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Density dependent catchability coefficient in the Georgia Strait salmon sport fishery Shardlow, Thomas Frost 1983

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DENSITY DEPENDENT CATCHABILITY COEFFICIENT IN THE GEORGIA STRAIT SALMON SPORT FISHERY by THOMAS FROST SHARDLOW Bachelor Of Science A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA August, 1983 (5) Thomas Frost Shardlow, ]_983 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e head o f my department o r by h i s o r h e r r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f The U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 DE-6 (2/79) i i ABSTRACT This investigation examined the relationship between the catch per unit of e f f o r t ( c / l ) of salmon anglers and salmon abundance. The relationship was determined by using simultaneous estimates of both c/f and salmon abundance (n). Salmon in th e i r natural marine environment were observed as they approached f i s h i n g gear. The components of catch, such as the number of f i s h approaching, attacking and b i t i n g the lure per unit of fished e f f o r t were video recorded. Salmon abundance was estimated simultaneously with catch and e f f o r t through the use of a chart recording echo sounder. The re s u l t s showed that catch per unit of e f f o r t (c/f) was not l i n e a r l y related to abundance (n) as in the simple expression. c/f = qn, where 'q', the c a t c h a b i l i t y c o e f f i c i e n t , i s a constant proportion of the abundance caught by a nominal unit of fi s h i n g e f f o r t . In t h i s study q was found not to be constant but to increase with abundance. This result contrasts with previous empirical studies of c a t c h a b i l i t y in which estimates of q declined with f i s h abundance. However, because only h i s t o r i c a l catch records had been used to estimate c a t c h a b i l i t y , few of the relevant d e t a i l s of the interaction between the f i s h i n g gear and f i s h were avail a b l e . In the present study, video recordings of f i s h encounters with fi s h i n g gear suggest that feeding f a c i l i t a t i o n between salmon may be the mechanism underlying increcised c a t c h a b i l i t y with abundance. iv TABLE OF CONTENTS L i s t of Figures v i L i s t of Tables ..... v i i i Acknowledgement ix Introduction ••• 1 The Fishery Investigated 5 Materials and Methods 6 Observing and Recording Catch Information 6 Attack Behaviour 7 Fishing Gear 8 Density Estimation 9 Abundance Calculation 11 Sampling Locations 12 Results 16 Salmon Behaviour 16 Test Fishing Versus Sport Fishing •••• 1& S t r a t i f i c a t i o n of Samples 19 ' Time S t r a t i f i c a t i o n 19 Depth S t r a t i f i c a t i o n 20 V i s i b i l i t y S t r a t i f i c a t i o n 23 Comparisons Between Locations 24 Non-Linearity 24 Discussion 25 Relation Between q and N 26 F a c i l i t a t i o n 27 Alternative Explanations 29 Catchability Coefficient 32 Comparison With Other Studies 34 Variation Between Sampling Locations 39 Summary 41 References 42 Notes 44 v i LIST OF FIGURES FIGURE TITLE PAGE 1 Diagram of towed video unit 51 2 Diagram of towed video unit and fis h i n g gear assembly 52 3 Echo-gram example . .. 53 4a Example of echo-gram of salmon targets taken during c a l i b r a t i o n t r i a l s 54 4b Example of echo-gram of salmon and dogfish taken during c a l i b r a t i o n t r i a l s 55 5 Chart of sampling locations 56 6 Chart of S u t i l Point 57 7 Chart of Gower Point 58 8 Chart of Active Pass 59 9 Chart of Cowichan Bay 60 10 Chart of Cape Mudge 61 11 Comparison of slopes between Gower Point and Active Pass 62 12 Regression of encounters against standard abundance for Gower Point 63 13 Regression of encounters against standard abundance for Active Pass 64 14 Regression of encounters against standard abundance for Cowichan Bay 65 15 Regression of encounters against standard abundance for Cape Mudge 66 16 Regression of encounters against standard abundance for S u t i l Point 67 17 Regression of proportion of encounters against standard abundance for Gower Point 68 18 Regression of proportion of encounters against standard abundance for Active Pass 69 19 Graph of number of str i k e s versus number of v i i close encounters 70 20 Graph of number of f i s h hooked versus number of strikes 71 21 Number of close encounters against standard abundance at a l l locations 72 v i i i LIST OF TABLES TABLE TITLE PAGE 1 Comparison of catch for camera and control f i s h i n g lines 45 2 Comparison for test fishing and angler C.P.E 46 2 3 r values for temporal s t r a t i f i c a t i o n s ... 47 2 4 r values for depth s t r a t i f i c a t i o n s 48 5 Salmon density in association with herring schools 49 6 Encounter responses, f i s h stocks, f i s h hooked, and standard abundance by sampling location 50 ACKNOWLEDGEMENT I would f i r s t of a l l l i k e to thank my wife Susan for her continued patience and support. I also wish to express my gratitude and appreciation to the following: - to W.E. N e i l l who provided both opportunity and guidance, - to R. Hilborn for his valuable assistance formulating parts of the discussion, - to M. Ledbetter who's c r i t i c a l review of my work enhanced i t greatly, - to B. Gazey who assisted with s t a t i s t i c a l problems, - to G. Badgero, D. Peacock and C. Murray who assisted in f i e l d work, - to T.G. Northcote who was a member of my thesis committee, - to G. Parsons who drafted the diagram of the towed video unit, - to A. Wood and B. Masse of the Department of Fisheries and Oceans who helped substantially in the funding of thi s work, - to C.J. Walters who c r i t i c i s e d my work, - to C. Lauridson who reviewed the text for s p e l l i n g and grammatical errors, - to N.J. Wilimovsky for h i s hel p f u l comments, - to D. Chin who typed the manuscript. 1 INTRODUCTION C a t c h p e r u n i t o f e f f o r t i s o f t e n seen as an i n d e x o f r e l a t i v e f i s h a b u n d a n c e . The a p p l i c a t i o n o f t h i s i n d e x f o r t h e p u r p o s e o f m e a s u r i n g r e l a t i v e d e n s i t y o r abundance of f i s h s t o c k s had met, up t o t h e mid 1960's, w i t h l i t t l e s u c c e s s . L a r g e amounts o f u n e x p l a i n e d v a r i a b i l i t y i n e s t i m a t e s of a v e r a g e c a t c h p e r u n i t o f e f f o r t ( c / f ) made p r e d i c t i o n s o f r e l a t i v e abundance u n r e l i a b l e ( P a l o h e i m o and D i c k i e , 1964). C e n t r a l t o t h i s p r o b l e m , t h e n and now, i s an u n d e r s t a n d i n g o f how "q", t h e p r o p o r t i o n of t h e p o p u l a t i o n t h a t " i s c a u g h t p e r n o m i n a l u n i t o f e f f o r t , i s r e l a t e d t o abundance. In t h e s i m p l e s t f o r m u l a t i o n , q, t h e c a t c h a b i l i t y c o e f f i c i e n t , i s g i v e n a s a c o n s t a n t p r o p o r t i o n o f t h e abundance "N": c / f = qN where c / f i s t h e c a t c h p e r n o m i n a l u n i t o f f i s h i n g e f f o r t ( e . g . b o a t - d a y s , soak t i m e f o r n e t s , e t c . ) The r e l i a b i l i t y , t h e n , o f t h i s c / f i n d e x o f abundance i s o n l y a s good a s t h e d e g r e e t o w h i c h q i s c o n s t a n t o v e r N. The mechanisms w h i c h c o u l d t h e o r e t i c a l l y g i v e r i s e t o v a r i a b i l i t y i n q have been d i s c u s s e d by G u l l a n d (1964) and P a l o h e i m o and D i c k i e ( 1 9 6 4 ) . G u l l a n d l i s t e d s e v e r a l g e n e r a l c o n s i d e r a t i o n s w h i c h c o u l d c o n t r i b u t e t o v a r i a t i o n s i n q . Of t h e s e c o n s i d e r a t i o n s , changes i n q w i t h f i s h abundance i s g i v e n as t h e most i m p o r t a n t b e c a u s e t h i s t y p e o f v a r i a t i o n i n q " w i l l 2 change the whole shape of the r e l a t i o n between stock abundance and c/f from a proportional one to some type of curve" (Gulland 1964). Paloheimo and Dickie (1964) proposed a model based on the theoretical interaction between randomly di s t r i b u t e d schools of f i s h and fishermen searching for schools. Their prediction was that q i s not constant but varies inversely with N. Later investigations of how q varies with N such as those by MacCall (1976), Garrod (1977), and others, were based on comparisons of long term catch and e f f o r t records with population size estimates gained from cohort analysis. These empirical estimates of q produced results consistent with the predictions of Paloheimo and Dickie (1964). That i s , q was shown to vary inversely with N. However these studies, because they were concerned about models dealing with a whole f l e e t and estimates of whole cohort size, did not make direc t observations of f i s h interacting with fi s h i n g gear. This type of dir e c t observation i s important for several reasons. By making dire c t observations, the assumptions inherent in cohort analysis are avoided. Estimates of q gained through cohort analysis depend c r i t i c a l l y on assumptions about natural mortality rates (Ricker 1977 and Gulland 1977). Cohort analysis uses catch and escapement estimates and estimates of natural mortality to reconstruct the previous number of f i s h of a given age class within a f i s h population. This process begins with estimates of spawners ( f i n a l escapements), and includes the catch at a time prior to escapement to gain an estimate of the population size under f i s h i n g exploitation. Added to t h i s population estimate i s the number of f i s h l ost through age 3 s p e c i f i c natural mortality. Thus, the number of f i s h caught, escaping, and dying of natural causes during a given time comprises an estimate of age s p e c i f i c abundance of the cohort. The r a t i o of catch to cohort size provides an estimate of q. However, since age s p e c i f i c natural mortality i s never known with any certainty, a range of possible mortality rates i s assumed (Ricker 1977) and q w i l l vary depending on these assumptions about natural mortality. Moreover, Walters (pers. comm.) has recently found evidence to suggest that natural mortality rates themselves may vary with abundance. A direct measurement of abundance w i l l avoid these uncertainties. The use of dir e c t on-site measures of catch and abundance also avoids the problems associated with long term catch records that may show trends in q which are independent of density. Gulland (1964) points out that d i r e c t improvements in fi s h i n g technology continually increase the e f f i c i e n c y of a unit of e f f o r t . Therefore, for long term records, the units of e f f o r t are not d i r e c t l y comparable. Most previous studies of the c a t c h a b i l i t y c o e f f i c i e n t have used records spanning many years. In a more recent salmon fishery study by Peterman and Steer (1981), the problems involved in estimating abundance from cohort analysis were side stepped by summing catch escapement records to gain an estimate of abundance. In addition, these authors found no time trends in q and thus answer Gulland's (1964) concerns regarding changing catch e f f i c i e n c y over time. The results of their investigation again supported previous evidence that q declines with abundance. However, there are some problems with the methods used by Peterman and Steer. F i r s t in 4 their catch plus escapement estimate of abundance, there was a unstated proportion of escapement. The escapement enumeration in the fishery which they examined is notorious for i t s inaccuracy. This may explain why two'of the three non-linear relations between c/f and abundance reported by Peterman and Steer were not s i g n i f i c a n t . Second, by using catch records that compare annual or seasonal c/f with annual or seasonal records of abundance, much of the interaction of f i s h i n g e f f o r t with f i s h abundance i s l o s t . Fishermen w i l l encounter a wide range of densities or abundance during the course of their operations over r e l a t i v e l y short periods of time. The components of the behaviour of f i s h and the performance of fi s h i n g gear with variations in short term abundance are the factors that influence catch success, and therefore the c a t c h a b i l i t y c o e f f i c i e n t , most d i r e c t l y . The d e t a i l s of how these short term differences in density and behaviour can contribute to variations in q are not available in existing catch records. To my knowledge there are no published f i e l d studies that couple c/f with density estimates gained from direc t on-site observations. The purpose of this study i s to determine the r e l a t i o n between c/f and abundance for a s p e c i f i c fishery using a direct measure of abundance and catch success. The methods employed used direct on-site estimates of density and the components of catch success. Both measures were taken simultaneously during fis h i n g operations. 5 The Fishery Investigated The fishery investigated was the Georgia S t r a i t salmon sport fishery. The most common fi s h i n g method used in t h i s fishery is t r o l l i n g in which a lure i s towed through the water from small boats (average length 5 m) operating near shore (Argue et a l . 1980). The p r i n c i p l e of operation i s to attract and hook f i s h using a lure that resembles the natural prey items of salmon. The f i s h i s retrieved using a rod, r e e l , and monofilament l i n e . This fishery, in re l a t i o n to most commercial f i s h e r i e s , i s a special case. The times and location of f i s h i n g operations by sport fishermen are influenced as much by non-fis h i n g recreational objectives as they are by catch success (Byran 1974). However, the basic unit of f i s h i n g e f f o r t , hook and l i n e , has i t s counterpart in commercial salmon t r o l l i n g where many lures are t r o l l e d at the same time from larger vessels. Some of the mechanisms that a f f e c t catch success of sport t r o l l i n g , therefore, may also influence catch success in commercial hook and li n e f i s h e r i e s . 6 MATERIALS AND METHODS The b a s i c data r e q u i r e m e n t s f o r an e s t i m a t e of t h e r e l a t i o n between c a t c h per u n i t of e f f o r t and abundance i n v o l v e (1) a p r e c i s e d e f i n i t i o n and measure of one u n i t of e f f o r t , ( f ) , (2) a r e c o r d of c a t c h d u r i n g the u n i t of e f f o r t , ( c ) , and (3) an assessment of abundance (n) t h a t does not r e l y on c / f f o r a measurement of n, t h a t i s , an independent measure of abundance which i s c o u p l e d c l o s e l y i n time w i t h measures of c a t c h s u c c e s s . In t h i s s t u d y , e f f o r t i s d e f i n e d as the amount of time a s i n g l e hook and l i n e u n i t of s p o r t gear was t r o l l e d t h r ough the water. T h i s u n i t of e f f o r t was r e c o r d e d by the use of an underwater v i d e o camera and the b e h a v i o u r of f i s h near the gear was o b s e r v e d . O b s e r v i n g and R e c o r d i n g C a t c h I n f o r m a t i o n The b a s i c parameter, c a t c h per t r o l l i n g time d u r i n g a sample, can be e a s i l y e s t i m a t e d by s i m p l e c a t c h r e c o r d s . However, t h i s study was concerned w i t h a d e t a i l e d l o o k a t the i n t e r a c t i o n between the gear and the f i s h . These i n t e r a c t i o n s w i l l form the components of c a t c h s u c c e s s . An underwater v i d e o camera was d e v e l o p e d t o r e c o r d the b e h a v i o u r of salmon i n t h e i r n a t u r a l environment as they responded t o s p o r t f i s h i n g gear. I t was important t h a t the camera i n t e r f e r e as l i t t l e as p o s s i b l e w i t h the b e h a v i o u r of salmon toward the gear. For t h i s r e a s o n , the camera s e l e c t e d had the c a p a b i l i t y of p r o d u c i n g an image under n a t u r a l l y o c c u r r i n g ambient l i g h t c o n d i t i o n s found a t f i s h i n g d e p t h s . T h i s c a p a b i l i t y a l l o w e d the o p e r a t o r t o a v o i d the use of l i g h t s which 7 are normally used in underwater video operations. The camera was mounted on a sp e c i a l l y designed s t a b i l i z e r unit which enabled the assembly to be towed through the water in a stable attitude. (See s t a b i l i z e r design, Figure 1). The s t a b i l i z e r was constructed of clear a c r y l i c in an attempt to make i t less conspicuous to approaching f i s h . Sport fi s h i n g gear, attached to the end of monofilament nylon l i n e , was towed behind and attached to the camera assembly. The attachment was secured such that the terminal f i s h i n g gear was situated near the centre of the cameras' f i e l d of view (See Figure 2). A wide angle (90°) camera lens enabled observations of salmon approach behaviour up to approximately 9 metres behind the lens in clear water. An a u t o - i r i s adjusted for the changing l i g h t conditions encountered during sampling. The camera used was a Panasonic 1350. Images were recorded on a portable video tape recorder (Panasonic N.V. 3085) and monitored using a high resolution t e l e v i s i o n monitor (Panasonic W.V. 5300). This system was powered with a small portable gas generator. Attack Behaviour Attacks by individual salmon were categorized into various types of approach behaviours. These behaviours ranged from apparent d i s i n t e r e s t of f i s h which swam through the f i e l d of view and did not orient toward the lure, to dir e c t pursuit and b i t i n g of the lure. The basic behaviour c l a s s i f i c a t i o n s used in the analysis were encounters, close encounters, st r i k e s , and hooked. An 8 encounter involved a behaviour in which a salmon oriented and swam toward the l u r e . Often th i s pursuit was short l i v e d and the f i s h would swim away without coming in close contact with the l u r e . Close encounters included only those f i s h which came within close proximity of the lure (within 1 m). S tr ikes were recorded when a f i sh took the lure in i t s ' mouth. F i n a l l y , hooked involved cases where f i s h were caught by the hook and were re tr ieved using a rod and r e e l . F i sh ing Gear Conventional sport f i sh ing techniques were used to catch and re tr ieve salmon. This technique involved using 13.6 kg. test monofilament nylon l i n e to which was attached a f lasher and a lure or bait (Figure 2). The choice of terminal gear and lure type was made on the basis of the resu l t s of a cree l survey conducted at the f i sh ing l o c a t i o n . When sport fishermen were not at a locat ion the standard combination of f lasher and f l a s h t a i l was used for test f i s h i n g . The c r e e l survey included most anglers present when angler numbers were small (15 or less) and at least 20% of the anglers when the ir numbers were large (more than 30). Survey forms were issued to anglers while they were a c t i v e l y f i sh ing from the i r boats. Their f i s h i n g was not interrupted and the forms were recovered af ter approximately f i f teen minutes. Where subsamples of anglers were taken, the se lect ion of sample cases was a r b i t r a r i l y based on boat co lour . Guided fishermen and large pleasure craf t ( larger than 9 m) were excluded from the sample. 9 The most common lure type and flasher combination employed by the anglers was selected for test f i s h i n g . S i m i l a r l y , the depth range of fis h i n g by anglers at a location was used when test f i s h i n g . The survey also provided an estimate of the anglers catch per unit of e f f o r t . That estimate was used as a comparison with test f i s h i n g to esta b l i s h the compatibility of test f i s h i n g with normal sport f i s h i n g catch. Fishing was conducted from an 8 m boat t r o l l i n g at a speed of approximately two knots (based on a constant engine rpm of 1800). I selected sampling locations (see page 11) where currents or winds would not strongly influence ground speed. Density Estimation A hydro-acoustical estimate of salmon abundance was gained through the use of a chart recording echo sounder. This system consisted of a Furuno fm21d echo sounder with a frequency of 50 KHz. The towed transducer produced a beam angle of 42° and the acoustic impulse was modified with a time varied gain. The gain modification corrected for depth variations in target strengths. This system is designed to count individual f i s h targets (see Figure 3 for quary). A common problem in hydro-acoustical estimates of abundance is the species i d e n t i f i c a t i o n of echo targets. Often g i l l netting or other techniques are employed to determine proportionate species composition. In thi s study, such techniques were not used since individual f i s h could often be seen on the t e l e v i s i o n monitor and the echogram at the same 10 time. From these simultaneous observations echo targets could be i d e n t i f i e d as dogfish (Squalus acanthias) , herring (Clupea  harengus), and salmon (Oncorhynchus spp). Schooling herring were ea s i l y distinguished; however large herring (approx. 2 0 cm.) were observed i n d i v i d u a l l y in some locations both on the echogram and the monitor and could not be r e l i a b l y separated from salmon echo tracings. S i m i l a r l y , dogfish produced an echo target very similar to that produced by salmon. Samples in which dogfish or large non-schooling herring were evident were excluded from the analysis. Additional f i s h species were never observed on the video recordings except in one case where a greenling (Hexagrammos sp.) was seen as the camera passed very near the sea f l o o r . Target strength, the darkness and resolution of the image produced on the echogram, are said to be dependent, among other things, on the size of the swim bladder of the sonified f i s h . However, as previously mentioned, dogfish and salmon seemed to produce very similar echo tracings. In order to assure that these observations were not a result of a mismatch caused by a video observation of a di f f e r e n t f i s h than that recorded on the echo gram, a c a l i b r a t i o n of echo targets was conducted. The c a l i b r a t i o n consisted of a series of so n i f i c a t i o n s of anaesthetized salmon and dogfish suspended on a monofilament l i n e and harness in the water column. The results of this t r i a l (See Figure 4a and b) show that both f i s h produced similar targets. 11 Abundance Calculation Hydro-acoustical transects were divided into f i v e minute intervals during which time the volume so n i f i e d and the number of f i s h targets counted on the echo gram were recorded. This volume was further s t r a t i f i e d into f i v e fathom (9.1 m.) depths. Since the echo sounder s o n i f i e s a cone shaped volume, the volume sonified is not constant over depth. The volume sonified by strata i s : V = A S t i D D where V = volume of stratum i j A = cross sectional area of depth stratum i S = speed t r a v e l l e d during time in t e r v a l j t = duration of time interval j and A = (H 2 tan21°) - (h 2 tan2l°) where H = t o t a l depth sonified h = depth outside stratum i 21 = 1/2 sonar beam angle and density or number of f i s h per stratum i j = d i j 1 2 d = T /V i j i j i j where T = number of f i s h targets counted i j during distance S t j j The basic unit used in the analysis was the standard abundance which i s the number of f i s h targets (weighted for volume sonified) counted during a portion of a transect. Standard abundance 'D' was calculated as follows: m n n D = Z ( ( Z d / n ) ( I V ) ) i j i j j=1 i=1 i=1 Sampling Locations The areas and times of sampling i n i t i a l l y included 18 d i f f e r e n t locations within the S t r a i t of Georgia sampled from May through October 1978. These areas were located from Saanich Inlet and Cowichan Bay in the south to Cape Mudge near Campbell River in the north (Figure 5). Of the 18 locations, 5 were selected for continued monitoring on the following basis: (1) Free From Submerged Hazards. Reefs or abrupt projections from the ocean floor represented a hazard to the towed video camera. (2) Depth Less Than 30 Fathoms (55 m.). The model of echo sounder used was limited to 30 fathoms (55 m.) of depth s o n i f i c a t i o n . (3) Protected from Weather and Strong Currents. A turbulent water surface causes the sound impulse from the echo sounder to 13 t r a v e l in many di f f e r e n t directions, depending on the attitude of the transducer. As waves t i l t the towed s t a b i l i z e r unit, the attitude of the transducer i s changed and the echo trace becomes unreliable.. Fast moving currents also d i s t o r t echo recordings such that innumeration of f i s h i s not possible. (4) Sport Fishing Locations. The locations were fished by anglers. The presence of other units of gear allowed for a comparison of catch per unit e f f o r t between anglers and test f i s h i n g . Moreover, the fis h i n g techniques used by anglers were observed and test fishing techniques could then be adjusted to represent similar techniques. (5) Wide Range of Densities. A wide range of f i s h densities was required in order to determine the relationship between density and c/f. (6) Species Account. Locations that showed the persistent presence of non-salmon species were avoided since hydro-acoustical techniques are unable to d i s t i n g u i s h the species of individual targets. The commonly occurring non-salmon species were dogfish and herring. The five locations selected for continued monitoring are described below: (a) S u t i l Point. S u t i l Point, on the southern t i p of Cortes Island, receives l i t t l e sport f i s h i n g a c t i v i t y during the summer. Samples were taken at thi s location on June 22, 1978 and test fishing was conducted following the 10 fathom contour (Figure 6). One half of the eight samples taken were discarded due to the presence of dogfish. Water v i s i b i l i t y was very poor and f i s h could only be seen on the camera when they were 14 adjacent to the lure. At the time of sampling, there were no anglers f i s h i n g . A l l f i s h caught during sampling were coho. (b) Gowers Point. Gowers Point i s a popular salmon f i s h i n g s i t e situated at the entrance to Howe Sound in the southern S t r a i t of Georgia. Samples taken followed the 15 fathom contour of a drop off (Figure 7). This location was fished on August 4, 1978 at which time many anglers were present. The water v i s i b i l i t y was very good and clear video records of approaching f i s h were achieved. A l l f i s h caught by anglers and during test f i s h i n g were coho. (c) Active Pass. Active Pass is a common angling location situated between Mayne and Galiano Islands in the southern Gulf Island group. Samples were taken here on September 2nd and 3rd 1978 off the eastern entrance to the pass (Figure 8). Water v i s i b i l i t y was excellent. At the time of sampling, no anglers were present. The f i s h caught during sampling were chinook and coho. (d) Cowichan Bay. Samples from t h i s location were taken in the estuarine area off of the Cowichan River on September 8 and 17, 1978 (Figure 9) during a sport f i s h i n g closure. The water v i s i b i l i t y was f a i r . Coho and chinook salmon were caught at thi s location. (e) Cape Mudge. The waters in the v i c i n i t y of Campbell River are extensively used by anglers. The b e l l buoy off of Cape Mudge i s one of the most popular f i s h i n g s i t e s within the S t r a i t of Georgia. This sampling location was situated at the drop off of the shallow reef extending south east of Cape Mudge (Figure 10). Although this area was sampled intensively, only samples 15 taken on May 21, 22, 23, 25 and 29, 1978 were used. Other times showed the persistent presence of dogfish. There were many anglers fi s h i n g at thi s location when samples were taken. The water v i s i b i l i t y was excellent. Chinook, coho and pink salmon were caught in sampling and by anglers at this location. 16 RESULTS Salmon Behaviour Although the ways in which salmon approached the terminal gear were variable, there are some generalizations which can be described. Only on one occasion did salmon that were seen in the f i e l d of view of the camera not pursue the moving lure. This i s not to say that a l l salmon which saw the lure pursued i t , but rather that those salmon close enough to be seen through the camera were already in pursuit. The number of salmon approaching the lure at any one time varied from 1 to 4. Some approaches toward the lure lasted only a second or two, while other f i s h followed the lure for up to ten minutes. The salmon observed invariably approach from a position behind the lure however the intensity of pursuit was varied. In the most intense pursuits, i t seemed the decision to take the lure was made at some distance outside the camera's f i e l d of view. In these cases the f i s h made no apparent change in di r e c t i o n or velocity from the time i t was f i r s t seen to the time i t struck the lure. These approaches were very rapid and often resulted in a hooked f i s h . In other instances the salmon would approach at slower speeds and make several close passes by the lure, turning away and returning frequently. Some of these differences in approach can be ascribed to species differences. Pink salmon (Oncorhynchus qorbuscha) c h a r a c t e r i s t i c a l l y approached to a position very close (several cm) d i r e c t l y behind the lure and tracked the lure's path very c l o s e l y . This pursuit was ended either by a bite which was a simple opening and 17 closing of the fish ' s mouth about the lure or by swimming away. In contrast coho (Oncorhynchus kisutch) approached the lure quickly. However, instead of cl o s e l y following the path of the lure, they would bear off to one side just as they came close to the l i n e s , and would s t r i k e at the lure from the side or top. This difference in attack behavior can be attributed to the di f f e r e n t prey items taken by these species. Coho are more piscivorous than pink, the l a t t e r ' s diet being composed mainly of zooplankton. Other species observed to approach the lure included dogfish (Squalus acanthias) and large herring (Clupea harengus). Dogfish pursuit behaviour was marked by i t s ' lack of success. In most cases these f i s h were observed in groups of two to four. Although many repeated attempts were made by individual dogfish to capture the lure, their e f f o r t s to bite i t almost never succeeded. It i s d i f f i c u l t to imagine how th i s predator could capture free swimming f i s h after viewing t h i s clumsy pursuit behaviour of a slow moving lure. Perhaps the combination of a ve n t r a l l y located mouth and eyes set far back in the head have the e f f e c t of producing a bl i n d period just before capture of prey. When a dogfish b i t i t l i f t e d i t s head such that i t s mouth was nearer the lure. During t h i s head l i f t v i s u a l contact with the lure was most l i k e l y l o s t . F i n a l l y , on one occasion large individual herring were seen to capture the lure. Their pursuit behaviour was similar to the close tracking pursuit described for pink salmon. The lure was not pursued at other times when herring were observed in schools. 18 Test Fishing Versus Sport Fishing A main assumption in this study i s that the samples taken on the f i s h i n g grounds are representative of anglers' experiences of catch at various abundances of salmon. The use of an underwater camera i s a departure from normal sport f i s h i n g practice and the influence of the camera on salmon attack behaviour i s relevant in interpreting the results of catch success. The presence of the camera unit could p o t e n t i a l l y i n h i b i t the normal approach behaviour of salmon toward the lure. To assess any deterrent effect the camera may have had on catch a control l i n e was employed simultaneously with the camera l i n e during some of the test f i s h i n g bouts. The same terminal gear type and arrangement were used for both l i n e s . Table 1 compares catches on the two l i n e s . The n u l l hypothesis was that the control l i n e caught the same number of f i s h as the li n e with the camera. The probability associated with a catch as small as that observed on the camera when compared to the control i s .10 < P < .20. This result suggests that there may be some deterrent effect associated with the use of the camera when f i s h i n g . If a deterrent effect did ex i s t , then i t can be predicted that test f i s h catch per unit of e f f o r t would be lower on average than anglers c/f when both anglers and test f i s h i n g were conducted at the same time and place. Samples of the anglers catch and e f f o r t taken at the same locations and during the same times as test f i s h i n g were analysed for differences in c/f. Table 2 l i s t s ten separate pairs of study versus angler c/f values. Comparisons of the two c/f values showed that there was no evidence to suggest that the 19 two samples were d i f f e r e n t . In fact, angler and test fishing values are very similar (test mean c/f = .47, angler mean c/f = .44, Table 2). This suggests that test f i s h i n g samples can be used to represent the normal sport f i s h i n g dependence of catch success on salmon abundance, and that the p o s s i b i l i t y of a deterrent e f f e c t associated with the use of the camera was l i k e l y due to random v a r i a t i o n . Strat i f icat ion of Samples The next step in the analysis was to determine the appropriate size of the basic sampling unit. As discussed in the previous section, the intention of t h i s study was to look at the behaviour of salmon and the performance of units of fishing e f f o r t over short periods of time. The length of t h i s period of time was unknown at the beginning of the work. Time S t r a t i f i c a t i o n The smallest temporal sampling unit taken at each location was based on five minute intervals of simultaneous echo sounding of salmon density and video observation of salmon encountering the terminal gear. From th i s basic unit, larger samples were obtained by grouping density and encounter estimates from adjacent five minute i n t e r v a l s . A linear regression analysis of encounters against density was conducted for samples taken at f i v e , f i f t e e n , and t h i r t y minute i n t e r v a l s . A comparison of the variance associated with the regression for each time in t e r v a l was used to select the 20 sample size that would produce the maximum r 2 for a l l locations. It can be seen (Table 3) that by increasing the time in t e r v a l of each sample from five minutes to t h i r t y minutes, the v a r i a b i l i t y between encounters and density decreased. From t h i s analysis t h i r t y minutes was selected as the standard sample size for a l l cases. 1 Depth S t r a t i f i c a t i o n Recall that the depth echo sounded was t h i r t y fathoms (55 m) and the density estimate i s based on f i s h targets sonified throughout this water column. The amount of va r i a t i o n in encounters, that can be explained by density differences i s small for locations other than Gowers Point (Table 3). The p r o b a b i l i t y of a salmon responding to the lure depends on the distance between the f i s h and the bait. A salmon which i s a great distance from the lure cannot respond i f the lure i s beyond i t s ' range of perception. Even in cases where the lure i s within the fi s h ' s range of perception, other more a t t r a c t i v e prey items closer to the f i s h may cause i t to disregard the lure. To solve this problem some measure of the distance at which salmon approached the lure i s needed. Salmon which are well beyond th i s distance can be excluded from the density estimation since they are not part of the population which i s capable of approaching the gear. Direct measurement of the approach distance was not possible from video records since almost a l l f i s h seen by the camera were already in the process of approaching the gear. The alternative method used was to assume that the f i s h were 21 randomly distributed within the volume sampled and that the number of f i s h seen in the camera to approach the lure was d i r e c t l y proportional to the volume searched by the lure. The volume searched in th i s case is equivalent to the volume in which f i s h w i l l approach the lure and the radius of this volume i s taken as the average approach distance. This approach distance was calculated as follows: T/V = a/v and v = Va/T where T = number of f i s h targets counted in the volume echo sounded, V = the volume of the sample, a = the number of f i s h seen to approach the lure v = the volume searched by the lure. To calculate the maximum observed approach distance the highest recorded encounters/abundance r a t i o was used. This value was 84 approaches out of 427 targets observed at Active Pass (Table 5). The volume sonified in each sample was 240,000 m3. Therefore: 22 T = 427 V = 240,000 m3 a = 84 3 v = (240,000)(84) = 47213 m 427 and v = 7 r r 2d r = /47213 m 3/7rd where d = 1850 m, the distance t r a v e l l e d at 2 kts in 30 minutes (standard sample) and so r = 2.85 m. The value (r = 2.85) represents the maximum observed average radius of the volume in which salmon approached the lure. That i s , i t is the expected radius given the number of f i s h seen to approach the gear at a given density. It represents the average because the lure a t t r a c t s salmon from varying distances. Individual f i s h seen to approach the gear could have come from very near the camera or from some unknown maximum distance. Therefore, a l l salmon targets innumerated beyond th i s radius cannot be ruled as being beyond the range of possible response to the lure. The maximum observed average distance, however, does provide a guide to the magnitude of the maximum approach distance. From t h i s , an a r b i t r a r y l i m i t of five fathoms (9.1 m) radius about the lure was used to constitute the volume on which the density estimate was based. The 9.1 m l i m i t was taken to include a l l salmon that could p o t e n t i a l l y approach the gear as well as some f i s h which were at some reasonable point beyond that distance. 23 Table 4 shows the reduction in v a r i a b i l i t y accomplished by excluding distant f i s h from the sample volume. While Gowers Point showed no improvement,2 Cowichan Bay, and Active Pass p a r t i c u l a r l y showed less variable results when depth was s t r a t i f i e d . From this analysis the sample volume used in subsequent discussions includes only f i s h enumerated in the 9.1 m depth stratum above and below the depth of the lure . This depth s t r a t i f i c a t i o n , coupled with the time interval stratum of th i r t y minutes, constitutes the volume within which standard abundance was estimated. V i s i b i l i t y S t r a t i f i c a t i o n Differences in t u r b i d i t y or water c l a r i t y were observed between locations. In some locations i t was evident from reviewing video tapes that the observer could see f i s h at a greater distance from the camera than could be seen at more turbid locations. This, coupled with the observation that salmon approached to variable distances behind the camera, means that in locations where v i s i b i l i t y for the observer was high, f i s h could be seen approaching the lure that would not be observed when v i s i b i l i t y was low. Because no di r e c t measure of v i s i b i l i t y was made, standardizing samples for v i s i b i l i t y differences i s d i f f i c u l t . For this reason a procedure was adopted whereby those f i s h approached to within very close proximity of the lure (within 0.6 m) were taken to represent a "close approaching" f i s h . The remaining f i s h which could be seen to approach, but did not approach close enough to be seen i f the water c l a r i t y had been below the 0.6 m, were c l a s s i f i e d as "distant 24 approaches." For comparisons between locations only close approaches were used. Comparisons Between Locations The results of v i s i b i l i t y s t r a t i f i c a t i o n are shown in Figure 11 where close encounters are compared with density for two locations. The remaining locations have either a low pr o b a b i l i t y associated with the regression, or have variances too large to show differences between locations. Gowers Point and Active Pass show substantial differences in slopes of encounters against density (t = 1.78, p < .10) (Figure 11). This difference could be attributed to a variety of b i o l o g i c a l factors related to salmon stock, size, species composition, and seasonal attributes at each location. Non-Linearity Within some of the locations there i s a strong suggestion of a non-linear encounter response over the densities sampled. Figures 12, 13 and 14 show the regression of a second order polynomial y = ax + bx 2. These non-linear regressions are based on t o t a l encounters (close plus d i s t a n t ) . For Gower Point (Figure 12) the p r o b a b i l i t y of a non-linear r e l a t i o n i s p < .05. S i m i l a r l y , Active Pass showed the second term b to be p = .06 (Figure 13). Cowichan Bay, however, did not provide good evidence for departure from a linear r e l a t i o n (p = .54). The remaining two locations have either points over a limited range of densities, (Cape Mudge, see Figure 15), or have too few 25 samples (See Figure 16) to show a trend in encounter rates. Discussion In review, a basic assumption underlying the re l a t i o n between catch per e f f o r t (c/f) and f i s h abundance (N) i s that: c/f = qN (1) where the catch c o e f f i c i e n t 'q' i s some constant proportion of N. Empirical studies previous to thi s one have indicated, however, that q varies inversely with N. The results of the present study suggest the opposite, that i s , q increases with N. The inverse relation between q and N has been attributed to the effect of gear saturation (Rothschild 1976) when, for example, a net or trap becomes f u l l of f i s h and increases in abundance-cannot result in a higher catch rate. An asymptote in c/f i s reached. In the case of hook and l i n e f i s h i n g , saturation would occur when the hooks are occupied by f i s h at a rate faster than the f i s h can be removed. Thus, as abundance increases, the proportion of N caught per unit of e f f o r t decreases. However, in the sport fishery investigated, f i s h abundances never reached a le v e l where the catch of one f i s h impeded the catch of a second. The highest recorded catch per line-hour was 1 at Cape Mudge (Table 3). It takes approximately f i v e minutes to remove a f i s h from the li n e and resume f i s h i n g . Other f i s h e r i e s as well, such as the commercial t r o l l fishery, may seldom experience salmon 3 26 densities high enough to reduce c a t c h a b i l i t y . In fact, the opposite can be true. Catchability may increase with abundance over the range of abundance normally encountered by hook and li n e f i s h e r i e s Relation Between g and N The relationship under discussion in this study thus far has been between the number of approaches or attacks to the l i n e per unit e f f o r t (a/f) and standard density n (where n = some subset of N ) . a/f = Pn (2) P i s the fraction of the population n that i s attracted to the gear. To find c/f one needs to know the proportion of the attracted f i s h which are caught (p ). Therefore: o c/f = Pp n o and q = Pp (3) o For the present, p w i l l be assumed to be some constant value. o For Georgia S t r a i t Salmon, i t can be seen from the results shown in Figures 12 and 13 that P is not constant and that the 27 rate of a t t r a c t i n g f i s h increases with abundance. In e f f e c t , this means that the e f f i c i e n c y of the lure to attract f i s h increases with abundance. E f f i c i e n c y can depend on the volume swept or "searched" by the f i s h i n g gear. The problem now becomes to determine how a larger volume i s searched at high f i s h density than at low density. F a c i l i t a t i o n The concept of predators providing important signals that f a c i l i t a t e prey detection by conspecifics is well documented (See Curio 1976 for summary). Behavioral interactions between feeding f i s h have been commonly observed both informally by fishermen and experimentally by s c i e n t i s t s . There i s much anecdotal information available from anglers who were f i s h i n g from the shore of clear water streams or lakes and watched f i s h chase their lures. Almost any angler can recount observing several f i s h chasing his lure at the same time with one f i s h leading the chase and the other f i s h following behind. The suggestion here i s that the attack on the lure by one f i s h prompted other f i s h to attack. Commercial salmon t r o l l e r s , believing that one hooked coho or sockeye on the l i n e w i l l a t t r a c t others, w i l l delay r e t r i e v i n g hooked f i s h from the water. This type of attack or feeding f a c i l i t a t i o n has also been observed experimentally. Protasov (1970) c i t e s the work of Veronin (1957) as having experimentally established "that the visu a l link between f i s h of the same species during feeding i s based on conditional imitative reflexes". That i s , individuals in a conspecific group w i l l imitate the food getting behaviour 28 of nearby feeding f i s h . Experiments conducted by Veronin on Black Sea bass, pic k e r e l , perch, and roach, and studies by Markl (1972) on piranhas show that this phenomenon i s widely seen in fishes. It is reasonable to expect that salmon feeding in the ocean w i l l exhibit the same "imitative food-getting r e f l e x " . This f a c i l i t a t i o n can be interpreted as the transferring of information about the a v a i l a b i l i t y of food (the lure) from salmon near the lure to salmon at farther distances from the lure. This phenomenon in effect increases the volume swept by the lure as more f i s h are available to transfer information about the presence of food. In terms of equation (2) where a/f = Pn, P is not a constant proportion of n, but i s made up of two components; a 1 the i n i t i a l proportion attracted to the lure, plus a the 2 additional component of n attracted by the feeding response of a . Thus: -1 . a/f = a n + a n ( a n ) 1 2 1 a/f = a n + a a n 2 1 2 .1 l e t a a = b 2 1 a/f = a n + bn 2 (4) 1 This quadratic expression forms the b i o l o g i c a l model that was 29 applied to the data in the form of a second order polynomial regression of encounters against standard density. Figures 12 and 13 show that the second term (b) is s i g n i f i c a n t . Since P i s not constant with respect to n but increases with n, then a regresssion of P against n should show b from equation (4) to have a positive slope. By the preceeding argument: P = a + bn (5) 1 Figures 17 and 18 show that b i s p o s i t i v e (p < .05) for equation (5) as expected. Alternative Explanations 1. Error in Density Estimates. The non l i n e a r i t y found in Figures 12 and 13 could have arisen from "scatter" in the hydro-acoustical estimation of density. Submerged debris in the water may have produced signals that were interpreted as f i s h targets. This misinterpretation would mean that the y-intercept for the equation describing the r e l a t i o n between f i s h encounters and f i s h targets would f a l l somewhere below zero on the y axis. The regression analysis used constrained the l i n e through the o r i g i n . This constraint in the presence of "scatter" would produce a non linear r e l a t i o n when in fact none existed. 30 However, samples taken from Campbell River at low densities and low encounter rates ( S e e F i g . 10) suggest that the l i n e was near zero when density was low. Hence, the non-linear response i s not l i k e l y to have been produced by error in target i d e n t i f i c a t i o n . 2. Learning. An increasing rate of prey capture with increasing prey abundance has been attributed to learning (Holling 1966). With each capture, the predator, through practice, has become either better able to catch or to recognize prey (Se; Curio 1976) prey. In th i s study, for learning the predator (which in th i s case i s the author) to have occurred, f i s h i n g s t y l e or techniques would had to have been d i f f e r e n t at high densities and at low d e n s i t i e s . Fishing or sampling techniques, however were consistent over a l l d e n s i t i e s . Therefore, learning i s unlik e l y . 3. Changes in Fish Behaviour with Abundance. Changes in f i s h behaviour could cause an increase in approach rate for two reasons: F i r s t , the number of observations of an in d i v i d u a l f i s h 31 approaching the gear may increase. There i s a l i m i t to the l a t e r a l distance at which f i s h could be observed in the video camera. Therefore i t i s possible that an individual f i s h could enter the f i e l d of view more than once. I f the general swimming a c t i v i t y of salmon increased at high densities, then one could expect to see a given individual more frequently at high densities than at low dens i t i e s . This hypothesis is untestable with the data col l e c t e d during this study and as far as I am aware t h e r e is no e v i d e n c e t h a t f i s h e s e x h i b i t t h i s b e h a v i o u r . The.second cause i s related to the f i r s t . It can be assumed that the propensity of a salmon to approach a potential food item (the lure) increases with density. High densities or aggregations of salmon can be reasonably assumed to be associated with concentrations in salmon prey. An individual f i s h in association with concentrations of prey i s then l i k e l y to be a c t i v e l y feeding. Feeding salmon would have a greater propensity to approach a potential prey (the lure) than non-feeding salmon. Therefore at high densities of salmon there i s a greater l i k e l i h o o d of an approach than at low densities for any individual salmon. The result of t h i s feeding versus non-feeding behaviour would appear to be the same as the result of f a c i l i t a t i o n . However in the l a t e r case, no dir e c t f a c i l i t a t i o n occurred. In order to distinguish between the alternative mechanisms of f a c i l i t a t i o n and changes in feeding behaviour, a measure of the association of prey density and salmon density i s needed. Echo chart recordings kept a continuous record of targets encountered. Schools of small herring, a natural prey of salmon, 32 could be distinguished and examination of these transects showed no association between high salmon density and herring schools. Table 5 shows the mean number of salmon targets sonified in a stratum (5 fathom, 5 min.) where herring schools are located compared to strata in the same sample where herring schools are not evident. No s i g n i f i c a n t difference in salmon density was observed between the two cases. Catchabi1ity Coef f ic ient The discussion up to this point has concentrated on the dependence of the number of approaches to the lure by salmon on density, of salmon a/f = Pn (2) To determine the c a t c h a b i l i t y c o e f f i c i e n t , q, i t i s necessary to know (p ). o c/f = Pp n (3) o where q = Pp o p was defined as the proportion of the f i s h attracted that o were actually caught. However, p can be further decomposed to o 33 p = p * p where p is the proportion of s t r i k e s and p i s the 0 1 2 1 2 proportion of those f i s h which struck that were caught. This separation of p into two components has the advantage o of allowing for the separation of the b i o l o g i c a l and mechanical aspects in the catch component, p , the s t r i k e s per encounter, 1 is associated with b i o l o g i c a l variables such as hunger and the stimuli presented by d i f f e r e n t lure types, p can be thought of 2 as being associated with mechanical variables, such as probability of the hook being set once i t i s struck by a f i s h . Factors such as hook size , sharpness, and position r e l a t i v e to the lure and the f i s h ' s mouth, are important variables in t h i s case. Figure 19 shows str i k e s plotted against close encounters from which p = .14 ± .04 and Figure 20 shows the number hooked 1 depending on strikes to be p = .31 ± .06. q now has two' 2 additional components to those given in equation (3) where q = (a + bn)p 1 o now p = p * p o 1 2 and q = (a + bn) p p 1 1 2 (6) 34 This q, however i s an estimate of c a t c h a b i l i t y only for a very small volume of the S t r a i t of Georgia. Although i t i s beyond the scope of t h i s thesis, the f i n a l consideration in determining q for a whole fishery i s to know the relationship between n and N. In this study, n is the population of salmon in the standard sample volume of ocean and N i s the abundance of salmon in the t o t a l population or stock. The r e l i a b i l i t y of any estimate of q for an entire fishery such as the Georgia S t r a i t i s only as good as the u n d e r s t a n d i n g of how n r e l a t e s to N. In the Results section, much of the analysis was designed to select an appropriate n. S t r a t i f i c a t i o n of samples was conducted such that the number of approaches to the lure, and ultimately catch per unit e f f o r t , could be predicted over varying densities. It was assumed that the volume selected for n represented a volume within which the population of salmon was r e l a t i v e l y available to the f i s h i n g gear. Larger v e r t i c a l sample volumes showed no re l a t i o n between the number of salmon encounters and salmon abundance. Similar d i f f i c u l t i e s would arise when re l a t i n g the parameters of catch and density found within a sampling location to the larger volume of the Georgia S t r a i t . Figure 21 i l l u s t r a t e s the problem of attempting to predict q over large areas and times. This figure shows the highly variable result of combining the five d i f f e r e n t locations for a single estimate of q. 35 Comparison With Other Studies This study shows q to increase with density while a l l the others referenced show q to decrease with density. As discussed e a r l i e r , the Paloheimo and Dickie (1964) model was based on the interaction between fishermen searching for schools and changes in the number and radius of schools. These authors did assume, however, that once a school was located that a f i s h i n g vessel would catch a constant proportion of that school. Therefore, as Paloheimo and Dickie pointed out, the c a t c h a b i l i t y c o e f f i c i e n t was assumed to be a constant proportion of the density or the abundance in a school. It was beyond the scope of my study to consider the relationship between the size and number of salmon schools in Georgia S t r a i t and angler f i s h i n g success. My findings can, however, be interpreted as being the relationship between catch per unit of nominal e f f o r t and changes in density within a school or aggregation of salmon. In t h i s respect I have argued that q is not a constant but w i l l increase with l o c a l density. The effect of t h i s l a t t e r type of density dependent c a t c h a b i l i t y c o e f f i c i e n t would be most pronounced when o v e r a l l density, defined as the number of schools by Paoheimo and Dickie, and the densities encountered during f i s h i n g operations are sim i l a r . An example of t h i s situation occurs when ove r a l l abundance is low and salmon are aggregated at the common fi s h i n g locations. At high o v e r a l l abundance however, when the number of schools are numerous and the capacity of the f i s h i n g vessel to respond by increasing area searched i s limited, then th e o r e t i c a l l y an asymptote in catch per e f f o r t w i l l be reached 36 (Paloheimo and Dickie 1964). These authors discussed l i m i t a t i o n s in area searched and proposed several mechanisms which could produce an asymptote at high levels of o v e r a l l density. The effect then of having an increasing c a t c h a b i l i t y c o e f f i c i e n t at low density and a decreasing c a t c h a b i l i t y c o e f f i c i e n t at high densities is a sigmoid shaped function for catch per unit of e f f o r t over density. L i t t l e i s known about the nature of s p a t i a l heterogeneity of salmon in Georgia S t r a i t that could be related to this question. This type of sigmoid c u r v e must thus remain as speculation at this point.. However, Peterman and Steer (1981) have discussed the application of Holling's (1965) type II and type III functional responses to salmon sport f i s h e r i e s . The type III response is a sigmoid function while the type II response i s analagous to Paloheimo and Dickie's (1964) relationship for catch per unit of e f f o r t to density. Peterman and Steer ('1981) concluded that a type II and not a type III response best described their sport fishery. The f i t t i n g of a type II response by Peterman and Steer, however, could have resulted as an a r t i f a c t of large errors in the estimation of abundance (Shardlow and Hilborn, submitted). Moreover, as Shardlow and Hilborn (submitted) pointed out, the mechanism proposed by Peterman and Steer (1981) assumed that salmon density, at locations where fishermen could f i s h e f f e c t i v e l y , would remain constant regardless of annual changes in salmon abundance. In the fishing locations sampled in the current study, estimated density varied ten f o l d (Table 6). It seems unlikely that similar density fluctuations could not occur on a larger temporal or s p a t i a l scale than the scale sampled during 37 this study. In the present study the results are more analagous to a type III than a type II functional response because type III responses occur when the rate of e f f e c t i v e search, for example area searched by a predator, increases with prey density (Hassel 1978). It is argued in this current study that the search component or volume searched increased with density through the f a c i l i t a t i o n effect in salmon predatory behaviour. F i n a l l y , a l l functional responses e v e n t u a l l y r e a c h a point where increases in prey density do not result in any further increase in rate of prey capture. This plateau or maximum rate of prey capture can be reached when, as discussed by Holling (1965), the time required to handle and process prey consumes a l l the time available for predatory a c t i v i t y . Although such a plateau was c l e a r l y not found during this study, a plateau would be expected to occur when a l l the time spent f i s h i n g was taken up in hooking and ret r i e v i n g f i s h . A l t e r n a t i v e l y , a plateau could also be reached when additional f i s h attracted to the lure per unit time, a/f, could not be hooked because the lure was already occupied by a f i s h . This type of hook saturation was discussed by Rothschild (1964). I observed that i t took approximately f i v e minutes to retrieve a hooked salmon and resume f i s h i n g . Thus, the maximum rate of catch would be twelve f i s h per hour. The average rate of catch experienced during this study (Table 2) was .47 and .44 f i s h per lure hour for test f i s h i n g and anglers, respectively. However, by using the parameter estimates gained from Active Pass and Gowers Point, the standard abundance at which an 38 asymptote would be reached can be calculated as follows: 2 c/f = (a n + bn )P P 1 1 2 therefore; - 4b c/f P P 1 2 2b and substituting the following parameter estimates: a 1 b P 1 P 2 c/f Max* Active Pass .005 .00035 .04 .31 6 Gower Point .0015 -4 .75x10 .04 .31 6 *12 f i s h / l u r e hour = 6 f i s h for .5 hour standard sample. n i s found to be 1167 for Active Pass and 2530 for Gowers Point. That i s , the standard abundance at which an asymptote based on handling time could be reached i s 2.7 and 3.3 times higher than the maximum density found at Active Pass and Gowers Point, respectively (Figs. 12 and 13). These estimates, then, help to put into perspective the density range found in thi s study and the density .required to achieve maximum catch per lure hour in the sport fishery investigated. Of course the factors which can l i m i t catch per unit e f f o r t ( may be operating such as those proposed by Paloheimo and Dickie 39 (1964). Upper l i m i t s on the number of approaching f i s h which can occupy the space near the lure can also l i m i t approach rate (a/f) and therefore catch per e f f o r t . Further work in t h i s area would be required before quantitative predictions about catch success in relation to abundance could be made for the Georgia S t r a i t salmon sport fishery. Variation Between Sampling Location The reason underlying the v a r i a t i o n between locations i s not c l e a r . Error caused by the a b i l i t y of the observer to see approaching salmon has been accounted for by counting only those f i s h which could have been seen in a l l locations, that i s , close encounters only. Error resulting from inaccurate density assessment could have produced the observed differences. For example, i f non-salmon species were present at Gowers Point and not at Active Pass, and i f the re a l encounter rate had been i d e n t i c a l for both locations, then the slope for Gowers Point would appear lower than the slope for Active Pass. Error in i d e n t i f i c a t i o n of targets would lead the observer to include non-salmon, and therefore non-approaching f i s h , as part of the density. However no species other than salmon were observed on video tapes at either Gowers Point or Active Pass. S t i l l , the p o s s i b i l i t y remains that non-salmon species may have been present and gone undetected by video records. If i t i s assumed that density estimates are accurate, then a set of possible b i o l o g i c a l causes for d i f f e r e n t slopes can be examined. F i r s t , the steepness of the slope can represent a measure of fi s h i n g e f f i c i e n c y . E f f i c i e n t gear or lures w i l l "search" or 40 "sweep" a larger volume of water per unit e f f o r t than less e f f i c i e n t gear. Although there are no quantitative measurements available, I observed that the waters at Active Pass had better v i s i b i l i t y than at Gowers Point. From th i s observation i t can be presumed that the salmon at Active Pass could see the lure from a greater distance (a greater volume searched by the gear) than the salmon at Gowers Point, thereby hypothetically producing the higher proportion of f i s h attracted at Active Pass. Further evidence of the effect of water c l a r i t y on approach rate can be seen from S u t i l Point where both v i s i b i l i t y and proportion of f i s h attracted to the lure were the lowest of a l l locations. Second, seasonal differences in salmon behaviour may account for di f f e r e n t slopes. Gowers Point was fished in August 1978 while Active Pass was fished in October 1978. If such factors as aggressiveness of feeding intensity should vary seasonally then this could be r e f l e c t e d in aproach rate. Another p o s s i b i l i t y for d i f f e r e n t slopes i s maturity differences between salmon at d i f f e r e n t locations. It i s commonly observed by anglers that salmon near t h e i r spawning streams are d i f f i c u l t to catch. Gowers Point was l i k e l y composed of mostly Capilano River coho on t h e i r migration to the nearby river in August. Active Pass i s located c e n t r a l l y in Georgia S t r a i t and salmon here are more l i k e l y a further distance from their natal r i v e r s than salmon at Gowers Point. Unfortunately, a measure of gonadal development to indicate the stage of maturity of salmon at different locations was not taken during t h i s study. Size differences (an alternative measure of maturity) were not evident between Gowers Point, S u t i l Point, and Active 41 Pass. Therefore there is no evidence to support the notion that the f i s h at these two locations were of d i f f e r e n t maturities. F i n a l l y , the salmon species composition was not the same at both locations. Gowers Point had only coho salmon in evidence by cre e l survey and video observation while Active Pass had a mixture of spring and coho. It is not unreasonable to assume that chinook and coho would have d i f f e r e n t approach behaviours. The causes underlying the differences in c a t c h a b i l i t y between locations remains unknown. Further studies may show that c a t c h a b i l i t y is s p e c i f i c to any or a l l of species, location, and seasonal considerations. Summary This investigation examined the r e l a t i o n between the catch per unit of e f f o r t of salmon anglers and salmon abundance. The relationship was determined by using simultaneous estimates of .both c/f and salmon abundance. Previous investigations to determine the c a t c h a b i l i t y c o e f f i c i e n t , using h i s t o r i c a l catch records have suggested that c a t c h a b i l i t y varies inversely with abundance. However, many of the d e t a i l s of the interaction between fis h i n g gear and f i s h which result in the observed catch success were not available from catch records. In the present study, dir e c t observation of these d e t a i l s and subsequent analysis suggest that c a t c h a b i l i t y may increase with abundance. The suggested mechanism underlying the observed increase is feeding f a c i l i t a t i o n between salmon as they encounter the f i s h i n g gear. 42 REFERENCES Argue , A . W . , R. H i l b o r n , R . M . Peterman, M . E . S t a l e y , and C . J . W a l t e r s . 1982. S t r a i t of G e o r g i a ch inook and coho f i s h e r y . Can. B u l l . F i s h , and A q u a t . S c i . 211: 91 pp . B r y a n , R . C . 1974. The d imens ions of a s a l t - w a t e r spor t f i s h e r y t r i p or what do people look for in a f i s h i n g t r i p b e s i d e s f i s h ? C a n . Dept . E n v i r o n . , Dept . of F i s h e r i e s and Oceans , P a c i f i c R e g i o n , T e c h . R e p t . , P A C / T - 7 4 - 1 , 35 pp . C u r i o , E . 1976. The E t h o l o g y o f P r e d a t i o n . S p r i n g e r - V e r l a g , New Y o r k . 250 pp. G a r r o d , D . J . 1977. The N o r t h A t l a n t i c c o d . I n : J . A . G u l l a n d ( e d . ) , F i s h P o p u l a t i o n Dynamics . John Wi l ey and Sons , T o r o n t o , p . 216-242. G u l l a n d , J . A . 1964. C a t c h per u n i t e f f o r t as a measure of abundance. I n : J . A . G u l l a n d ( e d . ) , On the Measurement of Abundance of F i s h S t o c k s . Rapports et P r o c e s -Verbaux , C o n s e i l I n t e r n a t i o n a l pour 1 ' E x p l o r a t i o n de l a Mer , 155:152-163. G u l l a n d , J . A . 1977. The a n a l y s i s of data and development of models . I n : J . A . G u l l a n d ( e d . ) , F i s h P o p u l a t i o n Dynamics . John W i l e y and Sons, T o r o n t o , p . 67-95. H a s s e l , M . P . 1978. The Dynamics of Anthropod P r e d a t o r - P r e y Systems. P r i n c e t o n U n i v e r s i t y P r e s s . Monographs i n P o p u l a t i o n B i o l o g y 13, 237 p p . H o l l i n g , C S . 1965. The f u n c t i o n a l response of p r e d a t o r s to prey d e n s i t y and i t s r o l e i n mimicry and p o p u l a t i o n r e g u l a t i o n . Mem. E n t . Soc . Canada 45: 1-60. H o l l i n g , C S . 1966. The f u n c t i o n a l response of i n v e r t e b r a t e p r e d a t o r s to prey d e n s i t y . Mem. E n t . Soc . Canada 48:1-86. 43 Lindgren, B.W. 1976. S t a t i s t i c a l Theory. MacMillan, New York. 614 pp. MacCall, A.D. 1976. Density dependence of c a t c h a b i l i t y c o e f f i c i e n t in the C a l i f o r n i a P a c i f i c sardine, Sardinops saga cal r u l e a , Purse Seine Fishery. C a l i f o r n i a Cooperative Oceanic Fis h e r i e s Investigations Report 18:136-148. Markl, H. 1972. Aggression und Beuteverbalten bei Piranhas (Serrasalminae, Characidae). 2. Tierpsychol. 30: 190-216. Paloheimo, J.E. and L.M. Dickie. 1964. Abundance and f i s h i n g success. In: J.A. Gulland (ed.), On the Measurement of Abundance of Fish Stocks. Rapports et Proces-Verbaux, Conseil International pour 1'Exploration de l a Mer, 155:152-163. Peterman, R.M. and G. Steer. 1981. Relations between sport-fishery c a t c h a b i l i t y c o e f f i c i e n t and salmon abundance. Trans. Amer. Fish. Soc. 110: 585-593. Protasov, V.R. 1970. Vision and Near Orientation of F i s h . U.S. Dept. Interior and Nat. S c i . Found., Washington, D.C. English translation from Russian by I s r a e l Program for S c i e n t i f i c Translation. 175 pp. Ricker, W.E. 1977. The h i s t o r i c a l development. In: J.A. Gulland (ed.), Fish Population Dynamics, John Wiley and Sons, Toronto, p. 1-26. Rothschild, B.J. 1964. Fishing e f f o r t . In: J.A. Gulland (ed.), Fish Population Dynamics, John Wiley and Sons, Toronto, p. 1-26. Shardlow, T. and R. Hilborn. 1983. Comment on "Relation between sport-fishing c a t c h a b i l i t y c o e f f i c i e n t s and salmon abundance". Submitted to Trans. Amer. Fish . Soc. 12 pp. Veronin, L.G. 1957. Vision and Near Orientation of Fish. U.S. Dept. Interior and Nat. S c i . Found., Washington, D.C. English translation from Russian by I s r a e l Program for S c i e n t i f i c Translation. 175 pp. 44 NOTES 1With each increase in time interval the number of samples i s decreased and intervals greater than t h i r t y minutes produce a sample number too small for r e l i a b l e regression. 2The mean depth was ten fathoms at Gowers Point. Therefore, the t o t a l depth was equivalent to the depth of the standard sample. 3'Salmon' refers to coho and chinook salmon only. 45 TABLE 1 C o m p a r i s o n of c a t c h f o r camera and c o n t r o l f i s h i n g l i n e s 1 LOCATION CAMERA CATCH CONTROL P o r l i e r P a s s 1 0 A c t i v e P a s s 6 1 1 Cowichan Bay 1 2 S u t i l P o i n t 9 13 T h e t i s I s l a n d 0 1 Lambert C h a n n e l 0 1 TOTAL X 2 = 2.68, .10 < 17 p < .20 28 1 T h e s e c o m p a r i s o n s a r e o n l y f o r samples where t h e c o n t r o l and camera l i n e were f i s h e d s i m u l t a n e o u s l y . 46 TABLE 2 Comparison of for test fishing and angler catch per e f f o r t (c/f) TEST DIFFERENCE LOCATION FISHING ANGLERS in c/f (d) GOWERS POINT catch** 1 ( .25)* 1 1 (3.6) -3 .35 hrs. fished*** 4 3 PORLIER PASS catch 1 (.11) 6 ( .33) — .22 hrs. fished 9 18 CAPE MUDGE catch 6 (1.1) 35 ( .45) + .65 hrs. fished 5. 5 77 SUTIL POINT catch 1 (.5) 6 ( .92) _ .42 hrs. fished 2 6.i CAPE MUDGE catch 0 (0) 8 ( .29) — .29 hrs. fished 1 27 CAPE MUDGE catch 3 (2.0) 3 ( .20) + 1 .8 hrs. fished 1 . 5 15 CAPE MUDGE catch 2 (1.0) 3 ( . 18) + .82 hrs. fished 2 17 CAPE MUDGE catch 1 (1.0) 12 ( .82) + .18 hrs. fished 1 14.i LAMBERT catch 1 (.16) 7 ( .32) — .16 CHANNEL hrs. fished 6 22 TOTAL CATCH 16 (.47) 101 ( .44) TOTAL HRS > FISHED 34 226 -.11 MEAN DIFFERENCES (d) *CATCH PER EFFORT IN BRACKETS **CATCH = INCLUDES ALL SALMON SPECIES ***HRS. FISHED = LURE HOURS d = -.11 Sd = 1.32 t = /n - l d/Sd = -.23 p .58 (t test from Bernard W. Lindgren S t a t i s t i c a l Theory 1976) 47 TABLE 3 r 2 Values for temporal s t r a t i f i c a t i o n s TIME INTERVAL LOCATION 5 minutes 1 5 minutes 30 minutes 2 r P 2 r P 2 r P GOWERS POINT .42 < .05 .62 < .05 .89 < .05 ACTIVE PASS .32 < .05 .39 < .05 .32 < .05 COWICHAN BAY .16 < .05 .16 < .05 .24 .08 CAPE MUDGE .03 .07 .05 .12 .07 .16 SUTIL POINT .07 .44 .12 .19 .26 .24 TABLE 4 r 2 Values for depth s t r a t i f i c a t i o n TOTAL DEPTH STRATIFIED LOCATION VOLUME VOLUME 2 r P 2 r P GOWERS POINT .89 < .05 .89 < .05 ACTIVE PASS .32 < .05 • 7 1 < .05 COWICHAN BAY .24 .08 .31 < .05 CAPE MUDGE .07 .16 .04 .23 SUTIL POINT .26 .24 .3 .18 49 TABLE 5 Salmon density in association with herring schools LOCATION MEAN SALMON COUNT IN STRATA SHOWING HERRING SCHOOLS MEAN SALMON COUNT IN STRATA SHOWING NO HERRING Fraser Jetty 0 2.6 IONA 2.5 7.5 IONA 28th 1 .5 Po r l i e r Pass 0 .8 Po r l i e r Pass .5 2 Por1ier Pass 1 1 .6 Por l i e r Pass .33 1 .6 Por l i e r Pass 1 1 .75 Por l i e r Pass 1 0 Po r l i e r Pass 6 1 Po r l i e r Pass 0 .33 Por l i e r Pass 3 1 Por l i e r Pass 3 2.6 Gowers Point 3.6 2.6 Gowers Point 6 5.6 Gowers Point 4.5 5 TOTAL 33.43 36.48 50 TABLE 6 Encounter Responses, f i s h stocks, f i s h hooked, and standard abundance by sampling location ENCOUNTERS FISH FISH STANDARD LOCATION TOTAL CLOSE STRIKES HOOKED ABUNDANCE 14 1 4 0 0 385 S u t i l 1 7 17 1 1 889 Point 0 0 0 0 417 5 5 0 0 435 5 4 2 0 176 5 4 1 1 292 6 6 1 0 268 Gowers 13 5 0 0 374 Point 10 7 0 0 298 21 8 0 0 596 48 21 4 0 760 1 7 6 1 0 467 2 2 1 1 50 6 4 0 0 63 22 7 1 1 280 40 18 1 0 427 Active 9 6 0 0 168 Pass 21 13 3 2 232 28 1 1 1 1 332 7 3 1 0 180 46 7 1 0 328 84 20 2 1 427 46 3 1 0 588 29 3 0 0 866 1 4 0 0 0 595 Cowichan 8 2 1 1 383 Bay 2 1 0 0 523 0 0 0 0 571 0 0 0 0 216 1 1 0 0 68 7 0 0 0 433 8 5 0 0 48 2 0 0 0 72 10 8 7 2 100 6 1 1 0 20 4 1 1 1 68 Cape 16 6 6 2 52 Mudge 7 3 0 0 44 5 5 2 0 1 36 0 0 0 0 4 7 4 3 0 16 3 2 1 1 32 5 3 3 1 4 10 1 0 0 108 0 0 0 0 68 FIGURE 1 Diagram of towed video unit V ^ - ^ ^ ^ ^ ^ \ / (7) 12. WASHER I ALUM-II. BALLAST 1 LEAD 10. WASHER 1 NEOPR£N£ 9. R. CABLE MNT. 1 ALUM. a FR. CABLE MNT. 1 u , 7. STOP 1 u 6. MOUNTING PLATE 2 » 5. PIVOT MNT. 2. II 4. BRACE Z 3. DEFLECTOR 2 SHT. ACRYLIC 2. STABILIZER 1 II il 1. PLANER I ii » FISHERIES RESEARCH DRAWN BY:G. PARSONS PART DESCRIPTION REQ'D MATERIAL DESIGNED : T. SHARDLOW CHECKED:^ ^ « \ L . i— DATE: 790514 TRACED or SCALE: /.-s APPROVED: — j " ^r*ut\lhur-^ DRAW'G:/ of 13 52 FIGURE 2 Diagram of towed video unit and f i s h i n g gear assembly J 4 5 c m LEADER FIGURE 3 Echo-gram example Targets found near the bottom were not enumerated as i n groundfish examples shown above. 54 FIGURE 4a Example of echo-gram of salmon targets taken during c a l i b r a t i o n t r i a l s ANAESTHETIZED SALMON SUSPENDED FROM MONOFILAMENT HARNESS AT 3.6m., 12.7m. and 21.8m. TARGETS SHOWN IN CIRCLES 55 FIGURE 4b Example of echo-gram of salmon and dogfish taken during c a l i b r a t i o n t r i a l s F I G U R E 7 9.1 m^\\ G I B S O N S LANDING 36.6 S A M P L E A R E A 2 KM I 1 F I G U R E 10 Chart of Cape Mudge Q U A D R A I S L A N D < O II / I \) \: N V C A P E MUDGE v — \ i \ / I » m S A M P L E A R E A 2 KM 62 FIGURE 11 Comparison of slopes between Gower Point and Active Pass * A c t i v e pass, y=0.0+.037x, p ^ .05 + Gower p o i n t , y=0.0+.019x, p<.05 data from t a b l e 6 FIGURE 12 ' "" . Regression of encounters against standard abundance for Gower Point O CD i ! C D n rv CD n L D \ -n n r~) n a + 0 1GG 200 300 400 500 600 700 800 900 STANDARD ABUNDANCE Y=0+.0015x+.000075x' 2 r =.95 f o r a, p <.05 f o r b, p <.05 data from t a b l e 6 64 FIGURE 13 Regression of encounters against standard abundance for Active Pass to OL o CD CD_ CO o_ 3r o_ in o_ o UJ CNI CD-CD + / / / + / / / / + + / • + 0 1 00 200 300" 400 500 600 700 800 900 ST' .ARD ABUNDANCE y=0-f.005x+.00035x 2 r =.78 f o r a, p < .05 f o r b, p = .06 data from tabl e 6 FIGURE 14 Regression of encounters against standard abundance for Cowichan Bay GO o o o CD cdj o. n C D r~> L D o _ C D , o . 4-s + I- - - E T 0- 100 200 300 400 500 600 700 800 :900 STANDARD ABUNDANCE ! Y=.0071x+.000032x • 2 r =.32 f o r a, p = .06 f o r b, p = .54 data from t a b l e 6 F I G U R E 15 66 Regression of encounters against standard abundance for Cape Mudge 0 1 00 200 300 400 500 600 700 800 900 STANDARD ABUNDANCE data from t a b l e 6 67 FIGURE 16 Regression of encounters against standard, abundance for S u t i l Point O CO O 00 o c_> OO r~> r\ r~> CO n m n <^ n O J 0 100 200 300 400 500 600 700 800 900 STANDARD ABUNDANCE data from t a b l e 6 68 FIGURE 17 Regression of proportion of encounters against standard abundance for Gower Point CO O I ; — « rx o — STANDARD ABUNDANCE Y=0+. 000079X f o r b, p <.05 data from t a b l e 6 69 FIGURE 18 Regression of proportion of encounters against standard abundance for Active Pass CM STANDARD ABUNDANCE Y=.00033x r 2=.43 data from t a b l e 6 70 FIGURE 19 Graph of number of s t r i k e s versus number of close encounters O CN C O . C O . «—I CN. 00 LU , > »—< i cc — » _ , 1 C O " «=c i— o CO" C N -O G~ 20 © 3 © 2 40 • • 4 60 80 10 TOTAL CLOSE ENCOUNTERS l=Cowichan Bay Y=0.0+.14x 2=Sutil Point r 2=.33 3=CampbelT River 4=Gower Point 5=Active Pass FIGURE 20 Graph of number of f i s h hooked versus number of st r i k e s o r _ : : TOTAL STRIKES l=Cowichan bay 2=Suti1 point 3=Cape mudge 4=Gower point 5=Active pass Y=0.0+.314x 2 _ FIGURE 21 Number of close encounters against standard abundance at a l l locations L D C N C3_ OJ L O -irr + + ++ + + - + + + H- + ++ + + + * * + + + + i I - - r • • ( • — f -G 1G0 2G0 3G0 4G0 5G0 6G0 7 0 0 8G0 9 G 0 STANDARD ABUNDANCE to 

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