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Interspecific competition between rainbow trout (Salmo gairdneri Richardson) and redside shiners (Richardsonius… Johannes, Robert Earl 1959

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INTERSPECIFIC COMPETITION BETWEEN RAINBOW TROUT (Salmo g a i r d n e r i R i c h a r d s o n ) AND REDSIDE SHINERS ( R i c h a r d s o n i u s b a l t e a t u s ( R i c h a r d s o n ) ) IN TWO BRITISH COLUMBIA LAKES  by  ROBERT EARL JOHANNES  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE  i n the Department of Zoology  We a c c e p t t h i s t h e s i s as conforming t o t h e required  standard  THE UNIVERSITY OF BRITISH COLUMBIA September,  1959  i  ABSTRACT  Competition  i s d e f i n e d as t h e demand o f two  same e x t r i n s i c r e s o u r c e s i n excess o f The  o r more organisms f o r the  supply.  d i s t r i b u t i o n , movements, b e h a v i o u r and f o o d o f t r o u t and  shiners  i n P a u l and P i n a n t a n l a k e s were s t u d i e d i n o r d e r t o determine the i t e m ' s mechanisms o f i n t e r s p e c i f i c c o m p e t i t i o n between them. y e a r s were Wo two  compared  and  Data from r e c e n t  w i t h d a t a f o r y e a r s when t r o u t a l o n e i n h a b i t e d the l a k e .  i n t e r s p e c i f i c a g g r e s s i o n was  observed.  s p e c i e s were competing f o r space was  The p o s s i b i l i t y t h a t the  discounted.  Stomach c o n t e n t s o f s h i n e r s i n P i n a n t a n Lake r e v e a l e d a marked q u a l i t a t i v e d i u r n a l food c y c l e .  I n P a u l Lake, s h i n e r s have d r a s t i c a l l y  reduced t h e Gammarus p o p u l a t i o n r e l a t i v e t o i t s p r e - s h i n e r abundance. o v e r g r a z i n g was  This  caused by t h e c o n c e n t r a t i o n o f l a r g e numbers o f s h i n e r s o v e r  t h e s h o a l s where Gammarus a r e also; p r e s e n t i n t h e i r h i g h e s t c o n c e n t r a t i o n s and  the a b i l i t y o f s h i n e r s to pursue f o o d deeper i n t o t h e weeds and t o gra.ze  an a r e a more t h o r o u g h l y t h a n t r o u t . reduced  I h P i n a n t a n Lake s h i n e r s have apparently-  the d e n s i t y o f Daphnia to a p o i n t where t r o u t a r e unable  them as r a p i d l y a s i n p r e - s h i n e r y e a r s .  to feed  The ©bility o f b o t h s p e c i e s to  on utilize  many t y p e s of f o o d t e n d s t o reduce t h e i n t e n s i t y o f c o m p e t i t i o n . The  study demonstrates how  a p p r a i s a l of c o m p e t i t i o n .  f a l s e i m p l i c a t i o n s may  a r i s e from a d e l a y e d  I f o b s e r v a t i o n s had not been made on P a u l Lake  u n t i l a f t e r c o m p e t i t i o n had been observed  the importance o f Gammarus as  an  i t e m o f c o m p e t i t i o n would have p r o b a b l y been o v e r l o o k e d and t h e whole competitive r e l a t i o n s h i p  misconstrued.  I n c l u d e d among t h e b a s i c mechanisms of c o m p e t i t i o n i s the b y one  consumption  o r more organisms o f something i n s h o r t s u p p l y b e f o r e i t r e a c h e s  p o t e n t i a l h a b i t a t where i t would become a v a i l a b l e to another  a  organism o r group.  tLIL  EnvijWipBnfeai. factors and behaviour were shown to be important i n f l u ences i» the dynamics of competition.  The physical and biological environment  and the dltdLiRit^on and behaviour of competitors may bo i n states of continual flux.  Hence natural competitive relationships can be considerably more  complicated and variable than situations described by the most elastic of theoretical models.  In the  presenting  requirements  this  British  it  freely available  for  that  Columbia,  I agree  permission  copying  gain  shall  may  n o t be a l l o w e d  of  University  and study.  I  of this  without  at the  shall  copying  be g r a n t e d  or publication  fulfilment of  the Library  f o r extensive  my  s£#-&<<£M^  The University of British Vancouver Canada, Date  degree  o r by h i s r e p r e s e n t a t i v e s .  that  Department  that  f o rreference  scholarly, purposes  Department  i n partial  f o r an advanced  of  agree  thesis  Columbia,  make  further  of this  by t h e Head  thesis  o f my  I t i s understood  thesis  f o rfinancial  written  permission.  iii  ACKNOWLEDGEMENTS  F i n a n c i a l support f o r t h i s g r a n t s t o D r . P. A. L a r k i n ,  study came from N a t i o n a l Research C o u n c i l  I t i s a p l e a s u r e t o thank Dr. L a r k i n f o r h i s  enthusiasm and guidance. Much o f t h e c o n s t r u c t i o n o f a p p a r a t u s and c o l l e c t i o n o f f i e l d m a t e r i a l was  c a r r i e d out by E . S t e n t o n and J . A. Boon.  I am i n d e b t e d t o T. Miura and  E . W. R i c k e r f o r a i d i n g w i t h d a t a a n a l y s i s . A p p r e c i a t i o n i s extended t o those members o f t h e Kamloops detachment o f t h e B. C  #  Game Commission who gave t h e i r h e l p , p a r t i c u l a r l y Mr. P a t  Mulligan. Dr. C. C. L i n d s e y * s v a l u a b l e s u g g e s t i o n s a r e g r a t e f u l l y acknowledged. F i s h c o l l e c t i o n s and u n p u b l i s h e d d a t a from p a s t s t u d i e s o n P a u l and P i n a n t a n l a k e s were made a v a i l a b l e by t h e B. C. Game Commission.  iv  TABLE OF CONTENTS Page INTRODUCTION  . . . . . 1  DESCRIPTION OF STUDY SITE  4  METHODS  7  EVIDENCE FOR THE EXISTENCE OF COMPETITION.  9  A.  C o m p e t i t i o n f o r Space. . . . . . .  B.  C o m p e t i t i o n f o r Food (i)  Food A v a i l a b l e  9 . . . . . 1 0 .  i n P i n a n t a n and P a u l Lakes . . . . . . . .  10  ( i i ) The Food o f S h i n e r s  12  ( i i i ) The Food o f T r o u t  15  ( i v ) E v i d e n c e o f C o m p e t i t i o n f o r Food between Young T r o u t and Shiners . . . . .  . . . 18  FACTORS AFFECTING TIME AND PLACE OF COMPETITION.  A.  .22  D i s t r i b u t i o n and movement o f S h i n e r s (i)  S e a s o n a l Movements . . . . . . . . . .  (ii)  D i u r n a l Movements  .22 . . . . . . . .  .22 ..24  B.  D i s t r i b u t i o n and Movements o f T r o u t  25  C.  Time and P l a c e o f C o m p e t i t i o n  26  BEHAVIOURAL FACTORS I N COMPETITION  26  MECHANISMS OF COMPETITION FOR FOOD  33  SUMMARY  36  DISCUSSION  33  V  L I S T GF FIGURES  Figure  1,  Instantaneous  Page  growth r a t e ( l o g ^ g l o r k l e n g t h a t age -j_ minus n+  l o g f o r k l e n g t h a t a g e ) i n r e l a t i o n to f o r k l e n g t h a t the n  b e g i n n i n g o f the y e a r f o r rainbow t r o u t i n P a u l and P i n a n t a n lakes. •  2,  P e r c e n t t o t a l volume o f stomach c o n t e n t s o f s h i n e r s t a k e n a t 4 hour i n t e r v a l s o v e r 24  hours on the s h o a l s and a t 12  midnight  and 4 a»m. o f f s h o r e  3,  14  P r o b a b i l i t y o f o c c u r r e n c e o f Amphipods rainbow t r o u t , P a u l Lake, 1931  4»  i n stomach c o n t e n t s o f  t o 1957  21  Diagrams o f t h e D i s t r i b u t i o n o f Trout and S h i n e r s i n t h e O b s e r v a t i o n Pens when n o t F e e d i n g . . . . . .  5.  6  . . . . . . . . .  ..  ...31  Diagrams o f t h e D i s t r i b u t i o n o f Trout and S h i n e r s i n t h e O b s e r v a t i o n Pens when F e e d i n g . . . . . . .  ....32  vi  LIST OF TABLES  Table  Page  1»  L i m n o l o g i c a l c h a r a c t e r i s t i c s o f P a u l and P i n a n t a n l a k e s . . . .  2.  P e r c e n t T o t a l Volume Items i n P i n a n t a n Lake Dredgings i n June and August 195S  3.  • •  H  P e r c e n t c o m p o s i t i o n by volume o f stomach c o n t e n t s o f s h i n e r s i n P a u l and P i n a n t a n l a k e s i n v a r i o u s y e a r s . . . . . . . . . .  4.  4  15  Average volume o f stomach c o n t e n t o f P i n a n t a n Lake rainbow t r o u t a c c o r d i n g to month and l e n g t h group i n  1958. . . . . . . . . .  17  INTRODUCTION  I n t e r s p e c i f i c c o m p e t i t i o n i s an i m p o r t a n t i n f l u e n c e i n t h e b a l a n c e o f mixed s p e c i e s communities.  The r o l e o f i n t e r s p e c i f i c c o m p e t i t i o n i n f r e s h w a t e r  f i s h p o p u l a t i o n s i s a fundamental q u e s t i o n i n f i s h e r i e s b i o l o g y .  The problem  o f c o m p e t i t i o n o f u n d e s i r a b l e s p e c i e s w i t h d e s i r a b l e ones ( s p o r t s f i s h e s ) i s d e a l t w i t h a t some l e n g t h i n t h e l i t e r a t u r e o f pond f i s h Bennet,  1952j  and Smith,  1941J  R i c k e r and G o t t s c h a l k ,  194l)«  c u l t u r e ( f o r example  Smith and Swingle,  1939j Swingle  These p a p e r s d e a l m a i n l y w i t h t h e end r e s u l t s o f and  remedies f o r u n d e s i r a b l e c o m p e t i t i o n .  F i e l d studies of competition  almost  alwaya b e g i n a f t e r t h e e f f e c t s o f c o m p e t i t i o n a r e m a n i f e s t l y e v i d e n t .  The  b i o l o g y o f t h e c o m p e t i t o r s b e f o r e c o m p e t i t i o n o c c u r r e d and d u r i n g t h e f i r s t phases o f c o m p e t i t i o n i s l o s t t o t h e o b s e r v e r .  He i s s e r i o u s l y handicapped  i n p i c k i n g o u t t h e f e a t u r e s o f t h e c o m p e t i t o r s " b i o l o g y which d i f f e r those before competition occurred.  from  Hence he i s i l l - e q u i p p e d t o determine what  changes c o m p e t i t i o n has wrought and through what mechanisms.  As a r e s u l t ,  attempts t o d e s c r i b e t h e a c t u a l mechanisms o f i n t e r s p e c i f i c c o m p e t i t i o n i n f r e s h w a t e r f i s h e s have been c o n f i n e d almost e n t i r e l y t o l a b o r a t o r y experiments and a b s t r a c t , n e c e s s a r i l y s i m p l i f i e d mathematical models. S e v e r a l workers have p o i n t e d o u t t h e d i f f i c u l t i e s o f i n t e r s p e c i f i c competition i n freshwater f i s h e s .  Lagler  demonstrating  (1944)  emphasizes  t h a t t o demonstrate t h a t two s p e c i e s a r e drawing from a common food  supply  does not by i t s e l f prove t h a t c o m p e t i t i o n f o r f o o d e x i s t s , f o r i f t h e f o o d i s abundant and t h e p o p u l a t i o n s a r e kept r e l a t i v e l y low by o t h e r f a c t o r s , t h e " f e e d i n g c o u l d have l i t t l e e f f e c t on s u p p l y " .  Starret  (1950)  suggests t h a t  i t i s d i f f i c u l t t o e s t a b l i s h c o m p e t i t i v e r e l a t i o n s between s p e c i e s because t h e f i s h may change t h e i r d i e t s r a t h e r than e n t e r i n t o s e v e r e Larkin  (1955)  s t a t e s that_ i t  competition.  i s d i f f i c u l t to s e p a r a t e i n t e r s p e c i f i c  competition  2 as a f a c t o r i n p o p u l a t i o n c o n t r o l from o t h e r phenomena such as c a n n i b a l i s m mutual p r e d a t i o n .  Wide h a b i t a t range i n f r e s h w a t e r f i s h e s and t h e i r c o n s e -  quent f l e x i b i l i t y of f e e d i n g h a b i t s as w e l l as t h e i r f l e x i b l e growth r a t e high r e p r o d u c t i v e p o t e n t i a l a l s o obscure competitive Crossman  and  (1957) p o i n t s  and  relations.  out t h a t mathematical approaches to  competition  d e a l m a i n l y w i t h p o p u l a t i o n s i z e , b i r t h r a t e s , m o r t a l i t y r a t e s and  reproductive  p o t e n t i a l , and n e g l e c t the e f f e c t s o f the r e a c t i o n s o f f i s h e s to t h e  environment.  These r e a c t i o n s " p o s s i b l y c r e a t e d i f f e r e n t i n t e r a c t i o n s i n n e a r l y s i m i l a r environments".  The  i n f l u e n c e o f f a c t o r s such as d i s t r i b u t i o n , movements and  b e h a v i o u r upon t h e i n t e r a c t i o n of t h e c o m p e t i t o r s w i l l be d i s c u s s e d i n the following sections. The  e x a c t meaning of " c o m p e t i t i o n " has t r o u b l e d many a u t h o r s  i n s t a n c e Dobzhansky, 1950;  Udvardy, 1952;  L a r k i n , 1956)  need f o r a p r e c i s e d e f i n i t i o n o f t h e term.  who  (for  have v i s u a l i z e d  Andrewartha and B i r c h ,  (1954) and  Crombie  1954,  (1947) on  Larkin,  Udvardy, 1952;  1956;  competition  E l t o n and M i l l e r ,  and MacFadyen,  1957).  1954;  Nicholson  t h e o t h e r hand, p r e f e r t o c o n s i d e r a l l d e n s i t y  dependent a n i m a l i n t e r a c t i o n s ' i n c l u d i n g p a r a s i t i s m and p r e d a t i o n as phenomena.  the  P r e d a t i o n and p a r a s i t i s m a r e  c o n s i d e r e d as separate i n t e r a c t i o n s not f a l l i n g w i t h i n t h e bounds o f by most r e c e n t w r i t e r s (Solomon, 1949;  1  Most o f t h e s e w r i t e r s a c c e p t  competitive  the d e f i n i t i o n of c o m p e t i t i o n  "the demand, t y p i c a l l y a t the same time, o f more than one  as  o r g a n i s m f o r the  r e s o u r c e s o f the environment i n excess o f Immediate s u p p l y , "  (Larkin,  same  1956).  The l a c k o f agreement o v e r whether o r not p r e d a t i o n and p a r a s i t i s m c o n s t i t u t e c o m p e t i t i o n c e n t e r s around the i n t e r p r e t a t i o n of t h e word, N i c h o l s o n and Crombie c o n s i d e r t h e l i f e o f t h e h o s t o r p r e y to be a most o t h e r w r i t e r s do n o t . can be accomplished I f we  A c o n c e p t u a l s t a n d a r d i z a t i o n o f the term  "resource".  resource; competition  o n l y by r i d d i n g d e f i n i t i o n o f i t s a m b i g u i t y .  choose a d e f i n i t i o n which admits a l l c a t e g o r i e s o f  unfavorable  3 biological interactions,"it in  a community i n one way  (can be) argued t h a t e v e r y a c t i v i t y o f each  o r a n o t h e r c o n s t i t u t e s an a c t o f c o m p e t i t i o n  a l l o t h e r members o f t h e community," ( L a r k i n , b r o a d i n meaning as t o  1956).  with  The word i s r e n d e r e d  have l i t t l e v a l u e as a p r e c i s e s c i e n t i f i c term.  s t r u g g l e f o r e x i s t e n c e " s e r v e s j u s t as w e l l t o C o m p e t i t i o n may  animal  express t h i s general  so  "The  concept.  be c o n s i d e r e d as one o f t h e mechanisms i n t h e s t r u g g l e f o r  existence along with predation, p a r a s i t i s m e t c . The i n s e r t i o n o f one word i n t h e d e f i n i t i o n so t h a t i t reads, "....  demand f o r e x t r i n s i c r e s o u r c e s  helps c l a r i f y the concept.  problem o f whether l i f e i s a r e s o u r c e o r not i s s k i r t e d .  The  In a predatory  p a r a s i t i c r e l a t i o n s h i p t h e l i f e of t h e h o s t o r p r e y i s not e x t r i n s i c .  or  Pre-  d a t i o n and p a r a s i t i s m a r e t h u s c l e a r l y e x c l u d e d from c o m p e t i t i v e phenomena. C o m p e t i t i o n i s d e f i n e d i n t h i s paper as t h e demand o f more t h a n organism f o r t h e same e x t r i n s i c r e s o u r c e s i n excess o f s u p p l y . i n t o two  one  It i s divided  phases; c o m p e t i t i o n f o r space and c o m p e t i t i o n f o r f o o d .  P a u l and P i n a n t a n l a k e s near Kamloops, B r i t i s h Columbia, were chosen for of  t h i s study.  A t the t i m e o f s t u d y b o t h l a k e s c o n t a i n e d o n l y two  species  f i s h , t h e rainbow t r o u t , Salmo ^ a i r d n e r i , and t h e r e d s i d e s h i n e r , R i c h a r d -  s o n i u s b a l t e a t u s , p r o v i d i n g a r e l a t i v e l y simple example o f i n t e r s p e c i f i c competition. t h e two  L i m n o l o g i c a l and f i s h b i o l o g y s t u d i e s have been c a r r i e d on i n  l a k e s f o r many o f t h e l a s t t h i r t y y e a r s and p r o v i d e d much background  information f o r this investigation. ing  fish  the f i r s t  The b i o l o g y o f the t r o u t b e f o r e and  y e a r s a f t e r t h e i n t r o d u c t i o n o f s h i n e r s i s w e l l documented  durand  p r o v i d e s a unique o p p o r t u n i t y t o s t u d y the mechanisms o f c o m p e t i t i o n d u r i n g t h e i r development. rainbow t r o u t  P i n a n t a n Lake has s i n c e been p o i s o n e d  alone.  and r e s t o c k e d  with  4  DESCRIPTION OF STUDY SITE  P a u l and P i n a n t a n l a k e s a r e s i t u a t e d about twelve m i l e s n o r t h e a s t o f Kamloops, B. C ,  a t an a l t i t u d e o f about 2500 and 2860 f e e t r e s p e c t i v e l y .  l a k e s a r e about f o u r m i l e s a p a r t and connected  b y a stream, f l o w i n g from  P i n a n t a n i n t o P a u l , which o c c a s i o n a l l y s t o p s running i n t h e summer. has one o u t l e t and one major i n l e t . t h e summer a l s o e n t e r P a u l Lake. p h y s i c a l f e a t u r e s o f Paxil Lake. lakes.  The  Each l a k e  A few mountain streams which d r y up i n  L a r k i n e t a l (1950) have d e s c r i b e d t h e Rawson (1934) d i s c u s s e s t h e l i m n o l o g y o f b o t h  Table I l i s t s some l i m n o l o g i c a l c h a r a c t e r i s t i c s o f t h e two l a k e s .  Table I .  L i m n o l o g i c a l c h a r a c t e r i s t i c s o f P a u l and P i n a n t a n l a k e s .  P a u l Lake  Pinantan  Lake  Area  3*960 a c r e s  a 161 a c r e s  S h o r e l i n e development  *5.55 u n i t s  • 3.89 u n i t s  Maximum depth  4182  feet  o62  feet  Mean d e p t h  •112  feet  a31  feet  TDS  •216 p.p.m.  • 238 p.p.m.  « L a r k i n e t a l , 1950 * Crossman, 1957. OB. C. Game Commission, • Rawson, Rawson productivity.  (1934) d e s c r i b e s  unpubl.  1942.  P i n a n t a n Lake as h i g h l y e u t r o p h i c w i t h  high  He c o n s i d e r s P a u l Lake a s t y p i c a l l y o l i g o t r o p h i c , but w i t h  r e l a t i v e l y h i g h p r o d u c t i v i t y due t o an e x t e n s i v e s h o a l a r e a . Mottley  (1932) and  Crossman  (1957) d i s c u s s  the sequence o f events  i n P a u l Lake a f t e r t h e l i b e r a t i o n o f t r o u t f r y i n 1900. is  similar.  The h i s t o r y o f P i n a n t a n  There was a r a p i d b u i l d up o f a l a r g e u n d e r e x p l o i t e d p o p u l a t i o n .  The c o n s t r u c t i o n o f a good r o a d t o t h e l a k e s l e d e v e n t u a l l y t o d e p l e t i o n o f  5 t h e s t o c k s and fry#stock±ng programs were i n i t i a t e d . Sometime a f t e r 1930 t h e Redside s h i n e r , R i c h a r d s o n i u s introduced i n t o Pinantan.  b a l t e a t u s was  A b a r r i e r was p l a c e d i n t h e stream between  and P a u l t o exclude t h e s h i n e r s b u t by 1945  Pinantan  t h e y had i n v a d e d t h e l a t t e r .  A  subsequent marked drop i n t h e c a t c h p e r u n i t e f f o r t , a decrease i n t h e growth r a t e o f y e a r l i n g t r o u t and a n i n c r e a s e i n growth r a t e o f l a r g e t r o u t i n P a u l Lake (see F i g u r e 1) was a t t r i b u t e d t o t h e i r i n t e r a c t i o n with  1959)«  and L a r k i n , in  A n o t i c e a b l e decrease i n t h e percent  the catch occurred  1955  t o 56).  s h i n e r s (Crossman  o f one-year-old  fish  (34$ of t h e c a t c h i n 1946 to 49 compared w i t h 6.1$ i n  T h i s was a t t r i b u t a b l e t o t h e s l o w e r growth r a t e o f young t r o u t j  fewer o n e - y e a r - o l d s  a t t a i n e d s u f f i c i e n t s i z e t o e n t e r the f i s h e r y .  The d e p r e s s i o n i n t h e growth r a t e o f y e a r l i n g t r o u t reached a maximum in  1952.  Large t r o u t d i d not s t a r t f e e d i n g on s h i n e r s u n t i l about 195l»  succeeding  In  y e a r s s h i n e r s made up a s u c c e s s i v e l y l a r g e r q u a n t i t y o f t h e d i e t o f  t r o u t o v e r 10 i n c h e s f o r k l e n g t h . i n c r e a s e d a c c o r d i n g l y and surpassed  The growth r a t e o f t h e s e l a r g e r t r o u t that of pre-shiner years.  By 1956 t h e  growth r a t e o f t r o u t under t e n i n c h e s , which d i d n o t f e e d a p p r e c i a b l y on s h i n e r s , had a l s o i n c r e a s e d s l i g h t l y from t h e 1951 l e v e l b u t was s t i l l  lower  than i n pre-shiner years. MacLeod  (1957,  MS) s t a t e s t h a t t h e growth r a t e o f s m a l l t r o u t i n  P i n a n t a n i n 1952 was c o n s i d e r a b l y l o w e r t h a n t h e p r e - s h i n e r growth r a t e i n Paul. was  The growth r a t e o f P i n a n t a n t r o u t i n 1957,  c a l c u l a t e d by t h i s w r i t e r ,  found t o be s l i g h t l y h i g h e r than i n 1952 but s t i l l l o w e r than i n p r e -  s h i n e r y e a r s i n P a u l Lake, (see F i g u r e 1 ) .  U n f o r t u n a t e l y no p r e - s h i n e r  data  on t r o u t growth r a t e s i n P i n a n t a n Lake were a v a i l a b l e and t h e d e l e t e r i o u s e f f e c t s o f c o m p e t i t i o n w i t h s h i n e r s o n growth can o n l y be i n f e r r e d . m  1955 t h e I n s t i t u t e o f F i s h e r i e s , U n i v e r s i t y o f B r i t i s h Columbia,  i n i t i a t e d a program o f study o f t h e i n t e r a c t i o n o f t h e two s p e c i e s .  Crossman  10 FORK F i g u r e 1.  20 LENGTH (cm.) at BEGINNING Cf YEAR  30  Instantaneous growth r a t e (log-^Q f o r k l e n g t h a t age  n 4 >  ^ minus  l o g f o r k l e n g t h a t a g e ) i n r e l a t i o n to f o r k l e n t h a t the n  b e g i n n i n g o f the y e a r f o r rainbow t r o u t i n P a u l and Pinantan lakes. 1946 and 1932 P a u l L i k e c u r v e s from L a r k i n e t a l (1957) 1^55-56 c u r v e s from Grossman (1957) 1?52 c u r v e s f o r Pinantan Lake from \fecLeod (1957 MS).  7 (1957) dealt with the predation of trout on shiners and concluded that this predation had no significant influence on the size of the shiner population, but that i t resulted i n an increased growth rate of trout over ten inches long. The present study deals with competition between the two species. Because of the importance of the rainbow trout as a sports fish and because of the large amount of research done on i t s biology i n Paul Lake previous to the introduction of shiners, this study deals mainly with the effects of the shiners on trout.  The effects of the trout on shiners were studied only  casually. METHODS To establish the mechanisms involved i n competition between the two species the study was divided into two facets. 1.  What, when, whef«e and how much do shiners and trout eat?  2m  When the two species come i n contact how do they influence each other?  Competition for Food The stomach contents of 168 shiners caught i n gillnets and a dipnet over the summer were analyzed to evaluate the seasonal, diurnal and spatial differences i n their choice of food.  About 20 stomachs from shiners taken  i n each of 1946 and 1948 i n Pinantan Lake and i n 1950, 1952 and 1959 i n Paul Lake were also examined. The contents of stomachs from 335 trout caught by angling and gillnetting i n the Pinantan Lake throughout the summer were analyzed. To determine the qualitative and quantitative availability of food, three horizontal plankton tows at each of 3, 10 and 20 feet were made at noon and again at midnight during the f i r s t week i n September, 1958, with a ClarkeBumpus plankton sampler. Ten bottom dredgings on the shoal i n depths from 1 to  8 10 f e e t were t a k e n i n June and a g a i n i n August i n P i n a n t a n ,  P r e l i m i n a r y dredg-  i n g s i n the Chara w i t h an Ekman dredge p r o v e d u n s a t i s f a c t o r y . not c l o s e p r o p e r l y o v e r t h e weeds. samples. two  Consequently  a rake was  The dredge would  used t o g a t h e r  the  Each sample c o n s i s t e d o f the amount o f Chara t h a t would f i t i n t o  l i t r e container.  S i x - s i d e d , ( i . e . c o m p l e t e l y e n c l o s e d ) pens 3  b y 3'  1  a  by  3  1  were u s e d i n experiments t o s t u d y t h e u t i l i z a t i o n o f a known amount o f i n t r o d u c e d food.  F a c t o r s A f f e c t i n g t h e Time and P l a c e o f The  s e a s o n a l and d i u r n a l movements o f s h i n e r s i n P a u l Lake have been  d e s c r i b e d by Crossman (1957). 1958  24  S t u d i e s were c a r r i e d out on P i n a n t a n Lake i n  t o c o r r o b o r a t e and extend t h e s e f i n d i n g s .  throughout one  Competition  t h e summer.  D i r e c t o b s e r v a t i o n s were made  D i u r n a l movements were r e c o r d e d u s i n g two  s e t n e a r shore, t h e o t h e r o f f s h o r e , f o r one  gillnets,  hour every f o u r hours  throughout  hours. Crossman a l s o d i s c u s s e s the d i s t r i b u t i o n and movements of t r o u t i n  P a u l Lake.  D i r e c t o b s e r v a t i o n s and o v e r n i g h t g i l l n e t s e t s were made  throughout  t h e summer b o t h i n P a u l and P i n a n t a n l a k e s .  Behavioural Factors i n Competition S h i n e r s and 3 t o 6 i n c h t r o u t were h e l d i n e n c l o s u r e s i n the l a k e f o r o b s e r v a t i o n and f e e d i n g experiments.  F o u r - s i d e d pens, 6'  by 6*  by 5' deep and  b u i l t o f door s c r e e n on a wooden frame, were p l a c e d on Chara beds n e a r and anchored f i r m l y t o t h e bottom i n about 4 f e e t o f w a t e r .  shore  The bottoms o f the  pens were open, hence t h e e n c l o s e d f i s h were swimming over n a t u r a l Chara beds and had a c c e s s t o the bottom. I n o r d e r t o t e s t t h e p o s s i b l e e f f e c t of s h i n e r odor on t r o u t , channels  f l o w i n g i n t o a box c o n t a i n i n g t e n 3 to 6 i n c h t r o u t i n two  f e e t o f water was  constructed.  I n one  channel a s h i n e r was  tm  cubic  placed i n a wire  9 cage.  The  o t h e r c h a n n e l c o n t a i n e d no s h i n e r .  i n each o f the two  c u r r e n t s was  o b s e r v e d and  The  number o f t r o u t o r i e n t i n g  the t e s t r e p e a t e d t e n  times.  Many hours d u r i n g t h e summer were devoted t o d i r e c t o b s e r v a t i o n o f the two  s p e c i e s on and around the s h o a l i n P i n a n t a n  EVIDENCE FOR  Lake.  THE EXISTENCE OF COMPETITION  Ih the l i t e r a t u r e of f i s h e r i e s biology, competition i n t o t h r e e a s p e c t s ; c o m p e t i t i o n f o r food, Food i s a w e l l d e l i n e a t e d s u b d i v i s i o n .  space and  i s commonly d i v i d e d  spawning a r e a  Competition  f o r space however i s a  phrase o f t e n l o o s e l y a p p l i e d to any s i t u a t i o n where overcrowding o c c u r s . i s o f t e n i n s h o r t s u p p l y i s not  space p e r se, but  w i t h i n a c i r c u m s c r i b e d space,e.g. f e e d i n g a r e a shade, s u n l i g h t , oxygen, warmth, e t c . and thorough study o f c o m p e t i t i o n  1956).  (Larkin,  some r e s o u r c e  What  contained  (as d i s t i n c t from f o o d ) s h e l t e r ,  also, logically,  spawning a r e a .  Any  f o r space must s o r t from a wide range o f p o s s i -  b i l i t i e s j u s t what a t t r i b u t e s o f the space a r e i n c o n t e n t i o n .  A.  Competition The  f o r Space.  p o s s i b i l i t y t h a t t r o u t may  t h e y would o t h e r w i s e o f trout before  occupy was  not  exclude  s h i n e r s from p a r t s o f the  examined.  ( M o t t l e y and M o t t l e y , 1932)  A knowledge o f t h e  and  lake  distribution  a f t e r (Crossman, 1957)  the  i n t r o d u c t i o n o f s h i n e r s i n t o P a u l and P i n a n t a n l a k e s l e d the w r i t e r t o a l l but d i s c o u n t t h e p o s s i b i l i t y t h a t s h i n e r s exclude f o r m e r range.  t r o u t from any p a r t o f t h e i r  However, some s p e c i e s o f c y p r i n i d s have been shown t o  r e p e l l e d by t h e o d o r i n t h e water o f c e r t a i n o t h e r  be  s p e c i e s , ( H a s l e r , 1954)  and  t e s t s were c a r r i e d out to determine whether s h i n e r s might r e p e l t r o u t i n such a manner.  T r o u t made no d i s t i n c t i o n i n choosing  a s h i n e r and another c u r r e n t w i t h o u t  a shiner i n i t .  made; twenty by each of t e n 3 to 6 i n c h t r o u t . one  between a c u r r e n t c o n t a i n i n g  The  Two-hundred c h o i c e s s h i n e r was  switched  c u r r e n t to the o t h e r a f t e r one-hundred c h o i c e s had been made.  were  from  Exactly  10 one hundred movements were made i n t o each o f t h e two c u r r e n t s .  The p o s s i b i l i t y  t h a t s h i n e r s might, o v e r a p e r i o d o f time, " c o n d i t i o n " t h e water and t h u s  repel  t r o u t was n o t examined. I t i s h i g h l y u n l i k e l y t h a t t r o u t and s h i n e r s compete f o r spawning space. S h i n e r s a r e known t o spawn i n streams a s w e l l as i n l a k e s ( L i n d s e y , 1950) but i n P a u l and P i n a n t a n l a k e s i t appears t h a t most spawning t a k e s p l a c e i n t h e l a k e s on t h e s h o a l s *  A l l t r o u t spawning i s done i n t h e streams.  S h i n e r s have been o b s e r v e d t h i r t y y a r d s downstream f r o m t h e P i n a n t a n Lake o u t l e t  ( M c A l l i s t e r , unpub.) and n e a r t h e o u t l e t o f t h e stream f l o w i n g  i n t o P a u l Lake.  (Grossman, 1957)*  However, the s h i n e r s o b s e r v e d were few  and occupancy o f t h e s e a r e a s n o t p r o l o n g e d .  The p r e s e n t w r i t e r o b s e r v e d no  s h i n e r s i n the spawning stream a t any time d u r i n g h i s i n v e s t i g a t i o n . S h i n e r s s t a r t spawning between the end o f May and the end o f June ( L i n d s e y , 1950) j u s t a f t e r the t r o u t spawning r u n i s o v e r .  S h i n e r eggs a r e  b r o a d c a s t f e r t i l i z e d r a t h e r t h a n b u r i e d i n t h e g r a v e l and shed o n l y a t n i g h t ( L i n d s e y , 1950) u n l i k e t r o u t eggs.  No s h i n e r s have e v e r been o b s e r v e d w i t h  t r o u t i n Upper P a u l Creek. B.  C o m p e t i t i o n f o r Food (i)  Food A v a i l a b l e i n P i n a n t a n and P a u l Lakes Bottom Organisms T a b l e 2 shows t h e p e r c e n t o f t h e t o t a l volume o f v a r i o u s f o o d i t e m s  i n twenty P i n a n t a n d r e d g i n g s made o n t h e s h o a l ( i n t e n f e e t o f w a t e r o r l e s s ) . The  r e s u l t s a r e p r o b a b l y l e s s p r e c i s e t h a n they might  number o f d r e d g i n g s .  o f the small  A n i s o p t e r a l a r v a e were t h e most i m p o r t a n t s i n g l e i t e m  i n b o t h June and August.  P l a n a r i a dropped from 21% o f t h e t o t a l volume i n June  t o v i r t u a l absence i n August.  H y a l l e l a and Physa made up most o f t h e b a l a n c e  o f t h e d r e d g i n g s i n b o t h months. Chironomidae,  be because  S h i n e r eggs, Sphaeridae and t h e l a r v a e o f  Ephemeroptera, T r i c h o p t e r a and Enallagma were p r e s e n t i n s m a l l  numbers i n June and absent i n August.  The t o t a l volume o f bottom organisms i n  11 the dredgings i n June was 35*6 cc's while i t dropped steeply to 11.5 cc's In The difference i s statistically significant (p <  August.  .02). A l l groups  except Zygoptera larvae were present i n fewer numbers i n August than i n June. Table 2,  Percent Total Volume of Food Items i n Pinantan Lake Dredgings i n June and August 1958.  Item Anisoptera Planaria Physa larvae June  32.3  Aug.  74.8  27  Hyallela Pianorbis Trichoptera Hirudinea Zygoptera  11.7  3.4  17.4  4.3  7.1  1.1 2.6  Dredgings were richest i n volume and variety of organisms i n the shallowest ( l to 2 inches) water and visible organisms were strikingly absent from two dredgings taken i n water deeper than 8 feet i n both June and August. This i s i n marked contrast to Paul Lake where abundance of organisms, although similarly greatest near the surface, tapers off much more gradually; a l l groups of organisms i n the 0 to 5 meter depth zone were also found i n the 5 to 10 meter zone and one-third of them were present i n the 30 to 40 meter zone, (Larkin et a l , 1950). Rawson (1934) attributes his finding no bottom organisms below the thermocline i n Pinantan Lake to severe oxygen depletion.  Paul Lake  on the other hand has abundant oxygen at a l l depths. Larkin et a l (1950) recorded the bottom fauna of Paul Lake i n 1948 and 1949.  A l l groups found i n Pinantan Lake were present i n Paul Lake as well  as Gammarus, Lymnaea. and Oligochaetes.'.  Chironomids were the most abundant  organisms numerically at a l l depths, from 0 to 50 meters.  Percentage volumetric  analysis of the various groups was not recorded. A marked decline i n the numbers of Amphipods i n the lake since the introduction of shiners was noted.  |  12 Plankton P l a n k t o n tows made t h r o u g h the f i r s t week i n September 1958 showed t h a t s i x t y - f i v e p e r c e n t b y volume o f t h e p l a n k t o n i n t h e t o p 20 f e e t o f P i n a n t a n Lake ( e q u i v a l e n t , a p p r o x i m a t e l y ; t o t h e e p i l i m n i o n ) c o n s i s t e d o f Daphnia was Diaptomus.  and 25$  Aphanizomenon was unimportant i n p l a n k t o n tows made a t 3 f e e t  and 10 f e e t b u t made up 45$ o f t h e p l a n k t o n a t a depth o f twenty S i m i l a r l y Anabaena was unimportant the t o t a l volume.  feet.  except a t 20 f e e t where i t made up 10$ o f  About 5$ o f the m i d n i g h t tows a t a l l t h r e e depths  consisted  o f Chaoborus w h i l e none were r e c o r d e d i n 12 tows made d u r i n g d a y l i g h t . Other organisms noted were Ceratium. Asplanchna, S p i r o g y r a , Polyphemus  t  P a n d o r i n a . P r o t o c o c c u s , U l o t h r i x , O h r i o d o r u s , PedJastrum.Dynobrion, c y s t i s . Staurastrum, A s t e r i o n e l l a , Merismopedia, Simocephalus,  Tetradesmus  C l a d o p h o r a , Anuraea,  Colacium,  and C o n c h o s t r a c a .  Rawson (1934) d i s c u s s e s t h e p l a n k t o n i n P a u l Lake. Daphnia  Sphaero-  Diaptomus and  i n t h a t o r d e r were t h e organisms o f major importance.  Two h o r i z o n t a l  p l a n k t o n tows j u s t under t h e s u r f a c e and two t o t a l v e r t i c a l tows made i n August 1959 i n d i c a t e d t h i s was s t i l l t h e c a s e , (ii)  The Food o f S h i n e r s  I n p r e l i m i n a r y t e s t s groups o f s h i n e r s were h e l d f o o d l e s s f o r v a r i o u s l e n g t h s o f time b e f o r e t h e i r stomachs were examined.  Whereas 90$ o f t h e stomach  c o n t e n t s from f i s h k i l l e d immediately o n c a p t u r e were i d e n t i f i a b l e , o n l y 15.5$ were i d e n t i f i a b l e a f t e r t h e f i s h were h e l d f o o d l e s s f o r an hour and 3.7$ a f t e r two hours.  I t was c o n c l u d e d t h a t almost a l l f o o d i n t h e stomachs o f s h i n e r s  k i l l e d i m m e d i a t e l y on c a p t u r e had been e a t e n w i t h i n two hours o f c a p t u r e . Hence any d i u r n a l change i n f o o d h a b i t s c a n e a s i l y be d e t e c t e d i n t h e stomach contents.  S h i n e r s were sampled  e a r l y August.  a t f o u r hour i n t e r v a l s throughout the day i n  A marked d i u r n a l change i n foods was d i s c o v e r e d .  13 F i g u r e 2 shows t h e g r a d u a l s h i f t i n dominance from Daphnia t o d u r i n g t h e day and back t o Daphnia a t n i g h t i n the d i e t o f P i n a n t a n shiners.  The  stomachs o f  and 6.2%  algae  37  s h i n e r s taken a t midnight contained  ( m a i n l y S p i r u l i n a , S p i r o g y r a and  I n c o n t r a s t the stomachs o f 29 Daphnia and 69% The shoals.  algae  Lake  92.5$  Daphnia  Nodularia).  s h i n e r s t a k e n a t 5 P»m» c o n t a i n e d o n l y  19.1$  algae.  s h i f t i s more s t r i k i n g f o r o f f s h o r e f i s h t h a n f o r t h o s e on  the  A t m i g n i g h t o f f s h o r e f i s h had v i r t u a l l y no a l g a e i n t h e i r stomachs  w h i l e s h o a l f i s h had 14$ f i s h had 34$  a l g a e and &k% Daphnia.  a l g a e i n t h e i r stomachs w h i l e  But by 4 a.m.  s h o a l f i s h had  the  o n l y &%  offshore algae.  All  s h i n e r s move back t o t h e s h o a l at dawnj o f f s h o r e s h i n e r s were caught o n l y i n t h e m i d n i g h t and 4 a.m. Plankton  sets.  c o n s i s t i n g almost e n t i r e l y o f Daphnia p u l e x . made up  26.4#»  o f the t o t a l d i e t and a l g a e c o n t r i b u t e d  O t h e r food i n c l u d e d H y a l l e l a ,  v a r i o u s a q u a t i c and t e r r e s t r i a l i n s e c t s , P L a n o r b i s f o o d s e v e r composed more t h a n 12%  too w e l l d i g e s t e d t o be  P a u l and  Pinantan  taken f r o m 1946  Physa.  of the t o t a l d i e t .  1950b)  none o f the  t o 1959  269  these  of  s i n c e t h e i r p r e s e r v a t i o n made a c c u r a t e  been  contained a s i n g l e shiner. i n P a u l Lake by the w r i t e r and C. Game Commission  g r a d u a l d e t e r i o r a t i o n o f a l l but the 1959  stomachs  stomach a n a l y s i s d i f f i c u l t and  f i g u r e s i n T a b l e 3, below, a r e j u s t e s t i m a t e s .  A p p a r e n t l y t h e r e was  the no marked  q u a l i t a t i v e d i u r n a l v a r i a t i o n i n t h e f e e d i n g h a b i t s o f P a u l Lake s h i n e r s s i m i l a r to t h a t found i n Pinantan  one  s h i n e r stomachs from  i n v a r i o u s p r e v i o u s y e a r s i n b o t h l a k e s b y members o f the B. The  About 10%  s h i n e r c a n n i b a l i s m has  Stomachs of s h i n e r s c o l l e c t e d i n 1959  were a l s o examined.  None o f  identified.  I t i s i n t e r e s t i n g t o note t h a t w h i l e observed i n other l a k e s (Lindsey,  and  o f the d i e t o f a group o f f i s h caught a t  t i m e and t o g e t h e r t h e y made up o n l y 6.1% t h e f o o d was  58.1%  shiners.  C o l l e c t i o n s o f shiners taken at  • — DAPHNIA  100  A — ALGAE  NIGHT  O —OTHER  DARKNESS  REPRESENTED  SHADING.  6 I2midn.  FOODS  BY  ,  _1L  .  4am.  8am.  12 noon Fig.  4 pnr  8 pm..  2.  Percent total volume, of stomach contents of shiners taken at 4 hour intervals- over 24 hours on the shoals and at 12 midnight and 4 a.m. offshore* •' , * U n i d e n t i f i e d food not included.  I2midn;  15 2 p.m. and 2 a.m. b o t h c o n t a i n e d about 15$ Gammarus and 85$ t e r r e s t r i a l  insects.  The t i m e s o f day o f c a p t u r e were n o t r e c o r d e d f o r t h e o t h e r c o l l e c t i o n s . No a l g a e o r Daphnia was found i n the stomachs o f any P a u l Lake s h i n e r s . The  s t r i k i n g d i f f e r e n c e s i n t h e d i e t s o f s h i n e r s i n t h e two l a k e s i s n o t  unusual.  L i n d s e y (1950b) mentions t h e wide v a r i e t i e s o f f o o d e a t e n by s h i n e r s ,  i n d i f f e r e n t l a k e s and streams.  He s t a t e s furthermore t h a t i n l a k e s i n h a b i t e d  by both rainbow t r o u t and s h i n e r s , t h e s h i n e r s were found t o e a t a l l t y p e s o f food eaten by the t r o u t .  Table 3.  Percent c o m p o s i t i o n b y volume o f stomach c o n t e n t s o f s h i n e r s i n P a u l and P i n a n t a n l a k e s i n v a r i o u s y e a r s .  Gammarus  Terrestri* insects  Algae  Lake  Year  Daphnia  Month  Number of f i s h examined  Trout Fingerlir  #»  H  CD  ;  H  fi CD  ii July  1946  Pin.  22  Aug.  1948  Pin.  36  July  1950  Paul  20  Aug.  1952  Paul  4  Aug.  1959  Paul  17  5  90  5  90  10  70  30 50*  85  50  15 •  * F i n g e r l i n g t r o u t had been p l a n t e d i n t h e a r e a o f c a p t u r e o f t h e s e s h i n e r s t h e same day.  (ii)  Two o f t h e f o u r f i s h examined c o n t a i n e d t r o u t  fingerlings.  The Food o f T r o u t The  stomach c o n t e n t s o f 335 t r o u t t a k e n from P i n a n t a n Lake i n t h e  summer o f 1958  were examined. ' Daphnia c o n s t i t u t e d 63$ o f t h e f o o d o f t r o u t  o f 10 Inches f o r k l e n g t h and under.  The r e l a t i v e importance  o f Daphnia  16 d i m i n i s h e s a s t h e t r o u t grow l a r g e r .  Shiners c o n s t i t u t e d the major food item  o f t r o u t o v e r 14 i n c h e s l o n g (70$ b y volume f o r t h e summer) a l t h o u g h dropped m a r k e d l y i n importance  i n September w h i l e Daphnia and A n i s o p t e r a i n c r e a s -  ed p r o p o r t i o n a l l y i n importance. on s h i n e r s .  Bottom organisms,  they  F i s h under t e n i n c h e s l o n g f e d n e g l i g i b l y  m a i n l y d r a g o n f l y nymphs, made up t h e b u l k o f  the r e s t o f the food o f a l l t h r e e s i z e I n P a u l Lake i n 1955 and  1956  groups.* Crossman  (1957) r e p o r t s  i n importance  o f s h i n e r s and decrease i n importance  frow l a r g e r .  Bottom organisms,  a similar increase  o f plankton as the trout  assumed a g r e a t e r importance  than i n Pinantan  making up about 50$ o f t h e t o t a l d i e t . Few Gammarus o r H y a l l e l a were p r e s e n t i n e i t h e r P a u l o r P i n a n t a n Larkin et a l  1931,  (1950) p o i n t  trout.  out t h a t i n P a u l Lake t h i s i s i n marked c o n t r a s t to  a p r e - s h i n e r y e a r , when amphipods made up  39.8$ o f  t h e d i e t o f P a u l Lake  trout. Diaptomus were c o n s p i c u o u s l y absent from t h e stomachs o f b o t h t r o u t and s h i n e r s d e s p i t e t h e f a c t t h a t t h e y were one o f t h e two most abundant p l a n k t e r s i n b o t h l a k e s . The v e r y l o w a v a i l a b i l i t y t o salmonids o f C y c l o p s a v e r y s i m i l a r copepod - has a l s o been mentioned by Southern (1933), L i n d s t r o m (1955) and N i l s s o n (1955)« N i l s s o n r e p o r t s t h a t w h i l e h i g h p r o d u c t i o n o f p l a n k t o n s p e c i e s i s i n g e n e r a l connected w i t h h i g h consumption o f t h a t s p e c i e s b y c h a r , C y c l o p s composed a s much a s 90$ o f the p l a n k t o n i n Lake B l a j o n y e t c o n s t i t u t e d l e s s t h a n 1$ o f t h e d i e t o f c h a r . Diaptomus. though p r e s e n t i n s m a l l numbers i n t h e l a k e was n e v e r t a k e n i n t h e stomachs o f c h a r . A simple experiment e x p l a i n e d t h i s anomaly. A d u l t Daphnia and D i a p t omus were p l a c e d i n a g l a s s o f l a k e water. The w r i t e r attempted t o c a p t u r e s i n g l e i n d i v i d u a l s o f each s p e c i e s from t h e g l a s s w i t h a n eye d r o p p e r . Daphnia were r e l a t i v e l y easy t o c a t c h b u t i t was v i r t u a l l y i m p o s s i b l e t o c a p t u r e Diaptomus. They i n v a r i a b l y dodged t h e eye d r o p p e r w i t h g r e a t a g i l i t y . Undoubte d l y t h e y do t h e same when a f i s h approaches. T h i s r e a c t i o n appeared t o be a response a t l e a s t i n p a r t t o t h e s i g h t o f the approaching eye dropper. R i c k e r (1932) supposed t h a t t h e appearance o f C y c l o p s i n p l a n k t o n h a u l s i n C u l t u s Lake i n numbers r e l a t i v e l y t o o few was due t o a r h e o t r o p i c r e s p o n s e . The above o b s e r v a t i o n suggests t h a t t h i s o c c u r r e n c e was due a t l e a s t i n p a r t t o t h e a b i l i t y o f t h i s a n i m a l t o f l e e q u i c k l y o n s i g h t i n g an a p p r o a c h i n g o b j e c t .  Average volume of stomach content of Pinantan Lake rainbow trout according to month and length group i n 1958. Number i n bracket under length group i s number of trout i n that group for whole year. Number i n bracket beside month i s the number of trout of a l l sizes i n that month.  14-14* (97)  •  7  Number of stomachs with item  12  8  6  4  3  Percentage with item  40  27  Percentage of total volume  52  17  Number of stomachs with item  25  Percentage with item Percentage of total volume  1 10  3 ii  1  4  4  65  8 29  3 10  10  76  1  *•  3  5  20 13 10 •  9  86 14 43  + Ik  22  63  2  3  14  7 10 4  -  H <D O CO  10  2  9  18  *  29  2  33 16 12 12  3  2  23  54 26 20 20  5  3  4  33  28 18  * 14  5  •  9  29  19 16 16  7  1  5  24  64  42 36 36  16  4 11  4  68  10  15 1  1  2  9 1  •  -  21  4  2  mm  70 13 13 65  2  2  13  2 17  3  3  21  8 68 12 12  2 14  5 32 12  1  5  5  4  '  4  nymphs  13  0  CO  0  5  31 29 36 50 36 3 30 20 40  -  •  1 2 8 4 - - 7  Terrestrial insects  9  6 25  a,  Other bottom organ.  61  5  48  CD -P  Other insect larvae  S  1  7  2  Chironomid larvae  Percentage of total volume  8  9  H  Daphnia  6 11  8 24  2  CO  xi  Shiners  11  Terrestrial insects  69  Other bottom organ.  6  Other insect larvae  Percentage with item  -  4  Chironomid  2  Daphnia  4  Shiners  21  Miscellaneous  2  Other bottom organ.  Other insect larvae  Chironomid larvae  Antisoptera nymphs  Number of stomachs with item  Terrestrial insects  (116)  Daphnia  10-14"  Shiners  6-10" (122)  ITEM  (69 )  ' Miscellaneous  CL62)  &04)  Length group (fork length i n inches)  SEPTEMBER  AUGUST  Anisoptera  JULY  larvae  Table 4.  11 12 37 40 18 13  4  2  16  8  25  5  4  4  29  29  6  5  18 (iii)  Evidence of Competition  f o r Food between Young Trout  and  Shiners Pinantan Daphnia was  the major f o o d o f s h i n e r s and young t r o u t i n P i n a n t a n  T h i s i n i t s e l f i s not s u f f i c i e n t competition  Lake  conclusively.  evidence  As L a g l e r  t o demonstrate t h e e x i s t e n c e o f  (1947)  s t a t e d , i f the f o o d i s abundant  and the f i s h p o p u l a t i o n k e p t r e l a t i v e l y low by o t h e r f a c t o r s , f e e d i n g " L i t t l e e f f e c t on  dance o f Daphnia i n P i n a n t a n Their probable  the present  has  supply.  I t i s i m p o s s i b l e t o determine t h e e f f e c t s o f the  years.  Lake,  s h i n e r s on t h e abun-  Lake d i r e c t l y as t h e r e a r e no d a t a from p r e - s h i n e r  e f f e c t s however may  s i t u a t i o n w i t h one  be i n f e r r e d from a comparison o f  d e s c r i b e d by R i c k e r  (1936) i n  C u l t u s Lake.  R i c k e r showed t h a t the consumption o f Daphnia b y young sockeye saLmon i n 1934  i n C u l t u s Lake was  sufficient  t o reduce t h e a v a i l a b l e s u p p l y and  a r e d u c t i o n i n the growth r a t e o f the sockeye.  The  concentration of  cause  shiners  p r e s e n t a t n i g h t i n the p e l a g i c f e e d i n g ground i n P a u l Lake i s between 1 i n 6 c u b i c m e t e r s and 3 i n 1 c u b i c m e t e r ( c a l c u l a t e d u s i n g d a t a from  1953).  Lindsey,  D i r e c t o b s e r v a t i o n s o f the d e n s i t i e s o f s h i n e r s on the s h o a l s i n  b o t h P a u l and P i n a n t a n l a k e s suggested t h a t t h e i r c o n c e n t r a t i o n s i n t h e s e two  l a k e s were r o u g h l y  similar.  T h i s i s o f the same o r d e r of magnitude  the c o n c e n t r a t i o n o f sockeye i n C u l t u s Lake d u r i n g R i c k e r s 1  (from 2 i n 3 t o 2 i n 7 c u b i c Adult  observations  meters).  shiners i n Pinantan  Lake consumed about 2200 Daphnia p e r  hi t i m e s a s many p e r i n d i v i d u a l as t h e sockeye i n C u l t u s Lake a s by Ricker*  The  as  c o n c e n t r a t i o n o f Daphnia i n P i n a n t a n  day*—  estimated  Lake i n August  1958  T h i s e s t i m a t e i s based on p r e v i o u s l y d e s c r i b e d d i g e s t i o n r a t e experiments, v o l u m e t r i c stomach a n a l y s i s and a count o f the number o f Daphnia per c u b i c c e n t i m e t e r of p u r e Daphnia i n a t r o u t stomach. Stomach c o n t e n t s o f t r o u t r a t h e r t h a n s h i n e r s were used because the p h a r y n g e a l t e e t h o f s h i n e r s fragment t h e i r f o o d so much t h a t t h e enumeration of i n d i v i d u a l p l a n k e r s i s impossible.  19 was  per l i t r e — a b o u t i  about 1.2  t h e cone ent r a t i o n i n G u l t u s Lake.  o f t u r n o v e r o f p l a n k t o n i n P i n a n t a n may  The  rate  be somewhat h i g h e r than t h a t o f C u l t u s  however due t o i t s h i g h e r summer temperature and t o t a l d i s s o l v e d s o l i d  content.  Serious competition f o r food occurred a t the described l e v e l o f food f e e d e r s i n C u l t u s Lake.  I t may  and  be i n f e r r e d from t h e above comparison t h a t  t h e Daphnia i n P i n a n t a n Lake may  a l s o have been kept a t a r e l a t i v e l y low  by s h i n e r s , and t h a t s i n c e the f o o d requirements  of young rainbow t r o u t  level, are  s i m i l a r t o those o f sockeye f i n g e r l i n g s , c o m p e t i t i o n f o r f o o d o c c u r r e d i n P i n a n t a n Lake.  That the growth r a t e o f young P i n a n t a n Lake t r o u t was  i n fact  even slower t h a n the s h i n e r - r e t a r d e d growth r a t e o f young P a u l Lake t r o u t supports t h i s : v i e w . I n summary, t h e r e were p r o b a b l y no fewer t h a n £ the c o n c e n t r a t i o n o f s h i n e r s i n P i n a n t a n as t h e r e were sockeye i n C u l t u s Lake d u r i n g t h e y e a r s b e i n g compared.  These s h i n e r s a t e o v e r f o u r t i m e s as much Daphnia p e r  i n d i v i d u a l p e r day a s sockeye.  The  c o n c e n t r a t i o n o f Daphnia i n P i n a n t a n Lake  i s 5 t h a t o f C u l t u s Lake though t h e r a t e of p r o d u c t i o n i s p r o b a b l y g r e a t e r . C o m p e t i t i o n f o r f o o d was  demonstrated i n C u l t u s Lake.  i n f e r r e d i n P i n a n t a n Lake from t h i s  Competition  f o r food i s  comparison.  P a u l Lake C o m p e t i t i o n f o r food i n P a u l Lake has c e n t e r e d m a i n l y around Amphipods. Gammarus and  H y a l l e l a have d e c r e a s e d i n abundance i n P a u l Lake and  have  decreased markedly i n importance as t r o u t f o o d s i n c e t h e i n t r o d u c t i o n o f shiners. I h 1949,  amphipods i n bottom d r e d g i n g s  1931  abundant as i n t h e p r e - s h i n e r y e a r  were l e s s t h a n one  ( l a r k i n et a l ,  1950).  i n t r o d u c t i o n o f s h i n e r s i n t o P a u l Lake, amphipods made up trout.  In  1931  M o t t l e y and  stomach c o n t a i n e d  16?  Mottley  amphipods.  (1932) r e p o r t  Afterward, i n  third  Before  39.8$ of  as the  the food o f  t h a t the average t r o u t  1947—1948,  t h e y made up  only  20 9.4/5  o f the food o f t r o u t ( L a r k i n et a l ,  1950)  and i n 1955—1956 "even i n d i v i d -  u a l gammarids were r a r e l y p r e s e n t " i n t h e stomachs o f t r o u t . (Grossman,  1957).  F i g u r e 3 shows t h e d e c l i n e o f importance o f amphipods i n t h e d i e t o f  trout* I t was supposed t h a t s h i n e r s had cropped o f f t h e amphipods t o t h i s l o w level.  Pen f e e d i n g experiments were c a r r i e d out t o s u b s t a n t i a t e t h i s hypo-  thesis.  A f t e r 500 Gammarus had been i n t r o d u c e d i n t o each o f t h e h o l d i n g  boxes and a l l o w e d t o seek s h e l t e r i n Chara p l a c e d i n the bottom, t r o u t and s h i n e r s were i n t r o d u c e d .  A l l o t h e r v i s i b l e food had been washed f r o m t h e Chara.  A f t e r t w e n t y - f o u r h o u r s t h e boxes were l i f t e d and t h e remaining  Gammarus  counted. Ten 3 t o 6 i n c h t r o u t and t e n l i t o 3 i n c h s h i n e r s consumed a p p r o x i m a t e l y t h e same number o f Gammarus d u r i n g t h i s t i m e . Gammarus were r e c o v e r e d , c o n t a i n i n g no f i s h .  From t h e t r o u t pen 119  105 f r o m t h e s h i n e r pen and 315 from a c o n t r o l pen  Apparently,  b o t h s h i n e r s and t r o u t e a t l a r g e q u a n t i t i e s  o f Gammarus when t h e y a r e a v a i l a b l e . The p o t e n t i a l g r a z i n g i n t e n s i t y o f t h e s e times t h a t of t h e t r o u t population.  Lindsey  s h i n e r s on Gammarus i s many  (1953)  s h i n e r s i n P a u l Lake was t o o g r e a t f o r a c c u r a t e  s t a t e s t h a t the number o f  e s t i m a t i o n from f i n - c l i p p i n g  experiments b u t t h a t i n 1950 t h e number was somewhere between f i v e m i l l i o n and one  hundred m i l l i o n .  L a r k i n and Smith  to be " s e v e r a l m i l l i o n " . were about  16,000 t r o u t  Crossman  (1953) e s t i m a t e d  (1957) e s t i m a t e s  t h e number i n  1952  t h a t i n 1955-56 t h e r e  o v e r s i x i n c h e s i n l e n g t h i n P a u l Lake.  Even i f t h e r e  were f i v e t i m e s t h i s many t r o u t under s i x i n c h e s l o n g i n t h e l a k e t h e r e were p r o b a b l y a t l e a s t f i f t y t i m e s a s many s h i n e r s a s t r o u t i n t h e l a k e .  During  the summer months thousands o f s h i n e r s can o f t e n be seen a t one g l a n c e the shoal areas.  T h i s l a r g e number o f s h i n e r s i n t h e l a k e , t h e i r  t o e a t amphipods and t h e i r c o n g r e g a t i o n  o v e r the a r e a o f g r e a t e s t  along  readiness concentra-  1.0  Figure 3.  P r o b a b i l i t y o f occurrence of AmDhipods i n stomach contents of rainbow, trout, Paul Lake, 1931 to 1957.  22 tion of amphipods points to a major item of competition. FACTORS AFFECTING TIME AND PLACE OF COMPETITION A study of the distribution and movements of shiners and trout provides knowledge of the areas where both species occur and hence where competition i s most l i k e l y to take place. A.  Distribution and Movements of Shiners Crossman described the seasonal and diurnal movements of shiners i n  Paul Lake.  Their movements i n Pinantan Lake were similar althoughothe trends  were not so clear cut. (i)  SeasonalMovements Data on the distribution of shiners i n Pinantan Lake were derived from  direct observations made day by day during June through August 1958, as well as from gillnet sets on the shoal and offshore. The fish f i r s t appear on the shoals i n schools i n May or June.  In May  1957 McAllister (unpub.) observed a huge "school of 2-3" shiners" about 50 yards down the outlet stream.  Nothing i s known of their winter distribution.  Observations commenced on Pinantan Lake i n the middle of June.  The shiners  on the shoal exhibited the same vertical and horizontal stratification noted by Crossman (1957) i n Paul Lake and Lindsey (1950a) i n Rosebud Lake: the smallest shiners were closest to the surface and closest to shore; the large fish progressively deeper and farther offshore.  On cloudy days a l l but the  2 inch long, newly-hatched f r y moved into deeper water just off the shoal. The shiners were most frequently seen i n schools of from 30 to 500.  Occas-  ional individuals swimming lethargically by themselves were noted throughout the summer. Dead shiners were also seen frequently. Presumably some disease was affecting these fish, although no cause was found.  The incidence of the  tapeworm Ligula which varies from almost 100 to almost 0 per cent i n various  23 y e a r s was  n e g l i g i b l e i n 1958*  I n e a r l y J u l y t h e v e r t i c a l s t r a t i f i c a t i o n i n a l l but the 1958 t o break down g r a d u a l l y .  P r y s t i l l stayed i n segregated  w i t h i n about s i x i n c h e s o f t h e  surface.  f r y seemed  s c h o o l s o f 50 to  Otherwise, d i f f e r e n t s i z e d  fish  i n t e r m i n g l e d i n 1 to 8 f e e t o f water, 1 t o 4 f e e t from the s u r f a c e . i n J u l y the f r y g r a d u a l l y b r o k e up and moved f a r t h e r o f f t h e w i t h the l a r g e r f i s h .  Later  shoal mingling  By J u l y 20 t h e s c h o o l s were d i m i n i s h i n g n o t i c e a b l y i n  s i z e and number; the f i s h were v i s i b l e o f f s h o r e up to 30 i n deeper water.  100  The l a r g e s t f i s h l e f t the  y a r d s o r more and  shoals f i r s t .  An e s t i m a t e d  50$  o f t h e f r y remained on t h e s h o a l d u r i n g t h i s g e n e r a l o f f s h o r e movement. I n a c h a n n e l about e i g t h f e e t deep between an i s l a n d and t h e shore l a r g e s c h o o l s t a y e d about 3 ° f e e t o f f s h o r e , f o r almost a month, s h i f t i n g d u r i n g the day.  I n 0 to 3 f e e t of water was  o f water a s c h o o l o f l a r g e r f i s h  themselves out w i t h i n 10 t o 15  seconds, i n t o t h e t h r e e s e p a r a t e  The l a k e was  s i z e groups.  F i s h i n g and  poisoned  reformed and  gillnetting  i n e a r l y September.  s t r a t i f i e d on  the  Presumably t h e  f i s h would have moved o f f s h o r e en masse i n O c t o b e r as they had observed  three  very productive.  By the end o f August the s c h o o l s had shoals again.  The  but i n v a r i a b l y s o r t e d  These s c h o o l s were c o n t i n u o u s l y p r e y e d on by t r o u t . f o r t r o u t i n t h i s v i c i n i t y was  500  f i s h and i n 2 t o 8 f e e t  (about 3 i n c h e s and o v e r ) .  schools o f t e n i n t e r m i n g l e d through v e r t i c a l mixing  without  a s c h o o l o f about  f r y ; i n . 0 t o 8 f e e t o f water a s c h o o l o f o n e - y e a r - o l d  a  been  t o do i n p r e v i o u s y e a r s by the l o d g e owner and by Crossman (1957)  i n P a u l Lake.  Nothing  i s known o f t h e i r w i n t e r d i s t r i b u t i o n .  L a r k i n (pers.  comm.) has noted t h a t t h e y c o l l e c t around h o l e s cut i n t h e i c e d u r i n g  the  winter. I n J u l y and August s e v e r a l o b s e r v a t i o n t r i p s were made around the p e r i m e t e r o f the l a k e .  S h i n e r s were s t r i k i n g l y absent  from the s h o a l s o f  24 the s o u t h h a l f o f the l a k e , on a l l o c c a s i o n s .  One  c o u l d c r u i s e f o r hundreds  o f y a r d s a t a time i n t h i s a r e a w i t h o u t s e e i n g a s i n g l e s h i n e r w h i l e same t i m e t h e s h o a l o f the n o r t h h a l f was When t h e  almost c o n t i n u o u s l y  s h i n e r s were c o n c e n t r a t e d  on the  shoals  1  at  the  populated.  d u r i n g June and  early  J u l y , s c h o o l s of l a r g e ( o v e r 3 i n c h e s F. L.)  i n d i v i d u a l s were o f t e n seen  t o 40 y a r d s o f f s h o r e on t h e northwest s i d e .  They would o f t e n v i g o r o u s l y  dapple the s u r f a c e .  20  E i t h e r almost a l l the i n d i v i d u a l s i n the s c h o o l would  p a r t i c i p a t e i n t h i s a c t i v i t y o r none a t a l l . I f t h e y were f e e d i n g as no f o o d was d a p p l i n g b e h a v i o u r was  seen.  I t c o u l d not be  I n the  n o t e d o n l y o c c a s i o n a l l y and  ascertained  s c h o o l s n e a r shore t h i s was  c a r r i e d out o n l y  by  single i n d i v i d u a l s .  (ii)  D i u r n a l Movements  S i m i l a r to Crossman's f i n d i n g s , the  s h i n e r s were o b s e r v e d t o  out a l l o v e r the s u r f a c e waters (0 t o 25 f e e t deep) o f the l a k e as  spread nightfall  approached and move back t o shore at the f i r s t l i g h t o f morning. A t 11:30  p.m.,  June 25,  about o n e - t w e n t i e t h of the daytime numbers  were seen by f l a s h l i g h t on the s h o a l randomly d i s t r i b u t e d a c c o r d i n g to h o r i z o n t a l l y and v e r t i c a l l y , except f o r f r y which were s t i l l i n w i t h i n s i x inches of the surface.  At n i g h t s h i n e r s c o u l d be  the s u r f a c e s p o r a d i c a l l y a l l o v e r the l a k e .  size  schools  seen  dimpling  I t c o u l d n o t be a s c e r t a i n e d i f  t h e y were f e e d i n g . Two 1.  The  p o s s i b l e reasons f o r t h i s o f f s h o r e movement p r e s e n t  themselves;  s h i n e r s move out i n response to an upward m i g r a t i o n o f Daphnia  which t h e y f e e d e x t e n s i v e l y a t n i g h t . the shore a t n i g h t and  2.  on  They l o s e t h e i r o r i e n t a t i o n t o  swim about a t random h o r i z o n t a l l y .  L a r k i n e t a l (1950) mentions a s l i g h t downward m i g r a t i o n o f Daphnia i n P a u l Lake from 4:30  t o 10:30  from 7:30  a.m.  p.m.  t o 4:30  a.m.  and  a compensatory upward m i g r a t i o n  However even a t the peak o f t h e downward  25 m i g r a t i o n t h e p r o p o r t i o n o f Daphnia below 12 meters never exceeded 25$. Pinantan  Lake a s e r i e s of m i d n i g h t and noon p l a n k t o n  10 f e e t and  20 f e e t d i d not i n d i c a t e any  tows a t d e p t h o f 1 f o o t ,  s i g n i f i c a n t d i f f e r e n c e between n i g h t  and day i n t h e number of Daphnia i n t h e upper t e n f e e t o f water, the i n which the shiners are concentrated a l o s s o f o r i e n t a t i o n to t h e reasonable  B.  at n i g h t .  Hence the second  explanation,  theory.  D i s t r i b u t i o n and Movement o f  l e a s t i n 1952 any one  area  shore i n t h e dark appears t o be t h e more  Trout  Tagging s t u d i e s done by Crossman (1957) i n 1952  at  In  indicated that  t h e r e were no d i s c r e t e p o p u l a t i o n s o f t r o u t at any one  t i m e " (except  during the  spawning m i g r a t i o n )  "at place  i n P a u l Lake.  "The  t r o u t seemed t o move about f r e e l y from p l a c e to p l a c e o v e r the l e n g t h o f l a k e , a t times moving from one Observations  and  end t o the  other".  r e s u l t s o f g i l l n e t t i n g on b o t h P a u l and  Pinantan  l a k e s i n d i c a t e t h a t d u r i n g t h e summer the l a r g e t r o u t (10 i n c h e s and t e n d to s t a y around and,  the  i n P a u l , below t h e t h e r m o c l i n e .  Mottley  larger)  and  J f o t t l e y (1932) s t a t e t h a t t h e o l d e r f i s h i n P a u l Lake "seek g r e a t depths" during the  summer.  Recently planted hatchery  were seen however i n l a r g e numbers on t h e  f i n g e r l i n g s (2-5 i n c h e s  s h o a l s i n as l i t t l e  F.L.)  as a f o o t o f  water, o f t e n swimming i n company w i t h s c h o o l s of s h i n e r s , i n J u l y 1959» Large t r o u t were f r e q u e n t l y o b s e r v e d by Crossman (1957) and w r i t e r to make q u i c k dashes i n t o s h a l l o w water ( a s l i t t l e w h i l e c h a s i n g and f e e d i n g on s h i n e r s . water o f f t h e  s h o a l when t h e y had  as one  this  f o o t deep)  They r e t u r n e d i m m e d i a t e l y i n t o deeper  caught o r l o s t t h e i r p r e y .  This  c o u l d be o b s e r v e d almost c o n t i n u o u s l y i n r e s t r i c t e d a r e a s of the warm b r i g h t days i n l a t e J u l y and August i n b o t h l a k e s . movements a r e d e s c r i b e d i n d e t a i l by Crossman (1957)*  activity  shoals  These f e e d i n g  on  20  I G.  Time and. P l a c e of  Competition  The o n l y a r e a where t r o u t and numbers d u r i n g t h e summer was o f water.  s h i n e r s were b o t h o b s e r v e d i n  j u s t o f f t h e s h o a l s i n the upper t w e n t y f e e t  T r o u t but n o t s h i n e r s a l s o ranged below t h i s d e p t h .  were e v e r t a k e n a l o n g w i t h s h i n e r s i n t h e o f f s h o r e g j l l n e t was  suspended from t h e s u r f a c e and was  f e e d i n g by t r o u t was observations  No t r o u t  sets.  Almost no  o b s e r v e d o f f s h o r e by n i g h t ( o r day)  d u r i n g the  Lake.  Apparently  trout during t h e i r offshore night  May.  Lindsey  (1950a)  dormant i n the mud  gLllnet  surface summer  s h i n e r s do n o t encounter many  migrations.  L i t t l e i s known o f the d i s t r i b u t i o n o f s h i n e r s f r o m l a t e to  (The  e i g h t f e e t deep).  i n Pinantan  any  s t a t e s t h a t a few  September  s h i n e r s have been found l y i n g  and among bottom d e b r i s d u r i n g t h e w i n t e r .  Larkin (pers.  comm.) o b s e r v e d s h i n e r s g a t h e r i n g around h o l e s c u t i n t h e i c e i n P a u l L a k e . Except f o r t h i s one at  o b s e r v a t i o n s h i n e r s have not been seen on the  the s u r f a c e d u r i n g the w i n t e r .  i t e m f o r t r o u t o v e r 14 o f 78  1953  i n c h e s l o n g i n t h e summer, t h e y were found i n o n l y  (Crossman,  b u t i o n s of t r o u t and  1957)*  very  e n t e r e d P a u l Lake (Crossman, i n t e n s e d u r i n g the  T h i s would suggest t h a t the w i n t e r  s i m i l a r to what i t was  1957).  Probably  distri-  before  competition  shiners  The  had  f o r food i s not  winter.  BEHAVIOURAL FACTORS IN COMPETITION The p r e v i o u s s e c t i o n s have d e a l t w i t h what c o m p e t i t i o n where i t o c c u r s . important  Observation  of the b e h a v i o u r o f the two  i n o r d e r t o determine how  competition  i s f o r and  species i s  o c c u r s and,  i s more s u c c e s s f u l t h a n t h e o t h e r , what a b i l i t i e s  two  inches long taken i n  s h i n e r s i n P a u l Lake do not o v e r l a p a p p r e c i a b l y .  d i e t o f t h e s e t r o u t was  or  While s h i n e r s c o n s t i t u t e the major f o o d  stomachs t a k e n f r o m P a u l Lake t r o u t o v e r 14  December  shoals  i f one  species  o r habits i t possesses  J7 which make i t more s u c c e s s f u l . Crossman nature.  (1957)  s t a t e s t h a t he observed  no i n t e r s p e c i f i c aggression i n  The s h i n e r s however "appeared to be more e f f i c i e n t f e e d e r s and when  a t r o u t and s h i n e r d a r t e d a f t e r t h e same f o o d i t e m , t h e s h i n e r i n v a r i a b l y g o t it  and w h i l e s h i n e r s would move r i g h t i n t o t h e shore t o f e e d , t r o u t d i d n o t  come i n t o water s h a l l o w e r t h a n 15 i n c h e s . " I n t h e course o f ssummer o b s e r v a t i o n s t h e p r e s e n t w r i t e r saw many t r o u t 3 t o 5 i n c h e s l o n g swimming i n company w i t h , o r i n and out o f , s c h o o l s o f s h i n e r s i n 2 t o $ f e e t o f water.  There appeared t o be no i n t e r s p e c i f i c  a g r e s s i o n except i n one i n s t a n c e when a s i x i n c h t r o u t was "chased" f o r about t h r e e f e e t by a ljg i n c h s h i n e r . v i o u s t o one a n o t h e r .  Behaviour  s p e c i e s appeared t o be o b l i -  S h i n e r s however moved n o t i c e a b l y f a s t e r and hence ranged  o v e r a w i d e r a r e a p e r u n i t time  A.  Otherwise b o t h  i n s e a r c h o f food t h a n t r o u t d i d .  o f T r o u t i n a Pen M t h o u t  Shiners  An o b s e r v a t i o n pen was marked o f f h o r i z o n t a l l y i n n i n e e q u a l by s t r i n g s s t r e t c h e d a c r o s s t h e pen along the s u r f a c e . F.L.)  Trout  squares  (3 t o 5 i n c h e s  remained a t t h e s i d e s o r c o r n e r s o f t h i s open-bottom pen j u s t above o r  down among t h e dense weed growth, except when t h e y were f e e d i n g .  Observations  f o r s e v e r a l days showed t h a t a c t i v i t y was g r e a t e r d u r i n g b r i g h t weather.  In  t h e b r i g h t e s t p a r t o f t h e day, o u t o f t e n t r o u t , some s i n g l e i n d i v i d u a l would move from t h e a r e a under one square i n t o another  e v e r y one t o two minutes..  On d u l l days t h e r e was v i r t u a l l y no movement except when f o o d was i n t r o d u c e d . Only 4 o r 5 o f t h e t e n f i s h e x h i b i t e d t h i s b e h a v i o u r o v e r about tv*> dozen f i f t e e n minute o b s e r v a t i o n s s p r e a d o v e r s e v e r a l d a y s j some i n d i v i d u a l s remained c o n t i n u a l l y h i d d e n i n t h e weeds.  There was some e s t a b l i s h m e n t o f  t e r r i t o r i e s , and n i p p i n g o c c u r r e d a s o f t e n a s t h r e e t i m e s p e r minute i n b r i g h t sunlight.  The same t e r r i t o r y was n o t h e l d by a f i s h i n d e f i n i t e l y .  There  28 appeared t o be no unchanging  o r d e r o f dominance o r "peck o r d e r " a l t h o u g h t h e  l a r g e s t f i s h o f t h e t e n appeared t o dominate a l l the o t h e r s . i n d i v i d u a l v a r i a t i o n was seen i n t h e s e a c t i v i t i e s .  1  Considerable  I n some c a s e s one t r o u t  was o b s e r v e d c h a s i n g another t r o u t l a r g e r than i t s e l f .  (These o b s e r v a t i o n s  on dominance, n i p p i n g , t e r r i t o r i e s and t h e c o r r e l a t i o n o f l i g h t w i t h a c t i v i t y agree w e l l w i t h t h o s e o f S t r i n g e r and Hoar  intensity  (1954) i n t h e  laboratory). The f i s h p r e f e r r e d shaded a r e a s i n t h e pen about areas.  s i x t o one o v e r  sunlit  During o v e r c a s t days and i n t h e evenings t h e r e were no s i g n s o f movement  and t h e t r o u t went deeper among t h e weeds. There was n e v e r a t e r r i t o r y e s t a b l i s h e d near t h e c e n t e r o f t h e pen. O c c a s i o n a l l y a swimming t r o u t on " f i n d i n g i t s e l f " i n the m i d d l e o f t h e pen would appear f r i g h t e n e d and d a r t q u i c k l y f o r c o v e r among t h e weeks a t a s i d e o r c o r n e r o f t h e pen. The t r o u t were l e f t i n t h e pens f o r a week w i t h o u t any f o o d except t h e n a t u r a l f o o d t h e y might f i n d i n the. weeds.  A t the end o f t h i s time no f e e d i n g  was observed and i t was assumed t h a t t h e t r o u t had e a t e n a l l a v a i l a b l e food.  natural  About 2000 Gammarus, most o f which were a l i v e , were p l a c e d i n t h e pen  w i t h t h e t r o u t which were a t t h e time a l l down among t h e weeds.  The l a r g e s t  (about 5 i n c h e s F.L.) t r o u t immediately came o u t o f the weeds and made 54 f e e d i n g movements ( c o n s i s t i n g o f d a r t i n g a t and swallowing a Gammarus) i n the space o f f o u r minutes.  Three s m a l l e r t r o u t  (3 t o 4 i n c h e s ) s t a r t e d f e e d i n g  w i t h i n a minute b u t t h e y appeared more wary t h a n t h e l a r g e t r o u t and r e t u r n e d i m m e d i a t e l y t o t h e i r t e r r i t o r y among t h e weeds a f t e r each f e e d i n g movement. The l a r g e s t t r o u t o f t e n made s i x t o e i g h t f e e d i n g movements b e f o r e r e t u r n i n g t o his  home t e r r i t o r y .  W i t h i n f i v e minutes a l l t e n t r o u t were f e e d i n g w i t h v a r y i n g  degrees o f a c t i v i t y which seemed t o be r e l a t e d t o t h e i r s i z e . t r o u t would pursue t h e same shrimp and c o l l i d e .  Sometimes two  The f i s h o f t e n n i p p e d and  29  chased each o t h e r even when n e i t h e r f i s h was  i n i t s habitual territory.  A t t h e end o f f i f t e e n minutes few Gammarus were i n s i g h t and f e e d i n g a c t i v i t y had dropped t o t h r e e movements per minute.  The t r o u t o f t e n m i s t o o k  t h e t i p s o f Chara shoots f o r Gammarus, mouthed them, t h e n s p a t them o u t . Almost w i t h o u t e x c e p t i o n no t r o u t s t r a y e d f a r t h e r t h a n one  square i n any  d i r e c t i o n from i t s home t e r r i t o r y *  Food was  o f two  (3 to 4 i n c h e s ) o c c a s i o n a l l y s p a t out l a r g e  f e e t or l e s s .  Smaller t r o u t  u s u a l l y c a p t u r e d by a sudden dash  Gammarus t h r e e o r f o u r t i m e s b e f o r e f i n a l l y swallowing completely.  them, o r r e j e c t i n g them  The t r o u t took dead Gammarus r e s t i n g on t h e weeds as w e l l a s  Gammarus i n m o t i o n .  The t r o u t seemed t o f e e d a t random, making e r r a t i c  rushes  h e r e and t h e r e , o f t e n i g n o r i n g Gammarus n e a r them and r u s h i n g a f t e r o t h e r s f a r t h e r away.  B.  Behaviour o f S h i n e r s i n a Pen Without T r o u t U n l i k e t h e t r o u t , s h i n e r s s t a y e d i n one compact s c h o o l i n an a r e a o f  about 8 c u b i c f e e t j u s t above and among the t i p s o f the weeds.  O n l y when  f r i g h t e n e d by q u i c k movements o f t h e o b s e r v e r d i d t h e y s c a t t e r down i n t o weeds.  The  the  s c h o o l moved v e r y s l o w l y o v e r t h e whole a r e a o f the pen o f t e n  s t a y i n g more o r l e s s i n one s p o t f o r t e n m i n u t e s o r more b u t n e v e r seeming t o f a v o r any spot n e a r e r t h e s i d e s o r coarners t h a n the c e n t e r o f the pen.  No  f e e d i n g movements were seen a f t e r the f i r s t day i n the p e n — p r e s u m a b l y t h e y had c l e a n e d out the f o o d . was  No c o r r e l a t i o n o f a c t i v i t y w i t h l i g h t  intensity  noted. When Gammarus were i n t r o d u c e d s h i n e r s appeared  and m e t h o d i c a l l y t h a n t r o u t . n e a r them t h a t was  t o f e e d more e f f i c i e n t l y  They moved as a s c h o o l e a t i n g e v e r y Gammarus  v i s i b l e t o the o b s e r v e r as w e l l as making f e e d i n g movements  a t o b j e c t s too s m a l l f o r t h e o b s e r v e r t o see.  ( T h i s was  not so w i t h the t r o u t , ,  The o b s e r v e r c o u l d i n v a r i a b l y see t h e o b j e c t s on w h i c h they were f e e d i n g ) . The  s h i n e r s f e d more s l o w l y than the t r o u t , making a p p r o x i m a t e l y h a l f the number  30 o f f e e d i n g movements per i n d i v i d u a l p e r u n i t time as d i d t h e t r o u t .  They  pursued Gammarus s e v e r a l i n c h e s f u r t h e r down i n t o the weeds t h a n the t r o u t would venture  G.  a f t e r them.  They o f t e n spat o u t and r e j e c t e d l a r g e r Gammarus.  B e h a v i o u r o f Trout and There was  The  Together  no observed d i f f e r e n c e between t h e b e h a v i o u r o f the two  i n the p r e s e n c e o f one o r without  Shiners  a n o t h e r and t h e i r b e h a v i o u r when i n s e p a r a t e pens, w i t h  food. s h i n e r s , b e i n g a t f i r s t h i g h e r above the weeds, were on t h e whole  f a s t e r t o n o t i c e the Gammarus and  f a s t e r t o s t a r t f e e d i n g than t h e t r o u t .  the t r o u t s t a r t e d f e e d i n g t h e y ranged h i g h e r above t h e weeds t h a n the and t h e  shinens I n the  shiners  ranged l o w e r among the weeds t h a n the t r o u t , (see F i g u r e s 4 &  surface to feed.  S h i n e r s made none.  were s t i l l twelve l a r g e Gammarus f l o a t i n g on the  and  When  5).  space o f h a l f an hour however, t r o u t made o n l y t h r e e movements  r i g h t up t o the  nipped  species  F i v e hours l a t e r  surface.  and chased each o t h e r but n e v e r chased s h i n e r s .  s h i n e r would c o l l i d e w h i l e moving i n s e a r c h o f f o o d .  Trout  there  often  Occasionally a trout As the t r o u t were  l a r g e r and moving f a s t e r , the s h i n e r s were u s u a l l y pushed a s i d e i n t h i s collision. frequent. was  I n c i d e n t s o f t h i s n a t u r e appeared a c c i d e n t a l however and were i n I n o n e - h a l f hour o f i n t e r m i n g l e d f e e d i n g o f the two  o b s e r v e d to chase o r n i p a s h i n e r .  chased a f o u r i n c h t r o u t f o r about one  s p e c i e s no t r o u t  On one o c c a s i o n o n l y , one  two-inch  f o o t h o r i z o n t a l l y . Both, s h i n e r s  shiner  and  t r o u t o c c a s i o n a l l y m i s t o o k t h e t i p s of Chara p l a n t s f o r Gammarus. mouthed them, then r e j e c t e d them. O c c a s i o n a l l y when a t r o u t mouthed a Gammarus then s p a t i t out as  descr-  i b e d p r e v i o u s l y a s h i n e r would eat i t b e f o r e the t r o u t had a chance t o mouth it  again.  T h i s was  t h e o n l y t y p e o f o v e r t c o m p e t i t i o n f o r the  i t e m o b s e r v e d and was  seen o n l y t w i c e i n o n e - h a l f  hour.  same f o o d  Figure 4.  Diagrams of the D i s t r i b u t i o n of Trout and Shiners i n the Observation Pens when not Feedingo  Diag. A.  Schematic Side View.  Diag. B.  View Looking Down on Pen.  Trout remain at the sides or corners of the pen just above or down among the weeds. Shiners hover i n a compact school above and mong the weed t i p s , favoring neither center nor sides of the pen« Shiner <^==~ Trout •yTrout hidden i n weeds  Figure 5. Diagrams of the Distribution of Trout and Shiners i n the Observation Pens when Feeding.  Diag. A.  Schematic Side View.  Diag. B.  View Looking Down on Pen.  Trout range h i g h e r i n the water than s h i n e r s . S h i n e r s go deeper i n t o t h e weeds f o r f o o d than t r o u t . ( T r o u t p i c t u r e d i n t h e weeds have j u s t r e t u r n e d from f e e d i n g e x c u r s i o n and do not f e e d while i n the weeds) Shiners s t i l l s c h o o l though l e s s compactly than when not f e e d i n g . Trout do n o t s c h o o l . The two s p e c i e s i n t e r m i n g l e as though o b l i v i o u s t o each o t h e r . 0  Shiner -Trout Trout hidden i n weeds.  33 In subsequent o b s e r v a t i o n s the same a c t i v i t i e s were n o t e d .  S h i n e r s seemed  t o f e e d more q u i c k l y when t h e y had been s t a r v e d f o r a l o n g e r p r i o d o f  MECHANISMS OF COMPETITION FOR  time.  FOOD  I t has been s t a t e d t h a t t r o u t would not f o r a g e down among t h e weeds i n s e a r c h o f food l i k e  shiners.  A second l i n e o f e v i d e n c e  a p p l i c a b i l i t y o f t h i s o b s e r v a t i o n i n the l a k e .  supports t h e g e n e r a l  Gammarus  were abundant down  to a d e p t h o f 50 m e t e r s i n P a u l Lake i n p r e - s h i n e r y e a r s of low t r o u t abundance (Rawson,  1934)•  When t h e t r o u t p o p u l a t i o n was  i n c r e a s e d a s a r e s u l t o f heavy  s t o c k i n g , Gammarus below the Chara zone d e c r e a s e d m a r k e d l y i n abundance w h i l e i n t h e Chara zone t h e y d e c r e a s e d o n l y s l i g h t l y ( L a r k i n e t a l ,  1950).  Chara  appears t o p r o v i d e amphipods w i t h r e l a t i v e l y e f f e c t i v e s h e l t e r from the p r e d a t i o n of trout. I n t e r s p e c i f i c a g g r e s s i o n i s not a f a c t o r i n c o m p e t i t i o n f o r food between t r o u t and  shiners.  None was  observed i n the l a k e n o r i n the o b s e r v a t i o n pens.  There a r e f i v e o b s e r v a t i o n s which suggest  that trout are at a  disadvan-  t a g e when competing w i t h s h i n e r s f o r amphipods i n P a u l L a k e . 1.  S h i n e r s range deeper among t h e weeds i n s e a r c h o f f o o d t h a n  trout,  c r o p p i n g o f f many bottom organisms b e f o r e t h e y move out i n t o a r e a s i n which t h e y a r e a v a i l a b l e t o t r o u t . 2.  S h i n e r s appear t o be a b l e t o u t i l i z e food i t e m s s m a l l e s t i t e m s t a k e n by t r o u t .  Consequently  smaller than  t h e y may  the  graze o f f Large  numbers of smphipods b e f o r e t h e y reach a s i z e a t which t h e y become available to trout.  They may  a l s o g r a z e o f f a much h i g h e r p r o p o r t i o n  o f immature amphipods than t r o u t , l e a v i n g fewer to s u r v i v e l o n g enough t o reproduce  and r e p l e n i s h t h e amphipod p o p u l a t i o n .  b y s h i n e r s d u r i n g f e e d i n g experiments averaged a s t h o s e l e f t by  trout).  (Amphipods l e f t  t h r e e t i m e s as l a r g e  35 3.  I n t h e summer d u r i n g t h e day shoals. and  4.  i n deeper water not  so c l o s e t o the main source o f amphipods.  S h i n e r s a r e more m e t h o d i c a l for their  There a r e p r o b a b l y 50  Competition  f e e d e r s t h a n t r o u t , s e a r c h i n g an a r e a more  food. o r more s h i n e r s f o r e a c h t r o u t i n t h e  f o r Daphnia i n P i n a n t a n  lake.  Lake does not f a c i l i t a t e ' • study  d e s c r i p t i o n as r e a d i l y as the above s i t u a t i o n . however.  d i r e c t l y over the  T r o u t tend a l s o t o be n e a r the s h o a l s , but f a r t h e r o f f s h o r e  thoroughly 5.  s h i n e r s are c o n c e n t r a t e d  and  S e v e r a l f a c t s a r e apparent  S h i n e r s spread o u t i n t h e p e l a g i c a r e a s o f t h e l a k e o n l y a t n i g h t .  T r o u t appear t o be i n t h i s a r e a o n l y d u r i n g t h e day. t h e a n g l i n g success  This i s evidenced  by  o f f s h o r e d u r i n g the day w h i l e no t r o u t were ever caught i n  o f f s h o r e g i l l n e t s e t s d u r i n g the n i g h t .  Presumably the two  s p e c i e s do  not  o v e r l a p a p p r e c i a b l y i n t h e i r occupancy of t h e p e l a g i c -'feeding ^afea*. 1  A t t h e p r e s e n t l e v e l o f g r a z i n g t h e r e remained about 1200 p e r c u b i c meter i n t h e epili.mni.on i n l a t e summer. s h o r t because the p l a n k t o n was  Daphnia (2 c c  Presumably t r o u t were g o i n g  spread out r e l a t i v e l y d i f f u s e l y and t h e y were  unable t o f e e d on i t a s r a p i d l y as i n p r e - s h i n e r  years.  There were r e l a t i v e l y few amphipods i n P i n a n t a n Lake compared w i t h P a u l Lake when s h i n e r s were f i r s t i n t r o d u c e d , a c c o r d i n g t o Rawson (1934). He a t t r i b u t e d t h i s to. the absence o f any e x t e n s i v e Chara zone i n S i n c e t h i s time a v e r y r i c h and  Pinantan.  e x t e n s i v e Ghara zone has d e v e l o p e d .  This  p l a n t a f f o r d s amphipods c o n s i d e r a b l e p r o t e c t i o n f r o m the p r e d a t i o n o f t r o u t and i t i s p r o b a b l e  t h a t a l a r g e amphipod p o p u l a t i o n would have d e v e l o p e d  a l o n g w i t h t h e Chara had  s h i n e r s not been p r e s e n t t o graze them down.  has a l r e a d y been mentioned t h a t s h i n e r s p e n e t r a t e weeds much deeper i n  (It search  o f food t h a n t r o u t ) . I n P a u l Lake an e x t e n s i v e Chara zone and a l a r g e p o p u l a t i o n o f amphipod was  a l r e a d y p r e s e n t when s h i n e r s a n t e r e d the l a k e .  I t i s p o s s i b l e t h a t when  36 t h e amphipods i n P a u l Lake a r e reduced by s h i n e r s t o a l e v e l o f abundance comparable t o t h a t i n Pinantan main f o o d , Pinantan  Lake t h e s h i n e r s w i l l s h i f t t o Daphnia a s t h e i r  A second phase o f c o m p e t i t i o n p a r a l l e l i n g t h a t d e s c r i b e d i n  Lake might t h e n ensue.  Nilsson  (1955) r e p o r t s  t h a t c h a r i n Swedish  l a k e t u r n f r o m a d i e t o f bottom organisms t o p l a n k t o n when t h e f o r m e r become scarce. An a l t e r n a t i v e h y p o t h e s i s i s t h e p o s s i b l e development o f a new r e l a t i o n s h i p between t r o u t and s h i n e r s i n Paxil Lake u n p r e d i c t a b l e o n t h e b a s i s o f what has o c c u r r e d i n P i n a n t a n .  As Crossman  (1957) has  s t a t e d , r e a c t i o n s of animals  t o t h e environment may p o s s i b l y c r e a t e d i f f e r e n t i n t e r a c t i o n s i n n e a r l y environements.  similar  D i f f e r e n c e s between t h e two l a k e s , i . e . d e p t h , oxygen, p r o f i l e ,  may be such t h a t t h e f u t u r e r e l a t i o n s h i p o f t r o u t and s h i n e r s i n P a u l Lake w i l l have l i t t l e  i n common w i t h t h e s i t u a t i o n d e s c r i b e d i n P i n a n t a n  Lake.  SUMMARY  A.  Items Competed F o r T r o u t and s h i n e r s  Paul o r Pinantan  Lakes.  do not compete f o r space o r f o r spawning a r e a i n The reduced growth r a t e o f y e a r l i n g t r o u t s i n c e t h e  iitizissduetionof s h i n e r s i n t o t h e l a k e s i s t h e r e s u l t o f c o m p e t i t i o n  f o r food.  S h i n e r s have d r a s t i c a l l y reduced the abundance o f amphipods i n P a u l Lake. S h i n e r s and y e a r l i n g t r o u t compete f o r Daphnia i n P i n a n t a n  B.  Mechanisns o f  Lake.  Competition  Y e a r l i n g t r o u t and s h i n e r s show no b e h a v i o u r a l i n t e r f e r e n c e w i t h one a n o t h e r when swimming t o g e t h e r . when o n l y one s p e c i e s i s p r e s e n t . r o l e i n competition During  T h e i r b e h a v i o u r i s i d e n t i c a l w i t h what i t i s D i r e c t p h y s i c a l aggression plays a minimal  between t h e two s p e c i e s .  d a y l i g h t i n t h e summer t h e main c o n c e n t r a t i o n o f s h i n e r s i s o v e r  37 t h e f o o d - r i c h s h o a l areas whereas t h e t r o u t a r e somewhat deeper and f a r t h e r offshore.  S h i n e r s go deeper i n t o t h e weeds i n s e a r c h o f food t h a n do t r o u t .  They appear t o i n c l u d e i n t h e i r d i e t f o o d p a r t i c l e s s m a l l e r t h a n t h e s i z e e a t e n by t r o u t .  smallest  I t i s t h e s e f a c t o r s i n the s h i n e r ' s b i o l o g y and  distri-  b u t i o n p l u s i t s enormous numbers which have caused t h e d e p l e t i o n o f amphipods i n P a u l Lake and the d e c l i n e i n the growth r a t e o f young t r o u t . I n P i n a n t a n Lake s h i n e r s have a p p a r e n t l y reduced the d e n s i t y o f Daphnia i n t h e e p i l i m n i o n to a p o i n t where t r o u t a r e unable to f e e d on them as r a p i d l y as i n p r e - s h i n e r  years.  38 DISCUSSION  While t h e p r e s e n t study c o v e r s p a r t i c u l a r s i t u a t i o n s ^ i n o n l y two t h e r e s u l t s have b r o a d e r  significance.  lakes  The d a t a w i l l be c o n s i d e r e d below a s  t h e y a p p l y t o t h e phenomena o f i n t e r s p e c i f i c c o m p e t i t i o n i n g e n e r a l and  fish  competition i n p a r t i c u l a r . Competiton i n n a t u r e i s u s u a l l y f i r s t r e c o g n i z e d o n l y a f t e r i t s r e s u l t s have become a p p a r e n t .  U s u a l l y a n o t i c e a b l e change i n one  o f the  competing  p o p u l a t i o n s ( i . e . r e d u c t i o n , e x t i n c t i o n , e m i g r a t i o n ) must o c c u r b e f o r e o b s e r v e r becomes aware t h a t c o m p e t i t i o n i s t a k i n g p l a c e .  Discovering the  items and mechanisms o f c o m p e t i t i o n under t h e s e c i r c u m s t a n c e s t r y i n g t o understand  a n o v e l a f t e r r e a d i n g o n l y the l a s t  i s like  chapter.  The p r e s e n t s i t u a t i o n i n P a u l Lake s e r v e s to i l l u s t r a t e how i m p l i c a t i o n s may  the  a r i s e from a d e l a y e d a p p r a i s a l o f c o m p e t i t i o n .  false  In recent  y e a r s amphipods were s c a r c e i n t h e l a k e and formed o n l y a s m a l l f r a c t i o n o f t h e stomach c o n t e n t s o f t r o u t and  shiners.  On the b a s i s o f t h i s  observation  a l o n e an o b s e r v e r would h a r d l y s u s p e c t t h a t amphipods were t h e most item of competition.  What i s more, a s t h e r e i s r e l a t i v e l y l i t t l e o v e r l a p i n  the p r e s e n t f e e d i n g h a b i t s o f t h e two c o m p e t i t i o n f o r f o o d was example o f H a r t l e y ' s  important  s p e c i e s one might conclude  not o c c u r r i n g a t a l l .  (1948) statement  that  The p r e s e n t case i s an  t h a t , "the f i n d i n g o f d i f f e r e n t  i n d i f f e r e n t s p e c i e s i s not i r r e f u t a b l e p r o o f o f t h e absence o f  foods  competition  u n l e s s i t can be shown t h a t a l l s e l e c t i o n o f f o o d s i s by c h o i c e a l o n e f r o m d i v e r s e superabundant f o o d s t o c k s a l l e q u a l l y a c c e s s i b l e t o the studied."  species  O b s e r v a t i o n s made b e f o r e and d u r i n g the f i r s t phases o f  §©mp§ti-  t i o n showed t h a t s h i n e r s and t r o u t were not f e e d i n g by c h o i c e a l o n e .  They  have been f o r c e d by t h e i r d e p l e t i o n o f amphipods to r e p l a c e them i n t h e i r d i e t s w i t h o t h e r , presumably l e s s p r e f e r r e d f o o d s .  39 T h i s s h i f t i n t h e d i e t s o f t r o u t and s h i n e r s i n P a u l Lake s i n c e compet i t i o n began s e r v e s t o c a u t i o n a g a i n s t assuming t h a t the p r e s e n t f e e d i n g h a b i t s o f t h e two there.  s p e c i e s i n P i n a n t a n Lake t e l l t h e whole s t o r y o f  competition  P o s s i b l y t h e most important o r i g i n a l items and mechanisms o f  competi-  t i o n i n P i n a n t a n were q u i t e d i f f e r e n t from what they were a t the time o f this  study. Another i m p o r t a n t  consideration i n studying competition i s :  d i r e c t i o n does one l o o k t o f i n d causes and mechanisms?  i n what  Theoretical  approaches t o p o p u l a t i o n iris^actions d e a l m a i n l y w i t h the c h a r a c t e r i s t i c s o f t h e competing p o p u l a t i o n s , i . e . numbers o f c o m p e t i t o r s , b i r t h r a t e s , d e a t h r a t e s and r e p r o d u c t i v e p o t e n t i a l s .  They have f o c u s s e d l i t t l e a t t e n t i o n upon  the b e h a v i o u r o f the c o m p e t i t o r s and t h e m o d i f y i n g e f f e c t s o f t h e environment. The p r e s e n t study i n d i c a t e s t h a t b o t h t h e s e f a c t o r s may r a b l e b e a r i n g on the dynamics o f c o m p e t i t i o n .  Environmental  have c o n s i d e -  f a c t o r s such  l a k e morphology, p l a n t growth temperature and oxygen p r o f i l e s and i n t e n s i t y p l a y important competition.  as  light  r o l e s i n d e t e r m i n i n g the i t e m s , a r e a s and t i m e s o f  I n d i r e c t l y t h e y may  a l s o i n f l u e n c e the i n t e n s i t y o f c o m p e t i t i o n  b y d e t e r m i n i n g the d i s t r i b u t i o n o f t h e two  s p e c i e s and t h e i r degrees o f  overlap. Items o f c o m p e t i t i o n a r e a t l e a s t i n p a r t determined  by t h e i r  b i l i t y and t h i s i n t u r n i s s t r o n g l y i n f l u e n c e d b y p l a n t growth.  availa-  In Paul  Lake where Chara and c o n s e q u e n t l y amphipods were abundant, t h e main food i t e m i n c o n t e n t i o n was  amphipods.  On t h e o t h e r hand, i n P i n a n t a n Lake,  because o f t h e r e l a t i v e l y sparse Chara zone a t the time o f t h e i n t r o d u c t i o n of  s h i n e r s , few amphipods were p r e s e n t and competiton Environmental  c e n t e r e d around Daphnia.  f a c t o r s l i m i t i n g the t i m e s and areas o f competiton  are:  1.  the temporal  2.  the summer h y p o l i m n i a l oxygen d e f i c i t i n P i n a n t a n Lake which keeps  and  s p a t i a l d i s t r i b u t i o n of food,  40 fish i n the hypolimnion, 3»  High summer temperatures on the shoal apparently preventing trout from occupying i t more than momentarily,  4.  Some diurnal variable - probably light intensity - which causes shiners to occupy offshore waters and feed appreciably on B&phnia only at night. As well as these more of less constant or cyclic environmental influen-  ces there are also less predictable year tor year influences.  Some unknown  epidemic caused a mass mortality of shiners i n Paul Lake i n August 1958,  As  a consequence shiners were found far less frequently i n the stomach contents of trout i n 1959 than i n early 1958.  Amphipods were significantly more  abundant i n the trout diet i n 1959, presumably because there were fewer shiners to graze them down. Infestation by the tapeworm, Ligula. i n shiners varied from almost 100 to almost 0 percent i n the lakes from year to year.  Ligula-invested shiners  are not only sluggish and more vulnerable to the predation of trout, but also they have extremely small amounts of food i n their stomachs compared to uninfested f i s h .  Probably shiners consumed significantly less food per  individual i n years when they were heavily parasitized. Year to year differences i n r a i n f a l l and atmospheric temperature cause an almost two-fold difference i n the summer heat income of Paul Lake (Larkin et a l , 1950) and a five degree f&hrenheit difference i n the summer epilimnal temperature (McAllister, unpub.). Annual fluctuations of this magnitude might be expected to exert a significant influence upon food production and fish distribution - both factors i n the intensity of competition. Nilsson (1955) ascribed year to year differences i n the foods of trout and char i n three Swedish lakes at least i n part to differences i n temperature and lake levels i n different years.  41 Many o f t h e above f a c t o r s a c t t o make b o t h c o m p e t i t o r s and i t e m s  of  c o m p e t i t i o n non-randomly d i s t r i b u t e d b o t h s p a t i a l l y and t e m p o r a l l y i n a h i g h l y complicated f a s h i o n A more c a r e f u l s t u d y would have undoubtedly uncovered more environment a l l y c o n t r o l l e d a s p e c t s o f c o m p e t i t i o n i n P a u l and P i n a n t a n L a k e s *  Why,  f o r i n s t a n c e , a r e t r o u t not found o f f s h o r e l i k e s h i n e r s i n P i n a n t a n Lake during the night? A t h i r d o f t e n n e g l e c t e d f a c t o r i n t h e dynamics o f c o m p e t i t i o n i s t h e behaviour of the competitors.  S h i n e r s a r e more e f f i c i e n t f e e d e r s t h a n  trout;  t h e y seem t o a p p r e c i a t e s m a l l e r f o o d i t e m s ; t h e y go deeper i n t o t h e weeds after food.  The  s i t u a t i o n would be f u r t h e r c o m p l i c a t e d ,  as i t undoubtedly  i s between many p a i r s o f c o m p e t i t o r s , i f t h e r e were any  interspecific  b e h a v i o u r a l p a t t e r n s which f a v o r e d one  other.  s p e c i e s over t h e  C o m p e t i t i o n need not e n t a i l any p h y s i c a l c o n f l i c t ; t h e  competitors  need not even be i n the same p l a c e a t t h e same time t o compete.  (1949) n o t e d  Solomon  t h i s when he i n c l u d e d among t h e mechanisms o f c o m p e t i t i o n ,  o c c u p a t i o n o r consumption by e a r l i e r a r r i v a l s o f something i n l i m i t e d so t h a t l a t e comers are a u t o m a t i c a l l y e x c l u d e d o r d e p r i v e d . " s t u d y demonstrates t h i s s i t u a t i o n . d u r i n g the. n i g h t .  The  "the  supply  present  S h i n e r s eat p l a n k t o n o f f s h o r e o n l y  T r o u t a r e a p p a r e n t l y p r e s e n t i n t h e p e l a g i c f e e d i n g ground  o n l y d u r i n g t h e day.  A l s o , s h i n e r s e a t bottom organisms down among t h e  weeds b e f o r e t h e y r e a c h an a r e a where t h e y become a v a i l a b l e t o t r o u t .  This  suggests t h a t c o m p e t i t o r s may  be s e p a r a t e d even a s t e p f u r t h e r t h a n t h a t  p o i n t e d out by Solomon and we  can i n c l u d e among t h e b a s i c mechanisms o f  c o m p e t i t i o n , the consumption by one  o r more organisms o f something i n s h o r t  s u p p l y b e f o r e i t reaches a p o t e n t i a l h a b i t a t where i t would become available t o another organism o r g r o u p . Larkin  (1956) has  commented on t h e vague demarcation  of e c o l o g i c a l  42 zones i n f r e s h w a t e r  environements and  faunas w i t h i n t h e s e zones. t e r i z e d by more b r e a d t h complexity  the l a c k o f sharp d e m a r c a t i o n o f  "Freshwater communities would seem t o be  fish  charac-  t h a n h e i g h t i n t h e pyramid o f a f o o d c h a i n : a  i n horizontal organization."  c o u l d be imagined t h a n the p r e s e n t one.  No  b e t t e r example o f t h i s  Both s h i n e r s and t r o u t eat v i r t u a l l y  a l l t h e organisms a v a i l a b l e i n t h e l a k e s - i n c l u d i n g each o t h e r . t i m e s o f t h e day and y e a r f i s h may  At  different  be found l e a d i n g e i t h e r a p e l a g i c , a s h o a l ,  o r a bottom e x i s t e n c e w i t h t h e i r f o o d h a b i t s v a r y i n g a c c o r d i n g l y . h a b i t s o f rainbow t r o u t and  statement  The  feeding  s h i n e r s ( L i n d s e y , 1950a and b, and o t h e r s ) i n  v a r i o u s l a k e s , r e v e a l an enormous range o f d i e t a r y t o l e r a n c e .  The  ability  o f b o t h s p e c i e s to change t h e i r d i s t r i b u t i o n s and d i e t s t e n d s t o reduce t h e i n t e n s i t y of competition. P a u l Lake, b o t h t r o u t and t h i s new  While amphipods have been s e v e r e l y d e p l e t e d i n s h i n e r s have r e p l a c e d them i n t h e i r d i e t s .  That  d i e t i s not as s a t i s f a c t o r y a s t h e o l d one f o r young t r o u t may  i n f e r r e d from t h e i r slower growth r a t e - b u t l a r g e t r o u t now b e o f r e as a r e s u l t o f f e e d i n g on rence i n a b i l i t i e s o f l a r g e and completely  s h i n e r s (Crossman, 1957).  be  grow f a s t e r t h a n Here t h e  diffe-  s m a l l f i s h t o change t h e i r d i e t s r e s u l t s i n  o p p o s i t e e f f e c t s of c o m p e t i t i o n w i t h i n d i f f e r e n t s i z e groups o f  t h e same s p e c i e s . In conclusion, the present may  study i n d i c a t e s t h a t competition i n nature  be c o n t i n u a l l y s h i f t i n g i n i n t e n s i i y and  b i o l o g i c a l environment and  emphasis.  The p h y s i c a l and  the d i s t r i b u t i o n and b e h a v i o u r o f c o m p e t i t o r s  be i n s t a t e s o f c o n t i n u a l f l u x i n which case the  "niches" of the  competitors  cannot be c o n s i d e r e d c o n s t a n t a s i s a b a s i c assumption i n a l l t h e o r e t i c a l models.  Hence n a t u r a l c o m p e t i t i v e  complicated  r e l a t i o n s h i p s can be c o n s i d e r a b l y more  and v a r i a b l e t h a n s i t u a t i o n s d e s c r i b e d by the most e l a s t i c  e x i s t i n g models.  may  of  43 LITERATURE CITED Anderwartha, H. G. and L. C. Birch. 1954. The distribution and abundance of animals. Chicago, 782 PP. Bennett, G. W. 1944. The effect of species combinations on fish production. Trans. N. A. Wildlife Conf., 9$ 184-188. Crombie, A. C. 1947. Interspecific competition.  J. Animal Ecol., 16(1):  44-73. Crossman, E. J . 1957. Factors involved i n the predator-prey relationship of rainbow trout (Salmo gairdneri Richardson) and redside shiners (Richardsonius balteatus Richardson) i n Paul Lake, British Columbia. Ph.D. thesis, Univ. of British Columbia. Crossman, E. J . and P. A. Larkin. 1959. Results of planting yearling rainbow trout i n Paul Lake, British Columbia. Jour. Amer. Fish. Soc. for 1959. In press. Dobzhansky, T. 1950. Heredity, environment and evolution. Science, H I : 161-166. Elton, C. S. and R. S. Miller. 1954. An ecological survey of animal communities: with a practical system of classifying habitats by structural characters. J. Ecol. 42(2): 460-496. Hartley, P. H. T. 1948. Food and feeding relationships i n a community of freshwater fishes. J . Anim. Ecol., 17(1): 1-14. Hasler, A. D. 1954* Odour perception and orientation i n fishes. Res. Bd. Canada, 11(2): 107-129. Lagler, K. F. 1944. Problems of competition and predation. Wildlife Conf., 9: 212-219.  J . Fish.  Trans. N. A".  Larkin, P. A. 1956. Interspecific competition and population control i n freshwater fish. J . Fish. Res. Bd. Canada, 13(3): 327-342. Larkin, P. A« and S. B. Smith. 1953. Some effects of introduction of the redside shiner on the kamloops trout i n Paul Lake, British Columbia. Trans. Am. Fish. S o c , for 1953, 83: 161-175. Larkin, P. A., G« C. Anderson, W. A. Clemens, and D. C. G. Mackay. 1950. Production of Kamloops trout (Salmo gairdneri kamloops) i n Paul Lake, British Columbia. Sci. Publ., B. C. Game Dept., No. 1. Lindsey, C. C. 1950a. Structural variation as related to the ecology of the redside shiner Richardsonius balteatus (Richardson). M.A. thesis, Univ. of British Columbia. Lindsey, C. G. 1950b. The relation of the redside shiner to production of trout i n British Columbia. Sci. Rept., B. C. Game Commission. 8 pp. Lindsey, C.C. 1953* Variation in anal f i n ray count of the redside shiner Richardsonius balteatus (Richardson).  Canadian J . Zool. 31: 211 - 255  44 L i n d s t r o m , T. 1955* On t h e r e l a t i o n f i s h s i z e - f o o d s i z e , Freshw. Res. D r o t t n i n g h o l m . 33: 70-165. MacFadyen, A. 1957. London. McLeod, C. MS.  Animal E c o l o g y : Aims and Methods.  R e p t . from I n s t .  Putnam and Sons.  1957. The growth r a t e o f rainbow t r o u t i n P i n a n t a n I n s t i t u t e o f F i s h e r i e s , Univ. o f B r i t i s h Columbia.  Lake, B. C.  M o t t l e y , C. McC. 1932. The p r o p a g a t i o n o f t r o u t i n t h e Kamloops d i s t r i c t . T r a n s . Am. F i s h . Soc. f o r 1932. 62: 144-151. M o t t l e y , C. McC. and J . C. M o t t l e y . 1933. The f o o d o f Kamloops t r o u t . A n n u a l R e p t . B i o l . Bd. Canada. 1933: 91-92. N i c h o l s o n , A. J . 1954* An o u t l i n e o f t h e dynamics o f animal A u s t r . J . Z o o l . , 2(1): 9-65.  populations.  N i l s s o n , N. 1955. S t u d i e s o n t h e f e e d i n g h a b i t s o f t r o u t and c h a r i n n o r t h Swedish l a k e s . Rept. from I n s t . Freshw. R e s . D r o t t n i n g h o l m . 29:  108-111.  Rawson, D. S. 1934. P r o d u c t i v i t y s t u d i e s i n l a k e s o f t h e Kamloops r e g i o n , B r i t i s h Columbia. B u l l . B i o l . Bd. Canada, 42: 1-31. Rawson, D. S. 1942. A comparison a t some l a r g e A l p i n e l a k e s i n western Canada. E c o l o g y , 23. R i c k e r , W. E . tions.  1932. The u t i l i t y o f n e t s i n f r e s h - w a t e r p l a n k t o n i n v e s t i g a TCrans. Am. F i s h . S o c . f o r 1932. 62.  R i c k e r , ¥ . E . 1936. The f o o d and t h e f o o d s u p p l y o f sockeye salmon (Oncorhynchus nerka Walbaum) i n C u l t u s Lake, B r i t i s h Columbia. J . B i o l . Bd. Canada, 3(5): 450-4-68. R i c k e r , W. E . and J . G o t t s c h a l k . 1941. An experiment i n removing c o a r s e f i s h from a l a k e . T r a n s . Am. F i s h . Soc. f o r 1940. 70: 382-390. Smith, E . V. and H. S. Swingle. 1939. The r e l a t i o n s h i p between p l a n k t o n p r o d u c t i o n and f i s h p r o d u c t i o n i n ponds. T r a n s . Am. F i s h . S o c . f o r  1938.  68: 309-315.  Solomon, M. E . 1949. The n a t u r a l c o n t r o l o f a n i m a l p o p u l a t i o n s . E c o l . 18: 1-35. Southern, R. June,  1933.  1932.  The f o o d and growth o f brown t r o u t . C i t e d i n N i l s s o n , (1955).  J . Animal  Salmon & t r o u t Mag.  S t a r r e t , W. C. 1950. Food r e l a t i o n s h i p s o f t h e minnows o f t h e Des Moines R i v e r , Iowa. E c o l o g y , 31(2): 216-233. S t r i n g e r , G. E . and ¥ . S. Hoar, 1955. Aggressive behaviour Kamloops t r o u t . Canadian J o u r . Z o o l . 33: 48-160.  of underyearling  45 Swingle, H. S. and E . V. Smith. 1941* The management o f ponds f o r t h e p r o d u c t i o n o f game and pan f i s h . I n a Symposium on H y d r o b i o l o g y . Tihiv. W i s c o n s i n p r e s s , Madison, pp. 218-226. Udvardy, M. D, F. bird l i f e .  1951* The s i g n i f i c a n c e Qikos, 3 ( 1 ) : 98-123.  }  o f i n t e r s p e c i f i c competition i n  

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