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

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MECHANISMS OF FOOD RESOURCE PARTITIONING AND THE FORAGING STRATEGIES OF RAINBOW TROUT (Salmo g a i r d n e r i ) AND KOKANEE ( O n c o r h y n c h u s n e r k a ) I N MARION LAKE, B R I T I S H COLUMBIA  by KIM  D. HYATT  B. S c . UNIVERSITY OF WINDSOR, 1971  A THESIS SUBMITTED I N PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n t h e Department of ZOOLOGY  We a c c e p t t h i s t h e s i s a s c o n f o r m i n g t o t h e r e q u i r e d  THE UNIVERSITY OF BRITISH COLUMBIA  J u n e 1980 0  Kim D. Hyatt  standard  In p r e s e n t i n g t h i s  thesis  an advanced degree at  further  fulfilment  of  the  requirements  the U n i v e r s i t y of B r i t i s h Columbia, I agree  the L i b r a r y s h a l l make it I  in p a r t i a l  freely  available  for  this  thesis  f o r s c h o l a r l y purposes may be granted by the Head of my Department  of  this  thesis for  It  financial  g a i n s h a l l not  of  The U n i v e r s i t y o f B r i t i s h Columbia 2075  Wesbrook  Vancouver, V6T  Date  Place  Canada  1W5  0 J -  )H/g*  or  i s understood that copying or p u b l i c a t i o n  written permission.  Department  that  reference and study.  agree t h a t p e r m i s s i o n f o r e x t e n s i v e copying o f  by h i s r e p r e s e n t a t i v e s .  for  be allowed without my  ABSTRACT  This study was conducted to satisfy three objectives. The f i r s t 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 welldeveloped 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 outcomes of spatial segregation.  ii  iii 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 explanations 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 relativescarcity 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 mouthsize; 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 t e r ) 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, morphologicallyuniform 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 t h e i d e a s and a p p r o a c h t o e c o l o g y o f a g r o u p o f b i o l o g i s t s engaged i n s t u d y i n g t h e M a r i o n Lake e c o s y s t e m a s p a r t o f Canada's c o n t r i b u t i o n t o t h e 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  l a r g e l y d i s s o l v e d by t h e t i m e  s t u d y was begun, t h e l e g a c y  rny  t h i s g r o u p had  of 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 present F i n a n c i a l support  work.  came f r o m t h e C a n a d i a n I n t e r n a t i o n a l  B i o l o g i c a l P r o g r a m , t h e N a t i o n a l R e s e a r c h C o u n c i l o f Canada and the U n i v e r s i t y o f B r i t i s h Columbia, Department o f Zoology. Drs.  I . E . E f f o r d , C . J . W a l t e r s , T.G. N o 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 t h e e a r l y s t a g e s o f t h e r e s e a r c h w h i l e Drs. W a l t e r s , Northcote provided  and L i l e y r e v i e w e d  t h e t h e s i s and  helpful criticisms of early thesis drafts. Dr. J.D. M c P h a i l g r a c i o u s l y assumed t h e t a s k o f r e s e a r c h  supervisor a f t e r the departure provided  o f D r . E f f o r d f r o m U.B.C.  He  not only a kindred s p i r i t but also a mixture of advice,  c r i t i c i s m , h e a l t h y d o s e s o f encouragement and a n 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 t h e s t i m u l a t i o n and good o f a s u c c e s s i o n o f g r a d u a t e s t u d e n t s who b r o a d e n e d my o f a p p r o a c h e s t o b o t h s c i e n c e and l i v i n g . experiences and  cheer  understanding  Conversations  and s h a r e d  w i t h Ben S e g h e r s , George C a l e f , Wren G r e e n , M i k e S w i f t  R i c k Charnov were e s p e c i a l l y v  influential.  vi T h r o u g h o u t t h i s s t u d y I have r e l i e d on t h e s u p p o r t o f my  p a r e n t s who  n o u r i s h i n g my  early curiosity  F i n a l l y t o my  w i f e , A n n i c e , my  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  understanding w i t h o u t which completed.  unquestioning  about the behaviour of  animals.  t h a n k s f o r encouragement  and  t h i s s t u d y m i g h t n e v e r have been  TABLE OF CONTENTS  Page ii  ABSTRACT ACKNOWLEDGEMENTS  V  L I S T OF TABLES  xiii  XVi  L I S T OF FIGURES  CHAPTER 1 GENERAL INTRODUCTION, GENERAL METHODS AND MATERIALS INTRODUCTION  1  GENERAL METHODS AND MATERIALS  9  CHAPTER 2 THE SPECIES S P E C I F I C DIETARY PATTERNS OF TROUT AND KOKANEE I N MARION LAKE INTRODUCTION  17  METHODS  17  RESULTS  19  C o m p a r i s o n s between p r e y c o n t e n t s o f t r o u t and kokanee  19  C o m p a r i s o n s between p r e y c o n t e n t s o f p r e d a t o r s and t h e environment  31  DISCUSSION  39  SUMMARY  44 vii  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 with area  55  Diel a c t i v i t y patterns of predators  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 f o o d s e a r c h  A c t i v i t y p a t t e r n s o f emerging c h i r o n o m i d s DISCUSSION  .  62 63 64  The r o l e o f s p a t i a l and t e m p o r a l s e g r e g a t i o n i n p r o d u c i n g 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 o f s p a t i a l and t e m p o r a l s e g r e g a t i o n i n p r o d u c i n g d i f f e r e n c e s between t h e p r e y c o n t e n t s o f p r e d a t o r s and t h e e n v i r o n m e n t  69  Unexplained  71  dietary patterns  SUMMARY  73 CHAPTER 4 THE RELATIONSHIP BETWEEN FOOD-SEARCH BEHAVIOUR AND DIETARY PATTERNS OF TROUT AND KOKANEE 4-A.  INTRODUCTION  F I E L D DESCRIPTIONS 75  Page METHODS  78  RESULTS  79  Search techniques  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 s e 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 search 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 o f s e a r c h p o s i t i o n s o n d i e t a r y patterns  96  4-B.  LABORATORY COMPARISONS  INTRODUCTION  97  METHODS  99  RESULTS  103  E x p e r i m e n t 4.1 - r e a c t i v e d i s t a n c e and p r e y d e n s i t y E x p e r i m e n t 4.2 - r e a c t i v e d i s t a n c e , kokanee  trout  .... 103  versus 105  DISCUSSION  107  The r o l e o f s e a r c h p o s i t i o n s i n p r o d u c i n g d i e t a r y f e r e n c e s between t r o u t and k o k a n e e  dif-  The r o l e o f s e a r c h t e c h n i q u e s and s e a r c h p o s i t i o n i n p r o d u c i n g d i f f e r e n c e s between t h e p r e y c o n t e n t s o f p r e d a t o r s and t h e e n v i r o n m e n t Unexplained SUMMARY  dietary patterns  107  109 I l l 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 c a p t u r e  119  Comparisons o f p r e d a t o r morphologies  123  DISCUSSION  127  The  capture - success hypothesis  128  The  gill-raker,  129  The  attack-rate hypothesis  The m o u t h - s i z e , 5-B.  prey-size hypothesis  130  ingestion-success hypothesis  131  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 ,  prey-size hypothesis  Experiment  5.4, The m o u t h - s i z e ,  ingestion-success  hypothesis DISCUSSION 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  ...  142  147 150 150  xi 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 t h e p r e y c o n t e n t s o f t h e p r e d a t o r s and o f t h e e n v i r o n m e n t SUMMARY  155 156  CHAPTER 6 THE ROLE OF SHORT TERM EXPERIENCE I N 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 C h a o b o r u s  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 p r e y 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  Experiment  6.4, The s u c c e s s and s t r e n g t h - o f - r e s p o n s e  182  hypothesis  186  DISCUSSION The  165  191  range o f responses  by t r o u t and kokanee t o p r e y  ...  191  Species s p e c i f i c e f f e c t s o f experience 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 The e n v i r o n m e n t a l strategies  202 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  The r o l e o f p r e y s i z e , r e l a t i v e a b u n d a n c e , 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-selected foraging s t r a t e g i e s  208  217  The r o l e o f p r e y d i s t r i b u t i o n i n s h a p i n g s e a r c h and a t t a c k components o f t r o u t and k o k a n e e f o r a g i n g strategies  220  The r o l e o f t e m p o r a l p a t t e r n s o f p r e y r e n e w a l i n s h a p i n g s e a r c h components o f t r o u t and k o k a n e e f o r a g i n g strategies  229  The r o l e o f p r e y s i z e and abundance i n s h a p i n g 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 e l e m e n t s t h a t f u n c t i o n d u r i n g t h e 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  L I S T 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 k o k a n e e from M a r i o n L a k e . S e a s o n a l e x p l o i t a t i o n o f c h i r o n o m i d pupae by t r o u t and k o k a n e e . Taxonomic c o m p o s i t i o n o f f o o d i t e m s from 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 .  predators  The r e l a t i v e l e v e l o f 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 r i c h m o n d e n s i s by t r o u t and k o k a n e e . Taxonomic c o m p o s i t i o n o f p r e y e a t e n by s m a l l t r o u t and k o k a n e e c o l l e c t e d September 7, 1974. 2 Mean number o f amphipods p e r m v e r s u s t h e mean number p e r p r e d a t o r i n d i f f e r e n t months o f t h e year. D e p t h - d i s t r i b u t i o n s o f t r o u t and k o k a n e e u n d e r 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. North-trap versus south-trap captures of t r o u t and kokanee 1974-75. A c o m p a r i s o n o f t h e 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 p r e y a t i n s h o r e ( d e p t h s £2m) and o f f s h o r e ( d e p t h s ~>3m) l o c a t i o n s i n M a r i o n Lake. A c o m p a r i s o n o f t h e 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 p r e y i n a variety of sub-habitats. R e s i d e n c e t i m e and t h e t o t a l number o f 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 t h e h o v e r and search technique. A summary o f t h e 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 employed by t r o u t and kokanee f o r a g i n g i n t h e field. 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 t h e l a k e s u r f a c e on two o c c a s i o n s .  xiii  XIV  Page 14.  The s e a s o n a l e x p l o i t a t i o n o f c h i r o n o m i d l a r v a e by t r o u t and k o k a n e e .  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 e p h e m e r o p t e r a n ( m a y f l y ) nymphs i n l o w - d e n s i t y and h i g h - d e n s i t y e x p e r i m e n t s .  1 0 3  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 t r i c h o p t e r a n s consumed by t r o u t and k o k a n e e .  108  17.  T e c h n i q u e s o f a p p r o a c h and c a 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 p r e y i n t h e f i e l d .  119  18.  A l i s t o f p r e y t y p e s o b s e r v e d t o evoke s p e c i f i c a p p r o a c h and c a p t u r e t e c h n i q u e s i n t h e l a b o r a t o r y and i n t h e f i e l d .  124  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 p r e y used i n e x p e r i m e n t s to determine the a t t a c k s u c c e s s o f t r o u t and k o k a n e e on a v a r i e t y o f p r e y .  134  20.  A c o m p a r i s o n o f c a p t u r e s u c c e s s and i n g e s t i o n s u c c e s s o f " s i z e - m a t c h e d " t r o u t and k o k a n e e i n a t t a c k s on s m a l l , a g i l e p r e y .  138  21.  S p e c i e s c o m p o s i t i o n o f s m a l l z o o p l a n k t o n used e x p e r i m e n t 5.3 w i t h t r o u t and k o k a n e e .  22.  Means and 95% c o n f i d e n c e l i m i t s o f t h e maximum a t t a c k r a t e s p e r m i n u t e a c h i e v e d 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 densities.  145  23.  Means and 95% c o n f i d e n c e l i m i t s o f t h e t o t a l a t t a c k s p e r .5 h o u r s c o m p l e t e d 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 densities.  145  24.  94:  in  A c o m p a r i s o n o f c a p t u r e s u c c e s s and i n g e s t i o n s u c c e s s 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 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 prey.  144  148  25.  The e f f e c t o f e x p e r i e n c e w i t h p r e y 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 s u c c e s s o f t r o u t and kokanee  166  26.  A c o m p a r i s o n o f t h e t i m e t o f i r s t a t t a c k and o f 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 c h a o b o r u s l a r v a e .  170  C h a r a c t e r i s t i c s o f p r e y used t o p r e d i c t t h e expected order of 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 species of predators involved i n t r i a l s v a l u e s have been d e r i v e d .  used and t h e number f r o m w h i c h TFA  A summary o f t h e e l e m e n t s w h i c h d e f i n e t h e 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 k o k a n e e . A summary o f t h e a d a p t i v e complex t h a t h a s 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 k o k a n e e .  L I S T OF  FIGURES  Figure  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 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 M a r i o n L a k e , B.C.  20  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 c h i r o n o m i d pupae i n t h e i r d i e t s .  24  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 o f a l l p r e y e a t e n by s i z e - m a t c h e d t r o u t and kokanee.  26  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 s p . and H y a l e l l a sp) e a t e n by s i z e - m a t c h e d t r o u t and k o k a n e e .  29  6.  A comparison of molluscs H e l i s o m a spp size-matched  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 ( P i s i d i u m spp; p l a n o r b i d s n a i l s , and Menetus sp.) e a t e n by t r o u t and k o k a n e e .  30  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 o f w a t e r column p r e y e x p l o i t e d by s i z e - m a t c h e d t r o u t and k o k a n e e .  33  8.  A c o m p a r i s o n , on an a n n u a l b a s i s , o f t h e r e l a t i v e p r o p o r t i o n s o f p r e y i n t h e e n v i r o n m e n t and i n t h e d i e t o f t r o u t from M a r i o n L a k e , B.C.  35  9.  A c o m p a r i s o n , on an a n n u a l b a s i s , o f t h e r e l a t i v e p r o p o r t i o n s o f p r e y i n t h e e n v i r o n m e n t and i n t h e d i e t o f kokanee from M a r i o n L a k e , B.C.  36  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 o f amphipods i n t h e e n v i r o n m e n t and i n t h e d i e t s of the p r e d a t o r s .  38  11.  A c o n t o u r map.of M a r i o n Lake i n d i c a t i n g t h e l o c a t i o n s o f b o t h t r a p and o b s e r v a t i o n s i t e s .  49  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 k o k a n e e o b s e r v e d a t o n s h o r e and o f f s h o r e l o c a t i o n s i n Marion Lake.  56  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 t r o u t and k o k a n e e n e t t e d a t two h o u r i n t e r v a l s , o v e r t w e n t y - f o u r hour p e r i o d s a t v a r i o u s times o f the y e a r .  59  14.  A c o m p a r i s o n o f t h e r e l a t i v e numbers o f t r o u t and 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 hour p e r i o d s a t v a r i o u s times o f the y e a r .  61  15.  The mean number o f c h i r o n o m i d pupae o b t a i n e d i n s u r f a c e n e t - h a u l s t a k e n between e a r l y a f t e r n o o n and l a t e e v e n i n g on t h r e e s e p a r a t e d a t e s .  65  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 that maintain s p e c i f i c search p o s i t i o n s while v i s u a l l y scanning t h e bottom sediments f o r p r e y .  91  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 that maintain s p e c i f i c search p o s i t i o n s while v i s u a l l y scanning the lake surface f o r prey.  92  18.  The minimum p r e y 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 t h e s u b s t r a t e s o n w h i c h p r e y are l o c a t e d .  98  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 o f kokanee r e s p o n d i n g t o e i t h e r s t a t i o n a r y o r moving m a y f l y nymphs.  104  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 t o l a r g e and s m a l l p r e y t y p e s .  106  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 w e i g h t o f t r o u t and kokanee from M a r i o n L a k e .  125  The r e l a t i o n s h i p between s t a n d a r d l e n g t h and j a w gape o f t r o u t and kokanee f r o m M a r i o n L a k e .  126  23.  The r e l a t i o n s h i p between t h e d e n s i t y o f s m a l l z o o p l a n k t o n and t h e maximum a t t a c k r a t e s o f t r o u t and k o k a n e e .  141  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 o f p r e y used f o r t h e 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 k o k a n e e .  143  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 c h a o b o r u s l a r v a e a s p r e y and t h e t o t a l number o f 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 .  167  ^22.  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 c h a o b o r u s l a r v a e as p r e y and t h e t i m e t o i n i t i a t e t h e 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 c h a n g e s i n e x p e r i e n c e and p r e y d e n s i t y o n t h e t o t a l number o f a t t a c k s t h a t k o k a n e e i n i t i a t e on p a r t i c u l a r p r e y types i n a given t r i a l . The r e l a t i o n s h i p between t h e l e v e l o f a t t a c k s u c c e s s t h a t i n d i v i d u a l k o k a n e e have w i t h s p e c i f i c p r e y t y p e s and t h e t o t a l number o f a t t a c k s t h a t i n d i v i d u a l k o k a n e e 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 c h a n g e s i n e x p e r i e n c e and p r e y d e n s i t y on t h e t i m e t h a t i n d i v i d u a l kokanee t a k e 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 p r e y t y p e 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 % a t t a c k s u c c e s s 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 r e s p o n s e as i n d i c a t e d by t h e 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 three t r i a l s o f experience w i t h s p e c i f i c prey types. 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 e x p o s u r e t o a c o n s t a n t d e n s i t y o f not o n e c t i d s and t h e t o t a l number o f 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 e x p o s u r e t o n o t o n e c t i d s and t h e t i m e t o f i r s t attack of i n d i v i d u a l predators i n a given 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 t h e t o t a l number o f 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 e x p o s u r e t o D i a p t o m u s k e n a i and t h e t i m e 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 given 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 f r o m t h e l a k e s u r f a c e on t h e d i a m e t e r o f t h e c i r c u l a r a r e a w i t h i n w h i c h s u r f a c e p r e y may be d e t e c t e d .  The e f f e c t o f a p r e d a t o r s s e a r c h the s i z e of s e l e c t e d p o r t i o n s of instantaneous f i e l d of search.  position the  on  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 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 and the s t r a t e g i e s which organisms evolve i n response to 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 l o n g been i n t r i g u e d by d i f f e r e n c e s i n m o r p h o l o g y 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 t o comprehend t h e manner and e x t e n t o f s u c h d i f f e r e n c e s i s t o comprehend much o f t h e 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.  S c h o e n e r , 1974.  INTRODUCTION A l l organisms are faced  with  t h e common  challenge  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 e n e r g y i n t h e f o r m  of already  e n e r g y compounds o r e l s e t h e raw  materials This  synthesized  high  from w h i c h t h e y and new p r o t o p l a s m c a n be  r e s o u r c e a c q u i s i t i o n i s n e v e r a random p r o c e s s and a  great deal  of research  i n the b i o l o g i c a l sciences  i n t e r p r e t e d as a search f o r general and cell  synthesised.  may be  rules governing  energy a c q u i s i t i o n a t l e v e l s o f o r g a n i z a t i o n  nutrient  from t h e  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 t h e l e v e l o f  t h e w h o l e o r g a n i s m and a r e c o n c e r n e d w i t h actions  t h a t d e t e r m i n e t h e d i s t r i b u t i o n and abundance o f  organisms" fall  the study o f " i n t e r -  (Krebs, 1972).  A l a r g e number o f t h e s e  interactions  i n t o t h e c a t e g o r y o f n u t r i e n t and e n e r g y a c q u i s i t i o n .  Thus, e c o l o g i s t s t o o a r e 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 e n e r g y a c q u i s i t i o n a s w e l l a s i n e x p l a i n i n g  .  the nature o f 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 r e r e s p o n s i b l e  f o r producing the patterns ( f o r example , see d i s c u s s i o n i n Paine, 1969  or Schoener, 1971,  1974).  -Most v e r t e b r a t e s and many, i n v e r t e b r a t e predators are euryphagic  (see De R u i t e r , 1967  and Emlen, 1973  f o r reviews  that i s , they eat mixed d i e t s c o n t a i n i n g a number of d i f f e r e n t types of food organisms.  In t h e i r search f o r patterns of  n u t r i e n t or energy a c q u i s i t i o n by animals, e c o l o g i s t s have conducted  a g r e a t many s t u d i e s e i t h e r comparing the d i e t a r y  h a b i t s of c l o s e l y r e l a t e d species of predators which occur s y m p a t r i c a l l y or comparing the abundance of food types that are apparently a v a i l a b l e i n the h a b i t a t of a predator to the abundance of food types present i n the predator's d i e t .  These  s t u d i e s have shown repeatedly that (1.) c l o s e l y r e l a t e d species of predators acquire d i f f e r e n t sets of food items, even i n instances where great care has been taken to o b t a i n the predators from the same h a b i t a t s where they p o t e n t i a l l y have had access to an i d e n t i c a l s e t of foods (Root, and Enders, 1971; Lieberman, 1973; 1972;  Schoener, 1974;  Paine, 1963;  Gwynne and B e l l , 1968;  Keast and Webb, 1966)  e x h i b i t s what appears  and  1967;Pulliam Brown and  H e i n r i c h , 1976;  Tyler,  (2.) each species of predator  to be d e n s i t y independent  exploitation  of foods from the t o t a l complex of foods that i s apparently a v a i l a b l e i n a given environment (Root, 1967; 1975;  Gerking, 1962;  Levings, 1972;  E f f o r d and Tsumura, 1973;  Hespenheide,  Costa and Cummins,  Moore and Moore, 1976).  1972;  3 The  t y p e s o f p r e d a t o r s examined i n some o f t h e  best o f these s t u d i e s i n c l u d e b i r d s 1967;  ( T i n b e r g e n , 1960; R o o t ,  O r i a n s and H o r n , 1969; H a r t w i c k , 1973; H e s p e n h e i d e ,  1975)/  fish  ( s e e H y a t t , 1979 f o r r e v i e w ) , mammals ( E s t e s and  G o d d a r d , 1967; Gwynne and B e l l , 1968; S c h a l l e r , 1972; K r u u k , 1972;  Brown and L i e b e r m a n n , 1 9 7 3 ; R e i c h m a n i  1977),  v e r t e b r a t e s ( P a i n e , 1 9 6 3 ; Menge, 1 9 7 2 ; F e d o r e n k o , Having  demonstrated  and i n 1975).  d i f f e r e n c e s i n the prey  contents  o f p r e d a t o r s compared e i t h e r t o e a c h o t h e r o r t o t h e n a t u r a l environment,  an e c o l o g i s t i s f a c e d w i t h t h e p r o b l e m  of pro-  v i d i n g a n s w e r s t o q u e s t i o n s a b o u t why f o o d r e s o u r c e s a r e a c q u i r e d i n t h e way t h e y a r e . split  i n t o groups which  an e v o l u t i o n a r y a p p r o a c h ( B i r c h and  H i s t o r i c a l l y e c o l o g i s t s have  have used e i t h e r a m e c h a n i s t i c o r t o p r o v i d e answers f o r such  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 a p p r o a c h 1973;  questions  (Emlen,  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 t e r m s o f t h e o p e r a t i o n o f p r o x i m a l mechanisms.  The work o f b o t h H o l l i n g  (1964; 1 9 6 6 , 1968) and Ware  ( 1 9 7 1 , 1973) e x e m p l i f i e s t h i s a p p r o a c h .  To t h e s e  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  characteristics  of predators 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 some f o o d i t e m s more o f t e n t h a n o t h e r s . analysing the forces that operate Holling  acquire  F o r e x a m p l e , by  i n a mantid's  forearm,  (1964) p r e d i c t e d t h e s i z e o f p r e y t h a t c o u l d be g r a s p e d  4  best.  He then showed i n behavioural experiments that the  l a r g e s t number of feeding responses were d i s p l a y e d to prey of j u s t t h i s s i z e .  S i m i l a r l y Ware (1973) explored s e v e r a l  v i s u a l "mechanisms" that operate during search and d e t e c t i o n of prey by rainbow t r o u t i n the l a b o r a t o r y and  then used  r e s u l t s to e x p l a i n a number of patterns of prey intake  these  ex-  h i b i t e d by t r o u t that had been foraging f o r benthic i n v e r t e brates i n the  field.  Studies i n which a mechanistic approach has been used have o f t e n avoided  any attempt to provide i n s i g h t s i n t o  the nature of the e v o l u t i o n a r y f o r c e s that have acted to shape the o v e r a l l "design" of an organism.  Accordingly a second  group of e c o l o g i s t s has attempted to resolve questions about patterns o f food a c q u i s i t i o n by employing an e v o l u t i o n a r y or " s t r a t e g i c " approach (Schoener, 1971). (1965, 1971), MacArthur & Pianka  The work of Schoener  (1966), Emlen (1966), MacArthur  and Levins (1967), Root (1967), Royama (1970), MacArthur (1972), Hespenheide (1975), Diamond (1975), and Pianka to c i t e but a few, of  (1975),  c h a r a c t e r i z e s t h i s approach to s t u d i e s  food resource d i v i s i o n by predators.  To these  authors  explanations of s p e c i f i c d i e t a r y patterns l i e i n an a n a l y s i s of how  agents of n a t u r a l s e l e c t i o n , operating w i t h i n a p a r t i -  c u l a r environmental  context, have acted to shape the foraging  s t r a t e g i e s of predators as adaptive  (or optimal) s o l u t i o n s  to problems of energy arid n u t r i e n t a c q u i s i t i o n .  5  Although m e c h a n i s t i c or s t r a t e g i c approaches t o answer q u e s t i o n s a b o u t t h e s i g n i f i c a n c e o f a g i v e n  dietary  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 , t h e y need not be s i n c e s t u d i e s o f t h e 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  search  f o r a n s w e r s 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  re-  s o u r c e s a r e 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  way  t h a t they a r e .  i n the  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 t h e two a p p r o a c h e s 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  selective  p r e s s u r e t o maximize the net energy r e t u r n from f o r a g i n g been i n s h a p i n g examples c i t e d  the behaviour  of a v a r i e t y of animals  i n P y k e , P u l l i a m and  E r i c h s e n and Webber, 1977;  C h a r n o v , 1977:  Goss-Custard,  1977  (see  Krebs,  o r Zach, 1979).  I n t h e p r e s e n t s t u d y I have used b o t h m e c h a n i s t i c and approaches i n attempting  has  strategic  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  fishes inhabiting a small  fresh-  water ecosystem. The originated t h a t two  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  from o b s e r v a t i o n s by E f f o r d and  s i m i l a r species of f i s h  (rainbow  study  Tsumura (1973) 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 lake 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 t y p e s found  in their  diets  when compared e i t h e r t o e a c h o t h e r o r when compared t o t h e  6  r e l a t i v e abundance o f p r e y a p p a r e n t l y a v a i l a b l e i n t h e environment.  Although  lake  d i f f e r e n c e s i n the prey c o n t e n t s  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 e n v i r o n m e n t have been d e s c r i b e d  of  the n a t u r a l  i n numerous f i e l d  studies  ( s e e r e f e r e n c e s c i t e d a b o v e ) , i t i s o f t e n n o t c l e a r what i t i s p r e c i s e l y that favours-the a c q u i s i t i o n of a p a r t i c u l a r set  of prey  i t e m s by a g i v e n s p e c i e s o f p r e d a t o r .  This i s  c e r t a i n l y t r u e f o r t h e 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 h e t r o u t and  k o k a n e e s t u d i e d by E f f o r d and  Tsumura, t h u s one  m a j o r g o a l s o f the p r e s e n t  study  o f anatomy, p h y s i o l o g y and  behaviour  mechanisms w h i c h f a v o u r d i e t s by t r o u t and  of- t h e  i s t o d e t e r m i n e how i n t e r a c t as  proximal  the a c q u i s i t i o n o f s p e c i e s  kokanee i n the  Both H o l l i n g  (1966) and  elements  specific  field. De  R u i t e r (1967) have  e m p h a s i z e d t h a t the p r e d a t i o n p r o c e s s w h i c h g e n e r a t e s p a t t e r n s has a l i m i t e d  number o f s i m p l e b e h a v i o u r a l  dietary components  t h a t s e r v e as t h e common d e n o m i n a t o r s l i n k i n g a l l a n i m a l s i n t h e i r quest f o r food. l e a d s f r o m s e a r c h and of food to  Thus, the b e h a v i o u r a l c h a i n  detection to f i n a l  items p r o v i d e s a convenient  organize  rejection  framework a r o u n d w h i c h  However, few  s t u d i e s have a t t e m p t e d  to  morphological, behavioural or p h y s i o l o g i c a l charac-  t e r i s t i c s operate  t o shape d i e t a r y p a t t e r n s o v e r  sequence of f o r a g i n g e v e n t s Rather  i n g e s t i o n or  s t u d i e s aimed a t r e v e a l i n g t h e o r i g i n s o f d i v e r s e  .dietary patterns. e x p l o r e how  that  f o r single species of  the g e n e r a l t r e n d has been t o a t t e m p t  the  entire  predators.  explanations  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 t h e f i e l d  as a c o n s e q u e n c e  o f b i o l o g i c a l mechanisms t h a t o p e r a t e d u r i n g o n l y a  single  phase o f t h e f o r a g i n g c y c l e ( e g . d u r i n g f o o d h a n d l i n g , R e a r , 1962;  W e r n e r , 1974;  H e i n r i c h , 1976;  M u r t o n , 1971;. Ware, 1973;  during prey d e t e c t i o n ,  Z a r e t , 1972;  Zaret & 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 e a c h  stage  o f t h e f o r a g i n g c y c l e a r e commonly a s s e r t e d t o f o r m 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 t h e f o r a g i n g s t r a t e g y o f a given predator  ( K l o p f e r , 1973)  s t u d i e s w h i c h examine how  y e t t h e r e a r e few  experimental  i n t e r r e l a t e d sets of  adaptations  i n f l u e n c e the r a n g e o f p r e y t h a t p r e d a t o r s c a n  effectively  exploit.  I n d e e d , t h e r e a r e 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 t h a n l a b e l them as e i t h e r e n e r g y m a x i m i z e r s Consequently,  or time  to  minimizers  (Schoener,  1971).  the second major g o a l of  t h i s study  i s t o p r o v i d e a s y s t e m a t i c a s s e s s m e n t o f the  inter-  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 a s s e s s m e n t o f  the  n a t u r e o f t h e "match" between the p r e d a t o r ' s s t r u c t u r e s and behaviours  on t h e one  combinations  t h e i r c h o s e n h a b i t a t and  prey  on t h e o t h e r .  The 1 ) . The  hand and  thesis i s organized  i n t o seven chapters ( F i g .  r e m a i n d e r o f 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 t h r o u g h o u t  the s t u d y .  I n c h a p t e r two  I  p r o v i d e d e t a i l e d c o m p a r i s o n s 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.7  Y  THE ADAPTIVE NATURE OF TROUT AND KOKANEE FORAGING  STRATEGIES  A CH. 5 THE ROLE OF ATTACK BEHAVIOUR AND PREDATOR MORPHOLOGY  CH. 6 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 k o k a n e e i n t h e l a k e e n v i r o n m e n t . 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  mechanistic  a p p r o a c h t o t e s t h y p o t h e s e s a b o u t how p r o x i m a l f a c t o r s shape t h e 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 u n d e r n a t u r a l conditions.  This series of chapters  i s organized along the  same l i n e s a s t h e s e r i e s o f " d e c i s i o n s " e a c h p r e d a t o r must make w h i l e f o r a g i n g .  T h u s , c h a p t e r t h r e e d e a l s w i t h where  and when t h e p r e d a t o r s , c h o o s e t o 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 t h e 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 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  deals  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 t h e a t t a c k phase o f f o r a g i n g and c h a p t e r s i x d e a l s w i t h t h e 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 i n a l t e r i n g predator responses seven,  to prey.  Finally  i n chapter  I use a s t r a t e g i c a p p r o a c h t o r e a s s e s s r e s u l t s  the preceding  chapters  i n an a t t e m p t  prey  from  to 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 k o k a n e e i n t e r m s o f 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 addressed  to specific habitat-prey  adaptations  combinations.  GENERAL METHODS AND MATERIALS The  Study  Area P o p u l a t i o n s o f r a i n b o w t r o u t (Salmo g a i r d n e r i ) and  kokanee (Oncorhynchus nerka) i n Marion formed t h e f o c u s f o r t h e p r e s e n t s t u d y .  Lake, B r i t i s h Marion  as J a c o b ' 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 Vancouver i n a r e s e a r c h f o r e s t operated  Columbia  Lake ( a l s o known  50 k i l o m e t e r s e a s t o f  by t h e 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).  t e n y e a r s (1964-1974) t h e M a r i o n Lake e c o s y s t e m i n t e n s i v e study sponsored  was t h e f o c u s o f  by t h e N a t i o n a l R e s e a r c h  Council of  Canada, t h e U n i v e r s i t y o f B.C. and by t h e C a n a d i a n  section of  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.  Efford  For  (1972) r e v i e w e d  t h e g e n e r a l o b j e c t i v e s o f t h e program and a number o f a u t h o r s ( E f f o r d , 1 9 7 2 ; H a l l & H y a t t , 1974) have r e v i e w e d of research directed The  these  objectives.  b a s i n o f M a r i o n Lake i s 800 m l o n g and a b o u t 200 m  w i d e a t i t s maximum. north.  towards  the progress  The m a j o r i n l e t s t r e a m e n t e r s from t h e  D u r i n g t h e d r y summer s e a s o n , when t h e p r e s e n t s t u d y was  conducted,  t h e l a k e h a s a maximum d e p t h t h a t v a r i e s from 5 - 6 m,  a mean d e p t h o f 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  lake margin  along t h e 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 o f a boggy zone i n w h i c h considerably.  the water l e v e l  V e g e t a t i o n i n t h i s zone i s d o m i n a t e d  varies  by s e d g e s ,  t h e s m a l l s h r u b M y r i c a g a l e , and o t h e r s h r u b s and s m a l l t r e e s s u c h 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 The  (Alnus r u b r a ) .  w e s t e r n s h o r e l i n e h a s a s t e e p e r s l o p e and i s c o v e r e d by s t a n d s  o f r e d c e d a r ( T h u j a p l i c t a ) , w e s t e r n 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.) t o . t h e w a t e r s edge.  There a r e o n l y f o u r  types o f r o o t e d a q u a t i c p l a n t s t h a t a r e abundant.  These c o v e r  22% o f t h e l a k e a r e a ( D a v i e s , 1 9 7 0 ; N e i s h , 1971) and i n c l u d e P o t o m a g e t o n n a t a n s , 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 o f C h a r a g l o b u l a r i s a r e 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 t h e s o u t h end o f t h e 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 w a t e r d e e p e r t h a n 2 m b u t 80% the macrophytes occur  i n water  Approximately mud  l e s s t h a n 2 m deep.  78% o f t h e l a k e b o t t o m c o n s i s t s o f open  ( a deep f l o c c u l e n t o o z e known as g y t t j a ) .  s h a l l o w water  sediments  s h i f t t o much f i n e r s e d i m e n t s  a l g a e and d i a t o m s  During  filmentous  by  litter  d e r i v e d from weed There i s a g r a d u a l  by l e s s l i t t e r  i n the deeper  t h e summer a s p a r s e c o v e r o f  o c c u r s on t h e s u r f a c e o f open mud  deeper than 1 meter. a t h i c k mat  covered  surface of  i s covered  branches  emergent v e g e t a t i o n a r o u n d t h e l a k e .  areas of the l a k e .  The  ( d e p t h s l e s s t h a n 2 m)  composed o f l e a v e s , n e e d l e s , t w i g s and beds and  of  filamentous  areas  i n water  At depths of l e s s than 1 meter there i s o f t e n  o f b e n t h i c a l g a e composed o f s i n g l e c e l l e d  and  forms. M a r i o n 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  that 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  S_. g a i r d n e r i ; k o k a n e e , O.  nerka; three spine  G a s t e r o s t e u s a c u l e a t u s ) and  two  (rainbow  trout,  stickleback,  s a l a m a n d e r s ( 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 ; t h e n e o t o n o u s form o f t h e N o r t h w e s t e r n Ambystoma  gracile).  In a d d i t i o n to the p r e s e n t  dissertation,  s t u d i e s o f t h e p r e d a t o r s t h a t have been c o m p l e t e d  or 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 , and  s t i c k l e b a c k s (Sandercock,  1969;  N e i s h , 1970;  p r o g r e s s ) ; predatory behaviour of rainbow  1975);  t r o u t , salamanders  and d i e t a r y h a b i t s o f s a l a m a n d e r s ,  s t i c k l e b a c k s i n the f i e l d progress).  salamanders  McPhail, i n  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; Burko,  salamander,  Neish,  salmonids  ( E f f o r d & Tsumura, 1973;  and 1970;  and  Hyatt, i n  12  C h a r a c t e r i s t i c s Of Trout And Rainbow t r o u t and  Kokanee kokanee are both members of  family Salmonidae which i s composed of freshwater and  anadromous  f i s h e s that range widely i n the waters of the northern It  i s the dominant family of f i s h e s i n the northern  the  hemisphere.  waters of  North America, Europe and A s i a (Scott & Crossman, 1973). species are of great resources, for  and  Both  importance as commercial or r e c r e a t i o n a l  the rainbow t r o u t i s a standard  laboratory animal  a wide range of p h y s i o l o g i c a l i n v e s t i g a t i o n s .  The  literature  dealing with various aspects of the biology of these two i s massive and Key  species  I w i l l not attempt to summarize i t here. references  that may  background on rainbow t r o u t may  be consulted  for additional  be found i n Scott & Crossman,  1973  (general review of l i f e h i s t o r y c h a r a c t e r i s t i c s ) ; MacCrimmon,  1972  (review of native and present  1944;  Hartman and G i l l , 1968  Newman, 1960; behaviour);  ( h a b i t a t a s s o c i a t i o n s ) ; Jenkins,  Slaney and Northcote, 1974  Ware, 1971;  Key  g l o b a l d i s t r i b u t i o n ) ; Neave,  Bryan, 1972  references  that may  (social organization  (feeding  for additional  background on kokanee or sockeye salmon (the two  are  taxonomically  Scott and  Crossman,  1973  (general review of l i f e h i s t o r y c h a r a c t e r i s t i c s ) ; Nelson,  1968  ( n a t u r a l d i s t r i b u t i o n ) ; Vernon, 1957;  and Northcote, 1965;  Beach, 1974;  of lake dwelling p o p u l a t i o n s ) ; Hartman and Burgner, 1972;  and  behaviour).  be consulted  i n d i s t i n g u i s h a b l e ) i n c l u d e : F o e r s t e r , 1968;  1969;  Northcote, 1973;  Lorz  I r i z a r r y , 1975 ( c h a r a c t e r i s t i c s  F o e r s t e r , 1968;  Goodlad et a l . , 1974  Behnke,  1972,  (habitat  13  a s s o c i a t i o n s ) ; Newman, 1960; H o a r , 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 ) ; R a n k i n , 1978; E g g e r s ,  1978 ( f e e d i n g b e h a v i o u r ) ;  H o a r , 1976 ( e v o l u t i o n and p h y l o g e n t i c  affinities).  S t u d y o f t r o u t and kokanee i n t h e M a r i o n  Lake e c o s y s t e m  o f f e r e d many a d v a n t a g e s f o r t h e p u r s u i t o f knowledge a b o u t foraging behaviour.  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 a r e g e n e t i c a l l y life  i s o l a t e d and c o m p l e t e  their  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 o f which  Lake i s a p a r t . Marion  Marion  The a b s e n s e o f b o t h e x t e n s i v e m i g r a t i o n s by  Lake f i s h and o f gene f l o w f r o m o t h e r  p o t e n t i a l l y reduces  populations  the u n c e r t a i n t y , encountered  i n other studies,  about whether p a r t i c u l a r t r a i t s a r e 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 ( M o r s e , Balen, 1973).  1 9 7 1 ; B i r c h and E h r l i c h , 1967; Van  Growth r a t e s o f both s p e c i e s i n Marion  among t h e l o w e s t r e c o r d e d  across t h e i r geographic  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  Lake a r e  r a n g e and t h e r e  t o a l i m i t e d food  supply  ( s e e H a l l & H y a t t , 1974 f o r d e t a i l s ) , t h u s 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 s h o u l d be r i g o r o u s .  Detailed analysis  of the d i e t a r y h a b i t s of the predators  (Sandercock,  1969; E f f o r d  & Tsumura, 1973 and H y a t t , u n p u b l i s h e d  d a t a ) and e x t e n s i v e  data  on t h e 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 o f t h e i r ( H a m i l t o n , 1965; H a r g r a v e , McCauley, unpublished  prey  1969; M a t h i a s , 1 9 7 1 ; W i n t e r b o u r n , 1 9 7 1 ;  d a t a ) p r o v i d e d an a l m o s t  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 e c o s y s t e m c o n t e x t . The  a combination  nature o f the q u e s t i o n s asked o f l a b o r a t o r y and f i e l d  i n t h i s study required  investigation.  I carried  14 out the bulk o f 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 o f 1971 and 1 9 7 2 .  R e s u l t s f r o m t h i s work h e l p e d  the l a b o r a t o r y experiments  t o examine t h e f i n e s c a l e d e t a i l s o f  f o o d g a t h e r i n g and r e s p o n s e s s e a s o n a l changes.  shape  o f t h e p r e d a t o r s t o some m a j o r  I c o m p l e t e d most o f t h e l a b o r a t o r y work  d u r i n g t h e summers o f 1974 and 1 9 7 5 . C o l l e c t i o n Techniques Trap nets, g i l l  n e t s , s e i n e n e t s and a w i r e  were used t o o b t a i n f i s h f o r v a r i o u s p u r p o s e s .  cylinder  I used t h e w i r e  cylinder exclusively to obtain uninjured f i s h f o r laboratory experiments.  A t n i g h t , from a boat equipped  w i t h auto  head-  lights,  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 k o k a n e e i n s h a l l o w  water.  Because b o t h s p e c i e s e x h i b i t v e r y l o w a c t i v i t y  c o n d i t i o n s of darkness ( d i a m e t e r 1 m, d e p t h  i t was p o s s i b l e t o p l a c e a w i r e  under cylinder  2 m) o v e r 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 t h e l a b o r a t o r y . Laboratory Animals  and S t a n d a r d  Experimental  Procedures  T r o u t and k o k a n e e used i n l a b o r a t o r y e x p e r i m e n t s c a p t u r e d , handled  and m a i n t a i n e d  unless otherwise s p e c i f i e d .  were  under i d e n t i c a l c o n d i t i o n s  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 site.  Each a q u a r i u m 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 of cold  (10°C + 2 ) , s p r i n g - f e d w a t e r .  F i s h were m a i n -  t a i n e d on a d i e t o f c h i c k e n l i v e r when n o t i n v o l v e d i n e x p e r i m e n t s . A s e r i e s o f 200 l i t e r  aquaria (dimensions:  L=92 cm,  W=48 cm, D=46.5 cm) i n a n e a r b y l a b o r a t o r y b u i l d i n g s e r v e d a s experimental  "arenas"  f o r the m a j o r i t y o f feeding t r i a l s . A l l  l a b o r a t o r y experiments l i g h t and t e m p e r a t u r e  were c o n d u c t e d (10°C + 2 ) .  under c o n d i t i o n s o f c o n s t a n t  I l l u m i n a t i o n was p r o v i d e d by  a bank ( s e v e n 100 w a t t b u l b s ) o f i n c a n d e s c e n t l i g h t s mounted 30 cm above t h e w a t e r  surface.  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 t h e l i g h t s and t h e a r e n a , s e r v e d a s 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 t h e s i d e s o f t h e 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  p r e y were p r e s e n t e d .  D u r i n g a p r e - e x p e r i m e n t a l p e r i o d , e a c h p r e d a t o r was conditioned  t o f e e d f r e e l y a f t e r t r a n s f e r from  to the arena.  i t s home a q u a r i u m  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 t h e f i e l d .  The h u n g e r 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 e x p e r i m e n t s  by d e p r i v i n g t h e  p r e d a t o r s o f f o o d f o r 48 - 72 h o u r s .  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 , u n p u b l i s h e d  indicated that f i s h required  results)  40-60 h o u r s 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 The s t a n d a r d p r o c e d u r e  f o r experiments  ration.  consisted of  g a t h e r i n g t h e r e q u i r e d number and t y p e o f p r e y from M a r i o n  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 t o 48 h o u r s .  The p r e y were i n t r o d u c e d t o t h e e x p e r i m e n t a l a r e n a and  a l l o w e d 30 m i n u t e s t o d i s p e r s e b e f o r e t h e i n t r o d u c t i o n o f a predator.  S p e c i f i c aspects o f the predator's feeding  behaviour  were r e c o r d e d c h r o n o l o g i c a l l y o n a R u s t r a c , 4 c h a n n e l , c h a r t recorder.  Single feeding t r i a l s  u s u a l l y l a s t e d 30 m i n u t e s a t w h i c h  t i m e t h e p r e d a t o r was r e t u r n e d t o i t s home a q u a r i u m . was d r a i n e d and c l e a n e d f o l l o w i n g e a c h t r i a l r e c o v e r y and c o u n t i n g o f r e m a i n i n g  prey.  The a r e n a  to facilitate  16  Measurements On P r e d a t o r s And During  Prey  the pre-experimental  p e r i o d t r o u t and k o k a n e e  were a n a e s t h e t i z e d w i t h M S - 2 2 2 , w e i g h e d t o t h e 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 t h e n e a r e s t mm. were o b t a i n e d from p r e s e r v e d vernier calipers.  s a m p l e s o f p r e d a t o r s by u s i n g  sliding  L i v e i n v e r t e b r a t e s used i n e a c h f e e d i n g  were p h o t o g r a p h e d b e f o r e and a f t e r e a c h t r i a l , pan,  j a w measurements  c o n t a i n i n g a mm  rule f o r reference.  trial  i n a w h i t e enamel  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 d a t e w i t h  calipers  from t h e image o f t h e p r e y p r o j e c t e d o n t o a s c r e e n .  Measurements  on v e r y s m a l l p r e y s u c h as z o o p l a n k t o n were o b t a i n e d  from  preserved equipped  s a m p l e s by u s i n g a d i s s e c t i o n m i c r o s c o p e w i t h an o c u l a r m i c r o m e t e r .  (Wild  M-5),  17  CHAPTER 2 THE SPECIES S P E C I F I C 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 r e p e a t e d p a t t e r n s , not simply t o accumulate f a c t s . Repetitions of p a t t e r n s i n n a t u r e a r e 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 t h e means o f m a k i n g c o m p a r i s o n s w h i c h may t h e n s e r v e a s t h e s e e d s o f t e s t a b l e hypotheses. R. M a c A r t h u r , 1 9 7 2 .  INTRODUCTION Two t y p e s o f c o m p a r i s o n s have f r e q u e n t l y been used to The  draw i n f e r e n c e s a b o u t t h e f o r a g i n g b e h a v i o u r  of predators.  d i e t s o f s i m i l a r s p e c i e s a r e compared t o e a c h o t h e r , o r t h e  dietary composition  of a p a r t i c u l a r predator  apparent a v a i l a b i l i t y o f prey section I w i l l on t h e n a t u r e  i s compared t o t h e  i n the environment.  In t h i s  d e a l w i t h b o t h t y p e s o f c o m p a r i s o n s and comment o f t h e i n f e r e n c e s t h a t they  allow.  METHODS Initial  a n a l y s i s o f t r o u t and kokanee d i e t s f r o m  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 o f 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 o f 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 t h a n 15 cm l o n g . determining  T h u s , I was i n t e r e s t e d i n  w h e t h e r t h e 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 o f d i f f e r e n c e s i n the average s i z e o f  18 predators included 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 size.  limited  to predators of s i m i l a r  T h e r e f o r e , I a n a l y z e d some " s i z e - m a t c h e d "  predators  o b t a i n e d from e i t h e r t h e 1963-66 c o l l e c t i o n s o r from c o l l e c t i o n s t a k e n between 1972-76. me t o e x t e n d  the comparisons  new  The new c o l l e c t i o n s  allowed  t o f i s h o f s m a l l e r body s i z e  than  t h o s e 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 t h e t i m e s , l o c a t i o n s and t e c h n i q u e s o f c a p t u r e f o r t h e 1963-66 c o l l e c t i o n a r e a v a i l a b l e ( E f f o r d & Tsumura, 1 9 7 3 ) .  elsewhere  S p e c i m e n s from t h e 1972-76  collection,  used i n t h e 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 b o a t e q u i p p e d  with lights.  T h i s ensured  that trout  and k o k a n e e were matched n o t o n l y f o r s i z e b u t f o r l o c a t i o n ( w i t h i n an a r e a o f a few hundred s q u a r e m e t e r s ) , and t i m e o f c a p t u r e ( b e t w e e n 11 P.M. and 1 A.M.) a s w e l l . fish  The m a j o r i t y o f  i n t h e 1972-76 c o l l e c t i o n were t a k e n from d e p t h s  than 2 m i n t h e n o r t h e a s t c o r n e r o f Marion Lake. f i s h were i m m e d i a t e l y  killed  of l e s s  F r e s h l y sampled  i n 70% e t h a n o l and t h e n  transferred  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 meter.  ( W i l d M-5) e q u i p p e d  w i t h an o c u l a r m i c r o -  T h r e e measurements ( l e n g t h , head w i d t h , maximum body  w i d t h ) were t a k e n from e a c h p r e y .  S i z e frequency  distributions  o f p r e y a r e b a s e d 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 ) .  F r e q u e n c y d i s t r i b u t i o n s a r e b a s e d upon l e n g t h  when p r e y a r e o f r e a s o n a b l y u n i f o r m s h a p e .  In order to provide  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 a r e based upon a r e a when p r e y  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 , p e n t a g o n a l  etc...).  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 f o o d i t e m s i n t h e d i e t s o f t r o u t and k o k a n e e , I s o r t e d  fish,  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, J u n e 10/66  and A u g u s t  11/63.  P r e y measurements  were t a k e n from a t o t a l o f 41 p r e d a t o r s , w i t h no f e w e r t h a n 5 f i s h o f each s p e c i e s from e a c h month. o f f o o d i t e m s i n a sample o f f i s h considerably  Because the t o t a l  number  f o r any month v a r i e d  (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 s a m p l e s .  This guarantees  t h a t the food items  from  any month's s a m p l e s have an e q u a l i n f l u e n c e on t h e shape o f t h 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 c o n s t r u c t e d from d a t a p o o l e d f o r t h e t h r e e months.  RESULTS Between P r e d a t o r C o m p a r i s o n s E x a m i n a t i o n o f the taxonomic  c o m p o s i t i o n o f the d i e t s  by w e i g h t o r by numbers ( F i g . 2a & b) i n d i c a t e s t h a t o v e r c o u r s e o f a y e a r t r o u t and k o k a n e e e x p l o i t s i g n i f i c a n t l y p r o p o r t i o n s o f each m a j o r p r e y t y p e .  There  by t h e o t h e r .  The  four prey types  different  a r e numerous e x a m p l e s  o f p r e y t y p e s 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 b u t utilized  barely  (Trichoptera,  Amphipoda, O d o n a t a and p l a n o r b i d s n a i l s ) t h a t make up 69% w e i g h t ) o f t h e 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% o f t h e of prey i n the d i e t o f kokanee.  the  (by  weight  Over t h e c o u r s e o f a y e a r ,  c h i r o n o m i d l a r v a e , p u p a e , and c l a d o c e r a n s were more i m p o r t a n t dietary  i t e m s f o r kokanee ( 4 8 % by w e i g h t ) t h a n 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 p r e y t y p e s i n t h e d i e t s o f t r o u t and kokanee  from  M a r i o n L a k e , B.C.  (a) D i e t as % by w e i g h t (b) D i e t  as %. by numbers.  D a t a p o o l e d from s a m p l e s o f  c o l l e c t e d i n t h e months o f Nov., and Aug.  T r o u t and kokanee  matched f o r a n a l y s i s .  Feb., A p r i l ,  fish June  were n o t r i g o r o u s l y  size-  A l l tests for significant  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 of s i n g l e prey types by p r e d a t o r s a r e based upon t h e n o r m a l 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 ( S i e g e l , 1956).  ++ = .01, + ^ = .05. Tsumura, 1973. w  Data adopted from Efford and  TROUT  KOKANEE y  o  O F Dl E T BY W E I G H T  Notonect ids Cor i x i d s N e u r o p t e r a n larvae Pi s i d i u m S i mul ium l a r v a e Copepods Ephemeropt'eran larvae Plecopteran larvae Aquatic beetle adults Planorbid snaiIs Odonate larvae C ladocerans C h i r o n o m i d larvae Amphipods C h i r o n o m i d pupae Terrestrial insects Trichopteran larvae  J  40 ©  °/  Q  O F Dl E T BY  30  20  10  10  20  30  40  NUMBERS  A q u a t i c b e e t l e a d u l ts ++ E p h e m e r o p t e r a n l a r v a e ++ Simulium l a r v a e Terrestrial i n s e c t s . Pisidi um O d o n a t e larvae + + Copepods T r i c h o p t e r a n l a r v a e ++ P l a n o r b i d snai I s Amphipods Chironomid larvae Cladocerans C h i r o n o m i d pupae',+ +  N = 4430  N=3038  + +  + +  +  + +  + +  +  +  50  40  30  20  10  10  20  30  40  50  The degree of d i e t a r y s i m i l a r i t y between t r o u t and kokanee f l u c t u a t e s s u b s t a n t i a l l y from month to month.  For com-  parisons of monthly changes i n the degree of food s i m i l a r i t y I have used the overlap measure of M o r i s i t a (1959) and Horn (1966). The overlap c o e f f i c i e n t CX v a r i e s from zero when the samples are completely d i s c r e t e to one when the samples are i d e n t i c a l .  =  s 2? _ , x . y . ^"' i =. l~ i=i T x + z. y  where: s i s the t o t a l number of food categories x^ i s the proportion of the d i e t of predator x taken from the i t y . i s the p r o p o r t i o n o f the d i e t categoryy taken from the of predator  1  ±  ±  2  1  f o o d  i  Although  category  no s t a t i s t i c a l method i s a v a i l a b l e to t e s t the s i g n i f -  icance of C X , other authors 1975)  t h . food  (Zaret & Rand, 1971; Fedorenko,  have assumed that values equal to or greater than .60  represent s i g n i f i c a n t overlap. i n b i r d s i t i s not unusual  In comparisons of food overlap  f o r a l l values to be g r e a t e r than .60  (Orians & Horn, 1969; Pulliam & Enders, 1971) .  By t h i s  criterion  t r o u t and kokanee e x h i b i t c l o s e overlap i n only one month df the year (Table 1 ) .  22  TABLE 1.  by w e i g h t  Overlap Feb.(64) .018 Overlap  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 M a r i o n L a k e . Numbers i n b r a c k e t s i n d i c a t e t h e y e a r i n w h i c h t h e sample was t a k e n .  April(66)  June(66)  Aug. (63)  Nov.(63)  Pooled  .474  .463  .201  .736  .443  . 8 8 1  •  . 4 4 0  •  by numbers .245  .243  1 3 6  4 6 2  Some p r e y t y p e s s u c h as c h i r o n o m i d pupae a r e e a t e n i n l a r g e numbers by b o t h t r o u t and kokanee o v e r t h e c o u r s e o f a year.  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 t h e number o f pupae  e a t e n , t h e means o f t h e number o f pupae e a t e n i n any g i v e n month by t h e two p r e d a t o r s a r e n o t 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 n o t s i g n i f y t h a t t r o u t and kokanee a r e e q u a l l y likely  t o e x p l o i t c h i r o n o m i d pupae.  Indeed  the v a r i a n c e  around  t h e mean number o f pupae e a t e n by t r o u t o r kokanee t e n d s  to  obscure  to  t h e f a c t t h a t i n d i v i d u a l k o k a n e e a r e more l i k e l y  c o n t a i n r e l a t i v e l y l a r g e numbers o f pupae t h a n a r e trout.  individual  F o r e x a m p l e , d a t a p o o l e d from an e q u a l number o f  and k o k a n e e c a p t u r e d 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 o f c h i r o n o m i d pupae S m i r n o v two-sample t e s t , X p  = 27.03, degrees  <.001) t h a n t r o u t ( F i g u r e 3 ) .  trout  (Kolmogorov-  o f freedom =  A l t h o u g h o n l y 6% o f 163  2, trout  examined c o n t a i n e d more t h a n 40 c h i r o n o m i d 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 o r more pupae. more, 75 o f t h e 163  Further-  t r o u t examined c o n t a i n e d no pupae a t a l l  TABLE 2.  Month February April May June July August September October November December  S e a s o n a l e x p l o i t a t i o n o f c h i r o n o m i d pupae by t r o u t and k o k a n e e . N i s t h e s a m p l e s i z e and S.D. t h e s t a n d a r d d e v i a t i o n o f t h e means.  Mean number o f pupae p e r t r o u t 0.00 10.14 93.75 19.62 1.82 7.35 1.20 .31 3.92 0.00  S.D.  17.4 181.5 22.2 3.2 9.5 1.8 .9 9.2  -  Mean number o f pupae p e r kokanee .58 56.86 274.93 50.71 17.09 19.85 16.53 27.77 13.00 0.00  S.D. 1.0 36.3 246.0 55.9 21.0 32.6 19.8 22.0 21.4  -  N 12 7 16 21 11 40 15 13 24 4  D a t a p o o l e d from p r e d a t o r s c o l l e c t e d between 1963 and 1974 by H a m i l t o n ( u n p u b l i s h e d r e s u l t s ) , S a n d e r c o c k ( 1 9 6 9 ) , E f f o r d and Tsumura (1973) and H y a t t ( p r e s e n t s t u d y ) .  24  FIGURE 3.  A comparison  o f t h e p r o p o r t i o n s o f t r o u t and kokanee  that contain d i f f e r e n t q u a n t i t i e s of 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 k o k a n e e c a p t u r e d i n v a r i o u s months ( s e e T a b l e 2) between t h e y e a r s 1963 and 1974. o f t r o u t o r kokanee examined.  N = number  CO X  o < o  100  Ul  71 -  80  UJ  61  or a  51 5  N = 163  N = 163  9 I -100 90  co  KOKANEE  +  81 -  w  TROUT  -70 60  O z o  41-50  D£  31-40  X  o O a:  21 -  30  co 5  I I -  20  UJ  r  0 - 10  80  70  60  50  40  30 %  ; 20  10  0  OF PREDATORS  10  20  30  40  50  60  70  80  EXAMINED  to 0)  while  only  40 o f 163  kokanee examined f a i l e d  to  contain  c h i r o n o m i d 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 The  size-frequency  matched t r o u t and may  from t h e 1% o f  d i s t r i b u t i o n s of p r e y from s i z e -  kokanee are  be m i s l e a d i n g .  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  have i n d i c a t e d by number may  that  these s i z e s .  form as much as  50%  Thus, s m a l l d i f f e r e n c e s  f o r energy i n t a k e . proportion  of the  kokanee are  have c o n s i d e r a b l e  smallest  trout diet  food intake  a  by  of  significance greater  s i z e classes of  size classes  not  i n the  prey while  ( z o o p l a n k t o n ) more  s i z e s of prey e x p l o i t e d  e n t i r e l y due  to 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  examined f o r t h e  exploited.  Relative  proportions  o f the  by  i n the  W i t h i n a category of prey there are  A sample o f s i z e - m a t c h e d t r o u t and was  of  ( T a b l e 3) .  prey e x p l o i t e d .  1966  8%  only  Tsumura  i n s e c t s , s n a i l s , odonates, trichopterans)  Differences  differences  kokanee t a k e  total  Trout undoubtedly o b t a i n  kokanee e x p l o i t t h e intensively  of t h e i r d i e t  i n numbers a t t h i s end  of t h e i r d i e t from l a r g e  (terrestrial  8%  E f f o r d and  items c o n s t i t u t i n g only  t h e p r e y s i z e - d i s t r i b u t i o n may  and  s i m i l a r ( F i g . 4 ) , however, t h i s  Numerically trout obtain  t h e i r d i e t from p r e y o f  weight.  Items  type  of  frequently  i n d i v i d u a l s consumed.  k o k a n e e ( T a b l e 4)  s i z e - d i s t r i b u t i o n s of  from June,  amphipods  t o k o k a n e e , r a i n b o w t r o u t consume  large  trout  greater  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 size-frequency d i s t r i b u t i o n s of  a l l p r e y e a t e n by s i z e - m a t c h e d N = the t o t a l  t r o u t and k o k a n e e .  number o f p r e y i t e m s measured and  p o o l e d from a l l p r e d a t o r s examined. samples o f f i s h c o l l e c t e d and  August.  Data pooled  i n t h e months o f May,  from June,  KOKANEE N =  ~~i 40  r~ 30  1 20  1  1 10 %  OF  0 FOOD  1 10  ITEMS EATEN  1 20  1765  1 30  r 4(  TABLE 3.  Taxonomic c o m p o s i t i o n o f f o o d i t e m s 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  larvae  744  31.6  370  9.6  Chironomid  pupae  954  40.5  2,797  72.3  Chironomid  adults  278  11.8  97  2.5  76  3.2  511  13.2  37  1.0  23  .6  22  .6  13  .3  Zooplankton Pisidium sp. Terrestrial  insects  125  5.3  52  2.2  Trichoptera  46  2.0  Amphipoda  27  1.2  Odonata  20  .9  Other  34  1.4  Mollusca  Total  (snails)  items  2,356  3,870  28 c l o s e r examination o f the d a t a r e v e a l s t h a t t h i s  difference  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 o f amphipod  ( H y a l e l l a a z t e c a , s i z e s t o 4 mm)  k o k a n e e , and o f a l a r g e r amphipod s i z e s t o 10 mm)  (Crangonyx  by t r o u t ( T a b l e 4 ) .  richmondensis,  S i m i l a r l y , b e c a u s e kokanee  consume p r i m a r i l y t h e s m a l l b i v a l v e P i s i d i u m s p p . l e s s t h a n 2 mm)  by  (generally  and t r o u t c o n c e n t r a t e on p u l m o n a t e m o l l u s c s  ( H e l i s o m a and Menetus s p p . , s i z e s t o 10 mm),  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 f o r m o l l u s c s e a t e n by t h e p r e d a t o r s a r e significantly different  (Fig.5).  W i n t e r b o u r n * s (1971)  examination  o f 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 k o k a n e e , 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 caddis 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 species moulting t o the f i n a l  the s u c c e s s i o n o f  i n s t a r during the year.  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 t h o s e t h a t were  Kokanee 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 TABLE 4.  mm)..  The r e l a t i v e l e v e l o f 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 r i c h m o n d e n s i s by t r o u t and k o k a n e e .  No. examined Mean l e n g t h (cm) Range No. o f Crangonyx sp. eaten No. o f H y a l e l l a sp. eaten Hyalella/Crangonyx  Trout 12 14.9 10.5-16.7 96  Kokanee 9 15.1 12.8-16.5 21  89  77  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 ( C r a n g o n y x  s p . and H y a l e l l a  by s i z e - m a t c h e d t r o u t and k o k a n e e .  sp.) e a t e n  N = the t o t a l  number o f p r e y i t e m s p o o l e d f r o m a l l p r e d a t o r s examined.  D a t a o b t a i n e d from s a m p l e s o f f i s h  i n t h e month o f J u n e .  collected  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  s p . and Menetus sp.) e a t e n by s i z e - m a t c h e d  trout  and k o k a n e e .  N = t h e t o t a l number o f p r e y i t e m s p o o l e d  from a l l p r e d a t o r s e x a m i n e d . samples of f i s h c o l l e c t e d and  August.  D a t a o b t a i n e d from  i n t h e months o f May,  June  KOKANEE N = 41  i  1 60  1  r40  20 %  20  40  60  —r80  OF MOLLUSCS E A T E N  CO  o  31  Results i and  p r e s e n t e d so f a r have d e a l t m a i n l y w i t h  kokanee between 10  d i f f e r e n c e s are not s i z e range. collected  and  17  i n length.  However, d i e t a r y  r e s t r i c t e d s o l e l y to predators w i t h i n  S i z e - m a t c h e d , y o u n g - o f - t h e - y e a r t r o u t and  greater  ( z o o p l a n k t o n and  t h a n 95%  in diet  f a c t t h a t animals of both  species  o f t h e i r d i e t f r o m j u s t two  dipterans).  prey groups  Small t r o u t (average length  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 Sida c r y s t a l l i n a , s i g n i f i c a n t number o f d i p t e r a n s , few  water mites (Table 5 ) .  length  7.9  cm)  terrestrial  and  d i v e r s i t y of  a l m o s t no  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 w i t h i n 2-3  5.6  cm)  a  i n s e c t s and  By c o n t r a s t , s m a l l k o k a n e e  exploited a greater  l a r g e r numbers o f d i p t e r a n s  this  kokanee  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  c o m p o s i t i o n i n s p i t e of the obtained  cm  trout  a (average  zooplankton,  terrestrial  insects.  f i s h were sampled  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 -  habitats . Results  of size-frequency  a n a l y s i s of prey  (Fig.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  ( 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 s u c h as Bosmina s p . and s u c h as C y c l o p s sp.)  than t r o u t  prey copepods  do.  C o m p a r i s o n 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 V a r i o u s P r e y Types i n the N a t u r a l E n v i r o n m e n t and i n the D i e t s o f T r o u t and K o k a n e e . 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  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 o f p r e y i n M a r i o n Lake and  kokanee  apparent  are  availability  the p r e y e x p l o i t e d by e a c h s p e c i e s  of  32  TABLE 5.  Taxonomic c o m p o s i t i o n o f p r e y e a t e n 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 No. examined Mean l e n g t h  (cm)  Range Zooplankton Sida sp. Bosmina s p . Chydorus sp. Leptodora sp. Ceriodaphnia sp. Polyphemus s p . Alona sp. Cyclops sp. Mites sp. Ostracods  Kokanee  9  9  5.6  7.9  4.3-6.6  6.9-8.5  No. 1,219 2  %  No.  90.2 .1  184 166  1  .2  9  .7  31 16 39  2.3 1.2 2.9  6  .4  3 18  .2 1.3  %  5 1 2 2 291 5 2  19.7 17.8 1 . 1 .5 .1 .2 .2 31.2 .5 .2  103 124 24  11.0 13.3 2.6  9 3 2 1 2  1.0 .3 .2 .1 .2  Diptera Chironomid l a r v a e C h i r o n o m i d pupae Chironomid a d u l t s Other Amphipoda Trichoptera Odonata Ephemeroptera Terrestrial insects Total  1,345  933  33  FIGURE 7.  A comparison  of the size-frequency d i s t r i b u t i o n s of  w a t e r c o l u m n p r e y ( p r i m a r i l y z o o p l a n k t o n and c h i r o n o m i d pupae) e x p l o i t e d  by s i z e - m a t c h e d  t r o u t and k o k a n e e .  N = t h e t o t a l number o f p r e y i t e m s p o o l e d from a l l p r e d a t o r s examined. fish collected  D a t a o b t a i n e d from s a m p l e s o f  i n t h e same m i c r o h a b i t a t s and a t t h e  same t i m e o f d a y d u r i n g t h e month o f S e p t e m b e r .  TROUT N= 1345  3200 +  KOKANEE N= 933  2560-2720  3> 1920-2080 >UJ cr a.  o  1280-1440]  x — t o UJ  640-800  1u"  0-160  40  30  T  20  i  1  10  1  0  r  10  ~~T~ 20  30  "T  40  % OF PREY EATEN  Ul  u>  34 predator.  On an a n n u a l  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  b o t h t r o u t and k o k a n e e c o n s i s t s o f a r e l a t i v e l y prey types.  Furthermore,  s m a l l number o f  when compared t o t h e p r o p o r t i o n s o f p r e y  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 t h e i r w e i g h t s o r numbers  range o f prey  i n proportion to either  ( F i g s . 8 and 9 ) .  T h e r e a r e 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 o f 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 a b u n d a n t p r e y p r e s e n t i n the l a k e . of  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  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.  100 s p e c i e s o f c h i r o n o m i d s been f o u n d 1973) .  found  Out o f n e a r l y  i n M a r i o n L a k e , o n l y 19 have  i n t h e d i e t s o f t r o u t and k o k a n e e ( E f f o r d & Tsumura,  Some o f t h e most a b u n d a n t s p e c i e s by volume o r by number  do n o t a p p e a r i n t h e d i e t s o f f i s h a t a l l ( e . g . P e g a s t i e l l a 23% by no.; P h a e n o s p e c t r a  sp.A,  p r o t e x t u s & Polypedelium spp.,  29% by v o l . ) . The u t i l i z a t i o n o f many p r e y t y p e s b e a r s l i t t l e to  s e a s o n a l changes i n t h e i r apparent  availability.  relation  The i n t e n s e  u t i l i z a t i o n o f amphipods by t r o u t i n Aug. c o i n c i d e s w i t h  their  g r e a t e s t abundance ( T a b l e 6) b u t 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 t h e month o f J u n e when amphipod numbers a r e a t t h e i r  lowest l e v e l s . of  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  amphipod use a s compared t o a p p a r e n t  amphipod abundance.  The  peak u t i l i z a t i o n o f c h i r o n o m i d pupae o c c u r s i n May and J u n e f o r f i s h and by J u l y - A u g u s t t h e mean number o f c h i r o n o m i d pupae p e r stomach i n f i s h  i s l e s s t h a n one t e n t h t h e May-June v a l u e ( s e e  data i n Table 2 ) .  However, a l l t h e a v a i l a b l e e v i d e n c e  indicates  35  FIGURE 8.  A c o m p a r i s o n on an a n n u a l b a s i s o f t h e  relative  p r o p o r t i o n s o f prey i n the environment  and  o f t r o u t from M a r i o n L a k e , B.C. (b) % by w e i g h t . abscissa.  i n the d i e t  (a) % by number,  Note t h e c h a n g i n g s c a l e a l o n g t h e  D a t a 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 o f r e l a t i v e abundance.  prey  35a TROUT  ENVIRONMENT 1. SIALIS 2. EPHEMEROPTERA  2.  3. ODONATA  3.  4. OLIGOCHAETES 5. TRICHOPTERA  5.  6. CERATOPOGONIDS  6.  7. HIRUDINEA  .  8. PLANORBIDAE  8.  9. PIS1DIUM  9.  10. CHIRONOMID PUPAE  10.  11. AMPHIPODA  II.  12. CHIRONOMID LARVAE  12.  13. MEIOFAUNA  13.  loo  10  10  100  r  %  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  t i l l  20  40 60  %  NOTE: LOG SCALE ON ABSCISSA  BY WEIGHT'  36  FIGURE 9.  A c o m p a r i s o n on an a n n u a l b a s i s o f t h e 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 t h e  d i e t o f kokanee from M a r i o n L a k e , B.C.  (a) % by  number, (b) % by w e i g h t . along the a b s c i s s a .  Note t h e c h a n g i n g  scale  D a t a 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 o f r e l a t i v e prey abundance.  36a ENVIRONMENT  KOKANEE  1. SIALIS 2. MAYFLY 3. ODONATES  3.  4. OLIGOCHAETES  4.  5. CADDIS  5.  6. CERATOPOGONIDS  6.  7. LEECHES 8. PLANORBIDS 9. PISIDIUM  9.  10. CHIRONOMID R •  10.  11.  AMPHIPODS  II.  1.2. CHIRONOMID L.  12.  13. MEIOFAUNA  13.  "T  ~T~  T  100  "ioo  10  10  %  BY NUMBER  1. CHIRONOMID P. 2. PISIDIUM  2.  3. MEIOFAUNA  3.  4. CERATOPOGONIDS 5. EPHEMEROPTERA  5.  6. OLIGOCHAETES  6.  7.  AMPHIPODS  8. SIALIS 9. ODONATES 10.  10. CADDIS  II.  11. PLANORBIDS  12.  12. CHIRONOMID L. 13. HIRUDINEA  13. l l' I  l  60 40  I  20  10  TT  I %  NOTE •• LOG SCALE  ON ABSCISSA  I BY WEIGHT  II  H i l l  10  I  20  T—l  I |  4 0 60  37  t h a t t h e number o f c h i r o n o m i d s p u p a t i n g  and e m e r g i n g i n J u l y and  Aug. i s a s g r e a t o r g r e a t e r t h a n t h a t r e c o r d e d  i n May and J u n e  ( E f f o r d & Tsumura, 1 9 7 3 ; H a m i l t o n , 1 9 6 5 ; M c C a u l e y , p e r s . Many o f t h e d i f f e r e n c e s between p r e y i n c l u d e d  comm.). i n the  d i e t and t h o s e r a n d o m l y sampled from t h e l a k e a r e r e l a t e d t o organism s i z e .  I have a l r e a d y m e n t i o n e d t h a t when c a d d i s  are abundant as s m a l l , e a r l y , i n s t a r s ; t h e l a r g e f o u r t h and f i f t h  t r o u t continue  i n s t a r stages  larvae  to exploit  o f l e s s abundant s p e c i e s .  The same p a t t e r n emerges from a c o m p a r i s o n o f t h e amphipods e x p l o i t e d by t r o u t compared t o t h o s e p r e s e n t  i n the environment  ( F i g . 1 0 a ) . D u r i n g J u n e , t h e l a r g e s t s i z e c l a s s e s o f amphipods a r e many t i m e s more a b u n d a n t i n t h e s t o m a c h s o f t r o u t t h a n e x p e c t e d from t h e i r r e l a t i v e abundance i n t h e l a k e p o p u l a t i o n .  F o r kokanee  t h e 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 proportion of small  amphipods.  TABLE 6.  Mean number o f amphipods p e r m v e r s u s t h e mean number p e r p r e d a t o r i n d i f f e r e n t months o f t h e y e a r . Feb.  April  June  Aug.  Nov.  1594  1724  1100  2478  1657  .25  2.79  5.69  5.67  .50  12  14  24  45  18  Amphipods/Kokanee  .38  2.14  1.92  .28  2.58  Kokanee examined  18  7  12  24  19  Amphipods Amph i p o d s / T r o u t T r o u t examined  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 predators.  i n t h e e n v i r o n m e n t and i n t h e d i e t s o f t h e D a t a f o r s i z e s o f amphipods  present i n  the e n v i r o n m e n t were o b t a i n e d f r o m s a m p l e s d u r i n g J u n e o f 1969.  taken  D a t a f o r s i z e s o f amphipods  the d i e t s o f p r e d a t o r s were o b t a i n e d f r o m s a m p l e s taken  i n June o f  1966.  in  38a  ENVIRONMENT  TROUT  N = 560,  N = 209  1969  ,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 %  OF TOTAL AMPHIPODS PRESENT  "i—i—i—r 40 50  39  DISCUSSION A v a r i e t y o f c o m p a r i s o n s p r e s e n t e d above i n d i c a t e the  that  i n d i v i d u a l " s a m p l i n g " a c t i v i t i e s o f t r o u t , kokanee and  scientists  i n M a r i o n Lake e a c h r e s u l t i n t h e a c q u i s i t i o n o f  d i f f e r e n t sets of invertebrate "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, w h e r e , and how i n v e r tebrates  are obtained  f r o m t h e l a k e e n v i r o n m e n t by e a c h o f t h e s e  "predators". I t should  n o t be p a r t i c u l a r l y  s u r p r i s i n g that the  r e l a t i v e abundance o f i n v e r t e b r a t e p r e y t y p e s i n t h e d i e t s o f t r o u t and k o k a n e e do n o t c l o s e l y f o l l o w t h e a p p a r e n t abundance o f t h e s e same i n v e r t e b r a t e s  i n the lake environment given  that the  c h a r a c t e r i s t i c s o f t h e " s a m p l i n g g e a r " and p r o c e d u r e s used by s c i e n t i s t s and p r e d a t o r s  a r e i n h e r e n t l y so d i f f e r e n t . F o r  e x a m p l e , i t i s most o f t e n t h e c a s e t h a t man-made s a m p l i n g devices  a r e d e s i g n e d and employed t o o b t a i n s a m p l e s o f a w i d e  range o f i n v e r t e b r a t e s  i n proportion  i n the n a t u r a l environment.  to their actual densities  Thus p l a n k t o n  used i n l a k e s a r e r e l a t i v e l y n o n - s e l e c t i v e acquire  nets or benthic  grabs  i n t h a t they tend t o  t h e m a j o r i t y o f o r g a n i s m s w i t h i n t h e s p a c e 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  s u c h a s t r o u t and k o k a n e e  have been " d e s i g n e d " t h r o u g h 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 o f the t o t a l range o f i n v e r t e b r a t e prey that r e s i d e i n a given  e n v i r o n m e n t and may f r e q u e n t l y  acquire  some p r e y i t e m s r a t h e r t h a n 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 r e s p o n s e s , armour, t e x t u r e o r t a s t e .  Furthermore, while  the sampling  protocol  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 predator w i l l  shift  c o n t i n u o u s l y i n response  general environmental it  i s important  of prey  f i x e d , t h a t o f any t o s l i g h t changes i n  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 .  t o k e e p i n mind t h a t t h e a p p a r e n t  Thus,  availability  t y p e s a s 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  mechanical  device w i l l  availability  o f prey  o f t e n c o n s t i t u t e a poor index o f the  to a given  predator.  B e c a u s e t h e r e a r e s o many o b v i o u s d i f f e r e n c e s between t h e d e s i g n and o p e r a t i o n o f man-made s a m p l i n g " d e s i g n " and s a m p l i n g  procedures  p r e d i c t w i t h some c e r t a i n t y  d e v i c e s and t h e  of predators, i t i spossible to  t h a t d i f f e r e n c e s i n the prey  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 variety of predator c h a r a c t e r i s t i c s  sets  be t h e r e s u l t o f a  t h a t f u n c t i o n d u r i n g each  stage o f t h e 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 prey.  By c o n t r a s t , d i f f e r e n c e s i n t h e " d e s i g n " and o p e r a t i o n o f  t r o u t and k o k a n e e a s p r e d a t o r s a r e n o t a t a l l o b v i o u s  and t h u s  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 a b o u t t h e i d e n t i t y and relative  importance  of various predator c h a r a c t e r i s t i c s  w i l l promote t h e a c q u i s i t i o n  which  of species s p e c i f i c d i e t s .  For  e x a m p l e , i t i s n o t c l e a r w h e t h e r t r o u t and k o k a n e e i n t h e 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 different locations,  search f o r prey a t d i f f e r e n t  f o r p r e y by u s i n g d i f f e r e n t  t e c h n i q u e s , respond  once d e t e c t e d , a p p r o a c h o r c a p t u r e p r e y by u s i n g  times,  to different  factors  The p o t e n t i a l  prey  different  t e c h n i q u e s , o r choose t o i n g e s t o r r e j e c t d i f f e r e n t prey captured.  search  once  e x i s t s f o r any one o r a l l o f t h e s e  to c o n t r i b u t e t o the observed  p a t t e r n s o f prey  acquisition,  41  t h u s any a t t e m p t prey  t o 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  s e t s a c q u i r e d by t h e s e p r e d a t o r s i n t h e f i e l d must e n t a i l a  systematic enquiry  i n t o t h e r o l e t h a t each o f 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 a p p r o a c h t o a s s e s s t h e mechanisms t h a t f a v o u r 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 by p r e d a t o r s h a s n o t 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 studying the d i e t a r y habits of animals.  Having  established the  e x i s t e n c e o f 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 s i m p l e h y p o t h e s e s a s e x p l a n a t i o n s f o r t h e pattern.  The most common o f t h e s e a r e (1) t h a t a v a i l a b i l i t y ( i n  the sense o f p h y s i c a l p r o x i m i t y ) i s t h e main f a c t o r which  determines  t h e s e t o f p r e y 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 r e i n c r e d i b l e g e n e r a l i s t s that simply accept  food items as they  a p p e a r o r ( 2 ) t h a t a c t i v e s e l e c t i o n and a v o i d a n c e by p r e d a t o r s c o n t r o l s t h e s e t o f p r e y a c q u i r e d .  o f food  items  This implies that  p r e d a t o r s a r e phenomenally choosy about t h e items t h a t they e a t i n an e n v i r o n m e n t where a g r e a t d i v e r s i t y o f p r e y a r e s i m p l y for  the taking.  By t h e m s e l v e s ,  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 r e p e a t e d qualification Hutchinson,  there  expression  without  ( A l l a n , 1 9 4 2 ; Houde, 1967; S i e f e r t , 1968;  1 9 7 1 ; Cody, 1974 and E n g e l , 1976 t o name b u t a few)  has a c c o m p l i s h e d  l i t t l e other than t o obscure  the nature o f the  a c t u a l r a n g e o f mechanisms t h a t do c o n t r o l t h e non-random a c q u i s i t i o n o f p r e y by p r e d a t o r s i n t h e f i e l d . In  p r e v i o u s work ( H y a t t , 1 9 7 9 ) , I have s t r e s s e d t h a t  e a c h phenomenon o f non-random e x p l o i t a t i o n o f p r e y  s h o u l d 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 mechanisms  42  operating alone o r i n concert.  Here t h e 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 o f p r e y by s i z e w i l l Caddis l a r v a e ( T r i c h o p t e r a ) found  s e r v e a s an e x a m p l e .  i n the d i e t o f t r o u t are  c o n s i d e r a b l y l a r g e r t h a n t h o s e e x p l o i t e d by k o k a n e e .  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 a r e c a p a b l e o f h a n d l i n g prey than kokanee.  larger  This inference receives circumstantial  from t h e f a c t t h a t t o a l l o u t w a r d  appearances  support  trout are heavier-  b o d i e d and have l a r g e r j a w s t h a n kokanee o f s i m i l a r l e n g t h . ever, o t h e r hypotheses  p r o v i d e competing  How-  explanations f o r the  same d i e t a r y p a t t e r n . F o r e x a m p l e , n o t o n l y a r e t h e 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 t h a n t h o s e o b t a i n e d by kokanee b u t t h e y a l s o belong  t o s p e c i e s which  live primarily  t h e l a k e s h o r e ( W i n t e r b o u r n , 1971) . kokanee a r e r e s t r i c t e d lake.  i n weed beds a l o n g  The s p e c i e s e x p l o i t e d by  t o open s e d i m e n t  areas throughout the  T h u s , t h e 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 t h e p r e d a t o r s in  t u r n e x p o s e s 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 o f p r e y .  competing  hypotheses  which Similar  a r e 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 t h e  s p e c i e s - s i z e p a t t e r n s o f 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 a u t h o r s have s u g g e s t e d  b o t h p r e d a t o r s and p r e y from t h i s problem  but this w i l l  prey are r e l a t i v e l y  t h a t the sampling o f  identical locations eliminates  o n l y be t h e c a s e where p r e d a t o r s and  immobile  over a p e r i o d o f time t h a t i n c l u d e s  t h e most r e c e n t f o r a g i n g b o u t .  In the present study, as w e l l as  i n many o t h e r s , t h i s a s s u m p t i o n  i s seriously violated.  Predators  s u c h a s t r o u t and k o k a n e e a r e 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 n o t g u a r a n t e e  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 The d i f f i c u l t y for  from t h a t  location.  o f competing hypotheses as e x p l a n a t i o n s  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 u n i q u e t o t h i s  T i n b e r g e n (1960) e x p l a i n e d  study.  t h e non-random a c q u i s i t i o n o f p r e y  by  i n s e c t i v o r o u s b i r d s i n p i n e w o o d s as a c o n s e q u e n c e o f t h e f o r m a t i o n of a " s p e c i f i c search  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 m i g h t l e a r n t h e k e y 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 e n c o u n t e r s w i t h i t and t h e n b e g i n  exclusively would tend  f o r those to overlook  relatively rare. his  cues.  observations  other  to search  By c o n c e n t r a t i n g on one p r e y f o o d s , e s p e c i a l l y when t h e y  they were  Royama (1970) p o i n t e d o u t t h a t on t h e b a s i s o f c e r t a i n features of the b i r d s behaviour  i n c o n s i s t e n t with the search  image h y p o t h e s i s  and t h a t  were identical  d i e t a r y p a t t e r n s c o u l d be p r o d u c e d i f t h e b i r d s p o s s e s s e d p a r t i c u l a r f o r a g i n g movements w h i c h w o u l d 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 search  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 ( s e e K r e b s , 1973  for d i s c u s s i o n ) . From t h e s e  few e x a m p l e s , i t s h o u l d  be c l e a r t h a t  field  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 a r e most v a l u a b l e as s o u r c e s  of  h y p o t h e s e s a b o u t t h e b i o l o g i c a l mechanisms t h a t a r e i n v o l v e d i n c o n t r o l l i n g prey  acquisition.  and  s t u d i e s must be t h e m a j o r s o u r c e s  experimental  However a d d i t i o n a l o b s e r v a t i o n a l of c r i t i c a l  evidence t o t e s t the merit of p a r t i c u l a r explanations o r interpretations of a pattern. process  To some i t may  seem a  tedious  t o e l i m i n a t e o r i d e n t i f y t h e 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  exploitation,  b u t u n l e s s we u n d e r s t a n d t h e 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  their significance i n others. In order t o 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 t h e 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 M a r i o n Lake I w i l l , o v e r  the course  o f the next  few c h a p t e r s ,  examine where t r o u t and k o k a n e e c h o o s e t o f o r a g e , when t h e y t o f o r a g e , how t h e y how e x p e r i e n c e  search  f o r p r e y , how t h e y a t t a c k p r e y and  i n encountering  foraging behaviour  choose  v a r i o u s prey a l t e r s the predators  on s u b s e q u e n t o c c a s i o n s .  SUMMARY 1.  T r o u t 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 s e a s o n a l  b a s i s , even though  t h e 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 h a b i t a t s and a p p a r e n t l y had a c c e s s food  items 2.  from t h e same  t o an i d e n t i c a l s e t o f  ( F i g . 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 a r e f r e q u e n t l y r e l a t e d  to differences  i n m o r p h o l o g y 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 t h e s i z e s o f prey  (Figs. 4 t o 7). 3.  Predator  r e s p e c t t o prey  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  t y p e s t h a t have many f e a t u r e s i n common ( e . g .  t h e two s p e c i e s o f a m p h i p o d s , T a b l e  4 , F i g . 5; v a r i o u s  o f m o l l u s c s , F i g . 6 ; t h e many s p e c i e s o f c h i r o n o m i d s ) .  species  45  4. simply  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 r e l a t e d to d i f f e r e n c e s  kokanee a r e  i n the r e l a t i v e s i z e s o f  not  the  predators. 5. "density  T r o u t and  kokanee e x h i b i t pronounced p a t t e r n s  i n d e p e n d e n t " e x p l o i t a t i o n o f p r e y from t h e  of prey t h a t i s apparently ( F i g s . 8 and 6.  a v a i l a b l e i n the  F i e l d descriptions of d i e t a r y patterns  involved 7.  be  total  environment  a r e most  b i o l o g i c a l mechanisms  that  B e c a u s e e a c h 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  alone or together,  there  i d e n t i t y and  mechanisms t h a t a r e pattern.  valuable  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 .  f a v o u r e d 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 s s e s s the  complex  9).  as s o u r c e s o f h y p o t h e s e s a b o u t t h e are  natural  of  i s generally  a need t o  act  systematically  r e l a t i v e importance of the p a r t i c u l a r  responsible  f o r promoting a given  dietary  may  CHAPTER 3  THE RELATIONSHIP BETWEEN SPATIAL SEGREGATION, TEMPORAL SEGREGATION, AND DIETARY PATTERNS o f TROUT AND KOKANEE IN MARION LAKE INTRODUCTION S c h o e n e r (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 t h e most f r e q u e n t f o r m o f e c o l o g i c a l i n t e r r e s t r i a l c o m m u n i t i e s and  segregation  he p o i n t e d o u t t h a t  relatively  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 of 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 a u t h o r s  ( 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 w i d e t o l e r a n c e o f h a b i t a t t y p e and  thus  display extensive overlap while others K e a s t , 1970;  M o y l e , 1973;  w e l l developed  spatial  v e r t i c a l height i n  should  ( N i l s s o n , 1960,  Werner e t a l . , 1977)  have  1967;  observed  segregation along g r a d i e n t s of  depth,  t h e w a t e r c o l u m n , and v e g e t a t i o n s t r u c t u r e .  B e c a u s e many o f t h e p r e y 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 i n Marion  Lake ( e g . w a t e r c o l u m n , d e e p  shallow-water  sediments,  by t h e p r e d a t o r s  -water-sediments,  weed b e d s ) , h a b i t a t s e g r e g a t i o n  i s t h e f i r s t mechanism w h i c h may  to produce s p e c i e s s p e c i f i c  dietary patterns.  t h e f i r s t h y p o t h e s i s t h a t I have a t t e m p t e d spatial  habitats  operate  Therefore  to test i s that  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 e x h i b i t e d by t r o u t and Park  p a t t e r n s of prey  exploitatation  kokanee.  (1941) f i r s t d e v e l o p e d  the i d e a t h a t  activity  p e a k s o c c u r r i n g a t d i f f e r e n t t i m e s o f a 24 h o u r day c a n p r o d u c e a t y p e o f symmetry i n a n i m a l c o m m u n i t i t e s , some s p e c i e s b e i n g 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 r e known t o v a r y w i d e l y f o r b o t h m a r i n e water  fish  ( C a r l a n d e r & C l e a r y , 1949;  E n g e l & Magnuson, 1 9 7 6 ) .  (Hobson, 1972)  K e a s t , 1970;  activity  and  Emery,  fresh1973;  Aquatic invertebrates also e x h i b i t  con-  siderable 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;  1962;  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  Elliot,  1968) .  s y s t e m s ( B i d e r , 1962)  i n d i c a t e s t h a t t h e y have a c t i v i t y  c o i n c i d e w i t h those of t h e i r  cycles  t h a t t h e t i m i n g o f p r e d a t o r and  (1974) l e a v e s no d o u b t  invertebrate a c t i v i t y  i n f l u e n c e t h e p a t t e r n s o f p r e y a c q u i s i t i o n by f i s h .  cycles  t a k e n a t 1600  nymphs f o u n d of  of  bluegills  o b t a i n e d a t 0700  t o the o b s e r v a t i o n t h a t mayfly  i n t h e l a k e were p r e d o m i n a n t l y a c t i v e a t n i g h t .  the p o t e n t i a l importance of a c t i v i t y  considered  in this  c y c l e s , the second  Because  hypothesis  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  p r e d a t o r and p r e y a c t i v i t i e s p r o d u c e d i f f e r e n c e s i n t h e f o o d  found  i n t r o u t , kokanee and  the  that  h o u r s b u t t h a t t h e nymphs had  become t h e most i m p o r t a n t f o o d i t e m i n b l u e g i l l s This p a t t e r n corresponded  can  They f o u n d  no E p h e m e r o p t e r a ( m a y f l y nymphs) were p r e s e n t i n s a m p l e s o f  hours.  which  major prey.  A s t u d y by Baumann & K i t c h e l l  (Lepomis m a c r o c h i r u s )  Waters,  environment.  sequence items  METHODS Patterns of Habitat Depth  Occupation  Distribution I recorded  i n 120 and  the d i s t r i b u t i o n of f i s h  i n M a r i o n Lake  h o u r s o f d i v i n g d u r i n g t h e months o f May,  A u g u s t o f 1971  and  1972.  June, J u l y  Salmonids i n M a r i o n 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 b u t  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 o u t at s p e c i f i c stations  ( F i g . 11).  I made o b s e r v a t i o n s t h r o u g h o u t t h e  ( 0 5 0 0 - 2 1 0 0 h o u r s ) and d u r i n g any one  each s t a t i o n r e c e i v e d equal  s e r i e s of  e q u i p p e d 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 , reference-markers  a r e a and  graduated  H o r i z o n t a l markers enclosed  a one  a t 5 cm i n square meter  a f o u r s q u a r e m e t e r a r e a on t h e l a k e b o t t o m .  m a r k e r s e x t e n d e d f r o m d i a g o n a l l y opposed c o r n e r s o f bottom g r i d  t o the s u r f a c e .  could estimate of those  t h e s i z e , s p e c i e s i d e n t i t y and  f i s h p a s s i n g between and  t h e r e f o r e encompassed 4, 8, 12  over  this surface  vertical  position  the v a r i o u s markers. 4 m of water  and  & 16 c u b i c m e t e r s o f w a t e r .  d i m e n s i o n s o f the l a r g e s t s t a t i o n  (2 X 2 X 4 m)  s m a l l enough t h a t u n d e r t h e most u n f a v o r a b l e low l i g h t and  Vertical  A d i v e r p o s i t i o n e d a t the  S t a t i o n s i n c l u d e d d e p t h s o f 1, 2, 3, and  The  effort  observations.  E a c h s t a t i o n was  tervals.  day  were  conditions of  high t u r b i d i t y , a l l f i s h passing over  between t h e r e f e r e n c e m a r k e r s c o u l d be c o u n t e d and  and identified  A c o n t o u r map  o f M a r i o n Lake i n d i c a t i n g t h e  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  A, B, C and D r e p r e s e n t  sites.  t h e 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  respectively.  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 s o u t h locations  respectively.  trap-net  Lake  Outlet  vo 0»  to species.  T h i s ensured t h a t t h e v i s u a l s i g h t i n g s would  n o t be i n f l u e n c e d by d i u r n a l o r s e a s o n a l  changes i n t h e  detectability of f i s h . The  observation  procedure consisted of a d i v e r  moving i n t o p o s i t i o n a t t h e 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 m i n u t e s t o compensate f o r any d i s t u r b a n c e o f a n i m a l s i n t h e v i c i n i t y , and t h e n r e c o r d i n g a set of observations  f o r twenty minutes.  standard  I recorded  observa-  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 recorder  (Rustrak)  modified  f o r underwater  operation, f o r a continuous record of observations,  i n 1972.  I scored  i n 1971  t h e 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  to the nearest  5 cm i n t e r v a l , w h i l e  a n i m a l s as s u r f a c e  i n 1972 I  designated  ( w i t h i n 50 cm o f t h e w a t e r s u r f a c e ) ,  b o t t o m ( w i t h i n 50 cm o f t h e b o t t o m ) , o r m i d w a t e r . D i s t r i b u t i o n With Area The to detect  l o c a t i o n o f u n d e r w a t e r s t a t i o n s made i t p o s s i b l  some c h a n g e s i n t h e 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 o v e r t h e c o u r s e o f t h e 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 o p e r a t e d a t i n t e r v a l s t h r o u g h t h e p e r i o d May,1974 - May,1975. i n 1-2 m o f w a t e r a t t h e s o u t h  One t r a p was l o c a t e d  end o f t h e l a k e and t h e o t h e r  i n 1-2 m o f w a t e r a t t h e n o r t h end o f t h e l a k e , The t r a p s were i d e n t i c a l and  operation.  (Fig 11).  i n every respect o f c o n s t r u c t i o n  Thus, I a c c e p t e d d i f f e r e n c e s i n t h e p r o p o r -  t i o n s o f a n i m a l s c a u g h t a t t h e two l o c a t i o n s a s i n d i c a t i v e  51  of d i f f e r e n c e s i n s p a t i a l  distribution.  I n a d d i t i o n t o t h e t r a p p i n g and I s p e n t i n e x c e s s o f 100 collecting  hours i n a boat equipped w i t h  a r e n o t q u a n t i t a t i v e , my  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  agreement w i t h t h e d i s t r i b u t i o n s sampling  general  a t these  times  lights, Although  impression is in  i n d i c a t e d by q u a n t i t a t i v e  techniques.  Patterns of  Activity  Two  c o n d i t i o n s must be met  strongly influence differential  t r o u t and  kokanee.  Trout  and  different a c t i v i t y patterns  before  activity  a c t i v i t y o f one the o t h e r .  cycles  prey e x p l o i t a t i o n  by  k o k a n e e must e x h i b i t m a r k e d l y  (eg. n o c t u r n a l vs d i u r n a l )  t h e a c t i v i t y o f some p r e y g r o u p s must c o i n c i d e w i t h  activity  observations,  f i s h from a l l a r e a s o f t h e l a k e a t n i g h t .  these o b s e r v a t i o n s  will  underwater  and  the  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 t h a n w i t h  B e c a u s e t h e r e was  no  information concerning  the  c y c l e s of e i t h e r f i s h or i n v e r t e b r a t e s i n Marion  L a k e , 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 examination  of prey a c t i v i t y I analyzed  data  as a p r e r e q u i s i t e t o  any  cycles.  from g i l l - n e t c a p t u r e s  and  had  m o f w a t e r n e a r t h e e a s t e r n edge o f  i n 1-4  the l a k e to c a t c h f i s h . mesh s i z e s ( 2 . 5 , 3.75  and  The 5.0  Monofilament  fish  c o m p l e t e d between 1963 been p l a c e d  1966.  of  nets contained cm).  The  gill-nets  sections of  three  n e t s were examined  e v e r y two  hours f o r a t w e n t y - f o u r hour p e r i o d  in "activity"  c o u l d be d e t e c t e d .  and  November  April  1966,  May  1964,  from f i x e d s t a t i o n s  d a t a t o c h e c k 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 a c t i v i t y r a t h e r t h a n o f some o t h e r ability  was  1963.  Direct v i s u a l observations i n the l a k e p r o v i d e d  0  A t w e n t y - f o u r hour s e t  c o m p l e t e d f o r e a c h o f F e b r u a r y 1964, A u g u s t 1963  so t h a t c h a n g e s  to v i s u a l l y detect  and  foraging  f a c t o r (eg. changes i n  avoid  g i l l - n e t s under  conditions  of 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  gill-net  c a p t u r e s i n d i c a t e d a t e n d e n c y f o r kokanee t o be more a c t i v e than t r o u t during  t h e l a t e e v e n i n g and  night.  This  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 m i g h t w e l l p r o d u c e some o f t h e d i f f e r e n c e s i n p r e y e x p l o i t a t i o n by  fish i f activity  l e v e l s o f some p r e y g r o u p s 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 k o k a n e e more c l o s e l y . item by  i n the d i e t s o f t r o u t and  the p r e d a t o r s  lake.  C h i r o n o m i d pupae a r e an  The  during  kokanee and  they are  obtained  t i m e s o f a d u l t emergence f r o m  the  emergence o f a d u l t s f r o m 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 p l o i t a t i o n o f pupae by  t r o u t and  kokanee ( T a b l e  a l i k e l y p a t t e r n t o be p r o d u c e d by a s y n c h r o n o u s c y c l e s . To  important  test this possibility,  ex2.)  seemed  activity  I t o o k s a m p l e s t o document  the temporal  s e q u e n c e o f p u p a l emergence o n s e l e c t e d  d u r i n g 1972. A bow-amounted n e t ( o p e n i n g  dates  1 m X l m , mesh s i z e  2 0 0 y m ) was pushed a l o n g j u s t b e l o w t h e s u r f a c e i n f r o n t of a boat f o r 1 minute. a t each t i m e .  T h r e e r e p l i c a t e h a u l s were  Samples were p r e s e r v e d  sorted a t a l a t e r  taken  i n 7 0 % e t h a n o l and  date.  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 distributions  (bottom, midwater, surface) o f f i s h  1971-72 ( T a b l e 7 ) .  during  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 t h e p r e d a t o r s  ( H y a t t , M s ) , I have d e -  s i g n a t e d o b s e r v a t i o n s a s e i t h e r e a r l y summer o r l a t e summer b a s e d upon t h e r a n g e o f s u r f a c e t e m p e r a t u r e s The  encountered.  d e p t h d i s t r i b u t i o n s o f t r o u t ( T a b l e 7a & b)  a r e 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 1956)  f o r two i n d e p e n d e n t s a m p l e s ,  Siegel,  w h i l e t h e d e p t h d i s t r i b u t i o n s o f kokanee a r e n o t 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 t i o n s , b u t a r e s i g n i f a n t l y d i f f e r e n t between t h e s e  observa-  periods  ( \, , P < .001, d f = 2 ) . Due t o t h e s m a l l numbers o f k o k a n e e 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 p o o l e d obtained  u n d e r e a r l y summer c o n d i t i o n s and t h o s e  f o r late  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 d e p t h b u t i o n s o f t r o u t compared w i t h k o k a n e e .  the data  distri-  Depth d i s t r i b u t i o n s  TABLE 7.  a.  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 = t h e t o t a l number o f s i g h t i n g s .  E a r l y Summer  Date and Range of surface temperature  Trout  Kokanee  ; B  % M  % S  May 2 6 - J u l y 6/71 10-17 °C  73  23  4  May 24-31/72 10-16 *C  12  44  June 8-28/72 2 12-17 °C  43  A l l dates  % B  % M  % S  N  306  9  24  67  303  44  363  17  50  33  6  35  22  113  13  17  70  23  41  34  25  782  9  24  67  332  J u l y 29-Aug.l/71 23-27 °C  89  10  1  582  74  25  1  276  J u l y 18-28/72 19-25 °C  31  60  9  233  100  Aug.3-11/72 19-23 °C  62  28  10 1189  72  24  4  25  A l l dates  66  26  8 2004  76  23  1  316  b.  pooled  N  L a t e Summer  pooled  of t r o u t and kokanee d i f f e r s i g n i f i c a n t l y  (X , p <.001, df 2  =2) under both e a r l y and l a t e summer c o n d i t i o n s .  In e a r l y  summer kokanee occupy p r i m a r i l y surface h a b i t a t s while t r o u t occupy bottom ones.  In l a t e summer there are more t r o u t  than kokanee i n surface h a b i t a t s although  q u a l i t a t i v e l y trout  and kokanee e x h i b i t s i m i l a r depth d i s t r i b u t i o n s (Table 7b). D i s t r i b u t i o n With Area Proportions of t r o u t and kokanee observed water s t a t i o n s were not equal  ( F i g .12). Under e a r l y summer  c o n d i t i o n s i n both 1971 and 1972 I observed of t r o u t at shallow  a t under-  the m a j o r i t y  (1-2 m) , inshore s t a t i o n s , while the  g r e a t e s t number of kokanee were always observed (3-4m) s t a t i o n s f a r t h e s t o f f s h o r e .  a t the deep  Thus, t r o u t and kok.anee  e x h i b i t a remarkable degree o f segregation.  An increase  i n overlap of areas occupied by the two species under l a t e summer c o n d i t i o n s ( F i g . 12c) i s l a r g e l y the consequence o f a pronounced seasonal change i n areas occupied Under e a r l y summer observed  by t r o u t .  c o n d i t i o n s , fewer than 30% of a l l t r o u t  occurred a t o f f s h o r e s t a t i o n s but under l a t e summer  c o n d i t i o n s 48% o f a l l t r o u t occurred there (compare f i g . 12a with 12c). D i f f e r e n c e s i n the proportions o f f i s h detected at v a r i o u s l o c a t i o n s were not r e s t r i c t e d only to s t a t i o n s i n d i f f e r e n t depths of water.  The north and south  trap-nets  were l o c a t e d a t s i m i l a r depths (1.5m) and there were s i g n i f i c a n t d i f f e r e n c e s i n the proportions o f predators  captured  56  Figure  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 k o k a n e e o b s e r v e d a t o n s h o r e  (stations  A and B) and o f f s h o r e ( s t a t i o n s C and D) i n Marion Lake. observed during  N = the t o t a l  locations  number o f f i s h  a given set of observations.  Numbers i n b r a c k e t s  i n d i c a t e the range o f  lake surface temperatures attained each o b s e r v a t i o n s e t .  during  OBSERVATION STATION  > I  CO I  o I  AND THEIR ASSOCIATED WATER COLUMN D E P T H S > I -  o I  - ••". .». CD O  LOCATIONS  M  o CD  co Ul  co i . ro  o i  (m)  o i  OJ  CD O  H 33 O  Z II  c  OJ  o  o  ro O'  ro  —I  33  5p  ro co - i  o  2  ro o ->i -  z c  o  s? 1  CD  CD  ro  ro  o 2  o  o  cr CD  -M"  rn  w  *_  CD  Z  £ £  o m m  o  X  »• 5 *  i  m m  o Ul  CO  o  co  o  i n t h e s e n e t s d u r i n g a number o f s a m p l i n g 8a & b ) . end  intervals  I n g e n e r a l kokanee were more a b u n d a n t a t t h e  of the l a k e .  The  single exception to t h i s trend  u n d e r l a t e summer c o n d i t i o n s when the s o u t h n e t more kokanee t h a n t h e n o r t h n e t . presence  (Table north  occurred  captured  I a t t r i b u t e t h i s to  the  o f a few h u n d r e d k o k a n e e a t t h i s t i m e , i n a s p r i n g  l e s s t h a n 50 m away from t h e s o u t h n e t . a r e n o t as c l e a r .  On  two  occasions  Patterns for trout  t h e r e were more t r o u t  c a p t u r e d a t the n o r t h n e t and on two o t h e r s t h e r e were s i g n i f i c a n t d i f f e r e n c e s (Table Diel A c t i v i t y Patterns of  no  8b).  Predators  G i l l - n e t c a p t u r e s o v e r 24 h o u r s s u g g e s t  that neither  t r o u t o r kokanee a r e 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 (Fig. 13).  When c a p t u r e s a r e summed o v e r a l l s e a s o n s ,  there  i s c l e a r l y a t r e n d f o r k o k a n e e t o be most " a c t i v e " a t n i g h t and  f o r t r o u t t o be most " a c t i v e "  i n the day.  c o m p a r i s o n s ( F i g . 13a & b) s u g g e s t  Between s e a s o n  similar activity  patterns  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  trout.  I t i s necessary before attempting  any  t o examine two c r i t i c a l  i n t e r p r e t a t i o n o f the  assumptions  significance  of these r e s u l t s f o r p a t t e r n s of 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 capture 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 t o t h a t u n d e r n o c t u r n a l c o n d i t i o n s and as measured by g i l l - n e t c a p t u r e s  that  "activity"  i s a r e f l e c t i o n of foraging  a c t i v i t y r a t h e r t h a n some o t h e r t y p e o f  activity.  58  TABLE 8  N o r t h t r a p (Nth) v e r s u s South t r a p (Sth) C a p t u r e s o f t r o u t and kokanee 1974-75  Interval  J u n e 14 J u l y 17  Range o f s u r f a c e temperatures C  10-17  19-23  No o f t r a p p i n g d a y s  29  13  a.  Nth  Sth  69  29  70  30  No.  Kokanee captured  % captured Z (binomial test, Siegel, 1956)  3.98 **  b. T r o u t No.  captured  Nth  Sth  J u l y 17 - J u l y 30  Nth  O c t . 19 -Nov..6  May 5 - May 26 5-12  6-11  20  18 Sth  Nth  Sth  Nth  Sth  6  41  83  34  25  6  13  87  71  29  81  19  -  5.00 **  Nth  Sth  -  4.44 **  Nth  Sth  -3.21 **  Nth  Sth  383  257  96  41  131  136  82  72  % captured  60  40  70  30  49  51  53  47  Z  -  statistic  **  s i g n i f i c a n t a t t h e .01  NS  not s i g n i f i c a n t  4.96 **  level  4.58 **  -0.24 NS  -  0.73 NS  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 k o k a n e e n e t t e d a t two h o u r i n t e r v a l s , o v e r twenty-four  hour p e r i o d s d u r i n g  (b.) l a t e summer and ( c . ) p o o l e d  ( a . ) e a r l y summer over a l l dates.  A n e t t i n g s e r i e s was c o m p l e t e d d u r i n g e a c h o f F e b . 1964,  A p r i l , 1966; May, 1964; A u g . , 1 9 6 3 ; and  Nov.,  1 9 6 3 . N = t h e t o t a l number o f t r o u t o r  kokanee c a p t u r e d series.  during a p a r t i c u l a r netting  59a  Q  LxJ  \-  hUJ z: CO  or L±J m  O  KOKANEE  N = I30  ©  TROUT  N = I36  a. EARLY SUMMER  (MAY-JUNE)  2-  26  -  O  N=277  ©  N  b. L A T E SUMMER (AUGUST)  =58  2218-  Z>  AL  z  hhu_ o O  106 2  —1  22  -  O  N = 6 11  @  N = 284  1  1  1  1  T  i  1  r  1  r  c. TOTAL OVER A L L SEASONS  1814106  -  2  -  —i 4-6  1  1  -A. M-  8-10  n  1 12-2  1  1  P.M. 4 - 6  n  1 8-10  r  12-2  A. M.  I f d i f f e r e n c e s i n d a y and n i g h t v u l n e r a b i l i t y a r e related  t o some f a c t o r o t h e r  ability  to v i s u a l l y detect  than f i s h a c t i v i t y  and a v o i d  (eg. the  the n e t s ) , I expected  t h a t more i n d i v i d u a l s o f b o t h s p e c i e s w o u l d be c a p t u r e d a t n i g h t than i n the day.  Since  t h i s i s not the case, 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 c h a n g e s i n a c t i v i t y . not  immediately confirm  t h e second a s s u m p t i o n and a s t u d y  o f f i s h by D a r n e l l and M e i e r o t t o may be i n c o r r e c t .  I can  (1965) s u g g e s t s t h a t i t  They f o u n d t h a t f e e d i n g  a c t i v i t y , as  measured by t h e volume o f f o o d p r e s e n t i n t h e g u t , d i d n o t coincide with the periods  o f maximum "swimming" a c t i v i t y "  as measured by t h e v u l n e r a b i l i t y o f f i s h t o c a p t u r e i n t r a p s . The and  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  of trout  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 t h e  d i e l c y c l e , but they a r e l e s s s u b j e c t  t o the assumptions  that l i m i t the usefullness of g i l l - n e t , capture data. I wished t o t e s t the hypothesis t e m p o r a l sequence o f f o r a g i n g  that d i f f e r e n c e s i n the  a c t i v i t y alone could  affect  prey e x p l o i t a t i o n , t h e r e s u l t s presented here concern t r o u t and k o k a n e e t h a t were o b s e r v e d i n t h e same  and  habitats during  only  sub-habitats.  T h e r e f o r e c o m p a r i s o n s a r e l i m i t e d t o t r o u t and k o k a n e e in offshore-surface,  Because  foraging  e a r l y summer ( F i g . 14 a)i  i n offshore-bottom, habitats during  l a t e summer ( F i g .  14b) . Direct observations  i n d i c a t e t h a t i n e a r l y summer  b o t h 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  Figure  14. A comparison of and  kokanee  sixteen  the  relative  observed  at  hour p e r i o d s .  observed  (a.)  hour trout  in offshore-surface  summer  (b.)  bottom  habitats  indicate  two  numbers o f  trout  + one  during standard  late  intervals and  error  during  observed  summer. of  the  over  kokanee  habitats  and kokanee  trout  in  early  offshore-  Vertical means.  bars  61a  5^7  7-9 A M  9-11  11-1 —  1-3 — -  3-5 P M  5-7  7-9  62  e a r l y m o r n i n g f o l l o w e d by l o w e r of the day.  l e v e l s f o r the remainder  I n l a t e summer t h e r e a r e s h o r t i n t e r v a l s  t h e d a y when k o k a n e e a r e more a c t i v e t h a n t r o u t b u t , the e x c e p t i o n o f t h e e a r l y e v e n i n g  with  peak i n a c t i v i t y by k o k a n e e ,  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 The  during  are s i m i l a r .  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  T h e r e i s o f t e n some u n c e r t a i n t y i n t h e a s s e s s m e n t of whether a p r e d a t o r prey  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  (Beukema, 1968; M u r d o c h e t a l . 1 9 7 5 ) .  T h i s i s because  some o f t h e m o t o r 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 animals  are a l s o incorporated i n t o non-search  searching  activities  ( C u r i o , 1976) and b e c a u s e p r e d a t o r s engaged i n n o n - s e a r c h a c t i v i t e s are often instantaneously receptive to opportunities f o r capture o f prey  (eg.Schaller's observations of l i o n s ,  1972) . Although  i t was i m p o s s i b l e i n t h e p r e s e n t  study  t o a l w a y s 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 n o n - s e a r c h behaviours,  I c o n t e n d t h a t much o f t h e 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 M a r i o n Lake was d i r e c t e d t o w a r d s search.  prey  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 t y p e s o f e v i d e n c e . Laboratory  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 a readiness to search  indicate that f i s h  f o r and a t t a c k p r e y  t h e i r stomachs a r e n o t f i l l e d  to capacity.  exhibit  items as l o n g as Trout  and kokanee  i n M a r i o n Lake s e l d o m o b t a i n a s much a s 50% o f t h e d a i l y r a t i o n they a r e capable  o f consuming  ( S a n d e r c o c k , 1969; H y a t t ,  unpublished d a t a ) . ness of the gut)  Thus, h u n g e r ( o p e r a t i o n a l l y e q u a l s  s h o u l d s e r v e 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 search c o n t i n u o u s l y f o r prey. t r o u t and  Certainly  kokanee i n t h e f i e l d a p p e a r e d t o be r e c e p t i v e  throughout food  full-  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  items. Selective responsiveness to s t i m u l i  i s o f t e n the  most i m p o r t a n t f e a t u r e by w h i c h d i f f e r e n t t y p e s o f " g o a l oriented" behaviours ( H i n d e , 1966) .  On  s u c h as f o o d s e a r c h c a n be c a t e g o r i z e d  this basis too, daylight a c t i v i t i e s  t r o u t and kokanee i n M a r i o n Lake seemed t o be r e l a t e d 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 f i e l d were g e n e r a l l y d i r e c t e d p o t e n t i a l food items.  by f i s h i n t h e  During d a y l i g h t hours, d i r e c t  of time  defense,  T h i s was  t o r e c o g n i z e such b e h a v i o u r s  s e a r c h a c t i v i t i e s were o b s e r v e d  obser-  i n a c t i v i t e s t h a t were n o t  agonistic interactions, predator avoidance). t o an i n a b i l i t y  by  t o r e v e a l more t h a n  r e l a t e d t o food search (eg. r e s t i n g , t e r r i t o r i a l  due  pri-  towards s t i m u l i presented  v a t i o n s at underwater s t a t i o n s f a i l e d a n e g l i g i b l e investment  of  to involve large  not  since  non-  investments  o f t i m e by t r o u t and k o k a n e e 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  of year. A c t i v i t y P a t t e r n s o f Emerging  Chironomids  Results of surface-net hauls r e v e a l that 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 , b u t 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  t h e May, 18 and J u n e , 1 s a m p l e s ,  t h e p e a k s o f emergence  activity  o c c u r r e d from l a t e e v e n i n g t o n i g h t .  The peak o f  activity  f o r t h e much s m a l l e r J u n e , 9 emergence o c c u r r e d  in late afternoon. 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 d e g r e e o f d i e t a r y o v e r l a p ( C h a p t e r 2) and t h a t m a j o r d i f f e r e n c e s e x i s t between t h e a p p a r e n t availability  o f p r e y i n t h e 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 . G i v e n t h e 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 activity  o f t r o u t , kokanee and some o f t h e i r m a j o r 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  t e m p o r a l s e g r e g a t i o n may p l a y a r o l e p a t t e r n s o f prey  i n shaping the observed  exploitation.  The R o l e o f S p a t i a l and T e m p o r a l S e g r e g a t i o n i n P r o d u c i n g 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 L a r v a l o d o n a t e s , e p h e m e r o p t e r a n s and s n a i l s stitute in April  major  con-  f o o d s o f t r o u t i n e a r l y summer ( 3 7 % by w t .  and 1 2 % by w t . i n J u n e / 6 6 ) , b u t a r e b a r e l y r e p r e -  sented i n t h e d i e t o f kokanee ( a b s e n t i n A p r i l  and o n l y 2%  by w t . i n J u n e / 6 6 ) a t t h i s t i m e ( E f f o r d & Tsumura,  1973).  Because the m a j o r i t y o f these p r e y a r e found a t depths o f l e s s t h a n 2 m i n M a r i o n Lake ( T a b l e 9 ) , I s u g g e s t e d differential  that  e x p l o i t a t i o n b y t r o u t and kokanee m i g h t 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 c o m p a r i s o n o f t h e 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 p r e y a t i n s h o r e ( d e p t h s <2 m) and o f f s h o r e ( d e p t h s >3 m) l o c a t i o n s i n M a r i o n Lake  Abundance e x p r e s s e d as % o f t o t a l numbers on a m P r e y Group  Inshore  Chironomid  pupae  Chironomid  larvae  Amphipods  50-60  larvae  snails  Odonate l a r v a e Ephemeropteran  larvae  0  303  Data 1  *  40-50  Source  Hamilton,  1965  50  50  H a m i l t o n , 1965 McCauley, p e r s . comm. M a t h i a s , 1971 E f f o r d , unpubl. W i n t e r b o u r n , 1971  80  20  Delury,  87  13  87  13  82-90  Trichopteran Planorbid  70  Offshore  basis  10-18  1971.  2 * I n f e r r e d from the d i s t r i b u t i o n 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 t h e more a b u n d a n t s p e c i e s o f c h i r o n o m i d s w h i c h have c o m p l e t e d 50% o f t h e i r emergence by t h e end o f May a n d / o r a l l emergence by t h e end o f J u n e . Hamilton's data 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 a r e 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 t h e d i s t r i b u t i o n o f immature o d o n a t e s o r e p h e m e r o p t e r a n s . However, b o t h o f t h e s e g r o u p s a r e most a b u n d a n t i n c l o s e a s s o c i a t i o n w i t h t h e c o v e r 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 t h e 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 t h e abundance o f t h e s e 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 t h e predators.  R e s u l t s from t h i s s t u d y l e n d s t r o n g s u p p o r t  this hypothesis.  Undoubtedly the o c c u p a t i o n o f o f f s h o r e  a r e a s by t h e m a j o r i t y o f kokanee and  o f i n s h o r e a r e a s by t r o u t  creates 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 of prey.  to  Even f o r t h o s e few kokanee t h a t do f o r a g e  a r e a s , t h e o c c u p a t i o n o f p r i m a r i l y m i d w a t e r and h a b i t a t s ( T a b l e 7) w i l l  i n inshore surface  r e d u c e the p r o b a b i l i t y o f  b o t t o m p r e y s u c h as o d o n a t e s and  snails.  these  encountering  By c o n t r a s t , t h e  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 f a v o u r d e t e c t i o n and  will  e x p l o i t a t i o n of these prey types i n  e a r l y summer. The and o f for  o c c u p a t i o n o f o f f s h o r e a r e a s by  inshore  a r e a s by t r o u t p r o v i d e s a r e s o n a b l e  explanation  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 of other  t y p e s as w e l l . more t e r r e s t r i a l 5) and  1972)  i n s e c t s t h a n kokanee ( F i g . 2, T a b l e s  (water boatmen).  be r e s t r i c t e d and  abundant  Notonectids  and  3  and  and  c o r i x i d s a r e known  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 ,  i t is likely there.  prey  Over t h e c o u r s e o f t h e y e a r t r o u t e x p l o i t  t r o u t a l o n e e x p l o i t n o t o n e c t i d s (backswimmers)  corixids to  kokanee  that t e r r e s t r i a l  insects w i l l  D i e t s o f the s m a l l e s t t r o u t and  be most  kokanee  examined i n t h i s s t u d y c o n t a i n e d l a r g e numbers o f  zooplankton  ( T a b l e 5) b u t t r o u t e x p l o i t e d t h e c l a d o c e r a n S i d a  crystallina  almost  exclusively.  Kokanee t o o k r e l a t i v e l y modest numbers  o f S i d a s p . a l o n g w i t h many o t h e r s p e c i e s o f  zooplankton  s u c h as L e p t o d o r a s p .  T h i s d i f f e r e n c e may be t r a c e d t o t h e  close a s s o c i a t i o n of Sida sp. with l i t t o r a l  zone beds o f  l i l y p a d s w h i l e s p e c i e s such as L e p t o d o r a s p . a r e found i n t h e w a t e r column o f o f f s h o r e a r e a s ( H y a t t /  only  unpublished  data). Some d i f f e r e n c e s i n t h e s i z e - f r e q u e n c y o f prey source  distributions  used by t r o u t and kokanee most l i k e l y have i n habitat segregation.  their  F o r example, kokanee  e x p l o i t the s m a l l e r species o f molluscs  likely  ( 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 r e n o t found i n a p p r e c i a b l e numbers  i n the offshore l o c a t i o n s occupied  by kokanee ( D e l u r y , 1971) . Since observed during for  prey  t h e m a j o r i t y o f t r o u t and kokanee t h a t I t h e d a y were c o n t i n u o u s l y engaged  (see d e s c r i p t i o n s i n chapter  4),  i n searching  i t i s unlikely  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  species  i n f i s h d i e t s a r e 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 o f the p r e d a t o r s  o r t h e i r prey.  However,  a s y n c h r o n o u s 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 t h e p r e d a t o r s erences.  t o p r o d u c e some d i e t a r y d i f f -  F o r example t h e g r e a t e r e x p l o i t a t i o n o f  chironomid  pupae by k o k a n e e 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  greater occupation  o f midwater  and s u r f a c e h a b i t a t s where pupae a r e most v u l n e r a b l e t o 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 e v e n i n g and a t n i g h t combined w i t h emergence p e a k s by chironomid  pupae a t t h e s e t i m e s ( F i g . 15) c o u l d  certainly  c o n t r i b u t e t o t h e g r e a t e r e x p l o i t a t i o n o f pupae by kokanee during  some months o f t h e y e a r .  evidence supporting  However, i n g e n e r a l t h e  the idea that asynchronous foraging  a c t i v i t e s by t r o u t , kokanee and t h e i r p r e y p l a y a m a j o r 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 t h e 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 segregation  than the evidence suggesting  a s a m a j o r mechanism f a v o u r i n g  food  habitat  resource  partitioning.  The R o l e o f S p a t i a l S e g r e g a t i o n i n P r o d u c i n g Differences Between t h e P r o p o r t i o n s o f P r e y Types Observed i n the N a t u r a l E n v i r o n m e n t and i n t h e D i e t s o f T r o u t 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 t h e r e l a t i v e abundance o f p o t e n t i a l prey i n the lake  ( C h a p t e r 2, F i g s . 8 & 9) r e v e a l e d m a j o r  diff-  e r e n c e s between t h e a p p a r e n t a v a i l a b i l i t y o f p r e y i n t h e 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 .  t h i s chapter  provide  Results obtained i n  evidence that a l i k e l y reason these  d i f f e r e n c e s o c c u r i s t h a t t r o u t , kokanee and s c i e n t i s t s  each  i m p l e m e n t s a m p l i n g schemes w h i c h 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 order the  to c h a r a c t e r i z e the species composition  of  i n v e r t e b r a t e s t h a t o c c u p y an a v e r a g e s q u a r e m e t e r o f l a k e  70  bottom,  scientists  ( H a m i l t o n , 1965;  u n p u b l i s h e d d a t a ; B r y a n , 1971) schemes i n w h i c h zone was  E f f o r d and Tsumura, 1 9 6 8 - 6 9 ,  employed s t r a t i f i e d  random s a m p l i n g  t h e number o f b e n t h i c s t a t i o n s w i t h i n e a c h  depth  approximately p r o p o r t i o n a l t o the t o t a l lake area covered  by t h a t zone. twice monthly  T h u s , o f t h e 9 s t a t i o n s .sampled  and a v e r a g e d  b a s i s by H a m i l t o n ( 1 9 6 5 ) , 5 were l o c a t e d a t  o f l e s s t h a n 2 m w h i l e t h e r e m a i n i n g 4 were l o c a t e d a t between 2 m and on a m o n t h l y  6m.  taken  b a s i s o v e r a two y e a r i n t e r v a l by E f f o r d and  remaining 8 o r i g i n a t e d when a v e r a g e d  depths  depths  S i m i l a r l y , o f t h e 25 b e n t h i c g r a b s  17 on a v e r a g e o r i g i n a t e d  on a  Tsumura,  from d e p t h s o f 2 m o r l e s s w h i l e t h e  from d e p t h s between 2 m and 6 m.  these data are l i k e l y  to i n d i c a t e the  Although  relative  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 w h o l e ,  they are u n l i k e l y t o i n d i c a t e the  d e n s 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 encountered kokanee w h i l e f o r a g i n g .  relative  by t r o u t o r  For example, o v e r the course of a y e a r  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 s a m p l i n g  t o o b t a i n p r e y from d e p t h zones g r e a t e r t h a n 2 m .  By  effort  contrast,  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 s p e n t between 65 and 88% o f t h e i r t i m e f o r a g i n g  i n t h i s zone.  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  Thus, the s e t o f i s biased  t h o s e s p e c i e s t h a t a r e most a b u n d a n t i n s h a l l o w (< 2 m) habitats  ( e g . amphipods, s n a i l s , odonates)  i n v e r t e b r a t e p r e y a c q u i r e d by kokanee w i l l t h o s e s p e c i e s t h a t a r e most a b u n d a n t benthic h a b i t a t s (eg. chironomid  towards water  w h i l e the s e t o f be b i a s e d  towards  i n r e l a t i v e l y d e e p (> 2  larvae).  m)  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 benthic  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 u s e d by f o r a g i n g r a i n b o w t r o u t t h a n b y k o k a n e e , however d i f f e r e n c e s i n where " s a m p l e s " o f i n v e r t e b r a t e s were t a k e n w i t h i n a g i v e n d e p t h zone p r o b a b l y obtained  b i a s the s e t s o f i n v e r t e b r a t e s  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.  H a m i l t o n a s w e l l a s E f f o r d and Tsumura o b t a i n e d  F o r example,  a l l grab samples  w i t h i n a g i v e d e p t h zone from open mud l o c a t i o n s . o b t a i n i n g s a m p l e s f r o m weed-beds t h a t o c c u r a l o n g  They  avoided  the shoreline  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  often  s e a r c h e d i n and a r o u n d submerged weed-beds l o c a t e d a l o n g t h e lakeshore.  T h i s d i f f e r e n c e may be i m p o r t a n t  (1971) o b s e r v a t i o n  given  Winterbourn's  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 r e  p a r t i c u l a r l y a b u n d a n t i n weed-beds. representation of caddis  Thus, t h e apparent  over-  l a r v a e i n b e n t h i c g r a b s a m p l e s 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 a r e a s sampled by t r o u t and s c i e n t i s t s .  Unexplained Dietary  Patterns  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  proximity,  plays 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 prey, a r e many t r e n d s w h i c h a r e n o t a c c o u n t e d f o r by s p a t i a l Under l a t e summer c o n d i t i o n s , t h e r e  there  segregation.  i s a decline i n diet  o v e r l a p b e t w e e n t r o u t and k o k a n e e ( C h a p t e r 2, T a b l e 1.) d e s p i t e a considerable  increase  i n spatial overlap.  F o r f i s h g r e a t e r t h a n 10 cm  i n s i z e , t h e e x c l u s i v e use o f t r i c h o p t e r a n l a r v a e b y t r o u t and o f  72  cladocerans and  by kokanee c o n t r i b u t e s t o t h e d e c r e a s e i n d i e t  overlap  i s n o t a l o g i c a l outcome o f t h e o b s e r v e d p a t t e r n s o f h a b i t a t  segregation. Despite  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 l a r v a e than kokanee.  chironomid  These l a r g e l y b e n t h i c p r e y do n o t a p p e a r  t o 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 t h a n i n o f f s h o r e habitats  ( T a b l e 9 ) , t h u s , some o t h e r mechanism must o p e r a t e 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 t h e observation  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, b u t i t i s n o t an a d e q u a t e mechanism t o p r o d u c e t h e 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 p r e y by t r o u t .  This 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 trend a p p l i e s t o both a broad range o f a q u a t i c i n v e r t e b r a t e s (Chapter l i m i t e d groups such as z o o p l a n k t o n Habitat segregation a l i m i t e d success prey still  (Chapter  2, F i g . 5 ) .  and t i m i n g o f a c t i v i t i e s  are only  i n e x p l a i n i n g t h e non-random e x p l o i t a t i o n o f  from t h e l a k e e n v i r o n m e n t . have no e x p l a n a t i o n .  observations  2, F i g . 2) and t o more  Many o f t h e o b s e r v e d d i f f e r e n c e s  In the next chapter,  o f foraging behaviour  i n the f i e l d  I have combined with selected  l a b o r a t o r y experiments to t e s t the general hypothesis  that  d i f f e r e n c e s i n how t r o u t and k o k a n e e s e a r c h  will  a c c o u n t f o r some o f t h e a s y e t u n e x p l a i n e d random, p r e y - e x p l o i t a t i o n .  f o r prey  p a t t e r n s o f non-  73  SUMMARY 1.  T r o u t 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 segregation 2.  ( T a b l e s 7 & 8, F i g . 1 2 ) .  I n e a r l y summer kokanee o c c u p y 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 mainly 3.  b o t t o m ones ( T a b l e 7 ) .  I n l a t e summer t h e r e a r e more t r o u t t h a n kokanee  i n surface h a b i t a t s although  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 a r e a s s o c i a t e d w i t h m i d w a t e r and b e n t h i c h a b i t a t s ( T a b l e 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 o c c u p y 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 trend t o occupy  inshore  ( < 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  relatively l i t t l e  temporal segregation.  N e i t h e r s p e c i e s c a n 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 7.  ( F i g 13 & 1 4 ) .  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 k o k a n e e i n  M a r i o n Lake p l a y s a m a j o r r o l e  i n producing  predator-specific dietary  patterns. 8.  A number o f d i f f e r e n c e s between t h e a p p a r e n t  of i n v e r t e b r a t e prey  i n t h e l a k e e n v i r o n m e n t and t h e i r  availablity utilization  by t r o u t and k o k a n e e may be a t t r i b u t e d t o t h e f a c t t h a t t h e r e l a t i v e p r o p o r t i o n s o f prey obtained  by s c i e n t i s t s s a m p l i n g  a l l areas o f  t h e l a k e a r e n o t t h e 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 i n more r e s t r i c t e d areas (eg. weed-beds, o f f s h o r e benthic h a b i t a t s ) o f the l a k e . 9.  Many of the d i e t a r y d i f f e r e n c e s described  earlier  (Chapter 2) are not accounted f o r by e i t h e r s p a t i a l or temporal segregation  between t r o u t and kokanee.  75  CHAPTER 4 THE RELATIONSHIP BETWEEN FOOD-SEARCH BEHAVIOUR AND DIETARY PATTERNS OF TROUT AND KOKANEE I N 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 ( C h a p t e r 2) t h a t t r o u t and k o k a n e e i n M a r i o n that spatial  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  s e g r e g a t i o n ( o n a s c a l e o f m e t e r s ) p l a y s an  important r o l e i n shaping observed and  (Chapter  temporal  3).  foraging behaviour their  exploitation  However c o n s i d e r a t i o n o f o n l y  s e g r e g a t i o n by f i s h  patterns unexplained.  shaping  the p a t t e r n s o f prey  i n Marion  spatial  Lake l e f t many d i e t a r y  This implies that d i f f e r e n c e s i n the  o f t h e p r e d a t o r s must p l a y a m a j o r r o l e i n  diets.  De R u i t e r (1967) h a s d i v i d e d t h e f o r a g i n g b e h a v i o u r o f animals Events  i n t o four phases:  s e a r c h , a p p r o a c h , c a p t u r e , and i n g e s t i o n .  t h a t t a k e p l a c e d u r i n g any one o f t h e s e p h a s e s may have  a profound  i n f l u e n c e on t h e d i e t o f a p r e d a t o r .  In this  chapter,  I have s e t o u t t o t e s t t h e 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 in search behaviour  will  account  f o r d i f f e r e n c e s i n the types o f  p r e y a c q u i r e d by t r o u t and k o k a n e e .  To d e m o n s t r a t e t h i s r e q u i r e s  an a s s e s s m e n t o f w h e t h e r t h e r e i s a "match" between the- p r e d a t o r s 1  search behaviours behaviours)  and s p e c i f i c c h a r a c t e r i s t i c s  of their  (locations,  prey.  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 d i f f e r e n t aspects.  sizes,  f r o m many  A variety of studies indicate that microhabitat  76  s p e c i f i c search,  t h e 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 s e n s o r y p e r c e p t i o n particularly  and p r e y d e t e c t i o n a r e  important i n determining d i e t a r y patterns  of free  l i v i n g p r e d a t o r s ( M a c A r t h u r , 1958; R o o t , 1967; B a k e r , 1972; Ware, 1973). deals  I have d i v i d e d with  t h i s c h a p t e r i n t o two s e c t i o n s .  first  t h e f o o d - s e a r c h b e h a v i o u r o f t r o u t and k o k a n e e u n d e r  natural conditions.  The s e c o n d d e a l s  o f t r o u t and kokanee t o d e t e c t laboratory. and  The  with  various  the r e l a t i v e  abilities  n a t u r a l prey i n the  The i m p l i c a t i o n s o f r e s u l t s o n s e a r c h b e h a v i o u r  prey d e t e c t i o n , f o r d i e t a r y patterns  are discussed.  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 and S p e c i a l i z e d Search Techniques T h e r e i s no s t a n d 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 habitat, a sub-habitat studies  or a microhabitat  i t i s c l e a r t h a t they a r e not absolute  u n i t s m o s t commonly d i s t i n g u i s h e d scale.  and from a v a r i e t y o f  In the majority  relative  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  of studies, dealing  behaviour o f free-ranging  but rather  vertebrates,  with  the foraging  h a b i t a t s and  sub-habitats  a r e b i o p h y s i c a l u n i t s w h i c h c o v e r many s q u a r e m e t e r s o f s p a c e (eg. f o r e s t f l o o r , Microhabitats  f o r e s t canopy, l i t t o r a l  zone, b e n t h i c  sediments).  by c o n t r a s t a r e b i o p h y s i c a l u n i t s i n w h i c h a t l e a s t  some d i m e n s i o n s a r e measured on a s c a l e o f cm o r mm i n tree bark, the undersides of leaves,  (eg. cracks  t h e exposed l a y e r o f  s e d i m e n t a s compared t o t h e s u b - s u r f a c e s e d i m e n t s on a l a k e bottom).  Within  t h i s c o n t e x t each s u b - h a b i t a t  o f an a g g r e g a t e o f m i c r o h a b i t a t s , assortments of food.  generally  consists  w h i c h may c o n t a i n d i f f e r e n t  S p e c i a l i s e d search techniques which r e s u l t i n microh a b i t a t s p e c i f i c search by v e r t e b r a t e predators have been i d e n t i f i e d as the b a s i s f o r the occurrence of d i f f e r e n c e s between the d i e t s of s i m i l a r predators searching f o r food i n the same h a b i t a t s (MacArthur,  1958;  Root, 1967;  Baker, 1972),  as w e l l as the reason f o r why  a p a r t i c u l a r predator f a i l s to  take p o t e n t i a l prey i n p r o p o r t i o n to t h e i r r e l a t i v e abundance i n the environment (Royama, 1970;  Ware, 1973;  Moore and Moore,  1976). Root (1967) and Baker (1972) i n p a r t i c u l a r emphasized that d i f f e r e n c e s i n m i c r o h a b i t a t s searched by sympatric of  b i r d s served as the b a s i s f o r t h e i r d i f f e r e n t i a l  of  i n v e r t e b r a t e s i n f o r e s t and mud-flat  species  exploitation  habitats respectively.  For example, Root found t h a t Western f l y c a t c h e r s (Empidonax d i f f i c i l i s ) search a l a r g e area f o r prey from a " s e n t i n e l " p o s i t i o n on an exposed perch.  He suggested  that t h i s search  technique biased f l y c a t c h e r s to e x p l o i t l a r g e , a c t i v e i n s e c t s e i t h e r i n the a i r or from the surface of f o l i a g e .  A high  p r o p o r t i o n of l a r g e Hymenoptera and D i p t e r a i n the d i e t of f l y c a t c h e r s supported  his prediction.  In c o n t r a s t to f l y c a t c h e r s  p l a i n t i t m i c e (.Parus inornatus) o f t e n searched  f o r prey  by p u l l i n g apart flowers and f o l i a g e as w e l l as by probing beneath f o l i a g e .  Consequently,  t h e i r d i e t s contained a high  p r o p o r t i o n of small i n s e c t s which were concealed microhabitats.  i n these  These i n s e c t s d i d not appear commonly i n the  d i e t of f l y c a t c h e r s .  S i m i l a r l y , Baker (1972) noted that some  species of shorebirds (eg. short b i l l e d dowagers) searched  78  for  p r e y by p r o b i n g d e e p l y  while others for  i n t o the substrate with t h e i r  ( e g . l e s s e r y e l l o w l e g s ) , searched  o b j e c t s e x p o s e d on t h e mud s u r f a c e .  bills,  exclusively  He p r e s e n t e d  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 for  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 p r e y by t h e p r e d a t o r s . S i n c e many o f t h e s p e c i e s o f p r e y e x p l o i t e d by t r o u t  and k o k a n e e 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 ( e g . e p i b e n t h i c or sub-benthic), I hypothesized in  that specialised  search  techniques  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 m i g h t form t h e  b a s i s f o r some d i f f e r e n c e s i n t h e d i e t a r y p a t t e r n s o f t r o u t and k o k a n e e . ducted  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 c o n -  o b s e r v a t i o n s o f t r o u t and k o k a n e e f o r a g i n g u n d e r n a t u r a l  conditions.  METHODS L o c a t i o n s , t i m e s and g e n e r a l t e c h n i q u e s  of obtaining 3).  o b s e r v a t i o n s have been d e s c r i b e d e l s e w h e r e ( C h a p t e r at  w h i c h o b s e r v a t i o n s were c o n d u c t e d d i f f e r e d b o t h w i t h  to  d e p t h and d i s t a n c e from s h o r e .  Although  observations of foraging behaviours  o f t r o u t and k o k a n e e a s s o c i a t e d i n Marion Lake,  l o c a t i o n s a t w h i c h o b s e r v a t i o n s were c o n d u c t e d c o n t a i n e d 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,  I recorded  of l i t t e r , the search  a c t i v e l y foraging predators  respect  I d i d not obtain  w i t h t h e e n t i r e range o f m i c r o h a b i t a t s p r e s e n t  deposits, accumulations  Locations  clumps o f water  techniques  a  brush  lilies).  and p o s i t i o n s o f  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 d i f f e r e n t p o r t i o n s o f the food supply present i n d i f f e r e n t m i c r o h a b i t a t s .  When f i s h swam  i n t o view standard observations recorded were; the species i d e n t i t y , the number of i n d i v i d u a l s present, the s p e c i f i c substrate that i n d i v i d u a l s were oriented to, the water column p o s i t i o n of each animal (expressed i n cm from the nearest s u b s t r a t e ) , the c r u i s i n g v e l o c i t y o f randomly selected animals, the "search techniques" employed by i n d i v i d u a l f i s h , the r e l a t i v e s i z e s and i d e n t i t i e s of items that f i s h attacked and the sequence of events surrounding an a t t a c k .  To record those  events r e q u i r i n g an estimate of time, I used e i t h e r a four channel event recorder or a stopwatch. Because i n d i v i d u a l f i s h a t underwater w i t h i n view f o r only 15 seconds  stations  remained  to three minutes, i t was impossible  to determine e x a c t l y how many d i f f e r e n t i n d i v i d u a l s my observations involve.  However, I could o f t e n recognize i n d i v i d u a l  t r o u t due t o d i f f e r e n c e s i n s i z e or markings ( s c a r s , c o l o u r patterns, 200 to 300 t r o u t c a r r i e d colour coded tags) and kokanee frequently appeared hundred  i n groups c o n t a i n i n g up to a few  i n d i v i d u a l s , thus i t i s l i k e l y that the data c o l l e c t e d  involve s e v e r a l hundred  i n d i v i d u a l s of each species and that  the behaviours observed are r e p r e s e n t a t i v e of those g e n e r a l l y employed by t r o u t and kokanee i n Marion  Lake.  RESULTS Search  Techniques When p a r t i c u l a r behaviours are associated with s p e 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 a r e r e p e a t e d l y f o l l o w e d by a t prey c a p t u r e , i t i s l i k e l y  attempts  t h a t such b e h a v i o u r s a r e p a r t  of the process o f prey search.  In the present study, 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 e a c h o f t h e s e  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 s e p a r a t e t y p e s o f search.  1.  C r u i s e and  Search  T r o u t use t h i s t e c h n i q u e t o l o c a t e p r e y i n t h e w a t e r c o l u m n , a t t h e s e d i m e n t s u r f a c e and a t t h e l a k e s u r f a c e .  Kokanee  e x h i b i t t h i s f o r m o f p r e y s e a r c h o n l y when t h e y a r e f o r a g i n g i n t h e water, c o l u m n .  C r u i s e and s e a r c h i s t h e most ambiguous  o f the b e h a v i o u r s c l a s s i f i e d eye  there i s l i t t l e  swimming a c t i v i t y .  a s s e a r c h and t o t h e u n t r a i n e d  t o d i f f e r e n t i a t e t h e t e c h n i q u e from g e n e r a l The k e y c o m b i n a t i o n o f c h a r a c t e r i s t i c s  that i d e n t i f y the technique i s t h a t the predators 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  items.  However, i n a d d i t i o n t o t h e 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 a r e o t h e r c l u e s which  suggest t h a t the  predators are a c t i v e l y searching f o r prey. The f i r s t c l u e i s t h a t t h e 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 p r e y a r e l i k e l y t o be f o u n d .  When t r o u t employ t h i s t e c h n i q u e t o s e a r c h f o r  b e n t h i c p r e y , t h e y assume p o s i t i o n s t h a t v a r y from 0 - 50 cm above t h e s u b s t r a t e , b u t o r i e n t downwards  to face i t .  Ware  81  (1971) s u g g e s t e d t o 20°)  (about  s e r v e s t o d i r e c t the p r e d a t o r ' s v i s u a l a.xis o n t o  sediment. observer  that this characteristic incline  the  I n s u p p o r t o f t h i s , when t r o u t were v e r y c l o s e t o 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 t h e  e f f e c t c o n t i n u o u s eye movements as i f " s c a n n i n g " t h e surface.  10  When u s i n g c r u i s e and  an  fish sediment  search 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 a t t a c k ' s on p r e y from 45 t o 100 The  second  cm b e l o w t h e a i r - w a t e r i n t e r f a c e . c l u e t h a t c r u i s e and  search i s related  t h e p r o c e s s o f p r e y 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 a r y i n a way  t h a t suggests  they are v i s u a l l y scanning  immediate s u r r o u n d i n g s f o r f o o d .  to  velocities their  Swimming s p e e d s a r e 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 s l o w e r f o r f i s h o r i e n t e d t o b e n t h i c substrates (Table 10). T r o u t and k o k a n e e b o t h r e l y on v i s i o n d u r i n g s e a r c h (Ware, 1971;  Ali,  1959),  this reduction i n velocity h e t e r o g e n e i t y t h a t water  t h u s one o b v i o u s r e a s o n f o r  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  column and  bottom sediments  predator using v i s i o n to l o c a t e prey. of benthic substrates w i l l a t t e n t i o n per u n i t of ( e g . see B o y n t o n and must be l o w e r  here.  food  offer  a  The more complex b a c k g r o u n d  r e q u i r e a longer i n t e r v a l of p e r c e p t u a l  space  scanned i n o r d e r t o d e t e c t prey  Busch, 1956), consequently  search  velocities  82  TABLE 1 0 .  A c o m p a r i s o n o f t h e swimming v e l o c i t i e s o f t r o u t and k o k a n e e a c t i v e l y s e a r c h i n g f o r p r e y i n a variety of sub-habitats.  a. T r o u t  Mean (cm/sec)  Water column & lake surface  27.7  Sediment s u r f a c e  8.6  S.D.  Range 11.1- 50.0 4.2-  16.7  N  +  1.4  165  +  1.0  35  b. Kokanee Water column & lake surface Sediment s u r f a c e  TABLE 1 1 .  18.2  10.0- 50.0  + 2.1  45  6.6  5.0- 10.0  + 0.5  22  R e s i d e n c e t i m e and t h e t o t a l number o f 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 t h e h o v e r and search technique.  Time s p e n t i n a s i n g l e l o c a t i o n (minutes) 4 2 6 4 5  T o t a l number o f attacks initiated 6 3 4 1 2  83  2.  Hover and Trout  Search  use t h i s t e c h n i q u e  s u r f a c e and a t t h e l a k e s u r f a c e .  t o l o c a t e p r e y on t h e s e d i m e n t  The k e y c o m b i n a t i o n  c h a r a c t e r i s t i c s that i d e n t i f y the technique maintains  a stationary position  of  i s that the predator  (10 t o 20 cm above t h e b o t t o m  o r 50 t o 60 cm b e l o w the l a k e s u r f a c e ) and a t t a c k s o n l y prey  items.  slightly  recognizable  The l o n g a x i s o f t h e body may be h o r i z o n t a l o r  tilted  (10 t o 20°) t o w a r d s t h e s u b s t r a t e .  The  constant  p e r f o r m a n c e o f s l i g h t r e o r i e n t a t i o n movements t o t h e s u b s t r a t e , scanning  movements o f t h e e y e s and t h e i n i t i a t i o n o f a number  o f a t t a c k s on p r e y technique  items  t h a t move, d i s t i n g u i s h t h i s  from r e s t p o s t u r e s .  arc centered  search  P r e y may be a t t a c k e d w i t h i n an  between t h e p r e d a t o r ' s e y e s and s u b t e n d i n g  310 t o  320°. T r o 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 t h e lake surface.  S i x t y o f the seventy  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  t r o u t I have  observed  f o r benthic prey.  Trout  spend no more t h a n a few m i n u t e s i n a s i n g l e l o c a t i o n and there i s considerable v a r i a b i l i t y prey  (Table 1 1 ) .  at locating  On a number o f 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 o f i n d i v i d u a l f i s h and  i n t h e i r success  have f o u n d t h a t t h e y w i l l  from an above s u r f a c e  use t h e t e c h n i q u e  consecutively,  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 s q u a r e m e t e r o v e r 20 t o 30 m i n u t e s  techniques  area,  intervals.  No kokanee used t h i s t e c h n i q u e f i e l d observations.  location  d u r i n g any o f t h e  Kokanee c h a r a c t e r i s t i c a l l y use o n l y  which i n v o l v e constant  swimming  activity.  search  3.  T e s t and  Search  T h e r e i s l i t t l e d o u b t t h a t b o t h t r o u t and use t h i s t e c h n i q u e as a means t o l o c a t e f o o d i t e m s .  kokanee 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 t h e p r e d a t o r s a r e m o b i l e and t h e y i n i t i a t e many a t t a c k s on i t e m s t h a t a r e n o t p r e y o r g a n i s m s . trout w i l l  F o r example an  o r i e n t t o , r a p i d l y a p p r o a c h , g r a s p and  a s m a l l wood c h i p l y i n g on t h e s e d i m e n t t h i s i t e m may  o b j e c t i s mouthed and will  then r e j e c t e d .  Finally  abandon t h e i t e m , however i t may  b e f o r e making an a t t a c k on a n o t h e r .  then  surface.  be r e p e a t e d s e v e r a l t i m e s and each  reject  A t t a c k s on time  the  the p r e d a t o r  move o n l y a s h o r t d i s t a n c e The  specific  items  a t t a c k e d a r e u s u a l l y w i t h i n the s i z e range o f prey that w i l l  individual  n o r m a l l y be consumed by f i s h  items  i n the l a k e .  Trout  use  t h i s t e c h n i q u e o c c a s i o n a l l y a t t h e l a k e s u r f a c e b u t more f r e q u e n t l y when s e a r c h i n g f o r p r e y on t h e l a k e b o t t o m . p o s i t i o n s and  Search  swimming v e l o c i t i e s a r e i n d i s t i n g u i s h a b l e from  d e s c r i b e d above f o r t r o u t e m p l o y i n g Kokanee use t e s t and forage a t the l a k e s u r f a c e .  c r u i s e and  search.  s e a r c h e x c l u s i v e l y when t h e y  In c o n t r a s t to the c r u i s e  s e a r c h t e c h n i q u e t h a t t r o u t use most o f t e n a t t h e  and  surface,  t h e t e s t and s e a r c h t e c h n i q u e o f kokanee i s p e r f o r m e d  from  a s h o r t e r d i s t a n c e b e l o w t h e l a k e s u r f a c e ( u s u a l l y 5 t o 30 and a t s i g n i f i c a n t l y l o w e r v e l o c i t i e s Table 11). of  The  the tendency  those  (mean 18.2  cm p e r  cm)  second,  lower v e l o c i t i e s a r e p r o b a b l y a consequence f o r kokanee t o m o m e n t a r i l y  o b j e c t ( r a n g e 1 mm  t o 15 mm)  grasp every  f l o a t i n g on t h e s u r f a c e .  prey-size A  subtle  85  d i f f e r e n c e i n t h i s technique, i s t h a t kokanee f o r a g i n g o n l y once b e f o r e foraging  as p r a c t i s e d by  a t the s u r f a c e  moving o n .  T h i s may  t r o u t and  kokanee,  u s u a l l y t e s t an  be  item  a consequence o f  i n close association with other  kokanee, w h i l e  their  trout  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 This  Search  t e c h n i q u e was  prey a t the sediment s u r f a c e . slowly  (6.6  surface.  cm p e r  The  used o n l y by kokanee s e a r c h i n g The  s e c o n d - T a b l e 10)  f i s h takes a mouthfull  l i t t e r , v i g o r o u s l y mouths i t , and The  predator right  sediment  o f s e d i m e n t and  leaf  take another m o u t h f u l l  material.  q u i c k l y c a r r i e s i t through  t h e s m a l l c l o u d o f suspended m a t e r i a l and may  a t the  very  f o r c e f u l l y e j e c t s the  f o r w a r d m o t i o n o f the p r e d a t o r  predator  moves a l o n g  for  at this point  the  of sediment or 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 .  kokanee d e t e c t and  prey that  a r e e x p o s e d on  By  using  this  technique  the sediment  surface  p r e y w h i c h become e x p o s e d as a c o n s e q u e n c e o f the  disturbance.  substrate  When I have o b s e r v e d t h i s t e c h n i q u e used  under  l a b o r a t o r y c o n d i t i o n s , p r e y o f t e n escape because the  predator  fails  falling  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  debris. I have n e v e r o b s e r v e d r a i n b o w t r o u t employ technique to search and  only  f o r prey, although,  on v e r y  rare  this occasions,  i n t h e l a b o r a t o r y , I have o b s e r v e d them d i s t u r b  s e d i m e n t s by body movements r e m i n i s c e n t  of redd  digging.  To b r i e f l y  summarize t h e 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 b e t w e e n t h e two s p e c i e s , t r o u t a l o n e use h o v e r and  s e a r c h w h i l e g r a b and s e a r c h i s a t e c h n i q u e used  by k o k a n e e .  Although  two o f t h e t e c h n i q u e s  exclusively  (cruise & search,  t e s t & s e a r c h ) a r e s h a r e d by t r o u t and k o k a n e e , t h e y a r e n o t p r a c t i s e d w i t h e q u a l f r e q u e n c y and t h e same t e c h n i q u e s a r e n o t always 12).  employed t o l o c a t e p r e y i n t h e same s u b - h a b i t a t s ( T a b l e Search  habitat  t e c h n i q u e s employed by k o k a n e e a r e more s u b -  s p e c i f i c t h a n t h o s e used by t r o u t .  An E x a m i n a t i o n o f t h e D i s t r i b u t i o n o f S e a r c h P o s i t i o n s o f T r o u t and Kokanee i n R e l a t i o n t o L a k e S u r f a c e and B o t t o m 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  o f p r e y 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 t h e l a k e s u r f a c e o r l a k e bottom.  Several of the search  techniques  used by t h e 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 r a n g e o f s e a r c h p o s i t i o n s w i t h r e s p e c t t o the s u r f a c e b e i n g scanned f o r p r e y . Thus, I expected  t h a t t h e use o f d i f f e r e n t s e a r c h  techniques  as w e l l a s d i f f e r e n c e s i n t h e f r e q u e n c y o f use o f t h e same techniques 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 o f t r o u t and k o k a n e e when t h e y s e a r c h e d  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. . I n t h e f i e l d , t r o u t and kokanee o c c u p i e d a c o n t i n u o u s range o f s e a r c h p o s i t i o n s e x t e n d i n g through bottom.  the water  from t h e l a k e s u r f a c e ,  column t o t h e s e d i m e n t s u r f a c e a t t h e l a k e  In order to detect 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 s e a r c h p o s i t i o n s assumed by t h e p r e d a t o r s , I f i r s t had t o  87  TABLE 1 2 .  A summary o f t h e 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 employed by t r o u t and k o k a n e e f o r a g i n g i n t h e f i e l d .  Technique c r u i s e & search  L e v e l of Occurrence  S u b - h a b i t a t s Searched  Trout  Trout  Kokanee  +++  +  ++  N  t e s t & search  +  +++  S,W,B  grab & search  N  +++  -  hover & search  +++ ++ + N  v e r y common common occasional never observed  S - lake  surface  B - lake  bottom  w - w a t e r column  S,W,B S,B  Kokanee W  s,w B  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 o f 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 t h e 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  t o b e n t h i c and s u r f a c e p r e y  i n the  field.  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 P r e y and t h e S p a t i a l L i m i t s on B e n t h i c and Surface Search In  t h e f i e l d , a sudden change i n t h e d i r e c t i o n ,  v e l o c i t y o r f o r m o f swimming b e h a v i o u r was  of a searching  predator  u s u a l l y f o l l o w e d by an a t t a c k o n a p r e y o r g a n i s m .  Because  t h e 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 t h e i m m e d i a t e  vicinity  o f t h e b o t t o m o r w a t e r - c o l u m n r e f e r e n c e m a r k e r s was r e l a t i v e l y r a r e , and b e c a u s e p r e d a t o r s a l w a y s a p p r o a c h e d p r e y oblique angle  a t an  t o t h e s u b s t r a t e , I d i d n o t o b t a i n many p r e c i s e  estimates of r e a c t i v e distances t o prey.  However I d i d o b s e r v e  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 t h a n 50 cm  from t h e  s e d i m e n t s u r f a c e (measured a s t h e p e r p e n d i c u l a r d i s t a n c e between t h e s e d i m e n t s u r f a c e and t h e head o f t h e p r e d a t o r ) never attacked benthic prey, but predators c l o s e r than  this  f r e q u e n t l y made a t t a c k s . Laboratory  s t u d i e s have r e v e a l e d many o f t h e 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 react to prey.  S i z e , c o n t r a s t and movement o f p r e y  predators, (Protasov,  1968;  Ware, 1 9 7 1 ) ; e x p e r i e n c e  and s i z e o f t h e p r e d a t o r  (Beukema,  1968;  Ware, 1 9 7 1 ) ; and c o m p l e x i t y o f t h e b a c k g r o u n d e n v i r o n m e n t  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 distance.  These v a r i a b l e s a p p e a r t o i n t e r a c t i n t h e f i e l d  i n a way t h a t l i m i t s t h e s e a r c h that maintain surface. greater  f o r benthic  prey t o predators  p o s i t i o n s o f l e s s t h a n 50 cm f r o m t h e s e d i m e n t  The maximum r e a c t i v e d i s t a n c e t h a n 50 cm o n some o c c a s i o n s ,  a l w a y s a p p r o a c h e d p r e y a t an o b l i q u e The  to benthic  p r e y was  s i n c e t r o u t and k o k a n e e angle to the substrate.  maximum 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 p r e y l o c a t e d a t t h e l a k e s u r f a c e was much g r e a t e r prey on b e n t h i c  substrates.  Fish maintaining  much as 85 cm b e l o w t h e s u r f a c e On two o c c a s i o n s  I obtained  initiated  than t o  p o s i t i o n s as  a t t a c k s on p r e y .  s a m p l e s f o r s i z e measurements  o f t h e t a r g e t s t r o u t were a t t a c k i n g  (Table  1 3 ) ; t h u s I am  c e r t a i n t h a t t h e y a r e c a p a b l e o f r e s p o n d i n g from p o s i t i o n s t h a t a r e 80-85 cm b e l o w t h e s u r f a c e 6-10 s q u a r e 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  o f t h i s s i z e was m o d e s t l y g r e a t e r oblique  occurred.  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 t h e l a k e s u r f a c e o n two o c c a s i o n s  Reactive Distance (cm) Range  to prey  because o f t h e s l i g h t l y  a n g l e a t w h i c h a t t a c k s on s u r f a c e p r e y  TABLE 1 3 .  Mean  t o t a r g e t s no l a r g e r t h a n  Target Size  No. o f attacks  Maximum s u r f a c e No. a r e a (mm ) measured 2  70  65-85  5  10  20  50  30-80  10  64  40  Target identity Nuphar s e e d s adult  Diptera  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 k o k a n e e n e v e r i n i t i a t e d a t t a c k s o n l a k e s u r f a c e o r b e n t h i c p r e y from  distances  g r e a t e r t h a n 100 cm below t h e l a k e s u r f a c e o r 50 cm above t h e l a k e bottom.  Therefore,  by d e f i n i t i o n , o n l y p r e d a t o r s  t h e s e d i s t a n c e s were c o n s i d e r e d s u r f a c e and b e n t h i c  as a c t i v e l y searching  substrates respectively.  within lake  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 t h e d i s t r i b u t i o n s o f t r o u t and kokanee s e a r c h s u r f a c e and b e n t h i c  The  positions with respect  to lake  sub-habitats.  D i s t r i b u t i o n of Predator  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 lake surface o r bottom, t r o u t and k o k a n e e 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 o f s e a r c h positions.  The r e s u l t s p r e s e n t e d  observations  here a r e pooled  from a l l  c a r r i e d o u t i n b o t h 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 s e a r c h  f o r benthic  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 t h e s e d i m e n t s u r f a c e t r o u t a r e most o f t e n o b s e r v e d s e a r c h i n g  prey  from  while  f r o m p o s i t i o n s some  15 t o 30 cm o f f o f t h e b o t t o m ( F i g . 1 6 ) . T h i s s p e c i e s - s p e c i f i c pattern i s maintained  when t r o u t and k o k a n e e s e a r c h  s u r f a c e prey as w e l l . searching  Here kokanee a r e most f r e q u e n t l y o b s e r v e d  f o r s u r f a c e p r e y from p o s i t i o n s t h a t a r e 5 t o 30 cm  below t h e l a k e s u r f a c e . search  f o r lake  By c o n t r a s t , t h e m a j o r i t y o f t r o u t  f o r t h e s e p r e y from p o s i t i o n s t h a t a r e 45 t o 100 cm  below t h e l a k e s u r f a c e  (Fig.17).  91  FIGURE 16.  The  r e l a t i v e proportions  o f t r o u t and  kokanee  maintain  s p e c i f i c search  positions while  scanning  the bottom sediments f o r p r e y .  that  visually N =  the  t o t a l number o f t r o u t o r kokanee o b s e r v e d .  The  sequence o f s o l i d  spheres i n d i c a t e s the  o f the s m a l l e s t p r e y t h a t t r o u t o r kokanee detect on  size can  from a g i v e n p o s i t i o n .  S i z e s were  estimated  the b a s i s of d a t a presented  i n F i g . .18  (see  discussion for explanation).  MINIMUM DETECTABLE P A R T I C L E S I Z E  (mm ) 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 maintain  s p e c i f i c search  positions while  scanning  the lake surface f o r prey.  that  visually  N = the t o t a l  number o f t r o u t o r kokanee o b s e r v e d .  ..The s e q u e n c e o f s o l i d s p h e r e s i n d i c a t e s t h e r e l a t i v e s i z e o f t h e s m a l l e s t p r e y t h a t t r o u t o r kokanee may detect  from a g i v e n p o s i t i o n .  on t h e b a s i s o f d a t a p r e s e n t e d discussion f o r explanation).  S i z e s were  estimated  i n F i g . .18 ( s e e  93  DISCUSSION The P o t e n t i a l E f f e c t s o f S e 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 a r c h on D i e t a r y P a t e r n s T r o u t and k o k a n e e s h a r e some t e c h n i q u e s o f b u t o t h e r s a r e used by o n l y one  o f the p r e d a t o r s .  search  The  question  of whether a p a r t i c u l a r search 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 f o o d i t e m s b u t n o t o t h e r s c a n o n l y be if  answered  search techniques are considered i n r e l a t i o n t o the  c o n t e n t s o f t h e p r e d a t o r ' s d i e t s and the  prey  known c h a r a c t e r i s t i c s o f  prey. 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  kokanee r e s u l t i n some d e g r e e o f m i c r o h a b i t a t s p e c i f i c but are not e a s i l y r e l a t e d  and  search  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  c o m p o s i t i o n of the p r e d a t o r ' s d i e t s .  For example, grab  s e a r c h as p r a c t i s e d by k o k a n e e , opens up p o s s i b i l i t i e s  and 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 a r e n o t open t o t r o u t i e . p r e y may  be l o c a t e d by " f l u s h i n g "  beneath b e n t h i c sediments. used by t r o u t ( T a b l e 12) prey  them from c o n c e a l e d  By c o n t r a s t t h e s e a r c h  restrict  i n exposed p o s i t i o n s .  positions, techniques  them t o t h e d e t e c t i o n o f  In s p i t e of t h i s d i f f e r e n c e ,  two m a j o r c l a s s e s o f p r e y known t o spend most o f t h e i r c o n c e a l e d b e n e a t h t h e s e d i m e n t s u r f a c e (amphipods and are w e l l represented (Chapter 2 ) .  i n t h e d i e t s o f b o t h t r o u t and  the  time chironomids)  kokanee  T h i s simply emphasizes t h a t although d i f f e r e n c e s  i n foraging behaviour w i l l  frequently result i n differences in  t h e k i n d s o f f o o d s consumed by p r e d a t o r s , t h e y may  also result  94  i n t h e a c q u i s i t i o n o f t h e same f o o d s from s l i g h t l y  different  microhabitats. Kokanee a c q u i r e p r e y from b o t h c o n c e a l e d 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  positions.  T h i s a p p a r e n t l y d o e s n o t r e s u l t i n any q u a l i t a t i v e  differences  i n t h e d i e t s o f t h e p r e d a t o r s b u t may certain quantitiative differences.  s e r v e as t h e b a s i s f o r For e x a m p l e , c h i r o n o m i d  l a r v a e a r e an. i m p o r t a n t component o f t h e d i e t s o f b o t h  trout  and k o k a n e e , b u t k o k a n e e 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 t h a n t r o u t i n f o u r o u t o f f i v e months o f the y e a r including  (Table 14),  t h e month o f A u g u s t when b o t h p r e d a t o r s a r e 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 benthic s u b - h a b i t a t s (Chapter 3 ) . Coarse-scale habitat segregation f a i l e d  to explain  this  d i f f e r e n c e s i n c e c h i r o n o m i d l a r v a e a r e no more a b u n d a n t i n o f f s h o r e than i n onshore  TABLE 14.  The s e a s o n a l e x p l o i t a t i o n o f l a r v a e by t r o u t and k o k a n e e  1  Sample Date Nov. Feb. April June Aug.  l o c a t i o n s i n M a r i o n Lake ( C h a p t e r 3 ) .  Trout Larvae Larvae e x p l o i t e d expected 10 11 14 233 131  131 30 7 406 246  Larvae exploited 252 64 5 382 247  chironomid  Kokanee Larvae expected 131 45 12 209 132  Probability that trout exploit more l a r v a e t h a n k o k a n e e do p < .00003 p <,. 00003 N.S. p < .00003 p < .00003  N o r m a l a p p r o x i m a t i o n t o t h e 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 e x p e c t e d f r e q u e n c i e s a r e d e r i v e d f r o m t h e number o f t r o u t and kokanee p r e s e n t i n e a c h months sample and a s s u m i n g t h a t e a c h p r e d a t o r s h o u l d consume an e q u a l number of l a r v a e . D a t a from E f f o r d & Tsumura ( 1 9 7 3 ) .  95  On t h e l a k e b o t t o m , c h i r o n o m i d abundant prey  type  l a r v a e a r e t h e most  t o r e g u l a r l y r e a c h s i z e s g r e a t e r t h a n 2 mm.  Because the m a j o r i t y o f l a r v a e a r e s u b - b e n t h i c ,  tube-dwellers,  t r o u t , u s i n g h o v e r and s e a r c h o r c r u i s e and s e a r c h will  frequently f a i l  t o d e t e c t them. 2  abundance o f l a r v a e (5 t o 10 p e r cm unpublished  d a t a ) , I suggest  techniques,  However g i v e n t h e g r e a t i n l a t e summer, M c C a u l e y ,  t h a t a kokanee (12 cm i n l e n g t h )  w i t h a 6-8 mm mouth gape ( s e e C h a p t e r  5) and engaged i n  r e p e a t e d l y grabbing m o u t h f u l l s of sediment, w i l l high p r o b a b i l i t y of d i s t u r b i n g l a r v a e .  experience  a  These may be t h e n be  d e t e c t e d v i s u a l l y o r perhaps w i t h t h e a i d o f chemosensory d e t e c t o r s i n t h e mouth. It  i s c l e a r from t h e work o f I v l e v  concealment o f chironomid  (1961) t h a t t h e  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 . i n d i c a t e d t h a t the presence did  H i s experiments  o f s i l t on t h e b o t t o m o f an a q u a r i u m  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 of chironomid  l a r v a e t o carp (Cyprinus c a r p i o ) , b u t d r a s t i c a l l y reduced the c o n s u m p t i o n o f l a r v a e by r o a c h with the n o - s i l t The  ( R u t i l u s r u t i l u s ) , when compared  controls.  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  e s p e c i a l l y r e l e v a n t t o my s u g g e s t i o n t h a t d i f f e r e n t  (1972) a r e search  techniques 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 of chironomid l a r v a e by t r o u t and k o k a n e e .  T h e i r d e s c r i p t i o n s l e a v e no d o u b t  that other p a i r s of salmonids, e x i s t i n g  i n s y m p a t r y , use t h e  t e c h n i q u e s o f g r a b and s e a r c h , h o v e r and s e a r c h o r c r u i s e and s e a r c h 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  benthic  96  prey.  Furthermore,  they demonstrated  i n two s e t s o f l a b o r a t o r y e x p e r i m e n t s ,  t h a t g r a b and s e a r c h ( a s 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 t o h o v e r by Salmo c l a r k i ) a s a t e c h n i q u e  f o r locating either  l a r v a e u n d e r a sand and l e a f l i t t e r worms under a sand s u b s t r a t e .  and s e a r c h ( a s p r a c t i s e d chironomid  substrate, o r , Tubifex  This evidence o f f e r s strong  s u p p o r t f o r my c o n t e n t i o n t h a t t h e d i f f e r e n t s e a r c h  techniques  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 r e a s o n f o r t h e greater e x p l o i t a t i o n of chironomid with 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 S e a r c h on D i e t a r y P a t t e r n s The  l a r v a e by kokanee compared  Positions  use o f 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 and d i f f e r e n c e s  i n t h e f r e q u e n c y o f use o f t h e same t e 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 t h e d i s t r i b u t i o n s o f s e a r c h p o s i t i o n s o f t r o u t and k o k a n e e when t h e y f o r a g e a t t h e l a k e s u r f a c e o r b o t t o m . will  t h i s have on d i e t a r y p a t t e r n s ?  q u e s t i o n r e l i e s on t h e p o t e n t i a l  What e f f e c t  A g e n e r a l answer t o t h i s  i n t e r a c t i o n betwen t h e s e a r c h  p o s i t i o n s o f t h e 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 . Visual predators l i k e responses  t h a t a r e d e p e n d e n t on p r e y s i z e  this study). suggested  t r o u t and kokanee d i s p l a y a t t a c k (Ware, 1 9 7 1 ; H y a t t ,  On t h e 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  t h a t when t r o u t s e a r c h f o r b e n t h i c p r e y t h e y 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 t o c r e a t e a r e f u g e from d e t e c t i o n f o r p r e y t h a t a r e b e l o w a s p e c i f i c limit. apparent  size  A d a p t i n g Ware's r e s u l t s t o t h e p r e s e n t s t u d y , i t i s 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 t h e l o c a t i o n o f t h e p r e y , t h e l a r g e r t h e p r e y 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 t h e s e r e s u l t s a r e used t o  p r e d i c t t h e minimum s i z e s o f p r e y t h a t t h e p r e d a t o r s  may  detect  a t t h e l a k e s u r f a c e o r b o t t o m i t becomes a p p a r e n t t h a t t h e minimum p r e y s i z e s d e t e c t e d  by k o k a n e e s h o u l d  s m a l l e r than those detected  by t r o u t ( F i g s . 17 & 1 8 ) .  inference w i l l  be m i s l e a d i n g  be  considerably This  i n the event that 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 t h a n kokanee and r e s p o n d t o t h e same r a n g e o f p r e y s i z e s from a g r e a t e r d i s t a n c e . Protasov  (1968) r e p o r t e d  that mullet reacted  F o r example v i s u a l l y t o 15  mm p r e y a t a d i s t a n c e o f 25 cm w h i l e h o r s e m a c k e r e l t o i d e n t i c a l p r e y a t 65 cm. in the a b i l i t i e s  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  o f t r o u t and k o k a n e e t o d e t e c t  I c o n d u c t e d a number o f l a b o r a t o r y  4-B.  reacted  similar  prey  experiments.  LABORATORY COMPARISONS OF PREY DETECTION  INTRODUCTION I have s u g g e s t e d t h a t t r o u t and kokanee w i l l 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 they m a i n t a i n and  bottom.  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  A second p o s s i b i l i t y  that both species w i l l detect  i n the f i e l d To  because surface  i s t h a t t r o u t and kokanee  exhibit differences i n their abilities and  often  to v i s u a l l y detect  prey  t h e same r a n g e o f p r e y s i z e s  d e s p i t e the maintenance o f d i f f e r e n t search p o s i t i o n s .  t e s t t h i s hypothesis  I conducted l a b o r a t o r y experiments  t o compare t h e r e a c t i v e d i s t a n c e where RD i s d e f i n e d when t h e p r e d a t o r  (RD) o f t r o u t and k o k a n e e  as t h e d i s t a n c e between the f i s h  initiates  an a t t a c k .  and p r e y  98  FIGURE 18  The minimum p r e y 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 from t h e s u b s t r a t e s  distances  on w h i c h p r e y a r e l o c a t e d .  A d a p t e d from d a t a i n T a b l e 1. ( p . 95) o f Ware, 1 9 7 1 .  - i  i  20 SEARCH  1  1  40 POSITION  1  60 (c  1  r  .Like o t h e r i n v e s t i g a t o r s 1968),  I have assumed t h a t RD  the p r e d a t o r ' s a b i l i t y fish,  t h i s assumption  experiments  ( H o l l i n g , 1966;  Beukema,  to food items i s a f u n c t i o n o f  to v i s u a l l y d e t e c t food items. i s amply s u p p o r t e d  (1971) w h i c h d e m o n s t r a t e d  For  by Ware's e x t e n s i v e  t h a t RD  i s controlled  by p r e y c h a r a c t e r i s t i c s s u c h as s i z e , c o n t r a s t and movement b u t n o t by p r e d a t o r c h a r a c t e r i s t i c s s u c h as h u n g e r (= d e g r e e of  gut  fullness).  METHODS In  experiments  conducted  t o assess the  reactive  d i s t a n c e o f f i s h , t h e methods o f p r e y p r e s e n t a t i o n may  have  an i m p o r t a n t e f f e c t on t h e r e s u l t s o b t a i n e d .  I n most s t u d i e s ,  t h e p r e y a r e i n t r o d u c e d one  end o f a l o n g ,  a t a t i m e , a t one  n a r r o w t a n k , and o u t o f s i g h t o f t h e p r e d a t o r (Ware, Werner & H a l l , 1974; dimensions i n one  Confer & Blades, 1975).  1971;  Because o f  o f t h e e x p e r i m e n t a l t a n k , t h e p r e d a t o r must swim  d i r e c t i o n o n l y and w i l l The  i n e v i t a b l y approach  advantage of t h i s technique  the  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 p r o v i d e d by t h e e x p e r i m e n t e r The  disadvantages  prey.  i s t h a t i t guarantees  t h a t e a c h a t t a c k by t h e p r e d a t o r i s a c o n t r o l l e d e v e n t  tank).  the  (unless  unintentionally  when i n t r o d u c i n g t h e p r e y t o t h e  are t h a t a g r e a t d e a l of time i s  required to 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 predators 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 shape o f t h e t a n k f o r c e s t h e p r e d a t o r t o d e t e c t p r e y  are the  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 functions. In o r d e r t o a l l o w prey d e t e c t i o n a l o n g a l l a x i s o f o f t h e 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 a r e n a 45 cm) w h i c h d i d n o t f o r c e t h e f i s h t o r e s p o n d direction.  ( 9 2 x 47 x  i n o n l y one  To r e d u c e t h e t i m e 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  observations, I introduced single predators  into the experimental  a r e n a 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 . then recorded  the r e a c t i v e distances of f i s h  I  i n consecutive  a t t a c k s on p r e y . The  f r o n t and b o t t o m o f t h e 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 at  cm s q u a r e s . the beginning  By r e c o r d i n g a c o o r d i n a t e f r o m e a c h o f t h e s e and end o f an a t t a c k , I c o u l d measure t h e  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 e a c h a t t a c k , a t t h e end o f an experimental  session.  A number o f f e a t u r e s a i d e d i n an a c c u r a t e  determination of the coordinates. was a l w a y s c l e a r l y  The d i s c o v e r y o f a p r e y  item  i n d i c a t e d by a sudden change o f v e l o c i t y  o r o f b o t h v e l o c i t y and o r i e n t a t i o n by t h e p r e d a t o r . t h e f i s h were h i g h l y c o n d i t i o n e d t o t h e p r e s e n c e  Because  o f an o b s e r v e r  i t was p o s s i b l e t o f o l l o w t h e i r movements f r o m a p o s i t i o n a s c l o s e as t h e f r o n t o f t h e t a n k w o u l d a l l o w . t h e modest d i m e n s i o n s  o f the experimental  This along  tank  with  ( 9 2 x 47 x 45 cm)  a l l o w e d t h e 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 t h e f i s h and a v o i d p r o b l e m s o f p a r a l l a x .  F i n a l l y , the  shadow c a s t e o n t h e t a n k b o t t o m by t h e moving o r s t a t i o n a r y f i s h s e r v e d a s a u s e f u l cue i n d e t e r m i n i n g - 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 .  Preliminary t r i a l s  w i t h an i n d e p e n d e n t o b s e r v e r p r o d u c e d a c c e p t a b l e e s t i m a t e s o f coordinates indicating artificial  . predator  the i n i t i a l  and  f i n a l p o s i t i o n s of  ( a p i e c e o f hose) moving a t  the  an  velocities  e q u i v a l e n t t o t h o s e e x h i b i t e d by t r o u t and k o k a n e e i n t h e experiments. It  i s c r i t i c a l t o t h e i n t e r p r e t a t i o n o f the  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 by f i s h  i n the arena  (DRD)  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 . p r e d a t o r s were h u n t i n g i t e m , i t was  prey  item a t a time  i n an a r e n a  the  t h a t c o n t a i n e d more t h a n  ( i e : consecutive responses  to prey  i n c i d e n t s of prey d e t e c t i o n ) .  one  one may  In  this  w o u l d s i m p l y r e p r e s e n t an i n d e x o f the d i s t a n c e  between c o n s e c u t i v e p r e y l o c a t i o n s and . d i s t a n c e a t w h i c h the f i s h d e t e c t p r e y . t h e a l t e r n a t i v e s t h a t DRD ability  Since  p o s s i b l e f o r them t o d e t e c t more t h a n  not r e p r e s e n t separate e v e n t , DRD  exhibited  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 o f s e p a r a t e e v e n t s .  prey  results  n o t a measure o f  the  To d i s t i n g u i s h between  s e r v e s as an i n d e x o f the  predator's  t o d e t e c t p r e y o r o f p r e y s p a c i n g , I examined t h e  o f p r e y d e n s i t y c h a n g e s on  . E x p e r i m e n t 4.1  DRD.  R e a c t i v e D i s t a n c e and  I f DRD  effect  Prey  Density  i s an i n d e x o f p r e y s p a c i n g , t h e r e  should  be a s i g n i f i c a n t l y g r e a t e r f r e q u e n c y o f l o n g r e a c t i v e d i s t a n c e s i n experiments  conducted  a t low p r e y d e n s i t i e s compared t o  102  high density experiments  ( i e : c l o s e r prey s p a c i n g ) .  not occur i f the d e t e c t i o n of prey proceeds separate events i n the arena.  (Ephemeroptera) of  20, 40, 100  and h a n d l e d  larvae.  as a s e r i e s o f  I r e c o r d e d DRD  w i t h each o f t h r e e kokanee (130-150 mm  i n separate  long) foraging  I n t h e a r e n a I used p r e y  and 200 p e r .42 m .  Experiment  4.2  densities  A l l p r e d a t o r s were m a i n t a i n e d  R e a c t i v e D i s t a n c e , T r o u t v s . Kokanee  measure o f t h e a b i l i t y  2.1  indicated  t h a t DRD  will  I proceeded  t o use DRD  s e r v e as a  of the p r e d a t o r s to d e t e c t prey  r e s u l t s ) r a t h e r t h a n as a measure o f p r e y s p a c i n g . i n comparing  (see  Therefore,  t r o u t and kokanee f o r  differences i n perceptual sensitivity  to prey.  Three  rainbow  t h r e e k o k a n e e , o b t a i n e d from M a r i o n Lake were used  in these experiments. I conducted  They r a n g e d  i n l e n g t h from 130-160  separate experiments f o r both species w i t h  d i f f e r e n t prey types.  The  l a r g e and s m a l l p r e y t y p e s  d a m s e l f l y l a r v a e and 3 mm  D a p h n i a ) span a r a n g e  prey s i z e s i m i l a r to that normally encountered i n Marion  f o r mayfly  methods).  Experiment  (16 mm  trials  i n a standard fashion before experimental t r i a l s  ( C h a p t e r 1, g e n e r a l  t r o u t and  This w i l l  Lake.  mm.  two used of  by t h e p r e d a t o r s  103 RESULTS  Experiment  4.1 The  R e a c t i v e D i s t a n c e and DRD  Prey Density  of kokanee, responding t o mayfly  larvae,  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  l o n g r e a c t i v e d i s t a n c e s (p 5>.98, K o l m o g o r o v - S m i r n o v ,  tailed-test,  S i e g e l 1956)  (Table 15).  Thus i t a p p e a r s  proceeds of  than i n h i g h - d e n s i t y experiments t h a t the d e t e c t i o n o f prey  as a s e r i e s o f s e p a r a t e e v e n t s i n s p i t e o f t h e  numerous p r e y i n t h e e x p e r i m e n t a l a r e n a .  d i f f e r e n c e between DRD  measures of prey d i s t r i b u t i o n ,  for  i n t h e low and  m o v i n g and  is a legitimate  presence  substantial  c o n s t i t u t e s a second  evidence supporting t h i s conclusion.  different  The  items  o f kokanee r e s p o n d i n g 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) of  one-  I f DRD  line  were s i m p l y  t h e y s h o u l d have been d r a m a t i c a l l y  h i g h - d e n s i t y t r i a l s and  s t a t i o n a r y prey.  identical  Thus, I c o n c l u d e t h a t  index o f the p r e d a t o r ' s a b i l i t i e s  DRD  to detect  prey.  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 t o s t a t i o n a r y e p h e m e r o p t e r a n ( m a y f l y ) nymphs i n l o w d e n s i t y (20 & 40 p r e y / . 4 2 s q u a r e m e t e r s ) and h i g h d e n s i t y (100 & 200 p r e y / . 4 2 s q u a r e m e t e r s ) e x p e r i m e n t s .  Reactive Distance 0-5  6-10  Density  11-15  16-20  21-25  (cm) 26-30 31-35  36-40  N  Number o f R e s p o n s e s  Low  7  16  15  5  1  2  0  High  0  7  15  8  2  0  1  0 1  46 34  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 mayfly  responding  t o e i t h e r s t a t i o n a r y o r moving  nymphs ( E p h e m e r o p t e r a ,  Centroptilum  s p . ) . N = t h e t o t a l number o f o b s e r v a t i o n s .  105 E x p e r i m e n t 4.2  Reactive  Distance-Trout  C o m p a r i s o n s o f DRD do  v s . Kokanee  ( F i g . 20) i n d i c a t e t h a t  n o t r e s p o n d more o f t e n t o p r e y a t g r e a t e r d i s t a n c e s  trout than  k o k a n e e ( A l l c o m p a r i s o n s p>.95, K o l m o g o r o v - S m i r n o v , o n e - t a i l e d test).  They a l s o i n d i c a t e t h a t l a r g e p r e y a r e a t g r e a t e r  o f d e t e c t i o n t h a n s m a l l p r e y (compare 3mm  risk  D a p h n i a t o 16 mm  o d o n a t e s ) and t h a t moving p r e y a r e a t g r e a t e r  r i s k of detection  t h a n s t a t i o n a r y p r e y 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 o d o n a t e s t o moving o n e s ) . B e c a u s e t r o u t and kokanee c o n s i s t e n t l y d i f f e r e n t search  p o s i t i o n s i n the f i e l d  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  maintain  and r e s p o n d t o  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 t h e same s i z e s o f p r e y i n t h e f i e l d . usually maintain  search  For instance  p o s i t i o n s t h a t a r e f u r t h e r t h a n 20 cm.  away f r o m t h e s e d i m e n t and c o n s e q u e n t l y s h o u l d 2 benthic  p r e y a s s m a l l a s 3mm  It  C o n v e r s e l y , kokanee  i s w i s e t o e x e r c i s e some c a u t i o n  Although the l a b o r a t o r y data c l e a r l y  I never observed a t t a c k s on b e n t h i c t h a n 50 cm.  minations of reactive distance  to the f i e l d indicate  occasionally attack  the s i z e o f l a r v a l odonates, from d i s t a n c e s  greater  should  i n the a p p l i c a t i o n  of reactive distance  t h a t b o t h t r o u t and kokanee s h o u l d  distances  detect  prey i n these s m a l l s i z e c l a s s e s .  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 situation.  seldom  ( t h e approximate s i z e o f the  s m a l l p r e y used i n e x p e r i m e n t 4 . 2 ) . frequently detect  , trout  greater  prey  t h a n 50 cm.,  prey i n the f i e l d  S i m i l a r l y Ware's (1971)  from deter-  i n the l a b o r a t o r y suggest  that  106  FIGURE 20  A c o m p a r i s o n o f the f r e q u e n c y d i s t r i b u t i o n s r e a c t i v e d i s t a n c e o f t r o u t and and  s m a l l p r e y t y p e s , (a.)  of  odonates, Enallagma  t h a t a r e moving o r s t a t i o n a r y (b.)  distance  large  reactive distance  k o k a n e e t o l a r g e p r e y (16 mm sp.)  kokanee t o  for  reactive  o f t r o u t t o l a r g e p r e y t h a t a r e moving  stationary  or  ( c . ) r e a c t i v e d i s t a n c e o f kokanee t o  s m a l l p r e y (3 mm a r e moving (d.)  z o o p l a n k t o n , D a p h n i a sp.)  that  r e a c t i v e d i s t a n c e of t r o u t to  prey t h a t are moving.  small  ^90T  107  very high 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 detected  f r o m g r e a t e r t h a n 70 cm  15 mm  are  by t r o u t , w h i l e my  seldom  field  2 observations  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  a t d i s t a n c e s g r e a t e r t h a n 85 cm.  may  be  detected  T h i s simply emphasizes  d i f f i c u l t y of e x a c t l y d u p l i c a t i n g f i e l d  the  c o n d i t i o n s i n the  l a b o r a t o r y , and w a r n s 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 determinations  o f 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  of 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  minimum s i z e o f d e t e c t a b l e p r e y  i n the  estimates  absolute  field.  DISCUSSION The R o l e o f S e a r c h P o s i t i o n s i n P r o d u c i n g 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 T r o u t and Kokanee There i s a g e n e r a l to  t r e n d f o r t r o u t from M a r i o n Lake  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  t h a n kokanee o f c o m p a r a b l e s i z e s do  (Chapter  in their diets 2, F i g . 4 ) .  t r e n d i s i n p a r t r e l a t e d t o t h e f a c t t h a t f o r many p r e y e a t e n by b o t h . t r o u t and  k o k a n e e , t r o u t consume a  This types  higher  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. amphipodsF i g . 5, m o l l u s c s - F i g . 6, presented  i n t h i s chapter Laboratory  zooplankton-Fig. h e l p e x p l a i n why  7).  Results  t h i s trend  e v i d e n c e ( E x p e r i m e n t 4.2)  t r o u t are unable to d e t e c t s m a l l prey k o k a n e e , y e t i n the f i e l d  suggested  that  at greater distances  t r o u t c o n s i s t e n t l y assumed  p o s i t i o n s t h a t were f u r t h e r away t h a n kokanee from l o c a t i o n s of prey  occurs.  a t the l a k e s u r f a c e o r bottom.  than  search  the Therefore,  108  a given s i z e c l a s s of a s i n g l e prey  t y p e must  experience  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 a n d , i n g e n e r a l , t h e minimum s i z e s o f b e n t h i c o r s u r f a c e  prey  detected  detected  by k o k a n e e w i l l  by t r o u t .  be much s m a l l e r t h a n t h o s e  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  small  H y a l e l l a s p . (3-4mm) make up a g r e a t e r p r o p o r t i o n o f t h e b e n t h i c amphipods i n t h e d i e t o f k o k a n e e , w h i l e l a r g e C r a n g o n y x sp.  (7-10mm) a r e more a b u n d a n t i n t h e d i e t o f t r o u t ( C h a p t e r  2,  F i g . 5 and T a b l e 4 ) . The and  search  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 p r e y  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 a c c o u n t 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 other benthic prey Small molluscs  types.  such as p i s i d i u m spp. a r e o f t e n absent from  the d i e t o f t r o u t b u t a r e u s u a l l y p r e s e n t  i n the d i e t o f  kokanee ( S a n d e r c o c k , 1969 and C h a p t e r 2, F i g . 5 ) . 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 )  Similarly,  i n the d i e t o f  kokanee a r e f r o m s m a l l s i z e c l a s s e s ( a s i n d e x e d  by head  capsule  width), while trout exploit a greater proportion of large caddis 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 k o k a n e e . D a t a from Winterbourn, 1971.  Head W i d t h (mm)  0.4-0.8  No. e a t e n by trout  39  No. e a t e n by kokanee  8  0.8-1.2  1.2-1.6  1.6-2.4  151  232  265  13  109  The R o l e o f S e a r c h T e c h n i q u e s and S e a r c h P o s i t i o n s i n P r o d u c i n g D i f f e r e n c e s Between t h e P r o p o r t i o n s Of P r e y Types O b s e r v e d i n t h e N a t u r a l E n v i r o n m e n t and i n t h e D i e t s o f T r o u t and Kokanee Knowledge g a i n e d search  concerning  positions not only provides  search  techniques  and  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 t h e p r e d a t o r s c o n t r i b u t e s t o a greater understanding  but also  o f why d i s c r e p a n c i e s  may e x i s t between t h e a p p a r e n t a v a i l a b i l i t y o f p o t e n t i a l p r e y i n t h e l a k e e n v i r o n m e n t and i n t h e d i e t s o f t r o u t and kokanee ( C h a p t e r 2, F i g ' s . 8 & 9 ) . In e s s e n c e , t h e d i s c r e p a n c i e s e x i s t because t h e sampling devices  ( e g . Ekman d r e d g e , K a j a k c o r e ,  Hargrave  s a m p l e r ) used by s c i e n t i s t s t o d e t e r m i n e t h e r e l a t i v e  abundance  o f i n v e r t e b r a t e s i n t h e l a k e do n o t remove s a m p l e s o f p r e y from t h e e n v i r o n m e n t i n t h e same way t h a t e i t h e r t r o u t o r kokanee d o .  F o r e x a m p l e , an Ekman d r e d g e removes s a m p l e s  of benthic prey without  d i s t i n g u i s h i n g whether they a r e  e x p o s e d on t h e mud s u r f a c e , c o n c e a l e d  j u s t beneath the s u r f a c e  o r b u r i e d u n d e r s e v e r a l mm o f s e d i m e n t . techniques  By c o n t r a s t ,  o f t r o u t a l l o w them t o o b t a i n o n l y t h o s e  search  benthic  p r e y t h a t a r e e x p o s e d on o r above t h e s e d i m e n t s u r f a c e and s e a r c h techniques  o f kokanee p r e v e n t them f r o m g a i n i n g a c c e s s t o p r e y  b u r i e d more t h a n a few mm b e n e a t h t h e s e d i m e n t . regular patterns  Since  there are  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 benthic  sediments ( B u r g i s e t a l . 1973),  t h e d e p t h o f p e n e t r a t i o n o f s a m p l i n g g e a r and  predators  b i a s e s them t o o b t a i n some p r e y b u t n o t 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 t h e same p r e y t y p e s .  T h i s may  explain  why t h e s i n g l e most a b u n d a n t s p e c i e s o f c h i r o n o m i d l a r v a e (by numbers o r by volume) p r e s e n t i n M a r i o n Lake h a s n e v e r recorded i n the d i e t s of the f i s h  ( E f f o r d & Tsumura,  been  1973).  Trout not only e x p l o i t a greater p r o p o r t i o n o f large prey than kokanee, b u t 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 of  individuals  a g i v e n p r e y t y p e r e l a t i v e t o t h e i r abundance i n t h e  environment.  Amphipods ( C h a p t e r 2, F i g . 5) and c a d d i s l a r v a e  ( W i n t e r b o u r n , 1971) a r e two p r e y t y p e s w h i c h d e m o n s t r a t e  this  pattern. Winterbourn  (1971) f o u n d  by t r o u t i n M a r i o n Lake b e l o n g e d i n s t a r o f each  species.  t h a t 72% o f t h e c a d d i s e a t e n  t o t h e l a s t o r second  He c o n c l u d e d  t h a t t r o u t were  from  last  "selecting"  the l a r g e s t s i z e s o f 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 a p p a r e n t when t h e s p e c i e s c o m p o s i t i o n o f c a d d i s e a t e n each month was r e l a t e d each o f t h e s p e c i e s .  t o the l i f e  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 p r e y e d  upon s e q u e n t i a l l y a s t h e i r l a s t i n s t a r s a p p e a r e d In  l a t e summer and e a r l y f a l l ,  importance  histories of  i n the l a k e .  c a d d i s l a r v a e were o f l e a s t  i n t h e d i e t o f t r o u t , a l t h o u g h t h e y were p r e s e n t  at  maximum abundance f o r t h e y e a r a s s m a l l , e a r l y - i n s t a r s t a g e s .  If  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 trout are  r e p r e s e n t a t i v e o f t h e t i m e s o f t h e y e a r when o b s e r v a t i o n s were not c a r r i e d o u t (evidence t h a t t h i s i s l i k e l y  will  be p r e s e n t e d  in  a l a t e r c h a p t e r ) , then s e a s o n a l changes i n t h e 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 species (eg. caddis) w i l l  as c h a n g e s i n s i z e s o f p r e y o c c u r a t d i f f e r e n t  occur  developmental  stages  (Ware, 1973) .  This 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 t h e s e a s o n a l and a p p a r e n t l y s i z e - d e p e n d e n t  exploitation of  b o t h c a d d i s l a r v a e and amphipods by t r o u t .  Unexplained  Dietary patterns Knowledge a b o u t s e a r c h t e c h n i q u e s , s e a r c h  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 t h e f i e l d h a s my a b i l i t y  positions  improved  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 o f p r e d a t o r s and e n v i r o n m e n t ,  b u t a number o f  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  at explanation. I f t r o u t and kokanee o b t a i n e d a l l o f t h e s m a l l - p r e y component o f t h e i r d i e t s from s u r f a c e o r b o t t o m s u b - h a b i t a t s , then s e a r c h p o s i t i o n s a l o n e 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 o f prey they o b t a i n .  that  However, many o f t h e s m a l l p r e y t h a t t r o u t and  k o k a n e e e x p l o i t o r i g i n a t e from t h e w a t e r  column.  In the l a t e r  summer months, k o k a n e e o f a v e r a g e 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 o f t h e i r f o o d ( 4 8 % by w e i g h t  i n Aug.) from s m a l l  (usually  l e s s t h a n 1 mm body l e n g t h ) c l a d o c e r a n s and c o p e p o d s i n the water  column ( E f f o r d & Tsumura, 1 9 7 3 ) .  found  Although i n  l a t e summer t r o u t spend a t l e a s t a s much t i m e a s kokanee i n the water  c o l u m n , z o o p l a n k t o n n e v e r c o n t r i b u t e more t h a n  (by w e i g h t )  of their d i e t .  0.5%  F o r t h e s m a l l e s t t r o u t and kokanee  which  a t t i m e s o b t a i n v i r t u a l l y a l l o f t h e i r f o o d from t h e  water  column ( C H a p t e r  2,  F i g . 7 & table 5),  there a r e major  d i f f e r e n c e s i n t h e 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 o f  zooplankton e x p l o i t e d .  No mechanism p r o p o s e d  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 m a i n t e n a n c e by t r o u t and k o k a n e e w i l l  o f s p e c i e s - s p e c i f i c , search p o s i t i o n s clearly  f a v o u r t h e d e t e c t i o n and  e x p l o i t a t i o n of a greater proportion of small caddis larvae by kokanee t h a n by t r o u t b u t i t d o e s n o t o f f e r a r e a d y 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 all  size classes relative to trout.  will  In the next c h a p t e r , I  address the general hypothesis t h a t d i f f e r e n c e s i n  m o r p h o l o g y and t h e b e h a v i o u r o f t r o u t and k o k a n e e d u r i n g a p p r o a c h , c a p t u r e and i n g e s t i o n o f p r e y w i l l of  these unexplained d i e t a r y  a c c o u n t f o r some  diferences.  SUMMARY 1.  T r o u t 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  search techniques (Table 12). 2. are  The s e a r c h t e c h n i q u e s t h a t t r o u t and k o k a n e e s h a r e  n o t p r a c t i s e d w i t h t h e same f r e q u e n c y and a r e n o t a l w a y s  employed t o l o c a t e p r e y w i t h i n t h e same s u b - h a b i t a t s . 3.  S e a r c h t e c h n i q u e s employed by k o k a n e e a r e more s u b -  h a b i t a t s p e c i f i c t h a n t h o s e used by t r o u t ( T a b l e 1 2 ) . 4;  The m a j o r i t y o f k o k a n e e s e a r c h f o r b e n t h i c p r e y  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 t h e s e d i m e n t  surface,  w h i l e t r o u t a r e most o f t e n o b s e r v e d s e a r c h i n g f o r b e n t h i c p r e y from p o s i t i o n s some 15 t o 30 cm o f f o f t h e b o t t o m  ( F i g . 16).  113  5.  The m a j o r i t y o f kokanee s e a r c h  f o r lake  surface  p r e y f r o m p o s i t i o n s t h a t a r e 5 t o 30 cm b e l o w t h e l a k e w h i l e t r o u t a r e most o f t e n o b s e r v e d 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 b e l o w t h e s u r f a c e 6.  Differences  i n the search  techniques  k o k a n e e r e s u l t i n some d e g r e e o f m i c r o h a b i t a t  7. for  (Fig.17).  o f t r o u t and  specific  b u t a r e n o t 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 ' in their  surface,  search  differences  diets. Differences  i n search  techniques  likely  account  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 predators  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  t h a n by t r o u t . 8. search  T r o u t and k o k a n e e 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 p o s i t i o n s i n t h e f i e l d and r e s p o n d t o i d e n t i c a l  prey  at s i m i l a r distances  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  t h e y 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 o f e i t h e r b e n t h i c 9.  o r lake surface prey i n the f i e l d .  The s a m p l i n g d e v i c e s  used by s c i e n t i s t s t o d e t e r m i n e  t h e 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 n o t d i s c r i m i n a t e b e t w e e n " p r e y " e x p o s e d o n t h e mud  surface,  l o c a t e d j u s t b e n e a t h t h e 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 u n d e r t h e mud s u r f a c e . and  When s e a r c h i n g  k o k a n e e u n d o u b t e d l y do d i s c r i m i n a t e i n t h i s  Therefore  fashion.  some d i s c r e p a n c i e s w i l l e x i s t between t h e a p p a r e n t  a v a i l a b i l i t y o f prey i n the lake and  f o r benthic prey, trout  ( a s measured by s c i e n t i s t s )  p a t t e r n s o f p r e y u t i l i z a t i o n by t r o u t and kokanee b e c a u s e  s c i e n t i s t s and f i s h do n o t use t h e same p r o c e d u r e s t o d e t e c t p r e y sampled from t h e l a k e  environment.  10.)  Differences  kokanee do n o t p r o v i d e  i n prey-search  b e h a v i o u r s o f t r o u t and  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 t h a n t r o u t (by no. o r by volume) f r o m r e l a t i v e l y s m a l l ( <1 mm)water-column o r f o r why k o k a n e e o b t a i n so few c a d d i s in their d i e t relative to trout.  prey  larvae of a l l sizes  115  CHAPTER 5 THE RELATIONSHIP BETWEEN ATTACK BEHAVIOURS, MORPHOLOGICAL CHARACTERISTICS AND DIETARY PATTERNS OF TROUT AND KOKANEE I N MARION LAKE  5-A.  Field Descriptions  INTRODUCTION Each s u c c e s s f u l a t t a c k o n p r e y by p r e d a t o r s u s u a l l y involves a s e r i e s of behaviours  which i n c l u d e o r i e n t a t i o n ,  a p p r o a c h , 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 o f t h e prey. prey  P r e d a t o r s c a n n o t s u c c e s s f u l l y a p p r o a c h and c a p t u r e item they d e t e c t nor can they s u c c e s s f u l l y manipulate  i n g e s t every prey behaviour  item they c a p t u r e .  and  I n t e r a c t i o n s between t h e  and m o r p h o l o g y o f a g i v e n p r e d a t o r d e t e r m i n e  an a t t a c k i s s u c c e s s f u l o r n o t .  every  whether  Consequently, d i f f e r e n c e s  between p r e d a t o r s i n b o t h a t t a c k b e h a v i o u r s 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 .  and  morphologies  Morphological  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 t h a n  behavioural  differences. S c h o e n e r (1974) h a s p o i n t e d o u t t h e c o n v e n i e n c e  and  occasional necessity o f estimating d i f f e r e n c e s i n resource utilization of species.  o f a n i m a l s by u s i n g m o r p h o l o g i c a l  characteristics  The most common i n d i c a t o r i s t h e s i z e o f t h e f e e d -  i n g s t r u c t u r e , w h i c h i s u s u a l l y c o r r e l a t e d w i t h mean f o o d hardness,  o r depth  i n some p r o t e c t i v e medium.  Many  size,  experimental  s t u d i e s s u p p o r t the c o n t e n t i o n logical  features  and  food  t h a t c o r r e l a t i o n s between morpho-  t y p e s do e x i s t .  r o d e n t s a p p e a r t o s e l e c t s e e d s on ( R o z e n z w e i g & S t e r n e r , 1970) . u t i l i z a t i o n by due  the b a s i s of ease of  Overlap i n nectar  bumblebee s p e c i e s  H e s p e n h e i d e , 1975;  handling  resource  ( H e i n r i c h , 1976)  t o d i f f e r e n c e s i n tongue l e n g t h .  e t a l . , 1976;  Small heteromyid  is slight  Numerous a u t h o r s  W i l l s o n , 1972;  (Grant  Rear,  1962)  have p r e s e n t e d e v i d e n c e 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 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 vs. small)  or texture Results  P u l l i a m and  (hard  vs.  sizes  to s i z e ( l a r g e  soft).  from t h e s e s t u d i e s a r e n o t w i t h o u t  E n d e r s (1971) f o u n d t h a t f i v e s p e c i e s o f  in  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  in  s p i t e of possessing  resources  a wide range of b i l l  exceptions finches  by  size,  s i z e s (9.3-18.9  mm).  I n R o o t ' s t h o r o u g h s t u d y (1967) o f f i v e f o l i a g e g l e a n i n g  birds,  p r e y s i z e was  bill  size.  n o t 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  H e s p e n h e i d e (1975) has  which species  warned t h a t many c a s e s e x i s t i n  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  q u i t e d i f f e r e n t d i e t a r y i t e m s due S i m i l a r l y , other in  species which possess s t r i k i n g  m o r p h o l o g y , may  again  not  for behavioural  by e x p e r i m e n t a l  to behavioural  exploit  differences. differences  exhibit striking differences in diet, reasons.  Field observations,  s t u d i e s o f f e r one  complemented  o f t h e b e s t ways i n w h i c h  a s s e s s t h e l i n k s between a t t a c k b e h a v i o u r s , c h a r a c t e r i s t i c s and  but  d i e t a r y h a b i t s of  morphological  predators.  I have i d e n t i f i e d a number o f d i f f e r e n c e s i n dietary patterns  o f t r o u t and  to  kokanee f r o m M a r i o n Lake  the (Chapter  117  Many o f t h e s e d i f f e r e n c e s have been a c c o u n t e d of  f o r on t h e b a s i s  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 ( C h a p t e r 3)  search behaviours  (Chapter 4),  or predator  however, the b a s i s f o r o t h e r 4).  d i e t a r y d i f f e r e n c e s r e m a i n s unknown ( s e e d i s c u s s i o n , C h a p t e r T h e r e f o r e , the c e n t r a l focus of t h i s chapter i s t o t e s t  the  general hypothesis that d i f f e r e n c e s i n predator attack behaviours will  interacting with differences i n predator  account  f o r some o f t h e s e u n e x p l a i n e d d i e t a r y  The  chapter i s divided  i n t o two  section i s essentially descriptive. techniques of approach  and  morphologies differences.  sections.  I will  The  d e s c r i b e the  c a p t u r e t h a t a r e used by t r o u t  kokanee d u r i n g a t t a c k s on p r e y  i n the f i e l d .  I will  and  a l s o des-  cribe d i f f e r e n c e s i n morphological c h a r a c t e r i s t i c s of p r e d a t o r s t h a t may  first  the  a f f e c t t h e i r attack success w i t h prey.  On  t h e b a s i s o f t h e s e d e s c r i p t i o n s I w i l l p r o p o s e a number o f hypotheses  concerning  t r o u t and k o k a n e e . of  t h e l i k e l y outcome o f a t t a c k s on p r e y  The  second  s e c t i o n o f the c h a p t e r  a s e r i e s of l a b o r a t o r y experiments  hypotheses.  Finally, I will  designed  to t e s t  consists these  d i s c u s s the s i g n i f i c a n c e o f  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  by  the  food-resource  p a r t i t i o n i n g e x h i b i t e d by t h e p r e d a t o r s i n t h e  field.  METHODS Attack  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 k o k a n e e were c o m p l e t e d on t h e i r s e a r c h b e h a v i o u r s .  a t t h e same t i m e as o b s e r v a t i o n s  D e t a i l s concerning  specific  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 be  o f obtaining observations can  f o u n d i n t h i s e a r l i e r work ( s e e C h a p t e r s 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 that the f i n e r  d e t a i l s o f a t t a c k s on p r e y i t e m s a r e seldom s e e n s i n c e t h e observer  i s u s u a l l y no c l o s e r t h a n .5 m t o t h e p r e d a t o r .  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  preliminary laboratory observations  For  during  of attack techniques  of  t r o u t and k o k a n e e e x p l o i t i n g a v a r i e t y o f p r e y .  Morphological 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 r a i n b o w t r o u t a p p e a r t o be s t o c k i e r t h a n kokanee o f 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 ,  a l s o a p p e a r t o be l e s s s t r e a m l i n e d  and t o p o s s e s s 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 versus  trout  body-length  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  their oral  cavities. I c o m p i l e d l e n g t h and w e i g h t d a t a on f i s h t a k e n f r o m  a summer t r a p p i n g p r o g r a m . from t r a p - n e t s anesthetized  T r o u t and kokanee were removed  ( i n w h i c h t h e y had been c a p t i v e s f o r 24-72 h o u r s )  with-MS 2 2 2 , measured f o r f o r k l e n g t h  e s t mm, and t h e n w e i g h e d on a gram b a l a n c e . recovery,  f i s h were r e l e a s e d  t o the lake.  r e l a t i o n s h i p between j a w w i d t h  and s t a n d a r d  t o the near-  A f t e r t a g g i n g and I determined the lengths of predators  from measurements t a k e n on s a m p l e s 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  as t h e h o r i z o n t a l d i s t a n c e  ( t o the nearest  calipers,  .1 mm) between t h e  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 . were a l s o c o n d u c t e d  on p r e s e r v e d  Gill-raker  examinations  fish.  RESULTS Techniques  o f A p p r o a c h and C a p t u r e Once a p r e y had been d e t e c t e d by a p r e d a t o r i n t h e  field, and  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 a p p r o a c h  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 p r e y  None o f t h e s e a p p r o a c h  (Table 1 7 ) .  and c a 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 s e a r c h t e c h n i q u e  ( C h a p t e r 4)  b u t some were used more  f r e q u e n t l y by t r o u t o r k o k a n e e .  TABLE 1 7 .  +++ ++ + N  T e c h n i q u e s o f a p p r o a c h and c a p t u r e used by t r o u t and k o k a n e e d u r i n g a t t a c k s o n p r e y i n the f i e l d .  Technique  L e v e l o f Occurrence Trout Kokanee  Rush Stalk Dart Jump Scrape  +++ + N ++ +  v e r y common common occasional never observed  ++ ++ ++ N +  1.  Rush A f t e r d e t e c t i o n of a food item the p r e d a t o r  briefly and  orients  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  j u s t b e f o r e t h e moment o f c o n t a c t opens i t s mouth t o e n g u l f  the p r e y .  T r o u t may  rush 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 ( C h a p t e r 4 ) . 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  The  rapid  are g e n e r a l l y  d e t e c t e d from l e s s t h a n 80 cm away, i s a c c o m p l i s h e d  by s e v e r a l  p o w e r f u l s t r o k e s o f t h e c a u d a l f i n . Rough e s t i m a t e s o f times taken t o cover the d i s t a n c e t o prey suggest a t t a i n v e l o c i t i e s o f 50 t o 150 invertebrate prey.  cm p e r second  that trout  during a rush  An e n t i r e a t t a c k s e q u e n c e ( o r i e n t ,  engulf) i s u s u a l l y completed  w i t h i n 2 to 3  on  approach,  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 a r e c l o s e r t o t h e l o c a t i o n s o f p r e y , t h e y have f e w e r o p p o r t u n i t i e s t o use t h i s t e c h n i q u e and when r u s h i n g p r e y t h e y do n o t 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  2.  appear  do.  Stalk 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  d e c e l e r a t e s by p e r f o r m i n g and p e l v i c f i n s . approach  typically  b r a k i n g movements w i t h b o t h p e c t o r a l  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  (1 t o 5 cm p e r s e c o n d ) i s a c c o m p l i s h e d  of p e c t o r a l f i n s c u l l i n g .  The  t h e f i n a l phase o f t h e a p p r o a c h  p r e d a t o r may  slow  w i t h the a i d  stop e n t i r e l y  before  when e i t h e r a r u s h ( t r o u t )  a d a r t ( k o k a n e e - see d e s c r i p t i o n below) w i l l be used t o t h e r e m a i n i n g d i s t a n c e ( u s u a l l y j u s t a few cm)  or  cover  t o the prey.  121  T r o u t do n o t use t h i s t e c h n i q u e o f a p p r o a c h and  I have o n l y o b s e r v e d  prey such as salamanders or  them u s e i t i n a p p r o a c h i n g v e r t e b r a t e (Ambystoma g r a c i l e , T a r i c h a g r a n u l o s a )  sticklebacks (Gasterosteus aculeatus).  approach  very o f t e n  Kokanee use t h i s  f r e q u e n t l y when t h e y a r e h u n t i n g f o r b e n t h i c p r e y 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 zooplankton 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,  k o k a n e e employ  d a r t t o c o v e r t h e f i n a l 2 t o 5 cm between t h e m s e l v e s types o f prey.  and c e r t a i n  D a r t b e g i n s from a s t a t i o n a r y p o s i t i o n .  p r e d a t o r f l e x e s i t s body i n t o an e x a g g e r a t e d  The  s i g m o i d p o s t u r e and  t h e n r a p i d l y t h r u s t s f o r w a r d w i t h mouth agape t o e n g u l f t h e prey.  The t e c h n i q u e d i f f e r s  p o s t u r e assumed  from r u s h o r s t a l k i n t h a t t h e  t o perform d a r t i s w e l l o u t s i d e o f the range o f  normal  swimming movements,  always  l e s s t h a n one body l e n g t h o f t h e p r e d a t o r , and t h e move-  ment i s c o m p l e t e d In  t h e d i s t a n c e c o v e r e d by d a r t i s  within a fraction of a  second.  t h e l a b o r a t o r y t h i s t e c h n i q u e was p r e d i c t a b l y e v o k e d  by copepods ( D i a p t o m u s k e n a i ) and l e s s commonly  by amphipods  ( H y a l e l l a a z t e c a ) w h i c h e x h i b i t marked a v o i d a n c e r e s p o n s e s t o approaches  by p r e d a t o r s .  In the f i e l d  kokanee commonly  exhibited  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 o n p r e y  in the f i e l d o r i n the laboratory.  122  4.  Jump T h i s movement i s o n l y e x e c u t e d by t r o u t when t h e y  a d u l t i n s e c t s above t h e l a k e s u r f a c e . the a t t a c k are the p r e d a t o r continue  initial  stages  i t t o b r e a k t h e s u r f a c e o f the w a t e r and  i n t o the a i r to c o n t a c t  and  capture  the p r e y .  to  small cm  c l e a r t h e w a t e r by more t h a n 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 p r e y s u c h as c a d d i s  adults.  Scrape W h i l e moving a l o n g  with  A  w h i c h 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  b e l o w t h e l a k e s u r f a c e may  • 5.  of  i d e n t i c a l t o r u s h , however t h e a c c e l e r a t i o n o f  allows  t r o u t (15 cm)  The  detect  a f i r m , smooth s u b s t r a t e , a  fish,  i t s mouth h e l d f l u s h t o t h e 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  this t a c t i c to  attack  s m a l l , s e s s i l e animals which are anchored to the underside  of  lily  and  l e a v e s , p l a n t s t e m s , submerged l o g s and  kokanee most f r e q u e n t l y use lily  l e a v e s w h i c h may  s q u a r e cm  the technique  moves from t h e u n d e r s i d e bed  and  Trout  t o o b t a i n p r e y from  p o s s e s s more t h a n 25 c l a d o c e r a n s  i n l a t e summer ( S t a r r , 1 9 7 3 ) .  single l i l y  limbs.  may  o f one  A predator  per  typically  l e a f t o the next w i t h i n a  move d i r e c t l y f r o m one  s e v e r a l m e t e r s away where i t w i l l  continue  bed  t o use  to another this  attack  technique. Kokanee a p p e a r e d t o use than t r o u t but  t h i s technique  t h i s i s l i k e l y because o n l y a s m a l l  less frequently proportion  o f t h e t o t a l number o f kokanee o b s e r v e d were i n i n s h o r e t h a t have a b u n d a n t weed beds and  areas  submerged s n a g s ( C h a p t e r 3 ) .  From b o t h f i e l d and  laboratory, observations,  summarized t h e p r e y t y p e s t h a t a r e d i f f e r e n t a p p r o a c h and observations  are  t a k e n by  q u a l i t a t i v e and  o f t e n t h e t a c t i c s a r e used o r t h e f u l l t h a t each p r e d a t o r  the p r e d a t o r s  capture techniques (Table  strictly  exploits.  I have  18).  using  These  do n o t r e v e a l  range of prey  However, i t i s a p p a r e n t  how  species that  d i f f e r e n t t a c t i c s a r e used w i t h d i f f e r e n t p r e y and  that  and  t h e same  k o k a n e e may  use  d i f f e r e n t techniques to attack  trout  prey.  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 s i g n i f i c a n t l y heavier from M a r i o n Lake. due  than kokanee o f e q u i v a l e n t  (Fig.  T h e r e f o r e d i f f e r e n c e s i n a p p e a r a n c e must  t o d i f f e r e n c e s i n t h e way  apportioned.  length  The  that a given  f a l s e impression  by  a p p e a r a n c e o f t h e head r e g i o n o f t r o u t .  the l e s s  In q u a n t i t a t i v e terms  kokanee of e q u i v a l e n t  k o k a n e e e x h i b i t jaw  o r a p p r o x i m a t e l y 30% s m a l l e r  size (Fig. 22).  I t w o u l d be  the p r e d a t o r s  w i d t h s t h a t a r e 3-4  than t r o u t of e q u i v a l e n t  jaw For  when f a c e d  handling  with a v a r i e t y of  mm  body  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  a m a j o r i n f l u e n c e on t h e c a p t u r e and by  than  spanning the s i z e range of most i n t e r e s t i n t h i s  s t u d y ( 1 2 0 - 1 6 0 mm)  size.  be  streamlined  t h i s d i f f e r e n c e i s e x p r e s s e d c l e a r l y by d i f f e r e n c e s i n t h e w i d t h s o f t r o u t and  21)  amount o f w e i g h t i s  t h a t t r o u t are h e a v i e r  size-matched kokanee i s i n p a r t c r e a t e d  predators  not  have  success experienced prey.  124  TABLE 18.  a.  A l i s t o f p r e y t y p e s o b s e r v e d t o evoke s p e c i f i c a p p r o a c h and c a p t u r e t e c h n i q u e s i n t h e l a b o r a t o r y ( L ) and f i e l d ( F ) .  Trout  Rush  Stalk  Jump  Scrape  amphipods (L & F)  salamanders (F)  chironomid a d u l t s (F)  cladocerans S i d a s p . (F)  caddis larvae (F)  stickleback (F)  caddis adults (F) damselfly a d u l t s (F)  d a m s e l f l y 1. (L) gyrinids (L & F) gerrids (F) sticklebacks (F) aquatic insect pupae ( L & F) notonectids (L) copepods & c l a d o c e r a n s ( L ) b.  Kokanee  Rush  Stalk  Dart  Scrape  amphipods (L & F) m a y 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) cladocerans Daphnia sp. (L) n o t o n e c t i d s (L)  copepods (L) amphipods (L & F)  copepods (L) amphipods (L & F)  cladocerans S i d a s p . ( L & F)  125  FIGURE 2 1 .  The r e l a t i o n s h i p between f o r k l e n g t h of  and wet w e i g h t  t r o u t and kokanee f r o m M a r i o n L a k e .  representative plotted  data points  Note  that  were r a n d o m l y drawn and  b u t N = t h e number o f t r o u t o r k o k a n e e  a c t u a l l y measured.  125a  o TROUT  Log Y = - 1.928 +2.925 Log X  N=I20  -1  5  r  r-—i  6  7  r 8  9  FORK  1  10 LENGTH  (cm)  1  r~  20  30  126  FIGURE 22.  The r e l a t i o n s h i p between s t a n d a r d l e n g t h of  t r o u t and kokanee from M a r i o n L a k e ,  t a k e n as t h e h o r i z o n t a l d i s t a n c e t i p s o f the p r e m a x i l l a r i e s . data points  and j a w gape j a w gape i s  between t h e p o s t e r i o r  Note t h a t  representative  have been p l o t t e d b u t N = t h e number o f  t r o u t o r kokanee a c t u a l l y m e a s u r e d .  9  T R O U T , Y= - . 5 5 6 7 + . 0 8 3 8 8 x , N= 122  O  KOKANEE, Y = - . 5 9 8 7  + . 0 6 2 3 4 x , N=98  STANDARD  LENGTH (mm)  CTi  127  O r d i n a r i l y k o k a n e e p o s s e s s 28 r a k e r s on t h e f i r s t g i l l length rakers  (Clemens and  25 kokanee and  ion  Wilby,  long,  serrated  t r o u t p o s s e s s 17  1961).  My  t o 21 medium  examination of  t r o u t from M a r i o n Lake i n d i c a t e s t h a t kokanee  have 28-32 g i l l 17-21.  arch, while  t o 40  r a k e r s on  Thus a l t h o u g h  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  k o k a n e e f r o m M a r i o n Lake e x h i b i t a  i n the range of g i l l  rakers normally  observed, they  reductstill  p o s s e s s many more t h a n t r o u t .  DISCUSSION A s i n g l e a t t a c k on proceed i n stages  a p r e y i s a complex e v e n t t h a t  from i n i t i a l o r i e n t a t i o n t h r o u g h  contact, capture, manipulation a predator.  and  In t h i s study,  which r e s u l t s i n the I operationally define  a t t a c k s u c c e s s as t h e p r o p o r t i o n o f p r e y i n g e s t e d detected predators  and and  then approached. prey during  any  approach,  i n g e s t i o n o r r e j e c t i o n by  A s u c c e s s f u l a t t a c k i s one  i n g e s t i o n of the prey.  to prey  A l t h o u g h i n t e r a c t i o n s between s t a g e o f an a t t a c k may  influence  o v e r a l l l e v e l s of attack success,  the d e s c r i p t i o n s  h e r e were n o t  a d e t a i l e d assessment of  intended  to provide  i n f l u e n c e o f e a c h a t t a c k s t a g e on a t t a c k s u c c e s s Beukema, 1968) , b u t  may  r a t h e r to provide  propose s p e c i f i c hypotheses concerning comes o f a t t a c k s t h a t t r o u t and  presented  (e.g.,  a b a s i s on w h i c h  to  d i f f e r e n c e s i n the  kokanee are l i k e l y  in encounters with a v a r i e t y of n a t u r a l prey.  the  to  out-  experience  128  The  Capture-success Capture  Hypothesis s u c c e s s i s o p e r a t i o n a l l y d e f i n e d here as t h e  p r o p o r t i o n o f prey t h a t a r e grasped  f i r m l y o r "secured" i n the  mouth o f a p r e d a t o r r e l a t i v e t o t h e t o t a l number o f p r e y 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 t h e c a p t u r e s u c c e s s h y p o t h e s i s , I have r e l i e d on t h e o b s e r v a t i o n s t h a t t r o u t and k o k a n e e sometimes use 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 on t h e same t y p e s o f p r e y 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 a p p r o a c h  and c a 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 of  prey  types  ( T a b l e 1 8 ) . F o r 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  "rush" during attacks  on copepods may e x p e r i e n c e l o w e r l e v e l s o f c a p t u r e t h a n kokanee u s i n g " s t a l k and d a r t " .  Field observations also  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 m i s s when j u m p i n g 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 n e v e r on s u r f a c e p r e y s u c h a s w a t e r s t r i d e r s the i n i t i a t i o n The  success  to catch  follows a "rush"  (Gerridae), i n spite of  o f numerous a t t a c k s by t r o u t . o b s e r v a t i o n s above s u g g e s t 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  likely  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 a r e 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 w h i c h avoidance  r e s p o n s e s d u r i n g a t t a c k s by f i s h .  exhibit  Such p r e y w i l l t a x  t h e 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 t o t h e p o i n t of  c a p t u r e , b u t once c a p t u r e d , a r e u n l i k e l y t o p r e s e n t much  difficulty  d u r i n g t h e m a n i p u l a t i o n and i n g e s t i o n p h a s e s o f  129 attack.  S i n c e a number o f t h e p r e y t y p e s c o n s t i t u t i n g  signifi-  c a n t p r o p o r t i o n s o f t r o u t and k o k a n e e d i e t s c o n f o r m t o t h e s e characteristics  ( 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  m i g h t f o r m t h e b a s i s f o r e x p l a n a t i o n s o f some o f t h e 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 k o k a n e e . capture success hypothesis c l e a r l y stated  T h e r e f o r e , the  i s that "differences  i n t h e c a p t u r e s u c c e s s o f t r o u t and k o k a n e e d u r i n g  encounters  w i t h s m a l l p r e y i n t h e f i e l d p l a y an i m p o r t a n t r o l e predator-specific,  diets."  The G i l l - r a k e r , P r e y - s i z e ,  Hypothesis  Kokanee p o s s e s s a g r e a t e r number o f l o n g g i l l - r a k e r s than t r o u t .  Kokanee i n M a r i o n Lake  e x p l o i t l a r g e r q u a n t i t i e s and ( C h a p t e r 2, F i g s . 2 and 7 ) . throughout  i n shaping  serrated  consistently  s m a l l e r zooplankton than t r o u t These d i f f e r e n c e s i n d i e t  t h e g e o g r a p h i c r a n g e o f t r o u t and k o k a n e e .  do  occur Thus,  t h i s s p e c i e s p a i r s e r v e s as a n o t h e r example o f t h e o b s e r v a t i o n by many a u 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 z o o p l a n k t o n t o a g r e a t e r e x t e n t as a of food than s p e c i e s which possess  fewer, s h o r t e r ,  Many a u t h o r s have s u g g e s t e d  gill-rakers.  that gill-rakers  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  source  function  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  for a critical  review of t h i s t o p i c ) .  I initially  t h a t kokanee i n M a r i o n Lake must o b t a i n more z o o p l a n k t o n  expected than  130  t r o u t by f i l t e r i n g The  i n response t o high d e n s i t i e s o f small prey.  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  a s an " a t t a c k "  p r o c e d u r e i n c l u d e t h e m a i n t e n a n c e o f a f u l l y - o p e n e d mouth f o r up t o a few s e c o n d s a t a t i m e d u r i n g p a s s a g e b y a f i s h a concentration of small plankton.  through  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 t h e f i e l d e x h i b i t e d t h e 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 t h e t a c t i c o f " f i l t e r i n g " small prey.  Instead, both species always operated i n a  " r a p t o r i a l " mode by p u r s u i n g Apparently  and g r a s p i n g  i n d i v i d u a l prey  some p l a n k t o n - f e e d i n g  fish  items.  always operate  i n r a p t o r i a l mode ( K j e l s o n , 1 9 7 1 ) , w h i l e o t h e r s c a n o p e r a t e i n either filtering  o r r a p t o r i a l modes (Leong & O ' C o n n e l l , 1969;  O'Connell & Z w e i f e l , 1972). of technique  For the l a t t e r  species, the choice  a p p e a r s t o be i n f l u e n c e d by t h e a b s o l u t e  density  o f t h e p r e y and t h e r e l a t i v e p r o p o r t i o n o f l a r g e t o s m a l l I d i d n o t take s i z e s of zooplankton observation  samples t o determine the d e n s i t i e s o r  present  i n the f i e l d during  sets, consequently the hypothesis  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 occasions  prey.  any o f t h e  t h a t kokanee  t h a n t r o u t by f i l t e r i n g  on  when t h e y e n c o u n t e r h i g h d e n s i t i e s o f s m a l l p r e y  r e m a i n s t o be t e s t e d u n d e r c o n t r o l l e d c o n d i t i o n s i n t h e l a b o r a t o r y .  The  Attack  Rate H y p o t h e s i s A l t h o u g h t r o u t and k o k a n e e o f t e n a p p e a r t o use t h e  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 , there 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 t h e number o f a t t a c k s initiated.  F o r e x a m p l e , when f o r a g i n g  i n the water column,  k o k a n e e were o b s e r v e d t o i n i t i a t e as many as 15 a t t a c k s a 2 meter d i s t a n c e w h i l e  t r o u t s e l d o m i n i t i a t e d more t h a n  Given the v e l o c i t i e s maintained  by t h e p r e d a t o r s  kokanee must a t t a i n a t t a c k  exceeding 1 per second w h i l e  t r o u t do  The  rates  not exceed 1 a t t a c k  d i f f e r e n c e i n a t t a c k r a t e s may  o f e i t h e r t r o u t and  be  kokanee c o n s i s t e n t l y h u n t i n g  for different  a higher  o r s h o r t e r a t t a c k t i m e s t h a n t r o u t on  t h e same p l a n k t o n i c  be  The  t r u e and  "attack" rate hypothesis  r a t e of  detection  assumes t h e l a t t e r c a s e  t h a t as a c o n s e q u e n c e kokanee may  greater q u a n t i t y of zooplankton  per  a consequence  t y p e s o f p r e y o r o f kokanee a c h i e v i n g  prey.  5.  at these  t i m e s ( C h a p t e r 4, T a b l e 1 0 ) /  3 seconds.  over  to  accumulate a  i n t h e i r d i e t compared  to  trout.  The  Mouth-size, Ingestion-success  Hypothesis  Kokanee e x h i b i t s m a l l e r mouth d i m e n s i o n s t h a n t r o u t o f s i m i l a r l e n g t h and f o r the capture likely  and  weight.  A l t h o u g h t h i s may  be  advantageous  r e t e n t i o n of s m a l l , a g i l e prey, i t i s a l s o  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  kokanee compared t o 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 i n g e s t i n g l a r g e "armoured" p r e y o n c e t h e y have been I f t h i s p r o v e s t o be the o u t s t a n d i n g included  for and  captured.  t h e c a s e i t w i l l h e l p e x p l a i n a number o f  d i f f e r e n c e s i n the types o f prey t h a t  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  are  kokanee.  132  5-B.  L a b o r a t o r y T e s t s of Hypotheses Concerning P r e y A t t a c k by T r o u t and Kokanee I have p r o p o s e d  f o u r hypotheses  concerning  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 k o k a n e e .  This  section  c o n s i s t s of a s e r i e s of l a b o r a t o r y experiments designed  to test  these  hypotheses.  METHODS Experiment  5.1  The  Capture-success  This experiment  Hypothesis  c o n s i s t s of feeding t r i a l s  i n which  t r o u t and 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 t o 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 ( D i a p t o m u s k e n a i ) , and c l a d o c e r a n s ( D a p h n i a p u l e x ) .  These p r e y were c h o s e n  so  t h a t d i f f e r e n c e s i n a t t a c k s u c c e s s by t h e p r e d a t o r s w o u l d depend upon a p p r o a c h  and  c a p t u r e t e c h n i q u e s r a t h e r t h a n on  during manipulation 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 of the s m a l l e s t prey type  for  feeding t r i a l s  visible  (D_. p u l e x ) had  selected  t o be l a r g e enough t o be  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  (92 x 47 x 45 cm)  difficulties  arena  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 M a r i o n L a k e . were o b t a i n e d from n e a r b y  D_. k e n a i  E u n i c e 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 to o b t a i n prey of 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.  T r o u t and k o k a n e e used  in trials  133  w i t h t h e above p r e y were s i z e - m a t c h e d .  T a b l e 19 l i s t s t h e  c h a r a c t e r i s t i c s o f b o t h p r e d a t o r s and p r e y used ment. for in  i n this  I followed standard procedures, o u t l i n e d e a r l i e r  experi(Chapter 1 ) ,  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 o f p r e d a t o r s t o t a k e p a r t experiments. To c o n d u c t  a trial,  I introduced a single predator  i n t o t h e a r e n a 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 prey.  I t h e n r e c o r d e d c a p t u r e s u c c e s s (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 s u c c e s s ( I S = # o f p r e y i n g e s t e d / # o f prey captured) o f predators d u r i n g encounters w i t h prey. A l t h o u g h m u l t i p l e t r i a l s were p e r f o r m e d prey combination are pooled  here.  Experiment  5.2  w i t h each p r e d a t o r -  (see Chapter 6 ) , r e s u l t s across a l l t r i a l s  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 z o o p l a n k t o n have maximum body d i m e n s i o n s  o f l e s s t h a n 3 mm.  Preliminary field  observa-  t i o n s l e d me t o s u g g e s t t h a t kokanee m i g h t be c a p a b l e o f s u s 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 concent r a t i o n s of small p l a n k t o n i c prey. I conducted  To t e s t t h i s h y p o t h e s i s ,  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 p u l e x 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 conditions.  I used  D. p u l e x ( 1 - 3 mm)  a s i e v e t o o b t a i n a l i m i t e d s i z e range o f  from l a b o r a t o r y c u l t u r e s .  P r e y 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 n o t r e p l a c e d a s t h e trial  proceeded.  TABLE 1 9 .  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 p r e y used i n e x p e r i m e n t s t o d e t e r m i n e t h e a t t a c k s u c c e s s o f t r o u t and kokanee on a v a r i e t y o f p r e y .  PREY  KOKANEE Mean Size (mm)  Identity  Range  NO. Used  Zooplankton  Mean Size ( cm)  Daphnia s p .  1.5  1.0-  2.5  5  11.4  Diaptomus k e n a i  2.0  1.0-  3.0  4  8.4  4  16.4  Chaoborus spp.  10.0  7.5-14.5  TROUT Range  No. Used  Mean Size (cm)  10 . 3 - 1 2 . 7  4  11.2  9.2-13.1  9.3  4  8.9  7.7-11.4  15.6-17.7  6  16.4  14.9-21.6  7.6-  6  *  8.7  Range  7.7-  9.2  Amphipods Hyalella azteca  4.6  3.5-  6.0  4  13.0  12.0-13.5  4  15.2  13.4-17.0  Crangonyx  8.3  8.0-  8.7  3  14.7  14.5-14.9  4  15.2  13.4-17.0  4  12.1  11.8-12.4  richmondensis  Others Notonecta undulata & Buenoa c o n f u s a  10 .5  6.5-14.0  4  16.4  15.6-17.7  Ephemeroptera ( C e n t r o p t i l u m sp.)  11.8  10 . 1 - 1 2 . 6  3  12.7  12.4-13.3  Odonata ( E n a l l a g m a s p . )  15.6  15.0-16.2  3  14u2  13.2-15.0  Trichoptera (species unknown).  15.0  10.0-16.5  8  14.8  13.4-16.0  Not-tested 4  15.0  Not  12.5-16.5  tested  two g r o u p s o f t r o u t were used i n t h e t e s t s w i t h Chaoborus s p p . i n o r d e r t o examine t h e 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 c a p t u r e and i n g e s t i o n s u c c e s s .  Any  a d v a n t a g e t h a t kokanee m i g h t d i s p l a y compared  to  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 p r e y 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 d e t e r m i n e d d i f f e r e n c e s i n b e h a v i o u r 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  have  i n foraging f o r zooplankton.  t h e two  predators  To p a r t i a l l y e l i m i n a t e  had  the  i n f l u e n c e of d i f f e r e n c e s i n foraging experience  on r e s u l t s i n  t h i s e x p e r i m e n t , 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 feeding  i n t h e f a l l o f 1973  and  provided  regime i n the l a b o r a t o r y u n t i l  ments i n J u n e o f  them w i t h an  identic  the time of the e x p e r i -  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 f e d 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 s h r i m p o r fresh zooplankton. s m a l l and  A l l of these food  were o b t a i n e d  A forced a i r bubbler during  relatively  i n t h e w a t e r column by t h e  helped  the f e e d i n g p e r i o d s .  e x p e r i m e n t 5.2  i t e m s were  predators.  k e e p t h e b r i n e s h r i m p suspended Thus, t r o u t and  were h i g h l y p r e c o n d i t i o n e d  kokanee used i n  t o f e e d upon z o o -  plankton.  E x p e r i m e n t 5.3 The  The  G i l l - r a k e r , Prey-size  observation  prey i n the f i e l d d i d not more numerous g i l l  Hypothesis  t h a t kokanee d i d not falsify  the h y p o t h e s i s  "filter" that  small  their  r a k e r s g i v e them an a d v a n t a g e o v e r t r o u t  in obtaining small prey.  Field observations  may  not  have  c o i n c i d e d w i t h the t i m e s 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 p r e y w h i c h a r e 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 for  t r i a l s w i t h s i z e matched t r o u t and k o k a n e e f o r a g i n g  a mixture of very small zooplankton at d e n s i t i e s ranging  from 7 t o 35 p e r l i t e r . p r e y from E u n i c e L a k e .  I o b t a i n e d t h e z o o p l a n k t o n used P r e y l a r g e r t h a n 1.5  from t h e l a b o r a t o r y s t o c k s by s i e v i n g . one  In each  r e p l i c a t e p o r t i o n o f t h e p r e y m i x t u r e was  stored and  mm  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  were e x c l u d e d set of  trials  s e t a s i d e and  of species composition  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 date.  were used  as  The o t h e r p o r t i o n s  i n t r i a l s with individual predators.  Experiment  5.4  The  Mouth-size, Ingestion-success Hypothesis  This experiment  c o n s i s t s of feeding t r i a l s  i n which  t r o u t o r k o k a n e e 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 t o 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 ( O d o n a t a ) , boatmen ( N o t o n e c t a ) , amphipods ( A m p h i p o d a ) , c h a o b o r u s ( D i p t e r a ) and m a y f l y nymphs ( E p h e m e r o p t e r a ) . tities Table  Specific  water  larvae iden-  o f p r e y , p r e y s i z e s and p r e d a t o r s i z e s a r e l i s t e d  in  19. I selected  the f i r s t f o u r prey types f o r t h i s  ment b e c a u s e t h e y p o s s e s s e d  combinations of  experi-  characteristics  ( l a r g e s i z e , t o u g h body c o v e r i n g s , o r s l o w movements) w h i c h improved  the l i k e l i h o o d  that d i f f e r e n c e s i n a t t a c k success  t h e p r e d a t o r s w o u l d depend on m a n i p u l a t i o n and than approach  and  capture techniques.  The  by  ingestion rather  l a s t two p r e y  types  were s e l e c t e d b e c a u s e t h e y were w e l l w i t h i n the s i z e r a n g e  of  137  t h e f i r s t f o u r p r e y t y p e s b u t t h e y l a c k e d t h e i r t o u g h , bodycoverings.  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 of  interactions  between p r e y s i z e , p r e y " t e x t u r e " , p r e d a t o r mouth s i z e 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  ( C h a p t e r 6)  r e s u l t s a c r o s s a l l t r i a l s have been p o o l e d  here.  In  but  a l l t r i a l s except those d e a l i n g w i t h caddis l a r v a e ,  t h e s p e c i e s named was on c a p t u r e and  the o n l y prey p r e s e n t .  I obtained  results  i n g e s t i o n s u c c e s s o f kokanee on t r i c h o p t e r a n s  from mixed p r e y t r i a l s  i n w h i c h e q u a l numbers o f o d o n a t e s ,  e p h e m e r o p t e r a n s and amphipods were a l s o p r e s e n t . likely  and  t o have i n f l u e n c e d  T h i s i s not  the r e s u l t s r e p o r t e d here.  RESULTS E x p e r i m e n t 5.1  The  Capture-success  Hypothesis  As e x p e c t e d , t h e o v e r a l l a t t a c k s u c c e s s ( p r e y i n g e s t e d to  p r e y 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 p r e y  l o n g ) was  c o n t r o l l e d p r i m a r i l y by  (<5  mm  i n t e r a c t i o n s between  p r e d a t o r s and p r e y d u r i n g t h e a p p r o a c h and  c a p t u r e phase o f  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 , t h e 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 s u c c e s s w i t h p r e y a f t e r c a p t u r e ( T a b l e 2 0 ) . T r o u t and kokanee b o t h used " r u s h " as t h e a p p r o a c h and  capture technique i n encounters w i t h small cladocerans  (p.  pulex).  The  a b s e n c e o f any a v o i d a n c e r e s p o n s e by  p r e y e n a b l e d b o t h t r o u t and kokanee t o a c h i e v e 100%  these  capture  TABLE 20.  Prey  A c o m p a r i s o n o f c a p t u r e s u c c e s s (CS) and i n g e s t i o n s u c c e s s ( I S ) 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 prey. TA = t o t a l a t t a c k s i n i t i a t e d . D a t a on CS and I S a r e expressed t o the nearest percentage p o i n t .  Identity  Mean in  size mm  KOKANEE  TROUT CS  IS  No. o f t r i a l s conducted w i t h individual fish  TA  CS  IS  TA  Trout  Kokanee  D. p u l e 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  success.  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 s u c c e s s i n e n c o u n t e r s w i t h c o p e p o d s (I). k e n a i ) . 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  e x h i b i t e d by t h e c o p e p o d s .  "Rush" was  This  responses  the o n l y approach  and  c a p t u r e t e c h n i q u e used by t r o u t , i n e n c o u n t e r s w i t h copepods,. .While k o k a n e e s h i f t e d  from " r u s h " d u r i n g t h e f i r s t  few a t t a c k s  on copepods t o a c o m b i n a t i o n o f " s t a l k - a n d - d a r t " f o r t h e r e m a i n d e The g r e a t e r c a p t u r e s u c c e s s t h a t k o k a n e e , 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 capture techniques.  t o t h i s s h i f t of approach  I t i s n o t c l e a r a t t h i s p o i n t why  trout  i n g e s t e d o n l y 25% o f t h e c o p e p o d s t h a t t h e y d i d c a p t u r e . 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  and  This  success  t h a t kokanee e x h i b i t e d w i t h t h e s e p r e y ( T a b l e 2 0 ) . T r o u t 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 a p p r o a c h encounters w i t h H y a l e l l a sp. responses The  and  capture techniques i n  These p r e y e x h i b i t e d  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 .  h i g h c a p t u r e s u c c e s s o f t r o u t and kokanee w i t h t h e p r e y  i n d i c a t e s t h a t the avoidance  r e s p o n s e o f amphipods i s n o t  e f f e c t i v e as t h a t o f copepods i n p r o m o t i n g by  avoidance  as  escape d u r i n g a t t a c k s  fish. 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 p r e y d u r i n g t h e a p p r o a c h attack.  and  c a p t u r e phase o f  These r e s u l t s g e n e r a l l y s u p p o r t t h e h y p o t h e s i s t h a t  s u c h i n t e r a c t i o n s may  assume an i m p o r t a n t r o l e i n s h a p i n g  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  Hypothesis  T r o u t and k o k a n e e e x p e r i e n c e d i d e n t i c a l l e v e l s o f c a p t u r e s u c c e s s i n e n c o u n t e r s w i t h c l a d o c e r a n s (D_. p u l e x ) , h o w e v e r , r e s u l t s from t h e p r e s e n t e x p e r i m e n t  indicate that 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 p r e y a t various densitites.  The means o f t h e maximum a t t a c k r a t e s  a c h i e v e d by kokanee a t a l l p r e y d e n s i t i e s a r e more t h a n t h o s e 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  double  ( F i g . 23).  Kokanee p u r s u e d D a p h n i a s p . by t r a c i n g a smooth p a t h t h a t f l o w e d from one c a p t u r e t o t h e n e x t w i t h  little  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 c h a n g e s o f a l i g n m e n t one  attack to 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  m i d w a t e r , s i g h t e d a p r e y , s t o p p e d and t h e n r u s h e d capture.  i n t o make a  A f t e r a capture t r o u t o f t e n stopped a b r u p t l y " t o  s e a r c h f o r and l i n e up on" t h e n e x t p r e y i t e m .  Thus, t h e 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 a r e l i k e l y of  from  a consequence  i n t e r a c t i o n s between a h i g h e r r a t e o f p r e y d e t e c t i o n and  s h o r t e r a t t a c k t i m e s on p r e y .  S i n c e t r o u t and kokanee used i n  t h i s e x p e r i m e n t were h i g h l y " p r e c o n d i t i o n e d " t o f e e d 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 s u p p o r t t h e h y p o t h e s i s t h a t kokanee may accumulate  a greater quantity o f zooplankton i n t h e i r  diet  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 o f s u c c e s s f u l a t t a c k on such  prey.  141  FIGURE 2 3 .  The r e l a t i o n s h i p between plankton  the d e n s i t y o f small  (1.5 mm D a p h n i a sp.) and t h e maximum a t t a c k  r a t e s o f t r o u t and k o k a n e e . t h e 95% c o n f i d e n c e  V e r t i c a l bars i n d i c a t e  i n t e r v a l s o f t h e means.  number o f t r o u t o r kokanee density.  zoo-  N = the  used i n t r i a l s a t e a c h  142  Experiment  5.3 It  flexibility  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 i s c e r t a i n t h a t some f i s h have t h e b e h a v i o u r a l  t o e x p l o i t z o o p l a n k t o n one a t a t i m e o r ,  depending  on t h e d e n s i t i e s and s i z e s o f p r e y , t o s w i t c h o v e r t o a mode o f o p e r a t i o n i n w h i c h many p r e y a r e " f i l t e r e d " simultaneously.  from t h e w a t e r  F i s h a t t a c k i n g p r e y by t h e l a t t e r method  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  exhibit  respiratory  movements. In at  s p i t e of the p r o v i s i o n o f very s m a l l prey  ( F i g . 24)  d e n s i t i e s r a n g i n g from 7 - 3 5 p e r l i t e r , n e i t h e r t r o u t o r  kokanee were i n d u c e d t o e x p l o i t z o o p l a n k t o n by " f i l t e r i n g " .  These  r e s u l t s f a i l t o s u p p o r t t h e 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 z o o p l a n k t o n t h a n t r o u t do a s  as c o n s e q u e n c e o f an a b i l i t y column.  t o f i l t e r s u c h p r e y from t h e w a t e r  However, t h e r e s u l t s do o f f e r s t r i k i n g  testimony to 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 r e s e v e n t o ten  t i m e s h i g h e r t h a n t h o s e a t t a i n e d by t r o u t ( T a b l e 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 t i m e s a s many a t t a c k s  on s m a l l z o o p l a n k t o n ( s e e T a b l e 21  f o r s p e c i e s composition) as  t r o u t (Table 2 3 ) . at  the average  G i v e n t h a t kokanee c a n f u n c t i o n c o n t i n u o u s l y  o f t h e i r maximum a t t a c k r a t e and a s s u m i n g 100%  c a p t u r e s u c c e s s , t h e y 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 p e r hour.  However a l i m i t e d amount o f d a t a  suggests  t h a t capture success i s about 70%, t h e r e f o r e the n e t i n t a k e p e r h o u r i s l i k e l y c l o s e r t o 1820 z o o p l a n k t o n .  T r o u t i n t h e 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 o f p r e y used J u l y 18 and J u l y 26 f o r t h e s m a l l - p l a n k t o n , t r i a l s w i t h t r o u t and  kokanee.  between feeding  N= 300  0-0.9  .2-.29  .4-.49  .6-.69  .8-.89  BODY LENGTH OF PREY (mm)  1.0-1.09  12-1.29  1.4-1.49  144  TABLE 2 1 .  S p e c i e s c o m p o s i t i o n ( a s % by number) o f s m a l l z o o p l a n k t o n used i n e x p e r i m e n t 5.3 w i t h t r o u t and k o k a n e e .  Identity Diaptomus t y r e l l i  Low  Density  7/1 i t e r  Medium D e n s i t y 27/liter  High D e n s i t y 35/liter  82  60  83  Bosmina s p .  9  17  9  Holopedium sp.  3  6  5  Polyphemus s p .  3  6  2  Daphnia sp.  0  0  1  Diaphanosoma s p .  2  10  1  Rotifers  0  1  0  Others  1  1  0  1,552  1,908  3,466  Sample s i z e  145  TABLE 2 2 .  Means and 95% c o n f i d e n c e l i m i t s o f t h e maximum a t t a c k r a t e s p e r m i n u t e a c h i e v e d 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 densities.  Density o f prey per l i t e r 7 27 35 Pooled  data  TABLE 2 3 .  Trout  Kokanee N  N 4.00 + 5.7  3  5.30 + 3.2  4  4.00 + 4.7  4  4.50 + 1.7  11  49.5  +  2 .1  4  35.3  + 17 .2  3  40.5  +  2  42.8 +  7 .0  9  Means and 95% c o n f i d e n c e l i m i t s o f t h e t o t a l a t t a c k s p e r .5 h o u r s c o m p l e t e d by t r o u t and k o k a n e e e x p l o i t i n g small zooplankton at various d e n s i t i e s .  Kokanee  Trout  Density o f prey per l i t e r  N  N  7  43 + 83  3  875 + 211  4  27  49 + 33  4  735 + 760  3  35  36  4  673  2  11  783 + 160  9  Pooled  data  ±  6 1  46 + 23  146  i n t e r v a l o f t i m e w o u l d 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 t h e y a t t a i n 100% and 70% c a p t u r e s u c c e s s One p r o b l e m  respectively.  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  that  t r o u t a r e p u t t i n g o u t 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 b u t choose n o t t o .  Certainly i n exploiting  D a p h n i a s p . t h e s e same t r o u t a v e r a g e d per minute,  b e t t e r t h a n 30 a t t a c k s  t h a t i s , almost 8 times t h e i r performance  smaller zooplankton.  o n t h e much  Given that t r o u t can detect the smaller  z o o p l a n k t o n , t h e r e i s no r e a s o n f o r them t o f a i l  i n attaining  s i m i l a r a t t a c k r a t e s on them. The e v i d e n c e a b o u t t h e d e t e c t a b i l i t y o f t h e s e p r e y f o r trout i s conflicting. was d i f f i c u l t  D u r i n g 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  to confirm that trout directed attacks at i n d i v i d u a l  p l a n k t e r s b e c a u s e t h e i r s i z e s a r e a t t h e l o w e r l i m i t o f a human observer'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  p r e y were v e r y c l o s e t o t h e f r o n t o f t h e a r e n a , 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 t h e y moved.  This  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 prey. o b s e r v a t i o n s s u g g e s t t h a t t h e r e i s some d i f f i c u l t y trout.  Many t r o u t e x h i b i t e d  suggests  Other  i n this f o r  intense search 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 high prey d e n s i t y  (35 p r e y p e r l i t e r )  A t t h e end o f s u c h  b u t made few a t t a c k s .  t r i a l s when l a r g e z o o p l a n k t o n ( e g . D a p h n i a s p . , C h a o b o r u s spp. e t c . . ) were i n t r o d u c e d i n t o t h e a r e n a , t h e t r o u t c o n t i n u e d to  s e a r c h i n t h e 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  in attack rate. in  These o b s e r v a t i o n s s u g g e s t  the present experiment  that s m a l l prey  a r e a t the margin o f t r o u t  used  visual  s e n s i t i v i t y b u t t h a t they a r e w e l l w i t h i n the d e t e c t i o n range o f kokanee. Regardless 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 , t h e p o i n t i s t h a t t h e y d o n ' t , w h i l e kokanee do.  Whether t r o u t a r e c h o o s i n g n o t t o a t t a c k o r a r e  incapable of e f f i c i e n t l y locating  t h e 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 o f 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 same, t h a t i s , kokanee w i l l  t e n d 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) f o o d i t e m s t o a g r e a t e r e x t e n t t h a n  Experiment  5.4  be t h e  trout.  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  R e s u l t s from t h i s e x p e r i m e n t  ( T a b l e 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 o f 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 e n c o u n t e r s w i t h l a r g e , i n v e r t e b r a t e p r e y 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 d u r i n g the approach  interactions  and c a p t u r e phase a s w e l l a s d u r i n g t h e  m a n i p u l a t i o n and i n g e s t i o n phase o f p r e y a t t a c k .  When l a r g e ,  armoured p r e y a r e i n v o l v e d , e v e n t s 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 s u c c e s s (compare I S 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 T a b l e 2 4 ) .  When l a r g e , s o f t - b o d i e d p r e y a r e i n v o l v e d , e v e n t s d u r i n g t h e approach of  and c a p t u r e phase a l o n e d e t e r m i n e  the d i f f e r e n t  a t t a c k s u c c e s s (compare CS t o I S f o r d i p t e r a n s and  ephemeropterans i n Table 2 4 ) .  levels  TABLE 24.  A c o m p a r i s o n o f c a p t u r e s u c c e s s (CS) and i n g e s t i o n s u c c e s s ( I S ) 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 d i f f e r e n t s p e c i e s o f invertebrate prey. TA = t o t a l a t t a c k s i n i t i a t e d . D a t a on CS and I S are rounded t o t h e n e a r e s t percentage p o i n t . 95% c o n f i d e n c e l i m i t s a r e i n d i c a t e d and a r e based on t h e normal a p p r o x i m a t i o n t o t h e binomial d i s t r i b u t i o n .  L a r g e , "Armoured", Prey Types  Mean Length i n mm  TROUT CS IS  Trichoptera ( s p e c i e s unknown)  15.0  Odonata (Enallagma boreale)  15.6  67+5  68+7  Notonecta (Notonecta u n d u l a t a , Buenoa confusa)  10.5  46+4  Amphipoda (Crangonyx richmondensis)  8.3  91+3  TA  CS  KOKANEE IS  TA  No. o f t r i a l s conducted w i t h individual fish Trout Kokanee  93-1-3  2+2  248  273  72+4  28+4  618  12  15  6+3  552  45+7  0+0  165  16  16  100+0  230  68+6  46+7  266  16  12  Large, Soft-bodied, P r e y Types D i p t e r a (Chaoborus a m e r i c a n u s , C. trevittatus) Ephemeroptera ( C e n t r o p t i l u m sp.)  *  10.0  11.8  93+1 * 80+4  100+0 100+0  1,934 441  86+2 100+0  60+3 100+0  1,990  18  794  t r o u t i n t h i s g r o u p were o n l y h a l f t h e s i z e o f t r o u t and kokanee "matched" f o r s i z e i n t h e o t h e r g r o u p s t h a t were t e s t e d w i t h Chaoborus s p p . See T a b l e 19 f o r p r e d a t o r s i z e s .  18  13  149  I suggested e a r l i e r that because kokanee e x h i b i t smaller, mouth dimensions  than t r o u t of s i m i l a r length, they might  experience a g r e a t e r degree of d i f f i c u l t y than t r o u t i n manipul a t i n g and ingesting l a r g e , armoured prey.  T h i s hypothesis i s  f i r m l y supported by the r e s u l t s presented here (Table 24 ). Kokanee and t r o u t e x h i b i t s i m i l a r l e v e l s of capture success i n encounters with large prey, however kokanee experience s u b s t a n t i a l l y g r e a t e r d i f f i c u l t y than t r o u t i n manipulating and ingesting many of these prey.  T h i s d i f f i c u l t y i s not due  to any  simple r e l a t i o n between prey s i z e and predator mouth s i z e .  For  example kokanee experience r e l a t i v e l y low i n g e s t i o n success on Crangonyx sp. (46%) as compared with the mayfly Centroptilum sp. (100%) even though the l a t t e r are 42% l a r g e r than Crangonyx sp.  This d i f f e r e n c e i s most l i k e l y associated with the amount of  "armour" possessed by the prey.  Crangonyx sp. possess a tough,  c h i t i n o u s exoskeleton which must o f f e r considerable r e s i s t a n c e to compression  during i n g e s t i o n .  r e l a t i v e l y s o f t - b o d i e d and  Centroptilum sp. by c o n t r a s t i s  flexible.  I t i s apparent from feeding t r i a l s conducted d a m s e l f l i e s and e s p e c i a l l y with notonectids (Nk and  with  undulata  confusa) that prey armour i s l e s s o f a d e t e r r e n t to  s u c c e s s f u l attacks by t r o u t .  In the notonectid t r i a l s , I  p u r p o s e f u l l y s e l e c t e d p a r t i c u l a r l y w e l l armoured, l a r g e prey which I thought would be unmanageable f o r both t r o u t and kokanee. goal was  This  completely r e a l i z e d with kokanee (0% IS) but l e s s so with  t r o u t (6% I S ) .  T h i s i s a l l the more remarkable  since the t r o u t  150 used i n t h e s e 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 t h a n t h e k o k a n e e used (12.1 cm v e r s u s 16.4 virtually  cm)  and  had  jaw w i d t h s t h a t were  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 . 2 2 ) .  T h u s , 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 d i m e n s i o n s to  appear  have an a d v a n t a g e o v e r kokanee i n i n g e s t i n g l a r g e , armoured  invertebrates.  DISCUSSION The  r e l a t i v e v u l n e r a b i l i t y of a s i n g l e prey type  to  d i f f e r e n t species of predators or of a v a r i e t y of prey types 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 d e t e r m i n e d "match" o r " m i s m a t c h " o f p r e d a t o r and p r e y  of  now  R e s u l t s from  the  be used t o r e s o l v e t h e g e n e r a l q u e s t i o n  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  w i t h d i f f e r e n c e s i n predator morphologies for  the  characteristics  d u r i n g t h e a t t a c k phase o f t h e f e e d i n g c y c l e . p r e s e n t c h a p t e r may  by  to  are l i k e l y  s p e c i f i c p a t t e r n s of food-resource p a r t i t i o n i n g  t r o u t and k o k a n e e from M a r i o n  interacting to  account  exhibited  by  Lake.  Explanations of Dietary Differences Between T r o u t and Kokanee Both  f i e l d and  laboratory observations indicate  some t e c h n i q u e s o f a p p r o a c h e i t h e r t r o u t o r kokanee.  and  capture of prey are s p e c i f i c  to  This should lead to 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  capture  g i v e g r e a t e r a c c e s s t o c e r t a i n p r e y f o r one p r e d a t o r another.  that  F o r example b o t h t r o u t and  techniques over  kokanee i n c l u d e l a r g e  151  numbers o f t h e l a r v a l , p u p a l and a d u l t s t a g e s o f c h i r o n o m i d s i n t h e i r d i e t s , b u t t r o u t consume a p p r o x i m a t e l y chironomid  a d u l t s a s kokanee d u r i n g  Table 3 ).  t w i c e a s many  t h e summer months ( C h a p t e r 2,  This d i e t a r y d i f f e r e n c e i s not obviously r e l a t e d t o  temporal segregation,  s p a t i a l segregation  (Chapter 3 ) , o r search  b e h a v i o u r s ( C h a p t e r 4) o f t r o u t and k o k a n e e , however e v i d e n c e from t h e p r e s e n t  chapter  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 attack behaviours. observation  I base t h i s s u g g e s t i o n  on t h e  t h a t t r o u t f r e q u e n t l y jump t o c a t c h a v a r i e t y o f  a e r i a l p r e y w h i l e kokanee a p p a r e n t l y  do n o t .  This should  allow  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 o f t h e a d u l t forms o f b o t h 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 w h i c h a r e much  more a b u n d a n t i n f l i g h t a few cm above t h e l a k e s u r f a c e  t h a n on  it. Differences  i n a p p r o a c h and c a p t u r e  techniques  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 r e s p o n s e s t o s m a l l such as zooplankton  and q u a n t i t y o f  zooplankton  by t r o u t and k o k a n e e i n t h e f i e l d . E f f o r d and Tsumura (1973) r e p o r t e d  (almost  prey  i n t h e l a b o r a t o r y form t h e b a s i s f o r e x p l a n a -  tions of d i f f e r e n c e s i n the kinds obtained  as w e l l  that  cladocerans  e n t i r e l y S i d a c r y s t a l l i n a ) were t h e d o m i n a n t f o r m o f  zooplankton  f o u n d i n t h e d i e t o f m o d e r a t e s i z e t r o u t (mean f o r k  l e n g t h 17.5 cm) from M a r i o n L a k e . that zooplankton  By c o n t r a s t t h e y i n d i c a t e d  i n t h e d i e t o f kokanee o f c o m p a r a b l e 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 of both cladocerans  ( 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 Cyclops b i c u s p i d a t u s ) .  I have p r e v i o u s l y  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 i s repeated and  confirmed  of  zooplankton  i n t h e d i e t s o f s m a l l t r o u t (mean f o r k l e n g t h 5.6  k o k a n e e (mean f o r k l e n g t h 7.9  cm)  cm)  from M a r i o n Lake ( C h a p t e r  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  of  copepods i n the d i e t o f t r o u t r e l a t i v e t o kokanee i s l i k e l y upon t h e 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  data  two  occur.  s p e c i f i c r e a s o n s f o r why  such a p a t t e r n should  F i r s t r e s u l t s from e x p e r i m e n t 5.1 a p p r o a c h and  capture  techniques  indicate that a s h i f t (from  "rush"  t r o u t i n e n c o u n t e r s w i t h one  Many s p e c i e s o f c o p e p o d s , i n c l u d i n g C_^  in dart")  success than  s p e c i e s o f copepod (EK  which e x h i b i t s avoidance responses during  identify  t o " s t a l k and  by k o k a n e e 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  based  kenai)  a t t a c k s by  predators.  bicuspidatus,  e x h i b i t avoidance responses that are q u a l i t a t i v e l y  similar  to  t h o s e p e r f o r m e d by D . k e n a i , t h u s d i f f e r e n c e s i n a p p r o a c h and  capture  and  higher  techniques  may  a l l o w kokanee g r e a t e r c a p t u r e  i n t a k e t h a n t r o u t o f 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  numbers o f copepods ( C ^ b i c u s p i d a t u s ) kokanee and  t h e dominance o f c l a d o c e r a n s  (S^  crystallina  i n t e r a c t i o n b e t w e e n the p r o b a b i l i t y o f a t t a c k and  size  kokanee.  greater  i n the d i e t of  i n t h e d i e t o f t r o u t i s based upon t h e d i f f e r e n c e s i n  t r o u t and  success  the  prey s i z e f o r  I have d e m o n s t r a t e d t h a t t r o u t o f m o d e r a t 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 r e s p o n s e s t o  p l a n k t o n i c p r e y s m a l l e r t h a n 1 mm  i n l e n g t h but  that size-matched  153  kokanee e x h i b i t w e l l d e v e l o p e d a t t a c k r e s p o n s e s t o p r e y l e s s t h a n 1 nun l o n g  (Experiment 5.3).  bicuspidatus  Since  t h e maximum s i z e o f  i n M a r i o n Lake i s l e s s t h a n 1.0  mm  C.  (Northcote  &  C l a r o t t o , 1 9 7 5 ) , 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 a b s e n t from t h e d i e t o f m o d e r a t e s i z e d t r o u t b u t  relative-  l y common i n t h e d i e t o f k o k a n e e .  By c o n t r a s t t h e  S. c r y s t a l l i n a  i n the l a k e a t s i z e s to  1.8  mm  and  i s commonly p r e s e n t  i t s dominance r e l a t i v e t o o t h e r  d i e t o f t r o u t i s u n d o u b t e d l y due s i z e , l o c a l c o n c e n t r a t i o n on t h e a b s e n c e o f any vertebrate  cladoceran  zooplankton  i n the  t o the c o m b i n a t i o n o f  the u n d e r s i d e  of l i l y  avoidance responses during  large  pads,  and  encounters  with  predators. The  evidence that 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 r e s p o n s e t h a n k o k a n e e o f s i m i l a r s i z e may  a l s o e x p l a i n why  S. c r y s t a l l i n a ) t h a t v e r y cm)  obtained  composed o f  on one  the  zooplankton  s m a l l t r o u t (mean f o r k l e n g t h  occasion  a l a r g e p r o p o r t i o n of the  zooplankton  i n length  while  (Sj^ c r y s t a l l i n a ,  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 cm)  5.6  were l a r g e l y from s i z e - c l a s s e s  i n d i v i d u a l s g r e a t e r t h a n .6 mm  (mean f o r k l e n g t h 7.9  (largely  C.  by s m a l l k o k a n e e  on t h e same o c c a s i o n  were from  c l a s s e s composed o f i n d i v i d u a l s l e s s t h a n .5 mm  size-  i n length  ( C h a p t e r 2, F i g . 7 ) . The  foregoing  i s no s i n g l e e x p l a n a t i o n  d i s c u s s i o n makes i t c l e a r t h a t f o r why  kokanee from M a r i o n Lake  e x p l o i t greater q u a n t i t i e s of zooplankton Indeed, t h i s general  trend  there  is likely  than t r o u t  favoured  by t h e  do. small  154  s i z e s o f 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 t h e l a k e  (Efford,  unpublished d a t a ) , the h i g h e r l e v e l s of capture success t h a t k o k a n e e may facility  e x p e r i e n c e w i t h c o p e p o d s , and  finally  the g r e a t e r  t h a t k o k a n e e 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 o f 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 t i m e ( E x p e r i m e n t s 5.2  and 5 . 3 ) .  interval  Kokanee do n o t o b t a i n more z o o p l a n k t o n  or  s m a l l e r z o o p l a n k t o n t h a n t r o u t as a c o n s e q u e n c e o f an  to  filter  t h e s e p r e y from t h e w a t e r column ( E x p e r i m e n t  ability  5.3).  T r o u t from M a r i o n 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 t h a n kokanee  of  In p r e v i o u s c h a p t e r s I  c o m p a r a b l e s i z e s do  have p o i n t e d o u t how ( C h a p t e r 3) and  (Chapter 2 ) .  d i f f e r e n c e s i n both s p a t i a l segregation  s e a r c h b e h a v i o u r s ( C h a p t e r 4) o f t r o u t  and  k o k a n e e promote t h i s p a t t e r n o f f o o d 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 t h e p r e d a t o r s and  t h e i r p r e y d u r i n g t h e a t t a c k phase o f  feeding cycle w i l l  promote t h i s p a t t e r n as w e l l .  The  the  larger  mouth s i z e and a p p a r e n t l y more p o w e r f u l jaw m u s c u l a t u r e  of  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 t o a c h i e v e h i g h e r  levels  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 c c e s s w i t h l a r g e , armoured  i n v e r t e b r a t e s ( E x p e r i m e n t 5 . 4 ) , t h u s , on a v e r a g e , kokanee  will  have t o i n i t i a t e many more a t t a c k s on s u c h p r e y t o a c h i e v e t h e same l e v e l s o f i n t a k e as t r o u t . amphipods (Crangonyx i s 2-3 or  sp.) o r o d o n a t e s  F o r p r e y s u c h as ( E n a l l a g m a sp.)  large the  figure  t i m e s as many a t t a c k s w h i l e on p r e y s u c h as n o t o n e c t i d s  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 .  Difficulties  155  during  the manipulation  and i n g e s t i o n o f l a r g e p r e y  probably  a c c o u n t f o r t h e v i r t u a l a b s e n c e o f t h e 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 (;>5mm i n l e n g t h )  (>5 mm i n d i a m e t e r ) ,  caddis  and l a r v a l o d o n a t e s ( >6 mm i n l e n g t h )  larvae from  t h e d i e t o f M a r i o n Lake kokanee ( C h a p t e r 2 ) .  D i f f e r e n c e s Between t h e P r o p o r t i o n s o f P r e y O b s e r v e d i n t h e N a t u r a l E n v i r o n m e n t and i n t h e D i e t s o f T r o u t 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 composition o f t h e i n v e r t e b r a t e community i n M a r i o n Lake have employed a v a r i e t y of devices core, plankton  (Ekman d r e d g e , H a r g r a v e s a m p l e r , K a j a k  pump, p l a n k t o n  Chapter 2 I pointed  net)  t o o b t a i n samples.  In  o u t t h a t t r o u t and kokanee do n o t a c q u i r e  p r e y from t h e n a t u r a l e n v i r o n m e n t i n t h e same p r o p o r t i o n s a s these other  types o f samplers do.  Studies  such as those  h e r e 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 s o . t h e s e man-made s a m p l i n g d e v i c e s  reported  In general  were d e s i g n e d and employed  to obtain invertebrates i n proportion to their actual d e n s i t i e s i n t h e n a t u r e a l e n v i r o n m e n t , t h u s t h e s a m p l e r s tend or omit r e l a t i v e l y ' f e w species o f aquatic  to "reject"  i n v e r t e b r a t e s on  the b a s i s o f d i f f e r e n c e s i n armour, s i z e o r escape By c o n t r a s t t r o u t and kokanee have been " d e s i g n e d "  behaviours. 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  portion  o f t h e 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 t h e l a k e and a s 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 i t e m s i n t h e i r d i e t s due t o a r m o u r , s i z e o r e s c a p e behaviours.  156  Thus f a r I have examined i n sequence t h e 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 , search b e h a v i o u r s and a t t a c k p r o c e d u r e s o f t r o u t and k o k a n e e M a r i o n Lake on t h e c o m p o s i t i o n o f t h e i r d i e t s .  from  This exercise  has p r o v i d e d e x p l a n a t i o n s o f how s p e c i f i c mechanisms may o p e r a t e to p r e d i s p o s e the p r e d a t o r s t o accumulate diets. of  species-specific  T h i s d o e s n o t mean t h a t a l l o f t h e i n t e r e s t i n g p a t t e r n s  p r e y 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 p r o d u c e them n o r have I examined a l l o f t h e m a j o r mechanisms i n v o l v e d i n s h a p i n g 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 t y p e s i s l i k e l y t o have a p r o f o u n d  i n f l u e n c e on t h e r e s p o n s e s o f b o t h  trout  and k o k a n e e d u r i n g a l l s t a g e s o f t h e f o o d g a t h e r i n g p r o c e s s . In  t h e n e x t c h a p t e r , I w i l l examine t h e 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 t h e r e s p o n s e s o f t r o u t and kokanee t o p r e y .  SUMMARY 1.  T r o u t and k o k a n e e sometimes use d i f f e r e n t a p p r o a c h and  c a p t u r e t e c h n i q u e s on t h e same t y p e s o f p r e y and i n d i v i d u a l l y use d i f f e r e n t a p p r o a c h of  and c a 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 d e g r e e s  s u c c e s s on d i f f e r e n t t y p e s o f p r e y 2.  they  Kokanee w i t h w e l l d e v e l o p e d  ( T a b l e s 17 and 1 8 ) . g i l l - r a k e r s do n o t o b t a i n  more z o o p l a n k t o n o r s m a l l e r z o o p l a n k t o n t h a n t r o u t as a c o n s e q u e n c e of  any a b i l i t y  t o f i l t e r s u c h s m a l l p r e y from t h e w a t e r  column  (Experiment 5.3). 3. or  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  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) 4.  z o o p l a n k t o n ( E x p e r i m e n t s 5.2 and 5 . 3 ) . Trout are e i t h e r incapable of e f f i c i e n t l y l o c a t i n g  s m a l l (<1 mm)  z o o p l a n k t o n o r t h e y c h o o s e t o i g n o r e them  5.3)/ w h i l e kokanee a r e q u i t e a d e p t a t e x p l o i t i n g 5.  (Experiment  these s m a l l prey.  E x p e r i m e n t s 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  f e e d on z o o p l a n k t o n s u g g e s t t h a t a d v a n t a g e s o f kokanee o v e r i n o b t a i n i n g z o o p l a n k t o n a r e l i k e l y due t o g e n e t i c a l l y 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 6.  trout  fixed  (Experiment 5.3).  Kokanee e x h i b i t j a w w i d t h s t h a t a r e 30% s m a l l e r t h a n  t h o s e p o s s e s s e d by t r o u t o f e q u i v a l e n t body s i z e 7.  very  ( F i g . 22).  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 t h e i n g e s t i o n o f r e l a t i v e l y l a r g e (>5 mm),  armoured  prey (Experiment 5 . 4 ) . 8.  E x p e r i m e n t s w i t h b o t h n o t o n e c t i d s and o d o n a t e s a s p r e y  i n d i c a t e t h a t p r e y 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 t h a n by kokanee ( E x p e r i m e n t 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 b o t h  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  involved i n the  a t t a c k phase o f f o r a g i n g by t r o u t and k o k a n e e s e r v e a s t h e b a s i s f o r e x p l a n a t i o n s o f a number o f d i f f e r e n c e s between t h e d i e t s o f free ranging predators.  These d i f f e r e n c e s i n c l u d e : t h e g r e a t e r  u t i l i z a t i o n o f a e r i a l p r e y by t r o u t , t h e i n c l u s i o n o f l a r g e numbers o f copepods i n t h e d i e t o f k o k a n e e b u t n o t o f t r o u t , t h e 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 k o k a n e e compared t o t r o u t , and t h e r e l a t i v e s c a r c i t y o f l a r g e armoured p r e y i n t h e d i e t o f k o k a n e e .  (>4 mm  body  length),  158  CHAPTER 6 THE ROLE OF SHORT TERM EXPERIENCE I N SHAPING THE RESPONSES OF TROUT AND KOKANEE TO PREY  INTRODUCTION Predators are s e l e c t i v e l y attentive to s t i m u l i are c h a r a c t e r i s t i c o f d i f f e r e n t prey i n f o r m a t i o n s a v e s an a n i m a l  from w a s t i n g  s t i m u l i which are unimportant matter  how l i m i t e d  types.  that  Selection of  time o r energy on  and t h i s w i l l be a d a p t i v e no  the r e p e r t o i r e o f response  (Manning, 1972).  Directed a t t e n t i o n i n higher v e r t e b r a t e s i s u s u a l l y brought about through manifests  t h e p r o c e s s o f l e a r n i n g i . e . , "a p r o c e s s  itself  by a d a p t i v e c h a n g e s i n i n d i v i d u a l b e h a v i o u r  a r e s u l t of experience" suggest  that  (Thorpe,  1956) .  as  A variety of studies  that s e l e c t i v e p e r c e p t i o n of responses  resulting  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 o f p r e y e x p l o i t a t i o n by p r e d a t o r s .  F o r example B r y a n 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 h a s a l o n g term e f f e c t  (on the o r d e r o f  weeks o r months) on p a t t e r n s o f f o o d 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 experimental  s t u d i e s w i t h b i r d s suggest  an i m p o r t a n t  r o l e o f 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 o f p r e y e x p l o i t a t i o n (Murton,  1 9 7 1 ; A l c o c k , 1 9 7 1 ; K e a r , 1962; R a b i n o w i t c h ,  1969) . Experiments  with three-spined s t i c k l e b a c k s (Gasterosteus  aculeatus) faced w i t h d i f f e r e n t combinations e x p e r i e n c e w i t h one p r e y  o f prey r e v e a l t h a t  simultaneously a l t e r s the r i s k o f others  159  (Beukema, 1 9 6 8 ) .  S t i c k l e b a c k s faced with combinations  s p . and  Enchytraeus  s p . "on  t h e e x p e c t a t i o n " o f l o c a t i n g t h e more p a l a t a b l e  Enchytraeus  sp. as prey c o n s i s t e n t l y r e j e c t e d  of Tubifex  s p . , s i n c e a t t h e moment o f 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 sp.  Enchytraeus  Therefore, predator experience with d i f f e r e n t prey  u n d e r n a t u r a l c o n d i t i o n s may of  Tubifex  types  commonly i n f l u e n c e t h e p r e y ' s  risk  exploitation. L e a r n i n g and h a b i t u a t i o n a r e i n v o l v e d i n d e t e r m i n i n g  what an a n i m a l w i l l o r w i l l  not eat or a v o i d .  i n d i r e c t l y a f f e c t the types of foods gathered on h a b i t a t s e l e c t i o n , t i m i n g o f a c t i v i t i e s , L e a r n i n g may  Learning through  search  may  i t s influence  techniques.  a l s o d i r e c t l y a f f e c t the t y p e s o f foods g a t h e r e d  by  i n f l u e n c i n g t h e p r o b a b i l i t y o f a t t a c k s on p r e y 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 have been c a p t u r e d .  the p r o b a b i l i t y of r e j e c t i o n of prey a f t e r These d i r e c t e f f e c t s o f l e a r n i n g a r e  focus of the p r e s e n t c h a p t e r .  they  the  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 k o k a n e e from M a r i o n 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 t h e ways t h a t s i m i l a r p r e y a r e l o c a t e d , a p p r o a c h e d , and manipulated I am  now  ( C h a p t e r s 3, 4 and  interested  5).  Because of these  differences,  i n t e s t i n g the g e n e r a l hypothesis t h a t  exposure to 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 for  t r o u t and k o k a n e e and  thus learned responses  consequence of t h i s e x p e r i e n c e w i l l to  experience  t o prey as a  s e r v e as a p o w e r f u l mechanism  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  field.  160  METHODS 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 C h a o b o r u s spp. L a r v a e on t h e P r e d a t o r y Responses o f T r o u t and Kokanee  I designed  t h i s experiment  pieces of i n f o r m a t i o n .  First,  and k o k a n e e w o u l d r e s p o n d w i t h which  p r e d a t o r s (4 kokanee and  I wished  to determine  to repeated exposure  on f i r s t e n c o u n t e r  from t o t a l a t t a c k s u c c e s s  t o p r o v i d e a number o f how  to a prey  t h e y have h i g h (>_ 80%)  (Chapter 5 ) .  trout type  but f a r  I used two g r o u p s o f  4 t r o u t ) c a p t u r e d f r e s h from  the  field.  A l l predators received i d e n t i c a l treatment with respect to c a p t u r e , h a n d l i n g and m a i n t e n a n c e p r o c e d u r e s . c a p t u r e from t h e f i e l d p r e d a t o r s i n 200  trials  feeding t r i a l s with  l i t e r a q u a r i a s t o c k e d w i t h 400  C h a o b o r u s spp. l a r v a e . 72 h o u r s  I conducted  Seven d a y s  Additional t r i a l s  u n t i l e a c h p r e d a t o r had  w i t h c h a o b o r u s l a r v a e as  completed  late  took p l a c e  after  individual  instar every  seven c o n s e c u t i v e  prey.  When w i l d c a u g h t p r e d a t o r s a r e 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 n o t o n l y by  e x p e r i e n c e g a i n e d w i t h p a r t i c u l a r p r e y t y p e s b u t a l s o by experience gained which  from e x p o s u r e  t o an e x p e r i m e n t a l  i n t h i s s t u d y i n c l u d e d h a n d l i n g and  a q u a r i u m t o an e x p e r i m e n t a l a r e n a .  procedure,  t r a n s f e r from a "home"  I wished  to observe  e f f e c t s o f e x p e r i e n c e w i t h p r e y on p r e d a t o r s r a t h e r t h a n e f f e c t s o f 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 o f the procedure.  To m i n i m i z e  the the  experimental  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 o f p r e d a t o r h a n d l i n g and during t h i s experiment.  the  maintenance  F o r e x a m p l e , t h e d i p n e t , used t o c a p t u r e  161  fish  f o r t r a n s f e r , f i t t h e home a q u a r i u m i n a way t h a t I c o u l d  always  obtain individual predators without resorting  pursuit.  to forced  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  transferred  always  from t h e home a q u a r i u m t o t h e e x p e r i m e n t a l a r e n a i n  a red p l a s t i c bucket.  I n order t o assess the success o f these  m e a s u r e s I compared t h e r e s p o n s e s  on t h e f i r s t  t r i a l with  chaoborus l a r v a e , o f newly c a p t u r e d kokanee, t o t h e responses of  experienced kokanee.  The f o u r a n i m a l s  g r o u p 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 the experienced  i n a t least f i v e feeding  trials  w i t h s m a l l amphipods ( H y a l e l l a a z t e c a ) a s p r e y . As p r e d a t o r s a c c u m u l a t e  experience with p a r t i c u l a r  prey,  t h e y may e x h i b i t c h a n g e s i n : c a p t u r e s u c c e s s , f r e q u e n c y o f p r e y 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 attack i n a given t r i a l .  variety of potential  i n d i c e s e x i s t t o document t h e i n f l u e n c e o f  experience w i t h prey on p r e d a t o r responses. present experiment  Thus a  I r e l i e d on t h e  t o provide i n f o r m a t i o n t o assess the r e l a t i v e  s e n s i t i v i t y o f t h e s e 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 in  further  Experiment  experiments.  6.2  The 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 e s o f Kokanee t o a V a r i e t y o f B e n t h i c P r e y  This experiment  was d e s i g n e d  Types  t o examine t h e e f f e c t s o f  c h a n g e s i n b o t h p r e y 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 o n t h e responses  o f 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 o u t 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 four species o f benthic prey  ( H y a l e l l a sp., Centroptilum s p . ,  Crangonyx s p . , and E n a l l a g m a  sp.).  The s i z e s o f p r e d a t o r s  162  and p r e y Table two  used i n t h i s e x p e r i m e n t  19, C h a p t e r  5).  are recorded elsewhere  A l l p r e d a t o r s used had  identical pre-conditioning trials  experimental  a r e n a and  w i t h mixed prey  6.1  least  i n the the  Each p r e d a t o r r e c e i v e d a t l e a s t  consecutive feeding t r i a l s r e s u l t s of experiment  completed a t  t h u s were q u i t e f a m i l i a r w i t h  experimental procedures.  (see  four  w i t h s p e c i f i c prey types s i n c e suggested  t h a t the e f f e c t s  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 r e s p o n s e s  of  were a l w a y s  apparent  by t h i s p o i n t . 2 I used p r e y d e n s i t i e s o f 20, 40, 70 and respectively  i n consecutive t r i a l s .  predator responses  I n t h i s way  100  per  .42  m  I c o u l d examine  t o c h a n g e s i n p r e y 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 p r e s e n t e d w i t h an e q u a l o p p o r t u n i t y t o accumulate e q u i v a l e n t l e v e l s of experience w i t h the v a r i o u s prey  t y p e s by t h e end  of t r i a l f o u r .  across a l l four t r i a l s ,  I was  T h u s , by p o o l i n g  results  a b l e t o examine the r e s p o n s e  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 . total attacks initiated  and  i n a g i v e n t r i a l as s t a n d a r d experience with prey. trial  is likely  time to i n i t i a t e  number o f p r e y  eaten,  the f i r s t a t t a c k  i n d i c e s of p r e d a t o r responses  to  Because time t o f i r s t a t t a c k i n a g i v e n  t o be a f f e c t e d by p r e y s i z e and  activity, I collected prey  I recorded  levels  of  i n f o r m a t i o n on t h e s e c h a r a c t e r i s t i c s  t y p e s used i n t r i a l s  w i t h kokanee.  of  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 : Effects of E x p e r i e n c e w i t h L a r g e , Armoured P r e y on T r o u t and Kokanee.  Young o r a d u l t v e r t e b r a t e s w h i c h  f o r a g e 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 f o o d 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 p r e y must be b r o a d .  Inevitably this  responsiveness w i l l r e s u l t i n encounters u n f i t to eat.  Repeated exposure  w i t h prey  broad  items t h a t are  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 d e c r e m e n t i n responsiveness to a repeated or constant stimulus 1956).  In previous feeding t r i a l s  involving a v a r i e t y of  ( C h a p t e r 5 ) , t r o u t and k o k a n e e e x p e r i e n c e d i n g e s t i o n s u c c e s s ( 6 % and notonectids.  I designed  (Thorpe, prey  the lowest l e v e l s  of  0% r e s p e c t i v e l y ) w i t h l a r g e , armoured 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 o f a t t a c k s u c c e s s w i l l r e d u c e  the  r e s p o n s i v e n e s s o f t r o u t and k o k a n e e t o p r e y . I conducted and  i n d i v i d u a l feeding t r i a l s with four trout  f o u r kokanee ( s e e T a b l e 19 f o r s i z e s ) .  i n t h i s experiment  had  completed  at l e a s t three pre-conditioning  t r i a l s w i t h c h a o b o r u s l a r v a e as p r e y t h u s a l l p r e d a t o r s were f u l l y laboratory procedures.  Each p r e d a t o r used  i n the e x p e r i m e n t a l  experienced w i t h the  standard  At the b e g i n n i n g of each t r i a l ,  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 t h e  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  lasting  .5 h o u r s )  a supplementary  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  conducted four days.  f o o d s u p p l y a t any  40 l a r g e  experimental  arena.  B e c a u s e t h e s e p r e y were s e l d o m e a t e n and  arena,  (each arena.  the f i s h d i d not r e c e i v e  time d u r i n g the experiment,  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 e v e r y 24 h o u r s T h i s ensured  t h a t responses  for  by t h e p r e d a t o r s t o t h e  I  164 p r e y would be a c o n s e q u e n c e o f a c c u m u l a t e d  experience r a t h e r than  a c o n s e q u e n c e o f any l o n g term e f f e c t s o f s t a r v a t i o n . number o f p r e y 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 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 response  and t i m e t o i n i t i a t e  as standard i n d i c e s o f p r e d a t o r  to experience with notonectids.  Experiment  6.4  The S u c c e s s and S t r e n g t h o f Response 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 c e w i t h S m a l l A g i l e P r e y on T r o u t and Kokanee  I have d e m o n s t r a t e d  p r e v i o u s l y t h a t kokanee a r e  more e f f e c t i v e p r e d a t o r s t h a n t r o u t i n i n i t i a l 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 repeated exposure  i s designed  contacts with  (Chapter 5 ) .  Therefore,  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 a s compared t o k o k a n e e .  in  I recorded  This  experiment  t o t e s t t h e h y p o t h e s i s t h a t when s u c h d i f f e r e n c e s  s u c c e s s w i t h s m a l l p r e y e x i s t , kokanee w i l l d e v e l o p  response  t h a n t r o u t , upon r e p e a t e d e x p o s u r e  t o these  I used D i a p t o m u s k e n a i a s t h e p r e y  a stronger  prey.  i n this  experiment  b e c a u s e t h e y a r e l a r g e enough (mean l e n g t h i n t h i s e x p e r i m e n t 2 mm)  was  t o a l l o w e a s y o b s e r v a t i o n o f p r e d a t o r s u c c e s s and b e c a u s e  t r o u t and k o k a n e e 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 o f c a p t u r e  success  w i t h t h e s e p r e y upon i n i t i a l  with  contact.  In a single t r i a l  these  p r e y , t r o u t a c h i e v e d o n l y 8% c a p t u r e s u c c e s s w h i l e k o k a n e e 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 s u c c e s s by p r e d a t o r s i n  t h e s e t r i a l s was c l e a r l y r e l a t e d response  t o the pronounced  avoidance  o f p_. k e n a i when a t t a c k e d by f i s h . Eight size-matched  t r o u t and k o k a n e e (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 ) s e r v e d a s p r e d a t o r s .  Up u n t i l  the time  165  of  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 c a p t u r e from t h e f i e l d and s u b s e q u e n t h a n d l i n g and m a i n t e n a n c e p r o c e d u r e s o f p_. k e n a i i n each t r i a l conducted hours  i n the laboratory.  The i n i t i a l d e n s i t y  was s e t a t 100 p e r 200 l i t e r s .  I  h a l f h o u r 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 e v e r y 24  f o r four days.  I r e c o r d e d t h e number o f p r e y e a t e n ,  a t t a c k s , t o t a l r e j e c t i o n s , and t i m e t o i n i t i a t e in a given t r i a l  as t h e s t a n d a r d  total  the f i r s t a t t a c k  i n d i c e s of 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 D_. k e n a i .  RESULTS Experiment  6.1  E x p e r i e n c e w i t h Chaoborus  Larvae  T r o u t and k o k a n e e b o t h 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  in initial  t h e r e i s no e v i d e n c e ability  f o r any s i g n i f i c a n t  t o capture o r handle  repeated exposure  t r i a l s w i t h c h a o b o r u s l a r v a e and improvement i n t h e i r  these prey as a consequence o f  t o them ( T a b l e 25 a and b ) . T h i s d o e s n o t  mean t h a t " e x p e r i e n c e " h a s no e f f e c t o n t h e p r e d a t o r ' s 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 t h e t o t a l of  a t t a c k s t h a t t r o u t and k o k a n e e make from one t r i a l  next  response number t othe  ( F i g . 2 5 ) . T h i s t r e n d may be a c o n s e q u e n c e o f e i t h e r a n  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 c o n s e q u e n c e o f an increased a b i l i t y  t o process chaoborus l a r v a e .  Both a r e  possible. Animals  s u c h a s 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 o f n o v e l foods as e x p e r i e n c e w i t h these ( R o z i n , 1969) .  increases  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 p r e s u m a b l y a  166  TABLE 25.  a. Prey  The e f f e c t o f e x p e r i e n c e w i t h p r e y 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 s u c c e s s o f t r o u t and k o k a n e e . Numbers i n b r a c k e t s i n d i c a t e t h e t o t a l number o f 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 . Attack 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 .  T r o u t A t t a c k S u c c e s s ( a s %) Identity  Zooplankton Daphnia sp. Diaptomus k e n a i Chaoborus s p p . Amphipods Hyalella azteca Crangonyx richmondensis  Mean Size mm 1.5 2.0 10 .0 4.6 8.3  Others Odonata (Enallagma boreale)  15.6  Notonectids  10 .5  b.  Trial 1  ( ( (  (  91 300) 91 120)  (  75 100)  (  4 79)  (  Kokanee A t t a c k S u c c e s s (as  Zooplankton Daphnia sp.  1.5  Diaptomus k e n a i  2.0  Chaoborus s p p . Amphipods Hyalella azteca Crangonyx richmondensis Others Odonata (Enallagma boreale) Notonectids Ephemeroptera ( C e n t r o p t i l u m sp.)  10.0  4.6 8.3  15.6 10.5 11.8  100 404) 8 231) 86 85)  Trial 2 100 337) 12 155) 92 212)  ( ( (  Trial 4  100 (668) 18 ( 97) 97 (305)  100 (361) 8 ( 12) 95 (261)  —  —  —  —  -  -  -  -  —  —  —  —  8 75)  2 ( 42)  7 ( 57)  100 (811) 36 (980) 89 (449)  100 (574) 39 (805) 87 (449)  98 133) 16 49)  92 (225) 60 ( 20)  95 (378) 43 ( 28)  49 35) 0 56) 75 59)  26 84) 0 8) 60 141)  31 (121) 0 ( 8) 56 (197)  43 ( 75) 0 ( 2) 61 (203)  (  -  -  89 83) 63 49)  (  92 (1,071)  -  100 859) 24 1,262) 85 366)  (  -  100 558)  -  -  100 225) 18 (1,224) 86 ( 308)  (  (  -  -  —  -  %)  (  (  Trials > 4  -  -  (  Trial 3  -  (  86 867)  —  (  78 27)  (  38 47) — —  (  54 71)  167  FIGURE 25.  The r e l a t i o n s h i p b e t w e e n c o n s e c u t i v e  feeding  w i t h c h a o b o r u s l a r v a e a s p r e y and t h e t o t a l o f a t t a c k s t h a t t r o u t and k o k a n e e i n i t i a t e given  trial.  trials number  in a  T r o u t 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  O  KOKANEE  Y= 18.21 + 9.64x Y= 8 2 . 9 8 + 15.25 x  o  CONSECUTIVE FEEDING TRIALS WITH CHAOBORUS LARVAE  168  consequence o f a " t e s t i n g " p r o c e d u r e which a l l o w s the a n i m a l s run m i n i m a l r i s k of o b t a i n i n g a l e t h a l dose of a n o v e l is  toxic.  I t i s most l i k e l y  on c o n s e c u t i v e  t h a t the  trend to increase  t h a t the  had  initial  f o r seven days p r i o r t o the  t h a t t h e s m a l l t r o u t (mean l e n g t h 8.7  parallel  physiology.  I b a s e 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 been s t a r v e d  that  attacks  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 to  c h a n g e s i n some a s p e c t o f d i g e s t i v e t r a c t anatomy o r  c h a o b o r u s and  food  to  predators trial  cm)  with  used i n  t h e e x p e r i m e n t e x h i b i t e d f e w e r a t t a c k s w h i c h i n c r e a s e d more s l o w l y from t r i a l  to t r i a l  (mean l e n g t h 16.4  t h a n was  cm).  a t t a c k s from t r i a l  The  i d e a t h a t the  to t r i a l  accomodating p r e d a t o r  the case w i t h the l a r g e r kokanee  i s due  increase  to a gradual  in total  process  of  stomachs t o l a r g e r volumes of food  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  after  these  observations. The  progressive  the f i r s t a t t a c k  (TFA)  t h a t b o t h t r o u t and  d e c l i n e i n the time taken to  on p r e y i n the a r e n a ( F i g . .26)  kokanee respond p o s i t i v e l y to the  of a highly acceptable  p r e y and  A comparison of values  o f TFA  k o k a n e e and  inexperienced  i n TFA  the response of the p r e d a t o r s  for  TFA  T a b l e 26)  and  predators  of r e l a t i v e l y  on  the  first  on  The  inexperienced  one  combination  procedures.  26)  trial  indicates i s due  procedure  s i m i l a r values  trial  indicates  experienced  t r o u t (Table  t o the e x p e r i m e n t a l  t o responses to the p r e y . of experienced  laboratory  between l a b o r a t o r y  k o k a n e e and  t h a t most o f t h e v a r i a b i l i t y  t h a n due  standard  initiate  to rather  obtained  (Group I I kokanee,  predators  on  trial  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  w i t h c h a o b o r u s l a r v a e as p r e y and initiate given  the  trial.  ordinate.  f i r s t a t t a c k by  the  feeding time  by  to  t r o u t o r kokanee i n a  N o t e the c h a n g i n g s c a l e on  Curves f i t t e d  trials  eye.  the  170  (Fig.  26) s u g g e s t s  procedure  t h a t the e f f e c t s o f the standard  l a r g e l y d i s a p p e a r by t r i a l  c h a n g e s i n TFA a f t e r t r i a l experience w i t h prey  TABLE 26.  Group I  experimental  two w i t h p r e d a t o r s .  Thus,  two a r e most l i k e l y a c o n s e q u e n c e o f  alone.  A comparison of the time t o f i r s t a t t a c k (TFA) and o f t o t a l a t t a c k s by p r e d a t o r s o n 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 w e r e .  Kokanee  TFA ( s e c ) A t t a c k s .  Group I TFA ( s e c )  Trout  Group I I Kokanee  Attacks  TFA ( s e c )  Attacks  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 h o u r s .  The d e c r e a s e  i n TFA from t r i a l  two t h r o u g h  trial  five  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 acceptable prey t o b o t h t r o u t and k o k a n e e . may  F u r t h e r , i t a p p e a r s t h a t c h a n g e s i n TFA  s e r v e 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 p r e y on p r e d a t o r r e s p o n s e  levels.  171  E x p e r i m e n t 6.2  Attack  R e s p o n s e s o f Kokanee t o E x p e r i e n c e V a r i e t y o f B e n t h i c Prey Types.  with a  Success In  g e n e r a l kokanee do n o t g a i n an i m p r o v e d a b i l i t y t o  capture o r handle b e n t h i c prey (Table 25b).  Although  types  i n consecutive  mean i n g e s t i o n s u c c e s s  trials  across a l l t r i a l s  i s more v a r i a b l e f o r k o k a n e e e x p o s e d t o Crangonyx s p . and E n a l l a g m a s p . t h a n f o r t h o s e e x p o s e d t o H y a l e l l a s p . and C e n t r o p t i l u m s p . (Table 25b), a t t a c k success does n o t 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 o f the e f f e c t s o f e x p e r i e n c e  on  predator responses t o prey.  Total  Attacks 2 A t an i n i t i a l  d e n s i t y o f 20 p r e y p e r .42 m a l l  k o k a n e e c o m p l e t e d s i m i l a r numbers o f a t t a c k s on t h e f o u r of benthic prey.  types  I n 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 s p . and C e n t r o p t i l u m s p . ( F i g . 27 a and b ) , b u t a h i g h l y v a r i a b l e r e s p o n s e t o Crangonyx s p . and E n a l l a g m a 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 c o n s e q u e n c e o f r e s p o n s e s t o p r e y d e n s i t y , i f t h e a b i l i t i e s o f kokanee t o p r o c e s s are f u l l y  l a r g e Crangonyx s p . and E n a l l a g m a s p .  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  k o k a n e e , i n t r i a l s w i t h H y a l e l l a s p . and C e n t r o p t i l u m s p . , are n o t 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 . pieces o f evidence  suggest that t h i s i s not the case.  Various First,  Kokanee g e n e r a l l y were n o t c o n s u m i n g a maximum r a t i o n o f e i t h e r  172  FIGURE 27.  The e f f e c t s o f c h a n g e s i n e x p e r i e n c e and p r e y d e n s i t y on t h e t o t a l number o f a t t a c k s  that  kokanee i n i t i a t e on p a r t i c u l a r p r e y t y p e s i n a given  trial.  Crangonyx sp. o r Enallagma sp. i n these used i n the C e n t r o p t i l u m 12.7  cm)  than those  l e n g t h 14.7 should  cm)  trials.  The  s p . t r i a l s were s m a l l e r (mean l e n g t h  used i n t r i a l s w i t h Crarrgonyx  and  kokanee  therefore their total  sp.  (mean  i n t a k e of prey  (by volume)  have been l e s s t h a n f o r k o k a n e e f e e d i n g on C r a n g o n y x  I n s h a r p 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 sp.  (mean l e n g t h 11.8  (mean l e n g t h 8.3  mm)  i n g e s t i o n success  mm)  were consumed by k o k a n e e .  Given the  o f k o k a n e e w i t h C r a n g o n y x s p . and  i n t a k e ) consumed by t h e p r e d a t o r s  i n two  ( a s wet  s p . t r i a l s w o u l d have t o have i n i t i a t e d their guts).  Enallagma sp. d i d not p r e c l u d e  attacks during handling  the  Crangonyx  t h e i r making a d d i t i o n a l The  longest  t i m e s on t h e s e p r e y were s e l d o m g r e a t e r t h a n 15  seconds  and were u s u a l l y on  .5 h o u r p e r i o d o f s i n g l e t r i a l s .  the o r d e r of 2 - 4 seconds.  kokanee i n t r i a l s w i t h Crangonyx s p . and  In a d d i t i o n ,  Enallagma sp.  a g r e a t d e a l o f t i m e engaged i n s e e m i n g l y " a i m l e s s " about the aquarium. e x h i b i t any T h i s was  reach  a l s o apparent  t h a t t h e t i m e t a k e n by kokanee t o h a n d l e i n d i v i d u a l s p . and  trials  Crangonyx  attacks to  I t was  average  weight  pre-conditioning  116  I n some t r i a l s  spent  swimming  i n d i v i d u a l kokanee f a i l e d  r e s p o n s e t o e i t h e r Crangonyx s p . o r E n a l l a g m a  n o t due  t o any g e n e r a l  b e c a u s e a t t h e end  of t r i a l s  sp.  information  I have c a l c u l a t e d t h a t k o k a n e e i n t h e  saturation ( i . e . to f i l l  Centroptilum  than the r e l a t i v e l y s m a l l e r Crangonyx  on t h e number o f Crangonyx s p . e q u i v a l e n t s  w i t h mixed prey  sp.  lack of responsiveness  i n w h i c h t h e r e had  been no  to  to  sp. stimuli  response,  t h e i n t r o d u c t i o n o f a few m o s q u i t o o r c h a o b o r u s l a r v a e was  always  174  f o l l o w e d by i m m e d i a t e a t t a c k and i n g e s t i o n . t h a t kokanee a r e more s t r o n g l y p r e d i s p o s e d  T h e r e f o r e , i t seems t o a t t a c k some p r e y  ( H y a l e l l a s p . and C e n t r o p t i l u m sp.) t h a n o t h e r s s p . and E n a l l a g m a  (Crangonyx  sp.) and t h a t p o s s i b l y p r e d a t o r  experience  w i t h some p r e y h a s a g r e a t e r i n f l u e n c e o v e r r e s p o n s e  levels  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 I have a r g u e d  Success  above t h a t t h e u p p e r l i m i t on t o t a l  a t t a c k s made on p r e y by p r e d a t o r s i n t h e s e t r i a l s  i s not a  consequence o f s a t u r a t i o n o f 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 capacities.  i n t h e amount o f t i m e a v a i l a b l e o r o f t h e i r g u t The r e l e v a n t q u e s t i o n t h e n i s what d o e s c o n t r o l t h e  t o t a l number o f a t t a c k s t h a t t h e s e p r e d a t o r s make on s p e c i f i c prey  types?  suggest which  R e s u l t s from a l a t e r e x p e r i m e n t  (see Experiment  6.3)  t h a t when kokanee a r e r e p e a t e d l y e x p o s e d t o p r e y w i t h  t h e y 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 s u c c e s s t h e y make  v e r y few a t t a c k s , t h u s , I s u g g e s t  t h a t the l e v e l s o f a t t a c k  s u c c e s s 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 p r e y may i n f l u e n c e t h e t o t a l number o f a t t a c k s t h a t t h e y a r e w i l l i n g determine  whether o r n o t t h i s i s the case  attacks i n i t i a t e d  t o make.  I have p o o l e d t h e t o t a l  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 p r e y  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  To  types  t h i s value against %  a t t a c k s u c c e s s 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 t h e same four 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 u n m i s t a k a b l e  r e l a t i o n s h i p between t h e l e v e l o f a t t a c k  s u c c e s s 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 t h e 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 t h e l e v e l o f a t t a c k s u c c e s s t h a t i n d i v i d u a l k o k a n e e have w i t h s p e c i f i c p r e y  types  and t h e t o t a l number o f a t t a c k s t h a t i n d i v i d u a l k o k a n e e i n i t i a t e on t h e s e p r e y t y p e s o v e r f o u r c o n s e c u t i v e trials.  1. C e n t r o p t i l u m s p . 2. E n a l l a g m a 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. N o t o n e c t a 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 ( E x p e r i m e n t 6.3) b e c a u s e t h e r e s p o n s e by kokanee t o these prey i s b a s i c a l l y independent o f prey  density.  ARCSIN  OF %  INGESTION  SUCCESS  176  t h a t t h e y make on s p e c i f i c p r e y t y p e s .  My i n t e r p r e t a t i o n o f  t h e s e r e s u l t s r e l i e s on t h e a s s u m p t i o n t h a t t h e p r e d a t o r s do make " c h o i c e s " about whether o r n o t t o a t t a c k i n d i v i d u a l prey. p a r t i c u l a r l y strong piece o f evidence  A  t h a t t h i s i s t h e case  c o n s i s t s o f t h e t r e n d s i n t i m e t o f i r s t a t t a c k (TFA) by kokanee on p r e y  i n consecutive feeding  , Time t o F i r s t A t t a c k  trials.  (TFA)  T h e r e 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 k o k a n e e e x p o s e d to  i n c r e a s i n g d e n s i t i e s o f H y a l e l l a s p . and C e n t r o p t i l u m s p .  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 t h e i d e a  t h a t H y a l e l l a s p . and C e n t r o p t i l u m s p . a r e h i g h l y a c c e p t a b l e as p r e y by k o k a n e e . for  T h e r e i s no w e l l d e v e l o p e d  d e c l i n e i n TFA  k o k a n e e e x p l o i t i n g e i t h e r Crangonyx s p . o r E n g a l l a g m a s p .  (Fig.  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  t h e p r o b a b i l i t y o f p r e y d e t e c t i o n by p r e d a t o r s . a b s o l u t e v a l u e o f TFA i s d e t e r m i n e d  I n i t i a l l y the  by p r e y c h a r a c t e r i s t i c s  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  such  t h e n be  f u r t h e r i n f l u e n c e d by p r e y d e n s i t y , f a m i l i a r i t y o f t h e p r e d a t o r s w i t h the experimental procedure with the prey.  On t r i a l  and f a m i l i a r i t y o f t h e p r e d a t o r s  one o f t h i s e x p e r i m e n t  e x c e p t p r e y 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 . s p e c i f i c prey c h a r a c t e r i s t i c s expected types.  a l l factors By  examining  i t i s p o s s i b l e to p r e d i c t the  o r d e r o f TFA f r o m l o w e s t t o h i g h e s t f o r t h e f o u r  prey  177  FIGURE 29.  The e f f e c t s o f c h a n g e s i n e x p e r i e n c e and p r e y  density  on t h e t i m e t h a t i n d i v i d u a l kokanee t a k e 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 p r e y t y p e s i n a g i v e n trial.  N o t e t h e s c a l e d i f f e r e n c e s between t h e  ordinates fitted  o f a & b and c & d r e s p e c t i v e l y .  by e y e .  Curves  178  Prey used and  i n t h i s experiment  were a l l o f s i m i l a r c o l o u r  low c o n t r a s t , t h u s , t h e s e 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  little  t o the o b s e r v e d  detectability  variability  i n TFA  i n c r e a s e s l i n e a r l y w i t h an  kokanee s h o u l d r e s p o n d  to Enallagma  sp.,  values.  Because  i n c r e a s e i n prey  prey size,  Centroptilum  sp.,  sp. i n order o f i n c r e a s i n g  TFA.  Crangonyx s p . , and  Hyalella  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 o f time  p r e y spend moving and  on  t h i s b a s i s a l o n e kokanee s h o u l d  t o Crangonyx s p . , H y a l e l l a Enallagma o f TFA  sp., Centroptilum sp.,  sp. i n o r d e r of i n c r e a s i n g  in determining  assume t h a t movement and  To p r e d i c t  TFA  and  the o r d e r  I have a r b i t r a r i l y a v a l u e o f 100.  produced according  an  i s possible  the o r d e r o f TFA  and  important  (note  that  ( s e e Ware, 1973),  on p r e y p r e d i c t e d h e r e ) .  a s s i g n e d the l a r g e s t and  t h e most a c t i v e  Next, I have awarded the r e m a i n i n g  f o r s i z e and movement r e l a t i v e respectively.  analysis  size  are e q u a l l y  that t h e i r e f f e c t s are a d d i t i v e  a l t h o u g h a more s o p h i s t i c a t e d use would n o t a l t e r  size  respond  and  as a consequence o f the combined e f f e c t s o f p r e y  movement, I w i l l  its  TFA.  prey  prey p o i n t s  t o the l a r g e s t and most a c t i v e  F i n a l l y , by a d d i n g  index which i n d i c a t e s  the two  prey  v a l u e s t o g e t h e r I have  the o r d e r o f the f o u r p r e y  t o i n c r e a s i n g v a l u e s o f TFA  that  (Table 27).  types  17?  TABLE 27.  C h a r a c t e r i s t i c s o f p r e y used t o p r e d i c t t h e e x p e c t e d o r d e r o f TFA p r o c e e d i n g f r o m smallest to largest. See t e x t f o r e x p l a n a t i o n o f p o i n t s awarded. Proportion of time spent moving  Points awarded  100  .01*  1  101  11.8  76  .05*  7  83  Crangonyx. s p .  8.3  53  .74**  100  153  H y a l e l l a sp.  4.6  29  .26**  35  64  Prey  Mean size I d e n t i t y . mm  E n a l l a g m a s p . 15.6 Centroptilum sp.  Points awarded  Total points awarded  * 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 , p r e y o b s e r v e d a t 10 C ** v a l u e s o b t a i n e d f r o m e x t e n s i v e s t u d i e s by Ware, 1 9 7 1 ; p r e y o b s e r v e d a t 10°C.  On t h e b a s i s o f t o t a l p o i n t s a c c u m u l a t e d ,  the predicted  o r d e r o f TFA, a s a c o n s e q u e n c e o f t h e i n t e r a c t i o n b e t w e e n p r e y s i z e and movement, i s C r a n g o n y x s p . , E n a l l a g m a s p . , and H y a l e l l a s p . the order observed Enallagma  T h i s i s almost e x a c t l y the reverse o f  i . e . , Centroptilum sp., Hyalella sp.,  s p . , and Crangonyx s p . r e s p e c t i v e l y .  does n o t appear  sp.,Centroptilum  t o be c o n s i s t e n t l y  Thus, TFA  i n f l u e n c e d by f a c t o r s  related  to the d e t e c t a b i l i t y o f prey. 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 s p . a r e s e v e n t i m e s 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 s p . because Crangonyx s p . a r e l a r g e r and spend more t i m e moving when e x p o s e d .  T h u s , he  180  successfully predicted that trout recognize  Crangonyx  more e a s i l y , a t t a c k them f r o m f u r t h e r away and o f t e n t h a t t h e y do  H y a l e l l a sp.  Crangonyx s p . s h o u l d H y a l e l l a s p . do. Crangonyx s p . a v e r a g e TFA  (673  sec)  (8 sec)  conclusion  across  a l l four t r i a l s .  i n t h e same way  here not o n l y  This The  t h a t t r o u t do.  Attack TFA  poorly  pre-  armoured  t h a t the e f f e c t s o f experience  sp.  with  tendency.  Success does not  a p p e a r 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  f a c t o r s r e l a t e d to prey d e t e c t a b i l i t y s i z e , movement).  Instead,  i t seems t h a t p r e d a t o r  attacks.  The  in  w i t h v a r i o u s p r e y t y p e s may  l e v e l of a t t a c k success experienced  i s not a p p r o p r i a t e  attack success across  by  (e.g. prey d e n s i t y , prey experience  some p r e y a f f e c t s t h e " w i l l i n g n e s s " o f the a n i m a l s t o  It  The  i n d i c a t e t h a t kokanee are  these prey i n t e n s i f y r a t h e r than reduce t h i s  trials  inescapable  s p . r a t h e r t h a n l a r g e , w e l l armoured C r a n g o n y x  Enallagma sp., but  and  i  than t h e i r  t r i a l one.  t o a t t a c k s m a l l H y a l e l l a s p . and  Centroptilum  TFA  to  i s t h a t kokanee do n o t r e s p o n d t o H y a l e l l a s p .  r e s u l t s presented  and  times longer  t o H y a l e l l a s p . on  r e l a t i v e t o Crangonyx s p .  disposed  from k o k a n e e t h a n  k o k a n e e e x h i b i t an a v e r a g e TFA  t h a t i s 84  d i f f e r e n c e i s maintained  a t t a c k them more  F o r t h e same r e a s o n s  e l i c i t a s h o r t e r TFA  Instead  sp.  initiate  by t h e  be p a r t i c u l a r l y  t o compare p r e d a t o r  with  predators  important. TFA  a l l p r e y t y p e s t e s t e d on b o t h t r o u t  kokanee b e c a u s e o f the g r e a t d i f f e r e n c e s i n p r e y d e n s i t y  values  and  and and  c h a r a c t e r i s t i c s o f p r e y used i n the v a r i o u s e x p e r i m e n t s ( T a b l e  28).  TABLE 28.  Prey  C h a r a c t e r i s t i c s o f t h e p r e y s p e c i e s used and t h e number o f 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 w h i c h TFA v a l u e s have been d e r i v e d .  Mean size mm  Identity  C h a o b o r u s spp."*" H y a l e l l a sp.  10.0  2  2 Crangonyx s p .  2  N o t o n e c t i d s spp. 4 Diaptomus k e n a i  1 2 3 4  Experiment Experiment Experiment Experiment  6.1 6.2 6.3 6.4  transparent.  Number o f p r e d a t o r s used Trout Kokanee  400  400  400  4  4  a c t i v e , low c o n t r a s t .  20  40  70  100  0  4  s t a t i o n a r y , medium contrast.  20  40  70  100  0  3  a c t i v e , low c o n t r a s t .  20  40 - 70  100  0  3  15.6  s t a t i o n a r y , medium contrast.  20  40  70  100  0  3  10 .5  a c t i v e , high c o n t r a s t .  40  40  40  40  4  4  2.0  a c t i v e , high c o n t r a s t .  100  100  100  100  4  4  11.8 8.3  Enallagma sp.  stationary,  Prey d e n s i t y i n consecutive t r i a l s T-l T-2 T-3 T-4 400  4.6  Centroptilum sp.  Other characteristics  182  However, i t i s w o 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 r e t o prey w i t h which success. of  this  t h e y have a g i v e n l e v e l o f  The s t a n d a r d d e v i a t i o n o f TFA i s used h e r e a s a n i n d e x variability.-  B e c a u s e much o f t h e v a r i a b i l i t y  t r i a l s w i t h prey i s produced familiarity  in initial  by t h e p r e d a t o r ' s l a c k o f  w i t h the experimental procedure  (Experiment  6.1), I  have c o n s i d e r e d a s r e p l i c a t e s o n l y r e s p o n s e s r e c o r d e d i n t h e final  t h r e e t r i a l s p r e d a t o r s completed The  r e s u l t s from t h i s  w i t h any p r e y  type.  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 l o n g a s t r o u t and k o k a n e e e x p e r i e n c e an a t t a c k s u c c e s s o f 30% or  b e t t e r w i t h p r e y , they w i l l respond  f a s h i o n from t r i a l  to t r i a l .  i n a highly consistent  However, i f a t t a c k s u c c e s s  falls  much b e l o w 30% t h e 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 their  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 t h e  experimental  Experiment  Attack  arena.  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  Success In  four consecutive t r i a l s with notonectids, small  t r o u t and k o k a n e e f a i l ability  t o e x h i b i t any improvement i n t h e i r  t o s u c c e s s f u l l y h a n d l e and i n g e s t n o t o n e c t i d s ( T a b l e 2 5 ) .  T r o u t e x h i b i t h i g h e r as w e l l a s more v a r i a b l e s u c c e s s w i t h t h e s e p r e y t h a n kokanee d o , a l t h o u g h e v e n t r o u t a r e 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 . i n d i c a t e t h a t the low l e v e l of  Observations  o f success i s p r i m a r i l y a consequence  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,  prey'  183  FIGURE 30.  The  r e l a t i o n s h i p between % a t t a c k s u c c e s s 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 response  of  as i n d i c a t e d by t h e 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 of experience w i t h s p e c i f i c prey types.  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 p r e y t y p e s T a b l e 28).  Curve f i t t e d  by  eye.  (see  183a CO  o  IOOOH  UJ  cr  900 H  < Q  >  800-  O  700 H  O  600 A  co or  o  500H  UJ  y-  40oH  Ll_ O  LZ  < >  UJ Q  300-  200 H  < o  loo  H  CO  T  r~  10  T  T" 30  T  I  50 50  T 70  % ATTACK SUCCESS OF INDIVIDUAL  T 90  T  PREDATORS  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 . t r o u t (21.6  cm)  no d i f f i c u l t y  C a s u a l o b s e r v a t i o n s on  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  experiment.  Total  Attacks Trout e x h i b i t higher l e v e l s of response one  experience  i n i n g e s t i n g n o t o n e c t i d s i n t h e s i z e r a n g e used i n  this  on t r i a l  larger  t h a n kokanee do  ( F i g . 31).  to notonectids  T h e r e f o r e , they  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 p r e y k o k a n e e do o r w i t h i n t h e s h o r t s p a n o f one  trial  type  either  than  there i s a  d i f f e r e n t i a l e f f e c t 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 on t h e o f t r o u t and k o k a n e e t o i n i t i a t e responses  attacks.  Consideration of  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  s i v e n e s s o f kokanee t o t h e s e " p r e y " d e c r e a s e s . to recognize t h i s prey over  i n t e r v a l between t r i a l s  and  a v e r y low l e v e l , g i v e n such  the  i n d i c a t e s t h a t as  t h e amount 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 c r e a s e s the  m a i n t a i n the a b i l i t y  tendency  respon-  T h u s , kokanee the 24  hour  are capable of reducing responses unrewarding  prey.  to  This supports  h y p o t h e s i s t h a t the process of h a b i t u a t i o n w i l l operate  the  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 handle  and  to  ingest. The  response  o f t r o u t o v e r the f o u r t r i a l s  i s quite  u n l i k e t h a t o f kokanee.  I n s p i t e o f an a t t a c k s u c c e s s t h a t n e v e r  e x c e e d s 8%  be as low as 2%,  to  and w h i c h may  t r o u t continue to  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 t h a n kokanee do.  of t r o u t to h a b i t u a t e to notonectids i s s u r p r i s i n g , since Benfield  (1972) has  The  respond failure  especially  reported t h a t rainbow t r o u t q u i c k l y  185  FIGURE 3 1 .  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 e x p o s u r e t o 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 t h e t o t a l number o f a t t a c k s t h a t t r o u t and kokanee initiate  i n a given  .5 h o u r t r i a l .  N = number o f  i n d i v i d u a l predators tested i n consecutive  trials.  V e r t i c a l b a r s i n d i c a t e + one s t a n d a r d e r r o r o f t h e 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 ) that 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 tendencies repeated  t r o u t and k o k a n e e v a r y d r a m a t i c a l l y i n t h e i r  t o a t t a c k l a r g e armoured p r e y  l i k e notonectids  e x p o s u r e t o them, t h e r e a r e some s i m i l a r i t i e s  responses.  B o t h t r o u t and kokanee f a i l  This suggests  predators  i n their  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 o v e r f o u r c o n s e c u t i v e 32).  t h a t a t the beginning  trials (Fig.  o f each t r i a l  both  are equally r e l u c t a n t to attack notonectids.  t h e l o n g TFA r e c o r d e d  during  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  not a consequence o f d e t e c t i o n d i f f i c u l t i e s  Certainly  trials is  since prey  such as  H y a l e l l a sp. which a r e l e s s than h a l f the s i z e o f n o t o n e c t i d s and  o f l o w e r c o n t r a s t ( T a b l e 28) were a l w a y s a t t a c k e d  s h o r t e r times  E x p e r i m e n t 6.4  Attack  ( F i g . 29 a ) .  R e s p o n s e s t o D i a p t o m u s k e n a i : The E f f e c t s of 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.  Success Kokanee a r e a p p r o x i m a t e l y  in  i n much  c a p t u r i n g D_. k e n a i .  observed  t w i c e as s u c c e s s f u l as t r o u t  I n sharp c o n t r a s t t o the s i t u a t i o n  f o r the m a j o r i t y o f prey  t y p e s 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 t h r o u g h abilities  t o c a p t u r e jD. k e n a i .  t h e i r capture  success  by t r i a l  t r o u t , t h i s was a c c o m p l i s h e d  trial  three i n t h e i r  B o t h p r e d a t o r s more t h a n  double  t h r e e (Table 2 5 ) . I n t h e case o f  through  refinement  of a basic pattern  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 o f e x p o s u r e t o n o t o n e c t i d s and t h e t i m e t o f i r s t of individual predators i n a given t r i a l . l i n e j o i n s t h e means. the o r d i n a t e .  attack  The s o l i d  Note t h e changing s c a l e along  188  of  approach  and a t t a c k w h i l e f o r k o k a n e e t h e i n c r e a s e d s u c c e s s  r e l i e d on a c o m p l e t e  change i n t h e b a s i c p a t t e r n o f a p p r o a c h and  a t t a c k between t r i a l  one and t r i a l  D. k e n a i t r o u t e x h i b i t e d  three.  During t r i a l s w i t h  t h e same b e h a v i o u r a s t h e y 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  p r e y , s t o p , and t h e n l u n g e t o make a c a p t u r e a f t e r w h i c h  they  o f t e n s t o p a b r u p t l y " t o s e a r c h f o r and l i n e up o n " t h e n e x t item (see Chapter  5).  Although refinement o f these  results i n significant  prey  tactics  improvement i n t h e a b i l i t y o f t r o u t t o  c a p t u r e D. k e n a i , t h e v a s t m a j o r i t y o f JJ. k e n a i a r e s t i l l s u c c e s s f u l i n evading capture. to  T h i s i s b e c a u s e JJ. k e n a i  respond  the o n r 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 t h e s i d e ,  s u c c e s s f u l l y evading In  thus  capture.  previous experiments  ( C h a p t e r 5) w i t h o t h e r  p l a n k t o n (Daphnia s p . , Diaptomus t y r e l l i  e t c . . . ) , kokanee  zoopursued  p r e y by swimming i n a smooth p a t h t h a t f l o w e d 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 c h a n g e s  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  t o JJ. k e n a i t h e s e t a c t i c s change d r a s t i c a l l y .  After  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  exposure sighting 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 t h e a i d o f g e n t l e s c u l l i n g by pectoral fins. flex  When w i t h i n 3 - 5 cm o f JJ. k e n a i , k o k a n e e s t o p ,  i n t o a s i g m o i d p o s t u r e , and t h e n r a p i d l y r e l e a s e t o t h r u s t  f o r w a r d and make t h e c a p t u r e .  This setof t a c t i c s i s also f a r  from c o m p l e t e l y s u c c e s s f u l b u t d o e s a l l o w k o k a n e e t o a t t a i n a l e v e l o f s u c c e s s t h a t i s g r e a t e r t h a n d o u b l e t h e maximum e x h i b i t e d by t r o u t ( T a b l e 2 5 ) .  189  Total Attacks Kokanee e x h i b i t h i g h e r l e v e l s o f r e s p o n s e D_. k e n a i on t r i a l  than t r o u t to  one ( F i g . 3 3 ) . T h e r e f o r e , t h e y 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 t h e s h o r t span o f a s i n g l e t r i a l  there i s a d i f f e r e n t i a l  e f f e c t o f e x p e r i e n c e w i t h D_. k e n a i on t h e t e n d e n c y 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 . (i.e.  Given the magnitude of the d i f f e r e n c e  a n a v e r a g e o f 60 a t t a c k s v e r s u s more t h a n 300 a t t a c k s by  kokanee) t h e f o r m e r seems more  likely.  Over t h e c o u r s e o f f o u r c o n s e c u t i v e t r i a l s b o t h and k o k a n e e e x h i b i t a t e n d e n c y a t t a c k s i n i t i a t e d on D. k e n a i .  trout  t o r e d u c e t h e t o t a l number o f However, I w i s h t o s t r e s s  that  e v e n on t h e l a s t t r i a l k o k a n e e a r e h i g h l y r e s p o n s i v e t o (more t h a n 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 a l m o s t h a b i t u a t i o n and make few r e s p o n s e s a t a l l .  kenai  complete  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 of  kokanee.  t o t h e p r e y i s d i f f e r e n t from  that  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 n e v e r r e j e c t e d any o f t h e 1188 D. k e n a i  t h a t t h e y managed t o c a p t u r e .  The r e s p o n s e 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 t h e o n l y p o t e n t i a l s o u r c e o f f o o d o v e r t h e f o u r day d u r a t i o n o f t h e e x p e r i m e n t . from t h i s e x p e r i m e n t l e n d s o l i d initial  The r e s u l t s  s u p p o r t t o t h e h y p o t h e s i s t h a t an  d i f f e r e n c e i n c a p t u r e s u c c e s s o f t r o u t and kokanee  exploiting  s m a l l , a g i l e p r e y i s a c c o m p a n i e d by t h e d e v e l o p m e n t o f  a more i n t e n s e p o s i t i v e r e s p o n s e by kokanee upon r e p e a t e d to  the prey.  exposure  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 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 t h e t o t a l number o f a t t a c k s t h a t t r o u t and k o k a n e e initiate  i n a given t r i a l .  N = number o f t r o u t o r  k o k a n e e t e s t e d on c o n s e c u t i v e t r i a l s .  Vertical  i n d i c a t e + one s t a n d a r d e r r o r o f t h e means.  bars  KOKANEE  1  I  I  —I  2 3 4 CONSECUTIVE T R I A L S WITH D. KENAI AS P R E Y  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 e x p o s e d 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 total attacks.  already indicated  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  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 b i l i t y o f jp_. k e n a i as p r e y f o r k o k a n e e . an i n i t i a l d e c l i n e (compare t r i a l s f o l l o w e d by a d r a m a t i c  one  i n c r e a s e , which  and  TFA  of t r o u t  attack this  over  acceptexhibits  two o f F i g . 3 4 ) ,  suggests  t h a t as  become more e x p e r i e n c e d w i t h D. k e n a i , t h e y become l e s s to  by  trout willing  prey.  DISCUSSION The  Range o f R e s p o n s e s by T r o u t and Kokanee t o P r e y T h e r e a r e numerous e x a m p l e s i n t h e l i t e r a t u r e  papers d e a l i n g w i t h q u e s t i o n s concerning the r e l a t i v e a b i l i t y o f p r e y t o p r e d a t o r s ( s e e Edmunds, 1974  accept-  for references).  Most o f t h e s e have c o n c e n t r a t e d on t h e e f f e c t s o f a stimulus dimension  of  single  ( 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 d i m e n s i o n s  a r e " i n t e g r a t e d and  summated" by p r e d a t o r s i n t h e c o u r s e o f t h e i r items (Eisenberg & Leyhausen, 1972).  The  inspections of  stimulus  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 predators.  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 p r e y  e l i c i t a range of response highly negative. of  t h a t v a r i e s from v e r y p o s i t i v e  I t i s the p r e d a t o r ' s assessment of the  the t o t a l s t i m u l u s s e t which w i l l  response  to a p a r t i c u l a r prey  type.  determine  prey  to  may to significance  i t s ultimate  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 e x p o s u r e t o D i a p t o m u s k e n a i and t h e t i m e t o f i r s t a t t a c k o f t r o u t and k o k a n e e i n a g i v e n The s o l i d  l i n e j o i n s t h e means.  s c a l e along the o r d i n a t e .  trial.  Note the changing  193  In many c a s e s t h e f i n a l a s s e s s m e n t proceeds without d i f f i c u l t y .  o f p r e y by p r e d a t o r s  F o r e x a m p l e , 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 t h e  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 with particularly  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 t h e l a t t e r . d a t a on t o t a l a t t a c k s o r t i m e t o i n i t i a t e develop r e l i a b l e responses which  first  On t h e b a s i s o f a t t a c k s , kokanee  i n d i c a t e t h a t prey such as  C h a o b o r u s s p p . , H y a l e l l a s p . , C e n t r o p t i l u m s p . , 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 a r e j u s t as r e a d i l y  identified  as u n a c c e p t a b l e .  p o s i t i v e r e s p o n s e s by t r o u t s u g g e s t t h a t t h e y r e a d i l y  Strong,  identify  C h a o b o r u s s p p . , Crangonyx s p . , and H y a l e l l a s p . a s acceptable prey. A second i n which  s i t u a t i o n encountered  t h e i n f o r m a t i o n c o n t e n t o f t h e s t i m u l i p r e s e n t e d by  p r e y i s c l e a r and r e s u l t s mistakes.  i n t h e p r e d a t o r s c o n s i s t e n t l y making  The b e s t known e x a m p l e s o f t h i s a r e c a s e s i n w h i c h  p a l a t a b l e i n s e c t s mimic t h e appearance u n p a l a t a b l e ones (Rettenmeyer, escape  by p r e d a t o r s i s t h a t  and b e h a v i o u r o f h i g h l y  1 9 7 0 ; B r o w e r , 1969) and t h u s  h i g h l e v e l s o f 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 Shepherd,  1965).  from t h e i r m o d e l s ( S e x t o n , 1960; Duncan &  A l t h o u g h 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 continue t o 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 a r e 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 .  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 p r e y  t h a t a r e n o v e l more  r e a d i l y than they l e a r n t o r e j e c t those resembling p a l a t a b l e prey response  (Brower,  For example,  1958; S h e t t l e w o r t h , 1972) .  familiar The c o n t i n u e d  of t r o u t t o notonectids i n s p i t e o f very low i n g e s t i o n  s u c c e s s may f a l l  i n t o t h i s c l a s s of response,  that i s , notonectids  present 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 , high c o n t r a s t , and p r o v o c a t i v e movements. be r e l i a b l e  Many o f t h e s e s t i m u l i  i n d i c a t o r s o f a wide range o f prey  receptivity  will  that are highly  a c c e p t a b l e t o t r o u t , thus t h e i r continued response may r e p r e s e n t an u n a v o i d a b l e  visual  to notonectids  cost of maintaining a high state of  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  acceptable  prey. 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 present 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  difficult  problems o f 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 stimulus sets that contain c o n f l i c t i n g predators.  It isdifficult  information content f o r  t o know e x a c t l y how t o s t u d y  this  p r o b l e m ( s e e S u t h e r l a n d , 1 9 6 4 ) , b u t i n f o r m a t i o n from t h e p r e s e n t s t u d y as w e l l as o t h e r s s u g g e s t s phenomenon.  t h a t i t may be a f a i r l y common  For example, t h e responses  o f kokanee t o Crangonyx  s p . and E n a l l a g m a s p . a r e ambiguous a t b e s t . aspects of t r o u t responses TFA  values suggest  upon r e p e a t e d  Similarly  some  t o n o t o n e c t i d s a r e ambiguous.  a continued reluctance t o attack notonectids  encounters,  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 o f a t t a c k s on t h e s e p r e y w i t h i n s i n g l e hour  trials.  Their  half  195  T r o u t r e s p o n s e s t o D_. k e n a i a r e 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 readiness  t o a t t a c k t h e s e p r e y , b u t t h e n i n l a t e r t r i a l s TFA v a l u e s d r a m a t i c a l l y suggesting  j u s t the opposite.  Upon i n i t i a l  increase 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 captures;  however t h e s e a r e n o t c a r r i e d t h r o u g h t o i n g e s t i o n and  many o f t h e p r e y a r e r e j e c t e d . e a s i l y associated with strong g i v e n an a p p r o p r i a t e  These p r e d a t o r  responses are not  s t i m u l i f r o m any o f t h e p r e y , r a t h e r ,  predator-prey  combination,  evoke h i g h l y c o n s i s t e n t r e s p o n s e s .  According  t h e s e same p r e y t o Ware  (1971),  t r o u t r e s p o n d 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 s p . and  i n the present  and  negative  study,  kokanee r e g i s t e r e d unambiguous p o s i t i v e  r e s p o n s e s t o D. k e n a i and n o t o n e c t i d s  respectively.  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 t o by o t h e r p r e d a t o r s reported  one  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 .  K r u u k (1972)  t h a t he c o u l d n o t d e t e c t a n y d i f f e r e n c e s i n a n i m a l s o f  species  attacked  S c h a l l e r (1972)  t h a t he was u n a b l e t o d i s c o v e r t h e f a c t o r s w h i c h t r i g g e r e d  sustained reported  a r e by no means o b v i o u s .  completion  t h a t were a t t a c k e d  by h y e n a s .  experienced  compared t o t h o s e t h a t were n o t  Mech (1970) and Haber ( p e r s . comm.) have  similar difficulties  i n determining  the c r i t i c a l  s t i m u l i w h i c h l e a d w o l v e s , h o l d i n g p r e y s u c h a s moose a t b a y , to a b r u p t l y cease a t t a c k s . Predators  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 s t r e a m o f  s t i m u l i a s s o c i a t e d w i t h p r e y from t h e moment o f d e t e c t i o n a t l e a s t t h e moment o f i n g e s t i o n . behavioural  o r chemical  Many  until  morphological,  c h a r a c t e r i s t i c s o f prey a r e 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 s e n s e , b e c a u s e t h e y s e r v e  as  t h a t u n c o u p l e o r d i s - i n t e g r a t e an e f f e c t i v e a p p r o a c h and p r o c e d u r e by p r e d a t o r s .  F o r e x a m p l e , N e i l and  Cullen  stimuli attack  (1974)  presented  e v i d e n c e t o s u g g e s t t h a t s c h o o l s o f p r e y a c t upon ambush  predators  s u c h as p i k e  ( E s o x l u c i u s ) by  complex s e q u e n c e o f a t t a c k by c a u s i n g with  the  avoidance r e a c t i o n s  together  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 t h e g o a l  catching prey. fluviatilis) by  interfering with  For p u r s u i t predators  the s c h o o l s  f o r c i n g the p r e d a t o r s  pursuit.  By e x t e n s i o n  s u c h as p e r c h  (Perca  appeared t o d i s r u p t the a t t a c k to c o n t i n u a l l y switch targets  t o the p r e s e n t  study  to b e l i e v e that prey t e x t u r e or a s l i g h t l y  i t i s not awkward  of  sequence during  too d i f f i c u l t  handling  p r o c e d u r e w i l l d e t e r k o k a n e e 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 r e y s u c h as Crangonyx s p . n o r  i s i t u n l i k e l y that stimuli  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  chain that trout  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 p r e y terrestrial predator  insects.  T h i s s u g g e s t s t h a t t h e r e may t o l e a r n c e r t a i n t h i n g s and  be p r e d a t o r not  related predators are p o s s i b l e .  conditions.  others.  Prey  the e f f e c t s of experience  i n the p r e s e n t  similar  specific predispositions  S p e c i f i c E f f e c t s of Experience with In c o n s i d e r i n g  are  w i t h which  s t i m u l i a c q u i r e c o n t r o l o f b e h a v i o u r under s i m i l a r  Species  or  C e r t a i n l y i t i s c l e a r that there  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  from  study,  These a r e t h a t a n i m a l s may  two  on c l o s e l y  extreme p o s i t i o n s  have a h i g h l y  generalized a b i l i t y to l e a r n or that animals possess s p e c i a l i z e d  197  abilities  to l e a r n o n l y c e r t a i n t h i n g s .  o g i s t s have h i s t o r i c a l l y  Experimental psychol-  s t r e s s e d t h e f o r m e r v i e w , b u t from  an  e c o l o g i c a l p e r s p e c t i v e t h e l a t t e r v i e w makes more s e n s e , t h a t i s , a n i m a l s s h o u l d 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 i m p o r t a n t f o r them t o l e a r n u n d e r 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 t h e y may  n o t l e a r n o t h e r c o m p a r a b l e t a s k s a t a l l i f t h e s e have  place in their lives 1971;  i n n a t u r e ( S e l i g m a n , 1970;  Rozin & Kalat,  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 r e s p o n s e s  t r o u t and k o k a n e e t o t h e same p r e y were s u r p r i s i n g perhaps  s h o u l d n o t have been.  b e h a v i o u r may  learn.  t o me,  but  f e a t u r e of  T h i s must be t r u e o f a b i l i t i e s  T r o u t and k o k a n e e l e a r n n o t t o a t t a c k some s m a l l ,  ( s u c h as D_. k e n a i ) and  some l a r g e , armoured p r e y  as n o t o n e c t i d s ) r e s p e c t i v e l y .  by  E t h o l o g i s t s have p o i n t e d o u t t h a t  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  morphology o r p h y s i o l o g y .  prey  no  The  to agile  (such  e f f e c t s o f e x p e r i e n c e seem t o  o p 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 to a t t a c k or i n g e s t these prey.  These p r e d i s p o s i t i o n s  be e i t h e r a c o n s e q u e n c e o f g e n e t i c a l l y d e t e r m i n e d 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 field  The  by t h e p r e d a t o r s under  t o such prey are i n n a t e .  i n d i c a t e t h a t responses  V a r i o u s a u t h o r s ( H i n d e , 1966;  have i n d i c a t e d t h a t i n n a t e b e h a v i o u r  to habituation.  Smith  i n 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  1972)  differences or  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 p e r f o r m a n c e  present experiments.  may  (1973) f o u n d  i s highly  Figler,  resistant  t h a t the a t t a c k responses  young s h r i k e s t o m o d e l s d i d n o t h a b i t u a t e , e v e n i n t h e a b s e n c e  of  198  of  any t a n g i b l e r e w a r d  and s h e c o n c l u d e d  r e s p o n s e s were i n n a t e . 5.3 and 5 . 4 )  (Experiment  The e x p e r i m e n t s  that their attack conducted  earlier  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 o f j u v e n i l e  t r o u t and k o k a n e e t o z o o p l a n k t o n s u g g e s t 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 of  t r o u t and k o k a n e e may be g e n e t i c a l l y d e t e r m i n e d , 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 establish this  experimental tests are required to  point.  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 T r o u t and Kokanee Many m o t o r p a t t e r n s i n v o l v e d i n s e a r c h , a p p r o a c h ,  attack  and m a n i p u l a t i o n o f p r e y may be common t o t h e 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 t h e form o f s u c h p a t t e r n s w h i c h  t h e n may g e n e r a t e  substantial  d i f f e r e n c e s i n p a t t e r n s o f p r e y e x p l o i t a t i o n by p r e d a t o r s .  There  a r e no s p e c i f i c d i f f e r e n c e s between t h e c o m p o s i t i o n o f t r o u t and k o k a n e e d i e t s , o r between t h e p r e y c o n t e n t s o f t h e s e p r e d a t o r s and t h e e n v i r o n m e n t , of  which  I can a t t r i b u t e s o l e l y to the e f f e c t s  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 general experience  o p e r a t e 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 ness o f p r e d a t o r s t o prey t h a t a r e encountered.  will  the responsive-  Thus, t h e  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 o n 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 1)  amplify the trends i d e n t i f i e d p r e v i o u s l y f o r :  k o k a n e e t o 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  zooplankton 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 p r e y t y p e s s u c h a s caddis  larvae  (Trichoptera), dragonfly  (Odonata), aquatic (e.g. p l a n o r b i d 3)  insect adults  and d a m s e l f l y  nymphs  (e.g. n o t o n e c t i d s ) ,  molluscs  s n a i l s ) and l a r g e amphipods ( C r a n g o n y x s p . ) ,  t r o u t t o " o v e r e x p l o i t " and k o k a n e e t o " u n d e r e x p l o i t "  large  prey items r e l a t i v e t o t h e i r e n v i r o n m e n t a l abundance, 4)  trout to contain a disproportionate i t e m s and k o k a n e e t o c o n t a i n  number o f l a r g e  a disproportionate  prey  number o f  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 i r o n m e n t a l abundance. I n an e a r l i e r r e v i e w a r t i c l e f i s h dietary habits, I stressed  ( H y a t t , 1979) , d e a l i n g  the point that patterns  random e x p l o i t a t i o n o f p r e y s h o u l d  be c o n s i d e r e d  with  o f non-  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 w h i c h o p e r a t e a l o n e o r in concert. by v a r i o u s  T h i s v i e w was c o n t r a s t e d i n v e s t i g a t o r s of sponsoring  with  t h e more common p r a c t i s e  one mechanism o r a n o t h e r a s  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  patterns.  I n t h e p r e s e n t s t u d y i t i s a p p a r e n t t h a t some o f t h e differences i n dietary patterns  between t r o u t and k o k a n e e a r e n o t  t h e r e s u l t o f s i n g l e mechanisms b u t r a t h e r a r e p r o d u c e d by e n t i r e s e t s o f mechanisms w h i c h o p e r a t e a t e a c h s t a g e o f t h e b e h a v i o u r a l chain for do  involved  i n food  gathering.  t r o u t t o e x p l o i t l a r g e r p r e y t h a n kokanee o f e q u i v a l e n t  trend size  ( C h a p t e r 2) i s f a v o u r e d n o t 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 ( C h a p t e r 3) and s e a r c h by  F o r example, t h e g e n e r a l  the i n a b i l i t y  p o s i t i o n s (Chapter 4),  but also  o f t r o u t to e f f e c t i v e l y capture the s m a l l e s t  c l a s s e s o f prey (Chapter 5 ) , the i n a b i l i t y  size  o f kokanee t o e f f e c t i v e l y  h a n d l e l a r g e , armoured p r e y ( C h a p t e r 5 ) , and f i n a l l y by t h e  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 t h e r e s p o n s i v e n e s s o f t h e s e p r e d a t o r s t o l a r g e and s m a l l p r e y r e s p e c t i v e l y  (this  chapter).  SUMMARY 1.  The  e f f e c t s o f e x p e r i e n c e on s u b s e q u e n t r e s p o n s e s  t r o u t and k o k a n e e t o p r e y a r e s i m i l a r f o r p r e y w i t h w h i c h 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 (Experiment 2.  ingestion  of  the success  6.1). B o t h t r o u t and k o k a n e e d e v e l o p 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 o f r e s p o n s e s p e c i e s w i t h which attack success.  to single  prey  the p r e d a t o r s experience g r e a t e r than  30%  Lower l e v e l s o f a t t a c k s u c c e s s 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 o f r e s p o n s e by t h e 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  3.  6.2).  T r o u t and k o k a n e e e x h i b i t an i n i t i a l  l a r g e , well-armoured e v e n t u a l l y respond  notonectids.  reluctance to attack  However, i n any  trial  trout  t o n o t o n e c t i d s more s t r o n g l y t h a n k o k a n e e  do.  W i t h r e p e a t e d e x p o s u r e , kokanee r a p i d l y h a b i t u a t e t o t h e s e unrewarding  p r e y b u t t r o u t do n o t i n s p i t e o f v e r y l o w a t t a c k  success (Experiment 4.  6.3).  In response  t o 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 a p p r o a c h improves t h e i r c a p t u r e success.  and c a p t u r e t a c t i c s  Trout r e f i n e t h e i r basis  and c a p t u r e t a c t i c s , b u t do n o t employ e n t i r e l y new response  t o e x p e r i e n c e w i t h D. k e n a i ( E x p e r i m e n t  which approach  ones i n 6.4).  201  5.  In g e n e r a l ,  attack  relatively  reject  small,  exhibit to  a  ignore  or  large,  agile  strong  ones  exhibit  reject 6.2,  (Experiments  to  these  relatively  6.3  and  a  strong  well-armoured,  p r e d i s p o s i t i o n to  (Experiments intensify  trout  6.4).  differences.  prey  6.3  attack  large,  and  predisposition and  to  6.4).  small,  of  or  Kokanee  agile  well-armoured  The e f f e c t s  ignore  to  prey  and  ones  experience  act  202  CHAPTER 7 TROUT AND KOKANEE FORAGING - THE STRATEGIC POINT OF VIEW  Much o f e v o l u t i o n a r y b i o l o g y i s t h e w o r k i n g o u t o f an a d a p t a t i o n i s t p r o g r a m . 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 e a c h a s p e c t o f an o r g a n i s m s m o r p h o l o g y , p h y s i o l o g y , and b e h a v i o u r h a s 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 p r o b l e m p r o v i d e d by t h e e n v i r o n m e n t . The r o l e o f 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 a b o u t how e a c h p a r t f u n c t i o n s a s an adaptive device. I n p r a c t i s e an a d a p t a t i o n i s t p r o g r a m 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 t h e o r g a n i s m and o f t h e e n v i r o n m e n t and t h e n r e l a t i n g t h e d e s c r i p t i o n s by f u n c t i o n a l statements. :  L e w o n t i n , 1978.  INTRODUCTION T r o u t and k o k a n e e f r o m M a r i o n 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 . chapters  In previous  I have d e a l t w i t h why t h i s i s s o i n t e r m s o f p r o x i m a l  "mechanisms" b u t have i g n o r e d ary p o i n t o f view.  Therefore,  why t h i s i s s o from an e v o l u t i o n the purpose of t h i s chapter  d e v e l o p a d i s c u s s i o n w h i c h w i l l p l a c e r e s u l t s from chapters  i n t o an e v o l u t i o n a r y  i s to  previous  context.  Each s p e c i e s p o s s e s s e s a v a r i e t y o f s t r a t e g i e s ( e g . 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 ) w h i c h c o n s i s t o f t h e sum o f b o t h t h e 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 t h r o u g h n a t u r a l s e l e c t i o n t o m a x i m i z e t h e f i t n e s s of i n d i v i d u a l s .  Environmental d i v e r s i t y  ingredient that favours strategies.  This  i s the e s s e n t i a l  the e v o l u t i o n o f species  i s because the m e t a b o l i c  specific  and g e n e t i c  203 resources  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 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 , come a t t h e e x p e n s e o f e n e r g y and i n f o r m a t i o n adaptations  addressed t o d i f f e r e n t environmental c h a r a c t e r i s t i c s ,  (Cody and Diamond, 1 9 7 5 ) . any  trait  t h a t c o u l d go i n t o a n o t h e r s e t o f  Hence t h e b e n e f i t s d e r i v e d  a r e w e i g h e d by n a t u r a l s e l e c t i o n a g a i n s t  maintaining  that t r a i t  and a l s o a g a i n s t  from  the costs of  t h e "abandoned" b e n e f i t s  of a l t e r n a t i v e t r a i t s . I n 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 t h e morphological by  and b e h a v i o u r a l  traits  t h a t c o n t r o l food " s e l e c t i o n "  t r o u t and kokanee i n M a r i o n Lake (summarized i n T a b l e 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  exclusive  foraging  to gather  s t r a t e g i e s which s u i t these predators  food most e f f e c t i v e l y  f r o m two d i f f e r e n t e n v i r o n m e n t s .  One  c o n s e q u e n c e o f t h i s i s t h a t t r o u t and k o k a n e e a r e p r e a d a p t e d t o g a t h e r d i f f e r e n t f o o d s e v e n i n t h e same e n v i r o n m e n t ( e g . 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  Lake).  I w i l l d e v e l o p t h e c h a p t e r i n two p a r t s . 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 general habitat-prey foraging  The f i r s t  nature o f the  c o m p l e x e s t h a t have shaped t r o u t and kokanee  strategies.  The s e c o n d p a r t c o n s i s t s o f a d i s c u s s i o n  c o n c e r n i n g t h e e v o l u t i o n and a d a p t i v e  significance of specific  e l e m e n t s o f t r o u t and k o k a n e e f o r a g i n g s t r a t e g i e s .  TABLE 29. A summary o f the "elements" which define the foraging s t r a t e g i e s o f t r o u t and kokanee.  TROUT  KOKANEE  Within lake d i s t r i b u t i o n c o n s i s t e n t l y skewed towards onshore and benthic habitats  Within lake d i s t r i b u t i o n c o n s i s t e n t l y skewed towards offshore and surface o r water column h a b i t a t s  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 l e v e l s throughout d a y l i g h t hours. R e l a t i v e l y i n a c t i v e a t night.  A c t i v i t y peaks i n e a r l y morning and l a t e afternoon. Foraging e f f o r t s are continuous throughout the d a y l i g h t hours. R e l a t i v e l y i n a c t i v e a t night.  Do not "track" the environmental abundance o f prey very c l o s e l y  .Track the environmental abundance o f p o t e n t i a l prey much more c l o s e l y than t r o u t do  SOURCE • Table 7. and F i g . 12 F i g ' s . 13 and 14.  Fig's. 8 and 9  Greatest p r o p o r t i o n o f d i e t (by wt.) obtained from r e l a t i v e l y l a r g e s i z e c l a s s e s of prey (eg. 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, caddis and other f i s h )  Greatest proportion o f d i e t (by wt.) obtained from r e l a t i v e l y small s i z e c l a s s e s o f prey (eg. chironomid pupae, zooplankton etc...)  F i g ' s . 4, 5, 6, and 7  U s u a l l y forage as s o l i t a r y i n d i v i d u a l s but may forage w i t h i n groups on some occasions  Usually forage i n groups except when searching i n benthic habitats a t 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  V i s i o 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 o r i n t e r m i t t e n t swimming a c t i v i t y  Employ a v a r i e t y of search techniques which always involve constant swimming activity  Table 12. and text  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 , " t e s t " r e l a t i v e l y few inanimate o b j e c t s f o r t h e i r " p o t e n t i a l as prey" and maintain search p o s i t i o n s t h a t are a t r e l a t i v e l y g r e a t distance from the substrates that are scanned  Area intensive searchers i e . maintain r e l a t i v e l y low search v e l o c i t i e s , " t e s t " large numbers of small inanimate objects f o r t h e i r p o t e n t i a l as food and maintain search p o s i t i o n s that are r e l a t i v e l y c l o s e to the substrates that are scanned (see text f o r f u r t h e r explanation)  Table 10. F i g ' s . 16 and 17  Reactive d i s t a n c e t o a v a r i e t y of prey of d i f f e r e n t s i z e s (up to 16 mm) i s u s u a l l y l e s s than 70 cm  Reactive distance t o a v a r i e t y of prey of d i f f e r e n t s i z e s (up to 16 mm) i s u s u a l l y l e s s than 70 cm  F i g . 20  Employ a v a r i e t y of approach and capture techniques to take b e n t h i c , water column, surface and a e r i a l prey  Employ a v a r i e t y of approach and capture techniques to take benthic, water column, and surface prey but do not e x p l o i t a e r i a l prey  Table 17.  Possess r e l a t i v e l y l a r g e mouths ( i e . jaw widths are c l o s e to 7-8% of standard length) and p o o r l y developed gill-rakers  Possess r e l a t i v e l y small mouths ( i e . jaw widths are c l o s e to 5-6% of standard length) and w e l l developed g i l l - r a k e r s  F i g . 22 and text  Exhibit a predisposition to attack r e l a t i v e l y l a r g e , well-armoured prey and tend to ignore o r r e j e c t s m a l l , a g i l e ones  E x h i b i t a p r e d i s p o s i t i o n to attack r e l a t i v e l y small, a g i l e prey and to ignore or r e j e c t l a r g e , armoured ones  Experiments 5.2, 5.3 and 5.4  Table 29 - continued  E x h i b i t r e l a t i v e l y high l e v e l s o f manipulation and i n g e s t i o n success w i t h l a r g e , armoured and morphologically d i v e r s e prey  E x h i b i t r e l a t i v e l y low l e v e l s o f manipulation and ingestion success w i t h l a r g e , armoured and morphologically d i v e r s e prey  Table 24.  E x h i b i t v a r i a b l e a t t a c k and ingestion success w i t h r e l a t i v e l y s m a l l , a g i l e prey  E x h i b i t g e n e r a l l y high attack and i n g e s t i o n success with small, a g i l e prey  Tables 20 and 22  Maximum a t t a c k r a t e s on s m a l l , a g i l e , prey a t high d e n s i t i e s are r e l a t i v e l y low ( i e . average 1860 a t t a c k s per hour on Daphnia a t d e n s i t i e s o f s l i g h t l y greater than 2 per l i t e r )  Maximum attack rates on s m a l l , a g i l e , prey a t high d e n s i t i e s are r e l a t i v e l y high ( i e . average more than 3000 attacks per hour on Daphnia).  E x h i b i t a r e l a t i v e l y high prey-size threshold f o r an e f f e c t i v e a t t a c k response on very s m a l l prey  E x h i b i t a r e l a t i v e l y low prey-size threshold f o r an e f f e c t i v e attack response on very small prey  Table 22  Develop and maintain high l e v e l s o f r e c e p t i v i t y t o prey w i t h which they experience g r e a t e r than 30% a t t a c k success. Responses more v a r i a b l e a t lower l e v e l s of success.  Develop and maintain high l e v e l s o f r e c e p t i v i t y t o prey w i t h which they have greater than 30% attack success. Responses are more v a r i a b l e a t lower l e v e l s of success  F i g . 30  Maintain high l e v e l s o f response t o some large prey i n s p i t e o f low i n g e s t i o n success  Habituate r a p i d l y t o large prey which o f f e r low net energy returns and/or p o s s i b l y high r i s k o f damage during ingestion  F i g . 23  F i g . 31  Table 29 - continued  Habituate r a p i d l y t o some small prey i n s p i t e of a p o t e n t i a l f o r p o s i t i v e net energy returns  Maintain high l e v e l s of response to small prey i n general  F i g . 33  o  208  THE ENVIRONMENTAL CONTEXT FOR TROUT AND KOKANEE FORAGING STRATEGIES In and  each i n s t a n c e t h e 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  kokanee must u l t i m a t e l y r e l a t e t o a s p e c i f i c  context.  What i s t h i s c o n t e x t ?  characteristics related kokanee observed  environmental  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  t o t h e f e e d i n g b e h a v i o u r o f t r o u t and  i n t h i s s t u d y have e v o l v e d o r a t l e a s t been  m a 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 t h e h a b i t a t s and prey t h a t a r e found w i t h i n M a r i o n Lake.  However, a d a p t a t i o n s  do n o t e v o l v e from s c r a t c h ( L e w o n t i n , 1 9 7 8 , H o r r i d g e  1977),  t h u s , t h e k i n d s o f 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 t h e f o o d r e s o u r c e s o f M a r i o n Lake  will  have been l i m i t e d t o ones t h a t a r e e x t r e m e l y  s i m i l a r t o those  of  t h e elements  their  immediate a n c e s t o r s .  Consequently,  making up t h e 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 k o k a n e e i n M a r i o n Lake must be r e l a t e d n o t o n l y t o s p e c i f i c of  characteristics  h a b i t a t s and p r e y f o u n d w i t h i n M a r i o n Lake b u t a l s o t o  h a b i t a t and p r e y 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 t o 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 t h e 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 r e s p o n d  t o i n M a r i o n Lake compared t o t h o s e commonly  c o n s i d e r e d t o be i m p o r t a n t t o t h e s p e c i e s a c r o s s geographic of  range w i l l  help i d e n t i f y the general  t h e h a b i t a t - p r e y c o m p l e x e s t h a t have l i k e l y  maintained  characteristics shaped and  t h e 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 k o k a n e e . In  13.3  their  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  (area  h a , maximum d e p t 6 m), M a r i o n 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 s t r e a m s do n o t r e p r e s e n t  a single  h a b i t a t - p r e y complex t o r a i n b o w t r o u t o r k o k a n e e . presented  i n t h i s study  observations  Results  ( C h a p t e r 3) as w e l l as a d d i t i o n a l  ( H a l l & Hyatt,  1974;  Hyatt,  unpublished  data)  i d e n t i f y r a i n b o w t r o u t from M a r i o n Lake as p r e d a t o r s  that  most commonly use h a b i t a t s f o u n d w i t h i n s m a l l s t r e a m s o r the l i t t o r a l  zone o f the l a k e .  O u t s i d e o f M a r i o n Lake  r a i n b o w 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  in a variety  o f l o c a t i o n s i n c l u d i n g : b r o o k s and  rivers  Neave, 1944;  Rawstron, 1972),  Hartman & G i l l ,  1968;  ponds ( B e r s t & McCombie, 1 9 7 5 ) , l a k e s Cartwright,  1961;  (Hart, 1973).  (Metzelaar,  However b e h a v i o u r a l  responses of t r o u t to  J e n k i n s , 1969)  responses to both s a l i n i t y (McCrimmon, 1972)  ( H o a r , 1976)  1960;  and p h y s i o l o g i c a l and  temperature  suggest t h a t t r o u t are p r i m a r i l y animals  of streams, intermediate of  1956;  t h e open o c e a n  h a b i t a t f e a t u r e s s u c h as o v e r h e a d c o v e r (Newman, McCrimmon & K w a i n , 1966;  small  (Larkin et a l .  Tody, 1 9 6 4 ) , e s t u a r i e s and  1929;  s i z e d r i v e r s and  the l i t t o r a l  zone  lakes. Benthic  and  littoral  h a b i t a t s o f s t r e a m s and  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 f a u n a t h a t may as p r e y f o r t r o u t .  Some o f t h e  important  molluscs  and  Hynes, 1970; reflected  a v a r i e t y of aquatic Wetzel,  i n the  1975).  potentially  groups  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 ,  lakes serve  include  amphipods, c r a y f i s h ,  insecta ( f o r reviews  This taxonomic d i v e r s i t y  see is  i n v e r t e b r a t e s by a w i d e r a n g e o f body s i z e s  however t h e v a r i o u s s i z e s o f p r e y a r e n o t e q u a l l y a b u n d a n t .  210  F o r e x a m p l e , w i t h i n t h e t o p cm o f a " t y p i c a l " sediment average  i n the l i t t o r a l  square meter of  zone o f M a r i o n L a k e , t h e r e i s an  ( b a s e d upon m o n t h l y  samples taken over a y e a r , Hoebel,  u n p u b l i s h e d d a t a ) o f 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  (rotifers,  c o p e p o d s , m i t e s , c l a d o c e r a n s and nematodes) between  100  2 mm  ym and  in size.  F o r p r e y g r e a t e r t h a n 2 mm  in size,  t h e a v e r a g e numbers p e r s q u a r e m e t e r a r e g e n e r a l l y l e s s 50,000 ( b a s e d upon m o n t h l y et  than  samples taken over a y e a r , E f f o r d  a l . , unpublished d a t a ) .  S i m i l a r p a t t e r n s o f abundance  v e r s u s s i z e e x i s t f o r t h e 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. drift  B i s h o p and Hynes (1969) sampled  the  invertebrate  i n a s t r e a m , on a 24 h o u r b a s i s , o n c e a month f o r a  y e a r and  f o u n d t h a t more t h a n 96% o f t h e p r e y p r e s e n t were  s m a l l e r t h a n 5 mm  i n length.  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 e d i v e r s i t y o f p o t e n t i a l p r e y i n b e n t h i c and is reflected  i n the d i e t o f rainbow t r o u t .  l o c a t i o n s or developmental  terrestrial  i n s e c t s ( S w i f t , 1970;  i n s e c t l a r v a e (Tody, 1964;  littoral  habitats  At v a r i o u s times,  s t a g e s t r o u t d i e t s may  by z o o p l a n k t o n ( J o h n s o n and H a s l e r , 1954;  that  be  dominated  A n t i p a , 1974),  Northcote, 1973), a q u a t i c  Crossman and L a r k i n , 1959;  Tippets  and M o y l e , 1 9 7 8 ) , l a r g e c r u s t a c e a n s s u c h 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 Crossman and L a r k i n , 1 9 5 9 ) .  ( L e o n a r d and L e o n a r d ,  Regardless of t h i s  dietary  d i v e r s i t y there are g e n e r a l trends i n rainbow t r o u t habits.  1946,  dietary  F o r example as r a i n b o w t r o u t i n c r e a s e i n body  t h e y 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  size,  211  invertebrates  ( t r o u t l e s s t h a n 10 cm i n s i z e ) , t o l a r g e r  crustaceans or aquatic and  finally  fish,  i n s e c t a ( t r o u t l e s s t h a n 20 cm i n s i z e ) ,  t o a d i e t d o m i n a t e d by l a r g e p r e y i t e m s s u c h a s  squid o r c r a y f i s h ( t r o u t greater  t h a n 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 items present i n benthic  and l i t t o r a l  prey  h a b i t a t s , t r o u t , over  most o f t h e i r d e v e l o p m e n t a l h i s t o r y , d e r i v e t h e b u l k o f t h e i r food  ( g r e a t e r t h a n 4 mm)  from r e l a t i v e l y l a r g e  prey  items.  T r o u t from M a r i o n Lake c o n f o r m t o t h i s p a t t e r n s i n c e t h e y do not  t r a c k t h e e n v i r o n m e n t a l abundance o f p r e y v e r y  ( C h a p t e r 2,) b u t r a t h e r d e r i v e  the b u l k o f t h e i r food  r e l a t i v e l y l a r g e ( g r e a t e r t h a n 4 mm), a r m o u r e d , invertebrates  closely  (eg. s n a i l s , caddis  from  benthic  l a r v a e , o d o n a t e nymphs and  amphipods). The habitat-prey  d e t a i l s p r e s e n t e d above i n d i c a t e t h a t t h e complex t h a t t r o u t r e s p o n d t o i n t h e M a r i o n  Lake s y s t e m ( s t r e a m and l a k e s h o r e p r e y s u c h as b e n t h i c is  habitats, relatively  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  large  insects)  t h e same a s t h a t w h i c h h a s been i m p o r t a n t t o t h e s p e c i e s  in general.  Therefore,  I suggest that the foraging  o f t r o u t from M a r i o n Lake i s l i k e l y r e p r e s e n t a t i v e of the species  i n general  and t h a t i t h a s e v o l v e d  strategy of that  primarily  in response t o c e r t a i n s e l e c t i v e pressures c h a r a c t e r i s t i c of the l i t t o r a l It  and b e n t h i c  i s my c o n t e n t i o n  regions  o f b o t h r i v e r s and l a k e s .  t h a t t r o u t " s e e " an e n v i r o n m e n t  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  containing  prey that are r e l a t i v e l y  212  dispersed  within a given  foraging patch.  I will  argue  t h i s type o f environment has s e l e c t i v e l y favoured  adaptations  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 dispersed  prey a t the cost o f t h e i r a b i l i t i e s  that  large  to effectively  e x p l o i t small, c o n t a g i o u s l y - d i s t r i b u t e d prey.  Because p r e y  t h a t t r o u t o b t a i n most o f t h e i r e n e r g y f r o m a r e r e l a t i v e l y dispersed  (see discussion below), I w i l l  r e f e r t o the foraging  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 . I n s h a r p c o n t r a s t t o r a i n b o w t r o u t , M a r i o n Lake kokanee a r e a l m o s t 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  less  habitats.  In s p i t e o f the f a c t that  t h a n 35% o f t h e s u r f a c e a r e a o f M a r i o n L a k e l i e s contours a t depths greater in  o v e r bottom  t h a n 2 m, kokanee a r e f o u n d l a r g e l y  t h i s a r e a ( C h a p t e r 3 ) . O u t s i d e o f M a r i o n L a k e , k o k a n e e and  s o c k e y e s a l m o n ( f r o m w h i c h kokanee a r e  taxonomically  i n d i s t i n g u i s h a b l e ) o c c u p y a l i m i t e d r a n g e 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 t h e  open o c e a n .  the confines  Within  o f a l a k e , k o k a n e e and  j u v e n i l e s o c k e y e u s u a l l y o c c u p y t h e o f f s h o r e , u p p e r and m i d d l e l a y e r s o f w a t e r ( F o e r s t e r , 1968; Hartman & B u r g n e r , 1 9 7 2 ; 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  associated  h a b i t a t s (Northcote  with benthic  & L o r z , 1966;  Chapman e t a l . , 1967) o f o f f s h o r e w a t e r s f o r s h o r t p e r i o d s o f t i m e , however t h e r e a r e few r e c o r d s p r o l o n g e d use o f t h e l i t t o r a l p r i o r t o maturation. habitat features  of populations  t h a t make  zone o f l a k e s o r o f r u n n i n g  Behavioural  waters  r e s p o n s e s o f 0. n e r k a t o  s u c h a s c o v e r (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 1952; F o e r s t e r , 1968;  ( H o a r , 1976)  H y a t t , Ms.  and t e m p e r a t u r e  (Brett,  i n prep.) suggest t h a t o u t s i d e  o f t h e b r e e d i n g s e a s o n 0. n e r k a i s p r i m a r i l y a d a p t e d t o t h e upper w a t e r s o f l a r g e l a k e s and t h e open o c e a n . The p r e y complex  o f the upper w a t e r s 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 dominated  o r j u v e n i l e s o c k e y e salmon i s  by j u s t t h r e e m a j o r g r o u p s .  These i n c l u d e  rotifers,  c l a d o c e r a n s and copepods ( f o r r e v i e w s see H u t c h i n s o n , 1967; Wetzel, 1975).  The  size-frequency d i s t r i b u t i o n of  invertebrate  p r e y 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 o c c u p i e d by t r o u t . potential  For example,  and b e n t h i c  habitats  a l t h o u g h more t h a n 15% o f t h e  i n v e r t e b r a t e p r e y ( o r g a n i s m s g r e a t e r t h a n 100  b e n t h i c h a b i t a t s o f M a r i o n Lake commonly f a l l w i t h i n r a n g i n g from 2-30  mm,  m)  sizes  i n v e r t e b r a t e p r e y l a r g e r t h a n 2 mm  are  v i r t u a l l y a b s e n t from t h e w a t e r c o l u m n .  T h i s same p a t t e r n  i s repeated  Freshwater  f o r the m a j o r i t y o f l a k e s .  ( e g . M y s i s r e l i c t a , L e p t o d o r a s p . , Chaoborus  invertebrates  spp.  and p o n t o p o r e i a sp.) w h i c h commonly e x c e e d s i z e s o f 4 and w h i c h may  mm  o c c u r i n the upper w a t e r s 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 s u c h p r e y a r e l a r g e l y  a b s e n t f r o m t h e l a k e s t h a t kokanee found i n .  in  and j u v e n i l e s o c k e y e a r e  I n c a s e s where s u c h p r e y a r e p r e s e n t i n t h e  upper  waters of l a k e s , i t i s u s u a l l y only f o r a short i n t e r v a l  during  t h e 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 f r o m d e e p w a t e r 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 s o c k e y e salmon use s m a l l ( l e s s t h a n 2 mm)  and  juvenile  crustacean zooplankton  214  as t h e i r m a j o r s o u r c e o f f o o d .  A number o f s t u d i e s  (Narver,  1970;  Woody, 1 9 7 2 ; Horak and T a n n e r , 1 9 6 3 ; D a v i s and W a r r e n ,  1970;  M c D o n a l d , 1973) i n d i c a t e t h a t 80% o r more o f a n n u a l  food  i n t a k e ( b y v o l u m e ) may c o n s i s t o f l i m n e t i c  zooplankton.  Some s t u d i e s (Chapman e t a l . , 1 9 6 7 ; N o r t h c o t e and L o r z , 1966; G o o d l a d e t a l . , 1974) r e p o r t t h a t c h i r o n o m i d  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 o f t h e d i e t , however b e n t h i c and  terrestrial  i n s e c t s a r e seldom r e p o r t e d  i n the d i e t of t h i s The  still  as s i g n i f i c a n t  species.  pronounced d i e t a r y s h i f t s t h a t o c c u r i n rainbow  t r o u t as they i n c r e a s e kokanee.  invertebrates  i n body s i z e a r e l a r g e l y a b s e n t i n  Kokanee ( o r s o c k e y e salmon) a s l a r g e as 30 cm may  r e l y almost e n t i r e l y on s m a l l  ( l e s s t h a n 3 mm)  column p r e y f o r t h e b u l k o f t h e i r f o o d . (Hanamura, 1966; L e B r a s s e u r ,  water  A number o f a u t h o r s  1966) have i n d i c a t e d t h a t e v e n  o c e a n d w e l l i n g s o c k e y e a t s i z e s between 30 and 50 cm consume m o s t l y s m a l l amphipods and copepods f r o m t h e w a t e r c o l u m n . 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 k o k a n e e may  rely  on r e l a t i v e l y l a r g e w a t e r colmn p r e y s u c h a s M y s i s s p . f o r food  ( s i z e s g r e a t e r t h a n 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  populations  containing  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  t h a n 40 cm  (Northcote,  1 9 7 2 ; I r i z a r r y , 1975) .  I n t h e open o c e a n , s o c k e y e  l a r g e r t h a n 50 cm i n l e n g t h consume p r e d o m i n a n t l y s q u i d and s m a l l f i s h  (LeBrasseur,1966 ).  euphasiids,  T h u s , o v e r most o f  t h e i r , d e v e l o p m e n t a l h i s t o r y , k o k a n e e ( a n d s o c k e y e salmon) a r e pelagic predators small-bodied  that gather  t h e i r food  from  relatively  p r e y f o u n d i n t h e upper w a t e r s o f l a k e s o r t h e  215  open o c e a n .  Kokanee from M a r i o n Lake p a r t i a l l y c o n f o r m t o  t h i s p a t t e r n s i n c e t h e y d e r i v e much o f t h e i r f o o d water-column prey (eg. chironomid the  The with  pupae, z o o p l a n k t o n ) however  inclusion of q u a n t i t i e s of benthic  d i e t represents  from s m a l l ,  a s i g n i f i c a n t departure  invertebrates i n their from t h e g e n e r a l  pattern.  h a b i t a t - p r e y complex t h a t kokanee a r e f a c e d  i n M a r i o n Lake e x h i b i t s a number o f f e a t u r e s t h a t  from t h o s e n o r m a l l y  e n c o u n t e r e d by t h e s p e c i e s .  differ  First less  t h a n 30% o f t h e l a k e a r e a c a n be c l a s s e d a s l i m n e t i c i n character  ( i e . open w a t e r l a c k i n g c o v e r s u c h a s l o g s , b r u s h  o r weed b e d s ) .  Next t h e zooplankton  community i n M a r i o n Lake  i s e p h e m e r a l and i s w e l l d e v e l o p e d o n l y d u r i n g period  i n l a t e summer ( E f f o r d , 1 9 7 0 ) .  e x h i b i t s a seasonal  a 2-3 month  F i n a l l y M a r i o n Lake  t e m p e r a t u r e r e g i m e w h i c h " f o r c e s " kokanee  t o abandon s u r f a c e w a t e r s ( H y a t t Ms. i n p r e p . ) and t o f o r a g e in offshore  ( d e p t h s g r e a t e r t h a n 2 m), b e n t h i c - h a b i t a t s i n  l a t e summer ( C h a p t e r 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 t h e r e s p o n s e s o f kokanee t o t h e h a b i t a t - p r e y complex i n M a r i o n Lake s u g g e s t a f o r a g i n g s t r a t e g y w h i c h 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 t h e o f f s h o r e and u p p e r w a t e r s o f l a r g e l a k e s and i n some c a s e s t h e open o c e a n . I t i s my c o n t e n t i o n s o c k e y e salmon) p e r c e i v e  t h a t kokanee (and i m p l i c i t l y  an e n v i r o n m e n t c o n t a i n i n g  relatively  high d e n s i t i e s of s m a l l , low-energy, prey that are c o n t a g i o u s l y d i s t r i b u t e d within a given  foraging patch.  t h i s t y p e o f e n v i r o n m e n t has f a v o u r e d  I will  adaptations  argue  that  which enable  216  kokanee to e x p l o i t r e l a t i v e l y  small, contagiously-distributed  prey a t the expense of t h e i r a b i l i t i e s dispersed  prey.  to e x p l o i t l a r g e ,  B e c a u s e p r e y t h a t kokanee o b t a i n most o f  t h e i r e n e r g y from a r e c o n t a g i o u s l y d i s t r i b u t e d ( s e e below), I w i l l a  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  C-strategy. It  i s important  to note a t the o u t s e t of t h i s  _ t h a t t h e c o n c e p t s o f C and but  discussion  D s t r a t e g i e s are not  a r e m e a n i n g f u l o n l y by c o m p a r i s o n .  absolute,  A given 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 c o m p a r i s o n w i t h organism.  Thus I w i l l  possess adaptations t o the C - s t r a t e g y  a r g u e t h a t r a i n b o w t r o u t as  of kokanee.  that in designating  The  u t i l i t y as w e l l as  t h i s p o i n t of view e x a c t l y  i n Stearns,  to t h e i r r e p r o d u c t i v e  1976).  By d e v e l o p i n g  D-selection with respect  s t r a t e g i e s o f kokanee and i s p o s s i b l e to recognize  g r e a t e r d e t a i l and or behavioural  K-  strategies  the concepts of  to the  foraging  t r o u t , I hope t o d e m o n s t r a t e t h a t i t complementary a s s o c i a t i o n s of  p a r t i c u l a r sets of environmental conditions. specific  the  parallels  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  describe  species  o r g a n i s m s as e i t h e r r - s t r a t e g i s t s o r  s t r a t e g i s t s with respect  . C - s e l e c t i o n and  a  another  w h i c h 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  weakness i n a p p l y i n g  (reviewed  discussion  I will  biological to now  f e a t u r e s o f e a c h h a b i t a t - p r e y complex i n t h e n a t t e m p t t o show t h a t  morphological  c h a r a c t e r i s t i c s associated with foraging  t r o u t o r kokanee a r e a d a p t a t i o n s  to these  features.  by  217  THE ROLE OF PREY. SIZE,' RELATIVE ABUNDANCE AND PHYSICAL FEATURES OF 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 FORAGING' STRATEGIES IN TROUT OR KOKANEE In t h e upper w a t e r s o f l a k e s c o n t a i n i n g  fish,  large,  i n v e r t e b r a t e p r e y (body s i z e s g r e a t e r t h a n 4mm) a r e s c a r c e compared t o t h e numbers f o u n d i n b e n t h i c  and l i t t o r a l  habitats,  however s m a l l p r e y (body s i z e s between 100 ym and 2mm) a r e abundant.  Kokanee a s w e l l as t h e m a j o r i t y o f o t h e r  freshwater  f i s h e s i n h a b i t i n g t h e u p p e r and o f f s h o r e w a t e r s o f t e m p e r a t e zone l a k e s o b t a i n t h e b u l k o f t h e i r d i e t s f r o m s m a l l p r e y i t e m s ( l e s s t h a n 2 mm body s i z e ) .  relatively Due t o t h e  a b s e n c e o f any d e t a i l e d a n a l y s i s o f t h e b i o l o g i c a l and p h y s i c a l conditions that c o n t r o l the e v o l u t i o n o f successful s t r a t e g i e s , I can only suggest that t h i s s i t u a t i o n because t h e r e a r e v e r y which p r o v i d e returned  occurs  few p o t e n t i a l c o m b i n a t i o n s o f a d a p t a t i o n s  a s e l e c t i v e l y advantageous balance o f energy  t o e n e r g y expended f o r a p r e d a t o r  habitats for large-bodied  searching  but extremely rare prey.  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 e n t l y favours  foraging  the evolution o f C-selected  open w a t e r Thus, i t  prey c o n s i s t -  strategies for  s u c h a s kokanee t h a t must o b t a i n t h e i r f o o d  predators  from t h e h a b i t a t -  p r e y complex a s s o c i a t e d w i t h t h e open w a t e r s o f t e m p e r a t e zone l a k e s . In benthic  or l i t t o r a l  h a b i t a t s o f lakes o r streams,  l a r g e , i n v e r t e b r a t e p r e y (body s i z e s g r e a t e r t h a n 4 mm) a r e a b u n d a n t compared t o t h e numbers f o u n d 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 a r e s t i l l  relatively  s c a r c e compared t o t h e  numbers o f s m a l l p r e y (body s i z e s l e s s t h a n 2 mm), many  species  218  of freshwater  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 , o v e r most o f t h e i r d e v e l o p m e n t a l from r e l a t i v e l y l a r g e prey i t e m s . of large-bodied evolutionary  prey i n benthic  Thus, t h e g r e a t e r  h a b i t a t s has  large, dispersed  concentrate  abundance  apparently  " o p p o r t u n i t i e s " to include D-selected  s t r a t e g i e s ( i n which predators  histories,  expanded  foraging  on e x p l o i t i n g r e l a t i v e l y  prey) i n a d d i t i o n to C-selected  foraging  s t r a t e g i e s . B e c a u s e r a i n b o w t r o u t a r e among t h e s p e c i e s  that  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 o f l a r g e p r e y i n t h e i r d i e t s , I consider T h e r e may  them t o be be  any  D-strategists.  number o f r e a s o n s f o r why  conditions  i n t h e s t r e a m o r l a k e h a b i t a t s o c c u p i e d by  t r o u t have  the balance of n a t u r a l s e l e c t i o n i n favour  o f the  of a D-strategy. observation  The  one  o r ; in.many p o p u l a t i o n s , w i t h i n running crustacean end  proportions  These h a b i t a t s o f t e n l a c k  z o o p l a n k t o n t h a t c h a r a c t e r i z e s the  presence of other  the  t h e i r e n t i r e developmental h i s t o r y  water h a b i t a t s .  o f the p r e y r e s o u r c e  evolution  s u g g e s t e d h e r e r e l i e s on  t h a t t r o u t a l w a y s spend s i g n i f i c a n t  tipped  small-but-abundant  distribution in lakes.  s m a l l prey types s t i l l  Although  concealed beneath benthic  substrates  normally  ( H y n e s , 1970)  and  thus i s  waters.  T h i s l i k e l y r e d u c e s 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 o r the e x p l o i t a t i o n o f  s m a l l p r e y by v i s u a l p r e d a t o r s  the  streams i s l a r g e l y  much l e s s e x p o s e d t h a n t h e m i c r o f a u n a o f s t a n d i n g  the e v o l u t i o n o f a d a p t a t i o n s  the  guarantees that  abundance o f p r e y o f 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 d i s t r i b u t e d , t h e m i c r o f a u n a o f r i v e r s and  the  s u c h as t r o u t .  Of  favour  relatively  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 t h e d r i f t o f s t r e a m s and  t r o u t do  the m a j o r i t y of t h e i r food invertebrates  (Jenkins  number o f s t u d i e s  supply  enter  appear to f r e q u e n t l y i n streams from  obtain  drifting  e t a l . , 1 9 7 0 ) , however r e v i e w o f  (references  suggests that small-bodied  i n B i s h o p and  Hynes,  p r e y ( l e s s t h a n 2 mm)  a  1969)  are  relatively  i n a c c e s s i b l e t o t r o u t even when s u s p e n d e d i n t h e w a t e r column o f a s t r e a m o r r i v e r as The of d e t e c t i n g  s m a l l s i z e of them a g a i n s t  d r i f t i n g d e t r i t u s , and before  drift. i n d i v i d u a l prey, the  difficulty  a background of s i m i l a r s i z e s of  the s h o r t time a v a i l a b l e f o r response  c u r r e n t s c a r r y prey out of a t t a c k range l i k e l y  against  s p e c i a l i z a t i o n on  trout.  The  s m a l l p r e y by p r e d a t o r s  such  a l t e r n a t i v e that natural s e l e c t i o n w i l l  the r e t e n t i o n of a d a p t a t i o n s  d i s t r i b u t i o n i n flowing water h a b i t a t s . p r e y complex o f l a k e s  end  A l t h o u g h the  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  below).  habitat-  that favour  the  differential  prey w i t h i n l i t t o r a l  incompatible  with  flowing  foraging  those t h a t  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 discussion  resource  i s somewhat d i f f e r e n t f r o m t h a t o f  s t r a t e g y here because a d a p t a t i o n s  is  relatively  of the prey  waters, I suggest that t r o u t r e t a i n a D-selected  h a b i t a t s of l a k e s are  as  favour  t o e x p l o i t the l a r g e ,  r a r e p r e y i t e m s w h i c h c h a r a c t e r i z e one  select  and  benthic  favour (see  220  THE ROLE OF PREY DISTRIBUTION I N SHAPING SEARCH AND ATTACK COMPONENTS OF TROUT AND KOKANEE FORAGING STRATEGIES In of  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  z o o p l a n k t o n commonly r e a c h 200-300 a n i m a l s p e r l i t e r and  o c c a s i o n a l l y exceed  5000 p e r l i t e r .  The d e n s i t y o f z o o p l a n k t o n  i n l a k e s i n h a b i t e d by O. n e r k a i s u s u a l l y r e p o r t e d a s falling  between 50-100 a n i m a l s p e r l i t e r  Fo.erster,  1968).  (references i n  U n l i k e b e n t h i c p r e y , t h e m a j o r i t y o f which  are u s u a l l y concealed, zooplankton a r e v i r t u a l l y exposed w i t h i n a f o r a g i n g p a t c h . of  always  Thus i f t h e a v e r a g e d e n s i t y  z o o p l a n k t o n i n a g i v e n f o r a g i n g p a t c h were 50 p e r l i t e r ,  t h e n t h e d i s t a n c e between s e q u e n t i a l e n c o u n t e r s by kokanee w i t h p r e y w o u l d be o n t h e o r d e r o f a few cm ( i e . 5 cm. o r less).  I t i s important t o emphasize t h a t t h i s v a l u e  likely  r e p r e s e n t s t h e 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  will  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 g e a r , moving l i n e a r l y  indicates.  T h i s i s because  sampling  t h r o u g h many c u b i c m e t e r s o f w a t e r , i s  not responsive 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 fish  are.  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 ,  prey a r e l i k e l y ( C h a p t e r 4)  multiple  t o be w i t h i n t h e r e a c t i v e d i s t a n c e o f a kokanee  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 p e r l i t e r b a s i s t h e d e n s i t i e s o f i n v e r t e b r a t e p r e y f o u n d w i t h i n t h e t o p 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 o f l a k e s may  commonly  exceed  t h e d e n s i t i e s o f i n v e r t e b r a t e s p r e s e n t i n t h e upper of  lakes.  waters  F o r e x a m p l e , w i t h i n M a r i o n Lake t h e r e a r e on  average  5 3.0-4.0x10  p o t e n t i a l prey  ( o r g a n i s m s between 100 ym  and  -2  15 mm  i n size  ) m  and more t h a n 80% o f t h e s e a r e  w i t h i n t h e t o p cm o f s e d i m e n t  found  (Efford et a l . , unpublished 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 t h e 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 t h e d e n s i t y of  b e n t h i c p r e y a r e n o t u s e f u l f o r even rough  of  t h e d i s t a n c e between s e q u e n t i a l e n c o u n t e r s w i t h b e n t h i c  p r e y by p r e d a t o r s s u c h as t r o u t . of  calculations  T h i s i s because the m a j o r i t y  b e n t h i c p r e y a r e n o r m a l l y c o n c e a l e d 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 s u c h as  trout.  Because the p r e y t h a t a r e s u b j e c t to d e t e c t i o n a r e those in  the d r i f t  suspended  o f s t r e a m s o r on t h e s u r f a c e o f 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 o r l a k e s , i t i s the d e n s i t i t e s o f these p r e y t h a t w i l l g e n e r a l l y d e f i n e t h e d i s t a n c e between p o t e n t i a l e n c o u n t e r s w i t h p r e y by t r o u t . B i s h o p 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 o f b e n t h i c p r e y i n t h e 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 ( a v e r a g e g r e a t e r t h a n 3.4x10 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 o f t h e  benthos time.  (.0002 - .004%) was  p r e s e n t i n the d r i f t  Therefore, d e n s i t i e s of d r i f t i n g  exceeded  an a v e r a g e  one seldom  i n t e r v a l once p e r  Furthermore  o f a l l p r e y were l a r g e r t h a n 5 mm  a t any  invertebrates  ( t a k e n o v e r a 24 hour  month) o f one p r e y p e r 20 l i t e r s . 3.5%  organisms  because o n l y  i n s i z e , the d e n s i t y  222  o f l a r g e r p r e y was  g e n e r a l l y l e s s than 1 per  500  liters.  t h e Speed R i v e r f l o w s  t h r o u g h a g r i c u l t u r a l l a n d and  r e l a t i v e l y productive  compared t o s t r e a m s i n h a b i t e d by  i n t h e P a c i f i c N o r t h w e s t t h e above v a l u e s than the average d e n s i t i e s of  is trout  are probably  invertebrate d r i f t  higher  encountered  t r o u t t h r o u g h o u t much o f t h e i r n a t i v e r a n g e .  Translated  a spatial  average  distance on  s c a l e , these values  s u g g e s t t h a t on  between p o t e n t i a l e n c o u n t e r s w i t h p r e y by  stream d r i f t  would be on  the o r d e r  o f 14-20  a d d i t i o n i f only l a r g e prey ( s i z e s g r e a t e r considered  the d i s t a n c e  Since  into  the  trout  cm.  by  feeding  In  t h a n 5 mm)  are  between p o t e n t i a l e n c o u n t e r s w o u l d  o f t e n be a m e t e r o r more. G i v e n the o b s e r v a t i o n s d i s t r i b u t i o n , abundance and  above, i t i s apparent t h a t  s i z e s of prey i n l i m n e t i c or  w a t e r h a b i t a t s have p r o v i d e d  rather different spatial  scales  the e v o l u t i o n o f s e a r c h  The  r e l a t i v e l y contagious d i s t r i b u t i o n of prey w i t h i n l i m n e t i c  should  be  trout or  flowing  for  h a b i t a t s , where t h e r e may  b e h a v i o u r s by  the  kokanee.  thousands of prey per c u b i c  h a v e " f a v o u r e d t h e e v o l u t i o n and  continued  maintenance  of area i n t e n s i v e search  t e c h n i q u e s by k o k a n e e .  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 of prey w i t h i n  By  contrast flowing  w a t e r 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 m  , should  search  have f a v o u r e d t h e e v o l u t i o n o f a r e a  t e c h n i q u e s by The  prey  extensive  trout.  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  while searching  meter,  kokanee e x h i b i t  f o r p r e y i n M a r i o n Lake a r e h i g h l y  with t h i s p o i n t of view.  F o r e x a m p l e , t r o u t and  compatible  kokanee  obtain  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 t h e l a k e s u r f a c e , h o w e v e r ,  when f o r a g i n g a t t h e 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 a r e 45-100 cm below t h e 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 b e l o w t h e 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 t h e p r e d a t o r s  below t h e 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 e a w i t h i n w h i c h , on t h e o r e t i c a l g r o u n d s , p r e y may be d e t e c t e d . T h i s i s b e c a u s e beyond an a n g l e o f 97° 12' c o m p l e t e r e f l e c t i o n of l i g h t occurs this reflection  and t h e p r e d a t o r s w i l l  internal  see o n l y  ( F i g .35).  For p r e d a t o r s  f o r a g i n g a t the lake surface i n very  s h a l l o w w a t e r , t h e r e f l e c t i o n beyond 97° 12' w o u l d c o n s i s t o f a distorted  image o f o b j e c t s o n t h e l a k e b o t t o m , w h i l e i n d e e p e r  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 o f 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 s e a r c h p o s i t i o n 75 cm b e l o w t h e s u r f a c e , a "window" w i t h a d i a m e t e r  o f 170 cm, w h i l e a kokanee a t o n l y  25 cm below t h e s u r f a c e w i l l o n l y 1/3 t h i s d i a m e t e r  searches  experience  a s e a r c h window  ( 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 a r e 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 t h e s u r f a c e b u t t h e y g a i n t h e 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 o f l a k e s u r f a c e p e r u n i t time  f o r relatively large  prey.  T h i s i s e s p e c i a l l y a p p a r e n t 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 r e 55% g r e a t e r t h a n o f kokanee when s e a r c h i n g  those  i n t h e w a t e r column ( T a b l e 1 0 ) .  When c o m b i n e d , t h e e f f e c t o f s e 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  t h e l a k e s u r f a c e on t h e d i a m e t e r a r e a w i t h i n w h i c h , on p r e y may  be d e t e c t e d .  o f the  t h e o r e t i c a l grounds, surface Beyond 97°  12'  i n t e r n a l r e f l e c t i o n of l i g h t occurs will  see o n l y t h i s r e f l e c t i o n .  o f 170  cm and  circular  complete  and  Search  57 cm a r e r e a s o n a b l y  the  predators  field  diameters  representative  v a l u e s g i v e n t h e 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  lake  s u r f a c e (see F i g . 1 7 ) . The  solid  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 at  t h a t t r o u t o r kokanee c o u l d  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  field  of search,  detect  instantaneous  v a l u e s o f t h e 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 p r e y 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 t h e r e a c t i v e d i s t a n c e o f t r o u t to very h i g h c o n t r a s t prey o f Ware, 1 9 7 1 ) .  According  p o s i t i o n e d a t 75 cm  ( s e e F i g . 3, P.  t o these  results, a trout  below t h e l a k e s u r f a c e w o u l d  r e q u i r e a p r e y o f a t l e a s t 45  s q u a r e mm  e l i c i t a r e s p o n s e a t t h e edge o f the field  i n area  the s u r f a c e would respond t o prey of l e s s than of  search.  to  instantaneous  o f s e a r c h , w h i l e a kokanee a t o n l y 25 cm  s q u a r e mm  104  a t t h e edge o f t h e i n s t a n t a n e o u s  from 3  field  KOKANEE-AREA  INTENSIVE  SEARCH  VELOCITY = 18.2 ^  cm/s ,  4 5 mm 2  VELOCITY = 27.7  SEARCH  FIELDS DRAWN TO S C A L E  cm/s  225  d i f f e r e n c e s a l l o w t r o u t t o s e a r c h a p p r o x i m a t e l y 15 t i m e s t h e a r e a t h a t kokanee do i n t h e same i n t e r v a l .  Therefore a t  the lake s u r f a c e the D-strategy o f t r o u t i n v o l v e s area e x t e n s i v e s e a r c h f o r l a r g e p r e y w h i l e t h e C - s t r a t e g y o f kokanee i n v o l v e s area intensive search f o r small prey. Unlike the s i t u a t i o n a t the lake surface, the search field  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 t h e s u b s t r a t e s u r f a c e (Fig.  36 a  and b ) , however t r o u t m a i n t a i n s e a r c h  positions  t h a t a r e o n a v e r a g e 30 cm away from t h e l a k e b o t t o m when t h e y f o r a g e 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 s e a r c h p o s i t i o n s t h a t a r e o n l y 5 cm away from t h e b o t t o m ( C h a p t e r 4, F i g . 1 6 ) . B e c a u s e 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 10), t r o u t w i l l  still  (Table  scan a s l i g h t l y l a r g e r area o f l a k e  b o t t o m p e r u n i t t i m e t h a n k o k a n e e however t h e d i s t i n c t i o n between a r e a e x t e n s i v e s e a r c h and a r e a i n t e n s i v e s e a r c h i s hardly  significant. The  essential  n e a r b o t t o m s e a r c h p o s i t i o n s o f kokanee a r e  i f they a r e 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  (ie.  < 1 mm  ) as those n o r m a l l y e x p l o i t e d  i n limnetic  b u t why s h o u l d t r o u t i n M a r i o n Lake m a i n t a i n s e a r c h such  t h a t they s a c r i f i c e the a b i l i t y  habitats,  positions  t o d e t e c t s m a l l prey  e v e n though t h e y do n o t g a i n t h e a d v a n t a g e o f 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 p r e y a s was t h e c a s e f o r t h e same behaviour  a t the lake surface?  One k e y may l i e i n t h e a s s u m p t i o n  t h a t t h e b e n t h i c s e a r c h b e h a v i o u r o f 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 substrates.  In l a k e s such as Marion  t h i s i s a reasonable assumption, but i n the flowing  water  h a b i t a t s commonly o c c u p i e d by t r o u t t h e b o t t o m i s h i g h l y i r r e g u l a r due t o t h e p r e s e n c e o f r o c k - c o b b l e Bottom i r r e g u l a r i t i e s the search  substrates.  i n flowing water h a b i t a t s w i l l  f i e l d of a predator  block  t h a t i s c l o s e t o the bottom  more t h a n f o r one t h a t s e a r c h e s f r o m a p o s i t i o n f u r t h e r o f f t h e b o t t o m (compare F i g . 36 c and d ) . An a d d i t i o n a l a d v a n t a g e for  a predator  assuming a h i g h e r  i s that the search  p o s i t i o n i n t h e w a t e r column  f i e l d may t h e n t a k e i n b o t h s u r f a c e and  benthic  substrates, given  36 d ) .  T h e r e f o r e i n f l o w i n g w a t e r h a b i t a t s w h i c h have  an  an a p p r o p r i a t e  water depth ( F i g . played  important r o l e i n shaping the D-strategy o f t r o u t , the  maintenance of search bottom c o u l d search.  p o s i t i o n s some d i s t a n c e  s e r v e a s an a d a p t a t i o n  t o favour  away from t h e area  extensive  Thus, t r o u t i n M a r i o n Lake may r e s p o n d t o p r e y  on b e n t h i c  substrates  according  search  t o t h e 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 w a t e r 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 aspects o f the s p a t i a l  distribution  o f p r e y i n f l o w i n g w a t e r 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 p r o c e d u r e s used by t r o u t and k o k a n e e . trout frequently leap distances above t h e l a k e s u r f a c e  F o r example  o f one t o two body  lengths  a s p a r t o f an a t t a c k p r o c e d u r e t o  c a p t u r e a e r i a l p r e y w h i l e kokanee do n o t a p p e a r t o l e a p a t all  f o r prey (Chapter 5 ) .  The a b i l i t y o f t r o u t t o 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 inshore  227  h a b i t a t s o f l a k e s where t h e r e w i l l u s u a l l y be many  flying  i n s e c t s j u s t above t h e w a t e r ' s s u r f a c e t h r o u g h o u t t h e summer. T h i s i s e s p e c i a l l y a p p a r e n t i n M a r i o n Lake w h i c h i s s m a l l , s h a l l o w and s u r r o u n d e d 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 a r e commonly i n f l i g h t j u s t above t h e l a k e ' s s u r f a c e t h r o u g h o u t much o f t h e summer. The  f a i l u r e o f kokanee t o l e a p f o r a e r i a l p r e y d o e s  n o t a p p e a r t o be v e r y a d a p t i v e w i t h i n t h e c o n t e x t o f t h e M a r i o n Lake e c o s y s t e m .  A l t h o u g h kokanee o c c u p y  offshore  h a b i t a t s i n M a r i o n L a k e , t h e l a k e i s s m a l l enough t h a t  large  numbers o f p o t e n t i a l p r e y a r e i n f l i g h t j u s t above t h e 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 capture  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 that the u s u a l h a b i t a t o f k o k a n e e i s e i t h e r t h e upper w a t e r s o f o f f s h o r e a r e a s i n l a r g e l a k e s o r , a s s o c k e y e s a l m o n , t h e upper of are  the Northeast P a c i f i c .  In these l o c a t i o n s a e r i a l  waters prey  u n l i k e l y t o have c o n s t i t u t e d an i m p o r t a n t enough p r e y  r e s o u r c e t o f a v o u r t h e e v o l u t i o n o f such b e h a v i o u r .  228  FIGURE 36.  The 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 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 instantaneous f i e l d o f search.  (a)  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 5 cm o f f o f l a k e bottom. Diameter o f instantaneous f i e l d o f search e q u a l s a p p r o x i m a t e l y 98 cm i f t h e r e a c t i v e d i s t a n c e t o p r e y i s assumed t o be 50 cm.  (b)  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 30 cm o f f of l a k e b o t t o m . D i a m e t e r o f 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 i s r e d u c e d t o a p p r o x i m a t e l y 79 cm i f t h e r e a c t i v e d i s t a n c e t o p r e y i s assumed t o be 50  cm.  (c)  Predator maintaining search p o s i t i o n of approximately 5 cm away from t o p o f 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 instantaneous f i e l d o f s e a r c h i s o n l y a p p r o x i m a t e l y 40 cm a t t h e s t r e a m b o t t o m and w i l l n o t i n c l u d e t h e w a t e r s u r f a c e i n r i f f l e s d e e p e r t h a n 45 cm. A g a i n t h e 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)  Predator maintaining search p o s i t i o n approximately 30 cm away f r o m t o p o f 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 instantaneous f i e l d o f s e a r c h i s a p p r o x i m a t e l y 76 cm a t t h e b o t t o m and 64 cm a t t h e s t r e a m s u r f a c e . A s s u m p t i o n s a r e t h a t t h e 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 t h e r e a c t i v e d i s t a n c e t o p r e y 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 to the s p a t i a l d i s t r i b u t i o n o f prey,  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 o f water or  b e n t h i c h a b i t a t s o f l a k e s o r streams w i l l  have a c t e d t o  shape and m a i n t a i n some e l e m e n t s o f t h e f o r a g i n g of  column  strategies  t r o u t and k o k a n e e . 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 t h e c o n t i n u o u s r e n e w a l o f p r e y a t any f i x e d location. because  The r a t e o f r e n e w a l w i l l  o f stream c h a r a c t e r i s t i c s  v a r y between  locations  ( e g . water v e l o c i t y ,  bed m o r p h o l o g y ) a s w e l l a s between t i m e s b e c a u s e  stream-  o f prey  b e h a v i o u r ( s e e W a t e r s 1972 f o r r e v i e w ) h o w e v e r , t h e t i m e for  r e n e w a l o f d r i f t i n g p r e y a f t e r r e m o v a l by t r o u t  riffle to  habitats w i l l  a few m i n u t e s .  scale  within  g e n e r a l l y be on t h e o r d e r o f a few s e c o n d s  As i n s t r e a m s , t h e w i t h i n - p a t c h r e n e w a l  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 o f l a k e s may  be v e r y f a s t ( s e c o n d s t o m i n u t e s ) b u t h i g h l y v a r i a b l e i n b o t h s p a c e and t i m e . at  T h i s i s because  very close  (concealed benthic invertebrates)  ( d i s t a n c e s o f mm t o a few m) t o t h e l o c a t i o n s  from w h i c h t r o u t o b t a i n t h e i r p r e y . the  renewal  t h e 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 b o t t o m are  the sources f o r prey  p r e y exposed  potentially  Thus i f t r o u t d e p l e t e  on t h e sediment s u r f a c e , prey r e n e w a l can  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 b e n e a t h  t h e s e d i m e n t s u r f a c e move i n t o e x p o s e d  positions.  230  Unlike  the b e n t h i c  invertebrates or  i n s e c t s t h a t t r o u t commonly use  as f o o d , t h e  terrestrial  zooplankton  t h a t kokanee commonly e x p l o i t i n the u p p e r w a t e r s o f  lakes  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 . r e m o v a l o f p r e y f r o m a f o r a g i n g p a t c h by kokanee can r e l a t i v e l y c o m p l e t e and  ments" some d i s t a n c e Due  by  prey t h a t are  diel  patterns  i n deep w a t e r " c o m p a r t -  away ( m e t e r s t o h u n d r e d s o f  t o t h e s p a t i a l and  be  p r e y r e n e w a l w i l l have t o depend  upon b o t h mass movements o f s u r f a c e w a t e r s and of v e r t i c a l migration  Thus  meters).  temporal s c a l e of these events, prey  r e n e w a l w i t h i n a f o r a g i n g p a t c h i n open w a t e r s i s l i k e l y much s l o w e r ( m i n u t e s t o h o u r s ) t h a n i n the b e n t h i c h a b i t a t s of l a k e s , o r the s u r f a c e of flowing  and  inferences  water-column  habitats  drawn above a r e g e n e r a l l y  then d i f f e r e n t r a t e s of w i t h i n - p a t c h and  inshore-surface  waters. I f the  benthic  and  correct,  prey renewal i n l i m n e t i c ,  f l o w i n g w a t e r h a b i t a t s may  have a c t e d  the e v o l u t i o n o f some d i f f e r e n c e s i n t r o u t and  t o shape  kokanee  search  behaviour. For example, the frequently  i n v o l v e s the  foraging  s t r a t e g y of t r o u t i n streams  s e l e c t i o n of f i x e d  l o c a t i o n s from  w h i c h t h e b o t t o m , w a t e r - c o l u m n o r s u r f a c e may prey (Jenkins, 1969). trout in a r t i f i c i a l  Furthermore, laboratory  s t r e a m s ( R i n g l e r , 1979)  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 o n e s and  be  scanned  studies  on  indicate that  trout  p o s i t i o n s from l e s s p r o f i t a b l e  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  f o r m e r ( s e e Chapman & B j o r n n ,  for  1969).  the  T r o u t i n M a r i o n Lake  231  e x h i b i t t h e same b e h a v i o u r s s i n c e t h e y f r e q u e n t l y s e a r c h f o r l a k e s u r f a c e o r b e n t h i c p r e y by s c a n n i n g t h e s u b s t r a t e s from a stationary position f o r intervals lasting  up t o a few m i n u t e s .  I f few c a p t u r e s a r e made, t r o u t resume m o b i l e s e a r c h 4).  (Chapter  G i v e n t h a t f l o w i n g w a t e r ; n e a r s h o r e , l a k e - s u r f a c e ; and  benthic habitats e x h i b i t a p o t e n t i a l f o r rapid but highly  variable  renewal of prey, t h e maintenance o f s t a t i o n a r y search p o s i t i o n s by t r o u t may be v i e w e d  a s an a d a p t i v e p r o c e d u r e  t o assess the  r a t e o f w i t h i n - p a t c h p r e y r e n e w a l b e f o r e t h e 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 . s e e C h a r n o v , O r i a n s and H y a t t , 1976). The p r e y s e a r c h b e h a v i o u r o f k o k a n e e c o n t r a s t s s h a r p l y w i t h t h a t o f t r o u t s i n c e kokanee o n l y employ s e a r c h techniques that involve continuous f i x e d - v e l o c i t y ( C h a p t e r 4).  swimming  Thus, kokanee i n M a r i o n Lake do n o t pause l o n g  enough a t t h e l a k e o r s e d i m e n t 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 t h e r a t e o f w i t h i n - p a t c h p r e y r e n e w a l . kokanee a r e a d a p t e d  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  where w i t h i n - p a t c h p r e y r e n e w a l moving d i r e c t l y  T h i s may be b e c a u s e habitats  i s r e l a t i v e l y s l o w and where  through a s e r i e s o f 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 p r e y t o r e c o v e r w i t h i n a p a t c h where t h e y have been r e c e n t l y depleted.  232  THE ROLE OF PREY S I Z E AND ABUNDANCE I N 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 p o s s e s s  foraging  s t r a t e g i e s t h a t have been shaped t h r o u g h e v o l u t i o n a r y to environments c o n t a i n i n g and  relatively  reponses  large, dispersed  prey  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 only e x h i b i t adaptations  the d e t e c t i o n o f such prey b u t a l s o should  which  favour  d i s p l a y complementary  sets of adaptations  f o rdealing with  respectively during  t h e a t t a c k phase o f f o o d a c q u i s i t i o n .  Results this  from p r e v i o u s  chapters o f f e r considerable  prey  support f o r  idea. Trout possess r e l a t i v e l y  associated and  l a r g e and s m a l l  with high  l a r g e mouths w h i c h a r e  l e v e l s o f success i n the  manipulation  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  prey.  diverse  Trout also e x h i b i t a p r e d i s p o s i t i o n to attack  l a r g e , w e l l - a r m o u r e d p r e y and t o m a i n t a i n  high  relatively  l e v e l s o f response  t o l a r g e p r e y e v e n when i n g e s t i o n s u c c e s s i s l o w . P e r s i s t e n t a t t a c k s u n d o u b t e d l y pay o f f by e n s u r i n g  that large but rare  energy packages which r e q u i r e prolonged o r vigorous  handling  a r e n o t abandoned p r e m a t u r e l y by t r o u t . Adaptations which s u i t p r e y have a p p a r e n t l y  n o t been a c h i e v e d w i t h o u t  the b e n e f i t s o f a l t e r n a t e t r a i t s . relatively  trout f o r acquiring  Accordingly  poorly developed g i l l - r a k e r s  "abandoning" t r o u t possess  which are o f t e n  i n some way w i t h an a d v a n t a g e i n e x p l o i t i n g r e l a t i v e l y prey (see Hyatt,  1979 f o r d i s c u s s i o n ) .  large  associated small  Trout also lack the  233  h i g h d e g r e e o f s t r e a m l i n e d f o r m a t t a i n e d by kokanee and t h i s i s a c c o m p a n i e d by a r e l a t i v e l y on s m a l l , a g i l e p r e y .  i n f e r i o r maximum a t t a c k r a t e  F i n a l l y trout exhibit a behavioural  t e n d e n c y t o e i t h e r i g n o r e o r r e j e c t many t y p e s o f 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 .  This  behaviour  a p p e a r s even i n s i t u a t i o n s when t h e p r e y a r e 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 t h e o n l y o b v i o u s As C - s e l e c t e d  source o f food.  strategists i n limnetic habitats,  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 range o f prey and w i l l  sizes  ( i e . u s u a l l y b e t w e e n 100 ym and  s e l d o m have e x p e r i e n c e d  a r e more l i k e l y  4mm)  l a r g e p r e y - s i z e a s an o b s t a c l e  to the s u c c e s s f u l capture or i n g e s t i o n of prey. i n handling prey  narrow  Difficulties  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 t h e v e r y s m a l l b u t a b u n d a n t p r e y a t t h e "end" o f t h e p r e y - s i z e d i s t r i b u t i o n . mouth, w e l l d e v e l o p e d  The r e l a t i v e l y  small  g i l l - r a k e r s , and l o w 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 r e s p o n s e on i n v e r t e b r a t e s a r e 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 f a v o u r for e x p l o i t a t i o n of r e l a t i v e l y  adaptations  small, morphologically-uniform  p r e y by k o k a n e e . 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 have f a v o u r e d  the e v o l u t i o n of behaviours  r a t e s of prey  i n t a k e i n o r d e r t o meet t h e e n e r g y demands  o f n o r m a l g r o w t h and m e t a b o l i s m .  t h a t promote h i g h  Kokanee do a c h i e v e  high  maximum a t t a c k r a t e s compared t o t r o u t when c o n f r o n t e d high d e n s i t i e s of small planktonic prey. may be a c h i e v e d  also  with  High a t t a c k r a t e s  n o t o n l y a s a c o n s e q u e n c e o f a more s t r e a m l i n e d  234  body form and to favour: attack  greater maneuverability  a l s o through s e l e c t i o n  s h o r t d e c i s i o n times concerning whether or not  a given prey item,  p r e y and,  but  short handling  as a c o r o l l o r y , s h o r t g i v i n g up  p r e y t h a t p r e s e n t any  to  times with i n d i v i d u a l t i m e s on i n d i v i d u a l  " d i f f i c u l t y " during manipulation.  These  c h a r a c t e r i s t i c s a r e h i g h l y c o m p a t i b l e w i t h t h o s e t r a i t s known t o c o n s t i t u t e elements of the from M a r i o n Lake ( T a b l e  foraging  behaviour of  kokanee  29).  I n 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 e n a b l e kokanee  to 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 mutually  e x c l u s i v e o f t h o s e w h i c h w o u l d a l l o w them t o  exploit large, dispersed  prey.  The  s m a l l mouth s i z e o f k o k a n e e  i s c l e a r l y r e l a t e d t o t h e r e l a t i v e l y low  l e v e l s of  manipulation  and  i n g e s t i o n s u c c e s s t h a t t h e y e x p e r i e n c e w i t h l a r g e , armoured  and  morphologically  diverse prey.  possess a behavioural  Kokanee a l s o a p p e a r t o  p r e d i s p o s i t i o n to frequently  o r r e j e c t even t h o s e l a r g e p r e y t h a t l a b o r a t o r y i n d i c a t e they can an a d a p t i v e the  handle.  ignore  experiments  I suggest that t h i s i s l o g i c a l l y  measure t o r e d u c e t h e r i s k o f p h y s i c a l damage t o  f i n e l y d e v e l o p e d s t r u c t u r e o f g i l l - r a k e r s w h i c h may  useful  i n e x p l o i t i n g very  small  be  prey.  FORAGING STRATEGIES AS COMPLEMENTARY COMPONENTS OF L I F E HISTORY STRATEGIES A foraging strategy represents the adaptive  only a portion  complex t h a t c h a r a c t e r i z e s e a c h s p e c i e s  i s e s s e n t i a l t o remember t h a t t r e n d s d i r e c t i o n o f any  one  component may  i n the  and  of it  evolutionary  i n f l u e n c e the s e l e c t i v e  235  a d v a n t a g e s o f many o r a l l o f t h e o t h e r components ( F i g . 3 7 ) . F o r e x a m p l e , w a t e r 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 o n t h e m a i n t e n a n c e o f l a r g e group s i z e .  Given that there  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 column, then the elements o f a s p e c i e s will  evolve  with reference  w e l l as t o those exerted resource.  groups i n the water foraging  strategy  t o t h i s s e l e c t i v e pressure  by t h e b a s i c n a t u r e  as  o f the prey  S u b t l e d i f f e r e n c e s i n t h e r e p o n s e s o f t r o u t and  kokanee t o " p r e y " a r e 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, and  attack  t h e n r e j e c t p i e c e s o f t w i g o r armoured p r e y i n t h e w a t e r  column o r on t h e s e d i m e n t s u r f a c e .  Attacks  may be r e p e a t e d  final  takes place.  several times u n t i l  upon a s i n g l e  item  rejection or ingestion  When i n d i v i d u a l k o k a n e e f o r a g e w i t h i n g r o u p s  i n t h e water column, they t o o " t e s t " o b j e c t s f o r t h e i r p o t e n t i a l , h o w e v e r , t h e y do n o t r e p e a t e d l y I n s t e a d , a f t e r an i n i t i a l  food  attack single  items.  r e j e c t i o n , t h e y move on w i t h t h e  g r o u p and t h e n t e s t t h e n e x t i t e m e n c o u n t e r e d .  The t e n d e n c y  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 p r e y i t e m s i n t h e w a t e r column i s l i k e l y due t o t h e " 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 t h e m a i n g r o u p a s i t moves.  For. t r o u t w h i c h  a l o n e most o f t h e t i m e , no s u c h c o n s t r a i n t a p p l i e s . 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  forage  Single  " t e s t " prey j u s t  as t r o u t d o . Because o f the nature  o f t h e i n t e r a c t i o n s between  t h e components making up t h e 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 o u t l i n e of the i n t e r a c t i o n s between the basic s t r u c t u r e of the h a b i t a t and the s t r a t e g i e s which organisms evolve i n response and  competitors.  to food, predators  -HABITAT • STRUCTURE  ANTIPREDATOR STRATEGY  •^DISTRIBUTION AND ABUNDANCE OF FOOD  GROUP-*SIZE  ^ STRATEGIES FOR ' t SOCIAL ORGANIZATION  FORAGING •STRATEGIES  237  it  i s usually impossible  element of a f o r a g i n g  to s p e c i f y whether a  strategy  particular  i s the p r e c e d e n t f o r o r  the  antecedent of a given p a t t e r n of 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 pattern of antipredator behaviour.  I n t h e example a b o v e ,  I c o u l d have a r g u e d j u s t as e a s i l y t h a t w a t e r - c o l u m n favour  habitats  the e v o l u t i o n of a prey complex, which i n t u r n  the e v o l u t i o n o f f o r a g i n g t h a t t h e r e was  i n g r o u p s by p r e d a t o r s .  Then,  an o v e r w h e l m i n g a d v a n t a g e t o f o r a g i n g  i n the water column, the elements of a s p e c i e s s t r a t e g y would evolve  with reference  as w e l l as t o t h o s e e x e r t e d  by  favours  as  given groups  antipredator  to t h i s s e l e c t i v e  the n a t u r e of the  pressure  important  predators. A l t h o u g h the o r d e r s t r a t e g i e s c a n n o t be  f o r the e v o l u t i o n o f a  s p e c i f i e d w i t h any  species  certainty, I believe  t h a t i t i s c l e a r f r o m t h e p r e s e n t s t u d y t h a t the b a s i c o f the h a b i t a t d e t e r m i n e s the n a t u r e of the a d a p t i v e ( s u m m a r i z e d i n T a b l e 30.) by  t r o u t and  conclusion  Cullen  to reproduction  tridactyla).  with respect  (1957) r e a c h e d a  concerning the adaptive  with respect (Rissa  kokanee.  that evolves  complex to  foraging  similar  complex t h a t has  by c l i f f - n e s t i n g  structure  evolved  kittiwakes  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 o f d i e t over most of developmental h i s t o r y composed of r e l a t i v e l y l a r g e (>4mm) prey.  Wide range o f invertebrate prey s i z e s common i n benthic and l i t t o r a l habitats o f flowing water o r lakes. Small prey (<2mm) r e l a t i v e l y unavailable i n flowing water.  Large prey provide more favourable net r e t u r n of energy per u n i t foraging e f f o r t than small prey i n flowing water h a b i t a t s .  KOKANEE  Track environmental abundance o f prey more c l o s e l y than t r o u t do. Bulk o f d i e t over most of developmental h i s t o r y composed of r e l a t i v e l y small (<2mm) prey items.  Narrow range o f invertebrate prey s i z e s commonly present (100.^m-3mm) i n limnetic h a b i t a t s r e l a t i v e to benthic or l i t t o r a l zone h a b i t a t s . Large prey s i z e s (>3mm) r e l a t i v e l y unavailable due to v i r t u a l absence from plankton.  Small but abundant prey provide a more favourable net r e t u r n o f energy per u n i t foraging e f f o r t than large prey i n limnetic habitats.  Large prey are r e l a t i v e l y dispersed i e . on average distances between p o t e n t i a l encounters w i t h large prey (>2mm) may o f t e n be a meter or more.  Search procedures increase the area scanned per u n i t time and predispose the predators t o d e t e c t l a r g e but rare prey.  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 f o r their " p o t e n t i a l as prey" and maintain search p o s i t i o n s t h a t are a t r e l a t i v e l y great distance from the "substrates that are scanned.  Table 3 0 . - continued  KOKANEE  Maintain r e l a t i v e l y low v e l o c i t i e s , "test" large numbers of small inanimate objects f o r t h e i r potential as prey and maintain search positions that are r e l a t i v e l y close to the "substrates" that are scanned.  Small planktonic prey exhibit r e l a t i v e l y contagious d i s t r i b u t i o n s within a given foraging patch. On average distances between sequential encounters with small prey (<2mm) may be 5 cm o r l e s s .  Search procedures favour the detection o f large numbers of small prey through area intensive search.  TROUT  Stationary search positions often maintained as an alternative to mobile search.  Within-patch renewal rates of benthic and t e r r e s t r i a l prey highly v a r i a b l e i n both space and time. Potential e x i s t s f o r very rapid (seconds to minutes) prey renewal a f t e r depletion.  Procedure allows predator to assess within-patch, prey-renewal rates before a "decision" i s made to abandon a patch.  KOKANEE  Search techniques always involve continuous, f i x e d v e l o c i t y swimming.  Within-patch renewal rates of planktonic prey r e l a t i v e l y slow (minutes to hours) i n both space and time.  Continuous movement through a series o f patches r e s u l t s i n a higher rate of prey discovery than waiting within a patch f o r prey to recover a f t e r depletion.  to to vo  Table 30 - continued  ATTACK BEHAVIOUR TROUT  Commonly l e a p t o capture a e r i a l prey.  A e r i a l prey frequently very abundant w i t h i n a few cm o f the water's surface i n flowing water o r l i t t o r a l zone h a b i t a t s .  KOKANEE  Do not leap t o capture a e r i a l prey.  A e r i a l prey l a r g e l y absent near the surface of the offshore waters o f large lakes o r the open ocean.  TROUT  Possess r e l a t i v e l y l a r g e mouths and a behavioural p r e d i s p o s i t i o n t o attack relatively large, wellarmoured prey.  Invertebrate prey span a wide range o f s i z e s and many possess tough c h i t i n o u s body coverings.  Ensure r e l a t i v e l y high l e v e l s ofsuccess during manipulation and i n g e s t i o n of l a r g e , armoured o r morphologically d i v e r s e prey.  KOKANEE  Possess r e l a t i v e l y small mouths, w e l l developed g i l l - r a k e r s and a p r e d i s p o s i t i o n t o attack r e l a t i v e l y small, morphologically-uniform prey.  Invertebrate prey span a narrow range of s i z e s and are morphologically uniform r e l a t i v e t o diverse invertebrates of benthic and l i t t o r a l zone habitats.  Small mouth and g i l l rakers may a i d i n capture o r r e t e n t i o n of small, planktonic prey.  Access t o a source o f food that would be unavailable otherwise.  Table 30 - continued  TROUT  Maintain high l e v e l s o f response t o large prey even i f i n g e s t i o n success i s low. Tend t o ignore o r r e j e c t many types o f small planktonic prey.  Large, armoured prey may require vigorous o r prolonged handling before i n g e s t i o n but each prey represents a large quantity o f energy.  Persistence during attacks on l a r g e prey ensures that l a r g e but rare energy packages are not abandoned prematurely.  KOKANEE  Maintain high l e v e l s o f response t o small prey even i f capture success i s low. Tend t o ignore o r r e j e c t l a r g e , armoured prey upon i n i t i a l encounter and habituate rapidly to stimuli presented by l a r g e prey upon repeated encounters.  Small, morphologically uniform prey require very short "handling" times and each prey represents a small quantity o f energy. Large prey s i z e should seldom c o n s t i t u t e a problem during manipulation o r ingestion o f prey.  Ensures that predators maintain high r a t e s o f prey intake and a favourable net-energy r e t u r n . Rejection o f large prey may be an i n e v i t a b l e consequence of the set o f " r u l e s " kokanee use t o assess small prey ( i e . minimize handling time) o r a procedure which reduces the r i s k o f damage t o d e l i c a t e s t r u c t u r e s such as g i l l - r a k e r s .  242  SUMMARY 1.  T r o u t 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  o f f l o w i n g w a t e r s and 2.  Conditions  particular  of the b e n t h i c  littoral  zones of  lakes.  associated with flowing water h a b i t a t s i n  have f a v o u r e d  trout concentrate  and  complex  the e v o l u t i o n of a D-strategy  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  i n which  prey items f o r  the bulk of t h e i r energy r e q u i r e m e n t s . 3.  Adaptations  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 area-extensive  search;  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 p r e y ; mouth s i z e , and large  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  large  attack  prey. 4.  T r o u t 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  morphologically-uniform  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 where t h e y s e r v e 5.  as t h e s o l e s o u r c e o f  habitat-prey  complex o f t h e l i m n e t i c zone o f l a r g e l a k e s and,  6. favoured  Conditions  conditions  food.  Kokanee a r e a d a p t e d p r i m a r i l y t o t h e  t o t h e open w a t e r s o f t h e P a c i f i c  small,  as s o c k e y e s a l m o n ,  ocean.  a s s o c i a t e d w i t h open w a t e r h a b i t a t s have  the e v o l u t i o n of a C - s t r a t e g y  i n w h i c h kokanee  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 p r e y i t e m s f o r t h e  concentrate  majority  of t h e i r energy requirements. 7.  Adaptations  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 small 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 a p r e d i s p o s i t i o n to attack r e l a t i v e l y s m a l l , 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  search;  morphologically gill-rakers;  and  243  an a b i l i t y 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.  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