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Planktivorous feeding and habitat utilization of sympatric and experimentally segregated populations… Hume, Jeremy M. B. 1978

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PLANKTIVOROUS FEEDING AND HABITAT UTILIZATION OF SYMPATRIC AND EXPERIMENTALLY SEGREGATED POPULATIONS OF COASTAL CUTTHROAT TROUT (SALMO CLARKI CLARKI) AND DOLLY VARDEN CHAR (SALVELINUS MALMA) by JEREMY M. B. HUME B.Sc., U n i v e r s i t y of B r i t i s h Columbia, 1971 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept t h i s t h e s i s as conforming to the required standard. THE UNIVERSITY OF BRITISH COLUMBIA ' • AUGUST, 19 78 Jeremy M.B. Hume, 1978 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of Brit ish Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Zoology The University of Brit ish Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 D a t e August 4 , 1978 i I ABSTRACT Sympatric p o p u l a t i o n s of D o l l y Varden ( S a l v e l i n u s malma) and c u t t h r o a t (Salmo c l a r k i c l a r k i ) are s p a t i a l l y and food segregated i n Loon Lake, B r i t i s h Columbia. D o l l y Varden are found mainly i n the lower water column, while c u t t h r o a t are found i n the middle and upper water column. Zooplankton, the o n l y major prey type found i n both d i e t s , appear more important to the c u t t h r o a t . The purpose of t h i s study was to determine whether t h i s s e g r e g a t i o n was due to b e h a v i o u r a l i n t e r a c t i v e processes or g e n e t i c a l l y based s e l e c t i v e p r o c e s s e s . T h i s was i n v e s t i -gated by the s e g r e g a t i o n and t r a n s f e r r a l of the members of the two p o p u l a t i o n s to separate f i s h l e s s l a k e s . In a d d i t i o n , f i e l d s t u d i e s examined s e l e c t i v e f e e d i n g on zooplankton and l a b o r a t o r y s t u d i e s compared the f e e d i n g behaviour and a b i l i t i e s o f D o l l y Varden and c u t t h r o a t on three d i f f e r e n t types of zooplankton. L i t t l e change o c c u r r e d d u r i n g the two year sampling p e r i o d i n e i t h e r the d i e t or the v e r t i c a l d i s t r i b u t i o n of the two s p e c i e s , an i n d i c a t i o n t h a t the f i s h were not i n t e r a c t i v e l y segregated. However, the p h y s i c a l and b i o l o g i c a l charac-t e r i s t i c s of the lakes may have r e i n f o r c e d , a t l e a s t i n i t i a l l y , p r e v i o u s behaviour p a t t e r n s . As a r e s u l t , more time may be r e q u i r e d to p r o p e r l y assess the f i n a l r e s u l t s . Laboratory results indicate that primarily Chaoborous larvae and secondarily Daphnia pulex were the preferred prey types to both f i s h predators, mainly because of their poor escape response when compared to that of Diaptomus kenai. The abundance of these prey types i n the diet of lake-caught f i s h was generally i n accordance with the laboratory pref-erences. These results also demonstrated that cutthroat were more ef f i c i e n t planktivores than were Dolly Varden. i v TABLE OF CONTENTS. Page A b s t r a c t i i Table of Contents .' i v L i s t of F i g u r e s v i i L i s t of Tables x i i i Acknowledgements x v i i I. I n t r o d u c t i o n 1 I I . Study Area 5 A. D e s c r i p t i o n of the Lakes 5 B. I n v e r t e b r a t e Fauna 12 1. Crustaceous Zooplankton 12 2. Chaoborus 16 I I I . Methods and M a t e r i a l s 17 A. F i e l d Work 17 1. T r a n s f e r Methods.. 17 2. P o p u l a t i o n Estimates-LOon Lake... 19 3. Sampling Methods 19 (i) F i s h 19 ( i i ) Zooplankton 20 4. Laboratory A n a l y s i s 21 (i) F i s h Stomach Contents 21 ( i i ) Zooplankton 23 B. Laboratory Experiments 2 3 1. F i s h Holding F a c i l i t i e s . . . 23 2. Prey Items 24 3. Observation F a c i l i t i e s 25 4. Experimental Procedure 26 V IV. R e s u l t s 29 A. F i e l d 29 1. F i s h p o p u l a t i o n s 29 2. F i s h Age and Length Composition.. 32 3. F i s h Growth. 32 4. V e r t i c a l F i s h D i s t r i b u t i o n 40 (i) Loon Lake - C u t t h r o a t and D o l l y Varden 4 0 ( i i ) Eunice Lake - C u t t h r o a t 47 ( i i i ) K a t h e r i n e Lake - D o l l y Varden 50 5. D i e t 53 (i) C u t t h r o a t Trout 53 ( i i ) D o l l y Varden 58 6. F a c t o r s A f f e c t i n g D i e t 63 (i) F i s h Length 63 ( i i ) Depth Caught 67 7. Composition of Zooplankton i n the D i e t 71 (i) Seasonal Changes 71 ( i i ) Comparison with Lake Zooplankton 7 5 ( i i i ) S i z e D i s t r i b u t i o n of Zooplankton 8 2 8. Chaobrous i n the D i e t 93 B. L a b o r a t o r y 95 1. Movement and L o c a t i o n 9 5 2. P r e d a t i o n on S i n g l e Prey Types 97 V i (i) D i f f e r e n c e s between Prey Types 97 ( i i ) Changes wit h Experience 99 3. P r e d a t i o n on P a i r e d Prey Types...., 101 V. D i s c u s s i o n 108 VI. L i t e r a t u r e C i t e d -^ 22 V I I . Appendices 138 v i i LIST OF FIGURES. F i g u r e DESCRIPTION Page 1. Map of U.B.C. Research F o r e s t showing the l o c a t i o n of the study l a k e s 6 2. Map of Loon Lake showing sampling s i t e s 7 3. Map of Eunice Lake showing sampling s i t e s . . . . 8 4. Map of Katherine Lake showing sampling s i t e s . 9 5a. Hypsographic curves showing area of c r o s s s e c t i o n s taken a t v a r i o u s depths of the study lakes 10 5b. Hypsographic curves showing area of c r o s s s e c t i o n s taken at v a r i o u s depths of the study l a k e s expressed as a percentage of the s u r f a c e area 10 6. Mean body l e n g t h of some common crustacean macrozooplankters i n the l i m n e t i c r e g i o n of the study lakes 15 7a. Length d i s t r i b u t i o n of Loon Lake c u t t h r o a t captured i n t r a p n e t s and t r a n s f e r r e d to Eunice Lake 33 7b. Length d i s t r i b u t i o n of Loon Lake D o l l y Varden captured i n t r a p n e t s and t r a n s f e r r e d to K a therine Lake 33 8a. R e l a t i o n s h i p of weight and l e n g t h of c u t t h r o a t caught i n g i l l nets i n Loon and Eunice Lakes 36 8b. R e l a t i o n s h i p of weight and l e n g t h of D o l l y Varden caught i n g i l l nets i n Loon and' Katherine Lakes 36 v i i i 9a. R e l a t i o n s h i p of growth r a t e and i n i t i a l s i z e o f tagged and r e c a p t u r e d c u t t h r o a t i n Loon and Eunice Lakes 38 9b. R e l a t i o n s h i p of growth r a t e and i n i t i a l : s i z e of tagged and r e c a p t u r e d D o l l y Varden i n Loon and Katherine Lakes 38 10. V e r t i c a l d i s t r i b u t i o n d u r i n g the s p r i n g and summer of sympatric Loon Lake c u t -t h r o a t and D o l l y Varden i n nets s e t from surface t o bottom at the 5 and 10 m contours 41 11. V e r t i c a l d i s t r i b u t i o n d u r i n g the autumn and wi n t e r of sympatric Loon Lake c u t t h r o a t and D o l l y Varden i n nets s e t from s u r f a c e to bottom a t the 5 and 10 m contours 42 12. V e r t i c a l d i s t r i b u t i o n of a l l o p a t r i c Eunice Lake c u t t h r o a t i n nets from surface to bottom at the 5 and 10 m contours 4 8 13. V e r t i c a l d i s t r i b u t i o n of a l l o p a t r i c K a therine Lake D o l l y Varden i n nets s e t from surface to bottom at the 5 and 10 m contours. 51 14. Seasonal changes i n the d i e t of Loon Lake c u t t h r o a t d i v i d e d i n t o major prey c a t e g o r i e s a c c o r d i n g to l o c a t i o n of prey i n the water column 54 15. Seasonal changes i n the d i e t of Eunice Lake c u t t h r o a t d i v i d e d i n t o major prey c a t e g o r i e s a c c o r d i n g to l o c a t i o n of prey i n the water column 56 i x 16. Seasonal changes i n the d i e t of Loon Lake D o l l y Varden d i v i d e d i n t o major prey c a t e g o r i e s a c c o r d i n g t o l o c a t i o n of prey i n the water column 60 17. Seasonal changes i n the d i e t of Katherine Lake D o l l y Varden d i v i d e d i n t o major prey c a t e g o r i e s a c c o r d i n g t o l o c a t i o n of prey i n the water column 62 18a. D i f f e r e n c e s i n the d i e t of Eunice c u t t h r o a t of d i f f e r e n t body lengths 64 18b. D i f f e r e n c e s i n the d i e t of Loon c u t t h r o a t of d i f f e r e n t body lengths 64 19a. D i f f e r e n c e s i n the d i e t of Katherine D o l l y Varden of d i f f e r e n t body lengths 66 19b. D i f f e r e n c e s i n the d i e t of Loon D o l l y Varden of d i f f e r e n t body lengths 66 20a. D i f f e r e n c e s i n the p l a n k t i v o r o u s d i e t of Eunice c u t t h r o a t and Katherine D o l l y Varden of d i f f e r e n t body lengths 68 20b. D i f f e r e n c e s i n the p l a n k t i v o r o u s d i e t of Loon c u t t h r o a t and D o l l y Varden of d i f f e r e n t body lengths 6 8 21. Comparison of the d i e t of Eunice and Loon c u t t h r o a t caught i n the lower and upper h a l f o f the 5 and 10 m g i l l nets f i s h i n g from the s u r f a c e to the bottom of the lake 7 0 22. Seasonal changes i n the composition of zooplankton i n the d i e t of Loon and Eunice c u t t h r o a t 7 2 23. Seasonal changes i n the composition of zoo-plankton i n the d i e t of Loon and Katherine D o l l y Varden 74 24a. Comparison of the seasonal changes i n the percentage c o n t r i b u t i o n of Daphnia rosea to the p l a n k t i v o r o u s d i e t of the f i s h and to l i m n e t i c lake zooplankton p o p u l a t i o n s i n Loon, Eunice, and Katherine Lakes 76 2 4b. Comparison of the seasonal changes i n the percentage c o n t r i b u t i o n of Diaptomus kenai to the p l a n k t i v o r o u s d i e t of the f i s h and to the l i m n e t i c lake zooplankton popula-t i o n s i n Loon, Eunice, and Katherine Lakes... 76 25. Comparison of the seasonal changes i n the percentage c o n t r i b u t i o n of other zooplankters to the p l a n k t i v o r o u s d i e t of the f i s h and to the l i m n e t i c lake zooplankton p o p u l a t i o n i n Loon Lake 7 9 2 6a. Comparison of the seasonal changes i n the percentage c o n t r i b u t i o n of other zooplankters to the p l a n k t i v o r o u s d i e t of the f i s h and to the l i m n e t i c lake zooplankton p o p u l a t i o n i n K atherine Lake 81 26b. Comparison of the seasonal changes i n the percentage c o n t r i b u t i o n of other zooplankters to the p l a n k t i v o r o u s d i e t of the f i s h and to X I 36 88 the l i m n e t i c lake zooplankton p o p u l a t i o n of Eunice Lake 81 27. Comparison of the seasonal changes i n the l e n g t h d i s t r i b u t i o n of E K _ rosea i n the d i e t of c u t t h r o a t and i n the l i m n e t i c lake zooplankton of Eunice Lake 8 3 28. Comparison of the seasonal changes i n the l e n g t h d i s t r i b u t i o n of rosea i n the d i e t of c u t t h r o a t and i n the l i m n e t i c lake zooplankton of Loon Lake 29. Length d i s t r i b u t i o n of D_^  rosea i n 5 d i f f -e r e n t depth i n t e r v a l s i n May, 19 76 and August, 1975 30. Comparison of the l e n g t h d i s t r i b u t i o n of D. kenai i n the d i e t of c u t t h r o a t and i n •the l i m n e t i c zooplankton of Loon and Eunice Lakes 91 31a. S i n g l e prey type experiments: r e l a t i v e frequency of a t t a c k s by c u t t h r o a t and by D o l l y Varden on 3 prey types; Daphnia pulex, Diaptomus kenai., and 4 t h i n s t a r Chaoborus t r i v i t t a t u s . . 100 31b. S i n g l e prey type experiments: capture e f f i c i e n c y by c u t t h r o a t and by D o l l y Varden on 3 prey types; Daphnia pulex, Diaptomus kenai and 4th i n s t a r Chaoborus t r i v i t t a t u s . . . 100 32a. P a i r e d prey type experiment: cumulative captures of 4th i n s t a r C_^  t r i v i t t a t u s and D. pulex by c u t t h r o a t and D o l l y Varden 104 x i i 32b. P a i r e d prey type experiment: cumulative captures of 4th i n s t a r t r i v i t t a t u s and D. kenai by c u t t h r o a t and D o l l y Varden 104 33a. P a i r e d prey type experiment: cumulative a t t a c k s on D^ pulex and D^ kenai by c u t t h r o a t and D o l l y Varden 106 33b. P a i r e d prey type experiment: cumulative captures of D^ pulex and D^ kenai by c u t -t h r o a t and D o l l y Varden 106 34. P a i r e d prey type experiment: cumulative captures 2nd i n s t a r t r i v i t t a t u s and D.  pulex by c u t t h r o a t and D o l l y Varden 107 x i i i LIST OF TABLES. Table D e s c r i p t i o n Page l a . P h y s i c a l c h a r a c t e r i s t i c s of the study l a k e s . . 11 l b . Chemical c h a r a c t e r i s t i c s of the study l a k e s . . 11 2. Maximum summer d e n s i t y of some common cr u s t a c e a n macrozooplankton i n the study l a k e s b e f o r e f i s h t r a n s f e r s 14 3. Number arid date of f i s h t r a n s f e r s from Loon Lake t o Eunice and Katherine Lakes 31 4. Comparisons of the l e n g t h and weight of c u t t h r o a t and D o l l y Varden caught i n g i l l nets s e t i n the experimental lakes and i n Loon Lake 35 5. Number of Loon c u t t h r o a t caught i n the upper and lower halves of g i l l nets f i s h e d from s u r f a c e to bottom a t . the 5 and 10 m lake contours 44 6. Number of Loon D o l l y Varden caught i n the upper and lower h a l v e s of g i l l nets f i s h e d from s u r f a c e to bottom at the 5 and 10 m lake contours 4 5 7. Number of Eunice c u t t h r o a t caught i n the upper and lower halves of g i l l nets f i s h e d from s u r f a c e t o bottom at the 5 and 10 m lake contours 4 9 8. Number of Katherine D o l l y Varden caught i n the upper and lower halves of g i l l nets f i s h e d from s u r f a c e to bottom at the 5 and 10 m lake contours 52 x i v 9. Comparison of the mean lengths and standard d e v i a t i o n s of the Eunice Lake Daphnia i n the lake p o p u l a t i o n and i n the c u t t h r o a t d i e t . . . . 84 10. Comparison of the mean lengths and standard d e v i a t i o n s of the Loon Lake Daphnia i n the lake p o p u l a t i o n and i n the c u t t h r o a t d i e t . . . . 87 11. Comparison of Daphnia lengths from depths at which p l a n k t i v o r o u s f i s h were caught and a t a l l depths 89 12a. Comparison of the mean lengths and standard d e v i a t i o n s of the Loon Lake D^ _ kenai i n the lake p o p u l a t i o n and i n the c u t t h r o a t d i e t 92 12b. Comparison of the mean lengths and standard d e v i a t i o n s of the Eunice Lake D^ kenai i n the lake p o p u l a t i o n and i n the c u t t h r o a t d i e t 92 13. Mean number and frequency of occurrence of Chaoborus i n the stomach contents of Eunice c u t t h r o a t and Kath e r i n e D o l l y Varden 94 14. Mean time (minutes) spend moving i n 10 minute o b s e r v a t i o n p e r i o d s ; b e f o r e , d u r i n g , and a f t e r f e e d i n g by 3 f i s h of each s p e c i e s . . 96 15. Mean time (minutes) spent i n each v e r t i c a l t h i r d of the o b s e r v a t i o n tank d u r i n g o b s e r v a t i o n ; b e f o r e , d u r i n g , and a f t e r f e e d i n g , by 3 f i s h of each s p e c i e s 96 X V 16a. Comparison of f e e d i n g a b i l i t i e s between c u t t h r o a t and D o l l y Varden on 3 prey types... 98 16b. Comparison of f e e d i n g a b i l i t i e s between 3 prey types by c u t t h r o a t and by D o l l y Varden 98 17. Comparison of f e e d i n g by c u t t h r o a t and D o l l y Varden when presented w i t h p a i r e d prey types 103 18. U t i l i t y of Daphnia, D. k e n a i , and 4th i n s t a r (young) C^ t r i v i t t a t u s t o c u t t h r o a t and D o l l y Varden 115 xv i LIST OF APPENDIX TABLES. l e DESCRIPTION Page I. Computations f o r Schnabel p o p u l a t i o n estimate of subadult and a d u l t D o l l y Varden i n Loon Lake. . 138 I I . Computation f o r Schnabel p o p u l a t i o n estimate of subadult and a d u l t c u t t h r o a t t r o u t i n Loon Lake 139 I I I . Mean number, mean volume, and frequency of occurrence of prey items i n the d i e t of Loon c u t t h r o a t 140 IV. Mean number, mean volume, and frequency of occurrence of prey items i n the d i e t of Loon D o l l y Varden 141 V. Mean number, mean volume, and frequency of occurrence of prey items i n the d i e t of Eunice c u t t h r o a t 142 VI. Mean number, mean volume, and frequency of occurrence of prey items i n the d i e t of Katherine D o l l y Varden 143 ACKNOWLEDGEMENTS I am g r a t e f u l to my su p e r v i s o r , Dr. T. G. Northcote f o r h i s encouragement throughout the course of t h i s p r o j e c t and f o r h i s h e l p f u l comments on the manuscript. Dr. W. Smyly provided many u s e f u l ideas. Drs. W. E. N e i l l and C. J . Walters provided u s e f u l suggestions during the study and welcome c r i t i c i s m of the manuscript. I am p a r t i c u l a r l y t h a n k f u l to K. Tsumura, T. Johnston , and A. Bennett. A l l were of i n v a l u a b l e a s s i s t a n c e i n the f i e l d . K. Tsumura a l s o designed and b u i l t the trap nets used i n the study. T. Johnston provided u s e f u l c r i t i c i s m throughout the course of the study. A. Bennett performed much of the tedious l a b o r a t o r y a n a l y s i s of the many f i e l d samples. I wish to thank my w i f e , C o l l e e n , f o r her continuous support, encouragement, and e s p e c i a l l y , patience throughout my years i n graduate s t u d i e s . I am t h a n k f u l to the members of the U. B. C. Research Forest s t a f f f o r t h e i r help i n t h i s study. F i n a l l y , I wish to acknowledge the f i n a n c i a l a s s i s t a n c e provided by the Department of Zoology i n the form of teaching a s s i s t a n t s h i p s and by the N a t i o n a l Research C o u n c i l i n the form of a grant to my su p e r v i s o r . 1 I. INTRODUCTION Two sympatric n o n - i n t e r b r e e d i n g p o p u l a t i o n s occupying the same niche cannot c o e x i s t . T h i s i s termed c o m p e t i t i v e e x c l u s i o n by Harden (1960). E i t h e r d i f f e r e n c e s i n r e p r o d u c t i o n r a t e s w i l l l e a d t o the l o c a l e x t i n c t i o n of one s p e c i e s (Hutchinson, 1957) or, e c o l o g i c a l d i f f e r e n c e s w i l l be magnified by i n t e r a c t i o n s between the s p e c i e s l e a d i n g t o the oc c u p a t i o n of d i f f e r e n t n i c h e s . The l a t t e r process i s termed i n t e r a c t i v e s e g r e g a t i o n by Bryan (1956) and N i l s s o n (1967) and r e q u i r e s a degree of b e h a v i o u r a l p l a s t i c i t y as 'niche p r e f e r e n c e s ' change ;in the presence or absence of i n t e r a c t i n g s p e c i e s ( N i l s s o n , 1967) . These phenotypic a d a p t a t i o n s w i l l e v e n t u a l l y become s t a b i l i z e d i n the genotype of the excluded s p e c i e s (Hutchinson, 1957). Bryan (1956) and N i l s s o n (1967) term the genotypic a d a p t a t i o n s t o d i f f e r e n t n i c h e s as s e l e c t i v e segre-g a t i o n . S e l e c t i v e s e g r e g a t i o n may evolve not o n l y from the sympatric i n t e r a c t i v e p r ocesses, as o u t l i n e d , but a l s o i n c i d e n t -a l l y i n a l l o p a t r y (Armitage, MS 1973). Although these p r i n c i p l e s are e a s i l y e s t a b l i s h e d i n theory, they are d i f f i c u l t t o prove. Extreme c h a r a c t e r or e c o l o g i c a l divergence among sympatric s p e c i e s does not prove, any more than t h e i r absence d i s p r o v e s , the hypotheses (Zaret and Rand, 1971). The argument i s c i r c u l a r , and t h e r e f o r e , u n t e s t a b l e ( B i r c h and E h l r i c h , 1967). N e v e r t h e l e s s , as Zaret and Rand i n d i c a t e , the consequences can s t i l l be v a l i d and worth examining. One approach i s to make comparisons between communities t h a t vary by the presence or absence of a s i n g l e s p e c i e s . By examining changes i n the remaining s p e c i e s one can i n f e r c a u s a l r e l a t i o n s h i p s (Schoener, 1974). S t u d i e s on sympatric and a l l o p a t r i c p o p u l a t i o n s of t r o u t and char u t i l i z e t h i s method. N i l s s o n (1967) demonstrated s p a t i a l and d i e t a r y s e g r e g a t i o n between c o - e x i s t i n g p o p u l a t i o n s of brown t r o u t (Salmo t r u t t a ) and a r c t i c char ( S a l v e l i n u s a l p i n u s ) . Andrusak and Northcote (1971) showed t h a t a l l o p a t r i c D o l l y Varden ( S a l v e l i n u s malma) and c u t t h r o a t t r o u t (Salmo c l a r k i c l a r k i ) occupied the t o t a l water column of s m a l l lakes and fed on a wide range of b e n t h i c p l a n k t o n i c , and s u r f a c e organisms. However, c o - e x i s t i n g c u t t h r o a t and D o l l y Varden were s p a t i a l l y segregated. Sympatr c u t t h r o a t u s u a l l y occurred and fed near the lake s u r f a c e w h i l e sympatric D o l l y Varden o c c u r r e d deeper i n the water column and fed p r i m a r i l y on bottom organisms. Armitage (MS, 1973), st u d y i n g a l l o p a t r i c D o l l y Varden and D o l l y Varden sympatric w i t h c u t t h r o a t , had s i m i l a r r e s u l t s but was unable to show c h a r a c t e r displacement i n m o r p h o l o g i c a l f e a t u r e s except f o r p y l o r i c caeca numbers. Laboratory experiments showed t h a t D o l l y Varden were " o r i e n t e d " more towards the bottom and were more e f f i c i e n t benthic ' feeders than ..the . c u t t h r o a t w i t h which they c o - e x i s t e d . The c u t t h r o a t t r o u t were s u r f a c e - midwater " o r i e n t e d " and were more e f f i c i e n t s u r f a c e feeders than D o l l y Varden (Schutz and Northcote, 19 72). A l l o p a t r i c D o l l y Varden were l e s s s u b s t r a t e o r i e n t e d and •. more v a r i a b l e i n s p a t i a l p r e -ference than sympatric D o l l y Varden. Moreover, they took novel prey more r e a d i l y (Armitage, MS 1973) . These s t u d i e s on c u t t h r o a t and D o l l y Varden attempted to determine i f the s p a t i a l and d i e t a r y s e g r e g a t i o n e x h i b i t e d by sympatric p o p u l a t i o n s was due to i n t e r a c t i v e or s e l e c t i v e 3 p r o c e s s e s . The evidence presented was somewhat e q u i v o c a l , making demonstration of the process i n v o l v e d d i f f i c u l t . Schutz and Northcote (1972) p o s t u l a t e d t h a t i n t e r a c t i v e s e g r e g a t i o n would occur o n l y f o r a r e l a t i v e l y b r i e f time a f t e r the s t a r t of sympatry and r a p i d l y l e a d t o the g e n e t i c s t a b i l i z a t i o n of the niche p r e f e r e n c e s . However, Armitage concluded t h a t n e i t h e r process c o u l d be demonstrated by h i s morphometric or b e h a v i o u r a l s t u d i e s . In t h i s study I i n v e s t i g a t e d the problem of whether a l e a r n e d b e h a v i o u r a l process or a g e n e t i c process was i n v o l v e d w i t h the s p a t i a l and d i e t a r y s e g r e g a t i o n observed i n the sympatric p o p u l a t i o n of D o l l y Varden and c u t t h r o a t , i n Loon Lake, s t u d i e d by Armitage (MS 1973). A l l o p a t r y was a r t i f i c i a l l y induced on members of the sympatric Loon Lake f i s h p o p u l a t i o n s by s e g r e g a t i n g and t r a n s f e r r i n g the two s p e c i e s to separate f i s h l e s s l a k e s . I f the f i s h were i n t e r a c t i v e l y segregated, t h i s induced a l l o p a t r y should r e s u l t i n r a p i d expansion of t h e i r s p a t i a l d i s t r i b u t i o n and d i e t a r y h a b i t s . I f , i n s t e a d , they were s e l e c t i v e l y segre-gated, the induced a l l o p a t r y should not q u i c k l y produce major changes i n e i t h e r d i s t r i b u t i o n or d i e t . Of the three major h a b i t a t s and prey sources i n the c o a s t a l B.C. mountain lakes (benthos, l i m n e t i c zooplankton, and s u r f a c e i n s e c t s ) , Loon Lake c u t t h r o a t and D o l l y Varden showed major d i e t a r y o v e r l a p o n l y i n the l i m n e t i c zooplankton group.. Although zooplankton was u t i l i z e d by both f i s h s p e c i e s , i t formed a l a r g e r p o r t i o n of the c u t t h r o a t d i e t , 48% to 78%, than of the D o l l y Varden d i e t , 13% to 54% (Armitage, MS 1973). In a d d i t i o n , Schutz and Northcote (1972) found the c u t t h r o a t to be more s u r f a c e - midwater o r i e n t e d than the..... D o l l y Varden. Thus, f i e l d and l a b o r a t o r y evidence i n d i c a t e k t h a t c u t t h r o a t have an advantage i n midwater f e e d i n g , although the D o l l y Varden have a l s o kept the l i m n e t i c h a b i t a t as p a r t of t h e i r environment. The midwater h a b i t a t then i s one area, at l e a s t , where the evidence i n d i c a t e s i n t e r a c t i v e processes o c c u r r i n g between c u t t h r o a t and D o l l y Varden p o p u l a t i o n s . Consequently, my study focused on the p l a n k t i v o r o u s d i e t and f e e d i n g behaviour of both f i s h s p e c i e s . F i e l d s t u d i e s i n c l u d e d a d e t a i l e d examination of the s p e c i e s and s i z e com-p o s i t i o n of the zooplankton i n the d i e t . L a b o r a t o r y s t u d i e s compared the f e e d i n g behaviour, p r e f e r e n c e s and e f f i c i e n c y of c u t t h r o a t and D o l l y Varden on three d i f f e r e n t types of zooplankton: an i n s e c t l a r v a , Chaoborus t r i v i t t a t u s ; a c l a d o c e r a n , Daphnia pulex; and a copepod, Diaptomus k e n a i . 5 I I . STUDY AREA A. DESCRIPTION OF THE LAKES The study lakes are s i t u a t e d i n the U.B.C. Research F o r e s t (49°19'N, 122°34'W), 50 km e a s t of Vancouver and 10 km NNE of the town of Haney ( f i g u r e 1 ). The t r a n s f e r r e d f i s h o r i g i n a t e d from sympatric p o p u l a t i o n s i n Loon Lake ( f i g . 2 ) . C u t t h r o a t t r o u t were t r a n s f e r r e d to f i s h l e s s Eunice Lake ( f i g . 3 ) and D o l l y Varden were t r a n s f e r r e d to f i s h l e s s K atherine Lake ( f i g . 4 ) . A f o u r t h l a k e , Gwendoline, was l e f t f i s h l e s s as a c o n t r o l f o r other r e l a t e d zooplankton s t u d i e s (Northcote, Walters and Hume, i n p r e s s ) . A g e n e r a l d e s c r i p t i o n of the Research F o r e s t lakes i s given by Northcote and C l a r o t t o (1974). Table 1. l i s t s p h y s i c a l and chemical c h a r a c t e r i s t i c s of the lakes i n v o l v e d i n my study. The f i s h donor l a k e , Loon, at 3 40 m e l e v a t i o n i s 140 m lower than e i t h e r of the r e c i p i e n t l a k e s , which are at n e a r l y the same e l e v a t i o n . Loon i s by f a r the l a r g e s t and the deepest of the three l a k e s . I t s mean depth (26.4 m) i s n e a r l y twice t h a t of Eunice (15.8'm). Katherine i s much shallower (7.4 m mean depth) than e i t h e r of the other l a k e s . S h o r e l i n e development (D^) i s an i n d i c a t o r of the p o t e n t i a l l i t t o r a l area. Table 1 shows high D^ values f o r Loon and K a t h e r i n e with a c o m p a r a t i v e l y low value f o r Eunice. The hypsographic curve ( f i g . 5a) showing the lake area i n h e c t a r e s at v a r i o u s depths i n d i c a t e s a much more e x t e n s i v e l i t t o r a l area above 5 m i n Loon Lake than i n the other l a k e s . However, Loon has very l i t t l e of i t s t o t a l area above 5 m (13%) when compared to Katherine (47%) and Eunice (22%) ( f i g . 5b). C h e m i c a l l y a l l the lakes are s i m i l a r ( t a b l e l b ) , although 6 Figure 1. Map of U. B. 0. Research Forest showing the location of the study lakes. 0 100 200 M E T R E S F i g u r e 2. Map o f . L o o n Lake showing sample s i t e s . Depth contour i n metres. 8 Figure 3» Map of Eunice Lake showing sample sites. Depth contour in metres. Figure 4 . Map of Katherine Lake showing sample sites. Depth contour in metres. F i g u r e 5&* Hypsographic curves showing area o f cross s e c t i o n s . t a k e n a t var ious depths of the study l a k e s . F i g u r e 5b. Hypsographic curves showing area of cross s e c t i o n s taken a t v a r i o u s depths of the study lakes expressed as a percentage of the surface a r e a . 16 AREA (ha) 24 4_ % OF SURFACE AREA 4B L. 40 -J 0 KATHERINE 60 _ l 80 _J KATHERINE Table 1. (a) Physical characteristics of the study lakes (partly from Northcote and Glarotto, 1974). LOON EUNICE KATHERINE ELEVATION (m) 340 480 505 SURFACE AREA (ha) 50.7 18.2 23.6 VOLUME (m3 X 10^) 1336 288 175 SHORE LINE LENGTH (m) 5687 ... 2658 3002 SHORE LINE DEVELOPMENT, D_ 2.17 1.47 1.86 MAXIMUM DEPTH (m) 62 44 29 MEAN DEPTH (m) 26.4 15.8 7.4 (b) Chemical characteristics of the study lakes (from Northcote and Clarotto, 1974). LOON EUNICE KATHERINE T.D.S. (mg/l)13 32 16 15 PH b 6.4 - 6.7 6.4 6.6 COLOUR (Pt u n i t s ) b 5 15 10 - 15 TRANSPARENCY (SECCHI, m) 9 6-10 6.5 - 9 EPILIMNION DEPTH (late summer) (m) 7 - 8 4 - 6 6 HYPOLIMNION Og.min (mg/l) ca7 6 3.6 U. surface water 12 Loon Lake has about double the t o t a l d i s s o l v e d s o l i d (T.D.S.) content that the other two have. However, a l l T.D.S. values are l e s s than 46 ppm, the low mean value f o r c o a s t a l mountain lakes found by Northcote and L a r k i n (1956). Secchi d i s c readings are a l l between 5 and 10 m and a l l the lakes form a moderate thermocline throughout the summer. Hypolimnal oxygen, l e v e l s can be q u i t e low during l a t e summer i n a l l lakes but l e s s so i n Loon. A l l are s l i g h t l y a c i d i c and, except f o r Loon Lake, the waters are brown s t a i n e d , w i t h Pt values of 10 - 15. The geography of the f o r e s t area was described by E f f o r d (1967). G l a c i e r s covered the lake area repeatedly during the P l e i s t o c e n e , the most recent probably around 11,500 B.P. (Armstrong, 1957). The area i s t y p i c a l southern c o a s t a l mountain topography. Mountain lakes are surrounded by steep slopes and g r a n i t i c rock outcrops of quartz i o d i t e i n the north of the f o r e s t , w i t h gradual slopes of g l a c i a l t i l l i n the south (Roddick and Armstrong, 1956). E f f o r d (1967) describes the climate as m i l d and wet. From 1954 to 1964 mean monthly temperatures ranged from 1.7°C i n January to 16.2°C i n J u l y ; mean annual p r e c i p i t a t i o n was 2 40 cm (E f f o r d , 1967). Much of the p r e c i p i t a t i o n i n winter i s i n the form of snow, e s p e c i a l l y around the higher l a k e s . Katherine and Eunice Lakes u s u a l l y freeze over i n w i n t e r , but Loon Lake i s ice-covered only o c c a s i o n a l l y . B. INVERTEBRATE FAUNA 1. Crustaceous Zooplankton The common l i m n e t i c macrozooplankton of the lakes were described by Northcote and C l a r o t t o (1974) p r i o r to the f i s h t r a n s f e r program and are shown i n t a b l e 2. S i x copepod and seven cladoceran species are commonly found i n the research 13 l a k e s . Although p o p u l a t i o n d e n s i t i e s vary ( e s p e c i a l l y between y e a r s ) , the maximum summer d e n s i t y presented here i s a good i n d i c a t i o n of the r e l a t i v e numbers prese n t d u r i n g most of the f i s h sampling p e r i o d s . F i g u r e 6 shows the mean s i z e and range i n s i z e of some of the more common s p e c i e s both w i t h i n and between the l a k e s . In g e n e r a l , Eunice has much higher d e n s i t i e s of c l a d o c e r a than do Loon or K a t h e r i n e . The l a r g e s t , Leptodora k i n d t i i , i s p resent o n l y i n Loon Lake, where i t has been o c c a s i o n a l l y captured. The next l a r g e s t s p e c i e s , Daphnia rosea, i s much sm a l l e r i n Loon Lake than i n the o t h e r s . Medium-sized Holopedium gibberum are s i m i l a r i n s i z e i n a l l three lakes while the s m a l l e r Diaphanosoma brachyurum and the s t i l l s m a l l e r Bosmina l o n g i r o s t r i s are l a r g e r i n Loon Lake than i n the f i s h l e s s l a k e s . Northcote and C l a r o t t o a t t r i b u t e t h i s to s i z e s e l e c t i v e p r e d a t i o n on small i n d i v i d u a l s by Chaoborus sp. i n Eunice and Katherine Lakes. Amongst the copepods, the s m a l l e r Diaptomus oregonensis i n Loon Lake and D^ t y r e l l i i n Eunice are much more abundant than the l a r g e r D^ kenai or D^ l e p t o p u s . In Katherine Lake a l l copepods are few i n number. Only two taxa occur i n a l l three l a k e s . p_^_ kenai i s l a r g e r i n f i s h l e s s Eunice and Katherine Lakes w h i l e the c y c l o p o i d s are l a r g e r i n Loon. Recent data, gathered subsequent to the f i s h t r a n s f e r program, i n d i c a t e s some change i n the abundance of zooplankton i n the r e c e n t l y stocked l a k e s . As w e l l the s i z e range of some zooplankton has changed s i n c e f i s h i n t r o d u c t i o n (Northcote et al., i n press) . Daphnia rosea and D^ kenai are both more abundant and s m a l l e r s i n c e f i s h s t o c k i n g , e s p e c i a l l y i n l a t e summer and f a l l , w h i l e Bosmina has i n c r e a s e d i n s i z e . Table 2. Maximum summer density of some crustacean macrozooplankton in the study lakes before f i s h transfers (modified from Northcote and Clarotto, 197*0. SPECIES MAXIMUM SUMMER DENSITY (NUMBERS / 100 LITRES) LOON EUNICE KATHERINE CLADOGERA Leptodora k i n d i t t i a - -Daphnia rosea 72 233 12 Holopedium gibberum k6 12k ko Diaphanosoma brachyurum 35 253 11 Polyphemus pediculus 7.3 13 5.1 Ceriodaphnia pulchella 51 b -Bosmina longirostris 14-2 230 5k C0PEP0DA Diaptomus kenai 19° 70 8.9 D, leptopus - 0.5 2.5 D. oregonensis 69 - -D, t y r e l l i - 800 10 Gyclopoids d 9k 27 k.k YEAR SAMPLED 1969 1972 1973 1967 1969 1971 1972 1969 a. Present but no data on density. b. Occurs very rarely, c. Originally misidentified as D. leptopus. d. Two cyclopoid species have since been identified by W. Smylyj Cyclops bicuspidatus thomasi and Tropocyclops praslnus praslnus. Figure 6. Mean "body length (©) of some common crustacean macrozooplankters in the limnetic region of the study lakes. Vertical lines and short horizontal bars show range and 95$ confidence limits res-pectively (modified from Northcote and Clarotto, 1974). (Daphnia rosea; Katherine, 19 July, 15 August, 9 September, 1969; Eunice, 26 May, 28 June, 14 Sep-tember, 1967. 28 May, 9 July, 5 August, 10 Septem-ber, 1969, 5 September, 3 October, 1971? Loon, 4 June, 11 July, 6 August, 5 September, 1969, 8 July, 1972, 10 July, 22 September, 1973. Holopedlum gibberumt Katherine and Loon, as for Daphnia (none on 8 July, 1972); Eunice 26 May, 1967, 28 May, 9 July, 5 August, 1969, 6 June, 1972. Diaphanosoma brachyurum; Katherine and Loon, as for Daphnia; Eunice, 11 August, 14 September, 1967» 28 May, 9 July, 10 September, 1969, 3 August, 1971. Bosmina longirostris; Katherine and Loon, as for Daphnia; Eunice, 14 September, 196?, 28 May, 10 September, 1969, 23 July 1971. Cyclopoids: Katherine and Loon, as for Daphnia; Eunice, 14 September 1967, 28 May, 9 July, 10 Sep-tember, 1969. Diaptomus kenai; Katherine and Eunice, 28 June, 25 July, 21 August, 18 September, 16 October, 1974; Loon, 8 July 1972, 22 September, 1973.) KATHERINE EUNICE DAPHNIA LOON 1 f t * * 1-0 HOLOPEDIUM 10 DIAPHANOSOMA • 0 10 tt , » « 0 0 10 KATHERINE EUNICE BOSMINA LOON CYCLOPOIDS 10 DIAPTOMUS KENAI - 2. H I. 0 0 2. Chaoborus S w i f t and Fedorenko, i n a s e r i e s of papers, documented the abundance, d i e t , v e r t i c a l m i g r a t i o n , and f e e d i n g h a b i t s of the two p l a n k t o n i c chaoborid l a r v a e i n Eunice Lake, Chaobrous  t r i v i t t a t u s and the s m a l l e r americanus. Both of these s p e c i e s are p r e s e n t i n Katherine Lake (Northcote e t al., i n p r e s s ) , where they presumably have s i m i l a r l i f e h i s t o r i e s . The l a r g e f o u r t h and t h i r d i n s t a r s of these s p e c i e s v e r t i c a l l y migrate from a depth of approximately 7-10 m d u r i n g the day to the s u r f a c e at n i g h t . In Loon Lake n e i t h e r s p e c i e s occurs; but the s m a l l e r f l a v i c a n s i s p r e s e n t , though l a r g e l y b e n t h i c d u r i n g the day. I I I . METHODS AND MATERIALS A. FIELD WORK 1. T r a n s f e r Methods F i s h were captured a l i v e i n Loon Lake u s i n g two m o d i f i e d Beamish t r a p nets (Beamish, 1973) and by purse s e i n i n g . The s m a l l e r net had a t r a p 1.5 m long by 1.2 m wide and was 1.2 m deep. The 61.0 m c e n t r a l l e a d and the two 12.2 m wings were 2.4 m deep t a p e r i n g towards the t r a p . The l a r g e r net had a t r a p 1.8 m long by 1.5 m wide and was 1.5 m deep, w i t h a 76.2 m c e n t r a l l e a d and two 22.9 m wings. A l l leads were 4.9 m deep t a p e r i n g towards the t r a p . U n l i k e Beamish's nets these were c o n s t r u c t e d of 10 mm nylon- k n o t l e s s mesh wit h both f l o a t and l e a d l i n e s . A f t e r a summer ( i n 1974) of experimental f i s h i n g at v a r i o u s l o c a t i o n s on Loon Lake two s i t e s were chosen f o r the t r a n s f e r program ( f i g . 2). The l a r g e r net was p l a c e d near the o u t l e t w i t h i t s main l e a d s t r e t c h e d across the l a k e . The wings were n e a r l y p e r p e n d i c u l a r t o the c e n t r a l l e a d and approximately p a r a l l e l t o the shore. A l l leads were f l o a t i n g . The wings reached from the s u r f a c e to the bottom while there was a maximum of 2 m of water under the bottom of the c e n t r a l l e a d . A l l leads were anchored at the f r e e end of the f l o a t l i n e . The s m a l l e r net was s i t u a t e d i n the l a r g e s t l i t t o r a l area of the l a k e . I t s c e n t r a l l e a d s t r e t c h e d across the j u n c t i o n of the main lake and the southeastern o u t l e t arm. The wings were p l a c e d i n the same manner as f o r the l a r g e r net. A l l leads reached from the surface, to the bottom and were anchored at the f l o a t l i n e ends. The two t r a p nets f i s h e d both s i d e s of the c e n t r a l l e a d . Purse s e i n i n g was done from two open outboard-powered 18 boats u s i n g a 61.0 m net c o n s i s t i n g of a 48.8 m l e n g t h of 1.9 cm s t r e t c h e d mesh and a 12.2 m l e n g t h of 2.5 cm s t r e t c h e d mesh added f o r e x t r a l e n g t h . The net was 7.6 m deep. Both boats-proceeded t o the f i s h i n g area where one boat stopped and acted as an anchor p o i n t f o r the purse l e a d s . Using f u l l t h r o t t l e the net boat b a i l e d out the net i n a c i r c l e ; i t passed the leads to the anchor boat and c l o s e d the purse by drawing the purse l i n e at top speed through the r i n g s oh the bottom of the s e i n e . The purse seine was p u l l e d immediately i n t o the boat from the bottom up, u n t i l the captured f i s h were con c e n t r a t e d i n a pocked of the net. F i s h i n g areas s u i t a b l e f o r purse s e i n i n g were chosen by the o b s e r v a t i o n of f i s h r i s i n g to the s u r f a c e . Consequently, s e i n i n g was most e f f e c t i v e at dusk on windless days. Most s e i n i n g was done i n the o u t l e t arm and i n the southern end of the main l a k e . Only c u t t h r o a t t r o u t were caught by t h i s method. The f i s h were t r a n s f e r r e d by d i p net from e i t h e r the purse s e i n e or t r a p nets to 40 1 p l a s t i c p a i l s which were taken to the dock. When more than about twenty f i s h were caught they were p l a c e d i n a h o l d i n g pen i n the lake near the dock and d i p n e t t e d as needed. On shore the p a i l s were aerated and kept i n the shade. F i s h were a n e s t h e t i z e d i n MS-222, weighed, measured (fork l e n g t h ) , and p l a c e d i n t o 40 1 recovery p a i l s . A p o r t i o n of the f i s h were tagged w i t h f i n g e r l i n g tags (model FTF-69 F l o y Tag and Manufacturing Inc., S e a t t l e ) c o n s i s t i n g of a numbered 48 X 32 mm o v a l d i s c attached by a s o l i d v i n y l thread through the body musculature at the a n t e r i o r base of the d o r s a l f i n . 154 (9.8%) of the c u t t h r o a t and 178 (20.2%) of the D o l l y Varden were tagged. The f i s h were t r a n s f e r r e d 19 by truck i n the aerated 40 1 p a i l s to Eunice Lake (9.3 km) or to Katherine Lake (11.2 km). I f necessary, the temperature of the p a i l water was allowed to e q u i l i b r a t e w i t h that of the lake water. However, there was seldom more than a 1C° temp-erature d i f f e r e n c e . A f t e r e q u i l i b r a t i o n , the p a i l s were tipp e d over and the f i s h r eleased along the l i t t o r a l zone. The t o t a l t r a n s f e r time ranged from one to three hours, depending on the number of f i s h i n v o l v e d . No f i s h that appeared i n poor c o n d i t i o n a f t e r handling was t r a n s f e r r e d . During the complete t r a n s f e r operation there were only two D o l l y Varden (0.2%) and sixty-one c u t t h r o a t (3.9%) m o r t a l i t i e s . Five c u t t h r o a t were caged i n the Eunice Lake l i t t o r a l zone f o r 24 hours a f t e r t r a n s f e r . There were no m o r t a l i t i e s . 2. Population Estimates - Loon Lake The trap nets were used i n a tag-recapture program to estimate the population s i z e of subadult and a d u l t Loon Lake c u t t h r o a t and D o l l y Varden. Sampling took place from 3 May 1974 to 17 J u l y 1974; however, because of the decrease i n captures a f t e r mid-June, only those D o l l y Varden caught before 7 June 1974 and c u t t h r o a t caught before 25 June 1974 were used i n e s t i m a t i n g population s i z e . The population of both species of f i s h were estimated using the Schnabel method (Ricker, 1975). The equations used are o u t l i n e d i n appendices I and I I . 3. Sampling Methods (i) F i s h F i s h were sampled w i t h f l o a t i n g nylon monofilament g i l l nets 5 m and 10 m deep. Each net c o n s i s t e d of three 5 m long s e c t i o n s w i t h s t r e t c h e d mesh s i z e s of 25.4, 38.1, and 50.8 mm. H o r i z o n t a l marks every metre f a c i l i t a t e d the determination of the capture depth of each f i s h . The nets were f i s h e d p a r a l l e l to shore from s u r f a c e t o bottom, one a t the 5 m and one at the 10 m contour. Nets were s e t approximately once a month from J u l y 19 75 t o October 1975 and from May 1976 to August 1976. A d d i t i o n a l s e t s were made i n Katherine and Eunice Lakes i n October 1976 and, through the i c e , i n Loon Lake i n March 1976. Nets were s e t near dusk and p u l l e d at dawn the f o l l o w i n g morning. However, a f t e r September 1975, the Loon Lake nets were p u l l e d about midnight i n order to av o i d e x c e s s i v e l y high c a t c h e s . N e t t i n g s i t e s ( f i g u r e s 2, 3, and 4) were marked by permanent buoys du r i n g the f i s h i n g season and were adju s t e d to account f o r v a r i a t i o n s i n water l e v e l . Immediately a f t e r the nets were p u l l e d the f i s h were p i c k e d from them. A code number was assigned to each f i s h and marked on a waterproof tag i n s e r t e d i n i t s mouth. Records were kept on n e t t i n g data, the code number and the capture depth of each f i s h . The f o r k l e n g t h and the wet weight were recorded w i t h i n 2 hours a f t e r p i c k i n g . In the m a j o r i t y of cases the stomachs of the f i s h were removed and preserved s e p a r a t e l y i n 70% e t h a n o l although a few f i s h were l e f t i n t a c t . The f i s h were preserved i n 10% f o r m a l i n f o r s e v e r a l weeks. Then they were r i n s e d i n water f o r 24 hours and p l a c e d i n 37% i s o p r o p a n o l f o r s t o r a g e . ( i i ) Zooplankton Zooplankton were sampled w i t h Clarke-Bumpus sampling gear as o u t l i n e d i n Fedorenko and S w i f t (1972) u s i n g a number 10 mesh (18 3 microns) plankton net. The sampling gear was suspended from the s i d e of an open boat u s i n g a hand winch mounted on a board clamped t o the gunwales. The plankton hauls were made 21 o b l i q u e l y by s t e a d i l y r a i s i n g the net from the bottom depth to the top depth of the sampling i n t e r v a l . Biweekly tows.were made near midday at approximately 1 1/2 knots f o r 3 minutes down the middle of the main body of each lake (see f i g u r e s 2, 3 and 4). In Loon Lake, tows were made from 40-20, 20-10, 10-4, 4-2, and 2-0 m. In Katherine and Eunice Lakes, tows were made from 10-7, 7-4, 4-2, and 2-0 m, as w e l l as from 35-10 m i n Eunice Lake and 25-10 m i n Katherine Lake. A l s o , a l i t t o r a l tow was made i n a l l the lakes as near as p o s s i b l e to the s h o r e l i n e . I t was o f t e n d i f f i c u l t to approach c l o s e l y to the s h o r e l i n e because of the presence of submerged logs and snags. The samples were preserved i n a 6% s o l u t i o n of sugared f o r m a l i n (Haney and H a l l , 1972) f o r counting. 4. Laboratory A n a l y s i s (i) F i s h Stomach Contents The contents o f i n d i v i d u a l f i s h stomachs from the oesophagus to the p y l o r i c s p h i n c t e r were removed and p l a c e d i n 70% e t h a n o l . The m a t e r i a l was l a t e r i d e n t i f i e d , s o r t e d i n t o c a t e g o r i e s , and counted under 25-50 X m a g n i f i c a t i o n . The volume of each c a t e -gory was measured by volume displacement i n a graduated c e n t -r i f u g e tube (Andrusak, MS 1968), o r , f o r s m a l l e r amounts i n a 1 ml c a p a c i t y graduated c y l i n d e r accurate t o .05 ml. Volumes l e s s than .05 ml were recorded as .001 ml. Ca t e g o r i e s used were m o d i f i e d from (Andrusak, MS 1968) and Armitage (MS 1973). Large l i t t o r a l organisms, such as the pupae and nymphs of T r i c h o p t e r a , P l e c o p t e r a , and C o l e o p t e r a , and leeches, were separated from s m a l l e r benthos, such as mol l u s c s , gastropods, amphipods, and chironomid l a r v a e . Chaoborus l a r v a e were recorded as a separate midwater form as they are p l a n k t o n i c a t a l l times i n K a t h e r i n e and Eunice Lakes and at n i g h t i n Loon Lake (benthic d u r i n g the day). Crustaceous zooplankton were recorded as one category f o r comparison to other prey groupings and were a l s o i d e n t i f i e d to s p e c i e s f o r compar-is o n s w i t h i n the zooplankton. Other p l a n k t o n i c c a t e g o r i e s were d i p t e r a n pupae and a q u a t i c i n s e c t s (notonectids or c o r i x i d s ) . Water mites, recorded s e p a r a t e l y , although sometimes numerous, were never found i n measureable volumes. A l l v e r t e b r a t e s ( f i s h and salamanders) were recorded as one category, as were a l l t e r r e s t r i a l i n s e c t s . Since there was l i t t l e d e t r i t u s p r e s e n t , t h i s category was i g n o r e d . Percentage composition of prey types, both by frequency and volume, can be determined i n two ways. T o t a l percent i s determined by summing over a l l f i s h f o r each prey type and d i v i d i n g by the t o t a l sum of a l l prey types (times 100). Mean percent i s determined by summing the percentage composition of a given prey type i n the d i e t of each f i s h and d i v i d i n g by the number o f f i s h i n the d i e t ( W i ndell, 1971). T o t a l percent o v e r r e p r e s e n t s f i s h with r e l a t i v e l y l a r g e amounts of prey i n t h e i r stomachs ( u s u a l l y l a r g e f i s h ) , w h i le mean percent g i v e s equal r e p r e s e n t a t i o n to a l l f i s h r e g a r d l e s s of the amount of stomach c o n t e n t s . Only mean percentage values f o r both frequency and volume data are r e p o r t e d here. For s i m p l i c i t y , percentages based on volume are r e p o r t e d as "% v o l " and those based on frequency as "% num". In a d d i t i o n , the presence or absence of each category was used to determine the percentage occurrence of the c a t e g o r i e s as w e l l as the mean number of c a t e g o r i e s on each sampling date. • Empty stomachs were noted but not used f o r these c a l c u l a t i o n s . 23 ( i i ) Zooplankton Zooplankton were i d e n t i f i e d and counted u s i n g a d i s s e c t i n g scope at 25 X m a g n i f i c a t i o n . Numbers were estimated by c o u n t i n g two or three subsamples of 1/6 to 1/36 of the t o t a l sample, depending on the numbers i n the t o t a l sample. To determine the d e n s i t y of zooplankton a t each depth range the mean of the subsamples was m u l t i p l i e d by the subsample f a c t o r , d i v i d e d by the number of r e v o l u t i o n s of the C-B meter, and then m u l t i p l i e d by the a p p r o p r i a t e c o n v e r s i o n f a c t o r f o r the C-B used, to express d e n s i t y as numbers/100 1. T o t a l lake d e n s i t y , expressed as numbers/meter , was then c a l c u l a t e d by summing the product of each d e n s i t y times the h e i g h t of the o b l i q u e tow and m u l t i p l y i n g t h i s sum by 10. Measurement of cladocerans and of copepods were made as i n Northcote and C l a r o t t o (1975). For most c l a d o c e r a , the d i s t a n c e between the a n t e r i o r end and the head (ex c l u d i n g any helmut p r o j e c t i o n s present) and the p o s t e r i o r margin of the v a l v e s ( e x c l u d i n g spine) was measured. For copepods, the metasome and urosome (excluding caudal rami) was measured. Twenty-five animals of each s p e c i e s were measured, from each depth l e v e l where p o s s i b l e . A weighted mean and v a r i a n c e was then c a l c u l a t e d from measurements subsampled from each depth i n t e r v a l i n p r o p o r t i o n t o the r e l a t i v e abundance of the s p e c i e s at t h a t depth i n t e r v a l . Species abundance at a given depth i n t e r v a l was determined by m u l t i p l y i n g the d e n s i t y of the s p e c i e s by the volume of water i n the depth i n t e r v a l . B. LABORATORY EXPERIMENTS 1. F i s h Holding F a c i l i t i e s A l l f i s h used i n l a b o r a t o r y experiments were captured 2k i n May 1976 from Loon Lake u s i n g t r a p n e t s . They were h e l d i n blue f i b e r g l a s s tanks, 110 X 50 cm, w i t h a screened c e n t r a l d r a i n p i p e 36 cm high. The tanks were d i v i d e d across the width at the d r a i n pipe by p e r f o r a t e d b l a c k s h e e t i n g . One f i s h was kept i n each s e c t i o n of the tank. A constant flow of d e c h l o r i n a t e d water was p r o v i d e d to each tank at a r a t e of about 3 1/min. Water temperature d u r i n g the experimental p e r i o d v a r i e d from 8 t o 10°C as meas-ured by a maximum/minimum thermometer. Room l i g h t i n g was p r o v i d e d by f i v e 4 0 watt G.E. c o o l white f l u o r e s c e n t bulbs p l a c e d approximately 1.5 meters from the water s u r f a c e . Photoperiod, c o n t r o l l e d by a Paragon model 4003-OS time switch, was s e t f o r 14 hrs of l i g h t (from 7:00 to 21:00 hrs) and 10 hrs of darkness d a i l y . A l l tanks were covered with 65 mm s t r e t c h e d mesh nylon n e t t i n g . F i s h were maintained on a d i e t of chopped chicken l i v e r . 2. Prey Items A l l prey items, except f o r Daphnia, were captured w i t h nets towed at very low speeds i n order to reduce prey damage. As q u i c k l y as p o s s i b l e they were r e t u r n e d i n 4.5 1 c o n t a i n e r s to the l a b , where they were maintained i n environment chambers i n 19 1 a q u a r i a . A i r was bubbled s l o w l y i n t o each aquarium. The temperature was set f o r 6°C and the l i g h t i n g was s i m i l a r to t h a t f o r f i s h . Most animals were used w i t h i n 2-3 days of capture and none were used a f t e r 5 days. Daphnia rosea appeared to be very d e l i c a t e and c o u l d not be kept a l i v e d u r i n g the r e q u i r e d h a n d l i n g p e r i o d . Con-sequently, animals from a c u l t u r e of Daphnia pulex (from Deer Lake, B.C.) maintained by Dr. W.E. N e i l l were s u b s t i t u t e d 25 f o r Daphnia r o s e a . Problems s i m i l a r to those encountered w i t h Daphnia rosea were a l s o experienced d u r i n g an a b o r t i v e attempt to use Holopedium gibberum as a prey item. Diaptomus kenai and Chaoborus s u f f e r e d much lower m o r t a l i t y r a t e s d u r i n g the h a n d l i n g . In a l l cases, o n l y strong-swimming ap p a r e n t l y h e a l t h y i n d i v i d u a l s were used i n experiments. Prey items were s o r t e d i n i t a l l y t o s i z e c l a s s e s by g e n t l y s i e v i n g w i t h a graduated s e r i e s of mesh s i z e s . T h i s not only made subsequent s e l e c t i o n and capture of prey items e a s i e r but a l s o decreased the d e n s i t y of organisms i n the h o l d i n g a q u a r i a . A l l prey items were captured i n d i v i d u a l l y 2 u s i n g a s m a l l (2.2 cm.) l i f t i n g net made from 1.5 mm mesh nylon n e t t i n g mounted on a #1 p a i n t brush handle (from Dr. W. Smyly, pers. comm.). T h i s method e f f e c t i v e l y s o r t e d both Daphnia and Diaptomus kenai to one s i z e c l a s s o n l y (between 1.5 and 2.0 mm). Chaoborus i n s t a r s were s o r t e d by eye u s i n g the t e c h -niques o u t l i n e d i n Fedorenko and S w i f t (1972). A l l prey items were used on the same day as they were s o r t e d . 3. Observation F a c i l i t i e s A l l experiments were conducted i n two g l a s s - f r o n t e d o b s e r v a t i o n tanks. The tanks were c o n s t r u c t e d of 3/4 i n c h plywood, p a i n t e d grey. They h e l d 277 1 of water each w i t h i n s i d e dimensions 118 by 50 cm; water depth was 47 cm. L i g h t i n g was p r o v i d e d by a s i n g l e 40 watt G.E. c o o l white f l u o r e s c e n t l i g h t , 130 cm long, c e n t r a l l y s i t u a t e d 26 cm above the water s u r f a c e . L i g h t i n t e n s i t y was measured once d u r i n g the experiments with a G.M. Submarine Photometer (model 15M-02/1). Readings were 210-215 l u x 1 cm below the water s u r f a c e and 94-98 lux at 11 cm above the bottom. As 26 a comparison, measurements using the same photometer were taken at Eunice Lake on October 29, 1976 at 11:30 h r s . under a heavy cloud cover. Readings taken at depths of 1.5 and 5 m were 255 to 190 l u x and 105 to 68 lux r e s p e c t i v e l y ; these correspond to the top and bottom of the observation tank. Other sources of l i g h t and d i s t r a c t i o n s were screened o f f by black p l a s t i c sheeting hanging from c e i l i n g to f l o o r w i t h an opening to one s i d e . A constant flow of water (about 3 1/sec) was maintained between experiments. Inflow was from a pipe 10 cm above the water surface and outflow was from an overflow pipe at the opposite end of the tank. A e r a t i o n was provided between experiments. Both water flow and a e r a t i o n were stopped during the experiments to reduce disturbances and prey l o s s . 4. Experimental Procedure Before any experiments were conducted, a l l f i s h used were fed only i n the observation tanks on known numbers of chicken l i v e r p i e c e s . A f t e r one-half hour, o r , when a l l the l i v e r was eaten, whichever was e a r l i e r , the f i s h were t r a n s f e r r e d back to t h e i r h o l d i n g tanks and the remaining food counted. This procedure was repeated u n t i l the f i s h showed l i t t l e v a r i a t i o n i n the number of food items eaten. Approximately 15 f i s h underwent t h i s procedure. The 4 c u t t h r o a t and the 4 D o l l y Varden used i n the experiments were s e l e c t e d because they remained a l i v e , apparently healthy, and appeared to adapt q u i c k l y t o the handling procedure. During the experiments the f i s h were fed to s a t i a t i o n on chicken l i v e r i n the observation tank, approximately 72 hours before the f i r s t t r i a l . The t r i a l s of each experiment were spaced 24 hours apart u n t i l f i n i s h e d . The f i s h were not g i v e n other food u n t i l the day a f t e r the completion of each experiment. T h i s method presented p o s s i b l e c o m p l i c a t i o n s from hunger e f f e c t s but enhanced a d a p t a t i o n t o a new prey type. A l l t r i a l s were conducted between 1100 and 1730 hrs P.D.T. For a t r i a l , a f i s h was t r a n s f e r r e d by net to the o b s e r v a t i o n tank. 15-30 minutes were allowed f o r the f i s h to adapt. By the end of t h i s time the f i s h was e i t h e r hovering or swimming i n the water column. Prey items together w i t h 300 ml of water were poured g e n t l y along the s u r f a c e o f the water. The f i s h t y p i c a l l y r e a c t e d t o the prey i n t r o d u c t i o n procedure by r e t r e a t i n g t o a corner f o r 2-3 minutes. The pouring procedure and i n i t i a l f i s h behaviour ensured the d i s p e r s a l of prey animals throughout the tank before f e e d i n g s t a r t e d . Observations were made c l o s e to the g l a s s tank f r o n t (5-60 cm, depending on prey v i s i b i l i t y ) . To ensure s i g h t i n g of a l l the prey items under a t t a c k and to reduce p a r a l l a x , I moved from one end of the tank t o the other i n c o n j u n c t i o n with the f i s h under o b s e r v a t i o n . Observations were recorded on a Rustrak four t r a c k event r e c o r d e r (model 92) and timed w i t h a stopwatch. •.: A s e t of o b s e r v a t i o n s was made on the movement and l o c a t i o n of i n d i v i d u a l f i s h i n the o b s e r v a t i o n tank. Three f i s h of each s p e c i e s were observed and the time they spent i n motion and t h e i r l o c a t i o n i n the tank was recorded. The f i s h were p l a c e d i n the tank one-half hour before the s t a r t of viewing which took p l a c e b e f o r e , d u r i n g , and a f t e r f e e d i n g on 2nd i n s t a r Chaoborus l a r v a e . Each f i s h was observed f o r 10 minutes d u r i n g each p e r i o d f o r a t o t a l o f 30 minutes of o b s e r v a t i o n . 29 IV. RESULTS A. FIELD 1. F i s h P o p u l a t i o n s The data and c a l c u l a t i o n s used f o r determining p o p u l a t i o n s i z e s of sub-adult and a d u l t c u t t h r o a t and D o l l y Varden by means of the Schnabel method are given i n appendices I & I I . These r e s u l t e d i n the f o l l o w i n g estimates of p o p u l a t i o n s i z e of Loon Lake f i s h : 7,278 c u t t h r o a t , w i t h 95% c o n f i d e n c e i n t e r v a l s of 4,631 t o 10,917, and 3,112 D o l l y Varden, with 95% confidence i n t e r v a l s of 2,356 to 3,953. The assumptions on which these estimates are based are d i s c u s s e d i n R i c k e r (1975). Important to t h i s study are two major p o s s i b l e sources of e r r o r : (1) "non-random mixing of marked f i s h o r non-random sampling of the f i s h p o p u l a t i o n s ; and (2) p o s s i b l e r e c r u i t m e n t to the p o p u l a t i o n . The bulk of the f i s h i n g and marking e f f o r t was performed at the southern end of the lake w i t h the f i s h being r e l e a s e d near t h e i r capture p o i n t s . T h e r e f o r e , i f the f i s h d i d not migrate throughout the l a k e , the r e s u l t s would be low, an estimate of the southern p o p u l a t i o n o n l y . However, two r e c a p t u r e s were near the i s l a n d i n the northern end of the l a k e , an i n d i c a t i o n of p o s s i b l e e x t e n s i v e f i s h movement. These estimates do not i n c l u d e i n d i v i d u a l s l e s s than 100 mm, because no f i s h s m a l l e r than 70 mm was tagged, and few under 100 mm were re c a p t u r e d . As the t a g g i n g p e r i o d (May and June) i s one of r e l a t i v e l y r a p i d growth there may have been r e c r u i t m e n t from these s m a l l e r f i s h . T h i s occurrence would r e s u l t i n o v e r e s t i m a t i n g the t r u e p o p u l a t i o n s i z e . I f both species are e q u a l l y vulnerable to t r a p n e t t i n g during the sampling pe r i o d then the p o p u l a t i o n estimates w i l l at l e a s t r e f l e c t t h e i r r e l a t i v e p r o p o r t i o n s . This assumption appears reasonable as they are both found i n s i m i l a r h a b i t a t during the s p r i n g and are of s i m i l a r s i z e . I n i t i a l p o p u lation s i z e s i n Eunice and Katherine Lakes are known almost e x a c t l y since these lakes were f i s h l e s s before s t o c k i n g . F i s h were t r a n s f e r r e d to these lakes from Loon Lake i n the f a l l of 1974 and i n the s p r i n g and f a l l of 1975. In a d d i t i o n , a f u r t h e r t r a n s f e r was made to Katherine Lake i n the s p r i n g of 1976. Table 3 shows tha t most of the c u t t h r o a t ( t o t a l 1571) were t r a n s f e r r e d to Eunice Lake i n the f a l l of 1974 and s p r i n g of 1975. Catches of D o l l y Varden ( t o t a l 881) were smal l e r , n e c e s s i t a t i n g a longer t r a n s f e r p e r i o d . During e a r l y J u l y 1975 and 1976, t r o u t f r y were observed i n the i n l e t stream to Eunice Lake from Gwendoline Lake and i n Eunice Lake o f f the mouth of t h i s stream. L a t e r , i n October, 1976, small f i s h (79-115 mm) were caught i n a small mesh g i l l net (12.7 mm); presumably these were the young of the year, an i n d i c a t i o n of a s u c c e s s f u l breeding p o p u l a t i o n . No small D o l l y Varden were observed i n Katherine Lake or i t s stream system. No s u i t a b l e spawning creeks were found i n the Katherine Lake drainage area and no j u v e n i l e f i s h were captured. Yet, one lone male i n breeding c o n d i t i o n was observed i n a small i n l e t stream i n e a r l y October, 1975, and a number of females w i t h p a r t i a l l y reabsorbed eggs were captured. In a d d i t i o n , f i s h eggs were found i n a few October * see footnote page 131 31 Table 3. Numbers and dates of f i s h transfers from Loon Lake to Eunice and Katherine Lakes, FISH TRANSFERRED TO DATE EUNICE LAKE KATHERINE LAKE (CUTTHROAT) (DOLLY VARDEN) 197^  11 OCTOBER to 28 NOVEMBER 821 252 1975 13 MAY to 25 JUNE 611 329 7 OCTOBER to 7 NOVEMBER 139 108 1976 3 MAY to 7 JUNE - 192 TOTAL 1571 881 stomach samples of Katherine Lake D o l l y Varden. A l l are i n d i c a t i o n s of an attempt to spawn. 2. F i s h Age and Length Composition C u t t h r o a t t r o u t t r a n s f e r r e d to Eunice Lake ranged i n l e n g t h from 55 to 255 mm with a mean of rj. 175 mm. The l e n g t h frequency histogram ( f i g u r e 7 4a) shows three peaks at 85,155, and 195 mm. Scale a n a l y s i s by Armitage (MS 1973) and p r o b a b i l i t y a n a l y s i s (Cassie 1950, 1954) on the f a l l c a t c h data alone i n d i c a t e t h a t these peaks correspond to 1 +, 2 + and 3 + groups. P r o b a b i l i t y a n a l y s i s suggests t h a t those f i s h above 225 mm may be 4 + y e a r s . Few, i f any, 0 + c u t t h r o a t were caught although they were observed i n Loon Lake at the mouth of some i n l e t streams i n l a t e summer. On J u l y 2, 1974, e i g h t 0 + c u t t h r o a t , 22-36 mm long, were caught w i t h d i p nets at the mouth of an i n l e t stream. D o l l y Varden t r a n s f e r r e d to Katherine Lake had the same s i z e range as c u t t h r o a t t r o u t , w i t h a mean l e n g t h of 156 mm. The l e n g t h frequency diagram ( f i g u r e 7b) shows peaks at 65, 125, and 165 mm. Scale a n a l y s i s by Armitage (MS 1973) and p r o b a b i l i t y paper a n a l y s i s i n d i c a t e t h a t these are 1 + 2 +, and 3 + f i s h with the p o s s i b i l i t y t h a t those over 2 05 mm are 4 + years. No 0 + animals were captured or observed. Because of the r a d i c a l changes i n growth p a t t e r n s a f t e r t r a n s f e r , no attempt was made to age f i s h caught i n e i t h e r Eunice or Katherine Lakes. 3. F i s h Growth F i s h i n the experimental lakes grew more than those i n Loon Lake. T h i s i s demonstrated i n three ways; (1) changes Figure 7a. Length distribution of Loon Lake cutthroat captured in trapnets and transferred to Eunice Lake. Figure ?b. Length distribution of Loon Lake Dolly Varden captured in trapnets and transferred to Katherine Lake. A. CUTTHROAT 260-| 2 4 0 2 2 0 ' 2 0 0 4 180-1 g 160 U . 140 O K g 120 X Z 100' 8 0 6 0 4 0 ' 2 0 FALL CATCH SPRING CATCH 3 * 99 135 153 I7S FORK LENGTH (mm) B. DOLLY VARDEN 140-1 1204 7T 100 s m 1 6 0-| 40 20 0 FALL CATCH { s ^ j SPRING CATCH 3 * 96 98 119 136 186 179 FORK LENGTH (mm) 196 236 i n l e n g t h d i s t r i b u t i o n of the f i s h ; (2) changes i n the r e l a t i o n s h i p of f i s h weight to l e n g t h ; and (3) changes i n the growth r a t e s of i n d i v i d u a l f i s h . The f o l l o w i n g analyses of the f i r s t two f a c t o r s i n c o r p o r a t e o n l y f i s h captured i n the experimental mesh g i l l n e t s , i n order t o e l i m i n a t e p o s s i b l e d i f f e r e n c e s due to s e l e c t i v i t y of d i f f e r e n t capture methods. A n a l y s i s of i n d i v i d u a l growth r a t e s u t i l i z e d both g i l l net and t r a p net data as the sample s i z e s from the g i l l nets alone were inadequate. In Loon Lake, c u t t h r o a t had a maximum l e n g t h of 23 3 mm and a mean of 180.0 mm (ta b l e 4). Much l a r g e r c u t t h r o a t (up to 310 mm) were captured i n Eunice Lake. The mean l e n g t h of Eunice c u t t h r o a t (209.9 mm) was s i g n i f i c a n t l y (P< .05) gr e a t e r than the mean of the Loon c u t t h r o a t . S i m i l a r l y , i n Loon Lake, the mean and maximum l e n g t h of D o l l y Varden were 172.7 and 217 mm r e s p e c t i v e l y , w h i l e i n Katherine Lake the corresponding f i g u r e s were 237.8 and 337 mm. The mean s i z e of the Katherine D o l l y Varden was s i g n i f i c a n t l y (P<.05) gr e a t e r than t h a t of the Loon D o l l y Varden. Increases i n the mean and maximum lengths of the t r a n s f e r r e d f i s h were not the only changes observed i n f i s h s i z e . In the s m a l l e r l e n g t h c l a s s e s sampled by the g i l l n ets, t h e r e was l i t t l e d i f f e r e n c e i n the weight of the f i s h between the c o n t r o l and experimental l a k e s . However, i n the l a r g e r l e n g t h c l a s s e s , f i s h of a given l e n g t h tended to weigh more i n the experimental lakes than i n Loon Lake ( f i g . 8). F u n c t i o n a l r e g r e s s i o n of the logs of weight and leng t h (Ricker, 1973) of Eunice c u t t h r o a t r e s u l t e d i n a slope Table 4. Comparison of the length and weight of cutthroat and Dolly Varden caught in g i l l nets set in the experimental lakes and in Loon Lake, with T values. (H Q: means in:experimental lakes = mean in Loon Lake, two tailed.) n LENGTH (mm) T-TEST (df) Range min max Mean s CUTTHROAT Loon Eunice 218 214 111 116 233 310 180.0 209.9 18.4654 41.8528 7.272*** (430) DOLLY VARDEN Loon Katherine 25 125 112 134 217 337 172.0 237.2 24.8O5O 45.435 6.957*** (148) n WEIGHT (gm) T-TEST (df) Range min max Mean s CUTTHROAT Loon Eunice 220 198 11.4 14.1 142.8 324.0 56.7 U 8 . 7 14.4581 70.5040 10.359*** (416) DOLLY VARDEN Loon Katherine 43 140 13.4 42.0 83.6 368.6 49.6 I 6 I . 5 16.1161 87.6948 11.845*** (181) Probability that the means in the experimental lakes = mean in Loon Lake <.0 0 1 . 35a F i g u r e 8a. R e l a t i o n s h i p o f weight and l e n g t h of cut throat caught i n g i l l nets i n Loon and Eunice Lakes, wi th f u n c t i o n a l r e g r e s s i o n l i n e s . Data p o i n t s are omitted f o r c l a r i t y . F i g u r e 8b. R e l a t i o n s h i p o f weight and l e n g t h o f D o l l y V a r -den caught i n g i l l nets i n Loon and Katherine Lakes, with f u n c t i o n a l r e g r e s s i o n l i n e s . Data p o i n t s are omitted f o r c l a r i t y . A. 400, LOON y-4.414 + 2.730X n » 2 2 0 R 2 « . 9 I 4 EUNICE Y=-5.027 + 3.036X n» 203 R 2 » . 9 8 3 EUNICE CT v—— , , . 100 200 300 400 LENGTH (mm) i c * - , — — , , •00 200 300 400 LENGTH (mm) of. 3.036 w i t h 95% confidence, l i m i t s of iv,:Q54. This i s o m e t r i c growth (Ricker, 19 75) i n d i c a t e s that the t r a n s f e r r e d c u t t h r o a t were able to maintain the same growh ra t e s i n weight throughout the lengths sampled. The corresponding r e g r e s s i o n slope f o r Loon c u t t h r o a t i s 2.730 + .105, an i n d i c a t i o n that the l a r g e r c u t t h r o a t i n Loon Lake could not maintain the same rate of growth i n weight as the smaller f i s h . The d i f f e r e n c e s i n slopes i s not as c l e a r f o r the D o l l y Varden, p o s s i b l y because of the small sample s i z e of g i l l netted Loon D o l l y Varden a v a i l a b l e (n=4 3). The slope of the Katherine D o l l y Varden r e g r e s s i o n l i n e i n n e a r l y 3.0 (2.961 + .062), i n d i c a t i n g i s o m e t r i c growth. In Loon Lake, the slope i s much l e s s than i n Katherine Lake but the 95% confidence l i m i t s are r e l a t i v e l y l a r g e (slope = 2.816 + .305); t h i s makes i t d i f f i c u l t to conclude i f the true slope f o r the Loon population i s l e s s than that f o r Katherine. More evidence of the d i f f e r e n c e i n growth r a t e s between lakes i s found by examining the r a t e of growth of i n d i v i d u a l f i s h . Figures 9A and 9B compare the growth r a t e (mg/day) w i t h the i n i t i a l s i z e (gm) of tagged and recaptured f i s h from a l l three l a k e s . The f i g u r e s show the average d a i l y growth r a t e s of f i s h t h a t were recaptured one year a f t e r tagging and f i s h t hat were tagged i n the s p r i n g (May and June) and recaptured i n the f a l l (September and October) of the same year. However, f o r Eunice Lake, while some f i s h were tagged i n the f a l l and recaptured i n the s p r i n g (winter growth r a t e s ) , none were recaptured a f t e r a year's growth. 3 7 a Figure 9a. Relationship of growth rate and i n i t i a l size of tagged and recaptured cutthroat in Loon and Eunice Lakes. Regression lines are drawn when probability < .05 that the slope of the line i s equal to zero. Figure 9b. Relationship of growht rate and i n i t i a l size of tagged and recaptured Dolly Varden in Loon and Katherine Lakes. Regression lines are drawn when probability < .05 that the slope of the line i s equal to zero. A. CUTTHROAT LOON SUMMER (•——•) LOON YEARLY (O— — O) EUNICE SUMMER (A ) EUNICE WINTER (A ) 1602. 1 1392. 30 40 80 I N I T I A L W E I G H T (gm) B. OOLLY VARDEN LOON SUMMER (•-LOON YEARLY (O— -O) KATHERINE SUMMER (A) KATHERINE YEARLY (A ) 11 A IKK 1507. A A 30 4 0 90 INITIAL WEIGHT (gm) TO C O The growth r a t e s of the e x p e r i m e n t a l l y a l l o p a t r i c f i s h were g r e a t e r i n every i n s t a n c e than the maximum growth r a t e s of t h e i r sympatric p o p u l a t i o n s . In Eunice Lake, the l a r g e r c u t t h r o a t appeared t o grow f a s t e r than the s m a l l e r f i s h i n the summer, wh i l e , i n the wi n t e r , the re v e r s e appeared t r u e ( f i g . 9a). However, r e g r e s s i o n a n a l y s i s o f t h i s data i n d i c a t e s t h a t no change i n growth r a t e w i t h i n i t i a l weight ( p r o b a b i l i t y of s l o p e s =0, > .05) e x i s t s f o r e i t h e r s et of data. The average d a i l y growth r a t e s (both f o r one year and f o r the summer months only) of sympatric Loon Lake c u t t h r o a t ( f i g . 9a) were g r e a t e r f o r the s m a l l e r f i s h than f o r the l a r g e r f i s h ( p r o b a b i l i t y of slopes =0, <.01). Some of the l a r g e s t Loon c u t t h r o a t even had negative growth r a t e s . The r a t e of decrease w i t h s i z e i n y e a r l y growth was the same as f o r summer growth ( p r o b a b i l i t y of slop e s being =, > .05). But, the mean growth r a t e d u r i n g the summer was l a r g e r than d u r i n g the wint e r ( p r o b a b i l i t y of Y - i n t e r c e p t s being =, < .01) . In Katherine Lake a l s o , the l a r g e r a l l o p a t r i c D o l l y Varden appeared t o grow f a s t e r than the s m a l l e r f i s h , but d u r i n g the summer and on a y e a r l y b a s i s ( f i g . 9b). But, as f o r the Eunice c u t t h r o a t , r e g r e s s i o n a n a l y s i s i n d i c a t e s no s i g n i f i c a n t change i n growth r a t e s w i t h s i z e ( p r o b a b i l i t y of sl o p e s = 0, > .05). On the other hand, the l a r g e r sympatric D o l l y Varden i n Loon Lake ( f i g . 9b) grew at slower r a t e s than d i d the s m a l l e r sympatric f i s h ( p r o b a b i l i t y of slopes = 0,<.01). The slop e s of the summer and the y e a r l y r e g r e s s i o n ho l i n e s are not s i g n i f i c a n t l y d i f f e r e n t (P>.05) but the average d a i l y growth r a t e s i n the summer are l a r g e r than those based on a year's growth (P<.001). There was l e s s d i f f e r e n c e i n the growth r a t e s of l a r g e and smal l sympatric D o l l y Varden than i n those of l a r g e and smal l sympatric c u t t h r o a t t r o u t (P of summer sl o p e s b e i n g = <.01; P of y e a r l y slopes b e i n g =<.05). 4. V e r t i c a l F i s h D i s t r i b u t i o n (i) Loon Lake - C u t t h r o a t and D o l l y Varden The catches of D o l l y Varden i n Loon Lake were s m a l l e r than those of c u t t h r o a t t r o u t , making s t a t i s t i c a l a n a l y s i s of the data i m p o s s i b l e i n many cases. However, g e n e r a l trends are apparent. Data from Armitage (MS 1973) c o l l e c t e d w i t h the same g i l l nets i n an i d e n t i c a l manner are presented here t o a i d the o b s e r v a t i o n of those trends f o r D o l l y Varden and c u t t h r o a t . For the same purpose, data c o l l e c t e d i n September, 1973 by an undergraduate limnology c l a s s conducted by T.G. Northcote i s a l s o i n c l u d e d . In May, 1972 and again, i n May and June, 1976, c u t t h r o a t t r o u t caught c l o s e t o shore i n the 5m nets were found throughout the water column ( f i g 10). O f f s h o r e , i n the 10m net, w h i l e s t i l l c aptured throughout the water column, s i g n i f i c a n t l y more (P<.05) were captured i n the top h a l f than i n the bottom h a l f of the net (ta b l e 5). Most D o l l y Varden captured i n the onshore net i n May, 1972 were near the bottom ( t a b l e 6) although some were i n midwater ( f i g 10). In May and June, 1976, no D o l l y Varden were caught i n the 5m net. Off s h o r e , i n May 1972, only two D o l l y Varden were caught i n kOa Figure 10. Vertical distribution during the spring and summer of sympatric Loon Lake cutthroat (blank) and Dolly Varden (shaded) in nets set from surface to bottom at the 5 and 10 m contours. Catches expressed as catch per hour set. Dura-tion of set (E) is indicated. SPRING 2 9 NAY, B I I JUNE, l » T 6 E' 6.S HRS. I OFFSHORE C T « 4 5 JULY, 1972 E= 20.0 HRS. OFFSHORE CT«44 OV-9 7 JULY, B 7 AUGUST, 1979 E- 24.0 HRS. 1 N S HORJX CT«38 DV'I OFFSHORE C T ' 2 4 0 V 8 0.1 FISH/HOUR 0 N 2 3 4 9 6 -7 -8 -9 -!0-19 JULY, a 2 4 AUGUST, 1976 E« 5 .0 HRS. INSHORE CT«23 DV'O OFFSHORE C T « I 4 OV'O 41 a Figure 11. Vertical distribution during the autumn and winter of sympatric Loon Lake cutthroat (blank) and Dolly Varden (shaded) i n nets set from sur-face to bottom at the 5 and 10 m contours. Catches expressed as catch per hour set. Dura-tion of set (E) i s indicated. 28 SEPTEMBER, a 18 OCTOBER, 1970 EOO.OHRS. INSHORE CT-36 DV«I0 T -AUTUMN I OFFSHORE CT»42 DV«26 7-8-9-10-MID- SEPTEMBER, 1972 E=20.0 HRS. INSHORE CT=2I DV»0 OFFSHORE CT» 31 0V«I2 -WINTER-S MARCH, 1976 E>I5.0 HRS. LL INSHORE CT-12 DV=II OFFSHORE CT«38 OV D » 0.1 FISH / HOUR 22 SEPTEMBER, 1973 £•10.0 HRS. OFFSHORE CT' 24 DV*8 n 2 3 1 4 x 8 t- • & 6 -9-10-IS SEPTEMBER, Si 27 OCTOBER, I97B E« 8.0 HRS. n INSHORE CT^24 DV'I II 111 • 11111 • •< i • t OFFSHORE CT= 32 0V» 3 the bottom w h i l e i n 1976, the three captured o f f s h o r e were found both midwater and on the bottom. Inshore, c u t t h r o a t were caught throughout the water column i n J u l y , 1972, and i n J u l y and August, 1975 and 1976 ( f i g . 10). In the 1975 sample, more c u t t h r o a t were caught i n the bottom h a l f than i n the top h a l f of the 5m net (P < .05) while i n the other two years there was no d i f f e r e n c e ( t a b l e 5). In the o f f s h o r e , 10m net, c u t t h r o a t were captured throughout the water column i n the summers of 1972 and 1975; but, i n 1976, a l l but one were captured above the midpoint of the net. In 1975, s i g n i f i c a n t l y more of the c u t t h r o a t were were captured i n the lower h a l f o f the 10m net (P<.05) while i n 1972 th e r e was a more even d i s t r i b u t i o n throughout the o f f s h o r e water column (ta b l e 5). D o l l y Varden, when captured d u r i n g the summer, were caught e x c l u s i v e l y i n the bottom h a l f o f both the i n s h o r e and the o f f s h o r e nets (table 6 ) . In the 5m n e t s , the few D o l l y Varden captured were a s s o c i a t e d c l o s e l y w i t h the bottom. O f f s h o r e , the D o l l y Varden captured i n 1972 and 1975 were found p a r t l y i n the lower midwater area ( f i g . 10). No D o l l y Varden were captured i n J u l y or August, 1976. During the fo u r years sampled, c u t t h r o a t t r o u t were captured throughout the water column i n the autumn both inshore and o f f s h o r e ( f i g . 11). In September, 1972, more c u t t h r o a t were captured i n the lower h a l f of the 10m net than i n the upper (P<.05) w h i l e there was no d i f f e r e n c e i n the 5m net (table 5). No d i f f e r e n c e i n the d i s t r i b u t i o n of c u t t h r o a t between the top and bottom of e i t h e r the 5m or the Table 5. Nuaber of Loon cutthroat caught in the upper and lower halves of g i l l nets fished from surface to bottom at the 5 and 10 B lake contours with "X? tests. (HQi upper catch - lower catch, two-tailed.) DATE LOCATION OF CAPTURE CONTOUR UPPER LOWER "X? SPRING May, 1972 5 28 2 5 0.075 10 25 5 12.033* Hay and June, 1976 5 8 13 0.762 10 39 6 27.756* SUMMER July, 1972 5 15 19 0.265 10 21 23 0.023 July and August, 1975 5 8 30 11.605* 10 I* 35 8.163* July and August, 1976 5 10 13 0.174 10 13 1 8.643* AUTUMN September and October, 1970 5 20 16 0.250 10 27 15 2.881 Septeaber, 1972 5 12 9 0.190 10 7 24 8.250* September, 1973 10 21 14 1.028 September and October, 1975 5 14 10 0.375 10 18 14 0.281 WINTER March, 1976 5 5 7 O.O83 10 15 23 1.289 Probability of upper catch - lower catch <.05, Other differences are not significant* Table 6. Number of Loon Dolly Varden caught in the upper and lower halves of g i l l nets fished from surface to bottom at the 5 and 10 m lake contours with "X-2 tests. (H Q: upper catch = lower catch, two-tailed.) DATE CONTOUR LOCATION OF UPPER CAPTURE LOWER X 2 SPRING May, 1972 5 2 14 7.562 10 0 2 a May and June, 1976 5 0 0 a 10 2 1 a SUMMER July, 1972 5 0 1 a 10 0 9 7.111 July and August, 1975 5 0 1 a 10 0 7 5.143* July and August, 1976 5 0 0 a 10 0 0 a AUTUMN September and October, 1970 5 4 6 0.100 NS 10 4 21 10.240* September, 1972 5 0 0 a 10 0 12 IO.O83* September, 1973 10 0 8 6.125 September and October, 1975 5 0 1 a 10 0 3 a WINTER March, I976 5 10 1 5.818* 10 0 3 a Probability of upper catch - lower catch <.05« a Sample size i s too small for valid use of "X2 test. 10m nets was e x h i b i t e d i n any of the autumn samples from 1970, 1973, and 1975. The f a l l d i s t r i b u t i o n of D o l l y Varden ap p a r e n t l y changed somewhat between 1970 and 1975 ( f i g . 11). In 1970, D o l l y Varden were captured throughout the i n s h o r e water column i n l a t e September and mid-October. No s i g n i f i c a n t d i f f e r e n c e i n numbers between the upper and lower h a l f of the 5m net appeared. O f f s h o r e , although most D o l l y Varden were captured c l o s e to the bottom, a number were captured i n midwater. In September, 19 72, no D o l l y Varden were captured inshore while only one was captured onshore i n 1975. The i n s h o r e h a b i t a t was not sampled i n 1973. O f f s h o r e , D o l l y Varden were c o n f i n e d t o the bottom 3 meters i n 19 73, bottom 2 meters i n 1972, and the bottom meter i n 1975. Armitage thought t h a t the d i f f e r e n c e i n depth d i s t r i b u t i o n between mid-September, 1972, and l a t e September and mid-October, 19 70, was due to the d i f f e r e n t times of the year sampled. However, the 1975 samples were taken s t i l l l a t e r i n the season than those of 1970. Yet, no captures of D o l l y Varden i n the water column occ u r r e d as they d i d i n 1970. The March 1976 samples were taken under 10 cm of i c e t h a t covered a l l of the lake south of the i s l a n d . As the lake was not f r o z e n two weeks e a r l i e r , the i c e was the r e s u l t of a r e c e n t f r e e z i n g p e r i o d . Inshore, c u t t h r o a t were found mainly i n the lower water column ( f i g . 11) although there was no s i g n i f i c a n t d i f f e r e n c e i n d i s t r i b u t i o n between the top and bottom halves of the net. O f f s h o r e , they i n h a b i t e d the e n t i r e water column. In comparison, D o l l y Varden were found throughout the inshore water column ( f i g . 14) and were more abundant i n the upper h a l f of the 5m net. O f f s h o r e , fewer D o l l y Varden were caught and these, although not on the bottom, were i n the lower water column. In summary, although the D o l l y Varden were captured throughout the water column i n the s p r i n g , they were a s s o c i a t e d mainly w i t h the bottom. In the summer, a l l D o l l y Varden were caught i n the lower h a l f of the nets and most were captured o f f s h o r e . In the f a l l of 1970, although the D o l l y Varden were taken throughout the water column, the m a j o r i t y were captured o f f s h o r e , i n the bottom metre of net. In c o n t r a s t , the f a l l samples of 1972-1975 took D o l l y Varden onl y i n the bottom 3 metres of net. During the w i n t e r , D o l l y Varden were captured throughout the water column w i t h no apparent a s s o c i a t i o n with the bottom. On the other hand, c u t t h r o a t were g e n e r a l l y captured throughout the water column at a l l times of the year. Inshore they were never excluded from the bottom p o r t i o n of the water column. Of f s h o r e , only i n the summer of 1976 were they not found i n the bottom p o r t i o n of the water column. Large numbers of c u t t h r o a t were found both i n s h o r e and o f f s h o r e , ( i i ) Eunice Lake - C u t t h r o a t L i t t l e seasonal v a r i a t i o n i n the v e r t i c a l d i s t r i b u t i o n of the a l l o p a t r i c c u t t h r o a t t r o u t p o p u l a t i o n i s apparent. C u t t h r o a t were captured inshore a t the 5m contour throughout the water column on a l l sampling dates from May and June, through J u l y and August, to September and October ( f i g . 12). There were never any s i g n i f i c a n t d i f f e r e n c e s between the Figure 12. Vertical distribution of allopatric Eunice Lake cutthroat in nets set from surface to bottom at the 5 and 10 m contours. Catches expressed as catch per hour set. Duration of set (E) i s indicated. -SPRING-27 MAY, a 21 JUNE, 197* E'27.0 HRS. n INSHORE CT= 13 OFFSHORE CT»I2 0-I 2 3 7 S 9 10 Q «0.2 FISH / HOUR 0 I 2 3 8 4 7 8 9 10 SUMMER-3 JULY, a 6 AUGUST, I97B E> 21.0 HRS. INSHORE CT«I4 OFFSHORE CT=20 AUTUMN 8 SEPTEMBER, 8 30 OCTOBER, 1979 E*27.0HRS. INSHORE CT» 19 • n OFFSHORE CT»23 2O.JULY, S 29 AUGUST, 1978 E-24.0 HRS. TL INSHORE CT»20 6 OCTOBER, IB76 E> 19.9 HRS. INSHORE CT'28 OFFSHORE CT = 20 OFFSHORE CT«23 Table 7. Number of Eunice cutthroat caught in the upper and lower halves of g i l l nets fished from surface to bottom at the 5 and 10 m lake contours with X 2 tests. (H Q: upper catch = lower catch, two-tailed,) DATE LOCATION OF CAPTURE CONTOUR UPPER LOWER SPRING May and June, 1976 SUMMER July and August, 1975 July and August, 1976 AUTUMN 5 10 5 10 5 10 8 11 9 17 12 19 3 1 5 3 8 1 1.454 6.750* 0.643 8.450* O.450 14.450* September and October, 1975 5 6 13 1.895 10 14 7 1.714 October, 1976 5 14 14 0.035 10 20 3 11.130* Probability of upper catch = lower catch <.05, Other differences are not significant. 50 numbers c a u g h t i n t h e u p p e r and t h e l o w e r p o r t i o n s o f t h e 5m n e t ( t a b l e 7) . I n t h e n e t s a t t h e 10m c o n t o u r c u t t h r o a t w e r e t a k e n a l m o s t e x c l u s i v e l y i n t h e u p p e r 5m o f n e t ( f i g . 1 2 ) . More f i s h w e r e c a u g h t i n t h e u p p e r 5m (P <.05) on a l l s a m p l i n g d a t e s e x c e p t i n S e p t e m b e r and O c t o b e r , 19 75, when no d i f f e r e n c e b e t w e e n t h e two h a l v e s o f t h e n e t was a p p a r e n t ( t a b l e 7 ) . ( i i i ) K a t h e r i n e L a k e - D o l l y V a r d e n I n May and J u n e , 1976, D o l l y V a r d e n w ere f o u n d t h r o u g h o u t t h e w a t e r c o l u m n , b o t h i n s h o r e a nd o f f s h o r e ( f i g 1 3 ) . I n s h o r e , d u r i n g J u l y and A u g u s t o f 1975 and 1 9 7 6 , D o l l y V a r d e n w e r e c l o s e l y a s s o c i a t e d w i t h t h e b o t t o m a l t h o u g h a few w e r e c a p t u r e d i n t h e l o w e r m i d w a t e r c o l u m n . The f i s h o f f s h o r e w e r e l i k e w i s e a s s o c i a t e d w i t h t h e b o t t o m ; b u t t h e y w ere a l s o c a p t u r e d i n t h e w a t e r c o l u m n . I n t h e summer o f 19 75, no d i f f e r e n c e b e t w e e n t h e numbers c a u g h t i n t h e u p p e r a n d l o w e r h a l v e s o f t h e 10m n e t was f o u n d w h i l e i n 1976 s i g n i f i c a n t l y more (P <..05) w e r e c a p t u r e d i n t h e b o t t o m h a l f ( t a b l e 8 ) . F i s h w e r e t a k e n t h r o u g h o u t t h e w a t e r c o l u m n a b o v e t h e 5m c o n t o u r i n t h e f a l l o f 1975 and 1976 ( f i g 1 3 ) . T h e r e was no a p p a r e n t a s s o c i a t i o n w i t h t h e b o t t o m i n s h o r e a t t h i s t i m e . O f f s h o r e , D o l l y V a r d e n w e r e more c l o s e l y a s s o c i a t e d w i t h t h e b o t t o m t h a n a t any o t h e r t i m e . S i g n i f i c a n t l y more were caught i n the bottom h a l f o f the 10m net in" the f a l l o f both years (table. 8) . I n E u n i c e L a k e d u r i n g c a l m w e a t h e r c u t t h r o a t c o u l d o f t e n be s e e n r i s i n g t o t h e s u r f a c e . F i s h w e r e n e v e r o b s e r v e d r i s i n g i n K a t h e r i n e L a k e by m y s e l f o r by o t h e r s who s a m p l e d z o o p l a n k t o n on t h e l a k e e v e r y two weeks d u r i n g t h e i c e - f r e e 50a Figure 13. Vertical distribution of allopatric Katherine Lake Dolly Varden in nets set from surface to bottom at the 5 and 10 m contours. Catches expressed as catch per hour set. Duration of set (E) is indicated. SPRING 27 MAY, a 21 JUNE, 1976 OFFSHORE DV=2I Q> 0.2 FISH /HOUR 0-I • 2-3-7-6-9-•SUMMER-9 JULY, a 7 AUGUST, 1979 E =22.3 HRS. D fl INSHORE DV=7 20 JULY, a 24 AUGUST, 1976 E'24.0 HRS. AUTUMN 18 SEPTEMBER, a 30 OCTOBER, 1979 £* 30.0 HRS. A INSHORE DV=9 • J L OFFSHORE DV = II 8 OCTOBER, 1976 E • 15.8 HRS. INSHORE 0V'I3 OFFSHORE DV»I9 INSHORE 0V*3 OFFSHORE DV'IS Table 8. Number of Katherine Dolly Varden caught in the upper and lower halves of g i l l nets fished from surface to bottom at the 5 and 10 m lake contours with X tests. (H Q: upper catch = lower catch, two-tailed,) LOCATION OF CAPTURE DATE CONTOUR UPPER LOWER SPRING May and June, 19?6 5 3 3 a 10 7 14 1.714 SUMMER July and August, 1975 5 1 6 2.286 10 4 20 9.375* July and August, 1976 5 2 11 4.923* 10 4 11 2.400 AUTUMN September and October, 1975 5 2 8 2.500 10 0 22 20.045* October, 1976 5 1 2 a 10 1 12 7.692* Probability of upper catch = lower catch< .05. 2 a Sample size i s too small for valid use of X test. Other differences are not significant. p e r i o d (C. Walters, pers. comm.). T h i s supports the evidence c i t e d above of the D o l l y Varden's a s s o c i a t i o n w i t h the bottom. 5. D i e t (i) C u t t h r o a t In Loon Lake the midwater forms, zooplankton and d i p t e r a n pupae, were the main d i e t a r y components of the sympatric c u t t h r o a t ( f i g 14). That midwater prey comprised between 62.8% v o l and 89.9% v o l of t r o u t stomach contents i n the summer of 1975 (July and August) and the s p r i n g of 19 76 (May and June) r e s p e c t i v e l y i n d i c a t e s l i t t l e seasonal v a r i a t i o n . N u m e r i c a l l y , midwater forms composed a s t i l l g r e a t e r p r o p o r t i o n of the d i e t , r a n g i n g from 71.6% to 92.4%. Crustaceous zooplankton was the predominant midwater prey type found i n the sympatric c u t t h r o a t stomachs ( f i g 14) as evidenced by t h e i r presence i n the stomach contents of 90% o f a l l Loon c u t t h r o a t . Zooplankton averaged 52% v o l i n 1975 and. 75% vol i n 1976, r e a c h i n g a minimum of '36% (summer, 1975) and a maximum of 76.3% (summer, 1976). No c o n s i s t e n t seasonal v a r i a t i o n was apparent, w i t h the p o s s i b l e e x c e p t i o n t h a t fewer absolute numbers were taken d u r i n g the summer months (appendix I I I ) . The other midwater form, d i p t e r a n pupae, o c c u r r e d i n l a r g e q u a n t i t i e s i n the summer and f a l l of 1975 (26.8% v o l and 11.2% v o l r e s p e c t i v e l y ) but i n l e s s e r q u a n t i t i e s i n 1976 (6.9% v o l i n s p r i n g and 3.1% v o l i n summer). D i p t e r a n pupae were not found i n October or March samples, probably because 53a Figure 14. Seasonal changes in the diet of Loon Lake cutthroat divided into major prey categories according to location of prey in the water column. Midwater prey is subdivided into crustaceous zooplankton (blank) and others (over 99% dipteran pupae) (hatched). Benthic prey is subdivided into large benthos (hatched) and small benthos (blank). Sample size (N) is indicated. M E A N % VOLUME I 1 at a | z ] 10 " 9-n O M E A N % FREQUENCY S o 1 1 of t h e i r absence from the l a k e . Benthic organisms were prese n t i n s i g n i f i c a n t q u a n t i t i e s throughout a l l of the sampling p e r i o d s w i t h a low of 9.1% v o l i n the autumn of 1975 and a h i g h of 25% i n the w i n t e r of 1976 ( f i g 14). When averaged over both y e a r s , s i m i l a r p r o p o r t i o n s by volume of the l a r g e r , m o t i l e b e n t h i c forms, e s p e c i a l l y T r i c h o p t e r a and C o l e o p t e r a , and the s m a l l e r , l e s s m o t i l e forms were captured. However, n u m e r i c a l l y the s m a l l e r benthos composed 3.5 times more of the d i e t than d i d the l a r g e r benthos. In g e n e r a l , the l a r g e r i n s e c t i n v e r t e b r a t e s which predominated i n the s p r i n g d i e t . In g e n e r a l , the l a r g e r i n s e c t l a r v a e were"more-prevalent d u r i n g the summer^than were the other non-rinseet benthos which predominated in the spring diet. Small f i s h were found i n the stomachs of o n l y two l a r g e i n d i v i d u a l s (223 mm and 233 mm) i n September, 1975. These were the l a r g e s t f i s h caught i n the g i l l nets on Loon Lake. The i n v e r t e b r a t e midwater prey types, zooplankton d i p t e r a n pupae, and Chaoborus, a l s o formed a major p o r t i o n of the d i e t of the Eunice Lake c u t t h r o a t ( f i g 15). As w i t h the sympatric c u t t h r o a t l i t t l e seasonal v a r i a t i o n i n the p r o p o r t i o n of midwater forms i n the d i e t was apparent. N u m e r i c a l l y they ranged between 68.7% ( f a l l , 1976) and 77.8% ( f a l l , 1975). Although the v a r i a t i o n was g r e a t e r (41.3% to 66.1%) on a v o l u m e t r i c b a s i s , seasonal trends were s t i l l not apparent. Of the midwater prey types crustaceous zooplankton were the main prey item present i n the a l l o p a t r i c c u t t h r o a t d i e t ( f i g 15), o c c u r r i n g i n 69% of the f i s h each year. In 1975, 5 5 a F i g u r e 15. Seasonal changes i n the d i e t of Eunice Lake c u t -t h r o a t d i v i d e d i n t o major prey categories a c c o r -d i n g to l o c a t i o n o f prey i n the water column. Midwater prey i s subdivided i n t o crustaceous zooplankton ( b l a n k ) , Chaoborus la rvae (spot ted) , and others (over 9% d i p t e r a n pupae) (hatched) . Benthic prey i s subdivided i n t o large benthos (hatched) and s m a l l benthos ( b l a n k ) . Sample s i z e (N) IS i n d i c a t e d . they averaged 43% v o l and 58% num; i n 1976, 42% v o l and 63% num. Chaoborus, when present i n the l a k e , formed an important component of the c u t t h r o a t d i e t , composing 31.7% v o l and 43.9% num of the stomach contents i n the summer of 1975. They were found i n 70% of the f i s h examined. The lowest c o n t r i b u t i o n o f crustaceous zooplankton t o the d i e t (22.9% v o l and 41.9% num) was d u r i n g the p e r i o d of Chaoborus abundance. Other midwater forms (mainly chironomid pupae but a l s o o c c a s i o n a l l y n o t o n e c t i d s . composed the s m a l l e s t p o r t i o n of the d i e t . Though they were present i n the d i e t on a l l sampling dates ( f i g 18), they were most common i n the s p r i n g (13.1% v o l and 18.2% num), and found o n l y r a r e l y i n the f a l l of 1975 ( < 1 % num). On other sampling dates they formed . between 3% and 8% of the d i e t by volume and by frequency. T e r r e s t r i a l i n s e c t s were a l a r g e component of the d i e t throughout the sampling program. T h e i r p r o p o r t i o n i n the stomach contents of a l l o p a t r i c t r o u t ranged from 8.4% num (sp r i n g , 1976) to 30.3% num ( f a l l , 1976), averaging 15% f o r the two years. Because of t h e i r l a r g e s i z e i n r e l a t i o n to the n u m e r i c a l l y dominant zooplankton they composed an even l a r g e r v o l u m e t r i c p r o p o r t i o n of the d i e t w i t h a minimum of 11.7% ( f a l l , 1975) and a maximum of 51.4% ( f a l l , 1976), averaging 22% f o r the two y e a r s . The l a r g e b e n t h i c organisms (Odonata, C o l e o p t e r a , T r i c o p t e r a , and leeches) a l s o formed a major p r o p o r t i o n of the a l l o p a t r i c c u t t h r o a t d i e t . Only i n the f a l l of 1976 58 were they l e s s than 15% v o l or 10% num o f the d i e t . T h e i r maximum p r o p o r t i o n i n the d i e t was 24.2% v o l i n the summer of 1976. The s m a l l e r b e n t h i c organisms were present i n a p p r e c i a b l e q u a n t i t i e s (>1%) only i n the s p r i n g of 1976 and the summer of 1975, when they were s t i l l l e s s than 5% of the d i e t both by volume and frequency. I n t e r e s t i n g l y , water mites (Order Hydracarina) were found o c c a s i o n a l l y i n the d i e t of c u t t h r o a t from Eunice and Loon, as w e l l as i n t h a t of Loon D o l l y Varden. These were sometimes present i n s u b s t a n t i a l q u a n t i t i e s i n i n d i v i d u a l f i s h (eg. 550 i n one Eunice Lake c u t t h r o a t caught i n August, 1975). They ranged between .4 mm and 1.2 mm i n body diameter and were o f t e n c o l o u r e d b r i g h t r e d or b l u e . Water mites are con s i d e r e d t o be u n p a l a t a b l e and even poisonous (Hyatt, i n press) and only taken i n c i d e n t a l l y . In summary, sympatric and a l l o p a t r i c c u t t h r o a t were predominantly p l a n k t i v o r o u s d u r i n g the months sampled. Other midwater forms were l i k e w i s e h e a v i l y preyed upon, e s p e c i a l l y Chaoborus, i n Eunice Lake, when abundant d u r i n g the summer of 1975. Both t e r r e s t r i a l i n s e c t s and benthos were w e l l r e presented at c e r t a i n times of the year and c o n t r i b u t e d about equal biomass throughout the e n t i r e sampling p e r i o d . In Eunice Lake the l a r g e r benthos c o n t r i b u t e d much more by volume than d i d the s m a l l e r benthos w h i l e i n Loon Lake both were e q u a l l y represented over the e n t i r e year, ( i i ) D o l l y Varden The sample s i z e s of D o l l y Varden i n Loon Lake were s m a l l , 59 a c o n d i t i o n t h a t m a k e s g e n e r a l i z i n g a b o u t s e a s o n a l v a r i a t i o n s d i f f i c u l t . P o s s i b l y t h e d a t a a v a i l a b l e a r e b i a s e d t o w a r d s a f e w s u r f a c e a n d i n s h o r e - - o r i e n t e d f i s h a n d a r e n o t r e p r e s e n t a t i v e o f t h e p o p u l a t i o n a s a w h o l e . H o w e v e r , A r m i t a g e (MS 1 9 7 3 ) , who f i s h e d g i l l n e t s f r o m t h e s u r f a c e t o t h e b o t t o m a t 5 , 1 0 , a n d 20 m, f o u n d f e w f i s h n e a r t h e b o t t o m d e e p e r t h a n t h e 10 m c o n t o u r , t h e a r e a f i s h e d b y t h e n e t s i n t h i s s t u d y . I n s p i t e o f t h e s e l o w s a m p l e s i z e s , some o b s e r v a t i o n s c a n b e m a d e . M i d w a t e r a n d b e n t h i c o r g a n i s m s w e r e v i r t u a l l y t h e o n l y f o r m s f o u n d i n t h e D o l l y V a r d e n s t o m a c h s ( f i g 1 6 ) . No v e r t e b r a t e p r e y a n d o n l y o n e t e r r e s t r i a l i n s e c t was f o u n d i n t h e s t o m a c h c o n t e n t s , t h a t i n A u g u s t , 19 7 5 . M i d w a t e r p r e y f o r m e d t h e l a r g e s t p o r t i o n o f t h e d i e t i n t h e s p r i n g o f 1976 ( 7 7 . 3 % v o l ) , l e s s i n t h e summer a n d f a l l o f 1 9 7 5 ( 5 2 . 1 % v o l ) , a n d t h e l e a s t i n M a r c h , 1 9 7 6 ( 2 1 . 3 % v o l ) . Z o o p l a n k t o n w a s t h e m a j o r c o m p o n e n t o f t h e m i d w a t e r c l a s s i f i c a t i o n , r a n g i n g f r o m 2 1 . 3 % v o l t o 7 2 . 7 % v o l . H o w e v e r , d i p t e r a n p u p a e w e r e p r e s e n t i n l a r g e q u a n t i t i e s i n t h e summer o f 1 9 7 5 w i t h 2 9 . 1 % v o l a s c o m p a r e d t o 2 3 . 0 % v o l f o r z o o p l a n k t o n . B e n t h i c f o r m s w e r e t h e s m a l l e s t p o r t i o n o f t h e d i e t i n t h e s p r i n g ( 2 2 . 7 % v o l ) b u t i n c r e a s e d t h r o u g h t h e summer a n d f a l l t o p r e d o m i n a t e i n t h e w i n t e r ( 7 8 . 7 % v o l ) . The l a r g e r b e n t h i c o r g a n i s m s w e r e common i n t h e d i e t t h r o u g h o u t t h e s p r i n g , s u m m e r , a n d f a l l ( 1 7 . 3 % t o 2 3 . 1 % v o l ) b u t w e r e r a r e l y f o u n d i n t h e s t o m a c h c o n t e n t s o f f i s h c a u g h t i n t h e w i n t e r (<^1 .5% v o l a n d num) . K a t h e r i n e L a k e D o l l y V a r d e n o b t a i n e d t h e i r l a r g e s t v o l u m e o f f o o d f r o m t h e b e n t h i c c o m m u n i t y ( f i g 1 7 ) . The d i e t a v e r a g e d 59a F i g u r e 16. Seasonal changes i n the d i e t o f Loon Lake D o l l y Varden d i v i d e d i n t o major prey categories a c c o r -d i n g to l o c a t i o n o f prey i n the water column. Midwater prey i s subdivided i n t o crustaceous zooplankton (blank) and others (over 9% d i p -teran pupae) (hatched) . Benthic prey i s sub-d i v i d e d i n t o l a r g e benthos (hatched) and s m a l l benthos ( b l a n k ) . Sample s i z e (N) i s i n d i c a t e d . 09 61 56.4% v o l be n t h i c organisms i n 1975 and 73.6% v o l i n 1976. The s p r i n g d i e t c o ntained the l a r g e s t p r o p o r t i o n of benthos (89.0% v o l ) although l i t t l e food of any s o r t was found i n the stomachs of f i s h caught i n May, 1976. Three f i s h had empty stomachs while the other f i f t e e n had an average of onl y 2 prey items, the most abundant being l a r g e benthos. Benthos composed between 46% and 67% v o l at other times of the year o c c u r r i n g i n 83% of a l l f i s h sampled. In c o n t r a s t t o the sympatric D o l l y Varden d i e t , the l a r g e r prey types were u s u a l l y the major component of the b e n t h i c category ( f i g 17). Midwater forms made up o n l y a smal l p o r t i o n by volume o f the Katherine D o l l y Varden d i e t , averaging 33% over the two years of sampling. N u m e r i c a l l y they formed a l a r g e r p a r t of the d i e t but s t i l l averaged l e s s than 50% over the 2 y e a r s . They were of l e a s t importance i n the s p r i n g of 1976, while i n the summer and f a l l they composed between 35-50% v o l of the d i e t . Chaoborus were common i n the d i e t throughout the summer and f a l l of 1975 and i n the summer of 1976, composing 8.8% to 43.9% num of the d i e t at t h i s time. Crustaceous zooplankton formed l i t t l e of the d i e t bulk, averaging o n l y 15.6% and 12.9% v o l i n 1975 and 1976 r e s p e c t i v e l y . They were v i r t u a l l y absent i n J u l y , 1975 and i n May and June, 1976, o c c u r r i n g i n o n l y 2 f i s h d u r i n g each o f these months. Other midwater forms (Dipteran pupae and h o t o n e c t i d s were present o n l y i n s m a l l numbers i n the summer and f a l l of 1975. In 1976 d i p t e r a n pupae were found i n every sampling p e r i o d , r e a c h i n g a maximum of 18.0% v o l i n the f a l l of 1976. 6 1 a Figure i7. Seasonal changes in the diet of Katherine Lake Dolly Varden divided into major prey categories according to location of prey in the water column. Midwater prey i s subdivided into crustaceous zoo-plankton (blank). Chaoborus larvae (spotted), and others (over 95$ dipteran pupae) (hatched). Ben-thic prey i s subdivided into large benthos (hatched) and small benthos (blank). Sample size ( N ) i s indicated. MEAN % VOLUME m m m m m nnm w m z . -i . • i o D a _ m z c « E MEAN FREQUENCY > m 3 •o m o « o o 6 r < m a H ro CD a > m ] co c > o m •© a m 22 Zefl O ui o o ] 1Z. t = 3C ] ] 22 u C I i 3 9 63 T e r r e s t r i a l i n s e c t s were not a major component of the d i e t averaging 3% v o l i n both years. Only 7 f i s h out of the 115 f i s h sampled had any s u r f a c e i n s e c t s — 1 1 i n s e c t s i n t o t a l (appendix VI) . 6. F a c t o r s A f f e c t i n g D i e t . (i) F i s h Length A l l f i s h captured were combined by year and by l a k e , and then s o r t e d by lengths i n t o separate s i z e c l a s s e s f o r stomach content a n a l y s i s . The s i z e c l a s s e s were chosen, p a r t l y on the b a s i s of modes i n the length-frequency d i s t r i b u t i o n of the g i l l net catches, and p a r t l y i n an attempt t o ensure an a r b i t r a r y minimum of f i v e f i s h per s i z e c l a s s where p o s s i b l e . Eunice Lake c u t t h r o a t had the widest l e n g t h range a v a i l a b l e f o r comparison ( f i g 18a). Crustaceous zooplankton appeared important i n the d i e t of a l l s i z e c l a s s e s , although t h e r e was c o n s i d e r a b l e v a r i a t i o n , both w i t h i n , and between, s i z e c l a s s e s . There was a g e n e r a l i n c r e a s e i n the r e l a t i v e numbers of zooplankton with f i s h s i z e , from almost 50% i n the s m a l l e r c l a s s e s t o over 80% i n the l a r g e s t . T h i s t r e n d was not apparent i n the volume data where f i s h over 28 0 mm had both the l a r g e s t p r o p o r t i o n (89%) i n 1975, and the s m a l l e s t p r o p o r t i o n (31%) i n 1976, of zooplankton. S i m i l a r l y the other midwater forms, d i p t e r a n pupae and Chaoborus (when present i n 1975), were found i n a l l s i z e c l a s s e s and were not c o n s i s t e n t l y more abundant i n any one c l a s s . B enthic organisms, e s p e c i a l l y the l a r g e r ones, c o n s t i t u t e d more of the stomach con-t e n t s i n the middle s i z e c l a s s e s (140-279mm) than i n e i t h e r the l a r g e or the small c l a s s e s . T e r r e s t r i a l i n s e c t s were l e s s abundant i n the l a r g e s t s i z e c l a s s than i n the other t h r e e , 63a Figure 18a. Differences in the diet of Eunice cutthroat of different "body lengths. Diet divided into Jiiajor prey categories according to location of prey in the water column. Midwater prey i s subdivided into crustaceous zooplankton (blank), Chaoborus larvae (spotted), and others (hatched) t Benthic prey i s subdivided into large benthos (hatched) and small benthos (blank). Sample size (N) IS indicated. F i g u r e 1 8 b . D i f f e r e n c e s i n t h e d i e t o f L o o n c u t t h r o a t o f d i f f e r e n t b o d y l e n g t h s . D i e t d i v i d e d i n t o m a j o r p r e y c a t e g o r i e s a c c o r d i n g t o l o c a t i o n o f p r e y i n t h e w a t e r c o l u m n . M i d w a t e r p r e y i s s u b d i v i d e d i n t o c r u s t a c e o u s z o o p l a n k t o n ( b l a n k ) a n d o t h e r s ( h a t c h e d ) . B e n t h i c p r e y i s s u b d i v i d e d i n t o l a r g e b e n t h o s ( h a t c h e d ) a n d s m a l l b e n t h o s ( b l a n k ) . S a m p l e s i z e (N) i s i n d i c a t e d . A. EUNICE 23 0 23 0 hi 2 3 o > z < UJ 2 lOOr-TS -SO -25 -o1-*'[ 0»-<N) 1976 79-129 (mm) (13) o >. o • z UJ o UJ cr u. 8* ui 10 L 0 100 75 SO 25 0 25 0 SURFACE PREY VERTEBRATE PREY I I L Z Z L MIDWATER PREY ESSS3 BENTHIC PREY 1975 1976 1975 1976 140 - 219 ( mm) (46) (44) 220- 279 (mm) (3D (24) SURFACE PREY VERTEBRATE PREY MIDWATER PREY ssssa BENTHIC PREY ^ Esa ^ ESS JZ1 ( S 3 1975 280 - 310 (mm) (5) (9) B. LOON SURFACE PREY 60 r 2sr-JZL VERTEBRATE PREY Ul 2 5 < Ul 2 "[ 0L T5 • 50 -25 • 0 25 MIDWATER PREY 1975 1976 (N) M l — 138 (mm) (7) (5) >-o z Ui O Ul DC Ul z 10 L 0 100 75 50 25 ,o S3 BENTHIC PREY E 3 I I 1975 1976 1879 1976 140- 199 (mm) 201 - 233 (mm) (41) (85) fr) SURFACE PREY (5) JZL VERTEBRATE PREY MIDWATER PREY S 3 BENTHIC PREY _EE3 f ° ° 1 C7\ 4=-a l t h o u g h t h e r e was c o n s i d e r a b l e v a r i a t i o n w i t h i n t h e f i s h o v e r 280 mm l o n g . A s m i g h t be e x p e c t e d , v e r t e b r a t e s w e r e f o u n d m a i n l y i n f i s h o v e r 220 mm l o n g a n d c o m p r i s e d t h e l a r g e s t v o l u m e (25%) i n f i s h o v e r 280 mm i n 1 9 7 6 . Z o o p l a n k t o n was a l s o a b u n d a n t i n a l l s i z e s o f L o o n c u t t h r o a t , f o r m i n g a s l i g h t l y l a r g e r c o m p o n e n t o f t h e d i e t i n f i s h o v e r 140 mm i n l e n g t h ( f i g 1 8 b ) . N e i t h e r d i p t e r a n p u p a e n o r b e n t h o s v a r i e d c o n s i s t e n t l y w i t h f i s h s i z e , A s i n E u n i c e L a k e , t e r r e s t r i a l i n s e c t s w e r e m o r e i m p o r t a n t t o t h e s m a l l e r f i s h , w h i l e v e r t e b r a t e s w e r e f o u n d o n l y i n t h e l a r g e s t ( > 230 mm). B e n t h i c o r g a n i s m s , p a r t i c u l a r l y t h e l a r g e m o t i l e o n e s , w e r e t h e p r e d o m i n a n t p r e y t y p e ( 5 1 - 7 7 % v o l ) i n a l l K a t h e r i n e D o l l y V a r d e n s i z e c l a s s e s ( f i g 1 9 a ) . No c o n s i s t e n t v a r i a t i o n w i t h s i z e w a s e v i d e n t f o r b e n t h o s . T e r r e s t r i a l a n d a q u a t i c i n s e c t s , i n c l u d i n g d i p t e r a n p u p a e , a l l o f m i n o r i m p o r t a n c e , d i d n o t v a r y c o n s i s t e n t l y w i t h f i s h s i z e e i t h e r . I n 1 9 7 5 , s m a l l f i s h t e n d e d t o h a v e m o r e C h a o b o r u s l a r v a e ( 2 9 % ' v o l ) t h a n m e d i u m (23% v o l ) o r l a r g e f i s h (13% v o l ) . On t h e o t h e r h a n d , i n b o t h y e a r s l a r g e f i s h h a d m o r e z o o p l a n k t o n (mean o f 16% v o l ) t h a n m e d i u m (14% v o l ) o r s m a l l f i s h (9% v o l ) . A l t h o u g h t h e r e w e r e 31 m e d i u m - s i z e d f i s h , s a m p l e s i z e s o f L o o n L a k e D o l l y V a r d e n l o n g e r t h a n 2 00 mm o r s h o r t e r t h a n 130 mm w e r e v e r y s m a l l (3 a n d 4 , r e s p e c t i v e l y ) . H o w e v e r , t h e r e was l i t t l e v a r i a t i o n w i t h i n t h e s e s a m p l e s . Two t r e n d s w e r e a p p a r e n t . The p r o p o r t i o n o f z o o p l a n k t o n i n t h e d i e t d e c r e a s e d s t e a d i l y w i t h f i s h s i z e , f r o m 62% v o l f o r s m a l l 65a Figure 19a. Differences i n the d i e t of Katherine D o l l y Var-den of d i f f e r e n t body lengths. Diet d i v i d e d i n t o major prey categories according to l o c a t i o n of prey i n the water column. Midwater prey i s sub-div i d e d i n t o crustaceous zooplankton (blank), Chaoborus larvae (spotted), and others (hatched). Benthic prey i s subdivided i n t o large benthos (hatched) and small benthos (blank). Sample s i z e (N) i s indicated. Figure 19b. Differences i n the d i e t of Loon Dolly Varden of d i f f e r e n t body lengths. Diet divided i n t o major prey categories according to l o c a t i o n of prey i n the water column. Midwater prey Is subdivided i n t o crustaceous zooplankton (blank) and others (hatched). Benthic prey i s subdivided i n t o large benthos (hatched) and small benthos (blank). Sample s i z e (N) i s indicated. MEAN % FREQUENCY MEAN % VOLUME 99 67 f i s h , to 38% v o l f o r medium f i s h , to 4% v o l f o r f i s h greater than 200 mm; a l l benthic foods, e s p e c i a l l y the lar g e benthos category, became more abundant as f i s h s i z e increased ( f i g 19b) . The composition of the zooplankton p o r t i o n of the d i e t was a l s o examined wi t h respect to f i s h length-. There were no apparent trends of v a r i a t i o n s i n zooplankton composition f o r e i t h e r Eunice c u t t h r o a t or Katherine D o l l y Varden ( f i g 20a). D. rosea comprised the m a j o r i t y of the zooplankton d i e t f o r f i s h of a l l s i z e s while D. kenai formed v a r y i n g proportions of the d i e t with no r e l a t i o n s h i p to length. In Loon Lake, D. rosea was a l s o the dominant zooplankter present i n a l l f i s h lengths f o r both c u t t h r o a t and D o l l y Varden ( f i g 20b). However, D. kenai formed a l a r g e r p r o p o r t i o n of the zooplankton as f i s h s i z e increased f o r both f i s h s pecies. ( i i ) Depth Caught Only c u t t h r o a t were d i s t r i b u t e d evenly enough throughout the water column t o allow comparisons between the d i e t of f i s h caught at d i f f e r e n t depths. I n s u f f i c i e n t D o l l y Varden were a v a i l a b l e i n the upper water column to provide an adequate comparison w i t h those i n the lower h a l f i n e i t h e r Loon or Katerine Lakes. Only those net sets t h a t had f i s h i n both the upper and lower halves of the net were used i n the comparisons. A l l the 5 m net sets from both years t h a t met the above c r i t e r i a were combined f o r a n a l y s i s ; 10 m net sets were t r e a t e d s i m i l a r l y . Diet was d i v i d e d i n t o three broad c a t e g o r i e s — b e n t h o s , midwater forms, and t e r r e s t r i a l i n s e c t s . 67a Figure 20a. Differences in the planktivorous diet of Eunice cutthroat and Katherine Dolly Varden of different body lengths. The other zooplankton classifica-tion includes; Polyphemus pedicuius (P), Dia-phanosoma brachyurum (D), and unidentifiable material of cladoceran origin (u). Figure 20b. Differences in the planktivorous diet of Loon cut-throat and Dolly Varden of different body lengths. The other zooplankton classification includes: Holopedlum gibberum (H), Leptodora klndlttl (L), Diaphanosoma brachyurum (D), Polyphemus pedicuius (P), Bosmina longlrostris (B), Latona setlfera (s), Cyclopoid copepods (c), and unidentified material of cladoceran origin (u). MEAN % FREQUENCY MEAN % FREQUENCY 3 I — o 3 i 3 N —o 3 I y s 1.3 o -» X ro a D l~ >' Z o 3 - r s' o "1 > -t O z c CO PI X 3. I EE 5 z m 5 , 1 5 2 a O m t s s 3 — o 3 I I 3 — o 3 I TJ o z p n i — J m a o co O c X 31 > -t o K PI z MEAN % FREQUENCY MEAN % FREQUENCY I 4 i i -8 = 3 1 3 ! X PI a TJ > Z PC Si T o >• X z > a o piJ 5 o o TJ H O z C o g I o m 3 ? 3 5 H O 3K l i 'I] w • J O I" r 3i > TJ X a o PI > J TJ -( O . £ c CO Pl 8 O C H - I X a> 89 In Eunice Lake ( f i g 21), f i s h caught i n the lower h a l f of both nets had more benthos i n t h e i r d i e t than d i d f i s h caught i n the upper h a l f . T e r r e s t r i a l i n s e c t s were of almost equal importance i n the d i e t of the f i s h caught i n both s e c t i o n s of the 5 m net; however, i n the 10 m net they were more common i n the f i s h caught i n the upper p o r t i o n . Midwater forms were more predominant i n f i s h from the lower h a l f o f the 10 m net. The opp o s i t e r e l a t i o n s h i p was observed i n the 5 m net. S i m i l a r l y , i n Loon Lake midwater prey items i n the d i e t were more predominant i n the lower 10 m net f i s h than i n the upper f i s h . The r e v e r s e r e l a t i o n s h i p i n the 5 m net was a l s o observed i n Loon Lake. Benthic organisms formed a g r e a t e r p r o p o r t i o n of the d i e t of the lower 5 m net f i s h than of the upper f i s h . However, i n the 10 m net no b e n t h i c forms were observed i n the lower f i s h w h i le they composed over 13% v o l of the upper f i s h e s d i e t . T e r r e s t r i a l i n s e c t s were s l i g h t l y more common i n the d i e t o f lower 5 m net f i s h than i n t h a t o f the upper f i s h . In g e n e r a l , the depths of capture does not n e c e s s a r i l y i n d i c a t e the d i e t of an i n d i v i d u a l f i s h . While most f i s h caught at lower depths may have more benthos i n t h e i r stomach contents than f i s h caught i n the upper water column, these r e s u l t s show th a t t h i s ' i s not n e c e s s a r i l y t r u e . S i m i l a r statements can be made about the r e l a t i o n s h i p of depth of capture and t e r r e s t r i a l i n s e c t s . 69a Figure 21. Comparison of the diet of Eunice and Loon cutthroat caught in the lower (spotted) and upper ("blank) half of the 5 and 10 m g i l l nets fishing from the surface to the bottom of the lake. Samples are combined only from dates when fis h were caught i n both halves of the nets. Dates of samples are: Eunice; 5 m net, October, 1975t June, July and October, 1976; 10 m net, August, September, and October, 1975t August and October, 1976; Loon; 5 m net* September, 1975t May, July, and August, 1976; 10 m et, September and October, 1975t March, 1976. 70 5m N E T 10 m N E T 0 - 2 m , N> 21 2-5m, N « 2 4 E U N I C E 0-2 2-5 BEN 0-2 2 - 5 M I D 0-2 2-5 TERR Ul s 3 O > lOO-i 0 - 5 m , N» 27 5 - I O m , N « 1 7 8 0 -6 0 -4 0 -20-O 0-5 5-10 BEN 0-5 5"10 MID 0-5 5-K) TERR L O O N 0-2m, N=29 2-5m, N«20 0-2 2^5 BEN 0-2 2 -5 MID 0-2 2 - 5 TERR z 3 o 100 80 60 H 3? 4 0 i 20H 0-5m, N« I3 5-IOm, N=I6 0 -5 5-10 BEN 0-3 5-10 MID 0-5 5-10 TERR 7. Composition of Zooplankton i n the D i e t (i) Seasonal Changes The crustaceous zooplankton group was examined i n g r e a t e r d e t a i l than the other major prey groupings. A l l zooplankters were i d e n t i f i e d to s p e c i e s , when p o s s i b l e . Data are r e p o r t e d as a mean percentage of the zooplankton present i n the d i e t d u r i n g a gi v e n sampling p e r i o d and are based on f r e q u e n c i e s o n l y . Daphnia rosea and Diaptomus kenai were the major zooplankters consumed by the sympatric Loon c u t t h r o a t ( f i g 22). D. rosea predominated i n the c u t t h r o a t d i e t d u r i n g the s p r i n g , summer, and f a l l . D. rosea comprised 79.4% and 74.8% i n the s p r i n g and f a l l r e s p e c t i v e l y and were s l i g h t l y lower i n the two summer sampling p e r i o d s - 67.5% i n 1975 and 66.6% i n 1976. D. kenai was the predominant zooplankter i n the d i e t (52.5%) from the wint e r sampling p e r i o d . The r e s t of the year D. kenai comprised between 2.6% and 18.2% with the lowest v a l u e s o c c u r i n g i n the summer months. In the winte r , o n l y D. rosea and D. kenai o c c u r r e d i n the d i e t . During the other seasons, a number o f oth e r p l a n k t e r s , mostly c l a d o c e r a , were p r e s e n t i n the d i e t (up to 30.8% i n the summer). Holopedium gibberum was the most common of these forms ( f i g 22). While a few were prese n t i n the d i e t d u r i n g the s p r i n g (< 1.0%) , they were most common i n the summer and f a l l , composing from 5.4% to 10.5% of a l l zooplankton present. Other forms were o c c a s i o n a l l y important i n the p l a n k t i v o r o u s p o r t i o n of the d i e t . Two pr e d a t o r y cladocerans i n Loon Lake, the l a r g e (approximately 5 mm) 71a Figure 22. Seasonal changes in the composition of zooplankton in the diet of Loon and Eunice cutthroat. The other zooplankton classification includesj Holo-pedium glbberum (H), Leptodora klndittl (L), Diaphanosoma brachyurum (D), Polyphemus pediculus (p), Bosmina longlrostrls (B), Latona setifera. (s), Cylopoid copepods (c), and unidentified material of cladoceran origin (u). LOON LAKE CUTTHROAT EUNICE LAKE CUTTHROAT (ST) (34) (3 7) (40) (21) OIAPTOMUS (N) (25) (43) (38) (37) (29) 1978 SPflINO x r r L 1975 1976 SUMMER 8.gf, DAPHNIA  ROSEA 1975 FALL 1976 WINTER 80 o u 60 20 1976 SPRING 40 OTHER ZOOPLANKTON 204 1975 1976 SUMMER 1975 1976 AUTUMN u.Mc-2 P-»  I 1 Leptodora k i n d t i i and the smaller (<1 mm) but n o t i c e a b l e , at l e a s t to the human observer, (because of a l a r g e eyespot), Polyphemus pediculas were both present i n the d i e t throughout s p r i n g and summer (1-7%). The small (<.51 mm) Bosmina  l o n g i r o s t r i s was s p o r a d i c a l l y taken i n some months (up to 3.9% num). Diaphanosoma brachyurum (< .87 mm) was preyed upon i n the summer of 1975 (6.4%), and, to a c e r t a i n extent, i n September, 1975 (1.0%), but not at any other time. S i m i l a r l y , Latona s e t i f e r a was a common prey item i n the summer of 1976 (10.2%), but not any other dates. Cyclopoids were present i n the d i e t only during the s p r i n g of 1976, i n small q u a n t i t i e s (4.0%). In the s p r i n g , D. rosea composed l e s s than 45% of the a l l o p a t r i c Eunice Lake c u t t h r o a t d i e t ( f i g 22). During the summer t h i s p r o p o r t i o n rose to 66 - 78% and i n the f a l l to over 90% of the d i e t . As D. kenai was the only other zooplankter found i n q u a n t i t y i n the Eunice c u t t h r o a t d i e t , i t s p r o p o r t i o n of the d i e t showed a reverse t r e n d , f a l l i n g from a high of 59% i n the s p r i n g , to 18-29% i n the summer, and to 0-7% i n the f a l l . The other zooplankters present, Bosmina, Holopedium, and Polyphemus, occurred i n the gut contents of only one Eunice c u t t h r o a t each. Seasonal changes i n the composition of zooplankters i n the d i e t of sympatric D o l l y Varden were d i f f i c u l t to d i s c e r n because of the small sample s i z e s . In general, D. rosea was the dominant p l a n k t e r , v a r y i n g from 35.4% i n midsummer 1975 to 100% i n the f a l l 1975 ( f i g 23). During the spr i n g 1976, 73a Figure 23. Seasonal changes in the composition of zooplankton in the diet of Loon and Katherine Dolly Varden, The other zooplankton classification includes: Polyphemus pedicuius (p), Gyclopoid copepods (c), and unidentifiable material of cladoceran origin (U). LOON LAKE DOLLY VARDEN KATHERINE LAKE DOLLY VARDEN (7) (12) (4) (14) DIAPTOMUS KENAJ IN) in (2 4) (32) (26) (33) (16) 1976 1979 1975 SPRING SUMMER FALL 1976 WINTER DAPHNIA ROSEA 80 u 60' 204 OTHER ZOOPLANKTON o 40-20' 1976 SPRING 1975 1976 SUMMER 1979 1976 AUTUMN 7 0 . 8 % o f t h e p l a n k t o n w a s D. r o s e a . D . k e n a i c o m p o s e d a l e s s e r , b u t s t i l l s i g n i f i c a n t , p o r t i o n o f t h e d i e t , c o m p o s i n g 2 8 - 3 0 % i n t h e s p r i n g , summer a n d w i n t e r , b u t a b s e n t i n t h e f a l l , 1 9 7 5 . The o n l y o t h e r z o o p l a n k t o n f o u n d i n t h e g u t s o f D o l l y . V a r d e n w e r e c y c l o p o i d s , w h i c h c o m p o s e d 3 4 . 4 % o f t h e d i e t i n t h e summer o f 1 9 7 5 b u t w h i c h w e r e n o t f o u n d a t o t h e r t i m e s . I n t h e a l l o p a t r i c K a t h e r i n e L a k e D o l l y V a r d e n , D . r o s e a was t h e o n l y z o o p l a n k t e r f o u n d i n l a r g e n u m b e r s ( f i g 2 3 ) . D. k e n a i w a s f o u n d o n l y i n o n e f i s h i n 1 9 7 5 . D. k e n a i a n d P o l y p h e m u s w e r e f o u n d i n o n e f i s h e a c h . I n t h e summer o f 1 9 7 6 , d i g e s t i o n made i d e n t i f i c a t i o n o f some m a t e r i a l o f a p p a r e n t c l a d o c e r a n o r i g i n d i f f i c u l t . ( i i ) C o m p a r i s o n w i t h L a k e Z o o p l a n k t o n F i s h do n o t f e e d i n d i r e c t r e l a t i o n s h i p t o t h e a p p a r e n t a v a i l a b i l i t y o f p r e y i t e m s i n t h e e n v i r o n m e n t ( I v l e v , 1 9 6 1 ; B r o o k s a n d D o d s o n , 1 9 6 5 ) . R a t h e r , t h e y t e n d t o s e l e c t d i s p r o p o r t i o n a t e l y f r o m t h e p r e y p o p u l a t i o n c e r t a i n t y p e s b a s e d o n s i z e , s p e c i e s , d e n s i t y o r o t h e r f a c t o r s . I n t h i s s e c t i o n g r a p h i c a l c o m p a r i s o n s b e t w e e n t h e p e r c e n t a g e c o m p o s i t i o n o f t h e v a r i o u s p l a n k t o n f o r m s i n t h e l a k e a n d i n t h e d i e t w i l l b e p r e s e n t e d . No a t t e m p t t o c o n s t r u c t s e l e c t i v i t y i n d i c e s , s u c h a s I v l e v ' s ( 1 9 6 1 ) , h a s b e e n m a d e . A s h a s b e e n s h o w n , D. r o s e a c o m p r i s e d b e t w e e n 60% a n d 100% o f t h e z o o p l a n k t o n i n t h e d i e t o f a l l f i s h p o p u l a t i o n s e x c e p t L o o n D o l l y V a r d e n ( f i g 2 4 a ) f r o m J u l y t h r o u g h O c t o b e r i n b o t h y e a r s . A t t h i s t i m e , D . r o s e a c o m p r i s e d a much s m a l l e r p r o p o r t i o n o f t h e z o o p l a n k t o n p o p u l a t i o n s o f a l l t h r e e l a k e s , Figure 24a. Comparison of the seasonal changes in the per?-centage contribution of Daphnia rosea to the planktivorous diet of cutthroat (open circles and dashed line) and of Dolly Varden (triangles and dash-dot line) and to the limnetic lake zooplankton populations (solid circles and solid line) in Loon, Eunice, and Katherine Lakes. F i g u r e 24b. Comparison of the seasonal changes i n the p e r -centage c o n t r i b u t i o n of Diaptomus kenai to the plankt ivorous d i e t of cut throat (open c i r c l e s and dashed l i n e ) and ©f D o l l y Varden ( t r i a n g l e s and dash-dot l i n e ) and to the l i m n e t i c l a k e zooplankton populat ions ( s o l i d c i r c l e s and s o l i d l i n e ) i n Loon, Eunice , and Katherine Lakes. LOON \ \ / JULY | AUGUST | SEPT. | OCT. | MARCH ' MAY | JUNE | JULY | AUGUST | 1979 1976 JULY | AUGUST I SEPT. | OCT. | MAY | JUNE | JULY | AUGUST OCT. | 1979 1976 100-80 60 40-to-0 -KATHERINE — JULY | AUGUST | SEPT. | OCT. | MAY * | JUNE | JULY | AUGUST ^ OCT. | 1979 1976 B. DIAPTOMUS KENAI 100 80 60 40 20-0 100 80 20 0 100-SO' 60' 40 20-0 LOON ,» JULY | AUGUST | SEPT. | OCT. | MARCH 1 MAY | JUNE | JULY | AU6UST| 1975 1976 EUNICE JULY | AUGUST~[ SEPT. | OCT. 1979 JULY | AUGUST OCT. 1976 KATHERINE I AUGUST | SEPT. I OCT. I MAY \ JUNE I JU 1979 77 i n c r e a s i n g from lows of 3-19% i n J u l y up to highs of 24-59% i n October. (In Katherine Lake, D. rosea d e c l i n e d i n October.) Moreover, Loon c u t t h r o a t and D o l l y Varden had high p r o p o r t i o n s of D. rosea i n t h e i r d i e t i n wint e r and s p r i n g at a time when the lake p o p u l a t i o n was very low. In Eunice Lake, the p r o p o r t i o n s of D. rosea i n the d i e t were about the same as i n the lake i n May, and c o n s i d e r a b l y h i g h e r i n June. Katherine D o l l y Varden ate no zooplankton i n May, a time when l i t t l e zooplankton was presen t i n the lake (Northcote e t al., 1977). In June, Katherine D o l l y Varden d i d not eat any D. rosea which comprised 8-14% of the lake zooplankton p o p u l a t i o n . Thus, f o r Loon Lake throughout the year, and f o r the other l a k e s from J u l y onwards, D. rosea was grazed h e a v i l y , a p p a r e n t l y i n d i s p r o p o r t i o n to i t s abundance. During May i n Eunice and June i n Kat h e r i n e , i t was ap p a r e n t l y undergrazed. However, the i n f o r m a t i o n obtained from Clarke-Bumpus sampling equipment on zooplankton p o p u l a t i o n s i s probably not the same i n f o r m a t i o n a v a i l a b l e t o a f e e d i n g f i s h which c o u l d be c o n c e n t r a t i n g e x c l u s i v e l y on areas of higher d e n s i t y . The r e l a t i o n s h i p between the percentage composition o f D. kenai i n the stomach contents and i n the la k e s ( f i g 24b) i s not as c l e a r as f o r D. rosea . V a r i a t i o n s i n percentage composition of D. kenai i n sympatric c u t t h r o a t a p p a r e n t l y was not r e l a t e d t o v a r i a t i o n s i n the l a k e . D. ken a i was v i r t u a l l y absent from stomachs i n J u l y , 1975 and f l u c t u a t e d between 8.1% and 23.9% on the other three 1975 sample dates; i n the lake there was l i t t l e f l u c t u a t i o n w i t h a l l samples having l e s s than 4.5% D. k e n a i . In 1976 D. kenai decreased 78 s t e a d i l y i n the lake from a high of 13.4% i n March to approximately 6-10% from June 30 through October. In the cutthroat, however, D. kenai varied from over 50% to 0% with l i t t l e apparent relationship to lake plankton. The planktivorous d i e t of Loon Dolly Varden contained 30% D. kenai i n July and August 1975, while, during the same time period, there was a maximum of only 3.5% D. kenai i n the lake plankton. Si m i l a r l y , i n March and May, 1976 D. kenai comprised 28-32% of the planktivorous d i e t but only a maximum of 14.4% i n the lake plankton. Stomach samples were not available for other dates. D. kenai was predominant i n the die t of a l l o p a t r i c Eunice cutthroat i n the spring, a time when i t was also the most common lake plankter ( f i g 24b). D. kenai was not found i n the die t of Eunice cutthroat i n 1975 afte r July, even though i t s t i l l composed 6-10% of the lake plankton. In 197 6, the percentage contribution of D. Kenai was approximately the same i n the d i e t and i n the lake with the exception of July, when i t was much more common i n the d i e t . In Katherine D. kenai, although composing a substantial portion of the lake plankton from May to July, 197 6, was found i n stomachs on only two occasions - i n July, 1975 (1 fish) and i n June, 1976 (2 f i s h ) . Sympatric Loon Lake cutthroat were the only f i s h to take a number of other zooplankton forms ( f i g 25). A low percentage of Holopedium was consistently found i n the stomach contents of cutthroat, a percentage that approximated that i n the lakes with the exception of August, 1975, when Figure 25. Comparison of the seasonal changes in the percentage contri-bution of other zooplankters to the planktivorous diet of cut-throat (open circles.and dashed line) and of Dolly Varden (triangles and dash-dot line) and to the limnetic lake zoo-plankton. population (solid circles and so l i d line) in Loon Lake. i t was c o n s i d e r a b l y underrepresented i n the stomachs. Other p e l a g i c zooplankton, e s p e c i a l l y Bosmina and Diaphanosoma were taken o c c a s i o n a l l y , u s u a l l y i n a s m a l l e r p r o p o r t i o n than t h e i r occurrence i n the l a k e . In the other l a k e s , few s p e c i e s o t h e r than D. rosea and D. kenai were taken. Katherine D o l l y Varden, i n August, 1975, contained 11.4% Polyphemus ( f i g 26a) although i t was on l y 1.0% of the lake plankton. No other zooplankton were found i n the Katherine D o l l y Varden, though other plankton, such as Holopedium, were major forms i n Katherine Lake ( f i g 26a). Eunice c u t t h r o a t , i n October, 1975, had 5.2% Polyphemus wh i l e none were found i n the lake ( f i g 26b), an i n d i c a t i o n of the l a c k of e f f e c t i v e n e s s of Clarke-Bumpus sampling equipment f o r sampling a s m a l l , probably patchy, and mainly l i t t o r a l p o p u l a t i o n . Only a few (< 1%) Holopedium or Bosmina were found i n Eunice c u t t h r o a t stomachs, although, i n the l a k e , Holopedium and Bosmina reached 19.9% and 15.1% r e s p e c t i v e l y . In summary, both a l l o p a t r i c and sympatric f i s h c o n t a i n e d D. rosea i n g r e a t e r p r o p o r t i o n than i t s abundance i n the lake w h i l e changes i n the percentage abundance of lake D. rosea tended to be r e f l e c t e d i n the stomach c o n t e n t s . D. kenai i n sympatric D o l l y Varden and c u t t h r o a t was over-r e p r e s e n t e d i n the d i e t , but the wide f l u c t u a t i o n s were not o b v i o u s l y r e l a t e d to changes i n lake percentage abundance. In a l l o p a t r i c f i s h , D. kenai was o f t e n found i n s i m i l a r p r o p o r t i o n s i n the stomachs and i n the l a k e s , e s p e c i a l l y i n s p r i n g . Other p e l a g i c zooplankton were r a r e l y found i n Figure 26a. Comparison of the seasonal changes in the per-L; centage contribution of other zooplankters to the planktivorous diet of Dolly Varden (triangles and dash-dot line) and to the limnetic lake zoo-plankton population (circles and so l i d line) in Katherine Lake. Figure 26b. Comparison of the seasonal changes in the per-centage contribution of ©ther zooplankters to the planktivorous diet of cutthroat (triangles and dashed line) and to the limnetic lake zoo-plankton population (circles and solid line) in Eunice Lake. a l l o p a t r i c f i s h ; however, sympatric c u t t h r o a t ate a number of other s p e c i e s , p a r t i c u l a r l y Holopedium, u s u a l l y i n much lower p r o p o r t i o n s than found i n the l a k e . ( i i i ) S i z e D i s t r i b u t i o n of Zooplankton Adequate samples from stomach contents f o r measurements of zooplankton were sometimes d i f f i c u l t t o o b t a i n , because of e i t h e r low numbers or advanced d i g e s t i o n i n many stomachs. Consequently, data were gathered o n l y from c u t t h r o a t t r o u t on D. rosea (most dates) and D. ken a i (some d a t e s ) . As d i g e s t i o n r a t e s f o r a zooplankton s p e c i e s may be d i r e c t l y p r o p o r t i o n a l t o the le n g t h of the i n d i v i d u a l p l a n k t e r (Barrington, 1957), i t i s p o s s i b l e t h a t the measurement may be b i a s e d towards l a r g e r i n d i v i d u a l s . In an attempt to overcome t h i s problem, no measurements were taken whenever much d i g e s t i o n had o b v i o u s l y o c c u r r e d . Daphnia from Eunice Lake pl a n k t o n samples, averaged over a l l depths, u s u a l l y had bimodal l e n g t h d i s t r i b u t i o n s ( f i g 27). The mode f o r the l a r g e r animals corresponded to a d u l t females (_>1.2 mm) while the other mode f o r s m a l l e r D. rosea was mainly immature females, from June to August, or males (^1.0 mm), from September and October. Daphnia s i z e c l a s s e s u s u a l l y ranged from .6 mm (.4 mm r a r e l y ) to 2.0 mm (2.2 mm r a r e l y ) w i t h means between 1.0 mm and 1.19 mm. The l e n g t h d i s t r i b u t i o n o f Daphnia i n the stomach contents of Eunice c u t t h r o a t '.was u s u a l l y unimodal ( f i g 27) . For the most p a r t the s i n g l e modes corresponded t o the a d u l t females i n the lake p o p u l a t i o n , although, i n 1976, immatures and males were w e l l represented (> 5% i n t o t a l on a l l 83 Figure 27. Comparison of the seasonal changes i n the length distribution of D. rosea in the diet of cutthroat (triangles and dashed line) and in the limnetic lake zooplankton (circles and:solid line) of Eunice Lake. AUGUST I97S > o z U J r> o ui tr. JUNE 1976 JULY 1976 Table 9. Comparison of the mean lengths and standard deviations of the Eunice Lake Daphnia in the lake population and in the cutthroat diet, with T values. (HQ: mean lake length = mean diet length, two-tailed.) DATE LAKE CUTTHROAT T-TEST X s n X s n 1975 July 1.08 .4287 41 1.68 .3419 57 7.448 August 1.04 .3318 35 1.40 .2105 114 6.056 September 1.17 .3949 42 1.36 .1423 75 • ** 3.074 October 1.17 . 3 ^ 9 35 1.51 .1527 109 5.659 1976 June 1.15 .5132 27 1.41 .2915 30 2.300* July 1.12 .3930 32 1.24 .1481 60 1.707 NS August 1.19 .3001 44 1.29 .1562 75 •* 2.115 October 1.06 .4222 3k 1.35 .1620 100 _ . "X' X X 3.896 Probability that mean lake length = mean diet length < ,05. •%"X-Probability that mean lake length = mean diet length <C .01. Probability that mean lake length = mean diet length < .001. o c c a s i o n s ) . Although the .8 mm s i z e c l a s s of Daphnia was found o c c a s i o n a l l y i n the stomach samples, D. rosea u s u a l l y ranged from 1.0 to 2.2 mm i n l e n g t h . The mean l e n g t h of D. rosea i n a l l o p a t r i c c u t t h r o a t stomachs was always g r e a t e r than the l e n g t h of those i n the lake by at l e a s t .12 mm and up to .60 mm (t a b l e 9). These d i f f e r e n c e s were a l l s i g n i f i c a n t (P<.05), except i n J u l y , 1976. There was a g e n e r a l , although not a c o n s i s t e n t , t r e n d towards s m a l l e r d i f f e r e n c e s between the mean l e n g t h of D. rosea i n the lake and i n the c u t t h r o a t stomachs from s p r i n g to autumn. U n l i k e those i n Eunice Lake, D. rosea i n Loon Lake u s u a l l y had a unimodal l e n g t h d i s t r i b u t i o n . Lengths ranged from 0.4 to 1.4 mm, wit h means of 0.87 mm to 0.94 mm. In c o n t r a s t to Eunice c u t t h r o a t , those i n Loon d i d not s e l e c t the l a r g e r Daphnia from the l a k e . Rather, Daphnia i n the Loon c u t t h r o a t d i e t were unimodally d i s t r i b u t e d by l e n g t h , w i t h the modes e i t h e r approximating those of the lake plankton or o c c u r r i n g at a s m a l l e r s i z e c l a s s ( f i g 28). Daphnia i n the stomachs ranged from 0.4 mm to 1.2 mm (ta b l e 10). With the e x c e p t i o n of two dates, there was no s i g n i f i c a n t d i f f e r e n c e (P>.05) between the mean s i z e of Daphnia i n Loon Lake or i n i t s c u t t h r o a t . In August, 1975 and May, 1976, Daphnia were s i g n i f i c a n t l y s m a l l e r i n the c u t t h r o a t stomachs than i n the lake p l a n k t o n . One p o s s i b l e reason f o r t h i s unusual occurrence was t h a t the c u t t h r o a t were f e e d i n g at depths where the Daphnia were sm a l l e r than the lake average. F i g u r e 29 shows the s i z e Figure 28, Comparison of the seasonal changes in the length distribution of D. rosea i n the diet of cutthroat (triangles and dashed line) and in the limnetic lake zooplankton (circles and solid l i n e ) of Loon Lake, JULY, I97S 30 40 30-1 AUGUST, 1976 N FISH 108 LAKE 4 9 Table 10. Comparison of the mean lengths and standard deviations of the Loon Lake Daphnia in the lake population and in the cutthroat diet, with T values, (HQ: mean lake length = mean diet length, two-tailed.) DATE LAKE CUTTHROAT T-TEST X s n X s n 1975 July- .90 .1844 4 8 .88 .1898 59 0.753 August .91 .2318 4 5 .78 .2032 100 3.3861*** September .88 .2206 85 .88 .2043 51 0 October .91 .2045 54 .87 .1830 103 1.157 1976 May .87 .2239 44 .76 .I856 108 3.139** June .90 .2378 39 .85 .1923 55 1.244 July .90 .2124 40 .86 .2051 46 O.853 August .94 .1738 4 5 .92 .2332 108 0.470 Probability that mean lake length = mean diet length < .01. Probability that mean lake length = mean diet length <C .001. Other differences are not significant. Figure 29. Length distribution of D. rosea in 5 different dejith intervals in May, 1976 and August, 1975* The depth interval in which cutthroat containing measurable D, rosea were caught is indicated. Mean size of D. rosea in each depth interval is indicated by a short vertical line. Table 11. Comparison of Daphnia lengths from depths at which plankti-vorous f i s h were caught and at a l l depths, with T values, (H n: lengths are equal, two-tailed.) DATE DEPTHS WITH FISH ALL DEPTHS T-TEST X s n X s n August 1975 .87 .2437 36 .91 .2318. . 45 1.177 NS May 1976 .76 .2275 31 .87 .2238 44 2.005* * Probability that lengths from depths with f i s h = lengths at a l l depths < .05. 90 d i s t r i b u t i o n of Daphnia at depths where p l a n k t i v o r o u s c u t t h r o a t were captured, and at a l l other depths on these two dates. Daphnia at f i s h depths tended to be smaller on both dates, but only i n May, 1976 was t h i s d i f f e r e n c e s i g n i f i c a n t (table 11). Measurable specimens of D. kenai were d i f f i c u l t to f i n d i n the stomach contents. As a r e s u l t , only two dates each from Loon and Eunice were examined. The number of f i s h c o n t a i n i n g measurable D. kenai and the number measured were small (2-4 f i s h and 33-38 D. kenai) except i n October, 1975 i n Loon (8 f i s h and 107 D. k e n a i ) . In Eunice Lake measurements were taken i n J u l y , 19 75 and 1976. The length d i s t r i b u t i o n of the lake D. kenai was b a s i c a l l y unimodal ( f i g 30). Size c l a s s e s ranged from 1.4 mm to 2.6 mm i n both years, w i t h a 1975 mean of 2.12 mm and a 1976 mean of 1.96 mm (table 12). The length of D. kenai i n the a l l o p a t r i c c u t t h r o a t examined i n J u l y , 1975 ranged between 1.4 and 2.8 mm, w i t h a mean of 2.31 mm, .19 mm l a r g e r than the lake mean length. The range i n length of D. kenai i n the 1976 stomachs was smaller than i n 1975, 1.8 to 2.6 mm, as was the mean, 2.18 mm. In both years, D. kenai were s i g n i f i c a n t l y l a r g e r (P <.05) i n the c u t t h r o a t stomachs than i n the lake (table 12). In Loon Lake, D. kenai had a unimodal d i s t r i b u t i o n i n August, 1975, ranging from 1.2 mm to 2.0 mm, and a mean length of 1.78 mm. In October of the same year the d i s t r i b u t i o n was bimodal, ranging from 1.0 mm to 2.0 mm, w i t h a mean of 1.53 mm ( f i g 30). Modes f o r the large and small D. kenai 90a F i g u r e 30a. Comparison of the l e n g t h d i s t r i b u t i o n of D. kenai l n the d i e t of cut throat ( t r i a n g l e s and clashed l i n e ) and i n the l i m n e t i c zooplankton ( c i r c l e s and s o l i d l i n e ) of Eunice Lake. F i g u r e 30b. Comparison of the length d i s t r i b u t i o n of D. kenai i n the d i e t of cut throat ( t r i a n g l e s and dashed l i n e ) and i n the l i m n e t i c zooplankton ( c i r c l e s and s o l i d l i n e ) of Loon Lake. 91 B.LOON LAKE LENGTH (mm) LENGTH (mm) Table 12. (a) Comparison of the mean lengths and standard deviations of the Loon Lake D, kenai in the lake population and in the cutthroat diet, with T values, (H^: mean lake length = mean diet length, two-tailed,) DATE LAKE FISH T-TEST X s n X s n August 1975 1.78 .1187 50 1.76 .1561 33 0.609 NS October 1975 1.53 .2508 25 1.67 .1637 107 3.610 *** Table 12, (b) Comparison of the mean lengths and standard deviations of the Eunice Lake D, kenai in the lake population and in the cutthroat diet, with T values, (H^: mean lake length = mean diet length, two-tailed.) DATE LAKE FISH T-TEST X s n X s n -July 1975 2,12 .2803 73 2.31 .2701 24 2.872 ** July 1976 1.96 .3365 . 74 2.18 .1800 . 18 2.587 * Probability that mean lake length =» mean diet length < ,05, Probability that mean lake length = mean diet length < .01. Probability that mean lake length = mean diet length < .001, corresponded to a d u l t s and copepodites r e s p e c t i v e l y . D. kenai i n the stomach contents was unimodal on both dates. In August, 1975, the mean s i z e of D. kenai i n stomachs was n e a r l y i d e n t i c a l (.02 mm d i f f e r e n c e ) to t h a t of D. kenai i n the l a k e s ; the d i s t r i b u t i o n was very s i m i l a r . In October, 1975 c u t t h r o a t stomachs contained o n l y D. ken a i 1.4 mm or longer, w i t h a mean of 1.67 mm. The mean l e n g t h of D. kenai i n the October c u t t h r o a t stomachs was s i g n i f i c a n t l y g r e a t e r (P<.05) than i n the lakes by .24 mm (t a b l e 12). 8. Chaoborus i n the D i e t One p o s s i b l e reason f o r the l a c k of importance of zooplankton i n the J u l y , 1975 d i e t of Eunice and Katherine f i s h was the presence of two s p e c i e s o f Chaoborus l a r v a e , p r i o r t o and immediately a f t e r f i s h i n t r o d u c t i o n . Chaoborus were present i n l a r g e numbers i n C - B samples i n Gwendoline, Kat h e r i n e , and Eunice p r i o r to 1975, but, l a t e r i n 1975 and i n 1976, they were found i n l a r g e q u a n t i t i e s o n l y i n nearby f i s h l e s s Gwendoline Lake (Northcote et a l , i n press) . In Eunice over 62% of a l l f i s h examined i n J u l y and August, 1975 (t a b l e 13) contained l a r g e numbers of the 4th i n s t a r , C. t r i v i t a t u s ; however, Chaoborus were found only s p o r a d i c a l l y throughout the r e s t of the sampling p e r i o d s , an i n d i c a t i o n of the r a p i d d e c l i n e i n lake abundance of C. t r i v i t t a t u s . In K a t h e r i n e , most C. t r i v i t t a t u s (mainly 3rd i n s t a r s ) were taken by f i s h p r i o r t o October, 1975 (tab l e 13) but a few were s t i l l found i n many f i s h throughout 1976. C. americanus was not abundant i n the d i e t of e i t h e r f i s h Table 13. Mean number and frequency of occurrence of Chaoborus in the stomach contents of Eunice cutthroat and Katherine Dolly Varden. DATE n C. TRIVITTATUS C. AMERICANUS 2ND 3RD 4TH 3RD 4TH Mean A Mean „ Mean _ Mean « Mean ^  No. 0 c c u r No. ° C C U r No. ° C C U r No. ° C C U r No. ° C C U r EUNICE LAKE 1975 July 22 0.1 .04 111.9 .77 August 21 1.0 .14 37.8 .62 September 15 0.1 ,07 October 22 0,4 .23 0.1 .04 1976 May 13 2.3 .46 June 12 July 27 0.1 .07 August 11 1,8 .09 October 29 0.03 .03 KATHERINE LAKE 1975 July 21 0.2 .10 99.4 .71 4.8 .48 August 11 30.3 .18 0.05 .05 September 13 34.0 .69 2.2 .31 0.1 .09 October 20 6.6 .30 2.8 ,05 1976 May 15 0.1 .07 June 9 0.2 .11 July 14 August 12 1.8 .08 6.2 .42 October 16 0.2 .19 d u r i n g the sampling p e r i o d . However, plankton samples i n d i c a t e d a r a p i d d e c l i n e of t h i s s p e c i e s p r i o r t o the s t a r t of the f i s h sampling program. I t was p o s s i b l e t h a t C. americanus was h e a v i l y preyed upon p r i o r t o f i s h sampling. B. LABORATORY 1. Movement and L o c a t i o n The time spent by i n d i v i d u a l f i s h moving and i n the top, middle, and bottom t h i r d s of the tank was observed. Table 14 shows t h a t D o l l y Varden moved about the tank over 90% of the time. The presence of food d i d not s i g n i f i c a n t l y i n c r e a s e t h e i r moving time. On the other hand, c u t t h r o a t averaged approximately 11.7% of the o b s e r v a t i o n p e r i o d moving, w i t h a low b e f o r e f e e d i n g , r i s i n g d u r i n g f e e d i n g , and d e c r e a s i n g immediately afterwards, although there was no s i g n i f i c a n t d i f f e r e n c e between o b s e r v a t i o n p e r i o d s . During the f e e d i n g the c u t t h r o a t appeared to be o r i e n t e d towards the water column more than the D o l l y Varden, spending 60% of t h e i r time i n the middle t h i r d of the tank ( t a b l e 15). D o l l y Varden spent over h a l f of the time i n the top 2/3 of the tank, mostly i n the middle p o r t i o n (36%). Before and a f t e r f e e d i n g , both s p e c i e s a p p a r e n t l y p r e f e r r e d the bottom t h i r d of the tank. However, the D o l l y Varden tended to swim throughout the water column, w i t h l e s s time spent on the bottom than the c u t t h r o a t . The l a t t e r u s u a l l y remained near the bottom b e f o r e f e e d i n g (94%) , and, at no p e r i o d d i d they spend much time i n the upper t h i r d of the tank. Tattle 14. Mean time (minutes) spent moving in 10 minute observation periods, before, during, and after feeding by 3 f i s h of each species. MEAN TIME (MINUTES) BEFORE DURING AFTER CUTTHROAT .62 1.89 1.01 DOLLY VARDEN 9.27 9.74 9.75 Table 15. Mean time (minutes) spent in each vertical third of the observation tank during observation periods, before, during, and after feeding by 3 f i s h of each species. POSITION CUTTHROAT TOP MIDDLE BOTTOM DOLLY VARDEN TOP MIDDLE BOTTOM MEAN TIME (MINUTES) BEFORE DURING AFTER .03 .05 .18 .55 5.86 3.62 9.42 4.09 6.20 1.28 1.50 2.05 2.51 3.60 2.36 6.21 4.90 5.59 97 2. P r e d a t i o n on S i n g l e Prey Types (i) D i f f e r e n c e s Between Prey Types D o l l y Varden at t a c k e d and subsequently captured s i g n i f i c a n t l y more D. pulex and D. kenai than d i d c u t t h r o a t ( t a b l e 16a). T h i s p a t t e r n was repeated w i t h Chaoborus, but the d i f f e r e n c e s were not s i g n i f i c a n t (P>.05). However, D o l l y Varden missed an att a c k e d prey item more o f t e n than d i d c u t t h r o a t , i . e . D o l l y Varden had lower capture e f f i c i e n c i e s . They captured more prey simply because they a t t a c k e d more. Although some D. kenai e i t h e r escaped or were r e l e a s e d a f t e r capture by both c u t t h r o a t and D o l l y Varden, the l a t t e r were able to r e t a i n s i g n i f i c a n t l y more (P<.05) of them. During the f e e d i n g p e r i o d , both f i s h s p e c i e s made a t t a c k s on non-prey items, such as smal l p i e c e s of f l o a t i n g or submerged d e b r i s and a p a i n t brush h a i r stuck on the s i d e of one o b s e r v a t i o n tank. As with plankton, the D o l l y Varden a t t a c k e d more "non-prey" items than d i d the c u t t h r o a t (P< .05). The former had a maximum of 28 non-prey a t t a c k s i n one f e e d i n g s e s s i o n on D. ken a i w h i l e the l a t t e r had a maximum of o n l y 11. Both s p e c i e s a t t a c k e d Chaoborus at a hig h e r r a t e than they d i d e i t h e r of the other two prey types (t a b l e 16b). A l s o , t h e i r a t t a c k r a t e on Daphnia was hig h e r than t h a t on D. k e n a i , but, the d i f f e r e n c e s i n t h i s case were not s i g n i f i c a n t (P>.05). In d i r e c t r e l a t i o n to the a t t a c k r a t e , the capture r a t e was higher on Chaoborus, wi t h D. pulex and D. ken a i being captured at lower r a t e s . In t h i s case D o l l y Varden captured s i g n i f i c a n t l y more D. pulex than D. k e n a i . C u t t h r o a t e x h i b i t e d the same trend; but t h i s o b s e r v a t i o n i s not s t a t i s t i c a l l y Table 16. (a) Comparison of fishing abilities between cutthroat trout and Dolly Varden on 3 prey types; Beans of 4 fish in th* last 5 t r i a l s on each prey type. Means that are under-lined are not significantly different; a l l other paired means are significantly different (T test, F< .05 , two-tailed), 4TH INSTAR C. TRIVITTATUS D. PULEX D. KENAI NUMBER OF PREY 100 300 300 FISH CT DV CT DV CT DV ATTACKS (NO.) 75.0 100.5 70.0 172.5 51.8 124.5 CAPTURES (NO.) 72.0 88.8 65.O 141.2 27.3 87.8 CAPTURE EFFICIENCY (£ OF ATTACKS) 96.7 88.3 92.9 81.7 52.6 70.7 RETENTIONS (JS OF CAPTURES) 100 100 100 100 70.9 96.9 NON-PREY ATTACKS 1.8 14 4.3 15.5 3.8 22.5 Table 16. (b) Comparison of feeding abilities between 3 prey types by cutthroat and by Dolly Varden; means of 4 f i s h ln the last 5 trials on each prey type. Means that are under-lined are not significantly different; a l l other grouped means are significantly different (ANOVA and Duncan's Multiple Range Test, P<.05). CUTTHROAT DOLLY VARDEN .. . . C.TRIV. D. PULEX D. KENAI C. TRIV. D. PULEX D. KENAI NUMBER OF PREY 100 300 300 100 300 300 - A T 8 w AVAILABLE) ^ 6 - S J i M _ *00'* 'TOTAVAILABLE) - 3 * 2 4 * - . *™ 29.2 GROSS CAPTURE EFFI CXE1N CY (£ CAP./ATT.) 96.7 92.9 52.6 88.3 8I.7 70.7 RETAINED CAPTURES , 06 OF CA^RS) -122 100 70.9 J P O 100 96.9 RETAINED CAPTURES ~, u 91 - z « Ro •» hn n 9a •» (fo OF AVAILABLE ) 7 7 ^  -SLlZ - W O *7.0 28.3 FINAL CAPTURE ^°S£H CAPT. > - 7 W 35.2 88.3 8I.7 65.5 / ATTACKS) s i g n i f i c a n t . When at t a c k i n g . D. k e n a i , both f i s h e s had low capture e f f i c i e n c i e s ( c a p t u r e s / a t t a c k s ) . C u t t h r o a t captured about 53% of the D. kena i a t t a c k e d , as compared t o 90% f o r both Chaoborus and D. pulex while D o l l y Varden had a capture e f f i c i e n c y of 70% on D. kena i and over 80% on the o t h e r s . D o l l y Varden were s l i g h t l y more e f f i c i e n t on Chaoborus than on D. pulex w h i l e c u t t h r o a t captured both e q u a l l y w e l l . Both f i s h s p e c i e s r e t a i n e d a l l Chaoborus or D. pulex once captured, but l o s t a s i g n i f i c a n t number of D. kenai a f t e r capture. In many cases i t appeared t h a t the f i s h a c t i v e l y r e j e c t e d the Diaptomus once the prey was i n i t s mouth. ( i i ) Changes with Experience A f t e r the f i s h were c o n d i t i o n e d w i t h c h i c k e n l i v e r p i e c e s , Chaoborus were the f i r s t p l a n k t o n i c prey f e d to them i n the o b s e r v a t i o n tanks. I n i t i a l a t t a c k r a t e s were low f o r both f i s h s p e c i e s ( f i g 31a), l e s s than 20 a t t a c k s per 15 minute p e r i o d . However, by the t h i r d t r i a l , the r a t e s had i n c r e a s e d d r a m a t i c a l l y . In the l a s t t hree t r i a l s , D o l l y Varden maintained a t t a c k r a t e s w e l l over 95% on a l l a v a i l a b l e animals, and o f t e n captured a l l Chaoborus pres e n t w i t h i n 15 minutes. Although the c u t t h r o a t never a t t a i n e d as high an a t t a c k r a t e as the D o l l y Varden, they d i d maintain, d u r i n g the l a s t t hree t r i a l s , r a t e s between 75% and 80%. T h i s l a r g e i n c r e a s e may be due to the f i s h e s ' i n i t i a l i n e x p e r i e n c e w i t h p l a n k t o n i c food i n the l a b o r a t o r y . During the l a t e r experiments with D. rosea, and then D. 99a Figure 31a. Single prey type experiments: relative frequency of attacks by cutthroat (open symbols and solid lines) and by Dolly Varden (solid symbols;rand-i dashed lines) on 3 prey types; Daphnia pulex (c i r c l e ) , Diaptomus kenai(triangle) and 4th i n -star Chaoborus triv i t t a t u s (square). Values are the means of the same 4 f i s h of each species in each t r i a l . Figure 31b. Single prey type experiments: capture efficiency (captures per attack) by cutthroat (open symbols and solid lines) and by Dolly Varden (solid sym-bols and dashed lines) on 3 prey types; Daphnia  pulex (c i r c l e ) , Diaptomus kenai (triangle), and 4th instar Chaoborus triv i t t a t u s (square). Values are the means of the same 4 f i s h of each species in each t r i a l . 100 k e n a i , both f i s h s p e c i e s e x h i b i t e d an i n c r e a s e i n a t t a c k r a t e s , but l e s s than those shown i n the Chaoborus experiments. Both D o l l y Varden and c u t t h r o a t i n i t i a l l y a t t a c k e d D. k e n a i at a h i g h e r r a t e than they att a c k e d D. rosea, but, by the t h i r d or f o u r t h t r i a l , both a t t a c k e d the D. rosea a t a higher r a t e . The D o l l y Varden accomplished t h i s by i n c r e a s i n g t h e i r a t t a c k r a t e on D. r o s e a . In comparison, the c u t t h r o a t i n c r e a s e d t h e i r a t t a c k r a t e on D. rosea o n l y s l i g h t l y , but g r e a t l y decreased t h e i r a t t a c k r a t e on D. k e n a i i n the l a s t two t r i a l s . Capture e f f i c i e n c i e s were g e n e r a l l y high and constant on both Chaoborus and D. rosea ( f i g 31b). Capture e f f i c i e n c i e s ranged between 74% and 96% on both prey s p e c i e s . Both f i s h i n i t i a l l y had s i m i l a r capture e f f i c i e n c i e s , but by the l a s t t r i a l , c u t t h r o a t were more s u c c e s s f u l i n c a p t u r i n g both Chaoborus and D. rosea (P <.05) ( t a b l e 16a). Capture e f f i c i e n c i e s on D. kenai were very low i n the f i r s t t r i a l , 41% f o r D o l l y Varden and 34% f o r c u t t h r o a t . Capture e f f i c i e n c y i n c r e a s e d w i t h e x p e r i e n c e , more so f o r D o l l y Varden than f o r c u t t h r o a t . By t r i a l 5, D o l l y Varden had a success r a t e of 71%, s i g n i f i c a n t l y h i g h e r (P< .05) than the success r a t e of 52.6% f o r c u t t h r o a t ( t a b l e 13a). In s p i t e of t h i s improvement i n s u c c e s s f u l a t t a c k s , the D o l l y Varden demonstrated l i t t l e i n c r e a s e i n a t t a c k r a t e s a f t e r three t r i a l s , w h i le the c u t t h r o a t a c t u a l l y decreased t h e i r a t t a c k r a t e s i n the l a s t two t r i a l s . 3. P r e d a t i o n on P a i r e d Prey Types The experiments on s i n g l e prey types suggested t h a t 102 both f i s h e s would a t t a c k and capture prey i n the ord e r : 4th C. t r i v i t t a t u s , D. pulex, and D. k e n a i , although the o r d e r i n g between Daphnia and Diaptomus was u n c l e a r . Consequently a s e r i e s of p a i r e d prey o b s e r v a t i o n s were conducted. When presented w i t h a l l combinations o f p a i r e d prey (table 17), both f i s h e s attacked and captured t h e i r prey i n the f o l l o w i n g o r d e r : 4th i n s t a r C. t r i v i t t a t u s , D. pulex, and D. ken a i (P<.05). In a l l cases but one, both f i s h e s had a capture e f f i c i e n c y i n the same order (P<.05). The one e x c e p t i o n was the la c k of s i g n i f i c a n t d i f f e r e n c e i n at t a c k success by c u t t h r o a t on 4th C. t r i v i t t a t u s and D. pulex. These r e s u l t s c o n f i r m the trends e x h i b i t e d i n the s i n g l e prey experiments. To examine the e f f e c t of s i z e on the apparent "preference" of the f i s h f o r Chaoborus, another experiment was conducted. 2nd i n s t a r Chaoborus sp. (probably C. t r i v i t t a t u s ) was presented along w i t h D. pulex. Both prey items were of approximately the same l e n g t h . No s i g n i f i c a n t d i f f e r e n c e i n the number of a t t a c k s , the number of cap t u r e s , or the capture e f f i c i e n c i e s were observed (t a b l e 17). One aspect of the p a i r e d prey experiments not apparent from the f i n a l outcome as shown i n t a b l e 17 i s the t i m i n g of the a t t a c k s and captures on each s p e c i e s . When giv e n both 4th i n s t a r Chaoborus and D. pulex or D_. k e n a i , ( f i g 32) , the r a t e of a t t a c k and capture on Chaoborus was h i g h e s t i n the f i r s t few minutes of f e e d i n g (^ .8 captures/minute, <19 captures/minute, i n the f i r s t 4 minutes) f o r both f i s h , d e c r e a s i n g as the prey d e n s i t y decreased. The a t t a c k and Table 17. Comparison of feeding by cutthroat and Dolly Varden when presented with paired prey types; means of 4 f i s h of each species i n 2 t r i a l s (8 t r i a l s in tota l ) . Means that are underlined are not significantly different; a l l other paired means are significantly different (T test, P < .05» two-tailed). 4TH C. D. 4TH C. D,' D. D. 2ND C. D. TRIV. PULEX TRIV. KENAI KENAI PULEX TRIV. PULEX NO. OP PREY 100 100 100 100 100 100 100 100 ATTACKS (N) CT 8i-A 30-2 78.6 22.6 33.8 1 8.4 16.9 16.4 DV 113.0 56.3 113.1 ^.5 78.8 38.8 27.6 28.4 CAPTURES (N) OT 83.I 28.9 77.1 12.8 .32.2 7.6 15.8 15.9 DV 98.1 43.6 97.9 32.4 75.5 22.1 26.1 27.4 CAPTURE EFFICIENCY (#) GT 98.8 95.6 98.I 56.2 75.^  41.7 94.2 96.7 DV 87.1 77.8 86.6 72.5 96.1 58.1 96.2 94.5 Figure 32a. Paired prey type experiment: cumulative captures of 4th instar C. trivittatus (square) and D, pulex (circle) by cutthroat (open symbols and solid lines) and Dolly Varden (dashed line and solid symbols). Values are the means of 2 trials of 4 fish of each species. Figure 32b, Paired prey type experiment: cumulative captures of 4th instar G, trivittatus (square) and D. kenai (triangles) by cutthroat (open symbols and solid lines) and Dolly Varden (solid symbols and dashed lines). Values are the means of 2 trials of 4 fish of each species. capture r a t e on the other prey presented with Chaoborus was low i n i t i a l l y (<0.83 captures/minute, i n the f i r s t 10 minutes), i n c r e a s i n g g r e a t l y only a f t e r the Chaoborus capture r a t e decreased. However, the a t t a c k and capture r a t e on D. pulex and D. kenai (< 2 captures/minute, i n the second 10 minutes) was always lower than the maximum r a t e on Chaoborus. When the f i s h were presented with the ch o i c e between D. pulex or D. kenai ( f i g 33) the a t t a c k and capture r a t e s were approximately constant f o r most of the f e e d i n g p e r i o d , v a r y i n g between a low of 0.1 captures/minute f o r D. kenai by c u t t h r o a t and a high of 2.2 captures/minute f o r D. pulex by D o l l y Varden i n the f i r s t 20 minutes. In c o n t r a s t to the pre v i o u s experiment w i t h the 4th i n s t a r Chaoborus l a r v a e , there was no i n i t i a l p e r i o d of very h i g h a t t a c k and capture r a t e s , but, i n c e r t a i n cases, the a t t a c k and capture r a t e s decreased near the end of the f e e d i n g p e r i o d . The ch o i c e between 2nd i n s t a r Chaoborus and D. pulex r e s u l t e d i n a t t a c k and capture curves v e r y s i m i l a r to those on D. pulex and D. kenai an approximately constant c a p t u r e - r a t e on both s p e c i e s ( f i g 34). 1 0 5 a Figure 33a. Paired prey type experiment: eumulative attacks on D. pulex ( c i r c l e s ) and D, kenai ( t r i a n g l e s ) by cutthroat (open symbols and s o l i d l i n e s ) and D o l l y Varden ( s o l i d symbols and dashed l i n e s ) . Values are the means of 2 t r i a l s of 4 f i s h of each species. Figure 33b. Paired prey type experiment: cumulative captures of D, pulex ( c i r c l e s ) and D, kenai ( t r i a n g l e s ) by cutthroat (open symbols and s o l i d l i n e s ) and Do l l y Varden ( s o l i d symbols and dashed l i n e s ) . Values are the means of 2 t r i a l s of k f i s h of each species. A. 100, PREY ABUNDANCE D. PULEX ISO D. KENAI ISO 80J. co < < Ui > 4 0 J D.V. or. O.PULEX O.V. on 0. KENAI 1 14 18 TIME (min) B. CO, CO Ul K £ 60^ o. <t o Ul > 4 9 ^ < _l O 2 0 j .Jk_ — - % 14 18 TIME (min) 22 2 6 30 100, PREY ABUNDANCE D. PULEX 100 C. TRIVITATUS 100 tr TIME (min) Figure 34. Paired prey type experiment: cumulative captures of 2nd instar C. triv i t t a t u s (square) and D. pulex (circle) by cutthroat (open.symbols and so l i d lines) and Dolly Varden (solid symbols and dashed lines ) . Values are the means of 2 t r i a l s of 4 f i s h of each species. V. DISCUSSION In t h i s study I have t r i e d to determine some of the f a c t o r s i n f l u e n c i n g the h a b i t a t and food s e g r e g a t i o n of c u t t h r o a t and D o l l y Varden. To do t h i s , I examined i n the laboratory, aspects of p l a n k t i v o r o u s prey s e l e c t i o n and the comparative a b i l i t y of c u t t h r o a t and D o l l y Varden as p l a n k t i v o r e s . R e s u l t s of these experiments and of experiments by o t h e r s on b e n t h i c and s u r f a c e p r e d a t i o n w i l l be compared to f i e l d r e s u l t s of prey and h a b i t a t s e l e c t i o n by n a t u r a l l y sympatric and e x p e r i m e n t a l l y a l l o p a t r i c p o p u l a t i o n s of c u t t h r o a t and D o l l y Varden. P r e d i c t i o n s from c u r r e n t f o r a g i n g and c o m p e t i t i o n theory w i l l be d i s c u s s e d and compared to the r e s u l t s of t h i s study. The t h e o r i e s of o p t i m a l f o r a g i n g have been developed f o r the purpose of i n t e r p r e t i n g observed p a t t e r n s of f o r a g i n g by p r e d a t o r s and of p r e d i c t i n g the impact of p r e d a t o r s on prey p o p u l a t i o n s . McArthur and Pianka (196 6) t h e o r e t i c a l l y i n t e r p r e t e d niche s e g r e g a t i o n i n terms of optimal f o r a g i n g ; r e c e n t l y Werner and H a l l (1976) e x p e r i m e n t a l l y t e s t e d t h i s i n t e r p r e t a t i o n i n s u n f i s h . F i r s t , I w i l l d i s c u s s the p l a n k t i v o r o u s d i e t of c u t t h r o a t and D o l l y Varden i n terms of f o r a g i n g theory and how t h i s d i e t i s a f f e c t e d by v a r i o u s f a c t o r s . Then I w i l l proceed to examine h a b i t a t and prey s e l e c t i o n of the lake f i s h p o p u l a t i o n s i n s i m i l a r terms. One can v i s u a l i z e the members of a p o p u l a t i o n as t r a v e l l i from h a b i t a t patch to h a b i t a t patch. W i t h i n each homogenous h a b i t a t patch, the predator i s able to choose from a v a r i e t y 109 of prey o b j e c t s t h a t t y p i c a l l y vary as to s p e c i e s , s i z e , and abundance. Each prey form can be ranked as to i t s u t i l i t y to the p r e d a t o r . In most s t u d i e s to date, the net energy g a i n (energy g a i n / u n i t h a n d l i n g time) i s assumed to be the a p p r o p r i a t e form of u t i l i t y r a n k i n g , an assumption based on the argument t h a t , i f an animal has a f i x e d energy requirement or a f i x e d time f o r f e e d i n g , i t s optimum s t r a t e g y i s to forage i n order to maximize the net r a t e of energy i n t a k e . Energy must be i n c l u d e d i n f o r a g i n g theory; however, other c o n s i d e r a t i o n s , such as n u t r i t i o n or p r e d a t o r avoidance, may be important i n some systems (Eggers, 1977). The u t i l i t i e s of the prey types are assessed by determining the energy gained per u n i t h a n d l i n g time f o r each prey type. A l l prey types are then ranked i n o r d e r . The o p t i m a l d i e t i s determined by adding to the d i e t the food types of h i g h e s t u t i l i t y i n rank o r d e r . A prey type i s only i n c l u d e d i n the d i e t i f i t s r a t i o o f energy gained to h a n d l i n g time i s g r e a t e r than the r a t i o of t o t a l energy gained to t o t a l f o r a g i n g time f o r a l l prey types of h i g h e r u t i l i t y (Charnoff, 1976). Since f o r a g i n g time i n c l u d e s h a n d l i n g and s e a r c h i n g time, the i n c l u s i o n of a prey type i n the d i e t i s not dependent on i t s own abundance, but r a t h e r on the absolute abundance of a l l h i g h e r ranked prey types. As the abundance of higher u t i l i t y prey types decreases, prey types of lower u t i l i t y are i n c l u d e d i n the d i e t . U s u a l l y the u t i l i t y of prey i n c r e a s e s w i t h s i z e (Werner and H a l l , 1976), s u b j e c t to c e r t a i n c o n s t r a i n t s , such as changes i n h a n d l i n g time with s i z e . Werner (1974) showed t h a t , when prey s i z e i s small r e l a t i v e t o f i s h mouth s i z e , as i s the case with zooplankton and salmonids, h a n d l i n g time i s n e a r l y constant over a broad range of prey s i z e s . However, t h i s r e l a t i o n s h i p c o u l d vary with prey s p e c i e s . Length c h a r a c t e r i s t i c s of prey s p e c i e s , such as Daphnia and D. k e n a i , t h a t have d i f f e r e n t b a s i c shapes may not be an adequate estimate of u t i l i t y . A b e t t e r estimate i s mass, or, i f p o s s i b l e , the a c t u a l energy content. The mass of Daphnia and D. kenai i n Eunice and Loon Lakes and i n the l a b o r a t o r y experiments was estimated from the length-weight r e l a t i o n s h i p o b tained by M. S w i f t (pers. comm.) f o r these s p e c i e s i n Eunice Lake. Average c a l o r i c values per u n i t dry-weight were obtained f o r Daphnia sp. and Diaptomus sp. from Cummins and Wuycheck (1971) and used to c a l c u l a t e the energy content per prey item. From these c a l c u l a t i o n s the mean l e n g t h (1.68 mm) of Daphnia fed on by Eunice c u t t h r o a t i n J u l y 1975 had an energy content o n e - t h i r d as l a r g e as the energy content of the longer (mean l e n g t h = 2.31 mm) D. kenai fed on d u r i n g the same p e r i o d ( t a b l e 18). However, i n the l a b o r a t o r y experiments, Daphnia had c a l o r i c v a l u e s the same as, or s l i g h t l y h igher than, those of the D. kenai of the same l e n g t h (1.68 mm). The dry weight of 4th i n s t a r Chaoborus (from S w i f t , 1974) was much l a r g e r than e i t h e r of the other two s p e c i e s ; consequently, i t s energy content was more than 10 times as l a r g e as t h a t of D. kenai ( t a b l e 18). As a r e s u l t , i n the l a b o r a t o r y experiments the u t i l i t y 111 rankings were: 4th Chaoborus > D. kenai < D. rosea; however, i n the lake D. kenai were ranked h i g h e r than D. r o s e a . F i s h p r e f e r e n c e s i n the l a b o r a t o r y were: 4th Chaoborus > D. rosea > D. kenai; hence, other f a c t o r s than simple energy content of the prey a f f e c t s p r e d a t i o n on D. rosea and D. k e n a i . Energy gained from a prey type depends not only on the energy content of i n d i v i d u a l prey items but on two other major f a c t o r s a f f e c t i n g the p a t t e r n s of prey s e l e c t i o n (Eggers, 1977). These are: (1) d i f f e r e n c e s i n the r a t e of prey encounters due to v a r i a b l e prey v i s i b i l i t y ; (2) d i f f e r e n c e s i n the capture e f f i c i e n c y of a p r e d a t o r on prey. While environmental c o n s i d e r a t i o n s , such as the amount of l i g h t and water t u r b i d i t y , are important, prey v i s i b i l i t y i s p r i m a r i l y determined by the v i s u a l apparatus of the f i s h and by such prey c h a r a c t e r i s t i c s as c o n t r a s t , s i z e , motion, and d i s t a n c e from the p r e d a t o r . C o n t r a s t i s determined both by c o l o u r and by the b r i g h t n e s s of the o b j e c t and i t s background, Eggers (1977) reviews much of the l i t e r a t u r e on v i s i b i l i t y as i t a f f e c t s f i s h p r e d a t i o n and d i s c u s s e s the r e l a t i o n s h i p of the v a r i o u s f a c t o r s . Hester (1968) s t a t e s t h a t b r i g h t n e s s c o n t r a s t p l a y s the primary r o l e i n the v i s i b i l i t y of prey to a f i s h , p o s s i b l y because of the r a p i d a t t e n u a t i o n of the v i s i b l e spectrum i n water. However, Ginetz and L a r k i n (1973) demonstrate t h a t rainbow t r o u t choose d i f f e r e n t c o l o u r e d f i s h eggs not o n l y on the b a s i s of c o n t r a s t to t h e i r background but a l s o on the b a s i s of p r e f e r e n c e s u n r e l a t e d to background c o l o u r . The a b i l i t y of a f i s h t o d i s t i n g u i s h an o b j e c t from i t s background, the c o n t r a s t t h r e s h o l d , i s not constant but v a r i e s with the area of the image on the r e t i n a as w e l l as wi t h the amount of ambient l i g h t (Eggers, 1977). The r e a c t i o n d i s t a n c e of a f i s h to a prey item i s a mea-sure of the a b i l i t y of t h a t f i s h to d e t e c t the prey item (Ware 1971). W i t h i n a broad range of prey s i z e s the r e a c t i o n d i s -tance to a prey item does not vary g r e a t l y with s i z e i f the in h e r e n t c o n t r a s t of the prey i s very low, or i f t u r b i d i t y i s hig h or i f i l l u m i n a t i o n i s low (Eggers 1977) . At higher con-t r a s t l e v e l s and under b e t t e r viewing c o n d i t i o n s , the s u r f a c e area of the prey p l a y s a major r o l e i n i t s d e t e c t i o n by a f i s h . Movement of the prey item a l s o causes an i n c r e a s e i n the r e a c -t i o n d i s t a n c e by i n c r e a s i n g the amount of r e t i n a l area s t i m u l a t e d . The viewing c o n d i t i o n s f o r my l a b o r a t o r y experiments were good T u r b i d i t y was v i r t u a l l y n i l , and l i g h t c o n d i t i o n s were hi g h , as evidenced by my a b i l i t y to view prey items i n the o b s e r v a t i o n tanks e a s i l y . In t h i s study, the th r e e prey types used c o n t r a s t e d w e l l with the background. However,, p e r c e p t i o n by the human observer may be d i f f e r e n t than t h a t of the f i s h The head capsule and two p a i r s of a i r sacs on Chaoborus were e a s i l y v i s i b l e as were the eye spot and s l i g h t brown t i n g e of D. rosea and the red c o l o u r a t i o n of D. k e n a i . Of course, Chaoborus were much l a r g e r than e i t h e r of the other two prey items. The s u r f a c e of one s i d e of EK rosea, c a l c u l a t e d from data from S w i f t and Fedorenko (197 5) , i s about 1 1/2 times as great as t h a t of a D. kenai of the same l e n g t h . The 113 Chaoborus, although not t r a v e l l i n g any g r e a t d i s t a n c e , o f t e n changed d i r e c t i o n s by " t w i t c h i n g " , w h i l e the Daphnia f r e q u e n t l y moved by a short j e r k y motion. The D. kenai were s t a t i o n a r y f o r r e l a t i v e l y long p e r i o d s , o c c a s i o n a l l y moving i n one s w i f t l e a p . Both D o l l y Varden and c u t t h r o a t e x h i b i t e d a p r e f e r e n c e f o r the prey items i n the same order as the v i s i b i l i t y of the prey to myself. I f one assumes t h a t c o n t r a s t l e v e l s were s i m i l a r , then the prey items t h a t had the l a r g e s t s u r f a c e areas and moved the most, Chaoborus and D. rosea, were chosen over D. k e n a i , w h i l e Chaoborus, the l a r g e s t prey, was chosen over D. rosea . Theory suggests t h a t the sudden change i n the water c u r r e n t s c r e a t e d by a f i s h a t t a c k can cause a zooplankter to attempt an escape. Changes i n a c c e l e r a t i o n c r e a t e d by attempted captures w i t h p i p e t t e s has caused Cyclops abyssorum (Schroder, 1967), C. s c u t i f e r , Diaptomus a s h l a n d i i , and Heterocope s e p t e n t r i o n a l i s (Rankin, 1977) to e x h i b i t w e l l -developed escape responses. D. pulex, D. rosea, H. gibberum, and Bosmina do not e x h i b i t the same response, or, at l e a s t , are not as s u c c e s s f u l at escaping when sucked by a p i p e t t e (personal o b s e r v a t i o n ) . S w i f t and Fedorenko (1975) , when s t i m u l a t i n g D. kenai and Diaphanosoma wi t h a d i s s e c t i n g probe, e l i c i t e d escape responses of 3-10 cm i n a s i n g l e leap. S i m i l a r s t i m u l a t i o n o f Bosmina and Polyphemus caused no r e a c t i o n i n the p l a n k t e r s . Escape responses, when present, can cause sharp decreases i n the capture e f f i c i e n c y by f i s h . A t t a c k s on Chaoborus and 114 Daphnia t h a t e l i c i t e d l i t t l e or no observed response had an 80% t o 100% success r a t e . In c o n t r a s t , a t t a c k s on D. k e n a i were s u c c e s s f u l o n l y 50% to 70% o f the time. D. kenai o f t e n d a r t e d out of the path o f an a t t a c k i n g f i s h . Weighting the energy content o f i n d i v i d u a l s -of a prey type by the capture e f f i c i e n c y of the f i s h on t h a t prey type causes changes i n the d i f f e r e n c e s between the u t i l i t y r ankings of prey (t a b l e 18). In the l a b o r a t o r y the value o f a Daphnia to the c u t t h r o a t i s i n c r e a s e d t o almost three times the value of a D. kenai of the same l e n g t h . Daphnia a l s o i n c r e a s e d i n r e l a t i v e v alue to D o l l y Varden, but not to the same extent, because the D o l l y Varden have higher capture e f f i c i e n c i e s on D. kenai than the c u t t h r o a t do. In the f i e l d , a D. kenai of average s i z e s t i l l has a high e r u t i l i t y to the f i s h p r e d a t o r s than does a Daphnia of average s i z e , but the d i f f e r e n c e s between the two prey s p e c i e s i s reduced. In p a r t i c u l a r , the u t i l i t y of a D. ken a i to Eunice c u t t h r o a t becomes almost the same as the u t i l i t y of a Daphnia. As the energy content of 4th i n s t a r Chaoborus i s so l a r g e when compared t o t h a t of Daphnia and D. k e n a i , and, as they are e a s i l y captured, t h i s prey s p e c i e s r e t a i n s i t s h i g h u t i l i t y value when capture e f f i c i e n c i e s are c o n s i d e r e d . In the l a b o r a t o r y , the p r e f e r e n c e s i n prey types e x h i b i t e d by the f i s h are i n accordance w i t h the c a l c u l a t e d u t i l i t y r a n k i n g . In the study l a k e s , the examination o f the p l a n k t i v o r o u s d i e t o f c u t t h r o a t and D o l l y Varden r e v e a l s a g e n e r a l , but not p e r f e c t , agreement wi t h these c a l c u l a t i o n s . The f i r s t samples taken from Eunice and Ka t h e r i n e Lakes Table 18. Utility of Daphnia. D. kenai, and 4th Instar (young) 0. trivittatus to cutthroat and Dolly Varden, FISH PREY DRY ENERGY CAPTURE LENGTH WEIGHT CONTENT EFFICIENCY GROSS ENERGY GAIN ATTACK RATE ENERGY GAIN FEEDING PERIOD FORAGING FEEDING PERIOD ENERGY GAIN ACTIVITY (mm\ (ua*-) e « 1 ' x 1 0 " 3 captures cal.xto" 3 a t t a c k B  v ' . > 6 ' prey Item attacks prey Item feeding period time calories moving calories • feeding feeding foraging period period tine DAPHNIA Mean size ln Eunice Lake and In the laboratory CT DV 1.68 1.68 15 15 75.4 75.* 92.9 70.1 61.6 70 172.5 4.90 10.63 5.7 29.2 0.86 0.36 Mean size ln Loon Lake SV .78 .78 10.0 10.0 92.9 81.7 9.3 8.2 70 172.5 0.65 1.1*2 5.7 29.2 0.11 0.05 D. KENAI Mean size in Eunice Lake DV 2.31 2.31 38 38 218.2 218.2 35.2 65.5 76.8 142.9 51.8 124.5 3.98 17.79 5.7 29.2 0.70 0.61 Mean size in Loon Lake and In the laboratory cr DV 1.68 1.68 12 12 68.9 68.9 35.2 65.5 24.2 45.1 51.8 124.8 1.26 5.63 5.2 29.2 0.24 0.19 4th INSTAR C. TRIVITTATUS size ln the CT 600 2916.6 96.7 2863.9 75 214.8 5.7 37.68 lakes and In the laboratory SV 600 2916.6 88.3 2615.1 100.5 262.8 29.2 9.00 a dry weight of Daphnia and D. kenai from M. Swift (per. coma.); of C. trivittatus from H. Swift (MS, 1974). b calorio values from Cummins and Huycheck (1971)i mean calories per gm dry weight arei Daphnlds, 5028| Diaptomldae, 5741i Chaoborus sp. 4936. a f t e r the f i s h i n t r o d u c t i o n (July and August, 1975) showed l a r g e numbers of the h i g h l y v i s i b l e , l a r g e , and e a s i l y c aptured 3rd and 4th Chaoborus t r i v i t t a t u s i n the stomachs of both f i s h s p e c i e s . The e l i m i n a t i o n from the study l a k e s by l a t e f a l l , 1975, of both C. t r i v i t t a t u s and C. americanus i s probably d i r e c t l y due to f i s h p r e d a t i o n (Northcote e t a l . , i n p r e s s ) . When abundant i n the l a k e s , the l a r g e and e a s i l y captured Daphnia were u s u a l l y the dominant prey types i n the c u t t h r o a t stomachs. The percentage composition of Daphnia i n the f i s h stomachs was always higher than the percentage composition i n the l a k e s . F l u c t u a t i o n s i n the r e l a t i v e abundance of lake p o p u l a t i o n s was a l s o r e f l e c t e d by f l u c t u a t i o n s i n the d i e t composition of Daphnia. D. k e n a i , w i t h i t s h i g h u t i l i t y r a n k i n g, should have been preyed upon d i s p r o p o r t i o n a t e l y to i t s r e l a t i v e abundance i n Loon Lake. T h i s was not always the case. The sympatric Loon Lake c u t t h r o a t o f t e n had c o n s i d e r a b l y h i g h e r p r o p o r t i o n s of D. k e n a i i n t h e i r d i e t than were prese n t i n the l a k e ; but, on 3 of the 9 sampling dates, no- D. kenai were found i n the stomach contents, i n s p i t e of t h e i r presence i n the l a k e . The few samples of sympatric D o l l y Varden taken always had l a r g e p r o p o r t i o n s of D. kenai i n t h e i r d i e t . In Eunice Lake, the u t i l i t i e s of D. kenai and Daphnia are n e a r l y the same. Here, a l l o p a t r i c c u t t h r o a t o n l y had l a r g e numbers o f D. kenai i n t h e i r d i e t d u r i n g the s p r i n g and e a r l y summer. In t h i s p e r i o d , when Daphnia i s r e l a t i v e l y s c a r c e i n the l a k e , D. kenai and other copepods are much more abundant i n the lake than l a t e r i n the summer and f a l l . I t appears t h a t when Daphnia i s r e l a t i v e l y abundant, D. kenai i s not a t t a c k e d . A s i m i l a r p a t t e r n of s p r i n g p r e d a t i o n on D. kenai by c u t t h r o a t i s r e p o r t e d by Anudrusak (MS 1969) i n P l a c i d Lake. Only 2 a l l o p a t r i c K atherine D o l l y Varden ate any D. k e n a i ; however, these were d u r i n g the p e r i o d of h i g h l a k e abundance. Fora g i n g theory p r e d i c t s t h a t D. kenai would be pr e s e n t i n the diet of the f i s h whenever i t i s present i n the l a k e . The f a c t t h a t i t i s not c o u l d be due to a number of f a c t o r s . The c a l c u l a t i o n s of c a l o r i c value of the prey items c o u l d be i n e r r o r . D. k e n a i or D. rosea c o u l d have s i g n i f i c a n t l y d i f f e r e n t values per gram dry weight than r e p o r t e d by Cummins and Wuycheck (1971) f o r other s p e c i e s of these genera. D i f f e r e n c e s i n d i g e s t i o n r a t e s and e f f i c i e n c y on the two s p e c i e s c o u l d a f f e c t the u t i l i t y of the prey. Daphnia was o f t e n found i n more advanced s t a t e s o f d i g e s t i o n than the D. kenai i n the same f i s h stomach, although t h i s impression was not q u a n t i f i e d i n any way. On most sampling dates a f t e r e a r l y summer, D. kenai composed o n l y 1-4% of the a d u l t l i m n e t i c plankton p o p u l a t i o n . Perhaps, because of t h i s r a r i t y , they were not i d e n t i f i e d as food items. In the l a b o r a t o r y , a t t a c k r a t e s d i d i n c r e a s e w i t h experience, although o n l y to a s m a l l e x t e n t . A l t e r -n a t i v e l y , capture e f f i c i e n c i e s may have been lower than estimated (thereby d e c r e a s i n g u t i l i t y ) . In the l a b o r a t o r y , capture e f f i c i e n c i e s d i d i n c r e a s e c o n s i d e r a b l y w i t h experience. 118 S i z e s e l e c t i v e p r e d a t i o n i s e x h i b i t e d by the Eunice c u t t h r o a t on both Daphnia and D. kenai but not by the Loon c u t t h r o a t . S i z e s e l e c t i o n has been demonstrated i n a l a r g e number of p l a n k t i v o r o u s f i s h s t u d i e s (eg. Brooks and Dodson, 1965; G a l b r a i t h , 1967; Green, 1967; Brooks, 1968; Dodson, 1970) and can be e x p l a i n e d i n terms of optimum f o r a g i n g . L a r g e r prey items p r o v i d e more energy to the pr e d a t o r than s m a l l ones i f there i s l i t t l e or no change i n h a n d l i n g time. Eunice Lake Daphnia and D. kenai have both a l a r g e r mean s i z e and a l a r g e r maximum s i z e than do the Loon Lake p l a n k t e r s . In a d d i t i o n , t here i s a h i g h e r abundance of a l l s i z e c l a s s e s of Daphnia and D. kenai i n Eunice than i n Loon. Under these circumstances the optimum f o r a g i n g t a c t i c i n Eunice Lake i s to feed o n l y on the l a r g e r s i z e c l a s s e s and t o i n c l u d e no s i z e c l a s s where the r a t i o o f food value t o h a n d l i n g time exceeds the r a t i o of t o t a l food value to f o r a g i n g time. While there i s not s u f f i c i e n t data to t e s t t h i s theory i n Eunice Lake p r o p e r l y the evidence a v a i l a b l e i s i n agreement with the p r e d i c t i o n s o u t l i n e d . The mean s i z e of Daphnia and D. kenai i n Eunice Lake c u t t h r o a t i s l a r g e r than i n Loon c u t t h r o a t . Moreover, on many dates the minimum s i z e c l a s s of Daphnia found i n Eunice c u t t h r o a t i s l a r g e r than the mean s i z e of Daphnia found i n Loon c u t t h r o a t . While the d i f f e r e n c e between D. kenai s i z e c l a s s e s i n Loon and Eunice c u t t h r o a t stomachs are not as g r e a t , Loon c u t t h r o a t do eat s m a l l e r s i z e c l a s s e s . I f the d e n s i t y of prey i s low enough, t h e r e f o r e i n c r e a s i n g s e a r c h i n g time, the optimal f o r a g i n g t a c t i c i s to capture a l l the prey items encountered. I v l e v (1961) and Werner and H a l l (1974) have demonstrated t h i s p a t t e r n i n l a b o r a t o r y s t u d i e s on f i s h p r e d a t i o n . Once more, although there are not enough data to r i g o r o u s l y t e s t t h i s theory, the a v a i l a b l e evidence supports i t . The zooplankton samples taken d u r i n g the course of t h i s study (unpublished data) and Northcote and C l a r o t t o (19 74) i n d i c a t e t h a t most prey types, e s p e c i a l l y the l a r g e r ones ( i . e . Daphnia and D. k e n a i ) , are much l e s s abundant i n Loon than i n Eunice Lake. In a d d i t i o n , on every sampling date the Loon Lake c u t t h r o a t had fewer zooplankton i n t h e i r d i e t than d i d Eunice c u t t h r o a t of a s i m i l a r l e n g t h . In most cases, i t was not p o s s i b l e to demonstrate t h a t f i s h were caught at depths where the mean Daphnia s i z e was s m a l l e r than the l a k e mean. With one e x c e p t i o n the Loon c u t t h r o a t were caught over a wide range of depths w i t h v a r i a b l e Daphnia s i z e s . Loon c u t t h r o a t not o n l y feed on s m a l l e r Daphnia but they a l s o prey on a number of other s m a l l e r p l a n k t e r s . These i n c l u d e small s p e c i e s with a maximum s i z e l e s s than t h a t of an average Daphnia i n Loon; Bosmina, Polyphemus, and Latona, and a l s o the medium s i z e d s p e c i e s ; C y c l o p o i d sp., Diaphanosoma, and Holopedium. They were r a r e l y found i n the d i e t of any of the other three f i s h p o p u l a t i o n s . Loon Lake D o l l y Varden do not feed on these s p e c i e s , p o s s i b l y because the a v a i l a b i l i t y of b e n t h i c prey to D o l l y Varden p l a c e s them below the l i m i t s of an o p t i m a l d i e t . The success of a predator w i t h i n a h a b i t a t patch depends only p a r t i a l l y on the c h a r a c t e r i s t i c s of the h a b i t a t and the prey forms found t h e r e . A l s o of importance i s the a b i l i t y of the predator to forage w i t h i n t h a t h a b i t a t . A s u c c e s s f u l p r e d ator w i t h i n a h a b i t a t patch can reduce the abundance of h i g h r a n k i n g prey types and thereby reduce the u t i l i t y o f the e n t i r e h a b i t a t to p o t e n t i a l competitors (Schoener, 1974) to such an extent t h a t i t i s no longer p r o f i t a b l e f o r the competitors t o v i s i t t h a t h a b i t a t p a tch. Laboratory experiments and o b s e r v a t i o n s on s o l i t a r y c u t t h r o a t and D o l l y Varden p r o v i d e evidence of more s u c c e s s f u l l i m n e t i c f o r a g i n g by c u t t h r o a t . When o f f e r e d zooplankton as the s o l e food source, c u t t h r o a t spent the m a j o r i t y of the f o r a g i n g time i n the middle water column w h i l e the D o l l y Varden spent a h i g h e r percentage of the f o r a g i n g time i n the lower water column. Using f i s h from a sympatric p o p u l a t i o n , Schutz (MS 1969) had s i m i l a r r e s u l t s when he o f f e r e d s u r f a c e prey t o the c u t t h r o a t and D o l l y Varden. The c u t t h r o a t remained p r i m a r i l y i n the upper water column, o r i e n t e d towards the s u r f a c e while the D o l l y Varden f r e q u e n t l y r e t u r n e d to the bottom. He observed t h a t D o l l y Varden searched f o r be n t h i c prey from a p o s i t i o n much c l o s e r to the bottom than t h a t o f the c u t t h r o a t . When r e s t i n g , the D o l l y Varden were c l o s e l y a s s o c i a t e d w i t h the bottom w h i l e the c u t t h r o a t hovered i n the water column. My o b s e r v a t i o n s were d i f f e r e n t . Although the D o l l y Varden spent the m a j o r i t y of t h e i r time i n the bottom t h i r d of the tank, they f r e q u e n t l y v i s i t e d other p o r t i o n s o f the water column. On the o t h e r hand, c u t t h r o a t spent n e a r l y a l l of t h e i r time when not f o r a g i n g i n the bottom t h i r d of the water column. P o s s i b l e reasons f o r these d i f f e r e n c e s range from innate d i f f e r e n c e s i n the f i s h examined t o b e h a v i o u r a l d i f f e r e n c e s induced by v a r i a t i o n s 121 i n the h a n d l i n g and h o l d i n g procedures. C u t t h r o a t and D o l l y Varden used by myself and by Schutz were sympatric f i s h but from two d i f f e r e n t l a k e s , and t h e r e f o r e , w i t h d i f f e r e n t genotypes. On the other hand, the d i f f e r e n c e s may be a r t i f a c t s of the two o b s e r v a t i o n methods used. Schutz's o b s e r v a t i o n s were q u a l i t a t i v e ones, made i n the h o l d i n g tanks, while mine were q u a n t i t a t i v e , made i n the o b s e r v a t i o n tanks. Although the f i s h were experienced and a h a l f hour was allowed b e f o r e o b s e r v a t i o n , my r e s u l t s may be a f u n c t i o n o f the D o l l y Varden adapting b e t t e r to h a n d l i n g . F o r a g i n g methods on zooplankton d i f f e r e d markedly i n the two f i s h . D o l l y Varden, when s e a r c h i n g f o r zooplankton i n my experiments, were moving f o r v i r t u a l l y the whole f e e d i n g p e r i o d , p a s s i n g through a l l areas o f the o b s e r v a t i o n tank. T h i s high a c t i v i t y l e v e l i s r e l a t e d t o a c o r r e s p o n d i n g l y high a t t a c k r a t e on both prey and non-prey items, and subsequently a high absolute number of cap t u r e s . The c u t t h r o a t f o r a g i n g p a t t e r n i s c h a r a c t e r i z e d by long h o v e r i n g p e r i o d s i n midwater (up to 80% of the f o r a g i n g time) i n t e r -spersed w i t h slow c r u i s i n g . On l o c a t i n g a prey item, the c u t t h r o a t paused ( i f moving), o r i e n t e d themselves to the prey, and a t t a c k e d . Consequently, they a t t a c k e d and captured a s m a l l e r number of prey than d i d the D o l l y Varden. However, the c u t t h r o a t r a r e l y a t t a c k e d a non-prey item. S i m i l a r r e s u l t s f o r both c u t t h r o a t and D o l l y Varden were found by Schutz (MS 1969) i n b e n t h i c f e e d i n g . Schutz a l s o found t h a t c u t t h r o a t , when se a r c h i n g the s u b s t r a t e , swam at speeds near 3.0 cm/second, while the D o l l y Varden swam twice as f a s t at 122 speeds between 5.3 and 7.1 cm/second. D i f f e r e n c e s i n the capture e f f i c i e n c i e s of c u t t h r o a t and D o l l y Varden on zooplankters can cause d r a s t i c d i f f e r e n c e s i n the u t i l i t y rankings of these prey types to the p r e d a t o r . The ranking of Daphnia and D. k e n a i by c a l o r i c value alone i n d i c a t e s the same u t i l i t y of these two p l a n k t e r s to both p r e d a t o r s . I f the capture e f f i c i e n c i e s are i n c l u d e d i n the u t i l i t y r a n k i n g s , then Daphnia and Chaoborus become more p r o f i t a b l e t o c u t t h r o a t than t o D o l l y Varden because of the capture e f f i c i e n c y o f c u t t h r o a t on these s p e c i e s . However, D. k e n a i becomes more v a l u a b l e t o the D o l l y Varden, which i s more e f f i c i e n t at c a p t u r i n g t h a t prey type. Other p r e d a t o r c h a r a c t e r i s t i c s a f f e c t the value of the prey types to the p r e d a t o r s . Within the time p e r i o d of the f e e d i n g experiments, D o l l y Varden always a t t a c k e d and t h e r e f o r e captured more prey items than d i d c u t t h r o a t , i n s p i t e of t h e i r lower capture e f f i c i e n c i e s i n Daphnia and Chaoborus. T h e r e f o r e , w i t h i n the f e e d i n g p e r i o d , the D o l l y Varden had a l a r g e r gross energy g a i n than d i d the c u t t h r o a t . However, i n order t o achieve t h i s , they had much h i g h e r a c t i v i t y r a t e s . I f energy expended i n f o r a g i n g i s p r o p o r t i o n a l to the r a t e of a c t i v i t y , and, i f the r a t e of a c t i v i t y i s p r o p o r t i o n a l to the time spent moving, then the g a i n per u n i t of expenditure can be c a l c u l a t e d ( c a l o r i e s / m i n u t e of swimming). Although t h i s i s adequate f o r the needs of t h i s study, the c a l c u l a t i o n probably underestimates the energy expenditure of the D o l l y Varden which foraged at a f a s t e r speed than d i d the c u t t h r o a t i n Schutz's experiments (MS 1969). These c a l c u l a t i o n s are shown i n t a b l e 18 (page 115). 123 Table 18 demonstrates t h a t , i n the l a b o r a t o r y , c u t t h r o a t t r o u t g a i n at l e a s t 1 1/2 times as much energy from Chaoborus, Daphnia, or D. kenai per minute of f o r a g i n g a c t i v i t y than do the D o l l y Varden. D. kenai p r o v i d e e q u a l l y low r e t u r n s to both p r e d a t o r s . The d i f f e r e n c e s i n the swimming speeds recorded by Schutz on l y accentuate the d i f f e r e n c e s shown here. D i f f i c u l t i e s may be encountered by e x t r a p o l a t i n g these r e s u l t s t o the f i e l d s i t u a t i o n . The e f f e c t of h o l d i n g and o b s e r v i n g the f i s h i n r e l a t i v e l y s m a l l tanks may have- a f f e c t e d the swimming and f o r a g i n g a c t i v i t y of the f i s h and consequently the a t t a c k r a t e s on prey. However, i t i s l e s s l i k e l y t h a t capture e f f i c i e n c i e s would be a f f e c t e d by l a b o r a t o r y c o n d i t i o n s ; these l a b o r a t o r y estimate are probably an accurate r e f l e c t i o n of b a s i c d i f f e r e n c e s i n p l a n k t o n i c f e e d i n g a b i l i t i e s . I t seems u n l i k e l y t h a t the demonstrated d i f f e r e n c e s i n f e e d i n g behaviour are the r e s u l t o f e i t h e r h a n d l i n g o r p r e v i o u s l e a r n i n g experiences i n the l a k e environment. The f i s h were h e l d i n the l a b o r a t o r y f o r a t l e a s t 2 1/2 months b e f o r e exposure to p l a n k t o n i c food. Ware (1971) found t h a t p r e v i o u s l y experienced rainbow t r o u t r e a c t e d n a i v e l y t o s p e c i f i c prey items a f t e r a s i m i l a r time p e r i o d . While i n the l a b o r a t o r y , both f i s h s p e c i e s were t r e a t e d i d e n t i c a l l y ; maintenance f e e d i n g was done p r i m a r i l y w i t h chopped c h i c k e n l i v e r dropped to the bottom of the h o l d i n g tank. There i s no evidence t h a t suggests a p r e v i o u s l y l e a r n e d component to the f i s h behaviour at the s t a r t of the midwater f e e d i n g experiments. L e a r n i n g , i n f a c t , was e v i d e n t d u r i n g the course of the experiments, e s p e c i a l l y i n the f i r s t ones, w i t h Chaoborus. 124 The a t t a c k frequency i n c r e a s e d d r a m a t i c a l l y i n the f i r s t t h ree t r i a l s on Chaoborus and then remained constant. While i n c r e a s e s i n a t t a c k frequency on the other two prey types o c c u r r e d , these were much s m a l l e r . T h i s i n d i c a t e s t h a t the f i s h were l e a r n i n g to feed i n a h a b i t a t and on prey types u n f a m i l i a r t o them. N e v e r t h e l e s s , although the a b i l i t y o f a l l f i s h i n c r e a s e d with experience, the r e l a t i v e a b i l i t i e s of D o l l y Varden and c u t t h r o a t remained v i r t u a l l y unchanged throughout the course of the experiments. Other evidence of in n a t e d i f f e r e n c e s i n f e e d i n g a b i l i t y i n d i f f e r e n t environments i s p r o v i d e d by Schutz and Northcote (1972). Working w i t h a lake p o p u l a t i o n of sympatric c u t t h r o a t and D o l l y Varden, they found the D o l l y Varden t o be more s u c c e s s f u l ( c a p t u r e s / u n i t e f f o r t ) a t f e e d i n g on both exposed chironomid l a r v a e and b u r i e d T u b i f e x worms. The D o l l y Varden were more o r i e n t e d . to the bottom and spent more time s e a r c h i n g i t . In c o n t r a s t , when fe e d i n g on D r o s o p h i l a f l o a t i n g on the s u r f a c e , c u t t h r o a t were the more s u c c e s s f u l . When presented with s u r f a c e prey, the c u t t h r o a t remained a t the s u r f a c e f o r the e n t i r e f e e d i n g p e r i o d w h i l e the D o l l y Varden made frequent t r i p s to the bottom to search. When presented w i t h both b e n t h i c and s u r f a c e prey, the c u t t h r o a t captured a l l s u r f a c e prey b e f o r e s e a r c h i n g the bottom w h i l e D o l l y Varden made only b r i e f s u r f a c e t r i p s and f a i l e d to capture a l l s u r f a c e prey i n a 30 minute t r i a l . I n t e r a c t i o n s between the p r e d a t o r s i n p a i r e d experiments d i d not d i s c l o s e any d i f f e r e n c e s i n behaviour patterns from the s i n g l e p r e dator experiments. 125 In the homogeneous environment c o n s i s t i n g of o n l y one patch type, each prey item can be ranked by i t s u t i l i t y to a p r e d a t o r . The a d d i t i o n or removal of a competitor from such an environment does not a f f e c t the u t i l i t y o f the prey items, o n l y t h e i r abundance (Schoener, 1974) . In a heterogeneous environment composed of d i f f e r e n t h a b i t a t types, an important d i f f e r e n c e appears. The h a b i t a t patches can be u t i l i t y ranked, depending on the t o t a l u t i l i t y and abundance of the prey types w i t h i n each h a b i t a t . However, u n l i k e the prey type u t i l i t y , a competitor can change the r e l a t i v e u t i l i t i e s of whole h a b i t a t patches by a l t e r t i n g the abundance of higher ranked prey w i t h i n the h a b i t a t . I f the t o t a l u t i l i t y of a h a b i t a t patch i s lowered too f a r below other h a b i t a t s , then i t may no longer be i n c l u d e d i n an optimal d i e t (Werner and H a l l , 1976). Hence, we would expect a decrease i n the h a b i t a t s u t i l i z e d by a p r e d a t o r and the s e g r e g a t i o n of the competitors to d i f f e r e n t h a b i t a t s . Both competitors might s t i l l forage i n the same h a b i t a t patches i f the t o t a l u t i l i t y of a g i v e n patch i s h i g h enough to i n c l u d e i t s prey items i n the optimum d i e t of each s p e c i e s . Because the predator must forage more i n t e n s i v e l y i n some h a b i t a t s the number of lower ranked prey items i n the d i e t w i l l i n c r e a s e . I f a competitor i s removed from the environment, the u t i l i t i e s of the h a b i t a t patches w i l l change; one p o s s i b l e r e s u l t i s an i n c r e a s e i n the number of h a b i t a t s u t i l i z e d by the remaining p r e d a t o r . T h i s i n c r e a s e i n h a b i t a t u t i l i z a t i o n would occur only i f the predator had r e t a i n e d enough p l a s t i c i t y of behaviour to expand to p r e v i o u s l y u n u t i l i z e d h a b i t a t s ( i . e . i n t e r a c t i v e l y segregated). I f the s e g r e g a t i o n was maintained on a g e n e t i c b a s i s , the removal of a competitor would not r e s u l t i n niche expansion. However, i f t h i s does not occur, then the remaining p r e d a t o r w i l l drop the lower ranked prey types from i t s d i e t i n each h a b i t a t i t forages i n . Evidence suggests t h a t a l l o p a t r i c c u t t h r o a t i n Eunice Lake c o n t r a c t e d t h e i r h a b i t a t range s l i g h t l y , i n s t e a d o f remaining the same or expanding. Sympatric Loon c u t t h r o a t were u s u a l l y found throughout the whole water column both inshore and o f f s h o r e . O c c a s i o n a l l y , o f f s h o r e , c u t t h r o a t were even caught i n l a r g e numbers i n the lower r a t h e r than upper h a l f of the g i l l n e t s . At a l l times i n the year, Eunice Lake c u t t h r o a t occupied the whole inshore water column, but, o f f s h o r e , u n l i k e Loon c u t t h r o a t , they were found mainly i n the upper water column. D o l l y Varden i n Katherine Lake expanded i n t o the o f f s h o r e l i t t o r a l h a b i t a t but remained c l o s e l y a s s o c i a t e d w i t h the bottom benthos. In the summer and f a l l of 1975 and 1976, Katherine Lake D o l l y Varden were found i n s u b s t a n t i a l numbers both i n s h o r e and o f f s h o r e . In Loon Lake, they were r a r e l y found inshore d u r i n g the summer, and, i n the f a l l , they were only captured inshore i n s u b s t a n t i a l numbers d u r i n g 1970. In the s p r i n g , D o l l y Varden were found inshore i n both l a k e s , but, i n K a t h e r i n e , they were more abundant o f f s h o r e . In a l l cases, most D o l l y Varden were captured i n the lower h a l f of the net. Andrusak and Northcote (1971) and Armitage (MS 197 3) r e p o r t e d on n a t u r a l p o p u l a t i o n s of a l l o p a t r i c c u t t h r o a t and D o l l y Varden i n o t h e r c o a s t a l B.C. l a k e s whose d i s t r i b u t i o n s d i f f e r e d n o t i c e a b l y from those found i n Katherine or Eunice J Lakes. In Dickson Lake, a l l o p a t r i c D o l l y Varden were wi d e l y d i s t r i b u t e d throughout the water column, both i n s h o r e and o f f s h o r e , d u r i n g a l l seasons examined. In shallow (6 m) P l a c i d Lake, a l l o p a t r i c c u t t h r o a t were found i n the upper water column i n the s p r i n g , but, as the year progressed, they became more c l o s e l y a s s o c i a t e d w i t h the bottom. The d i e t of the f i s h p o p u l a t i o n r e f l e c t e d t h e i r v e r t i c a l d i s t r i b u t i o n . Sympatric c u t t h r o a t fed predominantly on midwater prey (mainly zooplankton, but a l s o chironemid pupae, on some d a t e s ) . Both t e r r e s t r i a l i n s e c t s and benthos were c o n s i s t e n t l y present i n the d i e t of the sympatric c u t t h r o a t , but, u s u a l l y not i n l a r g e q u a n t i t i e s . Armitage (MS 1973) had s i m i l a r r e s u l t s , although t e r r e s t r i a l i n s e c t s were more important i n the d i e t of c u t t h r o a t i n h i s study. Midwater forms were a l s o a dominant component of the a l l o p a t r i c Eunice Lake c u t t h r o a t d i e t . But, i n c o n t r a s t t o the sympatric c u t t h r o a t , benthos, e s p e c i a l l y the l a r g e m o t i l e benthos, were u s u a l l y w e l l r e presented i n the d i e t , averaging 21% v o l as compared to 13% v o l i n Loon c u t t h r o a t . Moreover, both f i s h and t e r r e s t r i a l i n s e c t s were more common i n the a l l o p a t r i c d i e t . T h i s higher percentage by volume of benthos and f i s h i n the d i e t of Eunice c u t t h r o a t can be e x p l a i n e d i n terms of d i f f e r e n c e s i n prey s i z e and f i s h s i z e . Eunice c u t t h r o a t 128 c o n t a i n a h i g h e r p r o p o r t i o n b y o c c u r r e n c e o f l a r g e b e n t h o s t h a n do t h e L o o n c u t t h r o a t . B e c a u s e t h e E u n i c e b e n t h o s i s l a r g e r i t c o n t r i b u t e s a h i g h e r v o l u m e t o t h e E u n i c e c u t t h r o a t d i e t t h a n do t h e s m a l l e r b e n t h o s t o t h e L o o n c u t t h r o a t d i e t . P i s c i v o r y i s p r i m a r i l y a f u n c t i o n o f f i s h s i z e . T h u s , b e c a u s e L o o n c u t t h r o a t a r e s m a l l e r , t h e y c a n n o t be e f f e c t i v e p r e d a t o r s on o t h e r v e r t e b r a t e s . I n b o t h l a k e s , o n l y c u t t h r o a t g r e a t e r t h a n 200 mm c o n t a i n e d f i s h i n t h e i r d i e t . I n summary, a l t h o u g h d i f f e r e n c e s do e x i s t i n t h e d i e t o f t h e two c u t t h r o a t p o p u l a t i o n s , t h e s e d i f f e r e n c e s may r e f l e c t d i f f e r e n c e s i n t h e h a b i t a t s a n d f i s h s i z e , a n d n o t c h a n g e s i n f o r a g i n g b e h a v i o u r . I n c o n t r a s t t o t h e c u t t h r o a t , s y m p a t r i c D o l l y V a r d e n consumed l a r g e amounts o f b e n t h i c o r g a n i s m s a n d s m a l l e r q u a n t i t i e s o f m i d w a t e r p r e y . Sample s i z e s f o r my d a t a a r e s m a l l ; h o w e v e r , s e a s o n a l c h a n g e s a r e i n a g r e e m e n t w i t h t h o s e r e p o r t e d b y A r m i t a g e (MS 19 73) . He f o u n d t h a t z o o p l a n k t o n w e r e i m p o r t a n t c o m p o n e n t s o f d i e t i n t h e s p r i n g b u t d e c l i n e d i n i m p o r t a n c e t h r o u g h o u t t h e summer and f a l l . The l a r g e m o t i l e b e n t h i c o r g a n i s m s a r e b y f a r t h e d o m i n a n t p r e y t y p e s i n t h e d i e t s o f a l l o p a t r i c K a t h e r i n e L a k e D o l l y V a r d e n . O n l y when C h a o b o r u s a r e p r e s e n t i n l a r g e numbers i s t h e t o t a l b e n t h o s i n t h e D o l l y V a r d e n d i e t l e s s t h a n 50%. The i m p o r t a n c e o f m i d w a t e r f o r m s i n t h e d i e t d e c l i n e i n l a t e summer and f a l l a s t h e y do f o r L o o n L a k e D o l l y V a r d e n . They a r e v i r t u a l l y a b s e n t i n t h e s p r i n g . The d i e t o f n a t u r a l l y a l l o p a t r i c D i c k s o n L a k e D o l l y V a r d e n , e x a m i n e d b y A n d r u s a k and N o r t h c o t e (1971) and A r m i t a g e (MS 1973) i s much b r o a d e r t h a n i n t h e K a t h e r i n e D o l l y V a r d e n . I n D i c k s o n L a k e , b e n t h o s compose only 6% v o l to 19% v o l of the D o l l y Varden d i e t while zooplankton and s u r f a c e i n s e c t s make up most of the remainder. T h i s evidence does not demonstrate t h a t e i t h e r c u t t h r o a t t r o u t or D o l l y Varden have r e t a i n e d enough p l a s t i c i t y i n t h e i r b e h a v i o u r a l p a t t e r n s t o expand i n t o unoccupied f e e d i n g n i c h e s . T h e i r s p a t i a l and d i e t a r y s e g r e g a t i o n i n Loon Lake may be due to N i l s s o n ' s (1967) s e l e c t i v e s e g r e g a t i o n hypo-t h e s i s . The l a b o r a t o r y s t u d i e s d i s c u s s e d e a r l i e r p r o v i d e evidence f o r in n a t e behaviour p a t t e r n s and f e e d i n g a b i l i t i e s . However, there e x i s t other f a c t o r s r e l a t e d t o the c h a r a c t e r -i s t i c s of the r e c i p i e n t l a k e s t h a t tend t o l i m i t h a b i t a t expansion and r e i n f o r c e p r e v i o u s b e h a v i o u r a l p a t t e r n s . Eunice Lake i s steep s i d e d w i t h a l a r g e l i m n e t i c zone and a smal l l i t t o r a l zone. Only 2 0% of the lake i s above the 5m contour. I t has an abundant p l a n k t o n i c prey p o p u l a t i o n o f Daphnia and D. k e n a i . Both of these prey types reach a l a r g e r s i z e than they do i n Loon Lake. In f a c t , two years a f t e r the f i s h i n t r o d u c t i o n , 30% of the Daphnia were s t i l l l onger than the maximum s i z e (1.4 mm) i n Loon Lake. Upon f i r s t e n t e r i n g Eunice Lake c u t t h r o a t would f i n d t h a t the m a j o r i t y of t h e i r environment was the l i m n e t i c h a b i t a t c o n t a i n i n g abundant high u t i l i t y prey types. Even the d i e t of f i s h caught i n the bottom h a l f o f the inshore net was over one-half zooplankton. In c o n t r a s t t o Loon and Eunice Lakes, Katherine Lake has a very e x t e n s i v e l i t t o r a l area. Nearly 50% of the bottom of Katherine Lake i s above the 5m contour. In the l i m n e t i c zone, 130 Daphnia and D. kenai are as l a r g e as i n Eunice Lake but are not as abundant, e s p e c i a l l y i n l a t e summer and f a l l . Although no data are a v a i l a b l e , the l a r g e l i t t o r a l zone should have l a r g e p o p u l a t i o n s of b e n t h i c organisms of hig h u t i l i t y t o D o l l y Varden. Armitage (MS 1973) examined a n a t u r a l l y a l l o p a t r i c p o p u l a t i o n of D o l l y Varden i n a s i m i l a r l a k e (Foley) w i t h e x t e n s i v e l i t t o r a l development. The d i e t and depth d i s t r i b u t i o n of the two p o p u l a t i o n s of D o l l y Varden are very s i m i l a r . F o l e y Lake f i s h d i d u t i l i z e s u r f a c e i n s e c t s t o a g r e a t e r extent than those of Ka t h e r i n e , but not as much as the a l l o p a t r i c D o l l y Varden i n the deeper Dickson Lake. I n i t i a l l y , when the f i s h were f i r s t i n t r o d u c e d i n t o the experimental l a k e s they found an overabundance of the prey types t h a t composed t h e i r primary d i e t under sympatric c o n d i t i o n s i n Loon Lake. However, because they are s e l e c t i v e p r e d a t o r s , they should reduce the abundance of the l a r g e r s i z e c l a s s e s , and, i f the p r e d a t i o n pressure i s high enough, they should e v e n t u a l l y e l i m i n a t e the l a r g e s t s i z e c l a s s e s and s p e c i e s i n t h e i r p r e f e r r e d h a b i t a t s . The r e d u c t i o n i n prey s i z e and abundance a f t e r the i n t r o d u c t i o n o f a f i s h p r edator i s w e l l documented i n lake communities (Brooks, 1968). Data presented here and i n Northcote e t a l . ( i n press) shows t h i s i s a l r e a d y happening i n Eunice and Kath e r i n e Lakes. As the pr e d a t o r s reduce the u t i l i t y of t h e i r p r e f e r r e d h a b i t a t , they w i l l add prey items of lower u t i l i t y t o t h e i r d i e t . I f the h a b i t a t and food s e l e c t i o n e x h i b i t e d by the f i s h i s of a b e h a v i o u r a l nature, then the new prey items w i l l be from other h a b i t a t s as w e l l as t h e i r own; but, i f the h a b i t a t and food c h o i c e s are due to a g e n e t i c b a s i s , then there w i l l be no i n c r e a s e i n the u t i l i z a t i o n of other h a b i t a t s . By the end of the sampling program, the u t i l i t i e s of the p r e f e r r e d prey items i n Eunice and Katherine Lakes were s t i l l h i g h i n comparison to Loon. Daphnia i n the Eunice c u t t h r o a t d i e t were l a r g e r than those i n the Loon d i e t ; s i m i l a r l y , K a therine D o l l y Varden had c o n s i d e r a b l y more o f the l a r g e benthos i n t h e i r d i e t than d i d Loon D o l l y Varden. Thus, i t appears t h a t , i f the impact of the f i s h on the lake prey p o p u l a t i o n i s h i g h enough to s e r i o u s l y a l t e r the t o t a l u t i l i t i e s of a l l the h a b i t a t s , then not enough time had elapsed by the end of the sampling p e r i o d to observe t h i s phenomenon. Consequently, f u r t h e r o b s e r v a t i o n s of changes i n the abundance and s i z e composition of the prey p o p u l a t i o n s and of the s p a t i a l and food s e l e c t i o n of the p r e d a t o r p o p u l a t i o n s i s needed to determine the f i n a l r e s u l t s of the experiment. A p o s s i b l e c o m p l i c a t i o n i s the l a c k of a b r e e d i n g p o p u l a t i o n of D o l l y Varden which were few i n number at the s t a r t and w i l l be reduced by both n a t u r a l and sampling mortality*. On the o t h e r hand, a r e p r o d u c i n g p o p u l a t i o n , such as the Eunice c u t t h r o a t p o p u l a t i o n , i n t r o d u c e s the p o s s i b i l i t y of g e n e t i c change i n f u t u r e g e n e r a t i o n s . * Since the time of writing M. Henderson has seen 5 Dolly Varden fry in qfetream entering Katherine Lake near the g i l l net sites. 2 f i s h that were captured measured 31.5 and 26.1 mm. VI. LITERATURE CITED Andrusak, H. 1968. I n t e r a c t i v e s e g r e g a t i o n between a d u l t D o l l y Varden ( S a l v e l i n u s malma) and c u t t h r o a t t r o u t (Salmo c l a r k i c l a r k i ) i n small c o a s t a l B r i t i s h Columbia l a k e s . M. Sc. t h e s i s . Deptartment Zoology. Univ. B r i t i s h Columbia. 76p. Andrusak, H., and T. G. Northcote. 1971. Segregation between a d u l t c u t t h r o a t t r o u t (Salmo c l a r k i ) and D o l l y Varden ( S a l v e l i n u s malma) i n sm a l l c o a s t a l B r i t i s h Columbia l a k e s . J . F i s h . Res. Bd. Canada 28: 1259-1268. Armitage, G. N. 1973. Character displacement and v a r i a b i l i t y i n l a c u s t r i n e sympatric and a l l o p a t r i c D o l l y Varden ( S a l v e l i n u s malma) p o p u l a t i o n s . M. Sc. t h e s i s . Depart-ment Zoology. Univ. B r i t i s h Columbia. 119 p. Armstrong, J . E. 1957. S u r f i c i a l geology o f New Westminster map area, B r i t i s h Columbia. Geol. Surv. Canada. Paper 57-5:1-25. B a r r i n g t o n , C. J . W. 1957. The al i m e n t a r y c a n a l and d i g e s t i o n , p. 109-161. In M. E. Brown (ed.) The ph y s i o l o g y of f i s h e s . V o l . I. Academic Press, Inc., New York. Beamish, R. J . 1973. 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Charnov, E. L. 1976. Optimal foraging: attack strategy of a mantid. Am. Nat. 110: 141-151. Cummins, K. W., and J . C. Wuycheck. 1971. C a l o r i c equivalents for investigations i n ecological energetics. Mitt. Internat. Verein. Limnol. 18: 1-158. Dodson, S. I. 1970. Complementary feeding niches sustained by s i z e - s e l e c t i v e predation. Limnol. and Oceanogr. 15: 131-137. Efford, I. E. 1967. Temporal and s p a t i a l differences i n phytoplankton productivity i n Marion Lake, B r i t i s h Columbia. J. Fish. Res. Bd. Canada. 24: 2283-2307. Eggers, D. M. 1977. The nature of prey selection by planktivorous f i s h . Ecology 58: 46-59. Fedorenko, A. Y., and M. C. Swift. 1972. Comparative biology of Chaoborus t r i v i t t a t u s i n Eunice Lake, B r i t i s h Columbia. Limnol. and Oceanogr. 17: 721-730. Galbraith, M. G. J r . 1967. Size selective predation on Daphnia by rainbow trout and yellow perch. Trans. Am. Fish. Soc. 96: 1-10. Ginetz, R. M. , and P. A. Larkin. 1973. Choice of colors of food items by rainbow trout (Salmo g a i r n e r i ) . J. Fish. Res. Bd. Canada 30: 229-234. Green, J. 1967. The d i s t r i b u t i o n and v a r i a t i o n of Daphnia  lumholtzii (Crustacea: Cladocera) i n r e l a t i o n to f i s h predation i n Lake Albert, East A f r i c a . J. Zool. 151: 181-197. Haney, J. F., and D. J . H a l l . 19 73. Sugarcoated Daphnia: a preservation technique for cladocera. Limnol. and Oceanogr. 18: 331-332. Hardin, G. 1960. The competitive exclusion p r i n c i p l e . Science: 131: 1292-1297. Hester, F. J. 1968. Visual contrast thresholds of the gold f i s h (Carassius auratus). Vision Research 8: 1315-1336 Hutchinson, G. E. 1957. Concluding remarks. Cold Spring Harbor Sym. Quant. B i o l . 22: 415-427. Hyatt, K. D. In press. Factors a f f e c t i n g the selec t i v e a c q u i s i t i o n of natural foods by fishes. In w. S. Hoar and D. J. Randall (eds.) Fish physiology. Academic Press, New York. Ivlev, V. S. 1961. Experimental ecology of the feeding of fishes. Yale University Press, New Haven. MacArthur, R. H., and E. R. Pianka. 1966. On optimal use of a patchy environment. Am. Nat. 100: 603-610. Nilsson, N. A. 1967. Interactive segregation between f i s h species, p. 295-313. In S. D. Gerking (ed.) The b i o l o g i c a l basis of freshwater f i s h production. Black-well S c i e n t i f i c Publ., Oxford. Northcote, T. G., and R. Clarotto. 1974. Limnetic macro-zooplanktoni and f i s h predation i n some coastal B r i t i s h Columbia lakes. Verh. Internat. Verein. Limnol. 19: 2378-2393. Northcote, T. G., and P. A. Larkin. 1956. Indices of productivity i n B r i t i s h Columbia lakes. J. Fi s h . Res. Bd. Canada 13: 515-540. Northcote, T. G., C. J. Walters, and J. M. B. Hume. In press. I n i t i a l impacts of experimental f i s h i n t r o -ductions on the macrozooplankton of small oligotrophic lakes. Verh. Internat. Verein. Limnol. Rankin, D. P. 19 77. Increased predation by juvenile sockeye salmon (Qhcorhyncus 'nerka W.albaum) r e l a t i v e to changes i n macrozooplankton abundance i n Babine Lake, B r i t i s h Columbia. M. Sc. thesis. Department Zoology. Univ. B r i t i s h Columbia. 101 p. Ricker, W. E. 1975. Computation and interpretation of b i o l o g i c a l s t a t i s t i c s of f i s h populations. B u l l . Fish. Bd. Canada 191: 1-382. Roddick, J. A., and J. E. Armstrong. 1956. P i t t Lake (Vancouver, east half) map. New Westminster D i s t r i c t . B r i t i s h Columbia. Geol. Surv. Canada. Map 8-1956. Schroder, R. 1967. Verhalten von Cyclops abyssorum i n der Stromung. Arch. Hyrobiol. Suppl. 33: 84-91. Schoener, T. W. 1974. Resource p a r t i t i o n i n g i n e c o l o g i c a l communities. S c i e n c e . 185: 27-39. Schutz, D. C. 196 9. An experimental study of f e e d i n g behaviour and i n t e r a c t i o n s of c o a s t a l c u t t h r o a t t r o u t (Salmo c l a r k i c l a r k i ) and D o l l y Varden ( S a l v e l i n u s  malma). M. Sc. t h e s i s . Department Zoology. Univ. B r i t i s h Columbia. 81 p. Schutz, D. C. and T. G. Northcote. 1972. An experimental study of f e e d i n g behaviour and i n t e r a c t i o n of c o a s t a l c u t t h r o a t t r o u t (Salmo c l a r k i c l a r k i ) and D o l l y Varden ( S a l v e l i n u s malma). J . F i s h . Res. Bd. Canada 29: 555-565. S w i f t , M. C. 1974. E n e r g e t i c s of v e r t i c a l m i g r a t i o n i n Chaoborus t r i v i t t a t u s l a r v a e . PhD. t h e s i s . Department Zoology. Univ. B r i t i s h Columbia. 142 p. S w i f t , M. C , and A. Y. Fedorenko. 1975. Some aspects of prey capture by Chaborus l a r v a e . Limnol. and Oceanogr. 20: 418-425. Ware, D. M. 19 71. P r e d a t i o n by rainbow t r o u t (Salmo g a i r d n e r i ) : the e f f e c t of experience. J . F i s h Res. Bd. Canada 28: 1847-1852. Werner, E. E. 1974. The f i s h s i z e , prey s i z e , h a n d l i n g time r e l a t i o n i n s e v e r a l s u n f i s h e s and some i m p l i c a t i o n s . J . F i s h . Res. Bd. Canda 31: 1531-1536. Werner, E. E., and D. J . H a l l . 1976. Niche s h i f t s i n s u n f i s h e s : experimental evidence and s i g n i f i c a n c e . Science 191: 404-406. 137 W i n d e l l , J . T. 1971. Food a n a l y s i s and r a t e of d i g e s t i o n , p. 215-226. In W. E. R i c k e r (ed.) Methods f o r a s s e s s -ment of f i s h p r o d u c t i o n i n f r e s h waters. I. B. P. Hand-book No. 3. B l a c k w e l l S c i e n t i f i c P u b l . , Oxford. Zaret, T. M., and A. S. Rand. 1971. Competition i n t r o p i c a l stream f i s h e s : support f o r the co m p e t i t i v e e x c l u s i o n p r i n c i p l e . Ecology 52: 336-342. 1 38 Appendix Table I. Computations for Schnabel population estimate of adult and subadult Dolly Varden in Loon Lake. Data from trapnet taggings and recaptures. DATE NUMBER CAUGHT G t RECAPTURES R t NO. DIED IN NETS D t MARKED RELEASES V D t MARKED FISH AT LARGE M t G t M t May 3 281 0 0 281 0 ,0 8 69 2 5 64 281 19.389 9 21 2 2 19 345 7,245 10 19 2 0 19 364 6,916 15 29 1 0 29 383 11,107 16 21 4 0 21 412 8,652 17 23 3 0 23 433 9,959 21 18 3 6 12 456 8,208 22 21 3 1 20 468 9,828 27 53 15 42 11 488 25,864 30 35 8 3 32 499 17,465 June 4 46 8 1 45 531 24,426 7 33 2 - - 576 19,008 TOTAL 53 168,067 Using Chapman's modification of Schnabel's short formula (Ricker, 1975) the population estimate of Dolly Varden in Loon Lake i s : l G t M t N -- 3112.4 I f R i s distributed as a Poisson variable the 95$ confidence limits (calculated from Appendix II in Ricker, 1975) are: 2356.5 to 3952.6 Appendix Table II, Computations for Schnabel population estimate of adult and subadult cutthroat in Loon Lake, Data from trapnet taggings and recaptures. DATE NUMBER RECAPTURES NO. DIED MARKED MARKED FISH CAUGHT IN NETS RELEASES AT LARGE C t R t D t V D t M t G t M t May 3 81 0 0 81 0 0 8 57 1 2 55 81 4,617 9 17 0 0 17 136 2,312 10 23 0 1 22 153 3.519 15 14 0 0 14 175 2,450 16 20 0 0 20 189 3,780 17 12 0 0 12 209 2,508 21 13 1 4 9 - 221 2,873 22 19 1 0 19 230 4,370 27 45 3 0 45 249 11,205 30 36 2 0 36 294 10,584 June 4 39 5 0 39 330 12,870 7 35 2 1 34 369 12,915 12 23 0 0 23 403 9,269 13 31 1 1 30 426 13,206 18 30 2 0 30 456 13,680 19 14 0 3 11 486 6,804 20 23 0 0 23 497 11,431 25 47 3 - - 520 24,440 TOTAL 21 152,833 Using Chapman's modification of Schnabel's short formula (Ricker, 1975) the population estimate of cutthroat in Loon Lake i s : N - 7277.8 95$ confidence limits are: 4631.3 to 10916.6 139a Appendix Table I I I . Mean number, mean volume, and frequency o f occurrence o f prey items i n the d i e t of Loon c u t t h r o a t . 1975 SPECIES DATE N 7 JULY IS 7 AUG. 21 15 SEPT. 25 SMALL BXHTKOS NU" VUL occ 5.69 .oca .692 a. 38 .008 .667 5.20 .002 .080 LARGS BENTHOS NUM VOL OCC 1.08 .023 .185 5.la .017 .238 .16 .006 .120 DIPTERAN PUPAE NUM VUL OCC 7.61 .031 .538 25.39 .027 .809 11.28 .015 .760 VERTEBRATES NUM VOL OCC 0 0 ,0H .096 .080 SURFACE PREY NUM VOL OCC 50.08 .069 .769 .38 .000 .oas 5.32 .073 .080 DIAPTOMUS KENAI NUM VOL OCC 2.00 .000 • 15a 322.a7 .017 .190 96.00 .012 .200 DAPHNIA ROSEA NUM VOL OCC 235.15 .017 .615 3«0.38 .023 .667 511.32 .077 .760 HOLOFEDIUH GIBB2RUM NUM VOL OCC 173.85 .006 .231 65.71 .007 ,23t. 108.00 .010 .200 LEPTODORA KINDITII NUM VOL OCC 13.5a .000 .15a 9.86 .003 .238 15.80 .00* .200 POLYPHEMUS PEDICUIDS NUM VOL OCC .38 .ooo .077 7.1« .000 .190 25.00 .002 .080 BOSMINA LONGIROSTRIS NUM VOL OCC .23 .000 .077 0 .10 .000 .oao DIAPHASOSOMA BRACHYURUM NUM VOL OCC 0 0 18.00 .002 .oao LATONA SBTIFERA NUM VOL OCC 0 0 0 UNIDENTIFIED CLADOCESANS NUM VOL OCC 0 15.?« .005 .090 0 CYCLCPOIDS NUM VOL OCC 0 0 0 Hkm KITES 4 (KYffiUCARINA) NUM VOL OCC .006 .062 .006 .113 .88 .000 .120 27 OCT. 15 .13 .000 .133 .13 .000 .333 3.87 .800 .OOU 002.66 .080 .533 1916.67 .31 0 1.000 4.00 .000 .133 1976 5 MARCH 25 MAY ?! JIIFgF 19 JULY 20 AUG. 21 17 2o 19 18 10.10 8.07 .10 12.06 3.?? .026 .oui .ooo .oao ..1"} ,S2« .176 . 100 .526 .111 .19 .35 .8S l.?6 .<>8 .002 .003 .015 .Onn .IICH .103 .059 .200 .210 .167 7.88 a.60 3.?7 2.5<> .050 .01 0 .(.08 .'in! 0 .588 .650 .a21 .6*7 6.29 2.15 .53 I.00 .027 .013 .004 0 .588 .350 .?o3 .3*9 557.33 260.60 57.89 .2? .250 .150 .»32 0 .619 0 .550 .053 ,'i55 151.05 1079.01 399.15 572.37 569.au .067 .18? .115 .198 .203 .762 .882 .750 .73? .HSi 2.9U .50 3a.21 31.11 .000 .0 00 ."08 . 1 a 0 ' 0.059 .050 .263 .333 1.05 16,3?., 51.7? .ooo ..io3 . o i l 0 0 .100 .368 .5^6 30.00 8.61 .008 ,<>»0 0 0 0 ,a21 ,?78 11. SH .003 0 0 0 . 1 05 0 57.53 .029 .210 6.67 ,ooa .056 30.00 .012 .050 ,75 .09(1 .150 2.68 , 0 ll 0 .105 . in n .056 .0 5 6 Appendix Table IV. Mean number, mean volume, and frequency of occurrence of prey items in the diet of Loon Dolly Varden. 1975 1976 „ « 7 JULY T UX. I* .j-t. 21 OCT. = JJ-" « »« " "j" LARGE BENTHOS N U M 2 . 3 5 5.00 1 2 a«2S0 toou > u i DIPTERAN N U M 2 . 6 7 8S"?!U 2 8 , ? , e i *.0** SURFACE PREY NUM .17 „ DIAPTOMUS NUM 166.R3 I*-*7. "inOa In75 • KENAI VOL . 0 5 0 . 0 0 0 q fl > l u J . „ „ 0 7#i*' fc7 1 5 . U O 6 U 3 . 7 5 2 0 0 . O f ) DAPHNIA NUM « 0 . 5 0 6 13J . 0 0 7 . 1 00 . 0 5 0 T^OSEA. VOL . 0 2 5 fl 0 'Ml U»* . 5 0 0 1 . 0 0 0 CVCXOPOIDS VOL ' " l o l l ' „ 0 0 0 OCC 0 .313 0 0 WATER KITES NUM . 1 7 2 1 . 6 7 S K ) «gL . :?g°7 :?2? o o o o o 1 41 a Appendix Table V. Mean number, mean volume, and frequency of occurrence of prey items in the diet of Eunice cutthroat. 1975 S P E C I E S DATE N 3 JULY 22 6 AUG. 21 15 S E P T . IS 30 OCT. 22 SHALL BENTHOS NUM VOL OCC 9.6S .007 .151 3.21 .003 .381 0 36 .000 .227 LARGS BENTHOS NUM VOL OCC 7.37 .166 .727 1.72 .196 .176 11.31 .331 .200 2.09 .098 .151 DIPTERAN PUPAE NUM VOL OCC 1.63 .015 .151 12.31 .022 . .521-.67 .000 .067 .!«' .000 . 1 36 VERTEBRATES NUM VOL OCC .05 .055 .015 .05 .048 .018 .13 .383 .133 0 SURFACE PREY NUM VOL OCC 23.11 .190 .682 .072 .333 18.53 .210 .733 .36 .005 .227 CORIXIDS & NOTONECTIDS NUM VOL OCC 2.36 .005 .109 .05 .000 .018 .07 .000 .067 0 CHAOBORUS SP. NUM 'VOL OCC 112.00 .52a .772 58.80 .193 .77? .07 .000 .067 .59 .000 .227 DIAPTOMUS KENAI NUM VOL OCC 95.09 .022 .273 2.57 .002 .095 0 .0 DAPHNIA ROSEA NUM VOL OCC 518.81 .066 .515 787.90 .086 .667 1228.00^  '.SH 1978.11 .971 .909 HOLOPEDIUM ' GIBBERUM NUM VOL OCC .05 .000 .015 0 0 0 POLYPHEMUS FEDICULUS NUM VOL OCC 0 0 0 31.09 .016 .015 BOSMINA LONGIROSTRIS NUM VOL OCC 0 2.38 .000 .oaa 0 0 UNIDENTIFIED CLADOCSRANS MIM VOL OCC 0 1.13 .ooo .095 0 0 CYCLOPOIDS NUM VOL OCC 0 .05 .000 .018 0 0 WATER KITES (HYDRACARINA) NUM VOL OCC .61 . 0 0 0 .227 J3.10 .021 .190 I' 0 ' 0 1976 27tMAY 22 JUNE 20 JULY 25 AUG. « OCT, •» u f c .67 . 0 9 3*Stt 0 0 ? .11011 .035 .538 .538 .385 4.00 .031 .151 ".222 .091 i . 3 i LOO 12. n.§« «;•;», •na - .yi -Ml : n ? 6.62 72.25 b.uo .18 21.79 .008 .321 .021 .000 .nui .162 .750 .107 .162 .517 .38 .l'3 .385 0 0 0 . 0 3 1 .15 13.33 1.15 19.09 11 . C 6 ,001 .167 .065 .128 .ni", .151 .833 .370 .515 . 7 9 3 . I 0 . I.: 0 0 0 0 0 0 . 1 6 9 2.38 .15 1.82 .03 .008 .000 ,000 .i>on ,400 0 .074 .091 .031 1816.15 313.33 691.14 .91 12.21 •»2, ,MTa6 .411 .000 .003 :2M .<"*> - 1 0 1 15.. 0 866.67 917.70 654.55 535.69^  •°5i ' i j ! : 7 i i : i 5 i •*•>" 0 0 u 0 o 0 0 o 0 0 o o 0 0 o ,000 ,091 0 .91 ,000 0 0 .091 Appendix Table VI. Mean number, mean volume, and frequency of occurrence of prey items in the diet of Katherine Dolly Varden. 1975 SPECIES DATE N 9 JULY 21 7 AUG. 11 15 SEPT. 13 SMALL BENTHOS NUM VOL OCC 1 .76 .034 .333 .36 .005 .182 10.SO .102 .062 LARGS BENTHOS NUM VOL OCC 17.29 .219 .667 11.10 .286 .050 .80 .161 .385 DIPTERAN PUPAE NUM VOL OCC 0.08 .012 .190 .18 .000 .091 .08 .000 .077 VERTEBRATES NUM VOL OCC 0 0 0 SURFACE PRE? NUM VOL OCC .05 .000 .006 .005 .182 0 CORIXIDS 4 NOTONECTIDS NUM VOL OCC .38 .002 .008 0 .08 .031 .077 CHAOBORUS SP NUM VOL OCC 1 00.53 .099 .710 SO.36 .033 .182 36.15 .036 .692 DIAPTOMUS KENAI NUM VOL OCC .10 .000 .008 0 0 DAPHNIA ROSEA NUM VOL OCC .10 .000 .OOB 365.55 .073 .505 668.06 .100 .308 POLYPHEMUS FEDICULUS NUM VOL OCC 0 15.73 .009 .091 0 UNIDENTIFIED CLADOCERANS NUM VOL UCC 0 0 0 FISH ECGS NUM VOL OCC 0 0 0 ' 1976 OCT. 27 MAY 22 JUNE 20 JULY 25 AUG. 8 OCT. 20 15 9 10 12 16 20.75 .73 2.56 1.71 .67 2.75 .060 .000 . 1 1 1 .000 .000 .fl<"6 .650 .267 .333 .210 ,?50 .125 2.60 .67 29.33 32.93 11.6/ I.OA .215 .007 1 .205 .557 .075 .Iff! .350 .600 .77B .571 .750 .038 .13 7.A9 5.50 5.00 .000 .006 .010 .037 0 .067 .222 .357 0 .250 .07 .800 0 .067 0 0 0 0 .11 .07 .25 .06 .006 .002 .Onj .0 00 0 0 .111 .071 .167 .06? .75 .013 0 0 0 0 0 ' .1»8 9.35 .07 .2? B.00 .19 .01 0 ,000 .000 - .008 .f^ c .300 .067 .111 0 .017 .188 20.oo .011 0 0 .??2 0 0 (i 36.05 269.03 16,75 15.50 15 .350 129 .000 .GOO 357 .333 .038 35.71 .010 0 0 .210 .80 .135 .100 0 

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