheries 3 1.3.1 Biology and Ecology 3 1.3.2 Fisheries theory 4 1.3.3 Informational constraints 5 1.3.4 Economics 6 1.4 Thesis outline 7 Chapter 2 Trends and Patterns in Elasmobranch Exploitation: An Overview of World Fisheries for Sharks, Rays and Relatives 10 2.1 Introduction. . 10 2.1.1 Organisation of this work 11 2.2 Characterisation of elasmobranch fisheries 12 2.2.1 The Official Statistics 12 2.2.1.1 Trends and outlooks by FAO Major Fishing Areas. . . . 12 iv 2.2.1.2 Catches by countries 16 2.2.2 Major Fisheries for Elasmobranchs 21 2.2.2.1 America 21 2.2.2.2 Europe 45 2.2.2.3 Africa and Indian subcontinent 62 2.2.2.4 Asia 72 2.2.2.5 Australian subcontinent 89 2.2.3 Bycatches and Discards of Elasmobranchs at Sea 96 2.2.3.1 Drift gillnet fisheries 97 2.2.3.2 Longline fisheries 123 2.2.3.3 Purse Seine Fisheries 151 2.2.3.4 Other miscellaneous fisheries 156 2.2.3.5 Overview. 157 2.3 Discussion 160 2.3.1 Current Situation of Elasmobranch Fisheries 160 2.3.2 Conservation of elasmobranchs 162 2.4 Summary and conclusions 164 Chapter 3 . Density-Dependent Fecundity in Elasmobranchs and Its Implications in Fisheries Management: A Deterministic Age-structured Simulation Model 167 3.1 Introduction 167 3.1.1 Biological characteristics of the group in relation to exploitation 167 3.1.2 Definition of the problem 168 3.2 Construction of the model 169 3.2.1 Model-building considerations 169 3.2.2 Biological considerations 170 3.2.2.1 Early life natural mortality 170 3.2.2.2 Life history functional relationships 170 3.2.3 General characteristics of the model 171 3.2.4 Formulation 172 3.2.5 Initial parameters 174 V 3.2.6 Sensitivity analysis 175 3.3 Results 176 3.3.1 Baseline run 176 3.3.2 Sensitivity analysis 179 3.3.3 The value of fecundity increases 181 3.3.3 A tropical paradigm with management applications 187 3.4 Discussion 191 3.4.1 Simulation results and documented changes in fecundity 191 3.4.2 Trade-off between fecundity and early life natural mortality. . . . 193 3.4.3 Compensatory mechanisms in elasmobranch populations 194 3.4.4 Considerations for fisheries management and research 194 3.4.5 From model to reality 195 Chapter 4. A Monte Carlo Analysis of Fishery Models for Sharks 197 4.1 Introduction 197 4.1.1 Problems for shark fisheries assessment and management. .. 197 4.1.2 Holden's view and modern methods in fisheries science 198 4.2 Methods 199 4.2.1 General approach 199 4.2.2 A set of simulation models of a shark population under exploitation 200 4.2.3 Spatial behaviour and its representation in the operating models 207 4.2.4 Fishery models examined 211 4.2.4.1 Surplus production models 212 4.2.4.2 Delay difference model 214 4.2.5 Benchmarks for model performance 220 4.2.6 Fitting fishery models to data 221 4.2.7 The types of trials performed 224 4.2.7.1 Nomenclature 224 4.2.7.2 Sequence of trials 224 4.3 Results 225 4.3.1 Simulated populations and fishery data time series 225 vi 4.3.2 Tests with the 'proportionality' operating populations 228 4.3.2.1 Tests of observation error assumptions and different versions of the Deriso-Schnute model 228 4.3.2.2 Tests between the Schaefer, Fox, and Deriso-Schnute model 230 4.3.2.3 The B0=K assumption 233 4.3.3 Changes in the spatial behaviour of the stock: hyperstability and hyperdepletion 233 4.3.4 Unproductive stocks 236 4.3.5 Trials with uninformative data 240 4.4 Discussion 244 4.4.1 Unsuccessful estimation assumptions and model misspecifications 244 4.4.2 Changes in the CPUE-biomass relationship 244 4.4.3 Performance of the estimation procedures, and choice of the best model 246 4.4.4 Significance for the assessment and management of real shark fisheries 249 4.4.5 The B0=K strategy 250 4.4.6 Summary and conclusions 250 Chapter 5 Elasmobranch Stock Assessment and Management in the Real World: The Multispecies Shark Fishery of Yucatan, Mexico 252 5.1 Introduction 252 5.2 The shark fishery of Yucatan: a typical case study 253 5.3 Methods 254 5.4 Results 256 5.5 Discussion 264 5.5.1 Shortcomings of the data 264 5.5.2 Harnessing uncertainty 267 5.5.3 How can we improve future assessments of the Yucatan shark fishery? 271 5.6 Summary and conclusions 273 R e f e r e n c e s 2 7 4 A P P E N D I X 1. Lis t o f c o m m o n a n d latin n a m e s o f the e l a s m o b r a n c h s m e n t i o n e d in the text 2 9 7 A P P E N D I X 2 . C a l c u l a t i o n o f true v a l u e s o f fopt a n d Copt for the o p e r a t i n g p o p u l a t i o n s 2 9 9 VIII List of tables. Table 2.1 Elasmobranch catches by FAO Statistical Area 1967-1991. Mean catch, variation and Index of Relative Production (IRP) are given for the last 25 yr, and catch trends for the last 10 yr. 13 Table 2.2 Reported world catches in commercial elasmobranch fisheries (thousand tonnes). (Data from Compagno, 1990 and FAO, unless otherwise indicated). (T.W.F. = total world fisheries, T.W.CLUP = total world clupeoid fisheries, T.ELAS = total world elasmobranch fisheries. EL/FISH = T.ELAS as % of T.W.F., CLUP/FISH = T.W.CLUP as % of T.W.F.). 18 Table 2.3 Sharks species considered in each of the USA east coast management unit (from NOAA 1991). 28 Table 2.4 Shark landings, in dressed weight (kg), west coast USA (adapted from Cailliet et al. 1993). 30 Table 2.5 Shark species found in the commercial fisheries of Mexico. 37 Table 2.6 Shark species reported in Spanish commercial fisheries (adapted from Munoz-Chapuli 1985 a,b). 61 Table 2.7 Percentage catches of sharks and rays according to fishing gear and zones in Taiwan (Prov. of China) and Malaysia (data from SEAFDEC 1988). 82 Table 2.8 Percentage catches of sharks and rays according to fishing gear and zones in Philippines and Thailand (data from SEAFDEC 1988). 87 Table 2.9 Estimation of shark bycatches in the Japanese salmon fisheries, based on information from research cruises. 102 Table 2.10 Alternative estimates of shark bycatches in Japanese salmon fisheries, based on Canadian research cruise (LeBrasseur et al. 1987). 102 Table 2.11 Effort statistics for the flying squid driftnet fishery in the North Pacific for the period 1988-1990 (from Yatsu et al. 1993, Gong et al. 1993 and Yeh & Tung 1993). 108 Table 2.12 Estimation of bycatches of elasmobranchs in 1990 Squid driftnet fishery based on reports of observer programme on board commercial vessel (INPFC 1991). ix 108 Table 2.13 Estimated bycatches of elasmobranchs in the 1990 North Pacific large-mesh driftnet based on reports of the observer programme 1990 (INPFC 1992). 113 Table 2.14 Reported bycatches of elasmobranchs in South Pacific driftnet fisheries. 116 Table 2.15 Elasmobranchs caught in Mediterranean driftnets (adapted from Northridge 1991). 120 Table 2.16 Summary of estimated bycatch of elasmobranchs in high seas driftnet fisheries. 122 Table 2.17 Catch rates and estimated total catch of sharks in the Spanish swordfish fishery. 135 Table 2.18 Shark species commonly caught by tuna longlining in the Indian Ocean (adapted from Sivasubramaniam, 1964). 139 Table 2.19 Estimated bycatch of sharks in tuna longline fisheries of the Central and South Pacific (SPC zone), based on the results of Strasburg (1958). 146 Table 2.20 Estimated bycatch of sharks in the North Pacific by the longline fleets of Japan and Korea, based on the results of Strasburg (1958). 150 Table 2.21 Selected estimates of shark bycatches in high seas longline fisheries. 150 Table 3.1 Results of sensitivity analysis of the model. B%- proportion of virgin biomass remaining after 100yr of fishing; N%= proportion of virgin population remaining after 100yr of fishing. Numbers in parentheses are percentage change from values of the baseline run. 180 Table 4.1 Types of trials performed with informative effort pattern and productive stock (age or recruitment = 7yr). A= additive observation error; M= multiplicative observation error; S= Schaefer; F=Fox; D=Deriso-Schnute; Dn=non-delay Deriso-Schnute; D**= misspecified Deriso-Schnute. 226 Table 4.2 Types of trials performed with clue proportional to biomass and an unproductive stock (age of recruitment = y). Key as in table 4.1. 220 Table 5.1. Results of first set of trials: Total Least Squares (TLS) fits of the Schaefer Model to CPUE and catch data for the entire data set of the Yucatan shark fishery. x 258 Table 5.2. Results of second set of trials: TLS fits of the Schaefer Model to CPUE and catch data for the downward portion of the time series (years 85-89) from the shark fishery of Yucatan. 261 Table A.1. Median and quartiles of true fopl and C o p ( values for the different operating populations as characterised by cpue-biomass relationship, stock-recruitment function, and age of entry to the fishery ("productivity"). 291 XI List of figures. Figure 2.1 World reported catch of elasmobranch fishes 1947-1991. (Data from FAO, SEAFDEC, Fishery Yearbooks for Taiwan Area, and Secretaria de Pesca). 13 Figure 2.2 Historical catches of elasmobranchs for the 25 major elasmobranch-fishing countries, arranged by geographical area. 20 Figure 2.3 Elasmobranch catches of the USA by major groups and regions as reported by FAO, during 1977-1991. 23 Figure 2.4 Elasmobranch catches from the east coast of the USA during 1980-1989. Bars represent shark fisheries. (Data from FAO and Hoff 1990). 23 Figure 2.5 Elasmobranch catches in the Pacific and Gulf of Mexico/Caribbean coasts of Mexico during 1977-1991 (sh = sharks). (Data from Secretaria de Pesca, Mexico). 35 Figure 2.6 Elasmobranch catches of Peru, by species groups, during 1977-1991 (Data from FAO). 41 Figure 2.7 Elasmobranch catches of Brazil, by species groups, during 1977-1991. (Data from FAO). 41 Figure 2.8 Elasmobranch catches of Argentina, by species groups, during 1977-1991. (Data from FAO). 47 Figure 2.9 Elasmobranch catches of Norway, by species groups, during 1978-1991. (Data from FAO). 47 Figure 2.10 Elasmobranch catches of former USSR, by species groups, during 1978-1991. (Data from FAO). 52 Figure 2.11 Elasmobranch catches of U.K., by country and species groups, during 1978-1991. (Data from FAO). 52 Figure 2.12 Elasmobranch catches of Ireland, by species groups, during 1978-1991. (Data from FAO). 57 Figure 2.13 Elasmobranch catches of France, by species groups, during 1978-1991. (Data from FAO). 57 Figure 2.14 Elasmobranch catches of Spain, by species groups, during 1978-1991. (Data from FAO). 61 Figure 2.15 Elasmobranch catches of Italy, by species groups, during 1978-1991 (Data from FAO). 63 Figure 2.16 Elasmobranch catches of Nigeria, by species groups, during 1977-1991. (Data from FAO). 65 Figure 2.17 Elasmobranch catches of Pakistan, by species groups, during 1977-1991. (Data from FAO). 65 Figure 2.18 Elasmobranch catches of India, by region, during 1977-1991. (Data from FAO). 70 Figure 2.19 Elasmobranch catches of Sri Lanka, by species groups, during 1977-1991 (Data from FAO). 70 Figure 2.20 Elasmobranch catches in different fisheries of Japan during 1976-1984 (S=sharks, B=batoids, IMongline). (Data from Taniuchi (1990) and Ishihara (1990)). 74 Figure 2.21 Elasmobranch catches of Japan, by species groups and region, during 1977-1991. (Data from FAO). 74 Figure 2.22 Elasmobranch catches of South Korea, by species groups and region, during 1977-1991. (Data from FAO). 77 Figure 2.23 Estimated shark catches for the People's Republic of China from fin exports, using 3% and 5% conversion factor. (Fin export data from P. Wongsawang, pers. comm.). 77 Figure 2.24 Elasmobranch catches of Taiwan, by species groups, during 1978-1990. (Data from FAO). 82 XIII Figure 2.25 Elasmobranch catches of Malaysia, by species groups and region, during 1976-1990 (E.P.M.=eastern peninsular Malaysia, W.P.M.=western peninsular Malaysia). (Data from SEAFDEC). 85 Figure 2.26 Elasmobranch catches of Philippines, by species groups and region, during 1976-1990. (Data from SEAFDEC). 85 Figure 2.27 Elasmobranch catches of Thailand, by species groups and region, during 1976-1991. (Data from SEAFDEC). 87 Figure 2.28 Elasmobranch catches of Indonesia, by species groups and region, during 1976-1990 (B=batoids, S=sharks). (Data from SEAFDEC). 90 Figure 2.29 Elasmobranch catches of Australia, by FAO statistical areas, during 1977-1991. (Data from FAO). 95 Figure 2.30 Elasmobranch catches of New Zealand, by species groups, during 1977-1991. (Data from FAO). 95 Figure 2.31 Generalized area of operation of the Japanese landbased and non-traditional (ex-mothership) fisheries in 1990. (Based on INPFC 1993). 99 Figure 2.32 Legal boundaries of the Japanese, Korean and Taiwanese flying squid driftnet fisheries. (Redrawn from Pella et al. 1993). 105 Figure 2.33 Area of operation of Japanese large-mesh drift net fishery. (Redrawn from Nakano etal. 1993). 111 Figure 2.34 South Pacific Commission statistical area. (Taken from Lawson 1991). 115 Figure 2.35 Effort distribution of Japanese longline fishery in the Atlantic Ocean in the 1980's. Keys indicate accumulated nominal hook numbers in thousands. (Redrawn from Nakano 1993). 125 Figure 2.36 Distribution of Korean long-line catches, no units given. (Redrawn from NFRDA 1988). 130 xiv Figure 2.37 Distribution of nominal CPUE of bigeye tuna (a) and albacore (b) in the deep and regular longline fisheries of Taiwan in the Atlantic Ocean, 1990. (Redrawn from Hsu and Liu 1992). 131 Figure 2.38 Distribution of effort (in thousands of hooks) by the Spanish swordfish longline fishery in the Atlantic Ocean during 1988-1991. (Redrawn from Mejuto et al. 1993). 135 Figure 2.39 Distribution of Taiwanese catch per unit effort of albacore by (a) regular and (b) deep longline fisheries during 1988 in the Indian Ocean. (Redrawn from Hsu and Liu 1990). 137 Figure 2.40 Distribution of longline effort in the SPC area during 1990, units not given. (Taken from Lawson 1991). 143 Figure 2.41 Major areas of Tuna purse seine fisheries in the world. 153 Figure 3.1 Values of the natural mortality coefficient used by the density dependent function of the model. 177 Figure 3.2 Growth of the simulated elasmobranch population according to parameters defined in the text. 177 Figure 3.3 Stationary (stable) structure of the simulated population at the asymptotic size (virgin population), defined by the mortality and natality schedules. 178 Figure 3.4 Decay of the simulated population under the baseline fishing mortality pattern. 178 Figure 3.5 Total numbers ( ) and recruitment (—) trends of the simulated shark population. The top histograms show the structure of the population every 10 years, a) F=0.05 times baseline, fecundity=5 times baseline; b) F=0.9 times baseline, fecundity=1.6 times baseline; c) F=3 times baseline, fecundity=1.6 times baseline. 182 Figure 3.6 Response surfaces of population numbers (left) and biomass (right) to different fishing regimes during 100 years, when fecundity is increased 100% from baseline values. Values of F are multipliers of the baseline fishing mortality. Note the steepness of both surfaces as F decreases, indicating the sensitivity of the model to changes in F. The axes in the figures have been switched to offer the best view of the surfaces. 184 XV Figure 3.7 Response surface of population numbers to changes in fecundity during a 100 years' period when F is 1.8 times the baseline value. Compare the flatness of the surface with that of figure 6. The axes in the figure have been switched to offer the best view of the surface. 184 Figure 3.8 Proportion of the virgin population after 100 yr of fishing (A/%) against fishing intensity (expressed as multipliers), for different fecundity increase multipliers (numerals on each line). ( — baseline fishing mortality; .... virgin population size in numbers). 185 Figure 3.9 Response surfaces of A/% (upper) and B% (lower) as a function of fecundity increases and F values (both plotted as multipliers). Y axis reversed in order to facilitate view. 186 Figure 3.10 Growth in weight of Carcharhinus falciformis from Yucatan, used for the calculation of population biomass. 189 Figure 3.11 Age-specific density-dependent mortality coefficients used for the simulation of the silky shark (Carcharhinus falciformis). 189 Figure 3.12. Forecasted evolution of the silky shark fishery of Yucatan under 4 different management scenarios: a) no change in estimated fishing mortality; b) a total ban of the bycatch of juveniles in the red grouper hook & line fishery; c) a reduction of 50% in the fishing mortality from the gillnet fishery for adults; d) a total ban of the gillnet fishery for adults. ( tot. numbers; — recruits; .... tot. biomass). 190 Figure 3.13 Proportion of the biomass (B%) and numbers (N%) from the virgin stock left after 100 yr of fishing under 4 management alternatives (numbered 1-4 in the Y axis), under initial and doubled fishing mortalities, for 3 different fecundities: Top- baseline fecundity; Centre- increase of 40% in fecundity; Bottom- increase of 100% in fecundity. (For explanation of the management alternatives see text). 192 Figure 4.1 Schematic representation of the approach taken in this study for testing the performance of three fisheries models for the estimation of assessment and management parameters for shark fisheries. The process is repeated 100 times (Monte Carlo simulations). 201 xvi Figure 4.2 Characteristics of the simulated age structured shark population. The two vulnerability schedules differ only in the age of entry to the fishery. 203 Figure 4.3 Effort patterns used to simulate the harvesting process in the operating models, a) High contrast effort, b) Uninformative effort. 206 Figure 4.4 Theoretical relationships between cpue and abundance. 208 Figure 4.5 Hyperstability (top), proportionality (centre) and hyperdepletion (bottom) as simulated with equation 4.9. (top, Q=5; centre, Q=1; bottom, Q=0.5). 210 Figure 4.6 Total biomass trajectory of the simulated shark populations simulated stock-recruitment data. Top figures, Beverton-Holt OP; Bottom figures, Ricker OP. Fishing starts at year 80. 227 Figure 4.7 Examples of one realization of biomass and cpue trends for different types of operating populations. 229 Figure 4.8 Modified box plots showing distribution of estimates of management and biomass assessment benchmarks for each estimation model using the multiplicative observation error assumption. Monte Carlo simulations performed with the proportionality OP, high contrast effort, and productive stock. Codes in the x axis correspond to letters used to name estimation procedures in table 4.1. 231 Figure 4.9 Examples of model fits to biomass time series. A) Beverton-Holt OP; B) Ricker OP; C) Beverton-Holt OP with the Bo=K assumption. Legend key: OP is true biomass, S is Schaefer model, F is Fox model,D is Deriso-Schnute model; and D** is Deriso-Schnute model with mis-specified stock-recruitment function. 232 Figure 4.10 Modified box plots showing distribution of estimates of management and biomass assessment benchmarks for each estimation model using multiplicative observation error and the B„=K assumptions. Monte Carlo simulations performed with the proportionality OP and the high contrast effort pattern. Codes in the x axis correspond to letters used to name estimation procedures in table 4.1. 234 xvii Figure 4.11 Modified box plots showing distribution of estimates of management and biomass assessment benchmarks for each estimation model using the multiplicative observation error assumption. Monte Carlo simulations performed with the Hyperstability and Hyperdepletion OPs and high contrast effort pattern. Codes in the x axis correspond to letters used to name estimation procedures in table 4.1.. ()= number of failed trials. 235 Figure 4.12 Examples of model fits to biomass time series for the Beverton-Holt OP. A) Hyperstability Case; B) Hyperdepletion Case. Legend key as in fig. 4.9. By chance, fig B shows a relatively good fit of the Fox model, however, note large uncertainty of TD values for this model in fig. 4.11. 237 Figure 4.13 Modified box plots showing distribution of estimates of management and biomass assessment benchmarks for each estimation model using the multiplicative observation error assumption. Monte Carlo simulations performed with the unproductive stock (age of recruitment =4yr) and proportionality OP. Codes in the x axis correspond to letters used to name estimation procedures in table 4.1. 238 Figure 4.14 Examples of model fits to biomass time series. A) Ricker OP with low productivity; B) Same as above, with uninformative effort pattern; C) As above, with Bo=K assumption. Legends as in Figure 4.9. 239 Figure 4.15 Modified box plots showing distribution of estimates of management and biomass assessment benchmarks for each estimation model using the B„=K and the multiplicative observation error assumptions. Monte Carlo simulations performed with the high contrast effort pattern, unproductive stock (age of recruitment =4yr) and the proportionality OP. Codes in the x axis correspond to letters used to name estimation procedures in table 4.1. 241 Figure 4.16 Modified box plots showing distribution of estimates of management and biomass assessment benchmarks for each estimation model using the multiplicative observation error assumption. Monte Carlo simulations performed with the uninformative effort pattern, the unproductive stock (age of recruitment =4yr) and the proportionality OP. Codes in the x axis correspond to letters used to name estimation procedures in table 4.1. 242 xviii Figure 4.17 Modified box plots showing distribution of estimates of management and biomass assessment benchmarks for each estimation model using the multiplicative observation error and the B„=K assumptions. Monte Carlo simulations performed with the uninformative effort pattern, the unproductive stock (age of recruitment =4yr) and the proportionality OP. Codes in the x axis correspond to letters used to name estimation procedures in table 4.1. 243 Figure 4.18 Distribution of errors around a q value of 0.3, according to the two observation error assumptions considered in the study: additive and multiplicative. All data are simulated with the multiplicative model. 245 Figure 5.1. Historical catches of the Yucatan shark fishery. R = Reconstructed from shark by-products; O = Official data, Ministry of Fisheries (25 species of which 7 are common). 255 Figure 5.2. CPUE time series for shark fisheries in two neighbouring localities of Yucatan. 255 Figure 5.3 Approximate range of action of the artisanal shark fisheries of Rio Lagartos and El Cuyo. Additional constrains are imposed by depth. Deployment of nets is not possible beyond 75 m. 257 Figure 5.4. Standardized CPUE time series for shark fisheries in two neighbouring localities of Yucatan. 257 Figure 5.5. Examples of fits to the entire CPUE series: a) 'good' visual fit, but high TLS score. b) Low TLS score, but poor visual fit at the beginning of the time series. 260 Figure 5.6. Examples of fits to the downward portion of the CPUE series (years 85-92) : a) unconstrained fit (trial 2.7); b) using the Bo-K constraint (trial 2.38). 263 Figure 5.7. Plot of CPUE and effort for the shark fishery of Yucatan, (effort estimated from the total catch and the available CPUE series). 266 Figure 5.8. The uncertainty around the MSY value is roughly bounded by MSY isolines of less than 2 (thousand tonnes). Pairs of estimated r and K values for the Schaefer model are plotted for runs using the Bo=K assumption (crosses) and for runs not using this assumption (circles). Inset shows the complete range of estimated parameter values. 268 XIX Figure 5.9 Scatter plot of r and q values obtained from fits with (crosses) and without (circles) the Bo=K assumption. The straight lines are lines of equal fopt values. 269 Figure 5.10 Uncertainty in the CPUE-Biomass relationship. Isolines of MSY = 1.5 thousand tonnes, for different values of the parameter beta representing proportionality (1), hyperdepletion (1.5) and hyperstability (0.5). Key to symbols as in figure 5.8. 270 Figure A.1 Modified box plots showing distribution of estimates of management and biomass assessment benchmarks for each estimation model using the two observation error assumptions. Monte Carlo simulations performed with the proportionality Beverton-Holt OP and high contrast effort. Codes in the x axis correspond to letters used to name estimation procedures in table 4.1.. For each estimation model, A means additive, M means multiplicative observation error assumption. ()= number of failed trials. 300 Figure A.2 Modified box plots showing distribution of estimates of management and biomass assessment benchmarks for each estimation model and the two observation error assumptions, using the B0=K assumption. Monte Carlo simulations performed with the proportionality Ricker OP and high contrast effort. Codes in the x axis correspond to letters used to name estimation procedures in table 4.1. For each estimation model, A means additive, M means multiplicative observation error assumption. ()= number of failed trials. 301 A c k n o w l e d g e m e n t s . x x F i n a n c i a l suppor t for a la rge part o f m y s tud ie s is gratefully a c k n o w l e d g e d to C O N A C y T , a n d the Instituto N a c i o n a l de la P e s c a , M e x i c o . M y t h a n k s a l s o to the F i s h e r i e s C e n t r e , U B C , a n d the B . C . Min i s t ry o f E n v i r o n m e n t ' s F i s h e r i e s R e s e a r c h B r a n c h , for s o m e add i t i ona l shor t - term suppor t in the form of a R e s e a r c h A s s i s t a n t s h i p . F o r the p repa ra t ion o f C h a p t e r 2, the fo l lowing p e r s o n s p r o v i d e d v a l u a b l e informat ion abou t the f i sher ies of the i r coun t r i e s o r f i sher ies u n d e r their expe r t i s e : M r . L e o n a r d o C a s t i l l o , Instituto N a c i o n a l de la P e s c a , M e x i c o C i ty , M e x i c o ; Dr . C h e - T s u n g C h e n , N a t i o n a l T a i w a n O c e a n Univers i ty ; Dr . P a u l i n e D a y a r a t n e , N a t i o n a l A q u a t i c R e s o u r c e s A g e n c y , C o l o m b o , S r i L a n k a ; M r . S h i g e t o H a s e , Nor th P a c i f i c A n a d r o m o u s F i s h C o m m i s s i o n , V a n c o u v e r , C a n a d a ; Dr . D a v i d Ho l t s , N a t i o n a l M a r i n e F i s h e r i e s S e r v i c e , L a J o l l a , U S A ; Dr . R o s a n g e l a L e s s a , U n i v e r s i d a d e F e d e r a l R u r a l do P e r n a m b u c o , R e c i f e , B r a z i l ; M r . J u l i o M o r o n , Indo-P a c i f i c T u n a D e v e l o p m e n t a n d M a n a g e m e n t P r o g r a m m e , C o l o m b o , S r i L a n k a ; Dr . R a m o n M u n o z - C h a p u l i , Un ive r s i t y o f M a l a g a , S p a i n ; Dr . S i g m u n d M y k l e v o l l , Institute of M a r i n e R e s e a r c h , B e r g e n , N o r w a y ; M r . La r ry J . P a u l , M A F F i s h e r i e s , W e l l i n g t o n , N . Z . ; M s . C h e e P h a i k E a n , F i s h e r i e s R e s e a r c h Institute at P e n a n g , M a l a y s i a ; Dr . A n d r e w R i c h a r d s , M r . P a u l T a u r i k i a n d M r . P a u l V . N i c h o l s , F o r u m F i s h e r i e s A g e n c y , H o n i a r a , S o l o m o n Is lands; M r . Pa i ro j S a i k l i a n g , D e p a r t m e n t of F i s h e r i e s , B a n g k o k , T h a i l a n d ; Dr . C a r o l u s M . V o o r e n , F u n d a c a o U n i v e r s i d a d e d o R i o G r a n d e , B r a z i l ; M s . P o u c h a m a r n W o n g s a w a n g , S o u t h e a s t A s i a n F i s h e r i e s D e v e l o p m e n t C e n t e r , S a m u t p r a k a r n , T h a i l a n d . Dr . T i m o t h y A . L a w s o n , S o u t h P a c i f i c C o m m i s s i o n , N o u m e a , N e w C a l e d o n i a , k ind ly con t r ibu ted m a p s for s o m e f igures . I w i s h to t hank m y s u p e r v i s o r Prof . T o n y J . P i t c h e r for h is suppor t a n d s u p e r v i s i o n . M y t h a n k s to Prof . C a r l J . W a l t e r s for h is helpful a d v i c e a n d a s s i s t a n c e wi th m a n y t e c h n i c a l matters , a n d for s o m e ha rd to get but v e r y fruitful d i s c u s s i o n s a b o u t m a n y a s p e c t s of f i sher ies a s s e s s m e n t a n d m a n a g e m e n t that h a d a b e a r i n g in c h a p t e r s 4 a n d 5. Prof . E m e r i t u s D o n L u d w i g a l s o p r o v i d e d helpful a d v i c e a n d useful d i s c u s s i o n s a b o u t M o n t e C a r l o a n a l y s i s a n d f i shery s y s t e m s . M y s i n c e r e a p p r e c i a t i o n to the r e m a i n i n g m e m b e r s of m y s u p e r v i s o r y c o m m i t t e e not yet m e n t i o n e d , Prof . L e s M . L a v k u l i c h , Prof . P a u l H . L e B l o n d , xxi Prof. J. Donald McPhajl, and Prof. Tony R.E. Sinclair, for their support and helpful advice to keep my work in focus. Mr. Cesareo Cabrera and his staff at Rio Lagartos, Yucatan, along with the personnel of Compania Pesquera Atlantida in El Cuyo, Yuc. allowed me access to their shark landing data. Their cooperation to obtain the fishery data for Chapter 5 is greatly acknowledged. Ms. Alida Bundy kindly gave useful comments to a large part of the text. Special mention of appreciation goes to my wife Ying Chuenpagdee for her continuous assistance in the collection of data, preparation of figures and tables, and final editing and typing of the thesis, as well as for her financial support during the last year of my studies that made it possible to finish this thesis. xxii To my wife, Ying, for her unconditional support and love, which made this work possible. XXIII P r e f a c e . T h i s d i sse r ta t ion c o m p r i s e s four m a i n chap t e r s w h i c h a r e e s s e n t i a l l y s e p a r a t e s tud ies . A l t h o u g h t h e s e s tud ie s a re l i nked by the c o m m o n subject o f f i she r i e s for s h a r k s a n d rays , they might be r e g a r d e d a s a d d r e s s i n g a different a s p e c t o f that subject . In part icular , C h a p t e r 2 is a c o m p r e h e n s i v e r ev i ew of wor ld ' s f i sher ies for e l a s m o b r a n c h s , w h i c h s h o u l d be c o n s i d e r e d a s a re fe rence s o u r c e for the rest o f the d i s se r t a t ion . C h a p t e r 2 w a s p u b l i s h e d by the F o o d a n d Agr i cu l t u r e O r g a n i z a t i o n o f the U n i t e d N a t i o n s du r ing 1 9 9 4 . P e r m i s s i o n h a s b e e n o b t a i n e d from F A O to i n c l u d e this w o r k in the present d i s se r t a t ion . T h e full c i ta t ion o f the d o c u m e n t is: Bonf i l , R . 1 9 9 4 . O v e r v i e w of W o r l d E l a s m o b r a n c h F i s h e r i e s . F A O F i s h e r i e s T e c h n i c a l P a p e r 3 4 1 . 1 1 9 p . F A O , R o m e . T h e r e m a i n i n g c h a p t e r s a d d r e s s m o r e quant i ta t ive a s p e c t s o f p o p u l a t i o n d y n a m i c s a n d f i sher ies a s s e s s m e n t . R e a d e r s in te res ted in popu la t ion d y n a m i c s s h o u l d refer to c h a p t e r 3 . T h o s e in te res ted in the theore t i ca l a s p e c t s o f a s s e s s m e n t m e t h o d s a r e a d v i s e d to go straight to C h a p t e r 4 . F ina l ly , r e ade r s l o o k i n g for a p rac t i ca l e x a m p l e o f a s s e s s m e n t of a rea l f i shery for s h a r k s s h o u l d g o direct ly to C h a p t e r 5. A list o f the sc ient i f ic a n d c o m m o n n a m e s of the s h a r k s a n d r ays m e n t i o n e d th roughou t this t hes i s is i n c l u d e d in a p p e n d i x 1. 1 C H A P T E R 1 E L A S M O B R A N C H S : A SPEC IF IC P R O B L E M O F F ISHERIES A S S E S S M E N T A N D M A N A G E M E N T . The most chivalrous fish of the ocean, To ladies forbearing and mild, Though his record be dark Is the man-eating shark Who will eat neither woman nor child... Wallace Irwin 1.1 Introduct ion. E l a s m o b r a n c h f i sher ies a re c o m i n g of a g e , a n d with t h e m , a n e e d for the r e s p o n s i b l e m a n a g e m e n t of t h e s e r e s o u r c e s . W h i l e s h a r k s a n d rays h a v e u s u a l l y b e e n s h u n n e d by mos t w e s t e r n cu l tu res a s unpa l a t ab l e o r u n d e s i r a b l e , they h a v e b e e n the f o c u s of important f i sher ies in o ther par ts of the w o r l d for a l ong t ime . A c c o r d i n g to c a t c h stat is t ics , e l a s m o b r a n c h s p l ay o n l y a s e c o n d a r y role in the w o r l d f i sher ies a r e n a . T h e g l o b a l r e c o r d e d c h o n d r i c h t h y a n c o m m e r c i a l c a t c h to ta l led 7 0 4 , 0 0 0 t in 1 9 9 1 , a n equ iva l en t to 0 . 7 % of the w o r l d f i sher ies du r ing that y e a r ( see s ec t ion 2 .2 .1 .1) ; e v e n c o n s i d e r i n g a n under - repor t ing l eve l of 5 0 % of the r e c o r d e d c a t c h , s h a r k s a n d rays c o m p r i s e o n l y a b o u t 1% of the w o r l d f i sher ies p roduc t ion . D e s p i t e this m o d e s t role, e l a s m o b r a n c h s c a n be o f p r ime impor t ance in s o m e r e g i o n s o f the w o r l d w h e r e they p resen t ly sus t a in s igni f icant f i she r ies . In coun t r i e s l ike S r i - L a n k a , P a k i s t a n , a n d A u s t r a l i a , e l a s m o b r a n c h s r ep resen t b e t w e e n 5 % a n d 9 % of the total f i sher ies p roduc t ion . In t hese coun t r i e s , a n d in o ther c a s e s o f loca l ly important e l a s m o b r a n c h f i she r i es (i.e. U S A At l an t i c s h a r k f ishery) , the a d e q u a t e a s s e s s m e n t a n d m a n a g e m e n t of the r e s o u r c e s s h o u l d be a major c o n c e r n . O v e r the last f ew d e c a d e s the a c c e p t a n c e of e l a s m o b r a n c h s a s f o o d a n d their impor t ance a s f i shery r e s o u r c e s h a v e g r o w n w o r l d w i d e , a l t hough for r e a s o n s d i s c u s s e d b e l o w , th is is not a l w a y s p roper ly ref lected in the f i shery stat is t ics . T w o m a i n fac tors h a v e c o n v e r g e d in the c rea t ion o f n e w marke t s for e l a s m o b r a n c h s a s f o o d a r o u n d the g l o b e . No tab ly , e n h a n c e d l eve l s o f i n c o m e in m a n y coun t r i e s a re appa ren t l y r e s p o n s i b l e for a n e n o r m o u s e x p a n s i o n o f m a r k e t s for shark-f in s o u p . T h i s c o m m o d i t y h a s s u p e r s e d e d its role a s a pure ly traditional Chinese dish to become a trendy gastronomic icon of wealth and power. Parallel to this, there are pressing needs to increase the catches of non-traditional species in order to replace many established fisheries that are currently overexploited (Garcia and Newton 1995). As a net result, elasmobranch fisheries continue to develop in many parts of the globe and the total catches of sharks and rays keep growing without any apparent levelling off. The problem with unchecked fishery exploitation or inadequate fisheries assessment and management is that they almost inevitably lead to stock collapse and ensuing socio-economic hardship. The NW Atlantic cod fishery is a recent bitter example of this reality. In the case of elasmobranchs, there are more causes for concern about their overexploitation than is usual with most fishery resources. Elasmobranchs and their fisheries possess particular characteristics that warrant their treatment as a specific problem of fisheries science deserving a careful and close attention. This thesis is conceived precisely with this in mind. 1.2 A note on taxonomy. The elasmobranchs are part of the Chondrichthyes. The Class Chondrichthyes comprises a diverse group of fishes whose most obvious common feature is the possession of a cartilaginous skeleton, as opposed to the bony skeleton of the Osteichthyes or bony fishes. The cartilaginous fishes form an ancient successful group dating back to the Devonian, in which basic models remain largely unchanged since their last large flourish during the Cretaceous. Despite their ancient origin, they possess some of the most acute and remarkable senses found in the animal kingdom, allowing them to coexist successfully with the more modern teleost designs. The chondrichthyans are grouped into two main subclasses: Holocephalii (Chimaeras or ratfishes and elephant fishes) with 3 families and approximately 37 species inhabiting cool and deep waters; and the Elasmobranchii which is a large and diverse group (including sharks and rays) with representatives in all types of environments, from fresh waters to the depths of marine trenches and from polar regions to warm tropical waters. The great majority of the commercially important species of chondrichthyans are elasmobranchs. The 3 latter r e c e i v e the i r n a m e from their p la ted gi l ls , w h i c h c o m m u n i c a t e to the ex te r io r by m e a n s of 5-7 o p e n i n g s . T h e c l a s s i f i ca t ion o f e l a s m o b r a n c h s is a subject of c o n t i n u o u s d e b a t e but t hey a re gene ra l l y d i v i d e d into 3 g r o u p s o f s h a r k s (i.e. s q u a l o m o r p h s , g a l e o m o r p h s a n d s q u a t i n o m o r p h s ) w h i c h i nc lude 3 0 f ami l i e s a n d a p p r o x i m a t e l y 3 6 8 s p e c i e s , a n d a g r o u p k n o w n a s the ba to ids , c o m p r i s i n g the rays , ska te s , t o r p e d o e s a n d s a w f i s h e s , a n d e m b r a c i n g a total o f 14-21 fami l i e s a n d a b o u t 4 7 0 s p e c i e s ( C o m p a g n o 1977 , 1984 ; S p r i n g e r a n d G o l d 1989) . F o r the p rac t i ca l p u r p o s e s of this thes i s , a l l the C h o n d r i c h t h y e s ( sha rks , ska t e s , r ays a n d c h i m a e r a s ) wi l l often be t rea ted toge ther u n d e r the n a m e " e l a s m o b r a n c h s " o r " sha rks a n d rays". A l t h o u g h this is a n inaccura t e te rm if t a k e n strictly, it s impl i f ies wr i t ing a n d r e a d i n g by a v o i d i n g u n c o m m o n or l eng thy t e rmino logy s u c h a s " c h o n d r i c h t h y a n s " o r " sharks , ska te s , r ays a n d c h i m a e r a s " e v e r y t ime I n e e d to m a k e re fe rence to the g r o u p . 1.3 P r o b l e m s for the A s s e s s m e n t a n d M a n a g e m e n t o f E l a s m o b r a n c h F i s h e r i e s . T h e e l a s m o b r a n c h s p resen t a n a r ray o f p r o b l e m s for f i sher ies a s s e s s m e n t a n d c o n s e r v a t i o n . T h e s e p r o b l e m s c a n be l o o s e l y c l a s s i f i ed a s t h o s e a s s o c i a t e d with the par t icular b i o l o g y a n d e c o l o g y o f t he se f i shes , p e r c e i v e d p r o b l e m s wi th f i sher ies theory, p r o b l e m s c a u s e d by informat iona l cons t ra in t s , a n d f inal ly p r o b l e m s that d e p e n d o n e c o n o m i c factors . 1.3.1 B i o l o g y a n d E c o l o g y . O n e o f the c h i e f p r o b l e m s f a c e d w h e n d e a l i n g with e l a s m o b r a n c h f i she r i es is that thei r b io log i ca l a n d e c o l o g i c a l profi les m a k e s t h e m h igh ly p rone to ove rexp lo i t a t i on . M o s t sha rk a n d m a n y ray s p e c i e s c a n be c l a s s i f i ed a s s t rong K s t ra tegis ts ( H o e n i g a n d G r u b e r 1990) : they are l o n g - l i v e d a n d this , toge ther with their t yp ica l s l o w growth , resul ts in a late a g e of first s e x u a l matura t ion , w h i c h c o m m o n l y r a n g e s b e t w e e n 3 a n d 2 5 y e a r s d e p e n d i n g o n the s p e c i e s . M o s t e l a s m o b r a n c h s h a v e ve ry l ow fecundi ty w h e n c o m p a r e d wi th b o n y f i shes o r m a r i n e inver tebra tes , the r ange of y o u n g p r o d u c e d by e a c h f e m a l e is b e t w e e n 2 a n d 125 per litter ( see Prat t a n d C a s e y 1 9 9 0 for a s u m m a r y of k e y l i fe-history cha rac t e r i s t i c s o f 4 sha rks ) . T h e c o m b i n a t i o n o f the a b o v e factors t r ans la tes into a l o w rep roduc t ive potent ia l a n d m e a n s that the product ivi ty a n d r e s i l i ence of e l a s m o b r a n c h s t o c k s is c o m p a r a t i v e l y low. A t the c o m m u n i t y l eve l , the top p reda tor n i che o c c u p i e d by m a n y s h a r k s r a i s e s the ques t ion of thei r i m p o r t a n c e a s regula tors o f o ther s p e c i e s ' dens i t i e s . W h a t a r e the impl i ca t ions of thei r r e m o v a l / d e p l e t i o n from the e c o s y s t e m ? A l t h o u g h it c o u l d be d e s i r a b l e to cont ro l sha rk p o p u l a t i o n s in v e r y spec i f i c s i tua t ions (i.e. b e c a u s e they c a n affect the e c o n o m y of important b e a c h resort a r e a s l ike N a t a l o r H a w a i i ) , it is l ikely that the i r r e m o v a l c o u l d br ing u n d e s i r a b l e e c o l o g i c a l a n d e c o n o m i c a l c o n s e q u e n c e s , a s d o c u m e n t e d in the c o a s t o f N a t a l by v a n d e r E ls t (1979) . It is v e r y difficult h o w e v e r , to a s s e s s the effects o f s h a r k dep le t ion in the e c o s y s t e m o r to k n o w w h i c h s t o c k s o f e l a s m o b r a n c h s a re ac tua l ly e n d a n g e r e d , w h e n there is insufficient informat ion abou t thei r b a s i c b i o l o g y a n d e c o l o g y , the s i z e a n d state of thei r s t ocks , a n d the rea l m a g n i t u d e of thei r exp lo i ta t ion . 1.3.2 F i s h e r i e s theory . It is c o m m o n be l i e f that a n o t h e r cons t ra in t for the a s s e s s m e n t a n d m a n a g e m e n t of s h a r k s a n d rays is p o o r d e v e l o p m e n t o f theory ( A n d e r s o n 1 9 9 0 , A n d e r s o n a n d T e s h i m a 1990) . F i s h e r i e s r e s e a r c h o n e l a s m o b r a n c h s h a s b e e n s c a n t y if not i n a d e q u a t e a n d to date there is no spec i f i c m e t h o d o l o g i c a l f r amework for a s s e s s i n g their f i she r i e s . F o r a start, su rp lus p roduc t ion m o d e l s h a v e b e e n t radi t ional ly d i s r e g a r d e d for the a s s e s s m e n t of s h a r k a n d ray f i sher ies for s e v e r a l r e a s o n s , wi thout au tho r s ac tua l ly e x a m i n i n g the sui tabi l i ty of the r ange of t he se m o d e l s to spec i f i c e l a s m o b r a n c h f i sher ies . S o m e au tho r s ( A n d e r s o n 1990 , A n d e r s o n & T e s h i m a 1990 , S i l v a 1993) be l i eve that p roduc t ion m o d e l s a re o f l imited use ma in ly b e c a u s e o f thei r l ack of b i o l o g i c a l reali ty (no a g e s t ructure , no expl ic i t a c c o u n t of growth a n d rep roduc t ive m o d e s , on ly a ve ry c r u d e incorpora t ion o f c o m p e n s a t o r y m e c h a n i s m s , etc.) . O the r s , l ike H o l d e n (1977) a n d W o o d et a l . (1979) , d i s m i s s su rp lus p roduc t ion m o d e l s a r g u i n g that e l a s m o b r a n c h b io logy v io l a t e s s o m e o f the a s s u m p t i o n s o f t he se m o d e l s ( see C h a p t e r 4) . T h e mul t i spec i f i c a n d mul t igea r nature of mos t sha rk a n d ray f i she r i es further c o m p l i c a t e s thei r a s s e s s m e n t . T a k e the f i sher ies for s h a r k s a n d rays in the t rop ics a s a n e x a m p l e (which inc iden ta l ly a c c o u n t for m o r e than 5 0 % of the repor ted w o r l d e l a s m o b r a n c h c a t c h e s ) . T h e s e 5 fisheries include a mixed catch of several species of sharks and rays; furthermore, these catches are often obtained with a great variety of gears and from several types of vessels. Multispecies fisheries present serious methodological problems because of their complex biological and technological interactions. As a consequence, the theoretical development of multispecies assessment and management is still lagging behind the rest of fisheries science (Hilborn and Walters 1992). In addition to this, the usage of multiple gears and fleets for the exploitation of any fish resource introduces another level of complexity for their assessment and management (e.g. standardisation of effort, the complications of allocating quotas to the various types of gears and vessels). 1.3.3 Informational constraints. There are several kinds of information deficiencies that make the assessment of elasmobranch fisheries very difficult. Probably the most widespread and pressing of them is the lack of adequate fishery statistics. The latter are not well maintained for elasmobranchs around the world, partly because of problems in species identification (specially for tropical species), partly for economic reasons. Most statistical records aggregate all skates and rays in a single group without further species identification, while shark catches are commonly split into two categories, large and small sharks (Chapter 2). In the worst cases, all elasmobranchs are reported together as a single item: "various elasmobranchs". Without statistics by species or species groups it is very difficult to implement most fishery models or to get any insight into the dynamics of the stocks. This sets obvious constraints on our ability to do assessment and management. The lack of statistics by species or species groups has a large economic component: it is not cost effective to sort catches by species when they all attain the same price. Nonetheless, it has been shown that whenever a specific market is developed for an elasmobranch species, catch information becomes readily available. Lack of information on some relevant areas of elasmobranch population dynamics is also a constraining factor for their assessment and management. First, stock-recruitment relationships have never been documented with hard data for any elasmobranch, although an almost proportional relationship is suspected due to the reproductive strategies of the group (Holden 1973, Hoff 1990). Secondly, there is a general shortage of hard evidence 6 abou t d e n s i t y - d e p e n d e n t m e c h a n i s m s regula t ing e l a s m o b r a n c h p o p u l a t i o n s i z e . Thi rd ly , the spa t ia l s t ructure a n d d y n a m i c s o f mos t s t o c k s a re a l m o s t total ly u n k n o w n . T h i s often o v e r l o o k e d i s s u e is o f par t icular impor t ance to f i sher ies m a n a g e m e n t both at the l oca l a n d in terna t ional l e v e l . Inadequa te k n o w l e d g e o f migra t ion routes , s t o c k de l imi ta t ion a n d m o v e m e n t ra tes a m o n g s t t he se s t o c k s / s u b s t o c k s , c a n s e r i o u s l y u n d e r m i n e o the rwi se " so l id" a s s e s s m e n t a n d m a n a g e m e n t r e g i m e s . F ina l ly , the difficulties in f ind ing a d e q u a t e m o d e l s for e l a s m o b r a n c h s a re e x a c e r b a t e d by c o n s i d e r a b l e g a p s in o u r u n d e r s t a n d i n g o f their b io logy . T h e life c y c l e s o f mos t s p e c i e s , e v e n in t e rms o f the b a s i c p a r a m e t e r s o f age , growth a n d r ep roduc t ion , h a v e just s tar ted to be u n v e i l e d du r ing the last fifteen y e a r s o r so , a n d o n l y for a handfu l o f e l a s m o b r a n c h s tocks , ma in ly t h o s e of c o m m e r c i a l i m p o r t a n c e in d e v e l o p e d coun t r i e s . T h i s s i tuat ion h a s h i n d e r e d the u s e of the m o r e "b io log ica l ly correct" age - s t ruc tu red m o d e l s . 1.3.4 E c o n o m i c s . S h a r k s a n d r ays a re not a h igh ly p r i c e d f i shery p roduc t (e .g . in the T a i w a n e s e gil lnet f i sher ies o f the C e n t r a l W e s t e r n Pac i f i c , p r i ces for sha rk in t runk at ta in o n l y 2 0 % a n d 6 0 % of the pr ice of w h o l e t unas a n d m a c k e r e l s r e spec t ive ly (Mi l l ing ton 1981)) . S o m e e x c e p t i o n s are sport f i sher ies , w h i c h c a n be o f c o n s i d e r a b l e e c o n o m i c v a l u e , ce r t a in s p e c i e s for w h o m a t rendy g a s t r o n o m i c d e m a n d h a s recent ly d e v e l o p e d in s o m e parts o f the w o r l d (i.e. m a k o a n d th r e she r s h a r k s in U S A ) , o r t hose s p e c i e s w h i c h unfor tunately a r e h igh ly - sough t o n l y for thei r teeth a n d j a w s , like the great whi te shark . S h a r k ' s fins for o r ien ta l s o u p s t and a s the o n l y h i g h l y - p r i c e d e l a s m o b r a n c h product ; a ki lo of top-qual i ty dry f ins c a n fetch m o r e than $ 1 0 0 U S . A n u m b e r o f the p r o b l e m s a s s o c i a t e d with e l a s m o b r a n c h exp lo i ta t ion c a n be t r a c e d b a c k to e c o n o m i c factors . In par t icular , two e c o n o m i c cons t ra in t s s u r r o u n d i n g e l a s m o b r a n c h f i sher ies c a u s e w h a t I c a l l the " t ragedy of sha rks" . First , r e s e a r c h a n d m a n a g e m e n t for s h a r k s a n d rays a re h a m p e r e d by the low e c o n o m i c v a l u e o f this g r o u p : in a t ime w h e n budge t s a l l o c a t e d for sc ient i f ic r e s e a r c h a n d for r e s o u r c e m a n a g e m e n t con t inue to d e c r e a s e , priori t ies a re g i v e n to r e s o u r c e s e c o n o m i c a l l y m o r e important than e l a s m o b r a n c h s , i.e. s a l m o n s , p r a w n s , tunas , etc. T h e s e c o n d , is the h igh pr ice a t ta ined by sha rk fins in the in terna t ional market . T h i s h igh pr ice s t imula tes i n c r e a s i n g explo i ta t ion a n d is a l s o the force b e h i n d "f inning" p rac t i ces in m a n y f i sher ies . W i t h i n th is context , "f inning" ( cons i s t ing in cutt ing-off the fins from the s h a r k a n d d u m p i n g the c a r c a s s to the s ea ) is a n e x c e s s i v e l y was te fu l habit w h i c h is unfortunately ve ry c o m m o n a m o n g f i s h e r m e n th roughout the w o r l d : w h e n s h a r k s a re c a u g h t a s a b y c a t c h , the ex t ra high-profi ts o b t a i n e d from cutt ing the fins off the s h a r k a re difficult to fo rgone in the n a m e o f c o n s e r v a t i o n . A p a r t from the e th ica l i s s u e s o f this p rac t ice ( m a n y t imes the s h a r k is d u m p e d still a l ive ) , f inning is r e s p o n s i b l e for h igh dea th rates o f s h a r k s at s e a . H e n c e , the d y n a m i c s o f the g e n e r a l l ow pr ice o f e l a s m o b r a n c h s a n d the h igh pr ice of s h a r k fins k e e p s h a r k s c a u g h t h o p e l e s s l y i n -b e t w e e n . A t present , this d i l e m m a s e e m s to h a v e no v i a b l e so lu t ion that is cons i s t en t wi th both e c o n o m i c a n d c o n s e r v a t i o n interests . In add i t ion , the e c o n o m i c incen t ive b e h i n d f inning p rac t i ces is indi rec t ly r e s p o n s i b l e for the fact that a subs tan t i a l part of the s h a r k c a t c h n e v e r m a k e s it to the official s tat is t ics; a s no sha rk c a r c a s s e s a re brought to port, in mos t c a s e s sha rk fin l a n d i n g s a re not c o n v e r t e d to l ive we igh t a n d a c c o u n t e d for. T h i s is p robab ly o n e of the major r e a s o n s w h y official s tat is t ics do not truly reflect the recent i n c r e a s e in e l a s m o b r a n c h explo i ta t ion w o r l d w i d e . 1.4 T h e s i s ou t l ine . C o n s i d e r i n g the r ange of p r o b l e m s a s s o c i a t e d with e l a s m o b r a n c h f i sher ies , it is not su rp r i s ing that there is a his tory o f fa i led sus ta inab i l i ty in thei r exp lo i t a t ion ( A n d e r s o n 1990 , a n d H o l d e n 1977) p rov ide a list o f fa i l ed e l a s m o b r a n c h f i sher ies ) . In recen t y e a r s h o w e v e r , there h a s b e e n g r o w i n g in ternat ional c o n c e r n o v e r the c o n s e r v a t i o n o f s o m e e l a s m o b r a n c h s tocks a n d it s e e m s that now, m o r e than ever , there is a n e e d for a m o r e de t a i l ed a p p r o a c h to the p r o b l e m of e l a s m o b r a n c h a s s e s s m e n t a n d m a n a g e m e n t . T h i s t he s i s is c o n c e i v e d wi thin the contex t o f e l a s m o b r a n c h s a s a s p e c i a l c a s e of f i sher ies m a n a g e m e n t requ i r ing spec i f i c a t tent ion. A l t h o u g h e a c h of t h e s e c h a p t e r s cons t i tu tes a s epa ra t e a n d i n d e p e n d e n t s tudy, they a re a l l l iked by the c o m m o n p u r p o s e o f cont r ibu t ing t o w a r d s the e s t a b l i s h m e n t o f g e n e r a l ru les for the ra t ional exp lo i ta t ion o f e l a s m o b r a n c h s . In this s e n s e , the c o n c e p t o f t a ck l i ng f i shery r e s o u r c e s in a t a x o n o m i c / s p e c i f i c b a s i s is not a novel ty . A s the w i d e s p r e a d app l i ca t i on o f t radi t ional f i shery m o d e l s to a l l t ypes o f r e s o u r c e s h a s f requent ly p r o v e n to be a n u n s u c c e s s f u l s t rategy, m o r e a n d m o r e f i shery sc ien t i s t s a re tu rn ing t o w a r d s d e v e l o p i n g m e t h o d s t a i lo red to the spec i f i c n e e d s o f the r e s o u r c e o f the i r c o n c e r n ( see I W C 1987 , Pun t 1988) . 8 T h e first s tudy, C h a p t e r 2 , s e r v e s two p u r p o s e s , it is a b a c k g r o u n d a n d f r a m e w o r k for the thes i s , a n d cons t i tu tes a major re fe rence s o u r c e for e l a s m o b r a n c h f i she r i e s w o r l d w i d e . T h i s o v e r v i e w in tegra tes in a s ing le v o l u m e the mos t important informat ion a v a i l a b l e abou t f i sher ies for e l a s m o b r a n c h s a r o u n d the w o r l d , a n d p r o v i d e s a p re l imina ry a n a l y s i s o f the g l o b a l s i tua t ion . It c o n t a i n s de ta i l ed a n a l y s e s o f e l a s m o b r a n c h f i she r i es in coun t r i e s that h a v e s ignif icant s h a r k a n d ray c a t c h e s , a n d g i v e s the first e v e r e s t ima te o f g l o b a l b y c a t c h e s in h i g h - s e a s f i she r i es o f the w o r l d . C h a p t e r 2 w a s p u b l i s h e d du r ing 1 9 9 4 by the F o o d a n d Agr i cu l t u r e O r g a n i z a t i o n o f the U n i t e d N a t i o n s , a s F A O F i s h e r i e s T e c h n i c a l P a p e r 3 4 1 , a n d h a s b e e n r e c e i v e d wi th great interest by the scient i f ic c o m m u n i t y . In C h a p t e r 3,1 cont r ibute t o w a r d s the u n d e r s t a n d i n g o f the popu l a t i on d y n a m i c s o f s h a r k s , a n d its re la t ion with exp lo i t a t ion . In this s tudy, a s i m p l e de te rmin i s t i c s imu la t i on m o d e l with expl ic i t a g e s t ructure is u s e d to a n a l y s e the effects of d e n s i t y - d e p e n d e n t fecundi ty u p o n the abil i ty o f a s h a r k popu la t i on to sus t a in f i sh ing mortal i ty. T h i s is d o n e by s imu la t i ng the na tura l g rowth of a popu la t ion a n d its l ong te rm trajectory u n d e r different s c e n a r i o s of explo i ta t ion a n d fecundi ty i n c r e a s e s . In add i t ion , the u s e o f the m o d e l a s a n a i d in m a n a g e m e n t d e c i s i o n m a k i n g is exempl i f i ed with a c a s e from a t rop ica l s h a r k f ishery. Ul t imate ly , this c h a p t e r i l lustrates the b io log i ca l l imita t ions that c o n s t r a i n m o s t e l a s m o b r a n c h f i sher ies . T h e s e a r c h for a d e q u a t e f i shery m o d e l s for the a s s e s s m e n t o f e l a s m o b r a n c h popu la t ions is t rea ted in C h a p t e r 4 . T h i s is a la rge M o n t e C a r l o a n a l y s i s o f the p e r f o r m a n c e o f three f i sher ies m o d e l s for the es t imat ion o f a s s e s s m e n t a n d m a n a g e m e n t p a r a m e t e r s of e l a s m o b r a n c h f i she r ies . T h i s m o d e l l i n g e x e r c i s e a i m s at d e t e r m i n i n g if s i m p l e f i shery m o d e l s c a n be u s e d for s h a r k a s s e s s m e n t a n d m a n a g e m e n t p a r a m e t e r e s t ima t ion , w h i c h m o d e l is best, a n d h o w robus t s u c h m o d e l s a re . A full age - s t ruc tu red s t o c h a s t i c s imu la t i on m o d e l of a sha rk popu la t i on is u s e d to gene ra t e c a t c h a n d c p u e da t a v i a a s t o c h a s t i c ha rves t ing s u b m o d e l . A total o f 10 different s c e n a r i o s i nc lud ing va r i a t i ons in s t o c k recrui tment re la t ionsh ips , f i shab le s tock s i z e , spa t ia l b e h a v i o u r o f the s h a r k s , a n d da t a qual i ty , a re a n a l y s e d to test the r o b u s t n e s s of the f i shery m o d e l s . T h e s e a re the s u r p l u s p roduc t ion m o d e l s o f S c h a e f e r (1957) a n d F o x (1971) , a n d the de lay-d i f fe rence m o d e l o f D e r i s o (1980) 9 a n d S c h n u t e (1985) . T h e M o n t e C a r l o a n a l y s i s i n c l u d e s s e v e r a l a l t e rna t ives for i m p l e m e n t i n g the es t ima t ions ; pe r fo rmance is e x a m i n e d by c o m p a r i n g the e s t i m a t e s of s tock a s s e s s m e n t a n d m a n a g e m e n t pa r ame te r s from e a c h f i shery m o d e l a g a i n s t the k n o w n v a l u e s of the s i m u l a t e d popu la t i ons . F ina l ly , C h a p t e r 5 is a n e x a m p l e o f the p rac t i ca l difficulties f a c e d du r ing rea l e l a s m o b r a n c h f i sher ies a s s e s s m e n t work . In this c h a p t e r I a n a l y s e the m u l t i s p e c i e s s h a r k f ishery of Y u c a t a n t h rough the app l i ca t i on of o n e of the f i shery m o d e l s s t u d i e d in c h a p t e r 4, a n d s u g g e s t s o m e p o s s i b l e so lu t ions to o v e r c o m e the s h o r t c o m i n g s o f the a v a i l a b l e da ta . 10 C H A P T E R 2 T R E N D S A N D P A T T E R N S IN E L A S M O B R A N C H EXPLOITATION: A N O V E R V I E W O F W O R L D F ISHERIES FOR S HARKS , R A Y S A N D RELATIVES. 2.1 Introduct ion. T h e a p p a r e n t fragility of e l a s m o b r a n c h s a n d the pas t h is tory o f c o l l a p s e s in their f i sher ies ( see A n d e r s o n (1990) for a rev iew) a re c a u s e s for c o n c e r n . T h i s is s p e c i a l l y true n o w that the c o n t i n u i n g i n c r e a s e in their c a t c h e s ( see 2.2) a n d the e v e r d e m a n d i n g marke t for sha rk fins m a y be e n d a n g e r i n g the sus ta inabi l i ty o f t h e s e f i sher ies . In recen t y e a r s , there h a s b e e n g r o w i n g in te rna t iona l c o n c e r n o v e r the state o f e l a s m o b r a n c h s tocks , a n d s o m e conse rva t i on i s t m o v e m e n t s a re start ing to r ing the a l a r m o v e r the plight o f s h a r k s a n d rays a r o u n d the w o r l d . Unfor tunate ly , whi l s t the e v e n t u a l a d o p t i o n o f a n y h a r s h uni la tera l c o n s e r v a t i o n m e a s u r e (i.e. e m b a r g o e s l ike t h o s e o f the t u n a - d o l p h i n c o n t r o v e r s y ) c o u l d h a v e nega t ive effects in the f i sher ies o f m a n y coun t r i e s for w h i c h e l a s m o b r a n c h s a re of c o n s i d e r a b l e impor t ance , the reali ty is that the p resen t i m p a c t s o f f i she r i es o n s h a r k a n d ray s tocks o n a g l o b a l s c a l e a re difficult to a s s e s s . T h i s h a p p e n s b e c a u s e there is not o n l y a l ack o f informat ion o n the s i z e of mos t e l a s m o b r a n c h s tocks , but b e c a u s e there is no readi ly a v a i l a b l e b a s i c informat ion abou t thei r f i sher ies w o r l d w i d e . M u c h of the ex i s t i ng information abou t s h a r k a n d ray f i sher ies is not on ly d i s p e r s e , but is u s u a l l y u n p u b l i s h e d a n d kept by t hose c o n c e r n e d with their s tudy o r m a n a g e m e n t in m a n y l abora to r i e s a r o u n d the w o r l d . E v e n the rea l m a g n i t u d e of the total w o r l d c a t c h e s o f s h a r k s a n d r ays is uncer ta in , ma in ly b e c a u s e o f p o o r k n o w l e d g e o f the total l eve l s of b y c a t c h e s a n d d i s c a r d s . T h i s c h a p t e r is o r i en t ed t o w a r d s a l l ev ia t ing the l ack o f b a s e l i n e informat ion a b o u t s h a r k a n d ray f i sher ies w o r l d w i d e . T h e p resen t o v e r v i e w of e l a s m o b r a n c h f i she r i es puts toge ther for the first t ime m o s t o f the ex i s t ing informat ion abou t the cha rac t e r i s t i c s a n d d ivers i ty o f thei r f i sher ies , the s p e c i e s u n d e r explo i ta t ion , the extent o f the c a t c h e s , the l e v e l o f b y c a t c h e s a n d d i s c a r d s in the h igh s e a s , a n d abou t m a n a g e m e n t m e a s u r e s cur ren t ly in use for e l a s m o b r a n c h f i sher ies . 11 2.1.1 O r g a n i s a t i o n of this work . S e c t i o n 2.2.1 part ia l ly d e s c r i b e s the s c a l e of g l o b a l e l a s m o b r a n c h f i sh ing by e x a m i n i n g the official s ta t is t ics w o r l d w i d e . T h i s s ec t ion c o n s i s t s o f a n o v e r v i e w of the c a t c h stat is t ics by F A O major f i sh ing a r e a s i nc lud ing short- term pro jec ted c a t c h e s , a n d a n o v e r v i e w of the t r ends in the mos t important f i sher ies for e l a s m o b r a n c h s in the w o r l d o n a coun t ry b a s i s . F o r this r ev iew, coun t r i e s wi th official e l a s m o b r a n c h c a t c h e s o f 1 0 , 0 0 0 t/yr o r m o r e are c a l l e d "major" e l a s m o b r a n c h - f i s h i n g coun t r i e s . S e c t i o n s 2 .2 .2 a n d 2 .2 .3 d e a l wi th the major f i sher ies for e l a s m o b r a n c h s , a n d the b y c a t c h e s a n d d i s c a r d s at s e a , r e spec t ive ly . A l t h o u g h it is difficult to d i s t i ngu i sh b e t w e e n d i rec ted a n d inc iden ta l f i sher ies , e s p e c i a l l y w h e n d e a l i n g with f i shes that a re s e l d o m ta rge ted and /o r c a u g h t a l o n e a s is the c a s e o f s h a r k s a n d rays , I wi l l u s e the fo l lowing t w o m a i n d i v i s i o n s for the t rea tment of e l a s m o b r a n c h c o m m e r c i a l f i sher ies : I wi l l treat u n d e r the n a m e "direct", a l l f i sher ies that target e l a s m o b r a n c h s , toge ther with a l l c o a s t a l f i she r i es a n d s m a l l s c a l e m u l t i s p e c i e s f i sher ies w h i c h c a t c h e l a s m o b r a n c h s inc iden ta l ly . T y p i c a l l y , the c a t c h e s from the se t w o s o u r c e s a re m i x e d toge ther in the official s ta t is t ics o f m o s t coun t r i e s a n d it b e c o m e s n e c e s s a r y to treat t h e m together . O n the o ther h a n d , there is a g r o u p of l a rge-s c a l e l ong - r ange f i sher ies that ma in ly target h igh v a l u e s p e c i e s o n the h igh s e a s . T h e s e f i sher ies v e r y f requent ly c a t c h e l a s m o b r a n c h s inc iden ta l ly but u s u a l l y d i s c a r d t he se b y c a t c h e s for v a r i o u s r e a s o n s . T h e y c o m p r i s e e s sen t i a l l y a different c a t e g o r y of f i sher ies in w h i c h the e l a s m o b r a n c h s are not on ly b e i n g w a s t e d , but the a c t u a l n u m b e r s of e l a s m o b r a n c h s c a u g h t a re a l s o poor ly k n o w n a n d usua l ly d o not m a k e it to the ca t ch s tat is t ics . M o s t c a s e s in this c a t e g o r y a re h i g h - s e a s la rge s c a l e f i she r i es wi th driftnets a n d long l ines c a r r i e d out by a few coun t r i e s a n d target ing h igh profile r e s o u r c e s s u c h a s tunids , bi l l f ishes , s a l m o n i d s a n d s q u i d . T h e s e f i sher ies a re s u s p e c t e d of c a u s i n g subs tan t i a l kills of e l a s m o b r a n c h s , m a i n l y s h a r k s . T h i s h a s r a i s e d c o n c e r n o v e r the c o n s e r v a t i o n of t he se f i shes , a l t hough o n a v e r y different s c a l e than the c o n c e r n o v e r m a r i n e m a m m a l s , w h i c h a re a l s o f requent ly t a k e n a s b y c a t c h e s in t he se f i sher ies . D e p e n d i n g o n the a m o u n t of informat ion a v a i l a b l e , the s p e c i e s , c a t c h e s , gea r s , f i sh ing units , loca l i t i es , l eve l s of explo i ta t ion a n d ex i s t ing m a n a g e m e n t o r c o n s e r v a t i o n m e a s u r e s , a re s u m m a r i s e d for e a c h c a s e . 2.2 C h a r a c t e r i s a t i o n o f e l a s m o b r a n c h f i sher ies . 12 2.2.1 T h e Official S ta t i s t ics . T h e da t a u s e d in this a n a l y s i s is t a k e n from official f i shery s ta t is t ics o f e a c h count ry . T h e first s o u r c e is the c o m p i l a t i o n o f C o m p a g n o (1990) w h o a n a l y s e d F A O da t a for the pe r iod 1 9 4 7 - 1 9 8 5 . F A O f igures s i n c e 1 9 7 0 h a v e b e e n u p d a t e d u s i n g F i s h e r i e s Y e a r b o o k s for 1 9 8 8 -1991 ( F A O 1 9 9 0 - 1 9 9 3 ) a n d da t a p r o v i d e d direct ly from the F A O sta t is t ica l d a t a b a s e ( D a v i d D i e , F A O , pe r s . c o m m . A u g u s t 2 , 1 9 9 3 ) . A d d i t i o n a l s o u r c e s a re : F i s h e r y S ta t i s t i ca l Bu l l e t ins for the S o u t h C h i n a S e a A r e a y e a r s 1 9 7 6 - 1 9 9 0 ( S E A F D E C 1 9 7 7 - 1 9 9 3 ) , the F i s h e r i e s Y e a r b o o k o f T a i w a n A r e a for 1 9 7 0 a n d 1 9 8 8 - 1 9 9 0 a n d the M e x i c a n F i s h e r y Sta t i s t ica l Y e a r b o o k s 1 9 7 6 - 1 9 9 0 (Sec re t a r i a de P e s c a 1 9 7 9 - 1 9 9 2 ) . Af te r the t h o r o u g h r e v i e w of F A O da ta d o n e by C o m p a g n o (1990) , the informat ion is he re u p d a t e d a n d e x p a n d e d , i nc lud ing expl ic i t ly the c a t c h e s of T a i w a n a n d e s t ima tes o f the c a t c h e s o f the P e o p l e ' s R e p u b l i c of C h i n a . 2.2.1.1 T r e n d s a n d o u t l o o k s by F A O M a j o r F i s h i n g A r e a s . To ta l w o r l d e l a s m o b r a n c h c a t c h e s repor ted for the pe r iod 1 9 4 7 - 1 9 9 1 (fig. 2 .1) a m o u n t e d to a r eco rd o f 7 0 4 , 0 0 0 t in 1 9 9 1 . R o u g h l y , four p e r i o d s wi th different t r ends c a n be identif ied. P o o r g rowth in c a t c h e s b e t w e e n 1947 a n d 1954 , a s u s t a i n e d i n c r e a s e o f p roduc t ion dur ing 1 9 5 5 - 1 9 7 3 f o l l o w e d by a p e r i o d of s l u g g i s h p roduc t ion for mos t o f the 70 ' s a n d finally r e n e w e d growth in c a t c h e s dur ing the last y e a r s 1 9 8 4 - 1 9 9 1 . C a t c h e s by F A O M a j o r F i s h i n g A r e a s from 1967 to 1991 a re s u m m a r i s e d in tab le 2 . 1 . A n at tempt is m a d e to rank t he se A r e a s a c c o r d i n g to thei r e l a s m o b r a n c h c a t c h e s . B e c a u s e the s i z e s , c o a s t l i n e l eng ths a n d h u m a n popu la t i ons o f e a c h A r e a v a r y no tab ly , a rough i n d e x of relat ive p roduc t ion w a s d e v i s e d for c o m p a r i s o n p u r p o s e s . T h i s i n d e x is de f ined a s the a v e r a g e total e l a s m o b r a n c h ca t ch o f e a c h A r e a d i v i d e d by the s q u a r e root o f the sur face of that A r e a in k m 2 . A better i n d e x might h a v e b e e n to u s e the e x t e n s i o n o f con t inen ta l she l f for e a c h A r e a , but it w a s not p o s s i b l e to ob ta in t he se da ta . Arbi t ra r i ly , v a l u e s o f the i n d e x b e l o w 5 w e r e c o n s i d e r e d indica t ive o f l ow relat ive p roduc t ion , t h o s e b e t w e e n 5 a n d 10 in te rmedia te a n d t h o s e of m o r e than 10 a s h igh . A d d i t i o n a l l y , the t r end in c a t c h e s dur ing the 13 Table 2.1 Elasmobranch catches by FAO Statistical Area 1967-1991. Mean catch, variation and Index of Relative Production (IRP) are given for the last 25 yr, and catch trends for the last 10 yr. (All weights in tonnes, live weight.) F . A . O . M a j o r F i sh ing A r e a s A r e a M e a n C a t c h C o e f f i c i e n t I.R.P. T r e n d 8 2 - 9 1 Million Km2 ' 0 0 0 t of V a r i a t i o n Avg Catch/SqrtAre ' 0 0 0 t/y 2 7 N E A t l a n t i c O c e a n 1 6 . 9 . 9 4 . 8 1 2 % 2 3 . 0 7 0 . 2 6 61 N W P a c i f i c O c e a n 2 0 . 5 1 0 2 . 3 1 0 % 2 2 . 6 0 - 0 . 2 9 51 W Indian O c e a n 3 0 . 2 9 7 . 6 1 9 % 1 7 . 7 5 1.16 21 N W A t l a n t i c O c e a n 5.2 2 6 . 5 5 7 % 1 1 . 6 1 5 .48 3 7 M e d i t e r r a n e a n & B l ack S e a s 3 .0 1 8 . 2 2 9 % 1 0 . 5 0 - 0 . 7 6 71 W C e n t r a l P a c i f i c O c e a n 3 3 . 2 59.1 3 8 % 1 0 . 2 6 5 . 0 0 41 S W A t l a n t i c O c e a n 1 7 . 6 3 4 . 2 3 0 % 8 . 1 5 0 . 6 0 5 7 E Indian O c e a n 2 9 . 8 4 2 . 9 3 2 % 7 . 8 7 1.34 3 4 E C e n t r a l A t l a n t i c O c e a n 1 4 . 0 2 8 . 6 2 9 % 7 . 6 3 - 0 . 6 5 8 7 S E P a c i f i c O c e a n 1 6 . 6 2 1 . 4 3 2 % 5 . 2 4 - 0 . 3 9 31 W C e n t r a l A t l a n t i c O c e a n 1 4 . 7 1 7 . 4 4 7 % 4 . 5 4 0 . 7 7 7 7 E C e n t r a l P a c i f i c O c e a n 5 7 . 5 21.1 3 4 % 2 . 7 9 0 . 0 8 81 S W Pac i f i c O c e a n 3 3 . 2 1 0 . 4 4 7 % 1.81 0 . 5 5 6 7 N E Pac i f i c O c e a n 7.5 4 . 8 6 0 % 1 .74 0 . 2 0 4 7 S E A t l a n t i c O c e a n 1 8 . 6 6.6 4 2 % 1.53 0 . 0 7 e u u - r r 700-H 600-H 400-H 300-H 200-H' 100-IllllllllllllllllllllllllllllIllllPllll 1947 1951 1955 1959 1963 1967 1971 1975 1979 1983 1987 1991 Years Figure 2.1 World reported catch of elasmobranch fishes 1947-1991. (Data from FAO, SEAFDEC, Fishery Yearbooks for Taiwan Area, and Secretaria de Pesca). 14 las ts 10 y e a r s r e c o r d e d for e a c h A r e a , is e x p r e s s e d a s the s l o p e o f a l i nea r r e g r e s s i o n fitted to the da t a by leas t s q u a r e s . In the W e s t e r n A t l a n t i c O c e a n , a l l the A r e a s h a v e fairly h igh i n c r e a s i n g t rends , e s p e c i a l l y A r e a 21 (Nor th W e s t At lan t ic ) w h i c h h a s the h ighes t i n c r e a s i n g t r end o v e r a l l . T h e s e three A r e a s s h o w s t rong va r i a t ions in thei r c a t c h e s . A r e a 21 h a d the h ighes t var iabi l i ty , wi th recent y e a r s a p p a r e n t l y r e c o v e r i n g p roduc t ion from a d r a m a t i c d r o p suf fered in the late 70 ' s fo l lowing h igh y i e l d s in the ea r ly 70 ' s . A r e a 21 h a d a m a r g i n a l l y h igh i n d e x of relat ive p roduc t ion ( IRP) , but c o n s i d e r i n g that a g o o d part o f this a r e a i n c l u d e s a rc t ic wa t e r s prac t ica l ly v o i d for f i sh ing , w e s h o u l d not e x p e c t a m u c h h i g h e r future I R P from this A r e a . In the W e s t e r n C e n t r a l A t l an t i c ( A r e a 31) , there w a s a t r end o f m o d e r a t e i n c r e a s e in c a t c h e s , w h i l e the I R P ind ica t ed a l ow e l a s m o b r a n c h y i e l d . T h i s a g r e e s with S t e v e n s o n (1982) w h o s u g g e s t s that e l a s m o b r a n c h r e s o u r c e s in this A r e a c o u l d h a v e b e e n under -u t i l i sed . P e r h a p s there is still a potent ia l for e x p a n s i o n o f c a t c h e s in th is A r e a , ma in ly for coun t r i e s o f the C a r i b b e a n r eg ion . F o r A r e a 41 (Sou th W e s t e r n At lan t i c ) , e l a s m o b r a n c h c a t c h e s a l s o s h o w a m o d e r a t e i n c r e a s i n g t rend after v a r i a b l e c a t c h e s in the 60 ' s . A v e r a g e c a t c h o f e l a s m o b r a n c h s in A r e a 41 is the h ighes t in the W e s t e r n A t l a n t i c but this is a l s o the larges t a r e a . H e n c e , it h a s o n l y a n in te rmedia te I R P . S m a l l i n c r e a s e s in c a t c h e s might still be p o s s i b l e he re in the future. In c o m p a r i s o n , c a t c h e s in A r e a 31 h a v e b e e n the lowes t in the W e s t e r n At l an t i c , w h i l e in the first hal f of the p e r i o d a n d du r ing the last t w o y e a r s , A r e a 21 h a d the h ighes t y i e l d s . F o r the E a s t e r n A t l an t i c O c e a n , A r e a 2 7 ( Nor th E a s t e r n At lan t ic ) h a d by far the larges t c a t c h e s in the A t l a n t i c a s w e l l a s the third larges t a n d the s e c o n d leas t v a r i a b l e c a t c h e s in the w o r l d . A c c o r d i n g to the I R P , this A r e a h a s the h ighes t p roduc t ion o f e l a s m o b r a n c h s w o r l d w i d e but further e x p a n s i o n s in the c a t c h e s s h o u l d p r o b a b l y not be e x p e c t e d . In fact, the c a t c h t r end hard ly i n c r e a s e d a s p roduc t ion h a s fa l len s i n c e 1 9 8 8 , p e r h a p s s h o w i n g that the h igh l eve l s o f explo i ta t ion in this A r e a a re not s u s t a i n a b l e . T h e C e n t r a l E a s t e r n A t l an t i c ( A r e a 34) s h o w s a m e d i u m leve l o f var ia t ion in e l a s m o b r a n c h p r o d u c t i o n . C a t c h e s in this A r e a i n c r e a s e d du r ing the ear ly 70 ' s but the recent t rend s h o w s a s l o w d e c l i n e . T h i s is a n A r e a with a n in te rmedia te I R P , thus a g o o d r e c o v e r y in c a t c h e s c o u l d be p o s s i b l e . F o r the M e d i t e r r a n e a n S e a ( A r e a 37) , p roduc t ion w a s re la t ively v a r i a b l e du r ing the p e r i o d e x a m i n e d , but its recen t t r end of d e c l i n i n g c a t c h e s is the s teepes t . B e c a u s e o f the s m a l l s i z e a n d the 15 h igh dens i ty of h u m a n se t t l ements of this A r e a , f i sh ing is in t ense a n d the I R P for e l a s m o b r a n c h s is the third h ighes t in the A t l an t i c O c e a n . V e r y l ikely , s h a r k a n d ray s t o c k s here a re c l o s e to full exp lo i t a t ion . In A r e a 4 7 (Sou th E a s t e r n At lan t i c ) c a t c h e s h a v e b e e n fairly v a r i a b l e . It h a s the s e c o n d s m a l l e s t m e a n c a t c h o f e l a s m o b r a n c h s a n d the lowes t I R P in the w o r l d , s h o w i n g the mos t poss ib i l i t i e s for i n c r e a s e d exp lo i ta t ion o f e l a s m o b r a n c h s in the future. F r o m the four A r e a s o f the E a s t e r n At lan t i c , A r e a 2 7 d o m i n a t e d the c a t c h e s with a n e l a s m o b r a n c h p roduc t ion s u p e r i o r to t hose o f the o ther three A r e a s together . T h e r e a re o n l y two F A O A r e a s in the Indian O c e a n . T h e W e s t e r n Indian O c e a n ( A r e a 51) h a s the s e c o n d h ighes t a v e r a g e y i e l d in the w o r l d . T h i s A r e a h a s s h o w n r e a s o n a b l y l ow var iabi l i ty in c a t c h e s . C a t c h e s i n c r e a s e d s tead i ly up to the ea r ly 70 ' s but fell d r ama t i ca l ly dur ing 1 9 8 3 . J u d g i n g from the recent i n c r e a s i n g t rend in p roduc t ion , the s i tua t ion s e e m s to be r e c o v e r i n g but c a t c h e s h a v e not yet r e a c h e d p r ev ious l eve l s . T h e I R P o f A r e a 51 is the third h ighes t in the w o r l d . M o s t o f the c a t c h e s in this A r e a a re t a k e n in the nor thern reg ion by P a k i s t a n , India a n d S r i - L a n k a . S t o c k s in the nor thern r eg ion might be c l o s e to ove r -exp lo i ta t ion , but g i v e n the large e x t e n s i o n o f this A r e a a n d the l o w c a t c h e s from its sou the rn por t ion, it might p resen t s o m e poss ib i l i t i e s for i n c r e a s i n g e l a s m o b r a n c h explo i ta t ion e s p e c i a l l y f rom o c e a n i c s p e c i e s . A r e a 57 (Eas t e rn Indian O c e a n ) s h o w s m o r e va r i ab l e c a t c h e s wi th a g r o w i n g t rend . It h a s a n in te rmedia te I R P a n d h i g h e r y i e l d s a re e x p e c t e d here . In the Indian O c e a n , A r e a 51 p r o d u c e s o n a v e r a g e m o r e than d o u b l e the c a t c h e s of A r e a 5 7 . In the W e s t e r n P a c i f i c O c e a n , c a t c h e s in A r e a 61 (North E a s t e r n Pac i f i c ) h a d a d e c r e a s i n g t rend a n d the lowes t var iabi l i ty of e l a s m o b r a n c h c a t c h e s in the w o r l d . T h i s A r e a h a d the h ighes t a v e r a g e y i e l d s in the w o r l d a n d the I R P w a s a c c o r d i n g l y v e r y h igh , marg ina l ly s e c o n d to that o f the Nor th E a s t e r n At l an t i c . There fo re , c a t c h e s in th is a r e a might not i n c r e a s e subs tan t i a l ly in the future a n d s t o c k s m a y e v e n be p resen t ly o v e r e x p l o i t e d . A r e a 71 (Cen t ra l W e s t e r n Pac i f ic ) s h o w e d the s e c o n d h ighes t i n c r e a s i n g t r end in c a t c h e s , r e a c h i n g in the last f ew y e a r s c a t c h e s five t imes t hose o f the mid -60 ' s . T h e I R P in this a r e a is re la t ively h igh a n d might indica te that y i e l d s c o u l d p r o b a b l y not be e x p a n d e d m u c h more . In the S o u t h E a s t e r n P a c i f i c ( A r e a 81) , c a t c h e s h a v e v a r i e d subs tan t ia l ly , wi th a l ow pos i t ive t rend in recen t c a t c h e s . A v e r a g e c a t c h e s a n d the I R P a r e v e r y low. O n e o f the p o s s i b l e r e a s o n s for this is the re la t ively s m a l l e x t e n s i o n o f coas t l i ne i n s ide th is A r e a , t oge the r with 16 c o r r e s p o n d i n g l y few h u m a n se t t l ements . T h e potent ia l o f this a r e a to s igni f icant ly i n c r e a s e c a t c h e s wi l l d e p e n d ma in ly o n the capab i l i t i e s of the s t o c k s o f o c e a n i c a n d d e e p wa te r e l a s m o b r a n c h s p e c i e s to sus t a in f i sher ies . O f the three A r e a s o f the W e s t e r n Pac i f i c , A r e a 61 is the m o s t important in e l a s m o b r a n c h explo i ta t ion h a v i n g p r o d u c e d o n a v e r a g e a lmos t twice the c a t c h e s o f A r e a 71 a n d abou t ten t i m e s t h o s e of A r e a 8 1 . F ina l ly , for the three a r e a s of the E a s t e r n Pac i f i c , A r e a 6 7 (North E a s t e r Pac i f i c ) h a s the s m a l l e s t a v e r a g e c a t c h e s a n d the h ighes t va r ia t ion in the w o r l d . T h e I R P is the s e c o n d s m a l l e s t of a l l A r e a s a n d the t rend o f recent c a t c h e s is m o d e r a t e l y pos i t ive . L a r g e r c a t c h e s c o u l d be o b t a i n e d here in the future. A r e a 77 (Cen t r a l E a s t e r n Pac i f i c ) h a s s o m e w h a t va r i ab l e c a t c h e s wi th a v e r y l ow i n c r e a s i n g t rend a n d a v e r y l ow I R P . A r e a 7 7 is the larges t in the w o r l d but the a s s o c i a t e d l ow h u m a n popu la t ion dens i ty might a c c o u n t fo r the l ow I R P . T h e potent ia l for i n c r e a s i n g c a t c h e s here is p r o b a b l y g o o d e s p e c i a l l y in C e n t r a l A m e r i c a n coun t r i e s a n d in the vas t o c e a n i c wa te r s . T h e S o u t h E a s t e r n P a c i f i c ( A r e a 87) is the o n l y A r e a o f the E a s t P a c i f i c wi th a nega t ive t rend in c a t c h e s a n d h a s a n in te rmedia te I R P . Fur ther i n c r e a s e s in the c a t c h e s s h o u l d be p o s s i b l e he re . O f the w h o l e E a s t e r n Pac i f i c , A r e a s 7 7 a n d 87 h a v e a l m o s t the s a m e a v e r a g e c a t c h e s du r ing this p e r i o d , a m o u n t i n g to abou t four t i m e s t h o s e o f A r e a 6 7 . A s s u m i n g that recen t c a t c h t r ends wi l l r e m a i n wi thout major c h a n g e s in e a c h o f F A O ' s M a j o r F i s h i n g A r e a s , r epor ted c a t c h e s o f e l a s m o b r a n c h s in the w o r l d c a n be e x p e c t e d to r e a c h b e t w e e n 7 5 5 , 0 0 0 t a n d 8 2 7 , 0 0 0 t by the y e a r 2 0 0 0 . T h e s e fo recas t s a re b a s e d o n 5 y e a r s tep " jackknife" l inea r r e g r e s s i o n a n a l y s e s o f e l a s m o b r a n c h c a t c h e s s i n c e 1967 in e a c h F A O M a j o r F i s h i n g A r e a . 2 .2 .1 .2 C a t c h e s by coun t r i e s . D a t a fo r the p e r i o d 1947 -1991 ind ica te that 2 6 coun t r i e s p resen t ly ha rves t o r h a v e h a r v e s t e d recent ly m o r e than 1 0 , 0 0 0 t/yr o f e l a s m o b r a n c h f i shes , i.e. the re a re 2 6 "major e l a s m o b r a n c h - f i s h i n g count r ies" . A l t h o u g h there a re no official s ta t is t ics for the e l a s m o b r a n c h c a t c h e s of the P e o p l e ' s R e p u b l i c o f C h i n a , they a l s o s u r p a s s 1 0 , 0 0 0 t/yr ( see s ec t ion 2 .2 .2 .4 . ) , a n d therefore qualify the P R C a s o n e o f the major e l a s m o b r a n c h - f i s h i n g na t ions . 17 Hi s to r i ca l c a t c h s tat is t ics f o r t h e 2 5 major e l a s m o b r a n c h - f i s h i n g c o u n t r i e s for w h i c h da t a are a v a i l a b l e , a re s h o w n in tab le 2 .2 . J a p a n h a s t radi t ional ly b e e n the o v e r a l l major f i sher of e l a s m o b r a n c h s in the w o r l d with a v e r a g e c a t c h e s of 6 5 , 0 0 0 t/yr. I n d o n e s i a , India, T a i w a n a n d P a k i s t a n fo l low with c a t c h e s b e t w e e n 3 3 , 0 0 0 t/yr a n d 4 3 , 0 0 0 t/yr. F r a n c e , the U K , the former U S S R a n d N o r w a y , report ha rves t s of b e t w e e n 2 1 , 0 0 0 t/yr a n d 2 7 , 0 0 0 t/yr. M e x i c o , B r a z i l , S o u t h K o r e a , N i g e r i a , P h i l i p p i n e s , S r i - L a n k a a n d P e r u c a u g h t b e t w e e n 11 ,000 t/yr a n d 1 8 , 0 0 0 t/y. A large g roup o f coun t r i e s f o r m e d by S p a i n , U S A , M a l a y s i a , A r g e n t i n a , T h a i l a n d , A u s t r a l i a , Italy, N e w Z e a l a n d a n d Ireland f o l l o w e d wi th a v e r a g e c a t c h e s b e t w e e n 4 , 0 0 0 a n d 1 0 , 0 0 0 t/y. E v e n t h o u g h there is great var iabi l i ty in the d e v e l o p m e n t o f i nd iv idua l e l a s m o b r a n c h f i sher ies , s o m e pat terns c a n be identif ied from t h e s e da ta . A b o u t o n e th i rd o f the major e l a s m o b r a n c h s - f i s h i n g coun t r i e s s h o w recent level l ing-off t r ends in the i r c a t c h e s , p robab ly s i g n a l l i n g full exp lo i ta t ion of s h a r k a n d ray r e s o u r c e s . S e v e n c o u n t r i e s s h o w fal l ing t r ends w h i l e n ine o the r s h a v e a definite r ise in c a t c h e s (fig. 2 .2) . E l a s m o b r a n c h p roduc t ion is s p e c i a l l y h igh in Indones i a , w h e r e c a t c h e s h a v e r o c k e t e d s i n c e the ea r ly 70 ' s wi th no s i gn for a s l o w - d o w n at a l l . T a i w a n , the U S A , S p a i n a n d India are o ther e x a m p l e s o f coun t r i e s with i n c r e a s i n g f i sher ies for s h a r k s a n d rays . J a p a n , h is tor ica l ly the l e a d e r in e l a s m o b r a n c h p roduc t ion , h a s a c l e a r t r end o f d e c r e a s i n g c a t c h e s . N o r w e g i a n c a t c h e s s h o w a c l e a r t r end o f i n c r e a s e until the ea r ly 60 ' s , but th is h a s s i n c e s w i t c h e d to a s h a r p d e c r e a s e . T h e s a m e is true for the fo rmer U S S R c a t c h e s , w h i c h h a d a g r o w i n g pe r iod from the ea r ly 60 ' s to the mid -70 ' s but h a v e subs tan t i a l ly d e c r e a s e d s i n c e , without r e c o v e r i n g to fo rmer l eve l s . C a t c h e s in the U K h a v e a v e r y sl ight a l m o s t impercep t ib l e d e c r e a s i n g t r end . P a k i s t a n h a d a powerfu l t rend of i n c r e a s i n g c a t c h e s until the late 70 ' s , but d rama t i ca l ly d r o p p e d in the ea r ly 80 ' s to m a k e a s l o w but s u s t a i n e d c o m e b a c k . T h e r ange of c a u s e s for the d e c r e a s i n g t r ends is not e a s y to find in a l l c a s e s , but p o s s i b l e e x p l a n a t i o n s for s o m e c a s e s a re g i v e n b e l o w in the ind iv idua l coun t ry a c c o u n t s (2.2.2) . T h e repor ted s tat is t ics ind ica te that dur ing the last 15 y e a r s s h a r k s h a v e b e e n s l ight ly m o r e important in the c a t c h e s t han o ther e l a s m o b r a n c h s . T h e a v e r a g e r epor t ed c a t c h o f s h a r k s a n d ba to ids is 2 8 5 , 4 3 3 t/yr a n d 1 8 0 , 1 9 6 t/yr r e spec t ive ly , wi th 1 9 0 , 1 5 9 t/yr repor ted a s "var ious e l a s m o b r a n c h s " . Af ter r ea l loca t ing the repor ted c a t c h e s in this last c a t e g o r y to 18 Table 2.2 Reported world catches in commercial elasmobranch fisheries (thousand tonnes, live weight). (Data from Compagno, 1990 and FAO, unless otherwise indicated). (T.W.F. = total world fisheries, T.W.CUPL = total world cupleoid fisheries, T.ELAS = total world elasmobranch fisheries. EL/FISH = T.ELAS as % of T.W.F., CUPL/FISH = T.W.CUPL as % of T.W.F.). YEAR T.W.F. T.W.CUP T.ELAS. EL/FISH CUPL/FISH USA MEX BRA PERU ARG USSR UK EIRE NORW SPAIN % % (pi 1947 1948 1949 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 20000 19600 20100 21100 23600 25200 25900 27600 28900 30500 31500 32800 36400 39500 43000 46400 47600 52000 52400 57300 60400 63900 62700 70388 70747 66121 62824 66597 66487 69930 69226 70596 71331 72141 74884 76810 77591 83989 86454 92822 94379 99016 100208 97434 96926 3481 3486 3724 4081 4392 5440 5500 5760 6410 7020 7230 7450 9060 10290 1 2620 14730 14930 18730 17442 19426 20308 21117 18786 22209 20241 14288 12073 14631 14373 15371 13043 14493 15790 16070 16920 17867 17455 19607 21101 23955 22375 24388 24800 22183 21407 201 211 245 204 197 203 204 194 270 280 310 300 300 320 370 380 400 400 405 433 444 476 502 508 482 519 583 549 586 544 556 600 603 609 612 617 568 598 623 630 666 694 . 679 695 704 1.0 1.1 1.2 1.0 0.8 0.8 0.8 0.7 0.9 0.9 1.0 0.9 0.8 0.8 0.9 0.8 0.8 0.8 0.8 0.8 0.7 0.7 0.8 0.7 0.7 0.8 0.9 0.8 0.9 0.8 0.8 0.9 0.8 0.8 0.8 0.8 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 17.4 17.8 18.5 19.3 18.6 21.6 21.2 20.9 22.2 23.0 23.0 22.7 24.9 26.1 29.3 31.7 31.4 36.0 33.3 33.9 33.6 33.0 30.0 31.6 28.6 21.6 19.2 22.0 21.6 22.0 18.8 20.5 22.1 22.3 22.6 23.3 22.5 23.3 24.4 25.8 23.7 24.6 24.7 22.8 22.1 13.1 12.8 11.2 6.1 12.8 3.1 2 2.8 2.8 3.3 14.3 16.6 16.6 16.6 5.7 9 9 8.6 8.6 6.3 7.3 7.3 7.3 1.7 1.5 1 1.8 2.2 1.7 4.1 4.7 5.9 11.1 11.2 11.0 11.7 12.4 9.3 11.9 12.1 15.2 17.2 20.4 34.6 35.5 4.1 4.5 5.6 4.6 3.6 3.4 3.5 4.4 5.1 5.3 6.5 6.3 8.9 9.1 9 8.4 14.1 16.6 14.3 16.1 15.6 21.5 24.6 26.6 35.7 34.6 31.4 34.1 33.3 29.4 27.9 34.6 33.1 38.1 34.0 4.6 5 5.9 7.6 8.9 10.6 13 12.5 12.6 12.6 3.2 15.6 9.5 9.9 6.1 7.3 9.3 21.9 23.3 25.8 31.3 29.1 25.2 29.6 25.7 27.8 24.3 24.9 24.7 25.2 1 1.4 1.2 1.3 1.1 2.5 3 4.5 3.3 3.5 3.4 4.2 7.2 3.8 5.4 5.1 6T 7.6 9.9 19.6 24.7 14.7 19 11.3 10.5 21.5 16.8 14.6 10.5 13.8 15.6 13.8 13.3 19.1 18.8 14.9 34.4 16.8 23.3 23.1 26.6 25.0 12.6 5.7 6.9 5.1 2.4 1 1.2 1.7 2.9 2.4 2.2 3.8 4.1 4.6 4 2.4 2.9 3.9 6.2 6.9 7.2 7.7 10.1 13.7 10.8 8.7 10 9.6 13.4 14.3 13.8 10.6 9.6 12.5 10.0 11.3 8.3 12.8 9.5 10.2 15.3 16.1 15.3 21.1 16.5 16.7 17.6 0.1 3.7 20.8 20.1 31.9 40.1 26.3 48.3 55.3 47.1 55.3 58.5 29.4 13.7 25.7 16.2 12.6 12.5 9.2 11.2 9.5 10.2 17.5 18.1 20.9 12.0 6.0 3.1 27.1 29.8 30.7 29.2 32.6 30.8 28.8 27.8 28.6 27.1 29.1 29.2 27.2 25.7 27.8 23.6 23.5 35.7 24.7 24.5 25.6 25.9 23.8 22.3 26.3 26.6 26 24.1 26.5 26.6 28.1 27.2 24.2 21.6 20.3 18.9 18.8 21.2 23.0 21.5 25.9 24.6 21.2 21.7 20.4 1.7 1.7 1.5 1.5 1.7 1.8 1.9 1.8 1.5 1.7 1.8 2.5 3.2 6.8 9.4 11.8 7.3 11.4 8.9 6.2 5.0 4.0 10.8 10.7 10 12 14 15.3 15.5 18.8 19.1 22.8 20.9 24.4 22 29 45.6 38.7 51.6 45.7 32.2 27.6 27.7 25.3 21.5 44.1 29.8 31.1 30.5 30.6 35.9 24.8 21.9 21.5 20.0 15.6 8.9 9.6 9.8 10.1 7.8 6.5 5.1 5.2 8.0 11.1 12.3 10.4 10.4 10.6 10.8 11.6 10.1 10.8 10.9 10.8 11.7 14.1 14.2 15.4 14 14.3 10.6 11.4 13.8 11.4 11.5 10.8 11.1 9.9 9.9 0 11.4 0 0.6 1 0.7 0.4 3.7 0.9 2.1 2.4 6.3 6.1 5.7 13.7 15.8 22.0 16.7 21.7 14.7 15.9 MEAN 57896 14357 455 0.8 24.4 %variation 43 46 37 14 20 % of worldwide elasmobranch catch, 1987-1991 % importance of elasmobranchs in country, 1987-1991 (p) data from Secretaria de Pesca (s) data from SEAFDEC (s/fl data from SEAFDEC and FAO (t/f) data from Fishery Yearbooks for Taiwan Area and FAO 9.8 76 3.57 0.42 17.4 72 4.88 2.36 16.4 55 3.69 3.00 11.7 72 2.71 0.29 8.8 59 2.54 3.19 22.7 75 1.75 0.11 25.7 14 3.31 2.63 4.3 80 1.03 3.03 21.4 56 1.21 0.44 9.8 57 2.65 1.22 19 Table 2.2 Continued YEAR ITALY FRA NIG PKST INDIA SRILK THAI (si MALAY (s) INDONE Is/f) S KOR JAPAN PHILIPP (s) TAIWA (t/f) AUST N ZEL 1947 20.5 1 73.2 1948 16 1.5 2 14.6 86.1 1949 16.7 9.1 3 118.5 1950 13.7 2 100.7 1951 13.5 2 85.7 1952 13.1 9.8 0.6 2 89.1 1953 14.4 10.8 15.9 0.7 2.2 10.5 97.4 10.7 1954 13.7 9.8 16 3.1 2.3 9.2 102.9 1955 14.9 11.7 20.4 2.5 1.6 10.8 97.2 1956 15.2 9.7 21.9 3 1.6 14.8 92.6 1957 15.2 17.6 23.1 3.9 3.1 12.2 93.8 1958 15.2 9.5 24.3 4.3 2.7 10.2 82.9 1959 15.1 9.8 23.5 4.3 2.8 7.6 86 16.5 1960 16.7 11.3 35.6 7.1 4.3 10.9 83.9 17.1 1961 34.3 9.4 33.6 8.5 4 3.2 8.7 78.3 18.9 1962 33.1 22 40.8 10.3 4.5 3.2 9.9 81.5 19.7 1963 35.5 0.3 25.2 43 12.1 5.1 4.4 9.4 77.4 17.1 1964 37.4 0.3 26.2 34.9 11.2 5.8 4.7 12.6 69 18.8 1965 29.5 28.2 31.4 11.8 12.4 4.6 66.9 20.2 1966 36.3 37.2 37.4 11.6 12.8 6.4 6.3 71.1 22.9 1967 33.1 38.4 29.6 16.3 8 7 5.6 67.5 26.0 1968 27.4 40.3 31.2 14.7 12.3 6.5 18 56 33.1 1969 39 42.5 8.75 18.8 59.3 32.8 1970 4.8 28.2 30.4 39.8 44.1 12.5 22.4 3.6 14.2 61.8 6.9 36.3 7.8 2.6 1971 5.0 25.2 9.4 41.8 41.3 9.8 12.5 6.4 10.3 12.3 50.2 7.3 39.7 7.4 3.1 1972 5.4 25.7 10.2 62.9 45.2 11.5 14.4 6.7 9.2 7.2 52.2 8.2 41.4 7.4 2.4 1973 4.6 27.3 10.4 74 60 17.9 13.6 7.7 16.3 19.3 49.4 9.0 38.1 3.0 2.6 1974 5.1 25.6 11.2 34.8 60.1 15.7 13.7 8.2 18.5 18.9 45.7 9.4 45.8 4.3 3.5 1975 4.8 23.9 12.5 36.6 61 13.1 12.1 8.5 27 22.5 46.2 10.4 62.4 2.9 3.0 1976 5.6 26.8 19.4 40.3 49.1 15.6 11.4 12.2 28.7 18.7 52.9 9.1 59.9 4.5 4.4 1977 5.6 23.2 19.9 64.1 45.6 11.3 12.2 12.2 29.5 17.4 59.7 8.9 56.4 6.9 5.3 1978 4.8 27.8 20.3 71.9 49.9 12.6 9.8 13.7 30.3 18.2 51.2 21.2 48.1 8.0 4.2 1979 4.5 31.9 20.9 74.7 40.9 12.8 9.3 11.9 33.3 19.0 53.0 9 43.7 7.5 4.4 1980 5.1 35.0 21.5 65.0 49.7 14.2 9.5 10.9 42.9 18.0 54.3 9.7 52.3 9.4 6.6 1981 3.9 42.0 11.9 62.9 50.0 21.3 10.2 11.5 43.2 21.5 49.0 12.6 43.7 9.5 7.3 1982 4.8 32.8 14.0 68.8 47.8 20.1 9.6 9.9 45 20.5 47.6 11.4 47.2 9.6 8.0 1983 6.5 39.2 12.0 18.2 51.4 19.2 8.5 10.3 49.9 22.3 43.7 8.2 43.5 9.4 9.9 1984 12.2 34.1 13.0 20.9 54.0 14.7 8.1 10 52.8 20.5 45.7 11.3 48.5 7.1 11.5 1985 14.3 33.1 14.2 29.5 50.5 15.1 9.2 10.3 54.3 22.9 39.4 10.9 55.8 7.5 11.1 1986 13.4 36.4 9.3 27.4 49.1 15.5 13.5 11.2 55.1 21.0 44.4 18.1 46 10.6 8.3 1987 9.8 36.6 9.5 28.6 57.9 16.1 14.4 11.7 58.2 16.2 42.9 16.2 50.1 13.5 9.5 1988 10.4 34.4 9.5 30.3 73.5 16.7 11.4 16.8 63.9 21.7 28.6 17.9 43.9 14.2 13.0 1989 8.4 34.0 6.9 27.6 66.3 17.0 11.2 13.4 74.9 20.8 33.9 19.0 54.8 8.3 10.8 1990 9.6 34.0 8.4 40.0 51.2 15.3 11.0 16.8 73.3 15.7 32.1 18.4 75.7 6.7 12.3 1991 13.7 25.7 7.2 45.1 52.9 18.4 11.8 16.9 79.8 17.3 33.8 19.0 68.6 7.6 13.7 7.4 26.7 12.6 33.0 41.6 11.9 8.4 9.4 42.7 15.2 65.2 12.4 39.9 7.9 7.2 47 33 54 63 36 47 62 43 49 34 34 37 42 36 53 1.51 4.79 1.21 4.99 8.78 2.42 1.74 2.20 10.18 2.67 4.98 2.63 8.52 1.46 1.73 1.89 3.78 2.92 7.42 1.72 8.76 0.43 2.46 2.41 0.66 0.31 0.85 3.50 4.80 2.19 20 1947 1951 1955 1959 1963 1967 1971 1975 1979 1983 1987 1991 80 1947 1951 1955 1959 1963 1967 1971 1975 1979 1983 1987 1991 ~° 40 NIGERIA •( f \ A / * / \ — - - — PAKISTAN . '. ' , / ' / \ • / / \ INDIA SRI-LANKA ' f \ ' '• I \ tv... • ; v , / i / . .„ - —.i.* 1947 1951 1955 1959 1963 1967 1971 1975 1979 1983 1987 1991 80 -THAILAND • MALAYSIA INDONESIA 1947 1951 1955 1959 1963 1967 1971 1975 1979 1983 1987 1991 Years -ARGENTINA 1947 1951 1955 1959 1963 1967 1971 1975 1979 1983 1987 1947 1951 1955 1959 1963 1967 1971 1975 1979 1983 1987 19 80 • AUSTRALIA -NEW ZEALAND 1947 1951 1855 1959 1963 1967 1971 1975 1979 1983 1987 1991 SOUTH KOREA - J A P A N \ / , . ' x PHILIPPINES —TAIWAN ' .\.. .ry ISfc j J \/ 1947 1951 1955 1959 1963 1967 1971 1975 1979 1983 1987 1991 Years Figure 2.2 Historical catches of elasmobranchs for the 25 major elasmobranch-fishing countries, arranged by geographical area. 21 e i ther s h a r k s o r rays with the a i d o f anc i l l a ry informat ion from s e c t i o n 2 .2 .2 . , a n d then spli t t ing the r e m a i n i n g 9 4 , 1 3 9 t/yr of "var ious e l a s m o b r a n c h s " in e q u a l parts , a total of 393 ,741 t/yr (about 59 .5 % of total e l a s m o b r a n c h s ) c a n be at t r ibuted to s h a r k s a n d 2 6 2 , 0 4 6 t/yr to s k a t e s a n d rays (about 39 .5 %) w h e r e a s o n l y l e s s t han 1% a re c h i m a e r a s a n d e l e p h a n t f i shes . 2 .2 .2 M a j o r F i s h e r i e s for E l a s m o b r a n c h s . T w o m a i n s o u r c e s w e r e u s e d for the informat ion in this s e c t i o n . First , l i terature o n the subject w a s c o n s u l t e d for e a c h c a s e a s e x t e n s i v e l y a s p o s s i b l e . M u c h informat ion p robab ly r e m a i n s in the form of u n p u b l i s h e d reports from different g o v e r n m e n t a l off ices a r o u n d the w o r l d . S e c o n d l y , in a n a t tempt to fill in s o m e o f the m a n y g a p s o f informat ion , a ques t i onna i r e w a s d e s i g n e d a n d sent to officers o r sc ien t i s t s in a l l the major e l a s m o b r a n c h -f i sh ing coun t r i e s . H o w e v e r the s u c c e s s of this a p p r o a c h w a s poor . T h e extent of p u b l i s h e d w o r k o n e l a s m o b r a n c h s in e a c h coun t ry a n d the l eve l o f r e s p o n s e to the ques t i onna i r e is ref lected in the quant i ty o f informat ion that is p r e s e n t e d u n d e r e a c h coun t ry ' s a c c o u n t . 2.2.2.1 A m e r i c a . U S A . G e n e r a l o v e r v i e w . T h e U S A is o n e o f the few coun t r i e s with r e a s o n a b l y de t a i l ed informat ion o n e l a s m o b r a n c h f i she r ies . N e v e r t h e l e s s , no c o m p r e h e n s i v e a c c o u n t o f t h e s e f i she r i es o n a na t iona l b a s i s s e e m s to exis t . M a i n f i sher ies for e l a s m o b r a n c h s o f the U S A h a v e t radi t ional ly b e e n cen t r ed o n s h a r k s , a l t h o u g h ba to ids h a v e a l s o b e e n f i shed th rough his tory. R a y s a n d ska t e s w e r e r e c o r d e d in U . S . c o m m e r c i a l c a t c h e s a s ea r ly a s 1916 (Mar t in a n d Z o r z i , 1993) , ma in ly a s a b y c a t c h o f m o r e important f i sher ies . H o w e v e r , the first d i r ec t ed f i sher ies for e l a s m o b r a n c h s in this coun t ry s e e m to h a v e b e e n t hose for the soupf in s h a r k Galeorhinus galeus (then zyopterus) in C a l i f o r n i a , a n d the f i shery for la rge s h a r k s off S a l e r n o in F l o r i d a . B o t h f l ou r i shed a s a c o n s e q u e n c e o f the h igh d e m a n d for sha rk l iver o i l in the 40 ' s -50 ' s a n d d i e d m a i n l y b e c a u s e o f the labora tory s y n t h e s i s o f v i t amin A in 1 9 5 0 . 2 2 Unti l recent ly a n d a c c o r d i n g to F A O stat is t ics , the c o m m e r c i a l c a t c h e s o f e l a s m o b r a n c h s in the U S A w e r e toge the r with t hose o f A r g e n t i n a , the leas t impor tan t a m o n g major e l a s m o b r a n c h - f i s h i n g coun t r i e s in A m e r i c a . H o w e v e r , this s i tua t ion h a s c h a n g e d s i n c e the ea r ly 90 ' s . E l a s m o b r a n c h p roduc t ion h a s v a r i e d c o n s i d e r a b l y for the last 4 0 y e a r s osc i l l a t ing a r o u n d 1 0 , 0 0 0 t/yr until the late 80 ' s . T w o p e r i o d s of v e r y l ow c a t c h e s w e r e 1 9 5 2 - 1 9 5 6 a n d 1 9 7 0 - 1 9 7 7 , w h i l e 1 9 5 8 - 1 9 6 0 s a w s o m e of the h ighes t y i e l d s . T h e p o s t - w a r p e a k o f 17 ,000 t h a s b e e n b r o k e n s i n c e 1 9 8 8 (fig. 2 .2) . C a t c h e s rapid ly i n c r e a s e d d u r i n g the m i d 70 ' s a n d s o a r e d in the mid -80 ' s . S t i l l , e l a s m o b r a n c h s a re o n l y a m i n o r f i shery in the U S A a s c a t c h e s dur ing 1987 -1991 a v e r a g e d on ly to 0 .42 % of the total f i she r i es p roduc t ion w h i l e they con t r ibu ted 3 . 5 7 % of the total repor ted e l a s m o b r a n c h c a t c h in the w o r l d (table 2 .2) . A c c o r d i n g to C o m p a g n o (1990) , the recent r ise in c a t c h e s might reflect a c h a n g e in c o n s u m e r p r e f e r e n c e s that h a s m a d e sha rk mea t f a s h i o n a b l e a n d a c c e p t a b l e to the pub l i c a s a di rect result of the i n f amous " J a w s " f i lms . T h i s w o u l d h a v e p r o m p t e d a w h o l e n e w g roup o f f i she r i es d i r ec t ed to s h a r k s in the U S A . A c c o r d i n g to C o o k (1990) , recen t c h a n g e s in in te rna t iona l shark-f in marke t s h a v e further i n c r e a s e d the d e m a n d for s h a r k s in the U S A . A m o n g s t t h e s e n e w f i sher ies , t hose for the t h r e s h e r shark , Alopias vulpinus, the P a c i f i c a n g e l s h a r k Squatina californica a n d the shortfin m a k o Isurus oxyrinchus, a re the mos t important in the w e s t coas t . F o r the G u l f of M e x i c o a n d eas t c o a s t o f U S A , mos t o f the recent ly r i s ing s h a r k f i sher ies h a v e a d ive r se c a t c h o f c o a s t a l s h a r k s , r epor ted a s unc l a s s i f i ed s h a r k s . T h i s difference in de ta i l o f the repor ted c a t c h e s o n both s i d e s o f the U S A p r o b a b l y fo l lows b e c a u s e o f the e x i s t e n c e of w e l l de f ined m a r k e t s a n d p r i ces for m a n y s p e c i e s o f e l a s m o b r a n c h s o n the w e s t coas t , w h i c h a re l a c k i n g in the eas t coas t . A c c o r d i n g to the N a t i o n a l O c e a n i c a n d A t m o s p h e r i c A d m i n i s t r a t i o n ( N O A A 1991) , in the eas t c o a s t f i sher ies o n l y m a k o s h a r k s h a v e a s e p a r a t e pr ice from the r e m a i n i n g "unc la s s i f i ed sharks" . D a t a from F A O s h o w s that up until 1 9 8 0 e l a s m o b r a n c h c a t c h e s in the U S A w e r e abou t e v e n l y d is t r ibuted in both s i d e s of the count ry . S i n c e 1981 h o w e v e r , the eas t c o a s t h a s con t r ibu ted the bulk of the c a t c h e s t h a n k s to a la rge e x p a n s i o n o f f i she r i es for s h a r k s a n d rays (fig. 2 .3) . T h i s n e w growth l ed to the recent imp lemen ta t i on o f m a n a g e m e n t s t ra teg ies for la rge s h a r k f i sher ies in the eas t coas t . O v e r a l l , the t w o m o s t important e l a s m o b r a n c h g r o u p s in the f i she r i es o f the U S A are the 23 40,000 35,000 5,000 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Years Sharks NE Sharks NW Sharks SE H | Skates NE Sharks SW • Batoids W Figure 2.3 Elasmobranch catches of the USA by major groups and regions as reported by FAO, during 1977-1991. 9,000 8,000-7,000-w 6,000 <D C 2 5,000-X5 c w 4,000 3 O ^ 3,000-2,000-1,000-0-mm m m 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 Years Skates N. Engl. — Dogfish N. Engl. G. of Mex. fJM] S. Atlant. Mid-Atlant. New Engl. | id-Atlant. £ Figure 2.4 Elasmobranch catches from the east coast of the USA during 1980-1989. Bars represent shark fisheries. (Data from FAO and Hoff 1990). 2 4 d o g f i s h e s (main ly Squalus acanthias) a n d the ska t e s . D o g f i s h a n d ska te c a t c h e s from the wa t e r s wi th in F A O A r e a 21 ( roughly c o r r e s p o n d i n g to the N e w E n g l a n d a n d M i d - A t l a n t i c r eg ions of the N a t i o n a l M a r i n e F i s h e r i e s S e r v i c e [ N . M . F . S . ] o f the U S A ) a n d dogf ish c a t c h e s in F A O A r e a 6 7 ( roughly c o r r e s p o n d i n g to the c o a s t s o f W a s h i n g t o n a n d O r e g o n ) h a v e d o m i n a t e d the e l a s m o b r a n c h p roduc t ion o f the coun t ry until v e r y recent ly . D o g f i s h c a t c h e s off the N o r t h e a s t U S A ( A r e a 21) w e r e the major part o f total e l a s m o b r a n c h c a t c h e s dur ing 1 9 7 9 - 1 9 8 3 , fell d o w n in 1984 a n d s l o w l y r e c o v e r e d s i n c e 1 9 8 5 . S k a t e c a t c h e s in this reg ion h a v e i n c r e a s e d t r e m e n d o u s l y s i n c e 1 9 8 3 , a n d w e r e the s e c o n d m o s t important g roup in 1 9 8 9 with a l m o s t o n e third of the total e l a s m o b r a n c h c a t c h e s o f the coun t ry (figs. 2.3 a n d 2.4) . D o g f i s h c a t c h e s off the nor thwes t U S A ( taken ma in ly in W a s h i n g t o n ) h a d fairly va r i ab l e y i e ld s , con t rac t ing du r ing the m i d 80 ' s , part ial ly r e c o v e r i n g in 1 9 8 6 - 1 9 8 7 on ly to s u b s e q u e n t l y fa l l . M o s t o f the dogf i sh o n the eas t c o a s t a n d s k a t e s o n both s i d e s o f the coun t ry a re t a k e n by t rawlers , w h i l e dogf i sh in the nor thwes t c o a s t a r e a p p a r e n t l y h a r v e s t e d with gi l lnets a n d t rawl nets . A l t h o u g h both rays a n d dogf i sh a re l ow p r i c e d r e s o u r c e s w h e n c o m p a r e d with s o m e o ther e l a s m o b r a n c h s (i.e. m a k o o r t h r e s h e r s h a r k s ) they a re a v a i l a b l e in s u c h la rge quant i t ies that they b e c o m e profitable for f i sh ing en te rp r i se s . T h e dogf i sh a n d ray r e s o u r c e s o f the U S A a p p e a r to be wi thout spec i f i c m a n a g e m e n t r e g i m e s . In the bes t c a s e s , s o m e s t o c k s a re i n c l u d e d in g e n e r a l m a n a g e m e n t s c h e m e s for g r o u n d fish r e s o u r c e s . G r u l i c h a n d D u P a u l (1987) es t imate that the s p i n y dogf i sh s t o c k s of the U S A eas t c o a s t c o u l d suppor t a harves t of abou t 2 4 , 0 0 0 t/yr in the mid -80 ' s . H o w e v e r , recent s tud ie s s u g g e s t that the b i o m a s s o f the Squalus acanthias s t ock s u s t a i n i n g mos t o f this f ishery, a l t h o u g h i n c r e a s i n g recent ly , is h igh ly va r i ab l e from y e a r to y e a r ( S i l v a 1993) . T h i s c o u l d m e a n that h igh l eve l s o f explo i ta t ion w o u l d not be s u s t a i n a b l e , p r even t ing s t e a d y s u p p l y to a la rge marke t . T h e E a s t C o a s t . T h r o u g h o u t this cen tury , the s ing le mos t important f i shery for s h a r k s in the E a s t C o a s t of U S A w a s that for la rge s h a r k s o f S a l e r n o F l o r i d a dur ing the p e r i o d 1 9 3 5 - 1 9 5 0 (major a c c o u n t s in S p r i n g e r [ 1 9 5 1 , 1 9 6 0 ] ) . T h e f i shery w a s c e n t r e d o n p roduc t ion o f v i t amin A from sha rk l iver o i l a n d w a s c l o s e d a s a c o n s e q u e n c e of the indus t r ia l s y n t h e s i s o f this v i t amin . F i n s a n d h i d e s w e r e a l s o u t i l i sed . T h e fleet w a s c e n t r e d at S a l e r n o but it u s u a l l y e x t e n d e d 2 5 o p e r a t i o n s w e s t to the M i s s i s s i p p i r iver dur ing s u m m e r , a n d after 1 9 4 5 it e x p a n d e d to i nc lude boa t s in the C a r o l i n a s , the F l o r i d a k e y s a n d the G u l f c o a s t o f F l o r i d a . T h e C a r i b b e a n a n d W e s t Indies a l s o p r o v i d e d c a t c h e s to this f ishery. In the later y e a r s , a p p r o x i m a t e l y half o f the c a t c h e s c a m e from the G u l f of M e x i c o . T h i s f i shery h a d up to 16 boa t s o f 12-15 .5 m ope ra t i ng concur ren t ly , e a c h f i sh ing with two bot tom l o n g l i n e s o f at leas t 2 0 0 h o o k s , in dep ths up to 91 m . F loa t i ng l ong l ines a n d bot tom gi l lnets w e r e a l s o u s e d o c c a s i o n a l l y . S a n d b a r s h a r k s , Carcharhinus plumbeus, c o m p o s e d m o s t o f the c a t c h e s , w h i c h p e a k e d at 10 ,514 s h a r k s in 1 9 4 7 . In recen t t imes , the s e c o n d mos t important e l a s m o b r a n c h f i sher ies in the U S A after dogf i sh a n d rays , a re the g r o w i n g f i sher ies for la rge s h a r k s in the G u l f o f M e x i c o a n d S o u t h At lan t i c . W h i l e c a t c h e s of la rge s h a r k s h a v e r e m a i n e d prac t ica l ly u n c h a n g e d in the M i d - A t l a n t i c a n d N e w E n g l a n d r eg ions , s h a r k c a t c h e s in the G u l f o f M e x i c o a n d S o u t h A t l a n t i c r eg ions u n d e r w e n t r ad i ca l c h a n g e s , wi th a n e ightfold i n c r e a s e in y i e l d from 1 9 8 4 to 1 9 8 9 (fig. 2.4) . T h i s t rend , c a u s e d ma in ly by the d e v e l o p m e n t o f a s tab le marke t , b e g a n in 1 9 8 5 w h e n f i s h e r m e n s tar ted to target s h a r k s with gi l lnets a n d l ong l ine s . T h e l a n d i n g o f p r e v i o u s l y d i s c a r d e d s h a r k b y c a t c h e s from o ther f i sher ies a l s o b e c a m e at t ract ive. A c c o r d i n g to N O A A (1991) , d i rec ted f i sher ies for s h a r k s in the ea s t c o a s t i nc lude : a monof i l amen t 18-64 c m m e s h driftnet f i shery appa ren t l y d i r ec t ed to s c h o o l i n g b lack t ip s h a r k s (Carcharhinus limbatus) in F lo r ida ; a M a y - N o v e m b e r gil lnet f i shery in the eas t c o a s t of F l o r i d a c a t c h i n g mos t ly g ray s h a r k s Carcharhinus spp . ; a driftnet f i shery for tunas , bi l l f ishes a n d s h a r k s in the At lan t i c , G u l f o f M e x i c o a n d C a r i b b e a n ; p e l a g i c l o n g l i n e s for tunas , bi l l f ishes a n d s h a r k s in the At lan t i c , C a r i b b e a n a n d G u l f o f M e x i c o (this f i shery d e p l o y s g e a r in a m e c h a n i s e d ope ra t i on invo lv ing large v e s s e l s a n d t h o u s a n d s o f h o o k s ) ; a recent f i shery for s h a r k s wi th bot tom long l i ne s se t s m a n u a l l y up to 100 h o o k s from e a c h s m a l l boat; a n d a p e l a g i c h o o k a n d l ine f i shery for tunas , bi l l f ishes a n d s h a r k s in the G u l f o f M a i n e , S o u t h N e w E n g l a n d a n d the M i d At lan t i c . L a w l o r a n d C o o k (1987) report that the s e a s o n a l E a s t F l o r i d a long l ine f i shery for s h a r k s is c a r r i ed out f rom boa t s 11-15 .5 m long wi th 2-4 f i s h e r m e n u s i n g bot tom a n d / o r sur face long l ines in 1-2 d a y s ope ra t i ons . T h e long l ine ' s ma in l i ne v a r i e s from 1.6 to 10 k m in length a n d is m a d e o f 4 .8-6 .4 ha rd - lay ta r red ny lon , f rom w h i c h 3 0 0 - 5 0 0 g a n g i o n s o f 3.6 m long 26 mul t i s t rand s t ee l c a b l e fal l , a r m e d with a 3/0 o r 3.5/0 s h a r k h o o k e a c h . B u o y s a re a t t ached to the m a i n l i n e o n 2 8 - 3 0 m l e a d e r s for bot tom long l i ne s a n d for p e l a g i c l o n g l i n e s with 10-30 m l e ade r s . B l u e f i s h , boni to , m a c k e r e l , mul le t a n d s q u i d a re the m o s t c o m m o n bait. A p p a r e n t l y , abou t 110 boa t s w o r k full-time a n d y e a r - r o u n d in this f i shery fo l l owing migra t ing s h a r k s a l o n g the coas t . A d d i t i o n a l informat ion ( N O A A 1991) i nd i ca t e s that 124 v e s s e l s target s h a r k s in the U S A eas t coas t , wi th long l ine r c a t c h e s du r ing 1 9 8 9 a d d i n g up to 6 ,140 t w h i l e gi l lnet ters c a u g h t 621 t. S o m e s h a r k s in the eas t c o a s t o f U S A are a l s o l a n d e d a s b y c a t c h in the fo l lowing f i sher ies : the G u l f o f M e x i c o t u n a f i sher ies ; the G u l f o f M e x i c o a n d sou th A t l a n t i c c o a s t s n a p p e r -g r o u p e r b o t t o m long l ine f ishery; swordf i sh gil lnet f i shery o f M a s s a c h u s e t t s a n d R h o d e Is land (up to 15 v e s s e l s ) a n d the gil lnet f i sher ies o f M a i n e , V i r g i n i a , N e w Y o r k a n d N e w J e r s e y . A c c o r d i n g to the repor ts o f Hoff (1990) a n d N O A A (1991) , the m a i n s p e c i e s c a u g h t in the S o u t h A t l an t i c a n d G u l f of M e x i c o with gi l lnets a re Carcharhinus plumbeus, C. limbatus, C. leucas, C. altimus, C. brevipinna, Galeocerdo cuvieri, Carcharias taurus, Negaprion brevirostris, Sphyrna lewini a n d S. mokarran. T h o s e c a p t u r e d wi th l o n g l i n e s a re ma in ly C . plumbeus, C. limbatus, C. isodon, C. acronotus, C. leucas, C. brevipinna, C. obscurus, Rhizoprionodon terraenovae, Carcharias taurus a n d Sphyrna lewini (a g l o s s a r y of e n g l i s h a n d latin s p e c i e s n a m e s is g i v e n in a p p e n d i x 1). R u s s e l l (1993) repor ts C . limbatus, Mustelus canis a n d Rhizoprionodon terraenovae a s the mos t c o m m o n s p e c i e s in sha rk long l ine r s in the nor thern G u l f o f M e x i c o . D a t a from N O A A (1991) s h o w s that e x - v e s s e l p r i ces for s h a r k s in the G u l f o f M e x i c o a n d sou theas t U S A a l m o s t d o u b l e d from a n a v e r a g e pr ice in cons t an t U S do l l a r s o f $ 0 . 5 7 / k g in 1 9 7 9 to $ 1 . 1 2 / k g in 1 9 8 6 , the a v e r a g e s i n c e 1983 b e i n g a p p r o x i m a t e l y $ 1.00/kg. M e a n w h i l e , the p r i ces for f ins h a v e r i sen nea r ly a n o r d e r of m a g n i t u d e s i n c e 1 9 8 5 . In g e n e r a l , h i g h e r p r i ces a re p a i d for d r e s s e d c a r c a s s e s a n d a l s o for s h a r k s f i shed in w a t e r s m o r e than 3 mi l e s from the c o a s t a s o p p o s e d to t hose caugh t ins ide the 3-mi le S ta te wa t e r s limit. O n l y the m a k o s h a r k a t ta ins a h i g h e r pr ice than the rest of the s p e c i e s w h i c h a re t rea ted a s "unc lass i f i ed shark". Hoff (1990) s t r e s s e s that important b y c a t c h e s o f s e v e r a l s p e c i e s o f s h a r k s a re t a k e n regular ly by the s h r i m p t rawl ing f i sher ies o f the north G u l f o f M e x i c o . Unfor tunate ly , mos t o f the c a t c h e s a re d i s c a r d e d b e c a u s e o f a l ack o f marke t for t h e m ( G M F M C 1980) . T h e on ly p u b l i s h e d e s t i m a t e s ( N O A A 1991) indica te that the inc iden ta l c a t c h o f s h a r k s in the G u l f o f 2 7 M e x i c o s h r i m p f i shery is o f abou t 2 , 8 0 0 t/yr. M o s t i nd iv idua l s a re j u v e n i l e s c a u g h t in nur se ry a r e a s a n d this kill might r ep resen t a n important threat for recru i tment to the b r e e d i n g s t o c k s in future y e a r s . E s c a p e m e n t o f l a rger s p e c i m e n s wi l l p r o b a b l y i n c r e a s e if the regu la t ions for the m a n d a t o r y u s e o f turtle e x c l u d e r d e v i c e s ( T E D ) are a p p r o v e d . O v e r a l l , total yea r ly d i s c a r d s o f s h a r k s in a l l f i sher ies o f the eas t c o a s t of U S A a v e r a g e d 1 6 , 0 0 0 1 ( N O A A 1 9 9 1 ) . T h e great i n c r e a s e in s h a r k explo i ta t ion both by c o m m e r c i a l a n d r ec rea t iona l f i s h e r m e n in the eas t c o a s t of the U S A l ed to the e s t a b l i s h m e n t of a m a n a g e m e n t r e g i m e b a s e d o n c a t c h quo t a s a n d b a g l imits s i n c e apr i l 1 9 9 3 . T h i s m a n a g e m e n t r e g i m e t ook a n ou t s t and ing 10+ y e a r s for i m p l e m e n t a t i o n d u e to, a m o n g o ther th ings , l a ck o f app rop r i a t e da ta o n a b u n d a n c e , b io logy , d is t r ibut ion, life his tory a n d c a t c h e s o f s h a r k s , n e e d e d for s tock a s s e s s m e n t . G i v e n c o n c e r n s abou t p o s s i b l e ove rexp lo i t a t ion of s h a r k s t o c k s du r ing the late 80 ' s , a n a s s e s s m e n t w a s pe r fo rmed with the v e r y l imited a v a i l a b l e in format ion . T h e s h a r k F i s h e r i e s M a n a g e m e n t P l a n d i v i d e s sha rk s p e c i e s in three m a n a g e m e n t units a c c o r d i n g to their habitat , a s s h o w n in tab le 2 .3 . T h e e s t ima ted l eve l s o f M S Y , w h i c h a p p e a r rather s m a l l w h e n c o m p a r e d wi th c a t c h e s in n e i g h b o u r i n g M e x i c o , a re a b o u t 3 ,400 t for la rge c o a s t a l s h a r k s a n d a b o u t 3 ,600 t for s m a l l c o a s t a l s h a r k s ( P a r r a c k 1 9 9 0 , N O A A 1991) . A n u m b e r of m a n a g e m e n t m e a s u r e s in effect s i n c e A p r i l 1 9 9 3 inc lude : 1 9 9 3 c o m m e r c i a l quo t a s (in d r e s s e d we igh t s ) o f 2 , 4 3 6 t for la rge c o a s t a l s h a r k s a n d 5 8 0 t for p e l a g i c s p e c i e s ; r ec rea t iona l b a g l imits of four sha rks /vesse l / t r ip for la rge c o a s t a l a n d p e l a g i c s h a r k s c o m b i n e d , a n d five s h a r k s / p e r s o n / d a y for s m a l l c o a s t a l s p e c i e s ; c o m m e r c i a l f i sh ing on ly by permit; fins l a n d e d in propor t ion to c a r c a s s e s ; r e l e a s e of s h a r k b y c a t c h e s e n s u r i n g m a x i m u m probabi l i ty of su rv iva l ; c o m p u l s o r y s u b m i s s i o n o f s a l e s r ece ip t s a n d l o g b o o k s from s e l e c t e d c o m m e r c i a l a n d rec rea t iona l opera tors ; a c c o m m o d a t i o n o f o b s e r v e r s in s e l e c t e d c o m m e r c i a l boa ts ; a n d b a n n i n g of s h a r k c a t c h e s for fore ign v e s s e l s in U S A wa te r s ( N M F S 1993) . T h e W e s t C o a s t . Ho l t s (1988) a n d Ca i l l i e t et a l . (1993) r ev i ew the s h a r k f i she r i es o f the w e s t c o a s t of the U S A . A s i d e from the s p i n y dogf i sh f i sher ies w h i c h d o m i n a t e the c a t c h e s o f the w e s t coas t , a n important g r o u p o f d i rec ted f i sher ies for s h a r k s s u d d e n l y ro se in C a l i f o r n i a at the e n d of the 70 ' s . H o w e v e r , s o m e o f t he se f i sher ies d e c l i n e d wi th in the fo l l owing ten y e a r s . T h e s e 28 Table 2.3 Sha rks spec ies cons idered in each of the U S A east coas t management unit (from N O A A 1991). F A O C o m m o n Name Scientif ic Name Large Coasta l Sharks Small Coasta l Sharks Pelagic Sharks Sandbar Blacktip Dusky Spinner Silky Bull B ignose Coppe r Ga lapagos Night Car ibbean reef Tiger Lemon Sandtiger Bigeye sand tiger Nurse Sca l loped hammerhead Great hammerhead Smooth- hammerhead Whale Basking Great White Atlantic sharpnose Car ibbean sharpnose Finetooth B lacknose Smalltail Bonnethead Sand devil Shortfin mako Longfin mako Porbeagle Thresher Bigeye thresher Blue Ocean ic whitetip Sharpnose sevengil l Bluntnose sixgill B igeye sixgill Carcharhinus plumbeus Carcharhinus limbatus Carcharhinus obscurus Carcharhinus brevipinna Carcharhinus falciformis Carcharhinus leucas Carcharhinus altimus Carcharhinus brachyurus Carcharhinus galapagensis Carcharhinus signatus Carcharhinus perezi Galeocerdo cuvier Negaprion brevirostris Carcharias taurus Odontaspis noronhai Ginglymostoma cirratum Sphyrna lewini Sphyrna mokarran Sphyrna zygaena Rhincodon typus Cetorhinus maximus Carcharodon carcharias Rhizoprionodon terraenovae Rhizoprionodon porosus Carcharhinus isodon Carcharhinus acronotus Carcharhinus porosus Sphyrna tiburo Squatina dumeril Isurus oxyrinchus Isurus paucus Lamna nasus Alopias vulpinus Alopias superciliosus Prionace glauca Carcharhinus longimanus Heptranchias perlo Hexanchus griseus Hexanchus vitulus 2 9 f i sher ies o r ig ina t ed m a i n l y a s a r e s p o n s e to c h a n g e s o f t r ends in c o n s u m e r p re fe rence w h i c h i n c r e a s e d d e m a n d a n d s p a r k e d a s o a r in p r i ce s for s o m e s p e c i e s . T o t a l c a t c h e s ( exc lud ing dogf ish) i n c r e a s e d th rough the late 70 ' s to a p e a k o f a b o u t 1,800 t in 1 9 8 2 but h a v e s i n c e v a r i e d wi th a d e c r e a s i n g t rend (table 2 .4) . Ca i l l i e t et a l . (1993) c o n s i d e r marke t f luctuat ions a n d suscep t ib i l i ty to overexp lo i t a t ion o f s o m e s t o c k s a s the m a i n r e a s o n s for d i m i n i s h i n g c a t c h e s . T h e first f i shery to r ise w a s that for the c o m m o n th re she r (Alopias vulpinus). T h i s w a s cen t r ed b e t w e e n S a n D i e g o a n d C a p e M e n d o c i n o , a n d ini t iated o p e r a t i o n s with 15 l a rge-m e s h driftnet v e s s e l s in 1 9 7 7 . E x - v e s s e l p r i ce s for this s p e c i e s i n c r e a s e d from U S $ 0 . 6 4 / k g in 1977 to U S $ 3 . 5 2 / k g in 1 9 8 6 . S o o n , the t h r e she r s h a r k f i shery w a s d i s p l a c e d by the m o r e v a l u a b l e sword f i sh f i shery a n d the t h r e she r sha rk b e c a m e a s e c o n d a r y target. T h i s l e a d to pol i t ical u p h e a v a l a n d s p u r i o u s m a n a g e m e n t o f the f i shery a n d r e su l t ed in the l o s s o f the t h r e she r p o p u l a t i o n s ( see B e d f o r d 1987 for a de t a i l ed a c c o u n t ) . C a t c h e s p e a k e d in 1 9 8 2 at 1 ,0831 w h e n m o r e than 2 0 0 v e s s e l s w e r e ope ra t ing , but s l o w l y d e c l i n e d thereafter. D u r i n g 1986 , a l imi ted a r e a a n d s e a s o n leg i s la t ion w a s p a s s e d , c a u s i n g a further d e c l i n e in c a t c h until the d i r ec t ed f i shery for this s p e c i e s w a s o u t l a w e d in O c t o b e r 1 9 9 0 . A t present , o n l y inc iden ta l c a t c h e s in the swordf i sh f i shery are permi t ted , a n d they a c c o u n t for a l m o s t 3 0 0 t/yr (D . Ho l t s , N M F S S o u t h w e s t F i s h e r i e s C e n t r e , pers . c o m m . , J u l y 2 2 , 1991) . T h r o u g h o u t mos t o f the f i shery c a t c h e s w e r e c o m p o s e d ma in ly o f y o u n g s h a r k s 1-2 y e a r s o l d , but a l s o i n c l u d e d a few A. superciliosus a n d A. pelagicus. B e d f o r d (1987) repor ts that marke t s a m p l i n g da t a s h o w e d d e c r e a s i n g m o d a l s i z e s th rough t ime a l o n g wi th d r o p p i n g C P U E i n d i c e s s i n c e the mid -80 ' s . U n p u b l i s h e d da t a (Hol ts , pers . c o m m . o p . cit.) s h o w s the m e a n length o f c a t c h e s c l ea r ly d e c l i n e d dur ing the s a m e p e r i o d . A n o t h e r important recen t d e v e l o p m e n t in the w e s t c o a s t w a s the f i shery for P a c i f i c a n g e l sha rk (Squatina californica). T h i s b e g a n a s a v e r y l o c a l i s e d ope r a t i on in S a n t a B a r b a r a in 1977 (166 kg l anded) , u n d e r w e n t great e x p a n s i o n in 1981 (158 t l a n d e d ) , r e a c h e d a p e a k in 1986 ( 5 6 3 1 l anded) a n d suffered a s t eady fall in the fo l lowing three y e a r s (121 t in 1989) (table 2.4; Ca i l l i e t et a l . 1993) . E x - v e s s e l p r i ces c l i m b e d from U S $ 0 . 3 3 / k g in 1978 to U S $ 0 . 9 9 / k g in 1984 (Hol ts 1988) . P a c i f i c a n g e l s h a r k s w e r e t a k e n initially a s b y c a t c h of the Pac i f i c hal ibut f i shery with bot tom set t r a m m e l nets . A s m a r k e t s a n d d e m a n d e x p a n d e d , they b e g a n to be t a rge ted with s i n g l e - w a l l e d gi l lnets m a d e o f ny lon twine ( N o . 2 4 to N o . 3 0 ) 30 CO cn CD CD -*—' CD -*—' CD CO o E p T3 CD •s. CO T J 3, < co Z) -•—« </) CO o o -I—» (/> </> 0) D) c c CO CO _c CO CN (D _Q CO LU < T - O C O O L O C O - T - C O O ^ N W I O T - O O CM" S-" -r-" <D h-" CO" CM" © in O) N S S CM T -CT) W r- i-OStMlO^T-CDCOS^ T-TtOT-inoo)TfOW Tt O CO" CM CD co" co" m" h-" O CD CM CM CO i-rj- CM CM CM S N CO T -LO CO CO CD T -LO i-OOJNOO'-Wt-(D O) If) i- N CM O ) CO i-co OWlf)Si-'tCOCO^OOTfOrN(OCOmoW r^ CNi^ Lor^ -h-cocococvj CM ^ s <o r CM •<-mtocococoh-i-Ti-coco coo T - T - tn LO* oi co" N-" co* in* T -" in i-" o" G ) CM O CNJ T— CO CO CO Tf CM T-co" -i-incococM^mcocMh-o ( D c o s m i -o icoton ' tn co_ o in CM co cn T - to CM O ) i- CO r-» O ) i-co in co o — co m m CM CM" cn cn co in cn CM CO T - O CM CJ) CM T * V- T - O O O O CO CM CM r O (O N CM CO CO CO CO CM N O O) CO tO CM" T - " r-" o~ T - CJ) CD CJ) T— m CD m coco^o-i-mo>T-coi-cvjcoiotoojo) coco-i-r-coi-cMCM lO ^ if) O) CM O CO o)-«tcococj)a)-r- inco-i-cocooocomoT-Tfoco T - CO •<-TI- TI- o (OCOOi-'tCO^^CM o n T - w s ^ o i m o o" cj)" CD" of co" in co" co" io cn m s "3- -3-r-- -i-lOOr-ir)COCOCM(DO)N 0)T-lf)CDNlf)COCOCO't C M i n c o m o c M O T f c o c o O CO CJ) CO i— ^ J -CM CO IT) CO N O CM CM co r- m T -o CO Tl- CO o o i-CM" T - " WCD^ NCDCDOCOCOCO cnr-LOCMcoT-T-cococM 0)(OOCMCOrfNNO CO" CT)" co" m" co" CM CM" o" 0) r- i- CM i- CJ) CM T-T- CO i- i- 1-iTtlDONOWOOOO) O C O C J ) O O O O O T J - C T ) C O T-inocoojr^ o>CMin COCMinCMCMCMOCMCM CMCDO)LOCNJCMi-0)CO CM" CJ) cn" o" r--" K co" TI-" CM" omco«tfor*-oo^rmoi-N CO CO T -co CM CO CO CM LO i- CO CO CD CO TJ- i- CM Tf CO O ) r O S r M T-" co" co" to" O " CO" CM" TJ- co in •<- o co i-co CO T-CO CO CJ) CT) o o co CD in co r T - T f If) O T-" m" CT)" T -" O O CT) O rf oooTj-moTi-oococoocDOr^in Tl- T- Tf Tf CT) CO CO o co ro cj) Tt CM in o o co co i-CT)" co* TI-" o" S t T -O O T I - O L O O O O O L O O O C M L O TI- co co cn m -c o _ o O) ! i D. O tn o 01^1^0^^000(00000 O OO CVJ CVJ CVJ CO CVJ CO Tf" CD 2 •£ = f II o £ E ^ o E S O ffl o 0) fu I £ = ? o o 3 O Ol O .= o = 5 .i? [ o m J j m i i K / j o x o i d i U u i i a o i Q D o E 31 3 6 6 - 5 4 9 m long a n d 13 m e s h e s d e e p ( m e s h s i z e s b e t w e e n 3 0 . 5 a n d 4 0 . 6 c m ) ( R i c h a r d s 1987) . V e s s e l s w e r e u s u a l l y t hose of the hal ibut f ishery, w h i c h u s e h y d r a u l i c g e a r re t r ievers . O p e r a t i o n s w e r e c e n t r e d in the S a n t a B a r b a r a - V e n t u r a r eg ion a n d the C h a n n e l I s lands in wa t e r s l e s s t han 2 0 m d e e p , no m o r e than 1.6 k m from s h o r e . In the o p i n i o n o f Ca i l l i e t et a l . (1993) , the d rop in c a t c h e s s i n c e 1986 is d u e to a c o m b i n a t i o n o f d e c l i n i n g ava i lab i l i ty o f the s p e c i e s a n d c h a n g e s in the marke t a s c h e a p e r impor t s o f s h a r k mea t b e c a m e a v a i l a b l e . T h e o n l y r egu la t ions a p p l i e d to th is f i shery a re still t h o s e w h i c h per ta in to the set-net f i shery for hal ibut in C a l i f o r n i a , neg l ec t i ng the n e e d for s e p a r a t e m a n a g e m e n t o f this e l a s m o b r a n c h r e s o u r c e . A shortfin m a k o (Isurus oxyrinchus) f i shery a l s o s tar ted in C a l i f o r n i a a s a v a l u a b l e b y c a t c h of the driftnet f i shery for swordf i sh a n d c o m m o n th re she r s h a r k o f the late 70 ' s . C a t c h e s i n c r e a s e d s t ead i ly from 1 9 7 7 th rough 1 9 8 2 w h e n they r e a c h e d 2 3 9 t, t hen u n d e r w e n t a pe r iod of l o w e r c a t c h e s p o s s i b l y at t r ibuted to c h a n g e s in f i sh ing s t ra tegy o r e n v i r o n m e n t a l cond i t i ons (Hol t s 1988) , but p e a k e d a g a i n in 1987 at 2 7 7 t. S i n c e then , c a t c h e s h a v e d e c l i n e d o n c e m o r e (table 2 .4) . T h e b y c a t c h of m a k o s in the driftnet f i shery is l ow a n d s i n c e 1988 a c l o s e l y con t ro l l ed e x p e r i m e n t a l f i shery w a s s tar ted wi th l o n g l i n e s target ing this s p e c i e s . U n d e r t h i s r eg ime , s ix v e s s e l s u s i n g 4 .8-8 .2 k m s t a in l e s s s t ee l c a b l e l ong l i ne s n e a r the sur face , a re a l l o w e d to fish in t ime /a r ea c l o s u r e s a w a y from spor t f i sh ing g r o u n d s . A d d i t i o n a l l y , a n 8 0 1 T A C h a s b e e n e s t a b l i s h e d a n d a marke t for the subs t an t i a l b lue sha rk b y c a t c h mus t be d e v e l o p e d to uti l ise this r e s o u r c e . B y c a t c h e s o f shortfin m a k o in the driftnet f i shery a re a l s o a l l o w e d . A l t h o u g h the shortfin m a k o f ishery is m a i n l y s u s t a i n e d by ve ry y o u n g s h a r k s a v e r a g i n g 9-14 kg d r e s s e d weight , there is no a p p a r e n t d e c l i n e in the m e a n s i z e o f the c a t c h e s , popu la t i ons look hea l thy a n d e v e n might be re la t ive ly u n e x p l o i t e d (Holts 1988 , Ca i l l i e t et a l . 1993) . In add i t ion to the three f i sher ies m e n t i o n e d a b o v e w h i c h const i tu te the m a i n "new" sha rk f i sher ies in the last 15 y e a r s o n the w e s t coas t , m a n y o ther e l a s m o b r a n c h s a re a l s o t a k e n c o m m e r c i a l l y , m a i n l y a s a b y c a t c h o f o ther f i sher ies . Mar t in a n d Z o r z i (1993) r e v i e w the ska te f i sher ies o f C a l i f o r n i a . S k a t e s (main ly Raja binoculata, R. inornata a n d R. rhina) h a v e b e e n f i shed in C a l i f o r n i a s i n c e at leas t 1916 , a v e r a g i n g 96 t a n d 1 1 . 8 % of the total c o m m e r c i a l e l a s m o b r a n c h c a t c h e s in C a l i f o r n i a pe r a n n u m . S a n F r a n c i s c o a n d M o n t e r e y a re the m a i n l a n d i n g ports m a k i n g o v e r 7 0 % of the total . T e c h n i c a l cons t ra in t s in the 32 p r o c e s s i n g limit m a r k e t a b l e s k a t e s to s i z e s o f up to 1 kg , therefore m o s t o f the l a n d i n g s o f R. binoculata a n d R. rhina a re c o m p o s e d of immatu re i nd iv idua l s . R o e d e l a n d R i p l e y (1950) s u g g e s t e d that the ska t e r e s o u r c e might be underu t i l i s ed , but it a l s o s e e m s to be p resen t ly mi s su t i l i s ed . A marke t for l a rger s k a t e s s h o u l d be d e v e l o p e d in o r d e r to o p t i m i s e u s e a n d m a n a g e m e n t o f t he se r e s o u r c e s . A n o t h e r s p e c i e s o f interest is the b lue s h a r k (Prionace glauca). Ho l t s (1988) a n d Ca i l l i e t et a l . (1993) s u m m a r i s e the a v a i l a b l e informat ion . T h e b lue s h a r k is a major inc iden ta l c a t c h of the driftnet f i shery o f C a l i f o r n i a a n d a m i n o r b y c a t c h o f the set-net f i she r i es for hal ibut a n d a n g e l s h a r k s . Morta l i ty e s t ima te s fo r the driftnet f i shery w e r e o f 1 5 , 0 0 0 - 2 0 , 0 0 0 (3001) s h a r k s a n n u a l l y in the ea r ly pe r iod , a l t hough c h a n g e s in g e a r d e s i g n h a v e a c c o u n t e d for r educ t ions in this mortal i ty. T h e e x p e r i m e n t a l long l ine f i shery for m a k o s h a r k s a l s o t a k e s inc iden ta l c a t c h e s o f b lue s h a r k s at a rate o f four b lue s h a r k s for e a c h m a k o . N e v e r t h e l e s s , the en fo rcemen t o f r ap id r e l e a s e o f l ive s h a r k s is e x p e c t e d to d e c r e a s e mortal i ty. A s m a l l e x p e r i m e n t a l l ong l ine f i shery with o n e v e s s e l took p l a c e du r ing 1 9 8 0 - 1 9 8 2 a n d c a t c h e s of b lue s h a r k s p e a k e d a r o u n d 90 t in 1 9 8 0 a n d 1981 (table 2 .4) . T h e m a i n cons t ra in t for the d e v e l o p m e n t o f a la rge s c a l e f i shery for b lue s h a r k s is the l ack o f a marke t . B l u e sha rk mea t is repor ted ly l e s s pa la t ab le than that o f o ther e l a s m o b r a n c h s . A t t e m p t s to initiate a f i shery for s a l m o n s h a r k s Lamna ditropis in A l a s k a n wa t e r s w a s repor ted by P a u s t (1987) but no further r e c o r d s of this w e r e found . T h e s i ng l e mos t important f i shery for e l a s m o b r a n c h s in the h is tory o f the w e s t c o a s t w a s the C a l i f o r n i a f i shery for soupf in s h a r k Galeorhinus galeus du r ing the 1 9 3 0 ' s - 1 9 4 0 ' s . R i p l e y (1946) g i v e s a de t a i l ed desc r ip t ion of this f i shery . S t i m u l a t e d by the d i s c o v e r y in 1937 that the soupf in s h a r k s o f that a r e a w e r e the r iches t s o u r c e o f h igh p o t e n c y v i t amin A in the w o r l d , the s u b s e q u e n t four y e a r s m a r k e d a t r e m e n d o u s i n c r e a s e in c a t c h e s w h i c h r e a c h e d o v e r e ight t i m e s t hose o f p r e - b o o m leve l s a n d a v e r a g e d a p p r o x . 3 ,400 t/yr. V e s s e l s from the nor thern hal ibut f i shery s w i t c h e d to sha rk f i sh ing a n d in a v e r y short p e r i o d a l l sorts of v e s s e l s mod i f i ed their ope ra t i ons a n d j o i n e d the f i shery to ta l l ing abou t 6 0 0 boa t s by 1 9 3 9 . Swift c h a n g e s in g e a r s from drift a n d set g i l lnets to m a c h i n e - h a n d l e d hal ibut l ong l ines a n d b a c k to "diver" g i l lnets a n d the pos te r io r m e c h a n i s a t i o n o f thei r o p e r a t i o n o c c u r r e d in a p e r i o d o f l e s s t han 3 y e a r s (de ta i led desc r ip t ion of g e a r in R o e d e l a n d R i p l e y 1950) . Nor the rn C a l i f o r n i a w a s the m a i n f i sh ing a r e a with m o r e than 7 0 % of the c a t c h e s , a l t hough 33 f ishing o c c u r r e d th roughou t the entire c o a s t mos t ly wi th in 7.8 k m from s h o r e in wa t e r s up to 144 m d e e p . S i n c e 1 9 4 1 , c a t c h e s p l u m m e t e d a n d n e v e r r e c o v e r e d the i r p e a k l eve l s . T h e d i s c o v e r y o f syn the t i c v i t amin A p r e v e n t e d efforts to rev ive this f i shery to its fo rmer glory, a l t hough a s m a l l f i shery h a s r e m a i n e d up to p resen t t imes . C a t c h e s s i n c e 1 9 7 6 f luctuated b e t w e e n 6 6 a n d 2 5 3 t/yr (table 2 .4) . Ac t iv i t i e s a re n o w c e n t r e d a r o u n d S a n D i e g o a n d O r a n g e c o u n t i e s (Hol ts 1988) w h e r e c a t c h e s are appa ren t l y a n inc iden ta l p roduct o f net f i sher ies for hal ibut a n d a n g e l shark . O n l y the g e n e r a l r egu la t ions for the latter f i sher ies "protect" soupf in s h a r k popu la t i ons . H o l d e n (1977) e s t i m a t e d the north P a c i f i c unexp lo i t ed s tock at 2 9 , 4 0 0 t, but it a p p e a r s that s t o c k s h a v e not yet r e c o v e r e d to the fo rmer l eve l s (Hol ts 1988) . H o w e v e r , no recent a s s e s s m e n t s h a v e b e e n d o n e for this s p e c i e s . F ina l ly , a short l ived s m a l l - s c a l e h a r p o o n f i shery for b a s k i n g s h a r k s (Cetorhinus maximus) took p l a c e dur ing the late 40 ' s in P i s m o B e a c h ( R o e d e l a n d R i p l e y 1950) but p e r i s h e d a l s o a s a c o n s e q u e n c e o f the fall of the l iver o i l industry. M e x i c o . S i n c e the mid -70 ' s , M e x i c a n e l a s m o b r a n c h f i sher ies h a v e b e e n the la rges t in A m e r i c a (fig. 2 .2) . A c c o r d i n g to F A O stat is t ics , there h a s b e e n a g e n e r a l t r end o f i n c r e a s e d c a t c h e s of e l a s m o b r a n c h s in M e x i c o , from the typ ica l 5 ,000 t/yr o f the 50 ' s to the recent y i e l d s v a r y i n g a r o u n d 3 0 , 0 0 0 t/yr s i n c e the ea r ly 80 ' s . J u d g i n g from the t r end o f the last ten y e a r s , M e x i c a n f i sher ies for s h a r k s a n d rays h a v e a t ta ined relat ive stabil i ty. E l a s m o b r a n c h s a re a re la t ively important r e s o u r c e in M e x i c o , m a k i n g 2 .36 % of the total na t iona l c a t c h e s dur ing 1 9 8 7 - 1 9 9 1 . T h i s is c o m p a r a b l e to o ther major e l a s m o b r a n c h - f i s h i n g coun t r i es , but is subs tan t ia l ly h ighe r than the 0.8 % cont r ibu t ion o f e l a s m o b r a n c h s to w o r l d f i sher ies in the last 10 y e a r s . E l a s m o b r a n c h explo i ta t ion in M e x i c o c a n be t r a c e d b a c k to at leas t the 1930 ' s , but de t a i l ed stat is t ics a re difficult to f ind before the mid -70 ' s . W a l f o r d (1935) repor ts " seve ra l tons" of sha rk fins from the w e s t c o a s t of M e x i c o b e i n g impor ted to C a l i f o r n i a e a c h y e a r a n d R i p l e y (1946) refers to M e x i c a n f i sher ies s u p p l y i n g s h a r k l iver o i l to the U S A indust ry . M a z a t l a n a n d G u a y m a s w e r e the m a i n l and ing points in the w e s t coas t . C a t c h e s p e a k e d at 9 ,000 t in 1944 but d e c l i n e d to 4 8 0 t in 1 9 5 3 , after the fall o f the s h a r k l iver o i l indust ry (Cas t i l lo 34 1990) . In the ea s t c o a s t du r ing the 40 ' s , a fleet ta rge t ing s h a r k s b a s e d at P r o g r e s o , Y u c a t a n h a d cha rac t e r i s t i c s s imi l a r to the fleet o f S a l e r n o , F l o r i d a , a n d c a u g h t up to 3 ,200 t/yr s i n c e 1950 ( G M F M C 1980) . M e x i c a n f i sher ies for e l a s m o b r a n c h s are c e n t r e d o n s h a r k s . B a t o i d s a r e s e l d o m exp lo i t ed but c o n s i d e r a b l e ( and u n k n o w n ) a m o u n t s a re d i s c a r d e d in the e x t e n s i v e t r awl ing ope ra t i ons for s h r i m p f i she r ies . A c c o r d i n g to da t a t a k e n from the M e x i c a n Min i s t ry o f F i s h e r i e s y e a r b o o k s for the p e r i o d 1 9 7 7 - 1 9 9 1 , s h a r k s a c c o u n t for 94 .8 % (29 ,036 t/yr) of e l a s m o b r a n c h c a t c h e s w h i l e ba to ids o n l y represen t 4 .2 % (1 ,272 t/yr). B e c a u s e o f its l a rge r sho re l i ne , the P a c i f i c c o a s t con t r ibu tes 6 0 % o f total s h a r k c a t c h e s w h i l e the r e m a i n i n g 4 0 % c o m e s from the G u l f o f M e x i c o a n d C a r i b b e a n . N o da t a o n c a t c h e s by s p e c i e s a re a v a i l a b l e . O n l y s m a l l s h a r k s ( those m e a s u r i n g l e s s t han 1.5 m T L w h e n c a u g h t a n d k n o w loca l ly a s c a z o n ) a n d large s h a r k s ( those l a rge r t han 1.5 m T L ) are r e c o r d e d in the s tat is t ics . L a r g e s h a r k s a re 6 0 % of total s h a r k c a t c h e s a n d 2/3 o f t he se a re c a u g h t in the P a c i f i c w h i l e on ly 1/3 are c a u g h t in the G u l f of M e x i c o a n d C a r i b b e a n . T h e r e m a i n i n g 4 0 % of the total sha rk c a t c h e s are s m a l l s h a r k s , 6 4 % c o m e from the P a c i f i c a n d 36 % from the eas t coas t . T h e r e is s o m e var iabi l i ty in the c a t c h e s o f la rge a n d s m a l l s h a r k s from e a c h coas t , but ove ra l l , M e x i c a n f i sher ies s e e m to h a v e r e a c h e d a n equ i l ib r ium dur ing the last 10 y e a r s (fig. 2 .5) . M e a n w h i l e , ba to id c a t c h e s a re s l o w l y a n d s t ead i ly e x p a n d i n g . M e x i c a n s h a r k f i she r i es a re la rge ly a r t i sana l , m u l t i s p e c i e s , mu l t i gea r f i she r ies . Bonf i l e t a l . (1990) , C a s t i l l o (1990) a n d Bonf i l (in press) s u m m a r i s e mos t o f the a v a i l a b l e informat ion o n e l a s m o b r a n c h f i she r i es in M e x i c o . It is e s t ima ted that a p p r o x i m a t e l y 2/3 o f the sha rk c a t c h is t a k e n by s m a l l - s c a l e f i she r ies . V e s s e l s a re g e n e r a l l y f i b r eg l a s s boa t s 7-9 m long with o u t b o a r d moto r s u s i n g e i ther gi l lnets o r l ong l ines d e p e n d i n g o n the c u s t o m s o f e a c h r eg ion . S o m e v e s s e l s of 14-20 m a re a l s o u s e d w h e r e a s o n l y a few v e s s e l s in e x c e s s o f 2 0 m take part in the f i shery . S ign i f ican t quant i t ies o f s h a r k s a n d r ays a re a l s o t a k e n a s inc iden ta l c a t c h e s o f l a r g e - s c a l e t rawl f i sher ies for s h r i m p or d e m e r s a l f i shes in both s i d e s of the count ry . L a r g e s c a l e f i sher ies for t unas a n d bi l l f ishes in both c o a s t s a l s o cont r ibute to the total c a t c h e s . S h a r k s a n d rays a re t radi t ional ly u s e d for f o o d in M e x i c o , e i ther f resh , f rozen o r m o r e c o m m o n l y , sa l t -d r ied . S h a r k fins a re a n important export , h i d e s a r e a l s o in tens ive ly u t i l i sed a n d m o s t offal is b u r n e d d o w n to fish m e a l . 35 40,000 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Years Pacific, large sh. Gulf/Car; large sh. %%%\ Pacific, small sh. | | Gulf/Car, small sh. H i Both; Batoids Figure 2.5 Elasmobranch catches in the Pacific and Gulf of Mexico/Caribbean coasts of Mexico during 1977-1991 (sh = sharks). (Data from Secretaria de Pesca, Mexico). 36 T h e m a i n f i sh ing g r o u n d s in the P a c i f i c a re c e n t r e d in the G u l f o f C a l i f o r n i a in the north a n d the G u l f of T e h u a n t e p e c in the sou th . H o w e v e r , mos t o f the a v a i l a b l e informat ion abou t t he se f i sher ies c o m e s from the nor thern coas t . Little is k n o w from the s h a r k f i sher ies in the G u l f of T e h u a n t e p e c apar t from the total c a t c h e s . In the nor thern r eg ion , s h a r k s are ma in ly c a u g h t with monof i l amen t l ong l i ne s of 1-2 k m a n d a p p r o x i m a t e l y 3 5 0 h o o k s , a l t hough s m a l l c a t c h e s a re t a k e n with v a r i o u s gi l lnets o f up to 2 k m in length . T h e r e a r e repor ts that s o m e 17 v e s s e l s , 4 4 m l o n g a n d u s i n g long l ines o f up to 2 , 0 0 0 h o o k s t a rge t ed s h a r k s a n d bi l l f ishes in the P a c i f i c c o a s t du r ing 1 9 8 7 . It is u n k n o w n if t h e s e v e s s e l s a re still in ope ra t ion . Hol t s (1988) s ta tes that a s imi l a r n u m b e r o f J a p a n e s e - M e x i c a n joint ven tu re long l ine r s c a u g h t 2 3 4 t/yr o f s h a r k s in B a j a C a l i f o r n i a dur ing 1 9 8 1 - 1 9 8 3 . O n the eas t , f i sh ing g r o u n d s s p a n the entire coa s t l i ne . D u r i n g 1 9 7 6 - 1 9 8 8 , V e r a c r u z a n d C a m p e c h e s h a r e d 58 % of the total s h a r k c a t c h w h i l e T a m a u l i p a s a n d Y u c a t a n m a d e ano the r 30 % . L o n g l i n e s a re preferred in V e r a c r u z a n d T a m a u l i p a s . G i l l n e t s o f 11-40 c m m e s h s i z e a r e the m a i n f i sh ing g e a r in the B a n k o f C a m p e c h e . A d d i t i o n a l l y , there is a subs tan t i a l b y c a t c h of ma in ly j uven i l e s h a r k s in the s e m i - i n d u s t r i a l i s e d long l ine f i sher ies for red g r o u p e r a n d red s n a p p e r of the C a m p e c h e B a n k but no e s t i m a t e s o f the b y c a t c h are a v a i l a b l e . T h e s p e c i e s c a u g h t in the different r eg ions o f the M e x i c a n c o a s t a n d the structure of s u c h c a t c h e s a re o n l y part ial ly k n o w n . M o s t r e s e a r c h h a s b e e n d o n e in the mouth of the G u l f of C a l i f o r n i a in the w e s t c o a s t a n d in the sou the rn S t a t e s o f C a m p e c h e , Y u c a t a n a n d Q u i n t a n a R o o in the eas t coas t . Important l a n d i n g s in o ther a r e a s of both c o a s t s h a v e b e e n ve ry poor ly s t ud i ed . A t leas t 4 4 s p e c i e s of s h a r k s are repor ted in the c o m m e r c i a l c a t c h e s o f M e x i c o . A v a i l a b l e informat ion ind ica te s 12 a s the mos t important for their cont r ibu t ion to the c a t c h e s in the a r e a of L a P a z , B a j a C a l i f o r n i a S u r a n d S i n a l o a , w h e r e a s there a r e 15 m a i n s p e c i e s in the G u l f of M e x i c o a n d C a r i b b e a n (table 2 .5) . M o s t o f the c a t c h e s o f l a rge s h a r k s cons i s t of Carcharhinus s p p , Sphyrna s p p a n d o ther c a r c h a r h i n i d s , w h i l e the s m a l l s h a r k c a t c h e s a re a mixture o f Mustelus s p p . a n d Rhizoprionodon spp . , wi th j u v e n i l e s o f the large s h a r k s s o m e t i m e s con t r ibu t ing a n important part o f the total . F o r the S i n a l o a c o a s t in the cen t ra l Pac i f i c Rhizoprionodon longurio, Sphyrna lewini, Nasolamia velox, Carcharhinus limbatus, C. falciformis, C. leucas a n d Galeocerdo cuvieri, a re the mos t impor tant s p e c i e s . G a l v a n -M a g a n a et a l . (1989) report Mustelus lunulatus, Heterodontus mexicanus a n d Sphyrna lewini Table 2.5 Shark species found in the commercial fisheries of Mexico. GULF OF MEXICO FAMILY SPECIES PACIFIC /CARIBBEAN Hexanchidae 1 Heptranchias perlo X 2 Hexanchus griseus X 3 Hexanchus vitulus X Echinorhinidae 4 Echinorhinus cookei X Squalidae 5 Centrophorus granulosus X 6 Centrophorus uyato X 7 Squalus cubensis X 8 Squalus mitsukurii X Squatinidae 9 Squatina californica X * Heterodontidae 10 Heterodontus mexicanus X * Ginglymostomatidae 11 Ginglymostoma cirratum x X * Rhiniodontidae 12 Rhiniodon typus X X Alopiidae 13 Alopias vulpinus X * 14 Alopias superciliosus X X Lamnidae 15 Isurus oxyrinchus x X Triakidae 16 Mustelus californicus x 17 Mustelus canis X * 18 Mustelus lunulatus X * 19 Mustelus sp. ? X 20 Triakis semifasciata X Carcharhinidae 21 Carcharhinus acronotus X* 22 Carcharhinus altimus X X 23 Carcharhinus brevipinna X * 24 Carcharhinus falciformis X * X * 25 Carcharhinus leucas X * X * 26 Carcharhinus limbatus X * X * 27 Carcharhinus longimanus X 28 Carcharhinus obscurus X X * 29 Carcharhinus perezi X 30 Carcharhinus plumbeus X * 31 Carcharhinus porosus X X 32 Carcharhinus signatus X 33 Galeocerdo cuvieri X * X * 34 Nasolamia velox X * 35 Negaprion acutidens X 36 Negaprion brevirostris X * 37 Prionace glauca X * 38 Rhizoprionodon longurio X * 39 Rhizoprionodon terraenovae X * Sphyrnidae 40 Sphyrna lewini X * X * 41 Sphyrna media X 42 Sphyrna mokarran X X * 43 Sphyrna tiburo X X * 44 Sphyrna zygaena X * Main species in the commercial catches. 38 a s the mos t important s h a r k s in the a r e a o f L a P a z , B . C . E x p e r i m e n t a l c a t c h e s o f long l ine r s in the P a c i f i c c a u g h t ma in ly pre-adul t a n d adul t Alopias vulpinus a n d Carcharhinus limbatus ( V e l e z et a l . 1989) . F o r the eas t coas t , the mos t important s p e c i e s a re Carcharhinus falciformis, C. leucas, C . obscurus, C . plumbeus, C. limbatus, Rhizoprionodon terraenovae, Sphyrna tiburo, Mustelus canis, C. brevipinna, Negaprion brevirostris, Sphyrna mokarran, Sphyrna lewini, Galeocerdo cuvieri a n d Ginglymostoma cirratum. W i t h the e x c e p t i o n s o f C . obscurus, C. plumbeus a n d Ginglymostoma cirratum, a l l the impor tant s p e c i e s of the eas t c o a s t a re k n o w n to be heav i ly exp lo i t ed a s j u v e n i l e s a n d s o m e t i m e s e v e n a s n e w b o r n s at least in s o m e part o f thei r r ange . T h e r e a r e o n l y a few p re l imina ry a s s e s s m e n t s o f the s ta tus of s o m e s h a r k s tocks for the eas t coas t . A l v a r e z (1988) reports that a c c o r d i n g to su rp lus p roduc t ion m o d e l s , the s t o c k s of Sphyrna tiburo a n d Rhizoprionodon terraenovae in Y u c a t a n a re c l o s e to op t ima l explo i ta t ion l eve l s ; resul ts o f the y ie ld-per- recrui t m o d e l s u g g e s t Sphyrna tiburo is at the o p t i m u m explo i ta t ion l eve l w h e r e a s Rhizoprionodon terraenovae s e e m s to be a l r e a d y o v e r e x p l o i t e d . F o r the p roduc t ion m o d e l s , c a t c h a n d effort w e r e e s t i m a t e d in a ve ry rough w a y a n d f o r t h e d y n a m i c m o d e l , g rowth a n d mortali ty w e r e e s t i m a t e d v i a length f r equency a n a l y s i s . Bonf i l (1990) , e s t i m a t e d growth v i a ve r t eb rae r e a d i n g s a n d u s i n g the y ie ld -pe r -recruit m o d e l d i a g n o s e d growth over f i sh ing for the Carcharhinus falciformis s tock of the C a m p e c h e B a n k . T h i s resul ts ma in ly from the v e r y h igh b y c a t c h e s o f n e w b o r n s a n d j u v e n i l e s o f this s p e c i e s in the l oca l red g r o u p e r f i shery . T h e r e h a v e b e e n a n u m b e r of p e r m a n e n t r e s e a r c h p r o g r a m m e s for s h a r k f i sher ies in M e x i c o s i n c e the ea r ly 80 ' s . D e s p i t e this , to date there is n o spec i f i c m a n a g e m e n t for e l a s m o b r a n c h f i she r i es in M e x i c o . A n u m b e r o f c o n c e r n s h a v e b e e n e x p r e s s e d abou t s o m e u n d e s i r a b l e p rac t i ces in the f i sher ies . A t least , Carcharhinus falciformis, C. acronotus, Rhizoprionodon terraenovae a n d Sphyrna tiburo a re b e i n g h e a v i l y e x p l o i t e d a s j u v e n i l e s in C a m p e c h e a n d Y u c a t a n , h e n c e o p e n i n g the poss ib i l i ty o f a future c o l l a p s e o f the s t ocks . Add i t i ona l l y , there a re s u g g e s t i o n s that s t rong d e c r e a s e s in the a b u n d a n c e of j u v e n i l e s of C . leucas, C. limbatus, C. acronotus, C. perezi a n d Negaprion brevirostris h a v e o c c u r r e d in s o m e c o a s t a l l a g o o n s o f the Y u c a t a n P e n i n s u l a d u e to h e a v y f i sh ing wi th set nets (Bonfi l in press). It is v e r y l ikely that this s i tuat ion is c o m m o n p l a c e in m o s t c o a s t a l l a g o o n s a l o n g the c o a s t o f M e x i c o . A d d i t i o n a l l y , the ki l l ing o f la rge quant i t ies o f p regnan t f e m a l e s of 39 Rhizoprionodon longurio in S i n a l o a , o n the w e s t coas t , is a n o t h e r c a u s e for c o n c e r n . A l t h o u g h informat ion is l imi ted , it is l ikely that m a n y s t o c k s in the G u l f s o f C a l i f o r n i a a n d T e h u a n t e p e c a r e c l o s e to the o p t i m u m explo i ta t ion l eve l o r e v e n o v e r f i s h e d . H o w e v e r , no a s s e s s m e n t s a re k n o w n to date in t hose a r e a s . L i m i t e d o r non-ex i s ten t informat ion abou t the s i z e o f the s t o c k s a n d abou t the ac tua l l eve l s o f mortal i ty m a k e s the a d e q u a t e a p p r a i s a l of the s ta tus o f M e x i c a n f i sher ies difficult. A s in o ther coun t r i e s , s o c i o - e c o n o m i c a n d heal th p r o b l e m s re la ted to the f i sher ies further c o m p l i c a t e the m a n a g e m e n t o f e l a s m o b r a n c h s in M e x i c o . T h e c h a n c e s o f cur ta i l ing the f i sh ing o f j u v e n i l e s h a r k s in M e x i c o wi l l be c o n s t r a i n e d by the p r o b l e m s re la ted to the a r t i sana l nature of m a n y of the f i sh ing fleets ( loss o f i n c o m e for la rge n u m b e r s of f i she rmen) a n d the h igh e s t e e m that s m a l l s h a r k s h a v e o n the M e x i c a n t ab le . T h e h i g h e r concen t r a t i on of h e a v y me ta l s in o l d e r s h a r k s a l s o m a k e s the ha rves t i ng of j u v e n i l e s preferab le . P e r u . F r o m the mid-s ix t i e s a n d until v e r y recent ly , the e l a s m o b r a n c h c a t c h e s o f P e r u w e r e the third la rges t in A m e r i c a a n d con t r ibu ted 2.71 % of the w o r l d e l a s m o b r a n c h c a t c h . N e v e r t h e l e s s , e l a s m o b r a n c h s a re of m i n o r impor t ance in P e r u v i a n f i she r i es a n d represen t o n l y 0 .29 % of the total f i shery p roduc t ion (table 2 .2) . T h e e l a s m o b r a n c h f i sher ies of P e r u h a d a fairly s t e a d y t rend o f s l o w d e v e l o p m e n t in the 50 ' s a n d ea r ly 60 ' s . S i n c e the mid-60 ' s c a t c h e s h a v e o s c i l l a t e d a r o u n d 1 8 , 0 0 0 t, p e a k i n g at m o r e t han 3 0 , 0 0 0 t in 1984 a n d u n e x p e c t e d l y c r a s h i n g in 1990-1991 (fig. 2 .2) . T h e r e m a y be a l ink b e t w e e n recent ly d e c l i n i n g e l a s m o b r a n c h c a t c h e s a n d the erupt ion o f c h o l e r a in P e r u d u r i n g 1 9 9 0 . A c c o r d i n g to F A O stat is t ics , e l a s m o b r a n c h y i e l d in P e r u is s t rongly d o m i n a t e d by s m o o t h -h o u n d s . D u r i n g the p e r i o d 1 9 7 7 - 1 9 9 1 , s m o o t h - h o u n d s of the g e n u s Mustelus w e r e the mos t important s p e c i e s in the e l a s m o b r a n c h c a t c h e s m a k i n g 56 % (10 ,219 t/yr) o f the total a n d a c c o u n t e d for 2 5 , 0 0 0 1 in 1984 w h e n r eco rd e l a s m o b r a n c h c a t c h e s o f P e r u r e a c h e d 3 4 , 4 0 0 t (fig. 2 .6) . S e v e r a l u n s p e c i f i e d rays m a k e 2 5 % (4 ,640 t/yr) o f the total c a t c h e s . T h e i r l a n d i n g s h a v e i n c r e a s e d s ignif icant ly s i n c e 1984 , m a k i n g t h e m the s e c o n d mos t important e l a s m o b r a n c h g roup in P e r u . Rhinobatosplaniceps a n d a n g e l s h a r k s Squatina s p p . a re a l s o important s p e c i e s in the c a t c h e s with a v e r a g e s o f 10 % (1 ,908 t/yr) a n d 3 % (560 t/yr) 4 0 re spec t ive ly . T h e y i e l d s o f t he se two g r o u p s s h o w e d v a r i a b l e t r ends in this pe r iod . A n a s s o r t e d g r o u p o f e l a s m o b r a n c h s m a d e the r e m a i n i n g 6 % (1 ,133 t/yr). A p a r t from F A O stat is t ics , no th ing e l s e is k n o w n abou t the e l a s m o b r a n c h f i she r i es o f P e r u . B r a z i l . B r a z i l i a n e l a s m o b r a n c h c a t c h e s a re the third h ighes t in A m e r i c a , after M e x i c o a n d the U S A . It a p p e a r s that B r a z i l i a n e l a s m o b r a n c h f i sher ies h a v e a t t a ined a re la t ive stabil i ty. Af ter a n s l o w but s t e a d y start t h rough the s ix t ies a n d a brief fall in the 70 ' s , the c a t c h e s o f s h a r k s a n d rays from B r a z i l s h o w a major l eap in the ea r ly 80 ' s . Y i e l d s h a v e v a r i e d s i n c e then , up to a 3 0 , 0 0 0 t m a x i m u m (fig. 2 .2) . S h a r k s a n d r ays con t r ibu ted 3 % to the total f i sher ies of B r a z i l du r ing 1987 -1991 w h i l e m a k i n g 3 .69 % of the w o r l d c a t c h e s o f e l a s m o b r a n c h s (table 2 .2) . B r a z i l i a n f i sher ies s tat is t ics d o not differentiate e l a s m o b r a n c h s by s p e c i e s . A t leas t 3 0 e l a s m o b r a n c h s a re c o m m o n in the c o m m e r c i a l c a t c h e s in the sou theas t , but m o s t o f the l a n d i n g s c o m e d r e s s e d wi thout guts , h e a d o r f ins, m a k i n g it difficult to sort by s p e c i e s ( T o m a s 1987) . S o m e o f the s p e c i e s repor ted for the c o m m e r c i a l c a t c h e s a re : Mustelus schmitti, Galeorhinus galeus, Prionace glauca, Isurus oxyrinchus, Squatina guggenheim, Squatina sp , Pristisspp., Rhinobatos percellens, R. horkelii, Dasyatis spp , Gymnura s p p a n d Myliobatis s p p . A c c o r d i n g to F A O da ta , du r ing the pe r iod 1 9 7 7 - 1 9 9 1 , B r a z i l i a n l a n d i n g s w e r e d o m i n a t e d by a n a s s o r t e d g r o u p of s p e c i e s c o r r e s p o n d i n g to 7 2 % (17 ,919 t/yr) o f the e l a s m o b r a n c h c a t c h . Y i e l d s for this g r o u p o f e l a s m o b r a n c h s g r e w rapid ly from l e s s t h a n 1,000 t in 1978 to m o r e than 2 3 , 0 0 0 t in 1 9 8 2 a n d h a v e r e m a i n e d c l o s e to 2 0 , 0 0 0 t/yr s i n c e then (fig. 2 .7) . A l l the s h a r k s k n o w n to o c c u r in B r a z i l i a n c a t c h e s a re i n c l u d e d in this g r o u p . A c c o r d i n g to Ba t i s t a (1988) , l a n d i n g s of Galeorhinus galeus h a v e i n c r e a s e d s i n c e 1 9 7 0 d u e to g r o w i n g act iv i t ies of t r awle r s in sou th eas t B r a z i l . T h e s e c o n d mos t impor tan t g r o u p du r ing this pe r iod w e r e the s k a t e s a n d rays cont r ibu t ing 17 % (4 ,254 t/yr) o f the c a t c h e s . L a n d i n g s of this g r o u p e x p a n d e d s lowly , a s w e l l a s t h o s e o f gui ta r f i shes Rhinobatos s p p . w h i c h a v e r a g e d 7 % (1 ,683 t/yr) o f the total e l a s m o b r a n c h c a t c h . S m a l l c a t c h e s o f s a w f i s h e s Pristis s p . h a v e b e e n s tead i ly l a n d e d a v e r a g i n g 4 % (1 ,014 t/yr) o f the c a t c h e s . 41 35,000 30,000 25,000 <g 20,000 c £ 15,000 10,000 5,000 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Year s Smoo th -hound H i A n g e l Shark Rays v a r - e lasm. Guitarfish Figure 2.6 E la smobranch ca tches of Peru , by spec ies groups, during 1977-1991 (Data from F A O ) . 35,000 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Years Var. Elasm. f^ffl Skates & rays Guitarfish Sawfish Figure 2.7 E la smobranch ca tches of Braz i l , by spec ies groups, during 1977-1991. (Data from F A O ) . 4 2 V o o r e n a n d Bet i to (1987) report o n at leas t 2 5 s p e c i e s o f s m a l l s h a r k s a n d 2 4 o f ba to ids for wa t e r s l e s s t han 100 m d e e p in the sou theas t e rn con t inen ta l shelf . S w e p t a r e a b i o m a s s e s t ima tes ind ica te 2 0 , 0 0 0 t a v a i l a b l e in win te r a n d 1 3 , 0 0 0 t in s u m m e r o f w h i c h 9 0 % are c o m p o s e d of 16 s m a l l s h a r k s a n d 8 ba to id s p e c i e s o f c o m m e r c i a l v a l u e . A p p a r e n t l y , the o n l y t radi t ional u s e for e l a s m o b r a n c h s in B r a z i l h a s b e e n for food , but G o c k s (1987) a n d J a c i n t o (1987) not s o m e efforts for the ut i l isat ion of h i d e s a n d o ther par ts . In the north o f B r a z i l , at leas t two k inds o f f i sher ies l a n d e l a s m o b r a n c h s (R . L e s s a , pers . c o m m . F e b r u a r y 1992) . A n indust r ia l long l ine f i shery for t u n a s wi th up to 5 0 % b y c a t c h e s of s h a r k s , c a p t u r e s m a i n l y Prionace glauca, Carcharhinus longimanus, Carcharhinus spp. , Sphyrna spp . , Isurus spp. , Alopias spp. , Pseudocarcharias kamoharai a n d Galeocerdo cuvieri. T h i s f i shery l a n d e d a n a v e r a g e of 144 t/yr of s h a r k s b e t w e e n 1 9 8 5 - 1 9 9 0 . A b o u t 6 0 % of t he se w e r e s h a r k s l e s s than 1.5 m T L . A r t i s a n a l f i sher ies for Cynoscium acoula a n d Scomberomorus s pp . c a t c h Carcharhinus porosus, Rhizoprionodon spp. , Sphyrna spp. , Isogomphodon oxyrhynchus a n d Pristis peroretti. T h e r e is a h igh i n c i d e n c e of j u v e n i l e s in this f i shery w h i c h is c a r r i e d out u s i n g s m a l l driftnets o f abou t 1 k m long a n d 6 m d e e p . O n the north s h o r e , b e t w e e n the A m a z o n r iver a n d Rec i f e , e l a s m o b r a n c h c a t c h e s m a y be a s high a s 6 0 % of the total in this a r t i sana l f ishery. Incidenta l c a t c h e s o f s m a l l s h a r k s a n d rays in the Brachyplatystoma, s h r i m p a n d s n a p p e r f i sher ies o f the north o f B r a z i l a re repor ted by E v a n g e l i s t a (1987) . A p p a r e n t l y mos t of the b y c a t c h e s w e r e fo rmer ly d i s c a r d e d but a re n o w b e g i n n i n g to be u t i l i sed . V o o r e n et a l . (1990) s u m m a r i s e informat ion o n d e m e r s a l f i she r i es for e l a s m o b r a n c h s dur ing 1 9 7 3 - 1 9 8 6 o n the con t inen ta l she l f off the sou the rn port o f R i o G r a n d e . E l a s m o b r a n c h s a c c o u n t for 7.3 % of the total c a t c h e s , 13.1 % of the otter t r awle r s c a t c h e s , 7.1 % of the p a i r e d t rawlers c a t c h e s a n d 5.4 % of s m a l l - s c a l e f i sher ies c a t c h e s . O t t e r t r a w l i n g ope ra t ions are c a r r i e d out wi th 4 4 0 - 4 8 0 H P boa t s in 11-13 d a y tr ips at d e p t h s b e t w e e n 4 0 - 1 0 0 m , w h i l e pa i r ed t r awl ing is d o n e by 3 4 0 - 3 7 0 H P boa t s in 9-11 d a y tr ips at d e p t h s l e s s than 4 0 m . S m a l l - s c a l e f i sher ies i nc lude b e a c h s e i n i n g a n d t r a m m e l nets in w a t e r s l e s s than 10 m d e e p a n d gi l lnet t ing by 11-16 m boa t s with 1 0 0 - 1 3 0 H P motors in w a t e r s 8-40 m d e e p . S m a l l s h a r k s a v e r a g e 4 6 . 3 % of e l a s m o b r a n c h c a t c h e s , w h i l e a n g e l s h a r k s , gui tar f i shes a n d rays a c c o u n t for 2 4 . 8 5 % , 2 4 . 5 % a n d 5 % respec t ive ly . Mustelus schmitti a n d Galeorhinus galeus m a k e mos t of the c a t c h e s of " c a c o e s " o r s m a l l s h a r k s , w h i c h s h o w i n c r e a s e d 4 3 l a n d i n g s from 1,414 t in 1 9 7 3 to 3 ,217 in 1 9 8 6 , but, a c c o r d i n g to S U D E P E (1990) l a n d i n g s d r o p p e d to 2 , 0 2 3 1 in 1 9 8 9 . T h e propor t ion o f s m a l l s h a r k s in the c a t c h e s o f the s m a l l - s c a l e a n d pa i r t r awle r f i shery i n c r e a s e d dur ing this p e r i o d but d e c r e a s e d in the otter t rawl f ishery. T h i s r e su l t ed in a n a l m o s t e q u a l propor t ion o f l a n d i n g s by e a c h type o f f i shery in 1 9 8 3 - 1 9 8 6 . T h e C P U E of s m a l l s h a r k s in t/trip of both t ypes o f t rawlers t e n d e d to i n c r e a s e th roughout the p e r i o d o f the s tudy. A n g e l sha rk (Squatina guggenheim a n d Squatina sp.) l a n d i n g s i n c r e a s e d from 8 2 2 1 in 1 9 7 3 to 1 ,7771 in 1986 . A s with the s m a l l s h a r k s , the propor t ion of c a t c h e s con t r ibu ted by s m a l l - s c a l e f i sher ies a n d pa i r t r awlers i n c r e a s e d w h i l e that o f otter t rawlers d e c r e a s e d . S t i l l , abou t 5 0 % of the total l a n d i n g o f a n g e l s h a r k s c o m e s from the latter. W h i l e C P U E o f a n g e l s h a r k s in otter t rawlers s h o w e d a n o v e r a l l i n c r e a s e , pa i r ed t rawlers ' C P U E i n c r e a s e d until 1 9 8 3 a n d d e c r e a s e d a f te rwards . L a n d i n g s o f gui tar fish Rhinobatos horkelii, v a r i e d b e t w e e n 6 0 0 t a n d 1,925 t. M o s t o f t h e s e c a m e from the s m a l l s c a l e f i sher ies (50 %) a n d pa i r ed t rawlers (32 % ) , w h i l e otter t r awle r s con t r ibu ted v e r y s m a l l c a t c h e s (13 % ) . D a t a o f C P U E s h o w e d a sl ight d e c r e a s e up to 1 9 8 2 for both t ypes of t rawlers , i n c r e a s i n g to 1984 a n d fa l l ing a g a i n af terwards . L a n d i n g s o f rays , ma in ly g e n u s Dasyatis a n d Gymnura a n d to a l e s s e r extent Myliobatis, g r e w from 36 t in 1 9 7 3 to 4 8 4 t in 1986 . S m a l l - s c a l e f i sher ies a v e r a g e d 18 % of t he se c a t c h e s , p a i r e d t r awl ing 53 % a n d otter t r awl ing 34 % . C P U E for rays in t rawl f i sher ies w e r e v a r i a b l e wi th a n i n c r e a s i n g t rend . T h e a p p a r e n t d e c l i n e o f s o m e o f t he se popu la t i ons in the last p e r i o d o f the a b o v e s tudy s e e m s to be c o n f i r m e d by a swi tch from t rawl ing to bot tom l o n g l i n e s a n d gi l lnets (the latter spec i f i ca l ly a i m e d at Squatina a n d Galeorhinus) w h i c h s tar ted in 1 9 8 6 d u e to d e c r e a s i n g C P U E ' s . T h i s swi tch w a s a l s o c o u p l e d with add i t i ona l f i sh ing for a n g e l s h a r k s by sh r imp t rawlers f rom o ther a r e a s du r ing the s h r i m p off s e a s o n ( C M . V o o r e n , U n i v e r s i d a d de R i o G r a n d e , pers . c o m m . D e c . 2 3 1991) . A m o r i m a n d Arfe l l i (1987) a n d Arfe l l i et a l . (1987) , report s o m e b y c a t c h e s o f la rge s h a r k s t a k e n in sou th a n d sou theas t wa t e r s by t u n a long l ine r s . Prionace glauca ( a ccoun t i ng for 33 % of total c a t c h e s o f this fleet in 1985) a n d Isurus oxyrinchus ( a c c o u n t i n g for 3.2 % of total c a t c h e s of this f i shery 1 9 7 1 - 1 9 8 5 ) a re c a u g h t th roughout the y e a r but m a i n l y du r ing A p r i l -J u l y a n d M a y - N o v e m b e r r e spec t ive ly . L a n d i n g s of b lue s h a r k s a r e c o m p o s e d ma in ly o f 2 0 -4 0 kg s h a r k s d r e s s e d w e i g h t (no h e a d , fins o r guts) a n d a c c o u n t e d for 5 5 3 t a n d 4 6 2 t in 1984 a n d 1 9 8 5 r e spec t ive ly . B l u e sha rk C P U E h a s v a r i e d from 0.4 k g / 1 0 0 h o o k s in 1971 44 (when their c ap tu re w a s a v o i d e d by sk ippe r s ) to 27 .6 k g / 1 0 0 h o o k s in 1 9 8 5 . Short f in m a k o c a t c h e s v a r i e d b e t w e e n 21 t (1971) a n d 7 3 1 (1981) , thei r m e a n w e i g h t in the c a t c h v a r y i n g b e t w e e n 4 2 kg a n d 6 0 kg th roughout 1 9 8 5 . T h e i r mea t is the m o s t v a l u e d a m o n g e l a s m o b r a n c h s in B r a z i l , a n d is u s e d loca l ly a s w e l l a s e x p o r t e d to U S A . A g o o d d e a l o f r e s e a r c h o n e l a s m o b r a n c h s is d o n e by s e v e r a l B r a z i l i a n Un ive r s i t i e s , g o v e r n m e n t a l a n d n o n - g o v e r n m e n t a l o r g a n i s a t i o n s . H o w e v e r , a c c o r d i n g to L e s s a (pers. c o m m . , op.ci t . ) , at p resen t there a re no m a n a g e m e n t m e a s u r e s for e l a s m o b r a n c h s in B r a z i l , a l t hough s o m e l o c a l g r o u p s in tend to ra i se g o v e r n m e n t a l a t tent ion into the s ta tus o f t he se f i sher ies . S o m e p l a n s to i m p l e m e n t repor t ing by s p e c i e s a r e a l s o u n d e r w a y . L e s s a points out that e l a s m o b r a n c h s t o c k s exp lo i t ed in the north c o a s t a r t i s ana l f i shery a re thought to be u n d e r e x p l o i t e d , t h o s e u t i l i sed by the t u n a long l ine f i shery a re s u s t a i n a b l y exp lo i t ed , w h e r e a s the sou th B r a z i l d e m e r s a l s t ocks a re a l m o s t su re ly o v e r e x p l o i t e d . A r g e n t i n a . A r g e n t i n a h a s s o m e o f the few e x p a n d i n g f i sher ies a m o n g major e l a s m o b r a n c h - f i s h i n g coun t r i e s in A m e r i c a . Af ter a t e m p o r a r y d rop in the late 40 ' s , a t t r ibuted to the g e n e r a l c o l l a p s e o f s h a r k l iver oi l f i sher ies a r o u n d the w o r l d , sha rk a n d ray y i e l d h a d a s l o w but s t e a d y growth from the ea r ly 50 ' s to the m i d 60 ' s (fig. 2 .2) . S i n c e 1 9 6 7 , y i e l d s f luctuated c l o s e l y to 1 0 , 0 0 0 1 but h a v e i n c r e a s e d s i n c e 1 9 8 1 . D e s p i t e the re la t ive ly l ow c a t c h e s , w h i c h a c c o u n t o n l y for 2 .54 % of the w o r l d e l a s m o b r a n c h c a t c h du r ing 1 9 8 7 - 1 9 9 1 , e l a s m o b r a n c h s a re r e a s o n a b l y important for A r g e n t i n e a n f i sher ies cont r ibu t ing 3 .19 % of the total y i e l d s dur ing this p e r i o d . T h i s is the h ighes t relat ive impor t ance for e l a s m o b r a n c h s in A m e r i c a n major e l a s m o b r a n c h - f i s h i n g coun t r i e s . D u r i n g the p e r i o d 1977-1991 the mos t important s p e c i e s in the e l a s m o b r a n c h c a t c h e s w e r e : Mustelus schmitti w h i c h a v e r a g e d 4 9 % (6 ,790 t/yr) o f the total e l a s m o b r a n c h c a t c h ; s e v e r a l rays at 2 0 % (2 ,722 t/yr), unc l a s s i f i ed e l a s m o b r a n c h s with 2 3 % (3 ,160 t/yr) a n d e l e p h a n t f i shes (Callorhinchus s p j wi th 8 % (1 ,048 t/yr). O f t he se g r o u p s , the s m o o t h - h o u n d s a n d "var ious e l a s m o b r a n c h s " h a d a g r o w i n g t rend in c a t c h e s w h i l e the e l e p h a n t f i shes a n d rays h a d a t e n d e n c y to d e c r e a s e . A r g e n t i n a is o n e of the few coun t r i e s wi th impor tant c a t c h e s of c h i m a e r i f o r m e s in the w o r l d (fig. 2 .8) . 4 5 C r e s p o a n d C o r c u e r a (1990) g ive a ve ry de t a i l ed desc r ip t ion o f the f i she r i e s for s h a r k s of C l a r o m e c o a n d N e c o c h e a , B u e n o s A i r e s P r o v i n c e . In this nor thern A r g e n t i n a f ishery, Galeorhinus galeus, Mustelus schmitti, Carchariastaurus a n d Squatina argentina a re caugh t with g i l lne ts . A b o u t 2 3 w o o d e n a n d iron v e s s e l s from 8-44.9 m in length t ake part in the f ishery. T h e y u s e ny lon monof i l amen t g i l lnets (2-3 m m twine) wi th m e s h s i z e s 19-21 c m a n d d i m e n s i o n s 55-71 m long 3.8 m d e e p pe r p a n e l (8-25 pane l s ) . T h e s e gi l lnets a r e set o n the bot tom b e t w e e n 0.5 a n d 2 5 n m from the c o a s t in dep ths v a r y i n g from 2 -70 m . T y p i c a l abou t 6-15 Squatina argentina a n d 1-20 o f the o ther s h a r k s a re c a u g h t pe r p a n e l . E x - v e s s e l p r i ces a re U S $ 3 - 4 / k g for u n d a m a g e d Galeorhinus d e s t i n e d for expor t (ma in ly to Italy) a n d U S $ 1 -2 .5 /kg for d a m a g e d o n e s that a re c o n s u m e d sa l t -dr ied in the l o c a l marke t . In a n u n u s u a l note, t h e se au tho r s report e x t e n s i v e d a m a g e to sha rk c a t c h e s by m a r i n e m a m m a l s ! S e a l ions bite off the be l ly o f e n t a n g l e d s h a r k s ea t ing on ly the l iver . M e n n i et a l . (1986) ind ica te the p r e s e n c e of m o r e s h a r k s s p e c i e s in nor thern A r g e n t i n a . In add i t ion to the s p e c i e s m e n t i o n e d a b o v e , they report Mustelus canis, M. fasciatus, Squalus blainvillei, S. cubensis a n d Notorhynchus cepedianus in the c o m m e r c i a l c a t c h e s of B u e n o s A i r e s p r o v i n c e . Mustelus schmitti a c c o u n t s for 9 2 % of the i r s h a r k s a m p l e s , at c o m m e r c i a l l and ing s i tes . T h e r e m a i n i n g s p e c i e s a re l e s s than 1 % of the s h a r k c a t c h e s e x c e p t S. cubensis w h i c h m a d e up 2 % . G o v e r n m e n t s tat is t ics o f s h a r k l a n d i n g s at M a r de l P l a t a port a v e r a g e d 5 ,890 t du r ing 1 9 7 1 - 1 9 8 0 , this is abou t 1/2 o f the a v e r a g e total e l a s m o b r a n c h c a t c h o f A r g e n t i n a du r ing that p e r i o d . A b o u t 9 3 % of this c a t c h is m a d e o f ' ga tuzos ' (p redominan t ly Mustelus schmitti, with s o m e quant i t ies o f M. canis a n d s m a l l n u m b e r s of M. fasciatus). ' C a z o n e s ' (main ly Galeorhinus galeus but i n c l u d i n g s o m e la rge M. canis) contr ibute the r e m a i n i n g 7 % . A p p a r e n t l y , the r e m a i n i n g s p e c i e s a re not r e c o r d e d in the s tat is t ics . 2 .2 .2 .2 E u r o p e . N o r w a y . N o r w a y h a s h a d s o m e o f the mos t important sha rk f i sher ies in the Nor th A t l an t i c . N o r w e g i a n c o m m e r c i a l f i she r i es for e l a s m o b r a n c h s h a v e b e e n quite v a r i a b l e s i n c e the e n d of W o r l d W a r II, wi th a n i n c r e a s i n g t rend up to 1 9 6 3 , f o l l o w e d by a g e n e r a l d e c r e a s e to l eve l s a r o u n d 46 7 ,500 t/yr s i n c e 1981 (fig. 2 .2) . C a t c h e s rose a g a i n in the last th ree y e a r s o n r eco rd . J u d g i n g from recent t rends , e l a s m o b r a n c h s a re not a n impor tan t f i shery r e s o u r c e for N o r w a y . S h a r k s a n d rays r ep resen t o n l y 0 .44 % of the total f i she r i es p roduc t ion of this count ry . M o r e o v e r , N o r w e g i a n e l a s m o b r a n c h f i sher ies o n l y cont r ibute 1.21 % to the w o r l d e l a s m o b r a n c h y i e l d 1987 -1991 (table 2 .2) . T h e larges t part o f the e l a s m o b r a n c h c a t c h e s h a s typ ica l ly b e e n s p i n y dogf i sh Squalus acanthias. N e v e r t h e l e s s , there w e r e important f i sher ies for p o r b e a g l e s in the 60 ' s a n d for b a s k i n g s h a r k s until the m i d 80 ' s . W h i l e m a r k e t i n g a n d e c o n o m i c a l cons t ra in t s have t radi t ional ly inh ib i ted b a s k i n g sha rk f i sher ies ( M a x w e l l 1 9 5 2 ; O ' c o n n o r 1 9 5 3 ; K u n s l i k 1988) , appa ren t l y the p o r b e a g l e , Lamna nasus, f i shery d e c l i n e d at leas t in part a s a result o f ove r -explo i ta t ion ( G a u l d 1 9 8 9 , M y k l e v o l l 1 9 8 9 a , A n d e r s o n 1990) . N o r w e g i a n e l a s m o b r a n c h f i sher ies s e e m to be r e c o v e r i n g after a p r o l o n g e d d e c l i n e . F o r t h e first t ime in a l m o s t 2 0 y e a r s , c a t c h t r ends are o n the r i se . F A O d a t a for the pe r iod 1 9 7 8 -1991 (fig. 2.9) s h o w c a t c h e s of s p i n y dogf i sh d e c l i n i n g from m o r e than 1 2 , 0 0 0 t in 1978 to 2 ,986 t in 1 9 8 6 then r ise up to 9 ,627 t in 1991 a n d a v e r a g i n g 5 ,715 t/yr (53 % of e l a s m o b r a n c h c a t c h e s for this per iod) . C a t c h e s o f b a s k i n g s h a r k s , Cetorhinus maximus, s h o w a pat tern s imi l a r to that of s p i n y dogf i sh c a t c h e s , a l t h o u g h the i r r e c o v e r y is more m o d e s t . B a s k i n g s h a r k c a t c h e s fell f rom 1 1 , 3 3 5 1 in 1 9 7 9 to o n l y 3 5 2 t in 1 9 8 7 , but w e r e of 1,932 t in 1 9 9 0 a n d a v e r a g e d 3 ,929 t/yr (36 %) du r ing this p e r i o d . R a y s c a t c h e s a re fairly s tab le at a r o u n d 1,115 t/yr (10 % ) . S m a l l quant i t ies o f p o r b e a g l e s a re still c augh t , a v e r a g i n g 6 7 t/yr ( less t han 1 % ) . A l t h o u g h there a re s o m e i n c o n g r u e n c i e s a m o n g different s o u r c e s o f da t a o n the N o r w e g i a n d i rec ted f i shery for p o r b e a g l e s o f the 60 ' s ( G a u l d 1989 ; A n d e r s o n 1990) , it is c l e a r that this f ishery c a u g h t la rge a m o u n t s o f s h a r k s . T h e s u m m a r y o f this f i shery is b a s e d o n A a s e n (1963) a n d M y k l e v o l l (1989a) . O p e r a t i o n s s tar ted a s a c o a s t a l act ivi ty but, s i n c e 1930 , the f i sh ing g r o u n d s s t ead i ly e x p a n d e d from N o r w e g i a n wa t e r s nor thwes t to the O r k n e y - S h e t l a n d a r e a a n d the F a r o e Is lands , then sou ther ly into Irish w a t e r s a n d f inal ly s t r e t ched to the A t l an t i c c o a s t s of C a n a d a a n d nor theas te rn U S A . Dis tan t w a t e r o p e r a t i o n s w e r e c a r r i e d out by s p e c i a l i s e d f r eeze r v e s s e l s 4 3 - 5 0 m long , d e p l o y i n g l ong l i ne s wi th up to 5 ,000 h o o k s in wa t e r s 10-30 m d e e p . S h a r k s l e s s than 10 kg in w e i g h t w e r e d i s c a r d e d b e c a u s e o f a l ack 47 (D C c o 25,000' 20,000 15,000 10,000 5,000 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Years Smooth-hound H Rays Elephant fish Var. elasm. Figure 2.8 E lasmobranch catches of Argentina, by spec ies groups, during 1977-1991. S c h o o l shark catches, too smal l to show (Data from F A O ) . 35,000-30,000-25,000-5,000-N O R W A Y 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Years Sp iny dogfish E H | Rays B a s k i n g shark Figure 2.9 E la smobranch catches of Norway, by spec ies groups, during 1978-1991. Porbeagle catches, too smal l to show (Data from F A O ) . 4 8 of marke t . T h e c o a s t a l fleet w a s c o m p o s e d o f w o o d e n boa t s 2 3 - 3 0 m l o n g w h i c h kept the c a t c h o n i ce . D r e s s e d c a r c a s s e s o f p o r b e a g l e s w e r e e x p o r t e d f rozen to Italy, w h i l e fins w e r e m a r k e t e d to the F a r Eas t . O n c e the N W At l an t i c p o r b e a g l e s t o c k s r e a c h e d unprof i table s i z e in 1 9 6 5 , the fleet m o v e d out to c a t c h m a k o s h a r k s in Nor th W e s t A f r i c a . P r e sen t ly , on ly b y c a t c h e s o f p o r b e a g l e s from p u r s e - s e i n i n g , t r awl ing a n d gi l lnet f i she r i es a re l a n d e d . N o r w e g i a n s d o not e v e n take their 2 0 0 t p o r b e a g l e T A C in E C w a t e r s . T h e b a s k i n g s h a r k f i shery ( d o c u m e n t e d by K u n z l i k 1988 a n d M y k l e v o l l 1 9 8 9 b ) h a s roots in the 16th cen tu ry w h e n the d r i ed str ips o f mea t w e r e u s e d a s f o o d . T h i s h a s t radi t ional ly b e e n a n impor tant d i r ec t ed f i shery in N o r w a y . Inspi red by a grea t d e m a n d for l iver o i l , the b ig e x p a n s i o n o f the f i shery s tar ted in 1 9 6 0 . S m a l l w o o d e n v e s s e l s 15 -25 m l o n g a n d a r m e d with h a r p o o n s , o p e r a t e d ma in ly du r ing A p r i l - A u g u s t . E x p e r i m e n t s to u s e the f lesh of b a s k i n g s h a r k s into f i s h m e a l a n d to g ive v a r i o u s u s e s to the h i d e s f a i l ed . C o n s e q u e n t l y , in a n attitude c o m p a r a b l e to that o f the "f inning" f i s h e r m e n o f o the r a r e a s , N o r w e g i a n f i s h e r m e n took just the l iver for o i l ex t rac t ion a n d d i s c a r d e d the c a r c a s s e s at s e a . L a t e l y they a l s o take the fins a n d expor t t h e m to the Or ient . D u r i n g the p e r i o d 1 9 5 9 - 1 9 8 0 , c a t c h e s r a n g e d b e t w e e n 1,266 a n d 4 , 2 6 6 s h a r k s pe r year , but h a v e d e c l i n e d s i n c e . E E C a g r e e m e n t s with N o r w a y l imi ted their c a t c h e s to a T A C of 4 0 0 t/yr of l ivers s i n c e 1 9 7 8 . T h i s c o r r e s p o n d s a p p r o x i m a t e l y to 2 , 4 0 0 t/yr w h o l e we igh t c o n s i d e r i n g l ivers to be 1/6 o f w h o l e weigh t . S o c i o -e c o n o m i c cons t ra in t s i nc lud ing l imited marke t s a n d a n a g e i n g fleet, a n d the errat ic dis t r ibut ion o f the s h a r k s a re identif ied a s r e a s o n s for the d e c l i n e o f the f i shery . T h e N o r w e g i a n f i shery for b a s k i n g s h a r k s d o e s not e v e n t ake the a l l o w e d c a t c h in E E C wa te r s . T h e oi l from the l ivers is s o l d for ex t rac t ion of s q u a l e n e , a h y d r o c a r b o n u s e d in c o s m e t i c a n d av ia t ion indus t r ies , but s i n c e r icher s o u r c e s h a v e b e e n f o u n d in d e e p - s e a s h a r k s of the g e n u s Centrophorus, the marke t for b a s k i n g s h a r k s is s h r i n k i n g . In g e n e r a l , the d y n a m i c s of N o r w e g i a n e l a s m o b r a n c h f i sher ies s e e m to be s t rongly i n f luenced by e c o n o m i c a n d s o c i a l factors ( M y k l e v o l l 1 9 8 9 a , 1 9 8 9 b , 1989c ) . M a n y of t he se f i sher ies in N o r w a y h a v e d e c l i n e d in part o r totally b e c a u s e of r e a s o n s ex te rna l to the d y n a m i c s o f the r e s o u r c e (e .g. marke t forces) . H o l d e n (1977) a n d M y k l e v o l l (1989d) s u m m a r i s e mos t o f the N o r w e g i a n f i shery for s p i n y dogf i sh Squalus acanthias in the nor theas t At lan t i c , w h i c h da t e s b a c k to 1 9 3 1 . E x p a n s i o n of the marke t s l ed to c a t c h e s of 8 ,767 t by 1 9 3 7 , a n d a p e a k o f a l m o s t 3 4 , 0 0 0 t in 1 9 6 3 . 4 9 S i n c e then , c a t c h e s h a v e s l o w l y fa l len to a l eve l o f l e s s t han 6 , 0 0 0 t in the 80 ' s . D u r i n g the pe r iod 1 9 5 0 - 1 9 7 0 , N o r w e g i a n long l ine r s f i shed ma in ly in the i r c o a s t a l w a t e r s dur ing win te r a n d in S c o t t i s h w a t e r s du r ing s u m m e r - a u t u m n . T h e f ishery a i m e d at expo r t i ng mos t o f the c a t c h e s to f ish a n d c h i p s s h o p s in E n g l a n d . Unt i l the ea r ly 70 ' s , this f i shery c o n s t r a i n e d the e x p a n s i o n o f the Br i t i sh f ishery, d u e to m o r e compe t i t i ve p roduc t s wi th l a rge r s i z e , better a p p e a r a n c e a n d l o w e r p r ice . In recent y e a r s , the migra t ion o f la rge n u m b e r s o f s p i n y dogf i sh into u n u s u a l l y nor thern par ts of N o r w a y h a s p r o d u c e d a n incen t ive for the f i shery . T h i s might a c c o u n t for the i n c r e a s e in c a t c h e s o b s e r v e d dur ing 1 9 8 9 - 1 9 9 1 . D u r i n g the first hal f o f this century , N o r w a y h a d a f i shery for g r e e n l a n d s h a r k s , Somniosus microcephalus, both a s a s p e c i a l i s e d act ivi ty a n d in c o m b i n a t i o n with s e a l i n g . J u d g i n g from the da ta r epor ted by M y k l e v o l l (1989c) , this f i shery p e a k e d in 1 9 1 7 , w h e n 1 7 , 0 4 9 hectol i t res o f l ivers w e r e l a n d e d . P r o b a b l y b e c a u s e of fal l ing marke t p r i ce s for the product , the f i shery c e a s e d in 1 9 6 0 . F i s h e r i e s for s k a t e s a n d rays h a v e n e v e r b e e n d e v e l o p e d a s a t a rge ted act ivi ty in N o r w a y , a l l c a t c h e s a re inc iden ta l to sp iny dogf i sh , l ing , ha l ibut a n d t rawl f i sher ies ( M y k l e v o l l 1 9 8 9 e ) . S k a t e s a n d rays of no c o m m e r c i a l v a l u e a n d s m a l l s p e c i m e n s are c o m m o n l y d i s c a r d e d D e s p i t e h a v i n g d e v e l o p e d s e v e r a l spec i f i c f i sher ies for s h a r k s , N o r w e g i a n r e s e a r c h in e l a s m o b r a n c h s h a s b e e n re la t ively poor . O u t of the three mos t impor tan t s h a r k f i sher ies of N o r w a y ( sp iny dogf i sh , p o r b e a g l e a n d b a s k i n g sha rks ) , o n l y the s p i n y dogf i sh w a s s tud ied in a n y dept in a r e s e a r c h p r o g r a m m e w h i c h l a s t ed from 1 9 5 8 to 1 9 8 0 . T h i s effort p r o d u c e d the first k n o w n a s s e s s m e n t of a n e l a s m o b r a n c h f i shery ( A a s e n 1964) . A a s e n e s t ima ted a m a x i m u m equ i l i b r ium y i e l d of 5 0 , 0 0 0 t/yr for w h a t he c o n s i d e r e d a s i n g l e s tock of sp iny dogf i sh for Nor the rn a n d W e s t e r n E u r o p e . B y 1 9 6 1 , this y i e l d l eve l w a s a l r e a d y s u r p a s s e d . P o r b e a g l e s w e r e briefly s t ud i ed w h i l e the f i shery w a s in e x p a n s i o n a n d this p r o d u c e d o n e of the first a t tempts to es t imate g rowth in s h a r k s from ver teb ra l r ings ( A a s e n 1963) . O n l y ve ry l imited r e s e a r c h w a s e v e r d o n e o n b a s k i n g s h a r k s . T h e r e is e v i d e n c e that N o r w e g i a n v e s s e l s t ake part in the o r a n g e r o u g h y f i sher ies o f N e w Z e a l a n d . H o w e v e r no de ta i l s abou t t he se act ivi t ies c o u l d be f o u n d . T h e f inal u s e g i v e n to the p r o b a b l y la rge b y c a t c h e s o f d e e p s e a s h a r k s in this f i shery is u n k n o w n ( see s ec t ion 2 .2 .3 .4 . ) . 5 0 F o r m e r U S S R . A l t h o u g h the U S S R d o e s not exis t a n y m o r e a s s u c h , the e l a s m o b r a n c h f i she r i es o f w h a t u s e d to be the S o v i e t U n i o n a re t rea ted here b e c a u s e o f the i m p o r t a n c e o f its c a t c h e s . F o r the s a k e o f s impl i f i ca t ion , the n a m e ' former U S S R ' wi l l be u s e d . F o r m e r U S S R f i she r i es for e l a s m o b r a n c h s w e r e not r e c o r d e d s e p a r a t e l y from the rest of the f i sher ies c a t c h e s o f this coun t ry before 1 9 6 4 in F A O y e a r b o o k s . S i n c e the b e g i n n i n g of r ecords , the c a t c h e s s o a r e d r e a c h i n g a total o f 5 9 , 0 0 0 1 in 1 9 7 5 , o n l y to fall d o w n a s sha rp ly a s they s o a r e d to a l eve l of abou t 2 0 , 0 0 0 t in 1 9 7 7 . S i n c e t hen , c a t c h e s h a v e b e e n quite uns tab le , v a r y i n g rough ly b e t w e e n 1 0 , 0 0 0 - 2 0 , 0 0 0 t/yr (fig. 2 .2) . D u e to the d i s a p p e a r a n c e of the S o v i e t U n i o n , c a t c h e s p l u m m e t e d in 1 9 9 0 - 1 9 9 1 . B e c a u s e o f the h u g e f i sher ies p roduc t ion o f the fo rmer U S S R , e l a s m o b r a n c h s con t r ibu ted o n l y 0.11 % of the total c a t c h e s for 1 9 8 7 - 1 9 9 1 , w h i c h is the lowes t a m o n g major e l a s m o b r a n c h - f i sh ing coun t r i e s . T h e former U S S R cont r ibu t ion to w o r l d e l a s m o b r a n c h f i sher ies w a s o f o n l y 1.75 % in the s a m e pe r iod . T h e e l a s m o b r a n c h c a t c h e s of the fo rmer U S S R w e r e m a d e up from con t r ibu t ions o f its e n o r m o u s f i sh ing fleet, w h i c h w o r k e d a r o u n d the w o r l d . A grea t va r i e ty o f s p e c i e s a re m a s k e d u n d e r the two m a i n h e a d i n g s that w e r e repor ted : r ays a n d v a r i o u s e l a s m o b r a n c h s . T h e e v e r c h a n g i n g cha rac te r i s t i c s o f fo rmer U S S R f i she r i es w h i c h d e p e n d e d la rge ly o n a g r e e m e n t s wi th v a r i o u s na t ions , m a k e s their a n a l y s i s difficult. D a t a from F A O (fig. 2 .10) s h o w that from 1978 to 1 9 9 1 , r ays a c c o u n t e d for 6 6 % (8,761 t/yr) of the total fo rmer U S S R e l a s m o b r a n c h c a t c h e s , w h i l e v a r i o u s e l a s m o b r a n c h s r e p r e s e n t e d 31 % (4 ,109 t/yr). S m a l l c a t c h e s o f Squalus acanthias a c c o u n t e d f o r t h e r e m a i n i n g 3 % of the total (327 t/yr). M o s t o f the e l a s m o b r a n c h c a t c h e s o f the fo rmer U S S R w e r e p robab ly t a k e n in la rge t r awl ing o p e r a t i o n s a s s u g g e s t e d by their l a rge c a t c h e s o f ba to ids . R a y s w e r e t a k e n m a i n l y in F A O a r e a s 21 (37 % ) , 4 7 (26 % ) , 2 7 (15 %) a n d 3 7 (10 % ) , wi th the r e m a i n i n g (12 %) t a k e n in a r e a s 34 , 4 1 , 51 a n d 7 1 . C a t c h e s o f v a r i o u s e l a s m o b r a n c h s c a m e chief ly f rom a r e a s 3 7 (37 % ) , 4 7 (31 %) a n d 34 (25 % ) , wi th the rest o f c a t c h e s (7 %) con t r ibu ted by a r e a s 2 7 , 5 1 , 71 a n d 8 1 . C a t c h e s o f t h e s e t w o g r o u p s in A r e a 3 7 c o r r e s p o n d to t h o r n b a c k ray Raja clavata a n d sp iny dogf i sh Squalus acanthias f i she r i es in the B l a c k 51 S e a . Ivanov a n d B e v e r t o n (1985) ind ica te that C r i m e a n a n d C a u c a s i a n f i s h e r m e n h a v e s p e c i a l i s e d f i sher ies for t h o r n b a c k ray a n d sp iny dogf i sh in the B l a c k S e a . T h o r n b a c k rays a re f i shed wi th ba i t ed long l ine a n d inc iden ta l ly c a u g h t in bo t tom gi l lne ts set for s p i n y dogf i sh . T h e latter is a l s o t a k e n by t rawl in the nor thwes te rn c o a s t o f the B l a c k S e a a n d by bot tom long l ines a n d f ixed ne ts a l o n g the c o a s t s o f C r i m e a a n d C a u c a s i a . Af te r the c o n t i n u o u s s h r i n k a g e o f e l a s m o b r a n c h c a t c h e s in fo rmer U S S R f i she r i es until 1 9 8 2 , c a t c h e s (main ly o f ba to id f i shes) w e r e s l o w l y i n c r e a s i n g a g a i n , w h e n pol i t ica l e v e n t s p rac t ica l ly shut d o w n a l l f i sher ies . U n i t e d K i n g d o m . T h e U n i t e d K i n g d o m h a s o n e o f the mos t s tab le e l a s m o b r a n c h f i she r i es in the w o r l d . T h e r eco rds s h o w a n a l m o s t s t e a d y t rend of c a t c h e s s l o w l y d e c r e a s i n g f rom 3 0 , 0 0 0 t/yr in the ea r ly pos t -wa r y e a r s , to the current l eve l o f abou t 2 2 , 0 0 0 t/yr (fig. 2 .2) . D u r i n g 1978-1991 total e l a s m o b r a n c h c a t c h e s v a r i e d b e t w e e n 2 0 , 0 0 0 t a n d 2 5 , 0 0 0 t m a i n l y b e c a u s e o f c h a n g e s in the c a t c h e s o f s p i n y dogf i sh Squalus acanthias w h i c h a v e r a g e d 6 3 % (13 ,820 t/yr) o f total e l a s m o b r a n c h c a t c h e s (fig. 2 .11) . A l m o s t 4 7 % pe rcen t o f s p i n y dogf i sh c a t c h e s dur ing this p e r i o d w e r e c a u g h t in E n g l a n d a n d W a l e s , wi th a n e q u a l a m o u n t c a u g h t in Sco t t i sh wa te r s , w h i l e the r e m a i n i n g 6 % c a m e from Nor the rn I re land. C a t c h e s o f rays a v e r a g e d 36 % (7 ,877 t/yr) o f a l l e l a s m o b r a n c h s a n d h a v e r e m a i n e d fairly cons tan t wi th a s l ight t e n d e n c y to i n c r e a s e . A p p r o x i m a t e l y 4 9 % of the ray c a t c h e s a r e t a k e n in S c o t l a n d a n d the s a m e a m o u n t in E n g l a n d - W a l e s , w h i l e Nor the rn I re land con t r ibu tes o n l y abou t 2 % . L e s s than 1 % of the total e l a s m o b r a n c h c a t c h of the U K is m a d e up o f S c y l i o r h i n i d s , S q u a l o i d s a n d u n s p e c i f i e d e l a s m o b r a n c h s . A s a g roup , c h o n d r i c h t h y a n s a re re la t ively important to U K f i sher ies m a k i n g up 2 .63 % of the total c a t c h e s du r ing 1 9 8 7 - 1 9 9 1 . H o l d e n (1977) s u m m a r i s e s informat ion for the s p i n y dogf i sh (Squalus acanthias) f i shery . T h i s s p e c i e s h a s b e e n f i shed in E n g l a n d s i n c e the b e g i n n i n g o f the cen tu ry but c a t c h e s d id not e x c e e d 2 , 8 5 0 t until 1 9 3 1 ; Sco t t i sh c a t c h e s a p p e a r o n r e c o r d in 1 9 5 4 . T o t a l sp iny dogf i sh c a t c h e s in the U K r e m a i n e d b e t w e e n 6 , 0 0 0 - 1 0 , 0 0 0 t/yr du r ing the 6 0 ' s a n d p e a k e d at 1 9 , 4 0 0 t in 1 9 7 8 . D u r i n g 1 9 5 0 - 1 9 7 0 mos t o f the s p i n y dogf i sh w e r e c a u g h t in a m o u n t s 52 30,000 25,000 20,000 15,000 10,000 5,000 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Years S. acanthias rrt+jj Rays Var. elasm. Figure 2.10 E lasmobranch ca tches of former U S S R , by spec ies groups, during 1978-1991. (Data from F A O ) . 30,000 i r——r" """-'i 1 T"""""""""""i 1 1 r 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Years Var. elasm. Scot. Rays Scot. Dogfish Eng. Dogfish Engl. Rays ^ | All N. Irel. Figure 2.11 E la smobranch ca tches of U . K . , by country and spec ies groups, during 1978-1991. (Data from F A O ) . 53 d ic ta ted by l o c a l marke t d e m a n d , o r e i ther inc iden ta l ly by t r awle r s a i m i n g for s p e c i e s s u c h a s c o d , h a d d o c k a n d h a k e . A c c o r d i n g to K u n z l i k (1988) , there w e r e f i sher ies for b a s k i n g s h a r k s Cetorhinus maximus in the U K d u r i n g the 40 ' s , ma in ly o n the w e s t c o a s t of S c o t l a n d . M o s t o f t h e s e f i sher ies w e r e short l ived b e c a u s e o f marke t ing difficulties a n d e c o n o m i c fa i lures ( M a x w e l l 1952) . B a s k i n g s h a r k s w e r e hun ted mos t ly dur ing the s u m m e r with h a n d o r w h a l i n g h a r p o o n s from v e s s e l s a d a p t e d from o ther f i sher ies , but c a t c h e s n e v e r s u r p a s s e d 3 0 0 s h a r k s pe r y e a r (approx . 6 0 0 t/yr). A s in the c a s e o f N o r w e g i a n a n d o ther b a s k i n g sha rk f i she r i es in the w o r l d , they w e r e ma in ly a i m e d at the l ivers . P r e s e n t c a t c h e s are m i n i m a l , s i n c e 1 9 8 3 , o n l y a s ing le boat f i shes oppor tun i s t i ca l ly for b a s k i n g s h a r k s in S c o t l a n d . P o r b e a g l e s h a r k s h a v e b e e n s p o r a d i c a l l y l a n d e d in s m a l l quant i t ies (no m o r e than 3 0 t/yr) in the U K , m a i n l y o n a n inc iden ta l b a s i s . T h e o n l y e x c e p t i o n is a s i ng l e oppor tun i s t i c event dur ing 1 9 8 7 - 1 9 8 8 w h e n p o r b e a g l e s w e r e u n u s u a l l y a b u n d a n t for a c o u p l e o f m o n t h s in the S h e t l a n d s a n d 3 5 - 4 5 t w e r e t a k e n in four m o n t h s ( G a u l d 1989) . A l t h o u g h U K c a t c h e s o f s k a t e s a n d rays a re l a rger in the Nor th S e a , m o s t o f the a v a i l a b l e informat ion is f rom the Irish S e a . Br i t i sh f i sher ies for s k a t e s a n d rays in the Irish S e a a re s u s t a i n e d m a i n l y by four s p e c i e s , in o r d e r o r i mpor t ance : Raja montagui, R. clavata, R. brachyura a n d R. naevus ( H o l d e n 1977) . F i s h i n g p r e s s u r e h a s a p p a r e n t l y c a u s e d a d e c l i n e in s o m e o f the l o c a l s t o c k s . B r a n d e r ( 1 9 7 7 , 1 9 8 1 ) c o n s i d e r s that s k a t e s a n d r ays o f the Irish s e a a re in n e e d o f i m m e d i a t e m a n a g e m e n t m e a s u r e s in o r d e r to a l l o w s t o c k s to r e c o v e r a n d attr ibutes the d i s a p p e a r a n c e o f Raja batis from the Irish s e a to e x c e s s i v e c o m m e r c i a l f i sh ing . A c c o r d i n g to da t a s u m m a r i s e d in R y l a n d a n d A j a y i (1984) , s t o c k s o f rays in the Br i s to l C h a n n e l w h i c h u s e d to p rov ide 2 7 % of the U K ray c a t c h , w e r e h a l v e d du r ing 1 9 6 4 -1974 . F o r the Nor th S e a , V i n t h e r a n d S p a r h o l t (1988) rough ly e s t ima te the b i o m a s s for R. radiata a n d for a l l r ays dur ing the m i d 80 ' s a s 1 6 0 , 0 0 0 - 2 5 2 , 0 0 0 t a n d 2 9 4 , 0 0 0 - 4 6 4 , 0 0 0 t r e spec t ive ly . D a t a p r e s e n t e d by t h e s e au tho r s s u g g e s t d e c l i n e s in the a b u n d a n c e of R. batis, R. clavata, R. naevus a n d i n c r e a s e s in a b u n d a n c e o f R. radiata. L a t e r e s t i m a t e s o f the b i o m a s s o f R. radiata a re of 1 0 0 , 0 0 0 t (Sparho l t a n d V i n t h e r 1991) . R e s e a r c h o n e l a s m o b r a n c h s is c o m p a r a t i v e l y a b u n d a n t in Br i t a in : h o w e v e r , m a n a g e m e n t 54 s e e m s to be a l o n g n e g l e c t e d a r e a . D u r i n g a p e r i o d of y e a r s , a g o o d a m o u n t o f r e s e a r c h w a s d e v o t e d to the s t o c k s of s p i n y dogf i sh in the U K ( H o l d e n 1 9 6 8 , H o l d e n a n d M e a d o w s 1962 , 1964) . H o w e v e r , desp i t e the g e n e r a l g u i d e l i n e s p r o p o s e d by H o l d e n b a s e d o n his a s s e s s m e n t o f the f ishery, it s e e m s that regula t ion w a s n e v e r i m p l e m e n t e d for this s t o c k . D e s p i t e the ava i lab i l i ty of a r e a s o n a b l e n u m b e r o f b a s i c s t ud i e s o n rays in the U K , there s e e m s to be n o m a n a g e m e n t spec i f i ca l ly d i r ec t ed to t h e s e f i shes . T h i s might be due , at leas t part ial ly, to the c o m p l i c a t i o n s o f set t ing m a n a g e m e n t r egu la t ions for mul t i spec i f ic f i sher ies ( e s p e c i a l l y bot tom-trawl f i sher ies) d u e to t e c h n i c a l in te rac t ions . I reland. E l a s m o b r a n c h f i she r i es of the Irish R e p u b l i c h a v e b e e n o f m i n o r i m p o r t a n c e w o r l d w i d e until 1 9 8 5 , w h e n c a t c h e s a t t a ined m o r e than 10 ,000 t/yr (fig. 2 .2) . In the p e r i o d 1987-1991 they on ly con t r ibu ted 1.03 % to the w o r l d c a t c h of e l a s m o b r a n c h s . D e s p i t e this l ow profile, e l a s m o b r a n c h f i sher ies h a v e b e e n re la t ively important for I re land in recent t imes , r ep re sen t ing 3 .03 % of the total f i sher ies p roduc t ion of this coun t ry . T h i s figure r anks re la t ively h igh c o m p a r e d to o ther major e l a s m o b r a n c h - f i s h i n g c o u n t r i e s (table 2.2) . R a y s h a v e b e e n exp lo i t ed for a l o n g t ime in I re land in s m a l l quant i t ies . S p i n y dogf i sh is the o ther m a i n e l a s m o b r a n c h a n d h a s g a i n e d m u c h at tent ion s i n c e the b e g i n n i n g of the 80 ' s . S i n c e 1 9 8 3 , s p i n y dogf i sh c a t c h e s h a v e m a d e the major p ropor t ion of the total e l a s m o b r a n c h c a t c h (fig. 2 .12) . D u r i n g the p e r i o d 1978 -1991 rays a n d dogf i sh w e r e equa l ly r e p r e s e n t e d in the e l a s m o b r a n c h c a t c h e s o f Ireland with 3 ,048 t/yr a n d 3 ,067 t/yr r e spec t ive ly . W h i l e the c a t c h e s of rays h a v e r e m a i n e d prac t ica l ly u n c h a n g e d s i n c e 1978 , dogf i sh c a t c h e s i n c r e a s e d t r e m e n d o u s l y in l e s s t han five y e a r s , h a d a s m a l l fall in 1986 a n d r e c o v e r e d a n d fell a g a i n after three y e a r s . S ta t i s t ics for 1989 -1991 s u g g e s t a relat ive stabili ty h a s b e e n a c h i e v e d in this f i shery . F a h y (1989a ,b , 1991) a n d F a h y a n d G l e e s o n (1990) c o v e r m o s t o f w h a t is k n o w n from the recent e l a s m o b r a n c h f i she r i es o f Ireland a n d mos t o f the fo l lowing informat ion is t a k e n from t h e s e s o u r c e s . R e c o r d i n g of ray l a n d i n g s g o e s b a c k to 1 9 0 3 . N o m o r e than 6 0 0 t/yr w e r e r e c o r d e d before 1940 , w h e n c a t c h e s b e g a n to r ise s l o w l y (partially d u e to i n c r e a s e d c o n s u m p t i o n in Ireland), up to the late 70 ' s w h e n they sha rp ly i n c r e a s e d r e a c h i n g 3 ,0001 in 1 9 8 5 . R a y c a t c h e s h a v e 55 t radi t ional ly b e e n t a k e n in grea tes t quant i t ies ( a round 5 0 % of the total) in the eas t c o a s t of Ireland, s i n c e 1 9 7 5 abou t 2 5 % h a s b e e n t a k e n from the north coas t , a n d the rest c a m e from the sou th a n d w e s t c o a s t s . M o s t o f the l a n d i n g s are not so r t ed by s p e c i e s but by a c a s u a l p r o c e s s de f ined by s imi la r i t ies in s i z e a n d a p p e a r a n c e . A t l eas t 18 t r awl ing v e s s e l s ca t ch rays from e a s t e r n Irish ports . Th i r t een otter t r awlers a n d four b e a m t rawle r s ope ra te from the sou theas t , but m o r e v e s s e l s a re s u s p e c t e d to t ake part in the f i shery . A l t h o u g h mos t o f t he se v e s s e l s c a t c h rays inc iden ta l ly to p r a w n s a n d bot tom te leos t s , a s m a l l ray f ishery a p p e a r s to be run o n a s e a s o n a l b a s i s by s o m e o f the s o u t h e a s t v e s s e l s . A t least n ine s p e c i e s o f rays a re found in the c o m m e r c i a l c a t c h e s . Raja brachyura, R. clavata, R. naevus a n d R. montagui, a re the mos t c o m m o n rays , rough ly in o r d e r o f impor t ance , a n d R. microocellata, R. batis, R. fullonica, R. undulata a n d R. alba, a re c a u g h t on ly s p o r a d i c a l l y . L a n d i n g s a re mos t ly c o m p o s e d of s m a l l ( less t han 6 0 c m T L ) a n d m e d i u m s i z e d rays (60-70 c m T L ) a c c o u n t i n g for 6 0 - 8 0 % of the we igh t . M o s t s p e c i e s a re totally recrui ted to the f i shery after a g e 2 , but R. naevus recrui ts at a g e 3. In the eas t coas t , at leas t 5 0 % of the c a t c h e s o f R. clavata a n d R. brachyura a re m a d e o f 0-2 y e a r o l d . To ta l mortal i ty e s t i m a t e s f o r t h e mos t important s p e c i e s m e n t i o n e d a b o v e r a n g e from 0 .54-0 .74 . A l t h o u g h the p o p u l a t i o n s a re heav i ly exp lo i t ed s p e c i a l l y in the s o u t h e a s t f ishery, they con t inue to p r o d u c e g o o d y i e l d s . T h e r e a re dogf i sh f i she r i es a l l a r o u n d the coun t ry but they h a v e c o n c e n t r a t e d o n the w e s t coas t . C a t c h e s w e r e h igh in the north ( C o . D o n e g a l ) du r ing 1 9 8 2 - 1 9 8 5 but con t r ibu t ions in the sou th ( C o . Ker ry) i n c r e a s e d du r ing 1 9 8 6 - 1 9 8 7 a s a result o f effort b e i n g shifted to the sou th d u e to d e c r e a s i n g c a t c h e s in the north. D o g f i s h e s in the pas t w e r e c o n s i d e r e d a n u i s a n c e , but n o w the f i shery is spec i f i ca l ly d i rec ted at t h e m . In the w e s t c o a s t f i sher ies , otter t r awlers f ish m a i n l y m a l e dogf i sh in wa t e r s s o m e t i m e s e x c e e d i n g 100 m d e e p , wh i l e monof i l amen t g i l lnets of 6.4 c m m e s h s i z e a re u s e d in s h a l l o w w a t e r s w h e r e they c a t c h great p ropor t ions o f p regnan t f e m a l e s . S p i n y dogf i sh in w e s t I re land fully recruit to the f ishery at a r o u n d 17 y r o f a g e a n d total mortal i ty coeff ic ients h a v e b e e n e s t i m a t e d at 0.24 a n d 0 .30 for f e m a l e s a n d m a l e s r e spec t ive ly . F a h y a n d G l e e s o n (1990) report that month ly C P U E o f gi l lnet ters in C a r r i g a h o l t h a s p l u m m e t e d by 8 0 - 9 0 % in a t w o - y e a r p e r i o d . A v a i l a b l e informat ion is insufficient to m a k e definite c o n c l u s i o n s abou t dep l e t i on o f the s t ocks , but it s e e m s that t h e s e a re c l o s e to b e i n g ove r f i shed . T o t a l f e m a l e s p a w n i n g b i o m a s s for C a r r i g a h o l t w a s e s t i m a t e d at 5 ,7001 by F a h y a n d G l e e s o n . M o s t o f the c a t c h e s a re d e s t i n e d 56 for expor t but no informat ion o n the g e n e s i s o f th is f i shery c o u l d be l o c a t e d . A f i shery for b a s k i n g s h a r k s b e g a n in 1947 at K e e m B a y o n the w e s t c o a s t of Ireland ( K u n z l i k 1988) . Initially h a r p o o n s a n d nets w e r e u s e d , but by 1951 o n l y ne ts w e r e u s e d . T h e s e w e r e e i ther e n c i r c l i n g ne ts o r en t ang l ing nets built o f s i s a l wi th m e s h s i z e s of 33 c m a n d set p e r p e n d i c u l a r to the s h o r e . Initially, the l iver w a s the o n l y target o f the f i shery but in later y e a r s f ins a n d mea t w e r e a l s o u s e d for h u m a n c o n s u m p t i o n . In 1 9 7 3 h a r p o o n s w e r e r e in t roduced to th is f i shery a n d a n o t h e r h a r p o o n f ishery s tar ted in the s o u t h eas t c o a s t of I reland. T h e w e s t c o a s t f i shery r e a c h e d its h ighes t y i e l d s ( a r ound 1,500 s h a r k s annua l ly ) dur ing the ea r ly 50 ' s a n d d e c l i n e d after 1 9 5 5 , p r o b a b l y a s a r e s p o n s e to the sh r ink ing marke t for l ivers w h i c h brought d o w n f i sher ies for s h a r k s a l l o v e r the w o r l d . C a t c h e s r e m a i n e d b e l o w 100 s h a r k s / y e a r dur ing mos t o f the p e r i o d 1 9 6 3 - 1 9 7 3 a n d i n c r e a s e d to a l m o s t 4 0 0 s h a r k s in 1 9 7 5 w h e n the last r e c o r d s w e r e t a k e n . S o m e tr ials to d e v e l o p a c o m m e r c i a l b lue s h a r k f i shery with l ong l i ne s off the sou th c o a s t o f I re land w e r e u n d e r w a y du r ing 1 9 9 0 ( C r u m m e y et a l . 1991) . F r a n c e . F r e n c h e l a s m o b r a n c h c a t c h e s s u g g e s t a n o t h e r re la t ively s tab le f i shery . T w o p e r i o d s of re la t ively s u s t a i n e d c a t c h e s are ident i f iable . F r o m 1948 to 1 9 6 0 c a t c h e s o s c i l l a t e d n e a r 15 ,000 t/yr, t hen shifted in 1961 to higher , m o r e va r i ab l e c a t c h e s a r o u n d 3 5 , 0 0 0 t/yr (fig. 2.2) . E l a s m o b r a n c h s represen t 3 .78 % of the total f i shery p roduc t ion o f F r a n c e , the h ighes t a m o n g E u r o p e a n coun t r i e s a n d rather h igh in g l o b a l t e rms . F r e n c h c a t c h e s m a k e 4 . 7 9 % of the w o r l d e l a s m o b r a n c h y i e l d . B e t w e e n 1 9 7 8 a n d 1 9 9 1 , F r e n c h c a t c h e s o f s k a t e s a n d "va r ious d o g f i s h e s " w e r e s t ab le . S p i n y dogf i sh , "va r ious e l a s m o b r a n c h s " a n d p o r b e a g l e c a t c h e s s h o w e d a s l ight d e c l i n i n g t rend (fig. 2 .13) . D u r i n g this pe r iod , s k a t e s a v e r a g e d 4 2 % (14 ,499 t/yr) of the total e l a s m o b r a n c h c a t c h e s w h i l e sp iny dogf i sh , v a r i o u s dogf i shes , v a r i o u s e l a s m o b r a n c h s a n d p o r b e a g l e s a v e r a g e d 3 2 % (10 ,806 t/yr), 18 % (6 ,139 t/yr), 6 % (2 ,103 t/yr) a n d 2 % (531 t/yr) r e spec t ive ly . S p i n y dogf i sh a n d s k a t e s a re c a u g h t by F r e n c h v e s s e l s ma in ly in the Nor theas t A t l an t i c but s m a l l c a t c h e s o f s k a t e s a re a l s o t a k e n in the nor thwes t A t l an t i c a n d the M e d i t e r r a n e a n S e a . A c c o r d i n g to G a u l d (1989) , a s m a l l flotil la o f F r e n c h v e s s e l s b a s e d 57 12,000 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Years S. acanthias Iftttl Rays Figure 2.12 E la smobranch ca tches of Ireland, by spec ies groups, during 1978-1991. (Data from F A O ) . 45,000 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Years L. nasus Var . e lasm. S. acanthias HjjtH Ska tes & Rays Var . dogfish Figure 2.13 E la smobranch ca tches of France, by spec ies groups, during 1978-1991. (Data from F A O ) . 58 in B r i t a n n i a spec i f i ca l ly target p o r b e a g l e s with l ong l i ne s in the B a y o f V i s c a y a n d Irish wa te r s , m a k i n g a b o u t 7 5 % of the total p o r b e a g l e c a t c h e s o f F r a n c e . T h e r e m a i n d e r is l a n d e d a s a b y c a t c h o f t rawl a n d s e i n e f i sher ies . T e t a r d ( 1 9 8 9 a , 1989b) s u m m a r i s e s informat ion abou t s h a r k a n d ba to id f i she r i es for F r a n c e . H i s informat ion a l l o w s further s e p a r a t i o n o f c a t c h stat is t ics into s p e c i e s o r s p e c i e s g r o u p s . T h e fo l lowing is f rom his a c c o u n t . T h e c a t c h o f ba to ids o f F r a n c e i n c l u d e s at leas t e ight s p e c i e s o f s k a t e s a n d rays . T h e s epa ra t i on o f ray s p e c i e s is p o s s i b l e b e c a u s e e a c h s p e c i e s at tains a different p r ice . Raja naevus a n d R. clavata a re the m o s t impor tant s p e c i e s a c c o u n t i n g r e s p e c t i v e l y for abou t 2 5 % a n d 17 % of the ba to id l a n d i n g s du r ing 1 9 7 8 - 1 9 8 7 . Raja montagui a n d a g roup f o r m e d by R. batis a n d R. oxyrinchus m a k e 4 % a n d 3 % respec t ive ly . Dasyatis pastinaca, Myliobatis aquila a n d Raja fullonica a re s p e c i e s of mino r impor t ance con t r ibu t ing o n l y 1 % of the c a t c h e s . F ina l ly a g roup o f unident i f ied rays m a k e s the r e m a i n i n g 5 0 % . M o s t o f the F r e n c h c a t c h e s of rays a re t a k e n a r o u n d the C e l t i c S e a a n d the E n g l i s h C h a n n e l a n d to s o m e extent in the Irish s e a a n d the Nor th o f the B a y of B i s c a y . R a y s a re mos t ly c a u g h t by bot tom t rawl ing o p e r a t i o n s . Raja clavata is ac t ive ly sough t for its h igh ly d e s i r e d meat . T e t a r d h ighl ights the a l m o s t c o m p l e t e d i s a p p e a r a n c e of R. alba f rom the c a t c h e s a n d the appa ren t l y d e c l i n i n g c a t c h e s o f R. clavata. M e a n w h i l e , y i e l d s of R. naevus s e e m to be i n c r e a s i n g . H e a l s o no tes that a n unc i t ed s tudy ind ica te s that the y i e l d p e r recruit o f R. naevus is at a n o p t i m a l v a l u e . J u d g i n g from T e t a r d s ' a ccoun t , it a p p e a r s that no m a n a g e m e n t regu la t ions a re in p l a c e for a n y o f t h e s e s p e c i e s in F r e n c h wa te r s . F r e n c h s h a r k l a n d i n g s a re chief ly c o m p o s e d o f sp iny dogf i sh a n d c a t s h a r k s . T h e latter a re ma in ly Scyliorhinus canicula wi th a m i n o r c o m p o n e n t o f S . stelaris. C a t s h a r k c a t c h e s are al l i nc iden ta l to t r awle r a n d longl ine f i sher ies a n d m a k e abou t 3 2 % of the sha rk c a t c h . T h e sp iny dogf i sh f i shery is o n e o f the few d i rec ted f i sher ies for s h a r k s in F r a n c e , a c c o u n t i n g for a l m o s t 5 7 % of a l l s h a r k l a n d i n g s . D u r i n g 1987 , a p p r o x i m a t e l y 2 7 l ong l ine r boa t s 8-25 m long (three o f t h e m with au toma t i c longl iners ) , w e r e potent ia l ly t a rge t ing s p i n y dogf i sh . H o w e v e r , abou t 8 0 % of the l a n d i n g s c a m e from bot tom t rawlers . T h e m a i n f i sh ing g r o u n d s of the F r e n c h f i shery for s p i n y dogf i sh a re the C e l t i c S e a , a n d former ly , Nor the rn Irish wa t e r s a n d the Nor th S e a . T o p e , Galeorhinus galeus, r a n k s third in i m p o r t a n c e a m o n g sha rk c a t c h e s with a b o u t 6 % of the total , but c a t c h e s s e e m to be in c l e a r d e c l i n e . T h e F r e n c h 59 f ishery for p o r b e a g l e s is a l s o a d i rec ted s h a r k f i shery w h i c h r e p r e s e n t s a b o u t 3 % of the sha rk c a t c h . S o m e shortfin m a k o s h a r k s a re c a u g h t inc iden ta l ly in the l ong l i ne s o f this f i shery . A b o u t 7 5 % of the l a n d i n g s a re from long l ine r s a n d the rest f rom t rawle rs . M a i n f i sh ing g r o u n d s for the F r e n c h p o r b e a g l e f i shery a re offshore w a t e r s f rom S p a i n to Ireland in winter , c l o s e r to s h o r e a n d a r o u n d the C h a n n e l I s lands in s p r i n g . S m o o t h - h o u n d s , Mustelus mustelus a n d M. asterias m a k e abou t 1 % of the total F r e n c h s h a r k c a t c h . S o m e mino r quant i t ies o f b lue sha rk a n d a n g e l shark , Squatina squatina, a r e l a n d e d inc identa l ly to long l ine a n d t rawl f i sher ies r e spec t ive ly . F r a n c e is both the major p r o d u c e r a n d impor te r o f sha rk in E u r o p e . B e c a u s e o f its h igh expor t s of m a i n l y p o r b e a g l e a n d tope to Italy, F r a n c e h a s a deficit o f s h a r k meat , thus impor ts h a v e i n c r e a s e d s i n c e 1 9 8 2 (9 ,000 t in 1986) . H o w e v e r , s o m e p r o b l e m s re la ted to m e r c u r y con ten t o f s h a r k mea t s e e m to limit F r e n c h expor t s to Italy c o n s t r a i n i n g the f i shery for p o r b e a g l e s h a r k s . T h e h o m e marke t is a l s o i n c r e a s i n g . T h e m e a t o f Lamna nasus, Squalus acanthias a n d Galeorhinus galeus h a s g o o d in ternal d e m a n d for h u m a n c o n s u m p t i o n a s "sa lmonet te" ( saumonet te ) in s c h o o l refector ies a n d res taurants . T h e internal d e m a n d for Squalus acanthias is not met by F r e n c h l a n d i n g s a n d c o n s i d e r a b l e quant i t ies h a v e b e e n impor t ed from the U K . S p a i n . S p a n i s h e l a s m o b r a n c h c a t c h e s w e r e s t eady dur ing 1947 -1971 w h e n y i e l d s v a r i e d wi th in a na r row b a n d o f 1 0 , 0 0 0 - 1 5 , 0 0 0 t/yr. T h i s w a s f o l l o w e d by a s u d d e n c o l l a p s e in the ea r ly 70 ' s a n d a later s l o w r e c o v e r y in the 80 ' s up to the recent l eve l o sc i l l a t i ng w i ld ly at 1 5 , 0 0 0 - 2 0 , 0 0 0 t/yr (fig. 2 .2) . E l a s m o b r a n c h s on ly m a k e 1.3 % of the total f i shery p roduc t ion o f S p a i n a n d contr ibute on ly 1.2 % of the w o r l d e l a s m o b r a n c h c a t c h (table 2 .2) . D i s a g g r e g a t e d da t a for the y e a r s 1978-1991 ind ica te s that the major r e a s o n for the recent i n c r e a s e in c a t c h e s are the ska te f i sher ies w h i c h h a v e g r o w n cons i s t en t l y in the pe r iod 1 9 8 0 - 1 9 8 7 (fig. 2 .14) . T h e bulk of s k a t e s c o m e s from o p e r a t i o n s in the N o r t h w e s t A t l an t i c ( ave rage o f 8 0 % of ska te c a t c h e s ) a n d the rest from the nor theas t e rn At l an t i c . N o informat ion o n the relat ive impor t ance of the s p e c i e s in the c a t c h e s is a v a i l a b l e . S h a r k c a t c h e s t a k e n m a i n l y in the Nor theas t A t l an t i c h a v e a l s o i n c r e a s e d in a s i m i l a r w a y . T h e s e 6 0 inc lude shortfin m a k o s Isurus oxyrinchus, p o r b e a g l e s Lamna nasus, s m a l l - s p o t t e d c a t s h a r k Scyliorhinus canicula a n d s o m e s q u a l o i d s . V a r i o u s s p e c i e s o f r ays a r e f i shed in s m a l l quant i t ies m a i n l y in the M e d i t e r r a n e a n S e a a l o n g with u n s p e c i f i e d e l a s m o b r a n c h s , w h i c h are a l s o c a u g h t in the cen t ra l e a s t e rn A t l an t i c ( F A O A r e a 37) . In th is a r e a , s k a t e s m a k e up 6 3 % (7 ,125 t/yr) a n d u n s p e c i f i e d s h a r k s 21 % (2 ,259 t/yr) o f e l a s m o b r a n c h c a t c h e s , the cont r ibut ion o f "var ious e l a s m o b r a n c h s " w a s o n l y 11 % (1 ,168 t/yr). A l l e l a s m o b r a n c h f i sher ies in S p a i n a re inc iden ta l c a t c h e s o f e i ther t r awl o r long l ine f i sher ies (R. M u n o z - C h a p u l i , pe rs . c o m m . J a n . 2 1992) . M u n o z - C h a p u l i ( 1985a ) repor ts o n the l a n d i n g s o f S p a n i s h c o m m e r c i a l bot tom t rawlers o p e r a t i n g in d e p t h s up to 5 0 0 m . Scyliorhinus canicula d o m i n a t e s the l a n d i n g s from the mou th o f the M e d i t e r r a n e a n , the sou the rn c o a s t o f S p a i n a n d c o a s t s o f nor thwes t A f r i c a . Centrophorus granulosus a n d Squalus blainvillei a re a l s o l a n d e d from the se a r e a s . In the en t r ance o f the M e d i t e r r a n e a n , Galeus melastromus is a l s o important in the c a t c h e s w h i l e a n o t h e r 11 s p e c i e s a re caugh t in s m a l l e r a m o u n t s in both r eg ions (table 2 .6) . M u n o z - C h a p u l i (1985b) repor ts that l a n d i n g s from long l ine v e s s e l s f i sh ing in the eas t e rn A t l an t i c f rom the A z o r e s to the C a p e V e r d e Is lands , a re d o m i n a t e d by Prionace glauca, Isurus oxyrinchus a n d Sphyrna zygaena, w h i l e 13 o ther s p e c i e s a re of m i n o r impor t ance (table 2 .6) . V e r y l ikely , both repor ts reflect not on ly the a b u n d a n c e of the s p e c i e s in s u c h a r e a s but a l s o the s e l e c t i o n o f s p e c i e s o n b o a r d . S p a n i s h sword f i sh long l ine r s c a u g h t 3 0 4 1 o f shortfin m a k o s a n d 2 0 1 o f p o r b e a g l e s from the north a n d cen t ra l eas t A t l an t i c dur ing 1984 (Mejuto 1985) . M a k o s w e r e m o r e a b u n d a n t dur ing S e p t - D e c a n d c a t c h e s w e r e ma in ly c o m p o s e d o f s h a r k s 1 0 0 - 2 4 0 c m F L with m a l e s more than d o u b l i n g the n u m b e r s of f e m a l e s . P o r b e a g l e c a t c h e s w e r e m o r e a b u n d a n t in M a r c h , S e p t e m b e r a n d O c t o b e r a n d ind iv idua l s w e r e mos t ly 1 5 0 - 2 2 5 c m F L , with m a l e s d o u b l i n g the n u m b e r s o f f e m a l e s . Italy. J u d g i n g from the h is tor ica l impor ts o f s h a r k s from N o r w a y (po rbeag l e s ) , F r a n c e (po rbeag l e s a n d tope) a n d A r g e n t i n a ( smoo th -hounds ) , e l a s m o b r a n c h m e a t s e e m s to be w e l l a p p r e c i a t e d in Italy. N o n e t h e l e s s , s h a r k s a n d rays h a v e b e e n for l o n g o f m i n o r impor t ance in Italian f i sher ies . C a t c h e s d id not e x c e e d 6 ,000 t/yr until the m i d 80 ' s w h e n m o r e than 10 ,000 t/yr w e r e t a k e n (fig. 2 .2) . Cur ren t ly , e l a s m o b r a n c h s r ep resen t o n l y 1.89 % of the total 61 25,000 20,000 15,000 10,000 5,000 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Years Porbeagle | I Sharks Rays N. Atlantic FFrT-H Rays rest of Atlan. Var. elasm. Figure 2.14 Elasmobranch catches of Spain, by species groups, during 1978-1991. (Data from FAO). Table 2.6 Shark species reported in Spanish commercial fisheries (adapted from Munoz-Chapuli 1985 a,b). Demersal Pelagic Hexanchus griseus Lamna nasus Heptranchias perlo Isurus oxyrinchus Squalus acanthias l.paucus S. blainvillei Alopias vulpinus Centrophorus granulosus A superciliosus C. lusitanicus Carcharhinus brevipinna Deania calcea C. falciformis Dalatias licha C. longimanus Squatina squatina C. obscurus S. aculeata C. plumbeus Galeus melastomus C. signatus Mustelus mustelus Prionace glauca M. asterias Galeorhinus galeus Sphyrna zygaena S. lewini 6 2 f ishery c a t c h e s o f Italy. Fu r the rmore , the Italian c a t c h o f s h a r k s a n d r ays m a k e s on ly 1.51 % of the w o r l d e l a s m o b r a n c h c a t c h (table 2 .2) . D u r i n g the p e r i o d 1 9 7 8 - 1 9 9 1 , s m o o t h - h o u n d s Mustelus s p p . a v e r a g e d 5 2 % (4 ,463 t/yr) of the e l a s m o b r a n c h c a t c h e s , r ays con t r ibu ted 38 % (3 ,340 t/yr) a n d v a r i o u s e l a s m o b r a n c h s 10 % (860 t/yr). C a t c h e s o f a l l e l a s m o b r a n c h g r o u p s g r e w s imi la r ly du r ing the e x p a n s i o n of the f i shery w h i c h p e a k e d in 1 9 8 5 a n d then d e c l i n e d to b o u n c e b a c k in 1990 -1991 (fig. 2 .15) . S m o o t h - h o u n d s w e r e a l l f i shed in M e d i t e r r a n e a n wa te r s , a l o n g wi th 91 % of the ray c a t c h . T h e rest o f r ays w e r e c a u g h t in F A O A r e a s 34 , 4 7 , 4 8 , 51 a n d 2 1 . C a t c h e s o f v a r i o u s e l a s m o b r a n c h s w e r e t a k e n in F A O A r e a 34 (70 %) a n d A r e a s 4 7 (7 % ) , 51 (16 %) a n d 41 (7 % ) . S m a l l c a t c h e s o f b lue s h a r k s Prionace glauca a re l a n d e d a s a b y c a t c h of the drift long l ine swordf i sh a n d a l b a c o r e f i sher ies of the G u l f o f T a r a n t o , w h e r e a v e r a g e s o f 14.5 t/yr a n d 4 t/yr w e r e l a n d e d r e spec t ive ly in e a c h f ishery dur ing 1978 -1981 ( D e M e t r i o e t a l . 1984) . D u r i n g th is p e r i o d , a n a v e r a g e of 12 boa t s f i shed for swordf i sh from A p r i l to A u g u s t u s i n g b e t w e e n 7 0 0 a n d 1 0 0 0 h o o k s ( M u s t a d no . 1) per boat . A d d i t i o n a l l y , a n a v e r a g e o f 4 4 boa t s f i shed for a l b a c o r e du r ing A u g u s t to D e c e m b e r with 2 , 0 0 0 h o o k s (3 c m long) pe r boat . D u e to the different h o o k s i z e a n d p robab ly a l s o to s e a s o n a l c y c l e s o f the s p e c i e s , the swordf i sh boa ts c a u g h t b lue s h a r k s of 2 5 kg a v e r a g e we igh t w h e r e a s b lue s h a r k s from the a l b a c o r e boa t s a v e r a g e d 3 k g . D e Met r io et a l . (1984) report that the mea t o f Prionace glauca is f raudulent ly s o l d in Italy a s Mustelus. It is therefore v e r y l ikely that the b lue sha rk ca t ch is p robab ly repor ted u n d e r Mustelus s p p . in the official s ta t is t ics . 2 .2 .2 .3 A f r i c a a n d Indian subcon t inen t . Information a b o u t e l a s m o b r a n c h f i sher ies in this r eg ion is par t icu lar ly s c a r c e . S ta t i s t ics from mos t o f t h e s e major e l a s m o b r a n c h - f i s h i n g coun t r i e s g ive little de ta i l o f the c a t c h c o m p o s i t i o n a n d literature s o u r c e s a re not on ly l imited but a l s o v e r y difficult to ob t a in . T h i s is p robab ly direct ly re la ted to the e c o n o m i c d e v e l o p m e n t of t he se coun t r i e s w h i c h a re l o c a t e d in o n e of the mos t e c o n o m i c a l l y d e p r e s s e d a r e a s of the w o r l d . N i g e r i a . N i g e r i a is the o n l y A f r i c a n nat ion with major e l a s m o b r a n c h f i she r ies . F A O stat is t ics for 63 16,000 14,000 12,000 10,000 8,000 6,000 4,000 2,000 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Years Mustelus spp. Rays Var. elasm. Figure 2.15 Elasmobranch catches of Italy, by species groups, during 1978-1991 (Data from FAO). 64 N i g e r i a a re p o o r a n d h a v e o n l y a p p e a r e d regula r ly s i n c e 1 9 7 0 . N i g e r i a h a s a n uns tab le f i shery with a n o v e r a l l t r end o f d e c r e a s i n g c a t c h e s fal l ing from the m o r e than 3 0 , 0 0 0 t/yr caugh t in the ea r ly 70 ' s to l e s s than 10 ,000 t s i n c e 1986 (fig. 2 .2) . H o w e v e r , wi thout a n y b a c k g r o u n d informat ion it is difficult to ven tu re into a n interpretat ion o f t h e s e fa l l ing c a t c h e s . D e s p i t e the fall in y i e l d s , e l a s m o b r a n c h s con t inue to be a re la t ive ly impor tant r e s o u r c e for N i g e r i a , recent ly ( 1 9 8 7 - 1 9 9 1 ) cont r ibut ing 2 .92 % of the total f i shery p roduc t ion of this count ry . T h e c a t c h of s h a r k s a n d rays of N i g e r i a con t r ibu tes o n l y 1.91 % to the w o r l d total . F A O da t a from 1977 -1991 s h o w that mos t of the c a t c h e s a re not r e c o r d e d by s p e c i e s . A g roup o f "var ious e l a s m o b r a n c h s " a c c o u n t s for 8 9 % (15 ,827 t/yr) of the c a t c h e s , wh i l e S q u a l i d a e a n d a g r o u p o f s k a t e s a n d rays a c c o u n t s for l e s s t han 1 % (7.6 t/yr) a n d abou t 10 % (1 ,703 t/yr) r e spec t i ve ly (fig. 2 .16) . N o further informat ion a b o u t the cha rac t e r i s t i c s of the f ishery, s p e c i e s , r e s e a r c h o r m a n a g e m e n t o f e l a s m o b r a n c h s in N i g e r i a c o u l d be found . P a k i s t a n . E l a s m o b r a n c h f i sher ies of P a k i s t a n h a v e b e e n fairly uns t ab l e . T h e y w e r e o f pr ime impor t ance o n a g l o b a l s c a l e until recent ly , w h e n y i e l d p l u m m e t e d . E l a s m o b r a n c h c a t c h e s of P a k i s t a n g r e w a l m o s t exponen t i a l l y from the late 40 ' s to a first p e a k o f abou t 7 5 , 0 0 0 t in 1973 , d r o p p e d a b o u t 5 0 % the fo l lowing three y e a r s a n d then r e c o v e r e d to p e a k l eve l s for a n o t h e r 6 y e a r s . Y i e l d c o l l a p s e d in 1 9 8 3 but h a s r e c o v e r e d o v e r the last 10 y e a r s to the p resen t l eve l s o f abou t 4 5 , 0 0 0 t (fig. 2 .2) . G i v e n the sca rc i ty o f d i rec t informat ion o n P a k i s t a n i f i she r i es it is v e r y difficult to a s s e s s the r e a s o n s for t h e s e d r a m a t i c c h a n g e s in e l a s m o b r a n c h c a t c h . T h e relat ive impor t ance o f e l a s m o b r a n c h s in P a k i s t a n is a m o n g the h ighes t in the w o r l d , r ep re sen t ing 7 .42 % of the total na t iona l c a t c h e s d u r i n g 1 9 8 7 - 1 9 9 1 . T h i s l eve l mus t h a v e b e e n at leas t d o u b l e dur ing the b o n a n z a o f the late 70 ' s . P a k i s t a n con t r ibu tes 4 .99 % of the w o r l d e l a s m o b r a n c h p roduc t ion (table 2 .2) . G r e y s h a r k s ( C a r c h a r h i n i d a e ) a n d ba to ids const i tute mos t o f the c a t c h e s , a v e r a g i n g 4 5 % (20 ,200 t/yr) a n d 5 4 % (24 ,380 t/yr) of the e l a s m o b r a n c h y i e l d r e s p e c t i v e l y d u r i n g 1 9 7 7 - 1 9 9 1 . S i n c e 1 9 8 7 , c a t c h e s o f s a w f i s h e s (Pr i s t idae) a n d gui ta r f i shes ( R h i n o b a t i d a e ) h a v e b e e n repor ted s e p a r a t e l y but they o n l y a c c o u n t for <1% a n d 1% of the e l a s m o b r a n c h c a t c h e s r e spec t ive ly (fig. 2 .17) . W h i l e g rey s h a r k s c a t c h e s d e c l i n e d s t ead i ly d u r i n g the late 70 ' s a n d 6 5 35,000 30,000 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Years WMA Var . e lasm. Batoids Figure 2.16 E la smobranch ca tches of Nigeria, by spec ies groups, during 1977-1991. (Data from F A O ) . 80,000 70,000 10,000 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Years Grey sharks Batoids Guitar & s a w fishes Figure 2.17 E la smobranch ca tches of Pakis tan , by spec ies groups, during 1977-1991. (Data from F A O ) . 6 6 ea r ly 80 ' s , ba to id c a t c h e s d r o p p e d abrupt ly by 4 3 , 0 0 0 1 in a s i ng l e y e a r (1983) thus c a u s i n g the ove ra l l c o l l a p s e . G r e y s h a r k s h a v e s i n c e b e e n the major g r o u p in the e l a s m o b r a n c h c a t c h e s . D e t a i l e d informat ion a b o u t P a k i s t a n i e l a s m o b r a n c h f i sher ies is v e r y poor . A report f rom the Indo P a c i f i c T u n a D e v e l o p m e n t a n d M a n a g e m e n t P r o g r a m m e ( I P T P 1991) is abou t the o n l y s o u r c e o f informat ion . A c c o r d i n g to this d o c u m e n t , the port o f K a r a c h i is the o n l y l a n d i n g site for the m e c h a n i s e d gil lnet fleet in the entire c o a s t o f the S i n d p r o v i n c e . S h a r k s a re c a u g h t ma in ly by p e l a g i c gil lnet boa t s f i sh ing a s far a s S o m a l i a a n d the w a t e r s off Y e m e n a n d O m a n , a l t h o u g h s m a l l c a t c h e s a re a l s o l a n d e d by bot tom gi l lnet ters w o r k i n g in c o a s t a l a r e a s of P a k i s t a n . T h e e s t i m a t e d n u m b e r of m e c h a n i s e d gi l lnet ters in P a k i s t a n in 1 9 8 9 w a s 3 9 4 v e s s e l s , 185 in S i n d p r o v i n c e a n d 2 0 9 in B a l u c h i s t a n p r o v i n c e . V e s s e l s in K a r a c h i r ange in s i z e s f rom 2 0 to 2 5 m in length a n d 5 to 7 m in brea th a n d u s e d i e s e l e n g i n e s o f 8 8 - 1 3 5 H P . T h e s e f i sher ies a re ve ry important s o c i o - e c o n o m i c a l l y , e m p l o y i n g c o n s i d e r a b l e n u m b e r s o f f i s h e r m e n . S m a l l boa t s ca r ry 15-17 c r e w a n d m a k e tr ips o f a b o u t 10 d a y s , w h e r e a s la rger boa ts ca r ry up to 2 5 f i s h e r m e n a n d s tay at s e a for 2 0 - 3 0 d a y s a n d o c c a s i o n a l l y 6 0 d a y s . C a t c h e s a re u s u a l l y sal t d r i ed in the la rger v e s s e l s a n d kept o n ice in the s m a l l e r o n e s . G i l l ne t s a re h a n d - w o v e n out o f mult i f i lament p o l y a m i d e twine a n d a r e 8 0 m e s h e s d e e p a n d 2 .5-9 k m l o n g ( a v e r a g e of abou t 5.2). M e s h s i z e s a re 10-16 c m a n d m a i n l i n e s of 14-16 m m d iamete r . S h a r k c a t c h e s a re so r t ed into eight different s i z e c a t e g o r i e s , but the s p e c i e s a re not s e p a r a t e d . Effort in this f i shery i n c r e a s e d from 2 3 , 0 0 0 f i sh ing d a y s in 1 9 8 8 to 2 8 , 0 0 0 in 1 9 8 9 then fell to 2 6 , 0 0 0 in 1 9 9 0 . E s t i m a t e s ind ica te that a b o u t 93 % of the sha rk c a t c h c o m e s from p e l a g i c driftnet v e s s e l s a s o p p o s e d to d e m e r s a l o r m i x e d f i sh ing v e s s e l s . T h e y i e l d o f s h a r k s for this driftnetting fleet is e s t i m a t e d at a b o u t 3 ,860 t/yr du r ing 1 9 8 8 - 1 9 9 0 . S h a r k y i e l d du r ing this p e r i o d w a s co r r e l a t ed with d i s t a n c e to f i sh ing g r o u n d s . T h e largest c a t c h e s c o m i n g from S o m a l i a n wa te r s , the mos t dis tant f i shery . S h a r k y i e l d s d e c r e a s e d by abou t 4 4 % from 1 9 8 9 to 1990 , a l t hough this d e c l i n e w a s not e x c l u s i v e to s h a r k c a t c h e s , te leos t ( tunas etc.) y i e l d s fell 3 2 % dur ing the s a m e p e r i o d . S o m e efforts to in t roduce longl ine f i sh ing for s h a r k s , r ays a n d o the r s p e c i e s in P a k i s t a n are s u m m a r i s e d by P r a d o a n d D r e w (1991) . A p p a r e n t l y gi l lnets a re m u c h m o r e f a v o u r e d in P a k i s t a n b e c a u s e o f thei r h ighe r c a t c h rates of v a l u a b l e s p e c i e s . 6 7 India. B y t radi t ion, India h a s h a d important f i sher ies for e l a s m o b r a n c h s . T h e f i sher ies h a d a re la t ively s t e a d y growth up to the m i d 70 ' s , f o l l owed by a p e r i o d o f stabil i ty in c a t c h e s du r ing mos t o f the 80 ' s , then a t r e m e n d o u s i n c r e a s e in c a t c h e s in 1 9 8 7 , resu l t ing in India b e c o m i n g o n e o f the top three e l a s m o b r a n c h p r o d u c e r s in the w o r l d du r ing the last ten y e a r s (fig. 2.2) . T h e i m p o r t a n c e o f the Indian y i e l d of s h a r k s a n d rays is h igh l igh ted w h e n w e c o n s i d e r it r ep resen t s 8 .78 % of the w o r l d e l a s m o b r a n c h c a t c h . S t i l l , d u e m a i n l y to large in land f i sher ies , e l a s m o b r a n c h s d o not rank v e r y h igh in the d o m e s t i c f i she r ies , m a k i n g o n l y 1.72 % of total f ish c a t c h e s o f India in 1 9 8 7 - 1 9 9 1 . C a t c h e s a r e not so r t ed into s p e c i e s o r t a x o n o m i c g r o u p s in the s tat is t ics but a re o n l y d i v i d e d into F A O a r e a s . A p p r o x i m a t e l y e q u a l a m o u n t s o f s h a r k s a n d rays (about 2 6 , 0 0 0 t/yr) w e r e o b t a i n e d in e a c h A r e a for the pe r iod 1 9 7 7 - 1 9 9 1 , wi th c a t c h e s from the w e s t c o a s t o n l y s l ight ly l a rge r t han t h o s e o f the eas t c o a s t dur ing 1977 -1991 (fig. 2 .18) . T h e r e is a re la t ive ly h igh n u m b e r o f p u b l i s h e d ar t ic les o n e l a s m o b r a n c h explo i ta t ion a n d ut i l isat ion in India, e s p e c i a l l y for the 80 ' s . A p p u k u t t a n a n d N a i r (1988) ind ica te that dur ing 1 9 8 3 - 1 9 8 5 s h a r k s c o m p r i s e d 5 5 % of the e l a s m o b r a n c h c a t c h o f the count ry . T h e m a i n f i sh ing a r e a s in o r d e r o f i m p o r t a n c e w e r e Gujarat , M a h a r a s h t r a , K e r a l a A n d h r a P r a d e s h , K a r n a t a k a a n d T a m i l N a d u . Important f i sh ing g r o u n d s for s h a r k s a r e r epor ted for A s h i k o d e , K e r a l a P r o v i n c e ( A n o n . 1983) . S h a r k s c a t c h e s a re a l l inc iden ta l to o ther f i sher ies in India (Appuku t t an a n d N a i r 1988) . S h a r k s a re m a i n l y t a k e n with l ong l i ne s w h i c h va ry r eg iona l ly in d e s i g n , but they a re a l s o t a k e n a s b y c a t c h o f t r awlers u s i n g d i s c o nets off Ra tnag i r i ( M a h a r a s h t r a ) , wi th bot tom set gi l lnets in P o r t o N o v o (Tami l N a d u ) a n d the s h r i m p t rawlers o f K e r a l a (Devara j & S m i t a 1988 ; S h a n t h a et a l . 1988 ; R a m a R a o et a l . 1989 ; K u l k o r n i & S h a r a n g d h e r 1990) . R a y s a re c a u g h t wi th bot tom set gi l lnets in Gujara t , nor thwes t India a n d C u d a l o r e a n d a re a b u n d a n t in the ou te r she l f a n d s l o p e off K e r a l a a n d K a r n a t a k t a ( D e v a d o s s 1978 ; Kun j ipa lu & K u t t a p p a n 1978 ; S u d a r s a n et a l . 1988) . D e v a d o s s (1984) i nd i ca t e s that ba to ids m a k e up to 1 0 % of b y c a t c h e s in C a l i c u t ; 9 0 % of the b y c a t c h c o m e s from t rawlers , 8% from gi l lnets a n d 2 % from h o o k a n d l ines . B o t h s h a r k s a n d rays a re a b u n d a n t in L a k s h a k w e e p a n d form 68 important b y c a t c h e s in t r awl ing f i sher ies in K r i s h n a p a t n a m ( S w a m i n a t h et a l . 1 9 8 5 ; J a m e s 1988) . R e c e n t repor ts ( D a h l g r e n 1992) ind ica te that d i r ec t ed f i sher ies for s h a r k s a re start ing to d e v e l o p o n a s e a s o n a l b a s i s o n the eas t c o a s t of India. A b o u t 5 0 0 v e s s e l s , both s a i l -p o w e r e d a n d m o t o r i s e d fish for s h a r k s with bot tom or drift l o n g l i n e s in the c o a s t s of O r i s s a A n d h r a P r a d e s h a n d T a m i l N a d u . B o t t o m long l i ne s a re u s u a l l y set in w a t e r s 8 0 - 1 5 0 m d e e p a n d o c c a s i o n a l l y a s d e e p a s 5 0 0 m . B u l l s h a r k s a n d t iger s h a r k s a re c o m m o n l y c a u g h t in bot tom l o n g l i n e s . T h e l ong l i ne s h a v e up to 4 0 0 h o o k s a n d the m e a t is u s u a l l y sa l t ed o n b o a r d . In O r i s s a a l o n e , abou t 2 0 0 boa t s a re e n g a g e d in drift l ong l i n ing o n a s e a s o n a l b a s i s ( D e c e m b e r - M a r c h ) . T h e mos t c o m m o n s p e c i e s in drift l ong l i ne s a re s i lky s h a r k s a n d s c a l l o p e d h a m m e r h e a d s h a r k s . C a t c h c o m p o s i t i o n da t a a re not read i ly a v a i l a b l e , but the m u l t i s p e c i e s nature o f t hese f i sher ies is ev iden t from the li terature. A p p u k u t t a n & N a i r (1988) report that m o r e than 2 0 s p e c i e s o f s h a r k s (main ly c a r c h a r h i n i d s a n d sphy rn ids ) a re k n o w n to be c o m m o n in the c a t c h e s . F r o m their d a t a for P a m b a n a n d K i l a k k a r a i , Rhizoprionodon acutus, R. oligolinx, Carcharhinus limbatus, C. sorrah, C . hemiodon, Sphyrna lewini a n d Eusphyra blochii s e e m to be the mos t impor tant s p e c i e s in the c a t c h e s . O t h e r s h a r k s f o u n d in Indian c a t c h e s are C . melanopterus a n d Scoliodon laticaudus ( D e v a d o s s 1988) . S o m e impor tan t ba to ids are : Dicerobatis eregoodoo, Rhynchobatus djiddensis, Rhinobatus granulatus, Himantura uarnak, H. bleekeri, Dasyatis sephen, D. jenkinsii, Aetobatus narinari, A. flagellum, Aetomylus nichofii a n d Mobula diabolus ( D e v a d o s s 1978 , 1983 ; Kun j ipa lu & K u t t a p p a n 1978) . A l t h o u g h l o c a l i s e d a s s e s s m e n t s o f the state o f the f i sher ies for e l a s m o b r a n c h s exis t ( S a n t h a n a k r i s h n a n 1 9 8 3 , K r i s h n a m o o o r t h i et a l . 1 9 8 6 , D e v a d o s s et a l . 1 9 8 8 , S u d a r s a n et a l . 1988) , there a re no o v e r a l l s tud ies o f the e l a s m o b r a n c h f i she r i es o f India (Appuku t t an & N a i r 1988) . D e v a d o s s (1983) repor ts ray r e s o u r c e s off C a l i c u t a p p a r e n t l y ove r f i shed by 1980 , w h i l e a c c o r d i n g to R e u b e n et a l . (1988) sha rk a n d ray r e s o u r c e s o f Nor theas t India w e r e still u n d e r e x p l o i t e d in 1 9 8 5 . D e v a d o s s et a l . (1988) pe r fo rmed l o c a l a s s e s s m e n t s u s i n g S c h a e f e r ' s m o d e l a n d p rov ide s u g g e s t i o n s o f effort i n c r e a s e / d e c r e a s e fo r the different a r e a s s tud ied . 6 9 T h e p resen t s i tua t ion in Indian e l a s m o b r a n c h f i sher ies n e e d s carefu l mon i to r ing . T h e h igh c a t c h e s o f e l a s m o b r a n c h s in India, w h i c h p e a k e d at 7 3 , 5 0 0 1 in 1 9 8 8 , s u g g e s t s a ve ry h igh l eve l o f exp lo i t a t ion . It is un l ike ly that s u c h large y i e l d s wi l l be s u s t a i n a b l e o v e r a l ong pe r iod of t ime, s p e c i a l l y u n d e r the light of the 1 9 8 3 c o l l a p s e o f n e i g h b o u r i n g P a k i s t a n i e l a s m o b r a n c h f i sher ies . S r i L a n k a . T h e e l a s m o b r a n c h f i sher ies o f S r i L a n k a a p p e a r o n r e c o r d s i n c e the ea r ly 50 ' s . T h e i r d e v e l o p m e n t h a s b e e n s l o w , g r o w i n g from l e s s than o n e t onne in 1 9 5 2 to the current l eve l of abou t 1 5 , 0 0 0 t/yr (fig. 2 .2) . S r i L a n k a n e l a s m o b r a n c h f i she r i es a re the s m a l l e s t a m o n g major e l a s m o b r a n c h - f i s h i n g coun t r i e s in the Indian O c e a n . H o w e v e r , s h a r k s a n d rays are quite impor tant at a na t iona l l eve l , cont r ibu t ing 8.76 % of the total c a t c h e s du r ing 1 9 8 7 - 1 9 9 1 . R e m a r k a b l y , th is is the h ighes t re lat ive impor t ance o f a n y e l a s m o b r a n c h f ishery in the w o r l d . T h e c a t c h o f s h a r k s a n d rays of S r i L a n k a r ep re sen t s 2 . 4 2 % of the w o r l d e l a s m o b r a n c h c a t c h for the pe r iod 1987-1991 (table 2 .2) . Information o n c a t c h c o m p o s i t i o n is ve ry p o o r for S r i L a n k a n e l a s m o b r a n c h f i sher ies . F A O da ta ind ica te s that c a t c h e s w e r e c o m m o n l y g r o u p e d in a s ing le "va r ious e l a s m o b r a n c h s " c a t e g o r y until 1 9 8 7 . S i n c e then , a n entry repor ted a s Carcharhinus falciformis cons t i tu tes the major part o f the c a t c h e s . O n the o ther h a n d , informat ion from the N a t i o n a l A q u a t i c R e s o u r c e s A g e n c y ( N A R A ) of S r i - L a n k a (P . D a y a r a t n e , N A R A , C o l o m b o , S r i L a n k a , pers . c o m m . F e b r u a r y 1992) ind ica te s that C . falciformis c o m p r i s e s o n l y 7 5 % of the sha rk c a t c h e s , wi th C . longimanus, C. sorrah, Sphyrna lewini, Alopias pelagicus a n d Isurus oxyrinchus, r a n k i n g h igh a m o n g the r e m a i n i n g 2 5 % ; h e n c e c a t c h e s repor ted a s C . falciformis by F A O here l o o s e l y l a b e l e d " C a r c h a r h i n i d s h a r k s " o n f igure 2 .19 . T h e r e a re few d i r ec t ed f i sher ies for e l a s m o b r a n c h s in S r i L a n k a . S o m e e s t ima te s (P . D a y a r a t n e , pers . c o m m . o p . cit.) indica te that a p p r o x i m a t e l y 8 5 % of the e l a s m o b r a n c h y i e l d s are b y c a t c h e s from o ther f i sher ies , w h i c h u s e ma in ly bot tom a n d drift g i l lne ts . B o t h the d i rec ted a n d inc iden ta l c a t c h e s o f e l a s m o b r a n c h s c o m e from s m a l l - s c a l e f i she r ies . Drifting sha rk l ong l i ne s a re u s e d in offshore ( >40 k m from shore ) E E Z w a t e r s in the d i r ec t ed f ishery. B o t t o m set g i l lne ts ope ra te in c o a s t a l a r e a s up to 2 5 k m from s h o r e (P . D a y a r a t n e 70 CD 80,000 70,000 60,000 50,000 40,000 30,000 20,000 10,000 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Years Var . e lasm. W coast Iftttil Var . e lasm. E coas t Figure 2.18 E la smobranch ca tches of India, by region, during 1977-1991. (Data from F A O ) . w CD C c o 25,000 20,000 15,000 10,000 5,000 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Years Var . e lasm. Carcharh in id sharks Figure 2.19 E la smobranch ca tches of Sr i Lanka , by spec ies groups, during 1977-1991 (Data from F A O ) . pers . c o m m . o p . cit .) . Pajot (1980) repor ts that e l a s m o b r a n c h s c o m p r i s e 2 6 . 6 2 % of the total c a t c h in w e i g h t for the l a r g e - m e s h s m a l l - s c a l e driftnet f i she r i es o f S r i L a n k a . T h e r e is s o m e de t a i l ed informat ion abou t the p e l a g i c t u n a f i she r i es o f S r i L a n k a in w h i c h subs tan t ia l a m o u n t s o f s h a r k s a re c a u g h t inc iden ta l ly . M o s t of t h e s e informat ion h a s b e e n p r o d u c e d by the I P T P / N A R A t u n a s a m p l i n g p r o g r a m m e ( I P T P 1 9 8 9 , D a y a r a t n e a n d M a l d e n i y a 1 9 8 8 , D a y a r a t n e a n d de S i l v a 1990 , D a y a r a t n e 1 9 9 3 a , b ) . T h e s a m p l i n g p r o g r a m m e s tar ted in K a n d a k u l i y a in the nor thwest , N e g o m b o in the w e s t a n d B e r u w a l a in the s o u t h w e s t c o a s t of S r i L a n k a dur ing 1986 . T w o add i t i ona l l o c a t i o n s w e r e a d d e d in the sou th c o a s t ( M a t a r a a n d H a m b a n t o t a ) in 1 9 8 7 . R o u g h l y , th ree t y p e s o f v e s s e l s ope ra te in the p e l a g i c t u n a f i sher ies : s m a l l o u t b o a r d m o t o r e d boa t s rough ly 5 m in length , s ta t ionary d i e s e l m o t o r e d v e s s e l s of abou t 9 m length a n d 3.5 t of d i s p l a c e m e n t , a n d the la rger 1 1 m long 11 t net t o n n a g e v e s s e l s with s ta t ionary d i e s e l motors . B y far, the m o s t n u m e r o u s are the 3.5 t v e s s e l s wi th abou t 2 , 0 0 0 units . T h e y usua l ly ca r ry a c r e w of four a n d abou t 4 0 p a n e l s o f net. O v e r 1,000 of t he se boa t s s p e n d m o r e than o n e d a y offshore pe r trip. In contrast , there a re o n l y 7 0 of the 11 t v e s s e l s , but t he se u s u a l l y ca r ry 5 0 - 6 0 p a n e l s of net a n d a re c a p a b l e of m a k i n g offshore tr ips 6-8 d a y s l o n g . G i l l n e t s a re the m o s t p o p u l a r g e a r a n d they h a v e b e e n u s e d for m a n y d e c a d e s by S r i L a n k a n f i s h e r m e n . E a c h p i e c e of net m e a s u r e s 5 0 0 x 1 0 0 m e s h e s o f s i z e 9 0 - 1 8 0 m m (140 -152 the m o s t c o m m o n ) , m a k i n g a total o f 3-4.5 k m of net p e r v e s s e l . In g e n e r a l , the y i e l d a n d c a t c h rate o f s h a r k s in this f i shery are quite va r i ab l e , but both h a v e a c l e a r i n c r e a s i n g t rend . To ta l s h a r k c a t c h g r e w from 1,569 t in 1 9 8 6 - 1 9 8 7 to 2 , 1 5 5 in 1 9 8 7 - 1 9 8 8 in the nor thwest , w e s t a n d s o u t h w e s t c o a s t s . F o r the w e s t a n d sou th c o a s t s , total s h a r k c a t c h e s i n c r e a s e d from 3 ,159 t to 4 , 3 7 4 t, to 8 ,676 t du r ing 1 9 8 9 - 1 9 9 1 . O v e r a l l sha rk c a t c h rates i n c r e a s e d from abou t 10 kg /day /boa t in 1986 to abou t 3 5 - 4 0 kg /boa t /day in 1 9 8 8 . T h e s e i n c r e a s e s in s h a r k y i e l d s a n d C P U E reflect e x p a n s i o n of the f i sh ing g r o u n d s to offshore a r e a s , i n c r e a s e in t ime spen t at s e a for e a c h trip a n d c h a n g e s in the f i sh ing g e a r w h i c h invo lve f ewer v e s s e l s f i sh ing o n l y wi th g i l lnets a n d m o r e v e s s e l s s w i t c h i n g to mul t ip le -gea r f i sh ing . T h e p e r c e n t a g e i m p o r t a n c e o f s h a r k s in the ca t ch o f e a c h g e a r type is abou t 15 % for driftnets, abou t 28 % for v e s s e l s u s i n g dr i f tne t s / longl ines /handl ines , abou t 4 0 % for dr i f tnets / longl ines/ t rol l l ines , a n d abou t 4 5 % in drif tnet/ longline v e s s e l s . E l a s m o b r a n c h c a t c h e s for e a c h g e a r type in 1991 w e r e : driftnet 3 1 3 t; drif tnet/ longline 3 ,569 t; dr i f tnet / longl ine/handl ine 5 1 3 t a n d drif tnet/ longline/troll l ine 1 ,1101. T h e s p e c i e s c o m p o s i t i o n o f s h a r k s in the p e l a g i c t u n a f i shery is d o m i n a t e d by grey 72 s h a r k s ( C a r c h a r h i n i d a e ) w h i c h const i tute abou t 8 5 % of the s h a r k c a t c h , f o l l owed by h a m m e r h e a d s (3.5 % ) , t h r e s h e r s h a r k s (1 % ) , m a c k e r e l s h a r k s (0.7 %) a n d o ther s h a r k s a n d rays c o m p r i s i n g the r e m a i n i n g 10.3 % . T h e c a t c h e s o f s h a r k s in this f i shery are e s t i m a t e d v i s u a l l y by we igh t . H o w e v e r , there a re current ly p l a n s to i n c l u d e three s p e c i e s of s h a r k s (Carcharhinus falciformis, C. longimanus a n d Prionace glauca) in the s a m p l i n g c a m p a i g n s in the n e a r future ( J . M o r o n , I P T P , pers . c o m m . D e c e m b e r 1993) . A c c o r d i n g to D a y a r a t n e (pers . c o m m . op . cit.) e l a s m o b r a n c h s a re at p resen t su s t a inab ly exp lo i t ed in S r i L a n k a . T h e r e are no m a n a g e m e n t m e a s u r e s for t h e s e f i sher ies , nor a re there a n y p resen t ly u n d e r c o n s i d e r a t i o n . S o far, there is no e v i d e n c e o f a n y c o n s e r v a t i o n p r o b l e m o r e n d a n g e r e d s p e c i e s . N o n e t h e l e s s , f igures s h o w that s h a r k s a n d rays represen t a n important f i shery for S r i - L a n k a a n d they s h o u l d be m a n a g e d carefu l ly . T h i s s u m m a r y ind ica te s that at leas t the p e l a g i c f i shery is p resen t ly at a d e v e l o p i n g s t age . It s e e m s this is the idea l t ime to start r e s e a r c h efforts a i m e d t o w a r d s the m a n a g e m e n t o f the r e s o u r c e . 2 .2 .2 .4 A s i a . J a p a n . A c c o r d i n g to r e c o r d e d stat is t ics , J a p a n h a s t a k e n the w o r l d ' s la rges t c a t c h e s of e l a s m o b r a n c h s , a l t hough t he se h a v e f o l l o w e d a c l e a r d e c r e a s i n g t r end after the initial e x p l o s i v e g rowth o f the late 40 ' s w h e n a r e c o r d 1 1 8 , 9 0 0 1 w e r e c a u g h t (fig. 2 .2) . D e s p i t e this con t rac t ion in c a t c h e s , J a p a n ' s e l a s m o b r a n c h y i e l d still r a n k s a m o n g the top s e v e n in the w o r l d wi th 3 7 , 3 0 0 t in 1991 a n d con t r ibu ted 4 .98 % of the total w o r l d e l a s m o b r a n c h c a t c h in the p e r i o d 1 9 8 7 - 1 9 9 1 . T h i s is quite h igh w h e n c o m p a r e d with m o s t o ther coun t r i e s . T a n i u c h i (1990) repor ts that the relat ive impor t ance of s h a r k s (which t radi t ional ly m a k e the majority of e l a s m o b r a n c h c a t c h e s ) d r o p p e d from 4 . 3 % of the total f ish c a t c h e s in 1 9 4 9 to 0 . 3 % in 1 9 8 5 . T a n i u c h i r e m a r k s that both a d e c r e a s e in the re la t ive c a s h v a l u e of e l a s m o b r a n c h s a n d a reduc t ion o f the J a p a n e s e e l a s m o b r a n c h s t o c k s s e e m r e s p o n s i b l e for the g e n e r a l d e c l i n e in t he se f i sher ies . A t present , e l a s m o b r a n c h s const i tu te 0.31 % of the total J a p a n e s e c a t c h e s , o n e o f the lowes t a m o n g major e l a s m o b r a n c h - f i s h i n g coun t r i e s (data from F A O for 1 9 8 7 - 1 9 9 1 ) . T a n i u c h i a l s o reports a s h a r p r educ t ion o f J a p a n e s e c a t c h e s o f s p i n y dogf i sh Squalus acanthias f rom m o r e than 5 0 , 0 0 0 t in 1 9 5 2 to l e s s than 73 10 ,000 t in 1 9 6 5 a n d poin ts out that this might r ep re sen t s a r educ t ion o f s p i n y dogf i sh s tocks ; c a t c h e s o f o ther s h a r k s h a s not f o l l o w e d the s a m e t rend . A s i d e from p o s s i b l e s tock reduc t ion , it s h o u l d be c o n s i d e r e d that a s J a p a n ' s e c o n o m y h a s e x p a n d e d t r e m e n d o u s l y s i n c e the pos t -wa r pe r iod , c h a n g e s in p u r c h a s e p o w e r might h a v e mod i f i ed c o n s u m e r p re fe rences thus e x p l a i n i n g the d e c r e a s e d d e m a n d for e l a s m o b r a n c h s . T h i s h y p o t h e s i s s e e m s to be c o n f i r m e d by the large a m o u n t s o f s h a r k s that a re d i s c a r d e d at s e a in v a r i o u s J a p a n e s e f i she r i es ( see b e l o w a n d next sec t ion) . J a p a n e s e e l a s m o b r a n c h y i e l d is chief ly a b y c a t c h o f o ther f i she r ies . S o m e e x c e p t i o n s are a t rawl f i shery for s k a t e s a n d rays in the E a s t C h i n a S e a , a s a l m o n s h a r k f i shery off nor theas t J a p a n in the O y a s h i o Front (Paus t , 1987) a n d a w in te r f i shery in H o k k a i d o for Raja pulchra ( I sh ihara 1990) . A d d i t i o n a l l y , s m a l l s c a l e c o a s t a l gi l lnet f i she r i es t ake up to 3 ,817 t o f s h a r k s , w h i c h a c c o u n t s for l e s s than 0 . 0 1 % of the total c o a s t a l gi l lnet c a t c h in J a p a n ( A n o n y m o u s 1986) . S e v e r a l t r ends c a n be ident i f ied in the da t a p r e s e n t e d by T a n i u c h i (1990) a n d I sh ihara (1990) for the pe r iod 1 9 7 6 - 1 9 8 5 (fig. 2 .20) . S h a r k s a c c o u n t e d for 8 3 % of the e l a s m o b r a n c h c a t c h e s o f J a p a n a n d ba to ids for 1 7 % ; at leas t 6 3 % of the sha rk c a t c h e s w e r e t a k e n a s b y c a t c h o f t u n a long l ine o p e r a t i o n s a r o u n d the w o r l d , w h i l e the r e m a i n i n g 3 7 % c a m e from v a r i o u s u n s p e c i f i e d s o u r c e s . O f the a v e r a g e 2 5 , 0 0 0 t/yr o f s h a r k s l a n d e d by the t u n a long l ine fleet, 5 8 % c a m e from offshore a r e a s , 3 3 % from the h igh s e a s a n d o n l y 9 % from c o a s t a l wa t e r s p r e s u m a b l y the J a p a n e s e E . E . Z . A d d i t i o n a l l y , sha rk c a t c h e s equ iva l en t to a p p r o x i m a t e l y 2.8 t imes the l a n d e d s h a r k b y c a t c h o f the long l ine t una f ishery is d i s c a r d e d at s e a . O f the a p p r o x i m a t e l y 9 ,000 t/yr c a t c h o f ba to ids , 5 0 % w a s f i shed in the E a s t C h i n a S e a , 3 5 % in H o k k a i d o a n d 8% in the S e a o f J a p a n . J a p a n ho ld s s o m e of the la rges t h i g h - s e a s f i sher ies for t u n a s a n d bi l l f i shes in the w o r l d . T h e s e p r o d u c e subs tan t i a l i nc iden ta l c a t c h e s of s h a r k s , s o m e of w h i c h a re u t i l i sed . A n a c c o u n t of t he se f i sher ies is g i v e n u n d e r s ec t ion 2 .2 .3 . D a t a from F A O for the p e r i o d 1977-1991 indica te that s h a r k s a re t a k e n m a i n l y in the nor thwes t P a c i f i c ( A r e a 61) w h e r e J a p a n e s e c a t c h e s a re rap id ly d e c l i n i n g (fig. 2 .21) . A p p r o x i m a t e l y 8 ,000 t/yr a re t a k e n in the rest o f the P a c i f i c wi th fairly cons t an t t rend a n d ve ry s m a l l c a t c h e s a re a l s o c a u g h t in the Indian a n d At l an t i c O c e a n s . A l l ba to id c a t c h e s c o m e from the nor thwes t Pac i f i c . 74 60,000 50,000 JAPAN 40,000 CD 30,000 + 20,000 10,000 1B. E.China Sea w B.Sea Japan • B. Hokkaido • S. II high seas W S. II offshore • S. II coastal s S. other gear Figure 2.20 Elasmobranch catches in different fisheries of Japan during 1976-1984 (S=sharks, B=batoids, INIongline). (Data from Taniuchi (1990) and Ishihara (1990)). 60,000 50,000 40,000 30,000 20,000 10,000 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Y e a r s B a t o i d s ^ S h a r k s N W P a c i f i c S h a r k s A t l an t i c S h a r k s R e s t Pac i f i c S h a r k s Indian Figure 2.21 Elasmobranch catches of Japan, by species groups and region, during 1977-1991. (Data from FAO). 75 D e t a i l e d informat ion o n the s p e c i e s c o m p o s i t i o n of the c a t c h e s is not a v a i l a b l e for J a p a n e s e stat is t ics after 1 9 6 8 . H o w e v e r , T a n i u c h i (1990) p resen t s da t a for the p e r i o d 1 9 5 1 - 1 9 6 7 a n d reports s p i n y dogf i sh Squalus acanthias a s the m a i n s p e c i e s in the c a t c h up to 1958 , f o l l o w e d in i m p o r t a n c e by b lue s h a r k Prionace glauca a n d s a l m o n s h a r k Lamna ditropis. T h e s a m e au tho r lists 2 5 sha rk s p e c i e s c a p t u r e d by t u n a long l ine v e s s e l s . C o n s i d e r i n g the large cont r ibut ion o f the s h a r k b y c a t c h e s o f long l ine f i sher ies to the total s h a r k c a t c h , a n d the r e s e a r c h c r u i s e s da t a repor ted by T a n i u c h i (1990) , the mos t impor tant s p e c i e s in the ove ra l l sha rk c a t c h e s in J a p a n s h o u l d be , by impor t ance , the b lue s h a r k Prionace glauca, the s i lky sha rk Carcharhinus falciformis, the o c e a n i c whi te t ip s h a r k C . longimanus a n d the shortfin m a k o Isurus oxyrinchus. H o w e v e r , this e s t imate might be affect by the s e l e c t i o n a n d d i s c a r d at s e a a n d by the s p e c i e s c o m p o s i t i o n of that part o f the total s h a r k c a t c h that d o e s not c o m e from the t u n a long l ine r s . In the E a s t C h i n a S e a , Raja boesemani, R. kwangtungensis a n d R. acutispina a re r e spec t ive ly the mos t important s p e c i e s in the ba to id c a t c h ( Y a m a d a 1986) . T h e u s e s g i v e n to e l a s m o b r a n c h s in J a p a n v a r y from h u m a n c o n s u m p t i o n o f mea t a n d e v e n car t i l age in v a r i o u s t radi t ional d i s h e s , to indust r ia l a n d m e d i c i n a l u s e s o f l iver o i l c o m p o u n d s a n d lea ther f rom h i d e s . H o w e v e r , J a p a n e s e f i s h e r m e n c o n s i d e r s h a r k s a n u i s a n c e a s they d a m a g e f i sh ing g e a r a n d h o o k e d t unas a n d bi l l f ishes , a n d a re e v e n c o n s i d e r e d compet i to r s for s o m e v a l u a b l e fish s t o c k s ( T a n i u c h i 1990) . N o m a n a g e m e n t m e a s u r e s a re k n o w n to exis t for e l a s m o b r a n c h f i sher ies in J a p a n . S o u t h K o r e a . T h e r e c o r d s o f S o u t h K o r e a n e l a s m o b r a n c h f i sher ies a re intermittent a n d l imi ted to F A O stat is t ics. H o w e v e r , this coun t ry h a s t a k e n m o r e than 10 ,000 t/yr o f e l a s m o b r a n c h s s i n c e at leas t 1948 a n d c a t c h e s s h o w a n i n c r e a s i n g t r end osc i l l a t i ng a r o u n d 2 0 , 0 0 0 t/yr s i n c e the mid-80 's ( f ig . 2 .2) . T h e recent c a t c h o f s h a r k s a n d rays o f S o u t h K o r e a con t r ibu tes 2 .67 % of the total w o r l d e l a s m o b r a n c h c a t c h (table 2 .2) . G i v e n the la rge f i she r i es p roduc t ion of S o u t h K o r e a , e l a s m o b r a n c h s a re o f m i n o r impor t ance r ep re sen t ing o n l y 0 .66 % of the total c a t c h e s ( 1 9 8 7 - 1 9 9 1 ) . T h e e l a s m o b r a n c h f i sher ies of S o u t h K o r e a are v e r y poor ly d o c u m e n t e d . T h e r e are no 76 reports o n c a t c h c o m p o s i t i o n by s p e c i e s . F r o m F A O da t a ( 1 9 7 7 - 1 9 9 1 ) , t w o major ca t ego r i e s a re identif ied a s ba to ids a n d "var ious e l a s m o b r a n c h s " , the latter p r o b a b l y referr ing to s h a r k s (fig. 2 .22) . D u r i n g th is pe r iod , ba to ids cons t i tu ted 7 3 % of the e l a s m o b r a n c h c a t c h a n d w e r e t a k e n chief ly in the P a c i f i c O c e a n (94 % ) , wi th s m a l l c a t c h e s in the A t l a n t i c (4 %) a n d the Indian O c e a n s (<1 % ) . V a r i o u s e l a s m o b r a n c h s w e r e a l s o t a k e n m a i n l y from the Pac i f i c O c e a n (88%) a n d in s m a l l quant i t ies from the A t l an t i c (9%) a n d Indian O c e a n s (3%). A l t h o u g h ba to ids c o m p r i s e the majority o f the e l a s m o b r a n c h c a t c h a c c o r d i n g to F A O stat is t ics , this o n l y r ep resen t s the ac tua l l a n d i n g s o f e l a s m o b r a n c h s a n d d o e s not inc lude d i s c a r d s . S o u t h K o r e a n marke t s may , to s o m e extent, in f luence the d i s c a r d p r o c e d u r e s o f e l a s m o b r a n c h s at s e a . T h e K o r e a n long l in ing tuna fleet is k n o w n to c a t c h a n d p robab ly d i s c a r d large n u m b e r s of s h a r k s in the h igh s e a s o f the w o r l d ( see s e c t i o n 2 .2 .3) . P e o p l e ' s R e p u b l i c o f C h i n a . T h e r e is no informat ion o n the e l a s m o b r a n c h f i sher ies o f the P e o p l e ' s R e p u b l i c o f C h i n a in F A O stat is t ics . A t t e m p t s to ob ta in informat ion direct ly from the f i she r i e s a g e n c y o f C h i n a r e c e i v e d the a n s w e r that no informat ion o n e l a s m o b r a n c h f i she r i es ex i s t s there . H o w e v e r , it is k n o w n that C h i n a h a s b e e n expor t ing i n c r e a s i n g quant i t ies o f s h a r k fins to H o n g K o n g dur ing the pas t few y e a r s , s o that a harves t of s h a r k s mus t exis t there e v e n a s a n inc iden ta l c a t c h . A rough es t imate b a s e d o n da ta from the S o u t h e a s t A s i a n F i s h e r i e s D e v e l o p m e n t C e n t e r ( S E A F D E C ) o n s h a r k fins expor t s into S o u t h e a s t A s i a n coun t r i e s (P . W o n g s a w a n g , S E A F D E C , S a m u t p r a k a n , T h a i l a n d , pers . c o m m . A p r i l 1992) i nd i ca t e s that C h i n a ' s sha rk c a t c h appa ren t l y g r e w from l e s s than 100 t in 1981 to s o m e w h e r e b e t w e e n 1 7 , 0 0 0 t a n d 2 8 , 0 0 0 1 in 1 9 9 1 , d e p e n d i n g o n w h i c h c o n v e r s i o n factor is u s e d for the e s t ima t ion (fig. 2 .23) . T h e s e f igures a re m i n i m u m es t ima tes o f the rea l c a t c h e s o f s h a r k s in C h i n a a s a n u n k n o w n part of the p roduc t ion might be c o n s u m e d d o m e s t i c a l l y . T h u s , the a c t u a l c a t c h e s a re e x p e c t e d to be m u c h h igher . A c c o r d i n g to C o o k (1991) , d u e to recen t r e l axa t ion in import a n d c o n s u m e r res t r ic t ions in C h i n a , d e m a n d for the t radi t ional s h a r k fin s o u p h a s s o a r e d , c rea t ing ex t ra d e m a n d for the product . In add i t ion to the e x p a n s i o n o f impor t s o f this c o m m o d i t y m e n t i o n e d by C o o k , this d e m a n d for sha rk fins mus t be c a u s i n g i n c r e a s e d explo i ta t ion o f e l a s m o b r a n c h s . Z h o w a n d W a n g (1990) p rov ide s o m e information that conf i rms the e x i s t e n c e o f c a t c h e s o f 77 to CD c c o 25,000 20,000 15,000 10,000 5,000 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Years Batoids Atlantic Batoids Indian Batoids Pacific Var.elasm. Atlantic • \ Var. elasm. Indian Var. elasm. Pacific Figure 2.22 Elasmobranch catches of South Korea, by species groups and region, during 1977-1991. (Data from FAO). 30,000-25,000-CHINA 20,000-CD | 15,000-O 10,000-5,000-1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Years Figure 2.23 Estimated shark catches for the People's Republic of China from fin exports, using 3% and 5% conversion factor. (Fin export data from P. Wongsawang, pers. comm.) 78 s h a r k s a n d r ays in the P e o p l e ' s R e p u b l i c o f C h i n a . S h a r k s a n d r ays a r e t a rge ted in s o m e i n s t a n c e s u s i n g driftnets, set g i l lne ts a n d l ong l ine s . T h e r e a re m o r e than 3.5 mi l l ion gi l lnets f i sh ing in C h i n e s e wa te r s . Driftnets r ange from 3 0 m m to 3 6 0 m m m e s h s i z e , but p robab ly t hose ta rge t ing e l a s m o b r a n c h s b e l o n g to the u p p e r part o f this r a n g e . Driftnets target s h a r k s in X i a p u a n d J in j i ang , Fuj ian P r o v i n c e . S e t gi l lnets o c c u r in m e s h s i z e s 3 0 - 3 2 0 m m a n d are u s e d in s h a l l o w wa te r s to target a m o n g m a n y o ther s p e c i e s , Triakis scyllium a n d Squalus fernandinus in H a i y a n g , S h a n d o n g P r o v i n c e . S e t l ong l i ne s o f different t y p e s are u s e d to c a t c h v a r i o u s e l a s m o b r a n c h s . T h e y al l v a r y b e t w e e n 3 8 8 a n d 5 0 0 m l o n g . Prionace glauca a n d Carcharhinus s p p . a re t a rge ted with l ong l i ne s in H u i ' a n , Fu j ian P r o v i n c e , "var ious sha rks" a re c a u g h t in Y a n g j i a n g , G u a n g d o n g P r o v i n c e a n d "var ious rays" in C h a n g d a o , S h a n d o n g P r o v i n c e . A var ia t ion of l ong l ines c a l l e d rol l ing l ines a re u s e d to c a t c h rays in H a i x i n , H e b e i p r o v i n c e , M i n h o u , Fuj ian P r o v i n c e , a n d R u d o n g , J i a n g s u P r o v i n c e . R o l l i n g l ines c o n s i s t o f non-ba i t ed s h a r p h o o k s na r rowly s p a c e d o n the m a i n l ine . T a i w a n . T h i s coun t ry h a s s o m e o f the mos t important e l a s m o b r a n c h c a t c h e s in the w o r l d , c o m p r i s e d ma in ly o f s h a r k s . T h e r e is no c o m p r e h e n s i v e informat ion o n e l a s m o b r a n c h c a t c h e s before the 70 ' s for T a i w a n , but da ta from the F i s h e r i e s Y e a r b o o k s o f T a i w a n A r e a ind ica te that large quant i t ies o f e l a s m o b r a n c h s h a v e b e e n h a r v e s t e d s i n c e the 50 ' s (fig. 2 .2) . To ta l e l a s m o b r a n c h c a t c h e s f luc tuated a r o u n d 4 5 , 0 0 0 t/yr dur ing the p e r i o d 1 9 7 9 - 1 9 8 8 . T h i s w a s f o l l o w e d by a subs tan t i a l i n c r e a s e o f c a t c h e s in 1 9 8 9 a n d e s p e c i a l l y 1 9 9 0 w h e n y i e l d s o a r e d to m o r e than 7 0 , 0 0 0 t a s a result of i n c r e a s e d c a t c h e s o f la rge s h a r k s (fig. 2 .24) . T h e s e va r i a t ions in c a t c h p r o b a b l y represen t c h a n g e s in the d i s c a r d ra tes o f the distant fleet. E l a s m o b r a n c h s c o m p r i s e d abou t 3.5 % of the total f i sher ies c a t c h e s o f T a i w a n from 1 9 8 7 -1 9 9 1 . T h e majori ty o f t h e s e c a t c h e s w e r e large s h a r k s , i.e. a p p r o x i m a t e l y 8 1 % of the total e l a s m o b r a n c h c a t c h du r ing 1 9 7 8 - 1 9 9 0 . S m a l l s h a r k s a c c o u n t for a p p r o x i m a t e l y 14 % , wh i l e rays a re of v e r y little impor t ance cont r ibu t ing on ly abou t 5 % . T h e m a i n s p e c i e s in the e l a s m o b r a n c h c a t c h are h a m m e r h e a d s h a r k s (Sphyrna lewini, S. zygaena), g rey s h a r k s (Carcharhinus plumbeus, C. falciformis), m a k o s h a r k s (Isurus oxyrinchus), b lue s h a r k s (Prionace glauca) a n d t h r e she r s h a r k s (Alopias superciliosus, A. pelagicus) ( C . T . C h e n , N a t i o n a l T a i w a n O c e a n Univers i ty , pers . c o m m . J a n u a r y 1992) . 79 M o s t o f the s h a r k c a t c h e s of T a i w a n a re o b t a i n e d ou t s ide T a i w a n e s e w a t e r s by the v a r i o u s fa r - seas t u n a f leets . D u r i n g 1 9 8 8 - 1 9 9 0 , a p p r o x i m a t e l y 8 5 % of the l a rge s h a r k a n d 7 0 % of the s m a l l s h a r k c a t c h e s c a m e from t h e s e ope ra t i ons . In contras t , m o s t o f the ray c a t c h e s (53%) in the s a m e p e r i o d w e r e t a k e n in T a i w a n e s e w a t e r s by l o c a l f i she r ies . T h e T a i w a n e s e fa r - seas fleet is difficult to mon i to r a s it o p e r a t e s in a l l the o c e a n s o f the w o r l d a n d is c o m p o s e d o f mul t ip le s i z e s a n d t ypes of v e s s e l s , s u c h a s l ong l ine r s , driftnetters, a n d pur se s e i n e r s (Ho , 1988) . It is k n o w n that important s h a r k c a t c h e s a re t a k e n by l a rge - sca l e driftnetters ta rge t ing s h a r k s spec i f i ca l ly in I n d o n e s i a n w a t e r s in the r eg ion o f the Ara fu ra , B a n d a a n d T i m o r S e a s . T a i w a n o p e r a t e d a n important f i shery for s h a r k s in Nor the rn a n d Nor th W e s t e r n A u s t r a l i a wa t e r s from 1 9 7 2 to 1 9 8 6 . T h i s w a s ma in ly c o m p o s e d of driftnetters se t t ing mult i f i lament ny lon nets o f b e t w e e n 3 a n d 16 k m long , 17-30 m d e e p a n d wi th m e s h s i z e s o f 1 4 0 - 1 7 0 m m . V e s s e l s r a n g e d in t o n n a g e b e t w e e n 160 a n d 3 8 0 t o n n e s ( O k e r a et a l . 1 9 8 1 , S t e v e n s 1990) . In add i t ion , T a i w a n e s e pa i r t rawlers f i sh ing for d e m e r s a l fish o b t a i n e d sha rk b y c a t c h e s in a p p r o x i m a t e l y the s a m e g r o u n d s o f the driftnetters. T h e c a t c h e s o f driftnetters w e r e 8 0 % s h a r k s ; o f t hese , Carcharhinus ti'I'stoni a n d C . sorrah w e r e the m a i n c o m p o n e n t ( 5 5 % of total c a t c h e s ) , the r e m a i n i n g part w e r e t u n a a n d m a c k e r e l ( S t e v e n s 1990) . A c c o r d i n g to O k e r a et a l . (1981) , b e t w e e n 3 ,500 a n d 1 4 , 8 0 0 t/yr o f s h a r k s w e r e t a k e n by t he se driftnetters du r ing the pe r iod 1 9 7 5 - 1 9 8 0 , h o w e v e r , S t e v e n s a n d D a v e n p o r t (1991) report c a t c h e s equ iva l en t to b e t w e e n 7 ,200 a n d 1 1 , 2 0 0 t/yr l ive w e i g h t for the s a m e pe r iod . M e a n w h i l e , c a t c h e s from pa i r t rawlers a v e r a g e d « 2 , 3 0 0 t/yr o f s h a r k s , wi th up to 7 ,300 t t a k e n in 1 9 7 4 ( O k e r a et a l . 1981) . L imi t s o n n u m b e r o f v e s s e l s a n d f i sh ing a r e a s a s w e l l a s a c a t c h q u o t a o f 7 ,000 t p r o c e s s e d we igh t w e r e i m p o s e d to this f i shery in 1 9 7 9 by the A u s t r a l i a n g o v e r n m e n t . T h e T a i w a n e s e sha rk driftnet fleet p u l l e d out o f the f i shery in 1986 fo l lowing the impos i t i on o f a m a x i m u m gil lnet length of 2 .5 k m by A u s t r a l i a n author i t ies , w h i c h l ed to unprof i table b u s i n e s s ( S t e v e n s 1990) ; T a i w a n h a s s i n c e c o n t i n u e d the f i shery in I n d o n e s i a n wa te r s . A t leas t 7 ,000 t/yr o f s h a r k s w e r e t a k e n by the T a i w a n e s e fleet in A u s t r a l i a n E E Z before 1987 , but it is u n k n o w n h o w m u c h they t ake p resen t ly in Indones i a . If the S E A F D E C f igures repor ted for T a i w a n e s e l a rge - sca l e gi l lnet s h a r k c a t c h e s c o r r e s p o n d so l e ly to the f i shery in Indones i an wa te r s , then 19 ,636 t w e r e t a k e n there in 1 9 8 7 . A l s o , b y c a t c h e s of s h a r k s in o ther important l a rge - sca l e T a i w a n e s e f i sher ies , for e x a m p l e the t una longl ine f ishery, the Indian O c e a n driftnet f i shery a n d Nor th P a c i f i c s q u i d driftnet f ishery, 80 might a c c o u n t for part o f the s h a r k c a t c h e s o f this coun t ry but t h e s e r e m a i n u n k n o w n . T h e s e h i g h - s e a s f i she r i es a re t rea ted in deta i l in s ec t ion 2 .2 .3 . A c c o r d i n g to d a t a from the F i s h e r i e s Y e a r b o o k s o f T a i w a n A r e a , du r ing 1 9 8 8 - 1 9 9 0 the m a i n f i sh ing loca l i t i es for la rge s h a r k s w e r e Han H s i e n a n d P i n g t u n g H s i e n a c c o u n t i n g for 3 2 % (2 ,109 t/yr) a n d 4 9 % (3 ,246 t/yr) o f the large s h a r k s c a u g h t in T a i w a n e s e w a t e r s . K e e l u n g H s i e n w a s the m a i n site for c a t c h e s of s m a l l s h a r k s a n d r ays with 3 7 % (991 t/yr) a n d 7 3 % (875 t/yr) o f the l o c a l c a t c h e s o f e a c h g roup respec t ive ly . M o s t o f the T a i w a n e s e sha rk c a t c h e s are t a k e n by l a r g e - s c a l e f i she r ies , par t icular ly with l ong l ine s . A c c o r d i n g to S E A F D E C da ta , abou t 9 0 % of the 9 ,529 t d o m e s t i c e l a s m o b r a n c h c a t c h e s ( those t a k e n in the S o u t h C h i n a S e a A r e a ) in 1988 c a m e from l a r g e - s c a l e f i sher ies . F o r s h a r k s , l a r g e - s c a l e l ong l i ne s a n d h o o k a n d l ines a c c o u n t e d for 6 2 % of the c a t c h e s wh i l e gi l lnets a n d otter t rawls a c c o u n t e d for l e s s than 2 0 % e a c h (table 2 .7) . O n l y 5 % of the sha rk c a t c h c a m e from s m a l l - s c a l e gil lnet f i sher ies a n d l e s s t han 1% from t raps a n d l ong l ine s . F o r rays , otter t rawl w a s the mos t important l a rge - sca l e g e a r wi th 2 3 % of the c a t c h , but g e a r c l a s s i f i ed a s l a r g e - s c a l e "others" p r o v i d e d 5 8 % . G i l l n e t s con t r ibu ted to 7 % of the s m a l l -s c a l e c a t c h . T h e r e m a i n i n g 1 1 % of ray c a t c h e s w a s t a k e n u s i n g s m a l l - s c a l e gi l lnets a n d t raps . It is u n k n o w n if a n y s tock a s s e s s m e n t h a s b e e n d o n e for the T a i w a n e s e f i sher ies . N e v e r t h e l e s s , e l a s m o b r a n c h s t o c k s in T a i w a n a re b e l i e v e d to be o v e r e x p l o i t e d a n d t iger s h a r k s (Galeocerdo cuvieri) a re c o n s i d e r e d e n d a n g e r e d s p e c i e s ( C . T . C h e n , pers . c o m m . op . cit.). D e s p i t e this , no m a n a g e m e n t m e a s u r e s exis t at p resen t o r a r e b e i n g c o n s i d e r e d in the n e a r future for the e l a s m o b r a n c h f i sher ies of T a i w a n . M a l a y s i a . M a l a y s i a n e l a s m o b r a n c h f i sher ies a re o n e of the s m a l l e s t a m o n g A s i a n major e l a s m o b r a n c h - f i s h i n g coun t r i e s toge ther with t hose o f the P h i l i p p i n e s a n d T h a i l a n d . M a l a y c a t c h e s o f s h a r k s a n d rays m a k e on ly 2 .46 % of the w o r l d e l a s m o b r a n c h c a t c h . T h e d e v e l o p m e n t o f the f i shery in M a l a y s i a s h o w s a s t e a d y t r end o f s l o w growth from 1961 until the current l eve l o f abou t 1 5 , 0 0 0 t/yr (fig 2 .2) . E l a s m o b r a n c h s r ep resen t 2 .2 % of the total 81 f ishery p roduc t ion o f M a l a y s i a . R a y s d o m i n a t e the c a t c h e s . S E A F D E C d a t a ind ica te that from 1 9 7 6 - 1 9 9 1 r a y s r e p r e s e n t e d a n a v e r a g e o f 6 0 % o f the e l a s m o b r a n c h y i e l d a n d s h a r k s the r e m a i n i n g 4 0 % . C a t c h e s o f s h a r k s s h o w e d ove ra l l a v e r y s l ight ly d e c l i n i n g t rend, wh i l e ray c a t c h e s e x p a n d e d m a i n l y from 1986-1991 (figure 2 .25) . M a i n s p e c i e s in the ray c a t c h e s a re Rhyncobatus djiddensis (which is p r o c e s s e d a s "shark f in" t oge the r wi th o ther ray s p e c i e s ) , Gymnura s p p . a n d Dasyatis s pp . Scoliodon sorrakowa, Chiloscyllium indicum a n d Sphyrna s p p . a r e the mos t c o m m o n s p e c i e s in the sha rk c a t c h e s ( C . P h a i k , pers . c o m m . F e b r u a r y 1 9 9 2 ) . E l a s m o b r a n c h c a t c h e s in M a l a y s i a a re p r e d o m i n a n t l y b y c a t c h e s o f t rawl f i sher ies ( 9 5 % of the ca tch) wi th o n l y a s m a l l a m o u n t t a k e n in s m a l l s c a l e d i r ec t ed f i she r i es (5%). In both c o a s t s o f P e n i n s u l a r M a l a y s i a a n d the S a b a h coas t , b e t w e e n 6 0 % a n d 7 0 % of the l oca l sha rk c a t c h e s w e r e t a k e n with t rawls , w h i l e t hose o f rays w e r e in the o r d e r o f 7 2 - 9 3 % . P u r s e s e i n e s c a u g h t l e s s than 1% of s h a r k s in P e n i n s u l a r M a l a y s i a . In the wa te r s of S a r a w a k , la rge s c a l e otter t rawls t ake 7 0 % of the l oca l c a t c h e s o f r ays a n d 3 0 % of those of s h a r k s . In th is a r e a , o the r v a r i o u s l a rge - sca l e g e a r s a c c o u n t e d for l e s s than 1% of c a t c h e s of both s h a r k s a n d rays . M a l a y s i a n s m a l l - s c a l e f i sher ies for e l a s m o b r a n c h s , a l t hough not a s impor tant a s l a rge - sca l e f i sher ies for thei r cont r ibu t ion to total e l a s m o b r a n c h c a t c h e s , a re v e r y d i v e r s e . D u r i n g 1988 , they c o m p r i s e d 7 0 % of the S a r a w a k sha rk c a t c h e s u s i n g m a i n l y gi l l nets (54%) a n d long l ines a n d h o o k a n d l ine (15%), with t raps m a k i n g a v e r y s m a l l con t r ibu t ion (table 2.7) . R a y w e r e t a k e n in s m a l l - s c a l e f i sher ies u s i n g h o o k & line a n d l o n g l i n e s (17%) a n d gi l lnets (11 % ) ; s m a l l c a t c h e s w e r e a l s o t a k e n with t raps . F o r both c o a s t s o f P e n i n s u l a r M a l a y s i a a n d S a b a h , s m a l l s c a l e f i she r i es wi th gi l l ne t s took b e t w e e n 1 5 % a n d 2 8 % o f the s h a r k c a t c h e s , w h i l e h o o k & l ine a n d l ong l i ne s a c c o u n t e d for abou t 9 % of the c a t c h in P e n i n s u l a r M a l a y s i a a n d 2 5 % in S a b a h . C a t c h e s of rays from s m a l l - s c a l e f i she r i es in S a b a h a n d the w e s t c o a s t o f P e n i n s u l a r M a l a y s i a w e r e t a k e n ma in ly by h o o k & line a n d l ong l i ne s a n d to a l e s s e r extent by gi l lnets t raps a n d o ther gear . T h e o p p o s i t e w a s f o u n d in the eas t c o a s t o f P e n i n s u l a r M a l a y s i a , w h e r e mos t o f the s m a l l cont r ibut ion (5%) o f s m a l l s c a l e f i sher ies to the total r ays c a t c h c a m e from gi l lne ts . B e c a u s e o f the inc iden ta l nature o f e l a s m o b r a n c h c a t c h e s , the m o s t impor tant f i sh ing 8 2 80,000 10,000 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 Years Large sharks Htttfl Smal l sharks Bato ids Figure 2.24 E la smobranch ca tches of Ta iwan , by spec ies groups, during 1978-1990. (Data from F A O ) . Table 2.7 Percentage ca tches of sharks and rays according to fishing gear and z o n e s in Ta iwan (Prov. of China) and M a l a y s i a (data from S E A F D E C 1988). TAIWAN PENINSULAR MALAYSIA INSULAR MALAYSIA TYPE OF FISHERY WEST C O A S T EAST C O A S T S A B A H SARAWAK AND G E A R S H A R K S RAYS S H A R K S RAYS S H A R K S RAYS S H A R K S RAYS S H A R K S RAYS LARGE S C A L E Purse seine - - 0 - 0 - - - - -Trawl - - 63 80 70 93 60 72 - -Otter trawl 11 23 - - - - - - 30 70 Gill net 17 7 - - - - - - - -Hook & line 62 0 - - - - - - - -Others 4 58 - - - - - - 0 0 S M A L L S C A L E Gill/drift net 5 4 28 4 20 5 15 - 54 11 Hook/long line 0 - 8 16 9 0 25 26 15 17 Trap 0 7 - 0 0 0 0 0 1 2 TOTAL CATCH (mt) 8588 941 1359 6125 1111 2303 910 596 1872 2546 83 g r o u n d s a re t h o s e o f the t r awl ing f ishery, ma in ly p e n i n s u l a r M a l a y s i a a n d S a r a w a k . A v e r a g e c a t c h e s for 1 9 7 6 - 1 9 8 9 ind ica te that s h a r k s a re t a k e n m a i n l y in S a r a w a k (1 ,869 t/yr o r 1 5 % of total e l a s m o b r a n c h ca tch) , the w e s t (1 ,363 t/yr o r 11 %) a n d eas t (1 ,169 t/yr o r 9%) c o a s t s of P e n i n s u l a r M a l a y s i a a n d in l o w e r quant i t ies in S a b a h (778 t/yr, 6%) . C a t c h t rends for s h a r k s in t h e s e a r e a s s h o w a d e c r e a s e in w e s t P e n i n s u l a r M a l a y s i a , re la t ive ly s u s t a i n e d y i e l d s in S a r a w a k a n d S a b a h a n d var iabi l i ty in eas t P e n i n s u l a r M a l a y s i a (fig. 2 .25) . T h e w e s t c o a s t o f P e n i n s u l a r M a l a y s i a is the mos t important f i sh ing a r e a for rays (3 ,457 t/yr, 2 8 % of total e l a s m o b r a n c h c a t c h e s ) f o l l o w e d by S a r a w a k (2 ,004 t/yr, 16%) a n d the eas t c o a s t o f P e n i n s u l a r M a l a y s i a (1324 t/yr, 11%) , wi th S a b a h con t r ibu t ing o n l y 5 7 3 t/yr (5%). T h e t rend o f the c a t c h e s ind ica te s g rowth in ray y i e l d s in both c o a s t s o f P e n i n s u l a r M a l a y s i a , re lat ive stabil i ty in S a b a h a n d s t rong var iabi l i ty in S a r a w a k . A t present , there a re no m a n a g e m e n t m e a s u r e s for e l a s m o b r a n c h s , a n d o n l y ray c a t c h e s a r e indi rec t ly con t ro l l ed v i a the l i c e n c e res t r ic t ions for t rawl f i sher ies . P h i l i p p i n e s . T h e e l a s m o b r a n c h c a t c h e s o f the P h i l i p p i n e s w e r e o f m i n o r i m p o r t a n c e before the late 70 ' s a n d a l t hough v a r i a b l e , s h o w a g r o w i n g t rend a n d recent stabil i ty a r o u n d 1 7 , 0 0 0 t/yr s i n c e 1986 (fig 2 .2) . S h a r k s a n d rays c o m p r i s e d o n l y 0 .85 % of the total na t iona l c a t c h e s . A c c o r d i n g to S E A F D E C da ta , r ays a re s l ight ly m o r e important t han s h a r k s in the c a t c h e s with a n a v e r a g e o f 5 3 % of the e l a s m o b r a n c h y i e l d s in the p e r i o d 1 9 7 7 - 1 9 9 1 ; the c a t c h e s o f both g r o u p s h a d a g r o w i n g t rend dur ing this p e r i o d . P h i l i p p i n e c a t c h e s a c c o u n t for 2 .63 % of the w o r l d e l a s m o b r a n c h y i e l d . J u d g i n g from the c a t c h e s of 1988 (17,8791) , s m a l l s c a l e f i sher ies p rov ide the large majority o f e l a s m o b r a n c h c a t c h e s in P h i l i p p i n e s (table 2 .8) . In L u z o n , l a rge s c a l e t r awle r s a c c o u n t e d for 3 0 % of the l oca l s h a r k c a t c h e s but on ly 6 % of rays , wi th p u r s e s e i n e r s cont r ibu t ing a r o u n d 3 % of both g roups ' c a t c h e s . In V i s a y a s , t rawls w e r e the m a i n g e a r in large s c a l e f i sher ies for r ays (23%) but a c c o u n t e d o n l y for 1% of s h a r k c a t c h e s . L a r g e s c a l e pu r se s e i n i n g took 1 1 % a n d 8% of the s h a r k a n d ray c a t c h e s r e s p e c t i v e l y in V i s a y a s . C a t c h e s from s m a l l - s c a l e f i sher ies for both s h a r k s a n d rays in L u z o n a n d for s h a r k s in V i s a y a s w e r e ma in ly t a k e n by h o o k & line a n d l ong l i ne s ( 3 8 % - 7 6 % ) but a l s o by g i l lne ts ( 8 % - 3 0 % ) . T h e oppos i t e h a p p e n s in V i s a y a s w h e r e gil lnet c a t c h e s o f r ays w e r e g rea te r t han t hose from 84 h o o k & line a n d long l ine ( 4 2 % v s . 2 2 % ) . In V i s a y a s a n d L u z o n , "other gear" m a d e s m a l l con t r ibu t ions (< 13%) to the c a t c h e s o f both s h a r k s a n d rays , a n d t raps w e r e u s e d to ob ta in m i n o r c a t c h e s o f r ays (< 8%). E l a s m o b r a n c h c a t c h e s in M i n d a n a o w e r e a l l f rom s m a l l s c a l e f i sher ies : g i l lne ts w e r e the m a i n g e a r for rays (81%) a n d h o o k a n d l ine for s h a r k s (57%). S m a l l s c a l e g e a r c l a s s i f i e d a s "other" w e r e the s e c o n d m o s t impor tant for c a t c h i n g both g r o u p s in M i n d a n a o ( 2 8 % of s h a r k s , 1 0 % of rays) . G i l l ne ts took 1 5 % of the s m a l l - s c a l e sha rk c a t c h e s a n d t raps l e s s than 1%. F o r rays , h o o k & line a n d l o n g l i n e s w e r e the third mos t impor tant g e a r in this a r e a with 7 % of the c a t c h e s a n d t raps a n d otter t rawls con t r ibu ted m i n i m u m c a t c h e s . T h e c o m p o s i t i o n o f ba to id a n d s h a r k c a t c h e s by a r e a in the P h i l i p p i n e s is s h o w n in figure 2 .26 b a s e d o n S E A F D E C da ta . M o s t o f the c a t c h e s o f both s h a r k s a n d r ays a re t a k e n in M i n d a n a o , a v e r a g i n g 3 ,185 t/yr (24% of total e l a s m o b r a n c h c a t c h e s ) a n d 2 , 7 2 4 t/yr (21%) respec t ive ly . T h e y i e l d o f s h a r k s a n d rays in M i n d a n a o h a s g r o w n s i n c e the late 70 ' s . L u z o n is the s e c o n d a r e a in impor t ance with 1,993 t/yr o f s h a r k s (15%) a n d 2 , 3 1 2 t/yr o f ba to ids (18%) . S h a r k c a t c h e s in L u z o n h a v e d e c r e a s e d from the l eve l s o f the late 70 ' s w h i l e ba to id y i e l d s h a v e g r o w n recent ly after a d e c r e a s e in the ea r ly 80 ' s . Y i e l d o f s h a r k s a n d rays in V i s a y a s is the lowes t in the P h i l i p p i n e s with a v e r a g e s o f 1,108 t/yr (8%) a n d 1,856 t/yr (14%) re spec t ive ly ; y i e l d s o f both g r o u p s d e c r e a s e d shor t ly du r ing the ea r ly 80 ' s . Little is k n o w n abou t the s p e c i e s c o m p o s i t i o n o f e l a s m o b r a n c h c a t c h e s in the P h i l i p p i n e s . War fe l a n d C l a g u e (1950) report t iger s h a r k s a s the p r ime c a t c h in s h a r k l ong l i ne s a r o u n d the P h i l i p p i n e s du r ing exp lo ra to ry f i sh ing . O t h e r s h a r k s f o u n d in the s u r v e y i nc lude at least s ix s p e c i e s c o r r e s p o n d i n g to the g e n u s Carcharhinus, p lus Sphyrna zygaena, Scyliorhinus torazame, Hexanchus griseus a n d a n unident i f ied nur se shark . T h e s p e c i e s t a k e n by gi l lnets w e r e Pristis cuspidatus a n d Rhynchobatus djiddensis. A d d i t i o n a l l y , E n c i n a (1977) reports o n a n b u d d i n g dogf i sh f i shery c a t c h i n g Squalus acanthias a n d Centrophorus s p p . a l l a r o u n d the P h i l i p p i n e s , pr imar i ly d i r ec t ed t o w a r d s s q u a l e n e o i l ex t rac t ion . T h a i l a n d . N o w o n e o f the m o r e m o d e s t major e l a s m o b r a n c h - f i s h i n g coun t r i e s in S o u t h e a s t A s i a , T h a i l a n d h a s a n e l a s m o b r a n c h f ishery that g r e w c o n s i d e r a b l y in the 6 0 ' s but d e c l i n e d s i n c e 85 20,000 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Years ^Sharks W.P.M. • Sharks E.P.M. MSharks Sabah USharks Sarawak • Rays W.P.M. EH Rays E.P.M. • Rays Sabah m Rays Sarawak Figure 2.25 Elasmobranch catches of Malaysia, by species groups and region, during 1976-1990 (E.P.M.=eastern penninsular Malaysia, W.P.M.=western penninsular Malaysia). (Data from SEAFDEC). o 25,000 20,000 15,000 10,000 5,000 1976 1978 • Sharks Luzon • Batoids Luzon 1980 1982 1984 1986 1988 1990 Years M Sharks Visayas M Sharks Mindanao ^ Batoids Visayas • Batoids Mindanao Figure 2.26 Elasmobranch catches of Philippines, by species groups and region, during 1976-1990. (Data from SEAFDEC). 86 the ea r ly 70 ' s (fig. 2.2) ma in ly a s a c o n s e q u e n c e o f ove r -exp lo i t a t ion by t r awle r s in the G u l f of T h a i l a n d ( M e n a s v e t a et a l . 1 9 7 3 , P o p e 1979) . M o r e recent ly , the re w e r e s i g n s of a n appa ren t r e c o v e r y but c a t c h e s fell a g a i n s i n c e 1988 a n d the p resen t state o f the s t o c k s is unce r t a in . S h a r k s a n d ba to ids represen t a m i n o r f i shery in T h a i l a n d con t r ibu t ing o n l y 0 .43 % of the total na t iona l f i shery p roduc t ion du r ing 1 9 8 7 - 1 9 9 1 , a n d o n l y 1.74 % of the w o r l d e l a s m o b r a n c h c a t c h (table 2 .2) . R a y s d o m i n a t e the c a t c h e s , a n d a re a b y c a t c h o f the p r e d o m i n a t e l y t r awl ing T h a i f ishery. S E A F D E C d a t a s h o w that a v e r a g e c a t c h e s o f rays for the p e r i o d 1 9 7 6 - 1 9 9 1 a c c o u n t e d for 64 % of the e l a s m o b r a n c h y i e l d , w h i l e the rest w e r e s h a r k s . E s t i m a t e s o f the T h a i D e p a r t m e n t o f F i s h e r i e s s h o w that a p p r o x i m a t e l y 9 5 % of the s h a r k c a t c h is m a d e up of s h a r k s s m a l l e r than 1.5 m T L , ma in ly Carcharhinus spp . , w h i l e the m a i n ba to id s p e c i e s in the c a t c h a re Dasyatis s p p . a n d v a r i o u s e a g l e r ays . (P . S a i k l i a n g , D . O . F . pers . c o m m . D e c e m b e r 1991) . T h e m a i n f i sh ing g r o u n d s for s h a r k s a n d rays a re in the G u l f o f T h a i l a n d . D u r i n g 1 9 7 6 - 1 9 8 9 c a t c h e s from the G u l f a v e r a g e d 2 , 9 5 5 t/yr o f s h a r k s ( 2 8 % of a l l e l a s m o b r a n c h s caugh t ) a n d 4 , 8 8 5 t/yr of r ays (46%) , w h i l e the A n d a m a n S e a o n l y p r o d u c e d 1,042 t/yr o f s h a r k s (10%) a n d 1,709 t/yr o f r ays (16%) . T h e r e w a s no t rend in s h a r k c a t c h e s d u r i n g this pe r iod in the G u l f o f T h a i l a n d but there w a s a d e c r e a s i n g t rend in the A n d a m a n S e a . R a y s c a t c h e s g r e w c o n s i d e r a b l y in the G u l f o f T h a i l a n d but s h o w e d no t r end in the A n d a m a n S e a (fig 2 .27) . T h a i e l a s m o b r a n c h f i sher ies a re chief ly a l a rge - sca l e act ivi ty. F r o m a total o f 11 ,438 t of e l a s m o b r a n c h s t a k e n in 1988 by T h a i l a n d , mos t of the c a t c h e s o n both c o a s t s o f the count ry c a m e from l a r g e - s c a l e t rawlers . Ot ter t rawls p r o v i d e d 6 3 % a n d 8 2 % respec t ive ly , of the s h a r k a n d ray c a t c h e s of the G u l f o f T h a i l a n d a n d 9 2 % a n d 6 4 % of t hose in the A n d a m a n S e a coas t . A d d i t i o n a l l y , pa i r t rawls in the G u l f o f T h a i l a n d took a r o u n d 1 0 % of both s h a r k s a n d rays (table 2 .8) . In the G u l f of T h a i l a n d , l a r g e - s c a l e g i l lnets a c c o u n t e d for 2 2 % of sha rk c a t c h e s but o n l y for 1% of t hose o f rays . Fur ther , pu r se s e i n e r s con t r ibu ted with v e r y s m a l l c a t c h e s o f both g r o u p s . In the A n d a m a n S e a , s m a l l s h a r k c a t c h e s w e r e t a k e n by l a r g e - s c a l e gil l nets . S m a l l - s c a l e e l a s m o b r a n c h f i she r i es in T h a i w a t e r s a re re la t ively impor tant for thei r c a t c h e s o f rays with gil l nets in the A n d a m a n S e a , w h e r e they contr ibute a l m o s t 3 0 % of the l oca l ray c a t c h e s . S m a l l c a t c h e s ( less t han 1 % to 7 % of loca l 87 Table 2.8 Percen tage ca tches of sharks and rays according to fishing gear and z o n e s in Phil ippines and Thai land (data from S E A F D E C 1988). PHILIPPINES THAILAND TYPE OF FISHERY LUZON VISAYAS MINDANAO GULF INDIAN OCEAN AND GEAR SHARKS RAYS SHARKS RAYS SHARKS RAYS SHARKS RAYS SHARKS RAYS LARGE SCALE Purse seine 3 2 11 8 - - 1 0 - -Trawl 30 6 1 23 - - 12 10 - 0 Otter trawl - . - - - - - 63 82 92 64 Gill net - - - - - - 22 1 4 -Hook & line 2 - - - - - - - - -Others - 0 0 - - - - - - -SMALL SCALE Otter trawl - - - 1 - 0 - - - -Gill/drift net 21 30 8 42 15 81 1 3 - 29 Hook/long line 38 42 76 22 57 7 0 4 4 7 Trap - 7 - 3 0 1 - - - -Others 6 12 3 4 28 10 - - - -TOTAL CATCH (mt) 1513 3132 1742 1924 3879 5689 3436 5963 408 1631 16,000 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Years P^ fl Sharks Gulf H i Sharks Andaman ffTTT Rays Gulf |>$&8 Rays Andaman Figure 2.27 E la smobranch ca tches of Thai land, by spec ies groups and region, during 1976-1991. (Data from S E A F D E C ) . 88 c a t c h e s ) of both g r o u p s a re t a k e n a l s o in s m a l l - s c a l e h o o k & l ine a n d long l ine f i sher ies in both c o a s t s . In the G u l f of T h a i l a n d , s m a l l - s c a l e gi l lnets t ake o n l y s m a l l c a t c h e s of s h a r k s a n d rays . T h e r e s e e m to be no recent s tock a s s e s s m e n t s for the a r e a . S t u d i e s b a s e d o n 1963 a n d 1 9 6 6 - 1 9 7 2 r e s e a r c h c r u i s e s ' s w e p t a r e a e s t ima te s ( M e n a s v e t a et a l . 1973) , i nd i ca t ed total s t and ing s t o c k s o f 2 , 8 8 0 t for s h a r k s , 4 , 4 0 4 t for r ays a n d 1,988 t for rh inoba t ids in the w h o l e G u l f o f T h a i l a n d , a s w e l l a s a n e s t i m a t e d 5 ,0001 potent ia l y i e l d for a l l e l a s m o b r a n c h s . T h e s tudy h igh l igh ted s e v e r e r educ t ions in s t and ing s t o c k s of r ays o v e r that pe r iod a n d c l a s s i f i ed e l a s m o b r a n c h s t o c k s a s "heav i ly exp lo i t ed , if not too h e a v i l y a l ready" . H o w e v e r , t h e se e s t ima te s might h a v e b e e n too c o n s e r v a t i v e a s total G u l f c a t c h e s o f e l a s m o b r a n c h s from T h a i l a n d a n d M a l a y s i a w e r e 10 ,439 t in 1 9 7 7 , 1 0 , 9 5 9 t in 1 9 7 8 a n d 7,621 in 1979 , ma in t a in ing a l eve l of a b o u t 8 ,000 t/yr for a n o t h e r 6 y e a r s , a n d r i s ing a b o v e 1 0 , 0 0 0 t/yr in the late 80 ' s . N e v e r t h e l e s s , the r educ t ions in c a t c h ra tes s h o w n by P o p e (1979) are e v i d e n c e that the s t o c k s o f both s h a r k s a n d rays h a v e i n d e e d d e c l i n e d d rama t i ca l l y in the a r e a . I ndones i a . T h e r e is no informat ion o n the e l a s m o b r a n c h f i sher ies o f I n d o n e s i a before 1 9 7 1 , but r eco rds s h o w they h a v e e x p a n d e d t r e m e n d o u s l y s i n c e then . I n d o n e s i a n e l a s m o b r a n c h f i sher ies have the h ighes t s u s t a i n e d growth rate o f a n y e l a s m o b r a n c h f i sh ing coun t ry a n d they are current ly the larges t in the w o r l d . I ndones i an c a t c h e s a m o u n t e d to a l m o s t 8 0 , 0 0 0 1 in 1991 a n d there a re no s i g n s yet o f a n y leve l l ing off (fig. 2 .2) . I n d o n e s i a n f i she r i es for s h a r k s a n d rays r ep resen t 10 .18 % of the wor ld ' s e l a s m o b r a n c h c o m m e r c i a l c a t c h . D e s p i t e this , e l a s m o b r a n c h s a re o f on ly m o d e r a t e impor t ance in Indones i a , con t r ibu t ing 2.41 % to the total f i sher ies o f this coun t ry in the p e r i o d 1 9 8 7 - 1 9 9 1 . C o n t r a r y to m o s t major e l a s m o b r a n c h f i sh ing coun t r i e s in the r eg ion , w h i c h harves t l a rger quant i t ies o f r ays t han s h a r k s o r s imi l a r quant i t ies of both , e l a s m o b r a n c h c a t c h e s in I n d o n e s i a a re d o m i n a t e d by s h a r k s , w h i c h a c c o u n t e d for 66 % of the a v e r a g e e l a s m o b r a n c h c a t c h e s du r ing 1 9 7 6 - 1 9 9 1 . A c c o r d i n g to S E A F D E C da t a (1976-1989) the mos t impor tant a r e a s for s h a r k f i sh ing in Indones i a a re in the w e s t e r n part o f the country , n a m e l y J a v a (9 ,727 t/yr o n a v e r a g e a n d 89 2 1 % of total e l a s m o b r a n c h y ie lds ) , S u m a t r a (7 ,837 t/yr, 17%) a n d K a l i m a n t a n (5 ,870 t/yr 12%) , wi th the ea s t e rn p r o v i n c e s o f B a l i - N u s a T e n g a r a , S u l a w e s i a n d M o l l u c a - l r i a n J a y a , a c c o u n t i n g for 1,796 t/yr (3 .8%), 3 ,157 t/yr (7%) a n d 1,983 t/yr (4 .2%) r e spec t ive ly . T h i s pattern is s i m i l a r for ba to id c a t c h e s , but in this c a s e S u m a t r a is at the top with 6 ,404 t/yr ( 1 3 % of total e l a s m o b r a n c h ca t ches ) , f o l l owed by J a v a with 4 , 6 7 0 t/yr (11 %) a n d K a l i m a n t a n with 2 , 9 8 7 t/yr (6%). In the east , S u l a w e s i r anks first wi th 1,329 t/yr (3%), B a l i - N u s a T e n g a r a s e c o n d wi th 9 5 7 t/yr (2%) a n d M o l l u c a - Irian J a y a third wi th 5 1 8 t/yr (1%). T h e c a t c h e s of s h a r k s a n d rays s h o w i n c r e a s i n g t rends o v e r the p e r i o d in a l l p r o v i n c e s (fig. 2 .28) . E a s t e r n p r o v i n c e s c o u l d be the mos t su i tab le for future i n c r e a s e s in the f i shery . H o w e v e r , in add i t ion to the I n d o n e s i a n c a t c h e s , la rge quant i t ies o f s h a r k s a r e a l s o h a r v e s t e d by T a i w a n e s e driftnet v e s s e l s in ea s t e rn I n d o n e s i a n wa t e r s s i n c e t he se fleet a b a n d o n e d the A u s t r a l i a n E E Z in 1 9 8 7 . T h e T a i w a n e s e v e s s e l s w e r e c a p a b l e o f t ak ing at leas t 7 ,000 t/yr of sha rks , a n d c a t c h e s in the a r e a b e t w e e n north A u s t r a l i a a n d I n d o n e s i a w e r e in the r eg ion of 2 5 , 0 0 0 t/yr before 1 9 7 9 ( S t e v e n s , 1990) . In the light o f the ove ra l l c a t c h e s o f e l a s m o b r a n c h s t a k e n in I n d o n e s i a n wa te r s , it is su rp r i s ing that y i e l d s from Indones i a k e e p g r o w i n g y e a r after y e a r . T h e r e a re appa ren t ly no r e s e a r c h o r m a n a g e m e n t p r o g r a m m e s for e l a s m o b r a n c h s in I n d o n e s i a a n d the ques t ion o f the sus ta inab i l i ty o f s h a r k f i sher ies in the a r e a b e c o m e s m o r e in t r iguing a n d re levant a s c a t c h e s k e e p g r o w i n g . M u c h at tention s h o u l d be p a i d to this f i shery if I n d o n e s i a h a s a n y interest in c o n t i n u i n g it into the next century . 2 .2 .2 .5 A u s t r a l i a n subcon t inen t . A u s t r a l i a . E l a s m o b r a n c h f i sher ies in A u s t r a l i a a re s m a l l a n d ba re ly c l a s s i f i ab l e a s "major f isher ies" , h a v i n g o n l y t empora r i l y p r o d u c e d m o r e than 1 0 , 0 0 0 t/yr du r ing the late 80 ' s (fig. 2 .2) . T h e y on ly cont r ibute 1.46 % to the w o r l d e l a s m o b r a n c h c a t c h ( 1 9 8 7 - 1 9 9 1 ) . N e v e r t h e l e s s , A u s t r a l i a n s h a r k f i sher ies a re a m o n g the mos t d o c u m e n t e d a n d o n e o f the few m a n a g e d e l a s m o b r a n c h f i sher ies in the w o r l d . T h i s is p robab ly di rect ly re la ted to the impor t ance of e l a s m o b r a n c h s in the c a t c h e s o f A u s t r a l i a n f i sher ies . F A O da t a for 1 9 8 7 - 1 9 9 1 indica te that e l a s m o b r a n c h s cont r ibute 4.8 % of the total f i sher ies o f A u s t r a l i a , the third h ighes t percent 90 80,000 i 1976 1978 1980 1982 1984 1986 1988 1990 Years ffl S.Sumatra O S.Java DS. Bali-N.Teng. MS. Kalimantan ^ S . Sulawesi S S . Molluca, Irian J. MB. Sumatra DB. Java • B. Bali-N.Teng. SB. Kalimantan EHB. Sulawesi HB. Molluca, Irian J. Figure 2.28 Elasmobranch catches of Indonesia, by species groups and region, during 1976-1990 (B=batoids, S=sharks). (Data from SEAFDEC). 91 impor t ance in the w o r l d . A d d i t i o n a l l y , t h e se a re ve ry o l d f i sher ies that form part o f the f i sh ing tradit ion o f the count ry . S t e v e n s (1990) r e v i e w s A u s t r a l i a n s h a r k f i she r i es a n d reports that thei r his tory da t e s b a c k to the e n d of the 19th century , w h e n f i sher ies for s c h o o l s h a r k l iver oi l a n d fins a l r e a d y ex i s t ed in sou theas t e rn A u s t r a l i a . F A O da t a a re not repor ted by s p e c i e s o r s p e c i e s g r o u p s a n d it is o n l y p o s s i b l e to get the g e o g r a p h i c a l c o m p o s i t i o n o f the c a t c h e s from this in format ion . T h e la rge majority o f the c a t c h e s c o m e from A r e a 5 7 p robab ly ref lect ing m a i n l y the s o u t h e r n s h a r k f i shery for Mustelus antarcticus a n d Galeorhinus galeus. S m a l l c a t c h e s of e l a s m o b r a n c h s c o m e from A r e a 81 w h i l e c a t c h e s in A r e a 71 a re neg l ig ib le (fig. 2 .29) . His to r ica l ly , the mos t important e l a s m o b r a n c h f ishery in A u s t r a l i a h a s b e e n the sou the rn sha rk f i shery w h i c h p r o v i d e s the major part of the total e l a s m o b r a n c h c a t c h e s of the count ry . Information for this par t icu lar f i shery is s u m m a r i s e d by W a l k e r (1988) , A n o n y m o u s (1989) a n d S t e v e n s (1990) . S c h o o l s h a r k s Galeorhinus galeus w e r e the o r ig ina l target s p e c i e s , at leas t s i n c e 1 9 2 7 , w h e n r eco rds b e g a n to be t a k e n regular ly . H o w e v e r , o the r s p e c i e s t a k e n in the f i shery a re the g u m m y sha rk Mustelus antarcticus, the s a w s h a r k s Pristiophorus cirratus a n d P. nudipinnis a n d the e l ephan t fish Callorhynchus millii. M a n a g e m e n t o f the f ishery b e g a n a s ea r ly a s 1 9 4 9 w h e n a m i n i m u m s i z e o f 91 c m T L w a s i n t roduced for s c h o o l s h a r k s in V i c t o r i a . P ro t ec t i on o f nur se ry a r e a s in c o a s t a l l a g o o n s f o l l o w e d later. T h e f i shery e x p a n d e d from c o a s t a l to offshore o p e r a t i o n s in the mid -40 ' s a n d c a t c h e s g r e w g radua l ly until 1 9 6 9 . Y i e l d w a s t empora r i ly r e d u c e d fo l lowing a c o m b i n e d effect o f the in t roduct ion of monof i l amen t gi l lnets a n d a b a n by the g o v e r n m e n t of V i c t o r i a o f s c h o o l s h a r k s l onge r than 104 c m T L d u e to i m p e r m i s s i b l y h igh c o n c e n t r a t i o n s of m e r c u r y in thei r meat . T h e in t roduct ion o f g i l lnets w a s i n t ended to boos t the d e c r e a s i n g c a t c h e s o f s c h o o l s h a r k s , but this a l s o brought abou t large b y c a t c h e s o f g u m m y s h a r k s , w h i c h w e r e p r e v i o u s l y r e g a r d e d a s a n u n d e s i r a b l e s p e c i e s . D u e to the s i z e restr ic t ions o n s c h o o l s h a r k s a n d the ava i lab i l i ty of g u m m y s h a r k s , the latter w e r e d i s p l a c e d a s the m a i n s p e c i e s in the f i shery . S o o n after, r e v i s e d s i z e l imits a l l o w e d s c h o o l s h a r k s b e t w e e n 7 1 - 1 1 2 c m T L to be t a k e n a g a i n in the V i c t o r i a n f i shery a n d total c a t c h e s rose o n c e m o r e a t ta in ing a p e a k o f 3 ,754 t ( d r e s s e d weight ) in 1 9 8 6 , with both s p e c i e s cont r ibu t ing a p p r o x i m a t e l y e q u a l par ts to the c a t c h . S i n c e then , c a t c h e s h a v e s l o w l y fa l len a s a result o f m a n a g e m e n t o f the f i shery . 92 M o s t o f the c a t c h in the sou the rn sha rk f i shery is t a k e n wi th m o n o f i l a m e n t gi l lnets a n d long l ines , but s m a l l c a t c h e s a re a l s o t a k e n by t rawlers . G i l l n e t s v a r y g e o g r a p h i c a l l y in m e s h s i z e but they a r e a l l b e t w e e n 15 c m ( legal m i n i m u m ) a n d 2 0 . 2 3 c m , wi th 17 .78 c m a s the mos t c o m m o n m e s h s i z e . G i l l n e t s a re typ ica l ly 1.7 m in height a n d g e a r e d with a h a n g i n g coeff icient o f 0.6 ( K i r k w o o d & W a l k e r 1986) . G i l l ne t s a c c o u n t for a b o u t 9 0 % of the g u m m y sha rk a n d a p p r o x i m a t e l y 7 5 % of the s c h o o l s h a r k c a t c h e s . L o n g l i n e s a r e typ ica l ly 10 k m long a n d r i gged wi th s e v e r a l h u n d r e d s o f h o o k s . A l t h o u g h l e s s impor tant for their cont r ibut ion to the total c a t c h e s , thei r u s a g e h a s g r o w n lately e s p e c i a l l y in T a s m a n i a . T h e mos t impor tant f i sh ing g r o u n d s for g u m m y s h a r k a re pr imar i ly in B a s s Strai t a n d s e c o n d a r i l y in S o u t h A u s t r a l i a . T h e oppos i t e w a s true for s c h o o l s h a r k until recent ly , w h e n T a s m a n i a n c a t c h e s a l m o s t e q u a l l e d t hose o f e a c h o f the o ther a r e a s . D u r i n g 1 9 8 7 , total sha rk c a t c h e s w e r e d is t r ibuted by g e a r a n d a r e a a s fo l lows : in B a s s Strait , g i l lne ts 4 7 . 3 % , l ong l i ne s 7 .4%; in S o u t h A u s t r a l i a , g i l lne ts 2 7 . 3 % , long l ines 1.3%; in T a s m a n i a , g i l lne ts 1 0 . 9 % , long l ines 10 .4% ( A n o n y m o u s 1989) . T h e sou the rn s h a r k f i shery is a m o d e l o f hones t c o n c e r n o v e r e l a s m o b r a n c h r e s o u r c e s . F i s h i n g effort h a s e x p a n d e d in a l l a r e a s w h i l e gil lnet C P U E (kg/km/hr) h a s d r o p p e d for both s p e c i e s . T h i s h a s recent ly l ed f i sher ies sc ien t i s t s to s u s p e c t that both s t o c k s a re in d e c l i n e . A s a result , a mon i to r ing p r o g r a m a n d a s p e c i a l r e s e a r c h g r o u p h a v e b e e n set up for the s tudy o f the f i shery a n d s e v e r a l projects a re b e i n g f u n d e d by the f i sh ing industry a n d g o v e r n m e n t a g e n c i e s . T h e a p p r o a c h is c o m p r e h e n s i v e , wi th r e s e a r c h s p a n n i n g from b io log i ca l s t ud i e s ( M o u l t o n et a l . 1992) a n d the cons t ruc t ion o f d a t a b a s e s to bu i ld ing spec i f i c s imu la t ion m o d e l s for the m a n a g e m e n t of the f i shery ( W a l k e r 1 9 9 2 , S l u c k z a n o w s k i et a l . 1993) a n d e c o n o m i c a n a l y s e s ( C a m p b e l l et a l . 1991) . T h e b i o l o g y o f the s p e c i e s is w e l l d o c u m e n t e d a n d s u g g e s t s s ing le b r e e d i n g popu la t i ons for e a c h s p e c i e s in the w h o l e a r e a . H o w e v e r , s o m e c o n c e r n s h a v e b e e n r a i s e d recent ly abou t the spa t i a l s tructure a n d d y n a m i c s o f the s t ocks . C u r r e n t inves t iga t ions concen t r a t e o n the spa t i a l d y n a m i c s o f the s tocks a n d the vu lnerab i l i ty of j uven i l e s c h o o l s h a r k s to c o m m e r c i a l a n d sport f i sher ies in nurse ry a r e a s o f T a s m a n i a . T h e recent c o n c e r n s abou t p o s s i b l e ove rexp lo i t a t i on o f the s t o c k s l e d to m a n a g e m e n t m e a s u r e s a i m e d at r e d u c i n g effort by a b o u t a 5 0 % th rough a n e l abora t e l i c e n s i n g p r o c e d u r e . Unfor tunate ly , long l ine effort w a s not c o n s i d e r e d in the s c h e m e a n d g r e w rap id ly a s a result of the res t r ic t ions i m p o s e d to gi l lnet ters , c a u s i n g that ove ra l l effort r educ t ions fell short o f e x p e c t e d l eve l s . T h e sou the rn s h a r k f i shery c o n t i n u e s 93 to be in t ens ive ly s t ud i ed a n d mon i to r ed . T h e r e is a l s o a s m a l l e r s h a r k f i shery in the sou th w e s t e r n a n d s o u t h e r n c o a s t o f W e s t e r n A u s t r a l i a . C a t c h e s a r e d o m i n a t e d by Furgaleus macki a n d Mustelus antarcticus but subs tan t ia l c a t c h e s of Carcharhinus obscurus a re a l s o o b t a i n e d ( L e n a n t o n et a l 1990) . C a t c h e s a re a b o u t 1,600 t/yr a n d repor ted ly 1 0 % of the A u s t r a l i a n c a t c h of g u m m y sha rk c o m e s from this f i shery . M a n a g e m e n t m e a s u r e s i nc lude l i c e n s e l imi ta t ions , g e a r restr ict ion a n d a recent b a n w h i c h prohibi ts sha rk f i sh ing in w a t e r s from S h a r k B a y no r thward to Nor th W e s t C a p e ( A n o n y m o u s 1992) . T h e nor thern A u s t r a l i a sha rk f i shery w a s ini t iated in 1 9 7 4 by T a i w a n e s e gi l lnet ters exp lo i t ing s h a r k s , t u n a a n d m a c k e r e l in offshore a r e a s o f the A r a f u r a s e a . T a i w a n e s e pair - t rawlers f i sh ing in the s a m e a r e a s a l s o c a u g h t s h a r k s a s b y c a t c h ( see a c c o u n t of T a i w a n e s e f i sher ies a b o v e ) . S h a r k s m a d e up a p p r o x i m a t e l y 8 0 % of the c a t c h wi th 5 5 % c o m p o s e d by Carcharhinus tilstoni a n d C . sorrah. A t the b e g i n n i n g o f the 80 ' s A u s t r a l i a n f i she rmen b e c a m e in te res ted in t he se r e s o u r c e s a n d s m a l l f i sher ies s p r e a d in i n s h o r e wa t e r s from the Nor the rn Terr i tory to the north o f W e s t e r n A u s t r a l i a a n d Q u e e n s l a n d . C a t c h c o m p o s i t i o n is s imi l a r to that o f the offshore T a i w a n e s e f i shery a n d l a n d i n g s h a v e f luc tua ted b e t w e e n 50 a n d 4 0 0 t/yr ( S t e v e n s 1990) . A l t h o u g h s t o c k s d e c l i n e d d u e to ove rexp lo i t a t i on by the T a i w a n e s e fleet, the latter m o v e d out to Indones i a in 1987 a n d the s t o c k s a re b e l i e v e d to be r e c o v e r i n g . N o m a n a g e m e n t m e a s u r e s for the s m a l l d o m e s t i c f i shery are thought n e c e s s a r y at the m o m e n t . T h i s f i shery h a s a l s o b e e n c l o s e l y m o n i t o r e d a n d s e v e r a l r e s e a r c h projects h a v e b e e n c o n d u c t e d by the Nor the rn Terr i tory D e p a r t m e n t o f P r i m a r y Industry a n d F i s h e r i e s a n d the C S I R O . T h e future d e v e l o p m e n t of a n A u s t r a l i a n sha rk f i shery in the north o f A u s t r a l i a is c o n s t r a i n e d by h igh c o n c e n t r a t i o n s of m e r c u r y a n d s e l e n i u m in mos t s p e c i e s o f c a r c h a r h i n i d s a n d s p h y r n i d s . L y l e (1984) e s t i m a t e d that o n l y 4 9 % of the c a t c h in w e i g h t c o u l d be re ta ined if the m a x i m u m leve l o f m e r c u r y is set to 0.5 m g / k g . In add i t ion , marke t res t r ic t ions h a v e p r e c l u d e d the c a t c h e s from en te r ing the m a i n marke t for s h a r k mea t in M e l b o u r n e ( R o h a n 1981) . S o m e recent a r r a n g e m e n t s h a v e b e e n m a d e in the nor thern s h a r k f i shery to prevent ove rexp lo i t a t i on . S e v e r a l e n d o r s e m e n t s h a v e b e e n a l l o c a t e d in different a r e a s u n d e r C o m m o n w e a l t h ju r i sd ic t ion s i n c e J a n u a r y 1 9 9 2 . 94 N e w Z e a l a n d . E l a s m o b r a n c h f i sher ies in N e w Z e a l a n d w e r e u n d e r 1 0 , 0 0 0 t/yr until recent ly . A l t h o u g h current c a t c h e s a re not m u c h larger , there is a n ove ra l l i n c r e a s i n g t r end in y i e l d s i n c e the late 70 ' s (fig. 2 .2) . E l a s m o b r a n c h f i sher ies a re m o d e r a t e l y important for N e w Z e a l a n d with c a t c h e s m a k i n g 2 .19 % of the total na t iona l f i shery p roduc t ion . N e w Z e a l a n d f i sher ies for s h a r k s a re a n o t h e r g o o d e x a m p l e of c o n t i n u o u s r e s e a r c h a n d m a n a g e m e n t . O n a g l o b a l s c a l e , t h e s e f i sher ies a re ve ry s m a l l , cont r ibu t ing o n l y 1.73 % of the w o r l d e l a s m o b r a n c h y i e l d (table 2 .2) . A c c o r d i n g to F A O da t a for 1 9 7 7 - 1 9 8 9 , the c a t c h e s of the different e l a s m o b r a n c h g r o u p s in N e w Z e a l a n d a re qui te va r i ab l e . Dogf i sh c a t c h e s (most ly Squalus acanthias) s h o w a t r e m e n d o u s i n c r e a s e w h i l e c a t c h e s o f s m o o t h - h o u n d s s h o w a c l e a r d e c l i n e . B a t o i d a n d e l e p h a n t f ish c a t c h e s g r e w m o d e r a t e l y a n d the c a t c h o f g r ey s h a r k s (most ly Galeorhinus galeus) g r e w c o n s i d e r a b l y then con t r ac t ed dur ing this p e r i o d (fig. 2 .30) . R e c e n t informat ion from the N . Z . Min i s t ry o f Agr i cu l tu re a n d F i s h e r i e s i nd i ca t e s that dur ing 1 9 8 9 - 1 9 9 2 , a p p r o x i m a t e l y 15 % of the c a t c h w a s c o m p o s e d o f e l ephan t f i shes (Callorhinchus milli) a n d c h i m a e r a s (Hidrolagus spp. ) , 18 % w a s tope (Galeorhinus galeus), 12.5 % w a s rig (Mustelus lenticulatus), 33 % w a s s p i n y dogf i sh (Squalus acanthias), 17.5 % w a s the s k a t e s Raja nasuta a n d R. innominata a n d the r e m a i n i n g 4 % w a s c o m p r i s e d by 13 s p e c i e s o f la rge o r d e e p w a t e r s h a r k s a n d at leas t th ree s p e c i e s o f ba to ids . A b o u t 4 0 % of the total e l a s m o b r a n c h y i e l d is a b y c a t c h o f t rawl f i sher ies , w h i l e the r e m a i n i n g 6 0 % is ma in ly t a k e n direct ly with l ong l ines a n d se tne ts . E l e p h a n t f i shes a re c a u g h t ma in ly in the c o a s t of C a n t e r b u r y a n d tope a n d r igs a re c a u g h t a l l a r o u n d N e w Z e a l a n d . F r a n c i s a n d S m i t h (1988) a n a l y s e the c a t c h e s o f rig a r o u n d N e w Z e a l a n d a n d s u m m a r i s e s o m e informat ion abou t this f ishery. T h e rig f i shery is s t rongly s e a s o n a l a n d c o n c e n t r a t e d dur ing the aus t r a l s p r i n g a n d s u m m e r mo n t h s . C a t c h e s a re mos t ly e x p o r t e d to A u s t r a l i a . A l m o s t 9 0 % of the c a t c h e s w e r e a b y c a t c h o f t r awl ing f i sher ies du r ing the m i d 60 ' s , but the i n c r e a s e in d e m a n d a n d in t roduct ion o f monof i l amen t g i l lnets c h a n g e d the pattern of explo i ta t ion a n d p resen t ly se tne ts a c c o u n t for 8 0 % of the l a n d i n g s o n this s p e c i e s . F r a n c i s a n d S m i t h report that C P U E d e c l i n e d in three o f the five z o n e s a n a l y s e d du r ing 1 9 7 4 - 1 9 8 5 9 5 16,000 14,000 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Years Var. elas. Area 57 | | Var. elas. Area 71 Var. elas. 81 Batoids All Areas Figure 2.29 E la smobranch ca tches of Austra l ia , by F A O statistical areas, during 1977-1991. (Data from F A O ) . Figure 2.30 E la smobranch ca tches of N e w Zea land , by spec ies groups, during 1977-1991 (Data from F A O ) . 96 a n d that in s e v e r a l a r e a s s tock s i z e s a p p e a r to be d o w n to o n e third o f the i r o r ig ina l s i z e s . P r e s u m a b l y , this is partly the r e a s o n for the impos i t i on o f m a n a g e m e n t r egu la t ions in this f ishery. M a n a g e m e n t m e a s u r e s for the m a i n e l a s m o b r a n c h s p e c i e s in N e w Z e a l a n d inc lude r ev i sab le T A C ' s , a p e r c e n t a g e of w h i c h g o to ITQ ho lde r s . F o r t h e y e a r 1 9 9 2 , T A C ' s w e r e 6 3 6 t for e l e p h a n t f i shes , 2 , 0 7 0 t for rig a n d 3 ,087 t for tope ( A n n a l a 1993) . In add i t ion , b a s k i n g s h a r k s c a n o n l y be t a k e n a s a b y c a t c h a n d there a re cur rent p r o p o s a l s to inc lude more e l a s m o b r a n c h s p e c i e s u n d e r the quo ta m a n a g e m e n t s y s t e m . R e s e a r c h in N e w Z e a l a n d h a s c o n c e n t r a t e d in rig a n d sp iny dogf i sh ( F r a n c i s a n d M a c e 1 9 8 0 , H a n c h e t 1988 , F r a n c i s 1989 , M a s s e y a n d F r a n c i s 1989 , H a n c h e t 1 9 9 1 , F r a n c i s a n d F r a n c i s 1993) . S o m e s m a l l quant i t ies of l ivers from d e e p w a t e r s q u a l o i d s h a r k s a re cur ren t ly ut i l i sed from the b y c a t c h e s o f the o r a n g e roughy (Hoplostethus atlanticus) d e e p t rawl f i sher ies of N e w Z e a l a n d ( K i n g & C l a r k 1987) , a l t hough large quant i t ies of the s h a r k s a r e a l s o d i s c a r d e d at s e a ( see next s e c t i o n o n b y c a t c h e s o f l a rge - sca l e f i sher ies ) . R e s u l t s f rom r e s e a r c h c r u i s e s indica te that the s tock o f t h e s e d e e p s e a s h a r k s c o u l d sus t a in y i e l d s o f no m o r e than 2 , 2 5 0 t/yr. 2 .2 .3 B y c a t c h e s a n d D i s c a r d s o f E l a s m o b r a n c h s at S e a . S e v e r a l l a r g e - s c a l e f i sher ies ope ra t i ng in the h i g h - s e a s a r o u n d the w o r l d a re k n o w n to cap tu re e l a s m o b r a n c h s , par t icular ly s h a r k s , a s a subs tan t i a l b y c a t c h . A l t h o u g h s h a r k s are re ta ined a n d ut i l i sed in s o m e of t he se f i sher ies , mos t f requent ly they a re s i m p l y th rown o v e r b o a r d , s o m e t i m e s after thei r v a l u a b l e fins h a v e b e e n c h o p p e d off. T h e su rv iva l c h a n c e s of t he se b y c a t c h e s v a r y d e p e n d i n g o n the type o f g e a r u s e d . T r a w l a n d gil l nets , a n d p e r h a p s pu r se s e i n e s too, a l m o s t ce r ta in ly c a u s e 1 0 0 % d e a t h s a m o n g s h a r k s caugh t . W h i l e su rv iva l is h ighe r in l ong l ines b e c a u s e they permit s h a r k s s o m e l imited m o v e m e n t a n d thus s o m e resp i ra t ion , su rv iva l rates d e p e n d h igh ly o n the m e t a b o l i s m a n d e n d u r a n c e o f i nd iv idua l s p e c i e s . O v e r a l l , it is b e l i e v e d that mos t o f the b y c a t c h e s o f s h a r k s in l a rge - sca l e f i sher ies face a v e r y h igh mortal i ty. T h i s might not be true in the c a s e of ba to ids w h i c h g e n e r a l l y h a v e ve ry different mobi l i ty r e q u i r e m e n t s in o r d e r to resp i re . 9 7 H o w e v e r , thei r c a t c h e s a re no rma l ly ve ry s m a l l in l a rge - sca l e h i g h - s e a s f i she r i es d u e to their m o r e d e m e r s a l habi t s . T h e a m o u n t o f e l a s m o b r a n c h s k i l led in l a rge - sca l e h igh s e a s f i she r i e s a n d the rate of d i s c a r d s a re poor ly u n d e r s t o o d a n d h a v e n e v e r b e e n s y s t e m a t i c a l l y a s s e s s e d . R e p o r t s o n the s h a r k s t a k e n by the coun t r i e s i n v o l v e d in t h e s e f i sher ies d o not reflect the rea l l eve l s of inc iden ta l c a t c h e s but mos t f requent ly o n l y r ep resen t the a m o u n t s r e t a ined . T h e major p u r p o s e of this s e c t i o n to p resen t the a v a i l a b l e informat ion o n the m o s t important l a rge-s c a l e f i sher ies o f the w o r l d a n d eva lua t e a s far a s p o s s i b l e the extent o f the i r e l a s m o b r a n c h b y c a t c h e s , the a m o u n t s t a k e n a n d the total d i s c a r d s . Unt i l ve ry recent ly , there w e r e two m a i n l a rge - sca l e f i she r i es c a t c h i n g a n d d i s c a r d i n g s ignif icant n u m b e r s o f e l a s m o b r a n c h s in thei r ope ra t i ons , n a m e l y driftnet a n d longl ine f i sher ies . D u e to in terna t ional p r e s s u r e s a n d fo l lowing U N reso lu t ion 44/225, a l l l a rge - sca l e driftnet f i sher ies w e r e p h a s e d out of in ternat ional wa t e r s at the e n d o f 1 9 9 2 . T h e y are still d i s c u s s e d here h o w e v e r , d u e to the impor t ance o f thei r b y c a t c h e s . In add i t ion to long l ine a n d driftnet f i sher ies , o the r l a rge - sca l e f i sher ies with m i n o r e l a s m o b r a n c h b y c a t c h e s (tuna pu r se s e i n e a n d po le a n d l ine f i sher ies) a re briefly d i s c u s s e d . T h e d e e p t rawl f i sher ies for o r a n g e r o u g h y a r e t rea ted briefly b e c a u s e o f thei r potent ia l h igh i m p a c t o n d e e p w a t e r sha rk popu la t ions . T h e fo l lowing a c c o u n t s focus o n a s s e s s i n g the s p e c i e s o f e l a s m o b r a n c h s c a u g h t a n d their c a t c h rates in e a c h of t h e s e f i sher ies . Inc identa l c a t c h e s a re e s t ima ted w h e r e no e s t i m a t e s a l r e a d y exis t a n d t h e s e are then c o m p a r e d wi th r epor ted l a n d i n g s for e a c h f i shery o r coun t ry in o r d e r to a s s e s s the quant i t ies o f e l a s m o b r a n c h s w a s t e d e a c h y e a r a n d not i n c l u d e d in the official s tat is t ics o f w o r l d f i sher ies . 2.2.3.1 Drift gi l lnet f i sher ies . F o r the last f ew d e c a d e s , s e v e r a l coun t r i e s , chief ly J a p a n , K o r e a a n d T a i w a n , d e v e l o p e d l a rge - sca l e f i sher ies u s i n g drift g i l lnets in the h igh s e a s o f m a n y o c e a n s . T y p i c a l l y , v e s s e l s d e p l o y e d s e v e r a l k i lomet res o f gil lnet w h i c h t r a p p e d v e r y efficiently the re la t ive ly d i s p e r s e d r e s o u r c e s t hey w e r e a i m i n g for. Unfor tunate ly , they a l s o c a p t u r e d m a n y o the r non-target s p e c i e s w h i c h w e r e c o m m o n l y d i s c a r d e d , s o m e t i m e s in v e r y la rge quant i t i es . T h e c o n c e r n of env i ronmen ta l i s t s o v e r the impac t o f drift g i l lnets o n o c e a n i c f a u n a h a s b e e n f o c u s e d 98 ma in ly in the m o r e a p p e a l i n g mar ine m a m m a l s . H o w e v e r , it is n o w k n o w n that s h a r k s w e r e a m o n g the mos t f requent ly c a u g h t non-target o r g a n i s m s in s o m e o f t h e s e f i sher ies . D e s p i t e this , little a t tent ion h a s b e e n p a i d to the effect o f drift g i l lnets o n s h a r k p o p u l a t i o n s . A l t h o u g h al l l a rge - sca l e driftnet f i sher ies h a v e s t o p p e d o n the h igh s e a s o f the w o r l d s i n c e D e c e m b e r 1992 , a n a t tempt is m a d e here to a s s e s s the m a g n i t u d e of the i r kil ls o f s h a r k s a n d rays . T h o u g h mos t o f this k ind o f mortal i ty h a s c e a s e d , its effects m a y still be felt o v e r s u b s e q u e n t g e n e r a t i o n s o f e l a s m o b r a n c h s . T h e de ta i l s g i v e n b e l o w s h o u l d both p rov ide important re fe rence informat ion a n d s t and a s t e s t imony to a recen t e n v i r o n m e n t a l i s s u e . In this s e c t i o n , I a n a l i s e the mos t important l a rge - sca l e driftnet f i she r ies . T h e desc r ip t ion of t he se f i sher ies is b a s e d pr imar i ly o n the recent r ev i ew by Nor th r idge (1991) a n d the bul le t ins o f the Internat ional Nor th P a c i f i c F i s h e r i e s C o m m i s s i o n ( I N P F C ) ( M y e r s et a l . 1 9 9 3 , Ito et a l . 1993) . R e a d e r s a re referred to t hose reports for m o r e de t a i l ed informat ion o n t he se f i sher ies . Nor th P a c i f i c O c e a n . Unt i l recent ly , there w e r e three m a i n l a rge - sca l e driftnet f i she r i es in the Nor th Pac i f i c , n a m e l y the s a l m o n f ishery, the f lying s q u i d f i shery a n d the l a r g e - m e s h f i shery for t unas a n d bi l l f ishes . T o g e t h e r , t h e s e three f i sher ies m a d e the Nor th P a c i f i c the m o s t heav i ly exp lo i t ed a r e a o f the w o r l d with driftnets. T h i s p robab ly ref lected the g e o g r a p h i c loca t ion of the three m a i n coun t r i e s i n v o l v e d in driftnetting o n a l a r g e - s c a l e . a) S a l m o n f ishery . T h e J a p a n e s e fleet w a s the larges t in this f i shery . C a n a d i a n a n d U S A f i s h e r m e n still ho ld c o n s i d e r a b l e n u m b e r s of boa ts , but a re res t r ic ted to s m a l l driftnets (< 5 0 0 m pe r v e s s e l ) a n d f i sh ing e x c l u s i v e l y in the c o a s t a l w a t e r s of their E E Z T h e r e w e r e two J a p a n e s e f i sher ies for s a l m o n . T h e "mothersh ip" f i shery that o p e r a t e d in the in terna t ional wa t e r s o f the Nor th Pac i f i c , sou th of the A l e u t i a n s a n d o n the B e r i n g S e a , a n d the l and b a s e d f ishery that o c c u r r e d in the h i g h - s e a s eas t o f J a p a n ( F i g . 2 .31) . In g e n e r a l , du r ing the pas t t w o d e c a d e s the J a p a n e s e s a l m o n f i shery s h o w e d a cons i s t en t 9 9 Figure 2.31 Gene ra l i zed a rea of operation of the J apanese landbased and non-traditional (ex-mothership) fisheries in 1990. ( B a s e d on I N P F C 1993). 100 d e c l i n e in effort that i n v o l v e d con t rac t ions in n u m b e r o f v e s s e l s , f i sh ing a r e a a n d f i sh ing s e a s o n . T h e m o t h e r s h i p f i shery c o n s i s t e d o f p r o c e s s i n g s h i p s that s u p p o r t e d s o m e 4 0 s m a l l e r "catcher" v e s s e l s . T h e f i sh ing g r o u n d s w e r e d i v i d e d in s u b a r e a s wi th different o p e n i n g a n d c l o s i n g s e a s o n s , a l t h o u g h the total t ime s p a n of the f i shery o n l y ran from M a y 31 to J u l y 3 1 . T h e f i shery c o n t r a c t e d its o p e r a t i o n s b a s i c a l l y d u e to p r e s s u r e s from the U S A , C a n a d a a n d the fo rmer U . S . S . R . D u r i n g the 1 9 9 0 a n d 1991 s e a s o n s , the o p e r a t i o n s w e r e c o n v e r t e d to a l a n d b a s e d f i shery ("non-tradi t ional" l a n d b a s e d f ishery) by e l imina t ing the m o t h e r s h i p boa ts . C a t c h e s p e a k e d in 1956 , w h e n a p p r o x i m a t e l y 9.3 mi l l ion t an s w e r e set, w h i l e o n l y 2 3 8 , 7 0 0 t ans w e r e set in 1 9 9 1 , the last y e a r of the f i shery ( F . A . J . 1991) . T a n s a r e i n d e p e n d e n t net p a n e l s w h i c h const i tu te the w o r k i n g unit of driftnets a n d a re typ ica l ly 4 5 - 5 0 m long (a l ready r igged) in the s a l m o n f ishery . T h e driftnet w a s 8-10 m d e e p a n d w a s c o n s t r u c t e d of ny lon monof i l amen t with m e s h s i z e s in the r ange of 1 2 1 - 1 3 0 m m , e a c h v e s s e l d e p l o y i n g a m a x i m u m of 15 k m of net in a d u s k - t o - d a w n ope ra t i on . In the l a n d b a s e d f ishery, two t ypes o f v e s s e l s w e r e k n o w n : c o a s t a l boa t s o f <30 G T a n d m e d i u m s i z e v e s s e l s o f 3 0 - 1 2 7 G T . Effort in this f i shery a l s o d e c l i n e d s ignif icant ly in the later d a y s . T h e n u m b e r s of v e s s e l s in the f i shery d e c l i n e d a n d the f i sh ing a r e a w a s r e d u c e d . C o a s t a l v e s s e l s p e a k e d in the mid -70 ' s at 1,400 units but du r ing 1 9 7 8 - 1 9 8 8 there w e r e on ly 6 7 8 (Nor thr idge 1991) . V e s s e l s o v e r 3 0 G T w e r e at thei r h ighes t n u m b e r s in 1 9 7 2 - 1 9 7 4 with 3 7 4 boa ts , but w e r e r e d u c e d to on ly 8 3 in 1991 ( M y e r s et a l . 1993) . T h e total n u m b e r o f se t s pe r s e a s o n p e a k e d at a p p r o x i m a t e l y 1 9 , 7 0 0 in 1966 but d e c l i n e d to abou t 4 , 1 0 0 (781 ,176 tans) in 1 9 8 9 ( F . A . J . 1990) , wi th o n l y 3 7 4 , 9 9 0 t ans set d u r i n g 1991 ( F . A . J . 1991) . T h e f i sh ing s e a s o n s p a n n e d from late M a y to the e n d o f J u n e du r ing the last y e a r s of the f ishery: G i l l n e t s o f the l a n d b a s e d f ishery w e r e s imi l a r to t h o s e o f the m o t h e r s h i p f i shery but with s m a l l e r m e s h s i z e s o f 110 -117 m m . C o a s t a l v e s s e l s o f <10 G T set l e s s t han 10 k m of net pe r night w h i l e offshore v e s s e l s d e p l o y e d up to 15 k m of net. D e t a i l e d repor ts o n the b y c a t c h e s o f non-target s p e c i e s in t h e s e f i she r i e s ( see Nor th r idge 1 9 9 1 , for a s u m m a r y ) a re s t rongly b i a s e d t o w a r d s s tud ies d e a l i n g wi th m a r i n e m a m m a l s a n d birds : s h a r k s a re m e n t i o n e d o n l y a s a s ide i s s u e . H o w e v e r , the F i s h e r i e s A g e n c y of J a p a n ( F . A . J . 1987 , 1 9 8 8 , 1989) reports b y c a t c h e s o f s e v e r a l non- target s p e c i e s in thei r driftnet 101 r e s e a r c h c r u i s e s for s a l m o n . T h e i r resul ts for the y e a r s 1 9 8 6 - 1 9 8 8 a re p r e s e n t e d in tab le 2 .9 , t oge the r wi th the e s t i m a t e d total b y c a t c h of s h a r k s t a k e n in the 1 9 8 9 s e a s o n w h e n a total of 1 ' 0 9 7 , 6 3 0 t ans w e r e set. B l u e s h a r k s are the m o s t f requent ly r epor ted s h a r k s p e c i e s . T h e total b y c a t c h in the f i shery for 1 9 8 9 is here e s t i m a t e d a s 1 1 , 4 9 2 s h a r k s o f e ight s p e c i e s o r a p p r o x i m a t e l y 108 t. T h e s e resul ts s h o u l d be t a k e n with c a u t i o n . First , the a r e a s s u r v e y e d in the r e s e a r c h c r u i s e s w e r e a p p a r e n t l y different from those o f the c o m m e r c i a l f i shery , a n d there w e r e s o m e ve ry s m a l l m e s h s i z e s a m o n g s o m e o f the r e s e a r c h driftnets. T h i s p r o b a b l y h a s a n effect o n the c a t c h rates o f mos t s p e c i e s , both th rough c h a n g e s in ca tchab i l i ty o f the g e a r a n d ava i lab i l i ty o f e a c h s p e c i e s (e .g . b lue s h a r k s a re not e x p e c t e d to be c a u g h t in the B e r i n g S e a in h igh n u m b e r s d u e to the i r m o r e t empera t e dis tr ibut ion) . Di rec t ex t r apo la t ions o f the r e s e a r c h da t a to the total f i shery might thus not be represen ta t ive o f the rea l s i tua t ion . S e c o n d l y , mos t of the c a t c h ra tes o f s h a r k s repor ted in tab le 2.9 s e e m too low c o m p a r e d wi th o ther s tud ies . A l t h o u g h there a re no o ther direct reports for the s a l m o n f ishery , resu l t s f rom C a n a d i a n r e s e a r c h c r u i s e s ( L e B r a s s e u r et a l . 1987) c a n be u s e d to de r ive a l te rna t ive c a t c h rates for s h a r k s . T h e s e r e s e a r c h c r u i s e s w e r e d e s i g n e d to a s s e s s the s a l m o n b y c a t c h e s o f the s q u i d f ishery but e m p l o y e d nets vir tual ly iden t ica l to t h o s e o f the c o m m e r c i a l s a l m o n f ishery. A c c o r d i n g l y , thei r resul ts c o u l d better reflect the c a t c h ra tes o f s h a r k s in the c o m m e r c i a l s a l m o n f ishery . T h e e s t ima te s o b t a i n e d for b lue a n d s a l m o n s h a r k s a re o n e o r d e r o f m a g n i t u d e h i g h e r than t hose c a l c u l a t e d from F . A . J , da ta , wi th v a l u e s o f 5 ,275 a n d 194 s h a r k s / 1 0 0 0 k m of net r e spec t ive ly (Tab le 2 .10) . In g e n e r a l t e rms , the total c a t c h o f s h a r k s in the J a p a n e s e s a l m o n f i she r i e s is b e l i e v e d to h a v e b e e n re la t ive ly s m a l l w h e n c o m p a r e d with o ther driftnet f i she r i es in the north Pac i f i c ( see be low) . E v e n c o n s i d e r i n g the a l ternat ive c a t c h rates o f 5 ,502 s h a r k s p e r 1000 k m of driftnet b a s e d o n C a n a d i a n r e s e a r c h da ta , s o m e 3 0 0 , 0 0 0 ind iv idua l s o r a p p r o x i m a t e l y 1,237 t of s h a r k s a re e s t i m a t e d to h a v e b e e n c a u g h t dur ing the 1 9 8 9 s e a s o n in th is f ishery. T h i s re la t ively s m a l l c a t c h is m a i n l y a funct ion o f the s i z e o f the f i shery , w h i c h a s p r ev ious ly m e n t i o n e d w a s con t rac t ing y e a r by yea r . A s a re fe rence point, a c c o r d i n g to S h i m a d a & N a k a n o (1992) , s o m e 3 4 , 0 0 0 la rge a n d adul t s a l m o n s h a r k s w e r e l a n d e d from the s a l m o n driftnet f i shery in J a p a n in 1 9 6 0 . Fu r the rmore , repor ts for the ea r ly 80 ' s ( P a u s t 1987) 102 Table 2.9 Est imation of shark bycatches in the J apanese sa lmon fisheries, based on information from research cruises . 1986 1987 1988 Catch Rate a) Estimated Numbers in Catch 1989 b) Likely weight (kg) c) Species (24,549 tans) (17,056 tans) (17,805 tans) (sharks/1 oookm) Landbased Mothership Total per shark in the catch Unid. Lamnidae 0 1 2 1.01 39 16 55 50 2,771 Lamna ditropis 25 26 23 24.91 973 394 1,367 50 68,359 Isurus oxyrinchus 13 1 2 5.39 210 85 296 50 14,780 Prionace glauca 142 188 79 137.69 5,378 2,179 7,556 2.42 d) 18,287 Squalus acanthias 73 33 8 38.38 1,499 607 2,106 2 4,212 Isistius brasiliensis 1 1 0 0.67 26 11 37 0.75 28 Mustelus manazo 1 0 2 1.01 39 16 55 2 111 Triakis scyllium 0 0 1 0.34 13 5 18 2 37 Totals 255 250 117 209.39 8,179 3,313 11,492 159.17 108,586 a) assuming 50m tans in research cruises b) based in effort reported by FAJ (1990) c) Considering sizes expected for 110-130 mm mesh d) Calculated from LeBrasseur et al. (1987) length frequency data, Pratt (1979) TL-FL relationship, and Strasburg (1958) L-W relationship. Table 2.10 Alternative estimates of shark bycatches in J apanese sa lmon fisheries, based on C a n a d i a n research cruise (LeBrasseur et al. 1987). Sharks caught Catch rate Estimated numbers Likely weight (kg) Species (618 tans) per/1000km of net in 1989 catch per shark in 1989 fishery Prionace glauca 163 5,275 289,504 2.42 a) 700,601 Lamna ditropis 6 194 10,657 50 532,830 Squalus acanthias 1 32 1,776 2 3,552 Total 170 5,502 301,937 54.42 1,236,983 a) Calculated from LeBrasseur et al. (1987) length frequency data, Pratt (1979) TL-FL relationship, and Strasburg (1958) L-W relationship. 103 ind ica te 2 5 , 0 0 0 s a l m o n s h a r k s (Lamna ditropis) w e r e t a k e n e a c h y e a r by the J a p a n e s e s a l m o n f i s h e r m e n in the cen t ra l A l e u t i a n r eg ion . C o n s i d e r i n g the a v a i l a b l e effort s tat is t ics a n d the c a t c h ra tes o b t a i n e d from L e B r a s s e u r et a l . (1987) , a total o f l e s s than 1,600 s a l m o n s h a r k s s h o u l d h a v e b e e n t a k e n in the a r e a sou th o f the A l e u t i a n s in 1 9 8 9 . T h i s s u g g e s t s that a r educ t ion o f abou t 9 5 % in s a l m o n s h a r k f i sh ing mortal i ty a c c o m p a n i e d the d e c l i n e o f the f i shery . A l t h o u g h there is not e n o u g h informat ion to a s s e s s the l eve l o f c a t c h e s a n d d i s c a r d s o f s h a r k s that t ook p l a c e in this f ishery, it is p o s s i b l e that s o m e o f the s a l m o n s h a r k s w o u l d have b e e n kept a n d u t i l i sed . T h i s is s u g g e s t e d by repor ts o f spec i f i c f i she r i es for this s p e c i e s t ak ing p l a c e in N E J a p a n e s e wa t e r s off the O y a s h i o front ( P a u s t 1 9 8 7 , A n o n . 1988) , w h i c h ind ica te that the s a l m o n sha rk is a p p r e c i a t e d by J a p a n e s e f i s h e r m e n . O n the o ther h a n d , the mot iva t ion to k e e p s a l m o n s h a r k s p r o b a b l y h a d to be w e i g h t e d aga in s t the ava i lab i l i ty o f s p a c e a n d the d a n g e r of s p o i l a g e of the v a l u a b l e c a t c h e s of s a l m o n in the v e s s e l ' s s to rage a r e a . In J u l y 1 9 9 1 , a l l J a p a n e s e s a l m o n driftnet f i sher ies in the h i g h - s e a s c e a s e d ac t iv i t ies . M o s t of the fleet w a s d i s b a n d e d a l t hough a m i n o r part w a s r e a l l o c a t e d to the R u s s i a n E E Z wa te r s v i a a joint ven tu re b e t w e e n J a p a n a n d R u s s i a . T h e r e is still no official informat ion a v a i l a b l e abou t this n e w s a l m o n driftnet f i shery but j u d g i n g from the c a l c u l a t i o n s m a d e a b o v e the b y c a t c h e s o f e l a s m o b r a n c h s s h o u l d be o f re la t ively l ow i m p o r t a n c e . b) F l y i n g s q u i d f i shery . S i n c e the late 70 ' s , a major driftnet f i shery for f lying s q u i d (Ommastrephes bartrami), w a s s tar ted c o n s e c u t i v e l y by J a p a n , K o r e a a n d T a i w a n (in o r d e r o f impor t ance ) in the C e n t r a l Nor th P a c i f i c . In 1 9 9 0 a l m o s t 7 4 0 v e s s e l s from the three na t ions w e r e o p e r a t i n g . Y a t s u et a l . (1993) s u m m a r i s e mos t of the informat ion a v a i l a b l e for J a p a n w h i c h w a s the first coun t ry to b e g i n f i sh ing for f lying s q u i d in the cen t ra l Nor th P a c i f i c in 1 9 7 8 . J a p a n l imited the n u m b e r o f v e s s e l s a n d the a r e a o p e n to this f i shery (fig. 2 .32) , wi th a north b o u n d a r y w h i c h m o v e d th rough the y e a r to a v o i d c a t c h e s o f s a l m o n s w h i c h w e r e p roh ib i ted to the entire f lying s q u i d f ishery . T h e r e w e r e two c a t e g o r i e s o f J a p a n e s e v e s s e l s : 6 0 - 1 0 0 104 G T a n d 1 0 0 - 5 0 0 G T . T h e f i sh ing s e a s o n ran from J u n e 1st to D e c e m b e r 31st , a l t hough two t ypes of l i c e n s e s for 7 a n d 4 m o n t h s w e r e i s s u e d wi th in the s e a s o n . T h e driftnets w e r e c o n s t r u c t e d o f ny lon monof i l amen t (yarn 0.5 m m ) a n d m e s h s i z e s in the r ange 1 0 0 - 1 3 5 m m , with 1 1 5 - 1 2 0 m m the mos t c o m m o n l y e m p l o y e d . R i g g e d t ans w e r e 9-10 m d e e p a n d 3 3 - 4 2 m in leng th . E a c h v e s s e l set b e t w e e n 15 a n d 5 0 k m of net, a l t h o u g h s o m e repor ts indica te that mos t c o m m o n se t s w e r e c l o s e to 5 0 k m . F o l l o w i n g J a p a n ' s init iative, K o r e a j o i n e d the f i shery in 1 9 7 9 ( see G o n g et a l . 1 9 9 3 for a full a c c o u n t ) . K o r e a n s q u i d driftnet v e s s e l s w e r e mos t ly o f c. 3 5 0 G T , but s o m e boa t s e x c e e d e d 4 0 0 G T . T h e K o r e a n fleet f i shed from A p r i l to ea r ly A u g u s t in a n a r e a par t ia l ly o v e r l a p p i n g with the J a p a n e s e f i sh ing g r o u n d s , a n d from ear ly A u g u s t to m i d - D e c e m b e r for s m a l l e r s q u i d to the ea s t o f J a p a n (fig. 2 .32) . K o r e a n driftnets h a d 5 0 m t ans wi th m e s h s i z e s of 7 6 -155 m m . In the m a i n f i sh ing a r e a they w e r e c o m m o n l y 1 0 5 - 1 1 5 m m , w h i l e t h o s e u t i l i sed in the g r o u n d s eas t o f J a p a n w e r e 86 -96 m m . A c c o r d i n g to G o n g et a l . (1993) , K o r e a n v e s s e l s d e p l o y e d a b o u t 2 8 k m of driftnet in the ea r ly 1980 ' s but i n c r e a s e d to a h igh o f 4 5 k m in 1 9 9 0 . Information o n the T a i w a n e s e s q u i d f i shery is s c a r c e a n d m o s t o f this a c c o u n t is b a s e d o n the brief c o m m u n i c a t i o n of Y e h a n d T u n g (1993) . T a i w a n j o i n e d the f i shery in 1980 . V e s s e l s ' s i z e r a n g e d from 1 0 0 - 7 0 0 G R T but mos t w e r e 2 0 0 - 3 0 0 G R T . Driftnetters l a rger than 4 0 0 G R T w e r e i n t roduced ma in ly in 1984 w h i l e t h o s e l a rge r t han 6 0 0 G R T en te red dur ing the 1 9 8 6 - 1 9 8 7 s e a s o n . T a i w a n e s e driftnets for s q u i d w e r e a p p a r e n t l y c o n s t r u c t e d of monof i l amen t n y l o n . T h e i r m e s h s i z e s r a n g e d from 7 6 - 1 2 0 m m , wi th e a c h tan m e a s u r i n g b e t w e e n 15 a n d 4 0 m in length . T y p i c a l total l engths o f driftnet d e p l o y e d pe r boat w e r e 3 1 -41 km (F i t zge ra ld et a l . 1993) . T a i w a n e s e v e s s e l s w e r e a l l o w e d to fish y e a r r ound ( P e l l a et a l . 1993) but the f i sh ing s e a s o n w a s appa ren t l y r e a l i s e d o n l y from J u n e to N o v e m b e r ( Y e h a n d T u n g 1993) in a n a r e a v e r y s imi l a r to the K o r e a n g r o u n d s but e x t e n d i n g w e s t w a r d to the J a p a n e s e E E Z (fig 2 .32) . Effort s ta t is t ics for t he se f i sher ies h a v e b e e n m a d e a v a i l a b l e o n l y v e r y recent ly . A c c o r d i n g to da ta p r o v i d e d by Y a t s u et a l . (1993) , G o n g et a l . (1993) a n d Y e h a n d T u n g (1993) , the total n u m b e r o f v e s s e l s from the three coun t r i e s in the s q u i d driftnet f i shery for the pe r iod 1 9 8 8 - 1 9 9 0 w e r e 7 9 2 , 7 8 4 a n d 7 3 7 respec t ive ly . D a t a o n the total n u m b e r o f t ans d e p l o y e d 105 106 by J a p a n a n d K o r e a a re a l s o a v a i l a b l e . Unfor tunate ly , T a i w a n e s e s ta t is t ics d o not s epa ra t e effort b e t w e e n the s q u i d f i shery a n d the l a r g e - m e s h driftnet f i shery ( e x a m i n e d in next sec t ion) a s the i r boa t s c a r r i e d both t ypes o f g e a r a n d d e p l o y e d e i ther o n e d e p e n d i n g o n the e x p e c t e d c a t c h . Fu r the rmore , T a i w a n e s e effort s tat is t ics a re g i v e n o n l y in total v e s s e l / d a y s f i shed (table 2 .11) . T h e total n u m b e r o f s t a n d a r d i s e d t ans set by the T a i w a n e s e fleet in the s q u i d f i shery c a n be e s t i m a t e d wi th the a i d o f c o m p a r a t i v e da ta o n typ ica l total l ength o f s e t s for v e s s e l s from e a c h coun ty . F i t z g e r a l d et a l . (1993) p rov ide e s t ima te s o f a total o f 51-61 k m of driftnet per J a p a n e s e v e s s e l a n d a total o f 31-41 k m pe r T a i w a n e s e v e s s e l . D a t a from Y a t s u et a l . (1993) i nd i ca t e s that J a p a n e s e v e s s e l s d e p l o y e d a n a v e r a g e o f 9 9 7 . 4 3 t ans (50 m e a c h ) pe r f i sh ing d a y du r ing 1 9 8 9 a n d 1 9 9 0 . T h e effort o f T a i w a n e s e v e s s e l s is he re a s s u m e d to be a l l o c a t e d e q u a l l y to the f lying n e o n s q u i d a n d the l a r g e - m e s h f i she r i e s . A s s u m i n g that the n u m b e r of t an s p e r v e s s e l is e q u a l in the J a p a n e s e a n d T a i w a n e s e f leets, total efforts of 4 ' 4 7 1 , 6 7 8 , 5 ' 616 , 888 a n d 3 ' 595 ,855 s t a n d a r d i s e d (50 m) t ans o f net a r e e s t i m a t e d here for the T a i w a n e s e fleet in the s q u i d f i shery for the y e a r s 1 9 8 8 - 1 9 9 0 respec t ive ly . A c c o r d i n g l y , total effort for the three coun t r i e s in this f i shery c a n be a p p r o x i m a t e d at 6 4 7 8 2 , 2 3 6 t ans (3 ' 239 ,112 km) for 1 9 8 9 a n d 5 0 ' 9 2 2 , 3 8 8 t an s (2 ' 546 ,119 km) for 1 9 9 0 . T h e r e a re s e v e r a l s o u r c e s o f informat ion o n c a t c h e s o f non- target s p e c i e s in t he se f i sher ies , chief ly f rom r e s e a r c h c r u i s e s a n d m o r e recent ly from o b s e r v e r p r o g r a m m e s . R e s u l t s from s o m e r e s e a r c h s u r v e y s e n a b l e a n a s s e s s m e n t o f c a t c h rates in n u m b e r s o f s h a r k s for b lue , s a l m o n a n d four o the r s p e c i e s o f s h a r k s , s i z e structure a n d c a t c h rate in k g / m for b lue s h a r k s , p e r c e n t a g e dis t r ibut ion by m e s h s i z e for b lue a n d u n s p e c i f i e d s h a r k s p e c i e s , a n d d i f ferences in b lue s h a r k c a t c h e s b e t w e e n sur face a n d s u b s u r f a c e s q u i d driftnets ( F A J 1 9 8 3 , M u r a t a a n d S h i n g u 1 9 8 5 , M u r a t a 1 9 8 6 , 1 9 8 7 , Rowle t t 1 9 8 8 , M u r a t a e t a l . 1 9 8 9 , Y a t s u 1989 , Ito et a l . 1990) . H o w e v e r , resul ts from t h e s e s u r v e y s suffer the s a m e p r o b l e m s of the s a l m o n f i shery r e s e a r c h s u r v e y s . J a p a n e s e a n d K o r e a n r e s e a r c h c r u i s e s uti l ise a var ie ty of m e s h s i z e s w h i c h e x t e n d e d a b o v e a n d b e l o w the s i z e r ange o f t h o s e u t i l i sed in the c o m m e r c i a l f i shery . T h e app l i ca t i on o f thei r resul ts is therefore v e r y l imi ted f o r t h e p u r p o s e of a s s e s s i n g total c a t c h e s o f non-target s p e c i e s . F a r m o r e useful informat ion c o m e s from the o b s e r v e r p r o g r a m m e s o n b o a r d c o m m e r c i a l 107 v e s s e l s . D a t a from J a p a n e s e o b s e r v e r s for 1988 ( F A J 1989) ind ica te c a t c h ra tes o f 5 3 6 b lue s h a r k s pe r 1 0 0 0 k m of net. H o w e v e r , co l l ec t ive da t a f rom J a p a n e s e , C a n a d i a n a n d U . S . o b s e r v e r s for 1 9 8 9 (Gje rnes et a l . 1990) report 8 1 4 b lue s h a r k s p e r 1 0 0 0 k m of net. D a t a for the 1 9 9 0 o b s e r v e r p r o g r a m m e ( I N P F C 1991) a re m o r e de t a i l ed a n d ind ica te that 12 e l a s m o b r a n c h s p e c i e s w e r e t a k e n a s b y c a t c h in the f i shery . T h e c a t c h rates for b lue s h a r k s w a s 7 1 8 / 1 0 0 0 k m of driftnet, f o l l o w e d by s a l m o n s h a r k s ( 5 5 / 1 0 0 0 k m of driftnet). O t h e r large s h a r k s s p e c i e s cap tu red , p e r h a p s by en t ang l ing , w e r e c o m m o n th re she r (Alopias vulpinus), shortfin m a k o (Isurus oxyrinchus), wh i te (Carcharodon carcharias) a n d b a s k i n g s h a r k (Cetorhinus maximus) (Tab le 2 .12) . O b s e r v e r da t a f rom the K o r e a n fleet for 1 9 9 0 e s t i m a t e d a c a t c h rate o f 3 2 . 0 8 s h a r k s a n d rays pe r 1 0 0 0 p o k s ( K o r e a n tans) w h i c h is equ iva l en t to 6 4 1 . 6 / 1 0 0 0 k m of net. T h i s f igure is s l ight ly low, c o m p a r e d to the 7 8 5 / 1 0 0 0 km of net e s t i m a t e d for the J a p a n e s e f ishery . D a t a o n f i shes in m o s t o f the o b s e r v e r p r o g r a m m e s for the Nor th P a c i f i c driftnet f i sher ies a re l ikely to be s l ight ly u n d e r e s t i m a t e d . O n l y d e c k e d a n i m a l s a r e t a k e n into accoun t , thus u n k n o w n n u m b e r s of ' d ropof f f i shes w e r e not i n c l u d e d in the r e c o r d s . D e s p i t e this , o b s e r v e r p r o g r a m m e s p rov ide the bes t a v a i l a b l e informat ion . T h e r e a re s o m e i n d e p e n d e n t e s t ima tes for e l a s m o b r a n c h b y c a t c h e s in the s q u i d driftnets. Y a t s u et a l . (1993) es t imate a total inc iden ta l c a t c h of 7 2 3 , 9 3 3 b lue s h a r k s , 5 6 , 0 2 9 s a l m o n s h a r k s a n d 1 1 , 3 2 2 v a r i o u s s h a r k s a n d rays for the J a p a n e s e fleet d u r i n g 1 9 9 0 , a m o u n t i n g to 7 , 4 1 5 1 . Y a t s u et a l . ' s e s t ima t ion m e t h o d t a k e s into c o n s i d e r a t i o n s o u r c e s o f var iabi l i ty for c r u i s e s a n d se ts s a m p l e d . H o w e v e r , thei r e s t ima te s o f b lue s h a r k b y c a t c h for 1 9 8 9 a re a l m o s t d o u b l e t h o s e for 1 9 9 0 h ighl igh t ing the var iabi l i ty of e s t ima t i ons a n d the c h a n g e s in f i sh ing effort du r ing the p e r i o d . W e t h e r a l l a n d S e k i (1992) u s i n g a stratified es t ima t ion arr ive at a total b y c a t c h o f 1.2-1.4 mi l l ion b lue s h a r k s for the J a p a n e s e f i shery du r ing 1 9 8 9 , w h i l e Nor th r idge (1991) e s t i m a t e d the total c a t c h of b lue s h a r k s for the entire f lying s q u i d f i shery dur ing 1 9 8 9 at 2 .44 mi l l ion i nd iv idua l s ( cons ide r ing the s a m e effort l e v e l o f 1 9 8 8 a n d c a t c h rates d e r i v e d from G j e r n e s et a l . [1990]). U s i n g the latest effort s tat is t ics a v a i l a b l e for 1 9 9 0 a n d the resul ts f rom the J a p a n - U S A o b s e r v e r p r o g r a m m e ( I N P F C 1991) , n u m b e r s of e l a s m o b r a n c h s by s p e c i e s a n d the l ikely 108 Table 2.11 Effort statistics for the flying squid driftnet fishery in the North Paci f ic for the period 1988-1990 (from Y a t s u et a l . 1993, G o n g et a l . 1993 and Y e h & T u n g 1993). Year Japan Korea Taiwan Total # boats 463 150 179 792 1988 days fished - - 14,010 total tans 36,055,567 24,594,370 # boats 460 157 167 784 1989 days fished 33,646 - 17,598 total tans 34,385,032 24,780,316 # boats 457 142 138 737 1990 days fished 23,656 - 11,266 total tans 22,769,857 24,556,676 Table 2.12 Est imat ion of bycatches of e lasmobranchs in 1990 S q u i d driftnet fishery based on reports of observer programme on board commerc ia l v e s s e l ( I N P F C 1991). Species Numbers observe Catch rate Numbers in Likely mean Weight in (2,281,896 Tans) per/1000km of net Total catch Weight (kg)* Total catch (kg) Unidentified shark 1,191 10 26,578 15? 398,672 Prionace glauca 81,956 718 1,828,915 7(1) 12,802,407 Lamna ditropis 6,263 55 139,764 38.7 (1) 5,408,866 Isurus oxyrinchus 71 0.622 1,584 40 63,377 Alopias vulpinus 48 0.421 1,071 40 42,846 Squalus acanthias 8 0.070 179 2 357 Carcharodon carcharias 7 0.061 156 50 7,811 Isistius brasiliensis 5 0.044 112 0.75 84 Euprotomicrus bispinatus 1 0.009 22 0.20 4 Cetorhinus maximus 1 0.009 22 500 11,158 Dasyatis violacea 8 0.070 179 10? 1,785 Dasyatis brevis 1 0.009 22 10? 223 Unidentified ray 8 0.070 179 10? 1,785 Totals 89,568 785 1,998,783 - 18,739,376 * considering sizes expected for 100-135 mm. mesh (1) from Yatsu et al. 1993 109 we igh t o f the i r c a t c h e s in the 1 9 9 0 s e a s o n a re e s t i m a t e d he re a n d s u m m a r i s e d in table 2 .12 . T h e s e e s t i m a t e s ind ica te that a total o f abou t 2 mi l l ion s h a r k s equ iva l en t to 18 ,739 t w e r e t a k e n in the w h o l e f i shery . A b o u t 12 ,802 t o f t h e s e o r 1.8 mi l l ion i nd iv idua l s w e r e mos t ly ve ry y o u n g b lue s h a r k s , w h i c h a c c o r d i n g to N a k a n o a n d W a t a n a b e (1992) c o r r e s p o n d mos t ly to s h a r k s 1-2 y e a r s o l d . U n l e s s o t h e r w i s e s ta ted , the e s t ima te s o f the ind iv idua l a v e r a g e w e i g h t for e a c h s p e c i e s a re b a s e d o n a p p r o x i m a t i o n s I m a d e c o n s i d e r i n g the re la t ively s m a l l m e s h s i z e of the nets . In a n y c a s e t h e s e w e i g h t s might be nega t ive ly b i a s e d . A c c o r d i n g l y , the p resen t resul ts c a n be t a k e n a s a m i n i m u m es t ima te o f the real c a t c h of e l a s m o b r a n c h s in the f i shery . O f the a p p r o x i m a t e l y 1 8 , 7 0 0 1 o f s h a r k s caugh t , s o m e 8 ,4001 w o u l d h a v e b e e n t a k e n by J a p a n , w h i l e K o r e a a n d T a i w a n w o u l d h a v e c a u g h t 9 ,000 t a n d 1,300 t r e spec t ive ly . A great p ropor t ion o f the e l a s m o b r a n c h b y c a t c h e s w e r e a p p a r e n t l y d u m p e d to the s e a . A s s e s s m e n t o f s h a r k c a t c h e s for the J a p a n e s e fleet in 1 9 8 9 u s i n g the s a m e p r o c e d u r e a s a b o v e p r o d u c e d e s t ima te s o f 1.8 mi l l ion s h a r k s with a total w e i g h t o f a l m o s t 12 ,654 t. T h e repor ted total t ake o f s h a r k s by the s q u i d fleet o f J a p a n d u r i n g 1 9 8 9 is 2 3 7 , 7 3 4 ind iv idua l s o n l y ( F A J 1990) . A s s u m i n g this f igure is e q u a l to the l a n d e d c a t c h o f s h a r k s , abou t 1.56 mi l l ion s h a r k s w e i g h i n g s o m e 1 0 , 9 0 0 t w e r e w a s t e d in the o p e r a t i o n . S o m e o f the a lmos t 9 5 , 0 0 0 s a l m o n s h a r k s e s t i m a t e d to be c a u g h t in the f ishery, w e r e p r o b a b l y u t i l i sed a s this s p e c i e s is m o r e a p p r e c i a t e d in J a p a n . A n a p p r a i s a l o f the a m o u n t o f e l a s m o b r a n c h s ac tua l ly d i s c a r d e d by the f leets o f T a i w a n a n d K o r e a is not p o s s i b l e d u e to the l ack o f informat ion o n their s h a r k l a n d i n g s from the s q u i d f ishery. T h e total e s t i m a t e d c a t c h o f e l a s m o b r a n c h for the K o r e a n a n d T a i w a n e s e fleets in 1989 , w o r k e d out in the s a m e m a n n e r , a m o u n t s to 9 ,120 t a n d 2 , 0 6 7 t r e spec t ive ly . T h e p resen t e s t i m a t e s of e l a s m o b r a n c h b y c a t c h e s a re ce r t a in ly r o u g h d u e to the l imitat ions of the a v a i l a b l e informat ion . T h e y do h o w e v e r highl ight the p r o b l e m s f o u n d w h e n t rying to a s s e s s the m a g n i t u d e o f the e l a s m o b r a n c h b y c a t c h a n d the p ropor t ions d u m p e d to the s e a . E s t i m a t e s o f total w e i g h t of the b y c a t c h a re v e r y sens i t i ve to the a v e r a g e w e i g h t s for e a c h s p e c i e s u s e d in the c a l c u l a t i o n s . T h i s is par t icular ly true in the c a s e o f b lue s h a r k w h i c h a c c o u n t s for m o s t of the b y c a t c h e s in n u m b e r s . Ideally, the a v e r a g e w e i g h t o f s h a r k s u s e d for a par t icu la r a r e a s h o u l d reflect the s i z e c o m p o s i t i o n for that a r e a b e c a u s e o f disc t inct s i z e s e g r e g a t i o n for m a n y s p e c i e s , h o w e v e r , this deta i l in d a t a is not a v a i l a b l e at present . Y a t s u et a l . (1993) u s e a n a v e r a g e we igh t o f 7 kg for b lue s h a r k s a n d this w a s fo l l owed 110 here f o r t h e c a l c u l a t i o n s of tab le 2 .12 , h o w e v e r , a l ternat ive c a l c u l a t i o n s b a s e d o n the length f r equency repor ts for b lue s h a r k s o f L e B r a s s e u r et a l . (1987) a n d m o r p h o m e t r i c e q u a t i o n s f o r t h e s p e c i e s p r o v i d e d by S t r a s b u r g (1958) a n d Pratt (1979) , p r o d u c e a n a v e r a g e we igh t of 2.4 kg / shark . T h e es t imate o f 2.4 kg / sha rk is cons i s t en t wi th the f ind ings o f B e r n a r d (1986) , M c k i n n e l l et a l . (1989) a n d M u r a t a et a l . (1989) , for ne ts with the s a m e cha rac t e r i s t i c s a s t h o s e from the c o m m e r c i a l s q u i d f ishery . T h e f igures d e r i v e d here a p p e a r s l ight ly o v e r e s t i m a t e d w h e n c o m p a r e d wi th a l ternat ive f igures . H o w e v e r , c o n s i d e r i n g that o b s e r v e r p r o g r a m m e s d o not t ake into a c c o u n t a n y sort of d ropou t s from the nets , the p resen t e s t ima te s c o u l d be c l o s e r to the rea l mortal i ty inflicted by the driftnets a n d s e r v e a s a n ind ica t ion o f the o r d e r o f m a g n i t u d e o f the p r o b l e m . If this is true, p r e v i o u s a p p r a i s a l s o f b lue s h a r k c a t c h e s in the w h o l e f i shery ( A n o n . 1988) s e e m to be h igh ly o v e r e s t i m a t e d . Efforts to m i n i m i s e the t ake of non-target s p e c i e s in the s q u i d driftnet f i shery w e r e met with u n s u c c e s s f u l resul ts . D a t a s u m m a r i s e d by G o n g et a l . (1993) for K o r e a n r e s e a r c h e x p e r i m e n t s ind ica te that s h a r k b y c a t c h e s c a n d rop by a s m u c h a s 4 1 % w h e n subsu r f ace driftnets a re u t i l i sed i n s t ead o f n o r m a l (surface) driftnets. Unfor tuna te ly , c a t c h o f the target s p e c i e s (neon f lying squ id) d r o p p e d by 7 3 % , p r o b a b l y m a k i n g o p e r a t i o n s with the subsu r f ace driftnets unprof i table . A s a result o f in terna t ional a g r e e m e n t s , the s q u i d driftnet f ishery o f the Nor th P a c i f i c c e a s e d to exis t at the e n d of 1 9 9 2 . c) L a r g e - m e s h driftnet f i shery . A l a r g e - m e s h driftnet f i shery for sk ip jack , mar l in , a l b a c o r e a n d o ther t u n a s w a s init iated in the h igh s e a s o f the Nor th P a c i f i c in the ear ly 70 ' s by J a p a n . H o w e v e r , th is f i shery c a m e to a n e n d o n D e c e m b e r 31 1 9 9 2 toge the r with a l l o the r h i g h - s e a s driftnet f i she r i es in the a r e a . T h i s f i shery h a d its o r ig ins in the c o a s t a l J a p a n e s e bluefin t u n a f i shery o f the 1840 ' s . B y the late 1980 ' s it c o v e r e d a n a r e a e x t e n d i n g from 1 4 0 ° E to 1 4 5 ° W (fig. 2 .33) . T h e f i sh ing g r o u n d s w e r e d i v i d e d into t w o s u b a r e a s . A sou the rn s u b a r e a o p e n to f i sh ing y e a r r o u n d a n d a nor thern s u b a r e a wi th por t ions c l o s e d to f i sh ing du r ing spec i f i c m o n t h s in o r d e r to a v o i d c a t c h e s of s a l m o n i d s . R e c e n t repor ts ind ica te this f i shery o p e r a t e d wi th v e s s e l s in the 100-111 112 5 0 0 G T range . N e t s with s m a l l m e s h e s w e r e o f ny lon monof i l amen t t w i n e s o f 1.2 m m d i a m e t e r w h i l e l a rger m e s h e d nets u s e d mult i f i lament a n d mul t i s t rand tw ines . A l t h o u g h m e s h s i z e w a s res t r ic ted to be >150 m m , m e s h e s a s s m a l l a s 113 m m w e r e r e c o r d e d ; mos t driftnets h a d m e s h e s o f 180 m m ( I N P F C 1992) . T a n s w e r e c o m m o n l y 3 3 - 3 6 m in length . J a p a n e s e boa t s w e r e res t r ic ted to d e p l o y a m a x i m u m of 12 k m of net at a t ime . A c c o r d i n g to recent f igures , 4 5 9 v e s s e l s f rom J a p a n took part in the l a r g e - m e s h driftnet f i shery in 1988 with a total c a t c h o f a p p r o x i m a t e l y 4 0 , 0 0 0 t. T a i w a n e s e v e s s e l s a l s o par t ic ipa ted in this f ishery, but informat ion is s c a r c e . A p p a r e n t l y , up to 123 v e s s e l s f rom T a i w a n took part in this f i shery dur ing 1 9 8 9 . T h e T a i w a n e s e f i sh ing s e a s o n s p a n n e d chief ly from J u n e to D e c e m b e r . A c c o r d i n g to last a v a i l a b l e f igures (F i t zge ra ld et a l . 1993) , J a p a n e s e v e s s e l s d e p l o y e d a total of 4 , 6 8 2 , 6 3 0 s t a n d a r d (50 m) t ans in this f i shery du r ing 1 9 9 0 . T a i w a n e s e effort is here a s s u m e d to be the s a m e of that e s t ima ted for the s q u i d driftnet f i shery ( see a b o v e ) d u e to the c o m b i n e d nature of t he se f i sher ies ( Y e h a n d T u n g 1993) . T h e c o m b i n e d effort o f both na t ions du r ing 1 9 9 0 w a s p r o b a b l y equ iva l en t to a total o f 4 1 3 , 9 2 4 k m of la rge m e s h driftnet. Information o n the k i n d s a n d n u m b e r s of e l a s m o b r a n c h s c a u g h t in th is f i shery h a s recent ly b e c o m e a v a i l a b l e t h rough the reports o f the in terna t ional o b s e r v e r s p r o g r a m m e ( I N P F C 1992) . C a t c h ra tes a n d e s t ima te s o f the total c a t c h e s o f s h a r k s a n d ba to ids b a s e d o n effort l eve l s repor ted for 1 9 9 0 ind ica te that abou t 1 5 0 , 0 0 0 s h a r k s o r 1,722 t, w e r e t a k e n a s b y c a t c h (table 2 .13) . T h e a v e r a g e w e i g h t s o f s o m e s p e c i e s a re t a k e n from r e s e a r c h c r u i s e s that u t i l i sed driftnets wi th m e s h s i z e s 1 5 0 - 1 8 0 m m ( F A J 1983) , w h i l e o the r s a re m y bes t p o s s i b l e ' gues s t ima te s ' for the c o r r e s p o n d i n g m e s h s i z e s . T h e e s t i m a t e d e l a s m o b r a n c h b y c a t c h rate of 3 6 6 f i sh /1000 k m f o r t h e l a r g e - m e s h driftnets is abou t ha l f that o f the s q u i d f ishery. T h i s difference is re la ted to the different se lec t iv i ty pat terns of the nets i n v o l v e d , wi th l a rger m e s h e s a l l o w i n g for g rea t e r e s c a p e m e n t o f s m a l l non-target s p e c i e s . B l u e sha rk c a t c h rates a re l e s s t han ha l f o f t h o s e o b s e r v e d in s q u i d driftnets, w h i l e c a t c h rate for s a l m o n s h a r k s a re e v e n lower . O n the o the r h a n d , the a v e r a g e s i z e of e a c h s p e c i m e n is e x p e c t e d to be la rger in the l a r g e - m e s h f i shery . F r o m the ove ra l l e s t i m a t e d c a t c h o f e l a s m o b r a n c h s in this f i shery in 1990 , a p p r o x i m a t e l y 9 7 4 t w o u l d have 11 Table 2.13 Es t imated bycatches of e lasmobranchs in the 1990 North Paci f ic large-mesh driftnet based on reports of the observer programme 1990 ( I N P F C 1992). Species Numbers observed (513,367 Tans) Catch rate per 1000km of net Numbers in Total catch Likely mean Weight (kg) Weight in Total catch (kg) Unidentified shark 57 12.00 4,967 2 5 ? 124,177 Prionace glauca 7,692 300 124,040 9.2(1) 1,141,168 Lamna ditropis 136 5.30 2,193 32.5 (1) 71,276 Isurus oxyrinchus 592 23 9,547 3 0 ? 286,395 Alopias vulpinus 6 0.23 97 167 (1) 16,158 Squalus acanthias 1 0.04 16 2.5 40 Carcharodon carcharias 35 1.36 564 47.7 (1) 26,922 Isistius brasiliensis 305 12 4,918 0.85 4,181 Euprotomicrus bispinatus 156 6.08 2,516 0.25 629 Cetorhinus maximus 2 0.08 32 550 17,738 Triakidae 3 0.12 48 3 145 Sphyrnidae 2 0.08 32 127 (1) 4,096 Dasyatis violacea 73 2.84 1,177 12? 14,126 Dasyatis brevis 8 0.31 129 12? 1,548 Unidentified ray 69 2.69 1,113 12? 13,352 Totals 9,137 366 151,390 - 1,721,953 (1) Derived from F.A.J. (1983). 114 b e e n t a k e n by J a p a n a n d 7 4 8 t by T a i w a n . T h e r e a re no e s t i m a t e s in the literature to c o m p a r e with the p resen t resul t s . Fu r the rmore , there a re no s tat is t ics f o r t h e a m o u n t s o f e l a s m o b r a n c h s l a n d e d from the l a r g e - m e s h f ishery in J a p a n o r T a i w a n that a l l o w a n es t imate of d i s c a r d s . J u d g i n g from the t r ends in o ther h i g h -s e a s f i sher ies , it is v e r y l ikely h o w e v e r , that mos t b y c a t c h e s o f s h a r k s w e r e not u t i l i sed but i n s t ead d i s c a r d e d at s e a . S o u t h P a c i f i c O c e a n . L a r g e - s c a l e driftnet f i sh ing s t o p p e d s i n c e 1991 in the S o u t h P a c i f i c . F o r m e r l y , J a p a n a n d T a i w a n f i shed chief ly for a l b a c o r e with l a r g e - m e s h driftnets ( see Nor th r idge (1991) for deta i ls ) . D u e to p r e s s u r e from c o a s t a l s ta tes in the a r e a , a n a g r e e m e n t w a s m a d e to te rminate t h e s e f i sher ies in the h i g h - s e a s o f the S o u t h P a c i f i c by 1 9 9 1 . It is not c l e a r if the a g r e e m e n t per ta ins o n l y to the wa t e r s of the S o u t h P a c i f i c C o m m i s s i o n ( S P C ) (fig. 2 .34) o r if it a l s o i n c l u d e s the E a s t e r n S o u t h Pac i f i c . J a p a n s t o p p e d a l l l a r g e - s c a l e driftnet f i sh ing in the a r e a in 1 9 9 0 ( N a g a o et a l . 1993) , but informat ion o n T a i w a n e s e v e s s e l s is not at h a n d . H o w e v e r , a v a i l a b l e informat ion s u g g e s t s that e l a s m o b r a n c h b y c a t c h in l a r g e - s c a l e driftnets in the S o u t h P a c i f i c s h o u l d at p resen t be nil o r neg l ig ib le e v e n if v e s s e l s from T a i w a n con t inue to f ish there . A br ief a c c o u n t of the few repor ts o f e l a s m o b r a n c h b y c a t c h e s in S o u t h P a c i f i c driftnet f i sher ies is p r e s e n t e d b e l o w . S o m e repor ts o f e l a s m o b r a n c h c a t c h rates in the S o u t h P a c i f i c a re g i v e n in tab le 2 .14 b a s e d o n da t a from S h a r p i e s et a l . (1990) a n d W a t a n a b e (1990) . T h e i r s o u r c e s o f informat ion are two r e s e a r c h c r u i s e s c o n d u c t e d in the T a s m a n S e a a n d the S u b - T r o p i c a l C o n v e r g e n c e Z o n e ( S T C Z ) to the eas t o f N e w Z e a l a n d b e t w e e n 3 0 ° a n d 4 5 ° S . C a t c h ra tes e s t ima ted from the se da t a a re 181 a n d 158 s h a r k s / 1 0 0 0 k m of net f o r t h e S T C Z a n d the T a s m a n S e a re spec t ive ly , o r 5 ,035 k g / 1 0 0 0 k m of net for the T a s m a n S e a . W h i l e total e l a s m o b r a n c h c a t c h rates c o u l d s e e m re la t ively s imi l a r a m o n g both a r e a s , s t rong d i f fe rences in c a t c h rates for ind iv idua l s p e c i e s a re ev iden t w h e n look ing at de t a i l ed informat ion (e .g . b lue s h a r k s are m o r e f requent ly c a u g h t in the S T C Z than in the T a s m a n S e a w h i l e the o p p o s i t e is true for m a k o sha rks ) . A d d i t i o n a l l y , the c a t c h rate for the T a s m a n S e a is h igh c o m p a r e d to da ta from Cof fey a n d G r a c e (1990) . T h e s e d i f ferences i l lustrate the res t r ic t ions f a c e d for 115 116 Table 2.14 Reported bycatches of elasmobranchs in South Pacific driftnet fisheries. STCZ (464 km of net)* TASMAN SEA (766 km of net) ** Numbers Catch rate Numbers Catch rate Mean Catch rate Species Caught (#/1000 km) Caught (#/1000 km) Weight (kg/1000 km) Cetorhinus maximus 1 1.31 Prionace glauca 70 150.86 22 28.72 70 2,001 Lamna nasus - - 3 3.92 - -Isurus oxyrinchus 10 21.55 66 86.16 31 2,663 Isistius brasiliensis - - 10 13.05 - -Sphyrna zygaena - - 3 3.92 95 371 Dasyatis violacea 4 8.62 16 20.89 - -Total 84 181.03 121 157.96 195 5,035 * Data from Watanabe (1990) ** Data from Sharpies et al. (1990) 117 ex t rapo la t ions from c a t c h rates to total b y c a t c h e s w h e n the c a t c h ra tes a r e b a s e d o n information l imi ted to a par t icu lar a r e a / f i s h e r y / s e a s o n . Cof fey a n d G r a c e (1990) o b s e r v i n g c o m m e r c i a l v e s s e l s e s t i m a t e d c a t c h rates o f 4 8 s h a r k s / 1 0 0 0 k m of net a n d a total b y c a t c h o f 3 ,500 s h a r k s in the T a s m a n S e a a r e a for the 1 9 9 0 s e a s o n . M u r r a y (1990) c o m p i l e s da t a f rom s e v e r a l s o u r c e s a n d p r o v i d e s information o n p e r c e n t a g e by w e i g h t o f s h a r k s in total c a t c h e s of J a p a n e s e r e s e a r c h c a m p a i g n s u s i n g three t ypes o f driftnets a l o n g with total effort for e a c h type o f net. W i t h this informat ion, sha rk c a t c h rates a re here c a l c u l a t e d a s s u m i n g 5 0 m tans : for a l b a c o r e nets , 16 ,362 k g / 1 0 0 0 k m ; for s l e n d e r t u n a nets , 14 ,618 k g / 1 0 0 0 k m ; a n d for pomfret nets , 21 ,781 k g / 1 0 0 0 k m . G i v e n the l ack o f e s t ima te s o f the total a m o u n t of nets d e p l o y e d in t h e s e f i sher ies sha rk b y c a t c h is e s t i m a t e d u s i n g the p e r c e n t a g e s o f s h a r k s to the total a l b a c o r e c a t c h for the a l b a c o r e ne ts men t ion a b o v e , a n d the repor ted a l b a c o r e c a t c h e s for driftnet f leets in the S o u t h P a c i f i c for 1 9 8 9 p r o v i d e d by L a w s o n (1991) . T h e g r o s s e s t i m a t e s o f total sha rk b y c a t c h e s a re : J a p a n , 3 ,462 t, K o r e a , 48 t a n d T a i w a n , 2 ,871 t. T h e s e f igures a d d up to 6,381 t a n d c o r r e s p o n d to the repor ted p e a k in a l b a c o r e driftnet c a t c h e s . H e n c e , total b y c a t c h l eve l s s h o u l d h a v e b e e n s m a l l e r in the ea r l i e r a n d later y e a r s o f the f i shery . T h e s e e s t i m a t e d c a t c h e s per ta in o n l y to the wa t e r s o f the S o u t h P a c i f i c C o m m i s s i o n (fig 2 .34) a n d are c r u d e e s t ima te s l imi ted by the a v a i l a b l e informat ion . F u r t h e r m o r e , it is u n k n o w n if the da ta c i t ed by M u r r a y (1990) u s e d for e s t ima t ing b y c a t c h p e r c e n t a g e s con t a in information from the w h o l e S o u t h P a c i f i c r eg ion o r o n l y from part o f it. G e o g r a p h i c a l va r i a t ions in a b u n d a n c e a re l ike ly to affect the b y c a t c h l eve l s c o n s i d e r a b l y . Wi thou t a n y informat ion abou t driftnetting ac t iv i t ies in the rest of the S o u t h P a c i f i c O c e a n , I c a n o n l y s p e c u l a t e that g iven the propor t ion o f the S o u t h P a c i f i c c o v e r e d by the S P C a r e a (about 2/3) , the b y c a t c h of e l a s m o b r a n c h s in the w h o l e S o u t h e r n P a c i f i c c o u l d h a v e b e e n 5 0 % m o r e than that c a l c u l a t e d he re for the S P C z o n e , o r a total o f 9 ,572 t. A l t h o u g h uncer ta in , this e l a s m o b r a n c h b y c a t c h l eve l is abou t hal f o f that for driftnets in the Nor th P a c i f i c O c e a n . Indian O c e a n . S e v e r a l coun t r i e s h a v e e x t e n s i v e driftnet f i sher ies in the Indian O c e a n . H o w e v e r , mos t of the c o a s t a l s ta tes in the a r e a e .g . India, P a k i s t a n , S r i L a n k a , o n l y f ish wi th in i n sho re wa t e r s with s m a l l a n d m e d i u m - s c a l e f i sher ies a l r e a d y t rea ted in s e c t i o n 2 .2 . T h e e l a s m o b r a n c h 118 c a t c h e s o f t h e s e c o a s t a l s ta tes a re a s s u m e d to be l a n d e d a n d therefore a l r e a d y repor ted in F A O stat is t ics . T a i w a n is the o n l y coun t ry k n o w n to h a v e l a rge - sca l e driftnet v e s s e l s f i sh ing in the in ternat ional w a t e r s o f the Indian O c e a n , but there is v e r y l imi ted informat ion a v a i l a b l e . T h i s t u n a f i shery s tar ted with o n e boat in 1 9 8 3 a n d g r e w to a total o f 139 v e s s e l s in 1 9 8 8 . F i s h i n g a p p a r e n t l y t a k e s p l a c e from N o v e m b e r to M a r c h with driftnets o f 2 0 0 - 2 2 0 m m m e s h s i z e , 2 0 - 2 4 m dep th wi th 2 0 - 2 5 o r 3 7 - 4 7 k m of net d e p l o y e d pe r v e s s e l . F i s h i n g is ma in ly ca r r i ed out in w a t e r s of the Nor th W e s t a n d S o u t h C e n t r a l Indian O c e a n . H s u a n d L i u (1991) report s h a r k s to be 2 3 . 7 6 a n d 2 9 . 5 7 % of the total c a t c h e s in n u m b e r s a n d we igh t r e spec t ive ly for the 1 9 8 6 - 1 9 8 7 f i sh ing s e a s o n , w h i l e du r ing 1 9 8 7 - 1 9 8 8 this d e c r e a s e d to 0 .52 a n d 2 . 0 7 % . A s no s ignif icant c h a n g e s in f i sh ing a r e a w e r e o b s e r v e d b e t w e e n both f i sh ing s e a s o n s , this r educ t ion in sha rk b y c a t c h e s mos t l ike ly reflects c h a n g e s in d i s c a r d rates . M u l t i p l y i n g the p e r c e n t a g e c o m p o s i t i o n o f s h a r k s to the r epor t ed total l a n d i n g s of 18,281 t in the 1 9 8 6 - 1 9 8 7 s e a s o n ( I P T P 1990) , s o m e 5 ,405 t o f s h a r k s a r e e s t ima ted to h a v e b e e n c a u g h t in the f i shery . A total sha rk c a t c h of 6 ,108 t is he re e s t i m a t e d for the 1 9 8 8 - 1 9 8 9 s e a s o n , a s s u m i n g that the n u m b e r of v e s s e l s i n c r e a s e d by 1 3 % from the 1 9 8 6 -1987 l e v e l . A t l an t i c O c e a n . Unt i l recent ly , the o n l y k n o w n l a rge - sca l e driftnet f i sher ies in the A t l a n t i c w e r e a F r e n c h a l b a c o r e f i shery a n d a n Italian swordf i sh f ishery. H o w e v e r , T a i w a n e s e driftnet v e s s e l s w e r e thought to ope ra t e a l s o in the A t l an t i c O c e a n dur ing the ea r ly 1990 ' s . M a n y o ther f i sher ies with g i l lnets ex is t a c r o s s the A t l a n t i c a n d M e d i t e r r a n e a n a n d in m a n y c a s e s they a m o u n t to large quant i t ies o f nets d e p l o y e d pe r night. H o w e v e r , mos t o f t h e s e f i she r i es a re l imi ted to c o a s t a l w a t e r s a n d fall out of the s c o p e o f this s e c t i o n . A s u m m a r y o f t h e s e s m a l l e r f i sher ies is g i v e n by Nor th r idge (1991) . T h e F r e n c h a l b a c o r e f i shery b e g a n in the B a y o f B i s c a y in 1986 a n d 3 7 v e s s e l s w e r e ope ra t i ng by 1 9 8 9 . T h e s e boa t s t ro l led dur ing the d a y a n d u s e d g i l lne ts du r ing the night. F i s h i n g took p l a c e du r ing J u n e to S e p t e m b e r a n d e x t e n d e d from the A z o r e s north a n d e a s t w a r d fo l lowing the m o v e m e n t s of a l b a c o r e . N e t s w e r e 2 0 - 3 6 m d e e p a n d 8 0 - 1 2 0 m m 119 m e s h s i z e ; the mos t s u c c e s s f u l w e r e 9 0 m m in m e s h . W h i l e F r e n c h repor ts indica te driftnets' l eng ths o f 2-6 k m pe r v e s s e l , G r e e n p e a c e c l a i m s that they a r e up to 2 0 k m l o n g . F o r s h a r k b y c a t c h e s , the o n l y a v a i l a b l e informat ion ind ica te s t hey w e r e in the o r d e r of 6-1 0 % . W o o d l e y a n d E a r l e (1991) o b s e r v i n g s e v e r a l F r e n c h boa t s report s h a r k s (most ly Prionace glauca) a s the mos t c o m m o n b y c a t c h , a m o u n t i n g to 6 . 2 % of the a l b a c o r e c a t c h . S h a r k s c a u g h t w e r e e s t i m a t e d to r ange b e t w e e n 4 0 - 2 5 0 c m but m o r e c o m m o n l y b e t w e e n c a . 1 2 5 - 2 0 0 c m . W o o d l e y a n d E a r l e add i t iona l ly es t imate c a t c h ra tes o f 1,750 to 3 ,520 s h a r k s / 1 0 0 0 k m of net ( inc lud ing dropouts ) toge ther wi th a total c a t c h o f 2 2 , 0 1 5 to 4 4 , 2 8 2 s h a r k s du r ing the 1991 F r e n c h a l b a c o r e f ishery. T h i s is equ iva l en t to s o m e c a . 4 3 0 - 8 6 5 t of s h a r k s a s s u m i n g a m e a n total length o f 175 c m for b lue s h a r k s . T h e y report a d i s c a r d of two s h a r k s at s e a but no further informat ion is a v a i l a b l e o n the d i s p o s i t i o n of the sha rk b y c a t c h e s in this f i shery . H o w e v e r , t he se s h a r k c a t c h e s c o u l d be a l r e a d y i n c l u d e d in the repor ted "var ious e l a s m o b r a n c h s " o f F r a n c e w h i c h a m o u n t to a l m o s t 1 0 , 0 0 0 t/yr. T h e u s e o f driftnets in Italian f i sher ies for t u n a a n d swordf i sh h a s a l o n g his tory, but it w a s until the 1980 ' s that the f i shery e x p a n d e d c o n s i d e r a b l y a s a c o n s e q u e n c e o f g o v e r n m e n t a l suppor t . A c c o r d i n g to Nor th r idge (1991) , this w a s o n e o f the la rges t driftnet f i sher ies in the w o r l d before it w a s b a n n e d . B y 1989 , 7 0 0 v e s s e l s w e r e par t ic ipa t ing , 9 0 % of t h e m us ing nets of 12-13 k m in total length with dep ths o f 2 8 - 3 2 m a n d m e s h s i z e s in the r ange 180-4 0 0 m m . A few v e s s e l s u s e d l e s s t han 6 k m of net, w h i l e a few o the r s m o r e than 2 0 k m . T h e f i shery p u r s u e d a l b a c o r e a n d swordf i sh from S i c i l y a n d C a l a b r i a to the L i g u r i a n S e a . W h i l e there is no informat ion o n c a t c h rates o f non-target s p e c i e s , s e v e r a l e l a s m o b r a n c h s h a v e b e e n repor ted to o c c u r in this f ishery. S p e c i e s c o m m o n l y c a u g h t i nc lude c o m m o n thresher , b lue a n d p o r b e a g l e s h a r k s , a s w e l l a s m a n t a a n d c o m m o n e a g l e rays . A n o t h e r three s h a r k s a r e r epor ted a s infrequent s p e c i e s a n d 10 m o r e a s o c c a s i o n a l s p e c i e s (table 2 .15) . It is u n k n o w n w h e t h e r mos t o f the c a t c h e s w e r e kept o r d i s c a r d e d , a n d it is i m p o s s i b l e to es t imate the a m o u n t of the total c a t c h from a v a i l a b l e in format ion . H o w e v e r , a la rge i n c r e a s e in l a n d i n g s o f s m o o t h - h o u n d s took p l a c e concu r r en t l y wi th the e x p a n s i o n of the driftnet f i shery a n d it is k n o w n that o ther s h a r k s a re c o m m o n l y s m u g g l e d loca l ly a s s m o o t h - h o u n d s (De Met r io et a l . 1984) . There fo re , it is p o s s i b l e that a c o n s i d e r a b l e part o f the s h a r k b y c a t c h from this f i shery w a s l a n d e d . R e c e n t repor ts s u g g e s t that there a re still s o m e driftnetters in the L i g u r i a n S e a u s i n g g e a r l eng ths a b o v e the permi t ted 2 .5 k m per v e s s e l in the A r e a ( I C C A T 1993a ) . 120 Table 2.15 E la smobranchs caught in Mediterranean driftnets (adapted from Northridge 1991). Common Name Scientific Name Species commonly caught Thresher shark Blue shark Porbeagle Manta ray Common eagle ray Infrequent species Basking shark Shortfin mako Smooth hammerhead Occasional species Bigeye thresher Spinner shark Blacktip shark Dusky shark Sandbar shark Great white shark Sharpnose sevengill shark Sand tiger shark Smalltooth sand tiger Hammerhead shark Tope Bull ray Alopias vulpinus Prionace glauca Lamna nasus Mobula mobular Myliobatis aquila Cetorhinus maximus Isurus oxyrinchus Sphyrna zygaena Alopias superciliosus Carcharhinus brevipinna C. limbatus C. obscurus C. plumbeus Carcharodon carcharias Heptranchias perlo Carcharias taurus Odontaspis ferox Sphyrna spp. Galeorhinus galeus Pteromylaeus bovinus 121 Nor th r idge (1991) r e v i e w s s e v e r a l reports o f T a i w a n e s e v e s s e l s f i sh ing wi th la rge driftnets in different a r e a s o f the A t l an t i c O c e a n . H o w e v e r , no further informat ion o n the i s s u e is a v a i l a b l e . O v e r v i e w of driftnet f i sher ies . H i g h - s e a s driftnet f i sher ies w e r e a n important s o u r c e o f e l a s m o b r a n c h b y c a t c h e s . T h e e s t ima tes p r e s e n t e d a b o v e s u g g e s t that the total e l a s m o b r a n c h b y c a t c h c o u l d h a v e b e e n b e t w e e n 3 .28 a n d 4.31 mi l l ion s h a r k s a n d rays pe r y e a r du r ing 1 9 8 9 - 1 9 9 1 , o r in the o rde r o f 2 0 , 0 0 0 - 3 8 , 0 0 0 t/yr. T o t a l d i s c a r d s of e l a s m o b r a n c h s at s e a from driftnet f i sher ies c o u l d h a v e b e e n a s h igh a s 3 0 , 5 0 0 t/yr, but a s s u m i n g al l T a i w a n e s e a n d F r e n c h c a t c h e s w e r e kept, d i s c a r d s c o u l d h a v e b e e n l o w e r at 2 0 , 8 0 3 t/yr. T h e e s t i m a t e s p r e s e n t e d here a re d e r i v e d by a d d i n g e s t ima te s for e a c h o f the f i sher ies p r e v i o u s l y d e s c r i b e d a n d ca r r i e s a l o n g a c c u m u l a t e d uncer ta in ty . In this s e n s e this o v e r a l l e s t i m a t e s s h o u l d be t rea ted with d i sc re t ion a n d u s e d o n l y a s a first a p p r o x i m a t i o n of the l eve l o f e l a s m o b r a n c h s r e m o v e d by driftnets w o r l d w i d e . T h e Nor th P a c i f i c f i sher ies w e r e wi thout a n y doubt the mos t in tens ive a n d therefore the mos t impor tant driftnet f i sher ies for thei r c a t c h e s a n d w a s t e o f s h a r k s a n d rays (table 2 .16) . In par t icular , the f lying s q u i d f i shery with its h igh c a t c h rates a n d m a s s i v e effort k i l led more e l a s m o b r a n c h s t han a n y o ther h i g h - s e a s driftnet f i shery e v e r k n o w n . For tuna te ly , for the p u r p o s e o f a p p r a i s i n g the w o r l d b y c a t c h e s of e l a s m o b r a n c h s by h i g h - s e a s driftnet f i sher ies , the Nor th P a c i f i c f i she r i es a c c o u n t e d for the larges t p ropor t ion o f the total a n d w e r e a l s o the best s t ud i ed driftnet f i sher ies . T h i s s o m e h o w r e d u c e s the l eve l o f uncer ta in ty in the e s t ima tes o f w o r l d c a t c h e s of e l a s m o b r a n c h s with driftnets. B l u e s h a r k s w e r e u n d o u b t e d l y the mos t c o m m o n a n i m a l c a u g h t in driftnet f i sher ies b e c a u s e of their h igh a b u n d a n c e in p e l a g i c habi ta ts . To ta l n u m b e r s t a k e n in 1 9 8 9 a r e e s t i m a t e d to a m o u n t to 2 .2-2 .5 mi l l ion s h a r k s . T h e y a re caugh t w o r l d w i d e v e r y f requent ly in la rger n u m b e r s t han a n y o the r e l a s m o b r a n c h . B l u e s h a r k s m a y w e l l be the m o s t t h rea t ened e l a s m o b r a n c h by t h e s e f i sher ies , but m u c h m o r e informat ion is n e e d e d to a sce r t a in the m a g n i t u d e o f the impac t o f thei r r e m o v a l . 122 Table 2.16 S u m m a r y of est imated bycatch of e lasmobranchs in high s ea s driftnet fisheries. Fishery Total catch in tonnes Total catch in Catch rates Lower level Upper level number of individuals (sharks/1000 km nets) North Pacific Ocean salmon(89) 108 - 1,237 11,492-300,000 210-5,502 squid (90) 7,415 - 18,739 2.0-2.44 Million 536-814 large mesh (90) - 1,722 - 151,390 366 South Pacific Ocean(89) 6,381 - 9,572 56,000-841,500* 48-181 Indian Ocean(89) - 6,108 - 537,000* -Atlantic Ocean(91) 430 - 865 22,000 - 44,000 1,750-3,520 Total 22,164 38,243 3,282,882-4,313,890 * from extrapolation of average weight of large mesh fishery 123 T h e different l eve l s o f uncer ta in ty s u r r o u n d i n g the e s t i m a t e d c a t c h ra tes for e a c h f ishery highl ight the i m p o r t a n c e o f c o o p e r a t i v e o b s e r v e r p r o g r a m m e s in h i g h - s e a s f i sher ies w o r l d w i d e : o n l y t h o s e f i sher ies that h a d o b s e r v e r p r o g r a m m e s , h a d e n o u g h informat ion to a l l o w der iva t ion o f r e a s o n a b l y g o o d e s t ima te s of e l a s m o b r a n c h b y c a t c h . A l s o , o n l y in t he se c a s e s w a s p o s s i b l e to h a v e direct informat ion o f the s p e c i e s caugh t . C o n s i d e r i n g this , the bes t e s t ima te s a re t hose for the Nor th P a c i f i c s q u i d a n d l a r g e - m e s h f i she r i es w h i c h w e r e the on ly f i she r i es wi th o b s e r v e r s o n b o a r d . In contras t , l a rge r uncer ta in t i es s u r r o u n d ca t ch rate a n d total kill e s t i m a t e s from the rest of the f i sher ies a n a l y s e d . A c o n s e q u e n c e o f the recent c l o s u r e o f a l l l a rge - sca l e driftnet f i she r i es in the h i g h - s e a s is that overn ight , the mortal i ty infl icted by t he se f i sher ies h a s c e a s e d . T h i s p r o v i d e s s o m e relief to m a n y p o p u l a t i o n s of bi rds , m a m m a l s a n d o ther m a r i n e f a u n a . Unfor tuna te ly it on ly p r o v i d e s a s m a l l b rea th ing s p a c e for e l a s m o b r a n c h s a n d par t icu la r s h a r k s , w h i c h con t inue to be c a u g h t inc iden ta l ly in v e r y large n u m b e r s in o the r h i g h - s e a s f i she r i es (see next sec t ion) . 2 .2 .3 .2 L o n g l i n e f i she r ies . T h e mos t impor tant l a rge - sca l e long l ine f i sher ies in the w o r l d a re t h o s e for t unas a n d bi l l f i shes . T h e s e f i sher ies inc lude the fleets o f s e v e r a l coun t r i e s a n d t ake p l a c e in a l l o c e a n s of the w o r l d . T e c h n o l o g i c a l i nnova t ions s u c h a s the u s a g e o f d e e p l o n g l i n e s a n d the in t roduct ion o f b las t f r eez ing capab i l i t i e s o n boa rd , a l l o w t h e m to be a m o n g s o m e of the mos t t e c h n i c a l l y s o p h i s t i c a t e d a n d e c o n o m i c a l l y important f i she r i es in the w o r l d , e s p e c i a l l y t hose s u p p l y i n g the e x c l u s i v e s a s h i m i marke t . L o n g l i n e s a re a re la t ively unse l ec t i ve g e a r a n d in m a n y c a s e s s h a r k s a c c o u n t for a large part of the b y c a t c h e s . R e g u l a r l y , s h a r k s a re d i s c a r d e d a s f r eeze r s p a c e is l imi ted a n d r e s e r v e d f o r t h e v a l u a b l e target s p e c i e s . T h e extent o f the e l a s m o b r a n c h b y c a t c h in l a rge-s c a l e long l ine o p e r a t i o n s is u n k n o w n a n d v e r y difficult to a s s e s s b e c a u s e mos t o f the in ternat ional b o d i e s e n g a g e d in the s tudy a n d regula t ion o f t h e s e f i she r i es (i.e. I C C A T , I P T P , S P C , I A T T C ) d o not expl ic i t ly i nc lude s h a r k s a s a n i tem in the i r s ta t is t ics o r r e s e a r c h . T h i s is further c o m p l i c a t e d by the a b s e n c e o f a c o m p r e h e n s i v e informat ion o f t he se f i sher ies o n a g l o b a l s c a l e . In this s ec t ion I a t tempt to s u m m a r i s e the m o s t impor tan t cha rac te r i s t i c s 124 of the major l a r g e - s c a l e long l ine f i sher ies a n d to p rov ide e s t i m a t e s o f the i r b y c a t c h e s a n d d i s c a r d s o f e l a s m o b r a n c h s . A t l an t i c O c e a n . J a p a n , T a i w a n , K o r e a a n d S p a i n h a v e the mos t important l a r g e - s c a l e long l ine f i sher ies in the A t l an t i c O c e a n . S e v e r a l coun t r i e s , l ike C a n a d a , C u b a , U S A , Italy, M o r o c c o , B r a z i l a n d o thers , h a v e long l ine f i sher ies in thei r o w n w a t e r s but thei r effort is v e r y s m a l l a n d in s o m e c a s e s the e l a s m o b r a n c h b y c a t c h is u t i l i sed a n d a l r e a d y i n c l u d e d in official s tat is t ics . M o s t o f the informat ion a v a i l a b l e abou t A t l an t i c h i g h - s e a s f i she r i es c o m e s from the Internat ional C o m m i s s i o n for the C o n s e r v a t i o n of A t l an t i c T u n a s ( I C C A T ) . T h e informat ion a v a i l a b l e is h o w e v e r o f v a r i a b l e qual i ty a n d this s h o u l d be c o n s i d e r e d w h e n interpret ing the fo l lowing s u m m a r i e s . a) J a p a n . J a p a n e s e long l ine r s h a v e f i shed for a l b a c o r e (Thunnus alalunga) a n d ye l lowf in t una (Thunnus albacares) in the A t l an t i c O c e a n s i n c e the m i d 1 9 5 0 ' s a n d for b i g e y e tuna (Thunnus obesus) s i n c e at leas t 1 9 6 1 . A c c o r d i n g to S u s u k i (1988) , the fleet g radua l ly e x p a n d e d its g e o g r a p h i c a l r ange from the E q u a t o r i a l W e s t e r n A t l a n t i c g r o u n d s in 1956 to vir tual ly a l l o f the A t l a n t i c by 1 9 7 0 (fig. 2 .35) . M o s t recent ly , b i g e y e t u n a c o m p r i s e s m o r e than half o f the total repor ted c a t c h e s a n d is spec i f i ca l ly t a rge ted wi th d e e p l ong l i ne s yea r -round in a vas t a r e a b e t w e e n 4 5 ° N a n d 4 5 ° S . T h e s e d e e p l o n g l i n e s w e r e i n t roduced by the J a p a n e s e f i shery in 1977 a n d they a l s o t ake w e l c o m e d b y c a t c h e s o f ye l lowf in t u n a a n d swordf i sh (Xiphias gladius). A d d i t i o n a l l y , s o m e effort is d i r ec t ed t o w a r d s bluefin t una (Thunnus thynnus) in the M e d i t e r r a n e a n S e a ( I C C A T 1991a ) . T h e total n u m b e r o f J a p a n e s e long l ine r s in the A t l an t i c du r ing 1 9 8 8 a n d 1 9 8 9 w a s repor tedly 183 a n d 2 3 9 v e s s e l s ( N R I F S F 1992) with a total effort of 6 8 ' 4 4 4 , 7 1 6 a n d 9 1 ' 3 9 5 , 9 1 5 h o o k s r e spec t i ve ly ( I C C A T 1992) . R e c e n t l y p u b l i s h e d d a t a ind ica te that n o m i n a l effort of the J a p a n e s e fleet c o n t i n u e s to g r o w in the A t l an t i c O c e a n , wi th 9 6 , 6 5 1 , 0 0 0 h o o k s set du r ing 1 9 9 0 ( U o z u m i 1993) . J a p a n e s e reports o f "other s p e c i e s " for 1988 a n d 1 9 8 9 are 125 60S'—'—'—'—'—'—'—'—'—'—'—'—'—'—'— 1—'—'— 1—'—'—'—'—'—' 8 0 W SO 40 20 O 2 0 E Figure 2.35 Effort distribution of J apanese longline fishery in the Atlant ic O c e a n in the 1980's. K e y s indicate accumula ted nominal hook numbers in thousands . (Redrawn from Nakano 1993). 126 3 6 6 a n d 5 0 0 t but there is no ind ica t ion if t h e s e f igures i n c l u d e a n y s h a r k s o r o ther e l a s m o b r a n c h s . H o o k i n g ra tes o f s h a r k s for different a r e a s of the A t l an t i c O c e a n w h e r e the c o m m e r c i a l J a p a n e s e long l ine r s ope ra t e a re i n a d e q u a t e l y d o c u m e n t e d . W i t h o n e e x c e p t i o n , mos t of the a v a i l a b l e informat ion per ta ins o n l y to J a p a n e s e long l in ing ac t iv i t ies in the Nor th W e s t At l an t i c . W i t z e l l (1985) e s t ima te s h o o k i n g rates o f s h a r k s in J a p a n e s e long l ine r s at 1.31 s h a r k s / 1 0 0 0 h o o k s (107 k g / 1 0 0 0 hooks ) for the G u l f o f M e x i c o a n d at 5 .98 s h a r k s / 1 0 0 0 h o o k s (378 k g / 1 0 0 0 h o o k s ) for the U S A At l an t i c C o a s t . T h e s e a r e m i n i m u m es t ima tes a s they a re b a s e d o n J a p a n e s e l o g b o o k informat ion a n d under - repor t ing is k n o w n to o c c u r ( N a k a n o 1993) . In fact, repor ts f rom o b s e r v e r s in J a p a n e s e long l ine r s f i sh ing in the G u l f o f M e x i c o ind ica te h i g h e r h o o k rates, o f 1.74 s h a r k s / 1 0 0 0 h o o k s ( L o p e z et a l . 1979) . A u (1985) d o c u m e n t s c a t c h ra tes of b e t w e e n 1 a n d 5 s h a r k s / 1 0 0 0 h o o k s a s the m o s t c o m m o n for J a p a n e s e long l ine r s in U S A wa te r s b a s e d o n da t a from o b s e r v e r s . A u a l s o reports abou t 2 0 sha rk s p e c i e s in the b y c a t c h e s . Hoff a n d M u s i c k (1990) p rov ide month ly n u m b e r s o f f i s h c a u g h t for 10 s h a r k g r o u p s a n d n u m b e r s o f se t s m a d e by J a p a n e s e long l ine r s in the U S E E Z in 1 9 8 7 . T h e y report 8 ,330 s h a r k s from m o r e than eight s p e c i e s t a k e n a s b y c a t c h e s in th is f i shery . B l u e s h a r k s c o m p r i s e a b o u t 8 5 % of the total in n u m b e r s , f o l l o w e d by p o r b e a g l e a n d shortfin m a k o s h a r k s but g ive no ind ica t ion of s h a r k s ' s i z e s o r w e i g h t s . A s s u m i n g a n a v e r a g e o f 2 ,206 h o o k s p e r set (de r ived from da t a o f L o p e z et a l . 1979) the total h o o k rate c a n be a c c o r d i n g l y e s t i m a t e d a s 7 .04 s h a r k s / 1 0 0 0 h o o k s . H o o k i n g ra tes repor ted by N a k a n o (1993) for s h a r k s in J a p a n e s e A t l a n t i c o p e r a t i o n s r ange from 1 to 4 .5 s h a r k s / 1 0 0 0 h o o k s , wi th a rough a v e r a g e o f a b o u t 2.1 s h a r k s / 1 0 0 0 h o o k s . N a k a n o lists 11 e l a s m o b r a n c h s (10 s h a r k s a n d 1 ray) identif ied d u r i n g a r e s e a r c h c ru i s e in the A t l an t i c du r ing the 1960 ' s but d o e s not g ive h o o k i n g rates by s p e c i e s . A l t h o u g h N a k a n o d e r i v e s s e p a r a t e e s t ima te s for the Nor th a n d S o u t h At l an t i c , t h e s e h o o k i n g rates a re u n d e r e s t i m a t e d b e c a u s e o f the c o m m o n under repor t ing o f s h a r k s in the l o g b o o k s . M o s t s k i p p e r s d o not report s h a r k s at a l l w h e r e a s o thers r eco rd o n l y s h a r k s o f e c o n o m i c v a l u e ( N a k a n o 1993) . Information o n s h a r k b y c a t c h e s of o ther long l ine o p e r a t i o n s s e e m to conf i rm the o rde r of 127 magn i tude o f the v a r i o u s h o o k rates e s t i m a t e d a b o v e for the J a p a n e s e f i shery . R e s e a r c h c r u i s e s o f the U S A in the Nor th A t l an t i c a re d o c u m e n t e d by S i v a s u b r a m a n i a m (1963) a n d B r a z i l i a n t u n a long l ine r s in the E q u a t o r i a l W e s t A t l an t i c by H a z i n et a l . (1990) . F r o m the first report, h o o k ra tes o f 10 .35 s h a r k s / 1 0 0 0 h o o k s c a n be d e r i v e d for a n a r e a in s ide 0 - 8 0 ° W a n d 3 0 - 4 0 ° N . A s m a l l e r a r e a ins ide this h a d c a t c h rates for b lue s h a r k s a n d o c e a n i c whitet ip s h a r k s (Carcharhinus longimanus) of 3 .32 a n d 2 .3 s h a r k s / 1 0 0 0 h o o k s r e spec t ive ly . F o r t h e B r a z i l i a n long l ine r s , a v e r a g e s c a n be c a l c u l a t e d from the h o o k ra tes for s i x s h a r k g r o u p s p r o v i d e d by H a z i n et a l . in 1° s q u a r e s off R i o G r a n d e d o Nor t e . T h e resu l t s indica te a n ove ra l l h o o k rate o f 8.66 s h a r k s / 1 0 0 0 h o o k s a n d c a n be further split into 3 .94 for b lue sha rks , 4 .17 for g rey s h a r k s (genus Carcharhinus), 0 .27 for m a k o s h a r k s , 0 .08 for t h r e she r sha rks , 0 .14 for c r o c o d i l e s h a r k s (Pseudocarcharias kamoharai) a n d 0 .06 for o c e a n i c whitet ip s h a r k s . M u c h h ighe r h o o k i n g rates of up to 41 .6 s h a r k s / 1 0 0 0 h o o k s c a n be found in m o r e c o a s t a l a r e a s ( B e r k e l e y a n d C a m p o s 1988) . Ex t r apo la t ing from t h e s e h o o k i n g rates for spec i f i c a r e a s to the total A t l a n t i c is d a n g e r o u s a s the dis t r ibut ion o f s h a r k s is not h o m o g e n e o u s in s p a c e a n d t ime . A d d i t i o n a l l y , two different k i n d s of g e a r ( regular a n d d e e p longl ine) a re u s e d in c o m m e r c i a l l ong l in ing a n d they h a v e different effect in the c a t c h e s ( G o n g et a l . 1 9 8 7 , 1989) . O n the o ther h a n d , this r ange o f h o o k i n g ra tes c a n be u s e d to p l a c e b o u n d a r i e s a r o u n d the e s t ima te s . F r o m the v a r i o u s repor ts l i s ted a b o v e , there s e e m s to be a g e n e r a l a g r e e m e n t that total h o o k i n g rate v a l u e s f o r t h e A t l a n t i c O c e a n r ange rough ly b e t w e e n 1 a n d 10 s h a r k s / 1 0 0 0 h o o k s . G i v e n the s c a r c e informat ion a v a i l a b l e , h o o k i n g rates d e r i v e d from Hoff a n d M u s i c k (1990) are u s e d here to es t imate total c a t c h e s of J a p a n e s e long l ine r s in the A t l a n t i c O c e a n . T h e y are the m o s t u p d a t e d b a s e d o n da ta from J a p a n e s e long l ine r s a n d fit w e l l the o v e r a l l r ange of h o o k rates a v a i l a b l e . B e c a u s e the s p e c i e s c o m p o s i t i o n o f s h a r k s c h a n g e s a c c o r d i n g to the f i sh ing g r o u n d s , a n d J a p a n e s e effort f igures c a n n o t be d i s a g g r e g a t e d , I m a k e not a t tempt to e s t ima te c a t c h rates o f ind iv idua l s p e c i e s fo r the w h o l e J a p a n e s e A t l a n t i c long l ine f ishery . T h e a p p r o x i m a t e we igh t o f the c a t c h is e s t i m a t e d wi th the f igures o f H a z i n et a l . (1990) o f 40 .91 kg pe r shark . T h e total c a t c h o f s h a r k s by J a p a n e s e long l ine r s dur ing 1 9 8 9 in the A t l a n t i c O c e a n rough ly e s t i m a t e d a s ou t l i ned a b o v e is of 6 4 3 , 4 2 7 s h a r k s o r 2 6 , 3 2 2 t. T h e e s t i m a t e s for 1 9 9 0 a re 128 6 8 0 , 4 2 3 s h a r k s o r 2 7 , 8 3 5 t. H o w e v e r , t h e s e a s s e s s m e n t s a re unce r t a in . T h e e s t ima te s for 1 9 8 9 c o u l d be subs tan t ia l ly s m a l l e r (14 ,6191) if c a l c u l a t e d u s i n g the 3 0 % ratio o f s h a r k s to total t una c a t c h e s s u g g e s t e d by T a n i u c h i (1990) , o r e v e n la rger (40 ,1491) if w e a s s u m e the a v e r a g e w e i g h t s r epor ted by W i t z e l l (1985) for the S o u t h E a s t A t l a n t i c U S A w a t e r s . O n the o ther h a n d , the a v e r a g e we igh t o f 40 .91 kg / sha rk u s e d he re s e e m s to be r e a s o n a b l e , a n d is s u p p o r t e d by the repor ts o f R o d r i g u e z et a l . (1988) o f a s t e a d y a v e r a g e we igh t o f 48 .9 kg / sha rk for the b y c a t c h e s o f the C u b a n longl ine fleet ope ra t i ng in the t rop i ca l A t l a n t i c dur ing 1 9 7 3 - 1 9 8 5 . W i t z e l l (1985) repor ts that the p e r c e n t a g e of s h a r k s k i l l ed in the J a p a n e s e long l ine f ishery is on ly o f 7.2 % in the A t l a n t i c U . S . coas t , t h a n k s to the m a n d a t o r y r e l e a s e o f a l l b y c a t c h e s a n d p r o b a b l y b e c a u s e mos t o f the c a t c h e s a re b lue s h a r k s . T h i s s p e c i e s , a s w e l l a s o ther c a r c h a r h i n i d s h a r k s , repor ted ly su rv ive s better in the l o n g l i n e s t han l a m n o i d s h a r k s ( S i v a s u b r a m a n i a m 1 9 6 3 , Hoff a n d M u s i c k 1990 , H a z i n et a l . 1990) . If this kill rate is c o m m o n for the w h o l e J a p a n e s e A t l an t i c f ishery, o n l y b e t w e e n 1,052 a n d 2 , 8 9 0 1 of s h a r k s d i e d dur ing 1 9 8 9 ope ra t i ons . H o w e v e r , o the r reports ind ica te that the U . S . - e n f o r c e d r e l e a s e of a l l s h a r k b y c a t c h e s in this f i shery is not f o l l o w e d th roughou t the entire A t l a n t i c ( N a k a n o 1993) . M o r e o v e r , the s p e c i e s c o m p o s i t i o n o f the b y c a t c h e s is k n o w n to c h a n g e lat i tudinal ly a n d this c o u l d a l te r the o v e r a l l su rv iva l ra tes . A d d i t i o n a l va r i a t i ons in the e s t i m a t e d b y c a t c h of e l a s m o b r a n c h s a re e x p e c t e d to be found if w e c o n s i d e r the mul t ip le a r e a s a n d t ypes of g e a r s u s e d by the J a p a n e s e long l ine r s a c r o s s the A t l a n t i c O c e a n . H o w e v e r , a s l ong a s m o r e de t a i l ed informat ion o n a r ea l , s e a s o n a l a n d g e a r - w i s e h o o k i n g ra tes is not a v a i l a b l e to a s s e s s t he se c h a n g e s , it wi l l be difficult to ob ta in better e s t ima te s . T h e repor ted c a t c h o f e l a s m o b r a n c h by J a p a n in the A t l a n t i c O c e a n in 1 9 8 9 is 1,540 t ( see s ec t ion 2.2) . T h i s f igure falls c l o s e to the l o w e r limit o f the v e r y a m p l e range of e l a s m o b r a n c h c a t c h e s e s t i m a t e d here . H o w e v e r , if w e take a n a v e r a g e o f the different e s t ima te s p r o v i d e d a b o v e , at least 15 ,466 t of s h a r k s w o u l d h a v e b e e n d i s c a r d e d . M o s t of t h e m w o u l d h a v e b e e n f inned pr ior to r e l ea se , a s a c k n o w l e d g e d by N a k a n o (1993) . b) K o r e a . T h e K o r e a n long l in ing fleet h a d 2 9 v e s s e l s ope ra t i ng in the A t l a n t i c in 1 9 8 8 a n d 33 dur ing 129 1 9 8 9 ( N F R D A 1992) . T h i s fleet u s e s d e e p long l ines d i r ec t ed m a i n l y at b i g e y e t u n a s i n c e 1980 . B o t h the n u m b e r o f v e s s e l s a n d the c a t c h e s o f K o r e a in the A t l a n t i c h a v e d e c r e a s e d s i n c e 1 9 7 7 . T h e s e v e s s e l s repor ted a n effort o f 2 1 , 9 6 8 , 1 9 8 h o o k s a n d a total "others" ca t ch of 9 4 4 t for 1 9 8 9 ( I C C A T 1992) . N o informat ion o n the s p e c i e s i n c l u d e d u n d e r "others" is a v a i l a b l e a n d no repor ts o f e l a s m o b r a n c h b y c a t c h e s for this par t icu la r f i shery a re k n o w n to exis t . A c c o r d i n g to the repor ted A t l an t i c f i sh ing g r o u n d s o f the K o r e a n fleet d u r i n g 1 9 8 3 - 1 9 8 5 ( N F R D A 1988) m o s t o f the effort is l o c a l i s e d b e t w e e n 2 0 ° N - 2 0 ° S (fig. 2 .36) . T h u s , it s e e m s more appropr i a t e to u s e the h o o k rates d e r i v e d a b o v e from H a z i n et a l . (1990) for the equa to r i a l A t l an t i c . T h i s rough e s t ima te s indica te that 1 9 0 , 2 4 5 s h a r k s (86 ,554 b lue s h a r k s , 9 1 , 6 0 7 g r e y s h a r k s , 5 ,932 m a k o s h a r k s , 1,758 t h r e s h e r s h a r k s , 3 ,076 c r o c o d i l e s h a r k s a n d 1,318 o c e a n i c whi te t ip sha rks ) o r s o m e 7 ,783 t w e r e c a u g h t du r ing 1 9 8 9 by K o r e a n long l ine r s in the A t l an t i c O c e a n . T h i s c o m p a r e s v e r y h igh to the repor ted 143 t of v a r i o u s e l a s m o b r a n c h s t a k e n in that y e a r by S o u t h K o r e a in the A t l a n t i c O c e a n ( F A O 1993) . P r e s u m a b l y a n e l a s m o b r a n c h d i s c a r d of at leas t 9 7 % is o c c u r r i n g in this f i shery . T h e propor t ion o f s h a r k s r e l e a s e d a l ive a n d the extent o f f inning p rac t i ce s in the K o r e a n f ishery are u n k n o w n . c) T a i w a n . L o n g l i n e r s f rom T a i w a n h a v e f i shed for a l b a c o r e in the S o u t h A t l a n t i c s i n c e at leas t 1967 a n d in the Nor th A t l a n t i c at leas t s i n c e 1 9 7 2 . T y p i c a l l y , m o r e t han 8 0 % of the i r c a t c h e s are a l b a c o r e , f o l l o w e d by b i g e y e tuna . D u r i n g 1 9 8 9 , T a i w a n d e p l o y e d 3.6 mi l l ion h o o k s in the Nor th A t l a n t i c a n d 6 8 . 7 mi l l ion in the S o u t h A t l an t i c ( I C C A T 1 9 9 1 b ) . A c c o r d i n g to H s u a n d L iu (1992) , in 1 9 9 0 this i n c r e a s e d to a total o f 99 .8 mi l l ion h o o k s , 17.4 a n d 82 .4 mi l l ion in the Nor th a n d S o u t h A t l an t i c r e spec t ive ly . O f these , 17.5 mi l l ion h o o k s w e r e from d e e p long l ines d i r ec t ed t o w a r d s b i g e y e a n d ye l lowf in tunas , w h i l e the r e m a i n i n g 82 .2 mi l l ion h o o k s w e r e r egu la r l ong l i ne s f i sh ing for a l b a c o r e p r inc ipa l ly in the S o u t h A t l a n t i c (fig. 2 .37) . H s u a n d L i u a l s o report a T a i w a n e s e c a t c h o f 7 3 6 t o f s h a r k s a n d o the r f i shes for 1 9 9 0 . D u r i n g 1 9 9 1 , the n u m b e r of v e s s e l s ope ra t i ng in the A t l a n t i c fell a b o u t 1 0 % . H o w e v e r , repor ted s h a r k b y c a t c h e s i n c r e a s e d to 1 ,4861 ( H s u a n d L i u 1993) . T h e repor ts of H s u a n d L i u (1993) ind ica te that the va r i a t ions in the repor ted b y c a t c h e s o f s h a r k s from this f i shery 130 c CO CO 1-•4— o in co 't_ CO sz m U— CD C TO C _o CO cn CD i_ TJ C CO CL CD CD TJ CO SZ -*—' , „ XI CO 1_ o o CO _Q CO TJ c CN CO CJ) -—- CJ) CO CO cz LJ 3 TJ ye an CO CO LO jo I o E UJ 2 Z) CP •awn CO TJ c CD mi or o cz ci M— CJ) o CJ) c o -an XI CO 1_ o -1—' i/> o b o c CO ro c\i -t—» < CD i_ CO Z3 sz CD Li_ c 132 a re d e t e r m i n e d by the s u c c e s s in the c a t c h o f the target s p e c i e s . In y e a r s w h e n t u n a c a t c h e s a re re la t ively low, v e s s e l s t e n d to k e e p a l a rge r p ropor t ion o f the s h a r k b y c a t c h . T h e repor ted c a t c h o f s h a r k s in this f i shery s e e m s v e r y s m a l l for the n u m b e r o f h o o k s d e p l o y e d by the T a i w a n e s e long l in ing fleet. T h e T a i w a n e s e fleet f i shes p r e d o m i n a n t l y in the S o u t h A t l an t i c a n d for this r e a s o n the h o o k i n g rates d e r i v e d from H a z i n et a l . (1990) s e e m m o r e appropr i a t e for the p u r p o s e o f e s t ima t ion . N e v e r t h e l e s s , a s a la rge part o f the effort t a k e s p l a c e in t empera t e wa t e r s I d o not a t tempt to b r ake d o w n the b y c a t c h e s into s p e c i e s . T h i s w a y , it is rough ly e s t i m a t e d that 8 6 4 , 2 6 8 s h a r k s w e r e c a u g h t in 1 9 9 0 (p robab ly equ iva len t to 3 5 , 3 5 7 t) by the T a i w a n e s e long l ine r s . T h e rea l quant i ty o f e l a s m o b r a n c h s t a k e n by T a i w a n from the A t l a n t i c O c e a n is u n k n o w n . T h e p resen t a n a l y s i s is cer ta in ly rough d u e to l imited informat ion . H o w e v e r , it s u g g e s t s that a m a s s i v e d i s c a r d o f a r o u n d 3 4 , 0 0 0 t o f s h a r k s c o u l d be t ak ing p l a c e in the f ishery. A s in the c a s e o f the o the r f i sher ies d o c u m e n t e d a b o v e , e s t ima t ing the a c t u a l n u m b e r of s h a r k s r e l e a s e d a l i ve a n d d i s c a r d e d d e a d is ve ry difficult wi th the a v a i l a b l e in format ion . d) S p a i n . T h e S p a n i s h long l ine f i shery for swordf i sh in the A t l an t i c c a n be t r a c e d b a c k to at leas t 1973 ( G a r c e s a n d R e y 1984) . F i s h i n g g r o u n d s for 1988-1991 w e r e c e n t r e d in the E a s t e r n At l an t i c b e t w e e n 5 5 ° N a n d 1 5 ° S (fig. 2 .38) , a l t hough s o m e f i sh ing h a s a l s o b e e n repor ted for the M e d i t e r r a n e a n . S u r f a c e l ong l i ne s a re u s e d in w a t e r s o f the Nor th A t l a n t i c but d e e p long l ines h a v e b e e n i n t r o d u c e d in the S o u t h e a s t A t l an t i c s i n c e the recent e x p a n s i o n o f the f ishery there . T h e d e e p l ong l i ne s a re cons i s t of b a s k e t s o f abou t 1,200 m of l ine b e t w e e n floats a n d h a v e s o m e 3 3 b r a n c h l ines 15 m in length with the d e e p e s t h o o k s r e a c h i n g d o w n to b e t w e e n 3 6 0 a n d 4 7 0 m ( R e y a n d M u n o z - C h a p u l i 1991) . T h e S p a n i s h fleet set 3 5 , 8 5 0 , 0 7 8 h o o k s in the A t l an t i c O c e a n a n d 7 , 6 8 3 , 5 8 0 h o o k s in the M e d i t e r r a n e a n S e a dur ing 1989 , with i n c r e a s e s of 6 .75 a n d 7 . 3 % dur ing 1 9 9 0 r e spec t ive ly ( I C C A T 1 9 9 1 a , 1992) . D e Me t r i o et a l . (1984) report h o o k i n g rates o f b lue s h a r k s in sword f i sh l ong l i ne s in the M e d i t e r r a n e a n o f 0 .014 s h a r k s / 1 0 0 0 h o o k s . H o w e v e r , the i r repor ts d o not c o n s i d e r o ther sha rk s p e c i e s o r the d i s c a r d s d o n e at s e a a n d a re thus b i a s e d t o w a r d s s m a l l h o o k rates. 133 R e y a n d A l o t (1984) p rov ide da t a from the S p a n i s h sword f i sh fleet in the w e s t e r n M e d i t e r r a n e a n . T h e y ind ica te h o o k i n g rates of 6 .34 b lue s h a r k s , 0 .32 shortfin m a k o s h a r k s , 0.21 s m o o t h h a m m e r h e a d s h a r k s (Sphyrna zygaena) a n d 0 .005 p e l a g i c rays , pe r 1000 h o o k s . Meju to (1985) repor ts C P U E v a l u e s o f 138 .8 , 17.5 a n d 1.1 k g / 1 0 0 0 h o o k s for b lue , shortfin m a k o a n d p o r b e a g l e s h a r k s r e spec t ive ly in the north a n d north w e s t e r n g r o u n d s of the S p a n i s h A t l an t i c swordf i sh fleet, b a s e d o n a s a m p l e of 2 0 0 tr ips d u r i n g 1984 . T h i s is equ iva len t to h o o k ra tes o f 13.7 , 0 .259 a n d 0 .016 s h a r k s / 1 0 0 0 h o o k s r e spec t i ve ly for e a c h s p e c i e s . T h e s e c a t c h rates t ake into c o n s i d e r a t i o n the d i s c a r d s at s e a o f b lue s h a r k s , w h i c h Meju to e s t ima te s at 6 8 . 4 % in weigh t . Meju to a l s o f inds a l inea r r e l a t ionsh ip b e t w e e n c a t c h e s of swordf i sh a n d d i s c a r d s o f b lue s h a r k s , w h i c h is d r iven by the l imi ta t ions in s to rage c a p a c i t y a n d low v a l u e o f b lue s h a r k s . H e points out that in m a n y c a s e s fins a re r e m o v e d before d i s c a r d i n g the s h a r k s . M o r e recent ly , Me ju to a n d Ig les ias (1988) p rov ide information from exp lo ra to ry sword f i sh long l in ing ca r r i ed out du r ing 1 9 8 6 in the W e s t e r n Nor th At lan t i c . F r o m their da ta , c a t c h rates o f 13.5 a n d 2 .05 s h a r k s / 1 0 0 0 h o o k s o r 168 a n d 6 1 . 7 k g / 1 0 0 0 h o o k s c a n be c a l c u l a t e d for b lue a n d shortfin m a k o s h a r k s r e spec t i ve ly . T h e e l a s m o b r a n c h b y c a t c h of S p a n i s h long l ine r s i n c l u d e s m o r e than the three s p e c i e s m e n t i o n e d a b o v e . M u n o s - C h a p u l i (1985b) reports 16 s p e c i e s o f s h a r k s in the l a n d i n g s of v e s s e l s f i sh ing b e t w e e n C a p e V e r d e Is land a n d the A z o r e s . T h e b lue shark , the shortfin m a k o a n d the s m o o t h h a m m e r h e a d sha rk Sphyrna zygaena, w e r e , in order , the mos t a b u n d a n t s p e c i e s in the c a t c h e s (table 2 .6 s ec t ion 2 .2 .2) . L i m i t e d informat ion from the sou the rn A t l an t i c f i sh ing g r o u n d s o f the S p a n i s h swordf i sh f i shery w h e r e d e e p l ong l i ne s a re u s e d , i nd ica te s important c h a n g e s in the s p e c i e s c o m p o s i t i o n . R e y a n d M u n o z - C h a p u l i (1991) report 14 e l a s m o b r a n c h s in th is a r e a from the c a t c h e s o f 16 nights of f i sh ing o f a s ing le c o m m e r c i a l longl iner . F r o m their da ta , a v e r a g e h o o k rates in s h a r k s / 1 0 0 0 h o o k s a re e s t ima ted a s 20 .6 for night s h a r k s Carcharhinus signatus, 6.3 for s i lky s h a r k s , 3.4 for b i g e y e t h r e s h e r s h a r k s , 2 .9 for b lue s h a r k s , 2 for dev i l r ays Mobula sp., 1.8 for shortfin m a k o s h a r k s , 0.3 for c o m m o n h a m m e r h e a d s h a r k s a n d l e s s than 0.3 e a c h for Sphyrna couardi, S. mokarran, S. zygaena, Centrophorus granulosus, Galeocerdo cuvieri, Isurus paucus a n d Carcharhinus plumbeus. T h e o v e r a l l h o o k rate o f 134 e l a s m o b r a n c h s is e s t i m a t e d at 38 .8 f i sh /1000 h o o k s , w h i c h is fairly h igh c o m p a r e d to that for S p a n i s h sword f i sh long l ine r s in the Nor th At l an t i c . T h e different a r e a s f i s h e d a n d g e a r s u s e d c o u l d e x p l a i n t he se d i s c r e p a n c i e s , but the l imi ted t ime w i n d o w a n d n u m b e r of ope ra t i ons o b s e r v e d by R e y a n d M u n o z - C h a p u l i c o u l d a l s o be a s igni f icant s o u r c e of b ia s . T h e total c a t c h o f s h a r k s in the S p a n i s h f i shery for 1 9 8 9 c a n be e s t i m a t e d u s i n g the resul ts of Meju to (1985) . H i s report d o e s not o n l y t ake into a c c o u n t the d i s c a r d s o f b lue s h a r k s a n d p rov ide c a t c h ra tes in we igh t , but it c o v e r s a l a rger t ime f rame a n d g e o g r a p h i c a r e a than o ther reports . H e n c e , for the effort l eve l s o f 1989 , a total o f m o r e than 6 0 8 , 0 0 0 s h a r k s w e i g h i n g s o m e 6 ,856 t w o u l d h a v e b e e n c a u g h t in th is f i shery (5 ,646 t in the A t l an t i c a n d 1,210 t in the M e d i t e r r a n e a n ; tab le 2 .17) . G i v e n the d i s c a r d rate o f 6 8 . 3 % for b lue s h a r k s in the Spanish sword f i sh fleet repor ted by Meju to , the total d i s c a r d of b lue s h a r k s from the S p a n i s h f i shery du r ing 1 9 8 9 c o u l d be a s h igh a s 4 , 1 3 4 t. T h e resul ts p r e s e n t e d a b o v e s h o u l d be u s e d with cau t ion a s they a re b a s e d o n e s t ima ted da t a c o m i n g from o n l y part o f the g e o g r a p h i c a l a r e a f i shed by the S p a n i s h fleet. T h e y are useful o n l y to get a n i d e a o f the extent o f the e l a s m o b r a n c h b y c a t c h e s a n d d i s c a r d s . Bet ter e s t ima te s that t ake into a c c o u n t o ther s p e c i e s p resen t in the c a t c h e s a n d the g e o g r a p h i c a l / s e a s o n a l va r i a t ions o f c a t c h rates a n d s p e c i e s c o m p o s i t i o n wi l l h a v e to awai t i m p r o v e d informat ion from this o r s imi l a r f i she r ies . Indian O c e a n . A n u m b e r of coun t r i e s fish for tun ids with l ong l i ne s in the w a t e r s o f the Indian O c e a n . T h e three p r inc ipa l long l ine fleets a re from J a p a n , K o r e a a n d T a i w a n , w h i c h e n t e r e d the f ishery in 1952 , 1 9 6 3 a n d 1966 respec t ive ly . Indian long l ine r s s tar ted f i sh ing for t unas in 1986 but thei r c a t c h e s , a s w e l l a s t hose from other few coun t r i e s , a r e v e r y s m a l l in c o m p a r i s o n ( I P T P 1990) . M o s t o f the informat ion abou t long l ine f i sher ies in the Indian O c e a n is a v a i l a b l e th rough the repor ts o f the Indo-Pac i f i c T u n a D e v e l o p m e n t a n d M a n a g e m e n t P r o g r a m m e ( I P T P ) . T h e J a p a n e s e fleet f i shed t rop ica l a r e a s for ye l lowf in , a l b a c o r e a n d b i g e y e t unas at the b e g i n n i n g of the f i shery but shifted to h ighe r la t i tudes for s o u t h e r n bluef in a n d b i g e y e tuna 135 80N e o 40 20 20 40 60S 80W 60 4 0 2 0 0 20E Figure 2.38 Distribution of effort (in thousands of hooks) by the Spanish swordfish longline fishery in the Atlantic Ocean during 1988-1991. (Redrawn from Mejuto et al. 1993). Table 2.17 Catch rates and estimated total catch of sharks in the Spanish swordfish fishery. Information from Mejuto (1985) Estimated total catch 1989 Numbers Weight (t) Hook rate CPUE Mediterranean (7.68 M hooks) Atlantic (35.8 M hooks) Species (17.344 M hooks) (sh/1000 h) (kg/1000 h Numbers Weight(t) Numbers Weight(t) Prionace glauca * 237,660 2,408 13.703 138.8 105,286 1,067 491,244 4,977 Isurus oxyrinchus 4,488 304 0.259 17.5 1,988 135 9,277 ' 628 Lamna nasus 272 20 0.016 1.1 120 9 562 41 Totals 242,420 2,732 14 158 107,395 1,210 501,083 5,646 * includes estimated discards (68.4%) 136 dur ing the 1970 ' s , w h i l e in t roduc ing d e e p long l in ing in t rop ica l w a t e r s at the s a m e t ime . J u d g i n g from d a t a repor ted to I P T P , J a p a n e s e long l ine r s h a v e d e c r e a s e d the i r effort from 106 .64 mi l l ion h o o k s in 1986 to 74 .861 mi l l ion h o o k s in 1 9 8 9 . T h e da t a r e c o r d s o f J a p a n e s e long l ine r s in the Indian O c e a n d o not report a n y sha rk b y c a t c h e s in the f i shery . H o w e v e r , in F A O y e a r b o o k s J a p a n reports 6 7 5 t of "var ious e l a s m o b r a n c h s " c a u g h t in the Indian O c e a n d u r i n g 1 9 8 9 . G i v e n that the o n l y J a p a n e s e f ishery t ak ing p l a c e in t h o s e w a t e r s is the t u n a long l ine f i shery (excep t for three n e w l y in t roduced pu r se s e ine r s ) , the e l a s m o b r a n c h c a t c h e s repor ted to F A O , a l t hough s m a l l , c a n be at t r ibuted m a i n l y to s h a r k b y c a t c h e s of the long l ine rs . T a i w a n e s e v e s s e l s t ake the larges t c a t c h e s o f a l b a c o r e but a l s o fish for ye l lowf in a n d b i g e y e t unas pr imar i ly u s i n g d e e p l ong l i ne s in t rop ica l wa t e r s (fig 2 .39) . T h e r e w e r e 199 v e s s e l s in the f i shery in 1 9 8 3 , 1 2 7 in 1 9 8 5 , a n d 187 in 1 9 8 8 . T h e total effort in n o m i n a l h o o k s dur ing 1988 w a s e s t i m a t e d at 107 mi l l ion by T a i w a n e s e r e s e a r c h e r s ( I P T P 1990) . O n the o ther h a n d , u n p u b l i s h e d da t a from I P T P indica te that T a i w a n c a u g h t 3 3 , 0 5 2 s h a r k s with a total we igh t o f 1,216 t in this pe r iod , wi th a total effort o f 1 3 0 , 2 3 5 , 7 4 2 h o o k s . F o r 1 9 8 9 t he se v a l u e s w e r e 1 8 8 , 6 1 5 s h a r k s o r 7 ,474 t wi th a n effort o f 1 3 6 , 4 1 8 , 2 9 6 h o o k s . K o r e a n long l ine r s ope ra t e pr imar i ly in the t rop ica l Indian O c e a n ta rge t ing b i g e y e a n d ye l lowf in t u n a s wi th d e e p long l ines (fig. 2 .36) . T h e n u m b e r o f v e s s e l s h a s v a r i e d c o n s i d e r a b l y , p e a k i n g in 1 9 7 5 at 185 , d e c r e a s i n g to 6 2 in 1 9 8 5 a n d r e a c h i n g 112 in 1988 ( I P T P 1990) . A c c o r d i n g to mos t recent a v a i l a b l e da t a for I P T P , K o r e a n v e s s e l s caugh t 10,851 s h a r k s in 1 9 8 7 a n d the effort w a s 3 5 , 7 4 8 , 2 9 2 h o o k s . T h e a b s e n c e o f J a p a n e s e repor ts of s h a r k s t a k e n in the f i shery m a k e it n e c e s s a r y to es t imate the i r b y c a t c h e s . A d d i t i o n a l l y , the a n a l y s i s of a p p a r e n t h o o k i n g ra tes d e r i v e d from the repor ts o f the K o r e a n a n d T a i w a n e s e f i sher ies w e r e too l o w c o m p a r e d wi th a l ternat ive da ta from the Indian O c e a n (see be low) a n d s imi l a r f i sher ies in o the r o c e a n s (i.e. A t l an t i c O c e a n ) . T h e h o o k i n g rates c a l c u l a t e d from the da ta r epor ted a b o v e a r e 1.38 s h a r k s / 1 0 0 0 h o o k s for T a i w a n in 1 9 8 9 a n d 0.3 s h a r k s / 1 0 0 0 h o o k s for K o r e a in 1 9 8 7 . I c o n s i d e r e d that t he se c a t c h ra tes reflect c o n s i d e r a b l e under - repor t ing a n d a r e therefore o f little u s e . T h e se l ec t ion of c a t c h e s a n d d i s c a r d o f s h a r k s in h i g h - s e a s t u n a f i she r i es is a v e r y c o m m o n prac t ice . In the fo l lowing p a r a g r a p h s , the a v a i l a b l e informat ion o n s h a r k c a t c h rates in the 137 20 40 60 80 100 120 140E 2 0 4 0 6 0 S O 1 0 0 1 2 0 1 4 0 E Figure 2.39 Distribution of Ta iwanese catch per unit effort of a lbacore by (a) regular and (b) deep longline fisheries during 1988 in the Indian O c e a n . (Redrawn from H s u and Liu 1990). 138 Indian O c e a n is a n a l y s e d to p rov ide a l ternat ive e s t ima te s o f the b y c a t c h e s of s h a r k s in t he se three t u n a f i sher ies . Information o n s h a r k b y c a t c h e s in the Indian O c e a n long l ine f i she r i es is re la t ive ly a b u n d a n t a n d a l l o w s for g e o g r a p h i c a l par t i t ioning in s o m e c a s e s . H o w e v e r , v i r tua l ly no reports i nc lude da ta o n h o o k i n g rates by s p e c i e s . T h e o n l y ind ica t ion o f s p e c i e s c o m p o s i t i o n c o m e s from T a n i u c h i (1990) w h o repor ts the p e r c e n t a g e o f e a c h s p e c i e s in the s h a r k b y c a t c h e s of r e s e a r c h t u n a long l ine r s from J a p a n . H i s resul ts ind ica te that 7 6 . 6 % a re b lue s h a r k s , 6 .6% s i lky s h a r k s , 6 . 5 % shortfin m a k o s h a r k s , 3 .4% o c e a n i c whi te t ip s h a r k s a n d 6 . 8 % unident i f ied s h a r k s . S i v a s u b r a m a n i a m (1963) p r o v i d e s da t a o n ea r ly r e s e a r c h o p e r a t i o n s by J a p a n e s e a n d T a i w a n e s e v e s s e l s that ind ica te b y c a t c h e s o f 10 .83 s h a r k s / 1 0 0 0 h o o k s f o r t h e eas t e rn Indian O c e a n (E o f 6 0 ° E ) . S i v a s u b r a m a n i a m (1964) , repor ts o n c o m m e r c i a l a n d r e s e a r c h o p e r a t i o n s for s i x a r e a s o f the Indian O c e a n a n d ind i ca t e s that a b o u t 2 0 s p e c i e s of s h a r k s o c c u r in the b y c a t c h e s , 11 of t he se s h a r k s (main ly c a r c h a r h i n i d s ) a re v e r y c o m m o n (table 2 .18) . T h e resul ts o f S i v a s u b r a m a n i a m indica te la t i tudinal c h a n g e s in s p e c i e s c o m p o s i t i o n of s h a r k s a n d h i g h e r h o o k i n g rates for s h a r k s north of the equa to r . H e repor ts that the f r equency d is t r ibut ions o f h o o k i n g rates for s h a r k s for s i x a r e a s o f the Indian O c e a n s h o w a r ange o f 0-4 to 4 4 . 1 - 4 9 s h a r k s / 1 0 0 0 h o o k s , wi th a m o d a l c l a s s o f 4 .1-8 s h a r k s / 1 0 0 0 h o o k s . M i m u r a et a l . (1963) report h o o k i n g rates by a r e a a n d s e a s o n that a v e r a g e 5.1 s h a r k s / 1 0 0 0 h o o k s ( range 2 .6-7 .3) . O n m o r e recent reports , P i l l a i a n d H o n m a (1978) p rov ide mon th ly c a t c h ra tes o f p e l a g i c s h a r k s in 1 0 ° x 2 0 ° s q u a r e s f o r t h e J a p a n e s e fleet in the Indian O c e a n that r ange b e t w e e n 0.1 a n d 5 0 s h a r k s / 1 0 0 0 h o o k s . V a r g h e s e (1974; c i t ed by P i l l a i a n d H o n m a (1978)) reports h o o k i n g rates a s h igh a s 84 s h a r k s / 1 0 0 0 h o o k s a n d a n a v e r a g e w e i g h t o f 5 7 kg / sha rk in the L a k s h a d w e e p S e a . A c c o r d i n g to S i l a s a n d P i l l a i (1982) , h o o k i n g ra tes o f s h a r k s in the Indian O c e a n v a r y from y e a r to y e a r a n d b e t w e e n a r e a s , the h ighes t b e i n g b e t w e e n 0.6 a n d 10 s h a r k s / 1 0 0 0 h o o k s . T h e y a l s o report that in the a r e a o f the S o u t h e a s t A r a b i a n S e a , s h a r k s w e r e 6 3 . 8 a n d 5 7 . 8 % of the total c a t c h in n u m b e r a n d w e i g h t r e spec t i ve ly a n d h a d a n a v e r a g e w e i g h t of 3 0 kg . S i v a s u b r a m a n i a m (1987) s u m m a r i s e s d a t a from F i s h e r i e s S u r v e y of India t u n a r e s e a r c h c r u i s e s in the sou th w e s t c o a s t o f India du r ing 1 9 8 3 - 1 9 8 6 . T h e s e resul ts ind ica te h o o k i n g rates of 17.6 s h a r k s / 1 0 0 0 h o o k s . J a m e s a n d P i l l a i (1987) r ev i ew add i t i ona l r e s e a r c h c r u i s e s in a r e a s of the S o u t h e a s t A r a b i a n S e a , A n d a m a n S e a , 139 Table 2.18 Shark species commonly caught by tuna longlining in the Indian Ocean (adapted from Sivasubramaniam, 1964). Sc ien t i f i c n a m e C a u g h t b y l o n g l i n e ( a p p r o x . m e a n wt.) Carcharhinus longimanus C. falciformis C. albimarginatus C. melanopterus Prionace glauca Isurus oxyrinchus Lamna ditropis Galeocerdo cuvieri Sphyrna spp. Alopias pelagicus A superciliosus 3 0 k g 6 0 k g 4 0 k g 3 5 k g 5 0 k g 7 5 k g 7 5 k g ? 7 5 k g 5 0 k g 1 0 0 k g 140 W e s t e r n B a y o f B e n g a l , a n d the E q u a t o r i a l R e g i o n sou th o f India, p r o v i d i n g f igures for the p e r c e n t a g e cont r ibu t ion o f s h a r k s to the total c a t c h a v e r a g i n g 3 9 . 8 % ( range 3 0 . 9 - 4 3 . 7 % ) . T h e y a l s o report h o o k i n g rates that a v e r a g e 16.4 s h a r k s / 1 0 0 0 h o o k s ( range 7 .4-29.7) in the S o u t h e a s t A r a b i a n S e a . J a m e s a n d J a y a p r a k a s h (1988) report t w o different s tud ies c o m p r i s i n g s e v e r a l a r e a s a r o u n d India that indica te h o o k i n g rates o f 8.43 s h a r k s / 1 0 0 0 h o o k s ( range 3.3-14) a n d cont r ibut ion o f s h a r k s to the c a t c h e s o f 3 2 . 1 % ( range 19 .6-44 .8) in o n e c a s e a n d h o o k i n g rates o f 7.6 s h a r k s / 1 0 0 0 h o o k s ( range abou t 1.5-9.5) a n d cont r ibu t ions of s h a r k s to the c a t c h o f 1 7 . 4 % in the other . S t e v e n s (1992) repor ts h o o k i n g rates of 8.3 b lue s h a r k s a n d 3.5 m a k o s h a r k s pe r 1 0 0 0 h o o k s for a T a i w a n e s e r e s e a r c h long l ine r in sou th W e s t e r n A u s t r a l i a . S t r o n g va r i a t i ons a re ev iden t in h o o k i n g ra tes a c r o s s the Indian O c e a n d e p e n d i n g o n loca t ion a n d s e a s o n . Ideally, a n o v e r a l l e s t imate of e l a s m o b r a n c h b y c a t c h e s s h o u l d be built at leas t c o n s i d e r i n g a r e a l d i f ferences . Unfor tunate ly , th is is not p o s s i b l e g i v e n the a g g r e g a t e d nature o f effort s tat is t ics for the fleets of J a p a n , K o r e a a n d T a i w a n . N e v e r t h e l e s s , there s e e m s to be a c o n s e n s u s a r o u n d 1-10 s h a r k s / 1 0 0 0 h o o k s a s the mos t c o m m o n h o o k i n g rate. To ta l c a t c h e s of s h a r k s in n u m b e r s for the w h o l e Indian O c e a n are thus rough ly e s t i m a t e d u s i n g a h o o k i n g rate o f 7 .96 s h a r k s / 1 0 0 0 h o o k s o b t a i n e d by a v e r a g i n g the v a l u e s d e r i v e d from S i v a s u b r a m a n i a m (1963) a n d M i m u r a et a l . (1963) . T h e s e v a l u e s d o not o n l y c o m e from da t a per ta in ing to mos t o f the Indian O c e a n but a l s o ma tch with the mos t c o m m o n h o o k i n g rates repor ted by different s o u r c e s . T h e a v e r a g e we igh t of s h a r k s t a k e n in the f i shery is e s t i m a t e d at 38.21 kg d e r i v e d from the we igh t a n d n u m b e r s o f s h a r k s r epor ted for T a i w a n e s e long l ine r s dur ing 1 9 8 8 a n d 1 9 8 9 . T h e e s t ima ted sha rk b y c a t c h e s for the last a v a i l a b l e effort l eve l s a re : 5 9 6 , 2 6 7 s h a r k s o r 2 2 , 7 8 3 1 for J a p a n dur ing 1 9 8 9 , 2 4 8 , 7 3 5 s h a r k s o r 1 0 , 8 7 9 t for K o r e a du r ing 1 9 8 7 a n d 1 ,086 ,572 s h a r k s o r 4 1 , 5 1 8 t for T a i w a n in 1 9 8 9 . T h e total c a t c h o f s h a r k s in the Indian O c e a n t u n a long l ine h i g h - s e a s f i shery c a n be e s t i m a t e d at a p p r o x i m a t e l y 1 ,931,574 s h a r k s o r 7 5 , 1 8 0 t. B a s e d o n the repor ted c a t c h e s o f e l a s m o b r a n c h s from e a c h coun ty , the c o r r e s p o n d i n g d i s c a r d s o f s h a r k s a re e s t i m a t e d at abou t 2 2 , 1 0 8 t by J a p a n , 9 ,089 t by K o r e a a n d 3 4 , 0 4 4 t by T a i w a n . T h e p e r c e n t a g e s o f t he se d i s c a r d s that su rv ive a re not k n o w n , but j u d g i n g from the reports o f S i v a s u b r a m a n i a m (1963 ; 1964) abou t 7 0 - 8 0 % of the d i s c a r d s o f c a r c h a r h i n i d s h a r k s a re e x p e c t e d to su rv ive if r e l e a s e d a l ive w h e r e a s h a m m e r h e a d s a n d m a k o s h a r k s 141 are d e a d o n the l ine mos t of the t ime . H o w e v e r , the rate o f f inning a l t h o u g h u n k n o w n , is e x p e c t e d to be h i g h . T h e v a r i o u s e s t ima t ions p r o v i d e d here are l imi ted by the var iab i l i ty o f h o o k i n g rates repor ted f o r t h e Indian O c e a n a n d the r o u g h n e s s o f the effort s ta t is t ics . T h e y s h o u l d be t a k e n with cau t ion a n d u s e d a s a first a p p r o x i m a t i o n to the l eve l o f e l a s m o b r a n c h b y c a t c h e s a n d d i s c a r d s in t h e s e f i she r ies . T r o p i c a l a n d S o u t h P a c i f i c . T h e r e a re s e v e r a l f leets f i sh ing for t u n a in this a r e a , h o m e to m a n y s m a l l i n su l a r coun t r i e s . H o w e v e r , mos t o f the long l ine o p e r a t i o n s a re c a r r i e d out, in o r d e r o f impor t ance , by J a p a n e s e , S o u t h K o r e a n , T a i w a n e s e a n d A u s t r a l i a n v e s s e l s . In g e n e r a l , t h e s e f i sher ies a re ve ry poor ly d o c u m e n t e d , m a k i n g it ve ry difficult to a s c e r t a i n e l a s m o b r a n c h b y c a t c h e s . M o s t of the a v a i l a b l e informat ion for the cen t ra l P a c i f i c a r e a is that submi t t ed to the S o u t h Pac i f i c C o m m i s s i o n ( S P C ) by the f i sh ing na t ions a n d m a d e a v a i l a b l e t h rough the F o r u m F i s h e r i e s A g e n c y ( F F A ) (P . T a u r i k i , F F A , P . O B o x 6 2 9 , H o n i a r a , S o l o m o n Is lands , pe r s . c o m m . J u n e 1992) . A d d i t i o n a l l y , A u s t r a l i a n a n d N e w Z e a l a n d s o u r c e s p rov ide s o m e information per ta in ing the i r E E Z ' s . T h e r e s e e m s to be a g a p in informat ion for t hose a r e a s of the e a s t e r n P a c i f i c w h e r e nei ther A u s t r a l i a no r N e w Z e a l a n d h a s ju r i sd ic t ion . Fu r the rmore , the c o v e r a g e o f the f i sh ing fleets by the F F A d a t a is part ia l ( L a w s o n 1991) . H e n c e , the effort l eve l s f o r t h e entire cen t ra l a n d sou th P a c i f i c a r e a a re u n k n o w n a n d s u s p e c t e d to be la rger t han t h o s e a v a i l a b l e th rough the s o u r c e s u s e d he re . T h e a r e a c o n s i d e r e d in this s ec t ion a s " T r o p i c a l a n d S o u t h Pac i f i c" , is that c o m p r i s e d by w a t e r s sou th o f 2 0 ° N . J a p a n e s e f i s h e r m e n s tar ted e x p e r i m e n t i n g wi th l ong l i ne s in the w e s t e r n cen t ra l P a c i f i c a s ear ly a s the 1920 ' s a n d h a d 7 2 v e s s e l s ac t ive by 1 9 3 9 . H o w e v e r the p e a k in the e x p a n s i o n of this f i shery o c c u r r e d dur ing the late 1960 ' s , c o v e r i n g mos t o f the cen t r a l a n d sou th P a c i f i c ( S u z u k i 1988 , L a w s o n 1991) . A t present , at leas t 4 0 6 v e s s e l s a re s u s p e c t e d to ope ra te in the r eg ion . T h e F F A d a t a b a s e ind ica te s that J a p a n d e p l o y s m o r e than 7 0 % of the total effort in the a r e a , wi th 3 1 , 1 4 3 f i sh ing d a y s in 1 9 8 9 . 142 T h e S o u t h K o r e a n long l ine fleet a p p e a r e d dur ing 1958 a n d is r epor ted to h a v e 124 v e s s e l s at p resen t in the a r e a . A c c o r d i n g to S o u t h K o r e a n r e c o r d s ( N F R D A 1988) , l ong l ine r s from this coun t ry fish l a rge ly for tun ids in the S o u t h P a c i f i c (fig. 2 .36) . T h e S o u t h K o r e a n effort in the F F A z o n e is repor ted a s 6 ,312 f i sh ing d a y s for 1 9 8 9 . T h e T a i w a n e s e fleet is poor ly d o c u m e n t e d a n d there is not e v e n a n es t ima te o f the n u m b e r of v e s s e l s in the r eg ion . A part ial c o v e r a g e o f the T a i w a n e s e effort i nd i ca t e s thei r p r e s e n c e in the w a t e r s north of P a p u a - N e w G u i n e a , but they are a l s o k n o w n to f ish a r o u n d Fiji a n d A m e r i c a n S a m o a ( L a w s o n 1991) . A c c o r d i n g to F F A da ta , the T a i w a n e s e fleet a c c u m u l a t e d a n effort o f 4 , 1 6 3 f i sh ing d a y s in 1 9 8 9 . T h e A u s t r a l i a n long l ine f i sher ies for t u n a e x p a n d e d in the 1980 ' s but date b a c k to the 1960 ' s . D u r i n g 1 9 8 9 , A u s t r a l i a n long- l ine rs put a total effort o f 2 , 2 4 4 f i sh ing d a y s (P . Tau r ik i , F F A , pers . c o m m ) . In add i t ion to the a b o v e m e n t i o n e d fleets, a few v e s s e l s from C h i n a , Fiji a n d T o n g a par t ic ipate in the f ishery. H o w e v e r , in 1 9 8 9 their effort o n l y a c c o u n t e d for a total of 5 5 8 f i sh ing d a y s . T h e g e o g r a p h i c a l dis t r ibut ion of total l ong l ine effort du r ing 1 9 9 0 a v a i l a b l e to the S P C is s h o w n in figure 2 .40 . M o s t o f the f i sh ing effort t a k e s p l a c e b e t w e e n 1 5 ° N a n d 1 5 ° S . T h e repor ted c a t c h o f s h a r k s for 1 9 8 9 in this a r e a w a s 4 2 6 t. A p p r o x i m a t e l y 3 7 5 t c o r r e s p o n d s to T a i w a n e s e , 3 5 t to S o u t h K o r e a n , a n d 12 t to J a p a n e s e v e s s e l s . A l t h o u g h the n u m b e r o f h o o k s d e p l o y e d by e a c h coun t ry w e r e not a v a i l a b l e , the total for a l l long l ine r s a m o u n t s to 9 8 , 8 3 2 , 5 0 0 h o o k s dur ing 1 9 8 9 . T h e n u m b e r o f h o o k s p e r coun t ry c a n be e s t i m a t e d u s i n g the repor ted f i sh ing d a y s o f e a c h fleet. T h e c o r r e s p o n d i n g e s t i m a t e d ca t ch rates in k g / 1 0 0 0 h o o k s a re 0 .167 for the J a p a n e s e , 2.5 for S o u t h K o r e a n , 4 0 . 5 for the T a i w a n e s e a n d 0 for the A u s t r a l i a n fleet. T h i s is e q u a l to a n o v e r a l l c a t c h rate o f 4.31 kg of sha rk pe r 1 0 0 0 h o o k s . S u c h m i n u s c u l e c a t c h rates , g r o s s l y e q u i v a l e n t to l e s s than 0.5 s h a r k s / 1 0 0 0 h o o k s w h e n c o m p a r e d to o ther reports , a re a l m o s t ce r t a in ly a result of s e v e r e under - repor t ing , p r e s u m a b l y d u e to d i s c a r d p r o c e s s e s . T h i s is ev iden t a l s o in the c r o s s -c o m p a r i s o n o f the e s t i m a t e d c a t c h rates for e a c h o f the m e n t i o n e d coun t r i e s . S a i k a a n d Y o s h i m u r a (1985) plot h o o k i n g rates for the mos t c o m m o n s h a r k s p e c i e s caugh t by J a p a n e s e r e s e a r c h long l ine r s in the w e s t e r n equa to r i a l P a c i f i c . T h e s e a r e a p p r o x i m a t e l y 144 0 -14 /1000 h o o k s for o c e a n i c whi te t ip a n d for s i lky s h a r k s , 0 - 1 6 / 1 0 0 0 h o o k s for b lue s h a r k s a n d 0 -2 /1000 h o o k s for shortfin m a k o s h a r k s . A n ove ra l l h o o k i n g rate o f 2 0 . 4 5 s h a r k s / 1 0 0 0 h o o k s c a n be e s t i m a t e d f o r w a t e r s b e l o w 2 2 ° N from the report o f S t r a s b u r g (1958) o n r e s e a r c h a n d c o m m e r c i a l c r u i s e s in the eas t e rn equa to r ia l P a c i f i c . T h i s c a n be further split into 4 .14 b lue , 5 .46 o c e a n i c whi te t ip , 10 .07 s i lky a n d 0 .78 unident i f ied s h a r k s , pe r 1 0 0 0 h o o k s . S t e v e n s (1992) repor ts o n the b y c a t c h e s of b lue a n d m a k o s h a r k s o f long l ine r s off T a s m a n i a n w a t e r s b a s e d o n informat ion c o l l e c t e d by o b s e r v e r s o n b o a r d J a p a n e s e v e s s e l s f i sh ing for sou the rn b l u e f i n t u n a (Thunnus maccoyii). H e p r o v i d e s h o o k i n g rates o f 10.4 b lue a n d 0.5 m a k o s h a r k s per 1 0 0 0 h o o k s a n d e s t ima te s that 1,594 m a k o a n d 3 4 , 0 0 0 blue s h a r k s w e i g h i n g 2 4 a n d 2 7 5 t r e spec t ive ly , a re c a u g h t e a c h f i sh ing s e a s o n in this f ishery. H o o k i n g rates for o ther s p e c i e s a re not a v a i l a b l e but S t e v e n s m e n t i o n s that thresher , po rbeag l e , s c h o o l (Galeorhinus galeus), b l a c k (Da/af/as licha), c r o c o d i l e (Pseudocarcharias kamoharai), h a m m e r h e a d , ve lve t dogf i sh (Zameus squamulosus) a n d g r ey (Carcharhinus) s h a r k s a re a l s o p resen t in the b y c a t c h e s o f J a p a n e s e long l ine r s in the A u s t r a l i a n F i s h e r y Z o n e . S t e v e n s a l s o p r o v i d e s da ta f o r t h e b y c a t c h e s of b lue s h a r k s in N e w Z e a l a n d wa te r s : J a p a n e s e a n d K o r e a n f i sher ies h a v e h o o k i n g rates of 4 .8 a n d 1.3 b lue s h a r k s / 1 0 0 0 h o o k s r e spec t ive ly in Nor the rn N e w Z e a l a n d , w h e r e a s the J a p a n e s e c a t c h rate in sou the rn N e w Z e a l a n d is 4 b lue s h a r k s / 1 0 0 0 h o o k s . S t e v e n s d r a w s at tent ion to the s t rong under - repor t ing of sha rk b y c a t c h e s in J a p a n e s e l o g b o o k s a n d repor ts that f ins a re cut off f rom the s h a r k s before b e i n g d i s c a r d e d . T h i s s u g g e s t s that mortal i ty in this f i shery might be e q u a l to the total b y c a t c h . R o s s a n d B a i l e y (1986) report h o o k i n g rates for m a k o s h a r k s in the K o r e a n a n d J a p a n e s e f i sher ies for a l b a c o r e o f nor thern N e w Z e a l a n d a n d f o r t h e s o u t h e r n N e w Z e a l a n d J a p a n e s e f ishery for sou the rn bluefin tuna . A v e r a g e s a re 0 .43 a n d 0 .34 s h a r k s / 1 0 0 0 h o o k s for the nor thern a n d sou the rn f i sher ies r e spec t ive ly . T h e total c a t c h o f m a k o s h a r k s c a n be e s t i m a t e d at 3 3 4 t p r o c e s s e d we igh t b a s e d o n their da ta , h o w e v e r , b e c a u s e abou t 5 0 % of the sha rk ' s w e i g h t is lost du r ing p r o c e s s i n g , the l ive w e i g h t o f the m a k o s h a r k b y c a t c h s h o u l d be a p p r o x i m a t e l y 6 6 8 1 . A l t h o u g h R o s s a n d B a i l e y d o not p rov ide further informat ion, it is s u s p e c t e d that this f igure o n l y r ep resen t s the repor ted c a t c h a n d d o e s not c o n s i d e r 145 d i s c a r d s . T h e total b y c a t c h o f s h a r k s in the S P C z o n e c a n be rough ly e s t i m a t e d u s i n g the f igures from S t r a s b u r g (1958) a n d a c o n s e r v a t i v e g u e s s of 2 0 kg / sha rk to c a l c u l a t e the total we igh t of the c a t c h . E v e n t h o u g h this c a t c h rate might a p p e a r to be c o m p a r a t i v e l y too h igh , the dis t r ibut ion o f effort in t he se f i sher ies ( see figure 2 .40) just if ies the u s a g e o f h o o k i n g rates from the E q u a t o r i a l P a c i f i c . T h e resul ts , p r e s e n t e d in t ab le 2 .19 , ind ica te that a p p r o x i m a t e l y 2 ,021 ,711 s h a r k s o r 4 0 , 4 3 4 1 w e r e c a u g h t in 1 9 8 9 o f w h i c h p r o b a b l y a l m o s t 5 0 % w e r e s i lky s h a r k s . J a p a n con t r ibu tes the majority o f the e l a s m o b r a n c h c a t c h e s a n d a l s o h a s the h ighes t d i s c a r d rate. T h e total d i s c a r d is e s t i m a t e d at 4 0 , 0 0 0 t. T h e s h a r k b y c a t c h e s in the w h o l e T r o p i c a l a n d S o u t h P a c i f i c might still be h igher . J u d g i n g from s i z e o f the s ta t is t ical a r e a c o v e r e d by the S P C (fig. 2 .34) a n d the m a p s o f C P U E of the S o u t h K o r e a n long l ine fleet for the y e a r s 1 9 8 3 - 1 9 8 5 (see f igure 2 .36) a n d c o n s i d e r i n g the part ial c o v e r a g e o f the S P C a r e a by F F A stat is t ics ( S P C 1991) , it is e s t i m a t e d that the S o u t h K o r e a n fleet d e p l o y e d twice a s m a n y h o o k s in the w h o l e cen t r a l a n d sou th P a c i f i c a s t hose repor ted by the F F A . T h i s a p p l i e s a l s o to the J a p a n e s e a n d T a i w a n e s e fleets. U n d e r this a s s u m p t i o n , the total c a t c h of s h a r k s in the cen t ra l a n d sou th P a c i f i c ou t s ide the S P C z o n e c o u l d be a r o u n d 1 ,097,288 s h a r k s o r 2 1 , 9 4 6 t; 16 ,422 t by J a p a n , 3 ,328 t by S o u t h K o r e a a n d 2 , 1 9 6 t by T a i w a n . T h e s e f igures a s s u m e a n ex t ra effort o f 9 2 , 5 9 8 , 1 7 3 h o o k s (1989) a n d a total h o o k i n g rate o f 11 .85 s h a r k s / 1 0 0 0 h o o k s . T h i s h o o k i n g rate tr ies to take into c o n s i d e r a t i o n the p o s s i b l e o c c u r r e n c e o f effort ou t s ide equa to r i a l a r e a s a n d w a s c a l c u l a t e d by a v e r a g i n g the total h o o k i n g rates o b t a i n e d from S t r a s b u r g (1958) for the equa to r ia l z o n e , f rom S t e v e n s (1992) for T a s m a n i a n wa te r s , a n d from R o s s a n d B a i l e y (1986) a n d S t e v e n s (1992) for N e w Z e a l a n d wa te r s . T h e s a m e e s t i m a t e d we igh t of 2 0 kg / sha rk a s a b o v e w a s u s e d . A c c o r d i n g l y , it is e s t i m a t e d that s o m e 6 2 , 3 8 0 1 o f s h a r k s w e r e c a u g h t a s b y c a t c h o f long l ine f i sher ies in the w h o l e cen t ra l a n d s o u t h P a c i f i c in 1 9 8 9 . A c c o r d i n g to F A O stat is t ics , the joint repor ted c a t c h o f e l a s m o b r a n c h s from the W e s t C e n t r a l , S o u t h W e s t e r n a n d S o u t h E a s t e r n P a c i f i c o f J a p a n , T a i w a n a n d K o r e a is on ly 4 , 4 0 9 t for 1 9 8 9 . T h e s e f igures s u g g e s t that s o m e 5 8 , 0 0 0 t o f s h a r k s m a y be d i s c a r d e d in t he se f i sher ies . T h e e s t ima te s o b t a i n e d a b o v e are re la t ively m o r e uncer ta in t han t h o s e c a l c u l a t e d in 146 Table 2.19 Es t imated bycatch of sharks in tuna longline fisheries of the Centra l and South Pacif ic ( S P C zone) , based on the results of Strasburg (1958). Strasburg's data Estimated Catch in 1989 Numbers caught Hook rate Total Japan S. Korea Taiwan Australia Species (216,172 hooks) (#/1000 hooks) numbers weight (t) weight (t) weight (t) weight (t) weight (t) Carcharhinus falciformis 2,176 10.07 994,854 19,897 13,950 2,827 1,865 1,005 Carcharhinus longimanus 1,181 5.46 539,946 10,799 7,571 1,535 1,012 546 Prionace glauca 896 4.14 409,646 8,193 5,744 1,164 768 414 Various sharks 169 0.78 77,266 1,545 1,083 220 145 78 Totals 4,422 20.46 2,021,711 40,434 28,349 5,746 3,789 2,043 147 p r ev ious s e c t i o n s for o ther h i g h - s e a s long l ine f i sher ies . T h i s is a n unfor tunate c o n s e q u e n c e of the l imi ted informat ion a v a i l a b l e , both abou t the rea l effort l eve l s o f e a c h fleet a n d abou t h o o k i n g ra tes in the S o u t h Pac i f i c . A g a i n , t h e se e s t ima te s c a n be u s e d for c o m p a r a t i v e p u r p o s e s a n d a s p re l imina ry informat ion to be r e v i s e d w h e n e v e r app rop r i a t e b a s e l i n e da ta b e c o m e a v a i l a b l e . Nor th P a c i f i c . T h i s is a n o t h e r r eg ion w h e r e long l ine f i sher ies a re v e r y poor ly d o c u m e n t e d . T h e reports of N F R D A conf i rm that K o r e a n long l ine r s f i shed in the cen t ra l north P a c i f i c du r ing 1 9 8 3 - 1 9 8 5 (fig. 2 .36) . F i g u r e s from S u z u k i (1988) a l s o indica te that the J a p a n e s e long l ine fleet c o v e r s g o o d part o f the Nor th P a c i f i c . H o w e v e r , T a i w a n e s e v e s s e l s d o not h a v e a h i g h - s e a s long l ine f i shery in this a r e a ( N a k a n o a n d W a t a n a b e 1992) . T h e r e a re no s tat is t ics a v a i l a b l e , at leas t in E n g l i s h , o f the a m o u n t of effort d e p l o y e d by long l ine r s in the Nor th Pac i f i c . N a k a n o a n d W a t a n a b e (1992) es t imate long l ine effort of the K o r e a n fleet at 14-19 mi l l ion h o o k s per /yr for the pe r iod 1 9 8 2 - 1 9 8 8 . U s i n g this e s t imate a n d s ta t is t ics f rom the F i s h e r y A g e n c y o f J a p a n they ar r ive at a total effort o f 2 5 8 ' 4 2 2 , 7 8 0 h o o k s d e p l o y e d du r ing 1988 in the entire Nor th P a c i f i c by J a p a n a n d K o r e a . T h e i r e s t imate o f a b o u t 3 , 2 7 4 , 6 0 9 b lue s h a r k s caugh t by long l ine f i sher ies in the Nor th P a c i f i c du r ing 1 9 8 8 is b a s e d o n lat i tudinal stratification o f effort a n d h o o k i n g rates . B e c a u s e of the g e o g r a p h i c a l c o v e r a g e c o n s i d e r e d in the p r e v i o u s s e c t i o n for the T r o p i c a l a n d S o u t h Pac i f i c , o n l y w a t e r s north of 2 0 ° N are i n c l u d e d here a s "Nor th Pac i f i c" . F r o m the da t a of N a k a n o a n d W a t a n a b e , it is p o s s i b l e to es t imate a total effort o f 1 0 5 ' 8 8 5 , 4 1 8 h o o k s a n d a b y c a t c h of 2 ' 9 6 4 , 5 0 0 b lue s h a r k s for this port ion o f the Nor th P a c i f i c du r ing 1 9 8 8 . D a t a d e r i v e d from the reports o f S t r a s b u r g (1958) p r o d u c e o v e r a l l h o o k i n g rates o f 18 .45 b lue , 0 .07 o c e a n i c whi te t ip a n d 0 .84 unident i f ied s h a r k s (total o f 19 .36 s h a r k s / 1 0 0 0 hooks ) f o r t h e e a s t e r n north P a c i f i c north o f 2 2 ° N . T a b u l a t e d d a t a f rom S i v a s u b r a m a n i a m (1963) a l l o w s the c a l c u l a t i o n o f h o o k i n g rates of 6 .79 b lue a n d 0 .35 o c e a n i c whi te t ip s h a r k s / 1 0 0 0 h o o k s for two c o m b i n e d a r e a s of the Nor th P a c i f i c a b o v e 2 0 ° N . S a i k a a n d Y o s h i m u r a (1985) p resen t d a t a o n s h a r k b y c a t c h e s of J a p a n e s e r e s e a r c h c r u i s e s f rom 1 9 4 9 - 1 9 7 9 in the W e s t e r n P a c i f i c . T h e i r m a p s of h o o k i n g rates indica te v a l u e s o f a p p r o x i m a t e l y 0-3 o c e a n i c 148 whitet ip , 0-0.5 s i lky , 0-2 shortfin m a k o a n d 0-30 b lue s h a r k s pe r 1 0 0 0 h o o k s for the reg ion north o f 2 0 ° N . T h e m o s t c o m m o n h o o k i n g rate v a l u e s plot ted for b lue s h a r k s a p p e a r to be a r o u n d 10 s h a r k s / 1 0 0 0 h o o k s , w h e r e a s t hose for the o ther three s p e c i e s p r o b a b l y a re l e s s than 1 s h a r k / 1 0 0 0 h o o k s . O n the o ther h a n d , N a k a n o et a l . (1985) report n u m b e r s of b lue s h a r k s c a u g h t a n d n u m b e r of s ta t ions s a m p l e d for long l ine c r u i s e s du r ing 1 9 7 8 - 1 9 8 2 in the w e s t e r n north P a c i f i c . T h e s e l ong l ines h a d b e t w e e n 1 5 0 0 - 1 8 0 0 h o o k s . A s s u m i n g a m e a n of 1,650 h o o k s p e r s ta t ion, h o o k i n g rates a v e r a g e d to 17 .62 b lue s h a r k s / 1 0 0 0 h o o k s , quite s imi l a r to the f igure c a l c u l a t e d from S t r a sbu rg ' s da ta . T h e e s t i m a t e d total b y c a t c h o f s h a r k s by t u n a l ong l i ne s in the Nor th P a c i f i c is c o m p a r a t i v e l y ve ry h igh . B a s e d o n the h o o k i n g rates d e r i v e d a b o v e from S t r a s b u r g (1958) a n d the effort e s t ima ted by N a k a n o a n d W a t a n a b e (1992) , a total o f 2 , 0 5 0 , 1 3 6 s h a r k s w o u l d h a v e b e e n caugh t dur ing 1 9 8 8 in the Nor th Pac i f i c . R o u g h l y , 1 '950,000 o f t h e s e w o u l d be b lue sha rks , 7 ,250 o c e a n i c whi te t ip s h a r k s a n d abou t 9 0 , 0 0 0 unident i f ied s h a r k s (table 2 .20) . T h e s e e s t ima te s a re c o n s e r v a t i v e w h e n c o m p a r e d to the e s t ima te s o f N a k a n o a n d W a t a n a b e for b lue s h a r k s t a k e n in the s a m e a r e a . A s s u m i n g a n a v e r a g e w e i g h t o f 2 0 kg / sha rk r e g a r d l e s s of s p e c i e s , the e s t i m a t e d total b y c a t c h w o u l d be of 4 1 , 0 0 0 1 . C a t c h e s by coun t ry are difficult to es t imate s i n c e it is i m p o s s i b l e to split by coun t ry the effort e s t i m a t e s o f N a k a n o a n d W a t a n a b e . A v e r y c r u d e es t imate b a s e d o n the p ropor t ions o f effort i nd i ca t e s that about 7 . 3 5 % of the c a t c h e s c o u l d per ta in to S o u t h K o r e a a n d the rest to J a p a n . T h e r e is no informat ion o n the d i s c a r d s of s h a r k s from t h e s e f i she r ies , o r the p e r c e n t a g e of s h a r k s r e l e a s e d a l i v e . G i v e n the par t i t ioning of F A O stat is t ical a r e a s in the P a c i f i c O c e a n , it is a l m o s t i m p o s s i b l e to a s s i g n c a t c h e s o f e l a s m o b r a n c h s repor ted by J a p a n a n d K o r e a , to that part o f the Nor th P a c i f i c a b o v e 2 0 ° N . H o w e v e r , e v e n the total r epor ted "var ious e l a s m o b r a n c h s " c a t c h e s for F A O a r e a s 6 1 , 6 7 a n d 7 7 by J a p a n a n d K o r e a (5 ,537 t a n d 2 ,927 t r e spec t ive ly ) , the e s t i m a t e d d i s c a r d w o u l d be of a b o u t 2 2 , 0 0 0 t. O v e r v i e w of long l ine f i sher ies . H i g h - s e a s long l ine f i she r i es for t unas a n d bi l l f i shes a re a ve ry la rge s o u r c e o f b y c a t c h e s a n d d i s c a r d s o f e l a s m o b r a n c h s in the w o r l d . D e s p i t e the uncer ta in ty s u r r o u n d i n g the different e s t ima t ions , it is ev iden t that the a m o u n t o f effort exe r t ed by l ong l i n ing f leets (wor ldwide total 149 of abou t 7 5 0 mi l l ion hooks /yr ) is the m a i n r e a s o n f o r t h e h igh b y c a t c h e s t ima te s . T h e best e s t ima tes a l l o w e d by the qual i ty o f b a s e l i n e informat ion, a re p r e s e n t e d in t ab le 2 . 2 1 . T h e g r a n d total o f e l a s m o b r a n c h s c a u g h t inc iden ta l ly by long l in ing f leets in a l l the h i g h - s e a s of the w o r l d is e s t i m a t e d at a l m o s t 8.3 mi l l ion f i shes o r a n a s t o n i s h i n g 2 3 2 , 4 2 5 t. T h i s r ep resen t s a l m o s t a third o f the total w o r l d c a t c h o f e l a s m o b r a n c h s r epor t ed in c o m m e r c i a l f i sher ies by F A O in 1 9 9 1 . T h e relat ive i m p o r t a n c e o f sha rk b y c a t c h e s in n u m b e r o f f i shes is a l m o s t e q u a l l y d is t r ibuted in the long l ine f i sher ies o f the w o r l d . T h e f i sher ies of the At l an t i c , Indian, T r o p i c a l a n d S o u t h P a c i f i c a n d Nor th P a c i f i c O c e a n s e a c h a c c o u n t for abou t 2 mi l l ion e l a s m o b r a n c h s . H o w e v e r , the total w e i g h t o f b y c a t c h e s in the A t l an t i c a n d Indian O c e a n s is e s t i m a t e d to be a lmos t d o u b l e that for the w h o l e P a c i f i c O c e a n (table 2 .21) . T h i s is d u e to the different m e a n w e i g h t s u s e d in the c a l c u l a t i o n s a n d d o e s not n e c e s s a r i l y r ep resen t a rea l dif ference in we igh t o f the c a t c h e s . Spec i f i ca l l y , the m e a n we igh t o f 2 0 k g / s h a r k u s e d for the Pac i f i c O c e a n f i sher ies is v e r y c o n s e r v a t i v e . T h e l eve l s of d i s c a r d a n d su rv iva l of r e l e a s e d s h a r k s a re a l s o unce r t a in . T h e a c c u m u l a t e d e s t ima tes o f d i s c a r d s from the long l ine f i sher ies t rea ted a b o v e a m o u n t to a total of 2 0 4 , 3 4 7 t. It is u n k n o w n w h a t propor t ion of t he se d i s c a r d s s u r v i v e s the g e a r but s o m e reports ind ica te it c o u l d be a s h igh a s 6 6 % ( B e r k e l e y a n d C a m p o s 1988) . N e v e r t h e l e s s , there are n u m e r o u s a c c o u n t s o f f inning in the li terature (Mejuto 1 9 8 5 , S t e v e n s 1 9 9 2 , N a k a n o 1993) a n d g i v e n the i n c r e a s e in sha rk fin p r i ces in the late 1980 ' s it w o u l d be n a i v e to think that a great part o f the s h a r k s r e l e a s e d wi l l ac tua l ly su rv ive . Fur the r r e s e a r c h is n e e d e d to clarify the extent o f the rea l kil ls o f s h a r k s in long l ine f i sher ies . T h e b y c a t c h e s o f b lue s h a r k s in long l ine f i sher ies is v e r y la rge . A l t h o u g h s p e c i e s b r e a k d o w n w a s not a l w a y s p o s s i b l e for r e a s o n s e x p l a i n e d a b o v e , a n a p p r o x i m a t i o n c a n be d o n e for t hose a r e a s w h e r e o n l y total s h a r k b y c a t c h w a s e s t i m a t e d if w e c o n s e r v a t i v e l y c o n s i d e r 4 0 % of the total b y c a t c h to be b lue s h a r k s . A d d i n g this f igure to the n u m b e r s a l r e a d y e s t ima ted of b lue s h a r k s c a u g h t in t h o s e f i sher ies w h e r e s p e c i e s b r e a k d o w n w a s p o s s i b l e , w e find a total e s t imate o f 4 ' 0 7 5 , 1 6 2 b lue s h a r k s caugh t inc iden ta l ly in the h i g h - s e a s long l ine f i sher ies of the w o r l d . 150 Table 2.20 Estimated bycatch of sharks in the North Pacific by the longline fleets of Japan and Korea, based on the results of Strasburg (1958). Strasburg's data* Estimated Catch in 1988 Numbers caught Hook rate Total Species (87,595 hooks) (sharks/1000 hooks) numbers weight (t) ** Prionace glauca 1,616 18.45 1,953,432 39,069 Carcharhinus longimanus 6 0.07 7,253 145 Various sharks 74 0.84 89,452 1,789 Totals 1,696 19.36 2,050,136 41,003 * for cruises north of 21 N ** assuming 20 kg/shark Table 2.21 Selected estimates of shark bycatches in high seas longline fisheries. Area Number of individuals Total catch in tonnes Atlantic Ocean 2,305,940 76,318 Indian Ocean 1,931,574 75,180 South/Central Pacific Ocean 1,996,350 39,927 North Pacific (above 20N) 2,050,135 41,000 Total 8,283,999 232,425 151 T h e o r d e r o f m a g n i t u d e o f p resen t e s t ima te s s e e m s to be in g e n e r a l a g r e e m e n t with p r ev ious a s s e s s m e n t s . A s a re fe rence point, T a n i u c h i (1990) e s t i m a t e s a total sha rk c a t c h from J a p a n e s e long l ine r s o f 9 0 , 0 0 0 t u s i n g a ratio o f sha rk -ca t ch / t a rge t - spec i e s c a t c h for the t u n a a n d billf ish long l ine f i shery . T h e w o r l d w i d e e l a s m o b r a n c h b y c a t c h e s t i m a t e d here for J a p a n e s e long l ine r s is o f 115,441 t. T h e r e is h o w e v e r a g o o d d e g r e e o f uncer ta in ty in t roduced by the qual i ty o f the b a s e l i n e informat ion a v a i l a b l e for the p re sen t e s t ima t ions . F o r e x a m p l e , the h o o k i n g ra tes u s e d here r ange b e t w e e n 7 . 0 4 - 2 0 . 4 5 s h a r k s / 1 0 0 0 h o o k s , w h e r e a s T a n i u c h i (1990) plots h o o k i n g ra tes for J a p a n e s e r e s e a r c h long l ine r s r a n g i n g b e t w e e n 2 .7 a n d 8 s h a r k s / 1 0 0 0 h o o k s . O n l y re l iab le effort f igures a n d u p d a t e d h o o k i n g rates by reg ion wi l l a l l o w to m a k e r e a s o n a b l y better e s t i m a t e s o f the b y c a t c h e s . In cont ras t wi th driftnet f i sher ies , there is no o b s e r v e r p r o g r a m m e for a n y o f the h i g h - s e a s long l ine f i she r i es in the w o r l d . T h i s a c c o u n t s for m u c h of the uncer ta in ty s u r r o u n d i n g the e s t ima te s of non- target s p e c i e s c a u g h t in long l ine f i sher ies . It is wor th no t ing that mos t of the in terna t ional t u n a o r g a n i z a t i o n s a n d the g o v e r n m e n t s o f long l ine f i sh ing na t ions m a n d a t i n g l o g b o o k repor ts f rom long l ine r f leets , still d o not requi re o r en fo rce the repor t ing of b y c a t c h e s o f s h a r k s o r o ther e l a s m o b r a n c h s . S o m e o f t h e s e o r g a n i z a t i o n s a re t ak ing s t eps to c h a n g e this s i tuat ion ( I C C A T 1 9 9 3 b , N a k a n o 1993) . T h i s s h o u l d he lp r e d u c e the uncer ta in ty a b o u t the rea l l eve l s o f b y c a t c h e s a n d d i s c a r d s in the n e a r future. C o n s i d e r i n g the c o m m o n under repor t ing o f e l a s m o b r a n c h s in long l ine r l o g b o o k s ( S t e v e n s 1 9 9 2 , N a k a n o 1993) , o b s e r v e r p r o g r a m m e s a re u n d o u b t e d l y the bes t w a y to t ack le this c r u c i a l informat ion p r o b l e m . 2 .2 .3 .3 P u r s e S e i n e F i s h e r i e s . M o s t o f the l a r g e - s c a l e pu r se s e i n e f i sher ies for t u n a o c c u r in t rop ica l w a t e r s w h e r e the rela t ively s h a l l o w s c h o o l i n g b e h a v i o u r o f s o m e t u n a s p e c i e s m a k e s t h e m e a s y to c a t c h . T h e m a i n s p e c i e s t a rge ted by pur se s e i n e a re the ye l lowf in (Thunnus albacares) a n d sk ip jack (Katsuwonus pelamis) t unas , a l t hough s o m e o ther s p e c i e s a r e a l s o c a p t u r e d in s m a l l e r quant i t ies . P u r s e s e i n e s a re v e r y unse l ec t i ve gea r s , s o that o the r f ish a n d non-f ish s p e c i e s (e.g. mar ine m a m m a l s ) c o m m o n l y a s s o c i a t e d with the t u n a s c h o o l s a r e f requent ly c a u g h t in the f i sh ing ope ra t i on . 152 M a j o r t u n a pu r se s e i n e f i sher ies a re fairly l o c a l i s e d . T h e y a r e c e n t r e d in four m a i n a r e a s (fig. 2 .41) : the E a s t e r n T r o p i c a l P a c i f i c ( E T P ) off M e x i c o d o w n to the north o f S o u t h A m e r i c a ; the W e s t e r n C e n t r a l P a c i f i c ( W C P ) from the P h i l i p p i n e s a n d P a p u a - N e w G u i n e a to P o l y n e s i a ; the w e s t e r n Indian O c e a n (WIO) a r o u n d the S e y c h e l l e s a n d the E a s t e r n T r o p i c a l A t l an t i c ( E T A ) a r o u n d the G u l f o f G u i n e a . A d d i t i o n a l l y , there is s o m e t u n a p u r s e s e i n i n g off V e n e z u e l a in the W e s t e r n A t l an t i c O c e a n . T h e E T P pur se s e i n i n g f ishery b e g a n du r ing the 1950 ' s . It e x p a n d e d la rge ly in the 1960 ' s a n d 1970 ' s but d e c l i n e d t empora r i ly in the ea r ly 1980 ' s . P r e s e n t l y , a b o u t 2 8 0 , 0 0 0 t o f ye l lowf in t u n a a re c a u g h t by pu r se s e i n e r s in this r eg ion ( S a k a g a w a a n d K l e i b e r 1992) . T h e fleet u s e d to be d o m i n a t e d by U S A v e s s e l s but s i n c e the ea r ly 1980 ' s m a n y o f t he se w e r e r ea l l oca t ed to the W C P f ishery a n d n o w M e x i c a n v e s s e l s a re d o m i n a n t . T u n a pu r se s e i n i n g w a s d e v e l o p e d in the W C P by J a p a n e s e a n d U S A v e s s e l s in the 1970 ' s . In cont ras t to the E T P , effort here is l a rge ly d i r ec t ed t o w a r d s sk ip j ack t u n a a l t hough ye l lowf in t u n a is a l s o c a u g h t in la rge a m o u n t s . T h e J a p a n e s e fleet f i shes ma in ly l o g -a s s o c i a t e d s c h o o l s w h e r e a s U S A boa t s concen t r a t e o n f r e e - s w i m m i n g s c h o o l s ( S a k a g a w a a n d K l e i b e r 1992) . K o r e a n a n d T a i w a n e s e pu r se s e i n e r s h a v e j o i n e d the f i shery s i n c e the late 1970 ' s ( S u z u k i 1988) . T h e s e four coun t r i e s c o m p r i s e the major part o f the fleet, with s m a l l e r n u m b e r s o f v e s s e l s ope ra t i ng u n d e r the f lags o f A u s t r a l i a , I n d o n e s i a , P h i l i p p i n e s , M a r s h a l l I s lands , N e w Z e a l a n d , S o l o m o n Is lands a n d the fo rmer U . S . S . R . T h e total ca t ch o f t unas by pu r se s e i n e r s in the W C P dur ing 1 9 8 9 w a s 5 7 6 , 2 0 4 1 , at leas t 7 3 % w a s sk ip jack t u n a ( L a w s o n 1991) . In the W I O , the f i shery w a s s tar ted by a M a u r i t i u s - J a p a n pu r se s e i n e r in 1 9 7 9 , f o l l o w e d by F r e n c h v e s s e l s in 1 9 8 0 . B y 1984 a l l the F r e n c h fleet m o v e d from the A t l a n t i c to the W I O , toge ther with part o f the S p a n i s h fleet. D u r i n g 1989 , F r a n c e , S p a i n , P a n a m a , J a p a n , Maur i t iu s , U . S . S . R . a n d C a y m a n Is land h a d 4 9 pur se s e i n e r s in this f i shery , wi th the first two coun t r i e s d o m i n a t i n g the fleet. T h e total c a t c h e s in the W I O w e r e s o m e 2 2 0 , 0 0 0 t o f t unas (yel lowfin a n d sk ip jack main ly , but a l s o s o m e b igeye ) in 1 9 8 9 ( I P T P 1990) . P u r s e s e i n e f i sh ing for t unas in the t rop ica l A t l an t i c w a s ini t ia ted by F r e n c h f i s h e r m e n in the ea r ly 1960 ' s in the c o a s t a l w a t e r s o f the G u l f o f G u i n e a . A f r i c a n c o a s t a l s ta tes , S p a n i s h , a n d 153 154 U S A fleets j o i n e d later. T h e f i shery e x p a n d e d to offshore a r e a s at the e n d o f the 1970 ' s a n d it cur rent ly a c c o u n t s for m o r e than 8 0 % of the ye l lowf in t u n a c a t c h e s o f the A t l a n t i c ( S u z u k i 1988) . A t p resen t the majority o f the c a t c h e s a re t a k e n by the S p a n i s h a n d F r e n c h - l v o r i a n -S e n e g a l e s e - M o r o c c a n ( F I S M ) fleets, wi th s m a l l a m o u n t s con t r ibu ted by V e n e z u e l a n , U . S . S . R . a n d J a p a n e s e boa ts . Y e l l o w f i n a n d sk ip jack a re the m a i n t a rge ted s p e c i e s , wi th m i n o r b y c a t c h e s o f b i g e y e t una . A total o f 1 6 7 , 8 0 0 1 o f t u n a s w e r e c a u g h t by pu r se s e i n e r s in the t rop ica l A t l a n t i c dur ing 1989 ; at leas t 9 0 % of this c a m e from the ea s t e rn At l an t i c ( I C C A T 1 9 9 1 a , 1 9 9 1 b , 1992) . Information o n the e l a s m o b r a n c h b y c a t c h e s in pu r se s e i n e t u n a f i she r i es is appa l l i ng ly s c a r c e . E v e n t h o u g h the p r e s e n c e o f s h a r k s in the pu r se s e i n e c a t c h e s is d o c u m e n t e d at least s i n c e the m i d - 1 9 6 0 ' s , it h a s r e c e i v e d v e r y p o o r a t tent ion in the l i terature. B a n e (1966) reports s e v e r a l l a rge s i lky a n d o ther s h a r k s , a n d m a n t a rays t a k e n in a pu r se s e i n e set off G a b o n in 1 9 6 1 . B a n e a l s o men t ions C . limbatus, C. plumbeus a n d Rhizoprionodon acutus are a s s o c i a t e d with t u n a s c h o o l s in the a r e a . Y o s h i m u r a a n d K a w a s a k i (1985) report 183 s i lky s h a r k s c a u g h t by pu r se s e i n e f i sh ing in the W C P a n d length f r e q u e n c y h i s t og rams indica te that mos t s i lky s h a r k s w e r e b e t w e e n 6 0 a n d 170 c m T L wi th a m o d e o f 1 1 0 - 1 3 0 c m T L . F o r t h e Indian O c e a n , L a B l a c h e a n d K a r p i n s k i (1988) b a s e d o n o b s e r v e r p r o g r a m m e da ta , report b y c a t c h rates o f 6% of the total c a t c h for pu r se s e i n e r s that h a d b y c a t c h e s . T h e y c o n s i d e r v a r i o u s te leos t s , i nc lud ing u n d e r s i z e d a n d d a m a g e d t una , a s part of the b y c a t c h . O c e a n i c whi te t ip s h a r k s w e r e the s e c o n d major part (12%) o f the b y c a t c h . T h e mos t de t a i l ed a c c o u n t of s h a r k s a s s o c i a t e d wi th t u n a s c h o o l s is p r o v i d e d by A u (1991) for the E T P . A c c o r d i n g to A u , s h a r k s form a s s o c i a t i o n s with ye l lowf in t u n a s that c o u l d be of a n oppor tun i s t i c p r e d a t o r - s c a v e n g e r nature . T h e a s s o c i a t i o n o f s h a r k s with ye l lowfin tunas , m e a s u r e d a s p e r c e n t a g e of se t s h a v i n g s h a r k s , is 4 0 % for l o g - a s s o c i a t e d tuna s c h o o l s , 6 - 2 1 % for free s c h o o l s a n d to 1 3 % for d o l p h i n - a s s o c i a t e d s c h o o l s . A p p a r e n t l y , t h e se a s s o c i a t i o n s are l imi ted by the s w i m m i n g s p e e d o f s h a r k s . T h e s i lky s h a r k w a s the mos t c o m m o n e l a s m o b r a n c h in the b y c a t c h e s with up to 5 0 0 i n d i v i d u a l s c a u g h t per set. V a r i o u s o ther c a r c h a r h i n i d s , o c e a n i c whitet ip , s p h y r n i d , a l o p i d , l a m n i d , b lue a n d w h a l e s h a r k s w e r e a l s o c a u g h t toge ther with s e v e r a l ba to ids a n d m o b u l i d s . Unfor tunate ly , A u ' s report fai ls to p rov ide a n y useful m e a s u r e of the n u m b e r s o f s h a r k s c a u g h t by pu r se s e i n e f i sher ies (i.e. c a t c h o f s h a r k s per unit o f effort, o r p ropor t ion o f e l a s m o b r a n c h c a t c h to t una 155 ca tch) . A l t h o u g h he lists a v e r a g e n u m b e r s o f e a c h s h a r k s p e c i e s p e r set, t h o s e v a l u e s r ep resen t o n l y pu r se s e i n e se t s that c a u g h t that par t icu la r s p e c i e s . Wi thou t a n y reference in his p a p e r to the total n u m b e r s o r w e i g h t s o f s h a r k s in the full s a m p l e , h is resul ts a re of ve ry l imi ted u s e for the p u r p o s e of e s t ima t ing sha rk b y c a t c h ra tes . T h e total b y c a t c h e s o f e l a s m o b r a n c h s in pu r se s e i n e f i sher ies c a n be e s t i m a t e d in a ve ry rough w a y u s i n g the informat ion o n sha rk a n d t u n a c a t c h p r o v i d e d by L a b l a c h e a n d K a r p i n s k i (1988) . F r o m their da ta , s h a r k c a t c h is c a l c u l a t e d to be 0.51 % of the total t una c a t c h kept by the pu r se s e ine r s . U s i n g this propor t ion a n d the repor ted t u n a c a t c h e s l is ted in e a c h f ishery, the e s t i m a t e d total c a t c h o f s h a r k s in pu r se s e i n e f i she r i es du r ing 1 9 8 9 is o f 6 ,345 t: 8 5 6 t in the t rop ica l At lan t i c , 1,122 t in the W e s t e r n Indian O c e a n , 2 , 9 3 9 1 in the W e s t e r n C e n t r a l P a c i f i c a n d 1,428 t in the E a s t e r n T r o p i c a l P a c i f i c . T h e a b o v e e s t i m a t e s a s s u m e that the a m o u n t o f s h a r k s c a u g h t is d i rec t ly p ropor t iona l to t u n a c a t c h e s . A l t h o u g h this is a ve ry w i l d a s s u m p t i o n , it is wor thwh i l e g i v i n g it a thought . P u r s e s e i n i n g is e s sen t i a l l y a n ac t ive f i sh ing m o d e that t a k e s a d v a n t a g e o f the s c h o o l i n g b e h a v i o u r o f f i sh . S h a r k s a re k n o w n to ga the r a r o u n d t u n a s c h o o l s , e s p e c i a l l y l o g -a s s o c i a t e d s c h o o l s ( A u 1991) s o that sha rk c a t c h wi l l a l w a y s d e p e n d o n tuna c a t c h . C o n t r a r y to the c a s e o f p a s s i v e g e a r s ( longl ine , driftnet), s h a r k c a t c h e s in pu r se s e i n e do not o c c u r wi thout t u n a c a t c h e s ( f i shermen n e v e r set the g e a r if there a re no t u n a s c h o o l s ) . A c c o r d i n g l y , it s e e m s m o r e appropr i a t e to relate the s h a r k c a t c h to the t u n a c a t c h rather than to a n effort v a r i a b l e (usua l ly d a y s at s e a for pu r se s e ine r s ) a s in the c a s e o f p a s s i v e g e a r s w h e r e there is compe t i t i on for a h o o k o r s p a c e in the gi l l net. T h e m a i n w e a k n e s s of the p resen t e s t ima t ions , is to b a s e the c a l c u l a t i o n s in a s ing le ( and poor ly representa t ive) a c c o u n t o f the propor t ion o f sha rk c a t c h to t una c a t c h in pu r se s e i n e ope ra t i ons , a n d the ex t rapo la t ion o f W e s t e r n Indian O c e a n da t a to o the r g e o g r a p h i c a l a r e a s . T h e s e a s s e s s m e n t s wi l l i m p r o v e o n l y w h e n m o r e informat ion o n c a t c h ra tes b e c o m e s a v a i l a b l e a n d a s o u r u n d e r s t a n d i n g o f the s e a s o n a l a n d spa t ia l c h a n g e s in the s h a r k - t u n a a s s o c i a t i o n s i n c r e a s e s . T h e r e a re n o r e c o r d s of the cond i t ion of the e l a s m o b r a n c h s c a u g h t in t u n a pu r se s e i n e ope ra t ions , but it is v e r y l ikely that a l l of t h e m die e i ther by suffocat ion o r c r u s h i n g , w h e n they do not m a n a g e to bite thei r w a y out o f the nets . A l t h o u g h B a n e (1966) reports that the 156 sha rk c a t c h e s w e r e s o l d at s h o r e in the G u l f o f G u i n e a , this s e e m s to be a n e x c e p t i o n u n d e r a n e x p e r i m e n t a l f i sh ing c a m p a i g n . M o s t o f the s h a r k c a t c h e s in c o m m e r c i a l t una pu r se s e i n e f i sher ies a re p r o b a b l y d i s c a r d e d . H o w e v e r , this c a n n o t be c o n f i r m e d from the a v a i l a b l e informat ion . 2 .2 .3 .4 O t h e r m i s c e l l a n e o u s f i sher ies . T h e p r e c e d i n g s e c t i o n s t rea ted t h o s e f i sher ies r e s p o n s i b l e for the la rges t b y c a t c h e s a n d d i s c a r d s o f e l a s m o b r a n c h s (main ly sha rks ) o n a g l o b a l s c a l e . H o w e v e r , there a re o ther f i sher ies w h i c h inc iden ta l ly t ake e l a s m o b r a n c h s a n d a re wor th m e n t i o n i n g . P o l e a n d l ine f i sher ies for t unas t ake s o m e s h a r k b y c a t c h e s w h i l e f i sh ing t u n a s c h o o l s ( A n d e r s o n a n d T e s h i m a 1990) . Unfor tunate ly , a l m o s t no th ing is k n o w n abou t the c a t c h rates. B a n e (1966) m e n t i o n s s h a r k s t a k e n by "tuna c l ippers . . . a t the su r face o n live bait", w h i c h s u g g e s t s po le a n d l ine f i sh ing : 131 s h a r k s w e r e t a k e n at s i x s ta t ions by this m e t h o d . It is p o s s i b l e , d u e to the g l o b a l s c a l e of po le a n d l ine f i sher ies for t u n a s that thei r b y c a t c h of s h a r k s c o u l d be s ignif icant , p e r h a p s in the o r d e r of m a g n i t u d e o f the b y c a t c h from pur se s e ine r s . O n the o ther h a n d , po le a n d l ine g e a r m a y a v o i d the cap tu re o f s h a r k s a n d su rv iva l of d i s c a r d s c o u l d be h igh . B o t h factors w o u l d m i n i m i s e the i m p a c t o f the po le a n d l ine f i shery o n s h a r k s . T h e o r a n g e r o u g h y (Hoplostethus atlanticus) f i shery o f N e w Z e a l a n d t a k e s d e e p wa te r s q u a l o i d s h a r k s a n d o ther e l a s m o b r a n c h s in thei r bot tom t rawl nets . A l t h o u g h there are no e s t ima tes o f c a t c h ra tes o r propor t ion o f the c a t c h e s of t h e s e s h a r k s for the c o m m e r c i a l t rawlers , there is s o m e informat ion from r e s e a r c h v e s s e l s . A t leas t 21 e l a s m o b r a n c h s (11 s e l a c h i a n s , 4 ba to ids a n d 6 h o l o c h e p h a l a n s ) h a v e b e e n ident i f ied in d e e p w a t e r t rawl s u r v e y s a r o u n d N e w Z e a l a n d ( R o b e r t s o n et a l . 1984) . T h e r e a r e e ight s q u a l o i d s h a r k s that w o u l d h a v e c o m m e r c i a l impor t ance , of w h i c h Deania calcea is the m o s t a b u n d a n t in the Nor th Is land, Etmopterus baxteri in the S o u t h Is land a n d Centroscymnus spp. in the cen t ra l a r e a s . S u r v e y s in the Nor th Is land indica te that Deania calcea cons t i tu tes a l a rger part o f the total c a t c h e s t han e i ther o f the mos t important c o m m e r c i a l s p e c i e s , the o r a n g e roughy a n d the hok i (Macruronus novaezelandiae), ( C l a r k a n d K i n g 1989) . A l t h o u g h c a t c h ra tes in c o m m e r c i a l t r awl ing o p e r a t i o n s s h o u l d be s m a l l e r than in r e s e a r c h c r u i s e s d u e to more 157 ta rge ted f i sh ing , it is p o s s i b l e that the b y c a t c h e s o f e l a s m o b r a n c h s cons t i tu te b e t w e e n 10 a n d 5 0 % of the o r a n g e roughy c a t c h e s . A c c o r d i n g to recen t F A O sta t is t ics , the o r a n g e roughy c a t c h e s in N e w Z e a l a n d wa t e r s w e r e o f a r o u n d 4 4 , 0 0 0 t/yr du r ing 1 9 8 4 - 1 9 8 9 . T h e total b y c a t c h o f s q u a l o i d s h a r k s c o u l d therefore be b e t w e e n 4 , 4 0 0 a n d 2 2 , 0 0 0 t/yr in this f ishery. K i n g a n d C l a r k (1987) es t imate the M S Y for t he se s h a r k s t o c k s a s 2 , 2 5 0 t/yr. Ev iden t ly , the current c a t c h e s e x c e e d by far the e s t i m a t e d M S Y . M o s t o f the c a t c h e s are th rown o v e r b o a r d a s there is no marke t for t h e m , a l t h o u g h s m a l l quant i t ies a re u s e d for f i s h m e a l a n d l iver o i l ex t rac t ion . G i v e n the dep th from w h i c h t h e s e s h a r k s a re brought up (600-1 ,200 m) a n d the type of g e a r e m p l o y e d , a l l c a t c h e s a re p r o b a b l y d e a d w h e n re turned to the s e a . T h e impac t o f this l eve l o f b y c a t c h o n the l o c a l s t o c k s o f d e e p - s e a s h a r k s is poor ly u n d e r s t o o d . O n e c a n pos tu la te that it is l ikely to be h igh ly d a m a g i n g a n d un l ike ly to l e a d to su s t a inab l e exp lo i t a t ion . H o w e v e r , this is difficult to verify w h e n there is vir tual ly no information a b o u t the a b u n d a n c e , b io logy a n d popu la t i on d y n a m i c s o f t he se d e e p w a t e r s p e c i e s . M o r e r e s e a r c h is n e e d e d o n the ac tua l l eve l s of b y c a t c h , s u r v i v a l o f d i s c a r d s a n d abou t the popu la t i on d y n a m i c s of t he se d e e p sha rk p o p u l a t i o n s . 2 .2 .3 .5 O v e r v i e w . Es t ima t ing the total b y c a t c h a n d d i s c a r d o f e l a s m o b r a n c h s in h i g h - s e a s f i she r i es w o r l d w i d e is difficult b e c a u s e ne i ther o f t he se p r o c e s s e s are a d e q u a t e l y d o c u m e n t e d . T h e rates of d i s c a r d , f inning a n d su rv iva l a re vir tual ly u n k n o w n . T h e r e a re la rge uncer ta in t i es abou t the c a t c h rates a n d effort l eve l s by r eg ion . W e s h o u l d e x p e c t qual i ta t ive a n d quanti tat ive va r i a t ions in the e l a s m o b r a n c h b y c a t c h e s wi th in e a c h o c e a n d u e to a r e a l a n d s e a s o n a l c h a n g e s in ava i lab i l i ty o f the different s p e c i e s . Unfor tunate ly , t h e s e s o u r c e s o f var iabi l i ty c o u l d not be t a k e n into a c c o u n t in the p resen t w o r k wi th the a v a i l a b l e informat ion . F o r t he se r e a s o n s , the resul ts o b t a i n e d here s h o u l d be u s e d with c au t i on a s t hey a r e o n l y indica t ive o f the o r d e r o f m a g n i t u d e o f the b y c a t c h e s . P r e s e n t resul ts ind ica te that a ve ry large a m o u n t o f e l a s m o b r a n c h s a r e c a u g h t inc identa l ly in the h i g h - s e a s f i sher ies o f the w o r l d . T h e e s t ima ted g r a n d total o f e l a s m o b r a n c h b y c a t c h 158 at the e n d o f the 1980 ' s , is a r o u n d 2 6 0 , 0 0 0 a n d 3 0 0 , 0 0 0 1 o r 11 .6 -12 .7 mi l l ion fish per yea r . M o s t of t h e s e c a t c h e s a re s h a r k s , p r edominan t l y b lue s h a r k s . L o n g l i n e f i she r i es a re the mos t important s o u r c e o f s h a r k kil ls in the h i g h - s e a s , ma in ly b e c a u s e of the m a g n i t u d e of thei r effort. T h e y contr ibute a b o u t 8 0 % o f the e s t i m a t e d total e l a s m o b r a n c h b y c a t c h in we igh t a n d abou t 7 0 % in n u m b e r s o f f i sh . T h e r e is la rge uncer ta in ty a r o u n d the b y c a t c h e s t ima te s for this type of f i she r ies . H o w e v e r , the f igures are b a s e d o n the bes t a v a i l a b l e information a n d they s e e m to c o m p a r e w e l l wi th the few re fe rence poin ts at h a n d . T h e fo rmer h i g h - s e a s driftnet f i sher ies r a n k e d s e c o n d for thei r con t r ibu t ion to the total e l a s m o b r a n c h b y c a t c h e s . S i n c e t h e s e f i sher ies w e r e t e r m i n a t e d w o r l d w i d e at the e n d of 1992 , they a re n o w o n e l e s s p r o b l e m to w o r r y abou t in t e r m s o f sea- l i fe c o n s e r v a t i o n . It w o u l d be in teres t ing to k n o w the fate o f the v e s s e l s former ly e n g a g e d in driftnet f i sher ies s i n c e 1 9 9 2 : it is p o s s i b l e that this effort h a s b e e n red i rec ted to f i she r i es w h i c h might still impac t e l a s m o b r a n c h s a n d the o ther s p e c i e s p r e v i o u s l y affected by gi l lnet t ing act iv i t ies . D i s c a r d s from h i g h - s e a s f i sher ies a l s o a p p e a r to be v e r y h igh . T h e f igures s u g g e s t that up to 2 3 0 , 0 0 0 - 2 4 0 , 0 0 0 t of e l a s m o b r a n c h s a re d i s c a r d e d e v e r y y e a r in the v a r i o u s h i g h - s e a s f i sher ies . M o s t o f the d i s c a r d s a re p robab ly d e a d , a l m o s t ce r ta in ly t h o s e c a u g h t in the driftnet, pu r se s e i n e a n d o r a n g e r o u g h y f i sher ies . F o r long l ine f i she r ies , su rv iva l d e p e n d s o n w h e t h e r f i s h e r m e n r e l e a s e s h a r k s read i ly a n d u n h a r m e d . N e v e r t h e l e s s , c o m m o n f inning p rac t i ces m a k e d u b i o u s that su rv iva l is h igh in long l ine o p e r a t i o n s . A v a i l a b l e informat ion o n pur se s e i n e a n d po le -and- l ine t u n a f i she r i es a n d the d e e p t rawl f i sher ies for o r a n g e r o u g h y m a k e it ve ry difficult to a s s e s s the i m p o r t a n c e o f thei r b y c a t c h e s of s h a r k s a n d rays . P r e s e n t l y , they s e e m to sha re a m i n o r part o f the total b y c a t c h of e l a s m o b r a n c h s but there is a b ig g a p in direct informat ion o n this subject . M o r e , b a s i c r e s e a r c h , is n e e d e d in this f ie ld . T h e r e is a n o t h e r subs tan t i a l s o u r c e of b y c a t c h a n d w a s t e o f s h a r k s a n d r ays a r o u n d the w o r l d . T h i s is the inc iden ta l c a t c h o f bot tom t rawl ing v e s s e l s f i sh ing for s h r i m p s a n d f i shes in con t inen ta l s h e l v e s a r o u n d the w o r l d . T h e a s s e s s m e n t o f the e l a s m o b r a n c h b y c a t c h e s 159 in t he se f i sher ies is out the s c o p e o f this w o r k pr imar i ly b e c a u s e o f the ex f reme difficulty in ga the r ing informat ion abou t t h e m , a n d the m a g n i t u d e o f the w o r k i n v o l v e d . T h e s e f i sher ies a re k n o w n to be o f h igh impac t to l oca l popu la t i ons , s p e c i a l l y in the c a s e o f rays ( see a c c o u n t s o f Br i t i sh a n d T h a i f i sher ies in s e c t i o n s 2 .2 .2 .2 a n d 2 .2 .2 .4) . S o m e of t he se c a t c h e s o f e l a s m o b r a n c h s a re l a n d e d a n d repor ted u n d e r official s ta t is t ics o f the f i sh ing count ry . H o w e v e r , a la rge propor t ion is just d u m p e d at s e a , a n d is n e v e r a c c o u n t e d for. S p e c i e s o f e l a s m o b r a n c h s u n d e r p r e s s u r e from h i g h - s e a s f i she r ies . B l u e s h a r k s a re the mos t c o m m o n e l a s m o b r a n c h c a u g h t inc iden ta l ly in h i g h - s e a s f i sher ies . P r e s e n t e s t i m a t e s ind ica te that 6 .2-6.5 mi l l ion b lue s h a r k s a re t a k e n a n n u a l l y w o r l d w i d e in t hese f i she r ies . A l t h o u g h this is appa ren t l y the first e s t imate o f total c a t c h e s for b lue s h a r k s in a l l h i g h - s e a s f i she r i es of the w o r l d , there a re a c o u p l e o f part ia l e s t i m a t e s w h i c h c a n be u s e d f o r c o m p a r i s o n . S t e v e n s (1992) e s t ima te s that the J a p a n e s e long l ine f i sher ies a n n u a l l y take a total o f 4 3 3 , 4 4 7 b lue s h a r k s . T h i s figure a p p e a r s s m a l l c o m p a r e d with that e s t ima ted here . H o w e v e r , he c o n s i d e r s a c o n s e r v a t i v e h o o k i n g rate o f o n l y 1 s h a r k / 1 0 0 0 h o o k s . N a k a n o a n d W a t a n a b e (1992) es t imate that a l l the h i g h - s e a s f i she r i e s o f the Nor th Pac i f i c O c e a n c a u g h t 5 mi l l ion b lue s h a r k s dur ing 1 9 8 8 . In this c a s e , thei r e s t ima te a p p e a r s h igh aga in s t the p resen t resul ts . H e n c e , the a s s e s s m e n t o f b lue s h a r k b y c a t c h pe r fo rmed here s e e m s to be wi th in r e a s o n a b l e v a l u e s . O u r current k n o w l e d g e p reven t s a n a s s e s s m e n t of the impac t that the r e m o v a l of 6 mil l ion b lue s h a r k s a n n u a l l y h a s o n h i g h - s e a s e c o s y s t e m s o r o n the b lue s h a r k popu la t i ons . T h e r e is vir tual ly no th ing k n o w n abou t the s i z e of the s tocks o f b lue s h a r k s a n y w h e r e in the w o r l d , a n d the b i o l o g y o f mos t s t o c k s is poor ly u n d e r s t o o d . N a k a n o a n d W a t a n a b e (1992) pe r fo rmed the o n l y a s s e s s m e n t k n o w n to date of the impac t o f h i g h - s e a s f i sher ies in b lue s h a r k s t o c k s . Af ter e s t ima t ing b y c a t c h e s a n d u s i n g cohor t a n a l y s i s , t hey c o n s i d e r that the ca t ch l eve l s du r ing the late 1980 ' s d id not h a v e a s ignif icant impac t o n the popu la t i ons of the Nor th P a c i f i c . H o w e v e r , W e t h e r a l l a n d S e k i (1992) a n d A n o n y m o u s (1992) c o n s i d e r that appropr i a t e informat ion is l a c k i n g for a n a s s e s s m e n t of this k i n d . M o r e r e s e a r c h is bad ly n e e d e d both to a s s e s s the rea l b y c a t c h l eve l s in e a c h f i shery a n d the i r i m p a c t s o n the different popu la t i ons . 160 S i l k y s h a r k s a re p r o b a b l y the s e c o n d mos t c o m m o n s h a r k b y c a t c h , s p e c i a l l y in long l ine a n d pur se s e i n e f i she r ies . A s in the c a s e o f b lue s h a r k s , app ropr i a t e in format ion is l a c k i n g to a s s e s s the i m p a c t s o f the r e m o v a l l eve l s . In a n y c a s e , thei r g rowth a n d reproduc t ion c o m p a r e poor ly to t h o s e of b lue s h a r k s , i.e. s i lky s h a r k s h a v e s l o w e r g rowth , a later s e x u a l matura t ion a n d a r e m u c h l e s s f e c u n d (see Pratt a n d C a s e y [1990] for a c o m p i l a t i o n of life his tory p a r a m e t e r s o f sha rk s ) . H e n c e , they a re e x p e c t e d to be l e s s res i l ient to explo i ta t ion than b lue s h a r k s . A g a i n , m o r e r e s e a r c h is n e e d e d before a n y c o n c l u s i o n s c a n be d r a w n abou t the effects of t h e s e f i sher ies o n s i lky s h a r k popu la t i ons . L o c a l s t o c k s of Deania calcea, Etmopterus baxteri a n d Centroscymnus spp. in N e w Z e a l a n d c o u l d be a d d e d to the list o f e l a s m o b r a n c h s u n d e r p o s s i b l e threat by l a r g e - s c a l e f i she r ies . 2.3 D i s c u s s i o n . 2.3.1 C u r r e n t S i tua t ion o f E l a s m o b r a n c h F i s h e r i e s . S e v e r a l fea tures w e r e identif ied th roughout this r ev i ew. F i s h e r i e s for s h a r k s a n d rays are ve ry c o m m o n th roughou t the w o r l d a n d ve ry d ive r se in r ega rd to the s p e c i e s t a k e n a n d to the t ypes o f f i sh ing g e a r s a n d v e s s e l s u s e d . Unfor tunate ly , this d ivers i ty con t r ibu tes to the difficulty for k e e p i n g the appropr i a t e s tat is t ics o f y i e l d a n d a b u n d a n c e e s s e n t i a l fo r the s tudy of t he se f i she r ies . T h i s is par t icular ly ev iden t in the sca rc i ty o f informat ion a v a i l a b l e for mos t of the c a s e s r e v i e w e d here . V e r y few coun t r i e s h a v e sufficient informat ion abou t thei r sha rk a n d ray f i sher ies for a s s e s s m e n t p u r p o s e s , a n d in mos t c a s e s the informat ion is still d i s p e r s e d . S ta t i s t i cs for e l a s m o b r a n c h s a r o u n d the w o r l d n e e d to be i m p r o v e d : c a t c h s h o u l d be repor ted by major s p e c i e s a n d s p e c i e s g roups ; the e l a s m o b r a n c h b y c a t c h from high s e a s l a rge - sca l e f i sher ies s h o u l d a l s o be c o m p u l s o r y repor ted . T h e latter c o u l d be a c h i e v e d by the e s t a b l i s h m e n t o f o b s e r v e r p r o g r a m m e s for mos t h i g h - s e a s f i she r ies . A d d i t i o n a l l y , m o r e c o m p i l a t i o n a n d r e v i e w w o r k n e e d s to be d o n e o n a coun t ry a n d r e g i o n a l b a s i s to set the g r o u n d for a bet ter a p p r a i s a l o f explo i ta t ion l eve l s a n d in o r d e r to m a k e a n ove ra l l a s s e s s m e n t o f the s ta tus o f e l a s m o b r a n c h s t o c k s a r o u n d the w o r l d . A n o t h e r impor tant cha rac te r i s t i c brought out by the r e v i e w is the p r e d o m i n a n t l y inc iden ta l nature o f the c a t c h e s o f e l a s m o b r a n c h s . T h e n u m b e r of f i she r i es w h i c h spec i f i ca l ly a n d pr imar i ly target s h a r k s o r rays a r o u n d the w o r l d c a n p r o b a b l y be c o u n t e d with f ingers . M o s t 161 of the c a s e s e x a m i n e d in the p r e c e d i n g s e c t i o n s indica te that the vas t majori ty o f f i sher ies for s h a r k s a n d rays , a n d su re ly m o s t o f the w o r l d c a t c h e s , a r e in fact the p roduc t o f f i sher ies for o the r s p e c i e s . T h i s m a k e s their a s s e s s m e n t but e s p e c i a l l y the i r m a n a g e m e n t ve ry difficult. F e w m a n a g e r s wi l l cons t r a in e c o n o m i c a l l y o r s o c i a l l y impor tant f i she r i es in o rde r to m a n a g e e l a s m o b r a n c h s s u s t a i n a b l y . T h e resul ts from the a n a l y s e s of y i e l d t r ends in e a c h F A O M a j o r F i s h i n g A r e a of s ec t ion 2.2.2.1 s u g g e s t that a n e x p a n s i o n o f the c a t c h e s c o u l d be a c h i e v e d in s o m e A r e a s a n d to a l e s s e r extent o n a g l o b a l s c a l e . N e v e r t h e l e s s , l o c a l s t o c k s in s e v e r a l par ts of the w o r l d (North Indian O c e a n , Nor th S e a , Nor th E a s t At lan t ic ) a re p r o b a b l y o v e r e x p l o i t e d a n d c a t c h e s there a re e x p e c t e d to d e c r e a s e . H o w e v e r , t he se a n a l y s e s a re v e r y r o u g h a n d mus t be u s e d with c a u t i o n . In th is context , a better i n d e x o f relat ive p roduc t ion ( IRP) c o u l d be d e v e l o p e d in o r d e r to m a k e a better "quick a n d dirty" a s s e s s m e n t o f the poss ib i l i t i e s for e l a s m o b r a n c h explo i ta t ion in the w o r l d . A s i m p l e i m p r o v e m e n t w o u l d be to incorpora te in the I R P the a r e a of con t inen ta l she l f o f e a c h M a j o r F i s h i n g A r e a in o r d e r to w e i g h t the ha rves t o f s h a r k s a n d rays t a k e n , in a s imi l a r w a y in w h i c h the total sur face o f s e a o f e a c h A r e a w a s u s e d here . T h e i n c r e a s i n g g l o b a l t r end in repor ted sha rk a n d ray c a t c h e s s u g g e s t s that ove ra l l y i e l d s c o u l d be e x p e c t e d to con t inue r i s ing a s there is no s i gn of d e c l i n e in y i e l d . T h i s w o u l d be m i s l e a d i n g if in terpre ted uncr i t ica l ly . A c l o s e r look at the e l a s m o b r a n c h s f i sher ies in v a r i o u s coun t r i e s r e v e a l s c h a n g e s in the t ypes of f i sher ies a n d s p e c i e s e x p l o i t e d . W h i l e s o m e f i sher ies for e l a s m o b r a n c h s c o l l a p s e , o thers a re d e v e l o p e d e l s e w h e r e . T h i s ind ica tes that explo i ta t ion l eve l s a re not b e i n g s u s t a i n e d in a l l c a s e s . A l m o s t 3 0 % of the major f i sh ing coun t r i e s a n a l y s e d in s e c t i o n 2 .2 .1 .2 s h o w a fa l l ing t r end o n c a t c h e s . It is v e r y l ikely that the i n c r e a s e in w o r l d c a t c h e s might h a v e c o m p o n e n t s o ther t han a n a b s o l u t e i n c r e a s e in c a t c h . P o s s i b l e r e a s o n s for a n a p p a r e n t i n c r e a s e c o u l d be i m p r o v e m e n t s in the repor t ing of c a t c h e s a n d i n c r e a s e d l a n d i n g s of b y c a t c h e s in o ther f i she r ies . T h e l i k e l i h o o d that e l a s m o b r a n c h s wi l l be s u s t a i n a b l y e x p l o i t e d in the n e a r future is not ve ry p r o m i s i n g . T h e r e is in g e n e r a l a l ack of m a n a g e m e n t a n d r e s e a r c h d i r ec t ed t o w a r d s t he se fragile r e s o u r c e s . T h i s r a i s e s s e r i o u s doub t s abou t the future o f s h a r k a n d ray f i sher ies . O n l y 3 out o f 2 6 major e l a s m o b r a n c h - f i s h i n g coun t r i e s (Aus t ra l i a , U S A a n d N e w Z e a l a n d ) are k n o w n to h a v e m a n a g e m e n t a n d r e s e a r c h p r o g r a m m e s for thei r e l a s m o b r a n c h f i sher ies . 162 Surp r i s ing ly , not o n e of t he se three coun t r i e s p l a y s a l e a d i n g role in w o r l d w i d e e l a s m o b r a n c h y i e l d . M o r e o v e r , fo r the few coun t r i e s that d o h a v e s o m e f i she r i es information this i nd ica te s a p p a r e n t p r o b l e m s o f over -exp lo i t a t ion for s o m e e l a s m o b r a n c h s t o c k s (e.g. sha rk f i sher ies in sou the r B r a z i l , in both c o a s t s of the U S A a n d in sou the rn Aus t r a l i a ) . Unfor tunate ly , m a n y of the coun t r i e s p l a y i n g the major ro les in e l a s m o b r a n c h f i sher ies w o r l d w i d e h a v e v e r y l imi ted o r non-ex is ten t r e s e a r c h p r o g r a m m e s a n d p robab ly no m a n a g e m e n t for t h e s e r e s o u r c e s . If this s i tuat ion c o n t i n u e s una t t ended , s t o c k s wi l l even tua l ly be d r iven to s u c h low popu la t ion l eve l s that f i sh ing wi l l p r o b a b l y c e a s e for a ve ry long t ime . A par t icu lar c a s e that n e e d s c l o s e moni to r ing is the f i shery in Indones i a , w h i c h h a s g r o w n inc red ib ly qu i ck ly in the last 2 0 y e a r s a n d wi l l p r o b a b l y c o l l a p s e d ramat i ca l ly in the a b s e n c e o f m a n a g e m e n t . W o r l d c a t c h e s o f e l a s m o b r a n c h s a re subs tan t ia l ly h i g h e r t h a n ref lec ted by the different k inds of official s ta t is t ics . S ta t i s t ics repor ted to F A O a m o u n t to just b e l o w 7 0 0 , 0 0 0 1 for 1 9 9 1 . T h e resul ts p r e s e n t e d here s u g g e s t that the total c a t c h (as o p p o s e d to l and ings ) c o u l d be c l o s e r to 1 mi l l ion t, if w e i nc lude the e s t i m a t e d c a t c h o f the P e o p l e ' s R e p u b l i c o f C h i n a a n d the b y c a t c h from l a r g e - s c a l e h i g h - s e a s f i sher ies . H o w e v e r t h i s f igure d o e s not t ake into a c c o u n t d i s c a r d s from i n n u m e r a b l e bot tom t rawl f i sher ies a r o u n d the w o r l d . R e c r e a t i o n a l f i sher ies a re a l s o not i n c l u d e d s i n c e there is little informat ion a v a i l a b l e . H o w e v e r , there are ve ry important r ec rea t iona l f i sher ies for e l a s m o b r a n c h s in spec i f i c par ts o f the w o r l d (e.g. U S A , S o u t h A f r i c a , A u s t r a l i a ) . Hoff a n d M u s i c k (1990) es t imate that the mortal i ty of s h a r k s in rec rea t iona l f i she r i es o f the ea s t e rn U S A a l o n e , c a n be m o r e than 1 0 , 0 0 0 t/yr. T h e rea l total l eve l o f s h a r k s , r ays a n d c h i m a e r a s c a u g h t a r o u n d the w o r l d is p r o b a b l y c l o s e r to 1.35 mi l l ion t o r m o r e pe r yea r , tw ice the official s tat is t ics . 2 .3 .2 C o n s e r v a t i o n of e l a s m o b r a n c h s . T h e b y c a t c h o f e l a s m o b r a n c h s in h i g h - s e a s f i sher ies a r o u n d the w o r l d s e e m s to be a major s o u r c e o f c o n c e r n for c o n s e r v a t i o n d u e to the v e r y h igh n u m b e r s of s h a r k s k i l l ed . B l u e s h a r k s in par t icu la r might be f ac ing ex t r eme p r e s su re in m a n y par ts o f the g l o b e b e c a u s e of t h e se f i sher ies , but m o r e spec i f i c s tud ies a re n e e d e d in o r d e r to a d d r e s s the rea l s i tua t ion. 163 T h e p o s s i b l e threat o f e l a s m o b r a n c h ove rexp lo i t a t ion from h i g h - s e a s f i she r i es is ac tua l ly on ly part o f a c o m p l e x t e c h n i c a l in terac t ion . T h e r e is subs tan t i a l g e a r a n d c a t c h d a m a g e c a u s e d by s h a r k s in mos t o f t h e s e f i sher ies ( T a n i u c h i 1 9 9 0 , S i v a s u b r a m a n i a m 1 9 6 3 , 1 9 6 4 , P i l l a i a n d H o n m a 1978 , B e r k e l e y a n d C a m p o s 1988) w h i c h t r ans la t e s di rect ly into e c o n o m i c los s for the f i sh ing indus t r ies . A p o s s i b l e w a y to s o l v e this d u a l p r o b l e m c o u l d be to instal l s h a r k deterrent d e v i c e s in p a s s i v e f i sh ing g e a r s ( these a c c o u n t for mos t of the e l a s m o b r a n c h kill) . T h e N a t a l S h a r k B o a r d in S o u t h A f r i c a is current ly tes t ing a p r o m i s i n g non- le tha l e l e c t r o a c o u s t i c s d e v i c e to protect ba the rs from s h a r k a t tacks . A n o t h e r poss ib i l i ty w o u l d be to d e s i g n n e w se lec t ive f i sh ing g e a r t h a t c o u l d subs tan t ia l ly r e d u c e s h a r k h o o k i n g rates . H o w e v e r , for the t ime be ing the o n l y v i a b l e a l ternat ive is the imp lemen ta t i on o f su i t ab le b y c a t c h quo t a s for e l a s m o b r a n c h s in the h i g h - s e a s f i sher ies of the w o r l d th rough in te rna t iona l a g r e e m e n t , a n d their en fo r cemen t v i a o b s e r v e r p r o g r a m m e s . T h e c o n c e r n o v e r e l a s m o b r a n c h explo i ta t ion a r i s e s not o n l y from theore t i ca l c o n s i d e r a t i o n s abou t thei r b i o l o g i c a l a n d e c o l o g i c a l traits, but a l s o for h i s to r ica l r e a s o n s . T h e r eco rd of f i sher ies for s h a r k s a n d rays i n c l u d e s s e v e r a l c a s e s o f c o l l a p s e a n d rap id ly fal l ing ca t ch rates, r e m i n d i n g us o f the fragility o f t he se r e s o u r c e s ( H o l d e n 1977) . D o c u m e n t e d a c c o u n t s inc lude the C a l i f o r n i a f i shery for soupf in s h a r k s a n d the s p i n y dogf i sh f i shery of Br i t i sh C o l u m b i a in the 40 ' s , the s c h o o l sha rk f i shery o f S o u t h e r n A u s t r a l i a in the 50 ' s , the p o r b e a g l e s h a r k f i shery in the N o r t h w e s t A t l an t i c a n d the s p i n y dogf i sh f i shery in the Nor th S e a dur ing the 6 0 ' s ( A n d e r s o n 1990) . A l t h o u g h the u n d e r l y i n g r e a s o n s for s o m e of t hese c o l l a p s e s a re part ly u n d e r s t o o d (and a re s o m e t i m e s i n d e p e n d e n t o f h igh l eve l s of exploi ta t ion) , a n d desp i t e the fact that d e c r e a s i n g C P U E ' s a re a na tura l cha rac te r i s t i c of f i sher ies d e v e l o p m e n t , t h e se fa i lures const i tute a w a r n i n g a g a i n s t c a r e l e s s explo i ta t ion in v i e w o f the s p e c i a l b i o l o g i c a l at tr ibutes o f s h a r k s a n d rays d i s c u s s e d a b o v e . Effective pro tec t ion o f s h a r k s a n d rays from the potent ia l i m p a c t s o f l a r g e - s c a l e f i sher ies is not a n i m p o s s i b l e task . T h e efforts of in ternat ional co l l abo ra t i on that r egu l a t ed the c a t c h e s of s a l m o n i d s , m a r i n e b i rds a n d mar ine m a m m a l s in the Nor th P a c i f i c O c e a n a n d the recent b a n n i n g o f a l l driftnet f i sher ies in the h i g h - s e a s of the w o r l d a re t e s t i m o n y to the reali ty of effective pro tec t ion for m a r i n e f a u n a . T h e s t rong p r e s su re that s o m e coun t r i e s a re i m p o s i n g 164 o n fleets that c o n t i n u e to t ake s o m e d o l p h i n s in pu r se s e i n e t u n a o p e r a t i o n s a re ano the r c l e a r e x a m p l e that, w h e n the wi l l is there , effective pro tec t ion c a n be a c h i e v e d . T h e r o a d to effective m a n a g e m e n t a n d protec t ion of e l a s m o b r a n c h s d e p e n d s la rge ly o n e d u c a t i o n a n d a w a r e n e s s . T h i s is the on ly w a y in w h i c h it wi l l be p o s s i b l e to s t imula te in f i s h e r m e n , sc ien t i s t s , the pub l i c a n d g o v e r n m e n t s the wi l l n e e d e d to a c h i e v e rea l protect ion a n d m a n a g e m e n t o f s h a r k s a n d rays . Efforts o f this type h a v e a l r e a d y met with s o m e s u c c e s s . T h e S o u t h A f r i c a n G o v e r n m e n t h a s recent ly p ro tec ted the whi te shark; the g o v e r n m e n t o f A u s t r a l i a forbids the ki l l ing o f g rey -nu r se s h a r k s a n d is c o n s i d e r i n g protect ion of whi te s h a r k s ; C a l i f o r n i a just p a s s e d leg i s la t ion b a n n i n g the c a t c h o f whi te s h a r k s in their wa te r s . V a r i o u s recent sc ient i f ic m e e t i n g s h a v e f o c u s e d o n the i s s u e o f e l a s m o b r a n c h c o n s e r v a t i o n . D u r i n g 1 9 9 1 , the in ternat ional m e e t i n g " S h a r k s D o w n U n d e r " w a s he ld in S i d n e y , A u s t r a l i a , f o c u s i n g at tent ion o n the n e e d f o r t h e c o n s e r v a t i o n o f e l a s m o b r a n c h s by hos t ing a C o n s e r v a t i o n W o r k s h o p a s the o p e n i n g event . T h e A m e r i c a n E l a s m o b r a n c h S o c i e t y h e l d a S y m p o s i u m o n C o n s e r v a t i o n o f E l a s m o b r a n c h s du r ing its 1991 m e e t i n g a n d is p resen t ly se t t ing up a C o n s e r v a t i o n C o m m i t t e e at the in te rna t iona l l e v e l . T h e S p e c i e s S u r v i v a l C o m m i s s i o n o f the I U C N h a s recent ly f o r m e d a S h a r k S p e c i a l i s t G r o u p . Ev iden t ly , in ternat ional c o n c e r n abou t the future of e l a s m o b r a n c h s a n d the extent o f the i r explo i ta t ion is s tar t ing to p ick up . T h i s even t s s u g g e s t that it might be p o s s i b l e to a c h i e v e p rope r f i sher ies m a n a g e m e n t for s h a r k s a n d rays in the n e a r future. H o w e v e r , wi thout s ignif icant i n c r e a s e s in fund ing to e n a b l e m o r e effective r e s e a r c h o n e l a s m o b r a n c h , the g o a l o f s u s t a i n a b l e explo i ta t ion might n e v e r be r e a c h e d . In add i t ion , the conf l ic t ing d e m a n d s of c o n s e r v a t i o n a n d the s o c i o - e c o n o m i c c o n c e r n s o f f i s h e r m e n a l s o requi re d i r ec t ed r e s e a r c h efforts. 2.4 S u m m a r y a n d c o n c l u s i o n s . E l a s m o b r a n c h f i sher ies a re a t radi t ional a n d c o m m o n act ivi ty o f little i m p o r t a n c e g loba l ly but p rov id ing important s o u r c e s o f ha rd cu r r ency , protein a n d e m p l o y m e n t to m a n y loca l c o m m u n i t i e s a r o u n d the w o r l d . T h e s e f i sher ies a re par t icular ly impor tan t in p l a c e s s u c h a s S r i L a n k a , P a k i s t a n a n d A u s t r a l i a . T h e type o f explo i ta t ion o f e l a s m o b r a n c h r a n g e s from s u b s i s t e n c e f i sher ies with a r t i s ana l g e a r s a n d v e s s e l s , a s is the c a s e o f s o m e s a i l - p o w e r e d boa t s in India, to the h ighly indus t r i a l i sed f i sher ies with l ong l ine s , g i l lne ts o r t rawls of l o n g -165 r ange f i sh ing na t ions l ike J a p a n , T a i w a n , S p a i n a n d the fo rmer S o v i e t U n i o n . T h e r e a re 2 6 coun t r i e s that c a n be c o n s i d e r e d major e l a s m o b r a n c h - f i s h e r s , that is they harves t o r h a v e recent ly h a r v e s t e d m o r e than 10 ,000 t/yr o f e l a s m o b r a n c h s . A m o n g these , J a p a n , I ndones i a , India, T a i w a n a n d P a k i s t a n h a v e the h ighes t a v e r a g e e l a s m o b r a n c h y i e ld s . A b o u t 3 0 % of t h e s e 26 coun t r i e s s h o w recent fa l l ing t r ends in y i e l d . T h e a n a l y s i s of I R P ' s (Index o f R e l a t i v e P roduc t ion ) by F A O M a j o r F i s h i n g A r e a s s u g g e s t s that further i n c r e a s e s in explo i ta t ion of s h a r k s a n d r ays c o u l d be p o s s i b l e in the S o u t h E a s t Pac i f i c ( A r e a 87) , Nor th E a s t P a c i f i c ( A r e a 67) a n d the S o u t h E a s t A t l a n t i c ( A r e a 47 ) . H o w e v e r this a n a l y s i s is v e r y s imp l i s t i c a n d n e e d s to be t a k e n wi th c a u t i o n . A l t h o u g h there a re s o m e c a s e s o f spec i f i c f i sher ies for e l a s m o b r a n c h s (south A u s t r a l i a n sha rk f ishery, f i sher ies for s h a r k s in A r g e n t i n a a n d M e x i c o , b a s k i n g s h a r k f i sher ies of N o r w a y , etc.), the la rger part o f the y i e l d s of s h a r k s a n d rays in the w o r l d a re p r o d u c e d a s inc iden ta l c a t c h e s in o ther f i sher ies . T h i s p o s e s par t icu lar p r o b l e m s for a s s e s s m e n t a n d m a n a g e m e n t for s e v e r a l t e c h n i c a l a n d e c o n o m i c r e a s o n s . Official f i sher ies s tat is t ics d o not p roper ly reflect the a m o u n t s o f s h a r k s a n d rays ac tua l ly h a r v e s t e d e v e r y y e a r in the wor ld ' s o c e a n s . A l t h o u g h official f igures report abou t 7 0 0 , 0 0 0 t/y of e l a s m o b r a n c h s c a u g h t at the e n d of the 80 ' s , the a c t u a l l eve l is at leas t o f 1 '000,000 o r p o s s i b l y 1 '350,000 t/yr. T h e b y c a t c h e s of s h a r k s in l a rge - sca l e h i g h - s e a s f i sher ies a r o u n d the w o r l d is ve ry large , a m o u n t i n g p o s s i b l y to a l m o s t 50 % of the repor ted c a t c h e s from c o m m e r c i a l f i sher ies . T h e n u m b e r s o f s h a r k s c a u g h t a n n u a l l y in t h e s e f i sher ies du r ing 1 9 8 9 - 1 9 9 1 a re roughly e s t ima ted here at abou t 11 .6 -12 .7 mi l l ion . T h e long l ine f i sher ies for t u n a s o f J a p a n , K o r e a a n d T a i w a n a c c o u n t for mos t of t he se b y c a t c h e s . M o r e de t a i l ed informat ion is n e e d e d to p roper ly a d d r e s s the m a g n i t u d e o f this p r o b l e m a n d its effects u p o n s h a r k popu la t ions . O b s e r v e r p r o g r a m m e s n e e d to be i m p l e m e n t e d s o o n for t h e s e f i she r i es in o r d e r to ob ta in re l iable informat ion abou t y i e l d s , d i s c a r d s , a n d the extent o f f inning p rac t i ce s . T h e r e a re s e r i o u s d e f i c i e n c i e s not o n l y in the repor t ing rates but a l s o in the h a n d l i n g o f the repor ted s tat is t ics . T h e stat is t ics d i sc r imina te ve ry poor ly the t y p e s o f e l a s m o b r a n c h s caugh t 166 in each fishery, and this is of particular concern because it makes more difficult appraisals of any kind. Fisheries statistics need to be improved both in coverage of the fisheries and the dissaggregation of species. This probably implies improvement of quality control in the catches to allow for development of new markets. There is a generalised lack of research and management for shark and ray fisheries even in major elasmobranch-fishing countries. Very few fisheries are under specific management and this is a reason for further concern over their sustainability. Management of elasmobranch fisheries should ideally start very early in the development of the fisheries given their extreme fragility and the difficulties in reducing fishing effort once it grows beyond optimal levels. 167 C H A P T E R 3 DENSITY-DEPENDENT FECUNDITY IN ELASMOBRANCHS AND ITS IMPLICATIONS IN FISHERIES MANAGEMENT: A DETERMINISTIC A G E - S T R U C T U R E D SIMULATION M O D E L Representation is a compromise with chaos. Bernard Berenson 3.1 Introduct ion. 3.1.1 B i o l o g i c a l cha rac t e r i s t i c s o f the g roup in re la t ion to exp lo i t a t ion . T h e evo lu t ion o f e l a s m o b r a n c h reproduc t ive s y s t e m s h a s r e su l t ed in s t ra teg ies that a re in great cont ras t wi th t h o s e of mos t b o n y f i shes . T e l e o s t s a re e x t r e m e l y f e c u n d , a n d they gene ra l l y p r o d u c e s e v e r a l mi l l ion e g g s w h i c h a re r e l e a s e d to the e n v i r o n m e n t with the h o p e that a n uncer ta in n u m b e r wi l l even tua l ly d e v e l o p into recrui ts . In contras t , e l a s m o b r a n c h s h a v e v e r y dis t inct r eproduc t ive s t ra teg ies c o m p a r a b l e in m a n y w a y s to m a m m a l i a n r ep roduc t ion : t hey concen t r a t e large efforts in p r o d u c i n g a l imi ted quant i ty o f offspr ing w h i c h are recru i ted to the popu la t i on a s s o o n a s they are bo rn . In c o m p a r i s o n wi th te leos ts , uncer ta in ty in recru i tment s h o u l d be great ly r e d u c e d in e l a s m o b r a n c h s ( H o l d e n 1973) . T h e resul t ing (inferred) c l o s e re la t ionsh ip b e t w e e n parent s t o c k a n d recrui tment in e l a s m o b r a n c h s h a s b e e n c o n s i d e r e d of fo remos t i m p o r t a n c e for the i r a s s e s s m e n t a n d m a n a g e m e n t by t h o s e w o r k i n g with f i sher ies for this g r o u p ( H o l d e n 1 9 6 8 , 1974) . E v e n t h o u g h e l a s m o b r a n c h rep roduc t ion h a s often b e e n c o n s i d e r e d a l imit ing factor for their s u s t a i n e d explo i ta t ion ( H o l d e n 1 9 7 3 , 1 9 7 7 , H o e n i g a n d G r u b e r 1 9 9 0 , Pratt a n d C a s e y 1990) , f ew s tud ie s h a v e ac tua l ly e x p l o r e d in detai l the extent of s u c h cons t ra in t s . T h e s tudy o f the impor t ance o f e l a s m o b r a n c h fecundi ty h a s a l s o r e l e v a n c e in the u n d e r s t a n d i n g o f e l a s m o b r a n c h popu la t ion d y n a m i c s par t icular ly in re la t ion to dens i ty -168 d e p e n d e n t m e c h a n i s m s . T h e a n a l y s e s of different s t o c k s o f s p i n y dogf i sh Squalus acanthias (to date the bes t s tud ied e l a s m o b r a n c h ) h a v e resu l t ed in a l te rna t ive h y p o t h e s e s e a c h suppor t i ng e i ther c h a n g e s in fecundi ty , g rowth , immigra t ion o r j u v e n i l e mortal i ty, a s the unde r ly ing c o m p e n s a t o r y m e c h a n i s m r e s p o n s i b l e for the relat ive r e s i l i e n c e o f this s p e c i e s to exp lo i ta t ion . O n the o n e h a n d , H o l d e n (1973) p r o p o s e s d e n s i t y - d e p e n d e n t c h a n g e s in r ep roduc t ion a s the c o m p e n s a t o r y m e c h a n i s m for the S c o t t i s h - N o r w e g i a n s t o c k o f s p i n y dogf i sh . T h i s is re inforced by the resul ts o f G a u l d (1979) w h o p re sen t s s o m e e v i d e n c e o f c h a n g e s in fecundi ty for the s a m e s tock. No tab ly , G a u l d ' s da t a o n l y d e m o n s t r a t e c h a n g e s in the n u m b e r of o v a p r o d u c e d pe r f ema le , but fail to p rov ide e v i d e n c e for i n c r e a s e d n u m b e r o f e m b r y o s per f e m a l e . In cont ras t to H o l d e n ' s v i e w , W o o d s et a l . (1979) s u g g e s t that dens i ty -d e p e n d e n t fecundi ty is not e n o u g h to p rov ide effective c o m p e n s a t i o n in the relat ively resi l ient Br i t i sh C o l u m b i a n sp iny dogf i sh s tock, a n d they f avour a r educ t ion in natural mortal i ty a s the c o m p e n s a t o r y m e c h a n i s m . F a h y (1989) a n a l y s e s a n d e x c l u d e s both h y p o t h e s e s in add i t i on to c o m p e n s a t o r y growth , a n d p r o p o s e s that r ap id immigra t ion / re -c o l o n i z a t i o n from u n - d e p l e t e d n e a r b y s t o c k s might be the a n s w e r to the r e c o v e r y of s o m e s tocks o f s p i n y dogf i sh . T h e r e is add i t i ona l da t a from o ther e l a s m o b r a n c h g r o u p s r a i s ing d o u b t s o v e r the v a l u e of fecundi ty i n c r e a s e s . U s i n g a n ana ly t i ca l m o d e l , B r a n d e r ( 1 9 8 1 ) d e m o n s t r a t e s that fecundi ty c h a n g e s h a v e m i n i m a l o r no effect in the capab i l i t y of adul t s t o c k s o f Irish S e a s k a t e s to suppor t h e a v y f i sh ing , a n d that j uven i l e su rv iva l is the k e y factor. W a l k e r (1992) , o n bu i ld ing a s imu la t ion m o d e l for the g u m m y sha rk (Mustelus antarcticus) f i shery o f s o u t h e r n A u s t r a l i a reports not f inding a n y e v i d e n c e o f c h a n g e s in fecundi ty a s a resul t o f exp lo i t a t ion . 3.1.2 Def in i t ion of the p r o b l e m . A s imu la t ion a p p r o a c h is a v i a b l e a l ternat ive to a n a l y s e the d y n a m i c s of a popu la t ion in a fast a n d c h e a p w a y . It offers the a d v a n t a g e o f b e i n g e a s i l y a d a p t e d to fit different s c e n a r i o s a n d c o u l d therefore be u s e d to test v a r i o u s h y p o t h e s e s . In th is s e n s e , it c o u l d a l s o p rov ide important informat ion for future r e s e a r c h - p l a n n i n g a n d m a n a g e m e n t . 169 D u r i n g the p re sen t s tudy I built a n age - s t ruc tu red m o d e l o f a n e l a s m o b r a n c h popu la t i on with the fo l lowing a i m s a) to eva lua t e the potent ia l e f fec t iveness o f c h a n g e s in e l a s m o b r a n c h fecundi ty a s a c o m p e n s a t o r y r e s p o n s e to i n c r e a s e d f i sh ing mortal i ty, a n d b) to a s s e s s the impl i ca t ions o f the resul ts , a n d the potent ia l u s e o f the m o d e l , for f i she r i es m a n a g e m e n t . 3.2 C o n s t r u c t i o n o f the m o d e l . 3.2.1 M o d e l - b u i l d i n g c o n s i d e r a t i o n s . A m o d e l is a n y rep resen ta t ion o r abs t rac t ion o f a g i v e n s y s t e m o r p r o c e s s . T h e type a n d c o m p l e x i t y o f m o d e l s d e p e n d s o n the f ie ld o f r e s e a r c h a n d the par t i cu la r p r o b l e m to be a n a l y s e d . In t e r m s o f H o l l i n g ' s (1978) c l a s s i f i ca t ion , p r o b l e m s in popu la t i on m o d e l l i n g gene ra l l y lay in the a r e a of l ow qual i ty/quant i ty o f re levant da t a . F o r the par t icu lar c a s e o f e l a s m o b r a n c h popu la t i ons , w e mus t c o n s i d e r in add i t ion to this , the p o o r u n d e r s t a n d i n g of the b i o l o g i c a l a n d f i sher ies p r o c e s s e s i n v o l v e d (e .g. the expl ic i t form of the s tock- recru i tment re la t ion, the u n d e r l y i n g d e n s i t y - d e p e n d e n c e m e c h a n i s m , o r t h e a c t u a l l eve l s o f explo i ta t ion of a par t icu lar s tock) . T h e c o m p l e x i t y o f a m o d e l (unde r s tood a s the n u m b e r o f v a r i a b l e s i nc luded ) is not a l w a y s direct ly re la ted to its pe r fo rmance a n d use fu lnes s . L u d w i g a n d W a l t e r s (1985 , 1989) g ive e x a m p l e s o f s i m p l e m o d e l s ou tpe r fo rming m o r e c o m p l e x o n e s , whi l s t H i l b o r n a n d W a l t e r s (1992) suppor t the e x i s t e n c e o f a n "opt imal" m o d e l s i z e , w h i c h wi l l be spec i f i c for e a c h c a s e s tudy. It s e e m s po in t l e s s to cons t ruc t a la rge a n d e x t r e m e l y de t a i l ed m o d e l that i nc ludes e v e r y s ing le v a r i a b l e that is l ikely to affect the s y s t e m . V e r y f requent ly , the uncer ta inty s u r r o u n d i n g the e s t ima t ion o f s o m e of t he se v a r i a b l e s , o n l y r e d u c e s the abi l i ty of the m o d e l to p r o d u c e useful informat ion . T h e sec re t in s u c c e s s f u l m o d e l bu i ld ing r e s i d e s in loca t ing the mos t important v a r i a b l e s a n d ignor ing the o n e s that d o not a d d s igni f icant ly to the pe r fo rmance o f the s y s t e m . O n the o ther h a n d , w e mus t a l s o c o n s i d e r the l eve l o f reso lu t ion n e e d e d from the m o d e l in re la t ion to m a n a g e m e n t o r in format iona l n e e d s . Starf ie ld a n d B l e l o c h (1986) r e c o m m e n d a c o m p r o m i s e o f s impl i f ica t ion for m o d e l bu i ld ing , a n d in their o w n w o r d s : " c h o o s i n g the appropr i a t e l eve l (of resolut ion) is t hus a p r a g m a t i c c o m p r o m i s e b e t w e e n the c o m p l e x i t y ... a n d the n e e d to s o l v e a p r o b l e m wi th l imi ted da t a a n d in a r e a s o n a b l e a m o u n t o f t ime.. ." . T h i s is p r e c i s e l y the p h i l o s o p h y a d o p t e d th roughout this 170 chap te r . 3 .2.2 B i o l o g i c a l c o n s i d e r a t i o n s . 3.2.2.1 E a r l y life na tura l mortal i ty. O n e o f the m o s t c o m m o n a s s u m p t i o n s in f i sher ies s c i e n c e is that the na tura l mortal i ty of a s tock r e m a i n s cons t an t t h rough t ime a n d a c r o s s a g e c l a s s e s ( C a d d y 1991) . H o w e v e r , not surpr i s ing ly , recen t r e s e a r c h ind ica tes that natural mortal i ty is not cons t an t a c r o s s a g e s in w i l d sha rk p o p u l a t i o n s ( M a n i r e & G r u b e r 1991) . S h a r k s a re a v e r y hea l thy g roup o f o r g a n i s m a n d to date there a re no a c c o u n t s o f mortal i ty d u e to d i s e a s e o r pa ra s i t i sm in w i l d sha rk p o p u l a t i o n s . A l t h o u g h mos t s h a r k s s p e c i e s h a v e ve ry f ew p reda to r s (main ly o ther sha rks ) a n d their na tura l mortal i ty l eve l s a re thought to be v e r y low, s h a r k p u p s a re e x p o s e d to re la t ively h igh na tura l morta l i t ies ma in ly d u e to p reda t ion from la rge r s h a r k s p e c i e s o r to c a n n i b a l i s m by the adul t part o f the popu la t i on . A s the y o u n g s h a r k s a p p r o a c h a ce r ta in s i z e t h r e sho ld (approx . 1 m T L is s u g g e s t e d by Brans te t te r (1990)) , the i r na tura l mortali ty d e c r e a s e s b e c a u s e they a re a b l e to de te r p reda to rs o r e s c a p e t h e m t h a n k s to i n c r e a s e d s w i m m i n g s p e e d . A l t h o u g h it w o u l d s e e m m o r e a d e q u a t e to u s e a s i z e - s p e c i f i c na tura l mortal i ty dur ing this s tudy, a n a g e - s p e c i f i c mortal i ty s c h e d u l e h a s b e e n c h o s e n in o r d e r to s impl i fy the m o d e l . Effect ively, this i m p l i e s a s s u m i n g that there a re no c h a n g e s in i nd iv idua l g rowth rate, e x c e p t w h e n this is expl ic i t ly m o d e l l e d . Na tu ra l mortal i ty for s h a r k s h a s b e e n e s t i m a t e d to r ange b e t w e e n 0 .048 a n d 0.2 ( H o l d e n 1968 , P a u l y 1 9 7 8 , G r a n t et a l . 1979 , Bonf i l 1990) . In the p resen t m o d e l , the na tura l mortal i ty coefficient for ea r ly j u v e n i l e s is set to a v a l u e 2-3 t i m e s l a rge r t han that c h o s e n for adul t s . Mortal i ty is r e d u c e d wi th i n c r e a s i n g a g e , t hen b e c o m e s nea r ly cons t an t for mos t preadul t a n d adul t a g e c l a s s e s . 3 .2 .2 .2 Life h is tory funct ional r e l a t ionsh ips . T h e great majori ty o f the c o m m e r c i a l l y important s h a r k s p e c i e s a re v i v i p a r o u s . In the 171 ba to ids , o n l y the s k a t e s a re o v i p a r o u s s p e c i e s o f s ign i f i cance to f i she r ies . There fo re , I wi l l cen t re m y a n a l y s i s o n v i v i p a r o u s s p e c i e s . T h e r e a re s e v e r a l reports o f pos i t ive cor re la t ion b e t w e e n m o t h e r s i z e a n d litter s i z e (the n u m b e r o f e m b r y o s in a litter) for v i v i p a r o u s s h a r k s . T h i s r e l a t ionsh ip b e t w e e n fecundi ty a n d mo the r s i z e m a y e i ther a s s u m e a l inea r ( O l s e n 1954 , P a r s o n s 1 9 8 3 , S i m p f e n d o r f e r 1992) or e x p o n e n t i a l ( W a l k e r 1984 , L e n a n t o n et a l . 1990) fo rm. In the p re sen t m o d e l , I a s s u m e a l inear r e la t ionsh ip b e c a u s e this a p p e a r s to be the mos t c o m m o n c a s e in the s h a r k s s o far s tud ied . S i z e at birth is a l s o pos i t ive ly co r r e l a t ed to mo the r s i z e in s o m e s h a r k s ( O l s e n 1984 , H a n c h e t 1 9 8 8 , P e r e s & V o o r e n 1991) . H o w e v e r , p r o d u c i n g l a rge r n e w b o r n s h a r k s might m e a n s m a l l e r litter s i z e s d u e to s p a c e l imita t ions in the f ema le b o d y cav i ty , a s repor ted for Rhizoprionodon terraenovae ( P a r s o n s 1983) . A pos i t ive co r re la t ion b e t w e e n m o t h e r s i z e a n d the s i z e o f the n e w b o r n s is a d v a n t a g e o u s b e c a u s e it c a n m e a n d e c r e a s e d mortali ty by preda t ion for l a rger p u p s . S m a l l e r n e w b o r n s wi l l not o n l y h a v e h i g h e r na tura l mortal i ty than la rger n e w b o r n s but a s s u m i n g e q u a l g rowth rates , they wi l l r e m a i n for l o n g e r u n d e r the s i z e t h r e s h o l d that a l l o w s t h e m to e s c a p e o r de te r p reda to rs . O n c e this is c o n s i d e r e d , the mos t important c o n s e q u e n c e of a cor re la t ion b e t w e e n m o t h e r s i z e a n d the s i z e o f p u p s at birth is that the a v e r a g e natura l mortal i ty of the first a g e c l a s s in a g i v e n y e a r wi l l be d e p e n d e n t o n the s i z e s tructure o f the f ema le parent s tock . H o w e v e r , the incorpora t ion of this r e la t ionsh ip b e t w e e n m o t h e r s i z e a n d ea r ly natural mortal i ty o f the p u p s is not a t t empted in the p resen t m o d e l . 3.2.3 G e n e r a l cha rac t e r i s t i c s of the m o d e l . A s a first s impl i f ica t ion , the m o d e l c o n s i d e r s on ly the f ema le part o f the popu l a t i on . T h i s is a c o m m o n prac t ice in d e m o g r a p h i c m o d e l s ( K r e b s 1978) a n d is b a s e d o n the a s s u m p t i o n that m a l e ava i lab i l i ty is n e v e r l imit ing for r ep roduc t ion . F o r t h e m o d e l , fer t i l izat ion t a k e s p l a c e at the start o f the y e a r a n d ges ta t ion t ime is exac t ly o n e y e a r . H e n c e , n e w b o r n s a re recrui ted to the popu la t ion at the b e g i n n i n g o f eve ry yea r . A ges ta t ion t ime of o n e y e a r is the mos t c o m m o n a m o n g s h a r k s ( see tab le 2 o f Pratt & C a s e y 1990) , s p e c i a l l y a m o n g c a r c h a r h i n i d s w h i c h a re u sua l ly the mos t important c o m m e r c i a l s p e c i e s in the t rop ics . 172 A deterministic approach is used in this study in order to keep the model tractable, and because of computational restrictions (the whole model is implemented in Quattro Pro 3.0 and run on a 386/25 PC computer). In addition, stochastic processes in fish populations are usually associated with recruitment, and viviparous sharks are likely to show small recruitment variation as compared to bony fishes, so that for the present purpose these can be ignored. In Chapter 4, a shark population model incorporating stochastic variability in recruitment is implemented on another platform (BASIC). The simulations are broken into three phases. In the first, the different parameters of the model are entered (natural mortality and fecundity arrays, number of age classes, etc.) and a new population is allowed to grow to its equilibrium size. During the second phase, fishing is introduced via an array of age-specific fishing mortalities. After a 100 year period of constant fishing, the size of the remaining stock is compared to the virgin population. In the third phase, fishing mortality, fecundity, or any other initial conditions are changed to simulate different type of stock or different exploitation and density-dependent response scenarios. The model is run again under these new conditions and the results of each run are stored for later analysis. For simplicity, the change in fecundity of the exploited population is simulated as an immediate response process by which fecundity is increased as soon as exploitation begins. 3.2.4 Formulation. The simulated population is a closed system with no emigration/immigration in which the number of fish N in a cohort of age a+1 during the year t+1 are calculated as: where Nta is the number of sharks of age a at the beginning of year t, and Ma is the natural mortality of fish aged a until the beginning of age a+1. The total number of fish in the population in year t+1 is given by: 173 3.2 where Amax denotes the maximum age after which all sharks die and R the number of recruits born into the population at the beginning of the year (i.e. number of sharks aged 0, assuming single pulse birth). The number of recruits in a year t+1 is determined by the number of individuals in each mature age class during the previous year (f) and surviving to the beginning of t+1, times the age specific fecundity: A m a x M„ 3.3 E J V a=A mat where q>a is average fecundity in number of female newborns per mother per year, and A is the age of first maturity of the population. Total biomass is calculated using the equation: Anax _ 3.4 a=0 where B is total biomass, and Wa is average weight at age. The population growth according to this model has an undesirable feature, it follows an exponential behaviour which is not realistic. There are two easy ways of including density dependence in order to simulate a more 'realistic' logistic growth. One is via a stock-recruitment relationship, and another is to incorporate density-dependent natural mortality; the second approach is chosen here. The usage of a stock-recruitment relationship was 174 d i s c a r d e d b e c a u s e it o b s c u r e s the effects of o n e of the v a r i a b l e s w e w a n t to c h a n g e in the m o d e l , i .e. d e n s i t y - d e p e n d e n t fecundi ty . A d d i t i o n a l l y , s tock- rec ru i tment r e l a t ionsh ips h a v e not b e e n d o c u m e n t e d yet for w i l d e l a s m o b r a n c h s . O n the o ther h a n d , the i n c l u s i o n o f d e n s i t y - d e p e n d e n t na tura l mortal i ty is a p l aus ib l e a l te rnat ive , often r e c o m m e n d e d for theore t ica l m o d e l s o f f i s h popu la t i ons ( R i c k e r 1940 ; K e s t e v e n 1947) . T h e fo l lowing equa t i on is u s e d to adjust the initial v a l u e s o f a g e spec i f i c na tura l mortali ty (from here o n w a r d s c a l l e d the b a s e l i n e natural mortal i ty): Mt,a=Ma(<*+PNt-l) 3.5 w h e r e Mta d e n o t e s the na tura l mortal i ty for s h a r k s of a g e a in y e a r r, a n d a, p a re cons tan t s . T h i s m a k e s the c h a n g e in M a l inear funct ion o f popu la t i on n u m b e r s w h i c h is the s imples t form of e x p r e s s i n g dens i ty d e p e n d e n c e (Beve r ton a n d Hol t 1957) . F o r the s e c o n d p h a s e o f the m o d e l , f i sh ing mortali ty is i n t r o d u c e d into the equa t ion for number s - a t - age : J V l , a Iyt,ac here F a is the a g e spec i f i c f i sh ing mortali ty coefficient , a n d F% a factor a l l o w i n g the b a s e l i n e f i sh ing mortal i ty v a l u e to be c h a n g e d interact ively for e a c h run o f the m o d e l in o r d e r to s imula te different s c e n a r i o s of f i sh ing intensi ty. In a n a l o g y wi th f i sh ing mortal i ty c h a n g e s , fecundi ty is i n c r e a s e d by mul t ip ly ing the b a s e l i n e fecundi ty v a l u e s by a fac tor d e p e n d i n g o n the d e s i r e d p e r c e n t a g e i n c r e a s e . 3.2.5 Initial p a r a m e t e r s . T h e b a s e l i n e input v a l u e s for the m o d e l du r ing the b a s e l i n e run, a re l i s ted b e l o w . T h e s e p a r a m e t e r v a l u e s w e r e c h o s e n to a p p r o x i m a t e the k n o w n fea tures o f s h a r k s ' b io logy , a n d 175 s imula t ing a f i shery that w o u l d ha rves t a l l a g e s but wi th s o m e l e v e l o f se lec t iv i ty in w h i c h ve ry y o u n g (smal l ) o r v e r y o l d ( large) s h a r k s are l e s s s u s c e p t i b l e o f b e i n g c a p t u r e d . A m a x = 2 5 y e a r s A m a , = 10 y e a r s F e c u n d i t y a r r ay = [0 ,0 ,0 ,0 ,0 ,0 ,0 ,0 ,0 ,0 ,0 .75 ,1 ,1 .5 ,2 ,2 .5 ,3 ,3 .5 ,4 ,4 .5 ,5 ,5 ,5 ,5 ,6 ,6 ,6 ] M (base l ine ) = 0.4 for a g e 0 M (base l ine ) = 0.3 for a g e 1 M (base l ine ) = 0.2 for a g e 2 M (base l ine ) = 0 .15 for a g e s 3-4 M (base l ine ) = 0 .12 for a g e s 5-6 M (base l ine ) = 0 .09 for a g e s 7-25 F = 0 .08 for a g e s 0-6 F = 0 .12 for a g e s 7-20 F = 0 .04 for a g e s 2 1 - 2 5 T h e v a l u e s for p a n d a for the d e n s i t y - d e p e n d e n t na tura l mortal i ty funct ion w e r e "hand-tuned" to a l l o w a popu la t i on growth that w o u l d r e s e m b l e a t yp ica l log i s t i c c u r v e w h i l e k e e p i n g the u p p e r a n d l o w e r l imits o f Ma wi th in r e a s o n a b l e b o u n d s . T h e p a r a m e t e r v a l u e s w e r e p=0.6, a n d a=1 x 10" 6 T h e a c t u a l M v a l u e s u s e d by the c a l c u l a t i o n s in the m o d e l at e a c h popu la t i on s i z e are i l lustrated in f igure 3 .1 . A l l s i m u l a t i o n s w e r e in i t ia l i sed by ' s tock ing ' 10 ,000 n e w recrui ts (age 0) pe r yea r , until the y e a r w h e n the first cohor t r e a c h e d maturi ty. F r o m then o n w a r d s , the popu la t i on w a s left to g r o w a l o n e . 3.2.6 Sens i t iv i ty a n a l y s i s . A sens i t iv i ty a n a l y s i s is the e a s i e s t w a y to inves t iga te the b e h a v i o u r o f the m o d e l u n d e r 176 different s c e n a r i o s , a n d to u n d e r s t a n d w h i c h a re the mos t impor tant v a r i a b l e s affect ing the output. F o r this p u r p o s e , both s m a l l a n d large c h a n g e s in the input v a r i a b l e s a re a n a l y s e d w h e n p o s s i b l e . T h e c h o i c e o f a d i m e n s i o n l e s s va r i ab l e a s the output is f requent ly r e c o m m e n d e d in o rde r to s implify the sens i t iv i ty a n a l y s i s in n o n l i n e a r m o d e l s (Starf ie ld & B l e l o c h 1986) . F o r t h e p resen t s tudy, s u c h a s t ra tegy is a l s o ve ry useful for c o m p a r i n g the resul ts f rom different runs . T w o n e w v a r i a b l e s a re in t roduced : N% is the ratio b e t w e e n total popu l a t i on n u m b e r s after 100 y e a r s o f f i sh ing a n d the v i rg in popu la t ion n u m b e r s ; s imi la r ly , B% e x p r e s s e s the s a m e c o n c e p t in t e rms o f total b i o m a s s . T h e s e two v a r i a b l e s p rov ide a n e a s y w a y to c o m p a r e the effect o f c h a n g e s in fecundi ty o n the l ong te rm sus ta inab i l i ty o f a f i shery u n d e r different s c e n a r i o s o f f i sh ing intensi ty. Fu r the rmore , c o m p a r i n g B% to N% a l s o g i v e s s o m e insight abou t p o s s i b l e c h a n g e s in popu la t ion s i z e structure a s a resul t o f exp lo i ta t ion . T h e sens i t iv i ty a n a l y s i s is s t ructured in the fo l lowing w a y : runs 1-7 test the s e p a r a t e effects o f different c h a n g e s in f i sh ing intensi ty a n d i n c r e a s e s in fecundi ty , a s c o m p a r e d to b a s e l i n e resul ts ; runs 8-9 e x p l o r e the effects o f r educ t ions in the a g e o f first s e x u a l matura t ion , w h e r e a s runs 10-12 look at s e l ec t ive f i sh ing pat terns; run 13 c o n s i d e r s the effect of h a v i n g a flat a g e dis t r ibut ion at the v i rg in popu la t ion s i z e ; runs 14-15 i n c l u d e c h a n g e s in the natural mortal i ty a s s u m p t i o n s of the m o d e l ; a n d finally runs 16-18 a r e d e s i g n e d to o b s e r v e the effect o f the n u m b e r o f a g e c l a s s e s of the popu la t ion (i.e. longevi ty ) . 3.3 R e s u l t s . 3.3.1 B a s e l i n e run . T h e popu la t i on resu l t ing from the b a s e l i n e run o f the m o d e l a t ta ins a n equ i l i b r ium s i z e of abou t 1.05 mi l l ion s h a r k s in 7 9 y e a r s (fig. 3.2). T h i s popu l a t i on d i s p l a y s the s tab le (stat ionary) a g e s tructure (fig. 3.3) e x p e c t e d for a v i rg in popu l a t i on free o f d i s t u r b a n c e s . F i s h i n g b e g i n s at this point u n d e r a cons t an t r e g i m e u s i n g the b a s e l i n e v a l u e s o f F. T h e fo l lowing 100 y e a r s w h e n the f i shery t ake p l a c e , s h o w a s l ight ly o sc i l l a t i ng d e c a y of the s tock (fig. 3.4). 177 MORTALITY PATTERN 1 0 0 0 0 2 1 0 0 0 0 4 1 0 0 0 0 6 1 0 0 0 0 8 1 0 0 0 0 1 0 1 0 0 0 0 1 1 0 0 0 0 3 1 0 0 0 0 5 1 0 0 0 0 7 1 0 0 0 0 9 1 0 0 0 0 1 1 1 0 0 0 0 N 5 - 6 ^ 7 - 8 — < — 9 - 2 5 AGE GROUPS Figure 3.1 V a l u e s of the natural mortality coefficient used by the density dependent function of the model . 1 2 O O O O 0 1 0 0 0 0 0 0 H 8 0 0 0 0 0 •30 II I I I I I I I I M l I I M 'T IT I I M i l l 0 Biomass Figure 3.2 Growth of the simulated e lasmobranch population according to parameters defined in the text. 178 0 2 4 6 1 3 5 7 10 12 14 16 18 20 22 24 9 1 1 13 1 5 17 19 21 23 25 age Figure 3.3 Stationary (stable) structure of the simulated population at the asymptotic s ize (virgin population), defined by the mortality and natality schedules . Figure 3.4 Decay of the simulated population under the basel ine fishing mortality pattern. 179 3.3.2 Sens i t iv i ty a n a l y s i s . T h e resul ts o f the sens i t iv i ty a n a l y s i s a re s h o w n in tab le 3 .1 . T h e first impor tant o u t c o m e from runs 1-7, is that the s i m u l a t e d popu la t ion is m u c h m o r e s ens i t i ve to c h a n g e s in f i sh ing mortali ty than to i n c r e a s e s in fecundi ty . A fecundi ty i n c r e a s e o f 4 0 0 % h a s l e s s impac t o n B% t han a 4 0 % d e c r e a s e in f i sh ing mortal i ty. N o t e that the effect o f fecundi ty i n c r e a s e s is felt m u c h m o r e in the popu la t ion n u m b e r s t han in t e rms o f b i o m a s s . T h i s is not su rp r i s ing c o n s i d e r i n g that the fecundi ty i n c r e a s e is t r ans la t ed di rect ly a s a g a i n in the n u m b e r of n e w b o r n s h a r k s , w h i c h propor t iona l ly cont r ibute l e s s to the bu lk o f the b i o m a s s . B e c a u s e f i sh ing mortal i ty is a s s u m e d cons t an t th roughout the 100 yr p e r i o d c o n s i d e r e d here , the g rea te r part o f the n e w b o r n s n e v e r g r o w s o l d e n o u g h to cont r ibute subs tan t i a l ly to the total popu la t ion b i o m a s s . T h e m o d e l is a l s o m o r e sens i t i ve to r educ t ions in the a g e o f first maturi ty t han to i n c r e a s e s in fecundi ty (runs 8 a n d 9). A 1 0 % d e c r e a s e in Amat h a s a s t ronge r effect t han a 2 0 % i n c r e a s e in fecundi ty . W e c a n e x p e c t r e d u c e d a g e of maturi ty t h rough d e n s i t y - d e p e n d e n t c h a n g e s in g rowth . In cont ras t to the i n c r e a s e s in fecundi ty , a n i n c r e a s e in growth h a s a pos i t ive impac t in the total b i o m a s s . T h e effects of s e l ec t ive f i sh ing indica te that protec t ing a f ew j u v e n i l e a g e c l a s s e s by e x c l u d i n g t h e m from the f i shery (run 10) is m o r e benef ic ia l in t e r m s of b i o m a s s a n d a lmos t a s benef ic ia l in n u m b e r s , a s protec t ing a l l o f the adul t s tock (run 12). In c o m p a r i s o n , f i sh ing o n l y in the m i d d l e por t ion of the a g e dis t r ibut ion h a s the l e s s pos i t ive effect for the c o n s e r v a t i o n o f the s tock . T h i s p robab ly h a p p e n s b e c a u s e a major part o f the reproduc t ive potent ia l o f the popu la t i on is e l i m i n a t e d by f i sh ing the l e s s f e c u n d but m o r e a b u n d a n t a g e c l a s s e s . R u n 13 s h o w s that the long- te rm s i z e of the popu la t ion is i n sens i t i ve to its v i rg in s i z e s t ructure. T h i s is l a rge ly a n effect o f the t ime s p a n c o n s i d e r e d in the p re sen t s imu la t i ons . A c c o r d i n g to d e m o g r a p h i c theory, a n y popu la t i on wi l l even tua l l y at tain a s tab le a g e dis t r ibut ion, r e g a r d l e s s o f its initial a g e s tructure, if it is subject for l o n g e n o u g h to a cons tan t s y s t e m of mortal i ty a n d natali ty s c h e d u l e s ( K r e b s 1978) . T h e sens i t iv i ty o f the m o d e l to different a s s u m p t i o n s abou t the na tura l mortal i ty (runs 14 a n d 15) is in a g r e e m e n t with the 180 Table 3.1 Results of sensitivity analysis of the model. B%= proportion of virgin biomass remaining after 10Oyr of fishing; N%= proportion of virgin population remaining after 10Oyr of fishing. Numbers in parentheses are percentage change from values of the baseline run. DEPARTURES FROM BASELINE RUN B% N% run # Changes in F Increase in fecundity Base-line None None 0.314 0.306 1 -95% 0% 0.966 (+208%) 0.963 (+215%) 2 -40% 0% 0.589 (+88%) 0.580 (+90%) 3 +40% 0% 0.082 (-74%) 0.079 (-74%) 4 +100% 0% 0.00045 (-99.9%) 0.0005 (-99.8%) 5 0% 20% 0.365 (+16%) 0.379 (+24%) 6 0% 60% 0.433 (+38%) 0.497 (+62%) 7 0% 400% 0.568 (+81%) 0.974 (+218%) 8 Age of maturity reduced 10% (Ama= 9) 0.376 (+20%) 0.400 (+31%) 9 50% {Am= 5) 0.549 (+75%) 0.843 (+175%) 10 Selective fishing Only ages 10-25 0.703 (+124%) 0.650 (+112%) 11 Only ages 7-16 0.601 (+91%) 0.582 (+90%) 12 Only ages 0-9 0.635 (+102%) 0.667 (+118%) 13 Flat age distribution (same numbers in each age class) at equilibrium 0.314 (0%) 0.306 (0%) 14 Baseline M = 0.12 for all ages 0.395 (+26%) 0.393 (+28%) 15 M not density-dependent 0.380 (+21%) 0.389 (+27%) 16 F = M = 0.12 for all ages; fecundity = 5 for all mature ages ( > 9yr) 0.465 0.464 17 F, M and fecundity as above. Only 21 age classes, Ama = 8yr 0.571 (+23%) ** 0.571 (+23%) ** 18 As above, but only 16 age classes, Amal= 6yr 0.672 (+45%) ** 0.672 (+45%) ** Percentage change from run number 16. resul ts o f the c h a n g e s in f i sh ing mortal i ty from the initial runs (1-7). 181 F ina l ly , the resul ts o f the relat ive impor t ance of longev i ty in the m o d e l ind ica te that shor ter l ived s p e c i e s , w h i c h h a v e ear l i e r a g e s at first maturi ty than l o n g e r l i ved o n e s , h a v e a h ighe r net r ep roduc t ive rate, a n d a re therefore better su i t ed to su s t a in exp lo i t a t ion (runs 16-18) . S u m m a r i s i n g the p r e v i o u s resul ts , o v e r the r ange o f v a l u e s a n a l y s e d the m o d e l is more sens i t i ve to va r i a t i ons in mortali ty a n d a g e o f first maturi ty t han to i n c r e a s e s in fecundi ty . If a l l the a s s u m p t i o n s o f the m o d e l ho ld , this s u g g e s t s that for s h a r k f i sher ies , r e s e a r c h efforts s h o u l d concen t r a t e o h get t ing g o o d e s t ima te s a n d con t ro l s o f mortal i ty, ra ther than o n de ta i l ed inves t iga t ions o f the fecundi ty of the s t ocks . 3.3.3 T h e v a l u e o f fecundi ty i n c r e a s e s . T h e b e h a v i o u r o f the m o d e l w a s further inves t iga ted in de ta i l wi th a n e x t e n s i v e a n a l y s i s w h i c h i n c l u d e d a total o f 160 runs u n d e r a w i d e r ange o f c o m b i n a t i o n s b e t w e e n f i sh ing mortali ty a n d theore t i ca l i n c r e a s e s in fecundi ty . F i g u r e 3.5 s h o w s three e x a m p l e s of the d y n a m i c s o f the m o d e l u n d e r s o m e of t he se s c e n a r i o s . T h e a g e s t ructure o f the popu la t ion is s a m p l e d e v e r y 10 y e a r s a n d is s h o w n at the top o f e a c h g r a p h . A n i n c r e a s e o f 4 0 0 % the b a s e l i n e fecundi ty c o m b i n e d with ve ry light f i sh ing (0.05 t i m e s the b a s e l i n e level) c a u s e s the popu la t ion to i n c r e a s e a n d at tain a n e w equ i l ib r ium (fig. 3 .5a) . H o w e v e r , c h a n g e s o f 1.6 t imes the b a s e l i n e fecundi ty p r o d u c e d e c r e a s e d equ i l i b r ium popu l a t i on s i z e s w h e n f i sh ing mortali ty i n c r e a s e s to 0.9 t imes the b a s e l i n e v a l u e (fig. 3 .5b) . Af te r 7 0 y e a r s o f h e a v y explo i ta t ion (F-tr ipled) , the o l d e r a g e c l a s s e s h a v e d i s a p p e a r e d from the popu la t ion (fig. 3 .5c) . It is wor th not ing that in the two last c a s e s a b o v e , the i n c r e a s e in fecundi ty h a s a n impac t o n recru i tment a n d o n the s i z e o f the popu la t i on du r ing the first f ew y e a r s , but a s t ime g o e s by this effect is m o r e o r l e s s d i lu ted a s a c o n s e q u e n c e o f h e a v y f i sh ing . T h e in terac t ion b e t w e e n f i sh ing mortali ty a n d fecundi ty is bet ter i l lus t ra ted t h rough d i a g r a m s s u c h a s f igure 3.6. T h i s s h o w s a t h r e e - d i m e n s i o n a l su r face plot o f popu l a t i on t rajectories for 10Oyr, o v e r a w i d e r ange o f f i sh ing morta l i t ies w h e n fecundi ty h a s i n c r e a s e d 1 0 0 % . T h e x-y p l ane s h o w s the c o r r e s p o n d i n g c o n t o u r d i a g r a m s with i s o l i n e s o f popu la t i on s i z e . T h e 3 D sur face o f popu la t i on n u m b e r s is s t e e p e r t han that o f b i o m a s s a n d h a s a r i s ing p e a k at 182 NUMBERS IN POPULATON EVERY 10 YEARS a) b) 3000000 => 2000000 O a. 1000000 > a m 0 10 20 30 40 50 60 70 YEARS OF FISHING NUMBERS IN POPULATION EVERY 10 YEARS 1250000 750000 O 500000 rrrrr > 0 10 20 30 40 50 60 70 80 90 100 YEARS OF FISHING NUMBERS IN POPULATION EVERY 10 Y E A R S 0 10 20 30 40 50 60 70 80 90 100 YEARS OF FISHING c) Figure 3.5 Total numbers ( ) and recruitment (—) trends of the simulated shark population. The top histograms show the structure of the population every 10 years, a) F=0.05 times baseline, fecundity=5 times baseline; b) F=0.9 times baseline, fecundity=1.6 times baseline; c) F=3 times baseline, fecundity=1.6 times baseline. 183 low f i sh ing in tens i t ies n e v e r o b s e r v e d in the b i o m a s s su r face . T h i s h a p p e n s b e c a u s e w h e n fecundi ty i n c r e a s e s , the y o u n g a g e c l a s s e s a c c o u n t p ropor t iona te ly m u c h m o r e in t e rms of n u m b e r s t han in b i o m a s s . T h e c o n t o u r d i a g r a m s s h o w n here c o u l d h a v e o ther app l i c a t i ons . B a s e d o n t h e m , it is p o s s i b l e to find e a s i l y the c o m b i n a t i o n o f f i sh ing mortal i ty that w o u l d a l l o w to sus t a in a popu la t ion at a d e s i r e d l eve l in the l o n g t e rm, if w e k n o w the i n c r e a s e in fecundi ty w e c a n e x p e c t a s a result o f exp lo i t a t ion . T h i s c o u l d be a rough g u i d e u s e d ei ther for p u r p o s e s o f c o n s e r v a t i o n a n d m a n a g e m e n t o f a r e s o u r c e , o r for the c u l l i n g o f sha rk popu la t ions . In a s imi l a r w a y , w e c a n look at the effect o f a w h o l e r ange o f fecundi ty i n c r e a s e s for a f ixed v a l u e o f f i sh ing mortal i ty u s i n g the s a m e type o f d i a g r a m s a s a b o v e . In f igure 3.7 popu la t ion a b u n d a n c e is plot ted aga in s t different i n c r e a s e s in fecundi ty w h e n f i sh ing mortal i ty is 1.8 t imes the b a s e l i n e v a l u e . T h i s figure i l lustrates ve ry w e l l the l imi ted long- te rm v a l u e of c h a n g e s in fecundi ty w h e n f i sh ing mortal i ty is h igh . A l t h o u g h there is a c o n s i d e r a b l e benefit in t e rms of popu la t i on n u m b e r s dur ing the first 10-15 y e a r s (pure ly con t r ibu ted by ve ry y o u n g a g e c l a s s e s ) , f i sh ing mortali ty rapid ly s m o o t h s d o w n the d i f fe rences , a n d after 100 y e a r s o f f i sh ing there is re la t ively little difference b e t w e e n the popu l a t i on s i z e s from the different fecundi ty i n c r e a s e s . T h e grea te r sensi t iv i ty o f the m o d e l to f i sh ing mortali ty than to fecundi ty i n c r e a s e s is a l s o i l lustrated w h e n c o m p a r i n g the f l a tness o f f igure 3.7 aga ins t the s t e e p n e s s o f f igure 3 .6a . T h e t e rmina l long- te rm effects (after 100 y r o f f ishing) of the different s c e n a r i o s c o n s i d e r e d s o far, a re s u m m a r i s e d in f igure 3.8. A t the b a s e l i n e l eve l o f f i sh ing mortal i ty, o n l y i n c r e a s e s in fecundi ty o f 4 to 5 t imes w o u l d a l l o w the popu la t i on to r e m a i n c l o s e to its v i rg in s i z e . A t this s a m e l eve l o f f i sh ing a n d in the a b s e n c e o f a n y i n c r e a s e in fecundi ty , the popu la t ion w o u l d be r e d u c e d to abou t 3 0 % the v i rg in s i z e . If w e c o n s i d e r 1 0 0 % a s the l ikely b io log i ca l limit of a n i n c r e a s e in fecundi ty in a rea l sha rk popu la t ion (fecundity factor o f 2 in the figure), w e c a n s e e that the popu la t ion w o u l d be a l m o s t w i p e d out just by a n i n c r e a s e in f i sh ing mortali ty o f the s a m e m a g n i t u d e (F factor o f 2) . F ina l ly , f igure 3.9 i l lustrates h o w c h a n g e s in fecundi ty a n d f i sh ing mortal i ty affect N% m u c h m o r e than they affect B%. In this c a s e the a x e s o f the d i a g r a m a re s w i t c h e d to o b s e r v e m o r e c l ea r ly the different effects o f i n c r e a s e d fecundi ty in popu la t i on n u m b e r s a n d b i o m a s s . 184 Figure 3.6 R e s p o n s e surfaces of population numbers (left) and b iomass (right) to different fishing regimes during 100 years , when fecundity is increased 100% from basel ine values. V a l u e s of F are multipliers of the basel ine fishing mortality. Note the s teepness of both surfaces as F decreases , indicating the sensitivity of the model to changes in F. T h e axes in the figures have been switched to offer the best v iew of the surfaces. Figure 3.7 R e s p o n s e surface of population numbers to changes in fecundity during a 100 years ' period when F is 1.8 times the baseline value. C o m p a r e the flatness of the surface with that of figure 6. The axes in the figure have been switched to offer the best v iew of the surface. 185 3.5 F m u l t i p l i e r Figure 3.8 Proportion of the virgin population after 100 yr of fishing (N%) against fishing intensity (expressed as mulipliers), for different fecundity increase multipliers (numerals on each line). ( — baseline fishing mortality; .... virgin population s ize in numbers) . 186 Figure 3.9 R e s p o n s e surfaces of N% (upper) and B% (lower) as a function of fecundity increases and F values (both plotted as multipliers). Y axis reversed in order to facilitate view. 3.3.3 A tropical paradigm with management applications. 187 In this section the model is used to analyse a real tropical shark fishery and to explore the possible management applications of the model. The data comes from the silky shark (Carcharhinus falciformis) fishery of Yucatan. This species grows at least to 23 years of age and females attain maturity at around 12 years. The sex ratio at birth is very close to 1:1, birth takes place during a well defined 2-3 month period, gestation time is approximately 12 months, and maximum fecundity is around 14 embryos perfemale (Bonfil 1990). There is no length-fecundity relationship described forthis species. Therefore, values were simulated by allocating a fecundity of 2 for the first time mothers (age 12) and increasing this by 2 with each year of age until a fecundity of 12 was reached. Fecundity is maintained in this level for most females with the exception of the last 2 age groups which have a fecundity of 14. By halving the above mentioned figures, only female embryos are considered in the actual modelling. The entire reproductive cycle is here assumed to take 1 year, although some studies suggest that females might have a protracted cycle with a year of rest between pregnancies (Branstetter 1987). The growth in weight shown in figure 3.10 is used for the calculation of biomass. The fishery of Yucatan is composed of two fleets. A small scale directed fishery with gillnets capturing large adult sharks, and a large scale red grouper fishery using hook and line that catches large quantities of young silky sharks incidentally (Bonfil et al. 1990). There are some estimates of natural and total mortality forthis stock (Bonfil 1990) based on Pauly's formula (Pauly 1980), and catch curves constructed through the ELEFAN programme, respectively. From these, age specific values of both parameters were 'guesstimated'forthe different portions of the population. The natural mortality value of 0.2 derived by Bonfil (1990) was thought to be representative of most part of the population. However, considering the high natural mortality of young large tropical sharks (Manire & Gruber 1991), baseline natural mortality values chosen for age classes 0-2 were of 0.5, 0.4, and 0.3 respectively. For ages 3-6, a value of 0.25 was adopted. The a and p parameters of the density-dependent mortality relationship were hand-tuned as described in the previous section. The values used for the silky shark were of 2.5 x 10"7 188 a n d 0 .55 r e spec t ive ly . T h e ac tua l natural mortal i ty v a l u e s u s e d by the m o d e l a re s h o w n in figure 3 .11 . To ta l mortal i ty v a l u e s e s t i m a t e d from the c a t c h c u r v e s w e r e u s e d to subt rac t the b a s e l i n e natural mortal i ty e s t ima te s a n d ob ta in the c o r r e s p o n d i n g e s t ima te s o f f i sh ing mortal i ty. T h e latter w e r e i n c l u d e d in the m o d e l a s b a s e l i n e f i sh ing mortal i ty v a l u e s a n d a re thought to reflect w e l l the relat ive m a g n i t u d e o f f i sh ing mortali ty for the different par ts o f the popu la t ion in this f i shery . T h e s e v a l u e s a re 0 .05 for a g e 0, 0 .08 for a g e 1, 0 .15 for a g e s 2-8 , 0 .05 for a g e s 9-16, a n d 0 .23 for the r e m a i n i n g a g e g r o u p s . T h e m o d e l is u s e d to e x p l o r e four different s c e n a r i o s wi th m a n a g e m e n t a l t e rna t ives in m i n d . T h e first c h o i c e ( s cena r io 1) ma in t a in s the status quo. S c e n a r i o 2 c o n s i d e r s protec t ing the juven i l e por t ion o f the s tock by totally b a n n i n g the b y c a t c h o f s h a r k s in the h o o k a n d line f ishery, thus effect ively r e d u c i n g the f i sh ing mortal i ty o f a g e s 0-9 to z e r o . S c e n a r i o 3 r e d u c e s by half the f i sh ing mortal i ty of the preadul t a n d adul t s h a r k s c a u g h t by the d i rec ted gil lnet f ishery, p e r h a p s by i m p o s i n g a T A C . H e n c e , mortal i ty f o r a g e s 18-23 is r e d u c e d to 0 . 1 1 . F ina l ly , s c e n a r i o 4 is a total b a n o f the d i r ec t ed f i shery wi th g i l lne ts w h i c h r e d u c e s to z e r o the f i sh ing mortal i ty o f the a g e c l a s s e s c o n s i d e r e d in s c e n a r i o 3. F igu re 3 .12 s h o w s the popu la t i on trajectory for the next 100 y e a r s u n d e r the four s c e n a r i o s . If f i sh ing is kept at the p resen t l eve l ( s cena r io 1), the popu la t i on wi l l d e c l i n e by 5 0 % in 10 y e a r s t ime a n d wi l l a l m o s t d i s a p p e a r in 100 y e a r s (fig. 3 .12a) . If the s tock is to be c o n s e r v e d , pro tec t ing the j uven i l e por t ion of the popu la t i on ( s c e n a r i o 2) is by far the best op t ion for m a n a g e m e n t . T h i s m e a s u r e a l l o w s the gil lnet f i shery to be m a i n t a i n e d wh i l e k e e p i n g the s tock v e r y c l o s e to the v i rg in popu la t ion l eve l (fig. 3 .12b) . T h i s result is not su rp r i s ing . Bonf i l (1990) reports a s i tuat ion o f g rowth ove r f i sh ing in the s i lky sha rk f i shery of Y u c a t a n . T h e m a n a g e m e n t a l ternat ive of s c e n a r i o 2 is effect ively equ iva l en t to the w e l l k n o w n effect o f i n c r e a s i n g the a g e o f recrui tment in a f i shery suffer ing g rowth over f i sh ing (in the contex t of the B e v e r t o n & Hol t type o f m o d e l s ) , w h i c h is e x e m p l i f i e d g r aph i ca l l y in f igures 1-5 of B e d d i n g t o n & C o o k (1983) . In contras t , a r educ t ion o f 5 0 % o r a total b a n n i n g of the gil lnet f i shery wi l l not p r o d u c e s ignif icant benefi ts t o w a r d s the l o n g t e rm c o n s e r v a t i o n of the s tock . A c t u a l l y , for the long- term s i z e of the popu la t i on , t h e s e m e a s u r e s a re a lmos t a s b a d a s ma in t a in ing the status quo. 1 4 0 1 2 0 0 2 4 6 8 10 12 14 16 18 2 0 2 2 2 4 A G E Figure 3.10 Growth in weight of Carcharhinus falciformis from Yucatan, used for the calculation of population biomass. Figure 3.11 Age-specific density-dependent mortality coefficients used for the simulation of the silky shark (Carcharhinus falciformis). 190 NUMBERS IN POPULATION EVERY 10 YEARS NUMBERS IN POPULATION EVERY 10 YEARS m 1500000 30 40 50 60 70 YEARS OF F6HING \- 30000000 30 40 50 60 70 YEARS OF FISHING a) b) NUMBERS IN POPULATION EVERY 10 YEARS NUMBERS IN POPULATION EVERY 10 YEARS g 1500000 °$ 500000 30 40 50 60 70 YEARS OF FISHING > E 1000000 °0 500000 30 40 50 60 70 YEARS OF FISHING c) d) Figure 3.12. Forecas ted evolution of the silky shark fishery of Y u c a t a n under 4 different management scenarios: a) no change in estimated fishing mortality; b) a total ban of the bycatch of juveniles in the red grouper hook & line fishery; c) a reduction of 5 0 % in the fishing mortality from the gillnet fishery for adults; d) a total ban of the gillnet fishery for adults. ( tot. numbers; — recruits; .... tot. biomass) . 191 A l t h o u g h the p re sen t resul ts a re subject to uncer ta in ty in the v a r i o u s e s t i m a t e s o f mortali ty, the c o n c l u s i o n d e r i v e d from the m o d e l abou t the g e n e r a l b e h a v i o u r o f this f i shery s h o u l d be re la t ively robus t in the s e n s e that e v e n if the ind iv idua l a g e spec i f i c v a l u e s of M might not be accu ra t e , the g e n e r a l pattern of f i sh ing mortal i ty for the different s e g m e n t s o f the popu la t ion is p r o b a b l y g o o d e n o u g h to war ran t c o n s i s t e n c y o f the p re sen t resul ts . T h e four m a n a g e m e n t s c e n a r i o s a re a l s o a n a l y s e d u n d e r different i n c r e a s e s in f i sh ing mortali ty a n d fecundi ty . T h r e e p o s s i b l e popu la t ion r e s p o n s e s a r e c o n s i d e r e d in t e rms of fecundi ty i n c r e a s e : no c h a n g e , a n i n c r e a s e o f 4 0 % , a n d a n i n c r e a s e o f 1 0 0 % . F o r e a c h popu la t ion r e s p o n s e , f i sh ing mortal i ty is e i ther left a s in the e s t i m a t e d status quo o r it is d o u b l e d . T h e four m a n a g e m e n t s c e n a r i o s a re t hen c o n s i d e r e d for e a c h of t he se s ix c o m b i n a t i o n s o f F a n d fecundi ty . T h e resul ts of this a n a l y s i s ind ica te that if the popu la t ion c o u l d ac tua l ly r e s p o n d with a fecundi ty i n c r e a s e o f 1 0 0 % u n d e r the status quo f i sh ing mortali ty, m a n a g e m e n t s c e n a r i o 4 w o u l d main ta in abou t a 5 0 % of the v i rg in s tock n u m b e r s a n d a b o u t 3 0 % of its b i o m a s s (figure 3 .13) . In a l l c a s e s , m a n a g e m e n t s c e n a r i o 2 is still the bes t op t ion for the c o n s e r v a t i o n o f the s tock . Fu r the rmore , th is is the o n l y m a n a g e m e n t s c e n a r i o a l l o w i n g for f i sh ing mortali ty to be d o u b l e d wi thout the s tock b e i n g vir tual ly d r iven to ex t inc t ion , i n d e p e n d e n t l y o f the l eve l of fecundi ty o f the popu l a t i on . 3.4 D i s c u s s i o n . 3.4.1 S i m u l a t i o n resul ts a n d d o c u m e n t e d c h a n g e s in fecundi ty . W h i l e the p resen t s tudy c o n s i d e r s fecundi ty i n c r e a s e s o f up to 4 0 0 % a s a p o s s i b l e c o m p e n s a t o r y m e c h a n i s m , the o n l y d o c u m e n t e d c a s e o f a p p a r e n t c h a n g e s in fecundi ty of a n e l a s m o b r a n c h popu la t i on ( G a u l d 1979) s u g g e s t s fecundi ty i n c r e a s e s o f o n l y b e t w e e n 2 2 % a n d 7 4 % for different a g e g r o u p s for the S c o t t i s h - N o r w e g i a n s t o c k o f s p i n y dogf ish (Squalus acanthias). Independen t ly of the s ign i f i cance o f G a u l d ' s f ind ings w h i c h d o not s e e m to s u g g e s t i n c r e a s e s in the ac tua l fertility o f the popu la t i on , they a r e far f rom the large fecundi ty i n c r e a s e s found here to be o f a n y v a l u e for c o m p e n s a t i o n a g a i n s t light f i sh ing ( 4 0 0 % at b a s e l i n e F ; tab le 3.1). G a u l d a l s o s t ud i ed the s p a c e l imi ta t ions of t h e s p i n y dogf i sh b o d y cav i ty (without a l l o w i n g a n y s p a c e for food) a n d f o u n d that up to a 1 3 0 % i n c r e a s e in fecundi ty c o u l d be af forded in t e rms o f v o l u m e by this s p e c i e s . H o w e v e r , he c o n s i d e r s that BASELINE FECUNDITY 192 4 L O 0.8 0.6 0.4 0.2 0.0 0.2 0.4 0.6 0.8 1.0 B% N% 40% FECUNDITY I N C R E A S E 1.0 0.8 0.6 0.4 0.2 0.0 0.2 0.4 0.6 0.8 1.0 B% N% 100% FECUNDITY I N C R E A S E 1.2 0.8 0.4 0.0 0.4 0.8 1.2 B% N% Figure 3.13 Proportion of the b iomass (B%) and numbers (N%) from the virgin stock left after 100y of fishing under 4 management alternatives (numbered 1-4 in the Y axis), under initial and doubled fishing mortalities, for 3 different fecundities: Top-basel ine fecundity; Centre- increase of 4 0 % in fecundity; Bot tom- increase of 100% in fecundity. (For explanation of the management alternatives see text). 193 physiological factors such as gas exchange between embryos and uterus will probably prevent such a high increase in fecundity. In placental species such as the majority of the carcharhinid sharks, the amount of reserves required by the mother to nurture two times the normal quantity of embryos might pose a limit on fecundity increases. The elimination of toxins would represent an additional problem. Oviparous species such as most skates and some orectolobiformid sharks do not have space limitations for eggs production, however, fecundity increases in these species would be probably limited by the speed at which eggs can be laid and by the energy required to substantially increase production of the high quality egg yolk that feeds the embryos throughout their development. The present analysis demonstrates that independently of the potential capability of elasmobranchs to substantially increase their fecundity, the value of such a strategy is seldom sufficient to compensate for increases in fishing mortality. Adequate fisheries management must take into consideration this biological constraint of elasmobranchs. 3.4.2 Trade-off between fecundity and early life natural mortality. Given the importance of mortality during the first years of life highlighted during the present results, it is interesting to speculate on the possible secondary effects of fecundity increases. As outlined during the initial model building considerations, the time span and the magnitude of the vulnerability to predators is a function of the size of newborn sharks. If increases in fecundity encounter physiological or spatial limitations, this can be partially solved by reducing the average size of the litter at birth. However, this would reduce the success of the density-dependent fecundity strategy of the species. The net result would be that the average natural mortality of the litter, which as suggested by the present results is more important than fecundity, would be probably increased by the reduction in size at birth. Future versions of the model, hopefully implemented on a more efficient platform, could incorporate the relationship between natural mortality of newborns and their size at birth. This could be achieved by calculating a weighted average coefficient of natural mortality for pups during a few of their early years of life, based on the size frequency distribution in the 194 litters. T h e f r e q u e n c y dis t r ibut ion o f s i z e s at birth w o u l d in turn be a funct ion o f the dens i ty o f the litter a n d the s tructure of the parent s tock . 3.4.3 C o m p e n s a t o r y m e c h a n i s m s in e l a s m o b r a n c h popu la t i ons . T h e p resen t s tudy h a s p r o v i d e d e v i d e n c e that i n c r e a s e d fecundi ty a l o n e is v e r y unl ike ly to be the factor a c c o u n t i n g for the relat ive r e s i l i ence o f s o m e e l a s m o b r a n c h s tocks . U n r e a s o n a b l y l a rge i n c r e a s e s in fecundi ty are effective to coun t e r ac t for the l o s s e s , on ly u n d e r v e r y light f i sh ing morta l i t ies . T h e S c o t t i s h - N o r w e g i a n s tock o f s p i n y dogf i sh , w h i c h h a s b e e n p r o p o s e d a s a c a s e of i n c r e a s e d fecundi ty d u e to exp lo i ta t ion is k n o w n to be u n d e r h e a v y f i sh ing s i n c e a l ong t ime a g o ( A a s e n 1 9 6 3 , H o l d e n 1968) , a n d it is doubtful that c h a n g e s in fecundi ty a s t h o s e repor ted by G a u l d (1979) c o u l d e x p l a i n its appa ren t r e s i l i ence . T h e f ind ings p r e s e n t e d here p rov ide further suppor t to the c o n c l u s i o n s o f B r a n d e r (1981): mortal i ty o f the y o u n g a g e c l a s s e s is m o r e important for the capab i l i t y o f a s tock to wi ths t and f i sh ing , t han the fecundi ty o f the s p e c i e s . M y s tudy further suppor t s the resul ts of W o o d s e t a l . (1979) , r ega rd ing the n e e d o f u n r e a s o n a b l y h igh i n c r e a s e s in fecundi ty in o rde r to c o m p e n s a t e for f i sh ing in the Br i t i sh C o l u m b i a n s p i n y dogf i sh s tock . It is m o r e p robab l e that the u n d e r l y i n g m e c h a n i s m i n v o l v e d in the e v e n t u a l s u c c e s s o f a n e l a s m o b r a n c h popu la t ion to w i t h s t a n d a f ishery, if any , is a c o m b i n a t i o n o f r e s p o n s e s in w h i c h not on ly fecundi ty i n c r e a s e s up to a r e a s o n a b l e limit, but a l s o g rowth s p e e d s up a n d r e d u c e s the a g e at first maturi ty. T h i s w o u l d p robab ly be c o m p l e m e n t e d wi th d e n s i t y - d e p e n d e n t natural mortal i ty o f the s tock, s p e c i a l l y f o r t h e ea r ly a g e s . In h igh ly m o b i l e s p e c i e s , the immigra t ion of i nd iv idua l s f rom n e a r b y s t o c k s c o u l d a l s o cont r ibute to c o m p e n s a t e for f i sh ing mortali ty. 3.4.4 C o n s i d e r a t i o n s for f i sher ies m a n a g e m e n t a n d r e s e a r c h . T h e p resen t m o d e l h ighl ights the n e e d to ob ta in m o r e a c c u r a t e e s t i m a t e s o f both natural a n d f i sh ing mortal i ty in o r d e r to be a b l e to m a k e better s i m u l a t i o n s o f a n e l a s m o b r a n c h s tock . T h e m o d e l a l s o s u g g e s t s that budge t i ng great a m o u n t s o f effort a n d m o n e y to ve ry de ta i l ed s tud ie s o f fecundi ty a n d the reproduc t ive c y c l e o f the s p e c i e s w o u l d be a w r o n g a l l oca t ion of r e s e a r c h r e s o u r c e s . 195 A l t h o u g h the p resen t m o d e l is a useful tool for a rough e v a l u a t i o n o f the o u t c o m e o f ve ry g e n e r a l m a n a g e m e n t a l te rna t ives , the a d e q u a t e m a n a g e m e n t o f a n e l a s m o b r a n c h popu la t ion th rough the ut i l izat ion of a m o d e l l ike this wi l l requi re major i m p r o v e m e n t s . First of a l l , the na tura l uncer ta in ty o f the popu la t ion d y n a m i c s a n d the f i sh ing p r o c e s s s h o u l d be inco rpo ra t ed by d o i n g a fully s t o c h a s t i c m o d e l . A m o d u l e l ink ing effort wi th f i sh ing mortali ty w o u l d be the bes t w a y to integrate in the m o d e l v a r i a b l e s s u s c e p t i b l e o f di rect m e a s u r e m e n t in a rea l f i shery s y s t e m . A n o t h e r important add i t ion for m a n a g e m e n t c o n s i d e r a t i o n s w o u l d be the i n c l u s i o n o f expl ic i t e s t ima tes o f the c a t c h , to g ive a n i d e a o f the s o c i o - e c o n o m i c impac t of the m a n a g e m e n t a l te rna t ives in the short a n d the l o n g t e rm. Unfor tunate ly , i nc lud ing a l l t h e s e i m p r o v e m e n t s w o u l d require the e s t ima t ion of a d d i t i o n a l p a r a m e t e r s a n d it w o u l d bu i ld o n the c o m p l e x i t y o f the m o d e l , a n d this is b e y o n d the s c o p e o f the present s tudy. If the a b o v e s u g g e s t i o n s a re even tua l ly i nco rpo ra t ed , the m o d e l c o u l d t hen be u s e d to better exp lo re the o u t c o m e o f different regula t ion m e a s u r e s d e p e n d i n g o n the priori t ies of the m a n a g e r s . F o r e x a m p l e , in the c a s e of the s i lky s h a r k s of Y u c a t a n , the a i m c o u l d be to main ta in the popu la t i on at not l e s s than 5 0 % of the v i rg in s tock . B y b a n n i n g the b y c a t c h of j u v e n i l e s f rom the h o o k a n d l ine f i shery a n d wi thout e x p e c t i n g a n y c h a n g e s in fecundi ty , the m a n a g e m e n t objec t ive c o u l d be a c h i e v e d e v e n by i n c r e a s i n g the c a t c h e s o f the d i rec ted gil lnet f i shery for la rge s h a r k s . T h e u s a g e o f this type o f m o d e l for short t e rm fo recas t s is not r e c o m m e n d e d . M o d e l a p p r o x i m a t i o n s in the short t e rm a re subject to osc i l l a t ing b e h a v i o u r , a s s h o w n dur ing the first 2 0 to 3 0 y e a r s o f f i sh ing in s o m e runs o f the m o d e l . A s the t ime s p a n is e x t e n d e d , t he se osc i l l a t i ons b e c o m e l e s s important a n d vir tual ly d i s a p p e a r , wi thout s e r i o u s effects o n the long- te rm output c o n s i d e r e d here (N%, B%). Fu r the rmore , the fo recas t s o f this type of m o d e l s for the first f ew d e c a d e s s h o u l d be ve ry sens i t i ve to input p a r a m e t e r s . 3.4.5 F r o m m o d e l to reality. T h e r e a re s e v e r a l a s s u m p t i o n s that still n e e d to be v a l i d a t e d o r mod i f i ed for the m o d e l to be m o r e rea l i s t ic (a l though here w e mus t r e m e m b e r that rea l i s t ic m o d e l s a re not a l w a y s better m o d e l s ) . First , the e x i s t e n c e o f a d e n s i t y - d e p e n d e n t na tura l mortal i ty re la t ionsh ip h a s 196 not b e e n c o n c l u s i v e l y d e m o n s t r a t e d yet in a n y e l a s m o b r a n c h a n d wi l l p r o b a b l y r e m a i n wi thout subs tan t i a l p roof for m a n y y e a r s . In add i t ion , the s h a p e o f th is d e n s i t y - d e p e n d e n t m e c h a n i s m c o u l d t ake o ther fo rms rather t han the l inea r funct ion u s e d he re ( W a l k e r 1992) . V e r y l ikely , a n i n c r e a s e in the fecundi ty of a v i v i p a r o u s s p e c i e s w o u l d h a v e to be regu la ted th rough a true d e n s i t y - d e p e n d e n t m e c h a n i s m a n d w o u l d invo lve a t ime l ag w h i c h c o u l d be c o n s i d e r a b l e for s p e c i e s with late first s e x u a l matura t ion , a n d not a s in the p resen t m o d e l in w h i c h fecundi ty i n c r e a s e is a n au toma t i c swi tch that turns o n a s s o o n a s the f i sh ing boa ts a p p e a r o n s c e n e . T h e expl ic i t r ep resen ta t ion of a s tock- recru i tment re la t ionsh ip c o u l d a d d a n e w d i m e n s i o n to the m o d e l . T h i s w o u l d be a n a l ternat ive for bu i ld ing a "real is t ic" popu l a t i on that g r o w s in a log is t ic f a s h i o n , wi thout u s i n g a d e n s i t y - d e p e n d e n t na tura l mortal i ty funct ion a s it w a s d o n e he re . It mus t be no ted , h o w e v e r , that a s tock- recru i tment r e l a t ionsh ip is in fact a ve ry spec i f i c c a s e o f d e n s i t y - d e p e n d e n t natural mortal i ty in w h i c h o n l y the mortal i ty o f the pre-recrui ts is c o n s i d e r e d . 197 C H A P T E R 4 A M O N T E C A R L O ANALYS IS O F F ISHERY M O D E L S FOR S HARKS . 4.1 Introduct ion. 4.1.1 P r o b l e m s for sha rk f i sher ies a s s e s s m e n t a n d m a n a g e m e n t . E l a s m o b r a n c h s , par t icular ly s h a r k s , a re b e l i e v e d to be i n c a p a b l e o f s u s t a i n i n g in tens ive f i sher ies ( see C h a p t e r s 1, 3) . U n d e r this c i r c u m s t a n c e it w o u l d be v e r y useful to find m e t h o d s to b a l a n c e b e t w e e n the explo i ta t ion o f s h a r k s a n d rays a n d the i r c o n s e r v a t i o n . Unfor tunate ly , the a s s e s s m e n t a n d m a n a g e m e n t o f e l a s m o b r a n c h f i she r i es h a s p r o v e d ex t r eme ly difficult. P e r c e i v e d p r o b l e m s o f l ack o f a d e q u a t e f i shery m o d e l s h a v e p r e c l u d e d the ut i l izat ion o f s i m p l e s u r p l u s p roduc t ion m o d e l s n e e d i n g o n l y c a t c h a n d C P U E da ta . O n the o ther h a n d , the shor t age o f appropr i a t e de t a i l ed b i o l o g i c a l / e c o l o g i c a l d a t a m a k e s difficult the a p p l i c a t i o n o f a s s e s s m e n t p r o c e d u r e s b a s e d o n age - s t ruc tu red da ta . T h e pe rcep t ion that su rp lu s -p roduc t i on m o d e l s a re not appropr i a t e for s h a r k s a n d rays s t e m s m a i n l y from the ea r ly w o r k o n e l a s m o b r a n c h f i sher ies . W h i l e s t udy ing f i sher ies for the Nor th S e a s p i n y dogf i sh Squalus acanthias, H o l d e n (1977) s ta ted that the a s s u m p t i o n s of su rp lus p roduc t ion m o d e l s do not h o l d r ega rd ing i m m e d i a t e r e s p o n s e in the rate of popu la t ion g rowth to c h a n g e s in popu la t ion a b u n d a n c e , a n d i n d e p e n d e n c e o f the rate of natural i n c r e a s e from the a g e c o m p o s i t i o n of the s tock . H o l d e n b a s e s t h e s e c o n c l u s i o n s ma in ly o n the t ime d e l a y s c a u s e d by the typ ica l ly l o n g r ep roduc t ive c y c l e s o f this s p e c i e s a n d the s u s p e c t e d v e r y direct re la t ionsh ip b e t w e e n s tock a n d recrui ts . B e c a u s e s u c h life his tory cha rac t e r i s t i c s a re c o m m o n to mos t e l a s m o b r a n c h s , this h a s c a u s e d su rp lu s -p roduc t ion m o d e l s to be d i s r e g a r d e d for a s s e s s i n g e l a s m o b r a n c h f i she r ies . In o ther w o r d s , H o l d e n ' s t hough t s h a v e b e e n e c h o e d in the w o r k s o f o ther sc ien t i s t s w h o o p t e d for the more de ta i l ed a p p r o a c h offered by age - s t ruc tu red m o d e l s (e .g. W o o d et a l . 1 9 7 9 , W a l k e r 1992) . 198 W h a t is often o v e r l o o k e d is that there a re a l s o p r o b l e m s in a p p l y i n g t h e s e rea l i s t ic a g e -s t ructured m o d e l s . A g e - s t r u c t u r e d da ta is m u c h m o r e difficult a n d e x p e n s i v e to ob ta in . Fu r the rmore , the life c y c l e s of mos t s p e c i e s , e v e n in t e rms o f the b a s i c p a r a m e t e r s of age , growth a n d r ep roduc t ion , h a v e just s tar ted to be u n v e i l e d du r ing the last 15 y e a r s , a n d this on ly in the c a s e o f a handful o f s t o c k s ( see Pratt a n d C a s e y (1990) for a r ev iew) . In add i t ion , there a re s o m e re levant a r e a s of e l a s m o b r a n c h popu la t i on d y n a m i c s w h i c h a re still l a rge ly a mys te ry . F o r e x a m p l e : direct ly d e r i v e d s tock- rec ru i tment r e l a t i onsh ips have n e v e r b e e n d o c u m e n t e d for a n y e l a s m o b r a n c h , a l t hough a ve ry s t rong re l a t ionsh ip is s u s p e c t e d d u e to the r ep roduc t ive s t ra teg ies o f the g roup ( H o l d e n d 1 9 7 3 , Hof f 1990) ; the s i z e , s tructure a n d spa t i a l d y n a m i c s o f mos t s t o c k s / s u b s t o c k s o f e l a s m o b r a n c h s a re a l m o s t totally u n k n o w n . Inadequa te k n o w l e d g e o f migra t ion routes , s tock de l imi ta t ion a n d m o v e m e n t rates a m o n g s t t h e m , c a n s e r i o u s l y u n d e r m i n e o the rwi se " so l id" a s s e s s m e n t s a n d m a n a g e m e n t r e g i m e s . 4 .1 .2 H o l d e n ' s v i e w a n d m o d e r n m e t h o d s in f i sher ies s c i e n c e . C o n s i d e r i n g the p r o b l e m s ou t l i ned a b o v e , there s e e m to be t w o m a i n w a y s to a p p r o a c h the p r o b l e m s of e l a s m o b r a n c h f i sher ies a s s e s s m e n t a n d m a n a g e m e n t . T h e first is to invest mi l l ions o f do l l a r s a n d wai t s e v e r a l y e a r s in o r d e r to co l l ec t sufficient age - s t ruc tu red da ta for a s s e s s m e n t , a n d to ga in a better u n d e r s t a n d i n g o f e l a s m o b r a n c h b i o l o g y that war ran t s conf ident u s a g e o f age - s t ruc tu red m o d e l s . T h e s e c o n d op t ion is to f ind a l ternat ive s i m p l e m o d e l s for e l a s m o b r a n c h s tudy a n d m a n a g e m e n t . In this s e n s e , it is wor thwhi l e r e c o n s i d e r i n g the v i e w s o f H o l d e n u n d e r the light o f n e w a d v a n c e s in f i she r i es s c i e n c e . A p a r a m o u n t o b s t a c l e f o r t h e u s e o f classic su rp lu s -p roduc t ion m o d e l s in the 60 ' s a n d part of the 70 ' s w a s the equ i l ib r ium const ra in t . B a c k then , sc ien t i s t s w e r e fo r ced to a s s u m e that popu la t ions w e r e in equ i l i b r ium at a l l explo i ta t ion l eve l s (i .e. that e v e r y c a t c h o b s e r v e d w a s sus t a inab le ) in o r d e r to s implify the p r o c e s s o f fitting s u r p l u s - p r o d u c t i o n m o d e l s to da ta . T h e d a n g e r o u s c o n s e q u e n c e s o f this a s s u m p t i o n are w e l l k n o w n a n d expl ic i t ly w a r n e d aga in s t in f i shery text b o o k s (P i t che r a n d Har t 1982 , H i lbo rn a n d W a l t e r s 1992) . H o w e v e r , the c o m p u t e r revolu t ion h a s h e l p e d to o v e r c o m e the equ i l i b r ium cons t ra in t t h rough non- l inea r op t imisa t ion rou t ines w h i c h a re a v a i l a b l e to vir tual ly a n y f ishery sc ient i s t in the w o r l d . T h e d ivers i ty o f a p p r o a c h e s this offers for fitting su rp lus p roduc t ion m o d e l s h a s t r ans la t ed into 199 a n e w e r a o f popula r i ty for the ut i l izat ion o f w h a t a re p resen t ly k n o w n a s dynamic su rp lus p roduc t ion m o d e l s (Punt 1988 , P r a g e r 1994 , P o l a c h e k et a l . 1993) . P e r h a p s the mos t in teres t ing o u t c o m e o f a l l this is the v i e w that mos t o f the p r o b l e m s a s s o c i a t e d with s u c c e s s f u l l y a p p l y i n g su rp lus p roduc t ion m o d e l s a re d u e to the qua l i ty o f the f i sher ies da ta (Hi lborn 1979) , a n d the f inding that s i m p l e su rp lus p roduc t ion f i shery m o d e l s c a n s o m e t i m e s perform better t han the m o r e e l abora t e a n d b io log ica l ly de t a i l ed age - s t ruc tu red a p p r o a c h e s ( L u d w i g a n d W a l t e r s 1 9 8 5 , 1989 , P u n t 1991) . T h e s e c h a l l e n g i n g a n d re la t ively n e w points o f v i e w o p e n the q u e s t i o n o f w h e t h e r d y n a m i c su rp lus p roduc t ion m o d e l s c a n be s u c c e s s f u l l y a p p l i e d to e l a s m o b r a n c h f i sher ies . In o ther w o r d s , w e n e e d to find out w h e t h e r g o o d qual i ty da t a a n d the e m a n c i p a t i o n from the equ i l ib r ium a s s u m p t i o n war r an t s the u s e o f su rp lus p roduc t ion m o d e l s for e l a s m o b r a n c h f i sher ies a s s e s s m e n t a n d m a n a g e m e n t . T h e p resen t s tudy a i m s t o w a r d s a d v a n c i n g o u r u n d e r s t a n d i n g o f e l a s m o b r a n c h f i sher ies a s s e s s m e n t a n d m a n a g e m e n t m e t h o d s by a n s w e r i n g three q u e s t i o n s : c a n s i m p l e m o d e l s that u se c a t c h a n d effort da t a be u s e d for sha rk p o p u l a t i o n s ? , if y e s , w h i c h is the bes t from two su rp lus p roduc t ion a n d o n e part ia l ly age - s t ruc tu red m o d e l s ? , a n d final ly, a re s u c h m o d e l s a p p l i c a b l e u n d e r different s c e n a r i o s o f s tock / recru i tment re la t ionsh ip , spa t ia l behav iou r , s tock product ivi ty , a n d da t a qua l i ty? (i.e. h o w robus t a r e t hey? ) . 4 .2 M e t h o d s . 4.2.1 G e n e r a l a p p r o a c h . T h e p resen t s tudy re l ies o n the "opera t ing m o d e l " a p p r o a c h o f L inha rd t a n d Z u c c h i n i (1986) . T h i s a p p r o a c h i n v o l v e s cons t ruc t ing a de ta i l ed , c r ed ib l e m o d e l to r ep re sen t h o w the s y s t e m b e h a v e s in reali ty (the operating model), i n c lud ing a l l re levant a s p e c t s o f b i o l o g i c a l a n d n o n -b i o l o g i c a l d y n a m i c s (i.e. age-s t ruc ture , f i sh ing s y s t e m s , da t a c o l l e c t i o n , etc.) . T h e s e c o n d s tep is to bu i ld estimation procedures for the s y s t e m , b a s e d u s u a l l y o n s i m p l e r m o d e l s that a re f eas ib l e for p rac t i ca l s i tua t ions . F ina l ly , the ope ra t ing m o d e l is u s e d to gene ra t e large da ta se ts in a s t o c h a s t i c manne r , a n d t he se da ta a re u s e d by the e s t ima t ion p r o c e d u r e s to find p a r a m e t e r e s t ima te s for e a c h da t a set. T h e s e e s t ima te s a re t hen c o m p a r e d to the ' rea l ' 2 0 0 v a l u e s from the o p e r a t i n g m o d e l , p rov id ing a direct m e a n s of e v a l u a t i n g the pe r fo rmance (bias , uncer ta in ty , p a r a m e t e r con found ing ) of e a c h es t ima t ion p r o c e d u r e . T h e m e t h o d u s e d to gene ra t e the la rge n u m b e r o f da t a se t s from the o p e r a t i n g m o d e l s is k n o w n a s a M o n t e C a r l o p r o c e s s . T h i s c o n s i s t s o f c r ea t ing da t a se t s that differ from e a c h o ther by the effect o f o n e o r m o r e t ypes o f r a n d o m errors , g e n e r a l l y a s s u m e d to be e i ther no rma l ly o r l o g n o r m a l l y d is t r ibuted . M o n t e C a r l o a n a l y s i s r ep re sen t s the state o f the art in f i sher ies a n d h a s b e e n e x t e n s i v e l y u s e d for tes t ing v a r i o u s t y p e s o f f i she r i es m o d e l s (Hi lborn 1 9 7 9 , L u d w i g a n d W a l t e r s 1 9 8 5 , P u n t 1988 , F o u r n i e r a n d W a r b u r t o n 1989 , P o l a c h e k et a l . 1993) . T h e p r o c e d u r e f o r t h e s tudy is s u m m a r i s e d in figure 4.1 a n d c o n s i s t s o n first cons t ruc t ing a s e r i e s o f s imu la t i on m o d e l s of s h a r k popu la t i ons i n v o l v i n g s t o c h a s t i c effects that reflect 'natural ' va r i a t i ons o f b io log i ca l p r o c e s s e s . A n in tegra ted s t o c h a s t i c f i shery s u b s y s t e m ( inc lud ing 'natural ' va r i a t ions in ca tchabi l i ty) is then u s e d to ha rves t the popu la t ion a n d gene ra t e t ime s e r i e s of c a t c h a n d C P U E da ta from e a c h s i m u l a t i o n . A total o f o n e h u n d r e d M o n t e C a r l o s i m u l a t i o n s (trials) a re pe r fo rmed with e a c h o p e r a t i n g m o d e l a n d the da ta are fed to e a c h o f the three f i shery es t imat ion m o d e l s u n d e r scru t iny . F ina l ly , the a s s e s s m e n t a n d m a n a g e m e n t p a r a m e t e r s o b t a i n e d from e a c h es t ima t ion m o d e l a r e c o m p a r e d aga ins t the rea l v a l u e s k n o w n from the ope ra t i ng m o d e l . T h i s c o m p a r i s s o n a l l o w s to ca l cu la t e pe r fo rmance s tat is t ics f o r t h e 100 tr ials . T h e s e stat is t ics ind ica te the p r e c i s i o n a n d b ias in p a r a m e t e r e s t ima te s for e a c h of the different e s t ima t ion p r o c e d u r e s . 4 .2 .2 A set o f s imu la t i on m o d e l s of a s h a r k popu la t ion u n d e r exp lo i t a t ion . T h e different s h a r k popu la t i on m o d e l s c o n s i d e r e d in this s tudy a r i s e f rom a s s u m i n g different t ypes of s t o c k recrui tment re la t ionsh ip a n d spa t ia l b e h a v i o u r (as r e p r e s e n t e d by the re la t ionsh ip b e t w e e n ca tchab i l i ty a n d s tock b i o m a s s ) . B e c a u s e t h e s e t w o traits s t and a s s o m e o f the l e s s k n o w n cha rac te r i s t i c s in the c a s e o f e l a s m o b r a n c h s a n d a re a l s o o f great r e l e v a n c e for the de te rmina t ion o f k e y f i sher ies pa r ame te r s , it is impor tant to exp lo re the effect o f uncer ta in ty in t h e s e traits o n the es t ima t ion of a s s e s s m e n t a n d m a n a g e m e n t p a r a m e t e r s for e l a s m o b r a n c h s . T h e two va r i a t ions o f the s tock recru i tment r e la t ionsh ip are the fami l ia r m o d e l s o f B e v e r t o n a n d Hol t (1957) a n d R i c k e r (1954) . T h e different spa t ia l 201 Operating Models (= "Populations") I Stochastic Shark Population Models (Harvest Model Estimation Procedures 100 Monte Carlo Simulations Estimates of Biomass Estimates of Management Parameters Summary Statistics Figure 4.1 Schematic representation of the approach taken in this study for testing the performance of three fisheries models for the estimation of assessment and management parameters for shark fisheries. The process is repeated 100 times (Monte Carlo simulations). 2 0 2 b e h a v i o u r m o d e l s a re de t a i l ed in the fo l lowing s ec t i on . A d d i t i o n a l l y , two s i z e s o f the f i shab le s tock a re c o n s i d e r e d for e a c h s h a r k popu la t ion by s e l e c t i n g o n e o f t w o p o s s i b l e a g e s at first cap tu re . T h i s m e a n s that for a g i v e n sha rk popu la t ion , e a c h ' f i shable s tock ' h a s a different rate o f i n c r e a s e a n d M S Y leve l . E a c h ope ra t ing m o d e l ( O M ) is c o m p o s e d o f a s i m u l a t e d popu la t ion a n d a par t icu la r f i sh ing s u b - m o d e l . In a g r e e m e n t wi th the m o d e l d e v e l o p e d in c h a p t e r three , o n l y the f e m a l e part of the popu la t ion is c o n s i d e r e d (ma le s are a s s u m e d not to be l imit ing for recrui tment) . T h e popu la t ion is s i m u l a t e d by a set of difference e q u a t i o n s . T h e s h a r k popu l a t i on m o d e l is fully age - s t ruc tu red (26 a g e c l a s s e s ) wi th a g e - s p e c i f i c fecundi ty a n d dens i ty d e p e n d e n t natural mortal i ty. V u l n e r a b i l i t y to the f i sh ing g e a r is a l s o a g e - s p e c i f i c a n d the t w o a l ternat ive a g e s of first cap tu re a re 4 y r a n d 7y r (fig. 4 .2) . G r o w t h is s l o w a n d s e x u a l maturi ty is a t ta ined re la t ively late (age 10) in a n at tempt to m i m i c the k n o w n b i o l o g y o f the c a r c h a r h i n i d s h a r k s , w h i c h a re the m o s t c o m m o n l y exp lo i t ed a r o u n d the w o r l d . T h e popu la t i on is m o d e l l e d in the fo l lowing m a n n e r : In the a b s e n c e o f f i sh ing , total n u m b e r of s h a r k s in the entire popu la t i on next y e a r / V t t 1 a re c a l c u l a t e d a c c o r d i n g to the set o f equa t ions : (4.1) dS, b+S, Beverton-Holt (4.2a) o r Ricker (4.2b), 2 0 3 Age fee. M vul(1) vul(2) Weight 0 0 0.4 0 0 0 . 0 0 5 1 0 0.3 0 0 0 .03 2 0 0.2 0 0 0 .07 3 0 0 .15 0 0 0 .14 4 0 0 .12 0 1 0.21 5 0 0.1 0 1 0.29 6 0 0 .09 0 1 0 .37 7 0 0 .09 1 1 0 .44 8 0 0 .09 1 1 0 .52 9 0 0 .09 1 1 0.58 10 0.8 0 .09 1 1 0 .64 11 1 0 .09 1 1 0 .69 12 1.5 0 .09 1 1 0 .74 13 2 0 .09 1 1 0.78 14 2.5 0 .09 1 1 0.81 15 3 0 .09 1 1 0 .84 16 3.5 0 .09 1 1 0 .87 17 4 0 .09 1 1 0 .89 18 4.5 0 .09 1 1 0.91 19 5 0 .09 1 1 0 .92 2 0 5 0 .09 1 1 0 .93 21 5 0 .09 1 1 0 .94 2 2 5 0 .09 1 1 0.95 23 6 0 .09 0 .97 0.97 0 .96 2 4 6 0 .09 0 .93 0 .93 0 .97 25 6 0 .09 0 .93 0 .93 0 .97 growth 1.2 n age Vulnerability and Mortality 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 age — • — M - - o - - v u l ( 1 ) vul(2) Figure 4.2 Characteristics of the simulated age structured shark population. The two vulnerability schedules differ only in the age of entry to the fishery. 204 max (4.3) S, Here, Nta is the number of organisms of age a at the beginning of year t, Ma is the natural mortality coefficient of fish aged a, A m a x denotes the maximum age after which all organisms die, Amat is the age of first sexual maturity, S t + 1 is the parental stock at the end of the year measured as total number of fertilised eggs or embryos produced, f?t+1 is the number of sharks actually recruited to the population at the beginning of the year (assuming single pulse birth), and <pa is the product of the age specific fecundity times the number of broods per year. Recruitment is described using either the Beverton and Holt (4.2a) or the Ricker recruitment model (4.2b), where d, b, a', b' are constants. Natural variation in the recruitment process is represented by the term ©, an independent normally distributed random variable with mean 0 and variance a2. As the model only tracks the female part of the population, age specific fecundities are set to about half the values commonly found in carcharhinid sharks. The number of broods per year is here assumed to be one, although in reality this number ranges from less than 0.5 to 2 or more depending on the species. For simplicity, the fishery submodel assumes a type I fishery (Ricker 1975) with a fishing season short enough to allow ignoring natural mortality during harvest. For each simulation, once the population stabilises at carrying capacity, the fishery subsystem produces 20 years of catch and CPUE data using the following equations: C,= L [ j y i - « (4.4) 2 0 5 CPUE=-t ' ft (4.5) w h e r e C t is total c a t c h in we igh t dur ing y e a r t, qa is the a g e - s p e c i f i c ca tchab i l i ty coeff icient for a g e a ( ca l cu l a t ed by mul t ip ly ing the ove ra l l b a s e l i n e ca tchab i l i ty coeff ic ient q=0.Z by the vulnerab i l i ty at a g e ; s e e f ig. 4.2), ft the total effort in y e a r t, Wa is the a v e r a g e w e i g h t at a g e a. Na tu ra l va r i a t ion in the f i sh ing s y s t e m is r e p r e s e n t e d by v, a n o r m a l l y d is t r ibuted r a n d o m va r i ab l e with m e a n 0 a n d v a r i a n c e a 2 . H i lbo rn (1979) h a s s h o w n that da ta cont ras t (i.e. the o c c u r r e n c e o f s t rongly d i s s i m i l a r v a l u e s in proximity) h a s a subs tan t i a l effect o n the abil i ty o f f i shery m o d e l s to o v e r c o m e p o s s i b l e c o n f o u n d i n g effects a n d p rov ide a c c u r a t e e s t ima te s of the p a r a m e t e r s . H e r e , two pat terns of f i sh ing effort, e a c h p rov id ing a different a m o u n t of contras t , w e r e u s e d in the ha rves t ing s u b m o d e l s . T h e first effort s e q u e n c e i n c l u d e d a n initial br ief p e r i o d o f light effort f o l l owed by a l onge r p e r i o d o f v e r y h igh effort, a n d then a v e r y l o n g r e c o v e r y p e r i o d o f l ow efforts (fig. 4.3a). S u c h a n effort pattern is k n o w n to p rov ide h igh cont ras t in both effort a n d C P U E ( L u d w i g a n d W a l t e r s 1985) . T h e s e c o n d , l e s s informat ive effort pat tern w a s u s e d in a subse t o f s i m u l a t i o n s to test the r o b u s t n e s s o f the es t ima t ion m o d e l s to da t a qua l i ty (fig. 4.3b). T h i s latter effort s e q u e n c e is s o m e w h e r e b e t w e e n a 'one w a y trip' a n d 'up a n d d o w n the i soc l ine ' o f H i l b o r n a n d W a l t e r s (1992) a n d c o n s i s t s o f a rap id ly i n c r e a s i n g effort at the b e g i n n i n g of the se r i e s , a n d s l o w l y d e c l i n i n g efforts a s c a t c h e s d i m i n i s h . D u r i n g f i sh ing , popu la t i on n u m b e r s are s i m u l a t e d with va r i a t i ons of e q u a t i o n s 4.1 a n d 4.3: max -(M0+qafte'Vt). (4.6) Nt+i=Z[Nt>ae o High contrast effort 2 j 1.8 --1.6 --0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 a) y e a r s Uninformative effort 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 b) years Figure 4.3 Effort patterns used to simulate the harvesting process in the operating models, a) High contrast effort, b) Uninformative effort. 2 0 7 max _„ (4.7) i y t M e Ja a=A, 'mat Given that estimates from the fishery models pertain only to the exploitable part of the population, the population model keeps track only of biomass corresponding to the exploited part of the stock. The equation used for biomass calculation is: A max S r ^ V a (4.8) a=k where 6, is total biomass in year r, and k is the age of recruitment to the fishing gear. A total of 100 different sets of values for the random variables v and ca were used to produce the 100 Monte Carlo simulations for each operating model. In order for the model to simulate small enough variations in agreement with the life history characteristics of elasmobranchs, these random variables both had standard deviation 0 .05 in all trials. 4.2 .3 Spatial behaviour and its representation in the operating models. Three types of spatial behaviour (characterised by the relationship between CPUE and abundance offish) were simulated in the operating models. Traditionally, fisheries models assume that fishing effort is distributed at random in relation to the fish (Hilborn and Walters 1992) resulting in a CPUE that is proportional to abundance. This is known as the proportionality assumption and is the cornerstone for the traditional usage of commercial CPUE data as an index offish abundance. Two other types of relationship between CPUE and abundance are shown in figure 4 .4 . Hyperstability occurs when catchability changes inversely with biomass, causing CPUE to remain high while abundance drops. The overall result is an artificial appearance of stability in relative abundance. This phenomenon is probably the most common in real fisheries and 208 STOCK SIZE Figure 4.4 Theoretical relationships between cpue and abundance. 2 0 9 is k n o w to be true for s p e c i e s with s t rong s c h o o l i n g b e h a v i o u r s u c h a s c l u p e i d s a n d s o m e tunids , o r w h e n fish c o n g r e g a t e at k n o w n t imes a n d p l a c e s for r ep roduc t ive p u r p o s e s (e .g. o r a n g e r o u g h y in the C h a t h a m R i s e of N.Z.). Hypers tab i l i ty is a l s o c o m m o n w h e n f i she rmen h a v e la rge h a n d l i n g t i m e s with r e spec t to s e a r c h t imes , a n d w h e n they d o not s e a r c h fish r a n d o m l y . In contras t , a s i tuat ion o f hype rdep l e t i on is c h a r a c t e r i s e d by a ca tchab i l i ty coeff icient that d rops rap id ly a s a b u n d a n c e d e c r e a s e s . In this c a s e , C P U E falls fas ter than the rea l a b u n d a n c e , c r ea t ing a n a p p e a r a n c e o f d e p l e t e d s t o c k s w h e n in real i ty a b u n d a n c e is still h igh . A l t h o u g h h y p e r d e p l e t i o n a p p e a r s to be a l e s s f requent s i tua t ion in rea l f i sher ies (Hi lborn a n d W a l t e r s 1992) it c a n be f o u n d w h e n fish h a v e vi r tual s a n c t u a r i e s (spat ial ly s e c l u d e d a r e a s u n a c c e s s i b l e to exploi ta t ion) a n d f i sh ing p r o c e e d s by dep l e t i ng t hose a r e a s w h e r e f ish a r e v u l n e r a b l e to the gear . T h e three t ypes of a b u n d a n c e - C P U E re la t ionsh ip ou t l i ned a b o v e c a n be m o d e l l e d by d e n s i t y - d e p e n d e n t ca tchab i l i ty funct ions . S e v e r a l m o d e l s of dens i t y d e p e n d e n t ca tchabi l i ty w e r e e x p l o r e d for the represen ta t ion of hype rdep l e t i on a n d hypers tab i l i ty in the ope ra t ing m o d e l s , i n c l u d i n g t hose o f F o u r n i e r a n d D o o n a n (1987) a n d C s i r k e (1989) . H o w e v e r the first m o d e l d id not r e p r o d u c e hypers tabi l i ty sat isfactor i ly a n d the latter c o u l d not r epresen t the h y p e r d e p l e t i o n c a s e . T h e funct ion c h o s e n f o r t h e p resen t s tudy (Wal t e r s pers . c o m m . ) is: w h e r e qa is the a g e - s p e c i f i c ca tchabi l i ty , Bt is b i o m a s s at t ime t, K the c a r r y i n g c a p a c i t y o r v i rgin b i o m a s s , a n d Q a cons tan t . A s s h o w n in f igure 4 .5 , w h e n Q = 1 ca tchab i l i ty is cons tan t a n d the re la t ionsh ip is the u s u a l propor t ional i ty m o d e l b e t w e e n C P U E a n d a b u n d a n c e , if Q > 1 ca tchab i l i ty i n c r e a s e s a s b i o m a s s d e c r e a s e s k e e p i n g C P U E h igh a n d w e h a v e hypers tabi l i ty . F i n a l l y the hype rdep l e t i on c a s e o c c u r s w h e n 0 < Q < 1 a n d ca tchabi l i ty i n c r e a s e s with b i o m a s s , h e n c e C P U E d r o p s rap id ly wi th b i o m a s s . q{B) = qa{ Q ) (4.9) 210 biomass Figure 4.5 Hyperstability (top), proportionality (centre) and hyperdepletion (bottom) as simulated with equation 4.9. (top, Q=5; centre, Q=1; bottom, Q=0.5). 211 D u r i n g the M o n t e C a r l o tr ials , equa t ion 4 .9 is i nco rpo ra t ed both in the s i m u l a t i o n s of the ope ra t i ng m o d e l w h i l e gene ra t i ng f i shery da ta , a n d in the e s t ima t ion p r o c e d u r e o f e a c h f ishery m o d e l c o n s i d e r e d . T h e fo rmer is a c c o m p l i s h e d by subst i tu t ing the v a l u e o f qa in equa t ion 4.4 by the v a l u e o f q(Bt). A s imi l a r p r o c e d u r e c a n be u s e d w h e n r e c o v e r i n g p a r a m e t e r s from the f i shery m o d e l s in the ca l cu l a t i on o f b i o m a s s e s f rom o b s e r v e d c a t c h e s u s i n g the c a t c h e q u a t i o n . T h i s a l l o w s to eva lua t e w h e t h e r the f i shery m o d e l s c a n proper ly es t imate the b e n c h m a r k p a r a m e t e r s for a s s e s s m e n t a n d m a n a g e m e n t w h i l e e s t ima t ing the add i t iona l p a r a m e t e r Q w h i c h def ines cond i t i ons o f h y p e r d e p l e t i o n o r hypers tabi l i ty . 4 .2 .4 F i s h e r y m o d e l s e x a m i n e d . G i v e n that a n a n a l y s i s o f this k ind is n u m e r i c a l l y v e r y in tens ive , a n d c o n s i d e r i n g the different s c e n a r i o s to be e x p l o r e d dur ing the s tudy, it w a s n e c e s s a r y to k e e p eve ry th ing wi th in a m a n a g e a b l e s i z e , t hus o n l y three f i shery m o d e l s w e r e i n c l u d e d for the s tudy . T h e s e are the d y n a m i c v e r s i o n s o f the su rp lus p roduc t ion m o d e l s o f S c h a e f e r (1954) a n d F o x (1970) , a n d the de lay-d i f fe rence m o d e l o f D e r i s o (1980) a n d S c h n u t e (1985) . T h e S c h a e f e r a n d F o x m o d e l s a re s p e c i a l c a s e s of the g e n e r a l i s e d s u r p l u s p roduc t ion m o d e l o f P e l l a a n d T o m l i n s o n (1969) , a n d this m o d e l w a s initially c o n s i d e r e d i n s t e a d . H o w e v e r , exp lora to ry tr ials i nd i ca t ed that its pe r fo rmance w a s inferior to e i ther the S c h a e f e r o r the F o x m o d e l s , p o s s i b l y d u e to the ex t ra p a r a m e t e r that h a s to be e s t i m a t e d in o r d e r to fit the P e l l a a n d T o m l i n s o n m o d e l . The re fo re , the latter m o d e l w a s a b a n d o n e d . T h e d e l a y di f ference m o d e l o f D e r i s o - S c h n u t e w a s c h o s e n for its intuitive sui tabi l i ty to the b i o l o g i c a l cha rac t e r i s t i c s of e l a s m o b r a n c h s , in par t icu lar b e c a u s e it t a k e s a c c o u n t o f r ep roduc t ive d e l a y s in the popu la t ion d y n a m i c s . C o m p a r e d to su rp lus p roduc t ion m o d e l s , the de lay-d i f fe rence m o d e l r ep resen t s the next s tep in soph i s t i ca t ion a n d b i o l o g i c a l r e a l i s m , thus b r idg ing a g a p b e t w e e n s u r p l u s p roduc t ion a n d age- s t ruc tu red m o d e l s . T h e de lay-d i f fe rence m o d e l is ve ry attractive b e c a u s e o f its relat ive s impl ic i ty . F o r a l l the a b o v e r e a s o n s , it s e e m s wor th tes t ing the D e r i s o - S c h n u t e a p p r o a c h a l o n g s i d e t hose o f S c h a e f e r a n d F o x in o r d e r to eva lua t e the pe r fo rmance o f a m o d e l c a r r y i n g s o m e age-s t ruc ture informat ion a g a i n s t the ' s impler ' a n d 'unreal is t ic ' s u r p l u s p roduc t ion m o d e l s . 2 1 2 4.2.4.1 S u r p l u s p roduc t ion m o d e l s . A l l su rp lus p roduc t ion m o d e l s a re b a s e d o n the s i m p l e a s s u m p t i o n that b i o m a s s d y n a m i c s of s ing le s t o c k s c a n be r e p r e s e n t e d by the fo l lowing se l f e x p l a n a t o r y e q u a t i o n : new biomass = old biomass + surplus production - catch (4.10) w h e r e surplus production = recruitment + growth - losses from natural mortality (4.11) T h e s e m o d e l s s impl i fy b i o l o g i c a l p r o c e s s e s by i nc lud ing na tura l s o u r c e s o f i n c r e a s e (addi t ions in the form of r ep roduc t ion a n d ind iv idua l b o d y growth) a n d l o s s (natural mortali ty by p reda t ion , i l l nes s , compe t i t i on , etc.) into the s ing le su rp lus p roduc t ion t e rm. T h e v a r i o u s su rp lus p roduc t ion m o d e l s differ o n l y in the par t icu lar form c h o s e n to r ep re sen t this p r o c e s s . Schaefer model. T h e su rp lus p roduc t ion m o d e l o f S c h a e f e r (1954) is b a s e d o n the log is t ic m o d e l for popu la t ion g rowth . T h e difference equa t ion form of the m o d e l is: BM=Bt+nKi-5)-c, <412> w h e r e B is b i o m a s s , r is the intr insic rate o f popu la t ion g rowth , K is the c a r r y i n g c a p a c i t y a n d C is c a t c h . A n a l y t i c a l de r iva t ions found e l s e w h e r e (e .g. P u n t 1988) , s h o w that f o r t h e S c h a e f e r m o d e l the m a n a g e m e n t b e n c h m a r k of op t ima l c a t c h ( C o p J o r m a x i m u m s u s t a i n a b l e y i e l d ( M S Y ) , a n d the s tock s i z e at M S Y (BMSY) a re g i v e n by the fo l lowing fo rmu lae : 213 r = r K ^opt 4 (4.13) Considering the form of the catch equation assumed in equation 4.4, we have: (4.15) substituting equations 4.13 and 4.14 in equation 4.15 and solving for fwe obtain an explicit form of the optimal effort (fopt): f = _ 2_ (4.16) Jopt- q Fox model. The model of Fox (1970) assumes a Gompertz form forthe surplus production function. A convenient expression of this model in difference equation form is given by Punt (1988) as: l n » (4.17) Bt+,=B+rB(l- *-)-Ct 2 1 4 The equations for optimal catch and biomass at MSY for this form of the Fox model are given by: _rKel °P< i n £ (4.18) B =Kel (4-19> By a similar procedure as outlined above for the Schaefer model, it can be shown that the optimal effort in this particular form of the Fox model is equal to: ln(l-—) hp-' (4.20) 4 .2 .4 .2 Delay difference model. The delay difference model developed by Deriso (1980) and generalised by Schnute (1985) has three key assumptions: 1) Growth in mean body weight at age, for all ages fully vulnerable to fishing gear, follows a linear relationship: Wa = a+pWa_t (4.21) where l/Vis average weight, the index a is age and a, p are constants. 2) All fish aged k and older are equally vulnerable to fishing (knife edge selection), and 3) All fish aged k and older have the same natural mortality rate. 2 1 5 If w e a s s u m e that the total su rv iva l rate c a n be d e c o m p o s e d into cons t an t na tura l su rv iva l a n d v a r i a b l e ha rves t su rv iva l w e h a v e : S t = v|/ (4.22) w h e r e S , is the total su rv iva l rate, h is ha rves t rate, a n d v|/ is the na tura l su rv iva l rate. W e c a n d e c o m p o s e the b i o m a s s equa t i on 4 .8 into the s u m of b i o m a s s e s o f a l l a g e s k+1 a n d o l d e r p lus the n u m b e r o f recrui ts Rt to the f i shery t imes their a v e r a g e w e i g h t (at a g e of recrui tment) wk: Anax _ ( 4 2 3 ) a=k G i v e n that e a c h Nta c a n be writ ten a s su rv ivo r s from last y e a r at a g e a-7, a n d e a c h o f the a v e r a g e w e i g h t s at a g e c a n be wri t ten u s i n g equa t i on 4 . 2 1 , subs t i tu t ing both into equa t ion 4 .23 a n d r ea r r ang ing g ive s : Bt = Sa*Y, Na-l,t-i+ P £ J W l W « - l > + W * * » (4.24) w h e r e the s u m s a re o v e r a g e s k+1 to a g e m a x i m u m . F a c t o r i n g out t e r m s that d o not d e p e n d o n a g e , resul ts in s u m s o v e r a g e s k a n d o l d e r for y e a r t-1, t hus : V - V I « ^ - I + S , - I P * M + W A (4.25) 2 1 6 To ta l n u m b e r s a re : Nt=s<-A-i+Rt ( 4 - 2 6 ) T h e last two e q u a t i o n s c o m b i n e d toge ther p r o d u c e the de lay-d i f fe rence m o d e l with the a d v a n t a g e that popu la t i on n u m b e r s c a n be c o m p l e t e l y i g n o r e d . T h i s is a c c o m p l i s h e d by first wri t ing the t e rm aNt1 a b o v e in t e rms o f recrui ts a n d su rv ivo r s from Nt,2. «Ar*r_1=o5,_2ArV2+o^_1 <4-27> T h e n in a s i m i l a r m a n n e r , the te rm aSt_2Nt_2 c a n be e x p r e s s e d in t e r m s of a b i o m a s s equa t ion for Bu1 a n d Nt_2 a s : aSt_2Nt_2 = Bt_, - pSt_2Bt_2-w^ ( 4 2 8 ) w h i c h if subs t i tu ted into equa t ion 4 .27 a n d the result subs t i tu ted into e q u a t i o n 4 .25 , finally g i v e s us , after s o m e a l g e b r a , the de lay-d i f fe rence equa t i on : A form of the de lay-d i f fe rence m o d e l r e c o m m e n d e d by S c h n u t e (1985) a v o i d s the expl ic i t c a l cu l a t i on o f a by u s i n g a subst i tu t ion b a s e d o n equa t i on 4 . 2 1 : Bt=St-XBt-l + PSt-A-l -9StlSt-2Bt-2 ~ PWk-lSt-lRt-l +WA (4-30) 2 1 7 w h i c h is the form e m p l o y e d in the presen t s tudy . R e c r u i t m e n t in the de lay-d i f fe rence m o d e l w a s p red ic t ed u s i n g the fo l l owing equa t ions : dS{_k+l Beverton & Holt (4 .31) Rt = St-k+l e R'cker (4 .32) w h e r e d is the m a x i m u m recrui tment rate w h e n St.k+1 is la rge , b is the s p a w n i n g s tock s i z e n e e d e d to p r o d u c e a recrui tment o f hal f the m a x i m u m , a ' is the m a x i m u m product ivi ty per s p a w n e r w h e n the s p a w n i n g s tock is low, a n d b' m e a s u r e s d e n s i t y - d e p e n d e n t r educ t ions in product ivi ty a s s tock s i z e i n c r e a s e s (Hi lborn a n d W a l t e r s 1992) . In the c a s e o f the D e r i s o - S c h n u t e m o d e l , m a n a g e m e n t b e n c h m a r k s c a n be found by iterat ion. T h i s is a c c o m p l i s h e d by t rying different explo i ta t ion ra tes to c a l c u l a t e C o p f f r o m o n e of the fo l lowing fo rmulae for equ i l ib r ium b i o m a s s S e (Hi lborn a n d W a l t e r s 1992) : D a b Beve r ton -Hol t . (4.33) ZJ , • e G (l-h) B _ [ a ' + l n q - ^ - I n G ] [*'(I-*)] R i c k e r ( 4 3 4 ) w h e r e 218 (4.35) G _ [ l - ( l - p W l - « ) + Pt 2(l-«) 23 W t - P ^ t - i W - " ) Once the optimal catch and corresponding optimal exploitation rate l/op(have been found, the optimal effort is calculated as: Fixing parameter values for the Deriso-Schnute model. Fitting the delay-difference model as expressed in equation 4.30 to catch and CPUE data requires estimating a total of seven parameters: p, wk, and are needed by the delay-difference equation; two additional parameters (d, b or a', b) are needed for the stock recruitment model; finally the catchability q and the initial stock size B0 are required in order to relate CPUE to biomass. The procedure is simplified by fixing four of the parameters (d or a', p, wk and prior to the Monte Carlo trials, thus avoiding the need to estimate them during model fitting. This leaves only three parameters to be estimated, B0, b (or b) and q. Because fitting the surplus production models requires estimation of only three parameters, this strategy gives a handicap to the Deriso-Schnute model and allows a fair comparison between models. All fixed parameters were derived from the data used to simulate the populations. The growth parameter p, was calculated from a Ford-Walford plot of the weights at age presented in figure 4.2, while wk was taken directly from the same data. The natural survival \\i is constant after age 6 (fig. 4.2), and is calculated as e'M. The estimation of the parameters cf and a'of the stock recruitment models can be avoided by assuming that the population was sustainable (i.e. at equilibrium) when exploitation began: at equilibrium, the losses due to natural mortality have to be equal to the gains in the form of new recruits. Such an assumption is true in our particular case, and is a 2 1 9 r e a s o n a b l e g u e s s for o the r c a s e s if w e k n o w the s tock h a s not b e e n f i s h e d p r e v i o u s to da ta ga the r ing . T h i s a l l o w s to fix o n e point in the s tock- recru i tment funct ion (the m a x i m u m leve l of recrui tment) e l imina t ing the n e e d to es t imate direct ly the p a r a m e t e r s d o r a'. T h u s , recrui tment at equ i l i b r ium R0 mus t be e q u a l to n u m b e r s d y i n g (A/ 0 (1 - vj/)). T h e p a r a m e t e r N0 is c a l c u l a t e d from the m o d e l es t imate of B0 a n d the a v e r a g e equ i l i b r i um we igh t equa t ion p r o v i d e d in p a g e 3 4 0 of H i l b o r n a n d W a l t e r s (1992) . T h i s is u s e d a s the recru i tment v a l u e in the first y e a r o f the ca l cu l a t i ons , b e c a u s e w e a re a s s u m i n g t h e s e recrui ts w e r e p r o d u c e d by the v i rg in s p a w n i n g popu la t i on . T h e p a r a m e t e r d o r a ' o f the s tock- recru i tment re la t ionsh ip ( m a x i m u m recrui tment level) c a n then be e x p r e s s e d in t e r m s o f B0 a n d the p a r a m e t e r b o r b' o f the s tock- recru i tment m o d e l : B e v e r t o n - H o l t (4.37) / ^0 / a = ^~) + BQb R i c k e r (4.38) b a v o i d i n g in this w a y their i n d e p e n d e n t e s t ima t ion . Non-delay model and misspecification of stock-recruitment relationship. T h e de lay-d i f fe rence m o d e l of D e r i s o - S c h n u t e i n c l u d e s a r ep roduc t ive d e l a y in the s tock / recru i tment r e la t ionsh ip of k-1 yr, that is , the a g e o f recru i tment m i n u s o n e yea r . In o rde r to s e e h o w impor tant it is to c o n s i d e r this d e l a y in the m o d e l , d u r i n g the initial s t a g e s of this s tudy I u s e d a modi f i ed m o d e l that c a l c u l a t e s recrui tment a s a funct ion of last yea r ' s b i o m a s s . I refer to this m o d e l modi f ica t ion a s the n o n - d e l a y D e r i s o - S c h n u t e m o d e l th roughout this s tudy. T h e o r ig ina l m o d e l c o n s i d e r i n g the r ep roduc t ive d e l a y is referred to a s the D e r i s o - S c h n u t e m o d e l o r the de lay-d i f fe rence m o d e l . T h e c h o i c e o f the expl ic i t s tock- recru i tment funct ion to be u s e d in the D e r i s o - S c h n u t e m o d e l 2 2 0 is a n o t h e r point o f uncer ta in ty dur ing m o d e l fitting. A c r o s s e x a m i n a t i o n o f the effects o f c h o o s i n g the w r o n g s tock- recru i tment funct ion for fitting the D e r i s o - S c h n u t e m o d e l w a s pe r fo rmed du r ing tr ials wi th s i m u l a t e d popu la t ions that h a d propor t iona l i ty b e t w e e n C P U E a n d b i o m a s s . T h i s a n a l y s i s i n v o l v e d gene ra t i ng da t a with o n e o f the two s tock- recru i tment funct ions a n d pe r fo rming the e s t ima t ions u s i n g the o ther (wrong) s tock- rec ru i tment funct ion in the D e r i s o - S c h n u t e m o d e l . T h i s a p p r o a c h p r o v i d e s a n oppor tun i ty to e v a l u a t e the effects of s u c h a m o d e l mi s spec i f i ca t i on in the p e r f o r m a n c e of the de lay-d i f fe rence m o d e l . 4.2.5 B e n c h m a r k s for m o d e l p e r f o r m a n c e . M o d e l p e r f o r m a n c e for s tock a s s e s s m e n t in this s tudy w a s m e a s u r e d by three b e n c h m a r k s of b i o m a s s e s t ima t ion : the s i z e o f the s tock at the b e g i n n i n g o f exp lo i ta t ion B0, the t e rmina l s tock s i z e after 2 0 y r o f f i sh ing B e n d , a n d the total e s t i m a t e d a b s o l u t e d d i s c r e p a n c y TD, a m e a s u r e of the b i o m a s s e s t ima te s for the entire 2 0 y r t i m e s e r i e s . D u r i n g e a c h s imula t ion trial, e s t ima t ion b i a s w a s thus c a l c u l a t e d for e a c h b e n c h m a r k a s that p ropor t ion o f its rea l v a l u e r e p r e s e n t e d by its c o r r e s p o n d i n g es t imate . In the c a s e o f B0 a n d S e n d t h i s is on ly the es t imate d i v i d e d by the c o r r e s p o n d i n g rea l v a l u e . T h e m e a s u r e o f p e r f o r m a n c e for the entire 2 0 y r t ime s e r i e s TD, is c a l c u l a t e d as : w h e r e t=1 c o r r e s p o n d s to the first y e a r of f i sh ing , 6 , is e s t i m a t e d b i o m a s s a n d Bt is the true v a l u e o f the b i o m a s s . D u r i n g the p resen t s tudy, the pe r fo rmance cr i ter ia for m a n a g e m e n t p a r a m e t e r es t imat ion of the f i shery m o d e l s a re the t radi t ional b e n c h m a r k s o f o p t i m a l effort fopt a n d o p t i m a l ca t ch Copt, e s t i m a t e d for e a c h O p e r a t i n g M o d e l a s in A p p e n d i x 4. T h e m e a s u r e o f e s t ima t ion b ias is the propor t ion o f the true m a n a g e m e n t b e n c h m a r k r e p r e s e n t e d by its c o r r e s p o n d i n g es t imate , just a s e x p l a i n e d a b o v e for the b i o m a s s p a r a m e t e r s . ™ B.-B (4.39) 221 4 .2 .6 Fit t ing f i shery m o d e l s to da ta . Error types and the TLS fitting procedure. T h e m e t h o d u s e d to fit the f i shery m o d e l s to e a c h t ime s e r i e s o f s i m u l a t e d f i shery da ta w a s the To ta l L e a s t S q u a r e s m e t h o d ( T L S ) of L u d w i g et a l . (1988) . T h i s p r o c e d u r e a t tempts to c o m b i n e t w o different a p p r o a c h e s for d e a l i n g with er rors du r ing fit o f m o d e l s to c a t c h a n d effort da ta . M o s t fitting p r o c e d u r e s h a v e t radi t ional ly a s s u m e d that depa r tu r e s from m o d e l p red ic t ions (the d i f fe rences b e t w e e n the da t a v a l u e s a n d the v a l u e s p r e d i c t e d by the m o d e l s ) a re at t r ibutable to na tura l var iabi l i ty in the catch-effort "obse rva t ion" s y s t e m . S u c h errors c a n a r i se d u e to r a n d o m n e s s in the ca tchabi l i ty o r in the effective effort a p p l i e d . T h i s is k n o w n a s the o b s e r v a t i o n error a s s u m p t i o n a n d h a s b e e n u s e d by P e l l a a n d T o m l i n s o n (1969) , But terwor th a n d A n d r e w (1984) , P o l a c h e k et a l . (1993) a n d P r a g e r (1994) , a m o n g others . A n a l te rna t ive a p p r o a c h is to a s s u m e the er rors to be a l l d u e to b i o l o g i c a l p r o c e s s e s s u c h a s recrui tment , g rowth , o r s u r v i v a l . T h i s l e s s c o m m o n prac t ice is k n o w n a s the p r o c e s s error a s s u m p t i o n a n d is f o u n d in the w o r k s o f S c h n u t e (1977) a n d L u d w i g a n d W a l t e r s (1985) . T h e m e t h o d o f T L S t a k e s a c c o u n t o f both t ypes o f e r rors by m i n i m i z i n g a w e i g h t e d s u m of s q u a r e s that i n v o l v e s both o b s e r v a t i o n errors a n d p r o c e s s e r rors : TLS = — Eco2 + - E v 2 1-A, r - A t (4.40) w h e r e vt a n d co, a re the o b s e r v a t i o n a n d p r o c e s s er rors r e spec t ive ly , a n d X is the ratio of the v a r i a n c e o f the v f t o the total v a r i a n c e vt + cot. F o l l o w i n g the resul ts o f L u d w i g et a l . (1988) a n d P u n t (1988) , the p a r a m e t e r X is f ixed at 0.5 b e c a u s e it c a n n o t be sat isfactori ly e s t i m a t e d from the da ta . T h e r e a re at leas t two different a s s u m p t i o n s abou t the form of the o b s e r v a t i o n error v t . L u d w i g et a l . (1988) r e c o m m e n d a s s u m i n g mul t ip l ica t ive er rors a s s o c i a t e d to the effective " o b s e r v e d " effort: 2 2 2 / =f e ' J obs J true (4.41) w h e r e fobs is the o b s e r v e d effort, is the true effort a n d v (the o b s e r v a t i o n error) is a no rma l ly d is t r ibuted r a n d o m va r i ab l e with m e a n 0 a n d v a r i a n c e a v 2 . A n o t h e r poss ib i l i ty is to a s s u m e add i t ive er rors p e r h a p s a s s o c i a t e d with va r i a t ions in ca tchab i l i ty , thus : 1obs=<ltrue+Vt (4.42) w h e r e qobs is the o b s e r v e d ca tchabi l i ty , the n o m i n a l ca tchab i l i ty a n d vt is a s a b o v e . A l t h o u g h da t a a r e s i m u l a t e d by the ope ra t i ng p o p u l a t i o n s u s i n g o n l y e q u a t i o n 4 .37 , both a s s u m p t i o n s w e r e i m p l e m e n t e d in the es t ima t ion p r o c e d u r e s du r ing ea r ly t r ials in o r d e r to eva lua t e the effects o f e a c h a s s u m p t i o n in p e r f o r m a n c e o f the T L S m e t h o d . T h e p r o c e s s e r rors a p p e a r a s part o f the b i o m a s s d y n a m i c s : (4.43) w h e r e p roduc t ion P, is a funct ion o f b i o m a s s B, C is the c a t c h , a n d cot is a no rma l ly dis t r ibuted r a n d o m v a r i a b l e wi th m e a n 0 a n d v a r i a n c e a w 2 . D u r i n g the a c t u a l fitting p r o c e s s , the v t ' s a re t rea ted a s add i t i ona l ( n u i s a n c e ) p a r a m e t e r s to be e s t ima ted , that is , the es t ima t ion p r o c e d u r e s e a r c h e s for the bes t v a l u e s o f vt. O n c e the o b s e r v e d effort o r ca tchab i l i ty is co r r ec t ed with t h e s e e s t i m a t e s a c c o r d i n g to 4.41 o r 4 .42 , the b i o m a s s c a n be e s t i m a t e d from the c a t c h equa t i on u s i n g the o b s e r v e d c a t c h a n d e s t ima tes o f q. F ina l ly , the p r o c e s s error cot c a n be e s t i m a t e d from 4 . 4 3 . N o t e that o n l y n-1 p r o c e s s e r rors c a n be e s t i m a t e d from e a c h t ime s e r i e s o f n y e a r s o f d a t a b e c a u s e both Bt+1 223 and Bt appear in 4.43. Initial biomass assumption. Normally, the size of the stock at the beginning of harvest (S0) is estimated by the fitting procedure together with the entire biomass series and the rest of the model parameters. However, it is possible to omit this additional parameter when there is information about the history of the fishery suggesting that the stock was at its virgin size when exploitation began. In the case of surplus production models, this implies assuming that the first biomass in each time series is equal to the carrying capacity of the system (B0=K) and incorporating this explicitly in the fitting algorithm. According to Punt (1991) the B0=K assumption can help improve results under some situations. The benefits of this simplification in the fitting process are investigated throughout most of the simulations. Non-linear search algorithms. Fitting the fishery models under test to the simulated data involves minimising equation 4.40 via some non-linear optimisation algorithm that searches for the best values of B0, b or b\ q and all vt's. Such an algorithm must be able to perform numerical calculation of derivatives due to the complicated functions obtained as a solution to equation 4.43 especially in the case of the delay-difference model. The first optimisation choice forthe present study was the Fletcher-Powell algorithm because of its speed and availability through free distribution programmes on disk. However, after a few trials it was evident that this algorithm could not converge to a solution for too many cases. The downhill simplex algorithm of Nelder and Mead (Press et al. 1990) was then implemented because of its renowned robustness. This algorithm improved the rate of success in finding solutions for difficult cases but at a great cost in time due to the slowness of the method. However, still the rate of success (convergence to a reasonable solution) was not satisfactory. The Marquardt algorithm was finally implemented and used with better success for the totality of the study. Still, for some types of trials success rates as low as 50% were observed. Estimation bias. 2 2 4 In m o d e l fitting, it is c u s t o m a r y to a s s e s s the b i a s o f a par t icu lar es t imator . Be t te r p a r a m e t e r e s t ima te s c a n be o b t a i n e d from point e s t ima tes after co r r ec t ion for b i a s . O n e o f the mos t p o p u l a r m e t h o d s o f a s s e s s i n g a n d co r rec t ing es t imat ion b i a s in recen t t i m e s is the boots t rap (Efron 1982) . H o w e v e r , this t e c h n i q u e i n v o l v e s a v e r y in tens ive p r o c e s s ( 250 -1000 s imu la t i ons pe r trial) w h i c h is ex t r eme ly e x p e n s i v e in c o m p u t i n g t ime . T h e nature of the present s tudy, i n v o l v i n g 100 tr ials per ope ra t i ng m o d e l , p r e v e n t e d u s i n g b i a s - co r r ec t ed e s t ima tes o f the m a n a g e m e n t a n d a s s e s s m e n t b e n c h m a r k s . N e v e r t h e l e s s , o the r s tud ies s u g g e s t that e s t ima t ion b i a s is neg l ig ib le at leas t for the S c h a e f e r s u r p l u s p roduc t ion m o d e l ( P r a g e r et a l . 1994) . 4 .2 .7 T h e t y p e s o f tr ials pe r fo rmed . 4.2.7.1 N o m e n c l a t u r e . T h r o u g h o u t this work , ope ra t ing m o d e l s wi l l be c a l l e d ' opera t ing p o p u l a t i o n s ' o r O P ' s to a v o i d c o n f u s i o n b e t w e e n s o m a n y t ypes o f m o d e l s : f ishery, e s t ima t ing , a n d ope ra t ing m o d e l s . O p e r a t i n g popu la t i ons d e s i g n a t e d o n l y by the n a m e o f the type o f s tock- recru i tment funct ion u s e d ( R i c k e r a n d B e v e r t o n - H o l t ) refer to ope ra t i ng p o p u l a t i o n s c o n s i d e r i n g C P U E -b i o m a s s propor t ional i ty a n d u s i n g the informative effort pat tern to g e n e r a t e the f i shery da ta . O p e r a t i n g p o p u l a t i o n s u s i n g the l e s s p roduc t ive f i shab le s tock a r e he re d e s i g n a t e d a s 'unproduc t ive ' , whi l s t t hose u s i n g the non- informat ive effort s e q u e n c e a r e k n o w n a s 'uninformat ive ' . F ina l ly , O P ' s d e s i g n a t i n g the h y p e r d e p l e t i o n a n d hypers tab i l i ty c a s e s a re n a m e d u s i n g p r e c i s e l y t h e s e two t e rms . 4 .2 .7 .2 S e q u e n c e o f t r ia ls . T h e different o p e r a t i n g popu la t i ons c o n s i d e r e d , a l o n g with the n u m b e r o f e s t ima t ion m o d e l s a n d the different va r i a t ions for the o b s e r v a t i o n error e s t ima t ion t e s t ed in this s tudy, m a d e the n u m b e r o f p o s s i b l e c o m b i n a t i o n s too la rge . C o m p u t a t i o n a l t ime cons t ra in t s a n d the n e e d to k e e p m a n a g e a b l e the a m o u n t o f resul ts to be a n a l y s e d , d ic ta ted a h i e r a r c h i c a l a p p r o a c h for c a r r y i n g out the t r ia ls . T h i s s t ra tegy a l l o w e d to i nc lude a l l O P ' s a n d e x a m i n e a l l p o s s i b l e va r ia t ions of the e s t ima t ion p r o c e d u r e w h i l e d i s c a r d i n g a l o n g the w a y p r o c e d u r e s that p r o v e d to be unsa t i s fac tory a n d d id not war ran t further a n a l y s i s . 2 2 5 First , the two a l te rna t ive a s s u m p t i o n s for the o b s e r v a t i o n er ror (mul t ip l ica t ive in effort o r addi t ive in ca tchab i l i ty , e q s . 4.41 a n d 4 .42) w e r e inves t iga t ed wi th the R i c k e r a n d B e v e r t o n -Holt O P ' s . T h e s e tr ials i n v o l v e d es t imat ing p a r a m e t e r s with the S c h a e f e r a n d F o x su rp lus p roduc t ion m o d e l s a n d the d e l a y a n d n o n - d e l a y v e r s i o n s o f the D e r i s o - S c h n u t e m o d e l . After c o m p a r i n g the p e r f o r m a n c e o f both o b s e r v a t i o n error a s s u m p t i o n s , o n l y the o p t i m a l o f t he se a s s u m p t i o n s (i.e. mul t ip l ica t ive) w a s kept for the r e m a i n i n g part o f the s tudy . Fur ther tr ials with the R i c k e r a n d B e v e r t o n - H o l t O P ' s i n c l u d e d the de lay-d i f fe rence m o d e l with m i s s p e c i f i e d s tock- recru i tment funct ion a n d the B0=K t r ials . F r o m this point o n w a r d s , on ly the S c h a e f e r , F o x a n d D e r i s o m o d e l s w e r e e v a l u a t e d . T h e tes ts wi th the hypers tabi l i ty a n d hype rdep l e t i on O P ' s i n c l u d e d on ly the mul t ip l ica t ive o b s e r v a t i o n er ror a s s u m p t i o n . A n o t h e r s tage o f t r ials d e s i g n e d to further test the r o b u s t n e s s o f the S c h a e f e r , F o x a n d d e l a y -difference m o d e l s i n v o l v e d s imu la t i ons with the unp roduc t ive O P ' s , bo th wi th a n d wi thout the B0=K a s s u m p t i o n . F ina l ly , t h e se three m o d e l s w e r e t e s t ed wi th unp roduc t ive a n d uninformat ive v e r s i o n s o f the R i c k e r a n d B e v e r t o n - H o l t O P ' s . A s u m m a r y o f the different t ypes o f t r ials a c c o r d i n g to the s e q u e n t i a l o rde r in w h i c h they w e r e pe r fo rmed is p r o v i d e d in t ab l e s 4.1 a n d 4 .2 . 4.3 R e s u l t s P r e l i m i n a r y tr ials wi th the D e r i s o - S c h n u t e m o d e l h igh l igh ted the i m p o r t a n c e of gett ing r e a s o n a b l y a c c u r a t e e s t ima te s of the f ixed p a r a m e t e r s that a re not e s t i m a t e d dur ing the non - l i nea r fit o f the m o d e l to da ta . E r ro r s in the v a l u e s o f the a g e o f recru i tment to the f i shery (wrong v a l u e by 1 o r 2 a g e s ) , the natural su rv iva l , o r the different w e i g h t s requ i red dur ing e s t ima t ion (wk, a n d a v e r a g e weigh t ) a l l r e su l t ed in b i a s s e d e s t ima tes of m a n a g e m e n t p a r a m e t e r s a n d v e r y frequent ly in fai lure of the e s t ima t ion p r o c e d u r e to c o n v e r g e . In fact, a great a m o u n t o f t ime h a d to be spen t f inding the right v a l u e s of the f ixed pa r ame te r s a n d c h e c k i n g the imp lemen ta t i on o f the m o d e l p r o c e d u r e in o r d e r to get r e a s o n a b l e resul ts wi th this m o d e l , d u e to its relat ive c o m p l e x i t y . 4.3.1 S i m u l a t e d popu la t i ons a n d f ishery da ta t ime s e r i e s . F igure 4.6 s h o w s the typ ica l t rajectories of popu la t ion growth for e a c h o f the two ope ra t ing popu la t ions ( R i c k e r a n d Beve r ton -Ho l t ) . T h i s g e n e r a l b e h a v i o u r w a s a l s o o b s e r v e d for the 2 2 6 Table 4.1 Types of trials performed with informative effort pattern and productive stock (age or recruitment = 7yr). A= additive observation error; M= multiplicative observation error; S=Schaefer; F=Fox; D=Deriso-Schnute; Dn=non-delay Deriso-Schnute; D**= misspecified Deriso-Schnute. Operating Population Estimation Procedure Spatial behaviour Stock-recruitment Assumptions Fishery models Propor-tionality Beverton & Holt A S, F, D, Dn M S, F, D, Dn, D** Ricker A S, F, D, Dn M S, F, D, Dn, D** Beverton & Holt M (B0=K) S, F, D, D** Ricker M (B0=K) S, F, D, D** Hyper Stability Beverton & Holt M S, F, D Ricker M S, F, D Hyper Depletion Beverton & Holt M S, F, D Ricker M S, F, D Table 4.2 Types of trials performed with clue proportional to biomass and an unproductive stock (age of recruitment = 4yr). Key as in table 4.1. Operating Population Estimation Procedure Effort pattern Stock-recruitment Assumptions Fishery models Informative Effort Beverton & Holt M S, F, D M (B0=K) S, F, D Ricker M S, F, D M (B0=K) S, F, D Uninformative Effort Beverton & Holt M S, F, D M (B0=K) S, F, D Ricker M S, F, D M (B0=K) S, F, D 227 2 2 8 hypers tab i l i ty a n d hype rdep l e t i on c a s e s , b e c a u s e the d i f fe rences in spa t i a l b e h a v i o u r b e t w e e n o p e r a t i n g popu la t i ons b e c o m e s ev iden t o n l y du r ing the exp lo i ta t ion p h a s e . E x a m p l e s o f the 2 0 y r t ime s e r i e s o f b i o m a s s a n d C P U E for four va r i a t i ons o f the B e v e r t o n a n d Hol t O P a re s h o w n in figure 4.7. T h e R i c k e r O P p o p u l a t i o n s b e h a v e d in a s imi l a r f a sh ion differing o n l y in a b s o l u t e v a l u e s . A n informative pattern ( L u d w i g a n d W a l t e r s 1985) o f r ap id s t o c k d e c l i n e f o l l o w e d by a r e c o v e r y w h e n f i sh ing s u b s i d e s w a s sat isfactor i ly r e p r o d u c e d u n d e r the propor t ional i ty a n d hype rdep l e t i on s c e n a r i o s . H o w e v e r , this w a s not the c a s e for the hypers tab i l i ty c a s e o r w h e n da t a w e r e g e n e r a t e d u s i n g the uninformat ive effort s e r i e s . T h e i n c r e a s e in ca tchabi l i ty a s s o c i a t e d wi th l o w e r b i o m a s s e s in the hypers tabi l i ty c a s e p r o d u c e d a l a rger f i sh ing mortal i ty desp i t e the t e rmina l mi ld effort u s e d , thus p reven t ing the r e c o v e r y o f the popu la t i on . T h i s f igure a l s o i l lustrates v e r y c l ea r ly the d i f ferences in the C P U E - b i o m a s s re la t ionsh ip for e a c h o f the three spa t i a l a s s u m p t i o n s in the O P . F o r the t w o 'proport ional i ty ' c a s e s , w h e r e q r e m a i n s cons t an t o v e r a l l s tock l eve l s , the e x p e c t e d c l o s e c o r r e s p o n d e n c e b e t w e e n C P U E a n d b i o m a s s is ev iden t . In contras t , for the hypers tab i l i ty a n d hype rdep l e t i on c a s e s the b i o m a s s trajectory is poor ly i n d e x e d by C P U E . 4.3.2 T e s t s wi th the 'proport ional i ty ' ope ra t ing popu la t ions . A l l e s t ima t ion resul ts th roughout this s tudy a re p r e s e n t e d a s mod i f i ed b o x plots s h o w i n g the dis t r ibut ion o f the b e n c h m a r k p a r a m e t e r s a s m e a s u r e d by the m e d i a n a n d quar t i l es . In e a c h of the fopt, C o p t B0 a n d B e n d b o x plots , the hor i zon ta l l ine at unity r e p r e s e n t s the a b s e n c e of b ias , i.e. a perfect a n s w e r . F o r the total e x p e c t e d d i s c r e p a n c y T D , the target v a l u e is z e r o (a perfect b i o m a s s fit). 4.3.2.1 T e s t s o f o b s e r v a t i o n error a s s u m p t i o n s a n d different v e r s i o n s o f the D e r i s o - S c h n u t e m o d e l . T h e p e r f o r m a n c e o f e s t ima t ion p r o c e d u r e s u s i n g the a s s u m p t i o n o f mul t ip l ica t ive o b s e r v a t i o n errors in effort is better in a l l c a s e s with the e x c e p t i o n o f s o m e of the b e n c h m a r k s for the D e r i s o - S c h n u t e m o d e l (figs. A . 1 a n d A . 2 , A p p e n d i x 1). No t o n l y the b ias is g rea te r in the la rge majority o f the e s t ima tes from p r o c e d u r e s u s i n g the add i t ive 2 2 9 230 observation error assumption, but the uncertainty in the estimates and the number of unsuccessful fits are also usually larger than for estimates obtained by using the multiplicative observation error assumption. This is particularly noticeable in the results for the Ricker OP. The additive error in catchability assumption was therefore abandoned after these trials. The non-delay Deriso-Schnute model is generally inferior to the delay-difference Deriso-Schnute model (which accounts for the reproductive delay). In particular, the former model could not be used to obtain estimates of the management benchmarks for the Beverton-Holt OP, and estimates for the Ricker OP are inferior to those from the delay-difference model (figs. A.1 and A.2, Appendix 1). The differences in biomass assessment between the two models are minor. The non-delay Deriso-Schnute model was discarded at this stage. 4.3.2.2 Tests between the Schaefer, Fox, and Deriso-Schnute model. The Deriso-Schnute model performs far better than any of the surplus production models for the estimation of management parameters, particularly for the estimation of optimal effort (fig. 4.8). While the Schaefer and Fox models are reasonably good for estimating Copt, their performance for f o p t estimation is dismal especially in the Fox model and with the exception of the Scheafer estimate for the Beverton-Holt OP. The three models led to satisfactory estimation of stock assessment benchmarks, with all estimates of B0 and Se n d having relatively small biasses. There are small negative biasses in the Deriso-Schnute model estimates of B0 and 6e n d. In contrast, estimates from both surplus production models have small positive biasses. Among the surplus production models, the Fox model performs better than the Schaefer model for biomass assessment. According to the total discrepancy, all models manage to obtain relatively good fits to the overall biomass time series (a TD of 2 indicates a 10% average absolute error) with the Schaefer model having the largest discrepancies. The Deriso-Schnute model appears to be more robust: its performance is more consistent across operating populations than in the Schaefer and Fox models. Notably, the biomass time series of the Ricker OP seemed to be more difficult to assess for the surplus production models, than those of the Beverton-Holt OP (figs. 4.9a and b). 231 S-B S-R F-B F-R D-B D-R D"-B D"-R S-B S-R F-B F-R D-B D-R D"-B D"-R 3.5 -3 -2.5 -2 -1.5 -1 -0.5 -0 -S-B S-R F-B F-R D-B D-R D"-B D"-R Figure 4.8 Modified box plots showing distribution of est imates of management and b iomass a s ses smen t benchmarks for each estimation model using the multiplicative observation error assumption. Monte Car lo simulations performed with the proportionality O P , high contrast effort, and productive stock. C o d e s in the x axis correspond to letters, used to name estimation procedures in table 4 .1 . 232 H 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Years 6 T OP S-Bo=K 5 -- _ 0 -I 1 1 — • — I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 c Years Figure 4.9 Examples of model fits to biomass time series. A) Beverton-Holt OP; B) Ricker OP; C) Beverton-Holt OP with the Bo=K assumption. Legend key: OP is true biomass, S is Schaefer model, F is Fox model,D is Deriso-Schnute model; and D** is Deriso-Schnute model with mis-specified stock-recruitment function. 2 3 3 F igu re 4.8 a l s o i l lustrates the c o n s e q u e n c e s o f m i s s p e c i f y i n g the s tock- rec ru i tmen t funct ion in the D e r i s o - S c h n u t e m o d e l . W h i l e b i o m a s s e s t ima te s a re o n l y s l ight ly d e g r a d e d by this mi s spec i f i ca t i on , the e s t ima te s of m a n a g e m e n t p a r a m e t e r s for the B e v e r t o n - H o l t O P a re no t i ceab ly d e g r a d e d (over +60%) w h e n the w r o n g s tock- rec ru i tment funct ion is a s s u m e d . 4.3.2.3 T h e B0-K a s s u m p t i o n . In g e n e r a l t e rms , u s i n g the B0=K s t ra tegy du r ing e s t ima t ion p r o d u c e s m i x e d resul ts (fig. 4.10). T h i s a s s u m p t i o n i m p r o v e s a l l e s t ima te s o f fopt for the S c h a e f e r a n d F o x m o d e l s , s l ight ly d e g r a d e s Copt e s t ima te s of the S c h a e f e r m o d e l but m a r g i n a l l y i m p r o v e s t hose of the F o x m o d e l . T h e B0=K a s s u m p t i o n e i ther d e g r a d e s o r d o e s not i m p r o v e resul ts for the D e r i s o - S c h n u t e m o d e l : it d o e s not l e a d to better e s t ima te s of m a n a g e m e n t p a r a m e t e r s for the B e v e r t o n - H o l t O P , a n d s l ight ly d e g r a d e s fopt e s t ima te s for the R i c k e r O P . O n the o ther h a n d , the B0=K a s s u m p t i o n l e a d s to i m p r o v e d p e r f o r m a n c e in b i o m a s s es t ima t ion for a l l m o d e l s (fig. 4.9c). T h e b i a s in B0 a n d S e n d , a n d the total d i s c r e p a n c y a re s ignif icant ly r e d u c e d for the three es t ima t ion p r o c e d u r e s . T h e B0=K a s s u m p t i o n is a n a l y s e d dur ing further tes ts o f the propor t ional i ty O P ' s b e c a u s e o f its potent ia l to i m p r o v e b i o m a s s a s s e s s m e n t e s t ima te s . 4.3.3 C h a n g e s in the spa t ia l b e h a v i o u r o f the s tock: hypers tab i l i ty a n d h y p e r d e p l e t i o n . T h e hypers tab i l i ty a n d hype rdep l e t i on c a s e s p resen t a f o rmidab l e p r o b l e m to a l l f i shery m o d e l s . In a l l hypers tab i l i ty c a s e s , the three m o d e l s perform d i s m a l l y for the es t imat ion of m a n a g e m e n t pa r ame te r s , wi th the e x c e p t i o n of the F o x m o d e l e s t ima t i ons o f C o p , ( f i g . 4.11). A n effort to i m p r o v e m a n a g e m e n t p a r a m e t e r e s t ima t ion c o n s i s t e d o f c a l c u l a t i n g the op t ima l explo i ta t ion rate (which is a m o r e s tab le p a r a m e t e r than fopt b e c a u s e o f its i n d e p e n d e n c e from q e s t ima tes ) . H o w e v e r , this o n l y h e l p s to i m p r o v e the p e r f o r m a n c e o f the F o x m o d e l with the B e v e r t o n - H o l t O P . B i o m a s s a s s e s s m e n t is a l s o v e r y p o o r in a l l e s t imat ion p r o c e d u r e s . W h i l s t mos t e s t ima te s of B0 a re sat isfactory, this is d u e to the u s a g e o f the B0=K a s s u m p t i o n in a l l hypers tabi l i ty a n d hype rdep l e t i on tr ials ( sec t ion 4 .2.3.2). N o t e that the e s t ima te s from the D e r i s o - S c h n u t e m o d e l h a v e l a rge r uncer ta in t i es t han the o ther m o d e l s in a l l c a s e s o f hypers tabi l i ty , s u g g e s t i n g a g rea te r difficulty o f the de lay-d i f fe rence m o d e l to c o p e with the ex t ra l o a d of e s t ima t ing the add i t i ona l p a r a m e t e r Q . W i t h the 2 3 4 5-B S-R F-B F-R D-B D-R S-B S-R F-B F-R D-B D-R 3-2 O 1-1.5-1 * 0.5 n . (3) Figure 4.10 Modified box plots showing distribution of est imates of management and b iomass a s ses smen t benchmarks for each estimation model using multiplicative observation error and the Bp-K assumptions . Monte Car lo simulations performed with the proportionality O P and the high contrast effort pattern. C o d e s in the x axis cor respond to letters u sed to name estimation procedures in table 4.1. 235 S-B S-R F-B F-R D-B D-R S-B S-R F-B F-R D-B D-R SB SR FB FR DB DR SB SR FB FR DB DR SB SR FB FR DB DR t T it . 1 . SB SR FB FR DB DR SB SR FB FR DB DR SB SR FB FR DB DR Hyperdepletion Figure 4.11 Modified box plots showing distribution of est imates of management and b iomass a s ses smen t benchmarks for each estimation model using the multiplicative observation error assumption. Monte Car lo simulations performed with the Hyperstability and Hyperdeplet ion O P s and high contrast effort pattern. C o d e s in the x axis correspond to letters used to name estimation procedures in table 4 .1 . . ()= number of failed trials. 2 3 6 e x c e p t i o n o f the D e r i s o - S c h n u t e e s t ima te s for the B e v e r t o n - H o l t O P , a l l e s t i m a t e s of Bend have re la t ively la rge nega t ive b i a s e s . T h e o v e r a l l p o o r p e r f o r m a n c e o f the three f ishery m o d e l s for b i o m a s s a s s e s s m e n t u n d e r hypers tabi l i ty c o n d i t i o n s (fig. 4 .12a ) t r ans la tes into ve ry h igh total d i s c r e p a n c y v a l u e s . M a n a g e m e n t p a r a m e t e r e s t ima te s for the hype rdep l e t i on O P a re a l s o v e r y p o o r for a l l e s t imat ion m o d e l s . A l t h o u g h the S c h a e f e r m o d e l a p p e a r s to p rov ide g o o d e s t ima te s of fopt fo r the B e v e r t o n - H o l t O P , the c o r r e s p o n d i n g Coptestimates a re h igh ly pos i t ive ly b i a s s e d . T h e F o x m o d e l suffers f rom h igh ly i m p r e c i s e e s t ima tes a n d the D e r i s o - S c h n u t e m o d e l h a s the wors t p e r f o r m a n c e of the three m o d e l s . B i o m a s s e s t ima t ion w a s a l s o d i s m a l for a l l m o d e l s (fig. 4 .12b) . T h e D e r i s o - S c h n u t e m o d e l is a b l e to p r o d u c e a g o o d b i o m a s s fit ( low TD s co re ) for the R i c k e r O P but its pe r fo rmance c o l l a p s e s for the B e v e r t o n - H o l t O P . 4 .3 .4 U n p r o d u c t i v e s t ocks . W h e n the o r ig ina l o p e r a t i n g popu la t i ons a re s i m u l a t e d shift ing the a g e o f entry to the f ishery t o w a r d s l o w e r v a l u e s (age 4 a s o p p o s e d to a g e 7) the product iv i ty o f the f i shab le s tock d e c r e a s e s . T h i s c a n be s e e n by a n a l y s i n g the v a l u e s of the c o r r e s p o n d i n g true op t ima l ca t ch a n d the o p t i m a l effort (table A . 1). T h e es t ima t ion o f m a n a g e m e n t p a r a m e t e r s for t hese unp roduc t ive O P ' s is bes t in the D e r i s o - S c h n u t e a n d then in the S c h a e f e r m o d e l (fig. 4 .13) . T h e F o x m o d e l pe r fo rms d i s m a l l y in the es t ima t ion o f fopt. T h e r educ t ion in s tock product ivi ty s e e m s to be re la t ively benef ic ia l for the p e r f o r m a n c e o f s u r p l u s p roduc t ion m o d e l s : in c o m p a r i s o n with the resul ts for the p roduc t ive O P , e s t ima te s o f m a n a g e m e n t pa r ame te r s gene ra l l y i m p r o v e d f o r t h e s u r p l u s p roduc t ion m o d e l s but s l ight ly de te r io ra ted f o r t h e de l ay -difference m o d e l . B i o m a s s a s s e s s m e n t for the unproduc t ive s tock s c e n a r i o is re la t ive ly g o o d in a l l m o d e l s . O v e r a l l , the D e r i s o - S c h n u t e m o d e l ou tper forms the s u r p l u s p roduc t ion m o d e l s , h a v i n g r e m a r k a b l y g o o d b i o m a s s fits (fig. 4 .14a ) . T h e su rp lus p roduc t ion m o d e l s h a d c o m p a r a t i v e l y la rger var iabi l i ty t han the D e r i s o - S c h n u t e m o d e l in thei r b i o m a s s e s t ima te s . No tab ly , the R i c k e r O P s e e m e d to p resen t m o r e c h a l l e n g e s to the su rp lus p roduc t ion m o d e l s than the B e v e r t o n - H o l t O P . -+- -+- -+- -+- -+- -+- -+-A - O M 9 10 11 12 13 14 15 16 17 18 19 20 years S F D -+- -+- -t-B 8 9 10 11 12 13 14 15 16 17 18 19 20 years Figure 4.12 Examples of model fits to biomass time series for the Beverton-Holt OP. A) Hyperstability Case; B) Hyperdepletion Case. Legend key as in fig. 4.9. By chance, fig B shows a relatively good fit of the Fox model, however, note large uncertainty of TD values for this model in fig. 4.11. 2 3 8 S-B S-R F-B F-R D-B D-R Figure 4.13 Modified box plots showing distribution of est imates of management and b iomass a s ses smen t benchmarks for each estimation model using the multiplicative observation error assumption. Monte Car lo simulations performed with the unproductive stock (age of recruitment =4yr) and proportionality O P . C o d e s in the x axis correspond to letters used to name estimation procedures in table 4 .1 . 239 OP s F D Figure 4.14 Examples of model fits to biomass time series. A) Ricker O P with low productivity; B) Same as above, with uninformative effort pattern; C) A s above, with the Bo=K assumption. Legends as in fig. 4.9 2 4 0 U s i n g the B0=K a s s u m p t i o n in the unproduc t ive s tock s c e n a r i o i m p r o v e s c o n s i d e r a b l y the resul ts o f the S c h a e f e r a n d F o x m o d e l s but h a s nega t ive effects in the p e r f o r m a n c e o f the D e r i s o - S c h n u t e m o d e l (fig. 4 .15) . M a n a g e m e n t p a r a m e t e r e s t i m a t e s for both O P w e r e a c c e p t a b l e for the two su rp lus p roduc t ion m o d e l s but not for the D e r i s o - S c h n u t e m o d e l ( spec ia l ly fopt for the B e v e r t o n Hol t O P ) . B i o m a s s a s s e s s m e n t a re a l s o i m p r o v e d for the S c h a e f e r a n d F o x m o d e l s but d e g r a d e s l ight ly for the D e r i s o - S c h n u t e m o d e l ( see TD s c o r e s ) . 4 .3 .5 Tr i a l s wi th uninformat ive da ta . F i g u r e 4 .16 s h o w s the nega t ive effect of h a v i n g da t a that d o not p r o v i d e sufficient contras t for p a r a m e t e r e s t ima t ion . A l t h o u g h t he se da t a per ta in to the re la t ive ly e a s i e r to fit unproduc t ive s tock O P , the pe r fo rmance o f a l l m o d e l s de te r io ra tes s ignif icant ly in c o m p a r i s o n wi th resul ts s h o w n in figure 4 . 1 3 . N o n e o f the three m o d e l s is a b l e to p rov ide u n b i a s e d e s t ima te s o f both m a n a g e m e n t b e n c h m a r k s : g o o d e s t i m a t e s o f fopt w e r e not m a t c h e d wi th g o o d e s t ima te s o f Copt. T h e es t ima t ion o f b i o m a s s suffered s imi l a r l o s s in pe r fo rmance for a l l m o d e l s (fig. 4 .14b) , wi th s o m e TD v a l u e s r e a c h i n g l eve l s a s h igh a s t hose f o u n d for the hypers tab i l i ty a n d hype rdep l e t i on c a s e s . U n d e r th is s c e n a r i o , on ly the D e r i s o - S c h n u t e m o d e l y i e l d s m o r e o r l e s s cons i s t en t b i o m a s s fits for both ope ra t ing popu la t ions . T h e B0=K a s s u m p t i o n h e l p s to i m p r o v e resul ts sufficiently o n l y fo r the D e r i s o - S c h n u t e m o d e l (fig. 4 .17) . E s t i m a t e s o f m a n a g e m e n t b e n c h m a r k s a re o n l y wi th in a c c e p t a b l e v a l u e s in the D e r i s o - S c h n u t e m o d e l , a n d in the F o x m o d e l B e v e r t o n - H o l t O P . U n d e r t h e difficult s i tua t ions of l ow cont ras t da ta , this s t ra tegy h e l p s i m p r o v e mos t b i o m a s s e s t i m a t e s i nc lud ing t hose of the D e r i s o m o d e l (fig. 4 .14c ) , w h i c h w e r e p r e v i o u s l y de te r io ra ted by this s t ra tegy in the unproduc t ive s t o c k with h igh cont ras t effort s c e n a r i o (figs. 4 .13 a n d 4 .15 ) . H o w e v e r , the i m p r o v e m e n t is not sufficient for the b i o m a s s e s t ima tes o f the F o x m o d e l for the R i c k e r O P . W h e n the B0=K a s s u m p t i o n is u s e d , o n c e a g a i n the D e r i s o - S c h n u t e m o d e l is the best e s t ima t ion p r o c e d u r e for both ope ra t i ng p o p u l a t i o n s pe r fo rming sat isfactor i ly for m a n a g e m e n t a n d a s s e s s m e n t p a r a m e t e r e s t ima t ion . 241 S-B S-R F-B F-R D-B D-R S-B S-R F-R D-B Figure 4 .15 Modif ied box plots showing distribution of est imates of management and b iomass a s ses smen t benchmarks for each estimation model using the B0=K and the multiplicative observation error assumptions. Monte Car lo simulations performed with the high contrast effort pattern, unproductive stock (age of recruitment =4yr) and the proportionality O P . C o d e s in the x axis correspond to letters used to name estimation procedures in table 4 .1 . 2 4 2 S-B S-R F-B F-R D-B D-R 01 m S-B S-R F-B F-R D-B D-R S-B S-R F-B F-R D-B D-R Figure 4.16 Modified box plots showing distribution of est imates of management and b iomass a s ses smen t benchmarks for each estimation model using the multiplicative observation error assumption. Monte Car lo simulations performed with the uninformative effort pattern, the unproductive stock (age of recruitment =4yr) and the proportionality O P . C o d e s in the x axis correspond to letters used to name estimation procedures in table 4 .1 . 2 4 3 F-B F-R D-B D-R Figure 4.17 Modified box plots showing distribution of est imates of management and b iomass a s sessment benchmarks for each estimation model using the multiplicative observation error and the BP=K assumptions. Monte Car lo simulations performed with the uninformative effort pattern, the unproductive stock (age of recruitment =4yr) and the proportionality O P . C o d e s in the x axis correspond to letters used to name estimation procedures in table 4 .1 . 2 4 4 4.4 D i s c u s s i o n . 4.4.1 U n s u c c e s s f u l e s t ima t ion a s s u m p t i o n s a n d m o d e l m i s s p e c i f i c a t i o n s . T h e r e a s o n w h y the u s a g e o f add i t ive o b s e r v a t i o n errors in the e s t ima t ion p r o c e d u r e is inferior to the mul t ip l ica t ive er ror a s s u m p t i o n is that the add i t ive er ror a s s u m p t i o n is a mi s spec i f i ca t i on that c a n n o t p roper ly m i m i c the s i m u l a t e d e r ror v a l u e s ( see f ig . 4 .18) . F o r a g i v e n v a l u e o f vt, equa t i on 4 .40 d e t e r m i n e s a dis t r ibut ion o f a e s t i m a t e s that d o e s not ma t ch the n o r m a l c u r v e s i m u l a t e d in the da ta . T h e failure o f the n o n - d e l a y v e r s i o n o f the D e r i s o - S c h n u t e m o d e l h igh l igh ts the impor t ance of a c o n s c i o u s a n d carefu l imp lemen ta t i on o f the de lay-d i f fe rence m o d e l . T h e poo r pe r fo rmance o f the m o d e l wi th this mi s spec i f i ca t i on , a d d s to the difficulties e n c o u n t e r e d dur ing the i n d e p e n d e n t e s t ima t ion o f the f ixed p a r a m e t e r s a n d o the r m o d e l imp lemen ta t i on de ta i l s d e s c r i b e d a b o v e (sec t ion 4 .2 .4 .2) a n d d e m o n s t r a t e s that the D e r i s o - S c h n u t e m o d e l a s i m p l e m e n t e d he re wi th s e v e r a l f ixed pa rame te r s , is p rone to fail w h e n there a re m o d e l depa r tu res . T h e e x p e r i e n c e g a i n e d dur ing this s tudy a l l o w s to p rov ide a f ew l ines o f a d v i c e to n e w u s e r s o f this m o d e l : w h e n e v e r the resul ts f rom the D e r i s o - S c h n u t e m o d e l a re ve ry sens i t ive to s tar t ing v a l u e s , o r w h e n the m o d e l c o n v e r g e s to a b s u r d p a r a m e t e r e s t ima tes o r c a n n o t c o n v e r g e at a l l , there is l ikely to be a w r o n g f ixed p a r a m e t e r v a l u e o r s o m e mis spec i f i ca t i on in the m o d e l . Unfor tunate ly , a s it h a s b e e n s h o w n here , there are a l s o m a n y i n s t a n c e s in w h i c h w r o n g spec i f i ca t ions o f the m o d e l wi l l p rov ide s o m e k ind o f a n s w e r s , u sua l ly g r o s s l y b i a s s e d , without a n y w a y to d i a g n o s e this s i tua t ion . No tab ly , mi s spec i f i ca t i on o f s tock- recru i tment re la t ionsh ip is l ikely to h a p p e n in m a n y i n s t a n c e s wh i l e w o r k i n g with rea l f ish s tocks , wi th potent ia l ly nega t ive c o n s e q u e n c e s o n the es t ima t ion o f m a n a g e m e n t p a r a m e t e r s . A s there is no w a y to detect t h e s e c a s e s from a s i m p l e a n a l y s i s of the resul ts , it is r e c o m m e n d e d to do a carefu l de te rmina t ion o f the p a r a m e t e r s to be f ixed , before i m p l e m e n t a t i o n o f the m o d e l . 4 .4 .2 C h a n g e s in the C P U E - b i o m a s s re la t ionsh ip . N o n e o f the three es t ima t ion p r o c e d u r e s e x a m i n e d he re is a b l e to c o p e with the c o m p l e x i t i e s o f the hype rdep l e t i on a n d hypers tabi l i ty s i tua t ions . T h i s is in a g r e e m e n t with 245 Figure 4.18 Distribution of errors around a q value of 0.3, according to the two observation error assumpt ions considered in the study: additive and multiplicative. A l l data are simulated with the multiplicative model . 2 4 6 s imi l a r s tud i e s that h a v e s h o w n the vir tual imposs ib i l i ty o f f inding v a l u e s o f the ext ra p a r a m e t e r Q w h i c h d e t e r m i n e s the d e g r e e of hype rdep l e t i on o r hypers tab i l i ty ( R e e d a n d S i m m o n s 1994) . T h e inabil i ty o f m o d e l s to cor rec t ly es t imate th is v a l u e is d u e to p a r a m e t e r c o n f o u n d i n g . In the p resen t context , this m e a n s that there is s i m p l y not e n o u g h informat ion in the da t a for the m o d e l to s epa ra t e the effects of Q from the effects o f p a r a m e t e r s that de t e rmine the s h a p e o f the p roduc t ion funct ion a n d the total s tock s i z e . T h e o n l y so lu t ion to this p r o b l e m is to p rov ide the m o d e l s wi th add i t i ona l in format ion . F o u r n i e r a n d D o o n a n (1987) s u c c e s s f u l l y fitted a g e n e r a l i s e d D e r i s o - S c h n u t e m o d e l to h y p e r d e p l e t i o n a n d hypers tabi l i ty da t a but they u s e d a s i m p l e r pa rame te r i s a t i on , a d d i t i o n a l informat ion o n a v e r a g e w e i g h t a n d length , a n d a B a y e s i a n a p p r o a c h for thei r e s t ima t i ons . T h e k e y to their s u c c e s s h o w e v e r , is the add i t i ona l informat ion e m b e d d e d in the a v e r a g e w e i g h t a n d length da ta . In the light o f the w o r k o f F o u r n i e r a n d D o o n a n , de lay-d i f fe rence m o d e l s s e e m to be a p r o m i s i n g a p p r o a c h for s i tua t ions o f b i o m a s s - d e p e n d e n t ca tchab i l i ty w h e n more informat ion than just total c a t c h a n d a n a b u n d a n c e i n d e x a re a v a i l a b l e f o r t h e a s s e s s m e n t . 4 .4 .3 P e r f o r m a n c e o f the es t ima t ion p r o c e d u r e s , a n d c h o i c e o f the bes t m o d e l . T h e p e r f o r m a n c e o f the su rp lus p roduc t ion m o d e l s du r ing the p resen t s tudy ind ica te s their inabil i ty to p r o d u c e u n b i a s e d e s t ima te s of m a n a g e m e n t p a r a m e t e r s for a s i m u l a t e d a g e -s t ruc tured s h a r k popu la t i on . P e r h a p s m o r e important ly , s u r p l u s p roduc t ion m o d e l s s h o w a pers is tent ( and f requent ly large) pos i t ive b i a s in thei r e s t ima te s o f m a n a g e m e n t p a r a m e t e r s o v e r a w i d e r a n g e o f s i tua t ions . T h e r e w a s o n l y o n e in s t ance w h e r e both m o d e l s pe r fo rmed r e a s o n a b l y w e l l for m a n a g e m e n t p a r a m e t e r e s t ima t ion , the unp roduc t i ve O P with h igh cont ras t d a t a a n d the B0=K a s s u m p t i o n . T h e r e is a l s o a s t r iking i n c o n s i s t e n c y in the b e h a v i o u r o f s u r p l u s p roduc t ion m o d e l s w h e n different s tock- rec ru i tment funct ions are u s e d . C h a n g e s from a B e v e r t o n - H o l t to a R i c k e r O P c a u s e d a shift o n the relat ive a c c u r a c y for b i o m a s s e s t ima t ion from s u r p l u s p roduc t ion m o d e l s . T h i s shift is c h a r a c t e r i s e d by a t e n d e n c y t o w a r d s l a rge r e s t ima tes , f requent ly l e a d i n g to g rea te r pos i t ive b i a s s e s a n d c o n s e q u e n t l y h ighe r TD's. T h i s w a s g e n e r a l l y a c c o m p a n i e d by a w o r s e n i n g of the e s t ima te s of m a n a g e m e n t p a r a m e t e r s , e s p e c i a l l y fopt, e x c e p t for the uninformat ive da ta s c e n a r i o . It is u n c l e a r if the different s c a l e o f e a c h ope ra t i ng popu la t i on w a s r e s p o n s i b l e f o r t h i s b e h a v i o u r . T h e S c h a e f e r m o d e l w a s a b l e to 2 4 7 cor rec t ly e s t ima te m a n a g e m e n t pa r ame te r s for mos t o f the B e v e r t o n - H o l t O P ' s (excep t w h e n da ta cont ras t w a s poor) but fa i led v e r y frequent ly with the R i c k e r O P ' s . T h i s s u g g e s t s the S c h a e f e r m o d e l c o u l d be u s e d for rough e s t ima te s o f m a n a g e m e n t p a r a m e t e r s for e l a s m o b r a n c h p o p u l a t i o n s in s i tua t ions of h igh cont ras t in the f i shery da ta , p r o v i d e d there is a fairly g o o d ind ica t ion that the s tock- recru i tment r e la t ionsh ip i n d e e d fo l lows a B e v e r t o n -Holt c u r v e . T h e p resen t s tudy s e e m s to be the first of its k ind spec i f i ca l ly o r i en t ed t o w a r d s sha rk popu la t ions , h o w e v e r it c a n be con t r a s t ed aga in s t o ther i nves t iga t ions c o m p a r i n g the app l icab i l i ty o f s u r p l u s p roduc t ion m o d e l s to age - s t ruc tu red p o p u l a t i o n s . M y resul ts a re in conflict wi th f ind ings from L u d w i g a n d W a l t e r s (1985) w h o report that a g e n e r a l i s e d R i c k e r su rp lus p roduc t ion m o d e l per forms r e m a r k a b l y w e l l for fopt e s t ima t ion u s i n g da t a g e n e r a t e d by the D e r i s o - S c h n u t e m o d e l . P u n t ( 1 9 8 8 , 1 9 9 1 ) a l s o repor ts s u r p l u s p roduc t ion m o d e l s a s the mos t re l iab le for m a n a g e m e n t o f C a p e h a k e r e s o u r c e s s i m u l a t e d wi th a fully a g e -s t ructured o p e r a t i n g popu la t i on . S i m i l a r pos i t ive resul ts a re r epor ted wi th a S c h a e f e r e s t imator for a swordf i sh age - s t ruc tu red s imu la t ion m o d e l ( P r a g e r et a l . 1994) . T h e d i v e r g e n c e o f the a b o v e s tud ies with m y resul ts r e o p e n s a n in teres t ing q u e s t i o n : a re the v i e w s o f H o l d e n abou t m o d e l i n a d e q u a c y r e s p o n s i b l e for the fai lure o f su rp lus p roduc t ion m o d e l s o b s e r v e d h e r e ? Unfor tunate ly , this q u e s t i o n c a n n o t be s o l v e d with a s i m p l e c o m p a r i s o n b e t w e e n the p resen t s tudy a n d resul ts f rom the l i terature. T h e ope ra t ing popu la t ions , f i shery m o d e l s , e s t ima t ion p r o c e d u r e s , m i n i m i z a t i o n a lgo r i t hms a n d the s t a n d a r d s for accep t ing / re j ec t ing m o d e l s a re different in e a c h c a s e , m a k i n g difficult to u n d e r s t a n d the true r e a s o n s for thei r d i v e r g i n g resul ts . L u d w i g a n d W a l t e r s (1985) u s e o n l y a p r o c e s s er ror es t imator , later f o u n d to be g r o s s l y b i a s s e d ( L u d w i g et a l . 1 9 8 8 , P o l a c h e k et a l . 1993) , whi l s t P r a g e r et a l . (1994) u s e d a de te rmin i s t i c o p e r a t i n g popu la t i on a n d a n o b s e r v a t i o n error es t imator . In o rde r to inves t iga te if s u r p l u s p roduc t ion m o d e l s a re i n a d e q u a t e for sha rk - l ike popu la t ions , a better a p p r o a c h w o u l d be to perform para l le l s imu la t i ons with s i m i l a r age - s t ruc tu red ope ra t ing m o d e l s , o n e s i m u l a t i n g a t yp ica l sha rk popu la t ion ( long l i ved , s l o w g r o w i n g , l ow fecundi ty , etc.) the o ther s i m u l a t i n g a t yp ica l b o n y fish popu la t ion (fast g rowth , ea r ly maturity, h igh reproduc t ive rate, etc.) , a n d u s i n g exac t ly the s a m e es t ima t ion p r o c e d u r e s , cont ras t in da ta a n d t e c h n i q u e s for m o d e l fitting. 248 There seems to be more hope forthe usage of surplus production models for shark biomass assessment than for management parameter estimation. Although generally inferior to the Deriso-Schnute model, both surplus production models are capable of estimating biomass benchmarks and obtaining good biomass fits for most of the scenarios analysed (excluding hyperstability and hyperdepletion). Notably, the Fox model performs slightly better for this task than the Schaefer model. The B0=K assumption proved to be a good remedy for the performance of surplus production models underthe difficult situations of uninformative data. In this context, it seems adequate to use either of the surplus production models, for the assessment of shark stocks when data forthe Deriso-Schnute model is not available or the model is not applicable (i.e. multispecies fisheries). Results presented here indicate that the Deriso-Schnute model is by far the best choice for biomass assessment as well as forthe estimation of management parameters in shark-like populations. This model consistently outperformed the Schaefer and Fox models for both Ricker and Beverton-Holt OP's. Moreover, its robustness (under the ideal conditions of a simulation study) was evidenced by its superior performance under different scenarios of stock productivity and quality of fisheries data. The clear superiority of the Deriso-Schnute model over surplus production models for the estimation of management benchmarks seems surprising under the light of findings from Ludwig and Walters (1985). According to their study, the Ricker surplus production model outperforms the Deriso-Schnute model for the estimation of optimal effort even when data are generated with the latter model. Punt (1988, 1991) also found that surplus production models perform better than the Deriso-Schnute model for fisheries management parameter estimation. Leaving aside the speculation that perhaps the Ricker surplus production model is also far superior to the models of Schaefer and Fox, this apparent contradiction highlights the limits of simulation results. Each study is valid under the particular conditions where it was carried out, and extrapolations must be done very cautiously. Undoubtedly, tackling this problem requires performing a study of very large proportions, in which all kinds of fishery models and estimation methods are tested at once underthe same conditions. Ideally, such a study would have to be done every time a new stock or resource needs to be assessed and managed. Forthe particular case of elasmobranch fisheries, this is probably the subject of a whole dissertation analogous to the work of Punt (1988) for South African hake 2 4 9 r e s o u r c e s . A n o t h e r so lu t ion , offering the a d v a n t a g e o f t ra in ing a n d c o m m u n i c a t i o n a m o n g different par t ies , is to o r g a n i s e this w o r k a s a n e x e r c i s e for a w h o l e g r o u p o f f i shery b io log i s t s a n d m a n a g e r s du r ing a f o c u s e d w o r k s h o p . Ul t imate ly , c h e c k i n g the of val id i ty o f the p resen t resul ts wi th m o r e tes ts w o u l d e m p h a s i s e the v i e w s o f H i l b o r n a n d W a l t e r s (1992) that it s h o u l d be c u s t o m a r y to rout inely perform M o n t e C a r l o a n a l y s e s o f ind iv idua l a s s e s s m e n t s , a n d to concen t r a t e efforts not in f inding a g o o d fit to the da t a but in f inding al l p o s s i b l e a l te rna t ive m o d e l s ( and pa rame te r s ) that c o u l d e x p l a i n s u c h d a t a e q u a l l y w e l l . 4 .4 .4 S i g n i f i c a n c e for the a s s e s s m e n t a n d m a n a g e m e n t o f rea l s h a r k f i she r i es . T h e resul ts from the p resen t s tudy might m i s l e a d the uncr i t ica l r eader . E v e n t h o u g h the D e r i s o - S c h n u t e m o d e l ou tpe r fo rmed both s u r p l u s p roduc t ion m o d e l s t h roughou t mos t of the s tudy, this d o e s not gua ra n t e e that the D e r i s o - S c h n u t e m o d e l wi l l p rov ide m o r e accu ra t e a n d re l iab le e s t i m a t e s o f b i o m a s s a n d m a n a g e m e n t b e n c h m a r k s t han s u r p l u s p roduc t ion m o d e l s u n d e r a rea l s i tua t ion . T h e appa ren t r o b u s t n e s s a n d rel iabi l i ty o f the m o d e l s e e m s to be c h a l l e n g e d by its sensi t iv i ty to w r o n g c h o i c e (es t imat ion) o f f ixed pa rame te r s , par t icular ly the a g e o f entry to the f ishery, h igh l igh ted du r ing the ea r ly exp lo ra to ry tr ials m e n t i o n e d at the e n d o f s ec t ion 4 .2 .4 .2 . T h e resul ts o f the s tock- rec ru i tment funct ion mi s spec i f i c a t i on ( sec t ion 4 .4 .1) a l s o ca s t doubt o n the app l i cab i l i t y o f this m o d e l in rea l s i tua t ions . T h e m a i n p r o b l e m is that the con t ro l l ed c o n d i t i o n s u n d e r w h i c h the D e r i s o - S c h n u t e m o d e l h a s b e e n t e s t ed here wi l l ha rd ly be met in a rea l f i shery . T h e p a r a m e t e r s k n o w n 'a priory' (actual ly e s t i m a t e d o r ' s to len ' from the s i m u l a t e d data) a n d f ixed he re for the de l ay -difference m o d e l a re p r e c i s e l y s o m e of the mos t uncer ta in in mos t rea l s h a r k s s t o c k s a n d m a n y fish popu l a t i ons : the v a l u e o f na tura l mortali ty, the type o f s tock recru i tment funct ion, a n d the e x a c t a g e o f entry to the f ishery. Unfor tunate ly , s l ight d e p a r t u r e s from the rea l v a l u e s p r o v e d to be b a d for e s t ima t ion dur ing the p resen t tes ts . In a l l l i k e l i h o o d , rea l a p p l i c a t i o n s o f the D e r i s o - S c h n u t e m o d e l wi l l be v u l n e r a b l e to l a rge b i a s in the es t imat ion of m a n a g e m e n t p a r a m e t e r s b e c a u s e of uncer ta in t ies in the f ixed p a r a m e t e r v a l u e s . In c o i n c i d e n c e with the p resen t s tudy, F o u r n i e r a n d D o o n a n (1987) find that er rors in the es t ima t ion o f the a g e o f recrui tment to the f i shery c a n b a d l y b i a s the resul ts from de l ay -difference m o d e l s , s p e c i a l l y w h e n this a g e is u n d e r e s t i m a t e d . G o o d de t e rmina t i ons of the 2 5 0 unde r ly ing type o f s tock- recru i tment re la t ionsh ip a re a l s o n e e d e d before a p p l y i n g the D e r i s o m o d e l to rea l f i she r ies . 4 .4 .5 T h e B0=K s t ra tegy. T h e effect o f u s i n g the B0=K s t ra tegy dur ing m o d e l fitting w a s not c l e a r in the p resen t s tudy. T h e benefi ts of this a s s u m p t i o n w e r e not g e n e r a l i s e d f o r t h e three m o d e l s , no r w e r e they cons i s t en t a c r o s s the different t ypes of t r ials . Fu r the rmore , this a s s u m p t i o n p r o d u c e d nega t ive effects in s o m e c a s e s . T h e s e resul ts a re in a g r e e m e n t with p r e v i o u s f ind ings . P u n t (1988) repor ts a su rp lus p roduc t ion m o d e l with the B0=K s t ra tegy a s the o p t i m a l m o d e l to e s t ima te m a n a g e m e n t p a r a m e t e r s for a h a k e f ishery . H o w e v e r , a n a l y s i s o f h is t abu l a t ed resul ts r e v e a l s that for a few c a s e s the s t ra tegy w a s de t r imenta l . P r a g e r et a l . (1994) f o u n d con t rad ic to ry resul ts a n d d id not r e a c h a n y c o n c l u s i o n s from a n a l y s e s o f a s i m i l a r c o n s t r a i n o n B0. T h e unpredic tab i l i ty o f the effect o f the B0=K s t rategy, s u g g e s t s a c a u t i o u s a p p r o a c h for its u s a g e . A t h o r o u g h e x a m i n a t i o n of its effects for the par t icu lar s y s t e m u n d e r s tudy v i a a M o n t e C a r l o a n a l y s i s is impera t ive . 4 .4 .6 S u m m a r y a n d c o n c l u s i o n s . T h e p resen t s tudy p r o v i d e s insight into s e v e r a l a r e a s o f f i she r i e s a s s e s s m e n t a n d m a n a g e m e n t : the app l icab i l i ty o f su rp lus p roduc t ion m o d e l s a n d de lay-d i f fe rence m o d e l s to sha rk p o p u l a t i o n s a n d their r o b u s t n e s s u n d e r different s c e n a r i o s ; the app l i cab i l i t y of su rp lus p roduc t ion m o d e l s to age - s t ruc tu red popu la t ions ; the effects o f different a s s u m p t i o n s abou t the form of the o b s e r v a t i o n error o n es t imat ion ; the sens i t iv i ty o f the de lay-d i f fe rence m o d e l to different m o d e l mi s spec i f i ca t ions ; a n d the abi l i ty of the three m o d e l s to es t imate pa r ame te r s in the a b s e n c e of propor t ional i ty b e t w e e n C P U E a n d b i o m a s s , a n d u n d e r s i tua t ions o f un informat ive da ta . In add i t ion , t he se resul ts a re a s tart ing point for further d e v e l o p m e n t s o n the a s s e s s m e n t a n d m a n a g e m e n t of e l a s m o b r a n c h f i sher ies . Future w o r k c o u l d bu i ld u p o n the f ind ings p r e s e n t e d he re by p u r s u i n g m o r e d e t a i l e d / c o m p r e h e n s i v e s tud ie s in spec i f i c a r e a s 251 highlighted through this work and abandoning futile approaches that have been shown here to be unsuccessful. None of the models analysed here should be used for estimation of stock size or management parameters under situations of suspected hyperdepletion or hyperstability when only catch and abundance data are available. The performance of the estimation procedures under this situations can lead to grossly biassed parameter estimates. The usage of surplus production models, in particular the Fox model, forthe estimation of management parameters in elasmobranch populations is not recommended. Prime reasons for this are large positive biasses in parameter estimation and lack of robustness. Although the Schaefer and Fox surplus production models are not recommended for management parameter estimation, they can be used more confidently for the rough estimation of stock size in shark fisheries that meet the proportionality requirement. For situations of uninformative fishery data, the B0=K strategy could help provide better biomass estimates. The Deriso-Schnute model is the most promising technique from those analysed here, for the estimation of assessment and management parameters in shark-like fisheries. This model provides the least biassed estimates of management and biomass benchmarks under most scenarios tested. This, however, is conditioned to stringent requirements: catchability should remain constant as abundance changes, and there must be a very good knowledge of life history parameters for the stock (i.e. parameters to be fixed in the model). 252 C H A P T E R 5 E L A S M O B R A N C H S T O C K A S S E S S M E N T A N D M A N A G E M E N T IN T H E R E A L W O R L D : T H E MULT ISPEC IES S H A R K FISHERY O F Y U C A T A N , MEX ICO. 5.1 Introduction. The practice of elasmobranch fisheries assessment frequently faces a large set of problems. First, there are many areas in which our knowledge of sharks is very limited, specially in relation to key population dynamic processes. Secondly, there is very limited practical experience in relation to proven methods for elasmobranch fisheries assessment. Additional problems seldom considered when discussing shark management are the definition of the size, distribution and dynamics of unit stocks, as well as the long-range seasonal migrations (specially in the case of large sharks) and other spatial dynamics. However, some of the problems related to population dynamics and model choice for elasmobranch fisheries assessment have already been discussed in previous chapters. Here I focus on a discussion of some practical problems likely to be found when attempting to do assessments of real elasmobranch fisheries, especially in the developing world (which produces 2/3 of the total world elasmobranch catch), and I suggest some alternatives for overcoming these problems. The discussion is led by an analysis of data from a real case study, the multispecies shark fishery of Yucatan, Mexico. A secondary objective is thus to do an assessment of this fishery, which is characterised by a typically poor database. As it happens, the only management recommendations that can be made at this stage are those aimed at improving our ability to do an assessment of this fishery. The measures suggested here can serve as general guidelines for solving similar problems likely to occur in many other tropical fisheries for elasmobranchs. 5.2 T h e s h a r k f i shery o f Y u c a t a n : a t yp ica l c a s e s tudy. 2 5 3 T h e o v e r v i e w p r e s e n t e d in c h a p t e r 2 i l lustrates h o w c o m m o n it is for mos t s h a r k a n d ray f i sher ies th roughou t the w o r l d to h a v e a ve ry l imi ted p o o l of informat ion for a s s e s s m e n t p u r p o s e s . In g e n e r a l , the o n l y s tat is t ics a v a i l a b l e a re a g g r e g a t e d c a t c h e s o f sha rk o r ray s p e c i e s (in the w o r s e c a s e s they a re both l u m p e d together) a n d t h e s e u s u a l l y con ta in no further deta i l (i .e. spa t ia l information) . C a t c h per unit effort ( C P U E ) o r f i she ry - independen t a b u n d a n c e i n d i c e s a re v e r y ha rd to find for e l a s m o b r a n c h f i she r ies , not o n l y in the d e v e l o p i n g w o r l d but o n a g l o b a l b a s i s (Bonfi l 1994) . T h e s h a r k f i shery o f Y u c a t a n is a pretty typ ica l e x a m p l e o f the a b o v e s i tua t ion , a n d thus the s h o r t c o m i n g s found for the a s s e s s m e n t o f this f i shery p r o b a b l y mirror the p r o b l e m s to be e n c o u n t e r e d in m a n y sha rk o r ray f i sher ies in the w o r l d . T h e sha rk f i shery o f Y u c a t a n h a s b e e n d e s c r i b e d by Bonf i l et a l . (1988 , 1990) . T h e r e are a p p r o x i m a t e l y 2 5 s p e c i e s in the c a t c h e s but s e v e n a re the mos t impor tant . S e v e r a l fleets contr ibute to the total s h a r k c a t c h : a s m a l l - s c a l e gil lnet f i shery s p r e a d a l o n g the c o a s t m a k e s up a large propor t ion o f the l and ings ; important c a t c h e s a re a l s o t a k e n inc iden ta l ly in the l a rge - sca l e h o o k a n d l ine finfish f i sher ies b a s e d at P r o g r e s o ; f inally a f ew v e s s e l s from the l a rge - sca l e f i shery cont r ibute add i t iona l c a t c h e s w h i l e e x c l u s i v e l y f i sh ing for s h a r k s with gi l lnets . T h e re l iab le r e c o r d o f s h a r k c a t c h e s in Y u c a t a n g o e s b a c k o n l y to 1 9 7 7 , w h e n the Min i s t ry o f F i s h e r i e s w a s e s t a b l i s h e d in M e x i c o . H o w e v e r , Bonf i l et a l . (1990) a s s e m b l e d h is tor ica l r e co rds b a c k to 1 9 5 6 b a s e d o n the app l i ca t i on o f c o n v e r s i o n fac tors to different sha rk by-produc t s from s e v e r a l s o u r c e s o f g o v e r n m e n t a l s tat is t ics . T h i s h i s to r ica l r e c o r d ind ica tes that sha rk c a t c h e s w e r e re la t ively s m a l l before 1 9 8 2 , s e l d o m r e a c h i n g m o r e than 6 0 0 t/y. C a t c h e s g r e w v e r y rap id ly after 1 9 8 0 a n d h a v e b e e n v a r y i n g a r o u n d s o m e 2 , 1 0 0 t/y s i n c e 1 9 8 2 (fig. 5.1). T h e app l i ca t i on o f V P A a n d re la ted m e t h o d s for the a n a l y s i s o f th is f i shery is not f eas ib le at the m o m e n t b e c a u s e o n l y o n e sha rk s p e c i e s from Y u c a t a n h a s b e e n a g e d (but not va l ida ted) a n d there a re no t ime s e r i e s da t a o n ca tch-a t -age for a n y s h a r k s p e c i e s . In fact, the s ing le s p e c i e s a p p r o a c h might not be the appropr i a t e p r o c e d u r e for this h ighly 2 5 4 mul t i spec i f ic f i shery in w h i c h se t t ing m a n a g e m e n t r e g i m e s for o n e s p e c i e s wi thout affecting the w h o l e o f the f i shery might be difficult to i m p l e m e n t . In a n y c a s e , the l a c k o f c a t c h a n d a b u n d a n c e informat ion for ind iv idua l s p e c i e s p r e c l u d e s the a p p l i c a t i o n o f e v e n the mos t s i m p l e s ing le s p e c i e s m o d e l s . T h e a b o v e cha rac t e r i s t i c s o f l a ck o f de ta i l ed informat ion by s p e c i e s , h igh s p e c i e s divers i ty in the c a t c h e s , a n d mul t ip le t ypes of g e a r s a n d v e s s e l s i n v o l v e d in the f i sh ing activi ty, a re c o m m o n p l a c e in m o s t of the t rop ica l e l a s m o b r a n c h f i sher ies o f the w o r l d . T h e r e is h o w e v e r o n e type o f informat ion that is read i ly a v a i l a b l e in Y u c a t a n , a n d p r o b a b l y e l s e w h e r e : a g g r e g a t e d total c a t c h da t a for s h a r k s by yea r . If s o m e k ind o f a b u n d a n c e i n d e x c o u l d be found , t hen a rough a s s e s s m e n t is p o s s i b l e by a p p l y i n g d y n a m i c s u r p l u s p roduc t ion m o d e l s to t he se a g g r e g a t e d - s p e c i e s da ta . A l t h o u g h a g g r e g a t e d - s p e c i e s s u r p l u s p roduc t ion m o d e l s h a v e s e v e r a l pitfalls ( see P a u l y 1979) they p rov ide a useful first ( and s o m e t i m e s the on ly pos s ib l e ) a p p r o x i m a t i o n o f the s ta tus of the s t o c k s a n d h a v e b e e n u s e d before , a m o n g others , by B r o w n et a l . (1976) for f i sher ies of the G e o r g e s B a n k , by R a l s t o n a n d P o l o v i n a (1982) for h a n d l i n e f i sher ies o f H a w a i i , by I C S E A F for the m a n a g e m e n t o f C a p e hake r e s o u r c e s o f s o u t h w e s t e r n A f r i c a (Hi lborn a n d W a l t e r s 1992) , a n d P o l a c h e c k et a l . (1993) for h a k e s off N a m i b i a . I o b t a i n e d informat ion o n sha rk c a t c h e s per f i sh ing trip from two f i sh ing c o m p a n i e s that a l l o w e d a c c e s s to thei r in ternal r eco rds a n d p a y m e n t s l ip s . T w o t ime s e r i e s o f C P U E w e r e thus built for two n e i g h b o u r i n g loca t ions o f the ea s t e rn Y u c a t a n coas t . D a t a from E l C u y o c o v e r e d the y e a r s 1 9 8 3 - 1 9 8 8 , w h e r e a s da ta from R i o L a g a r t o s w e r e for the y e a r s 1 9 8 8 -1992 (fig. 5.2). T h e proximi ty o f the two loca l i t i es a n d the r ad ius o f ac t ion o f t he se a r t i sana l v e s s e l s i nd i ca t e s that both c o m m u n i t i e s s h a r e a c o m m o n f i sh ing g r o u n d (fig. 5.3). T h u s , it w a s in p r inc ip le p l aus ib l e to try to pas te both s e r i e s toge the r to form a s i n g l e t ime s e r i e s of 10 yr . T h i s n e w t ime s e r i e s o f C P U E c o u l d then be u s e d a s a n i n d e x o f a b u n d a n c e for the m u l t i s p e c i e s s t o c k after m a k i n g the u s u a l l e ap o f faith o f a s s u m i n g that there is propor t ional i ty b e t w e e n c o m m e r c i a l C P U E a n d fish a b u n d a n c e . 5.3 M e t h o d s . In o r d e r to jo in t oge the r the two t ime se r i e s o f C P U E , the da t a first h a d to be s t a n d a r d i s e d . 255 3000 i 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 Years Figure 5.1. Historical catches of the Yucatan shark fishery. R = Reconstructed from shark by-products; O = Official data, Ministry of Fisheries (25 species of which 7 are common). 0 -E l C u y o R i o L a g a r t o s - - + 83 84 85 86 87 88 89 90 91 92 Year Figure 5.2. CPUE time series for shark fisheries in two neighbouring localities of Yucatan. 2 5 6 T h i s w a s d o n e by o b t a i n i n g a better m e a s u r e o f f i sh ing effort for both t ime se r i e s , a n d then p r o c e e d i n g to s t a n d a r d i s e the da ta . Interviews with f i s h e r m e n in both loca l i t i es r e v e a l e d that wh i l e a t yp ica l v e s s e l in R i o L a g a r t o s u s e d gi l lnets to ta l l ing 16 p a n e l s o f net o f a p p r o x . 4 0 fath. e a c h , E l C u y o v e s s e l s u s e d on ly eight o f t h e s e p a n e l s pe r boat d u r i n g the m i d 1980 ' s . B y t rans la t ing the C P U E for e a c h local i ty from catch-per- t r ip to ca tch-per -ne t , a n d then s t a n d a r d i z i n g the two t ime se r i e s , t h e se m a t c h e d perfect ly (fig. 5.4). T h e resu l t ing c o m p o s i t e h is tor ica l da t a set f o r t h e f i shery is c h a r a c t e r i s e d by a n i n c r e a s i n g t r end in C P U E du r ing the first two y e a r s , f o l l o w e d by a d e c r e a s i n g C P U E . T h e t e rmina l four y e a r s o f da ta s h o w no t rend in C P U E . A d y n a m i c S c h a e f e r s u r p l u s p roduc t ion m o d e l w a s fitted to the t ime s e r i e s o f total sha rk c a t c h e s for Y u c a t a n a n d t h e s e C P U E da ta , u s i n g the total leas t s q u a r e s m e t h o d ( L u d w i g et a l . 1988) . T h e t e c h n i c a l de ta i l s of this p r o c e d u r e a re g i v e n in the p r e v i o u s c h a p t e r ( sec . 4.2.6) T w o se t s o f t r ials w e r e pe r fo rmed . In the first set, the S c h a e f e r m o d e l w a s fitted to the entire t ime s e r i e s . F o r t h e s e c o n d set o f tr ials , the first two y e a r s o f C P U E da t a ( i nc r ea s ing C P U E ) w e r e left out o f the a n a l y s i s . A g r o w i n g C P U E ear ly in the t ime s e r i e s c a n be e x p l a i n e d in the p resen t con tex t by s e v e r a l a l ternat ive h y p o t h e s e s : i) a p e r i o d o f l ow effort that a l l o w s s tock r e c o v e r y after h e a v y explo i ta t ion ; ii) a n o p e n s y s t e m w h e r e there is net immigra t ion ; iii) a b s e n c e o f propor t ional i ty b e t w e e n C P U E a n d b i o m a s s ; o r iv) er rors ear ly in d e v e l o p m e n t o f the c a t c h repor t ing s y s t e m . A v a i l a b l e da ta s u g g e s t that the effort l eve l s w e r e not l ow in this f i shery du r ing the p h a s e o f g r o w i n g C P U E a n d that the s tock h a d not b e e n f i shed ha rd in pr ior t imes , s o the poss ib i l i ty of a r e c o v e r i n g s t o c k is e x c l u d e d . A l t e rna t i ve s ii) a n d iii) v io la te the a s s u m p t i o n s o f su rp lus p roduc t ion m o d e l s a n d c o u l d therefore not be e v a l u a t e d in the a n a l y s i s . O n the o ther h a n d , the repor t ing s y s t e m o f the f i sh ing c o m p a n i e s w h i c h p r o v i d e d the da t a h a d b e e n in p l a c e for m a n y y e a r s wi thout c h a n g e , a n d a s thei r r eco rds w e r e kept for b o o k - k e e p i n g p u r p o s e s , they are thought to be re l i ab le . 5.4 R e s u l t s . T a b l e 5.1 s h o w s the resul ts o f the first set of tr ials u s i n g the c o m p l e t e t ime s e r i e s of C P U E a n d c a t c h . A t t e m p t s to fit the S c h a e f e r m o d e l to t he se da t a p r o d u c e d h igh ly uncer ta in 257 Figure 5.3 Aproximate range of action of the artisanal shark fisheries of Rio Lagartos and El Cuyo. Additional constrains are imposed by depth. Deployment of nets is not possible beyond 75 m. 83 84 85 86 89 90 91 87 88 Year Figure 5.4. Standardized C P U E time series for shark fisheries in two neighbouring localities of Yucatan. 92 2 5 8 T a b l e 5.1. Resu l t s of first set of trials: Total Leas t S q u a r e s (TLS) fits of the S c h a e f e r mode l to C P U E and ca tch data for the 10 yr t ime-series from the Y u c a t a n shark fishery. Trial —Starting values Parameter estimates TLS number r K q r K q Bo score Constraints 1.1 0.2 25 0.01 0.019561 35.739452 0.009810 122.580 0.452700 none 1.2 0.03 30 0.01 0.002883 20.234309 0.002786 424.480 0.449500 none 1.3 0.3 30 0.01 0.002883 20.234309 0.002786 424.480 0.449500 none 1.4 0.3 50 0.005 0.028215 60.901740 0.006933 171.000 0.453100 none 1.5 0.2 100 0.01 0.000210 1.454529 0.002764 430.980 0.448767 none 1.6 0.2 25 0.01 0.200027 41.026883 0.027499 39.576 0.664143 none 1.7 0.25 10 0.1 0.878897 10.000000 0.138291 9.350 0.546733 K frozen 1.8 0.25 15 0.1 0.523457 15.000000 0.092199 13.700 0.515239 K frozen 1.9 0.523 20 0.092 0.347191 20.000000 0.066545 18.730 0.496242 K frozen 1.10 0.347 25 0.066 0.233732 25.000000 0.049758 24.850 0.483547 K frozen 1.11 0.2 25 0.05 0.200000 32.593812 0.035117 34.580 0.475505 r frozen 1.12 0.15 25 0.1 0.150000 39.000853 0.026613 45.400 0.469012 r frozen 1.13 0.15 25 0.09 0.476546 16.026933 0.090000 14.050 0.512209 q frozen 1.14 0.1 15 0.12 0.693299 12.007724 0.120000 10.720 0.533486 q frozen 1.15 0.05 50 0.05 0.215046 26.611502 0.050000 24.700 0.482166 q frozen 1.16 0.3 10 0.1 0.000000 51.030000 0.028000 0.404230 Bo=K 1.17 0.3 25 0.03 0.000000 50.925988 0.028467 0.404234 Bo=K 1.18 0.05 15 0.15 0.000000 51.149185 0.028333 0.404236 Bo=K 1.19 0.5 20 0.05 0.000000 50.952946 0.028446 0.404236 Bo=K 1.20 0.5 40 0.1 0.000000 40.000000 0.036652 0.413799 Bo=K, K frozen 1.21 0.1 75 0.01 0.000000 75.000000 0.018953 0.412556 Bo=K, K frozen 1.22 0.5 75 0.01 0.000000 75.000000 0.018953 0.412556 Bo=K, K frozen 1.23 0.3 35 0.01 0.082005 35.000000 0.041765 0.408888 Bo=K, K frozen 1.24 0.3 40 0.01 0.000000 40.000000 0.036478 0.410514 Bo=K, K frozen 1.25 0.3 45 0.01 0.000000 45.000000 0.032373 0.405681 Bo=K, K frozen 1.26 0.3 50 0.01 0.000000 50.000000 0.029017 0.404261 Bo=K, K frozen 1.27 0.3 50 0.05 0.131621 29.760354 0.050000 0.413875 Bo=K, q frozen 1.28 0.1 15 0.05 0.131207 29.768486 0.050000 0.413875 Bo=K, q frozen 1.29 0.3 50 0.03 0.000000 48.570130 0.030000 0.404439 Bo=K, q frozen 1.30 0.3 50 0.045 0.099888 32.884498 0.045000 0.410625 Bo=K, q frozen 1.31 0.15 100 0.05 0.150000 30.859003 0.047192 0.413867 Bo=K, r frozen 1.32 0.05 100 0.05 0.050000 42.219089 0.034365 0.406138 Bo=K, r frozen 1.33 0.025 45 0.03 0.025000 46.055521 0.031496 0.404955 Bo=K, r frozen 1.34 0.025 47 0.03 0.000000 50.909983 0.028478 0.404233 Bo=K 1.35 0.2 100 0.001 0.000000 361.720456 0.003638 0.467299 Bo=K 1.36 0.3 10 0.01 0.850560 10.000000 0.138877 0.523538 Bo=K, K frozen 1.37 0.3 15 0.01 0.472999 15.000000 0.097656 0.479044 Bo=K, K frozen 1.38 0.3 20 0.01 0.309643 20.000000 0.073396 0.437818 Bo=K,K frozen 1.39 0.3 25 0.01 0.198556 25.000000 0.059078 0.422255 Bo=K, K frozen 1.40 0.3 100 0.1 0.441921 15.514177 0.100000 0.461611 Bo=K, q frozen 1.41 0.3 10 0.1 0.000015 44.885062 0.000006 195534 1.197700 none 1.42 0.3 5 0.5 failed trial none 1.43 0.1 7 0.1 failed trial none 1.44 0.05 25 0.5 failed trial none 1.45 0.3 10 0.05 failed trial none 1.46 0.3 10 0.01 failed trial none 1.47 0.15 100 0.3 failed trial Bo=K 1.48 0.1 75 0.5 failed trial Bo=K, K frozen 2 5 9 p a r a m e t e r e s t ima te s w h i c h w e r e not un ique (final e s t ima te s s t rongly d e p e n d e n t o n the start ing e s t i m a t e s in the non - l i nea r s e a r c h p rocedu re ) . N o t i c e a b l y , s e v e r a l c o m b i n a t i o n s of r, K a n d q c o u l d e x p l a i n e q u a l l y w e l l the da ta , i.e. the v a l u e s o f the m i n i m i z a t i o n cr i ter ion are s imi l a r w h i l e p a r a m e t e r v a l u e s a re m a r k e d l y different. T h i s w a s s p e c i a l l y ev iden t dur ing u n c o n s t r a i n e d tr ials , w h e r e e s t ima te s o f r v a r i e d by up to two o r d e r s o f m a g n i t u d e with little c h a n g e in the fitting cr i ter ion (trials 1.1-1.5). In add i t ion , for s o m e tr ials the non- l inea r min imiza t i on just fa i l ed c o m p l e t e l y (trials 1.42-1.48). S t r a t eg ie s a i m e d at h e l p i n g the es t ima t ion w e r e u n s u c c e s s f u l . First , o n e p a r a m e t e r at a t ime w a s f rozen du r ing m i n i m i z a t i o n , but the fit p r o d u c e d b i o m a s s e s t i m a t e s that w e r e grea ter than K (trials 1.7-1.15). U s i n g the B0=K s t rategy, a l o n e o r in c o m b i n a t i o n wi th f r eez ing o n e of the pa rame te r s , p r o v i d e d the bes t r ange of v a l u e s for the m i n i m i z a t i o n cr i ter ion ( T L S b e t w e e n 0 .40 a n d 0 .42; tr ials 1.16-1.34). H o w e v e r , this d id not r e m o v e the uncer ta in ty in the resul ts , a n d v e r y f requent ly l ed to r e s t ima te s o f z e r o . A d d i t i o n a l l y , the bes t v a l u e s of the fitting cr i ter ion d id not c o r r e s p o n d with the bes t fits to the C P U E s e r i e s (fig. 5.5), ra i s ing doub t s a b o u t thei r rel iabil i ty. R e s u l t s from the tr ials b a s e d on ly o n da t a c o r r e s p o n d i n g to the eight y e a r s of d e c r e a s i n g C P U E a re s h o w n in t ab le 5.2. T h e effect of ignor ing the i n c r e a s i n g part o f the C P U E se r i e s w a s a n i m p r o v e m e n t in pe r fo rmance o f the T L S funct ion t o w a r d s l o w e r m i n i m a . H o w e v e r , this w a s la rge ly a n d effect o f f ewer v a l u e s b e i n g c o n s i d e r e d in the T L S s c o r e . In fact the s t ra tegy d id not r e m o v e the p r o b l e m s e n c o u n t e r e d before . T h e r e w e r e m a n y i n s t a n c e s in w h i c h the a n s w e r s w e r e n o n s e n s i c a l (r = 0; b i o m a s s e s m u c h l a rge r t han K). In fact, a l l the fits wi th the 'best ' T L S s c o r e s ( those b e t w e e n 0.16 a n d 0.18) r e su l t ed in v a l u e s of B0 g rea ter than K (trials 2 .3 -2 .7 , 2 . 1 7 - 2 . 2 1 , 2 .24 -2 .25 , 2 .28-2 .34) . In m a n y c a s e s the non- l inea r op t imisa t ion fa i led (trials 2 .46-2 .74) . A l t e rna t e ly f r eez ing o n e o f the p a r a m e t e r s a n d letting the o thers free du r ing es t ima t ion d id not e l imina te the a b o v e p r o b l e m s . D e s p i t e this , s o m e rela t ively w e l l b e h a v e d resul ts w e r e o b t a i n e d with the B0=K s t ra tegy (trials 2 .35-2 .45) . H o w e v e r , e v e n this s t ra tegy resu l t ed into uncer ta in ty in the p a r a m e t e r e s t i m a t e s ( spec ia l ly f o r r ) , r v a l u e s o f z e r o , a n d s o m e d e p e n d e n c e o f resul ts u p o n s tar t ing v a l u e s . In add i t ion , the T L S s c o r e s from the B0=K fits w e r e not the o v e r a l l l owes t a n d the e s t i m a t e d C P U E d id not ma t ch the da t a sufficiently w e l l (fig. 5.6). 260 <u 1.0 + 2" 0.8 0.6 0.4 0.2 0.0 observed estimated TLS = 0.4088 b) 83 84 85 86 87 88 Years — I — 89 90 —4-91 92 Figure 5.5. Examples of fits to the entire CPUE series: a) 'good' visual fit, but high TLS score (trial 1.6). b) Low TLS score, but poor visual fit at the beginning of the time series (trial 1.23). 261 Table 5.2. Results of second set of trials: TLS fits of CPUE and catch data for the downward portion of the time series (years 85-92) from the shark fishery of Yucatan. Trial —Starting value number r K q 2.1 0.2 100 0 2.2 0.2 100 0 2.3 0.1 50 0 2.4 0.15 25 0 2.5 0.4 50 0 2.6 0.05 100 0 2.7 0.05 100 0 2.8 0.5 100 0 2.9 0.5 100 0 2.10 0.2 100 0 2.11 0.2 100 0 2.12 0.5 10 0 2.13 0.5 40 0 2.14 0.25 55 0 2.15 0.2 55 0 2.16 0.2 100 0 2.17 0.05 100 0 2.18 0.01 45 0 2.19 0.005 45 0 2.20 0.01 100 0 2.21 0.1 100 0 2.22 0.2 100 0 2.23 0.3 100 0 2.24 0.3 100 0 2.25 0.3 100 0 2.26 0.3 100 0 2.27 0.3 100 0 2.28 0.05 100 0 2.29 0.05 40 0 2.30 0.1 75 0 2.31 0.05 75 0 2.32 0.05 75 0 2.33 0.05 75 0 2.34 0.2 50 0 2.35 0.05 100 0 2.36 0.3 100 0 2.37 0.05 25 0 2.38 0.5 50 0 2.39 0.01 25 0 2.40 0.01 50 0 2.41 0.5 100 0 2.42 0.5 25 0 2.43 0.2 100 0 2.44 0.2 45 0 2.45 0.3 100 0 2.46 0.25 5 0 2.47 0.1 8 0 2.48 0.15 9 0 2.49 0.15 10 0 2.50 0.15 15 0 Parameter estimates TLS r K q Bo score Constraints 0.000000 76.235340 0.035560 44.919 0.335130 none 0.000000 68.765576 0.035561 44.930 0.335130 none 0.056635 301.466232 0.001049 1652.334 0.169462 none 0.056417 104.694688 0.003060 566.742 0.171341 none 0.047788 136.044866 0.002087 815.991 0.171559 none 0.050290 295.266865 0.000992 1715.660 0.170342 none 0.055384 229.563378 0.001359 1276.732 0.169738 none 0.000000 87.221379 0.038150 44.964 0.351572 none 0.000000 73.026995 0.035561 44.940 0.335130 none 0.000000 72.890042 0.035566 44.934 0.335130 none 0.000000 66.021038 0.035565 44.952 0.335131 none 0.698674 10.000000 0.126087 14.845 0.308084 K frozen 0.000000 40.000000 0.037380 44.915 0.343349 K frozen 0.000000 55.000000 0.035561 44.933 0.335130 K frozen 0.000000 55.000000 0.038938 44.988 0.362495 K frozen 0.000000 100.000000 0.038926 45.001 0.362495 K frozen 0.050000 335.978637 0.000865 1966.673 0.170244 r frozen 0.010000 162.578179 0.000440 3859.212 0.173141 r frozen 0.005000 246.209719 0.000150 11363.246 0.173500 r frozen 0.010000 156.217911 0.000454 3783.909 0.173000 r frozen 0.100000 221.932509 0.002127 817.088 0.172706 r frozen 0.200000 36.071652 0.020644 85.801 0.191134 r frozen 0.300000 22 : 799622 0.040789 44.031 0.211097 r frozen 0.102745 50.372810 0.010000 174.736 0.176955 q frozen 0.080424 83.518130 0.005000 348.137 0.172791 q frozen 0.545006 12.222566 0.100000 18.584 0.269733 q frozen 2.553657 4.441209 0.300000 6.846 0.432039 q frozen 0.058387 165.024835 0.002000 851.029 0.171158 q frozen 0.022187 75.410945 0.002000 851.029 0.173978 q frozen 0.045419 136.430525 0.002000 851.029 0.171526 q frozen 0.067337 92.750194 0.004000 426.181 0.172800 q frozen 0.081621 70.560995 0.006000 290.278 0.173444 q frozen 0.090476 57.312158 0.008000 218.146 0.175086 q frozen 0.108509 43.766158 0.012000 145.907 0.178428 q frozen 0.043290 39.572226 0.034777 0.289850 Bo=K 0.041891 40.806946 0.033655 0.290269 Bo=K 0.048187 38.899841 0.035379 0.289851 Bo=K 0.044640 39.310043 0.034922 0.289876 Bo=K 0.009746 44.859617 0.030582 0.290192 Bo=K 0.010134 44.657819 0.030771 0.290196 Bo=K 0.000000 46.569233 0.029440 0.290406 Bo=K 0.000000 48.685076 0.027848 0.290799 Bo=K 0.000000 46.984687 0.029162 0.290416 Bo=K 0.000000 47.462990 0.028727 0.290478 Bo=K 2.805020 4.257789 0.300000 0.419515 Bo=K, q frozen 38.3939485 0.038195 0.30823266 Bo=K 73.9878294 0.019037 0.31627416 Bo=K 41.4014195 0.035247 0.30106583 Bo=K 40.7081229 0.035852 0.30103718 Bo=K 41.1427534 0.035449 0.30100048 Bo=K 262 Table 5.2. Continued Trial number —Starting values-r K q Parameter estimates r K q TLS Bo score Constraints 2.51 0.15 20 0.05 0.024643 41.6875158 0.03495 0.300655551 Bo=K 2.52 0.15 30 0.05 0.04394 39.4014528 0.03514 0.289970479 Bo=K 2.53 0.15 9 0.1 0.051294 38.3594583 0.035895 0.28986273 Bo=K, Vi=0.05 2.54 0.15 30 0.05 0.046412 39.1574264 0.035146 0.289848786 Bo=K, Vi=0.05 2.55 0.15 5 0.05 2.201323 5 0.234194 0.389843718 Bo=K, K frozen 2.56 0.15 7.5 0.05 1.297722 7.5 0.157274 0.386253703 Bo=K, K frozen 2.57 0.15 10 0.05 0.824437 10 0.130818 0.362737404 Bo=K, K frozen 2.58 0.15 12.5 0.05 0.598343 12.5 0.108831 0.33863909 Bo=K, K frozen 2.59 0.15 15 0.05 0.451837 15 0.092444 0.321597552 Bo=K, K frozen 2.60 0.15 20 0.05 0.283087 20 0.069817 0.303091551 Bo=K, K frozen 2.61 0.15 25 0.05 0.185024 25 0.055709 0.294965128 Bo=K, K frozen 2.62 0.15 30 0.05 0.120665 30 0.046221 0.2914473 Bo=K, K frozen 2.63 0.15 35 0.05 0.075169 35 0.039443 0.290109528 Bo=K, K frozen 2.64 0.15 40 0.05 0.041771 40 0.034382 0.289856726 Bo=K, K frozen 2.65 0.15 45 0.05 0.015441 45 0.030458 0.290156523 Bo=K, K frozen 2.66 0.15 50 0.05 0 50 0.027304 0.29079023 Bo=K, K frozen 2.67 0.15 60 0.05 0 60 0.02249 0.29435403 Bo=K, K frozen 2.68 0.5 25 0.001 0.198587 25 0.052056 0.30082778 Bo=K, K frozen, Vi 2.69 0.15 25 0.01 0.056376 126.986224 0.002535 0.171548756 none 2.70 0.2 10 0.3 failed trial none 2.71 0.3 20 0.03 failed trial none 2.72 0.15 10 0.01 failed trial K frozen 2.73 0.5 20 0.3 failed trial K frozen 2.74 0.3 20 0.1 failed trial K frozen 2.75 0.5 20 0.03 failed trial K frozen 2.76 0.1 20 0.03 failed trial K frozen 2.77 0.5 25 0.3 failed trial K frozen 2.78 0.05 25 0.03 failed trial K frozen 2.79 0.5 30 0.1 failed trial K frozen 2.80 0.25 35 0.03 failed trial K frozen 2.81 0.01 100 0.1 failed trial r frozen 2.82 0.5 50 0.3 failed trial Bo=K 2.83 0.2 100 0.5 failed trial Bo=K 2.84 0.01 100 0.3 failed trial Bo=K 2.85 0.05 10 0.05 failed trial none 2.86 0.05 10 0.2 failed trial none 2.87 0.05 100 0.05 failed trial none 2.88 0.05 50 0.05 failed trial none 2.89 0.05 40 0.05 failed trial none 2.90 0.5 10 0.2 failed trial none 2.91 0.5 10 0.05 failed trial none 2.92 0.2 45 0.2 failed trial none 2.93 0.05 100 0.05 failed trial r frozen 2.94 0.01 45 0.1 failed trial r frozen 2.95 0.005 45 0.05 failed trial r frozen 2.96 0.005 100 0.1 failed trial r frozen 2.97 0.005 100 0.05 failed trial r frozen 2.98 0.005 45 0.1 failed trial r frozen 2 6 3 O 3 Q. 1 1 1 1 0. 0 0 0 8 6 4 2 1 8 6 4 + 2 83 a ) observed estimated TLS= 0.16973 84 85 86 —I 1— 87 88 Y e a r s 89 90 91 92 3 Q. 8 6 + 4 2 1 8 6 4 2 + 83 b) observed estimated TLS= 0.28987 84 85 86 — l 1— 87 88 Y e a r s 89 90 91 92 Figure 5.6. Examples of fits to the downward portion of the CPUE series (years 85-92): a) unconstrained fit (trial 2.7); b) using the Bo-K constraint (trial 2.38). 2 6 4 5.5 D i s c u s s i o n . 5.5.1 S h o r t c o m i n g s o f the da ta . F a i l u r e s du r ing the first set of tr ials a re not su rp r i s ing . T h e first two d a t a poin ts in the t ime se r i e s def ine a n u p w a r d s t r end in C P U E (the m o d e l r e a d s this a s a b u n d a n c e ) that c anno t be e x p l a i n e d p roper ly in the light o f the effort informat ion e m b e d d e d in the da ta . O n l y a r e c o v e r i n g s tock (i.e. u n d e r l ow effort for a sufficiently l ong recent p e r i o d o f t ime after h e a v y exploi ta t ion) c a n s h o w a n i n c r e a s e in a b u n d a n c e cons i s t en t wi th the s u r p l u s p roduc t ion m o d e l . F a i l i n g to de tec t this f rom the da ta , the m o d e l e i ther p red ic t s b i o m a s s e s in e x c e s s o f K o r t r ies to ignore the u p w a r d s t rend in C P U E (fig. 5 .5b) . In fact, the ' u n e x p l a i n a b l e ' i n c r e a s i n g t r end in C P U E s h o u l d be e n o u g h to ca s t s e r i o u s doub t s a b o u t the rel iabil i ty o f the entire da ta set. It is ve ry p o s s i b l e that in te rac t ions b e t w e e n the spa t ia l d is t r ibut ion of f i she rmen ' s effort a n d the dis t r ibut ion o f s h a r k s is resul t ing in C P U E not b e i n g p ropor t iona l to fish a b u n d a n c e . S e v e r a l s c e n a r i o s w h e r e this c a n h a p p e n are l i s ted by H i l b o r n a n d W a l t e r s (1992) . T h e s e au thors s t ress that c o m m e r c i a l C P U E da ta are s e l d o m a n a d e q u a t e i n d e x o f a b u n d a n c e . F o r t h e s a k e o f this d i s c u s s i o n , I wi l l resort to a n o l d e x p l a n a t i o n for this i n c r e a s i n g C P U E at the b e g i n n i n g of the t ime s e r i e s w h i c h c o n s i d e r s that this t r end represen t a ' l ea rn ing pe r iod ' in w h i c h f i s h e r m e n w e r e b e c o m i n g m o r e a n d m o r e efficient; this l ea rn ing p e r i o d e n d s w h e n C P U E starts to d rop . W e c a n then justify i gno r ing the first two da t a points , a n d w o r k u n d e r the a s s u m p t i o n that the d o w n w a r d port ion o f the C P U E s e r i e s d o e s in fact r epresen t a n i n d e x o f a b u n d a n c e . M a n y of the p r o b l e m s e n c o u n t e r e d dur ing p a r a m e t e r e s t ima t ion a r e e x p e c t e d g i v e n the cha rac te r i s t i c s o f the da t a set. T h e r e a re s e v e r a l r e a s o n s w h y the Y u c a t a n da t a a re not ve ry useful for a s s e s s m e n t . First , this is a short t ime s e r i e s with o n l y t en , o r in the wors t c a s e eight da t a poin ts . It w o u l d be i n d e e d su rp r i s ing to get re l iab le a n s w e r s wi th s u c h a short t ime s e r i e s o f da t a . N e v e r t h e l e s s , the mos t de t r imenta l attribute o f t h e s e da t a is not the s m a l l quanti ty, but the p o o r qual i ty o f the informat ion they c o n t a i n . T h e r e is not e n o u g h cont ras t in the Y u c a t a n C P U E a n d c a t c h da t a to let the m o d e l differentiate c l ea r ly w h a t type of popu la t ion it is d e a l i n g wi th : is this a la rge s tock with v e r y l ow product ivi ty , a s m a l l a n d ve ry p roduc t ive s tock, o r s o m e t h i n g e l s e ? L u d w i g a n d W a l t e r s (1985) s u g g e s t a rule of 2 6 5 t h u m b for cont ras t in the da ta : rat ios o f at leas t 4 s h o u l d be o b s e r v a b l e in the t ime se r i e s b e t w e e n la rges t a n d s m a l l e s t s tock b i o m a s s a n d effort, in o r d e r to h a v e e n o u g h cont ras t for p a r a m e t e r e s t ima t ion o f su rp lus p roduc t ion m o d e l s . T h i s ratio is o n l y 2 .5 in the Y u c a t a n C P U E da ta . T h e l ack o f con t ras t in the Y u c a t a n da t a is a l s o ev iden t in the t e n d e n c y o f the m o d e l to return a r v a l u e o f z e r o . T h e S c h a e f e r m o d e l fai ls to es t imate r p rope r ly b e c a u s e the effects of r a r e not ev iden t in the ea r ly part o f o n e - w a y trip da t a s e r i e s . A c c o r d i n g to H i l b o r n (1979) , in o r d e r to ob ta in informat ion abou t r it is n e c e s s a r y first to b r ing d o w n the s tock to low l eve l s of a b u n d a n c e a n d then a l l o w it to r e c o v e r u n d e r fairly l o w f i sh ing effort. In the Y u c a t a n da ta , there is a b s o l u t e l y no r e c o v e r y pe r iod for the s tock . T h i s da t a set is i n d e e d a ve ry g o o d e x a m p l e o f wha t is k n o w n a s the o n e w a y trip: a t yp ica l pat tern o f c o n t i n u o u s l y g r o w i n g f i sh ing effort a c c o m p a n i e d by a d e c r e a s i n g C P U E (fig. 5.7). O n e w a y trips a re charac te r i s t i ca l ly difficult for es t imat ion p u r p o s e s b e c a u s e o f the i r l a c k o f informat ion abou t k e y fea tures o f the s tock . E v e n t h o u g h the resul ts o f u s i n g the B0=K s t ra tegy dur ing the s e c o n d se t o f tr ials offer the best T L S s c o r e s for a c r ed ib l e set o f resul ts , they s h o u l d be r e g a r d e d wi th s u s p i c i o n . T h e y do not o n l y rely total ly o n the B0=K s t ra tegy in o r d e r to m a k e s e n s e , but w h e n the fit is u n c o n s t r a i n e d , t h e s e resul ts still d e p e n d o n the s tar t ing v a l u e s a n d h a v e uncer ta in ty in their p a r a m e t e r e s t i m a t e s (par t icular ly f o r t h e p a r a m e t e r r w h i c h v a r i e s b e t w e e n 0.0 , 0 .009 , a n d 0 .044) . It w o u l d be ill a d v i s e d to set m a n a g e m e n t m e a s u r e s abou t the a d e q u a t e l eve l o f effort in this f i shery e x p e c t i n g to ob ta in the p red ic ted MSY b a s e d o n e s t i m a t e s o f the op t ima l effort d e r i v e d from the p resen t a n a l y s e s . M o s t l ikely s u c h r e c o m m e n d a t i o n s w o u l d not p r o d u c e the e x p e c t e d resul ts in the short run, not on ly b e c a u s e of the s h o r t c o m i n g s o f the S c h a e f e r m o d e l for m a n a g e m e n t p a r a m e t e r e s t ima t ion identif ied in the p r e v i o u s chap te r , but b e c a u s e of the uncer ta in ty s u r r o u n d i n g the resul ts o f fopt a n d MSY from t h e s e tr ials ( see be low) . M a n a g e m e n t p a r a m e t e r s d e r i v e d from su rp lus p roduc t ion m o d e l s a r e s p e c i a l l y sens i t i ve to v a l u e s o f r, the in t r ins ic rate o f i n c r e a s e (Hi lborn a n d W a l t e r s 1992) . D u e to the uncer ta in ty in r identif ied here , MSY a n d fopt e s t ima te s d e r i v e d from t h e s e resul ts wi l l be in t r ins ical ly unre l i ab le . 266 • 85 • 87 • 86 • 84 • 83 • 88 • 91 i i J 1 — • 89 • 92 1 1 • 90 —i 0 10 20 30 40 effort 50 60 70 80 Figure 5.7. Plot of CPUE and effort for the shark fishery of Yucatan, (effort estimated from the total catch and the available CPUE series) 2 6 7 5.5.2 H a r n e s s i n g uncer ta inty . In g e n e r a l , it is p o s s i b l e to k n o w or at leas t to set b o u n d s , to MSY and fopt e v e n t h o u g h the S c h a e f e r m o d e l p a r a m e t e r s a re not w e l l k n o w n . A sca t te r plot o f the r a n g e of r a n d K es t ima tes o b t a i n e d from different fits o f the m o d e l wi l l u s u a l l y p r o d u c e a w e l l de f ined p a r a m e t e r s p a c e c o r r e s p o n d i n g to a re la t ively n a r r o w b a n d o f MSY v a l u e s . S imi l a r l y , a scat ter plot o f r a g a i n s t q w i l l he lp set b o u n d s to the true v a l u e o f fopt. D u r i n g exp lora to ry tr ials , this m e t h o d w o r k e d w e l l w h e n a p p l i e d to fake d a t a wi th g o o d con t ras t g e n e r a t e d from a S c h a e f e r m o d e l a n d a n a l y s e d with the s a m e k ind of p r o c e d u r e u s e d f o r t h e Y u c a t a n da ta . F igu re 5.8 is a plot o f a l l r a n d K e s t ima te s from the s e c o n d set o f t r ials (8yr t ime se r i e s , tab le 5.2) e x c l u d i n g a l l fits p r o d u c i n g r v a l u e s o f z e r o . T h i s sca t te r plot de f ines a p a r a m e t e r s p a c e w h i c h s u g g e s t s that f o r t h e Y u c a t a n f ishery, mos t M S Y v a l u e s a re b e t w e e n 0 .25 a n d 4 t h o u s a n d t o n n e s , a l t hough the large majority o f the resul ts a r e u n d e r the 2 t h o u s a n d t o n n e s MSY i so l ine . Unfor tunate ly , the Y u c a t a n d a t a s h o w a p a t h o l o g i c a l s i tuat ion that l e a v e s still a lot o f uncer ta in ty a r o u n d the MSY e s t ima te s . T h e r e a re t w o p r o b l e m s with t he se da ta : first, the resul ts l e a d to MSY v a l u e s that c a n be a s l o w a s 0.2 o r a s large a s 4 ( t housand tonnes ) ; a n d s e c o n d , there a re two ve ry dist inct r-K t radeoffs in the resul ts . C i r c l e s ind ica te the resul ts from u n c o n s t r a i n e d fits, w h e r e a s c r o s s e s ind ica te resul ts from fits u s i n g the B0=K cons t ra in t . T h e r ange o f MSY v a l u e s o b t a i n e d wi th the Bo=K s t ra tegy s p a n s e v e r a l o r d e r s of m a g n i t u d e a n d t e n d to a MSY of z e r o a s K i n c r e a s e s t o w a r d s v a l u e s g rea te r t han 4 0 . B a s i c a l l y , it is still i m p o s s i b l e to s a y with cer ta in ty w h a t the MSY l eve l is for this f i shery . T h e s a m e pa tho log i ca l s i tuat ion c a n be o b s e r v e d for the se t s o f v a l u e s of r a n d q, w h i c h def ine two different t radeoffs d e p e n d i n g o n w h e t h e r the B0=K a s s u m p t i o n w a s u s e d o r not (fig. 5.9) A s imi l a r a n a l y s i s i l lustrates v e r y w e l l the effects o f a n o t h e r potent ia l s o u r c e o f uncer ta in ty in the Y u c a t a n da ta . F igu re 5 .10 is a sca t te r plot o f the s a m e resul ts o f t ab le 5.2, but this t ime the MSY i s o l i ne s a re c a l c u l a t e d u s i n g three different re la t ions b e t w e e n C P U E a n d b i o m a s s . T h e s e re la t ions a re de f ined by the fo l lowing e q u a t i o n : CPUE = w h e r e p is a cons t an t def in ing propor t ional i ty (p qB* = 1), h y p e r d e p l e t i o n (p >1) o r hypers tabi l i ty 2 6 8 0 20 40 60 80 100 120 140 K Figure 5.8. The uncertainty around the MSY value is roughly bounded by MSY isolines of less than 2 (thousand tonnes). Pairs of estimated r and K values forthe Schaefer model are plotted for runs using the Bo=K assumption (crosses) and for runs not using this assumption (circles). Inset shows the complete range of estimated parameter values. 269 1.00 - i -0.90 -0.80 4-0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 Figure 5.9 Scatter plot of r and q values obtained from fits with (crosses) and without (circles) the Bo=K assumption. The straight lines are lines of equal fopt values. 2 7 0 Figure 5.10. Uncertainty in the CPUE-Biomass relationship. Isolines of MSY* = 1.5 thousand tonnes, for different values of the parameter beta representing proportionality (1), hyperdepletion (1.5) and hyperstability (0.5). Numbers in each isoline are (3 values. Key to symbols as in figure 5.8. 271 (P <1). B y c a l c u l a t i n g the r a n d K v a l u e s that result in a MSY i so l ine o f v a l u e 1.5 t h o u s a n d t o n n e s u s i n g v a l u e s o f p o f 1, 0 .5 , a n d 1.5, it is s h o w n that the Y u c a t a n d a t a c o u l d fit a n y of t h e se l ines . F r o m this , it is ev iden t that the da ta c o u l d be b o u n d e d by a n infinite n u m b e r of MSY i s o l i ne s o f v a r i o u s v a l u e s , d e p e n d i n g o n w h a t is the true v a l u e o f p. 5.5.3 H o w c a n w e i m p r o v e future a s s e s s m e n t s o f the Y u c a t a n s h a r k f i she ry? M a n a g e m e n t a d v i c e for this s y s t e m h a s to be careful ly g e a r e d t o w a r d s i m p r o v i n g the a s s e s s m e n t o f the f i shery , i.e. w e n e e d to e n s u r e a bet ter cont ras t in the d a t a a n d w e n e e d to k n o w if C P U E is p ropor t iona l to a b u n d a n c e . In o ther w o r d s , it is n e c e s s a r y to expl ic i t ly d e s i g n e x p e r i m e n t a l m a n a g e m e n t a c t i o n s that w o u l d p rov ide us with i) c l e a r l y con t ras t ing effort l eve l s , a n d ii) a n a b u n d a n c e i n d e x that is truly p ropor t iona l to s tock s i z e . Unfor tunate ly , this imp l i e s t ak ing v e r y s e r i o u s d e c i s i o n s a b o u t f i sh ing pat terns for the o n c o m i n g y e a r s . T h e fo rmal a n d t h o r o u g h w a y to p r o c e e d for m a k i n g s u c h d e c i s i o n s is th rough the a p p l i c a t i o n of a d a p t i v e po l i cy a n a l y s i s (Wal te r s 1986) . H o w e v e r , d i s c u s s i n g adap t ive m a n a g e m e n t d e s i g n is a c o m p l e x top ic ou t s ide the s c o p e o f the p resen t s tudy. It s h o u l d suffice to s a y here that a p rope r s tat is t ical eva lua t i on o f a l l the l ike ly h y p o t h e s e s a n d a l l p o s s i b l e m a n a g e m e n t d e c i s i o n s s h o u l d be m a d e before a n y d e c i s i o n is t a k e n o v e r w h e t h e r o r not to set e x p e r i m e n t a l m a n a g e m e n t ac t ions , a n d before d e c i d i n g w h i c h adap t ive po l i c i e s , if any , s h o u l d be t r ied. M y a p p r o a c h here is thus to s i m p l y ou t l ine in ve ry b r o a d t e rms w h i c h a r e the t ypes of ac t i ons that w o u l d p rov ide the S c h a e f e r m o d e l wi th the informat ion it n e e d s for the p rope r a s s e s s m e n t o f the Y u c a t a n f i shery . T h e s i m p l e co l l e c t i on of a n o t h e r ten y e a r s of C P U E a n d c a t c h informat ion wi thout a n y p l a n n i n g o n the qual i ty o f t he se da t a wi l l o n l y p r o d u c e a 2 0 y r un informat ive t ime se r i e s . W h a t is n e e d e d a re f i sh ing pat terns that wi l l p rov ide e n o u g h informat ion a b o u t the s i z e of the s tock (K), the ca tchab i l i ty of the f i sh ing s y s t e m (qr), a n d the product iv i ty o f the s tock (r). T h e tr ials p r e s e n t e d a b o v e d e m o n s t r a t e d par t icu lar p r o b l e m s to e s t ima te the v a l u e of r. T h i s fact, a n d the i m p o r t a n c e o f r for m a n a g e m e n t p a r a m e t e r e s t ima t ion (MSY a n d f o p f) m a k e it natural to th ink first a b o u t ob t a in ing informat ion o n r. N e v e r t h e l e s s , to g a i n informat ion abou t r it w o u l d be d e s i r a b l e to first dr ive the s tock d o w n c o n s i d e r a b l y a n d then let it r ecove r . A p a r t f rom l e a d i n g us to the c o n d i t i o n s w e n e e d for ob t a in ing informat ion a b o u t r, this w o u l d p rov ide us with v a l u a b l e informat ion abou t K-q first. T h e r e c o m m e n d e d r ec ipe w o u l d 272 thus be, to fish harder for a few years in order to bring down the abundance in a relatively short time and thus provide the model with information about K and q (Hilborn, 1979). This should be followed by a significant reduction in effort for a number of years that allows the stock to rebuild. This phase of stock recovery would provide the model with very valuable information about the intrinsic rate of growth r. A test fishery would probably be the best way to monitor the recovery of the stock. The final question that needs to be addressed is whether the commercial CPUE data from this fishery are a good index of stock abundance. I have shown in the previous chapter that even the best fitting procedures for surplus production models fail to provide unbiased estimates whenever hyperdepletion or hyperstability are present. Under the current circumstances in the Yucatan fishery, the first step should be to establish a system for the collection of spatial effort and CPUE information. This should be followed by the mapping of this information in order to identify trends and patterns in the allocation of effort and its relation to shark distribution. This can in turn provide some clues about the relationship between abundance and CPUE. If the above analysis indicates a likelihood of hyperdepletion or hyperstability in this fishery, then it would be necessary to develop a careful and detailed fisheries data gathering system. One approach is to carry fishery-independent surveys of shark abundance; however the operational cost of this alternative would likely be prohibitive. Another, perhaps more feasible option, is to employ a good number of fishermen as sampling devices in an exercise on experimental fishing. Implementing this alternative will certainly require compensation to participating fishermen for their opportunity costs, but these would likely be much smaller than the costs of carrying fishery independent surveys and would offer an advantageous interaction between researchers, managers and fishermen which will prove to be essential forthe success of future management actions and policy making. In either case, the spatial distribution of the fishing effort has to be such that fishing takes place evenly throughout the entire range of distribution of the sharks and not only following the concentrations of sharks, which is probably what is happening in the fishery now, as this is the most common behaviour among fishermen that know their business. 273 5.6 Summary and conclusions. The limitations in the data encountered during the present study of the Yucatan shark fishery are likely to be found whenever stock assessment and management of elasmobranch fisheries is attempted in most tropical countries. In the great majority of the cases there will be no detailed fishery statistics by species and no biological information. The high cost of obtaining species-specific catch data and the complications of discriminating effort for a species in a fishery that is multispecific, are likely to constrain any attempts to perform the more sophisticated single-species analyses in tropical elasmobranch fisheries. I have shown that it is possible to obtain some CPUE information by direct search among the local fishing companies and use this for surplus production analyses that can provide a great deal of insight about the problems for assessing the fishery. This should also be a feasible way to get started in any other country with a significant elasmobranch fishery. The usage of aggregated-species surplus production models illustrated here should only be a first and rough step towards the assessment of elasmobranch fisheries. Their major benefit is perhaps to emphasise the pitfalls of the available information and this might help to set management strategies that allow for better information. For some cases with better data conditions, where fishery independent abundance information or better contrasted CPUE data is available, the kind of analyses illustrated here might prove to yield satisfactory results. For difficult cases, the fitting strategies used here (B0=K constraint; freezing one parameter at a time) might help in the analysis of the data. If such investigations stress problems of poor data contrast, then a formal analysis of adaptive management strategies should be carried out. In such a case, it should be kept in mind that the level of participation of fishermen into any organised data gathering process is going to be constrained by many cultural, social and economic factors not considered here. Further research including a case study of an actual attempt to implement the kind of policies suggested here for improving the data for assessment, would be a very important step forward to evaluate the validity and feasibility of such policies. 2 7 4 R e f e r e n c e s . A a s e n , O . 1 9 6 3 . N o r s k e p i g g h a m e r k n i n g e r 1 9 6 2 . F i s k e t s G a n g , Arg. 4 9 : 4 5 5 - 6 0 . A a s e n , 0 . 1 9 6 3 . L e n g t h a n d growth of the p o r b e a g l e (Lamna nasus, B o n a t e r r e ) in the Nor th W e s t A t l an t i c . F i s k D i r . S k r . S e r i e H a v u n d e r s o k e l s e r 13(6) :20-37 . A a s e n , O . 1 9 6 4 . T h e explo i ta t ion o f the s p i n y dogf i sh (Squalus acanthias L . ) in E u r o p e a n wa te r s . F i s k D i r . S k r . S e r i e H a v u n d e r s o k e l s e r 13 :5 -16 . A l v a r e z , J . H . 1 9 8 8 . A n a l i s i s de la p e s q u e r i a de c a z o n de la p e n i n s u l a de Y u c a t a n . C e n t r a de Inves t igac ion y de E s t u d i o s A v a n z a d o s de l Instituto P o l i t e c n i c o N a c i o n a l U n i d a d M e r i d a , M e x i c o . 135 pp . ( M . S . thes i s ) A m o r i m , A . F . a n d C A . Ar fe l l i . 1 9 8 7 . E s t u d o b i o l o g i c o - p e s q u e i r o d o c a c a o - a z u l , Prionace glauca, no s u d e s t e e s u l d o B r a s i l ( 1985 -1986) . Ill R e u n i a o d o G r u p o de T r a b a l h o S o b r e P e s c a e P e s q u i s a de T u b a r o e s e R a i a s no B r a s i l , F o r t a l e z a , B r a s i l . 28 J u l y -31 Ju ly , 1 9 8 7 . (Abstract ) A n d e r s o n , E . D . 1 9 9 0 . E s t i m a t e s o f large s h a r k c a t c h e s in the W e s t e r n A t l a n t i c a n d G u l f of M e x i c o , 1 9 6 0 - 1 9 8 6 . N O A A T e c h n i c a l R e p o r t N M F S 9 0 . pp . 4 4 3 - 4 5 4 . A n d e r s o n , E . D . a n d K . T e s h i m a . 1 9 9 0 . W o r k s h o p o n f i sher ies m a n a g e m e n t . In: H . L . Pratt Jr . , S . H . G r u b e r , a n d T. T a n i u c h i , (eds.) E l a s m o b r a n c h s a s l iv ing r e s o u r c e s : a d v a n c e s in the b io logy , e c o l o g y , s y s t em a t i c s , a n d the s ta tus o f f i she r i e s . N O A A T e c h . R e p . N M F S 9 0 . A n n a l a , J . H . (comp. ) . 1 9 9 3 . R e p o r t from the F i s h e r y A s s e s s m e n t P l e n a r y , M a y 1 9 9 3 : s tock a s s e s s m e n t s a n d y i e l d e s t ima tes . 241 p. ( U n p u b l i s h e d report h e l d in M A F F i s h e r i e s G r e t a P o i n t l ibrary, Wel l ing ton . ) A n o n y m o u s . 1 9 8 3 . N e w f i sh ing g r o u n d loca t ed off A z h i k o d e . S e a f o o d Expo r t J . ; v o l . 15 , no. 6, p .8 ; 1 9 8 3 . A n o n y m o u s . 1 9 8 6 . T h e t ypes o f f i sh caught , f i sh ing g r o u n d s , a n d net c l a s s i f i ca t i on . Y a m a h a F i s h e r y J o u r n a l 1986 . pp . 194 . A n o n y m o u s . 1 9 8 8 . R e p o r t o f the S e c r e t a r y o f C o m m e r c e to the C o n g r e s s o f the U n i t e d S t a t e s o n the nature , extent , a n d effects of driftnet f i sh ing in w a t e r s o f the Nor th P a c i f i c O c e a n , P u r s u a n t to sec t ion 4 0 0 5 of P u b l i c L a w 1 0 0 - 2 2 0 , the "driftnet impac t mon i to r ing , a s s e s s m e n t a n d con t ro l ac t of 1987" . 8 7 p . A n o n y m o u s . 1 9 8 9 . S ta tus report o n sou the rn sha rk f ishery . A u s t r a l i a n F i s h e r i e s . 48(7) , 14-19. A n o n y m o u s . 1 9 9 2 a . S h a r k b a n in the wes t . A u s t r a l i a n F i s h e r i e s . 51(12) :6 . A n o n y m o u s . 1 9 9 2 b . R e p o r t for p resen ta t ion to the uni ted na t ions pu r suan t to reso lu t ions 4 4 / 2 2 5 a n d 4 5 / 1 9 7 . Sc ien t i f i c r e v i e w o f Nor th P a c i f i c h igh s e a s driftnet f i sher ies , S i d n e y B . C . , J u n e 11-14 , 1 9 9 1 . 2 7 5 A p p u k i t t a n K . K . a n d K . P . Na i r . 1 9 8 8 . S h a r k r e s o u r c e s o f India, wi th no t e s o n b i o l o g y o f a few s p e c i e s . M a n g a l o r e , K a r n a t a k a . ; pp. 1 7 3 - 1 8 3 . Arfe l l i , C . A . , A . F . A m o r i m a n d E . S . R o d r i g u e s . 1 9 8 7 . E s t u d o b i o l o g i c o - p e s q u e i r o do a n e q u i m , Isurus oxyrinchus, no s u d e s t e e s u l d o B r a s i l ( 1 9 7 1 - 1 9 8 5 ) . Ill R e u n i a o d o G r u p o de T r a b a l h o S o b r e P e s c a e P e s q u i s a de T u b a r o e s e R a i a s no B r a s i l , F o r t a l e z a , B r a s i l . 2 8 J u l y - 31 Ju ly , 1 9 8 7 . (Abstract ) A u , D . W . K . 1 9 8 5 . S p e c i e s c o m p o s i t i o n in the J a p a n e s e long l ine f i shery off the sou the rn a n d ea s t e rn U n i t e d S t a t e s . C o l l . V o l . S c i . P a p . I C C A T 2 3 ( 2 ) : 3 7 6 - 3 8 5 . A u , D . W . K . 1 9 9 1 . P o l y s p e c i f i c nature o f t u n a s c h o o l s : shark , d o l p h i n , a n d s e a b i r d a s s o c i a t e s . F i s h . B u l l . U . S . 8 9 : 3 4 3 - 3 5 4 . B a n e , G . W . 1 9 6 6 . O b s e r v a t i o n s o n the s i lky shark , Carcharhinus falciformis, in the G u l f o f G u i n e a . C o p e i a , 1966 (2) :354-356 . Ba t i s t a , V . S . 1 9 8 8 . D e t e r m i n a c a o d a Idade e A n a l i s e de C r e s c i m e n t o d o C a c a o Mustelus schmitti'Springer 1 9 3 9 ( E l a s m o b r a n c h i i , T r i a k i d a e ) . d a P l a t a f o r m a C o n t i n e n t a l d o R i o G r a n d e d o S u l . M . S c . T h e s i s . U n i v . do R i o G r a n d e , B r a z i l . 9 9 pp . B e d d i n g t o n , J . 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( D o c u m e n t submi t t ed to the A n n u a l M e e t i n g o f the Internat ional Nor th P a c i f i c F i s h e r i e s C o m m i s s i o n , A n c h o r a g e , A l a s k a , N o v e m b e r 1986) D e p a r t m e n t of F i s h e r i e s a n d O c e a n s , F i s h e r i e s R e s e a r c h B r a n c h , Pac i f i c B i o l o g i c a l S ta t ion , N a n a i m o , B . C . , C a n a d a V 9 R 5 K 6 . B e v e r t o n , R . J . H . a n d S . J . Hol t . 1 9 5 7 . O n the d y n a m i c s o f e x p l o i t e d f ish popu la t i ons . F i s h . Invest. Min i s t . A g r i c . F i s h . F o o d G . B . (2 S e a F i sh . ) 19 :533 p. Bonf i l , R . S . 1 9 9 0 . Con t r ibu t ion to the f i sher ies b i o l o g y o f the s i lky s h a r k Carcharhinus falciformis ( B i b r o n 1839) from Y u c a t a n , M e x i c o . Un ive r s i t y C o l l e g e o f Nor th W a l e s , B a n g o r . M . S c . T h e s i s . 7 7 pp . Bonf i l , R . 1 9 9 4 . O v e r v i e w of W o r l d E l a s m o b r a n c h F i s h e r i e s . F A O F i s h . T e c h . P a p . 3 4 1 . F A O , R o m e . 119 p. 2 7 6 Bonf i l , R . (in press). E s t a d o de l c o n o c i m i e n t o de los r e c u r s o s de t i b u r o n e s de l G o l f o de M e x i c o y C a r i b e . in : A n a l i s i s y d i a g n o s t i c o de los r e c u r s o s p e s q u e r o s c r i t i cos de l G o l f o d e M e x i c o , (eds . D . F l o r e s - H e r n a n d e z , P . S a n c h e z - G i l , J . C . Se i jo a n d F . A r r e g u i n - S a n c h e z , E d s . ) E P O M E X S e r i e C i e n t i f i c a N o . 4 . Bonf i l , R . S . , D . D e A n d a , a n d R . M e n a . 1 9 9 0 . S h a r k F i s h e r i e s in M e x i c o : T h e c a s e of Y u c a t a n a s a n e x a m p l e , pp . 4 2 7 - 4 4 1 . In: H . L . Prat t Jr . , S . H . G r u b e r , a n d T. T a n i u c h i , (eds.) 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A g e , growth a n d reproduc t ive b io logy o f the s i lky shark , Carcharhinus falciformis, a n d the s c a l l o p e d h a m m e r h e a d , Shyrna lewini, f rom the nor thwes te rn G u l f o f M e x i c o . E n v i r o n . B i o l . F i s h . 19 (3): 1 6 1 - 1 7 3 . Branste t ter , S . 1 9 9 0 . E a r l y l ife-history impl i ca t ions o f s e l e c t e d c a r c h a r h i n o i d a n d l a m n o i d s h a r k s o f the N o r t h w e s t At l an t i c . In: H . L . Pratt Jr . , S . H . G r u b e r , a n d T. T a n i u c h i , (eds.) E l a s m o b r a n c h s a s l iv ing r e s o u r c e s : a d v a n c e s in the b io logy , e c o l o g y , s y s t e m a t i c s , a n d the s ta tus o f f i sher ies . N O A A T e c h . R e p . N M F S 9 0 : 17-28 . B r o w n , B . , J . B r e n n a n , M . G r a s s l e i n , E . H e y e r d h a l , a n d R . H e n n e m u t h . 1 9 7 6 . T h e effect of f i sh ing o n the m a r i n e fish b i o m a s s in the N o r t h w e s t A t l a n t i c f rom the G u l f of M a i n e to C a p e Ha t te ras . I C N A F R e s . B u l l . 12: 4 9 - 6 8 . But terwor th , D . S . , a n d P . A . A n d r e w . 1984 . D y n a m i c catch-effort m o d e l s for the h a k e s tocks in I C S E A F D i v i s i o n s 1.3 to 2 .2 C o l i n . S c i e n t . P a p . int. C o m m . S E . A t l . F i s h . 11 (1):29-5 8 . C a d d y , J . F . 1 9 9 1 . D e a t h rates a n d t ime in tervals : is there a n a l te rna t ive to the cons tan t na tura l mortal i ty a x i o m ? R e v i e w s in F i s h B i o l o g y a n d F i s h e r i e s , 1: 1 0 9 - 1 3 8 . Ca i l l i e t , G . M . , D . B . Ho l t s , a n d D . B e d f o r d , 1 9 9 3 . A r ev i ew of the c o m m e r c i a l f i sher ies for s h a r k s o n the W e s t C o a s t of the U n i t e d S ta t e s , p p . 1 3 - 2 9 in : S h a r k C o n s e r v a t i o n . P r o c e e d i n g s of a n Internat ional W o r k s h o p o n the C o n s e r v a t i o n o f E l a s m o b r a n c h s he ld at T a r o n g a Z o o , S y d n e y , A u s t r a l i a , 24 F e b r u a r y 1 9 9 1 . ( E d i t e d by J u l i a n P e p p e r e l l , J o h n W e s t a n d P e t e r W o o n ) . Z o o l o g i c a l P a r k s B o a r d o f N S W . C a m p b e l l , D . , T . Ba t t ag l ene , a n d S . P a s c o e . 1 9 9 1 . M a n a g e m e n t o p t i o n s f o r t h e sou the rn s h a r k f ishery . A n e c o n o m i c a n a l y s i s . A u s t r a l i a n B u r e a u o f A g r i c u l t u r a l a n d R e s o u r c e E c o n o m i c s . A u s t r a l i a n G o v e r n m e n t P u b l i s h i n g S e r v i c e , C a n b e r r a . 4 3 pp . 2 7 7 C a s t i l l o J . L . G . 1 9 9 0 . S h a r k f i sher ies a n d r e s e a r c h in M e x i c o - a r ev i ew . C h o n d r o s 2(1) :1-2. C l a r k , M . R . a n d K . J . K i n g . 1 9 8 9 . D e e p w a t e r fish r e s o u r c e s off the Nor th Is land, N e w Z e a l a n d : resul ts o f a t rawl su rvey , M a y 1 9 8 5 to J u n e 1 9 8 6 . N e w Z e a l a n d F i s h e r i e s T e c h n i c a l R e p o r t N o . 11 : 56 pp . Coffey , B T . a n d R . V . G r a c e . 1 9 9 0 . A p re l imina ry a s s e s s m e n t o f the i m p a c t s o f driftnet f i sh ing o n o c e a n i c o r g a n i s m s : T a s m a n S e a , S o u t h P a c i f i c , J a n u a r y 1 9 9 0 . B r i a n T. Cof fey a n d A s s o c i a t e s L i m i t e d : E . I . A . / S . D . F . ( G . P . 0 1 / 1 9 9 0 ) . 19p . ( M S ) C o m p a g n o , L . J . V . 1 9 7 7 . P h y l e t i c r e l a t ionsh ips o f l iv ing s h a r k s a n d r ays . A m . Z o o l . , 17(2): 3 0 2 - 2 2 . C o m p a g n o , L . J . V . 1 9 8 4 . F A O s p e c i e s c a t a l o g u e . V o l . 4 . S h a r k s o f the w o r l d . A n anno t a t ed a n d i l lus t ra ted c a t a l o g u e of sha rk s p e c i e s k n o w n to da te . P a r t s 1 & 2 . F A O F i s h . S y n o p . (125) V o l . 4 :1 -655 . C o m p a g n o , L . J . V . 1 9 9 0 . S h a r k explo i ta t ion a n d c o n s e r v a t i o n , pp . 3 9 1 - 4 1 4 . In: H . L . Pratt Jr . , S . H . G r u b e r , a n d T. T a n i u c h i , (eds.) E l a s m o b r a n c h s a s l iv ing r e s o u r c e s : a d v a n c e s in the b io logy , e c o l o g y , s y s t ema t i c s , a n d the s ta tus o f f i sher ies . N O A A T e c h . R e p . N M F S 90 . C o o k , S . F . 1 9 9 0 . T r e n d s in sha rk fin marke t s : 1980 , 1 9 9 0 a n d b e y o n d . C h o n d r o s . V o l . 2(1) :3-6. C r e s p o , E . A . , a n d J . F . C o r c u e r a . 1 9 9 0 . Interact ions b e t w e e n m a r i n e m a m m a l s a n d f i sher ies in s o m e a r e a s o f the c o a s t o f A r g e n t i n a a n d U r u g u a y . I W C D o c u m e n t n u m b e r S C / 0 9 0 / G 2 : 31 pp . C r u m m y , C , M . R o n a n a n d E . F a h y . 1 9 9 1 . 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D a t a r e c o r d o f f i shes a n d s q u i d s c a u g h t inc iden ta l ly in g i l lnets of J a p a n e s e s a l m o n r e s e a r c h v e s s e l s , 1 9 8 6 - 1 9 8 8 . ( D o c u m e n t submi t t ed to the Internat ional Nor th P a c i f i c F i s h e r y C o m m i s s i o n ) . ( M S ) . 2 7 9 F A J ( F i s h e r i e s A g e n c y o f J a p a n ) . 1 9 8 3 . S u r v e y o n the feasibi l i ty o f u s i n g sur face gi l lnets for pomfret a n d driftnets for f lying s q u i d r e s o u r c e s in the Nor th P a c i f i c O c e a n in 1 9 8 2 . ( D o c u m e n t submi t t ed to the Internat ional Nor th P a c i f i c F i s h e r y C o m m i s s i o n ) F A J ( F i s h e r i e s A g e n c y o f J a p a n ) . 1 9 8 9 . S u m m a r y o f o b s e r v a t i o n s for J a p a n e s e s q u i d driftnet f i shery in the north P a c i f i c in 1 9 8 8 . ( D o c u m e n t submi t t ed to the Internat ional Nor th P a c i f i c F i s h e r y C o m m i s s i o n ) . ( M S ) . 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F i s h e r i e s for R a y (Bato ide i ) in W e s t e r n s ta t is t ical a r e a v i i a , inves t iga ted th rough the c o m m e r c i a l c a t c h e s . Irish F i s h e r i e s Inves t iga t ions . S e r i e s B (Mar ine ) N o . 34 , 1 9 8 9 . 14 pp . F a h y , E . 1 9 8 9 c . T h e v a l u e of a l te ra t ions in fecundi ty to a n e x p l o i t e d s h a r k popu la t ion , i l lus t ra ted by Squalus acanthias ( L ) . I C E S . C M 1 9 8 9 / M i n i . N o . 2 . F a h y , E . 1 9 9 1 . T h e sou th e a s i e r ray Raja s p p . f ishery, wi th o b s e r v a t i o n s o n the growth of rays in Irish wa t e r s a n d their c o m m e r c i a l g r a d i n g . Irish F i s h e r i e s Invest igat ions . S e r i e s B (Mar ine ) N o . 37 , 1 9 9 1 . 18 pp . F a h y , E . a n d P . G l e e s o n . 1 9 9 0 . T h e pos t -peak -y i e ld gil lnet f i shery for s p u r d o g Squalus acanthias L . in W e s t e r n Ireland. Irish F i s h e r i e s Inves t iga t ions . 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F i n a l report o f s q u i d a n d b y c a t c h o b s e r v a t i o n s in the J a p a n e s e driftnet f i shery for n e o n f lying s q u i d (Ommastrephes bartami) J u n e - D e c e m b e r , 1 9 8 9 . O b s e r v e r P r o g r a m . Jo in t report of F i s h e r i e s A g e n c y o f J a p a n , C a n a d i a n D e p a r t m e n t s o f F i s h e r i e s a n d O c e a n s , U n i t e d S t a t e s N a t i o n a l M a r i n e F i s h e r i e s S e r v i c e , U n i t e d S t a t e s F i s h a n d Wild l i fe S e r v i c e . 114 p. G M F M C (Gul f o f M e x i c o F i s h e r i e s M a n a g e m e n t C o u n c i l ) . 1 9 8 0 . F i s h e r y m a n a g e m e n t p lan f o r t h e s h a r k a n d o ther e l a s m o b r a n c h f i shery of the G u l f o f M e x i c o . G M F M C . T a m p a , F l o r i d a . G o c k s , W . E . G . 1 9 8 7 . Indus t r i a l i zacao To ta l D o s T u b a r o e s e C a c o e s . 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T h e R e p o r t o f the D a t a P repa ra to ry M e e t i n g for S o u t h W e s t A t l a n t i c T u n a a n d T u n a - l i k e F i s h e r i e s . R e c i f e , P e r n a m b u c o , B r a z i l , J u l y 1 - 7 , 1 9 9 2 . pp . 1-19 In: C o l l e c t i v e V o l u m e of Sc ien t i f i c P a p e r s 40(2) . I C C A T , M a d r i d , S p a i n . I N P F C (Internat ional Nor th P a c i f i c F i s h e r i e s C o m m i s s i o n ) . 1 9 9 0 . F i n a l report o f s q u i d a n d b y c a t c h o b s e r v a t i o n s in the J a p a n e s e driftnet f i shery for n e o n f lying s q u i d {Ommastreph.es bartrami). J u n e - D e c , 1 9 8 9 . O b s e r v e r p r o g r a m m e joint report by the N a t i o n a l S e c t i o n s o f C . J . U . I N P F C (Internat ional Nor th P a c i f i c F i s h e r i e s C o m m i s s i o n ) . 1 9 9 1 . 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C a m p a n a c o m e r c i a l de p r o s p e c c i o n de a b u n d a n c i a de p e z e s p a d a , Xiphias gladius L , y e s p e c i e s a s o c i a d a s , e n a r e a s p r o x i m a s a G r a n d B a n k s . I C C A T C o l l . V o l . S c i . P a p s . 2 7 : 1 5 5 - 1 6 3 . Meju to , J . , B . G a r c i a a n d J . M . de la S e r n a . 1 9 9 3 . Ac t iv i ty o f the Spanish su r face long l ine fleet ta rge t ing swordf i sh (Xiphias gladius L.) in the At l an t i c , y e a r s 1988 to 1 9 9 1 , c o m b i n e d . I C C A T C o l l . V o l . S c i . P a p s . 40 (1 ) :393 -399 . M e n a s v e t a , D . , S . S h i n o , a n d S . C h u l l a s o r n . 1 9 7 3 . P e l a g i c f i shery r e s o u r c e s o f the S o u t h C h i n a S e a a n d p r o s p e c t s for thei r d e v e l o p m e n t . U N D P / F A O . S C S / D E V / 7 3 / 6 . 6 8 p p . M e n n i , R . C . , M B . C o u s s e a u , a n d A . R . G o s z t o n y i . 1986 . S o b r e la b i o l o g i a de los t iburones c o s t e r o s de la p r o v i n c i a de B u e n o s A i r e s . A n a l e s de la S o c i e d a d C i e n t i f i c a A r g e n t i n a 2 1 3 : 3 - 2 7 . Mi l l i ng ton , P . J . 1 9 8 1 . T h e T a i w a n e s e gil lnet f i shery in the A u s t r a l i a n F i s h i n g Z o n e : A p re l imina ry a n a l y s i s o f the first yea r ' s ope ra t i on , pp . 1 1 9 - 1 4 4 in: G r a n t , C . J . a n d D . G . W a l t e r (eds.) "Nor thern P e l a g i c F i s h S e m i n a r ( D a r w i n 1981)" . D e p a r t m e n t o f P r i m a r y Industry, A u s t r a l i a n G o v e r n m e n t P u b l i s h i n g S e r v i c e . C a n b e r r a . M i m u r a , K et a l . 1 9 6 3 . S y n o p s i s o f b i o l o g i c a l da ta o n ye l lowf in tuna , Neothunnus macropterus T e m m i n c h a n d S c h l e g e l , 1 8 4 2 . S p e c i e s s y n o p s i s N o . 10, F A O F i s h . R e p 6 (2 ) :319 -349 . M o u l t o n , P . L . , T.I . W a l k e r , a n d S . R . S a d d l i e r . 1 9 9 2 . A g e a n d growth o f g u m m y shark , Mustelus antarcticus G i i n t e r , a n d s c h o o l shark , Galeorhinus galeus ( L i n n e a u s ) , from sou the rn A u s t r a l i a n wa te r s , pp . 879 -911 in: " A g e de t e rmina t ion a n d growth in fish a n d o the r a q u a t i c an ima l s " , (ed . D . C . Smi th ) . A u s t . J . o f M a r . a n d F r e s h w a t e r R e s 43(5) : M u n o z - C h a p u l i , R . 1 9 8 5 a . A n a l i s i s de las c ap tu r a s de e s c u a l o s d e m e r s a l e s e n e l A t l an t i co N E ( 2 7 ° N - 3 7 ° N ) y m a r d e A l b o r a n ( M e d i t e r r a n e o O c c i d e n t a l ) . Inves t igac ion P e s q u e r a 49 (1 ) :121 -136 . 2 8 7 M u n o z - C h a p u l i , R . 1 9 8 5 b . A n a l i s i s de las c a p t u r a s de e s c u a l o s p e l a g i c o s e n e l A t l an t i co noror ien ta l ( 1 5 ° - 4 0 ° N ) . Inves t igac ion P e s q u e r a 49 (1 ) :67 -79 . M u r a t a , M . 1 9 8 6 . R e p o r t o n f i sh ing s u r v e y o n f lying s q u i d by the drift gi l lnet ters S h o y o M a r u , K u r o m o r i M a r u N o 38 a n d K a n k i M a r u N o . 58 in the Nor th P a c i f i c in 1 9 8 5 . F i s h e r i e s A g e n c y o f J a p a n . I N P F C . M u r a t a , M . 1 9 8 7 . R e p o r t o n f i sh ing s u r v e y o n f lying s q u i d by the drift gi l lnet ters S h o y o M a r u , K u r o m o r i M a r u N o 38 a n d K a n k i M a r u N o . 58 in the Nor th P a c i f i c in 1986 . ( D o c u m e n t submi t t ed to the A n n u a l M e e t i n g o f the Internat ional Nor th P a c i f i c F i s h e r i e s C o m m i s s i o n , V a n c o u v e r , C a n a d a , 1987 , O c t o b e r ) . 2 0 p . F i s h e r i e s A g e n c y of J a p a n , T o k y o , J a p a n 100 . M u r a t a , M . , Y . N a k a m u r a a n d H . S a i t o . 1 9 8 9 . R e p o r t o n f i sh ing s u r v e y o n f lying s q u i d by S h o y o M a r u , K a n k i M a r u N o . 3 a n d H o v o - M a r u N o . 78 in the Nor th P a c i f i c in 1988 . ( D o c u m e n t submi t t ed to the A n n u a l M e e t i n g o f the Internat ional Nor th Pac i f i c F i s h e r i e s C o m m i s s i o n , Sea t t l e , W a s h i n g t o n , 1 9 8 9 O c t o b e r ) . 2 5 p. F i s h e r i e s A g e n c y of J a p a n , H o k k a i d o R e g i o n a l F i s h e r i e s R e s e a r c h L a b o r a t o r y , 116 K a t s u r a k o i , K u s h i r o , J a p a n 0 8 5 . M u r a t a , M . a n d C . S h i n g u . 1 9 8 5 . R e p o r t o n f i sh ing s u r v e y o n f lying s q u i d by drift gi l lnetters O u m i - M a r u a n d K a n k i - M a r u N o . 58 in the Nor th P a c i f i c in 1 9 8 4 . F i s h e r i e s A g e n c y of J a p a n . I N P F C . M u r r a y , T . 1 9 9 0 . R e v i e w of r e s e a r c h a n d o f recent d e v e l o p m e n t s in S o u t h P a c i f i c a l b a c o r e f i sher ies , wi th e m p h a s i s o n l a rge - sca l e p e l a g i c driftnet f i sh ing . T h i r d S o u t h Pac i f i c A l b a c o r e R e s e a r c h W o r k s h o p . Inf. P a p . N o . 2 . S P C . N o u m e a , N e w C a l e d o n i a 9-12 Oc t . 1 9 9 0 . 2 7 pp . M y e r s , K . W . , C . K . Har r i s , Y . Ishida, L . M a r g o l i s a n d M . O g u r a . 1 9 9 3 . R e v i e w of the J a p a n e s e l a n d b a s e d driftnet s a l m o n f ishery in the W e s t e r n Nor th P a c i f i c O c e a n a n d the con t inen t o f or ig in of s a l m o n i d s in this a r e a . Internat ional Nor th P a c i f i c F i s h e r i e s C o m m i s s i o n . Bul le t in no . 5 2 . 86 pp . M y k l e v o l l , S . 1 9 8 9 a . N o r w a y ' s p o r b e a g l e f i shery . W o r k i n g d o c u m e n t p r e s e n t e d at the I C E S S t u d y G r o u p o n E l a s m o b r a n c h F i s h e r i e s , D u b l i n I reland, 2 6 - 2 8 A p r i l 1 9 8 9 . (mimeo) . M y k l e v o l l , S . 1 9 8 9 b . N o r w a y ' s b a s k i n g s h a r k f ishery. W o r k i n g d o c u m e n t p r e s e n t e d at the I C E S S t u d y G r o u p o n E l a s m o b r a n c h F i s h e r i e s , D u b l i n I re land, 2 6 - 2 8 A p r i l 1989 . ( m i m e o ) . M y k l e v o l l , S . 1 9 8 9 c . N o r w a y ' s g r e e n l a n d s h a r k f ishery. W o r k i n g d o c u m e n t p r e s e n t e d at the I C E S S t u d y G r o u p o n E l a s m o b r a n c h F i s h e r i e s , D u b l i n I re land, 2 6 - 2 8 A p r i l 1 9 8 9 . ( m i m e o ) . M y k l e v o l l , S . 1 9 8 9 d . N o r w a y ' s s p u r d o g f i sher ies . W o r k i n g d o c u m e n t p r e s e n t e d at the I C E S S t u d y G r o u p o n E l a s m o b r a n c h F i s h e r i e s , D u b l i n I re land, 2 6 - 2 8 A p r i l 1 9 8 9 . (mimeo) . 2 8 8 M y k l e v o l l , S . 1 9 8 9 e . N o r w a y ' s ska te a n d ray f ishery. W o r k i n g d o c u m e n t p r e s e n t e d at the I C E S S t u d y G r o u p o n E l a s m o b r a n c h F i s h e r i e s , D u b l i n I re land, 2 6 - 2 8 A p r i l 1 9 8 9 . ( m i m e o ) . N a g a o , K . , S . O t a , a n d J . H i r o n o . 1 9 9 3 . R e g u l a t i o n o f the J a p a n e s e h igh s e a s driftnet f i she r ies . Internat ional Nor th P a c i f i c F i s h e r i e s C o m m i s s i o n . Bu l l e t in no . 53(l) : 39 -44 . N a k a n o , H . 1 9 9 3 . A r ev i ew of the J a p a n e s e f i shery a n d r e s e a r c h o n s h a r k s in the At l an t i c O c e a n . I C C A T C o l l . V o l . S c i . P a p . 40 (2 ) :409 -412 . N a k a n o , H . , a n d Y . W a t a n a b e . 1 9 9 2 . Effect o f h i g h - s e a s driftnet f i she r i e s o n b lue sha rk s tock in the Nor th P a c i f i c . 15 pp . C o m p e n d i u m of d o c u m e n t s submi t t ed to the Sc ien t i f i c R e v i e w of Nor th P a c i f i c H i g h s e a s Driftnet F i s h e r i e s , S i d n e y , B . C . , C a n a d a , J u n e 11-14 , 1 9 9 1 , V o l . 1. N a k a n o , H . , M . M a k i h a r a , a n d K . S h i m a z a k i . 1 9 8 5 . Dis t r ibut ion a n d b i o l o g i c a l cha rac te r i s t i c s o f the b lue s h a r k in the cen t ra l north Pac i f i c . B u l l . F a c u l t y F i s h . , H o k k a i d o U n i v . 36(3) 9 9 - 1 1 3 . N F R D A (Na t iona l F i s h e r i e s R e s e a r c h a n d D e v e l o p m e n t A g e n c y o f K o r e a ) . 1 9 8 8 . A n n u a l report o f c a t c h a n d effort s tat is t ics a n d f i sh ing g r o u n d s for the K o r e a n t u n a longl ine f i shery 1 9 8 3 - 1 9 8 5 . 6 1 0 pp . N F R D A (Na t iona l F i s h e r i e s R e s e a r c h a n d D e v e l o p m e n t A g e n c y o f K o r e a ) . 1 9 9 2 . N a t i o n a l R e p o r t o f K o r e a , pp . 4 2 7 - 4 2 9 i n : I C C A T R e p o r t for b i e n n i a l p e r i o d , 1990-1991 Par t I (1990) : 4 5 5 pp . N M F S (Na t iona l M a r i n e F i s h e r i e s S e r v i c e ) . 1 9 9 3 . N e w ru les regula te s h a r k f i sh ing in U S At lan t i c , G u l f a n d C a r i b b e a n wa te r s . N e w s Bul le t in N R 9 3 - 1 5 . N M F S S o u t h e a s t R e g i o n a l Off ice , St . P e t e r s b u r g , F L . N O A A (Na t iona l O c e a n i c a n d A t m o s p h e r i c O r g a n i z a t i o n ) . 1 9 9 1 . F i s h e r y m a n a g e m e n t p lan for s h a r k s o f the A t l an t i c O c e a n . N M F S , N O A A . U . S . D e p a r t m e n t of C o m m e r c e . O c t o b e r 2 8 , 1 9 9 1 . 147 pp . Nor th r idge , S . P . 1 9 9 1 . Driftnet f i sher ies a n d their i m p a c t s o n non- target s p e c i e s : a w o r l d w i d e r ev i ew. F A O F i s h e r i e s T e c h n i c a l P a p e r , 3 2 0 : 1 1 5 p . N R I F S F (Na t iona l R e s e a r c h Institute o f F a r S e a s F i s h e r i e s of J a p a n ) . 1 9 9 2 . N a t i o n a l R e p o r t o f J a p a n , pp . 4 2 3 - 4 2 6 i n : I C C A T R e p o r t for b i enn i a l pe r iod , 1 9 9 0 - 1 9 9 1 Par t I (1990) : 4 5 5 pp . O ' c o n n o r , P . F . 1 9 5 3 . S h a r k - O ! L o n d o n , S e e k e r a n d W a r b u r g , 2 7 2 pp . O k e r a , W . , J . D . S t e v e n s , a n d J . S . G u n n . 1 9 8 1 . F i s h e r y s i tua t ion report: t rop ica l s h a r k s . Nor the rn P e l a g i c F i s h S e m i n a r ( D a r w i n 1981) . D e p a r t m e n t o f P r i m a r y Industry. F i s h e r i e s D i v i s i o n A u s t r a l i a n G o v e r n m e n t P u b l i s h i n g S e r v i c e . C a n b e r r a . 177 pp . 2 8 9 O l s e n , A . 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T h e reproduc t ive b io logy o f the A t l a n t i c s h a r p n o s e shark , Rhizoprionodon terraenovae ( R i c h a r d s o n ) . F i s h . B u l l . 8 (1): 6 1 - 7 3 . P a u l y , D . 1 9 7 8 . A cr i t ique o f s o m e literature da ta o n the g rowth , r ep roduc t ion a n d mortali ty o f the l a m n i d sha rk Cetorhinus maximus ( G u n n e r u s ) . I C E S C M . 1978/1-1:17 10p . P a u l y , D . 1 9 7 9 . T h e o r y a n d m a n a g e m e n t o f t rop ica l m u s l t i s p e c i e s s t o c k s . A rev iew, with e m p h a s i s o n the sou theas t A s i a n d e m e r s a l f i sher ies . I C L A R M S t u d i e s a n d R e v i e w s N o . 1. M a n i l a , 3 5 pp . P a u l y , D . 1 9 8 0 . O n the in te r re la t ionsh ips b e t w e e n natura l mortal i ty, g rowth pa rame te r s , a n d m e a n e n v i r o n m e n t a l t empera tu re in 175 fish s t ocks . J . C o n s . C I E M , 39 (2 ) :175- 192 . Paus t , B . C . 1 9 8 7 . T h e d e v e l o p i n g A l a s k a s a l m o n sha rk f i shery , i n : " S h a r k s a n inquiry into b io logy , behav iou r , f i sher ies , a n d use" (S . C o o k ed.) , pp . 1 7 3 - 1 7 8 . O r e g o n Sta te Un ive r s i t y E x t e n s i o n S e r v i c e . P e l l a , J . J . , a n d P . K . T o m l i n s o n . 1 9 6 9 . A g e n e r a l i z e d s tock p roduc t ion m o d e l . B u l l . Inter-A m . T r a p . T u n a C o m m . 1 3 : 4 1 9 - 4 9 6 . P e l l a , J . , R . R u m b a u g h , L . S i m o n , a n d M . D a h l b e r g . 1 9 9 3 . Inc identa l a n d i l l ega l c a t c h e s of s a l m o n i d s in the Nor th P a c i f i c driftnet f i sher ies . I . N . P . F . C . Bu l l e t in 53 (3 ) :325-358 . P e r e s , M . B . , a n d C . M . V o o r e n . 1 9 9 1 . S e x u a l d e v e l o p m e n t , r ep roduc t ive c y c l e , a n d fecundi ty o f the s c h o o l sha rk Galeorhinus galeus off s o u t h e r n B r a z i l . F i s h e r y B u l l . U . 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D a v e n p o r t . 1 9 9 1 . A n a l y s i s o f c a t c h da t a from the T a i w a n e s e gill-net f i shery off nor thern A u s t r a l i a , 1 9 7 9 - 1 9 8 6 . R e p . C S I R O M a r . L a b . 1991 (213) :55pp . S t e v e n s o n , D . K . 1 9 8 2 . U n a r ev i s ion de los r e c u r s o s m a r i n o s de la r eg ion de la c o m i s i o n de p e s c a p a r a e l A t l an t i co cen t ro -occ iden t a l ( C o p a c o ) . F A O , D o c u m e n t o s T e c n i c o s de P e s c a , N o . 2 1 1 . 143 pp . S t r a s b u r g , D . W . 1 9 5 8 . Dis t r ibu t ion , a b u n d a n c e , a n d habi t s o f p e l a g i c s h a r k s in the cen t ra l P a c i f i c O c e a n . F i s h . B u l l . U . S . F i s h . Wi ld l i fe S e r v . 5 8 : 3 3 5 - 3 6 1 . 2 9 4 S u d a r s a n , D . , M . E . J o h n a n d A . J o s e p h . 1 9 8 8 . A n a s s e s s m e n t o f d e m e r s a l s t o c k s in the s o u t h w e s t c o a s t of India with par t icular re fe rence to the exp lo i t ab l e r e s o u r c e s in ou te r con t inen ta l she l f a n d s l o p e . C M F R I s p e c , pub l . ; no . 4 0 , pp . 1 0 2 - 1 0 3 . S U D E P E ( S u p e r i n t e n d e n c i a d o D e s e n v o l v i m e n t o d a P e s c a ) . 1 9 9 0 . A n n u a l repor ts of f i sher r ies s tat is t ics . S U D E P E , R i o G r a n d e , (not s e e n , c i t ed in P e r e s a n d V o o r e n (1991)) . S u z u k i , S . 1 9 8 8 . S t u d y o f in teract ion b e t w e e n long l ine a n d pu r se s e i n e f i she r i es in ye l lowf in t una , Thunnus albacares (Bonna te r re ) . B u l l . F a r S e a s F i s h . R e s . L a b 2 5 : 7 3 - 1 4 4 . S w a m i n a t h , M . , T . E . S i v a p r a k a s a m , P . S . J o y a n d P . P r a v e e n . 1 9 8 5 . A s tudy o f mar ine f i shery r e s o u r c e s off K r i s h n a p a t n a m . C I F N E T ; C o c h i n (India); 1 9 8 5 ; 4 3 pp . T a n i u c h i , T . 1 9 9 0 . T h e role o f e l a s m o b r a n c h s in J a p a n e s e f i she r ies . N O A A T e c h . R e p . N M F S 9 0 : 4 1 5 - 4 2 6 . T e t a r d . 1 9 8 9 a . S p e c i e s r ev i ew. R a y s . W o r k i n g d o c u m e n t p r e s e n t e d at the I C E S E l a s m o b r a n c h S t u d y G r o u p M e e t i n g , D u b l i n I reland, A p r i l 1 9 8 9 . 13 pp . (ms) Te t a rd . 1 9 8 9 b . S p e c i e s r ev i ew. S h a r k s . W o r k i n g d o c u m e n t p r e s e n t e d at the I C E S E l a s m o b r a n c h S t u d y G r o u p M e e t i n g , D u b l i n I reland, A p r i l 1 9 8 9 . 14 pp . (ms) T o m a s , A . R . G . 1 9 8 7 . C h a v e t a x o n o m i c a p a r a c a n o e s e v i s c e r a d o s d o s u d e s t e d o B r a s i l . Ill R e u n i a o d o G r u p o de T r a b a l h o S o b r e P e s c a e P e s q u i s a de T u b a r o e s e R a i a s no B r a s i l , F o r t a l e z a , B r a s i l . 28 J u l y - 31 Ju ly , 1 9 8 7 . (Abst rac t ) v a n d e r Els t , R . P . 1 9 7 9 . A proliferat ion o f s m a l l s h a r k s in the s h o r e - b a s e d N a t a l sport f i shery . E n v . B i o l . F i s h . 4 (4 ) :349-362 . V a r g h e s e , G . 1 9 7 4 . S h a r k r e s o u r c e s of the L a c c a d i v e wa te r s . S e a f o o d e x p . Jour . , 6 (1) :65-6 8 . V e l e z , R . , D . M e d i z a v a l J . J . V a l d e z a n d N . A . V e n e g a s . 1 9 8 9 . P r o s p e c c i o n y P e s c a E x p l o r a t o r i a de R e c u r s o s P e s q u e r o s e n la Z o n a E c o n o m i c a E x c l u s i v a de l O c e a n o P a c i f i c o . I .N .P . C . R . I . P . , M e x i c o . 179 pp . V in the r , M . , a n d H . Spa rho l t . 1 9 8 8 . T h e b i o m a s s o f s k a t e s in the Nor th S e a . I C E S C M 1 9 8 8 / G : 4 8 . 2 6 pp . V o o r e n , C . M . , a n d R . Bet i to . 1 9 8 7 . C a c o e s e a r r a i a s d e m e r s a l s d o rio g r a n d e do s u l c o m o r e c u r s o s r e s q u e i r o s : b i o m a s s a , d is t r ibuigao por p ro fund idade , e m i g r a c o e s . Ill R e u n i a o d o G r u p o de T r a b a l h o S o b r e P e s c a e P e s q u i s a de T u b a r o e s e R a i a s no B r a s i l , F o r t a l e z a , B r a s i l . 2 8 J u l y - 31 Ju ly , 1 9 8 7 . (Abst rac t ) V o o r e n , C . M . , M . L . G . de A r a u j o , a n d R . Bet i to . 1 9 9 0 . A n a l i s e d a es ta t i s t i ca d a p e s c a de e l a s m o b r a n q u i o s d e m e r s a i s no porto de R i o G r a n d e , de 1 9 7 3 a 1 9 8 6 . C i e n c i a e C u l t u r a 4 2 ( 1 2 ) : 1 1 0 6 - 1 1 1 4 (in Port , wi th E n g . abst . ) 2 9 5 W a l f o r d , L . A . 1 9 3 5 . T h e s h a r k s a n d rays of C a l i f o r n i a . F i s h Bu l l e t in N o . 4 5 . C a l i f o r n i a D i v i s i o n o f F i s h a n d G a m e . 6 5 pp . W a l k e r , T . I. 1 9 8 4 . Inves t iga t ions o f the g u m m y shark , Mustelus antarcticus G u n t h e r , from S o u t h e r n A u s t r a l i a n wa te r s . In " P r o c e e d i n g s o f the S h a r k A s s e s s m e n t W o r k s h o p , S o u t h E a s t F i s h e r i e s C o m m i t t e e S h a r k R e s e a r c h G r o u p ' . ( D e p a r t m e n t of P r i m a r y Industry: C a n b e r r a . ) W a l k e r , T.I . 1 9 8 8 . T h e sou the rn sha rk f ishery . In: P r o c e e d i n g s o f the w o r k s h o p o n scient i f ic a d v i c e for f i sher ies m a n a g e m e n t : get t ing the m e s s a g e a c r o s s . E d . M . W i l l i a m s . A u s t r a l i a n G o v e r n m e n t P u b l i s h i n g S e r v i c e , C a n b e r r a . 7 8 pp . W a l k e r , T.I. 1 9 9 2 . F i s h e r y s imula t ion m o d e l for s h a r k s a p p l i e d to the G u m m y shark , Mustelus antarcticus Gun the r , from sou the rn A u s t r a l i a n w a t e r s . A u s t . J . M a r . F r e s h w a t e r R e s . , 1992 , 4 3 : 1 9 5 - 2 1 2 . Wa l t e r s , C . J . 1986 . A d a p t i v e m a n a g e m e n t o f r e n e w a b l e r e s o u r c e s . M a c M i l l a n P u b l i s h i n g C o . , N e w Y o r k . 3 7 4 pp . W a r f e l , H . E . a n d J . A . C l a g u e . 1 9 5 0 . S h a r k f i sh ing potent ia l i t ies o f the P h i l i p p i n e s e a s . R e s e a r c h R e p o r t 15. F i s h a n d Wildl i fe S e r v i c e . U n i t e d S t a t e s D e p a r t m e n t of the Interior. W a t a n a b e , Y . 1 9 9 0 . R e p o r t o n drop-out o b s e r v a t i o n s in drift-net f i sh ing . T h i r d S o u t h P a c i f i c A l b a c o r e R e s e a r c h W o r k s h o p . Inf. P a p . N o . 2 . S P C . N o u m e o , N e w C a l e d o n i a 9-12 Oc t . 1 9 9 0 . 8 pp . W e t h e r a l l , J . a n d M . S e k i . 1 9 9 2 . A s s e s s i n g i m p a c t s of Nor th P a c i f i c h igh s e a s driftnet f i sher ies o n P a c i f i c pomfret a n d s h a r k s : p r o g r e s s a n d p r o b l e m s . C o m p e n d i u m of d o c u m e n t s submi t t ed to the Sc ien t i f i c R e v i e w of Nor th P a c i f i c H i g h s e a s Driftnet F i s h e r i e s , S i d n e y , B . C . , C a n a d a , J u n e 11-14 , 1 9 9 1 , V o l . 2 . W i t z e l l , W . N . 1 9 8 5 . T h e inc iden ta l cap tu re o f s h a r k s in the A t l a n t i c U n i t e d S t a t e s F i s h e r y C o n s e r v a t i o n Z o n e by the J a p a n e s e t u n a long l ine fleet. N O A A T e c h . R e p . N M F S 3 1 : 2 1 - 2 2 . W o o d , C . C , K . S . K e t c h e n , a n d R . J . B e a m i s h . 1 9 7 9 . P o p u l a t i o n d y n a m i c s o f sp iny dogf i sh (Squalus acanthias) in Br i t i sh C o l u m b i a wa te r s . J . F i s h . R e s . B o a r d C a n . 3 6 : 6 4 7 - 6 5 6 . W o o d l e y T . H . a n d M . E a r l e . 1 9 9 1 . O b s e r v a t i o n o n the F r e n c h a l b a c o r e driftnet f i shery o f the N o r t h e a s t A t l an t i c . P r e l i m i n a r y report p r e p a r e d for G r e e n p e a c e Internat ional . 9 pp. Y a m a d a , U . 1 9 8 6 . R a j i d a e . in: F i s h e s o f the E a s t C h i n a S e a a n d the Y e l l o w S e a (O. O k a m u r a , ed.) p. 3 2 - 3 6 . S e i k a i R e g . F i s h . R e s . L a b . N a g a s a k i (in J a p a n e s e ) (not s e e n , c i t ed by Ish ihara 1990) 2 9 6 Y a t s u , A . 1 9 8 9 . C r u i s e report o f f lying s q u i d s u r v e y by the Wakatori Maru in J u l y / A u g u s t , 1 9 8 9 . ( D o c u m e n t submi t t ed to the A n n u a l M e e t i n g o f the Internat ional Nor th Pac i f i c F i s h e r i e s C o m m i s s i o n , Sea t t l e , W a s h i n g t o n , 1 9 8 9 Oc tobe r . ) 2 0 p . F i s h e r i e s A g e n c y of J a p a n , F a r S e a s F i s h e r i e s R e s e a r c h Labora to ry , 5-7-1 O r i d o , S h i m i z u , S h i z u o k a , J a p a n 4 2 4 . Y a t s u , A . , K . H i r a m a t s u a n d S . H a y a s e . 1 9 9 3 . Out l ine o f the J a p a n e s e s q u i d driftnet f i shery with no tes o n the by -ca t ch . Internat ional Nor th P a c i f i c F i s h e r i e s C o m m i s s i o n . Bul le t in no . 53( l ) : 5-24. Y e h , S . Y . , a n d I .H. T u n g . 1 9 9 3 . R e v i e w of T a i w a n e s e p e l a g i c s q u i d f i she r i es in the Nor th P a c i f i c . Internat ional Nor th P a c i f i c F i s h e r i e s C o m m i s s i o n . Bu l l e t in no . 53(l) : 7 1 - 7 6 . Y o s h i m u r a , H . a n d S . K a w a s a k i . 1 9 8 5 . S i l k y sha rk (Carcharhinus falciformis) in the t rop ica l w a t e r o f W e s t e r n P a c i f i c . R e p . J a p . G r o u p E l a s m o b r . S t u d i e s . N o . 2 0 , 1 9 8 5 . Z h o w , K . , a n d X . W a g . 1 9 9 0 . Br i e f r ev i ew of p a s s i v e f i sh ing g e a r s a n d inc iden ta l c a t c h e s o f s m a l l c e t a c e a n s in C h i n e s e w a t e r s . D o c u m e n t p r e s e n t e d to the Internat ional W h a l i n g C o m m i s s i o n W o r k s h o p o n mortali ty o f c e t a c e a n s in p a s s i v e f i sh ing nets a n d t raps . L a J o l l a , C a l i f o r n i a , O c t o b e r 1 9 9 0 . ( S C / 0 9 0 / 2 1 ) , 13 pp . 297 APPENDIX 1. List of common and latin names of the elasmobranchs mentioned in the text. Latin name Common name Latin name Common name Aetobatus narinari Spotted eagle ray Mustelus henlei Brown smooth-hound Aetomylaeus nichofii Banded eagle ray Mustelus lenticulatus Rig Alopias pelagicus Pelagic thresher shark Mustelus lunulatus Sicklefin smooth-hound Alopias superciliosus Bigeye thresher shark Mustelus manazo Starspotted smooth-hound Alopias vulpinus Common thresher shark Mustelus mustelus Smooth-hound Callorhynchus millii Elephant fish Mustelus schmitti Narrownose smooth-hound Carcharhinus acronotus Blacknose shark Myliobatis aquila Common eagle ray Carcharhinus albimarginatus Silvertip shark Nasolamia velox Whitenose shark Carcharhinus altimus Bignose shark Negaprion acutidens Sicklefin lemon shark Carcharhinus brachyurus Copper shark Negaprion brevirostris Lemon shark Carcharhinus brevipinna Spinner shark Notorynchus cepedianus Bradnose sevengill shark Carcharhinus falciformis Silky shark Odontaspis ferox Ragged-thooth shark Carcharhinus galapagensis Galapagos shark Odontaspis noronhai Bigeye sand tiger shark Carcharhinus hemiodon Pondicherry shark Prionace glauca Blue shark Carcharhinus isodon Finetooth shark Pristiophorus cirratus Longnose sawshark Carcharhinus leucas Bull shark Pristiophorus nudipinnis Shortnose sawshark Carcharhinus limbatus Blacktip shark Pseudocarcharias kamoharai Crocodile shark Carcharhinus longimanus Oceanic whitetip shark Pteromylaeus bovinus Bull ray Carcharhinus melanopterus Blacktip reef shark Raja alba White skate Carcharhinus obscurus Dusky shark Raja batis Blue skate Carcharhinus perezi Caribbean reef shark Raja binoculata Big skate Carcharhinus plumbeus Sandbar shark Raja brachyura Blonde ray Carcharhinus porosus Smalltail shark Raja clavata Thornback ray Carcharhinus signatus Night shark Raja fullonica Shagreen ray Carcharhinus sorrah Spot-tail shark Raja inornata California ray Carcharhinus tilstoni Australian blacktip shark Raja microocellata Painted ray Carcharias taurus Sandtiger shark Raja montagui Spotted ray Carcharodon carcharias Great White shark Raja naevus Butterfly skate Centrophorus granulosus Gulper shark Raja oxyrinchus Longnosed skate Centrophorus lusitanicus Lowfin gulper shark Raja radiata Thorny skate Centrophorus uyato Little gulper shark Raja rhina Longnose skate Cephaloscyllium ventriosum Swell shark Raja undulata Undulate ray Cetorhinus maximus Basking shark Rhincodon typus Whale shark Chiloscyllium indicum Slender bambooshark Rhinobatos granulatus Granulated guitarfish Dalatias licha Kitefin shark Rhinobatos horkelii Brazilian guitarfish Dasyatis brevis Whiptail stingray Rhinobatos percellens Chola guitarfish Dasyatis jenkinsii Pointed-nose stingray Rhinobatos planiceps Pacific guitarfish Dasyatis pastinaca Common stingray Rhizoprionodon acutus Milk shark 298 Latin name Common name Latin name Common name Dasyatis sephen Cowtail ray Rhizoprionodon longurio Pacific sharpnose shark Dasyatis violacea Pelagic stingray Rhizoprionodon oligolinx Grey sharpnose shark Deania calcea Birdbeak dogfish Rhizoprionodon porosus Caribbean sharpnose shark Echinorhinus cookei Brambell shark Rhizoprionodon terraenovae Atlantic sharpnose shark Euprotomicrus bispinatus Pigmy shark Rhynchobatus djiddensis whitespotted shovelnose Eusphyra blochii Winghead shark Scoliodon laticaudus Spadenose shark Furgaleus macki Whiskery shark Scoliodon sorrakowa Milk shark Galeocerdo cuvier Tiger shark Scyliorhinus canicula Lesser spotted dogfish Galeorhinus galeus Tope, school or soupfin shark Scyliorhinus stelaris Nursehound Galeus melastomus Blackmouth catshark Scyliorhinus torazame Cloudy catshark Ginglymostoma cirratum Nurse shark Somniosus microcephalus Greenland shark Heptranchias perfo Sharpnose sevengill shark Sphyrna lewini Scalloped hammerhead shark Heterodontus franciscanus Hornshark Sphyrna mokarran Great hammerhead shark Heterodontus mexicanus Mexican hornshark Sphyrna tiburo Bonnethead shark Hexanchus griseus Bluntnose sixgill shark Sphyrna zygaena Smooth hammerhead shark Hexanchus vitulus Bigeye sixgill shark Squalus acanthias Spiny or picked dogfish, spurdog Himantura bleekeri Whiptail stingray Squalus blainvillei Longnose spurdog Himantura uarnak Honeycomb stingray Squalus cubensis Cuban dogfish Isistius brasiliensis Cookiecutter shark Squalus fernandinus Japanese spurdog Isogomphodon oxyrhynchus Daggernose shark Squalus mitsukurii Shortspine spurdog Isurus oxyrinchus Shortfin mako shark Squatina aculeata Sawback angelshark Isurus paucus Longfin mako shark Squatina argentina Argentine angelshark Lamna ditropis Salmon shark Squatina californica Pacific angel shark Lamna nasus Porbeagle shark Squatina dumeril Sand devil shark Mobula diabolus Devil ray Squatina squatina Angelshark Mustelus antarcticus Gummy shark Triakis scyllium Banded houndshark Mustelus asterias Starry smooth-hound Triakis semifasciata Leopard shark Mustelus californicus Grey smooth-hound Mustelus canis Dusky smooth-hound Mustelus fasciatus Striped smooth-hound 2 9 9 A P P E N D I X 2 . C a l c u l a t i o n of true v a l u e s o f f o p t a n d C o p t for the o p e r a t i n g popu la t ions . A fo rmal e s t ima t ion o f the true v a l u e s o f the m a n a g e m e n t b e n c h m a r k s o f fopt a n d C o p t imp l i e s n u m e r i c a l c a l c u l a t i o n o f thei r v a l u e s by i teration for e a c h s ing l e s imu la t i on tr ial . T h i s is n e c e s s a r y b e c a u s e the s t o c h a s t i c nature o f e a c h s imu la t i on trial p r o d u c e s va r i a t ions in the par t icu lar l eve l o f effort n e e d e d to p r o d u c e a l ong te rm m a x i m u m y i e l d , r ende r ing the l eve l s o f f t a n d C o p t to be different for e a c h s i m u l a t e d popu l a t i on . T h e n u m e r i c a l s e a r c h c o n s i s t s of c h o o s i n g a cons tan t l eve l o f effort a n d f i sh ing the p o p u l a t i o n for a l ong pe r iod of t ime (300 y e a r s in this s tudy) then c a l c u l a t i n g the a v e r a g e c a t c h a f te r the n e w equ i l ib r ium (ca tches f o r t h e y e a r s 1 0 0 - 3 0 0 in this c a s e ) . T h i s p r o c e s s is r e p e a t e d t ry ing different l eve l s of effort in d i sc re t e s tep s i z e s . T h e l eve l o f effort p r o d u c i n g the h ighes t a v e r a g e long te rm c a t c h is the o p t i m a l effort a n d the c o r r e s p o n d i n g c a t c h is M S Y . D u e to the e n o r m o u s a m o u n t of c o m p u t a t i o n a l t ime d e m a n d e d for a p p l y i n g this a p p r o a c h to 100 M o n t e C a r l o s imu la t i ons for e a c h ope ra t i ng popu l a t i on , a shor tcut s t ra tegy w a s d e v i s e d . A total o f 11 different e s t ima tes o f the m a n a g e m e n t p a r a m e t e r s w e r e o b t a i n e d for e a c h o p e r a t i n g popu la t i on by u s i n g 11 different s e e d s to restart the p s e u d o - r a n d o m n u m b e r gene ra to r in the p r o g r a m . Inside e a c h o f the 11 tr ials , the s a m e s e q u e n c e o f p s e u d o -r a n d o m n u m b e r s w a s u s e d for e a c h o f the effort l eve l s by u s i n g the s a m e s e e d . T h e m e d i a n a n d quar t i les o f the 11 e s t ima te s o f fopt a n d C o p t w e r e s a v e d from e a c h ope ra t i ng popu la t ion a n d a re s h o w n in tab le A . 1 . G i v e n the s m a l l va r i a t ion in the e s t ima tes , the m e d i a n v a l u e s w e r e u s e d a s the true v a l u e s o f L a n d C t for e a c h o p e r a t i n g popu la t i on . T a b l e A . 1 M e d i a n a n d quar t i les of true fopt a n d C o p t v a l u e s for the different ope ra t ing popu la t i ons a s c h a r a c t e r i s e d by c p u e - b i o m a s s re la t ionsh ip , s tock- rec ru i tment funct ion a n d a g e of entry to the f i shery ("productivity"). Operating population Optimal effort Optimal Catch cpue/biomass S-R A k Median Quartiles Median Quartiles Proportional B&H 7yr .3720 .3700 .3720 .1978 .1973 1984 it Ricker 7yr .2640 .2625 .2645 .0573 .0572 0574 I I B&H 4yr .2799 .2789 .2809 .1827 .1824 1833 " Ricker 4yr .2050 .2040 .2050 .0535 .0533 0536 , Hyper Stability B&H 7yr .1960 .1940 .1965 .1982 .1977 1988 II Ricker 7yr .1750 .1745 .1760 .0575 .0574 0577 Hyper Depletion B&H 7yr .5890 .5630 .5920 .1979 .1976 1985 " Ricker 7yr .3750 .3730 .3780 .0574 .0572 0575 300 S-A S-M F-A F-M Dn-a Dn-M D-A D-M S-A S-M F-A F-M Dn-« Dn-M D-A D-M * -X-S-A S-M F-A F-M Diva Dn-M D-A D-M S-A S-M F-A F-M Dn-a Dn-M D-A D-M Figure A.1 Modified box plots showing distribution of estimates of management and biomass assessment benchmarks for each estimation model using the two observation error assumptions. Monte Carlo simulations performed with the proportionality Beverton-Holt OP and high contrast effort. Codes in the x axis correspond to letters used to name estimation procedures in table 4 .1 . . For each estimation model, A means additive, M means multiplicative observation error assumption. ()= number of failed trials. 301 S-A S-M F-A F-M Dn-A Dn-M D-A D-M S-A S-M F-A F-M Dn-A Dn-M D-A D-M TJ C <V ffi E S-A S-M F-A F-M Dn-A Dn-M D-A D-M S-A S-M F-A F-M Dn-A Dn-M D-A D-M S-A S-M (17) F-A (19) * * * * X F-M On-A Dn-M D-A D-M (35) (53) Figure A . 2 Modified box plots showing distribution of est imates of management and b iomass a s ses smen t benchmarks for each estimation model and the two observation error assumptions , using the B0=K assumption. Monte Car lo simulations performed with the proportionality Ricker O P and high contrast effort. C o d e s in the x axis correspond to letters used to name estimation procedures in table 4.1. For each estimation model , A means additive, M means multiplicative observation error assumpt ion. ()= number of failed trials.
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Resource Management and Environmental Studies
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Elasmobranch fisheries: status, assessment and management
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