Open Collections

UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Mechanisms of food resource partitioning and the foraging strategies of rainbow trout (Salmo gairdneri)… Hyatt, Kim D. 1980

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Notice for Google Chrome users:
If you are having trouble viewing or searching the PDF with Google Chrome, please download it here instead.

Item Metadata

Download

Media
831-UBC_1980_A1 H93.pdf [ 11.94MB ]
Metadata
JSON: 831-1.0095431.json
JSON-LD: 831-1.0095431-ld.json
RDF/XML (Pretty): 831-1.0095431-rdf.xml
RDF/JSON: 831-1.0095431-rdf.json
Turtle: 831-1.0095431-turtle.txt
N-Triples: 831-1.0095431-rdf-ntriples.txt
Original Record: 831-1.0095431-source.json
Full Text
831-1.0095431-fulltext.txt
Citation
831-1.0095431.ris

Full Text

MECHANISMS OF FOOD RESOURCE PARTITIONING AND THE FORAGING STRATEGIES OF RAINBOW TROUT (Salmo g a i r d n e r i ) AND KOKANEE (Oncorhynchus nerka) IN MARION LAKE, BRITISH COLUMBIA by KIM D. HYATT B. Sc. UNIVERSITY OF WINDSOR, 1971 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of ZOOLOGY We accept t h i s t h e s i s as conforming t o the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA June 1980 0 Kim D. Hyatt In presenting th is thes is in p a r t i a l fu l f i lment o f the requirements for an advanced degree at the Un ivers i ty of B r i t i s h Columbia, I agree that the L ibrary sha l l make it f ree ly ava i l ab le for reference and study. I fur ther agree that permission for extensive copying of th is thes is for scho lar ly purposes may be granted by the Head of my Department or by his representat ives . It is understood that copying or pub l i ca t ion of th is thes is for f inanc ia l gain sha l l not be allowed without my wri t ten permission. Department of The Univers i ty of B r i t i s h Columbia 2075 Wesbrook P l a c e V a n c o u v e r , Canada V6T 1W5 Date 0 J - )H/g* ABSTRACT This study was conducted to satisfy three objectives. The f i rst was to provide a detailed description of the differences between the prey contents of rainbow trout ( Salmo qairdneri ) and kokanee ( Oncorhynchus nerka ) compared either to each other or to the prey contents of the natural environment. The second was to determine how elements of anatomy, physiology and behaviour interact to promote the acquisition of species specific diets by trout and kokanee. The third was to precisely identify the foraging strategies of the two predators by assessing how different anatomical and behavioural characteristics serve as interrelated adaptations that suit each species to effectively use a specific habitat-prey complex. Matched samples of trout and kokanee from Marion Lake exhibit only modest A dietary overlap ( mean of CX= .462, range.136 to .881 ). Although kokanee appear to track the environmental abundance of prey more closely than trout, both predators exhibit pronounced patterns of "density independent" acquisition of prey from the total complex of prey that is apparently available in the lake. To test hypotheses about the factors that control these dietary patterns, I conducted a series of studies concerning where trout and kokanee choose to forage, when they choose to forage, how they search for prey, how they attack prey, and how experience in encountering various prey alters the predator's foraging behaviour. Temporal segregation of trout and kokanee foraging activities is not well-developed under field conditions and appears unlikely to promote strong patterns of food-resource partitioning. By contrast, spatial segregation is well-developed and clearly plays a major role in promoting the acquisition of relatively large numbers of nearshore benthic prey ( eg. planorbid snails or odonates ) by trout and of relatively large numbers of offshore, water-column prey ( eg. chironomid pupae ) by kokanee. Close inspection of the details of predator and prey distributions indicates that many aspects of food-resource partitioning are not logical out-comes of spatial segregation. i i i i i Differences in predator search-techniques do not determine the presence or absence of various prey types in predator diets, however, differences in predator search behaviours do bias them to obtain different quantities of particular classes of prey. Kokanee search procedures allow them to detect prey in both exposed and concealed locations while trout detect only exposed prey. While searching for benthic or lake-surface prey, kokanee maintain search positions which allow them to detect prey of smaller sizes than trout. This clearly favours the trend for kokanee to include greater quantities of small prey ( eg. Hyalella sp. ) in their diets than trout. Differences in search procedures do not explain why kokanee obtain a greater proportion of their diet than trout from small zooplankton ( £ 1 mm ) in late summer or why kokanee seldom exploit any of the relatively large ( > 4 mm ), armoured prey that are common in the diet of similar sized trout. Differences in both behavioural and morphological characteristics involved in the attack phase of foraging by trout and kokanee serve as the basis for explan-ations of a number of differences between the diets of free-ranging predators. These differences include: the greater utilization of aerial prey by trout, the inclusion of large numbers of copepods in the diet of kokanee but not of trout, the generally greater utilization of zooplankton by kokanee compared to trout, and the relative-scarcity of large ( > 4 mm ), armoured prey, in the diet of kokanee. A series of laboratory experiments was used to examine the extent to which short term experience might influence food-resource partitioning by trout and kokanee. These experiments offered convincing evidence that differential effects of experience will amplify the trends in resource partitioning already set in motion by differences in habitat selection, search procedures, and attack procedures. I argue that the morphological and behavioural traits that control food "selection" by trout and kokanee in Marion Lake are a consequence of the evolution of mutually exclusive foraging strategies. Trout are portrayed as D-strategists that concentrate on relatively large, dispersed prey for the bulk of their energy requirements. Adaptations which enable trout to differentially exploit large prey include: procedures for area-extensive search; a predisposition to attack relatively large, armoured-prey; large mouth-size; and persistent responses to opportunities to attack large prey. An inability to sustain high attack rates on small prey ( < 1 mm ) at high density ( 35 per l i ter ) and a tendency to ignore or reject such prey suggest that trout are not well-adapted to exploit relatively small, morphologically-uniform,prey. iv Kokanee are portrayed as C-strategists which concentrate on relatively small, contagiously-distributed prey for the bulk of their energy requirements. Adaptations which enable kokanee to differentially exploit small prey include: procedures for area-intensive search; a predisposition to attack relatively small, morphologically-uniform prey; small mouth-size; well-developed gill-rakers; and an ability to sustain high attack rates on small planktonic prey. Low ingestion success with a variety of large benthic prey and a tendency to ignore or reject such prey under laboratory conditions where they serve as the sole source of food are evidence that kokanee are not well adapted to exploit large, armoured-prey. Adaptations associated with the search, capture, manipulation and ingestion phases of the foraging cycle appear in each instance to be evolutionary responses to specific features of a given habitat-prey complex. C-selected or D-selected foraging strategies appear to be mutually exclusive evolutionary avenues down which trout and kokanee have been directed by the fundamental nature of a given habitat-prey complex. ACKNOWLEDGEMENTS I was i n i t i a l l y a t t r a c t e d t o U.B.C. by the idea s and approach t o ecolo g y o f a group o f b i o l o g i s t s engaged i n s t u d y i n g the Marion Lake ecosystem as p a r t o f Canada's c o n t r i b u t i o n t o the I n t e r n a t i o n a l B i o l o g i c a l Program. Although t h i s group had l a r g e l y d i s s o l v e d by the time rny study was begun, the l e g a c y o f t h e i r e f f o r t s i s c l e a r l y apparent i n the p r e s e n t work. F i n a n c i a l support came from the Canadian I n t e r n a t i o n a l B i o l o g i c a l Program, the N a t i o n a l Research C o u n c i l of Canada and the U n i v e r s i t y o f B r i t i s h Columbia, Department of Zoology. Drs. I.E. E f f o r d , C.J. W a l t e r s , T.G. No r t h c o t e and N.R. L i l e y a d v i s e d me d u r i n g the e a r l y stages o f the r e s e a r c h w h i l e Drs. W a l t e r s , N o r t h c o t e and L i l e y reviewed the t h e s i s and p r o v i d e d h e l p f u l c r i t i c i s m s of e a r l y t h e s i s d r a f t s . Dr. J.D. M c P h a i l g r a c i o u s l y assumed the t a s k of r e s e a r c h s u p e r v i s o r a f t e r the d e p a r t u r e of Dr. E f f o r d from U.B.C. He p r o v i d e d not o n l y a k i n d r e d s p i r i t but a l s o a m i x t u r e o f a d v i c e , c r i t i c i s m , h e a l t h y doses of encouragement and an i n c r e d i b l e amount o f p a t i e n c e t o l e t me f i n d my own way. I p r o f i t e d g r e a t l y from the s t i m u l a t i o n and good cheer of a s u c c e s s i o n o f graduate s t u d e n t s who broadened my understanding o f approaches t o both s c i e n c e and l i v i n g . C o n v e r s a t i o n s and shared e x p e r i e n c e s w i t h Ben Seghers, George C a l e f , Wren Green, Mike S w i f t and R i c k Charnov were e s p e c i a l l y i n f l u e n t i a l . v vi Throughout t h i s study I have r e l i e d on the u n q u e s t i o n i n g support of my p a r e n t s who were i n l a r g e p a r t r e s p o n s i b l e f o r n o u r i s h i n g my e a r l y c u r i o s i t y about the behaviour of a n i m a l s . F i n a l l y t o my w i f e , A n n i c e , my thanks f o r encouragement and understanding w i t h o u t which t h i s study might never have been completed. TABLE OF CONTENTS Page ABSTRACT i i ACKNOWLEDGEMENTS V LIST OF TABLES x i i i LIST OF FIGURES X V i CHAPTER 1 GENERAL INTRODUCTION, GENERAL METHODS AND MATERIALS INTRODUCTION 1 GENERAL METHODS AND MATERIALS 9 CHAPTER 2 THE SPECIES SPECIFIC DIETARY PATTERNS OF TROUT AND KOKANEE IN MARION LAKE INTRODUCTION 17 METHODS 17 RESULTS 19 Comparisons between prey c o n t e n t s o f t r o u t and kokanee 19 Comparisons between prey c o n t e n t s o f p r e d a t o r s and the environment 31 DISCUSSION 39 SUMMARY 44 v i i vi i i Page CHAPTER 3 THE RELATIONSHIP BETWEEN SPATIAL SEGREGATION, TEMPORAL SEGREGATION AND DIETARY PATTERNS OF TROUT AND KOKANEE INTRODUCTION 46 METHODS 48 RESULTS 52 Depth d i s t r i b u t i o n 53 D i s t r i b u t i o n w i t h area 55 D i e l a c t i v i t y p a t t e r n s o f p r e d a t o r s 57 The r e l a t i o n between d i u r n a l a c t i v i t y and food search . 62 A c t i v i t y p a t t e r n s of emerging chironomids 63 DISCUSSION 64 The r o l e of s p a t i a l and temporal s e g r e g a t i o n i n producing d i e t a r y d i f f e r e n c e s between t r o u t and kokanee 64 The r o l e of s p a t i a l and temporal s e g r e g a t i o n i n producing d i f f e r e n c e s between the prey c o n t e n t s of p r e d a t o r s and the environment 69 Unexplained d i e t a r y p a t t e r n s 71 SUMMARY 73 CHAPTER 4 THE RELATIONSHIP BETWEEN FOOD-SEARCH BEHAVIOUR AND DIETARY PATTERNS OF TROUT AND KOKANEE 4-A. FIELD DESCRIPTIONS INTRODUCTION 75 Page METHODS 78 RESULTS 79 Search t e c h n i q u e s 79 Search p o s i t i o n s 90 DISCUSSION The p o t e n t i a l e f f e c t s o f se a r c h t e c h n i q u e s and m i c r o h a b i t a t - s p e c i f i c s e arch on d i e t a r y p a t t e r n s 93 The p o t e n t i a l e f f e c t of se a r c h p o s i t i o n s on d i e t a r y p a t t e r n s 96 4-B. LABORATORY COMPARISONS INTRODUCTION 97 METHODS 99 RESULTS 103 Experiment 4.1 - r e a c t i v e d i s t a n c e and prey d e n s i t y .... 103 Experiment 4.2 - r e a c t i v e d i s t a n c e , t r o u t v e r s u s kokanee 105 DISCUSSION 107 The r o l e o f search p o s i t i o n s i n producing d i e t a r y d i f -f e r e n c e s between t r o u t and kokanee 107 The r o l e o f search t e c h n i q u e s and search p o s i t i o n i n producing d i f f e r e n c e s between the prey c o n t e n t s of p r e d a t o r s and the environment 109 Unexplained d i e t a r y p a t t e r n s I l l SUMMARY 112 X Page CHAPTER 5 THE RELATIONSHIP BETWEEN ATTACK BEHAVIOURS, MORPHOLOGICAL CHARACTERISTICS AND DIETARY PATTERNS OF TROUT AND KOKANEE 5-A. FIELD DESCRIPTIONS INTRODUCTION 115 METHODS 117 RESULTS 119 Techniques o f approach and ca p t u r e 119 Comparisons o f p r e d a t o r morphologies 123 DISCUSSION 127 The c a p t u r e - success h y p o t h e s i s 128 The g i l l - r a k e r , p r e y - s i z e h y p o t h e s i s 129 The a t t a c k - r a t e h y p o t h e s i s 130 The mouth-size, i n g e s t i o n - s u c c e s s h y p o t h e s i s 131 5-B. LABORATORY TESTS OF ATTACK HYPOTHESES METHODS 132 RESULTS 137 Experiment 5.1, The c a p t u r e - s u c c e s s h y p o t h e s i s 137 Experiment 5.2, The a t t a c k - r a t e h y p o t h e s i s 140 Experiment 5.3, The g i l l - r a k e r , p r e y - s i z e h y p o t h e s i s ... 142 Experiment 5.4, The mouth-size, i n g e s t i o n - s u c c e s s h y p o t h e s i s 147 DISCUSSION 150 E x p l a n a t i o n s o f d i e t a r y d i f f e r e n c e s between t r o u t and kokanee 150 x i Page E x p l a n a t i o n s o f d i f f e r e n c e s between the prey c o n t e n t s of the p r e d a t o r s and of the environment 155 SUMMARY 156 CHAPTER 6 THE ROLE OF SHORT TERM EXPERIENCE IN SHAPING THE RESPONSES OF TROUT AND KOKANEE TO PREY INTRODUCTION 158 METHODS 160 RESULTS Experiment 6.1, The e f f e c t s o f e x p e r i e n c e w i t h Chaoborus 165 Experiment 6.2, The e f f e c t s o f e x p e r i e n c e w i t h b e n t h i c prey 171 Experiment 6.3, The h a b i t u a t i o n h y p o t h e s i s 182 Experiment 6.4, The success and s t r e n g t h - o f - r e s p o n s e h y p o t h e s i s 186 DISCUSSION 191 The range of responses by t r o u t and kokanee to prey ... 191 Spec i e s s p e c i f i c e f f e c t s o f e x p e r i e n c e w i t h prey 196 Consequences f o r p a t t e r n s o f prey a c q u i s i t i o n 198 SUMMARY 200 CHAPTER 7 TROUT AND KOKANEE FORAGING, THE STRATEGIC POINT OF VIEW INTRODUCTION 202 The environmental c o n t e x t f o r t r o u t and kokanee f o r a g i n g s t r a t e g i e s 208 The r o l e of prey s i z e , r e l a t i v e abundance, and p h y s i c a l f e a t u r e s o f the environment i n promoting C - s e l e c t e d o r D-s e l e c t e d f o r a g i n g s t r a t e g i e s 217 The r o l e of prey d i s t r i b u t i o n i n shaping search and a t t a c k components o f t r o u t and kokanee f o r a g i n g s t r a t e g i e s 220 The r o l e of temporal p a t t e r n s o f prey renewal i n shaping search components of t r o u t and kokanee f o r a g i n g s t r a t e g i e s 229 The r o l e o f prey s i z e and abundance i n shaping m o r p h o l o g i c a l and b e h a v i o u r a l elements t h a t f u n c t i o n d u r i n g the a t t a c k phase o f C - s e l e c t e d o r D - s e l e c t e d f o r a g i n g s t r a t e g i e s 232 SUMMARY 242 REFERENCES 244 LIST OF TABLES Food o v e r l a p c a l c u l a t i o n s f o r t r o u t and kokanee from Marion Lake. Seasonal e x p l o i t a t i o n o f chironomid pupae by t r o u t and kokanee. Taxonomic c o m p o s i t i o n o f food items from p r e d a t o r s used i n f o o d - s i z e d e t e r m i n a t i o n s . The r e l a t i v e l e v e l of e x p l o i t a t i o n o f H y a l e l l a  a z t e c a and Crangonyx richmondensis by t r o u t and kokanee. Taxonomic c o m p o s i t i o n of prey eaten by s m a l l t r o u t and kokanee c o l l e c t e d September 7, 1974. 2 Mean number of amphipods per m v e r s u s the mean number per p r e d a t o r i n d i f f e r e n t months o f the y e a r . D e p t h - d i s t r i b u t i o n s of t r o u t and kokanee under e a r l y summer and l a t e summer c o n d i t i o n s i n Marion Lake. N o r t h - t r a p v e r s u s s o u t h - t r a p c a p t u r e s of t r o u t and kokanee 1974-75. A comparison of the r e l a t i v e abundance i n e a r l y summer ( A p r i l - J u n e ) o f prey a t i n s h o r e (depths £2m) and o f f s h o r e (depths ~>3m) l o c a t i o n s i n Marion Lake. A comparison of the swimming v e l o c i t i e s o f t r o u t and kokanee a c t i v e l y s e a r c h i n g f o r prey i n a v a r i e t y o f s u b - h a b i t a t s . Residence time and the t o t a l number of a t t a c k s i n i t i a t e d by f i v e t r o u t u s i n g the hover and se a r c h t e c h n i q u e . A summary of the d i f f e r e n c e s i n search t e c h n i q u e s employed by t r o u t and kokanee f o r a g i n g i n the f i e l d . R e a c t i v e d i s t a n c e o f t r o u t t o t a r g e t s o f known s i z e a t the l a k e s u r f a c e on two o c c a s i o n s . x i i i XIV Page 14. The seasonal e x p l o i t a t i o n of chironomid l a r v a e 94: by t r o u t and kokanee. 15. D i s t r i b u t i o n s o f r e a c t i v e d i s t a n c e o f kokanee to 1 0 3 s t a t i o n a r y ephemeropteran (mayfly) nymphs i n low - d e n s i t y and h i g h - d e n s i t y experiments. 16. Head c a p s u l e w i d t h s ( i n mm) o f l a r v a l 108 t r i c h o p t e r a n s consumed by t r o u t and kokanee. 17. Techniques o f approach and c a p t u r e used by t r o u t 119 and kokanee d u r i n g a t t a c k s on prey i n the f i e l d . 18. A l i s t of prey types observed t o evoke s p e c i f i c 124 approach and c a p t u r e t e c h n i q u e s i n the l a b o r a t o r y and i n the f i e l d . 19. C h a r a c t e r i s t i c s o f p r e d a t o r s and prey used i n 134 experiments to determine the a t t a c k success o f t r o u t and kokanee on a v a r i e t y o f p r e y . 20. A comparison of c a p t u r e success and i n g e s t i o n 138 success o f " s i z e - m a t c h e d " t r o u t and kokanee i n a t t a c k s on s m a l l , a g i l e p r e y . 21. S p e c i e s c o m p o s i t i o n of s m a l l z o o p l a n k t o n used i n 144 experiment 5.3 w i t h t r o u t and kokanee. 22. Means and 95% confidence l i m i t s o f the maximum 145 a t t a c k r a t e s per minute achieved by t r o u t and kokanee e x p l o i t i n g s m a l l z o o p l a n k t o n a t v a r i o u s d e n s i t i e s . 23. Means and 95% c o n f i d e n c e l i m i t s o f the t o t a l 145 a t t a c k s per .5 hours completed by t r o u t and kokanee e x p l o i t i n g s m a l l zooplankton a t v a r i o u s d e n s i t i e s . 24. A comparison of c a p t u r e success and i n g e s t i o n 148 success o f "size-matched" t r o u t and kokanee i n , a t t a c k s on d i f f e r e n t s p e c i e s o f i n v e r t e b r a t e p r e y . 25. The e f f e c t o f e x p e r i e n c e w i t h prey i n s u c c e s s i v e 166 f e e d i n g t r i a l s on a t t a c k success o f t r o u t and kokanee 26. A comparison of the time t o f i r s t a t t a c k and of t o t a l a t t a c k s by e x p e r i e n c e d and n a i v e p r e d a t o r s on chaoborus l a r v a e . 170 C h a r a c t e r i s t i c s o f prey used to p r e d i c t the expected o r d e r o f times to f i r s t a t t a c k . C h a r a c t e r i s t i c s of prey s p e c i e s used and the number of p r e d a t o r s i n v o l v e d i n t r i a l s from which TFA v a l u e s have been d e r i v e d . A summary o f the elements which d e f i n e the f o r a g i n g s t r a t e g i e s o f t r o u t and kokanee. A summary o f the a d a p t i v e complex t h a t has e v o l v e d w i t h r e s p e c t t o f o r a g i n g by t r o u t and kokanee. LIST OF FIGURES F i g u r e Page 1. Study o r g a n i z a t i o n 8 2. A comparison of the r e l a t i v e p r o p o r t i o n s o f 20 v a r i o u s prey types i n the d i e t s of t r o u t and kokanee from Marion Lake, B.C. 3. A comparison of the p r o p o r t i o n s o f t r o u t 24 and kokanee t h a t c o n t a i n d i f f e r e n t q u a n t i t i e s o f chironomid pupae i n t h e i r d i e t s . 4. A comparison of the s i z e - f r e q u e n c y d i s t r i b u t i o n 26 of a l l prey eaten by size-matched t r o u t and kokanee. 5. A comparison of the s i z e - f r e q u e n c y d i s t r i b u t i o n s 29 o f amphipods (Crangonyx sp. and H y a l e l l a sp) eaten by size-matched t r o u t and kokanee. 6. A comparison of the s i z e - f r e q u e n c y d i s t r i b u t i o n s 30 of m o l l u s c s ( P i s i d i u m spp; p l a n o r b i d s n a i l s , Helisoma spp and Menetus sp.) eaten by size-matched t r o u t and kokanee. 7. A comparison of the s i z e - f r e q u e n c y d i s t r i b u t i o n s 33 of water column prey e x p l o i t e d by size-matched t r o u t and kokanee. 8. A comparison, on an annual b a s i s , o f the r e l a t i v e 35 p r o p o r t i o n s o f prey i n the environment and i n the d i e t o f t r o u t from Marion Lake, B.C. 9. A comparison, on an annual b a s i s , o f the r e l a t i v e 36 p r o p o r t i o n s o f prey i n the environment and i n the d i e t o f kokanee from Marion Lake, B.C. 10. A comparison of the s i z e - f r e q u e n c y d i s t r i b u t i o n s 38 of amphipods i n the environment and i n the d i e t s of the p r e d a t o r s . 11. A contour map.of Marion Lake i n d i c a t i n g the 49 l o c a t i o n s o f both t r a p and o b s e r v a t i o n s i t e s . 12. A comparison of the r e l a t i v e p r o p o r t i o n s o f t r o u t 56 and kokanee observed a t onshore and o f f s h o r e l o c a t i o n s i n Marion Lake. xvi xv i i Page 13. A comparison of the r e l a t i v e p r o p o r t i o n s o f 59 t r o u t and kokanee n e t t e d a t two hour i n t e r v a l s , o v e r twenty-four hour p e r i o d s a t v a r i o u s times of the y e a r . 14. A comparison of the r e l a t i v e numbers o f t r o u t and 61 kokanee observed a t two hour i n t e r v a l s , over s i x t e e n -hour p e r i o d s a t v a r i o u s times of the y e a r . 15. The mean number o f chironomid pupae o b t a i n e d i n 65 s u r f a c e n e t - h a u l s taken between e a r l y a f t e r n o o n and l a t e evening on t h r e e s e p a r a t e d a t e s . 16. The r e l a t i v e p r o p o r t i o n s o f t r o u t and kokanee 91 t h a t m a i n t a i n s p e c i f i c s e a r c h p o s i t i o n s w h i l e v i s u a l l y scanning the bottom sediments f o r p r e y . 17. The r e l a t i v e p r o p o r t i o n s o f t r o u t and kokanee 92 t h a t m a i n t a i n s p e c i f i c s e a r c h p o s i t i o n s w h i l e v i s u a l l y scanning the l a k e s u r f a c e f o r p r e y . 18. The minimum prey s i z e r e q u i r e d t o e l i c i t an 98 a t t a c k by t r o u t i n se a r c h p o s i t i o n s a t v a r i o u s d i s t a n c e s from the s u b s t r a t e s on which prey are l o c a t e d . 19. Frequency d i s t r i b u t i o n s o f r e a c t i v e d i s t a n c e s 104 of kokanee responding t o e i t h e r s t a t i o n a r y o r moving mayfly nymphs. 20. A comparison of the frequency d i s t r i b u t i o n s f o r 106 r e a c t i v e d i s t a n c e o f t r o u t and kokanee t o l a r g e and s m a l l prey t y p e s . 21. The r e l a t i o n s h i p between f o r k l e n g t h and wet weight 125 of t r o u t and kokanee from Marion Lake. ^22. The r e l a t i o n s h i p between standard l e n g t h and jaw 126 gape of t r o u t and kokanee from Marion Lake. 23. The r e l a t i o n s h i p between the d e n s i t y o f 141 s m a l l z o o p l a n k t o n and the maximum a t t a c k r a t e s o f t r o u t and kokanee. 24. The s i z e - f r e q u e n c y d i s t r i b u t i o n of prey 143 used f o r the s m a l l - p l a n k t o n , f e e d i n g - t r i a l s w i t h t r o u t and kokanee. 25. The r e l a t i o n s h i p between c o n s e c u t i v e f e e d i n g 167 t r i a l s w i t h chaoborus l a r v a e as prey and the t o t a l number of a t t a c k s t h a t t r o u t and kokanee i n i t i a t e i n a g i v e n t r i a l . The r e l a t i o n s h i p between c o n s e c u t i v e f e e d i n g t r i a l s w i t h chaoborus l a r v a e as prey and the time t o i n i t i a t e the f i r s t a t t a c k by t r o u t o r kokanee i n a g i v e n t r i a l . The e f f e c t s o f changes i n e x p e r i e n c e and prey d e n s i t y on the t o t a l number of a t t a c k s t h a t kokanee i n i t i a t e on p a r t i c u l a r prey types i n a g i v e n t r i a l . The r e l a t i o n s h i p between the l e v e l of a t t a c k success t h a t i n d i v i d u a l kokanee have w i t h s p e c i f i c prey types and the t o t a l number of a t t a c k s t h a t i n d i v i d u a l kokanee i n i t i a t e on these prey types over f o u r c o n s e c u t i v e t r i a l s . The e f f e c t s o f changes i n e x p e r i e n c e and prey d e n s i t y on the time t h a t i n d i v i d u a l kokanee take to i n i t i a t e t h e i r f i r s t a t t a c k on s p e c i f i c prey types i n a g i v e n t r i a l . The r e l a t i o n s h i p between % a t t a c k success o f i n d i v i d u a l t r o u t and kokanee and t h e i r v a r i a b i l i t y o f response as i n d i c a t e d by the standard d e v i a t i o n o f the time t o f i r s t a t t a c k over the l a s t t h r e e t r i a l s o f ex-p e r i e n c e w i t h s p e c i f i c prey t y p e s . The r e l a t i o n s h i p between c o n s e c u t i v e t r i a l s o f exposure to a c o n s t a n t d e n s i t y o f no-t o n e c t i d s and the t o t a l number of a t t a c k s t h a t t r o u t and kokanee i n i t i a t e i n a g i v e n .5 hour t r i a l . The r e l a t i o n s h i p between c o n s e c u t i v e t r i a l s o f exposure to n o t o n e c t i d s and the time t o f i r s t a t t a c k o f i n d i v i d u a l p r e d a t o r s i n a g i v e n t r i a l . The r e l a t i o n s h i p between c o n s e c u t i v e t r i a l s o f exposure t o a c o n s t a n t d e n s i t y o f Diaptomus k e n a i and the t o t a l number of a t t a c k s t h a t t r o u t and kokanee i n i t i a t e i n a g i v e n t r i a l . The r e l a t i o n s h i p between c o n s e c u t i v e t r i a l s o f exposure to Diaptomus k e n a i and the time t o f i r s t a t t a c k o f t r o u t and kokanee i n a g i v e n time. The e f f e c t o f a p r e d a t o r ' s v e r t i c a l d i s t a n c e from the l a k e s u r f a c e on the diameter of the c i r c u l a r area w i t h i n which s u r f a c e prey may be d e t e c t e d . T h e e f f e c t o f a p r e d a t o r s s e a r c h p o s i t i o n o n t h e s i z e o f s e l e c t e d p o r t i o n s o f t h e i n s t a n t a n e o u s f i e l d o f s e a r c h . An o u t l i n e o f t h e i n t e r a c t i o n s b e t w e e n t h e b a s i c s t r u c t u r e o f t h e h a b i t a t a n d t h e s t r a -t e g i e s w h i c h o r g a n i s m s e v o l v e i n r e s p o n s e t o f o o d , p r e d a t o r s and c o m p e t i t o r s . 1 CHAPTER 1 GENERAL INTRODUCTION, GENERAL METHODS AND MATERIALS B i o l o g i s t s have long been i n t r i g u e d by d i f f e r e n c e s i n morphology and h a b i t among c l o s e l y r e l a t e d s p e c i e s , f o r to comprehend the manner and e x t e n t o f such d i f f e r e n c e s i s to comprehend much of the n a t u r a l c o n t r o l o f o r g a n i c d i v e r s i t y . T.W. Schoener, 1974. INTRODUCTION A l l organisms are faced w i t h the common c h a l l e n g e o f a c q u i r i n g and p r o c e s s i n g n u t r i e n t s and energy i n the form o f a l r e a d y s y n t h e s i z e d h i g h energy compounds o r e l s e the raw m a t e r i a l s from which they and new protoplasm can be s y n t h e s i s e d . T h i s r e s o u r c e a c q u i s i t i o n i s never a random p r o c e s s and a g r e a t d e a l of r e s e a r c h i n the b i o l o g i c a l s c i e n c e s may be i n t e r p r e t e d as a se a r c h f o r g e n e r a l r u l e s g o v e r n i n g n u t r i e n t and energy a c q u i s i t i o n a t l e v e l s of o r g a n i z a t i o n from the c e l l t o the ecosystem. E c o l o g i s t s u s u a l l y work a t o r above the l e v e l o f the whole organism and are concerned w i t h the study o f " i n t e r -a c t i o n s t h a t determine the d i s t r i b u t i o n and abundance o f organisms" (Krebs, 1972). A l a r g e number of these i n t e r a c t i o n s f a l l i n t o the c a t e g o r y o f n u t r i e n t and energy a c q u i s i t i o n . Thus, e c o l o g i s t s too are i n t e r e s t e d i n i d e n t i f y i n g p a t t e r n s o f n u t r i e n t and energy a c q u i s i t i o n as w e l l as i n e x p l a i n i n g . the nature of the b i o l o g i c a l i n t e r a c t i o n s t h a t a re r e s p o n s i b l e for producing the patterns (for example , see discussion in Paine, 1969 or Schoener, 1971, 1974). -Most vertebrates and many, invertebrate predators are euryphagic (see De Ruiter, 1967 and Emlen, 1973 for reviews that i s , they eat mixed diets containing a number of di f f e r e n t types of food organisms. In their search for patterns of nutrient or energy acquisition by animals, ecologists have conducted a great many studies either comparing the dietary habits of closely related species of predators which occur sympatrically or comparing the abundance of food types that are apparently available in the habitat of a predator to the abundance of food types present in the predator's d i e t . These studies have shown repeatedly that (1.) closely related species of predators acquire different sets of food items, even in instances where great care has been taken to obtain the pre-dators from the same habitats where they potentially have had access to an identical set of foods (Root, 1967;Pulliam and Enders, 1971; Schoener, 1974; Paine, 1963; Brown and Lieberman, 1973; Gwynne and B e l l , 1968; Heinrich, 1976; Tyler, 1972; Keast and Webb, 1966) and (2.) each species of predator exhibits what appears to be density independent exploitation of foods from the total complex of foods that i s apparently available in a given environment (Root, 1967; Hespenheide, 1975; Gerking, 1962; Levings, 1972; Costa and Cummins, 1972; Efford and Tsumura, 1973; Moore and Moore, 1976). 3 The types o f p r e d a t o r s examined i n some o f the b e s t of these s t u d i e s i n c l u d e b i r d s ( T i n b e r g e n , 1960; Root, 1967; O r i a n s and Horn, 1969; H a r t w i c k , 1973; Hespenheide, 1975)/ f i s h (see H y a t t , 1979 f o r r e v i e w ) , mammals ( E s t e s and Goddard, 1967; Gwynne and B e l l , 1968; S c h a l l e r , 1972; Kruuk, 1972; Brown and Liebermann, 1973; Reichmani 1977), and i n -v e r t e b r a t e s ( P a i n e , 1963; Menge, 1972; Fedorenko, 1975). Having demonstrated d i f f e r e n c e s i n the prey c o n t e n t s o f p r e d a t o r s compared e i t h e r t o each o t h e r o r t o the n a t u r a l environment, an e c o l o g i s t i s faced w i t h the problem o f p r o -v i d i n g answers to q u e s t i o n s about why food r e s o u r c e s a re a c q u i r e d i n the way they a r e . H i s t o r i c a l l y e c o l o g i s t s have s p l i t i n t o groups which have used e i t h e r a m e c h a n i s t i c o r an e v o l u t i o n a r y approach t o p r o v i d e answers f o r such q u e s t i o n s ( B i r c h and E h r l i c h , 1966; H o l l i n g , 1968). The m e c h a n i s t i c o r r e d u c t i o n i s t approach (Emlen, 1973; H o r r i d g e , 1977) i n v o l v e s a s e a r c h f o r e x p l a n a t i o n s o f d i e t a r y p a t t e r n s i n terms of the o p e r a t i o n o f p r o x i m a l mech-anisms. The work o f both H o l l i n g (1964; 1966, 1968) and Ware (1971, 1973) e x e m p l i f i e s t h i s approach. To these authors e x p l a n a t i o n s f o r s p e c i f i c d i e t a r y p a t t e r n s l i e i n an a n a l y s i s o f how b e h a v i o u r a l , a n a t o m i c a l o r p h y s i o l o g i c a l c h a r a c t e r i s t i c s o f p r e d a t o r s i n f l u e n c e the p r o b a b i l i t y t h a t they w i l l a c q u i r e some food items more o f t e n than o t h e r s . For example, by a n a l y s i n g the f o r c e s t h a t o p e r a t e i n a mantid's forearm, H o l l i n g (1964) p r e d i c t e d the s i z e of prey t h a t c o u l d be grasped 4 best. He then showed in behavioural experiments that the largest number of feeding responses were displayed to prey of just this size. Similarly Ware (1973) explored several visual "mechanisms" that operate during search and detection of prey by rainbow trout in the laboratory and then used these results to explain a number of patterns of prey intake ex-hibited by trout that had been foraging for benthic inverte-brates in the f i e l d . Studies in which a mechanistic approach has been used have often avoided any attempt to provide insights into the nature of the evolutionary forces that have acted to shape the overall "design" of an organism. Accordingly a second group of ecologists has attempted to resolve questions about patterns of food acquisition by employing an evolutionary or "strategic" approach (Schoener, 1971). The work of Schoener (1965, 1971), MacArthur & Pianka (1966), Emlen (1966), MacArthur and Levins (1967), Root (1967), Royama (1970), MacArthur (1972), Hespenheide (1975), Diamond (1975), and Pianka (1975), to c i t e but a few, characterizes this approach to studies of food resource d i v i s i o n by predators. To these authors explanations of sp e c i f i c dietary patterns l i e in an analysis of how agents of natural selection, operating within a p a r t i -cular environmental context, have acted to shape the foraging strategies of predators as adaptive (or optimal) solutions to problems of energy arid nutrient acquisition. 5 Although m e c h a n i s t i c o r s t r a t e g i c approaches t o answer q u e s t i o n s about the s i g n i f i c a n c e of a g i v e n d i e t a r y p a t t e r n have g e n e r a l l y been used i n a m u t u a l l y e x c l u s i v e f a s h i o n , they need not be s i n c e s t u d i e s o f the p r o x i m a l mechanisms which promote r e s o u r c e d i v i s i o n w i l l o f t e n p r o v i d e i n f o r m a t i o n c o n t a i n i n g a v a r i e t y o f c l u e s t h a t may be used i n the s e a r c h f o r answers t o e v o l u t i o n a r y q u e s t i o n s d e a l i n g w i t h why r e -sources are c u r r e n t l y p a r t i t i o n e d by p r e d a t o r s i n the way t h a t they a r e . Indeed t h e . r e c e n t a p p l i c a t i o n of a combination o f the two approaches i n s t u d i e s o f " o p t i m a l f o r a g i n g " by p r e d a t o r s has begun t o r e v e a l how important the s e l e c t i v e p r e s s u r e to maximize the net energy r e t u r n from f o r a g i n g has been i n shaping the behaviour o f a v a r i e t y o f animals (see examples c i t e d i n Pyke, P u l l i a m and Charnov, 1977: Krebs, E r i c h s e n and Webber, 1977; G o s s - C u s t a r d , 1977 or Zach, 1979). In the p r e s e n t study I have used both m e c h a n i s t i c and s t r a t e g i c approaches i n a t t e m p t i n g t o p r o v i d e e x p l a n a t i o n s f o r the s p e c i e s s p e c i f i c d i e t s o f two f i s h e s i n h a b i t i n g a s m a l l f r e s h -water ecosystem. The s t i m u l u s f o r the i n i t i a t i o n of t h i s study o r i g i n a t e d from o b s e r v a t i o n s by E f f o r d and Tsumura (1973) t h a t two s i m i l a r s p e c i e s o f f i s h (rainbow t r o u t , Salmo g a i r d n e r i ; kokanee, Oncorhynchus nerka) i n h a b i t i n g a s m a l l , s u b a l p i n e l a k e i n B r i t i s h Columbia d i s p l a y e d s u b s t a n t i a l d i f f e r e n c e s i n the p r o p o r t i o n s o f v a r i o u s prey types found i n t h e i r d i e t s when compared e i t h e r t o each o t h e r o r when compared t o the 6 r e l a t i v e abundance of prey a p p a r e n t l y a v a i l a b l e i n the l a k e environment. Although d i f f e r e n c e s i n the prey c o n t e n t s of c l o s e l y r e l a t e d p r e d a t o r s o r o f p r e d a t o r s and the n a t u r a l environment have been d e s c r i b e d i n numerous f i e l d s t u d i e s (see r e f e r e n c e s c i t e d above), i t i s o f t e n not c l e a r what i t i s p r e c i s e l y t h a t f a v o u r s - t h e a c q u i s i t i o n o f a p a r t i c u l a r s e t o f prey items by a g i v e n s p e c i e s o f p r e d a t o r . T h i s i s c e r t a i n l y t r u e f o r the d i e t a r y p a t t e r n s e x h i b i t e d by the t r o u t and kokanee s t u d i e d by E f f o r d and Tsumura, thus one of- the major g o a l s o f the p r e s e n t study i s to determine how elements o f anatomy, p h y s i o l o g y and behaviour i n t e r a c t as p r o x i m a l mechanisms which favour the a c q u i s i t i o n o f s p e c i e s s p e c i f i c d i e t s by t r o u t and kokanee i n the f i e l d . Both H o l l i n g (1966) and De R u i t e r (1967) have emphasized t h a t the p r e d a t i o n p r o c e s s which generates d i e t a r y p a t t e r n s has a l i m i t e d number of s i m p l e b e h a v i o u r a l components t h a t serve as the common denominators l i n k i n g a l l animals i n t h e i r quest f o r f o o d . Thus, the b e h a v i o u r a l c h a i n t h a t l e a d s from s e a r c h and d e t e c t i o n t o f i n a l i n g e s t i o n o r r e j e c t i o n o f food items p r o v i d e s a c o n v e n i e n t framework around which t o o r g a n i z e s t u d i e s aimed a t r e v e a l i n g the o r i g i n s o f d i v e r s e . d i e t a r y p a t t e r n s . However, few s t u d i e s have attempted t o e x p l o r e how m o r p h o l o g i c a l , b e h a v i o u r a l or p h y s i o l o g i c a l c h a r a c -t e r i s t i c s o p e r a t e to shape d i e t a r y p a t t e r n s over the e n t i r e sequence of f o r a g i n g events f o r s i n g l e s p e c i e s o f p r e d a t o r s . Rather the g e n e r a l t r e n d has been to attempt e x p l a n a t i o n s 7 o f d i e t a r y p a t t e r n s o f p r e d a t o r s from the f i e l d as a consequence o f b i o l o g i c a l mechanisms t h a t operate d u r i n g o n l y a s i n g l e phase o f the f o r a g i n g c y c l e (eg. d u r i n g food h a n d l i n g , Rear, 1962; Werner, 1974; H e i n r i c h , 1976; d u r i n g prey d e t e c t i o n , Murton, 1971;. Ware, 1973; Z a r e t , 1972; Z a r e t & K e r f o o t , 1975). T h i s trend i s u n f o r t u n a t e s i n c e the m o r p h o l o g i c a l , b e h a v i o u r a l and p h y s i o l o g i c a l c h a r a c t e r i s t i c s t h a t i n t e r a c t a t each stage o f the f o r a g i n g c y c l e are commonly a s s e r t e d t o form p a r t o f an a d a p t i v e complex t h a t d e f i n e s the f o r a g i n g s t r a t e g y o f a g i v e n p r e d a t o r ( K l o p f e r , 1973) y e t t h e r e are few e x p e r i m e n t a l s t u d i e s which examine how i n t e r r e l a t e d s e t s o f a d a p t a t i o n s i n f l u e n c e the range of prey t h a t p r e d a t o r s can e f f e c t i v e l y e x p l o i t . Indeed, t h e r e are few s t u d i e s a v a i l a b l e which d e f i n e the f o r a g i n g s t r a t e g i e s o f p r e d a t o r s more p r e c i s e l y than t o l a b e l them as e i t h e r energy maximizers or time m i n i m i z e r s (Schoener, 1971). Consequently, the second major g o a l of t h i s study i s to p r o v i d e a s y s t e m a t i c assessment of the i n t e r -r e l a t e d a d a p t a t i o n s t h a t d e f i n e the f o r a g i n g s t r a t e g i e s o f t r o u t and kokanee. To do t h i s r e q u i r e s an assessment of the n a t u r e of the "match" between the p r e d a t o r ' s s t r u c t u r e s and b e h a v i o u r s on the one hand and t h e i r chosen h a b i t a t and prey combinations on the o t h e r . The t h e s i s i s o r g a n i z e d i n t o seven c h a p t e r s ( F i g . 1 ) . The remainder of t h i s c h a p t e r d e a l s w i t h g e n e r a l methods and m a t e r i a l s used throughout the s t u d y . In c h a p t e r two I p r o v i d e d e t a i l e d comparisons o f the d i e t a r y p a t t e r n s e x h i b i t e d STUDY ORGANIZATION CH. 2 DIETARY PATTERNS DESCRIPTIONS CH. 3 Y THE ROLE OF SPATIAL OR TEMPORAL SEGREGATION CH. 4 1 THE ROLE OF PREDATOR SEARCH BEHAVIOUR CH. 5 THE ROLE OF ATTACK BEHAVIOUR AND PREDATOR MORPHOLOGY CH. 6 CH.7 Y THE ADAPTIVE NATURE OF TROUT AND KOKANEE FORAGING STRATEGIES A THE ROLE OF SHORT TERM EXPERIENCE 9 by f r e e r a n g i n g t r o u t and kokanee i n the l a k e environment. In c h a p t e r s t h r e e through s i x I use a s t r i c t l y m e c h a n i s t i c approach to t e s t hypotheses about how p r o x i m a l f a c t o r s shape the d i e t a r y p a t t e r n s t h a t t r o u t and kokanee e x h i b i t under n a t u r a l c o n d i t i o n s . T h i s s e r i e s o f c h a p t e r s i s o r g a n i z e d a l o n g the same l i n e s as the s e r i e s o f " d e c i s i o n s " each p r e d a t o r must make w h i l e f o r a g i n g . Thus, c h a p t e r t h r e e d e a l s w i t h where and when the predators, choose to f o r a g e , c h a p t e r f o u r d e a l s w i t h how the p r e d a t o r s s e a r c h f o r p r e y , c h a p t e r f i v e d e a l s w i t h i n t e r a c t i o n s between m o r p h o l o g i c a l and b e h a v i o u r a l c h a r a c t e r i s t i c s d u r i n g the a t t a c k phase o f f o r a g i n g and ch a p t e r s i x d e a l s w i t h the r o l e o f s h o r t term e x p e r i e n c e w i t h prey i n a l t e r i n g p r e d a t o r responses t o p r e y . F i n a l l y i n c h a p t e r seven, I use a s t r a t e g i c approach to r e a s s e s s r e s u l t s from the p r e c e d i n g c h a p t e r s i n an attempt t o p r e c i s e l y d e f i n e the f o r a g i n g s t r a t e g i e s o f t r o u t and kokanee i n terms of i n t e r -r e l a t e d s e t s o f m o r p h o l o g i c a l and b e h a v i o u r a l a d a p t a t i o n s addressed t o s p e c i f i c h a b i t a t - p r e y c o m b i n a t i o n s . GENERAL METHODS AND MATERIALS The Study Area P o p u l a t i o n s of rainbow t r o u t (Salmo g a i r d n e r i ) and kokanee (Oncorhynchus nerka) i n Marion Lake, B r i t i s h Columbia formed the focus f o r the p r e s e n t s t u d y . Marion Lake ( a l s o known as Jacob's Lake) i s s i t u a t e d a p p r o x i m a t e l y 50 k i l o m e t e r s e a s t of Vancouver i n a r e s e a r c h f o r e s t operated by the U n i v e r s i t y o f B r i t i s h Columbia ( l a t i t u d e 49° 19° N, l o n g i t u d e 122° 33' W). For ten y e a r s (1964-1974) the Marion Lake ecosystem was the focus o f i n t e n s i v e study sponsored by the N a t i o n a l Research C o u n c i l of Canada, the U n i v e r s i t y of B.C. and by the Canadian s e c t i o n o f the I n t e r n a t i o n a l B i o l o g i c a l Program. E f f o r d (1972) reviewed the g e n e r a l o b j e c t i v e s of the program and a number of authors ( E f f o r d , 1972; H a l l & H y a t t , 1974) have reviewed the p r o g r e s s o f r e s e a r c h d i r e c t e d towards these o b j e c t i v e s . The b a s i n o f Marion Lake i s 800 m long and about 200 m wide a t i t s maximum. The major i n l e t stream e n t e r s from the n o r t h . During the d r y summer season, when the p r e s e n t study was conducted, the l a k e has a maximum depth t h a t v a r i e s from 5 - 6 m, a mean depth of 2 - 2.5 m and i s a p p r o x i m a t e l y 10 h e c t a r e s i n a r e a . The l a k e margin along the e a s t e r n shore i s i l l - d e f i n e d and c o n s i s t s of a boggy zone i n which the water l e v e l v a r i e s c o n s i d e r a b l y . V e g e t a t i o n i n t h i s zone i s dominated by sedges, the s m a l l shrub M y r i c a g a l e , and o t h e r shrubs and s m a l l t r e e s such as salmon b e r r y (Rubus s p e c t a b i l i s ) and a l d e r (Alnus r u b r a ) . The western s h o r e l i n e has a s t e e p e r s l o p e and i s covered by stands of red cedar (Thuja p l i c t a ) , western hemlock (Tsugo h e t e r o p h y l l a ) and w i l l o w ( S a l i x spp.) to. the waters edge. There are o n l y f o u r types of roo t e d a q u a t i c p l a n t s t h a t are abundant. These cover 22% o f the l a k e area ( D a v i e s , 1970; N e i s h , 1971) and i n c l u d e Potomageton natans, P_. e d i h y d r u s , I s o e t e s o c c i d e n t a l i s and Nuphar p o l y s e p h a l a . Dense beds of Chara g l o b u l a r i s are found i n a s s o c i a t i o n w i t h a l a r g e s p r i n g a t the south end of the l a k e . 11 I s o e t e s o c c i d e n t a l i s o c c u r s i n water deeper than 2 m but 80% of the macrophytes occur i n water l e s s than 2 m deep. Appr o x i m a t e l y 78% o f the l a k e bottom c o n s i s t s o f open mud (a deep f l o c c u l e n t ooze known as g y t t j a ) . The s u r f a c e o f s h a l l o w water sediments (depths l e s s than 2 m) i s covered by l i t t e r composed of l e a v e s , n e e d l e s , t w i g s and branches d e r i v e d from weed beds and emergent v e g e t a t i o n around the l a k e . There i s a g r a d u a l s h i f t to much f i n e r sediments covered by l e s s l i t t e r i n the deeper areas of the l a k e . During the summer a sparse cover o f f i l a m e n t o u s a l g a e and diatoms o c c u r s on the s u r f a c e of open mud areas i n water deeper than 1 meter. At depths o f l e s s than 1 meter t h e r e i s o f t e n a t h i c k mat of b e n t h i c a l g a e composed of s i n g l e c e l l e d and f i l m e n t o u s forms. Marion Lake c o n t a i n s f i v e s p e c i e s o f v e r t e b r a t e p r e d a t o r s t h a t are r e s i d e n t . These i n c l u d e t h r e e f i s h e s (rainbow t r o u t , S_. g a i r d n e r i ; kokanee, O. nerka; t h r e e s p i n e s t i c k l e b a c k , G a s t e r o s t e u s a c u l e a t u s ) and two salamanders ( P a c i f i c c o a s t newt, T a r i c h a g r a n u l o s a ; the neotonous form of the Northwestern salamander, Ambystoma g r a c i l e ) . In a d d i t i o n to the p r e s e n t d i s s e r t a t i o n , s t u d i e s o f the p r e d a t o r s t h a t have been completed o r are i n p r o g r e s s i n c l u d e ; p o p u l a t i o n dynamics of the s a l m o n i d s , salamanders and s t i c k l e b a c k s (Sandercock, 1969; N e i s h , 1970; M c P h a i l , i n p r o g r e s s ) ; p r e d a t o r y behaviour of rainbow t r o u t , salamanders and s t i c k l e b a c k s under l a b o r a t o r y c o n d i t i o n s (Ware, 1971; N e i s h , 1970; Burko, 1975); and d i e t a r y h a b i t s o f salamanders, salmonids and s t i c k l e b a c k s i n the f i e l d ( E f f o r d & Tsumura, 1973; H y a t t , i n p r o g r e s s ) . 12 Characteristics Of Trout And Kokanee Rainbow trout and kokanee are both members of the family Salmonidae which i s composed of freshwater and anadromous fishes that range widely in the waters of the northern hemisphere. It is the dominant family of fishes in the northern waters of North America, Europe and Asia (Scott & Crossman, 1973). Both species are of great importance as commercial or recreational resources, and the rainbow trout is a standard laboratory animal for a wide range of physiological investigations. The l i t e r a t u r e dealing with various aspects of the biology of these two species is massive and I w i l l not attempt to summarize i t here. Key references that may be consulted for additional background on rainbow trout may be found in Scott & Crossman, 1973 (general review of l i f e history chara c t e r i s t i c s ) ; MacCrimmon, 1972 (review of native and present global d i s t r i b u t i o n ) ; Neave, 1944; Hartman and G i l l , 1968 (habitat associations); Jenkins, 1969; Newman, 1960; Slaney and Northcote, 1974 (social organization and behaviour); Ware, 1971; Bryan, 1972 (feeding behaviour). Key references that may be consulted for additional background on kokanee or sockeye salmon (the two are taxonomically indistinguishable) include: Foerster, 1968; Scott and Crossman, 1973 (general review of l i f e history chara c t e r i s t i c s ) ; Nelson, 1968 (natural d i s t r i b u t i o n ) ; Vernon, 1957; Northcote, 1973; Lorz and Northcote, 1965; Beach, 1974; I r i z a r r y , 1975 (characteristics of lake dwelling populations); Foerster, 1968; Behnke, 1972, Hartman and Burgner, 1972; Goodlad et a l . , 1974 (habitat 13 a s s o c i a t i o n s ) ; Newman, 1960; Hoar, 1976 ( s o c i a l o r g a n i z a t i o n and b e h a v i o u r ) ; Rankin, 1978; Eggers, 1978 ( f e e d i n g b e h a v i o u r ) ; Hoar, 1976 ( e v o l u t i o n and p h y l o g e n t i c a f f i n i t i e s ) . Study of t r o u t and kokanee i n the Marion Lake ecosystem o f f e r e d many advantages f o r the p u r s u i t of knowledge about f o r a g i n g b e h a v i o u r . The two s p e c i e s c o n s t i t u t e s e l f s u s t a i n i n g p o p u l a t i o n s which are g e n e t i c a l l y i s o l a t e d and complete t h e i r l i f e c y c l e s w i t h i n the c o n f i n e s o f the watershed of which Marion Lake i s a p a r t . The absense of both e x t e n s i v e m i g r a t i o n s by Marion Lake f i s h and of gene f l o w from o t h e r p o p u l a t i o n s p o t e n t i a l l y reduces the u n c e r t a i n t y , encountered i n o t h e r s t u d i e s , about whether p a r t i c u l a r t r a i t s are l o c a l l y a d a p t i v e o r r e l a t e d t o some o t h e r h a b i t a t (Morse, 1971; B i r c h and E h r l i c h , 1967; Van B a l e n , 1973). Growth r a t e s of both s p e c i e s i n Marion Lake are among the lo w e s t recorded a c r o s s t h e i r geographic range and t h e r e i s l i t t l e doubt t h a t t h i s i s r e l a t e d t o a l i m i t e d food s u p p l y (see H a l l & H y a t t , 1974 f o r d e t a i l s ) , thus s e l e c t i o n f o r a l o c a l l y a d a p t i v e f o r a g i n g s t r a t e g y should be r i g o r o u s . D e t a i l e d a n a l y s i s o f the d i e t a r y h a b i t s of the p r e d a t o r s (Sandercock, 1969; E f f o r d & Tsumura, 1973 and H y a t t , unpublished data) and e x t e n s i v e d a t a on the d i s t r i b u t i o n , abundance and p r o d u c t i o n l e v e l s of t h e i r prey ( H a m i l t o n , 1965; Hargrave, 1969; M a t h i a s , 1971; Winterbourn, 1971; McCauley, unpublished data) p r o v i d e d an almost unprecedented o p p o r t u n i t y t o r e l a t e d e t a i l s o f d i e t a r y p a t t e r n s and f o r a g i n g behaviour t o a known ecosystem c o n t e x t . The nature of the q u e s t i o n s asked i n t h i s study r e q u i r e d a combination o f l a b o r a t o r y and f i e l d i n v e s t i g a t i o n . I c a r r i e d 14 out the b u l k of the q u a n t i t a t i v e f i e l d o b s e r v a t i o n s d u r i n g the summers of 1971 and 1972. R e s u l t s from t h i s work helped shape the l a b o r a t o r y experiments t o examine the f i n e s c a l e d e t a i l s o f food g a t h e r i n g and responses of the p r e d a t o r s t o some major se a s o n a l changes. I completed most of the l a b o r a t o r y work d u r i n g the summers of 1974 and 1975. C o l l e c t i o n Techniques Trap n e t s , g i l l n e t s , s e i n e n e t s and a w i r e c y l i n d e r were used to o b t a i n f i s h f o r v a r i o u s purposes. I used the w i r e c y l i n d e r e x c l u s i v e l y t o o b t a i n u n i n j u r e d f i s h f o r l a b o r a t o r y experiments. At n i g h t , from a boat equipped w i t h auto head-l i g h t s , i t was p o s s i b l e t o l o c a t e t r o u t and kokanee i n s h a l l o w water. Because both s p e c i e s e x h i b i t v e r y low a c t i v i t y under c o n d i t i o n s of darkness i t was p o s s i b l e t o p l a c e a w i r e c y l i n d e r (diameter 1 m, depth 2 m) over them, d i p n e t them i n t o a h o l d i n g c o n t a i n e r , and c a r r y them u n i n j u r e d t o the l a b o r a t o r y . L a b o r a t o r y Animals and Standard E x p e r i m e n t a l Procedures Trout and kokanee used i n l a b o r a t o r y experiments were c a p t u r e d , handled and m a i n t a i n e d under i d e n t i c a l c o n d i t i o n s u n l e s s o t h e r w i s e s p e c i f i e d . F i s h were housed i n d i v i d u a l l y i n 40 - 80 l i t e r g l a s s and s t a i n l e s s s t e e l a q u a r i a a t a l a k e s i d e s i t e . Each aquarium r e c e i v e d n a t u r a l i l l u m i n a t i o n and a c o n s t a n t supply o f c o l d (10°C + 2 ) , s p r i n g - f e d water. F i s h were main-t a i n e d on a d i e t of c h i c k e n l i v e r when not i n v o l v e d i n experiments. A s e r i e s o f 200 l i t e r a q u a r i a (dimensions: L=92 cm, W=48 cm, D=46.5 cm) i n a nearby l a b o r a t o r y b u i l d i n g served as e x p e r i m e n t a l "arenas" f o r the m a j o r i t y o f f e e d i n g t r i a l s . A l l l a b o r a t o r y experiments were conducted under c o n d i t i o n s o f c o n s t a n t l i g h t and temperature (10°C +2). I l l u m i n a t i o n was p r o v i d e d by a bank (seven 100 watt bulbs) o f incandescent l i g h t s mounted 30 cm above the water s u r f a c e . A sheet o f w h i t e , t r a n s l u c e n t , p l a s t i c ; p o s i t i o n e d between the l i g h t s and the arena, served as a d i f f u s e r . F l a t - w h i t e p a r t i t i o n s , arranged around the s i d e s o f the arena p r o v i d e d a c o n s t a n t background a g a i n s t which prey were p r e s e n t e d . During a p r e - e x p e r i m e n t a l p e r i o d , each p r e d a t o r was c o n d i t i o n e d t o feed f r e e l y a f t e r t r a n s f e r from i t s home aquarium t o the arena. T h i s p r o c e s s u s u a l l y r e q u i r e d one t o two weeks f o l l o w i n g c a p t u r e from the f i e l d . The hunger l e v e l o f t r o u t and kokanee was s t a n d a r d i z e d b e f o r e a l l experiments by d e p r i v i n g the p r e d a t o r s of food f o r 48 - 72 hours. R e s u l t s from o t h e r s t u d i e s (Ware and H y a t t , unpublished r e s u l t s ) i n d i c a t e d t h a t f i s h r e q u i r e d 40-60 hours a t 10° C t o c o m p l e t e l y d i g e s t a s a t i a t i o n r a t i o n . The standard procedure f o r experiments c o n s i s t e d of g a t h e r i n g the r e q u i r e d number and type of prey from Marion Lake and h o l d i n g them i n p l a s t i c c o n t a i n e r s , w i t h o u t f o o d , f o r up to 48 hours. The prey were i n t r o d u c e d t o the e x p e r i m e n t a l arena and all o w e d 30 minutes to d i s p e r s e b e f o r e the i n t r o d u c t i o n of a p r e d a t o r . S p e c i f i c a s p e c t s o f the p r e d a t o r ' s f e e d i n g behaviour were recorded c h r o n o l o g i c a l l y on a R u s t r a c , 4 c h a n n e l , c h a r t r e c o r d e r . S i n g l e f e e d i n g t r i a l s u s u a l l y l a s t e d 30 minutes a t which time the p r e d a t o r was r e t u r n e d t o i t s home aquarium. The arena was d r a i n e d and cleaned f o l l o w i n g each t r i a l t o f a c i l i t a t e r e c o v e r y and co u n t i n g of remaining p r e y . 16 Measurements On P r e d a t o r s And Prey During the p r e - e x p e r i m e n t a l p e r i o d t r o u t and kokanee were a n a e s t h e t i z e d w i t h M S - 2 2 2 , weighed t o the n e a r e s t gram, and measured f o r t o t a l l e n g t h t o the n e a r e s t mm. jaw measurements were o b t a i n e d from p r e s e r v e d samples of p r e d a t o r s by u s i n g s l i d i n g v e r n i e r c a l i p e r s . L i v e i n v e r t e b r a t e s used i n each f e e d i n g t r i a l were photographed b e f o r e and a f t e r each t r i a l , i n a w h i t e enamel pan, c o n t a i n i n g a mm r u l e f o r r e f e r e n c e . Body l e n g t h s o r w i d t h s , where r e q u i r e d , were o b t a i n e d a t a l a t e r date w i t h c a l i p e r s from the image of the prey p r o j e c t e d onto a s c r e e n . Measurements on very s m a l l prey such as zoo p l a n k t o n were o b t a i n e d from p r e s e r v e d samples by u s i n g a d i s s e c t i o n microscope ( W i l d M - 5 ) , equipped w i t h an o c u l a r micrometer. 17 CHAPTER 2 THE SPECIES SPECIFIC DIETARY PATTERNS OF TROUT AND KOKANEE IN MARION LAKE To do s c i e n c e i s t o s e a r c h f o r repeated p a t t e r n s , not s i m p l y t o accumulate f a c t s . R e p e t i t i o n s of p a t t e r n s i n nature are u s u a l l y i m p e r f e c t and t h i s g i v e s us the means o f making comparisons which may then serve as the seeds of t e s t a b l e hypotheses. R. MacArthur, 1972. INTRODUCTION Two types o f comparisons have f r e q u e n t l y been used t o draw i n f e r e n c e s about the f o r a g i n g behaviour of p r e d a t o r s . The d i e t s o f s i m i l a r s p e c i e s a re compared t o each o t h e r , o r the d i e t a r y c o m p o s i t i o n of a p a r t i c u l a r p r e d a t o r i s compared t o the apparent a v a i l a b i l i t y o f prey i n the environment. In t h i s s e c t i o n I w i l l d e a l w i t h both types o f comparisons and comment on the nature o f the i n f e r e n c e s t h a t they a l l o w . METHODS I n i t i a l a n a l y s i s o f t r o u t and kokanee d i e t s from 1963-1966 ( E f f o r d & Tsumura, 1973) i n d i c a t e d s u b s t a n t i a l d i f f e r -ences i n t h e i r p a t t e r n s of food e x p l o i t a t i o n . The o r i g i n a l a n a l y s i s was based upon t r o u t of 10-25 cm i n l e n g t h w h i l e kokanee were g e n e r a l l y l e s s than 15 cm l o n g . Thus, I was i n t e r e s t e d i n de t e r m i n i n g whether the s p e c i e s s p e c i f i c d i e t a r y p a t t e r n s were p r i m a r i l y a consequence of d i f f e r e n c e s i n the average s i z e of 18 p r e d a t o r s i n c l u d e d i n the o r i g i n a l a n a l y s i s o r i f the d i f f e r e n c e s would p e r s i s t g i v e n comparisons l i m i t e d t o p r e d a t o r s of s i m i l a r s i z e . T h e r e f o r e , I an a l y z e d some "size-matched" p r e d a t o r s o b t a i n e d from e i t h e r the 1963-66 c o l l e c t i o n s o r from new c o l l e c t i o n s taken between 1972-76. The new c o l l e c t i o n s a l l o w e d me t o extend the comparisons t o f i s h of s m a l l e r body s i z e than those examined by E f f o r d & Tsumura. D e t a i l s c o n c e r n i n g the t i m e s , l o c a t i o n s and te c h n i q u e s o f c a p t u r e f o r the 1963-66 c o l l e c t i o n a r e a v a i l a b l e elsewhere ( E f f o r d & Tsumura, 1973). Specimens from the 1972-76 c o l l e c t i o n , used i n the p r e s e n t a n a l y s i s , were c o l l e c t e d by d i p - n e t t i n g a t n i g h t from a boat equipped w i t h l i g h t s . T h i s ensured t h a t t r o u t and kokanee were matched not o n l y f o r s i z e but f o r l o c a t i o n ( w i t h i n an area o f a few hundred square m e t e r s ) , and time of cap t u r e (between 11 P.M. and 1 A.M.) as w e l l . The m a j o r i t y o f f i s h i n the 1972-76 c o l l e c t i o n were taken from depths of l e s s than 2 m i n the n o r t h e a s t c o r n e r of Mari o n Lake. F r e s h l y sampled f i s h were immediately k i l l e d i n 70% e t h a n o l and then t r a n s f e r r e d f o r l o n g e r term s t o r a g e t o 10% f o r m a l i n . To o b t a i n s i z e - f r e q u e n c y d a t a on p r e y , I used a d i s s e c t i o n microscope ( W i l d M-5) equipped w i t h an o c u l a r m i c r o -meter. Three measurements ( l e n g t h , head w i d t h , maximum body width) were taken from each pr e y . S i z e frequency d i s t r i b u t i o n s o f prey a r e based on e i t h e r body l e n g t h s o r a r e a ( a r e a = l e n g t h x maximum w i d t h ) . Frequency d i s t r i b u t i o n s are based upon l e n g t h when prey a re of r e a s o n a b l y uniform shape. In o r d e r to p r o v i d e g r e a t e r a c c u r a c y , d i s t r i b u t i o n s are based upon area when prey 19 i n c l u d e a v a r i e t y o f shapes ( t u b u l a r , s p h e r i c a l , pentagonal e t c . . . ) . To c o n s t r u c t r e p r e s e n t a t i v e s i z e - f r e q u e n c y d i s t r i b u t i o n s o f food items i n the d i e t s of t r o u t and kokanee, I s o r t e d f i s h , matched as c l o s e l y as p o s s i b l e f o r s i z e , from c o l l e c t i o n s taken on May 26/64, June 10/66 and August 11/63. Prey measurements were taken from a t o t a l of 41 p r e d a t o r s , w i t h no fewer than 5 f i s h of each s p e c i e s from each month. Because the t o t a l number of food items i n a sample of f i s h f o r any month v a r i e d c o n s i d e r a b l y (range 213-1228), I a p p l i e d a w e i g h t i n g f a c t o r t o each month's samples. T h i s guarantees t h a t the food items from any month's samples have an equal i n f l u e n c e on the shape of the s i z e - f r e q u e n c y d i s t r i b u t i o n c o n s t r u c t e d from d a t a pooled f o r the t h r e e months. RESULTS Between P r e d a t o r Comparisons Examination of the taxonomic c o m p o s i t i o n o f the d i e t s by weight o r by numbers ( F i g . 2a & b) i n d i c a t e s t h a t over the course of a year t r o u t and kokanee e x p l o i t s i g n i f i c a n t l y d i f f e r e n t p r o p o r t i o n s of each major prey type. There are numerous examples of prey types e x p l o i t e d e x t e n s i v e l y by one p r e d a t o r but b a r e l y u t i l i z e d by the o t h e r . The f o u r prey types ( T r i c h o p t e r a , Amphipoda, Odonata and p l a n o r b i d s n a i l s ) t h a t make up 69% (by weight) o f the d i e t o f t r o u t c o n t r i b u t e o n l y 17% of the weight of prey i n the d i e t o f kokanee. Over the course o f a y e a r , chironomid l a r v a e , pupae, and c l a d o c e r a n s were more important d i e t a r y items f o r kokanee (48% by weight) than f o r t r o u t ( 8% by w e i g h t ) . 20 FIGURE 2. A comparison of the r e l a t i v e p r o p o r t i o n s o f v a r i o u s prey types i n the d i e t s of t r o u t and kokanee from Marion Lake, B.C. (a) D i e t as % by weight (b) D i e t as %. by numbers. Data pooled from samples o f f i s h c o l l e c t e d i n the months of Nov., Feb., A p r i l , June and Aug. Trout and kokanee were not r i g o r o u s l y s i z e -matched f o r a n a l y s i s . A l l t e s t s f o r s i g n i f i c a n t d i f f e r e n c e s i n e x p l o i t a t i o n o f s i n g l e prey types by p r e d a t o r s are based upon the normal a p p r o x i m a t i o n t o the b i n o m i a l d i s t r i b u t i o n ( S i e g e l , 1956). ++ w = .01, + ^ = .05. Data adopted from Efford and Tsumura, 1973. K O K A N E E TROUT yo OF Dl ET BY WEIGHT Notonect i ds Cor i x i d s Neurop te ran larvae Pi s i d i u m S i mul ium la rvae C o p e p o d s E p h e m e r o p t ' e r a n l a r v a e P l e c o p t e r a n l a r v a e A q u a t i c bee t l e a d u l t s P l a n o r b i d sna i I s Odona te larvae C l a d o c e r a n s Ch i ronomid larvae Amph ipods Ch i ronomid pupae Ter res t r i a l i n s e c t s Trichopteran larvae J 40 30 20 10 10 20 30 40 © °/Q OF Dl ET BY NUMBERS Aqua t i c beet le adul ts E p h e m e r o p t e r a n l a r v a e S imu l ium l a r v a e + + Ter res t r i a l i n s e c t s + + . P i s i d i u m + Odona te larvae ++ ++ + + C o p e p o d s T r i c h o p t e r a n l a r v a e P lano rb id snai I s + + A m p h i p o d s + + Chironomid l a r vae + + C l a d o c e r a n s Ch i ronomid pupae' ++ ,+ + 50 40 N = 4430 N=3038 30 20 10 10 20 30 40 50 The degree of dietary s i m i l a r i t y between trout and kokanee fluctuates substantially from month to month. For com-parisons of monthly changes in the degree of food s i m i l a r i t y I have used the overlap measure of Morisita (1959) and Horn (1966). The overlap coe f f i c i e n t CX varies from zero when the samples are completely discrete to one when the samples are i d e n t i c a l . where: s i s the t o t a l number of food c a t e g o r i e s s 2? _ , x . y . x^ i s the p r o p o r t i o n of the d i e t = "^'. ~ 1 of predator x taken from the i t T x ± + z . y ± 2 f o o d category i = l i = i y . i s the proportion of the diet 1 of predator y taken from the i th. food category Although no s t a t i s t i c a l method i s available to test the s i g n i f -icance of C X , other authors (Zaret & Rand, 1971; Fedorenko, 1975) have assumed that values equal to or greater than .60 represent s i g n i f i c a n t overlap. In comparisons of food overlap in birds i t i s not unusual for a l l values to be greater than .60 (Orians & Horn, 1969; Pulliam & Enders, 1971) . By this c r i t e r i o n trout and kokanee exhibit close overlap in only one month df the year (Table 1). 22 TABLE 1. Food o v e r l a p c a l c u l a t i o n s f o r t r o u t and kokanee from Marion Lake. Numbers i n b r a c k e t s i n d i c a t e the year i n which the sample was t a k e n . Overlap by weight Feb.(64) A p r i l ( 6 6 ) June(66) Aug. (63) Nov.(63) Pooled .018 .474 .463 .201 .736 .443 O v e r l a p by numbers .243 .245 . 8 8 1 • 1 3 6 . 4 4 0 • 4 6 2 Some prey types such as chironomid pupae are eaten i n l a r g e numbers by both t r o u t and kokanee over the course of a y e a r . Because of i n d i v i d u a l v a r i a b i l i t y i n the number of pupae e a t e n , the means o f the number of pupae eaten i n any g i v e n month by the two p r e d a t o r s are not s i g n i f i c a n t l y d i f f e r e n t (Table 2 . ) . However, t h i s does not s i g n i f y t h a t t r o u t and kokanee are e q u a l l y l i k e l y t o e x p l o i t chironomid pupae. Indeed the v a r i a n c e around the mean number of pupae eaten by t r o u t o r kokanee tends to obscure the f a c t t h a t i n d i v i d u a l kokanee are more l i k e l y t o c o n t a i n r e l a t i v e l y l a r g e numbers of pupae than are i n d i v i d u a l t r o u t . For example, d a t a pooled from an equal number o f t r o u t and kokanee captured between 1963 and 1974 i n d i c a t e s t h a t kokanee e a t s i g n i f i c a n t l y l a r g e r numbers of chironomid pupae (Kolmogorov-Smirnov two-sample t e s t , X = 27.03, degrees of freedom = 2, p <.001) than t r o u t ( F i g u r e 3 ) . Although o n l y 6% o f 163 t r o u t examined c o n t a i n e d more than 40 chironomid pupae i n t h e i r g u t s , 28% o f 163 kokanee examined c o n t a i n e d 40 or more pupae. F u r t h e r -more, 75 of the 163 t r o u t examined c o n t a i n e d no pupae at a l l TABLE 2. Seasonal e x p l o i t a t i o n o f chironomid pupae by t r o u t and kokanee. N i s the sample s i z e and S.D. the standard d e v i a t i o n o f the means. Mean number of Mean number of Month pupae per t r o u t S.D. pupae per kokanee S.D. N February 0.00 .58 1.0 12 A p r i l 10.14 17.4 56.86 36.3 7 May 93.75 181.5 274.93 246.0 16 June 19.62 22.2 50.71 55.9 21 J u l y 1.82 3.2 17.09 21.0 11 August 7.35 9.5 19.85 32.6 40 September 1.20 1.8 16.53 19.8 15 October .31 .9 27.77 22.0 13 November 3.92 9.2 13.00 21.4 24 December 0.00 - 0.00 - 4 Data pooled from p r e d a t o r s c o l l e c t e d between 1963 and 1974 by Hamilton (unpublished r e s u l t s ) , Sandercock (1969), E f f o r d and Tsumura (1973) and Hyatt (present s t u d y ) . 24 FIGURE 3. A comparison of the p r o p o r t i o n s o f t r o u t and kokanee t h a t c o n t a i n d i f f e r e n t q u a n t i t i e s o f chironomid pupae i n t h e i r d i e t s . Data pooled f o r equal numbers o f t r o u t and kokanee capt u r e d i n v a r i o u s months (see Table 2) between the y e a r s 1963 and 1974. N = number of t r o u t o r kokanee examined. CO X o < o w co Ul or a UJ 5 O z o D£ X o O a: UJ co 5 100 + 9 I -100 81 - 90 71 - 8 0 61 - 7 0 51 - 60 4 1 - 5 0 3 1 - 4 0 21 - 30 I I - 20 0 - 10 TROUT N = 163 r KOKANEE N = 163 80 70 60 50 40 30 ; 20 10 0 10 20 3 0 4 0 50 60 70 80 % OF PREDATORS EXAMINED to 0) w h i l e o n l y 40 of 163 kokanee examined f a i l e d t o c o n t a i n chironomid pupae. S i z e - f r e q u e n c y D i s t r i b u t i o n s o f Food Items The s i z e - f r e q u e n c y d i s t r i b u t i o n s of prey from s i z e -matched t r o u t and kokanee are s i m i l a r ( F i g . 4 ) , however, t h i s may be m i s l e a d i n g . N u m e r i c a l l y t r o u t o b t a i n 8% o f t h e i r d i e t from the l a r g e s t s i z e - c l a s s e s of prey w h i l e kokanee take o n l y 1% o f t h e i r d i e t from prey of these s i z e s . E f f o r d and Tsumura have i n d i c a t e d t h a t items c o n s t i t u t i n g o n l y 8% of t r o u t d i e t by number may form as much as 50% o f the t o t a l food i n t a k e by weight. Thus, s m a l l d i f f e r e n c e s i n numbers a t t h i s end of the prey s i z e - d i s t r i b u t i o n may have c o n s i d e r a b l e s i g n i f i c a n c e f o r energy i n t a k e . Trout undoubtedly o b t a i n a g r e a t e r p r o p o r t i o n of t h e i r d i e t from l a r g e s i z e c l a s s e s o f prey ( t e r r e s t r i a l i n s e c t s , s n a i l s , odonates, t r i c h o p t e r a n s ) w h i l e kokanee e x p l o i t the s m a l l e s t s i z e c l a s s e s (zooplankton) more i n t e n s i v e l y (Table 3) . D i f f e r e n c e s i n the s i z e s of prey e x p l o i t e d by t r o u t and kokanee are not e n t i r e l y due t o d i f f e r e n c e s i n the type of prey e x p l o i t e d . W i t h i n a c a t e g o r y o f prey t h e r e a r e f r e q u e n t l y d i f f e r e n c e s i n the s i z e - d i s t r i b u t i o n s of i n d i v i d u a l s consumed. A sample of size-matched t r o u t and kokanee (Table 4) from June, 1966 was examined f o r the s i z e - d i s t r i b u t i o n s of amphipods e x p l o i t e d . R e l a t i v e t o kokanee, rainbow t r o u t consume g r e a t e r p r o p o r t i o n s o f the l a r g e s i z e - c l a s s e s o f amphipods ( F i g . 5 ) . A 26 FIGURE 4 . A comparison of the s i z e - f r e q u e n c y d i s t r i b u t i o n s of a l l prey eaten by size-matched t r o u t and kokanee. N = the t o t a l number of prey items measured and pooled from a l l p r e d a t o r s examined. Data pooled from samples of f i s h c o l l e c t e d i n the months o f May, June, and August. KOKANEE N = 1765 ~~i r~ 1 1 1 1 1 1 r 4 0 30 20 10 0 10 20 3 0 4( % OF FOOD ITEMS EATEN TABLE 3. Taxonomic c o m p o s i t i o n of food items from p r e d a t o r s used i n f o o d - s i z e d e t e r m i n a t i o n s . Trout Kokanee No. examined 25 16 Mean l e n g t h (cm) 14.8 14.0 Range 11.5-17.5 12.0-15.8 % No. % Chironomid l a r v a e 744 31.6 370 9.6 Chironomid pupae 954 40.5 2,797 72.3 Chironomid a d u l t s 278 11.8 97 2.5 Zooplankton 76 3.2 511 13.2 P i s i d i u m sp. 37 1.0 T e r r e s t r i a l i n s e c t s 125 5.3 23 .6 M o l l u s c a ( s n a i l s ) 52 2.2 T r i c h o p t e r a 46 2.0 Amphipoda 27 1.2 22 .6 Odonata 20 .9 Other 34 1.4 13 .3 T o t a l items 2,356 3,870 28 c l o s e r e x a m i n a t i o n of the d a t a r e v e a l s t h a t t h i s d i f f e r e n c e o r i g i n a t e s l a r g e l y from d i f f e r e n t i a l e x p l o i t a t i o n o f a s m a l l s p e c i e s of amphipod ( H y a l e l l a a z t e c a , s i z e s t o 4 mm) by kokanee, and of a l a r g e r amphipod (Crangonyx r i c h m o n d e n s i s , s i z e s t o 10 mm) by t r o u t (Table 4 ) . S i m i l a r l y , because kokanee consume p r i m a r i l y the s m a l l b i v a l v e P i s i d i u m spp. ( g e n e r a l l y l e s s than 2 mm) and t r o u t c o n c e n t r a t e on pulmonate m o l l u s c s (Helisoma and Menetus spp., s i z e s to 10 mm), the s i z e -frequency d i s t r i b u t i o n s f o r m o l l u s c s eaten by the p r e d a t o r s are s i g n i f i c a n t l y d i f f e r e n t ( F i g . 5 ) . Winterbourn*s (1971) examination of t r i c h o p t e r a n s , e x p l o i t e d by t r o u t and kokanee, r e v e a l e d t h a t t r o u t e x p l o i t the l a r g e s t s i z e c l a s s e s of c a d d i s l a r v a e a v a i l a b l e ( s i z e s t o 20 mm) a t a l l times of the y e a r . The c a d d i s l a r v a e p r e s e n t i n t r o u t stomachs a c c u r a t e l y r e f l e c t e d the s u c c e s s i o n o f s p e c i e s m o u l t i n g t o the f i n a l i n s t a r d u r i n g the y e a r . Kokanee e x p l o i t e d v e r y few c a d d i s l a r v a e and those t h a t were taken belonged t o the s m a l l e s t s p e c i e s i n the l a k e ( O e c e t i s i n c o n s p i c u a , body l e n g t h s t o 4 mm).. TABLE 4. The r e l a t i v e l e v e l of e x p l o i t a t i o n o f H y a l e l l a a z t e c a and Crangonyx  richmondensis by t r o u t and kokanee. Trout Kokanee No. examined 12 9 Mean l e n g t h (cm) 14.9 15.1 Range 10.5-16.7 12.8-16.5 No. o f Crangonyx 96 21 sp. eaten No. o f H y a l e l l a 89 77 sp. eaten H y a l e l l a / C r a n g o n y x 1.0/1.1 3.7/1.0 29 FIGURE 5. A comparison of the s i z e - f r e q u e n c y d i s t r i b u t i o n s o f amphipods (Crangonyx sp. and H y a l e l l a sp.) eaten by size-matched t r o u t and kokanee. N = the t o t a l number of prey items pooled from a l l p r e d a t o r s examined. Data o b t a i n e d from samples of f i s h c o l l e c t e d i n the month o f June. 30 FIGURE 6. A comparison o f the s i z e - f r e q u e n c y d i s t r i b u t i o n s o f m o l l u s c s ( P i s i d i u m spp.; p l a n o r b i d s n a i l s , Helisoma sp. and Menetus sp.) eaten by size-matched t r o u t and kokanee. N = the t o t a l number of prey items pooled from a l l p r e d a t o r s examined. Data o b t a i n e d from samples of f i s h c o l l e c t e d i n the months of May, June and August. i 1 1 r-60 40 20 KOKANEE N = 41 20 40 60 —r-80 % OF MOLLUSCS EATEN CO o 31 R e s u l t s presented so f a r have d e a l t m a i n l y w i t h t r o u t i and kokanee between 10 and 17 cm i n l e n g t h . However, d i e t a r y d i f f e r e n c e s are not r e s t r i c t e d s o l e l y t o p r e d a t o r s w i t h i n t h i s s i z e range. Size-matched, young-of-the-year t r o u t and kokanee c o l l e c t e d i n Sept./74 e x h i b i t i n t e r e s t i n g d i f f e r e n c e s i n d i e t c o m p o s i t i o n i n s p i t e of the f a c t t h a t animals of both s p e c i e s o b t a i n e d g r e a t e r than 95% o f t h e i r d i e t from j u s t two prey groups ( z o o p l a n k t o n and d i p t e r a n s ) . Small t r o u t (average l e n g t h 5.6 cm) i n t e n s i v e l y e x p l o i t e d the c l a d o c e r a n S i d a c r y s t a l l i n a , a s i g n i f i c a n t number of d i p t e r a n s , t e r r e s t r i a l i n s e c t s and a few water m i t e s (Table 5 ) . By c o n t r a s t , s m a l l kokanee (average l e n g t h 7.9 cm) e x p l o i t e d a g r e a t e r d i v e r s i t y o f z o o p l a n k t o n , l a r g e r numbers of d i p t e r a n s and almost no t e r r e s t r i a l i n s e c t s . These d i f f e r e n c e s a r e s u r p r i s i n g s i n c e the f i s h were sampled w i t h i n 2-3 hours of each o t h e r from v i r t u a l l y the same m i c r o -h a b i t a t s . R e s u l t s of s i z e - f r e q u e n c y a n a l y s i s of prey ( F i g . 7 ) , r e v e a l t h a t kokanee, i n s p i t e of t h e i r l a r g e r s i z e i n t h i s sample, i n t e n s i v e l y e x p l o i t s m a l l e r s i z e c l a s s e s of prey ( p r i m a r i l y s m a l l c l a d o c e r a n s such as Bosmina sp. and copepods such as C y c l o p s sp.) than t r o u t do. Comparisons of the R e l a t i v e P r o p o r t i o n s o f V a r i o u s Prey Types i n the N a t u r a l Environment and i n the D i e t s of Trout and Kokanee. The d i e t a r y d i f f e r e n c e s between t r o u t and kokanee are c l o s e l y p a r a l l e l e d by d i f f e r e n c e s between the apparent a v a i l a b i l i t y o f prey i n Marion Lake and the prey e x p l o i t e d by each s p e c i e s o f 32 TABLE 5. Taxonomic c o m p o s i t i o n o f prey eaten by s m a l l t r o u t and kokanee c o l l e c t e d September 7, 1974. Trout Kokanee No. examined 9 9 Mean l e n g t h (cm) 5.6 7.9 Range 4.3-6.6 6.9-8.5 Zooplankton No. % No. % S i d a sp. 1,219 90.2 184 19.7 Bosmina sp. 2 .1 166 17.8 Chydorus sp. 1 . 1 Leptodora sp. 5 .5 C e r i o d a p h n i a sp. 1 .1 Polyphemus sp. 1 .2 2 .2 Alona sp. 2 .2 Cyclops sp. 291 31.2 M i t e s sp. 9 .7 5 .5 Ostracods 2 .2 D i p t e r a Chironomid l a r v a e 31 2.3 103 11.0 Chironomid pupae 16 1.2 124 13.3 Chironomid a d u l t s 39 2.9 24 2.6 Other Amphipoda 9 1.0 T r i c h o p t e r a 6 .4 3 .3 Odonata 2 .2 Ephemeroptera 3 .2 1 .1 T e r r e s t r i a l i n s e c t s 18 1.3 2 .2 T o t a l 1,345 933 33 FIGURE 7. A comparison of the s i z e - f r e q u e n c y d i s t r i b u t i o n s of water column prey ( p r i m a r i l y z o o p l a n k t o n and chironomid pupae) e x p l o i t e d by size-matched t r o u t and kokanee. N = the t o t a l number of prey items pooled from a l l p r e d a t o r s examined. Data o b t a i n e d from samples of f i s h c o l l e c t e d i n the same m i c r o h a b i t a t s and a t the same time of day d u r i n g the month o f September. 3200 + 2560-2720 3> 1920-2080 >-UJ cr a. o 1280-1440] x t— o UJ 640-800 0-160 40 TROUT N= 1345 30 T 20 1 u" KOKANEE N= 933 i 1 1 r 10 0 10 % OF PREY EATEN ~~T~ 20 30 " T 40 U l u> 34 p r e d a t o r . On an annual b a s i s , a l a r g e p r o p o r t i o n o f the d i e t o f both t r o u t and kokanee c o n s i s t s of a r e l a t i v e l y s m a l l number of prey t y p e s . Furthermore, when compared t o the p r o p o r t i o n s o f prey a p p a r e n t l y a v a i l a b l e i n the l a k e , i t i s c l e a r t h a t each p r e d a t o r does not e x p l o i t the f u l l range of prey i n p r o p o r t i o n to e i t h e r t h e i r weights o r numbers ( F i g s . 8 and 9 ) . There are numerous examples of i n t e n s e u t i l i z a t i o n of p o o r l y r e p r e s e n t e d prey types and of f a i l u r e s t o use o t h e r n u m e r i c a l l y abundant prey p r e s e n t i n the l a k e . The same s i t u a t i o n p r e v a i l s w i t h r e s p e c t t o u t i l i z a t i o n o f i n d i v i d u a l s p e c i e s w i t h i n major prey groups. Out o f n e a r l y 100 s p e c i e s of chironomids found i n Marion Lake, o n l y 19 have been found i n the d i e t s of t r o u t and kokanee ( E f f o r d & Tsumura, 1973) . Some of the most abundant s p e c i e s by volume o r by number do not appear i n the d i e t s of f i s h a t a l l (e.g. P e g a s t i e l l a sp.A, 23% by no.; Phaenospectra p r o t e x t u s & P o l y p e d e l i u m spp., 29% by v o l . ) . The u t i l i z a t i o n o f many prey types bears l i t t l e r e l a t i o n t o s e a s o n a l changes i n t h e i r apparent a v a i l a b i l i t y . The i n t e n s e u t i l i z a t i o n of amphipods by t r o u t i n Aug. c o i n c i d e s w i t h t h e i r g r e a t e s t abundance (Table 6) but t r o u t e x p l o i t e q u a l l y l a r g e numbers i n the month of June when amphipod numbers are a t t h e i r l o w e s t l e v e l s . Kokanee d i s p l a y s i m i l a r d i s c r e p a n c i e s i n i n t e n s i t y o f amphipod use as compared t o apparent amphipod abundance. The peak u t i l i z a t i o n of chironomid pupae o c c u r s i n May and June f o r f i s h and by J u l y - A u g u s t the mean number of chi r o n o m i d pupae per stomach i n f i s h i s l e s s than one t e n t h the May-June v a l u e (see d a t a i n Table 2 ) . However, a l l the a v a i l a b l e evidence i n d i c a t e s 35 FIGURE 8. A comparison on an annual b a s i s o f the r e l a t i v e p r o p o r t i o n s o f prey i n the environment and i n the d i e t of t r o u t from Marion Lake, B.C. (a) % by number, (b) % by weight. Note the changing s c a l e along the a b s c i s s a . Data m o d i f i e d a f t e r E f f o r d and Tsumura (1973) p l u s u n p u b l i s h e d r e c o r d s of r e l a t i v e prey abundance. 35a 1. SIALIS 2. EPHEMEROPTERA 3. ODONATA 4. OLIGOCHAETES 5. TRICHOPTERA 6. CERATOPOGONIDS 7. HIRUDINEA . 8. PLANORBIDAE 9. PIS1DIUM 10. CHIRONOMID PUPAE 11. AMPHIPODA 12. CHIRONOMID LARVAE 13. MEIOFAUNA 100 ENVIRONMENT 10 2. 3. 5. 6. 8. 9. 10. II. 12. 13. TROUT r 10 loo % BY NUMBER 1. CHIRONOMID PUPAE 2. PISIDIUM 3. MEIOFAUNA 4. CERATOPOGONIDS 5. EPHEMEROPTERA 6. OLIGOCHAETES 7 AMPHIPODA 8. SIALIS 9. ODONATA 10. TRICHOPTERA 11. PLANORBIDAE 12. CHIRONOMID LARVAE 13. HIRUDINEA l i i 60 40 20 t i l l 40 60 % BY WEIGHT' NOTE: LOG SCALE ON ABSCISSA 36 FIGURE 9. A comparison on an annual b a s i s of the r e l a t i v e p r o p o r t i o n s o f prey i n the environment and i n the d i e t o f kokanee from Marion Lake, B.C. (a) % by number, (b) % by weight. Note the changing s c a l e along the a b s c i s s a . Data m o d i f i e d a f t e r E f f o r d and Tsumura (1973) p l u s unpublished r e c o r d s o f r e l a t i v e prey abundance. 36a ENVIRONMENT KOKANEE 1. SIALIS 2. MAYFLY 3. ODONATES 4. OLIGOCHAETES 5. CADDIS 6. CERATOPOGONIDS 7. LEECHES 8. PLANORBIDS 9. PISIDIUM 10. CHIRONOMID R • 11. AMPHIPODS 1.2. CHIRONOMID L. 13. MEIOFAUNA "T 100 T 10 3. 4. 5. 6. 9. 10. II. 12. 13. ~T~ 10 "ioo % BY NUMBER 1. CHIRONOMID P. 2. PISIDIUM 3. MEIOFAUNA 4. CERATOPOGONIDS 5. EPHEMEROPTERA 6. OLIGOCHAETES 7. AMPHIPODS 8. SIALIS 9. ODONATES 10. CADDIS 11. PLANORBIDS 12. CHIRONOMID L. 13. HIRUDINEA l l ' I l I 60 40 20 10 2. 3. 5. 6. 10. II. 12. 13. TT I I % BY WEIGHT I I H i l l 10 I 20 T — l I | 40 60 NOTE • LOG SCALE ON ABSCISSA 37 t h a t the number of chironomids pupating and emerging i n J u l y and Aug. i s as g r e a t o r g r e a t e r than t h a t recorded i n May and June ( E f f o r d & Tsumura, 1973; H a m i l t o n , 1965; McCauley, p e r s . comm.). Many o f the d i f f e r e n c e s between prey i n c l u d e d i n the d i e t and those randomly sampled from the l a k e are r e l a t e d t o organism s i z e . I have a l r e a d y mentioned t h a t when c a d d i s l a r v a e are abundant as s m a l l , e a r l y , i n s t a r s ; t r o u t c o n t i n u e t o e x p l o i t the l a r g e f o u r t h and f i f t h i n s t a r s t a g e s of l e s s abundant s p e c i e s . The same p a t t e r n emerges from a comparison o f the amphipods e x p l o i t e d by t r o u t compared t o those p r e s e n t i n the environment ( F i g . 10a). During June, the l a r g e s t s i z e c l a s s e s o f amphipods are many times more abundant i n the stomachs of t r o u t than expected from t h e i r r e l a t i v e abundance i n the l a k e p o p u l a t i o n . For kokanee the p a t t e r n i s d i f f e r e n t ( F i g . 10b) and i n c l u d e s a h i g h e r p r o p o r t i o n of s m a l l amphipods. TABLE 6. Mean number of amphipods per m ve r s u s the mean number per p r e d a t o r i n d i f f e r e n t months o f the y e a r . Feb. A p r i l June Aug. Nov. Amphipods 1594 1724 1100 2478 1657 Amph ipods/Trou t .25 2.79 5.69 5.67 .50 Trout examined 12 14 24 45 18 Amphipods/Kokanee .38 2.14 1.92 .28 2.58 Kokanee examined 18 7 12 24 19 38 FIGURE 10. A comparison of the s i z e - f r e q u e n c y d i s t r i b u t i o n s of amphipods i n the environment and i n the d i e t s o f the p r e d a t o r s . Data f o r s i z e s of amphipods p r e s e n t i n the environment were o b t a i n e d from samples taken d u r i n g June of 1969. Data f o r s i z e s o f amphipods i n the d i e t s of p r e d a t o r s were o b t a i n e d from samples taken i n June of 1966. 38a ENVIRONMENT N = 560, 1969 TROUT N = 209 ,1966 E E o c — >> O X) LU N CO Q O Q. X CL 12 H I0H T — i — i — i — r 10 20 30 " i — i — i — r 40 50 % OF TOTAL AMPHIPODS PRESENT 39 DISCUSSION A v a r i e t y o f comparisons presented above i n d i c a t e t h a t the i n d i v i d u a l "sampling" a c t i v i t i e s o f t r o u t , kokanee and s c i e n t i s t s i n Marion Lake each r e s u l t i n the a c q u i s i t i o n o f d i f f e r e n t s e t s of i n v e r t e b r a t e "prey". U l t i m a t e l y such d i f f e r e n c e s must be r e l a t e d t o d i f f e r e n c e s i n when, where, and how i n v e r -t e b r a t e s are o b t a i n e d from the l a k e environment by each o f these " p r e d a t o r s " . I t should not be p a r t i c u l a r l y s u r p r i s i n g t h a t the r e l a t i v e abundance of i n v e r t e b r a t e prey types i n the d i e t s o f t r o u t and kokanee do not c l o s e l y f o l l o w the apparent abundance o f these same i n v e r t e b r a t e s i n the l a k e environment g i v e n t h a t the c h a r a c t e r i s t i c s of the "sampling gear" and procedures used by s c i e n t i s t s and p r e d a t o r s are i n h e r e n t l y so d i f f e r e n t . For example, i t i s most o f t e n the case t h a t man-made sampling d e v i c e s are designed and employed t o o b t a i n samples of a wide range o f i n v e r t e b r a t e s i n p r o p o r t i o n t o t h e i r a c t u a l d e n s i t i e s i n the n a t u r a l environment. Thus p l a n k t o n nets o r b e n t h i c grabs used i n l a k e s are r e l a t i v e l y n o n - s e l e c t i v e i n t h a t they tend to a c q u i r e the m a j o r i t y o f organisms w i t h i n the space sampled by the d e v i c e . By c o n t r a s t p r e d a t o r s such as t r o u t and kokanee have been "designed" through n a t u r a l s e l e c t i o n t o e x p l o i t a r e l a t i v e l y l i m i t e d p o r t i o n of the t o t a l range of i n v e r t e b r a t e prey t h a t r e s i d e i n a g i v e n environment and may f r e q u e n t l y a c q u i r e some prey items r a t h e r than o t h e r s due t o s u b t l e d i f f e r e n c e s i n c h a r a c t e r i s t i c s such as prey s i z e , c o l o u r , escape responses, armour, t e x t u r e o r t a s t e . Furthermore, w h i l e the sampling p r o t o c o l 40 used by s c i e n t i s t s i s o f t e n a r b i t r a r i l y f i x e d , t h a t o f any p r e d a t o r w i l l s h i f t c o n t i n u o u s l y i n response t o s l i g h t changes i n g e n e r a l e n v i r o n m e n t a l c o n d i t i o n s o r prey c h a r a c t e r i s t i c s . Thus, i t i s important t o keep i n mind t h a t the apparent a v a i l a b i l i t y o f prey types as measured by a s c i e n t i s t u s i n g a p a r t i c u l a r m echanical d e v i c e w i l l o f t e n c o n s t i t u t e a poor index o f the a v a i l a b i l i t y o f prey t o a g i v e n p r e d a t o r . Because t h e r e a re so many obvio u s d i f f e r e n c e s between the d e s i g n and o p e r a t i o n of man-made sampling d e v i c e s and the "de s i g n " and sampling procedures of p r e d a t o r s , i t i s p o s s i b l e to p r e d i c t w i t h some c e r t a i n t y t h a t d i f f e r e n c e s i n the prey s e t s a c q u i r e d by s c i e n t i s t s and p r e d a t o r s w i l l be the r e s u l t of a v a r i e t y o f p r e d a t o r c h a r a c t e r i s t i c s t h a t f u n c t i o n d u r i n g each stage of the f o r a g i n g c y c l e from s e a r c h t o f i n a l i n g e s t i o n o f pr e y . By c o n t r a s t , d i f f e r e n c e s i n the " d e s i g n " and o p e r a t i o n o f t r o u t and kokanee as p r e d a t o r s are not a t a l l obvious and thus t h e r e i s c o n s i d e r a b l e u n c e r t a i n t y about the i d e n t i t y and r e l a t i v e importance of v a r i o u s p r e d a t o r c h a r a c t e r i s t i c s which w i l l promote the a c q u i s i t i o n of s p e c i e s s p e c i f i c d i e t s . For example, i t i s not c l e a r whether t r o u t and kokanee i n the f i e l d a c q u i r e s p e c i e s s p e c i f i c d i e t s because they s e a r c h f o r prey i n d i f f e r e n t l o c a t i o n s , s e a r c h f o r prey a t d i f f e r e n t t i m e s , s e a r c h f o r prey by using d i f f e r e n t t e c h n i q u e s , respond t o d i f f e r e n t prey once d e t e c t e d , approach o r ca p t u r e prey by us i n g d i f f e r e n t t e c h n i q u e s , o r choose to i n g e s t o r r e j e c t d i f f e r e n t prey once c a p t u r e d . The p o t e n t i a l e x i s t s f o r any one o r a l l o f these f a c t o r s to c o n t r i b u t e to the observed p a t t e r n s o f prey a c q u i s i t i o n , 41 thus any attempt to t r u l y understand the b a s i s f o r the d i f f e r e n t prey s e t s a c q u i r e d by these p r e d a t o r s i n the f i e l d must e n t a i l a s y s t e m a t i c e n q u i r y i n t o the r o l e t h a t each of these f a c t o r s p l a y . The need f o r a s y s t e m a t i c approach to asses s the mechanisms t h a t favour d i f f e r e n t i a l a c q u i s i t i o n of prey by p r e d a t o r s has not been g e n e r a l l y r e c o g n i z e d by f i e l d e c o l o g i s t s s t u d y i n g the d i e t a r y h a b i t s of a n i m a l s . Having e s t a b l i s h e d the e x i s t e n c e of a p a r t i c u l a r d i e t a r y p a t t e r n many e c o l o g i s t s have r e l i e d upon one o r two sim p l e hypotheses as e x p l a n a t i o n s f o r the p a t t e r n . The most common o f these are (1) t h a t a v a i l a b i l i t y ( i n the sense of p h y s i c a l p r o x i m i t y ) i s the main f a c t o r which determines the s e t of prey o b t a i n e d and i m p l i c i t i n t h i s t h a t p r e d a t o r s a re i n c r e d i b l e g e n e r a l i s t s t h a t s i m p l y accept food items as they appear o r (2) t h a t a c t i v e s e l e c t i o n and avoidance o f food items by p r e d a t o r s c o n t r o l s the s e t of prey a c q u i r e d . T h i s i m p l i e s t h a t p r e d a t o r s are phenomenally choosy about the items t h a t they e a t i n an environment where a g r e a t d i v e r s i t y o f prey are s i m p l y t h e r e f o r the t a k i n g . By themselves, n e i t h e r o f these views has much s c i e n t i f i c m e r i t and t h e i r repeated e x p r e s s i o n w i t h o u t q u a l i f i c a t i o n ( A l l a n , 1942; Houde, 1967; S i e f e r t , 1968; Hutc h i n s o n , 1971; Cody, 1974 and E n g e l , 1976 t o name but a few) has accomplished l i t t l e o t h e r than t o obscure the natu r e of the a c t u a l range o f mechanisms t h a t do c o n t r o l the non-random a c q u i s i t i o n of prey by p r e d a t o r s i n the f i e l d . In p r e v i o u s work ( H y a t t , 1979), I have s t r e s s e d t h a t each phenomenon o f non-random e x p l o i t a t i o n of prey should be con s i d e r e d t o be a f u n c t i o n o f s e v e r a l p o t e n t i a l mechanisms 42 o p e r a t i n g a l o n e o r i n c o n c e r t . Here the p a t t e r n o f p r e d a t o r s p e c i f i c e x p l o i t a t i o n of prey by s i z e w i l l serve as an example. Caddis l a r v a e ( T r i c h o p t e r a ) found i n the d i e t o f t r o u t a re c o n s i d e r a b l y l a r g e r than those e x p l o i t e d by kokanee. The s i m p l e s t i n f e r e n c e t o make i s t h a t t r o u t are capable of h a n d l i n g l a r g e r prey than kokanee. T h i s i n f e r e n c e r e c e i v e s c i r c u m s t a n t i a l support from the f a c t t h a t t o a l l outward appearances t r o u t are h e a v i e r -bodied and have l a r g e r jaws than kokanee of s i m i l a r l e n g t h . How-e v e r , o t h e r hypotheses p r o v i d e competing e x p l a n a t i o n s f o r the same d i e t a r y p a t t e r n . For example, not o n l y are the c a d d i s l a r v a e t h a t t r o u t o b t a i n l a r g e r than those o b t a i n e d by kokanee but they a l s o belong t o s p e c i e s which l i v e p r i m a r i l y i n weed beds along the l a k e s h o r e (Winterbourn, 1971) . The s p e c i e s e x p l o i t e d by kokanee are r e s t r i c t e d t o open sediment areas throughout the l a k e . Thus, the p a t t e r n d e s c r i b e d above may be e x p l a i n e d by i n v o k i n g d i f f e r e n c e s i n h a b i t a t s e l e c t i o n by the p r e d a t o r s which i n t u r n exposes them t o d i f f e r e n t d i s t r i b u t i o n s of prey. S i m i l a r competing hypotheses are a p p l i c a b l e as e x p l a n a t i o n s f o r the s p e c i e s - s i z e p a t t e r n s of m o l l u s c e x p l o i t a t i o n by t r o u t and kokanee. V a r i o u s authors have suggested t h a t the sampling o f both p r e d a t o r s and prey from i d e n t i c a l l o c a t i o n s e l i m i n a t e s t h i s problem but t h i s w i l l o n l y be the case where p r e d a t o r s and prey are r e l a t i v e l y immobile over a p e r i o d o f time t h a t i n c l u d e s the most r e c e n t f o r a g i n g bout. In the p r e s e n t s t u d y , as w e l l as i n many o t h e r s , t h i s assumption i s s e r i o u s l y v i o l a t e d . P r e d a t o r s such as t r o u t and kokanee are h i g h l y m o b i l e , t h u s , c a p t u r e a t a p a r t i c u l a r l o c a t i o n does not guarantee t h a t food items they 43 c o n t a i n were a l s o o b t a i n e d from t h a t l o c a t i o n . The d i f f i c u l t y o f competing hypotheses as e x p l a n a t i o n s f o r a g i v e n d i e t a r y p a t t e r n i s by no means unique t o t h i s s t u d y . Tinbergen (1960) e x p l a i n e d the non-random a c q u i s i t i o n of prey by i n s e c t i v o r o u s b i r d s i n pinewoods as a consequence of the f o r m a t i o n o f a " s p e c i f i c s e a r c h image" f o r p a r t i c u l a r p r e y . By t h i s he meant t h a t b i r d s might l e a r n the key v i s u a l c h a r a c t e r i s t i c s o f a prey a f t e r s e v e r a l encounters w i t h i t and then b e g i n to s e a r c h e x c l u s i v e l y f o r those cues. By c o n c e n t r a t i n g on one prey they would tend t o o v e r l o o k o t h e r foods, e s p e c i a l l y when they were r e l a t i v e l y r a r e . Royama (1970) p o i n t e d out t h a t on the b a s i s of h i s o b s e r v a t i o n s c e r t a i n f e a t u r e s of the b i r d s behaviour were i n c o n s i s t e n t w i t h the search image h y p o t h e s i s and t h a t i d e n t i c a l d i e t a r y p a t t e r n s c o u l d be produced i f the b i r d s possessed p a r t i c u l a r f o r a g i n g movements which would r e s u l t i n a c o n c e n t r a t i o n of t h e i r s e a rch e f f o r t s i n p a r t i c u l a r l o c a t i o n s (see Krebs, 1973 f o r d i s c u s s i o n ) . From these few examples, i t should be c l e a r t h a t f i e l d d e s c r i p t i o n s of d i e t a r y p a t t e r n s are most v a l u a b l e as sources o f hypotheses about the b i o l o g i c a l mechanisms t h a t are i n v o l v e d i n c o n t r o l l i n g prey a c q u i s i t i o n . However a d d i t i o n a l o b s e r v a t i o n a l and e x p e r i m e n t a l s t u d i e s must be the major sources of c r i t i c a l e vidence t o t e s t the m e r i t of p a r t i c u l a r e x p l a n a t i o n s o r i n t e r p r e t a t i o n s of a p a t t e r n . To some i t may seem a t e d i o u s p r o c e s s t o e l i m i n a t e o r i d e n t i f y the b i o l o g i c a l mechanisms a c t u a l l y r e s p o n s i b l e f o r s p e c i f i c p a t t e r n s o f prey e x p l o i t a t i o n , but u n l e s s we understand the u n d e r l y i n g mechanisms i t w i l l be 44 i m p o s s i b l e t o p r e d i c t p a t t e r n s i n many i n s t a n c e s o r t o understand t h e i r s i g n i f i c a n c e i n o t h e r s . In o r d e r to t e s t hypotheses about the f a c t o r s t h a t c o n t r o l the d i e t a r y p a t t e r n s e x h i b i t e d by t r o u t and kokanee i n Marion Lake I w i l l , o v e r the course of the next few c h a p t e r s , examine where t r o u t and kokanee choose t o f o r a g e , when they choose t o f o r a g e , how they search f o r p r e y , how they a t t a c k prey and how e x p e r i e n c e i n en c o u n t e r i n g v a r i o u s prey a l t e r s the p r e d a t o r s f o r a g i n g behaviour on subsequent o c c a s i o n s . SUMMARY 1. Trout and kokanee e x h i b i t s p e c i e s s p e c i f i c d i e t a r y p a t t e r n s on both an annual and a sea s o n a l b a s i s , even though the p r e d a t o r s were c o l l e c t e d s i m u l t a n e o u s l y from the same h a b i t a t s and a p p a r e n t l y had access t o an i d e n t i c a l s e t o f food items ( F i g . 2 ) . 2. P a t t e r n s o f prey e x p l o i t a t i o n t h a t a r e s p e c i f i c t o e i t h e r t r o u t o r kokanee are f r e q u e n t l y r e l a t e d t o d i f f e r e n c e s i n morphology and e s p e c i a l l y t o d i f f e r e n c e s i n the s i z e s o f prey ( F i g s . 4 t o 7 ) . 3 . P r e d a t o r s p e c i f i c d i e t a r y p a t t e r n s p e r s i s t even w i t h r e s p e c t t o prey types t h a t have many f e a t u r e s i n common (e.g. the two s p e c i e s of amphipods, Table 4 , F i g . 5; v a r i o u s s p e c i e s o f m o l l u s c s , F i g . 6 ; the many s p e c i e s of c h i r o n o m i d s ) . 45 4. D i e t a r y d i f f e r e n c e s between t r o u t and kokanee are not s i m p l y r e l a t e d to d i f f e r e n c e s i n the r e l a t i v e s i z e s of the p r e d a t o r s . 5. Trout and kokanee e x h i b i t pronounced p a t t e r n s o f " d e n s i t y independent" e x p l o i t a t i o n o f prey from the t o t a l complex of prey t h a t i s a p p a r e n t l y a v a i l a b l e i n the n a t u r a l environment ( F i g s . 8 and 9 ) . 6. F i e l d d e s c r i p t i o n s o f d i e t a r y p a t t e r n s are most v a l u a b l e as sources of hypotheses about the b i o l o g i c a l mechanisms t h a t are i n v o l v e d i n c o n t r o l l i n g prey a c q u i s i t i o n . 7 . Because each p a t t e r n o f non-random, p r e y - a c q u i s i t i o n may be favoured by a v a r i e t y o f b i o l o g i c a l mechanisms t h a t may a c t alone o r t o g e t h e r , t h e r e i s g e n e r a l l y a need to s y s t e m a t i c a l l y a s s e s s the i d e n t i t y and r e l a t i v e importance of the p a r t i c u l a r mechanisms t h a t are r e s p o n s i b l e f o r promoting a g i v e n d i e t a r y p a t t e r n . CHAPTER 3 THE RELATIONSHIP BETWEEN SPATIAL SEGREGATION, TEMPORAL SEGREGATION, AND DIETARY PATTERNS of TROUT AND KOKANEE IN MARION LAKE INTRODUCTION Schoener (1974) has suggested t h a t s e p a r a t i o n by h a b i t a t i s the most f r e q u e n t form of e c o l o g i c a l s e g r e g a t i o n i n t e r r e s t r i a l communities and he p o i n t e d out t h a t r e l a t i v e l y l i t t l e d a t a are a v a i l a b l e to reach f i r m c o n c l u s i o n s about p a t t e r n s o f e c o l o g i c a l s e g r e g a t i o n i n a q u a t i c communities. Some authors ( L a r k i n , 1956; Chapman, 1966) have s t r e s s e d t h a t f r e s h w a t e r f i s h e s o f the north-temperate zone e x h i b i t r e l a t i v e l y wide t o l e r a n c e o f h a b i t a t type and thus should d i s p l a y e x t e n s i v e o v e r l a p w h i l e o t h e r s ( N i l s s o n , 1960, 1967; Keast, 1970; Moyle, 1973; Werner e t a l . , 1977) have observed w e l l developed s p a t i a l s e g r e g a t i o n along g r a d i e n t s of d e p t h , v e r t i c a l h e i g h t i n the water column, and v e g e t a t i o n s t r u c t u r e . Because many of the prey e x p l o i t e d by t r o u t and kokanee e x h i b i t c l o s e a s s o c i a t i o n s w i t h s p e c i f i c h a b i t a t s i n Marion Lake (eg. water column, deep -water-sediments, s h a l l o w - w a t e r sediments, weed b e d s ) , h a b i t a t s e g r e g a t i o n by the p r e d a t o r s i s the f i r s t mechanism which may o p e r a t e t o produce s p e c i e s s p e c i f i c d i e t a r y p a t t e r n s . T h e r e f o r e the f i r s t h y p o t h e s i s t h a t I have attempted t o t e s t i s t h a t s p a t i a l s e g r e g a t i o n (between depths o r s u b - h a b i t a t s ) i s 47 r e s p o n s i b l e f o r the s p e c i e s s p e c i f i c p a t t e r n s o f prey e x p l o i t a t a t i o n e x h i b i t e d by t r o u t and kokanee. Park (1941) f i r s t developed the i d e a t h a t a c t i v i t y peaks o c c u r r i n g a t d i f f e r e n t times o f a 24 hour day can produce a type of symmetry i n animal communitites, some s p e c i e s being n o c t u r n a l , some d i u r n a l , and o t h e r s c r e p u s c u l a r . Temporal p a t t e r n s o f a c t i v i t y a r e known t o vary w i d e l y f o r both marine (Hobson, 1972) and f r e s h -water f i s h ( C a r l a n d e r & C l e a r y , 1949; K e a s t , 1970; Emery, 1973; Engel & Magnuson, 1976). A q u a t i c i n v e r t e b r a t e s a l s o e x h i b i t con-s i d e r a b l e v a r i a b i l i t y i n d i e l a c t i v i t y p a t t e r n s (Moon, 1940; Waters, 1962; E l l i o t , 1968) . Work w i t h p r e d a t o r y v e r t e b r a t e s i n t e r r e s t r i a l systems ( B i d e r , 1962) i n d i c a t e s t h a t they have a c t i v i t y c y c l e s which c o i n c i d e w i t h those of t h e i r major pr e y . A study by Baumann & K i t c h e l l (1974) l e a v e s no doubt t h a t the t i m i n g of p r e d a t o r and i n v e r t e b r a t e a c t i v i t y c y c l e s can i n f l u e n c e the p a t t e r n s o f prey a c q u i s i t i o n by f i s h . They found t h a t no Ephemeroptera (mayfly nymphs) were p r e s e n t i n samples of b l u e g i l l s (Lepomis macrochirus) taken a t 1600 hours but t h a t the nymphs had become the most important food item i n b l u e g i l l s o b t a i n e d a t 0700 hours. T h i s p a t t e r n corresponded t o the o b s e r v a t i o n t h a t mayfly nymphs found i n the l a k e were predo m i n a n t l y a c t i v e a t n i g h t . Because of the p o t e n t i a l importance of a c t i v i t y c y c l e s , the second h y p o t h e s i s c o n s i d e r e d i n t h i s c h a p t e r i s t h a t d i f f e r e n c e s i n the temporal sequence of p r e d a t o r and prey a c t i v i t i e s produce d i f f e r e n c e s i n the food items found i n t r o u t , kokanee and the environment. METHODS P a t t e r n s o f H a b i t a t Occupation Depth D i s t r i b u t i o n I recorded the d i s t r i b u t i o n o f f i s h i n Marion Lake i n 120 hours of d i v i n g d u r i n g the months of May, June, J u l y and August of 1971 and 1972. Salmonids i n Marion Lake g e n e r a l l y a v o i d an a c t i v e l y moving d i v e r but i g n o r e a s t a t i o n a r y one, t h u s , most d i u r n a l o b s e r v a t i o n s were c a r r i e d out at s p e c i f i c s t a t i o n s ( F i g . 1 1 ) . I made o b s e r v a t i o n s throughout the day (0500-2100 hours) and each s t a t i o n r e c e i v e d e q u a l e f f o r t d u r i n g any one s e r i e s o f o b s e r v a t i o n s . Each s t a t i o n was equipped w i t h h o r i z o n t a l and v e r t i c a l p l a s t i c , r e f e r e n c e - m a r k e r s graduated a t 5 cm i n -t e r v a l s . H o r i z o n t a l markers e n c l o s e d a one square meter area and a f o u r square meter a r e a on the l a k e bottom. V e r t i c a l markers extended from d i a g o n a l l y opposed c o r n e r s o f t h i s bottom g r i d t o the s u r f a c e . A d i v e r p o s i t i o n e d a t the s u r f a c e c o u l d e s t i m a t e the s i z e , s p e c i e s i d e n t i t y and v e r t i c a l p o s i t i o n o f those f i s h p a s s i n g between and over the v a r i o u s markers. S t a t i o n s i n c l u d e d depths of 1, 2, 3, and 4 m of water and t h e r e f o r e encompassed 4, 8, 12 & 16 c u b i c meters of water. The dimensions of the l a r g e s t s t a t i o n (2 X 2 X 4 m) were s m a l l enough t h a t under the most u n f a v o r a b l e c o n d i t i o n s o f low l i g h t and h i g h t u r b i d i t y , a l l f i s h p a s s i n g over and between the r e f e r e n c e markers c o u l d be counted and i d e n t i f i e d A contour map o f Marion Lake i n d i c a t i n g the l o c a t i o n s o f both t r a p and o b s e r v a t i o n s i t e s . A, B, C and D r e p r e s e n t the 1, 2, 3 and 4 meter o b s e r v a t i o n s t a t i o n s r e s p e c t i v e l y . H and I r e p r e s e n t s u r f a c e o b s e r v a t i o n g r i d s . N and S r e p r e s e n t n o r t h and south t r a p - n e t l o c a t i o n s r e s p e c t i v e l y . L a k e O u t l e t vo 0» t o s p e c i e s . T h i s ensured t h a t the v i s u a l s i g h t i n g s would not be i n f l u e n c e d by d i u r n a l o r sea s o n a l changes i n the d e t e c t a b i l i t y o f f i s h . The o b s e r v a t i o n procedure c o n s i s t e d of a d i v e r moving i n t o p o s i t i o n a t the s u r f a c e , above a s t a t i o n , then w a i t i n g f o r . f i v e minutes to compensate f o r any d i s t u r b a n c e o f animals i n the v i c i n i t y , and then r e c o r d i n g a st a n d a r d s e t o f o b s e r v a t i o n s f o r twenty minutes. I recorded o b s e r v a -t i o n s by hand on a p l e x i g l a s s s l a t e i n 1971 and used a f o u r -channel event r e c o r d e r (Rustrak) m o d i f i e d f o r underwater o p e r a t i o n , f o r a c o n t i n u o u s r e c o r d of o b s e r v a t i o n s , i n 1972. I scored the v e r t i c a l p o s i t i o n o f each f i s h s i g h t e d i n 1971 to the n e a r e s t 5 cm i n t e r v a l , w h i l e i n 1972 I d e s i g n a t e d animals as s u r f a c e ( w i t h i n 50 cm o f the water s u r f a c e ) , bottom ( w i t h i n 50 cm o f the bottom), o r midwater. D i s t r i b u t i o n With Area The l o c a t i o n o f underwater s t a t i o n s made i t p o s s i b l to d e t e c t some changes i n the d e p t h - h a b i t a t d i s t r i b u t i o n s o f f i s h over the course of the summer. Two t r a p s ( f y k e nets w i t h 6mm mesh and 17 m l e a d s ) were operated a t i n t e r v a l s through the p e r i o d May,1974 - May,1975. One t r a p was l o c a t e d i n 1-2 m of water a t the south end of the l a k e and the o t h e r i n 1-2 m of water a t the n o r t h end of the l a k e ( F i g 1 1 ) . , The t r a p s were i d e n t i c a l i n every r e s p e c t o f c o n s t r u c t i o n and o p e r a t i o n . Thus, I accepted d i f f e r e n c e s i n the p r o p o r -t i o n s o f animals caught a t the two l o c a t i o n s as i n d i c a t i v e 51 o f d i f f e r e n c e s i n s p a t i a l d i s t r i b u t i o n . In a d d i t i o n t o the t r a p p i n g and underwater o b s e r v a t i o n s , I spent i n excess of 100 hours i n a boat equipped w i t h l i g h t s , c o l l e c t i n g f i s h from a l l areas o f the l a k e a t n i g h t . Although these o b s e r v a t i o n s are not q u a n t i t a t i v e , my g e n e r a l i m p r e s s i o n of the d i s t r i b u t i o n o f the p r e d a t o r s a t these times i s i n agreement w i t h the d i s t r i b u t i o n s i n d i c a t e d by q u a n t i t a t i v e sampling t e c h n i q u e s . P a t t e r n s o f A c t i v i t y Two c o n d i t i o n s must be met b e f o r e a c t i v i t y c y c l e s w i l l s t r o n g l y i n f l u e n c e d i f f e r e n t i a l prey e x p l o i t a t i o n by t r o u t and kokanee. Trout and kokanee must e x h i b i t markedly d i f f e r e n t a c t i v i t y p a t t e r n s (eg. n o c t u r n a l vs d i u r n a l ) and the a c t i v i t y o f some prey groups must c o i n c i d e w i t h the a c t i v i t y o f one s p e c i e s o f p r e d a t o r more c l o s e l y than w i t h the o t h e r . Because t h e r e was no i n f o r m a t i o n c o n c e r n i n g the a c t i v i t y c y c l e s of e i t h e r f i s h o r i n v e r t e b r a t e s i n Marion Lake, i t was most e f f i c i e n t t o t e s t f o r d i f f e r e n c e s i n the a c t i v i t y p a t t e r n s o f the p r e d a t o r s as a p r e r e q u i s i t e t o any examination o f prey a c t i v i t y c y c l e s . I a n a l y z e d d a t a from g i l l - n e t c a p t u r e s o f f i s h completed between 1963 and 1966. Monofilament g i l l - n e t s had been p l a c e d i n 1-4 m of water near the e a s t e r n edge of the l a k e to c a t c h f i s h . The nets c o n t a i n e d s e c t i o n s o f three mesh s i z e s (2.5, 3.75 and 5.0 cm). The nets were examined every two hours f o r a t w e n t y - f o u r hour p e r i o d so t h a t changes 0 i n " a c t i v i t y " c o u l d be d e t e c t e d . A t w e n t y - f o u r hour s e t was completed f o r each of February 1964, A p r i l 1966, May 1964, August 1963 and November 1963. D i r e c t v i s u a l o b s e r v a t i o n s from f i x e d s t a t i o n s i n the l a k e p r o v i d e d d a t a to check t h a t a c t i v i t y as measured by v u l n e r a b i l i t y t o g i l l - n e t s i s a r e f l e c t i o n o f f o r a g i n g a c t i v i t y r a t h e r than o f some o t h e r f a c t o r (eg. changes i n a b i l i t y t o v i s u a l l y d e t e c t and a v o i d g i l l - n e t s under c o n d i t i o n s o f changing l i g h t ) . Prey P r e l i m i n a r y e x a m i n a t i o n of d a t a from g i l l - n e t c a p t u r e s i n d i c a t e d a tendency f o r kokanee to be more a c t i v e than t r o u t d u r i n g the l a t e evening and n i g h t . T h i s suggested t h a t d i f f e r e n c e s i n a c t i v i t y c y c l e s might w e l l produce some of the d i f f e r e n c e s i n prey e x p l o i t a t i o n by f i s h i f a c t i v i t y l e v e l s o f some prey groups p a r a l l e l e d t h a t o f e i t h e r t r o u t o r kokanee more c l o s e l y . Chironomid pupae are an i m p o r t a n t item i n the d i e t s o f t r o u t and kokanee and they are o b t a i n e d by the p r e d a t o r s d u r i n g times of a d u l t emergence from the l a k e . The emergence of a d u l t s from pupae seemed t o e x h i b i t pronounced d i e l v a r i a t i o n s , t h u s , d i f f e r e n c e s , i n ex-p l o i t a t i o n o f pupae by t r o u t and kokanee (Table 2.) seemed a l i k e l y p a t t e r n t o be produced by asynchronous a c t i v i t y c y c l e s . To t e s t t h i s p o s s i b i l i t y , I took samples t o document the temporal sequence o f pupal emergence on s e l e c t e d dates d u r i n g 1972. A bow-amounted net (opening 1 m X lm, mesh s i z e 200ym) was pushed along j u s t below the s u r f a c e i n f r o n t o f a boat f o r 1 minute. Three r e p l i c a t e h a u l s were taken a t each t i m e . Samples were p r e s e r v e d i n 70% e t h a n o l and s o r t e d a t a l a t e r d a t e . RESULTS Depth D i s t r i b u t i o n I completed s i x s e t s o f o b s e r v a t i o n s on the depth d i s t r i b u t i o n s (bottom, midwater, s u r f a c e ) o f f i s h d u r i n g 1971-72 (Table 7 ) . Because temperature s t r o n g l y i n f l u e n c e s h a b i t a t s e l e c t i o n by the p r e d a t o r s ( H y a t t , Ms), I have de-s i g n a t e d o b s e r v a t i o n s as e i t h e r e a r l y summer o r l a t e summer based upon the range of s u r f a c e temperatures encountered. The depth d i s t r i b u t i o n s o f t r o u t (Table 7a & b) are s i g n i f i c a n t l y d i f f e r e n t between a l l s e t s o f o b s e r v a t i o n s (p<.001, d f = 2, X f o r two independent samples, S i e g e l , 1956) w h i l e the depth d i s t r i b u t i o n s o f kokanee are not s i g n i -f i c a n t l y d i f f e r e n t w i t h i n e i t h e r e a r l y o r l a t e summer ob s e r v a -t i o n s , but are s i g n i f a n t l y d i f f e r e n t between these p e r i o d s ( \, , P < .001, d f = 2 ) . Due t o the s m a l l numbers o f kokanee s i g h t e d d u r i n g some s e t s o f o b s e r v a t i o n s , I pooled the d a t a o b t a i n e d under e a r l y summer c o n d i t i o n s and those f o r l a t e summer c o n d i t i o n s t o t e s t f o r d i f f e r e n c e s i n depth d i s t r i -b u t i o n s o f t r o u t compared w i t h kokanee. Depth d i s t r i b u t i o n s TABLE 7. Depth d i s t r i b u t i o n s o f t r o u t and kokanee B = bottom, M = midwater, S = s u r f a c e and N = the t o t a l number of s i g h t i n g s . a. E a r l y Summer Date and Range of s u r f a c e temperature May 2 6 - J u l y 6/71 10-17 °C May 24-31/72 10-16 *C June 8-28/72 2 12-17 °C A l l dates pooled Trout K o k a n e e ; B % M % S N % B % M % S N 73 23 4 306 9 24 67 303 12 44 44 363 17 50 33 6 43 35 22 113 13 17 70 23 41 34 25 782 9 24 67 332 b. Late Summer J u l y 29-Aug.l/71 23-27 °C J u l y 18-28/72 19-25 °C Aug.3-11/72 19-23 °C A l l dates pooled 89 10 1 582 74 25 1 276 31 60 9 233 100 62 28 10 1189 72 24 4 25 66 26 8 2004 76 23 1 316 of trout and kokanee d i f f e r s i g n i f i c a n t l y (X2, p <.001, df =2) under both early and late summer conditions. In early summer kokanee occupy primarily surface habitats while trout occupy bottom ones. In late summer there are more trout than kokanee in surface habitats although qu a l i t a t i v e l y trout and kokanee exhibit similar depth distributions (Table 7b). Distribution With Area Proportions of trout and kokanee observed at under-water stations were not equal (Fig .12). Under early summer conditions in both 1971 and 1972 I observed the majority of trout at shallow (1-2 m) , inshore stations, while the greatest number of kokanee were always observed at the deep (3-4m) stations farthest offshore. Thus, trout and kok.anee exhibit a remarkable degree of segregation. An increase in overlap of areas occupied by the two species under late summer conditions (Fig. 12c) i s largely the consequence of a pronounced seasonal change in areas occupied by trout. Under early summer conditions, fewer than 30% of a l l trout observed occurred at offshore stations but under late summer conditions 48% of a l l trout occurred there (compare f i g . 12a with 12c). Differences in the proportions of f i s h detected at various locations were not restricted only to stations in different depths of water. The north and south trap-nets were located at similar depths (1.5m) and there were s i g n i -ficant differences in the proportions of predators captured 56 F i g u r e 12 A comparison of the r e l a t i v e p r o p o r t i o n s o f t r o u t and kokanee observed a t onshore ( s t a t i o n s A and B) and o f f s h o r e ( s t a t i o n s C and D) l o c a t i o n s i n Marion Lake. N = the t o t a l number of f i s h observed d u r i n g a g i v e n s e t of o b s e r v a t i o n s . Numbers i n b r a c k e t s i n d i c a t e the range of l a k e s u r f a c e temperatures a t t a i n e d d u r i n g each o b s e r v a t i o n s e t . OBSERVATION STATION LOCATIONS AND THEIR ASSOCIATED WATER COLUMN DEPTHS (m) > CO o o I I I I - • • " . M . » . o CD O o ro O ' CD co Ul H 33 O c z c CD rn w ro o o CO o ro o ->i -CD - M " 2 £ CD £ o m m > co I i - . ro o i OJ o i CD O o ro o *_ o ro o o o co o Z —I II 33 OJ p ro 5 co - i 2 s? 1 cr CD Z X »• 5 * i m m Ul i n these n e t s d u r i n g a number of sampling i n t e r v a l s (Table 8a & b ) . In g e n e r a l kokanee were more abundant a t the n o r t h end of the l a k e . The s i n g l e e x c e p t i o n t o t h i s t r e n d o c c u r r e d under l a t e summer c o n d i t i o n s when the south net c a p t u r e d more kokanee than the n o r t h n e t . I a t t r i b u t e t h i s t o the presence o f a few hundred kokanee a t t h i s t i m e , i n a s p r i n g l e s s than 50 m away from the south n e t . P a t t e r n s f o r t r o u t are not as c l e a r . On two o c c a s i o n s t h e r e were more t r o u t captured a t the n o r t h net and on two o t h e r s t h e r e were no s i g n i f i c a n t d i f f e r e n c e s (Table 8b). D i e l A c t i v i t y P a t t e r n s o f P r e d a t o r s G i l l - n e t c a p t u r e s over 24 hours suggest t h a t n e i t h e r t r o u t o r kokanee are s t r i c t l y n o c t u r n a l , d i u r n a l o r c r e p u s c u l a r ( F i g . 1 3 ) . When c a p t u r e s are summed over a l l seasons, t h e r e i s c l e a r l y a t r e n d f o r kokanee to be most " a c t i v e " a t n i g h t and f o r t r o u t to be most " a c t i v e " i n the day. Between season comparisons ( F i g . 13a & b) suggest s i m i l a r a c t i v i t y p a t t e r n s f o r kokanee but a s h i f t from morning t o l a t e a f t e r n o o n i n peak a c t i v i t y by t r o u t . I t i s n e c e s s a r y t o examine two c r i t i c a l assumptions b e f o r e a t t e m p t i n g any i n t e r p r e t a t i o n o f the s i g n i f i c a n c e of these r e s u l t s f o r p a t t e r n s o f prey a c q u i s i t i o n . These are t h a t v u l n e r a b i l i t y t o c a p t u r e d u r i n g the d a y l i g h t hours i s s i m i l a r to t h a t under n o c t u r n a l c o n d i t i o n s and t h a t " a c t i v i t y " as measured by g i l l - n e t c a p t u r e s i s a r e f l e c t i o n o f f o r a g i n g a c t i v i t y r a t h e r than some o t h e r type of a c t i v i t y . 58 TABLE 8 North t r a p (Nth) v e r s u s South t r a p (Sth) Captures o f t r o u t and kokanee 1974-75 I n t e r v a l June 14 J u l y 17 Oct. 19 May 5 J u l y 17 - J u l y 30 -Nov..6 - May 26 Range of s u r f a c e temperatures C No of t r a p p i n g days 10-17 29 19-23 13 6-11 18 5-12 20 a. Kokanee No. captured % captured Z ( b i n o m i a l t e s t , S i e g e l , 1956) Nth Sth Nth Sth Nth Sth Nth Sth 69 70 29 30 3.98 ** 6 13 41 87 - 5.00 ** 83 71 34 29 - 4.44 ** 25 81 6 19 - 3 . 2 1 ** b. Trout No. cap t u r e d % captured Z s t a t i s t i c Nth Sth Nth Sth Nth Sth Nth Sth 383 257 60 40 - 4.96 ** 96 41 131 136 70 30 49 51 4.58 ** - 0 . 2 4 NS 82 53 72 47 - 0.73 NS ** s i g n i f i c a n t a t the .01 l e v e l NS not s i g n i f i c a n t A comparison of the r e l a t i v e p r o p o r t i o n s o f t r o u t and kokanee n e t t e d a t two hour i n t e r v a l s , over twenty-four hour p e r i o d s d u r i n g (a.) e a r l y summer (b.) l a t e summer and (c.) pooled over a l l d a t e s . A n e t t i n g s e r i e s was completed d u r i n g each o f Feb. 1964, A p r i l , 1966; May, 1964; Aug., 1963; and Nov., 1963. N = the t o t a l number of t r o u t o r kokanee captured d u r i n g a p a r t i c u l a r n e t t i n g s e r i e s . 59a O KOKANEE N = I30 © TROUT N = I36 a. EARLY SUMMER (MAY-JUNE) Q LxJ 2 -\-h-UJ z: CO 26 -or L±J 2 2 -m 18-Z> z AL 1 0 -h-O 6 -h-u_ 2 -o 22 -1 8 -1 4 -1 0 -6 -2 -O N=277 © N =58 b. L A T E SUMMER (AUGUST) O N = 6 11 @ N = 284 —1 1 1 1 1 T c. TOTAL OVER ALL SEASONS i 1 r —i 1 1 n 1 1 1 n 1 r 1 r 4 - 6 8-10 12-2 4 - 6 8-10 12-2 -A. M- P.M. A. M. I f d i f f e r e n c e s i n day and n i g h t v u l n e r a b i l i t y are r e l a t e d t o some f a c t o r o t h e r than f i s h a c t i v i t y (eg. the a b i l i t y t o v i s u a l l y d e t e c t and a v o i d the n e t s ) , I expected t h a t more i n d i v i d u a l s of both s p e c i e s would be captu r e d a t n i g h t than i n the day. S i n c e t h i s i s not the c a s e , I suggest t h a t c a p t u r e s do r e f l e c t d i e l changes i n a c t i v i t y . I can not immediately c o n f i r m the second assumption and a study of f i s h by D a r n e l l and M e i e r o t t o (1965) suggests t h a t i t may be i n c o r r e c t . They found t h a t f e e d i n g a c t i v i t y , as measured by the volume of food p r e s e n t i n the g u t , d i d not c o i n c i d e w i t h the p e r i o d s o f maximum "swimming" a c t i v i t y " as measured by the v u l n e r a b i l i t y o f f i s h t o ca p t u r e i n t r a p s . The d a t a c o l l e c t e d by d i r e c t o b s e r v a t i o n o f t r o u t and kokanee a c t i v i t y a r e l i m i t e d t o o n l y a p o r t i o n o f the d i e l c y c l e , but they a re l e s s s u b j e c t t o the assumptions t h a t l i m i t the u s e f u l l n e s s o f g i l l - n e t , c a p t u r e d a t a . Because I wished t o t e s t the h y p o t h e s i s t h a t d i f f e r e n c e s i n the temporal sequence o f f o r a g i n g a c t i v i t y a l o ne c o u l d a f f e c t prey e x p l o i t a t i o n , the r e s u l t s presented here concern o n l y t r o u t and kokanee t h a t were observed i n the same s u b - h a b i t a t s . T h e r e f o r e comparisons a re l i m i t e d t o t r o u t and kokanee f o r a g i n g i n o f f s h o r e - s u r f a c e , h a b i t a t s d u r i n g e a r l y summer ( F i g . 14 a)i and i n o f f s h o r e - b o t t o m , h a b i t a t s d u r i n g l a t e summer ( F i g . 14b) . D i r e c t o b s e r v a t i o n s i n d i c a t e t h a t i n e a r l y summer both t r o u t and kokanee e x h i b i t maximum a c t i v i t y l e v e l s i n .61 F i g u r e 1 4 . A c o m p a r i s o n o f t h e r e l a t i v e n u m b e r s o f t r o u t a n d k o k a n e e o b s e r v e d a t two h o u r i n t e r v a l s o v e r s i x t e e n h o u r p e r i o d s . ( a . ) t r o u t and k o k a n e e o b s e r v e d i n o f f s h o r e - s u r f a c e h a b i t a t s d u r i n g e a r l y summer ( b . ) t r o u t and k o k a n e e o b s e r v e d i n o f f s h o r e -b o t t o m h a b i t a t s d u r i n g l a t e summer. V e r t i c a l b a r s i n d i c a t e + o n e s t a n d a r d e r r o r o f t h e m e a n s . 61a 5^7 7-9 9 -11 11-1 1-3 3-5 5-7 7-9 A M — — - P M 62 e a r l y morning f o l l o w e d by lower l e v e l s f o r the remainder of the day. In l a t e summer t h e r e a re s h o r t i n t e r v a l s d u r i n g the day when kokanee are more a c t i v e than t r o u t b u t , w i t h the e x c e p t i o n o f the e a r l y evening peak i n a c t i v i t y by kokanee, the a c t i v i t y p a t t e r n s o f the p r e d a t o r s are s i m i l a r . The R e l a t i o n Between D i u r n a l A c t i v i t y and Food Search There i s o f t e n some u n c e r t a i n t y i n the assessment of whether a p r e d a t o r i s a c t i v e l y engaged i n s e a r c h i n g f o r prey (Beukema, 1968; Murdoch e t a l . 1975). T h i s i s because some o f the motor p a t t e r n s e x h i b i t e d by a c t i v e l y s e a r c h i n g animals are a l s o i n c o r p o r a t e d i n t o non-search a c t i v i t i e s ( C u r i o , 1976) and because p r e d a t o r s engaged i n non-search a c t i v i t e s a r e o f t e n i n s t a n t a n e o u s l y r e c e p t i v e t o o p p o r t u n i t i e s f o r c a p t u r e of prey (eg. S c h a l l e r ' s o b s e r v a t i o n s of l i o n s , 1972) . Although i t was i m p o s s i b l e i n the p r e s e n t study t o always d i s t i n q u i s h c l e a r l y between s e a r c h and non-search b e h a v i o u r s , I contend t h a t much of the d i u r n a l a c t i v i t y o f t r o u t and kokanee i n Marion Lake was d i r e c t e d towards prey s e a r c h . T h i s c o n t e n t i o n i s based upon v a r i o u s types o f e v i d e n c e . L a b o r a t o r y work by Beukema (1968) on s t i c k l e b a c k s and my own l a b o r a t o r y o b s e r v a t i o n s i n d i c a t e t h a t f i s h e x h i b i t a r e a d i n e s s t o s e a r c h f o r and a t t a c k prey items as lon g as t h e i r stomachs are not f i l l e d t o c a p a c i t y . Trout and kokanee i n Marion Lake seldom o b t a i n as much as 50% o f the d a i l y r a t i o n they a re capable o f consuming (Sandercock, 1969; H y a t t , unpublished d a t a ) . Thus, hunger ( o p e r a t i o n a l l y e q u a l s f u l l -ness of the gut) should serve as a s t r o n g m o t i v a t i o n f o r the p r e d a t o r s t o s e a r c h c o n t i n u o u s l y f o r p r e y . C e r t a i n l y t r o u t and kokanee i n the f i e l d appeared t o be r e c e p t i v e throughout the d a y l i g h t p e r i o d t o o p p o r t u n i t i e s t o a t t a c k food items. S e l e c t i v e r e s p o n s i v e n e s s t o s t i m u l i i s o f t e n the most important f e a t u r e by which d i f f e r e n t types o f " g o a l o r i e n t e d " b e h a v i o u r s such as food s e a r c h can be c a t e g o r i z e d (Hinde, 1966) . On t h i s b a s i s t o o , d a y l i g h t a c t i v i t i e s of t r o u t and kokanee i n Marion Lake seemed t o be r e l a t e d p r i -m a r i l y t o food s e a r c h s i n c e o v e r t responses by f i s h i n the f i e l d were g e n e r a l l y d i r e c t e d towards s t i m u l i presented by p o t e n t i a l food i t e m s . During d a y l i g h t hours, d i r e c t obser-v a t i o n s a t underwater s t a t i o n s f a i l e d t o r e v e a l more than a n e g l i g i b l e investment of time i n a c t i v i t e s t h a t were not r e l a t e d t o food s e a r c h (eg. r e s t i n g , t e r r i t o r i a l d e f e n s e , a g o n i s t i c i n t e r a c t i o n s , p r e d a t o r a v o i d a n c e ) . T h i s was not due to an i n a b i l i t y t o r e c o g n i z e such b e h a v i o u r s s i n c e non-search a c t i v i t i e s were observed t o i n v o l v e l a r g e investments of time by t r o u t and kokanee d u r i n g o t h e r p o r t i o n s o f the d i e l c y c l e , a t o t h e r l o c a t i o n s i n the l a k e , o r a t o t h e r times o f y e a r . A c t i v i t y P a t t e r n s o f Emerging Chironomids R e s u l t s o f s u r f a c e - n e t h a u l s r e v e a l t h a t chironomid pupae d i s p l a y d i s t i n c t i n t e r v a l s f o r maximum emergence on any g i v e n d a t e , but c o n s i d e r a b l e v a r i a b i l i t y i n times of maximum emergence on d i f f e r e n t d a t e s ( F i g . 15 a, b, c ) . In the May, 18 and June, 1 samples, the peaks o f emergence a c t i v i t y o c c u r r e d from l a t e evening to n i g h t . The peak o f a c t i v i t y f o r the much s m a l l e r June, 9 emergence o c c u r r e d i n l a t e a f t e r n o o n . DISCUSSION I have p r e v i o u s l y e s t a b l i s h e d t h a t t r o u t and kokanee e x h i b i t o n l y a modest degree o f d i e t a r y o v e r l a p (Chapter 2) and t h a t major d i f f e r e n c e s e x i s t between the apparent a v a i l a b i l i t y o f prey i n the l a k e and t h e i r u t i l i z a t i o n by p r e d a t o r s . Given the p a t t e r n s o f h a b i t a t o c c u p a t i o n and a c t i v i t y o f t r o u t , kokanee and some of t h e i r major prey, i t i s apparent t h a t s p a t i a l s e g r e g a t i o n and t o a l e s s e r e x t e n t temporal s e g r e g a t i o n may p l a y a r o l e i n shaping the observed p a t t e r n s o f prey e x p l o i t a t i o n . The Role of S p a t i a l and Temporal S e g r e g a t i o n i n Producing D i e t a r y D i f f e r e n c e s Between Trout and Kokanee L a r v a l odonates, ephemeropterans and s n a i l s con-s t i t u t e major foods of t r o u t i n e a r l y summer (37% by wt. i n A p r i l and 12% by wt. i n June/66), but are b a r e l y r e p r e -sented i n the d i e t o f kokanee (absent i n A p r i l and o n l y 2% by wt. i n June/66) a t t h i s time ( E f f o r d & Tsumura, 1973). Because the m a j o r i t y of these prey a re found a t depths of l e s s than 2 m i n Marion Lake (Table 9 ) , I suggested t h a t d i f f e r e n t i a l e x p l o i t a t i o n by t r o u t and kokanee might be based 65 FIGURE 15. The mean number of chironomid pupae obtained in surface net hauls taken between early afternoon and late evening on three separate dates. Numbers in brackets indicate the range of three replicate samples. AVERAGE NUMBER OF CHIRONOMID PUPAE OBTAINED PER NET HAUL 66 TABLE 9 A comparison o f the r e l a t i v e abundance i n e a r l y summer ( A p r i l - J u n e ) o f prey a t i n s h o r e (depths <2 m) and o f f s h o r e (depths >3 m) l o c a t i o n s i n Marion Lake Abundance expressed as % of t o t a l numbers on a m b a s i s Prey Group Inshore O f f s h o r e Data Source Chironomid pupae 70 0 303 1* H a m i l t o n , 1965 Chironomid l a r v a e 50-60 40-50 Ha m i l t o n , 1965 McCauley, p e r s . comm. Amphipods 82-90 10-18 M a t h i a s , 1971 E f f o r d , unpubl. T r i c h o p t e r a n l a r v a e 50 50 W i n t e r b o u r n , 1971 P l a n o r b i d s n a i l s 80 20 D e l u r y , 1971. Odonate l a r v a e 87 13 2 * I n f e r r e d from the d i s t r i b u t i o n Ephemeropteran l a r v a e 87 13 of s u i t a b l e cover I have c o n s i d e r e d o n l y the more abundant s p e c i e s o f chironomids which have completed 50% of t h e i r emergence by the end o f May and/or a l l emergence by the end of June. Hamilton's d a t a f o r 0 - 1.9 m and 4-6 m were used t o r e p r e s e n t i n s h o r e and o f f s h o r e areas r e s p e c t i v e l y . ^* There are no good q u a n t i t a t i v e d a t a c o n c e r n i n g the d i s t r i b u t i o n of immature odonates o r ephemeropterans. However, both of these groups are most abundant i n c l o s e a s s o c i a t i o n w i t h the cover o f f e r e d by weed-beds ( P e a r l s t o n e , 1971 and p e r s o n a l o b s e r v a t i o n s ) . T h e r e f o r e , I have used the p a t t e r n s o f weed-bed, d e p t h - d i s t r i b u t i o n ( D a v i e s , 1970; N e i s h , 1971) t o i n f e r the abundance of these two groups. i n p a r t upon some form o f h a b i t a t s e g r e g a t i o n between the p r e d a t o r s . R e s u l t s from t h i s study l e n d s t r o n g support t o t h i s h y p o t h e s i s . Undoubtedly the o c c u p a t i o n o f o f f s h o r e areas by the m a j o r i t y o f kokanee and of i n s h o r e areas by t r o u t c r e a t e s d i f f e r e n t o p p o r t u n i t i e s f o r e x p l o i t a t i o n o f these p r e y . Even f o r those few kokanee t h a t do forage i n i n s h o r e a r e a s , the o c c u p a t i o n o f p r i m a r i l y midwater and s u r f a c e h a b i t a t s (Table 7) w i l l reduce the p r o b a b i l i t y o f e n c o u n t e r i n g bottom prey such as odonates and s n a i l s . By c o n t r a s t , the c o n c e n t r a t i o n o f t r o u t i n p o s i t i o n s c l o s e to the bottom w i l l f avour d e t e c t i o n and e x p l o i t a t i o n o f these prey types i n e a r l y summer. The o c c u p a t i o n o f o f f s h o r e areas by kokanee and of i n s h o r e areas by t r o u t p r o v i d e s a r e s o n a b l e e x p l a n a t i o n f o r s i g n i f i c a n t d i f f e r e n c e s i n e x p l o i t a t i o n o f o t h e r prey types as w e l l . Over the course of the year t r o u t e x p l o i t more t e r r e s t r i a l i n s e c t s than kokanee ( F i g . 2, Tables 3 and 5) and t r o u t alone e x p l o i t n o t o n e c t i d s (backswimmers) and c o r i x i d s (water boatmen). N o t o n e c t i d s and c o r i x i d s are known to be r e s t r i c t e d t o i n s h o r e h a b i t a t s i n the l a k e ( L o v e l y , 1972) and i t i s l i k e l y t h a t t e r r e s t r i a l i n s e c t s w i l l be most abundant t h e r e . D i e t s of the s m a l l e s t t r o u t and kokanee examined i n t h i s study c o n t a i n e d l a r g e numbers o f zooplankton (Table 5) but t r o u t e x p l o i t e d the c l a d o c e r a n S i d a c r y s t a l l i n a almost e x c l u s i v e l y . Kokanee took r e l a t i v e l y modest numbers of S i d a sp. along w i t h many o t h e r s p e c i e s o f z o o p l a n k t o n such as Leptodora sp. T h i s d i f f e r e n c e may be t r a c e d t o the c l o s e a s s o c i a t i o n o f S i d a sp. w i t h l i t t o r a l zone beds of l i l y p a d s w h i l e s p e c i e s such as Leptodora sp. a r e found o n l y i n the water column o f o f f s h o r e areas ( H y a t t / unpublished d a t a ) . Some d i f f e r e n c e s i n the s i z e - f r e q u e n c y d i s t r i b u t i o n s o f prey used by t r o u t and kokanee most l i k e l y have t h e i r source i n h a b i t a t s e g r e g a t i o n . For example, kokanee l i k e l y e x p l o i t the s m a l l e r s p e c i e s o f m o l l u s c s ( P i s i d i u m spp.) r a t h e r than l a r g e r p l a n o r b i d s n a i l s because the l a t t e r a re not found i n a p p r e c i a b l e numbers i n the o f f s h o r e l o c a t i o n s o c c u p i e d by kokanee ( D e l u r y , 1971) . S i n c e the m a j o r i t y o f t r o u t and kokanee t h a t I observed d u r i n g the day were c o n t i n u o u s l y engaged i n s e a r c h i n g f o r p r e y (see d e s c r i p t i o n s i n c h a p t e r 4 ) , i t i s u n l i k e l y t h a t d i f f e r e n c e s i n the presence o r absence o f p a r t i c u l a r prey s p e c i e s i n f i s h d i e t s are due t o d i f f e r e n c e s i n d i u r n a l a c t i v i t y l e v e l s of the p r e d a t o r s o r t h e i r p r e y . However, asynchronous a c t i v i t e s may combine w i t h p a t t e r n s o f h a b i t a t s e g r e g a t i o n by the p r e d a t o r s to produce some d i e t a r y d i f f -e r e n c e s . For example the g r e a t e r e x p l o i t a t i o n o f chi r o n o m i d pupae by kokanee as compared t o t r o u t ( F i g . 3) i s c o n s i s t e n t w i t h the former p r e d a t o r s g r e a t e r o c c u p a t i o n o f midwater and s u r f a c e h a b i t a t s where pupae are most v u l n e r a b l e to d e t e c t i o n d u r i n g emergence. In a d d i t i o n the tendency f o r kokanee t o e x h i b i t h i g h e r a c t i v i t y l e v e l s than t r o u t i n the l a t e evening and a t n i g h t combined w i t h emergence peaks by chironomid pupae a t these times ( F i g . 15) c o u l d c e r t a i n l y c o n t r i b u t e to the g r e a t e r e x p l o i t a t i o n o f pupae by kokanee d u r i n g some months of the y e a r . However, i n g e n e r a l the evidence s u p p o r t i n g the i d e a t h a t asynchronous f o r a g i n g a c t i v i t e s by t r o u t , kokanee and t h e i r prey p l a y a major r o l e i n c o n t r o l l i n g d i e t a r y d i f f e r e n c e s e x h i b i t e d by the p r e d a t o r s i s much l e s s c o n v i n c i n g than the evidence s u g g e s t i n g h a b i t a t s e g r e g a t i o n as a major mechanism f a v o u r i n g food r e s o u r c e p a r t i t i o n i n g . The Role o f S p a t i a l S e g r e g a t i o n i n Producing D i f f e r e n c e s Between the P r o p o r t i o n s o f Prey Types Observed i n the N a t u r a l Environment and i n the D i e t s o f Trout and Kokanee Comparisons o f the p r o p o r t i o n s o f prey i n the d i e t s o f t r o u t and kokanee w i t h the r e l a t i v e abundance o f p o t e n t i a l prey i n the l a k e (Chapter 2, F i g s . 8 & 9) r e v e a l e d major d i f f -erences between the apparent a v a i l a b i l i t y o f prey i n the l a k e and t h e i r u t i l i z a t i o n by the p r e d a t o r s . R e s u l t s o b t a i n e d i n t h i s c h a p t e r p r o v i d e evidence t h a t a l i k e l y reason these d i f f e r e n c e s o ccur i s t h a t t r o u t , kokanee and s c i e n t i s t s each implement sampling schemes which e x h i b i t d i f f e r e n c e s i n s p a t i a l coverage. In o r d e r to c h a r a c t e r i z e the s p e c i e s c o m p o s i t i o n o f the i n v e r t e b r a t e s t h a t occupy an average square meter o f l a k e 70 bottom, s c i e n t i s t s ( H a m i l t o n , 1965; E f f o r d and Tsumura, 1968-69, unpublished d a t a ; Bryan, 1971) employed s t r a t i f i e d random sampling schemes i n which the number of b e n t h i c s t a t i o n s w i t h i n each depth zone was a p p r o x i m a t e l y p r o p o r t i o n a l t o the t o t a l l a k e area covered by t h a t zone. Thus, of the 9 s t a t i o n s .sampled and averaged on a t w i c e monthly b a s i s by Hamilton (1965), 5 were l o c a t e d a t depths of l e s s than 2 m w h i l e the remaining 4 were l o c a t e d a t depths between 2 m and 6m. S i m i l a r l y , o f the 25 b e n t h i c grabs taken on a monthly b a s i s over a two year i n t e r v a l by E f f o r d and Tsumura, 17 on average o r i g i n a t e d from depths of 2 m o r l e s s w h i l e the remaining 8 o r i g i n a t e d from depths between 2 m and 6 m. Although when averaged these d a t a are l i k e l y t o i n d i c a t e the r e l a t i v e d e n s i t i e s o f each type of b e n t h i c i n v e r t e b r a t e p r e s e n t i n the l a k e as a whole, they are u n l i k e l y t o i n d i c a t e the r e l a t i v e d e n s i t i e s o f b e n t h i c i n v e r t e b r a t e s encountered by t r o u t o r kokanee w h i l e f o r a g i n g . For example, over the course of a year s c i e n t i s t s a p p l i e d between 32 and 44% o f t h e i r sampling e f f o r t to o b t a i n prey from depth zones g r e a t e r than 2m. By c o n t r a s t , kokanee s e a r c h i n g f o r b e n t h i c i n v e r t e b r a t e s spent between 65 and 88% of t h e i r time f o r a g i n g i n t h i s zone. Thus, the s e t o f b e n t h i c i n v e r t e b r a t e s a c q u i r e d by s c i e n t i s t s i s b i a s e d towards those s p e c i e s t h a t a r e most abundant i n s h a l l o w (< 2 m) water h a b i t a t s (eg. amphipods, s n a i l s , odonates) w h i l e the s e t o f i n v e r t e b r a t e prey a c q u i r e d by kokanee w i l l be b i a s e d towards those s p e c i e s t h a t are most abundant i n r e l a t i v e l y deep (> 2 m) b e n t h i c h a b i t a t s (eg. chironomid l a r v a e ) . 71 The s p a t i a l p a t t e r n e s t a b l i s h e d by s c i e n t i s t s f o r sampling b e n t h i c i n v e r t e b r a t e s c o i n c i d e s more c l o s e l y w i t h t h a t used by f o r a g i n g rainbow t r o u t than by kokanee, however d i f f e r e n c e s i n where "samples" o f i n v e r t e b r a t e s were taken w i t h i n a g i v e n depth zone p r o b a b l y b i a s the s e t s of i n v e r t e b r a t e s o b t a i n e d by s c i e n t i s t s and t r o u t i n d i f f e r e n t ways. For example, Hamilton as w e l l as E f f o r d and Tsumura o b t a i n e d a l l grab samples w i t h i n a g i v e depth zone from open mud l o c a t i o n s . They avoided o b t a i n i n g samples from weed-beds t h a t occur along the s h o r e l i n e because the p l a n t s i n t e r f e r e d w i t h the o p e r a t i o n o f the sampling gear. By c o n t r a s t , rainbow t r o u t observed i n t h i s study o f t e n searched i n and around submerged weed-beds l o c a t e d along the l a k e s h o r e . T h i s d i f f e r e n c e may be important g i v e n Winterbourn's (1971) o b s e r v a t i o n t h a t many s p e c i e s o f c a d d i s l a r v a e a re p a r t i c u l a r l y abundant i n weed-beds. Thus, the apparent o v e r -r e p r e s e n t a t i o n o f c a d d i s l a r v a e i n b e n t h i c grab samples may a l s o be p a r t i a l l y based upon d i f f e r e n c e s i n areas sampled by t r o u t and s c i e n t i s t s . Unexplained D i e t a r y P a t t e r n s Although a v a i l a b i l i t y , i n the sense o f s p a t i a l p r o x i m i t y , p l a y s a major r o l e i n the d i f f e r e n t i a l a c q u i s i t i o n o f p r e y , t h e r e are many trends which are not accounted f o r by s p a t i a l s e g r e g a t i o n . Under l a t e summer c o n d i t i o n s , t h e r e i s a d e c l i n e i n d i e t o v e r l a p between t r o u t and kokanee (Chapter 2, Table 1.) d e s p i t e a c o n s i d e r a b l e i n c r e a s e i n s p a t i a l o v e r l a p . For f i s h g r e a t e r than 10 cm i n s i z e , the e x c l u s i v e use of t r i c h o p t e r a n l a r v a e by t r o u t and o f 72 c l a d o c e r a n s by kokanee c o n t r i b u t e s t o the decrease i n d i e t o v e r l a p and i s not a l o g i c a l outcome o f the observed p a t t e r n s o f h a b i t a t s e g r e g a t i o n . D e s p i t e the g r e a t e r p r o p o r t i o n o f time spent f o r a g i n g i n bottom h a b i t a t s , t r o u t e x p l o i t s u b s t a n t i a l l y fewer chironomid l a r v a e than kokanee. These l a r g e l y b e n t h i c prey do not appear to be p r e s e n t at higher d e n s i t i e s i n i n s h o r e than i n o f f s h o r e hab-i t a t s (Table 9 ) , thus, some o t h e r mechanism must operate t o produce the d i f f e r e n c e . S p a t i a l s e g r e g a t i o n o f f e r e d an e x p l a n a t i o n f o r the o b s e r v a t i o n t h a t kokanee e x p l o i t s m a l l e r m o l l u s c s than t r o u t do, but i t i s not an adequate mechanism t o produce the g e n e r a l t r e n d f o r e x p l o i t a t i o n o f l a r g e r prey by t r o u t . T h i s i s e s p e c i a l l y c l e a r g i v e n t h a t the tr e n d a p p l i e s t o both a broad range of a q u a t i c i n v e r t e b r a t e s (Chapter 2, F i g . 2) and to more l i m i t e d groups such as zooplankton (Chapter 2, F i g . 5 ) . H a b i t a t s e g r e g a t i o n and t i m i n g of a c t i v i t i e s are o n l y a l i m i t e d success i n e x p l a i n i n g the non-random e x p l o i t a t i o n o f prey from the l a k e environment. Many o f the observed d i f f e r e n c e s s t i l l have no e x p l a n a t i o n . In the next c h a p t e r , I have combined o b s e r v a t i o n s o f f o r a g i n g behaviour i n the f i e l d w i t h s e l e c t e d l a b o r a t o r y experiments to t e s t the g e n e r a l h y p o t h e s i s t h a t d i f f e r e n c e s i n how t r o u t and kokanee search f o r prey w i l l account f o r some o f the as y e t un e x p l a i n e d p a t t e r n s o f non-random, p r e y - e x p l o i t a t i o n . 73 SUMMARY 1. Trout and kokanee e x h i b i t w e l l d e f i n e d p a t t e r n s o f s p a t i a l s e g r e g a t i o n ( T a b l e s 7 & 8, F i g . 1 2 ) . 2. In e a r l y summer kokanee occupy p r i m a r i l y s u r f a c e h a b i t a t s w h i l e t r o u t occupy m a i n l y bottom ones (Table 7 ) . 3. In l a t e summer t h e r e a re more t r o u t than kokanee i n s u r f a c e h a b i t a t s a l t h o u g h the m a j o r i t y o f i n d i v i d u a l s of both s p e c i e s are a s s o c i a t e d w i t h midwater and b e n t h i c h a b i t a t s (Table 7 ) . 4. Kokanee e x h i b i t a c o n s i s t e n t t r e n d t o occupy o f f s h o r e ( > 2 m) h a b i t a t s ( F i g . 1 2 ) . 5. Trout e x h i b i t a c o n s i s t e n t t r e n d t o occupy i n s h o r e ( < 2 m) h a b i t a t s ( F i g . 1 2 ) . 6. A c t i v i t y p a t t e r n s o f t r o u t and kokanee i n d i c a t e r e l a t i v e l y l i t t l e temporal s e g r e g a t i o n . N e i t h e r s p e c i e s can be c l a s s e d as s t r i c t l y n o c t u r n a l , d i u r n a l o r c r e p u s c u l a r ( F i g 13 & 14). 7. S p a t i a l s e g r e g a t i o n between t r o u t and kokanee i n Marion Lake p l a y s a major r o l e i n pr o d u c i n g p r e d a t o r - s p e c i f i c d i e t a r y p a t t e r n s . 8. A number of d i f f e r e n c e s between the apparent a v a i l a b l i t y o f i n v e r t e b r a t e prey i n the l a k e environment and t h e i r u t i l i z a t i o n by t r o u t and kokanee may be a t t r i b u t e d t o the f a c t t h a t the r e l a t i v e p r o p o r t i o n s of prey o b t a i n e d by s c i e n t i s t s sampling a l l areas o f the l a k e are not the p r o p o r t i o n s t h a t t r o u t and kokanee encounter when foraging in more restricted areas (eg. weed-beds, offshore benthic habitats) of the lake. 9. Many of the dietary differences described e a r l i e r (Chapter 2) are not accounted for by either spatial or temporal segregation between trout and kokanee. 75 CHAPTER 4 THE RELATIONSHIP BETWEEN FOOD-SEARCH BEHAVIOUR AND DIETARY PATTERNS OF TROUT AND KOKANEE IN MARION LAKE 4-A. FIELD DESCRIPTIONS INTRODUCTION I have p r e v i o u s l y e s t a b l i s h e d (Chapter 2) t h a t t r o u t and kokanee i n Marion Lake d i s p l a y s p e c i e s s p e c i f i c d i e t s and t h a t s p a t i a l s e g r e g a t i o n (on a s c a l e o f meters) p l a y s an important r o l e i n shaping the p a t t e r n s o f prey e x p l o i t a t i o n observed (Chapter 3 ) . However c o n s i d e r a t i o n o f o n l y s p a t i a l and temporal s e g r e g a t i o n by f i s h i n Marion Lake l e f t many d i e t a r y p a t t e r n s u n e x p l a i n e d . T h i s i m p l i e s t h a t d i f f e r e n c e s i n the f o r a g i n g behaviour of the p r e d a t o r s must p l a y a major r o l e i n shaping t h e i r d i e t s . De R u i t e r (1967) has d i v i d e d the f o r a g i n g behaviour of animals i n t o f o u r phases: s e a r c h , approach, c a p t u r e , and i n g e s t i o n . Events t h a t take p l a c e d u r i n g any one of these phases may have a profound i n f l u e n c e on the d i e t of a p r e d a t o r . In t h i s c h a p t e r , I have s e t out to t e s t the g e n e r a l h y p o t h e s i s t h a t d i f f e r e n c e s i n s e a rch behaviour w i l l account f o r d i f f e r e n c e s i n the types o f prey a c q u i r e d by t r o u t and kokanee. To demonstrate t h i s r e q u i r e s an assessment of whether t h e r e i s a "match" between the- p r e d a t o r 1 s search b e h a v i o u r s and s p e c i f i c c h a r a c t e r i s t i c s ( l o c a t i o n s , s i z e s , b e h a v i o u r s ) o f t h e i r p r e y . Food s e a r c h by p r e d a t o r s may be c o n s i d e r e d from many d i f f e r e n t a s p e c t s . A v a r i e t y of s t u d i e s i n d i c a t e t h a t m i c r o h a b i t a t 76 s p e c i f i c s e a r c h , the use o f s p e c i a l i s e d s e a r c h t e c h n i q u e s , and i n t e r a c t i o n s between sensory p e r c e p t i o n and prey d e t e c t i o n a re p a r t i c u l a r l y i m portant i n d e t e r m i n i n g d i e t a r y p a t t e r n s o f f r e e l i v i n g p r e d a t o r s (MacArthur, 1958; Root, 1967; Baker, 1972; Ware, 1973). I have d i v i d e d t h i s c h a p t e r i n t o two s e c t i o n s . The f i r s t d e a l s w i t h the food-search behaviour of t r o u t and kokanee under n a t u r a l c o n d i t i o n s . The second d e a l s w i t h the r e l a t i v e a b i l i t i e s o f t r o u t and kokanee to d e t e c t v a r i o u s n a t u r a l prey i n the l a b o r a t o r y . The i m p l i c a t i o n s o f r e s u l t s on s e a r c h behaviour and prey d e t e c t i o n , f o r d i e t a r y p a t t e r n s are d i s c u s s e d . M i c r o h a b i t a t S p e c i f i c Search and S p e c i a l i z e d Search Techniques There i s no stand a r d d e f i n i t i o n f o r what c o n s t i t u t e s a h a b i t a t , a s u b - h a b i t a t o r a m i c r o h a b i t a t and from a v a r i e t y o f s t u d i e s i t i s c l e a r t h a t they are not a b s o l u t e but r a t h e r r e l a t i v e u n i t s most commonly d i s t i n g u i s h e d by d i f f e r e n c e s o f b i o p h y s i c a l s c a l e . In the m a j o r i t y o f s t u d i e s , d e a l i n g w i t h the f o r a g i n g behaviour o f f r e e - r a n g i n g v e r t e b r a t e s , h a b i t a t s and s u b - h a b i t a t s are b i o p h y s i c a l u n i t s which cover many square meters o f space (eg. f o r e s t f l o o r , f o r e s t canopy, l i t t o r a l zone, b e n t h i c s e d i m e n t s ) . M i c r o h a b i t a t s by c o n t r a s t a re b i o p h y s i c a l u n i t s i n which a t l e a s t some dimensions are measured on a s c a l e of cm o r mm (eg. c r a c k s i n t r e e bark, the un d e r s i d e s of l e a v e s , the exposed l a y e r of sediment as compared t o the s u b - s u r f a c e sediments on a l a k e bottom). W i t h i n t h i s c o n t e x t each s u b - h a b i t a t g e n e r a l l y c o n s i s t s o f an aggregate of m i c r o h a b i t a t s , which may c o n t a i n d i f f e r e n t assortments of f o o d . Specialised search techniques which result in micro-habitat specific search by vertebrate predators have been iden t i f i e d as the basis for the occurrence of differences between the diets of similar predators searching for food in the same habitats (MacArthur, 1958; Root, 1967; Baker, 1972), as well as the reason for why a particular predator f a i l s to take potential prey in proportion to their relative abundance in the environment (Royama, 1970; Ware, 1973; Moore and Moore, 1976). Root (1967) and Baker (1972) in particular emphasized that differences in microhabitats searched by sympatric species of birds served as the basis for their d i f f e r e n t i a l exploitation of invertebrates in forest and mud-flat habitats respectively. For example, Root found that Western flycatchers (Empidonax  d i f f i c i l i s ) search a large area for prey from a "sentinel" position on an exposed perch. He suggested that this search technique biased flycatchers to exploit large, active insects either in the a i r or from the surface of foliage. A high proportion of large Hymenoptera and Diptera in the diet of flycatchers supported his prediction. In contrast to flycatchers pla i n titmice (.Parus inornatus) often searched for prey by pulling apart flowers and foliage as well as by probing beneath foliage. Consequently, their diets contained a high proportion of small insects which were concealed in these microhabitats. These insects did not appear commonly in the diet of flycatchers. Similarly, Baker (1972) noted that some species of shorebirds (eg. short b i l l e d dowagers) searched 78 f o r prey by probing deeply i n t o the s u b s t r a t e w i t h t h e i r b i l l s , w h i l e o t h e r s (eg. l e s s e r y e l l o w l e g s ) , searched e x c l u s i v e l y f o r o b j e c t s exposed on the mud s u r f a c e . He presented evidence t h a t t h i s d i f f e r e n c e i n search procedure served as the b a s i s f o r d i f f e r e n t i a l d e t e c t i o n and a c q u i s i t i o n o f prey by the p r e d a t o r s . S i n c e many o f the s p e c i e s o f prey e x p l o i t e d by t r o u t and kokanee e x h i b i t m i c r o h a b i t a t d i f f e r e n c e s (eg. e p i b e n t h i c o r s u b - b e n t h i c ) , I h y p o t h e s i z e d t h a t s p e c i a l i s e d s e a r c h t e c h n i q u e s i n c o n c e r t w i t h m i c r o h a b i t a t s p e c i f i c s e a r c h might form the b a s i s f o r some d i f f e r e n c e s i n the d i e t a r y p a t t e r n s o f t r o u t and kokanee. To p r o v i d e d a t a t o t e s t t h i s h y p o t h e s i s , I con-ducted o b s e r v a t i o n s o f t r o u t and kokanee f o r a g i n g under n a t u r a l c o n d i t i o n s . METHODS L o c a t i o n s , times and g e n e r a l t e c h n i q u e s o f o b t a i n i n g o b s e r v a t i o n s have been d e s c r i b e d elsewhere (Chapter 3 ) . L o c a t i o n s at which o b s e r v a t i o n s were conducted d i f f e r e d both w i t h r e s p e c t t o depth and d i s t a n c e from sho r e . Although I d i d not o b t a i n o b s e r v a t i o n s of f o r a g i n g b e h a v i o u r s of t r o u t and kokanee a s s o c i a t e d w i t h the e n t i r e range of m i c r o h a b i t a t s p r e s e n t i n Marion Lake, l o c a t i o n s a t which o b s e r v a t i o n s were conducted c o n t a i n e d a v a r i e t y o f m i c r o h a b i t a t f e a t u r e s (eg. open sediment, brush d e p o s i t s , a c c u m u l a t i o n s of l i t t e r , clumps of water l i l i e s ) . I r e corded the se a r c h t e c h n i q u e s and p o s i t i o n s o f a c t i v e l y f o r a g i n g p r e d a t o r s i n g r e a t d e t a i l because I wished t o d e t e c t any c o n s i s t e n t d i f f e r e n c e s between t r o u t and kokanee 79 which would promote access to different portions of the food supply present in diff e r e n t microhabitats. When f i s h swam into view standard observations recorded were; the species identity, the number of individuals present, the spe c i f i c substrate that individuals were oriented to, the water column position of each animal (expressed in cm from the nearest substrate), the cruising velocity of randomly selected animals, the "search techniques" employed by individual f i s h , the rela t i v e sizes and id e n t i t i e s of items that f i s h attacked and the sequence of events surrounding an attack. To record those events requiring an estimate of time, I used either a four channel event recorder or a stopwatch. Because individual f i s h at underwater stations remained within view for only 15 seconds to three minutes, i t was impossible to determine exactly how many different individuals my observa-tions involve. However, I could often recognize individual trout due to differences in size or markings (scars, colour patterns, 200 to 300 trout carried colour coded tags) and kokanee frequently appeared in groups containing up to a few hundred individuals, thus i t is l i k e l y that the data collected involve several hundred individuals of each species and that the behaviours observed are representative of those generally employed by trout and kokanee in Marion Lake. RESULTS Search Techniques When particular behaviours are associated with spe c i f i c e n v i r o n m e n t a l c o n d i t i o n s and are r e p e a t e d l y f o l l o w e d by attempts a t prey c a p t u r e , i t i s l i k e l y t h a t such b e h a v i o u r s are p a r t of the p r o c e s s o f prey s e a r c h . In the p r e s e n t s t u d y , prey d e t e c t i o n and a t t a c k by f i s h commonly o c c u r r e d w i t h i n a s e r i e s o f r e c o g n i z a b l y d i f f e r e n t b e h a v i o u r a l and e n v i r o n m e n t a l c o n t e x t s . For convenience, I have a r b i t r a r i l y d e s i g n a t e d each o f these b e h a v i o u r a l - e n v i r o n m e n t a l a s s o c i a t i o n s as se p a r a t e types o f s e a r c h . 1. C r u i s e and Search Trout use t h i s technique to l o c a t e prey i n the water column, a t the sediment s u r f a c e and a t the l a k e s u r f a c e . Kokanee e x h i b i t t h i s form o f prey search o n l y when they a r e f o r a g i n g i n the water, column. C r u i s e and search i s the most ambiguous o f the beha v i o u r s c l a s s i f i e d as search and t o the u n t r a i n e d eye t h e r e i s l i t t l e to d i f f e r e n t i a t e the technique from g e n e r a l swimming a c t i v i t y . The key combination o f c h a r a c t e r i s t i c s t h a t i d e n t i f y the technique i s t h a t the p r e d a t o r s are mobile and t h e i r responses are r e s t r i c t e d t o r e c o g n i z a b l e prey i t e m s . However, i n a d d i t i o n t o the i n i t i a t i o n o f a t t a c k s d i r e c t e d towards p r e y , t h e r e are o t h e r c l u e s which suggest t h a t the p r e d a t o r s are a c t i v e l y s e a r c h i n g f o r pr e y . The f i r s t c l u e i s t h a t the p r e d a t o r s m a i n t a i n a f i x e d o r i e n t a t i o n t o c e r t a i n " s u b s t r a t e s " where prey are l i k e l y t o be found. When t r o u t employ t h i s technique to s e a r c h f o r b e n t h i c p r e y , they assume p o s i t i o n s t h a t v a r y from 0 - 50 cm above the s u b s t r a t e , but o r i e n t downwards t o f a c e i t . Ware 81 (1971) suggested t h a t t h i s c h a r a c t e r i s t i c i n c l i n e (about 10 t o 20°) s e r v e s to d i r e c t the p r e d a t o r ' s v i s u a l a.xis onto the sediment. In support o f t h i s , when t r o u t were v e r y c l o s e to an o b s e r v e r i n the f i e l d i t was p o s s i b l e t o see t h a t the f i s h e f f e c t c o n t i n u o u s eye movements as i f "scanning" the sediment s u r f a c e . When u s i n g c r u i s e and s e a r c h a t the l a k e s u r f a c e , t r o u t m a i n t a i n a h o r i z o n t a l p o s t u r e and make f r e q u e n t attack's on prey from 45 t o 100 cm below the a i r - w a t e r i n t e r f a c e . The second c l u e t h a t c r u i s e and s e a r c h i s r e l a t e d t o the p r o c e s s o f prey d e t e c t i o n i s t h a t p r e d a t o r swimming v e l o c i t i e s v a r y i n a way t h a t suggests they are v i s u a l l y scanning t h e i r immediate s u r r o u n d i n g s f o r f o o d . Swimming speeds are s i m i l a r f o r f i s h f o r a g i n g a t the l a k e s u r f a c e o r i n the water column, but movement i s s u b s t a n t i a l l y slower f o r f i s h o r i e n t e d t o b e n t h i c s u b s t r a t e s ( Table 10). Trout and kokanee both r e l y on v i s i o n d u r i n g food search (Ware, 1971; A l i , 1959), thus one obvious reason f o r t h i s r e d u c t i o n i n v e l o c i t y i s the r e l a t i v e l e v e l of s u b s t r a t e h e t e r o g e n e i t y t h a t water column and bottom sediments o f f e r a p r e d a t o r u s i n g v i s i o n t o l o c a t e p r e y . The more complex background o f b e n t h i c s u b s t r a t e s w i l l r e q u i r e a l o n g e r i n t e r v a l of p e r c e p t u a l a t t e n t i o n per u n i t o f space scanned i n o r d e r t o d e t e c t prey (eg. see Boynton and Busch, 1956), c o n s e q u e n t l y search v e l o c i t i e s must be lower here. TABLE 10. A comparison of the swimming v e l o c i t i e s o f t r o u t and kokanee a c t i v e l y s e a r c h i n g f o r prey i n a v a r i e t y o f s u b - h a b i t a t s . 82 a. Trout Water column & l a k e s u r f a c e Sediment s u r f a c e Mean (cm/sec) 27.7 8.6 Range S.D. 11.1- 50.0 + 1.4 4.2- 16.7 + 1.0 N 165 35 b. Kokanee Water column & l a k e s u r f a c e Sediment s u r f a c e 18.2 6.6 10.0- 50.0 + 2.1 45 5.0- 10.0 + 0.5 22 TABLE 11. Residence time and the t o t a l number of a t t a c k s i n i t i a t e d by f i v e t r o u t using the hover and search t e c h n i q u e . Time spent i n a s i n g l e l o c a t i o n (minutes) T o t a l number of a t t a c k s i n i t i a t e d 4 2 6 4 5 6 3 4 1 2 83 2. Hover and Search Trout use t h i s technique to l o c a t e prey on the sediment s u r f a c e and a t the l a k e s u r f a c e . The key combination o f c h a r a c t e r i s t i c s t h a t i d e n t i f y the technique i s t h a t the p r e d a t o r m a i n t a i n s a s t a t i o n a r y p o s i t i o n (10 t o 20 cm above the bottom o r 50 t o 60 cm below the l a k e s u r f a c e ) and a t t a c k s o n l y r e c o g n i z a b l e prey items. The long a x i s o f the body may be h o r i z o n t a l o r s l i g h t l y t i l t e d (10 t o 20°) towards the s u b s t r a t e . The c o n s t a n t performance of s l i g h t r e o r i e n t a t i o n movements t o the s u b s t r a t e , scanning movements o f the eyes and the i n i t i a t i o n o f a number of a t t a c k s on prey items t h a t move, d i s t i n g u i s h t h i s s e a r c h technique from r e s t p o s t u r e s . Prey may be a t t a c k e d w i t h i n an a r c c e n t e r e d between the p r e d a t o r ' s eyes and subtending 310 t o 320°. Tro u t r a r e l y use t h i s t a c t i c f o r s e a r c h i n g a t the l a k e s u r f a c e . S i x t y o f the seventy t r o u t I have observed u s i n g t h i s t a c t i c were s e a r c h i n g f o r b e n t h i c p r e y . Trout spend no more than a few minutes i n a s i n g l e l o c a t i o n and t h e r e i s c o n s i d e r a b l e v a r i a b i l i t y i n t h e i r s u ccess a t l o c a t i n g prey (Table 1 1 ) . On a number of o c c a s i o n s , I have f o l l o w e d the a c t i v i t i e s of i n d i v i d u a l f i s h from an above s u r f a c e l o c a t i o n and have found t h a t they w i l l use the technique c o n s e c u t i v e l y , i n s e v e r a l l o c a t i o n s , w i t h i n a 25 t o 50 square meter a r e a , over 20 t o 30 minutes i n t e r v a l s . No kokanee used t h i s technique d u r i n g any o f the f i e l d o b s e r v a t i o n s . Kokanee c h a r a c t e r i s t i c a l l y use o n l y s e a r c h t e c h n i q u e s which i n v o l v e c o n s t a n t swimming a c t i v i t y . 3. Test and Search There i s l i t t l e doubt t h a t both t r o u t and kokanee use t h i s t echnique as a means to l o c a t e food items. The key combination of c h a r a c t e r i s t i c s t h a t i d e n t i f y the technique i s t h a t the p r e d a t o r s are mobile and they i n i t i a t e many a t t a c k s on items t h a t are not prey organisms. For example an i n d i v i d u a l t r o u t w i l l o r i e n t t o , r a p i d l y approach, g r a s p and then r e j e c t a s m a l l wood c h i p l y i n g on the sediment s u r f a c e . A t t a c k s on t h i s item may be repeated s e v e r a l times and each time the o b j e c t i s mouthed and then r e j e c t e d . F i n a l l y the p r e d a t o r w i l l abandon the i t e m , however i t may move o n l y a s h o r t d i s t a n c e b e f o r e making an a t t a c k on another. The s p e c i f i c items a t t a c k e d are u s u a l l y w i t h i n the s i z e range of prey items t h a t w i l l n o r m a l l y be consumed by f i s h i n the l a k e . T r o u t use t h i s technique o c c a s i o n a l l y a t the l a k e s u r f a c e but more f r e q u e n t l y when s e a r c h i n g f o r prey on the l a k e bottom. Search p o s i t i o n s and swimming v e l o c i t i e s are i n d i s t i n g u i s h a b l e from those d e s c r i b e d above f o r t r o u t employing c r u i s e and s e a r c h . Kokanee use t e s t and s e a r c h e x c l u s i v e l y when they forage a t the l a k e s u r f a c e . In c o n t r a s t to the c r u i s e and search technique t h a t t r o u t use most o f t e n a t the s u r f a c e , the t e s t and s e a r c h technique of kokanee i s performed from a s h o r t e r d i s t a n c e below the l a k e s u r f a c e ( u s u a l l y 5 t o 30 cm) and a t s i g n i f i c a n t l y lower v e l o c i t i e s (mean 18.2 cm per second, Table 1 1 ) . The lower v e l o c i t i e s a re p r o b a b l y a consequence of the tendency f o r kokanee to momentarily g r a s p every p r e y - s i z e o b j e c t (range 1 mm t o 15 mm) f l o a t i n g on the s u r f a c e . A s u b t l e 85 d i f f e r e n c e i n t h i s t e c h n i q u e , as p r a c t i s e d by t r o u t and kokanee, i s t h a t kokanee f o r a g i n g a t the s u r f a c e u s u a l l y t e s t an item o n l y once b e f o r e moving on. T h i s may be a consequence o f t h e i r f o r a g i n g i n c l o s e a s s o c i a t i o n w i t h o t h e r kokanee, w h i l e t r o u t are o f t e n s o l i t a r y (see d i s c u s s i o n i n Chapter 7 ) . ~ 4. Grab and Search T h i s technique was used o n l y by kokanee s e a r c h i n g f o r prey a t the sediment s u r f a c e . The p r e d a t o r moves along v e r y s l o w l y (6.6 cm p e r second - Table 10) r i g h t a t the sediment s u r f a c e . The f i s h t a k e s a m o u t h f u l l of sediment and l e a f l i t t e r , v i g o r o u s l y mouths i t , and f o r c e f u l l y e j e c t s the m a t e r i a l . The forward motion o f the p r e d a t o r q u i c k l y c a r r i e s i t through the s m a l l c l o u d of suspended m a t e r i a l and a t t h i s p o i n t the p r e d a t o r may take another m o u t h f u l l of sediment o r t u r n through o 180 t o reexamine the s e t t l i n g d e b r i s . By using t h i s technique kokanee d e t e c t prey t h a t are exposed on the sediment s u r f a c e and prey which become exposed as a consequence o f the s u b s t r a t e d i s t u r b a n c e . When I have observed t h i s technique used under l a b o r a t o r y c o n d i t i o n s , prey o f t e n escape because the p r e d a t o r f a i l s to d i s c r i m i n a t e s u c c e s s f u l l y between prey and f a l l i n g d e b r i s . I have never observed rainbow t r o u t employ t h i s t echnique t o search f o r p r e y , a l t h o u g h , on very r a r e o c c a s i o n s , and o n l y i n the l a b o r a t o r y , I have observed them d i s t u r b sediments by body movements r e m i n i s c e n t of redd d i g g i n g . To b r i e f l y summarize the d i f f e r e n c e s i n s e a r c h t e c h n i q u e s between the two s p e c i e s , t r o u t alone use hover and search w h i l e grab and search i s a technique used e x c l u s i v e l y by kokanee. Although two of the techni q u e s ( c r u i s e & s e a r c h , t e s t & search) are shared by t r o u t and kokanee, they are not p r a c t i s e d w i t h equal frequency and the same te c h n i q u e s are not always employed to l o c a t e prey i n the same s u b - h a b i t a t s (Table 12). Search t e c h n i q u e s employed by kokanee are more sub-h a b i t a t s p e c i f i c than those used by t r o u t . An Examination o f the D i s t r i b u t i o n o f Search P o s i t i o n s of Trout and Kokanee i n R e l a t i o n t o Lake S u r f a c e and Bottom S u b - h a b i t a t s D i r e c t o b s e r v a t i o n s suggested t h a t the m a j o r i t y of prey d i s c o v e r e d by t r o u t and kokanee o c c u r r e d e i t h e r a t the l a k e s u r f a c e o r l a k e bottom. S e v e r a l of the se a r c h t e c h n i q u e s used by the p r e d a t o r s i n v o l v e d a d i f f e r e n t range of search p o s i t i o n s w i t h r e s p e c t to the s u r f a c e being scanned f o r p r e y . Thus, I expected t h a t the use of d i f f e r e n t s e a r c h t e c h n i q u e s as w e l l as d i f f e r e n c e s i n the frequency o f use o f the same tec h n i q u e s might r e s u l t i n d i f f e r e n t d i s t r i b u t i o n s o f search p o s i t i o n s of t r o u t and kokanee when they searched f o r prey l o c a t e d a t the l a k e s u r f a c e o r bottom. . In the f i e l d , t r o u t and kokanee occ u p i e d a con t i n u o u s range of se a r c h p o s i t i o n s e x t e n d i n g from the l a k e s u r f a c e , through the water column t o the sediment s u r f a c e a t the l a k e bottom. In o r d e r to d e t e c t d i f f e r e n c e s i n the d i s t r i b u t i o n s o f search p o s i t i o n s assumed by the p r e d a t o r s , I f i r s t had t o 87 TABLE 12. A summary o f the d i f f e r e n c e s i n se a r c h t e c h n i q u e s employed by t r o u t and kokanee f o r a g i n g i n the f i e l d . L e v e l of Occurrence S u b - h a b i t a t s Searched Technique Trout Kokanee Trout Kokanee c r u i s e & search +++ + S,W,B W hover & se a r c h ++ N S,B -t e s t & search + +++ S,W,B s,w grab & search N +++ - B +++ ve r y common ++ common + o c c a s i o n a l S -w -B -l a k e s u r f a c e water column l a k e bottom N never observed 88 d e f i n e which p o r t i o n s o f the e n t i r e range of search p o s i t i o n s were r e l a t e d t o s e a r c h f o r prey l o c a t e d i n a s s o c i a t i o n w i t h b e n t h i c s u b s t r a t e s and l a k e s u r f a c e r e s p e c t i v e l y . To do t h i s I r e l i e d on o b s e r v a t i o n s o f the maximum r e a c t i v e d i s t a n c e (Ware, 1971) o f p r e d a t o r s to b e n t h i c and s u r f a c e prey i n the f i e l d . R e a c t i v e D i s t a n c e t o P o t e n t i a l Prey and the S p a t i a l L i m i t s on B e n t h i c and Su r f a c e Search In the f i e l d , a sudden change i n the d i r e c t i o n , v e l o c i t y o r form o f swimming behaviour of a s e a r c h i n g p r e d a t o r was u s u a l l y f o l l o w e d by an a t t a c k on a prey organism. Because the c o i n c i d e n c e o f an a t t a c k o c c u r r i n g i n the immediate v i c i n i t y o f the bottom o r water-column r e f e r e n c e markers was r e l a t i v e l y r a r e , and because p r e d a t o r s always approached prey a t an o b l i q u e angle t o the s u b s t r a t e , I d i d not o b t a i n many p r e c i s e e s t i m a t e s of r e a c t i v e d i s t a n c e s t o prey. However I d i d observe t h a t f i s h m a i n t a i n i n g p o s i t i o n s f u r t h e r than 50 cm from the sediment s u r f a c e (measured as the p e r p e n d i c u l a r d i s t a n c e between the sediment s u r f a c e and the head of the p r e d a t o r ) never a t t a c k e d b e n t h i c p r e y , but p r e d a t o r s c l o s e r than t h i s f r e q u e n t l y made a t t a c k s . L a b o r a t o r y s t u d i e s have r e v e a l e d many o f the v a r i a b l e s t h a t i n f l u e n c e the d i s t a n c e a t which f i s h , as v i s u a l p r e d a t o r s , r e a c t t o p r e y . S i z e , c o n t r a s t and movement of prey ( P r o t a s o v , 1968; Ware, 1971); e x p e r i e n c e and s i z e of the p r e d a t o r (Beukema, 1968; Ware, 1971); and c o m p l e x i t y o f the background environment 89 (Ware, 1971) , a l l have a s u b s t a n t i a l i n f l u e n c e on r e a c t i v e d i s t a n c e . These v a r i a b l e s appear t o i n t e r a c t i n the f i e l d i n a way t h a t l i m i t s the search f o r b e n t h i c prey to p r e d a t o r s t h a t m a i n t a i n p o s i t i o n s of l e s s than 50 cm from the sediment s u r f a c e . The maximum r e a c t i v e d i s t a n c e to b e n t h i c prey was g r e a t e r than 50 cm on some o c c a s i o n s , s i n c e t r o u t and kokanee always approached prey a t an o b l i q u e angle to the s u b s t r a t e . The maximum r e a c t i v e d i s t a n c e of t r o u t and kokanee t o prey l o c a t e d a t the l a k e s u r f a c e was much g r e a t e r than to prey on b e n t h i c s u b s t r a t e s . F i s h m a i n t a i n i n g p o s i t i o n s as much as 85 cm below the s u r f a c e i n i t i a t e d a t t a c k s on pre y . On two o c c a s i o n s I o b t a i n e d samples f o r s i z e measurements of the t a r g e t s t r o u t were a t t a c k i n g (Table 13); thus I am c e r t a i n t h a t they are capable of responding from p o s i t i o n s t h a t are 80-85 cm below the s u r f a c e t o t a r g e t s no l a r g e r than 6-10 square m i l l i m e t r e s . The maximum r e a c t i v e d i s t a n c e to prey of t h i s s i z e was modestly g r e a t e r because of the s l i g h t l y o b l i q u e angle a t which a t t a c k s on s u r f a c e prey o c c u r r e d . TABLE 13. R e a c t i v e d i s t a n c e o f t r o u t to t a r g e t s of known s i z e a t the l a k e s u r f a c e on two o c c a s i o n s R e a c t i v e D i s t a n c e Target S i z e (cm) Mean Range No. o f Maximum s u r f a c e No. Target a t t a c k s area (mm2 ) measured i d e n t i t y  70 65-85 5 10 20 Nuphar seeds 50 30-80 10 64 40 a d u l t D i p t e r a 90 Under n a t u r a l c o n d i t i o n s , t r o u t and kokanee never i n i t i a t e d a t t a c k s on l a k e s u r f a c e o r b e n t h i c prey from d i s t a n c e s g r e a t e r than 100 cm below the l a k e s u r f a c e o r 50 cm above the l a k e bottom. T h e r e f o r e , by d e f i n i t i o n , o n l y p r e d a t o r s w i t h i n these d i s t a n c e s were c o n s i d e r e d as a c t i v e l y s e a r c h i n g l a k e s u r f a c e and b e n t h i c s u b s t r a t e s r e s p e c t i v e l y . Given these r e s t r i c t i o n s , i t i s now p o s s i b l e t o compare the d i s t r i b u t i o n s o f t r o u t and kokanee search p o s i t i o n s w i t h r e s p e c t t o l a k e s u r f a c e and b e n t h i c s u b - h a b i t a t s . The D i s t r i b u t i o n of P r e d a t o r Search P o s i t i o n s While f o r a g i n g f o r prey a t the l a k e s u r f a c e o r bottom, t r o u t and kokanee e x h i b i t d i f f e r e n t d i s t r i b u t i o n s of search p o s i t i o n s . The r e s u l t s presented here are pooled from a l l o b s e r v a t i o n s c a r r i e d out i n both e a r l y and l a t e summer i n t e r v a l s (see Chapter 3 ) . The m a j o r i t y o f kokanee search f o r b e n t h i c prey from p o s i t i o n s t h a t are w i t h i n 5 cm of the sediment s u r f a c e w h i l e t r o u t are most o f t e n observed s e a r c h i n g from p o s i t i o n s some 15 t o 30 cm o f f of the bottom ( F i g . 1 6 ) . T h i s s p e c i e s - s p e c i f i c p a t t e r n i s m a i n t a i n e d when t r o u t and kokanee se a r c h f o r l a k e s u r f a c e prey as w e l l . Here kokanee are most f r e q u e n t l y observed s e a r c h i n g f o r s u r f a c e prey from p o s i t i o n s t h a t are 5 t o 30 cm below the l a k e s u r f a c e . By c o n t r a s t , the m a j o r i t y o f t r o u t search f o r these prey from p o s i t i o n s t h a t a r e 45 t o 100 cm below the l a k e s u r f a c e ( F i g . 1 7 ) . 91 FIGURE 16. The r e l a t i v e p r o p o r t i o n s o f t r o u t and kokanee t h a t m a i n t a i n s p e c i f i c s e a r c h p o s i t i o n s w h i l e v i s u a l l y scanning the bottom sediments f o r p r e y . N = the t o t a l number of t r o u t o r kokanee observed. The sequence of s o l i d spheres i n d i c a t e s the s i z e o f the s m a l l e s t prey t h a t t r o u t o r kokanee can d e t e c t from a g i v e n p o s i t i o n . S i z e s were e s t i m a t e d on the b a s i s o f d a t a presented i n F i g . .18 (see d i s c u s s i o n f o r e x p l a n a t i o n ) . MINIMUM DETECTABLE PARTICLE S I Z E ( m m 2 ) vo I—1 92 FIGURE 17. The r e l a t i v e p r o p o r t i o n s o f t r o u t and kokanee t h a t m a i n t a i n s p e c i f i c s e a r c h p o s i t i o n s w h i l e v i s u a l l y scanning the l a k e s u r f a c e f o r p r e y . N = the t o t a l number of t r o u t o r kokanee observed. ..The sequence o f s o l i d spheres i n d i c a t e s the r e l a t i v e s i z e of the s m a l l e s t prey t h a t t r o u t o r kokanee may d e t e c t from a g i v e n p o s i t i o n . S i z e s were e s t i m a t e d on the b a s i s o f d a t a presented i n F i g . .18 (see d i s c u s s i o n f o r e x p l a n a t i o n ) . 93 DISCUSSION The P o t e n t i a l E f f e c t s o f Search Techniques and M i c r o h a b i t a t S p e c i f i c Search on D i e t a r y P a t e r n s Trout and kokanee share some techniq u e s o f s e a r c h but o t h e r s are used by o n l y one of the p r e d a t o r s . The q u e s t i o n of whether a p a r t i c u l a r s e a rch technique b i a s e s a p r e d a t o r t o l o c a t e some food items but not o t h e r s can o n l y be answered i f s earch t e c h n i q u e s are c o n s i d e r e d i n r e l a t i o n t o the prey c o n t e n t s of the p r e d a t o r ' s d i e t s and known c h a r a c t e r i s t i c s o f the p r e y . D i f f e r e n c e s i n the s e a r c h t e c h n i q u e s o f t r o u t and kokanee r e s u l t i n some degree of m i c r o h a b i t a t s p e c i f i c s e a r c h but are not e a s i l y r e l a t e d t o any q u a l i t a t i v e d i f f e r e n c e s i n the c o m p o s i t i o n o f the p r e d a t o r ' s d i e t s . For example, grab and search as p r a c t i s e d by kokanee, opens up p o s s i b i l i t i e s f o r prey l o c a t i o n i n m i c r o h a b i t a t s t h a t are not open t o t r o u t i e . prey may be l o c a t e d by " f l u s h i n g " them from concealed p o s i t i o n s , beneath b e n t h i c sediments. By c o n t r a s t the s e a r c h t e c h n i q u e s used by t r o u t (Table 12) r e s t r i c t them t o the d e t e c t i o n o f prey i n exposed p o s i t i o n s . In s p i t e of t h i s d i f f e r e n c e , the two major c l a s s e s o f prey known t o spend most o f t h e i r time concealed beneath the sediment s u r f a c e (amphipods and chironomids) are w e l l r e p r e s e n t e d i n the d i e t s of both t r o u t and kokanee (Chapter 2 ) . T h i s s i m p l y emphasizes t h a t a l t h o u g h d i f f e r e n c e s i n f o r a g i n g behaviour w i l l f r e q u e n t l y r e s u l t i n d i f f e r e n c e s i n the k i n d s o f foods consumed by p r e d a t o r s , they may a l s o r e s u l t 94 i n the a c q u i s i t i o n o f the same foods from s l i g h t l y d i f f e r e n t m i c r o h a b i t a t s . Kokanee a c q u i r e prey from both concealed and exposed p o s i t i o n s w h i l e t r o u t o b t a i n them from o n l y exposed p o s i t i o n s . T h i s a p p a r e n t l y does not r e s u l t i n any q u a l i t a t i v e d i f f e r e n c e s i n the d i e t s of the p r e d a t o r s but may serve as the b a s i s f o r c e r t a i n q u a n t i t i a t i v e d i f f e r e n c e s . For example, chironomid l a r v a e are an. important component of the d i e t s of both t r o u t and kokanee, but kokanee e x p l o i t s i g n i f i c a n t l y more l a r v a e than t r o u t i n f o u r out o f f i v e months of the year (Table 1 4 ) , i n c l u d i n g the month of August when both p r e d a t o r s are n o r m a l l y c l o s e l y a s s o c i a t e d w i t h b e n t h i c s u b - h a b i t a t s (Chapter 3 ) . C o a r s e - s c a l e h a b i t a t s e g r e g a t i o n f a i l e d t o e x p l a i n t h i s d i f f e r e n c e s i n c e chironomid l a r v a e are no more abundant i n o f f s h o r e than i n onshore l o c a t i o n s i n Marion Lake (Chapter 3 ) . TABLE 1 14. The s e a s o n a l e x p l o i t a t i o n o f chironomid l a r v a e by t r o u t and kokanee P r o b a b i l i t y t h a t Trout Kokanee t r o u t e x p l o i t Sample Date Larvae e x p l o i t e d Larvae expected Larvae e x p l o i t e d Larvae expected more l a r v a e than kokanee do Nov. 10 131 252 131 p < .00003 Feb. 11 30 64 45 p <,. 00003 A p r i l 14 7 5 12 N.S. June 233 406 382 209 p < .00003 Aug. 131 246 247 132 p < .00003 Normal a p p r o x i m a t i o n to the b i n o m i a l d i s t r i b u t i o n , one t a i l e d t e s t , S i e g e l , 1956. The expected f r e q u e n c i e s are d e r i v e d from the number of t r o u t and kokanee p r e s e n t i n each months sample and assuming t h a t each p r e d a t o r s h o u l d consume an equal number of l a r v a e . Data from E f f o r d & Tsumura (1973). 95 On the l a k e bottom, chironomid l a r v a e are the most abundant prey type to r e g u l a r l y reach s i z e s g r e a t e r than 2 mm. Because the m a j o r i t y o f l a r v a e are s u b - b e n t h i c , t u b e - d w e l l e r s , t r o u t , u s i n g hover and se a r c h o r c r u i s e and se a r c h t e c h n i q u e s , w i l l f r e q u e n t l y f a i l t o d e t e c t them. However g i v e n the g r e a t 2 abundance of l a r v a e (5 t o 10 per cm i n l a t e summer, McCauley, unpublished d a t a ) , I suggest t h a t a kokanee (12 cm i n le n g t h ) w i t h a 6-8 mm mouth gape (see Chapter 5) and engaged i n r e p e a t e d l y g r a b b i n g m o u t h f u l l s of sediment, w i l l e x p e r i e n c e a hi g h p r o b a b i l i t y o f d i s t u r b i n g l a r v a e . These may be then be d e t e c t e d v i s u a l l y o r perhaps w i t h the a i d of chemosensory d e t e c t o r s i n the mouth. I t i s c l e a r from the work o f I v l e v (1961) t h a t the concealment of chironomid l a r v a e under s i l t has d i f f e r e n t consequences f o r d i f f e r e n t s p e c i e s o f p r e d a t o r s . H i s experiments i n d i c a t e d t h a t the presence of s i l t on the bottom o f an aquarium d i d not s u b s t a n t i a l l y a l t e r the a v a i l a b i l i t y o f chironomid l a r v a e t o carp ( C y p r i n u s c a r p i o ) , but d r a s t i c a l l y reduced the consumption of l a r v a e by roach ( R u t i l u s r u t i l u s ) , when compared w i t h the n o - s i l t c o n t r o l s . The o b s e r v a t i o n s o f Schutz and N o r t h c o t e (1972) are e s p e c i a l l y r e l e v a n t to my s u g g e s t i o n t h a t d i f f e r e n t s e a r c h t e c h n i q u e s r e s u l t i n d i f f e r e n t i a l e x p l o i t a t i o n o f chironomid l a r v a e by t r o u t and kokanee. T h e i r d e s c r i p t i o n s l e a v e no doubt t h a t o t h e r p a i r s o f salmo n i d s , e x i s t i n g i n sympatry, use the tec h n i q u e s o f grab and s e a r c h , hover and se a r c h o r c r u i s e and search as s p e c i e s - s p e c i f i c a d a p t a t i o n s f o r l o c a t i n g b e n t h i c 96 p r e y . Furthermore, i n two s e t s o f l a b o r a t o r y experiments, they demonstrated t h a t grab and search (as p r a c t i s e d by S a l v e l i n u s malma) was g r e a t l y s u p e r i o r to hover and se a r c h (as p r a c t i s e d by Salmo c l a r k i ) as a technique f o r l o c a t i n g e i t h e r chironomid l a r v a e under a sand and l e a f l i t t e r s u b s t r a t e , o r , T u b i f e x worms under a sand s u b s t r a t e . T h i s evidence o f f e r s s t r o n g support f o r my c o n t e n t i o n t h a t the d i f f e r e n t s e a rch t e c h n i q u e s used by t r o u t and kokanee c o n s t i t u t e a l i k e l y reason f o r the g r e a t e r e x p l o i t a t i o n o f chironomid l a r v a e by kokanee compared w i t h t r o u t i n Marion Lake. The P o t e n t i a l E f f e c t s o f Search P o s i t i o n s on D i e t a r y P a t t e r n s The use of d i f f e r e n t s e a rch t e c h n i q u e s and d i f f e r e n c e s i n the frequency o f use of the same te c h n i q u e s l e a d s t o d i f f e r e n c e s i n the d i s t r i b u t i o n s of se a r c h p o s i t i o n s of t r o u t and kokanee when they forage a t the l a k e s u r f a c e o r bottom. What e f f e c t w i l l t h i s have on d i e t a r y p a t t e r n s ? A g e n e r a l answer to t h i s q u e s t i o n r e l i e s on the p o t e n t i a l i n t e r a c t i o n betwen the s e a r c h p o s i t i o n s o f the p r e d a t o r s and v i s u a l d e t e c t i o n o f p r e y . V i s u a l p r e d a t o r s l i k e t r o u t and kokanee d i s p l a y a t t a c k responses t h a t a re dependent on prey s i z e (Ware, 1971; H y a t t , t h i s s t u d y ) . On the b a s i s o f l a b o r a t o r y o b s e r v a t i o n s , Ware suggested t h a t when t r o u t s e arch f o r b e n t h i c prey they m a i n t a i n a f i x e d s e a r c h p o s i t i o n and t h a t t h i s f u n c t i o n s to c r e a t e a re f u g e from d e t e c t i o n f o r prey t h a t a re below a s p e c i f i c s i z e l i m i t . Adapting Ware's r e s u l t s t o the p r e s e n t s t u d y , i t i s apparent t h a t the, f u r t h e r a p r e d a t o r ' s s e a r c h p o s i t i o n i s from the l o c a t i o n o f the p r e y , the l a r g e r the prey must be t o 97 e l i c i t an a t t a c k ( F i g . 1 8 ) . I f these r e s u l t s are used t o p r e d i c t the minimum s i z e s of prey t h a t the p r e d a t o r s may d e t e c t a t the l a k e s u r f a c e o r bottom i t becomes apparent t h a t the minimum prey s i z e s d e t e c t e d by kokanee should be c o n s i d e r a b l y s m a l l e r than those d e t e c t e d by t r o u t ( F i g s . 17 & 18). T h i s i n f e r e n c e w i l l be m i s l e a d i n g i n the event t h a t t r o u t possess g r e a t e r v i s u a l a c u i t y than kokanee and respond to the same range of prey s i z e s from a g r e a t e r d i s t a n c e . For example Pr o t a s o v (1968) r e p o r t e d t h a t m u l l e t r e a c t e d v i s u a l l y t o 15 mm prey a t a d i s t a n c e of 25 cm w h i l e horse mackerel r e a c t e d t o i d e n t i c a l prey a t 65 cm. To t e s t f o r p o s s i b l e d i f f e r e n c e s i n the a b i l i t i e s of t r o u t and kokanee t o d e t e c t s i m i l a r prey I conducted a number of l a b o r a t o r y experiments. 4-B. LABORATORY COMPARISONS OF PREY DETECTION INTRODUCTION I have suggested t h a t t r o u t and kokanee w i l l o f t e n d e t e c t a d i f f e r e n t range of prey s i z e s i n the f i e l d because they m a i n t a i n d i f f e r e n t search p o s i t i o n s a t the l a k e s u r f a c e and bottom. A second p o s s i b i l i t y i s t h a t t r o u t and kokanee e x h i b i t d i f f e r e n c e s i n t h e i r a b i l i t i e s to v i s u a l l y d e t e c t prey and t h a t both s p e c i e s w i l l d e t e c t the same range of prey s i z e s i n the f i e l d d e s p i t e the maintenance o f d i f f e r e n t s e a r c h p o s i t i o n s . To t e s t t h i s h y p o t h e s i s I conducted l a b o r a t o r y experiments t o compare the r e a c t i v e d i s t a n c e (RD) o f t r o u t and kokanee where RD i s d e f i n e d as the d i s t a n c e between the f i s h and prey when the p r e d a t o r i n i t i a t e s an a t t a c k . 98 FIGURE 18 The minimum prey s i z e r e q u i r e d t o e l i c i t an a t t a c k by t r o u t i n s e a r c h p o s i t i o n s a t v a r i o u s d i s t a n c e s from the s u b s t r a t e s on which prey are l o c a t e d . Adapted from d a t a i n Table 1. (p. 95) o f Ware, 1971. - i i 1 1 1 1 r 20 40 60 S E A R C H POSITION (c .Like o t h e r i n v e s t i g a t o r s ( H o l l i n g , 1966; Beukema, 1968), I have assumed t h a t RD t o food items i s a f u n c t i o n o f the p r e d a t o r ' s a b i l i t y t o v i s u a l l y d e t e c t food items. For f i s h , t h i s assumption i s amply supported by Ware's e x t e n s i v e experiments (1971) which demonstrated t h a t RD i s c o n t r o l l e d by prey c h a r a c t e r i s t i c s such as s i z e , c o n t r a s t and movement but not by p r e d a t o r c h a r a c t e r i s t i c s such as hunger (= degree of gut f u l l n e s s ) . METHODS In experiments conducted t o a s s e s s the r e a c t i v e d i s t a n c e o f f i s h , the methods of prey p r e s e n t a t i o n may have an important e f f e c t on the r e s u l t s o b t a i n e d . In most s t u d i e s , the prey are i n t r o d u c e d one a t a t i m e , a t one end of a l o n g , narrow tank, and out o f s i g h t of the p r e d a t o r (Ware, 1971; Werner & H a l l , 1974; Confer & B l a d e s , 1975). Because o f the dimensions o f the e x p e r i m e n t a l t a n k , the p r e d a t o r must swim i n one d i r e c t i o n o n l y and w i l l i n e v i t a b l y approach the p r e y . The advantage of t h i s t echnique i s t h a t i t guarantees t h a t each a t t a c k by the p r e d a t o r i s a c o n t r o l l e d event ( u n l e s s the prey are " d e t e c t e d " w i t h the a i d of s t i m u l i u n i n t e n t i o n a l l y p r o v i d e d by the experimenter when i n t r o d u c i n g the prey t o the t a n k ) . The d i s a d v a n t a g e s are t h a t a g r e a t d e a l of time i s r e q u i r e d t o o b t a i n r e p l i c a t e o b s e r v a t i o n s , the p r e d a t o r s are o f t e n d i s t u r b e d by h a n d l i n g between s i n g l e a t t a c k s , and the shape of the tank f o r c e s the p r e d a t o r t o d e t e c t prey along 100 o n l y the a x i s o f the v i s u a l f i e l d i n which b i n o c u l a r v i s i o n f u n c t i o n s . In o r d e r to a l l o w prey d e t e c t i o n along a l l a x i s o f of the v i s u a l f i e l d , I used an e x p e r i m e n t a l arena (92 x 47 x 45 cm) which d i d not f o r c e the f i s h t o respond i n o n l y one d i r e c t i o n . To reduce the time r e q u i r e d t o o b t a i n r e p l i c a t e o b s e r v a t i o n s , I i n t r o d u c e d s i n g l e p r e d a t o r s i n t o the e x p e r i m e n t a l arena when i t c o n t a i n e d a p o p u l a t i o n of w e l l d i s p e r s e d p r e y . I then recorded the r e a c t i v e d i s t a n c e s o f f i s h i n c o n s e c u t i v e a t t a c k s on pre y . The f r o n t and bottom o f the e x p e r i m e n t a l tanks were marked o f f i n a n u m e r i c a l and a l p h a b e t i c a l l a t t i c e o f 2.5 cm squares. By r e c o r d i n g a c o o r d i n a t e from each o f these a t the begi n n i n g and end of an a t t a c k , I c o u l d measure the r e a c t i v e d i s t a n c e i n v o l v e d i n each a t t a c k , a t the end of an e x p e r i m e n t a l s e s s i o n . A number of f e a t u r e s a i d e d i n an a c c u r a t e d e t e r m i n a t i o n of the c o o r d i n a t e s . The d i s c o v e r y o f a prey item was always c l e a r l y i n d i c a t e d by a sudden change of v e l o c i t y o r of both v e l o c i t y and o r i e n t a t i o n by the p r e d a t o r . Because the f i s h were h i g h l y c o n d i t i o n e d t o the presence o f an o b s e r v e r i t was p o s s i b l e to f o l l o w t h e i r movements from a p o s i t i o n as c l o s e as the f r o n t o f the tank would a l l o w . T h i s along w i t h the modest dimensions o f the e x p e r i m e n t a l tank (92 x 47 x 45 cm) al l o w e d the o b s e r v e r enough freedom o f movement t o v i s u a l l y t r a c k the f i s h and a v o i d problems of p a r a l l a x . F i n a l l y , the shadow c a s t e on the tank bottom by the moving o r s t a t i o n a r y f i s h served as a u s e f u l cue i n determining- i t s t r u e p o s i t i o n 101 w i t h r e s p e c t to the bottom c o o r d i n a t e s . P r e l i m i n a r y t r i a l s w i t h an independent o b s e r v e r produced a c c e p t a b l e e s t i m a t e s o f the c o o r d i n a t e s i n d i c a t i n g the i n i t i a l and f i n a l p o s i t i o n s o f an a r t i f i c i a l . p r e d a t o r (a p i e c e o f hose) moving a t v e l o c i t i e s e q u i v a l e n t t o those e x h i b i t e d by t r o u t and kokanee i n the experiments. I t i s c r i t i c a l t o the i n t e r p r e t a t i o n o f the r e s u l t s t h a t the d i s t r i b u t i o n of r e a c t i v e d i s t a n c e v a l u e s (DRD) e x h i b i t e d by f i s h i n the arena i s a consequence of prey d e t e c t i o n t a k i n g p l a c e as a s e r i e s of s e p a r a t e e v e n t s . However, t h e r e i s an a l t e r n a t e i n t e r p r e t a t i o n t h a t must be d e a l t w i t h . S i n c e the p r e d a t o r s were h u n t i n g i n an arena t h a t c o n t a i n e d more than one prey i t e m , i t was p o s s i b l e f o r them t o d e t e c t more than one prey item a t a time ( i e : c o n s e c u t i v e responses to prey may not r e p r e s e n t s e p a r a t e i n c i d e n t s of prey d e t e c t i o n ) . In t h i s e vent, DRD would s i m p l y r e p r e s e n t an index of the d i s t a n c e between c o n s e c u t i v e prey l o c a t i o n s and not a measure of the . d i s t a n c e a t which the f i s h d e t e c t p r e y . To d i s t i n g u i s h between the a l t e r n a t i v e s t h a t DRD s e r v e s as an index of the p r e d a t o r ' s a b i l i t y t o d e t e c t prey o r of prey s p a c i n g , I examined the e f f e c t o f prey d e n s i t y changes on DRD. .Experiment 4.1 R e a c t i v e D i s t a n c e and Prey D e n s i t y I f DRD i s an index of prey s p a c i n g , t h e r e s h o u l d be a s i g n i f i c a n t l y g r e a t e r frequency o f long r e a c t i v e d i s t a n c e s i n experiments conducted a t low prey d e n s i t i e s compared t o 102 h i g h d e n s i t y experiments ( i e : c l o s e r prey s p a c i n g ) . T h i s w i l l not occur i f the d e t e c t i o n o f prey proceeds as a s e r i e s o f s e p a r a t e events i n the arena. I recorded DRD i n separate t r i a l s w i t h each of t h r e e kokanee (130-150 mm long) f o r a g i n g f o r m a y f l y (Ephemeroptera) l a r v a e . In the arena I used prey d e n s i t i e s of 20, 40, 100 and 200 per .42 m . A l l p r e d a t o r s were m a i n t a i n e d and handled i n a s t a n d a r d f a s h i o n b e f o r e e x p e r i m e n t a l t r i a l s (Chapter 1, g e n e r a l methods). Experiment 4.2 R e a c t i v e D i s t a n c e , Trout v s . Kokanee Experiment 2.1 i n d i c a t e d t h a t DRD w i l l serve as a measure of the a b i l i t y o f the p r e d a t o r s to d e t e c t prey (see r e s u l t s ) r a t h e r than as a measure of prey s p a c i n g . T h e r e f o r e , I proceeded t o use DRD i n comparing t r o u t and kokanee f o r d i f f e r e n c e s i n p e r c e p t u a l s e n s i t i v i t y to p r e y . Three rainbow t r o u t and t h r e e kokanee, o b t a i n e d from Marion Lake were used i n these experiments. They ranged i n l e n g t h from 130-160 mm. I conducted s e p a r a t e experiments f o r both s p e c i e s w i t h two d i f f e r e n t prey t y p e s . The l a r g e and s m a l l prey types used (16 mm d a m s e l f l y l a r v a e and 3 mm Daphnia) span a range of prey s i z e s i m i l a r t o t h a t n o r m a l l y encountered by the p r e d a t o r s i n Marion Lake. 103 RESULTS Experiment 4.1 R e a c t i v e D i s t a n c e and Prey D e n s i t y The DRD o f kokanee, responding t o m a y f l y l a r v a e , i n l o w - d e n s i t y experiments, d i d not e x h i b i t a g r e a t e r frequency of long r e a c t i v e d i s t a n c e s (p 5>.98, Kolmogorov-Smirnov, one-t a i l e d - t e s t , S i e g e l 1956) than i n h i g h - d e n s i t y experiments (Table 15). Thus i t appears t h a t the d e t e c t i o n o f prey items proceeds as a s e r i e s of s e p a r a t e events i n s p i t e o f the presence of numerous prey i n the e x p e r i m e n t a l arena. The s u b s t a n t i a l d i f f e r e n c e between DRD o f kokanee responding t o e i t h e r s t a t i o n a r y o r moving nymphs ( F i g . 19) c o n s t i t u t e s a second l i n e of evidence s u p p o r t i n g t h i s c o n c l u s i o n . I f DRD were s i m p l y measures of prey d i s t r i b u t i o n , they should have been d r a m a t i c a l l y d i f f e r e n t i n the low and h i g h - d e n s i t y t r i a l s and i d e n t i c a l f o r moving and s t a t i o n a r y p r e y . Thus, I conclude t h a t DRD i s a l e g i t i m a t e index of the p r e d a t o r ' s a b i l i t i e s t o d e t e c t p r e y . TABLE 15 D i s t r i b u t i o n s o f r e a c t i v e d i s t a n c e o f kokanee to s t a t i o n a r y ephemeropteran (mayfly) nymphs i n low d e n s i t y (20 & 40 prey/.42 square meters) and h i g h d e n s i t y (100 & 200 prey/.42 square meters) experiments. R e a c t i v e D i s t a n c e (cm) 0-5 6-10 11-15 16-20 21-25 26-30 31-35 36-40 N Number of Responses 7 16 15 5 1 2 0 0 46 0 7 15 8 2 0 1 1 34 D e n s i t y Low High 104 FIGURE 19 Frequency d i s t r i b u t i o n s o f r e a c t i v e d i s t a n c e o f kokanee responding t o e i t h e r s t a t i o n a r y o r moving ma y f l y nymphs (Ephemeroptera, C e n t r o p t i l u m s p . ) . N = the t o t a l number of o b s e r v a t i o n s . 105 Experiment 4.2 R e a c t i v e D i s t a n c e - T r o u t v s . Kokanee Comparisons of DRD ( F i g . 20) i n d i c a t e t h a t t r o u t do not respond more o f t e n t o prey a t g r e a t e r d i s t a n c e s than kokanee ( A l l comparisons p>.95, Kolmogorov-Smirnov, o n e - t a i l e d -t e s t ) . They a l s o i n d i c a t e t h a t l a r g e prey a re a t g r e a t e r r i s k o f d e t e c t i o n than s m a l l prey (compare 3mm Daphnia t o 16 mm odonates) and t h a t moving prey are a t g r e a t e r r i s k of d e t e c t i o n than s t a t i o n a r y prey o f i d e n t i c a l s i z e (compare s t a t i o n a r y odonates to moving o n e s ) . Because t r o u t and kokanee c o n s i s t e n t l y m a i n t a i n d i f f e r e n t s e arch p o s i t i o n s i n the f i e l d and respond t o i d e n t i c a l prey a t s i m i l a r d i s t a n c e s i n the l a b o r a t o r y , I conclude t h a t they w i l l e x p e r i e n c e d i f f e r e n t p r o b a b i l i t i e s o f d e t e c t i n g the same s i z e s o f prey i n the f i e l d . For i n s t a n c e , t r o u t u s u a l l y m a i n t a i n search p o s i t i o n s t h a t are f u r t h e r than 20 cm. away from the sediment and consequently should seldom d e t e c t 2 b e n t h i c prey as s m a l l as 3mm (the approximate s i z e of the s m a l l prey used i n experiment 4.2). C o n v e r s e l y , kokanee should f r e q u e n t l y d e t e c t prey i n these s m a l l s i z e c l a s s e s . I t i s wise to e x e r c i s e some c a u t i o n i n the a p p l i c a t i o n of l a b o r a t o r y d e t e r m i n a t i o n s o f r e a c t i v e d i s t a n c e t o the f i e l d s i t u a t i o n . Although the l a b o r a t o r y d a t a c l e a r l y i n d i c a t e t h a t both t r o u t and kokanee should o c c a s i o n a l l y a t t a c k prey the s i z e of l a r v a l odonates, from d i s t a n c e s g r e a t e r than 50 cm., I never observed a t t a c k s on b e n t h i c prey i n the f i e l d from d i s t a n c e s g r e a t e r than 50 cm. S i m i l a r l y Ware's (1971) d e t e r -m i n a t i o n s o f r e a c t i v e d i s t a n c e i n the l a b o r a t o r y suggest t h a t 106 FIGURE 20 A comparison of the frequency d i s t r i b u t i o n s f o r r e a c t i v e d i s t a n c e o f t r o u t and kokanee to l a r g e and s m a l l prey t y p e s , (a.) r e a c t i v e d i s t a n c e o f kokanee to l a r g e prey (16 mm odonates, Enallagma sp.) t h a t are moving or s t a t i o n a r y (b.) r e a c t i v e d i s t a n c e of t r o u t to l a r g e prey t h a t are moving o r s t a t i o n a r y (c.) r e a c t i v e d i s t a n c e o f kokanee to s m a l l prey (3 mm z o o p l a n k t o n , Daphnia sp.) t h a t are moving (d.) r e a c t i v e d i s t a n c e o f t r o u t to s m a l l prey t h a t are moving. ^90T 107 v e r y h i g h c o n t r a s t t a r g e t s (white c y l i n d e r s of l i v e r o f f e r e d a g a i n s t a b l a c k background) of a p p r o x i m a t e l y 15 mm are seldom d e t e c t e d from g r e a t e r than 70 cm by t r o u t , w h i l e my f i e l d 2 o b s e r v a t i o n s i n d i c a t e t h a t t a r g e t s o f o n l y 10 mm may be d e t e c t e d a t d i s t a n c e s g r e a t e r than 85 cm. T h i s s i m p l y emphasizes the d i f f i c u l t y o f e x a c t l y d u p l i c a t i n g f i e l d c o n d i t i o n s i n the l a b o r a t o r y , and warns a g a i n s t the e x p e c t a t i o n t h a t l a b o r a t o r y d e t e r m i n a t i o n s of r e a c t i v e d i s t a n c e , combined w i t h f i e l d e s t i m a t e s o f search p o s i t i o n s , w i l l l e a d t o p r e d i c t i o n s o f the a b s o l u t e minimum s i z e of d e t e c t a b l e prey i n the f i e l d . DISCUSSION The Role o f Search P o s i t i o n s i n Producing S p e c i f i c D i e t a r y D i f f e r e n c e s Between Trout and Kokanee There i s a g e n e r a l t r e n d f o r t r o u t from Marion Lake to i n c l u d e a g r e a t e r p r o p o r t i o n . o f l a r g e prey i n t h e i r d i e t s than kokanee of comparable s i z e s do (Chapter 2, F i g . 4 ) . T h i s t r e n d i s i n p a r t r e l a t e d t o the f a c t t h a t f o r many prey types eaten by b o t h . t r o u t and kokanee, t r o u t consume a h i g h e r p r o p o r t i o n o f l a r g e i n d i v i d u a l s than kokanee (eg. amphipods-F i g . 5, m o l l u s c s - F i g . 6, z o o p l a n k t o n - F i g . 7 ) . R e s u l t s presented i n t h i s c h a p t e r h e l p e x p l a i n why t h i s t r e n d o c c u r s . L a b o r a t o r y evidence (Experiment 4.2) suggested t h a t t r o u t are unable to d e t e c t s m a l l prey a t g r e a t e r d i s t a n c e s than kokanee, y e t i n the f i e l d t r o u t c o n s i s t e n t l y assumed se a r c h p o s i t i o n s t h a t were f u r t h e r away than kokanee from the l o c a t i o n s o f prey a t the l a k e s u r f a c e o r bottom. T h e r e f o r e , 108 a g i v e n s i z e c l a s s of a s i n g l e prey type must ex p e r i e n c e d i f f e r e n t p r o b a b i l i t i e s o f d e t e c t i o n by t r o u t o r kokanee and, i n g e n e r a l , the minimum s i z e s of b e n t h i c o r s u r f a c e prey d e t e c t e d by kokanee w i l l be much s m a l l e r than those d e t e c t e d by t r o u t . Thus, i t i s no l o n g e r s u r p r i s i n g t h a t s m a l l H y a l e l l a sp. (3-4mm) make up a g r e a t e r p r o p o r t i o n o f the b e n t h i c amphipods i n the d i e t o f kokanee, w h i l e l a r g e Crangonyx sp. (7-10mm) are more abundant i n the d i e t o f t r o u t (Chapter 2, F i g . 5 and Table 4 ) . The i n t e r a c t i o n between v i s u a l d e t e c t i o n o f prey and search p o s i t i o n s o f p r e d a t o r s may a l s o account f o r d i f f e r e n c e s i n p a t t e r n s o f e x p l o i t a t i o n o f o t h e r b e n t h i c prey t y p e s . S m a l l m o l l u s c s such as p i s i d i u m spp. are o f t e n absent from the d i e t o f t r o u t but are u s u a l l y p r e s e n t i n the d i e t o f kokanee (Sandercock, 1969 and Chapter 2, F i g . 5 ) . S i m i l a r l y , the m a j o r i t y o f c a d d i s l a r v a e ( T r i c h o p t e r a ) i n the d i e t o f kokanee are from s m a l l s i z e c l a s s e s (as indexed by head c a p s u l e w i d t h ) , w h i l e t r o u t e x p l o i t a g r e a t e r p r o p o r t i o n o f l a r g e c a d d i s l a r v a e (Table 1 6 ) . TABLE 16. Head c a p s u l e w i d t h s (mm) o f l a r v a l t r i c h o p t e r a n s consumed by t r o u t and kokanee. Data from Winterbourn, 1971. Head Width (mm) 0.4-0.8 0.8-1.2 1.2-1.6 1.6-2.4 No. eaten by t r o u t 39 151 232 265 No. eaten by kokanee 8 13 109 The Role of Search Techniques and Search P o s i t i o n s i n Producing D i f f e r e n c e s Between the P r o p o r t i o n s Of Prey Types Observed i n the N a t u r a l Environment and i n the D i e t s of Trout and Kokanee Knowledge gained c o n c e r n i n g s e a r c h t e c h n i q u e s and sea r c h p o s i t i o n s not o n l y p r o v i d e s i n s i g h t s i n t o the reasons f o r d i e t a r y d i f f e r e n c e s between the p r e d a t o r s but a l s o c o n t r i b u t e s t o a g r e a t e r understanding o f why d i s c r e p a n c i e s may e x i s t between the apparent a v a i l a b i l i t y o f p o t e n t i a l prey i n the l a k e environment and i n the d i e t s of t r o u t and kokanee (Chapter 2, F i g ' s . 8 & 9 ) . In essence, the d i s c r e p a n c i e s e x i s t because the sampling d e v i c e s (eg. Ekman dredge, Kajak c o r e , Hargrave sampler) used by s c i e n t i s t s t o determine the r e l a t i v e abundance of i n v e r t e b r a t e s i n the l a k e do not remove samples of prey from the environment i n the same way t h a t e i t h e r t r o u t o r kokanee do. For example, an Ekman dredge removes samples of b e n t h i c prey w i t h o u t d i s t i n g u i s h i n g whether they a re exposed on the mud s u r f a c e , concealed j u s t beneath the s u r f a c e o r b u r i e d under s e v e r a l mm o f sediment. By c o n t r a s t , s e a r c h t e c h n i q u e s o f t r o u t a l l o w them to o b t a i n o n l y those b e n t h i c prey t h a t are exposed on o r above the sediment s u r f a c e and se a r c h t e c h n i q u e s o f kokanee pr e v e n t them from g a i n i n g access t o prey b u r i e d more than a few mm beneath the sediment. S i n c e t h e r e are r e g u l a r p a t t e r n s i n the v e r t i c a l d i s t r i b u t i o n o f d i f f e r e n t prey types w i t h i n b e n t h i c sediments ( B u r g i s e t a l . 1973), the depth o f p e n e t r a t i o n o f sampling gear and p r e d a t o r s b i a s e s them t o o b t a i n some prey but not o t h e r s o r t o o b t a i n 110 d i f f e r e n t p r o p o r t i o n s o f the same prey t y p e s . T h i s may e x p l a i n why the s i n g l e most abundant s p e c i e s of chironomid l a r v a e (by numbers o r by volume) p r e s e n t i n Marion Lake has never been recorded i n the d i e t s of the f i s h ( E f f o r d & Tsumura, 1973). Trout not o n l y e x p l o i t a g r e a t e r p r o p o r t i o n o f l a r g e prey than kokanee, but a l s o o v e r e x p l o i t l a r g e s i z e d i n d i v i d u a l s of a g i v e n prey type r e l a t i v e t o t h e i r abundance i n the environment. Amphipods (Chapter 2, F i g . 5) and c a d d i s l a r v a e (Winterbourn, 1971) are two prey types which demonstrate t h i s p a t t e r n . Winterbourn (1971) found t h a t 72% o f the c a d d i s eaten by t r o u t i n Marion Lake belonged t o the l a s t o r second from l a s t i n s t a r o f each s p e c i e s . He concluded t h a t t r o u t were " s e l e c t i n g " the l a r g e s t s i z e s of c a d d i s a v a i l a b l e a t a l l times of y e a r . T h i s was e s p e c i a l l y apparent when the s p e c i e s c o m p o s i t i o n of c a d d i s eaten each month was r e l a t e d t o the l i f e h i s t o r i e s o f each of the s p e c i e s . D i f f e r e n t s p e c i e s o f c a d d i s were preyed upon s e q u e n t i a l l y as t h e i r l a s t i n s t a r s appeared i n the l a k e . In l a t e summer and e a r l y f a l l , c a d d i s l a r v a e were of l e a s t importance i n the d i e t of t r o u t , a l t h o u g h they were p r e s e n t at maximum abundance f o r the year as s m a l l , e a r l y - i n s t a r s t a g e s . I f the d i s t r i b u t i o n o f search p o s i t i o n s observed f o r t r o u t are r e p r e s e n t a t i v e of the times of the year when o b s e r v a t i o n s were not c a r r i e d out (evidence t h a t t h i s i s l i k e l y w i l l be pr e s e n t e d i n a l a t e r c h a p t e r ) , then seasonal changes i n the p r o b a b i l i t y of d e t e c t i o n of s i n g l e prey s p e c i e s (eg. c a d d i s ) w i l l occur as changes i n s i z e s of prey o c c u r a t d i f f e r e n t developmental stages (Ware, 1973) . T h i s i s a p l a u s i b l e e x p l a n a t i o n t o account f o r the s e a s o n a l and a p p a r e n t l y size-dependent e x p l o i t a t i o n of both c a d d i s l a r v a e and amphipods by t r o u t . Unexplained D i e t a r y p a t t e r n s Knowledge about search t e c h n i q u e s , s e a r c h p o s i t i o n s and m i c r o h a b i t a t - s p e c i f i c s e a r c h i n the f i e l d has improved my a b i l i t y t o e x p l a i n the b a s i s f o r d i f f e r e n c e s i n the prey c o n t e n t s of p r e d a t o r s and environment, but a number of s i g n i f i c a n t d i e t a r y p a t t e r n s c o n t i n u e t o r e s i s t my attempts a t e x p l a n a t i o n . I f t r o u t and kokanee o b t a i n e d a l l o f the s m a l l - p r e y component of t h e i r d i e t s from s u r f a c e o r bottom s u b - h a b i t a t s , then search p o s i t i o n s a lone would generate the observed d i f f e r e n c e s i n the s i z e - f r e q u e n c y d i s t r i b u t i o n s of prey t h a t they o b t a i n . However, many o f the s m a l l prey t h a t t r o u t and kokanee e x p l o i t o r i g i n a t e from the water column. In the l a t e r summer months, kokanee of average s i z e o b t a i n a l a r g e p r o p o r t i o n of t h e i r food (48% by weight i n Aug.) from s m a l l ( u s u a l l y l e s s than 1 mm body l e n g t h ) c l a d o c e r a n s and copepods found i n the water column ( E f f o r d & Tsumura, 1973). Although i n l a t e summer t r o u t spend a t l e a s t as much time as kokanee i n the water column, zooplankton never c o n t r i b u t e more than 0.5% (by weight) of t h e i r d i e t . For the s m a l l e s t t r o u t and kokanee which a t times o b t a i n v i r t u a l l y a l l of t h e i r food from the water column (CHapter 2, F i g . 7 & t a b l e 5 ) , t h e r e are major d i f f e r e n c e s i n the s p e c i e s and s i z e - c l a s s c o m p o s i t i o n of zooplankton e x p l o i t e d . No mechanism proposed thus f a r w i l l s a t i s f a c t o r i l y e x p l a i n these d i f f e r e n c e s . The maintenance o f s p e c i e s - s p e c i f i c , s e a r c h p o s i t i o n s by t r o u t and kokanee w i l l c l e a r l y favour the d e t e c t i o n and e x p l o i t a t i o n of a g r e a t e r p r o p o r t i o n of s m a l l c a d d i s l a r v a e by kokanee than by t r o u t but i t does not o f f e r a ready e x p l a n a t i o n f o r why kokanee e x p l o i t so few c a d d i s l a r v a e i n a l l s i z e c l a s s e s r e l a t i v e t o t r o u t . In the next c h a p t e r , I w i l l address the g e n e r a l h y p o t h e s i s t h a t d i f f e r e n c e s i n morphology and the behaviour of t r o u t and kokanee d u r i n g approach, c a p t u r e and i n g e s t i o n of prey w i l l account f o r some of these unexplained d i e t a r y d i f e r e n c e s . SUMMARY 1. Trout and kokanee e x h i b i t some s p e c i e s s p e c i f i c s e a r ch t e c h n i q u e s (Table 12). 2. The search t e c h n i q u e s t h a t t r o u t and kokanee share are not p r a c t i s e d w i t h the same frequency and a r e not always employed t o l o c a t e prey w i t h i n the same s u b - h a b i t a t s . 3. Search t e c h n i q u e s employed by kokanee are more sub-h a b i t a t s p e c i f i c than those used by t r o u t (Table 12). 4; The m a j o r i t y o f kokanee search f o r b e n t h i c prey from p o s i t i o n s t h a t a r e w i t h i n 5 cm o f the sediment s u r f a c e , w h i l e t r o u t are most o f t e n observed s e a r c h i n g f o r b e n t h i c prey from p o s i t i o n s some 15 t o 30 cm o f f o f the bottom ( F i g . 1 6 ) . 113 5. The m a j o r i t y o f kokanee search f o r l a k e s u r f a c e prey from p o s i t i o n s t h a t a re 5 t o 30 cm below the l a k e s u r f a c e , w h i l e t r o u t are most o f t e n observed s e a r c h i n g f o r these prey from p o s i t i o n s some 45 t o 100 cm below the s u r f a c e ( F i g . 1 7 ) . 6. D i f f e r e n c e s i n the se a r c h t e c h n i q u e s o f t r o u t and kokanee r e s u l t i n some degree o f m i c r o h a b i t a t s p e c i f i c s e a r c h but are not e a s i l y r e l a t e d t o any q u a l i t a t i v e ' d i f f e r e n c e s i n t h e i r d i e t s . 7. D i f f e r e n c e s i n s e a r c h t e c h n i q u e s l i k e l y account f o r c e r t a i n q u a n t i t a t i v e d i f f e r e n c e s i n the d i e t s of the p r e d a t o r s such as the g r e a t e r u t i l i z a t i o n o f chironomid l a r v a e by kokanee than by t r o u t . 8. Trout and kokanee c o n s i s t e n t l y m a i n t a i n d i f f e r e n t s e a r c h p o s i t i o n s i n the f i e l d and respond t o i d e n t i c a l prey a t s i m i l a r d i s t a n c e s i n the l a b o r a t o r y ( F i g . 2 0 ) , t h e r e f o r e they must e x p e r i e n c e d i f f e r e n t p r o b a b i l i t i e s o f d e t e c t i n g the same s i z e s of e i t h e r b e n t h i c o r l a k e s u r f a c e prey i n the f i e l d . 9. The sampling d e v i c e s used by s c i e n t i s t s t o determine the r e l a t i v e abundance o f b e n t h i c i n v e r t e b r a t e s i n the l a k e do not d i s c r i m i n a t e between "prey" exposed on the mud s u r f a c e , l o c a t e d j u s t beneath the mud s u r f a c e , o r l o c a t e d s e v e r a l mm under the mud s u r f a c e . When s e a r c h i n g f o r b e n t h i c p r e y , t r o u t and kokanee undoubtedly do d i s c r i m i n a t e i n t h i s f a s h i o n . T h e r e f o r e some d i s c r e p a n c i e s w i l l e x i s t between the apparent a v a i l a b i l i t y o f prey i n the l a k e (as measured by s c i e n t i s t s ) and p a t t e r n s o f prey u t i l i z a t i o n by t r o u t and kokanee because s c i e n t i s t s and f i s h do not use the same procedures t o d e t e c t prey sampled from the l a k e environment. 10.) D i f f e r e n c e s i n p r e y - s e a r c h b e h a v i o u r s of t r o u t and kokanee do not p r o v i d e a ready e x p l a n a t i o n f o r why kokanee o b t a i n a g r e a t e r p r o p o r t i o n o f t h e i r d i e t than t r o u t (by no. o r by volume) from r e l a t i v e l y s m a l l ( <1 mm)water-column prey o r f o r why kokanee o b t a i n so few c a d d i s l a r v a e o f a l l s i z e s i n t h e i r d i e t r e l a t i v e t o t r o u t . 115 CHAPTER 5 THE RELATIONSHIP BETWEEN ATTACK BEHAVIOURS, MORPHOLOGICAL CHARACTERISTICS AND DIETARY PATTERNS OF TROUT AND KOKANEE IN MARION LAKE 5-A. F i e l d D e s c r i p t i o n s INTRODUCTION Each s u c c e s s f u l a t t a c k on prey by p r e d a t o r s u s u a l l y i n v o l v e s a s e r i e s of beha v i o u r s which i n c l u d e o r i e n t a t i o n , approach, c o n t a c t , c a p t u r e , m a n i p u l a t i o n and i n g e s t i o n of the prey. P r e d a t o r s cannot s u c c e s s f u l l y approach and ca p t u r e every prey item they d e t e c t nor can they s u c c e s s f u l l y m a n i p u l a t e and i n g e s t every prey item they c a p t u r e . I n t e r a c t i o n s between the behaviour and morphology o f a g i v e n p r e d a t o r determine whether an a t t a c k i s s u c c e s s f u l o r not. Consequently, d i f f e r e n c e s between p r e d a t o r s i n both a t t a c k b e h a v i o u r s and morphologies may promote d i v e r g e n t d i e t a r y p a t t e r n s . M o r p h o l o g i c a l d i f f e r e n c e s and t h e i r r e l a t i o n s h i p t o r e s o u r c e p a r t i t i o n i n g by p r e d a t o r s have been s t u d i e d more e x t e n s i v e l y than b e h a v i o u r a l d i f f e r e n c e s . Schoener (1974) has p o i n t e d out the convenience and o c c a s i o n a l n e c e s s i t y o f e s t i m a t i n g d i f f e r e n c e s i n r e s o u r c e u t i l i z a t i o n o f animals by us i n g m o r p h o l o g i c a l c h a r a c t e r i s t i c s o f s p e c i e s . The most common i n d i c a t o r i s the s i z e of the f e e d -ing s t r u c t u r e , which i s u s u a l l y c o r r e l a t e d w i t h mean food s i z e , hardness, o r depth i n some p r o t e c t i v e medium. Many e x p e r i m e n t a l s t u d i e s support the c o n t e n t i o n t h a t c o r r e l a t i o n s between morpho-l o g i c a l f e a t u r e s and food types do e x i s t . Small heteromyid ro d e n t s appear t o s e l e c t seeds on the b a s i s o f ease of h a n d l i n g (Rozenzweig & S t e r n e r , 1970) . O v e r l a p i n n e c t a r r e s o u r c e u t i l i z a t i o n by bumblebee s p e c i e s ( H e i n r i c h , 1976) i s s l i g h t due t o d i f f e r e n c e s i n tongue l e n g t h . Numerous authors (Grant e t a l . , 1976; Hespenheide, 1975; W i l l s o n , 1972; Rear, 1962) have presented evidence t h a t b i r d s w i t h d i f f e r e n t b i l l s i z e s e x p l o i t foods t h a t are d i f f e r e n t w i t h r e s p e c t to s i z e ( l a r g e v s . s m a l l ) o r t e x t u r e (hard v s . s o f t ) . R e s u l t s from these s t u d i e s are not w i t h o u t e x c e p t i o n s P u l l i a m and Enders (1971) found t h a t f i v e s p e c i e s o f f i n c h e s i n o l d - f i e l d h a b i t a t s d i d not d i v i d e food r e s o u r c e s by s i z e , i n s p i t e of p o s s e s s i n g a wide range of b i l l s i z e s (9.3-18.9 mm). In Root's thorough study (1967) o f f i v e f o l i a g e g l e a n i n g b i r d s , prey s i z e was not c l o s e l y r e l a t e d t o e i t h e r body s i z e o r b i l l s i z e . Hespenheide (1975) has warned t h a t many cases e x i s t i n which s p e c i e s are q u i t e s i m i l a r m o r p h o l o g i c a l l y but e x p l o i t q u i t e d i f f e r e n t d i e t a r y items due t o b e h a v i o u r a l d i f f e r e n c e s . S i m i l a r l y , o t h e r s p e c i e s which possess s t r i k i n g d i f f e r e n c e s i n morphology, may not e x h i b i t s t r i k i n g d i f f e r e n c e s i n d i e t , a g a i n f o r b e h a v i o u r a l reasons. F i e l d o b s e r v a t i o n s , complemented by e x p e r i m e n t a l s t u d i e s o f f e r one of the b e s t ways i n which to assess the l i n k s between a t t a c k b e h a v i o u r s , m o r p h o l o g i c a l c h a r a c t e r i s t i c s and d i e t a r y h a b i t s o f p r e d a t o r s . I have i d e n t i f i e d a number of d i f f e r e n c e s i n the d i e t a r y p a t t e r n s o f t r o u t and kokanee from Marion Lake (Chapter 117 Many o f these d i f f e r e n c e s have been accounted f o r on the b a s i s of e i t h e r p r e d a t o r s p a t i a l s e g r e g a t i o n (Chapter 3) o r p r e d a t o r s e a r c h b e h a v i o u r s (Chapter 4 ) , however, the b a s i s f o r o t h e r d i e t a r y d i f f e r e n c e s remains unknown (see d i s c u s s i o n , Chapter 4 ) . T h e r e f o r e , the c e n t r a l f o c u s o f t h i s c h a p t e r i s t o t e s t the g e n e r a l h y p o t h e s i s t h a t d i f f e r e n c e s i n p r e d a t o r a t t a c k b e h a v i o u r s i n t e r a c t i n g w i t h d i f f e r e n c e s i n p r e d a t o r morphologies w i l l account f o r some of these u n e x p l a i n e d d i e t a r y d i f f e r e n c e s . The c h a p t e r i s d i v i d e d i n t o two s e c t i o n s . The f i r s t s e c t i o n i s e s s e n t i a l l y d e s c r i p t i v e . I w i l l d e s c r i b e the t e c h n i q u e s of approach and c a p t u r e t h a t a r e used by t r o u t and kokanee d u r i n g a t t a c k s on prey i n the f i e l d . I w i l l a l s o des-c r i b e d i f f e r e n c e s i n m o r p h o l o g i c a l c h a r a c t e r i s t i c s o f the p r e d a t o r s t h a t may a f f e c t t h e i r a t t a c k success w i t h p r e y . On the b a s i s of these d e s c r i p t i o n s I w i l l propose a number of hypotheses concerning the l i k e l y outcome of a t t a c k s on prey by t r o u t and kokanee. The second s e c t i o n o f the c h a p t e r c o n s i s t s o f a s e r i e s of l a b o r a t o r y experiments designed t o t e s t these hypotheses. F i n a l l y , I w i l l d i s c u s s the s i g n i f i c a n c e o f the f i n d i n g s f o r e x p l a n a t i o n s of p a t t e r n s of f o o d - r e s o u r c e p a r t i t i o n i n g e x h i b i t e d by the p r e d a t o r s i n the f i e l d . METHODS A t t a c k Techniques D e s c r i p t i o n s o f the a t t a c k t e c h n i q u e s o f f r e e r a n g i n g t r o u t and kokanee were completed a t the same time as o b s e r v a t i o n s on t h e i r s e a rch b e h a v i o u r s . D e t a i l s c o n c e r n i n g s p e c i f i c l o c a t i o n s , t i m e s , and t e c h n i q u e s o f o b t a i n i n g o b s e r v a t i o n s can be found i n t h i s e a r l i e r work (see Chapters 3 and 4 ) . One problem w i t h d i r e c t o b s e r v a t i o n i n the f i e l d i s t h a t the f i n e r d e t a i l s of a t t a c k s on prey items are seldom seen s i n c e the ob s e r v e r i s u s u a l l y no c l o s e r than .5 m t o the p r e d a t o r . For t h i s reason I w i l l i n c l u d e some i n f o r m a t i o n o b t a i n e d d u r i n g p r e l i m i n a r y l a b o r a t o r y o b s e r v a t i o n s o f a t t a c k t e c h n i q u e s o f t r o u t and kokanee e x p l o i t i n g a v a r i e t y o f pre y . M o r p h o l o g i c a l C h a r a c t e r i s t i c s S u p e r f i c i a l l y rainbow t r o u t appear t o be s t o c k i e r than kokanee of s i m i l a r l e n g t h . From a d o r s a l a s p e c t , t r o u t a l s o appear t o be l e s s s t r e a m l i n e d and t o possess a more massive jaw musculature than kokanee. Thus, I a n t i c i p a t e d t h a t s i g n i f i c a n t d i f f e r e n c e s would e x i s t i n t h e i r b o d y -length v e r s u s weight r e l a t i o n s h i p s as w e l l as i n the s t r u c t u r e o f t h e i r o r a l c a v i t i e s . I compiled l e n g t h and weight d a t a on f i s h taken from a summer t r a p p i n g program. Trout and kokanee were removed from t r a p - n e t s ( i n which they had been c a p t i v e s f o r 24-72 hours) a n e s t h e t i z e d with-MS 222, measured f o r f o r k l e n g t h t o the near-e s t mm, and then weighed on a gram b a l a n c e . A f t e r t a g g i n g and r e c o v e r y , f i s h were r e l e a s e d t o the l a k e . I determined the r e l a t i o n s h i p between jaw w i d t h and standard l e n g t h s o f p r e d a t o r s from measurements taken on samples o f f i s h t h a t had been s t o r e d i n 10% f o r m a l i n . Jaw w i d t h was measured w i t h V e r n i e r c a l i p e r s , as the h o r i z o n t a l d i s t a n c e ( t o the n e a r e s t .1 mm) between the 119 p o s t e r i o r t i p s o f the p r e m a x i l l a r i e s . G i l l - r a k e r examinations were a l s o conducted on pr e s e r v e d f i s h . RESULTS Techniques o f Approach and Capture Once a prey had been d e t e c t e d by a p r e d a t o r i n the f i e l d , I c o u l d d i s t i n g u i s h between f i v e t e c h n i q u e s o f approach and c a p t u r e t h a t were used d u r i n g a t t a c k s on prey (Table 1 7 ) . None of these approach and ca p t u r e t e c h n i q u e s were s p e c i f i c t o o n l y one search technique (Chapter 4) but some were used more f r e q u e n t l y by t r o u t o r kokanee. TABLE 17. Techniques o f approach and ca p t u r e used by t r o u t and kokanee d u r i n g a t t a c k s on prey i n the f i e l d . Technique L e v e l o f Occurrence T r o u t Kokanee Rush S t a l k Dart Jump +++ + N ++ ++ ++ ++ N + Scrape + +++ very common ++ common + o c c a s i o n a l N never observed 1. Rush A f t e r d e t e c t i o n o f a food item the p r e d a t o r o r i e n t s b r i e f l y t o f a c e the t a r g e t , a c c e l e r a t e s to reach i t s l o c a t i o n and j u s t b e f o r e the moment of c o n t a c t opens i t s mouth to e n g u l f the p r e y . Trout may r u s h prey w h i l e o p e r a t i n g i n hover-and - s e a r c h o r c r u i s e - a n d - s e a r c h modes (Chapter 4 ) . The r a p i d a c c e l e r a t i o n t o reach i n v e r t e b r a t e p r e y , which are g e n e r a l l y d e t e c t e d from l e s s than 80 cm away, i s accomplished by s e v e r a l p o w e r f u l s t r o k e s of the c a u d a l f i n . Rough e s t i m a t e s of times taken t o cover the d i s t a n c e t o prey suggest t h a t t r o u t a t t a i n v e l o c i t i e s of 50 t o 150 cm per second d u r i n g a rush on i n v e r t e b r a t e p r e y . An e n t i r e a t t a c k sequence ( o r i e n t , approach, e n g u l f ) i s u s u a l l y completed w i t h i n 2 t o 3 seconds. Because kokanee m a i n t a i n s e a r c h p o s i t i o n s t h a t are c l o s e r t o the l o c a t i o n s o f p r e y , they have fewer o p p o r t u n i t i e s t o use t h i s t echnique and when r u s h i n g prey they do not appear t o a t t a i n the h i g h v e l o c i t i e s t h a t t r o u t do. 2. S t a l k A f t e r s i g h t i n g a p r e y , a swimming p r e d a t o r t y p i c a l l y d e c e l e r a t e s by p e r f o r m i n g b r a k i n g movements w i t h both p e c t o r a l and p e l v i c f i n s . A f t e r a b r i e f pause f o r o r i e n t a t i o n , a slow approach (1 t o 5 cm per second) i s accomplished w i t h the a i d o f p e c t o r a l f i n s c u l l i n g . The p r e d a t o r may s t o p e n t i r e l y b e f o r e the f i n a l phase of the approach when e i t h e r a r u s h ( t r o u t ) o r a d a r t (kokanee - see d e s c r i p t i o n below) w i l l be used t o cover the remaining d i s t a n c e ( u s u a l l y j u s t a few cm) t o the p r e y . 121 Tr o u t do not use t h i s technique of approach v e r y o f t e n and I have o n l y observed them use i t i n approaching v e r t e b r a t e prey such as salamanders (Ambystoma g r a c i l e , T a r i c h a g r a n u l o s a ) o r s t i c k l e b a c k s ( G a s t e r o s t e u s a c u l e a t u s ) . Kokanee use t h i s approach f r e q u e n t l y when they are hu n t i n g f o r b e n t h i c prey i n the f i e l d o r f o r c e r t a i n s p e c i e s o f zoo p l a n k t o n i n the l a b o r a t o r y . 3. Dart A f t e r completing a s t a l k i n g approach, kokanee employ d a r t t o cover the f i n a l 2 t o 5 cm between themselves and c e r t a i n types o f pre y . Dart begins from a s t a t i o n a r y p o s i t i o n . The p r e d a t o r f l e x e s i t s body i n t o an exaggerated sigmoid p o s t u r e and then r a p i d l y t h r u s t s forward w i t h mouth agape t o e n g u l f the pr e y . The technique d i f f e r s from r u s h o r s t a l k i n t h a t the p o s t u r e assumed to perform d a r t i s w e l l o u t s i d e of the range of normal swimming movements, the d i s t a n c e covered by d a r t i s always l e s s than one body l e n g t h of the p r e d a t o r , and the move-ment i s completed w i t h i n a f r a c t i o n of a second. In the l a b o r a t o r y t h i s t echnique was p r e d i c t a b l y evoked by copepods (Diaptomus kenai) and l e s s commonly by amphipods ( H y a l e l l a a zteca) which e x h i b i t marked avoidance responses t o approaches by p r e d a t o r s . In the f i e l d kokanee commonly e x h i b i t e d t h i s movement w h i l e f o r a g i n g f o r b e n t h i c p r e y . I never observed t r o u t use d a r t d u r i n g a t t a c k s on prey i n the f i e l d o r i n the l a b o r a t o r y . 122 4. Jump T h i s movement i s o n l y executed by t r o u t when they d e t e c t a d u l t i n s e c t s above the l a k e s u r f a c e . The i n i t i a l s t a g e s o f the a t t a c k are i d e n t i c a l t o r u s h , however the a c c e l e r a t i o n o f the p r e d a t o r a l l o w s i t t o break the s u r f a c e o f the water and t o c o n t i n u e i n t o the a i r to c o n t a c t and c a p t u r e the p r e y . A s m a l l t r o u t (15 cm) which a c c e l e r a t e s from a d i s t a n c e o f 50 t o 80 cm below the l a k e s u r f a c e may c l e a r the water by more than a body l e n g t h when l e a p i n g f o r a e r i a l prey such as c a d d i s a d u l t s . • 5. Scrape While moving along a f i r m , smooth s u b s t r a t e , a f i s h , w i t h i t s mouth h e l d f l u s h t o the s u b s t r a t e s u r f a c e , c a r r i e s out repeated b i t i n g movements. T r o u t use t h i s t a c t i c t o a t t a c k s m a l l , s e s s i l e animals which are anchored t o the underside of l i l y l e a v e s , p l a n t stems, submerged l o g s and l i m b s . Trout and kokanee most f r e q u e n t l y use the technique t o o b t a i n prey from l i l y l e a v e s which may possess more than 25 c l a d o c e r a n s per square cm i n l a t e summer ( S t a r r , 1973). A p r e d a t o r t y p i c a l l y moves from the underside of one l e a f t o the next w i t h i n a s i n g l e l i l y bed and may move d i r e c t l y from one bed to another s e v e r a l meters away where i t w i l l c o n t i n u e t o use t h i s a t t a c k t e c h n i q u e . Kokanee appeared t o use t h i s technique l e s s f r e q u e n t l y than t r o u t but t h i s i s l i k e l y because o n l y a s m a l l p r o p o r t i o n o f the t o t a l number of kokanee observed were i n i n s h o r e areas t h a t have abundant weed beds and submerged snags (Chapter 3 ) . From both f i e l d and l a b o r a t o r y , o b s e r v a t i o n s , I have summarized the prey types t h a t are taken by the p r e d a t o r s u s i n g d i f f e r e n t approach and c a p t u r e t e c h n i q u e s (Table 1 8 ) . These o b s e r v a t i o n s are s t r i c t l y q u a l i t a t i v e and do not r e v e a l how o f t e n the t a c t i c s are used o r the f u l l range of prey s p e c i e s t h a t each p r e d a t o r e x p l o i t s . However, i t i s apparent t h a t d i f f e r e n t t a c t i c s are used w i t h d i f f e r e n t prey and t h a t t r o u t and kokanee may use d i f f e r e n t t e c h n i q u e s t o a t t a c k the same pr e y . Comparisons of P r e d a t o r Morphologies In s p i t e of t h e i r s t o c k i e r appearance t r o u t are not s i g n i f i c a n t l y h e a v i e r than kokanee of e q u i v a l e n t l e n g t h ( F i g . 21) from Marion Lake. T h e r e f o r e d i f f e r e n c e s i n appearance must be due to d i f f e r e n c e s i n the way t h a t a g i v e n amount of weight i s a p p o r t i o n e d . The f a l s e i m p r e s s i o n t h a t t r o u t are h e a v i e r than size-matched kokanee i s i n p a r t c r e a t e d by the l e s s s t r e a m l i n e d appearance of the head r e g i o n of t r o u t . In q u a n t i t a t i v e terms t h i s d i f f e r e n c e i s expressed c l e a r l y by d i f f e r e n c e s i n the jaw w i d t h s of t r o u t and kokanee of e q u i v a l e n t s i z e ( F i g . 2 2 ) . For p r e d a t o r s spanning the s i z e range of most i n t e r e s t i n t h i s study ( 1 2 0 - 1 6 0 mm) kokanee e x h i b i t jaw w i d t h s t h a t a r e 3-4 mm o r a p p r o x i m a t e l y 30% s m a l l e r than t r o u t o f e q u i v a l e n t body s i z e . I t would be s u r p r i s i n g i f such d i f f e r e n c e s d i d not have a major i n f l u e n c e on the c a p t u r e and h a n d l i n g success e x p e r i e n c e d by the p r e d a t o r s when faced w i t h a v a r i e t y o f prey. 124 TABLE 18. A l i s t of prey types observed t o evoke s p e c i f i c approach and ca p t u r e t e c h n i q u e s i n the l a b o r a t o r y (L) and f i e l d ( F ) . a. Trout Rush amphipods (L & F) c a d d i s l a r v a e (F) d a m s e l f l y 1. (L) g y r i n i d s (L & F) g e r r i d s (F) s t i c k l e b a c k s (F) S t a l k salamanders (F) s t i c k l e b a c k (F) Jump chironomid a d u l t s (F) c a d d i s a d u l t s (F) d a m s e l f l y a d u l t s (F) Scrape c l a d o c e r a n s S i d a sp. (F) a q u a t i c i n s e c t pupae (L & F) n o t o n e c t i d s (L) copepods & c l a d o c e r a n s (L) b. Kokanee Rush amphipods (L & F) may f l y nymphs (L) d a m s e l f l y 1. (L) i n s e c t pupae (L & F) cl a d o c e r a n s Daphnia sp. (L) n o t o n e c t i d s (L) S t a l k copepods (L) amphipods (L & F) Dart copepods (L) amphipods (L & F) Scrape c l a d o c e r a n s S i d a s p . (L & F) 125 FIGURE 21. The r e l a t i o n s h i p between f o r k l e n g t h and wet weight of t r o u t and kokanee from Marion Lake. Note t h a t r e p r e s e n t a t i v e d a t a p o i n t s were randomly drawn and p l o t t e d but N = the number of t r o u t o r kokanee a c t u a l l y measured. 125a o TROUT Log Y = - 1.928 +2.925 Log X N=I20 -1 r r-—i r 1 1 r~ 5 6 7 8 9 10 20 3 0 F O R K L E N G T H (cm) 126 FIGURE 22. The r e l a t i o n s h i p between standard l e n g t h and jaw gape of t r o u t and kokanee from Marion Lake, jaw gape i s taken as the h o r i z o n t a l d i s t a n c e between the p o s t e r i o r t i p s o f the p r e m a x i l l a r i e s . Note t h a t r e p r e s e n t a t i v e d a t a p o i n t s have been p l o t t e d but N = the number o f t r o u t o r kokanee a c t u a l l y measured. 9 TROUT, Y= - .5567 + .08388 x , N= 122 O KOKANEE, Y= - .5987 + .06234x, N=98 STANDARD LENGTH (mm) CTi 127 O r d i n a r i l y kokanee possess 28 t o 40 l o n g , s e r r a t e d r a k e r s on the f i r s t g i l l a r c h , w h i l e t r o u t possess 17 t o 21 medium l e n g t h r a k e r s (Clemens and W i l b y , 1961). My e x a m i n a t i o n o f 25 kokanee and t r o u t from Marion Lake i n d i c a t e s t h a t kokanee have 28-32 g i l l r a k e r s on the f i r s t g i l l a r c h and t h a t t r o u t have 17-21. Thus alth o u g h kokanee from Marion Lake e x h i b i t a r e d u c t -i o n i n the range of g i l l r a k e r s n o r m a l l y observed, they s t i l l p ossess many more than t r o u t . DISCUSSION A s i n g l e a t t a c k on a prey i s a complex event t h a t may proceed i n stages from i n i t i a l o r i e n t a t i o n through approach, c o n t a c t , c a p t u r e , m a n i p u l a t i o n and i n g e s t i o n or r e j e c t i o n by a p r e d a t o r . A s u c c e s s f u l a t t a c k i s one which r e s u l t s i n the i n g e s t i o n of the prey. In t h i s s t u d y , I o p e r a t i o n a l l y d e f i n e a t t a c k success as the p r o p o r t i o n of prey i n g e s t e d t o prey d e t e c t e d and then approached. Although i n t e r a c t i o n s between p r e d a t o r s and prey d u r i n g any stage of an a t t a c k may i n f l u e n c e o v e r a l l l e v e l s o f a t t a c k s u c c e s s , the d e s c r i p t i o n s presented here were not intended t o p r o v i d e a d e t a i l e d assessment of the i n f l u e n c e of each a t t a c k stage on a t t a c k success ( e . g . , Beukema, 1968) , but r a t h e r to p r o v i d e a b a s i s on which to propose s p e c i f i c hypotheses conce r n i n g d i f f e r e n c e s i n the o u t -comes of a t t a c k s t h a t t r o u t and kokanee are l i k e l y t o e x p e r i e n c e i n encounters w i t h a v a r i e t y o f n a t u r a l p r e y . 128 The Capture-success H y p o t h e s i s Capture success i s o p e r a t i o n a l l y d e f i n e d here as the p r o p o r t i o n of prey t h a t are grasped f i r m l y o r "secured" i n the mouth of a p r e d a t o r r e l a t i v e t o the t o t a l number of prey approached o r pursued a f t e r d e t e c t i o n and o r i e n t a t i o n . To d e r i v e the captur e success h y p o t h e s i s , I have r e l i e d on the o b s e r v a t i o n s t h a t t r o u t and kokanee sometimes use d i f f e r e n t approach and ca p t u r e t e c h n i q u e s on the same types o f prey and t h a t they i n d i v i d u a l l y use d i f f e r e n t approach and ca p t u r e t e c h n i q u e s w i t h v a r y i n g degrees o f success on d i f f e r e n t t ypes o f prey (Table 18). For example p r e l i m i n a r y l a b o r a t o r y o b s e r -v a t i o n s suggest t h a t t r o u t u t i l i z i n g " r u s h " d u r i n g a t t a c k s on copepods may e x p e r i e n c e lower l e v e l s of ca p t u r e success than kokanee using " s t a l k and d a r t " . F i e l d o b s e r v a t i o n s a l s o i n d i c a t e t h a t t r o u t f r e q u e n t l y miss when jumping t o c a t c h a d u l t c a d d i s and t h a t s u c c e s s f u l c a p t u r e never f o l l o w s a "r u s h " on s u r f a c e prey such as water s t r i d e r s ( G e r r i d a e ) , i n s p i t e o f the i n i t i a t i o n of numerous a t t a c k s by t r o u t . The o b s e r v a t i o n s above suggest t h a t d i f f e r e n c e s i n the approach and c a p t u r e t e c h n i q u e s used by t r o u t and kokanee i n the f i e l d l i k e l y a l t e r the o v e r a l l success o f a t t a c k s on prey t h a t are r e l a t i v e l y s m a l l , f a s t , o r a g i l e and which e x h i b i t avoidance responses d u r i n g a t t a c k s by f i s h . Such prey w i l l tax the r e f l e x e s and m a n e u v e r a b i l i t y o f a p r e d a t o r up to the p o i n t of c a p t u r e , but once c a p t u r e d , a re u n l i k e l y t o p r e s e n t much d i f f i c u l t y d u r i n g the m a n i p u l a t i o n and i n g e s t i o n phases o f 129 a t t a c k . S i n c e a number of the prey types c o n s t i t u t i n g s i g n i f i -c a n t p r o p o r t i o n s of t r o u t and kokanee d i e t s conform t o these c h a r a c t e r i s t i c s ( e . g . , z o o p l a n k t o n , amphipods, m a y f l i e s ) , I hoped t h a t r e s u l t s c o n f i r m i n g d i f f e r e n t i a l c a p t u r e success might form the b a s i s f o r e x p l a n a t i o n s o f some of the u n e x p l a i n e d d i e t a r y d i f f e r e n c e s between t r o u t and kokanee. T h e r e f o r e , the c a p t u r e success h y p o t h e s i s c l e a r l y s t a t e d i s t h a t " d i f f e r e n c e s i n the c a p t u r e success of t r o u t and kokanee d u r i n g encounters w i t h s m a l l prey i n the f i e l d p l a y an important r o l e i n shaping p r e d a t o r - s p e c i f i c , d i e t s . " The G i l l - r a k e r , P r e y - s i z e , H y p o t h e s i s Kokanee possess a g r e a t e r number of long s e r r a t e d g i l l - r a k e r s than t r o u t . Kokanee i n Marion Lake c o n s i s t e n t l y e x p l o i t l a r g e r q u a n t i t i e s and s m a l l e r zooplankton than t r o u t do (Chapter 2, F i g s . 2 and 7 ) . These d i f f e r e n c e s i n d i e t o c c u r throughout the geographic range of t r o u t and kokanee. Thus, t h i s s p e c i e s p a i r s e r v e s as another example of the o b s e r v a t i o n by many au t h o r s t h a t f i s h w i t h w e l l - d e v e l o p e d and numerous g i l l - r a k e r s r e l y on zooplankton t o a g r e a t e r e x t e n t as a source o f food than s p e c i e s which possess fewer, s h o r t e r , g i l l - r a k e r s . Many authors have suggested t h a t g i l l - r a k e r s f u n c t i o n much l i k e a s i e v e o r f i l t e r d u r i n g f e e d i n g and thus c o n t r o l the s i z e range of p l a n k t o n i c prey t h a t p r e d a t o r s r e t a i n (see H y a t t , 1979 f o r a c r i t i c a l r eview o f t h i s t o p i c ) . I i n i t i a l l y expected t h a t kokanee i n Marion Lake must o b t a i n more zoo p l a n k t o n than 130 t r o u t by f i l t e r i n g i n response t o h i g h d e n s i t i e s o f s m a l l p r e y . The movements t h a t c h a r a c t e r i z e f i l t e r i n g as an " a t t a c k " procedure i n c l u d e the maintenance o f a f u l l y - o p e n e d mouth f o r up t o a few seconds a t a time d u r i n g passage b y a f i s h through a c o n c e n t r a t i o n of s m a l l p l a n k t o n . To my s u r p r i s e n e i t h e r kokanee o r t r o u t i n the f i e l d e x h i b i t e d the c h a r a c t e r i s t i c f e e d i n g movements a s s o c i a t e d w i t h the t a c t i c of " f i l t e r i n g " s m a l l p r e y . I n s t e a d , b oth s p e c i e s always operated i n a " r a p t o r i a l " mode by pur s u i n g and g r a s p i n g i n d i v i d u a l prey items. A p p a r e n t l y some p l a n k t o n - f e e d i n g f i s h always operate i n r a p t o r i a l mode ( K j e l s o n , 1971), w h i l e o t h e r s can operat e i n e i t h e r f i l t e r i n g o r r a p t o r i a l modes (Leong & O'Connell, 1969; O'Connell & Z w e i f e l , 1972). For the l a t t e r s p e c i e s , the c h o i c e o f technique appears t o be i n f l u e n c e d by the a b s o l u t e d e n s i t y o f the prey and the r e l a t i v e p r o p o r t i o n of l a r g e to s m a l l p r e y . I d i d not take samples t o determine the d e n s i t i e s o r s i z e s of zoop l a n k t o n p r e s e n t i n the f i e l d d u r i n g any o f the o b s e r v a t i o n s e t s , c o n s e q u e n t l y the h y p o t h e s i s t h a t kokanee e x p l o i t more and s m a l l e r z o o p l a n k t o n than t r o u t by f i l t e r i n g on o c c a s i o n s when they encounter h i g h d e n s i t i e s o f s m a l l prey remains t o be t e s t e d under c o n t r o l l e d c o n d i t i o n s i n the l a b o r a t o r y . The A t t a c k Rate Hypothesis Although t r o u t and kokanee o f t e n appear t o use the same a t t a c k t e c h n i q u e s when f o r a g i n g i n the f i e l d , t h e r e are f r e q u e n t l y q u a n t i t a t i v e d i f f e r e n c e s i n the number o f a t t a c k s i n i t i a t e d . For example, when f o r a g i n g i n the water column, kokanee were observed t o i n i t i a t e as many as 15 a t t a c k s over a 2 meter d i s t a n c e w h i l e t r o u t seldom i n i t i a t e d more than 5. Given the v e l o c i t i e s m a i n t a i n e d by the p r e d a t o r s a t these times (Chapter 4, Table 10)/ kokanee must a t t a i n a t t a c k r a t e s exceeding 1 per second w h i l e t r o u t do not exceed 1 a t t a c k per 3 seconds. The d i f f e r e n c e i n a t t a c k r a t e s may be a consequence of e i t h e r t r o u t and kokanee c o n s i s t e n t l y h u n t i n g f o r d i f f e r e n t types of prey o r of kokanee a c h i e v i n g a h i g h e r r a t e of d e t e c t i o n o r s h o r t e r a t t a c k times than t r o u t on the same p l a n k t o n i c prey. The " a t t a c k " r a t e h y p o t h e s i s assumes the l a t t e r case to be t r u e and t h a t as a consequence kokanee may accumulate a g r e a t e r q u a n t i t y o f zooplankton i n t h e i r d i e t compared t o t r o u t . The M o u t h - s i z e , I n g e s t i o n - s u c c e s s H y p o t h e s i s Kokanee e x h i b i t s m a l l e r mouth dimensions than t r o u t o f s i m i l a r l e n g t h and w e i g h t. Although t h i s may be advantageous f o r the c a p t u r e and r e t e n t i o n of s m a l l , a g i l e p r e y , i t i s a l s o l i k e l y t h a t i t r e s u l t s i n a g r e a t e r degree of d i f f i c u l t y f o r kokanee compared to t r o u t i n s u c c e s s f u l l y m a n i p u l a t i n g and i n g e s t i n g l a r g e "armoured" prey once they have been c a p t u r e d . I f t h i s proves to be the case i t w i l l h e l p e x p l a i n a number of the o u t s t a n d i n g d i f f e r e n c e s i n the types o f prey t h a t a r e i n c l u d e d i n the d i e t s of f r e e - r a n g i n g t r o u t and kokanee. 132 5-B. L a b o r a t o r y T e s t s o f Hypotheses Concerning Prey A t t a c k by Trout and Kokanee I have proposed f o u r hypotheses c o n c e r n i n g the r e l a t i o n s h i p s between a t t a c k b e h a v i o u r s , m o r p h o l o g i c a l c h a r a c t e r -i s t i c s and d i e t a r y p a t t e r n s o f t r o u t and kokanee. T h i s s e c t i o n c o n s i s t s o f a s e r i e s o f l a b o r a t o r y experiments designed t o t e s t these hypotheses. METHODS Experiment 5.1 The Capture-success Hypothesis T h i s experiment c o n s i s t s of f e e d i n g t r i a l s i n which t r o u t and kokanee were g i v e n separate o p p o r t u n i t i e s to a t t a c k s m a l l amphipods ( H y a l e l l a a z t e c a ) , copepods (Diaptomus k e n a i ) , and c l a d o c e r a n s (Daphnia p u l e x ) . These prey were chosen so t h a t d i f f e r e n c e s i n a t t a c k success by the p r e d a t o r s would depend upon approach and c a p t u r e t e c h n i q u e s r a t h e r than on d i f f i c u l t i e s d u r i n g m a n i p u l a t i o n or i n g e s t i o n of prey. A second c o n s i d e r a t i o n was t h a t a l l i n d i v i d u a l s o f the s m a l l e s t prey type s e l e c t e d f o r f e e d i n g t r i a l s (D_. pulex) had to be l a r g e enough to be v i s i b l e i n a l l l o c a t i o n s w i t h i n the e x p e r i m e n t a l arena (92 x 47 x 45 cm) t o an o u t s i d e o b s e r v e r . I o b t a i n e d H_. a z t e c a f r e s h from Marion Lake. D_. k e n a i were o b t a i n e d from nearby Eunice Lake and D. p u l e x were c u l t u r e d i n the l a b o r a t o r y s p e c i f i c a l l y f o r these t r i a l s . I used a graded s e r i e s of s i e v e s t o o b t a i n prey o f uniform s i z e s f o r i n t r o d u c t i o n i n t o the e x p e r i m e n t a l arena. Trout and kokanee used i n t r i a l s 133 w i t h the above prey were size-matched. Table 19 l i s t s the c h a r a c t e r i s t i c s of both p r e d a t o r s and prey used i n t h i s e x p e r i -ment. I f o l l o w e d standard procedures, o u t l i n e d e a r l i e r (Chapter 1 ) , f o r c a p t u r e , h a n d l i n g and p r e p a r a t i o n of p r e d a t o r s t o take p a r t i n experiments. To conduct a t r i a l , I i n t r o d u c e d a s i n g l e p r e d a t o r i n t o the arena when i t c o n t a i n e d a p o p u l a t i o n o f w e l l d i s p e r s e d p r e y . I then recorded c a p t u r e success (CS = # o f c a p t u r e s / # o f p u r s u i t s ) and i n g e s t i o n success ( I S = # o f prey i n g e s t e d / # o f prey captured) o f p r e d a t o r s d u r i n g encounters w i t h p r e y . Although m u l t i p l e t r i a l s were performed w i t h each p r e d a t o r -prey combination (see Chapter 6 ) , r e s u l t s a c r o s s a l l t r i a l s are pooled here. Experiment 5.2 The A t t a c k Rate Hypothesis The m a j o r i t y o f f r e s h w a t e r zooplankton have maximum body dimensions o f l e s s than 3 mm. P r e l i m i n a r y f i e l d o b s e r v a -t i o n s l e d me to suggest t h a t kokanee might be capable o f sus-t a i n i n g h i g h e r a t t a c k r a t e s than s i m i l a r s i z e d t r o u t on concen-t r a t i o n s o f s m a l l p l a n k t o n i c p r e y . To t e s t t h i s h y p o t h e s i s , I conducted t r i a l s w i t h t r o u t and kokanee f o r a g i n g f o r Daphnia pulex a t d i f f e r e n t d e n s i t i t e s under c o n t r o l l e d c o n d i t i o n s . I used a s i e v e to o b t a i n a l i m i t e d s i z e range o f D. pulex (1-3 mm) from l a b o r a t o r y c u l t u r e s . Prey removed by p r e d a t o r s d u r i n g a f e e d i n g t r i a l were not r e p l a c e d as the t r i a l proceeded. TABLE 19. C h a r a c t e r i s t i c s o f p r e d a t o r s and prey used i n experiments to determine the a t t a c k success of t r o u t and kokanee on a v a r i e t y o f prey. PREY KOKANEE TROUT I d e n t i t y Zooplankton Mean S i z e (mm) Range NO. Used Mean S i z e ( cm) Range No. Used Mean S i z e (cm) Range Daphnia sp. 1.5 1 .0- 2.5 5 11.4 10 .3-12.7 4 11.2 9 .2-13 .1 Diaptomus k e n a i 2.0 1 .0- 3.0 4 8.4 7 .6 - 9.3 4 8.9 7 .7-11 .4 Chaoborus spp. 10.0 7 .5-14 .5 4 16.4 15.6-17.7 6 16.4 14 .9-21 .6 Amphipods 6 * 8.7 7 . 7 - 9.2 H y a l e l l a a z t e c a 4.6 3 . 5 - 6.0 4 13.0 12.0-13.5 4 15.2 13 .4-17 .0 Crangonyx richmondensis 8.3 8 . 0 - 8.7 3 14.7 14.5-14.9 4 15.2 13 .4-17 .0 Others Notonecta u n d u l a t a & Buenoa c o n f u s a 10 .5 6 .5-14 .0 4 16.4 15.6-17.7 4 12.1 11.8-12.4 Ephemeroptera ( C e n t r o p t i l u m sp.) 11.8 10 .1-12.6 3 12.7 12.4-13.3 N o t - t e s t e d Odonata (Enallagma sp.) 15.6 15 .0-16 .2 3 14u2 13.2-15.0 4 15.0 12 .5-16 .5 T r i c h o p t e r a ( s p e c i e s unknown). 15.0 10 .0-16 .5 8 14.8 13.4-16.0 Not t e s t e d two groups o f t r o u t were used i n the t e s t s w i t h Chaoborus spp. i n orde r t o examine the w i t h i n s p e c i e s e f f e c t o f p r e d a t o r s i z e on capture and i n g e s t i o n s u c c e s s . Any advantage t h a t kokanee might d i s p l a y compared t o t r o u t i n o b t a i n i n g p l a n k t o n i c prey c o u l d r e s u l t e i t h e r from g e n e t i c a l l y determined d i f f e r e n c e s i n behaviour and phenotype o r from d i f f e r e n c e s i n e x p e r i e n c e the two p r e d a t o r s have had i n f o r a g i n g f o r z o o p l a n k t o n . To p a r t i a l l y e l i m i n a t e the i n f l u e n c e o f d i f f e r e n c e s i n f o r a g i n g e x p e r i e n c e on r e s u l t s i n t h i s experiment, I c o l l e c t e d j u v e n i l e t r o u t and kokanee from the f i e l d i n the f a l l of 1973 and p r o v i d e d them w i t h an i d e n t i c f e e d i n g regime i n the l a b o r a t o r y u n t i l the time of the e x p e r i -ments i n June of 1974. During the l a b o r a t o r y c o n d i t i o n i n g p e r i o d , t r o u t and kokanee were fed e x c l u s i v e l y e i t h e r f r o z e n b r i n e shrimp o r f r e s h z o o p l a n k t o n . A l l o f these food items were r e l a t i v e l y s m a l l and were o b t a i n e d i n the water column by the p r e d a t o r s . A f o r c e d a i r b u b b l e r helped keep the b r i n e shrimp suspended d u r i n g the f e e d i n g p e r i o d s . Thus, t r o u t and kokanee used i n experiment 5.2 were h i g h l y p r e c o n d i t i o n e d to feed upon zoo-p l a n k t o n . Experiment 5.3 The G i l l - r a k e r , P r e y - s i z e Hypothesis The o b s e r v a t i o n t h a t kokanee d i d not " f i l t e r " s m a l l prey i n the f i e l d d i d not f a l s i f y the h y p o t h e s i s t h a t t h e i r more numerous g i l l r a k e r s g i v e them an advantage over t r o u t i n o b t a i n i n g s m a l l p r e y . F i e l d o b s e r v a t i o n s may not have c o i n c i d e d w i t h the times o r l o c a t i o n s o f h i g h d e n s i t i e s o f s m a l l p l a n k t o n i c prey which are most l i k e l y to e l i c i t the 136 t e c h n i q u e . To p r o v i d e a more c r i t i c a l t e s t o f the h y p o t h e s i s , I conducted t r i a l s w i t h s i z e matched t r o u t and kokanee f o r a g i n g f o r a m i x t u r e of v e r y s m a l l z o o p l a n k t o n a t d e n s i t i e s r a nging from 7 t o 35 per l i t e r . I o b t a i n e d the z o o p l a n k t o n used as prey from Eunice Lake. Prey l a r g e r than 1.5 mm were excluded from the l a b o r a t o r y s t o c k s by s i e v i n g . In each s e t o f t r i a l s one r e p l i c a t e p o r t i o n of the prey m i x t u r e was s e t a s i d e and s t o r e d i n 30% e t h a n o l f o r d e t e r m i n a t i o n s o f s p e c i e s c o m p o s i t i o n and s i z e d i s t r i b u t i o n s of prey a t a l a t e r d a t e . The o t h e r p o r t i o n s were used i n t r i a l s w i t h i n d i v i d u a l p r e d a t o r s . Experiment 5.4 The M o u t h - s i z e , I n g e s t i o n - s u c c e s s Hypothesis T h i s experiment c o n s i s t s o f f e e d i n g t r i a l s i n which t r o u t o r kokanee were g i v e n s e p a r a t e o p p o r t u n i t i e s to a t t a c k c a d d i s l a r v a e ( T r i c h o p t e r a ) , d a m s e l f l y nymphs (Odonata), water boatmen ( N o t o n e c t a ) , amphipods (Amphipoda), chaoborus l a r v a e ( D i p t e r a ) and mayfly nymphs (Ephemeroptera). S p e c i f i c i d e n -t i t i e s of p r e y , prey s i z e s and p r e d a t o r s i z e s are l i s t e d i n Table 19. I s e l e c t e d the f i r s t f o u r prey types f o r t h i s e x p e r i -ment because they possessed combinations o f c h a r a c t e r i s t i c s ( l a r g e s i z e , tough body c o v e r i n g s , o r slow movements) which improved the l i k e l i h o o d t h a t d i f f e r e n c e s i n a t t a c k success by the p r e d a t o r s would depend on m a n i p u l a t i o n and i n g e s t i o n r a t h e r than approach and c a p t u r e t e c h n i q u e s . The l a s t two prey types were s e l e c t e d because they were w e l l w i t h i n the s i z e range o f 137 the f i r s t f o u r prey types but they l a c k e d t h e i r tough, body-c o v e r i n g s . I b e l i e v e d t h a t r e s u l t s from t r i a l s w i t h these prey might y i e l d g r e a t e r i n s i g h t i n t o the nature o f i n t e r a c t i o n s between prey s i z e , prey " t e x t u r e " , p r e d a t o r mouth s i z e and i n g e s t i o n success of the p r e d a t o r s . M u l t i p l e t r i a l s were com-p l e t e d w i t h each p r e d a t o r - p r e y c o m b i n a t i o n (Chapter 6) but r e s u l t s a c r o s s a l l t r i a l s have been pooled here. In a l l t r i a l s except those d e a l i n g w i t h c a d d i s l a r v a e , the s p e c i e s named was the o n l y prey p r e s e n t . I o b t a i n e d r e s u l t s on c a p t u r e and i n g e s t i o n success of kokanee on t r i c h o p t e r a n s from mixed prey t r i a l s i n which equal numbers o f odonates, ephemeropterans and amphipods were a l s o p r e s e n t . T h i s i s not l i k e l y t o have i n f l u e n c e d the r e s u l t s r e p o r t e d here. RESULTS Experiment 5.1 The Capture-success Hypothesis As e x p e c t e d , the o v e r a l l a t t a c k success (prey i n g e s t e d t o prey a t t a c k e d ) o f t r o u t and kokanee w i t h s m a l l prey (<5 mm long) was c o n t r o l l e d p r i m a r i l y by i n t e r a c t i o n s between p r e d a t o r s and prey d u r i n g the approach and c a p t u r e phase of a t t a c k s i n c e , w i t h one e x c e p t i o n , the p r e d a t o r s e x p e r i e n c e d 100 p e r c e n t i n g e s t i o n success w i t h prey a f t e r c a p t u r e (Table 2 0 ) . T r o u t and kokanee both used " r u s h " as the approach and c a p t u r e technique i n encounters w i t h s m a l l c l a d o c e r a n s (p. p u l e x ) . The absence o f any avoidance response by these prey enabled both t r o u t and kokanee t o achieve 100% c a p t u r e TABLE 20. A comparison o f c a p t u r e success (CS) and i n g e s t i o n success (IS) of "size-matched" t r o u t and kokanee i n a t t a c k s on s m a l l , a g i l e p r e y . TA = t o t a l a t t a c k s i n i t i a t e d . Data on CS and IS are expressed t o the n e a r e s t percentage p o i n t . No. o f t r i a l s conducted w i t h TROUT KOKANEE i n d i v i d u a l f i s h Prey I d e n t i t y Mean s i z e i n mm CS IS TA CS IS TA Trout Kokanee D. pule x 1.5 100 100 2,328 100 100 2,469 16 16 D. k e n a i 2.0 11 25 495 28 100 4,271 15 16 H. a z t e c a 4.6 91 100 300 94 100 819 4 16 s u c c e s s . Both p r e d a t o r s e x p e r i e n c e d the l o w e s t l e v e l s o f c a p t u r e success i n encounters w i t h copepods (I). kenai) . T h i s may be a t t r i b u t e d t o the h i g h l y e f f e c t i v e escape responses e x h i b i t e d by the copepods. "Rush" was the o n l y approach and c a p t u r e technique used by t r o u t , i n encounters w i t h copepods,. .While kokanee s h i f t e d from "rush" d u r i n g the f i r s t few a t t a c k s on copepods t o a combination of " s t a l k - a n d - d a r t " f o r the remainde The g r e a t e r c a p t u r e success t h a t kokanee, compared t o t r o u t , e x p e r i e n c e w i t h copepods i s due t o t h i s s h i f t o f approach and c a p t u r e t e c h n i q u e s . I t i s not c l e a r a t t h i s p o i n t why t r o u t i n g e s t e d o n l y 25% o f the copepods t h a t they d i d c a p t u r e . T h i s i s e s p e c i a l l y s u r p r i s i n g i n view o f the h i g h i n g e s t i o n s u c c e s s t h a t kokanee e x h i b i t e d w i t h these prey (Table 20). Trout used " r u s h " w h i l e kokanee employed e i t h e r " r u s h " o r " s t a l k - a n d - d a r t " as approach and c a p t u r e t e c h n i q u e s i n encounters w i t h H y a l e l l a sp. These prey e x h i b i t e d avoidance responses a t the i n s t a n t o f p h y s i c a l c o n t a c t w i t h the p r e d a t o r s . The h i g h c a p t u r e success o f t r o u t and kokanee w i t h the prey i n d i c a t e s t h a t the avoidance response o f amphipods i s not as e f f e c t i v e as t h a t of copepods i n promoting escape d u r i n g a t t a c k s by f i s h . R e s u l t s presented here i n d i c a t e t h a t a t t a c k success i s c o n t r o l l e d by s p e c i f i c b e h a v i o u r a l i n t e r a c t i o n s between p r e d a t o r s and prey d u r i n g the approach and c a p t u r e phase of a t t a c k . These r e s u l t s g e n e r a l l y support the h y p o t h e s i s t h a t such i n t e r a c t i o n s may assume an important r o l e i n shaping p r e d a t o r - s p e c i f i c d i e t s i n the f i e l d (see d i s c u s s i o n ) . 140 Experiment 5.2 The A t t a c k - r a t e H y p o t h e s i s Trout and kokanee expe r i e n c e d i d e n t i c a l l e v e l s o f ca p t u r e success i n encounters w i t h c l a d o c e r a n s (D_. pulex) , however, r e s u l t s from the p r e s e n t experiment i n d i c a t e t h a t the p r e d a t o r s s u s t a i n d i f f e r e n t a t t a c k r a t e s on these prey a t v a r i o u s d e n s i t i t e s . The means o f the maximum a t t a c k r a t e s a c h ieved by kokanee a t a l l prey d e n s i t i e s are more than double those a t t a i n e d by t r o u t o f s i m i l a r s i z e ( F i g . 23). Kokanee pursued Daphnia sp. by t r a c i n g a smooth path t h a t flowed from one ca p t u r e t o the next w i t h l i t t l e i n t e r r u p t i o n and w i t h v e r y p r e c i s e changes o f alignment from one a t t a c k t o the next. T r o u t , by c o n t r a s t , o f t e n s c u l l e d i n midwater, s i g h t e d a p r e y , stopped and then rushed i n t o make a c a p t u r e . A f t e r a c a p t u r e t r o u t o f t e n stopped a b r u p t l y " t o sea r c h f o r and l i n e up on" the next prey i t e m . Thus, the h i g h e r a t t a c k r a t e s o f kokanee compared t o t r o u t are l i k e l y a consequence of i n t e r a c t i o n s between a h i g h e r r a t e of prey d e t e c t i o n and s h o r t e r a t t a c k times on pre y . S i n c e t r o u t and kokanee used i n t h i s experiment were h i g h l y " p r e c o n d i t i o n e d " t o feed on z o o p l a n k t o n , these d i f f e r e n c e s are l i k e l y based upon g e n e t i c a l l y f i x e d morpho-l o g i c a l and b e h a v i o u r a l t r a i t s . These r e s u l t s support the h y p o t h e s i s t h a t kokanee may accumulate a g r e a t e r q u a n t i t y o f zooplankton i n t h e i r d i e t compared t o t r o u t by s u s t a i n i n g h i g h e r r a t e s of s u c c e s s f u l a t t a c k on such prey. 141 FIGURE 23. The r e l a t i o n s h i p between the d e n s i t y o f s m a l l zoo-p l a n k t o n (1.5 mm Daphnia sp.) and the maximum a t t a c k r a t e s o f t r o u t and kokanee. V e r t i c a l bars i n d i c a t e the 95% c o n f i d e n c e i n t e r v a l s of the means. N = the number of t r o u t o r kokanee used i n t r i a l s a t each d e n s i t y . 142 Experiment 5.3 The G i l l - r a k e r , P r e y - s i z e Hypothesis I t i s c e r t a i n t h a t some f i s h have the b e h a v i o u r a l f l e x i b i l i t y t o e x p l o i t z o oplankton one a t a time o r , depending on the d e n s i t i e s and s i z e s of p r e y , t o s w i t c h over t o a mode o f o p e r a t i o n i n which many prey are " f i l t e r e d " from the water s i m u l t a n e o u s l y . F i s h a t t a c k i n g prey by the l a t t e r method e x h i b i t a p e c u l i a r g u l p i n g a c t i o n t h a t resembles exaggerated r e s p i r a t o r y movements. In s p i t e of the p r o v i s i o n o f v e r y s m a l l prey ( F i g . 24) at d e n s i t i e s r a n g i n g from 7 - 3 5 per l i t e r , n e i t h e r t r o u t o r kokanee were induced t o e x p l o i t z o oplankton by " f i l t e r i n g " . These r e s u l t s f a i l t o support the h y p o t h e s i s t h a t kokanee w i t h w e l l developed g i l l - r a k e r s e x p l o i t more zooplankton than t r o u t do as as consequence o f an a b i l i t y t o f i l t e r such prey from the water column. However, the r e s u l t s do o f f e r s t r i k i n g t estimony t o the s u p e r i o r " a b i l i t y " o f kokanee i n f o r a g i n g f o r s m a l l z o o p l a n k t o n . Kokanee a t t a i n maximum a t t a c k r a t e s t h a t a re seven t o ten times h i g h e r than those a t t a i n e d by t r o u t (Table 22 ) and i n a h a l f hour f e e d i n g s e s s i o n average e i g h t e e n times as many a t t a c k s on s m a l l zooplankton (see Table 21 f o r s p e c i e s composition) as t r o u t (Table 2 3 ) . Given t h a t kokanee can f u n c t i o n c o n t i n u o u s l y a t the average of t h e i r maximum a t t a c k r a t e and assuming 100% ca p t u r e s u c c e s s , they c o u l d o b t a i n a p p r o x i m a t e l y 2600 s m a l l zooplankton per hour. However a l i m i t e d amount of d a t a suggests t h a t c a p t u r e success i s about 70%, t h e r e f o r e the net i n t a k e per hour i s l i k e l y c l o s e r t o 1820 zo o p l a n k t o n . T r o u t i n the same 143 FIGURE 24. The s i z e - f r e q u e n c y d i s t r i b u t i o n of prey used between J u l y 18 and J u l y 26 f o r the s m a l l - p l a n k t o n , f e e d i n g t r i a l s w i t h t r o u t and kokanee. N= 300 0-0.9 .2-.29 .4-.49 .6-.69 .8-.89 1.0-1.09 12-1.29 1.4-1.49 BODY LENGTH OF PREY (mm) 144 TABLE 21. S p e c i e s c o m p o s i t i o n (as % by number) of s m a l l z o o p l a n k t o n used i n experiment 5.3 w i t h t r o u t and kokanee. Low Density I d e n t i t y 7/1 i t e r Diaptomus t y r e l l i 82 Bosmina sp. 9 Holopedium sp. 3 Polyphemus sp. 3 Daphnia sp. 0 Diaphanosoma sp. 2 R o t i f e r s 0 Others 1 Sample s i z e 1,552 Medium D e n s i t y High D e n s i t y 2 7 / l i t e r 3 5 / l i t e r 60 83 17 9 6 5 6 2 0 1 10 1 1 0 1 0 1,908 3,466 145 TABLE 22. Means and 95% c o n f i d e n c e l i m i t s of the maximum a t t a c k r a t e s per minute achieved by t r o u t and kokanee e x p l o i t i n g s m a l l z o o p l a n k t o n a t v a r i o u s d e n s i t i e s . D e n s i t y o f prey per l i t e r 7 27 35 Pooled d a t a Trout N 4.00 + 5.7 3 5.30 + 3.2 4 4.00 + 4.7 4 4.50 + 1.7 11 Kokanee N 49.5 + 2 .1 4 35.3 + 17 .2 3 40.5 + 2 42.8 + 7 .0 9 TABLE 23. Means and 95% c o n f i d e n c e l i m i t s of the t o t a l a t t a c k s per .5 hours completed by t r o u t and kokanee e x p l o i t i n g s m a l l z o o p l a n k t o n a t v a r i o u s d e n s i t i e s . D e n s i t y o f prey per l i t e r T r o ut N Kokanee N 7 43 + 83 3 875 + 211 4 27 49 + 33 4 735 + 760 3 35 36 ± 6 1 4 673 2 Pooled d a t a 46 + 23 11 783 + 160 9 146 i n t e r v a l of time would e x p l o i t o n l y 270 and 189 z o o p l a n k t o n i f I assume they a t t a i n 100% and 70% c a p t u r e success r e s p e c t i v e l y . One problem w i t h t h i s c o n c l u s i o n i s the i m p l i c i t assumption t h a t t r o u t are p u t t i n g out a maximum e f f o r t t o a t t a c k p l a n k t o n d u r i n g these t r i a l s . An a l t e r n a t e i n t e r p r e t a t i o n i s t h a t t r o u t c o u l d perform b e t t e r but choose not t o . C e r t a i n l y i n e x p l o i t i n g Daphnia sp. these same t r o u t averaged b e t t e r than 30 a t t a c k s per minute, t h a t i s , almost 8 times t h e i r performance on the much s m a l l e r z o o p l a n k t o n . G i v e n t h a t t r o u t can d e t e c t the s m a l l e r z o o p l a n k t o n , t h e r e i s no reason f o r them t o f a i l i n a t t a i n i n g s i m i l a r a t t a c k r a t e s on them. The evidence about the d e t e c t a b i l i t y o f these prey f o r t r o u t i s c o n f l i c t i n g . During most t r i a l s w i t h s m a l l p l a n k t o n i t was d i f f i c u l t t o c o n f i r m t h a t t r o u t d i r e c t e d a t t a c k s a t i n d i v i d u a l p l a n k t e r s because t h e i r s i z e s are a t the lower l i m i t of a human ob s e r v e r ' s v i s u a l r e s o l u t i o n . On some o c c a s i o n s when t r o u t and prey were very c l o s e to the f r o n t of the arena, I d e f i n i t e l y saw a t t a c k s d i r e c t e d a t s m a l l z o o p l a n k t o n when they moved. T h i s suggests t h a t t r o u t are capable o f v i s u a l l y d e t e c t i n g these p r e y . Other o b s e r v a t i o n s suggest t h a t t h e r e i s some d i f f i c u l t y i n t h i s f o r t r o u t . Many t r o u t e x h i b i t e d i n t e n s e s e a r c h behaviour throughout the h a l f hour d u r a t i o n o f t r i a l s conducted a t h i g h prey d e n s i t y (35 prey per l i t e r ) but made few a t t a c k s . At the end of such t r i a l s when l a r g e z o o p l a n k t o n (eg. Daphnia sp., Chaoborus spp. e t c . . ) were i n t r o d u c e d i n t o the are n a , the t r o u t c o n t i n u e d t o s e a rch i n the same f a s h i o n b u t d i s p l a y e d a d r a m a t i c i n c r e a s e 147 i n a t t a c k r a t e . These o b s e r v a t i o n s suggest t h a t s m a l l prey used i n the p r e s e n t experiment are a t the margin of t r o u t v i s u a l s e n s i t i v i t y but t h a t they are w e l l w i t h i n the d e t e c t i o n range o f kokanee. Re g a r d l e s s o f whether t r o u t c o u l d a t t a i n h i g h e r a t t a c k r a t e s on s m a l l z o o p l a n k t o n , the p o i n t i s t h a t they don't, w h i l e kokanee do. Whether t r o u t are choosing not to a t t a c k o r are i n c a p a b l e of e f f i c i e n t l y l o c a t i n g the s m a l l e r z o o p l a n k t o n due t o p r o p e r t i e s of t r o u t sensory c a p a c i t i e s , the r e s u l t s w i l l be the same, t h a t i s , kokanee w i l l tend t o e x p l o i t z o o p l a n k t o n and o t h e r s m a l l (<,1mm) food items t o a g r e a t e r e x t e n t than t r o u t . Experiment 5.4 The Mo u t h - s i z e , I n g e s t i o n - s u c c e s s H y p o t h e s i s R e s u l t s from t h i s experiment (Table 24 ) i n d i c a t e t h a t the o v e r a l l a t t a c k success ( p r o p o r t i o n of prey i n g e s t e d t o prey a t t a c k e d ) o f t r o u t and kokanee d u r i n g encounters w i t h l a r g e , i n v e r t e b r a t e prey i s c o n t r o l l e d by p r e d a t o r - p r e y i n t e r a c t i o n s d u r i n g the approach and ca p t u r e phase as w e l l as d u r i n g the m a n i p u l a t i o n and i n g e s t i o n phase of prey a t t a c k . When l a r g e , armoured prey are i n v o l v e d , events d u r i n g m a n i p u l a t i o n and i n g e s t i o n o f prey p l a y a g r e a t e r r o l e than those d u r i n g approach and c a p t u r e i n d e t e r m i n i n g a t t a c k success (compare IS t o CS f o r t r i c h o p t e r a n s , odonates, n o t o n e c t i d s and amphipods i n Table 24). When l a r g e , s o f t - b o d i e d prey are i n v o l v e d , events d u r i n g the approach and captur e phase alone determine the d i f f e r e n t l e v e l s o f a t t a c k success (compare CS t o IS f o r d i p t e r a n s and ephemeropterans i n Table 24). TABLE 24. A comparison of c a p t u r e success (CS) and i n g e s t i o n success (IS) o f "size-matched" t r o u t and kokanee i n a t t a c k s on d i f f e r e n t s p e c i e s o f i n v e r t e b r a t e p r e y . TA = t o t a l a t t a c k s i n i t i a t e d . Data on CS and IS are rounded to the n e a r e s t percentage p o i n t . 95% co n f i d e n c e l i m i t s are i n d i c a t e d and are based on the normal approximation t o the b i n o m i a l d i s t r i b u t i o n . Large, "Armoured", Prey Types Mean Length i n mm TROUT CS IS TA KOKANEE CS IS TA No. o f t r i a l s conducted w i t h i n d i v i d u a l f i s h T rout Kokanee T r i c h o p t e r a ( s p e c i e s unknown) Odonata (Enallagma  b o r e a l e ) 15.0 15.6 93-1-3 2+2 67+5 68+7 273 72+4 28+4 248 618 12 16 15 Notonecta (Notonecta u n d u l a t a , Buenoa confusa) 10.5 Amphipoda (Crangonyx  richmondensis) 8.3 46+4 6+3 91+3 100+0 552 45+7 0+0 230 68+6 46+7 165 266 16 16 12 Large, S o f t - b o d i e d , Prey Types D i p t e r a (Chaoborus  americanus, C. t r e v i t t a t u s ) 10.0 93+1 100+0 1,934 * 80+4 100+0 441 86+2 100+0 1,990 18 18 Ephemeroptera ( C e n t r o p t i l u m sp.) 11.8 60+3 100+0 794 13 * t r o u t i n t h i s group were o n l y h a l f the s i z e of t r o u t and kokanee "matched" f o r s i z e i n the o t h e r groups t h a t were t e s t e d w i t h Chaoborus spp. See Table 19 f o r p r e d a t o r s i z e s . 149 I suggested e a r l i e r that because kokanee exhibit smaller, mouth dimensions than trout of similar length, they might experience a greater degree of d i f f i c u l t y than trout in manipu-lating and ingesting large, armoured prey. This hypothesis i s firmly supported by the results presented here (Table 24 ). Kokanee and trout exhibit similar levels of capture success in encounters with large prey, however kokanee experience substantially greater d i f f i c u l t y than trout in manipulating and ingesting many of these prey. This d i f f i c u l t y i s not due to any simple relation between prey size and predator mouth size. For example kokanee experience r e l a t i v e l y low ingestion success on Crangonyx sp. (46%) as compared with the mayfly Centroptilum sp. (100%) even though the l a t t e r are 42% larger than Crangonyx sp. This difference i s most l i k e l y associated with the amount of "armour" possessed by the prey. Crangonyx sp. possess a tough, chitinous exoskeleton which must offer considerable resistance to compression during ingestion. Centroptilum sp. by contrast i s r e l a t i v e l y soft-bodied and f l e x i b l e . It i s apparent from feeding t r i a l s conducted with damselflies and especially with notonectids (Nk undulata and confusa) that prey armour i s less of a deterrent to successful attacks by trout. In the notonectid t r i a l s , I purposefully selected p a r t i c u l a r l y well armoured, large prey which I thought would be unmanageable for both trout and kokanee. This goal was completely realized with kokanee (0% IS) but less so with trout (6% IS). This i s a l l the more remarkable since the trout 150 used i n these t r i a l s were a p p r e c i a b l y s m a l l e r than the kokanee used (12.1 cm v e r s u s 16.4 cm) and had jaw w i d t h s t h a t were v i r t u a l l y i d e n t i c a l t o those o f the l a r g e r kokanee ( F i g . 22). Thus, s m a l l e r t r o u t p o s s e s s i n g e q u i v a l e n t jaw dimensions appear to have an advantage over kokanee i n i n g e s t i n g l a r g e , armoured i n v e r t e b r a t e s . DISCUSSION The r e l a t i v e v u l n e r a b i l i t y o f a s i n g l e prey type to d i f f e r e n t s p e c i e s of p r e d a t o r s o r o f a v a r i e t y o f prey types t o a s i n g l e s p e c i e s o f p r e d a t o r w i l l o f t e n be determined by the "match" o r "mismatch" of p r e d a t o r and prey c h a r a c t e r i s t i c s d u r i n g the a t t a c k phase of the f e e d i n g c y c l e . R e s u l t s from the p r e s e n t c h a p t e r may now be used t o r e s o l v e the g e n e r a l q u e s t i o n of whether d i f f e r e n c e s i n p r e d a t o r a t t a c k b e h a v i o u r s i n t e r a c t i n g w i t h d i f f e r e n c e s i n p r e d a t o r morphologies are l i k e l y t o account f o r s p e c i f i c p a t t e r n s of f o o d - r e s o u r c e p a r t i t i o n i n g e x h i b i t e d by t r o u t and kokanee from Marion Lake. E x p l a n a t i o n s o f D i e t a r y D i f f e r e n c e s Between Trout and Kokanee Both f i e l d and l a b o r a t o r y o b s e r v a t i o n s i n d i c a t e t h a t some te c h n i q u e s of approach and c a p t u r e of prey are s p e c i f i c t o e i t h e r t r o u t o r kokanee. T h i s should l e a d t o d i f f e r e n c e s i n d i e t a r y h a b i t s because s p e c i f i c approach and c a p t u r e t e c h n i q u e s g i v e g r e a t e r access to c e r t a i n prey f o r one p r e d a t o r over another. For example both t r o u t and kokanee i n c l u d e l a r g e 151 numbers of the l a r v a l , pupal and a d u l t stages o f chironomids i n t h e i r d i e t s , but t r o u t consume a p p r o x i m a t e l y t w i c e as many chironomid a d u l t s as kokanee d u r i n g the summer months (Chapter 2, Table 3 ). T h i s d i e t a r y d i f f e r e n c e i s not o b v i o u s l y r e l a t e d t o temporal s e g r e g a t i o n , s p a t i a l s e g r e g a t i o n (Chapter 3 ) , o r search b e h a v i o u r s (Chapter 4) o f t r o u t and kokanee, however evidence from the p r e s e n t c h a p t e r i n d i c a t e s i t may be r e l a t e d t o d i f f e r e n c e s i n t h e i r a t t a c k b e h a v i o u r s . I base t h i s s u g g e s t i o n on the o b s e r v a t i o n t h a t t r o u t f r e q u e n t l y jump to c a t c h a v a r i e t y o f a e r i a l prey w h i l e kokanee a p p a r e n t l y do not. T h i s should a l l o w t r o u t t o o b t a i n g r e a t e r q u a n t i t i e s than kokanee of the a d u l t forms of both a q u a t i c and t e r r e s t r i a l i n s e c t s which are much more abundant i n f l i g h t a few cm above the l a k e s u r f a c e than on i t . D i f f e r e n c e s i n approach and ca p t u r e t e c h n i q u e s as w e l l as q u a n t i t a t i v e d i f f e r e n c e s i n a t t a c k responses to s m a l l prey such as zooplankton i n the l a b o r a t o r y form the b a s i s f o r ex p l a n a -t i o n s of d i f f e r e n c e s i n the k i n d s and q u a n t i t y o f zooplankton o b t a i n e d by t r o u t and kokanee i n the f i e l d . E f f o r d and Tsumura (1973) r e p o r t e d t h a t c l a d o c e r a n s (almost e n t i r e l y S i d a c r y s t a l l i n a ) were the dominant form o f zooplankton found i n the d i e t of moderate s i z e t r o u t (mean f o r k l e n g t h 17.5 cm) from Marion Lake. By c o n t r a s t they i n d i c a t e d t h a t zooplankton i n the d i e t of kokanee o f comparable s i z e s (mean f o r k l e n g t h 14.0 cm) was composed o f s i g n i f i c a n t q u a n t i t i e s o f both c l a d o c e r a n s ( p r i m a r i l y S^ c r y s t a l l i n a ) and copepods 152 ( p r i m a r i l y C yclops b i c u s p i d a t u s ) . I have p r e v i o u s l y confirmed t h a t t h i s d i f f e r e n c e i n the taxonomic c o m p o s i t i o n o f z o o p l a n k t o n i s repeated i n the d i e t s of s m a l l t r o u t (mean f o r k l e n g t h 5.6 cm) and kokanee (mean f o r k l e n g t h 7.9 cm) from Marion Lake (Chapter 2, Table 5 ) . The dominance o f c l a d o c e r a n s and the s c a r c i t y o f copepods i n the d i e t o f t r o u t r e l a t i v e to kokanee i s l i k e l y based upon the i n t e r a c t i o n o f s e v e r a l f a c t o r s , however my d a t a i d e n t i f y two s p e c i f i c reasons f o r why such a p a t t e r n should o c c u r . F i r s t r e s u l t s from experiment 5.1 i n d i c a t e t h a t a s h i f t i n approach and c a p t u r e t e c h n i q u e s (from " r u s h " t o " s t a l k and d a r t " ) by kokanee a l l o w s them t o a t t a i n g r e a t e r c a p t u r e success than t r o u t i n encounters w i t h one s p e c i e s o f copepod (EK k e n a i ) which e x h i b i t s avoidance responses d u r i n g a t t a c k s by p r e d a t o r s . Many s p e c i e s o f copepods, i n c l u d i n g C_^  b i c u s p i d a t u s , e x h i b i t avoidance responses t h a t are q u a l i t a t i v e l y s i m i l a r to those performed by D.kenai, thus d i f f e r e n c e s i n approach and c a p t u r e t e c h n i q u e s may a l l o w kokanee g r e a t e r c a p t u r e success and h i g h e r i n t a k e than t r o u t of copepods i n g e n e r a l . The second e x p l a n a t i o n f o r the appearance o f g r e a t e r numbers of copepods (C^ b i c u s p i d a t u s ) i n the d i e t o f kokanee and the dominance of c l a d o c e r a n s ( S ^ c r y s t a l l i n a i n the d i e t o f t r o u t i s based upon the d i f f e r e n c e s i n the i n t e r a c t i o n between the p r o b a b i l i t y o f a t t a c k and prey s i z e f o r t r o u t and kokanee. I have demonstrated t h a t t r o u t o f moderate s i z e (mean f o r k l e n g t h 11.2 cm) e x h i b i t weak a t t a c k responses t o p l a n k t o n i c prey s m a l l e r than 1 mm i n l e n g t h but t h a t size-matched 153 kokanee e x h i b i t w e l l developed a t t a c k responses to prey l e s s than 1 nun long (Experiment 5.3). S i n c e the maximum s i z e of C.  b i c u s p i d a t u s i n Marion Lake i s l e s s than 1.0 mm ( N o r t h c o t e & C l a r o t t o , 1975), i t i s no l o n g e r s u r p r i s i n g t h a t C y c l o p s i s l a r g e l y absent from the d i e t of moderate s i z e d t r o u t but r e l a t i v e -l y common i n the d i e t of kokanee. By c o n t r a s t the c l a d o c e r a n S. c r y s t a l l i n a i s commonly p r e s e n t i n the l a k e a t s i z e s to 1.8 mm and i t s dominance r e l a t i v e t o o t h e r z o o p l a n k t o n i n the d i e t of t r o u t i s undoubtedly due t o the c o m b i nation o f l a r g e s i z e , l o c a l c o n c e n t r a t i o n on the underside o f l i l y pads, and the absence o f any avoidance responses d u r i n g encounters w i t h v e r t e b r a t e p r e d a t o r s . The evidence t h a t t r o u t e x h i b i t a l a r g e r p r e y - s i z e t h r e s h o l d f o r an e f f e c t i v e a t t a c k response than kokanee of s i m i l a r s i z e may a l s o e x p l a i n why the zooplankton ( l a r g e l y S. c r y s t a l l i n a ) t h a t v e r y s m a l l t r o u t (mean f o r k l e n g t h 5.6 cm) o b t a i n e d on one o c c a s i o n were l a r g e l y from s i z e - c l a s s e s composed of i n d i v i d u a l s g r e a t e r than .6 mm i n l e n g t h w h i l e a l a r g e p r o p o r t i o n of the zooplankton (Sj^ c r y s t a l l i n a , C.  b i c u s p i d a t u s , Bosmina l o n g i r o s t r i s ) o b t a i n e d by s m a l l kokanee (mean f o r k l e n g t h 7.9 cm) on the same o c c a s i o n were from s i z e -c l a s s e s composed of i n d i v i d u a l s l e s s than .5 mm i n l e n g t h (Chapter 2, F i g . 7 ) . The f o r e g o i n g d i s c u s s i o n makes i t c l e a r t h a t t h e r e i s no s i n g l e e x p l a n a t i o n f o r why kokanee from Marion Lake e x p l o i t g r e a t e r q u a n t i t i e s o f zooplankton than t r o u t do. Indeed, t h i s g e n e r a l t r e n d i s l i k e l y favoured by the s m a l l 154 s i z e s of most s p e c i e s o f z o o p l a n k t o n p r e s e n t i n the l a k e ( E f f o r d , u n p u b l i s h e d d a t a ) , the h i g h e r l e v e l s of c a p t u r e success t h a t kokanee may e x p e r i e n c e w i t h copepods, and f i n a l l y the g r e a t e r f a c i l i t y t h a t kokanee have compared t o t r o u t i n g a t h e r i n g l a r g e numbers of s m a l l z o o p l a n k t o n w i t h i n a s p e c i f i e d time i n t e r v a l (Experiments 5.2 and 5.3). Kokanee do not o b t a i n more zooplankton o r s m a l l e r zooplankton than t r o u t as a consequence o f an a b i l i t y t o f i l t e r these prey from the water column (Experiment 5.3). Trout from Marion Lake i n c l u d e g r e a t e r numbers o f l a r g e (>5 mm), armoured, prey types ( c a d d i s l a r v a e , odonate l a r v a e , amphipods, p l a n o r b i d s n a i l s ) i n t h e i r d i e t than kokanee of comparable s i z e s do (Chapter 2 ) . In p r e v i o u s c h a p t e r s I have p o i n t e d out how d i f f e r e n c e s i n both s p a t i a l s e g r e g a t i o n (Chapter 3) and search b e h a v i o u r s (Chapter 4) o f t r o u t and kokanee promote t h i s p a t t e r n of food r e s o u r c e p a r t i t i o n i n g . R e s u l t s from t h i s c h a p t e r i n d i c a t e t h a t i n t e r a c t i o n s between the p r e d a t o r s and t h e i r prey d u r i n g the a t t a c k phase of the f e e d i n g c y c l e w i l l promote t h i s p a t t e r n as w e l l . The l a r g e r mouth s i z e and a p p a r e n t l y more po w e r f u l jaw musculature o f t r o u t r e l a t i v e t o kokanee a l l o w t r o u t to a c h i e v e h i g h e r l e v e l s of m a n i p u l a t i o n and i n g e s t i o n success w i t h l a r g e , armoured i n v e r t e b r a t e s (Experiment 5.4), t h u s , on average, kokanee w i l l have t o i n i t i a t e many more a t t a c k s on such prey t o a c h i e v e the same l e v e l s o f i n t a k e as t r o u t . For prey such as l a r g e amphipods (Crangonyx sp.) o r odonates (Enallagma sp.) the f i g u r e i s 2-3 times as many a t t a c k s w h i l e on prey such as n o t o n e c t i d s or c a d d i s l a r v a e the f i g u r e w i l l be even h i g h e r . D i f f i c u l t i e s 155 d u r i n g the m a n i p u l a t i o n and i n g e s t i o n o f l a r g e prey p r o b a b l y account f o r the v i r t u a l absence o f the l a r g e s t amphipods ( > 8 mm i n l e n g t h ) , m o l l u s c s (>5 mm i n d i a m e t e r ) , c a d d i s l a r v a e (;>5mm i n length) and l a r v a l odonates ( >6 mm i n len g t h ) from the d i e t of Marion Lake kokanee (Chapter 2 ) . D i f f e r e n c e s Between the P r o p o r t i o n s o f Prey Observed i n the N a t u r a l Environment and i n the D i e t s o f Trout and Kokanee E c o l o g i s t s concerned w i t h d e s c r i b i n g the c o m p o s i t i o n o f the i n v e r t e b r a t e community i n Marion Lake have employed a v a r i e t y o f d e v i c e s (Ekman dredge, Hargrave sampler, Kajak c o r e , p l a n k t o n pump, p l a n k t o n net) t o o b t a i n samples. In Chapter 2 I p o i n t e d out t h a t t r o u t and kokanee do not a c q u i r e prey from the n a t u r a l environment i n the same p r o p o r t i o n s as these o t h e r types o f samplers do. S t u d i e s such as those r e p o r t e d here h e l p t o e x p l a i n p r e c i s e l y why t h i s i s so. In g e n e r a l these man-made sampling d e v i c e s were designed and employed t o o b t a i n i n v e r t e b r a t e s i n p r o p o r t i o n t o t h e i r a c t u a l d e n s i t i e s i n the n a t u r e a l environment, thus the samplers tend t o " r e j e c t " o r omit r e l a t i v e l y ' f e w s p e c i e s o f a q u a t i c i n v e r t e b r a t e s on the b a s i s of d i f f e r e n c e s i n armour, s i z e o r escape b e h a v i o u r s . By c o n t r a s t t r o u t and kokanee have been "designed" through n a t u r a l s e l e c t i o n t o accommodate a r e l a t i v e l y l i m i t e d p o r t i o n o f the t o t a l range o f p o t e n t i a l i n v e r t e b r a t e prey t h a t r e s i d e i n the l a k e and as t h i s c h a p t e r shows may f r e q u e n t l y f a i l t o i n c l u d e items i n t h e i r d i e t s due to armour, s i z e o r escape b e h a v i o u r s . 156 Thus f a r I have examined i n sequence the p o t e n t i a l i n f l u e n c e of s p a t i a l s e g r e g a t i o n , temporal s e g r e g a t i o n , s e a r c h b e h a v i o u r s and a t t a c k procedures of t r o u t and kokanee from Marion Lake on the c o m p o s i t i o n o f t h e i r d i e t s . T h i s e x e r c i s e has p r o v i d e d e x p l a n a t i o n s of how s p e c i f i c mechanisms may operat e t o p r e d i s p o s e the p r e d a t o r s t o accumulate s p e c i e s - s p e c i f i c d i e t s . T h i s does not mean t h a t a l l o f the i n t e r e s t i n g p a t t e r n s o f prey e x p l o i t a t i o n have been s u c c e s s f u l l y l i n k e d t o the mechanisms t h a t produce them nor have I examined a l l of the major mechanisms i n v o l v e d i n shaping d i e t a r y h a b i t s . For example, e x p e r i e n c e i n d e a l i n g w i t h v a r i o u s prey types i s l i k e l y t o have a profound i n f l u e n c e on the responses o f both t r o u t and kokanee d u r i n g a l l stages o f the food g a t h e r i n g p r o c e s s . In the next c h a p t e r , I w i l l examine the i n f l u e n c e o f s h o r t term e x p e r i e n c e on the responses o f t r o u t and kokanee t o p r e y . SUMMARY 1. T r o u t and kokanee sometimes use d i f f e r e n t approach and cap t u r e t e c h n i q u e s on the same types of prey and i n d i v i d u a l l y they use d i f f e r e n t approach and captur e t e c h n i q u e s w i t h v a r y i n g degrees of success on d i f f e r e n t types of prey ( T a b l e s 17 and 1 8 ) . 2. Kokanee w i t h w e l l developed g i l l - r a k e r s do not o b t a i n more zooplankton o r s m a l l e r zooplankton than t r o u t as a consequence of any a b i l i t y t o f i l t e r such s m a l l prey from the water column (Experiment 5.3). 3. Kokanee c o n s i s t e n t l y e x h i b i t e i t h e r h i g h e r a t t a c k r a t e s o r g r e a t e r a t t a c k success than t r o u t i n encounters w i t h s m a l l 157 (<3 mm) zooplankton (Experiments 5.2 and 5.3). 4. Trout are e i t h e r i n c a p a b l e of e f f i c i e n t l y l o c a t i n g v e r y s m a l l (<1 mm) zo o p l a n k t o n o r they choose to i g n o r e them (Experiment 5.3)/ w h i l e kokanee are q u i t e adept a t e x p l o i t i n g these s m a l l p r e y . 5. Experiments w i t h t r o u t and kokanee " p r e c o n d i t i o n e d " t o feed on zooplankton suggest t h a t advantages of kokanee over t r o u t i n o b t a i n i n g z o o p l a n k t o n are l i k e l y due to g e n e t i c a l l y f i x e d m o r p h o l o g i c a l and b e h a v i o u r a l c h a r a c t e r i s t i c s (Experiment 5.3). 6. Kokanee e x h i b i t jaw w i d t h s t h a t a r e 30% s m a l l e r than those possessed by t r o u t o f e q u i v a l e n t body s i z e ( F i g . 2 2 ) . 7. Trout d i s p l a y a h i g h l y s i g n i f i c a n t advantage over kokanee i n the i n g e s t i o n of r e l a t i v e l y l a r g e (>5 mm), armoured prey (Experiment 5 . 4 ) . 8. Experiments w i t h both n o t o n e c t i d s and odonates as prey i n d i c a t e t h a t prey armour i s l e s s o f a d e t e r r e n t t o s u c c e s s f u l a t t a c k s by t r o u t than by kokanee (Experiment 5 . 4 ) . 9. Q u a l i t a t i v e and q u a n t i t a t i v e d i f f e r e n c e s i n both b e h a v i o u r a l and m o r p h o l o g i c a l c h a r a c t e r i s t i c s i n v o l v e d i n the a t t a c k phase of f o r a g i n g by t r o u t and kokanee serve as the b a s i s f o r e x p l a n a t i o n s of a number of d i f f e r e n c e s between the d i e t s of f r e e r a n g i n g p r e d a t o r s . These d i f f e r e n c e s i n c l u d e : the g r e a t e r u t i l i z a t i o n of a e r i a l prey by t r o u t , the i n c l u s i o n of l a r g e numbers of copepods i n the d i e t of kokanee but not of t r o u t , the g e n e r a l l y g r e a t e r u t i l i z a t i o n o f z o o p l a n k t o n by kokanee compared t o t r o u t , and the r e l a t i v e s c a r c i t y o f l a r g e (>4 mm body l e n g t h ) , armoured prey i n the d i e t of kokanee. 158 CHAPTER 6 THE ROLE OF SHORT TERM EXPERIENCE IN SHAPING THE RESPONSES OF TROUT AND KOKANEE TO PREY INTRODUCTION P r e d a t o r s are s e l e c t i v e l y a t t e n t i v e t o s t i m u l i t h a t are c h a r a c t e r i s t i c of d i f f e r e n t prey t y p e s . S e l e c t i o n o f i n f o r m a t i o n saves an animal from w a s t i n g time o r energy on s t i m u l i which are unimportant and t h i s w i l l be a d a p t i v e no matter how l i m i t e d the r e p e r t o i r e of response (Manning, 1972). D i r e c t e d a t t e n t i o n i n h i g h e r v e r t e b r a t e s i s u s u a l l y brought about through the proc e s s of l e a r n i n g i . e . , "a proc e s s t h a t m a n i f e s t s i t s e l f by a d a p t i v e changes i n i n d i v i d u a l behaviour as a r e s u l t of e x p e r i e n c e " (Thorpe, 1956) . A v a r i e t y o f s t u d i e s suggest t h a t s e l e c t i v e p e r c e p t i o n of responses r e s u l t i n g from e x p e r i e n c e w i t h v a r i o u s s t i m u l i may i n f l u e n c e p a t t e r n s of prey e x p l o i t a t i o n by p r e d a t o r s . For example Bryan and L a r k i n (1972) c l a i m t h a t e x p e r i e n c e has a long term e f f e c t (on the o r d e r o f weeks or months) on p a t t e r n s o f food s p e c i a l i z a t i o n by t r o u t . S i m i l a r l y e x p e r i m e n t a l s t u d i e s w i t h b i r d s suggest an important r o l e of l e a r n i n g i n g e n e r a t i n g non-random p a t t e r n s of prey e x p l o i t a t i o n (Murton, 1971; A l c o c k , 1971; Kear, 1962; R a b i n o w i t c h , 1969) . Experiments w i t h t h r e e - s p i n e d s t i c k l e b a c k s ( G a s t e r o s t e u s  a c u l e a t u s ) faced w i t h d i f f e r e n t combinations o f prey r e v e a l t h a t e x p e r i e n c e w i t h one prey s i m u l t a n e o u s l y a l t e r s the r i s k o f o t h e r s 159 (Beukema, 1968). S t i c k l e b a c k s faced w i t h combinations o f T u b i f e x sp. and Enchytraeus sp. as prey c o n s i s t e n t l y r e j e c t e d T u b i f e x sp. "on the e x p e c t a t i o n " o f l o c a t i n g the more p a l a t a b l e Enchytraeus sp., s i n c e a t the moment of r e j e c t i o n o f T u b i f e x sp., the f i s h e s c o u l d not s i m u l t a n e o u s l y p e r c e i v e Enchytraeus sp. T h e r e f o r e , p r e d a t o r e x p e r i e n c e w i t h d i f f e r e n t prey types under n a t u r a l c o n d i t i o n s may commonly i n f l u e n c e the prey's r i s k of e x p l o i t a t i o n . L e a r n i n g and h a b i t u a t i o n are i n v o l v e d i n d e t e r m i n i n g what an animal w i l l o r w i l l not eat o r a v o i d . L e a r n i n g may i n d i r e c t l y a f f e c t the types of foods gathered through i t s i n f l u e n c e on h a b i t a t s e l e c t i o n , t i m i n g of a c t i v i t i e s , s e a r ch t e c h n i q u e s . L e a r n i n g may a l s o d i r e c t l y a f f e c t the types o f foods gathered by i n f l u e n c i n g the p r o b a b i l i t y o f a t t a c k s on prey t h a t a r e d i s c o v e r e d and by i n f l u e n c i n g the p r o b a b i l i t y o f r e j e c t i o n o f prey a f t e r they have been c a p t u r e d . These d i r e c t e f f e c t s o f l e a r n i n g are the focus of the p r e s e n t c h a p t e r . I have a l r e a d y e s t a b l i s h e d t h a t t r o u t and kokanee from Marion Lake d i s p l a y c o n s i d e r a b l e d i v e r g e n c e i n the ways t h a t s i m i l a r prey are l o c a t e d , approached, and manipulated (Chapters 3, 4 and 5 ) . Because of these d i f f e r e n c e s , I am now i n t e r e s t e d i n t e s t i n g the g e n e r a l h y p o t h e s i s t h a t exposure t o i d e n t i c a l prey o f t e n c o n s t i t u t e s a d i f f e r e n t e x p e r i e n c e f o r t r o u t and kokanee and thus l e a r n e d responses to prey as a consequence of t h i s e x p e r i e n c e w i l l serve as a p o w e r f u l mechanism to produce p r e d a t o r s p e c i f i c p a t t e r n s of prey e x p l o i t a t i o n i n the f i e l d . 160 METHODS Experiment 6.1 The E f f e c t s o f Experience w i t h Chaoborus spp. Larvae on the P r e d a t o r y Responses of Trout and Kokanee I designed t h i s experiment t o p r o v i d e a number of p i e c e s of i n f o r m a t i o n . F i r s t , I wished t o determine how t r o u t and kokanee would respond to repeated exposure t o a prey type w i t h which on f i r s t encounter they have h i g h (>_ 80%) but f a r from t o t a l a t t a c k success (Chapter 5 ) . I used two groups of p r e d a t o r s (4 kokanee and 4 t r o u t ) captured f r e s h from the f i e l d . A l l p r e d a t o r s r e c e i v e d i d e n t i c a l treatment w i t h r e s p e c t to c a p t u r e , h a n d l i n g and maintenance procedures. Seven days a f t e r c a p t u r e from the f i e l d I conducted f e e d i n g t r i a l s w i t h i n d i v i d u a l p r e d a t o r s i n 200 l i t e r a q u a r i a stocked w i t h 400 l a t e i n s t a r Chaoborus spp. l a r v a e . A d d i t i o n a l t r i a l s took p l a c e every 72 hours u n t i l each p r e d a t o r had completed seven c o n s e c u t i v e t r i a l s w i t h chaoborus l a r v a e as p r e y . When w i l d caught p r e d a t o r s are used i n l a b o r a t o r y experiments, t h e i r responses w i l l be a f f e c t e d not o n l y by e x p e r i e n c e gained w i t h p a r t i c u l a r prey types but a l s o by the e x p e r i e n c e gained from exposure t o an e x p e r i m e n t a l procedure, which i n t h i s study i n c l u d e d h a n d l i n g and t r a n s f e r from a "home" aquarium t o an e x p e r i m e n t a l a r e n a . I wished t o observe the e f f e c t s of e x p e r i e n c e w i t h prey on p r e d a t o r s r a t h e r than the e f f e c t s of e x p e r i e n c e w i t h o t h e r a s p e c t s of the e x p e r i m e n t a l procedure. To minimize the l a t t e r , I took g r e a t care t o s t a n d a r d i z e a l l a s p e c t s of p r e d a t o r h a n d l i n g and maintenance d u r i n g t h i s experiment. For example, the d i p n e t , used t o c a p t u r e 161 f i s h f o r t r a n s f e r , f i t the home aquarium i n a way t h a t I c o u l d always o b t a i n i n d i v i d u a l p r e d a t o r s w i t h o u t r e s o r t i n g t o f o r c e d p u r s u i t . A f t e r g e n t l e n e t t i n g , i n d i v i d u a l p r e d a t o r s were always t r a n s f e r r e d from the home aquarium t o the e x p e r i m e n t a l arena i n a red p l a s t i c bucket. I n o r d e r to assess the success of these measures I compared the responses on the f i r s t t r i a l w i t h chaoborus l a r v a e , o f newly cap t u r e d kokanee, t o the responses of e x p e r i e n c e d kokanee. The f o u r animals i n the exper i e n c e d group had p r e v i o u s l y p a r t i c i p a t e d i n a t l e a s t f i v e f e e d i n g t r i a l s w i t h s m a l l amphipods ( H y a l e l l a a zteca) as prey . As p r e d a t o r s accumulate e x p e r i e n c e w i t h p a r t i c u l a r p r e y , they may e x h i b i t changes i n : c a p t u r e s u c c e s s , frequency o f prey r e j e c t i o n , t o t a l a t t a c k s i n i t i a t e d , r e a c t i v e d i s t a n c e o r the time taken t o i n i t i a t e the f i r s t a t t a c k i n a g i v e n t r i a l . Thus a v a r i e t y o f p o t e n t i a l i n d i c e s e x i s t t o document the i n f l u e n c e o f ex p e r i e n c e w i t h prey on p r e d a t o r responses. I r e l i e d on the pr e s e n t experiment t o p r o v i d e i n f o r m a t i o n t o asses s the r e l a t i v e s e n s i t i v i t y o f these i n d i c e s and t h e i r p o t e n t i a l f o r a p p l i c a t i o n i n f u r t h e r experiments. Experiment 6.2 The E f f e c t s o f Experience on Responses o f Kokanee t o a V a r i e t y o f B e n t h i c Prey Types T h i s experiment was designed t o examine the e f f e c t s o f changes i n both prey d e n s i t y and p r e d a t o r e x p e r i e n c e on the responses of kokanee t o a v a r i e t y o f b e n t h i c i n v e r t e b r a t e s . I c a r r i e d out i n d i v i d u a l f e e d i n g t r i a l s w i t h t h i r t e e n kokanee and f o u r s p e c i e s o f b e n t h i c prey ( H y a l e l l a sp., C e n t r o p t i l u m sp., Crangonyx sp., and Enallagma s p . ) . The s i z e s o f p r e d a t o r s 162 and prey used i n t h i s experiment are recorded elsewhere (see Table 19, Chapter 5 ) . A l l p r e d a t o r s used had completed a t l e a s t two i d e n t i c a l p r e - c o n d i t i o n i n g t r i a l s w i t h mixed prey i n the e x p e r i m e n t a l arena and thus were q u i t e f a m i l i a r w i t h the e x p e r i m e n t a l procedures. Each p r e d a t o r r e c e i v e d a t l e a s t f o u r c o n s e c u t i v e f e e d i n g t r i a l s w i t h s p e c i f i c prey types s i n c e r e s u l t s of experiment 6.1 suggested t h a t the e f f e c t s of e x p e r i e n c e on v a r i o u s p r e d a t o r responses were always apparent by t h i s p o i n t . 2 I used prey d e n s i t i e s o f 20, 40, 70 and 100 per .42 m r e s p e c t i v e l y i n c o n s e c u t i v e t r i a l s . In t h i s way I c o u l d examine p r e d a t o r responses t o changes i n prey d e n s i t y from t r i a l t o t r i a l and p r e d a t o r s were presented w i t h an equal o p p o r t u n i t y to accumulate e q u i v a l e n t l e v e l s of e x p e r i e n c e w i t h the v a r i o u s prey types by the end of t r i a l f o u r . Thus, by p o o l i n g r e s u l t s a c r o s s a l l f o u r t r i a l s , I was a b l e to examine the response of kokanee t o e x p e r i e n c e w i t h p r e y , minus the i n f l u e n c e of changes i n prey d e n s i t y between t r i a l s . I recorded number of prey e a t e n , t o t a l a t t a c k s i n i t i a t e d and time to i n i t i a t e the f i r s t a t t a c k i n a g i v e n t r i a l as s t a n d a r d i n d i c e s of p r e d a t o r responses t o e x p e r i e n c e w i t h p r e y . Because time t o f i r s t a t t a c k i n a g i v e n t r i a l i s l i k e l y t o be a f f e c t e d by prey s i z e and l e v e l s of a c t i v i t y , I c o l l e c t e d i n f o r m a t i o n on these c h a r a c t e r i s t i c s o f prey types used i n t r i a l s w i t h kokanee. Experiment 6.3 The H a b i t u a t i o n H y p o t h e s i s : E f f e c t s o f E x p e r i e n ce w i t h Large, Armoured Prey on Trout and Kokanee. Young o r a d u l t v e r t e b r a t e s which forage on t h e i r own must d e a l w i t h many d i f f e r e n t food s i t u a t i o n s , t h e r e f o r e t h e i r r e s p o n s i v e n e s s t o prey must be broad. I n e v i t a b l y t h i s broad r e s p o n s i v e n e s s w i l l r e s u l t i n encounters w i t h prey items t h a t are u n f i t t o e a t . Repeated exposure of p r e d a t o r s to such prey i s commonly p r e d i c t e d t o r e s u l t i n h a b i t u a t i o n , t h a t i s , a decrement i n r e s p o n s i v e n e s s t o a repeated o r c o n s t a n t s t i m u l u s (Thorpe, 1956). In p r e v i o u s f e e d i n g t r i a l s i n v o l v i n g a v a r i e t y o f prey (Chapter 5 ) , t r o u t and kokanee experienced the l o w e s t l e v e l s o f i n g e s t i o n success (6% and 0% r e s p e c t i v e l y ) w i t h l a r g e , armoured n o t o n e c t i d s . I designed the p r e s e n t experiment to t e s t the h y p o t h e s i s t h a t low l e v e l s of a t t a c k success w i l l reduce the r e s p o n s i v e n e s s of t r o u t and kokanee t o prey. I conducted i n d i v i d u a l f e e d i n g t r i a l s w i t h f o u r t r o u t and f o u r kokanee (see Table 19 f o r s i z e s ) . Each p r e d a t o r used i n t h i s experiment had completed a t l e a s t t h r e e p r e - c o n d i t i o n i n g t r i a l s w i t h chaoborus l a r v a e as prey i n the e x p e r i m e n t a l a r e n a , thus a l l p r e d a t o r s were f u l l y e x p e r i e n c e d w i t h the standard l a b o r a t o r y procedures. At the b e g i n n i n g of each t r i a l , 40 l a r g e n o t o n e c t i d s (mean l e n g t h 10.5 mm) were p l a c e d i n the e x p e r i m e n t a l arena. Each p r e d a t o r r e c e i v e d f o u r c o n s e c u t i v e t r i a l s (each l a s t i n g .5 hours) o f e x p e r i e n c e w i t h n o t o n e c t i d s i n the arena. Because these prey were seldom eaten and the f i s h d i d not r e c e i v e a supplementary food supply a t any time d u r i n g the experiment, I conducted t r i a l s w i t h i n d i v i d u a l p r e d a t o r s every 24 hours f o r f o u r days. T h i s ensured t h a t responses by the p r e d a t o r s to the 164 prey would be a consequence o f accumulated e x p e r i e n c e r a t h e r than a consequence of any lon g term e f f e c t s of s t a r v a t i o n . I recorded number of prey e a t e n , t o t a l a t t a c k s i n i t i a t e d and time t o i n i t i a t e the f i r s t a t t a c k i n a g i v e n t r i a l as standard i n d i c e s o f p r e d a t o r response t o e x p e r i e n c e w i t h n o t o n e c t i d s . Experiment 6.4 The Success and S t r e n g t h of Response H y p o t h e s i s : E f f e c t s o f Experience w i t h Small A g i l e Prey on Trout and Kokanee I have demonstrated p r e v i o u s l y t h a t kokanee are more e f f e c t i v e p r e d a t o r s than t r o u t i n i n i t i a l c o n t a c t s w i t h a v a r i e t y o f s m a l l p l a n k t o n i c prey (Chapter 5 ) . T h e r e f o r e , repeated exposure t o such prey w i l l c o n s t i t u t e a d i f f e r e n t e x p e r i e n c e f o r t r o u t as compared t o kokanee. T h i s experiment i s designed t o t e s t the h y p o t h e s i s t h a t when such d i f f e r e n c e s i n success w i t h s m a l l prey e x i s t , kokanee w i l l develop a s t r o n g e r response than t r o u t , upon repeated exposure to these p r e y . I used Diaptomus k e n a i as the prey i n t h i s experiment because they are l a r g e enough (mean l e n g t h i n t h i s experiment was 2 mm) t o a l l o w easy o b s e r v a t i o n o f p r e d a t o r success and because t r o u t and kokanee e x p e r i e n c e d i f f e r e n t l e v e l s of ca p t u r e success w i t h these prey upon i n i t i a l c o n t a c t . In a s i n g l e t r i a l w i t h these p r e y , t r o u t achieved o n l y 8% c a p t u r e success w h i l e kokanee a t t a i n e d 18% c a p t u r e s u c c e s s . The l a c k o f t o t a l success by p r e d a t o r s i n these t r i a l s was c l e a r l y r e l a t e d t o the pronounced avoidance response of p_. k e n a i when a t t a c k e d by f i s h . E i g h t size-matched t r o u t and kokanee (mean l e n g t h s 8.9 and 8.4 cm r e s p e c t i v e l y ) served as p r e d a t o r s . Up u n t i l the time 165 o f the experiment a l l p r e d a t o r s had r e c e i v e d i d e n t i c a l treatment w i t h r e s p e c t t o ca p t u r e from the f i e l d and subsequent h a n d l i n g and maintenance procedures i n the l a b o r a t o r y . The i n i t i a l d e n s i t y of p_. k e n a i i n each t r i a l was s e t a t 100 per 200 l i t e r s . I conducted h a l f hour t r i a l s w i t h i n d i v i d u a l p r e d a t o r s every 24 hours f o r f o u r days. I recorded the number of prey e a t e n , t o t a l a t t a c k s , t o t a l r e j e c t i o n s , and time t o i n i t i a t e the f i r s t a t t a c k i n a g i v e n t r i a l as the stand a r d i n d i c e s of p r e d a t o r response t o ex p e r i e n c e w i t h D_. k e n a i . RESULTS Experiment 6.1 Experience w i t h Chaoborus Larvae Trout and kokanee both a t t a i n h i g h c a p t u r e and i n g e s t i o n success i n i n i t i a l t r i a l s w i t h chaoborus l a r v a e and th e r e i s no evidence f o r any s i g n i f i c a n t improvement i n t h e i r a b i l i t y t o ca p t u r e o r handle these prey as a consequence o f repeated exposure t o them (Table 25 a and b ) . T h i s does not mean t h a t " e x p e r i e n c e " has no e f f e c t on the p r e d a t o r ' s response l e v e l s s i n c e t h e r e i s c l e a r l y an i n c r e a s e i n the t o t a l number of a t t a c k s t h a t t r o u t and kokanee make from one t r i a l t o the next ( F i g . 25). T h i s t r e n d may be a consequence o f e i t h e r an i n c r e a s e d w i l l i n g n e s s t o make a t t a c k s o r a consequence o f an i n c r e a s e d a b i l i t y t o p r o c e s s chaoborus l a r v a e . Both are p o s s i b l e . Animals such as l a b o r a t o r y r a t s commonly e x h i b i t an i n c r e a s e d i n t a k e of n o v e l foods as e x p e r i e n c e w i t h these i n c r e a s e s ( R o z i n , 1969) . The g r a d u a l l y i n c r e a s e d i n t a k e i s presumably a 1 6 6 TABLE 25. The e f f e c t o f e x p e r i e n c e w i t h prey i n s u c c e s s i v e f e e d i n g t r i a l s on a t t a c k success o f t r o u t and kokanee. Numbers i n b r a c k e t s i n d i c a t e the t o t a l number of a t t a c k s f o r a p a r t i c u l a r t r i a l . A t t a c k success i s the p r o p o r t i o n of prey eaten t o the number a t t a c k e d . a. Trout A t t a c k Success (as %) Mean T r i a l T r i a l T r i a l T r i a l T r i a l s Prey I d e n t i t y S i z e 1 2 3 4 > 4 mm Zooplankton Daphnia sp. 1.5 100 100 100 100 100 ( 404) ( 337) (668) (361) ( 558) Diaptomus k e n a i 2.0 8 12 18 8 -( 231) ( 155) ( 97) ( 12) -Chaoborus spp. 10 .0 86 92 97 95 92 ( 85) ( 212) (305) (261) (1,071) Amphipods H y a l e l l a a z t e c a 4.6 91 - - - -( 300) - - - -Crangonyx 8.3 91 - - - -richmondensis ( 120) — — — — Others Odonata 15.6 75 - - - -(Enallagma b o r e a l e ) ( 100) — — — — N o t o n e c t i d s 10 .5 4 8 2 7 — ( 79) ( 75) ( 42) ( 57) -b. Kokanee A t t a c k Success (as %) Zooplankton Daphnia sp. 1.5 100 100 100 100 -( 225) 859) (811) (574) -Diaptomus k e n a i 2.0 18 24 36 39 -(1,224) 1,262) (980) (805) -Chaoborus spp. 10.0 86 85 89 87 86 ( 308) 366) (449) (449) ( 867) Amphipods H y a l e l l a a z t e c a 4.6 89 98 92 95 — ( 83) 133) (225) (378) -Crangonyx 8.3 63 16 60 43 78 richmondensis ( 49) 49) ( 20) ( 28) ( 27) Others Odonata 15.6 49 26 31 43 38 (Enallagma b o r e a l e ) ( 35) 84) (121) ( 75) ( 47) N o t o n e c t i d s 10.5 0 0 0 0 — ( 56) 8) ( 8) ( 2) — Ephemeroptera 11.8 75 60 56 61 54 ( C e n t r o p t i l u m sp.) ( 59) 141) (197) (203) ( 71) 167 FIGURE 25. The r e l a t i o n s h i p between c o n s e c u t i v e f e e d i n g t r i a l s w i t h chaoborus l a r v a e as prey and the t o t a l number of a t t a c k s t h a t t r o u t and kokanee i n i t i a t e i n a g i v e n t r i a l . T r out used were 8.7 cm i n mean l e n g t h . Kokanee used were 16.4 cm i n mean l e n g t h . H TROUT Y= 18.21 + 9.64x O KOKANEE Y= 82 .98+ 15.25 x o CONSECUTIVE FEEDING TRIALS WITH CHAOBORUS LARVAE 168 consequence of a " t e s t i n g " procedure which a l l o w s the animals to run minimal r i s k o f o b t a i n i n g a l e t h a l dose of a n o v e l food t h a t i s t o x i c . I t i s most l i k e l y t h a t the t r e n d t o i n c r e a s e a t t a c k s on c o n s e c u t i v e t r i a l s w i t h chaoborus l a r v a e i s r e l a t e d t o p a r a l l e l changes i n some aspect of d i g e s t i v e t r a c t anatomy o r p h y s i o l o g y . I base t h i s c o n c l u s i o n upon the o b s e r v a t i o n s t h a t the p r e d a t o r s had been s t a r v e d f o r seven days p r i o r t o the i n i t i a l t r i a l w i t h chaoborus and t h a t the s m a l l t r o u t (mean l e n g t h 8.7 cm) used i n the experiment e x h i b i t e d fewer a t t a c k s which i n c r e a s e d more s l o w l y from t r i a l t o t r i a l than was the case w i t h the l a r g e r kokanee (mean l e n g t h 16 .4 cm). The i d e a t h a t the i n c r e a s e i n t o t a l a t t a c k s from t r i a l t o t r i a l i s due t o a g r a d u a l p r o c e s s of accomodating p r e d a t o r stomachs t o l a r g e r volumes of food a f t e r a p e r i o d of " s t a r v a t i o n " i s c o n s i s t e n t w i t h both of these o b s e r v a t i o n s . The p r o g r e s s i v e d e c l i n e i n the time taken to i n i t i a t e the f i r s t a t t a c k (TFA) on prey i n the arena ( F i g . .26) i n d i c a t e s t h a t both t r o u t and kokanee respond p o s i t i v e l y to the combination o f a h i g h l y a c c e p t a b l e prey and s t a n d a r d l a b o r a t o r y procedures. A comparison of v a l u e s of TFA between l a b o r a t o r y e x p e r i e n c e d kokanee and i n e x p e r i e n c e d kokanee and t r o u t (Table 26) i n d i c a t e s t h a t most of the v a r i a b i l i t y i n TFA on the f i r s t t r i a l i s due t o the response of the p r e d a t o r s t o the e x p e r i m e n t a l procedure r a t h e r than due t o responses to the p r e y . The s i m i l a r v a l u e s o b t a i n e d f o r TFA o f e x p e r i e n c e d p r e d a t o r s on t r i a l one (Group I I kokanee, Table 26) and of r e l a t i v e l y i n e x p e r i e n c e d p r e d a t o r s on t r i a l two 169 FIGURE 26. The r e l a t i o n s h i p between c o n s e c u t i v e f e e d i n g t r i a l s w i t h chaoborus l a r v a e as prey and the time to i n i t i a t e the f i r s t a t t a c k by t r o u t o r kokanee i n a g i v e n t r i a l . Note the changing s c a l e on the o r d i n a t e . Curves f i t t e d by eye. 170 ( F i g . 26) suggests t h a t the e f f e c t s o f the stan d a r d e x p e r i m e n t a l procedure l a r g e l y d i s a p p e a r by t r i a l two w i t h p r e d a t o r s . Thus, changes i n TFA a f t e r t r i a l two are most l i k e l y a consequence o f e x p e r i e n c e w i t h prey a l o n e . TABLE 26. A comparison of the time to f i r s t a t t a c k (TFA) and of t o t a l a t t a c k s by p r e d a t o r s on chaoborus l a r v a e . Group I . p r e d a t o r s were not f a m i l i a r w i t h the e x p e r i m e n t a l procedure w h i l e Group I I p r e d a t o r s were. Group I Kokanee Group I Trout Group I I Kokanee TFA (sec) Attacks. TFA (sec) A t t a c k s TFA (sec) A t t a c k s 416 96 290 36 26 ;89 223 102 720 36 9 141 *1800 9 800 44 20 94 686 95 660 35 65 103 X 781 76 617 338 30 107 S.D. 705 44 226 44 24 24 t h i s i s a minimum v a l u e s i n c e t r i a l s l a s t e d o n l y .5 hours. The decrease i n TFA from t r i a l two through t r i a l f i v e i n d i c a t e s t h a t chaoborus l a r v a e are a h i g h l y a c c e p t a b l e prey t o both t r o u t and kokanee. F u r t h e r , i t appears t h a t changes i n TFA may serve as an e s p e c i a l l y s e n s i t i v e i n d i c a t o r o f the e f f e c t s o f e x p e r i e n c e w i t h prey on p r e d a t o r response l e v e l s . 1 7 1 Experiment 6.2 Responses o f Kokanee t o Exp e r i e n c e w i t h a V a r i e t y o f B e n t h i c Prey Types. A t t a c k Success In g e n e r a l kokanee do not g a i n an improved a b i l i t y t o ca p t u r e o r handle b e n t h i c prey types i n c o n s e c u t i v e t r i a l s ( Table 25b). Although mean i n g e s t i o n success a c r o s s a l l t r i a l s i s more v a r i a b l e f o r kokanee exposed t o Crangonyx sp. and Enallagma sp. than f o r those exposed t o H y a l e l l a sp. and C e n t r o p t i l u m sp. (Table 25b), a t t a c k success does not appear t o be a p a r t i c u l a r l y s e n s i t i v e index of the e f f e c t s o f e x p e r i e n c e on p r e d a t o r responses to prey . T o t a l A t t a c k s 2 At an i n i t i a l d e n s i t y o f 20 prey per .42 m a l l kokanee completed s i m i l a r numbers of a t t a c k s on the f o u r types o f b e n t h i c p r e y . In subsequent t r i a l s , as prey d e n s i t y i n c r e a s e s , t h e r e i s a c l e a r l y d e f i n e d i n c r e a s e i n t o t a l a t t a c k s by kokanee on H y a l e l l a sp. and C e n t r o p t i l u m sp. ( F i g . 27 a and b ) , but a h i g h l y v a r i a b l e response t o Crangonyx sp. and Enallagma sp. ( F i g . 27 c and d ) . These r a t h e r d i f f e r e n t r e s u l t s c o u l d a l l be a consequence of responses t o prey d e n s i t y , i f the a b i l i t i e s o f kokanee t o pr o c e s s l a r g e Crangonyx sp. and Enallagma sp. are f u l l y s a t u r a t e d a t the lowest prey d e n s i t y , w h i l e those o f kokanee, i n t r i a l s w i t h H y a l e l l a sp. and C e n t r o p t i l u m s p., are not s a t u r a t e d even a t the h i g h e s t prey d e n s i t y . V a r i o u s p i e c e s o f evidence suggest t h a t t h i s i s not the c a s e . F i r s t , Kokanee g e n e r a l l y were not consuming a maximum r a t i o n of e i t h e r 172 FIGURE 27. The e f f e c t s o f changes i n e x p e r i e n c e and prey d e n s i t y on the t o t a l number of a t t a c k s t h a t kokanee i n i t i a t e on p a r t i c u l a r prey types i n a g i v e n t r i a l . Crangonyx sp. o r Enallagma sp. i n these t r i a l s . The kokanee used i n the C e n t r o p t i l u m sp. t r i a l s were s m a l l e r (mean l e n g t h 12.7 cm) than those used i n t r i a l s w i t h Crarrgonyx sp. (mean l e n g t h 14.7 cm) and t h e r e f o r e t h e i r t o t a l i n t a k e of prey (by volume) should have been l e s s than f o r kokanee f e e d i n g on Crangonyx sp. In sharp c o n t r a s t t o t h i s e x p e c t a t i o n , many more l a r g e C e n t r o p t i l u m sp. (mean l e n g t h 11.8 mm) than the r e l a t i v e l y s m a l l e r Crangonyx sp. (mean l e n g t h 8.3 mm) were consumed by kokanee. Given the average i n g e s t i o n success o f kokanee w i t h Crangonyx sp. and i n f o r m a t i o n on the number of Crangonyx sp. e q u i v a l e n t s (as wet weight i n t a k e ) consumed by the p r e d a t o r s i n two p r e - c o n d i t i o n i n g t r i a l s w i t h mixed prey I have c a l c u l a t e d t h a t kokanee i n the Crangonyx sp. t r i a l s would have to have i n i t i a t e d 116 a t t a c k s t o reach s a t u r a t i o n ( i . e . t o f i l l t h e i r g u t s ) . I t was a l s o apparent t h a t the time taken by kokanee t o handle i n d i v i d u a l Crangonyx sp. and Enallagma sp. d i d not p r e c l u d e t h e i r making a d d i t i o n a l a t t a c k s d u r i n g the .5 hour p e r i o d of s i n g l e t r i a l s . The l o n g e s t h a n d l i n g times on these prey were seldom g r e a t e r than 15 seconds and were u s u a l l y on the o r d e r of 2 - 4 seconds. In a d d i t i o n , kokanee i n t r i a l s w i t h Crangonyx sp. and Enallagma sp. spent a g r e a t d e a l of time engaged i n seemingly " a i m l e s s " swimming about the aquarium. In some t r i a l s i n d i v i d u a l kokanee f a i l e d t o e x h i b i t any response t o e i t h e r Crangonyx sp. o r Enallagma sp. T h i s was not due t o any g e n e r a l l a c k o f r e s p o n s i v e n e s s t o s t i m u l i because a t the end of t r i a l s i n which t h e r e had been no r esponse, the i n t r o d u c t i o n of a few mosquito or chaoborus l a r v a e was always 174 f o l l o w e d by immediate a t t a c k and i n g e s t i o n . T h e r e f o r e , i t seems t h a t kokanee are more s t r o n g l y p r e d i s p o s e d to a t t a c k some prey ( H y a l e l l a sp. and C e n t r o p t i l u m sp.) than o t h e r s (Crangonyx sp. and Enallagma sp.) and t h a t p o s s i b l y p r e d a t o r e x p e r i e n c e w i t h some prey has a g r e a t e r i n f l u e n c e over response l e v e l s than changes i n prey d e n s i t y do. The R e l a t i o n Between T o t a l A t t a c k s and A t t a c k Success I have argued above t h a t the upper l i m i t on t o t a l a t t a c k s made on prey by p r e d a t o r s i n these t r i a l s i s not a consequence of s a t u r a t i o n of e i t h e r the p r e d a t o r s a b i l i t i e s t o handle prey i n the amount of time a v a i l a b l e o r of t h e i r gut c a p a c i t i e s . The r e l e v a n t q u e s t i o n then i s what does c o n t r o l the t o t a l number of a t t a c k s t h a t these p r e d a t o r s make on s p e c i f i c prey types? R e s u l t s from a l a t e r experiment (see Experiment 6.3) suggest t h a t when kokanee are r e p e a t e d l y exposed t o prey w i t h which they have l i t t l e c a p t u r e o r i n g e s t i o n success they make very few a t t a c k s , t h u s , I suggest t h a t the l e v e l s o f a t t a c k success e x p e r i e n c e d by p r e d a t o r s w i t h prey may i n f l u e n c e the t o t a l number of a t t a c k s t h a t they are w i l l i n g t o make. To determine whether o r not t h i s i s the case I have pooled the t o t a l a t t a c k s i n i t i a t e d by i n d i v i d u a l p r e d a t o r s on s i n g l e prey types i n f o u r c o n s e c u t i v e t r i a l s and p l o t t e d t h i s v a l u e a g a i n s t % a t t a c k success e x p e r i e n c e d by i n d i v i d u a l p r e d a t o r s o v e r the same f o u r t r i a l s . R e s u l t s from t h i s a n a l y s i s ( F i g . 28) r e v e a l t h a t t h e r e i s an unmistakable r e l a t i o n s h i p between the l e v e l of a t t a c k success e x p e r i e n c e d by i n d i v i d u a l p r e d a t o r s and the t o t a l a t t a c k s 175 FIGURE 28. The r e l a t i o n s h i p between the l e v e l of a t t a c k success t h a t i n d i v i d u a l kokanee have w i t h s p e c i f i c prey types and the t o t a l number of a t t a c k s t h a t i n d i v i d u a l kokanee i n i t i a t e on these prey types over f o u r c o n s e c u t i v e t r i a l s . 1. C e n t r o p t i l u m sp. 2. Enallagma b o r e a l e , 3. Crangonyx r i c h m o n d e n s i s , 4. H y a l e l l a a z t e c a , 5. Notonecta u n d u l a t a o r Buenoa c o n f u s a . Note t h a t I have i n c l u d e d r e s u l t s from t r i a l s w i t h n o t o n e c t i d s (Experiment 6.3) because the response by kokanee to these prey i s b a s i c a l l y independent o f prey d e n s i t y . A R C S I N O F % I N G E S T I O N S U C C E S S 176 t h a t they make on s p e c i f i c prey t y p e s . My i n t e r p r e t a t i o n o f these r e s u l t s r e l i e s on the assumption t h a t the p r e d a t o r s do make " c h o i c e s " about whether o r not to a t t a c k i n d i v i d u a l p r e y . A p a r t i c u l a r l y s t r o n g p i e c e o f evidence t h a t t h i s i s the case c o n s i s t s of the tre n d s i n time t o f i r s t a t t a c k (TFA) by kokanee on prey i n c o n s e c u t i v e f e e d i n g t r i a l s . , Time t o F i r s t A t t a c k (TFA) There i s a p r o g r e s s i v e d e c l i n e i n TFA o f kokanee exposed t o i n c r e a s i n g d e n s i t i e s of H y a l e l l a sp. and C e n t r o p t i l u m sp. on c o n s e c u t i v e t r i a l s ( F i g . 29 a and b ) . T h i s s u p p o r t s the i d e a t h a t H y a l e l l a sp. and C e n t r o p t i l u m sp. a r e h i g h l y a c c e p t a b l e as prey by kokanee. There i s no w e l l developed d e c l i n e i n TFA f o r kokanee e x p l o i t i n g e i t h e r Crangonyx sp. o r Engallagma sp. ( F i g . 29 c and d ) . TFA w i l l be i n f l u e n c e d by f a c t o r s t h a t a f f e c t the p r o b a b i l i t y o f prey d e t e c t i o n by p r e d a t o r s . I n i t i a l l y the a b s o l u t e v a l u e of TFA i s determined by prey c h a r a c t e r i s t i c s such as s i z e , a c t i v i t y , c o l o u r and c o n t r a s t . TFA w i l l then be f u r t h e r i n f l u e n c e d by prey d e n s i t y , f a m i l i a r i t y o f the p r e d a t o r s w i t h the e x p e r i m e n t a l procedure and f a m i l i a r i t y o f the p r e d a t o r s w i t h the prey . On t r i a l one o f t h i s experiment a l l f a c t o r s except prey c h a r a c t e r i s t i c s were h e l d c o n s t a n t . By examining s p e c i f i c prey c h a r a c t e r i s t i c s i t i s p o s s i b l e to p r e d i c t the expected o r d e r of TFA from l o w e s t t o h i g h e s t f o r the f o u r prey t y p e s . 177 FIGURE 29. The e f f e c t s o f changes i n e x p e r i e n c e and prey d e n s i t y on the time t h a t i n d i v i d u a l kokanee take t o i n i t i a t e t h e i r f i r s t a t t a c k on s p e c i f i c prey types i n a g i v e n t r i a l . Note the s c a l e d i f f e r e n c e s between the o r d i n a t e s o f a & b and c & d r e s p e c t i v e l y . Curves f i t t e d by eye. 178 Prey used i n t h i s experiment were a l l of s i m i l a r c o lour and low c o n t r a s t , thus, these c h a r a c t e r i s t i c s w i l l c o n t r i b u t e l i t t l e to the observed v a r i a b i l i t y i n TFA v a l u e s . Because prey d e t e c t a b i l i t y i ncreases l i n e a r l y with an increase i n prey s i z e , kokanee should respond to Enallagma sp., Centroptilum sp., Crangonyx sp., and H y a l e l l a sp. i n order of i n c r e a s i n g TFA. Prey d e t e c t a b i l i t y i s a l s o enhanced by the p r o p o r t i o n of time that prey spend moving and on t h i s b a s i s alone kokanee should respond to Crangonyx sp., H y a l e l l a sp., Centroptilum sp., and Enallagma sp. i n order of i n c r e a s i n g TFA. To p r e d i c t the order of TFA as a consequence of the combined e f f e c t s of prey s i z e and movement, I w i l l assume that movement and s i z e are e q u a l l y important i n determining TFA and that t h e i r e f f e c t s are a d d i t i v e (note that although a more s o p h i s t i c a t e d a n a l y s i s i s p o s s i b l e (see Ware, 1973), i t s use would not a l t e r the order of TFA on prey p r e d i c t e d h e r e ) . I have a r b i t r a r i l y assigned the l a r g e s t and the most a c t i v e prey a value of 100. Next, I have awarded the remaining prey p o i n t s f o r s i z e and movement r e l a t i v e to the l a r g e s t and most a c t i v e prey r e s p e c t i v e l y . F i n a l l y , by adding the two values together I have produced an index which i n d i c a t e s the order of the four prey types according to i n c r e a s i n g values of TFA (Table 27). 17? TABLE 27. C h a r a c t e r i s t i c s o f prey used t o p r e d i c t the expected o r d e r of TFA proceeding from s m a l l e s t to l a r g e s t . See t e x t f o r e x p l a n a t i o n of p o i n t s awarded. Prey I d e n t i t y Mean s i z e . mm P o i n t s awarded P r o p o r t i o n o f time spent moving P o i n t s awarded T o t a l p o i n t s awarded Enallagma sp. 15.6 100 .01* 1 101 C e n t r o p t i l u m sp. 11.8 76 .05* 7 83 Crangonyx. sp. 8.3 53 .74** 100 153 H y a l e l l a sp. 4.6 29 .26** 35 64 * v a l u e s o b t a i n e d i n t h i s s t u d y , prey observed a t 10 C ** v a l u e s o b t a i n e d from e x t e n s i v e s t u d i e s by Ware, 1971; prey observed a t 10°C. On the b a s i s o f t o t a l p o i n t s accumulated, the p r e d i c t e d o r d e r o f TFA, as a consequence o f the i n t e r a c t i o n between p r e y s i z e and movement, i s Crangonyx sp., Enallagma sp., C e n t r o p t i l u m sp., and H y a l e l l a sp. T h i s i s almost e x a c t l y the r e v e r s e o f the o r d e r observed i . e . , C e n t r o p t i l u m s p., H y a l e l l a sp., Enallagma s p., and Crangonyx sp. r e s p e c t i v e l y . Thus, TFA does not appear t o be c o n s i s t e n t l y i n f l u e n c e d by f a c t o r s r e l a t e d t o the d e t e c t a b i l i t y o f pre y . By c o n d u c t i n g a more e l a b o r a t e a n a l y s i s , Ware (1973) i n d i c a t e d t h a t i n d i v i d u a l Crangonyx sp. are seven times more v u l n e r a b l e t o a t t a c k by t r o u t than H y a l e l l a sp. because Crangonyx sp. a re l a r g e r and spend more time moving when exposed. Thus, he 180 s u c c e s s f u l l y p r e d i c t e d t h a t t r o u t r e c o g n i z e Crangonyx sp. more e a s i l y , a t t a c k them from f u r t h e r away and a t t a c k them more o f t e n t h a t they do H y a l e l l a sp. For the same reasons Crangonyx sp. s h o u l d e l i c i t a s h o r t e r TFA from kokanee than H y a l e l l a sp. do. I n s t e a d kokanee e x h i b i t an average TFA t o Crangonyx sp. (673 sec) t h a t i s 84 times l o n g e r than t h e i r i average TFA (8 sec) t o H y a l e l l a sp. on t r i a l one. T h i s d i f f e r e n c e i s m a i n t a i n e d a c r o s s a l l f o u r t r i a l s . The i n e s c a p a b l e c o n c l u s i o n i s t h a t kokanee do not respond t o H y a l e l l a sp. r e l a t i v e t o Crangonyx sp. i n the same way t h a t t r o u t do. The r e s u l t s p resented here not o n l y i n d i c a t e t h a t kokanee are p r e -d i s p o s e d t o a t t a c k s m a l l H y a l e l l a sp. and p o o r l y armoured C e n t r o p t i l u m sp. r a t h e r than l a r g e , w e l l armoured Crangonyx sp. and Enallagma sp., but t h a t the e f f e c t s o f e x p e r i e n c e w i t h these prey i n t e n s i f y r a t h e r than reduce t h i s tendency. TFA and A t t a c k Success TFA does not appear t o be c o n s i s t e n t l y i n f l u e n c e d by f a c t o r s r e l a t e d t o prey d e t e c t a b i l i t y (e.g. prey d e n s i t y , prey s i z e , movement). I n s t e a d , i t seems t h a t p r e d a t o r e x p e r i e n c e w i t h some prey a f f e c t s the " w i l l i n g n e s s " of the animals t o i n i t i a t e a t t a c k s . The l e v e l of a t t a c k success e x p e r i e n c e d by the p r e d a t o r s i n t r i a l s w i t h v a r i o u s prey types may be p a r t i c u l a r l y i m p o r t a n t . I t i s not a p p r o p r i a t e t o compare p r e d a t o r TFA v a l u e s and a t t a c k success a c r o s s a l l prey types t e s t e d on both t r o u t and kokanee because of the g r e a t d i f f e r e n c e s i n prey d e n s i t y and c h a r a c t e r i s t i c s o f prey used i n the v a r i o u s experiments (Table 28). TABLE 28. C h a r a c t e r i s t i c s o f the prey s p e c i e s used and the number of p r e d a t o r s i n v o l v e d i n t r i a l s from which TFA v a l u e s have been d e r i v e d . Mean s i z e Other Prey d e n s i t y i n Number o f p r e d a t o r s c o n s e c u t i v e t r i a l s used Prey I d e n t i t y mm c h a r a c t e r i s t i c s T - l T-2 T-3 T-4 Tro u t Kokanee Chaoborus spp."*" 10.0 s t a t i o n a r y , t r a n s p a r e n t . 400 400 400 400 4 4 2 H y a l e l l a sp. 4.6 a c t i v e , low c o n t r a s t . 20 40 70 100 0 4 C e n t r o p t i l u m sp. 11.8 s t a t i o n a r y , medium c o n t r a s t . 20 40 70 100 0 3 2 Crangonyx sp. 8.3 a c t i v e , low c o n t r a s t . 20 40 - 70 100 0 3 2 Enallagma sp. 15.6 s t a t i o n a r y , medium c o n t r a s t . 20 40 70 100 0 3 N o t o n e c t i d s spp. 10 .5 a c t i v e , h i g h c o n t r a s t . 40 40 40 40 4 4 4 Diaptomus k e n a i 2.0 a c t i v e , h i g h c o n t r a s t . 100 100 100 100 4 4 1 Experiment 6.1 2 Experiment 6.2 3 Experiment 6.3 4 Experiment 6.4 182 However, i t i s wo r t h w h i l e t o examine how v a r i a b l e p r e d a t o r responses a re t o prey w i t h which they have a g i v e n l e v e l of su c c e s s . The standard d e v i a t i o n o f TFA i s used here as an index of t h i s v a r i a b i l i t y . - Because much o f the v a r i a b i l i t y i n i n i t i a l t r i a l s w i t h prey i s produced by the p r e d a t o r ' s l a c k o f f a m i l i a r i t y w i t h the e x p e r i m e n t a l procedure (Experiment 6.1), I have c o n s i d e r e d as r e p l i c a t e s o n l y responses recorded i n the f i n a l t h r e e t r i a l s p r e d a t o r s completed w i t h any prey type. The r e s u l t s from t h i s a n a l y s i s ( F i g . 30) i n d i c a t e t h a t as long as t r o u t and kokanee e x p e r i e n c e an a t t a c k success of 30% or b e t t e r w i t h p r e y , they w i l l respond i n a h i g h l y c o n s i s t e n t f a s h i o n from t r i a l t o t r i a l . However, i f a t t a c k success f a l l s much below 30% the p r e d a t o r s d i s p l a y tremendous v a r i a b i l i t y i n t h e i r w i l l i n g n e s s t o make a t t a c k s upon i n t r o d u c t i o n t o the e x p e r i m e n t a l arena. Experiment 6.3 The H a b i t u a t i o n H y p o t h e s i s : Responses t o N o t o n e c t i d s A t t a c k Success In f o u r c o n s e c u t i v e t r i a l s w i t h n o t o n e c t i d s , s m a l l t r o u t and kokanee f a i l t o e x h i b i t any improvement i n t h e i r a b i l i t y t o s u c c e s s f u l l y handle and i n g e s t n o t o n e c t i d s (Table 25). Trout e x h i b i t h i g h e r as w e l l as more v a r i a b l e success w i t h these prey than kokanee do, a l t h o u g h even t r o u t are o n l y m a r g i n a l l y s u c c e s s f u l a t u s i n g these prey as a food s o u r c e . O b s e r v a t i o n s i n d i c a t e t h a t the low l e v e l of success i s p r i m a r i l y a consequence of d i f f i c u l t i e s i n m a n i p u l a t i n g these l a r g e , well-armoured, p r e y ' 183 FIGURE 30. The r e l a t i o n s h i p between % a t t a c k success o f i n d i v i d u a l t r o u t and kokanee and t h e i r v a r i a b i l i t y o f response as i n d i c a t e d by the s t a n d a r d d e v i a t i o n o f the time t o f i r s t a t t a c k o v e r the l a s t t h r e e t r i a l s o f e x p e r i e n c e w i t h s p e c i f i c prey t y p e s . Data d e r i v e d from t r i a l s w i t h seven d i f f e r e n t prey types (see Table 28). Curve f i t t e d by eye. 183a CO o UJ cr < Q O O co or o UJ < o CO IOOOH 900 H > 800-700 H 600 A 500H y- 40oH L l _ O LZ 300-< > UJ Q 200 H loo H T T " 30 T I 50 T T T r~ 10  50 70 % ATTACK SUCCESS OF INDIVIDUAL PREDATORS T 90 184 by r e l a t i v e l y s m a l l p r e d a t o r s . Casual o b s e r v a t i o n s on l a r g e r t r o u t (21.6 cm) i n the l a b o r a t o r y i n d i c a t e t h a t they e x p e r i e n c e no d i f f i c u l t y i n i n g e s t i n g n o t o n e c t i d s i n the s i z e range used i n t h i s experiment. T o t a l A t t a c k s Trout e x h i b i t h i g h e r l e v e l s of response to n o t o n e c t i d s on t r i a l one than kokanee do ( F i g . 31). T h e r e f o r e , they e i t h e r have a g r e a t e r p r e d i s p o s i t i o n t o a t t a c k t h i s prey type than kokanee do o r w i t h i n the s h o r t span of one t r i a l t h e r e i s a d i f f e r e n t i a l e f f e c t of e x p e r i e n c e w i t h n o t o n e c t i d s on the tendency of t r o u t and kokanee to i n i t i a t e a t t a c k s . C o n s i d e r a t i o n o f the responses of the p r e d a t o r s over a l l f o u r t r i a l s i n d i c a t e s t h a t as the amount of e x p e r i e n c e w i t h n o t o n e c t i d s i n c r e a s e s the respon-s i v e n e s s of kokanee t o these "prey" d e c r e a s e s . Thus, kokanee m a i n t a i n the a b i l i t y t o r e c o g n i z e t h i s prey over the 24 hour i n t e r v a l between t r i a l s and are capable of r e d u c i n g responses to a very low l e v e l , g i v e n such unrewarding p r e y . T h i s s u p p o r t s the h y p o t h e s i s t h a t the p r o c e s s of h a b i t u a t i o n w i l l operate t o reduce the p r o b a b i l i t y o f kokanee e x p l o i t i n g prey which are d i f f i c u l t t o handle and i n g e s t . The response of t r o u t over the f o u r t r i a l s i s q u i t e u n l i k e t h a t o f kokanee. In s p i t e o f an a t t a c k success t h a t never exceeds 8% and which may be as low as 2%, t r o u t c o n t i n u e to respond t o n o t o n e c t i d s a t much h i g h e r l e v e l s than kokanee do. The f a i l u r e o f t r o u t to h a b i t u a t e t o n o t o n e c t i d s i s s u r p r i s i n g , e s p e c i a l l y s i n c e B e n f i e l d (1972) has r e p o r t e d t h a t rainbow t r o u t q u i c k l y 185 FIGURE 31. The r e l a t i o n s h i p between c o n s e c u t i v e t r i a l s o f exposure to a c o n s t a n t d e n s i t y o f n o t o n e c t i d s and the t o t a l number of a t t a c k s t h a t t r o u t and kokanee i n i t i a t e i n a g i v e n .5 hour t r i a l . N = number of i n d i v i d u a l p r e d a t o r s t e s t e d i n c o n s e c u t i v e t r i a l s . V e r t i c a l bars i n d i c a t e + one standard e r r o r o f the means. DU - 1 CONSECUTIVE TRIALS WITH NOTONECTIDS 186 h a b i t u a t e t o prey ( G y r i n i d s , Dineutes d i s c o l o r ) t h a t are u n p a l a t a b l e due t o d e f e n s i v e c h e m i c a l s e c r e t i o n s . . Time t o F i r s t A t t a c k (TFA) Although t r o u t and kokanee v a r y d r a m a t i c a l l y i n t h e i r t e n d e n c i e s t o a t t a c k l a r g e armoured prey l i k e n o t o n e c t i d s d u r i n g repeated exposure to them, t h e r e are some s i m i l a r i t i e s i n t h e i r responses. Both t r o u t and kokanee f a i l t o e x h i b i t any s i g n i f i c a n t r e d u c t i o n i n TFA on n o t o n e c t i d s over f o u r c o n s e c u t i v e t r i a l s ( F i g . 32). T h i s suggests t h a t a t the b e g i n n i n g of each t r i a l both p r e d a t o r s are e q u a l l y r e l u c t a n t t o a t t a c k n o t o n e c t i d s . C e r t a i n l y the long TFA recorded f o r i n d i v i d u a l s on c o n s e c u t i v e t r i a l s i s not a consequence of d e t e c t i o n d i f f i c u l t i e s s i n c e prey such as H y a l e l l a sp. which are l e s s than h a l f the s i z e of n o t o n e c t i d s and of lower c o n t r a s t (Table 28) were always a t t a c k e d i n much s h o r t e r times ( F i g . 29 a ) . Experiment 6.4 Responses to Diaptomus k e n a i : The E f f e c t s o f D i f f e r e n t i a l Success w i t h S m a l l , A g i l e Prey. A t t a c k Success Kokanee are a p p r o x i m a t e l y t w i c e as s u c c e s s f u l as t r o u t i n c a p t u r i n g D_. k e n a i . In sharp c o n t r a s t t o the s i t u a t i o n observed f o r the m a j o r i t y o f prey types t e s t e d , t r o u t and kokanee do d i s p l a y a c o n s i s t e n t improvement through t r i a l t h r e e i n t h e i r a b i l i t i e s t o ca p t u r e jD. k e n a i . Both p r e d a t o r s more than double t h e i r c a p t u r e success by t r i a l t h r e e (Table 25). In the case of t r o u t , t h i s was accomplished through r e f i n e m e n t of a b a s i c p a t t e r n 187 FIGURE 32. The r e l a t i o n s h i p between c o n s e c u t i v e t r i a l s of exposure to n o t o n e c t i d s and the time t o f i r s t a t t a c k o f i n d i v i d u a l p r e d a t o r s i n a g i v e n t r i a l . The s o l i d l i n e j o i n s the means. Note the changing s c a l e along the o r d i n a t e . 188 of approach and a t t a c k w h i l e f o r kokanee the i n c r e a s e d success r e l i e d on a complete change i n the b a s i c p a t t e r n o f approach and a t t a c k between t r i a l one and t r i a l t h r e e . During t r i a l s w i t h D. k e n a i t r o u t e x h i b i t e d the same behaviour as they had i n t r i a l s w i t h Daphnia sp., t h a t i s , they s c u l l i n midwater, s i g h t a pr e y , s t o p , and then lunge t o make a ca p t u r e a f t e r which they o f t e n s t o p a b r u p t l y " t o search f o r and l i n e up on" the next prey item (see Chapter 5 ) . Although r e f i n e m e n t o f these t a c t i c s r e s u l t s i n s i g n i f i c a n t improvement i n the a b i l i t y o f t r o u t to capture D. k e n a i , the v a s t m a j o r i t y o f JJ. k e n a i are s t i l l s u c c e s s f u l i n evading c a p t u r e . T h i s i s because JJ. k e n a i respond t o the onr u s h i n g p r e d a t o r by d a r t i n g r a p i d l y t o the s i d e , thus s u c c e s s f u l l y evading c a p t u r e . In p r e v i o u s experiments (Chapter 5) w i t h o t h e r zoo-p l a n k t o n (Daphnia sp., Diaptomus t y r e l l i e t c . . . ) , kokanee pursued prey by swimming i n a smooth path t h a t flowed from one c a p t u r e to the next w i t h l i t t l e i n t e r r u p t i o n and w i t h v e r y p r e c i s e changes of a l l i g n m e n t from c a p t u r e t o c a p t u r e . With repeated exposure to JJ. k e n a i these t a c t i c s change d r a s t i c a l l y . A f t e r s i g h t i n g a p r e y , a c r u i s i n g kokanee t y p i c a l l y d e c e l e r a t e s and approaches s l o w l y e i t h e r by g l i d i n g o r w i t h the a i d of g e n t l e s c u l l i n g by p e c t o r a l f i n s . When w i t h i n 3 - 5 cm of JJ. k e n a i , kokanee s t o p , f l e x i n t o a sigmoid p o s t u r e , and then r a p i d l y r e l e a s e to t h r u s t forward and make the c a p t u r e . T h i s s e t o f t a c t i c s i s a l s o f a r from c o m p l e t e l y s u c c e s s f u l but does a l l o w kokanee to a t t a i n a l e v e l of success t h a t i s g r e a t e r than double the maximum e x h i b i t e d by t r o u t ( T able 25). 189 T o t a l A t t a c k s Kokanee e x h i b i t h i g h e r l e v e l s of response than t r o u t to D_. k e n a i on t r i a l one ( F i g . 3 3 ) . T h e r e f o r e , they e i t h e r possess a g r e a t e r p r e d i s p o s i t i o n t o a t t a c k t h i s prey type than t r o u t o r w i t h i n the s h o r t span o f a s i n g l e t r i a l t h e r e i s a d i f f e r e n t i a l e f f e c t of e x p e r i e n c e w i t h D_. k e n a i on the tendency o f t r o u t and kokanee t o i n i t i a t e a t t a c k s . Given the magnitude of the d i f f e r e n c e ( i . e . an average of 60 a t t a c k s v e r s u s more than 300 a t t a c k s by kokanee) the former seems more l i k e l y . Over the course of f o u r c o n s e c u t i v e t r i a l s both t r o u t and kokanee e x h i b i t a tendency t o reduce the t o t a l number of a t t a c k s i n i t i a t e d on D. k e n a i . However, I wish t o s t r e s s t h a t even on the l a s t t r i a l kokanee are h i g h l y r e s p o n s i v e t o k e n a i (more than 200 a t t a c k s ) w h i l e t r o u t e x h i b i t almost complete h a b i t u a t i o n and make few responses a t a l l . Even on o c c a s i o n s when t r o u t a t t a c k and s u c c e s s f u l l y c a p t u r e D_. k e n a i t h e r e i s evidence t h a t t h e i r response t o the prey i s d i f f e r e n t from t h a t o f kokanee. Out o f a t o t a l o f 55 D. k e n a i s u c c e s s f u l l y c a p t u r e d , t r o u t r e j e c t e d 41. Kokanee never r e j e c t e d any o f the 1188 D. k e n a i t h a t they managed to c a p t u r e . The response o f t r o u t i s p a r t i c u l a r l y s u r p r i s i n g g i v e n t h a t JJ. k e n a i were the o n l y p o t e n t i a l source of food over the f o u r day d u r a t i o n of the experiment. The r e s u l t s from t h i s experiment l e n d s o l i d s u p port t o the h y p o t h e s i s t h a t an i n i t i a l d i f f e r e n c e i n captu r e success of t r o u t and kokanee e x p l o i t i n g s m a l l , a g i l e prey i s accompanied by the development of a more i n t e n s e p o s i t i v e response by kokanee upon repeated exposure t o the pre y . 190 FIGURE 33. The r e l a t i o n s h i p between c o n s e c u t i v e t r i a l s o f exposure to a c o n s t a n t d e n s i t y o f Diaptomus k e n a i and the t o t a l number of a t t a c k s t h a t t r o u t and kokanee i n i t i a t e i n a g i v e n t r i a l . N = number o f t r o u t o r kokanee t e s t e d on c o n s e c u t i v e t r i a l s . V e r t i c a l bars i n d i c a t e + one standard e r r o r o f the means. KOKANEE 1 I —I I 2 3 4 CONSECUTIVE TRIALS WITH D. KENAI AS PREY 191 Time t o F i r s t A t t a c k R e s u l t s from TFA f o r p r e d a t o r s exposed t o JJ. k e n a i g e n e r a l l y r e f l e c t the p a t t e r n s of response a l r e a d y i n d i c a t e d by t o t a l a t t a c k s . Kokanee e x h i b i t a w e l l d e f i n e d d e c l i n e i n TFA over the f o u r t r i a l s , a response which c o n f i r m s the r e l a t i v e a c c e p t -a b i l i t y o f jp_. k e n a i as prey f o r kokanee. TFA of t r o u t e x h i b i t s an i n i t i a l d e c l i n e (compare t r i a l s one and two of F i g . 3 4 ) , f o l l o w e d by a d r a m a t i c i n c r e a s e , which suggests t h a t as t r o u t become more ex p e r i e n c e d w i t h D. k e n a i , they become l e s s w i l l i n g t o a t t a c k t h i s p r e y . DISCUSSION The Range of Responses by Trout and Kokanee to Prey There are numerous examples i n the l i t e r a t u r e of papers d e a l i n g w i t h q u e s t i o n s c o n c e r n i n g the r e l a t i v e a c c e p t -a b i l i t y o f prey t o p r e d a t o r s (see Edmunds, 1974 f o r r e f e r e n c e s ) . Most of these have c o n c e n t r a t e d on the e f f e c t s o f a s i n g l e s t i m u l u s dimension ( e . g . t a s t e , c o l o u r , s i z e ) even though i t i s c e r t a i n t h a t many s t i m u l u s dimensions are " i n t e g r a t e d and summated" by p r e d a t o r s i n the course o f t h e i r i n s p e c t i o n s o f prey items ( E i s e n b e r g & Leyhausen, 1972). The s t i m u l u s dimensions t h a t r e p r e s e n t a g i v e n prey are not of uniform s i g n i f i c a n c e t o p r e d a t o r s . P o t e n t i a l s t i m u l i a s s o c i a t e d w i t h each prey may e l i c i t a range of response t h a t v a r i e s from v e r y p o s i t i v e to h i g h l y n e g a t i v e . I t i s the p r e d a t o r ' s assessment of the s i g n i f i c a n c e of the t o t a l s t i m u l u s s e t which w i l l determine i t s u l t i m a t e response to a p a r t i c u l a r prey t y p e . 192 FIGURE 3 4 . The r e l a t i o n s h i p between c o n s e c u t i v e t r i a l s o f exposure to Diaptomus k e n a i and the time t o f i r s t a t t a c k o f t r o u t and kokanee i n a g i v e n t r i a l . The s o l i d l i n e j o i n s the means. Note the changing s c a l e along the o r d i n a t e . 193 In many cases the f i n a l assessment of prey by p r e d a t o r s proceeds w i t h o u t d i f f i c u l t y . For example, many caged and w i l d p r e d a t o r s l e a r n a f t e r o n l y a few t r i a l s t o a v o i d a t t a c k i n g u n p a l a t a b l e i n s e c t s o r t o pursue p a l a t a b l e ones. When t h i s i s the case, i t i s o f t e n p o s s i b l e t o a s s o c i a t e the p r e d a t o r s response w i t h p a r t i c u l a r l y s t r o n g s t i m u l i such as noxious c h e m i c a l s e c r e t i o n s and, s e c o n d a r i l y , b r i g h t c o l o u r s i n the former o r l a r g e s i z e and " p r o v o c a t i v e " movements i n the l a t t e r . On the b a s i s o f d a t a on t o t a l a t t a c k s o r time t o i n i t i a t e f i r s t a t t a c k s , kokanee develop r e l i a b l e responses which i n d i c a t e t h a t prey such as Chaoborus spp., H y a l e l l a sp., C e n t r o p t i l u m sp., and D. k e n a i are e a s i l y d i s c r i m i n a t e d as a c c e p t a b l e , w h i l e prey such as n o t o n e c t i d s are j u s t as r e a d i l y i d e n t i f i e d as u n a c c e p t a b l e . S t r o n g , p o s i t i v e responses by t r o u t suggest t h a t they r e a d i l y i d e n t i f y Chaoborus spp., Crangonyx sp., and H y a l e l l a sp. as a c c e p t a b l e p r e y . A second s i t u a t i o n encountered by p r e d a t o r s i s t h a t i n which the i n f o r m a t i o n content of the s t i m u l i presented by prey i s c l e a r and r e s u l t s i n the p r e d a t o r s c o n s i s t e n t l y making m i s t a k e s . The b e s t known examples of t h i s are cases i n which p a l a t a b l e i n s e c t s mimic the appearance and behaviour of h i g h l y u n p a l a t a b l e ones (Rettenmeyer, 1970; Brower, 1969) and thus escape h i g h l e v e l s of p r e d a t i o n because p r e d a t o r s f a i l t o d i f f e r e n t i a t e mimics from t h e i r models (Sex t o n , 1960; Duncan & Shepherd, 1965). Although seldom s t u d i e d , i t i s j u s t as l i k e l y t h a t unacceptable prey items c o n t i n u e to e l i c i t p r e d a t o r responses because these prey p r e s e n t s t i m u l i t h a t are s i m i l a r t o those 194 possessed by f a m i l i a r and h i g h l y a c c e p t a b l e p r e y . For example, some b i r d s l e a r n t o r e j e c t u n p a l a t a b l e prey t h a t are n o v e l more r e a d i l y than they l e a r n to r e j e c t those resembling f a m i l i a r p a l a t a b l e prey (Brower, 1958; S h e t t l e w o r t h , 1972) . The co n t i n u e d response of t r o u t to n o t o n e c t i d s i n s p i t e o f v e r y low i n g e s t i o n success may f a l l i n t o t h i s c l a s s of response, t h a t i s , n o t o n e c t i d s p r e s e n t t r o u t w i t h s t i m u l i which i n c l u d e l a r g e s i z e , h i g h v i s u a l c o n t r a s t , and p r o v o c a t i v e movements. Many o f these s t i m u l i w i l l be r e l i a b l e i n d i c a t o r s of a wide range of prey t h a t a r e h i g h l y a c c e p t a b l e t o t r o u t , thus t h e i r c o n t i n u e d response to n o t o n e c t i d s may r e p r e s e n t an unavoi d a b l e c o s t o f m a i n t a i n i n g a h i g h s t a t e o f r e c e p t i v i t y t o s t i m u l i o r d i n a r i l y a s s o c i a t e d w i t h such a c c e p t a b l e p r e y . A f i n a l and r a t h e r p o o r l y e x p l o r e d s i t u a t i o n i s t h a t i n which prey p r e s e n t p r e d a t o r s w i t h p a r t i c u l a r l y d i f f i c u l t problems of d i s c r i m i n a t i o n which a r i s e as a consequence o f prey s t i m u l u s s e t s t h a t c o n t a i n c o n f l i c t i n g i n f o r m a t i o n c o n t e n t f o r p r e d a t o r s . I t i s d i f f i c u l t t o know e x a c t l y how to study t h i s problem (see S u t h e r l a n d , 1964), but i n f o r m a t i o n from the p r e s e n t study as w e l l as o t h e r s suggests t h a t i t may be a f a i r l y common phenomenon. For example, the responses o f kokanee to Crangonyx sp. and Enallagma sp. are ambiguous a t b e s t . S i m i l a r l y some as p e c t s of t r o u t responses t o n o t o n e c t i d s are ambiguous. T h e i r TFA v a l u e s suggest a c o n t i n u e d r e l u c t a n c e t o a t t a c k n o t o n e c t i d s upon repeated e n c o u n t e r s , however, they i n e v i t a b l y do respond w i t h a l a r g e number of a t t a c k s on these prey w i t h i n s i n g l e h a l f hour t r i a l s . 195 T rout responses t o D_. k e n a i are a l s o ambiguous. I n i t i a l l y , TFA v a l u e s d e c l i n e s u g g e s t i n g an i n c r e a s e i n r e a d i n e s s t o a t t a c k these p r e y , but then i n l a t e r t r i a l s TFA v a l u e s i n c r e a s e d r a m a t i c a l l y s u g g e s t i n g j u s t the o p p o s i t e . Upon i n i t i a l exposure t o JJ. k e n a i , t r o u t make numerous a t t a c k s and many s u c c e s s f u l c a p t u r e s ; however these are not c a r r i e d through t o i n g e s t i o n and many o f the prey are r e j e c t e d . These p r e d a t o r responses are not e a s i l y a s s o c i a t e d w i t h s t r o n g s t i m u l i from any o f the p r e y , r a t h e r , g i v e n an a p p r o p r i a t e p r e d a t o r - p r e y c o m b i n a t i o n , these same prey evoke h i g h l y c o n s i s t e n t responses. A c c o r d i n g t o Ware (1971), t r o u t respond i n a c o n s i s t e n t l y p o s i t i v e f a s h i o n t o Crangonyx sp. and i n the p r e s e n t s t u d y , kokanee r e g i s t e r e d unambiguous p o s i t i v e and n e g a t i v e responses to D. k e n a i and n o t o n e c t i d s r e s p e c t i v e l y . S t i m u l i t h a t t r i g g e r and s u s t a i n a t t a c k s to c o m p l e t i o n by o t h e r p r e d a t o r s are by no means o b v i o u s . S c h a l l e r (1972) r e p o r t e d t h a t he was unable to d i s c o v e r the f a c t o r s which t r i g g e r e d s u s t a i n e d a t t a c k s on g i r a f f e s o r b u f f a l o by l i o n s . Kruuk (1972) r e p o r t e d t h a t he c o u l d not d e t e c t any d i f f e r e n c e s i n ani m a l s o f one s p e c i e s t h a t were a t t a c k e d compared t o those t h a t were not a t t a c k e d by hyenas. Mech (1970) and Haber ( p e r s . comm.) have exp e r i e n c e d s i m i l a r d i f f i c u l t i e s i n d e t e r m i n i n g the c r i t i c a l s t i m u l i which l e a d wolves, h o l d i n g prey such as moose a t bay, t o a b r u p t l y cease a t t a c k s . P r e d a t o r s r e c e i v e and i n t e r p r e t a c o n t i n u o u s stream of s t i m u l i a s s o c i a t e d w i t h prey from the moment of d e t e c t i o n u n t i l a t l e a s t the moment of i n g e s t i o n . Many m o r p h o l o g i c a l , b e h a v i o u r a l o r ch e m i c a l c h a r a c t e r i s t i c s o f prey a re undoubtedly 196 r e t a i n e d , i n an e v o l u t i o n a r y sense, because they serve as s t i m u l i t h a t uncouple o r d i s - i n t e g r a t e an e f f e c t i v e approach and a t t a c k procedure by p r e d a t o r s . For example, N e i l and C u l l e n (1974) presented evidence to suggest t h a t s c h o o l s of prey a c t upon ambush p r e d a t o r s such as p i k e (Esox l u c i u s ) by i n t e r f e r i n g w i t h the complex sequence of a t t a c k by c a u s i n g avoidance r e a c t i o n s t o g e t h e r w i t h i r r e l e v a n t b e h a v i o u r s t h a t were i n a p p r o p r i a t e t o the g o a l of c a t c h i n g p r e y . For p u r s u i t p r e d a t o r s such as perch ( P e r c a  f l u v i a t i l i s ) the s c h o o l s appeared t o d i s r u p t the a t t a c k sequence by f o r c i n g the p r e d a t o r s t o c o n t i n u a l l y s w i t c h t a r g e t s d u r i n g p u r s u i t . By e x t e n s i o n t o the p r e s e n t study i t i s not too d i f f i c u l t t o b e l i e v e t h a t prey t e x t u r e o r a s l i g h t l y awkward h a n d l i n g procedure w i l l d e t e r kokanee from e f f e c t i v e l y e x p l o i t i n g l a r g e p rey such as Crangonyx sp. n o r i s i t u n l i k e l y t h a t s t i m u l i from s m a l l , a g i l e prey types d i s r u p t the b e h a v i o u r a l c h a i n t h a t t r o u t o r d i n a r i l y depend upon t o e x p l o i t l a r g e r b e n t h i c prey o r t e r r e s t r i a l i n s e c t s . C e r t a i n l y i t i s c l e a r t h a t t h e r e are p r e d a t o r s p e c i f i c d i f f e r e n c e s i n the r e a d i n e s s w i t h which s i m i l a r s t i m u l i a c q u i r e c o n t r o l o f behaviour under s i m i l a r c o n d i t i o n s . T h i s suggests t h a t t h e r e may be p r e d a t o r s p e c i f i c p r e d i s p o s i t i o n s t o l e a r n c e r t a i n t h i n g s and not o t h e r s . S p e c i e s S p e c i f i c E f f e c t s o f Experience w i t h Prey In c o n s i d e r i n g the e f f e c t s o f e x p e r i e n c e on c l o s e l y r e l a t e d p r e d a t o r s i n the p r e s e n t s t u d y , two extreme p o s i t i o n s are p o s s i b l e . These are t h a t animals may have a h i g h l y g e n e r a l i z e d a b i l i t y t o l e a r n o r t h a t animals possess s p e c i a l i z e d 197 a b i l i t i e s to l e a r n o n l y c e r t a i n t h i n g s . E x p e r i m e n t a l p s y c h o l -o g i s t s have h i s t o r i c a l l y s t r e s s e d the former view, but from an e c o l o g i c a l p e r s p e c t i v e the l a t t e r view makes more sense, t h a t i s , animals should be p r e d i s p o s e d t o l e a r n t h i n g s t h a t i t i s important f o r them t o l e a r n under n a t u r a l c o n d i t i o n s e s p e c i a l l y w e l l , w h i l e they may not l e a r n o t h e r comparable t a s k s a t a l l i f these have no p l a c e i n t h e i r l i v e s i n nature (Seligman, 1970; R o z i n & K a l a t , 1971; S h e t t l e w o r t h , 1972). The d i f f e r e n t i a l e f f e c t s o f e x p e r i e n c e on responses by t r o u t and kokanee t o the same prey were s u r p r i s i n g t o me, but perhaps should not have been. E t h o l o g i s t s have p o i n t e d out t h a t behaviour may be as s p e c i e s c h a r a c t e r i s t i c as any f e a t u r e of morphology o r p h y s i o l o g y . T h i s must be t r u e of a b i l i t i e s t o l e a r n . Trout and kokanee l e a r n not to a t t a c k some s m a l l , a g i l e prey (such as D_. kenai) and some l a r g e , armoured prey (such as n o t o n e c t i d s ) r e s p e c t i v e l y . The e f f e c t s o f e x p e r i e n c e seem t o op e r a t e by a m p l i f y i n g an e x i s t i n g , a l t h o u g h weak, p r e d i s p o s i t i o n not t o a t t a c k o r i n g e s t these prey. These p r e d i s p o s i t i o n s may be e i t h e r a consequence of g e n e t i c a l l y determined d i f f e r e n c e s o r o f d i f f e r e n c e s i n e x p e r i e n c e accumulated by the p r e d a t o r s under f i e l d c o n d i t i o n s p r i o r t o t h e i r c a p t u r e and performance i n the p r e s e n t experiments. The f a i l u r e of t r o u t to h a b i t u a t e to n o t o n e c t i d s as o b j e c t s f o r a t t a c k may i n d i c a t e t h a t responses t o such prey are i n n a t e . V a r i o u s authors (Hinde, 1966; F i g l e r , 1972) have i n d i c a t e d t h a t i n n a t e behaviour i s h i g h l y r e s i s t a n t t o h a b i t u a t i o n . Smith (1973) found t h a t the a t t a c k responses o f young s h r i k e s to models d i d not h a b i t u a t e , even i n the absence 198 o f any t a n g i b l e reward and she concluded t h a t t h e i r a t t a c k responses were i n n a t e . The experiments conducted e a r l i e r (Experiment 5.3 and 5 .4 ) and i n v o l v i n g p r e c o n d i t i o n i n g of j u v e n i l e t r o u t and kokanee to zoo p l a n k t o n suggest t h a t a t t a c k p r e d i s p o s i t i o n s o f t r o u t and kokanee may be g e n e t i c a l l y d e termined, a l t h o u g h I r e c o g n i z e t h a t more c r i t i c a l e x p e r i m e n t a l t e s t s are r e q u i r e d t o e s t a b l i s h t h i s p o i n t . The Consequences o f S p e c i e s S p e c i f i c E f f e c t s o f Experience f o r P a t t e r n s o f Prey A c q u i s i t i o n by Trout and Kokanee Many motor p a t t e r n s i n v o l v e d i n s e a r c h , approach, a t t a c k and m a n i p u l a t i o n o f prey may be common to the members o f two c l o s e l y r e l a t e d s p e c i e s , however, l e a r n i n g may l e a d t o d i v e r g e n c e i n the form of such p a t t e r n s which then may generate s u b s t a n t i a l d i f f e r e n c e s i n p a t t e r n s of prey e x p l o i t a t i o n by p r e d a t o r s . There are no s p e c i f i c d i f f e r e n c e s between the c o m p o s i t i o n o f t r o u t and kokanee d i e t s , o r between the prey c o n t e n t s o f these p r e d a t o r s and the environment, which I can a t t r i b u t e s o l e l y t o the e f f e c t s of s h o r t term e x p e r i e n c e w i t h p r e y . In g e n e r a l e x p e r i e n c e w i l l o perate t o a m p l i f y d i e t a r y d i f f e r e n c e s by a l t e r i n g the r e s p o n s i v e -ness of p r e d a t o r s to prey t h a t a re encountered. Thus, the d i f f e r e n t i a l e f f e c t s o f e x p e r i e n c e on r e s p o n s i v e n e s s o f t r o u t and kokanee w i l l a m p l i f y the t r e n d s i d e n t i f i e d p r e v i o u s l y f o r : 1) kokanee to e x p l o i t a g r e a t e r q u a n t i t y and a g r e a t e r v a r i e t y of z o o p l a n k t o n than t r o u t do, 199 2) t r o u t t o e x p l o i t more l a r g e , armoured prey types such as c a d d i s l a r v a e ( T r i c h o p t e r a ) , d r a g o n f l y and d a m s e l f l y nymphs (Odonata), a q u a t i c i n s e c t a d u l t s (e.g. n o t o n e c t i d s ) , m o l l u s c s (e.g. p l a n o r b i d s n a i l s ) and l a r g e amphipods (Crangonyx s p . ) , 3) t r o u t t o " o v e r e x p l o i t " and kokanee t o " u n d e r e x p l o i t " l a r g e prey items r e l a t i v e t o t h e i r e n v ironmental abundance, 4) t r o u t t o c o n t a i n a d i s p r o p o r t i o n a t e number of l a r g e prey items and kokanee to c o n t a i n a d i s p r o p o r t i o n a t e number of s m a l l prey items r e l a t i v e t o t h e i r e n v ironmental abundance. In an e a r l i e r r e v iew a r t i c l e ( H y a t t , 1979) , d e a l i n g w i t h f i s h d i e t a r y h a b i t s , I s t r e s s e d the p o i n t t h a t p a t t e r n s o f non-random e x p l o i t a t i o n of prey should be c o n s i d e r e d t o be a f u n c t i o n o f s e v e r a l p o t e n t i a l b i o l o g i c a l mechanisms which ope r a t e alone o r i n c o n c e r t . T h i s view was c o n t r a s t e d w i t h the more common p r a c t i s e by v a r i o u s i n v e s t i g a t o r s of spons o r i n g one mechanism o r another as the s o l e d r i v i n g f o r c e behind p a r t i c u l a r d i e t a r y p a t t e r n s . In the p r e s e n t study i t i s apparent t h a t some of the d i f f e r e n c e s i n d i e t a r y p a t t e r n s between t r o u t and kokanee are not the r e s u l t of s i n g l e mechanisms but r a t h e r are produced by e n t i r e s e t s of mechanisms which operate a t each stage of the b e h a v i o u r a l c h a i n i n v o l v e d i n food g a t h e r i n g . For example, the g e n e r a l t r e n d f o r t r o u t t o e x p l o i t l a r g e r prey than kokanee o f e q u i v a l e n t s i z e do (Chapter 2) i s favoured not o n l y by d i f f e r e n c e s i n h a b i t a t s e l e c t i o n (Chapter 3) and search p o s i t i o n s (Chapter 4 ) , but a l s o by the i n a b i l i t y o f t r o u t to e f f e c t i v e l y c a p t u r e the s m a l l e s t s i z e c l a s s e s o f prey (Chapter 5 ) , the i n a b i l i t y o f kokanee to e f f e c t i v e l y handle l a r g e , armoured prey (Chapter 5 ) , and f i n a l l y by the 200 d i f f e r e n t i a l e f f e c t s o f e x p e r i e n c e on the r e s p o n s i v e n e s s o f these p r e d a t o r s t o l a r g e and s m a l l prey r e s p e c t i v e l y ( t h i s c h a p t e r ) . SUMMARY 1. The e f f e c t s o f e x p e r i e n c e on subsequent responses o f t r o u t and kokanee to prey are s i m i l a r f o r prey w i t h which the p r e d a t o r s a t t a i n s i m i l a r l e v e l s o f c a p t u r e and i n g e s t i o n success (Experiment 6.1). 2. Both t r o u t and kokanee develop and m a i n t a i n r e l a t i v e l y c o n s t a n t and u s u a l l y h i g h l e v e l s of response t o s i n g l e prey s p e c i e s w i t h which the p r e d a t o r s e x p e r i e n c e g r e a t e r than 30% a t t a c k s u c c e s s . Lower l e v e l s of a t t a c k success r e s u l t i n h i g h l y v a r i a b l e l e v e l s of response by the p r e d a t o r s t o s i n g l e s p e c i e s of prey (Experiment 6.2). 3. Trout and kokanee e x h i b i t an i n i t i a l r e l u c t a n c e t o a t t a c k l a r g e , well-armoured n o t o n e c t i d s . However, i n any t r i a l t r o u t e v e n t u a l l y respond to n o t o n e c t i d s more s t r o n g l y than kokanee do. With repeated exposure, kokanee r a p i d l y h a b i t u a t e t o these unrewarding prey but t r o u t do not i n s p i t e of v e r y low a t t a c k success (Experiment 6.3). 4. In response to e x p e r i e n c e w i t h s m a l l , a g i l e , JJ_. k e n a i , kokanee e x h i b i t a change i n approach and c a p t u r e t a c t i c s which improves t h e i r c a p t u r e s u c c e s s . Trout r e f i n e t h e i r b a s i s approach and c a p t u r e t a c t i c s , but do not employ e n t i r e l y new ones i n response t o e x p e r i e n c e w i t h D. k e n a i (Experiment 6 . 4 ) . 201 5 . I n g e n e r a l , t r o u t e x h i b i t a s t r o n g p r e d i s p o s i t i o n t o a t t a c k r e l a t i v e l y l a r g e , w e l l - a r m o u r e d , p r e y and t o i g n o r e o r r e j e c t s m a l l , a g i l e o n e s ( E x p e r i m e n t s 6 . 3 a n d 6 . 4 ) . K o k a n e e e x h i b i t a s t r o n g p r e d i s p o s i t i o n t o a t t a c k s m a l l , a g i l e p r e y a n d t o i g n o r e o r r e j e c t r e l a t i v e l y l a r g e , w e l l - a r m o u r e d o n e s ( E x p e r i m e n t s 6 . 2 , 6 . 3 and 6 . 4 ) . T h e e f f e c t s o f e x p e r i e n c e a c t t o i n t e n s i f y t h e s e d i f f e r e n c e s . 202 CHAPTER 7 TROUT AND KOKANEE FORAGING - THE STRATEGIC POINT OF VIEW Much of e v o l u t i o n a r y b i o l o g y i s the working out o f an a d a p t a t i o n i s t program. E v o l u t i o n a r y : b i o l o g i s t s assume t h a t each aspect o f an organisms morphology, p h y s i o l o g y , and behaviour has been molded by n a t u r a l s e l e c t i o n as a s o l u t i o n t o a problem p r o v i d e d by the environment. The r o l e of the e v o l u t i o n a r y b i o l o g i s t i s then t o c o n s t r u c t a p l a u s i b l e argument about how each p a r t f u n c t i o n s as an a d a p t i v e d e v i c e . In p r a c t i s e an a d a p t a t i o n i s t program i s c o n s t r u c t e d by c r e a t i n g d e s c r i p t i o n s o f the organism and of the environment and then r e l a t i n g the d e s c r i p t i o n s by f u n c t i o n a l statements. Lewontin, 1978. INTRODUCTION Trout and kokanee from Marion Lake e x p l o i t s t a t i s t i c a l l y d i f f e r e n t s e t s o f food r e s o u r c e s . In p r e v i o u s c h a p t e r s I have d e a l t w i t h why t h i s i s so i n terms of p r o x i m a l "mechanisms" but have ignored why t h i s i s so from an e v o l u t i o n -ary p o i n t o f view. T h e r e f o r e , the purpose of t h i s c h a p t e r i s t o develop a d i s c u s s i o n which w i l l p l a c e r e s u l t s from p r e v i o u s c h a p t e r s i n t o an e v o l u t i o n a r y c o n t e x t . Each s p e c i e s possesses a v a r i e t y o f s t r a t e g i e s (eg. f e e d i n g , r e p r o d u c t i v e , a n t i p r e d a t o r ) which c o n s i s t o f the sum of both the f i x e d and f a c u l t a t i v e a d a p t a t i o n s t h a t have been shaped through n a t u r a l s e l e c t i o n t o maximize the f i t n e s s o f i n d i v i d u a l s . Environmental d i v e r s i t y i s the e s s e n t i a l i n g r e d i e n t t h a t f a v o u r s the e v o l u t i o n o f s p e c i e s s p e c i f i c s t r a t e g i e s . T h i s i s because the m e t a b o l i c and g e n e t i c 203 r e s o u r c e s t h a t go i n t o one s e t o f a d a p t a t i o n s , addressed t o p a r t i c u l a r e n vironmental c h a r a c t e r i s t i c s , come a t the expense of energy and i n f o r m a t i o n t h a t c o u l d go i n t o another s e t o f a d a p t a t i o n s addressed t o d i f f e r e n t e n v i r o n m e n t a l c h a r a c t e r i s t i c s , (Cody and Diamond, 1975). Hence the b e n e f i t s d e r i v e d from any t r a i t are weighed by n a t u r a l s e l e c t i o n a g a i n s t the c o s t s o f m a i n t a i n i n g t h a t t r a i t and a l s o a g a i n s t the "abandoned" b e n e f i t s o f a l t e r n a t i v e t r a i t s . In t h i s c h a p t e r my b a s i c argument w i l l be t h a t the m o r p h o l o g i c a l and b e h a v i o u r a l t r a i t s t h a t c o n t r o l food " s e l e c t i o n " by t r o u t and kokanee i n Marion Lake (summarized i n Table 29 ) are a consequence o f the e v o l u t i o n o f m u t u a l l y e x c l u s i v e f o r a g i n g s t r a t e g i e s which s u i t these p r e d a t o r s to g a t h e r food most e f f e c t i v e l y from two d i f f e r e n t environments. One consequence o f t h i s i s t h a t t r o u t and kokanee are preadapted t o ga t h e r d i f f e r e n t foods even i n the same environment (eg. b e n t h i c and l i t t o r a l h a b i t a t s of Marion L a k e ) . I w i l l develop the c h a p t e r i n two p a r t s . The f i r s t i n v o l v e s the i d e n t i f i c a t i o n o f the g e n e r a l nature of the h a b i t a t - p r e y complexes t h a t have shaped t r o u t and kokanee f o r a g i n g s t r a t e g i e s . The second p a r t c o n s i s t s of a d i s c u s s i o n c o n c e r n i n g the e v o l u t i o n and a d a p t i v e s i g n i f i c a n c e o f s p e c i f i c elements of t r o u t and kokanee f o r a g i n g s t r a t e g i e s . TABLE 29. A summary of the "elements" which define the foraging strategies of trout and kokanee. TROUT KOKANEE SOURCE Within lake d i s t r i b u t i o n consistently skewed towards onshore and benthic habitats A c t i v i t y peaks i n e a r l y morning a f t e r which foraging e f f o r t s assume lower but consistent levels throughout daylight hours. Relatively inactive at night. Within lake d i s t r i b u t i o n consistently • Table 7. skewed towards offshore and surface or and water column habitats F i g . 12 A c t i v i t y peaks i n early morning and la t e Fig's. 13 afternoon. Foraging e f f o r t s are continuous and 14. throughout the daylight hours. Relatively inactive at night. Do not "track" the environmental abundance of prey very c l o s e l y .Track the environmental abundance of potential prey much more closely than trout do Fig's. 8 and 9 Greatest proportion of d i e t (by wt.) obtained from r e l a t i v e l y large size classes of prey (eg. t e r r e s t r i a l insects, s n a i l s , odonates, caddis and other fish) Greatest proportion of d i e t (by wt.) obtained from r e l a t i v e l y small size classes of prey (eg. chironomid pupae, zooplankton etc...) Fig's. 4, 5, 6, and 7 Usually forage as s o l i t a r y individuals but may forage within groups on some occasions Usually forage in groups except when searching i n benthic habitats at which time they are s o l i t a r y Unpublished observations V i s i o n used as the primary sensory mode during food search Visio n used as the primary sensory mode during food search A l i , 1959; Ware, 1971; Hyatt, present study Table 29 - continued Employ a v a r i e t y of search techniques which may involve constant or intermittent swimming a c t i v i t y Area extensive searchers i e . maintain r e l a t i v e l y high search v e l o c i t i e s , "test" r e l a t i v e l y few inanimate objects for t h e i r "potential as prey" and maintain search positions that are at r e l a t i v e l y great distance from the substrates that are scanned Reactive distance to a variety of prey of d i f f e r e n t sizes (up to 16 mm) i s usually l e s s than 70 cm Employ a variety of approach and capture techniques to take benthic, water column, surface and a e r i a l prey Possess r e l a t i v e l y large mouths ( i e . jaw widths are close to 7-8% of standard length) and poorly developed g i l l - r a k e r s Exhibit a predisposition to attack r e l a t i v e l y large, well-armoured prey and tend to ignore or reject small, a g i l e ones Employ a variety of search techniques which always involve constant swimming a c t i v i t y Table 12. and text Area intensive searchers i e . maintain Table 10. r e l a t i v e l y low search v e l o c i t i e s , "test" Fig's. 16 large numbers of small inanimate objects and 17 f o r t h e i r potential as food and maintain search positions that are r e l a t i v e l y close to the substrates that are scanned (see text for further explanation) Reactive distance to a variety of prey F i g . 20 of d i f f e r e n t sizes (up to 16 mm) i s usually less than 70 cm Employ a variety of approach and capture Table 17. techniques to take benthic, water column, and surface prey but do not exploit a e r i a l prey Possess r e l a t i v e l y small mouths ( i e . jaw F i g . 22 and widths are close to 5-6% of standard text length) and well developed g i l l - r a k e r s Exhibit a predisposition to attack r e l a t i v e l y small, a g i l e prey and to ignore or reject large, armoured ones Experiments 5.2, 5.3 and 5.4 Table 29 - continued Exhibit r e l a t i v e l y high l e v e l s of manipulation and ingestion success with large, armoured and morphologically diverse prey Exhibit variable attack and ingestion success with r e l a t i v e l y small, a g i l e prey Maximum attack rates on small, a g i l e , prey at high densities are r e l a t i v e l y low ( i e . average 1860 attacks per hour on Daphnia at densities of s l i g h t l y greater than 2 per l i t e r ) Exhibit a r e l a t i v e l y high prey-size threshold f o r an ef f e c t i v e attack response on very small prey Develop and maintain high levels of rec e p t i v i t y to prey with which they experience greater than 30% attack success. Responses more variable at lower levels of success. Maintain high l e v e l s of response to some large prey i n spite of low ingestion success Exhibit r e l a t i v e l y low levels of Table 24. manipulation and ingestion success with large, armoured and morphologically diverse prey Exhibit generally high attack and Tables 20 ingestion success with small, a g i l e and 22 prey Maximum attack rates on small, a g i l e , prey at high densities are r e l a t i v e l y F i g . 23 high ( i e . average more than 3000 attacks per hour on Daphnia). Exhibit a r e l a t i v e l y low prey-size Table 22 threshold for an effective attack response on very small prey Develop and maintain high levels of F i g . 30 rece p t i v i t y to prey with which they have greater than 30% attack success. Responses are more variable at lower levels of success Habituate rapidly to large prey which o f f e r low net energy returns and/or F i g . 31 possibly high r i s k of damage during ingestion Table 29 - continued Habituate r a p i d l y to some small prey Maintain high levels of response F i g . 33 i n spite of a potential f o r p o s i t i v e to small prey i n general net energy returns o 208 THE ENVIRONMENTAL CONTEXT FOR TROUT AND KOKANEE FORAGING STRATEGIES In each i n s t a n c e the f o r a g i n g s t r a t e g i e s o f t r o u t and kokanee must u l t i m a t e l y r e l a t e to a s p e c i f i c e n vironmental c o n t e x t . What i s t h i s c o n t e x t ? B e h a v i o u r a l and m o r p h o l o g i c a l c h a r a c t e r i s t i c s r e l a t e d t o the f e e d i n g behaviour of t r o u t and kokanee observed i n t h i s s tudy have e v o l v e d o r a t l e a s t been ma i n t a i n e d most r e c e n t l y w i t h r e s p e c t t o the h a b i t a t s and prey t h a t are found w i t h i n Marion Lake. However, a d a p t a t i o n s do not ev o l v e from s c r a t c h (Lewontin, 1978, Ho r r i d g e 1977), t h u s , the k i n d s of f o r a g i n g s t r a t e g i e s t h a t t r o u t and kokanee have e v o l v e d t o e x p l o i t the food r e s o u r c e s o f Marion Lake w i l l have been l i m i t e d t o ones t h a t a re extremely s i m i l a r t o those of t h e i r immediate a n c e s t o r s . Consequently, the elements making up the f o r a g i n g s t r a t e g i e s o f t r o u t and kokanee i n Marion Lake must be r e l a t e d not o n l y t o s p e c i f i c c h a r a c t e r i s t i c s o f h a b i t a t s and prey found w i t h i n Marion Lake but a l s o t o h a b i t a t and prey c h a r a c t e r i s t i c s t h a t t r o u t and kokanee have responded to i n o t h e r ecosystems over e v o l u t i o n a r y time. A b r i e f summary o f the h a b i t a t - p r e y f e a t u r e s t h a t t r o u t and kokanee respond t o i n Marion Lake compared t o those commonly c o n s i d e r e d t o be important t o the s p e c i e s a c r o s s t h e i r g eographic range w i l l h e l p i d e n t i f y the g e n e r a l c h a r a c t e r i s t i c s o f the h a b i t a t - p r e y complexes t h a t have l i k e l y shaped and mai n t a i n e d the f o r a g i n g s t r a t e g i e s of t r o u t and kokanee. In s p i t e of i t s l i m i t e d p h y s i c a l dimensions ( a r e a 13.3 ha, maximum dept 6 m), Marion Lake and i t s 2-5 m i l e s 209 o f a c c e s s i b l e t r i b u t a r y streams do not r e p r e s e n t a s i n g l e h a b i t a t - p r e y complex to rainbow t r o u t or kokanee. R e s u l t s presented i n t h i s study (Chapter 3) as w e l l as a d d i t i o n a l o b s e r v a t i o n s ( H a l l & H y a t t , 1974; H y a t t , unpublished data) i d e n t i f y rainbow t r o u t from Marion Lake as p r e d a t o r s t h a t most commonly use h a b i t a t s found w i t h i n s m a l l streams o r the l i t t o r a l zone of the l a k e . O u t s i d e of Marion Lake rainbow t r o u t form s e l f s u s t a i n i n g p o p u l a t i o n s i n a v a r i e t y o f l o c a t i o n s i n c l u d i n g : brooks and r i v e r s ( M e t z e l a a r , 1929; Neave, 1944; Hartman & G i l l , 1968; Rawstron, 1972), s m a l l ponds ( B e r s t & McCombie, 1975), l a k e s ( L a r k i n e t a l . 1956; C a r t w r i g h t , 1961; Tody, 1964), e s t u a r i e s and the open ocean ( H a r t , 1973). However b e h a v i o u r a l responses o f t r o u t to h a b i t a t f e a t u r e s such as overhead cover (Newman, 1960; McCrimmon & Kwain, 1966; J e n k i n s , 1969) and p h y s i o l o g i c a l responses to both s a l i n i t y (Hoar, 1976) and temperature (McCrimmon, 1972) suggest t h a t t r o u t are p r i m a r i l y animals o f streams, i n t e r m e d i a t e s i z e d r i v e r s and the l i t t o r a l zone o f l a k e s . B e n t h i c and l i t t o r a l h a b i t a t s o f streams and l a k e s c o n t a i n a d i v e r s e i n v e r t e b r a t e fauna t h a t may p o t e n t i a l l y serve as prey f o r t r o u t . Some of the important groups i n c l u d e nematodes, o l i g o c h a e t e s , l e e c h e s , o s t r a c o d s , amphipods, c r a y f i s h , m o l l u s c s and a v a r i e t y o f a q u a t i c i n s e c t a ( f o r r eviews see Hynes, 1970; W e t z e l , 1975). T h i s taxonomic d i v e r s i t y i s r e f l e c t e d i n the i n v e r t e b r a t e s by a wide range of body s i z e s however the v a r i o u s s i z e s o f prey are not e q u a l l y abundant. 210 For example, w i t h i n the top cm of a " t y p i c a l " square meter of sediment i n the l i t t o r a l zone of Marion Lake, t h e r e i s an average (based upon monthly samples taken over a y e a r , Hoebel, unpublished data) of a p p r o x i m a t e l y 300,000 p o t e n t i a l prey ( r o t i f e r s , copepods, m i t e s , c l a d o c e r a n s and nematodes) between 100 ym and 2 mm i n s i z e . For prey g r e a t e r than 2 mm i n s i z e , the average numbers per square meter are g e n e r a l l y l e s s than 50,000 (based upon monthly samples taken over a y e a r , E f f o r d e t a l . , u n p ublished d a t a ) . S i m i l a r p a t t e r n s o f abundance ve r s u s s i z e e x i s t f o r the i n v e r t e b r a t e s p r e s e n t as d r i f t i n streams. Bishop and Hynes (1969) sampled the i n v e r t e b r a t e d r i f t i n a stream, on a 24 hour b a s i s , once a month f o r a year and found t h a t more than 96% of the prey p r e s e n t were s m a l l e r than 5 mm i n l e n g t h . R e s u l t s from a h o s t o f s t u d i e s make i t c l e a r t h a t the d i v e r s i t y o f p o t e n t i a l prey i n b e n t h i c and l i t t o r a l h a b i t a t s i s r e f l e c t e d i n the d i e t o f rainbow t r o u t . At v a r i o u s t i m e s , l o c a t i o n s o r developmental s t a g e s t r o u t d i e t s may be dominated by zooplankton (Johnson and H a s l e r , 1954; A n t i p a , 1974), t e r r e s t r i a l i n s e c t s ( S w i f t , 1970; N o r t h c o t e , 1973), a q u a t i c i n s e c t l a r v a e (Tody, 1964; Crossman and L a r k i n , 1959; T i p p e t s and Moyle, 1978), l a r g e c r u s t a c e a n s such as c r a y f i s h ( M e t z e l a a r , 1929); m o l l u s c s , l e e c h e s , o r f i s h (Leonard and Leonard, 1946, Crossman and L a r k i n , 1959). Regardless o f t h i s d i e t a r y d i v e r s i t y t h e r e are g e n e r a l t r e n d s i n rainbow t r o u t d i e t a r y h a b i t s . For example as rainbow t r o u t i n c r e a s e i n body s i z e , they u s u a l l y e x h i b i t d i e t a r y s h i f t s from s m a l l - b o d i e d 211 i n v e r t e b r a t e s ( t r o u t l e s s than 10 cm i n s i z e ) , t o l a r g e r c r u s t a c e a n s o r a q u a t i c i n s e c t a ( t r o u t l e s s than 20 cm i n s i z e ) , and f i n a l l y t o a d i e t dominated by l a r g e prey items such as f i s h , s q u i d o r c r a y f i s h ( t r o u t g r e a t e r than 30 cm i n s i z e ) . Thus, i n s p i t e o f the preponderance o f s m a l l - b o d i e d prey items p r e s e n t i n b e n t h i c and l i t t o r a l h a b i t a t s , t r o u t , over most of t h e i r developmental h i s t o r y , d e r i v e the bu l k of t h e i r food from r e l a t i v e l y l a r g e ( g r e a t e r than 4 mm) prey i t e m s . T rout from Marion Lake conform t o t h i s p a t t e r n s i n c e they do not t r a c k the env i r o n m e n t a l abundance of prey v e r y c l o s e l y (Chapter 2,) but r a t h e r d e r i v e the b u l k of t h e i r food from r e l a t i v e l y l a r g e ( g r e a t e r than 4 mm), armoured, b e n t h i c i n v e r t e b r a t e s (eg. s n a i l s , c a d d i s l a r v a e , odonate nymphs and amphipods). The d e t a i l s presented above i n d i c a t e t h a t the h a b i t a t - p r e y complex t h a t t r o u t respond t o i n the Marion Lake system (stream and l a k e s h o r e h a b i t a t s , r e l a t i v e l y l a r g e prey such as b e n t h i c i n v e r t e b r a t e s and t e r r e s t r i a l i n s e c t s ) i s the same as t h a t which has been important to the s p e c i e s i n g e n e r a l . T h e r e f o r e , I suggest t h a t the f o r a g i n g s t r a t e g y o f t r o u t from Marion Lake i s l i k e l y r e p r e s e n t a t i v e of t h a t o f the s p e c i e s i n g e n e r a l and t h a t i t has e v o l v e d p r i m a r i l y i n response t o c e r t a i n s e l e c t i v e p r e s s u r e s c h a r a c t e r i s t i c o f the l i t t o r a l and b e n t h i c r e g i o n s o f both r i v e r s and l a k e s . I t i s my c o n t e n t i o n t h a t t r o u t "see" an environment c o n t a i n i n g low d e n s i t i e s o f l a r g e , e n e r g y - r i c h prey t h a t are r e l a t i v e l y 212 d i s p e r s e d w i t h i n a g i v e n f o r a g i n g p a t c h . I w i l l argue t h a t t h i s type o f environment has s e l e c t i v e l y favoured a d a p t a t i o n s which enable t r o u t t o d i f f e r e n t i a l l y e x p l o i t r e l a t i v e l y l a r g e d i s p e r s e d prey a t the c o s t o f t h e i r a b i l i t i e s t o e f f e c t i v e l y e x p l o i t s m a l l , c o n t a g i o u s l y - d i s t r i b u t e d p r e y . Because prey t h a t t r o u t o b t a i n most o f t h e i r energy from a r e r e l a t i v e l y d i s p e r s e d (see d i s c u s s i o n b e l o w ) , I w i l l r e f e r t o the f o r a g i n g s t r a t e g y o f t r o u t as a D - s t r a t e g y . In sharp c o n t r a s t t o rainbow t r o u t , Marion Lake kokanee are almost e x c l u s i v e l y i n h a b i t a n t s o f o f f s h o r e , water-column and b e n t h i c h a b i t a t s . In s p i t e of the f a c t t h a t l e s s than 35% o f the s u r f a c e a r e a o f Marion Lake l i e s o v e r bottom con t o u r s a t depths g r e a t e r than 2 m, kokanee are found l a r g e l y i n t h i s a r e a (Chapter 3 ) . Ou t s i d e of Marion Lake, kokanee and sockeye salmon (from which kokanee are t a x o n o m i c a l l y i n d i s t i n g u i s h a b l e ) occupy a l i m i t e d range o f h a b i t a t s most commonly a s s o c i a t e d w i t h l a r g e o l i g o t r o p h i c l a k e s and the open ocean. W i t h i n the c o n f i n e s o f a l a k e , kokanee and j u v e n i l e sockeye u s u a l l y occupy the o f f s h o r e , upper and middle l a y e r s o f water ( F o e r s t e r , 1968; Hartman & Burgner, 1972; Goodlad e t a l . , 1974). O c c a s i o n a l l y kokanee may be c l o s e l y a s s o c i a t e d w i t h b e n t h i c h a b i t a t s ( N o r t h c o t e & L o r z , 1966; Chapman e t a l . , 1967) o f o f f s h o r e waters f o r s h o r t p e r i o d s o f ti m e , however t h e r e a re few r e c o r d s o f p o p u l a t i o n s t h a t make prolonged use of the l i t t o r a l zone of l a k e s o r of running waters p r i o r t o m a t u r a t i o n . B e h a v i o u r a l responses o f 0. nerka t o h a b i t a t f e a t u r e s such as cover (Newman, 1960) and p h y s i o l o g i c a l 213 responses to both s a l i n i t y (Hoar, 1976) and temperature ( B r e t t , 1952; F o e r s t e r , 1968; H y a t t , Ms. i n prep.) suggest t h a t o u t s i d e o f the b r e e d i n g season 0. nerka i s p r i m a r i l y adapted t o the upper waters of l a r g e l a k e s and the open ocean. The prey complex of the upper waters o f l a k e s t y p i c a l l y o c c u p i e d by kokanee or j u v e n i l e sockeye salmon i s dominated by j u s t t h r e e major groups. These i n c l u d e r o t i f e r s , c l a d o c e r a n s and copepods ( f o r reviews see Hutchinson, 1967; W e t z e l , 1975). The s i z e - f r e q u e n c y d i s t r i b u t i o n of i n v e r t e b r a t e prey i n the upper waters o f l a k e s i s s e v e r e l y t r u n c a t e d i n comparison t o t h a t of the l i t t o r a l and b e n t h i c h a b i t a t s o c c u p i e d by t r o u t . For example, a l t h o u g h more than 15% of the p o t e n t i a l i n v e r t e b r a t e prey (organisms g r e a t e r than 100 m) i n b e n t h i c h a b i t a t s o f Marion Lake commonly f a l l w i t h i n s i z e s r a n ging from 2-30 mm, i n v e r t e b r a t e prey l a r g e r than 2 mm are v i r t u a l l y absent from the water column. T h i s same p a t t e r n i s r e p e a t e d f o r the m a j o r i t y o f l a k e s . Freshwater i n v e r t e b r a t e s (eg. M y s i s r e l i c t a , L eptodora sp., Chaoborus spp. and p o n t o p o r e i a sp.) which commonly exceed s i z e s of 4 mm and which may occur i n the upper waters of l a k e s are c l e a r l y the e x c e p t i o n , not the r u l e . Indeed such prey are l a r g e l y absent from the l a k e s t h a t kokanee and j u v e n i l e sockeye are found i n . In cases where such prey are p r e s e n t i n the upper waters of l a k e s , i t i s u s u a l l y o n l y f o r a s h o r t i n t e r v a l d u r i n g the d i e l c y c l e when v e r t i c a l m i g r a t i o n from deep water o c c u r s . V a r i o u s s t u d i e s i n d i c a t e t h a t kokanee and j u v e n i l e sockeye salmon use s m a l l ( l e s s than 2 mm) c r u s t a c e a n zooplankton 214 as t h e i r major source o f food . A number of s t u d i e s ( N a r v e r , 1970; Woody, 1972; Horak and Tanner, 1963; Davis and Warren, 1970; McDonald, 1973) i n d i c a t e t h a t 80% o r more of annual food i n t a k e (by volume) may c o n s i s t of l i m n e t i c z o o p l a n k t o n . Some s t u d i e s (Chapman e t a l . , 1967; N o r t h c o t e and L o r z , 1966; Goodlad e t a l . , 1974) r e p o r t t h a t chironomid pupae make up a s u b s t a n t i a l p r o p o r t i o n of the d i e t , however b e n t h i c i n v e r t e b r a t e s and t e r r e s t r i a l i n s e c t s are seldom r e p o r t e d as s i g n i f i c a n t i n the d i e t of t h i s s p e c i e s . The pronounced d i e t a r y s h i f t s t h a t o ccur i n rainbow t r o u t as they i n c r e a s e i n body s i z e are l a r g e l y absent i n kokanee. Kokanee ( o r sockeye salmon) as l a r g e as 30 cm may s t i l l r e l y almost e n t i r e l y on s m a l l ( l e s s than 3 mm) water column prey f o r the b u l k of t h e i r f o o d . A number of authors (Hanamura, 1966; L e B r a s s e u r , 1966) have i n d i c a t e d t h a t even ocean d w e l l i n g sockeye a t s i z e s between 30 and 50 cm consume mos t l y s m a l l amphipods and copepods from the water column. At s i z e s g r e a t e r than 30 cm, l a k e - d w e l l i n g kokanee may r e l y on r e l a t i v e l y l a r g e water colmn prey such as Mysis sp. f o r food ( s i z e s g r e a t e r than 3 mm), however p i s c i v o r y i s r a r e l y observed even i n those f r e s h w a t e r p o p u l a t i o n s c o n t a i n i n g i n d i v i d u a l kokanee which a t t a i n s i z e s g r e a t e r than 40 cm (N o r t h c o t e , 1972; I r i z a r r y , 1975) . In the open ocean, sockeye l a r g e r than 50 cm i n l e n g t h consume predominantly e u p h a s i i d s , s q u i d and s m a l l f i s h (LeBrasseur,1966 ). Thus, over most o f t h e i r , developmental h i s t o r y , kokanee (and sockeye salmon) are p e l a g i c p r e d a t o r s t h a t g a t h e r t h e i r food from r e l a t i v e l y s m a l l - b o d i e d prey found i n the upper waters o f l a k e s o r the 215 open ocean. Kokanee from Marion Lake p a r t i a l l y conform t o t h i s p a t t e r n s i n c e they d e r i v e much of t h e i r food from s m a l l , water-column prey (eg. chironomid pupae, zooplankton) however the i n c l u s i o n of q u a n t i t i e s of b e n t h i c i n v e r t e b r a t e s i n t h e i r d i e t r e p r e s e n t s a s i g n i f i c a n t d e p a r t u r e from the g e n e r a l p a t t e r n . The h a b i t a t - p r e y complex t h a t kokanee are faced w i t h i n Marion Lake e x h i b i t s a number of f e a t u r e s t h a t d i f f e r from those n o r m a l l y encountered by the s p e c i e s . F i r s t l e s s than 30% o f the l a k e area can be c l a s s e d as l i m n e t i c i n c h a r a c t e r ( i e . open water l a c k i n g cover such as l o g s , brush o r weed beds). Next the zooplankton community i n Marion Lake i s ephemeral and i s w e l l developed o n l y d u r i n g a 2-3 month p e r i o d i n l a t e summer ( E f f o r d , 1970). F i n a l l y Marion Lake e x h i b i t s a s e a s o n a l temperature regime which " f o r c e s " kokanee t o abandon s u r f a c e waters ( H y a t t Ms. i n prep.) and t o fo r a g e i n o f f s h o r e (depths g r e a t e r than 2 m), b e n t h i c - h a b i t a t s i n l a t e summer (Chapter 3 . ) . In s p i t e of these d i f f e r e n c e s , I b e l i e v e t h a t the responses o f kokanee to the h a b i t a t - p r e y complex i n Marion Lake suggest a f o r a g i n g s t r a t e g y which c l e a r l y bears the i m p r i n t of e v o l u t i o n i n response t o the h a b i t a t - p r e y complex t h a t c h a r a c t e r i z e s the o f f s h o r e and upper waters of l a r g e l a k e s and i n some cases the open ocean. I t i s my c o n t e n t i o n t h a t kokanee (and i m p l i c i t l y sockeye salmon) p e r c e i v e an environment c o n t a i n i n g r e l a t i v e l y h i g h d e n s i t i e s of s m a l l , low-energy, prey t h a t are c o n t a g i o u s l y d i s t r i b u t e d w i t h i n a g i v e n f o r a g i n g p a t c h . I w i l l argue t h a t t h i s type of environment has favoured a d a p t a t i o n s which enable 216 kokanee to e x p l o i t r e l a t i v e l y s m a l l , c o n t a g i o u s l y - d i s t r i b u t e d prey a t the expense of t h e i r a b i l i t i e s t o e x p l o i t l a r g e , d i s p e r s e d p r e y . Because prey t h a t kokanee o b t a i n most of t h e i r energy from are c o n t a g i o u s l y d i s t r i b u t e d (see d i s c u s s i o n b e l o w ) , I w i l l r e f e r t o the f o r a g i n g s t r a t e g y o f kokanee as a C - s t r a t e g y . I t i s important to note a t the o u t s e t o f t h i s d i s c u s s i o n _ t h a t the concepts of C and D s t r a t e g i e s are not a b s o l u t e , but are meaningful o n l y by comparison. A g i v e n organism i s more o r l e s s o f a D - s t r a t e g i s t o n l y by comparison w i t h another organism. Thus I w i l l argue t h a t rainbow t r o u t as a s p e c i e s possess a d a p t a t i o n s which i d e n t i f y them as D - s t r a t e g i s t s r e l a t i v e t o the C - s t r a t e g y o f kokanee. The u t i l i t y as w e l l as the weakness i n a p p l y i n g t h i s p o i n t o f view e x a c t l y p a r a l l e l s t h a t i n d e s i g n a t i n g organisms as e i t h e r r - s t r a t e g i s t s o r K-s t r a t e g i s t s w i t h r e s p e c t to t h e i r r e p r o d u c t i v e s t r a t e g i e s (reviewed i n S t e a r n s , 1976). By d e v e l o p i n g the concepts of . C - s e l e c t i o n and D - s e l e c t i o n w i t h r e s p e c t to the f o r a g i n g s t r a t e g i e s of kokanee and t r o u t , I hope t o demonstrate t h a t i t i s p o s s i b l e to r e c o g n i z e complementary a s s o c i a t i o n s of b i o l o g i c a l t r a i t s which c o n s t i t u t e f o r a g i n g s t r a t e g i e s addressed t o p a r t i c u l a r s e t s o f e n v i r o n m e n t a l c o n d i t i o n s . I w i l l now d e s c r i b e s p e c i f i c f e a t u r e s of each h a b i t a t - p r e y complex i n g r e a t e r d e t a i l and then attempt t o show t h a t m o r p h o l o g i c a l o r b e h a v i o u r a l c h a r a c t e r i s t i c s a s s o c i a t e d w i t h f o r a g i n g by t r o u t or kokanee are a d a p t a t i o n s t o these f e a t u r e s . 217 THE ROLE OF PREY. SIZE,' RELATIVE ABUNDANCE AND PHYSICAL FEATURES OF THE ENVIRONMENT IN PROMOTING C - s e l e c t e d o r D - s e l e c t e d FORAGING' STRATEGIES IN TROUT OR KOKANEE In the upper waters o f l a k e s c o n t a i n i n g f i s h , l a r g e , i n v e r t e b r a t e prey (body s i z e s g r e a t e r than 4mm) are s c a r c e compared to the numbers found i n b e n t h i c and l i t t o r a l h a b i t a t s , however s m a l l prey (body s i z e s between 100 ym and 2mm) a r e abundant. Kokanee as w e l l as the m a j o r i t y o f o t h e r f r e s h w a t e r f i s h e s i n h a b i t i n g the upper and o f f s h o r e waters o f temperate-zone l a k e s o b t a i n the b u l k of t h e i r d i e t s from r e l a t i v e l y s m a l l prey items ( l e s s than 2 mm body s i z e ) . Due t o the absence of any d e t a i l e d a n a l y s i s o f the b i o l o g i c a l and p h y s i c a l c o n d i t i o n s t h a t c o n t r o l the e v o l u t i o n o f s u c c e s s f u l f o r a g i n g s t r a t e g i e s , I can o n l y suggest t h a t t h i s s i t u a t i o n o c c u r s because t h e r e are v e r y few p o t e n t i a l combinations o f a d a p t a t i o n s which p r o v i d e a s e l e c t i v e l y advantageous b a l a n c e o f energy r e t u r n e d t o energy expended f o r a p r e d a t o r s e a r c h i n g open water h a b i t a t s f o r l a r g e - b o d i e d but extremely r a r e p r e y . Thus, i t appears t h a t the extreme s c a r c i t y o f l a r g e - b o d i e d prey c o n s i s t -e n t l y f a v o u r s the e v o l u t i o n o f C - s e l e c t e d s t r a t e g i e s f o r p r e d a t o r s such as kokanee t h a t must o b t a i n t h e i r food from the h a b i t a t -prey complex a s s o c i a t e d w i t h the open waters of temperate-zone l a k e s . In b e n t h i c or l i t t o r a l h a b i t a t s o f l a k e s o r streams, l a r g e , i n v e r t e b r a t e prey (body s i z e s g r e a t e r than 4 mm) are abundant compared t o the numbers found i n l i m n e t i c h a b i t a t s . Although l a r g e prey are s t i l l r e l a t i v e l y s c a r c e compared t o the numbers of s m a l l prey (body s i z e s l e s s than 2 mm), many s p e c i e s 218 o f f r e s h w a t e r f i s h t h a t feed upon b e n t h i c prey o b t a i n the b u l k o f t h e i r d i e t s , over most of t h e i r developmental h i s t o r i e s , from r e l a t i v e l y l a r g e prey i t e m s . Thus, the g r e a t e r abundance o f l a r g e - b o d i e d prey i n b e n t h i c h a b i t a t s has a p p a r e n t l y expanded e v o l u t i o n a r y " o p p o r t u n i t i e s " t o i n c l u d e D - s e l e c t e d f o r a g i n g s t r a t e g i e s ( i n which p r e d a t o r s c o n c e n t r a t e on e x p l o i t i n g r e l a t i v e l y l a r g e , d i s p e r s e d prey) i n a d d i t i o n t o C - s e l e c t e d f o r a g i n g s t r a t e g i e s . Because rainbow t r o u t are among the s p e c i e s t h a t i n c l u d e a d i s p r o p o r t i o n a t e l y h i g h number of l a r g e prey i n t h e i r d i e t s , I c o n s i d e r them t o be D - s t r a t e g i s t s . There may be any number of reasons f o r why c o n d i t i o n s i n the stream or l a k e h a b i t a t s occupied by t r o u t have t i p p e d the balance of n a t u r a l s e l e c t i o n i n favour of the e v o l u t i o n o f a D - s t r a t e g y . The one suggested here r e l i e s on the o b s e r v a t i o n t h a t t r o u t always spend s i g n i f i c a n t p r o p o r t i o n s o r ; in.many p o p u l a t i o n s , t h e i r e n t i r e developmental h i s t o r y w i t h i n running water h a b i t a t s . These h a b i t a t s o f t e n l a c k the c r u s t a c e a n z o oplankton t h a t c h a r a c t e r i z e s the small-but-abundant end of the prey r e s o u r c e d i s t r i b u t i o n i n l a k e s . Although the presence of o t h e r s m a l l prey types s t i l l guarantees t h a t the abundance of prey of d i f f e r e n t s i z e s i n r i v e r s i s l o g n o r m a l l y d i s t r i b u t e d , the m i c r o f a u n a of r i v e r s and streams i s l a r g e l y concealed beneath b e n t h i c s u b s t r a t e s (Hynes, 1970) and thus i s much l e s s exposed than the m i c r o f a u n a of s t a n d i n g w a ters. T h i s l i k e l y reduces the p r o b a b i l i t y t h a t s e l e c t i o n w i l l f a v o u r the e v o l u t i o n o f a d a p t a t i o n s f o r the e x p l o i t a t i o n o f r e l a t i v e l y s m a l l prey by v i s u a l p r e d a t o r s such as t r o u t . Of course 219 s i g n i f i c a n t q u a n t i t i e s o f t h i s m i c r o f a u n a must s t i l l e n t e r the d r i f t o f streams and t r o u t do appear t o f r e q u e n t l y o b t a i n the m a j o r i t y of t h e i r food s u p p l y i n streams from d r i f t i n g i n v e r t e b r a t e s ( J e n k i n s e t a l . , 1970), however review o f a number of s t u d i e s ( r e f e r e n c e s i n Bishop and Hynes, 1969) suggests t h a t s m a l l - b o d i e d prey ( l e s s than 2 mm) a r e r e l a t i v e l y i n a c c e s s i b l e to t r o u t even when suspended i n the water column of a stream or r i v e r as d r i f t . The s m a l l s i z e of i n d i v i d u a l p r e y , the d i f f i c u l t y o f d e t e c t i n g them a g a i n s t a background of s i m i l a r s i z e s of d r i f t i n g d e t r i t u s , and the s h o r t time a v a i l a b l e f o r response b e f o r e c u r r e n t s c a r r y prey out o f a t t a c k range l i k e l y s e l e c t a g a i n s t s p e c i a l i z a t i o n on s m a l l prey by p r e d a t o r s such as t r o u t . The a l t e r n a t i v e t h a t n a t u r a l s e l e c t i o n w i l l f a v o u r i s the r e t e n t i o n of a d a p t a t i o n s t o e x p l o i t the l a r g e , r e l a t i v e l y r a r e prey items which c h a r a c t e r i z e one end of the prey r e s o u r c e d i s t r i b u t i o n i n f l o w i n g water h a b i t a t s . Although the h a b i t a t -prey complex of l a k e s i s somewhat d i f f e r e n t from t h a t of f l o w i n g w a t e r s , I suggest t h a t t r o u t r e t a i n a D - s e l e c t e d f o r a g i n g s t r a t e g y here because a d a p t a t i o n s t h a t favour the d i f f e r e n t i a l a c q u i s i t i o n of l a r g e d i s p e r s e d prey w i t h i n l i t t o r a l and b e n t h i c h a b i t a t s of l a k e s are i n c o m p a t i b l e w i t h those t h a t favour e x p l o i t a t i o n of s m a l l , c o n t a g i o u s l y - d i s t r i b u t e d prey (see d i s c u s s i o n below). 220 THE ROLE OF PREY DISTRIBUTION IN SHAPING SEARCH AND ATTACK COMPONENTS OF TROUT AND KOKANEE FORAGING STRATEGIES In the upper waters o f l a k e s , p o p u l a t i o n d e n s i t i e s of z ooplankton commonly reach 200-300 animals per l i t e r and o c c a s i o n a l l y exceed 5000 per l i t e r . The d e n s i t y o f zooplankton i n l a k e s i n h a b i t e d by O. nerka i s u s u a l l y r e p o r t e d as f a l l i n g between 50-100 animals per l i t e r ( r e f e r e n c e s i n Fo.erster, 1968). U n l i k e b e n t h i c p r e y , the m a j o r i t y o f which are u s u a l l y c o n c e a l e d , zooplankton are v i r t u a l l y always exposed w i t h i n a f o r a g i n g p a t c h . Thus i f the average d e n s i t y of z o o plankton i n a g i v e n f o r a g i n g p a t c h were 50 per l i t e r , then the d i s t a n c e between s e q u e n t i a l encounters by kokanee w i t h prey would be on the o r d e r of a few cm ( i e . 5 cm. o r l e s s ) . I t i s important to emphasize t h a t t h i s v a l u e l i k e l y r e p r e s e n t s the maximum w i t h i n - p a t c h d i s t a n c e between c o n s e c u t i v e prey s i n c e f i s h w i l l o f t e n encounter h i g h e r d e n s i t i e s o f prey than l i m n e t i c sampling gear i n d i c a t e s . T h i s i s because sampling gear, moving l i n e a r l y through many c u b i c meters of water, i s not r e s p o n s i v e t o f i n e s c a l e v a r i a t i o n s i n prey d e n s i t y but f i s h a r e . Thus a t any i n s t a n t , i n l i m n e t i c h a b i t a t s , m u l t i p l e prey are l i k e l y t o be w i t h i n the r e a c t i v e d i s t a n c e o f a kokanee (Chapter 4) s e a r c h i n g w i t h i n a p a t c h ( p h y s i c a l s c a l e cm t o m) c o n t a i n i n g p r e y . On a per l i t e r b a s i s the d e n s i t i e s o f i n v e r t e b r a t e prey found w i t h i n the top cm o f b e n t h i c s u b s t r a t e s o f e i t h e r 221 streams o r the l i t t o r a l zone of l a k e s may commonly exceed the d e n s i t i e s of i n v e r t e b r a t e s p r e s e n t i n the upper waters of l a k e s . For example, w i t h i n Marion Lake t h e r e are on average 5 3.0-4.0x10 p o t e n t i a l prey (organisms between 100 ym and -2 15 mm i n s i z e ) m and more than 80% o f these are found w i t h i n the top cm of sediment ( E f f o r d e t a l . , u n p u blished d a t a ) . T h i s i s e q u i v a l e n t t o 2.4-3.2x10 prey per l i t e r . However, u n l i k e the s i t u a t i o n f o r l i m n e t i c p r e y , d a t a on the d e n s i t y of b e n t h i c prey are not u s e f u l f o r even rough c a l c u l a t i o n s of the d i s t a n c e between s e q u e n t i a l encounters w i t h b e n t h i c prey by p r e d a t o r s such as t r o u t . T h i s i s because the m a j o r i t y of b e n t h i c prey are n o r m a l l y concealed and thus are not s u b j e c t t o d e t e c t i o n by v i s u a l p r e d a t o r s such as t r o u t . Because the prey t h a t a r e s u b j e c t to d e t e c t i o n are those suspended i n the d r i f t of streams o r on the s u r f a c e of b e n t h i c s u b s t r a t e s i n e i t h e r streams or l a k e s , i t i s the d e n s i t i t e s o f these prey t h a t w i l l g e n e r a l l y d e f i n e the d i s t a n c e between p o t e n t i a l encounters w i t h prey by t r o u t . Bishop and Hynes (1969) r e p o r t e d t h a t a l t h o u g h d e n s i t i e s of b e n t h i c prey i n the Speed R i v e r ( s o u t h e r n O n t a r i o , 5 Canada) were v e r y h i g h (average g r e a t e r than 3.4x10 organisms m between J u l y and D e c ) , o n l y a s m a l l p r o p o r t i o n of the benthos (.0002 - .004%) was p r e s e n t i n the d r i f t a t any one time. T h e r e f o r e , d e n s i t i e s o f d r i f t i n g i n v e r t e b r a t e s seldom exceeded an average (taken over a 24 hour i n t e r v a l once per month) o f one prey per 20 l i t e r s . Furthermore because o n l y 3.5% o f a l l prey were l a r g e r than 5 mm i n s i z e , the d e n s i t y 222 o f l a r g e r prey was g e n e r a l l y l e s s than 1 per 500 l i t e r s . S i n ce the Speed R i v e r f l o w s through a g r i c u l t u r a l l a n d and i s r e l a t i v e l y p r o d u c t i v e compared t o streams i n h a b i t e d by t r o u t i n the P a c i f i c Northwest the above v a l u e s are p r o b a b l y h i g h e r than the average d e n s i t i e s of i n v e r t e b r a t e d r i f t encountered by t r o u t throughout much of t h e i r n a t i v e range. T r a n s l a t e d i n t o a s p a t i a l s c a l e , these v a l u e s suggest t h a t on average the d i s t a n c e between p o t e n t i a l encounters w i t h prey by t r o u t f e e d i n g on stream d r i f t would be on the o r d e r of 14-20 cm. In a d d i t i o n i f o n l y l a r g e prey ( s i z e s g r e a t e r than 5 mm) are c o n s i d e r e d the d i s t a n c e between p o t e n t i a l encounters would o f t e n be a meter or more. Given the o b s e r v a t i o n s above, i t i s apparent t h a t the d i s t r i b u t i o n , abundance and s i z e s o f prey i n l i m n e t i c or f l o w i n g water h a b i t a t s have p r o v i d e d r a t h e r d i f f e r e n t s p a t i a l s c a l e s f o r the e v o l u t i o n o f search b e h a v i o u r s by t r o u t o r kokanee. The r e l a t i v e l y c o n t a g i o u s d i s t r i b u t i o n o f prey w i t h i n l i m n e t i c h a b i t a t s , where t h e r e may be thousands of prey per c u b i c meter, should have"favoured the e v o l u t i o n and c o n t i n u e d maintenance of area i n t e n s i v e search t e c h n i q u e s by kokanee. By c o n t r a s t the r e l a t i v e l y d i s p e r s e d d i s t r i b u t i o n o f prey w i t h i n f l o w i n g water h a b i t a t s , where t h e r e w i l l g e n e r a l l y be o n l y a few prey m , should have favoured the e v o l u t i o n o f area e x t e n s i v e s e a r c h t e c h n i q u e s by t r o u t . The c h a r a c t e r i s t i c s t h a t t r o u t and kokanee e x h i b i t w h i l e s e a r c h i n g f o r prey i n Marion Lake are h i g h l y c o m p a t i b l e w i t h t h i s p o i n t o f view. For example, t r o u t and kokanee o b t a i n 223 s u b s t a n t i a l q u a n t i t i e s o f prey from the l a k e s u r f a c e , however, when f o r a g i n g a t the l a k e s u r f a c e t r o u t m a i n t a i n s e a r c h p o s i t i o n s t h a t are 45-100 cm below the s u r f a c e w h i l e kokanee c o n s i s t e n t l y search from p o s i t i o n s t h a t a r e o n l y 5-30 cm below the s u r f a c e (Chapter 4, F i g . 1 7 ) . The v e r t i c a l d i s t a n c e o f the p r e d a t o r s below the l a k e s u r f a c e a f f e c t s the diameter of the c i r c u l a r a r ea w i t h i n which, on t h e o r e t i c a l grounds, prey may be d e t e c t e d . T h i s i s because beyond an angle o f 97° 12' complete i n t e r n a l r e f l e c t i o n of l i g h t o c c u r s and the p r e d a t o r s w i l l see o n l y t h i s r e f l e c t i o n ( F i g . 3 5 ) . For p r e d a t o r s f o r a g i n g a t the l a k e s u r f a c e i n ve r y s h a l l o w water, the r e f l e c t i o n beyond 97° 12' would c o n s i s t of a d i s t o r t e d image o f o b j e c t s on the l a k e bottom, w h i l e i n deeper water the r e f l e c t i o n would i n c l u d e o n l y o b j e c t s found i n the water column. The s i g n i f i c a n c e of t h i s i s t h a t a t r o u t , m a i n t a i n i n g a search p o s i t i o n 75 cm below the s u r f a c e , searches a "window" w i t h a diameter of 170 cm, w h i l e a kokanee a t o n l y 25 cm below the s u r f a c e w i l l e x p e r i e n c e a se a r c h window o n l y 1/3 t h i s diameter ( F i g . 3 5 ) . Thus, by m a i n t a i n i n g p o s i t i o n s t h a t are f u r t h e r below the s u r f a c e than those o f kokanee, t r o u t s a c r i f i c e the a b i l i t y t o d e t e c t s m a l l prey on the s u r f a c e but they g a i n the c a p a c i t y t o s e a r c h a much l a r g e r area of l a k e s u r f a c e per u n i t time f o r r e l a t i v e l y l a r g e p r e y . T h i s i s e s p e c i a l l y apparent when i t i s r e c a l l e d t h a t t r o u t m a i n t a i n swimming v e l o c i t i e s t h a t a re 55% g r e a t e r than those o f kokanee when s e a r c h i n g i n the water column (Table 10). When combined, the e f f e c t o f se a r c h p o s i t i o n and v e l o c i t y 224 FIGURE 35. The e f f e c t o f a p r e d a t o r ' s v e r t i c a l d i s t a n c e from the l a k e s u r f a c e on the diameter of the c i r c u l a r a r ea w i t h i n which, on t h e o r e t i c a l grounds, s u r f a c e prey may be d e t e c t e d . Beyond 97° 12' complete i n t e r n a l r e f l e c t i o n o f l i g h t o c c u r s and the p r e d a t o r s w i l l see o n l y t h i s r e f l e c t i o n . Search f i e l d d i ameters o f 170 cm and 57 cm are r e a s o n a b l y r e p r e s e n t a t i v e v a l u e s g i v e n the s e a r c h p o s i t i o n s t h a t t r o u t and kokanee u s u a l l y assume when f o r a g i n g a t the l a k e s u r f a c e (see F i g . 1 7 ) . The s o l i d spheres i n d i c a t e the r e l a t i v e s i z e s o f the s m a l l e s t prey t h a t t r o u t o r kokanee c o u l d d e t e c t a t p a r t i c u l a r l o c a t i o n s w i t h i n the i n s t a n t a n e o u s f i e l d of s e a r c h , v a l u e s of the minimum d e t e c t a b l e t a r g e t s i z e f o r s t a t i o n a r y prey were e s t i m a t e d by u s i n g Ware's r e s u l t s .on the r e a c t i v e d i s t a n c e o f t r o u t t o v e r y h i g h c o n t r a s t prey (see F i g . 3, P. 104 o f Ware, 1971). A c c o r d i n g t o these r e s u l t s , a t r o u t p o s i t i o n e d a t 75 cm below the l a k e s u r f a c e would r e q u i r e a prey o f a t l e a s t 45 square mm i n area to e l i c i t a response a t the edge of the i n s t a n t a n e o u s f i e l d of s e a r c h , w h i l e a kokanee a t o n l y 25 cm from the s u r f a c e would respond t o prey o f l e s s than 3 square mm a t the edge of the i n s t a n t a n e o u s f i e l d of s e a r c h . K O K A N E E - A R E A I N T E N S I V E S E A R C H VELOCITY = 18.2 c m / s ^ , 45 mm 2 VELOCITY = 27.7 c m / s SEARCH FIELDS DRAWN TO S C A L E 225 d i f f e r e n c e s a l l o w t r o u t to s e a r c h a p p r o x i m a t e l y 15 times the area t h a t kokanee do i n the same i n t e r v a l . T h e r e f o r e a t the l a k e s u r f a c e the D- s t r a t e g y of t r o u t i n v o l v e s area e x t e n s i v e search f o r l a r g e prey w h i l e the C - s t r a t e g y of kokanee i n v o l v e s area i n t e n s i v e s e arch f o r s m a l l p r e y . U n l i k e the s i t u a t i o n a t the l a k e s u r f a c e , the s e a r c h f i e l d a t the l a k e bottom i s t h e o r e t i c a l l y g r e a t e s t f o r a p r e d a t o r p o s i t i o n e d r i g h t a t o r j u s t above the s u b s t r a t e s u r f a c e ( F i g . 36 a and b ) , however t r o u t m a i n t a i n search p o s i t i o n s t h a t are on average 30 cm away from the l a k e bottom when they forage f o r b e n t h i c p r e y , w h i l e kokanee g e n e r a l l y m a i n t a i n search p o s i t i o n s t h a t a r e o n l y 5 cm away from the bottom (Chapter 4, F i g . 1 6 ) . Because t r o u t swim a p p r o x i m a t e l y 30% f a s t e r than kokanee w h i l e s e a r c h i n g f o r b e n t h i c prey (Table 1 0 ) , t r o u t w i l l s t i l l scan a s l i g h t l y l a r g e r area o f l a k e bottom p e r u n i t time than kokanee however the d i s t i n c t i o n between area e x t e n s i v e search and area i n t e n s i v e s e a r c h i s h a r d l y s i g n i f i c a n t . The near bottom s e a r c h p o s i t i o n s o f kokanee are e s s e n t i a l i f they are t o s u c c e s s f u l l y d e t e c t prey as s m a l l 2 ( i e . < 1 mm ) as those n o r m a l l y e x p l o i t e d i n l i m n e t i c h a b i t a t s , but why should t r o u t i n Marion Lake m a i n t a i n search p o s i t i o n s such t h a t they s a c r i f i c e the a b i l i t y t o d e t e c t s m a l l prey even though they do not g a i n the advantage of an e n l a r g e d f i e l d - o f - s e a r c h f o r l a r g e prey as was the case f o r the same behaviour a t the l a k e s u r f a c e ? One key may l i e i n the assumption t h a t the b e n t h i c s e a r c h behaviour of t r o u t has e v o l v e d i n 226 response to r e l a t i v e l y f l a t s u b s t r a t e s . In l a k e s such as Marion t h i s i s a reas o n a b l e assumption, but i n the f l o w i n g water h a b i t a t s commonly oc c u p i e d by t r o u t the bottom i s h i g h l y i r r e g u l a r due to the presence of r o c k - c o b b l e s u b s t r a t e s . Bottom i r r e g u l a r i t i e s i n f l o w i n g water h a b i t a t s w i l l b l o c k the search f i e l d of a p r e d a t o r t h a t i s c l o s e t o the bottom more than f o r one t h a t searches from a p o s i t i o n f u r t h e r o f f the bottom (compare F i g . 36 c and d ) . An a d d i t i o n a l advantage f o r a p r e d a t o r assuming a h i g h e r p o s i t i o n i n the water column i s t h a t the s e a r c h f i e l d may then take i n both s u r f a c e and b e n t h i c s u b s t r a t e s , g i v e n an a p p r o p r i a t e water depth ( F i g . 36 d ) . T h e r e f o r e i n f l o w i n g water h a b i t a t s which have pl a y e d an important r o l e i n shaping the D- s t r a t e g y o f t r o u t , the maintenance of search p o s i t i o n s some d i s t a n c e away from the bottom c o u l d serve as an a d a p t a t i o n t o fa v o u r a rea e x t e n s i v e s e a r c h . Thus, t r o u t i n Marion Lake may respond to prey s e a r c h on b e n t h i c s u b s t r a t e s a c c o r d i n g t o the same s e t o f " r u l e s " d i c t a t e d by s e l e c t i o n i n f l o w i n g water h a b i t a t s . C e r t a i n s p e c i a l i z e d a s p e c t s o f the s p a t i a l d i s t r i b u t i o n o f prey i n f l o w i n g water o r l i t t o r a l zone h a b i t a t s as compared t o l i m n e t i c ones may a l s o have i n f l u e n c e d the e v o l u t i o n of a t t a c k procedures used by t r o u t and kokanee. For example t r o u t f r e q u e n t l y l e a p d i s t a n c e s of one t o two body l e n g t h s above the l a k e s u r f a c e as p a r t of an a t t a c k procedure t o ca p t u r e a e r i a l prey w h i l e kokanee do not appear t o l e a p a t a l l f o r prey (Chapter 5 ) . The a b i l i t y o f t r o u t to l e a p f o r a e r i a l prey i s h i g h l y advantageous i n streams o r the i n s h o r e 2 2 7 h a b i t a t s of l a k e s where t h e r e w i l l u s u a l l y be many f l y i n g i n s e c t s j u s t above the water's s u r f a c e throughout the summer. T h i s i s e s p e c i a l l y apparent i n Marion Lake which i s s m a l l , s h a l l o w and surrounded by p r o d u c t i v e f o r e s t l a n d s . Both t e r r e s t r i a l and a q u a t i c i n s e c t s are commonly i n f l i g h t j u s t above the l a k e ' s s u r f a c e throughout much of the summer. The f a i l u r e of kokanee to l e a p f o r a e r i a l prey does not appear t o be v e r y a d a p t i v e w i t h i n the c o n t e x t of the Marion Lake ecosystem. Although kokanee occupy o f f s h o r e h a b i t a t s i n Marion Lake, the l a k e i s s m a l l enough t h a t l a r g e numbers of p o t e n t i a l prey are i n f l i g h t j u s t above the s u r f a c e even i n these l o c a t i o n s . The absence of w e l l developed c a p t u r e procedures f o r a e r i a l prey i s not s u r p r i s i n g g i v e n t h a t the u s u a l h a b i t a t of kokanee i s e i t h e r the upper waters o f o f f s h o r e areas i n l a r g e l a k e s o r , as sockeye salmon, the upper waters o f the N o r t h e a s t P a c i f i c . In these l o c a t i o n s a e r i a l p rey are u n l i k e l y t o have c o n s t i t u t e d an important enough prey r e s o u r c e t o favour the e v o l u t i o n of such b e h a v i o u r . 228 FIGURE 36. The e f f e c t of a p r e d a t o r ' s s e a r c h p o s i t i o n on the s i z e o f s e l e c t e d p o r t i o n s o f the i n s t a n t a n e o u s f i e l d o f s e a r c h . (a) P r e d a t o r m a i n t a i n i n g search p o s i t i o n 5 cm o f f o f l a k e bottom. Diameter o f i n s t a n t a n e o u s f i e l d o f search equals a p p r o x i m a t e l y 98 cm i f the r e a c t i v e d i s t a n c e to prey i s assumed to be 50 cm. (b) P r e d a t o r m a i n t a i n i n g search p o s i t i o n 30 cm o f f of l a k e bottom. Diameter of i n s t a n t a n e o u s f i e l d o f s e a r ch i s reduced t o a p p r o x i m a t e l y 79 cm i f the r e a c t i v e d i s t a n c e t o prey i s assumed to be 50 cm. (c) P r e d a t o r m a i n t a i n i n g s e a r c h p o s i t i o n o f a p p r o x i m a t e l y 5 cm away from top of i r r e g u l a r , r o c k - c o b b l e s u b s t r a t e w i t h i n a stream r i f f l e . Diameter of i n s t a n t a n e o u s f i e l d of search i s o n l y a p p r o x i m a t e l y 40 cm a t the stream bottom and w i l l not i n c l u d e the water s u r f a c e i n r i f f l e s deeper than 45 cm. Again the assumption i s t h a t the r e a c t i v e d i s t a n c e t o prey i s 50 cm. (d) P r e d a t o r m a i n t a i n i n g s e a r c h p o s i t i o n a p p r o x i m a t e l y 30 cm away from top of i r r e g u l a r , r o c k - c o b b l e s u b s t r a t e w i t h i n a stream r i f f l e . Diameter of i n s t a n t a n e o u s f i e l d of search i s a p p r o x i m a t e l y 76 cm a t the bottom and 64 cm a t the stream s u r f a c e . Assumptions are t h a t the r i f f l e i s a p p r o x i m a t e l y 65-75 cm deep and t h a t the r e a c t i v e d i s t a n c e t o prey i s 50 cm. 229 THE ROLE OF TEMPORAL PATTERNS OF PREY RENEWAL IN SHAPING SEARCH COMPONENTS OF TROUT AND KOKANEE FORAGING STRATEGIES In a d d i t i o n t o the s p a t i a l d i s t r i b u t i o n o f p r e y , t h e i r temporal d i s t r i b u t i o n w i t h i n patches of water column or b e n t h i c h a b i t a t s of l a k e s o r streams w i l l have acted t o shape and m a i n t a i n some elements of the f o r a g i n g s t r a t e g i e s of t r o u t and kokanee. W i t h i n streams o r r i v e r s , f l o w i n g water p r o v i d e s a mechanism f o r the continuous renewal of prey a t any f i x e d l o c a t i o n . The r a t e o f renewal w i l l v a r y between l o c a t i o n s because of stream c h a r a c t e r i s t i c s (eg. water v e l o c i t y , stream-bed morphology) as w e l l as between times because o f prey behaviour (see Waters 1972 f o r review) however, the time s c a l e f o r renewal of d r i f t i n g prey a f t e r removal by t r o u t w i t h i n r i f f l e h a b i t a t s w i l l g e n e r a l l y be on the o r d e r of a few seconds t o a few minutes. As i n streams, the w i t h i n - p a t c h renewal r a t e s of the prey o f t r o u t i n the l i t t o r a l zone of l a k e s may be v e r y f a s t (seconds to minutes) but h i g h l y v a r i a b l e i n both space and time. T h i s i s because the sources f o r prey renewal a t the l a k e s u r f a c e ( t h e v a s t " r e s e r v o i r " o f t e r r e s t r i a l and a i r b o r n e i n s e c t s ) and l a k e bottom (concealed b e n t h i c i n v e r t e b r a t e s ) are v e r y c l o s e ( d i s t a n c e s o f mm t o a few m) t o the l o c a t i o n s from which t r o u t o b t a i n t h e i r p r e y . Thus i f t r o u t d e p l e t e the prey exposed on the sediment s u r f a c e , prey renewal can p o t e n t i a l l y take p l a c e almost i n s t a n t a n e o u s l y i f prey concealed a few mm beneath the sediment s u r f a c e move i n t o exposed p o s i t i o n s . 230 U n l i k e the b e n t h i c i n v e r t e b r a t e s o r t e r r e s t r i a l i n s e c t s t h a t t r o u t commonly use as f o o d , the z o o p l a n k t o n t h a t kokanee commonly e x p l o i t i n the upper waters of l a k e s are v i r t u a l l y always exposed w i t h i n a f o r a g i n g p a t c h . Thus removal of prey from a f o r a g i n g p a t c h by kokanee can be r e l a t i v e l y complete and prey renewal w i l l have to depend upon both mass movements of s u r f a c e waters and d i e l p a t t e r n s o f v e r t i c a l m i g r a t i o n by prey t h a t are i n deep water "compart-ments" some d i s t a n c e away (meters to hundreds of m e t e r s ) . Due to the s p a t i a l and temporal s c a l e of these e v e n t s , prey renewal w i t h i n a f o r a g i n g p a t c h i n open waters i s l i k e l y much slower (minutes to hours) than i n the b e n t h i c and i n s h o r e - s u r f a c e h a b i t a t s of l a k e s , o r the s u r f a c e and water-column h a b i t a t s o f f l o w i n g w a t e r s . I f the i n f e r e n c e s drawn above are g e n e r a l l y c o r r e c t , then d i f f e r e n t r a t e s o f w i t h i n - p a t c h prey renewal i n l i m n e t i c , b e n t h i c and f l o w i n g water h a b i t a t s may have acted t o shape the e v o l u t i o n of some d i f f e r e n c e s i n t r o u t and kokanee se a r c h b e h a v i o u r . For example, the f o r a g i n g s t r a t e g y of t r o u t i n streams f r e q u e n t l y i n v o l v e s the s e l e c t i o n o f f i x e d l o c a t i o n s from which the bottom, water-column or s u r f a c e may be scanned f o r prey ( J e n k i n s , 1969). Furthermore, l a b o r a t o r y s t u d i e s on t r o u t i n a r t i f i c i a l streams ( R i n g l e r , 1979) i n d i c a t e t h a t t r o u t can d i s t i n g u i s h p r o f i t a b l e f e e d i n g p o s i t i o n s from l e s s p r o f i t a b l e ones and tend t o p e r i o d i c a l l y s h i f t l o c a t i o n s to occupy the former (see Chapman & B j o r n n , 1969). Trout i n Marion Lake 231 e x h i b i t the same beha v i o u r s s i n c e they f r e q u e n t l y search f o r l a k e s u r f a c e o r b e n t h i c prey by scanning the s u b s t r a t e s from a s t a t i o n a r y p o s i t i o n f o r i n t e r v a l s l a s t i n g up to a few minutes. I f few c a p t u r e s are made, t r o u t resume mobile search (Chapter 4). Given t h a t f l o w i n g water; nearshore, l a k e - s u r f a c e ; and b e n t h i c h a b i t a t s e x h i b i t a p o t e n t i a l f o r r a p i d but h i g h l y v a r i a b l e renewal of prey , the maintenance o f s t a t i o n a r y s e arch p o s i t i o n s by t r o u t may be viewed as an a d a p t i v e procedure t o assess the r a t e of w i t h i n - p a t c h prey renewal b e f o r e the p r e d a t o r s abandon a g i v e n f o r a g i n g l o c a t i o n ( e g . see Charnov, O r i a n s and H y a t t , 1976). The prey s e a r c h behaviour o f kokanee c o n t r a s t s s h a r p l y w i t h t h a t of t r o u t s i n c e kokanee o n l y employ search t e c h n i q u e s t h a t i n v o l v e c o ntinuous f i x e d - v e l o c i t y swimming (Chapter 4). Thus, kokanee i n Marion Lake do not pause lo n g enough a t the l a k e o r sediment s u r f a c e t o o b t a i n i n f o r m a t i o n on the r a t e of w i t h i n - p a t c h prey renewal. T h i s may be because kokanee are adapted to forage p r i m a r i l y w i t h i n l i m n e t i c h a b i t a t s where w i t h i n - p a t c h prey renewal i s r e l a t i v e l y slow and where moving d i r e c t l y through a s e r i e s of patches w i l l always r e s u l t i n a h i g h e r r a t e of prey d i s c o v e r y than w a i t i n g f o r prey to r e c o v e r w i t h i n a patch where they have been r e c e n t l y d e p l e t e d . 232 THE ROLE OF PREY SIZE AND ABUNDANCE IN SHAPING MORPHOLOGICAL AND BEHAVIOURAL CHARACTERISTICS THAT FUNCTION DURING THE ATTACK PHASE OF C - s e l e c t e d o r D-s e l e c t e d FORAGING STRATEGIES I f t r o u t and kokanee r e a l l y possess f o r a g i n g s t r a t e g i e s t h a t have been shaped through e v o l u t i o n a r y reponses t o environments c o n t a i n i n g r e l a t i v e l y l a r g e , d i s p e r s e d prey and s m a l l , c o n t a g i o u s l y - d i s t r i b u t e d prey r e s p e c t i v e l y , then the p r e d a t o r s should not o n l y e x h i b i t a d a p t a t i o n s which favour the d e t e c t i o n of such prey but a l s o should d i s p l a y complementary s e t s of a d a p t a t i o n s f o r d e a l i n g w i t h l a r g e and s m a l l prey r e s p e c t i v e l y d u r i n g the a t t a c k phase of food a c q u i s i t i o n . R e s u l t s from p r e v i o u s c h a p t e r s o f f e r c o n s i d e r a b l e support f o r t h i s i d e a . Trout possess r e l a t i v e l y l a r g e mouths which are a s s o c i a t e d w i t h h i g h l e v e l s o f success i n the m a n i p u l a t i o n and i n g e s t i o n o f l a r g e , armoured o r m o r p h o l o g i c a l l y d i v e r s e p r e y . Trout a l s o e x h i b i t a p r e d i s p o s i t i o n t o a t t a c k r e l a t i v e l y l a r g e , well-armoured prey and t o m a i n t a i n h i g h l e v e l s o f response t o l a r g e prey even when i n g e s t i o n success i s low. P e r s i s t e n t a t t a c k s undoubtedly pay o f f by e n s u r i n g t h a t l a r g e but r a r e energy packages which r e q u i r e prolonged o r v i g o r o u s h a n d l i n g are not abandoned p r e m a t u r e l y by t r o u t . A d a p t a t i o n s which s u i t t r o u t f o r a c q u i r i n g l a r g e prey have a p p a r e n t l y not been achieved w i t h o u t "abandoning" the b e n e f i t s of a l t e r n a t e t r a i t s . A c c o r d i n g l y t r o u t possess r e l a t i v e l y p o o r l y developed g i l l - r a k e r s which are o f t e n a s s o c i a t e d i n some way w i t h an advantage i n e x p l o i t i n g r e l a t i v e l y s m a l l prey (see H y a t t , 1979 f o r d i s c u s s i o n ) . Trout a l s o l a c k the 233 h i g h degree of s t r e a m l i n e d form a t t a i n e d by kokanee and t h i s i s accompanied by a r e l a t i v e l y i n f e r i o r maximum a t t a c k r a t e on s m a l l , a g i l e p r e y . F i n a l l y t r o u t e x h i b i t a b e h a v i o u r a l tendency t o e i t h e r i g n o r e o r r e j e c t many types of s m a l l p l a n k t o n i c prey t h a t kokanee f i n d h i g h l y a c c e p t a b l e . T h i s behaviour appears even i n s i t u a t i o n s when the prey are p r e s e n t a t v e r y h i g h d e n s i t i e s and r e p r e s e n t the o n l y o b v i o u s source o f f o o d . As C - s e l e c t e d s t r a t e g i s t s i n l i m n e t i c h a b i t a t s , kokanee w i l l have g e n e r a l l y e x p l o i t e d a r e l a t i v e l y narrow range of prey s i z e s ( i e . u s u a l l y between 100 ym and 4mm) and w i l l seldom have exper i e n c e d l a r g e p r e y - s i z e as an o b s t a c l e t o the s u c c e s s f u l c a p t u r e o r i n g e s t i o n of prey. D i f f i c u l t i e s i n h a n d l i n g prey are more l i k e l y t o have been a s s o c i a t e d w i t h an i n a b i l i t y t o r e t a i n the v e r y s m a l l but abundant prey a t the "end" of the p r e y - s i z e d i s t r i b u t i o n . The r e l a t i v e l y s m a l l mouth, w e l l developed g i l l - r a k e r s , and low p r e y - s i z e t h r e s h o l d f o r an e f f e c t i v e a t t a c k response on i n v e r t e b r a t e s are l o g i c a l outcomes o f n a t u r a l s e l e c t i o n o p e r a t i n g t o favour a d a p t a t i o n s f o r e x p l o i t a t i o n of r e l a t i v e l y s m a l l , m o r p h o l o g i c a l l y - u n i f o r m prey by kokanee. High d e n s i t i e s o f u n i f o r m l y low-energy prey should a l s o have favoured the e v o l u t i o n of be h a v i o u r s t h a t promote h i g h r a t e s of prey i n t a k e i n o r d e r t o meet the energy demands of normal growth and metabolism. Kokanee do achieve h i g h maximum a t t a c k r a t e s compared to t r o u t when c o n f r o n t e d w i t h h i g h d e n s i t i e s o f s m a l l p l a n k t o n i c p r e y . High a t t a c k r a t e s may be achieved not o n l y as a consequence o f a more s t r e a m l i n e d 234 body form and g r e a t e r m a n e u v e r a b i l i t y but a l s o through s e l e c t i o n t o f a v o u r : s h o r t d e c i s i o n times co n c e r n i n g whether or not to a t t a c k a g i v e n prey i t e m , s h o r t h a n d l i n g times w i t h i n d i v i d u a l prey and, as a c o r o l l o r y , s h o r t g i v i n g up times on i n d i v i d u a l prey t h a t p r e s e n t any " d i f f i c u l t y " d u r i n g m a n i p u l a t i o n . These c h a r a c t e r i s t i c s are h i g h l y c o m p a t i b l e w i t h those t r a i t s known t o c o n s t i t u t e elements of the f o r a g i n g behaviour of kokanee from Marion Lake (Table 29). In many r e s p e c t s the a d a p t a t i o n s t h a t enable kokanee t o e f f e c t i v e l y e x p l o i t s m a l l , c o n t a g i o u s l y - d i s t r i b u t e d prey are m u t u a l l y e x c l u s i v e of those which would a l l o w them t o e x p l o i t l a r g e , d i s p e r s e d p r e y . The s m a l l mouth s i z e o f kokanee i s c l e a r l y r e l a t e d t o the r e l a t i v e l y low l e v e l s of m a n i p u l a t i o n and i n g e s t i o n s u ccess t h a t they e x p e r i e n c e w i t h l a r g e , armoured and m o r p h o l o g i c a l l y d i v e r s e prey. Kokanee a l s o appear t o possess a b e h a v i o u r a l p r e d i s p o s i t i o n t o f r e q u e n t l y i g n o r e o r r e j e c t even those l a r g e prey t h a t l a b o r a t o r y experiments i n d i c a t e they can handle. I suggest t h a t t h i s i s l o g i c a l l y an a d a p t i v e measure to reduce the r i s k o f p h y s i c a l damage t o the f i n e l y developed s t r u c t u r e of g i l l - r a k e r s which may be u s e f u l i n e x p l o i t i n g very s m a l l p r e y . FORAGING STRATEGIES AS COMPLEMENTARY COMPONENTS OF LIFE HISTORY STRATEGIES A f o r a g i n g s t r a t e g y r e p r e s e n t s o n l y a p o r t i o n o f the a d a p t i v e complex t h a t c h a r a c t e r i z e s each s p e c i e s and i t i s e s s e n t i a l t o remember t h a t t r e n d s i n the e v o l u t i o n a r y d i r e c t i o n o f any one component may i n f l u e n c e the s e l e c t i v e 235 advantages o f many o r a l l of the o t h e r components ( F i g . 3 7). For example, water column h a b i t a t s , l a c k i n g c o v e r , may favour an a n t i p r e d a t o r s t r a t e g y t h a t r e l i e s on the maintenance o f l a r g e group s i z e . G iven t h a t t h e r e i s an overwhelming a n t i p r e d a t o r advantage t o m a i n t a i n i n g groups i n the water column, then the elements of a s p e c i e s f o r a g i n g s t r a t e g y w i l l e v o l v e w i t h r e f e r e n c e t o t h i s s e l e c t i v e p r e s s u r e as w e l l as to those e x e r t e d by the b a s i c nature of the prey r e s o u r c e . S u b t l e d i f f e r e n c e s i n the reponses o f t r o u t and kokanee t o "prey" are l i k e l y r e l a t e d t o j u s t such i n t e r a c t i o n s . I n d i v i d u a l t r o u t o f t e n o r i e n t t o , approach, a t t a c k and then r e j e c t p i e c e s o f twig o r armoured prey i n the water column o r on the sediment s u r f a c e . A t t a c k s upon a s i n g l e item may be repeated s e v e r a l times u n t i l f i n a l r e j e c t i o n o r i n g e s t i o n t a k e s p l a c e . When i n d i v i d u a l kokanee forage w i t h i n groups i n the water column, they too " t e s t " o b j e c t s f o r t h e i r food p o t e n t i a l , however, they do not r e p e a t e d l y a t t a c k s i n g l e items. I n s t e a d , a f t e r an i n i t i a l r e j e c t i o n , they move on w i t h the group and then t e s t the next item encountered. The tendency t o make o n l y a s i n g l e a t t a c k upon p o t e n t i a l prey items i n the water column i s l i k e l y due t o the " n e c e s s i t y " o f m a i n t a i n i n g c o n t a c t w i t h the main group as i t moves. For. t r o u t which forage alone most of the ti m e , no such c o n s t r a i n t a p p l i e s . S i n g l e kokanee i n the l a b o r a t o r y w i l l r e p e a t e d l y " t e s t " prey j u s t as t r o u t do. Because of the nature of the i n t e r a c t i o n s between the components making up the v a r i o u s s t r a t e g i e s o f a s p e c i e s , 236 FIGURE 37. An outline of the interactions between the basic structure of the habitat and the strategies which organisms evolve in response to food, predators and competitors. - H A B I T A T • STRUCTURE •^DISTRIBUTION AND ABUNDANCE OF FOOD FORAGING •STRATEGIES ANTIPREDATOR STRATEGY GROUP-*-SIZE ^ STRATEGIES FOR ' t SOCIAL ORGANIZATION 237 i t i s u s u a l l y i m p o s s i b l e t o s p e c i f y whether a p a r t i c u l a r element of a f o r a g i n g s t r a t e g y i s the precedent f o r o r the antecedent of a g i v e n p a t t e r n o f s o c i a l o r g a n i z a t i o n or a p a r t -i c u l a r p a t t e r n o f a n t i p r e d a t o r b e h a v i o u r . In the example above, I c o u l d have argued j u s t as e a s i l y t h a t water-column h a b i t a t s favour the e v o l u t i o n of a prey complex, which i n t u r n f a v o u r s the e v o l u t i o n o f f o r a g i n g i n groups by p r e d a t o r s . Then, g i v e n t h a t t h e r e was an overwhelming advantage t o f o r a g i n g as groups i n the water column, the elements of a s p e c i e s a n t i p r e d a t o r s t r a t e g y would ev o l v e w i t h r e f e r e n c e t o t h i s s e l e c t i v e p r e s s u r e as w e l l as t o those e x e r t e d by the n a t u r e of the i m p o r t a n t p r e d a t o r s . Although the o r d e r f o r the e v o l u t i o n o f a s p e c i e s s t r a t e g i e s cannot be s p e c i f i e d w i t h any c e r t a i n t y , I b e l i e v e t h a t i t i s c l e a r from the p r e s e n t study t h a t the b a s i c s t r u c t u r e o f the h a b i t a t determines the nature of the a d a p t i v e complex (summarized i n Table 30.) t h a t e v o l v e s w i t h r e s p e c t to f o r a g i n g by t r o u t and kokanee. C u l l e n (1957) reached a s i m i l a r c o n c l u s i o n c o n c e r n i n g the a d a p t i v e complex t h a t has e v o l v e d w i t h r e s p e c t t o r e p r o d u c t i o n by c l i f f - n e s t i n g k i t t i w a k e s ( R i s s a t r i d a c t y l a ) . TABLE 30. A SUMMARY OF THE ADAPTIVE COMPLEX THAT HAS EVOLVED WITH RESPECT TO FORAGING BY TROUT AND KOKANEE. PREDATOR CHARACTERISTIC ENVIRONMENTAL CHARACTERISTIC ADAPTIVE SIGNIFICANCE OF PREDATOR CHARACTERISTIC DIET TROUT Do not "track" the environmental abundance of prey very c l o s e l y . Bulk of d i e t over most of developmental histo r y composed of r e l a t i v e l y large (>4mm) prey. KOKANEE Track environmental abundance of prey more clo s e l y than trout do. Bulk of d i e t over most of developmental history composed of r e l a t i v e l y small (<2mm) prey items. Wide range of invertebrate prey sizes common i n benthic and l i t t o r a l habitats of flowing water or lakes. Small prey (<2mm) r e l a t i v e l y unavail-able i n flowing water. Narrow range of invertebrate prey sizes commonly present (100.^ m-3mm) i n limnetic habitats r e l a t i v e to benthic or l i t t o r a l zone habitats. Large prey sizes (>3mm) r e l a t i v e l y unavailable due to v i r t u a l absence from plankton. Large prey provide more favourable net return of energy per unit foraging e f f o r t than small prey i n flowing water habitats. Small but abundant prey provide a more favour-able net return of energy per unit foraging e f f o r t than large prey i n limnetic habitats. SEARCH BEHAVIOUR TROUT Maintain r e l a t i v e l y high search v e l o c i t i e s , " t e s t " r e l a t i v e l y few inanimate objects for t h e i r "potential as prey" and maintain search positions that are at r e l a t i v e l y great distance from the "substrates that are scanned. Large prey are r e l a t i v e l y dispersed i e . on average distances between potential encounters with large prey (>2mm) may often be a meter or more. Search procedures increase the area scanned per unit time and predispose the predators to detect large but rare prey. Table 3 0 . - continued KOKANEE Maintain relatively low velocities, "test" large numbers of small inanimate objects for their potential as prey and maintain search positions that are relatively close to the "substrates" that are scanned. Small planktonic prey exhibit relatively contagious distributions within a given foraging patch. On average distances between sequential encounters with small prey (<2mm) may be 5 cm or less. Search procedures favour the detection of large numbers of small prey through area intensive search. TROUT Stationary search positions often maintained as an alternative to mobile search. KOKANEE Search techniques always involve continuous, fixed-velocity swimming. Within-patch renewal rates of benthic and terrestrial prey highly variable in both space and time. Potential exists for very rapid (seconds to minutes) prey renewal after depletion. Within-patch renewal rates of planktonic prey relatively slow (minutes to hours) in both space and time. Procedure allows predator to assess within-patch, prey-renewal rates before a "decision" i s made to abandon a patch. Continuous movement through a series of patches results in a higher rate of prey discovery than waiting within a patch for prey to recover after depletion. to to vo Table 30 - continued TROUT KOKANEE TROUT KOKANEE ATTACK BEHAVIOUR Commonly leap to capture a e r i a l prey. Do not leap to capture a e r i a l prey. Possess r e l a t i v e l y large mouths and a behavioural predisposition to attack r e l a t i v e l y large, w e l l -armoured prey. Possess r e l a t i v e l y small mouths, wel l developed g i l l - r a k e r s and a predisposition to attack r e l a t i v e l y small, morphologically-uniform prey. A e r i a l prey frequently very abundant within a few cm of the water's surface i n flowing water or l i t t o r a l zone habitats. Access to a source of food that would be unavailable otherwise. A e r i a l prey largely absent near the surface of the offshore waters of large lakes or the open ocean. Invertebrate prey span a wide range of sizes and many possess tough chitinous body coverings. Invertebrate prey span a narrow range of sizes and are morphologically uniform r e l a t i v e to diverse invertebrates of benthic and l i t t o r a l zone habitats. Ensure r e l a t i v e l y high levels ofsuccess during manipulation and ingestion of large, armoured or morphologically diverse prey. Small mouth and g i l l -rakers may aid i n capture or retention of small, planktonic prey. Table 30 - continued TROUT Maintain high l e v e l s of response to large prey even i f ingestion success i s low. Tend to ignore or reject many types of small planktonic prey. KOKANEE Maintain high l e v e l s of response to small prey even i f capture success i s low. Tend to ignore or reject large, armoured prey upon i n i t i a l encounter and habituate rapidly to s t i m u l i presented by large prey upon repeated encounters. Large, armoured prey may require vigorous or prolonged handling before ingestion but each prey represents a large quantity of energy. Small, morphologically uniform prey require very short "handling" times and each prey represents a small quantity of energy. Large prey size should seldom constitute a problem during manipulation or ingestion of prey. Persistence during attacks on large prey ensures that large but rare energy packages are not abandoned prematurely. Ensures that predators maintain high rates of prey intake and a favourable net-energy return. Rejection of large prey may be an inevitable consequence of the set of "rules" kokanee use to assess small prey ( i e . minimize handling time) or a procedure which reduces the r i s k of damage to delicate structures such as g i l l - r a k e r s . 242 SUMMARY 1. Trout are p r i m a r i l y adapted t o the h a b i t a t - p r e y complex o f f l o w i n g waters and of the b e n t h i c and l i t t o r a l zones of l a k e s . 2. C o n d i t i o n s a s s o c i a t e d w i t h f l o w i n g water h a b i t a t s i n p a r t i c u l a r have favoured the e v o l u t i o n of a D - s t r a t e g y i n which t r o u t c o n c e n t r a t e on r e l a t i v e l y l a r g e , d i s p e r s e d prey items f o r the b u l k o f t h e i r energy r e q u i r e m e n t s . 3. A d a p t a t i o n s which enable t r o u t to d i f f e r e n t i a l l y e x p l o i t l a r g e prey i n c l u d e : procedures f o r a r e a - e x t e n s i v e s e a r c h ; a p r e d i s p o s i t i o n t o a t t a c k r e l a t i v e l y l a r g e , armoured prey; l a r g e mouth s i z e , and p e r s i s t e n t responses to o p p o r t u n i t i e s to a t t a c k l a r g e p r e y . 4. Trout are not w e l l adapted t o e x p l o i t r e l a t i v e l y s m a l l , m o r p h o l o g i c a l l y - u n i f o r m prey s i n c e the p r e d a t o r s e x h i b i t a tendency t o i g n o r e o r r e j e c t such prey even under l a b o r a t o r y c o n d i t i o n s where they serve as the s o l e source of f o o d . 5. Kokanee are adapted p r i m a r i l y t o the h a b i t a t - p r e y complex o f the l i m n e t i c zone of l a r g e l a k e s and, as sockeye salmon, t o the open waters of the P a c i f i c ocean. 6 . C o n d i t i o n s a s s o c i a t e d w i t h open water h a b i t a t s have favoured the e v o l u t i o n of a C - s t r a t e g y i n which kokanee c o n c e n t r a t e on s m a l l , c o n t a g i o u s l y - d i s t r i b u t e d prey items f o r the m a j o r i t y o f t h e i r energy r e q u i r e m e n t s . 7. A d a p t a t i o n s which enable kokanee to d i f f e r e n t i a l l y e x p l o i t s m a l l prey i n c l u d e : procedures f o r a r e a - i n t e n s i v e s e a r c h ; a p r e d i s p o s i t i o n t o a t t a c k r e l a t i v e l y s m a l l , m o r p h o l o g i c a l l y uniform p r e y ; s m a l l mouth s i z e ; w e l l - d e v e l o p e d g i l l - r a k e r s ; and 243 an ability to sustain high attack rates on small, planktonic prey. 8. Kokanee are not well adapted to exploit large, armoured prey since the predators exhibit a tendency to ignore or reject such prey even under laboratory conditions where these prey serve as the sole source of food. 9-. Adaptations associated with search, approach or pursuit, and manipultation and ingestion phases of the feeding process appear in each instance to be evolutionary responses to specific features of a given habitat-prey complex. 10. C-selected or D-selected foraging strategies appear to be mutually exclusive evolutionary avenues down which kokanee and trout have been directed by the fundamental nature of a given habitat-prey complex. 244 REFERENCES A l c o c k , J . . 1971. I n t e r s p e c i f i c d i f f e r e n c e s i n a v i a n f e e d i n g behaviour and the e v o l u t i o n o f B a t e s i a n mimicry. Behaviour 40: 1-10. A l i , M.A. 1959. The o c u l a r s t r u c t u r e , r e t i n o m o t o r and photo-b e h a v i o u r a l responses of j u v e n i l e P a c i f i c salmon. Can. J . Z o o l . 37: 965-996. A l l e n , K.R. 1942. Comparisons of bottom faunas as sources of a v a i l a b l e f i s h food. Trans. Amer. F i s h . Soc. 71: 275-283. A n t i p a , R. 1974. Food h a b i t s of l a c u s t r i n e salmonids i n Washington S t a t e i n r e l a t i o n t o i n f e c t i o n s w i t h l a r v a e o f the bass tapeworm ( P r o t e o c e p h a l u s a m b l o p l i t e s ) Trans. Amer. F i s h . Soc. 103: 811-814. Baker, M.C. 1972. S t o c h a s t i c p r o p e r t i e s of the f o r a g i n g behaviour o f s i x s p e c i e s o f m i g r a t o r y s h o r e b i r d s . Behaviour 45: 242-270. Bauman, P.C. & K i t c h e l l , J.F. 1974. D i e l p a t t e r n s of d i s t r i b u t i o n and f e e d i n g o f b l u e g i l l (Lepomis macrochirus) i n Lake Wingra, W i s c o n s i n . Trans. Am. F i s h Soc. 103: 255-r260. Beach, D.R. 1974. E v a l u a t i o n o f f i s h p o p u l a t i o n s i n Anderson Ranch R e s e r v o i r . Lake & R e s e r v o i r I n v e s t i g a t i o n s , P r o j e c t F-53-R-9 Job I I I - C . Idaho F i s h and Game Dept. Behnke, R.J. 1972. The s y s t e m a t i c s o f salmonid f i s h e s o f r e c e n t l y g l a c i a t e d l a k e s , j . F i s h . Res. Bd. Can. 29: 639-671. B e n f i e l d , E.F. 1972. A d e f e n s i v e s e c r e t i o n o f Dineutes d i s c o l o r ( C o l e o p t e r a : G y r i n i d a e ) . Ann. Ent. Soc. Am. 65: 1324-1327. B e r s t , A.H. & McCombie, A.M. 1975. Rainbow t r o u t and s p l a k e i n a sou t h e r n O n t a r i o r e s e r v o i r . Res. Rept. No. 96 M i n i s t r y o f N a t u r a l Resources, O n t a r i o . Beukema, J . J . 1968. P r e d a t i o n by the t h r e e - s p i n e d s t i c k l e b a c k ( G a s t e r o s t e u s a c u l e a t u s ) . The i n f l u e n c e o f hunger and ex p e r i e n c e . Behaviour 30: 1-126. B i d e r , J.R. 1962. Dynamics and the t e m p e r o - s p a t i a l r e l a t i o n s of a v e r t e b r a t e community. Ecology 43: 316-320. 245 B i r c h , L.C. & E h r l i c h , P.R. 1967. E v o l u t i o n a r y h i s t o r y and p o p u l a t i o n b i o l o g y . Nature 214: 349-352. B i s h o p , J.E. and Hynes, H.B.N. 1969. Downstream d r i f t o f the i n v e r t e b r a t e fauna i n a stream ecosystem. A r c h . H y d r o b i o l 66: 56-90. Boynton, R.M. & BUSH, W.R. 1956. R e c o g n i t i o n o f forms a g a i n s t a complex background. J . Opt. Sc i e n c e Am. 46: 758-764. B r e t t , J.R. 1952. Temperature t o l e r a n c e i n young P a c i f i c salmon genus, Oncorhynchus. J . F i s h . Res. Bd. Can. 9: 265-323. L e B r a s s e u r , R.J. 1966. Stomach c o n t e n t s of salmon and s t e e l -head t r o u t i n the N o r t h e a s t e r n P a c i f i c Ocean. J . F i s h Res. Board Can. 23:85-100. Brocksen, R J W D a v i s , G.E. & Warren, C.E. 1970. A n a l y s i s of t r o p h i c p r o c e s s e s on the b a s i s o f den s i t y dependent f u n c t i o n s , pp. 468-498 In J.H. S t e e l e (ed.) Marine Food C h a i n s , U n i v e r s i t y o f C a l i f o r n i a P r e s s , B e r k e l e y . Brower, L.P. 1969. E c o l o g i c a l c h e m i s t r y . S c i e n t . Amer. 220: 22-29. Brower, J.V.Z. 1958. E x p e r i m e n t a l s t u d i e s o f mimicry i n some North American b u t t e r f l i e s . I I I . Danaus g i l i p p u s  b e r e n i c e and L i m e n i t i s a r c h i p p u s f l o r i d e n s i s . E v o l u t i o n 12: 273-285. Brown, J.H. & G.A. Lieberman 1973. Resource u t i l i z a t i o n and c o e x i s t e n c e o f s e e d - e a t i n g d e s e r t r o d e n t s i n sand dune h a b i t a t s . Ecology 54: 788-797. Bryan, A. 1971. Some a s p e c t s of the b e h a v o i u r a l e c o l o g y o f two amphipods. M.Sc. T h e s i s , U n i v e r s i t y o f B r i t i s h Columbia, Vancouver, Canada. Bryan, J.E. 1971. Prey s p e c i a l i z a t i o n by i n d i v i d u a l t r o u t l i v i n g i n a stream and ponds. Ph.D. T h e s i s , U n i v e r s i t y o f B r i t i s h Columbia, Vancouver, Canada. Bryan, J . & L a r k i n , P.A. 1972. Food s p e c i a l i z a t i o n by i n d i v i d u a l t r o u t . «J. F i s h . Res. Bd. Can. 29: 1615-1624. Burg i s , M.J., D a r l i n g t o n , J.P.E.C., Dunn, I.G., Ganf, G.G., Gwahaba, J . J . and McGowan, L.M. 1973. The biomass and d i s t r i b u t i o n o f organisms i n Lake George, Uganda. P r o c . R. Soc. London B. 184: 271-298. Burko, T. 1975. S i z e - s e l e c t i v e p r e d a t i o n by the t h r e e - s p i n e d s t i c k l e b a c k . M.Sc. T h e s i s - U n i v e r s i t y o f B r i t i s h Columbia Vancouver, Canada. C a r l a n d e r , K.D. & C l e a r y , R.E. 1949. The d a i l y a c t i v i t y p a t t e r n s o f some f r e s h w a t e r f i s h . Am. Mid. Nat. 41: 447-452. C a r t w r i g h t , J.W. 1961. I n v e s t i g a t i o n of the rainbow t r o u t o f Kootenay Lake, B r i t i s h Columbia, w i t h s p e c i a l r e f e r e n c e t o the Lardeau R i v e r , B r i t i s h Columbia F i s h and Game Branch Manage. P u b l . 7:46 p. Chapman, D.W. 1966. Food and space as r e g u l a t o r s o f salmonid p o p u l a t i o n s i n streams. Amer. Nat. 100: 345-357. Chapman, D.W., Campbell, H.J. and Fort u n e , J . D . J r . 1967. Summer d i s t r i b u t i o n and food of Kokanee and t r o u t i n E l k Lake, Oregon. Trans. Am. F i s h . Soc. 96: 308-312. Chapman, D.W. and B j o r n n , T.C. 1969. D i s t r i b u t i o n o f salmonids i n streams, w i t h s p e c i a l r e f e r e n c e t o food and f e e d i n g . In T.G. Nor t h c o t e (ed.) H.R. M a c M i l l a n Symposium on Salmon and Trout i n Streams. U n i v e r s i t y o f B r i t i s h Columbia, Vancouver, Canada. Charnov, E.L., O r i a n s , G.H. and H y a t t , K. 1976. E c o l o g i c a l i m p l i c a t i o n s of r e s o u r c e d e p r e s s i o n . Amer. Nat. 110: 247-259. Clemens, W.A. and W i l b y , G.V. 1961. F i s h e s o f the P a c i f i c Coast o f Canada. F i s h . Res. Board Can. B u l l . No. 68. Cody, M.L. 1974. C o m p e t i t i o n and the s t r u c t u r e o f b i r d communities. P r i n c e t o n U n i v e r s i t y P r e s s , P r i n c e t o n , N.J. Cody, M.L. & Diamond, J.M. 1975. pp. 1-12 i n Ecology and E v o l u t i o n of Communities. Harvard U n i v e r s i t y P r e s s , Cambridge, Mass. Con f e r , J.L. and B l a d e s , P.T. 1975. Omnivorous zooplankton and p l a n k t i v o r o u s f i s h . L i m n o l . and Oceanogr. 20: 571-579 C o s t a , R.R. and Cummins, K.W. 1972. The c o n t r i b u t i o n o f Leptodora and o t h e r zooplankton t o the d i e t o f v a r i o u s f i s h . Am. M i d i . Nat. 87: 559-564. Crossman, E . J . and L a r k i n , P.A. 1959. Y e a r l i n g l i b e r a t i o n s and change o f food as a f f e c t i n g rainbow t r o u t y i e l d i n Pa u l Lake, B.C. Trans. Amer. F i s h . Soc. 88: 36-44. 247 C u l l e n , E. 1957. A d a p t a t i o n s i n the K i t t i w a k e to c l i f f - n e s t i n g . I b i s 99: 275-302. C u r i o , E. 1976. The E t h o l o g y o f P r e d a t i o n . S p r i n g e r - V e r l a g , N.Y. D a r n e l l , R.M. & M e i r o t t o , R.R. 1965. D i u r n a l p e r i o d i c i t y i n the b l a c k b u l l h e a d ( I c t a l u r u s m e l a s ) . Trans. Am. F i s h . Soc. 94: 1-8. D a v i e s , G.S. 1970. P r o d u c t i v i t y of macrophytes i n Marion Lake, B.C. J . F i s h . Res. Bd. Can. 27: 71-81. D e l u r y , R.T. 1971. The h o s t p a r a s i t e system of Menetus  c o o p e r i (Gastropoda: P l a n o r b i d a e ) and Megalodiscus  microphagus (Trematoda: Paramphistomatidae) i n Marion Lake, B r i t i s h Columbia. B.Sc. T h e s i s , U n i v e r s i t y o f B r i t i s h Columbia, Vancouver. De R u i t e r , L. 1967. The f e e d i n g behaviour of v e r t e b r a t e s i n the n a t u r a l environment. In "Handbook of P h y s i o l o g y " ( J . F i e l d e d . ) , V o l . 1, S e c t . 6, pp. 97-116. Am. P h y s i o l . S o c , Washington, D.C. Diamond, J.M. 1975. Assembly o f s p e c i e s communities, pp. 342-444 In M.L. Cody and J.M. Diamond ( e d s . ) . Ecology and E v o l u t i o n of Communities. Duncan, C.J. and Sheppard, P.M. 1965. Sensory d i s c r i m i n a t i o n and i t s r o l e i n the e v o l u t i o n of B a t e s i a n mimicry. Behaviour 24: 269-282. Edmunds, M. 1974. Defense i n A n i m a l s : A survey o f A n t i p r e d a t o r Defenses. Longmans, Green, New York. E f f o r d , I.E. 1972. An i n t e r i m r e v iew o f the Marion Lake P r o j e c t . i n Z. Kajak & A. H i l l b r i c h t - Ilkowska ( e d s . ) . P r o d u c t i v i t y Problems of Freshwaters. Warszawa - Krakow, 1972. E f f o r d , I.E. & Tsumura, K. 1973. A comparison of the food o f salamanders and f i s h i n Marion Lake, B.C. Trans. Am. F i s h . Soc. 102: 33-47. Eggers, D.M. 1979. L i m n e t i c f e e d i n g behaviour of j u v e n i l e sockeye salmon i n Lake Washington and p r e d a t o r avoidance. L i m n o l . and Oceanogr. 23: 1114-1125. E i s e n b e r g , J.F. & P. Leyhausen 1972. The p h y l o g e n e s i s o f p r e d a t o r y behaviour i n mammals. Z. T i e r p s y c h o l . 30: 59-93. E l l i o t t , J.M. 1968. The d a i l y a c t i v i t y p a t t e r n s o f M a y f l y nymphs. J . Z o o l . Lond. 155: 201-221. Emery, A. 1973. P r e l i m i n a r y comparisons of day and n i g h t h a b i t s o f f r e s h w a t e r f i s h i n O n t a r i o Lakes. J . F i s h . Res. Bd. Can. 30: 761-774. Emlen, J.M. 1966. The r o l e o f time and energy i n food p r e f e r e n c e . Amer. Natur. 100: 611-617. Emlen, J.M. 1973. Ecology: An E v o l u t i o n a r y Approach. Addison - Wesley P u b l i s h i n g Co., Don M i l l s . E n g e l , S. & Magnuson, J . J . 1976. V e r t i c a l and h o r i z o n t a l d i s t r i b u t i o n of coho salmon (O. k i s u t c h ) , y e l l o w perch (P. f l a v e s c e n s ) and C i s c o (Coregonus a r t e d i i ) i n P a l l e t t e Lake, W i s c o n s i n . J . F i s h . Res. Bd. Can. 33: 2710-2715. E s t e s , R.D. & Godard, J . 1967. Prey s e l e c t i o n and h u n t i n g behaviour of the A f r i c a n w i l d dog. J . o f W i l d l i f e Management 31: 52-70. Fedorenko, A.Y. 1975. I n s t a r and s p e c i e s - s p e c i f i c d i e t s i n two s p e c i e s o f Chaoborus Limnol & Oceanogr. 20: 238-249. F i g l e r , M.H. 1972. The r e l a t i o n between e l i c i t i n g s t i m u l u s s t r e n g t h and h a b i t u a t i o n o f the t h r e a t d i s p l a y i n male Siamese f i g h t i n g f i s h , B e t t a splendens. Behaviour 42: 63-96. F o e r s t e r , R.E. 1968. The Sockeye Salmon. B u l l e t i n 162. F i s h . Res. Bd. Can. G a l b r a i t h , M.G. 1967. S i z e s e l e c t i v e p r e d a t i o n on Daphnia by rainbow t r o u t and y e l l o w p e r c h . Trans. Am. F i s h . Soc. 96: 1-10. G e r k i n g , S.D. 1962. P r o d u c t i o n and food u t i l i z a t i o n i n a p o p u l a t i o n o f b l u e g i l l s u n f i s h . E c o l . Mon. 32: 31-78. Goodlad, J.C., G j e r n e s , T.W. & Brannon, E.L. 1974. F a c t o r s a f f e c t i n g sockeye salmon (Oncorhynchus nerka) growth i n f o u r l a k e s of the F r a s e r R i v e r system. J . F i s h . Res. Bd. Can. 31: 871-892. Goss-Custard, J.D. 1977. Optimal f o r a g i n g and the s i z e s e l e c t i o n o f worms by redshank, T r i n g a t o t a n u s . Anim. Behav. 25: 10-29. Gra n t , P.R., Grant, B.R., Smith, J.M.N., Abb o t t , I . I . , & Abbott, L.K. 1976. Darwin's F i n c h e s : P o p u l a t i o n V a r i a t i o n and N a t u r a l S e l e c t i o n . P r o c . Nat. Acad. S c i . U.S.A. 73: 257-261. 249 Gwynne, M.D. & B e l l , R.H.V. 1968. S e l e c t i o n o f v e g e t a t i o n components by g r a z i n g u n g u l a t e s i n the S e r e n g e t i N a t i o n a l Park. Nature: 220: 390-393. H a l l , K.G. & H y a t t , K.D. 1974. Marion Lake (IBP) - from b a c t e r i a t o f i s h . J . F i s h . Res. Bd. Can. 31: 893-911. Ham i l t o n , A.L. 1965. An a n a l y s i s o f a f r e s h w a t e r b e n t h i c community w i t h s p e c i a l r e f e r e n c e t o the chironomidae. Ph.D. T h e s i s . U.B.C, Vancouver. Hanamura, N. 1966. Salmon of the North P a c i f i c Ocean - P a r t I I I . A r e v i e w o f the l i f e h i s t o r y o f North P a c i f i c Salmon 1. Sockeye salmon i n the f a r e a s t . I n t . North P a c i f i c F i s h e r i e s Commission, B u l l e t i n No. 18. Hargrave, B.T. 1969. E p i b e n t h i c a l g a l p r o d u c t i o n and community r e s p i r a t i o n i n the sediments of Marion Lake. J . F i s h . Res. Bd. Can. 26: 2003-2026. H a r t , J.L. 1973. P a c i f i c F i s h e s o f Canada. F i s h . Res. Board Can. B u l l . No. 180. Hartman, G.F. & G i l l , C.A. 1968. D i s t r i b u t i o n o f j u v e n i l e s t e e l h e a d and c u t t h r o a t t r o u t (Salmo g a i r d n e r i and S. c l a r k i c l a r k i ) w i t h i n streams i n southwestern B r i t i s h Columbia. J . F i s h . Res. Bd. Can. 25: 33-48. Hartman, W.L. & Burgner, R.L. 1972. Limnology and f i s h e c o l o g y o f sockeye salmon l a k e s o f the w o r l d . J . F i s h . Res. Bd. Can. 29: 699-715. H a r t w i c k , E.B. 1973. For a g i n g S t r a t e g y of the b l a c k o y s t e r -c a t c h e r . Ph.D. T h e s i s , U n i v e r s i t y o f B r i t i s h Columbia, Vancouver, Canada. H e i n r i c h , B. 1976. Resource p a r t i t i o n i n g among some e u s o c i a l i n s e c t s . Ecology 57: 874-889. Hespenheide, H.A. 1975. Prey c h a r a c t e r i s t i c s and p r e d a t o r n i c h e w i d t h pp. 158-180 in M.L. Cody & J.M. Diamond eds. Ecology & E v o l u t i o n of Communities. Harvard U n i v e r s i t y P r e s s , Cambridge, Mass. Hinde, R.A. 1966. Animal Behaviour. M c G r a w - H i l l , N.Y. Hoar, W.S. 1976. Smolt t r a n s f o r m a t i o n : e v o l u t i o n , b e h a v i o u r , p h y s i o l o g y . J . F i s h . Res. Bd. Can. 33: 1233-1252. Hobson, E.S. 1968. P r e d a t o r y behaviour o f some shore f i s h e s i n the G u l f of C a l i f o r n i a . Res. Rep. U.S. F i s h . W i l d l . Serv. 73: 1-92. H o l l i n g , C S . 1964. The a n a l y s i s o f complex p o p u l a t i o n p r o c e s s e s . Can. Entomol. 96: 335-347. H o l l i n g , C S . 1968. The t a c t i c s o f a p r e d a t o r . Symp. Roy. Entomol. Soc. London 4: 47-58. H o l l i n g , C S . 1966. The f u n c t i o n a l response o f i n v e r t e b r a t e p r e d a t o r s to prey d e n s i t y . Mem. Entomol. Soc. Canada 48: 1-86. Horak, D.H. & Tanner, H.A. 1963. The use o f v e r t i c a l g i l l n e t s i n s t u d y i n g f i s h depth d i s t r i b u t i o n i n H o r s e t o o t h R e s e r v o i r , C o l o r a d o . Trans. Amer. F i s h . Soc. 93: 137-145. Horn, H.S. 1966. Measurement o f " o v e r l a p " i n comparative e c o l o g i c a l s t u d i e s . Am. Nat. 100: 419-424. H o r r i d g e , G.A. 1977. M e c h a n i s t i c t e l e o l o g y and e x p l a n a t i o n i n n e uroethology. B i o s c i e n c e 27: 725-732. Houde, E.D. 1967. Food of p e l a g i c young of the w a l l e y e , S t i z o s t e d i o n v i t r e u m v i t r e u m , i n Oneida Lake, New York, Trans. Am. F i s h . Soc. 96: 17-24. Hut c h i n s o n , B.P. 1971. The e f f e c t of f i s h p r e d a t i o n on the zooplankton o f t e n Adirondack l a k e s w i t h p a r t i c u l a r r e f e r e n c e to the a l e w i f e , A l o s a pseudoharengus. Trans. Am. F i s h . Soc. 100: 323-335. Hut c h i n s o n , G.E. 1967. A T r e a t i s e on Limnology. I I . I n t r o d u c t i o n t o Lake B i o l o g y and the Limnoplankton. John W i l e y and Sons I n c . , New York. 1115pp. H y a t t , K.D. 1979. Feeding S t r a t e g y . Chapter 2 pp. 71-119 i n W.S. Hoar and D.G. R a n d a l l (eds.) F i s h P h y s i o l o g y V o l . 8. Academic P r e s s , N.Y. Hynes, H.B.N. 1970. The Ecology o f Running Waters. U n i v e r s i t y of Toronto P r e s s , Toronto. 555p. I r i z a r r y , R.A. 1975. F i s h e r i e s i n v e s t i g a t i o n s i n P r i e s t and Upper P r i e s t Lakes. Lake & R e s e r v o i r I n v e s t i g a t i o n s . Job Performance Report P r o j e c t F-53-R-10 Job X l l - a . Idaho F i s h & Game Dept. I v l e v , V.S. 1961. E x p e r i m e n t a l Ecology of the Feeding o f F i s h e s . New Haven: Y a l e U n i v e r s i t y P r e s s . 302pp. J e n k i n s , T.M. J r . 1969. S o c i a l s t r u c t u r e , p o s i t i o n c h o i c e and m i c r o d i s t r i b u t i o n o f two t r o u t s p e c i e s (Salmo t r u t t a and Salmo g a i r d n e r i ) r e s i d e n t i n mountain streams. Anim. Behav. Monogr. 2: 57-123. 251 J e n k i n s , T.M.Jr., Feldmeth, C R . and E l l i o t t , G.V. 1970. Feeding of rainbow t r o u t (Salmo g a i r d n e r i ) i n r e l a t i o n t o abundance of d r i f t i n g i n v e r t e b r a t e s i n a mountain stream. J . F i s h . Res. Board Can. 27: 2356-2361. Johnson, W.E. and H a s l e r , A.D. 1954. Rainbow t r o u t p r o d u c t i o n i n d y s t r o p h i c l a k e s . J . W i l d l . Mgt. 18: 113-134. Kear, J . 1962. Food s e l e c t i o n i n f i n c h e s w i t h s p e c i a l r e f e r e n c e to i n t e r s p e c i f i c d i f f e r e n c e s . P r o c . Z o o l . Soc. Lond. 138: 163-204. Keas t , A. and Webb, D. 1966. Mouth and body form r e l a t i v e to the f e e d i n g e c o l o g y i n the f i s h fauna of a s m a l l l a k e , Lake O p i n i c o n , O n t a r i o . J . F i s h . Res. Board Can. 23: 1845-1867. Keas t , A. 1970. Food s p e c i a l i z a t i o n and b i o e n e r g e t i c i n t e r r e l a t i o n s i n the f i s h faunas of some s m a l l O n t a r i o waterways, in J.H. S t e e l e (ed.) Marine Food Chains. K j e l s o n , M.A. 1971. S e l e c t i v e p r e d a t i o n by a f r e s h w a t e r p l a n k t i v o r e , the t h r e a d f i n shad (Dorosoma pete n e n s e ) . Ph.D. T h e s i s , U n i v e r s i t y o f C a l i f o r n i a , D a v i s . K l o p f e r , P.H. 1973. B e h a v i o r a l Aspects of Ecology 2nd ed. P r e n t i c e - H a l l I n c . , Englewood C l i f f s , N.J. Krebs, C.J. 1972. Ecology: The e x p e r i m e n t a l a n a l y s i s o f d i s t r i b u t i o n and abundance. Harper & Row, New York. Krebs, J.R. 1973. B e h a v i o u r a l a s p e c t s o f p r e d a t i o n pp. 73-111 i n P.G. Bateson and P.H. K l o p f e r (eds.) P e r s p e c t i v e s i n E t h o l o g y . Krebs, J.R., E r i c h s e n , J.T., and Webber, M.I. 1977. Optimal prey s e l e c t i o n i n the g r e a t t i t (Parus m a j o r ) . Anim. Behav. 25: 30-38. Kruuk, H. 1972. The Spotted Hyena. U n i v e r s i t y o f Chicago P r e s s . L a r k i n , P.A. 1956. I n t e r s p e c i f i c c o m p e t i t i o n and p o p u l a t i o n c o n t r o l i n f r e s h w a t e r f i s h . J . F i s h . Res. Bd. Can. 13: 327-342. L a r k i n , P.A., Terpenning, J.G., & P a r k e r , R.R. 1956. S i z e as a determinant o f growth r a t e i n rainbow t r o u t . Trans. Am. F i s h . Soc. 86: 84-96. L o r z , H.W. & N o r t h c o t e , T.G. 1965. F a c t o r s a f f e c t i n g stream l o c a t i o n and t i m i n g and i n t e n s i t y o f e n t r y by spawning kokanee (Oncorhynchus nerka) i n t o an i n l e t o f N i c o l a Lake, B.C. J . F i s h . Res. Bd. Can. 22: 665-687. 252 Leonard, J.W. and Leonard, F.A. 1946. An analysis of the feeding habits of rainbow trout and lake trout in Birch Lake, Cass County, Michigan. Trans. Amer. Fish. Soc. 76: 301-314. Leong, R.J.H. and O'Connell, C P . 1969. A laboratory study of particulate and f i l t e r feeding of the northern anchovy (Engraulis mordax). J . Fish. Res. Board Can. 26: 557-582. Levings, CD. 1972. A study of temporal change in a marine benthic community with particular reference to predation by Pseudopleuronectes americanus. Ph.D. Thesis, Dalhousie Univ., Halifax, Nova Scotia. Lewontin, R.C. 1978. Adaptation. Scient. Am. 239: 212-230. Lovely, D.S. 1972. A study of the shallow water predators Notonecta undulata and Buenoa confusa (Hemiptera: Notonectidae) in Marion Lake. BSc. Thesis, U.B.C. Vancouver. MacArthur, R.H. 1958. Population ecology of some warblers of northeastern coniferous forests. Ecology 39: 599-619. MacArthur, R.H. 1972. Geographical Ecology, Harper & Row, New York. MacArthur, R.H. and Pianka, E.R., 1966. On optimal use of a patchy environment. Amer. Natur. 100: 603-609. MacArthur, R.H. and Levins, R. 1967. The limiting s i m i l a r i t y of convergence and divergence of coexisting species. Am. Nat. 101; 377-385. MacCrimmon, H.R. & Kwain, W. 1966. Use of overhead cover by rainbow trout (Salmo gairdneri) exposed to a series of l i g h t i n t e n s i t i e s . J . Fish. Res. Bd. Can. 23:983-990. MacCrimmon, H.R. 1972. World d i s t r i b u t i o n of rainbow trout (Salmo gairdneri). J . Fish Res. Board Can. 28: 663-704. Manning, A. 1972. An Introduction to Animal Behaviour 2nd. ed. Edward Arnold Ltd., London. Mathias, J.A. 1971. Energy flow and secondary production of the amphipods Hyalella azteca and Crangonyx richmondensis  occidentalis in Marion Lake. J . Fish. Res. Bd. Can. 28: 711-726. McDonald, J . 1973. Diel v e r t i c a l movements and feeding habits of underyearling sockeye salmon (Oncorhynchus nerka), at Babine Lake, B.C. Fish. Res. Board Can. Tech. Rept. No. 378. 253 Mech, L.D. 1970. The Wolf: The Ecology and Behaviour of an Endangered S p e c i e s , Garden C i t y , N.Y. N a t u r a l H i s t o r y P r e s s . Menge, B.A. 1972. Foraging s t r a t e g y of a s t a r f i s h i n r e l a t i o n to a c t u a l prey a v a i l a b i l i t y and env i r o n m e n t a l p r e d i c t a b i l i t y . E c o l . Mon. 242: 25-50. M e t z e l a a r , J . 1929. The food of t r o u t i n M i c h i g a n . Trans. Am. F i s h . Soc. 59: 146-152. Moon, H.P. 1940. An i n v e s t i g a t i o n o f the movements of f r e s h -water i n v e r t e b r a t e fauna. J . Anim. E c o l . 9: 76-83. Moore, J.W. and Moore, I.A. 1976. The b a s i s of food s e l e c t i o n i n f l o u n d e r s , P l a t i c h t h y s f l e s u s ( L . ) , i n the Severn E s t u a r y . J . F i s h . B i o l . 9: 139-156. M o r i s i t a , M. 1959. Measuring of i n t e r s p e c i f i c a s s o c i a t i o n and s i m i l a r i t y between communities. Mem. Fac. S c i . Kyushu U n i v e r s i t y Ser. E. ( B i o l o g y ) 3: 65-80. Morse, D.H. 1971. The i n s e c t i v o r o u s b i r d as an a d a p t i v e s t r a t e g y . Ann. Rev. E c o l & S y s t e m a t i c s 2 : 177-200. Moyle, P.B. 1973. E c o l o g i c a l s e g r e g a t i o n among t h r e e s p e c i e s of minnows ( C y p r i n i d a e ) i n a Minnesota Lake. Trans. Amer. F i s h . Soc. 102: 794-805. Murdoch, W.W., Avery, S. and Smyth, M.E.B. 1975. S w i t c h i n g i n p r e d a t o r y f i s h Ecology 56: 1094-1105. Murton, R.K. 1971. The s i g n i f i c a n c e o f a s p e c i f i c s e a rch image i n the f e e d i n g behaviour o f the wood-pigeon. Behaviour 40: 10-42. Nar v e r , D.W. 1970. D i e l v e r t i c a l movements and f e e d i n g o f u n d e r y e a r l i n g sockeye salmon and the l i m n e t i c zooplankton i n Babine Lake, B.C. J . F i s h . Res. Board Can. 27: 281-316. Neave, F. 1944. R a c i a l c h a r a c t e r i s t i c s and m i g r a t o r y h a b i t s o f Salmo g a i r d n e r i . J . F i s h . Res. Bd. Can 6: 245-251 N e i l l , S.R. and C u l l e n , J.M. 1974. Experiments on whether s c h o o l i n g by t h e i r prey a f f e c t s the h u n t i n g behaviour of cephalopod and f i s h p r e d a t o r s . F. Z o o l . London 172: 549-569. N e i s h , I . e . 1970. A comparative a n a l y s i s o f the f e e d i n g behaviour of two salamander p o p u l a t i o n s i n Marion Lake, B.C. PhD. T h e s i s , U.B.C. 254 Neish, I.C. 1971. Comparison of size, structure and dis -tributional patterns of two salamander populations in Marion Lake, B.C. J . Fish. Res. Bd. Can. 28: 49-58. Nelson, J.S. 1968. Distribution and nomenclature of North American kokanee, Oncorhynchus nerka. J . Fish. Res. Bd. Can. 25: 409-414. Newman, M.A. 1960. A comparative study of the resi d e n t i a l behaviour of juvenile salmonids. Ph.D. Thesis, University of B r i t i s h Columbia, Vancouver, Canada. 295pp. Nilsson, N.A. 1960. Seasonal fluctuations in the food segregation of trout, char and whitefish in 14 north-Swedish lakes. Rep. Inst. Freshwater. Res. Drottningholm. Nilsson, N.A. 1967. Interactive segregation between f i s h species iri S.D. Gerking (ed.) The Biological Basis of Freshwater Fish Production, Blackwells, Oxford. Northcote, T.G. 1973. Some impacts of man on Kootenay Lake and i t s salmonids. Great Lakes Fish. Comm. Tech Rept. No. 25. Northcote, T.G. & Lorz, H.W. 1966. Seasonal and d i e l changes in food of adult kokanee (Oncorhynchus nerka in Nicola Lake, B.C. J . Fish. Res. Bd. Can 23: 1259-1263. Northcote, T.G. and R. Claratto, 1975. Limnetic macrozooplankton and f i s h predation in some coastal B r i t i s h Columbia lakes. Verh Internat. Verein Limnol. 19: 2378-2393. O'Connell, C P . and Zweifel, J.R. 1972. A laboratory study of particulate and f i l t e r feeding of the P a c i f i c mackerel, Scomber japonicus. U.S. Fish W i l d l i f e Serv., Fish Bull 70: 973-978. Orians, G.S. and Horn, H.S. 1969. Overlap in foods and foraging of four species of blackbirds in the potholes of Central Washington. Ecology 5o: 930-939. 1963. Trophic relationships of eight sympatric predatory gastropods. Ecology 44: 63-73. Paine, R.T. 1969. A note on trophic complexity and community s t a b i l i t y . Am. Nat. 103: 91-93. Park, 0. 1941. Concerning community symmetry. Ecology 22: 164-167. p e a r l s t o n e , P.M. 1971. O b s e r v a t i o n s of a n a t u r a l popul of d a m s e l f l y l a r v a e . Msc. T h e s i s U.B.C Vancouver. 255 P u l l i a m , H.R. & Enders , F. 1971. The f e e d i n g e c o l o g y o f f i v e s y m p a t r i c f i n c h s p e c i e s . Ecology 52: 557-566. Pyke, G.H. P u l l i a m , H.R. and Charnov, E.L. 1977. Optimal f o r a g i n g : A s e l e c t i v e r e view of t h e o r y and t e s t s . Quart. Rev. B i o l . 52: 137-154. R a b i n o w i t c h , V. 1969. The r o l e o f e x p e r i e n c e i n the development and r e t e n t i o n o f seed p r e f e r e n c e s i n zebra f i n c h e s . Behaviour 33: 222-236. Rankin, D.P. 1977. Increased p r e d a t i o n by j u v e n i l e sockeye salmon (Oncorhynchus nerka) r e l a t i v e t o changes i n macrozooplankton abundance i n Babine Lake, B r i t i s h Columbia. M.Sc. T h e s i s , U n i v e r s i t y o f B r i t i s h Columbia, Vancouver, Canada, l O l p . Rawstron, R.R. 1972. H a r v e s t , s u r v i v a l and c o s t o f two domestic s t r a i n s o f tagged rainbow t r o u t s t o c k e d i n Lake B e r r y e s s a , C a l i f o r n i a . C a l . F i s h & Game 58: 44-49. Reichman, O.J. 1977. O p t i m i z a t i o n of d i e t s through food p r e f e r e n c e s by Heteromyid r o d e n t s . E c o l o g y 58: 454-457. Rettenmeyer, C.W. 1970. I n s e c t Mimmicry. Ann. Rev. Ent. 15: 43-74. R i n g l e r , N.H. 1979. Prey s e l e c t i o n by d r i f t - f e e d i n g brown t r o u t (Salmo t r u t t a ) . J . F i s h Res. Board Can. 36: 392-403. Root, R.B. 1967. The n i c h e e x p l o i t a t i o n p a t t e r n o f the b l u e -gray g n a t c a t c h e r . E c o l . Mon. 37: 317-347. Royama, T. 1970. F a c t o r s g o v e r n i n g the h u n t i n g b e h a v i o r and s e l e c t i o n o f food by the g r e a t t i t , Parus major. J . Anim. E c o l . 39: 619-668. Rozenzweig, M.L. & S t e r n e r , P.W. 1970. P o p u l a t i o n e c o l o g y o f d e s e r t rodent communities. Body s i z e and seed-husking as bases f o r heteromyid c o e x i s t e n c e . Ecology 51: 217-224. R o z i n , P. 1969. A d a p t i v e food sampling p a t t e r n s i n v i t a m i n d e f i c i e n t r a t s . J . Comp. P h y s i o l . P s y c h o l . 69: 126-132. R o z i n , P. and K a l a t , J.W. 1971. S p e c i f i c hungers and p o i s o n avoidance as a d a p t i v e s p e c i a l i z a t i o n s o f l e a r n i n g . P s y c h o l . Rev. 78: 459-486. 256 Sandercock, F.K. 1969. B i o e n e r g e t i c s o f the rainbow t r o u t (Salmo g a i r d n e r i ) and the kokanee (Oncorhynchus nerka) p o p u l a t i o n s o f Marion Lake, B.C. Unpublished PhD. T h e s i s U.B.C., Vancouver. S c h a l l e r , G.B. 1972. The S e r e n g e t i L i o n . U n i v . o f Chicago P r e s s . Schoener, T.W. 1965. The e v o l u t i o n o f b i l l s i z e d i f f e r e n c e s among, s y m p a t r i c , c o n g e n e r i c s p e c i e s o f b i r d s . E v o l u t i o n 19: 189-213 Schoener, T.W. 1974. Resource p a r t i t i o n i n g i n e c o l o g i c a l communites. S c i e n c e 185: 27-39. Schut z , D.C. and N o r t h c o t e , T.G. 1972. An e x p e r i m e n t a l study o f fe e d i n g behaviour and i n t e r a c t i o n o f c u t t h r o a t t r o u t (Salmo  c l a r k i ) and D o l l y Varden ( S a l v e l i n u s malma). J . F i s h . Res. Bd. Can. 29: 555-565. S c o t t , W.B. and Crossman, E.J. 1973. The f r e s h w a t e r f i s h e s o f Canada. F i s h . Res. Bd. Can. B u l l . No. 184. Seligman, M.E.P. 1970. On the g e n e r a l i t y o f the laws o f l e a r n i n g . Psych. Rev. 77: 406-418. Sexton, O.J. 1960. E x p e r i m e n t a l s t u d i e s o f a r t i f i c i a l b a t e s i a n mimics. B e h a v i o u r , 15: 244-252. S h e t t l e w o r t h , S.J. 1972. C o n s t r a i n t s on l e a r n i n g , Advances i n the Study o f Behaviour 4: 1-68. S i e f e r t , R.E. 1968. Rep r o d u c t i v e b e h a v i o r , i n c u b a t i o n and m o r t a l i t y o f eggs, and p o s t l a r v a l food s e l e c t i o n i n the white c r a p p i e . Trans. Am. F i s h . Soc. 97: 252-259. S i e g e l , S. 1956. Nonparametric S t a t i s t i c s For the B e h a v i o r a l S c i e n c e s . M c G r a w - H i l l , N.Y. Sla n e y , P.A. & N o r t h c o t e , T.G. 1974. E f f e c t s of prey abundance on d e n s i t y and t e r r i t o r i a l behaviour of young rainbow t r o u t (Salmo g a i r d n e r i ) i n l a b o r a t o r y stream c h a n n e l s . J . F i s h . Res. Board Can. 31:1201-1209. Smith, S.M. 1973. Prey a t t a c k behaviour i n loggerhead s h r i k e s Behaviour 44: 113-141. S t a r r , P.J. 1973. C o l o n i z a t i o n o f l i l y p a d s by S i d a c r y s t a l l i n a i n Marion Lake, B.C. M. Sc. T h e s i s , U n i v e r s i t y o f B r i t i s h Columbia, Vancouver, Canada. 74 pp. S t e a r n s , S.C. 1976. L i f e h i s t o r y t a c t i c s : A re v i e w o f the i d e a s . Quart. Rev. B i o l . 51: 3-47. 257 S u t h e r l a n d , N.S. 1964. The l e a r n i n g of d i s c r i m i n a t i o n s by an i m a l s . Endeavour 23: 148-152. S w i f t , M.C. 1970. A q u a l i t a t i v e and q u a n t i t a t i v e study o f t r o u t food i n C a s t l e Lake, C a l i f o r n i a . C a l i f . F i s h and Game 56: 109-120. Thorpe, W.H. 1956. L e a r n i n g and I n s t i n c t i n Anim a l s . Harvard U n i v e r s i t y P r e s s , Cambridge. Ti n b e r g e n , L. 1960. The n a t u r a l c o n t r o l o f i n s e c t s i n p i n e woods, I , F a c t o r s i n f l u e n c i n g the i n t e n s i t y of p r e d a t i o n by s o n g b i r d s . A r c h i v . N e e r l . de Z o o l . 13: 265-343. T i p p e t s , W.E. and P.B. Moyle, 1978. E p i b e n t h i c f e e d i n g by rainbow t r o u t (Salmo g a i r d n e r i ) i n the McCloud R i v e r , C a l i f o r n i a . J . o f Anim. E c o l . 47: 549-559. Tody, W.H. 1964. An i n v e s t i g a t i o n o f the success of rainbow t r o u t p o p u l a t i o n s i n t e n l a k e s r e l a t i v e t o l i m i t i n g e n v i r o n m e n t a l f a c t o r s . Unpublished PhD. T h e s i s , U n i v e r s i t y of M i c h i g a n . T y l e r , A.V. 1972. Food r e s o u r c e d i v i s i o n among n o r t h e r n marine demersal f i s h e s . J . F i s h . Res. Bd. Can. 29: 997-1003. Van B a l e n , J.H. 1973. A comparative study of the bre e d i n g e c o l o g y of the Great t i t (Parus major) i n d i f f e r e n t h a b i t a t s . Ardea 61: 1-93. Vernon, E.H. 1957. Morphometric comparison o f t h r e e r a c e s o f kokanee (Oncorhynchus nerka) w i t h i n a l a r g e B r i t i s h Columbia l a k e . J . F i s h . Res. Bd. Can. 14: 573-598. Ware, D.M. 1971. The p r e d a t o r y behaviour of rainbow t r o u t (Salmo g a i r d n e r i ) . PhD. T h e s i s , U.B.C, Vancouver. Ware, D.M. 1973. R i s k of e p i b e n t h i c prey t o p r e d a t i o n by rainbow t r o u t (Salmo g a i r d n e r i ) . J . F i s h . Res. Bd. Can. 30: 787-797. Waters, T.F. 1962. D i u r n a l p e r i o d i c i t y i n the d r i f t of stream i n v e r t e b r a t e s . Ecology 43: 316-320. Werner, E.E. 1974. The f i s h s i z e , prey s i z e , h a n d l i n g time r e l a t i o n i n s e v e r a l s u n f i s h e s and some i m p l i c a t i o n s . J . F i s h . Res. Board. Can. 31: 1531-1536. Werner, E.E. and H a l l , D.J. 1974. Optimal f o r a g i n g and the s i z e s e l e c t i o n o f prey by the b l u e g i l l s u n f i s h . Ecology 55: 1042-1052. 258 Werner, E.E., H a l l , D.J., L a u g h l i n , D.R. Wagner, D.J., Wilsmann, L.A., & Funk, F.C. 1977. H a b i t a t p a r t i t i o n i n g i n a f r e s h w a t e r f i s h community. J . F i s h . Res. Bd. Can. 34: 360-370. W e t z e l , R.G. 1975. Limnology. W.B. Saunders Co., P h i l a d e l p h i a Pa. 743 p. W i l l s o n , M.F. 1972. Seed s i z e p r e f e r e n c e s i n f i n c h e s . W i l s o n B u l l . 84: 449-455. Winterbourn, M.J. 1971. The l i f e h i s t o r i e s and t r o p h i c r e l a t i o n -s h i p s of the T r i c h o p t e r a of Marion Lake, B.C. Can. J . Z o o l . 49: 623-635. Woodey, J.C. 1972. D i s t r i b u t i o n , f e e d i n g and growth of j u v e n i l e sockeye salmon i n Lake Washington. PhD. T h e s i s , U n i v e r s i t y of Washington. Zach, R. 1979. S h e l l d r o p p i n g : d e c i s i o n making and o p t i m a l f o r a g i n g i n Northwestern crows. Behaviour 68: 106-117. Z a r e t , T.M. 1972. P r e d a t o r s , i n v i s i b l e prey and the nat u r e o f polymorphism i n the C l a d o c e r a . Limnol and Oc. 17: 171-184. Z a r e t , T.M. & Rand, A.S. 1971. C o m p e t i t i o n i n t r o p i c a l stream f i s h e s : Support f o r the c o m p e t i t i v e e x c l u s i o n p r i n c i p l e . Ecology 52: 336-342. Z a r e t , T.M. & K e r f o o t , C. 1975. F i s h p r e d a t i o n on Bosmina  l o n g i r o s t r i s : Body s i z e s e l e c t i o n v e r s u s v i s i b i l i t y s e l e c t i o n . Ecology 56: 232-237. 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            data-media="{[{embed.selectedMedia}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
https://iiif.library.ubc.ca/presentation/dsp.831.1-0095431/manifest