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An analysis of prey detection in cutthroat trout (Salmo clarki clarki) and Dolly Varden charr (Salvelinus… Henderson, Michael Andrew 1982

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AN ANALYSIS OF PREY DETECTION IN CUTTHROAT TROUT (SALMO CLARKI CLARKI) AND DOLLY VARDEN CHARR (SALVELINUS MALMA) by MICHAEL ANDREW HENDERSON B . S c , U n i v e r s i t y of Western O n t a r i o , 1974 M . S c , U n i v e r s i t y of Manitoba, 1977 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept t h i s t h e s i s as conforming t o the r e q u i r e d standard  THE UNIVERSITY OF BRITISH COLUMBIA J u l y 1982 (c) M i c h a e l Andrew Henderson, 1982  In p r e s e n t i n g  t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of  requirements f o r an advanced degree at the  the  University  of B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make it  f r e e l y a v a i l a b l e f o r reference  and  study.  I further  agree t h a t p e r m i s s i o n f o r e x t e n s i v e copying of t h i s t h e s i s f o r s c h o l a r l y purposes may department or by h i s or her  be granted by the head of representatives.  my  It i s  understood t h a t copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain  s h a l l not be allowed without my  permission.  Department of  dLoo/e^tj  The U n i v e r s i t y of B r i t i s h 1956 Main Mall Vancouver, Canada V6T 1Y3  DE-6  (3/81)  Columbia  written  i i  ABSTRACT Laboratory Varden charr  feeding experiments showed that sympatric D o l l y  ( S a l v e l i n u s malma) were able to l o c a t e and  prey t a r g e t s at lower trout  (Salmo  i r r a d i a n c e l e v e l s than sympatric c u t t h r o a t  clarki  clarki)  Reaction d i s t a n c e (RD)  which  irradiance prey nr  photons  s"  were  for  1  their  irradiance  threshold  targets 2  but  level  (VIT),  not  charr  an  (3.0 x 1 0  nr  photons of prey  2  s~  and  photons n r  16  for trout).  1  was  poorer.  3.0  2  s"  visually  x 10  1  increased  from  15  (3.G  photons n r  that  of  charr  presence  of  red  for  irradiance  yellow and  Reaction increases  in  blue  i n c r e a s e i n RD was the percent  levels  (SIL) which produced the for charr and  6.6  of  size,  sympatric  central  x  At  10  18  followed  in  all  of t r o u t  At a given  in both s p e c i e s occurred  trout  and  in  the  d e c r e a s i n g order  charr  increased  movement and c o n t r a s t .  greatest in t r o u t .  i n both s p e c i e s .  The  the same prey type.  i n c r e a s e in RD was  i n c r e a s e i n RD was  for  by  irradiance.  distance prey  1  The VIT and SIL were independent  i r r a d i a n c e l e v e l the l a r g e s t RD  SIL  1  s"  2  i r r a d i a n c e l e v e l s g r e a t e r than the VIT of t r o u t , the RD  green,  x 10 *  type and prey c h a r a c t e r i s t i c s i n both s p e c i e s .  exceeded  a  i r r a d i a n c e l e v e l below  detected  t r o u t ) to a s a t u r a t i o n i r r a d i a n c e l e v e l maximum RD  acuity  of both s p e c i e s to a r t i f i c i a l and n a t u r a l  prey t a r g e t s i n c r e a s e d as visual  consume  At  or  The  with percent  Between the VIT and  the  g r e a t e r at higher i r r a d i a n c e above  the  SIL  the  percent  constant. portion  and a l l o p a t r i c  of  retinas  populations  of t r o u t and charr from were  examined  by  light  microscopy. cone  The  two  t r o u t p o p u l a t i o n s had a s i m i l a r d e n s i t y of  c e l l s and higher  c h a r r had  the lowest  intermediate  than e i t h e r charr p o p u l a t i o n .  cone  density.  cell  density  Rod  cell  t r e n d , being highest  in sympatric  allopatric  intermediate  cone  t r o u t and  cell  mosaic,  s i n g l e cones, was  and  Sympatric  allopatric  d e n s i t y showed the  c h a r r , lowest in  charr  opposite  in sympatric  allopatric  and  charr.  The  c o n s i s t i n g of a r e g u l a r a r r a y of double  the same i n a l l four p o p u l a t i o n s .  Both  and cone  types were s m a l l e s t in the two  trout populations, largest  in the  sympatric  intermediate  in a l l o p a t r i c  charr.  g r e a t e s t i n sympatric  charr,  The  c h a r r p o p u l a t i o n and  degree of r e t i n a l  least  in  charr.  the In  sympatric  summation was  t r o u t p o p u l a t i o n s and summary,  and  histological  allopatric  in a l l o p a t r i c  indicated  that  the highest l e v e l of  charr were the most  sensitive  to  irradiance conditions. Foraging  irradiance  v e l o c i t y of sympatric  increased  species. diel  studies  t r o u t possessed  v i s u a l a c u i t y while sympatric low  intermediate  reaching  a  t r o u t and charr i n c r e a s e d as maximum  From f o r a g i n g v e l o c i t y , RD  irradiance  water searched  regime  visually  c h a r r on a mid-summer Below  the  b u r i e d prey  VIT,  and  at  the SIL of each  information  in Loon Lake I estimated for two  on  the  the volume of  n a t u r a l prey types by t r o u t  and  day. only charr were able to l o c a t e and consume  t a r g e t s , presumably by employing t h e i r  chemosensory  system. Differences sympatric  in  v i s u a l and  t r o u t and charr and  n o n - v i s u a l feeding behaviour  in r e t i n a l s t r u c t u r e s of  in  sympatric  iv  and a l l o p a t r i c on  field  habits.  t r o u t and charr are g e n e r a l l y as  studies  of  their  expected  vertical distribution  and  based feeding  V  TABLE OF CONTENTS  ABSTRACT  i i  LIST OF TABLES  vii  LIST OF FIGURES  x  LIST OF PLATES  xiii  ACKNOWLEDGEMENTS  xiv  1.0 I n t r o d u c t i o n  1  2.0 MATERIALS AND METHODS  9  2.1 Laboratory  Studies  2.1.1 V i s u a l Feeding  9 Experiments  9  2.1.2 Foraging V e l o c i t y  20  2.1.3 Non-Visual  20  Foraging Behaviour  2.2 I r r a d i a n c e L e v e l s i n Loon Lake  22  2.3 Eye H i s t o l o g y  22  3.0 R e s u l t s  24  3.1 General Feeding 3.2 E f f e c t  Behaviour  of Experience  24  on Reaction Distance  3.3 E f f e c t of Food D e p r i v a t i o n and D a i l y Food Ration  25 on  Reaction Distance  28  3.4 E f f e c t of Temperature on Reaction Distance  30  3.5 E f f e c t of F i s h S i z e on Reaction Distance  33  3.6 E f f e c t of I n f r a r e d I r r a d i a n c e on Reaction Distance 3.7  Effect  of  Irradiance  . 33  C h a r a c t e r i s t i c s on Reaction  Distance  36  3.7.1 I r r a d i a n c e L e v e l  36  vi  3.7.2 I r r a d i a n c e Q u a l i t y  39  3.8 E f f e c t of Prey C h a r a c t e r i s t i c s on Reaction Distance  42  3.8.1 Prey S i z e  42  3.8.2 Prey Movement  46  3.8.3 Prey Contrast  51  3.9 Reaction D i s t a n c e t o N a t u r a l Prey  54  3.10 R e t i n a l H i s t o l o g y  66  3.10.1 Rod and Cone C e l l Density  72  3.10.2 Cone C e l l S i z e  74  3.10.3 Degree of Summation  77  3.10.4 Cone C e l l Mosaic  79  3.10.5 Feeding I m p l i c a t i o n s  81  3.11 E f f e c t  of I r r a d i a n c e L e v e l on Foraging V e l o c i t y  ... 84  3.12 D i e l V a r i a t i o n of I r r a d i a n c e i n Loon Lake  88  3.13 A Model of V i s u a l Prey Searching P o t e n t i a l  94  3.14 Non-Visual Foraging Behaviour  98  4.0 D i s c u s s i o n 4.1 Sensory Behaviour  101 i n Trout and Charr  101  4.1.1 V i s u a l  101  4.1.1.1 C h a r a c t e r i s t i c s of the I r r a d i a n c e  106  4.1.1.2 C h a r a c t e r i s t i c s of the Prey  115  4.1.2 Chemical  120  4.2 R e t i n a l S t r u c t u r e i n Trout and Charr  124  4.3 General  132  5.0 References  138  LIST OF TABLES  Table  1.  Effect  distance  of  (cm  Varden charr  food  ±  95%  to 3  mm  deprivation CL)  2.  distance Varden  Effect  of  artificial  daily  (cm ± 95% CL) charr  to  3  1 9  Table 3. E f f e c t distance  of  (cm  Varden charr irradiance Table  4.  distance Varden  95%  to 3  mm  CL)  of  (cm ± 95% CL) charr  to  3  of  mm  infrared  cutthroat  artificial  4.2  presence  at an  s"  31  1  reaction  t r o u t and D o l l y targets 2  s~  and c h a r r ,  an 32  reaction  and  Dolly  prey t a r g e t s  at an  or  2  s"  34  1  absence  (A) of (cm ± 95%  t r o u t and D o l l y Varden charr  to  3  mm  prey t a r g e t s . In the presence and absence of  i r r a d i a n c e the V i t a - L i g h t  x 10  at  1  on mean  photons n r (P)  target  trout  artificial 1 9  Dolly  mean  prey  length  2  reaction  and  i r r a d i a n c e on mean r e a c t i o n d i s t a n c e  CL) of c u t t h r o a t  infrared  on  photons n r  body  i r r a d i a n c e l e v e l of 4.0 x 1 0 Table 5. E f f e c t of the  prey  an 29  1  photons n r  artificial  total  s"  trout  of c u t t h r o a t  1 9  at  r a t i o n on mean  temperature  l e v e l of 4.0 x 1 0  Effect  1 9  targets 2  artificial  water  ±  prey  cutthroat  i r r a d i a n c e l e v e l of 4.0 x 1 0  reaction  t r o u t and D o l l y  photons nr  food  of  mm  mean  of c u t t h r o a t  i r r a d i a n c e l e v e l of 4.0 x 1 0 Table  on  1 7  and 3.0 x 1 0  1 5  irradiance  photons i r r  2  s  _ 1  level  was  f o r the t r o u t  respectively  Table 6. E f f e c t of  artificial  35 prey  target  size  on the  viii  maximum  reaction  distance  (cm) of c u t t h r o a t  t r o u t and  D o l l y Varden charr  47  Table 7. Maximum r e a c t i o n d i s t a n c e and  Dolly  Diaptomus  Varden charr  cm)  various kenai s  trout  to 1.4-1.6 mm Daphnia rosea and  kenai  Table 8. Linear (RD;  (cm) of c u t t h r o a t  60  regression  of  equations f o r r e a c t i o n  cutthroat  t r o u t and D o l l y Varden charr to  s i z e s (X; mm) of at  distance  Daphnia  rosea  and  an i r r a d i a n c e l e v e l of 4.0 x 1 0  1 9  Diaptomus  photons  m~  2  64  1  Table 9. E f f e c t of movement of n a t u r a l prey t a r g e t s on mean reaction distance Dolly  (cm ± 95% CL) of c u t t h r o a t  trout  and  Varden charr at an i r r a d i a n c e l e v e l of 4.0 x 1 0  photons n r rosea  2  s " . Moving  (M) and s t a t i o n a r y  1  and Diaptomus  kenai  (S)  1 9  Daphnia  were 1.4 - 1.6 mm i n length (N  ~ = 1 00) Table  65  10.  Effect  of  contrast  mean r e a c t i o n d i s t a n c e and  Dolly  10  photons n r  19  Daphnia length Table  rosea  trout  2  s " . High 1  (H)  and Diaptomus  and  kenai  low  (L)  contrast  were 1.4 - 1.6 mm i n  (N = 100)  67  11. A comparison of the d e n s i t y  of c u t t h r o a t  95%  (cm ± 95% CL) of c u t t h r o a t  Varden charr at an i r r a d i a n c e l e v e l of 4.0 x  and rod c e l l s  Table  of n a t u r a l prey t a r g e t s on  (± 95%  CL)  of  i n the r e t i n a s of sympatric and a l l o p a t r i c t r o u t and D o l l y Varden charr  (N = 100). ... 73  12. A comparison of c r o s s - s e c t i o n a l cone c e l l CL)  at  cone  the  outer  segment e l l i p s o i d  level  s i z e (± i n the  ix  retinas  of  sympatric  and  allopatric  populations  of  c u t t h r o a t t r o u t and D o l l y Varden c h a r r (N = 100) Table  13.  type  A  78  comparison of the d e n s i t y of n u c l e i per c e l l  (± 95% CL) and r a t e s of  summation  (photoreceptor:  b i p o l a r : ganglion c e l l s )  i n the r e t i n a s of sympatric and  allopatric  of  populations  Varden charr Table  14.  A  cutthroat  trout  and D o l l y  (N = 100)  qualitative  histological  estimates  80 comparison of  visual  of  behavioural  acuity  and  and  visual  s e n s i t i v i t y under low i r r a d i a n c e c o n d i t i o n s i n sympatric and a l l o p a t r i c p o p u l a t i o n s of c u t t h r o a t t r o u t and  Dolly  Varden charr Table  83  15. Number of s t r i k e s and number of b u r i e d  prey  targets  captured  (±  95% CL) i n s t i l l  artificial and moving  water by D o l l y Varden charr at an i r r a d i a n c e l e v e l than 6.1 x 10 * photons n r 1  2  s  -1  (N = 100)  less 99  X LIST OF FIGURES  Figure  1. A contour map of Loon Lake showing the i r r a d i a n c e  sampling s t a t i o n . Depths i n meters Figure  10  2. R e l a t i v e photon d i s t r i b u t i o n over the wavelength  spectrum of V i t a - L i g h t Figure  3. R e l a t i v e photon d i s t r i b u t i o n over the  spectrum yellow Figure  of  Vita-Light  passed  through  wavelength  blue,  green,  and red f i l t e r s  15  4. E f f e c t of experience on the r e a c t i o n d i s t a n c e  five cutthroat artificial 10  13  t r o u t and f i v e D o l l y Varden charr  2  mean  to 3 mm  prey t a r g e t s at an i r r a d i a n c e l e v e l of 4.0 x  photons n r s " . Each datum  19  of  of  50  1  observations.  point  represents  Vertical lines  the  i n d i c a t e the  range Figure  26  5. E f f e c t of the q u a n t i t y  reaction  distance  D o l l y Varden charr  (±  95%  to 1, 3  of i r r a d i a n c e on the CL)  and  of c u t t h r o a t 5  mm  mean  t r o u t and  artificial  prey  t a r g e t s . Sample s i z e per datum p o i n t equals 100. Figure  37  6. E f f e c t of i r r a d i a n c e q u a l i t y on the mean r e a c t i o n  distance charr level  (± 95% CL) of c u t t h r o a t  t o 3 mm a r t i f i c i a l was  1.03 x 1 0  1 9  prey  t r o u t and D o l l y Varden  targets.  The  irradiance  photons n r s " . Sample s i z e per 2  1  datum p o i n t equals 100 Figure  40  7. E f f e c t of the s i z e of a r t i f i c i a l  the mean r e a c t i o n d i s t a n c e  prey t a r g e t s  (± 95% CL) of c u t t h r o a t  on  trout  xi  and  Dolly  Varden  c h a r r . The i r r a d i a n c e l e v e l used f o r  each curve i s s p e c i f i e d i n photons n r  2  s  _ 1  . Sample  size  per datum point equals 100 Figure  8.  Effect  of  43  movement  of  3  mm a r t i f i c i a l  t a r g e t s on the mean r e a c t i o n  distance  cutthroat  Varden  trout  and  Dolly  (±  95%  prey  CL) of  charr at d i f f e r e n t  l e v e l s of i r r a d i a n c e . Sample s i z e per datum p o i n t  equals  100 Figure  48 9. E f f e c t of inherent  c o n t r a s t of  3  mm  prey t a r g e t s on the mean r e a c t i o n d i s t a n c e cutthroat  trout  and  Dolly  Varden  artificial  (± 95% CL) of  charr at d i f f e r e n t  l e v e l s of i r r a d i a n c e . Sample s i z e per datum p o i n t  equals  100 Figure  52 10. Mean r e a c t i o n d i s t a n c e  trout  and  Diaptomus  Dolly  kenai  irradiance  Varden  charr  1.4 to 1.6 mm  levels.  (± 95% CL)  Sample  to  in  of  cutthroat  Daphnia  rosea and  length  at d i f f e r e n t  s i z e per datum p o i n t  equals  100  56  Figure  11.  Diaptomus of  Effect kenai  cutthroat  of  the  size  of  Daphnia  rosea  on the mean r e a c t i o n d i s t a n c e trout  and D o l l y  Varden  t  i r r a d i a n c e l e v e l of 4.0 x 1 0  1 9  photons n r  (± 95% CL)  charr 2  s  and  - 1  at .  an  Sample  s i z e per datum p o i n t equals 100 Figure  12. Swimming v e l o c i t y (cm nr  trout  1  and  artificial Figure  61  13.  Dolly  Varden  charr  ± 95% CL) of c u t t h r o a t while f o r a g i n g on 3 mm  prey t a r g e t s at d i f f e r e n t i r r a d i a n c e l e v e l s . Depth-time  isopleths  of  irradiance  levels  85  xii  (photons 28-29, Figure  nr  2  s~ )  in  1  Loon  Lake on J u l y 8-9 and J u l y  1980  14.  (photons  89  The  relationship  nr  s~ )  2  in  1  between  Loon Lake on J u l y 8-9,  v i s u a l s e n s i t i v i t y of c u t t h r o a t charr.  The  upper  t r o u t and  l i n e f o r each species  depth at which the i r r a d i a n c e l e v e l to  maximize r e a c t i o n d i s t a n c e .  depth where  the  irradiance  irradiance threshold. Figure  irradiance  See text  is  during  that p o r t i o n  i d e n t i f i e s the  just  sufficient  The lower l i n e marks the  levels  match  the  visual  for d e t a i l s  of the 24 h p e r i o d level  reaction  and  details  Varden  92 t r o u t and  f o r Daphnia rosea and Diaptomus  when the i r r a d i a n c e distance  1980 and  Dolly  15. Volume of water searched by c u t t h r o a t  D o l l y Varden charr  level  was  on J u l y 8-9,  sufficient  foraging  to  kenai 1980  maximize  v e l o c i t y . See text f o r 96  xi i i  LIST OF PLATES  Plate  1. Transverse s e c t i o n  trout.  1  pigment), 2 nuclear  visual  nuclear l a y e r , cell Plate  cell  external  layer,  4  of  of  retina  of  layer  (containing  limiting  membraine,  external  plexiform  6 internal plexiform  cutthroat epithelial 3  external  layer, 5 internal  layer,  7  ganglion  l a y e r , 8 nerve f i b r e l a y e r . X 330 2.  Transverse s e c t i o n  of the r e t i n a of c u t t h r o a t Plate  the  3. T a n g e n t i a l s e c t i o n the  retina  through the v i s u a l c e l l  layer  t r o u t . C cone, R rod. X 500.  through the v i s u a l  of c u t t h r o a t  D-double cone c e l l . X 500  68  trout. S-single  cell  70  layer  cone c e l l , 75  xiv  ACKNOWLEDGEMENTS I express thanks to my  supervisor  and  friend  Dr.'  T.G.  Northcote f o r h i s support and encouragement d u r i n g the course of this are  study.  H i s comments on e a r l i e r d r a f t s of the manuscript  greatly appreciated.  I wish a l s o to thank D r s . M.C. Healey,  W. E. N e i l l ,  J.D. McPhail and  supervisory  committee  and  C.J. Walters  made  many  of  Montreal  for their  teaching me the science of h i s t o l o g y . L. Berg and  and  P. W i t h l e r  served  on  h e l p f u l comments.  e s p e c i a l l y g r a t e f u l to Dr. M.A. A l i and h i s University  who  associates  guidance  and  my I am  at the  patience in  Thanks are a l s o given  to  f o r t h e i r a s s i s t a n c e i n the l a b o r a t o r y  field. A very s p e c i a l thank you goes to my wife, Debbie, who while  busy with her own graduate work always found spirits  up,  and  to  time  to  keep  my  our daughter Lindsay who -waited f o r us to  f in i sh. Finally, in  general  I wish to thank no one i n p a r t i c u l a r but at  the  Institute  of  everyone  Animal Resource Ecology f o r  p r o v i d i n g a very s t i m u l a t i n g environment  i n which to work.  1  1 .0 INTRODUCTION The  diversity  of food organisms consumed by f i s h  than that f o r any other group of  vertebrates  (Nikolsky  Generally  the d i e t a r y h a b i t s of f i s h are c l a s s i f i e d  the major  prey  resulted  in  herbivores,  types the  they  consume  feeding on phytoplankton  detritivores, feeding  on  insects  and  feeding on decaying benthic other  (Hyatt  identification  of  and other  invertebrates,  of  prey  first  (Holling  generated by the prey.  fish  into  these  food  to t h e i r vast  array  little  insight  1966).  To  detect  must be  In high  sequence i s the a  prey  responsive  i n t o the  to  detection  the  sensory  the  stimuli  i r r a d i a n c e environments most  appear to r e l y on t h e i r v i s u a l sensory  food  terrestrial  of these animals that determine  step i n any predatory  on d i e l  material;  acquisition.  c a p a b i l i t i e s of a predator  Studies  groups—  plant  zooplankton,  i t provides  adaptations  t h e i r p a t t e r n s of food The  broad  1970).  feeding p e r i o d i c i t i e s show that i n many s p e c i e s  Laboratory  and  (Vinyard  1976)  O'Brien  the  reaction  under  c o n d i t i o n s , as w e l l as the manipulation visual characteristics  1966,  s t u d i e s i n v o l v i n g the measurement of  feeding r a t e s ( A l i 1959) and  fish  system f o r l o c a t i n g prey.  i s passed i n t o the gut only d u r i n g the day (Woodhead  Blaxter  has  v e r t e b r a t e s ; and omnivores, feeding on both  behaviours,  environment-specific  This  organic m a t e r i a l ; c a r n i v o r e s ,  type c a t e g o r i e s i s a way of b r i n g some order foraging  1963).  i n terms of  1979).  four  p l a n t and animal m a t e r i a l . While grouping  of  i s greater  a  distance  range  of  of models with  (Saxena 1966) i l l u s t r a t e  the  to  prey  irradiance different importance  2  of the v i s u a l system f o r prey d e t e c t i o n in f i s h . While  the  basic  plan of the eye  in a l l f i s h  there are a number of s t r u c t u r a l adaptations which sensitivity  of  species  living  i n low  i s the same, enhance  i r r a d i a n c e environments.  These i n c l u d e t u b u l a r shaped eyes (Marshall 1979), the of  a  retinal  tapetum  r e t i n a l summation cells  ( A l i and A n c t i l  (Walls  1942)  and  a high d e n s i t y  evolution  s e l e c t i v e pressures, alternative  there  In  i r r a d i a n c e environments,  silty  fish  the  changes  important  rocky  environments for prey  most  extreme  cave f i s h on  in  case in  of  fish  rhomboides  cues  retinal  other  example  in low  structure  which  c r e v i c e s , on muddy bottoms, in  (Carr  et  i s found  a l . 1976)  can o r i e n t a t e p r e c i s e l y to stimuli.  sensory  locating  Exploratory  (McBride et a l . 1962)  behaviour and cod  such  low  m o d a l i t i e s have become chemical  senses.  (Walls  and which r e l y 1942).  I c t a l u r u s spp.  items  by  i n sharks (Brawn  In  in the numerous species of  prey and  food  various  in lower a c u i t y .  l o c a t i o n , most notably the  for  the  living  i n which the eye has almost disappeared  chemical  rod  during  between  r i v e r s and e s t u a r i e s or in very deep water.  irradiance  The  live  that  i n a compromise  increase s e n s i t i v i t y also result Many  long  has been an i n t e r p l a y of competing  resulting  advantages.  of  s t r u c t u r e of s p e c i e s  from d i f f e r e n t p h o t i c environments i t appears of  presence  1977), a high degree of  (Lockett 1977). By comparing the eye  course  the  1969)  Lagodon  (Atema  following  1971)  chemical  (Tester 1963), salmon increases  in  the  presence of the a p p r o p r i a t e v e r t e b r a t e or i n v e r t e b r a t e e x t r a c t s . In  a few  s p e c i e s of f i s h e l e c t r i c a l and mechanical  stimuli  3  are a l s o used clavata  for locating  prey.  Scyliorhinus  sp. and  can l o c a t e t h e i r prey, Pleuronectes p l a t e s s a , s o l e l y on  the b a s i s of e l e c t r i c a l  s t i m u l i generated  1971). Other types of f i s h , p a r t i c u l a r l y using sound s t i m u l i  (Nelson and Gruber  C o n s i d e r a b l e comparative function  of  sensory  little  sensory  known  prey  (Kalmijn  sharks, can d e t e c t prey  1963, Banner 1972).  systems i n f i s h with major d i f f e r e n c e s i n and  is  by the  i n f o r m a t i o n e x i s t s on the form and  t h e i r p h y l o g e n e t i c background about  how  spatial  distribution.  systems  sympatry.  This  is a  particularly  important  s t u d i e s on resource p a r t i o n i n g between  Yet,  have evolved i n  c l o s e l y r e l a t e d or e c o l o g i c a l l y s i m i l a r s p e c i e s which  are  Raja  occur  in  consideration in  animals.  These  studies  based on Gause's p r i n c i p l e that two species cannot c o - e x i s t  i n d e f i n i t e l y on the same l i m i t i n g numerous  studies  have  resource  revealed  ecological  d i f f e r e n c e s between seemingly  similar  Hartley  MacArthur  1953,  Betts  1955,  (Harden 1960). and  sympatric 1958,  behavioural  s p e c i e s (e.g. Glova  attempt has been made t o look f o r d i f f e r e n c e s  While  in  1978), no  the  sensory  c a p a c i t i e s of these same animals. Throughout  northern  examples of salmonid spatial  and  species  dietary  1978). Cutthroat t r o u t as  trout)  and  British  in  Waters  as  Columbia  Varden charr)  sympatry  (Newman 1965,  ( Salmo c l a r k i  Dolly  h e r e a f t e r r e f e r r e d to coastal  and  regions there are numerous  living  segregation  Hartman 1965, Everhart  to  temperate  and  showing  1956, K a l l e b e r g 1958, Nilsson  1967,  Hume  c l a r k i ; hereafter referred  charr commonly  (  Salvelinus occur  malma;  together  lakes (Andrusak and Northcote  in  1970).  4  When sympatric these f i s h are s p a t i a l l y  segregated  during  the  s p r i n g and summer. The t r o u t are found i n the s u r f a c e waters and feed p r i m a r i l y on zooplankton and surface, i n s e c t s . The c h a r r are found  i n deeper  waters and feed on benthic organisms  l e s s e r extent zooplankton. Zooplankton common i n the d i e t of both portion  of  the d i e t of t r o u t  54%; Armitage the  fish  entire  column  i s the only prey resource  although  s i t u a t i o n s both  (Andrusak  behaviour of sympatric t r o u t and that  trout  fish  and Northcote  In a s e r i e s of l a b o r a t o r y experiments  found  a  larger  occupy  and feed on a wide range of b e n t h i c ,  p l a n k t o n i c and s u r f a c e organisms  (1972)  i t forms  (48 t o 78%) than of c h a r r (13 t o  1973). In a l l o p a t r i c water  and t o a  charr,  examining Schutz  1971).  the feeding  and  Northcote  captured a g r e a t e r number of s u r f a c e  prey per u n i t searching time than d i d c h a r r . When presented with exposed the  benthic prey, the rate of prey capture  charr.  Unlike  charr,  trout  was  greater i n  were unable t o l o c a t e b u r i e d  benthic prey. Although both s p e c i e s showed a decrease  i n r a t e of  prey capture with d e c r e a s i n g i r r a d i a n c e l e v e l s , the e f f e c t much more pronounced Differences  in trout.  in  the  spatial  distribution  behaviour of sympatric t r o u t and c h a r r are c l e a r l y While  and  little  i s known about  systems  of  s p e c i e s have evolved t o allow them t o forage i n t h e i r  to  Without  f o r food  the processes i n v o l v e d i n prey  d e t e c t i o n , p a r t i c u l a r l y how the sensory  environments.  feeding  demonstrated.  these d i f f e r e n c e s may reduce d i r e c t c o m p e t i t i o n  resources,  was  these  two  respective  t h i s type of i n f o r m a t i o n , i t i s d i f f i c u l t  understand and p r e d i c t the  nature  of  food  acquisition  in  5  cohabiting The  species. objective  of t h i s study i s to determine whether  are any d i f f e r e n c e s i n the v i s u a l and of  sympatric  differences  trout in  and  their  charr  spatial  chemoreceptive  that  are  there  abilities  r e l a t e d to observed  distribution  and  diet.  The  hypotheses to be t e s t e d are as f o l l o w s :  (i)  At  high  i r r a d i a n c e l e v e l s , the d i s t a n c e at which  prey t a r g e t s can f i r s t in  be detected  visually  sympatric t r o u t than i n sympatric  charr.  ( i i ) The minimum i r r a d i a n c e l e v e l r e q u i r e d prey  detection  sympatric  is  lower  e f f e c t on the d i s t a n c e  first  detected  At high  visually  irradiance  have  the  at which prey t a r g e t s are  i n sympatric t r o u t and c h a r r .  irradiance levels,  an  increase  i n the  movement or c o n t r a s t of prey t a r g e t s r e s u l t s i n  a greater  If  than i n  trout.  same  size,  for visual  i n sympatric charr  ( i i i ) Changes i n the q u a l i t y of  (iv)  i s greater  increase  in  the  t a r g e t s are f i r s t  detected  than i n sympatric  charr.  distance visually  t e s t s of hypotheses one through three  at  which  i n sympatric  these trout  r e v e a l any d i f f e r e n c e s  6  i n the v i s u a l a b i l i t i e s of these two  species  of sympatric t r o u t and  will  (v) There are  be examined to t e s t the  differences  sympatric t r o u t and in  charr  r e t i n a s of  examined to t e s t the  (vi)  retinal  that can  There  hypothesis:  structure  explain  of  differences that  are  trout  and  charr  will  be  hypothesis:  are  differences  a l l o p a t r i c charr  differences  retina  in t h e i r s p a t i a l d i s t r i b u t i o n .  allopatric  between sympatric and and  in  the  t h e i r v i s u a l r e a c t i o n to prey t a r g e t s and  r e l a t e d to d i f f e r e n c e s  As w e l l , the  charr,  allopatric  in  retinal  trout  that that are  and  structure sympatric  r e l a t e d to observed  in t h e i r s p a t i a l d i s t r i b u t i o n .  If d i f f e r e n c e s e x i s t i n the v i s u a l a b i l i t i e s  of t r o u t and  charr,  the next step w i l l  be to determine the p o t e n t i a l volume of water  searched  by each species  per  day  f o r d i f f e r e n t prey types.  The  hypothesis to be t e s t e d i s :  ( v i i ) There i s no d i f f e r e n c e  in the  searched v i s u a l l y per day  sympatric t r o u t and  for  prey.  by  volume  of  water charr  7  A t e s t of t h i s hypothesis w i l l the  tests  of  information visual  hypotheses  on the d i e l  foraging  be t e s t e d are as  require one  information  through  obtained  three,  as  well  i r r a d i a n c e l e v e l s in Loon Lake  v e l o c i t y of t r o u t and  c h a r r . The  as  and  the  hypotheses to  follows:  ( v i i i ) There i s no d i f f e r e n c e in the number per  from  day  that  sympatric  trout  and  of  hours  charr can  forage  visually.  (ix)  There i s no d i f f e r e n c e  in  v e l o c i t y of sympatric t r o u t and  The  potential  prey d e t e c t i o n t e s t i n g the  the  visual  charr.  importance of the chemoreceptive system f o r  in sympatric t r o u t and  charr w i l l  be evaluated  charr  not  by  hypothesis:  (x)  Sympatric  trout  and  do  chemoreceptive system f o r l o c a t i n g prey  If  foraging  hypothesis  ix i s r e j e c t e d , the  use  their  targets.  f o l l o w i n g hypothesis w i l l  be  tested:  (xi)  The  use  prey t a r g e t s  of the chemoreceptive system for l o c a t i n g in  sympatric  trout  and  charr  is  not  8  c o n t r o l l e d by the ambient  Tests  of  the  effectiveness sympatric environment  above of  trout  the  hypotheses visual  irradiance  will  and  level.  be used to e v a l u a t e the  chemoreceptive  systems  of  and charr f o r l o c a t i n g prey i n r e l a t i o n to the  in which these f i s h  live.  9  2.0 MATERIALS AND METHODS Trout obtained  and from  Loon  Forest, B r i t i s h trap  nets  charr  used Lake,  Columbia  behavioural  located  in  experiments  the  were  U. B. C. Research  ( F i g . 1). Most f i s h  were  caught  with  in the e a r l y s p r i n g . O c c a s i o n a l l y a d d i t i o n a l animals  were c o l l e c t e d with g i l l being  in  transported  to  nets i n the  the  University  animals were held o u t s i d e i n  1300  L  temperature v a r i e d between 7.6 and  2.J_  summer  and  fall.  of B r i t i s h fibreglass  After  Columbia tanks.  the  Water  12.1° C over the year.  Laboratory S t u d i e s  2_.J_.J_ V i s u a l Feeding Experiments Experiments  in  which  fish  used  their  l o c a t e prey t a r g e t s were d e f i n e d as v i s u a l In  these  the  fish,  target  experiments  v i s u a l system to  feeding  experiments.  measurements of the r e a c t i o n d i s t a n c e of  the maximum d i s t a n c e between the  when the t a r g e t was  first  fish  and  the  s i g h t e d , were made. A sequence  of  observable, r e p e a t a b l e behaviours e x h i b i t e d by the f i s h  a  prey  target  was  sighted  (see  determine the p o s i t i o n of the f i s h All tank for port  section  *  3.1)  were used to  from  a  used  as  a  o b s e r v i n g the f i s h . The unpainted s e c t i o n was covered  with a one-way m i r r o r . The fluorescent  Plexiglass  20 * 30 cm. The tank was p a i n t e d f l a t white except  one s t r i p along the length of one side which was for  when  i n these experiments.  b e h a v i o u r a l o b s e r v a t i o n s were made  300  prey  Vita-Light  radiation  source  tubes suspended  was  provided  by  50 cm above the s u r f a c e  1. A contour map of Loon Lake showing the i r r a d i a n c e sampling s t a t i o n . Depths i n meters.  12  of  the water. The tubes were encased  where  the  amount  with a diaphram  and  flow-through  box  and q u a l i t y of i r r a d i a n c e c o u l d be r e g u l a t e d filters.  Observations  l e v e l s were made using an i n f r a r e d A  i n an i r r a d i a n c e - p r o o f  system  at  low  irradiance  i r r a d i a n c e source and viewer.  replaced  the  water  in  the  tank  approximately once every 27 minutes. The  spectral  irradiance  emission  source  used  emissions o c c u r r e d at wavelength this  properties  of  the  i n t h i s study are shown i n F i g . 2. Peak  450  nm,  dropping  off rapidly  decreased and slowly as the wavelength  point.  81%  as the  i n c r e a s e d from  The amount of i r r a d i a n c e emitted at 350 and 725 nm  r e l a t i v e to the amount of i r r a d i a n c e emitted at 450 and  Vita-Light  respectively.  The  spectral  nm  distribution  V i t a - L i g h t source was s i m i l a r to that of n a t u r a l  light  was  25  of the on the  e a r t h ' s s u r f a c e at mid-day (Young 1974). The l i n e v o l t a g e of the irradiance  source  was  held  constant  to  ensure the s p e c t r a l  d i s t r i b u t i o n d i d not change. S p e c t r a l t r a n s m i s s i o n c h a r a c t e r i s t i c s of the to  manipulate  wavelength in  the  irradiance  quality  are  blue,  green,  yellow  wavelength  and range  red  filters,  over  occurred was v a r i a b l e among the f i l t e r s ,  used  i n F i g . 3. The  of maximum t r a n s m i s s i o n was 420, 530, 590 and 640  Although the t o t a l  200  shown  filters  which  nm  respectively. transmission  i t was always  l e s s than  nm. Measurements  quantum meter sensor.  of  (model  Measurements  i r r a d i a n c e l e v e l s were made with a L i - C o r 185-A) and an attached  underwater  were made 0.2 cm above the water  quantum surface.  2. R e l a t i v e photon d i s t r i b u t i o n spectrum of V i t a - L i g h t .  over the wavelength  RELATIVE o  o  o  o  p  PHOTON o  NUMBER o  p  o  o  3. R e l a t i v e photon d i s t r i b u t i o n over the wavelength spectrum of V i t a - L i g h t passed through blue, green, yellow and red f i l t e r s .  400  500  600  700  400  WAVELENGTH  500  (nm)  600  700  17  The s p e c t r a l composition of through water  various  coloured f i l t e r s  surface using  (model  an  1  types  mm  artifical Loon  of  Light  prey, one a r t i f i c a l  in  experiments.  diameter and  prey. Daphnia Lake  using  Cylinders  a 183  head  and  kenai  beyond  surface at positions  introduced  to  rosea  and  Diaptomus  the  above  the  tank  the  other  end.  along  the  length  at one end and The  tubes  of  the  were  end  of  measured.  All  tank at a d i s t a n c e through  tubes  1 cm below the water  were tank.  randomized between  located  at  six  The tube used f o r successive  trials.  a l l experiments, except those used to examine the e f f e c t s of  to  rest  on  the  (see below), the  recorded.  reaction  distance  The p o s i t i o n of prey t a r g e t s and f i s h  detected the t a r g e t s was back of the tank.  prey  bottom of the tank. I f a f i s h  t a r g e t before i t came to r e s t , the  the  kenai  anterior  observation  prey motion on r e a c t i o n d i s t a n c e came  obtained  (1975). The length of Daphnia  c o u l d detect them v i s u a l l y  i n t r o d u c t i o n of prey was In  ,  the base of the s p i n e . For Diaptomus kenai , the  which the f i s h  extending  chicken  m mesh tow net, were the n a t u r a l  metasome and urosome (excluding caudal rami) was were  of  long were used as the  Diaptomus  as i n Northcote and C l a r o t t o  prey  cm above the  spectroradiometer  rosea was d e f i n e d as the d i s t a n c e between the the  passed  and two n a t u r a l , were  1,3 or 5 mm  rosea and  prey. Measurements of Daphnia made  Vita-Light  was measured 0.2  International  used in the v i s u a l feeding  from  and  700).  Three  liver  Vita-Light  determined from a s c a l e  targets sighted a was  not  when they  first  located  along  18  The ability  effect  2  motion  of t r o u t and  pinning cm  of  the  charr  to  detect  tests  i n d i c a t e d that the in  the  n a t u r a l prey, Daphnia rosea placing  the  s t a r t of an The  animals  and  The  inherent  inherent  liver  contrast  targets  (C  on  5  food.  70 s" . 1  respond to  Motion , was  by  The  mm  in  the  stopped by  reaction distance the  inherent  liver  the  (C  the  the  target  Therefore:  of a r t i f i c a l prey were used.  = 0.75).  water soaked l i v e r  Low  were produced by soaking  the  target.  It  was  assumed  that  any  the amount or type of chemical s t i m u l i r e l e a s e d  distance  quantitatively,  of prey  t a r g e t s , reddish-brown in c o l o u r , were used for  by  rosea  trout  (C ) i s equal to  T h i s produced a white c o l o u r e d  in  f r e s h and  of  contrast  ( B ) ( l e Grand 1967).  = 0.12)  differences  Daphnia  of  in the amount of i r r a d i a n c e r e f l e c t e d by  the high c o n t r a s t prey  reaction  of  c o n t r a s t of a t a r g e t  in water for 24-h.  Unsoaked  rate  Diaptomus kenai  examined by a l t e r i n g  l e v e l s of  liver  examined  in 30° C water f o r 3 minutes p r i o r to  (L) r e l a t i v e to the background  contrast  a  the  experiment.  charr was  difference  was  f i s h would not  absence  e f f e c t of prey c o n t r a s t  targets.  prey t a r g e t s on  them  c l e a r P l e x i g l a s s p l a t f o r m moved at  the moving p l a t f o r m  Two  artifical  prey on a small v e r t i c a l l y moving p l a t f o r m .  Preliminary  and  of  and was  t a r g e t s had  of t r o u t or c h a r r . The Diaptomus increased  kenai, by  no  effect  on  the  degree of c o n t r a s t  although  not  of  measured  immersing them b r i e f l y  in a  19  s a t u r a t e d s o l u t i o n of Sudan Black B  (water  insoluable  stain).  P r e l i m i n a r y ' experiments  i n d i c a t e d that changing the c o n t r a s t of  artifical  prey  and  palatibility All  natural  targets  d i d not  animals  to  their  t o the f i s h . used  in  visual  feeding  maintained i n the o b s e r v a t i o n tank f o r a prior  affect  the  experiments  minimum  one  week  s t a r t of an experiment. During t h i s p e r i o d , they  were given d a i l y a l l o t m e n t s of f r e s h  liver  amount  quantity  they  of  were  would  consume.  The  in  excess and  of the  quality  of  i r r a d i a n c e was set at the same l e v e l to be used i n the f o l l o w i n g experiment. In a l l cases, the animals light-8-h  dark  were  placed  initial  transfer  of  a  fish  from  o b s e r v a t i o n tank, the water temperature Subsequently,  a  16-h  i r r a d i a n c e regime except during the 24-h p e r i o d  preceding an experiment when i r r a d i a n c e was the  on  water  temperature  changed at a maximum r a t e of 1° C  in per  continuous. the  holding  i n each the  was  During to the  the  same.  o b s e r v a t i o n tank was  day  until  the  desired  experimental temperature was achieved. In  a l l visual  feeding experiments at l e a s t  f i v e t r o u t and  f i v e charr were used. These animals ranged i n s i z e  from 18.1  to  24.3 cm except i n experiments designed t o examine the e f f e c t s of fish fish  s i z e on r e a c t i o n d i s t a n c e . In the l a t t e r size  set of experiments  ranged from 10.7 to 23.2 cm. Unless otherwise noted,  each datum p o i n t r e p r e s e n t s the  mean  of  100  replicates.  number of r e p l i c a t e s per f i s h ranged from 14 to 20.  The  20  2.J_.2 F o r a g i n g V e l o c i t y Foraging  velocity  of  t r o u t and charr was measured i n the  same o b s e r v a t i o n tank used f o r the v i s u a l At  the  start  of  i n t o the tank and t h i s was  intervals  until  the  experiment  a d d i t i o n proved s u f f i c i e n t the  The  velocity  distance  p e r i o d was cm,  experiments  traversed  recorded. Ten f i s h ,  of each s p e c i e s were used  feeding,  at  one  minute  terminated. T h i s r a t e of  irradiance  and  earlier  for visual  were  x 10 * and 4.0 1  by  searching  behaviour  conducted  under s i x  x 10  19  photons  at  each  temperature  irradiance history  identical  to  to 20.1  level.  of f i s h that  The  used in  described  Non-Visual Foraging Behaviour designed to determine i f t r o u t or charr  c o u l d l o c a t e a r t i f i c a l prey t a r g e t s i n s t i l l  water at s u b - v i s u a l  i r r a d i a n c e l e v e l s . The bottom of the o b s e r v a t i o n tank was a  grid  cm. Ten  f r e s h 3 mm  the  to  0.55  l i v e r prey t a r g e t s were p l a c e d underneath the  g r a v e l on randomly s e l e c t e d squares while a f i s h was of  marked  of 1 cm squares and then covered to a depth of 2 cm  with g r a v e l . Stone s i z e s of the g r a v e l v a r i e d from 0.20  end  2  feeding experiments.  T h i s experiment was  in  m~  each f i s h d u r i n g a 10-minute  ranging in s i z e from 18.6  f o r a g i n g v e l o c i t y experiments was  2.\_.3  was  to maintain the  i r r a d i a n c e l e v e l s between 6.1 1  repeated  was  fish.  Foraging  s" .  experiments.  each experiment an a r t i f i c a l prey t a r g e t  introduced  of  feeding  h e l d at one  tank by a removable opaque p a r t i t i o n . The  s t a r t of  21  each experiment was marked by During  the  the  removal  was  located  It  the  was  assumed  observation  tank  At  the  was drained  end  In a second set of  other  tank  was  c o n d i t i o n s were  motion  was  experiments replaced  as  in  the  still  of  each  the l i v e r . water  i n the  the  previous  experiment.  Water  c r e a t e d by p l a c i n g two water pumps at e i t h e r end of  including  i t s length.  intake and o u t l e t , were completely  on 1 0 min p r i o r to the s t a r t of an experiment which the  removal  of the p a r t i t i o n  were l e f t on f o r the d u r a t i o n  turned  was  marked  r e s t r a i n i n g the f i s h and they  of the experiment. The  pumps d i d  not c r e a t e any noise or v i b r a t i o n which c o u l d be detected observer the  nor  The  submerged.  Each pump moved 7 . 3 L of water per minute. The pumps were  by  a  by t u r b u l e n t moving water. A l l  the tank and three at e q u i d i s t a n t p o i n t s along pumps,  that  and the tank and  g r a v e l washed to remove any odours a s s o c i a t e d with  observation  partition.  when the f i s h began snapping at and moving  the g r a v e l immediately above the t a r g e t . experiment  the  next ten minutes the number of prey t a r g e t s l o c a t e d  and consumed by the f i s h was recorded. target  of  by the  any apparent e f f e c t s on the f o r a g i n g behaviour of  fish. In a t h i r d set of experiments the pumps  the  liver  targets  were  placed  in  sealed  were  removed  transparent  c y l i n d e r s . Again a l l other c o n d i t i o n s were as d e s c r i b e d first  and glass  i n the  experiment. All  sub-visual  non-visual  foraging  experiments  i r r a d i a n c e l e v e l s , using  were c a r r i e d out at  10 t r o u t and 1 0 c h a r r .  22  2.2 I r r a d i a n c e L e v e l s  i n Loon Lake  D i e l changes i n the i r r a d i a n c e l e v e l s measured at  1-h  in  Loon  Lake  were  at s t a t i o n 1 ( F i g . 1) on J u l y 8-9 and J u l y 28-29, 1981 intervals.  Irradiance  levels  were  recorded  at  1 m  i n t e r v a l s i n the upper 40 m of the water column using the L i - C o r quantum  meter  described  e a r l i e r . Percent  s u r f a c e c o n d i t i o n s were recorded  at  the  c l o u d cover and water same  time  irradiance  measurements were made.  2.3 Sympatric t r o u t used  in  were  (19.2 - 20.8 cm) and charr  histological  Allopatric  trout  obtained  (Andrusak  studies  were  from  Placid  and  (18.5  Dickson  Lake  -  20.7  cm)  respectively  1968). Ten animals of each s p e c i e s were c o l l e c t e d from a l i v e to  the  University  of  where they were held at 10° C on a 16-h l i g h t  i r r a d i a n c e regime f o r seven days. At the end of animal was beheaded while eye  (19.7 - 21.4 cm)  c o l l e c t e d from Loon Lake.  (18.0 - 19.6 cm) and charr  each lake and t r a n s p o r t e d Columbia  Eye H i s t o l o g y  one  British 8-h dark  week  each  i n a l i g h t s - a d a p t e d s t a t e and the r i g h t  removed. The cornea and lens of each eye was removed and the  whole  eye  cup  was  fixed  i n Bouin's f o r 48-h. The eye cup was  then t r a n s f e r e d to 70% ethanol  and held f o r f u r t h e r  processing.  P r i o r to embedding, a 1 mm s e c t i o n of m a t e r i a l l y i n g at the i n t e r s e c t i o n of the d o r s a l - v e n t r a l and the  temporal-nasal  axes  of  eye cup was removed and dehydrated. T h i s t i s s u e , c o n t a i n i n g  the r e t i n a , was then embedded i n  Spurr's  medium.  One-half  m  23  transverse  and t a n g e n t i a l s e c t i o n s were cut on a R e i c h e r t OM U3  ultramicrotome. Sections, were s t a i n e d i n methylene b l u e . Q u a n t i t a t i v e e v a l u a t i o n of the r e t i n a was accomplished a micrometer f i t t e d counts  to the o c u l a r of a compound microscope.  of the d e n s i t i e s of rod, cone, b i p o l a r and g a n g l i a  were taken from t a n g e n t i a l s e c t i o n s each  (Ali  Anctil  Ten cells  1976) of  eye. Measurements of the diameter of s i n g l e cones, and the  longest and s h o r t e s t axes of e l l i p t i c a l l y were  and  with  made  in  the  region of the e l l i p s o i d outer  cones of each type were measured i n each Comparison of non-visual  foraging  shaped  reaction  distance,  behaviour  r e t i n a l h i s t o l o g y i n sympatric  cones  segments. Ten  retina. foraging  i n sympatric and a l l o p a t r i c  were made by a n a l y s i s of v a r i a n c e .  double  velocity  and  t r o u t and charr and trout  and  charr  24  3.0 RESULTS  _3.J_ General Feeding Trout brought  and  charr  into  variable  the  among  did  not  laboratory.  animals,  was  Behaviour  immediately This  begin feeding when  latent  period,  although  generally greater for trout  c h a r r . The mean number of days to f i r s t  than  feeding ± 95% c o n f i d e n c e  l i m i t s of 20 animals of each s p e c i e s was  12 ± 2.1  and  6 ±  1.8  days f o r t r o u t and c h a r r , r e s p e c t i v e l y . When  the  animals  began  using t h e i r v i s u a l system was  preceded  to feed i t was  to l o c a t e prey  targets.  Each  by the f i s h o r i e n t a t i n g to and v i s u a l l y  the prey t a r g e t . At the same time the forward animal  obvious they were  stopped  attack  fixing  on  of  the  movement  and the p e c t o r a l f i n s f l a r e d out. These events,  which occurred over approximately 0.5  s, were used  to  identify  the p o s i t i o n of the f i s h when the t a r g e t a p p a r e n t l y f i r s t visible  to  t a r g e t was have  i t . The d i s t a n c e between t h i s p o s i t i o n and the prey used to d e f i n e r e a c t i o n d i s t a n c e . S i m i l a r  been a s s o c i a t e d with the f i r s t  Salvelinus  gibbosus (Werner  (Confer and  Hall  fontinalis and  Blades  1974,  (Confer 1975)  behaviours  s i g h t i n g of prey t a r g e t s i n  S a l v e l i n u s namaycush ( K e t t l e and O'Brien 1978),  became  1978, et  and  Confer  al.  al.  1978), Lepomis  Lepomi s  V i n y a r d and O'Brien  et  macrochi rus  1976)  and used to  prey  target  d e f i n e r e a c t i o n d i s t a n c e of these s p e c i e s . F o l l o w i n g the s i g h t i n g of an fish  would  approaches  move  rapidly  towards  artificial  i t . On the f i r s t  the f i s h would not touch or consume the  the  s e r i e s of  prey.  After  25  two  • t o three  days of t h i s type of behaviour the f i s h began to  consume the t a r g e t s but i n most expelled.  It  was  only  cases  after  prey  they targets  were  immediately  had been present  c o n t i n u o u s l y f o r f i v e to seven days that the f i s h would  consume  and r e t a i n them. Trout  and  charr  moved back and f o r t h along the l e n g t h of  the o b s e r v a t i o n chamber while s e a r c h i n g f o r prey t a r g e t s  resting  on the bottom. O c c a s i o n a l l y the f i s h , p a r t i c u l a r l y c h a r r ,  would  s t r i k e at non-prey these  targets  t a r g e t s such as small stones and twigs. Often  were  consumed and r e j e c t e d s e v e r a l times before  the f i s h resumed i t s s e a r c h i n g behaviour. While the behaviour of the f i s h at the time of  attack  located  visually,  consumption  non-prey  t a r g e t s e v e r a l times  sensory  stimuli  the  indicated and  suggested  targets  were  being  r e j e c t i o n of a s i n g l e that  other  types  of  were used i n making the d e c i s i o n of whether or  not t o r e t a i n the item.  3.2 The 18  Ef f e c t of Experience on React ion  Di stance  r e a c t i o n d i s t a n c e of t r o u t and charr was  recorded f o r  days a f t e r they passed through the l a t e n t phase and began to  consume and r e t a i n prey t a r g e t s . was  short  i n both  species  Initially,  but  increased  reaction  t o a maximum with  i n c r e a s i n g p r e v i o u s f e e d i n g experience ( F i g . 4 ) . Trout a  maximum  days a f t e r more  reaction  distance  the i n i t i a t i o n  rapid,  reaching  distance  attained  t o prey t a r g e t s approximately 12  of f e e d i n g . The response of charr  their  was  maximum i n s i x days. The range i n  estimates of r e a c t i o n d i s t a n c e i n both  species  decreased  when  4. E f f e c t of experience on the r e a c t i o n d i s t a n c e of f i v e c u t t h r o a t t r o u t and f i v e D o l l y Varden charr to 3 mm a r t i f i c i a l prey t a r g e t s at an i r r a d i a n c e l e v e l of 4.0 x 10 photons m~ s"'. Each datum point represents the mean of 50 o b s e r v a t i o n s . V e r t i c a l l i n e s i n d i c a t e the range. 1 9  2  27  90 i  i  80  •  I •  i •  , i I •  I •  I  i  |  T i •  60  E  40  O  LU O IZ <  20  CUTTHROAT  TROUT  co r  0 40  O O < LU  T  • I  •I TI ? I '  •  '  *  '*  30  cr 20  10  DOLLY VARDEN C H A R R  o  io  DAYS A F T E R FIRST  15  FEEDINGS  28  they  reached While  t h e i r maximum r e a c t i o n d i s t a n c e . both  trout  d i s t a n c e w i t h i n the values for  were  and  18-day  different  charr feeding  x  10  photons  1 9  corresponding subsequent  value  nr  2  the  absolute  s~  t a r g e t s at an i r r a d i a n c e l e v e l of was  1  f o r charr  experiments  experiment,  ( F i g . 4 ) . The maximum r e a c t i o n d i s t a n c e  t r o u t feeding on 3 mm prey  4.0  a t t a i n e d a maximum r e a c t i o n  had  a  approximately  was  35  minimum  cm. of  85  cm.  The  A l l f i s h used i n 15  days  feeding  of prey  targets  experience. Capture captured  success,  a measure of the percent  and consumed on the  first  t r o u t and charr during the i n i t i a l no  significant  change  (P < .05)  strike,  was  monitored  18 days of f e e d i n g . There was i n capture  success  s p e c i e s over  t h i s p e r i o d . Capture success was g r e a t e r  than  averaging  of  3.3  charr  in  97 and 82% r e s p e c t i v e l y over  in either in trout  the d u r a t i o n  the experiment.  Ef f e c t of Food Depr i v a t ion and D a i l y Food Rat ion on React ion Di stance The  r e a c t i o n d i s t a n c e of t r o u t and charr to a r t i f i c i a l  t a r g e t s was recorded a f t e r p e r i o d s of food from  0  to  deprivation  after  at  ranging  96 h (Table 1). There was no s i g n i f i c a n t d i f f e r e n c e  (P < .05) i n r e a c t i o n d i s t a n c e of e i t h e r s p e c i e s to targets  prey  an  irradiance  food was withheld  l e v e l of 4.0 x 1 0  1 9  3  mm  prey  photons n r s "  f o r d i f f e r e n t p e r i o d s of time up  2  1  to 96  hours. The  e f f e c t of the d a i l y  food r a t i o n  f o r t r o u t and charr on  29  Table  1.  E f f e c t of food d e p r i v a t i o n on mean r e a c t i o n (cm ± 95% CL) charr  of  cutthroat  to 3 mm a r t i f i c i a l  radiance  t r o u t and D o l l y Varden  prey  l e v e l of 4.0 x 1 0  1 9  targets  at  an i r -  photons n r s " .  Number of hours of food  24  distance  48  2  1  deprivation  Species  0  72  96  Trout  82.6± 1.6  83.2± 1.2  82.5± 2.4  82.9± 1.8  83.1± 1.7  Charr  34.8± 1.8  33.9± 1.4  33.7± 2.0  34.6± 2.0  34.6± 1.6  30  t h e i r r e a c t i o n d i s t a n c e to a r t i f i c i a l (Table 2 ) .  prey t a r g e t s was  Ration l e v e l s of 1, 3 and 5% of f i s h wet weight were  t e s t e d . The f i s h were maintained on each r a t i o n days  prior  to  the  start  significant difference charr  (P < .05)  experiment.  in  the  different  There  reaction  ration  l e v e l s and p e r i o d s of food d e p r i v a t i o n had no  greater  the  an  or  reaction  at  of  level for  trout  the  3.4  distance  of  d i s t a n c e of t r o u t or c h a r r , r e a c t i o n d i s t a n c e  was  earlier.  on Reaction Di stance  trout  and  ^charr  are  spatially  d u r i n g the summer (Andrusak and Northcote 1971, Hume  1978). During t h i s p e r i o d the  trout  are  concentrated  in  the'  and upper p o r t i o n s of the water column while the benthic  o r i e n t a t e d charr are found i n deeper water. The thermocline Clarotto  in  1974)  Loon  Lake  indicates  environments of d i f f e r e n t Because dependent  of  the  visual a b i l i t i e s out  at 5, 10, 15 commonly  no  on  Sympatric p o p u l a t i o n s of  carried  was  effect  Ef f e c t of Temperature  segregated  seven  r a t i o n l e v e l s t e s t e d . While  (P < .05) i n t r o u t as d e s c r i b e d  middle  examined  presence  of  a  at approximately 8 m (Northcote and that  these  species  are  living  in  temperature. potential  importance  of  temperature  i n t r o u t and c h a r r , experiments  were  to determine the r e a c t i o n d i s t a n c e of both s p e c i e s and  20° C  (Table 3),  a  range  in  temperatures  encountered i n Loon Lake. F i s h used i n t h i s study were  a c c l i m a t e d to the experimental water temperature f o r a p e r i o d of seven days p r i o r to the s t a r t of the experiment. Neither  species  showed any s i g n i f i c a n t change (P < .05) i n r e a c t i o n d i s t a n c e  to  31  Table 2.  E f f e c t of d a i l y food r a t i o n on mean r e a c t i o n (cm ± 95% CL) charr  to 3 mm  of  trout  a r t i f i c i a l prey t a r g e t  l e v e l of 4.0 x 1 0  Daily  cutthroat  19  photons m~  2  distance  and D o l l y at an  Varden  irradiance  s" . 1  food r a t i o n as a percent of f i s h wet weight  Species  1  3  5  Trout  84.0  ± 2.2  83.4  ± 2.0  Charr  33.7  ± 2.0  34.3 ± 1.9  84.1  ±  1.2  33.9 ±  1.5  32  Table 3.  E f f e c t of water temperature on mean r e a c t i o n (cm ± 95% CL) of charr  to 3 mm  cutthroat  artificial  diance l e v e l of 4.0 x 1 0  trout prey  1 9  and  D o l l y Varden  targets  photons n r  2  distance  at an  irra-  s" . 1  Water temperature (°C)  10  Spec i e s  15  20  Trout  84.0 ± 1.1  83.4  ± 1.8  83.9 ± 1.5  83.1  ±  1.7  Charr  34.2 ± 1.8  34.2 ± 1.5  33.6 ± 2.1  34.7 ±  1.4  33  3 mm prey t a r g e t s at the d i f f e r e n t water temperatures.  3.5 Trout  Ef f e c t of F i s h Size and  charr,  on React ion  ranging  in  size  Distance from 10.7 to 23.2 cm  ( t o t a l length) were examined to determine i f there were any s i z e dependent  changes  in  visual  abilities  (Table 4 ) . Over  the  s i z e - r a n g e examined there was no s i g n i f i c a n t change (P < .05) i n reaction The  d i s t a n c e to 3 mm a r t i f i c a l  possibility  exists  (younger) animals  is  that  less  prey t a r g e t s i n e i t h e r  the  visual  effective  system  although  of  this  fish.  smaller was  not  tested.  3.6  E f f e c t of I n f r a r e d  Observation the  observation  An i n f r a r e d  determine  i f the  artifical  The  eye  required  irradiance  fish  reaction  required  Subsequently irradiance  reaction level  (Table 5 ) . Neither (P > .05) i rradiance.  to  in  but  sensitive  distance  produce  distance  of  reaction  to  trout  the and  of  was  measured  in  at  irradiance  any the  infrared charr  levels  a maximum r e a c t i o n  showed  distance  method  i t was necessary  irradiance  with an i n f r a r e d  species  another  l e v e l s but f i r s t were  l e v e l s below  source and viewer permitted  prey t a r g e t s was measured at  those  Distance  and c h a r r at i r r a d i a n c e  at low i r r a d i a n c e  irradiance.  than  trout  s e n s i t i v i t y of the human  surveillance.  to  of  I r r a d i a n c e on React ion  to less  distance. the  same  source added  significant  change  presence of i n f r a r e d  34  Table  4.  Effect (cm  of t o t a l body l e n g t h on mean r e a c t i o n  ± 95% CL) of c u t t h r o a t  c h a r r to 3 mm diance  artificial  l e v e l of 4.0 x 1 0  trout prey  1 9  and  Body length  Species  10.0-14.9  1.0  Trout  83.7 ±  Charr  34.7 ± 1.2  Dolly  targets  photons n r  2  distance  at an  Varden irra-  s" . 1  (cm)  15.0-19.9  20.0-24.5  83.1 ± 1.3  84.4 ± 1.2  33.211.9  34.011.7  35  Table 5.  E f f e c t of the presence (P) or absence i r r a d i a n c e on mean r e a c t i o n d i s t a n c e cutthroat ficial of  (cm ± 95% CL) of  t r o u t and D o l l y Varden charr  prey t a r g e t s .  infrared  level  (A) of i n f r a r e d  was  In the  irradiance 4.2  x 10  17  the  presence  1 5  artiabsence  irradiance  photons nr  s~  2  1  respectively.  Trout  Charr  p  57.2±2.3  and  Vita-Light  and 3.0 x 1 0  for the t r o u t and c h a r r ,  to 3 mm  A  57.4  '~  ±  1.9  P  17.5 ± 1-7  A  17.1  ±1.4  36  3 . 2 E f f e c t of I r r a d i a n c e  on:  The  rate at which a predator w i l l  1)  the  prey d e n s i t y ,  predator and first  the prey and  becomes  following the  C h a r a c t e r i s t i c s on React ion Di stance  2) the  3) the  visible  to  the  effects  preceding  of the  two  experiments  experimental c o n d i t i o n could  be  2 . 2 * 1  Irradiance  and  ( F i g . 5). In increasing  visually,  which  provide  of  the  the prey  1966).  information  and  establish  e f f e c t s of  charr. a  The on  i r r a d i a n c e , amount trout  to  in which the  and The  "standard"  these  variables  Level  irradiance both  level  species  irradiance  to  a  visual  saturation  irradiance  saturation 18  prey  than  2  1  distance  targets  Both the  threshold  s" ,  s i m i l a r in t r o u t and  irradiance  irradiance  photons nr  was  reaction charr  increased  with  a v i s u a l irradiance threshold,  independent of prey s i z e within The  r e l a t i o n s h i p between  reaction  from  maximum r e a c t i o n d i s t a n c e .  had  (Holling  used  i r r a d i a n c e l e v e l below which  greater  predator  q u a l i t a t i v e nature of the  distance  x 10  at  distance  were  depends  assessed.  The  6.6  distance  c h a r a c t e r i s t i c s of  reaction  prey  r e l a t i v e v e l o c i t y between  experiments were designed to  q u a l i t y , on  and  encounter  were  detected  l e v e l which produced  saturation were  not  the  irradiance  species  the  level  specific  and  the p r e c i s i o n of t h i s study. l e v e l f o r t r o u t was  more than two  orders  the corresponding value f o r charr  a v i s u a l irradiance threshold  of 3.0  x 10  15  approximately of  magnitude  ( F i g . 5). Trout photons n r  2  s~ , 1  5. E f f e c t of the q u a n t i t y of i r r a d i a n c e on the mean r e a c t i o n d i s t a n c e (± 95% CL) of c u t t h r o a t t r o u t and D o l l y Varden charr to 1, 3 and 5 mm a r t i f i c i a l prey targets. Sample s i z e per datum p o i n t equals 100.  39  while the s i m i l a r m~  2  value  was  3.0  x  10 *  photons  1  s" . 1  3_.7.2 I r r a d i a n c e Changes differences charr  Quality  in  irradiance  quality  (P < .05) i n the  to  distance  3  mm  prey  resulted  reaction  targets  This  ;and 27.1  was  cm  significant  of  trout  ( F i g . 6 ) . The l a r g e s t  followed  with  in  distance  f o r t r o u t , 58.1 cm, occurred i n  irradiance. 33.0  f o r charr  green,  reaction  presence  of red  by r e a c t i o n d i s t a n c e s  of 49.4,  yellow  the  and  and  blue  irradiance,  respectively. Qualitatively, distance  in  reaction  in  irradiance same,  charr  distance  . followed  the e f f e c t of i r r a d i a n c e q u a l i t y on r e a c t i o n was  the  occurring decreasing  ( F i g . 6 ) . While,  reaction distance  same as i n t r o u t with the l a r g e s t  i n the presence order the  of c h a r r  by  reaction  distance was  to  green,  qualitative  found  irradiance  yellow response  in  prey t a r g e t s  coloured  irradiance  significantly  reaction  distance  when  Vita-Light  irradiance  source was u n a l t e r e d  the  red  and was  blue the  f o r each q u a l i t y of i r r a d i a n c e  t e s t e d was approximately 30% of that species  of  spectral  less  trout.  In  both  i n the presence of (P < .05)  composition (Fig. 5).  than  of the  6. E f f e c t of i r r a d i a n c e q u a l i t y on the mean r e a c t i o n d i s t a n c e (± 95% CL) of c u t t h r o a t t r o u t and D o l l y Varden charr to 3 mm a r t i f i c i a l prey t a r g e t s . The i r r a d i a n c e l e v e l was 1.03 x 1 0 photons n r s " . Sample s i z e per datum point equals 100. 1 9  2  1  f/  WAVELENGTH  (nm)  42  3.8 E f f e c t of Prey C h a r a c t e r i s t i c s on Reaction  Distance  3.8.j_ Prey S i z e The  previous  quality  experiments have shown that the  quantity  and  of i r r a d i a n c e e f f e c t s the v i s u a l , a b i l i t i e s of t r o u t and  charr when measured i n terms of t h e i r targets.  For  above  in l i t t l e distance  to  prey  both s p e c i e s , there was an i r r a d i a n c e l e v e l below  which the v i s u a l system was not another  reaction distance  which i n c r e a s e s  increase  used  f o r prey  detection  i n the i r r a d i a n c e l e v e l  i n r e a c t i o n d i s t a n c e . As w e l l ,  the  and  resulted reaction  of t r o u t and charr changed under d i f f e r e n t q u a l i t i e s of  i rradiance. While  the  quantity  v i s u a l predators'  and q u a l i t y of i r r a d i a n c e present  environment w i l l a f f e c t i t s a b i l i t y  to  in a detect  prey t a r g e t s , so w i l l c h a r a c t e r i s t i c s of the t a r g e t s themselves.. A  t a r g e t must subtend some c r i t i c a l  before  i twill  be  visible  (Ware  t a r g e t s can be seen at greater contrast predator are  1971).  distances  Consequently,  than small  large  t a r g e t s . The  and movement of a t a r g e t a l s o a f f e c t s the a b i l i t y of a to detect  i t so that high c o n t r a s t  g e n e r a l l y v i s i b l e at greater  stationary  targets  experiments,  the  (Ware  Reaction charr  distances  1971).  distance  of t r o u t and charr increased  2  s~  1  the  moving  targets  than low c o n t r a s t or following  set  of  or greater  was examined.  with prey s i z e  ( F i g . 7 ) . At s a t u r a t i o n  photons m"  In  or  e f f e c t of prey s i z e , movement and c o n t r a s t on  the r e a c t i o n d i s t a n c e  and  angle of the predator's eye  in  both  irradiance levels,  for trout  and  3.0 x 1 0  trout  6.6 x 1 0 1 6  1 8  photons  7. E f f e c t of the s i z e of a r t i f i c i a l prey t a r g e t s on the mean r e a c t i o n d i s t a n c e (± 95% CL) of c u t t h r o a t t r o u t and D o l l y Varden c h a r r . The i r r a d i a n c e l e v e l used f o r each curve i s s p e c i f i e d i n photons nr s " . Sample s i z e per datum p o i n t equals 100. 2  1  45  nr  2  s"  or  1  s i z e was  maximal  (P < 0.5)  with  data were The was  not  greater  for  and  c h a r r , r e a c t i o n d i s t a n c e for any  did  not  show  a  significant  prey  increase  i n c r e a s i n g i r r a d i a n c e l e v e l . Consequently,  these  pooled. i n c r e a s e i n r e a c t i o n d i s t a n c e with  linear  increase  in  ( F i g . 7). Trout reaction  showed a  i n c r e a s i n g prey  proportionally  d i s t a n c e between prey  3 mm  reversed  l a r g e s t increase in r e a c t i o n d i s t a n c e  the  o c c u r r i n g between prey The  relative  a  54.5%  increase  was prey or  nr  2  s~  in  36.3%. Charr  showed a 50.2%  than  3.0  x  i r r a d i a n c e l e v e l of 3.0  10  x 10  15  of  charr for each prey obtained  the  photons n r  18  distance  nr  photons n r  (L  trout  1  as prey  size  x  10  maximum  (Rm)(L) ) + (L)  16  2  s"  2  s"  1  1  the  while  at  as to an  corresponding  24.9%. r e a c t i o n d i s t a n c e of t r o u t  s i z e at s a t u r a t i o n  =  s"  at i r r a d i a n c e l e v e l s equal  irradiance  using the Michaelis-Menton model:  RD  2  increase i n reaction distance  photons  16  i n c r e a s e in r e a c t i o n d i s t a n c e was Estimates  irradiance  increase i n r e a c t i o n d i s t a n c e  s i z e i n c r e a s e d from 1 to 5 mm greater  x 10  At  i r r a d i a n c e l e v e l of 3.0  the corresponding  1  ( F i g . 7).  reaction  At an  and  as the'amount of i r r a d i a n c e  level  to or greater than 6\6  i n c r e a s e d from 1 to 5 mm. photons  was  in s i z e .  s i z e s decreased  below the s a t u r a t i o n  l e v e l s equal showed  5 mm  trend  increase in the r e a c t i o n d i s t a n c e of t r o u t  charr to l a r g e r prey decreased  3 and  This  in s i z e  3 and  charr,  in s i z e .  greater  than they d i d between prey in  5 mm  1 and  size  levels  and were  46  where RD = the  reaction  distance  of a f i s h to a  prey t a r g e t (cm) Rm = the maximum r e a c t i o n distance to a prey t a r g e t L = the i r r a d i a n c e minus  the  (cm) l e v e l of the experiment  irradiance  reaction distance m-  l e v e l where the  equals zero  (photons  s- )  2  L  of a f i s h  1  = the  half  saturation  irradiance  level  minus the i r r a d i a n c e l e v e l where the r e a c t i o n distance  A  non-linear  equals zero (photons n r s ~ ) 2  least  squares  program  was  1  used to o b t a i n  estimates of the parameters Rm and L  . For a l l prey s i z e s  the  estimated  of t r o u t was greater  than  maximum  that of charr  3.8.2  Prey Movement  greater  10  16  distance  by more than a f a c t o r of two (Table 6 ) .  Trout reacted  same  reaction  (P < .05) d i s t a n c e  visual  properties  photons n r  greater  to moving prey t a r g e t s  2  s~  1  than s t a t i o n a r y  at i r r a d i a n c e  than or equal to 6.6 x 1 0  1 8  an  distance  increase  a  significantly  prey t a r g e t s  l e v e l s greater  ( F i g . 8 ) . At s a t u r a t i o n  of movement of the 3 mm prey t a r g e t s reaction  from  than 3.0 x  irradiance  photons n r was  with the  2  levels,  s " , the e f f e c t  additive,  1  increasing  by approximately 21 cm. T h i s corresponded to  in reaction distance  of 25.4% above that  found f o r  47  Table 6.  E f f e c t of  artificial  reaction distance Varden  (cm)  prey t a r g e t s i z e on of  cutthroat  the maximum  t r o u t and  Dolly  charr.  Prey s i z e  (mm)  Species  1  3  5  Trout  57.9  80.4  89.9  Charr  25.5  33.0  43. 1  48  Fig.  8. E f f e c t of movement of 3 mm a r t i f i c i a l prey t a r g e t s on the mean r e a c t i o n d i s t a n c e (± 95% CL) of c u t t h r o a t t r o u t and D o l l y Varden charr at d i f f e r e n t l e v e l s of irradiance. Sample s i z e per datum point equals 100.  50  stationary  prey  subsaturation  targets.  irradiance  The  increase  l e v e l s was  in r e a c t i o n d i s t a n c e  l e s s pronounced. When moving  prey t a r g e t s  were presented to t r o u t at an  4.2  photons m"  cm  x  10  17  or 8.8%  Although at an  greater the  (P >  Charr greater  targets,  a l s o reacted  in  of charr  approximately 32 and  to  a  increase  visual  charr  1  the  irradiance  difference moving  s"  targets  was  1  5.1  targets.  greater  significantly  ( F i g . 8). At  The  unaffected  x 10  to s t a t i o n a r y  37 cm,  in  but  saturation  and  decreased saturation  of c h a r r ,  reaction  2  s" ,  moving prey  as the  was  irradiance  photons n r  17  the  1  the  targets  corresponded r e s u l t of prey  e f f e c t of prey t a r g e t movement  irradiance  visual  targets  r e s p e c t i v e l y . This  of c h a r r  threshold  significantly  r e s u l t i n g from prey movement  below the last  (P > .05)  targets. was  not  than to s t a t i o n a r y  found for t r o u t , the  decreased 2  were  in r e a c t i o n d i s t a n c e  reaction distance  photons n r  s"  2  than or equal to 4.2  was  irradiance  photons m"  16  to moving prey t a r g e t s at  distances  reaction distance  the  stationary  of t r o u t to moving  these data  found in t r o u t  l e v e l s , greater  movement. As  x 10  reaction distance  l e s s than that  16.7%  for  by  of  .05).  (P < .05)  increase  found  mean r e a c t i o n d i s t a n c e  i r r a d i a n c e l e v e l of 3.0  different  on  that  level  increased  1  than  than to s t a t i o n a r y  irradiance  s~ , r e a c t i o n d i s t a n c e  2  at  level  as  irradiance  tested  to  3.0  x  above  no  of 10  15  the  significant  stationary  threshold  by movement of the prey  amount  l e v e l . At  there was  distance  the  of t r o u t  targets.  and and  51  3.8.3 Prey The the  Contrast  response of t r o u t and charr  prey  targets  ( F i g . 9)  was q u a l i t a t i v e l y  response to changes i n prey levels  greater  reacted  to  high  contrast  to low c o n t r a s t  reaction  movement  distance  (inherent greater  (inherent of  to  10  52.8  photons  targets  at  distance 52.1%  the  s" ,  2  was  1  same  in reaction  were  reaction  (P < .05) than  they  equal  to  6.6  while high c o n t r a s t produced  a  x  prey  reaction  distance  resulted  when  and  high c o n t r a s t  4.2  prey  x  10  1 7  difference  in  prey t a r g e t s a l s o photons  n r s~ 2  1  of 47.9 and 57.1 cm f o r low and  t a r g e t , r e s p e c t i v e l y . T h i s corresponded to a 19.2%  i n r e a c t i o n d i s t a n c e when high  i n place of low c o n t r a s t  contrast  contrast  relative  to a high and low c o n t r a s t  in a reaction distance  difference  high  used. As the amount of i r r a d i a n c e decreased below  d e c l i n e d . An i r r a d i a n c e l e v e l of  used  trout  1  = 0.71) 3 mm prey  than or  level  distance  s a t u r a t i o n l e v e l , the absolute  increase  s*  c o n t r a s t prey t a r g e t s at  cm  irradiance  2  of 80.3 cm. The d i f f e r e n c e , 27.5 cm, corresponded to a  increase  targets the  nr  irradiance  = 0.20) prey t a r g e t s . The  low  greater  of  s i m i l a r to t h e i r  photons n r  distance  saturation irradiance levels, 18  1 6  contrast  contrast  trout  contrast  ( F i g . 8 ) . At  than or equal to 3.0 x 1 0  t a r g e t s at a s i g n i f i c a n t l y did  to changes i n  (P > .05) prey  in  targets  targets  were  t a r g e t s . There was no s i g n i f i c a n t  reaction at  contrast  an  distance  irradiance  to  high  and  low  l e v e l of 3.0 x 10'  6  photons nr s " . 2  Charr  1  reacted  significantly  to  greater  high  contrast  distance  prey  targets  at  a  (P < .05) than they d i d to low  9. E f f e c t of inherent c o n t r a s t of 3 mm a r t i f i c i a l prey t a r g e t s on the mean r e a c t i o n d i s t a n c e (± 95% CL) c u t t h r o a t t r o u t and D o l l y Varden charr at d i f f e r e n t l e v e l s of i r r a d i a n c e . Sample s i z e per datum p o i n t equals TOO.  54  contrast to 3.0  t a r g e t s at a l l i r r a d i a n c e l e v e l s g r e a t e r  x 10  contrast  photons n r  16  s"  2  ( F i g . 9). While an  1  of the prey t a r g e t s r e s u l t e d in an  distance,  the  absolute  and  charr  to  contrast  x 10  low  contrast  distance  was  reaction  distance  decreased  32.5  as  the  a  same 24.5%  high  amount  and of  (P > .05)  contrast  contrast either  had  prey  no  The distance  level The  d i f f e r e n c e in prey  targets  decreased 2  s"  below  there  1  Differences  on the v i s u a l  high  reaction  in  the  was  in reaction distance  React ion Distance  preceeding of  a f f e c t e d by  experiments  trout  and  the q u a n t i t y  s i z e , movement and to  nr  With  of  to  prey  no high  target  irradiance threshold  of  species.  3.9  was  cm.  contrast  photons  6  targets.  effect  irradiance  than or  distance  26.1  irradiance  significant low  in r e a c t i o n  l e s s than that  reaction was  low  x 10'  and  i n the  increase  increase.  s a t u r a t i o n l e v e l . At 3.0 difference  the  1  targets  the  cm, to  s~ ,  2  prey  prey t a r g e t s at  increase  irradiance l e v e l s , greater  photons n r  17  equal  r e l a t i v e change was  found in t r o u t . At s a t u r a t i o n equal to 4.2  than or  whether  s i m i l a r to  zooplankters, Diaptomus kenai Lake t r o u t and  and  the  the  indicated to  Prey that  artifical  quality  c o n t r a s t . The  establish  t a r g e t s was  charr  to N a t u r a l  of  o b j e c t i v e of the distance  response  to  Daphnia  and  in the  s p r i n g and  prey  to n a t u r a l prey  and  prey.  Two  the copepod  were used. Both were common prey items charr  was  f o l l o w i n g work  artifical rosea  reaction  prey t a r g e t s  irradiance  reaction  cladoceran  the  of  summer (Hume 1978).  Loon  55  The  visual  irradiance  threshold  Daphnia and Diaptomus 1.4-1.6 photons  nr  feeding  s"  2  ( F i g . 10),  1  on 1 to  increase  in  5  mm  the i r r a d i a n c e  The  irradiance  in  the  reaction  length  same  artificial  produced a r e a c t i o n d i s t a n c e Daphnia.  mm  for trout was  level  prey  distance  3.0  cm  on  x  found  targets  l e v e l to 3.0 x 1 0 of 4.2  feeding  10  1 5  for trout  ( F i g . 5 ) . An photons n r s "  1 6  2  for trout  feeding  1  on  of t r o u t t o Diaptomus a t t h i s  l e v e l was not s i g n i f i c a n t l y d i f f e r e n t (P > 0.5)  than  that  found f o r Daphnia. At i r r a d i a n c e l e v e l s greater  than 3.0 x  10  photons  difference  16  (P < .05)  nr  in  s~ ,  2  there  1  the  was  reaction  a  significant  distance  of  z o o p l a n k t e r s . The average r e a c t i o n d i s t a n c e at nr  irradiance s"  2  l e v e l s between 4.2 x 1 0  was 16.2  1  distance  to  %  greater  to  the  Diaptomus. At the highest  t h i s study, 4.0 x 1 0 trout  than  Daphnia  1 9  photons n r  and  2  1 7  trout  to  the  of t r o u t to  and 4.0 x 1 0  irradiance  Daphnia photons  1 9  corresponding  two  reaction  l e v e l used i n  s " , the r e a c t i o n d i s t a n c e of 1  Diaptomus  was  23.8  and  21.3  cm,  respect i v e l y . Charr  showed  an  increase  zooplankters as the i r r a d i a n c e photons 10  s~  ,  1  2  s  _ 1  the  level  visual  charr  established (Fig.  feeding  on  f o r charr  reaction  distance  increased  from 3.0  irradiance threshold,  , the s a t u r a t i o n  v i s u a l irradiance threshold  for  two  2  photons n r  16  The  nr  in  irradiance level  and s a t u r a t i o n  t o both x  10  1 4  t o 3.0 x ( F i g . 10).  irradiance  level  the z o o p l a n k t e r s were the same as those feeding  on  artificial  5 ) . While the change i n r e a c t i o n d i s t a n c e z o o p l a n k t e r s was q u a l i t a t i v e l y s i m i l a r  to  prey  targets  of charr that  of  to the trout,  10. Mean r e a c t i o n d i s t a n c e (± 95% CL) of c u t t h r o a t t r o u t and D o l l y Varden charr to Daphnia rosea and Diaptomus kenai 1.4 to 1.6 mm i n length at d i f f e r e n t irradiance levels. Sample s i z e per datum p o i n t equals 100.  /  58  there  were  important  response. The 10  photons  15  t r o u t . The the  visual  two  rrr  difference  species  l e v e l f o r charr  in  the  irradiance threshold  s"  2  differences  was  lower  1  than  i n the even  being 6.60  of charr  greater,  was  irradiance  the  the  2.39  x  l e v e l between  saturation  photons n r  18  of  the corresponding value for  saturation  x 10  magnitude  s"  2  irradiance  l e s s than  1  that  found in t r o u t . Differences the  two  in the  z o o p l a n k t e r s were a l s o  species  reacted  (P < .05) levels,  reaction distance charr.  to Diaptomus was  distance  of  being 2.6  times greater  was  distance of 3.0  The charr  irradiance to  no  the  somewhat g r e a t e r ,  two  in  2  s"  Diaptomus  in r e a c t i o n d i s t a n c e not  the  of  4.1  in reaction distance  averaged  saturation  reaction cm  charr. reaction  trout  same. At  1.9  cm  level  distance  of  of  trout  greater  l e v e l s the charr  or a r e a c t i o n d i s t a n c e  to  and  saturation  ( F i g . 10).  irradiance  was  levels  irradiance  found for Diaptomus. At  zooplankters  reaction  1  z o o p l a n k t e r s was  the  times  reaction  in the  f o r Daphnia 9.0%  in  1.9  the  corresponds to a r e a c t i o n d i s t a n c e  difference  both  irradiance  irradiance  (P > .05)  to  distance  the average  z o o p l a n k t e r s at an  l e v e l s the d i f f e r e n c e and  greater  than the corresponding value for  to the  two  While  saturation  difference  significant difference  photons n r  15  Daphnia  at  charr  of t r o u t to Daphnia was The  relative difference  to  ( F i g . 10).  t r o u t to Diaptomus at s a t u r a t i o n  of charr  x 10  of t r o u t and  to Daphnia at a s i g n i f i c a n t l y  than that of  distance  evident  than they d i d to Diaptomus the  greater  There  reaction distance  This than  average the  to Daphnia  two 47.8%  59  greater than found f o r Diaptomus. These data reaction to  of  comparison  zooplankter  i s made  Estimates  the  to  the  species  of the maximum r e a c t i o n  the  at  least  of  when  the  cladocerans and  experiments.  to both zooplankters 10  encountered,  between  copepods used i n these  Fig.  that  d i s t a n c e of t r o u t i s l e s s s e n s i t i v e than that of charr  the type  charr  suggest  distance  were obtained  Michaelis-Menton  of  trout  by f i t t i n g  model  and  the data i n  (Table 7 ) . For  both  Daphnia and Diaptomus the maximum r e a c t i o n d i s t a n c e of t r o u t was approximately Both  trout  distance (Fig.  three times that found f o r c h a r r .  as  the  11). With  Daphnia  or  departure  and  charr  showed  of  Daphn ia  size trout  Diaptomus,  from  an  increase  and  of  feeding on  the  A  linearity  increase  occurred  was  2.25  s i z e s was not l i n e a r  rapidly  when  mm.  While  the  comparable to prey  to  those  differences  derived  in  change  prey  the  the  in  targets  ( F i g . 7), these data were  not  using the n a t u r a l prey  range  of  sizes  tested.  t a r g e t s were a l l 1 mm i n diameter and 1, 3 or 5  mm long. The length of n a t u r a l prey the  slight  when t r o u t were feeding on  d i s t a n c e of t r o u t and charr to a r t i f i c i a l  t a r g e t s due Artificial  linear.  Diaptomus was i n c r e a s e d from 1.25 to 1.75 mm than when  of d i f f e r e n t directly  increased  feeding on Daphnia and charr  i t was i n c r e a s e d from 1.75 to reaction  reaction  Diaptomus  Diaptomus, r e a c t i o n d i s t a n c e i n c r e a s i n g more size  in  targets  was  restricted  to  i n t e r v a l betwen 0.75 and 2.25 mm. Regressions  of  reaction  s i g n i f i c a n t d i f f e r e n c e i n slope  distance  on  prey  (P < .05) between  s i z e showed a both  species  60  Table 7.  Maximum r e a c t i o n d i s t a n c e (cm) of Dolly  Varden  Diaptomus  Species  charr  to  c u t t h r o a t t r o u t and  1.4-1.6 mm Daphnia rosea and  kenai.  Daphnia rosea  Trout  37. 1  Charr  12.7  Diaptomus  28.6 8.8  kenai  11. E f f e c t of the s i z e of Daphnia rosea and Diaptomus kenai on the mean r e a c t i o n d i s t a n c e ( ± 9 5 % CL) of c u t t h r o a t t r o u t and D o l l y Varden charr a t an i r r a d i a n c e l e v e l of 4.0 x 1 0 photons n r s " . Sample s i z e per datum point equals 100. 19  2  1  30  CUTTHROAT  TROUT  25  20  E o 15  _  r^phnia rosea  UJ o  Diaptomus kenai  5 » r00  O <  LU  _ 18  or  DOLLY  VARDEN  CHARR  15  10  5 h  05  2.0  1.5  1.0  PREY  SIZE  ( mm )  63  of  fish  and  prey  (Table 8 ) . The  d i s t a n c e f o r t r o u t and greatest  where  charr  rate of i n c r e a s e in r e a c t i o n  with  increasing  prey  size  Daphnia were used as prey. For both prey  the r a t e of i n c r e a s e in r e a c t i o n d i s t a n c e was  greater  was  types,  in  trout  than in c h a r r . The  r e a c t i o n d i s t a n c e of t r o u t and  charr a l s o was  by the presence or absence of movement in the two and  their  trout  l e v e l of c o n t r a s t with  and  charr  significantly stationary  reacted  greater  Daphnia  to  i n d i v i d u a l s although  at  moving  zooplankters  (P < .05)  than  the response was  they  in  reaction  increase  found  Diaptomus  individuals. similar  cm  distance.  The  for  f o r moving The  animals,  corresponding  45.1%. The  being  moving 171.1%  response  in magnitude to t h e i r  distance  did  increase  to  to  73.7%  increase in in  reaction  greater  to  than to  stationary  moving Diaptomus  response to Daphnia with  was  reaction  the s t a t i o n a r y form. Q u a l i t a t i v e l y , the e f f e c t of prey  movement  reaction  artificial  Diaptomus 44.8%  cm  found f o r  the  moving  charr  than  mm  g r e a t e r than was  on  to  of  a  Daphn i a , r e a c t i o n d i s t a n c e greater  a  more pronounced  d i s t a n c e of t r o u t to moving Diaptomus (Table 9) was  moving  at  s a t u r a t i o n i r r a d i a n c e l e v e l s i n c r e a s e d from 13.7  r e a c t i o n d i s t a n c e of charr was  the  species  r e a c t i o n d i s t a n c e of t r o u t to 1.4-1.6  for s t a t i o n a r y animals to 23.8 increase  prey  respect to the background. Both  distance  in t r o u t (Table 9). The  affected  distance  ( F i g . 8) and  quantitatively,  of t r o u t and  n a t u r a l prey  charr was  types  the same f o r  (Table 9)  although  the i n c r e a s e i n r e a c t i o n d i s t a n c e as the  of prey movement was  greater  (P < .05)  when n a t u r a l prey  result targets  64  Table 8 .  Linear  regression  (RD; cm) of various  2  Species  Trout  Charr  for  reaction  distance  c u t t h r o a t t r o u t and D o l l y Varden charr to  sizes  kenai at an m"  equations  (X; mm)  irradiance  of Daphnia rosea and Diaptomus level  of 4.0 x 1 0  19  photons  s" . 1  Prey  Equation  r  RD =  8.1  +  10.4  X  0.93  Diaptomus kenai  RD =  12.3  +  5.7  X  0.81  Daphnia  RD =  +  7.6  X  0.95  +  5.0  X  0.91  Daphnia  rosea  rosea  Diaptomus kenai  RD =  0. 1 -0.2  2  65  Table 9. E f f e c t  of  movement  reaction distance  of  natural  prey t a r g e t s on mean  (cm ± 95% CL) of c u t t h r o a t t r o u t and  D o l l y Varden charr at an i r r a d i a n c e l e v e l of 4.0 x 1 0 photons nr  2  s~ . 1  Moving  rosea and Diaptomus  kenai  1 9  (M) and s t a t i o n a r y (S) Daphnia were  1.4 - 1.6 mm i n l e n g t h  (N = 100).  Daphnia  Spec i e s  rosea  Diaptomus  S  M  Trout  23.8  ± 1.6  13.7 ± 1.8  Charr  11.9 ± 1.7  8.2 ± 1.0  M  kenai  S  ± 1.0  8.5 ± 1.8  8.4 ± 1.4  5.8 ± 1.1  23.1  66  were used. The contrast in  response of t r o u t and  1.4-1.6 mm Daphnia reaction  24.4%  reacted distance  in  distance  was  an  increase  (Table  trout  of both species  significantly  distance  irradiance levels and  to  i n the  of Daphnia was s i m i l a r t o t h e i r response to an increase  movement. Reaction  their  charr  to  low  greater  contrast  contrast  (P < .05)  increasing  to high c o n t r a s t Diaptomus at  a  significantly  greater  (P < .05) than they d i d to low c o n t r a s t animals,  Gross eye morphology teleostean  structures  examination of reveals  in  pattern  trout (Walls  and  charr  1942)  are  these  little  present  and  is  1961).  retinas  at  between  of  A l l major  a  magnification  the  two  of  species.  10X  A more  retina,  is  t r o u t and charr are q u a l i t a t i v e l y s i m i l a r  c o n s i s t of s e v e r a l l a y e r s  (Plate 2), the  virtually  below. The  and  the  i n both t r o u t and charr and an  structures  difference  resembles  d e t a i l e d comparison of one of these s t r u c t u r e s , the given  distance  Histology  i d e n t i c a l to that found i n Oncorhynchus ( A l i ocular  there  to the two forms.  3. H) R e t i n a l  typical  22.7  r e s p e c t i v e l y . While t r o u t a l s o  was no s i g n i f i c a n t d i f f e r e n c e (P > .05) i n the r e a c t i o n of charr  than  forms at s a t u r a t i o n  1.0), r e a c t i o n d i s t a n c e  and c h a r r ,  to high  located  second order  i n t e r n a l nuclear  in  ( P l a t e 1). The  the v i s u a l c e l l  neurones,  the  bipolar  rods  and  cones  l a y e r , are connected to cells,  found  l a y e r . Ganglion d e n d r i t e s make synaptic  i n the contact  Table  10.  Effect  of  contrast  reaction distance and 4.0  Dolly x 10  1.4  (cm ± 95% CL)  Varden  charr  photons n r  19  contrast  of n a t u r a l prey t a r g e t s on mean  Daphnia  - 1.6 mm  i n length  and  Diaptomus  Diaptomus  L  H  (H)  and low (L) kenai  were  (N = 100).  Daphnia rosea  Spec i e s  High  1  rosea  trout  at an i r r a d i a n c e l e v e l of  s" .  2  of c u t t h r o a t  kenai  L  H  Trout  29.2  ±  1.3  23.8  ±  1.6  Charr  14.8 ±  1.3  11.9 ±  1.5  1.6  21.3  ±  1.0  ± 1.1  8.4  ±  1.4  24.9 ± 9.7  68  Plate  1. Transverse s e c t i o n of the r e t i n a of c u t t h r o a t trout. 1 v i s u a l c e l l layer (containing e p i t h e l i a l pigment), 2 e x t e r n a l l i m i t i n g membrane, 3 e x t e r n a l nuclear l a y e r , 4 e x t e r n a l p l e x i f o r m l a y e r , 5 i n t e r n a l nuclear l a y e r , 6 i n t e r n a l p l e x i f o r m l a y e r , 7 ganglion c e l l l a y e r , 8 nerve f i b r e l a y e r . X 330.  70  Plate  2. Transverse s e c t i o n through the v i s u a l c e l l l a y e r of the r e t i n a of c u t t h r o a t t r o u t . C cone, R rod. X 500.  72  with  b i p o l a r axons i n the same l a y e r while axons of the g a n g l i a  c e l l s converge to form the o p t i c  3.j_0.j_ Rod and Cone C e l l  Density  Two types of v i s u a l c e l l s , in  the  vertebrate  tract.  retina  rods and cones,  (Walls  commonly  1942). Under high i r r a d i a n c e  c o n d i t i o n s , cone c e l l s are used to d e t e c t and transmit energy r e c e i v e d by eye to Animals  living  in  the  visual  centers  of  irradiant  the  cone  1942). Increases i n  c e l l d e n s i t y are o f t e n a s s o c i a t e d with  increases in v i s u a l  cells  irradiant low  animals  i n low i r r a d i a n c e environments (Walls  a c u i t y and r e d u c t i o n s i n s e n s i t i v i t y Rod  brain.  environments with high l e v e l s of i r r a d i a n c e  g e n e r a l l y have a high d e n s i t y of cone c e l l s r e l a t i v e to living  occur  are  responsible  information  for  common  detecting  i n animals  1942). High  living  transmitting  densities  of  rod  i n low i r r a d i a n c e environments  (Bowmaker 1976) are a s s o c i a t e d with poor high  and  1942).  to the v i s u a l centers of the b r a i n under  i r r a d i a n c e c o n d i t i o n s (Walls  cells,  to i r r a d i a n c e (Walls  visual  acuity  degree of s e n s i t i v i t y to low i r r a d i a n c e c o n d i t i o n s  and  a  (Gruber  1977). The charr  d e n s i t i e s of v i s u a l c e l l s  i n the r e t i n a s of  trout  and  were high, rods outnumbering cones by more than one order  of magnitude (Table 11). Cone d e n s i t y i n the r e t i n a of sympatric t r o u t was higher by  a  ratio  of  (P < .05) than cone d e n s i t y i n sympatric more  p o p u l a t i o n of sympatric density  found  in  than trout  allopatric  2:1. was  While  cone  slightly  trout,  the  density higher  charr i n the  than  difference  the  was not  73  Table  11.  A comparison of the rod c e l l s  density  (± 95% CL) of cone and  i n the r e t i n a s of sympatric  and a l l o p a t r i c  of c u t t h r o a t t r o u t and D o l l y Varden charr (N = 100).  Density  Cone  Spec i e s  Sympatric  (.0024  mm" ) 2  Rod  (Loon Lake)  Trout  21.2 ± 2.3  280.3 ± 6.8  Charr  9.0 ± 1.8  311.2 ± 7.2  A l l o p a t r ic Trout  ( P l a c i d Lake)  20.4 ± 1.9  283.3 ± 7.1  Charr  (Dickson  14.4 ± 1.7  302.7 ± 6.4"  Lake)  74  statistically density  significant  between  (P > .05).  allopatric  and  A  comparison  sympatric  charr  s i g n i f i c a n t d i f f e r e n c e (P < .05), the d e n s i t y of allopatric  population  being higher  than  found  of  cone  revealed  cones in the  in  a the  sympatric  populat i o n . D i f f e r e n c e s in rod d e n s i t y were evident among only the  p o p u l a t i o n s examined (Table  r e t i n a of sympatric the  density  previously  t r o u t was  found noted  in  11). The  for  cone  density  no s i g n i f i c a n t d i f f e r e n c e  between  populations  sympatric  and a l l o p a t r i c  the  than trend  populations.  in  rod  V i s u a l a c u i t y depends, in p a r t , on the d e n s i t y of cones  in  Cone C e l l  retina  charr.  Size  (Tamura & Wisby 1963). In g e n e r a l , animals by  density,  poor  density  allopatric  density or  acute v i s i o n are c h a r a c t e r i z e d  will  in these two  and  in the  trout  3.JJD.2  the  opposite  (P > .05)  sympatric  of  lower (P < .05)  charr,  There was  of  d e n s i t y of rods  significantly  sympatric  two  while (Walls  animals  with  retinas acuity  with  a  have  with high  the  number that can occur  cone  a lower cone  1942). As the c r o s s - s e c t i o n a l area of  determine  very  the  cones  in a u n i t area, cone  s i z e can be used as a measure of v i s u a l a c u i t y . Two and  types of cones were i d e n t i f i e d  charr.  The  cross-section ( P l a t e 3).  more  abundant  surrounded  Measurements  l e n g t h of the long and  the of  in the r e t i n a s of  trout  double  cones,  elliptical  circular  shaped  single  the  diameter  cones  of s i n g l e . c o n e s  short axes of double cones were  in  used  and to  75  Plate  3. T a n g e n t i a l s e c t i o n through the v i s u a l c e l l l a y e r the r e t i n a of c u t t h r o a t t r o u t . S - s i n g l e cone c e l l , D-double cone c e l l . X 500.  i  ir*  r i *^  •»  S  77  d e s c r i b e cone s i z e . D i f f e r e n c e s in cone s i z e e x i s t e d among the four of  fish  examined.  smaller  (P < .05)  sympatric  Single  than  cones in sympatric  single  charr (Table  cones  12). No  found  trout.  A  sympatric  the  size  of  retina  and  single  and a l l o p a t r i c c h a r r showed a  (P < .05), cone s i z e in sympatric found  the  40.0% of  s i g n i f i c a n t d i f f e r e n c e (P >  in the s i z e of s i n g l e cones in sympatric of  t r o u t were  in  existed  comparison  populations  .05)  allopatric  cones  significant  charr being 22.2%  between  difference greater  than  in the a l l o p a t r i c p o p u l a t i o n . S i m i l a r d i f f e r e n c e s e x i s t e d  in the s i z e of double cones among axes  of  four  the double cones of sympatric  shorter  (P < .05)  double  cones  in length than  found  axes of double longer  the  (P < .05)  in  than  charr  sympatric  Both  t r o u t were s i g n i f i c a n t l y  the corresponding  in sympatric  cones  populations.  (Table  charr  axes  of  the  12). As w e l l , the  were  significantly  those of a l l o p a t r i c c h a r r . There was  s i g n i f i c a n t d i f f e r e n c e (P > .05)  in the length  between sympatric  trout populations.  and a l l o p a t r i c  of  either  no  axis  3^.j_0.3 Degree of Summation The on one  number  bipolar  of v i s u a l c e l l s cell  the  converge  on  summation  ( A l i and Wagner 1980).  increases,  one  and  an  a l s o experiences 1980).  ganglion c e l l  animal  (rods and  number  of  cones) that converge bipolar  cells  that  i s used to d e f i n e the degree of As  the  degree  of  summation  becomes more s e n s i t i v e to i r r a d i a n c e but  a decrease  in v i s u a l  acuity  (Ali  and  Wagner  78  Table  12.  A  comparison  of  c r o s s - s e c t i o n a l cone  (± 95% CL) at the the  outer  r e t i n a s of sympatric  cell  segment e l l i p s o i d and a l l o p a t r i c  size  level in  populations  of c u t t h r o a t t r o u t and D o l l y Varden charr (N = 100).  Cone s i z e  (um)  Single Species  Sympatric  cone  Double cone L  S  (Loon Lake)  Trout  5. 5  +  0. 1  15. 4  +  0. 3  8 .8  +  0.  Charr  7. 7  +  0. 1  20. 9  +  0. 2  1 3.2  +  o.:  A l l o p a t r ic Trout  ( P l a c i d Lake)  5. 4  +  0. 2  15. 4  +  0. 3  8 .9  +  0.:  Charr  (Dickson  6. 3  +  0. 1  17. 1  +  0. 2  10 .3  +  0.  Lake)  *L - long a x i s , S - short a x i s  79  The  density  of  r e t i n a of sympatric  v i s u a l , b i p o l a r and  t r o u t was  from that of sympatric  charr  summation  in  cells the  was  corresponding  sympatric  rates  were  ganglion  and  cell  13). The  was  t r o u t which had  78.1:1. There was  (P <  of  the  no  visual  charr  where  significant  of the c e l l  visual,  each ganglion c e l l  charr  types  population  charr had  51.9  .05)  was  the  as  charr p o p u l a t i o n was  visual cells  for  and  (P <  r a t i o of the number of v i s u a l c e l l s  i n the sympatric  in  summation  bipolar  d e n s i t i e s were s i g n i f i c a n t l y d i f f e r e n t and a l l o p a t r i c  .05)  39.7  t r o u t p o p u l a t i o n s and  Estimates  i n the  r e s u l t i n g degree of  compared to sympatric  allopatric  of summation. The  while a l l o p a t r i c  (Table  i n the d e n s i t y of any  similar.  in the sympatric degree  ratio  (P > .05)  significantly different  sympatric  f o r each ganglion c e l l  difference the  lower  ganglion c e l l s  each  to  78.1:1, ganglion  cell.  3.J_0.4 Cone C e l l Mosaic The through  distribution histological  surface  sections  taken  i n the r e t i n a when viewed parallel  to  in  the d i s t r i b u t i o n of cone c e l l s  other  animals  distinct  groupings  Animals  possessing  live  cone c e l l s  are used to d e s c r i b e the cone c e l l mosaic  In some animals while  of  highly  or  ordered  lines  to animals  living  appears  i n low  retinal 1957). random,  into highly  (Engstrom  1963).  cone c e l l mosaic u s u a l l y  i n environments r e c e i v i n g l a r g e amounts  contrast  (Lyall  these c e l l s are organized  such as squares a  the  of  irradiance  in  i r r a d i a n c e environments where  80  Table  13.  A comparison of the d e n s i t y of n u c l e i (± 95% CL) and r a t e s  of  b i p o l a r : ganglion c e l l s ) and  allopatric  summation  Nuclei  type  (photoreceptor:  i n the r e t i n a s of  populations  D o l l y Varden charr  per c e l l  sympatric  of c u t t h r o a t t r o u t and  (N = 100).  (.0024  mm' ) 2  Photo-  Rate  receptor  Bipolar  Trout  301.5±5.1  125.9±4.2  7.6±0.9  39.7:16.6:1.0  Charr  320.214.9  91.514.0  4.1±0.8  78.1:22.3:1.0  303.714.6  127.114.7  8.3±1.2  36.6:15.3: 1 .0  311.6±3.9  106.8±4.1  6.0±0.7  51.9:17.8:1.0  Spec i e s  Ganglion  of  summation  Sympatr i c (Loon Lake)  Allopatric Trout (Placid  Lake)  Charr (Dickson Lake)  81  the cone c e l l s o f t e n 1963).  Increases  have  no  identifiable  pattern  (Engstrom  i n the degree of o r g a n i z a t i o n of the cone  mosaic have been a s s o c i a t e d with b e t t e r v i s u a l a c u i t y 1963)  and movement p e r c e p t i o n  (Lyall  1957, A n c t i l  cell  (Engstrom  1969, B a t h e l t  1970). Q u a l i t a t i v e l y , there was no d i f f e r e n c e mosaic  among  in  the  cone  the four p o p u l a t i o n s of f i s h examined. T a n g e n t i a l  s e c t i o n s passing through the cone e l l i p s o i d s r e v e a l e d a of  double cones arranged  in  a l l fish  examined  same, the area occupied surrounding  a  ( P l a t e 3 ) . Although by one  unit  ( i . e . four  unit  in the  t r o u t were compared. The u n i t area  cone  cell,  greater  these  There  allopatric  significantly  of  charr.  in  in a l l o p a t r i c  double  of  the  t r o u t was smaller i n area  sympatric area  unit  (P > .05)  was  the mosaic was the  units  mosaic  _3._1_0.5 Feeding  (P < .05)  was no d i f f e r e n c e  when  sympatric  i n sympatric  (P < .05) than the corresponding  charr area  s i z e d e s c r i b e d above (Table 12).  Implications  rod and cone d e n s i t y (Table  and  and  among the four p o p u l a t i o n s was the r e s u l t of  Three c h a r a c t e r i s t i c s of the r e t i n a examined i n t h i s  summation  mosaic  c h a r r . D i f f e r e n c e s i n the area of one u n i t of the  d i f f e r e n c e s i n cone c e l l  of  cones  s i n g l e cone) was d i f f e r e n t when comparisons were  from the r e t i n a of sympatric one  pattern  i n squares around a c e n t r a l s i n g l e cone  made between some of the p o p u l a t i o n s . One  than  cell  (Table  11), cone s i z e  (Table  12) and degree  13) show d i f f e r e n c e s between sympatric  charr while a f o u r t h , the cone  cell  mosaic  study,  i s the  trout same.  82  Sympatric and  trout  have  a higher  cone d e n s i t y ,  lower degree of summation than  These  differences  suggest  found  in  sympatric  the  i s operative  Reaction  behavioural distance  reaction  is  under lower  irradiance  distance  irradiance levels,  experiments d e s c r i b e d a  measure  and  distance  than  declined  in  below the  saturation irradiance  (Fig.  better  prey  sympatric  targets  charr.  of  trout  was  level,  higher  saturation  both  artificial  at  a  reaction  the  than  greater distance  visual  that  irradiance  found  for  charr  5).  the  allopatric  populations  sympatric  the  s t r u c t u r e and  populations  allopatric  counterparts There  was  suggested  populations  difference  in  it  was  any  assumed  to  be  the  same  that  found  their  the  in  in  irradiance sympatric  (Table four  the  14).  retinal  allopatric  s e n s i t i v i t y of a l l o p a t r i c as  close  behaviour  alone  of  the  p o s s i b l e to make  to  evidence  obtained  charr,  s e n s i t i v i t y to  c h a r a c t e r i s t i c s examined between sympatric and Consequently, the a c u i t y and  were  feeding  relative  based on h i s t o l o g i c a l no  distance  of t r o u t and  q u a l i t a t i v e comparisons of a c u i t y and for  longer  At  ( F i g . 10)  Although  a  14).  both species as the amount of i r r a d i a n c e decreased  c o r r e l a t i o n between r e t i n a l the  to  obtained  (Table  acuity,  acuity.  While no measurements of r e a c t i o n for  conditions  results  earlier  visual  sympatric t r o u t reacted  ( F i g . 5)  threshold  of  indicating  natural  charr.  that the v i s u a l system of  than that of sympatric t r o u t . T h i s supports the from  cone s i z e  that sympatric t r o u t possess b e t t e r  v i s u a l a c u i t y than sympatric charr and sympatric charr  smaller  trout.  trout  was  sympatric  83  Table  14.  A qualitative logical  estimates  sensitivity sympatric  comparison of b e h a v i o u r a l and h i s t o -  under and  of  visual  low  acuity  irradiance  and  visual  conditions  in  a l l o p a t r i c p o p u l a t i o n s of c u t t h r o a t  t r o u t and D o l l y Varden c h a r r .  Sensitivity  Acuity  Histo-  BehaSpec i e s  vioural  logical  Histo-  Behavioural  logical  Sympatr ic (Loon Lake) Trout  High  High  Poor  Poor  Charr  Low  Low  Good  Good  Allopatric Trout  High  Poor  ( P l a c i d Lake) Charr (Dickson Lake)  Intermediate  Intermediate  84  population.  Estimates  of  (Table  12) and degree of  charr  were  obtained  cone  density  summation  (Table  found to be intermediate  f o r sympatric  allopatric  charr  trout  and  lower  11), cone s i z e  13)  in  allopatric  between the same estimates  charr.  This  suggested  have b e t t e r v i s u a l a c u i t y and a higher  i r r a d i a n c e t h r e s h o l d than sympatric a  (Table  visual  irradiance  charr but poorer  threshold  than  that visual  acuity  either  and trout  populat i o n .  3^. J_J_ Ef f ect of I r r a d i a n c e L e v e l on Foraging Veloc i t y The depends  r a t e at which a v i s u a l in  part  on  the  predator  relative  velocity  Although both species of f i s h used i n their  zooplankton  zooplankter  and  this  encounter  study  as  well  as by a  per u n i t time i s so small that i t may be ignored. In  charr  experiments  at d i f f e r e n t  the  foraging  velocities  searched  of  i r r a d i a n c e l e v e l s were measured.  T h i s i n f o r m a t i o n was used to c a l c u l a t e the r e l a t i v e water  prey  between the two.  prey move, the r e l a t i v e d i s t a n c e covered  the f o l l o w i n g s e t of trout  will  by t r o u t and charr  volumes  f o r d i f f e r e n t prey  of  types (see  s e c t i o n 3.8). Q u a l i t a t i v e l y , the r e l a t i o n s h i p between and  irradiance  l e v e l was s i m i l a r  In both s p e c i e s f o r a g i n g v e l o c i t y increases  in  the  irradiance  foraging  i n t r o u t and charr i n c r e a s e d to  level.  a  velocity ( F i g . 12).  maximum  with  The amount of i r r a d i a n c e  r e q u i r e d to produce the maximum f o r a g i n g v e l o c i t y was 6.6 x 1 0 and  3.0 x 1 0  These  1 6  photons i r r  irradiance  levels  2  s"  1  1 8  i n t r o u t and c h a r r , r e s p e c t i v e l y .  correspond  to  those  producing  the  12. Swimming v e l o c i t y (cm irr ± 95% CL) of c u t t h r o a t and D o l l y Varden charr while f o r a g i n g on 3 mm a r t i f i c i a l prey t a r g e t s at d i f f e r e n t i r r a d i a n c e l e v e l s . 1  V6  IRRADIANCE  LEVEL (photons r i v s ) 1  87  maximum  reaction  distance  in  these two s p e c i e s  lowest estimate of f o r a g i n g v e l o c i t y i n irradiance  level  of  3.0  x  10  i r r a d i a n c e l e v e l producing  the  charr  x  was  lower  at  6.1  f o r a g i n g v e l o c i t y in both level  less  than  trout  photons  1 5  minimum  10 *  occurred nr  occurred  an  1  velocity  in  s " . The lowest  2  species  at  s ~ , while the  2  foraging  photons m"  1  ( F i g . 5 ) . The  1  at  an  irradiance  or equal to t h e i r v i s u a l i r r a d i a n c e  threshold  (Fig. 5). At m"  2  s"  irradiance the  1  foraging  than that of charr estimates  of  levels of  foraging  at  threshold, trout  remained  showing b r i e f periods foraging moving  behaviour continuously,  behaviour  both  fish  x  10  one  assumed  (see s e c t i o n 3.8).  continued  1 7  photons (P < .05)  levels  the  was  They  the  to  show  some  their visual irradiance very  position,  of movement. in  irradiance  below  behaviour in  even  4.2  f o r charr exceeded (P. < .05)  levels  the a s s o c i a t e d  usually  lower  velocity  irradiance  than  v e l o c i t y t r o u t was greater  ( F i g . 12). At  those f o r t r o u t . Although movement  greater  different.  only  The  occassionally  d i d not  exhibit  any  presence of food. The c h a r r ,  their  sub-visual  searching  88  3.j_2 D i e l V a r i a t i o n of I r radiance i n Loon Lake There depths  were  marked d i e l changes i n i r r a d i a n c e at d i f f e r e n t  Loon  Lake  in  measurements  (July  ( F i g . 13). The  first  set  8-9) were made on a c l o u d l e s s  of  diel  day while the  second s e t ( J u l y 28-29) were made i n the presence of 100% cover.  Despite  the d i f f e r e n c e s  i n cloud  cover the i s o p l e t h s of  i r r a d i a n c e over depth were s i m i l a r on the major  difference  occurring  in  the  two  upper  days,  the  level  J u l y 8-9 was approximately an order of magnitude greater 28-29. While the amount of i r r a d i a n c e  of a lake  i s known  (Hutchinson detect  to  decrease  with  irradiance  increasing  the  on  than on  reaching the surface  1957), these d i f f e r e n c e s are small  i n sub-surface waters due to  only  meter of the water  column between 12:00 and 14:00 h, where the i r r a d i a n c e  July  cloud  cloud  cover  and d i f f i c u l t to  rapid  attenuation  of  with depth. The p r e c i s i o n of i r r a d i a n c e measurements  made i n Loon Lake were adequate f o r the purposes of t h i s study. Irradiance ranged  from  l e v e l s over the upper 40 m of the  10 " 1  to  10  2 1  photons m"  2  s"  1  water  column  ( F i g . 13). Maximum  values at any depth occurred between 13:00 and 15:00 h while the lowest measurements were made between 24:00 maximum  and  03:00  h. The  rate of change i n i r r a d i a n c e occurred at dusk and dawn.  No measurements of i r r a d i a n c e were made below 1.0 x 10 * photons 1  nr  2  s" . 1  D i e l change i n the depth at which the i r r a d i a n c e Loon visual (Fig.  Lake  was equal to the s a t u r a t i o n  irradiance threshold 12)  are  shown  for trout  i n F i g . 14.  level  in  i r r a d i a n c e l e v e l and the and  charr  Estimates  on of  July  8-9  saturation  13. Depth-time i s o p l e t h s of i r r a d i a n c e l e v e l s (photons n r s ~ ) i n Loon Lake on J u l y 8-9 and J u l y 28-29, 1980. 2  1  <?0  91  i r r a d i a n c e l e v e l s and v i s u a l from the b e h a v i o u r a l  i r r a d i a n c e t h r e s h o l d s were  feeding experiments ( F i g . 5 ) .  Irradiance l e v e l s required for v i s u a l saturation were  present  21:00 h  equal  The  maximum  depth  was 16.7 m at approximately  to the v i s u a l  between  in  trout  i n the water column of Loon Lake between 05:30 and  ( F i g . 13).  occurred  obtained  at  which  this  value  14:00 h. I r r a d i a n c e  levels  i r r a d i a n c e t h r e s h o l d of t r o u t exceeded 40  09:00 and 18:00 h. Surface  m  i r r a d i a n c e l e v e l s were below  t h i s t h r e s h o l d between 22:30 and 03:30 h. Sufficient water and  column  i r r a d i a n c e was present for  i n at l e a s t part  of  s a t u r a t i o n of the eye of charr between 04:00  and 22:00 h ( F i g . 14). T h i s i r r a d i a n c e l e v e l exceeded  during  the  09:00  always greater  the  to  16:00  h  than the v i s u a l  40  m  p e r i o d . I r r a d i a n c e l e v e l s were  i r r a d i a n c e t h r e s h o l d of charr  in  some part of the water column. The minimum depth reached by t h i s irradiance  level  was  approximately  5.0  m  between 24:00 and  03:00 h. These data  show that there  lower water column i n midsummer sufficient  to  saturate  the  i s a considerable where the eye  of  p o r t i o n of the  irradiance  h.  operation they charr.  While  irradiance-  levels  are  inadequate  of the t r o u t v i s u a l system during much  never  decline  below  the  is  charr but not t r o u t . The  d i f f e r e n c e between these l e v e l s exceeds 20 m between 20:00  level  visual  of  07:00  and  'for the the  night  i r r a d i a n c e t h r e s h o l d of  14. The r e l a t i o n s h i p between i r r a d i a n c e l e v e l (photons n r s~ ) i n Loon Lake on J u l y 8-9, 1980 and v i s u a l s e n s i t i v i t y of c u t t h r o a t t r o u t and D o l l y Varden charr. The upper l i n e for each s p e c i e s i d e n t i f i e s the depth at which the i r r a d i a n c e l e v e l i s j u s t s u f f i c i e n t to maximize r e a c t i o n d i s t a n c e . The lower l i n e marks the depth where the i r r a d i a n c e l e v e l match the v i s u a l irradiance threshold. See text f o r d e t a i l s . 2  1  09 00 :  I2 00 :  14:00  I8 00 :  TIME  (h)  06^00 OSCH  94  3.j_3 A Model of V i s u a l Prey Searching P o t e n t i a l The p o t e n t i a l a r r a y of prey ultimately  limited  by  its  available  prey  detection  components of the feeding process learning  such  behaviour of the predator  as  The  a  abilities.  prey  Other and  may  alter  the  defined  by  of the p r e d a t o r .  number of prey d e t e c t e d by a v i s u a l f i s h predator i s a  volume  of  water  searched  d i s t a n c e of the predator and section  is  handling  framework  f u n c t i o n of the volume of water searched and the The  predator  (Beukema 1968)  nature of t h i s a r r a y but only w i t h i n the the prey d e t e c t i o n a b i l i t i e s  to  I  is  its  a  prey  density.  product of the  foraging  reaction  velocity.  In  this  have estimated the volume of water searched by  trout  and c h a r r f o r v a r i o u s s i z e s of Daphnia rosea and Diaptomus kenai on a t y p i c a l mid-summer day. The model used  VS where VS RD  (  2  (m ) 3  maxium r e a c t i o n d i s t a n c e to a prey section  (m)  3.4)  d i s t a c e moved by a predator while f o r a g i n g (m)  T  form:  x RD ) x D x T  volume of water searched  (see D  i s of the  (see s e c t i o n  3.6)  number of hours per day l e v e l was  sufficient  r e a c t i o n d i s t a n c e and (see  s e c t i o n s 3.6  and  that the  irradiance  to maximize both foraging velocity 3.7)  (h)  95  Two  assumptions  foraging  of  the  this  model  to  a  but  sufficient  to  For any searched  only  reaction  when  maximize  while  distance  of  the  the  both  irradiance reaction  level  is  distance  and  velocity.  s i z e of e i t h e r  per  that  given type of prey and 2) that f o r a g i n g i s  continuous  foraging  1)  f i s h search out a c y l i n d r i c a l volume the  r a d i u s of which equals the fish  are:  prey  day by t r o u t was  type,  the  volume  of  water  g r e a t e r than the volume searched  by charr ( F i g . 15). When averaged  over a l l prey s i z e s  and  both  prey types, t r o u t searched a volume of water 378.7% g r e a t e r than the corresponding volume searched by c h a r r . Both t r o u t and c h a r r searched  a  l a r g e r volume of water per day  Diaptomus of a s i m i l a r trout  for  1.25  mm  s i z e . The  Daphnia was  than was  searched for 1.25  length,  the  34.7%  volume  greater  difference  in  than  3  searched  a volume 36.1%  Diaptomus. For  greater  of water searched by t r o u t f o r Daphnia  was  volume  searched  prey  by  in  for  1.75  Diaptomus.  The  f o r Daphnia  g r e a t e r i n c h a r r than t r o u t . Charr searched  volume 137.0% greater f o r 1.25 mm  m,  water  the volume of water searched per day  and Diaptomus was  1.75  195.8  of  mm  the  mm  volume  f o r Daphnia than f o r  mm  Daphnia and  a  171.3% g r e a t e r f o r  Daphnia than f o r Diaptomus of a s i m i l a r  s i z e . Both  trout  and charr showed an i n c r e a s e i n the volume of water searched per day with i n c r e a s i n g prey  size.  15. Volume of water searched by c u t t h r o a t t r o u t and D o l l y Varden charr f o r Daphnia rosea and Diaptomus kenai during that p o r t i o n of the 24 h p e r i o d on J u l y 8-9, 1980 when the i r r a d i a n c e l e v e l was s u f f i c i e n t to maximize r e a c t i o n d i s t a n c e and f o r a g i n g v e l o c i t y . See text for detaiIs.  77  2 8 0  r  0.7  1.0  "~ 1.5  PREY  SIZE  2.0  (mm)  2.5  98  2 * 1 1 Trout the  and  Non-Visual Foraging  charr d i d not  irradiance  level  irradiance threshold  was  Behaviour  react v i s u a l l y to prey t a r g e t s when less  than or equal to t h e i r v i s u a l  ( F i g . 5). At  these  sub-visual  irradiance  l e v e l s t r o u t remained almost s t a t i o n a r y in mid-water in c o n t r a s t to  the  charr  which  tank, o c c a s i o n a l l y bottom.  This  continued  capturing  indicated  to move through the  prey  that  that  charr  had  were  The  behaviour of  different  than  charr the  located v i s u a l l y  at  sub-visual  behaviour  ( s e c t i o n 3.1).  shown The  settled  using  sensory system f o r l o c a t i n g prey at these low  observation  a  when  levels.  levels  prey of  the  non-visual  irradiance  irradiance  body  to  was  t a r g e t s were  the  animal  was  t i l t e d at a 45° angle r e l a t i v e to the bottom of the tank and i t s chin  was  c l o s e to but  not  touching  the bottom. At the same time  the head moved c o n t i n u a l l y from s i d e to s i d e . The of experiments was detect  prey  designed to determine i f t r o u t or charr  targets  at  irradiance  consumed present  10-minute  and (Table  stopped and  feeding  average of 4.2  charr  10 buried  15). When a prey t a r g e t  was  If the t a r g e t was consumed. strikes  l i v e r prey  u s u a l l y moved backwards 2 to 3 cm.  t a r g e t . The  g r a v e l was present  If i t was  in the  not,  located  located  by the animal p i c k i n g up a mouthful of g r a v e l the  in t h i s m a t e r i a l  and  targets  the  T h i s was  charr  followed  in the v i c i n i t y  immediately e x p e l l e d i t was  t a r g e t had  of  from the mouth.  the charr would make two  same l o c a t i o n . If the  could  of chemoreception.  sessions  of the  set  l e v e l s below t h e i r v i s u a l  i r r a d i a n c e t h r e s h o l d , presumably by use During  following  retained  and  to three more  still  not  been  99  Table  15.  Number of s t r i k e s and number of prey  targets  captured  buried  (± 95% CL)  artificial  in  still  and  moving water by D o l l y Varden charr at an i r r a d i a n c e level  l e s s than 6.1 x 10 * photons n r 1  Time  2  s'  1  (N =100).  Spent  Water  Prey  Searching  Condition  Type  (min)  Still  Liver  10  Still  Glass  10  0  0  Moving  Liver  10  3.8 ± 1.4  1.1 ± 0.6  Strikes  16.3 ± 5.1  Captured  4.2 ± 2.7  1 00  located  the  animal  would move on, c o n t i n u i n g  manner d e s c r i b e d e a r l i e r . Charr d i d not make the  liver  t a r g e t s were enclosed  any  in a s i m i l a r s e r i e s of experiments, d i d  either  liver  of  the  charr  in c a p t u r i n g  based on whether the t a r g e t was  Charr averaged 15).  16.3  strikes  per  10  average number of s t r i k e s per  feeding  in  the  l i v e r prey  minute  feeding  water to 3.8  water,  still  session  of 25.8  (P < .05)  water  (Table  %. targets  from t h e i r The  s e s s i o n d e c l i n e d from  16.3  the  capture  success the  less  (P < .05)  r a t e was  similar.  ability  of  l i v e r prey t a r g e t s , but once l o c a t e d had  their a b i l i t y  the  15).  significantly  i n d i c a t e d that moving water reduced l o c a t e the  targets  in moving water. While the number of prey  in moving water was  still  to  t a r g e t s over  success  significantly different in  still  15).  respond  number of s t r i k e s made by charr on b u r i e d l i v e r  response to the same t a r g e t s  captured  not  An average consumption r a t e of 4.2  in moving water was  in  when  consumed f o l l o w i n g a s t r i k e .  same time i n t e r v a l r e s u l t e d in a capture The  strikes  or g l a s s prey t a r g e t s .  Success  (Table  in the  in g l a s s c y l i n d e r s (Table  Trout,  was  to search  to capture  the  targets.  than This  charr  no e f f e c t  to on  101  4.0  DISCUSSION  4.j_ Sensory Behaviour  i n Trout and Charr  4. K l V i s u a l Prior  to  experiments, the  discussion  of  target  from the v i s u a l feeding  a comment i s r e q u i r e d on two  experimental d e s i g n . The  response  results  first  important  radiation  sighted.  The  second  i n quantum u n i t s rather  photometric Many  a  relates than  fish to  the  when  more  (Northmore  and  Yager  of  its  environment,  stereotyped  s i g h t e d was  responses their  1975). These methods, which  are  report  based  on  on  the  input-output  r e l a t i o n s h i p s . In t h i s study the v i s u a l stimulus was  was  of  conventional  animals p e r c e i v e  have been used as ways of i n d u c i n g an animal to  the  prey  measurements  p s y c o p h y s i c a l methods i n v o l v i n g both r e f l e x  surroundings  while  a  units.  and t r a i n i n g have been used to study how  nature  of  concerns the use of a r e f l e x  f o r i d e n t i f y i n g the l o c a t i o n of is  aspects  the  input  response of the f i s h when a prey  target  the output. T h i s response was  used  to  measure  the f i s h ' s power of v i s u a l p e r c e p t i o n . Reflex  methods are very a t t r a c t i v e because  the animal does not wane over time. T h i s i s not other  psycophysical  Consequently, irradiance  methods  (Blough  and  r e f l e x responses have been used to  the response of the  case  with  Yager  1972).  study  several  dependent behaviours in f i s h . They have been used to  t e s t an h y p o t h e s i s  relating  to  the  spatial  distribution  of  1 02  Stizostedion preference spectral  vitreum  sensitivity  in  auratus  Gasterosteus  Clupea  (Scherer  i n Lepomis macrochirus  Carassius  (1964)  vitreum  measured  (Vinyard and O'Brien  Lepomi s  qibbosus  (Cronly-Dillon  aculeatus  1975), to e s t a b l i s h  and  1976)  (Grundfest Muntz  least  amount  of  1965)  and  Blaxter  i r r a d i a n c e r e q u i r e d by  harengus for f e e d i n g , a v o i d i n g a b a r r i e r and  Despite the apparent  and  1932),  ( C r o n l y - D i l l o n and Sharma 1968).  the  prey  phototaxis.  s i m p l i c i t y of the r e f l e x method  there  are a number of c o n s i d e r a t i o n s which must precede i t s use. it  is  easily  necessary  that the response  observed.  This  accuracy  of  the  is  being measured be c l e a r  important  because  r e f l e x method i s l i m i t e d by the accuracy can measure the response.  trout  visual  Secondly,  charr it  to  is  important  some p r e d i c t a b l e way distance accessing  of  their  the  r e s u l t s obtained using a  In  this  developed  these  irradiance reflex  with and  response  with of  criteria.  that v a r i e s i n reaction  to be a u s e f u l response  abilities  the  response  with changes i n the s t i m u l u s . The  visual of  meets  The  to choose a response  t r o u t and charr proved  characteristics  generate  stimuli  and  ultimately  which the experimenter and  First  respect  to  the prey. I d e a l l y , should  be  used  for the the to  hypotheses which can be t e s t e d using d i f f e r e n t methods. study, and  hypotheses  tested  based  related on  to r e t i n a l  results  from  s t r u c t u r e were  the  behavioural  experiments. Throughout in photons m"  2  u n i t s was  t h i s study measurements of i r r a d i a n c e were made s"  1  between 400  and  700  nm.  The  choice of quantum  based on the nature of p h o t o b i o l o g i c a l  processes.  In  1 03  its  interaction  with  stream of p a r t i c l e s , discrete  amount  matter r a d i a t i o n behaves as i f i t were a  known as photons. of  energy  frequency. I f a photon comes photosensitive its  material  energy to one  reach an assume  quantum  is  contact  per u n i t  of  the  e x c i t e d s t a t e and  the  absorbed  s~  of 800 by  1  appropriate transfer  convert  will  the  determined  energy by  r e a c t i o n . In photosynthesis  required  its  receptor  for every molecule of oxygen  four photons are  on the order  an  to  a  . If enough photons  time  the  i t has  liberated  at  (Ramsay 1966). In the human  eye  the minimum number of photons r e q u i r e d  1  with  s i g n a l s . T h i s t h r e s h o l d w i l l be  efficiency  carries  proportional  molecule of that m a t e r i a l  been c a l c u l a t e d that  s"  photon  (e.g. a v i s u a l pigment) i t w i l l  electronically  into neural  least  in  i r r a d i a n c e receptor an  which  Each  for p e r i p h e r a l v i s i o n  is  at the cornea, corresponding to about  the rods (P-irenne  1956). These  suggest that p h o t o b i o l o g i c a l processes"are  more  80  considerations likely  to  be  understood i f r a d i a t i o n i s expressed in terms of photon numbers. Unfortunately  most  measurements of i r r a d i a n c e r e l a t i n g  f i s h behaviour are made in photometric u n i t s . Photometric are  r e l a t i v e measures of the q u a n t i t y  s p e c t r a l s e n s i t i v i t y of any pigments teleost  i t contains. species  pigments  as  (Ali those  eye  units  of r a d i a t i o n based on  s p e c t r a l s e n s i t i v i t y of the average human eye  (Arnold  depends on the  type  1974). of  found  Wagner in  1975a) the  none  human  eye.  had  the  irradiance  available  nor  the  amount  The  518 same  Consequently  photometric measurements of i r r a d i a n c e measure n e i t h e r the amount of  the  visual  In a survey of the v i s u a l pigments of and  to  that  total  can  be  104  s e l e c t i v e l y absorbed by d i f f e r e n t organisms. Prior  to  the  start  experiments, i t was inherent of the  in  identify  the experimental design,  perception  two  important to  i r r a d i a n c e and  distance  of the main s e r i e s of v i s u a l  of  the  fish  and,  f i s h s i z e and  characteristics  excluding c h a r a c t e r i s t i c s  the prey, which might  affect  consequently,  to prey t a r g e t s . Based on  features,  any  feeding  the  their  s t u d i e s from the  visual reaction  literature,  water temperature were of p a r t i c u l a r  concern. In  general,  eye  development  appears to be  incomplete in  young f i s h . These f i s h are o f t e n c h a r a c t e r i z e d by an optic  tectum  B l a x t e r and 1963,  Staines  Blaxter  different  with higher  are  The  Jones  the  1967)  adult  and  form  1975)  from the a d u l t  cone  (Ahlbert  their  and  (Blaxter  charr,  ranging  reaction  and  1975).  4). As a r e s u l t ,  form by the end  1969,  which are When  1975),  from 10.0  distance i t was This  most developmental s t u d i e s where the eye  (Ali  mosaics  1959,  these  reduced are  1967).  were f u l l y developed by t h i s stage.  i t s adult  (Ali  s p e c t r a l s e n s i t i v i t i e s that  (Blaxter  s i z e of t r o u t and  (Table  response  1968,- 1969,  compared with v i s u a l behaviour, they c o r r e l a t e  (Blaxter  no e f f e c t on  targets  1970), no retinomotor  irradiance thresholds  visual acuity different  and  from  differences  had  (Sharma 1975), a lack of rods ( B l a x t e r  incomplete  of the yolk  to  to 23.4  artifical  cm, prey  assumed that t h e i r  eyes  is  in  with  has  been shown to reach  agreement  sac or metamorphosis stage  Wagner 1975b). While some v i s u a l responses in f i s h  change in o l d e r animals, for example bottom c o l o u r  selection  do in  105  Salmo  qairdneri  changes  probably r e l a t e d t o respond"  a  at  the  age  physiological  of  or  14 months, they are  ecological  rather than any change i n v i s u a l p e r c e p t i o n  MacCrimmon  "urge  to  (Kwain and  1969).  It has been known f o r some time that temperature can a f f e c t the v i s u a l a b i l i t i e s of a p o i k i o t h e r m (Denton and Pirenne  1954).  One known temperature r e l a t e d response  fusion  frequency.  When  a  f i s h , each f l a s h  flickering light  elicits  electroretinogram. a l e v e l w i l l be  one  i s the  flicker  stimulus i s presented t o a  response  when  measured  as  an  I f the frequency of the f l a s h e s i s i n c r e a s e d  reached  electroretinogram  fuse  where so  individual  that  components  of the  i t i s impossible to r e l a t e the  stimulus to the response. The frequency at which t h i s occurs called  the  flicker  that the f l i c k e r temperature salar  in  Kobyashi pigment  (Ali  The  speed  attain  dependent,  increases  auratus  A l i 1964),  cones  temperature  frequency  Carassius  1967). and  f u s i o n frequency . I t has been demonstrated  fusion  (Hanyu and  is  and at a  with  increasing  (Hanyu and A l i 1963), Salmo Lepomis  which  the  light-adapted  gibbosus  ( A l i and  retinal  epithelial  state  is  also  o c c u r r i n g f a s t e r at higher temperatures  1975). Over a range of temperatures from 5 to 20° C  distance  of  trout  and charr to a r t i f i c a l  change (Table 3 ) . While these r e s u l t s cannot hypotheses fusion  the  reaction  prey t a r g e t s d i d not be  used  to  test  r e l a t e d to the e f f e c t s of temperature on the f l i c k e r  frequency  light-adapted  or  state,  the  rate  at  which  an  eye  attains  a  they do suggest that v i s u a l p e r c e p t i o n i n  106  t r o u t and c h a r r , at l e a s t as i t i s measured i n temperature  this  study,  is  independent.  _4._1_._1_._1_ C h a r a c t e r i s t i c s of the I r r a d i a n c e The  visual  system  receives s u f f i c i e n t the  environment.  Northcote visual  of  an  animal  information Results  i n the form of  of  a laboratory  (1972) suggest that both t r o u t  system  for  irradiance  from  study by Schutz and  and  charr  use  their  l o c a t i n g prey t a r g e t s . Yet, at l e a s t during  part of the year, charr receives  i s only u s e f u l when i t  i n h a b i t a deep water  environment  which  much l e s s i r r a d i a n c e than the environment i n h a b i t e d by  shallow water t r o u t  (Andrusak and Northcote,  1971 ) . R e s u l t s  my study are used to t e s t the hypotheses that  the  from  quantity  of  i r r a d i a n c e does not a f f e c t the d i s t a n c e at which t r o u t and charr can  detect prey t a r g e t s and that  the d i s t a n c e at which t r o u t and  f o r any q u a n t i t y charr  can  first  of i r r a d i a n c e detect  prey  t a r g e t s i s the same. Qualitatively,  the  the  reaction distance  and  n a t u r a l prey t a r g e t s  reaction below  distance  which  saturation of  prey  effect  of the amount of i r r a d i a n c e on  of t r o u t and charr  to a r t i f i c i a l  ( F i g . 10) i s the same. In both  increases targets  from a v i s u a l i r r a d i a n c e are  not  detected  (Fig.  species, threshold  visually  i r r a d i a n c e l e v e l above which i n c r e a s e s  5)  to  a  i n the amount  i r r a d i a n c e have l i t t l e e f f e c t on r e a c t i o n d i s t a n c e . While the  form of t h i s r e l a t i o n s h i p i s the same i n the two s p e c i e s ,  there  are major d i f f e r e n c e s i n the magnitude and other c h a r a c t e r i s t i c s of  the  response. ' The  visual  irradiance  threshold  and  the  107  saturation two  irradiance level  orders  of  f o r c h a r r are approximately one  magnitude  corresponding values 3.0  for  x 10  lower  trout.  respectively  Also,  photons m~  at  and  than  irradiance  the  levels  greater  than  of t r o u t  i s s i g n i f i c a n t l y g r e a t e r (p<.05) than that of c h a r r .  16  The magnitude of the decreasing  s " , the r e a c t i o n d i s t a n c e  2  change  irradiance levels  1  in  reaction  distance  i l l u s t r a t e s the importance  under of t h i s  r e l a t i o n s h i p when e s t i m a t i n g f i s h f e e d i n g r a t e s . If a f i s h cruising  predator,  then  a  50%  a  decrease i n r e a c t i o n d i s t a n c e  decreases the volume searched by a f a c t o r of f o u r . If is  is  the  fish  an ambush p r e d a t o r , consequently searching a volume of water  approximated  by a hemisphere (O'Brien e_t a_l. 1976),  the  volume  searched decreases by a f a c t o r of e i g h t . Reductions  in  reaction  distance  may  have  important  consequences f o r capture success. Once a prey moves o u t s i d e  the  reactive  in f a c t escaped  and  As  the  volume  of a predator the prey has  the predator must probability  of  begin trout  v i s u a l l y decreases levels.  Using  as  the  searching  again.  charr  capturing  or  irradiance  same  argument,  result,  a prey they d e t e c t  decreases the  a  below  p r o b a b i l i t y of a prey  escaping the r e a c t i v e volume of a c h a r r at s a t u r a t i o n levels  i s greater  than  in  trout  due  saturation  to  the  irradiance  lower  maximum  reaction distance in charr. There the  are two other methods which can be used to determine  influence  analysis  of  of  irradiance  stomach  on  visual  prey  contents over a 24-h  although there are numerous d i f f i c u l t i e s  period  in  detection.  An  i s one method  interpreting  this  108  type  of  data  (Jenkins  and Green  u s u a l l y r e l a t e feeding a c t i v i t y  1977). As w e l l these s t u d i e s  to time of day which can only be  used as a r e l a t i v e measure of i r r a d i a n c e . The most common p a t t e r n found i n t h i s type of a n a l y s i s i s a d i u r n a l feeding (Muzinic  rhythm.  For  example,  1931), Pleuronectes p l a t e s s a  alalunqa  (Iverson  1962) have the  adult  Clupea  (Hempel  greatest  harenqus  1956) and Thunnus  amount  of  food  in  t h e i r gut at dusk and dawn and the l e a s t at n i g h t . I t i s assumed in  these  s t u d i e s that the amount of food i s r e l a t e d to feeding  a c t i v i t y and dependent on the i r r a d i a n c e More exact  level.  information i s obtained from l a b o r a t o r y  feeding  experiments conducted over a range of i r r a d i a n c e l e v e l s . In most examples  of  this  type  of  work,  radiation  is  photometric u n i t s . For purposes of comparison with it  i s assumed that  nr  s"  2  (Blaxter  1  activity  1 f o o t - c a n d l e i s equal to 5.56  s"  2  declines  in  1  range  The  Lota l o t a  range  (Girsa  in  kisutch,  0. nerka,  10  13  four  photons n r  2  10  in  declines  from  1 6  1 3  to 1 0 to  and 10 " 1  ( F i g . 5)  10  1 2  1 6  of  photons over  Pacific  photons  17  photons n r to  10 * 1  nr  2  s"  1  which feeding salmon,  1  is  2  s~  1  in  photons  (Blaxter activity  Oncorhynchus  O. keta and 0. gorbuscha i s between  10  1 7  to  ( A l i 1959, B r e t t and Groot 1963). The range  in i r r a d i a n c e l e v e l s over which r e a c t i o n trout  x 10  study,  1961). The d e c l i n e f o r most v i s u a l  irradiance  species  s~  10  (Hunter 1968)  feeders occurs between 1970).  this  in  1970). The i r r a d i a n c e l e v e l s over which feeding  Trachurus symmetricus nr  measured  higher  than  a c t i v i t y d e c l i n e s in the c l o s e l y  distance  declines  in  the range over which feeding related  Pacific  salmon.  The  109  decline 10 "  in  reaction distance  photons n r  1  s~ ,  2  within  1  of charr  the  occurrs  range feeding  between 1 0 activity  and  16  declines  in P a c i f i c salmon. While  reaction  activity,  distance  is  not  a  measure  of  feeding  u s u a l l y expressed as number of prey captured per  time, i t i s a c o r r e l a t e of feeding distance  of  activity.  As  the  the  rate at which  a predator decreases, so to w i l l  i t encounters prey and  therefore,  the  rate  10  and  unit  at  reaction  which  it  can  consume prey. Between i r r a d i a n c e s in  trout  and  10  distance  declines  Pacific  salmon  irradiance  l e v e l s of  and  16  10 * photons rn" 1  13  shift  from  a light and  17  the  eye  suggests  s"  ability  the  to  reduction  to prey t a r g e t s  i s the  discussion  of  quantity  of i r r a d i a n c e  another  important  v i s u a l perception. hypothesis sensitivity  that of  eyes  the  of  nr  2  s" .  Salmo  1  l e v e l s between 10 * and 1  irradiance  range where  from a s t a t e of l i g h t to dark prey  decreases.  This  in t r o u t from  and a  state.  reaction distance  i s incomplete. The  findings  and  that  r e s u l t of the eyes s h i f t i n g  qualitatively trout  reaction  in reaction distance  characteristic The  1  i n charr  photons  1  capture  l i g h t adapted to a dark adapted Any  10 "  ( A l i 1961). Over the  1  their  that  1  s"  2  to a dark adapted s t a t e at  irradiance  of P a c i f i c salmon s h i f t s  adaptation,  charr  2  photons n r  15  ( F i g . 5). A l i (1959) r e p o r t s  l e v e l s between 1 0  photons n r  10 s"  2  s a l a r show a s i m i l a r s h i f t at 10  18  of in  there  charr  to  only  in terms of  q u a l i t y or  colour  the is  i r r a d i a n c e which a f f e c t s this is  study  support  no d i f f e r e n c e  different  in  colours  the the of  1 10  irradiance distance  when  sensitivity  i s measured i n terms of r e a c t i o n  ( F i g . 6 ) . In both s p e c i e s r e a c t i o n d i s t a n c e i s g r e a t e s t  in the presence of red i r r a d i a n c e f o l l o w e d i n by  green,  yellow  and  decreasing  blue i r r a d i a n c e . Although  s i m i l a r , the magnitude of the response  in  the  order  qualitatively  two  species i s  d i f f e r e n t . For any c o l o u r of i r r a d i a n c e the r e a c t i o n d i s t a n c e of trout  i s g r e a t e r than that of c h a r r . Early  the  i n v e s t i g a t o r s thought  relationship  dependent response predict  between and  that peaks i n curves d e s c r i b i n g  the  magnitude  irradiance  the wavelength  colour  found  that  v a r i e s depending related  to  the  on  irradiance be  used  to  (am) of the v i s u a l  i s not  true.  Blaxter  shape of the curve f o r Clupea harenqus  whether  phototaxis,  this  an  could  of maximum absorbance  pigments i n the eye. I t now appears (1964)  of  the  response  feeding  or  being  measured  perception  is  of a b a r r i e r .  Northmore and Muntz (1974) obtained the same type of r e s u l t when trying  to  Scardinius  establish  the  erythrophthalmus  study, the type of curve animal light  spectral from  sensitivity behavioural  obtained  when  the  was measured to moving bars of l i g h t , of  various  different.  It  widths  seems that  and  to  depending  data. response  light  of the  stimuli  curves  on the experimental c o n d i t i o n s . to  (Muntz may  be  Consequently  can  relative  index of s i m i l a r i t y  i n the v i s u a l pigments of d i f f e r e n t  animals  subjected  same experimental c o n d i t i o n s rather  the  used  was  s p e c t r a l s e n s i t i v i t y curves  to  be  this  s t a t i o n a r y bars of  sensitivity  only  for  In  i n f i s h and many other animals  1975b), a wide v a r i e t y of s p e c t r a l obtained  diffuse  curve  provide  a  111  than a b s o l u t e estimates of am. The photopic s p e c t r a l s e n s i t i v i t y curves ( F i g . 6) that  trout  and  Although s c o p t i c determined  charr  have  the  (dark-adapted)  f o r these  two  same  cone  sensitivity  As  i n the P a c i f i c  curves  pigments. were  not  s p e c i e s , the v i s u a l pigments of the  rods have been measured from r e t i n a l e x t r a c t s 1965) .  visual  indicate  (Munz  and  Beatty  salmon (Beatty 1966) two pigments are  found i n the rods of t r o u t and c h a r r , r e t i n e n e one and two  with  am of 503 and 527 nm, r e s p e c t i v e l y . It  i s g e n e r a l l y accepted that v i s u a l pigments are adapted  to the p h o t i c environment 1966) .  Two  hypotheses  i n which have  v i s u a l pigments are adapted sensitivity  hypothesis,  adapted  the  to  been  animal  put  that  This  hypothesis  visual  of  The f i r s t , the pigments  q u a l i t y of the ambient  accounts  photons  fishes  Munz  cartilagenous fishes and  Dartnall  Warren  (Denton  1956,  1957,  1971) and whales (McFarland  the wavelength  1958)  (Lythgoe  1971). The v a r i e t y of the  visual  pigments  of maximum t r a n s m i s s i o n of i r r a d i a n c e i n  the p o r t i o n of the water column where the animal l i v e s suggests  teleost  1957,  and Shaw 1956), p i n n e p i d s  s p e c i e s and l o c a t i o n s f o r which the am of matches  (Clarke  f o r the am of v i s u a l pigments including  and  are  irradiance in  for many s p e c i e s i n a wide v a r i e t y of groups (Denton  (Lythgoe  forward to e x p l a i n how  such a way as to c a t c h the g r e a t e s t number 1936).  lives  to t h e i r environment.  suggests  spectral  the  i t i s an e c o l o g i c a l l y  strongly  rather than a p h y l o g e n e t i c a l l y  based phenomenon. Although the s e n s i t i v i t y  hypothesis  works  well  i n some  1 12  s i t u a t i o n s i t does not account f o r the d i s t r i b u t i o n of a l l known fish  v i s u a l pigments  These  ( D a r t n a l l and Lythgoe 1965, Lythgoe  discrepencies  considering  l e d Lythgoe  sensitivity  (1966)  to  suggest  detect  objects  by  their  contrast  with  background. The c o n t r a s t h y p o t h e s i s shows that cases  the  visibility  of  a  in  to the  some  special  from the s p e c t r a l q u a l i t y  of these hypotheses would p r e d i c t  live  pigments  respect  but  of  light.  Either charr  possible  t a r g e t can be i n c r e a s e d by having  v i s u a l pigments whose am i s o f f s e t the ambient  that  alone i s m i s l e a d i n g as the f u n c t i o n of  the eye i s not merely to c a t c h as many photons as to  1972).  in  different  should  be  photic  different  that  environments,  i f t r o u t and their  a l s o . The s i m i l a r i t y  visual  in scoptic  v i s u a l pigments i s not d i f f i c u l t  to e x p l a i n . These pigments  used  1975b). During these p e r i o d s i n  at  dusk  Loon Lake  and  both  dawn (Muntz  trout  and  charr  occupy  the  surface  are  waters  (Andrusak and Northcote 1971). During  the  summer  in  the daytime t r o u t a r e concentrated  near surface waters while  charr  (Andrusak  1971). As a r e s u l t of a t t e n u a t i o n the  and  Northcote  often  occupy  s p e c t r a l composition and wavelength of maximum irradiance water  at the surface i s very d i f f e r e n t  (Duval e_t a_l. 1973),  yet  the  greater  transmission  from that  photopic  depths  visual  of  i n deeper pigments  appear t o be the same ( F i g . 6 ) . The most probable e x p l a n a t i o n of this  discrepancy  distribution segregated  of  lies these  in two  an  understanding species.  of  Although  the  annual  vertically  i n the summer, sympatric t r o u t and charr do not show  11 3  any d i f f e r e n c e  in  distribution  (Andrusak and Northcote Therefore, difference changes  at  times  at  other  times  other  than  summer  there  in  the  Yoshikami  am  of  their  visual  1970).  The  pigments  photic  environment  to occur  (Tsin  pigments  in  pigments  It  appears  time when segregated  i m p l i c a t i o n of t h i s  i s at l e a s t  irradiance i n a decrease  While the  change  i s subjected  that  the  (Bridges  to  a  new  50 days  photopic  visual  f o r the change to occur. The  f o r p a r t of the  year,  probably  distribution  gillnets,  some t r o u t are caught Hume  1978).  the  optimum  ones  f o r the  i n which they l i v e . T h i s would  i n v i s u a l prey d e t e c t i o n a b i l i t i e s .  there are s t a t i s t i c a l l y  vertical  vertical  not  environment  the summer, s e g r e g a t i o n  1971,  (Beatty 1966,  daytime d u r i n g the summer, the photopic v i s u a l pigments  of t r o u t or charr or both are  result  seasonal  but t h i s change takes approximately  1978).  be no  t r o u t and charr are e i t h e r u n a l t e r a b l e or there i s  not s u f f i c i e n t  ambient  year  of Salmo g a i r d n e r i can be  i n the l a b o r a t o r y i f the animal  the  should  i n the photopic pigments. Some f i s h e x h i b i t  altered  in  the  1971, T.G. Northcote, unpublished d a t a ) .  Schwanzara 1967) while others show no seasonal and  of  some  significant  of sympatric  i s not  charr are caught  Consequently,  When  fishing  with  near the s u r f a c e and  (Andrusak  the  in  t r o u t and charr during  complete.  i n deeper water  differences  and  Northcote  p o s s i b i l i t y e x i s t s that  there i s both a shallow and deep water form of each s p e c i e s . I f this only  i s true and i f the method of c o l l e c t i n g the  the animals  samples  shallow or deep water form of each s p e c i e s , then  provides a t h i r d explanation  f o r the  similarity  of  this  photopic  11 4  visual  pigments i n t r o u t and c h a r r . R e s u l t s from t h i s and other  s t u d i e s make t h i s p o s s i b i l i t y u n l i k e l y . S t u d i e s on the distribution (Andrusak same  of  sympatric  and Northcote  lake  from  trout  and  1971) and Loon  which  animals  in  charr Lake  vertical  from Marion Lake (Hume  1978),  the  t h i s study were c o l l e c t e d ,  p r o v i d e no i n f o r m a t i o n suggesting there i s more than one form of each s p e c i e s . Secondly, other responses measured i n using  the  same  animals  trout  living  in  a  of  low  charr  i s characteristic  i r r a d i a n c e environment  i s c h a r a c t e r i s t i c of an animal l i v i n g  environment. sympatric  Thirdly, histological  trout  and  charr  s t r u c t u r e with no o v e r l a p results  of  stomach  show  between  analyses  on  the  surface  had  recently  deeper  waters,  contained  an  while that of  of  the  retinas  of  clear differences in retinal the  two  Loon  been  depths while the stomachs of other  of  i n a high i r r a d i a n c e  studies  species.  Lake  s t r o n g l y suggest that some i n d i v i d u a l s of near  study  or animals c o l l e c t e d at the same time  suggest the v i s u a l system animal  this  both  trout  Finally, and charr  species  caught  f e e d i n g at c o n s i d e r a b l e  individuals  caught  i n the  prey types c h a r a c t e r i s t i c of s u r f a c e  water (T. G. Northcote, unpublished d a t a ) . An is  i n t e r e s t i n g f e a t u r e of the s p e c t r a l  that  their  Although there  peak are  photopic  visual  described  earlier,  wavelengths 1974),  has  occurs  sensitivity  in  the  difficulities  in  identifying  the  am  of  on  behavioural  studies  as  pigments this  also  based peak  been  red i r r a d i a n c e  curves  sensitivity  in  (Fig. 6).  the  longer  found i n Perca f l a v e s c e n s (Cameron  S c a r d i n i u s erythrophthalmus  (Northmore  and  Muntz  1974)  1 15  and  Carassius  auratus  (Yager  b e h a v i o u r a l techniques. The sensitivity  adaptive  seems to r e s u l t from the  coloured  by  available  irradiance  1975b). T h i s  substances that  shift  the  f a c t that the  of  of  far-red  lakes are  usually  s p e c t r a l d i s t r i b u t i o n of Muntz  wavelength nm  of  at the  maximum surface  setting  (Efford  transmission to 560  nm  at  1967),  shifts  18 m  from  (Duval  e_t  1973).  Visual and  predators  movement of an  can  object  e f f e c t of three of these contrast  on  the  Prey  d i s t i n g u i s h the  (Horridge  1968,  s i z e , form,  Prazdnikova  characteristics,  reaction  distance  of  size,  are  sensitive  or c o n t r a s t  at which they are  cod,  reaction  detected by  the  distance  to  food t a r g e t s  to  and  target  10 mm.  Lepomis  as the  More r e c e n t l y qibbosus  increase  and  in  lead size,  distance  charr. a  relationship  s i z e . In t h i s study in r e a c t i o n targets  distance increased  same r e l a t i o n s h i p has and  Blades  are  charr  the  a f f e c t the  The and  charr  trout  establish  s i z e of the  the  (Confer  do not  t r o u t and  first  Gadus morhua , showed an  artifical  and  differences  of prey t a r g e t s  first  Brawn (1969) was between  1969).  to changes in a l l three c h a r a c t e r i s t i c s and  to r e j e c t i o n of the hypothesis that movement  contrast  movement  trout  examined in t h i s study. R e s u l t s i n d i c a t e that  in  variety  significance  geological  _4.J_.J_._2 C h a r a c t e r i s t i c s of the  1  a  i s the case in Eunice Lake, a lake adjacent to Loon  approximately 470 al.  using  to longer wavelengths (Lythgoe 1975,  Lake with a s i m i l a r o r i g i n and where  1967)  1975),  adult to from  been found Lepomis  1 16  macrochirus  (Vinyard and O'Brien  Salvelinus  namaycush  (Kettle  1976,  Werner and H a l l  and O'Brien  1978)  1974)  and  when presented  with v a r i o u s s i z e s and s p e c i e s of copepods and c l a d o c e r a n s . Q u a l i t a t i v e l y , the r e s u l t s from described  above.  The  (Fig.  11)  increases.  s t u d i e s on the r e a c t i o n d i s t a n c e results  reported  ( F i g . 7)  While of  2  rate  parallel  those  and  natural  there are no co-habiting  than  3.0  x  species,  10  prey  comparative  here show c l e a r d i f f e r e n c e s between t r o u t  c h a r r . At i r r a d i a n c e l e v e l s g r e a t e r nr  study  r e a c t i o n d i s t a n c e of both t r o u t and c h a r r  i n c r e a s e s as the s i z e of a r t i f i c i a l targets  my  the and  photons  16  s " , the r e a c t i o n d i s t a n c e of t r o u t to any prey s i z e and the 1  of i n c r e a s e i n r e a c t i o n d i s t a n c e with i n c r e a s i n g prey  size  i s g r e a t e r than i n c h a r r ( F i g . 7 ) . At i r r a d i a n c e l e v e l s equal to or l e s s than 3.0  x 10  of charr exceeds  that of t r o u t .  The  reaction  independent t h i s study  photons  16  distance  nr  of  2  s " , the  reaction  1  trout  distance  and c h a r r become n e a r l y  of prey s i z e at the lowest i r r a d i a n c e l e v e l s used in ( F i g . 7). V i n y a r d and O'Brien  (1976) found  the  same  r e l a t i o n s h i p f o r Lepomi s macrochi rus feeding on v a r i o u s s i z e s of Daphnia  pulex  at low l e v e l s of i r r a d i a n c e . They suggested  the decreased e f f e c t irradiance  levels  of prey s i z e on r e a c t i o n was  another sensory system of  the  amount  shown that mechanism theory, he  it to  the  result  distance  f o r prey d e t e c t i o n which was  not  necessary  low  of the animal s w i t c h i n g to  of i r r a d i a n c e . More r e c e n t l y , Eggers is  at  that  to  invoke  independent (1977) has  another  sensory  e x p l a i n these r e s u l t s . In an a n a l y s i s of c o n t r a s t  showed  that  visual  reaction  distance  under  low  11 7  i r r a d i a n c e c o n d i t i o n s was The  effect  contrast charr  of  independent  changes  in  is  increases  similar  as  (Table  9)  to  the  amount  of of  trout  and  movement  to  artificial  prey  response to changes i n the amount of contrast  (Fig.  and  contrast  movement  10) of zooplankton prey i s the  the e f f e c t of i n c r e a s e s on r e a c t i o n d i s t a n c e  in  amount  i s g r e a t e s t at  i r r a d i a n c e l e v e l s . At a l l i r r a d i a n c e l e v e l s ,  movement  proportionally t r o u t than  The  targets  greater  the v i s u a l i r r a d i a n c e t h r e s h o l d of t r o u t , i n c r e a s e s  amount of  and  the amount of movement ( F i g . 8) or c o n t r a s t ( F i g .  same. In both t r o u t and charr  saturation  movement and  e f f e c t of changes i n prey s i z e .  of both s p e c i e s  9) i n c r e a s e s . The  than  the  size.  of prey t a r g e t s on the r e a c t i o n d i s t a n c e  reaction distance  of  of prey  or  contrast  of  prey  greater  increase  in  the  charr.  These  results  targets  i n the  produce  a  r e a c t i o n d i s t a n c e of  parallel  those  of  earlier  studies. Ware  (1971)  found  that  the r e a c t i o n d i s t a n c e  t r o u t , Salmo g a i r d n e r i , to moving prey t a r g e t s was to  of  rainbow  greater  than  s t a t i o n a r y prey and that over the range of prey s i z e s t e s t e d  (3-15 mm) Blades  the e f f e c t of t a r g e t motion (1975)  showed  gibbosus to Mesocyclops  that  the  edax was  of a s i m i l a r s i z e . M. edax was study  suggesting  prey t a r g e t s  that  increases  Confer and Blades larger  reaction  was  additive.  reaction greater  Confer  distance  than to other  of Lepomis copepods  the most a c t i v e prey used i n t h i s  the r e a c t i o n d i s t a n c e with i n c r e a s e s  of L. gibbosus to  in t a r g e t movement.  (1975) found that Lepomis gibbosus had  distance  to  and  the  darkly  pigmented  a  forms of  1 18  Daphnia  magna than to the l i g h t l y  pigmented  Daphnia  pulex  of  similar  size. Similar  f i n d i n g s have been reported  by K e t t l e  O'Brien  (1978),  found  distance  Salvelinus greater  who  namaycush  to  that  the  reaction  zooplankton  attributed in the  the  difference  zooplankton  distance Daphnia  of  Salvelinus  pulex f e l l  containing  from  the  namaycush  between  the  zooplankton from the f i s h and  of  fish.  to the high c o n c e n t r a t i o n  forms  and  from a f i s h l e s s lake  than to zooplankton from a lake  fishless  lake.  distance  f i s h l e s s lakes  was They  of pigment Reaction  to i n t e r m e d i a t e l y  reaction  a  pigmented found  ( K e t t l e and  with  O'Brien  1978). Size, ability  movement  and  contrast  of t r o u t and charr  of prey t a r g e t s  to l o c a t e them. An  these prey c h a r a c t e r i s t i c s increased  the  trout.  This  h i g h l y developed The  indicates  the  in  in  terms  volume  visual of  water scanned visibility irradiance It  will  of in  i s more  rate  has  important  at which the two  i s the same  in  species  both  species  of water searched becomes p r o p o r t i o n a l  in by  distance  always greater  discrimination  the  square of the r e a c t i o n d i s t a n c e . discrimination  i n any of  that v i s u a l d i s c r i m i n a t i o n  encounter prey. I f swimming speed then  was  the  in t r o u t than i n c h a r r .  difference  implications  increase  reaction  the f i s h to them and the. r e a c t i o n d i s t a n c e the  affected  trout trout  be greater  l e v e l s greater  is d i f f i c u l t  to  As the r e s u l t of b e t t e r  an increase due  to the  to  i n the absolute volume of  some  factor  than the increase  than 3.0 determine  visual  x 10  16  which  increasing  shown by charr  photons m~  2  of  prey  the  at  s" . 1  three  prey  119  characteristics  examined  on v i s u a l p e r c e p t i o n proportional  i n t h i s study has the g r e a t e s t  i n t r o u t and c h a r r .  change i n r e a c t i o n d i s t a n c e  A  comparison  of  the answer as there  i s no s i n g l e s c a l e which can be used to d e s c r i b e change  detection  the  least  important  characteristic  i s prey movement. While an increase  increases  the  reaction  distance  with  in  in  prey  visual  movement  of t r o u t and c h a r r , the f i s h  a l s o respond to non-moving prey. S i m i l a r  results  are  obtained  Salmo g a i r d n e r i (Ware 1971) and Gadus morhua (Brawn 1969). The  e f f e c t s of prey s i z e and c o n t r a s t cannot be considered  s e p a r a t e l y . The performence of any v i s u a l system the minimum c o n t r a s t t h r e s h o l d  (Hester  is  restricted  required for target detection  1968, Le Grand 1967). T h i s t h r e s h o l d  i s not constant but  decreases as the angle subtened at the eye (a measure by the t a r g e t i n c r e a s e s decreases  with  detected  lower l i m i t  of  i n c r e a s i n g ambient i r r a d i a t i o n . The upper l i m i t  by  the  i s set by the minimum angle  r e t i n a l photoreceptors  (Hester  that  by  the  eye.  subtending  cones (Table retina  be with  i t probably i s  r e l a t e d to e i t h e r the minimum c o n t r a s t t h r e s h o l d or the angle  can  While i t cannot be determined  c e r t a i n t y what causes the poorer a c u i t y i n c h a r r ,  can  1968). The  i s e s t a b l i s h e d by the minimum c o n t r a s t that  distinguished  size)  (Ware 1971). The c o n t r a s t t h r e s h o l d a l s o  of the c o n t r a s t t h r e s h o l d be  i n the  characteristics. Probably  by  the  r e s u l t i n g from changes  i n each prey c h a r a c t e r i s t i c does not provide  three  effect  minimum  the eye they can d e t e c t . The lower d e n s i t y of  11) and the  of sympatric  charr  higher (Table  degree  of  summation  i n the  13) r e l a t i v e to t r o u t suggests  120  that  i t may be r e l a t e d to the l a t t e r .  4.__._2 Chemical Thus f a r , my d i s c u s s i o n has been r e s t r i c t e d to aspects  of  prey  detection  l e v e l s above t h e i r appear  to  visual  and on  threshold  species  depend  on  more  f o r i n t e r p r e t i n g the nature of t h e i r  some s p e c i e s  show a s h i f t  behaviour i n f i s h  system f o r c o l l e c t i n g  than  surroundings  While  most  studies  of  i n d i c a t e the importance of the v i s u a l  information  from  the  environment,  some  l i v i n g under low i r r a d i a n c e c o n d i t i o n s .  Results sub-visual system  from t h i s study  support  the  hypothesis  that  f o r l o c a t i n g prey t a r g e t s but r e j e c t the same hypothesis  threshold  charr  can  successfully  their locate  visual buried  irradiance liver  prey  in  glass  t a r g e t s but do not respond to the same t a r g e t s sealed cylinders  (Table  laboratory  evidence suggesting  visual  at  i r r a d i a n c e l e v e l s t r o u t do not use t h e i r chemosensory  f o r c h a r r . At i r r a d i a n c e l e v e l s below  and  15). Schutz and Northcote  chemosensory  that  Although no n o n - v i s u a l used i n t h i s study  when  (1972) present  foraging  charr  under  high  systems  c o n d i t i o n s , the chemosensory system  charr  one  the importance of the chemosensory system p a r t i c u l a r l y f o r  species  prey.  species  from one mode to another depending  immediate environmental c o n d i t i o n s .  show  both  r e l y e x c l u s i v e l y on t h e i r v i s u a l system f o r l o c a t i n g  mode  sensory  visual  t r o u t and c h a r r . At i r r a d i a n c e  irradiance  prey t a r g e t s . However, a l l sensory  in  the  to  detect  use  some their  irradiance  buried  benthic  f o r a g i n g behaviour was observed i n the  irradiance  level  exceeded  121  their  visual  irradiance  t h r e s h o l d , the prey were never  from view of the p r e d a t o r . The with  the  large  sediment  numbers  epibenthic time  during  of  and  the  benthic  and  organisms,  side  side  t a r g e t they  1971)  consisting  both  stop  and  begin  steep  close  the  behaviours  the environment  the tank moving t h e i r  the  predicted  if  snapping  results  at  the  sediment.  The  water depends e n t i r e l y  in a c o n c e n t r a t i o n  are  suggests related  in s t i l l searching  it  is  water  gradient  that to  the  two  non-visual  the c o n c e n t r a t i o n of the is  detected  in  (Table 15) although there i s no  behaviour.  assumed  that  chemosensory system to l o c a t e prey. present  This the  chemosensory system (Andrusak  is  what  would  the  stimulus  i t s distribution  to l o c a t e the prey. Consequently,  Northcote  1971),  is  is  i s very  i n c h a r r , which move i n t o the l i t t o r a l and  be  c h a r r are using t h e i r  Although  in the t u r b u l e n t water,  uneven making i t d i f f i c u l t  night  from  and snapping when the g r a d i e n t becomes steep.  than  change i n  at  head  success of c h a r r in l o c a t i n g prey i n t u r b u l e n t water i s  less  still  their  day.  s t i m u l u s , swimming o c c u r r i n g when the stimulus  much  that  to the prey and decreases with d i s t a n c e  away from the source. T h i s  The  of  along the bottom. When they come c l o s e to a prey  on d i f f u s i o n . The d i f f u s i o n  foraging  that  suggest  of a chemical stimulus in s t i l l  is  associated  e x h i b i t two types of n o n - v i s u a l s e a r c h i n g behaviour. they swim throughout  which  are  i n the summer and  be used throughout  Initially  movement  charr  daytime  Northcote  chemosensory system may  to  that  i n f a u n a l forms, are found i n t h e i r guts at t h i s  (Andrusak  Charr  fact  hidden  probably  the zone less  1 22  e f f e c t i v e d u r i n g p e r i o d s of heavy wave a c t i o n . The p o s s i b i l i t y for  locating  of charr using  zooplankton  is  their  chemosensory  remote. Zooplankton  r e l e a s e chemical cues to which f i s h can  respond  system  are known to  (Johannes  and  Webb 1970). However, u n l i k e most benthic prey which are f i x e d in space, trail  a zooplankter a c t s as an i r r e g u l a r moving source and i t s i s e a s i l y d i s t u r b e d by water movements. The  locate  a  source  in  such  a  case  only  way  i s to search the water mass  c o n t a i n i n g the stimulus and r e l y on other, more d i r e c t i o n a l (e.g. v i s i o n ) i n f i n d i n g the exact source It use are it  cues  location.  i s reasonable to assume that the sensory  systems  charr  for prey d e t e c t i o n , l i k e other aspects of feeding behaviour, a d a p t i v e to the environment is  difficult  chemosensory  to  and  benthic  prey  access  i n which the animal l i v e s .  the  relative  importance  to  the  (Andrusak  animal.  and  While  Northcote  charr  1971)  n e c e s s a r i l y detected by the chemosensory system.  be detected v i s u a l l y  and may  1972). That charr choose to use  locating  the  their  consume are  and  (Schutz and  visual  system  prey when i r r a d i a n c e l e v e l s are high and the prey  Northcote  success i n c h a r r i s (82.0%)  than  not  sediment-water  are exposed i s w e l l documented i n t h i s study and by the work Schutz  the  Some p o r t i o n of  interface  1971)  above  they  just  Northcote  of  do  the benthic community i s at or (Ware  Yet,  v i s u a l systems in terms of the amount of food  they make a v a i l a b l e  for  to  for  (1972).  much prey  higher  As  well, for  the  visually  of  rate of capture detected  l o c a t e d with the chemoreceptive  (25.8%). T h i s suggests that use of the v i s u a l system  prey system  i s the most  123  e f f e c t i v e means of d e t e c t i n g prey However, as night approaches becomes  increasingly  in charr. the  unreliable  visual  as  a  During t h i s p e r i o d t h e i r v i s u a l system  system  of  charr  prey-sensing m o d a l i t y . functions  only  in  upper few meters of the water column ( F i g . 13) and the low of  irradiance  potential  at  prey.  extensively  on  this It  time l i m i t s t h e i r  seems  chemical  reasonable cues  l o c a t i n g those prey under low  emitted  irradiance  1976)  means  of  level  r e a c t i o n d i s t a n c e to  that by  they  may  rely  potential  prey i n  conditions.  Trout are known to be capable of d e t e c t i n g chemical (Jahn  stimuli  yet they d i d not use t h e i r chemosensory system locating  prey i n t h i s study  the  as  (Table 14). P o s s i b l y  a the  higher c o n c e n t r a t i o n of p l a n k t o n i c prey in the s u r f a c e waters of Loon Lake that they i n h a b i t as w e l l as t h e i r more system  provides  a sufficient  locating  visual  food source hence e l i m i n a t i n g  need f o r using other, probably l e s s for  acute  effective  sensory  systems  prey. Although not i d e n t i f y i n g the sensory  system  used f o r d e t e c t i n g prey, Andrusak (1968) d i d f i n d that the gut  fullness  of  sympatric  g r e a t e r than that of summer.  trout  sympatric  charr  consistently during  the  mean  40 - 50%  spring  and  T h i s o b s e r v a t i o n p r o v i d e s support f o r the argument that  the a v a i l a b i l i t y of food to t r o u t may charr.  It  relative while  was  the  be g r e a t e r than  that  for  does not a s s i s t though in d i s t i n g u i s h i n g between the  importance  of p o s i t i o n of the f i s h  i n the water  f o r a g i n g and the type and s t r u c t u r e of the sensory  column system  employed f o r l o c a t i n g prey i n determining food a v a i l a b i l i t y .  1 24  4.2 The  Ret i n a l S t r u c t u r e  behavioural  revealed  studies  in Trout  with  and  Charr  sympatric  trout  levels  than  reacted  trout  to prey t a r g e t s  (i.e.  the  visual  at  lower  poorer. The  irradiance  system of charr  s e n s i t i v e to i r r a d i a n c e than that of t r o u t ) but  i s more  their acuity  h i s t o l o g i c a l s t u d i e s were used as a means of  t e s t i n g the behavioural organization.  The  r e s u l t s but  hypothesis  d i f f e r e n c e s in r e t i n a l which  sensitivity five  charr  major d i f f e r e n c e s i n t h e i r response to v i s u a l s t i m u l i .  In summary, charr  charr  and  can  tested  structure  account  at  a was  between  for  different  observed  that  further  level  there  sympatric  are  trout  differences  of  the  of no and  in their  to i r r a d i a n c e or a c u i t y . Based on an examination  characteristics  was  of  r e t i n a which have been r e l a t e d to  e i t h e r the  l e v e l of s e n s i t i v i t y or a c u i t y in other  species,  and  density,  the degree of  cone  retinal  (Dickson  Lake)  and  was  acuity  a l l o p a t r i c populations is  known.  also  were  found  the  Unlike  summer,  throughout  Northcote  1971).  most evident charr,  rejected.  the  trout  ( P l a c i d Lake) and Although  visual  determined for animals from  sympatric p o p u l a t i o n s  most  of  surface and  the  difference  between the two  unlike  and  examined.  not  allopatric  The  size  t h e i r v e r t i c a l d i s t r i b u t i o n i n the  taken most f r e q u e n t l y at the during  cone  r e t i n a l s t r u c t u r e of a l l o p a t r i c  sensitivity  column  types,  summation, the hypothesis was  The charr  cone  forms  sympatric  rod  charr  trout  and  water  column  water  where t r o u t  in  deeper  are  water  charr were commonly (Andrusak  and  i n v e r t i c a l d i s t r i b u t i o n was of  charr,  with  allopatric  form, undergoing a d i e l  vertical  125  migration  and  commonly taken in the upper 5 m during  As a r e s u l t of d i f f e r e n c e s in t h e i r the  two  forms  of  each  species  i r r a d i a n c e regimes. Consequently, related  to  the  was  exposed  retinal  correlate  of  examined  by in  allopatric  to  structure  visual  be d i f f e r e n t between the two  differences  testing  the  different which  is  retinal  t r o u t and  structure  sympatric  and  is  sensitivity  forms. T h i s  hypothesis  to d i f f e r e n c e s i n t h e i r v e r t i c a l was  are  distribution  amount of i r r a d i a n c e in the environment and  used in t h i s study as a a c u i t y , may  vertical  daylight.  that  between  and  possibility  there  are  sympatric  no and  a l l o p a t r i c charr that r e l a t e  distribution.  The  hypothesis  rejected. The  charr  density  of  rods and  i s s i m i l a r to that  1976).  The  greatest  populations (Table  and  11).  populations  has  other  cone  the l e a s t  The  of  cones in the r e t i n a of t r o u t teleosts  density  is  important  in  and  Anctil  found in the two  in the p o p u l a t i o n  difference  (Ali  cone  of  implications  in  trout  sympatric  density  charr  among the  terms  of  and  four  visual  acuity. Visual The  acuity  is  minimum separable  animal can  resolve  determined in part by the cone d e n s i t y .  angle  (a  measure  ( ) i s determined as  of  visual  acuity)  follows:  (1)  where F = f o c a l d i s t a n c e of the lens which i s  2.25  an  126  (Matthiessen's r a t i o ) times the r a d i u s of the l e n s 0.25 = degree of shrinkage during h i s t o l o g i c a l procedures n = number of cones per 0.01  This  model  they f a l l (Tamura  assumes  2  that image l i n e s can only be r e s o l v e d when  on cones separated by at l e a s t one and  Wisby  1963).  The  separable angle, 1.8 minutes, while  mm  smallest  occurs  in  unstimulated  estimate the  sympatric charr have the l a r g e s t estimate at 2.9  better  i n sympatric  t r o u t than sympatric  separable angle f o r a l l o p a t r i c that  f o r the sympatric  separable  angle  of  more  trout,  2.3  minutes. t r o u t and  "trout-like"  charr  This  that  have  and  in  High d e n s i t i e s of rods are a s s o c i a t e d activity  a  either  form  with  of  g r e a t . Consequently,  trout  identified  types  1971).  the d e n s i t y of  sympatric  i t i s u n l i k e l y that rod d e n s i t y i s  ranging  i n the t e l e o s t  and  charr  (Table 11), the d i f f e r e n c e i s not  to d i f f e r e n c e s i n i r r a d i a n c e s e n s i t i v i t y Cones  to  vertical  nocturnal  ( A l i e_t a l . 1977). Although  of  minimum  corresponds  terms  rods i s s l i g h t l y higher i n both a l l o p a t r i c and than  acuity  i s s i m i l a r to  d i s t r i b u t i o n and feeding mode (Andrusak and Northcote  crepuscular  minutes.  value i s intermediate  charr  behaviour  trout  c h a r r . The minimum  1.9 minutes,  form. A l l o p a t r i c  between that of sympatric their  of minimum  sympatric  T h i s suggests, as d i d the b e h a v i o u r a l experiments, is  cone  from  single  retina  (Anctil  related  i n these two s p e c i e s . to quadruple 1.969). Based  have been on  their  1 27  relative  proportion  i n shallow and deep-water species  been a t t r i b u t e d d i f f e r e n t degrees of s e n s i t i v i t y The  predominant  theory  irradiance conditions  has  been  increases  that  to  they have irradiance.  sensitivity  from s i n g l e to  to  quadruple  (Willmer . 1953, O'Connell 1963, Engstrom 1963, Ahlbert  low  cones  1975, A l i  et a l . 1977, A l i and A n c t i l 1977). S i n g l e and double cones are present i n the r e t i n a of and  charr  type  (Plate  is different  sympatric  and  populations  between  allopatric  cone s i z e (Table  the  3). Although the a c t u a l d e n s i t y sympatric charr,  of each cone  and  charr  and  the r e s u l t of d i f f e r e n c e s i n  12), the p r o p o r t i o n  of each type  in  a l l four  i s the same. The r a t i o of double to s i n g l e cones i n  r e t i n a of t r o u t and c h a r r ,  Salmo  trout  trout  salar  4 : 1, i s the  and Salmo t r u t t a (Ahlbert  same  as  that  in  1976). The s i m i l a r i t y i n  the  r a t i o of double cones to s i n g l e cones i n the r e t i n a of t r o u t  and  charr  in  i r r a d i a n c e s e n s i t i v i t y between sympatric t r o u t and charr nor  suggests that cone type cannot e x p l a i n  is i t related  to  differences  in  the  vertical  between sympatric and a l l o p a t r i c forms of each The  for  the eye t o gather a s u f f i c i e n t  to make r e l i a b l e p r e d i c t i o n s retina.  similarity required  As  is  between  the two  in  number of photons  statistics,  populations,  the  distribution  i n nature. I t i s  about the nature of  case  difference  species.  b a s i c problem i n v i s i o n i s s t a t i s t i c a l  necessary  the  the  the  image  the greater the  larger  the  to d i s t i n g u i s h them. In v i s u a l terms then, a high  of  acuity  can  only  or f i n e d i s c r i m i n a t i o n  occur i n a high i r r a d i a n c e  on  sample level  of s i z e , movement or c o n t r a s t environment.  1 28  In a low premium,  i r r a d i a n c e environment  it  is  not  possible  where  photons  are  at  to r e s o l v e f i n e d e t a i l . In t h i s  s i t u a t i o n the primary concern must be to enhance s e n s i t i v i t y irradiance.  One  way  to  a  accomplish  this  is  to  to i n c r e a s e the  e f f e c t i v e capture area of each r e c e p t o r . One  means of i n c r e a s i n g the e f f e c t i v e  photoreceptor  is  to  increase  capture  cones  of  crepuscular  St i z o s t e d i o n  and  of  a  the c r o s s - s e c t i o n a l area of the  r e c e p t o r . T h i s occurs in the f a m i l y Percidae The  area  vitreum  ( A l i e_t a_l. (1977).  vitreum  which  n o c t u r n a l f e e d i n g forays (Ryder  l a r g e r than the cones of Perca f l a v e s c e n s  which  1977) is  makes are much  a  daytime  feeder. The c r o s s - s e c t i o n a l s i z e of both double and smallest  in  the  two  trout  p o p u l a t i o n of sympatric s i z e between sympatric more  sensitive  to  populations  of  to low  t r o u t and charr suggests  low  by  i n the  charr  conditions  The  intermediate  than  sympatric  charr.  water column than the sympatric  taken  higher  the cone  sensitive This  the o b s e r v a t i o n s of Andrusak and Northcote  effective  are  i r r a d i a n c e c o n d i t i o n s and p a r a l l e l s  which show a l l o p a t r i c charr are commonly  The  that  a l l o p a t r i c charr suggests that they are l e s s  irradiance  supported  largest  charr (Table 12). The d i f f e r e n c e i n cone  r e s u l t s of the b e h a v i o u r a l experiments. size  and  s i n g l e cones i s  is  (1971) in  the  form.  capture area of a photoreceptor can a l s o  be  enlarged by connecting a number of r e c e p t o r s i n the same area of the r e t i n a together so they f u n c t i o n as a u n i t . The spatial  integration  in  the  retina  concept  of  i s u s u a l l y r e f e r r e d to as  129  summation.  It  is  photoreceptor number of  cells  bipolar  irradiance is  measured  in  (rods cells  increases  terms  of  the  number  of  and cones) per b i p o l a r c e l l and per  ganglion  cell.  the  Sensitivity  to  with the degree of summation while a c u i t y  reduced. In general the degree  living  in  low  summation  in a  vitreum  much  vitreum  summation  ( A l i and A n c t i l  demersal  fish  Anctil  and  the  summation was in  low  Perca  highest  turbid a  in  fish  1977). In a study of  of  much higher  environment  higher  (1969) i d e n t i f i e d an  extent  irradiance  more have  between the b r i g h t n e s s of the lives  is  i r r a d i a n c e or t u r b i d environments. S t i z o s t e d i o n  canadense which l i v e Stizostedion  of  environment summation.  r a t e of 20  f l u v i a t i l i s or A c e r i n a cernua  and  inverse c o r r e l a t i o n in  which  Ahlbert  than  retinal  pelagic  the  (1975)  in Lucioperca lucioperca,  environments,  than  fish found  which  in the c l o s e l y  live  related  which are c h a r a c t e r i s t i c  of  high i r r a d i a n c e environments. Sympatric  charr show the h i g h e s t l e v e l of summation i n t h i s  study with a r a t i o of one 78.1  visual  approximately  cells  ganglion c e l l  (Table  13). The  1:16:38, occurs  to 22.3  lowest  in the two  bipolar c e l l s  to  l e v e l of summation,  trout populations. This  suggests,  as do the r e s u l t s of the b e h a v i o u r a l experiments, that  sympatric  charr are more s e n s i t i v e to low  than  sympatric  trout  and  conditions  but t h e i r a c u i t y i s poorer. The  summation and hence s e n s i t i v i t y acuity  irradiance  to low  l e v e l of  i r r a d i a n c e c o n d i t i o n s and  in a l l o p a t r i c charr i s intermediate between that of t r o u t  sympatric  c h a r r . Again,  the d i f f e r e n c e in r e t i n a l  structure  130  between  a l l o p a t r i c and sympatric  c h a r r appears to be r e l a t e d to  d i f f e r e n c e s i n the v e r t i c a l d i s t r i b u t i o n and feeding the two p o p u l a t i o n s The  (Andrusak and Northcote  mosaic-like  arrangement  of  habits  1971).  the  visual cells  r e t i n a of some t e l e o s t s has been known f o r more than (Ryder  of  i n the  a  century  1895). The major elements of v i s u a l c e l l mosaics are the  cones. Rods do not u s u a l l y show any p a t t e r n of r e g u l a r i t y  except  in very young f i s h  visual  cell  mosaics  1963)  and  (Wagner 1974).  have  North  instances  been  Various  described  American  patterns  of  f o r many Europeon  teleosts  (Anctil  1969).  (Engstrom In  some  these p a t t e r n s have been r e l a t e d to the behaviour and  feeding h a b i t s of the f i s h  (Anctil  1970,  1975). In the past, o r d e r l y cone  Wagner 1972, A h l b e r t  mosaics  have  appears t h i s mosaics  been is  are  associated  not  true  probably  d i s c r i m i n a t i o n of movement The (Plate  that  (Campbell an  Dathe  1969,  Bathelt cell  with good v i s u a l a c u i t y . I t now 1975)  adaptational  and  that  advantage  orderly f o r the  (Wagner 1978).  cone mosaic of sympatric  t r o u t and charr  is  the  same  3) except the a c t u a l area of one u n i t (four double c e l l s  surrounding of  1969,  their  one s i n g l e c e l l ) larger  although  movement  than  cone s i z e  i s l a r g e r i n the c h a r r , the (Table  12). Consequently i t appears  sympatric  charr  are  less  sensitive  sympatric  t r o u t ( F i g . 8) i t i s u n r e l a t e d to the  cone mosaic. I t i s more l i k e l y that the decreased movement i n charr  r e s u l t s from t h e i r  r a t e of summation, which reduces nothing  result  prey  s e n s i t i v i t y to  l a r g e r cone s i z e and higher  the  i s known about the r e l a t i v e  to  level  of  acuity.  While  s e n s i t i v i t y of sympatric and  131  allopatric  trout  and  sympatric  movement, the s i m i l a r i t y forms  of  each  sensitivity  i n the  species  and  allopatric  cone  mosaic  suggests  that  charr  suggest  that,  on  s e n s i t i v e to low i r r a d i a n c e acuity  i s better.  behavioural cellular  cone  any  of  between  a relative  conditions  difference in  and  provide  sympatric  charr  parallel an  but  the  two  species  explanation  cone  size  when  in  sympatry.  when measured  a  vertical  differences  distribution  in retinal  trout  in  sympatric  trout  t r o u t most common  (Andrusak and Northcote in  irradiance  insufficient structure.  level  to  be  most  of  these  allopatric  and  feeding  and  charr.  t r o u t . While the v e r t i c a l d i s t r i b u t i o n  allopatric  terms  mode There  s t r u c t u r e between sympatric and  p o p u l a t i o n s during the summer i s not i d e n t i c a l small,  Retinal  and l e v e l of summation. Again,  intermediate between that of sympatric  allopatric  at the  charr i s intermediate between that of  t r o u t and c h a r r , at l e a s t  having  no  their  those of the  d i f f e r e n c e s r e l a t e to known b e h a v i o u r a l d i f f e r e n c e s ,  are  trout  b a s i s , t r o u t are l e s s  than  observations  allopatric  density,  charr  the two  f o r d i f f e r e n c e s i n the v e r t i c a l d i s t r i b u t i o n and  feeding mode of  sympatric  These  experiments  level  structure  between  to movement i s u n r e l a t e d to the cone mosaic.  Differences in r e t i n a l structure and  charr to prey  common  of  these  two  the d i f f e r e n c e i s  between  4  and 5 m and  i n the upper 3 m of the water column  1971). I t i s l i k e l y experienced  reflected  in  by  that the d i f f e r e n c e  these  two p o p u l a t i o n s i s  differences  in  retinal  132  4.3 Behavioural  and  General  histological  evidence  v i s u a l system of sympatric c h a r r i s rival  demands  a  f o r s e n s i t i v i t y to low  compromise  is  to  between  the  i r r a d i a n c e c o n d i t i o n s and  a c u i t y . The v i s u a l system of t r o u t on the sensitive  suggests that the  other  hand  is  less  i r r a d i a n c e c o n d i t i o n s than the c h a r r ' s but a c u i t y  better. During the summer sympatric t r o u t and charr are segregated,  trout as  found near the s u r f a c e and c h a r r at g r e a t e r depths,  deep  as 25 m (Andrusak  i r r a d i a n c e l e v e l s decrease magnitude  and Northcote by  depth at which the i r r a d i a n c e reaction  distance  in t r o u t  water column than the s i m i l a r advantage unit  1970). Over t h i s  approximately  ( F i g . 13). Throughout  three  orders  is  just  sufficient  to  maximize  i s approximately 20 m higher in the depth  for  charr.  in  moving  above  the  depth  where  waters  There  is  their  reaction  model  in  was  developed  to  estimate  the  the volume of water  Diaptomus kenai d u r i n g one day  Daphnia  rosea  i n the summer ( F i g . 15). For  both f i s h the volume of water searched i n c r e a s e d with prey The  no  to maximize t h e i r r e a c t i o n d i s t a n c e .  searched by t r o u t and c h a r r f o r v a r i o u s s i z e s of and  of  most of the d a y l i g h t hours, the  d i s t a n c e i s g r e a t e s t . At the same time t r o u t must remain  A  depth  f o r charr i n terms of the volume of water searched per  time  upper  often  l a r g e s t volumes were searched f o r Daphnia  size.  rosea and f o r both  prey s p e c i e s t r o u t searched a g r e a t e r volume than c h a r r . The estimates of volume of water searched p r o v i d e s relative  only  a  index of the d a i l y s e a r c h i n g p o t e n t i a l as the r e a c t i o n  1 33  d i s t a n c e of the f i s h to the prey to  i n the l a b o r a t o r y  is  be the same as i n the f i e l d . D i f f e r e n c e s i n water  s p e c t r a l composition and amount  of  background  unlikely turbidity,  irradiance  and  s p e c t r a l r e f l e c t i v i t y of prey s u r f a c e s between the f i e l d and  the  l a b o r a t o r y w i l l c o n t r i b u t e to these d i f f e r e n c e s . The model makes two assumptions of  t r o u t and c h a r r . F i r s t  about  the feeding behaviour  i t i s assumed that both f i s h search a  c y l i n d r i c a l volume of water while f o r a g i n g where the the and  cylinder  equals the r e a c t i o n d i s t a n c e to a given prey  s i z e . While  this  is  the  most  common  e s t i m a t i n g the volume of water searched it  is  unlikely  is  not  the  made  path  of  nasal  and  temporal  c i r c l e the only requirement water searched  in the two  is different  in  directions.  f o r e s t i m a t i n g the  species.The s i m i l a r i t y  second  in the  way  Salvelinus  the  dorsal,  Although  assumption about lake.  both  the  not be a  relative  volume  trout  and  charr  prey t a r g e t s suggests t h i s i s the case.  assumption  of  the model i s that s e a r c h i n g i s  continuous but only when the i r r a d i a n c e l e v e l maximize  the  i s that the c r o s s - s e c t i o n a l shape be the same  d e t e c t and approach The  in  1975)  (1978) show that  c r o s s - s e c t i o n of the search path i n t r o u t and charr may  of  type  (Confer and Blades  search  of  a c i r c l e but a polygon and the d i s t a n c e from  the eye to the edge of the polygon ventral,  assumption  to be t r u e . Confer et. al.  c r o s s - s e c t i o n a l shape through namaycush  radius  reaction  distance  i s more d i f f i c u l t  to  is  sufficient  to  and s e a r c h i n g v e l o c i t y . T h i s  evaluate  as  little  is  known  the short term movements of e i t h e r of these s p e c i e s in the Certainly  the swimming speeds measured i n t h i s study  and  1 34  used i n the model are w i t h i n the range that  can  for  i n muscular f a t i g u e  long  p e r i o d s of time without  in salmon and charr stunted, It  seldom  is likely  resulting  (Beamish 1980). The f i s h  be  in  reaching s i z e s greater than  that food s u p p l i e s are marginal  maintained  Loon  Lake  are  20 cm (Hume 1978). r e q u i r i n g searching  at a l l times when i r r a d i a n c e i s s u f f i c i e n t . T h e o r e t i c a l and experimental ecologically co-exist 1904,  similar  species  like  i n d e f i n i t e l y on the same  Lotka . 1925,  Volterra  reduce or e l i m i n a t e competition resource)  sympatric  trout  and  limiting  1926,  shown  in  f o r food  (or any  trout  and  (1967) i d e n t i f i e s two types  salmonid  charr  segregation  refers  to  d i f f e r e n c e s s u f f i c i e n t l y great to be  do  between  Differences  in  behaviour  are  common  this,  different  of  (eg. i n  which implies  food  through i n t e r a c t i o n  species  at  parts  segregation  which  ecologically  the sense of e v o l u t i o n i s t s . R e s u l t s from t h i s study segregation  other  1971).  species  s e l e c t i o n ) are o f t e n magnified  selective  cannot  (Grinnell  communities. I n t e r a c t i v e segregation  that e c o l o g i c a l d i f f e r e n c e s between habitat  two  Gause 1934, Park 1962). To  of the water column (Andrusak and Northcote  occur  charr  resource  during the s p r i n g and summer by occupying  Nilsson  that  s p e c i e s must p a r t i t i o n t h e i r environment i n  e i t h e r space or time. Sympatric least  s t u d i e s have  or  while  have evolved isolated suggest  in that  t r o u t and charr i n Loon Lake i s s e l e c t i v e .  retinal  morphology  f u l l y expressed  and  consequently  i n s o l i t a r y animals  visual  and probably  evolved as s p e c i f i c b e n e f i t s towards the c o e x i s t e n c e of the two species.  This  conclusion  i s supported  by the r e s u l t s of other  1 35  s t u d i e s on these Northcote  two  (1972)  s p e c i e s . In a l a b o r a t o r y study  observed  between s o l i t a r y t r o u t differences  were  and  not  differences  in  charr  Loon  magnified  r e c e n t l y Hume (1978) separated and  charr  from  Loon  two-year sampling either and  the  from  and  Schutz  feeding  behaviour  Lake  and  interaction.  transferred  sympatric  p e r i o d there were  no  populations.  segregation was  due  Laboratory  Again  More trout  f i s h l e s s lakes. After a marked  differences  d i e t or v e r t i c a l d i s t r i b u t i o n between the  allopatric  these  through  Lake to separate  and  the  in  sympatric  implication  was  that  to g e n e t i c a l l y based s e l e c t i v e f o r c e s .  studies  show  that  trout  are very  aggressive  towards c h a r r . When p a i r e d , t r o u t always dominate charr and when f e e d i n g , t r o u t u s u a l l y take a s i g n i f i c a n t l y of  the  available  food  than charr  invade  a  in  that  when  behaviour,  are able to  terms of t h e i r own  occupy  requirements  and  to other p o r t i o n s of the aquatic environment. If then  allopatric  Andrusak  and  and Northcote  sympatric (1971) found  g r e a t e s t number of t r o u t were taken the  same  for  sympatric 2 m,  show  in r e t i n a l  allopatric  difference  was  and  d i f f e r e n c e , l e s s than no  trout  and  aggressive  the  "optimal  restrict this  there should be no d i f f e r e n c e in the v e r t i c a l  between  1972;  trout  f i s h l e s s lake, t r o u t , through t h e i r  h i g h l y competitive habitat"  proportion  (Schutz and Northcote  Rosenau 1978). Consequently, i t i s l i k e l y charr  greater  charr  is  true  distribution  populations.  While  that the depth at which the during the  allopatric  small. Results  summer  was  not  populations,  the  from  this  study  s t r u c t u r e between sympatric  t r o u t i n d i c a t i n g that the v i s u a l  system  of  and  animals  136  from  both  populations  i s most e f f e c t i v e  in the same p o r t i o n of  the water column. Charr, vertical  u n l i k e t r o u t , show major d i f f e r e n c e s  distribution,  between sympatric  feeding  and a l l o p a t r i c  habits  and  in  summertime  retinal structure  p o p u l a t i o n s . In terms of  these  three c h a r a c t e r s a l l o p a t r i c charr are more " t r o u t - l i k e " than  the  sympatric  form. T h i s supports  and  suggests  that i f charr evolve  the t r o u t dominance hypothesis independently  of t r o u t then  can  "choose", u n r e s t r i c t e d , that p o r t i o n of the aquatic  in  which  f o r c e d c h a r r , when sympatric  occupy the deeper water. T h i s , in  to low  for  turn,  has  with t r o u t , to  resulted  a v i s u a l system in charr which i s very  in  the  sensitive  irradiance conditions.  Implicit between  i n the  sympatric  above trout  in  charr and However, Northcote  the  competing  results 1972,  from  Hume  pressures  morphological  the  concept  charr  that  segregation  need not always be  Interactive  segregation  may  e a r l y stage of c o e x i s t e n c e with t r o u t  out  selective  is and  i n t e r a c t i v e or s e l e c t i v e . occur  habitat  to l i v e . Consequently, i t seems that the dominance of  t r o u t over charr has  selection  charr  and  with  them  this  study  1978)  clearly  cause  for  the  and  genetic  that  changes  b e h a v i o u r a l c h a r a c t e r s and  indeed  dominating  available  others  suggest  either  result  food.  (Schutz with in  and time,  various  in s e l e c t i v e  segregation. Although  the v i s u a l systems of t r o u t and charr seem c l o s e l y  r e l a t e d to t h e i r c h a r a c t e r i s t i c the  obvious  question  summertime s p a t i a l  i s what about other times  positioning,  of the year when  1 37  s e g r e g a t i o n breaks down (Andrusak and Northcote 1970)?  Although  eye  changing  pigments  may  vary  seasonally  environmental c o n d i t i o n s  in  response  to  (Beatty 1966), eye morphology does not.  Consequently eye morphology cannot be optimal at  a l l times  the  and l i k e other  year.  Fish  must  perform  many  functions  animals one of the most important i s the North  temperate  fish  put  on  s p r i n g and summer when the water (Kelso  and  Ward  systems of t r o u t maximize  1977).  their efficiency  of  i s warm and  Therefore  food  i s abundant  i t appears that the v i s u a l such  a  way  i n the time and environment  thereby  food.  most of t h e i r growth during the  and charr have evolved i n  i s most abundant and survival.  procurement  of  maximize  growth,  as to  when food  fecundity  and  138  5.0  REFERENCES  A h l b e r t , I.-B. 1975. O r g i n i z a t i o n of the cone c e l l s in the r e t i n a e of some t e l e o s t s i n r e l a t i o n to t h e i r feeding h a b i t s . Ph.D. T h e s i s . U n i v e r s i t y of Stockholm. 29 p. A h l b e r t , I.-B. 1976. 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