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Competitive interactions between juvenile sockeye salmon (Oncorhynchus nerka) and limnetic zone sticklebacks… O'Neill, Sandra M. 1986

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COMPETITIVE INTERACTIONS BETWEEN JUVENILE SOCKEYE SALMON (ONCORHYNCHUS NERKA) AND LIMNETIC ZONE STICKLEBACKS (GASTEROSTEUS ACULEATUS) by Sandra M. O'Neill B.Sc. (Hon.), Memorial U n i v e r s i t y o f Newfoundland, 1981 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September 1986 ©Sandra M. O'Neill, 1986 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of The University of B r i t i s h Columbia 1956 M a i n M a l l Vancouver, Canada V6T 1Y3 DE-6 C3 /81) i i A b s t r a c t E v i d e n c e s u p p o r t i n g t h e h y p o t h e s i s t h a t c o m p e t i t i v e i n t e r a c t i o n s o c c u r between j u v e n i l e s o c keye sa lmon and l i m n e t i c s t i c k l e b a c k s i n an o l i g o t r o p h i c l a k e was o b t a i n e d by m a n i p u l a t i n g f i s h abundance , s i z e and s p e c i e s c o m b i n a t i o n s i n l i m n e t i c zone e n c l o s u r e s i n Kennedy L a k e , B r i t i s h C o l u m b i a . Sockeye and s t i c k l e b a c k s were s t o c k e d a t d e n s i t i e s w i t h i n t h e range known t o commonly o c c u r i n B r i t i s h C o l u m b i a c o a s t a l l a k e s . In a l l f i s h - t r e a t m e n t e n c l o s u r e s , f i s h g r a z i n g p r e s s u r e q u i c k l y r e du ced t h e abundance o f l a r g e z o o p l a n k t e r s and t h en m a i n t a i n e d z o o p l a n k t o n c ommun i t i e s c o n t a i n i n g p r i m a r i l y n a u p l i i and r o t i f e r s , p r e y se l dom consumed by f i s h . Mean s i z e and abundance o f z o o p l a n k t o n w i t h i n a f i s h l e s s c o n t r o l e n c l o s u r e rema ined a t s i g n i f i c a n t l y h i g h e r l e v e l s t han t h o s e i n Kennedy Lake o r i n t e s t e n c l o s u r e s . D i e t s o f f i s h samp led f r om e n c l o s u r e s a t t h e end o f f e e d i n g t r i a l s d e m o n s t r a t e d t h a t s o c keye f e e d i n g by t h e m s e l v e s ( a l l o p a t r i c s o c k e y e ) consumed l a r g e r f o o d i t e m s t han s o c keye f e e d i n g w i t h s t i c k l e b a c k s ( s y m p a t r i c s o c k e y e ) , whereas s y m p a t r i c s t i c k l e b a c k s consumed l a r g e r f o o d i t e m s t han a l l o p a t r i c s t i c k l e b a c k s . I c o n c l u d e t h a t i n many o l i g o t r o p h i c c o a s t a l l a k e s , where f o od s u p p l i e s a r e l i m i t e d , s o c keye and s t i c k l e b a c k s may compete by e x p l o i t i n g t h e z o o p l a n k t o n c ommun i t i e s t o t h e i r mu tua l d i s a d v a n t a g e . F u r t h e r , c o m p e t i t i v e i n t e r a c t i o n s between s o c keye and s t i c k l e b a c k s r e s u l t i n d i e t a r y n i c h e s h i f t s by s o c keye i n a l l o p a t r y v s . s y m p a t r y . N i c h e s h i f t s by s t i c k l e b a c k s may i n v o l v e bo t h i n t e r - and i n t r a s p e c i f i c c o m p e t i t i v e i n t e r a c t i o n s . i i i TABLE OF CONTENTS Page l I ABSTRACT v L IST OF TABLES v i 11 L IST OF FIGURES ACKNOWLEDGMENTS 1.0 GENERAL INTRODUCTION 2 .0 MATERIALS AND METHODS 2 .1 S t u d y S i t e 2 .2 E n c l o s u r e E x p e r i m e n t s 2 . 3 E x p e r i m e n t a l D e s i g n and P r o c e d u r e s 3 .0 THE INFLUENCE OF SOCKEYE AND STICKLEBACKS ON ZOOPLANKTON i 5 COMMUNITY STRUCTURE: TESTING FOR EXPLOITATIVE COMPETITION 3 .1 I n t r o d u c t i o n 15 3 .2 M a t e r i a l s and Methods 15 3 . 2 . 1 Da t a a n a l y s i s 15 16 49 3 .3 R e s u l t s 3 .4 D i s c u s s i o n 3 . 4 . 1 The e f f e c t o f s o c keye and s t i c k l e b a c k s on t he s i z e s and 49 s p e c i e s c o m p o s i t i o n o f z o o p l a n k t o n c ommun i t i e s 3 . 4 . 2 C o m p e t i t i o n i n o l i g o t r o p h r i c l a k e s 51 3 . 4 . 3 S t i c k l e b a c k s and so ckeye p r o d u c t i v i t y 54 4 . 0 NICHE SHIFTS BY SOCKEYE AND STICKLEBACKS 5 6 4 . 1 I n t r o d u c t i o n 5g 4 . 2 M a t e r i a l s and Methods 57 58 4 . 2 . 1 Da ta a n a l y s i s i v 4 . 3 R e s u l t s D U 4 . 3 . 1 C o n d i t i o n o f f i s h g u t s 6 0 4 . 3 . 2 D i e t n i c h e s h i f t s : f o od consumed by a l l o p a t r i c v s . 6o s y m p a t r i c s o c k eye o r s t i c k l e b a c k s 4 . 3 . 3 D i e t a r y o v e r l a p between s o ckeye and s t i c k l e b a c k s 74 4 . 3 . 4 P r e y a v a i l a b i l i t y 76 4 . 3 . 5 B iomass o f p r e y p e r f i s h 80 4 . 4 D i s c u s s i o n 82 5.0 SUMMARY 95 6 .0 BIBLIOGRAPHY 96 V LIST OF TABLES Page Table 1. S i ze (fork length of sockeye and standard length of 9 s t i ck lebacks) and numbers of f i s h used in enclosure experiments. Table 2. Comparisons of species d i s t r i b u t i o n , to ta l 12 zooplankton abundance and zooplankton mean s i ze in two r e p l i c a t e v e r t i c a l hau ls . Table 3. Re la t ionsh ips between head length and head width to 13 to ta l body length of chironomid l a rvae . Table 4. Results of nonparametric ANOVA on zooplankton s i zes 19 from experiments with (a) sockeye year l i ngs and adul t s t i c k l e b a c k s , (b) underyearl ing sockeye and s t i ck lebacks and (c) sockeye and s t i ck leback f r y . Table 5 a . Zooplankton community c h a r a c t e r i s t i c s in enclosures 20 before and a f t e r treatment with a l l o p a t r i c groupings of sockeye y e a r l i n g s and adult s t i c k l e b a c k s . Table 5 b . Zooplankton community c h a r a c t e r i s t i c s in enclosures 21 before and a f t e r treatment with sympatric groupings of sockeye y e a r l i n g s and adult s t i c k l e b a c k s . T a b l e 6 . Z o o p l a n k t o n community c h a r a c t e r i s t i c s i n e n c l o s u r e s b e f o r e and a f t e r t r e a t m e n t w i t h v a r i o u s d e n s i t i e s o f sockeye^ y e a r l i n g s and a d u l t s t i c k l e b a c k s . T a b l e 7. Z o o p l a n k t o n community c h a r a c t e r i s t i c s i n e n c l o s u r e s b e f o r e and a f t e r t r e a t m e n t w i t h v a r i o u s d e n s i t i e s o f u n d e r y e a r l i n g s o c keye and s t i c k l e b a c k s . T a b l e 8a. Z o o p l a n k t o n community c h a r a c t e r i s t i c s i n e n c l o s u r e s b e f o r e and a f t e r t r e a t m e n t w i t h a l l o p a t r i c g r o u p i n g s o f u n d e r y e a r l i n g s o c keye and s t i c k l e b a c k s . T a b l e 8b. Z o o p l a n k t o n communi ty c h a r a c t e r i s t i c s i n e n c l o s u r e s b e f o r e and a f t e r t r e a t m e n t w i t h s y m p a t r i c m i x t u r e s o f u n d e r y e a r l i n g s o c keye and s t i c k l e b a c k s . T a b l e 9a. Z o o p l a n k t o n communi ty c h a r a c t e r i s t i c s i n e n c l o s u r e s b e f o r e and a f t e r t r e a t m e n t w i t h a l l o p a t r i c g r o u p i n g s o f s o c k e y e and s t i c k l e b a c k f r y . T a b l e 9b. Z o o p l a n k t o n community c h a r a c t e r i s t i c s i n e n c l o s u r e s b e f o r e and a f t e r t r e a t m e n t w i t h s y m p a t r i c g r o u p i n g s o f s o c k e y e and s t i c k l e b a c k f r y . T a b l e 10. Z o o p l a n k t o n communi ty c h a r a c t e r i s t i c s i n e n c l o s u r e s b e f o r e and a f t e r t r e a t m e n t w i t h v a r i o u s d e n s i t i e s o f s o c k e y e and s t i c k l e b a c k f r y . T a b l e 1 1 . D i e t c h a r a c t e r i s t i c s o f s o c keye y e a r l i n g s and a d u l t s t i c k l e b a c k s i n a l l o p a t r y and s y m p a t r y . T a b l e 12 . D i e t c h a r a c t e r i s t i s o f u n d e r y e a r l i n g s o c keye and s t i c k l e b a c k s i n a l l o p a t r y and s y m p a t r y . T a b l e 1 3 . D i e t c h a r a c t e r i s t i c s o f s o c keye and s t i c k l e b a c k f r y i n a l l o p a t r y and s y m p a t r y . T a b l e 14 . R e s u l t s o f n o n p a r a m e t r i c ANOVA on s i z e s o f p r e y consumed by f i s h i n f i s h t r e a t m e n t s f r om (a) y e a r l i n g , (b) u n d e r y e a r l i n g , and ( c ) f r y e x p e r i m e n t s . T a b l e 1 5 . D i e t o v e r l a p measurements by t a x a and s i z e between s o c keye and s t i c k l e b a c k s i n a l l o p a t r y f r om (a) y e a r l i n g , (b) u n d e r y e a r l i n g , and ( c ) f r y e x p e r i m e n t s . T a b l e 16 . Mean s i z e (mm) and m a j o r t a x a o f p r e y (by number) consumed v s . t h o s e a v a i l a b l e i n o pen -wa t e r a r e a s o f f i s h t r e a t m e n t e n c l o s u r e s f r om (a) y e a r l i n g , (b) u n d e r y e a r l i n g and f r y e x p e r i m e n t s . T a b l e 17 . R e s u l t s o f n o n p a r a m e t r i c ANOVA on b i omass o f p r e y consumed by f i s h i n f i s h t r e a t m e n t s f r om (a) y e a r l i n g s , (b) u n d e r y e a r l i n g , and ( c ) f r y e x p e r i m e n t s . v i i i LIST OF FIGURES Page F igure 1. Map of Kennedy Lake, Vancouver I s l a n d , B.C. 4 F igure 2. Diagram of exper imenta l enc l o su re . 7 F igure 3. Temporal comparison of zooplankton mean s i z e , 18 t o t a l abundance and copepod abundance in a f i s h i e s s enc losure and in Kennedy Lake. (Heavy s t i c k l e b a c k i nvas ion cons i s t ed of 1 salmonid en te r i ng from June 23 to J u l y 7. F igure 4 . Frequency d i s t r i b u t i o n s (as %) of zooplankton by 24 s i z e i n enc losures before ( a , c, e) and a f t e r t reatment (b , d , f ) w i t h a l l o p a t r i c and sympatr i c groupings of sockeye y e a r l i n g s and adu l t s t i c k l e b a c k s . F igure 5. Frequency d i s t r i b u t i o n s (as %) of zooplankton by 25 taxa i n enc losures before (a , c, e) and a f t e r (b, d , f ) t reatment w i th a l l o p a t r i c and sympatr ic groupings of sockeye y e a r l i n g s and adu l t s t i c k l e b a c k s . F igure 6. Comparison of frequency d i s t r i b u t i o n s of 27 zoop lankton by s i z e sampled from a f i s h i e s s enc losure ( a , b ) , Kennedy Lake ( c , d ) , and pooled r e s u l t s from enc losures t r ea t ed w i t h a l l o p a t r i c and sympatr ic combinat ions of sockeye y e a r l i n g s and adu l t s t i c k l e b a c k s (e, f ) . F i g u r e 7. Compa r i s on o f f r e q u e n c y d i s t r i b u t i o n s o f z o o p l a n k t o n by t a x a samp led f r om a f i s h l e s s e n c l o s u r e , ( a , b) Kennedy Lake ( c , d ) , and p o o l e d r e s u l t s f r om e n c l o s u r e s t r e a t e d w i t h a l l o p a t r i c and s y m p a t r i c c o m b i n a t i o n s o f s o c k eye y e a r l i n g s and a d u l t s t i c k l e b a c k s ( e , f ) . F i g u r e 8. F r e quen cy d i s t r i b u t i o n s (as %) o f z o o p l a n k t o n by s i z e i n e n c l o s u r e s b e f o r e ( a , c , e) and a f t e r ( b , d , f ) t r e a t m e n t w i t h a l l o p a t r i c and s y m p a t r i c g r o u p i n g s o f u n d e r y e a r l i n g s o c keye and j u v e n i l e s t i c k l e b a c k s . F i g u r e 9. F r e quen c y d i s t r i b u t i o n s (as %) o f z o o p l a n k t o n by t a x a i n e n c l o s u r e s b e f o r e ( a , c , e) and a f t e r ( b , d , f ) t r e a t m e n t w i t h a l l o p a t r i c and s y m p a t r i c g r o u p i n g s o f u n d e r y e a r l i n g s o c keye and j u v e n i l e s t i c k l e b a c k s . F i g u r e 10. Compa r i s on o f f r e q u e n c y d i s t r i b u t i o n s o f z o o p l a n k t o n by s i z e sampled f r om a f i s h l e s s e n c l o s u r e ( a , b ) , Kennedy Lake ( c , d ) , and p o o l e d r e s u l t s f r om e n c l o s u r e s t r e a t e d w i t h a l l o p a t r i c and s y m p a t r i c c o m b i n a t i o n s o f u n d e r y e a r l i n g s o c keye and j u v e n i l e s t i c k l e b a c k s ( e , f ) . F i g u r e 11. Compa r i s on o f f r e q u e n c y d i s t r i b u t i o n s o f z o o p l a n k t o n by t a x a samp led f r om a f i s h l e s s e n c l o s u r e ( a , b ) , Kennedy Lake ( c , d ) , and p o o l e d r e s u l t s f r om e n c l o s u r e s t r e a t e d w i t h a l l o p a t r i c and s y m p a t r i c c o m b i n a t i o n s o f , u n d e r y e a r l i n g s o c keye and j u v e n i l e s t i c k l e b a c k s ( e , f ) . F igu re 12. Frequency d i s t r i b u t i o n s (as %) of zooplankton by s i z e i n enc losures before ( a , c, e) and a f t e r (b, d , f ) treatement w i t h a l l o p a t r i c and sympatr i c groupings of sockeye f r y and j u v e n i l e s t i c k l e b a c k s . F igure 13. Frequency d i s t r i b u t i o n s (as %) of zooplankton by taxa i n enc losures before ( a , c, e) and a f t e r (b, d, f ) t reatment w i t h a l l o p a t r i c and sympatr i c groupings of sockeye f r y and j u v e n i l e s t i c k l e b a c k s . F igure 14. Comparison of s i z e frequency d i s t r i b u t i o n s of zoop lankton sampled from a f i s h l e s s enc losure ( a , b ) , Kennedy Lake ( c , d ) , and pooled r e s u l t s from enc losures t r ea t ed w i th a l l o p a t r i c and sympatr i c combinat ions of sockeye f r y and j u v e n i l e s t i c k l e b a c k s (e , f ) . F igure 15. Comparison of frequency d i s t r i b u t i o n s of taxonomic c l a s se s of zooplankton sampled from a f i s h l e s s enc losure ( a , b ) , Kennedy Lake ( c , d ) , and pooled r e s u l t s from enc losures t r ea t ed w i th a l l o p a t r i c and sympatr i c combinat ions of sockeye f r y and j u v e n i l e s t i c k l e b a c k s (e , f ) . F igure 16. S i z e frequency d i s t r i b u t i o n s (as %) of prey items consumed v s . prey items a v a i l a b l e to those f i s h e s i n enc losures a t the end of exper iment, by (a) sockeye y e a r l i n g s and adu l t s t i c k l e b a c k s (b) underyear l i ng sockeye and j u v e n i l e s t i c k l e b a c k s , and (c) sockeye f r y and j u v e n i l e s t i c k l e b a c k s . F igure 17. Length-weight r e l a t i o n s h i p s of prey i t ems. Klekowski and Shuskina re ference was taken from Edmondson and Winberg, 1971. F igure 18. Frequency d i s t r i b u t i o n s (by s i z e and taxa) of prey items consumed by sockeye y e a r l i n g s in a l l o p a t r y and sympatry. F i g u r e . 19. Frequency d i s t r i b u t i o n s (by s i z e and taxa) of prey items consumed by adu l t s t i c k l e b a c k s i n a l l o p a t r y and sympatry. F igure 20. Frequency d i s t r i b u t i o n s (by s i z e and taxa) of prey items consumed by underyear l i ng sockeye in a l l o p a t r y and i n sympatry. F igure 21 . Frequency d i s t r i b u t i o n s (by s i z e and taxa) of prey items consumed by underyear l i ng s t i c k l e b a c k s i n a l l o p a t r y and in sympatry. F igure 22. Frequency d i s t r i b u t i o n s (by s i z e and taxa) of prey items consumed by sockeye f r y in a l l o p a t r y and sympatry. xii Page F igure 23. Frequency d i s t r i b u t i o n s (by s i z e and taxa) of prey 7 3 i tems consumed by s t i c k l e b a c k f r y in a l l o p a t r y and sympatry. F igure 24. Frequency d i s t r i b u t i o n s of zooplankton by s i z e i n 7 7 va r i ous f i s h treatment enc losures a t the end of each exper iment . F igure 25. Frequency d i s t r i b u t i o n s of zooplankton by taxa i n 7 8 va r i ous f i s h treatment enc losures a t the end of each exper iment . ACKNOWLEDGEMENTS I would l i k e to thank my supe rv i so r a t the P a c i f i c B i o l o g i c a l S t a t i o n (PBS), Dr . K. D. Hya t t , f o r h i s gu idance, en l i gh t en i ng d i s c u s s i o n s , c on s t r u c t i v e c r i t i c i s m s and f r i e n d s h i p throughout a l l phases of t h i s r e sea r ch . I would a l s o l i k e to thank my supe rv i so r a t UBC, Dr. C. C. L indsey f o r h i s encouragement and support and to the committee members, Dr . W. E. N e i l l and Dr. T. G. Nor thco te , f o r c r i t i c a l l y rev iewing t h i s manusc r ip t . Spec i a l thanks are extended to s t a f f PBS and to the many f r i e n d s tha t helped w i th the endless months of f i e l d work. In p a r t i c u l a r , Robert Cu l b e r t , James R a d z i u l , Ch r i s Moore, Lau r i e Lacovetsky and Mike Wright were e s p e c i a l l y h e l p f u l . A l s o , I would l i k e to thank my husband, Dr. T. Quinn, f o r h i s moral support and e d i t o r i a l comments tha t helped improve t h i s manuscr ip t . F i n a l l y , I wish to thank the l a d i e s from the Mi com o f f i c e a t PBS f o r t yp ing and r e - t yp i ng the many d r a f t s of t h i s manusc r ip t . F i n an c i a l support f o r t h i s study was prov ided by the Department of F i s h e r i e s and Oceans, PBS, Nanaimo, B r i t i s h Columbia. Personal support was prov ided by a Natura l Sc iences and Eng ineer ing Research Counc i l o f Canada (NSERC) post-graduate s c h o l a r s h i p , a B r i t i s h Columbia Sc ience Counc i l Graduate Award in Eng ineer ing and Technology and a NSERC grant awarded to Dr. C. C. L indsey . 1 1.0 GENERAL INTRODUCTION Numerous i n v e s t i g a t i o n s have demonstrated an inve rse r e l a t i o n s h i p between the abundance of j u v e n i l e sockeye salmon (Oncorhynchus nerka) per un i t area in lakes and t h e i r ra tes of growth (Burgner 1962; Mathisen and Kerns 1964; Ruggles 1965; Johnson 1965; Rogers 1973; Goodlad et a l . 1974). In an a n a l y s i s of dens i ty-dependent aspec ts of t r o ph i c p rocesses , Brocksen et a l . (1970) repor ted an inve rse r e l a t i o n s h i p between the biomass of young sockeye and the biomass of t h e i r food from severa l w ide l y d i s t r i b u t e d o l i g o t r o p h i c l a k e s . Evidence from these s tud i e s as we l l as others (B re t t 1971, 1983) i s pe rsuas ive tha t the growth ra te and p o s s i b l y s u r v i v a l (LeBrasseur et a l . 1968) of young sockeye i s f o od - l im i t e d w i t h i n a wide range of nursery l a k e s . Given t h i s c o n d i t i o n , compet i t i on f o r l i m i t e d food resources has g e n e r a l l y been regarded as an important process i n f l u e n c i n g the p roduc t ion of sockeye salmon in l a k e s . In many lakes j u v e n i l e sockeye are the most numerous l i m n e t i c f i s h spec ies ( Foe r s te r 1968; Hartman and Burgner 1972; Rogers 1973), thus i n t r a s p e c i f i c compet i t i on has u s ua l l y been cons idered to p lay the major , i f not e x c l u s i v e , r o l e i n c o n t r o l l i n g j u v e n i l e sockeye p roduc t ion (Burgner, i n p r e s s ) . Although o f ten suggested as p o t e n t i a l l y impor tant , ev idence tha t i n t e r s p e c i f i c compet i t i on f o r food i n f l uences sockeye p roduc t i on i s l a r g e l y c i r c ums t an t i a l ( e . g . Krog ius and Krokh in 1956; Hartman and Burgner 1972) or negat ive (Rogers 1973). In a recent study of 17 lakes a long the B r i t i s h Columbia coas t , Hyatt and Stockner (1985) used n u t r i e n t a d d i t i o n experiments to demonstrate tha t j u v e n i l e sockeye growth i s both f o o d - l i m i t e d and s t r ong l y i n f l uenced by v a r i a t i o n s i n the abundance of a l l l i m n e t i c f i s h . S ince th reesp ine s t i c k l e b a c k 2 (Gasterosteus acu lea tus ) were numer i ca l l y dominant in the l i m n e t i c zones of approx imate ly h a l f of the lakes s t u d i e d , i n t e r s p e c i f i c compet i t i on between j u v e n i l e sockeye and l imne t i c s t i c k l e b a c k s cou ld exe r t cons i de rab l e con t ro l over sockeye product ion i n many nursery l a k e s . S t i c k l e b a c k s may compete w i th sockeye d i r e c t l y by removing food items tha t would be consumed by sockeye ( e x p l o i t a t i v e c ompe t i t i o n ) , or i n d i r e c t l y by a l t e r i n g the amount of food a v a i l a b l e to sockeye v i a over t or sub t l e behav ioura l i n t e r a c t i o n s ( i n t e r f e r en ce c ompe t i t i o n ) . In t h i s study I p rov ide f u r t h e r i n s i g h t i n t o the mechanisms med ia t ing compet i t i ve i n t e r a c t i o n s between sockeye and s t i c k l e b a c k s by man ipu la t i ng f i s h abundance, s i z e and spec ies combinat ions in exper imenta l enc losures mainta ined in the l i m n e t i c zone of Kennedy Lake, B r i t i s h Co lumbia. Zooplankton communities sub jec t to f i s h p redat ion are examined to a s c e r t a i n i f sockeye and s t i c k l e b a c k s e x p l o i t a t i v e l y i n f l u ence the s i z e and spec i es compos i t ion of the food resource . D i e t s of sockeye and s t i c k l e b a c k s , i n a l l o p a t r y and in sympatry w i th each o the r , are examined f o r n iche s h i f t s which may occur as a consequence of i n t e r s p e c i f i c compet i t i on (P ianka 1981). 3 2.0 MATERIALS AND METHODS 2.1 Study S i t e Th is study was c a r r i e d out a t Kennedy Lake (49°06'N, 125°33'W) loca ted on the west coast of Vancouver I s l a n d , B r i t i s h Columbia ( F i g . 1 ) . The lake i s 4 m above sea l e v e l , 6,475 ha in area w i th a mean depth of 38 m (Ruther fo rd et a l . 1986). I t has two major arms (Main and Clayoquot) connected by a narrow (100 m) channel through which there appears to be very l i t t l e exchange of e i t h e r n u t r i e n t s or f i s h . In common w i t h the ma j o r i t y of warm monomictic lakes of the outer B r i t i s h Columbia coas t , Kennedy Lake i s c ha r a c t e r i z ed by a shor t water res idence t ime (1.1 y r ) , low phosphorus concen t ra t i on (1 .2 -2 .0 ug TP«L _ 1 i n an unt reated s t a t e ) , and extremely o l i g o t r o p h i c c ond i t i o n s (1 .08-2.01 ug TCh l » l _ 1 un t rea ted) (S tockner and Shor t reed 1985). The major bas ins of the lake are deep and l i t t o r a l zone hab i t a t (<5 m depth) i s r e s t r i c t e d to approx imate ly 3.2% of the l a k e ' s sur face a r ea . Beach s e i ne s , minnow t raps and a mid-water t r a w l , used i n con junc t i on w i t h an echo sounder, have been used to assess the f i s h popu l a t i on s . In dec reas ing order of abundance, the f i s h spec ies known to occupy the lake are s t i c k l e b a c k s , sockeye salmon, p r i c k l y s c u l p i n (Cottus a s pe r ) , peamouth chub (My loche i l u s c a u r i n u s ) , cu t th roa t t r o u t (Salmo c l a r k i c l a r k i ) and coho salmon (0 . k i s u t c h ) . The l imne t i c f i s h and zooplankton community of the l ake i s r e l a t i v e l y s imp le ; j u v e n i l e sockeye and s t i c k l e b a c k s are the only f i s h spec ies which r e g u l a r l y occupy o f f sho re waters of the lake to e x p l o i t zoop lankton as a major food source . The l i m n e t i c zooplankton commmunity i s s imp le . The smal l c l adoce ran , Bosmina coregoni l o ng i s p i n a (Deevey and Deevey) i s very F i g . 1 . M a p o f K e n n e d y L a k e , V a n c o u v e r I s l a n d , 3 . C . 5 abundant whereas the l a r g e r c ladocerans (Diaphanosoma s p . , S ida s p . , and Polyphemus sp. ) are r a r e . Daphnia spp. appear to be absent from the l a k e . Cyc l opo id copepods (D iacyc lops thomasi) and r a r e l y Cyc lops  verna l i s ( F i s h e r ) ) are more numerous than the ca l ano i d copepods (Diaptomus oreqonens is ( L i l l j e b o r g ) and Diaptomus kenai ( F i s h e r ) . K e r a t e l l a sp . and K e l i c o t t i a sp . are r o t i f e r s which r e g u l a r l y ach ieve high seasonal abundance. Neomysis merced is , a mys id , i s the l a r g e s t zoop l ank te r r o u t i n e l y sampled in the l i m n e t i c zooplankton community (K. D. Hya t t , unpub l . d a t a ) . Both arms of the lake have been t r ea ted w i th i no rgan i c n u t r i e n t s a t va r i ous t imes s ince 1978 as par t of the Lake Enrichment Program (Hyatt and Stockner 1985) and s tud ies are underway to determine the responses of both t a rge t ( j u v e n i l e sockeye) and non-target spec ies to lake f e r t i l i z a t i o n . The experiments d i s cussed below were conducted in C layoquot Arm i n 1983. The lake was f e r t i l i z e d i n 1983, but there i s no reason to suspect t h i s would a f f e c t the r e s u l t s of these exper iments . 2.2 Enc losure Experiments To t e s t whether l i m n e t i c sockeye and s t i c k l e b a c k s i n f l u en ce the zoop lankton community to t h e i r mutual d isadvantage and thereby compete e x p l o i t a t i v e l y , and to t e s t f o r d i e t a r y n iche s h i f t s tha t cou ld be due to compe t i t i ve i n t e r a c t i o n s , i t i s necessary to examine the zooplankton communities and f i s h d i e t s f o r each f i s h spec ies s epa ra t e l y and together i n s i m i l a r env i ronments. J j i s i t u enc losures were chosen as the t e s t apparatus as f i s h cou ld be separated f o r study ye t a l lowed access to s i m i l a r food s u p p l i e s . 6 Experiments were conducted i n 5 x 5 x 7 m po lye thy lene enc losures ( F i g . 2 ) . Enc losures of t h i s s i z e are l a rge enough to avo id pa t ch inesss of zooplankton d i s t r i b u t i o n ye t smal l enough to avo id s t rong "edge e f f e c t s " (Stephenson et a l . 1984). P r e - f a b r i c a t e d enc losures were assembled on the sur face of a l a rge f l o a t i n g p l a t f o r m . Each enc losures was f o l ded in acco rd ian f ash ion a long the v e r t i c a l a x i s and as i t was f o l d ed , 1 inch aluminium p i p i ng was added around the i n s i d e per imeter of the enc losures a t 5 m and 7 m depths to ma in ta in enc losure shape and volume upon submergence. A c o l l a r of 4 x 4 cedar beams padded w i th sheets of styrofoam was t i e d to the i n s i d e su r face of the enc losures f o r f l o t a t i o n . To help s ink the enc losures a t the time of i n s t a l l a t i o n , lead weights were t i e d to ex te rna l tabs l o ca ted on the corner of the enc l o su re s , 5 m below the s u r f a c e . To i n s t a l l each enc l o su re , l i n e s l ead ing to p r e v i o u s l y p o s i t i o ned anchors were a t tached to one s ide of the wooden c o l l a r and the enc losure was then pushed o f f the sur face of the f l o a t i n g p l a t f o rm i n t o the wate r . A 0.5 x 0.5 m opening i n the bottom of each enc losure enabled i t to f i l l r a p i d l y w i th lake water . A f t e r the enc losures f i l l e d w i th wate r , the oppos i te s i de of the wooden c o l l a r was anchored and SCUBA d i v e r s covered the opening i n the bottom w i t h a 40 urn p lankton mesh, exc lud ing f u r t h e r zooplankton or f i s h exchanges. Enc losures were suspended about 2 to 4 m above the lake bottom in a p ro tec ted cove of the l a k e . 2.3 Exper imental Design and Procedures A s e r i e s of experiments was conducted to determine i f (1) sockeye and s t i c k l e b a c k s feed ing by themselves ( a l l o p a t r i c t reatments) i n f l uenced t h e i r r e spec t i ve zooplankton communities s i m i l a r l y to each ^ — B L A C K PLASTIC F i g . 2 . D i a g r a m o f e x p e r i m e n t a l e n c l o s u r e . 8 other and to the zooplankton community produced when both f i s h spec ies feed together i n one treatment (sympatr ic t r ea tmen t s ) , (2) to determine i f sockeye and s t i c k l e b a c k s in a l l o p a t r y changed t h e i r d i e t s when compared w i th d i e t s of sockeye and s t i c k l e b a c k s i n sympatry. In each experiment a "replacement s e r i e s " des ign (de Wit 1960) was used i n which the t o t a l d ens i t y of f i s h in the enc losures was kept cons tan t . The three spec ies r a t i o s used were: 100% sockeye, 50% sockeye/50% s t i c k l e b a c k s and 100% s t i c k l e b a c k s . The 50:50 sympatr i c t reatment was r e p l i c a t e d to prov ide equal numbers of each f i s h spec ies in a l l o p a t r i c and sympatr i c s i t u a t i o n s f o r s t a t i s t i c a l a n a l y s i s . A f i s h l e s s con t ro l enc losure was added, making a t o t a l of f i v e enc losures and fou r t reatments (Table 1 ) . The spec ies and s i z e d i s t r i b u t i o n s of zooplankton in the enc losures were monitored before and a f t e r f i s h t rea tments . Environments and popu la t i ons vary i n t ime and space so , us ing g u i d e l i n e s suggested by Connel l (1983), experiments were conducted i n s p r i n g , e a r l y summmer and f a l l of 1983 and u t i l i z e d three d i f f e r e n t s i z e s of j u v e n i l e sockeye and l imne t i c s t i c k l e b a c k s a v a i l a b l e dur ing those seasons (see Table 1 ) . Due to heat ing of the ep i l imn i on and water s t r a t i f i c a t i o n , exper iments were not conducted dur ing the summer when su r face water temperatures (range from 21-26°C) were high enough to adve r se l y a f f e c t j u v e n i l e sockeye. The p h y s i o l o g i c a l l y opt imal temperature f o r sockeye i s 15°C (B r e t t 1971). F i sh were c o l l e c t e d from the lake by us ing a midwater t rawl (Gjernes 1979) and kept i n a ho ld ing pen f o r two days. Hea l thy f i s h were then removed, weighed or measured and stocked i n t o enc l o su re s . The d en s i t y of f i s h i n the enc losures was approx imate ly tw ice the dens i t y in Kennedy Lake i n 1983 (2700 smolts or 5400 f r y * h a _ 1 , see Table 1) but T a b l e 1 . S i z e ( f o r k l e n g t h o f sockeye and s t a n d a r d l e n g t h o f s t i c k l e b a c k s ) and numbers o f f i s h used i n e n c l o s u r e e x p e r i m e n t s . No. o f f i s h per e n c l o s u r e i ng E x p e r i m e n t a l g roup (and mean s i z e ) Date ( 1 9 8 3 ) dens i t y ( f i s h - h a " 1 ) Al l o p a t r i c sockeye Al l o p a t r i c s t i c k I e b a c k s Sympatr i c r e p l i c a t e 1 Sympatr i c r e p l i c a t e 2 Sockeye y e a r l i n g s (x=64 mm) A p r i l 15-27 4000 10 sockeye 10 s t i c k I e b a c k s 5 sockeye + 5 sockeye + A d u l t s t i c k l e b a c k s (x=52 mm) 5 s t i eke 1 backs 5 s t i c k 1 e b a c k s U n d e r y e a r 1 i n g sockeye (x=51 mm) Oc tober 10-30 4000 10 sockeye 10 s t i c k 1 e b a c k s 5 sockeye + 5 sockeye + J u v e n i l e s t i c k l e b a c k s (x=38 mm) 5 s t i c k 1 e b a c k s 5 s t i ck1ebacks Sockeye f r y (x=28 mm) May 7-26 9600 24 sockeye 24 s t i c k 1 e b a c k s 12 sockeye + 12 sockeye + J u v e n i l e s t i c k l e b a c k s (x=33 mm) 12 s t i ck1ebacks 12 s t i ck1ebacks to 10 w i t h i n the range of d e n s i t i e s observed in other years (Hyat t , unpubl ished d a t a ) . The day a f t e r f i s h i n t r o d u c t i o n s , enc losures were inspected by SCUBA d i v e r s and f i s h which had d ied (approx imate ly 10%) were rep laced w i th spec ies of s i m i l a r s i z e s . F i sh were a l lowed to feed f o r a pe r i od of about two to three weeks in enc losure t r i a l s . At approx imate ly weekly i n t e r v a l s , i n t eg ra t ed 7 m v e r t i c a l p lankton samples were c o l l e c t e d w i th the a i d of the c l o t h e s l i n e and p u l l e y mechanism tha t was set-up across the sur face of the enc losure and enabled sampl ing from the centre of each enc l o su re . A 0.25 m 2 b lack c y l i n d e r cone net (100 urn mesh) was used to c o l l e c t the samples. Samples were used to compare the e f f e c t s of the d i f f e r e n t f i s h t reatments on the zooplankton communit ies. Because the lake conta ined a lower f i s h dens i t y (2700 smolts - 5400 fry«ha _ 1 ) than the f i s h treatment enc l o su re s , i t s zoop lankton community should serve as an in te rmed ia te comparison between f i s h t reatments and the f i s h l e s s con t ro l enc l o su re . P lankton samples were preserved i n a su c ro se -bu f f e r ed , 4% f o rma l i n s o l u t i o n (Haney and Ha l l 1973) f o r l a t e r a n a l y s i s . P lankton samples were subsampled accord ing to the gu i d e l i n e s of Edmondson (1971) and McCauley (1984). A l l zooplankton w i t h i n a subsample were i d e n t i f i e d to genus or spec ies and t o t a l body length ( length exc lud ing sp i ne s , antennae and setae) was measured w i th the a i d of an e l e c t r o n i c ca i i pe r -m ic rocompute r arrangement (Spru les et a l . 1981). A po r t i on of the remaining sample was scanned f o r l a rge or rare z oop l ank t e r s . The t o t a l number of zoop lank te r s per cub ic meter was c a l c u l a t e d f o r both subsample and scan observa t ions and then summed. Because the net sampled approx imate ly 1% of the enc losure volume, sampling probably d i d not a l t e r the zooplankton community. Rep l i c a t e haul contents were very s i m i l a r , 11 thus a s i n g l e haul was cons idered to be r ep re sen ta t i ve of the zooplankton w i t h i n the enc losures (see Table 2 ) . At the end of t reatment , f i s h were removed from the enc losures and preserved f o r gut content a n a l y s i s . F i s h were f i x e d i n 10% bu f f e red f o rma l i n and a f t e r a t l e a s t 1 week were t r a n s f e r r e d to 50% ethanol f o r another week. F i n a l l y the f i s h were preserved i n 95% ethanol f o r a t l e a s t f i v e weeks a t which time they were weighed and measured ( t o t a l l eng th of s t i c k l e b a c k s and fo rk length of sockeye) . Stomachs were removed and scored f o r f u l l n e s s and d i g e s t i v e s t a t e . Prey items were i d e n t i f i e d and measured w i th the e l e c t r o n i c c a l i p e r . When prey items were broken or d i g e s t ed , undigested hard body pa r t s were used to es t imate the t o t a l l engths of the p rey . Tota l body lengths of ch ironomid l a r vae were es t imated from the length and width of the heads (see Table 3 f o r c onve r s i o n s ) . In a l l but f i v e of the stomachs, the t o t a l contents were i d e n t i f i e d and enumerated, thus reducing subsampling e r r o r s a s so c i a t ed w i t h gut content a n a l y s i s . Enc losures used in May and October exper iments had been p r e v i o u s l y sub jec ted to exper imenta l man ipu la t i ons ( e . g . y e a r l i n g and adu l t s t i c k l e b a c k a d d i t i o n s ) , and a c co rd i ng l y t h e i r zoop lankton communities d i f f e r e d from tha t of Kennedy Lake. To r e - e s t a b l i s h s i m i l a r zoop lankton communities between exper iments , c o l l e c t i o n s of h o r i z on t a l p lankton tows taken from w i t h i n the lake were in t roduced i n t o a l l but the f i s h l e s s c on t r o l en c l o su re s . Heavy wave a c t i v i t y dur ing storms i n e a r l y May caused corners of some of the enc losures to t ea r thereby pe rm i t t i ng the en t ry of f i s h from the l a k e . To e f f e c t i v e l y remedy i nvas ions by f i s h i n t o the en c l o su r e s , corners of the enc losures were mended and a l l f i s h were Table 2. Comparisons of species d i s t r i bu t i on , tota l zooplankton abundance and zooplankton mean s ize in two rep l icate ve r t i ca l hauls. (DIA = Diaphanosoma, BOS = Bosmina, CYC = Diacyclops. DPT = Diaptomus, NP = naup l i i , ROT = Rot i f e r s , LGB = Miscellaneous large prey) Zooplankton species d i s t r i bu t i on (percent) Total Mean abundance s ize Repl icate No. DIA BOS CYC DPT NP ROT LGB (#/m3) (mm) 1 .09 43.83 21.91 8.41 20.48 5.03 0.25 6845 0.29 2 .03 44.82 18.22 6.20 26.73 3.90 0.10 6004 0.29 13 Table 3. Re l a t i o n sh i p s between head length and head w id th to t o t a l body length of chironomid l a r v a e . (TL = t o t a l l eng th , HL = head l eng th , HW = head w i d t h . ) . A l l r e l a t i o n s h i p s are s i g n i f i c a n t a t p = 0 .01 . Sub- fami ly Equat ion r N Tanypodinae TL = 6.17 HL-0.64 0.95 9 Chironominae TL = 17.5 HL-1.68 0.90 16 Tanypodinae and Chironominae TL = 16.7 HW-0.13 0.88 25 14 removed by SCUBA d i v e r s or a se ine ne t . A d d i t i o n a l l y , the f i s h l e s s con t ro l enc losure was enc losed by an outer b a r r i e r of 1/4" ny lon mesh ne t t i n g which prec luded f u r t h e r en t ry by f i s h throughout the s e r i e s of exper iments even in the event of f u r t h e r t ea r s deve lop ing i n the en c l o su r e . 15 3.0 THE INFLUENCE OF SOCKEYE AND STICKLEBACKS ON ZOOPLANKTON COMMUNITY STRUCTURE: TESTING FOR EXPLOITATIVE COMPETITION 3.1 I n t r oduc t i on Numerous i n v e s t i g a t o r s have documented tha t p l ank t i v o r ous f i s h are s i z e - s e l e c t i v e predators (see reviews in Ha l l e t a l . 1976; M i l l s and Schiavone 1982). F i sh p redat ion i n f l uences both the spec ies and s i z e compos i t ion of zoop lankton communities d i r e c t l y , by removing the l a rge he r b i v o r e s , or i n d i r e c t l y , by removing the l a rge predaceous zoop lank te rs t ha t consume the sma l l e r herb ivorous zoop l ank t e r s . Consequent ly , the s i z e and spec ies compos i t ion of zooplankton communities are s h i f t e d towards sma l l e r mean body s i z e s in environments con ta i n i ng p l ank t i v o r ous f i s h . Theo r e t i c a l as we l l as emp i r i c a l ev idence suggests tha t reduc t i ons i n the mean body s i z e s of the zooplankton food resource w i l l f r e quen t l y reduce the scope f o r growth in popu la t i ons of p l ank t i v o r ous f i s h i n genera l ( M i l l s and Schiavone 1982) and sockeye in p a r t i c u l a r (Eggers 1977, 1982). Consequent ly , I concluded tha t examinat ions of the r e l a t i v e e f f e c t s of p l ank t i v o r ous sockeye and s t i c k l e b a c k s on the s t r u c t u r e of zoop lankton communities i n f i e l d enc losures cou ld be used to t e s t the hypothes i s tha t d i r e c t food removal i s one mechanism tha t mediates i n t e r s p e c i f i c compet i t i on between the two f i s h s pe c i e s . Zooplankton samples c o l l e c t e d from f i s h treatment enc l o su res , be fore and a f t e r t reatment , were examined to determine i f sockeye and s t i c k l e b a c k s a f f e c t each o t h e r ' s food supp ly . F i sh d i e t s were examined to determine i f f i s h p redat ion was respons ib l e f o r any observed changes i n zoop lankton communit ies. 16 3.2 M a t e r i a l s and Methods The methods were desc r i bed in Sec t i on 2. 3.2.1 Data a n a l y s i s A Kolmogorov-Smirnov two-sample t e s t ( S i e g e l , 1956; he r ea f t e r r e f e r r e d to as K-S t e s t ) , was used to determine i f s i z e frequency d i s t r i b u t i o n s of zooplankton decreased s i g n i f i c a n t l y dur ing the treatment p e r i o d . There fo re , f o r each enc losure and the lake sample, s i z e f requency d i s t r i b u t i o n s generated from zooplankton samples c o l l e c t e d a t the s t a r t of the treatment pe r i od were compared w i th t h e i r r e spec t i v e s i z e frequency d i s t r i b u t i o n s from samples c o l l e c t e d a t the end. Net e f f i c i e n c y was assumed to be r e l a t i v e l y constant (Hya t t , unpub. data) and t he re f o r e obse rva t i ons were not ad jus ted f o r v a r i a t i o n in net e f f i c i e n c y . A K r u s k a l - W a l l i s nonparametr ic a n a l y s i s of va r i ance (ANOVA) was used to t e s t whether mean zooplankton s i z e s among treatments were d i f f e r e n t from each other a t the s t a r t and end of the exper iment, r e s p e c t i v e l y . For s t a r t and end samples, body length measurements from i n d i v i d u a l zoop lank te r s i n samples c o l l e c t e d from the va r i ous t reatments were ranked by i n c r ea s i ng s i z e and ana lyzed w i th a s i n g l e f a c t o r ( i . e . t reatment) ANOVA. (A s i n g l e f a c t o r ANOVA app l i e d to ranked data approximates a K r u s k a l - W a l l i s t e s t ; SAS S t a t i s t i c a l Package, 1985.) Tukey 's m u l t i p l e comparison of means t e s t was used to determine homogeneous se ts of ranked means among the t rea tments . 3.3 Resu l t s Zooplankton mean s i z e , abundance of a l l spec ies of zooplankton ( i . e . t o t a l abundance) and number of copepods were a l l s u b s t a n t i a l l y g r ea t e r i n the enc losure w i thout f i s h than in the l a ke , except a t t imes 17 when the enc losure was breached and f i s h invaded ( F i g . 3 ) . A l though abso lu te numerical d i f f e r e n c e s were l a r g e , q u a l i t a t i v e l y s i m i l a r seasonal t rends in t o t a l abundance appear to have taken p lace i n the lake and the f i s h l e s s enc l o su re , v a l i d a t i n g the use of enc losures to c rea te a l l o p a t r i c and sympatr i c environments tha t are q u a l i t a t i v e l y s i m i l a r to the l a k e . Experiment 1. sockeye y e a r l i n g s and adu l t s t i c k l e b a c k s ( A p r i l 15-27, 1983) To i n i t i a t e t h i s exper iment, f i s h were exc luded from a l l enc losures f o r 12 days to assess enc losure i n t e g r i t y p r i o r to s t o c k i n g . A l though zooplankton communities i n the lake and the enc losures p r i o r to f i s h i n t r o du c t i o n s appear to have s i m i l a r s i z e s and taxonomic d i s t r i b u t i o n s , s i g n i f i c a n t d i f f e r en c e s e x i s t ( K r u s k a l - W a l l i s t e s t ; see Table 4 a ) . Two of the f i s h treatment enc losures conta ined s i g n i f i c a n t l y sma l l e r zoop lank te r s than the o ther two f i s h treatments and f i s h l e s s c on t r o l enc losures (Tukey's Procedure; see Table 4 a ) . Zoop lankters sampled from the lake were s i g n i f i c a n t l y sma l l e r than those sampled from the enc losures (Tukey's Procedure; Table 4a) as the enc losures were devo id of f i s h f o r 12 days a l l ow ing the s i z e of zooplankton i n the enc losures to i n c r e a s e . Mean s i z e was approx imate ly 0.5 mm (range of means 0.44 to 0.56 mm) in the enc losures and 0.34 in the lake (Tables 5, 6 ) . In a l l enc l o su res , D. thomasi was the dominant s pe c i e s , f o l l owed by n a u p l i i and then B. co regon i , whereas the lake conta ined main ly n a u p l i i and D. thomas i . The t o t a l zooplankton abundance a l s o v a r i e d among enc losures (Table 5, 6 ) . Approx imate ly two weeks a f t e r the i n t r o du c t i o n of f i s h (sockeye mean l eng th = 64 mm, s t i c k l e b a c k mean length = 52 mm) i n t o f ou r of the exper imenta l enc l o su res , s i z e frequency d i s t r i b u t i o n s of zooplankton F i s h l e s s C o n t r o l Lake Sample Heavy S t i c k l e b a c k Invas ion Into "F ish less C o n t r o l ' t^^SS^ l L i g h t S a l m o n i d I n v a s i o n Into "F ish less C o n t r o l " e 0.60 • E 0.55-UJ ISI 0.50 • <r> 2 0.45 • < UJ 2 0.40 NO 0.35 • h-2 0.30 < _J Q_ 0.25 O O NJ 0.20 • 120.000 • ro C 108.000 • 5ANCE 96.000 • 5ANCE 84.000 -ABUNt 72.000 • "ON 60,000 • .ANKT 48.000 -ZOO PL 36.000 • TOTAL 2^.000 -.2.000 • W/////WA E3 F i g . 3. Temporal c o m p a r i s o n o f z o o p l a n k t o n mean s i z e , t o t a l abundance and copepod abundance i n a f i s h l e s s e n c l o s u r e and i n Kennedy L a k e . (Heavy s t i c k l e b a c k i n v a s i o n c o n s i s t e d o f 24 f i s h e n t e r i n q f r o m A p r i l 27 t o June 16 . L i g h t i n v a s i o n c o n s i s t e d o f 1 s a l m o n i d e n t e r i n o f r o m June 23 t o J u l y 7 . ) u 2 < Q 2 Z> CD < Q O CL O 80.000 -50,000 30.000 20,000 15.000 10,000 -7,000 5,000 3,000 2,000 1,000 N W \ A M J J A S O N DATE (1983) 19 Table 4. Results of nonparametric ANOVA on zooplankton sizes from experiments with (a) sockeye yearlings and adult sticklebacks, (b) underyearling sockeye and (c) sockeye and stickleback fry. Within each sampling time of each experiment, treatments are listed in descending order of magnitude of ranked means. Replicate treatments are numbered are in parentheses. Homogeneous sets of means are bracketed. Sampling time Experiment Start Finish (a) Sockeye yearlings and Adult Sticklebacks (F=120.32, df=5, p<.0001) Sockeye Fishless control Sockeye and Sticklebacks (1) Sticklebacks | Sockeye and Sticklebacks (2)] Lake sample ^ (F=13.61,df=5, p<.0001) Lake sample Fishless control Sockeye Sockeye and Sticklebacks (2) Sockeye and Sticklebacks (1) Sticklebacks (b) Underyearling Sockeye and Sticklebacks (c) Sockeye and Sticklebacks fry (Replicate 2 of sympatric treatment and fishless control excluded due to fish invasion) (F=67.19, df=5, p<.0001) Fishless control — Sockeye and Sticklebacks (2) Sockeye Lake sample Sockeye and Sticklebacks (1) Sticklebacks (F=13.89, df=4, p<.0001) Sockeye ~~" Sockeye and sticklebacks Lake sample Sticklebacks Fishless control (F=77.98,df=5, p<.001) Fishless control Z Lake sample Z3 Sticklebacks Sockeye Sockeye and Sticklebacks (2) Sockeye and Sticklebacks (1) (F=7.15, df=3, p<.0001) Lake sample ZD Sockeye and Sticklebacks (2) Sockeye Sticklebacks 20 Table 5a . Zooplankton community c h a r a c t e r i s t i c s i n enc losures before and a f t e r t reatment w i t h a l l o p a t r i c groupings of sockeye y e a r l i n g s and adu l t s t i c k l e b a c k s . A l l o p a t r i c F i sh Treatments Sockeye S t i c k l e b a c k s Mean Major No. Mean Major No. No./m3 s i z e taxa copepods No./m3 s i z e taxa copepods Before 17277 0.56 D iacyc lops 14178 5564 0.45 D iacyc lops 3688 ( A p r i l 15) A f t e r 3781 0.19 Naup l i i 716 4362 0.25 Naup l i i 954 ( A p r i l 27) R o t i f e r s R o t i f e r s Summary 78% decrease i n t o t a l abundance 95% decrease i n copepod numbers 22% decrease in t o t a l abundance 74% decrease i n copepod numbers 21 Table 5b. Zooplankton community c h a r a c t e r i s t i c s i n enc losures before and a f t e r t reatment w i t h sympatr i c mix tures of sockeye y e a r l i n g s and adu l t s t i c k l e b a c k s . Sympatr ic F i sh Treatments Rep l i c a t e 1 Rep l i c a t e 2 Mean Major No. Mean Major No. No./m3 s i z e taxa copepods No./m3 s i z e taxa copepods Before 7143 0.53 D iacyc lops 5531 9226 0.44 D iacyc lops 7072 ( A p r i l 15) A f t e r 6549 0.19 Naup l i i 436 2944 0.29 Naup l i i 1042 ( A p r i l 27) Bosmina R o t i f e r s Summary 8% decrease in t o t a l abundance 68% decrease i n t o t a l abundance 92% decrease in copepod numbers 85% decrease i n copepod numbers T a b l e 6 . Z o o p l a n k t o n communi ty c h a r a c t e r i s t i c s in e n c l o s u r e s b e f o r e and a f t e r t r e a t m e n t w i t h v a r i o u s d e n s i t i e s o f sockeye y e a r l i n g s and a d u l t s t i c k l e b a c k s . 0 f i s h / h e c t a r e 2700 f i s h / h e c t a r e 4000 f i i s h / h e c t a r e ( f i s h l e s s c o n t r o l ) ( l a k e s a m p l e ) ( f i s h t r e a t m e n t s p o o l e d ) Mean No. Mean No. Mean Mean No. Time No. /m3 s i z e t a x a copepods No. /m3 s i z e t a x a copepods No . /m3 s i z e t a x a copepods B e f o r e 13623 0 . 5 6 D i a c y c l o p s 7372 6331 0 . 3 4 N a u p l i i 2132 9802 0 . 5 1 D i a c y c l o p s 30469 A p r i l 15 0 i acycI ops A f t e r 59698 0 . 3 7 Bosmina 6156 10281 0 . 2 9 N a u p l i i 2242 4409 0 . 2 2 N a u p l i i 3148 A p r ! 1 27 Bosmi na Rot i f e r s Summary 338% i n c r e a s e i n t o t a l abundance 62% i n c r e a s e in t o t a l abundance 55% d e c r e a s e i n t o t a l abundance 16% < decrease i n copepods 5% i n c r e a s e in copepods 90% d e c r e a s e i n copepods ro 23 w i t h i n each f i s h - t r ea tmen t enc losure were s i g n i f i c a n t l y sma l l e r (K-S, df=2, x 2 =418.42, 130.42, 340.00 and 13.21 r e s p e c t i v e l y ; p<.001 f o r a l l f i s h t reatments - F i g . 4 ) . The mean s i z e of zooplankton i n enc losures a t the s t a r t of the experiment decreased from a range of 0.44-0.56 mm to a range of 0.19-0.25 mm at the end of the experiment (Table 5 ) , wh i l e the major taxonomic groups present changed from D. thomasi to n a u p l i i and r o t i f e r s ( F i g . 5 ) . S l i g h t to major decreases (8-78 %) in t o t a l zooplankton abundance were a l s o observed. At the end of the treatment p e r i o d , the ranked mean s i z e s of zoop lankton >0.35 mm were not s i g n i f i c a n t l y d i f f e r e n t among f i s h t reatments (Tukey's Procedure; Table 4 a ) . Therefore the enc losures con ta i n i ng f i s h were more s i m i l a r to each other a t the end of the treatment pe r i od than at the beginn ing of the treatment p e r i o d , demonstrat ing the un i form e f f e c t of sockeye and s t i c k l e b a c k g raz i ng on the s i z e s t r u c t u r e of zooplankton communit ies. To determine i f the s h i f t s i n the f i s h - t r ea tmen t enc l o su res , from l a rge prey a t the s t a r t o f the experiment to sma l l e r prey a t the end, were due to f i s h p r eda t i on , the zooplankton community changes i n the f i s h - t r e a tmen t enc losures were compared w i th changes i n the lake and the f i s h l e s s c o n t r o l , which conta ined h a l f the dens i t y of f i s h and zero f i s h , r e s p e c t i v e l y . I t appears tha t the s i z e s h i f t i n the f i s h treatments may not have been e n t i r e l y due to f i s h p redat ion s ince the s i z e d i s t r i b u t i o n of zoop lankton w i t h i n the f i s h l e s s enc losure a l s o decreased s i g n i f i c a n t l y (K-S, df=2, X 2 =110.08; p<.001) and the mean s i z e decreased from 0.56 mm at the s t a r t to 0.37 mm a f t e r approx imate ly two weeks. During t h i s t ime, zoop lankton s i z e d i s t r i b u t i o n s in the lake sample a l s o decreased s l i g h t l y 24 B E F O R E T R E A T M E N T 6 0-1 4 0 -2 0 -N = 4 0 6 (a) > o z LU D O UJ or X = 0.56 N - 4 3 7 (b) .25 .45 .65 .85 1.12 1.75 5.0 A L L O P A T R I C S O C K E Y E N = 2 7 3 (C) X? 0.45 A F T E R T R E A T M E N T N = 5 4 8 (d) 0.25 .25 ;45 .65 .85 1.12 1.75 5.0 S I Z E (m m ) A L L O P A T R I C S T I C K L E B A C K S N = 7 3 2 (e) X = 0.48 N = 9 0 3 ( f ) X= 0.22 .25 A5 .65 .85 1.12 !.75 5.0 S Y M P A T R I C S O C K E Y E + S T I C K L E B A C K S Fig. 4. Frequency distributions (as %) of zooplankton by size in enclosures before (a, c, e) and after treatment '(b, d, f) with allopatric and sympatric groupings of sockeye yearlings and adult sticklebacks. (*Two replicates from sympatric treatments have been pooled.) B E F O R E ^ * / M 3 = 1 7 2 7 7 N a 4 0 6 rt*/M3 = 5 5 6 4 N - 2 7 3 * * / M = 8 1 8 5 N = 7 3 2 100 -| 5 0 >-O z LU 3 O LU o r 100 -| A F T E R 1 2 DAYS # / M 3 = 3781 ^/M 3 = 4362 50-N = 903 (f) 4747 9 i> C-SOCKEYE (5?- 64mm) STICKLEBACK S O C K E Y E 6 (X= 52mm) S T I C K L E B A C K S KEY f - R O T I F E R S ^ - N A U P L I I I k - C0PEP00S ^ - B O S M I N A C O T H E R C L A D O C E R A N S MISCELLANEOUS R ' LARGE PREY U - U N K N O W N F i g . 5. F requency d i s t r i b u t i o n s (as %) o f z o o p l a n k t o n by t a x a i n e n c l o s u r e s b e f o r e ( a , c , e) and a f t e r ( b , d , f ) t r e a t m e n t w i t h a l l o p a t r i c and s y m p a t r i c g r o u p i n g s o f sockeye y e a r l i n g s and a d u l t s t i c k l e b a c k s . (*Two r e p l i c a t e s f r o m s y m p a t r i c t r e a t m e n t s have been p o o l e d . ) but s i g n i f i c a n t l y (K-S, df=2, x 2 =14.19; p<.001). The mean s i z e decreased from 0.34 to 0.29 mm ( F i g . 6 ) . A l though in a l l t reatments and the l a k e , s i z e d i s t r i b u t i o n s of zoop lankton decreased s i g n i f i c a n t l y dur ing the exper iment , i n both the f i s h l e s s con t ro l and the l a ke , the s h i f t to a sma l l e r s i z e of zooplankton was ma in ly due to inc reases in B. coreqoni ( F i g . 7a-b, 7 c - d ) , whereas in the f i s h t reatments the s i z e decrease was due to i n c reases i n numbers of n a u p l i i or r o t i f e r s ( F i g . 7e- f ) and decreases i n the number of copepods. The inc rease i n Bosmina abundance was f a r g r ea te r i n the f i s h l e s s con t ro l than in the l a k e . F i s h p reda t i on was an important f a c t o r i n determin ing zoop lankton s i z e s , as by the end of the treatment pe r i od ranked mean s i z e s of zoop lankton from the f i s h l e s s con t ro l and the lake were s i g n i f i c a n t l y l a r g e r than those from the f i s h t reatments (Tukey's Procedure, Table 4 a ) . By the end of the exper iment, h igher f i s h abundance ( i . e . f i s h l e s s con t ro l < lake < f i s h t reatment) was a s soc i a t ed w i t h zoop lankton communities c ha r a c t e r i z ed by s u c c e s s i v e l y sma l l e r mean body s i z e (0 .37 , 0.29 and 0.22 mm, r e s p e c t i v e l y ) i n d i c a t i n g tha t f i s h p reda t i on i n f l uenced the s i z e s t r u c t u r e of the zooplankton communities ( F i g s . 6, 7 ) . Fu r the r evidence that f i s h p redat ion a f f e c t s zoop lankton community s t r u c t u r e i s ev ident as zooplankton abundance inc reased d r a m a t i c a l l y (333%) i n the f i s h l e s s enc losure and moderate ly (62%) in the lake (Table 6 ) . By c o n t r a s t , t o t a l zoop lankton abundance decreased an average 55% in the f i s h t rea tments , which had a f i s h d en s i t y approx imate ly double tha t i n the l a k e . Abso lu te i nc reases i n r o t i f e r numbers occur red i n a l l enc losures and i n the l a k e . However, r e l a t i v e to 2 7 B E F O R E T R E A T M E N T O O O x 28 n N 3 3 5 9 A F T E R 12 D A Y S T R E A T M E N T (b) N = 257 (d) N = 1888 (f) X = 0.29 X = 0 .22 .25 .45 .65 .85 1.12 1.75 5.0 .25 .45 .65 .85 1.12 1.75 5.0 .25 .45 .65 .85 1.12 1.75 5.0 S I Z E ( m m ) F I S H L E S S C O N T R O L L A K E S A M P L E F I S H T R E A T M E N T S ( 0 F I S H / H A ) ( 2 7 0 0 F I S H / H A ) ( 4 0 0 0 F I S H / H A ) F i g . 6. Compar ison o f f r e q u e n c y d i s t r i b u t i o n s of zooplankton by s i z e sampled f r o m a f i s h l e s s e n c l o s u r e (a, b ) , Kennedy Lake ( c , d ) , and p o o l e d r e s u l t s f r o m e n c l o s u r e s treated with a l l o p a t r i c and s y m p a t r i c c o m b i n a t i o n s o f sockeye y e a r l i n g s and a d u l t s t i c k l e b a c k s ( e , f ) . 28 O O O # / M 3 = 1 3 6 2 3 N = 315 BEFORE # / M 3 = 6 3 3 1 N = 4 8 8 (c) M E A N # / M 3 = 9 8 0 2 N = 1411 ro rr UJ cn 2 # / M = 5 9 6 9 8 N= 3 5 9 F I S H L E S S C O N T R O L ( 0 F I S H / H A ) A F T E R 12 D A Y S # / M 3 = 10281 N= 2 5 7 (d) L A K E S A M P L E ( 2 7 0 0 F I S H / H A ) M E A N # / M 3 = 4 4 0 9 N = I 8 8 8 (f) >CT" 1 u F I S H T R E A T M E N T S ( 4 0 0 0 F I S H / H A ) F i g . 7. Compar ison o f f r e q u e n c y d i s t r i b u t i o n s of zooplankton by t a x a sampled f r o m a f i s h l e s s e n c l o s u r e , (a, b) Kennedy Lake ( c , d ) , and p o o l e d r e s u l t s f rom e n c l o s u r e s t r e a t e d w i t h a l l o p a t r i c and s y m p a t r i c c o m b i n a t i o n s o f sockeye y e a r l i n g s and a d u l t s t i c k l e b a c k s ( e , f ) . other z oop l ank t e r s , r o t i f e r s were not numer i ca l l y important i n the f i s h l e s s con t ro l (<2%) or i n the lake (<14%). Experiment 2. underyear l i ng sockeye and s t i c k l e b a c k s (October 10-30, 1983) At the s t a r t of t h i s exper iment, s i g n i f i c a n t d i f f e r e n c e s e x i s t e d among s i z e s of zooplankton from communities sampled from the enc losures and the lake but cons ide rab le s i m i l a r i t i e s were ev ident (Tukey's Procedure; Table 4b ) . The con t ro l enc l o su re , which had been f i s h l e s s s i n ce e a r l y June, conta ined s i g n i f i c a n t l y l a r g e r mean s i z e s of zoop lank te r s than the o ther enc losures and the lake (Tukey's Procedure -Table 4b ) , supported a h igher abundance of zoop lank ton , and conta ined most ly l a rge D iacyc lops (Table 7 ) . The f i s h treatment enc losures had zooplankton which were s i m i l a r to each other in s i z e and in a l l but one of the enc losures (Rep l i c a t e 2 of the sympatr ic t r ea tmen t s ) , n a u p l i i were the numer i c a l l y dominant zoop lank te r f o l l owed by D i a cy c l op s . Rep l i c a t e 2 of the sympatr i c treatment was more l i k e the lake sample, as B. coreqoni and n a u p l i i dominated the community. Mean s i z e of zooplankton i n a l l f ou r f i s h treatment enc losures was approx imate ly 0.32 mm (0.25-0.33 range of means) 0.26 mm in the lake and 0.53 mm in the f i s h l e s s enc losure (see Tables 7 and 8 ) . A f t e r 20 days of treatments w i th underyear l i ng sockeye (mean length = 51 mm) and s t i c k l e b a c k s (mean length = 38 mm), s i g n i f i c a n t s h i f t s to sma l l e r s i z e d i s t r i b u t i o n s of zooplankton w i t h i n an enc losure were noted i n three of the four f i s h treatment enc losures (K-S, df=2, x2=44.io, 29.31 and 35.57; P<.001 f o r a l l t rea tments , F i g . 8 ) . The o v e r a l l s i z e d i s t r i b u t i o n of zooplankton from the a l l o p a t r i c s t i c k l e b a c k T a b l e 7 . Z o o p l a n k t o n communi ty c h a r a c t e r i s t i c s i n e n c l o s u r e s b e f o r e and a f t e r t r e a t m e n t w i t h v a r i o u s d e n s i t i e s o f u n d e r y e a r I Ing sockeye and s t i c k l e b a c k s . 0 f i s h / h e c t a r e 2700 f i s h / h e c t a r e 4000 f i s h / h e c t a r e ( f i s h l e s s c o n t r o l ) ( l a k e samp le ) ( f i s h t r e a t m e n t s p o o l e d ) Mean No. Mean No. Mean Mean No. Time No . /m3 s i z e t a x a copepods No. /m3 s i z e t a x a copepods No . /m3 s i z e t a x a copepods B e f o r e 50540 0 . 5 3 D i a c y c l o p s 37221 22429 0 . 2 6 Bosmina 4029 13711 0 . 3 2 N a u p l i i 15109 Oct 10 Naup1i i A f t e r 89642 0 . 4 9 Bosmina 41002 22881 0 . 3 6 D i a c y c l o p s 11700 15160 0 . 2 6 Bosmina 9158 Oct 30 Summary 77% i n c r e a s e i n t o t a l abundance 2% i n c r e a s e in t o t a l abundance 11% i n c r e a s e i n t o t a l abundance 10% i n c r e a s e i n copepods 190% i n c r e a s e in copepods 39% d e c r e a s e i n copepods CO o 31 Table 8a . Zooplankton community c h a r a c t e r i s t i c s i n enc losures before and a f t e r treatment w i th a l l o p a t r i c groupings of underyea r l i ng sockeye and s t i c k l e b a c k s . F i sh t reatments Sockeye S t i c k l e b a c k s Mean Major No. Mean Major No. No./m3 s i z e taxa copepods No./m3 s i z e taxa copepods Before 7700 0.32 Naup l i i 2328 5983 0.25 Naup l i i 2134 (October 10) D iacyc lops D iacyc lops A f t e r 12554 0.24 Bosmina 1384 8011 0.29 Bosmina 1453 (October 30) Summary 63% inc rease in t o t a l abundance 34% inc rease in t o t a l abundance 41% decrease i n copepod numbers 32% decrease i n copepod numbers 32 Table 8b. Zooplankton community c h a r a c t e r i s t i c s i n enc losures before and a f t e r treatment w i th sympatr ic mixtures of underyear l i ng sockeye and s t i c k l e b a c k s . Sympatr ic f i s h t reatments Rep l i c a t e 1 Rep l i c a t e 2 Mean Major No. Mean Major No. No./m3 s i z e taxa copepods No./m3 s i z e taxa copepods Before 23553 0.33 Naup l i i 6791 17608 0.33 Bosmina 3856 (October 10) D iacyc lops A f t e r 21910 0.26 Naup l i i 3942 18165 0.25 Bosmina 2379 (October 30) Naup l i i Summary 6% decrease in t o t a l abundance 3% inc rease i n t o t a l abundance 42% decrease i n copepod numbers 38% decrease i n copepod numbers 33 60-i 40-N= 345 X = 0.32 z LU O L U or 55 6 0 - | N = 386 40-, = 0.24 (a) (b) .25 .45.65 B5 IJ2 175 5.0 ALLOPATRIC SOCKEYE BEFORE TREATMENT N=405 (c) X = 0.25 AFTER TREATMENT N » 5 5 3 (d> .25 .45 .65 .85 1.12 1.75 5.0 SIZE (mm) ALLOPATRIC STICKLEBACKS N = 1129 X= 0,33 N = 642 X= 0.26 (e) (f) .25 .45 65 .85 1.12 1.75 5.0 SYMPATRIC SOCKEYE + STICKLEBACKS Fig. 8. Frequency distributions (as %) of zooplankton b y size in enclosures before (a, c, e) and after (b, d, f) treatment with a l l o p a t r i c and sympatric groupings of underyearling sockeye and juvenile sticklebacks. (*Two replicates from sympatric treatments have been pooled.) 34 t reatment was not s i g n f i c a n t l y sma l l e r (K-S, df=2, x 2 =1.729, P>.05) but the p r opo r t i on of zoop lank te rs >.5 mm was s i g n i f i c a n t l y l e s s between the s t a r t and f i n i s h of the experiment ( d i f f e r en ce between p ropo r t i ons t e s t , Zar 1974; df= », Z=2.1059, .05>P>.02). The s h i f t to sma l l e r zoop lank te rs in the f i s h treatment enc losures was l e s s dramat ic than i n Experiment 1 but r e s u l t e d from a s i m i l a r phenomenon: removal of the l a rge copepods (average 39% d e c l i n e , see F i g . 8, 9 ) . S l i g h t changes in t o t a l zoop lankton abundance were noted i n a l l three f i s h t reatments (see Table 8 ) . At the end of the treatment p e r i o d , the ranked mean s i z e s of zoop lankton were very s i m i l a r to each o the r . Three of the fou r f i s h - t r e a tmen t enc losures were not s i g n i f i c a n t l y d i f f e r e n t from each o ther w i th respect to s i z e and the f ou r th enc l o su re , the a l l o p a t r i c s t i c k l e b a c k t reatment , was not s t a t i s t i c a l l y d i f f e r e n t from the a l l o p a t r i c sockeye treatment but i t was s i g n i f i c a n t l y d i f f e r e n t from the two sympatr i c f i s h t rea tments . Wi th in the f i s h l e s s con t ro l enc l o su re , zooplankton s i z e s decreased s l i g h t l y (mean s i z e 0.53 to 0.49 mm) but s i g n i f i c a n t l y (K-S, df=2, P<.001) ( F i g . 10 ) . Un l i k e the f i s h treatment en c l o su r e s , the decrease was due to a d i s p r opo r t i o na t e inc rease i n the number of Bosmina and not a decrease in abso lu te copepod abundance (copepods inc reased 10%). Dur ing the t ime covered by Exp. 2, the s i z e d i s t r i b u t i o n of zoop lankton was not s i g n i f i c a n t l y sma l l e r i n the lake (K-S, df=2, x 2 =0; P>.05), however, there was a s h i f t i n taxonomic compos i t ion from B. coreqoni to D i a c y c l o p s . Abso lute copepod abundance in the lake inc reased by 190% ( F i g . l l c - d ) . At the end of the f i s h treatment p e r i o d , zoop lankton from the f i s h l e s s con t ro l were s i g n i f i c a n t l y l a r g e r than those from the lake which were s i g n i f i c a n t l y l a r g e r than those from the f i s h treatment enc losures BEFORE S O C K E Y E STICKLEBACK S O C K E Y E & ( X = 5 l m m ) ( X = 3 8 m m ) S T I C K L E B A C K S KEY -ST* ROTIFERS N A U P L I I % - BOSMINA OTHER C -C L A D O C E R A N S 'W - C O P E P O D S if MISCELLANEOUS R - LARGE PREY U - UNKNOWN F i g . 9. F requency d i s t r i b u t i o n s (as %) o f z o o p l a n k t o n by t a x a i n e n c l o s u r e s b e f o r e ( a , c , e) and a f t e r ( b , d , f ) t r e a t m e n t w i t h a l l o p a t r i c and s y m p a t r i c g r o u p i n g s o f u n d e r y e a r l i n g sockeye and j u v e n i l e s t i c k l e b a c k s . (*Two r e p l i c a t e s f r o m s y m p a t r i c t r e a t m e n t s have been p o o l e d . ) o o o IM = 317 B E F O R E T R E A T M E N T N = 2 9 2 ( c ) N = 1879 X = 0 . 2 6 X = 0 .32 N = 5 0 9 A F T E R 2 0 D A Y S T R E A T M E N T (b) N = 2 7 9 (d) N = 1581 (f) X = 0 . 3 6 X = 0 . 2 6 .25 45 .65 .85 1.12 1.75 5.0 F I S H L E S S C O N T R O L ( 0 F I S H / H A ) .25 4 5 .65 8 5 1.12 1.75 5.0 .25 4 5 .65 .85 1.12 1.75 5.0 SIZE ( m m ) L A K E S A M P L E ( 2 7 0 0 F I S H / H A ) F I S H T R E A T M E N T S ( 4 0 0 0 F I S H / H A ) F i g . 1 0 . Compar ison o f f r e q u e n c y d i s t r i b u t i o n s o f z o o p l a n k t o n by s i z e sampled f r o m a f i s h l e s s e n c l o s u r e ( a , b ) , Kennedy Lake ( c , d ) , and p o o l e d r e s u l t s f r o m e n c l o s u r e s t r e a t e d w i t h a l l o p a t r i c and s y m p a t r i c c o m b i n a t i o n s o f u n d e r y e a r l i n g sockeye and j u v e n i l e s t i c k l e b a c k s ( e , f ) . 37 B E F O R E # / M 3 = 5 0 5 4 0 # / M 3 = 2 2 4 2 9 M E A N # / M 3 = 13711 N» 317 N= 2 9 2 N= 1879 O O o x A F T E R 2 0 DAYS to E # / M 3 = 8 9 6 4 2 # / M 3 = 22881 M E A N #/ M 3 = 15160 FISHLESS C O N T R O L L A K E S A M P L E F I S H T R E A T M E N T S (0 F I S H / H A ) ( 2 7 0 0 F I S H / H A ) ( 4 0 0 0 F I S H / H A ) F i g . 1 1 . Compar ison o f f r e q u e n c y d i s t r i b u t i o n s o f o f z o o p l a n k t o n by t a x a sampled f r o m a f i s h l e s s e n c l o s u r e ( a , b ) , Kennedy Lake ( c , d ) , and p o o l e d r e s u l t s f r o m e n c l o s u r e s t r e a t e d w i t h a l l o p a t r i c and s y m p a t r i c c o m b i n a t i o n s o f , u n d e r y e a r l i n g sockeye and j u v e n i l e s t i c k l e b a c k s ( e , f ) . 38 (Tukey's Procedure - Table 4b ) . Fu r t he r , as was the case i n Exp. 1, h igher f i s h d en s i t y i n t h i s experiment was a s soc i a t ed w i th zooplankton communities c ha r a c t e r i z ed by p r o g r e s s i v e l y sma l l e r mean body s i z e , i n d i c a t i n g f i s h p redat ion was respons ib l e f o r the change i n zooplankton communities from the f i s h - t r ea tmen t enc l o su re s . From s t a r t to end of the exper iments , t o t a l zooplankton abundance inc reased 77% in the f i s h l e s s enc losure whereas i t decreased i n the f i s h t reatment enc l o su res , f u r t h e r i n d i c a t i n g tha t f i s h p reda t i on was l a r g e l y r e spons i b l e f o r the observed changes i n the zoop lankton community (Table 7 ) . Tota l zooplankton abundance d i d not change i n the lake which conta ined a f i s h d en s i t y between tha t i n the f i s h treatment enc losures and the f i s h l e s s con t ro l enc l o su re . Experiment 3. sockeye and s t i c k l e b a c k f r y (May 7-26, 1983) At the s t a r t of t h i s exper iment, s i g n i f i c a n t d i f f e r e n c e s e x i s t e d i n s i z e s of zooplankton among the enc losures and the l a k e , but r e s u l t s of Tukey's Procedure (Table 4 c ) , i n d i c a t e tha t p a r t i c u l a r groups of means were not s i g n i f i c a n t l y d i f f e r e n t from each o the r . F i sh treatment enc losures conta ined s i m i l a r communities of smal l zoop lank te r s dominated by r o t i f e r s , n a u p l i i and smal l copepods (Table 9 ) . The lake sample was s i m i l a r to the enc losures but had approx imate ly h a l f the t o t a l zoop lankton dens i t y and fewer copepods (Table 10) . J u s t p r i o r to the s t a r t o f t h i s experiment a breach in the f i s h l e s s c on t r o l enc losure permi t ted the ent ry of f ou r s t i c k l e b a c k s (1600 f i sh»ha - 1 ) such tha t the mean s i z e of zooplankton in the f i s h l e s s enc losure was s i m i l a r to the other enc l o su res , but B. c o regon i , not 39 Table 9a . Zooplankton community c h a r a c t e r i s t i c s i n enc losures before and a f t e r t reatment w i t h a l l o p a t r i c groupings of sockeye and s t i c k l e b a c k f r y . A l l o p a t r i c f i s h t reatments Sockeye S t i c k l e b a c k s Mean Major No. Mean Major No. No./m3 s i z e taxa copepods No./m3 s i z e taxa copepods Before 23408 0.27 Naup l i i 7790 (May 7) R o t i f e r s 24795 0.22 R o t i f e r s 5586 Naupl i i A f t e r 3022 0.29 Naup l i i 1071 (May 26) 2902 0.27 Naup l i i 962 Bosmina Summary 87% decrease in t o t a l abundance 88% decrease i n t o t a l abundance 86% decrease in copepod numbers 83% decrease i n copepod numbers 40 Table 9b. Zooplankton community c h a r a c t e r i s t i c s i n enc losures before and a f t e r t reatment w i th sympatr ic mix tures of sockeye and s t i c k l e b a c k f r y . Sympatr ic f i s h t reatments Rep l i c a t e 1 Rep l i c a t e 2 Mean Major No. Mean Major No. No./m3 s i z e taxa copepods No./m3 s i z e taxa copepods Before 21680 0.24 R o t i f e r s 6380 (May 7) Naup l i i A f t e r 1184* 0.21* N a u p l i i * 253* (May 26) Summary 95% decrease in t o t a l abundance 96% decrease i n copepod numbers 30362 0.24 R o t i f e r s 7353 Naupl i i 7052 0.26 Naup l i i 1765 Bosmina 77% decrease i n t o t a l abundance 76% decrease i n copepod numbers • Invas ion of 30 f i s h , May 20-26, r e s u l t s of r e p l i c a t e s not poo led . T a b l e 10. Z o o p l a n k t o n communi ty c h a r a c t e r i s t i c s i n e n c l o s u r e s b e f o r e and a f t e r t r e a t m e n t w i t h v a r i o u s d e n s i t i e s o f sockeye ond s t i c k l e b a c k f r y . 0 f i s h / h e c t a r e 5400 f i s h / h e c t a r e 9600 f i s h / h e c t a r e ( f i s h l e s s c o n t r o l ) ( l a k e s a m p l e ) ( f i s h t r e a t m e n t s p o o l e d * ) Mean No. Mean Mean No. Mean Mean No. Time No. /m3 s i z e t a x a copepods No. /m3 s i z e t a x a copepods No . /m3 s i z e t a x a copepods B e f o r e 72254 0 . 2 2 Bosmina 2280 12525 0 . 2 1 May 7 A f t e r n o t a p p l i c a b l e owing t o f i s h 30267 0 . 2 7 May 27 i n v a s i o n R o t i f e r 1506 26188 0 . 2 4 R o t i f e r 20729 N a u p l i i N a u p l i i Bosmina 5947 4325 0 . 2 7 N a u p l i i 3798 N a u p I i i Bosmina Summary ? 142% i n c r e a s e in t o t a l abundance 83% d e c r e a s e i n t o t a l abundance 295% i n c r e a s e in copepods 82% d e c r e a s e i n copepods • R e p l i c a t e 1 o f s y m p a t r i c t r e a t m e n t e x c l u d e d due t o i n v a s i o n o f 30 f i s h May 2 0 - 2 6 . 42 n a u p l i i and r o t i f e r s , was numer i ca l l y dominant. The t o t a l abundance of zoop lankton in the breached " f i s h l e s s " con t ro l was s t i l l d r a m a t i c a l l y h igher than i n o ther enc losures and the lake (Table 10) , as the l e ve l of f i s h p reda t i on was low. A f t e r t reatment , s i z e d i s t r i b u t i o n s of zooplankton w i t h i n f i s h t reatment enc losures were not s i g n i f i c a n t l y sma l l e r (K-S, df=2, x 2 ranges 0-3 .13; P>.05 f o r a l l compar isons) . A l l enc losures con ta i n i ng f i s h (mean s i z e of sockeye 28 mm, mean s i z e of s t i c k l e b a c k s 33 mm) d i sp l ayed a d e c l i n e in o v e r a l l zooplankton abundance (average 83%, Table 9a,b) a long w i th a dramat ic decrease i n r o t i f e r numbers, which r e su l t e d i n a s l i g h t i nc rease i n the zooplankton mean s i z e ( F i g . 12) . The number of copepods de c l i n ed (average 82%) in p ropo r t i on to the o v e r a l l zooplankton decrease (Table 9; F i g . 13) . The s i z e frequency d i s t r i b u t i o n of zooplankton from the lake d i d not change s i g n i f i c a n t l y (K-S, df=2, x 2 =.0059; P<.001) from the s t a r t to the end of the exper iment. The mean s i z e inc reased s l i g h t l y from 0.21 mm to 0.27 mm ( F i g . 14c-d) and B. coregoni dominated numer i ca l l y ( F i g . 15d). Un l i ke the f i s h treatments however, dur ing the same pe r i od samples from the lake showed a l a rge inc rease in t o t a l zoop lankton abundance (142%) and an even g rea te r inc rease in copepod numbers (295%) (Table 10) . I d e a l l y , to ensure tha t d i f f e r e n c e s i n numerical abundances of zoop lankton between f i s h treatments and the lake were due to d i f f e r e n c e s i n f i s h abundance and not other seasonal t rends in zooplankton p r o d u c t i v i t y r e s u l t i n g from a r t i f i c i a l l y enc l o s i ng the zooplankton community, i t i s necessary to i n c lude comparisons w i th the f i s h l e s s c o n t r o l . Un fo r tuna te l y these data are unava i l ab l e as 15 f i s h 43 60 - i N= 478 .25 .45.65 .85 1.12 L75 5.0 ALLOPATRIC SOCKEYE B E F O R E T R E A T M E N T N = 567 (c) = 0.22 A F T E R T R E A T M E N T N = 474 (d) 0.27 I I I r — .25 .45 .65 .85 1.12 1.75 5.0 S I Z E ( m m ) ALLOPATRIC STICKLEBACKS N = 406 (e) X = 0.24 N = 420 (f) X = 0,26 .25 .45 .65 .85 1.12 1.75 5.0 SYMPATRIC SOCKEYE + STICKLEBACKS F i g . 1 2 . F requency d i s t r i b u t i o n s (as %) o f z o o p l a n k t o n by s i z e i n e n c l o s u r e s b e f o r e ( a , c , e) and a f t e r ( b , d , f ) t r e a t e m e n t w i t h a l l o p a t r i c and s y m p a t r i c g r o u p i n g s o f sockeye f r y and j u v e n i l e s t i c k l e b a c k s . ( * R e p l i c a t e s y m p a t r i c t r e a t m e n t s were no t p o o l e d as 30 f i s h i n v a d e d e n c l o s u r e , May 2 0 - 2 6 . ) B E F O R E S O C K E Y E S T I C K L E B A C K S O C K E Y E S ( X = 2 8 m m ) ( X = 3 3 m m ) S T I C K L E B A C K S K E Y ? - R O T I F E R S C O P E P O D S N A U P L I I •P t. c -B O S M I N A O T H E R C L A D O C E R A N S R -U -M I S C E L L A N E O U S L A R G E P R E Y U N K N O W N F i g . 13 . F requency d i s t r i b u t i o n s (as %) o f z o o p l a n k t o n by t a x a i n e n c l o s u r e s b e f o r e ( a , c , e) and a f t e r ( b , d , f ) t r e a t m e n t w i t h a l l o p a t r i c and s y m p a t r i c g r o u p i n g s o f sockeye f r y and j u v e n i l e s t i c k l e b a c k s . ( * R e p l i c a t e s y m p a t r i c t r e a t m e n t s were no t p o o l e d as 30 f i s h i n v a d e d e n c l o s u r e , May 2 0 - 2 6 . ) 45 O O O ro N = 951 B E F O R E T R E A T M E N T N = 311 (c) N = 1451 = 0 . 2 2 X - 0.21 or UJ CO i o -6-2-A F T E R 19 D A Y S T R E A T M E N T (b) N = 3 4 8 NOT AVAILABLE (d) = 0 . 27 N = 1380 (f) X = 0 . 27 .25 45 .65 .85 1.12 1.75 5.0 .25 4 5 .65 .85 1.12 1.75 5.0 .25 4 5 .65 .85 i.12 1.75 5.0 SIZE (mm) F I S H L E S S C O N T R O L ( 0 F I S H / H A ) L A K E S A M P L E ( 5 0 0 0 F I S H / H A ) FISH T R E A T M E N T S ( 9 6 0 0 F I S H / H A ) F i g . 14 . Compar ison o f s i z e f r e q u e n c y d i s t r i b u t i o n s o f z o o p l a n k t o n sampled f r o m a f i s h l e s s e n c l o s u r e ( a , b ) , Kennedy Lake ( c , d ) , and p o o l e d r e s u l t s f r o m e n c l o s u r e s t r e a t e d w i t h a l l o p a t r i c and s y m p a t r i c c o m b i n a t i o n s o f sockeye f r y and j u v e n i l e s t i c k l e b a c k s ( e , f ) . (*A11 f i s h t r e a t m e n t s e x c e p t one r e p l i c a t e o f a s y m p a t r i c w h i c h was i n v a d e d by 3 0 f i s h , May 2 0 - 2 6 , were p o o l e d . ) 46 B E F O R E O O o # / M ° = 7 2 2 5 4 N = 9 5 1 (a) 0 III if # / M J N = 3 1 1 1 2 5 2 5 (c) M E A N # / M = 2 6 1 8 8 N = 1 4 5 1 (e) cr LU QD I 2 1 0 8 H 6 4 -2 -0 (b) N O T A V A I L A B L E F I S H L E S S C O N T R O L ( 0 F I S H / H A ) A F T E R 19 D A Y S # / M 3 = 3 0 2 6 7 N = 3 4 8 ( d ) L A K E S A M P L E ( 5 0 0 0 F I S H / H A ) M E A N y ^ / M = 4 3 2 5 N = I 3 8 0 ( f ) W t. c F I S H T R E A T M E N T S ( 9 6 0 0 F I S H / H A ) F i g . 15 . Compar ison o f f r e q u e n c y d i s t r i b u t i o n s o f t a x o n o m i c c l a s s e s o f z o o p l a n k t o n sampled f r o m a f i s h l e s s e n c l o s u r e ( a , b ) , Kennedy Lake ( c , d ) , and p o o l e d r e s u l t s f r o m e n c l o s u r e s t r e a t e d w i t h a l l o p a t r i c and s y m p a t r i c c o m b i n a t i o n s o f sockeye f r y and j u v e n i l e s t i c k l e b a c k s ( e , f ) . (*A11 f i s h t r e a t m e n t s e x c e p t one r e p l i c a t e o f a s y m p a t r i c t r e a t m e n t wh ich was i n v a d e d by 30 f i s h , May 2 0 - 2 6 , were p o o l e d . ) (approx imate ly 6000 sticklebacks»ha - 1) invaded the enc losure a t some time a f t e r i n i t i a t i o n of the exper iment, and compromised the s t a tus of the f i s h l e s s con t ro l r e s u l t s . Regard less , i t appears f i s h p reda t i on had an e f f e c t on zooplankton community s i z e as a t the end of the treatment s i z e s of zoop lankton sampled from f i s h treatment enc losures were no longer s i g n i f i c a n t l y d i f f e r e n t from each other ( i . e . a f t e r 19 days of t r ea tmen t ) , and they were s i g n i f i c a n t l y sma l l e r than zooplankton from the lake which had a lower dens i t y of f i s h (Tukey's Procedure - Table 4 c ) . I t i s u n l i k e l y tha t the major reduc t ions i n zooplankton numbers tha t occur red i n f i s h treatment enc losures were due to enc losure e f f e c t s a lone as i n o ther experiments major reduc t ions i n t o t a l zoop lankton abundance on ly occur red when f i s h were present i n the enc l o su re s . Prey s i z e consumed v s . prey s i z e a v a i l a b l e Prey items consumed by f i s h on the l a s t day of treatment v s . zoop lankton remaining i n the enc losures p rov ide es t imates of p redato r s e l e c t i v i t y . Because zooplankton communities were s t r u c t u r ed s i m i l a r l y i n a l l f i s h treatments of each experiment ( i . e . a l l f i s h t reatments a t the end of a p a r t i c u l a r experiment had s i m i l a r s i z e s and spec ies of zoop l ank ton) , f i s h d i e t s were pooled across a l l f i s h t reatments and f i s h spec ies i n each exper iment. The mean s i z e of food consumed by sockeye y e a r l i n g s and adu l t s t i c k l e b a c k s as we l l as by underyear l i ng sockeye and s t i c k l e b a c k s was l a r g e r than the mean s i z e of zooplankton a v a i l a b l e ( F i g . 16a, 16b, r e s p e c t i v e l y ) . Thus, i n these exper iments , where the zooplankton communities had a l ready s h i f t e d to sma l l e r forms over 12 and 20 day per iods r e s p e c t i v e l y , f i s h consumed the l a r g e s t prey items Z O O P L A N K T O N A V A I L A B L E n • 1888 (a) x • 0.22 F i g . 16 . S i z e f r e q u e n c y d i s t r i b u t i o n s (as %) o f p r e y i t e m s consumed v s . p r e y i t e m s a v a i l a b l e h t o t h o s e f i s h i n e n c l o s u r e s a t t h e end o f t h e _ e x p e r i m e n t , ( a ) sockeye y e a r l i n g s and a d u l t s t i c k l e b a c k s ( b ) u n d e r y e a r l i n g sockeye and Z O O P L A N K T O N C O N S U M E D j u v e n i l e s t i c k l e b a c k s , and ( c ) sockeye f r y and N - 2 5 9 5 j u v e n i l e s t i c k l e b a c k s . ( * One r e n l i c a t e f r o m ( * F I S H * 3 4 ) s y m p a t r i c t r e a t m e n t d e l e t e d due t o i n v a s i o n o f ,0 9 5 30 f i s h , May 2 0 - 2 6 . ) m x • 0.95 x l U l l l u .25 .45 .65 .85 1.12 1.75 25.0 S I Z E (mm) Z O O P L A N K T O N A V A I L A B L E n • 1581 (b ) x • 0.26 Z O O P L A N K T O N C O N S U M E D n • 3164 (* f i sh - 23) 1 x • 0.53 .25 .45 .65 .85 1.12 1.75 >5.0 SIZE (mm) Z O O P L A N K T O N A V A I L A B L E n « 1380 : o * I., Z O O P L A N K T O N C O N S U M E D N • 2 8 4 9 !)C FISH • 2 9 I • • • • • I I .45 .55 .35 l.!2 SIZE (mm) a v a i l a b l e . S i z e s e l e c t i v e f i s h p reda t i on appears to have been the mechanism by which zooplankton s i z e s were reduced. With the except ion of a few la rge prey i tems, f i s h i n the sockeye and s t i c k l e b a c k f r y experiment consumed prey roughly in p ropo r t i on to t h e i r a v a i l a b i l i t y ( F i g . 16c) . A c c o r d i ng l y , f i s h p redat ion appears to have reduced the t o t a l abundance of zoop lankton in t e s t enc losu res (compare F i g s . 14c, f ) , w i thout app re c i ab l y i n f l u e n c i n g spec ies or s i z e composi t ions ( F i g s . 14, 15) . 3.4 D i s cus s i on 3.4.1 The e f f e c t of sockeye and s t i c k l e b a c k s on the s i z e s and spec ies compos i t ion of zooplankton communities W i th in each of the three experiments tha t u t i l i z e d three s i z e c a t ego r i e s of sockeye and l i m n e t i c s t i c k l e b a c k s , zoop lankton communities were i n f l uenced s i m i l a r l y by sockeye and s t i c k l e b a c k s , r ega rd less of whether they were i n a l l o p a t r i c or sympatr ic environments, i n d i c a t i n g tha t they a f f e c t each o t h e r ' s food supply i n the same way. In l e s s than three weeks, the mean s i z e of zooplankton decreased in a l l f i s h - t r ea tmen t enc losures con ta i n i ng sockeye y e a r l i n g s and adu l t s t i c k l e b a c k s (Experiment 1) as we l l as underyear l i ngs of both spec ies (Experiment 2 ) . A s soc i a t ed w i th t h i s s i z e s h i f t was a decrease in abso lu te numbers of copepods (90% and 39% r e s p e c t i v e l y ) and inc reases in numbers of n a u p l i i and r o t i f e r s . R e l a t i v e to the f i s h l e s s c o n t r o l , t o t a l zooplankton abundance in f i s h treatment enc losures decreased moderate ly (average 55%) i n the y e a r l i n g experiment and inc reased s l i g h t l y (average 11%) i n the unde ryea r l i ng exper iment. Zooplankton mean s i z e was not a f f e c t e d by f i s h p redat ion from f r y of e i t h e r spec ies (Experiment 3) a l though abso lu te abundances of a l l zoop lank te rs were g r e a t l y decreased (average 83%). In g ene r a l , r e s u l t s from a l l three exper iments support the view tha t both the quan t i t y and q u a l i t y of zoop lankton p o t e n t i a l l y a v a i l a b l e as food to sockeye and s t i c k l e b a c k s are sub jec t to r ap id reduc t i ons under e x p l o i t a t i o n by e i t h e r s pe c i e s . A d d i t i o n a l support tha t f i s h p redat ion was r e spons i b l e f o r the observed changes in zooplankton samples c o l l e c t e d from the f i s h treatment enc losures i s ev ident as s i m i l a r changes were not observed i n the f i s h l e s s c o n t r o l . In con t r a s t to the f i s h treatment enc l o su re s , zoop lankton samples from the f i s h l e s s con t ro l enc losures in Experiment 1 and 2 showed on ly s l i g h t decreases in zooplankton s i z e , minor changes in copepod abundance and moderate to high inc reases in t o t a l zoop lankton abundance dur ing the treatment p e r i o d . In the f r y experiment a f i s h l e s s con t ro l was unava i l a b l e , but , lower t o t a l zooplankton and copepod abundances in the f i s h treatments as compared w i th the lake suggest tha t h igher f i s h d en s i t y in the enc losures (9600 h a - 1 v s . 5400 h a - 1 ) had dep le ted prey d e n s i t i e s . Fu r the r evidence of the dramat ic e f f e c t of f i s h p reda t i on on zooplankton s i z e and abundance was apparent from the heavy f i s h i nvas i on of the " f i s h l e s s " enc losure ( Ap r i l 27-June 16, see F i g . 3 ) . During t h i s t ime zooplankton abundances dropped from a high of 72,000/m 3 to a low of 5700/m 3 and from a mean s i z e of 0.55 mm to 0.23 mm. Except when invaded, the f i s h l e s s con t ro l conta ined s i g n i f i c a n t l y more l a r g e - s i z e d zoop lank te r s than the other enc l o su res , l e av i ng l i t t l e doubt of the s i g n i f i c a n c e of the e f f e c t of f i s h p redat ion on zooplankton community s t r u c t u r e . Most of the changes i n the s i z e d i s t r i b u t i o n of zooplankton in the con t ro l enc losure occurred through l a rge inc reases in abundances of Bosmina ( F i g s . 6-7, 10-11) . Increases in Bosmina abundance may be a s so c i a t ed w i t h the secondary e f f e c t s of reduced f i s h p r e d a t i o n . S i z e - s e l e c t i v e feed ing behaviour by sockeye and s t i c k l e b a c k s removes a d i s p r opo r t i o na t e number of the l a r g e r a v a i l a b l e taxa (eg. copepods) as we l l as the l a r g e s t i n d i v i d u a l s w i t h i n the sma l l e r taxa (eg. Bosmina). In the absence of f i s h p r eda t i on , more of the l a r g e r s i z ed Bosmina would s u r v i v e to m a t u r i t y , and due to t h e i r shor t l i f e span and h igh f e cund i t y numerical abundances would q u i c k l y i n c r e a se . 3.4.2 Compet i t ion in o l i g o t r o p h i c lakes Three cond i t i on s are necessary to demonstrate the occurrence of compet i t i on (P ianka 1983) and are o u t l i n e d below w i th respec t to l i m n e t i c s t i c k l e b a c k s competing w i th j u v e n i l e sockeye f o r f ood . (1) Sockeye and l imne t i c s t i c k l e b a c k s must feed on a common resource tha t i s i n shor t supp ly . Research by Krog ius and Krokh in (1956b), Greenbank and Nelson (1959 as reported by Hartman and Burgner 1972), Rogers (MS 1961), Rogers (1968), Eggers e t a l . (1978), and Hyatt (unpub l . d a t a ) , p rov ides ample evidence tha t sympatr ic popu la t i ons of sockeye and l i m n e t i c dwe l l i ng s t i c k l e b a c k s feed on s i m i l a r food resources , p a r t i c u l a r l y on copepods ( e . g . , Cyc lops spp.) and c ladocerans ( e . g . , Bosmina spp.) as we l l as emergent i n s e c t s . S tud ies by B r e t t (1971, 1983) and Hyatt and Stockner (1985) i n d i c a t e tha t sockeye growth in t y p i c a l B.C. sockeye lakes i s l i m i t e d due to an i n s u f f i c i e n t food supp ly . B r e t t (1971) demonstrated tha t ac tua l weights of j u v e n i l e sockeye in Babine Lake are approx imate ly 105% l e s s than the weights tha t sockeye were c a l c u l a t e d to be capable of a ch i ev ing under a s i m i l a r temperature regime but fed a f u l l r a t i o n , i n d i c a t i n g tha t the quan t i t y of food a v a i l a b l e in the lake was f a r l e s s than the amount needed to s a t i a t e the f i s h . Fur ther he suggested tha t zoop lankton popu la t i ons are reduced and f u r t h e r cropping i s e n e r g e t i c a l l y non-economical f o r sockeye. B r e t t (1983, p. 5) gene ra l i z ed tha t " . . . the f r e shwate r , l a c u s t r i n e per iod of the f i n g e r l i n g i s one of re tarded growth and r e s t r i c t e d r a t i o n . " (2) The amount of food tha t i s a v a i l a b l e i s determined by consumer use, i . e . s t i c k l e b a c k s must q u a l i t a t i v e l y or q u a n t i t a t i v e l y a f f e c t the food a v a i l a b l e to sockeye and v i c a v e r s a . The r e s u l t s i n d i c a t e tha t s t i c k l e b a c k s and sockeye are " t r oph i c analogues" such tha t s t i c k l e b a c k can a f f e c t the type and amount of food a v a i l a b l e to sockeye by d i r e c t consumption of the resource . The express ion " t r o ph i c analogues" i s not meant to imply tha t sockeye and s t i c k l e b a c k s have i d e n t i c a l feed ing h a b i t s , g iven the s p e c i e s - s p e c i f i c d i f f e r e n c e s i n body morphology ( e . g . mouth shape, gape s i z e and eye d iameter) and swimming behav iour . One spec ies may be more e f f i c i e n t than the o the r , but sockeye and s t i c k l e b a c k s f un c t i o n as t r oph i c analogues as they a f f e c t each o t h e r ' s food supply w i t h i n a very shor t pe r i od of t ime (12-20 days ) , r e l a t i v e to the amount of t ime they feed in a growing season (approx imate ly 200 d a y s ) . The r e s u l t s i n d i c a t e tha t these f i s h are p o t e n t i a l compe t i t o r s . The i n f l uence of these f i s h on the s i z e and spec ies composi t ion of the zooplankton seems to increase w i th f i s h s i z e . Sockeye and s t i c k l e b a c k s l a r g e r than the f r y stage a f f e c t both the quan t i t y and q u a l i t y ( i . e . s i z e ) of zooplankton whereas f r y a f f e c t on ly the q u a n t i t y . F i sh i n exper iments u t i l i z i n g the l a r g e s t f i s h ( i . e . sockeye y e a r l i n g s and adu l t s t i c k l e b a c k s ) had a g rea te r e f f e c t on the q u a l i t y of the zooplankton than d i d f i s h in the experiment w i th underyea r l i ng sockeye and s t i c k l e b a c k s . However, f a i l u r e of underyear l i ng and f r y stages to reduce s i z e s of zooplankton as much as y e a r l i n g sockeye and adu l t s t i c k l e b a c k s may be an a r t i f a c t of the sma l l e r s i z e s of zoop lankton present a t the s t a r t o f these experiments ( i . e . there was very l i t t l e scope f o r an o v e r a l l reduc t ion in s i z e to o c c u r ) . I f these r e s u l t s are not an a r t i f a c t , t h i s suggests the p o t en t i a l f o r compet i t i on to inc rease w i t h f i s h s i z e . (3) The sockeye 's f i t n e s s must be a f f e c t ed by the resource shor tage . I f l i m n e t i c s t i c k l e b a c k s a c t u a l l y compete w i th sockeye, then the reduced a v a i l a b i l i t y of food must reduce sockeye f i t n e s s . In o l i g o t h r o p h i c l a k e s , the weight tha t sockeye smolts ach ieve in 1 year of growth i s p o s i t i v e l y i n f l uenced by the amount of zoop lankton a v a i l a b l e per l i m n e t i c f i s h (Hyatt and Stockner 1985). Reviewing the work of o t he r s , Hyatt and Stockner argued tha t inc reased marine s u r v i v a l and reduced age a t re tu rn of adu l t s are c o r r e l a t e d w i th an inc rease i n s i z e a t which smolts migrate ( u sua l l y a f t e r 1 year in f reshwater) to the ocean environment. Thus, sockeye f i t n e s s seems to be decreased by reduc t i ons i n w i t h i n - l a k e growth tha t have r e su l t ed from lowered food a v a i l a b i l i t y . A l l o f the cond i t i ons necessary to demonstrate compet i t i ve i n t e r a c t i o n s between j u v e n i l e sockeye and l i m n e t i c s t i c k l e b a c k s in o l i g o t r o p h i c l akes have been met. Sockeye and s t i c k l e b a c k s feed on a common resource , zoop lankton , e s p e c i a l l y copepods and la rge c l adoce rans , and t h i s resource i s i n shor t supp ly . The zooplankton a v a i l a b l e to sockeye i s i n f l uenced by the presence of s t i c k l e b a c k s , as both spec ies are s i z e - s e l e c t i v e predators and consume the l a r g e s t zoop lank te rs f i r s t , 54 l e a v i n g t h e s m a l l e r z o o p l a n k t e r s . F o r e xamp l e , copepods p r e s e n t i n f i s h t r e a t m e n t e n c l o s u r e s a t t h e s t a r t o f t h e e x p e r i m e n t were no l o n g e r p r e s e n t a t t h e end o f t h e t r e a t m e n t . Because t he f i s h were s i z e - s e l e c t i v e p r e d a t o r s , s m a l l p r e y i t ems were a s s e s s e d as e n e r g e t i c a l l y l e s s v a l u a b l e t h an l a r g e p r e y i t e m s . A d d i t i o n a l l y , s o c k eye and s t i c k l e b a c k s r educe t h e abundance o f z o o p l a n k t o n . The r e d u c t i o n i n t h e e n e r g e t i c a l l y most v a l u a b l e p o r t i o n o f t h e f o od s u p p l y and r educed abundance o f z o o p l a n k t o n w i l l p r o b a b l y r e s u l t i n r educed s o c keye f i t n e s s . I c o n c l u d e t h a t s o c keye and l i m n e t i c s t i c k l e b a c k s compete e x p l o i t a t i v e l y f o r f o o d bu t add t h a t t h e i n t e r a c t i o n i s d y n a m i c ; i n t e n s i t y o f i n t e r a c t i o n i n c r e a s e s w i t h t h e s i z e and o v e r a l l abundance o f f i s h e s s t u d i e d . 3 . 4 . 3 S t i c k l e b a c k s and s o ckeye p r o d u c t i v i t y The f i n d i n g s o f t h i s s t u d y a r e r e l e v e n t t o s t u d i e s o f s o c keye p r o d u c t i o n i n c o a s t a l B .C . l a k e s . So ckeye p r o d u c t i v i t y i s l i n k e d t o l a k e p r o d u c t i v i t y , w h i c h may v a r y due t o n a t u r a l , u n c o n t r o l l e d e v e n t s ( e . g . y e a r l y c l i m a t i c v a r i a t i o n s ) , o r due t o c o n t r o l l e d e v e n t s such as a d d i t i o n s o f f e r t i l i z e r s ( S t o c k n e r and H y a t t 1 9 8 4 ) . Due t o d i f f e r e n c e s i n t h e i r l i f e c y c l e s , s t i c k l e b a c k s have a s h o r t e r g e n e r a t i o n t i m e t han s o c keye and t h e r e f o r e a r e p o t e n t i a l l y c a p a b l e o f a more r a p i d n u m e r i c a l r e s p o n s e t o changes i n l a k e p r o d u c t i v i t y . N u m e r i c a l r e s p o n s e s by s o ckeye w i l l no t o c c u r f o r two t o f o u r y e a r s . Through d i r e c t c o m p e t i t i o n f o r f o o d r e s o u r c e s , s t i c k l e b a c k s , e s p e c i a l l y t h o s e i n h a b i t i n g t h e l i m n e t i c zone and f e e d as t r o p h i c a n a l o g u e s t o s o c k e y e , have t he p o t e n t i a l t o r educe o r e l i m i n a t e ne t b e n e f i t s o f i n c r e a s e d l a k e p r o d u c t i v i t y t o s o c k e y e v i a t h e i r r a p i d numer i c r e s p o n s e s . The f a c t o r s wh i c h t i p t h e compet i t i ve edge in favour of one spec ies or the o ther are not c u r r e n t l y we l l understood. However, r e s u l t s from the present study i n d i c a t e tha t i n many B.C. coas ta l l akes the zooplankton communities of the l i m n e t i c zone w i l l be i n f l uenced by both j u v e n i l e sockeye and s t i c k l e b a c k s . The p roduc t i on dynamics of these two spec ies are probab ly in terdependent . 4.0 NICHE SHIFTS BY SOCKEYE AND STICKLEBACKS 4.1 I n t r oduc t i on The resources a spec ies uses, where i t f i n d s them, and the methods by which i t harvests them, l o o s e l y de f i ne i t s n i che , and n iche s h i f t s then are c on t r a c t i on s or expansions of a s pe c i e s ' n iche (Diamond, 1978). I n t e r s p e c i f i c compet i t i on may r e s u l t i n n iche s h i f t s by a spec ies as a compet i to r may a l t e r another spec i e s ' n iche through e x p l o i t a t i v e or behav ioura l mechanisms tha t a l t e r the resources a v a i l a b l e to tha t s p e c i e s . However, n iche s h i f t s are not on ly a consequence of compet i t i on but may a l s o be induced by p redat ion and a b i o t i c f a c t o r s (Connel l 1975). To demonstrate tha t a n iche s h i f t i s a d i r e c t consequence of compe t i t i on , the n iche of a spec ies ( e . g . type of food or h ab i t a t u t i l i z e d ) in an a l l o p a t r i c environment must con t r a c t when the spec ies i s p laced i n sympatry w i th i t s p o t e n t i a l compet i t o r . Other than the a d d i t i o n or removal of a compet i to r , environmental f a c t o r s must be the same i n both the a l l o p a t r i c and sympatr ic environments (P i anka , 1981). Fu r t he r , Hanson and Leggett (1985), contend tha t the observed n iche s h i f t must be accompanied by a reduc t ion i n f i t n e s s ( e . g . decreased growth r a t e , f e cund i t y or s u r v i v a l ) . In Sec t i on 3, evidence was presented to suggest tha t j u v e n i l e sockeye and th reesp ine s t i c k l e b a c k s compete e x p l o i t a t i v e l y by a f f e c t i n g the s tand ing crop and s i z e s t r u c t u r e of l imne t i c zooplankton communit ies. The purpose of t h i s study i s to t e s t f o r n iche s h i f t s by sockeye or s t i c k l e b a c k s as f u r t h e r evidence of i n t e r s p e c i f i c competiton and to e l abo ra te on the po s s i b l e mechanisms which mediate i n t e r a c t i o n s between these f i s h . Consequent ly , the d i e t s of sockeye and s t i c k l e b a c k s feed ing i n a l l o p a t r y and in sympatry are examined to t e s t whether sockeye or s t i c k l e b a c k s a l t e r t h e i r d i e t s ( i . e . e x h i b i t a n iche s h i f t ) when i n the presence of a p o t en t i a l compet i to r . 4.2 M a t e r i a l s and Methods The methods f o r these experiments were o u t l i n e d i n Sec t i on 2. B r i e f l y , three se ts of experiments were conducted to t e s t f o r d i e t n iche s h i f t s by a l l o p a t r i c and sympatr ic groupings of j u v e n i l e sockeye or th reesp ine s t i c k l e b a c k s e s t ab l i s h ed i n l imne t i c - zone enc l o su re s . The s i z e s of f i s h used d i f f e r e d f o r each exper iment. F i r s t , zoop lankton samples were c o l l e c t e d from enc losures and the lake to assess abundance of the food resou rces . Then, a l l o p a t r i c and sympatr i c groups of sockeye and s t i c k l e b a c k s were in t roduced i n t o the enc losures and were a l lowed to feed f o r 2-3 weeks and then removed. D i e t s of sockeye and s t i c k l e b a c k s were examined to determine i f e i t h e r spec ies e x h i b i t e d a s h i f t i n d i e t compos i t ion between a l l o p a t r i c and sympatr i c t rea tments . When f i s h were removed, zooplankton samples were aga in taken to appra i se the s t a t e of the food resource . Zooplankton abundances i n enc losures a t the end of the treatment per iods were assumed to be r ep r e sen t a t i v e of the po s s i b l e prey items a v a i l a b l e to the f i s h in the open water column at tha t t ime. To t e s t whether f i s h d i e t s were determined by presence or absence of a compet i tor or by v a r i a t i o n s in prey a v a i l a b i l i t y among enc l o su r e s , d i e t s of f i s h were then compared w i th the prey items a v a i l a b l e to them in the water column. A d d i t i o n a l l y , comparisons of prey s i z e s eaten v s . prey s i z e s a v a i l a b l e were used to t e s t f o r evidence of 58 po s s i b l e s i z e s e l e c t i v e feed ing behav ior by these f i s h . 4.21 Data a n a l y s i s To determine i f the s i z e s of prey items consumed w i t h i n each experiment were s i g n f i c a n t l y d i f f e r e n t among f i s h t rea tments , prey items from d i e t s of f i s h were ranked by i n c r ea s i ng s i z e and ana lysed w i th a one-way nonparametr ic ANOVA (same as a K r u s k a l - Wall i s t e s t ) . Tukey's comparison of means t e s t was app l i ed to a l l s i g n i f i c a n t d i f f e r e n c e s . Horn 's ove r l ap index (1966) was used to t e s t the hypothes i s tha t n iche over l ap ( i . e . d i e t ) between sockeye and s t i c k l e b a c k s was g r ea t e r i n a l l o p a t r y than i n sympatry. R = Z(XJ + y i ) log (XJ + y i ) - ZXJ log xi - Zyi log yi 0 (X + Y) log (X + Y) - X log X - Y log Y Ro i s the index of over lap where X and Y represent the t o t a l abundance of prey items consumed by sockeye and s t i c k l e b a c k s , r e s p e c t i v e l y , i n a p a r t i c u l a r treatment and x^nd y i represent the abundance of prey item i i n t h e i r d i e t s . S ince i nc reases i n zooplankton biomass per f i s h cou ld r e s u l t i n inc reased f i s h growth (Hyatt and S tockner , 1985), prey biomass i n guts was e s t ima ted . Weights of prey consumed by f i s h were es t imated on the bas i s of pub l i shed and unpubl ished length-we ight r e l a t i o n s h i p s of prey ( F i g . 17) . A one-way nonparametr ic ANOVA on the ranked es t imates of biomass per gut was performed f o r each experiment to assess i f the biomass of food per f i s h va r i ed among enc l o su res . Tukey's Procedure was app l i e d to a l l s i g n i f i c a n t means. 1-0.0 1.0 -0.1 -E X 2 o.oi H LU o 0 . 0 0 1 -N E O M Y S I S SIDA a D I A P H A N D S O M A 4 CHIRONIMID L A R V A E 1. 0 . 2 4 8 2 ( ( L - 0 . 7 4 ) / 3 . 4 7 9 ) 2 7 5 9 RANKIN (D.F0.) 2. I . 2 5 ( 0 . 0 0 5 I ( L ) 2 - 3 2 ) S M O C K 0 9 8 0 ) * 3. I . 2 5 ( 0 . 0 0 5 4 ( L ) 2 - 4 3 ) S M O C K ( 1 9 8 0 ) * 4. 0 . 0 9 2 ( L ) 2 - 4 4 9 E D M O N S O N 8i W I N B E R G (197 I ) + " 5. O . I 2 4 ( L ) 2 - 1 8 1 E D M O N S O N S W I N B E R G (1971) 6. 0 . 0 5 5 ( L ) 2 - 7 3 K L E K O W S K I 8> SHUSHKINA ( 1 9 6 6 ) 8 10 L E N G T H ( m m ) Fig. 17. Length-weight relationships of prey items. (Klekowski and Shushkina reference was taken from Edmondson and Winberg, 1971). (*Wet weight estimated: wet weight = 1.25 dry weight. * Diaphanosoma equation used for Sida.) 60 4.3 Results 4 .3 .1 Condi t ion of f i s h guts For each experiment, f i s h were removed from the enclosures on one of two consecutive days at about the same time, l a te morning to l a te afternoon in the y e a r l i n g and underyearl ing experiments (Exp. 1, 2 ) , and in the afternoon and night in the f r y experiment (Exp. 3 ) . Gut contents from the y e a r l i n g and underyearl ing experiments cons isted of p a r t l y d igested or non-digested food p a r t i c l e s that were r e a d i l y i d e n t i f i a b l e (see Tables 11, 12). The gut contents from the f r y experiment were in an advanced s ta te of d iges t ion but were s t i l l i d e n t i f i a b l e (Table 13). In the y e a r l i n g experiment, stomachs were greater than 1/2 f u l l with the exception of the f i s h in sympatry. A l l sympatric sockeye guts and a number of sympatric s t i ck leback guts were less than 1/2 f u l l (Table 11). A l l but one treatment of the underyear l ing experiment (Exp. 2) contained f i s h with guts that were at least 1/2 to 3/4 f u l l and only in the sympatric sockeye treatment were the major i ty of guts completely f u l l (Table 12). Guts from f r y (Exp. 3) var ied wi th in and across treatments but in general they were 1/2 f u l l or l ess (Table 13). 4 .3 .2 Diet niche s h i f t s : food consumed by a l l o p a t r i c v s . sympatric sockeye or s t i c k l e b a c k s . Experiment #1. sockeye year l i ngs X adul t s t i ck lebacks (Apr i l 15-Apri l 27) Sockeye: A niche s h i f t was observed for y e a r l i n g sockeye feeding in a l l o p a t r i c vs . sympatric treatments. Sockeye in a l l o p a t r y consumed s i g n i f i c a n t l y la rger prey items than sockeye in sympatry (Table 14a, - see F i g . 18a-b). The major i ty of the a l l o p a t r i c sockeye d i e t Table 11. D ie t c h a r a c t e r i s t i c s of sockeye y e a r l i n g s and adu l t s t i c k l e b a c k s in a l l o p a t r y and sympatry. Homogeneous se ts of means from Tukey 's a n a l y s i s on mean f i s h - d i e t weights are bracketed (see l a s t row of t a b l e ) . FISH TREATMENTS SOCKEYE STICKLEBACKS ALLOPATRIC SYMPATRIC ALLOPATRIC SYMPATRIC N (FISH) 8 7 9 10 MEAN FISH SIZE (mm) 63.4 64.7 54.0 50.9 TOTAL # PREY CONSUMED 98 207 1045 1612(1247)* MEAN PREY SIZE (mm) 6.09 0.85 0.72 0.81 # PREY ITEMS PER GUT 12.3 29.6 116.1 161.2 AVERAGE GUT FULLNESS 1/2 to 3/4 1/4 1/2 to 3/4 1/2 GUT CONDITION undigested p a r t l y d iges ted p a r t l y d iges ted p a r t l y d iges ted AVERAGE DIET WEIGHT PER GUT (mg) \ 25.91 1 1 3.14 2.22 7.30 I *Sub-sampled, N=1247. Table 12. D ie t c h a r a c t e r i s t i c s of underyear l i ng sockeye and s t i c k l e b a c k s i n a l l o p a t r y and sympatry. Homogeneous se ts of means from Tukey's a n a l y s i s on mean f i s h - d i e t weights are bracketed (see l a s t row of t a b l e , broken l i n e i n d i c a t e s mean i s not i n c l u d ed ) . FISH TREATMENTS SOCKEYE STICKLEBACKS ALLOPATRIC SYMPATRIC ALLOPATRIC SYMPATRIC N (FISH) 5 6 5 7 MEAN FISH SIZE (mm) 51.8 50.9 37.7 38.5 TOTAL # PREY CONSUMED* 1419(664) 6280(1384) 389 760(659) MEAN PREY SIZE (mm) 0.59 0.50 0.49 0.66 # PREY ITEMS PER GUT 283.8 1046.7 77.8 108.6 AVERAGE GUT FULLNESS 1/2 to 3/4 3/4 to 1 1/2 1/2 GUT CONDITION p a r t l y d iges ted p a r t l y d iges ted p a r t l y d iges ted p a r t l y d iges ted AVERAGE DIET WEIGHT PER GUT (mg) 1 7.17 | 16.17 "1 1.00 3.46 1 *I f gut contents were sub-sampled, N i s noted i n b r a cke t s . Table 13. D ie t c h a r a c t e r i s t i c s of sockeye and s t i c k l e b a c k f r y i n a l l o p a t r y and sympatry. Homogeneous se ts of means from Tukey's a n a l y s i s on mean f i s h - d i e t we ights are bracketed (see l a s t row of t a b l e ) . FISH TREATMENTS SOCKEYE STICKLEBACKS ALLOPATRIC SYMPATRIC ALLOPATRIC SYMPATRIC N (FISH) 7 7 8 7 MEAN FISH SIZE (mm) 28.2 28.7 34.5 33.1 TOTAL # PREY CONSUMED 167 119 1968 449 MEAN PREY SIZE (mm) 1.06 0.66 0.37 1.36 # PREY ITEMS PER GUT 23.9 17.0 246.0 64.1 AVERAGE GUT FULLNESS 1/4 to 3/4 1/4 >l/2 1/2 GUT p a r t l y d iges ted and CONDITION d iges ted d iges ted d iges ted p a r t l y d iges ted AVERAGE DIET WEIGHT PER GUT (mg) I 1.21 0.79 2.69 3.05 Table 14. Resu l t s of nonparametr ic ANOVA on s i z e s of prey consumed by f i s h in f i s h t reatments from (a) y e a r l i n g , (b) unde ryea r l i ng , and (c) f r y exper iments . W i th in each exper iment, t reatments are l i s t e d w i t h means ranked i n order of descending magnitude. Homogeneous sets of means are b racke ted . Mean s i z e Order of magnitude of of prey Experiment ranked means i n d i e t N (a) Sockeye y e a r l i n g s (F=651.55, df=3, p<.001) and adu l t s t i c k l e b a c k s A l l o p a t r i c sockeye 6 * 0 9 98 Symatr ic sockeye 0.85 207 Sympatr ic s t i c k l e b a c k s 0.81 1612 A l l o p a t r i c s t i c k l e b a c k s ^ . 0.72 1043 (b) Underyear l i ng sockeye (F=92.50, df=3, p<.001) and s t i c k l e b a c k s Sympatr ic s t i c k l e b a c k s " ^ j 0.66 761 A l l o p a t r i c sockeye | 0.59 1501 Sympatr ic sockeye — 0.50 6283 A l l o p a t r i c s t i c k l e b a c k s 0.49 398 (c) Sockeye and (F=159.41, df=3, p<.000) s t i c k l e b a c k f r y Sympatr ic s t i c k l e b a c k s ~ 1.36 486 A l l o p a t r i c sockeye 1.06 183 Sympatr ic sockeye 0.66 119 A l l o p a t r i c s t i c k l e b a c k s 0.37 2061 *Rep l i c a t e 2 of the sympatr ic treatment was excluded due to f i s h i n v a s i o n . 65 (85%) cons i s t ed of l a r g e , m isce l l aneous prey ( i e . ma in ly Neomysis and a few i n s e c t pupae), a long w i th a few copepods (10%) (percentages r e f e r to numerical percentages) ( F i g . 18c) . Only 20% of the sympatr i c sockeye d i e t c ons i s t ed of m isce l l aneous l a rge p rey , ( i . e . ma in ly i n s e c t l a rvae and some Neomysis) as compared w i th the 85% found i n stomachs from sockeye i n a l l o p a t r y . The bulk of the sympatr ic sockeye d i e t was composed of the smal l z oop l ank te r , Bosmina (46%), and copepods (33%) ( F i g . 18c -d ) . Copepods, because of t h e i r l a r g e r s i z e , would c on t r i bu t e cons i de rab l y to the weight of the d i e t (F igure 17) . S t i c k l e b a c k s : S t i c k l e b a c k s in sympatry w i th sockeye and in a l l o p a t r y consumed s i m i l a r s i z e s and taxa of zooplankton (Table 14a, see F i g . 19) . Only 33% of the a l l o p a t r i c s t i c k l e b a c k d i e t c ons i s t ed of m isce l l aneous l a rge prey ( i e . ma in ly chironomid l a rvae and a few i n s e c t pupae), whereas 55% of the d i e t was composed of copepods ( F i g . 19c) . Sympatr ic s t i c k l e b a c k s d i e t s cons i s t ed of on ly 6% by number as m i s ce l l aneous l a rge prey ( i . e . main ly chironomid l a rvae and a few Neomysis), and aga in copepods were numer i ca l l y dominant (68%) ( F i g . 19d). O v e r a l l , prey s i z e s consumed by s t i c k l e b a c k s in e i t h e r treatment and by sockeye in sympatry were not s i g n i f i c a n t l y d i f f e r e n t from each o the r ; on ly the a l l o p a t r i c sockeye conta ined s i g n i f i c a n t l y l a r g e r d i e t a r y items (Tukey's Procedure; Table 14a) . There fo re , a d i e t n iche s h i f t was observed f o r sockeye but not f o r s t i c k l e b a c k s . Experiment #2. underyear l i ng sockeye and s t i c k l e b a c k s (October 10-0ctober 30) Sockeye: As w i th y e a r l i n g sockeye from Exp. 1, a d i e t n iche s h i f t was observed by underyear l i ng sockeye feed ing i n a l l o p a t r i c v s . 66 F i g . 18. Frequency d i s t r i b u t i o n s (by siz e and taxa) of prey items consumed by sockeye yearlings in al l o p a t r y and sympatry. 67 Fig. 19. Frequency d i s t r i b u t i o n s (by size and taxa) of prey items consumed by adult sticklebacks in allopatry and sympatry. ALLOPATRIC STICKLEBACKS (9) (a) (c) 6 0 -, 3 0 A 2 0 A i o H X = 0.72 Np = l 0 4 5 5 0 A 3 0 A 2 0 A 10 H CD LU O rr LU Q_ > UJ r> 30 u. 20 SYMPATRIC STICKLEBACKS (10) (b) X= 0.81 N p=247 3 >€ TAXA SIZE (mm) Increasing size sympatr i c t rea tments; sympatr ic sockeye consumed s i g n i f i c a n t l y sma l l e r prey than a l l o p a t r i c sockeye (Table 14b; F i g . 20 a - b ) . A l l o p a t r i c sockeye ate most ly l a rge c l adocerans , main ly chydor ids (80% by number) and a few Bosmina (17% by number) w i th minimal numbers of copepods and m isce l l aneous l a rge prey ( F i g . 20b) . Sympatr ic sockeye consumed fewer of the l a rge chydor ids (37%) than d i d a l l o p a r i c sockeye (80%) and copepods became the dominant prey item (42% by number). The p ropo r t i ons of Bosmina and the misce l l aneous l a rge prey were s i m i l a r f o r sockeye i n e i t h e r treatment ( F i g . 20 c - d ) . The magnitude of the d i e t n iche s h i f t was l e s s than tha t observed f o r y e a r l i n g s as the d i f f e r e n c e between the s i z e s of food consumed by sockeye in a l l o p a t r y v s . sympatry was not as l a r g e . S t i c k l e b a c k s : In marked con t r a s t to the pa t t e rn f o r sockeye, s t i c k l e b a c k s from sympatr i c t reatments consumed s i g n i f i c a n t l y l a r g e r food items than s t i c k l e b a c k s i n the a l l o p a t r i c treatment (Table 14b; see F i g . 21 a - b ) , thereby d i s p l a y i n g a n iche s h i f t i n the oppos i te d i r e c t i o n to tha t of sockeye. Large c ladocerans composed on ly 29% of the a l l o p a t r i c s t i c k l e b a c k d i e t . Bosmina was the most abundant spec ies (45%), f o l l owed by l a rge c ladocerans (29%), copepods (23%) and a few m isce l l aneous l a rge p rey . The taxonomic composi t ion of d i e t s from s t i c k l e b a c k s i n sympatr i c and a l l o p a t r i c t reatments were s i m i l a r but sympatr i c s t i c k l e b a c k s consumed more l a rge c ladocerans and fewer Bosmina (48% and 25% r e s p e c t i v e l y vs 29% and 46%). P ropor t i ons of copepods and m isce l l aneous l a rge prey were s i m i l a r i n the d i e t s of sympatr ic and a l l o p a t r i c s t i c k l e b a c k s ( F i g . 21 c - d ) . The n iche s h i f t observed in unde ryea r l i ng s t i c k l e b a c k s was not observed i n adu l t s t i c k l e b a c k s (Exp. 1 ) . < r— O OL UJ 0_ >-O 30 (a) A L L O P A T R I C S O C K E Y E (5) 9 0 , ( C ) X =0.59 Np = 6 6 4 8 0 -2 0 -S Y M P A T R I C S O C K E Y E (6) X =0.50 Np= 1384 0.1 0.3 0.5 0.7 S I Z E (mm) T A X A • Increasing size F i g . 20. Frequency d i s t r i b u t i o n s (by size and taxa) of prey items consumed by underyearling sockeye in a l l o p a t r y and in sympatry. y J F i g . 21. Frequency d i s t r i b u t i o n s (by siz e and taxa) of prey items consumed by underyearling sticklebacks in a l l o p a t r y and in sympatry. ALLOPATRIC STICKL EBACKS ( 7 ) <-> S Y M P A T R I C S T I C K L E B AC K S ( 7 ) rr S I Z E (mm) Increasing size 71 Experiment #3. sockeye f r y X s t i c k l e b a c k f r y (May 7-May 26) Sockeye: S i m i l a r l y to Exp. 1 and 2 tha t conta ined sockeye l a r g e r than the f r y s tage , a d i e t n iche s h i f t was observed by sockeye f r y feed ing i n a l l o p a t r i c and sympatr ic t rea tments . Sockeye i n a l l o p a t r y consumed s i g n i f i c a n t l y l a r g e r prey items than sockeye in sympatry (Table 14c; see F i g . 22 a - b ) . F o r t y - f ou r percent of the a l l o p a t r i c sockeye d i e t (by number) was composed of m isce l l aneous l a rge prey ( i . e . ma in ly i n se c t pupae and ch i ronomid l a rvae) and some Bosmina (38%). (Percentages r e f e r to prey numbers.) Small chydor ids c on s t i t u t ed 17% of the d i e t wh i l e copepods were v i r t u a l l y absent ( <1% - F i g . 22c ) . Fewer of the m i sce l l aneous l a rge prey ( i . e . main ly chironomid l a r v a e , i n s e c t pupae and a few Neomysis) were eaten by sympatr ic sockeye (21% v s . 44% f o r a l l o p a t r i c sockeye) but more of the sma l l e r chydor ids (32% vs 17% f o r a l l o p a t r i c sockeye) were ea ten . The p ropo r t i ons of Bosmina and copepods eaten by sockeye were s i m i l a r i n both a l l o p a t r i c and sympatr i c treatments ( F i g . 22d) . S t i c k l e b a c k s : S i m i l a r l y to Experiment 2, a n iche s h i f t was observed by s t i c k l e b a c k f r y feed ing i n a l l o p a t r i c v s . sympatr i c t rea tments ; sympatr i c s t i c k l e b a c k s consumed s i g n i f i c a n t l y l a r g e r prey items than a l l o p a t r i c s t i c k l e b a c k s (Table 14; see F i g . 23 a - b ) . A l l o p a t r i c s t i c k l e b a c k s consumed few of the m isce l l aneous l a rge prey and a l a rge p ropo r t i on of sma l l e r chydor ids (65%). Bosmina composed 15% of the a l l o p a t r i c s t i c k l e b a c k d i e t and few copepods were consumed ( F i g . 23 c ) . In sympatry, s t i c k l e b a c k s ate g rea te r p ropor t i ons of m isce l l aneous l a rge prey (49%) and fewer of the smal l chydor ids than d i d s t i c k l e b a c k s i n a l l o p a t r y and as a r e s u l t , sympatr ic s t i c k l e b a c k s consumed l a r g e r mean s i z ed p rey . The p ropor t i ons of Bosmina eaten by s t i c k l e b a c k s were F,llL 22' F r e q l j e" cy distributions (by size and taxa) of prey items consumed by sockeye fry in allopatry and sympatry. A L L O P A T R I C S O C K E Y E ( 7 ) (c) X =1.06 Np=l67 501 40^ S Y M P A T R I C S O C K E Y E (7) S I Z E (mm) Increasing size 73 Fig. 23. Frequency distributions (by size and taxa) of prey items consumed by stickleback fry in allopatry and sympatry. A L L O P A T R I C S T I C K L E B A C K S (7) (c) 50-i 4 0 -3 0 -UJ 2 0 -< NT 10-UJ CJ or 11 I 0-^  U J CL. >-O z UJ o or 4 0 i S Y M P A T R I C S T I C K L E B A C K S (7) (d) 0.1 0.3 0.5 0.7 l ] 6 3 S I Z E (mm) T A X A Increasing size s i m i l a r i n both a l l o p a t r y and sympatry. In both Exp. 2 and 3, the n iche s h i f t s observed i n sockeye were in the oppos i te d i r e c t i o n to those observed in s t i c k l e b a c k s . 4 .3 .3 D i e t a r y over lap between sockeye and s t i c k l e b a c k s Accord ing to compet i t i on theo ry , when resources are l i m i t i n g , two spec ies tha t occupy the same niche cannot c oex i s t i n d e f i n i t e l y (Hard in 1960). In a l l o p a t r y , p o t e n t i a l compet i tors should show s i g n i f i c a n t n iche over lap as i t i s a p r e r e q u i s i t e to e x p l o i t a t i v e compet i t i on (P i anka , 1981) but i n sympatry, t h e i r n iches should d i v e r ge . MacArthur (1972) and Schoener (1974c) have noted tha t a compet i to r can cause another spec ies to drop hab i t a t s but not food items from i t s n i c he . The re fo re , h ab i t a t s should d ive rge i n sympatry, but i n un i form environments d i e t s may cont inue to show sub s t an t i a l o ve r l a p . Th is phenomenon was r e f e r r ed to as the n iche compression hypothes is (MacArthur and W i l s on , 1967). In two of the three experiments conducted, d i e t s of sockeye and s t i c k l e b a c k s i n a l l o p a t r y over lapped g r e a t l y (Table 15) . D i e t s of a l l o p a t r i c adu l t s t i c k l e b a c k s and y e a r l i n g sockeye over lapped s l i g h t l y bu t , d i e t s a t the end of the 12 day feed ing t r i a l were not n e c e s s a r i l y i n d i c a t i v e of what prey items were consumed throughout the feed ing t r i a l s . In a l l three exper iments, d i e t s of sockeye and s t i c k l e b a c k s i n sympatry e x h i b i t e d g rea te r over lap than tha t observed f o r the f i s h i n a l l o p a t r y . These r e s u l t s c on t r ad i c t p r e d i c t i o n s of compet i t i on theo ry . A d d i t i o n a l l y , i n the underyear l i ng and f r y experiments (Exp. 2 and 3) the Table 15. D i e t over lap measurements by taxa and s i z e between sockeye and s t i c k l e b a c k s in a l l o p a t r y from (a) y e a r l i n g , (b) unde r yea r l i n g , and (c) f r y exper iments . (Horn's Index was used - weighted fo rmu la . ) Taxa over lap S i z e over lap Experiment A l l o p a t r y Sympatry A l l o p a t r y Sympatry (a) Y ea r l i n g sockeye .117 .925 .312 .857 and adu l t s t i c k l e b a c k s (b) Underyear l i ng sockeye .790 .923 .926 .982 and s t i c k l e b a c k s (c) Sockeye and s t i c k l e b a c k f r y .819 .913 .913 .866 s i z e s of zoop lankton consumed by a l l o p a t r i c sockeye and sympatr i c s t i c k l e b a c k s were l a rge and more s i m i l a r to each o ther than f i s h i n o ther t reatments (Table 14) . Thus, i t appears tha t a l l o p a t r i c sockeye and sympatr i c s t i c k l e b a c k s occupy very s i m i l a r n i ches . 4 .3 .4 Prey a v a i l a b i l i t y Food a v a i l a b i l i t y (as determined by sampling the enc losures w i th a tow-net) was assessed to determine i f f i s h d i e t s were a f un c t i on of food a v a i l a b i l i t y . O v e r a l l , zooplankton samples c o l l e c t e d w i th a tow net i n d i c a t e d tha t s i z e and taxa of prey a v a i l a b l e as food items were s i m i l a r among a l l f i s h treatments of each exper iment. For each exper iment , a t the time f i s h were removed from the enc l o su re s , the s i z e s of zoop lankton w i t h i n the f i s h treatment enc losures were not s i g n i f i c a n t l y d i f f e r e n t from each other ( F i g . 24 - see Sec t i on 3, Table 4 f o r d e t a i l s ) . The on ly except ion was the a l l o p a t r i c s t i c k l e b a c k treatment of the underyear l i ng experiment (Exp. 2) which was s i g n i f i c a n t l y l a r g e r than the sympatr ic f i s h treatments but not the a l l o p a t r i c sockeye t reatment . However, zooplankton s i z e s in the a l l o p a t r i c s t i c k l e b a c k treatment were s i g n i f i c a n t l y sma l l e r from the lake and f i s h l e s s enc l o su re . The the smal l d i f f e r e n c e s in mean prey s i z e among the f i s h treatment enc losures i n d i c a t e tha t they were more s i m i l a r to each o ther than to lake or f i s h l e s s enc l o su re . The p ropo r t i ons of the va r i ous zooplankton taxa were s i m i l a r among enc losures w i t h i n each exper iment, however, the p ropor t i ons of Bosmina and n a u p l i i v a r i e d s l i g h t l y among treatments ( F i g . 25 ) . F i s h consumed cons ide rab le p ropor t i ons of m isce l l aneous l a rge prey and chydo r i d s , a l though these prey items were not commonly sampled in the water column (Table 16), so the p lankton tow-net d i d not f u l l y F i g . 2 4 . F r e q u e n c y d i s t r i b u t i o n s o f z o o p l a n k t o n by s i z e i n v a r i o u s f i s h t r e a t m e n t e n c l o s u r e s a t t h e e n d o f e a c h e x p e r i m e n t . Yea r l i n g E x p e r i m e n t LU < LU a or LU a. >-o LU 3 a LU or U n d e r y e a r l i n g E x p e r i m e n t N = 3 8 6 X = 0 . 2 4 F r y E x p e r i m e n t N . = 4 8 6 X = 0 . 2 9 0.1 0 5 0.1 0.5 1.011.7 1.3 S I Z E ( m m ) 78 Fig. 25. Frequency distributions of zooplankton by taxa in various f i s h treatment enclosures at the end of each experiment. UJ (S> < o or LU > u z LU o LU rr LU > LU o o CO CO o < m UJ _ i o t -co LU > LU o o CO (C CO u < CD LU _ J CO Yearling Experiment Vm3=378 Underyearling Experiment Fry Experiment */m3=3022 T A X A n c r e a s i n g size T a b l e 1 6 . Mean s i z e (mm) and m a j o r t a x a o f p r e y ( b y n u m b e r ) c o n s u m e d v s . t h o s e a v a i l a b l e i n o p e n - w a t e r a r e a s o f f i s h t r e a t m e n t e n c l o s u r e s f r o m ( a ) y e a r l i n g , ( b ) u n d e r y e a r I i n g a n d ( c ) f r y e x p e r i m e n t s . W i t h i n e a c h e x p e r i m e n t , t r e a t m e n t s a r e l i s t e d w i t h t h e i r means i n o r d e r o f d e s c e n d i n g m a g n i t u d e f o r s i z e o f p r e y c o n s u m e d . Mean s i z e and m a j o r t a x a Mean s i z e a n d m a j o r t a x a E x p e r i m e n t T r e a t m e n t o f p r e y i n d i e t o f p r e y i n e n c l o s u r e s ( a ) Y e a r 1 i ng A 1 l o p a t r i c s o c k e y e 6 . 0 9 - •Neomy s i s 0 . 1 9 - •naup I i i s o c k e y e and Sympa t r i c s o c k e y e 0 . 8 5 - B o s m i n a and c o p e p o d s 0 . 19 and 0 . 2 9 - n a u p l i i adu 11 Sympa t r i c s t i c k 1 e b a c k s 0 . 8 1 - c o p e p o d s 0 . 1 9 and 0 . 2 9 - n a u p 1 i i s t i c k 1 e b a c k s A 1 1opa t r i c s t i c k 1 e b a c k s 0 . 7 2 - c o p e p o d s 0 . 2 5 - n a u p 1 i i ( b ) U n d e r y e a r l i n g Sympa t r i c s t i c k 1 e b a c k s 0 . 6 6 - l a r g e c 1 a d o c e r a n s 0 . 25 and 0 . 2 6 - n a u p 1 i i s o c k e y e and A l l o p a t r i c s o c k e y e 0 . 5 9 - l a r g e c l a d o c e r a n s 0 . 2 4 - Bosm i na s t i c k 1 e b a c k s Sympa t r i c s o c k e y e 0 . 5 0 - c o p e p o d s 0 . 25 and 0 . 2 6 - n a u p 1 i i A 1 1 o p a t r i c s t i c k 1 e b a c k s 0 . 4 9 - Bosmi na 0 . 2 9 - Bosm i na ( c ) S o c k e y e and Sympa t r I c s t i c k 1 e b a c k s 1 . 3 6 - m i s c . l a r g e p r e y 0 . 21 and 0 . 2 6 - n a u p 1 i i s t i c k 1 e b a c k f r y A 1 l o p a t r i c s o c k e y e 1 . 0 6 - m i s c . l a r g e p r e y 0 . 2 9 - n o u p 1 i i S y m p a t r i c s o c k e y e 0 . 6 6 - B o s m i n a , c h y d o r i d s 0 . 21 and 0 . 2 6 - n a u p 1 i i A1 l o p a t r i c s t I c k 1 e b a c k s 0 . 3 7 - c h y d o r i ds 0 . 2 7 - n a u p 1 i i assess food a v a i l a b i l i t y . Wi th in each treatment of the y e a r l i n g and f r y experiment (Exps. 1, 3 ) , some of the f i s h stomachs sampled conta ined a t l e a s t a few Neomysis. Thus i t i s apparent tha t Neomysis were present i n a l l f i s h treatment enc l o su re s . Chironomid l a rvae and pupae were o f ten noted in zoop lankton samples as " ra re i tems" ye t they were o c c a s i o n a l l y p l e n t i f u l i n the f i s h ' s stomachs. Large s i z ed c l adoce rans , most ly l a rge chydor ids and S ida sp . were p l e n t i f u l i n the d i e t s of underyea r l i ng f i s h and smal l chydor ids were p l e n t i f u l i n the d i e t s of sockeye and s t i c k l e b a c k f r y (Exps. 2, 3 r e s p e c t i v e l y ) ye t these zoop lank te r s were very rare i n the zooplankton samples taken from the enc l o su re s . Chydor ids and S ida are u sua l l y found i n the l i t t o r a l areas of Kennedy Lake (Hyat t , pe r s . comm.) and the presence of these organisms in f i s h present and t h e i r r a r i t y i n the cen t r a l water column suggests they were present a t unknown d e n s i t i e s near the edges of the enc l o su re s . Neomysis, ch ironomid l a rvae and chironomid pupae may a l s o have been abundant a t unknown d e n s i t i e s near the s ides or bottoms of the en c l o su r e s . 4 .3 .5 Biomass of prey per f i s h Biomass of prey per gut was examined to determine i f those f i s h tha t ate l a r g e r food items (eg. a l l o p a t r i c sockeye) consumed more food per f i s h which cou ld t r a n s l a t e i n t o inc reased f i s h growth (Paloheimo and D i c k i e , 1966) . Sockeye in a l l o p a t r y consumed more food per gut than sockeye i n sympatry i n two of the three exper iments, however, on ly i n the y e a r l i n g experiment was the d i f f e r e n c e s t a t i s t i c a l l y s i g n i f i c a n t (Table 17a) . In the underyear l i ng experiment (Exp. 2 ) , a l though s i g n i f i c a n t l y l a r g e r prey items were consumed by a l l o p a t r i c sockeye, sympatr i c sockeye consumed s i g n i f i c a n t l y more food (by weight) (Table 17b). At the end of Table 17. Resu l t s of nonparametr ic ANOVA on biomass of prey consumed by f i s h i n f i s h t reatments from (a) y e a r l i n g , (b) unde r yea r l i n g , and (c) f r y exper iments . W i th in each exper iment, t reatments are l i s t e d w i t h means i n order of descending magnitude. Homogeneous se ts of means are b racke ted . Mean prey Order of magnitude of biomass per Experiment ranked means f i s h (mg) N (a) Sockeye y e a r l i n g s and (F=13.54, d f - 3 , p<.0001) adu l t s t i c k l e b a c k s (b) Underyear l i ng sockeye and s t i c k l e b a c k s (c) Sockeye and s t i c k l e b a c k f r y * A l l o p a t r i c sockeye 25.91 1 8 Sympatr ic s t i c k l e b a c k s 7.30 10 Sympatr ic sockeye 3.14 7 A l l o p a t r i c s t i c k l e b a c k s 2.22 9 (F=5.15, df=3, p<.009) Sympatr ic sockeye 16.17" 6 A l l o p a t r i c sockeye 7.17 5 Sympatr ic s t i c k l e b a c k s 3.46 7 A l l o p a t r i c s t i c k l e b a c k s 1.00 5 (F -1 .35 , df=3, p>.2814) Sympatr ic s t i c k l e b a c k s 3.05 7 A l l o p a t r i c s t i c k l e b a c k s 2.69 8 A l l o p a t r i c sockeye 1.21 7 Sympatr ic sockeye 0.79 7 *F i sh from Rep l i c a t e 2 of the sympatr ic t reatments were exc luded due to a f i s h i nvas i on i n tha t enc l o su re . the unde ryea r l i ng exper iment, sympatr ic sockeye had f u l l e r stomachs than sockeye and from the sympatr ic t reatment . However, i n t e r p r e t a t i o n of the s i g n i f i c a n c e of weight of food i n the gut i s confounded by d i f f e r e n c e s i n d i g e s t i o n s t a tus of the gut contents among enc l o su re s , d i f f e r e n c e s in gut f u l l n e s s r e l a t ed to v a r i a t i o n i n time of f i s h removal (3-5 h r s ) , and d i f f e r e n c e s i n f i s h s i z e . A d d i t i o n a l l y , the sample s i z e s used i n these exper iments were s m a l l . The mean biomass of prey per f i s h gut was always h igher i n sympatr i c s t i c k l e b a c k s than a l l o p a t r i c s t i c k l e b a c k s , but the d i f f e r e n c e s were not s t a t i s t i c a l l y s i g n i f i c a n t . 4.4 D i s cus s i on Evidence tha t i n t e r s p e c i f i c compet i t i on may produce n iche s h i f t s i n f reshwater f i s h communities has been de r i ved both from obse rva t i ona l s t ud i e s under uncon t r o l l ed f i e l d c ond i t i on s as we l l as from exper imenta l s t ud i e s i n v o l v i n g c o n t r o l l e d i n t r odu c t i o n s or removals of f i s h i n t o lakes or enc l o su re s . In uncon t r o l l ed obse rva t i ona l s t u d i e s , i n s i t u resource use by a spec ies i n a l l o p a t r y ( i . e . s p a t i a l l y or tempora l l y segregated from p o t e n t i a l c ompe t i t o r s ) , and in sympatry w i th i t s p o t e n t i a l compet i t o r , are examined f o r va r i ous d i f f e r e n c e s ( e . g . N i l s s o n , 1967; Andrusak and Nor thco te , 1971; N i l s s on and Nor thcote , 1981; Schmit t and Coyer, 1983; Hume and Nor thco te , 1985;) . Un fo r tuna te l y e c o l o g i c a l c o n d i t i o n s , ( e . g . p r eda t i o n , food d i s t r i b u t i o n ) in these "na tura l exper iments" i n e v i t a b l y vary between the a l l o p a t r i c and sympatr ic environments, and t h i s confounds sepa ra t i on of j u s t the e f f e c t s of compet i tors on n iche c h a r a c t e r i s t i c s (Conne l l , 1980; Schmitt and Coyer, 1983; Hanson and Legget t , 1985). F i e l d enc losure experiments i n v o l v i n g the c r e a t i o n of s i m i l a r environments can be used to reduce or e l im i na t e c o n d i t i o n s , o ther than compe t i t i on , which may a l s o r e s u l t i n spec ies n iche s h i f t s (Connel l 1983 and Schoener 1983). The a r t i f i c i a l components of these exper iments (eg. enc losure e f f e c t s ) must be taken i n t o c ons i de r a t i on i n the f i n a l i n t e r p r e t a t i o n of the exper imenta l r e s u l t s (Connel l 1975, 1983). Werner and Ha l l (1976, 1977, 1979) concluded tha t observed n iche s h i f t s among congener ic sun f i shes were due to compe t i t i on . However, i n t e r p r e t a t i o n of t h e i r r e s u l t s have been quest ioned g iven tha t unna tu r a l l y high d e n s i t i e s of f i s h were used and, i n some cases , experiments d i d not con t ro l f o r changes in f i s h dens i t y (Maiorana, 1977; Hanson and Legget t , 1985). From a n a l y s i s of f i s h in exper imenta l enc l o su res , Hanson and Leggett (1985, 1986) presented a conv inc ing case f o r the importance of i n t e r - and i n t r a s p e c i f i c compet i t i on to the growth and d i e t i n ye l l ow perch (Perca f l a ves cens ) and pumpkinseed (Lepomis q i bbosus ) . However, these authors d i d not repor t on whether e x p l o i t a t i v e or i n t e r f e r en ce compet i t i ve mechanisms were ope r a t i n g . In my s tudy, sockeye from a l l three exper iments d i s p l a yed a d i e t s h i f t , as sockeye i n a l l o p a t r y fed on l a rge s i z ed zoop lank te r s but those i n sympatry w i th s t i c k l e b a c k s consumed s i g n i f i c a n t l y sma l l e r z oop l ank t e r s . Converse ly , s t i c k l e b a c k s feed ing i n a l l o p a t r y consumed sma l l e r food p a r t i c l e s than d i d s t i c k l e b a c k s feed ing w i th sockeye (Exp. 2 and 3 ) , a d i e t s h i f t i n the oppos i te d i r e c t i o n of tha t f o r sockeye. As w e l l , the i n t e n s i t y of the n iche s h i f t s , assessed as the d i f f e r e n c e between the s i z e of food and the p ropo r t i on of edge hab i t a t prey items consumed by a p a r t i c u l a r spec ies of f i s h in a l l o p a t r y v s . sympatry, i nc reased w i th the s i z e of f i s h u t i l i z e d i n the exper iments , Exps. 1, 2, 84 and 3 r e s p e c t i v e l y . Adu l t s t i c k l e b a c k s in a l l o p a t r y d i d not consume s i g n i f i c a n t l y sma l l e r prey items than adu l t s t i c k l e b a c k s i n sympatry ( E x p . l ) . The d i e t d i f f e r e n c e s observed between treatments of sockeye or s t i c k l e b a c k s mainta ined in a l l o p a t r y or i n sympatry cou ld have occurred as a consequence of e i t h e r ( i ) d i f f e r e n c e s between the enc losure t reatments i n the r e l a t i v e and/or abso lu te abundances of va r i ous prey s i z e s or taxa or ( i i ) compet i t i ve i n t e r a c t i o n s w i t h i n and between f i s h s p e c i e s . Comparisons of samples obta ined from the water column of each enc losure i n d i c a t ed on ly minor d i f f e r e n c e s i n the s i z e and taxonomic compos i t ion of p rey . Consequent ly , d i e t d i f f e r e n c e s between a l l o p a t r i c and sympatr i c f i s h treatments were not a consequence of d i f f e r e n c e s in the i n ve r t eb r a t e communities sampled i n the va r i ous enc l o su re s . Zooplankton abundance va r i e d between a l l o p a t r i c and sympatr i c t r ea tments . Thus, f i s h w i t h i n the enc losure encountered d i f f e r e n t abso lu te numbers of p r o f i t a b l e prey items and t h i s may have a f f e c t e d the f i s h e s ' d i e t c ho i c e . Predators can a f f o r d to be more s e l e c t i v e when p r o f i t a b l e prey are abundant (see review in Krebs 1978). There was no evidence tha t d i f f e r e n c e s in abso lu te prey abundances i n f l uenced the observed d i e t s h i f t because a t l e a s t one of the two sympatr i c f i s h t reatment enc losures conta ined h igher t o t a l zooplankton abundances than the a l l o p a t r i c sockeye f i s h treatment enc l o su re . A c on s i s t en t pre ference of Bosmina by sockeye cou ld a l s o e x p l a i n the apparent n iche s h i f t s , g iven the v a r i a t i o n i n abundances of Bosmina among enc l o su re s . A high abundance of Bosmina, r e l a t i v e to l a r g e r p rey , might r e s u l t i n a pre ference f o r Bosmina as sockeye capture success w i t h these smal l prey i s 100% v s . 35% f o r l a r g e r prey l i k e copepods (Hyatt 1980). Al though open-water zooplankton communities were s i m i l a r i n a l l f i s h treatment enc l o su res , the abundances of smal l zoop l ank te r s such as Bosmina va r i e d among t rea tments . In Experiment 1, the a l l o p a t r i c - s o c k e y e enc losure conta ined very few Bosmina compared w i th o ther enc l s ou re s , thus , assuming the above prey pre ference hypo thes i s , the n iche s h i f t cou ld be r e a d i l y e xp l a i n ed . The hypothes is however, i s not supported in a l l exper iments . A l l o p a t r i c sockeye from Experiment 2 d i d not s e l e c t i v e l y acqu i re Bosmina a l though they were abundant i n the enc losure (59%). Net hauls obta ined from the open water column of each enc losure d i d not f u l l y represent the e n t i r e prey community e xp l o i t e d by a l l combinat ions of a l l o p a t r i c and sympatr i c f i s h . A l l o p a t r i c sockeye i n exper iments 1, 2, and 3 c o n s i s t e n t l y e xp l o i t e d l a rge prey types tha t were v i r t u a l l y absent i n water column samples. Th is suggests tha t these prey (Neomysis, Chydorus, chironomid l a rvae and i n s e c t pupae) were l i k e l y obta ined by sockeye from e i t h e r the s ides or bottoms of the enc l o su re s . A l though the abundances of prey a long the edge of the enc losures ( i . e . the l a rge m isce l l aneous prey and the l a rge c ladocerans) were unknown, i t seems u n l i k e l y tha t these prey items would have been more abundant in the a l l o p a t r i c sockeye enc losure than in the sympatr i c enc losures a t the s t a r t o f the treatment i n a l l three exper iments . The assignment of the a l l o p a t r i c f i s h treatment to enc losures i n each experiment was random and r e s u l t e d i n the use of a d i f f e r e n t enc losure f o r the a l l o p a t r i c treatment i n each exper iment. A d d i t i o n a l l y , i n Exp. 3 and Exp. 2 both sympatr ic s t i c k l e b a c k s and a l l o p a t r i c sockeye consumed equal p ropo r t i ons of the l a rge prey i t ems . Large prey items were present i n the enc losures from Exp. 3 and 2, tha t conta ined a l l o p a t r i c sockeye and sympatr i c sockeye and s t i c k l e b a c k s , and these prey were abundant enough to be a major food f o r both f i s h s pe c i e s . A l though the abundances of l a rge prey items probably were not tha t d i f f e r e n t among treatments at the s t a r t of each exper iment, i t i s not known whether sockeye and s t i c k l e b a c k s e xp l o i t e d the prey items at s i g n i f i c a n t l y d i f f e r e n t r a t e s . The abundance of these prey items may have v a r i e d among f i s h treatment enc losures a t the end of the treatment p e r i o d . In summary, there were some d i f f e r e n c e s in the r e l a t i v e and abso lu te abundances of va r i ous prey taxa and s i z e s among treatment en c l o su r e s , these d i f f e r e n c e s are not c ons i s t en t among treatments and d i d not appear to be great enough to e xp l a i n the pa t te rns of d i e t change observed f o r both sympatr ic spec ies treatments r e l a t i v e to t h e i r a l l o p a t r i c c oun te rpa r t s . A c co r d i ng l y , the evidence and arguments presented here support the view that d i e t changes observed i n the m a j o r i t y of experiments were a consequence of c ompe t i t i on . The compet i t i ve i n t e r a c t i o n s cou ld be e x p l o i t a t i v e and/or b ehav i o u r a l . P r e v i ou s l y ( s e c t i on 3 ) , I concluded tha t sockeye and s t i c k l e b a c k s from o l i g o t r o p h i c lakes compete by e x p l o i t a t i v e compe t i t i on , as i n a l l f i s h t rea tments , zooplankton communities were s i m i l a r l y changed. W i th in each exper iment, p redat ion by sockeye and s t i c k l e b a c k s produced s i m i l a r zooplankton communities i n a l l f i s h treatment enc losures and t he r e f o r e one would expect s i m i l a r f i s h d i e t s ac ross a l l t rea tments . However, the f i s h d i e t s va r i ed s i g n i f i c a n t l y among t reatments of each exper iment . Food w i t h i n the stomachs of these f i s h a t the end of a 2-3 week treatment pe r i od i s not n e c e s s a r i l y i n d i c a t i v e of prey consumed throughout the treatment p e r i o d . As reported in Sec t i on 3, i n a l l three exper iments the abso lu te numbers of copepods present i n a l l f i s h t reatment enc losures de c l i n ed d r a m a t i c a l l y dur ing the treatment pe r i od but i nc reased or decreased on ly s l i g h t l y i n the f i s h l e s s con t ro l and in Kennedy Lake (approx imate ly h a l f the dens i t y of f i s h used i n these expe r imen ts ) . F i sh p redat ion appears to have reduced the number of copepods i n a l l f i s h treatment enc l o su re s . At the end of the treatment p e r i o d , a l l o p a t r i c sockeye never conta ined app rec i ab l e numbers of copepods i n t h e i r d i e t , but i t i s most probable tha t copepods were consumed by both a l l o p a t r i c sockeye and s t i c k l e b a c k s a t some po in t dur ing the i n i t i a l t reatment per iods before the zooplankton communities were reduced i n s i z e . D ie t s of f i s h a t the end of the treatment per iods appear to be independent of the uni form d i s t r i b u t i o n s of open-water zoop lank ton . Knowledge of d i f f e r e n c e s in the s p a t i a l d i s t r i b u t i o n s of prey items prov ides the bas i s f o r some specu l a t i on on the s p e c i f i c reasons f o r the v a r i a t i o n i n f i s h d i e t s among the f i s h t rea tments . Zooplankton samples c o l l e c t e d from the cen t r a l water column of the enc losures ( he rea f t e r r e f e r r ed to as the open-water area) conta ined few of the m isce l l aneous l a rge prey and la rge c l adocerans , tha t were common in the stomachs of some f i s h from each f i s h t reatment . M i s ce l l aneous l a rge prey and chydor ids were probab ly concentrated near the s i des or bottom of the enc losures where they would have escaped capture by water column net h au l s . The re fo re , the d i e t s observed i n sockeye and s t i c k l e b a c k s are probab ly c o i n c i den t w i th the m i c r o -hab i t a t the f i s h were feed ing i n , edge h ab i t a t or open water h a b i t a t . S i m i l a r reduc t ions in l a rge zoop lank te rs i n a l l f i s h t reatments of p a r t i c u l a r experiments suggest tha t sockeye and s t i c k l e b a c k s e x p l o i t s i m i l a r types of open-water prey and f unc t i on as t r o p h i c ana logues. As time passes and the resources become reduced ( e . g . a v a i l a b l e prey are sma l l e r ) some of these f i s h i n c l ude more of the edge h ab i t a t prey items in t h e i r d i e t . At the end of the treatment p e r i o d , the p a r t i c u l a r m i c r o -hab i t a t tha t a f i s h feeds in changes depending on whether i t i s feed ing in an a l l o p a t r i c or sympatr i c t rea tment . With the exper imenta l des ign used in t h i s s tudy, the l e ve l of i n t r a s p e c i f i c compet i t i on decreased f o r each f i s h spec ies when tes ted in sympatr i c t rea tments . There fo re , the d i e t swi tches observed f o r sockeye and s t i c k l e b a c k s feed ing in a l l o p a t r y v s . sympatry, cou ld be due to (1) the presence of a compet i tor i n the sympatr ic t reatment or (2) reduc t i on in the l e v e l o f i n t r a s p e c i f i c c ompe t i t i on . M i t t l e ba ch (1981) and Werner e t a l . (1983) presented evidence suggest ing tha t sun f i s h were capable of assess ing the r e l a t i v e p r o f i t a b i l i t i e s of d i f f e r e n t hab i t a t s and cou ld moni tor the dynamics of t h e i r r e sou r ces . There fo re , the a dd i t i o n of an i n t r a - or i n t e r s p e c i f i c compet i to r which can a l t e r the hab i t a t p r o f i t a b i l i t i e s or resource a v a i l a b i l i t y , may be assessed by f i s h such tha t t h e i r f o rag ing behaviour i s m o d i f i e d . S i m i l a r l y , the m i c r o - hab i t a t s tha t sockeye and s t i c k l e b a c k s feed i n may be determined through the assessment of which of the two m i c r o - hab i t a t s i s more p r o f i t a b l e to them. An assessment of changes in hab i t a t p r o f i t a b i l i t i e s may occur i n two ways. I f one f i s h spec ies i s more e f f e c t i v e a t feed ing in a p a r t i c u l a r h a b i t a t , then the abundance or s i z e s of p a r t i c u l a r prey spec ies may dec rease . Thus the f i s h change t h e i r assessment of the hab i t a t p r o f i t a b i l i t y , r e l a t i v e to o ther h a b i t a t s . A l t e r n a t i v e l y , i n t r a -or i n t e r s p e c i f i c behav ioura l i n t e r a c t i o n s ( i . e . over t or pass ive i n t e r a c t i o n s ) among f i s h cou ld r e s u l t i n a decrease i n f o rag ing e f f e c t i v e n e s s ( e . g . l e s s time spent feed ing) and the hab i t a t w i l l be assessed as l e s s p r o f i t a b l e , r e l a t i v e to other h a b i t a t s . In t h i s s tudy, sockeye feed ing by themselves apparen t l y assessed the edge hab i t a t as the more p r o f i t a b l e one, whereas sockeye feed ing w i th s t i c k l e b a c k s assessed the open-water h ab i t a t to be more p r o f i t a b l e . The oppos i te assessment was observed f o r underyear l i ng and s t i c k l e b a c k f r y ; sympatr ic s t i c k l e b a c k s assessed the edge hab i t a t as the most p r o f i t a b l e , but a l l o p a t r i c s t i c k l e b a c k s assessed the open-water hab i t a t as the most p r o f i t a b l e . The unknown in fo rmat i on needed to understand these d i e t s h i f t s i s whether sockeye or s t i c k l e a c k s forage more e f f e c t i v e l y on prey items in the edge h a b i t a t . I f s t i c k l e b a c k s are more e f f e c t i v e feed ing in edge hab i t a t s than sockeye, s t i c k l e b a c k s would have reduced the abundance of prey items i n the edge h a b i t a t . At the end of the treatment p e r i o d , the abundance of prey items i n the edge hab i t a t would have been lowest i n the a l l o p a t r i c s t i c k l e b a c k t rea tments , i n te rmed ia te i n the sympatr i c t rea tments , and h ighes t i n the a l l o p a t r i c sockeye t rea tments , and t h i s would r e a d i l y e xp l a i n the d i e t s h i f t s . S t i c k l e b a c k s i n a l l o p a t r y would have h e a v i l y e x p l o i t e d the edge hab i t a t of t h e i r enc losure such tha t the open-water h ab i t a t became more p r o f i t a b l e to them. The edge hab i t a t of the sympatr i c enc l o su re s , which conta ined h a l f the number of s t i c k l e b a c k s present i n the a l l o p a t r i c t reatment , would have been l e s s e x p l o i t e d and more p r o f i t a b l e to sympatr ic s t i c k l e b a c k s than the open-water h a b i t a t . The m i c r ohab i t a t s h i f t s by a l l o p a t r i c v s . sympatr ic sockeye ( i . e . from edge to open-water hab i t a t ) would have occurred due to reduced abundance of prey items in the edge hab i t a t of the sympatr i c t rea tment . The above i n t e r p r e t a t i o n i n d i c a t e s tha t i n t e r s p e c i f i c compet i t i on w i t h s t i c k l e b a c k s causes sockeye to change t h e i r f o rag ing behav iour whereas i n t r a s p e c i f i c compet i t i on among s t i c k l e b a c k s i n f l uences t h e i r f o rag ing behav iour . In Kennedy Lake, sockeye always occupy the l i m n e t i c zone of the l a k e , whereas s t i c k l e b a c k s occupy the l i m n e t i c zone dur ing October to June and the l i t t o r a l zone in the summer. While feed ing i n the l i t t o r a l zone, s t i c k l e b a c k s eat zooplankton and aqua t i c and t e r r e s t r i a l i n s e c t s (K. D. Hya t t , unpubl ished d a t a ) . The f o rag ing behaviour a s so c i a t ed w i th f eed ing on l i t t o r a l zone prey and the s ides of the enc losures i s more s i m i l a r than tha t a s so c i a t ed w i th feed ing in the open-water h a b i t a t . I suggest s t i c k l e b a c k s are be t t e r adapted f o r feed ing on the edges of the enc losures than sockeye. A l t e r n a t i v e l y , i f sockeye and s t i c k l e b a c k s e x p l o i t the edge hab i t a t e q u a l l y , the abundances of prey items in the edge hab i t a t s would have been s i m i l a r among a l l f i s h t rea tments , then behav ioura l i n t e r a c t i o n s among these f i s h was probably the mechanism induc ing n iche s h i f t s . I n t e r f e rence compet i t i on cou ld have a f f e c t e d the amount of t ime spent feed ing or the p a r t i c u l a r space a f i s h feeds i n and thereby the f i s h e s changes i t s assessment of the q u a l i t y and quan t i t y of food a v a i l a b l e f o r consumption. The f i s h ' s assessment of the r e l a t i v e h ab i t a t p r o f i t a b i l i t i e s changes even though the food does not a c t u a l l y change. Thus the m i c r o -hab i t a t s h i f t s by sockeye i n a l l o p a t r i c v s . sympatr i c t reatments would have been due to a behav ioura l i n t e r a c t i o n w i t h s t i c k l e b a c k s i n sympatry, such tha t s t i c k l e b a c k s h indered sockeyes ' a c q u i s i t i o n of prey items i n the edge h a b i t a t . Sockeye would have assessed the r e l a t i v e change in hab i t a t p r o f i t a b i l i t i e s even though the ac tua l abundances of the resources would not have changed. By c o n t r a s t , i n sympatr ic t rea tments , sockeye do not appear to h inder the s t i c k l e b a c k s ' a c q u i s i t i o n of l a r g e r more va luab l e prey i tems. Moreover, an inc reased dens i t y of s t i c k l e b a c k s i n the a l l o p a t r i c t reatment compared w i th the sympatr ic t reatment , appears to have l i m i t e d the s i z e of prey consumed by s t i c k l e b a c k s , p o s s i b l y due to pass ive behav ioura l i n t e r a c t i o n s . I n t r a s p e c i f i c compet i t i on i s more l i m i t i n g to s t i c k l e b a c k s than i s i n t e r s p e c i f i c c ompe t i t i on . In Exp. 1, a du l t s t i c k l e b a c k s i n a l l o p a t r i c and sympatr ic enc losures consumed s i m i l a r prey s i z e s but t h i s does not n e c e s s a r i l y mean compet i t i on d i d not occu r . However, the absence of a n iche s h i f t cou ld i n d i c a t e tha t i n t r a - and i n t e r s p e c i f i c competive pressures were s i m i l a r , suggest ing tha t y e a r l i n g sockeye are b e t t e r ab le to compete w i th adu l t s t i c k l e b a c k s than are sma l l e r sockeye w i th sma l l e r s t i c k l e b a c k s . Th is i n t e r p r e t a t i o n i s c on s i s t en t w i th observa t i ons from Sec t i on 3 tha t suggest the i n t e n s i t y of the compet i t i ve i n t e r a c t i o n inc reases w i th f i s h s i z e . Thus, i n both of the above i n t e r p r e t a t i o n s , I conclude tha t i n t e r s p e c i f i c compet i t i ve i n t e r a c t i o n s between sockeye and s t i c k l e b a c k s i n sympatry cause sockeye to swi tch t h e i r d i e t to i n c lude more of the s m a l l e r , l e s s p r o f i t a b l e p rey . The d i e t s h i f t observed by s t i c k l e b a c k s feed ing in a l l o p a t r y v s . sympatry appears to be due to i n t r a s p e c i f i c compet i t i ve i n t e r a c t i o n s among s t i c k l e b a c k s . The ac tua l mechanism, e x p l o i t a t i v e or b ehav i ou r a l , r espons ib l e f o r the n iche s h i f t s i s unknown, a l though these two mechanisms are not n e c e s s a r i l y mutua l l y e x c l u s i v e . I d e a l l y , comparisons of sockeye and s t i c k l e b a c k growth in a l l o p a t r i c and sympatr i c f i s h treatments would best determine the e f f e c t of i n t e r s p e c i f i c and i n t r a s p e c i f i c compet i t i on (Hanson and Legget t , 1985). Due to the shor t du ra t i on of these experiments and the hand l i n g - s t r e s s i nvo l ved in measuring f i s h length and we igh t s , growth data were not ob ta i ned . However, data from coas ta l o l i g o t r o p h i c lakes i n d i c a t e tha t i nc reases in zooplankton s tand ing crop (expressed as t o t a l zoop lankton biomass per f i s h ) are p o s i t i v e l y c o r r e l a t e d w i th sockeye smolt weight (Hyatt and S tockner , 1985). The inc reased prey biomass i s l i k e l y to p o s i t i v e l y a f f e c t sockeye growth and f i t n e s s (see d i s c u s s i o n of sockeye f i t n e s s i n Sec t i on 3 ) . In E x p . l and Exp. 3, the consumption of l a r g e r prey by a l l o p a t r i c sockeye r e su l t e d h igher t o t a l prey biomass per gu t , suggest ing tha t a l l o p a t r i c sockeye fa red b e t t e r than sympatr ic sockeye. I n t e r s p e c i f i c compet i t i on w i th s t i c k l e b a c k s may have r e su l t e d i n lower growth ra tes f o r sympatr i c v s . a l l o p a t r i c sockeye i f the du ra t i on of the exper iments had been excuted f o r a longer t ime . The mean biomass of prey per f i s h gut was always h igher in sympatr ic s t i c k l e b a c k s than a l l o p a t r i c s t i c k l e b a c k s , suggest ing tha t sympatr ic s t i c k l e b a c k s f a red b e t t e r than a l l o p a t r i c s t i c k l e b a c k s . I f sympatr ic s t i c k l e b a c k s obta ined a h igher biomass of food and exper ienced h igher growth ra tes than a l l o p a t r i c s t i c k l e b a c k s , i n t r a s p e c i f i c compet i t i on would e xp l a i n the n iche s h i f t s observed by s t i c k l e b a c k s . S i m i l a r s e r i e s of events were demonstrated i n s t ud i e s by Werner and Ha l l (1976, 1977, 1979). In a l l o p a t r y , green sun f i s h and b l u e g i l l s ranked vegetated hab i t a t s as more p r o f i t a b l e than open-water h a b i t a t s , but because green sun f i s h were more e f f i c i e n t a t feed ing in vegetated h ab i t a t s than were b l u e g i l l s , i n sympatry i t was more p r o f i t a b l e f o r b l u e g i l l s to consume prey from the open-water h a b i t a t . Hanson and Leggett (1986) reported tha t ye l l ow perch feed ing by themselves consumed most ly l a rge macro inver tebra tes whereas in enc losures w i t h pumpkinseed they consumed fewer macro inver tebra tes and more m i c r o i n v e r t e b r a t e s . In the same treatment comparisons, the d i e t s of pumpkinseed d i d not change. These authors d i d not assess the a v a i l a b i l i t y of m i c r o i n ve r t eb r a t e s but the macro inver tebra te prey were not s i g n i f i c a n t l y d i f f e r e n t among enc l o su res . Pumpkinseed and ye l l ow perch appear to have no i n f l uence on the abundance of macro inver tebra tes i n the enc losure used i n t h e i r s tudy . Hanson and Leggett suggested tha t pumpkinseed apparen t l y reduced the a v a i l a b i l i t y of l a rge prey types to y e l l ow perch but they do not propose how the pumpkinseed would have reduced the a v a i l a b i l i t y of these prey i tems. I suggest tha t behav ioura l i n t e r a c t i o n s among ye l l ow perch and pumpkin seed may have a l t e r e d the y e l l ow pe r ch ' s access to the l a rge prey i tems. Overt behav ioura l i n t e r a c t i o n s were not observed by the authors but pass i ve behav ioura l i n t e r a c t i o n s may have occu r red . A l though the m i c r o -hab i t a t cho ices a v a i l a b l e i n the enc losures were an a r t i f a c t , they may be viewed as analogues of the v e r t i c a l m i c r o - hab i t a t s f i s h may choose between i n the l i m n e t i c zone of the l a k e . S t i c k l e b a c k s spend most of t h e i r t ime i n the upper 10 meters of the water column whereas sockeye are d i s t r i b u t e d deeper and on ly migrate to the sur face waters to feed a t dusk . V e r t i c a l m ig ra to ry behaviour i n sockeye may have evo lved f o r severa l reasons (see Levy, i n press) but under c e r t a i n c i rcumstances i t may serve to reduce compet i t i on between sockeye and s t i c k l e b a c k s . In c on c l u s i o n , the data presented i n t h i s study i n d i c a t e tha t s t i c k l e b a c k i n t e r a c t w i th sockeye and reduce sockeye 's access to the l a r g e r a v a i l a b l e food i tems. I n t r a s p e c i f i c compet i t i on among s t i c k l e b a c k s appears to reduce the number of l a rge prey items i n t h e i r d i e t s . Examinat ion of the prey taxa invo lved suggests tha t d i e t s h i f t s are most probab ly a consequence of changes in the d i s t r i b u t i o n s of sockeye and s t i c k l e b a c k s r e l a t i v e to the m i c r o - hab i t a t s of p rey . 95 5.0 SUMMARY (1) Prey a c q u i s i t i o n by sockeye and s t i c k l e b a c k s in a l l exper iments was h i g h l y s i z e - s e l e c t i v e . (2) Sockeye and s t i c k l e b a c k s competed e x p l o i t a t i v e l y f o r f ood , as w i t h i n 12-20 days both spec ies s i m i l a r l y changed t h e i r zoop lankton communities through removal of l a rge s i z e z oop l ank t e r s . (3) A l l o p a t r i c sockeye in each of three exper iments consumed prey tha t were s i g n i f i c a n t l y l a r g e r than d i d sockeye i n sympatry w i th s t i c k l e b a c k s . By con t r a s t a l l o p a t r i c s t i c k l e b a c k s i n two out of th ree exper iments consumed prey tha t were s i g n i f i c a n t l y sma l l e r than d i d s t i c k l e b a c k s i n sympatry w i th sockeye. (4) Sympatr ic sockeye and s t i c k l e b a c k s i n general d i s p l a yed g rea te r d i e t a r y ove r l ap (by prey s i z e or taxonomic compos i t ion) than d i d a l l o p a t r i c sockeye and s t i c k l e b a c k s . (5) Resu l t s presented in t h i s study support the hypothes is tha t d i e t s h i f t s by sockeye in a l l o p a t r y v s . sockeye in sympatry w i th s t i c k l e b a c k s were a consequence of i n t e r s p e c i f i c c ompe t i t i on . In c o n t r a s t , d i e t s h i f t s by s t i c k l e b a c k s in a l l o p a t r y v s . sympatry w i th sockeye appear to have been a consequence of i n t r a s p e c i f i c c ompe t i t i on . 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Diet:" niche s h i f t s by sockeye salmon (Oncorhynchus nerka) and threespine stickleback (Gasterosteus aculeatus) in response to competition, (submitted) 

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