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Increased predation by Juvenile Sockeye Salmon (Oncorhynchus Nerka Walbaum) relative to changes in Macrozooplankton… Rankin, David Paul 1977

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INCREASED PREDATION BY JUVENILE SOCKEYE SALMON (ONCORHYNCHUS NERKA WALBAUM) RELATIVE TO CHANGES IN MACROZOOPLANKTON ABUNDANCE IN BABINE LAKE, BRITISH COLUMBIA ty DAVID PAUL RANKIN B.Sc, University of Victoria, 1973 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA February, 1977 © David Paul Rankin In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced deg ree at 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 ha t the 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 a g r e e 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 the Head o f my Depar tment o r by h i s 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 not 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 . Depar tment o f ^-^otp/l06a. s/ 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 Place Vancouver, Canada V6T 1W5 i ABSTRACT A two year study was initiated in 1973 to examine effects of substantial (3.8 fold; from a 1962-66 mean of 39 million to about 150 million in 1973 and 197*0 increases in sockeye (Oncorhynchus nerka) Walbaum) fry numbers on zooplankton abundance in Babine Lake. Several lake areas and stationsware sampled for zooplankton bimonthly from May to October during 1973 and 197*+ and compared to data gathered between 1958 and 1962 prior to a large scale enhancement program for sockeye stocks. Zoo-plankton biomass had decreased up to 70$ in some areas of the lake during 1973i but only kOf0 in 197^ . Decreases in numbers were also evident. Although seasonal changes in fry diet followed changes in zooplankton species abundance, feeding was selective. The less abundant but larger forms, Daphnia and Heterocope together comprised 70?5 of the diet during summer, while Cyclops and Diaptomus formed the bulk (87$) of the diet in late f a l l . Significant decreases in Daphnia and Diaptomus abundance and increases in nauplii-early copepodite abundance had occurred by 1973. The increased 197^  zooplankton abundance relative to 1973 was attributed to decreased mid-summer fry numbers in the lake. Field data suggested low Diaptomus numbers contributed to much higher fry mortality (about double in 197*0 compared to 1973. An experimental study of species selectivity by sockeye fry indicated that they selected Cyclops and Diaptomus adults. The larger copepods, Heterocope and Epischura, were rejected by fry encountering zoo-plankton for the first time. Copepodites and nauplii were rejected, but less so when preferred prey were scarce. Prey activity, in my experiments, could not be used to predict predation vulnerability and hence the species i i selectivity displayed by the fry. Light and temperature had l i t t l e effect on Cyclops, Dlaptomus and Heterocope activity. i i i . i TABLE OF CONTENTS Page Abstract • • i Table of Contents • • i i i List of Figures »v List of Tables i x Acknowledgements x I. Introduction 1 II. Study Area.. 4 A. Babine Lake. 4 B. Spawning Channel Development , 4 C. Sockeye Fry Distribution in Babine Lake......... ..10 III. Materials and Methods 13 A. Lake Program..... 13 Zooplankton Sampling, 1973 13 Zooplankton Sampling, 1974 13 Laboratory Analysis. .13 Analysis of 1973 Stomach Samples 14 Data Analysis. .15 Comparison of Pre- and Postenhancement Data 15 Estimating Heterocope Abundance 17 B. Laboratory Experiments ,18 Electivity 18 Prey Activity i .20 Prey Susceptibility , 23 IV. Results , ,, ,, 24 A. Lake Program 24 Changes in Average Zooplankton Abundance Between Years.24 Iv. Biomass and Numbers. .24 Species Abundance 24 Seasonal Changes in Zooplankton Abundance 34 Biomass and Numbers .34 Taxa. «•.«.............. .....»•-. 41 Zooplankton Composition in Sockeye Fry Diets . . . . 5 0 B. Laboratory Experiments. 5? Size Range of Available Prey ..57 Electivity 57 Prey Activity and Susceptibility.. .........66 Activity 66 Susceptibility .66 V. Discussion ....71 A. Lake Program 71 Effects of Fry Predatlon on Zooplankton Abundance .71 Other Causes of Reduced Zooplankton Abundance...........73 Fry Selectivity in Babine Lake 77 Changes in Zooplankton Species Abundance .79 B. Laboratory Experiments..... 80 Electivity 81 Prey Activity and Susceptibility. 82 VI. Conclusions ...85 VII. Management Implications ..87 VIII. Literature Cited 88 IX. Appendices. 92 X. A. Relative Catching Efficiencies of Clarke-Bumpus and Miller Samplers 92 v. LIST OF FIGURES Figure Pag© 1. Babine Lake showing sampling stations and areas....................5 2. Estimated i n i t i a l main arm sockeye fry populations (1962-1976) and resultant smolt output in millions 8 3. Fry distribution within the Main Arm of Babine Lake, based on McDonald (1969). Area locations given in Figure 1....... 11 4. Observation apparatus used to determine activity rates and conduct predation experiments... 21 5. The prey seizing apparatus (a modified Pasteur pipette) used in zooplankton predation experiments... 21 6. Average seasonal zooplankton biomass and numbers in Babine Lake Areas 1-5 during 1958-'62 and 1973-,7i* (the average represents the area under a seasonal abundance curve divided by the length of the sampling season in days). Vertical bars represent 2 S.E. .......25 7. Average seasonal zooplankton biomass and numbers in Babine Lake Areas 1-5 during 1958-'62, 1973 and 1974. Vertical bars represent 2 S.E. ,, .....27 8a. Average seasonal Cyclops, Diaptomus and Daphnia abundance in Babine Lake Areas 1-5. Vertical bars represent 2 S.E 29 8b. Average seasonal nauplli-early copepodite Heterocope and Bosmlna abundance in Babine Lake Areas 1-5. Vertical bars represent 2 S.E. .,31 v i . 9. Seasonal changes in mean zooplankton biomass in Babine Lake Areas 1, 2, 4 and 5 during i958-'62 and 1973-'74. Vertical bars represent 2 S.E 35 10. Seasonal changes in zooplankton biomass in Babine Lake Areas 2 and 4 during 1958, '60, '62, * 73 and '74 37 11. Seasonal changes in zooplankton numbers in Babine Lake Areas 2 and 4 during 1958, '60, '62, '73 and '74 39 12a. Seasonal changes in Cyclops, Diaptomus and Heterocope numbers in Babine Lake Area 2 during 1958, '60, '62, '73 and '74 42 12b. Seasonal changes in Cyclops, Diaptomus and Heterocope numbers in Babine Lake Area 4 during 1958, '60, '62, '73 and '74 44 13a. Seasonal changes in nauplii-early copepodite, Bosmlna and Daphnia numbers in Babine Lake Area 2 during 1958, '60, •62, '73 and '74 ...46 13b. Seasonal changes in nauplii-early copepodite, Bosmlna and Daphnia numbers in Babine Lake Area 4 during 1958, '60, •62, '73 and 74 48 14. Zooplankton composition of sockeye fry diets in late August and late September of 1967 and 1973 (sample size in brackets). The fry were caught in the Main Arm of Babine Lake , 51 15. Seasonal changes in zooplankton composition of sockeye fry diets in the Main Arm of Babine Lake during 1967 (McDonald 1973)» Carats and numbers indicate sampling dates and sample sizes respectively 53 v i i . 16. The zooplankton composition of sockeye fry diets in different regions of the Main Arm of Babine Lake during late August - early September and late September -early October of 1973 (sample size in brackets).. 55 17. Size frequency distribution of zooplankton species encountered by fry during electivity experiments (n = 40),. 58 18. Mean total zooplankton density at different zooplankton density indices, G =» 1,2 and 4,5 (see methods). Vertical bars represent 95% confidence limits.....,,,.. , 60 19. Electivity values of zooplankton encountered by fed fish at different zooplankton densities. The horizontal scale represents a zooplankton density index increasing from 1 to 5 (see Fig. 18) . . . , 6 2 20. Electivity values of zooplankton encountered by starved fish at different zooplankton densities. The horizontal scale represents a zooplankton density index increasing from 1 to 5 (see Fig. 18) .64 21. Gopepod activity rates at different light intensities and temperatures. Horizontal bars represent 95% confidence limits of log transformed data. Vertical bars indicate range. 22. Zooplankton susceptibility to " a r t i f i c i a l " predation. Horizontal bars represent 95% confidence limits; vertical bars, the range , , v i i i , —2 23. Relationship between zooplankton biomass (mg. dry wt, m. day ~ ) and primary productivity (mg, G, m. , day ) in several oligotrophic sockeye producing lakes in British Columbia and Alaska (Great Central Lake before and after fertilization; Owikeno Lake (Narver 1969) adjusted according to Stockner and Shortreed (1974); five areas of Babine Lake's Main Arm in 1973) 75 24. Regression of vertical haul biomass (mg. dry wt. m~^ ) on 5 meter-strata biomass (mg. dry wt. m*"^) in 1973......... 93 25. Regression of vertical haul biomass (mg. dry wt. m~^ ) on 0-5 meter oblique haul biomass (mg. dry wt. m~^ ) in 1974........95 26. Regression of biomass estimates (mg. dry wt. m~^ ) of a Clarke-Bumpus sampler on those of a Miller sampler in 1974 98 27. Regression of Clarke-Bumpus catches (no. L ) on those of the Miller sampler,,... , 100 ix. LIST OF TABLES Table Page 1. Volume contributed by individual Babine Lake macro-zooplankton relative to Diaptomus ashlandi being assigned a unit volume of 1 ....16 X. ACKNOWLEDGMENTS I would like to thank my supervisor, Dr. T. G. Northcote for his assistance and helpful comments during preparation of the thesis, Messrs. F. Jordan, J. Martel, I. Miki and J. Weir of Fisheries and Marine Service provided logistical support and aided in sample collec-tion. Their help was greatly appreciated. The accomodation and support supplied by Messrs. R. M. Ginetz and I. McClean of the Fisheries and Marine Service at Fulton River was greatly appreciated. I would particularly like to thank Mr. A, Facchin for his assis-tance in the field and for processing most of the samples. Mr. S. Borden and Mrs. D. Lauriente of the Biology Data Centre (U.B.C.) provided expert advice on computer programming, I am indebted to Mr, H. D. Smith and Dr. J. G. Stockner for their encouragement, suggestions and criticisms during the study and preparation of the thesis. I would also like to thank my parents for their support during my years in university. This work was supported by a Fisheries and Marine Service grant (65-1621) to Dr. T. G. Northcote... 1. INTRODUCTION Recent studies have shown that both invertebrate and vertebrate predation can alter characteristics of fresh water macrozooplankton communi-ties (Brooks and Dodson 1965; Dodson 1970; Hall 1964; Warshaw 1972; Wells 1970). Invertebrate!predators such as Chaoborus, Diaptomus and Leptodora may reduce zooplankton prey densities, alter size frequency distributions or effect species composition in small lakes through size-selective predation (Sprules 1972). Fish and other aquatic vertebrates (such as salamanders) produce similar but more dramatic results in small lakes (Hutchinson 1971). Analogous changes have been observed in large fresh water lakes. Wells (1970) reported changes in the Lake Michigan zooplankton community resulting from size-selective predation by increased numbers of the alewife, Alosa  pseudoharengus (Wilson). Such increases in abundance of a single predator species makes i t relatively easy to account for changes in the prey community. However, interpretation of results may also be confused by inadequate knowledge of fish distribution and feeding (Northcote and Clarotto 1975), effects of competitors or subtle Interactions within the prey community. Increases in planktdvorous sockeye salmon fry (Oncorhynchus nerka Walbaum) have occurred in Babine Lake as a result of a salmon enhancement program (Dill 1968). The annual i n i t i a l fry population was increased from a 1962-66 mean of 39 million to 150 million in 1974. Young sockeye in Babine Lake reside in the pelagic zone for one year where they feed on a variety of zooplankton, chiefly Bosmlna coregoni (Baird), Cyclops scutlfer (Sars), Daphnia longispina (Leydig), Diaptomus ashlandi (Marsh) and Hetero-cope septentrionalis (juday and Muttkowskl) (Johnson 1961; McDonald 1969; Narver 1970). 2. Babine Lake produces up to 90% of the Skeena River sockeye run which is the second largest in British Columbia. Because sockeye stocks were declining (Shepard and Withler 1968), researchers sought ways of bolstering production. Johnson (MS 1965a) hypothesized that Babine Lake's main arm could support up to four times the average annual numbers of fry existing there during the period 1956-63. Johnson's basic assumptions are para-phrased as follows: 1. Spawning ground availability limited main arm sockeye production. 2. Sockeye fry did not disperse widely but remained in basins adjacent to their natal streams. 3. Sockeye fry densities were five times higher and zooplankton biomass much lower in the North Arm - Nilkitkwa Lake area than those in the main arm, yet no significant differences in fish size existed between these regions. Based on Johnson's work, the Federal Government built three spawning channels to increase spawning ground area. However, fry appeared to feed selectively (Narver 1970) and contrary to the second assumption, moved out of basins adjacent to their natal streams (McDonald 1969). Concentrated predation by fry in certain areas led to a concern that the main arm could not support the increased fry populations. Food limitations could have several effects: increased mortality, decreased size of seaward migrants or increased numbers of two-year migrants (Foerster 1954). Since there appears to be a positive relationship between seaward migrant size and their ocean survival (Johnson 1965c) numbers of returning adults available to the Skeena River fishery could be seriously reduced. Thus i t became necessary to gain an improved estimate of the lake's carrying capacity for 3. juvenile sockeye. To examine possible impacts of sockeye salmon enhance-ment on Babine Lake zooplankton and when or where food availability might limit production, I compared zooplankton biomass, numbers and species composition during pre- and postenhancement periods (1958-62, 1973-74 respectively). If there has been an effect, decreases in zooplankton biomass and species composition changes, related to fry feeding behaviour, might be expected in areas where predation was concentrated. Fry feeding behaviour was examined experimentally for two reasons. First, l i t t l e is known about early phases of fry feeding. Second, Increased fry numbers may induce changes in zooplankton density and in species composi-tion through selective feeding. Therefore experiments were designed to determine how feeding selectivity might vary with changes in zooplankton density and composition. Predator selectivity has been shown to depend on prey density and composition (Chizar and Hindell 1973; Ivlev 1961). As preferred prey density increases selectivity decreases and avoidance of non-preferred forms increases. Selective feeding may also depend on prey activity rates (Czaplicki and Porter 1974; Herzog and Burghardt 1974; Ivlev 196l) and more active prey may be more or less susceptible to predation. There were two objectives to this work} 1. To determine what effect increased sockeye fry numbers had on regional zooplankton abundance and species composition in Babine Lake. 2. To examine experimentally aspects of fry feeding behaviour which might increase the impact of predation on zooplankton. Fry feeding patterns selectivity experiments were 4. compared with those formed in the lake and effects of zooplankton density, size and activity on selectivity were examined, STUDY AREA Babine Lake Babine Lake (Fig, l ) , elevation 711 m,is a large (surface area 491 km ) oligotrophlc lake located about 159 km northwest of Prince George, British Columbia. The lake is divided into three regions based on morphometry, duration of ice cover and productivity. The North Arm and Morrison Arm, which are quite shallow (mean depth 18,7 m and 11.4 m respectively), are ice covered for approxiamtely 6 weeks longer than the rest of the lake. Lower primary productivity occurs in these narrow sheltered regions because of earlier stratification and shallower (< 6 m) mixed layer depth (Stockner and Shortreed 1974). Zooplankton densities are also lower, perhaps a result of higher concentrations of juvenile sockeye (Johnson 196l), lower primary production or both. The Main Arm of the lake (mean depth 68 m) receives more wind induced mixing and upwelling. Stratification occurs later and the mixed layer is much deeper (15-20 m) resulting in increased but regionally variable primary productivity (Stockner and Shortreed 1974), This in turn leads to regional disparities in zooplankton densities. On the basis of sockeye fry distribution (McDonald 1969) the main arm has been subdivided into five areas (Fig. l ) . Spawning Channel Development Three spawning channels were constructed on two tributaries to the main arm (two at Fulton River, one at Pinkut Creek, Fig. l ) . These 5. Figure 1. Babine Lake showing sampling stations and areas. 6. 7. channels were expected to accomodate approximately 240,000 spawning adults and subsequently produce 125 million fry (Dill 1968). The fir s t channel produced fry in 1966 and a l l three were operating by 1968. By 1973 the i n i t i a l fry population had increased to four times pre-1966 levels (Fig. 2). Smolt production, accompanying the increase in fry numbers, rose to approximately 80 million in 1973-74, but declined markedly in 1975 (Fig. 2). 8. Figure 2. Estimated i n i t i a l main arm sockeye fry populations (1962-1976) and resultant smolt output in millions. 9. 10. Sockeye Pry Distribution in Babine Lake Upon entering the lake from Fulton River and Pinkut Greek in the spring of 1966, sockeye fry migrated south (McDonald 1969). By the end of June approximately 70% of the i n i t i a l fry population was concentrated in Areas 4 and 5 (Fig. 1 and 3). This was followed by a period of north-ward dispersal. The fry were distributed evenly between Areas 2, 3 and 4 by the middle of August, but by October most were in Area 2 (Fig. 3). Their distribution patterns from early November until the following May when the majority migrate to sea are.;poorly understood. Although year to year variability is to be expected, patterns observed in 1966 are assumed to represent a general trend. During the summer and early f a l l the fry undergo diel vertical movements, in addition to horizontal migrations (Narver 1970), the former becoming more pronounced as the lake stratifies. Fry ascend to the surface at dusk from below the thermocline (35-55 m in the Main Arm) to feed (McDonald 1973; Narver 1970). This is followed by a night descent to about 12 m and predawn ascent to feed before moving to deeper daytime depths. There is l i t t l e or no feeding by the fish during the day. 11. Figure 3. Fry distribution within the Main Arm of Babine Lake, based on McDonald (1969). Area locations given in Figure 1. Time P e r i o d 1 June 25 - July 27 Time Per iod 2 Aug. 16 - Sept . 9 Time P e r i o d 3 O c t . 6 - Oct . 25 Year 13. MATERIALS AND METHODS LAKE PROGRAM Zooplankton Sampling, 1973 Samples were collected bimonthly (May-October) from ten depths (0.5, 1.5, 7.5, 12.5. 18.0, 22.0, 26 .0, 31.0, 36.0, 42.0m) at each of several stations (see Fig. 1) along the main axis of the lake using modified Miller samplers, with #10 (153yW ) mesh nets, towed at 1.5 m/sec. The samplers with mouth diameters of 12 cm, were enlarged versions of that described by Miller (1961). Sampling depths were confirmed using a Furuno FUG 400W echo sounder. The barge and equipment used in sample collection was described by Anderson and Narver (1968). Tow length varied between 3 and 6 minutes depending on zooplankton and algae abundance (later in the season nets became rapidly clogged with algae, predominantly Tabellaria fenestrata Kiitz). In addition to the ten horizontal strata sampled a vertical haul was taken from a depth of 40 m or the lake bottom i f the lake was shallower. Three percent formalin was used to preserve samples. Zooplankton Sampling, 1974 Samples were collected at 19 stations southeast of Fulton River (see Fig. l ) . Sampling technique differed from that of 1973 in that an oblique haul from the surface to 5 m (most of the zooplankton was in the upper gimeters) replaced horizontal tows. The depth of the sampler was increased by 1 m increments while being towed at 1.5 m/sec. Two length varied between 3 and 6 minutes. Sucrose was added to the formalin to prevent body distortion of cladocerans (Haney and Hall 1972). Laboratory Analysis Each sample was split several times in a Folsom plankton splitter. One half of the sample was used :for zooplankton dry weight determination and the remainder was subdivided (generally between a 1/128 and a 1/256 split) until the sub-sample contained sufficient plankters to be counted in 20 minutes (generally about 500 - 600 organisms). The following dominant types were enumerated: Bosmina coregoni  Cyclops scutifer*, Daphnla longispina, Dlaptomus ashlandi*, Epischura  nevadensls (Lilljeborg), Heterocope septentrionalis, Holopediuia gibberum (Zaddach) and copepod nauplii. A more complete species l i s t of Babine Lake zooplankton is provided by Johnson (1965b). The fractions used for dry weight were filtered onto pre-ashed, pre-weighed 4.25 cm Reeve-Engel GFC filters and dried at 65°C for 48 hr, then weighed on a Mettler H 18 balance. Each sample was then ashed at 600°C for four hours. Ash free dry weights were calculated using Johnson's (1964) formula. Analysis of 1973 Stomach Samples Juvenile sockeye salmon (samples available, were taken during August - October in Areas 1-5) were supplied by J. G. McDonald of the Pacific Biological Station and had been preserved in 10$ formalin for approximately 14 months. The wet weight and fork length of each fish was recorded prior to dissection. Stomachs removed from the fish were divided into cardiac and pyloric sections at the major fold. Each section was weighed intact, then weighed again with contents removed to determine wet weight of the latter. The contents were immersed in 37$ isopropyl alcohol for 24 hrs. to break up binding mucoid material. Zooplankton 1. Adults and copepodites counted separately. 15. types were enumerated as previously described and total volume of each estimated using a technique modified from Hellawell and Abel (1971). A grid was placed under the glass slide and the number of squares counted instead of using a micro projector. Percent composition by volume was calculated using the volume estimates given in Table I. Mean percent abundance of each zooplankton taxon was determined after an angular trans-formation to achieve normal distribution of the data (Sokal and Rolf 1969). In order to compare diets during pre- and postenhancement periods the results of a 1967 fry stomach analysis (McDonald 1973) are presented along with the 1973 fry stomach analysis. Data Analysis Comparison of Pre- and Postenhancement Data Clarke-Bumpus and Miller samplers, used during study periods 1958-'62 and 1973-'7*+ respectively, were compared to determine their relative sampling efficiency (Appendix A). A conversion factor, applied to the 1973_,74 biomass data, allowed comparison of biomass levels during the two periods. Changes in zooplankton abundance were determined, in part, by comparing integrated areas under seasonal biomass curves, averaged over several stations in each region during the pre- and postenhancement study periods (l958-'62, 1973-'74). Seasonal total zooplankton and species abun-dance curves were treated in a similar fashion. In addition, 1958-'62 values represent five year means since the among year variability was needed to make the comparisons. Areas divided by sampling season length (days), which varied from year to year, gave estimates of average abundance per day. Each zooplankton type was graphed separately and ordinate scales were varied depending on abundance levels. 16. Table I. Volume contributed by individual Babine Lake macrozooplankton relative to Dlaptomus ashlandi being assigned a unit volume of 1. Species Relative Relative Volume (1) Volume (2) Bosmina coregoni 1.5 1.4 Cyclops scutifer 1.5 .7 Daphnia longisplna 2.5 2.5 Dlaptomus ashlandi 1.0 1.0 Epischura nevadensis - 4.1 Heterocope septentrionalis 12.0 16.2 Holopedium gibberum - 7.0 (1) from Narver (1969) (2) Calculated from 1973 zooplankton and stomach samples using a method adapted from Hellawell and Abel (1971). 17. Differences ln analytical technique between study periods necessi-tated combining the 1973-74 Cyclops and Diaptomus adult and copepodite counts prior to comparing data of these periods. In addition, copepodite counts for these two genera were combined with copopod nauplii counts. Seasonal abundance patterns in 1958. I960 and 1962, representing years of high and low abundance, in Areas 2 and 4 were compared to patterns observed in 1973 and 1974. Areas 2 and 4 were sampled in both study periods and patterns were generally similar to those in Areas 1 and 5 respectively (Johnson 1965b), although densities in these regions were lower than in Area 2 and 4. Seasonal abundance patterns compared to seasonal changes in sockeye fry diets in 1967 and 1973t gave a rough measure of feeding selectivity. Estimating Heterocope Abundance Daytime samples from the surface were not representative of the food complex encountered by fry at night. One of the major food species, Heterocope migrates from a daytime depth of 20 - 30 m to the surface at dusk (Narver 1970). Thus a technique was required whereby abundance of Heterocope sampled during the day could be expressed in terms of the abun-dance of other zooplankton in the upper 5 m at night. Since sampling techniques varied between study periods, methods of estimating Heterocope abundance also had to be modified. During the 1958-'62 period zooplankton were caught during the day by a series of oblique hauls as deep as 80 m (Johnson 1965b). Heterocope concentrations were estimated by summing the concentrations from oblique hauls taken between 10 and 80 m at each station. Heterocope percent abundance was then calculated by comparing summed concentrations to that of other 18. zooplankton in the upper five meters. In 1973-74 Heterocope abundance was determined by applying 40 m vertical haul concentrations to estimates of total abundance obtained from 0-5 meter oblique hauls in 1974. Since no oblique hauls were taken ln 1973» i t was necessary to assess abundance in the 0-5 mtstrata by some other means. Total abundance calculated from 40 m vertical hauls (1974) was regressed on total abundance estimated by 0-5 m tows taken in 1974, giving the following relationship: Y - .50(X) + 1.14, r - .93, n - 50 where Y • vertical haul estimate of total abundance (no. l i t e r " 1 ) X » oblique haul estimate of total abundance (no. l i t e r " 1 ) Vertical haul abundance estimates (1973) were then converted to oblique haul estimates and 1973 Heterocope relative abundance calculated based on derived estimates. LABORATORY EXPERIMENTS Electivity A random sample of the 1974 Fulton River sockeye fry (mean weight » .15 gmf mean length =» .30 cm) run was subdivided into two groups. Group 1 fish, kept in filtered (5 ^ Aquapure filter) Fulton River water, were fed frozen Babine Lake zooplankton every 24 hours and were referred to as fed fish. Group 2 fish, kept in unfiltered Fulton River water, fed on material entering from the river. The amount of food, of a size suitable for fry to feed on, entering Group 2 tanks was negligible. These fry were referred to as starved fish. 19. Prior to the start of an experiment (at approximately 2030 hrs PST) several 19 l i t e r tanks were f i l l e d with filtered river water. One fish (either fed or starved) was placed in each tank and allowed to acclima-tize for two hours. Fed fish had been without food for 24 hours and starved fish had empty stomachs when the experiments were started based on stomach analysis of control fish. During the experiments fry were unable to see each other and were fed either littoral or limnetic plankton. The two plankton groups were chosen for the following reasons. First, young sockeye fry appeared to spend some time in the littoral zone of the lake (McDonald 1969) and second, i t was thought that the litt o r a l species composition might be different from that in the limnetic zone. A sample of either limnetic or litt o r a l plankton was inspected to determine species composition. Samples, approximating Diaptomus concentrations of 20, 50, 75$ 100 or 500 per l i t e r (represented by the zooplankton density index, 1-5), were then added to each tank. These produced mean total zooplankton concentrations ranging from about 100 to 6,200 per li t e r . Diaptomus was chosen as a zooplankton density Indicator because of its abundance, conspicuousness and importance as a food organism in the lake. Two tanks containing fish but no food were used as controls. The fish were allowed to feed for 15 minutes then killed and preserved in 10% formalin. In total, 20 combinations of fish group (fed or starved), zooplankton group (limnetic or littoral) and food density (c 1T5) were used, each replicated three times. Specimens of each zooplankton species were measured, using an ocular micrometer on a dissecting microscope, to determine length frequency distributions. Gopepods were measured from the rostrum to the distal ends 20. of the caudal setae and cladocerans from the head to the distal end of the posterior spine(s). Experimental fish were weighed (mean wt, - .15 g"0 and, stomach contents analysed as outlined above except that only the cardiac: section of the stomachs were examined. Zooplankton volume corrections (see column 3, Table i ) were then applied to the counts. Electivity coefficients (ivlev 1961) were calculated for each prey genus from mean values of replicates (angular transformation) ln each experimental combination . Values of g range from +1 (denoting positive selection) through 0 (no selection) to - 1 (negative selectivity or avoidance). Data analysis was facilitated by combining the results involving the limnetic and litt o r a l zooplankton groups. This was done because the two plankton groups differed l i t t l e in species composition, and recent evidence suggested most fry spent l i t t l e time in the litt o r a l zone (K. Simpson personal communication). In addition only the feeding results involving the significantly different low (C » 1,2) and high (C = 4,5) zooplankton densities were analized. Prey Activity Live Cyclops, Dlaptomus and Heterocope were placed in separate 250 ml beakers and acclimated at 4, 8 and l6°C for four hours. They were then placed in separate petri dishes under an observation apparatus and observed through a magnifier (Fig. 4) . The number of "starts" (commencement after cessation of swimming activity) per minute was recorded for ten 2. Electivity Index (E) -where r is the proportion of a particular prey type in the diet p is the portion of a particular prey type in the environment 21. Figure 4. Observation apparatus used to determine activity rates and conduct predatlon experiments. Figure 5. The prey seizing apparatus (a modified Pasteur pipette) used in zooplankton predatlon experiments. 22. Reducer (Dis =1.1 cm) B u l b Volume * 2 m l Bulb Volume wi th S p h e r e - U r n f 10:4mm (Heterocope 10 :1.2mm. fcyclop^ Diaptomus) 23. i n d i v i d u a l s of each species a t the three d i f f e r e n t acclimation temperatures and three l i g h t i n t e n s i t i e s (220, 350 and 560 lux- 3). Prey S u s c e p t i b i l i t y Zooplankton used i n predation experiments were sorted i n t o f i v e 4 4 groups: C. s c u t l f e r , D. ashlandi , and H. s e p t e n t r i o n a l i s . Samples were acclimated i n darkness a t l6°G f o r four hours. Each species was then placed separately i n an observation apparatus ( F i g . 4) at l6°G and 5&0 lux, and the time I required, a c t i n g as a l i g h t adapted "experienced" predator, to remove ten i n d i v i d u a l s with a modified pipe t t e ( F i g . 5) was recorded. Each experiment was r e p l i c a t e d ten times f o r each species. The sequence of experiments was chosen a t random to minimize the e f f e c t of immediately previous experience a t catching that species i n determining my subsequent success as a predator. The dimensions of prey s e i z i n g apparatus, a c l e a r ' Pasteur pipette, are given i n FigineS. The pipe t t e mouth diameter was adjusted depending on prey s i z e (1.2 mm f o r Cyclops and Diaptomus, 4 mm f o r Heterocope). Suction bulb volume was reduced from 2 ml to 1.3 ml to lower the chances of capturing prey not near the pipe t t e mouth. 3. Approximate l i g h t i n t e n s i t y was measured with a Photovolt Corp. F200 meter s e n s i t i v e between 300 and 650 with a peak at 375. 4. With and without eggs. 24. RESULTS LAKE PROGRAM Changes in Average Zooplankton Abundance Between Years Biomass and Numbers Average biomass levels, calculated from integrated areas under seasonal biomass curves (see methods), were significantly lower in 1973^ ?4 than 1958-162 (Pig. 6)^ The most significant decreases 70%) were observed i n Areas 1, 2 and 4. Average numbers were also lower in 1973-*74 than 1958-'62 (except i n Area 5) (Pig. 6) . The 1973 and 1974 biomass and numbers considered separately, differed i n a similar manner from 1958-'62 (Fig. 7). Biomass levels i n 1973 were considerably lower (Ci60%) than i n 1958-'62;(and 1974), particularly i n Areas 1, 2 and 5. Numbers were also lower in 1973 (Fig. 7). In 1974 biomass levels were significantly lower than in 1958-'62 (Fig. 7). Decreased biomass (*2k0%) was most evident in Areas 2 and 4. Although generally lower, numbers in 1974 were not s i g n i f i -cantly different from 1958-'62, Species Abundance The abundance of Cyclops, Daphnia, Diaptomus, Heterocope and nauplii-copepodites of a l l stages differed most between study periods (Fig. 8a, b). Cyclops abundance was significantly lower i n Areas 2'and 4 during 1973 than i n 1958-'62. There was l i t t l e difference between 1974 levels and those of 1958-'62 and the trend for Cyclops numbers to increase from north to south observed i n 1958-62 was also more apparent i n 1974 than 1973 (Fig. 8at). 5. Two years separated by a dash Indicate data averaged over more than one season (year). Data points with non-overlapping confidence intervals were considered significantly different. 25. Figure 6. Average seasonal zooplankton biomass and numbers in Babine Lake Areas 1-5 during 1958-'62 and 1973-'74 (the average represents the area under a seasonal abundance curve divided by the length of the sampling season in days). Vertical bars represent 2. S.E. 125-r 1 0 0 -7 5 -5 0 -2 5 -o 50-40-30-20-10-0 o i 2 3 4 Lake Areas (Fig.l) 1 9 5 8 - 6 2 (•) 1973-74 (O) 27. Figure 7. Average seasonal zooplankton biomass and numbers in Babine Lake Areas 1-5 during 1958-'62, 1973 and 1974. Vertical bars represent 2. S.E. 125 1 0 0 -7 5 -50 - -2 5 -i A _ l _ A I O A i 50 r 4 0 -3 0 -2 0 -1 0 -0 -o A 5 2 3 4 Lake A r e a s (Fig.1) 1 9 5 8 - 6 2 , 1973 1974 ( A ) (o) 29. Figure 8a. Average seasonal Cyclops. Diaptomus and Daphnia abundance in Babine Lake Areas 1-5. Vertical bars represent 2 S.E. Cyclops scutifer 30. 3 0 T 2 0 I 10 f I . i - i i i i i i —J i i_ -J L l _ _ l 1 L. . . o E 3 0 - r 2 0 Diaptomus ashlandi 10+ * i _ l I L_ 4-r 2 f Daphnia longispina i • J i i_ J i _i i i_ Lake Areas 1958- 62 (•) 1973 (A) 1974 (o) 31. Figure 8b. Average seasonal nauplii-early copepodite, Heterocope and Bosmlna abundance in Babine Lake Areas 1-5. Vertical bars represent 2 S.E. 10 + 5 + Naup l i i copepodites _ l I I I I I 1 1 L . T o 1 - 1 — I — « -32. E Z3 .15-.10 .06 Heterocope septentrionalis 1 i 5 _l__k I I I I 1 L . • _ J 1_ A 9 - j — i — 3 + B o s m i n o c o r e g o n i Q • j i i_ 5 Lake Areas 1958 - 62(») 1973 (A) 1974 (O) 33. Diaptomus, which was more (70$ abundant than Cyclops in 1958-'62, was reduced to a greater (40%) degree than Cyclops in 1973, particularly ln Areas 1, 2 and 4 (Fig. 8a). Average 1973 Diaptomus abundance in most areas was only slightly higher than Cyclops. Diaptomus increased in 1974 and observed values were not significantly different from those in 1958-'62. As with Cyclops, the trend for numbers to increase from north to south evident in 1958-'62 was more pronounced in 1974 than 1973 (Fig.8a). Daphnia, which was less abundant than either Cyclops or Diaptomus, was also significantly lower in 1973 than 1958T'62 (Fig. 8a). Its abundance increased in 1974 relative to 1973, but was s t i l l lower than in 1958-'62 although the difference was not significant. Unlike previous groups examined, nauplii-early copepodite abun-dance had generally changed l i t t l e in 1973 relative to 1958-'62. In 1974 nauplii-copepodite numbers were significantly higher than in 1958-'62 or 1973. Heterocope and Bosmlna abundance varied less between periods (Fig, 8b), Heterocope, which was much less abundant than the other zoo-plankters mentioned above, was more numerous in Area 1 in 1973 than 1958-'62 but tended to be lower in Areas 4 and 5. In 1974, Heterocope abun-dance in Areas 2 and 3 was lower than in 1973, but relative to 1958-'62, lower in a l l regions (particularly Area 2; Fig. 8b). Bosmlna showed l i t t l e significant variation in 1973 relative to 1958-'62 except in Area 1 (Fig. 8b). Concentrations in 1974 generally were significantly higher than those of 1973. but differed l i t t l e from 1958-S2 except in Area 5. Bosmlna concentrations in Area 5 during 1974, were considerably higher than in 1958-62 or 1973. 34. Seasonal Changes in Zooplankton Abundance Seasonal changes in zooplankton abundance during 1958-/62 and 1974 were compared to determine when zooplankton were most abundant and to see i f there have been major changes in seasonal patterns between study-periods. Biomass and Numbers Biomass generally peaked in July and August, but reached maxima earlier in the southern than northern areas during the five year period, 1958-'62 (Fig. 9). Year to year variation in biomass levels was greater in northern than southern areas. In the 1973-'74 period there was no evidence of a seasonal progression in timing of biomass peaks from south to north. May - mid June biomass in 1973-'74 appeared lower than in 1958-'62 (particularly in Areas 2 and 4) and biomass tended to peak later in the season than observed previously (Fig. 9). During 1958, I960 and 1962 biomass levels on the average were 20% higher in Area 4 than in Area 2 (Fig. 10). A similar trend was observed in 1973, "but not 1974. Relatively stable and high biomass levels in Area 2 during 1974 contrasted with the fluctuating lower levels of 1973. Numbers peaked earlier than biomass due to the presence of large numbers of juveniles (Fig. 11). Seasonal changes in numbers were similar in a l l areas during 1958, '60, '62 and '73, but generally peaked later in 1974 than the other years mentioned and remained relatively high through late summer (Fig. 11). Numbers in Area 2 during early June of 1974 were lower than in 1973 and the decline in numbers in mid-June of 1974 paralleled that of the biomass seen in Figure 10. 35. Figure 9. Seasonal changes in mean zooplankton "biomass in Babine Lake Areas 1, 2, k and 5 during 1958-'62 and 1973-'?2*. Vertical bars represent 2 S.E. ART A I 300-p 00 + 20 + 10 May J i r* J J y . Ai^usJ Seprentier October 1958 - 62 1973 -'74 A R E A 2 W°T T 37. Figure 10. Seasonal changes in zooplankton "biomass in Babine Lake Areas 2 and 4 during 1958,'60, '62, '73, and '74. 1 0 0 0 1 S 1 0 0 t Area 2 Area 4 M o n t h s I958 (•) I960 (A) I962 (O) 1973 (A) 1974 10) CO 39. Figure 11. Seasonal changes in zooplankton numbers in Babine Lak Areas 2 and 4 during 1958, '60, '62, 73 and 1 9 5 8 ( • ) I 9 6 0 (A) (962 (O) M J J A S 0 1 9 7 3 ( A ) 1 9 7 4 (O) 41. Taxa Cyclops was most abundant in June during 1958, '60 and '62 "but small f a l l peaks also occurred in Areas 2 and 4, (Pig. 12a, b). Both spring and f a l l blooms were prolonged in Area 4. Similar patterns were not as evident in 1973 and 1974, since spring blooms were apparently prolonged. Although 1974 Cyclops abundance was higher than in 1973, in mid-June of 1974, Area 2 numbers had apparently declined sharply from early June levels (Pig. 12a). Seasonal changes in Diaptomus abundance patterns had altered somewhat between study periods in that adults appeared more abundant in mid-summer in the latter period (Fig. 12a, b). While early summer and f a l l blooms were recorded in Area 2, only a single late June bloom was observed in Area 4 during 1958, '60, and '62. In 1973 and 1974, single blooms in late July were observed in Areas 2 and 4. Area 2 Diaptomus was considerably lower in spring of 1974 than 1973* although higher during the rest of the season. Heterocope, which was less abundant than Cyclops or Diaptomus, was most numerous in July - August but blooms were also observed in September - October during 1958 and 1962. In Area 2 the 1973 summer peak was prolonged and in 1974, absent. However, in Area 4 both peaks were present but occurred somewhat earlier than in 1958-'62 (Fig. 12a, b). Nauplii-early copepodites were most abundant in June of 1958, *60 and '62 but smaller f a l l peaks were also observed in September - October (Fig. 13a, b). In 1973 and 1974, spring peaks were more prolonged in both areas than in 1958, '60 or '62. The 1973 and 1974 f a l l blooms apparently were larger and occurred earlier in Area 4 than in Area 2. Both cladocerans, Bosmlna and Daphnia, showed similar fluctuations 42. Figure 12a. Seasonal changes i n Cyclops, Diaptomus and Heterocope numbers i n Babine Lake Area 2 during 1958, '60, '62, '73 and '74. 44. Figure 12b. Seasonal changes in Cyclops. Diaptomus and Heterocope numbers in Babine Lake Area 4 during 1958, '60, '62, '73 and '74. 1 9 5 8 I960 I 9 6 Z (•) (O) 1 9 7 3 (a ) 1 ! I M (O) 46. Figure 13a. Seasonal changes in nauplii-early copepodite, Bosmina and Daphnla numbers in Babine Lake Area 2 during 1958, '60, •62, '73 and '74. 48. Figure 13b. Seasonal changes in nauplii-early eopepodite, Bosmina and Daphnia numbers in Babine Lake Area 4 during 1958, '60, '62, '73 and '74. 50. in abundance being most numerous in July - August during 1958, '60 and •62 (Fig. 13a, b). Similar patterns were observed in 1973-74, however Daphnia was greatly reduced, particularly in Area 4 during 1973. Zooplankton Composition in Sockeye Fry Diets A comparison of the late August and late September diets of fry between 1967 and 1973 indicate that significant changes in composition have occurred (Fig. 14). In late August of 1973, Daphnia and Dlaptomus had increased in dietary importance relative to 1967. Epischura and Heterocope, which together formed \ % of the diet in 1967, were present only in small proportions in 1973. Changes in the early f a l l diet between 1967 and 1973 were more striking than those observed in late August, Daphnia, Epischura and Heterocope which together dominated the late September diet of 1967 were almost completely superseded by Dlaptomus and Cyclops in 1973. McDonald (1973) indicated Daphnia and Heterocope formed the bulk of the diet during the 1967 sampling season (Fig. 15). Heterocope dominated fry diets in early summer and f a l l ( Cg 60%), During late summer Daphnia replaced Heterocope as the major food organism ( C£ 1%) in the diet. The other cladoceran, Bosmina, was utilized to a limited extent ( CL %) in early summer and f a l l . Of the two numerically dominant zooplankters in the lake,(see Fig. 8a, b), Dlaptomus was utilized to a limited extent, particularly later in the June - October period, while Cyclops was not. Examination of fish taken from different areas in late August of 1973 indicates the prevalence of Daphnia in the diets was fairly wide spread in the main arm, (Fig. 16). However, Dlaptomus formed a large proportion of the diet in Areas 1 and 2. By late September, Dlaptomus dominated the diet in a l l areas of themain arm. The other numerically dominant 51. Figure 14. Zooplankton composition of sockeye fry diets in late August and late September of 1967 and 1973 (sample size in brackets). The fry were caught in the Main Arm of Babine Lake. 52. Iqte - August late - Septe mber 100 c o o a. 6 o O c a> o v_ CL. 5 0 4-0 * * * • * • • « t- < * • * * • • 4- 4 i. 4 • •• • • • • • • • * • • 4 • * «• • • 4 4 W W n j i n u 1973 (3 2) B o smino Cyc lops Daphnia Diaptomus E p i s c h u r a Hete rocope P a r t i a l l y d i g e s t e d m a t e r i a l 53. Figure 15. Seasonal changes in zooplankton composition of sockeye fry diets in the Main Arm of Babine Lake during 196? (McDonald 1973). Carats and numbers indicate sampling dates and sample sizes respectively. 54. 1 0 0 55. • * Figure 16. The zooplankton composition of sockeye fry diets in different regions of the Main Arm of Babine Lake during late August -early September and late September - early October of 1973 (sample size in brackets). 56. Lat* August - Early September B o s m i n a C y c l o p s Daphnio D iaptomus E p i s c h u r a par t ia l l y d i g e s t e d m a t e r i a l H e t e r o c o p e 57. zooplankton, Cyclops, was a more fimporfcantfood organism in late September than late August. The dietary importance of Cyclops varied between Areas, being generally more important in the northern main arm than in the south. LABORATORY EXPERIMENTS Size Range of Available Prey The size range of prey, available to fry, was bounded by the small ( .8 mm) Dlaptomus and Cyclops copepodites and Bosmina, and by the large copepod, Heterocope (x = 4.3 mm, Fig. 17). Adult Dlaptomus and Cyclops f e l l within the range bounded by their copepodites and the medium sized (x •» 2.57 mm) Daphnia. Epischura (J? =» 2.84 mm) was one of the larger prey available to fry during feeding experiments (Fig. 17). Electivity The mean total zooplankton densities (represented by zooplankton density indices 1, 2, 4 and 5; see methods) which each predator theoretically encountered are shown in Figure 18. Both predator groups, (fed and starved), encountered similar densities although starved fish encountered a greater range of densities than fed fish. Significant differences in density did not exist within zooplankton groups 1, 2, and 4, 5; however, low (1, 2) and high (4,5) densities were significantly ;different. There was a consistent predatlon pattern in a l l electivity experiments (Fig. 19,20). Adult Cyclops and Dlaptomus were selected (E> 0), while other forms were avoided. Because Bosmina and Daphnia were not abundant, electivity was not related to total prey density and these species are not represented in the figures. Fed fish generally showed increased electivity for Cyclops and Dlaptomus adults and Cyclops copepodites as total prey density rose. Electivity of Dlaptomus copepodites decreased while a l l other forms (nauplii, Heterocope, Epischura) were avoided ( E = -1), 58. Figure 17. Size frequency distribution of zooplankton species encountered by fry during electivity experiments (n = 40). 59. 12 8 4 B. COREGONI X=88 DAPHNIA LONGISPINA X = 2.57 i n=40 r-i , n-40 1 I I I 1 ! r 0 5 10 20 30 40 D. ASHLANDI X-1.12 CD -O e • c >. u c; 0). CT 0) 24 22 18 14 12 8 4 X-.80 + n=40 0 5 n=40 10 20 E. NEVADENSIS X=2.84 i n=40 ao H. SEPTRIONALIS X=4.30 40 n-50 50 14 12 8 4 I-C'SCUTIFER X=.84 X=1.87 n-40 0 5 / / / 10 20 | | adult copepodite Size (mm) 6o. Figure 18. Mean total zooplankton density at different zooplankton density indices, C = 1, 2 and k, 5 (see methods). Vertical bars represent 95% confidence limits. 61. 20,000 10, 000 4-Fed Fish in c ' Q c o <=, o QL O O N O H . c o C D 2 I ooo Starved F ish OO-L | 1 H-2 4 5 Z o o p l a n k t o n D e n s i t y I n d e x 62. Figure 19. Electivity values of zooplankton encountered by fed fish at different zooplankton densities. The horizontal scale represents a zooplankton density index increasing from 1 to 5 (see Fig. 18). C scutifer D. ashlandi (adults) (adults) Copepod H. septentrionalis E. nevadensis naupl i i 64. Figure 20. E l e c t i v i t y values of zooplankton encountered by starved f i s h at d i f f e r e n t zooplankton d e n s i t i e s . The h o r i z o n t a l scale represents a zooplankton density index in c r e a s i n g from 1 to 5 (see F i g . 18). C. sculifer D. oshlandi (adultt) (adultt) Ul ON 66. Starved f i s h displayed increased selectivity for Cyclops and Dlaptomus adults as total prey density rose. Electiveties of Cyclops and Dlaptomus copepodites fluctuated below 0 but were generally lowest when their relative abundance was highest. Nauplii, Heterocope and Epischura (in most cases) were avoided (E » - l ) . Prey Activity and Susceptibility Activity Cyclops and Heterocope were much more active than Dlaptomus (Fig. 21). Cyclops' characteristic horizontal movements were punctuated by short periods of "rest", hence a high ac t i v i t y rate. No significant variations l n Cyclops activity were noted except at 4°C where activity was lower between 350 and 560 lux and at 560 lux where activity increased with temperature (see Fig. 21), Heterocope activity, similar to that of Cyclops, was reduced at 350 lux and 8°C. Dlaptomus, unlike Cyclops and Heterocope, moved vertically and long periods of "rest" were punctuated by bursts of activity, resulting in low ac t i v i t y rates. There were no significant light or temperature induced variations in Dlaptomus act i v i t y . Susceptibility Predatlon susceptibility to the pipette differed l i t t l e among genera and between individuals with or without eggs (Fig. 22), except for egg bearing Cyclops which were 1.4 times more susceptible to predation than Cyclops without eggs. The apparent differences in Dlaptomus with and without eggs were not significant. Vulnerability of the much larger Heterocope was similar to that of the other genera, however the increased pipette aperature size may have affected results. According to Bernoulli's principle, increased aperature size should reduce intake velocity at the pipette mouth. Reductions in velocity could reduce capture success. 67. Figure 21. Gopepod activity rates at different light intensities and temperatures. Horizontal bars represent 9% confidence limits of log transformed data. Vertical bars indicate range. 68. 560 ^ -C yd ops Heterocope Diaptomus 220 lux 1 5 0 T Temp. 100 50 1 6 ' C i 1 , 1 : 1 8 C 150 -r 1 0 0 -50+1 .1 4 350 150 100 50 _4 y- _| 1- -\ \ 16 C 150-100-50 + -I r 200 000 400 500 600 L i g h t I n t e n s i t y ( L u x ) 560 150 100 50 5 10 15. T e m p e r a t u r e ( 0 C ) 20 69. Figure 22. Zooplankton susceptibility to " a r t i f i c i a l " predation. Horizontal bars represent 95% confidence limits; vertical bars, the range. 2 0 0 Cyclops Diaptomus Heterocope r— 1 i 1 -1 5 0 eggs no 1 0 0 " eggs no <pggs edjgs no eggs r 5 0 " Prey Type 71. DISCUSSION LAKE PROGRAM Effects of Fry Predatlon on Zooplankton Abundance in Babine Lake The lower zooplankton biomass levels of 1973-74* relative to 1958-*62 appears to be related to the significant increase in fry numbers during the last decade. Main Arm August fry densities had probably increased from a 1962-*66 mean of 940 fish/ha to 36OO fish/ha in 19736. According to a relationship derived by comparing zooplankton biomass and fry densities in several Babine Lake areas (Johnson 1961, Fig. 3c), such an increase in Main Arm fry densities could produce a 30 - 40$ decrease in zooplankton biomass. The actual decrease observed in 1973 was much greater (up to 65$) and appeared to result from prolonged concentrated predatlon ln certain lake areas. In Babine Lake (Johnson 1965c) and in other lakes (Galbraith 1967; Goodlad, Gjernes and Brannon 1974; Hutchinson 1971) high or increased planktivore densities were accompanied by low or decreased zooplankton biomass. Reduced zooplankton biomass likely results from selective removal of larger individuals from the zooplankton community. Reduction in biomass in Area 1 in 1973 relative to those of 1958-*62 deserve special mention because of estimated low fry densities, i.e. low predatlon pressure (see Fig. 3). Zooplankton biomass was reduced 70$ in this region which has been shown to be one of low primary productivity (Stockner and Shortreed 1975) and zooplankton biomass (Johnson 1965b). . These density.estimates were based on i n i t i a l fry populations of 39 and 150 million respectively, an average annual 1962-,73 mortality (z) of 0.65, and the August fry distribution pattern described by McDonald (I960). 72. Thus, this area may have been unable to support large fry densities. Increased predation could have resulted from year to year variations in fry distribution or perhaps from increased immigration into Area 1 as a result of high fry concentrations in adjacent areas (Johnson 1965c). Decreased food concentrations in adjacent areas might also have caused increased immigration as suggested for Great Central Lake by Barraclough and Robinson (1972). Zooplankton numbers were also reduced in 1973. Other workers (e.g. Archibald 1975* Brooks and Dodson 1965) have noted under fish predation, in small lakes, that decreases in large zooplankton forms were accompanied by increases in smaller zooplankton; the smaller forms had been formerly preyed upon or out-competed by the larger zooplankton. Such an increase in the density of small forms may not have occurred in Babine Lake for several reasons e.g. the reduction of larger zooplankton may not have been sufficient to reduce competition or predation within the zoo-plankton community or the smaller copepods and cladocerans may have been also utilized by the fry. The increased 1974 zooplankton biomass relative to 1973, probably resulted from heavy fry mortality. The numbers of fry entering the lake in April and May of 1973 and 1974 were comparable; however, available data (see Fig. 2) indicated the fry-smolt mortality rate in 1974 (Z = 1.4) was double that in 1973 (Z = .6). Because 1974 summer biomass levels were higher than those of 1973. mortality likely occurred early in the season, possibly as a result of food shortages. Since spring biomass levels were similar in 1973 and 1974, food shortages may have resulted from low Diaptomus concentrations. These in 1974 were $0% lower in Area 2 where most fry congregate early in the season. The depressed Diaptomus stocks may reflect 73. overcropping or low production or both, either of which could cause appreciable spring mortality. My experiments indicated Diaptomus to be a major food item early in the season as observed in Lake Washington (Woodey 1972). Low Diaptomus concentrations may also have led to compen-satory feeding on Cyclops as suggested by reduced Cyclops densities in the spring of 1974. As Cyclops is an intermediate host for the parasite Eubothrlum salvellni greater parasite loading and increased incidence of infection may have occurred, affecting survival (Norbert Boyce unpublished data). The probability of sockeye fry encountering abundant food early ln the season and minimizing the risk of early food limitation, is much greater i f dispersal is towards the southern Main Arm than north; year to year fluctuations in spring biomass levels were less in the southern regions than north. Southern regions of the Main Arm are more productive than those in the north (Johnson 1965b; Stockner and Shortreed 1975) and this discrepancy has apparently existed for several decades (Stockner 1975). However in spite of observed dispersal patterns, recent increases in fry numbers appear to have increased the chances of food limitation as discussed above. Other Causes of Reduced Zooplankton Abundance It appears likely, based on the preceding, that increased sockeye fry numbers have significantly reduced zooplankton abundance in Babine Lake. However other factors may have contributed to the declines; among them, zooplankton patehiness and decreased primary production. Zooplankton patches have been observed in Babine Lake (W. E. Johnson, personal communication), and in other lakes where their distribution 74. was greatly influenced by wind induced currents (e.g. Dumont 1967). In Babine Lake these patches may range in width from .4 to 1.3 km and ln length from 1.0 to 5.0 km (Johnson 1965b), p. 64). Location of 1973 sampling stations outside spatially and temporally persistent patches in Babine Lake may have contributed to the 1973 estimates of decline but not to those of 1974 because of the extended coverage that year. However, because of the degree of wind-induced mixing in the lake and the length of tows, I do not feel patchiness could be a major cause for the marked decrease in zooplankton abundance following fry.-enhancement in the lake. Some other factor such as reduced primary productivity may have contributed to the observed results. There is a positive relationship between primary productivity and zooplankton biomass levels in several oligotrophic sockeye-producing lakes in British Columbia, and Alaska (Fig. 23). It is evident that Babine and Karluk Lakes were almost an order of magnitude 7 more productive than both Great Central and Owikeno Lakes . The relationship between primary production and zooplankton biomass suggests that a decline in primary production between study periods could have caused the lower 1973 and 1974 biomass in Babine Lake. However, primary production appears to have differed l i t t l e between study periods (Stockner 1975). Therefore I feel this can be discounted as an important influence on zooplankton densities between 1958-'62 and 1973-'74. Similarly the increase in 1974 zooplankton abundance, relative to 1973» could be attributed to increased primary productivity. There is 7. Primary productivity observed by Stockner and Shortreed (1974) in Babine Lake was double that observed by Narver (1969). Based on this evidence, Narver's (1969) estimate of primary production in Owikeno Lake was doubled to give a better f i t of Narver's data with those observed elsewhere. 75. Figure 23. Relationship between zooplankton biomass (mg dry wt. —2 -1 -2 -1 m . day ) and primary productivity (mg Q.m . day ) in several oligotrophic sockeye producing lakes in British Columbia and Alaska (Great Central Lake before and after fertilization; Owikeno Lake (Narver 1969) adjusted according to Stockner and Shortreed (197*0, areas of Babine Lake's Main Arm in 1973). 76. I 00 T G r e a t C e n t r a l L a k e A O w i k e n o L a k e © B a b i n e L a k e O K a r l u k L a k e .1 I 1 — — i - i 1—c—i 1 — , — t — i — i — i 1 1 • 1 — • — i 1 1 — ^ — i — y — » 10 50 100 1000 P r i m a r y P r o d u c t i v i t y ( m g C - m - d a y ) 77. l i t t l e information available, other than Secchi depths, regarding primary production in 1974. However Stockner and Shortreed (1974) demonstrated a negative relationship (r = - 0 . 9 ) between increasing annual areal primary production and Secchi depth in Babine Lake during 1973. If a similar relationship,held in 1974 and primary production were similar in both years, one would expect l i t t l e difference in Secchi depths between.1973 and 1974. The average 1974 Secchi depths were d% deeper than in 1973. Therefore i t appears unlikely that increased primary production between 1973 and 1974 resulted in the observed increase in zooplankton biomass. Fry Selectivity in the Lake More specific effects of increased fry numbers on zooplankton abundance were explored by comparing Babine Lake fry diets in 1967 and 1973 and comparing dietary patterns to zooplankton species abundance between study periods. Changes in 1973 diets relative to 1967, i.e. more Cyclops and Diaptomus, and fewer Daphnia and Heterocope, may reflect real changes in abundance and composition between study periods as discussed in the following section. In spite of ;the fact that only a small number of 1973 fry were examined, the apparent paucity of Heterocope in 1973 diets remains unexplained in view of its apparent abundance in the zooplankton. Seasonal changes in I967 fry diets reflected changes in zooplankton species abundance in the lake; however, fry feeding was also selective in that relatively rare zooplankton were taken. Any comparison of fry diets and zooplankton abundance assessed at different times involves certain 78. risks, i.e. the zooplankton sampled may be different from that encountered by the fish (O'Brien and Vinyard 1974). However, i t was felt that the zooplankton data base for 1958-62 was sufficient to permit the following qualitative comparisons. Cyclops and Diaptomus adults were abundant throughout a l l years and, as experimental evidence suggests, utilized heavily by fry early in the season (there are no spring fry samples from Babine Lake)y However, the most abundant organisms in spring were copepod nauplii and they were not taken. The fi r s t appearance of Daphnia and Heterocope in the fry diets of 1967 coincided with their f i r s t occurrence in the summer plankton of 1958-62. Assuming no changes in plankton composition between 1958-62 and 1967, fry appeared to be selective feeders in summer. Daphnia and Heterocope together comprised 70% of the diet in 1967, but less than 7% of zooplankton numbers during 1958-62. Pry displayed greater selectivity for Heterocope than Daphnia because of Heterocope*s lower relative abundance (Hetero- cope, 0.2$; Daphnia, 6.3^). However as Daphnia and Heterocope abundance declined in f a l l , more Diaptomus were taken. Cyclops which was not recorded in the f a l l 1967 diets, probably appeared later following further declines in Heterocope abundance. Regional disparities in 1973 diet composition suggests variations in zooplankton species abundance between areas. In late August, 1973, apparently fewer Daphnia were consumed in the north (Area 1) than south (Area 5), implying reduced availability (abundance) ln the north (seasonal abundance data suggests that this was the case). A similar, but much less striking trend, was observed with Diaptomus in the late September diet. In this case, the northward decline in dietary importance was accompanied by an apparent increase in Diaptomus availability (abundance) during this period. 79. Changes in Zooplankton Species Abundance In the previous section the pre- and ppost-enhancement diet composition of fry was discussed; the next step was to examine changes in zooplankton species abundance between the two study periods. Changes in species abundance were related to observed fry diets, particularly the southern regions of the Main Arm. Given observed feeding patterns of fry and recorded increases in fry numbers, the greatest decreases could be expected in Daphnia and Heterocope abundance. Smaller decreases could be expected in Cyclops and Dlaptomus numbers. Reductions in 1973 Daphnia abundance (80$), relative to 1958-'62 , suggests a significant cropping effect particularly in Areas 2, 4 and 5» while 1974 abundance relative to 1973 suggest decreased cropping. Contrary to expectations, 1973 Heterocope abundance in northern areas had not decreased from 1958-'62 levels. Fry distribution may have concentrated predatlon in Areas 4 and 5 during July, and August when Hetero-cope was most abundant in Areas 1 and 2. Since reductions &n Areas 4 and 5 were not significant, i t suggested that analytical techniques used to compensate for Heterocope's diurnal migrations may have overestimated 1973 Heterocope abundance in a l l areas. Decreased 1974 abundance (50$) relative to 1958-62 suggested cropping had affected Heterocope abundance particularly in Area 2. Heavier predatlon may have reduced 1973 values below those of 1974 as observed for other zooplankton genera in Babine Lake. As expected, Cyclops and Dlaptomus were less affected than Daphnia, but reductions in the order of 50$ were observed in some areas. Cyclops was generally less affected by predatlon than Dlaptomus. In electivity and vulnerability experiments, Cyclops differed l i t t l e from Dlaptomus so clues as to the mechanisms could not be obtained. Three 80. factors may be instrumental in lessening the predatlon impact on Cyclops. First, since Cyclops were 35% fewer than Dlaptomus in 195&-'62 fry may not encounter Cyclops as frequently as they would Dlaptomus. Second, Qyclops may reproduce before predatlon becomes intense (large numbers of copepodites were observed early in the season, ensuring to some degree recruitment of the next generation). Third, after some experience, fry may select Dlaptomus over Cyclops. Cyclops and Diaptomus reductions in 1973 suggested heavier utilization by fry than in 1967, in response to decreased abundance of preferred Daphnia and Heterocope. Analysis of fry stomach contents in 1973 suggested that such a shift in diet composition occurred. Daphnia and Heterocope did form a smaller proportion of the diet in 1973 than in 1967. The striking, rather unexpected, increases in 1973 and 197^  nauplii-early copepodites in some areas may have occurred because they were too small to be utilized by fry. My laboratory experiments suggested nauplii were "avoided" (E<0) even at high densities. Increased abundance of nauplii may also reflect increased fecundity and reduced competition within the zooplankton community (Archibald 1975; Nelll 1975). Continuing reduction in numbers of the larger zooplankton could induce selection for smaller zooplankton, less accessible to fry, in the lake. This could have serious implications on the future carrying capacity of Babine Lake. LABORATORY EXPERIMENTS Selective feeding of sockeye salmon fry appears to have decreased abundance of certain zooplankton species in Babine Lake. The experiments discussed below were designed to determine how selective feeding (or electivity) was influenced by variations in zooplankton composition, density, size and activity. 81. Electivity Both predator groups (fed and starved) encountering live zoo-plankton for the f i r s t time, selected prey of intermediate length, i.e. Cyclops and Diaptomus adults. In most cases the proportions of these prey in the diets remained relatively constant as total prey density changed. Declines in the relative densities of Cyclops and Diaptomus adults/resulted in Increased electivity as observed by Ivlev (1961). Tinbergen (I960) hypothesized that selectivity of experienced predators persisted at low prey densities until the predator was no longer able to maintain a specific searching image. Experimental results suggest that lake fry feed heavily on Cyclops and Diaptomus adults early in the season. A similar feeding pattern by sockeye fry has been observed in Lake Washington (Woodey 1973). Cyclops and Diaptomus copepodites were avoided (E< 0) by both predator groups. Fed fish avoided Cyclops copepodites less as the relative abundance of more suitably sized prey decreased. Ivlev (1961) noted a similar response during the "continuous destruction of the food complex" by carp (Cyprinus carploLUtinaeus.)'. Fed fish took the less abundant and smaller Diaptomus copepodites more readily than Cyclops copepodites accounting for the higher electivity of Diaptomus. Selectivity of Cyclops and Diaptomus copepodites by starved fish did not appear related to total prey density. However, starved fish generally avoided these prey less when the relative abundance of smaller, less preferred quarry (nauplii) was high and that of larger prey was low. Copepod nauplii, the most abundant but smallest prey available, were not taken, by either predator group. This may be attributed to small prey size, i.e. nauplii e l i c i t a shorter predator reactive distance 82. than the larger, more visible, copepodites and adults (Confer and Blades 1975). Low abundance of Cyclops and Diaptomus adults in the lake may result in increased predation on the smaller copepodite and nauplii. The large mobile zooplankton, Heterocope and Epischura, were seldom eaten by fry in my experiments. Sockeye fry, caught in July of I967 (mean wt. «= .40 gm) in Babine Lake, contained 60% Heterocope (McDonald 1973). These fish were 166$ larger than test fry and had several weeks experience feeding on zooplankton. The behaviour and large size of Heterocope (4.30 mm), compared to Diaptomus (1.1 mm) suggests that small fry (such as those used in ray experiments) required some experience in order to handle Heterocope. However, my prey activity and susceptibility experiments indicated l i t t l e difference in vulnerability between these large prey and the smaller Cyclops and Diaptomus. Although results from replicated electivity experiments varied considerably, fry were consistent in selecting Cyclops and Diaptomus adults while avoiding copepodites, copepod nauplii and the larger copepods. Several factors might contribute to replicate variability, but the most important were considered to be differences in the degree of starvation . (Laurence(1972) found starvation reduced activity thus reducing the ability of larval bass to search for and catch food), prey density and composition. Prey Activity and Susceptibility Of the three genera examined, only Cyclops showed any consistent change in activity in response to variations in temperatures; activity increased with increasing temperature at high light intensity. McNaught and Hasler (1964) noted a similar response in Daphnia, which Lochhead (1961) described as orthokinetic. The response may be brought on by increased metabolic rates and decreasing water Viscosity. 83. Activity of the three genera examined did not vary significantly in response to light. This suggests that there is likely a very low diurnal change in predation vulnerability in areas with similar tempera-tures. Only Cyclops may be affected where temperature does change. However, since Heterocope undergoes extensive diurnal vertical migrations in Babine Lake (Narver 1970), i t appears capable of responding to changes in light intensity and may undergo changes in vulnerability to predation. Assuming that increased activity reduces vulnerability at 560 lux and l6°C, i t was expected that Cyclops would be least vulnerable to capture, followed by Heterocope and Dlaptomus. Predation experiments suggested this but differences in vulnerability were not significant largely because avoidance reactions of Dlaptomus reduced its catchability. Increasing the "predator" mouth size to accomodate Heterocope reduced intake velocities so that the relative vulnerability of these animals may have been under-estimated. If external egg sacs increased copepod vulnerability by impeding movement, one might expect selection of egg carrying females during heavy predation. Vallin (1969) noted the presence of Large numbers of female copepods with eggs in the diet of the Cisco, Coregonus albula. Cyclops was significantly more vulnerable when carrying eggs but Dlaptomus was not. Since Diaptomus egg sacs are more symetrical than those of Cyclops, there may be less drag, thus explaining the lack of difference between individuals with and without eggs. Singarajah (1975) suggested that escape depends on the copepod's ability to detect currents by means of receptors, which Marshall and Orr (1955) implied were in the antennules. The a r t i f i c i a l predator used here, and the one used by Singarajah (1975). create suction currents whereas a m. sockeye fry might be expected to produce a hydrostatic wave. Flemlnger and Clutter (1965) suggest that copepods can detect such hydrostatic waves, allowing the zooplankton some chance of escape once attacked by a fry. The effect of copepod activity on vulnerability to predation was examined as a mechanism which might predict sockeye fry selectivity; high vulnerability and selectivity of copepods were assumed to accompany low activity. Since the vulnerability of the three genera tested were not sufficiently different, activity could not be used in this case to predict selectivity. However activity might s t i l l influence selectivity in two ways; fir s t , high copepod activity rates might increase encounter rates between sockeye fry and its copepod prey. Second, high activity might increase detectability (Mayly 1970). Considering these factors, sockeye fry might be expected to select Cyclops and Heterocope over Dlaptomus. Selection of the more active, much larger, Heterocope has been observed in Babine Lake. 85. CONCLUSIONS The 3.8 fold Increase in Babine Lake sockeye fry numbers between the study periods 1958-'62 and 1973-'74 was accompanied by several changes in the zooplankton population. Outstanding among these were: 1. Decreases in 1973 zooplankton biomass were much greater {^6%) than the 30 - kQfo expected on the basis of earlier work (Johnson 1961) and may have resulted from concentrated predation in certain lake areas for prolonged periods and selective removal of the larger zooplankton. 1 2. Biomass levels were higher; in 1974 than 1973 even though i n i t i a l fry numbers were similar in both years. Fry-smolt mortality was higher in 1974 than in 1973, and i f occurring early in the season could have resulted in lower predation pressure. Food shortages, especially low Dlaptomus concentrations, may have contributed to the increased mortality. 3. Selective predation by increased numbers of fry has led to decreased abundance of Daphnia in 1973 and 1974 and Heterocope in 1974, relative to 1958-'62. Reduced numbers of Heterocope in the 1973 diet in spite of its apparent abundance in the lake, along with known feeding preferences, suggested Heterocopesnumbers have been overestimated. Cyclops and Dlaptomus abundance were less affected than those of Daphnia because they were apparently avoided during most of the season. Decreased abundance of preferred larger forms may have led to increased predation o n Cyclops and Dlaptomus in 1973 compared to 196?. The increases in nauplii-early copepodite abundance in 1973 and 1974 relative to 1958-'62 may result from avoidance by fry, increased fecundity of surviving 86. zooplankton or "both. Continued exploitation of larger zooplankton may result in selection of smaller forms, less accessible to fry. 4, Biomass levels also appeared to be influenced by the availability of primary food organisms. The southern regions of Babine Lake generally had higher primary productivity (Stockner and Shortreed 1975) and consequently higher biomass concentrations than those in the north. 5. Historically there appears to have been selection for fry migrating f i r s t into the southern, more productive, regions since zooplankton were more abundant early in the season there than they were in the north. Conclusions, which may be drawn from feeding experiments concerning the influence of prey abundance and activity on sockeye fry selectivity, are as follows: 1. Experiments confirmed that juvenile sockeye select Cyclops and Diaptomus adults in spring but also suggested that juvenile stages of these genera were selected i f relatively abundant and prey of a preferred size were scarce. 2. Some growth and experience may be necessary before fry can prey upon the larger zooplankton, e.g. Daphnia and Heterocope, which later form an important part of their diet. 3. In my experiments, prey activity alone could not be used to predict vulnerability and selectivity since prey with very different activity rates appeared equally vulnerable. High prey activity might increase selection by fry by raising fry-prey encounter rates and chances of detection. MANAGEMENT IMPLICATIONS 87. Johnson's hypothesis that the main arm of Babine Lake could support up to a four-fold increase in fry numbers appears to hold in 1972 and 1973 "but not in 1974. The chances of food limitation of fry production have greatly increased particularly in Areas 1, 4 and 5 where the most significant decreases in zooplankton biomass have occurred. However, one must bear in mind that fry distribution appears density dependent; greater dispersal at higher fry densities might reduce the chances of food shortages occurring because of localized predation. Continuing exploitation of large zooplankton, which has resulted in significant decreases in biomass, may lead to selection for smaller forms, e. g. Cyclops and Dlaptomus, less accessible to fry. Low abundance of the larger zooplankters could result in compensatory feeding on these smaller forms and consequently reduced growth. Increased predation on Cyclops and Dlaptomus during summer and winter may produce food shortages for the next generation of fry entering the lake since both genera are utilized heavily early in the season. At 1972-'74 fry production levels, a combination of high fry densities and low spring zooplankton abundance in the future may result in occasional years of high mortality. Averaged over several years, fluctuating yearly mortality would likely reduce potential smolt production from Babine Lake. Fry production increased over present levels could accelerate the processes described above. 88. LITERATURE CITED Anderson, Bruce C. and David W. Narver 1968. Techniques for sampling sampling zooplankton and underyearllng sockeye salmon in the midwater of large lakes. Fish. Res. Board Can. Ms No. 1009. Archibald, C. P. 1975. Experimental observations of the effects of predation by Goldfish (Carasslus auratus) on the zooplankton of a small saline lake. J. Fish. Res. Board Can. 32(9): 1589-1594. Barraclough, William E. and Douglas G. Robinson 1972. The fertilization of Great Central Lake III. Effect on juvenile sockeye salmon. Fish. Bull. 70(1): 37-48. Brooks, John L. and Stanley I. Dodson 1965. Predation, body size and composition of plankton. Science 150: 28-35. Chizar, David and J. T. Windell 1973. Predation by blue g i l l sunfish (Lepomis macrochlrus Lafinesque) upon meal worms larvae (Tenebrio  molltor). Anim. Behav. 21(3): 536-543. Confer, John L. and P. I. Blades 1975. Omnivorous zooplankton and plankti-vorous fish. Limnol. and Oceangr. 20(4): 571-579. Czaplicki, James A. and R, H. Porter 1974. Visual cues mediating the selection of Goldfish (Carasslus auratus) by two species of Natrix. J. Herpetol 8(2): 129-1347^ D i l l , Laurence M. 1968. The Babine Lake Salmon Development Program. Progress Report to March 31, 1968. Joint Publication of Dept. Fish. (Canada) and Fish. Res. Board Can. Dodson, Stanley I. 1970. Complementary feeding niches sustained by size-selective predation. Limnol. Oceanogr. 15: 131-137. 1974. Zooplankton competition and predation and experimental test of the size-frequency hypothesis. Ecol, 55: 605-613. Dumont,, Henri J. 1967. A five day study of patchiness in Bosmina corregoni, Baird in a shallow eutrophic lake, Mem, 1st , Ital. Idrobiol. 22: 81-103. Fleminger, A. and R. I, Clutter 1965. Avoidance of towed nets by zoo-plankton, Limnol. Oceanogr, 10(1): 96-104, Foerster, Russell E, 1954. On the relation of adult sockeye salmon (Oncorhynchus nerka) returns to known smolt seaward migrations. J. Fish. Res. Board Can. 11(4): 339-350. 89. Foerster, Russell E. 1968. The sockeye salmon (Oncorhynchus nerka). Fish. Res. Board Can. Bull. No. 162. 422 p. Galbraith, Merle G. Jr. 1967. Size-selective predation on Daphnia by Rainbow Trout and Yellow Perch. Trans. Am. Fish. Soc. Vol. 96(1): 1-10. Goodland, J. G., T. W. Gjernes and E. L. Brannan 1974. Factors affecting sockeye salmon (Oncorhynchus nerka) growth in four lakes of the Fraser River system. J. Fish. Res. Board Can. 31: 871-892. Hall, Donald J. 1964. An experimental approach to the dynamics of a natural population of Daphnia galeata mendotae. Ecol. 45: 94-112. Haney, James F. and Donald J. Hall 1972. Sugar-coated Daphnia: A preserva-tion technique for Cladocera. Limnol. Oceanogr. 18(2): 331-333. Hellawell, J. M. and R. Abel 1971. A rapid volumetric method for the analysis of the food of fishes. J. Fish. Biol. 3: 29-37. Herzog, Harold A., Jr. and Gordon M. Burghardt 1974. Prey movement and predatory "behaviour of juvenile western yellow-bellied racers, Golubes constrictor mormon. Herpetologia 30(3)i: 285-289. Hoag, Stephen H. 1968. The food of juvenile sockeye salmon, 0. nerka, related to the xooplankton in Iliamna Lake, Bristol Bay, Alaska. M.Sc. Thesis, College of Fisheries, Univ. of Wash. , 72 p. Hutchinson, B. P. 1971. The effect of fish predation on the zooplankton of ten Adirondach Lakes with particular reference to the Alewife, Alosa psuedohorengus. Trans. Amer, Fish. Soc. 100(2): 325-335. Ivlev, V. S, 1961, Experimental ecology of the feeding of fishes (Transl. from Russian) Yale Unlv, Press, New Haven, Conn, 302 p. Johnson, Waldo E. 1958. Density and distribution of young sockeye salmon (Oncorhynchus nerka) throughout a multi-basin lake system. J. Fish. Res. Board Can. 15: 961-987. 1961. Aspects of the ecology of a pelagic, zooplankton-eating fish. Verh. Inter. Verein. Limnol. 14: 727-731. 1964. Qualitative aspects of the pelagic, entomostrachan zooplankton of a multi-basin lake system over a 6-year period. Verh. Inter. Verein. Limnol. 15: 727-734. 1965a. A review of the biological evidence bearing on the potential of Babine Lake as a sockeye salmon producer. Unpubl. Manuscript. 1965b. Quantitative studies of the pelagic entomostrachan zooplankton of Babine Lake and Nilkitkwa Lake, 1955-1963: Methods, Stations and basic data. Fish. Res. Board Can. Ms No. 821. 90. Johnson, Waldo E. 1965c On mechanisms of self-regulation of population abundance in Oncorhynchus nerka. Mitl. Inter. Verein. Limnol. 13: 66-84. Laurence, Geoffrey C. 1972. Comparative swimming abilities of fed and starved larval largemouth bass (Mlcropterus salmoides). J. Fish. Biol. 4: 73-78. LeBrasseur, Robin J., C. D. McAllister, J. D. Fulton and 0. D. Kennedy, 1967. Selection of a zooplankton net for coastal observations. Fish. Res. Board Can. Tech. Report No. 37. and Owen D. Kennedy 1972. The fertilization of Great Central Lake II. Zooplankton standing stocks. Fish. Bull. 70(1): 25-36. Lochhead, John H. 1961. Locomotion. Gh. 9. In. T. H. Waterman (ed.) The physiology of Crustacea, Vol. II. Sense organs, integration and behaviour. Academic Press, New York, 681 p, McDonald, Jack G, 1969. Distribution, growth and survival of sockeye fry (Orrcorhynchus nerka) produced in natural and a r t i f i c i a l stream environments. J. Fish. Res. Board Can. 26: 229-267. _________________ 1973. Diel vertical movements and feeding habits of underyearling sockeye salmon (Oncorhynchus nerka), at Babine Lake, B. C. Fish. Res. Board Can. Tech. Report No. 378. McNaught, Donald C. and A. D. Hasler, 1964. Rate of movement of populations of Daphnia in relation to changes in light intensity. J. Fish. Res. Board Can. 21(2): 291-318, Marshall, S. M, and A. P. Orr 1955* The biology of a marine copepod Calanus flnmarchlcus (Gunnerus). Oliver and Boyd, London, 188 p. Mayly, E. I. 1970. The influence of predation on the adult sex ratios of two copepod species. Limnol Oceanogr. 15(4): 566-573. Miller, D. 1961. A modification of the small Hardy plankton samples for simultaneous high-speed plankton hauls. Bull. Mar. Ecol. 5: 165-172. Narver, David W. 1970. Diel vertical movements and feeding of underyearling sockeye salmon and the limnetic zooplankton in Babine Lake, British Columbia. J. Fish. Res. Board Can. 27: 281-316. Neill, William E. 1975. Experimental studies of microcrustacean competition, community composition and efficiency of resource utilization. Ecology 56(4): 809-826. Northcote, Thomas G. and Regina Glarotto 1974. Limnetic macrozooplankton and fish predation in small Coastal British Columbia lakes. Verh. Internat. Verein. Limnol. 19(3): 2378-2393. 91. O'Brien, W. John and G. L. Vinyard 1974. Comment on the use of Ivlev's Electivity Index with planktivorous fish. J. Fish. Res. Board Can. 31: 1427-1429. Shepard, M. P. and F. C. Withler 1958. Spawning stock size and resultant production for Skeena Sockeye. J. Fish. Res. Board Can. 15(5)s 1007-1025. Singaraja, K. V. 1975. Escape reactions of zooplankton: Effects of light and turbulence. J. Mar. Biol. Ass. U.K. 55(3): 627-740. Sokal, Robert R. and F. James Rohlf 1969. Biometry. The principles and practice of statistics in biological research. W. H. Freeman and Company, San Francisco. 776 p. Sprules, W. Gary 1972. Effects on size-selective predation and food competition on high altitude zooplankton communities. Ecology 53: 376-386. Stockner, John G. 1975. Phytoplankton heterogeneity and paleolimnology of Babine Lake, British Columbia, Canada. Verh. Internat. Verein. Limnol 19: 2236-2250. and K. R. S. Shortreed 1974. Phytoplankton succession and primary production in Babine Lake, British Columbia. Fish. Res. Board Can. Tech. Report No. 417. and K. R. S. Shortreed 1975. Phytoplankton succession and Primary Production in Babine Lake, British Columbia. J. Fish. Res. Board Can. Vol. 32(12): 2413-2427. Tinbergen, Lukas I960, The natural control of insects in pine woods 1. Factors influencing the intensity of predation by song birds. Arch. Neerlandaises Zool, 13: 265-343. Tranter, D. J. and A. C. Heron I965. Filtration characteristics of Clarke-Bumpus samplers. Aust. J. Mar. Freshwat. Res. 16: 281-291. and P. E. Smith 1968. Filtration Performance In. D. J. Tranter (ed.) Reviews on zooplankton sampling methods UNESCO 174 p. Vallin, S. 1969. Cisco feeding biology in Lanibar Fjord. Information from the Freshwater Laboratory, Drottingholm, 7. Warshaw, Stephen J. 1972. Effects of alewives (Alosa pseudoharengus) on the zooplankton of Lake Wonoskopomuc, Connecticut. Limnol. Oceanogr. 17(6): 816-825. Wells, LaRue 1970. Effects of Alewife predation on zooplankton populations in Lake Michigan. Limnol Oceanogr, 15(4): 556-565. Woodey, James C. 1972. Distribution, feeding and growth of juvenile sockeye salmon in Lake Washington. Ph.D. Thesis College of Fisheries, Univ. Wash, 207 p. APPENDIX A 92. Relative Catching Efficiencies of Clarke-Bumpus and Miller Zooplankton Samplers To asses the impact of sockeye fry on Babine Lake zooplankton i t was necessary to make quantitative comparisons of the different sampling techniques used in the study periods 1958-'62, 1973, 1974. Relative catching efficiencies of dissimilar plankton nets has received much attention (e.g. Fleminger and Clutter 1965; Lebrasseur, McAllister, Fulton and Kennedy 1967), but to my knowledge no one has compared the Clarke-Bumpus and Miller samplers. The catch of a plankton net is determined principally by the zooplankton's response to hydrostatic waves preceding approaching nets, the time required to respond (if at a l l ) , and the direction of the response. Tranter and Smith (1968) found the design of the sampler to be important in determining the nature of the preceding pressure wave and the filtration coefficient which determines how much water passes through the sampler. This Appendix discusses the relative efficiency of the Clarke-Bumpus and Miller samplers during three sampling procedures (horizontal, oblique and vertical tows). Methods Biomass estimates from vertical hauls in 1973 were regressed on those from 5 meter strata to determine i f a significant relationship existed (Fig. 24). The 1974 vertical haul biomass estimates were regressed on corresponding oblique hauls (0-5 m),(Fig. 25), thus permitting conversion of 1973 five meter estimates to 1974 "oblique units". Regression analysis of catches of simultaneous oblique tows with Clarke-Bumpus and Miller 93. Figure 24. Regression of vertical haul "biomass (mg. dry wt. m~J) on 5 meter-strata biomass (mg. dry wt. m"-^) in 1973. 94. LOG Y = 0.53 * 0.6l*L0G X n=39 , r= 0.61 1CO0-B I O M A S S C5 I vCTER H A U L ) 95. Figure 25. Regression of vertical haul biomass (mg. dry wt. m"-5) on 0-5 meter oblique haul biomass (mg. dry wt, m J) in 1974. 96. BIOMASS 1 0 - 5 OBLIQUE HAUL ) 97. samplers (each with a #10, 1153/u'net) determined their relative efficiency (Fig. 26, 2 7 . ) . Results and Discussion Figure 24 shows the relationship between the biomass estimates from 1973 vertical and 5 meter strata hauls while Figure 25 describes the relationship between the biomass estimates from 1974 vertical and 0-5 meter oblique hauls. Observed variability may result from concentration of zoo-plankton in the upper 5 meters, contagious distribution and fouling of nets by Tabellaria fenestrata. The tests show that the Miller sampler gave about a 36$ higher biomass estimate than the Glarke-Bumpus (see Fig. 26). Unlike the Miller, the Glarke-Bumpus has several obstructions preceding the sampler mouth. Resultant pressure waves may be detected by zooplankton, allowing the more mobile forms to avoid the net. The hydrodynamics peculiar to each sampler may also contribute to observed discrepancies in estimates. Tranter and Heron (1965) reported that their American Glarke-Bumpus with a 7 0 n e t had a filtration efficiency (F) of O.896 at velocities of 1.5 m / s e c . i . e . g i t filtered 89.6$ of the theoretical maximum Samplers of the Miller design may have F. values of 1.0 (Tranter and Smith 1968). In spite of the avoidance capabilities of the organisms i t appears that the Glarke-Bumpus samples less water than the Miller and thus underestimates the abundance of zooplankton. 8. Filtration efficiency, the relationship between volume filtered and volume swept by the mouth of the net, is described by the formula: p . W A-D where: W is the Volume filtered; A, the mouth area; and D, the distance towed. 98. Figure 26. Regression of biomass estimates (mg. dry wt. m."-5) of a Clarke-Bumpus sampler on those of a Miller sampler. Y = It.43 + 0.4062*X n=l4, r=0.80 300-Figure 2?, Regression of Glarke-Bumpus catches (no. L ) on those of the Miller sampler. w 3 p. e 3 cn Q) M (T) rH O r l -t-> o rH O Y=0.23 + 0.-68*X n-15. r=0.93 w . ZO* P< 6 3 | a nJ rH U Q) tr 10 •H co 3 6 O •+•> P. y-q.40 + o.70*x n-15, r=0.98 Cyclops/Litre ( M i l l e r ) Diaptomus/Litre ( M i l l e r ) Y=0.03 + 0.71*X 'n-15, r=0.85 a PP I eu U rt rH O CD U -P •H C •H 6 W O FP Y—0.11 + 0.96*X n-14, r=0.89 Daphnia/Litre ( M i l l e r ) d- 3- 1- »• 6- 7 . t- j. Bosmina/Litre ( M i l l e r ) 


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