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UBC Theses and Dissertations

Specializations for alternate trophic niches by two forms of threespine stickleback, Gasterosteus, co-existing… Bentzen, Paul 1982

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SPECIALIZATIONS FOR ALTERNATE TROPHIC NICHES BY TWO FORMS OF THREESPINE STICKLEBACK, GASTEROSTEUS, CO-EXISTING IN ENOS LAKE, VANCOUVER ISLAND by PAUL BENTZEN B . S c , M c G i l l U n i v e r s i t y , M o n t r e a l , 1 9 7 8 5 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF: MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA May 1982 ( c ) P a u l B e n t z e n , 1982 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 Z - O O l O O V The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date y j L ^ - r , ht?~ DE-6 (3/81) i i ABSTRACT Two m o r p h o l o g i c a l l y and e c o l o g i c a l l y d i s t i n c t forms of t h r e e s p i n e s t i c k l e b a c k ( G a s t e r o s t e u s ) , a " l i m n e t i c " form and a " b e n t h i c " form, c o - e x i s t i n Enos Lake on Vancouver I s l a n d . I used t h r e e e x p e r i m e n t s t o compare the f e e d i n g performance of the two forms, t o t e s t the h y p o t h e s i s t h a t l i m n e t i c s a re adapted t o p l a n k t i v o r y and b e n t h i c s a re adapted t o f o r a g i n g on b e n t h i c s u b s t r a t e s . The r e s u l t s support t h i s c o n c l u s i o n . When t e s t e d f o r maximum prey s i z e , b e n t h i c s were a b l e t o consume l a r g e r prey ( r e l a t i v e t o t h e i r body s i z e ) than l i m n e t i c s . B e n t h i c s were a l s o more s u c c e s s f u l than l i m n e t i c s i n f o r a g i n g on a b e n t h i c s u b s t r a t e . When a l l o w e d t o fo r a g e on a d e t r i t u s s u b s t r a t e f o r a f i x e d time i n t e r v a l , b e n t h i c s of both sexes c a p t u r e d more prey than male l i m n e t i c s , d e s p i t e the f a c t t h a t male l i m n e t i c s d i r e c t e d as many f e e d i n g s t r i k e s a t the s u b s t r a t e as d i d the b e n t h i c s . Female l i m n e t i c s d i d not f o r a g e on the s u b s t r a t e a t a l l . In c o n t r a s t , l i m n e t i c s were more s u c c e s s f u l i n f e e d i n g on p l a n k t o n than b e n t h i c s . When h e l d i n mesh e n c l o s u r e s suspended i n the water column of Enos Lake, l i m n e t i c s consumed more p l a n k t o n than b e n t h i c s . " S m a l l " l i m n e t i c s (26-35 mm s t a n d a r d l e n g t h ) consumed n e a r l y f o u r t i m e s more p l a n k t o n than " l a r g e " l i m n e t i c s (44-50 mm s t a n d a r d l e n g t h ) . The s m a l l l i m n e t i c s were a l l e i t h e r mature females or immature males; the l a r g e l i m n e t i c s were a l l mature males. These r e s u l t s , a l o n g w i t h m o r p h o l o g i c a l , b i o c h e m i c a l , e c o l o g i c a l and b e h a v i o u r a l data o b t a i n e d i n o t h e r s t u d i e s , support the c o n c l u s i o n t h a t the two forms of G a s t e r o s t e u s i n Enos Lake a re d i s t i n c t b i o l o g i c a l s p e c i e s . The r e s u l t s of t h i s study a l s o support another (unexpected) c o n c l u s i o n : mature male and female l i m n e t i c s a l s o d i f f e r i n f e e d i n g b e h a v i o u r . Female l i m n e t i c s appear t o be almost t o t a l l y p l a n k t i v o r o u s ; whereas, male l i m n e t i c s a re i n t e r m e d i a t e between female l i m n e t i c s and b e n t h i c s (both sexes) i n f e e d i n g b e h a v i o u r . i i i TABLE OF CONTENTS ABSTRACT i i LIST OF TABLES i v LIST OF FIGURES v ACKNOWLEDGEMENTS v i G e n e r a l i n t r o d u c t i o n 1 Maximum Prey S i z e 9 I n t r o d u c t i o n 9 R e s u l t s 12 D i s c u s s i o n 17 F o r a g i n g on a N a t u r a l B e n t h i c S u b s t r a t e 20 I n t r o d u c t i o n 20 Methods 20 R e s u l t s 22 D i s c u s s i o n 27 F o r a g i n g on P l a n k t o n 34 I n t r o d u c t i o n 34 Methods 35 R e s u l t s 39 D i s c u s s i o n 45 Ge n e r a l D i s c u s s i o n 54 Ref e r e n c e s . 59 LIST OF TABLES T a b l e 1. R e l a t i o n s h i p between maximum prey s i z e and s t a n d a r d l e n g t h of l a b o r a t o r y - r e a r e d s t i c k l e b a c k s 15 T a b l e 2. R e s u l t s of a n a l y s e s of c o v a r i a n c e on maximum pr e y s i z e and s t a n d a r d l e n g t h . ; 17 T a b l e 3. Performance of f i s h t e s t e d i n _ b e n t h i c f o r a g i n g e x periment 24 T a b l e 4. S t a n d a r d l e n g t h s of f i s h t e s t e d i n b e n t h i c f o r a g i n g experiment 25 T a b l e 5. R e s u l t s of t e s t s f o r c o r r e l a t i o n s between s t a n d a r d l e n g t h s of f i s h t h a t f e d and f o r a g i n g performance ........26 T a b l e 6. Zooplankton c o n c e n t r a t i o n s c a l c u l a t e d from l a k e and e n c l o s u r e samples 40 T a b l e 7. Approximate s i z e s of p l a n k t e r s i n l a k e and e n c l o s u r e samples 41 T a b l e 8. Number of p l a n k t o n i c prey i n stomachs of f i s h t h a t c o n t a i n e d prey 42 T a b l e 9. Comparison of the numbers of p l a n k t o n i c p r e y i n stomachs of s m a l l l i m n e t i c s , l a r g e l i m n e t i c s and b e n t h i c s t h a t c o n t a i n e d prey v44 T a b l e 10. C o r r e l a t i o n s between l o g number of prey per stomach, and s t a n d a r d l e n g t h and e s t i m a t e d g i l l r a k e r space f o r l i m n e t i c s 47 V LIST OF FIGURES F i g u r e 1. Photographs of l i m n e t i c s and b e n t h i c s 3 F i g u r e 2. Frequency d i s t r i b u t i o n of g i l l r a k e r numbers f o r Enos Lake s t i c k l e b a c k s 5 F i g u r e 3. R e l a t i o n s h i p between g i l l r a k e r l e n g t h and s t a n d a r d l e n g t h of l i m n e t i c s and b e n t h i c s 6 F i g u r e 4. R e l a t i o n s h i p between maximum prey s i z e and s t a n d a r d l e n g t h of l a b o r a t o r y - r e a r e d s t i c k l e b a c k s 14 F i g u r e 5. Maximum prey s i z e v e r s u s s t a n d a r d l e n g t h f o r 6 w i l d s t i c k l e b a c k s 16 F i g u r e 6. Drawing of e n c l o s u r e used i n p l a n k t o n f e e d i n g exper iment 36 F i g u r e 7. R e l a t i o n s h i p between number of p l a n k t o n i c prey i n the stomachs of l i m n e t i c s , and the s t a n d a r d l e n g t h of f i s h t h a t consumed them 46 F i g u r e 8. R e l a t i o n s h i p between g i l l r a k e r s p a c i n g and the s t a n d a r d l e n g t h of b e n t h i c s and l i m n e t i c s ....52 v i ACKNOWLEDGEMENTS I would l i k e t o thank my s u p e r v i s o r , Dr. J . D. M c P h a i l , f o r many l o n g d i s c u s s i o n s t h a t s t i m u l a t e d my i n t e r e s t i n the s t i c k l e b a c k s of Enos Lake, and f o r p r o v i d i n g the means f o r me t o study them. •I owe s p e c i a l thanks t o my f r i e n d and p a r t n e r i n the f i e l d , Mark Ridgway. Mark's study of the b r e e d i n g b e h a v i o u r of Enos Lake s t i c k l e b a c k s c o n t r i b u t e d t o my own p e r s p e c t i v e on these f i s h ; moreover, h i s company i n the f i e l d h e l p e d make the work a l o t of f u n . Thanks a l s o t o Dennis Lassuy, Steve Campana and Ma r v i n Rosenau f o r a s s i s t a n c e i n the f i e l d . Together we l e f t our mark on l o c a l p o p u l a t i o n s of abalone and salmon. Dr. J . N. M. Smith o f f e r e d v a l u a b l e a d v i c e a t key st a g e s d u r i n g the stu d y ; he a l s o reviewed the t h e s i s . Dave Z i t t i n of the B i o s c i e n c e s Data C e n t r e i n i t i a t e d me t o the j o y s of UNIX. H i s h e l p at the da t a a n a l y s i s stage of t h i s study was i n v a l u a b l e . Mr. N e i l S c o t t k i n d l y p e r m i t t e d us t o work a t Enos Lake. I am e s p e c i a l l y g r a t e f u l t o him f o r a l l o w i n g us t o s t a y a t h i s beach c a b i n d u r i n g the w i n t e r months, thus s p a r i n g us much mi s e r y i n the f i e l d and l o n g n i g h t s i n the A r l i n g t o n . F i n a l l y , a s p e c i a l thanks t o my f a m i l y f o r moral and m a t e r i a l s u p p o r t , and c o n s i d e r a b l e p a t i e n c e . P e r s o n a l support d u r i n g t h i s study was p r o v i d e d by a N a t u r a l S c i e n c e s and E n g i n e e r i n g Research C o u n c i l graduate s c h o l a r s h i p , and r e s e a r c h support was f u r n i s h e d by an NSERC o p e r a t i n g g r a n t . 1 GENERAL INTRODUCTION The t h r e e s p i n e s t i c k l e b a c k ( G a s t e r o s t e u s a c u l e a t u s ) i s a m o r p h o l o g i c a l l y v a r i a b l e s p e c i e s t h a t o c c u p i e s a wide v a r i e t y of f r e s h w a t e r and marine h a b i t a t s w i t h i n i t s h o l a r c t i c range. Much of the m o r p h o l o g i c a l v a r i a b i l i t y e x h i b i t e d by t h i s s p e c i e s i s c l i n a l , or a s s o c i a t e d w i t h major h a b i t a t t y p e s , ( i . e . marine v e r s u s f r e s h w a t e r s or l o t i c v e r s u s l e n t i c e n v i r o nments) (Munzing, 1963; Hagen and G i l b e r t s o n , 1972; Coad and Power, 1974; Moodie and Reimchen, 1976; G r o s s , 1978). A growing body of ev i d e n c e s u g g e s t s t h a t the s t r i k i n g m o r p h o l o g i c a l d i f f e r e n c e s found between s t i c k l e b a c k p o p u l a t i o n s are best e x p l a i n e d by n a t u r a l s e l e c t i o n , p a r t i c u l a r l y l o c a l s e l e c t i v e p r e s s u r e s e x e r t e d by v a r y i n g assemblages of p r e d a t o r s and c o m p e t i t o r s (Moodie, 1972b; Hagen and G i l b e r t s o n , 1973, B e l l , 1976; Reimchem, 1980). In some a r e a s , however, d i f f e r e n t forms of G. a c u l e a t u s appear t o occur t o g e t h e r (Hagen, 1967; N a r v e r , 1969; M i l l e r and Hubbs, 1969; Hagen and G i l b e r t s o n , 1972; Semler, 1971; Moodie, 1972a; Kynard, 1978). Most such i n s t a n c e s i n v o l v e e i t h e r s i m p l e polymorphism or p a r a p a t r y ( g e o g r a p h i c a l l y a d j a c e n t p o p u l a t i o n s i n e s s e n t i a l l y n o n - o v e r l a p p i n g c o n t a c t ) . In a few s m a l l l a k e s i n c o a s t a l B r i t i s h Columbia, however, m o r p h o l o g i c a l l y d i v e r g e n t p o p u l a t i o n s of s t i c k l e b a c k s occur i n sympatry ( i . e . they co-e x i s t i n the same l a k e ) . Enos Lake (49017'N 124009'W), on the e a s t e r n shore of Vancouver I s l a n d , i s such a l a k e . In Enos Lake, 1 2 " l i m n e t i c " and " b e n t h i c " forms of G a s t e r o s t e u s c o e x i s t . The names ( b e n t h i c and l i m n e t i c ) a l l u d e t o t h e i r use of space: l i m n e t i c s s c h o o l l o o s e l y i n the water column d u r i n g most of the y e a r , whereas b e n t h i c s u s u a l l y occur i n c l o s e a s s o c i a t i o n w i t h the s u b s t r a t e (Bentzen et_ a_l. , i n p r e p ) . The b e n t h i c and l i m n e t i c forms are d e s c r i b e d i n d e t a i l i n M c P h a i l ( i n p r e p . ) ; however, the f o l l o w i n g c u r s o r y d e s c r i p t i o n i s s u f f i c i e n t t o c h a r a c t e r i z e the two forms. The l i m n e t i c form i s c h a r a c t e r i z e d by a s l e n d e r body, a narrow snout, and v e r y l a r g e p r o t r u s i v e eyes (F-ig.1A). Apart from the s e a s o n a l b r e e d i n g c o l o u r of males, l i m n e t i c s are t y p i c a l l y c o u n t e r - c o l o u r e d ( i . e . s i l v e r y s i d e s and a dark d o r s a l s u r f a c e ) . The b e n t h i c form a t t a i n s a l a r g e r s i z e than the l i m n e t i c form and i s c h a r a c t e r i z e d by a r e l a t i v e l y deep body, a broad snout, and n o n - p r o t r u s i v e eyes ( F i g . l B ) . In c o l o u r , b e n t h i c s are t y p i c a l l y o l i v e or m o t t l e d brown. L i m n e t i c s and b e n t h i c s a l s o d i f f e r s t r i k i n g l y i n the morphology of t h e i r g i l l r a k e r s (a t r o p h i c t r a i t ) . In l i m n e t i c s the number of g i l l r a k e r s on the l e a d i n g edge of the f i r s t b r a n c h i a l a r c h ranges from 22 to 26, whereas the range i s between 17 and 21 i n b e n t h i c s ( F i g . 2 ) . The g i l l r a k e r s are l o n g i n l i m n e t i c s and r e l a t i v e l y s h o r t i n b e n t h i c s ( F i g . 3 ) . M o r p h o l o g i c a l ( M c P h a i l , i n p r e p . ) , b i o c h e m i c a l ( W i t h l e r , 1980) and b e h a v i o u r a l (Ridgway, i n prep.) e v i d e n c e i n d i c a t e s t h a t the l i m n e t i c and b e n t h i c forms i n Enos Lake are g e n e t i c a l l y i s o l a t e d from one a n o t h e r , and thus comprise two s e p a r a t e p o p u l a t i o n s ; however, m o r p h o l o g i c a l i n t e r m e d i a t e s between the 3 F i g u r e 1. Photographs of l i m n e t i c s and b e n t h i c s . A. L i m n e t i c male and female (male on t o p ) . B. B e n t h i c male and female (male on bottom). 5 Figure 2. Frequency d i s t r i b u t i o n of g i l l r a k e r numbers f o r Enos Lake s t i c k l e b a c k s . N = 70 b e n t h i c s , 70 l i m n e t i c s . E l ' IB-15. .. 15. .. J_ 1— Z G-3- .. 0 BENTHICS LIMNETICS 3 1G. 17- IB-19' EO. SI- EE- S3- E4- E5- EG- E7- EB NUMBER OF GILLRAKERS 6 Figure 3 . R e l a t i o n s h i p between g i l l r a k e r l e n g t h and standard length of l i m n e t i c s and b e n t h i c s . L i n e s represent f u n c t i o n a l r e gressions (N = 6 8 b e n t h i c s , 6 8 l i m n e t i c s ) . Benthics....(+) L i m n e t i c s . . . ( x ) STANDARD LENGTH CMM) 7 two forms, though r a r e , do o c c u r ( M c P h a i l i n p r e p ) . F u r t h e r m o r e , l a b o r a t o r y c r o s s e s between the two forms y i e l d r o b u s t , f e r t i l e o f f s p r i n g t h a t a r e m o r p h o l o g i c a l l y i n t e r m e d i a t e between the p a r e n t a l t y p e s ( M c P h a i l , p e r s . com.). The l a s t two o b s e r v a t i o n s suggest t h a t the m o r p h o l o g i c a l (and g e n e t i c ) i n t e g r i t y of the two forms i n Enos Lake must be m a i n t a i n e d by s e l e c t i o n a g a i n s t h y b r i d s . T h i s study d e s c r i b e s the f e e d i n g b e h a v i o u r of the two s y m p a t r i c forms of G a s t e r o s t e u s i n Enos Lake and a t t e m p t s t o determine i f l i m n e t i c s are p r i m a r i l y adapted t o f o r a g i n g on s m a l l z o o p l a n k t o n i n the water column, and i f b e n t h i c s are adapted t o f o r a g i n g on the l a r g e r prey t y p i c a l of l i t t o r a l and b e n t h i c e n v i r o n m e n t s . C e r t a i n l y , the e l o n g a t e , c l o s e l y spaced g i l l r a k e r s , and p e l a g i c h a b i t of l i m n e t i c s suggest a p l a n k t i v o r o u s l i f e s t y l e , whereas the more r o b u s t p r o p o r t i o n s of b e n t h i c s , as w e l l as w e l l as t h e i r tendency t o a s s o c i a t e w i t h the b e n t h i c s u b s t r a t e , suggest t h a t they are b e n t h i c f o r a g e r s . In a d d i t i o n , t h e r e i s an a p p a r e n t l y analagous s i t u a t i o n i n another c o a s t a l l a k e (Paxton Lake, on "Texada I s l a n d ) . In t h i s l a k e " b e n t h i c " and " l i m n e t i c " forms of G a s t e r o s t e u s t h a t are m o r p h o l o g i c a l l y s i m i l a r (but not i d e n t i c a l ) t o the Enos Lake forms a l s o c o - e x i s t . L a r s o n (1976) showed t h a t i n Paxton Lake the b e n t h i c form f o r a g e d p r i m a r i l y on macrobenthos, w h i l e the l i m n e t i c form f e d m a i n l y on z o o p l a n k t o n . L a r s o n ' s study w i l l be r e f e r r e d t o throughout t h i s t h e s i s . 8 I used two approaches to t e s t the hypothesis that l i m n e t i c s and benthics are adapted t o , and occupy, d i f f e r e n t t r o p h i c niches. In one approach I used a survey of stomach contents to document the general feeding behaviour of the two forms i n the lake. This study w i l l be reported elsewhere; however, p r e l i m i n a r y r e s u l t s i n d i c a t e that i n Enos Lake, l i m n e t i c s feed l a r g e l y on zooplankton, whereas benthics eat more "benthic" prey (Bentzen, unpub. da t a ) . In the second approach I compared the feeding behaviour and c a p a b i l i t i e s of the two forms through c o n t r o l l e d feeding experiments. This second approach forms the basis of my t h e s i s and attempts to t e s t three s p e c i f i c p r e d i c t i o n s about benthics and l i m n e t i c s . These p r e d i c t i o n s are as f o l l o w s : 1) Maximum prey s i z e . At a given s i z e , benthics should be capable of eating l a r g e r prey than l i m n e t i c s . 2) Foraging on a n a t u r a l benthic s u b s t r a t e . Given r e l a t i v e l y large (but sub-maximal) prey on a n a t u r a l substrate benthics should have greater feeding success than l i m n e t i c s . 3) Foraging on small zooplankton. Given small zooplanktonic prey, l i m n e t i c s should experience greater feeding success • than benthics. The bases for each of these p r e d i c t i o n s are developed f u r t h e r i n the body of the t h e s i s . 9 MAXIMUM PREY SIZE I n t r o d u c t i o n The p r e d i c t i o n t h a t b e n t h i c s t i c k l e b a c k s can consume l a r g e r prey than l i m n e t i c s t i c k l e b a c k s i s c e n t r a l t o the h y p o t h e s i s t h a t the two forms are adapted t o f o r a g i n g i n d i f f e r e n t e n v i r o n m e n t s . Z o o p l a n k t e r s i n the l i m n e t i c zone of Enos Lake r a r e l y exceed 1 mm i n l e n g t h , but p o t e n t i a l prey i n l i t t o r a l a r e a s i n c l u d e a v a r i e t y of i n v e r t e b r a t e s p e c i e s t h a t a t t a i n s i z e s an o r d e r of magnitude l a r g e r i n l i n e a r d i m e n s i o n s . T h i s d i f f e r e n c e i n the s i z e s of prey a v a i l a b l e i n the two environments s h o u l d m a n i f e s t i t s e l f i n a c o r r e s p o n d i n g d i f f e r e n c e i n the a b i l i t y of the two forms t o d e a l w i t h l a r g e p r e y . To c o n t r o l f o r e f f e c t s of e x p e r i e n c e on maximum prey s i z e , I t e s t e d t h i s p r e d i c t i o n w i t h l a b o r a t o r y - r e a r e d b e n t h i c s and l i m n e t i c s . I a l s o t e s t e d progeny of r e c i p r o c a l F1 c r o s s e s t o e v a l u a t e h y b r i d performance r e l a t i v e t o the two p a r e n t a l phenotypes. F i n a l l y , I t e s t e d w i l d caught b e n t h i c s and l i m n e t i c s t o supplement r e s u l t s o b t a i n e d w i t h the l a b o r a t o r y - r e a r e d f i s h . Methods Amphipods ( H y a l e l l a a z t e c a ) a re among the l a r g e s t prey consumed by both t y p e s of Enos Lake s t i c k l e b a c k s . T h i s s p e c i e s , however, was f a i r l y d i f f i c u l t t o o b t a i n i n adequate numbers and 10 a l a r g e enough s i z e range. I n s t e a d of H y a l e l l a , I used the e s t u a r i n e amphipod, Eogammarus c o n f e r v i c o l o s , as e x p e r i m e n t a l p r e y . T h i s s p e c i e s can be c o l l e c t e d i n l a r g e numbers and i n a wide range of s i z e s . Though p r i m a r i l y a b r a c k i s h water organism, i t t o l e r a t e d f r e s h water d u r i n g the s o r t i n g and e x p e r i m e n t a l p r o c e d u r e s w i t h n e g l i g i b l e m o r t a l i t y . C l e a r l y , Eogammarus i s not a normal prey of Enos s t i c k l e b a c k s ; however, i t resembles H y a l e l l a c l o s e l y i n g r o s s morphology and g e n e r a l a voidance response. The amphipods were c o l l e c t e d w i t h d i p n e t s i n the Squamish r i v e r e s t u a r y , and h e l d i n b r a c k i s h water u n t i l use. To f a c i l i t a t e s o r t i n g and h a n d l i n g , b o t t l e d c a r b o n a t e d water was used t o a n a e s t h e t i z e the amphipods. Each amphipod was measured w i t h an o c u l a r micrometer and a s s i g n e d t o a s i z e c a t e g o r y on the b a s i s of body l e n g t h . I d e f i n e d body l e n g t h as the d i s t a n c e from the base of the antennae t o the base of the uropods, w i t h the abdomen s l i g h t l y f l e x e d . Most of the amphipods assumed t h i s c h a r a c t e r i s t i c bent p o s t u r e by themselves when a n a e s t h e t i z e d ; the few t h a t d i d not were r e a d i l y molded i n t o t h i s p o s t u r e w i t h f i n e f o r c e p s . The amphipods ranged from 4.4 mm t o 16.4 mm i n body l e n g t h and were d i v i d e d up i n t o 17 ranked s i z e c a t e g o r i e s . Each c a t e g o r y spanned 0.7 mm. B o r d e r l i n e i n d i v i d u a l s were a s s i g n e d t o the s m a l l e r of the two s i z e c a t e g o r i e s i n q u e s t i o n . A l l amphipods were a l l o w e d t o r e c o v e r from the e f f e c t s of the a n a e s t h e t i c b e f o r e they were p r e s e n t e d t o s t i c k l e b a c k s . 11 The f i s h used i n t h i s experiment were h e l d i n d i v i d u a l l y i n bare 20 1 aquariums. The a q u a r i a were s e p a r a t e d by opaque p a r t i t i o n s p l a c e d between aquariums, and b l a c k c u r t a i n s ( p e r f o r a t e d o n l y by a s m a l l v i e w i n g h o l e ) draped i n f r o n t . The bottoms of the a q u a r i a were p a i n t e d a u n i f o r m brown, and the i n s i d e seams of each aquarium were f i l l e d w i t h a smooth bead of s i l i c o n e s e a l a n t t o e l i m i n a t e c r e v i c e s i n which amphipods c o u l d h i d e . The water temperature i n the t e s t a q u a r i a remained a t 190 * 10 C f o r the d u r a t i o n of the expe r i m e n t . S i n c e the l a b o r a t o r y - r e a r e d s t i c k l e b a c k s were i n e x p e r i e n c e d w i t h prey more c h a l l e n g i n g than Tubi fex or the s t a n d a r d l a b o r a t o r y r a t i o n (ground f i s h ) , I f e d a l l t e s t f i s h amphipods from the s m a l l e s t s i z e c a t e g o r i e s f o r s e v e r a l days. T h i s was done t o accustom the f i s h t o t h i s prey t y p e . D u r i n g prey s i z e t r i a l s , t h r e e amphipods were s i m u l t a n e o u s l y p r e s e n t e d t o each f i s h . The t r i a l s were conducted at a p p r o x i m a t e l y the same time each day. Two of the t e s t amphipods were of a s i z e one s i z e c a t e g o r y l a r g e r than the l a r g e s t t h a t the t e s t f i s h had p r e v i o u s l y consumed. The t h i r d amphipod was of a s i z e two s i z e c a t e g o r i e s s m a l l e r than the two l a r g e r amphipods. A f t e r a p p r o x i m a t e l y 90 m i n u t e s , the uneaten amphipods were r e c o v e r e d , r e - a n a e s t h e t i z e d and re-measured. S i n c e amphipods of a d j a c e n t s i z e c a t e g o r i e s were never used i n the same t r i a l , t h e r e was no problem r e a s s i g n i n g amphipods t o the proper s i z e c a t e g o r i e s . The r e s u l t s of a t r i a l were a n a l y s e d o n l y i f a t l e a s t one 1 2 amphipod was e a t e n . E x p e r i m e n t a l t r i a l s ended when each f i s h had consumed amphipods of a g i v e n s i z e c a t e g o r y a t l e a s t t w i c e , and had f a i l e d t o consume amphipods of the next l a r g e r s i z e c l a s s f o r f o u r c o n s e c u t i v e days. R e s u l t s D u r i n g the co u r s e of the experiment most s t i c k l e b a c k s improved markedly i n t h e i r a b i l i t y t o handle and consume amphipods. At f i r s t the m a j o r i t y of l a b o r a t o r y - r e a r e d s t i c k l e b a c k s r e f u s e d a l l but the s m a l l e s t amphipods o f f e r e d ; however, t h i s i n i t i a l p e r i o d of r e l u c t a n c e was f o l l o w e d by a phase i n which most f i s h a c c e p t e d amphipods of s t e a d i l y i n c r e a s i n g s i z e . When exposed t o amphipods of a new, l a r g e r s i z e c a t e g o r y , s t i c k l e b a c k s o f t e n i n i t i a l l y f a i l e d t o eat the prey . U s u a l l y , however, i n one or two days they a q u i r e d e i t h e r the m o t i v a t i o n or the a b i l i t y t o d e a l the new s i z e c a t e g o r y . I f the new prey s i z e c a t e g o r y was not consumed i n t h r e e days, the s t i c k l e b a c k s r a r e l y mastered t h a t s i z e c a t e g o r y even w i t h p r o l o n g e d exposure. Thus, f o u r c o n s e c u t i v e " f a i l u r e s " s e r v e d as a c o n s e r v a t i v e c r i t e r i o n f o r e s t a b l i s h i n g maximum prey s i z e . G e n e r a l l y , the maximum s i z e of amphipods consumed i n c r e a s e d w i t h f i s h s i z e . S i n c e both amphipods and s t i c k l e b a c k s e x h i b i t a p p r o x i m a t e l y i s o m e t r i c growth, i t i s r e a s o n a b l e t o assume a l i n e a r r e l a t i o n s h i p between s t i c k l e b a c k s i z e and maximum prey s i z e . T h e r e f o r e , I used l i n e a r r e g r e s s i o n s t o d e s c r i b e the 1 3 r e l a t i o n s h i p between s t i c k l e b a c k l e n g t h and maximum prey s i z e , and a n a l y s e s of c o v a r i a n c e t o t e s t f o r d i f f e r e n c e s between groups. R e g r e s s i o n s of prey s i z e on s t a n d a r d l e n g t h of the l a b o r a t o r y - r e a r e d s t i c k l e b a c k s were s t a t i s t i c a l l y s i g n i f i c a n t (p= 0.017 and 0.014) f o r both b e n t h i c (BB) and l i m n e t i c (LL) c r o s s e s , but 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 (p > 0.1) f o r e i t h e r of the r e c i p r o c a l F1 c r o s s e s . The f a i l u r e of the F1 r e g r e s s i o n s t o a c h i e v e s t a t i s t i c a l s i g n i f i c a n c e can be a t t r i b u t e d i n p a r t t o the narrow range of s i z e s of F1 s t i c k l e b a c k s t e s t e d (Table 1); however, they a l s o appear t o r e f l e c t a g r e a t e r v a r i a b i l i t y i n the prey h a n d l i n g a b i l i t i e s i n the F1 f i s h than i n e i t h e r p a r e n t a l t y p e . Mean maximum prey s i z e s f o r the two r e c i p r o c a l F1 c r o s s e s d i d not d i f f e r s i g n i f i c a n t l y (F=1.39, p= 0.256, d.f.=1,16). When maximum prey s i z e data f o r the two F1 c r o s s e s were p o o l e d , the r e s u l t i n g r e g r e s s i o n was s t a t i s t i c a l l y s i g n i f i c a n t (p= 0.032). For t h i s reason I used the combined F1 " r e g r e s s i o n t o compare the h y b r i d c r o s s e s w i t h p a r e n t a l c r o s s e s . The r e s u l t s of these comparisons are summarized i n T a b l e 2 and F i g . 4 . L a b o r a t o r y -r e a r e d b e n t h i c s consumed s i g n i f i c a n t l y l a r g e r amphipods than l a b o r a t o r y - r e a r e d l i m n e t i c s ( p < 0 . 0 0 l ) , whereas F1 h y b r i d s tended t o consume amphipods t h a t were i n t e r m e d i a t e i n s i z e between those consumed by the p a r e n t a l t y p e s . However, the s i z e d i s t r i b u t i o n of amphipods taken by F1 h y b r i d s was h e a v i l y skewed 14 Fi g u r e 4. R e l a t i o n s h i p between maximum prey s i z e and standard ,. length of l a b o r a t o r y - r e a r e d s t i c k l e b a c k s . Lines represent p r e d i c t i v e r e g r e s s i o n s . Benthic x benthic c r o s s ...BB( +) Limnetic x l i m n e t i c c r o s s . . . L L ( x ) Pooled benthic x l i m n e t i c and l i m n e t i c x benthic c r o s s e s . . . F 1 ) STANDARD LENGTH OF TEST FISH (MM) Table 1. R e l a t i o n s h i p between maximum prey s i z e and standard length of l a b o r a t o r y - r e a r e d s t i c k l e b a c k s . P r o b a b i l i t i e s < 0.05 are considered s i g n i f i c a n t . Size range ( i n P r o b a b i l i t y mm) of f i s h t e s t e d . that slope=0 2 CROSS N MIN MAX r SLOPE p BB 7 37 59 0.71 0.180 0.017 LL 8 36 51 0.67 0.221 0.014 LB 10 45 50 0.18 0.219 BL 8 39 50 0.23 0.223 F1 18 39 50 0.51 0.125 0.032 (LB+BL) towards the s i z e range consumed by the benthic c r o s s . Thus, the maximum s i z e s of amphipods eaten by F1 s t i c k l e b a c k s were s i g n i f i c a n t l y l a r g e r than those consumed by l i m n e t i c s (p<0.00l) but not s i g n i f i c a n t l y smaller than those eaten by benthics (p=0.191). The r e s u l t s obtained with w i l d caught benthics and l i m n e t i c s p a r a l l e l e d those obtained with l a b o r a t o r y - r e a r e d f i s h . The three benthics were able to consume l a r g e r amphipods, r e l a t i v e to t h e i r body s i z e , than were the three l i m n e t i c s ( F i g . 5 ) . Unlike the l a b o r a t o r y - r e a r e d f i s h , however, none of the w i l d benthics h e s i t a t e d when f i r s t exposed to amphipods in the F i g u r e 5 . Maximum prey s i z e v e r s u s s t a n d a r d l e n g t h f o r s t i c k l e b a c k s . B e n t h i c s . . . . ( + ) L i m n e t i c s . . . ( x ) 1 7 Ta b l e 2. R e s u l t s of a n a l y s e s of c o v a r i a n c e on maximum prey s i z e and s t a n d a r d l e n g t h . P r o b a b i l i t i e s < 0.05 are c o n s i d e r e d s i g n i f i c a n t . TEST SLOPE P = INTERCEPT P = BB X LL 0.661 <0.00 1 BB X F1 0.461 0 . 1 9 1 F1 X LL 0.249 <0.001 l a b o r a t o r y . From the s t a r t of the experiment a l l t h r e e r e a d i l y s e i z e d and a t e r e l a t i v e l y l a r g e amphipods. In c o n t r a s t , the w i l d l i m n e t i c s were more l i k e t h e i r l a b o r a t o r y - r e a r e d c o u n t e r p a r t s and d i s p l a y e d some i n i t i a l r e l u c t a n c e t o a t t a c k any but the s m a l l e s t amphipods o f f e r e d . Thus, whereas the w i l d b e n t h i c s were p r o b a b l y a l r e a d y p r e c o n d i t i o n e d t o d e a l w i t h l a r g e and somewhat r e c a l c i t r a n t p r e y , t h i s was p r o b a b l y not t r u e of the w i l d l i m n e t i c s a t the s t a r t of the ex p e r i m e n t . D i s c u s s i o n For f i s h t h a t swallow t h e i r prey whole, mouth s i z e and maximum prey s i z e a re known t o be c l o s e l y r e l a t e d ( H y a t t , 1979). On t h i s b a s i s , Keast and Webb (1966) suggested t h a t i t might be u s e f u l t o measure the mouth gapes of c o e x i s t i n g f i s h s p e c i e s i n ord e r t o g a i n i n f e r e n c e s about t h e i r e c o l o g i c a l ( i . e . t r o p h i c ) 18 r e l a t i o n s h i p s . W h i l e b e n t h i c and l i m n e t i c s t i c k l e b a c k s of e q u i v a l e n t s i z e do appear to d i f f e r i n gape, t h i s d i f f e r e n c e i s s m a l l r e l a t i v e t o the measurement e r r o r (Bentzen, unpub. d a t a ) . N e v e r t h e l e s s , my r e s u l t s c l e a r l y i n d i c a t e t h a t b e n t h i c s a r e a b l e t o consume l a r g e r prey ( r e l a t i v e t o t h e i r body s i z e ) than l i m n e t i c s . L a r s o n (1976) o b t a i n e d s i m i l a r r e s u l t s f o r Paxton Lake s t i c k l e b a c k s . Paxton Lake b e n t h i c s consumed l a r g e r amphipods ( H y a l l e l a a z t e c a ) than l i m n e t i c s of e q u i v a l e n t body l e n g t h . A l t h o u g h t h e i r mouth w i d t h s were not measured, F1 h y b r i d s were i n t e r m e d i a t e between the p a r e n t a l forms i n o t h e r a s p e c t s of t h e i r morphology ( M c P h a i l , i n p r e p . ) . I f maximum prey s i z e i s a f u n c t i o n of mouth s i z e , then F1 f i s h might be exp e c t e d t o consume prey i n t e r m e d i a t e i n s i z e between those taken by the p a r e n t a l c r o s s e s . In f a c t , the s i z e of the amphipods consumed by the F l f i s h was skewed h e a v i l y towards the s i z e consumed by the pure b e n t h i c c r o s s . T h i s o b s e r v a t i o n may r e f l e c t a dominance r e l a t i o n s h i p i n genes c o n t r o l l i n g mouth s i z e or some o t h e r f a c t o r i n f l u e n c i n g prey h a n d l i n g a b i l i t y . U ndoubtedly, the d i f f e r e n c e s i n maximum prey s i z e o b s e r v e d between b e n t h i c s and l i m n e t i c s r e f l e c t a r e a l d i f f e r e n c e i n prey h a n d l i n g a b i l i t y . These maximum prey s i z e s , however, p r o b a b l y do not c o r r e s p o n d d i r e c t l y t o the upper l i m i t s on prey s i z e t h a t p r e v a i l i n the w i l d . Burko (1975) r e p o r t e d two c a t e g o r i e s of "maximum prey s i z e " f o r a p o p u l a t i o n of t h r e e s p i n e s t i c k l e b a c k s . 19 He found that even though l a r g e r prey were a v a i l a b l e , the l a r g e s t prey items consumed in the w i l d were u s u a l l y 20% smaller than the biggest prey the s t i c k l e b a c k s of equivalent s i z e could consume i n the l a b o r a t o r y . 20 FORAGING ON A NATURAL BENTHIC SUBSTRATE I n t r o d u c t i o n Amphipods p r e s e n t e d t o s t i c k l e b a c k s i n a b a r r e n , r e s t r i c t e d environment p r o v i d e an o b j e c t i v e e s t i m a t e of the upper s i z e l i m i t f o r prey; however, they o f f e r l i t t l e i n d i c a t i o n of the outcome of s i m i l a r e n c o u n t e r s under n a t u r a l c i r c u m s t a n c e s . In n a t u r e p o t e n t i a l r e f u g e s a re numerous. An i n t e n d e d prey may escape c a p t u r e by d i v i n g i n t o the s u b s t r a t e or c l i n g i n g t o a s o l i d s u r f a c e . To p r o v i d e a more r e a l i s t i c t e s t of the f e e d i n g a b i l i t i e s of Enos Lake s t i c k l e b a c k s , I added a n a t u r a l d e t r i t u s s u b s t r a t e a l o n g w i t h b e n t h i c prey of sub-maximal s i z e t o a l a b o r a t o r y tank. The p r e d i c t i o n remained the same: b e n t h i c s w i l l be more s u c c e s s f u l than l i m n e t i c s at d e a l i n g w i t h b e n t h i c p r e y . Methods E x p e r i m e n t a l t r i a l s were conducted i n a 87 1 (32 x 34 x 76 cm) aquarium. The f o r a g i n g s u b s t r a t e c o n s i s t e d of p a r t i c u l a t e l e a f and wood l i t t e r o b t a i n e d from the l i t t o r a l zone of Deer Lake, a s m a l l n a t u r a l l a k e i n Vancouver. T h i s d e t r i t a l mix was added t o the aquarium t o form a s u b s t r a t e l a y e r 1.5 - 2.0 cm t h i c k . The prey organisms used were those macrobenthic i n v e r t e b r a t e s a l r e a d y p r e s e n t i n the sediment: amphipods 21 ( H y a l e l l a a z t e c a ) , l e e c h e s ( H i r u d o SJD. ) , and i s o p o d s ( A s e l l u s sp_. ) . Prey numbers i n the t e s t aquarium were not counted; however, H y a l e l l a was the most abundant m a c r o - i n v e r t e b r a t e p r e s e n t . H y a l e l l a ranged from almost m i c r o s c o p i c s i z e t o a maximum of 7.5 mm i n body l e n g t h . To m i n i m i z e p o t e n t i a l v i s u a l d i s t r a c t i o n s t h a t might a f f e c t the b e h a v i o u r of the t e s t f i s h , the ends and b a c k s i d e of the aquarium were p a i n t e d l i g h t g rey. I l l u m i n a t i o n was p r o v i d e d by a s i n g l e 40 watt f l u o r e s c e n t b u l b suspended 50 cm above the aquarium. Water temperature remained a t 190 * 10 C throughout the e x p e r i m e n t . To observe f i s h d u r i n g f o r a g i n g t r i a l s , I s t a t i o n e d myself i n semidarkness 1.5 metres away from the aquarium, r e m a i n i n g as m o t i o n l e s s as p o s s i b l e d u r i n g the t r i a l . B e f o r e use i n f o r a g i n g t r i a l s , s t i c k l e b a c k s of both forms were m a i n t a i n e d i n a s i n g l e 195 1 h o l d i n g aquarium immediately a d j a c e n t t o the t e s t aquarium. At the b e g i n n i n g of each t r i a l , a s t i c k l e b a c k was d i p n e t t e d from the h o l d i n g tank and t r a n s f e r r e d to the t e s t aquarium. The b e h a v i o u r of the i n t r o d u c e d s t i c k l e b a c k was then o b s e r v e d . The time between the i n t r o d u c t i o n of the f i s h and i t s f i r s t f o r a g i n g s t r i k e was noted; the s t i c k l e b a c k was then a l l o w e d to f o r a g e f o r a f u r t h e r 15 minute p e r i o d , d u r i n g which the number of f e e d i n g s t r i k e s were counted. F o l l o w i n g t h i s 15 minute f o r a g i n g bout, the s t i c k l e b a c k was removed from the t e s t aquarium, b r a i n p i t h e d , and s e t a s i d e f o r l a t e r e x a m i n a t i o n of i t s stomach c o n t e n t s . S t i c k l e b a c k s t h a t 22 f a i l e d t o b e g i n f o r a g i n g w i t h i n 15 minutes were removed and r e p l a c e d by another f i s h . L i m n e t i c s and b e n t h i c s were t e s t e d i n a l t e r n a t i o n , and f o u r t o s i x t r i a l s were conducted a t a p p r o x i m a t e l y the same time each day. Immediately a f t e r the l a s t e x p e r i m e n t a l t r i a l of each day, the f i s h r e m a i n i n g i n the h o l d i n g tank were f e d to s a t i a t i o n on the s t a n d a r d l a b o r a t o r y d i e t . Thus, f i s h used i n the experiment were a l l d e p r i v e d of food f o r 22 - 24 hours p r i o r t o b e i n g t e s t e d . Stomach c o n t e n t s were s o r t e d , counted and r e c o r d e d a l o n g w i t h sex and s t a n d a r d l e n g t h f o r each t e s t f i s h . The body l e n g t h s of amphipods, and the l e n g t h and w i d t h of the i s o p o d c a r a p a c e s were measured. To guard a g a i n s t d e p l e t i o n of prey numbers i n the t e s t aquarium, each day I r e p l a c e d the prey t h a t were consumed w i t h an e q u a l number of l i v e i n v e r t e b r a t e s of the same s p e c i e s and of s i m i l a r s i z e . R e s u l t s A t o t a l of 20 b e n t h i c s and 22 l i m n e t i c s were t e s t e d . A l l of the b e n t h i c s f o r a g e d r e a d i l y i n the t e s t aquarium, as d i d 15 l i m n e t i c s . A l l of the l i m n e t i c s t h a t f e d were males; of the 7 l i m n e t i c s t h a t d i d not f e e d , 6 were fem a l e s . S i n c e the b e h a v i o u r of the f i s h t h a t d i d not fee d appeared o t h e r w i s e t o be normal, i t seems u n l i k e l y t h a t t h e i r f a i l u r e t o f o r a g e was the r e s u l t of 23 d i s c o m f o r t or f r i g h t . S e v e r a l of the females t h a t f a i l e d t o forage on the s u b s t r a t e d i d chase s t r a y c y c l o p o i d copepods t h a t were p r e s e n t i n the water column of the aquarium. I t i s a l s o u n l i k e l y t h a t the seven l i m n e t i c s f a i l e d t o p e r c e i v e the " i n t e n d e d " prey i n t h i s e x p e r i m e n t . G a s t e r o s t e u s has e x c e l l e n t v i s i o n (Wootton, 1976), and even t o the human eye the s u b s t r a t e teemed w i t h amphipods and ot h e r i n v e r t e b r a t e s . S t i c k l e b a c k s t h a t d i d f e e d o f f the s u b s t r a t e d i d not h e s i t a t e l o n g b e f o r e s t r i k i n g a t p r e y . The e l a p s e d time between i n t r o d u c t i o n and the s t a r t of f o r a g i n g was never l o n g e r than 6 minutes, and 75% of the b e n t h i c s (both sexes) and 67% of the male l i m n e t i c s began t o fora g e w i t h i n two mi n u t e s . Among the f i s h t h a t f o r a g e d , b e n t h i c s were g e n e r a l l y more s u c c e s s f u l than l i m n e t i c s ( T a b l e 3 ) ; however, the l i m n e t i c s averaged more s t r i k e s at the s u b s t r a t e d u r i n g the 15 minute f o r a g i n g i n t e r v a l than the b e n t h i c s ( l i m n e t i c x = 27.0, b e n t h i c x = 21.6). T h i s d i f f e r e n c e i n the number of s t r i k e s a t the s u b s t r a t e was not s i g n i f i c a n t (Mann Whitney U t e s t , p > 0.1) but the b e n t h i c s c a p t u r e d more prey than the l i m n e t i c s ( b e n t h i c x = 8.6, l i m n e t i c x = 4.9, U t e s t , p < 0.01). The performance of the two forms d i f f e r e d more s t r i k i n g l y when t h e i r s u c c e s s r a t e s (number of prey eaten d i v i d e d by number of s t r i k e s ) were compared. B e n t h i c s averaged 0.45 prey per f e e d i n g s t r i k e , whereas male l i m n e t i c s averaged 0.013 prey each 24 Tab l e 3. Performance (by groups) of f i s h t e s t e d i n b e n t h i c f o r a g i n g e xperiment. A. D e s c r i p t i v e s t a t i s t i c s , BENTHICS --• Males Females (N=9) S.E. (N=1 1 ) x S.E, Both sexes (N=20) x S.E. LIMNETICS Males ( N=15) x S.E. Number of s t r i k e s 2 2 . 8 2 . 2 1 2 0 . 6 .3.51 2 1 . 6 2 . 1 3 2 7 . 0 3.71 Number of prey consumed 9.3 1.85 8.0 0.91 8.6 0.95 4.9 0.69 Success (prey per s t r i k e ) 0.46 0.117 0.44 0.050 0.45 0.058 0.20 0.30 B. S t a t i s t i c a l comparisons (Mann Whitney U T e s t ) . A l l comparisons (except b e n t h i c male vs female) are one-t a i l e d . Numbers i n p a r e n t h e s e s i n d i c a t e v a l u e of U. P r o b a b i l i t i e s < 0.05 are c o n s i d e r e d s i g n i f i c a n t . Number of s t r i k e s Number of prey consumed Success (prey per s t r i k e ) MALE BENTHICS VS FEMALE BENTHICS P = >0. 1 0 (31.5) >0 . 1 (45.0) >0. 1 (41 .0) MALE LIMNETICS VS MALE BENTHICS P = >0.05 (64.0) <0.025 (30.0) <0.025 (29.0) FEMALE BENTHICS P = >0.05 (113.0) <0.01 (32.5) <0.001 (15.5) ALL BENTHICS P = >0.05 ( 1 16.0) <0.0 1 (62.5) <0.001 (44.5) (p < 0.001, Mann-Whitney U t e s t ) . The 20 b e n t h i c s used i n t h i s t e s t spanned a c o n s i d e r a b l e 25 s i z e range, whereas the s i z e d i s t r i b u t i o n of the l i m n e t i c s ( p a r t i c u l a r l y the sub-group t h a t foraged) was more l i m i t e d ( Table 4 ) . To check i f body s i z e i n f l u e n c e d f o r a g i n g performance, I t e s t e d f o r c o r r e l a t i o n s between the f i s h ' s s t a n d a r d l e n g t h and 3 o t h e r v a r i a b l e s : number of f e e d i n g s t r i k e s , number of prey c a p t u r e d , and c a p t u r e s u c c e s s r a t e . For both forms the number of prey c a p t u r e d and the c a p t u r e success r a t e showed a weak tendency t o i n c r e a s e w i t h s i z e of f i s h t e s t e d ; however, these t r e n d 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 ( Table 5 ) . Table 4. St a n d a r d l e n g t h s of f i s h t e s t e d i n b e n t h i c f o r a g i n g e x p e r i m e n t . STANDARD LENGTHS (MM) CATEGORY N MIN MAX MEAN B e n t h i c s (males) 9 B e n t h i c s ( f e m a l e s ) 1 1 B e n t h i c s (both sexes) 20 L i m n e t i c s (males) 16 L i m n e t i c s ( f e m a l e s ) 6 40 56 47.7 41 62 49.6 40 -62 48.8 40 48 45.5 34 42 37.5 26 T a b l e 5. R e s u l t s of t e s t s f o r c o r r e l a t i o n s between s t a n d a r d l e n g t h s of f i s h t h a t f e d and f o r a g i n g performance i n b e n t h i c f o r a g i n g e x p e r i m e n t . BENTHICS LIMNETICS 2 2 r P r p Number of s t r i k e s # 0.047 >0.1 0.040 >0.1 Number of prey consumed # 0.076 >0.1 0.208 0.088 Success (prey per s t r i k e ) ## 0.118 >0.1 0.210 0.085 #...square r o o t t r a n s f o r m e d ## a r c s i n t r a n s f o r m e d The two forms d i d not d i f f e r much i n t h e i r s e l e c t i o n of pr e y . Amphipods were the dominant prey consumed by both forms (79% f o r b e n t h i c s and 83% f o r l i m n e t i c s ) . Leeches and i s o p o d s formed the remainder (19% and 2% of the prey f o r b e n t h i c s and and 13% and 4% of the prey f o r l i m n e t i c s ) . S i z e s e l e c t i o n of prey was a l s o not a f a c t o r i n t h i s e x p e r i m e n t , s i n c e the mean s i z e of amphipods eaten was the same f o r both forms (3.4 mm body l e n g t h ) . In t h i s experiment the l a r g e s t amphipod consumed (by a b e n t h i c ) was 5.3 mm i n body l e n g t h ; t h i s was s m a l l e r than the b i g g e s t amphipods consumed by the s m a l l e s t l i m n e t i c t e s t e d i n my maximum prey s i z e experiment ( F i g . 5 ) . L i m n e t i c s and b e n t h i c s a l s o d i f f e r e d l i t t l e i n the way they s e c u r e d prey from the s u b s t r a t e . Both forms adopted the head-27 down p o s t u r e t y p i c a l of b e n t h i c f o r a g e r s (see H y a t t , 1979), o r i e n t i n g the body a t 30 - 70 degrees t o the h o r i z o n t a l w h i l e s t u d y i n g the s u b s t r a t e . A f e e d i n g s t r i k e c o n s i s t e d of a sudden l u n g e , f o l l o w e d by a wrenching b i t e a t the i n t e n d e d t a r g e t . The s t i c k l e b a c k b r i e f l y mouthed the prey (and a s s o c i a t e d d e b r i s ) •, and then spat out both prey and d e b r i s w e l l above the s u b s t r a t e . The s t i c k l e b a c k then attempted to s e i z e the prey b e f o r e i t reached the s u b s t r a t e . L i m n e t i c s appeared t o succeed l e s s o f t e n then b e n t h i c s i n wrenching the i n t e n d e d prey l o o s e from the s u b s t r a t e . They a l s o seemed t o l o s e more prey than the b e n t h i c s a t the " s p i t t i n g o u t " s t a g e . A p p a r e n t l y , b e n t h i c s were b e t t e r a t subduing prey through "mouthing", thus r e d u c i n g the p r o b a b i l i t y of the prey e s c a p i n g d u r i n g d u r i n g the s o r t i n g p r o c e s s . P i s c u s s i o n These r e s u l t s c o n f i r m the p r e d i c t i o n t h a t b e n t h i c s a r e b e t t e r than l i m n e t i c s i n d e a l i n g w i t h l a r g e prey on a n a t u r a l s u b s t r a t e . However, they a l s o p r o v i d e a t l e a s t p r e l i m i n a r y e v i d e n c e f o r another (unexpected) c o n c l u s i o n : mature male and female l i m n e t i c s d i f f e r i n t h e i r b e n t h i c f o r a g i n g performance. Female l i m n e t i c s show no i n c l i n a t i o n t o fe e d on b e n t h i c p r e y , whereas male l i m n e t i c s are more l i k e b e n t h i c s i n t h i s r e g a r d . Given the o p p o r t u n i t y male l i m n e t i c s r e a d i l y s e a r c h f o r , and a t t a c k , b e n t h i c p r e y . 28 T h i s o b s e r v a t i o n p r o b a b l y r e f l e c t s a genuine d i f f e r e n c e i n the t r o p h i c a d a p t a t i o n of. male and female l i m n e t i c s t h a t c o r r e s p o n d s t o d i f f e r e n c e s i n t h e i r r e p r o d u c t i v e b e h a v i o u r . In Enos Lake male l i m n e t i c s defend t e r r i t o r i e s , n e s t s and s u c c e s s i v e broods of eggs and f r y from e a r l y May t o mid J u l y . T h i s a c t i v i t y a l l t a k e s p l a c e a l o n g the extreme l i t t o r a l f r i n g e of the l a k e , i n water r a n g i n g from 20 t o 100 cm i n depth (Ridgway, i n p r e p . ) . L i f e i n such an environment might be expected t o fa v o u r both a p r o c l i v i t y f o r , and an a b i l i t y t o d e a l w i t h , b e n t h i c p r e y . In c o n t r a s t , female l i m n e t i c s o n l y e n t e r the l i t t o r a l zone t o breed (Ridgway, i n p r e p . ) , and at o t h e r times s c h o o l i n the l i m n e t i c zone, where they f o r a g e on z o o p l a n k t o n . T h i s c r o s s - o v e r between r e p r o d u c t i v e and t r o p h i c h a b i t s may a l s o be viewed i n another l i g h t . B r e e d i n g male s t i c k l e b a c k s of any t y p e , i n c l u d i n g Enos Lake l i m n e t i c s , w i l l c h a l l e n g e and f r e q u e n t l y a t t a c k a n y t h i n g ( i n c l u d i n g s m a l l i n v e r t e b r a t e s ) (Wootton 1976, Ridgway, i n prep.) t h a t moves w i t h i n t h e i r t e r r i t o r i e s . Both my maximum prey s i z e and b e n t h i c f o r a g i n g e x p e r i m e n t s were conducted d u r i n g the summer and the male s t i c k l e b a c k s t h a t I used were not o n l y mature, but a l s o i n r e p r o d u c t i v e c o n d i t i o n . U n l i k e f e m a l e s , which were not a g g r e s s i v e , males were pugnacious i n t h e i r r e sponses t o both o t h e r f i s h and i n v e r t e b r a t e s . T h i s was most e v i d e n t i n the maximum prey s i z e e x p e r i m e n t . When p r e s e n t e d w i t h l a r g e amphipods, l i m n e t i c males t y p i c a l l y lunged f o r w a r d i m m e d i a t e l y 29 t o g r a p p l e w i t h the p r e y . In the same s i t u a t i o n female l i m n e t i c s tended t o huddle i n the back of t h e i r a q u a r i a , though when g i v e n enough time they d i d e v e n t u a l l y eat the amphipods. In c o n t r a s t , however, female l i m n e t i c s adapt r e a d i l y t o consuming T u b i f e x o f f the bottoms of a q u a r i a . T u b i f e x are p a s s i v e p r e y , whereas amphipods, l i k e o t h e r w e l l - a r m o u r e d a r t h r o p o d s , s t r u g g l e v i g o r o u s l y when mouthed by a p r e d a t o r . Thus, the d i f f e r e n c e i n the tendency of male and female l i m n e t i c s t o f o r a g e on the r o b u s t b e n t h i c prey o f f e r e d i n t h i s experiment may be p a r t l y a consequence of d i f f e r e n c e s i n t h e i r i n h e r e n t a g g r e s s i v e n e s s . I f t h i s i s so, then the h i g h l e v e l s of t e s t o s t e r o n e r e s p o n s i b l e f o r the a g g r e s s i v e n e s s of males (Wootton, 1976) a l s o i n f l u e n c e f o r a g i n g b e h a v i o u r . T h i s s u g g e s t i o n does n o t , however, a p p l y t o b e n t h i c s t i c k l e b a c k s . The male and female b e n t h i c s I t e s t e d d i d not d i f f e r s i g n i f i c a n t l y i n any a s p e c t of t h e i r f o r a g i n g performance (Table 3 ) . A p p a r e n t l y , no r e p o r t s of such e f f e c t s of r e p r o d u c t i v e hormones on f o r a g i n g b e h a v i o u r of f i s h e x i s t i n the l i t e r a t u r e . Not s u r p r i s i n g l y , however, hunger has'been shown t o be an i m p o r t a n t p h y s i o l o g i c a l d e t e r m i n a n t of f o r a g i n g performance. Beukema (1968) found t h a t i n c r e a s e d l e v e l s of hunger m o t i v a t e d t h r e e s p i n e s t i c k l e b a c k s t o s e a r c h more i n t e n s i v e l y and a t t a c k h i g h e r p r o p o r t i o n s of the prey they encountered than when they were somewhat s a t i a t e d . F i f t e e n - s p i n e d s t i c k l e b a c k s ( S p i n a c h i a  s p i n a c h i a ) a l s o took l o n g e r t o swallow t h e i r prey as t h e i r hunger d e c r e a s e d ( K i s l a l i o g l u and G i b s o n , 1976a,b). S i m i l a r 30 o b s e r v a t i o n s have been made on ot h e r s p e c i e s of f i s h ( f o r a r e v i e w , see R i n g l e r , 1 9 7 9 ) . In both my f e e d i n g e x p e r i m e n t s w i t h b e n t h i c prey I s t a n d a r d i z e d the food d e p r i v a t i o n i n t e r v a l a t a p p r o x i m a t e l y 24 ho u r s . Thus, hunger i s u n l i k e l y t o have been a c o n f o u n d i n g i n f l u e n c e on my r e s u l t s . L a r s o n (1972) found t h a t f o r both b e n t h i c and l i m n e t i c s t i c k l e b a c k s from Paxton Lake the e f f e c t s of hunger (as c h a r a c t e r i z e d by Beukema's ( 1 9 6 8 ) c r i t e r i a ) s t a b i l i z e d a f t e r 24 hours of food d e p r i v a t i o n . L a r s o n (1976) compared the amount of time l i m n e t i c and b e n t h i c s t i c k l e b a c k s from Paxton Lake spent s e a r c h i n g f o r prey on a l e a f l i t t e r s u b s t r a t e under c o n t r o l l e d c o n d i t i o n s i n the l a b o r a t o r y . He found t h a t b e n t h i c s spent more time s e a r c h i n g the l e a f l i t t e r than l i m n e t i c s . U n f o r t u n a t e l y , as elsewhere i n h i s st u d y , L a r s o n made no r e f e r e n c e t o the sex of the f i s h he t e s t e d . • D i f f e r e n c e s i n e x p e r i m e n t a l d e s i g n between L a r s o n ' s b e n t h i c f o r a g i n g experiment and my experiment make d i r e c t comparisons i m p o s s i b l e . However, my r e s u l t s g i v e no i n d i c a t i o n t h a t male Enos Lake l i m n e t i c s d i f f e r a p p r e c i a b l y from b e n t h i c s i n t h e i r i n c l i n a t i o n t o s e a r c h f o r and a t t a c k b e n t h i c p r e y . The male l i m n e t i c s t h a t I t e s t e d appeared to i n v e s t i g a t e the d e t r i t u s s u b s t r a t e w i t h the same i n t e n s i t y , and p u n c t u a t e d t h e i r s e a r c h w i t h as many f e e d i n g s t r i k e s , as d i d the b e n t h i c s . The major 31 d i f f e r e n c e I found between male l i m n e t i c s and b e n t h i c s was i n t h e i r r a t e of s u ccess at f o r a g i n g . The b e n t h i c s were more adept than the male l i m n e t i c s a t m a n i p u l a t i n g and subduing l a r g e p r e y . A p p a r e n t l y , t h e i r s u p e r i o r i t y i n t h i s r e g a r d r e s u l t s from h a v i n g more p o w e r f u l , and perhaps l a r g e r , mouths than l i m n e t i c s . I t i s noteworthy t h a t the p o s i t i v e c o r r e l a t i o n between "success r a t e " f o r t e s t s t i c k l e b a c k s and t h e i r s t a n d a r d l e n g t h s approached s t a t i s t i c a l s i g n i f i c a n c e more c l o s e l y f o r male l i m n e t i c s than f o r b e n t h i c s (Table 5) . The c i r c u m s c r i b e d s i z e d i s t r i b u t i o n of male l i m n e t i c s (11 out of 15 were e i t h e r 46 or 47 mm i n l e n g t h ) makes t h i s o b s e r v a t i o n a l l the more remarkable. The amphipods t h a t were the dominant prey i n t h i s experiment may have approached the " r e a l i s t i c " upper l i m i t s of prey s i z e more c l o s e l y f o r male l i m n e t i c s than f o r b e n t h i c s . By " r e a l i s t i c " I mean the l i m i t s on prey s i z e t h a t p r e v a i l under n a t u r a l c o n d i t i o n s , r a t h e r than i n b a r r e n l a b o r a t o r y a q u a r i a . My l i m i t e d o b s e r v a t i o n s on the f o r a g i n g performance of b e n t h i c and l i m n e t i c s t i c k l e b a c k s can be compared t o H y a t t ' s (1980) e x t e n s i v e study of the f o r a g i n g s t r a t e g i e s and c a p a b i l i t i e s of rainbow t r o u t and Kokanee. H y a t t compared rainbow t r o u t (Salmo g a i r d n e r i ) and Kokanee (Oncorhynchus nerka) i n M a r i o n Lake, a s m a l l c o a s t a l l a k e . Kokanee are p r i m a r i l y p l a n k t i v o r o u s throughout most of t h e i r range, but i n Marion Lake macrobenthos forms a major p o r t i o n of t h e i r d i e t . Rainbow t r o u t i n M a r i o n Lake a l s o f o r a g e on b e n t h i c p r e y ; however, rainbow t r o u t tend t o s e l e c t l a r g e , armoured, e p i b e n t h i c p r e y , whereas 32 Kokanee take prey t h a t a re s m a l l or r e l a t i v e l y unarmoured. In e x p e r i m e n t a l t r i a l s , H y a t t found t h a t a l t h o u g h t h e i r mouth s i z e s d i d not appear t o be l i m i t i n g , Kokanee e x p e r i e n c e d v e r y l i t t l e s u c c e s s i n m a n i p u l a t i n g l a r g e armoured pr e y ; whereas, w i t h i d e n t i c a l prey rainbow t r o u t f a r e d much b e t t e r . Kokanee, however, d i d b e t t e r a t c a p t u r i n g and s w a l l o w i n g prey i f the prey were l a r g e but r e l a t i v e l y unarmoured. H y a t t a l s o found t h a t Kokanee were b e t t e r than rainbow t r o u t at e x p l o i t i n g some t y p e s of b e n t h i c p r e y . Kokanee employed a "grab and s o r t " method of s i f t i n g t hrough bottom sediments, and c o n s e q u e n t l y o b t a i n e d more s m a l l , hidden prey than the rainbow t r o u t . T h i s was because the t r o u t tended t o r e a c t o n l y t o exposed p r e y . Other o b s e r v a t i o n s support the view t h a t f i s h t h a t feed on s m a l l c o n c e a l e d prey a re s e l e c t e d f o r d i f f e r e n t a d a p t a t i o n s than f i s h t h a t f o r a g e on l a r g e r , exposed p r e y . I v l e v (1961) compared the f e e d i n g s u c c e s s of v a r i o u s b e n t h i c f o r a g e r s on c h i r o n o m i d l a r v a e t h a t were e i t h e r exposed on a bare s u b s t r a t e or c o n c e a l e d under a l a y e r of s i l t . Concealment had l i t t l e e f f e c t on the su c c e s s of some p r e d a t o r s ( i . e . c a r p , 'Cyprinus c a r p i o ) , but d r a s t i c a l l y reduced the f e e d i n g s u c c e s s of o t h e r s p e c i e s ( i . e . r o a c h , R u t i l u s r u t i l u s ) . In a s i m i l a r v e i n , Schutz and N o r t h c o t e (1972) found t h a t D o l l y Varden ( S a l v e l i n u s malma) were more e f f e c t i v e a t f i n d i n g and consuming c o n c e a l e d c h i r o n o m i d l a r v a e than c u t t h r o a t t r o u t (Salmo c l a r k i i ) . A g a i n , the D o l l y Vardens used a grab and s o r t t e c h n i q u e and the c u t t h r o a t s d i d n o t . 33 The method used t o sec u r e prey by both male l i m n e t i c s and b e n t h i c s i n my b e n t h i c f o r a g i n g experiment resembled the grab and s o r t t e c h n i q u e s of D o l l y Vardens and Kokanee; however the s t i c k l e b a c k s were o r i e n t i n g on c l e a r l y v i s i b l e p r e y . A v a r i e t y of c o n c e a l e d or i n c o n s p i c u o u s b e n t h i c p r e y , ( i . e . c h i r o n o m i d l a r v a e and o s t r a c o d s ) do occur i n the d i e t s of both b e n t h i c and l i m n e t i c Enos Lake s t i c k l e b a c k s ( B e n t z e n , u n p u b l i s h e d d a t a ) . A l t h o u g h my r e s u l t s c l e a r l y i n d i c a t e t h a t b e n t h i c s a re s u p e r i o r t o l i m n e t i c s when f o r a g i n g on c o n s p i c u o u s , e p i b e n t h i c p r e y , they say l i t t l e about how the two forms measure up when f e e d i n g on s m a l l , c o n c e a l e d b e n t h i c p r e y . T h i s i s a q u e s t i o n worthy of f u t u r e e x a m i n a t i o n . 34 FORAGING ON PLANKTON I n t r o d u c t i o n I put s t i c k l e b a c k s i n t o mesh e n c l o s u r e s suspended i n the water column of Enos Lake t o t e s t the h y p o t h e s i s t h a t l i m n e t i c s are b e t t e r adapted t o p l a n k t i v o r y than b e n t h i c s . T h i s experiment i s the c o u n t e r p a r t t o the b e n t h i c f o r a g i n g experiment d e s c r i b e d p r e v i o u s l y . A g a i n , I attempted t o compare the performance of the two forms of s t i c k l e b a c k s , but t h i s time i n a s i t u a t i o n r e s e m b l i n g t h a t i n which l i m n e t i c s n o r m a l l y f o r a g e . The l i m n e t i c zone of Enos Lake i s c h a r a c t e r i z e d by v e r y s m a l l z o o p l a n k t o n and t u r b i d , h i g h l y s t a i n e d water (Bentzen, unpub. d a t a ) . Rather than attempt t o r e c r e a t e these c o n d i t i o n s i n the l a b o r a t o r y , I surrounded s m a l l "chunks" of the l i m n e t i c environment w i t h net e n c l o s u r e s t h a t were d e s i g n e d t o keep s t i c k l e b a c k s i n but a l l o w z o o p l a n k t o n t o move i n and o u t . To overcome any i n i t i a l d i s t r e s s from b e i n g handled or c o n f i n e d the s t i c k l e b a c k s were l e f t i n the e n c l o s u r e s f o r s e v e r a l days. I then sampled p l a n k t o n both w i t h i n and o u t s i d e of the e n c l o s u r e s t o v e r i f y t h a t t h e r e were z o o p l a n k t o n i n the e n c l o s u r e s , and to see how the c o n c e n t r a t i o n s of p l a n k t o n w i t h i n the e n c l o s u r e s compared to c o n c e n t r a t i o n s i n the s u r r o u n d i n g l a k e . F i n a l l y , I examined the stomach c o n t e n t s of the f i s h h e l d i n the e n c l o s u r e s , t o o b t a i n r e l a t i v e i n d i c e s of the e x t e n t t o 35 which i n d i v i d u a l s of the two forms f e d on p l a n k t o n . Methods Three e n c l o s u r e s were c o n s t r u c t e d of woven n y l o n n e t t i n g (Marion T e x t i l e s I n c . , 3 x 6 mm o v a l mesh). The mesh was dyed l i g h t green t o reduce i t s v i s u a l c o n t r a s t w i t h s u r r o u n d i n g l a k e water. The e n c l o s u r e s were c y l i n d r i c a l bags, open a t the t o p and c l o s e d a t the bottom. They were two metres deep and 80 cm i n d i a m e t e r . Hoops of one i n c h f l e x i b l e PVC p i p e were sewn i n t o the bottom and top of each e n c l o s u r e t o p r o v i d e shape. The bottoms of the e n c l o s u r e s were weighted w i t h a p p r o x i m a t e l y 200 g of l e a d and the top s were buoyed w i t h p l a s t i c f l o a t s . The e n c l o s u r e s were anchored i n a p p r o x i m a t e l y 4 m of water (Fig..6). S t i c k l e b a c k s were c o l l e c t e d by t r a p p i n g and n i g h t - l i g h t i n g . Ten f i s h of each form were added t o each e n c l o s u r e . An e f f o r t was made to i n c l u d e a v a r i e t y of s i z e s and r e p r e s e n t a t i v e s of both sexes i n each group. The s t i c k l e b a c k s were l e f t i n the e n c l o s u r e s f o r 5 f u l l days. D u r i n g the m i d - a f t e r n o o n of the s i x t h day, the s t i c k l e b a c k s were removed i n r a p i d s u c c e s s i o n from the bags and k i l l e d and p r e s e r v e d i n 10% f o r m a l i n . C l o s e i n s p e c t i o n r e v e a l e d no s i g n of v o m i t i n g d u r i n g the k i l l i n g p r o c e s s . Immediately b e f o r e the s t i c k l e b a c k s were removed from each e n c l o s u r e , a p l a n k t o n sample was taken from the middl e of each 36 F i g u r e 6. Drawing of e n c l o s u r e used i n p l a n k t o n f e e d i n g e x p e r i m e n t . 37 i 38 e n c l o s u r e w i t h a S c h i n d l e r - P a t a l a s p l a n k t o n t r a p f i t t e d w i t h a 64 micron s c r e e n . Three p l a n k t o n samples were a l s o taken from the l a k e i n the immediate v i c i n i t y of the e n c l o s u r e s a t a depth of one metre. The stomach c o n t e n t s of each f i s h were counted and the f o l l o w i n g c a t e g o r i e s r e c o r d e d : n a u p l i i of copepods, copepods and c o p e p o d i t e s (mostly c y c l o p o i d , some h a r p a c t i c o i d ) , c l a d o c e r a n s ( b o s m i n i d s and c h y d o r i d s ) and r o t i f e r s ( a l l K e r a t e l l a ) . Measurements of s t a n d a r d l e n g t h and e s t i m a t e s of g i l l r a k e r s p a c i n g were a l s o made f o r each f i s h . T h i s was done t o t e s t the p o s s i b i l i t y t h a t f i s h s i z e or g i l l r a k e r s p a c i n g might be c o r r e l a t e d w i t h the r e l a t i v e performance of the f i s h i n t h i s e x p e r i m e n t . G i l l r a k e r s p a c i n g was e s t i m a t e d t o the n e a r e s t hundredth of a m i l l i m e t r e w i t h an o c u l a r micrometer f i t t e d t o a W i l d d i s s e c t i n g m i c r o s c o p e . The z o o p l a n k t o n samples from both the l a k e and e n c l o s u r e s were s o r t e d i n t o the c a t e g o r i e s l i s t e d above. T o t a l c o u n t s of a l l c a t e g o r i e s (except r o t i f e r s ) were made. Large numbers,-and a tendency t o clump t o g e t h e r , made t o t a l c o u n t s of r o t i f e r s i m p r a c t i c a l . I n s t e a d , s i n c e r o t i f e r s were not an im p o r t a n t prey of s t i c k l e b a c k s i n the e n c l o s u r e s (see r e s u l t s ) , I used a rough index of r o t i f e r abundance based on combined c o u n t s of subsamples from each of the o r i g i n a l samples. T h i s was a low e s t i m a t e , s i n c e I d i d not attempt t o count a l l the r o t i f e r s t h a t s t u c k t o g e t h e r i n clumps. 39 The t i m i n g of t h i s experiment proved t o be i m p o r t a n t . I f i r s t t r i e d i t i n J u l y 1980. A c o m b i n a t i o n of h i g h s u r f a c e water temperature (220 C) and a g g r e s s i v e males r e s u l t e d i n the death of most of the s m a l l l i m n e t i c s i n the e n c l o s u r e s . In mid October I ran the experiment a g a i n . The water was c o o l e r , but i t was u n u s u a l l y murky w i t h suspended o r g a n i c m a t t e r and poor i n z o o p l a n k t o n . A p p a r e n t l y the l a k e was u n d e r g o i n g t u r n o v e r . The stomachs of f i s h used i n t h i s t r i a l c o n t a i n e d d e t r i t u s , but few i d e n t i f i a b l e p r e y . I t r i e d a t h i r d time a t the end of March 1981. T h i s was about a month b e f o r e the b e g i n n i n g of the b r e e d i n g season. T h i s time the water temperature was moderate (100 C ) , males were not a g g r e s s i v e and z o o p l a n k t o n were p r e s e n t i n r e a s o n a b l e numbers. The r e s u l t s of t h i s t r i a l a r e p r e s e n t e d below. R e s u l t s The p r e y : Zooplankton were p r e s e n t i n r e l a t i v e l y low abundance both i n s i d e and o u t s i d e of the e n c l o s u r e s . The p a t t e r n was the same i n each e n c l o s u r e (Table 6A): r o t i f e r s were c l e a r l y n u m e r i c a l l y dominant w i t h c o n c e n t r a t i o n s of at l e a s t 60 per 1, f o l l o w e d by n a u p l i i (x = 11.5 per 1), copepods and c o p e p o d i t e s (x = 4.4 per 1), and c l a d o c e r a n s (x = 2.7 per 1). T h i s o r d e r of n u m e r i c a l abundance was the r e v e r s e of the ranked s i z e o r d e r of the i n d i v i d u a l 40 T a b l e 6. Zooplankton c o n c e n t r a t i o n s ( p l a n k t o n / l i t r e ) c a l c u l a t e d from samples. #1 A. ENCLOSURES #2 #3 mean f o r t h r e e e n c l o s u r e s Rot i f e r s >6.0 >60 >60 >60 N a u p l i i 10.7 12.2 11.7 11.5 Copepod(i t e ) s 3.8 4.8 4.5 4.4 C l a d o c e r a n s 2.7 2.9 2.4 2.7 #1 B. LAKE #2 #3 mean f o r t h r e e samples Rot i f e r s >60 >60 >60 >60 N a u p l i i 10.8 29. 1 14.6 18.2 C o p e p o d ( i t e ) s 5.4 13.9 5.7 8.3 C l a d o c e r a n s 0.2 0.2 0.0 •0.1 p l a n k t e r s t h a t made up each of the f o u r c a t e g o r i e s (Table 7 ) . On average c l a d o c e r a n s were the l a r g e s t p l a n k t o n , f o l l o w e d by c o p e p o d ( i t e ) s , n a u p l i i and f i n a l l y , r o t i f e r s . The t h r e e p l a n k t o n samples t a k e n d i r e c t l y from the l a k e i n 41 Table 7. Approximate s i z e s ( i n micrograms) of p l a n k t e r s i n l a k e and e n c l o s u r e samples. CATEGORY mean s i z e s t a n d a r d e r r o r C l a d o c e r a n s 0.5 * 0.01 C o p e p o d ( i t e ) s 0.3 * 0.02 N a u p l i i 0.2 ** Rot i f e r s 0.1 ** *...Based on c o n v e r s i o n of l e n g t h measurements f o r 30 i n d i v i d u a l s t o w e i g h t s u s i n g r e g r e s s i o n formulae d e v e l o p e d by C.J. W a l t e r s ( u n p u b l i s h e d r e s u l t s ) . * * . . E s t i m a t e from C.J. W a l t e r s ( u n p u b l i s h e d r e s u l t s ) . the v i c i n i t y of the e n c l o s u r e s d i f f e r e d from the e n c l o s u r e samples i n some r e s p e c t s (Table 6A,B). A g a i n , r o t i f e r s were the most abundant, f o l l o w e d r a t h e r d i s t a n t l y by n a u p l i i , copepods and c o p e p o d i t e s , and c l a d o c e r a n s . The o r d e r of abundance was the same as i n the e n c l o s u r e s ; however, n a u p l i i and c o p e p o d ( i t e ) s were more abundant i n the open l a k e samples than i n the e n c l o s u r e samples. C o n v e r s e l y , c l a d o c e r a n s were l e s s abundant i n the open l a k e samples than i n the e n c l o s u r e samples. C o n c e n t r a t i o n s of immature and mature copepods may have been lower i n the e n c l o s u r e s than i n the open l a k e as a r e s u l t of e i t h e r a v o i d a n c e of the e n c l o s u r e by these p l a n k t e r s , or g r a z i n g p r e s s u r e e x e r t e d by the s t i c k l e b a c k s i n the e n c l o s u r e s , or some c o m b i n a t i o n of these f a c t o r s . C l a d o c e r a n s , on the o t h e r hand, must have been a t t r a c t e d t o the e n c l o s u r e s . 42 The p r e d a t o r s : In the e n c l o s u r e s l i m n e t i c s f e d on z o o p l a n k t o n t o a g r e a t e r e x t e n t than b e n t h i c s . The r e s u l t s from each e n c l o s u r e show the same o v e r a l l , p a t t e r n (Table -8). Of the 30 f i s h of each form t h a t T a ble 8. Mean number of prey i n stomachs of b e n t h i c s and l i m n e t i c s t h a t c o n t a i n e d p r e y , and o n e - t a i l e d Mann Whitney U t e s t comparisons of means. ---BENTHICS-— — -LIMNETICS PREY TYPE ENC.# N X S. E. N X S.E. P= 1 5 1 .20 0. 58 9 47.7 8.9 <0. 05 CLADOCERANS 2 6 0.16 — 8 47.6 12.6 — — 3 4 0.75 c. 2b 1 0 21.4 5.0 <0. 01 combined 15 0.67 0. 23 27 37.9 .5.5 — 1 5 1 .20 0. 37 9 25.3 8.4 0. 002 COPEPOD(ITE)S 2 6 1.17 0. 1 7 8 18.4 6.3 <0. 001 3 4 0.75 0. 25 10 23. 9 6.7 <0. 01 combined 1 5 1 .07 0. 1 5 27 22.9 4.0 1 5 0.0 — — 9 42.7 16.5 — NAUPLII 2 6 0.0 --— 8 30.5 13.2 — — 3 4 0.0 --— 10 34.9 13.1 — — combined 1 5 0.0 — 27 36.2 8. 1 1 5 0.0 — — 9 16.3 8.8 — — ROTIFERS 2 6 0.16 --— 8 2.4 1 .3 — — 3 4 0.0 --— 10 0.9 0.5 --— combined 1 5 0.0 — — 27 6.5 3.2 1 5 2.4 0. 75 9 1 32. 0 30.7 <0. 01 ALL PREY 2 6 1 .5 0. 34 8 99.4 26.2 <0. 001 3 4 1 .5 0. 33 10 81.1 22.5 0. 001 combined 1 5 1 .8 0. 30 27 103.5 15.3 — were t e s t e d , 27 of the l i m n e t i c s and 15 of the b e n t h i c s , had z o o p l a n k t o n i n t h e i r stomachs. The 27 l i m n e t i c s averaged 103 prey per stomach; whereas, the 15 b e n t h i c s averaged 1.8 prey per 43 stomach. C l a d o c e r a n s were n u m e r i c a l l y the most abundant prey i n the stomachs of l i m n e t i c s (x = 37.9), f o l l o w e d by n a u p l i i (x = 36.2), copepods and c o p e p o d i t e s (x = 22.9), and f i n a l l y r o t i f e r s (x = 6.5). The few z o o p l a n k t o n consumed by b e n t h i c s were m o s t l y c l a d o c e r a n s , c o p e p o d i t e s and mature copepods. Body s i z e proved t o be an i m p o r t a n t f a c t o r i n the performance of l i m n e t i c s : s m a l l f i s h tended t o eat more z o o p l a n k t o n than t h e i r l a r g e r c o u n t e r p a r t s . The l i m n e t i c s I t e s t e d f e l l i n t o two d i s t i n c t groups on t h i s b a s i s ( T able 9 ) . The l a r g e l i m n e t i c s were a l l mature males ( p r o b a b l y 2 y e a r s and o l d e r i n age) and were a l l g r e a t e r than 43 mm i n l e n g t h . The s m a l l l i m n e t i c s were a l l l e s s than 36 mm i n l e n g t h . T h i s group c o n s i s t e d of immature one year o l d males and mature females (1-2 y e a r s of a g e ) . The stomachs of the s m a l l l i m n e t i c s averaged 137.5 p r e y . In c o n t r a s t , the l a r g e male l i m n e t i c s averaged o n l y 39.5 p r e y . In a d d i t i o n , the t h r e e l i m n e t i c s t h a t d i d not c o n t a i n food when they were r e c o v e r e d from the e n c l o s u r e s belonged t o the " l a r g e male" group. The 30 b e n t h i c s t e s t e d spanned an even g r e a t e r range of body s i z e s than t h e i r l i m n e t i c c o u n t e r p a r t s (Table 9 ) ; however, w i t h i n the b e n t h i c group t h e r e was no c o r r e l a t i o n between body s i z e and f e e d i n g performance. The mean s i z e of the 15 b e n t h i c s w i t h food i n t h e i r stomachs (45mm) was s l i g h t l y s m a l l e r than the mean s i z e of the 15 f i s h w i t h o u t food (48mm), but t h i s d i f f e r e n c e was not s i g n i f i c a n t (F=1.0 p = 0.33, d.f.= 1,28). 44 Table 9. Comparison of the numbers of prey found i n stomachs of s m a l l l i m n e t i c s , l a r g e l i m n e t i c s and b e n t h i c s t h a t c o n t a i n e d p r e y . Numbers i n p a r e n t h e s e s denote the t o t a l number of f i s h t e s t e d i n each group. St a n d a r d l e n g t h (mm) GROUP DESCRIPTION N min max mean Number of c o n t a i n e d stomach prey i n mean S.E, S m a l l l i m n e t i c s 16 (16) 26 35 30 147.5 18.2 Large (male) l i m n e t i c s 11 (14) 44 50 45 39.5 9.2 B e n t h i c s (both sexes) 15 (30) 34 61 45 1 . 8 My e s t i m a t e s of mean g i l l r a k e r space f o r the 30 l i m n e t i c s were l i n e a r l y c o r r e l a t e d w i t h t h e i r s t a n d a r d l e n g t h s ( r = . 9 3 l ) . T h e r e f o r e , the s u p e r i o r performance of the s m a l l l i m n e t i c s may r e f l e c t the a b i l i t y of f i s h w i t h narrow g i l l r a k e r s p a c i n g t o consume more s m a l l p l a n k t o n i c prey than f i s h w i t h wider g i l l r a k e r s p a c i n g . S i z e and g i l l r a k e r s p a c i n g showed a s i m i l a r r e l a t i o n s h i p w i t h prey number. For l i m n e t i c s , s t a n d a r d l e n g t h s and e s t i m a t e d g i l l r a k e r space were i n v e r s e l y c o r r e l a t e d f o r a l l prey c a t e g o r i e s (except r o t i f e r s ) w i t h the number of prey consumed. These c o r r e l a t i o n c o e f f i c i e n t s were s i g n i f i c a n t when dat a from the t h r e e e n c l o s u r e s were p o o l e d , and prey number was l o g t r a n s f o r m e d . C o r r e l a t i o n c o e f f i c i e n t s were h i g h e s t f o r 45 n a u p l i i , f o l l o w e d by c o p e p o d ( i t e ) s , and f i n a l l y c l a d o c e r a n s (Table 10). F i g u r e 7 shows the r e g r e s s i o n s f o r l i m n e t i c s of l o g prey number on s t a n d a r d l e n g t h f o r n a u p l i i , c o p e p o d ( i t e ) s and c l a d o c e r a n s . The s l o p e of the r e g r e s s i o n f o r n a u p l i i i s s t e e p e r than t h a t of e i t h e r of the o t h e r two r e g r e s s i o n s . T h i s d i f f e r e n c e i s s i g n i f i c a n t f o r the n a u p l i i - c o p e p o d ( i t e ) comparison (p = 0.01), and approaches s t a t i s t i c a l s i g n i f i c a n c e f o r the n a u p l i i - c l a d o c e r a n s comparison (p = 0.070). The s l o p e of the c o p e p o d ( i t e ) - c l a d o c e r a n r e g r e s s i o n s do not d i f f e r s i g n i f i c a n t l y ( p = 0.485). S i m i l a r r e l a t i o n s h i p s a p p l y t o the weaker a s s o c i a t i o n between prey number and e s t i m a t e d g i l l r a k e r space. T h e r e f o r e , the i n v e r s e r e l a t i o n s h i p between the number of prey consumed and body l e n g t h (or g i l l r a k e r space) i s more pronounced f o r n a u p l i i than f o r the somewhat l a r g e r c o p e p o d ( i t e ) s and c l a d o c e r a n s . D i s c u s s i o n The r e s u l t s of the e n c l o s u r e experiment complement the r e s u l t s of the b e n t h i c f o r a g i n g e x p e r i m e n t . The e v i d e n c e a g a i n p o i n t s t o the c o n c l u s i o n t h a t Enos Lake s t i c k l e b a c k s are d i f f e r e n t i a t e d i n t o t h r e e t r o p h i c groups. In the e n c l o s u r e experiment however, the r a n k i n g i n terms of f o r a g i n g s u c c e s s of the t h r e e groups i s the r e v e r s e of t h a t encountered w i t h l a r g e 46 F i g u r e 7. R e l a t i o n s h i p between number of p l a n k t o n i c prey i n the stomachs of l i m n e t i c s , and the standard l e n g t h of f i s h t h a t consumed them. L i n e s represent p r e d i c t i v e r e g r e s s i o n s (N = 27 f i s h ) . Cladocerans.... (A) Copepod(ite)s..(B) Naupl i i . . . . (C) 100.x • STANDARD LENGTH OF TEST FISH CMM) 47 T a b l e 10. C o r r e l a t i o n s between l o g number of prey per stomach, and s t a n d a r d l e n g t h and e s t i m a t e d g i l l r a k e r space f o r l i m n e t i c s . P r o b a b i l i t i e s < 0.05 a r e c o n s i d e r e d s i g n i f i c a n t . v e r s u s v e r s u s e s t i m a t e d s t a n d a r d l e n g t h g i l l r a k e r space PREY TYPE N r P r P Rot i f e r s 1 3 0. .377 >0. . 1 0. .452 >0. . 1 N a u p l i i 23 0. .805 <0. .0001 0. . 729 <0. .0001 C o p e p o d ( i t e ) s 26 0, .453 0. .02 0. . 323 >0. . 1 C l a d o c e r a n s 27 0. .555 0, .0027 0, .59 0. .0012 A l l Prey 27 0, .697 <0, .0001 0. .653 0. .0002 e p i b e n t h i c p r e y . The b e n t h i c s were a l l f a i l u r e s a t f e e d i n g on the s m a l l p l a n k t o n a v a i l a b l e i n the e n c l o s u r e s . H a l f of the b e n t h i c s had no food i n t h e i r stomachs, and the o t h e r h a l f averaged 1.8 prey per f i s h . Mature male l i m n e t i c s f a r e d b e t t e r ; 79% i n t h i s group had food i n t h e i r stomachs, and the average was 39.5 prey per f i s h . S m a l l l i m n e t i c s were the most s u c c e s s f u l group i n f o r a g i n g i n the e n c l o s u r e s ; they averaged 137.5 prey per f i s h . T h i s group spanned a range of s i z e s t y p i c a l of mature female and immature male l i m n e t i c s . The stomachs of b e n t h i c s , and mature male l i m n e t i c s , were e i t h e r empty or almost empty; i n c o n t r a s t , the stomachs of s m a l l l i m n e t i c s were a l l e i t h e r f u l l , or almost f u l l . 48 The r e s u l t s of t h i s experiment cannot be e x p l a i n e d by d i f f e r e n c e s i n the mechanism by which the d i f f e r e n t groups of s t i c k l e b a c k s f e d on p l a n k t o n . A l t h o u g h some p l a n k t i v o r o u s f i s h are a b l e t o maximize t h e i r f e e d i n g e f f i c i e n c y by s w i t c h i n g from p a r t i c u l a t e f e e d i n g to f i l t e r f e e d i n g as the s i t u a t i o n w a r r a n t s (Holonov and Tash, 1978), t h r e e s p i n e s t i c k l e b a c k s a r e s t r i c t l y p a r t i c u l a t e f e e d e r s (Wootton, 1976). When I o f f e r e d s t i c k l e b a c k s of both forms dense swarms of newly hatched A r t e m i a n a u p l i i i n the l a b o r a t o r y , they d i d not s w i t c h t o f i l t e r f e e d i n g but p e r s i s t e d i n consuming the n a u p l i i one a t a t i m e . The g e n e r a l s u p e r i o r i t y as p l a n k t i v o r e s of l i m n e t i c s compared t o b e n t h i c s c o u l d be p a r t i a l l y the r e s u l t of a c o r r e s p o n d i n g s u p e r i o r i t y i n v i s u a l a c u i t y . A n a l y t i c a l models of the dynamics of p l a n k t i v o r y d e v e l o p e d by Ware (1975) and Eggers (1977) i n d i r e c t l y s t r e s s the importance of v i s u a l a c u i t y f o r p l a n k t i v o r e s . Both models o p e r a t e on the assumption t h a t the number of prey consumed i n a g i v e n amount of time i s p r o p o r t i o n a l t o the volume of water s e a r c h e d . T h i s , i n t u r n , i s a f u n c t i o n of swimming speed and the s i z e of the v i s u a l f i e l d . The e x t e n t of the v i s u a l f i e l d i s d i r e c t l y r e l a t e d t o the v i s u a l a c u i t y of the p r e d a t o r , as w e l l as a v a r i e t y of e x t r i n s i c f a c t o r s , i n c l u d i n g l i g h t i n t e n s i t y and q u a l i t y , water c l a r i t y , the s i z e of prey and t h e i r i n h e r e n t c o n t r a s t w i t h the background l i g h t i n g and c o l o u r a t i o n (Lythgoe 1966, V i n y a r d and O'Brien 1976, Werner and H a l l 1974, Confer and B l a d e s 1975). Thus, the 49 murky water and somewhat monochromatic l i g h t combined w i t h the s m a l l , r e l a t i v e l y t r a n s p a r e n t prey t y p i c a l of both my e n c l o s u r e s and the l i m n e t i c zone of Enos Lake, might s e l e c t f o r v i s u a l a c u i t y i n l i m n e t i c s . In c o n t r a s t , v i s u a l a d a p t a t i o n t o the p e r c e p t i o n of r e l a t i v e l y l a r g e prey under the t w i l i g h t c o n d i t i o n s t h a t p r e v a i l a l o n g much of the b e n t h i c s u b s t r a t e i n Enos Lake might f a v o u r v i s u a l s e n s i t i v i t y over v i s u a l a c u i t y i n b e n t h i c s . C e r t a i n l y the r e l a t i v e l y l a r g e s i z e and the f o r w a r d p o s i t i o n i n g of the eyes i n l i m n e t i c s suggest a h i g h l e v e l of v i s u a l a c u i t y and w e l l developed b i n o c u l a r p e r c e p t i o n . The more l a t e r a l l y p l a c e d eyes of b e n t h i c s may be i n f e r i o r i n t h i s r e g a r d . S i n c e v i s u a l a c u i t y g e n e r a l l y i n c r e a s e s w i t h s i z e i n f i s h e s ( B l a x t e r , 1975; Nakamura, 1968) the s u p e r i o r performance i n the e n c l o s u r e s of the s m a l l l i m n e t i c s compared t o the l a r g e male l i m n e t i c s cannot be e x p l a i n e d i n terms of v i s u a l a c u i t y . A more l i k e l y e x p l a n a t i o n i n v o l v e s changes w i t h s i z e i n g i l l r a k e r s p a c i n g among l i m n e t i c s . I f g i l l r a k e r s a c t l i k e a s i e v e i n r e t a i n i n g p r e y , then f i n e r s p a c i n g of g i l l r a k e r s s h o u l d f a v o u r a g r e a t e r r e t e n t i o n of s m a l l p r e y . In c r i t i c a l r e v i e w s of the l i t e r a t u r e on g i l l r a k e r morphology and f u n c t i o n , K l i e w e r (1970) and H y a t t (1979) both c o n c l u d e d t h a t the ' s i e v e ' analogy i s an o v e r s i m p l i f i c a t i o n . Both a u t h o r s however, based t h e i r views on f i e l d and l a b o r a t o r y s t u d i e s where f i s h had the c h o i c e of a wide range of prey s i z e s . For e n e r g e t i c reasons (Eggers, 1977; Werner and H a l l , 1974) i n such c a s e s , p a r t i c u l a t e p l a n k t i v o r e s w i l l 50 s e l e c t the l a r g e s t z o o p l a n k t o n a v a i l a b l e . Thus, any i n t e r p r e t a t i o n of g i l l r a k e r f u n c t i o n on the b a s i s of prey consumption i n such e x p e r i m e n t s i s confounded by s i z e s e l e c t i o n on the p a r t of the p r e d a t o r . However, when the range of a v a i l a b l e prey s i z e s i s l i m i t e d the r e s u l t s may be e a s i e r to i n t e r p r e t . For example, Wankowski (1979) found a c l o s e c o r r e l a t i o n between mean g i l l r a k e r s p a c i n g and the minimum.size of graded food p e l l e t s t h a t j u v e n i l e A t l a n t i c salmon were a b l e to r e t a i n . In the e n c l o s u r e s s m a l l p l a n k t o n were the o n l y food a v a i l a b l e and thus s t i c k l e b a c k s had the c h o i c e of a t t e m p t i n g t o eat s m a l l prey or not e a t i n g a t a l l . Under such c o n d i t i o n s , the mechanics of g i l l r a k e r f u n c t i o n might supersede s i z e s e l e c t i v i t y as a dominant f a c t o r g o v e r n i n g prey consumption. N a u p l i i were the s m a l l e s t prey consumed i n any numbers i n the e n c l o s u r e s . Body w i d t h i n t h i s prey c a t e g o r y was a p p r o x i m a t e l y 0.1-0.15 mm. T h i s c o r r e s p o n d s c l o s e l y t o the g i l l r a k e r space I e s t i m a t e d f o r the s m a l l e s t group of l i m n e t i c s t e s t e d ( F i g . 9 ) . Such s m a l l prey might o f f e r a g r e a t e r c h a l l e n g e t o t h e ' r e t e n t i o n a b i l i t y of the g i l l r a k e r a p p a r a t u s than the l a r g e r c o p e p o d ( i t e ) s and c l a d o c e r a n s t h a t were a l s o consumed i n r e l a t i v e l y l a r g e numbers. C o r r e l a t i o n c o e f f i c i e n t s f o r the r e l a t i o n s h i p between prey number and e s t i m a t e d g i l l r a k e r space were h i g h e r f o r n a u p l i i than f o r e i t h e r c o p e p o d ( i t e ) s or c l a d o c e r a n s (Table 10). The c o r r e l a t i o n between the number of n a u p l i i consumed and e s t i m a t e d g i l l r a k e r space was not as h i g h as the c o r r e s p o n d i n g c o r r e l a t i o n 51 between n a u p l i i and the body l e n g t h (Table 10); however, t h i s was p r o b a b l y a consequence of measurement e r r o r . G i l l r a k e r space i s h a rd t o measure. Thus body l e n g t h of l i m n e t i c s t i c k l e b a c k s i s p r o b a b l y a b e t t e r p r e d i c t o r of f u n c t i o n a l g i l l r a k e r space than my a t t e m p t s at d i r e c t measurement. G i l l r a k e r s p a c i n g may have been a f a c t o r i n the f a i l u r e of b e n t h i c s t o feed s u c c e s s f u l l y i n the e n c l o s u r e s ; however, s i n c e the g i l l r a k e r s p a c i n g of many b e n t h i c s was s i m i l a r t o the g i l l r a k e r s p a c i n g i n l a r g e male l i m n e t i c s ( F i g . 8 ) s p a c i n g a l o n e cannot account f o r the d i s p a r i t y i n performance between the two forms. In h i s study of Paxton Lake s t i c k l e b a c k s , L a r s o n (1976) a l s o compared the a b i l i t y of l i m n e t i c s and b e n t h i c s t o d e a l w i t h p l a n k t o n i c p r e y . H i s e x p e r i m e n t a l approach, however, d i f f e r e d f u n d a m e n t a l l y from the one adopted i n t h i s s t u d y . L a r s o n p r e s e n t e d l a r g e (1.09 + 0.14 mm) Daphnia i n v a r y i n g c o n c e n t r a t i o n s to i n d i v i d u a l s t i c k l e b a c k s i n 35 1 a q u a r i a and found t h a t w i t h these prey the performance of l i m n e t i c s was s u p e r i o r t o t h a t of b e n t h i c s . I attempted t o repeat L a r s o n ' s experiment w i t h Enos Lake b e n t h i c s and l i m n e t i c s ; however, my r e s u l t s were d i f f e r e n t than those r e p o r t e d by L a r s o n . He found t h a t the f e e d i n g r a t e s of l i m n e t i c s i n e x p e r i m e n t a l a q u a r i a were c o n s i s t e n t l y h i g h e r than the f e e d i n g r a t e s of the b e n t h i c s . L i m n e t i c s spent l e s s time s e a r c h i n g f o r prey between s u c c e s s i v e c a p t u r e s , and m a n i p u l a t e d (swallowed) the Daphnia f a s t e r than 52 Figure 8 . The r e l a t i o n s h i p between g i l l r a k e r spacing and the standard length of benthics and l i m n e t i c s . L i n e s represent f u n c t i o n a l regressions (N = 30 b e n t h i c s , 30 l i m n e t i c s ) . Benthics....(+) L i m n e t i c s . . . ( x ) 9 d i—i ID 030 _ 0«E5 1 0-EO 1 0-15 1 0-10 1 0.05 E5- 30 - 35. 40 - 45 . 50 - 55- GO- G5-STANDARD LENGTH DF FISH CMM) 53 most b e n t h i c s . The m a n i p u l a t i o n t i m e s ranged between a p p r o x i m a t e l y 1.2 t o 2.5 seconds f o r l i m n e t i c s , and between a p p r o x i m a t e l y 1.8 t o 8.8 seconds f o r b e n t h i c s . A l s o , m a n i p u l a t i o n times tended t o decrease w i t h i n c r e a s e s i n body l e n g t h i n l i m n e t i c s but i n c r e a s e w i t h i n c r e a s e s i n body l e n g t h of b e n t h i c s . In c o n t r a s t t o L a r s o n ' s d a t a , Enos Lake b e n t h i c s consumed l a r g e Daphn i a more r a p i d l y than l i m n e t i c s ; however, the m a n i p u l a t i o n times f o r both forms were so f l e e t i n g t h a t I c o u l d not measure them, a l t h o u g h they c e r t a i n l y d i d not b e g i n t o approach the m u l t i - s e c o n d i n t e r v a l s r e p o r t e d by L a r s o n . I f i n d my r e s u l t s e a s i e r t o e x p l a i n than those r e p o r t e d by L a r s o n . Daphnia of one m i l l i m e t r e s i z e are not p a r t i c u l a r l y s m a l l prey f o r s t i c k l e b a c k s (Hynes,. 1950; L a r s o n , 1976; Burko, 1975). L o c a t i n g such prey i n the c l e a r water of the g e n e r o u s l y l i t and c o n f i n e d q u a r t e r s of e x p e r i m e n t a l a q u a r i a i s u n l i k e l y t o c h a l l e n g e the v i s u a l a b i l i t i e s of e i t h e r form. E q u a l l y , prey of t h i s type a r e not l i k e l y t o s t r a i n the m a n i p u l a t i v e a b i l i t i e s of e i t h e r type of s t i c k l e b a c k . Once caught, Daphn i a a re r e l a t i v e l y p a s s i v e p r e y , and t h e r e f o r e s w a l l o w i n g t i m e s s h o u l d be m i n i m a l . F i n a l l y , l a r g e Daphnia are not r e p r e s e n t a t i v e of the p o t e n t i a l z o o p l a n k t o n prey i n the l i m n e t i c zone of Enos Lake. In Enos Lake, Bosmina an o r d e r of magnitude s m a l l e r i n s i z e are the dominant c l a d o c e r a n s . 54 GENERAL DISCUSSION The phenomenon of s y m p a t r i c , m o r p h o l o g i c a l l y and e c o l o g i c a l l y d i v e r g e n t p o p u l a t i o n s of f i s h e s i s not l i m i t e d t o Enos and Paxton l a k e s , or even t o g a s t e r o s t e i d s . S t r i k i n g p a r a l l e l s occur i n w h i t e f i s h (Coreqonus). S y m p a t r i c p a i r s of m o r p h o l o g i c a l l y d i s t i n c t , but c l o s e l y r e l a t e d forms occur i n l a k e s s c a t t e r e d throughout the b o r e a l l a t i t u d e s of E u r a s i a ( B e r g , 1948; Svardson, 1957) and N o r t h America (Kennedy, 1943; L i n d s e y , 1962; Fenderson, 1964; B o d a l y , 1979; K i r k p a t r i c k and S e l a n d e r , 1979). S i m i l a r p a i r s i n the r e l a t e d genus Prosopium a l s o occur i n s e v e r a l l a k e s i n A l a s k a (McCart, 1970). In each case the p a t t e r n i s s i m i l a r : a "low" g i l l r a k e r form and a " h i g h " g i l l r a k e r form c o e x i s t . The h i g h g i l l r a k e r form i s p r i m a r i l y p l a n k t i v o r o u s , whereas the low g i l l r a k e r form e a t s a g r e a t e r p r o p o r t i o n of l a r g e b e n t h i c p r e y . The h i g h g i l l r a k e r forms tend t o be s m a l l ("stunted" or "dwarf") and s h o r t - l i v e d compared t o t h e i r low g i l l r a k e r c o u n t e r p a r t . S i m i l a r s i t u a t i o n s a l s o occur among o t h e r s a l m o n i d s . Brown t r o u t Salmo t r u t t a ) and A r c t i c c h a r r ( S a l v e l i n u s a l p i n u s ) occur as m o r p h o l o g i c a l l y d i v e r g e n t and t r o p h i c a l l y s e g r e g a t e d p a i r s i n a number of n o r t h e r n European l a k e s ( t r o u t : A l l e n d o r f e t a l . , 1976; c h a r : N i l s o n and F i l i p s s o n , 1971; Nyman, 1972; H e n r i c s o n and Nyman, 1976). 55 Svardson (1957, 1970) a t t r i b u t e d the e x i s t e n c e of- h i g h and low g i l l r a k e r forms of Coregonus i n S c a n d i n a v i a t o a l l o p a t r i c d i v e r g e n c e i n s e p a r a t e g l a c i a l r e f u g e s , and McCart (1970) used the same e x p l a n a t i o n t o account f o r h i g h and low g i l l r a k e r forms of Prosopium i n A l a s k a . In c o n t r a s t , K i r k p a t r i c k and S e l a n d e r (1979) i n t e r p r e t e d the presence of C. c l u p e a f o r m i s p a i r s i n t h r e e w i d e l y s e p a r a t e d a r e a s of N o r t h America (Yukon, e a s t e r n O n t a r i o and Maine) as e v i d e n c e of s e v e r a l independent e v o l u t i o n s of the two forms. T h i s r e p e a t e d p a t t e r n of t r o p h i c a l l y d i v e r g e n t s y m p a t r i c p a i r s i n g a s t e r o s t e i d s , c o r e g o n i d s , and salmonids suggests t h a t they may a l l have e v o l v e d through a s i m i l a r p r o c e s s . Such p a i r s a l l occur i n f o r m e r l y g l a c i a t e d a r e a s , and hence a l l are of p o s t - P l e i s t o c e n e age (at l e a s t i n terms of t h e i r c o e x i s t e n c e ) . I t i s p o s s i b l e t h a t such p a i r s are the p r o d u c t s of _in_ s i t u d i v e r g e n c e by the o r i g i n a l c o l o n i z i n g forms i n t o a l t e r n a t e t r o p h i c n i c h e s . The e x i s t e n c e o f . such p a i r s , u s u a l l y i n the absence of g e n e t i c i n c o m p a t i b i l i t y mechanisms, p o i n t s t o the presence of c o n t i n u a l s e l e c t i v e p r e s s u r e a g a i n s t i n t e r m e d i a t e s . T h i s i m p l i e s t h a t the m o r p h o l o g i c a l -- and b e h a v i o u r a l --s p e c i a l i z a t i o n s of the p a i r s r e p r e s e n t a l t e r n a t e , and m u t u a l l y e x c l u s i v e a d a p t a t i o n s . U n f o r t u n a t e l y , o n l y m o r p h o l o g i c a l d a t a are a v a i l a b l e f o r most known s y m p a t r i c p a i r s . B e h a v i o u r a l d a t a are l a c k i n g f o r a l l but the g a s t e r o s t e i d p a i r s i n Paxton and Enos Lakes. 56 My o b s e r v a t i o n s i n d i c a t e t h a t the two forms i n Enos Lake d i f f e r s u b s t a n t i a l l y i n t h e i r f e e d i n g b e h a v i o u r s and a b i l i t i e s . C l e a r l y , l i m n e t i c s a re adapted to f o r a g i n g on s m a l l z o o p l a n k t o n i c p r e y , and b e n t h i c s a re adapted t o f o r a g i n g on the l a r g e r prey t y p i c a l of b e n t h i c s u b s t r a t e s . Mature male and female l i m n e t i c s a l s o appear t o d i f f e r from one another i n t h e i r t r o p h i c a d a p t a t i o n s . The performance of mature male l i m n e t i c s i n the two f o r a g i n g e x p e r i m e n t s was i n t e r m e d i a t e between the w h o l e l y p l a n k t i v o r o u s l i m n e t i c females and the almost e q u a l l y n o n - p l a n k t i v o r o u s b e n t h i c s . I t i s u n c l e a r whether t h i s d i f f e r e n c e between the sexes i s an a r t i f a c t of the tendency f o r female l i m n e t i c s t o be s m a l l e r than males, or i f i t i s the r e s u l t of g e n e t i c a l l y "programmed" d i f f e r e n c e s i n f e e d i n g b e h a v i o u r between the sexes. The i s s u e d e s e r v e s f u r t h e r s t u d y . Among p e l a g i c , p l a n k t i v o r o u s f i s h e s , t h i s t r o p h i c s e x u a l dimorphism may be the consequence of a problem p e c u l i a r t o l i m n e t i c s t i c k l e b a c k s : males must spend the p r o d u c t i v e summer months i n a " b e n t h i c " environment, engaged i n the e n e r g e t i c a l l y demanding t a s k s of d e f e n d i n g t e r r i t o r i e s and progeny. The o b s e r v a t i o n t h a t no comparable s e x u a l l y d i m o r p h i c t r o p h i c d i f f e r e n c e s occur i n male and female b e n t h i c s adds- f u r t h e r weight t o the b a s i c c o n c l u s i o n of t h i s s t u d y : i n terms of t r o p h i c a d a p t a t i o n , the two forms a r e c l e a r l y d i f f e r e n t " s p e c i e s " . Thus, the r e s u l t s of my study a r e consonant w i t h the a v a i l a b l e m o r p h o l o g i c a l ( M c P h a i l i n p r e p ) , b i o c h e m i c a l ( W i t h l e r , 1980) and r e p r o d u c t i v e (Ridgway, i n prep.) d a t a . They a l l 57 i n d i c a t e t h a t i n Enos Lake l i m n e t i c and b e n t h i c s t i c k l e b a c k s a c t as good " b i o l o g i c a l s p e c i e s " . My r e s u l t s a l s o agree i n g e n e r a l , i f not i n d e t a i l , w i t h L a r s o n ' s (1976) o b s e r v a t i o n s on Paxton s t i c k l e b a c k s . L a r s o n d i d not r e p o r t on the sex of the f i s h he s t u d i e d , thus the p o s s i b i l i t y remains t h a t male and female Paxton l i m n e t i c s a l s o d i f f e r i n f e e d i n g b e h a v i o u r . T h i s p o s s i b i l i t y s h o u l d be checked. The maximum prey s i z e experiment i n d i c a t e s h y b r i d s are i n t e r m e d i a t e i n t h e i r c a p a c i t y t o p r o c e s s l a r g e b e n t h i c p r e y , and s i n c e h y b r i d s have i n t e r m e d i a t e g i l l r a k e r numbers ( M c P h a i l , i n p r e p . ) , they are p r o b a b l y a l s o i n t e r m e d i a t e i n t h e i r c a p a c i t y t o p r o c e s s s m a l l z o o p l a n k t o n . Thus, as l o n g as the d e n s i t i e s of the p a r e n t a l forms i n Enos Lake remain h i g h , m o r p h o l o g i c a l i n t e r m e d i a t e s between l i m n e t i c s and b e n t h i c s p r o b a b l y w i l l be at a s e l e c t i v e d i s a d v a n t a g e compared to the p a r e n t a l forms. I t i s p o s s i b l e t h a t such i n t e r m e d i a t e s might s u c c e s s f u l l y adopt a " g e n e r a l i s t " f e e d i n g s t r a t e g y , but t h i s seems u n l i k e l y . Werner et a l . (1981) have shown t h a t even among a (presumably) m o r p h o l o g i c a l l y u n i f o r m t e s t p o p u l a t i o n of b l u e g i l l s (Lepomis  m a c r o c h i r u s ) , i n d i v i d u a l s t h a t f a i l e d t o s p e c i a l i z e on e i t h e r p l a n k t o n or benthos were l e s s s u c c e s s f u l at f e e d i n g than those t h a t s p e c i a l i z e d . W i t h i n f i v e weeks of t h e i r i n t r o d u c t i o n t o a pond b a r r e n of o t h e r f i s h l i f e , most of the b l u e g i l l s s p e c i a l i z e d as e i t h e r p l a n k t o n f e e d e r s or benthos f e e d e r s . T h i s r a p i d t r o p h i c d i v e r g e n c e w i t h i n a p o p u l a t i o n may c o r r e s p o n d t o 58 the f i r s t s t e p i n the p r o c e s s t h a t has c u l m i n a t e d i n the e v o l u t i o n of the two forms of G a s t e r o s t e u s i n Enos Lake. 59 REFERENCES A l l e n d o r f , F.W., N. Ryman, A. Stennek and G. S t a h l . 1976. G e n e t i c v a r i a t i o n i n S c a n d i n a v i a n p o p u l a t i o n s of brown t r o u t (Salmo t r u t t a L . ) : e v i d e n c e of g e n e t i c a l l y d i s t i n c t s y m p a t r i c p o p u l a t i o n s . H e r e d i t a s 82:19-24. . B e l l , M.A. 1976. 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