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Effects of prey abundance on distribution, density and territorial behavior of young rainbow trout in… Slaney, Pat A. 1972

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EFFECTS OF PREY ABUNDANCE ON DISTRIBUTION, DENSITY AND TERRITORIAL BEHAVIOR OF YOUNG RAINBOW TROUT IN STREAMS b y PAT A. SLANEY B . S c , U n i v e r s i t y o f B r i t i s h C o lumbia, 1968 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n t h e Department o f 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 t h e r e q u i r e d s t a n d a r d THE UNIVERS-ITY OF BRITISH COLUMBIA 1972 f In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make i t freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It i s understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department The University of British Columbia Vancouver 8 , Canada Date i i ABSTRACT The o b j e c t o f t h i s s t u d y was t o t e s t t h e h y p o t h e s i s t h a t p r e y abundance i n l a r g e p a r t r e g u l a t e s d i s p e r s i o n , t e r r i -t o r y s i z e and a g g r e s s i v e b e h a v i o r o f young r a i n b o w t r o u t d u r i n g t h e s t r e a m r e a r i n g phase o f t h e i r l i f e h i s t o r y . I n l a b o r a t o r y t e s t c h a n n e l s , where age 0+ f r y were i n -t r o d u c e d i n t o c h a n n e l s r e c e i v i n g t h r e e d i f f e r e n t amounts o f p r e y and p e r m i t t e d t o e m i g r a t e v o l u n t a r i l y , d e n s i t y o f f r y r e -mained h i g h e s t a t t h e h i g h e s t p r e y l e v e l . A l s o , t h e d i s t r i b u -t i o n o f f r y was p o s i t i v e l y a s s o c i a t e d w i t h a g r a d i e n t i n p r e y abundance. B o t h t e r r i t o r y s i z e and f r e q u e n c y o f a g g r e s s i v e e n c o u n t e r v a r i e d i n v e r s e l y w i t h p r e y l e v e l ; t h e h i g h e r t h e p r e y l e v e l , t h e s m a l l e r t h e t e r r i t o r y and t h e l o w e r t h e f r e q u e n c y o f a g g r e s s i v e e n c o u n t e r . E m i g r a t i o n from t h e t e s t c h a n n e l s was n e i t h e r as r a p i d nor as marked when p r e y l e v e l was a b r u p t l y r e -duced, compared t o when f r y were i n i t i a l l y i n t r o d u c e d t o t h e d i f f e r e n t p r e y l e v e l s . However, f r e q u e n c y o f a g g r e s s i v e en-c o u n t e r s i g n i f i c a n t l y i n c r e a s e d when t h e p r e y l e v e l was de-c r e a s e d and s i g n i f i c a n t l y d e c r e a s e d when t h e p r e y was i n c r e a s e d . I n Loon O u t l e t Creek, t h e abundance o f p r e y was p o s i -t i v e l y a s s o c i a t e d w i t h summer f i s h biomass i n one s t u d y s e c t i o n , w h i l e a p o s i t i v e a s s o c i a t i o n was not a p p a r e n t f o r a second s t u d y s e c t i o n . The second, l o c a t e d c l o s e r t o Loon Lake, on t h e ave r a g e t e n d e d t o have t h r o u g h o u t t h e summer a h i g h e r f i s h biomass and higher prey density. In addition, the spring lakeward migra-t i o n of juvenile rainbow trout was negatively c o r r e l a t e d with prey density and p o s i t i v e l y c o r r e l a t e d with temperature. I t i s suggested that i n the natural stream habitat, the density of f r y and juvenile rainbow trout i s strongly i n -fluenced by prey density e s p e c i a l l y when associated with the metabolic e f f e c t s of temperature and f i s h s i z e . i v TABLE OF CONTENTS Page TITLE PAGE i ABSTRACT i i TABLE OF CONTENTS i v LIST OF FIGURES v i i LIST OF TABLES . . . i x LIST OF APPENDICES . . X ACKNOWLEDGEMENTS x i I INTRODUCTION 1 I I DESCRIPTION OF LABORATORY AND FIELD STUDY AREAS . . . . . 4 L a b o r a t o r y S t u d y A r e a 4. F i e l d S t u d y A r e a . . . . . . . 4 Geography o f t h e Loon Lake System . . . . 4 Temperature, Water L e v e l and D i s c h a r g e . 6 Spawning and F r y Emergence 7 J u v e n i l e M i g r a t i o n P a t t e r n s . . . . . . . 7 O u t l e t Creek S t u d y S e c t i o n s 7 I I I MATERIALS AND METHODS 11 L a b o r a t o r y S t u d y . . . . . . . . . . . . . 11 Exper i m e n t 1 . . . . . . . . . . . . . . 11 Exper i m e n t 2 17 Exper i m e n t 3 17 F i e l d S t u d y . . . . . . . . . . 18 V Page P r e y D e n s i t y and F i s h S t a n d i n g Crop . . . 18 G e n e r a l Methods 18 D r i f t Measurements 19 Stomach A n a l y s i s . . . . . . . . . . . 20 P o p u l a t i o n E s t i m a t e s 20 P r e y D e n s i t y and Lakeward M i g r a t i o n o f 1+ and 2++ J u v e n i l e s 21 IV RESULTS 22 L a b o r a t o r y R e s u l t s 22 Exp e r i m e n t 1 22 D e n s i t y 22 D i s t r i b u t i o n . . . . . 22 T e r r i t o r y S i z e and A g o n i s t i c B e h a v i o r 25 Experiment 2 28 I n i t i a l B e h a v i o r and D i s t r i b u t i o n . . . 28 T e r r i t o r y S i z e . . . . . . . . . . . . 31 D e n s i t y , E m i g r a t i o n and D i s t r i b u t i o n . 31 Biomass and Growth . . . . . 34 F e e d i n g B e h a v i o r 34 Expe r i m e n t 3 . . . . . . . . . . . . . . 37 F i e l d R e s u l t s 39 P r e y D e n s i t y i n R e l a t i o n t o F i s h S t a n d i n g Crop 39 P r e y C o m p o s i t i o n i n F r y Stomachs . . . 39 Summer D r i f t D e n s i t y o f P r e y Organisms and F i s h S t a n d i n g Crop . . 40 v i Page D r i f t D e n s i t y o f P r e y Organisms i n R e l a t i o n t o Lakeward M i g r a t i o n o f 1+ and 2++ J u v e n i l e s 44 P r e y C o m p o s i t i o n i n J u v e n i l e Stomachs . 44 D r i f t D e n s i t y and J u v e n i l e Lakeward M i g r a t i o n 44 V DISCUSSION 48 L a b o r a t o r y S t u d y . . . . . . 48 F i e l d S t u d y 53 The E f f e c t s o f Stream E n r i c h m e n t . . . . . 57 T e r r i t o r i a l B e h a v i o r and P r e y S e l e c t i o n . . 59 BIBLIOGRAPHY 63 APPENDIX 68 v i i L I S T OF FIGURES FIGURE Page 1. The Loon Lake system showing l o c a t i o n s o f t h e two t r a p s a t t h e O u t l e t Creek 5 2. Maximum and minimum w a t e r t e m p e r a t u r e s , w a t e r l e v e l s , and d i s c h a r g e s a t Loon O u t l e t C r e e k ; 1968, 1969, and 1970. 8 3. S e a s o n a l p a t t e r n s o f l a k e w a r d m i g r a t i o n o f c u m u l a t i v e numbers o f r a i n b o w t r o u t a t Loon O u t l e t Creek . . . . . 9 4. M i d d l e s u b s e c t i o n o f S e c t i o n I I I , Loon O u t l e t Creek, August 25, 1967. 10 5. Upper and l o w e r s u b s e c t i o n s o f S e c t i o n IV, Loon O u t l e t Creek, August 25, 1967 10 6. M i d d l e and l o w e r p a r t o f t e s t c h a n n e l a t Loon Creek H a t c h e r y , J u l y , 1970 12 7. G r i d a t upper end o f . t e s t c h a n n e l a t Loon Creek H a t c h e r y , J u l y , 1970 12 8. The number o f f r y r e m a i n i n g i n t e s t c h a n n e l s a t t h r e e p r e y l e v e l s 23 9. D i s t r i b u t i o n o f f r y w i t h i n t h e t h r e e t e s t c h a n n e l s 24 10. Number o f a g g r e s s i v e e n c o u n t e r s p e r f i v e m i n u t e i n t e r v a l a t t h r e e p r e y l e v e l s . . . . . . . . . 29 11. The r e l a t i o n s h i p between p r e y d e n s i t y and mean t e r r i t o r y s i z e i n t h r e e t e s t c h a n n e l s 3.1-12. The e f f e c t o f t h r e e d i f f e r e n t p r e y l e v e l s on t h e d e n s i t y o f rai n b o w t r o u t f r y . . . . . . . . 32': 13. D i s t r i b u t i o n o f f r y w i t h i n t e s t c h a n n e l s a t 3 p r e y l e v e l s on day 8 . . . . 33 14. F r y t e r r i t o r i a l m o s a i c s a t t h r e e p r e y l e v e l s . . 36 15. F r e q u e n c y o f a g g r e s s i v e e n c o u n t e r a t two d i f -f e r e n t p r e y l e v e l s , no e m i g r a t i o n p e r m i t t e d . . 38 v i i i FIGURE Page 16. P e r c e n t a g e c o m p o s i t i o n o f f r y stomach c o n t e n t s , summer, 1967, S e c t i o n I I I , Loon O u t l e t Creek • • 40 17. P e r c e n t a g e c o m p o s i t i o n o f f r y stomach c o n t e n t s , summer, 1967, S e c t i o n IV, Loon O u t l e t Creek . . 41 18. D r i f t d e n s i t y o f p r e y o r g a n i s m s , and f i s h biomass - S e c t i o n s I I I and IV, Loon O u t l e t Creek, 1967 43 19. P e r c e n t a g e c o m p o s i t i o n o f j u v e n i l e stomach c o n t e n t s , 1970, S e c t i o n IV, Loon O u t l e t C r e e k . . 45 20. P r e y d e n s i t y and p r e y l e n g t h , mean d a i l y t e m p e r a t u r e , and d a i l y upstream movement o f 1+ and 2++ j u v e n i l e r a i n b o w t r o u t i n Loon O u t l e t Creek, 1970 . . . . . 46 L I S T OF TABLES TABLE Page I 1967 Loon O u t l e t Creek w a t e r t e m p e r a t u r e and e s t i m a t e d d i s c h a r g e . . . . . . . . . . . . 6 I I P r e - e x p e r i m e n t a l h o l d i n g c o n d i t i o n s f o r f r y . . 13 I I I E x p e r i m e n t a l p h y s i c a l c o n d i t i o n s 14 IV E x p e r i m e n t a l d e s i g n : f i s h numbers, d u r a t i o n , p r e y 15 V The mean number o f f r y i n t h e upper and l o w e r h a l v e s of t h e t e s t c h a n n e l s 25 V I T e r r i t o r y s i z e o f r a i n b o w t r o u t f r y d u r i n g p r e y a d d i t i o n and a f t e r p r e y a d d i t i o n i n t h e h i g h p r e y c h a n n e l . . . . . . . 27 V I I The number o f f r y h o l d i n g i n t h e u p stream and downstream h a l v e s o f t h e c h a n n e l s a t t h r e e p r e y l e v e l s 34 V I I I T o t a l biomass, mean i n d i v i d u a l w e i g h t , and growth o f f r y a t t h r e e p r e y l e v e l s . . . . 35 LIST OF APPENDICES APPENDIX Page I R e g r e s s i o n o f f r y w e i g h t on f r y l e n g t h . F r y s e i n e d from S e c t i o n I I I and IV, summer, 1967 68 I I Movement o f j u v e n i l e s t h r o u g h Trap 1 and 2, 1970 69 I I I D r i f t p e r c e n t a g e c o m p o s i t i o n - p r e y t y p e o r g a n i s m s o n l y - S e c t i o n I I I and IV, Loon O u t l e t Creek: summer, 1967 . . 70 IV R e g r e s s i o n o f p r e y s i z e on f i s h s i z e . B a e t i s sp 71 V R e g r e s s i o n o f p r e y s i z e on f i s h s i z e . C h i r o n o m i d a e . . . . . . . . . 72 V I R e g r e s s i o n o f p r e y s i z e on f i s h s i z e . S i m u l i d a e . . . . . . . . 73 V I I One-way a n a l y s i s o f v a r i a n c e on i n d i v i d u a l f r y w e i g h t s a t t h r e e p r e y l e v e l s , E x p e r i m e n t 2 74 x i ACKNOWLEDGEMENTS Fin a n c i a l support f o r t h i s study was made a v a i l a b l e through the Fish and W i l d l i f e Branch of B r i t i s h Columbia, and the National Research Council of Canada. Mr. R.A.H. Sparrow, F i s h and W i l d l i f e Br. B i o l o g i s t i n charge of Fi s h Culture, granted permission to u t i l i z e Loon Creek Hatchery f a c i l i t i e s . Such cooperation was g r e a t l y appreciated. Numerous colleagues and associates inclu d i n g Messrs. G. Armitage, R. Land, R. Norman, G. Northcote, R. Sjolund and e s p e c i a l l y A. Tautz gave assistance with various aspects of the study and equipment, and deserve thanks. I would l i k e to acknowledge Mr. R. Peterson f o r a s s i s -tance and f o r making a v a i l a b l e data on size and age d i s t r i b u -t i o n of f i s h moving through the traps operated at Loon Outlet. Dilores Lauriente gave assistance i n s e t t i n g up com-puter programs. Drs. I. Efford, J . Krebs, R. L i l e y , and G. Scudder read the manuscript and made valuable suggestions during the course of the study. It i s a great pleasure to express my gratitude to my supervisor, Dr. T.G. Northcote, f o r h i s stimulating suggestions and enthusiasm during the e n t i r e study, and preparation of the manuscript. F i n a l l y , I would l i k e to express my appreciation to x i i my wife, Linda, for her patience and assistance i n the labora-tory and f i e l d , and for typing of the manuscript. y I INTRODUCTION T e r r i t o r i a l b e h a v i o r i s r e c o g n i z e d as one mechanism i n f l u e n c i n g t h e d e n s i t y o f a n i m a l p o p u l a t i o n s (Wynne-Edwards, 1962); E t k i n , 1963; Kawanabe, 1969; K r e b s , 1 971). I n p a r t i c u -l a r , t h i s mechanism i s emphasized f o r s t r e a m d w e l l i n g s a l m o n i d s ( K a l l e b e r g , 1958; A l l e n , 1969; Chapman and B j o r n n , 1969; Mac-fadden, 1969). That such b e h a v i o r can l i m i t j u v e n i l e p o p u l a -t i o n d e n s i t y was d e m o n s t r a t e d i n coho salmon, Oncorhynchus  k i s u t c h , (Chapman, 1962; Mason and Chapman, 1 9 6 5 ) . An i m p o r t a n t c h a r a c t e r i s t i c o f t e r r i t o r i a l i t y i s s i z e o f t h e d e f e n d e d a r e a . K a l l e b e r g (1958) i n d i c a t e d t h a t i t s s i z e i s a f u n c t i o n o f a number o f f a c t o r s i n c l u d i n g v i s u a l . i s o l a t i o n from c o h a h i t i n g i n d i v i d u a l s , s i z e o f f i s h , and p o s -s i b l y i n i t i a l d e n s i t y . . A n o t h e r f a c t o r o f more r e c e n t c o n s i d e r a t i o n i s abun-dance o f p r e y o r g a n i s m s . The i n f l u e n c e o f t h i s f a c t o r on p o p u l a t i o n d e n s i t y , t e r r i t o r y s i z e , and a g g r e s s i v e b e h a v i o r i t s e l f , i s c o n t r o v e r s i a l . F o r example, a number o f a u t h o r s r e p o r t e d an i n c r e a s e i n a g g r e s s i o n when f i s h were f e d t o e x c e s s o r when f r e q u e n c y o f f e e d i n g was i n c r e a s e d ( K e e n l e y s i d e and Yamamotto, 1962; Newman, 19 6 5 ) . A l s o Chapman (1962) o b s e r v e d t h a t f e e d i n g coho j u v e n i l e s t o e x c e s s w i t h b r i n e s hrimp d i d not a l t e r t h e r e s i d e n t d e n s i t y i n an a r t i f i c i a l 2 stream c h a n n e l . C o n v e r s e l y , Symons (1968) f o u n d when a l t e r -n a t i n g t h r e e days s t a r v a t i o n w i t h t h r e e days f e e d i n g t h a t a g g r e s s i o n i n A t l a n t i c salmon p a r r was h i g h e s t d u r i n g s t a r v a -t i o n p e r i o d s . A l s o Mason and Chapman (196 5) i n a s t u d y o f coho f r y i n two stream c h a n n e l s , r e p o r t e d t h a t t h e c h a n n e l w i t h t h e l e a s t space s u p p o r t e d 6 3 % more f i s h . S i n c e i t was d i s c o v e r e d t h a t 7 3% more d r i f t was coming i n t o t h i s p a r t i c u l a r c h a n n e l i t was s u g g e s t e d t h a t t h e h i g h e r f o o d l e v e l l e d t o an i n c r e a s e i n r e a r i n g c a p a c i t y . F i n a l l y , Symons ( i n p r e s s ) e m p l o y i n g an a r t i f i c i a l s tream c h a n n e l i n a s t u d y o f A t l a n t i c salmon p a r r , f o u n d t h a t t h e number o f s u b o r d i n a t e t e r r i t o r i e s was r e d u c e d d u r i n g f o o d s c a r c i t y , a l t h o u g h no s i g n i f i c a n t change i n t h e t e r r i t o r y s i z e o f dominant p a r r was demonstra-t e d . C o n s e q u e n t l y , i t was t h e purpose o f t h i s s t u d y t o f i r s t t e s t e x p e r i m e n t a l l y w i t h i n a l a b o r a t o r y s i t u a t i o n t h e f o l l o w i n g h y p o t h e s e s : 1. D e n s i t y and d i s t r i b u t i o n o f u n d e r y e a r l i n g r a i n b o w t r o u t a r e i n f l u e n c e d by p r e y l e v e l . 2. T e r r i t o r y s i z e and a g g r e s s i v e b e h a v i o r a r e a f f e c t e d by p r e y l e v e l . S e c ondly, a second phase o f s t u d y was d e s i g n e d t o t e s t t h e f o l l o w i n g : 3. S e a s o n a l changes o c c u r i n p r e y d e n s i t y i n a t r o u t r e a r i n g s t r e a m . 4. Changes i n f r y and j u v e n i l e t r o u t movement and s t a n d i n g c r o p (biomass) a r e a s s o c i a t e d w i t h s e a s o n a l changes i n p r e y d e n s i t y ( i . e . , a t e s t o f t h e l a b o r a t o r y r e s u l t s ) . F o r a number o f r e a s o n s t h e Loon Lake system was chosen f o r t h i s s t u d y . I t c o n t a i n s o n l y one f i s h s p e c i e s , n a t i v e r a i n b o w t r o u t (Salmo g a i r d n e r i R i c h a r d s o n ) , w h i c h r e -s i d e as young from a few days t o s e v e r a l y e a r s i n t r i b u t a r y streams ( i n l e t s and an o u t l e t ) , p r i o r t o e n t e r i n g t h e l a k e . F u r t h e r m o r e , d o c u m e n t a t i o n o f t r o u t m i g r a t i o n p a t t e r n s , l e n g t h o f r e s i d e n c e , and p o s s i b l e mechanisms o f m i g r a t i o n has been made t h e r e ( L i n d s e y , N o r t h c o t e and Hartman, 1959; N o r t h c o t e , 1962; Hartman, N o r t h c o t e and L i n d s e y , 1962). I n a d d i t i o n , , a j u v e n i l e t r o u t p r o d u c t i o n s t u d y was i n p r o g r e s s on t h e streams ( P e t e r s o n , u n p u b l i s h e d d a t a ) , and l a b o r a t o r y f a c i l i t i e s were n e a r b y and a v a i l a b l e . . I I DESCRIPTION OF LABORATORY AND FIELD STUDY AREAS LABORATORY STUDY AREA Laboratory experiments were c a r r i e d out at the Loon Creek Hatchery, approximately 7 km south-west of the outl e t of Loon Lake (Figure 1). The primary source of water f o r the hatchery i s a natural spring with a stable, well oxygenated discharge of approximately 20 l i t e r s per second and a near constant temperature of 9.6 C (range 9.2 - 9.9 C). Experi-ments were c a r r i e d out i n a series of channels within the hatchery. FIELD STUDY AREA Geography of the Loon Lake System Loon Lake i s located i n a narrow v a l l e y at an eleva-t i o n of 860 m, 19.3 km east of Clinton, B r i t i s h Columbia. A more d e t a i l e d description of the lake system i s given i n Northcote (1962). The main i n l e t stream (Inlet Creek) drains a series of small lakes to the north-east and south-east. Much of the stream v a l l e y i s a g r i c u l t u r a l grassland, although about one ha l f of the stream margin i s l i n e d with deciduous trees. The outlet (Outlet Creek) leaves the lake v i a a marsh section 180 m i n length. Thereafter i t s flow increases and t h e s t r e a m g r a d i e n t a v e r a g e s 1 m p e r 200 m f o r t h e n e x t 1000 m e t e r s . A t t h i s p o i n t t h e O u t l e t Creek i s j o i n e d by H i h i u m Creek, and t h e g r a d i e n t g r a d u a l l y i n c r e a s e s downstream. Most o f t h e s t r e a m m a r g i n i s l i n e d w i t h d e c i d u o u s v e g e t a t i o n a l t h o u g h c o n i f e r s t e n d t o be dominant between 500 and 800 m from t h e l a k e . Temperature, Water L e v e l and D i s c h a r g e N o r t h c o t e (196 2) p r e s e n t e d i n f o r m a t i o n on t e m p e r a t u r e and d i s c h a r g e f o r major streams i n t h e Loon System. More r e -c e n t l y , d a i l y t e m p e r a t u r e s were r e c o r d e d a t Loon O u t l e t Creek -1968, 360 m from t h e l a k e , s t a t i o n A; 1969 and 1970, 640 m and 1000 m from t h e l a k e , s t a t i o n s B and C r e s p e c t i v e l y ( F i g u r e 2 ) . O n l y i n t e r m i t t e n t r e c o r d s were a v a i l a b l e f o r 1967 ( T a b l e I ) . Water l e v e l and d i s c h a r g e a r e a l s o g i v e n f o r 1968, 1969, 1970 ( F i g u r e 2 ) . E s t i m a t e s were o n l y a v a i l a b l e f o r 1967 d i s c h a r g e ( T a b l e I ) . T a b l e I 1967 Loon O u t l e t Creek w a t e r t e m p e r a t u r e s and e s t i m a t e d d i s c h a r g e . , Date J u l y 17 Aug 8 Aug 24 Sept 9 D i s t a n c e from l a k e m 700 700 200 | 200 D i s c h a r g e L / sec 180 t o 200 130 120 , 120 Time h r 0900 & 2100 0900 & 1800 1000 & 1630 0900 & 1630 Temperature C 14.4 & 19.4 16.0 & 21.0 14.0 & 18.6 9.0 & 13.0 i 7 Spawning and F r y Emergence , One major spawning a r e a i s a t t h e O u t l e t Creek ( F i g u r e 1) where r e c e n t r e c o r d s i n d i c a t e t h e s p r i n g r u n i s 10,000 t o 12,000 a d u l t s . The r u n e n t r a n c e r e a c h e s a maximum a t t h e end o f A p r i l and t h e e x i t o f spawners ( s p e n t ) o c c u r s d u r i n g May and June (Hartman e t a l . , 1962). Emergence' i s f i r s t e v i d e n t a t about mid-June and t a p e r s o f f i n l a t e J u l y . J u v e n i l e M i g r a t i o n P a t t e r n s J u v e n i l e m i g r a t i o n p a t t e r n s were e x t e n s i v e l y s t u d i e d and d i s c u s s e d by N o r t h c o t e (1962) where s e a s o n a l and d i e l p a t t e r n s o f movement, upstream and downstream, w i t h i n t h e Loon system a r e summarized. As p a r t o f a j u v e n i l e p r o d u c t i o n s t u d y a t t h e O u t l e t f u r t h e r i n f o r m a t i o n r e g a r d i n g l a k e w a r d m i g r a t i o n was c o l l e c t e d by t h e F i s h e r i e s R e s e a r c h S e c t i o n o f t h e 1 B r i t i s h C olumbia F i s h and W i l d l i f e B r a n c h d u r i n g 1968, 1969, and 1970. These d a t a a r e graphed as c u m u l a t i v e t o t a l s i n o r d e r t h a t p e r i o d s o f maximum l a k e w a r d m i g r a t i o n r a t e s can be a s c e r t a i n e d ( F i g u r e 3 ) . i O u t l e t Creek S t u d y S e c t i o n s \ Two 100 m s t u d y s e c t i o n s , d e s i g n a t e d I I I and I V were i used- i n t h i s s t u d y and a r e r e s p e c t i v e l y 180 and 600 m down-stream from Loon Lake ( F i g u r e s 4 and 5 ) . ! 2 Maximum and minimum water temperatures, water l e v e l s , d i s c h a r g e s a t Loon O u t l e t Creek; 1968, 1969, and 1970 (see t e x t f o r s t a t i o n l o c a t i o n s ) . F i g . 3 Seasonal p a t t e r n s of lakeward m i g r a t i o n of cumulative numbers of rainbow t r o u t (age 0+, 1+, 2++) at Loon Outl e t Creek .Trap 1 fc)0©) 1968, 1969; Trap 1 (OO) and 2 (AA) 1970. F i g u r e 4 M i d d l e s u b s e c t i o n o f S e c t i o n I I I , Loon O u t l e t Creek, August 25, 1967. F i g u r e 5 Upper (A) and l o w e r (B) s u b s e c t i o n s o f S e c t i o n IV, Loon O u t l e t Creek, August 25, 1967. I l l MATERIALS AND METHODS LABORATORY STUDY A s e r i e s o f t h r e e e x p e r i m e n t s was c o n d u c t e d i n t h e Loon Creek H a t c h e r y i n o r d e r t o t e s t e f f e c t s o f p r e y l e v e l on f r y d e n s i t y , d i s t r i b u t i o n and a g g r e s s i v e b e h a v i o r . E x p e r i m e n -t a l c o n d i t i o n s a r e l a r g e l y o u t l i n e d i n T a b l e s I I , I I I , and I V . Exp e r i m e n t 1 The p r i m a r y purpose o f t h i s e x p e r i m e n t was t o t e s t t h e e f f e c t o f p r e y o r g a n i s m abundance on f r y d e n s i t y . A c c o r d i n g l y , f r y w e r e " c o l l e c t e d from Loon I n l e t Creefc and t e s t e d a t t h r e e d i f f e r e n t p r e y l e v e l s ( T a b l e I V ) . A t t h e h i g h and i n t e r m e d i a t e p r e y l e v e l s , each o f t h e f o u r r e p l i c a -t i o n s was c a r r i e d out f o r a d u r a t i o n o f 48 h o u r s . Owing t o d i f f i c u l t i e s i n m a i n t a i n i n g t h e f e e d i n g regime, p r e y a d d i t i o n was t e r m i n a t e d a t t h i s t i m e . T h e r e a f t e r (up t o 85 h r ) p r e y i t e m s e n t e r e d o n l y from w i t h i n t h e t e s t c h a n n e l i t s e l f and w i t h h a t c h e r y w a t e r . P r e y i t e m s a t t h e low p r e y l e v e l e n t e r e d a t a l l t i m e s w i t h t h e h a t c h e r y w a t e r . A t t h e h i g h and i n t e r m e d i a t e p r e y l e v e l s , t h e p r e y c o m p o s i t i o n was a p p r o x i m a t e l y 50% Daphnia p u l e x , 4 0 % Diaptcmus  t y r e l l i , 2% Chironomidae l a r v a e , 2% S i m u l i d a e l a r v a e , 1% B a e t i s sp. nymphs, and 5% m i s c e l l a n e o u s o r ganisms ( e . g . w a t e r m i t e s ) . T h i s c o m p o s i t i o n u n a v o i d a b l y v a r i e d between F i g u r e 6 M i d d l e and l o w e r p a r t o f t e s t c h a n n e l a t Loon Creek H a t c h e r y , J u l y , 1970. F i g u r e 7 G r i d a t upper end o f t e s t c h a n n e l a t Loon Creek H a t c h e r y , J u l y , 1970. 13 T a b l e I I P r e - e x p e r i m e n t a l h o l d i n g c o n d i t i o n s f o r f r y D e s c r i p t i o n Exp. 1 Exp. 2 Exp. 3 S t o c k Loon I n l e t Loon O u t l e t Loon O u t l e t S i z e r a n g e 24 t o 45 mm 24 t o 25 mm 30 t o 35 mm aver a g e 30 mm R e a r i n g Loon Creek L o c a t i o n Loon I n l e t H a t c h e r y same as 2 H o l d i n g t i m e 5 days t o t o ' b u t t o n 30 days f r o m p r i o r t o 14 days from up 1 s t a g e 1 b u t t o n up 1 e x p e r i m e n t c o l l e c t i o n f r o m egg s t a g e s t a g e H o l d i n g 100/m 2 o f 250/m 2 o f 175/m 2 o f d e n s i t y f lume f l u m e f l u m e H o l d i n g ca 0.5 ca 0.5 ca 0.5 v e l o c i t y cm/sec cm/sec cm/ sec P h o t o p e r i o d 16-17 h r 15 h r 15 h r l i g h t l i g h t l i g h t F e e d i n g regime 10 gm 15 gm c u t 10 gm h a t c h e r y i n h o l d i n g h a t c h e r y T u b i f e x / d a y / f o o d , 5 gm fo o d , 5 gm m2 o f flu m e T u b i f e x p e r stream d r i f t / (10 m i n. day p e r m 2 day/m 2 flume i n t e r v a l s ) f l u m e 14 T a b l e I I I E x p e r i m e n t a l p h y s i c a l c o n d i t i o n s (see a l s o F i g u r e s 6 and 7 ) . D e s c r i p t i o n Exp. 1 Exp. 2 Exp. 3 S i z e o f t e s t c h a n n e l width=37 cm l e n g t h -300 cm 37 cm 390 cm 37 cm 39 cm S u b s t r a t e s i m u l a t e d stream bottom, (sand, g r a v e l , s t o n e s ) same as 1 sand o n l y P e r c e n t c h a n n e l c o v e r e d 30% t o p 100% s i d e s , ends 25% t o p 100% s i d e s , ends 0% t o p 100% s i d e s , ends Average wa t e r v e l o c i t y 4.5 cm/sec 6.5 cm/sec 6.5 cm/sec Average w a t e r d e p t h 9.5 cm 11.0 cm 11.0 cm P h o t o p e r i o d 16 t o 17 h r l i g h t 17 h r l i g h t 17 h r l i g h t O b s e r v a t i o n g r i d 40 x 37 cm m i d - c h a n n e l 40 x 37 cm upper end Temperature 9.6 C range 9.2 -9.9 9.6 C 9.4 C 15 T a b l e I V E x p e r i m e n t a l d e s i g n : f i s h numbers, d u r a t i o n , p r e y D e s c r i p t i o n Exp. 1 Exp..2 Exp. 3 I n i t i a l numbers 120/channel 100/channel 1 0 / t e s t a r e a Method o f i n t r o d u c t i o n Time e l a p s e d from i n t r o -d u c t i o n t o t r a p s opened D u r a t i o n Food t y p e Food d i s p e n s e r H a t c h e r y w a t e r 10/10 min randomly a s s i g n e d 2 h r 48 hr-added p r e y and 85 h r no addir-t i o n a l p r e y (per r e p l i -c a t i o n ) Loon O u t l e t stream d r i f t m a i n l y z o o -p l a n k t o n m a n u a l l y once/15 min 19 h r / d a y not f i l t e r e d 10/10 min randomly a s s i g n e d 43.5 h r 10 days T u b i f e x , ca c u t 0.5 mm t o 3 mm 1 0 / t e s t a r e a r andomly a s s i g n e d no t r a p s 5 m i n p e r r e p l i c a t i o n T u b i f e x , c a c u t 0.5 mm t o 1.5 mm a u t o m a t i c f e e d e r , o nce/ 10 min, 24 h r / day f i l t e r e d . 1 mm a p e r a -t u r e mesh same as 2 same as 2 P r e y q u a n t i t y H i g h p r e y l e v e l I n t e r m e d i a t e p r e y l e v e l Low p r e y l e v e l 4000/15 min e s t . 100 mg d r y wt 1500/15 min e s t . 33 mg d r y wt 12/15 min e s t . 1 mg d r y wt 250 mg wet wt same a s 2 / 10 min 50 mg wet wt / 10 min 10-15 mg wet wt/10 min (ap p r o x . 8 t o 10 i t e m s / mg) 25 mg wet wt / 10 min . 16 r e p l i c a t i o n s , but not markedly so. Invertebrates were c o l l e c t e d with a Surber sampler from the upper part of Loon Outlet i n the evening p r i o r to each r e p l i c a t i o n . Enough hatchery water was then added to organisms to bring the density to 1 ml/25 ml solu t i o n as de-termined by s e t t l e d volume. Approximately h a l f of the organ-isms were a l i v e when introduced to the top of the channels. A l l t e s t channels received d r i f t i n g organisms from the hatchery water which was sampled during l a t e afternoon with a 0.1 mm aperature net placed over the channel i n l e t . The samples were analyzed by subsampling and recording a l l organ-isms over one mm length. An a d d i t i o n a l 75 ml and 25 ml per 15 min prey solution were added to the high prey and i n t e r -mediate prey channels resp e c t i v e l y . I n l e t f r y u t i l i z e d i n t h i s experiment were s i m i l a r i n size to that observed within Loon In l e t Creek. D i s t r i b u t i o n of f r y within sections of the t e s t channel was plotted from an overhead area during times of least emigration. Emigration was recorded by monitoring movement out of the test channels i n t o upstream and downstream traps (Figure 9). These were checked every two hours, except during the night and a l l emigrating f r y were placed i n t o holding pens. Residual numbers of f r y were recorded as i n i t i a l number minus those emigrating from the test area since m o r t a l i t y was n e g l i g i b l e . Measurements of t e r r i t o r y s i z e (horizontal dimensions only) were obtained as based upon the adopted d e f i n i t i o n of 17 t e r r i t o r y ; i . e . , a defended area (Noble, 1939). For t h i s purpose a g r i d was located i n mid-channel and plots made of points of aggressive encounter ( i . e . chases followed by nips, or chases; not d i s p l a y a l ) . Plots were continued u n t i l a t e r -r i t o r i a l boundary could be discerned (10 to 30 min.) and measured by planimetry. In parts of the t e r r i t o r i a l periphery where interactions were infrequent, such as at rock or channel margins, foraging points were u t i l i z e d as boundary i n d i c a t o r s . Owing to the high rate of earl y emigration and a c t i v i t y , t e r r i t o r y measurements were made near termination of prey addition. Experiment 2 The second experiment repeated the f i r s t but with out-l e t f r y . The hypothesis was also tested that there was a growth difference among f r y at d i f f e r e n t prey l e v e l s . Both p r i o r to and during the experiment f i s h received weighed allotments of Tubifex dispensed at the upstream end of the holding areas and test channels by automatic feeding devices. The experiment was i n i t i a t e d two days a f t e r advanced alevins f i r s t began to appear photopositive ('button up 1 stage). Experiment 3 To t e s t i f frequency of aggressive encounter changed with prey abundance, an experiment was designed whereby den-s i t y was held constant (no emigration permitted from a te s t area) using two prey l e v e l s . 18 Four s m a l l t e s t a r e a s were c o n s t r u c t e d w i t h i n one h a t c h e r y c h a n n e l ( T a b l e I I I ) . The f i r s t was one m from t h e water i n l e t and p r e y d i s p e n s i n g l o c a t i o n , t h e l a s t 1.2 m downstream from t h e f i r s t . P r e y q u a n t i t y r e a c h i n g t h e f i r s t a r e a was o n l y s l i g h t l y h i g h e r t h a n a t t h e l a s t . A f t e r two days o f o b s e r v a t i o n on f o u r groups o f 10 f r y , t h e groups were r e p l a c e d v / i t h new f r y and t h e o b s e r v a t i o n s r e p e a t e d . P r i o r t o r e m o v a l o f t h e s e new groups o f f r y , each p r e y l e v e l was changed ( i . e . s h i f t e d from low t o h i g h l e v e l and v i c e v e r s a ) and f u r t h e r o b s e r v a t i o n s made o v e r 24 h o u r s . FIELD STUDY P r e y D e n s i t y and F i s h S t a n d i n g Crop G e n e r a l Methods I n 1967, f r y p o p u l a t i o n e s t i m a t e s , d r i f t samples, benthos samples, and f r y stomach samples were t a k e n i n s t u d y s e c t i o n s w i t h i n O u t l e t and I n l e t C r e e k s . These samples were a n a l y z e d d u r i n g 1969 and 1970 and t h e d a t a u t i l i z e d , i n p a r t , t o t e s t f o r a c o r r e l a t i o n between d e n s i t y o f d r i f t i n g p r e y organisms and s t a n d i n g c r o p o f f r y w i t h i n t h e two O u t l e t s t u d y s e c t i o n s . From d i r e c t o b s e r v a t i o n ( l a b o r a t o r y and i n Loon Creek) as w e l l as e v i d e n c e from t h e l i t e r a t u r e ( E l l i o t t , 1967a, b, 1968 a, b, 1970; J e n k i n s , 1969; J o h n s t o n , 1967; Mundie, 1969) d r i f t o r g a n i s m s were c o n s i d e r e d f a r more i m p o r t a n t t h a n benthos as a f o o d s o u r c e f o r young r a i n b o w t r o u t . F o r t h e s e r e a s o n s 19 only d r i f t i n g organisms, rather than benthos were examined herein. D r i f t Measurements D r i f t was sampled for two 10 minute i n t e r v a l s at three times during 24 hours (1600 hr, 2400 hr, and 0800 h r ) . A stan-dard Clarke-Bumpus plankton sampler (0.1 mm mesh aperature) was used at the center and side of the stream at the upstream end of each study section. The volume sampled was calculated by the following formula: V = v(a) (t) (k) 3 whereby V i s volume sampled i n m , v i s current v e l o c i t y i n 2 m/sec, a- i s area f i l t e r e d i n m , t i s time i n s e c , and k i s a percent acceptance constant (0.94). Each sample was emptied i n t o a white tray; large or-ganisms picked out, enumerated and the remainder of the sample examined under a di s s e c t i n g microscope. A l l v i s i b l e organisms were counted and size ranges were recorded. Organisms occurring i n f r y stomachs were measured to the nearest .1 mm ( t o t a l length from mouth to anus) by means of a c a l i b r a t e d ocular micrometer. Subsamples, when required, were removed with a pipet; the three subsample counts tested f o r randomness using the method reported by Lund, et a l . (1958). Any organism was con-sidered as a possible prey i f i t made up greater than 0.5% of the t o t a l summer stomach composition or greater than 1% on any one sampling day. Of these organisms, those comprising less 20 than 2.5% of each end of the size frequency d i s t r i b u t i o n (stomach contents) were not included. The subsample mean was adjusted according to the above c r i t e r i o n , m u l t i p l i e d by the d i l u t i o n factor where necessary, and added to the number of any larger organisms removed p r i o r to subsampling ( s i m i l a r l y adjusted so as not to include extremes i n the size d i s t r i b u t i o n ) giving t o t a l prey organisms per sample. Stomach Analysis Ten f r y were captured with seines from each study section approximately three hours a f t e r each d r i f t sampling. Attempts were made to c o l l e c t f r y only from undisturbed areas. Stomachs were dissected under a microscope, organisms were measured to the nearest .1 mm and i d e n t i f i e d as f a r as f e a s i b l e . Population Estimates Mark and recapture population estimates were made f o r each 100 meter section, one to two days p r i o r to d r i f t and stomach sampling. To calculate the t o t a l population numbers, the formula (Bailey, 1951): M(C + 1) R + 1 was used where N i s the population estimate, M i s the number marked, C i s the number captured, and R i s the number recap-tured with marks. To t h i s estimate 95% confidence l i m i t s were applied using the graphical method of Adams (1951). 21 Population estimates were converted to biomass using a weight-length conversion (corrected f or preservation weight changes) 2 and expressed as gm/m . Prey Density and Lakeward Migration of 1+ and 2++ Juveniles The hypothesis was tested that lakeward migration of juveniles from Loon Outlet i s inversely correlated to abundance of d r i f t i n g prey. Four samplers were constructed (modified from Cushing, 1964), with a mouth 10.3 cm square, approximately a 0.16 mm aperature net, and f i t t e d with a 0.1 mm mesh c o l l e c t -ing cup. Each was set to sample the stream from top to bottom for 30 minutes at the side as w e l l as middle areas. V e l o c i t y at the sampler mouth was measured and volume f i l t e r e d c alcu-lated f o r each. Samples were taken jus t before dawn, mid-morning, mid-afternoon and just a f t e r dusk, and were repeated every 10 to 12 days from spring to autumn. Information on juvenile trout movement through Traps 1 and 2 and t h e i r approximate o r i g i n was obtained d a i l y from spring t o autumn (Appendix I I ) . Stomach contents of a group of 5 to 10 juvenile trout were analyzed monthly. These f i s h were seined from an area above the d r i f t sampling section. A l l f i s h were measured and a l l organisms i d e n t i f i e d and measured. The same size c r i t e r i o n was used as described previously, but the c r i t e r i o n f or or-ganism type was that an organism was recorded at leas t once i n the stomach samples. IV RESULTS LABORATORY RESULTS Experiment 1 Density Addition of prey organisms to the channels had a marked eff e c t upon the re s i d u a l number of f r y (Figure 8 ) . Within 15 to 20 hours a f t e r the experiment started density i n the channels began to s t a b i l i z e ; the greater the prey abundance, the greater the density of f r y . After 48 hr no further prey were added and emigration rates again tended to increase, e s p e c i a l l y at high prey l e v e l . However, v a r i a b i l i t y was high, and t h i s change was 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 . High v a r i a b i l i t y could have been caused by a number of factors including conditioning of f i s h (some were held and reused i n r e p l i c a t i o n s ) and variable composition or q u a l i t y of prey. In a p i l o t experiment where prey were continuously siphoned into two te s t channels f or 27 hr (HP = 94, LP = 26) before feeding was stopped i t took 117 add i t i o n a l hours f or f r y density i n the high prey channel to decline to the same l e v e l as that i n the low prey channel. D i s t r i b u t i o n Fry dispersed i n a l l t e s t channels with no obvious F i g u r e 8 The number o f f r y r e m a i n i n g i n t e s t c h a n n e l s a f t e r t r a p s opened a t t h r e e p r e y l e v e l s : LP = low prey, IP = i n t e r -m e d i a t e pr e y , HP = h i g h p r e y . V e r t i c a l l i n e s i n d i c a t e 95% c o n f i d e n c e l i m i t s o f f o u r r e p l i c a t i o n s . Arrows i n d i c a t e t h e p o i n t i n t i m e when p r e y a d d i t i o n was t e r -m i n a t e d . 24 H P 1 P L P TRAP AREA COVER AREA 23 hr ' ' » I t t i f t f f t ' f , f f r t f f f t ' f » t t f t i i 1 1 1 f » f r f ::::::::::::::::::::::::::::::::: \ x /\ f v\ / \ 27 hr 1 1 il i ' f • » t » ! 1 ' r ' t 1 ' 1 . » , t 1 • f » » 1 1 t f » r f f F i g . 9 D i s t r i b u t i o n of f r y w i t h i n the three t e s t channels ( h i g h , i n t e r m e d i a t e , and low prey) a t 23 and 27 h r a f t e r t r a p s opened.' 25 c l u s t e r i n g a r o u n d t h e a r e a w h e r e p r e y w e r e d i s p e n s e d ( F i g u r e 9 ) . I n d i v i d u a l f r y d e f e n d e d a n a r e a s u r r o u n d i n g a home s t a t i o n . A n a p p a r e n t g r a d i e n t i n f r y d i s t r i b u t i o n was o b s e r v e d , e s p e c i a l l y i n t h e i n t e r m e d i a t e p r e y c h a n n e l . A c o m p a r i s o n was made o v e r f o u r o b s e r v a t i o n p e r i o d s b e t w e e n t h e number i n t h e u p p e r h a l f o f t h e c h a n n e l s a n d t h e l o w e r h a l f o f t h e c h a n n e l s ( T a b l e V ) . T a b l e V The mean number o f f r y i n t h e u p p e r a n d l o w e r h a l v e s o f t h e t e s t c h a n n e l s . U p p e r L o w e r H i g h P r e y 20.5 16.5 I n t e r m e d i a t e P r e y 17.0 7.3 * Low P r e y 4.5 3.0 * ( X 2 = S i g n i f i c a n t a t p = .05) Due t o s e t t l i n g o u t o f p r e y , d r i f t d e n s i t y was n e c e s s a r i l y h i g h e r i n t h e u p p e r p a r t t h a n t h e l o w e r p a r t o f t h e h i g h a n d i n t e r m e d i a t e c h a n n e l s . I n t h e l a t t e r , f r y d e n s i t y was a l s o s i g n i f i c a n t l y h i g h e r i n t h e u p p e r h a l f . T e r r i t o r y S i z e a n d A g o n i s t i c B e h a v i o r A s s o o n a s a l l f r y w e r e i n t r o d u c e d i n t o t h e c h a n n e l s , i t was o b s e r v e d t h a t t h e r e was a h i g h f r e q u e n c y o f movement i n u p s t r e a m a n d d o w n s t r e a m d i r e c t i o n s . The mean f r e q u e n c y o f movement, a s m e a s u r e d b y t h e number o f f r y m o v i n g p a s t a 15 cm 26 s t r i p i n f i v e minutes, was HP = 15, IP = 28, and LP = 36 for f i v e r e p l i c a t e d observation periods. Many f r y already were defending areas and those moving past were not attacked unless they exhibited p o s i t i v e l y rheotatic behavior and progressed close to the substrate. Most moved i n the upper part of the water column or near the channel margin. A c t i v i t y was p a r t i c u -l a r l y high i n the low prey channel making t e r r i t o r i a l measure-ment d i f f i c u l t . T e r r i t o r i a l mosaics were quickly established a f t e r the i n i t i a l high a c t i v i t y period, and most f r y maintained t h e i r home stations throughout the low emigration period of the experiment. However, within the mosaics not a l l adjacent i n -dividuals were equally dominant; i . e . , subordinate i n d i v i d u a l s tended to avoid interactions with adjacent t e r r i t o r y holders. No t o t a l subordination ( t o t a l submission) was observed but dominant f r y would i n i t i a t e encounters much more frequently. During the low emigration period subordinates occasionally departed from t h e i r home stations and attempted to take up residence elsewhere. In addition to these, approximately 5 to 10% of the f r y i n the channels tended to move from l o c a t i o n to l o c a t i o n (transients). Their movement coincided with d i s -play, nipping or chasing from adjacent t e r r i t o r i a l holders. Such f r y were e a s i l y distinguished by t h e i r l i g h t e r pigmenta-t i o n . Aggressive behavior patterns were s i m i l a r to those reported i n other juvenile salmonids (Stringer and Hoar, 1955; Jenkins, 1969). Nipping and chasing occurred more frequently 27 t h a n d i s p l a y ( e . g . , l a t e r a l d i s p l a y ) . However, such d i s p l a y f r e q u e n t l y t o o k p l a c e when f r y p a s s e d by o t h e r f r y , and n i p -p i n g and c h a s i n g more f r e q u e n t l y when a d j a c e n t t e r r i t o r y h o l d e r s i n t r u d e d i n a n o t h e r t e r r i t o r y . T e r r i t o r y s i z e was p l o t t e d one hour p r i o r t o t e r m i n a -t i o n and two t o t h r e e h o u r s a f t e r t e r m i n a t i o n o f p r e y a d d i t i o n . Data were o b t a i n e d o n l y f o r t h e more dominant i n d i v i d u a l s because o f t i m e l i m i t a t i o n s . R e s u l t s ( T a b l e VT) su g g e s t t h a t dominant i n d i v i d u a l s i n c r e a s e d t h e s i z e o f t h e i r t e r r i t o r i e s by a t l e a s t two f o l d , a l t h o u g h i t was not d e f i n i t e l y c o n f i r m e d t h a t t h e same i n d i v i -d u a l s were r e c o r d e d i n a l l measurements. T a b l e V I T e r r i t o r y s i z e o f r a i n b o w t r o u t f r y d u r i n g p r e y a d d i t i o n ( a t 47 h r ) and a f t e r p r e y a d d i t i o n ( a t 51 h r ) i n t h e h i g h p r e y c h a n n e l ( g r i d ) . S i m i l a r numbers o f f r y were p r e s e n t d u r i n g b o t h p e r i o d s . A p p r o x i m a t e A g g r e s s i v e T e r r i t o r y s i z e F r y s i z e mm E n c o u n t e r s cm 2 (mean and (mean) p e r 15 min ra n g e (mean and range) 47 h r (5 f r y ) 31 10.6 (8 t o 13) 65 (54 t o 89) 51 h r (2 f r y ) 31 14 (12 t o 16) 180 (138 t o 221) Few q u a n t i t a t i v e measurements o f t e r r i t o r y s i z e were made i n t h e i n t e r m e d i a t e p r e y c h a n n e l . One i n d i v i d u a l p r i o r t o t e r m i n a t i o n o f p r e y a d d i t i o n d e fended an a r e a o f a p p r o x i m a t e l y 160 cm (13 i n t e r a c t i o n s p e r 15 min) and i n -28 2 creased t h i s to 475 cm (25 interactions per 15 min) a f t e r termination. Aggressive a c t i v i t y was very low within the low prey channel and t e r r i t o r y sizes were not plotted. Experiment 2 I n i t i a l Behavior and D i s t r i b u t i o n Traps were closed f or a 43.5 hr period a f t e r the f i r s t f r y introduction. When f r y were i n i t i a l l y introduced they went into rock crevices and cover areas i n the upper part of the channel (less than 10 moved i n t o the area of the trap entrance). After 7.5 hr the f r y were d i s t r i b u t e d throughout more than one hal f of the channel. Neither feeding nor aggressive behavior was evident during 15 minute observation periods. At 23 hours from introduction both feeding and aggressive encounters occurred but aggressive encounters were infrequent (Figure 10). A l l f r y moved about frequently. Such movements were 2 p r i m a r i l y r e s t r i c t e d to a 600 cm area, and each f r y ' s range of movement completely overlapped that of adjacent i n d i v i d u a l s . By day 1 of the experiment ( i . e . the day the traps were opened) f r y were d i s t r i b u t e d throughout the channels. By day 2, interactions were frequent but apparently less so with i n the high, prey channel (Figure 10). Overlapping of movement range (associated with feeding) was s t i l l evident, p a r t i c u l a r l y at the high prey l e v e l , although t h i s was less evident by day 3. Both di s p e r s a l within channels and r e s t r i c t i o n of feeding 29 ffi'o Q. •27 O - n i l m. HP IP LP 20 hr PRIOR TO TRAPS OPENED HP IP LP HP IP LP HP IP LP 2 5 8 DAYS AFTER TRAPS OPENED 10 Number of aggressive encounters per five minute interval with time elapsed after traps opened. Points are individual observations, bars indicate means of six observations. 30. movements c o i n c i d e d w i t h t h e i n i t i a t i o n o f a g g r e s s i v e b e -h a v i o r . T e r r i t o r y S i z e T e r r i t o r y s i z e s , p l o t t e d on d a y 8 a n d 9 a t t h e u p p e r e n d s o f t h e c h a n n e l s , w e r e s i g n i f i c a n t l y d i f f e r e n t a t t h e t h r e e p r e y l e v e l s ( F i g u r e 1 1 ) . The h i g h e r t h e a b u n d a n c e o f p r e y t h e s m a l l e r was t h e t e r r i t o r y s i z e , a l t h o u g h t h e s h a p e o f t h e c u r v e i n d i c a t e s t h e r e i s a l o w e r l i m i t i n s i z e b e y o n d w h i c h t h e r e i s no f u r t h e r i n f l u e n c e b y p r e y . D e n s i t y , E m i g r a t i o n a n d D i s t r i b u t i o n 2 A s i g n i f i c a n t d i f f e r e n c e (X = 6 2 . 8 , 2 d f , p < .05) i n t h e d e n s i t y o f f r y a t t h e t h r e e p r e y l e v e l s a p p e a r e d b y t h e s e c o n d d a y , d e m o n s t r a t i n g t h a t t h e g r e a t e r t h e p r e y a b u n d a n c e , t h e g r e a t e r t h e d e n s i t y o f f r y ( F i g u r e 1 2 ) . The d i s t r i b u t i o n o f f r y w i t h i n t h e c h a n n e l was s i m i l a r t o t h a t o f e x p e r i m e n t 1 w h e r e f r y w e r e d i s p e r s e d t h r o u g h o u t t h e l e n g t h o f a l l c h a n n e l s ( F i g u r e 1 3 ) . The d i s t r i b u t i o n showed a g r a d i e n t i n numbers f r o m t h e u p s t r e a m t o t h e d o w n s t r e a m e n d . A c o m p a r i s o n o f t h e numbers i n t h e u p s t r e a m h a l f a n d down-s t r e a m h a l f o f t h e o b s e r v e d a r e a d e m o n s t r a t e s t h a t m o r e f r y h e l d t e r r i t o r i e s i n t h e u p p e r a r e a ( T a b l e V I I ) . M o s t o f t h e p r e y s e t t l e d i n t h e u p p e r t h i r d o f t h e c h a n n e l . T h u s , f r y d i s t r i b u t i o n was p o s i t i v e l y a s s o c i a t e d w i t h t h e g r a d i e n t i n p r e y d e n s i t y . 3VI o x 4 CM E o LU 3 N CO I 2 DC DC UJ «- 1 0 .1 .2 . 3 PREY DENSITY gm/m 3 AT TOP OF CHANNELS . 4 F i g . I i The r e l a t i o n s h i p between prey d e n s i t y and mean t e r r i t o r y s i z e a t the top of three t e s t channels. Prey q u a n t i t y averaged over h r t o o b t a i n d e n s i t y i n gm per m3. V e r t i c a l l i n e s i n d i c a t e 95$ Confidence l i m i t s . F i g u r e 12 The e f f e c t o f t h r e e d i f f e r e n t p r e y l e v e l s on t h e d e n s i t y o f rainbow t r o u t f r y : LP = low prey, IP = i n t e r m e d i a t e p r e y , HP = h i g h p r e y . S m a l l e r graphs i n d i c a t e f r y s i z e f r e q u e n c y p r i o r t o i n t r o d u c t i o n and a f t e r t e n days w i t h i n t e s t c h a n n e l s . 33 H P 1/\J I P n L P /\| UPSTREAM TRAPS COVER AREA t FRY NEAR SUBSTRATE l FRY NEAR SURFACE f FRY UNDER GRID •DOWNSTREAM TRAPS V . " 1 » t t t r V ' f f ' t ' t t f t » • I t t t t t » ! f * f f 1 t f t t t » t f t f V \ / F i g . 13 D i s t r i b u t i o n of f r y w i t h i n t e s t channels a t 3 prey l e v e l s on day 8. 34 T a b l e VTI The number o f f r y h o l d i n g i n t h e upstream and downstream h a l v e s o f t h e c h a n n e l s a t t h r e e p r e y l e v e l s . 2 Upstream Downstream X l e v e l o f s i g n i f i c a n c e H i g h P r e y 34 11 .001 I n t e r m e d i a t e P r e y 21 6 .01 Low P r e y 9 3 .10 Most o f t h e p r e y s e t t l e d i n t h e upper t h i r d o f t h e c h a n n e l . Thus, f r y d i s t r i b u t i o n was p o s i t i v e l y a s s o c i a t e d w i t h t h e g r a d i e n t i n p r e y d e n s i t y . Biomass and Growth S i m i l a r t o t h e p r e v i o u s r e s u l t s , t h e r e was a l a r g e d i f f e r e n c e i n t o t a l biomass o f f r y a t t h e t h r e e p r e y l e v e l s ( T a b l e V I I I ) . Growth appeared not t o be t h e same a t a l l p r e y l e v e l s . T h i s was t e s t e d w i t h a one way a n a l y s i s o f v a r i a n c e (Appendix I I ) , i n d i c a t i n g t h a t t h e t h r e e mean i n d i v i d u a l d r y w e i g h t s were not t h e same (P .< .05, but > .01). However, t h e d i f f e r e n c e i s not l a r g e , as a l s o i n d i c a t e d by t h e s i z e f r e -quency d i s t r i b u t i o n ( F i g u r e 1 1 ) . F e e d i n g B e h a v i o r I n d i v i d u a l s u t i l i z e d a home s t a t i o n from where p r e y f o r a g i n g and a g g r e s s i v e e n c o u n t e r s were p r i m a r i l y i n i t i a t e d 35 T a b l e V T I I T o t a l biomass (wet and d r y w e i g h t ) , mean i n d i -v i d u a l w e i g h t , and growth o f f r y a t t h r e e p r e y l e v e l s . T o t a l Biomass T o t a l Biomass I n d i v i d u a l Mean d r y wt. mg wet wt. mg mean d r y Growth, wt. mg % body S. E. d r y wt/ day** H i g h P r e y 1,648 12,148 24 + .7 3.0% I n t e r m e d i a t e P r e y 886 6,598 22 ±. .5 2.0% Low P r e y 327 2,102* 24 ± .8 3.0% * a p p r o x i m a t e l y 10% w a t e r l o s s i n f r e e z i n g . ** b a s e d on an i n i t i a l mean d r y w e i g h t o f 18.0 ± .35 mg. ( F i g u r e 1 4 ) . Many s t r i k e s o r g r a s p s a t p r e y i t e m s were w i t h i n a s h o r t d i s t a n c e (3 t o 4 cm) fr o m t h i s l o c a t i o n , b u t t r i p s t o t h e p e r i p h e r y , where e n c o u n t e r s were o b s e r v e d , were a l s o e v i d e n t . F r y r a r e l y a t t e m p t e d t o f e e d beyond t h e i r t e r r i t o r y . Home s t a t i o n s u s u a l l y were one s p e c i f i c l o c a t i o n t o w h i c h f r y would a l w a y s r e t u r n , e s p e c i a l l y when t e r r i t o r i e s were s m a l l ( f o o d l e v e l s h i g h ) . A t t h e l a r g e r t e r r i t o r y s i z e s (low f o o d l e v e l s ) home s t a t i o n s were n o t as d i s t i n c t nor as s m a l l . F o r example, near t h e upstream end o f t h e h i g h p r e y c h a n n e l , home 2 s t a t i o n s c o n s t i t u t e d an a r e a o f 4 t o 7 cm whereas a t t h e downstream end o f t h e low p r e y c h a n n e l , t h e y were 15 t o 18 2 cm . Home s t a t i o n s a l s o were c l o s e l y a s s o c i a t e d w i t h t h e s u b s t r a t e , t h e m a j o r i t y a p p r o x i m a t e l y 1 cm above i t . A l t h o u g h l a r g e q u a n t i t i e s o f p r e y i t e m s , i n b o t h I** Fry t e r r i t o r i a l mosaics at three prey levels: LP = low prey, IP = intermediate prey, HP = high prey (Experiment 2, day 8). Fry occupying unstable surface ter r i t o r i e s are not plotted (see Fig.13 for locations). f r y retreating under grid peripheral feeding locations | fry occupying territories near substrate © interaction locations during 10 to JO min periods 37 experiments 1 and 2, se t t l e d among the substrate i n t e r s t i c e s , f r y d id not a c t i v e l y feed on these items except when disturbed i o f f the bottom and d r i f t i n g . Experiment 3 The frequency of aggressive encounters (nips, and chases not followed by nips) was s i g n i f i c a n t l y greater at the lower of two prey l e v e l s when density was held constant (Figure 15). The r e s u l t s also show that there was a higher frequency of encounter i n the f i r s t f i v e hours than l a t e r i n the experiment. This i s perhaps r e l a t e d to i n i t i a l hunger l e v e l and the establishment of a dominance - subordination system. A f t e r t h i r t y two and t h i r t y s i x hours the two prey l e v e l s were reversed within the same groups of f r y . This manipulation had a very s i g n i f i c a n t e f f e c t on the number of aggressive encounters; reversing the frequency of encounters within the o r i g i n a l groups p a r t i c u l a r l y i n the s h i f t from high prey to low prey (Figure 15). Although the s h i f t was made just a f t e r the change from day to night i t was very un-l i k e l y that f r y were able to v i s u a l l y feed since i t was dark within the hatchery at night. There was also a delay i n sub-sidence of encounter frequency when low prey was s h i f t e d to high prey. Number and size of t e r r i t o r i e s did not appear to be obviously d i f f e r e n t under the two feeding regimes ( t e r r i t o r y sizes were not measured). The number of f r y holding d e f i n i t e 38 0 10 20 30 40 50 hr TIME FROM FRY INTRODUCTION F i g . 15 Frequency of aggr e s s i v e encounter a t two d i f f e r e n t prey l e v e l s - Low Prey and High Prey, no e m i g r a t i o n p e r m i t t e d . Arrows i n d i c a t e when prey l e v e l was reversed and v e r t i c a l l i n e s i n d i c a t e 95% confidence l i m i t s . 39 t e r r i t o r i e s ranged from four to seven, varying somewhat be-tween r e p l i c a t i o n s and treatments. Such v a r i a b i l i t y appeared to be associated v/ith the degree of dominance of one or two f r y within the test areas. T e r r i t o r i a l f r y i n the upper part of the t e s t area appeared to be much more dominant than ones further back. There were also f r y which were designated as transient subor-dinates and intransient subordinates. The transient subor-dinates appeared more numerous i n the high prey treatment (HP = 12/4 t e s t areas, LP = 4/4 test areas) and the i n t r a n -sient subordinates appeared to be present more often i n the low prey treatment. These stationary i n d i v i d u a l s were p r i m a r i l y positioned along the downstream margins and p a r t i t i o n s . Both types of f r y were submissive; never e l i c i t i n g an attack. FIELD RESULTS Prey Density i n Relation to Fish Standing Crop Prey Composition i n Fry Stomachs Fry were sampled for stomach contents during the summer (1967) at Section I I I and Section IV, 180 and 600 m downstream from the lake respectively. Results (Figures 16 and 17) show that organisms from the family Chironomidae and from the genus. Baetis made up a large proportion of prey items. T e r r e s t r i a l organisms were generally of much less s i g n i f i c a n c e i n the d i e t than aquatic organisms. 40 JULY 20 AUG. 25 AUG.11 SEPT. 9 1 . CHIRONOMIDAE LARVAE 2. BAST 13 sp NYMPHS 3. CHIRONOMIDAS ADULTS ^. CHIRONOMIDAS PUPAE 5 . SIMULIDAS LARVAE 6. GAMARUS l a c u s t r i s UJ TERRESTRIAL 7. HYDROPSYCHS sp LARVAE 8. TRICOPTSRA PUPAE 9. SIKULIDAE PUPAE 1 0 . SIMULIDAE ADULTS 1 1 . BAETIS sp ADULTS 1 5 . MISCELLANEOUS © NUMBER OF FRY F i g . 16 Percentage composition of f r y stomach contents, summer, 1967, Section I I I , Loon Outlet Creek. Only organisms of greater than 2$ composition included. Three sampling times of day are combined. 41 JULY 20 AUG.25 AUG . 11 SEPT.9 CHIR0K0KTDAE LARVAE 2. BAST13 sp NYMPHS 3. CHIRONOMIDAS ADULTS k. CHIRONOMTDAE PUPAE 5. SIMULIDAE LARVAE 7. HYDRO PSYCHE sp LARVAE 9. SIMULIDAE PUPAE 11. BASTIS sp ADULTS 12. HYDROPTILTDAE LARVAE 13. HYGR03IIDAE LARVAE lk. SLMIDAS LARVAE 15. MISCELLANEOUS © NUMBER OF FRY • F i g . 17 m TERRESTRIAL Percentage composition of f r y stomach c o n t e n t s , summer, 1967, S e c t i o n IV, Loon O u t l e t Creek. Only organisms of g r e a t e r than 2$ composition i n c l u d e d . Three sampling times of day are combined. 42 The most obvious difference between the two study sections i s that Gammarus l a c u s t r i s was never found i n stom-achs of f r y from Section IV, but made up a s i g n i f i c a n t per-centage of the prey during at least one sampling period i n stomachs from Section I I I . There were also more t e r r e s t r i a l organisms taken i n Section IV during the two l a t e summer sampling periods than during the same periods i n Section I I I . In comparing the four sampling periods over the summer, there were fewer Chironomids and Baetis sp taken by f r y as the summer progressed, p a r t i c u l a r l y i n Section IV. Summer D r i f t Density of Prey Organisms and Fish  Standing Crop Summer fluctuations i n d r i f t density of prey organisms are evident within both stream study sections (Figure 18). Prey density appears highest i n the early August sampling period i n both Sections I I I and IV and lowest i n the J u l y sampling period. Generally, Section I I I located closest^ to the lake had a greater abundance of d r i f t i n g prey organisms entering i t , compared to the Section IV 600 m from the lake (Appendix I I I : percent composition of d r i f t i n g prey). For Section IV night (2400 hr) samples are combined with day samples (1600, 0800 hr) since r e s u l t s were s i m i l a r . There was also a change over the summer i n f i s h stand-ing crop (biomass) within each study section, although the two sections appear d i s s i m i l a r i n J u l y (Figure 18). The J u l y estimate i s crude ( i . e . exceptionally large confidence l i m i t s ) JULY AUGUST SEPT. F i g . 18 D r i f t density of prey organisms, and f i s h biomass - Section I I I (upper) and IV (lower), Loon Outlet Creek, 1967. V e r t i c a l l i n e s indicate 95$ confidence l i m i t s . Dots indicate range. 44 because of very low recaptures. Fish were d i f f i c u l t to seine during t h i s period owing to the high water l e v e l and stream morphometry. Thereby some marked f r y probably emigrated from the census section causing an over-estimate. The other e s t i -mates are considered to be much more r e l i a b l e (also juvenile biomass was n e g l i g i b l e a f t e r J u l y ) . 2 In Section IV the biomass of f i s h per m p o s i t i v e l y correlates with the d r i f t i n g prey density, and i n Section I I I a s i m i l a r c o r r e l a t i o n may pertain, although less c l e a r l y , due to uncertainty of the Ju l y f i s h population estimate. Section I I I tended to have a higher f i s h biomass than Section IV except at the September sampling period. D r i f t Density of Prey Organisms i n Relation to Lakeward  Migration of 1+ and 2++ Juveniles. Prey Composition i n Juvenile Stomachs Chironomidae and Baetis sp. became less important i n la t e spring and early summer (Figure 19). Also t e r r e s t r i a l organisms were of l i t t l e s i g n i f i c a n c e u n t i l the J u l y sampling period. Unfortunately sample sizes were small thus l i m i t i n g further i n t e r p r e t a t i o n . D r i f t Density and Lakeward Migration A s i g n i f i c a n t reduction i n the density of d r i f t i n g prey occurred between l a t e May and early June (Figure 20). Later i n June further reduction occurred so that the prey 2 3 density was 7.3 per m compared to 24.0 per m on May 26 to 27. 45 MARCH 19 JUNE 9 MAY 27 JULY 15 ED ' TERRESTRIAL 1. CHIRONOMIDAS LARVAE 2. CHIRONOMIDAE PUPAE 3 . CHIRONOMIDAS ADULTS SIMULIDAE LARVAE 5 . BAST IS SD NYMPHS 6. SIMULIDAE ADULTS 7. BASTIS sp ADULTS 8. GAMMARUS l a c u s t r i s 9. SALM3 g a l r d n e r i EGGS 10. TIPULIDAE LARVAE 1 1 . HYDROPSYCHS sp LARVAE 12. MISCELLANEOUS DIPTERA ADULTS 1 3 . HEMIPTERA NYMPHS 1*4-. COLEOPTERA ADULTS 15 . CULICOIDSS so LARVAE 16. TRICOPTSRA ADULTS 17. FORMIC TDAE ADULTS 18. OTHER HYMSNOPTERA ADULTS 19. EMPIDAE ADULTS 20. MISCELLANEOUS AQUATIC INSECTS F i g . 19 Percentage composition of j u v e n i l e stomach c o n t e n t s , 1970, S e c t i o n I V , Loon O u t l e t Creek. © = Sample s i z e 46 < Q 20 | APRIL 10 20 MAY 10 20 JUNE 10 20 JULY F i g , 20 Prey d e n s i t y and prey l e n g t h , mean d a i l y temperature, and d a i l y upstream movement of 1+ & 2++ j u v e n i l e rainbow t r o u t i n Loon O u t l e t Creek (Trap 1 ) , 1970. V e r t i c a l l i n e s on d r i f t d e n s i t y are 95$ confidence l i m i t s . 47 In a d d i t i o n , the mean s i z e of p r e y i n the d r i f t samples was s m a l l e r i n June and J u l y than i n the May samples. E m i g r a t i o n o f J u v e n i l e t r o u t s t a r t e d near the end of May r e a c h i n g a maximum d a i l y r a t e about June 20 and was v i r -t u a l l y n e g l i g i b l e a f t e r J u l y 20 ( F i g u r e 20). Mean d a i l y temperature i n c r e a s e d s t e a d i l y , w i t h a few f l u c t u a t i o n s , from s p r i n g i n t o mid-summer. J u v e n i l e f i s h movement was t h e r e f o r e n e g a t i v e l y c o r r e l a t e d w i t h d e c r e a s i n g d r i f t d e n s i t y and p o s i -t i v e l y c o r r e l a t e d w i t h i n c r e a s i n g temperature. V DISCUSSION LABORATORY STUDY The r e s u l t s of the laboratory experiments, i n general, support the hypothesis that prey abundance a f f e c t s density and d i s t r i b u t i o n of underyearling (0+) trout. When f r y were introduced i n t o d i f f e r e n t prey l e v e l s , the r e s u l t s were c l e a r ; the abundance of prey organisms, during the time of study, had a s i g n i f i c a n t e f f e c t on the f r y density ( i . e . , increased prey; increased density of f r y ) . Accordingly, the recorded d i s t r i b u t i o n of f r y " w i t h i n a l l channels (experiment 1 and.2) also supported the tested hypothesis. I t would be expected, i f higher prey abundance permitted higher f r y density, that a gradient i n f r y numbers would be p o s i t i v e l y associated with a gradient i n prey abundance. This was most apparent when Tubifex was u t i l i z e d as a prey organism because i t lacked m o t i l i t y and s e t t l e d out more r a p i d l y than the invertebrate composition used i n the f i r s t experiment. However, when the prey l e v e l was s h i f t e d from high to a lower l e v e l (experiment 1) the influence upon f r y density or a l t e r n a t e l y on emigration, was not as marked. However, a number of factors could be involved which were not apparent i n the previous s i t u a t i o n . F i r s t l y , temperature was 9.6 C so that i t would require about 12 hr to empty 7 5% of the stomach 49 (Brett and Higgs, 1970) and the greatest increase i n voluntary food intake, as a measure of hunger, would be seven to eleven hours a f t e r f a s t i n g (Brett, 1971). I f emigration was rela t e d to hunger l e v e l , then there may be a delay i f f r y were f u l l y satiated. Also, i t may require a r e l a t i v e l y long period of time f o r i n d i v i d u a l f i s h to depart from t e r r i t o r i e s they were holding. Moreover, prey were s t i l l present within the channel, d r i f t i n g upwards from the substrate, so that prey density d id not s h i f t as r a p i d l y as i t would appear. F i n a l l y , i t i s possible that the length of time i n holding had a suppressing effect (conditioning) on aggression and movement (Keenleyside and Yamamotto, 1962). C e r t a i n l y a s h i f t i n prey abundance had a s i g n i f i c a n t e f f e c t on the frequency of aggressive encounter. This was p a r t i c u l a r l y s i g n i f i c a n t when the prey l e v e l was reduced (experiment 3, Figure 15). Accepting that there was a s i g n i f i c a n t e f f e c t of prey abundance on f r y density, the more important question to r a i s e i s the mechanism or causal f a c t o r ( s ) . In t h i s regard, t e r r i -t ory s i z e was found to be larger at lower prey l e v e l s (experi-ment 2). This implies that prey abundance d i r e c t l y a f f e c t s the t e r r i t o r y s i z e and aggressiveness of f r y . However, the change i n t e r r i t o r y size was not observed to occur p r i o r to any emigration, but instead as emigration proceeded. Thereby, no cause and effe c t r e l a t i o n s h i p can be d e f i n i t e l y established; i.e.., the observed difference i n size could possibly be r e s u l t rather than a cause. 50 The f i r s t e x p e r i m e n t a t l e a s t s u g g e s t e d t h a t t h e more d o m i n a n t f r y i n c r e a s e d t h e i r t e r r i t o r y s i z e s a f t e r a s h i f t i n p r e y a b u n d a n c e b u t p r i o r t o a n y s i g n i f i c a n t e m i g r a -t i o n . R e g a r d l e s s , i t may n o t b e e x p e c t e d t h a t t h e s i z e o f t e r r i t o r i e s s h o u l d i m m e d i a t e l y i n c r e a s e , s i n c e p r e s s u r e b y a d j a c e n t f r y w o u l d l i m i t e x p a n s i o n . T h e r e i s o t h e r e v i d e n c e , h o w e v e r , t h a t i n d i c a t e s a g g r e s s i o n i s a t l e a s t p a r t o f t h e m e c h a n i s m i n v o l v e d . Where e m i g r a t i o n was p r e v e n t e d ( e x p e r i e n t 3) a n d t h e r e b y d e n s i t y was c o n s t a n t , t h e f r e q u e n c y o f a g g r e s s i v e e n c o u n t e r was s i g -n i f i c a n t l y l e s s a t t h e h i g h e r p r e y l e v e l . M o r e o v e r , a s p r e -v i o u s l y i n d i c a t e d t h e r e was a s i g n i f i c a n t i n c r e a s e i n t h e f r e -q u e n c y o f a g g r e s s i v e e n c o u n t e r when p r e y a b u n d a n c e was d e -c r e a s e d a n d v i c e v e r s a . A l t h o u g h e v i d e n c e p r e s e n t e d s u p p o r t s t h e i n f e r e n c e t h a t y o u n g t r o u t a l t e r t e r r i t o r y s i z e r e l a t i v e t o p r e y a b u n d a n c e ( i . e . , v i a c h a n g e i n a g g r e s s i v e n e s s ) , t h e r e i s s t i l l a n a l t e r n a t e e x p l a n a t i o n . I t c a n be a r g u e d t h a t c h a n g e i n p r e y a b u n d a n c e a l t e r s o n l y t h e r e a c t i v e d i s t a n c e o f f r y t o p r e y , t h e r e b y i n c r e a s i n g t h e p r o b a b i l i t y o f f i s h a g g r e s s i v e l y e n c o u n t e r i n g a d j a c e n t f r y , a n d no i n c r e a s e d a g g r e s s i v e n e s s p e r s e . T h e s e t w o p o s s i b i l i t i e s a r e e x p l o r e d t h e o r e t i c a l l y a s f o l l o w s ; A a n d B: 51 Prey Density Hunger Level ( i . e . , p h y siological input - o r a l and v i s u a l factors - stomach f u l l n e s s - blood nutrient l e v e l ) I Reactive distance Aggressiveness to prey i . e . , reactive distance to cohabiting f i s h Size of Defended Area Density of Fry Thereby, any e f f e c t on density could proceed by pathway A or B, or both A and B. There i s some evidence that pathway A i s not the only explanation. For example, prey items were introduced only every 10 minutes so that during the intervening period (7 to 8 minutes) there were less prey items d r i f t i n g . During t h i s period aggressive encounters were frequently i n i t i a t e d from the home stat i o n , the majority not associated with feeding movements. If the increase i n aggression was only the r e s u l t of the feeding acts t h i s would not have been observed. On t h i s basis, and because the l i m i t s of the defended area and the feeding area l a r g e l y coincided, i t i s suggested both pathway A and B are involved. Thus, when prey density s h i f t s to a lower l e v e l , the only means by which an i n d i v i d u a l f i s h can receive a comparable quantity of prey, i s to increase the size of the feeding area. However, t h i s i s not possible unless adjacent f i s h are forced to reduce t h e i r areas; i . e . , the reactive distance to f i s h would also have to change. Thereby, an adjacent subordinate's t e r r i t o r y i s necessarily reduced, at the same time the f r e -quency of encounter increasing and prey density decreasing. These factors together, then possibly necessitate the emigra-t i o n of subordinates. The r e s u l t of the weight frequency d i s t r i b u t i o n s within the three channels did not demonstrate either equal growth at the three prey l e v e l s , nor a large difference i n growth. However, s i m i l a r growth may not be expected since more energy would be expended to s t r i k e prey at larger distances from the home s t a t i o n . Thus, an i n d i v i d u a l by expanding i t s t e r r i t o r y may grasp the same amount of prey and f i l l i t s stomach to the same f u l l n e s s , but there would be higher energy costs. For example, Brett (1964) found oxygen consumption i n sockeye was approximately doubled (at 10 C) by increasing the swimming speed from 2 times the body length to 3 times the body length. On t h i s basis, some difference i n growth may be expected, but why growth at the high prey l e v e l was the same as the low prey l e v e l and d i f f e r e n t from the intermediate l e v e l i s not c l e a r . Perhaps at the low prey l e v e l only those f r y i n i t i a l l y larger 53 remained within the channel. FIELD STUDY One of the objects of the f i e l d study was to ascer-t a i n i f f l u c t u a t i o n s i n prey density took place within a rearing stream. Results from both the 1967 summer and 1970 spring to summer d r i f t samples indicate that such f l u c t u a t i o n s do occur. This conclusion i s reached with the reservation that measurements of d r i f t density may have underestimated the t e r -r e s t r i a l component of the d r i f t , f o r two reasons. F i r s t l y , the p o s s i b i l i t y e x i s t s that juvenile trout may feed on a num-ber of t e r r e s t r i a l insects when these insects are either f l y i n g near the surface or depositing eggs rather than while they are d r i f t i n g . Thereby, the d r i f t samplers would not measure t h i s part. Secondly, i t i s also probable that d r i f t samplers did not adequately sample surface d r i f t i n g material owing to current patterns set up by the sampler causing d e f l e c t i o n of surface items. One way to ascertain the r e l a t i v e degree of p o t e n t i a l error i s to consider the percentage composition of t e r r e s t r i a l items. In the 1967 stomachs, t e r r e s t r i a l insects were of .negligible importance i n Section I I I and of l i m i t e d importance i n Section IV except i n September. Moreover, the majority of t e r r e s t r i a l insects found within stomachs was one extremely small species (mean size 1.5 mm) so that composition by percent weight would be much l e s s . S i m i l a r l y , i n 1970 spring to summer, i 54 t e r r e s t r i a l insects accounted for less than 18% of the die t u n t i l J u l y where they made up the majority of the d i e t . There-by, any error due to an underestimation of t h i s component i s probably small except i n the summer period of the 1970 sampling. The cause of the f l u c t u a t i o n i n d r i f t density was not d i r e c t l y v e r i f i e d but i s probably caused by at least three f a c t o r s . In la t e spring and l a t e r summer, pupation and adult emergence of insect fauna i s a regular occurrence (Mundie, personal communication). Secondly, the trout spawning popu-l a t i o n during the spring undoubtedly has a s i g n i f i c a n t e f f e c t on d r i f t density. Accordingly, Hildebrand (1971) found a 66% decrease i n the density of benthic invertebrates, r e l a t i v e to controls, a f t e r coho salmon had u t i l i z e d a stream area f o r spawning purposes. Moreover, the juvenile population i t s e l f may have contributed to the reduction and fl u c t u a t i o n s i n d r i f t density. Peterson (1966) reported evidence of t h i s occurrence, at least for d r i f t rates of ephemeroptera nymphs. The other object of the f i e l d study was to ascertain i f f l u c t u a t i o n s i n d r i f t density were associated with r e l a t i v e changes i n density and emigration of f r y and ju v e n i l e s . The re s u l t s , i n general, were compatible with and supported the laboratory findings; i . e . , both density (biomass) per unit area, and emigration of juveniles from the stream tended to correlate with measured d r i f t density. Such a c o r r e l a t i o n was p a r t i c u l a r l y evident during the spring to summer period between d r i f t density and f i n g e r -l i n g emigration (lake-ward migration) i n Loon Creek Outlet. "I 55 Here a strong inverse r e l a t i o n s h i p was apparent. Moreover, considering the associated reduction i n mean prey s i z e , the reduction i n d r i f t density (numbers) would be much more s i g -n i f i c a n t i n terms of weight. Northcote (1962, 1969) emphasized the importance of temperature, based on re s u l t s from laboratory and f i e l d experi-ments and from c o r r e l a t i o n analysis, for inducing emigration of various salmonids from rearing areas. Accordingly, the negative c o r r e l a t i o n between prey density and emigration of fi n g e r l i n g s could be considered to be of a spurious nature (Figure 20). However, Northcote (1962) noted that the mechanism involved of how temperature may regulate emigration i s not yet established and suggested i t could involve accelera-t i o n of metabolism and a c t i v i t y at higher temperatures. I f the work of Brett et a l . (1969) on the growth rate of f i n g e r l i n g sockeye salmon (Oncorhynchus nerka) i s considered, the r e s u l t s of t h i s study are very compatible with those of Northcote (1962). Brett et a l . (1969) concluded that the optimal temperature for growth s h i f t e d to a lower temperature as r a t i o n size was decreased (swimming speed ranged from 9 to 15 cm/sec). At Loon Outlet i t i s apparent that when a r e -duction i n prey density occurred, average temperature con-tinued to increase (Figure 20). Brett et a l . (1969) indicated that i f the r a t i o n was reduced from a maximum of 6% to 1.5% of the body weight per day ( i . e . , a 4 f o l d reduction) and at the same time the tem-perature increased by only 5 C to 15 C, then negative growth 56 would begin. In Loon Outlet Creek the average d a i l y tempera-ture was observed to r i s e from 10 C on May 20 to 20 C on June 20. Thereby, i f prey density decreased by four f o l d , then just on the basis of growth ( i . e . , with no behavioral adjustment) a s i g n i f i c a n t l e v e l of negative growth would be the r e s u l t . Accordingly, there i s evidence that suggests aggres-sive behavior and space u t i l i z a t i o n are influenced by tempera-ture. For example, Northcote (196 2) reported that f r y s h i f t e d back and f o r t h more ( i . e . , became more active) at higher tem-peratures. Also Hartman (1965) reported a marked decrease i n aggression of young steelhead (Salmo gairdneri) and coho s a l -mon when temperature was low. Therefore, on the basis of the laboratory r e s u l t s of t h i s study, the implication of Brett's study, the r e s u l t s of Northcote 1s study and the implication of Hartman 1s observations, i t i s believed that behavioral adjustment occurs (as :previously described), leading to numerical adjustment of f i s h to the abundance of prey organisms. Moreover, there i s also the factor of increasing f i s h size i t s e l f which increases metabolic demands (Brett, 1965; Ivlev, 1960). (Brett's analysis (1969) standardizes t h i s factor i n terms of percent body weight per day). Thereby, a greater amount of prey w i l l be required, as the spring pro-gresses, i n order to maintain a high comparable growth rate. Loon Outlet juveniles apparently double i n weight over the 57 spring months (Fish & W i l d l i f e data on f i l e ) and recently A l l a n (1969) emphasized the r e l a t i o n between t e r r i t o r y s i z e and f i s h size among, salmonids; the average area per f i s h being proportional to f i s h weight. This i s also consistent with the concept of food determined t e r r i t o r i a l i t y . Consequently, i t i s suggested that emigration (lake-ward migration) i s a r e s u l t of three i n t e r r e l a t e d a d d i t i v e factors, a l l of which p o t e n t i a l l y would cause growth rate re-duction without behavioral adjustment: increased f i s h s i z e , increasing temperature, and reduction i n prey density. On the basis of laboratory experiments, reported herein, prey abun-dance a f f e c t s t e r r i t o r i a l behavior and thereby i t i s suggested t h i s mechanism of behavioral adjustment causes displacement of subordinate i n d i v i d u a l s . Accordingly, Symons ( i n press) demonstrated that juvenile subordinate A t l a n t i c salmon were displaced from a test channel when the r a t i o n was markedly reduced. Further, t h i s may not require that a l l j uveniles migrating to the lake are subordinates. The prey density may drop to such a low l e v e l i n any part of the stream ( r e l a t i v e to temperature) that i t may become en e r g e t i c a l l y i n e f f i c i e n t for dominant i n d i v i d u a l s to remain, even i f p a r t i c u l a r l y large feeding areas are u t i l i z e d . THE EFFECTS OF STREAM ENRICHMENT Further supporting evidence of the e f f e c t of prey abundance on salmonid density may be i n d i r e c t l y i n f e r r e d from stream f e r t i l i z a t i o n and enrichment studies. Few studies have 58 been made on the effects of inorganic f e r t i l i z a t i o n of streams, but those recorded demonstrate an ef f e c t on salmonid density. For example, Huntsman (1948) observed a f t e r placing sacks of inorganic f e r t i l i z e r along a stream margin, there were large increases i n algae, insect fauna, and f i s h numbers. The maximum ef f e c t was 50 m downstream, with no ef f e c t above where the f e r t i l i z e r was placed. S i m i l a r l y , M i l l s (1969) observed that p r i o r to f e r t i l i z a t i o n of a loch, the ou t l e t 2 stream had a density of .2 stocked f r y / m , where the density 2 aft e r f e r t i l i z a t i o n was .9 stocked f r y / m . The following 2 year the density was .8 f r y / m and the density of yearlings was more than doubled compared to that of previous years. Other stocked streams nearby did not change from a s i m i l a r 2 i n i t i a l density ( i . e . , approximately .2 gm / m ). A s i m i l a r effect may also have occurred at Loon I n l e t Creek where the overwintering population was previously small (745 f i n g e r l i n g s migrated from the I n l e t Creek i n the Spring, 1953). In the l a s t 15 years, inorganic f e r t i l i z e r has been used i n large amounts on the surrounding a g r i c u l t u r a l land. Recent observations (1969 and 1970) on numbers of f i s h present i n the f a l l and spring suggest that density i s now higher. Correspondingly i n a study by Warren et a l . (1964), whereby a stream was enriched with sucrose, a very large i n -crease i n insect production was demonstrated, e s p e c i a l l y Chironomidae. However, no f a c i l i t y was included to allow for f i s h d i s p e r s a l . Although a seven f o l d increase i n production of cutthroat trout (Salmo c l a r k i ) was attained r e l a t i v e to the 59 control, no appreciation of natural population dynamics of the population could be achieved. Clearly, i n studies where emi-gration i s prevented such as Warren et a l . (1964) or Backiel and LeCren (1967) i t would be precarious to extrapolate the re s u l t s to natural populations without ample consideration o f dispersion. TERRITORIAL BEHAVIOR AND PREY SELECTION The r e s u l t s of t h i s study are, i n general, compatible with those of Symons (1968) and Symons (i n press), and also with the e a r l i e r observations of Mason and Chapman (1965). Moreover, Magnuson (196 2) also demonstrated i n Medaka that aggression was at a lower frequency when food was abundant. In contrast, some authors report that aggression i s highest when f i s h are fed to excess; Newman (1956) rainbow trout; Keenleyside and Yamamotto (1962) A t l a n t i c salmon. Moreover, Chapman (1962)found that feeding coho salmon to excess with brine shrimp, did not a l t e r f r y density i n a stream channel}. I n i t i a l l y there appears to be an i n c o m p a t i b i l i t y be-tween these observations and those of t h i s study. However, perhaps the designation 'excess' i s misleading. For example, both Chapman and Newman only used one feeding per 24 hours. Keenleyside and Yamamotto used a frequency of one to four feedings per '24 hours. Brett (1971), recording the amount of food consumed i n r e l a t i o n to deprivation time as a measure of hunger, reported that the maximal increase i n hunger occurred 60 seven to eleven hours from f a s t i n g . I f aggression i s related-to hunger, then i t may he expected that aggression would be high unless f i s h were fed continuously, keeping hunger at a low l e v e l . However, complete absence of food also reduces aggression, at least i n Medaka (Magnuson, 196 2) so that per-haps the v i s u a l stimulus of food i s necessary to e l i c i t aggression, or maintain i t at a ce r t a i n l e v e l . F i n a l l y , further tests of the ef f e c t of prey abundance on t e r r i t o r i a l behavior and density would be most meaningful i f the factors prey selection and duration i n holding are taken i n t o account. The question raised i s just how r i g i d are i n d i v i d u a l stream salmonids i n selection of prey? I f i n -dividuals s p e c i a l i z e on s p e c i f i c prey items can these f i s h r e a d i l y switch to other prey when the density of the i n i t i a l prey decreases? Observations at the Loon Creek Hatchery indicate that f r y removed from streams would not s i g n i f i c a n t l y feed on dry hatchery foods or cut tubifex (or whole) u n t i l at least a week had elapsed since the f i s h were captured. These same f i s h would r e a d i l y feed on stream insect fauna. A s i m i l a r observa-t i o n was made with respect to i n i t i a t i o n of feeding on zoo-plankters, although only about two days holding was necessary. Moreover, f r y i n i t i a l l y reared on stream d r i f t within the hatchery tended not to feed on cut tubifex, p a r t i c u l a r l y i f even small amounts of stream d r i f t were present. On the other hand, Bryan (personal communication) demonstrated by means of laboratory feeding experiments and 61 repeated sampling of marked stream dwelling trout (S. gairdneri) that f i s h are not r i g i d l y f i x e d i n prey s e l e c t i o n ; that trout do at least switch to items previously found infrequently i n stomachs. In Loon Outlet Creek, zooplankters usually comprise a s i g n i f i c a n t percentage (by weight and numbers) of the d r i f t composition, although t h i s can be intermittent i n presence. ) For example, i t was n e g l i g i b l e during the 1970 sampling, but of very marked occurrence i n the summer d r i f t , 1967. From the stomach analysis of Loon Outlet f i s h , zooplankton was v i r t u a l l y not u t i l i z e d as a food source, yet made up a s i g n i f i c a n t per-centage (Cladocerans only) i n the stomachs of lake dwelling juveniles and adults (Hartman, 1954). Consequently, consider-ing t h i s problem of prey s e l e c t i o n i t may be' of paramount im-portance to determine the effects on s o c i a l behavior of densi-t i e s of d i f f e r e n t prey types and of previous feeding h i s t o r y . Another factor that requires consideration i s the . effect of holding and crowding for extended periods of time. For example, Keenleyside and Yamamotto (196 2) demonstrated that juvenile A t l a n t i c salmon that were held for s i x weeks and then introduced for the f i r s t time into an observation aquarium, showed much reduced nipping frequencies over a two day period. Fish held for a period of 10 days p r i o r to introduction showed only a small reduction i n nipping frequency. Consequently, i t may be misleading to u t i l i z e f i s h that have either been held i n crowded conditions for more than a few weeks, or w i l d or domestic f i s h o r i g i n a t i n g from hatcheries. . In conclusion, i t was emphasized by Kalleberg (1958) that t e r r i t o r i a l i t y i s a c h a r a c t e r i s t i c evolved as a food supply mechanism. Accordingly, McFadden (1969) hypothesized that a change i n t e r r i t o r y size r e l a t i v e to food abundance would be h i g h l y adaptive, ensuring enough food for a s i g n i f i - . cant l e v e l of growth whether a salmonid inhabits a productive or unproductive stream; i . e . , i n contrast to the density dependent growth found i n salmonids inhabiting lakes — e.g., Johnston, 1965. The r e s u l t s of t h i s study, i n general, support the concept of a food linked s p a c i a l requirement i n a t e r r i -t o r i a l salmonid — Salmo g a i r d n e r i . BIBLIOGRAPHY Adams, L., 1951. Confidence l i m i t s f o r the Peterson or Lincoln index used i n animal population studies. J . W i l d l i f e Management, 15: 13-19. Al l e n , K.R., 1969. Limitations on production i n Salmonid populations i n streams, p. 3-18. In T.G. Northcote, (ed.) Salmon and trout i n streams. H.R. MacMillan lectures i n f i s h e r i e s . University of B r i t i s h Columbia. Backiel, T. and E.D. LeCren, 1967. Some density r e l a t i o n s h i p s for f i s h populations parameters, p. 216-293. In S.D. Gerking, (ed.), The B i o l o g i c a l basis of Freshwater f i s h production. Oxford: Blackwell, 291-293. Bailey, N.J.J., 1951. On estimating the size of mobile popu-l a t i o n s from recapture data, Biometrika 38: 293-306. Brett, J.R., 1964. The resp i r a t o r y metabolism and swimming performance of young sockeye salmon, J . Fish. Res. Bd. Canada, 21: 1183-1226. 1965. The r e l a t i o n of size to rate of oxygen con-sumption and sustained swimming speed of sockeye salmon (Oncorhynchus nerka), J . Fis h . Res. Bd. Canada, 22: 1491-1501. 1971. Satiation time, Appetite, and maximum food intake of sockeye salmon (Oncorhynchus nerka), 28 (3): 409-415. Brett, J.R., J.E. Shelbourn and C.T. Shoop, 1969. Growth rate and body composition of f i n g e r l i n g sockeye salmon, Oncorhynchus nerka, i n r e l a t i o n to temperature and r a t i o n s i z e . J . Fish. Res. Bd. Canada, 26(9): 2363-2394. Brett, J.R. and D.A. Higgs, 1970. Effect of temperature on the rate of gas t r i c digestion i n f i n g e r l i n g sockeye salmon, Oncorhynchus nerka. J . Fish. Res. Bd. Canada, 27(10) : 1767-1779. Chapman, D.W., 1962. Aggressive behavior of juvenile coho salmon as a cause of emigration. J . Fish. Res. Bd. Canada, 19: 1047-1080. 64 Chapman, D.W. and T.C. Bjornn, 1969. D i s t r i b u t i o n of salmonids i n streams with special reference to food and feeding, p. 153-176. In T.G. Northcote, (ed.), Salmon and trout i n . . streams. H.R. MacMillan lecture i n f i s h e r i e s . University of B r i t i s h Columbia. Cushing, C.E., 1964. An apparatus f or sampling d r i f t i n g or-ganisms i n streams. J . W i l d l i f e Management, 28 (3):592-594. E l l i o t , J.M., 1967. The food of trout (Salmo t r u t t a ) i n a Dartmoor stream. J . Applied Ecol., 4:- 59-72. 1967. Invertebrate d r i f t i n a Dartmoor stream. Arch. Hydrobiol., 63(2): 202-237. 1968. The l i f e h i s t o r i e s and d r i f t i n g of t r i c o p t e r a i n a Dartmoor stream. J . Anim. Ecol., 37: 615-6 25. 1968. The d a i l y a c t i v i t y patterns of mayfly nymphs (Ephemeroptera). J . Zool. London, 155: 201-221. 1970. Di e l changes i n invertebrate d r i f t and the food of trout, Salmo t r u t t a , L.J. Fish. B i o l . , - 2: 161-165. Etkin, W., 1963. Co-operation and competition, i n s o c i a l be-havior, p. 1-34. In W. Etkin (ed.), Social behavior and  organization among vertebrates. Univ. Chicago, 307 pp. Hartman, G.F., 1954. Con t r o l l i n g factors i n the feeding of Kamloops trout (Salmo gairdneri) i n Loon Lake B r i t i s h Columbia. B.A. thesis, Univ. of B r i t i s h Columbia, 40 pp. 196 5. The role of behavior i n the ecology and i n t e r -action of underyearling coho salmon (Oncorhynchus kisutch) and steelhead trout (Salmo g a i r d n e r i ) . J . Fish. Res. Bd. Canada, 22: 1035-1081. Hartman, G.F., T.G. Northcote, and CC. Lindsey. 1962. Com-parison of i n l e t and outlet spawning.runs of rainbow trout i n Loon Lake, B r i t i s h Columbia, J . Fis h . Res. Bd. Canada, 19 (2): 173-200. Hildebrand, S.G., 1971. The eff e c t s of coho spawning on the benthic invertebrates of the P l a t t e River, Benzie County, Michigan. Trans. Amer. Fis h . Soc, 100 (1): 61-68. Huntsman, A.G., 1948. F e r t i l i t y and f e r t i l i z a t i o n of streams. J . F i s h . Res. Bd. Canada, 7: 248-253. I 65 Ivlev, V.S., 1960. Z e i t s c h r i f t fur Fischerei und deren H i l l s -wissen schaften N.F. #3/4 281-289. Bestimmungsmethods der von dem wachsenden Fisch ausgenutzten Fultermengen. Trans. Gerking, S.D. 1961. F.R.B. Trans. #374. Method of e s t i -mating the food u t i l i z e d by growing f i s h . Jenkins, T.M. J r . , 1969. Soc i a l structure, p o s i t i o n choice and mi c r o d i s t r i b u t i o n of two trout species (Salmo t r u t t a and Salmo gairdneri) resident i n mountain streams. Anim. Behav. Mon., 2 (2): 57-123. Johnston, J.M., 1967. Food and feeding habits of coho salmon and steelhead trout i n Worthy Creek, Washington. M.Sc. the s i s , Univ. of Washington. Johnston, W.E., 1965. On mechanisms of s e l f - r e g u l a t i o n of population abundance i n Oncorhynchus nerka. M i t t . i n t . Ver. Limnol., 13: 66-87. Kalleberg, H., 1958. Observations i n a stream tank of t e r r i -t o r i a l i t y and competition i n juvenile salmon and tr o u t . Inst. Freshw. Res., Drottningholm, Rept. 39: 55-98. Kawanabe, H., 1969. The signi f i c a n c e of s o c i a l structure i n production of the "Ayu", Plecoglossus a l t i v e l i s , p. 243-251. Keenleyside, M.H.A. and F.T. Yamamotto., 1962. T e r r i t o r i a l behavior of juvenile A t l a n t i c salmon (Salmo salar L.). Behavior, 19: 139-169. Krebs, J.R., 1971. T e r r i t o r y and breeding density i n the great t i t , Parus ma j or L., Ecol. 52 (1): 2-22. Lindsey, C.C., T.G. Northcote and G.F. Hartman., 1959. Homing of rainbow trout to i n l e t and outlet spawning at Loon Lake, B r i t i s h Columbia. J . Fish. Res. Bd. Can., 16: 695-719. Lund, J . , K i p l i n g , and E.D. LeCren., 1958. The inverted micro-scope method of estimating a l g a l numbers and the s t a t i s -t i c a l basis of estimates by counting. Hydrobiol., 11: 143-170. Magnuson, J . J . , 1962. An analysis of aggressive behavior, growth, and competition f or food and space i n Medaka (Oryzias l a t i p e s (Pisces, Cyprinodontidae)). Canadian J . Zool., 40 (2): 314-363. Mason, J.C., D.W. Chapman, 1965. Significance of early emer-gence, environmental rearing capacity, and behavioral ecology of juvenile coho salmon i n stream channels, J . Fish Res. Bd. Canada, 22: 173-190. 6 6 M i l l s , D.H., 1969. The s u r v i v a l of j u v e n i l e A t l a n t i c salmon and brown t r o u t i n some S c o t t i s h streams, p. 217-228. In T.G. Northcote, (ed.), Salmon and t r o u t i n streams. H.R. MacMill a n Lectures i n f i s h e r i e s . U n i v e r s i t y of B r i t i s h Columbia. Mundie, J.H., 1969. E c o l o g i c a l i m p l i c a t i o n s of the d i e t of J u v e n i l e coho i n streams, p. 135-152. In T.G. Northcote (ed.), Salmon and t r o u t i n streams. H.R. MacMillan l e c t u r e s i n f i s h e r i e s . U n i v e r s i t y of B r i t i s h Columbia. McFadden, J.T., 1969. Dynamics and r e g u l a t i o n of Salmonid pop u l a t i o n s i n streams, p. 313-329. In T.G. Northcote, (ed.), Salmon and t r o u t i n streams. H.R. MacMill a n l e c t u r e s i n f i s h e r i e s . U n i v e r s i t y of B r i t i s h Columbia. Newman, M.A., 1956. S o c i a l behavior and i n t e r s p e c i f i c compe-t i t i o n i n two t r o u t s p e c i e s . P h y s i o l . Z o o l . 29: 64-81. Noble, G.K., 1939. The experimental animal from the n a t u r a l -i s t ' s p o i n t of view. Amer. N a t u r a l i s t , 73: 113-126. Northcote, T.G., 1962. M i g r a t o r y behavior of j u v e n i l e rainbow t r o u t , Salmo g a i r d n e r i , i n o u t l e t and i n l e t streams of Loon Lake B r i t i s h Columbia. J . F i s h . Res. Bd. Canada, 19 ( 2 ) : 201-270. 1969. Patterns and mechanisms i n the lakeward migra-t o r y behavior of j u v e n i l e t r o u t , p. 183-203. In T.G. Northcote, (ed.), Salmon and t r o u t i n streams. H.R. Mac-M i l l a n l e c t u r e s i n f i s h e r i e s . Univ. of B r i t i s h Columbia. Peterson, G.R., 1966. The r e l a t i o n s h i p of i n v e r t e b r a t e d r i f t abundance t o the standing crop of benthic organisms i n a small stream. M.Sc. t h e s i s , Univ. of B r i t i s h Columbia, 39 p. S t r i n g e r G.E., and W.S. Hoar, 1955. Aggressive behavior of un d e r y e a r l i n g Kamloops t r o u t . Canadian J . Zool, 33: 148-160. Symons, P.E.K., 1968. Increase i n aggression and i n st r e n g t h of the s o c i a l h i e r a r c h y among j u v e n i l e A t l a n t i c salmon deprived of food. J . F i s h . Res. Bd. Canada, 25: 2387-2401. ( i n p r e s s ) . B e h a v i o r a l adjustment of p o p u l a t i o n d e n s i t y t o a v a i l a b l e food by j u v e n i l e A t l a n t i c salmon. 67 Warren, C.E., J.H. Wales, G.E. Davis, and P. Doudoroff., 1964. Trout production i n an experimental stream enriched with sucrose. J . W i l d l i f e Management, 28 (4): 617-660. Wynne-Edwards, V.C, 1962. Animal dispersion i n r e l a t i o n to s o c i a l behavior. Hafner. N.Y. 653 pp. APPENDICES 68 Appendix I . Regression of f r y weight on f r y length (logarithms). Fry seined from Section I I I and IV, summer 1967. Y = -5-5479 + 1-G580X N = 198 2.0 + 2*5 3*0 3.5 4.O 4.5 LOG FRY LENGTH mm Appendix I I . Movement of juveniles through Trap 1 and 2, 1970. Shading Indicates origin of juveniles 15 22 29 5 12 19 26 3 10 17 24 31 7 14 21 28 21 28 4 11 18 25 2 9 16 23 30 6 13 20 27 3 MAY I JUNE ~~1 JULY T A U G . 1. CHIRONOMIDAE LARVAE 2. BABTI3 sp. NYMPHS 3. CHIRONOMIDAE ADULTS 4. CHIRONOMIDAE PUPAE 5. SIMULIDAE LARVAE 9. 6. GAMMARUS l a c u s t r i s 10. 7. HYDROPSYCHE sp LARVAE 11. 8. SIMULIDAE PUPAE 12. SIMULIDAE ADULTS NEMDURIDAE NYMPHS HYDROPTILIDAE LARVAE OTHER COLEOTERA LARVAE 13. ELMTDAE LARVAE 14. MISCELLANEOUS DIPTERA © SAMPLE SIZE Appendix I I I . D r i f t percentage composition - prey type organisms o n l y - Secti o n I I I (upper) and IV ( l o w e r ) , Loon O u t l e t Creek; summer, 1967. From l e f t t o r i g h t : J u l y 20, Aug. 11, Aug. 25, and Sept. 9 • Appendix IV. Regression of prey s i z e (Baeti3 sp) on f i s h s i z e . Slope i s s i g n i f i c a n t a t p = .0001 7 1 BAETIS Y = 1-4374 + N = 273 12.0 + 11.0.. 10.0.. 9.0.. 8 . 0 1 0*0 E5»0 50*0 75*0 100*0 1S5»0 FI3H SIZE ihM) Appendix V. Regression of prey s i z e (Chironomidae) on f i s h s i z e . Slope i s s i g n i f i c a n t at p = .0001 72 CHIRONOvlIDAE Y = 2-4114 + 0< IS.OJ. N M E N = 523 5 0 - 0 75.0 1 0 0 . 0 125-0 FISH SIZE (MM) 7 3 Appendix V I . Regression of prey s i z e ( S i m u l i d a e ) on f i s h s i z e . Slope i s s i g n i f i c a n t a t p = .0001 SIKO_IDAE Y = 2-547G + 0-0122X M l i . O . . 10.0.. 9.0.. 8*0. 7»0. 6.0. 5*0. 4*0. 3.0. S.O. N = 271 j^^Ss X X 0*0 50.0 75.0 100-0 1E5-0 FISH SIZE (MM) 74 Appendix VTI One-way analysis of variance on i n d i v i d u a l f r y weights at three prey l e v e l s , Experiment 2 Source Sum of squares d.f. Mean squares Between 72.6 2 36.3 Within 486.8 59 8.3 Total 559.4 61 - Value of F i s 4.4. - The p r o b a b i l i t y of F being larger than 4.4 i s .017. - Ba r l e t t ' s t e s t indicates that the p r o b a b i l i t y that the variances are homogenous i s .137. 

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