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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  presenting  this  thesis  an a d v a n c e d d e g r e e a t the L i b r a r y I  further  for  agree  scholarly  by h i s of  shall  this  written  the U n i v e r s i t y  make  it  freely  that permission  for  It  financial  for  of  The U n i v e r s i t y  British  2075 Wesbrook Place Vancouver, Canada V6T 1W5  of  Columbia,  British  Columbia  for  extensive  the  requirements  reference copying of  I agree and this  that  not  copying or  for that  study. thesis  by t h e Head o f my D e p a r t m e n t  gain shall  ^-^otp/l06a. s/ of  of  is understood  permission.  Department  fulfilment  available  p u r p o s e s may be g r a n t e d  representatives. thesis  in p a r t i a l  or  publication  be a l l o w e d w i t h o u t my  i ABSTRACT A two year study was initiated in 1973  to examine effects of  (3.8 fold; from a 1962-66 mean of 39 million to about 150 in 1973 and 197*0 increases in sockeye (Oncorhynchus nerka)  substantial million  Walbaum) fry numbers on zooplankton abundance in Babine Lake. Several lake areas and stationsware sampled for zooplankton bimonthly from May to October during 1962  1973 and 197*+  and comparedtodata gathered between  1958 and  prior to a large scale enhancement program for sockeye stocks. Zoo-  plankton biomass had decreased up to 70$ i n some areas of the lake during  1973i but only kOf  0  in  197^.  Decreases in numbers were also evident.  Although seasonal changes in fry diet followed changes i n zooplankton species abundance, feeding was selective.  The less abundant but larger  forms, Daphnia and Heterocope together comprised summer, while Cyclops and Diaptomus late f a l l .  70?5  of the diet during  formed the bulk (87$) of the diet in  Significant decreases i n Daphnia and Diaptomus abundance and  increases in nauplii-early copepodite abundance had occurred by 1973. increased 197^  zooplankton abundance relative to 1973  The  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 zooplankton for the f i r s t time.  Copepodites and nauplii were rejected, but  less so when preferred prey were scarce. Prey activity, i n my experiments, could not be used to predict predation vulnerability and hence the species  ii selectivity displayed by the fry. Light and temperature had l i t t l e effect on Cyclops, Dlaptomus and Heterocope activity.  iii. i  TABLE OF CONTENTS Abstract  Page  •  Table of Contents  • •  i  •  i i i  L i s t 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  C.  Sockeye Fry Distribution in Babine Lake.........  III.  ,  4 ..10  Materials and Methods  13  A.  13  Lake Program..... Zooplankton Sampling, 1973  13  Zooplankton Sampling, 1974  13  Laboratory Analysis. Analysis of 1973  .13  Stomach Samples  14  Data Analysis.  B.  .15  Comparison of Pre- and Postenhancement Data  15  Estimating Heterocope Abundance  17  Laboratory Experiments  ,18  Electivity  18  Prey Activity  i  Prey Susceptibility IV.  Results A.  .20 ,  ,  ,,  23 ,,  Lake Program Changes in Average Zooplankton Abundance Between  24 24  Years.24  Iv. Biomass and Numbers.  .24  Species Abundance  24  Seasonal Changes i n Zooplankton Abundance  34  Biomass and Numbers  .34  Taxa. «•.«.............. .....»•-.  41 ....50  Zooplankton Composition in Sockeye Fry Diets B.  Laboratory Experiments.  5?  Size Range of Available Prey  ..57  Electivity Prey Activity and Susceptibility..  57 .........66  Activity Susceptibility V.  Discussion A. Lake Program Effects of Fry Predatlon on Zooplankton Abundance  66 .66 ....71  71 .71  Other Causes of Reduced Zooplankton Abundance...........73 Fry Selectivity in Babine Lake Changes in Zooplankton Species Abundance B. Laboratory Experiments.....  VI. VII. VIII. IX. X.  77 .79 80  Electivity  81  Prey Activity and Susceptibility.  82  Conclusions Management Implications  ...85 ..87  Literature Cited  88  Appendices.  92  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  3.  8  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 21  and conduct predation experiments... 5.  The prey seizing apparatus (a modified Pasteur pipette) 21  used in zooplankton predation experiments... 6.  Average seasonal zooplankton biomass and numbers in Babine Lake Areas 1 - 5 during 1958-'62 and 1973- 7 * (the average ,  i  represents the area under a seasonal abundance curve divided by the length of the sampling season in days). Vertical bars represent 2 S.E. 7.  .......25  Average seasonal zooplankton biomass and numbers in Babine Lake Areas 1 - 5 during 1958-'62, 1973 and 1974. Vertical bars represent 2 S.E.  8a.  .....27  Average seasonal Cyclops, Diaptomus and Daphnia abundance in Babine Lake Areas 1 - 5 .  8b.  ,,  Vertical bars represent 2 S.E  29  Average seasonal nauplli-early copepodite Heterocope and Bosmlna abundance i n Babine Lake Areas 1 - 5 . Vertical bars represent 2 S.E.  .,31  vi.  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  10.  35  Seasonal changes in zooplankton biomass in Babine Lake Areas 2 and 4 during 1958, '60, '62, * 73 and '74  11.  37  Seasonal changes in zooplankton numbers in Babine Lake Areas 2 and 4 during 1958, '60, '62, '73 and '74  12a.  39  Seasonal changes in Cyclops, Diaptomus and Heterocope numbers in Babine Lake Area 2 during 1958, '60, '62, '73 and '74  12b.  42  Seasonal changes in Cyclops, Diaptomus and Heterocope numbers in Babine Lake Area 4 during 1958, '60, '62, '73 and '74  13a.  44  Seasonal changes in nauplii-early copepodite, Bosmlna and Daphnia numbers in Babine Lake Area 2 during 1958, '60, •62,  13b.  '73 and '74  ...46  Seasonal changes in nauplii-early copepodite, Bosmlna and Daphnia numbers in Babine Lake Area 4 during 1958, '60, •62,  14.  '73 and 74  48  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  15.  ,  51  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  vii.  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)..  17.  55  Size frequency distribution of zooplankton species encountered by fry during electivity experiments (n = 40),.  18.  58  Mean total zooplankton density at different zooplankton density indices, G =» 1,2 and 4,5 (see methods).  Vertical  bars represent 95% confidence limits.....,,,.. 19.  ,  60  Electivity values of zooplankton encountered by fed f i s h at different zooplankton densities.  The horizontal  scale represents a zooplankton density index increasing ...,62  from 1 to 5 (see Fig. 18) 20.  Electivity values of zooplankton encountered by starved f i s h at different zooplankton densities.  The horizontal  scale represents a zooplankton density index increasing from 1 to 5 21.  (see Fig. 18)  .64  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  ,  ,  viii,  23.  Relationship between zooplankton biomass (mg. dry wt, m.—2 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 f e r t i l i z a t i o n ; Owikeno Lake (Narver 1969) adjusted according to Stockner and Shortreed (1974); five areas of Babine Lake's Main Arm i n 1973) 24.  75  Regression of vertical haul biomass (mg. dry wt. m~^) on 5 meter-strata biomass (mg. dry wt. m*"^) in 1973.........  25.  93  Regression of vertical haul biomass (mg. dry wt. m~^) on 0 - 5 meter oblique haul biomass (mg. dry wt. m~^) i n 1974........95  26.  Regression of biomass estimates (mg. dry wt. m~^) of a Clarke-Bumpus sampler on those of a Miller sampler in 1974  27.  Regression of Clarke-Bumpus catches (no. L the Miller sampler,,...  ,  98  ) on those of 100  ix. LIST OF TABLES Table 1.  Page Volume contributed by individual Babine Lake macrozooplankton 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 collection.  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 assistance in the f i e l d 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 communities (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 i n 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 ( D i l l 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 Heterocope septentrionalis (juday and Muttkowskl) (Johnson 1961; McDonald 1969; Narver 1970).  2. Babine Lake produces up to 90% of the Skeena River sockeye run which i s 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 paraphrased 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. several effects:  Food limitations could have  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).  I f 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. F i r s t , l i t t l e i s known about early phases of fry feeding.  Second, Increased  fry numbers may induce changes in zooplankton density and in species composition 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 i s 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 i s 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 ( D i l l 1968). The f i r 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 in Areas 4 and 5 (Fig. 1 and 3). ward dispersal.  concentrated  This was followed by a period of north-  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 u n t i l the following May when the majority migrate to sea are.;poorly understood. Although year to year variability i s 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 d i e l 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 i s followed by a night descent to about 12 m and predawn ascent to feed before moving to deeper daytime depths.  There i s 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  Period 1  June 25 - J u l y 27  Time  Period  2 A u g . 16 - S e p t . 9  Year  Time  Period 3  Oct. 6 - Oct. 25  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, 2 6 . 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). echo sounder.  Sampling depths were confirmed using a Furuno FUG 400W 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 s p l i t 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) u n t i l 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 i s provided by Johnson (1965b). The fractions used for dry weight were filtered onto pre-ashed, pre-weighed 4.25 cm Reeve-Engel GFC f i l t e r s 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 i n 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 f i s h was recorded prior to dissection.  Stomachs removed from the f i s h 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 i n Table I. Mean percent abundance of each zooplankton taxon was determined after an angular transformation 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 abundance curves were treated i n 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 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  -  4.1  Epischura nevadensis Heterocope septentrionalis Holopedium gibberum  (1) (2)  Relative Volume (1)  12.0  -  16.2 7.0  from Narver (1969) Calculated from 1973 zooplankton and stomach samples using a method adapted from Hellawell and Abel (1971).  17. Differences ln analytical technique between study periods necessitated 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, i n 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 i n terms of the abundance 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, where  r - .93,  n - 50  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 gm  f  mean length =» .30 cm) run was subdivided into two groups.  Group 1 fish, kept in filtered (5 ^  Aquapure f i l t e r ) Fulton River water,  were fed frozen Babine Lake zooplankton every 24 hours and were referred to as fed f i s h .  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. referred to as starved fish.  These fry were  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 f i l t e r e d river water. One f i s h (either fed or starved) was placed in each tank and allowed to acclimatize 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 l i t t o r a l or limnetic plankton. The two plankton groups were chosen for the following reasons. First, young sockeye fry appeared to spend some time in the l i t t o r a l zone of the lake (McDonald 1969) and second, i t was thought that the l i t t o r a l species composition might be different from that in the limnetic zone. A sample of either limnetic or l i t t 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 l i t e r .  Diaptomus was chosen as a zooplankton density Indicator  because of i t s 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 k i l l e d and preserved in 10% formalin.  In total, 20 combinations of fish group (fed or starved),  zooplankton group (limnetic or l i t t o r a l ) 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 f i s h 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 l i t t 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 l i t t 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 p  i s the proportion of a particular prey type i n the diet i s the portion of a particular prey type in the environment  21.  Figure 4.  Figure 5.  Observation apparatus used to determine activity rates and conduct predatlon experiments.  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 B u l b Volume w i t h 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 o f each s p e c i e s a t t h e t h r e e d i f f e r e n t a c c l i m a t i o n and t h r e e l i g h t i n t e n s i t i e s  Prey  (220,  350  and 560  temperatures  lux- ). 3  Susceptibility Zooplankton u s e d i n p r e d a t i o n experiments were s o r t e d i n t o  4 groups:  five  4  C. s c u t l f e r , D. a s h l a n d i , and H. s e p t e n t r i o n a l i s .  Samples were  a c c l i m a t e d i n darkness a t l6°G f o r f o u r h o u r s .  Each s p e c i e s was t h e n p l a c e d  s e p a r a t e l y i n an o b s e r v a t i o n a p p a r a t u s ( F i g . 4)  a t l6°G and 5&0  l u x , and  t h e time I r e q u i r e d , a c t i n g as a l i g h t adapted " e x p e r i e n c e d " p r e d a t o r , t o remove t e n i n d i v i d u a l s w i t h a m o d i f i e d p i p e t t e ( F i g . 5) E a c h experiment was o f experiments was  was r e c o r d e d .  r e p l i c a t e d t e n times f o r each s p e c i e s .  The  chosen a t random t o m i n i m i z e t h e e f f e c t o f immediately  p r e v i o u s e x p e r i e n c e a t c a t c h i n g t h a t s p e c i e s i n d e t e r m i n i n g my s u c c e s s as a p r e d a t o r .  subsequent  The dimensions o f p r e y s e i z i n g a p p a r a t u s , a  ' Pasteur p i p e t t e , a r e given i n FigineS. a d j u s t e d depending on p r e y s i z e (1.2 Heterocope).  sequence  S u c t i o n b u l b volume was  The p i p e t t e mouth d i a m e t e r  mm  clear  was  f o r C y c l o p s and Diaptomus, 4 mm f o r  reduced from 2 ml t o 1.3  ml t o lower  t h e chances o f c a p t u r i n g prey n o t n e a r t h e p i p e t t e mouth.  3.  Approximate l i g h t i n t e n s i t y was measured w i t h a P h o t o v o l t Corp. F200 meter s e n s i t i v e between 300 and 650 w i t h a peak a t 375.  4.  With and w i t h o u t eggs.  24. RESULTS LAKE PROGRAM Changes i n Average Zooplankton Abundance Between Years Biomass and Numbers Average biomass l e v e l s , calculated from integrated areas under seasonal biomass curves (see methods), were s i g n i f i c a n t l y lower i n 1973^?4 than 1958-162 (Pig. 6)^  The most s i g n i f i c a n t decreases  observed i n Areas 1, 2 and 4.  70%) were  Average numbers were also lower i n 1973-*74  than 1958-'62 (except i n Area 5) (Pig. 6 ) .  The 1973 and 1974 biomass and  numbers considered separately, d i f f e r e d i n a s i m i l a r manner from 1958-'62 ( F i g . 7). Biomass l e v e l s i n 1973 were considerably lower  (Ci60%)  than i n  1958-'62;(and 1974), p a r t i c u l a r l y i n Areas 1, 2 and 5. Numbers were a l s o lower i n 1973 ( F i g . 7). In 1974 biomass l e v e l s were s i g n i f i c a n t l y lower than i n 1958-'62 ( F i g . 7). Areas 2 and 4.  Decreased biomass  (*2k0%)  was most evident i n  Although generally lower, numbers i n 1974 were not s i g n i f i -  cantly d i f f e r e n t from 1958-'62,  Species Abundance The abundance o f Cyclops, Daphnia, Diaptomus, Heterocope and nauplii-copepodites of a l l stages d i f f e r e d most between study periods (Fig. 8a, b ) . Cyclops abundance was s i g n i f i c a n t l y 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  l e v e l s and those o f 1958-'62 and the trend f o r Cyclops numbers t o increase from north to south observed i n 1958-62 was also more apparent i n 1974 than 1973 ( F i g . 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 s i g n i f i c a n t l y d i f f e r e n t .  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 100755025-  o  504030-  i  2010o  0 2  3  Lake Areas (Fig.l) 1 9 5 8 - 6 2 (•) 1973-74  (O)  4  27.  Figure 7.  Average seasonal zooplankton biomass and numbers in Babine Lake Areas 1-5 during 1958-'62, 1973 and 1974. bars represent 2. S.E.  Vertical  125  I  1007550-25-  i  O A  A  A  _l_  50 r  40-  5  3020-  o  10-  A  0 2 Lake 1958-62, 1973  (A)  1974  (o)  3 A r e a s (Fig.1)  4  i  29.  Figure 8a.  Average seasonal Cyclops. Diaptomus and Daphnia abundance in Babine Lake Areas 1-5.  Vertical bars represent 2 S.E.  Cyclops 3 0  20  30.  scutifer  T  I  10 f  i  I. -i  i  i  i  Diaptomus  i  —J  i  i  i_  -J  L  l _  1  _l  L...  ashlandi  30-r  o 20  10+  i  *  E  _l  4-r  2  Daphnia  I  L_  longispina  f  i J  J i • i  _i i_  Lake Areas 1958- 62 (•) 1973  (A)  1974  (o)  i  i_  31.  Figure 8b.  Average seasonal nauplii-early copepodite, Heterocope and Bosmlna abundance in Babine Lake Areas 1-5. bars represent 2 S.E.  Vertical  Nauplii  32.  copepodites  T  10 +  o  1 5+  _l  .15-  I  Heterocope  I  I  I  I  1  1  L.  - 1 — I — « -  septentrionalis  .10 .06  1 _l__k  E  Z3  Bosmino  i I  I  5 I  • I  1  L.  _J  1_  A  9  - j — i —  coregoni  Q  3+  5 • j  i  i_  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, ln Areas 1, 2 and 4 (Fig. 8a).  particularly  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 i n 1958'62.  As with Cyclops, the trend for numbers to increase from north to  south evident i n 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 abundance 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 i n 1973, but relative to 1958-'62, lower i n 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 studyperiods. Biomass and Numbers Biomass generally peaked i n 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 i n 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). a l l areas during 1958,  '60, '62  Seasonal changes in numbers were similar i n 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 i n 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-'?*. Vertical 2  bars represent 2 S.E.  ART A I  300-p  00 +  20 +  10  May  Jir*  JJy.  Ai^usJ  Seprentier  October  1958 - 62  1973 -'74  AREA 2  W°T  T  37.  Figure 10.  Seasonal changes in zooplankton "biomass i n Babine Lake Areas 2 and 4 during 1958,'60, '62, '73, and '74.  Area 2  10001  100  t  S Area 4  Months  I958 (•)  1973  I960 ( A )  1974 10)  I962  (O)  (A)  CO  39.  Figure 11.  Seasonal changes in zooplankton numbers in Babine Lak Areas 2 and 4 during 1958,  '60, '62, 73  and  M  1 9 5 8  ( • )  I 9 6 0  (A)  (962 (O)  J  J  A  1973(A)  1 9 7 4  (O)  S  0  41.  Taxa Cyclops was most abundant in June during 1958, '60 and '62 "but small f a l l peaks also occurred i n Areas 2 and 4, (Pig. 12a, b). and f a l l blooms were prolonged in Area 4.  Both spring  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 i n 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 ' 6 2 . In 1973 and 1974, single blooms i n  late July were observed in Areas 2 and 4.  Area 2 Diaptomus was considerably  lower i n 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. and in 1974, absent.  In Area 2 the 1973 summer peak was prolonged  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 i n both  areas than in 1958, '60 or ' 6 2 . 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.  F i g u r e 12a.  S e a s o n a l changes i n C y c l o p s , Diaptomus and Heterocope numbers i n Babine Lake A r e a 2 d u r i n g 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 i n n a u p l i i - e a r l y copepodite, Bosmina and Daphnla numbers i n Babine Lake Area 2 during 1958,  •62, '73 and '74.  '60,  48.  Figure 13b.  Seasonal changes i n n a u p l i i - e a r l y eopepodite, Bosmina and Daphnia numbers i n Babine Lake Area 4 during 1958,  '73 and '74.  '60,  '62,  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 i n Area 4 during 1973. Zooplankton Composition in Sockeye Fry Diets A comparison of the late August and late September diets of f r y between 1967 and 1973 indicate that significant changes i n 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 i n 1967, were present only in small proportions i n 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 i n 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 u t i l i z e d to a limited extent ( CL %) i n 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 f i s h taken from different areas in late August of 1973 indicates the prevalence of Daphnia in the diets was f a i r l y 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 f r y 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.  I q t e - August  late - Septe mber  100  * ** • *  c o o a. 6 Oo  •  •  t- < *  «  • * *  •  •  4- 4 i. 4 • •• •  5 0 4-  •  c a> o  •  • • • *  •  •  v_  4  •  CL.  • •  «•  4  * •  4  W W  njinu  0  1973 (3 2)  B o smino  Cyclops  Diaptomus  Epischura  Partially  digested  material  Daphnia Heterocope  53.  Figure 15.  Seasonal changes i n zooplankton composition of sockeye fry diets i n the Main Arm of Babine Lake during 196? (McDonald 1973). Carats and numbers indicate sampling dates and sample sizes respectively.  54.  100  55.  • * Figure 16.  The zooplankton composition of sockeye fry diets i n 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 -  Cyclops  Bosmina  Diaptomus  partially  Early September  Epischura  digested  material  Daphnio Heterocope  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.  DAPHNIA LONGISPINA  B. COREGONI X=88  12  X = 2.57 ,  n=40  8 4  i  0 5  r-i  20  10 1  III  1  n-40  !  r  30  H. SEPTRIONALIS  E. NEVADENSIS  D. ASHLANDI  40  X-1.12  24  X=4.30  22 CD  -O  e • c  >. u c;  X=2.84 i  18 14 12  0).  8  CT 0)  4  X-.80 + n=40  0 5  C'SCUTIFER 14  |  copepodite  n-40  8 4 I///  0 5  10  | adult  X=1.87  X=.84  12  40  ao  20  10  n-50  n=40  n=40  20 Size (mm)  50  6o.  Figure 18.  Mean total zooplankton density at different zooplankton density indices, C = 1, 2 and k,  5 (see methods).  bars represent 95% confidence limits.  Vertical  61.  20,000  Starved  Fed F i s h  10, 000 4-  in  c' Q c  o <=,  o  I  ooo  QL O O  N  O  H  . c o CD  2  OO-L  |  2  1  H4  5  Zoo  plankton  Density  Index  Fish  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 (adults)  D. ashlandi  Copepod  (adults)  nauplii  H. septentrionalis  E. nevadensis  64.  F i g u r e 20.  E l e c t i v i t y v a l u e s o f z o o p l a n k t o n encountered by s t a r v e d a t d i f f e r e n t zooplankton d e n s i t i e s . represents  5  a zooplankton d e n s i t y  (see F i g . 18).  fish  The h o r i z o n t a l s c a l e  i n d e x i n c r e a s i n g from 1 t o  C. sculifer (adultt)  D. oshlandi (adultt)  Ul  ON  66. Starved f i s h displayed increased s e l e c t i v i t y f o r Cyclops and Dlaptomus adults as t o t a l prey density rose.  E l e c t i v e t i e s of Cyclops and  Dlaptomus copepodites fluctuated below 0 but were generally lowest when t h e i r r e l a t i v e abundance was highest.  N a u p l i i , Heterocope and Epischura  (in most cases) were avoided (E » - l ) . Prey A c t i v i t y and S u s c e p t i b i l i t y Activity Cyclops and Heterocope were much more a c t i v e than Dlaptomus (Fig. 21).  Cyclops' c h a r a c t e r i s t i c horizontal movements were punctuated  by short periods of "rest", hence a high a c t i v i t y rate.  No s i g n i f i c a n t  v a r i a t i o n s l n Cyclops a c t i v i t y were noted except at 4°C where a c t i v i t y  was  lower between 350 and 560 lux and a t 560 l u x where a c t i v i t y increased with temperature (see F i g . 21),  Heterocope a c t i v i t y , s i m i l a r to that o f Cyclops,  was reduced a t 350 lux and 8°C.  Dlaptomus, unlike Cyclops and Heterocope,  moved v e r t i c a l l y and long periods of " r e s t " were punctuated by bursts of a c t i v i t y , r e s u l t i n g i n low a c t i v i t y rates.  There were no s i g n i f i c a n t  l i g h t or temperature induced variations i n Dlaptomus a c t i v i t y . Susceptibility Predatlon s u s c e p t i b i l i t y to the pipette d i f f e r e d l i t t l e among genera and between i n d i v i d u a l s with or without eggs ( F i g . 22), except f o r egg bearing Cyclops which were 1.4 times more susceptible to predation than Cyclops without eggs.  The apparent differences i n Dlaptomus with and without  eggs were not s i g n i f i c a n t .  V u l n e r a b i l i t y of the much l a r g e r Heterocope  was s i m i l a r to that of the other genera, however the increased pipette aperature s i z e may have affected r e s u l t s .  According to Bernoulli's  p r i n c i p l e , increased aperature s i z e should reduce intake v e l o c i t y at the pipette mouth.  Reductions i n v e l o c i t y 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. Temp. 220 lux 150  ^-C yd ops  560  16'C  i  T  1  100  Heterocope 50  Diaptomus ,  8 C  :  1  350  150 -r  150  100-  100  50+1  1  50  .1  4 _4  _|  y-  16 C  -\  10  15.  \  560  150-  150  100-  100  50  50 +  -I  200  1-  000 Light  400  500  Intensity(Lux)  r  600  5  Temperature (  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  r—  Diaptomus  i  1  Heterocope 1 -  eggs 150  no eggs  no eggs  no <pggs edjgs  100"  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 1973 . 6  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$) and appeared to result from prolonged concentrated predatlon ln certain lake areas. In Babine Lake (Johnson 1965c) and in other lakes (Galbraith 1967; Goodlad, Gjernes and Brannon 1974; Hutchinson 1971) high or increased planktivore densities were accompanied by low or decreased zooplankton biomass.  Reduced zooplankton  biomass likely results from selective removal of larger individuals from the zooplankton community. Reduction in biomass in Area 1 i n 1973 relative to those of 1958-*62 deserve special mention because of estimated low fry densities, i.e. low predatlon pressure (see Fig. 3). Zooplankton biomass was reduced 70$ i n 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 i n fry distribution or perhaps from increased immigration into Area 1 as a result of high fry concentrations i n 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 i n 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 zooplankton 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 i n 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 compensatory feeding on Cyclops as suggested by reduced Cyclops densities in the spring of 1974.  As Cyclops i s 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, i s much greater i f dispersal i s 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 i n 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 i s a positive relationship  between primary productivity and zooplankton biomass levels in several oligotrophic sockeye-producing lakes in British Columbia, and Alaska (Fig. 23). It i s 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. 7.  There i s  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. m  —2  . day  -1  ) and primary productivity (mg Q.m  -2  . day  -1  )  in several oligotrophic sockeye producing lakes in British Columbia and Alaska (Great Central Lake before and after f e r t i l i z a t i o n ; Owikeno Lake (Narver 1969) adjusted according to Stockner and Shortreed Lake's Main Arm in 1973).  (197*0,  areas of Babine  76.  I 00  T  Great  .1 I 10  1 — — i  -i  1—c—i  A  Owikeno  Lake  ©  Babine  Lake  O  Karluk  Lake  1—,—t—i—i—i  50  Primary  Central  100  1  Productivity  1  •  1—•—i  Lake  1  (mgC-m-day  1—^—i—y—»  )  1000  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 i n Babine Lake during 1973.  I f 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 The average 1974 Secchi depths were d% deeper than in 1973.  1974.  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 i t s 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 f i s h (O'Brien and Vinyard 1974).  However, i t was f e l t 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, u t i l i z e d 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 f i 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 (Heterocope, 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 i n 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$), relative to 1958' 6 2 , suggests a significant cropping effect particularly in Areas 2, 4 and 5» while 1974 abundance relative to 1973 suggest decreased cropping. Contrary to expectations, 1973 Heterocope abundance in northern areas had not decreased from 1958-'62 levels.  Fry distribution may have  concentrated predatlon in Areas 4 and 5 during July, and August when Heterocope was most abundant in Areas 1 and 2.  Since reductions &n Areas 4 and 5  were not significant, i t suggested that analytical techniques used to compensate for Heterocope's diurnal migrations may have overestimated 1973 Heterocope abundance in a l l areas. Decreased 1974 abundance (50$) 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 i n lessening the predatlon impact on Cyclops. First, since Cyclops were 35% fewer than Dlaptomus i n 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 i n 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 i n  1967. The striking, rather unexpected, increases i n 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; N e l l l 1975). Continuing reduction i n numbers of the larger zooplankton could induce selection for smaller zooplankton, less accessible to fry, i n 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 i n Babine Lake.  The experiments  discussed below were designed to determine how selective feeding (or  81. electivity) was influenced by variations in zooplankton composition, density, size and activity. Electivity Both predator groups (fed and starved) encountering live zooplankton 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 i n 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 f i s h 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 f i s h 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 i s likely a very low  diurnal change in predation vulnerability in areas with similar temperatures.  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 i t s catchability. Increasing the "predator" mouth size to accomodate Heterocope reduced intake velocities so that the relative vulnerability of these animals may have been underestimated. 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 i n 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; f i r s t , high copepod activity rates might increase encounter rates between sockeye fry and i t s 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. 1.  Outstanding among these were:  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 i n 1973 and 1974 and Heterocope i n 1974, relative to 1958-'62.  Reduced numbers of Heterocope i n the 1973 diet in spite of  i t s 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 i n 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.  87. MANAGEMENT IMPLICATIONS Johnson's hypothesis that the main arm of Babine Lake could support up to a four-fold increase i n fry numbers appears to hold i n 1972 and 1973 "but not i n 1974. The chances of food limitation of fry production have greatly increased particularly i n Areas 1, 4 and 5 where the most significant decreases i n zooplankton biomass have occurred.  However, one  must bear i n 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 i n 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 i n 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 i n 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 planktivorous 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 sizeselective 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 i n a shallow eutrophic lake, Mem, 1st, I t a l . Idrobiol.  22:  81-103.  Fleminger, A. and R. I, Clutter 1965. Avoidance of towed nets by zooplankton, 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. J r . 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 preservation 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 f i s h 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, zooplanktoneating 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 a b i l i t i e s 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. I I . 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. N e i l l , 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 i n 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 ClarkeBumpus 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.  92. APPENDIX A 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 i s determined principally by the zooplankton's response to hydrostatic waves preceding approaching nets, the time required to respond ( i f 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 f i l t r a t i o n coefficient which determines how much water passes through the sampler. This Appendix discusses the relative efficiency of the ClarkeBumpus 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~ ) on J  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-  BIOMASS  C5 I v C T E R H A U L  )  95.  Figure 25.  Regression of vertical haul biomass (mg. dry wt. m"-) on 5  0-5 meter oblique haul biomass (mg. dry wt, m ) in 1974. J  96.  BIOMASS  1 0 - 5 OBLIQUE HAUL )  97. samplers (each with a #10, 1153/u'net) determined their relative efficiency (Fig. 26,  27.).  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 f i l t r a t i o n efficiency (F) of O.896 at velocities of 1.5 m / s e c . i . e . g i t f i l t e r e d 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 GlarkeBumpus 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, i s described by the formula: p . W A-D where: W i s the Volume filtered; A, the mouth area; and D, the distance towed.  98.  Figure 26.  Regression of biomass estimates (mg. dry wt. m."-) of a 5  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 of the Miller sampler.  ) on those  Y=0.23 + 0.-68*X w 3 p. e  n-15.  w  . ZO*  P< 6 3  r=0.93  y-q.40 + o.70*x n-15,  |  3 cn Q) M  r=0.98  a  nJ  rH  U  (T) rH O  tr  Q) 10  •H rl  -t->  co 3 6 O  •+•>  P.  o  rH O  Cyclops/Litre  (Miller)  Y=0.03 + 0.71*X 'n-15, r=0.85  Diaptomus/Litre  (Miller)  a Y—0.11 + 0.96*X  PP I  eu  n-14, r=0.89  U rt rH O CD U -P •H  C  •H  6  W O FP  d-  Daphnia/Litre  (Miller)  3-  1-  »•  Bosmina/Litre  6-  7.  t-  (Miller)  j.  


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