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Specializations for alternate trophic niches by two forms of threespine stickleback, Gasterosteus, co-existing… Bentzen, Paul 1982

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S P E C I A L I Z A T I O N S FOR ALTERNATE TROPHIC NICHES BY TWO THREESPINE STICKLEBACK, GASTEROSTEUS, CO-EXISTING  FORMS OF  IN ENOS LAKE,  VANCOUVER ISLAND by PAUL BENTZEN B.Sc,  McGill University,  Montreal,1978  5 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF: MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We a c c e p t t h i s t h e s i s a s c o n f o r m i n g t o t h e required standard  THE UNIVERSITY OF B R I T I S H COLUMBIA May 1982 (c)  Paul Bentzen,  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  be granted by the head of  department or by h i s or her  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  Z-OOlOOV  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  Date  DE-6  (3/81)  yjL^-r,  Columbia  ht?~  written  i i  ABSTRACT Two m o r p h o l o g i c a l l y and e c o l o g i c a l l y d i s t i n c t forms of threespine stickleback ( G a s t e r o s t e u s ) , a " l i m n e t i c " form and a " b e n t h i c " f o r m , c o - e x i s t i n Enos L a k e on V a n c o u v e r Island. I u s e d t h r e e e x p e r i m e n t s t o compare t h e f e e d i n g p e r f o r m a n c e o f t h e two forms, t o t e s t the hypothesis that l i m n e t i c s a r e adapted t o p l a n k t i v o r y and b e n t h i c s a r e adapted to foraging on benthic substrates. The r e s u l t s s u p p o r t t h i s c o n c l u s i o n . When t e s t e d f o r maximum p r e y s i z e , b e n t h i c s were a b l e t o consume l a r g e r p r e y (relative to their body s i z e ) t h a n l i m n e t i c s . B e n t h i c s were a l s o more s u c c e s s f u l t h a n l i m n e t i c s i n f o r a g i n g on a benthic substrate. When a l l o w e d t o f o r a g e on a d e t r i t u s s u b s t r a t e f o r a f i x e d t i m e i n t e r v a l , b e n t h i c s o f b o t h s e x e s c a p t u r e d more p r e y t h a n male limnetics, despite the fact that male limnetics directed a s many f e e d i n g s t r i k e s a t the substrate as d i d the benthics. F e m a l e l i m n e t i c s d i d n o t f o r a g e on t h e s u b s t r a t e a t all. I n c o n t r a s t , l i m n e t i c s were more s u c c e s s f u l i n f e e d i n g on p l a n k t o n than b e n t h i c s . When h e l d i n mesh e n c l o s u r e s s u s p e n d e d in t h e water column o f Enos L a k e , l i m n e t i c s consumed more p l a n k t o n than b e n t h i c s . " S m a l l " l i m n e t i c s (26-35 mm standard length) consumed nearly f o u r t i m e s more p l a n k t o n t h a n " l a r g e " l i m n e t i c s (44-50 mm s t a n d a r d l e n g t h ) . The s m a l l l i m n e t i c s were a l l e i t h e r m a t u r e f e m a l e s o r immature m a l e s ; t h e l a r g e l i m n e t i c s were a l l m a t u r e m a l e s . These results, along with morphological, biochemical, ecological and b e h a v i o u r a l data obtained i n other s t u d i e s , s u p p o r t t h e c o n c l u s i o n t h a t t h e two f o r m s of G a s t e r o s t e u s in Enos L a k e a r e d i s t i n c t b i o l o g i c a l s p e c i e s . The r e s u l t s o f t h i s study a l s o s u p p o r t a n o t h e r ( u n e x p e c t e d ) c o n c l u s i o n : mature male and f e m a l e l i m n e t i c s a l s o d i f f e r i n f e e d i n g b e h a v i o u r . Female limnetics appear t o be a l m o s t t o t a l l y p l a n k t i v o r o u s ; w h e r e a s , male l i m n e t i c s a r e i n t e r m e d i a t e between female l i m n e t i c s and b e n t h i c s (both sexes) i n feeding behaviour.  iii  TABLE OF CONTENTS  ABSTRACT  i i  L I S T OF TABLES  iv  L I S T OF FIGURES  v  ACKNOWLEDGEMENTS General  vi  introduction  1  Maximum P r e y S i z e  9  Introduction  9  Results  12  Discussion  17  Foraging  on a N a t u r a l  Benthic  Substrate  20  Introduction  20  Methods  20  Results  22  Discussion  27  Foraging  on P l a n k t o n  34  Introduction  34  Methods  35  Results  39  Discussion  45  General Discussion References  54 .  59  L I S T OF TABLES  Table  1.  Relationship  between  maximum  prey  size  and  standard l e n g t h of l a b o r a t o r y - r e a r e d s t i c k l e b a c k s Table  2. R e s u l t s o f a n a l y s e s o f c o v a r i a n c e on maximum  s i z e and s t a n d a r d l e n g t h Table  15  3.  Performance  of  prey  .; fish  tested  17 in_ b e n t h i c  foraging  experiment Table  4.  24  Standard  lengths  of  fish  tested  in  benthic  f o r a g i n g experiment Table  5. R e s u l t s o f t e s t s  l e n g t h s of f i s h Table and Table  25  6.  that  f o r c o r r e l a t i o n s between s t a n d a r d  f e d and f o r a g i n g performance  Zooplankton  concentrations calculated  ........26  from  lake  e n c l o s u r e samples 7.  Approximate  40 sizes  of  plankters  in  lake  and  e n c l o s u r e samples  41  T a b l e 8. Number o f p l a n k t o n i c  prey  i n stomachs of f i s h  that  c o n t a i n e d prey Table  9.  42  Comparison  stomachs  of  o f t h e numbers o f p l a n k t o n i c p r e y i n  small  limnetics,  large  limnetics  and  benthics that contained prey Table  10.  Correlations  stomach, and s t a n d a r d space  for limnetics  between length  v  log and  number estimated  44  of prey per gillraker 47  V  L I S T OF  Figure Figure  1. P h o t o g r a p h s of 2.  FIGURES  l i m n e t i c s and  Frequency d i s t r i b u t i o n  benthics  3  of g i l l r a k e r  numbers f o r  Enos L a k e s t i c k l e b a c k s Figure  3.  standard Figure  4.  standard Figure wild Figure  5  Relationship l e n g t h of  between  l i m n e t i c s and  Relationship l e n g t h of  gillraker  between  length  and  benthics maximum  6 prey  size  and  laboratory-reared sticklebacks  5. Maximum p r e y s i z e v e r s u s s t a n d a r d  length  14 for  6  sticklebacks 6.  Drawing  16 of  enclosure  used i n p l a n k t o n  feeding  exper iment Figure the fish Figure  36  7. R e l a t i o n s h i p between number of p l a n k t o n i c p r e y  in  s t o m a c h s of  of  l i m n e t i c s , and  the  standard  length  t h a t consumed them 8.  standard  Relationship  46 between g i l l r a k e r  l e n g t h of b e n t h i c s  and  limnetics  spacing  and  the ....52  vi  ACKNOWLEDGEMENTS I w o u l d l i k e t o t h a n k my s u p e r v i s o r , D r . J . D. M c P h a i l , f o r many long discussions that stimulated my interest i n the s t i c k l e b a c k s o f Enos L a k e , a n d f o r p r o v i d i n g t h e means f o r me t o s t u d y them. •I owe s p e c i a l t h a n k s t o my f r i e n d a n d p a r t n e r i n t h e f i e l d , Mark Ridgway. Mark's s t u d y o f t h e b r e e d i n g b e h a v i o u r o f Enos L a k e s t i c k l e b a c k s c o n t r i b u t e d t o my own p e r s p e c t i v e on these fish; m o r e o v e r , h i s company i n t h e f i e l d h e l p e d make t h e work a l o t of f u n . Thanks a l s o t o D e n n i s L a s s u y , Steve Campana a n d Marvin Rosenau f o r assistance i n the f i e l d . T o g e t h e r we l e f t our mark on l o c a l p o p u l a t i o n s o f a b a l o n e a n d s a l m o n . D r . J . N. M. S m i t h o f f e r e d v a l u a b l e a d v i c e a t key stages during t h e s t u d y ; he a l s o r e v i e w e d t h e t h e s i s . Dave Z i t t i n o f t h e B i o s c i e n c e s D a t a C e n t r e i n i t i a t e d me t o t h e j o y s o f UNIX. His help at the data a n a l y s i s stage of t h i s study was invaluable. Mr. N e i l S c o t t k i n d l y p e r m i t t e d us t o work a t Enos Lake. I am e s p e c i a l l y g r a t e f u l t o h i m f o r a l l o w i n g u s t o s t a y a t h i s beach c a b i n d u r i n g t h e w i n t e r months, thus sparing us much misery i n t h e f i e l d and long n i g h t s i n t h e A r l i n g t o n . Finally, a m a t e r i a l support,  s p e c i a l thanks and c o n s i d e r a b l e  t o my family patience.  f o r moral and  Personal support during this study was p r o v i d e d by a Natural Sciences and E n g i n e e r i n g Research Council graduate s c h o l a r s h i p , and r e s e a r c h s u p p o r t was f u r n i s h e d by an NSERC operating grant.  1  GENERAL INTRODUCTION  The  threespine  stickleback  morphologically variable freshwater of  the  clinal,  variability  or a s s o c i a t e d w i t h major fresh  (Munzing, 1974;  species that occupies  waters  or  M o o d i e and R e i m c h e n ,  evidence found  suggests  that  selection,  types,  (i.e.  versus  lentic  environments)  1972;  1976; G r o s s ,  1978). A growing  populations  particularly assemblages  (Moodie,  Hagen  Reimchem,  of  and  are  local  Coad  t o occur  Kynard,  in e s s e n t i a l l y  Gilbertson,  together  explained  and  1973,  forms  of  (Hagen, 1967; N a r v e r ,  1 9 7 8 ) . Most s u c h  by  pressures competitors  Bell,  non-overlapping  British  in  the  e a s t e r n shore  G.  1976;  Columbia,  aculeatus  1969; M i l l e r a n d 1971;  instances involve either  ( geographically adjacent  Moodie, simple  populations  c o n t a c t ) . I n a few s m a l l l a k e s i n  however,  p o p u l a t i o n s of s t i c k l e b a c k s occur exist  Power,  1980).  polymorphism or parapatry  coastal  marine  body o f  selective  Hubbs, 1969; Hagen a n d G i l b e r t s o n , 1972; S e m l e r , 1972a;  and  best  predators  I n some a r e a s , h o w e v e r , d i f f e r e n t appear  species i s  Gilbertson,  e x e r t e d by v a r y i n g 1972b;  e x h i b i t e d by t h i s  Much  the s t r i k i n g morphological d i f f e r e n c e s  between s t i c k l e b a c k  natural  range.  habitat  lotic  1963; Hagen and  isa  a wide v a r i e t y of  and marine h a b i t a t s w i t h i n i t s h o l a r c t i c  morphological  versus  (Gasterosteus aculeatus)  in  same l a k e ) . Enos L a k e  morphologically divergent sympatry  ( i . e . they  co-  (49017'N 124009'W), on t h e  o f V a n c o u v e r I s l a n d , i s s u c h a l a k e . I n Enos L a k e ,  2  1 " l i m n e t i c " and names  "benthic"  (benthic  and  forms  of  limnetic)  Gasterosteus  allude  to  coexist.  their  use  of  The  space:  l i m n e t i c s s c h o o l l o o s e l y i n t h e w a t e r c o l u m n d u r i n g most o f year, the  whereas  substrate  limnetic  benthics u s u a l l y occur (Bentzen  forms  however, the  are  et_  the  ,  described  following  characterize  a_l.  two  in  (F-ig.1A). Apart  f o r m s . The  l i m n e t i c s are a  dark  than  from the  typically  body, a  broad  snout,  c o l o u r , b e n t h i c s are and  benthics (a  trophic  gillrakers  on  the  r a n g e s f r o m 22 benthics relatively  short  1980)  and  t h a t the  l i m n e t i c and  isolated  from  populations;  trait).  one  (in prep.);  sufficient  large  breeding  to  protrusive  c o l o u r of  (i.e. silvery  males,  sides  and  brown.  the  the  first  are  long  In  Limnetics  i n t h e m o r p h o l o g y of  of  deep  (Fig.lB).  range i s between  their  number  of  branchial arch 17 and  21  in  i n l i m n e t i c s and  (Fig.3).  prep.),  (Ridgway,  benthic  eyes  limnetics  gillrakers  another,  however,  very  or m o t t l e d  In  edge  (McPhail, in  behavioural  is  and  form a t t a i n s a l a r g e r s i z e  strikingly  in benthics  Morphological  olive  whereas the  The  in McPhail  non-protrusive  leading  t o 26,  (Fig.2).  benthic  typically  also differ  benthic  i s c h a r a c t e r i z e d by a r e l a t i v e l y  and  gillrakers  and  counter-coloured  l i m n e t i c f o r m and  The  l i m n e t i c form i s c h a r a c t e r i z e d  seasonal  d o r s a l s u r f a c e ) . The  the  prep).  description  by a s l e n d e r body, a n a r r o w s n o u t , eyes  in close association with  in detail  cursory  the  biochemical  i n prep.) evidence  forms i n Enos Lake are and  thus  morphological  comprise  two  (Withler, indicates genetically separate  i n t e r m e d i a t e s between  the  3  Figure  1.  P h o t o g r a p h s o f l i m n e t i c s and  A.  Limnetic  B.  Benthic  male and f e m a l e male and f e m a l e  benthics.  (male on t o p ) . (male on b o t t o m ) .  5  F i g u r e 2. Frequency d i s t r i b u t i o n of g i l l r a k e r numbers f o r Enos Lake s t i c k l e b a c k s . N = 70 b e n t h i c s , 70 l i m n e t i c s .  El'  BENTHICS  LIMNETICS  IB15. .. 15. .. J_ 1—  Z  G3- .. 0  3  1G. 17- IB-19' EO. SI- EE- S3- E4- E5- EG- E7- EB  NUMBER OF GILLRAKERS  6  Figure 3. R e l a t i o n s h i p between g i l l r a k e r l e n g t h and s t a n d a r d l e n g t h of l i m n e t i c s and b e n t h i c s . Lines represent functional r e g r e s s i o n s (N = 6 8 b e n t h i c s , 6 8 l i m n e t i c s ) . Benthics....(+) Limnetics...(x)  STANDARD LENGTH  CMM)  7  two  f o r m s , t h o u g h r a r e , do o c c u r  laboratory crosses offspring  that  between t h e  are  (McPhail  two  forms y i e l d  morphologically  ( M c P h a i l , p e r s . c o m . ) . The  suggest  the  two  morphological  Furthermore,  robust,  intermediate  p a r e n t a l types that  in prep).  (and  last  f o r m s i n Enos L a k e must be m a i n t a i n e d  by  between  two  genetic)  fertile the  observations  integrity  selection  of  the  against  hybrids.  This sympatric  study  describes  f o r m s of G a s t e r o s t e u s  determine  if  limnetics  small zooplankton adapted  to  benthic  column,  l a r g e r prey  lifestyle,  benthics,  as  the b e n t h i c  well  "benthic"  morphologically  the b e n t h i c limnetic  is  an  (Paxton  and  co-exist.  if  typical  Larson  form f o r a g e d  form f e d m a i n l y  r e f e r r e d to throughout t h i s  to  benthics of  closely  are  spaced  suggest  proportions  benthic  a of  foragers.  Island).  to  the  In  Enos  macrobenthos,  zooplankton.  Larson's  are Lake Lake  while  study  in  this  that  ( 1 9 7 6 ) showed t h a t i n P a x t o n  thesis.  and  analagous s i t u a t i o n  identical)  on  on are  littoral  f o r m s of G a s t e r o s t e u s  primarily on  attempts  limnetics  L a k e , on "Texada  (but not  two  tendency to a s s o c i a t e w i t h  apparently  "limnetic"  similar  and  of  their  the  adapted to f o r a g i n g  w h e r e a s t h e more r o b u s t  as w e l l as  there  coastal lake  also  habit  of  and  elongate,  s u b s t r a t e , suggest t h a t they  addition,  another  pelagic  the  behaviour Lake  primarily  environments. C e r t a i n l y ,  planktivorous  forms  the  feeding i n Enos  water  f o r a g i n g on  and  lake  are  i n the  gillrakers,  In  the  will  the be  8  I used two approaches t o t e s t the h y p o t h e s i s t h a t l i m n e t i c s and b e n t h i c s a r e niches.  In  adapted  t o , and  occupy,  This  study  will  be  of the two forms  reported  elsewhere;  p r e l i m i n a r y r e s u l t s i n d i c a t e t h a t i n Enos Lake, l a r g e l y on z o o p l a n k t o n , (Bentzen,  unpub.  f e e d i n g behaviour controlled basis  of  predictions  trophic  one approach I used a survey of stomach c o n t e n t s t o  document the g e n e r a l f e e d i n g behaviour lake.  different  my  limnetics  feed  In the second approach I compared the  and c a p a b i l i t i e s experiments.  thesis  however,  whereas b e n t h i c s eat more " b e n t h i c " prey  data).  feeding  i n the  and  of  the  two  forms  through  T h i s second approach forms the  attempts  to  test  three  specific  about b e n t h i c s and l i m n e t i c s . These p r e d i c t i o n s a r e  as f o l l o w s : 1)  Maximum prey s i z e . At  capable 2)  a  given  of e a t i n g l a r g e r prey than  benthics  a  natural  should have g r e a t e r f e e d i n g success F o r a g i n g on s m a l l z o o p l a n k t o n .  prey, l i m n e t i c s s h o u l d e x p e r i e n c e  should  be  limnetics.  F o r a g i n g on a n a t u r a l b e n t h i c s u b s t r a t e .  l a r g e (but sub-maximal) prey on  3)  size,  than  Given  relatively  substrate  benthics  limnetics.  Given  small  zooplanktonic  greater feeding  success • than  benthics. The  bases f o r each of these p r e d i c t i o n s a r e developed  the body of the t h e s i s .  further in  9  MAXIMUM PREY S I Z E  Introduct ion  The  prediction  prey  than  that  the  that  limnetic two  are  exceed  1 mm  areas  include  in  i n length,  a  variety  to  the  but  of  invertebrate in  difference  prey  environments difference  the  should in  the  sizes  of  manifest ability  foraging  potential  in  different  zone o f Enos L a k e prey  in  littoral  species that  linear  attain  dimensions.  available  itself  larger  to the hypothesis  limnetic  s i z e s an o r d e r o f m a g n i t u d e l a r g e r in  c a n consume  i s central  adapted  Zooplankters  rarely  sticklebacks  sticklebacks  forms  environments.  benthic  in  in  a  This  the  two  corresponding  o f t h e two f o r m s t o d e a l  with  large  prey.  To I  control  tested  f o r e f f e c t s o f e x p e r i e n c e on maximum p r e y  this  prediction  limnetics.  I also  evaluate  hybrid  tested  with  progeny of  performance  phenotypes. F i n a l l y ,  laboratory-reared  I tested  reciprocal  relative  b e n t h i c s and  F1  the  crosses  to  two  parental  w i l d c a u g h t b e n t h i c s and  limnetics  to supplement r e s u l t s o b t a i n e d w i t h  to  size,  the laboratory-reared  fish.  Methods  Amphipods consumed  (Hyalella azteca)  are  among  the  by b o t h t y p e s o f Enos L a k e s t i c k l e b a c k s .  h o w e v e r , was f a i r l y  difficult  to obtain  largest This  prey  species,  i n a d e q u a t e numbers  and  10  a  large  enough  size  range.  Instead  e s t u a r i n e a m p h i p o d , Eogammarus prey.  This  species  wide range of it  can  be  of H y a l e l l a ,  confervicolos,  as  I used  experimental  c o l l e c t e d i n l a r g e numbers and  s i z e s . Though p r i m a r i l y a b r a c k i s h w a t e r  t o l e r a t e d f r e s h water d u r i n g  the  sorting  the  and  in a  organism,  experimental  p r o c e d u r e s w i t h n e g l i g i b l e m o r t a l i t y . C l e a r l y , Eogammarus i s not a  normal  prey  of  Hyalella closely  Enos  in  sticklebacks;  gross  morphology  however, and  i t resembles  general  avoidance  response.  The river  amphipods  estuary,  facilitate  were c o l l e c t e d w i t h d i p n e t s  and  sorting  and  used to a n a e s t h e t i z e w i t h an  held  in  handling,  of  characteristic the fine  few  f l e x e d . Most bent  t h a t d i d not  forceps.  The  posture  by  were r e a d i l y amphipods  Each  spanned  All  t o the  amphipods  anaesthetic  0.7  s m a l l e r of t h e were  before  b o t t l e d carbonated  water  was  allowed  they  base of t h e the  two  measured on  the  the d i s t a n c e  from  uropods, with  amphipods  t h e m s e l v e s when  the  assumed  this  anaesthetized;  molded i n t o t h i s p o s t u r e  into  mm.  was  to a s i z e category  r a n g e d f r o m 4.4  were d i v i d e d up  assigned  To  body l e n g t h as  of  body l e n g t h and category  use.  assigned  the antennae t o the  abdomen s l i g h t l y  until  water  the amphipods. Each amphipod  o c u l a r m i c r o m e t e r and  base  Squamish  brackish  b a s i s of body l e n g t h . I d e f i n e d the  i n the  mm  17 r a n k e d s i z e  Borderline  to recover  16.4  mm  from the  in  categories.  individuals  size categories  were p r e s e n t e d  to  with  in  were  question.  e f f e c t s of  to s t i c k l e b a c k s .  the  11  The f i s h bare  20  1  partitions  u s e d i n t h i s e x p e r i m e n t were h e l d aquariums.  placed  (perforated  only  The  aquaria  between  were  aquariums,  by a s m a l l v i e w i n g  inside  hide.  uniform  prey  laboratory  fish.  in front. and  challenging  ration  (ground  to  accustom  than  fish),  the f i s h  Tubi fex  several  to t h i s prey  than  the  largest  standard  fish  amphipods  days.  type. During presented  t r i a l s were c o n d u c t e d a t a p p r o x i m a t e l y  larger  inexperienced the  I fed a l l test  e a c h d a y . Two o f t h e t e s t a m p h i p o d s were category  of  that  smaller  approximately  than  the  two  and  re-measured.  results  of a t r i a l  to  each time size  the  test  fish  had  two  size  amphipods.  After  recovered,  S i n c e amphipods of  were a n a l y s e d  size  one  s i z e c a t e g o r i e s were n e v e r u s e d i n t h e same t r i a l ,  The  prey  size  larger  problem r e a s s i g n i n g amphipods t o the proper  was  a  90 m i n u t e s , t h e u n e a t e n a m p h i p o d s were  re-anaesthetized  This  t h e same  p r e v i o u s l y consumed. The t h i r d a m p h i p o d was o f a s i z e categories  could  r e m a i n e d a t 190  or  t h r e e a m p h i p o d s were s i m u l t a n e o u s l y The  the  w i t h a smooth bead o f  t h e l a b o r a t o r y - r e a r e d s t i c k l e b a c k s were more  The  experiment.  from t h e s m a l l e s t s i z e c a t e g o r i e s f o r  trials,  curtains  brown,  The w a t e r t e m p e r a t u r e i n t h e t e s t a q u a r i a  Since  done  black  draped  in  by opaque  s e a l a n t t o e l i m i n a t e c r e v i c e s i n which amphipods  * 10 C f o r t h e d u r a t i o n o f t h e  with  and  seams o f e a c h a q u a r i u m were f i l l e d  silicone  separated  hole)  b o t t o m s o f t h e a q u a r i a were p a i n t e d a  individually  adjacent  t h e r e was  no  size categories.  only  i f at least  one  12  a m p h i p o d was e a t e n . consumed had for  Experimental  amphipods of a g i v e n  failed  to  trials  e n d e d when e a c h f i s h h a d  size category  at least  twice,  and  consume a m p h i p o d s o f t h e n e x t l a r g e r s i z e c l a s s  four consecutive  days.  Results  During improved  the  course  markedly  amphipods.  At  of  in  the  their  first  experiment  ability  the  to  majority  refused  however, t h i s  i n i t i a l p e r i o d of r e l u c t a n c e  in  increasing  which  most  fish  sticklebacks  handle  of  sticklebacks  phase  a l l but  most  and  consume  laboratory-reared  t h e s m a l l e s t amphipods o f f e r e d ;  accepted  was  followed  amphipods  of  by  a  steadily  s i z e . When e x p o s e d t o a m p h i p o d s o f a new, l a r g e r  size  category,  sticklebacks often  initially  failed  t o eat  the  Usually,  however,  o r two d a y s t h e y  aquired  e i t h e r the  in  one  motivation  or the a b i l i t y  new  size  prey  sticklebacks prolonged  category  rarely  t h e new s i z e c a t e g o r y .  was  not  consumed  mastered  that  size  criterion  i n three  category  e x p o s u r e . Thus, f o u r c o n s e c u t i v e  a conservative  with  to deal  "failures"  t h e maximum s i z e o f a m p h i p o d s consumed  fish  Since  approximately  isometric  growth,  and  sticklebacks  i t i s reasonable  linear  r e l a t i o n s h i p between s t i c k l e b a c k s i z e  size.  Therefore,  I  days, the  f o r e s t a b l i s h i n g maximum p r e y  both amphipods  used  linear  and  regressions  I f the  even  Generally, size.  prey.  with  served as size.  increased exhibit  t o assume a  maximum  prey  t o describe the  13  r e l a t i o n s h i p b e t w e e n s t i c k l e b a c k l e n g t h and maximum and  analyses  of  covariance  to  test  prey  size,  f o r d i f f e r e n c e s between  groups.  Regressions  of  laboratory-reared (p= 0.017 crosses either  size  sticklebacks  ,  standard  were  to  attributed  in  sticklebacks  achieve part  tested  a greater  t h e F1 f i s h  Mean  to  than  the  (Table  The  narrow 1);  and  the  F1  significance  can  be  of  F1  range  of  i n the prey handling  comparisons  with  are  benthics  consume  consumed  crosses. in  were  pooled,  significant  significantly  by  the  that  were  parental  The  results  T a b l e 2 and F i g . 4 .  l i m n e t i c s (p<0.00l),  amphipods  consumed  distribution  parental  summarized  laboratory-reared  those  crosses  the  (p= 0 . 0 3 2 ) .  r e a s o n I u s e d t h e c o m b i n e d F1 " r e g r e s s i o n t o compare t h e  hybrid crosses  to  in  (F=1.39, p= 0.256, d . f . = 1 , 1 6 ) . When  was s t a t i s t i c a l l y  reared  abilities  type.  resulting this  sizes  they a l s o appear t o  f o r t h e two F1 c r o s s e s  regression  (LL)  of  maximum p r e y s i z e d a t a  For  significant  p r e y s i z e s f o r t h e two r e c i p r o c a l F1  significantly  the  (p > 0.1) f o r  failure  however,  in either parental  of  limnetic  significant  statistical  variability  maximum  not d i f f e r  crosses.  length  statistically (BB)  b u t were n o t s t a t i s t i c a l l y  of t h e r e c i p r o c a l F1  reflect  on  a n d 0.014) f o r b o t h b e n t h i c  regressions  did  prey  larger  of  Laboratory-  amphipods  w h e r e a s F1 h y b r i d s  intermediate types.  than tended  i n s i z e between  However,  o f a m p h i p o d s t a k e n by F1 h y b r i d s  these  the  size  was h e a v i l y  skewed  14  F i g u r e 4. R e l a t i o n s h i p between maximum p r e y s i z e and s t a n d a r d ,. l e n g t h of l a b o r a t o r y - r e a r e d s t i c k l e b a c k s . Lines represent predictive regressions. B e n t h i c x b e n t h i c c r o s s ...BB( +) Limnetic x limnetic cross...LL(x) P o o l e d b e n t h i c x l i m n e t i c and l i m n e t i c x b e n t h i c c r o s s e s . . . F 1 )  STANDARD LENGTH OF TEST FISH  (MM)  Table 1.  R e l a t i o n s h i p between maximum prey s i z e and s t a n d a r d l e n g t h of l a b o r a t o r y - r e a r e d s t i c k l e b a c k s . P r o b a b i l i t i e s < 0.05 are c o n s i d e r e d s i g n i f i c a n t . S i z e range ( i n mm) of f i s h t e s t e d .  Probability t h a t slope=0 2  CROSS  N  MIN  MAX  r  SLOPE  p  BB  7  37  59  0.71  0.180  0.017  LL  8  36  51  0.67  0.221  0.014  LB  10  45  50  0.18  0.219  BL  8  39  50  0.23  0.223  F1 (LB+BL)  18  39  50  0.51  towards  the s i z e range consumed by the b e n t h i c c r o s s . Thus, the  maximum  sizes  significantly  of  amphipods  0.125  eaten  by  F1  0.032  sticklebacks  were  l a r g e r than those consumed by l i m n e t i c s (p<0.00l)  but not s i g n i f i c a n t l y  smaller  than  those  eaten  by  benthics  (p=0.191).  The  results  obtained  with  l i m n e t i c s p a r a l l e l e d those o b t a i n e d The  three  relative  benthics to  their  were  able  body  size,  to  wild  caught  benthics  with laboratory-reared consume  than  larger  and fish.  amphipods,  were the t h r e e l i m n e t i c s  ( F i g . 5 ) . U n l i k e the l a b o r a t o r y - r e a r e d f i s h , however, none of  the  w i l d b e n t h i c s h e s i t a t e d when f i r s t exposed to amphipods  the  in  Figure 5. Maximum p r e y sticklebacks. Benthics....(+) Limnetics...(x)  size versus  standard length for  17  T a b l e 2. R e s u l t s o f a n a l y s e s o f c o v a r i a n c e on maximum p r e y s i z e and s t a n d a r d l e n g t h . P r o b a b i l i t i e s < 0.05 a r e c o n s i d e r e d significant. TEST  SLOPE P =  BB X L L  0.661  <0.00 1  BB X F1  0.461  0.191  F1  0.249  X LL  laboratory.  From  the start  INTERCEPT P =  <0.001  of t h e experiment a l l three  readily  s e i z e d and a t e r e l a t i v e l y  l a r g e amphipods. In c o n t r a s t , the w i l d  l i m n e t i c s were more l i k e  their  and  displayed  some  initial  laboratory-reared reluctance  counterparts  t o a t t a c k any b u t t h e  s m a l l e s t amphipods o f f e r e d . Thus, whereas t h e w i l d b e n t h i c s probably  already  recalcitrant  preconditioned  prey,  this  l i m n e t i c s a t the s t a r t  was  to deal with probably  l a r g e and  not  true  were  somewhat  of t h e w i l d  of the experiment.  Discussion  For  fish  that  swallow t h e i r  prey  whole,  mouth  size  maximum p r e y s i z e a r e known t o be c l o s e l y r e l a t e d ( H y a t t , On  t h i s b a s i s , K e a s t a n d Webb ( 1 9 6 6 ) s u g g e s t e d t h a t  useful  t o measure t h e mouth g a p e s o f c o e x i s t i n g f i s h  order  to  gain  i n f e r e n c e s about t h e i r  and  1979).  i t m i g h t be species  in  ecological ( i . e . trophic)  18  relationships. equivalent  While  Nevertheless, consume  t o t h e measurement my r e s u l t s c l e a r l y  larger  l i m n e t i c s . Larson Lake  prey  limnetic  (Hyallela azteca)  of  i n gape, t h i s d i f f e r e n c e i s unpub.  data).  i n d i c a t e that benthics are able  (relative  Paxton  sticklebacks  e r r o r (Bentzen,  (1976) o b t a i n e d  sticklebacks.  amphipods  and  s i z e do a p p e a r t o d i f f e r  small r e l a t i v e  to  benthic  to  similar  Lake  than  their  body s i z e )  results  benthics  limnetics  for  Paxton  consumed  of  than  larger  equivalent  body  length.  Although were  their  mouth w i d t h s  were n o t m e a s u r e d , F1 h y b r i d s  i n t e r m e d i a t e between t h e p a r e n t a l forms i n o t h e r  t h e i r morphology ( M c P h a i l , i n p r e p . ) .  I f maximum p r e y  function  fish  of  consume p r e y  mouth  size,  intermediate  in size  p a r e n t a l c r o s s e s . In f a c t , the  then  F1  between  size  m i g h t be e x p e c t e d those  taken  may  relationship  mouth  factor  genes  i n f l u e n c i n g prey  controlling handling  reflect size  between b e n t h i c s a n d l i m n e t i c s r e f l e c t  correspond  prevail  T h e s e maximum p r e y directly  size"  a  by t h e  dominance  o r some o t h e r  size  observed  a r e a l d i f f e r e n c e i n prey  s i z e s , however, p r o b a b l y  t o t h e upper l i m i t s  i n t h e w i l d . B u r k o (1975)  "maximum p r e y  the  ability.  U n d o u b t e d l y , t h e d i f f e r e n c e s i n maximum p r e y  handling a b i l i t y .  to  by  F l f i s h was skewed h e a v i l y t o w a r d s t h e s i z e consumed  in  is a  t h e s i z e o f t h e a m p h i p o d s consumed by  pure b e n t h i c c r o s s . T h i s o b s e r v a t i o n  not  aspects of  reported  on p r e y  two  size  categories  do  that of  f o r a p o p u l a t i o n of t h r e e s p i n e s t i c k l e b a c k s .  19  He  found  that  even  though  larger  prey  were a v a i l a b l e , the  l a r g e s t prey items consumed i n the w i l d were u s u a l l y 20% s m a l l e r than the b i g g e s t  prey the s t i c k l e b a c k s of e q u i v a l e n t  consume i n the l a b o r a t o r y .  size  could  20  FORAGING ON A NATURAL BENTHIC  SUBSTRATE  Introduction  Amphipods p r e s e n t e d environment limit  provide  an  to sticklebacks objective  of  similar  encounters  nature p o t e n t i a l  refuges are  escape  by  solid  capture  diving  little  restricted  of t h e upper indication  of  size the  under n a t u r a l c i r c u m s t a n c e s . In  numerous.  prey  may  i n t o the s u b s t r a t e or c l i n g i n g  to a  An  intended  surface.  To p r o v i d e a more r e a l i s t i c of  estimate  f o r prey; however, they o f f e r  outcome  i n a barren,  Enos L a k e s t i c k l e b a c k s ,  test  of t h e  feeding  abilities  I added a n a t u r a l d e t r i t u s  along w i t h benthic prey  of  tank.  r e m a i n e d t h e same: b e n t h i c s w i l l be more  The  prediction  s u c c e s s f u l than  sub-maximal  size  to  a  substrate  limnetics at dealing with benthic  laboratory  prey.  Methods  Experimental  t r i a l s were c o n d u c t e d  cm)  aquarium.  leaf  a n d wood l i t t e r o b t a i n e d f r o m  Lake,  a  The  i n a 87 1 (32 x 34 x  f o r a g i n g s u b s t r a t e c o n s i s t e d of p a r t i c u l a t e the  littoral  zone  of  small n a t u r a l lake i n Vancouver. This d e t r i t a l  added t o t h e a q u a r i u m t o form a s u b s t r a t e l a y e r thick.  The  invertebrates  76  prey already  organisms present  used in  were the  1.5  those sediment:  -  Deer  mix was 2.0  cm  macrobenthic amphipods  21  (Hyalella  azteca ) ,  leeches  sp_. ) . P r e y numbers  in  however,  was  Hyalella  ( H i r u d o SJD. ) , and  the  test  the  most  present. Hyalella  ranged  from  maximum of 7.5  i n body  length.  mm  To m i n i m i z e p o t e n t i a l the  behaviour  aquarium single  of  aquarium.  stationed  To  myself  bulb  microscopic  suspended  fish  a t 190  1.5  size  in foraging  maintained  trials,  in a single  195  stickleback the  was  stickleback  aquarium.  above  * 10 C  trials,  away  s t i c k l e b a c k s of  was  t h e n o b s e r v e d . The  was  then a l l o w e d to f o r a g e f o r a f u r t h e r  later  was  noted;  t h e number of f e e d i n g s t r i k e s were  15 m i n u t e  foraging  from the t e s t aquarium,  e x a m i n a t i o n of i t s  stomach  bout,  the  contents.  15  the  minute  counted.  stickleback  b r a i n p i t h e d , and  a  introduction  stickleback  removed  strike  trial,  introduced  its  this  foraging  forms  transferred  the  time between the  the  immediately  and  p e r i o d , d u r i n g which  first  of  I  trial.  both  the f i s h  Following  from  At t h e b e g i n n i n g o f e a c h  behaviour  the  throughout  foraging  1 h o l d i n g aquarium  The  the  p r o v i d e d by a cm  metres  a  affect  b a c k s i d e of  d i p n e t t e d f r o m t h e h o l d i n g t a n k and  test  to  t h a t might  50  during  semidarkness  adjacent to the t e s t aquarium.  of  macro-invertebrate  t h e ends and  remained  counted;  r e m a i n i n g as m o t i o n l e s s as p o s s i b l e d u r i n g t h e  B e f o r e use  to  almost  (Asellus  not  g r e y . I l l u m i n a t i o n was  observe in  abundant  fish,  temperature  the experiment.  were  test  fluorescent  Water  were  visual distractions  were p a i n t e d l i g h t  40 w a t t  aquarium,  the  aquarium  isopods  was  set aside for  Sticklebacks  that  22  failed  to  begin  foraging  r e p l a c e d by a n o t h e r  within  15 m i n u t e s were removed and  fish.  L i m n e t i c s and b e n t h i c s were t e s t e d i n a l t e r n a t i o n , and f o u r to s i x t r i a l s  were c o n d u c t e d  day.  Immediately  the  fish  remaining  the standard were  after  at approximately  the l a s t experimental  trial  of each  each day,  i n t h e h o l d i n g t a n k were f e d t o s a t i a t i o n  laboratory diet.  a l l deprived  t h e same t i m e  of  Thus, f i s h  food  for  used  i n the  22 - 24 h o u r s  on  experiment  prior  to being  tested.  Stomach c o n t e n t s were s o r t e d , c o u n t e d with  sex  and  standard  length  for  each  l e n g t h s o f a m p h i p o d s , and t h e l e n g t h and carapaces numbers were  were  measured.  To  guard  and test  width  recorded fish. of  along  The body  the  isopod  a g a i n s t d e p l e t i o n of prey  i n t h e t e s t a q u a r i u m , e a c h day I r e p l a c e d t h e p r e y  consumed w i t h an e q u a l number o f l i v e  same s p e c i e s and o f s i m i l a r  that  i n v e r t e b r a t e s of t h e  size.  Results  A total  o f 20 b e n t h i c s and 22 l i m n e t i c s were t e s t e d . A l l o f  the b e n t h i c s f o r a g e d r e a d i l y limnetics. limnetics  A l l of  the l i m n e t i c s  aquarium,  that t h e i r  did  15  Since the behaviour  t h a t d i d not feed appeared o t h e r w i s e  seems u n l i k e l y  as  t h a t f e d were m a l e s ; o f t h e 7  t h a t d i d n o t f e e d , 6 were f e m a l e s .  of t h e f i s h it  i n the t e s t  failure  to  be  normal,  t o f o r a g e was t h e r e s u l t o f  23  discomfort forage were  or  fright.  Several  on t h e s u b s t r a t e present  unlikely  that  "intended" vision  in  prey  seven  limnetics  amphipods and o t h e r  long  that  before  introduction  and  d i d feed  averaged foraging  the  the s t a r t  substrate  that  strikes  interval  x = 21.6).  the  fish  This  of f o r a g i n g (both  within  at  the  difference  c a p t u r e d more  performance  when t h e i r of  in  feeding  strike,  d i d not  time  sexes)  and  between  67%  substrate  of  were g e n e r a l l y the  the  during  number  more  limnetics  t h e 15 m i n u t e  ( l i m n e t i c x = 27.0,  the  than 6  two m i n u t e s .  benthic  of s t r i k e s a t the  (Mann W h i t n e y U t e s t , p > 0.1) b u t  prey  than  the  limnetics  (benthic  p < 0.01).  o f t h e two f o r m s d i f f e r e d more  success rates  s t r i k e s ) were  the  substrate  substrate  3 ) ; however,  than the b e n t h i c s  x = 8.6, l i m n e t i c x = 4.9, U t e s t ,  The  perceive  was n e v e r l o n g e r  benthics  (Table  was n o t s i g n i f i c a n t  benthics  o f f the  foraged,  than l i m n e t i c s more  to  s t r i k i n g a t p r e y . The e l a p s e d  male l i m n e t i c s began t o f o r a g e  successful  failed  invertebrates.  m i n u t e s , a n d 75% o f t h e b e n t h i c s  Among  that  water column of t h e aquarium. I t i s a l s o  1 9 7 6 ) , a n d even t o t h e human e y e t h e  Sticklebacks hesitate  failed to  i n t h i s e x p e r i m e n t . G a s t e r o s t e u s has e x c e l l e n t  (Wootton,  teemed w i t h  the females that  d i d chase s t r a y c y c l o p o i d copepods  the  the  of  strikingly  (number o f p r e y e a t e n d i v i d e d by number  compared.  Benthics  averaged  0.45  prey  per  w h e r e a s male l i m n e t i c s a v e r a g e d 0.013 p r e y  each  24  T a b l e 3.  P e r f o r m a n c e (by g r o u p s ) o f f i s h foraging experiment.  tested i n benthic  A. D e s c r i p t i v e s t a t i s t i c s , Males (N=9)  BENTHICS --• Females (N=1 1 )  S.E. Number of s t r i k e s Number of p r e y consumed  x  S.E,  LIMNETICS Males  Both sexes (N=20) x  (N=15)  S.E.  x  S.E.  22.8  2.21  20.6  .3.51  21.6  2.13  27.0  3.71  9.3  1.85  8.0  0.91  8.6  0.95  4.9  0.69  0.20  0.30  Success (prey per s t r i k e ) 0.46 0.117  0.44 0.050  0.45 0.058  B. S t a t i s t i c a l c o m p a r i s o n s (Mann W h i t n e y U T e s t ) . A l l c o m p a r i s o n s ( e x c e p t b e n t h i c male v s f e m a l e ) a r e onet a i l e d . Numbers i n p a r e n t h e s e s i n d i c a t e v a l u e o f U. P r o b a b i l i t i e s < 0.05 a r e c o n s i d e r e d s i g n i f i c a n t .  Number of s t r i k e s Number of p r e y consumed Success (prey per s t r i k e )  MALE BENTHICS VS FEMALE BENTHICS P =  LIMNETICS VS ALL FEMALE BENTHICS BENTHICS P = P =  >0. 1 0 (31.5)  >0.05 (64.0)  >0.05 (113.0)  >0.05 ( 1 16.0)  >0 . 1 (45.0)  <0.025 (30.0)  <0.01 (32.5)  <0.0 1 (62.5)  >0. 1 (41 .0)  <0.025 (29.0)  <0.001 (15.5)  <0.001 (44.5)  (p < 0.001, M a n n - W h i t n e y U  The  MALE MALE BENTHICS P =  test).  20 b e n t h i c s u s e d i n t h i s  test  spanned  a  considerable  25  size  range,  (particularly (Table  4).  performance, standard strikes, both  whereas the  size  sub-group  To I  the  check tested  length  and  distribution  that  foraged)  i f body  size  for correlations 3  other  variables:  of the l i m n e t i c s was  more  influenced between number  foraging  the of  fish's feeding  number o f p r e y c a p t u r e d , a n d c a p t u r e s u c c e s s r a t e .  forms  t h e number o f p r e y c a p t u r e d a n d t h e c a p t u r e  r a t e showed a weak  tendency  to  increase  with  size  t e s t e d ; h o w e v e r , t h e s e t r e n d s were n o t s t a t i s t i c a l l y  4. S t a n d a r d foraging  l e n g t h s of f i s h experiment.  tested  i n benthic  STANDARD LENGTHS (MM) CATEGORY  N  MIN  MAX  40  56  47.7  41  62  49.6  1 1  40  -62  48.8  20  40  48  45.5  34  42  37.5  Benthics (males)  (females) Benthics (both sexes) Limnetics (males) Limnetics (females)  MEAN  9  Benthics  16 6  For  success of  fish  significant  (Table 5 ) .  Table  limited  26  T a b l e 5. R e s u l t s o f t e s t s f o r c o r r e l a t i o n s b e t w e e n s t a n d a r d l e n g t h s of f i s h t h a t f e d and f o r a g i n g performance in benthic foraging experiment. BENTHICS r  Number of s t r i k e s  #  2  LIMNETICS P  r  2  p  0.047  >0.1  0.040  >0.1  0.076  >0.1  0.208  0.088  Success (prey p e r s t r i k e ) ## 0.118 >0.1 #...square r o o t t r a n s f o r m e d ## arc s i n transformed  0.210  0.085  Number o f p r e y consumed #  The  two  forms  did  not d i f f e r  much i n t h e i r  p r e y . A m p h i p o d s were t h e d o m i n a n t p r e y consumed (79%  for  formed and  by  both  benthics  and 8 3 % f o r l i m n e t i c s ) .  Leeches  the remainder  ( 1 9 % and 2% o f t h e p r e y  for  13%  and  4%  of t h e prey  p r e y was a l s o n o t a f a c t o r  for limnetics).  Size  i n t h i s experiment,  length).  In t h i s experiment  b e n t h i c ) was 5.3 mm  t h i s was  s i z e experiment  from  the  mean  (3.4 mm  body  than  tested  (by a the i n my  (Fig.5).  L i m n e t i c s and b e n t h i c s a l s o d i f f e r e d prey  and  s e l e c t i o n of  smaller  b i g g e s t a m p h i p o d s consumed by t h e s m a l l e s t l i m n e t i c maximum p r e y  benthics  t h e l a r g e s t a m p h i p o d consumed  i n body l e n g t h ;  forms  and i s o p o d s  since  s i z e o f a m p h i p o d s e a t e n was t h e same f o r b o t h f o r m s  secured  s e l e c t i o n of  the s u b s t r a t e . Both  little forms  i n t h e way t h e y  adopted  t h e head-  27  down p o s t u r e t y p i c a l orienting  the  of  body  benthic  foragers  (see  Hyatt,  a t 30 - 70 d e g r e e s t o t h e h o r i z o n t a l  s t u d y i n g the s u b s t r a t e . A f e e d i n g s t r i k e c o n s i s t e d of lunge,  f o l l o w e d by a w r e n c h i n g  stickleback and  briefly  1979),  mouthed  a  while sudden  b i t e at the intended t a r g e t .  the prey  (and  associated  The  d e b r i s ) •,  t h e n s p a t o u t b o t h p r e y and d e b r i s w e l l a b o v e t h e s u b s t r a t e .  The  stickleback  then  attempted  to  seize  the prey before i t  reached the s u b s t r a t e .  L i m n e t i c s appeared to succeed wrenching seemed  l e s s o f t e n then  benthics  t h e i n t e n d e d p r e y l o o s e f r o m t h e s u b s t r a t e . They  in also  t o l o s e more p r e y t h a n t h e b e n t h i c s a t t h e " s p i t t i n g o u t "  s t a g e . A p p a r e n t l y , b e n t h i c s were b e t t e r a t s u b d u i n g "mouthing", thus r e d u c i n g the p r o b a b i l i t y d u r i n g d u r i n g the s o r t i n g  of t h e  prey  prey  through escaping  process.  Pi scuss ion  These better  results  confirm  than l i m n e t i c s  substrate. evidence female  However,  f o r another limnetics  the  prediction  i n d e a l i n g w i t h l a r g e prey they  also  (unexpected)  differ  in their  provide  Given  male  limnetics  on  to feed  a  least  on  natural  preliminary  mature  benthic foraging  male  readily  and  performance.  benthic  a r e more l i k e b e n t h i c s i n t h i s  t h e o p p o r t u n i t y male l i m n e t i c s  attack, benthic prey.  at  conclusion:  F e m a l e l i m n e t i c s show no i n c l i n a t i o n whereas  that benthics are  search  prey, regard.  f o r , and  28  This the  observation probably  trophic  adaptation  corresponds Enos  to  Lake  of  the  limnetics  broods  This a c t i v i t y  of  lake,  in  t o favour  school  zone  territories,  type,  both  ranging in  from  such  (Wootton  light.  Enos  Lake  anything  Ridgway,  in  prep.)  were  during  conducted  s t i c k l e b a c k s t h a t I u s e d were condition.  prey  amphipods,  on  be  to deal  times  zooplankton.  size  only  limnetic  invertebrates) within  benthic  their  foraging  summer  and  t h e male  mature,  but  also  females, their  which  was  When  presented lunged  not  to  both  most e v i d e n t  forward  in  were  responses  This  males t y p i c a l l y  of  c h a l l e n g e and  moves  and  the  Unlike  experiment.  sticklebacks  small  size  not  invertebrates.  male  that  a g g r e s s i v e , m a l e s were p u g n a c i o u s i n  maximum  forage  (including  experiments  and  might  f o r , a n d an a b i l i t y  limnetics, will  prey  fish  t o 100 cm i n d e p t h  environment  Breeding  B o t h my maximum  other  fringe  (Ridgway, i n p r e p . ) , and a t o t h e r  territories.  reproductive  and  between r e p r o d u c t i v e a n d t r o p h i c h a b i t s may  i n another  1976,  20  an  a proclivity  t o breed  attack  nests  In  In c o n t r a s t , female l i m n e t i c s o n l y enter t h e  including  frequently  that  t h e extreme l i t t o r a l  i n t h e l i m n e t i c z o n e , where t h e y  a l s o be v i e w e d  limnetics  reproductive behaviour.  defend  Life  This cross-over  any  female  e g g s a n d f r y f r o m e a r l y May t o m i d J u l y .  water  with, benthic prey. littoral  and  a l l takes place along  (Ridgway, i n p r e p . ) . expected  of. male  differences in their  male  successive  r e f l e c t s a genuine d i f f e r e n c e i n  with  i n the large  immediately  29  to grapple tended  w i t h the prey.  to huddle  enough t i m e  they  however, female the  bottoms  of  I n t h e same s i t u a t i o n  i n t h e back of t h e i r  female  limnetics  a q u a r i a , t h o u g h when g i v e n  d i d e v e n t u a l l y eat the amphipods. In l i m n e t i c s adapt r e a d i l y aquaria.  Tubifex  contrast,  to consuming T u b i f e x o f f  are  passive  well-armoured  prey,  amphipods,  like  other  vigorously  when  mouthed by a p r e d a t o r . T h u s , t h e d i f f e r e n c e i n  t h e t e n d e n c y of male and robust  benthic  prey  female  limnetics  i s so, then  the  aggressiveness  not  of  performance  forage may  on  the  be p a r t l y  inherent aggressiveness.  males  (Wootton,  This suggestion  sticklebacks.  differ  struggle  a If  t h e h i g h l e v e l s of t e s t o s t e r o n e r e s p o n s i b l e f o r  foraging behaviour. benthic  to  o f f e r e d i n t h i s experiment  c o n s e q u e n c e of d i f f e r e n c e s i n t h e i r this  arthropods,  whereas  The  significantly  r e p r o d u c t i v e hormones on  female  any  (Table 3). Apparently,  also influence  does n o t , however, a p p l y  male and in  1976)  benthics I tested did  aspect no  to  of  their  r e p o r t s of s u c h  f o r a g i n g behaviour  of  fish  foraging effects  exist  of  i n the  literature.  Not important Beukema  h o w e v e r , h u n g e r h a s ' b e e n shown t o be  physiological  determinant  (1968)  threespine higher  surprisingly,  found  of  of t h e p r e y  also  hunger d e c r e a s e d  took  longer  (Kislalioglu  intensively  they encountered  were somewhat s a t i a t e d . F i f t e e n - s p i n e d spinachia)  performance.  t h a t i n c r e a s e d l e v e l s of h u n g e r  s t i c k l e b a c k s t o s e a r c h more  proportions  foraging  to and  Gibson,  their  motivated  and  attack  t h a n when  sticklebacks  swallow  an  they  (Spinachia  prey  1976a,b).  as  their  Similar  30  observations review,  have  been  see R i n g l e r ,  In  both  standardized  my the  benthic of  on  hunger  food  experiments  benthic  (for a  d e p r i v a t i o n i n t e r v a l at approximately  24  i s unlikely results.  to  Larson  with  have  benthic  been  (1972)  by  (1976) c o m p a r e d t h e  sticklebacks  a  confounding  found t h a t the  (1968)  Beukema's  f o r both effects criteria)  deprivation.  amount  of  time  limnetic  from Paxton Lake spent s e a r c h i n g  substrate  under c o n t r o l l e d  l a b o r a t o r y . He f o u n d t h a t b e n t h i c s litter  of f i s h  I  a f t e r 24 h o u r s of f o o d  a leaf l i t t e r  study,  species  prey  (as c h a r a c t e r i z e d  Larson  leaf  other  and l i m n e t i c s t i c k l e b a c k s from Paxton Lake  stabilized  on  my  on  1979).  feeding  h o u r s . Thus, hunger influence  made  f o r prey  conditions  i n the  s p e n t more t i m e s e a r c h i n g t h e  than l i m n e t i c s . U n f o r t u n a t e l y ,  L a r s o n made no r e f e r e n c e  and  to  the  as elsewhere i n h i s  sex  of  the  fish  he  tested.  •Differences foraging  i n experimental  experiment  and  design  However, my r e s u l t s g i v e  Enos  limnetics  differ  t o search  f o r and  inclination limnetics  that  substrate  with  with  many  as  I  tested  no  appreciably attack  indication  that  from b e n t h i c s  benthic  prey.  male  in their The  male  appeared t o i n v e s t i g a t e the d e t r i t u s  t h e same i n t e n s i t y , feeding  benthic  my e x p e r i m e n t make d i r e c t c o m p a r i s o n s  impossible. Lake  between L a r s o n ' s  strikes,  and p u n c t u a t e d  their  as d i d t h e b e n t h i c s .  search  The m a j o r  31  difference their  I f o u n d b e t w e e n m a l e l i m n e t i c s and  rate  than the  of  success at  foraging.  The  benthics  was  in  b e n t h i c s were more a d e p t  m a l e l i m n e t i c s a t m a n i p u l a t i n g and  subduing large  prey.  A p p a r e n t l y , t h e i r s u p e r i o r i t y i n t h i s r e g a r d r e s u l t s from h a v i n g more p o w e r f u l , and noteworthy for  that  p e r h a p s l a r g e r , mouths t h a n l i m n e t i c s . I t  the  p o s i t i v e c o r r e l a t i o n between " s u c c e s s  test sticklebacks  and  statistical  significance  for  ( T a b l e 5)  benthics  male  limnetics  (11  were  the  approached the closely mean  for  the  out  My benthic (1980)  rather  prey  than  extensive  Marion  trout  in  The  of  By  may  have  size  more  "realistic" under  aquaria.  foraging  performance  of  the  foraging  strategies  trout  and  Kokanee.  Hyatt  small  also  f o r a g e on  compared  to  l a k e . Kokanee are  t h e i r r a n g e , but of  nerka)  primarily  in Marion  p r e y ; however,  l a r g e , armoured, e p i b e n t h i c  prey,  and  compared  t h e i r d i e t . Rainbow  benthic  of  Hyatt's  Kokanee ( O n c o r h y n c h u s  coastal  I  natural  study  a  of  length)  be  and  than  amphipods  prey  prevail  the  mm  can  rainbow  tend to s e l e c t  limits  on  t h r o u g h o u t most of  Lake  47  experiment  laboratory  macrobenthos forms a major p o r t i o n in  or  sticklebacks  (Salmo g a i r d n e r i )  planktivorous  this  that  in barren  rainbow t r o u t  Lake,  in  size  of  Marion  46  than f o r b e n t h i c s .  observations  limnetic  approached  size distribution  remarkable.  upper  capabilities  in  more  prey  male l i m n e t i c s  lengths  rate"  c l o s e l y f o r male l i m n e t i c s  15 were e i t h e r  a l l the  on  standard  circumscribed  "realistic"  limited and  of  dominant  limits  conditions,  more  . The  makes t h i s o b s e r v a t i o n that  their  is  Lake trout  rainbow whereas  32  Kokanee  take  prey  t h a t a r e s m a l l or r e l a t i v e l y  experimental t r i a l s , did  Hyatt  found  unarmoured. In  t h a t a l t h o u g h t h e i r mouth  n o t a p p e a r t o be l i m i t i n g , K o k a n e e e x p e r i e n c e d  success  in  identical  manipulating  prey  large  rainbow  armoured  trout  fared  prey;  much  very  large  but  relatively  Kokanee were b e t t e r t h a n of  benthic  sifting  prey.  through  s m a l l , hidden trout  tended  Other  unarmoured.  rainbow  better.  and c o n s e q u e n t l y  than t h e rainbow  o b s e r v a t i o n s support  fish  t h a t f o r a g e on l a r g e r ,  the  feeding  success  under a l a y e r of s i l t .  drastically  prey.  on a b a r e  Concealment  had  Ivlev  adaptations (1961)  than  compared  s u b s t r a t e or concealed little  effect  the f e e d i n g success of other  (1972)  that  Dolly  Varden  vein,  trout  on  the  Schutz  species( i . e . and N o r t h c o t e  ( S a l v e l i n u s malma) were more  e f f e c t i v e a t f i n d i n g and c o n s u m i n g c o n c e a l e d  used a grab and s o r t  the  ( i . e . c a r p , 'Cyprinus c a r p i o ) , but  r u t i l u s ) . In a s i m i l a r  cutthroat  o b t a i n e d more  t h a t f i s h t h a t f e e d on  for different  roach, R u t i l u s  than  types  prey.  t h e view  exposed  exposed  predators  reduced  found  some  that  o f v a r i o u s b e n t h i c f o r a g e r s on c h i r o n o m i d  l a r v a e t h a t were e i t h e r  some  found  t r o u t . T h i s was b e c a u s e  t o r e a c t o n l y t o exposed  of  i f the prey  Kokanee e m p l o y e d a " g r a b a n d s o r t " method o f  small concealed prey a r e s e l e c t e d  success  Kokanee,  Hyatt also  trout at e x p l o i t i n g  bottom sediments,  prey  little  whereas, w i t h  however, d i d b e t t e r a t c a p t u r i n g and s w a l l o w i n g prey were  sizes  chironomid  larvae  (Salmo c l a r k i i ) . A g a i n , t h e D o l l y  Vardens  t e c h n i q u e and t h e c u t t h r o a t s d i d n o t .  33  The  method u s e d  benthics and  sort  in  my  t o s e c u r e p r e y by b o t h m a l e  benthic f o r a g i n g experiment  t e c h n i q u e s o f D o l l y V a r d e n s and  sticklebacks  were  orienting  on c l e a r l y  of c o n c e a l e d o r i n c o n s p i c u o u s larvae  benthic  a n d o s t r a c o d s ) do o c c u r  limnetic  Enos L a k e  Although  Kokanee; visible prey,  (Bentzen,  little  small, future  examination.  benthic  however  the  prey. A v a r i e t y  (i.e.  chironomid  unpublished  data).  e p i b e n t h i c prey,  a b o u t how t h e two f o r m s m e a s u r e up  concealed  the grab  indicate that benthics are superior  t o l i m n e t i c s when f o r a g i n g on c o n s p i c u o u s , say  resembled  and  i n t h e d i e t s o f b o t h b e n t h i c and  sticklebacks  my r e s u l t s c l e a r l y  limnetics  prey.  This  when  feeding  they on  i s a q u e s t i o n worthy of  34  FORAGING ON PLANKTON  Introduct ion  I p u t s t i c k l e b a c k s i n t o mesh e n c l o s u r e s water  c o l u m n o f Enos L a k e t o t e s t  the hypothesis  are b e t t e r adapted t o p l a n k t i v o r y than i s the counterpart  to the benthic  previously. Again,  I attempted  two  forms  resembling  The small  of  unpub. d a t a ) . R a t h e r in  the  sticklebacks  To  turbid,  than attempt  enclosures  this  time  is  overcome  any  initial  sticklebacks  s e v e r a l days.  I then  enclosures  to  were  that  in  a  situation  forage.  characterized  to recreate  these  by  very  (Bentzen, conditions  s m a l l "chunks" of t h e l i m n e t i c that  were  left  designed  to  keep  t o move i n and o u t .  distress  sampled p l a n k t o n verify  described  h i g h l y s t a i n e d water  i n but a l l o w zooplankton  confined the  the  but  l a b o r a t o r y , I surrounded  environment w i t h net  experiment  f o r a g i n g experiment  zone o f Enos L a k e and  limnetics  benthics. This  t h a t i n which l i m n e t i c s normally  zooplankton  that  i n the  t o compare t h e p e r f o r m a n c e o f t h e  sticklebacks,  limnetic  suspended  in  from b e i n g handled or the  enclosures  for  both w i t h i n and o u t s i d e of  t h e r e were z o o p l a n k t o n  i n the  e n c l o s u r e s , a n d t o s e e how t h e c o n c e n t r a t i o n s o f p l a n k t o n w i t h i n t h e e n c l o s u r e s compared t o lake.  Finally,  concentrations  in  I examined t h e stomach c o n t e n t s  in the enclosures, to obtain r e l a t i v e  the  surrounding  of t h e f i s h  i n d i c e s of the  extent  held to  35  w h i c h i n d i v i d u a l s o f t h e two f o r m s f e d on p l a n k t o n .  Methods  Three  enclosures  (Marion T e x t i l e s  were  c o n s t r u c t e d o f woven n y l o n n e t t i n g  I n c . , 3 x 6 mm  o v a l m e s h ) . The  mesh  was  dyed  light  green t o reduce i t s v i s u a l c o n t r a s t w i t h surrounding  water.  The e n c l o s u r e s were c y l i n d r i c a l  b a g s , open a t t h e t o p and  c l o s e d a t t h e b o t t o m . They were two m e t r e s d e e p diameter. bottom  Hoops o f one i n c h f l e x i b l e  and  t o p of each e n c l o s u r e  of t h e e n c l o s u r e s were w e i g h t e d and  t h e t o p s were b u o y e d w i t h  were a n c h o r e d  fish  of  sexes  enclosures sixth  in  each  for 5 f u l l  day,  s h a p e . The  floats.  4 m of water  cm  the  days.  bottoms  200 g o f l e a d  The  enclosures  night-lighting.  sticklebacks  revealed  no  and  effort  representatives  The s t i c k l e b a c k s were l e f t During  the  mid-afternoon  were removed i n r a p i d  and p r e s e r v e d sign  of  in  (Fig..6).  by t r a p p i n g and  of s i z e s  group.  f r o m t h e bags and k i l l e d inspection  80  e a c h f o r m were a d d e d t o e a c h e n c l o s u r e . An  was made t o i n c l u d e a v a r i e t y both  to provide  with approximately  S t i c k l e b a c k s were c o l l e c t e d Ten  and  PVC p i p e were sewn i n t o t h e  plastic  i n approximately  lake  in  10%  vomiting  of  i n the of  the  succession  formalin.  Close  during the k i l l i n g  process.  Immediately enclosure,  a  before plankton  t h e s t i c k l e b a c k s were removed f r o m s a m p l e was t a k e n  from the middle  each  of each  36  Figure  6.  experiment.  Drawing  of  enclosure  used  in  plankton  feeding  37  i  38  enclosure with a Schindler - Patalas plankton 64 m i c r o n the  s c r e e n . Three p l a n k t o n  trap fitted  s a m p l e s were  l a k e i n t h e immediate v i c i n i t y  also  with a  taken  of the e n c l o s u r e s a t a  from depth  of one m e t r e .  The  stomach c o n t e n t s  of each  following categories recorded: copepodites  (mostly  (bosminids  and  Measurements  fish  nauplii  and  standard  length  that  fish  size  c o r r e l a t e d with the r e l a t i v e experiment.  Gillraker  and  or  estimates  of g i l l r a k e r  T h i s was done t o t e s t  gillraker  performance of  spacing  ( a l l Keratella).  was  spacing the  estimated  to  The  zooplankton  samples from both  c a t e g o r i e s (except  tendency  to  impractical. of  of  subsamples  clump  together,  made  Instead, since r o t i f e r s  rotifer from  abundance each  above.  Total  of  i n clumps.  total  the nearest fitted  to  a  counts  of r o t i f e r s  were n o t an i m p o r t a n t  on  the o r i g i n a l  combined  of  numbers,-and a  counts  (see r e s u l t s ) ,  based  e s t i m a t e , s i n c e I d i d not attempt stuck together  this  t h e l a k e and e n c l o s u r e s  r o t i f e r s ) were made. L a r g e  s t i c k l e b a c k s i n the enclosures  index  in  be  microscope.  were s o r t e d i n t o t h e c a t e g o r i e s l i s t e d all  the  might  fish  h u n d r e d t h o f a m i l l i m e t r e w i t h an o c u l a r m i c r o m e t e r Wild dissecting  and t h e  of copepods, copepods and  rotifers  s p a c i n g were a l s o made f o r e a c h f i s h . possibility  counted  c y c l o p o i d , some h a r p a c t i c o i d ) , c l a d o c e r a n s  chydorids)  of  were  prey  I used a rough counts  of  s a m p l e s . T h i s was a low  t o count a l l the r o t i f e r s  that  39  The first  t i m i n g of t h i s  tried  i t in July  temperature of  (220  experiment 1980.  C) and  unusually  murky  of f i s h prey.  1981.  was  This  breeding (100 in  season.  water  a  T h i s time  organic  trial  I tried a third about  was  l a k e was  used i n t h i s  identifiable  of h i g h s u r f a c e  month  at  before  the water  numbers. The  results  death  October  but  it  matter  and  poor i n  turnover.  the the  end  was  zooplankton trial  but  of  few  March the  moderate  were  are  was  The  b e g i n n i n g of  temperature  of t h i s  water  i n the  contained detritus,  time  I  cooler,  undergoing  C ) , m a l e s were not a g g r e s s i v e and  reasonable  important.  i n t h e e n c l o s u r e s . I n mid  suspended  z o o p l a n k t o n . A p p a r e n t l y the stomachs  be  a g g r e s s i v e males r e s u l t e d  a g a i n . The  with  to  A combination  most of t h e s m a l l l i m n e t i c s  I ran the experiment  proved  present  presented  below.  Results  The  prey:  Zooplankton and  were p r e s e n t  in relatively  o u t s i d e of t h e e n c l o s u r e s . The  enclosure  (Table 6A):  low a b u n d a n c e b o t h  p a t t e r n was  r o t i f e r s were c l e a r l y  w i t h c o n c e n t r a t i o n s of a t l e a s t  60 p e r  (x  copepodites  = 11.5  per  cladocerans was  the  1 ) , c o p e p o d s and  (x = 2.7  reverse  of  per  1,  the  ranked  size  t h e same i n e a c h  n u m e r i c a l l y dominant followed  1 ) . T h i s o r d e r of  inside  (x = 4.4  by  nauplii  per  1),  and  numerical  abundance  o r d e r of the  individual  40  T a b l e 6.  Zooplankton concentrations c a l c u l a t e d from samples.  A.  (plankton  /  litre)  ENCLOSURES  mean f o r three enclosures  #1  #2  #3  Rot i f e r s  >6.0  >60  >60  >60  Naupli i  10.7  12.2  11.7  11.5  Copepod(i t e ) s  3.8  4.8  4.5  4.4  Cladocerans  2.7  2.9  2.4  2.7  B.  LAKE  mean f o r three samples  #1  #2  #3  Rot i f e r s  >60  >60  >60  >60  Naupli i  10.8  29. 1  14.6  18.2  Copepod(ite)s  5.4  Cladocerans  0.2  13.9 0.2  5.7  8.3  0.0  •0.1  p l a n k t e r s t h a t made up e a c h o f t h e f o u r c a t e g o r i e s average cladocerans copepod(ite)s,  The  three  were  nauplii  the  largest  and f i n a l l y ,  plankton  plankton,  (Table  7 ) . On  followed  by  rotifers.  samples taken  directly  from t h e l a k e i n  41  Table  7.  Approximate s i z e s ( i n micrograms) of p l a n k t e r s i n l a k e and e n c l o s u r e samples.  CATEGORY  mean  standard  size  error  Cladocerans  0.5 *  0.01  Copepod(ite)s  0.3 *  0.02  Naupli i  0.2 **  Rot i f e r s  0.1 **  * . . . B a s e d on c o n v e r s i o n o f l e n g t h m e a s u r e m e n t s f o r 30 i n d i v i d u a l s t o w e i g h t s u s i n g r e g r e s s i o n f o r m u l a e d e v e l o p e d by C . J . W a l t e r s (unpublished r e s u l t s ) . * * . . E s t i m a t e from C . J . W a l t e r s ( u n p u b l i s h e d r e s u l t s ) .  the v i c i n i t y samples  of  the  enclosures  i n some r e s p e c t s  (Table  differed  6A,B). A g a i n ,  most a b u n d a n t , f o l l o w e d r a t h e r d i s t a n t l y and  copepodites,  same  as  in  were more  the  lake  Concentrations lower  in  the  samples  open  lake  than  in  the  enclosures  than  of the e n c l o s u r e  of  these  must have been a t t r a c t e d  were t h e copepods  o f a b u n d a n c e was t h e and c o p e p o d ( i t e ) s than  i n the  were l e s s a b u n d a n t i n  the  enclosure  copepods  may  samples. have  been  i n t h e open l a k e a s a r e s u l t of by t h e s e p l a n k t e r s , o r g r a z i n g  e x e r t e d by t h e s t i c k l e b a c k s  combination  enclosure  nauplii,  samples  cladocerans  of immature and mature  e i t h e r avoidance pressure  in  the  rotifers  t h e e n c l o s u r e s ; however, n a u p l i i  samples. Conversely,  open  by  and c l a d o c e r a n s . The o r d e r  abundant  enclosure  from  i n the enclosures, or  factors. Cladocerans, to the enclosures.  on t h e o t h e r  some hand,  42  The  predators:  In t h e e n c l o s u r e s l i m n e t i c s extent  than  on  zooplankton  b e n t h i c s . The r e s u l t s  same o v e r a l l , p a t t e r n  T a b l e 8.  fed  greater  f r o m e a c h e n c l o s u r e show t h e  ( T a b l e -8). Of t h e 30 f i s h  -- -BENTHICS— N X S. E.  ENC.#  of each form t h a t  — c. 2b  0. 23  47.7 47.6 21.4 37.9  8.9 12.6 5.0 .5.5  0. 37 0. 1 7 0. 25 0. 1 5  9 8 10 27  25.3 18.4 23. 9 22.9  8.4 6.3 6.7 4.0  9 8 10 27  42.7 30.5 34.9 36.2  16.5 13.2 13.1 8. 1  9 8 10 27  16.3 2.4 0.9 6.5  8.8 1 .3 0.5 3.2  9 8 10 27  1 32. 0 99.4 81.1 103.5  5 6 4 15  1 .20 0.16 0.75 0.67  1 2 3 combined  5 6 4 15  1 .20 1.17 0.75 1 .07  1 2 3 combined  5 6 4 15  0.0 0.0 0.0 0.0  1 ROTIFERS 2 3 combined  5 6 4 15  0.0 0.16 0.0 0.0  —— -— -— — —— -— -— ——  1 2 3 combined  5 6 4 15  2.4 1 .5 1 .5 1 .8  0. 75 0. 34 0. 33 0. 30  COPEPOD(ITE)S  -LIMNETICS N— X S.E. 9 8 10 27  1 2 3 combined  CLADOCERANS  ALL PREY  a  Mean number o f p r e y i n s t o m a c h s o f b e n t h i c s a n d l i m n e t i c s t h a t c o n t a i n e d p r e y , and o n e - t a i l e d Mann W h i t n e y U t e s t c o m p a r i s o n s o f means.  PREY TYPE  NAUPLII  to  0. 58  were t e s t e d ,  27 o f t h e l i m n e t i c s a n d 15  zooplankton  in  their  stomachs.  of  the  30.7 26.2 22.5 15.3  P= <0. 05  ——  <0. 01  —  0. 002 <0. 001 <0. 01  — —— —— —— — — -— <0. 01 <0. 001 0. 001  benthics,  —  had  The 27 l i m n e t i c s a v e r a g e d 103  p r e y p e r s t o m a c h ; w h e r e a s , t h e 15 b e n t h i c s a v e r a g e d 1.8 p r e y p e r  43  s t o m a c h . C l a d o c e r a n s were n u m e r i c a l l y t h e most a b u n d a n t p r e y the  stomachs  of  limnetics  (x = 3 6 . 2 ) , c o p e p o d s and rotifers  (x = 6 . 5 ) .  (x = 3 7 . 9 ) ,  copepodites  The  few  followed  (x = 2 2 . 9 ) ,  in  by  nauplii  and  finally  z o o p l a n k t o n consumed by b e n t h i c s  were m o s t l y c l a d o c e r a n s , c o p e p o d i t e s and m a t u r e c o p e p o d s .  Body  size  performance  of  zooplankton tested The  proved  to  limnetics:  than t h e i r  fell  into  in  small  two  important fish  factor  tended  counterparts.  distinct  age)  and  groups  to  The  in  the  eat  more  limnetics  on t h i s b a s i s  were a l l g r e a t e r t h a n 43 mm  l i m n e t i c s were a l l l e s s  years  of  137.5  age).  The  prey. In a d d i t i o n , when  they  the " l a r g e male"  The body within  t h a n 36 mm  (1-2  averaged  t h e l a r g e male l i m n e t i c s a v e r a g e d  only  to  group.  limnetic  w i t h food i n t h e i r the  stomachs 15  not s i g n i f i c a n t  greater  The  no c o r r e l a t i o n  range  between  mean s i z e o f t h e  (45mm) was  fish  even  of  c o u n t e r p a r t s (Table 9 ) ; however,  t h e r e was  feeding performance.  d i f f e r e n c e was  group  the t h r e e l i m n e t i c s t h a t d i d not c o n t a i n  the b e n t h i c group  of  The  were r e c o v e r e d f r o m t h e e n c l o s u r e s b e l o n g e d  s i z e s than t h e i r  size  This  of t h e s m a l l l i m n e t i c s  30 b e n t h i c s t e s t e d s p a n n e d an  and  and  in length.  in length.  I  (Table 9).  y e a r o l d m a l e s and m a t u r e f e m a l e s  stomachs  p r e y . In c o n t r a s t ,  food  mean  small  larger  c o n s i s t e d of immature one  size  an  l a r g e l i m n e t i c s were a l l m a t u r e m a l e s ( p r o b a b l y 2 y e a r s  older  39.5  be  without  slightly food  (F=1.0 p = 0.33,  15 b e n t h i c s  s m a l l e r than  (48mm), d.f.=  body  but 1,28).  the this  44  Table  9.  C o m p a r i s o n o f t h e numbers o f p r e y f o u n d i n s t o m a c h s o f s m a l l l i m n e t i c s , l a r g e l i m n e t i c s and b e n t h i c s t h a t c o n t a i n e d p r e y . Numbers i n p a r e n t h e s e s d e n o t e t h e t o t a l number o f f i s h t e s t e d i n e a c h g r o u p .  Standard GROUP DESCRIPTION  N  min  length  Number o f p r e y conta ined in stomach  (mm)  max  mean  mean  S.E,  Small 16 (16)  26  35  30  147.5  18.2  limnetics  11 (14)  44  50  45  39.5  9.2  Benthics (both sexes)  15 (30)  34  61  45  1.8  limnetics Large  (male)  My  estimates  were l i n e a r l y Therefore, reflect  o f mean g i l l r a k e r  correlated with their  the  standard  of f i s h w i t h  more  gillraker  s p a c i n g . S i z e and g i l l r a k e r  prey  small  narrow  consume  and  lengths  with  prey  than  log  from  (r=.93l).  spacing  fish  with  s p a c i n g showed  p r e y number. F o r l i m n e t i c s ,  estimated g i l l r a k e r categories  planktonic  gillraker  a  standard  s p a c e were i n v e r s e l y c o r r e l a t e d  (except  rotifers)  with  the  consumed. T h e s e c o r r e l a t i o n c o e f f i c i e n t s were data  limnetics  s u p e r i o r p e r f o r m a n c e of t h e s m a l l l i m n e t i c s  the a b i l i t y  relationship  s p a c e f o r t h e 30  number  transformed.  Correlation  coefficients  were  to  wider similar lengths  for a l l of prey  significant  t h e t h r e e e n c l o s u r e s were p o o l e d , and p r e y  may  number  highest  when was for  45  nauplii, (Table  followed  by  copepod(ite)s,  and  finally  10).  F i g u r e 7 shows t h e r e g r e s s i o n s f o r l i m n e t i c s number  on  standard  length  for  c l a d o c e r a n s . The  s l o p e of t h e  than  either  that  of  nauplii  ,  of  regression for nauplii  of  the  other the  two  significant  for  comparison  (p  = 0 . 0 1 ) , and  approaches s t a t i s t i c a l  the  copepod(ite)  significantly  Similar between the  - cladocerans -  cladoceran  is  and  steeper  -  This  copepod(ite) significance  (p = 0 . 0 7 0 ) . The  regressions  do n o t  slope differ  ( p = 0.485).  relationships  prey  number  and  apply  to  estimated  the  weaker  gillraker  association  space.  i n v e r s e r e l a t i o n s h i p between t h e number of p r e y  body l e n g t h ( o r g i l l r a k e r than  nauplii  comparison  prey  regressions.  is  for the n a u p l i i  log  copepod(ite)s  difference  of  cladocerans  Therefore,  consumed  s p a c e ) i s more p r o n o u n c e d f o r  f o r t h e somewhat l a r g e r c o p e p o d ( i t e ) s and  and  nauplii  cladocerans.  Discussion  The  results  of  the  r e s u l t s of t h e b e n t h i c points  to  the  differentiated experiment  experiment  foraging experiment.  conclusion into  enclosure  three  that trophic  Enos  The  Lake  groups.  h o w e v e r , t h e r a n k i n g i n t e r m s of  complement evidence  again  sticklebacks In  the  are  enclosure  foraging success  t h e t h r e e g r o u p s i s t h e r e v e r s e of t h a t e n c o u n t e r e d  the  with  of  large  46  Figure 7. R e l a t i o n s h i p between number of p l a n k t o n i c p r e y i n the stomachs of l i m n e t i c s , and the s t a n d a r d l e n g t h of f i s h that consumed them. L i n e s r e p r e s e n t p r e d i c t i v e r e g r e s s i o n s (N = 27 fish). C l a d o c e r a n s . . . . (A) Copepod(ite)s..(B) Naupl i i . . . . (C)  100.x  •  STANDARD LENGTH OF  TEST FISH  CMM)  47  Table  10.  C o r r e l a t i o n s b e t w e e n l o g number of p r e y p e r s t o m a c h , and s t a n d a r d l e n g t h and e s t i m a t e d g i l l r a k e r s p a c e f o r l i m n e t i c s . P r o b a b i l i t i e s < 0.05 a r e c o n s i d e r e d significant. versus standard length  PREY TYPE  versus estimated g i l l r a k e r space  N  r  Rot i f e r s  13  0..377  >0. . 1  0..452  >0. . 1  Naupli i  23  0..805  <0. .0001  0.. 729  <0. .0001  Copepod(ite)s  26  0,.453  0..02  0.. 323  >0. . 1  Cladocerans  27  0..555  0,.0027  0,.59  0..0012  All  27  0,.697  0..653  0..0002  Prey  epibenthic  prey.  The  <0, .0001  in  benthics  their  averaged  1.8  79%  in  this  was  39.5  no  food  prey per  in  fish.  g r o u p had  prey per  fish.  the  enclosures. s t o m a c h s , and  M a t u r e male l i m n e t i c s  food Small  in their  fish.  female and  and  Half  the  the other  half  fared  better;  the  average  l i m n e t i c s were t h e most s u c c e s s f u l averaged  137.5  prey  T h i s g r o u p s p a n n e d a r a n g e of s i z e s t y p i c a l  of m a t u r e  immature male l i m n e t i c s . The  benthics,  m a t u r e male l i m n e t i c s , were e i t h e r  stomachs  of  empty o r a l m o s t  empty; i n  contrast,  t h e s t o m a c h s o f s m a l l l i m n e t i c s were a l l e i t h e r  or  full.  almost  on  of  s t o m a c h s , and  group i n f o r a g i n g i n the e n c l o s u r e s ; they per  P  b e n t h i c s were a l l f a i l u r e s a t f e e d i n g  the s m a l l p l a n k t o n a v a i l a b l e had  r  P  full,  48  The  r e s u l t s of  differences  in  sticklebacks  f e d on  are  (Holonov  m e c h a n i s m by  cannot  which the  to  Tash,  explained  e f f i c i e n c y by  feeding  as  1978), t h r e e s p i n e  the  situation  sticklebacks  b o t h f o r m s d e n s e swarms of  newly h a t c h e d A r t e m i a  persisted  they  i n consuming the  The  general  compared  to  not  switch  n a u p l i i one  superiority  benthics  corresponding  as  could  to  at a  sticklebacks  the  result  of  d e v e l o p e d by Ware (1975) and  stress  the  i m p o r t a n c e of  in  t o the  v o l u m e of  water searched. T h i s , i n  acuity  of  factors, the  1976,  visual field  the  predator,  including  s i z e of  lighting  swimming s p e e d and  light  p r e y and  and  Hall  the  s i z e of  is directly as  well  t h e i r inherent  1974,  assumption  given  amount  the  C o n f e r and  of  time turn,  to  the  for the is is a The  visual  a v a r i e t y of e x t r i n s i c  quality,  contrast  (Lythgoe  that  visual field.  related as  i n t e n s i t y and  colouration  Werner and  a  the  Eggers  visual acuity  consumed  the  but  s u p e r i o r i t y i n v i s u a l a c u i t y . A n a l y t i c a l models planktivory  partially  prey  e x t e n t of  feeding  limnetics  number  of  in  of  B o t h m o d e l s o p e r a t e on  function  nauplii  time.  planktivores.  proportional  strictly  a  indirectly  of  warrants  are  filter  planktivores  be  from  of  d y n a m i c s of  (1977)  did  of  fish  switching  1 9 7 6 ) . When I o f f e r e d  laboratory,  by  planktivorous  (Wootton,  the  feeders  filter  be  d i f f e r e n t groups  p l a n k t o n . A l t h o u g h some  feeding  and  particulate  the  the  experiment  a b l e to maximize t h e i r feeding  particulate  of  this  1966, Blades  with  water the  clarity, background  V i n y a r d and 1975).  O'Brien  Thus,  the  49  murky  water  small,  relatively  and  the  acuity  and  somewhat m o n o c h r o m a t i c l i g h t transparent  l i m n e t i c zone of Enos in  limnetics.  perception  of  conditions  that  benthics.  of  visual  prey  much of t h e  sensitivity  relatively  placed  for  visual  adaptation  under  the  benthic  to  of  benthics  the  twilight  substrate  in  over v i s u a l a c u i t y  l a r g e s i z e and a  the  high  may  be  in  forward  level  w e l l d e v e l o p e d b i n o c u l a r p e r c e p t i o n . The  eyes  the  enclosures  select  the eyes i n l i m n e t i c s suggest  a c u i t y and  laterally  large  of b o t h my  might  contrast,  visual the  typical  Lake,  p r e v a i l along  Certainly  positioning  In  relatively  Enos L a k e m i g h t f a v o u r  visual  prey  combined w i t h  inferior  of more  in  this  regard.  Since  visual  acuity generally increases with size  (Blaxter,  1975;  enclosures  of t h e  limnetics  c a n n o t be  likely  small  explanation  spacing  among  retaining greater  prey,  Hyatt  on  field  and  Hall,  superior performance i n  involves  changes If  Both  with  gillrakers  spacing  small prey.  gillraker  1974)  For  i n such  the  In  t h a t the  energetic cases,  act  critical  m o r p h o l o g y and  authors  size  of g i l l r a k e r s  large  male  in  gillraker  like  a sieve in  should  favour  reviews  of  function, Kliewer 'sieve' analogy  however, based t h e i r  l a b o r a t o r y s t u d i e s where f i s h had sizes.  to  the  i n t e r m s of v i s u a l a c u i t y . A more  (1979) b o t h c o n c l u d e d  r a n g e of p r e y and  explained  then f i n e r  oversimplification.  the  l i m n e t i c s compared  limnetics.  r e t e n t i o n of  literature and  Nakamura, 1968)  in fishes  reasons  the c h o i c e (Eggers,  particulate  a the  (1970) is  an  views  on  of a wide  1977;  planktivores  Werner will  50  select  the  largest  interpretation consumption on t h e  of  interpret.  For  correlation graded  to  retain.  In  example,  food p e l l e t s  the  limited  s m a l l prey  (1979)  plankton  s t i c k l e b a c k s had  range  of  be e a s i e r t o a  close  the minimum.size  salmon  were  the  f u n c t i o n might supersede prey  the  i n any  This  the  were  able  only  food  t h e c h o i c e of a t t e m p t i n g  as a d o m i n a n t f a c t o r g o v e r n i n g  Body w i d t h  prey  to  o r n o t e a t i n g a t a l l . Under s u c h c o n d i t i o n s , t h e  m e c h a n i c s of g i l l r a k e r  smallest  of  selection  found  s p a c i n g and  any  size  t h e r e s u l t s may  Wankowski  small  Thus,  basis  when  that juvenile A t l a n t i c  enclosures thus  the  However,  between mean g i l l r a k e r  a v a i l a b l e and eat  is  on  i s c o n f o u n d e d by  predator.  sizes  available.  function  experiments  the  prey  of  gillraker  i n such  part  available  of  zooplankton  prey  consumed  i n t h i s prey category  corresponds  closely  size  consumption.  was  selectivity  Nauplii  were  numbers i n t h e e n c l o s u r e s . approximately  t o the g i l l r a k e r  0.1-0.15  mm.  space I e s t i m a t e d f o r  t h e s m a l l e s t g r o u p of l i m n e t i c s t e s t e d ( F i g . 9 ) . S u c h s m a l l p r e y might o f f e r a g r e a t e r c h a l l e n g e t o t h e ' r e t e n t i o n a b i l i t y gillraker  apparatus  than  the  larger  of  the  copepod(ite)s  and  c l a d o c e r a n s t h a t were a l s o consumed i n r e l a t i v e l y Correlation  coefficients  number  estimated  than  and  for  the  gillraker  f o r e i t h e r c o p e p o d ( i t e ) s or  relationship  s p a c e was  between  s p a c e were h i g h e r cladocerans  c o r r e l a t i o n between t h e number o f n a u p l i i gillraker  l a r g e numbers.  for nauplii  (Table  10).  consumed and  n o t as h i g h a s t h e c o r r e s p o n d i n g  prey  The  estimated  correlation  51  between was  nauplii  probably  i s hard  t h e body l e n g t h  (Table  10); however,  this  a c o n s e q u e n c e of measurement e r r o r . G i l l r a k e r  space  t o m e a s u r e . Thus body l e n g t h of l i m n e t i c s t i c k l e b a c k s i s  probably my  and  a b e t t e r p r e d i c t o r of  attempts  at d i r e c t  Gillraker  functional gillraker  space  than  measurement.  s p a c i n g may  have been a f a c t o r i n t h e  failure  b e n t h i c s t o feed s u c c e s s f u l l y i n the e n c l o s u r e s ; however, the  gillraker  gillraker cannot  spacing  spacing  of  many  benthics  was  similar  since to  i n l a r g e male l i m n e t i c s ( F i g . 8 ) s p a c i n g  account f o r the d i s p a r i t y  of  the  alone  i n p e r f o r m a n c e between t h e  two  forms.  In h i s study  of P a x t o n  a l s o compared the a b i l i t y planktonic  prey.  fundamentally presented  from  that  superior  (1.09  these  +  0.14  with  Enos  in  this  mm)  1  attempted  those  to  higher  than  L i m n e t i c s spent l e s s time and  manipulated  the  in  Larson varying  aquaria  repeat  limnetics;  and  feeding  searching  between  Larson's my  He  found  aquaria  were  r a t e s of t h e  f o r prey  was  however,  r e p o r t e d by L a r s o n .  f e e d i n g r a t e s of l i m n e t i c s i n e x p e r i m e n t a l  consistently  differed  study.  Daphnia  with  t h e p e r f o r m a n c e of l i m n e t i c s  L a k e b e n t h i c s and than  (1976)  benthics to deal  s t i c k l e b a c k s i n 35  prey  Larson  approach, however,  adopted  t o t h a t of b e n t h i c s . I  r e s u l t s were d i f f e r e n t  captures,  l i m n e t i c s and  one  to i n d i v i d u a l  with  experiment  t h a t the  of  sticklebacks,  experimental  the  large  concentrations found  His  Lake  benthics. successive  (swallowed) the Daphnia f a s t e r  than  52  Figure 8 . The r e l a t i o n s h i p between g i l l r a k e r s p a c i n g and t h e s t a n d a r d l e n g t h of b e n t h i c s and l i m n e t i c s . Lines represent f u n c t i o n a l r e g r e s s i o n s (N = 30 b e n t h i c s , 30 l i m n e t i c s ) . Benthics....(+) Limnetics...(x)  030  _  0«E5 1  9  0-EO 1  0-15 1  d  i—i  0-10  1  ID  0.05  E5-  30 - 35.  40 - 45 . 50 - 55-  STANDARD LENGTH DF FISH  GO-  CMM)  G5-  53  most  benthics.  The  approximately  1.2  to  approximately  1.8  to  manipulation  times  length of  manipulation 2.5  seconds  8.8 to  between  for  decrease  between  benthics.  Also,  w i t h i n c r e a s e s i n body  increase with increases in  body  length  benthics.  In large  contrast Daphn i a  manipulation  to Larson's  more times  d a t a , Enos L a k e b e n t h i c s consumed  rapidly f o r both  than  limnetics;  they  approach  intervals  the  multi-second  r e s u l t s e a s i e r to e x p l a i n  Daphnia for  of  one  millimetre  sticklebacks  (Hynes,.  1950;  i n the c l e a r  confined  quarters  of  this  t y p e a r e not  either  abilities  water  p r e y , and  Finally,  l a r g e Daphnia a r e not prey  1976;  the  cladocerans.  by  to  I  find  Larson.  small  prey  1975).  of t h e g e n e r o u s l y  l i t and  aquaria  is  form.  unlikely  to  E q u a l l y , prey  of  the m a n i p u l a t i v e a b i l i t i e s  of  Daphn i a a r e  limnetic  relatively  s h o u l d be  r e p r e s e n t a t i v e of  B o s m i n a an o r d e r of m a g n i t u d e  dominant  reported  t h e r e f o r e swallowing times  in  begin  Burko,  of e i t h e r  to s t r a i n  passive  Lake,  Larson,  t y p e of s t i c k l e b a c k . Once c a u g h t ,  zooplankton  not  r e p o r t e d by L a r s o n .  those  experimental  likely  did  the  that I could  s i z e a r e not p a r t i c u l a r l y  such prey  the v i s u a l  certainly  than  Locating  challenge  however,  f o r m s were so f l e e t i n g  n o t measure them, a l t h o u g h  my  ranged  f o r l i m n e t i c s , and  seconds  tended  i n l i m n e t i c s but  times  the  minimal. potential  zone of Enos L a k e . smaller  in  size  I n Enos are  the  54  GENERAL DISCUSSION  The  phenomenon  e c o l o g i c a l l y divergent Enos  and  Paxton  p a r a l l e l s occur  of  populations  lakes,  or  distinct,  lakes scattered  throughout  1948;  Lindsey,  Svardson,  1962;  but  1979).  the  Similar  a l s o occur  in several lakes  gillraker  coexist.  low  of l a r g e b e n t h i c  t o be  small  ("stunted"  their  low  gillraker  Similar trout  1976; and  char:  Nyman,  prey.  of  in  Eurasia  (Kennedy,  1943; and  r e l a t e d genus P r o s o p i u m  gillraker  form  1970).  In  f o r m and  a  each "high"  form i s p r i m a r i l y eats  a  high g i l l r a k e r  " d w a r f " ) and  Arctic  divergent  northern  N i l s o n and 1976).  of  Kirkpatrick  (McCart,  The  of  short-lived  greater  forms  tend  compared  to  counterpart.  Salmo t r u t t a ) and  number  America 1979;  gillraker  Striking pairs  latitudes  high g i l l r a k e r  s i t u a t i o n s a l s o occur  as m o r p h o l o g i c a l l y a  North  a "low"  Sympatric  to  r e l a t e d forms occur  Bodaly,  The  proportion  or  and  and  limited  gasterosteids.  boreal  in Alaska  is similar:  p l a n k t i v o r o u s , whereas the  to  p a i r s i n the  case the p a t t e r n form  f i s h e s i s not  closely  1964;  Selander,  morphologically  (Coreqonus).  1957)  Fenderson,  of  even  in w h i t e f i s h  morphologically  (Berg,  sympatric,  charr  and  among o t h e r  salmonids.  Brown  (Salvelinus alpinus)  occur  trophically  European l a k e s Filipsson,  1971;  segregated  pairs in  ( t r o u t : A l l e n d o r f et a l . , Nyman,  1972;  Henricson  55  Svardson low  (1957,  gillraker  divergence  1970)  a t t r i b u t e d t h e e x i s t e n c e of- h i g h  f o r m s of C o r e g o n u s i n  in  separate  glacial  t h e same e x p l a n a t i o n t o a c c o u n t  Scandinavia  r e f u g e s , and f o r h i g h and  of P r o s o p i u m i n A l a s k a . I n c o n t r a s t , (1979)  interpreted  three widely  the  separated  presence  areas  of N o r t h  O n t a r i o and  M a i n e ) as e v i d e n c e  of t h e two  forms.  This  repeated  pattern  they  may occur  It  age  through  (at l e a s t  i s p o s s i b l e t h a t such  divergence trophic  C.  by  the  pairs  original  n i c h e s . The  and  and  are  the  incompatibility  morphological  s p e c i a l i z a t i o n s of t h e p a i r s exclusive  adaptations.  Unfortunately,  f o r most known s y m p a t r i c  are  lacking  for  Enos  Lakes.  but  hence  forms  sympatric  Such  a l l  that pairs  are  of  of  _in_  situ  into alternate usually  in  the  mechanisms, p o i n t s t o  the  against  intermediates.  --  behavioural  and  represent a l t e r n a t e ,  are a v a i l a b l e  a l l  in  eastern  suggests  process.  pairs,  of c o n t i n u a l s e l e c t i v e p r e s s u r e the  Selander  (Yukon,  products  colonizing  presence  that  and  forms  i n t e r m s of t h e i r c o e x i s t e n c e ) .  of  implies  gillraker  clupeaformis pairs  a similar  absence  This  low  salmonids  e x i s t e n c e o f . such  genetic  (1970) u s e d  America  in formerly glaciated areas,  post-Pleistocene  McCart  of t r o p h i c a l l y d i v e r g e n t  in gasterosteids, coregonids,  all  allopatric  of s e v e r a l i n d e p e n d e n t e v o l u t i o n s  pairs  a l l have e v o l v e d  to  Kirkpatrick of  and  and  mutually  only morphological pairs.  Behavioural  the g a s t e r o s t e i d p a i r s  --  i n Paxton  data data and  56  My o b s e r v a t i o n s differ  i n d i c a t e t h a t t h e two f o r m s  substantially  Clearly,  in their  limnetics  zooplanktonic l a r g e r prey  are  prey,  in  feeding behaviours  adapted  to  Enos  Lake  and a b i l i t i e s .  foraging  on  small  a n d b e n t h i c s a r e a d a p t e d t o f o r a g i n g on t h e  typical  of  benthic  substrates.  female l i m n e t i c s a l s o appear t o d i f f e r  Mature  male  f r o m one a n o t h e r  and  in their  t r o p h i c a d a p t a t i o n s . The p e r f o r m a n c e o f m a t u r e male l i m n e t i c s i n the  two  wholely  foraging  experiments  was  intermediate  p l a n k t i v o r o u s l i m n e t i c f e m a l e s and  non-planktivorous  benthics.  It  is  the  female  limnetics  result  of  behaviour  to  be  genetically  pelagic,  than  "programmed"  between t h e s e x e s .  Among  consequence  sticklebacks:  males  whether  this for  males, or i f i t i s the  differences  fishes, of  the  equally  of the tendency  The i s s u e d e s e r v e s  planktivorous  d i m o r p h i s m may be t h e limnetic  smaller  almost  unclear  d i f f e r e n c e between t h e s e x e s i s an a r t i f a c t  between  a  in  further  this  feeding study.  trophic  problem  sexual  peculiar  must s p e n d t h e p r o d u c t i v e  to  summer  months i n a " b e n t h i c " e n v i r o n m e n t , e n g a g e d i n t h e  energetically  demanding  progeny.  tasks  observation  that  differences  occur  of  defending  no  trophic  comparable  in  weight t o the b a s i c  territories  male  and  conclusion  adaptation,  the  sexually  two  female  of  this  forms  " s p e c i e s " . T h u s , t h e r e s u l t s o f my s t u d y available morphological 1980)  and  reproductive  (McPhail  and  dimorphic  in  trophic  b e n t h i c s adds- f u r t h e r study: are  in  clearly  terms  prep.)  data.  of  different  a r e consonant w i t h  i n prep), biochemical  (Ridgway,  The  the  (Withler, They a l l  57  i n d i c a t e t h a t i n Enos L a k e l i m n e t i c and b e n t h i c as good " b i o l o g i c a l  species".  My r e s u l t s a l s o a g r e e i n g e n e r a l , Larson's not  (1976) o b s e r v a t i o n s  report  on  possibility differ  sex  remains  in  i f not  in  to  of  the  fish  he  studied,  This p o s s i b i l i t y  size  their  experiment  should  indicates  capacity to process  they a r e probably  process  thus  in  Enos  hybrids  numbers  selective  possible  t h a t such  "generalist" et  disadvantage  a l . (1981)  morphologically  have  Lake  remain  high,  morphological  specialized. Within  that  test  rapid  trophic  even  fish  plankton  divergence  a  failed  to  specialize  feeders  most  on  introduction of  Werner  (Lepomis  s u c c e s s f u l a t f e e d i n g than  life,  a  (presumably)  p o p u l a t i o n of b l u e g i l l s  f i v e weeks o f t h e i r  other  s p e c i a l i z e d as e i t h e r  be a t  adopt  among  uniform  of  successfully  shown  b e n t h o s were l e s s  barren  might  seems u n l i k e l y .  plankton  pond  will  s t r a t e g y , but t h i s  i n d i v i d u a l s that  that  capacity  compared t o t h e p a r e n t a l forms. I t i s  macrochirus), or  prey,  Thus, as long as t h e d e n s i t i e s of  intermediates  feeding  are  (McPhail,  in their  i n t e r m e d i a t e s between l i m n e t i c s a n d b e n t h i c s p r o b a b l y a  the  be c h e c k e d .  large benthic  also intermediate  small zooplankton.  the p a r e n t a l forms  with  on P a x t o n s t i c k l e b a c k s . L a r s o n d i d  s i n c e h y b r i d s have i n t e r m e d i a t e g i l l r a k e r  in prep.),  detail,  t h a t male a n d f e m a l e P a x t o n l i m n e t i c s a l s o  maximum p r e y  intermediate and  the  i n feeding behaviour.  The  sticklebacksact  the  either those to  a  bluegills  or benthos f e e d e r s .  w i t h i n a p o p u l a t i o n may c o r r e s p o n d  This to  58  the f i r s t  step  in  evolution  of t h e two  the  process  that  has  f o r m s of G a s t e r o s t e u s  culminated i n Enos  Lake.  in  the  59  REFERENCES  A l l e n d o r f , F.W., N. Ryman, A. S t e n n e k and G. S t a h l . 1976. G e n e t i c v a r i a t i o n i n S c a n d i n a v i a n p o p u l a t i o n s o f brown t r o u t ( S a l m o t r u t t a L . ) : e v i d e n c e of g e n e t i c a l l y d i s t i n c t s y m p a t r i c p o p u l a t i o n s . H e r e d i t a s 82:19-24. . Bell,  M.A. 1976. 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