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Predatory behaviour of rainbow trout (Salmo gairdneri) Ware, Daniel Morris 1971

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THE  PREDATORY BEHAVIOUR OF RAINBOW (SALMO G A I R D N E R I )  TROUT  by DANIEL  B.Sc,  University  A THESIS THE  M . WARE  of  SUBMITTED  IN  REQUIREMENTS DOCTOR OF in  the  British  C o l u m b i a , 1966  FULFILLMENT  PARTIAL FOR THE  DEGREE  OF  OF  PHILOSOPHY Department of  ZOOLOGY  Ule a c c e p t required  this  thesis  as  conforming  to  the  standard  THE  UNIVERSITY  OF  BRITISH  JANUARY,  1971  COLUMBIA  In p r e s e n t i n g t h i s t h e s i s  in p a r t i a l  f u l f i l m e n t o f the r e q u i r e m e n t s  an advanced degree at the U n i v e r s i t y of B r i t i s h C o l u m b i a , I agree the L i b r a r y I  further  s h a l l make i t f r e e l y  available for  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  r e f e r e n c e and copying of  this  of  this thesis for  It  is understood that copying or  thesis  Department o f  Z 6 D L O 6 V  The U n i v e r s i t y o f B r i t i s h Columbia Vancouver 8. Canada  Date  or  publication  f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my  written permission.  that  study.  f o r s c h o l a r l y purposes may be g r a n t e d by the Head of my Department by h i s r e p r e s e n t a t i v e s .  for  ABSTRACT The predatory behaviour of rainbow t r o u t was s t u d i e d to identify  some of the major f a c t o r s  response  to p r e y .  sp.  and H y a l e l l a  that  Two b e n t h i c - l i v i n g sp.  were s e l e c t e d  In some experiments,  as  influence their  amphipods Cranqonyx representative  a r t i f i c i a l food was  prey.  utilized.  Rainbow t r o u t adopt a searching p o s i t i o n some 10 to 15 cm from a s u b s t r a t e and l o c a t e can of  food v i s u a l l y .  detect only organisms that are exposed. a complex s u b s t r a t e ,  with g r e a t e r  (74%) than s t a t i o n a r y  success  will  react  presence  targets  The d i s t a n c e  (39$)  and a c t i v i t y of the o b j e c t  ambient i l l u m i n a t i o n , the s u b s t r a t e . palatable  food,  through l e a r n i n g .  with  from which t r o u t  to food was shown to be dependent upon the  inherent c o n t r a s t  but  In the  they  t r o u t were able to recognize moving prey  the same v i s u a l c h a r a c t e r i s t i c s .  of  As a r e s u l t ,  as well  size, as  the  t u r b i d i t y of the water and complexity  After  6 to 7 days of experience with a new  t r o u t can i n c r e a s e  their  reactive  distance  A general system of equations was developed  to d e s c r i b e the e f f e c t  of each of these parameters  on r e a c t i v e  distance. On the average, t r o u t s u c c e s s f u l l y  capture 82$ of the prey  they a t t a c k .  In the l a b o r a t o r y ,  maximum l e v e l  when the d e n s i t y of prey was i n c r e a s e d to 240 animals  per  sq.  m.  decreasing  the rate of capture reached a  I r r e s p e c t i v e of the abundance of food, however, hunger m o t i v a t i o n was found to depress  the  predator's  rate of capture as was the presence of a s u b s t r a t e i n which t h e -  prey could conceal The e f f e c t  themselves.  of water temperature  on the v e r t i c a l and  horizontal  movements of Cranqonyx and H y a l e l l a was a l s o  examined.  The v e r t i c a l  a c t i v i t y of both prey  e x p o n e n t i a l l y with a r i s e i n temperature.  increased  In c o n t r a s t ,  C. was suggested to be the optimum temperature movement of exposed A general hypothesis  that  invertebrate perceptual  animals.  the s e l e c t i v e  groups  level.  the f i s h as well  of  t h e i r prey,  by t r o u t ,  occurs at  The model considered the predatory as  the  e x p l o i t a t i o n of 4 major  i n Marion Lake,  the d e n s i t y  and p h y s i c a l  the  behaviour  characteristics  and was able to p r e d i c t with some accuracy  occurrence of d i f f e r e n t  foods  in trout  the  stomachs.  The model was a l s o able to account  f o r the s i z e -  e x p l o i t a t i o n of Cranqonyx and H y a l e l l a , the  changes i n the v u l n e r a b i l i t y of these s p e c i e s , that  the  s i m u l a t i o n model was developed to t e s t  of  selective  for  10°  the l e s s numerous Cranqonyx was captured  and the just  as  seasonal fact frequently  as H y a l e l l a . Trout r e q u i r e a t h r e s h o l d rate of capture /  min.) to maintain a s p e c i f i c  not a t t a i n  this  converge,  temporarily,  more abundant.  p a t t e r n of s e a r c h .  t h r e s h o l d they w i l l  other hunting p a t t e r n s .  If  they do  switch t h e i r a t t e n t i o n  As a r e s u l t ,  to  the p o p u l a t i o n should  i n t o areas i n which food i s  Since t r o u t  responsiveness to prey,  (about 2 captures  relatively  can also l e a r n to i n c r e a s e  both of these c h a r a c t e r i s t i c s  their would  improve t h e i r hunting e f f i c i e n c y . The r e s u l t s will  and a c t i v e ,  living  species.  discussed.  tend  smaller,  Moreover, i f a v i s u a l it  may not detect  l e s s than a c r i t i c a l s i z e .  these c o n c l u s i o n s i s  predators  prey that  success than  maintains a searching p o s i t i o n ,  food organisms  visual  may e x p l o i t , 1 a r g e  with g r e a t e r  a c t i v e or l e s s conspicuous  predator  of  study i n d i c a t e that  d i s c o v e r and,subsequently  to be exposed less  of t h i s  benthic-  The s i g n i f i c a n c e  ABSTRACT  i  LIST OF TABLES  vii  LIST OF FIGURES  ix  ACKNOWLEDGEMENTS  xii  GENERAL INTRODUCTION  1 SECTION I  THE EFFECT OF PREY DENSITY, PREY SIZE AND THE PRESENCE OF A SUBSTRATE ON THE FEEDING BEHAVIOUR OF TROUT INTRODUCTION  3  METHODS' AND MATERIALS  5  RESULTS  1 1  GENERAL FEEDING BEHAVIOUR  11  The Effect of Hunger and Prey Density on the Rate of Attack  12  The Relationship Between the Rate of Attack and Prey Size  18  THE EFFECT OF A SUBSTRATE ON THE ATTACK RATE  22  PREY CAPTURE SUCCESS  30  THE THRESHOLD RATE OF PREY CAPTURE AND THE SEARCHING PATTERN  3  4  DISCUSSION  39  SUMMARY  4 4  THE  EFFECT  OF E X P E R I E N C E ON THE R E S P O N S E OF TROUT U N F A M I L I A R PREY  TO  INTRODUCTION  47  METHODS  48  AND M A T E R I A L S  RESULTS THE TO  50 CHARACTERISTICS UNFAMILIAR  INITIAL  R E S P O N S E OF  TROUT  PREY  THE  EFFECT  THE  SPECIFICITY  THE  EXTINCTION  ATTENTION  OF THE  50  OF E X P E R I E N C E ON R E A C T I V E OF THE  DISTANCE  , 53  R E S P O N S E OF C O N D I T I O N E D  AND R E - D E V E L O P M E N T OF R E A C T I V E  TROUT.  57  D I S T A N C E 63  COMPETITION  63  DISCUSSION  66 SECTION PREY  ACTIVITY  AND  III VULNERABILITY  INTRODUCTION  73  METHODS AND M A T E R I A L S  73  FIELD  STUDIES  LABORATORY  73  STUDIES  75  RESULTS THE E F F E C T OF WATER T E M P E R A T U R E ON THE A C T I V I T Y OF CRANGQNYX AND H Y A L E L L A THE E F F E C T AMPHIPODS  OF T E M P E R A T U R E ON THE  76 VERTICAL  ACTIVITY  76 OF E X P O S E D  83  A SIMULATION MODEL OF THE PREDATORY BEHAVIOUR OF TROUT INTRODUCTION  87  METHODS AND MATERIALS  ....89  RESULTS  92  THE CHARACTERISTICS OF THE VISUAL RESPONSE OF TROUT TO PREY  ,92  The Relation Between Prey Size and Contrast Threshold  92  The Relation Between the Ambient Illumination, Visual Angle and Contrast Threshold  99  The Effect of Prey Movement on Reactive Distance  102  The Relation Between the Background, Reactive Distance and Target Recognition Success ...105 The Effect of Prey Activity and the Searching Position on the Width of the Path of Search  1°  THE ATTACK MODEL APPLICATIONS OF THE MODEL  9  H5 1  1  8  The selective Exploitation of Amphipods, Odonates, Planorbids and Caddis 118 The Size Selective Exploitation of Amphipods Seasonal  124  Changes in the Exploitation of Amphipodsl29  DISCUSSION  131  SUMMARY  i 4 1  BIBLIOGRAPHY  144  APPENDIX  152 I. II. III.  LIST OF SYMBOLS FOR THE ATTACK MODEL  152  PHYSICAL CHARACTERISTICS OF MARION LAKE ..154 POPULATION CHARACTERISTICS OF CRANGONYX AND HYALELLA  156  LIST OF TABLES SECTION  I  Table 1.  Characteristics substrates  Table 2.  A representation conducted  Table 3.  Average lengths and dry weights of prey used i n the p r e d a t i o n experiments  10  Table 4.  Regression constants for the rate of attack at time (t) with respect to the hunger l e v e l of the t r o u t and the d e n s i t y of prey 30 seconds e a r l i e r  17  Table 5.  The r e l a t i o n s h i p between r e a c t i v e prey s i z e  21  Table 6.  A comparison of the e f f e c t of the d i f f e r e n t s u b s t r a t e treatments on the p r o p o r t i o n of Crangonyx and H y a l e l l a that were exposed and captured during an experiment  Table 7.  of the experimental of the  8  experiments  9  distance  and  The p r o p o r t i o n of the t o t a l number of s t r i k e s which s u c c e s s f u l l y terminated with the capture of prey 32 SECTION  II  Table 1.  The r e l a t i o n s h i p between the i n i t i a l conditioned reactive distance  and  Table 2.  The e f f e c t of hunger on the r e a c t i v e of 3 t r o u t c o n d i t i o n e d to white prey  distance  SECTION Table 1.  Table 2.  28  56 71  III.  The v e r t i c a l d i s p e r s a l a c t i v i t y of Crangonyx and H y a l e l l a , i n Marion Lake, with respect to s e v e r a l environmental c o n d i t i o n s  78  A. The r e l a t i o n s h i p between the ambient water temperature and the instantaneous p r o p o r t i o n of Cranqonyx and H y a l e l l a exposed at or above the mud-water i n t e r f a c e  86  B. The r e l a t i o n s h i p between the ambient water temperature and the p r o p o r t i o n of time exposed amphipods spend moving 86  LIST OF TABLES SECTION IV  Page  Table 1.  The r e l a t i o n s h i p average r e a c t i v e  between prey s i z e and the distance  Table 2.  A comparison of s e v e r a l documented values of the minimum d e t e c t a b l e c o n t r a s t and the minimum v i s u a l angle of) d i f f e r e n t animals . . . A  Table 3.  The e f f e c t of background d i v e r s i t y r e c o g n i t i o n success  on prey  Table 4.  The e f f e c t  on  of background d i v e r s i t y  95  u u  J-07  reactive  distance  J-08  Table 5.  Values for the parameters of the attack modelJ.14  Table 6.  The p o p u l a t i o n c h a r a c t e r i s t i c s of the odonates, c a d d i s , and p l a n o r b i d s for s e v e r a l s e l e c t e d months during an ' a v e r a g e ' year A. A comparison between the expected and observed percentage occurence of d i f f e r e n t prey groups i n t r o u t stomachs  Table 7.  B. A comparison between the observed and p r e d i c t e d percentage occurence of d i f f e r e n t prey groups i n t r o u t stomachs  120 122  123  Table 8.  A comparison of the f i t between the expected, p r e d i c t e d and a c t u a l s i z e composition of Cranqonyx found i n t r o u t stomachs i n the month of November 128  Table 9.  Sensitivity  of the attack model to  selected  parameters  -133 APPENDIX  Table 1.  Temporal changes i n the s i z e s t r u c t u r e amphipods  i n Marion Lake  of .158  LIST  Figure Figure  Figure  Figure  1. 2.  3.  4.  The p r o g r e s s i v e attack  OF  FIGURES  SECTION  I  decline  in  1  Page  the  rate  1  The s e a r c h i n g v e l o c i t y o f Rainbouu in several control experiments The a v e r a g e populations equilibrium  of 6  trout 20  time reguired for 4 experimental o f C r a n g o n y x t o r e a c h an l e v e l of exposure  23  The r e l a t i o n s h i p b e t w e e n t h e number o f Crangonyx captured i n d i f f e r e n t h a b i t a t s a f t e r 50 m i n u t e s e x p o s u r e t o t r o u t p r e d a t i o n a n d their i n i t i a l density  26  Figure  51 The r e l a t i o n s h i p b e t w e e n t h e n u m b e r o f H y a l e l l a c a p t u r e d i n d i f f e r e n t h a b i t a t s a f t e r 50 m i n u t e s e x p o s u r e t o t r o u t a n d t h e i r i n i t i a l d e n s i t y . . . 27  Figure  6.  Figure  Figure  Figure  7.  8.  9.  The r e l a t i o n s h i p b e t w e e n prey and.the a t t a c k rate  Figure  Figure  Figure  1. 2.  3.  density  of  exposed 33  A schematic r e p r e s e n t a t i o n of the p a t t e r n of b e n t h i c s e a r c h i n g b e h a v i o u r by t r o u t d u r i n g a n experiment  36  The r e l a t i o n s h i p p r e y c a p t u r e and hunger  37  between the t h r e s h o l d r a t e the p r e d a t o r ' s s t a t e of  of  The r e l a t i o n s h i p b e t w e e n t h e e x p e r i m e n t a l s u b s t r a t e and t h e r a t e of e x t i n c t i o n o f t h e benthic searching p a t t e r n a f t e r the i n i t i a l phase of complete a t t e n t i o n SECTION  Figure  the  The and  e f f e c t of experience r e a c t i v e d i s t a n c e of  The e f f e c t o f e x p e r i e n c e d i s t a n c e of 6 t r o u t  38  II on t h e f e e d i n g t i m e 2 groups of t r o u t . . . . on  the  reactive 55  The e f f e c t o f s w i t c h i n g t r o u t , c o n d i t i o n e d t o white p r e y , to prey w i t h d i f f e r e n t l e v e l s of contrast  4 . The r e - d e v e l o p m e n t 4 trout  of  the  52  reactive  distance  62  of 6  4  X  SECTION III  Page  Figure 1. The effect of water temperature on the proportion of Cranggnyx that are exposed at or above the mud-water interface  79  Figure 2. The effect of water temperature on the proportion of Hyalella that are exposed at or above the mud-water interface  80  Figure 3. The relationship between the actual density of amphipods and their vulnerable density  82  Figure 4. The relationship between the water temperature and the average amount of time exposed Cranqonyx and Hyalella spend moving 85 SECTION IV Figure 1. A diagramatic representation of the relationship between the contrast threshold and the visual angle  97  Figure 2. A comparison between the observed reactive distance of 4 trout, exposed to different sizes of prey, and the calculated reactive distance .. 103 Figure 3. The effect of target movement on reactive distance  1 0 4  Figure 4. The geometric relationship between the radius of the reactive f i e l d , the trout's searching position and the width of their searching path along the sediment  HI  Figure 5. A schematic flow diagram of the parameters and computational steps in the attack model  H3  Figure 6. A comparison of the observed, expected and the predicted d i s t r i b u t i o n of different size classes of Hyalella found in trout stomachs at two different sampling periods  127  Figure 7. A comparison of the simulated and observed trend in the exploitation of Cranqonyx and Hyalella, by trout in Marion Lake  .130  F i g u r e 1.  The average seasonal in Marion,, Lake  trend i n water  temperature  F i g u r e 2.  The r e l a t i v e d e n s i t y of H y a l e l l a and Crangonyx i n Marion ' Lake  155 157  ACKNOWLEDGEMENTS  I am e s p e c i a l l y supervisor,  for h i s i n t e r e s t  the course of t h i s Drs.  P. L a r k i n ,  Mr. N. G i l b e r t manuscript. study, of  study.  C. H o l l i n g , for o f f e r i n g  to D r . Ian E . E f f o r d , my and encouragement  during  I mould also l i k e to thank R. L i l e y ,  T. Northcote and  v a l u a b l e c r i t i c i s m of  Many people have aided me throughout  the this  I would p a r t i c u l a r l y l i k e to acknowledge the help  Mr. P. Pearlstone and Mr. B . D e l u r y . Financial  Biological and  grateful  support  came from the Canadian I n t e r n a t i o n a l  Program, the N a t i o n a l Research Council of Canada  the U n i v e r s i t y of B r i t i s h  Zoology.  Columbia,  Department of  GENERAL INTRODUCTION Many laboratory studies have been conducted to describe the feeding behaviour of animals (De Ruiter, 1966); very few of these,however, have attempted to predict how natural of predators  will  populations  exploit d i f f e r e n t species of prey.  Until very recently, the method of how to systematically relate laboratory studies to the f i e l d was rather elusive. 1966  In  however, Holling described what he called the "experimental  components analysis".  This approach i s based upon the premise  that a b i o l o g i c a l process, into fragments.  such as predation,  can be broken down  These components can then be studied under  controlled conditions to elucidate their importance and relationship with other parts of the process.  The structure of the resulting  system i s not assumed to be a complete explanation but rather i s designed to be continually modified of new observations  and experimental r e s u l t s .  or description in the face  In essence then,  the system becomes a working hypothesis that can be tested either in the laboratory or on natural  populations.  In this study I have attempted, through laboratory experiments, to analyse the feeding behaviour of rainbow trout.  The f i r s t  section of the study i s devoted to a general description of their behaviour and some of the major components which affect i t .  This  analysis i s continued  in the second section and takes the form  of a single question:  can trout learn to a l t e r their response  to prey?  In the third section, I w i l l describe the effect of  water temperature on the a c t i v i t y patterns of two species of amphipods (Cranqonyx richmondensis and Hyalella azteca) that are  natural  prey of  rainbow t r o u t ,  and develop  a vulnerability  submodel. The f i n a l visual  section  characteristics  detection.  will  examine the r e l a t i o n s h i p  of prey i n general  and the process of  Various aspects from the manuscript  will  coupled i n t o a s i m u l a t i o n model to attempt to account selective especially  exploitation  of  the amphipods,  several  major  by the t r o u t  between  invertebrates,  the prey  then be for'the but  p o p u l a t i o n i n Marion Lake.  THE EFFECT OF PREY DENSITY, PREY SIZE AND THE PRESENCE OF A SUBSTRATE ON THE FEEDING BEHAVIOUR OF TROUT INTRODUCTION  fish  In most aquatic  systems i t  tend to e x p l o i t  exposed  conspicuous  species  other factors, will  appears as  prey to a g r e a t e r  (Grimas,  1963;  Allen,  extent  1941).  can be d i v i d e d i n t o 3 s p e c i f i c the c h a r a c t e r i s t i c s  exploit  prey.  that  than l e s s  There are  however, besides the degree of exposure  determine the rate predators  1)  a generalization  which  These  factors  categories:  of the prey ( i e .  size,  density  and b e h a v i o u r ) , 2)  the feeding behaviour of the predator behaviour,  and the mechanisms i t  and capture 3)  (i.e.  utilizes  searching to  locate  food),  the c h a r a c t e r i s t i c s  of the environment (i.e.  ambient i l l u m i n a t i o n ,  temperature  the  and p h y s i c a l  structure). Allen of  prey.  dispersal at  (1941) s t r e s s e d the importance of the He suggested that  since different  and b e h a v i o u r a l p a t t e r n s ,  different  densities,  p r o b a b i l i t y of being detected The t a c t i c s  that  s p e c i e s have  are d i f f e r e n t  they are not l i k e l y  1966).  These t a c t i c s ,  s i z e s and e x i s t  predators.  employ to l o c a t e  a l s o c o n t r i b u t e to determining p r e d a t i o n r a t e s Holling,  and handle food  (Ivlev,  Holling,  1966;  Beukema,  1961;  however, may be modified through  hunger m o t i v a t i o n and i n some cases through l e a r n i n g 1965;  different  to have the same  and captured by  predators  characteristics  1968).  (Holling,  There i s predation  is  also considerable affected  ambient i l l u m i n a t i o n and  by environmental f a c t o r s ( A l i , 1959;  1966;  Johannes  and L a r k i n ,  The purpose of t h i s factors:  of  Hunter,  1968;  that  such as  the  Blaxter,  1968a)  the p h y s i c a l complexity of the environment ( i v l e v ,  Macan,  and  evidence to i n d i c a t e  l)  rainbow t r o u t  1961).  study was to examine the e f f e c t  prey d e n s i t y ,  4) the presence  1961;  2) prey s i z e ,  3)  of 4  hunger m o t i v a t i o n  of a s u b s t r a t e on the feeding behaviour  (Salmo q a i r d n e r i ) .  r e p l i c a t e d with 2 d i f f e r e n t  Feeding experiments  s p e c i e s of amphipods as  were  prey  (Cranqonyx r.ichmondensis and H y a l e l l a azteca) . The study was d i v i d e d i n t o 4 parts to construct  a descriptive  equation of the e f f e c t  these components on the behaviour of t r o u t . sections  will  The f i r s t  while the f i n a l  examine the r e l a t i o n s h i p between the t r o u t ' s searching  of each of 3  c o n s i d e r the i n f l u e n c e of the aspects mentioned  above on the rate of prey capture,  and  systematically  behaviour.  section  rate of  will  capture  Four  rainbow  Columbia, 13.4  to  were  in  both  temperature background avoiding of  of  I  and  tank  Lake  to  This insure  The  at  are  on  the and  The it  the  the  of  grey  fine  2 C). was  Two  experiments. the  water  Although  the  'controlled'  experiments  The  by  in  colored hand,  at  the  by  the  same  to  was  test  Table  facilitate would of  control  surface.  The  litter  composed o f  very the  examined  6% a n d  large 15%  was  the  most  of  the  tank.  floor  settle  the  of  The  sediment  and t h e  effect  1.  treatment  entire  the  characteristics  were  screening fine  to  respectively,  litter  activity  trout  tanks,  from  between  aquarium.  employed  covered,  substrate  feeding  behaviour  were  other  very  length  predation  it  British  P.S.T.).  covered  the  glass  described  a bare,  was n e c e s s a r y that  natural,  conducting  was o b t a i n e d both  10 C ( -  in  isolation  for  on p r e d a t i o n .  litter  as  in  gallon)  substrates  simply  remove  by  and  held  Lake,  ranged  experimental  1400 h r s  bottom.  material  disturbed  and  Marion  They  utilized  was  treatments  stick  This  to  light  utilized  matter.  (50  were  holding  treatments,  of  and  was  complexity  was  of  complex  fish  different  pieces the  Each  (1200 to  these  II  study.  illumination  physical  substrate  this  was m a i n t a i n e d  day  each of  the  from  in  tanks  direct  Four  obtained  a 227 l i t r e  identical In  time  used  1 7 . 0 cm.  experiments other  trout,  from  coarse recovery rapidly  Marion particulate of  prey  if  it  a minimum o f  3  predator. at  was  different  densities  of prey i n each of the 4 s u b s t r a t e  P r e d a t i o n experiments s p e c i e s of amphipods  were r e p l i c a t e d independently f o r both (Table 2 ) .  The response of t r o u t  j u v e n i l e Cranqonyx was a l s o i n v e s t i g a t e d II  treatment.  treatments.  As a r e s u l t ,  the e f f e c t  but only  with those obtained when adult  i n the  of prey s i z e  could be assessed by comparing the r e s u l t s  to litter  on p r e d a t i o n  of these  experiments  Cranqonyx were i n the same  situation. To observe  the feeding  behaviour of t r o u t ,  in  s t a t e s of hunger m o t i v a t i o n , and yet i n s u r e that capture  enough prey to reach s a t i a t i o n ,  Cranqonyx and H y a l e l l a were u t i l i z e d . i n d i c a t e d that about  they would not  different  densities  Preliminary  the same s t a t e ,  s i z e d adult  Cranqonyx (Table 3 ) .  To reach  the d e n s i t y of each prey was  these r e s p e c t i v e  limits.  The range i n d e n s i t y of  Cranqonyx was i d e n t i c a l to that are both of s i m i l a r s i z e  juvenile  weight  The hunger l e v e l of the t r o u t was s t a n d a r d i z e d  period lasted  indicated  that  as  long as  the f i s h  below t h e i r s a t i a t i o n  an experiment  96 hour's).  as  they  (Table  3).  by d e p r i v i n g  ( i n a few  instances  Some p r e l i m i n a r y  results  r e q u i r e d 50 to 60 hours at 10 C. to  completely d i g e s t a s a t i a t i o n they were able to ingest  exceed  choosen f o r H y a l e l l a ,  and to some extent  them of food for 48 hours before  smaller, regulated  so the number captured during an experiment did not  this  consuming  they would have to capture over 200  Therefore,  of  results  the t r o u t would become s a t i a t e d a f t e r  90 standard  Hyalella.  different  ration.  Since the amount of  food  in v i r t u a l l y every experiment was well  level,  48 hours of d e p r i v a t i o n should have  been adequate to c l e a r between s u c c e s i v e  the digestive- t r a c t  of a l l  food  material  feedings.  The experimental  procedure c o n s i s t e d  of g a t h e r i n g  the  r e g u i r e d number of prey from Marion Lake and holding them, without  food,  in p l a s t i c  feeding  tank was prepared  substrate material  containers  for up to 24 hours.  by adding a standard  and spreading  it  sample of  uniformly over the bottom.  The prey were then i n t r o d u c e d and allowed 60 minutes to before feeding  a predator  was r e l e a s e d .  Specific  aspects of the  behaviour were recorded c h r o n o l o g i c a l l y on a  4 channel r e c o r d e r  (Model  The experiments  The  disperse troutJs  Rustrack,  921).  were terminated a f t e r  50 minutes at  which  time any prey remaining i n the tank were recovered and counted. This residual  d e n s i t y was s u b t r a c t e d  from the i n i t i a l  to determine the number of amphipods c a p t u r e d .  The recovery  technique was t e s t e d and was found to be 97% to 100^ i n r e c o v e r i n g prey,  therefore,  l o s s of animals during t h i s  density  efficient  no c o r r e c t i o n was made for any  operation.  Treatment  No. o b j e c t s or depth *  Average object size  Surface2 a r e a (cm )  2  0  Control  Area o f tank bottom c o v e r e d (cm )  4180  0  Litter  I  34  (6x1x1) cm.  4700  253  Litter  II  47  (9x2x1) cm.  5487  640  Fine  Litter  4mm,  *see  text  (0.4-0.7)  mm.  4180 +  4180  CD  TABLE of  2.  A representation  a substrate  figures  and p r e y  indicate  the  of  density  number o f  the on  experiments  conducted  to  determine  the  behaviour  of  trout.  replicate  Prey Adult  conducted  with  column  different  Cranqonyx  Adult  Hyalella  Juvenile  Cranqonyx  70  100  200  20  40  70  100  200  40  100  200  -  2  2  -  -  2  2  1  2  2  -  -  -  1  2  2  1  -  2  2  2  -  -  -  II  -  2  2  2  -  2  2  2  2  2  Litter  -  2  -  2  2  2  2  -  -  -  Litter  I  Litter  exp.  22  fish.  Density  40  Control  No.  experiments  The  effect  20  Substrate  Fine  feeding  the  -  2  -  2  27  6  TABLE 3. used  Average lengths  i n the p r e d a t i o n  *X Length (mm) *  Crangonyx  Juvenile Adult  *  of  i n d i v i d u a l prey  experiments.  Prey  Adult  and dry weights  Crangonyx  Hyalella  the range i n l e n g t h i s  X Dry weight  8.1  +_  1  2.6  4.6  _+  1  0.5  5.7  +  1  1.0  indicated  (mg)  RESULTS GENERAL FEEDING BEHAVIOUR Rainbow t r o u t verified fish.  by observing  In e i t h e r  distinct direct for  attack.  case,  This is  other v i s u a l  visual  d k irect  predators  (Messenger,  they search  A fish will  search  an angle ( l a t e r a l )  to i t s  the t r o u t  will  it will  orient  This  it  searching  A complete  Rainbow t r o u t  is  slightly  1957). p o s i t i o n with  d e t e c t most  targetsat  Once a t a r g e t  has  is  before  r a p i d and i s  of the p r e y .  position  before  it  followed  The predator resumes  a t t a c k sequence may r e q u i r e only 2 seconds.  are  not always  between prey and ' s i m i l a r ' litter.  it  to face the o b j e c t and then  will  strike  laboratory  i t with b i n o c u l a r v i s i o n  When the a t t a c k o c c u r s ,  then r e t u r n to i t s  since  characteristic  and engulfment  will  1966).  i n the  (Polyak,  immediately by a s t r i k e  stick  either  path of s e a r c h .  pause momentarily to f i x a t e  of  observed  1968;'Hoi1ing,  downward to face i t .  from t h i s  As a r e s u l t ,  hunting.  and has been  a x i s onto the sediment,  monocular v i s i o n . -  attacking.  and a r a p i d ,  from a p o s i t i o n some 10 to 15 cm  to the l o n g i t u d i n a l body a x i s  been detected  three  i n c l i n e (about 10 to 20 degrees) might be to  their visual  obligue  fixation,  a common p a t t e r n  above the s u b s t r a t e but o r i e n t characteristic  experimental  t h e i r a t t a c k response comprised  hunt f o r benthic prey,  i n the f i e l d ,  T h i s was  the behaviour of both wild and  steps; o r i e n t a t i o n ,  When t r o u t or  appear to l o c a t e food v i s u a l l y .  objects,  When they react  at and r e j e c t  successful  it  in discriminating  they w i l l  strike  at  to an inanimate o b j e c t  several  times  before  their  pieces they attack  response i s  terminated.  r e l y upon e i t h e r distinguish  tactile  between  Even t h i s  This indicates  food and i n e d i b l e  that  as  food they w i l l  It  is  of  long as  found that feeding  i n more d e t a i l  is  affected  the amount of  acj l i b i t u m a f t e r  was i n v e r s e l y  related  and r e l y upon v i s i o n to This  Ht  =  locate  supposition  below.  the feeding  Attack  m o t i v a t i o n of  by food d e p r i v a t i o n .  Ishiwata  (1968b)  food rainbow t r o u t  would consume,  various  food  meal.  m o t i v a t i o n to feed,  (1)  hunt from a p o s i t i o n  periods  of  deprivation,  to the amount which remained i n the  stomach from the previous animal's  trout  feeding  Hunger and Prey Density on the Rate of .(Control Experiments)  well documented that  many animals  objects.  detect only exposed a n i m a l s .  be examined  probably  or chemical d i s c r i m i n a t i o n to  some d i s t a n c e from the sediment  The E f f e c t  trout  very cursory d e s c r i p t i o n of t h e i r  behaviour i m p l i e s  will  that  it  1 - ( Vt /  If  hunger i s  defined as an  may be expressed  Vmax)  as:  where (H ) i s t  t h e l e v e l of hunger at  stomach c a p a c i t y stomach at  and (Vt) i s  time ( t ) . l)  (numerically,  0) when i t  This expression  of  allows one to q u a n t i f y time i f  the amount of  however,  and Patten,  1965).  hunger i s  immediately a f t e r  eguation  (l)  is  known.  feedbacks  ration,  It  is  m o t i v a t i o n (Ruch  such as  hunger w i l l  or  alternately,  With these r e s t r i c t i o n s  i n mind,  relationship rainbow  the food consumed during t h e i r previous  ad 1ibitum u n t i l prey).  tract.  trout.  or the maximum stomach c a p a c i t y  the f i s h was determined by holding them i n i s o l a t i o n  from the d i g e s t i v e  for  blood sugar  ingested  behaviour of  also,  to the complex  the s t a t e of  any food i s  between hunger and the feeding  it  does not account  can be used to i n v e s t i g a t e the  The s a t i a t i o n  and minimal  feeding  it  assumes that  from the stomach.  be g r e a t e s t  m o t i v a t i o n at any  with respect  For example,  change cleared  gut  knowito a f f e c t  but i m p l i c i t l y  empty,  feeding  food i n i t s  levels  food in the  convenient in that  an animals  time l a g s i n p h y s i o l o g i c a l  the  full.  extremely s i m p l i s t i c  mechanisms which are  (V ) is max  therefore,  when the stomach i s is  (t),  the amount of  Hunger w i l l ,  (numerically,  time  all  meal had been passed  They were then allowed to  they reached s a t i a t i o n  until  of  (stopped  Since the number of amphipods captured  feed  attacking  during t h i s  period  and of  the average weight food  each f i s h  o f a s i n g l e animal  consumed  could  these  experiments i n d i c a t e d  after  consuming  regardless  230 mg.  o f whether  were known, t h e amount  be e s t i m a t e d .  that  the t r o u t  dry weight,  t h e y were  The r e s u l t s o f  became  satiated  ( 1 S.E. _+ 12 mg.  f e d adult  ),  Cranqonyx o r  Hyalella.  In  the predation  replaced. the  Therefore,  predator's  progressed. at  level  according  by  linear  multiple divided  regression.  with  period  could  that  weight  had been  as w e l l  could  For t h i s  experiments  and were  captured  a t one i n t e r v a l  ( t ) could  l ) as b o t h point  were known  experiment  o f p r e y and  i n the succeeding  up t o t h a t  o f each o f t h e t e s t p r e y  analysed  a n a l y s i s , an  i n t e r v a l s so t h e d e n s i t y  (equation  as  a f f e c t the rate  o f hunger a t t h e b e g i n n i n g  be e s t i m a t e d  were n o t  as t h e e x p e r i m e n t  the c o n t r o l  the r a t e o f a t t a c k  The l e v e l  prey  o f prey  to t h e type o f prey  i n t o 30 s e c o n d  t + l ) .  prey  therefore,  o f hunger o f t h e t r o u t  correlated (  the density  o f hunger d e c l i n e d  attack,  were p o o l e d  state  both  captured  E i t h e r of these aspects  which t r o u t  was  experiments,  be  interval  o f each  time  t h e number o f  and t h e a v e r a g e  (Table 3 ) .  A regression  analysis  i n d i c a t e d that  prey d e n s i t y and hunger  were p o s i t i v e l y c o r r e l a t e d with the attack other words,  as  the d e n s i t y of prey decreased  diminished but at a r a t e that level  of  rate  hunger.  (Table 4 ) .  the r a t e of  was dependent upon the  An example of the p r o g r e s s i v e  attack  rate from one s e r i e s of experiments  Figure  1.  Observation suggested that hunger and the r a t e of attack  is  the negative  mantids  (Holling,  trout's  illustrated  feedback  in  between  might have been due to an  Hunger has a s i m i l a r e f f e c t  attack  d e c l i n e i n the  i n the amount of time t r o u t use to handle food as diminishes.  In  increase  t h e i r hunger  on handling time i n  1966).  On the b a s i s of the c o n t r o l experiments, between prey d e n s i t y  (PD), hunger ( )  (RA) was found to be adequately  the  relationship  and the r a t e of  d e s c r i b e d by the  attack  regression  eguation: (2) The e f f e c t s  RA =  b1  (PD)  +  b2  (Ht)  of prey d e n s i t y and hunger are  by the constants  (b^)  and (b^)  > (K) i s  +  K  indicated,  respectively,  the Y - i n t e r c e p t of  the  regression. In the c o n t r o l experiments larger  the value of  when Crangonyx were prey (Table 4 ) .  both s p e c i e s were at  comparable d e n s i t i e s ,  Crangonyx more r a p i d l y than H y a l e l l a . for  this  result  is  explored below.  (b^)  was  significantly  This means that when trout  attacked  One p o s s i b l e  explanation  ure  1.  The  (30 s e c o n d 2 control prey the  was  progressive intervals).  experiments 200  Hyalella.  rate of attack  attack  decline The  o f one  rate of another  the i n i t i a l  closed  fish;  fish.  of  attack  r e s u l t s presented are  i n which The  i n the r a t e  circles  t h e open  from  density  of  indicate  circles,  the  .CO  9.  .CM  O  ••  f-—• c o o  oi  .O  o o o o  •CO  o o  CD  ©  o o  > X  •CD  o  o o o  •CM  o  CO  o ro  i — T  CD  o  •  -  m m — r  o  «  CM  o  •  r  TABLE  Regression  4.  time (t)  with respect  constants for the r a t e of attack  to the hunger l e v e l of the t r o u t and  the d e n s i t y of prey 3D seconds e a r l i e r . indicates (b^)  the e f f e c t  the e f f e c t  standard  at  The constant  of prey d e n s i t y on the rate of  of hunger and (K) the y - i n t e r c e p t .  d e v i a t i o n of each of the constants and the  (b^)  attack; The partial  2  correlation  coefficients  (r)  are  amount of v a r i a b i l i t y accounted indicates all  indicated.  (Exp.)  is  the  for by each r e g r e s s i o n .  the number of time i n t e r v a l s  the experiments  R  from each  total (n)  obtained by p o o l i n g treatment.  I HULL M Exp  Prey  n  D e n s i t y (b^)  Hunger (b2)  K  R  -0.207  0.66  -0.13  0.83  -0.178  0.57  -0.05  0.88  -0.19  0.62  -0.13  0.50  -0.13  0.62  2  Control Hyalella  9  93  0.0024 + 0.0002 0.71  r Cranqonyx  4  50  0.62  0.0045 + 0.0005  0.77 Litter  Hyalella  6  78  r  0.0023 + 0.0003  I 0.345 + 0.072 0.49  0 .62  Cranqonyx  6  52  0.0075 + 0.0005  0.145 _+ 0.035 0.51  0.89  r  Litter Hyalella  6  65  0.0028 + 0.0002 0.81  r Cranqonyx  6  66  Cranqonyx  0.0026 + 0.0003 0.68  r  r  0.360 + 0.044  0.78  r  Juv.  0.474 + 0.064  6  51  0.0053 + 0.0009 0.65  II 0.241 + 0.067 0.42 0.326 + 0.162 0.65 0.223 + 0.061 0.46  The R e l a t i o n s h i p The largely  rate a predator by:  l)  at  prey  1966).  from which i t  The fact  will  determined react,  2)  the  v e l o c i t y between the that  Cranqonyx was  predator  attacked  rate than H y a l e l l a , could be accounted f o r i f  the searching  v e l o c i t y of the f i s h ,  they attacked  was dependent  were  Attack and Prey Size •  encounter food i s  and 3) the r e l a t i v e  (Holling,  a faster  will  the d i s t a n c e  d e n s i t y of prey, and  Between the Rate of  or the d i s t a n c e  either  from which  upon the type of prey to which they  exposed. Both s p e c i e s of amphipods move r e l a t i v e l y slowly with  respect  to t r o u t ;  predator  therefore,  in t h i s  case,  the v e l o c i t y of  w i l l ; . c o n t r i b u t e most to determining the r a t e of  encounter.  The range i n v e l o c i t y at which t r o u t  searched  Cranqonyx, was determined i n each of the c o n t r o l (fig.  2).  took to cover a known d i s t a n c e  characteristic  velocity  searching  position.  at which the f i s h searched  there was no apparent The d i s t a n c e  can  detect  An independent set  to i n v e s t i g a t e  the p o s s i b i l i t y  was r e l a t e d  to prey s i z e .  I introduced a single  when i t  was  Although the average measured,  changed. will  react  to prey  will  Brawn (1969) found that  l a r g e prey from a c o n s i d e r a b l y  smaller prey.  trout  it  from which a predator  a l s o determine the r a t e of a t t a c k .  the amount of  f o r H y a l e l l a was not  i n d i c a t i o n that  for  experiments  This component was expressed simply as  time the predator in i t s  the  greater distance  of experiments  that  the r e a c t i v e  cod than  was conducted distance  In order to measure t h i s  prey of known s i z e i n t o one of the  of distance, experimental  tanks  before  defined trout  releasing  a fish.  as the d i s t a n c e  between the predator  i n i t i a t e d an a t t a c k .  (Table 5 ) c l e a r l y dependent  was attacked  In a l l other r e s p e c t s , animals  are  very  at  except  conditions,  experiments  of r e a c t i o n  is  might be of  the  twice the r a t e as H y a l e l l a . these  similar. every amphipod was exposed  an experiment was t e r m i n a t e d .  s t a t e of hunger of the t r o u t of a l i t t e r  or both might be somewhat i n the next  their size,  affected  substrate,  behaviour of the p r e d a t o r ,  considered  and prey when the  t h e i r s i z e and a c t i v i t y ,  the d e n s i t y of prey,  the presence  was  Crangonyx, the l a r g e r  almost  In the c o n t r o l experiments captured before  the d i s t a n c e  This r e l a t i o n s h i p  to e x p l a i n why adult  two s p e c i e s ,  distance  The r e s u l t s of these  i n d i c a t e that  upon prey s i z e .  sufficient  The r e a c t i v e  the d i s p e r s a l altered.  section.  Under these as well  as  the  the r a t e of a t t a c k .  however,  and  the  In  feeding  behaviour of the' prey,  These p o s s i b i l i t i e s  are  ure  2.  several  The  s e a r c h i n g v e l o c i t y of  control  explanat i o n .  experiments.  See  rainbouu t r o u t text  for  in  further  SEARCHING VELOCITY (cm/sec)  TABLE prey the  5. size.  The r e l a t i o n s h i p The p r e y  were l i v e  number o f o b s e r v a t i o n s  average  reactive  significant  between r e a c t i v e Cranqonyx.  obtained  from  d i s t a n c e i s expressed  figure.  The 95$ c o n f i d e n c e  d i s t a n c e and  (n) i n d i c a t e s  2 fish.  The  to the nearest i n t e r v a l s are  p resented.  Prey Length (mm)  n  Mean r e a c t i v e d i s t a n c e (cm)  4  9  18+3  5  17  22+2  6  9  28 + 5  7  21  28+3  9  28  35+3  THE EFFECT OF A SUBSTRATE ON THE ATTACK RATE (LITTER EXPERIMENTS) Before a f i s h was.released the experimental disperse.  tank and were allowed one hour i n which to  When l i t t e r was present  themselves  some amphipods would conceal  immediately by moving under any o b j e c t  Other animals would move about cover or e l s e f a i l e d this  the prey were i n t r o d u c e d i n t o  behaviour i s  that were exposed  for some time before  to move under cover at  that  at  they encountered.  any i n s t a n t  all.  they took  The r e s u l t  of  i n time the number of prey  was determined by the r a t e at which animals  were both l e a v i n g and e n t e r i n g concealment. An experiment was conducted with adult if so,  they would e s t a b l i s h  Cranqonyx to determine  an e q u i l i b r i u m l e v e l of exposure,  how long i t would take.  Four p o p u l a t i o n s of amphipods were  i n t r o d u c e d i n t o separate c o n t a i n e r s of water Each v e s s e l identical  had 18% of the bottom area  to that  composing the l i t t e r  p o p u l a t i o n s were then observed  45 minutes a f t e r  e q u i l i b r i u m was reached  (25 cm in d i a m e t e r ) .  covered with s t i c k I and II  for over an hour.  the p r o p o r t i o n of animals that were exposed the f i r s t  and i f  substrates. Without  3).  The  exception,  declined rapidly in  they were i n t r o d u c e d u n t i l  (fig.  litter  an apparent  Although the l e n g t h of  time  concealed animals remained under cover proved to be r e l a t i v e l y long,  some d i d re-expose themselves.  continually it  exposed  or c o n c e a l e d .  was concluded that  d i s p e r s e before  The same animals were not  On the b a s i s of these  observations,  the hour i n which the prey were allowed to  an experiment was l i k e l y ' s u f f i c i e n t  p o p u l a t i o n to reach an e q u i l i b r i u m l e v e l of exposure  for the before  test the  ure  3.  The average time r e q u i r e d for  populations of  exposure.  four  experimental  of Cranqonyx to reach an • e q u i l i b r i u m ' l e v e l The data p o i n t s  The curve was f i t t e d  are means of 4  by i n s p e c t i o n .  replicates.  TIME (MIN) AFTER INTRODUCTION  predator  was i n t r o d u c e d .  The feeding behaviour of t r o u t was e s s e n t i a l l y when a s u b s t r a t e was present In both cases,  as  i t was in the c o n t r o l  the l i t t e r experiments  During the 50 minute d u r a t i o n of  the t r o u t d i d not d i s t u r b  of l i t t e r  to f i n d  animals that were exposed.  food,  Figures  the type of s u b s t r a t e . densities  It  is  were concealed and , t h e r e f o r e . w e r e more cover area  was  These r e s u l t s  captured  4 and 5 i l l u s t r a t e  evident that  a greater  the sediment  they only  r e l a t i o n s h i p between the number of prey that  experimental  number of  were captured and  at  each of  (figs.  i n v u l n e r a b l e to p r e d a t i o n when  4 and 5) were f i t t e d  by l i n e a r  were d e s c r i b e d adequately  the p r o p o r t i o n of animals which i s  that were exposed  given by the slope of the l i n e ,  the range of d e n s i t i e s of adult  the  Cranqonyx and Hyalel1 a  by a  l i n e which passed through the o r i g i n (Table 6A).  (PE),  the  available.  and without e x c e p t i o n ,  that  situation.  the f i s h maintained a searching p o s i t i o n and  responded v i s u a l l y to p r e y .  or move pieces  the same  used.  In the l i t t e r  II  regression  straight  This  indicates  i n each  treatment  was constant  over  s e r i e s the number  and j u v e n i l e Cranqonyx that were captured when both  p o p u l a t i o n s were of comparable s i z e  was not d i f f e r e n t  so  these  two sets of data were p o o l e d . The l e n g t h of time an amphipod r e q u i r e s  to l o c a t e  should be d i r e c t l y p r o p o r t i o n a l to the amount of s t i c k If  the amount of cover area  is  increased  of  time an animal remains concealed or exposed  cover litter.  but the average l e n g t h does not  then the p r o p o r t i o n of prey i n the p o p u l a t i o n that  are  change, exposed  should  diminish  accordingly.  relationship  between  of  were c a p t u r e d  prey  that  This was  may  not  the  be  less  i n the  vulnerable  the  number o f  the  attack If  analysed  i n the  t h i s . : case, density  of  determine feeding  r a t e of  fine  i f the  that  are  of  the  by  should  because  i t was  attacks  t r o u t d i r e c t e d at  Table  of  the  different  the  of  exception than  this,  the  then  results,  only  the  the  the  litter  and  of  values  was  and the  in  i n the  K litter  the  analysed  between objects.  summarized  constants  obtained  b^,  not  'similar'  the  through  b^,  discriminate  other  regression  affected  obtained  treatment  the  to  a n a l y s i s of  experiments are  those  i s no  values  in  i n terms o f  indirectly  regression  to  prey  to  therefore  experiments i s  directly  than  to a c c u r a t e l y  litter  there  control  other  litter  In b o t h c a s e s ,  significantly  Other  fine  difficult  results  4.  The  and  substrate.  litter  a substrate  was  Nevertheless,  (t), i t is possible  time  Cranqonyx  Hyalella  at  similar  situation.  the  there  Fewer  attack  of  as  i s expressed  fish  control  type  the  a multiple  be  present.  while  to  proportion  density  If i t d i d not,  obtained  experiments  as  presence of  behaviour  density.  prey  was  inverse  the  experiments.  the  during  population  vulnerable  prey  The  litter  were v u l n e r a b l e  attack  and  6 B).  (Table  II t r e a t m e n t ,  same f a s h i o n  the  area  the  explanation,however,  r a t e were r e l a t e d to  the  cover  interaction  that  explain  when a s u b s t r a t e  litter  i n the  prey  amount o f  a complete  a substrate-species  were c a p t u r e d  T h i s might  are  not  control,  I treatment  with  adult  indication  that  a litter  in  with  Cranqonyx. substrate  ure  4.  The  captured  relationship  in different  between t h e  habitats,  number o f  after  50  Cranqonyx  minutes  exposure 2  to  trout  predation,  (A)  closed  the  open c i r c l e s  the  closed  adult  circles  juvenile  I.  indicate  ( B . l mm  Cranqonyx  their  represent  litter  circles  Cranqonyx  and  (4.6  initial the  (B)  control the  experiments  (C)  the  per  0.42  situation,  litter  i n l e n g t h ) and mm).  density  II  fine  and  treatment,  conducted the  m  open litter  with  circles, series.  .  INITAL DENSITY  Figure  5.  The r e l a t i o n s h i p  captured to t r o u t ,  in different  between the number of H y a l e l l a  habitats after  and t h e i r i n i t i a l  the c o n t r o l II treatment  (B) the l i t t e r  50 minutes  d e n s i t y per 0.42 I treatment  and (D) the f i n e l i t t e r  exposure  M.  (C) the treatment.  (A)  litter  50  100  150  INITIAL DENSITY  200  TABLE  6.  A comparison of the e f f e c t  s u b s t r a t e treatments  of the  on the p r o p o r t i o n of  Cranqonyx and  H y a l e l l a that were exposed  and subsequently  during an experiment.  indicates  experiments; the s l o p e , part  (r)  A, treatment  the c o r r e l a t i o n c o e f f i c i e n t ;  s l o p e s that  l i n e are  l e v e l or l e s s .  In part  B,  (t)  is  captured  the number of  or p r o p o r t i o n of prey that  same v e r t i c a l 0.05  (n)  different  is  student's  is  were c a p t u r e d .  are not bracketed  significantly  (V ^)  (PE)  different  In  by the at  the variance of the  the slope.  t.  A. V u l n e r a b i l i t y w i t h i n  species  Hyalella Substrate Treatment  Vb 9 6 6 6  Control Litter I L i t t e r II Fine l i t t e r  0.99 0.99 0.98 0.90  PE  .0010 .0003 .0050 .0014  0.98 0 .791 0.16  .0000 .0133 .0040 .0016  1.001 0 .90J 0 .431 0.31J  0.661  Cranqonyx Control Litter I L i t t e r II Fine L i t t e r  4 6 12 6  B.  Control Litter I L i t t e r II Fine l i t t e r  1.00 0.97 0.91 0.97  V u l n e r a b i l i t y between  13 12 18 12  a Not s i g n i f i c a n t b S i g n i f i c a n t at or l e s s than 0.05  a 1.069a 24.350b 2.710b  level  species  altered  the behaviour of the t r o u t .  of  the prey was the primary f a c t o r  of  animals  that were c a p t u r e d .  between the s u b s t r a t e , the as  that  behaviour  determined the number  Therefore,  the  the v u l n e r a b l e d e n s i t y  relationship of amphipods and  rate of a t t a c k can be d e s c r i b e d by modifying equation  (2)  follows:  (3)  where (PE) i s The e f f e c t in  The concealment  the l i t t e r  adult  RA  =  b1  (PD)  (PE)  +  b2  (H )  +  K  the p r o p o r t i o n of the p o p u l a t i o n exposed of prey s i z e on the attack experiments.  r a t e was a l s o  Regardless of the type of  Cranqonyx were attacked  (Table  considerably,faster  6A).  evident  substrate,  than e i t h e r  j u v e n i l e Cranqonyx or H y a l e l l a .  I mentioned e a r l i e r  that  explanation  could be due to the  relationship  for t h i s o b s e r v a t i o n  between prey s i z e and r e a c t i v e of i n f o r m a t i o n that  is  distance.  consistent  The a d d i t i o n a l  with t h i s  supposition  Hyafrella and j u v e n i l e Cranqonyx are both about and were attacked  at  i d e n t i c a l rates  (Table  4).  is  the  piece that  the same s i z e  PREY CAPTURE SUCCESS The success predators three b a s i c components, approach and s t r i k e  at  have i n c a p t u r i n g  namely, prey  completely s u c c e s s f u l  not  that  induce t r o u t  1966).  visual  time i f  In the l i t t e r  attacked  relatively  few p i e c e s of l i t t e r .  however,  i n the f i n e l i t t e r  treatment  influence  to s u c c e s s f u l l y  another s e c t i o n Once t r o u t always  executed strike  recognize a prey  Every s t r i k e as  some f a i l  efficiency  71% of  the r e s t were d i r e c t e d  this  their at  amphipods.  by imparing t h e i r aspect i s  be t r e a t e d  (orient  ability  far beyond  i n more d e t a i l  toward i t )  they  in approaching to w i t h i n  they attempt,  to capture  of t r o u t  could be determined. experiments,  fish  in  (IV).  completely s u c c e s s f u l  distance.  the  the d i v e r s i t y of a s u b s t r a t e can  food,  paper and w i l l  'waste'  This was not the case,  as only about  success of t r o u t ,  discriminate  the scope of t h i s  experiments,  the same s i z e and c o l o r as  demonstrates that  the capture  they could  they attacked many  I and II  a t t a c k s were d i r e c t e d toward p r e y ; about  This  cues other than movement w i l l  inanimate o b j e c t s .  Although t h i s  not  attack inanimate o b j e c t s .  amount of searching  p i e c e s of l i t t e r  recognize,  Trout are  to a t t a c k but a l s o suggests that  a substantial  to  i n d i s c r i m i n a t i n g between prey and other  they w i l l  indicates  their a b i l i t y  (Holling,  'similar1, targets; only  food depends upon  prey.  however,  is  not  are  striking perfectly  Table 7 i n d i c a t e s  i n the experiments  i n which  The r e s u l t s of an a d d i t i o n a l  i n which the prey were 11 mm adult  set  the  this of c o n t r o l  Cranqonyx,  are  also  presented. These data  (Table 7)  show that  r e g a r d l e s s of the type of  s u b s t r a t e t r o u t were extremely s u c c e s s f u l Since s t r i k e if  success  (CS) was the major f a c t o r  a prey would be c a p t u r e d ,  different  treatments  RC  At t h i s  =  point,  the rate of capture  a single  which t r o u t ,  [b2  (PD) (PE)  the e f f e c t s  regression  capture.  in different  capture litter of  is  This assumption,  i n the f i r s t II  exposed  experiments prey.  (Ht)  of prey s i z e ,  restricted there i s  of course,  30 seconds  is  the rate  can capture  at  food.  to the rate of  not true and i s  In t h i s  example,  refuted  the rate of  of the c o n t r o l , l i t t e r  l i t t l e question that  prey approached  sq.  d e n s i t y of prey,  K ]CS  prey d e n s i t y ,  no l i m i t  (average = 0.67)  r a t e of capture  follows:  I and  p l o t t e d as a f u n c t i o n of the d e n s i t y  There i s  about 240/  the i m p l i c i t  +  as  by the assumption of  approached a maximum value  Therefore,  (3)  this  success have bean i n c o r p o r a t e d  i n F i g u r e 6.  is  b2  s t a t e s of hunger,  which i m p l i e s that  by the data presented  +  eguation which d e s c r i b e s  T h i s d e s c r i p t i o n however, linearity,  determined  (RC) i n the  and adding i t to equation  s u b s t r a t e complexity and capture into  that  can be d e s c r i b e d simply by t r e a t i n g  component as a constant (4)  i n c a p t u r i n g amphipods.  m. (100  as  cannot surpass t h i s l i m i t  (4)  is  r e g a r d l e s s of  or the p r e d a t o r ' s  rate  the d e n s i t y of  amphipods i n the  r e s t r i c t i o n of equation  their size,  the capture  tank). that  the  the  s t a t e of hunger.  TABLE  7.  The p r o p o r t i o n of the t o t a l  successfully  terminated  number of s t r i k e s which  with the capture of p r e y .  indicates  the number of experiments.  Species  Average Length (mm)  N  Mean s t r i k e success  95% confidence i n t e r v a l of mean  Control Hyalella  5.7  7  0.907  0.812  1.000  Crangonyx  8.1  5  0.909  0.778  1 .000  Cranganyx  10.8  9  0.902  0 .837  0 .967  1.000  Litter  I  Hyalella  5.7  6  0.882  0.753 -  Crangonyx  8.1  6  0 .886  0.716 - 1.000  Litter  II  Hyalella  5.7  6  0 .820  0 .656  0 .984  Crangonyx  4.6  6  0.833  0.669  0.997  Crangonyx  8.1  6  0.864  0.716  1.000  Figure  6.  The r e l a t i o n s h i p  prey and the a t t a c k  rate.  between the d e n s i t y of  exposed  The r a t e of a t t a c k was  determined over a 30 second time p e r i o d .  In each case,  the p r e d a t o r ' s l e v e l of hunger was maximal with respect to equation  (l).  The data p o i n t s  the c o n t r o l , l i t t e r  I and l i t t e r  both Cranqonyx and H y a l e l l a .  were obtained 2 experiments  from with  THE THRESHOLD RATE OF PREY CAPTURE AND THE SEARCHING PATTERN Searching  behaviour w i l l decrease rapidly once an animal's  hunger motivation i s satiated (Beukema, 1968; Holling, 1966). Other factors however, might alter both the duration and nature of searching before this occurs. When a trout was released at the start of a feeding experiment i t would immediately move to the bottom, adopt i t s c h a r a c t e r i s t i c searching position, and begin hunting for amphipods. diagramatically i l l u s t r a t e s the pattern of benthic behaviour during a typical experiment.  Figure 7  searching  As indicated, irrespective  of the density of exposed prey the predator would devote a l l of i t s attention for some time to hunting for amphipods.  Eventually  however, this attention was disrupted and began to wane.  During  t h i s phase of the experiment the f i s h would s h i f t i t s searching position and move higher up into the water column to hunt for food, or else hold a stationary position for a few minutes. either case i t devoted less time to hunting prey.  In  the substrate for  Although in every situation the i n i t i a l searching pattern  was disrupted before the experiment was terminated,  the duration  of this phase was related to the number of vulnerable prey.  The  trout shifted their attention sooner when the density of prey was low. This poses the question then as to the mechanism which might be responsible for causing a predator to disrupt one searching pattern and switch i t s attention to another pattern ( i . e . hunting for prey in the water column) or behaviour.  It  seems tenable that an animal would search as  t h i s behaviour was r e i n f o r c e d but that  it  long as  would s h i f t  a t t e n t i o n to other forms of hunting or behaviours i f of  food intake f e l l  toward z e r o .  r a t e of capture that  then they w i l l  alter  the  rate  Perhaps there i s a t h r e s h o l d  t r o u t must exceed i f  hunting for b e n t h i c p r e y .  its  they are to continue  If they cannot a t t a i n  this threshold  t h e i r behaviour.  This hypothesis can be t e s t e d by determining i f the of  capture,  amphipods,  when the t r o u t f i r s t  was r e l a t i v e l y constant  the p r e d a t o r ' s on.  These data  Hyalella,  are presented  the capture .008)  t h r e s h o l d was  relationship  hunger.  Nevertheless,  ( P = 0.04  there was a s i g n i f i c a n t  ) between the t h r e s h o l d and  i n e i t h e r i n s t a n c e once the rate of  below an average of 0.051  captures  t r o u t moved away from the bottom of the tank. a critical  modified  by hunger.  trout's  This was not true however, when the f i s h were  positive  there i s  reasonably  i r r e s p e c t i v e of the  to adult Crangonyx; i n t h i s case,  food  per second the This i n d i c a t e s  that  r a t e of capture although the t h r e s h o l d may be  Once the i n i t i a l continually  feeding  i n F i g u r e 8.  (mean = 0.058 _+ 1 5 . E .  intake f e l l  i r r e s p e c t i v e of  from the experiments with adult Crangonyx and  degree of hunger. exposed  (threshold)  for  s t a t e of hunger or the type of prey i t was  When H y a l e l l a were prey, constant  d i s r u p t e d t h e i r search  rate  searching p a t t e r n was d i s r u p t e d ,  changed t h e i r v e r t i c a l p o s i t i o n .  the  fish  They would move  up i n t o the water column for some time and then r e v e r t back to  ure  A schematic  7.  representation  of b e n t h i c s e a r c h i n g experiment.  behaviour by t r o u t  (CSP) i n d i c a t e s  (ISP)  prey,  to hunting for  p a t t e r n waned,  (PD)  illustrates  d e c l i n e i n the d e n s i t y of  and (TRC) i s  produces  during an  r e p r e s e n t s the phase i n which the  b e n t h i c searching the p r o g r e s s i v e  pattern  the phase i n which the  f i s h were completely a t t e n t i v e amphipods,  of the  exposed  the d e n s i t y of prey which  the t h r e s h o l d r a t e of  capture.  0 available time spent searching for  amphipods  Figure  8.  The r e l a t i o n s h i p  of prey capture The r e g r e s s i o n adult See  between the t h r e s h o l d  and the p r e d a t o r ' s s t a t e of  rate  hunger.  l i n e i n d i c a t e d i n the experiments  Cranqonyx i s  significant  text for f u r t h e r  at  explanation.  the 0.04  level.  with  Threshold Rate of Attack • • . • O  fNj  I  I  O CD  I  - L  I  O  i  4^  i i  Figure  9 .  The r e l a t i o n s h i p between the experimental  s u b s t r a t e and the r a t e of e x t i n c t i o n of the b e n t h i c searching p a t t e r n a f t e r attention.  the i n i t i a l  phase of  Only the r e s u l t s obtained with  are p r e s e n t e d .  experiments,  (C) L i t t e r II  Hyalella  The v e r t i c a l bars i n d i c a t e the 95%  confidence i n t e r v a l s of each of the means. control  complete  (B)  Litter I  experiments.  (A)  experiments,  (A)  A  i  i  10  10  | 1© -k  1 ^ 1  ©  © © ©  r~i—i—r~i—rn—j 20  30  40  50  20  30  40  50  T—i—i—i—i—n—I 10  20  30  i I  40  50  TIME INTERVAL (MIN) AFTER COMPLETE ATTENTION  .  hunting over the s u b s t r a t e . reinforced  for searching  they devoted to t h i s of  the experiment The presence  feeding  Since they were no longer being  for amphipods,  the amount of  behaviour waned throughout the remainder  (fig. 7 ) . of a l i t t e r  s u b s t r a t e did i n f l u e n c e  behaviour of the t r o u t  i n one sense,  r a t e of e x t i n c t i o n of the benthic searching to the complexity of the s u b s t r a t e r e t u r n to the bottom to search the s u b s t r a t e was  time  ( f i g . 9).  the  however, because the p a t t e r n was  related  The t r o u t would  for food more f r e q u e n t l y when  diversified.  DISCUSSION  Two fundamental  processes,  determine the food organisms supply. prey w i l l  The t a c t i c s restrict  prey d e t e c t i o n and capture  which comprise an a n i m a l s '  a predator  will  not only the types  u t i l i z e to of animals  locate it  can  will  food  attack  but also w i l l  effectively. receptor its  If  to l o c a t e  feeding  food i t may be somewhat l e s s  illumination  characteristic  that  sensory  restricted  salmonids,  could not capture prey u n t i l  exceeded  search  in  which r e l y predominantly  A l i (1959) demonstrated  visually,  can  can use more than one type of  a c t i v i t y than predators  upon v i s i o n . feed  it  determine where and when i t  the rod t h r e s h o l d .  of the environment w i l l  both i n space and time on the feeding  which  the ambient  Therefore,  impose  this  restrictions  a c t i v i t y of  visual  predators. Despite t h i s apparent to using v i s i o n to l o c a t e range and p r e c i s e of  a target.  visual larvae.  systems such as  can a c c u r a t e l y  true even for f a i r l y  for predators  size,  Prazordkova, 1969).  a long  fix  the p o s i t i o n  unsophisticated  Rilling  detect  only  et a_L, 1959),  this  with more developed v i s u a l  can d i s c r i m i n a t e 4 v i s u a l  form,  is  advantages  those possessed by mantids and dragonfly  ( P r i t c h a r d , 1965;  Most v e r t e b r a t e s  detect  it  Although these animals can e f f e c t i v e l y  not true  object;  there are d e f i n i t e  food. For example,  mechanism that  This i s  moving t a r g e t s is  drawback,  contrast,  a broader spectrum of  properties  and motion ( H o r r i d g e ,  T h i s i m p l i e s that  receptors. of an  1968;  they should be able  food organisms  to  simply because they  could e f f e c t  a d e c i s i o n to attack  quality  prey movement.  than  Many t h e o r i s t s  have assumed  on the b a s i s of some other  that  predators  will  encounter  food i n d i r e c t p r o p o r t i o n to the abundance of each prey organism. If  this  is  true,  distance. system is  that  almost  react It  One of the b a s i c  is  visual  then the predator must react  the d i s t a n c e  of any v i s u a l  r e q u i r e d to d i s c r i m i n a t e an o b j e c t  p r o p o r t i o n a l to the s i z e of the t a r g e t .  predators to l a r g e  such as  rainbow trout  prey from a g r e a t e r  was demonstrated  that  dependent upon prey s i z e attacked  characteristics  from a f i x e d  and that  adult Cranqonyx f a s t e r  have the o p p o r t u n i t y to  distance  the d i s t a n c e  Therefore,  of  this  than s m a l l e r  prey.  r e a c t i o n of t r o u t  is  could e x p l a i n why they  than e i t h e r  H y a l e l l a or j u v e n i l e  Cranqonyx. The searching p o s i t i o n that  rainbow t r o u t adopt when they  hunt for b e n t h i c prey was only considered s u p e r f i c i a l l y study.  This behaviour,however, has an i n t e r e s t i n g i m p l i c a t i o n ,  A predator  can only detect  a prey i f the height of i t s  position  is  l e s s than the d i s t a n c e  attack.  If  this  condition is  it  requires  capture.  There i s  to rainbow t r o u t  not met . then some small  some evidence that (section  this  are  subsequently,  i n f e r e n c e may apply  a reactive  and a s p e c i f i c  distance  sufficient  that  searching p o s i t i o n ,  components of the feeding behaviour of t r o u t .  characteristics  size  IV).  Both of these c h a r a c t e r i s t i c s , dependent upon prey s i z e  searching  to r e l e a s e an  c l a s s e s of prey may be i n v u l n e r a b l e to d e t e c t i o n and  basic  in this  is are  These same  to e x p l a i n the well documented  field  observation  exploit a  l a r g e prey  threshold size  also  pointed  rely  primarily  adopt  salmonids  out t h a t  Sheperd,  living  can that  capture both be  upon v i s i o n  to detect  food  positions.  t o behave  similarly  1970) t h i s  Within prey  that  tends  The  t o be benthic-  indication impared t h e  there  i s either  must  exist  t h a t can  swallow  their  t o be an optimum  successfully.  (1959) have d e m o n s t r a t e d  a predator  o f organism which  is likely most  o f prey  however,  by p r e d a t o r s  there  that  larger  Both  capture or small  Holling  success than  size.  process  of predation  o f prey,  the predator's  w h i c h l e a r n i n g can a l t e r  or  prey.  Holling  i s not o n l y  d e t e c t i o n and c a p t u r e  to  density  tend  (Schutz,  inconspicuous  to the s i z e  can be c a p t u r e d  components o f p r e y density  do n o t  species of  significantly  In t h e o r y ,  to d i m i n i s h i f the prey  optimum  fish  trout  t h e r e was no  considered  manipulated  (1964) and D i x o n  other  to rainbow  the types  study,  limit  range,  (1961)  and do n o t a p p e a r t o  Since  o f many  also affects  of trout.  this  feeding  below  1961).  of the prey  successfully  sized  predators  In t h e p r e s e n t  success  Ivlev  may e x p l a i n uuhy t h e y  (Smithi,  an u p p e r and l o w e r  food.  the  searching  success  the s i z e  1968).  bottom  organisms  exploit.  1961; B r o o k s , effective  ineffective  Capture  t o consume o t h e r s  very  appear  relatively  and f r e q u e n t l y f a i l  (ivlev,  pronounced  1969;  t h a t many s p e c i e s o f f i s h d i s p r o p o r t i o n a t e l y  success,  hunger m o t i v a t i o n  the behaviour  (1966) has d i s c u s s e d  and hunger m o t i v a t i o n  dependent  upon t h e  but a l s o the and t h e d e g r e e  of either  the predator  t h e i n f l u e n c e o f prey  on t h e f u n c t i o n a l  response  of  predators. identical be  The a c t i o n f o r trout,  reiterated  stimulates  here  of these  therefore,  beyond  components p r o v e d their  stressing  the attack rate  t o be  significance  that  w i l l not  increasing  prey  density  w h i l e d i m i n i s h i n g hunger m o t i v a t i o n  J  antagonizes of  trout  II).  searching, as w e l l .  responses this  will  will  1966).  completely  analagous  reinforced  (threshold  of search.  i s undoubtly  rate  i f they  streams  (Jenkins, also  suggests  areas  that  i n which  will  some i n d i v i d u a l s ,  will  shift  suggested  their  t o be of trout  1969).  position,  o f prey  f o r s t r i k i n g at  i n the feeding p o s i t i o n  By s i m p l y  the density  i n the attack  a r e not s u f f i c i e n t l y  has been  more v u l n e r a b l e t o a t t a c k .  If  signals  a r e not r e i n f o r c e d  o f capture) they  T h i s feedback  into  to o t h e r  of trout i s  in  converge  terminate  The s e a r c h i n g b e h a v i o u r  f o r t h e changes  temporarily  naturally  " ( R i l l i n g et  because  same f e e d b a c k  could  adopts.  a r e n o t rewarded stop  will  responding  responsible mountain  i n turn  sensitive  wane i f t h e y  simply  later  characteristics  o f hunger w i l l  I f they  they  1959; Holling,  This  be t r e a t e d  i s e v i d e n t , f o r example,  of mantids.  'dummy' t a r g e t  the behaviour  t h e r a t e o f c a p t u r e , which  behaviour  intermittently;  pattern  will  a l l of these  the c e s s a t i o n  this  Most  response  by l e a r n i n g  that  the searching p a t t e r n a predator  Although  al,  The p o s s i b i l i t y  In any c a s e ,  to determine  influence  a  effect.  c o u l d be a l t e r e d  (Section operate  this  trout  prey  randomly  locate  could  are r e l a t i v e l y shifting  areas o f prey  i s h i g h enough t o s u r p a s s  their  abundance.  the t h r e s h o l d  rate  of  area. into  capture, Given  a  enough  a specific  searching  then  the  time,  most o f  region  or  at  pattern,such  as  feeding  may the  least  remain  feeding  population  adopt  the  i n the  could  same  uuater  in  the  converge  relative  column o r  over  substrate. The  phenomenon o f  animals  (Neish,  1941).  Holling'(l959a)  an  predator  immediate  function they  as  will  1970;  convergence  I 9 6 0 ) as  pointed  that  out  response,  a stabilizing to  been  Tinbergen,  numerical  tend  has  such  component  counteract  any  reported  well  as  of  that  converging, the  serious  other  fish  predators  as  for  can  invoke  will  community  imbalance  (Allen,  because  in  prey  abundance. In  conclusion,  pattern  of  implies  that  column  and  which prey since an  search the  will are  the  i f they  suggests  tend  to  they  exposed  a n i m a l s but  smaller  or  that  not  will  converge  relatively  response that  are  population  t r o u t were f o u n d  attack  observation  that  being  react  was  through  temporarily  more v u l n e r a b l e  to  to o n l y  dependent  to  into  exposed  effective  predators  would  be  relatively  the  water  areas  and  size,  ineffective  in  In a d d i t i o n ,  prey  of  their  reinforced  attack.  upon p r e y  be  shift  sufficiently  disperse  should  more c r y p t i c  trout will  displayed  this .  large, in  capturing  species.  SUMMARY l)  In  experiments without  that  both prey  their  r a t e of  density attack.  and  a litter  the  These  substrate,  hunger m o t i v a t i o n  two  components a r e  i t was of  shown  trout  affect  antagonistic  since  the former  depresses  2)  i n c r e a s e s the attack  r a t e while the  latter  it.  The r e a c t i v e  d i s t a n c e of  prey s i z e .  This could e x p l a i n  the l a r g e s t  prey,  faster  rainbow t r o u t  is  dependent  why they attacked adult  than e i t h e r  H y a l e l l a or  upon  Cranqonyx,  juvenile  Cranqonyx.  3)  When a s u b s t r a t e was p r e s e n t ,  concealed  themselves.  inversely  related  will  4)  to the amount of cover a r e a .  to the d i v e r s i t y  their attack,  of the  There was no c o n s i s t e n t  a substrate d i r e c t l y altered The concealment  i n d i c a t i o n that the feeding  that  inversely  the presence of  behaviour of the prey was the primary  factor  of  rainbow t r o u t  the s u b s t r a t e .  In the l a b o r a t o r y , exceeds 0.051  experiments.  to capture prey was shown  of both the s i z e of the t e s t The l a t t e r  t h e i r success i n d i s c r i m i n a t i n g  6)  r a t e was  trout.  to be independent of  trout  behaviour of  The a b i l i t y  diversity  Since  substrate.  that determined the outcome of the l i t t e r  5)  prey  The p r o p o r t i o n that were exposed was  detect only exposed prey,  related  both s p e c i e s of  trout  prey and the  however,  did  impare  at  a rate  amphipods.  must be r e i n f o r c e d  captures per second  if  they are  to  maintain  a specific  searching p a t t e r n .  t h r e s h o l d they w i l l patterns will of  7)  the  they do not a t t a i n  switch t h e i r a t t e n t i o n  or b e h a v i o u r s .  wane at  If  Once t h i s o c c u r s ,  a rate that  is  this  to other hunting the o r i g i n a l  pattern  i n v e r s e l y dependent upon the d i v e r s i t y  substrate.  Due to 4 major c h a r a c t e r i s t i c s i) ii) iii) iv)  of t h e i r feeding  the dependence of the r e a c t i v e d i s t a n c e on prey s i z e , the searching  position,  the fact and  they w i l l  that  the t h r e s h o l d rate of  rainbow t r o u t are l i k e l y  attack  only exposed  to converge i n t o areas i n which prey predators  p r e y ; butshould be c o n s i d e r a b l y l e s s e f f e c t i v e  exploiting  prey,  capture  are r e l a t i v e l y abundant,should be e f f e c t i v e exposed  behaviour:  s m a l l e r or l e s s conspicuous  species.  of in  large,  SECTION  THE  II  E F F E C T OF E X P E R I E N C E ON THE R E S P O N S E OF TROUT TO UNF AMI L I A R PREY  INTRODUCTION The  concept  considerable ecology and  learn they  to  it  was  since  not  attention  "searching in  their  other  has  been  demonstrated  vertebrates Croze, To the by  date,  learning these  general that  the of  and  reinforced Both  The  many  to  will  to  (i960).  He  but  can  that,  if  shift  their  features  would  hunt  existence  1 9 6 8 ; De R u i t e r ,  and  animals  prey,  these  by . e x p e r i m e n t a l  behaviour  Tinbergen  they  of  attracted  animal  that  predators  1970).  model  to  account operated of  of  that the  prey of  the  of  this  behaviour  examine  distance  demonstrated phenomenon  most  feeding  to  palatability the  aim of  considerations  experience  affect  by  has  of  studies  be  with  "maximum  this  behaviour  on  various  1952; H o l l i n g ,  1959a;  1970).  components  With  the  (Beukema,  of  responsiveness  objects.  (Croze,  fields  suggested  a d a p t i v e s i n c e they would e n a b l e efficiency"  image"  proposed  have  continually  to  the  first  then,  increase  are  the  attention  since  others  of  the in  for  work  that  resulting  mind,  has  appear  been to  (1965)  the  learning  process.  through  a system  of  and  predator's  its model  searching  the  reaction was  to  state  had  to  reveal  affected  implications.  developed He  feedbacks  prey.  sufficient  image and  be  ecological  Holling  to  of  suggested between  hunger  Simulation account  important  a  for  to  studies the  implications  with respect  to the s e l e c t i v e  p a l a t a b l e and l e s s p a l a t a b l e The experiments examine of  determine i f  described  i n t h i s section were designed  to a r t i f i c i a l ,  but p a l a t a b l e prey,  t h e i r behaviour.  of  H o l l i n g ' s model to t e s t  and to  The r e s u l t s were then i n t e r p r e t e d  f o r the e f f e c t  if  i t was s u f f i c i e n t l y  of l e a r n i n g on the feeding  to  response  a s s o c i a t i v e l e a r n i n g could be an important  of  of  prey.  some of the f a c t o r s which might i n f l u e n c e the  rainbow t r o u t  account  advantages of mimicy between  component i n terms  general  to  behaviour  trout.  METHODS AND MATERIALS The rainbow t r o u t  selected  for t h i s  from 11 to 14 cm ( l to 2 years o l d ) Marion Lake,  B r i t i s h Columbia.  completely n a i v e , commercial  standardized  by c u t t i n g  ranged  and were obtained  To i n s u r e  the experimental  chicken l i v e r .  study  prey  that  in length from  the f i s h  were  were formed from  The shape and s i z e of the prey were  c y l i n d r i c a l p i e c e s of l i v e r 3 mm by 5  mm i n l e n g t h . Conditioning different individual (30  experiments  situations.  were conducted i n two e n t i r e l y  In the f i r s t  naive f i s h were placed  x 12 x 20 cm) that  g l a s s aquarium. through the tank.  Six  of experiments  i n t o a small  was suspended  standard  set  (A),  holding chamber  i n a 50 g a l l o n  (227  prey were then s c a t t e r e d at  A f t e r the food had been introduced the  liter) random trout  was released from the holding area.  An experiment lasted for  20 minutes and was considered to represent one 'day* of experience regardless of whether the animal: fed . or not. were conducted  Experiments  every 48 hours u n t i l the amount of time the f i s h  required to locate and capture a l l 6 prey s t a b i l i z e d .  Between  successive experiments the predators were held i n i s o l a t i o n and without  food.  A l l other experiments (B) were conducted  in a large rectangul  tank (180 x 16 x 30 cm) which had a small holding area at one end.  This chamber was separated from the remainder of the tank  by an opaque, s l i d i n g p a r t i t i o n .  One side of the tank was  marked o f f in 1 cm intervals so that the distance from which trout would react to prey could be estimated. For each feeding experience a f i s h was transferred to the holding chamber in the experimental  tank.  Before the  predator was released a single prey was placed near the opposite end of the tank.  The reactive distance was defined as the  distance between the predator and prey when the f i s h attacked. After a prey had been captured, the trout was returned to the holding chamber while another piece of food was introduced.  A  single day of experience consisted of s i x successive captures. Once these were complete, the predator was returned to i t s holding tank and deprived of food u n t i l the next test period 48 hours l a t e r . In some (B) experiments the trout were exposed to prey that contrasted d i f f e r e n t l y with the background.  The level of  target in  contrast  a saturated  degrees by  of  of  light  (le  is  In  white  the  dark  grey  was  black.  C.)  and  the  the  were the  both  it  prey or  background  carefully was  is  of  grey  the  Black to  the  with  could  time.  in  both  prey  sets  contrast  because  the  (tank  )  the  bottom  water  affected  in  sense of  experiments,  illumination  to  insure  that  by  changes  in  to  (tank  (0.3,  visual of  the  either bottom)  ( 1 0 _+ ft-candles)  coefficient,  the any  amount  throughout  temperature  (attenuation  term,  backround  background  water  the  the  the  relative  the  experiments,  to  prey  reproduced  Although  respect  words,  white  different  be  difference  a relative  a high  standard Several  black)  staining as  B.  in  controlled  not  Sudan  the  used  had  of  staining  a target  black  sets  by  defined  other  turbidity  fish  of  by  1967),  standard  2  length  reflected  of  (light  commonly  paper.  In  solution  the  Grand,  this  changed  contrast  varying  contrast,  was  0.50)  acuity  these  of  conditions.  RESULTS EXPERIMENT THE  C H A R A C T E R I S T I C S OF THE I N I T I A L OF TROUT TO U N F A M I L I A R PRE Y  When t r o u t of  their  some f i s h  will  experience  will  react.  The  are  feeding  they  required  exposed  behaviour  to  unfamiliar  change  with  it,  others  number it  would  duration  of  of  require  this  prey  for  exposure  as  a group  the of  aspects  Although  the  e x p o s u r e s an  was d e f i n e d  phase  several  stimulus  repeated  successive  attack  RESPONSE  experience.  i n v e s t i g a t e an u n f a m i l i a r  The  before  average  A  first  time  before  they  individual latent 9 test  phase. fish  was 4 d a y s and r a n g e d Once t h e l a t e n t the  fish  Four  continued  separate  orientation,  fail  to complete  attack,  develop this  their  capture of prey:  prey  many  an a t t a c k s e q u e n c e . fail  to f o l l o w  to capture  to average  prey.  two f u r t h e r  of the l a t e n t  a complete  experience. l)  3) a t t a c k , and 4) s t r i k e .  to the t e s t  and t h e n  termination  the behaviour of  t o change as t h e y a c q u i r e d f u r t h e r  to r e a c t  but f a i l  was f o u n d  p e r i o d uuas t e r m i n a t e d ,  2) f i x a t i o n ,  began  prey  1 t o as h i g h as 11.  steps preceed  fish  fixate  from  Once t h e  individuals  Some a n i m a l s through  would  would  visually  w i t h an a t t a c k , o r  The d u r a t i o n o f t h i s  phase  days o f e x p e r i e n c e a f t e r t h e  period.  Although  a t t a c k sequence  rapidly  may n o t be t r u e i f i t i s r e l a t i v e l y  trout  will  i f a prey  apparently  i s palatable  unpalatable  (Sheperd,  1970). After the as  the trout  amount o f t i m e they  they  component  of t h e i r  6 days o f e x p o s u r e .  the  attack rate  l)  the d e n s i t y of prey,  predator In the  velocity  2)  and 3)  experiments  of the f i s h  a complete  to capture with  them  attack pattern,  a l l 6 prey ( f i g .l ) .  behaviour  Holling  i s determined  and p r e y ,  the present  basis  took  became more f a m i l i a r  some o t h e r after  established  diminished Evidently,  was s t i l l  changing  (1966) d e m o n s t r a t e d  primarily  by t h r e e  the r e l a t i v e  factors:  velocity  c o n s i d e r a b l y from  between t h e  the predator's d i s t a n c e of r e a c t i o n . experience  or t h e i r  c o u l d have a f f e c t e d  reactive  distance.  o f o b s e r v a t i o n , i t d i d n o t a p p e a r as i f t h e i r  changed  that  one e x p e r i m e n t  either  On t h e velocity  to the next.  Therefore,  Figure. 1.  The e f f e c t  and r e a c t i v e circles fish  distance  on the feeding  of 2 groups  of t r o u t .  6 standard  presented.  (white)  The closed  test  circles  prey;  distance  trout.  given i n the  is  took 9 the  show the  average change i n the r e a c t i v e Further explanation  time  The open  i n d i c a t e the average amount of time i t  to capture  range i s  of experience  of 6 d i f f e r e n t text.  _ 7c 6E — 5CD  E 41•- 2 - 1 "D  0  CD  LL  -70 J —60  6  CD  o  I i  o o  1  0  I  I  1 2 o oT To oT "i—r T 1 T T T r r 1 2 •3 4 5 6 7 8 9 Days of e x p e r i e n c e  -50 -40 -30 — 20 -10  £ 1 a, ^ g  cc  the  effect  of  experience  on  their  reactive distance  was  examined.  EXPERIMENT B THE  In prey the to  EFFECT OF  these  i n the  experiments  rectangular  l a t e n t phase, 16  EXPERIENCE ON  their  consecutive the  intitial  with  f u r t h e r experience  considerably required to  required attack  higher  between  that  the  group of feeding  in  in  that  fish period  suggests  their  factor  of  ( f i g . 2).  time of  cases  40  to  a trout  (4)  aspects,  vary  up  increased  at  the  animal  develop  l e a r n i n g can  for  exposures to  One  these  through  indicated  stabilized  (about  required  recorded  standard  individual  In most  Both o f  passed  results  every  to  a  prey) however,  the  initial  response,  considerably  individuals.  apparent  This  The  reactive distance.  the  was  i t finally  experience  process  Nevertheless, is  experience.  somewhat more t i m e . and  group  reactive distance  level  a maximum  distance  indicate  After this  before  DISTANCE  t r o u t were e x p o s e d  reactive distance  6 to 7 days o f  develop  6 naive  tank.  days of  that  REACTIVE  feeding  the  change  data  are  pooled  that that  the  time).  was as  of  required  the  fish  by  the  acguired  r e a c t i o n could  increase  and  averaged, i t  in reactive distance  i s i n v e r s e l y c o r r e l a t e d with  distance  behind  i f these  in their  the  first  of  r a t e of  second  duration group  experience,  have been  the  the  attack  of  (fig. an  the l ) .  increase  causal (decrease  In the process  of l e a r n i n g the f i r s t  have with new prey are l i k e l y their  response.  d RD  =  experience  a (RD M  (a)  maximum d i s t a n c e T h i s expression  is  X  a r a t e constant and (RD = ) max  is  from which a c o n d i t i o n e d animal w i l l integrates  M  R D  The average value of  =  RD  (R^max)  known,  standard r e g r e s s i o n  max  the attack,  ( 1  e  }  day of experience  the value of  logarithmic transformation.  "  a (E)  was c a l c u l a t e d by p o o l i n g a l l  techniques i f  by grouping a l l the data  of  to,  i n F i g u r e 2 for the l a s t  t h i s parameter i s  if  as:  - RD) A  -  data  on  the d i s t a n c e of r e a c t i o n f o r a given l e v e l  (E),  trout  to have the g r e a t e s t e f f e c t  This hypothesis can be expressed  d~T  where (RD) i s  few experiences  (l)  (a) is  i n F i g u r e 2.  Once  can be estimated by first  A regression  (Table l ) .  the  l i n e a r i z e d by a  analysis  The r e s u l t s  was conducted showed that  these data were transformed they could be d e s c r i b e d adequately  by a s t r a i g h t  line  ( r = 0.90  ) but that  the l i n e d i d not pass  through the o r i g i n .  Therefore equation ( l )  was modified  i n c l u d e an i n t e r c e p t  (b).  and (b) are  The value of  (a)  to presented  i n Table 1. In most cases,  the t r o u t were able to double t h e i r  initial  Figure of  2-.  The e f f e c t  6 trout.  of experience on the r e a c t i v e  The t e s t prey were ' w h i t e '  Each data point represents the range i s  ( 5 mm i n l e n g t h ) .  a mean of 6 r e p l i c a t e  i n d i c a t e d for s e v e r a l  distance  days of  observations,  experience.  100-  9  8  80604020-  T • 1  .  T  t  1  100-1  '.1  I IIIIII III II II I I  I I II I I ! I 1 I I I I I I  E  o  e  3  CD  g 8005  5  "O  60-  o 40> o 2003 CD  I  .1  T T  *** •  1  I • .1  i i i ! IIIIIIIIIII I  I I I I I I I I 1 II ! I I I  0C  * -1  .• *  10O-I 7  8060-  • 1  40H 20-  T  #  •  T •• •  J T• 1  T  .T-  .I  T  • • ••2 •  o 1  1  I  I I I I I II M I I ! II 2 4 6 8 10 12 14  I I I ! I II I II ! I I I 2 4 6 8 10 12 14 16  Days of experience  TABLE  1.  The r e l a t i o n s h i p  conditioned  between t h e i n i t i a l  reactive distance  (RD  ).  The  (RDT) and  simulated  max prey The  were  'white'  ( h i g h c o n t r a s t ) and 5 mm  average values  by  pooling  in  the t e x t .  Fish  o f ( K ) , ( a ) and  a l l the data.  RDT  Further  ( b ) were  in length. obtained  explanation  RD  i s given  K  (cm)  max (cm)  1  20  56  2.8  3  26  64  2.5  4  24  56  2.3  7  36  58  1.6  8  32  56  1.8  9  40  74  1.8  Average RDT RD  max  ?  Parameter =  29  =  61  K  Values  = 2 . 0  a  =  0.466  b  =  -0.038  naive r e a c t i v e d i s t a n c e a f t e r  6 days of e x p e r i e n c e .  If  the  maximum d i s t a n c e from which a c o n d i t i o n e d a n i m a l ; w i l l is  assumed to be a constant  response two  (RDT) (Table l )  parameters  RDmax  By s u b s t i t u t i n g  eguation  be expressed  than i t s  (K) of i t s  initial  then the r e l a t i o n s h i p between  these  is simply:  (2)  can  function  react  -  K (RDT)  (2)  into ( l ) ,  of  experience  i n terms of an a n i m a l ' s naive response  maximum r e a c t i v e d i s t a n c e .  (3)  the e f f e c t  RD  =  K (RDT)  The r e s u l t  ( 1 - e "  a  (  E  rather  is,  )  +  b  )  THE SPECIFICITY OF THE ATTACK RESPONSE OF CONDITIONED TROUT Since t r o u t can i n c r e a s e t h e i r learning,  r e a c t i v e d i s t a n c e through  the q u e s t i o n which a r i s e s i s  their  response  to a prey?  visual  systems  r e c e i v e at  just  Most v e r t e b r a t e least  how s p e c i f i c  and some i n v e r t e b r a t e  4 d i s t i n c t pieces  l)  size,  Therefore a target  is  not j u s t one stimulus but r a t h e r i s a  of at  least  these  an animal could use s e v e r a l learned a s s o c i a t i o n . than  others.  3) c o n t r a s t  of i n f o r m a t i o n  about any o b j e c t :  composite set  2) form,  is  and 4) v e l o c i t y .  4 visual properties.  Undoubtly,  i f not a l l of these cues to form a  Some cues however, might be more important  To answer standard  this  question,  I decided to c o n d i t i o n t r o u t  white prey and then switch them to another object with  identical  physical properties  except for  response of a c o n d i t i o n e d animal i s reaction  to a 'new' o b j e c t  However, to t e s t  I choose  (1968) as  IF the  then  well  its  experience.  the l e v e l of prey  of the attack  response,  contrast  another  consideration. as others  have documented that  animal must detect a t h r e s h o l d l e v e l of c o n t r a s t  can d i s c r i m i n a t e  the c o n t r a s t further level  not s p e c i f i c  to a l t e r  must be taken i n t o  Hester  it  since  contrast.  should not change with  for the s p e c i f i c i t y  variable  visual  to  an o b j e c t  of a t a r g e t w i l l  away from i t .  of c o n t r a s t  it  from the background.  before  Underwater,  appear to a t t e n u a t e as one moves  As a r e s u l t ,  if  can be detected  than one with l e s s c o n t r a s t .  a  an o b j e c t  has a high  from a g r e a t e r  Consequently,  distance  i f trout  are  c o n d i t i o n e d to a white t a r g e t and then switched to one with lower c o n t r a s t reaction  (i.e.  black prey)  should be d i f f e r e n t .  from a g r e a t e r d i s t a n c e . specificity  but i s  The question  is,  to one t a r g e t  The white prey should be  T h i s does not i n d i c a t e  predictable  however,  (i.e.  will  white),  the r e a c t i o n of t r o u t ,  conditioned  change as  should react to the  'new' o b j e c t  of  reaction  if  their  response mechanics.  If  day;  attacked  on the b a s i s of v i s u a l  with a 'new' p r e y .  the f i r s t  t h e i r maximum d i s t a n c e of  they acquire  t h e i r response i s  experience  not s p e c i f i c  then they  from a "maximum d i s t a n c e "  response i s  specific,  should improve with e x p e r i e n c e .  then t h e i r In e i t h e r  on  distance  case,  if  the c o n t r a s t  0 f the  'neui' prey i s  the maximum r e a c t i v e  distance  lower than the o r i g i n a l then  should be  less.  Nine t r o u t were c o n d i t i o n e d to standard their  reactive  experience.  distance  were assigned  a control  On the f i r s t  For the t r o u t of t a r g e t  average d i s t a n c e control  contrast  to black p r e y .  latent  pronounced.  of  response  experience,  to dark grey prey,  was apparent  they (figs.  however,  because t h e i r  change i n the behaviour of On the f i r s t  from the h o l d i n g area  i t would r e a c t .  original  the  Their  the  of r e a c t i o n was c o n s i d e r a b l y l e s s than the  had to be released  that  immediately.  to  ( f i g . 3A).  exposed  before  the f i s h t r a n s f e r r e d  and from a maximum d i s t a n c e  exposed  There was a n o t i c e a b l e trout  The remaining animal  they acguired a d d i t i o n a l  without h e s i t a t i o n  the e f f e c t  Two f i s h  (white).  day of exposure  d i d not change as  3B, 3 C ) .  random, to be switched  or black p r e y .  to black p r e y .  and dark grey prey reacted  attacked  at  to each type of prey, with the exception that 4  f i s h were t r a n s f e r r e d  light  dark grey,  until  for 4 s u c c e s s i v e days of  They were then a s s i g n e d ,  to e i t h e r a l i g h t grey,  served as  stabilized  white prey  This lag  day every  individual  an average of 20 times  i n t e r v a l corresponds  p e r i o d , although i n t h i s  the  case,  to the  i t was not  Part of the e x p l a n a t i o n may be due to the  the experimental  as  fact  environment was extremely simple and that  'new' prey r e t a i n e d many of the p h y s i c a l the o r i g i n a l o b j e c t .  The second and by far  characteristics the most  pronounced change i n behaviour was i n the r e a c t i v e  distance  page 60 omitted in page numbering  component. that  These data  the attack  are shown i n F i g u r e 3 D and c l e a r l y  distance  increased  s i g n i f i c a n t l y as  the  indicate  fish  a c q u i r e d more e x p e r i e n c e . The behaviour of the f i s h t r a n s f e r r e d be summarized by saying that (searching'for the new prey, of  therefore,  something e l s e 1 .  level  (a)  and the i n i t i a l  prey  (RDT) as  from equation  that  treating response,  and the  independent of  contrast  i n F i g u r e 3 D . Using the  from p r e v i o u s  experiments  from which t r o u t attacked  point, (3).  (a)  (Table black  the change i n t h e i r response was  Since there  is  a reasonably  and p r e d i c t e d trend (curve,  close  fig. 3 D ) ,  the rate of l e a r n i n g was not. a f f e c t e d  by  contrast. To summarize,  these experiments  the response of c o n d i t i o n e d trout  the maximum d i s t a n c e target of  distance  a starting  t h i s demonstrates  distance  Their conditioned  (K) are  presented  and (b)  between the observed  prey  they were not  that both the rate of l e a r n i n g  l),  predicted  their  to  specific.  can be t e s t e d with the data  fit  that  of the c o n d i t i o n e d response  (K),  to be  Once they began to respond  i n the tank i d e n t i c a l l y .  was somewhat  estimates of  appear  however, they were able to i n c r e a s e  The hypothesis  l)  they i n i t i a l l y  r e a c t i o n which demonstrates  every o b j e c t  to black prey can  contrast,  learning.  trout w i l l  react  and 3 ) prey c o n t r a s t  demonstrate is  three  points:  somewhat s p e c i f i c , 2 )  to prey i s  dependent upon  does not a f f e c t  the  rate  F i g u r e 3.  The e f f e c t  white prey,  to prey with d i f f e r e n t  The data p o i n t s each group. the f i r s t  c o n d i t i o n e d to  levels  i n d i c a t e the mean r e a c t i v e  of  contrast.  distance  of  The 9b% confidence l i m i t s of each mean on  and l a s t  (A) white prey grey prey,  of s w i t c h i n g t r o u t ,  day of experience  (control),  (D) black  prey.  are shown.  (B) l i g h t grey prey,  (C) dark  A  70.-1  605 0 -  T  T •  60-  1 •  I  50-  4 0 -  40-  30-  30-  2 0 H  20-  10-  2  '"•I  i.  1  4  6  8  I IIIIIIII I 2  10  70-  7 0 - j  60-  6 0 -  5 0 H  50-  40-  40-  2 0 -  T  1 0 H  11 I I I I I I I I  30-  B  70-  T • "I  T • i  4  6  8  10  D  T  1  3020-  1 0 -  T/ •  1  1 0 H  II IIIIIII I 2  4  6  8  10  I II II IIII I 2  Days of e x p e r i e n c e  4 6 8 1 0  THE EXTINCTION AND RE-DEVELOPMENT OF REACTIVE DISTANCE To examine the e f f e c t reinforcement  on r e a c t i v e  distance,  c o n d i t i o n e d to black prey experience  for 90 days.  of long-term d e p r i v a t i o n of  (fig.  white p r e y .  reactive  d i s t a n c e was recorded  Upon re-exposure to black prey for s e v e r a l  the g r o u p ' s i n i t i a l  response when they were naive  (20  took  similar  their  s u c c e s s i v e days.  different  from t h e i r  In a d d i t i o n ,  to re-develop  re-  they  a' c o n d i t i o n e d  to the 5 to 6 days they o r i g i n a l l y  ( f i g . 3D). Although the e f f e c t  of short-term  d e p r i v a t i o n of  on r e a c t i v e  d i s t a n c e was not examined i n d e t a i l ,  experiments  indicated  for  fed  response a f t e r  cm).  r e q u i r e d 4 to 5 days of experience  further  p e r i o d they were  exposure (18 cm) was not s i g n i f i c a n t l y  response which i s  previously  3D) were d e p r i v e d of  During t h i s  standard  F i g u r e 4 shows that  the 4 f i s h  trout  reinforc  ene set  of  can maintain a maximum response  up to 14 days without r e i n f o r c e m e n t .  Therefore,  p e r i o d of d e p r i v a t i o n between 14 and 90 days i s  some  sufficient  to  reduce the r e a c t i v e  d i s t a n c e back to the o r i g i n a l l e v e l (RDT)  when the animal was  naive.  ATTENTION COMPETITION The experiments with the e f f e c t a predator  described  of experience  thus  far  have been concerned  on the d i s t a n c e of  was exposed to one type of p r e y .  If  r e a c t i o n when i t were  faced  F i g u r e 4. of  The re-development  4 trout.  The prey were  The data p o i n t s reaction. are  shown.  represent  of the r e a c t i v e  distance  'black' (5 mm i n  length).  the average d i s t a n c e of  The 95% confidence  l i m i t s of the means  The curve was f i t t e d  by eye.  40-1  T •1  E o 30CD O  C  03  inr I  20-  CD  >  o 10-  CTj CD  DC  IX  T  i 2  i—i—i—i—i—m—i—i—i 4 6 8 D a y s of e x p e r i e n c e  10  12  with a s i t u a t i o n  i n which i t  could encounter other  familiar  o b j e c t s then some form of a t t e n t i o n . c o m p e t i t i o n , or might o c c u r .  For example,  the d i s t a n c e  trout w i l l  interference react  one prey might be somewhat diminished i f  they attempted  become g e n e r a l l y  forms.  responsive  To examine t h i s  to a l t e r n a t e  possibility, prey u n t i l  to to  4 t r o u t were c o n d i t i o n e d to  low c o n t r a s t  (black)  their reactive  distance  stabilized.  They were then switched to a s i t u a t i o n  in which  each time they were released they could encounter e i t h e r a b l a c k , white or dark grey prey, were i d e n t i c a l except recent  experience  with equal p r o b a b i l i t y .  for t h e i r c o n t r a s t .  to the a l t e r n a t e  prey from an average d i s t a n c e  of  );  statistically  of  and were  the t r o u t attacked  the presence  reaction.  1 S . E . +_ 2.7  significant,  black  of 32 cm ( n - 15; 1 S . E . +_  when a l t e r n a t e s were p r e s e n t ,  37 cm ( n a 16;  that  types  fish  somewhat f a m i l i a r with them.  Before they were switched,  1.0  Although t h e i r most  had been confined to black o b j e c t s the  had been p r e v i o u s l y exposed therefore  These t a r g e t s  ).  they reacted  This d i f f e r e n c e  therefore,  of a l t e r n a t e food  from a  it  is  distance  not  must be concluded  did not a f f e c t  the  distance  DISCUSSION Experience with an u n f a m i l i a r prey w i l l components of the feeding behaviour of the l a t e n c y of t h e i r response, complete attack of  sequence,  however, i s  (1968) demonstrated  of  him to i n f e r that reaction,  searching  such changes, if  to a novel  especially  a predator  capable  increase  its  study demonstrates  latter Beukema also  food and  i n the  distance  was to develop,  develop'  reactive  distance  a  each time a p r e d a t o r ,  for that o b j e c t .  however, before  any f u r t h e r .  encountered before  That i s ,  the new attack  the o r i g i n a l l e v e l . the rate of  that  of l e a r n i n g , a t t a c k s a p a l a t a b l e  c o n d i t i o n must be met,  If  prey i t  response  another prey must be  distance  d i m i n i s h e s back to  the rate of l e a r n i n g i s  faster  than  response e x t i n c t i o n then even a few contacts  development of a searching antagonistic  rates w i l l  r e q u i r e d before  (maximum r e a c t i v e  image.  with  to promote the  The d i f f e r e n c e  between these  determine the d e n s i t y of prey  a predator  will  One c r i t i c a l  the attack  a r e l a t i v e l y rare prey could be s u f f i c i e n t  is  All  image.  which i s  two  l)  the  the most s i g n i f i c a n t .  were exposed  were necessary  The present  can  importance,  these b e h a v i o u r a l a l t e r a t i o n s  occured when s t i c k l e b a c k s led  rainbow t r o u t :  and 3) the r e a c t i v e d i s t a n c e .  by far  that  several  2) the development of a  these aspects have some t h e o r e t i c a l  observation  alter  can form a searching  that  image  distance).  Many prey p o p u l a t i o n s  tend to be polymorphic with  respect  to c o l o r ,  form, or some other v i s u a l  showed that  if  predators  quality.  Croze (1970)  d i s c r i m i n a t e between morphs then a  polymorphic p o p u l a t i o n w i l l  be l e s s v u l n e r a b l e to a t t a c k than  a monomorphic p o p u l a t i o n of the same d e n s i t y . however,  may not be a p p l i c a b l e  (Holling,  1965).  the extent prey i s  In any case,  by,  image.  predator.  result  treated  will  react  to only one form.  the predator  a maximum d i s t a n c e .  In the present  they were able to increase a low l e v e l of  study,  These c h a r a c t e r i s t i c s learn  (1965) as  model i s  that  are  mentioned e a r l i e r ,  model of the l e a r n i n g p r o c e s s .  every 48  relatively  to t h i s  is  the concept of i n t e r f e r e n c e , s t i m u l i could a f f e c t  paths.  alternate  of  new pathways of l e a r n i n g or the performance of Interference  is  The  i n which the  of  ones.  that  hours).  e i t h e r the r a t e of  supported  to  scarce.  developed a general of  pathways of a s s o c i a t i v e l e a r n i n g do not  formed independently of e x i s t i n g  will  'new' prey  One of the assumptions  but are  existing  but  they have the p o t e n t i a l  to d i s c o v e r polymorphic prey that Holling  specific  (6 encounters  imply that  predation  the response of  t h e i r responsiveness to  reinforcement  will  to each morph from  c o n d i t i o n e d t r o u t was shown to be somewhat  at  be minimized when  Although t h i s  does not react  the  be g r e a t e s t  i n maximum p r o t e c t i o n for the p o p u l a t i o n ,  a l s o be low i f  of  i d e n t i c a l l y by a  On the other hand, p r e d a t i o n w i l l  the predator  densities  the s p e c i f i c i t y  from p r e d a t i o n w i l l  when the morphs are similar' enough to be  abundant  decrease the r i s k of a  among other t h i n g s ,  The r i s k  extremely  over a wide range of  to which polymorphism w i l l  affected  searching  when prey are  This c o n c l u s i o n  the  interact alternative presence development  already  by some data  for  humans.  Shiffrin  (1970) reported  t h a t an i n d i v i d u a l would r e c a l l related  interference,  however, prey;  which i s  (Beukema,  both animals  1968)., had to  under more n a t u r a l  is  for  trout  nor for conditions,  conditions predators w i l l as  detect  conflicting  Thus the p o s s i b i l i t y  of  stimuli  interference  increased.  or ignore any prey i t  by H o l l i n g  encountered.  for  prey.  palatable,  the s h i f t  if  dependent  in this  reactive  considerable  is  would attack  it  has  the set  or lowering the  level.  If  the prey  lowered with each  unpalatable,  then the  distance is  postulated s t a t e of  threshold to be  hunger  o v e r t l y expressed by a change  distance. between the p a l a t a b i l i t y  a t t a c k t h r e s h o l d of t r o u t  will  is  is  upon the p r e d a t o r ' s  threshold  The r e l a t i o n s h i p  fish  it  Since the r e a c t i v e  functionally  general  then the a t t a c k t h r e s h o l d  s u c c e s s i v e encounter; raised.  threshold  Learning operates by r a i s i n g  a t t a c k t h r e s h o l d from some i n i t i a l  that  For an attack to o c c u r ,  must be hungrier than the attack  that  also assumed  determined whether i t  predator  in  task.  recognize only 2 or 3 types of  the hunger l e v e l of a predator  is  inversely  Under the experimental  The l e a r n i n g model proposed  is  item was  but one p o s s i b l e form of  both a wider v a r i e t y of prey as well from the environment.  probability  a s s o c i a t e d with the  could not be demonstrated  sticklebacks  the  a particular  to the number of elements  Attention competition,  that  was not examined.  evidence to i n d i c a t e  ingest  is  dependent  of prey and the  However, there  that  the amount of  upon i t s  palatability.  is  food many Both  Sheperd  (1970) and Ishiwata  consume c o n s i d e r a b l y cease f e e d i n g .  less unpalatable  Hence, there  t h r e s h o l d of many animals  is  other hand, the r e l a t i o n s h i p distance  (l968e) have shown that  is  food before  little  related  they v o l u n t a r i l y  doubt that  the attack  palatability.  between hunger and  has not been adequately  fish  On the  reactive  documented for many predatory  species. Even though H o l l i n g ' s model w i l l of  r e a c t i o n should i n c r e a s e  with p a l a t a b l e prey there of  is  as  trout  predict.that a c q u i r e more  some question  the mechanism through which l e a r n i n g i s  Beukema reactive  (1968) presented distance  of  as  I also  short  food t r o u t  does not change t h e i r  affects  to allow r e j e c t i o n of the hypothesis from which a l l predators  should be c a r e f u l l y  cannot be demonstrated  a predator's  through i t s  trout  responsiveness  the e f f e c t  examined.  If  sufficiently  that  hunger  will  react,  of hunger on the proposed  the reactive relationship  pathway i n which experience  responsiveness,  rather  directly  than i n d i r e c t l y  hunger m o t i v a t i o n .  Irrespective that  the  then the l e a r n i n g model must be modified  to i n c o r p o r a t e an a l t e r n a t e affects  operate.  term changes i n the amount of  data do lead one to suggest that distance  to  was s t a b l e over  Although n e i t h e r of these s t u d i e s are  the d i s t a n c e  generality  have some p r e l i m i n a r y  data which suggests that  detailed  to the  some evidence which i m p l i e d that  a wide range of hunger l e v e l s .  (Table 2 ) .  distance  experience  proposed  conditioned sticklebacks  have ingested  the  of the mechanism of l e a r n i n g ,  can double t h e i r attack d i s t a n c e  if  it  was shown  they  acquire  sufficient  experience.  Therefore,  they have the p o t e n t i a l inference is Hamilton,  unpublished d a t a ;  of  alternate  prey.  sectors.of  area w i l l  motivational  often  t h e i r hunting a c t i v i t i e s  levels  of i n d i v i d u a l  that  predators  corresponds  and  can be expected  to  the  in the feeding behaviour of many of the  image reported by Croze (1970).  crows s h i f t  i n which they w i l l  This  prey as well as  they acquire experience,. ref 1 ect  before  organisms.  to promote l e a r n i n g .  In c o n c l u s i o n , the changes  the searching  i n the same  i n the experiences  the r a t e they encounter d i f f e r e n t  t r o u t , as  to  However, t h i s does not  feed on d i f f e r e n t  because v a r i a t i o n s  c o n d i t i o n s necessary  of  c o n t a i n j u s t 2 or  to be a complete e x p l a n a t i o n s i n c e animals  not s u p r i s i n g ,  1941;  This phenomenon could be explained to some  the environment.  affect  (Allen,  i n t h e i r gut i n c o n j u n c t i o n with a number  specific  relative  This  communication)  salmonids often  restricted  is  prey.  studies  personal  degree i f predators  appear  exploit  field  Bryan,  individual  3 main food organisms  certain conditions prevail  to s e l e c t i v e l y  supported by s e v e r a l  which found that  if  t h e i r a t t e n t i o n the:re i s  not react  to the l a t e n t  He showed a lag phase  to an u n f a m i l i a r o b j e c t .  phase f o r t r o u t .  characteristic  This  However, once they  d i s c o v e r new prey both animals d i s p l a y a c a p a c i t y  to l e a r n  quickly.  Although t r o u t appear  to r e q u i r e somewhat more time to become  completely  this  responsive  they were exposed  is  difficult  to a d i f f e r e n t  Another c h a r a c t e r i s t i c  to determine because  schedule of  of the searching  reinforcement. image i s  that  it  TABLE  2.  conditioned the  The  effect  to  proportion  of  'white' prey of  the  that  t h e r e was  indicates  that  the  before  experiment.  Fish  gut  Hunger index  no  the  (5 mm).  stomach t h a t  indicates  an  hunger on  food  The  i n the  half  full.  (n)  indicates  gut. The  the  distance  hunger  i s empty.  was  n  reactiv/e  index  of  3  describes  An  index of  1.0  An  index of  0.5  fish  were f e d  number o f  1.0  12  84  +  3.3  3  0.5  6  94  +  1.5  5  1.0  12  5  0.5  9 9  7 9 ++  3.6  6  73  +  4.8  1.0  12  83  +  3.2  0.5  6  90  +  1.8  one  hour  experiments  Mean r e a c t i v e d i s t a n c e (cm)  3  trout,  _+ 1  S.E  is  relatively  as w e l l , react this  specific.  This i s  true to some extent  for  trout  s i n c e the group c o n d i t i o n e d to white,\prey d i d not  immediately when they were switched to black prey. case,  because the  the l a t e n t  p e r i o d was not very pronounced,  'new' o b j e c t  of  the o r i g i n a l p r e y .  as  Croze pointed out,  can be t r a n s f e r r e d  if  r e t a i n e d many of the  Nevertheless that  the o r i g i n a l image  image i s is  possibly  characteristics  these experiments  the searching  In  indicate,  specific  no longer  but  reinforced.  SECTION  III  PREY ACTIVITY AND VULNERABILITY INTRODUCTION The experiments only attack exposed  d e s c r i b e d i n Section prey.  Therefore,  d i s t i n g u i s h between the a c t u a l and  the v u l n e r a b l e  it  I showed that is  essential  trout  to  d e n s i t y of a prey p o p u l a t i o n  density.  Cranqonyx richmondensis and H y a l e l l a azteca  are  important  prey of the t r o u t p o p u l a t i o n i n Marion Lake; both s p e c i e s burrowing amphipods  and. tend to spend much of the time  concealed w i t h i n the sediment.  In'-general,  In t h i s s e c t i o n ,  at  I will  the mud-water  as well actively  as  are exposed  the average amount of time exposed  moving over the sediment  r e s u l t s of these experiments vulnerability  will  submodel that w i l l  intervals  interface.  examine the e f f e c t  on the p r o p o r t i o n of amphipods that  actually  the a c t i v i t y of  these animals appears to be confined to the short i n which they are exposed  are  of water  temperature  (vertical  activity)  i n d i v i d u a l s spend  (horizontal a c t i v i t y ) .  The  form the b a s i s of a prey be i n t e g r a t e d  with the main  a t t a c k model i n S e c t i o n IV.  METHODS AND MATERIALS Field  Studies The v e r t i c a l  a c t i v i t y of  Cranqonyx and H y a l e l l a was observed  i n both the l a b o r a t o r y and the f i e l d .  In the f i e l d  studies,  cores  of  sampler, with as dishes  sediment were removed from Marion Lake with ,a d e s c r i b e d by Hargrave l i t t l e disturbance  (20  cm d i a m e t e r ) .  fauna was not a l t e r e d  as  (1970), possible  The n a t u r a l  density.  The dishes  i n some d i s h e s ,  2 to 5 times above  experimental During  the  were then p l a c e d back i n t o the  being t r a n s f e r r e d ,  to a c c l i m a t e before  stacking  complement of bottom  l a k e to maintain them under ambient temperature After  transferred  into glass  with the exception that  the number of Crangonyx was i n c r e a s e d natural  and were  illumination.  the animals were allowed 24 hours  observations  series consisted  and  were i n i t i a t e d .  of 4 r e p l i c a t e  Each  cores  the study p e r i o d both the i n c i d e n t r a d i a t i o n  r e c o r d i n g p y r o h e l i o g r a p h ) and the water  temperature  of  sediment.  (Belfort, were  monitor ed. The v e r t i c a l a c t i v i t y of Cranqonyx and Hyalel1 a was examined with respect p o p u l a t i o n that  to the average p r o p o r t i o n of  was exposed  d u r i n g 15 c o n s e c u t i v e ,  above the mud-water  10 second o b s e r v a t i o n  Observations were conducted s e v e r a l  times  (0800 to 1700 h r s . P ' i S . T . ) and were repeated consecutive  each  interface  periods.  throughout the day for  for up to 4  days.  Eleven completely independent s e t s of  experiments were  conducted during the months of May, June and J u l y .  After  the  t e r m i n a t i o n of each of these s e r i e s the number of amphipods in  each core was determined by s o r t i n g  Preliminary t r i a l s complete r e c o v e r y .  i n d i c a t e d that  through the  sediment.  t h i s method would produce  Laboratory  Studies  In the l a b o r a t o r y , of  exposed  that  the v e r t i c a l  amphipods was observed at  ranged  from 5 to 20 C.  as w e l l  as  4 different  temperatures  For each experiment,  were removed from Marion Lake,  sorted,  amphipods  and then i s o l a t e d by  s p e c i e s i n t o separate c o n t a i n e r s of sediment. the sediment  the a c t i v i t y  In t h i s  case  had been screened to remove a l l other macroinver-  tebrates. Three r e p l i c a t e at  each experimental  conducted,  p o p u l a t i o n s of each s p e c i e s were observed temperature.  Before o b s e r v a t i o n s  the animals were allowed 24 hours to a c c l i m a t e  the experimental  conditions.  The number of animals  c o n t a i n e r was c a r e f u l l y c o n t r o l l e d ( e q u i v a l e n t Cranqonyx, it  fell  were  or 200 to 800 H y a l e l l a p e r .  w i t h i n the n a t u r a l  sq.  to  i n each  to 100 to 300  m.) to i n s u r e  that  range i n d e n s i t y of each s p e c i e s  (Appendix I'll). Throughout the experiments, was maintained at standardized temperature  about  at 10 hours  the background  10 f t - c a n d l e s ^ a n d  illumination  the l e n g t h of day  (0900 to 1800 hrs P . S . T . ) .  The water  was c o n t r o l l e d to w i t h i n 1 C. of the d e s i r e d  temperature.  test  In order to avoid the p o s s i b i l i t y of oxygen  d e p l e t i o n or s t r a t i f i c a t i o n  the water i n each c o n t a i n e r was  slowly c i r c u l a t e d . Hyalella  is  and was not fed  a deposit  feeding  (artificially);  and was fed dead b r i n e shrimp.  species  (Hargrave  Cranqonyx, however, In t h i s  case,  , is  1970) carnivorous  the amount of  food  provided was always  i n excess of what the p o p u l a t i o n s ,would  consume between s u c c e s s i v e f e e d i n g s . s p e c i e s during the experiments Observations  was l e s s than 5%.  on the v e r t i c a l  H y a l e l l a were conducted before  The m o r t a l i t y of both  a c t i v i t y of Crangonyx and  Cranqonyx was fed and were  repeated  every 5 minutes for up to one hour.  activity  was expressed i n terms of the average p r o p o r t i o n of  time exposed this  i n d i v i d u a l s spent  case, o b s e r v a t i o n s  throughout the  Their horizontal  moving over the sediment.  were conducted at  irregular  In  intervals  day. RESULTS  THE EFFECT OF WATER TEMPERATURE ON THE VERTICAL OF CRANGONYX AND HYALELLA Under both n a t u r a l and  and l a b o r a t o r y  c o n d i t i o n s , Cranqonyx  H y a l e l l a spend much of the day buried below the mud-water  interface.  When i n d i v i d u a l s are  concealed  in this  they tend to remain i n a c t i v e f o r some time before themselves; earlier  this  (Section  the instantaneous fairly  constant  appreciably  correlated  re-expose  I).  observations  Intensive  p r o p o r t i o n of animals  with the ambient  indicated  p e r i o d of time but  a strong  was  changed  environmental c o n d i t i o n s .  analysis  of the f i e l d s t u d i e s showed significantly  environmental parameters  One of the most s i g n i f i c a n t  that  that was exposed  a c t i v i t y of amphipods was  with s e v e r a l  Although there i s  they  behaviour was pointed out  over a short  the v e r t i c a l  fashion  characteristic  A multiple regression that  ACTIVITY  f a c t o r s was water  (Table  l).  temperature.  c o r r e l a t i o n between the l e v e l of  incident  r a d i a t i o n and the time of day o b s e r v a t i o n s  conducted, affected  the ambient water temperature  by d i e l  throughout the year t h i s  Therefore,  of Marion Lake changes  characteristic  considerably  of the environment could  changes i n the a c t i v i t y p a t t e r n s of the amphipods.  the apparent  and the degree of controlled  significantly  changes i n the l e v e l of i l l u m i n a t i o n .  Since the temperature  induce seasonal  was not  were  r e l a t i o n s h i p between water  exposure of both p o p u l a t i o n s  temperature  was t e s t e d under  conditions.  Laboratory s t u d i e s v e r i f i e d the f i e l d o b s e r v a t i o n instantaneous dependent.  p r o p o r t i o n of animals that If  were conducted both the l a b o r a t o r y  turn out to be very s i m i l a r  interesting  as  the l a b o r a t o r y  compared to the f i e l d It  the observed  by the exponential  UP.  where (VP^) i s instant  temperature  =  em3i  relationship  temperature temperature  and f i e l d This  is  artificial  between  could  equation:  (  T  )  "  m  i  (i)  exposed  at  (M3^) and(M4^) are constants and (T) i s  in C. ( f i g .  temperature  the mud-water i n t e r f a c e  the p r o p o r t i o n of s p e c i e s  i n time,  1 and 2 ) .  s t u d i e s were extremely  and the p r o p o r t i o n of amphipods at  (1)  (figs.  the  experiments.  was found that  be d e s c r i b e d  was  the f i e l d data i s grouped a c c o r d i n g to the  when o b s e r v a t i o n s results  was exposed  that  1 and 2;  any the  Table 2 A ) .  Although there was some i n d i c a t i o n that  the i l l u m i n a t i o n  and time of day might have i n f l u e n c e d the ' v e r t i c a l  movements  TABLE  1.  Hyalella,  The v e r t i c a l  i n Marion Lake,  conditions. were  dispersal  (n)  indicates  activity  with respect  to  of  Cranqonyx and  several  environmental  the number of days i n which o b s e r v a t i o n s  conducted.  Variable  Correlation  Coefficient  Cranqonyx Illumination  0 .229  1.56  Temp era tu re  0.511  5.10  Time of  day  -  *  0.70  0.102  ( n = 46 )  Hyalella Illumination  0.576  4.65  Temperature  0.372  3.09*  0.384  3.09  Time of  day  -  ( n = .43 )  *  significant  at  or l e s s  than 0.01  level  *  *  ure  1.  The e f f e c t  of water temperature on the  p r o p o r t i o n of Cranqonyx that are exposed at or above the mud-uiater i n t e r f a c e . i n d i c a t e the r e s u l t s experiments;  The s o l i d  circles  obtained from l a b o r a t o r y  the open t r i a n g l e s ,  field  experiments.  The 95% confidence i n t e r v a l s of each of the means are  indicated.  (See  Table 2A)  Figure  2 .  The e f f e c t  of mater temperature on the  p r o p o r t i o n of H y a l e l l a  that are exposed at or  above the mud-uuater i n t e r f a c e . i n d i c a t e the r e s u l t s experiments; The 95$  The s o l i d  circles  obtained i n the l a b o r a t o r y  the open t r i a n g l e s ,  field  experiments.  confidence i n t e r v a l s of each of the means  are i n d i c a t e d .  (See  Table 2A)  0  CO O CL X  «  o  w  Hyalella  0 c  .40-  o •4—'  >  03  Q-.30-  o  CL 0  .. .20c  o  o .10H a o  T—i—r—i—r—i—i—i—i  5 Water  10 15 20 t e m p e r a t u r e (CO  25  of  Cranqonyx and e s p e c i a l l y  possibilities  m. that  III),  of  the d e n s i t y  of amphipods,  are p o t e n t i a l l y v u l n e r a b l e to  Marion Lake at coupling  none of  these  by s i z e ,  per  were followed up.  An expression sq.  H y a l e l l a (Table l ) ,  different  equation  (l)  trout  predation in  times of the year can be d e r i v e d by  with the a c t u a l  the s i z e composition of pattern  d e n s i t y of prey  each p o p u l a t i o n  seasonal  temperature  (Appendix I I ) .  equation  ( 2 ) which can be designated  as  (D^, Appendix  (P ^ .) and the The r e s u l t  is  a prey v u l n e r a b i l i t y  submodel.  (2)  UN. .  where (UN^.) i s of  size  (j)  structure  that  are  =  D. P. .  r  e  M3. (T)  -  IY14  1  1  , J  the number of amphipods of s p e c i e s exposed.  The seasonal  of Ccanqonyx and H y a l e l l a i s  range i n the  (i), size  summarized i n Appendix  III. The profound e f f e c t vertical in  water temperature  has on the  a c t i v i t y of the amphipods i n Marion Lake i s  Figure 3.  As i n d i c a t e d ,  between the a c t u a l animals  that  that  very l i t t l e  relationship  d e n s i t y of e i t h e r s p e c i e s and the number of  are exposed  throughout the y e a r . was estimated  there i s  illustrated  and v u l n e r a b l e to a t t a c k from t r o u t  The v u l n e r a b l e segment of  from eguation ( 2 ) .  each p o p u l a t i o n  ure  3. The r e l a t i o n s h i p between the a c t u a l  amphipods (black  (white histograms)  histograms).  exposed  on the sediment  temperature  and t h e i r v u l n e r a b l e d e n s i t y  The v u l n e r a b l e segment  p o p u l a t i o n corresponds  d e n s i t y of  of  to the number of animals as a r e s u l t  (Appendix II)  each that  are  of the ambient water  (A) H y a l e l l a  (B) Cranqonyx  No. amphipods  per  sq. m. ( X i o o )  THE EFFECT OF TEMPERATURE ON THE ACTIVITY OF EXPOSED AMPHIPODS Cranqonyx  tend to move almost c o n t i n u o u s l y when exposed.  They do stop p e r i o d i c a l l y , however, to feed or to grasp at p i e c e s of l i t t e r .  In c o n t r a s t ,  i n a c t i v e at  a l l times.  Hyalella  a deposit  is  Hyalella  Hargrave  Thus, when t h i s s p e c i e s i s its  (1970) demonstrated  feeding s p e c i e s and must ingest  q u a n t i t i e s of sediment to meet i t s  of  tend to be r e l a t i v e l y  energetic  exposed i t  that large  requirements.  appears to spend most  time e i t h e r feeding or i n v o l v e d i n other  activities  which seem to r e q u i r e l i t t l e movement. In the l a b o r a t o r y , was also a f f e c t e d  the a c t i v i t y of exposed i n d i v i d u a l s  by temperature.  The r e s u l t s  (fig.  4)  suggest  that 1 0 ° C . i s ^ t h e optimum for Crangonyx, above or below t h i s , their o n  a c t i v i t y d e c l i n e d somewhat.  Unfortunately, o b s e r v a t i o n s  Hyalel1 a were confined to temperatures  temperatures  higher than t h i s ,  above 10 C.  At  t h e i r a c t i v i t y also declined  which suggests that they may have an optimum temperature which will  i s • s i m i l a r to Cranqonyx.  alter  Although the water  the movement of both s p e c i e s ,  temperature  Cranqonyx was always  more a c t i v e than H y a l e l l a . Assuming that amphipods are most a c t i v e at of  water temperature on t h e i r g e n e r a l l e v e l  1 0 ° C . the  of a c t i v i t y can be  d e s c r i b e d by the p a r a b o l a :  (S)  PA.  M5. l  +  M6. l  (T) '  v  -  effect  M7. l  (T2)  i n which case, species  (i)  PA^ i s  will  are constants  the p r o p o r t i o n of time an i n d i v i d u a l  spend moving when exposed;  The  M5^, M6^, and M7^  that can be estimated by f i t t i n g  F i g u r e 4 by m u l t i p l e r e g r e s s i o n results  the data i n  (Table 2 B ) .  presented above demonstrate that  water temperature w i l l  alter  are v u l n e r a b l e to attack  of  the ambient  the p r o p o r t i o n of animals  from t r o u t as well  the a c t i v i t y  of  exposed amphipods.  of  temperature on the movements of v u l n e r a b l e animals because  trout w i l l distance  react  It was necessary  as  that  to moving prey from a c o n s i d e r a b l y  than s t a t i o n a r y o b j e c t s of the same s i z e .  s i g n i f i c a n c e of t h i s o b s e r v a t i o n w i l l next  to c o n s i d e r the  section.  effect  greater The  become apparent  i n the  Figure  4.  The r e l a t i o n s h i p between water  and the average amount of time exposed (A) and H y a l e l l a (B) temperature  spend moving.  temperature Cranqonyx  The optimum  for the a c t i v i t y of both s p e c i e s  assumed to be 10 C.  (See  Table 2B)  is  Water  temperature C O  TABLE  2  A.  i• The  relationship  between  and t h e i n s t a n t a n e o u s  t h e ambient  proportion  water  temperature  of Cranqonyx  (T)  and H y a l e l l a  2 exposed  a t o r above  t h e mud-water  the  amount  of v a r i a b i l i t y  and  (P) i s t h e p r o b a b i l i t y  interface. (VP^).  accounted that  f o r by t h e  the slope  (R ) i s  regression  i s zero  (no  correlation). Species  (p  N3.( )  M4(p  R  P  2  Cranqonyx  0.138  4.63  0.81  0.0027  Hyalella  0.180  5.13  0.84  0.010  The  relationship  and  the average  moving. the  between  t h e ambient  proportion  (R ) i s t h e a m o u n t  of time  water  exposed,  of v a r i a b i l i t y  temperature  (T)  amphipods  spend  accounted  f o r by  regression.  Species  (^)  W5(.)  M6(.)  + 1 SE  |Y17( . ) + ISE l  R  2  Cranqonyx  0.39  0.069  0.069  0.0033  0.0027  0.58  Hyalella  0.01  0.039  0.049  0.0017  0.0019  0.34  SECTION IV A SIMULATION MODEL OF THE PREDATORY BEHAVIOUR OF TROUT INTRODUCTION One of the most common observations of t r o p h i c ecology that  many animals do not e x p l o i t prey i n d i r e c t p r o p o r t i o n to  their  abundance and,  therefore,  1955;  Ivlev  Ivlev  and  is  devised  exploit  , 1961).  feed s e l e c t i v e l y  was well aware of t h i s phenomenon  the term ' e l e c t i v i t y ' to d e s c r i b e  different  (Lindstrom,  food organisms.  i n d i c e s or s i m i l a r expressions  Although  are useful  how animals  'electivity'  as d e s c r i p t i v e  statements,  they, provide no insight into, or e x p l a n a t i o n of, the mechanisms responsible are  for  identified,  generalizations  'selective' there i s  predation.  little  Until  hope of a r r i v i n g at  evolutionary b i o l o g i s t s  (i.e.  ecologists  transfer).  that  (i.e.  energy  sufficient  Batesian  process:  l)  l e a r n i n g behaviour,  could r e s u l t  These components operate process and some, animals.  such as  Prey d e t e c t i o n ,  Ivlev  both to  evidence i n the l i t e r a t u r e  prey d e t e c t i o n ,  of  mimicry) and community  the a c t i o n of any one of three b a s i c  feeding  a set  to account f o r t h i s phenomenon, which, as  (1961) pointed out, has tremendous s i g n i f i c a n c e  There i s  these mechanisms  to suggest  components of  the  2) prey h a n d l i n g , and 3)  in ' s e l e c t i v e '  at d i f f e r e n t  predation;  stages i n the  feeding  l e a r n i n g , may not be common to a l l however, i s  a fundamental  the feeding of a l l animals, except, perhaps,  filter  stage i n feeders.  Every sensory  system i s  limited in i t s  i n f o r m a t i o n and depending upon i t s toward d e t e c t i n g For example,  certain  capacity  to  receive  mode of o p e r a t i o n  is  biased  types of s i g n a l s  the process of v i s u a l  Hester,  of a t a r g e t  1968); while chemoreceptors  are  c o n c e n t r a t i o n and nature of the s t i m u l u s . 'selectively' the s t i m u l i  detect some s p e c i e s i f  emitted by p r e y .  If  stimulus').  discrimination is  s e n s i t i v e to both the s i z e and c o n t r a s t 1967;  ('adequate  highly ( l e Grand,  s e n s i t i v e to  Predators  will  they react d i f f e r e n t l y  food s e l e c t i o n  can be  to as  perceptual  (Dixon, 1959;  At some p o i n t ,  successfully  Holling,  1964)  upon prey s i z e .  has been looked at  there  this  level.  If  mechanical Finally,  alter  capture  many animals  this  prey  that  can also  can be explained on  success i t  have the c a p a c i t y  In the l e a r n i n g  a food i s <3f major  importance.  objects  I960) but w i l l  p a l a t a b l e prey  likely  can be r e f e r r e d  to  selection.  Beukema, 1968).  Prop,  food s e l e c t i o n  detect.  i n which  Therefore s e l e c t i o n  to l e a r n and thereby  t h e i r response to prey through experience  unpalatable  1961)  food i s  tends to be an optimum s i z e d  the b a s i s of d i f f e r e n t i a l as  (Ivlev,  every prey they  In the animals  can be handled most s u c c e s s f u l l y . operate at  pursue  the a b i l i t y of an animal to capture  to be dependent  can be  selection.  Most predators cannot or capture  to  explained  simply on the b a s i s of the process of d i s c r i m i n a t i o n i t referred  the  (Croze,  1970;  process the p a l a t a b i l i t y Most animals  ( H o l l i n g , 1965;  Morrell  will  of  l e a r n to  and Turner,  avoid  1970;  i n c r e a s e t h e i r responsiveness to more  (Section  II;  Beukema,  1968).  Therefore,  learning  could a l s o be r e s p o n s i b l e of  f o r the d i s p r o p o r t i o n a t e  some prey by p r e d a t o r s .  referred  to as b e h a v i o u r a l  In t h i s s e c t i o n  I will  S e l e c t i o n at  t h i s l e v e l can be  selection. examine the process of prey  d e t e c t i o n and r e c o g n i t i o n , and t e s t the hypothesis selective  exploitation  e x p l o i t a t i o n of s e v e r a l  invertebrate  that  prey  the  (especially  the amphipods) by the t r o u t p o p u l a t i o n i n Marion Lake can be e x p l a i n e d at that w i l l  Since the s i m u l a t i o n model  this  hypothesis  will  not c o n s i d e r  of hunger or l e a r n i n g , among other t h i n g s ,  it  is  intended to be a complete d e s c r i p t i o n of the predatory  behaviour of t r o u t . it  level.  be developed to t e s t  the e f f e c t s not  the p e r c e p t u a l  could be e a s i l y  The model however, was s t r u c t u r e d  so  that  modified to i n c o r p o r a t e these components  more i n f o r m a t i o n became  as  available.  METHODS AND MATERIALS'" Nine rainbow t r o u t which ranged i n l e n g t h from 11 to 14 cm.  were obtained from Marion Lake.  After  the f i s h were  to the 'laboratory a month of p r e l i m i n a r y experiments conducted to habituate  transferrer,  were  them to being handled and to c o n d i t i o n  them to respond to a r t i f i c i a l  food.  The t e s t prey were formed from p i e c e s of chicken l i v e r . Preliminary  experiments  food to be p a l a t a b l e as  i n d i c a t e d that  the f i s h considered  they would a v i d l y consume i t ;  respond to other  'less palatable'  foods  this  readily.  the experiments,  the form of the prey was s t a n d a r d i z e d  this  they would not Throughout  (rectangular)  The c h a r a c t e r i s t i c s of altered,  however,  with d i f f e r e n t altered  target  to determine  visual  c o n t r a s t and motion were  how t r o u t  properties.  simply by changing  width constant  size,  would react  to prey  The s i z e of the prey was  t h e i r l e n g t h and holding  their  (3 n.m).  The inherent c o n t r a s t of a t a r g e t  (C Q )  can be defined  as i  the d i f f e r e n c e with respect  i n luminous f l u x  to the background C o  Two l e v e l s  liver  and  (le  'black'  of Sudan Black  ('white').  'black'  Grand, 1967).  In one case,  B (water i n s o l u b l e  That  the prey r e t a i n e d  For the  the n a t u r a l  color  The inherent c o n t r a s t of both the  with a n e u t r a l  density  conditions  the background  (tank bottom)  filter.  (Photovolt,  Under the  reflected  'white'  0.3  experimental ft.-candles,  c o n t r a s t of the undyed.prey was found to be  and that of the  'black'prey,  Once the f i s h  is,  pieces of  stain).  prey was determined with a photometer  the inherent  (L )  by immersing them i n a s a t u r a t e d  model 200)  food,  by the o b j e c t  R-L* R  =  other l e v e l of c o n t r a s t of  (R)  of c o n t r a s t were examined.  l i v e r were s t a i n e d solution  reflected  0.67  0.14.  had been c o n d i t i o n e d to respond  to  artificial  the d i s t a n c e from which they would react was observed  a large rectangular plexiglass. an opaque,  tank  At one end, sliding  (180  x 16 x 30 cm),  there was a small  partition  constructed  in  of  dear  holding chamber with  that was used to i s o l a t e the  fish  before an experiment.  While the f i s h was i n the holding area  a prey of known s i z e and c o n t r a s t was placed The t r o u t  was returned  was i n t r o d u c e d .  each f i s h  A f t e r an a t t a c k was completed  to the holding area while another  Up to 10 s u c c e s s i v e a t t a c k s were  during a t e s t  The hunger l e v e l of  clear  all  the f i s h was s t a n d a r d i z e d between  T h i s was known to be s u f f i c i e n t  An a d d i t i o n a l  that  d i s t a n c e was not a f f e c t e d  the r e a c t i v e  changes i n hunger.  If  the maximum amount of the same d i s t a n c e as  s e r i e s of t e s t s  Therefore  per s e c ) .  Control  respond only i f  'white'  on r e a c t i v e  completely  meal from the demonstrated term  or  they reacted  was u n l i k e l y to have  'black'  d i s t a n c e was  affected  examined  t a r g e t s on a small  to s i m u l a t e a slow moving animal that  with the e f f e c t  d i s t a n c e and prey r e c o g n i t i o n  platform  (3 mm  the f i s h would  the p l a t f o r m was supporting dealing  from  attacked.  t e s t s demonstrated  The experiments  successive  change i n hunger  of prey motion on r e a c t i v e  that moved v e r t i c a l l y  adopting  a 48 to 72 hour p e r i o d  the s l i g h t  the d i s t a n c e from which the t r o u t  standard  by  by short  food they could i n g e s t ,  that occurred during an experiment  by p l a c i n g  between  the predators were fed up to 50% of  they would a f t e r  food d e p r i v a t i o n .  The e f f e c t  prey  recorded  time to  the food consumed during the p r e v i o u s  digestive tract.  of  the  period.  a 48 to 72 hour p e r i o d of food d e p r i v a t i o n experiments.  it  The p o s i t i o n of the food was randomized  successive t r i a l s . for  tank.  was then r e l e a s e d and the d i s t a n c e from which  would react was r e c o r d e d . predator  i n t o the  food. of background  success were  diversity  conducted  under the same c o n d i t i o n s as in t h i s  case,  a diverse  the background uuas a l t e r e d  substrate.  scattering  small,  as  the prey  of  the tank.  those d e s c r i b e d above. ('broken')  Except to  simulate  The element of d i v e r s i t y uuas created by  'black'  pebbles,  (5 mm; c o n t r a s t  the same s i z e  = 0.14),  The mean d i s t a n c e  and  contrast  uniformly over the bottom  between adjacent  pebbles uuas  i n the order of 0.5 cm. Before a predator uuas r e l e a s e d , at  random i n t o the tank.  both s t a t i o n a r y  a single  The d i s t a n c e  prey was placed  from the t r o u t  and moving t a r g e t s as w e l l  as  their ability  r e c o g n i z e prey under these c o n d i t i o n s was r e c o r d e d . in  attacked to  A failure  r e c o g n i t i o n was considered to have o c c u r r e d i f a f i s h passed  by a p o t e n t i a l therefore, that  t a r g e t without attacking*. R e c o g n i t i o n s u c c e s s ,  was defined as  the r a t i o of the number of  attacks  were i n i t i a t e d to the number of o p p o r t u n i t i e s the  had to d i s c o v e r p r e y .  These experiments  fish  were r e p l i c a t e d over  6 days to determine i f the performance of the t r o u t would improve with  experience. RESULTS  THE CHARACTERISTICS OF THE VISUAL RESPONSE OF RAINBOW TROUT TO PREY The R e l a t i o n s h i p Between Prey Size and Contrast Threshold Since the aim of describe trout,  this  section is  to develop a model to  the response of a v i s u a l p r e d a t o r ,  to prey i t would be d e s i r a b l e  therefore,  interpret  such as rainbow  to seek g e n e r a l i t y and,  the process of prey  d e t e c t i o n i n terms  of  a general  theory of v i s u a l d i s c r i m i n a t i o n .  The inherent c o n t r a s t amount of l i g h t i t when the d i s t a n c e to s c a t t e r i n g bakcground, further  of an object  reflects  with respect  between i t  contrast  will  away from the o b j e c t .  (1967) as well distance  as o t h e r s  and apparent  exponential  appear Duntley  contrast  C  (1963) and l e Grand the r e l a t i o n s h i p between  can be d e s c r i b e d  by the  negative  a  =  C  6 (X)  oe  the apparent (X),  Cq  =  the inherent c o n t r a s t  6  =  the r a t e of e x t i n c t i o n of t a r g e t  3  The a t t e n u a t i o n  contrast  coefficient  (6)  terms of the wavelengths of r e f l e c t e d wavelengths  are absorbed  of a t a r g e t at of a t a r g e t  distance  (X = 0 ) , contrast.  should be s p e c i f i e d  in  l i g h t because longer  most r a p i d l y incwater (Sverdrup  et  al,  However, the a d d i t i o n of t h i s component would add  considerable  of  Due  l i g h t by the  =  C  simply  zero.  equation:  <X>  1942).  is  to d i m i n i s h as one moves  have found that  -  where  reflected  the  to the background  and an observer  and a b s o r p t i o n of  this  was defined as  complexity to the model, t h e r e f o r e ,  i n terms of the t o t a l  (6)  a t t e n u a t i o n of l i g h t  was defined  irrespective  wavelength. The rate of a t t e n u a t i o n was determined by measuring  inherent density  contrast filter)  of the prey  the  (photometer with a n e u t r a l  under the standard  experimental  conditions  as  well  as  t h e i r apparent  Equation of  this  (l)  contrast  at a d i s t a n c e  was then solved to estimate  parameter  and unstained  of 1 meter.  (6).  The value  was found to be about 0.50  for both dyed  targets.  In the study of v i s u a l d i s c r i m i n a t i o n i t to  express the s i z e  it  subtends  is  defined  of a t a r g e t  i s conventional  i n terms of the v i s u a l  with the r e t i n a of an o b s e r v e r .  T h i s angle  S  where (TD) i s the d i s t a n c e  =  tan 8  = TD/X  the l e n g t h of diameter of a t a r g e t and (X) between i t  In the present  and an  study  the r e l a t i o n s h i p between the v i s u a l  when they were attacked  reactive  distance  distance  4 trout  contrast,  0.14)  transformed (2)  is  is  to a v i s u a l  rate  The apparent  (Ca).  The average  contrast  distance  The r e s u l t s  (6)  angle.  s i z e s of prey  the  (inherent  These data  were  (RD) for  It was a l s o p o s s i b l e  of each t a r g e t ,  because the inherent c o n t r a s t  the a t t e n u a t i o n  reactive  a visual  angle by s u b s t i t u t i n g (S).  they  if  to d i f f e r e n t  and s o l v i n g for  contrast  can be d e s c r i b e d  i n d i c a t e d i n Table 1.  estimate the apparent attacked,  expressed as  reacted  is  observer.  angle prey subtended with t r o u t and the apparent  equation  (s)  as:  (2)  presented  angle  when the  (X) i n to fish  of the prey (C Q ) and  were known.  contrast  was c a l c u l a t e d  (RD) for  (X) i n equation  of t h i s t r a n s f o r m a t i o n  by s u b s t i t u t i n g (l)  and s o l v i n g  (CT) are  the for  presented  TABLE  1.  average of  The r e l a t i o n s h i p  reactive  the b l a c k prey  when  d i s t a n c e (RD). and  size  (TD) and t h e  (CT) i s t h e a p p a r e n t  (S) i s t h e v i s u a l  t h e y were a t t a c k e d .  replicate  between p r e y  The r e s u l t s  experiments with 4 f i s h ,  angle  they  were o b t a i n e d  contrast  subtended from  (n) i n d i c a t e s t h e t o t a l  number o f o b s e r v a t i o n s .  TD  *  (m)  n  RD  (m) + 1 S.E.  CT  S (min)  .0020  20  .20  .017  .127  34  .0035  20  .30  .017  .120  40  .005  20  .32  .016  .119  54  .006  20  .37  .019  .116  56  .009  20  .44  .022  .112  70  .012  20  .49  .018  .109  84  .015  20  .52  .018  .108  100  i n h e r e n t c o n t r a s t (C  =  0.14)  i n Table  1.  Although the t r o u t distance  than s m a l l e r  (Table l ) .  responded to l a r g e  prey from a  greater  t a r g e t s the r e l a t i o n s h i p was not l i n e a r  Another way of s t a t i n g  is  that  there was an i n v e r s e r e l a t i o n between the v i s u a l angle  that  a prey subtended and i t s attacked.  This i s  apparent  (CT  . ) that min  If  the apparent  is  that  (CT) when i t  illustrated  was  in Figure  1.  f u n c t i o n , however, that are not  from the experimental data  characteristic  then i t  contrast  diagramatically  There are two l i m i t s to t h i s apparent  this observation  there i s  (Table l ) .  The f i r s t  a minimum l e v e l of  contrast  can j u s t be d i s c r i m i n a t e d by a visual animal, 1 ° contrast  of a t a r g e t does not exceed t h i s  cannot be detected  a minimum v i s u a l angle  (S  (fig.  l).  Secondly,  there i s  . ) that must be subtended  level also  before  mm a t a r g e t can be d i s c r i m i n a t e d . system 1968;  is  restricted  l e Grand, If  The performance of any v i s u a l  by these two l i m i t s of r e s o l u t i o n  (Hester,  1967).  these a p p r o p r i a t e  l i m i t s are d e f i n e d ,  the  relationship  d e p i c t e d i n F i g u r e 1 can be d e s c r i b e d by the negative  expotentual  equation: (5)  CT  In which case, contrast anqle y in  (S  v  =  (CIYl)e  L - B (ln(S))  (L) and (B) are constants  t h r e s h o l d when a prey subtends . ). min'  The r e s t r i c t i o n s  and (ClYl) i s  the minimum v i s u a l  of equation 3 were M  f i g u r e 1 and can be summarized as  the  follows:  illustrated  Figure  1.  A d i a g r a m a t i c r e p r e s e n t a t i o n of the r e l a t i o n s h i p  between the c o n t r a s t t h r e s h o l d , or the apparent  contrast  a t a r g e t must have i n order to be d i s c r i m i n a t e d , and the visual  angle i t  subtends with the eye of an o b s e r v e r .  two l i m i t s to t h i s f u n c t i o n  are  (CT . ) and (S  . );  111 JL I I  is  the minimum l e v e l  while (S target (Cffl) i s  . ) is min  of c o n t r a s t  the s m a l l e s t ,  can subtend and s t i l l the c o n t r a s t  the minimum v i s u a l Curve (A) s i m u l a t e s curve (B),  The (CT , )  I I I .1* I t  111 JL I t  that can be d e t e c t e d ,  or minimum v i s u a l angle a 3 be d i s c r i m i n a t e d .  The point  t h r e s h o l d for a t a r g e t which subtends  angle at a s p e c i f i e d l e v e l the f u n c t i o n  at a somewhat higher  of  at a low l e v e l level.  illumination. of  illumination  S min Visual  S max angle  1)  (S)  cannot be l e s s than (S  2)  (CT) cannot be l e s s than  Some a d d i t i o n a l i n f o r m a t i o n i s the constants present  study,  contrast of  of equation  r e q u i r e d , however,  can be e s t i m a t e d .  contrast  Therefore I have assumed that  that t r o u t can d i s c r i m i n a t e  (Nakamura, 1968;  Yamanouchi,  In  they represent  values  the data  (CT) i s  for t r o u t ,  is  (after  1956;  analysis  be adequately estimated  the minimum  Hester,  1968)  and  Tamura, 1957).  weremmeasured d i r e c t l y f o r  the c o n t r a s t  fish  by e x t r a p o l a t i o n .  Once t h i s  of equation  In (CM/CT) against l n ( s ) .  demonstrated  that  d e s c r i b e d by a s t r a i g h t  values of  threshold  a v i s u a l angle of 5 minutes of  the slope of the l i n e and (l_) i s  regression  is  systems.  obtained the remaining parameters  d e r i v e d by r e g r e s s i n g (8)  0.05  have been found for  regressed against In (S),  can be estimated,  each  i n Table 1 are transformed to logarithms and  (Cffl) when a t a r g e t subtends  is  that  have well developed v i s u a l If  Values f o r  i n the order of 5 minutes  Although n e i t h e r of these parameters  that  In the  however, have been reported for other animals  t h e i r minimum v i s u a l angle i s  trout,  before  angle nor the minimum  t h r e s h o l d of t r o u t were o b t a i n e d .  (Table 2 ) .  of arc  ),  (CTm^n).  n e i t h e r the minimum v i s u a l  these parameters,  that  (3)  .  (3)  arc value  can be  In which case,  the Y - i n t e r c e p t . A  the data  i n Table 1 could  l i n e ( r = 0.94  (B) and (L) are presented  ) ; the  i n Table 5.  The R e l a t i o n Between the Ambient I l l u m i n a t i o n , V i s u a l Angle and Contrast Threshold It  is  well documented that  also affects illustrated  the process of v i s u a l i n Figure 1,  between the v i s u a l of  the background i l l u m i n a t i o n  where curve  (B),  This  (A) s i m u l a t e s the  angle and c o n t r a s t  i l l u m i n a t i o n , and curve  higher l e v e l .  discrimination.  t h r e s h o l d at  the f u n c t i o n at  Thus, f o r any angle l e s s than  is  relations  a low l e v e l  a somewhat  (S  ) the  contrast  max required for d i s c r i m i n a t i o n w i l l illumination  is  raised.  decrease i f  T h i s means that  the  a visual  be able to detect prey from a g r e a t e r d i s t a n c e at of  illumination.  a specified ambient  range.  This r e l a t i o n s h i p , Before  however,  any o p t i c a l  background predator higher  only holds  will levels  over  system can f u n c t i o n  i l l u m i n a t i o n must surpass some lower t h r e s h o l d  the  (^mj_n)«  On the other hand, once the i l l u m i n a t i o n reaches some upper level  ( r ' m a x ) the system w i l l  perform maximally.  i n c r e a s e i n the background illumination. : w i l l performance. levels are  Several  of i l l u m i n a t i o n that  presented  i n Table  The i n f l u e n c e of its  reported  effect  not improve  this  estimates of the upper and lower  affect  the v i s u a l  a c u i t y of  fish  2. i l l u m i n a t i o n can be d e s c r i b e d  on the component  (1967) have shown that  Any f u r t h e r  (CM).  Hester  (CM) d i m i n i s h e s at  through  (1968) and l e Grand a decreasing  rate i f  the background i l l u m i n a t i o n i s r a i s e d from the l i m i t of s c o t o p i c v i s i o n (R . ) to (R ). T h i s r e l a t i o n s h i pK can be approximated v mm' max by the negative e x p o n e n t i a l  equation:  TABLE  2.  A comparison of s e v e r a l  documented values of  d e t e c t a b l e contrast  (CT . ) and the minimum v i s u a l  different  (R • ) is v min'  is  animals.  the lowest  l e v e l of  the l i m i t  the minimum  angle (S  . ) of  of s c o t o p i c v i s i o n  and (R  i l l u m i n a t i o n which produces maximum  visual  ) max'  acuity.  XI)  R . min (b)  0;5  30  3.0  X  IO"7  l e Grand,  20.0  10  1.0  X  IO"2  Hester,  1.0  X  ID"  Blaxter,  1.0  X  io"  4  Ali,  1.0  X  io"  4  Blaxter,  Animal  CT . min  S . min (a)  Human  0.01  Goldfish  0.05  25.0  Herring Salmon  R  1 -• 10  (6sp;)  Plaice Marine T e l e o s t s  -  mackerel  a  angle expressed i n minutes of  b  the l e v e l  of  4  15.0 5.0  S k i p j a c k Tuna Jack  4.0  Source  illumination  in  10  1.0  X  IO"3  Nakamura,  12  1.0  X  io"  Hunter,  arc. ft-candles  1968 1968 b  1959  Tamura,  7  1967  1968 1957 1968 1968  a  CM =  ( 4 )  where (K^) i s when i t  the c o n t r a s t  subtends  illumination is (Rm^n);  K1  "  A  (  R  )  a prey must have to be attacked  the minimum v i s u a l angle and the ambient at  (A) i s  e  the p r e d a t o r ' s  scotopic l i m i t  simply a r a t e constant and (R) i s  of  vision  the l e v e l  of  illumination. The value of data for  (Table 5 ) . trout,  it  Although ( l ^ )  at  ft-candles) Equation  single  the standard l e v e l into  (4)  (R  =  (4).  can now be coupled with  i l l u m i n a t i o n and prey s i z e on the apparent r e q u i r e to d i s c r i m i n a t e a t a r g e t .  CT.., =  the value of  of i l l u m i n a t i o n  expression which i n c l u d e s the e f f e c t  ( 5 )  (1968)  was not s p e c i f i c a l l y measured  can be estimated by s u b s t i t u t i n g  (CM) (Table 5 ) 0.03  (A) was c a l c u l a t e d from H e s t e r ' s  K  e  (3)  to o b t a i n a  of both the ambient contrast  The r e s u l t  trout  is,  -[A R] + [L - B ( In  (S))]  The i n c l u s i o n of ambient i l l u m i n a t i o n , h o w e v e r , imposes two additional  restraints  on equation ( 5 )  mentioned i n c o n j u n c t i o n with 3)  a target (R . ) .  4) '  K, reaches 1  min'  other than those  already  (3):  cannot be detected i f  (R) i s  l e s s than  a minimum value when (R) = (R  max  ).  Now that the r e l a t i o n s h i p between the i l l u m i n a t i o n , visual it  is  angle and the c o n t r a s t necessary  the  t h r e s h o l d has been d e s c r i b e d ,  to convert t h i s expression  i n t o the average  distance trout w i l l react to d i f f e r e n t sizes of prey. transformation  This  can be accomplished by i t e r a t i v e l y solving  equations ( l ) and  ( 5 ) u n t i l a distance  s a t i s f i e s the equality (C  =  CT).  (X) i s found which  In the model, a computer  a program was  designed to undertake this  Figure 2 presents distance  operation.  a comparison between the actual reactive  (Table l ) and the distance that was  case, by i t e r a t i v e l y solving ( l ) and  generated in each  ( 5 ) . The close agreement  between the observed and calculated distance of attack indicates that the equations that have been developed do describe  the  experimental results reasonably well and that l i t t l e accuracy has been lost by transforming  the o r i g i n a l data.  The  other  point i l l u s t r a t e d in Figure 2 i s that the distance of reaction i s tending  towards a maximum.  The upper l i m i t to the attack  distance w i l l occur when the apparent contrast of the target is equal to the minimum level of contrast the animal can discriminate (CT  . ) .  min  The Effect of Prey Movement on Reactive  Distance  It has been shown that the distance from which'trout w i l l attack depends upon several c h a r a c t e r i s t i c s of a prey as well as the environment.  The equations that were develqped  to describe these effects were based upon the reaction of trout to stationary prey. was  Therefore  an additional series of experiments  conducted to determine i f the predators  d i f f e r e n t l y to moving prey.  would respond  The prey in this case had a high  Figure  2.  distance  A comparison between the observed (data p o i n t s )  s i z e s of prey, (curve). (1)  and the c a l c u l a t e d  The l a t t e r  and ( 5 ) .  of 4 t r o u t ,  exposed reactive  bars.  to  See text  for  further  different  distance  uuas obtained by s o l v i n g  equations  explanation.  95$ confidence l i m i t s of the means are vertical  reactive  The  i n d i c a t e d by the  TTT  Target  length (mm)  Figure  3.  The e f f e c t  distance. reactive different solid  of t a r g e t movement on  The open c i r c l e s distance sizes  circles  (4 f i s h )  (inherent  indicate  the average  for stationary contrast  reactive  prey of  = 0.67).  shouu the average r e a c t i v e  The  distance  for moving prey of the same s i z e and c o n t r a s t . 95% confidence  l i m i t s of the means are  The  indicated.  E o  140120H  CD  £ 10003 •+-> 80CO CD > -t—•  O 03 CD  DC  60-  j.  2  T  2  T O  T  O  4020«  1——1  5 10 T a r g e t l e n g t h (mm)  level  of c o n t r a s t  (0.67).  In the f i r s t  experiment,  prey of d i f f e r e n t they would react  sizes,  4 t r o u t were exposed  and the average d i s t a n c e  was r e c o r d e d .  Once these t r i a l s  to  stationary  from which were completed  the f i s h were switched to moving prey with the same l e v e l of contrast. attack  The r e s u l t s  (fig.  3)  c l e a r l y show that  moving prey from a s i g n i f i c a n t l y g r e a t e r  stationary  o b j e c t s with the same v i s u a l  over the range i n s i z e  trout  distance  properties.  that was i n v e s t i g a t e d ,  will than  At l e a s t  the e f f e c t  of  t a r g e t motion was a d d i t i v e because the t r o u t would react  to  moving prey 22 cm f u r t h e r away than they would to  stationary  t a r g e t s of the same s i z e . If object  the d i s t a n c e  of a given s i z e  then the e f f e c t expressed  and c o n t r a s t  (j)  attack  is  of motion on the d i s t a n c e  a  stationary  defined as  (^j)>  of r e a c t i o n can be  as:  (6)  where,  from which t r o u t w i l l  (MC) i s  R'. J  =  + IY1C  the increment e f f e c t  For the purposes additive effect  R. J  of t h i s paper,  of motion i s  the t a r g e t and the background  of motion. I will  assume that  the  independent of the v e l o c i t y of illumination.  The R e l a t i o n Between the Background, R e a c t i v e and Prey R e c o g n i t i o n Success When a prey was the only object  Distance  i n the tank and was  contrasted  against a f l a t ,  evenly i l l u m i n a t e d surface  the t r o u t mere 100$ s u c c e s s f u l of  whether i t  was s t a t i o n a r y  was d i v e r s i f i e d  ('broken')  in recognizing i t or moving.  regardless  However, i f  i n the sense that  ('smooth')  the background  other s i m i l a r  but  non-prey o b j e c t s were s c a t t e r e d over the s u r f a c e to break up the u n i f o r m i t y , then the t r o u t ' s  ability  recognize prey might be somewhat  impared.  might a l s o a l t e r possibilities Hyalella,  the r e a c t i v e  distance.  are worth i n v e s t i g a t i n g  as well  as  to d i s c r i m i n a t e  A 'broken' surface Both of  these  because Cranqonyx and  other i n v e r t e b r a t e s  i n Marion Lake,  exceedingly  c r y p t i c and l i v e i n a s s o c i a t i o n  background,  the  with a very  demonstrate that  have i n d i s c r i m i n a t i n g p r e y .  and moving t a r g e t s ,  <in r e c o g n i z i n g moving p r e y .  a later  was not the only change  when the background i s  as  The e f f e c t  be i n c o r p o r a t e d  a reduction in recognition  that  occurred,  was a l s o diminished by a f a c t o r  contrast  considerably of  the  into  the  stage.  In these experiments,  apparent  in discovering  although they were  background on r e c o g n i t i o n success w i l l model at  which  Under these  the f i s h were l e s s than 100$ s u c c e s s f u l  more s u c c e s s f u l  diverse  the presence of other o b j e c t s can d i m i n i s h  the success t r o u t  both s t a t i o n a r y  are  sediment.  Table 3 presents the r e s u l t s of some experiments  conditions,  or  a proportionality  of about  'broken' trout  before  they w i l l  constant  the r e a c t i v e  success  distance  4 (Table 4 ) .  Apparently,  r e q u i r e a higher l e v e l of  attack.  If  (E)  which d e s c r i b e s the  is  defined  distance  TABLE  3.  probability  The e f f e c t that  of background d i v e r s i t y on the  trout u/ill  successfully  a 5 mm prey ( i n h e r e n t c o n t r a s t the number of r e p l i c a t e  = 0.14).  experiments  recognize (n)  indicates  (4 f i s h ) .  confidence i n t e r v a l s of the means are  The 95%  presented.  Target  n  X P r o b a b i l i t y of recognition  Stationary  8  0.39 _+ 0.12  Moving  8  0.74 _+ 0.25  TABLE  4.  The e f f e c t  of the background on r e a c t i v e  In each case,  the prey were 5 mm and had an inherent  of 0 . 1 4 .  indicates  (n)  from 4 t r o u t ; distance in  (E)  is  the number of o b s e r v a t i o n s  the p r o p o r t i o n a l d i f f e r e n c e  i n a ' b r o k e n ' environment uiith respect  distance. contrast obtained  i n the to the  reactive distance  a 'smooth' environment.  Target  Background  n  Mean Reactive Distance (cm)  Stationary  'smooth'  44  35.0 _+ 2.0  Stationary  'broken'  52  8 .0 +_ 1.9 E = 0.23  *  the 95% confidence i n t e r v a l s of the means are  indicated.  from which t r o u t w i l l  react  i n a ' b r o k e n ' environment with  respect to t h e i r response when prey are 'smooth'  background,  then equation  express the d i s t a n c e (RDj) as  of  =  (R.)E,  for s t a t i o n a r y  (B)  RD.  =  (R.)E,  if  prey s i z e ,  very s u p e r f i c i a l l y .  the e f f e c t  as well  as  Ideally,  of the background i s  investigate  the f u n c t i o n a l  however, the estimated  considered to approximate  the  reactive one  independent relationship  degrees of complexity and the d i s t a n c e  For this study,  The experimental  prey and,  the prey i s moving.  between the background and  has been t r e a t e d  reaction.  of  in a ' b r o k e n ' environment  RD.  between d i f f e r e n t  is  can be modified to  (7)  should determine i f of  against a  follows:  The r e l a t i o n s h i p distance  reaction  (6)  contrasted  value of  of (E)  the c o n d i t i o n s i n Marion Lake.  background attempted  to s i m u l a t e  the  diversity  sediment.  The E f f e c t of Prey A c t i v i t y and the Searching P o s i t i o n on the Width of the Path of Search Trout, almost  l i k e most t e l e o s t s ,  encompasses a f u l l  the d e n s i t y of cones  have a f i e l d of v i s i o n  360',degrees.  i n the r e t i n a i s  In most  that  fishes,however,  not completely uniform  therefore  the v i s u a l  (Tamura, that it  1957;  field  Hester,  the v i s u a l  field  is  not i n fact  1968).  Nevertheless,  can be d e s c r i b e d as  has been shown that  of a prey,  reactive  field  must be q u a l i f i e d .  prey  there  is  it  is  responsive  defined  as  a spherical  distance  assume  a s o l i d sphere. is  the dimensions of  about  for  a predator  The radius or ( R D . ) .  Since  dependent upon  In other words,  field  to t h i s o b j e c t .  the r e a c t i v e  I will  the r e a c t i v e d i s t a n c e  the v i s u a l c h a r a c t e r i s t i c s  (j)  completely s p h e r i c a l  of t h i s  the every  i n whibh field  is  At the moment I  J  am only c o n s i d e r i n g s t a t i o n a r y  prey.  Rainbow t r o u t c h a r a c t e r i s t i c a l l y  adopt  a searching  position  some 10 to 15 cm above the sediment when they hunt f o r b e n t h i c food organisms.  Although t h i s may not appear  I suggested i n another study might i n fact  c r e a t e a refufe  of  the p r e d a t o r ' s  it  requires  I)  that  f o r some p r e y .  searching p o s i t i o n ever  For i f  zero to  (fig.  phenomenon.  sweeps along the bottom w i l l 4).  distance  animals w i l l  a predator moves f u r t h e r away from the sediment the path i t  the height  This i n f e r e n c e , however, i s only  c o n d i t i o n . o f a more general  Once t h i s occurs prey ( j )  significant  t h i s behaviour  exceeds the  to d i s c r i m i n a t e prey then those  i n v u l n e r a b l e to a t t a c k . limiting  (Section  to be very  be the  That i s ,  as  the width of  d i m i n i s h and approach will  be i n v u l n e r a b l e  attack. Since we are only  trout w i l l living  effectively  animals,  interested  i n the amount of sediment a  search when i t  is  hunting for bottom  the width of t h i s path ( E P . ) w i l l  be determined  ure of  4.  The geometric  the r e a c t i v e  position  (SP)  field  r e l a t i o n s h i p between the (RDj),  and the e f f e c t i v e  path along the sediment is  the t r o u t ' s  (EPj).  radius  searching  width of t h e i r  searching  The p o s i t i o n of the  simulated by the s o l i d c i r c l e i n the center of  reactive  field.  In order to be a t t a c k e d ,  must be w i t h i n the path of  search.  a prey  fish  the (j)  by the simple geometric  r e l a t i o n s h i p between the radius of  reactive  field  (^Dj)  position  (SP)  (fig.  (9)  FR . J  =  ( RD,2  EP .  =  2 FR.  EP.  =  2 (RD  since then,  .(10)  a r ,  d the height of the p r e d a t o r ' s  4).  That  The e f f e c t model at  -  SP 2 ) ^  -  2  SP 2 )  7  J  of prey motion can be i n c o r p o r a t e d i n t o the  t h i s point i n a r a t h e r simple way by weighting the  r a d i u s of the r e a c t i v e prey that  searching  is,  J  3  the  are a c t i v e  field  (PA.).  according to the p r o p o r t i o n of In other words,  if  (RD.) and  (RDj) are r e s p e c t i v e l y ,  the r a d i i of the r e a c t i v e  moving  and s t a t i o n a r y  prey,  reactive  field  for a prey  RD.j  =  [(PA.  (11)  EP. .  (j)  field  then the average radius of s p e c i e s  (ij  ) ( RD? ) ]  *  [(  2 (RD.  SP 2 )  is  for  of the  given by:  1-PA. )  ( RDj )]  hence,  To summarize,  =  the path a t r o u t w i l l different  distance this  prey.  submodel  section, are:  -  a s e r i e s of eguations  to account for the e f f e c t  for  2  of s e v e r a l  2  have been developed  variables  on the width of  sweep along the sediment when i t These components comprise the  (fig.  5).  The major eguations  searches  reactive  derived in  ure  5.  A schematic  computational designated  steps i n the attack  model.  and  The components  by (A) comprise the prey v u l n e r a b i l i t y submodel;  those designated submodel.  flow diagram of the parameters  by (B)  The s u b s c r i p t  comprise the r e a c t i v e (j)  refers  defined  according to t h e i r s i z e  species  (i).  The parameter  distance  to a c l a s s of prey,  and i n h e r e n t c o n t r a s t ,  names are l i s t e d  of  i n Appendix  I.  B  T  DCi) VDCi)  PCip VD  Ci.p  4 RECi.j)  V Y I  1'  RAC i j ) R C Ci , p  2 J  RCCi)[  CSCi )  indicates  an e s t i m a t e d  (R)  the value  for  trout  chosen  (after  Parameter  value.  In t h e c a s e o f t h e i l l u m i n a t i o n  represents the estimated  v a l u e o f (R ) max  A l i , 1959).  Cranqonyx  General  Hyalella  Source  A  0.356  Hester,  B  0.210  Table 1  C  0.14  o  0.14  1968  0.14  cm  0.22  es  0.84  Section  I  CT min  0.05  Hester,  1968  E  0.23  Table 4  0.25  *  0.19  Table 1  K  l  L 1*13  0.138  0.180  Section I  W4  4.63  5.13  Section I  W5  0.39  0.01  Section I  M6  0.069  0.039  Section I  M7  0.0033  0.0017  Section I 3  me  0.22  Figure  R min  0.001  Ali,  R  1.0  RS  0.39  Table  3  RS'  0.74  Table  3  0.91  Hargraves,  5.0  Table  0.10  Section  (Lake)  6  min SP VY  10.0  1959  2 I  1969  the a t t e n u a t i o n (1)  C a  =  C o  the apparent (5)  The  CT  =  contrast  r e q u i r e d to e l i c i t  Kx e - t A >  ]  R  d i s t a n c e (X) which s a t i s f i e s  be found by s o l v i n g as  of prey c o n t r a s t , - (6) X e  the r e a c t i v e  contrast  (j).  (6)  reaction  if  (Rj)  f°  L  r  R!  +  J  the background i s  =  CT) can  then defined  s t a t i o n a r y .'prey, .of  a  of prey motion i s  R.  (C  This d i s t a n c e i s  The e f f e c t  •  BUn(S))]  L-  the e q u a l i t y  and (5).  distance  J  However,  (lj  +  an a t t a c k ,  s i z e and  given by,  MC.  broken,  then the d i s t a n c e of  is:  (7)  HD.  =  (Rj)E,  if  the prey i s  (8;  RDj  =  (Rj'jE,  if  the prey i s  stationary  or  Finally,  moving.  the width of the path of search was expressed  (11)  EPij.  =  2 (RDij  -  as  SP 2 ) 2  THE ATTACK MODEL Holling the  (1966) has shown that  rate predators capture  prey,  4 b a s i c components  they  are:  determine  1)  the d e n s i t y of v u l n e r a b l e prey,  2)  the width of the path of  3)  prey r e c o g n i t i o n and capture  4)  the p r e d a t o r ' s  The seasonal  changes i n the d e n s i t y of III)  section  as  (Section  Although t h i s field that  I)  the attack model i s  verified  These o b s e r v a t i o n s , shift  sporadic  of search  trout  sec.  is  is  trout  that  is  only a r e l a t i v e l y  prey,  small  i n d i c a t e d that  this  of an hour  successfully  p r o p o r t i o n of animals  component with respect  of  of  sediment.  the average v e l o c i t y -  w i t h i n the path they sweep along the sediment.  this  freguently  specific  The r e s u l t  i n the course  trout w i l l  estimate.  trout  assumed to be reasonable  success i n r e c o g n i z i n g prey (RS^)  situation,  not an unreasonable  Depending upon the type of background and the a c t i v i t y of d i f f e r e n t  4 cm/sec.  search only 10 l i n e a r meters  it  be  do search slowly and  a r a t h e r crude estimate of  i n the f i e l d  search  food was determined  i n an a r t i f i c i a l  for any l e n g t h of time.  hunting behaviour i s  Although t h i s  for  p o s i t i o n and do not maintain a  a f i s h may e f f e c t i v e l y  of  that  however, a l s o  their vertical  pattern  search  was measured  a v e l o c i t y of 4 cm /  amphipods  synthesized.  and was found to average about  parameter  observations  vulnerable  the remaining aspects w i l l  The v e l o c i t y at which t r o u t earlier  and  and the width of the path of  have a l r e a d y been c o n s i d e r e d ; in this  success,  velocity.  in Marion Lake ( S e c t i o n  treated  search,  that  value.  relative discover are  Their  can be estimated  actually average  by weighting  to the p r o p o r t i o n of animals  that  are  active  ( P A ^ ) and the a b i l i t y of t r o u t  moving and non-moving p r e y .  (14)  RS. =  where (RS 1 ) a predator (Table  and (RS) will  are  capture  In which case,  (RS')j  This  fragment  the p r o b a b i l i t i e s  that  target  approached 1966).  i n approaching both H y a l e l l a the s t r i k e s  (Section  is  I).  predators  This  Messenger, 1959;  are  slow moving, t h e r e f o r e ,  100%  capture  prey  observation  prey tend to be very s u c c e s s f u l animals  faster  (Holling,  moving t a r g e t s  l e s s than 15 mm i n l e n g t h and are the component of  the r a t e of prey capture  the prey v u l n e r a b i l i t y ( S e c t i o n with the s e a r c h i n g  in  In Marion Lake, most of the b e n t h i c -  be added to the a t t a c k model and t r e a t e d  submodels  striking  Trout are  with the general  but not n e c e s s a r i l y  Braum, 1967).  invertebrates  to w i t h i n  slow moving, or s t a t i o n a r y  1968)  is  and Crangonyx, and on  they attempt  consistent  which pursue  subduing r e l a t i v e l y  (Dixon,  to be c o n s i d e r e d and that  (Holling,  the average 84% of  case,  (RS)]  r e p r e s e n t s the p r o b a b i l i t y  be s u c c e s s f u l l y  and then captured  successful  living  [(1-PA.)  respectively,  component remains  a prey w i l l  1966;  +  recognize a moving and s t a t i o n a r y  success.  distance  that  recognize both  3).  One f i n a l  that  [(PA.)  to  capture as  fairly  success can  a constant.  In which  (RC^j) can be d e r i v e d by combining III)  velocity  and r e a c t i v e  distance  (VY), prey r e c o g n i t i o n  (RS^)  and capture  success  RC. . = PEP.  (12)  This step completes  (CS) components.  .  (VY)  UN.  the attack  The r e s u l t  is,  . IRS. CS model.  As i t  stands,  it  is  not a complete d e s c r i p t i o n of the p r e d a t i o n process because  it  does not c o n s i d e r the amount of time t r o u t  food,  the e f f e c t  of hunger m o t i v a t i o n , or l e a r n i n g .  aspects have been shown to a f f e c t (Section is  I and I I ) .  to t e s t  spend handling  Nevertheless,  the concept that  amphipods,as  well  as  explained at  the p e r c e p t u a l  accomplishes  t h i s aim.  t h e i r feeding  behaviour  the purpose of t h i s  the s e l e c t i v e  several  These  section  e x p l o i t a t i o n of  other i n v e r t e b r a t e s , c a n l e v e l . • T h i s model  be  subsequently  APPLICATIONS OF THE MODEL The s e l e c t i v e E x p l o i t a t i o n of the Amphipods, Odonates, P l a n o r b i d s and C a d d i s . A previous examination of f i s h stomachs Tsumara,  unpublished data)  the year t r o u t  feed  Four major groups, (principally and  i n d i c a t e d that  extensively  throughout most of  upon b e n t h i c  the amphipods,  ( E f f o r d and  invertebrates.  the odonates,  the  Banksiola c r o t c h i ) and the p l a n o r b i d s  Helisoma), account f o r about  input to the t r o u t p o p u l a t i o n .  60% of the t o t a l These four groups  caddis (Menetus  energy can  therefore  be s i n g l e d out as average d e n s i t y the amphipods,  the most  important p r e y .  Some data on the  and s i z e of each of these animals, are b r i e f l y summarized for s e v e r a l  other than selected  months i n Table 6 . Although there their vertical that  is  activity,  planorbids,  similar  tend to l i v e at  on the o t h e r hand, w i l l  the mud-water  the p l a n o r b i d s  (Delury, personal  have a v e r t i c a l  the caddis  The  is  somewhat  communication).  limited observations,  to H y a l e l l a , but that  remain exposed.  Neither of  move below the mud-water  and may have an a c t i v i t y p a t t e r n that  Guided by these rather  that  some evidence which suggests  appear to burrow i n t o the sediment.  to the amphipods  identical  is  i n f o r m a t i o n concerning  or can be found i n areas of v e g e t a t i o n .  these i n v e r t e b r a t e s  interface  quantitative  there  the odonates and caddis  interface,  that  little  I have assumed  a c t i v i t y pattern  that  the odonates and caddis  is  always  I have a l s o assumed (on the b a s i s of some data) spend most of the time moving, while the  and odonates are r e l a t i v e l y  planorbids  inactive.  The eguations developed above were t r a n s c r i b e d  into Fortran  and a computer s i m u l a t i o n was conducted to p r e d i c t the rate of  these prey could be captured by t r o u t .  then compared with the a c t u a l observed  p a t t e r n of  during the months of February,  November.  These r e s u l t s were exploitation  May, June,  that  was  August and  These were the only months i n which stomach samples  were taken from the t r o u t unpublished  each  data).  population  ( E f f o r d and Tsumara,  TABLE  6.  caddis an  The p o p u l a t i o n  and p l a n o r b i d s  'average '  (ML) i s  year.  characteristics  for several  (MD) i s  of the odonates,  selected  tiean id e n s i t y  the  i  are  (no./sq.  m.),  the instantaneous  the mean l e n g t h (mm ) and (PA) i s  p r o p o r t i o n of prey that  months during  active.  Prey Group Month  Odonates  Caddis  (1)  Planorbids  (2)  MD  ML  PA  1  40  4  *  10  1  63  5  *  4  10  1  63  5  *  0  1  10  1  57  5  *  0  7  10  1  50  4  *  MD  ML  PA  MD  ML  PA  Feb  10  10  0  9  7  May  4  12  0  4  June  5  10  0  Aug  4  8  Nov  5  11  *  assumed to f o l l o w the same a c t i v i t y  pattern  as  Data Sources (1)  Pearlstone  (2)  Ulinterbourn (pers,  (3)  Lee  (1967)  (3)  (pers,  com. ); com.)  Hamilton (1965).  Hyalella.  In the s i m u l a t i o n the water  temperature  d e n s i t y and average s i z e of each prey III;  remainder,  are  (amphipods,  each of these v a r i a b l e s  The other parameter  summarized i n Table 5.  Appendix  values  the  during the p e r i o d  r e q u i r e d i n the model  At each time i n t e r v a l the model  simulated  the rate t r o u t  could capture  searching  for one hour.  The p r e d i c t e d occurence of  organisms  (expressed as  a percentage) was c a l c u l a t e d  these  II),  Table 6) were changed to correspond to  average c o n d i t i o n of in question.  (Appendix  different  prey by food from  results.  Table 7 A presents a comparison between the e x p l o i t a t i o n and an expected the premise  that  trout  t h e i r abundance.  d i s t r i b u t i o n that  capture  There i s  prey i n d i r e c t  little  doubt that  observed is  based upon  p r o p o r t i o n to there i s an  extremely poor c o r r e l a t i o n between the observed  and  distributions,  caddis.  Therefore,  at  was feeding  especially least  f o r the amphipods and  during these months,  is  considerably  better  was more accurate  dispersal  is  that  shown i n Tablse 7 B. agreement  occurence of p r e y .  and caddis  population  selectively.  The p a t t e r n of e x p l o i t a t i o n simulation  the t r o u t  expected  that  was p r e d i c t e d by the  In t h i s  case there  between the observed  Part of the e x p l a n a t i o n  and p r e d i c t e d  why the s i m u l a t i o n  i n accounting for the frequency of it  behaviour,  is  took i n t o c o n s i d e r a t i o n  their  amphipods vertical  t h e i r average s i z e and a c t i v i t y .  TABLE  7 A.  A comparison between the expected  percentage occurence of d i f f e r e n t The expected exploited observed  p r o p o r t i o n to i t s  d i s t r i b u t i o n of  Odonates  9  8  0.6  14  7  9  0.2  39  3  25  0.3  4  3  34  5  7  5  90  10  0.6  7  38  96  16  0.1  20  96  51  0.4  47  May  49  90  June  70  Aug Nov  57%  0  a  Mean d e v i a t i o n from  E  31%  a  observed  Planorbids  3  0.8  95  the  38  7  7  is  E  0.8  Feb  (0)  0  E  E  each prey was  density.  0  0  stomachs.  prey.  Caddis  Amphipods  Month  prey groups i n t r o u t  (E) d i s t r i b u t i o n assumes that  in direct  and observ/ed  10%  a  16%  a  TABLE  7 B.  predicted in trout  A comparison between the observed  (P) percentage occurence of d i f f e r e n t stomachs.  (o)  and  prey groups  The p r e d i c t e d d i s t r i b u t i o n was  generated  from the s i m u l a t i o n model.  Amphipods  Caddis  Odonates  Planorbids  0  P  0  P  0  P  0  P  Feb  7  21  47  48  7  30  38  1  May  49  50  34  30  7  17  9  2  June  70  70  10  16,  7  9  14  4  Aug  38  94  16  2  9  2  39  22  Nov  20  22  51  56  25  20  4  Month  14%  a  a  Mean d e v i a t i o n from  6%  a  observed  9%  2  a 18%  a  For example, great deal exposed.  the amphipods are  moving;  These c h a r a c t e r i s t i c s to a t t a c k .  spend a  totally hence,  exposed  will  density.  In t h i s  for the occurence of case,  it  is  that  trout  can not be o v e r r u l e d  detail.  (Section  II).  detect  Nevertheless,  that  important  In a d d i t i o n ,  l e a r n to s e l e c t i v e l y  the  probable  and h o r i z o n t a l a c t i v i t y of t h i s group i s  and shouOia be examined i n f u r t h e r possibility  are  they are more v u l n e r a b l e than one would expect  very a c c u r a t e l y .  the v e r t i c a l  to lower t h e i r  and tend to spend most of the time a c t i v e l y  The s i m u l a t i o n d i d not account planorbids  tend  a c t i v e when  On the other hand, the caddis  simply on the b a s i s of t h e i r  is  small,  of time concealed and are only moderately  vulnerability large,  relatively  the  planorbids since  there  c o n s i d e r a b l y more i n f o r m a t i o n concerning the behaviour of  Cranqonyx and H y a l e l l a the e x p l o i t a t i o n of these  populations  can be explored i n more d e t a i l . The Size There are of  Selective  E x p l o i t a t i o n of Amphipods  three s p e c i f i c  characteristics  e x p l o i t a t i o n of amphipods by the t r o u t  Lake.  The f i r s t ,  is  that  different  Secondly,  changes s e a s o n a l l y ,  p o p u l a t i o n i n Marion  p r o p o r t i o n to  even though H y a l e l l a i s  more numerous than Cranqonyx, the l a t t e r more f r e q u e n t l y .  Finally,  pattern  s i z e c a t e g o r i e s of H y a l e l l a  and Cranqonyx are not consumed i n d i r e c t abundance.  to the  is  the e x p l o i t a t i o n  their  about 7 times  captured  slightly  of both s p e c i e s  becoming more pronounced in the summer and  falling  to a lower lev/el i n the s p r i n g and l a t e f a l l .  question and i f  is,  so,  can the model account f o r any of these  what are the major f a c t o r s  year was d i v i d e d i n t o 24 two week i n t e r v a l s . each p e r i o d the ambient water  the d e n s i t y of both s p e c i e s (considered  temperature  (Appendix III)  i n Table  In a d d i t i o n ,  amphipods.  not l i k e l y  and the s i z e composition  should d i s p r o p o r t i o n a t e l y  captured  trout  some small  size  these two c h a r a c t e r i s t i c s  to attack others  stomachs  were taken.  because the g r e a t e s t  indicate  the number of animals  In t h i s  each s i z e c l a s s was e x p l o i t e d  figure, that  classes  suggest are  size. s i z e c l a s s e s of  i n the months of June and November.  were s e l e c t e d  search  a t t a c k l a r g e prey but  below a c r i t i c a l  periods  searching  i n d i v i d u a l s of the same s p e c i e s .  they may not detect  Therefore,  parameter  5.  F i g u r e 6 shows the frequency -"of d i f f e r e n t Hyalella  follow  The other  depending upon the average d i s t a n c e  from the sediment  II),  l a r g e prey should be more  v u l n e r a b l e to attack than smaller  trout  (Appendix  r e l a t i o n s h i p between the width of the  path and prey s i z e i m p l i e s that  that  'average'  At the beginning  these parameters.  f o r the model are l i s t e d  The p o s i t i v e  of  an  i n s i z e c l a s s e s of 1 mm) were changed to  the average trend i n each of values  observations  involved?  In the s i m u l a t i o n conducted for amphipods,  of  The  number of  the expected  These trout  curves  should have been found i f  i n p r o p o r t i o n to i t s  abundance.  The observed  d i s t r i b u t i o n demonstrates that  were captured more f r e q u e n t l y animals  less  frequency  calculated  number of H y a l e l l a that  In both months  (fig.  between the observed that  found in t r o u t  The p r e d i c t e d actual  each  size  captured. 6) there was a s i g n i f i c a n t distributions  the t r o u t were s e l e c t i v e l y  Hyalel l a l e s s  difference  (X>0.05) which  exploiting  On the other hand, the  stomachs  and that  by m u l t i p l y i n g the  and expected  s i z e c l a s s e s of prey.. p r e d i c t e d that  all.  were found by the r a t i o  c l a s s was p r e d i c t e d to be  indicates  than one would expect  than 3 mm were not found at  curves were  large Hyalella  different  simulation  than 3 mm should not have been  and was able to account  for  the  occurence of other s i z e c l a s s e s of prey to the extent observed  and p r e d i c t e d d i s t r u b t i o n s  are not  that  significantly  different. A s i m i l a r comparison of the observed, s i z e composition of Cranqonyx i s this  case,  c l o s e as different that  the f i t  H y a l e l l a but i s  from the observed.  2 mm was the s m a l l e s t this  presented  i n Table 8.  of the p r e d i c t e d d i s t r i b u t i o n i s  i t was for  could d e t e c t ,  p r e d i c t e d and  still  not  supposition  In  not  as  significantly  Although the s i m u l a t i o n  s i z e c l a s s of  expected  predicted  Cranqonyx that  trout  could not be t e s t e d because  during the months i n which stomach samples were taken,  all  the  Cranqonyx i n Marion Lake were g r e a t e r than 2 mm i n l e n g t h . In any case,  it  appears as  prey s i z e and r e a c t i v e  distance,  i f the r e l a t i o n s h i p as well  as  between  the propensity of  ure  6. A comparison of the observed  expected  (solid  triangles),  circles)  d i s t r i b u t i o n of d i f f e r e n t  Hyalella  found i n t r o u t  sampling p e r i o d s .  circles  and the p r e d i c t e d  stomachs  See text  (solid  at  (open  s i z e c l a s s e s of two  for further  different explanation  TABLE  8.  A comparison of the f i t  predicted  (P) and a c t u a l  found i n t r o u t expected  their  different  respective  (E),  s i z e composition of Cranqonyx  stomachs i n the month of November.  number i s  capturing  (0)  between the expected  based upon the assumption size classes  in d i r e c t  field densities.  that  The trout  were  p r o p o r t i o n to  The p r e d i c t e d  number  is  based upon the r e s u l t s of a s i m u l a t i o n .  Size class (mm)  *  Number (0)  Number (P)  Number (E)  4  0  0  0  5  2  1  1  6  2  9  11  7  10  10  11  8  12  8  7  9  9  6  6  10  2  4  3  11  4  2  1  Significantly  Chi-sguared  ( 0 - P )  =  7.4  Chi-squared  ( 0 - E )  =  11.2  different  at 0.05  level  trout  to maintain a s e a r c h i n g p o s i t i o n are  to account for t h e i r s e l e c t i v e and  the existence  can  detect.  Seasonal  sufficient  e x p l o i t a t i o n of l a r g e  of a t h r e s h o l d or minimum s i z e  Changes i n the E x p l o i t a t i o n of Amphipods  times of the year,  by searching  for one hour,  This s i m u l a t i o n shows that changes  abundance.  Cranqonyx reaches  For example,  the end of June (Appendix I I I ) ,  is  e a r l y i n September.  most abundant at  although Crangonyx i s  in  simply i n t h e i r seasonal  and yet i s  The same i s  always  a peak d e n s i t y  at  most v u l n e r a b l e to  true f o r H y a l e l l a ,  it  part of the summer ( J u l y  Moreover,  l e s s numerous than H y a l e l l a ,  both s p e c i e s are j u s t as  to capture throughout most of the year,  For  presented  approximately one month l a t e r .  s i m u l a t i o n suggests that  c o n s i d e r a b l y more  different  the end of August, but apparently i s ""more  v u l n e r a b l e to attack  is  is  at  the v u l n e r a b i l i t y of both  s p e c i e s does not r e f l e c t  attack  amphipods  of prey they  The number of amphipods a t r o u t could capture,  F i g u r e 7.  mechanisms  and August).  except At t h i s  the  vulnerable  i n the  late  time, Crangonyx  susceptible.  each month in which samples were a v a i l a b l e ,  contents of the t r o u t were analysed with respect  the stomach  to the average  number of Cranqonyx and H y a l e l l a that were found i n the  gut.  These data are superimposed over the simulated v u l n e r a b i l i t y curves  i n F i g u r e 7.  the observed  With the exception of the A p r i l  trend i n e x p l o i t a t i o n c l o s e l y  sample,  f o l l o w s the  simulated  Figure  7.  observed  A comparison of the simulated (data p o i n t s )  (curve)  and  trend i n the e x p l o i t a t i o n of  Cranqonyx and H y a l e l l a , by t r o u t ,  i n Marion Lake.  Number  captures  cn  JL  per  hour  O  o JL  J o I  11  • i i i  O  —*  CD 13 CD CQ  EL  o ID <  o c_ i—r  ZT  C_  >  CO  o  I—i—i—i—i—r~ No. a m p h i p o d s  oe [  in t r o u t  stomachs  trend.  A comparison of these r e s u l t s a l s o shows that Cranqonyx  was p r e d i c t e d to be j u s t as which,  with one e x c e p t i o n ,  v u l n e r a b l e to capture d i d i n fact  occur.  Therefore on the b a s i s of these data i t s i m u l a t i o n model can account exploitation just of  as  as well  frequently  prey,  namely,  significantly  the fact  Hyalella.  seems as  that  Cranqonyx i s  E v i d e n t l y , the  t h e i r s i z e and r e l a t i v e  can apparently  differences  if  the  the seasonal p a t t e r n of captured  characteristics  activity  contribute  to determining the r i s k of p r e d a t i o n .  characteristics considerable  as  as  for  as H y a l e l l a  be important  These  enough to  override  in density.  DISCUSSION One of the advantages of developing models of b i o l o g i c a l processes such as  predation  importance of d i f f e r e n t can then be expressed as or in the  is  that  components. hypotheses  the d i s t a n c e  trout  s i z e and inherent c o n t r a s t  is  the t u r b i d i t y of the water  (6).  the water were i n c r e a s e d  diminish,  inferences  and t e s t e d e i t h e r  can detect  experimentally  dependent If  prey of a  to some extent  the e x t i n c t i o n  then the r e a c t i v e  Subseguently, will  all  the c o n t r a s t  upon  d i s t a n c e would to prey  t h r e s h o l d for d i s c r i m i n a t i o n .  e l s e being e q u a l ,  a t t a c k prey at  specific  coefficient  simply because t r o u t would have to be c l o s e r  i n order to detect  predators  Any r e s u l t i n g  apparent  field.  For example,  of  one can examine the  i n murky waters  visual  a slower r a t e than they could  i n c l e a r e r water.  The question which a r i s e s however, i s  how s i g n i f i c a n t  the c l a r i t y of the environment to  is  just  predation?  The s i m u l a t i o n model was able to account for the e x p l o i t a t i o n of  amphipods as well  be a reasonable If  this is  as  several  abstraction  therefore,  i t may  of the feeding behaviour of  the case then i t  sensitivity  other prey,  trout.  i s worthwhile to examine the  of the model to some of i t s  components.  Table 9 presents the change i n the v u l n e r a b i l i t y of Crangonyx and H y a l e l l a that were a l t e r e d .  r e s u l t e d when s e v e r a l  In conducting these s i m u l a t i o n s ,  components  I arbitrarily  i n c r e a s e d the value of each parameter by 10% of the 'real'  value  (Table 5 ) .  each of these attendant of  The d i f f e r e n c e changes  i n the rate of  produced i n d i c a t e s  p r e d a t i o n to these parameters.  capture  the s e n s i t i v i t y  The components of the p r e d a t i o n  model can be p a r t i t i o n e d i n t o 3 c a t e g o r i e s : characteristics,  estimated  2) prey c h a r a c t e r i s t i c s ,  l ) environmental  and 3)  predator  characteristics.  Environmental  Characteristics  To answer the g u e s t i o n of  Cranqonyx and H y a l e l l a  t u r b i d i t y of the water; as an i n c r e a s e (Table  is  earlier,  the v u l n e r a b i l i t y  r e l a t i v e l y i n s e n s i t i v e to the  a change i n (6)  was not as  important  i n e i t h e r the ambient i l l u m i n a t i o n or  temperature  9).  Despite the fact predator w i l l at  raised  that  the searching a c t i v i t y of a v i s u a l  be r e s t r i c t e d  l e v e l s above the l i m i t  by the l e v e l  of v i s i o n  of i l l u m i n a t i o n ,  even  (R . ) the i l l u m i n a t i o n  could  TABLE  9.  S e n s i t i v i t y of the attack  parameters.  Each parameter  'real*  (Table 5 ) .  value  model to  uuas i n c r e a s e d  selected  by 10% of  its  In the case of the l e a r n i n g  component, the width of the path of search was doubled. The p e r i o d of s i m u l a t i o n was the January  Parameter  1-15  interval.  % change i n the attack Cranqonyx  A. Temperature  Turbidity  Environmental C h a r a c t e r i s t i c s  (T)  Illumination  (R)  ^ ,(6)  Hyalella  .  B.  22  33  22  20  - 2 Prey  -  5  Characteristics  Inherent Contrast  (C ) o  43  128  Vertical  (IYI3)  22  24  (M7)  17  14  Predator  Characteristics  Activity  Horizontal  activity C.  Learning Recognition Success  (RS)  50  50  22  24  rate  indirectly Hamilton  influence  the searching  (1965) and Hyatt  (personal  i n the l a t e summer (August), Lake,  displays a distinct  morning  in the water  shifted hrs.  After t h i s  inactive trout  since  contain  in t h e i r  The foraging closely into  coincides  this  a general  some f i s h  column u n t i l may become  decline  of t r o u t ,  As a r e s u l t ,  i n the a v a i l a b i l i t y  of  population food  Therefore,  lower l e v e l s  have  2300 or 2400 relatively food  during the day shift  in different  i n the  s e c t o r s of  feeding  the  (Hester  should be able to feed  as  An object  present a higher l e v e l of  t a r g e t on the sediment  environment.  morning and l a t e  they can i n the water column.  trout  will  not detect prey on the lake bottom  the evening sky w i l l  than a s i m i l a r  By  could be i n response to changes  will  against  shift  i n the amount of  the general  evening)  as  will  migration of chironomid pupae  of low i l l u m i n a t i o n ( e a r l y  effectively  feeding  i n the water column  During periods trout  early  f o r benthic p r e y .  L i t t l e m i g r a t i o n occurs  p o s i t i o n of the t r o u t  In the  stomachs.  with the d i e l  1965).  i n Marion  most of the p o p u l a t i o n  i n the water  activity,  region.  (Hamilton,  is  found that  appear to be  however,  time the p o p u l a t i o n  there  adopts.  column on chironomid pupae.  (2000 hrs)  back to feeding  pattern.  most animals  and hunt s p o r a d i c a l l y  the l a t e afternoon  population  feeding  Throughout the rest of the day, their position  an animal  communication)  the t r o u t  diel  (0500 to 0900 hrs)  predominantly  pattern  contrasted contrast  and T a y l o r ,  on l i m n e t i c prey  of i l l u m i n a t i o n than they r e q u i r e  to hunt  1965). at  for  benthic-living however,  animals.  As the lev/el of i l l u m i n a t i o n  and the m i g r a t i o n of chironomids begins  the p o p u l a t i o n may be able to feed more l a k e bottom; i n which case, to t h i s  region.  experiments not  to  rises,  subside,  e f f i c i e n t l y on the  their attention  could be  shifted  T h i s s u p p o s i t i o n was suggested by e a r l i e r  (Section  I)  which demonstrated  maintain a searching p a t t e r n  above a c r i t i c a l r a t e  (about  unless  that  trout  they are  2 captures  will  reinforced  per m i n u t e ) .  Changes i n the ambient temperature  can a l s o be  expected  to i n f l u e n c e the v u l n e r a b i l i t y of prey such as amphipods. this  case,  r i s i n g temperatures  animals  that  are exposed  exposed  i n d i v i d u a l s (Section  more s u c c e s s f u l a greater  will  as well  increase  as  the number of  the l e v e l of a c t i v i t y of  III).  Therefore t r o u t w i l l  i n r e c o g n i z i n g these prey and w i l l  distance.  In  be  attack  from  The major r o l e that water temperature  plays  i n the i n t e r a c t i o n between the t r o u t and the amphipods i n Marion Lake i s  apparent  i n F i g u r e 7.  both Cranqonyx and H y a l e l l a seasonal  temperature  In g e n e r a l , temperature the most animals  is  pattern  seasonal  closely  The v u l n e r a b i l i t y of c o r r e l a t e d with the  (Appendix  and d i e l  II).  changes  and the ambient i l l u m i n a t i o n  important f a c t o r s (Fry, 1947;  vulnerability  Thome,  to p r e d a t i o n .  that  affect  1969)  i n the water  may well  be two of  the a c t i v i t y of  and subsequently  aquatic  their  Prey C h a r a c t e r i s t i c s  The  sensitivity  physical  and  a n a l y s i s (Table  behavioural  the  of  the  time concealed  at  the  mud-water i n t e r f a c e . species  (Berglund, any  be  to o t h e r  Straskraba,  that  (Allen,  1941;  concealment  species  results  and  the  the  day  will  vary  depending  and,  In  this  changes i n the  o f a sudden  population  on  the  example,  the  H y a l e l l a begin  the  increase the  suddenly except  drops.  that  predation  rise  of  prey  predators. 1963)  Several  have the  i s supported  The  i t occurs  of  patterns,  by  suggested  risk the  May  however,  size  of  experimental  of  when t h e  i n June.  a  prey  in Figure  This  vulnerability  In b o t h  population.  for  rise of  7.  Cranqonyx  i n response  same phenomenon i s a p p a r e n t latter  the  curves  temperature.  perhaps  changes i n p o p u l a t i o r  is illustrated  April  and  factors  s t r u c t u r e of  vulnerability  lake  2)  composition  i n March and  ambient  part  size  i n the  simulated  to  i n the  latter  shift  rate of  and  until  objects  behaviour  upon a number o f o t h e r  density  effect  much  at  of  seasonally,  l) alterations in activity  The  the  greatly diminish  s u c h as  3)  spend  under  a result  Grimas,  inference  also  simulation.  Prey v u l n e r a b i l i t y even d u r i n g  or  several  invertebrates  proportion  1961;  b e h a v i o u r would This  As  from v i s u a l  Huruska,  to a t t a c k .  aquatic  will  Cranqonyx  respect,  1965).  a small  prey  sediment  In t h i s  s u s c e p t i b l e to a t t a c k  authors  of  H y a l e l l a and  e i t h e r i n the  i n s t a n t i n time o n l y  will  a  capture.  is similar  1968;  i n d i c a t e s that  characteristics  affect  these  r a t e of  9)  to continues  Cranqonyx  for Hyalella  cases  the  decline  in  the r a t e of capture  is  appearance of j u v e n i l e s  due to r e p r o d u c t i o n and the  i n the p o p u l a t i o n .  v u l n e r a b i l i t y of Cranqonyx was not the  v u l n e r a b i l i t y of  until  about one month a f t e r  depressed for very l o n g ,  H y a l e l l a did not begin to r i s e the onset of  In a d d i t i o n to the importance of and  size,  the inherent c o n t r a s t  its  r i s k of being a t t a c k e d .  10$ i n c r e a s e  in contrast  of a prey w i l l  Cranqonyx and H y a l e l l a , but e s p e c i a l l y  it  is  known that  c r y p t i c animals  being d i s c o v e r e d by v i s u a l species, just  the s e n s i t i v i t y  predators  than more  significant  every parameter changed  distance  encounter every s i z e c l a s s of p r e y .  and prey s i z e  the  affect  the rate  Therefore,  2).  become trout  seemingly  minor changes i n some major components can have a especially  the  (fig.  of r e a c t i o n w i l l  a m p l i f i e d because i t w i l l  on prey r i s k ,  was  t h i s would tend to have a  In a d d i t i o n , a change i n the d i s t a n c e  effect  that  on a small animal due to the form of  r e l a t i o n s h i p between r e a c t i v e  considerably  demonstrates  predation.  Since most of these a l t e r a t i o n s  effect  to  H y a l e l l a proved to be  to a change i n almost  component,  Although  conspicuous  of the model to c o n t r a s t  simulation,  distance  a  are l e s s s u s c e p t i b l e  In the s e n s i t i v i t y  reactive  analysis,  the l a t t e r .  component could be to  investigated.  affect  the v u l n e r a b i l i t y  how important t h i s  more responsive  behaviour  also  In the s e n s i t i v i t y  markedly r a i s e d  again  breeding.  concealment  of  well  Although the  significant  i n the case of a small  animal.  Predator  Characteristics  One of the most behaviour of t r o u t some d i s t a n c e  important c h a r a c t e r i s t i c s  is  the fact  will  ever,  fail  that  the height of the  exceeds the d i s t a n c e  to d i s c o v e r  characteristic;  if  some small  was s u f f i c i e n t  than 3 mm were not captured p r o p e n s i t y of t r o u t  are like  less  of  (fig.  6).  In g e n e r a l ,  the  position will  i n the p o p u l a t i o n  In c o n t r a s t ,  larger  Crangonyx, wi I I have s i z e c l a s s e s i n v u l n e r a b l e to  trout  (Section  formulated on the b a s i s that  II).  react  conditioned trout w i l l ,  can i n c r e a s e  it  to p r e y , i t w i l l  predation  rate than one that  Several  experimental  is  less  was  it  simulated  Obviously, i f a  i n which i t  food at  is  that a  of l e a r n i n g can be  the area about  be able to d i s c o v e r  is  Since an animal that  from h a l f the d i s t a n c e  the e f f e c t  greater  extremely  prey  their  simulates  by doubling the width of the path of s e a r c h . can i n c r e a s e  that  The attack model  the predator  c o n d i t i o n e d to recognize amphipods.  predator  tend  reproduction.  responsiveness to prey  conditioned w i l l  This  H y a l e l l a , because throughout  be animals  Under c e r t a i n c o n d i t i o n s  not  searching  to e x p l a i n why Hyalel1 a l e s s  than the t h r e s h o l d s i z e .  only during periods of  It  r e a c t i o n then t r o u t  to maintain a searching  always  food.  s i z e c l a s s e s of p r e y .  to favour a small a n i m a l , s u c h as the year there w i l l  feeding  they maintain a p o s i t i o n  from the sediment when they hunt for  was mentioned e a r l i e r , position  that  of the  a  responsive  will  respond  substantially (Table  s t u d i e s have v e r i f i e d that  9).  learning is  advantageous process through which predators  could  an  increase  their  Several their of  visual  learn  energy  characteristics response  s e a r c h on  can  rate of  intake  of the b e h a v i o u r  to prey,  to a l t e r  their  response  to f o r a g e i n d i f f e r e n t  feed  either  limnetic  the d r i f t  i n streams.  searching  position  prey  visually  the water  will  be  sector their the  suggests  trout  of the  will  before they  be  they  may In  will  in this  any  case,  whether  is relatively  be  hunting  in  this  prey  then  because  'broken'. must be  and  Thus closer  a searching  position  prey.  i n t h e w a t e r column o r o v e r temporarily into  upon a t h r e s h o l d r a t e o f  efficiency.  more  of  to  field  diminished, largely  more abundant b e c a u s e  responsiveness  hunt  benthic-living  classes  to converge  the c h a r a c t e r i s t i c  to i n c r e a s e t h e i r  feed  a  respond  reactive  maintain  on  'smooth'  when t h e y  relatively  s i n c e they  as  can  to f e e d i n g i n  i n r e c o g n i z i n g f o o d and  trout  are l i k e l y  i s dependent  addition,  their  will  somewhat  admirably  they  maintain  presents a their  they  are  as w e l l  adapted  f e e d on  that  they  a s u b s t r a t e and  l)  pattern  example,  they  1970).  namely,  fact  that  For  that  are b e t t e r  d i s c o v e r some s m a l l s i z e  which  In  be  Moreover,  they  search  over  If they  case  substrate, food  fact  the  organisms  i n r e c o g n i z i n g prey  successful  attack.  not  hunt  will  3)  imply  maximize the a r e a o f  efficiency  less  to prey,  the  they  environment.  hunting  and  S i n c e water e s s e n t i a l l y  most s u c c e s s f u l  background  they  that  of t r o u t ,  environments,  However,  Croze,  the dependence o f t h e i r  or benthic l i v i n g  when t h e y  column.  bafekground,  2)  a threshold rate of capture,  adapted on  (Beukema, 1968;  their  capture  of learning  will  to prey  thereby  and  areas  a  in  pattern of (Section  enable  I).  individuals  further  improve  In c o n c l u s i o n , the p r e d i c t i o n s generated model do not refute of  the hypothesis  Cranqonyx and Hyalel1 a,  Lake,  by the t r o u t  This hypothesis  exploitation  p o p u l a t i o n i n Marion  was expressed i n the attack model  appears to be s u f f i c i e n t  e x p l o i t a t i o n of d i f f e r e n t observation  that  to account  f o r the  disproportionate  s i z e c l a s s e s of prey as  Cranqonyx i s  j u s t as  well  as  the  v u l n e r a b l e to a t t a c k  Hyal el 1 a d e s p i t e a 7 f o l d ' . d i f f e r e n c e are  the s e l e c t i v e  can be explained by the process of prey r e c o g n i t i o n and  detection. and  that  by the s i m u l a t i o n  as  in t h e i r densities .  There  two p r i n c i p a l reasons why Cranqonyx i s imore v u l n e r a b l e  attack;  i n the f i r s t  place,  it  spends c o n s i d e r a b l y  it  will  it  is  a larger  this-  and attacked  replicate  i n the v u l n e r a b i l i t y of amphipods.  case i s  temperature. the sediment temperature  p r i m a r i l y due to the seasonal Very few amphipods are i n the winter months.  r i s e s more animals w i l l  c o n s i d e r a b l y more a c t i v e . populations  will  Therefore from a  than H y a l e l l a .  The attack model was a l s o able to pattern  and secondly  more time moving when exposed.  be recognized more s u c c e s s f u l l y  greater distance  animal  r i s e since  the seasonal  The explanation change i n water  exposed  and moving over  However, as  the water  be exposed  and ;  trout w i l l  be more s u c c e s s f u l  discover  in  them from a g r e a t e r  distance. therefore,  in  Therefore the v u l n e r a b i l i t y of both  r e c o g n i z i n g these prey and w i l l  This study,  to  demonstrates that  i n t e r a c t i o n between the behaviour of a v i s u a l  due to  the  predator  and  the c h a r a c t e r i s t i c s affiected  than  its  prey,  predation  is  by the d e n s i t y of a food organism,  influenced factors  of  by i t s  size,  are l i k e l y  not only  but also w i l l  a c t i v i t y and c o n t r a s t . '  to be j u s t  as  be  These  i f not c o n s i d e r a b l y  more important  density.  SUMMARY 1.  In order to t e s t  the hypothesis  d e t e c t i o n and r e c o g n i t i o n i s selective  that  the process of prey  sufficient  to e x p l a i n  e x p l o i t a t i o n of prey by t r o u t ,  conducted to i d e n t i f y some of the f a c t o r s their 2.  the  a study was that  affect  visual discrimination.  The d i s t a n c e related  trout w i l l  to prey s i z e .  developed to d e s c r i b e  react  was found to be n o n - l i n e a r l y  A general  system of equation was  the process of visual) d i s c r i m i n a t i o n  in terms of the r e l a t i o n s h i p between the s i z e and the contrast  a target  must have before  it  can be  detected  of the background i l l u m i n a t i o n on  contrast  apparent  (attacked). 3.  The e f f e c t  d i s c r i m i n a t i o n was not examined.  This component was  however, on the b a s i s of r e s u l t s documented i n the 4.  Rainbow t r o u t w i l l distance  react  than s t a t i o n a r y  considered,  literature.  to moving t a r g e t s from a g r e a t e r prey.  Irrespective  of the s i z e of  5.  the  test  (22  cm).  A  prey,  the  effect  'broken' background  recognize  prey  distance effect  and  of  reduced  incorporated  conditions  the  simulated  and by  the  m o t i o n uuas  ability  significantly  f o r b o t h moving  was  target  of  diminished  non-moving  assuming  diversity  trout  to  their  reactive  targets.  that of  constant  the  the  This  experimental  natural  lake  substrate.  6.  A general of  attack  several  invertebrate  planorbids) population  7.  Although  i n Marion  for  the  An  stomachs,  i t does not  ii)  their  of  caddis  exploitat  and  by  reasonably  accurate  adequately  disproportionate  the  trout  able  to  f o r the  smallest  the  fact  that  amphipods,  unable  H y a l e l l a even  though  caddis  account  explanation.  predicted  the  3  C r a n q o n y x and  (the  threshold  size  i s captured  Hyalella;  different  model was  H y a l e l l a consumed by Cranqonyx  to  e x p l o i t a t i o n of  amphipods  in  i n some months.  on  c l a s s e s of account  the  a complete  trout predation  the  of  i t was  size  as  the  amphipods,  planorbids  offer  additional simulation  i)  the  model was  occurence of  characteristics  simulate  (odonates,  percentage occurence  odonates i n t r o u t high  groups  to  Lake.  proposed  the  developed  principally  and  Therefore, 8.  but  the  predicting  model was  also  (3mm)  or  trout), as  frequently  i t i s 7 times l e s s abundant  iii)  the seasonal  p a t t e r n to the e x p l o i t a t i o n of both  species. 9.  In g e n e r a l ,  the r e s u l t s  of t h i s study  that  the s e l e c t i v e  will  tend to favour s m a l l ,  general  pressure exerted  lead to the c o n c l u s i o n  by v i s u a l  predators  c r y p t i c a l l y c o l o r e d animals,  r e d u c t i o n i n a c t i v i t y and concealment b e h a v i o u r .  a  BIBLIOGRAPHY All,  M.A. 1959.  The o c u l a r  behavioural Can. J . Allen,  Berglung,  Studies on the b i o l o g y of the e a r l y (Salmo s a l a r ) .  Animal E c o l .  T. 196B.  1968.  Rep.  Inst.  by brown t r o u t  Freshw.  Predation by the t h r e e - s p i n e d  and e x p e r i e n c e . J.H.S.  habits.  Res.  48:76-101.  (Gasterosteus aculeatus L . )  Blaxter,  Feeding  stages  10:47-76.  i n a pond.  Drottningholm J.J.  2.  The i n f l u e n c e of p r e d a t i o n  on A s e l l u s  Beukema,  salmon.  37:965-996.  of the salmon J.  retinomotor and photo-  responses of j u v e n i l e p a c i f i c  Zool.  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Grimas,  Res.  U. 1963.  of  the environment  Studies,  Lab.  68:  Biol.  Grand,  Y. l e ,  1967.  Press. Hamilton,  A.L.  food.  space v i s i o n .  Bloomington. (VIS 1965.  367  utilization  Zool.  Ph. D. t h e s i s ,  Bidr.  Epibenthic algal  Res.  British  Chironomidae. Columbia,  production  and  i n the sediments of Marion  Lake.  J.  1970.  The u t i l i z a t i o n  H y a l e l l a azteca  reference to the  benthic  p.  community r e s p i r a t i o n Fish.  University  p.  the U n i v e r s i t y of  B. C. 74  , B. T . 1969c.  Indiana  An a n a l y s i s of a freshwater  community with s p e c i a l  Hargrave  fish  and  497-503.  Form and  Vancouver,  P u b l . Ont.  on the a v a i l a b i l i t y  degree of bottom animals as 35:  55,  activity.  5-62.  Reflections  Uppsala.  Ser.  on animal  Bd. Canada, of b e n t h i c  (Amphipoda).  J.  26:  2003-2026  microflora  by  Animal E c o l .  39:  427-437. Hester,  F.  and J .  Taylor,  Fisheries Hester,  F.J.  1968.  1965.  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The f u n c t i o n a l  density  1966.  of simple types of  The a n a l y s i s of complex p o p u l a t i o n  Can. E n t . 96: 1965.  revealed  of small-mammal p r e d a t i o n of the European  pine s a w f l y . 1959b.  as  Fish.  Res.  Bd. Canada  feeding 25:  393-407. Huruska,  V. 1961.  An attempt at  a direct  investigation  of  the  i n f l u e n c e of the carp stock  on the bottom fauna  of  two ponds.  Verein.  732-736.  Verh.  Internat.  Limnol.  14:  Ivlev,  U . S . 1961.  Experimental ecology of the feeding  fishes. Ishiwata,  Yale U n i v e r s i t y P r e s s ,  N. 1968b.  III.  Jap.  1968e. UI.  Jenkins,  Degree of hunger and s a t i a t i o n  Soc.  Sci. Fish.  34:  Jap.  T.IY1. 1969.  604-607.  Soc.  affecting  Sci. Fish.  Social  34:  structure,  and Salmo q a i r d n e r i )  R . E . and P . A . L a r k i n .  and Rainbow t r o u t Columbia l a k e s . Lee,  E . B . MS 1967. family  species  trutta  i n mountain streams.  Competition for  (Richardsonius  (Salmo q a i r d n e r i ) J.  (Salmo  57-123.  1961.  between Redside s h i n e r s  amount.  p o s i t i o n choice and  resident  Animal Behav. Monog. 2:  satiation  fishes.  785-791.  m i c r o d i s t r i b u t i o n of two t r o u t  Johannes,  amount.  E c o l o g i c a l s t u d i e s on the feeding of  External factors  Bull.  New Haven.  E c o l o g i c a l s t u d i e s of the feeding of  fishes. Bull.  of  Fish.  Res.  food  balteatus)  i n two B r i t i s h  Bd. Canada 18 : 203-220.  Some aspects of the energy budget  Planorbidae i n Marion Lake,  B.C.  of  BSc.  the Thesis.  The U n i v e r s i t y of B r i t i s h Columbia, Uancouver, B . C . 37p. Lindstrom,  T . 1955. Rep.  On the r e l a t i o n f i s h  Inst.  Macan, T . T . 1966.  Freshw. Res.  Drottingholm 36:  size.  133-147.  The i n f l u e n c e of p r e d a t i o n on the bottom  fauna of a moorland f i s h pond. 61:  s i z e - food  432-452.  Arch.  Hydrobiol.  Mathias,  J . A . MS 1967.  P o p u l a t i o n e n e r g e t i c s of two amphipod  s p e c i e s i n Marion Lake. of Messenger,  B r i t i s h Columbia, Vancouver,  J.B.  1968.  The v i s u a l  Sepia o f f i c i n a l i s . Morrell,  G.M. and J . R . I.  I.  Anim. Behav.  The U n i v e r s i t y  74p.  the 16:  cuttlefish, 324-357.  Experiments  The response of wild b i r d s  1970.  B.C.  attack of  T u r n e r . 1970.  Behaviour 36: Neish,  M.Sc. t h e s i s .  i n mimicry:  to a r t i f i c i a l  116-130.  A comparative  a n a l y s i s of  the  feeding  behaviour of two salamander p o p u l a t i o n s Lake,  B . C . Ph. D t h e s i s ,  Columbia, Nakamura,  Vancouver,  E . L . 1968.  T.R., R.J.  B . C . 108  Brasseur,  J.D.  Polyak,  of  s i z e and c o n c e n t r a t i o n  blooms.  Oceanog.  S.L. of  Prazdnikova,  1957.  N . V . 1969.  visual in  fish  Fisheries  of  grazing  phytoplankton 10-17.  system.  The U n i v e r s i t y  Chicago.  Pecularities  images by f i s h , (1967),  Some  zooplankton  Japan. 23:  The v e r t e b r a t e v i s u a l  Chicago Press,  41-49.  F u l t o n , 1967.  on the c e l l  Soc.  Katsuwonus  Copeia:  o b s e r v a t i o n s on the dependence  J.  British  p.  V i s u a l a c u i t y of two tuna  le  i n Marion  the U n i v e r s i t y of  pel amis and Euthynnus a f f i n s . Parsons,  prey.  trans.  and W i l d l i f e ,  of  the d i c t i n c t i o n of  from Behaviour and  reception  R.M. Howland, Bureau of Washington, D . C .  Sports  Pritchard,  G. 1965.  Prey capture  Anisoptera). Prop,  N. 1960.  Can. J .  by dragonfly  Zool.  43:  P r o t e c t i o n against b i r d s  larvae  271-289. and p a r a s i t e s i n  some s p e c i e s of T e n t h r e d i n i d l a r v a e . Zool. Rilling,  S.,  13:  (Odonata:  Arch,  neerl.  380-447.  H. M i t t e l s t a e d t  and K . D . Roeder.  1959.  r e c o g n i t i o n i n the preying mantis.  Prey  Behaviour  14:  164-184. Ruch,  T . C . and H . D . P a t t e n .  1965.  Physiology and  biophysics.  W.B. Saunders Co. P h i l a d e l p h i a . Ruiter,  L . de.  1952.  stick  Some experiments  caterpillars.  1966. natural  Behaviour 4:  environment.  D . C . MS 1969.  _I_n Handbook of Section  An experimental  6 (l): study  malma).  M.Sc. T h e s i s ,  Columbia, Sheperd,  physiology  of the  feeding  cutthroat  and d o l l y vardetn  trout  (Salvelinus  the U n i v e r s i t y of  British  Vancouver,B. C.  B . G . MS 1970. rainbow t r o u t nerka)  i n the  96-116.  behaviour and i n t e r a c t i o n of c o a s t a l (Salmo c l a r k i c l a r k i )  of  222-232.  Feeding behaviour of v e r t e b r a t e s  (Am. P h y s i o l . Soc.) Schutz,  on the camouflage  Aspects  of the feeding  (Salmo q a i r d n e r i )  of Marion Lake,  the U n i v e r s i t y of  behaviour of  and kokanee  B r i t i s h Columbia.  the  (Onchoryncus  B.Sc.  Thesis,  B r i t i s h Columbia, Vancouver, B . C .  Shiffrin,  R.M. 1970. failure?  Smith,  as  food. V e r h . M. 1965.  Verein.  Sci.  M . J . 1969.  14:  F i s h . 22:  91-110.  L . 1960.  T . 1956.  Int.  and R . H . F l e m i n g . 1942.  perception  The  1087  in f i s h , Bull.  p. especially  Jap.  536-557.  (Trichoptera) .  The dynamics of i n s e c t  i n pine woods.  722-726.  106-127.  Behaviour of the caddis  Lend.. 44:  fontinalis)  Mitt.  power and accomodation.  (Curtis)  Arch.  The v i s u a l  Microcanthus s t r i q a t u s Pubis.  Limnol.  i n ponds and streams.  stellatus  Yamanouchi,  (Salvelinus  lake  of f i s h on the number of  A study of v i s u a l  on r e s o l v i n g  retrieval  natural  P r e n t i c e - H a l l , Englewood c l i f f s .  Tamura, T . 1957.  Tinbergen,  Verein.  The e f f e c t  L m i n o l . 13:  oceans.  Thome,  Int.  H.W., M.w1. Johnson,  Soc.  in a f e r t i l i z e d  u t i l i z a t i o n by t r o u t  invertebrates  Sverdrup,  t r a c e e r o s i o n or  168: 1601-1603.  Bottom fauna  and i t s  Straskraba,  Science  1961.  IYI.IAI.  Forgetting:  neer.  Zool.  a c u i t y of  fly larva  Potamphylax  P r o c . R. E n t .  and b i r d 13:  populations  259-472.  the c o r a l  fish,  (Cuvier and V a l e n c i e n n e s ) .  Seto. marine B i o l .  Lab.  5:--133-156.  Soc.  Appendix I  A LIST OF SYMBOLS FOR THE ATTACK MODEL Inherent c o n t r a s t  o  of prey  CS  Success of the predator i n c a p t u r i n g prey i t  CT  The apparent c o n t r a s t an attack  CT  min  The minimum l e v e l  of a prey that  of c o n t r a s t  ( no. /  sq.  is  has  attacked.  r e q u i r e d to  elicit  the predator can d i s c r i m i n a t e  D  Prey d e n s i t y  m. )  E  A p r o p o r t i o n a l i t y constant d e s c r i b i n g the e f f e c t background on r e a c t i v e d i s t a n c e .  TP  The width of the encounter path swept along the sediment  MC  The motion c o n s t a n t , the a d d i t i v e e f f e c t on the attack d i s t a n c e (m)  of target movement  PA  The p r o p o r t i o n of exposed prey moving at  any i n s t a n t  R min R max  The l i m i t of s c o t o p i c v i s i o n  R  The l e v e l  RA  The  rate of attack  RC  The  r a t e of capture  RE  The  rate of encounter ( n o . / h r . )  RD  The r e a c t i v e or attack r e a c t i v e f i e l d (m)  "RS  Prey r e c o g n i t i o n success  S  The v i s u a l angle a prey subtends with the predator arc )  ( ft-candles  of the (m)  i n time  )  The lowest l e v e l of i l l u m i n a t i o n which produces maximum visual acuity ( ft-candles ) of ambient i l l u m i n a t i o n  ( ft-candles  )  ( no./ hr. ) ( n o . / hr. )  distance,  The rate of a t t e n u a t i o n of t a r g e t c o e f f i c i e n t of the water)  also  the r a d i u s of the  contrast,  ( min. of  (the e x t i n c t i o n  SP  -  The d i s t a n c e of the predator searching p o s i t i o n (m)  from the sediment,  its  T  -  The ambient water temperature  VD  -  The v u l n e r a b l e d e n s i t y of prey  UP  -  The p r o p o r t i o n of prey exposed at or above the mud-water i n t e r f a c e at any i n s t a n t i n time  VY  -  The p r e d a t o r ' s  ( C ) (no./ sq.  m. )  average searching v e l o c i t y (m/ h r . )  THE PHYSICAL CHARACTERISTICS OF MARION LAKE  Efford  (1967) and Hargrav/e.  (1969c) presented  d e s c r i p t i o n of the b a s i c p h y s i c a l and chemical of  Marion Lake.  a detailed  characteristics  The d e s c r i p t i o n which f o l l o w s w i l l  to some of the more important  features  reported  be confined  by these  authors. Marion Lake i s  a small,  depth of 2.4 meters. is  shallow c o a s t a l  The primary p r o d u c t i o n i n the water column  extremely low throughout the year.  the p e r s i s t e n t result, 0.91;  f l u s h i n g of the lake  the t u r b i d i t y of the water Hargrave1969c)  is  This i s  undoubtly due to  (Dickman, 1968). (extinction  consistently  year due to the amount of p a r t i c u l a t e into  lake with a mean  As a  coefficient,  high throughout  matter  that  is  the  washed  the lake and not. because of changes i n the abundance of  phytopiankton . Although the lake becomes are  subject to warming i n the s p r i n g and summer.  fluctuations  i n the r a t e of temperature  the b a s i c p a t t e r n The sediment as  thermally s t r a t i f i e d ,  an extremely  however,  is  the same ( f i g .  f l o c u l e n t ooze  matter  material).  regions  Despite  change from year to  year  1).  i n Marion Lake can be b a s i c a l l y  very d i v e r s e  the p a r t i c u l a t e and other  is  all  (gyttja).  characterized  The s u b s t r a t e  surface  with respect to the s i z e and shape of  (stick  litter,  chironomid l a r v a l  cases,  ure  1.  The average  seasonal trend i n water  i n Marion Lake , recorded over 5 years  temperature  (1963-1968) in the  region of the lake l e s s than 3 meters.  20"=  MONTH  APPENDIX  III  THE POPULATION CHARACTERISTICS OF CRANGONYX AND HYALELLA IN MARION LAKE  Several  s t u d i e s have been conducted on the amphipods i n  Marion Lake for a c o n s i d e r a b l e Mathias, that  1967;  Bryan, unpublished d a t a ) .  Cranqonyx i s ,  on the average,  than H y a l e l l a and tends the l a k e .  number of years  about  These data 7 times  Crangonyx,  less  the l a r g e r  of the two s p e c i e s ,  f i e l d densities  pooled to e s t a b l i s h  a general  and the s i z e s t r u c t u r e year.  in the  generally  tend to be very s i m i l a r  Therefore the data from a l l  'average'  throughout  zone.  The timing of the r e p r o d u c t i o n of both s p e c i e s as well  year.  abundant  concentrated  a brood of young i n the summer about one month before  relative  1965;  indicate  to be f a i r l y evenly d i s t r i b u t e d  H y a l e l l a , on the other hand, i s  shallow l i t t o r a l  (Hamilton,  the a v a i l a b l e  Hyalella. as  their  from year  to  sources were  d e s c r i p t i o n of the d e n s i t y  (Table l )  produces  (fig.  of each p o p u l a t i o n over an  2)  Figure 2.  The r e l a t i v e density of Hyalella (A) and  Cranqonyx (B) in Marion Lake.  NUMBER o o  OF CD o o  AMPHIPODS—PER- SQUARE o o o  o o  CO o o  ro ro o o  METER ro  0) o o  TABLE Marion  1.  Temporal  changes  i n the s i z e  structure  o f amphipods. i n  Lake. Total  MONTH  Body  Length  (mm) Cranqonyx  Hyalella  L  X  H  L  X  H  Jan  2.0  4.2  5.5  5.0  7.5  11.0  Feb  2.0  4.2  6.0  6.0  9.4  12.0  Mar  2.2  4.3  6.0  6.5  9.2  12.0  Apr  2.5  4.5  6.0  7.0  9.1  12.0  May  2.5  4.5  6.5  1.0  3.7  11.0  Jun  3.5  4.8  6.0  2.0  4.1  8.0  Jul  1.5  2.5  6.0  2.0  4.5  9.0  Aug  1 .5  3.1  6.0  3.0  5.7  9.0  Sep  1.5  3.7  6.5  4.0  6.3  10.0  Oct  2.0  4.1  6.5  5.0  7.3  10.0  Nov  2.0  4.1  6.0  5.0  8.1  11.0  Dec  2.0  4.2  6.0  5.0  7.6  12.0  L = lower  limit  to  size  range  to  size  range  X = mean s i z e H = upper  limit  

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