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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 Richardson) AND REDSIDE SHINERS (Richardsonius balteatus (Richardson)) 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 accept t h i s t h e s i s as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September, 1959 i ABSTRACT Competition i s defined as the demand of two or more organisms f o r the same e x t r i n s i c resources i n excess of supply. The d i s t r i b u t i o n , movements, behaviour and food of t r o u t and shiners i n Paul and Pinantan lakes were studied i n order to determine the item's and mechanisms of i n t e r s p e c i f i c competition between them. Data from recent years were compared with data f o r years when trout alone inhabited the l a k e . Wo i n t e r s p e c i f i c aggression was observed. The p o s s i b i l i t y that the two species were competing f o r space was discounted. Stomach contents of shiners i n Pinantan Lake revealed 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 . In Paul Lake, shiners have d r a s t i c a l l y reduced the Gammarus population r e l a t i v e to i t s pre-shiner abundance. This overgrazing was caused by the concentration of large numbers of shiners over the shoals where Gammarus are also; present i n t h e i r highest concentrations and the a b i l i t y of shiners to pursue food deeper i n t o the weeds and to gra.ze an area more thoroughly than t r o u t . Ih Pinantan Lake shiners have apparently-reduced the density o f Daphnia to a point where trout are unable to feed on them as r a p i d l y as i n pre-shiner years. The ©bility o f both species to u t i l i z e many types of food tends to reduce the i n t e n s i t y of competition. The study demonstrates how f a l s e i mplications may a r i s e from a delayed a p p r a i s a l of competition. I f observations had not been made on Paul Lake u n t i l a f t e r competition had been observed the importance o f Gammarus as an item of competition would have probably been overlooked and the whole competitive r e l a t i o n s h i p misconstrued. Included among the basic mechanisms of competition i s the consumption by one or more organisms of something i n short supply before 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 a v a i l a b l e to another organism or group. tLIL EnvijWipBnfeai. factors and behaviour were shown to be important influ-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. I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y , p u r p o s e s may be g r a n t e d by t h e Head o f my Depa r t m e n t o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f s£#-&<<£M^ V a n c o u v e r Canada, The U n i v e r s i t y o f B r i t i s h C o l u m b i a , Date i i i ACKNOWLEDGEMENTS F i n a n c i a l support f o r t h i s study came from National Research Council grants to Dr. P. A. Lark i n , I t i s a pleasure to thank Dr. Larkin f o r h i s enthusiasm and guidance. Much of the construction of apparatus 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. Stenton and J . A. Boon. I am indebted to T. Miura and E. W. Ricker f o r a i d i n g with data a n a l y s i s . Appreciation i s extended to those members of the Kamloops detachment o f the B. C # Game Commission who gave t h e i r help, p a r t i c u l a r l y Mr. Pat Mulligan. Dr. C. C. Lindsey*s valuable suggestions are 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 unpublished data from past studies on Paul and Pinantan lakes were made a v a i l a b l e by the B. C. Game Commission. i v TABLE OF CONTENTS Page INTRODUCTION . . . . . 1 DESCRIPTION OF STUDY SITE 4 METHODS 7 EVIDENCE FOR THE EXISTENCE OF COMPETITION. 9 A. Competition f o r Space. . . . . . . 9 B. Competition f o r Food . . . . . 1 0 . ( i ) Food Available i n Pinantan and Paul Lakes . . . . . . . . 10 ( i i ) The Food of Shiners 12 ( i i i ) The Food of Trout 15 ( i v ) Evidence of Competition f o r Food between Young Trout and Shiners . . . . . . . . 18 FACTORS AFFECTING TIME AND PLACE OF COMPETITION. .22 A. D i s t r i b u t i o n and movement o f Shiners .22 ( i ) Seasonal Movements . . . . . . . . . . . . . . . . . . .22 ( i i ) D iurnal Movements . . 2 4 B. D i s t r i b u t i o n and Movements of Trout 25 C. Time and Place o f Competition 26 BEHAVIOURAL FACTORS IN COMPETITION 26 MECHANISMS OF COMPETITION FOR FOOD 33 SUMMARY 36 DISCUSSION 33 V LIST GF FIGURES Figure Page 1, Instantaneous growth rate (log^g l o r k length at agen+-j_ minus l o g fork length at age n) i n r e l a t i o n to fork length at the beginning of the year f o r rainbow trout i n Paul and Pinantan lakes. • . . . . . . . . . 6 2, Percent t o t a l volume of stomach contents of shiners taken at 4 hour i n t e r v a l s over 24 hours on the shoals and at 12 midnight and 4 a»m. offshore 14 3, P r o b a b i l i t y of occurrence o f Amphipods i n stomach contents of rainbow t r o u t , Paul Lake, 1931 to 1957 21 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 Feeding. . . . . . . . ...31 5. 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 Feeding. . . . . . . . . . . 3 2 v i LIST OF TABLES Table Page 1» Limnological c h a r a c t e r i s t i c s of Paul and Pinantan lakes. . . . 4 2. Percent T o t a l Volume Items i n Pinantan Lake Dredgings i n June and August 195S • • H 3. Percent composition by volume of stomach contents of shiners i n Paul and Pinantan lakes i n various years. . . . . . . . . . 15 4. Average volume of stomach content of Pinantan Lake rainbow trou t according to month and length group i n 1958. . . . . . . . . . 17 INTRODUCTION I n t e r s p e c i f i c competition i s an important i n f l u e n c e i n the balance of mixed species communities. The r o l e of i n t e r s p e c i f i c competition i n freshwater f i s h populations i s a fundamental question i n f i s h e r i e s biology. The problem of competition of undesirable species with desirable ones (sports f i s h e s ) i s de a l t with at some length i n the l i t e r a t u r e of pond f i s h culture ( f o r example Bennet, 1952j Ricker and Gottschalk, 1941J Smith and Swingle, 1939j Swingle and Smith, 194l)« These papers deal mainly with the end r e s u l t s of and remedies f o r undesirable competition. F i e l d studies of competition almost alwaya begin a f t e r the e f f e c t s of competition are man i f e s t l y evident. The biology of the competitors before competition occurred and during the f i r s t phases of competition i s l o s t to the observer. He i s s e r i o u s l y handicapped i n p i c k i n g out the features of the competitors" biology which d i f f e r from those before competition occurred. Hence he i s i l l - e q u i p p e d to determine what changes competition has wrought and through what mechanisms. As a r e s u l t , attempts to describe the act u a l mechanisms of i n t e r s p e c i f i c competition i n freshwater f i s h e s have been confined almost e n t i r e l y to laboratory experiments and abstract, n e c e s s a r i l y s i m p l i f i e d mathematical models. Several workers have pointed out the d i f f i c u l t i e s of demonstrating i n t e r s p e c i f i c competition i n freshwater f i s h e s . Lagler (1944) emphasizes that to demonstrate that two species are drawing from a common food supply does not by i t s e l f prove t h a t competition f o r food e x i s t s , f o r i f the food i s abundant and the populations are kept r e l a t i v e l y low by other f a c t o r s , the "feeding could have l i t t l e e f f e c t on supply". St a r r e t (1950) suggests that i t i s d i f f i c u l t to e s t a b l i s h competitive r e l a t i o n s between species because the f i s h may change t h e i r d i e t s rather than enter i n t o severe competition. La r k i n (1955) states that_ i t i s d i f f i c u l t to separate 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 population c o n t r o l from other phenomena such as cannibalism and mutual predation. Wide habitat range i n freshwater f i s h e s and t h e i r conse-quent f l e x i b i l i t y of feeding habits as w e l l as t h e i r f l e x i b l e growth rate and high reproductive p o t e n t i a l a l s o obscure competitive r e l a t i o n s . Crossman (1957) points out that mathematical approaches to competition deal mainly with population s i z e , b i r t h rates, m o r t a l i t y rates and reproductive p o t e n t i a l , and neglect the e f f e c t s of the reactions of f i s h e s to the environment. These reactions "possibly create d i f f e r e n t i n t e r a c t i o n s i n nearly s i m i l a r environments". The influence of f a c t o r s such as d i s t r i b u t i o n , movements and behaviour upon the i n t e r a c t i o n of the competitors w i l l be discussed i n the following sections. The exact meaning of "competition" has troubled many authors ( f o r 1 instance Dobzhansky, 1950; Udvardy, 1952; Larkin, 1956) who have v i s u a l i z e d the need f o r a precise d e f i n i t i o n of the term. Predation and p a r a s i t i s m are considered as separate i n t e r a c t i o n s not f a l l i n g within the bounds of competition by most recent writers (Solomon, 1949; Udvardy, 1952; E l t o n and M i l l e r , 1954; Andrewartha and Birch, 1954, Larkin, 1956; and MacFadyen, 1957). Nicholson (1954) and Crombie (1947) on the other hand, p r e f e r to consider a l l density dependent animal interactions' including parasitism and predation as competitive phenomena. Most of these writers accept the d e f i n i t i o n of competition as "the demand, t y p i c a l l y at the same time, of more than one organism f o r the same resources of the environment i n excess of Immediate supply," (Larkin, 1956). The lack of agreement over whether or not predation and p a r a s i t i s m constitute competition centers around the i n t e r p r e t a t i o n of the word, "resource". Nicholson and Crombie consider the l i f e of the host or prey to be a resource; most other w r i t e r s do not. A conceptual standardization of the term competition can be accomplished only by r i d d i n g d e f i n i t i o n o f i t s ambiguity. I f we choose a d e f i n i t i o n which admits a l l categories of unfavorable 3 b i o l o g i c a l i n t e r a c t i o n s , " i t (can be) argued that every a c t i v i t y of each animal i n a community i n one way or another c o n s t i t u t e s an act of competition with a l l other members of the community," (Larkin, 1956). The word i s rendered so broad i n meaning as t o have l i t t l e value as a precise s c i e n t i f i c term. "The struggle f o r existence" serves j u s t as w e l l t o express t h i s general concept. Competition may be considered as one of the mechanisms i n the struggle f o r existence along with predation, parasitism etc. The i n s e r t i o n of one word i n the d e f i n i t i o n so that i t reads, ".... demand for e x t r i n s i c resources helps c l a r i f y the concept. The problem of whether l i f e i s a resource or not i s s k i r t e d . In a predatory or p a r a s i t i c r e l a t i o n s h i p the l i f e of the host or prey i s not e x t r i n s i c . Pre-dation and parasitism are thus c l e a r l y excluded from competitive phenomena. Competition i s defined i n t h i s paper as the demand of more than one organism f o r the same e x t r i n s i c resources i n excess of supply. I t i s d i v i d e d i n t o two phases; competition f o r space and competition f o r food. Paul and Pinantan lakes near Kamloops, B r i t i s h Columbia, were chosen f o r t h i s study. At the time of study both lakes contained only two species of f i s h , the rainbow tr o u t , Salmo ^ a i r d n e r i , and the redside shiner, Richard-sonius balteatus, providing a r e l a t i v e l y simple example of i n t e r s p e c i f i c f i s h competition. Limnological and f i s h b i o l o g y studies have been c a r r i e d on i n the two lakes f o r many of the l a s t t h i r t y years and provided much background information f o r t h i s i n v e s t i g a t i o n . The biology o f the t r o u t before and dur-i n g the f i r s t years a f t e r the i n t r o d u c t i o n of shiners i s w e l l documented and provides a unique opportunity to study the mechanisms of competition during t h e i r development. Pinantan Lake has since been poisoned and restocked with rainbow t r o u t alone. 4 DESCRIPTION OF STUDY SITE Paul and Pinantan lakes are s i t u a t e d about twelve miles northeast of Kamloops, B. C , at an a l t i t u d e of about 2500 and 2860 f e e t r e s p e c t i v e l y . The lakes are about four miles apart and connected by a stream, flowing from Pinantan i n t o Paul, which o c c a s i o n a l l y stops running i n the summer. Each lake has one o u t l e t and one major i n l e t . A few mountain streams which dry up i n the summer also enter Paul Lake. La r k i n et a l (1950) have described the p h y s i c a l features of Paxil Lake. Rawson (1934) discusses the limnology o f both lakes. 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 of the two l a k e s . Table I. Limnological c h a r a c t e r i s t i c s of Paul and Pinantan lakes. Paul Lake Pinantan Lake Area 3*960 acres a 161 acres Shoreline development *5.55 u n i t s • 3.89 u n i t s Maximum depth 4182 f e e t o62 f e e t Mean depth •112 f e e t a31 f e e t TDS •216 p.p.m. • 238 p.p.m. « Lar k i n et a l , 1950 * Crossman, 1957. OB. C. Game Commission, unpubl. • Rawson, 1942. Rawson (1934) describes Pinantan Lake as h i g h l y eutrophic with high p r o d u c t i v i t y . He considers Paul Lake as t y p i c a l l y o l i g o t r o p h i c , but with r e l a t i v e l y high p r o d u c t i v i t y due to an extensive shoal area. Mottley (1932) and Crossman (1957) discuss the sequence o f events i n Paul Lake a f t e r the l i b e r a t i o n of trou t f r y i n 1900. The h i s t o r y of Pinantan i s s i m i l a r . There was a rapid b u i l d up of a large underexploited population. The construction o f a good road to the lakes l e d eventually to depletion of 5 the stocks and fry#stock±ng programs were i n i t i a t e d . Sometime a f t e r 1930 the Redside shiner, Richardsonius balteatus was introduced i n t o Pinantan. A b a r r i e r was placed i n the stream between Pinantan and Paul to exclude the shiners but by 1945 they had invaded the l a t t e r . A subsequent marked drop i n the catch per un i t e f f o r t , a decrease i n the growth rate of y e a r l i n g trout and an increase i n growth rate of l a r g e t r o u t i n Paul Lake (see Figure 1) was a t t r i b u t e d to t h e i r i n t e r a c t i o n with shiners (Crossman and Larkin, 1959)« A noticeable decrease i n the percent of one-year-old f i s h i n the catch occurred (34$ of the catch i n 1946 to 49 compared with 6.1$ i n 1955 to 56). This was a t t r i b u t a b l e to the slower growth rate of young t r o u t j fewer one-year-olds a t t a i n e d s u f f i c i e n t s i z e to enter the f i s h e r y . The depression i n the growth rate of y e a r l i n g t r o u t reached a maximum i n 1952. Large trout d i d not s t a r t feeding on shiners u n t i l about 195l» In succeeding years shiners made up a successively l a r g e r quantity of the d i e t of trout over 10 inches fork length. The growth rate of these l a r g e r t r o u t increased accordingly and surpassed that of pre-shiner years. By 1956 the growth rate of trout under ten inches, which d i d not feed appreciably on shiners, had also increased s l i g h t l y from the 1951 l e v e l but was s t i l l lower than i n pre-shiner years. MacLeod (1957, MS) states that the growth rate of small t r o u t i n Pinantan i n 1952 was considerably lower than the pre-shiner growth rate i n Paul. The growth rate of Pinantan trout i n 1957, c a l c u l a t e d by t h i s w r i t e r , was found t o be s l i g h t l y higher than i n 1952 but s t i l l lower than i n pre-shiner years i n Paul Lake, (see Figure 1 ) . Unfortunately no pre-shiner data on t r o u t growth rates i n Pinantan Lake were a v a i l a b l e and the delet e r i o u s e f f e c t s of competition with shiners on growth can only be i n f e r r e d . m 1955 the I n s t i t u t e of F i s h e r i e s , U n i v e r s i t y of B r i t i s h Columbia, i n i t i a t e d a program of study o f the i n t e r a c t i o n of the two species. Crossman 10 20 30 FORK LENGTH (cm.) at BEGINNING Cf YEAR Figure 1. Instantaneous growth rate (log-^Q fork length at age n 4 > ^ minus l o g fork length at age n) i n r e l a t i o n to fork lenth at the beginning of the year f o r rainbow trout i n Paul and Pinantan lakes. 1946 and 1932 Paul Like curves from Larkin et a l (1957) 1^55-56 curves from Grossman (1957) 1?52 curves 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 in 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 its biology in 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 in 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 in contact how do they influence each other? Competition for Food The stomach contents of 168 shiners caught in gillnets and a dipnet over the summer were analyzed to evaluate the seasonal, diurnal and spatial differences in their choice of food. About 20 stomachs from shiners taken in each of 1946 and 1948 in Pinantan Lake and in 1950, 1952 and 1959 in Paul Lake were also examined. The contents of stomachs from 335 trout caught by angling and gillnetting in 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 in September, 1958, with a Clarke-Bumpus plankton sampler. Ten bottom dredgings on the shoal in depths from 1 to 8 10 f e e t were taken i n June and again i n August i n Pinantan, Preliminary dredg-ings i n the Chara with an Ekman dredge proved u n s a t i s f a c t o r y . The dredge would not close properly over the weeds. Consequently a rake was used to gather the samples. Each sample consisted of the amount of Chara that would f i t i n t o a two l i t r e container. Six-sided, ( i . e . completely enclosed) pens 31 by 3' by 31 were used i n experiments to study the u t i l i z a t i o n of a known amount of introduced food. Factors A f f e c t i n g the Time and Place of Competition The seasonal and d i u r n a l movements of shiners i n Paul Lake have been described by Crossman (1957). Studies were c a r r i e d out on Pinantan Lake i n 1958 to corroborate and extend these f i n d i n g s . D i r e c t observations were made throughout the summer. Diurnal movements were recorded using two g i l l n e t s , one set near shore, the other offshore, f o r one hour every four hours throughout 24 hours. Crossman also discusses the d i s t r i b u t i o n and movements of trout i n Paul Lake. Direct observations and overnight g i l l n e t sets were made throughout the summer both i n Paul and Pinantan l a k e s . Behavioural Factors i n Competition Shiners and 3 to 6 i n c h t r o u t were held i n enclosures i n the lake f o r observation and feeding experiments. Four-sided pens, 6' by 6* by 5' deep and b u i l t of door screen on a wooden frame, were placed on Chara beds near shore and anchored f i r m l y to the bottom i n about 4 f e e t of water. The bottoms of the pens were open, hence the enclosed f i s h were swimming over n a t u r a l Chara beds and had access to the bottom. In order to t e s t the possible e f f e c t of shiner odor on t r o u t , tm channels flowing i n t o a box containing ten 3 to 6 inch t r o u t i n two cubic f e e t o f water was constructed. In one channel a shiner was placed i n a wire 9 cage. The other channel contained no shiner. The number of tr o u t o r i e n t i n g i n each of the two currents was observed and the t e s t repeated ten times. Many hours during the summer were devoted t o d i r e c t observation of the two species on and around the shoal i n Pinantan Lake. EVIDENCE FOR 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 s commonly divided i n t o three aspects; competition f o r food, space and spawning area (Larkin, 1956). Food i s a w e l l delineated subdivision. Competition f o r space however i s a phrase o f t e n l o o s e l y applied to any s i t u a t i o n where overcrowding occurs. What i s often i n short supply i s not space per se, but some resource contained within a circumscribed space,e.g. feeding area (as d i s t i n c t from food) s h e l t e r , shade, sunlight, oxygen, warmth, etc. and a l s o , l o g i c a l l y , spawning area. Any thorough study of competition f o r space must sort 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 of the space are i n contention. A. Competition f o r Space. The p o s s i b i l i t y that trout may exclude shiners from parts of the lake they would otherwise occupy was not examined. A knowledge of the d i s t r i b u t i o n o f trout before (Mottley and Mottley, 1932) and a f t e r (Crossman, 1957) the i n t r o d u c t i o n of shiners i n t o Paul and Pinantan lakes l e d the w r i t e r to a l l but discount the p o s s i b i l i t y that shiners exclude trout from any part o f t h e i r former range. However, some species of c y p r i n i d s have been shown to be r e p e l l e d by the odor i n the water of c e r t a i n other species, (Hasler, 1954) and t e s t s were c a r r i e d out to determine whether shiners might r e p e l trout i n such a manner. Trout made no d i s t i n c t i o n i n choosing between a current containing a shiner and another current without a shiner i n i t . Two-hundred choices were made; twenty by each of ten 3 to 6 i n c h t r o u t . The shiner was switched from one current to the other a f t e r one-hundred choices had been made. Ex a c t l y 10 one hundred movements were made i n t o each of the two currents. The p o s s i b i l i t y that shiners might, over a period of time, "condition" the water and thus r e p e l trout was not examined. I t i s h i g h l y u n l i k e l y that t r o u t and shiners compete f o r spawning space. Shiners are known to spawn i n streams as w e l l as i n lakes (Lindsey, 1950) but i n Paul and Pinantan lakes i t appears that most spawning takes place i n the lakes on the shoals* A l l trout spawning i s done i n the streams. Shiners have been observed t h i r t y yards downstream from the Pinantan Lake o u t l e t ( M c A l l i s t e r , unpub.) and near the o u t l e t of the stream flowing i n t o Paul Lake. (Grossman, 1957)* However, the shiners observed were few and occupancy of these areas not prolonged. The present w r i t e r observed no shiners i n the spawning stream at any time during h i s i n v e s t i g a t i o n . Shiners s t a r t spawning between the end of May and the end o f June (Lindsey, 1950) j u s t a f t e r the trout spawning run i s over. Shiner eggs are broadcast f e r t i l i z e d rather than buried i n the gravel and shed o n l y at night (Lindsey, 1950) unlike trout eggs. No shiners have ever been observed with trout i n Upper Paul Creek. B. Competition f o r Food ( i ) Food Av a i l a b l e i n Pinantan and Paul Lakes Bottom Organisms Table 2 shows the percent of the t o t a l volume of various food items i n twenty Pinantan dredgings made on the shoal ( i n ten feet of water or l e s s ) . The r e s u l t s are probably l e s s p r e c i s e than they might be because of the small number of dredgings. Anisoptera larvae were the most important si n g l e item i n both June and August. Pl a n a r i a dropped from 21% of the t o t a l volume i n June to v i r t u a l absence i n August. H y a l l e l a and Physa made up most of the balance of the dredgings i n both months. Shiner eggs, Sphaeridae and the larvae of Chironomidae, Ephemeroptera, Trichoptera and Enallagma were present i n small numbers i n June and absent i n August. The t o t a l volume of bottom organisms i n 11 the dredgings in June was 35*6 cc's while i t dropped steeply to 11.5 cc's In August. The difference is statistically significant (p < .02). A l l groups except Zygoptera larvae were present in fewer numbers in August than in June. Table 2, Percent Total Volume of Food Items in Pinantan Lake Dredgings in June and August 1958. Item Anisoptera Planaria Physa Hyallela Pianorbis Trichoptera Hirudinea Zygoptera larvae June 32.3 27 11.7 3.4 7.1 1.1 Aug. 74.8 17.4 4.3 2.6 | Dredgings were richest in volume and variety of organisms in the shallowest ( l to 2 inches) water and visible organisms were strikingly absent from two dredgings taken in water deeper than 8 feet in both June and August. This is in 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 in the 0 to 5 meter depth zone were also found in the 5 to 10 meter zone and one-third of them were present in the 30 to 40 meter zone, (Larkin et al, 1950). Rawson (1934) attributes his finding no bottom organisms below the thermocline in Pinantan Lake to severe oxygen depletion. Paul Lake on the other hand has abundant oxygen at a l l depths. Larkin et al (1950) recorded the bottom fauna of Paul Lake in 1948 and 1949. A l l groups found in Pinantan Lake were present in 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 in the numbers of Amphipods in the lake since the introduction of shiners was noted. 12 Plankton Plankton tows made through the f i r s t week i n September 1958 showed that s i x t y - f i v e percent by volume of the plankton i n the top 20 f e e t of Pinantan Lake (equivalent, approximately; to the epilimnion) consisted of Daphnia and 25$ was Diaptomus. Aphanizomenon was unimportant i n plankton tows made at 3 f e e t and 10 feet but made up 45$ of the plankton at a depth of twenty f e e t . S i m i l a r l y Anabaena was unimportant except at 20 fee t where i t made up 10$ of the t o t a l volume. About 5$ of the midnight tows at a l l three depths consisted o f Chaoborus while none were recorded i n 12 tows made during d a y l i g h t . Other organisms noted were Ceratium. Asplanchna, Spirogyra, Polyphemus t Pandorina. Protococcus, Ul o t h r i x , Ohriodorus, PedJastrum.Dynobrion, Sphaero- c y s t i s . Staurastrum, A s t e r i o n e l l a , Merismopedia, Cladophora, Anuraea, Colacium,  Simocephalus, Tetradesmus and Conchostraca. Rawson (1934) discusses the plankton i n Paul Lake. Diaptomus and Daphnia i n that order were the organisms of major importance. Two h o r i z o n t a l plankton tows j u s t under the surface and two t o t a l v e r t i c a l tows made i n August 1959 in d i c a t e d t h i s was s t i l l the case, ( i i ) The Food of Shiners In preliminary t e s t s groups of shiners were held foodless f o r various lengths of time before t h e i r stomachs were examined. Whereas 90$ o f the stomach contents from f i s h k i l l e d immediately on capture were i d e n t i f i a b l e , only 15.5$ were i d e n t i f i a b l e a f t e r the f i s h were held foodless f o r an hour and 3.7$ a f t e r two hours. I t was concluded th a t almost a l l food i n the stomachs of shiners k i l l e d immediately on capture had been eaten w i t h i n two hours of capture. Hence any d i u r n a l change i n food habits can e a s i l y be detected i n the stomach contents. Shiners were sampled at four hour i n t e r v a l s throughout the day i n e a r l y August. A marked d i u r n a l change i n foods was discovered. 13 Figure 2 shows the gradual s h i f t i n dominance from Daphnia to algae during the day and back to Daphnia at night i n the d i e t of Pinantan Lake shiners. The stomachs of 37 shiners taken at midnight contained 92.5$ Daphnia and 6.2% algae (mainly S p i r u l i n a , Spirogyra and Nodularia). I n contrast the stomachs of 29 shiners taken at 5 P»m» contained only 19.1$ Daphnia and 69% algae. The s h i f t i s more s t r i k i n g f o r offshore f i s h than f o r those on the shoals. At mignight offshore f i s h had v i r t u a l l y no algae i n t h e i r stomachs while shoal f i s h had 14$ algae and &k% Daphnia. But by 4 a.m. the offshore f i s h had 34$ algae i n t h e i r stomachs while shoal f i s h had only &% algae. A l l shiners move back to the shoal at dawnj offshore shiners were caught only i n the midnight and 4 a.m. sets. Plankton c o n s i s t i n g almost e n t i r e l y of Daphnia pulex. made up 58.1% of the t o t a l d i e t and algae contributed 26.4#» Other food included H y a l l e l a , various aquatic and t e r r e s t r i a l i n s e c t s , PLanorbis and Physa. None o f these foods ever composed more than 12% o f the d i e t of a group of f i s h caught at one time and together they made up only 6.1% of the t o t a l d i e t . About 10% of the food was too w e l l digested to be i d e n t i f i e d . I t i s i n t e r e s t i n g to note that while shiner cannibalism has been observed i n other lakes (Lindsey, 1950b) none of the 269 shiner stomachs from Paul and Pinantan taken from 1946 to 1959 contained a s i n g l e shiner. Stomachs of shiners c o l l e c t e d i n 1959 i n Paul Lake by the w r i t e r and i n various previous years i n both lakes by members of the B. C. Game Commission were also examined. The gradual d e t e r i o r a t i o n of a l l but the 1959 stomachs since t h e i r preservation made accurate stomach an a l y s i s d i f f i c u l t and the f i g u r e s i n Table 3, below, are j u s t estimates. Apparently there was 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 the feeding habits of Paul Lake shiners s i m i l a r to that found i n Pinantan shiners. C o l l e c t i o n s o f shiners taken at 100 • — DAPHNIA A — A L G A E O — O T H E R FOODS NIGHT DARKNESS REPRESENTED BY SHADING. , 6 _1L I2midn. . 4 a m . 8 a m . 12 noon F i g . 2. 4 pnr 8 pm.. I2midn; 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* •' , * Unidenti f ied food not included. 15 2 p.m. and 2 a.m. both contained about 15$ Gammarus and 85$ t e r r e s t r i a l i n s e c t s . The times o f day of capture were not recorded f o r the other c o l l e c t i o n s . No algae or Daphnia was found i n the stomachs of any Paul Lake shiners. The s t r i k i n g d i f f e r e n c e s i n the d i e t s of shiners i n the two lakes i s not unusual. Lindsey (1950b) mentions the wide v a r i e t i e s of food eaten by shiners, i n d i f f e r e n t lakes and streams. He states furthermore that i n lakes inhabited by both rainbow trou t and shiners, the shiners were found to eat a l l types of food eaten by the t r o u t . Table 3. Percent composition by volume o f stomach contents of shiners i n Paul and Pinantan lakes i n various years. H #» CD ;H fi CD i i Month Year Lake Number of f i s h examined Daphnia Algae Terrestri* insects Gammarus Trout Fingerlir J u l y 1946 P i n . 22 5 90 5 Aug. 1948 P i n . 36 90 10 J u l y 1950 Paul 20 70 30 Aug. 1952 Paul 4 50* 50 Aug. 1959 Paul 17 85 15 • * F i n g e r l i n g t r o u t had been planted i n the area of capture of these shiners the same day. Two of the four f i s h examined contained tro u t f i n g e r l i n g s . ( i i ) The Food of Trout The stomach contents of 335 trout taken from Pinantan Lake i n the summer of 1958 were examined. ' Daphnia co n s t i t u t e d 63$ of the food of t r o u t of 10 Inches f o r k length and under. The r e l a t i v e importance of Daphnia 16 diminishes as the trout grow l a r g e r . Shiners constituted the major food item o f trout over 14 inches long (70$ by volume f o r the summer) although they dropped markedly i n importance i n September while Daphnia and Anisoptera 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. F i s h under ten inches long f e d n e g l i g i b l y on shiners. Bottom organisms, mainly dragonfly nymphs, made up the bulk of the r e s t of the food of a l l three s i z e groups.* In Paul Lake i n 1955 and 1956 Crossman (1957) reports a s i m i l a r increase i n importance of shiners and decrease i n importance of plankton as the trout frow l a r g e r . Bottom organisms, assumed a greater importance than i n Pinantan making up about 50$ of the t o t a l d i e t . Few Gammarus or H y a l l e l a were present i n e i t h e r Paul or Pinantan t r o u t . L a r k i n et a l (1950) point out that i n Paul Lake t h i s i s i n marked contrast to 1931, a pre-shiner year, when amphipods made up 39.8$ of the d i e t o f Paul Lake t r o u t . Diaptomus were conspicuously absent from the stomachs of both t r o u t and shiners despite the f a c t that they were one of the two most abundant plankters i n both lakes. The very low a v a i l a b i l i t y to salmonids of Cyclops -a very s i m i l a r copepod - has also been mentioned by Southern (1933), Lindstrom (1955) and Nilsson (1955)« Nilsson reports that while high production of plankton species i s i n general connected with high consumption of that species by char, Cyclops composed as much as 90$ of the plankton i n Lake Blajon yet constituted l e s s than 1$ of the d i e t of char. Diaptomus. though present i n small numbers i n the lake was never taken i n the stomachs of char. A simple experiment explained t h i s anomaly. Adult Daphnia and Diapt- omus were placed i n a glass of lake water. The w r i t e r attempted to capture single i n d i v i d u a l s of each species from the glass with an eye dropper. Daphnia were r e l a t i v e l y easy to catch but i t was v i r t u a l l y impossible to capture Diaptomus. They i n v a r i a b l y dodged the eye dropper with great a g i l i t y . Undoubt-edly they do the same when a f i s h approaches. This reaction appeared to be a response at l e a s t i n part to the sight of the approaching eye dropper. Ricker (1932) supposed that the appearance of Cyclops i n plankton hauls i n Cultus Lake i n numbers r e l a t i v e l y too few was due to a rheotropic response. The above observation suggests that t h i s occurrence was due at l e a s t i n part to the a b i l i t y of t h i s animal to f l e e q u i c k l y on si g h t i n g an approaching o b j e c t . Table 4. Average volume of stomach content of Pinantan Lake rainbow trout according to month and length group in 1958. Number in bracket under length group i s number of trout in that group for whole year. Number in bracket beside month i s the number of trout of a l l sizes i n that month. Length group (fork length in inches) ITEM JULY &04) AUGUST CL62) SEPTEM (69 BER ) Shiners Daphnia Antisoptera nymphs Chironomid larvae Other insect larvae Other bottom organ. Terrestrial insects Miscellaneous Shiners Daphnia CO xi H 2 CD -P a, 0 Chironomid larvae Other insect larvae Other bottom organ. Terrestrial insects CO 0 H <D O CO Shiners Daphnia Anisoptera nymphs Chironomid larvae Other insect larvae Other bottom organ. Terrestrial insects ' Miscellaneous ' 6-10" (122) Number of stomachs with item 2 21 4 2 4 - 2 9 7 48 8 24 8 1 5 10 - 21 4 4 2 mm 11 12 Percentage with item 6 69 11 6 11 - 6 25 13 86 14 43 2 9 18 - 70 13 13 7 - 37 40 Percentage of total volume S 61 9 • 7 - + Ik 22 63 9 1 • * 29 2 - 65 2 2 1 - 18 13 10-14" (116) Number of stomachs with item 12 8 6 4 3 - 2 3 14 33 16 12 12 3 2 13 2 17 3 3 2 - 4 2 Percentage with item 40 27 20 13 10 - 7 10 23 54 26 20 20 5 3 21 8 68 12 12 8 - 16 8 Percentage of total volume 52 17 • 1 10 - 4 4 33 28 18 * 14 5 • 2 5 32 12 1 4 - 25 5 14-14* (97) Number of stomachs with item 25 3 i i 1 4 4 - 9 29 19 16 16 7 1 5 14 4 5 5 5 - - 4 4 Percentage with item 65 8 29 3 10 10 - 24 64 42 36 36 16 4 11 31 29 36 50 36 - 29 29 Percentage of total volume 76 1 9 *• 3 5 - 4 68 10 15 1 1 2 3 30 20 40 - • - - 6 5 18 ( i i i ) Evidence of Competition f o r Food between Young Trout and Shiners Pinantan Lake Daphnia was the major food of shiners and young trou t i n Pinantan Lake, This i n i t s e l f i s not s u f f i c i e n t evidence to demonstrate the existence of competition c o n c l u s i v e l y . As Lagler (1947) stated, i f the food i s abundant and the f i s h population kept r e l a t i v e l y low by other f a c t o r s , feeding has "Li t t l e e f f e c t on supply. I t i s impossible to determine the e f f e c t s o f the shiners on the abun-dance of Daphnia i n Pinantan Lake d i r e c t l y as there are no data from pre-shiner years. Their probable e f f e c t s however may be i n f e r r e d from a comparison o f the present s i t u a t i o n with one described by Ricker (1936) i n Cultus Lake. Ricker showed that the consumption of Daphnia by young sockeye saLmon i n 1934 i n Cultus Lake was s u f f i c i e n t to reduce the a v a i l a b l e supply and cause a reduction i n the growth rate of the sockeye. The concentration of shiners present a t night i n the pelagic feeding ground i n Paul Lake i s between 1 i n 6 cubic meters and 3 i n 1 cubic meter ( c a l c u l a t e d using data from Lindsey, 1953). D i r e c t observations of the d e n s i t i e s of shiners on the shoals i n both Paul and Pinantan lakes suggested that t h e i r concentrations i n these two lakes were roughly s i m i l a r . This i s of the same order of magnitude as the concentration o f sockeye i n Cultus Lake during Ricker 1 s observations (from 2 i n 3 t o 2 i n 7 cubic meters). Adult shiners i n Pinantan Lake consumed about 2200 Daphnia per day*— hi times as many per i n d i v i d u a l as the sockeye i n Cultus Lake as estimated by Ricker* The concentration of Daphnia i n Pinantan Lake i n August 1958 This estimate i s based on p r e v i o u s l y described d i g e s t i o n rate experiments, volumetric stomach a n a l y s i s and a count of the number of Daphnia per cubic centimeter of pure Daphnia i n a trout stomach. Stomach contents of trout rather than shiners were used because the pharyngeal teeth of shiners fragment t h e i r food so much that the enumeration of i n d i v i d u a l plankers i s impossible. 19 was about 1.2 per l i t r e — a b o u t i the cone ent r a t i o n i n Gultus Lake. The rate of turnover of plankton i n Pinantan may be somewhat higher than that of Cultus however due to i t s higher summer temperature and t o t a l dissolved 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 and feeders i n Cultus Lake. I t may be i n f e r r e d from the above comparison that the Daphnia i n Pinantan Lake may also have been kept at a r e l a t i v e l y low l e v e l , by shiners, and that since the food requirements of young rainbow t r o u t are s i m i l a r to those o f sockeye f i n g e r l i n g s , competition f o r food occurred i n Pinantan Lake. That the growth rate of young Pinantan Lake trout was i n f a c t even slower than the shiner-retarded growth rate of young Paul Lake trout supports this: view. In summary, there were probably no fewer than £ the concentration of shiners i n Pinantan as there were sockeye i n Cultus Lake during the years being compared. These shiners ate over four times as much Daphnia per i n d i v i d u a l per day as sockeye. The concentration of Daphnia i n Pinantan Lake i s 5 that of Cultus Lake though the rate of production i s probably greater. Competition f o r food was demonstrated i n Cultus Lake. Competition f o r food i s i n f e r r e d i n Pinantan Lake from t h i s comparison. Paul Lake Competition f o r food i n Paul Lake has centered mainly around Amphipods. Gammarus and Hyallela have decreased i n abundance i n Paul Lake and have decreased markedly i n importance as trout food since the introduction of shiners. Ih 1949, amphipods i n bottom dredgings were l e s s than one t h i r d as abundant as i n the pre-shiner year 1931 ( l a r k i n et a l , 1950). Before the i n t r o d u c t i o n of shiners i n t o Paul Lake, amphipods made up 39.8$ of the food of t r o u t . In 1931 Mottley and Mottley (1932) report that the average trout stomach contained 16? amphipods. Afterward, i n 1947—1948, they made up only 20 9.4/5 of the food of t r o u t (Larkin 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 present" i n the stomachs of trout. (Grossman, 1957). Figure 3 shows the decline of importance of amphipods i n the d i e t of trout* I t was supposed that shiners had cropped o f f the amphipods to t h i s low l e v e l . Pen feeding experiments were c a r r i e d out to substantiate t h i s hypo-t h e s i s . A f t e r 500 Gammarus had been introduced i n t o each o f the holding boxes and allowed t o seek s h e l t e r i n Chara placed i n the bottom, t r o u t and shiners were introduced. A l l other v i s i b l e food had been washed from the Chara. A f t e r twenty-four hours the boxes were l i f t e d and the remaining Gammarus counted. Ten 3 to 6 i n c h t r o u t and ten l i to 3 i n c h shiners consumed approximate-l y the same number of Gammarus during t h i s time. From the trout pen 119 Gammarus were recovered, 105 from the shiner pen and 315 from a c o n t r o l pen containing no f i s h . Apparently, both shiners and t r o u t eat la r g e q u a n t i t i e s of Gammarus when they are a v a i l a b l e . The p o t e n t i a l grazing i n t e n s i t y of these shiners on Gammarus i s many times that of the trout population. Lindsey (1953) states t h a t the number o f shiners i n Paul Lake was too great f o r accurate estimation from f i n - c l i p p i n g experiments but t h a t i n 1950 the 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 . Larkin and Smith (1953) estimated the number i n 1952 to be "sev e r a l m i l l i o n " . Crossman (1957) estimates that i n 1955-56 there were about 16,000 t r o u t over s i x inches i n length i n Paul Lake. Even i f there were f i v e times t h i s many trout under s i x inches long i n the lake there were probably at l e a s t f i f t y times as many shiners as t r o u t i n the lake. During the summer months thousands o f shiners can often be seen at one glance along the shoal areas. This l a r g e number o f shiners i n the lake, t h e i r readiness to eat amphipods and t h e i r congregation over the area of greatest concentra-1.0 Figure 3. Probability of 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 likely to take place. A. Distribution and Movements of Shiners Crossman described the seasonal and diurnal movements of shiners in Paul Lake. Their movements in Pinantan Lake were similar althoughothe trends were not so clear cut. (i) SeasonalMovements Data on the distribution of shiners in 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 firs t appear on the shoals in 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 is known of their winter distribution. Observations commenced on Pinantan Lake in the middle of June. The shiners on the shoal exhibited the same vertical and horizontal stratification noted by Crossman (1957) in 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 fry moved into deeper water just off the shoal. The shiners were most frequently seen in 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 in various 23 years was n e g l i g i b l e i n 1958* In e a r l y J u l y the 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 f r y seemed to break down gradually. Pry s t i l l stayed i n segregated schools of 50 to 100 within about s i x inches of the surface. Otherwise, d i f f e r e n t s i z e d f i s h intermingled i n 1 to 8 f e e t of water, 1 to 4 feet from the surface. L a t e r i n J u l y the f r y gradually broke up and moved f a r t h e r o f f the shoal mingling with the l a r g e r f i s h . By J u l y 20 the schools were diminishing noticeably i n s i z e and number; the f i s h were v i s i b l e offshore up to 30 yards or more and i n deeper water. The l a r g e s t f i s h l e f t the shoals f i r s t . An estimated 50$ of the f r y remained on the shoal during t h i s general offshore movement. In a channel about e i g t h f e e t deep between an i s l a n d and the shore a l a r g e school stayed about 3° f e e t offshore, f o r almost a month, without s h i f t i n g during the day. In 0 to 3 f e e t of water was a school of about 500 f r y ; in.0 to 8 f e e t of water a school of one-year-old f i s h and i n 2 to 8 f e e t of water a school of l a r g e r f i s h (about 3 inches and over). The three schools o f t e n intermingled through v e r t i c a l mixing but i n v a r i a b l y sorted themselves out within 10 to 15 seconds, i n t o the three separate s i z e groups. These schools were continuously preyed on by t r o u t . F i s h i n g and g i l l n e t t i n g f o r trout i n t h i s v i c i n i t y was very productive. By the end of August the schools had reformed and s t r a t i f i e d on the shoals again. The lake was poisoned i n e a r l y September. Presumably the f i s h would have moved offshore en masse i n October as they had been observed to do i n previous years by the lodge owner and by Crossman (1957) i n Paul Lake. Nothing i s known of t h e i r winter d i s t r i b u t i o n . L a r k i n (pers. comm.) has noted that they c o l l e c t around holes cut i n the i c e during the winter. In J u l y and August several observation t r i p s were made around the perimeter of the l a k e . Shiners were s t r i k i n g l y absent from the shoals of 24 the south h a l f of the lake, on a l l occasions. One could c r u i s e f o r hundreds of yards at a time i n t h i s area without seeing a s i n g l e shiner while at the same time the shoal of the north h a l f was almost continuously populated. When the shiners were concentrated on the shoals 1 during June and e a r l y July, schools of large (over 3 inches F. L.) i n d i v i d u a l s were often seen 20 to 40 yards offshore on the northwest s i d e . They would o f t e n v i g o r o u s l y dapple the surface. 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 school 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 or none at a l l . I t could not be ascertained I f they were feeding as no food was seen. In the schools near shore t h i s dappling behaviour was noted only o c c a s i o n a l l y and was c a r r i e d out only by single i n d i v i d u a l s . ( i i ) D i urnal Movements Si m i l a r to Crossman's f i n d i n g s , the shiners were observed to spread out a l l over the surface waters (0 to 25 f e e t deep) of the lake as n i g h t f a l l approached and move back to shore at the f i r s t l i g h t of morning. At 11:30 p.m., June 25, about one-twentieth of the daytime numbers were seen by f l a s h l i g h t on the shoal randomly d i s t r i b u t e d according to s i z e 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 schools within s i x inches of the surface. At night shiners could be seen dimpling the surface s p o r a d i c a l l y a l l over the lake. I t could not be ascertained i f they were feeding. Two p o s s i b l e reasons f o r t h i s offshore movement present themselves; 1. The shiners move out i n response to an upward migration of Daphnia on which they feed extensively at n i g h t . 2. They l o s e t h e i r o r i e n t a t i o n t o the shore at night and swim about at random h o r i z o n t a l l y . L a r k i n et a l (1950) mentions a s l i g h t downward migration of Daphnia i n Paul Lake from 4:30 to 10:30 a.m. and a compensatory upward migration from 7:30 p.m. to 4:30 a.m. However even at the peak of the downward 25 migration the proportion of Daphnia below 12 meters never exceeded 25$. In Pinantan Lake a s e r i e s of midnight and noon plankton tows at depth of 1 foot, 10 f e e t and 20 feet d i d not i n d i c a t e any s i g n i f i c a n t d i f f e r e n c e between night and day i n the number of Daphnia i n the upper ten f e e t of water, the area i n which the shiners are concentrated at night. Hence the second explanation, a l o s s of o r i e n t a t i o n to the shore i n the dark appears to be the more reasonable theory. B. D i s t r i b u t i o n and Movement of Trout Tagging studies done by Crossman (1957) i n 1952 i n d i c a t e d that "at l e a s t i n 1952 there were no d i s c r e t e populations o f t r o u t at any one place at any one time" (except during the spawning migration) i n Paul Lake. "The t r o u t seemed to move about f r e e l y from place to place over the length of the lake, at times moving from one end to the other". Observations and r e s u l t s of g i l l n e t t i n g on both Paul and Pinantan lakes i n d i c a t e that during the summer the large trout (10 inches and l a r g e r ) tend to stay around and, i n Paul, below the thermocline. Mottley and Jfottley (1932) state that the older f i s h i n Paul Lake "seek great depths" during the summer. Recently planted hatchery f i n g e r l i n g s (2-5 inches F.L.) were seen however i n large numbers on the shoals i n as l i t t l e as a foot o f water, o f t e n swimming i n company with schools of shiners, i n J u l y 1959» Large tro u t were frequently observed by Crossman (1957) and t h i s w r i t e r to make quick dashes i n t o shallow water (as l i t t l e as one f o o t deep) while chasing and feeding on shiners. They returned immediately i n t o deeper water o f f the shoal when they had caught or l o s t t h e i r prey. This a c t i v i t y could be observed almost continuously i n r e s t r i c t e d areas of the shoals on warm b r i g h t days i n l a t e J u l y and August i n both l a k e s . These feeding movements are described i n d e t a i l by Crossman (1957)* 20 I G. Time and. Place of Competition The only area where trout and shiners were both observed i n any numbers during the summer was j u s t o f f the shoals i n the upper twenty fe e t of water. Trout but not shiners also ranged below t h i s depth. No t r o u t were ever taken along with shiners i n the offshore g j l l n e t s e t s . (The g L l l n e t was suspended from the surface and was eight f e e t deep). Almost no surface feeding by trout was observed offshore by night (or day) during the summer observations i n Pinantan Lake. Apparently shiners do not encounter many trout during t h e i r offshore night migrations. L i t t l e i s known of the d i s t r i b u t i o n of shiners from l a t e September to May. Lindsey (1950a) states that a few shiners have been found l y i n g dormant i n the mud and among bottom debris during the winter. L a r k i n (pers. comm.) observed shiners gathering around holes cut i n the i c e i n Paul Lake. Except f o r t h i s one observation shiners have not been seen on the shoals or at the surface during the winter. While shiners c o n s t i t u t e the major food item f o r trout over 14 inches long i n the summer, they were found i n only two of 78 stomachs taken from Paul Lake t r o u t over 14 inches long taken i n December 1953 (Crossman, 1957)* This would suggest that the winter d i s t r i -butions of trout and shiners i n Paul Lake do not overlap appreciably. The d i e t of these t r o u t was very s i m i l a r to what i t was before shiners had entered Paul Lake (Crossman, 1957). Probably competition f o r food i s not intense during the winter. BEHAVIOURAL FACTORS IN COMPETITION The previous sections have dealt with what competition i s for and where i t occurs. Observation of the behaviour of the two species i s important i n order to determine how competition occurs and, i f one species i s more successful than the other, what a b i l i t i e s o r habits i t possesses J7 which make i t more succe s s f u l . Crossman (1957) states that he observed no i n t e r s p e c i f i c aggression i n nature. The shiners however "appeared to be more e f f i c i e n t feeders and when a trout and shiner darted a f t e r the same food item, the shiner i n v a r i a b l y got i t and while shiners would move r i g h t i n t o the shore to feed, t r o u t d i d not come i n t o water shallower than 15 inches." In the course o f ssummer observations the present w r i t e r saw many trout 3 to 5 inches long swimming i n company with, or i n and out of, schools of shiners i n 2 to $ f e e t o f water. There appeared to be no i n t e r s p e c i f i c agression except i n one instance when a s i x inch trout was "chased" f o r about three f e e t by a ljg i n c h shiner. Otherwise both species appeared to be o b l i -vious to one another. Shiners however moved noticeably f a s t e r and hence ranged over a wider area per u n i t time i n search of food than trout d i d . A. Behaviour of Trout i n a Pen Mthout Shiners An observation pen was marked o f f h o r i z o n t a l l y i n nine equal squares by s t r i n g s stretched across the pen along the surface. Trout (3 to 5 inches F.L.) remained at the sides o r corners of t h i s open-bottom pen j u s t above or down among the dense weed growth, except when they were feeding. Observations f o r several days showed that a c t i v i t y was greater during b r i g h t weather. In the b r i g h t e s t part of the day, out of ten t r o u t , some single i n d i v i d u a l would move from the area under one square i n t o another every one to two minutes.. On d u l l days there was v i r t u a l l y no movement except when food was introduced. Only 4 or 5 of the ten f i s h e x hibited t h i s behaviour over about tv*> dozen f i f t e e n minute observations spread over several daysj some i n d i v i d u a l s remained c o n t i n u a l l y hidden i n the weeds. There was some establishment of t e r r i t o r i e s , and nipping occurred as of t e n as three times per minute i n b r i g h t su n l i g h t . The same t e r r i t o r y was not held by a f i s h i n d e f i n i t e l y . There 28 appeared to be no unchanging order o f dominance o r "peck order" although the 1 l a r g e s t f i s h of the ten appeared t o dominate a l l the others. Considerable i n d i v i d u a l v a r i a t i o n was seen i n these a c t i v i t i e s . In some cases one trout was observed chasing another trout l a r g e r than i t s e l f . (These observations on dominance, nipping, t e r r i t o r i e s and the c o r r e l a t i o n of l i g h t i n t e n s i t y with a c t i v i t y agree w e l l with those of Stringer and Hoar (1954) i n the lab o r a t o r y ) . The f i s h preferred shaded areas i n the pen about s i x t o one over s u n l i t areas. During overcast days and i n the evenings there were no signs o f movement and the trout went deeper among the weeds. There was never a t e r r i t o r y e stablished near the center of the pen. Occasionally a swimming trout on " f i n d i n g i t s e l f " i n the middle of the pen would appear frightened and dart quickly f o r cover among the weeks at a side or corner of the pen. The t r o u t were l e f t i n the pens f o r a week without any food except the nat u r a l food they might f i n d i n the. weeds. At the end of t h i s time no feeding was observed and i t was assumed that the t r o u t had eaten a l l a v a i l a b l e n a t u r a l food. About 2000 Gammarus, most of which were a l i v e , were placed i n the pen with the t r o u t which were at the time a l l down among the weeds. The l a r g e s t (about 5 inches F.L.) t r o u t immediately came out of the weeds and made 54 feeding movements (c o n s i s t i n g of dart i n g at and swallowing a Gammarus) i n the space of four minutes. Three smaller t r o u t (3 to 4 inches) started feeding wit h i n a minute but they appeared more wary than the large t r o u t and returned immediately to t h e i r t e r r i t o r y among the weeds a f t e r each feeding movement. The l a r g e s t t r o u t often made s i x to eight feeding movements before returning to his home t e r r i t o r y . Within f i v e minutes a l l ten t r o u t were feeding with varying degrees of a c t i v i t y which seemed to be r e l a t e d to t h e i r s i z e . Sometimes two trout would pursue the same shrimp and c o l l i d e . The f i s h o f t e n nipped and 2 9 chased each other even when neither f i s h was i n i t s h a b i t u a l t e r r i t o r y . At the end of f i f t e e n minutes few Gammarus were i n s i g h t and feeding a c t i v i t y had dropped to three movements per minute. The t r o u t o f t e n mistook the t i p s of Chara shoots f o r Gammarus, mouthed them, then spat them out. Almost without exception no trout strayed f a r t h e r than 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 u s u a l l y captured by a sudden dash of two f e e t or l e s s . Smaller t r o u t (3 to 4 inches) o c c a s i o n a l l y spat out large Gammarus three or four times before f i n a l l y swallowing them, or r e j e c t i n g them completely. The t r o u t took dead Gammarus r e s t i n g on the weeds as w e l l as Gammarus i n motion. The tro u t seemed to feed at random, making e r r a t i c rushes here and there, o f t e n ignoring Gammarus near them and rushing a f t e r others f a r t h e r away. B. Behaviour of Shiners i n a Pen Without Trout Unlike the trout, shiners stayed i n one compact school i n an area of about 8 cubic f e e t j u s t above and among the t i p s of the weeds. Only when frightened by quick movements of the observer d i d they s c a t t e r down in t o the weeds. The school moved very slowly over the whole area o f the pen often staying more or l e s s i n one spot f o r ten minutes or more but never seeming to favor any spot nearer the sides or coarners than the center of the pen. No feeding movements were seen a f t e r the f i r s t day i n the pen—presumably they had cleaned out the food. No c o r r e l a t i o n of a c t i v i t y with l i g h t i n t e n s i t y was noted. When Gammarus were introduced shiners appeared to feed more e f f i c i e n t l y and methodically than t r o u t . They moved as a school eating every Gammarus near them that was v i s i b l e t o the observer as w e l l as making feeding movements at objects too small f o r the observer to see. (This was not so with the trout,, The observer could i n v a r i a b l y see the objects on which they were feeding). The shiners fed more slowly than the trout, making approximately h a l f the number 30 of feeding movements per i n d i v i d u a l per u n i t time as d i d the t r o u t . They pursued Gammarus several inches f u r t h e r down i n t o the weeds than the t r o u t would venture a f t e r them. They often spat out and rejected l a r g e r Gammarus. G. Behaviour of Trout and Shiners Together There was no observed d i f f e r e n c e between the behaviour of the two species i n the presence o f one another and t h e i r behaviour when i n separate pens, with or without food. The shiners, being at f i r s t higher above the weeds, were on the whole f a s t e r to notice the Gammarus and f a s t e r to s t a r t feeding than the t r o u t . When the trout started feeding they ranged higher above the weeds than the shiners and the shinens ranged lower among the weeds than the t r o u t , (see Figures 4 & 5). In the space o f h a l f an hour however, trou t made only three movements r i g h t up to the surface to feed. Shiners made none. Fi v e hours l a t e r there were s t i l l twelve large Gammarus f l o a t i n g on the surface. Trout o f t e n nipped and chased each other but never chased shiners. Occasionally a t r o u t and shiner would c o l l i d e while moving i n search of food. As the trout were l a r g e r and moving f a s t e r , the shiners were u s u a l l y pushed aside i n t h i s c o l l i s i o n . Incidents of t h i s nature appeared a c c i d e n t a l however and were i n -frequent. In one-half hour o f intermingled feeding of the two species no t r o u t was observed to chase or nip a shiner. On one occasion only, one two-inch shiner chased a four inch trout for about one foot h o r i z o n t a l l y . Both, shiners and trout o c c a s i o n a l l y mistook the t i p s of Chara plants f o r Gammarus. mouthed them, then rejected them. Occasionally when a t r o u t mouthed a Gammarus then spat i t out as descr-ibed p r e v i o u s l y a shiner would eat i t before the t r o u t had a chance to mouth i t again. This was the only type of overt competition f o r the same food item observed and was seen only twice i n one-half hour. Figure 4. Diagrams of the Distribution of Trout and Shiners in 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 tips, favoring neither center nor sides of the pen« Shiner <^ ==~ Trout •yTrout hidden in weeds Figure 5. Diagrams of the Distribution of Trout and Shiners in the Observation Pens when Feeding. Diag. A. Schematic Side View. Diag. B. View Looking Down on Pen. Trout range higher i n the water than shiners. Shiners go deeper into the weeds for food than t r o u t . (Trout pictured i n the weeds have just returned from feeding excursion and do not feed while i n the weeds) 0 Shiners s t i l l school though l e s s compactly than when not feeding. Trout do not school. The two species intermingle as though o b l i v i o u s to each other. Shiner -Trout Trout hidden in weeds. 33 In subsequent observations the same a c t i v i t i e s were noted. Shiners seemed to feed more qui c k l y when they had been starved f o r a longer p r i o d of time. MECHANISMS OF COMPETITION FOR FOOD It has been stated that trout would not forage down among the weeds i n search o f food l i k e shiners. A second l i n e of evidence supports the general a p p l i c a b i l i t y of t h i s observation i n the l a k e . Gammarus were abundant down to a depth of 50 meters i n Paul Lake i n pre-shiner years of low trout abundance (Rawson, 1934)• When the trout population was increased as a r e s u l t of heavy stocking, Gammarus below the Chara zone decreased markedly i n abundance while i n the Chara zone they decreased only s l i g h t l y (Larkin et a l , 1950). Chara appears to provide amphipods with 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 predation of t r o u t . I n t e r s p e c i f i c aggression i s not a f a c t o r i n competition f o r food between trout and shiners. None was observed i n the lake nor i n the observation pens. There are f i v e observations which suggest that trout are at a disadvan-tage when competing with shiners f o r amphipods i n Paul Lake. 1. Shiners range deeper among the weeds i n search of food than t r o u t , cropping o f f many bottom organisms before they move out i n t o areas i n which they are a v a i l a b l e to t r o u t . 2. Shiners appear to be able to u t i l i z e food items smaller than the smallest items taken by t r o u t . Consequently they may graze o f f Large numbers of smphipods before they reach a s i z e at which they become avai l a b l e to t r o u t . They may also graze o f f a much higher proportion of immature amphipods than t r o u t , l e a v i n g fewer to survive long enough to reproduce and r e p l e n i s h the amphipod population. (Amphipods l e f t by shiners during feeding experiments averaged three times as large as those l e f t by t r o u t ) . 35 3. In the summer during the day shiners are concentrated d i r e c t l y over the shoals. Trout tend also to be near the shoals, but f a r t h e r offshore and i n deeper water not so c l o s e to the main source o f amphipods. 4. Shiners are more methodical feeders than t r o u t , searching an area more thoroughly f o r t h e i r food. 5. There are probably 50 or more shiners f o r each trout i n the lake. Competition f o r Daphnia i n Pinantan Lake does not f a c i l i t a t e ' • study and 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 . Several f a c t s are apparent however. Shiners spread out i n the pelagic areas of the lake only at night. Trout appear to be i n t h i s area only during the day. This i s evidenced by the angling success offshore during the day while no trout were ever caught i n offshore g i l l n e t sets during the night. Presumably the two species do not overlap appreciably i n t h e i r occupancy of the pelagic -'feeding1 ^ afea*. At the present l e v e l of grazing there remained about 1200 Daphnia (2 cc per cubic meter i n the epili.mni.on i n l a t e summer. Presumably t r o u t were going short because the plankton was spread out r e l a t i v e l y d i f f u s e l y and they were unable to feed on i t as r a p i d l y as i n pre-shiner years. There were r e l a t i v e l y few amphipods i n Pinantan Lake compared with Paul Lake when shiners were f i r s t introduced, according t o Rawson (1934). He a t t r i b u t e d t h i s to. the absence of any extensive Chara zone i n Pinantan. Since t h i s time a very r i c h and extensive Ghara zone has developed. This plant a f f o r d s amphipods considerable p r o t e c t i o n from the predation of t r o u t and i t i s probable that a l a r g e amphipod population would have developed along with the Chara had shiners not been present to graze them down. ( I t has already been mentioned that shiners penetrate weeds much deeper i n search of food than t r o u t ) . In Paul Lake an extensive Chara zone and a l a r g e population of amphipod was already present when shiners antered the lake. I t i s possible that when 36 the amphipods i n Paul Lake are reduced by shiners to a l e v e l o f abundance com-parable to that i n Pinantan Lake the shiners w i l l s h i f t to Daphnia as t h e i r main food, A second phase o f competition p a r a l l e l i n g that described i n Pinantan Lake might then ensue. Nilsson (1955) reports that char i n Swedish lake turn from a d i e t of bottom organisms to plankton when the former become scarce. An a l t e r n a t i v e hypothesis i s the possible development o f a new r e l a t i o n -ship between trout and shiners i n Paxil Lake unpredictable on the b a s i s of what has occurred i n Pinantan. As Crossman (1957) has stated, reactions of animals to the environment may p o s s i b l y create d i f f e r e n t i n t e r a c t i o n s i n nearly s i m i l a r environements. Differences between the two lakes, i . e . depth, oxygen, p r o f i l e , may be such that the future r e l a t i o n s h i p of trout and shiners i n Paul Lake w i l l have l i t t l e i n common with the s i t u a t i o n described i n Pinantan Lake. SUMMARY A. Items Competed For Trout and shiners do not compete f o r space or f o r spawning area i n Paul o r Pinantan Lakes. The reduced growth rate of y e a r l i n g t r o u t since the iitizissduetionof shiners i n t o the lakes i s the r e s u l t of competition f o r food. Shiners have d r a s t i c a l l y reduced the abundance o f amphipods i n Paul Lake. Shiners and y e a r l i n g t r o u t compete f o r Daphnia i n Pinantan Lake. B. Mechanisns of Competition Yearling t r o u t and shiners show no behavioural interference with one another when swimming together. Their behaviour i s i d e n t i c a l with what i t i s when only one species i s present. D i r e c t p h y s i c a l aggression plays a minimal r o l e i n competition between the two species. During daylight i n the summer the main concentration of shiners i s over 37 the food-rich shoal areas whereas the trout are somewhat deeper and f a r t h e r offshore. Shiners go deeper i n t o the weeds i n search of food than do t r o u t . They appear to include i n t h e i r d i e t food p a r t i c l e s smaller than the smallest s i z e eaten by t r o u t . I t i s these f a c t o r s i n the shiner's biology and d i s t r i -bution plus i t s enormous numbers which have caused the depletion of amphipods i n Paul Lake and the decline i n the growth rate o f young t r o u t . In Pinantan Lake shiners have apparently reduced the density of Daphnia i n the epilimnion to a point where t r o u t are unable to feed on them as r a p i d l y as i n pre-shiner years. 38 DISCUSSION While the present study covers p a r t i c u l a r s i t u a t i o n s ^ i n only two lakes the r e s u l t s have broader s i g n i f i c a n c e . The data w i l l be considered below as they apply to the phenomena of i n t e r s p e c i f i c competition i n general and f i s h competition i n p a r t i c u l a r . Competiton i n nature i s u s u a l l y f i r s t recognized only a f t e r i t s r e s u l t s have become apparent. Usually a noticeable change i n one o f the competing populations ( i . e . reduction, e x t i n c t i o n , emigration) must occur before the observer becomes aware that competition i s taking place. Discovering the items and mechanisms of competition under these circumstances i s l i k e t r y i n g to understand a novel a f t e r reading only the l a s t chapter. The present s i t u a t i o n i n Paul Lake serves to i l l u s t r a t e how f a l s e i mplications may a r i s e from a delayed a p p r a i s a l of competition. In recent years amphipods were scarce i n the lake and formed only a small f r a c t i o n of the stomach contents of trout and shiners. On the basis of t h i s observation alone an observer would hardly suspect that amphipods were the most important item of competition. What i s more, as there i s r e l a t i v e l y l i t t l e overlap i n the present feeding habits of the two species one might conclude that competition f o r food was not occurring at a l l . The present case i s an example of Hartley's (1948) statement that, "the f i n d i n g of d i f f e r e n t foods i n d i f f e r e n t species i s not i r r e f u t a b l e proof of the absence o f competition unless i t can be shown that a l l s e l e c t i o n of foods i s by choice alone from diverse superabundant food stocks a l l equally accessible to the species studied." Observations made before and during the f i r s t phases of §©mp§ti-t i o n showed that shiners and trout were not feeding by choice alone. They have been forced by t h e i r d epletion of amphipods to replace them i n t h e i r d i e t s with other, presumably l e s s preferred foods. 39 This s h i f t i n the d i e t s of trout and shiners i n Paul Lake since compe-t i t i o n began serves to caution against assuming that the present feeding habits of the two species i n Pinantan Lake t e l l the whole story of competition there. P o s s i b l y the most important o r i g i n a l items and mechanisms o f competi-t i o n i n Pinantan were quite d i f f e r e n t from what they were at the time of t h i s study. Another important consideration i n studying competition i s : i n what d i r e c t i o n does one look to f i n d causes and mechanisms? T h e o r e t i c a l approaches to population ir is^actions deal mainly with the c h a r a c t e r i s t i c s of the competing populations, i . e . numbers of competitors, b i r t h rates, death rates and reproductive p o t e n t i a l s . They have focussed l i t t l e a t t e n t i o n upon the behaviour of the competitors and the modifying e f f e c t s o f the environment. The present study i n d i c a t e s that both these f a c t o r s may have conside-rable bearing on the dynamics of competition. Environmental f a c t o r s such as lake morphology, plant growth temperature and oxygen p r o f i l e s and l i g h t i n t e n s i t y play important r o l e s i n determining the items, areas and times of competition. I n d i r e c t l y they may also influence the i n t e n s i t y of competition by determining the d i s t r i b u t i o n of the two species and t h e i r degrees of overlap. Items of competition are at l e a s t i n part determined by t h e i r a v a i l a -b i l i t y and t h i s i n turn i s strongly influenced by plant growth. In Paul Lake where Chara and consequently amphipods were abundant, the main food item i n contention was amphipods. On the other hand, i n Pinantan Lake, because of the r e l a t i v e l y sparse Chara zone at the time of the i n t r o d u c t i o n of shiners, few amphipods were present and competiton centered around Daphnia. Environmental f a c t o r s l i m i t i n g the times and areas o f competiton are: 1. the temporal and s p a t i a l d i s t r i b u t i o n of food, 2. the summer hypolimnial oxygen d e f i c i t i n Pinantan Lake which keeps 40 fish in 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 in Paul Lake in August 1958, As a consequence shiners were found far less frequently in the stomach contents of trout in 1959 than in early 1958. Amphipods were significantly more abundant in the trout diet in 1959, presumably because there were fewer shiners to graze them down. Infestation by the tapeworm, Ligula. in shiners varied from almost 100 to almost 0 percent in 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 in their stomachs compared to uninfested fish. Probably shiners consumed significantly less food per individual in years when they were heavily parasitized. Year to year differences in rainfall and atmospheric temperature cause an almost two-fold difference in the summer heat income of Paul Lake (Larkin et al, 1950) and a five degree f&hrenheit difference in 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 in the intensity of competition. Nilsson (1955) ascribed year to year differences in the foods of trout and char in three Swedish lakes at least i n part to differences in temperature and lake levels in different years. 41 Many of the above f a c t o r s act t o make both competitors and items of competition non-randomly d i s t r i b u t e d both s p a t i a l l y and temporally i n a hi g h l y complicated fashion A more c a r e f u l study would have undoubtedly uncovered more environmen-t a l l y c o n t r o l l e d aspects of competition i n Paul and Pinantan Lakes* Why, f o r instance, are tro u t not found offshore l i k e shiners i n Pinantan Lake during the night? A t h i r d o ften neglected f a c t o r i n the dynamics of competition i s the behaviour of the competitors. Shiners are more e f f i c i e n t feeders than tr o u t ; they seem to appreciate smaller food items; they go deeper i n t o the weeds a f t e r food. The s i t u a t i o n would be f u r t h e r complicated, as i t undoubtedly i s between many pa i r s of competitors, i f there were any i n t e r s p e c i f i c behavioural patterns which favored one species over the other. Competition 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 ; the competitors need not even be i n the same place at the same time t o compete. Solomon (1949) noted t h i s when he included among the mechanisms of competition, "the occupation or consumption by e a r l i e r a r r i v a l s of something i n l i m i t e d supply so that l a t e comers are automatically excluded or deprived." The present study demonstrates t h i s s i t u a t i o n . Shiners eat plankton offshore only during the. n i g h t . Trout are apparently present i n the pelagic feeding ground only during the day. Also, shiners eat bottom organisms down among the weeds before they reach an area where they become a v a i l a b l e to t r o u t . T h i s suggests that competitors may be separated even a step f u r t h e r than that pointed out by Solomon and we can include among the b a s i c mechanisms of competition, the consumption by one or more organisms of something i n short supply before i t reaches a p o t e n t i a l habitat where i t would become available t o another organism or group. Lark i n (1956) has commented on the vague demarcation of e c o l o g i c a l 42 zones i n freshwater environements and the l a c k of sharp demarcation of f i s h faunas w i t h i n these zones. "Freshwater communities would seem to be charac-t e r i z e d by more breadth than height i n the pyramid of a food chain: a complexity i n h o r i z o n t a l organization." No b e t t e r example of t h i s statement could be imagined than the present one. Both shiners and trout eat v i r t u a l l y a l l the organisms a v a i l a b l e i n the lakes - i n c l u d i n g each other. At d i f f e r e n t times of the day and year f i s h may be found leading e i t h e r a p e l a g i c , a shoal, or a bottom existence with t h e i r food habits varying accordingly. The feeding habits of rainbow trout and shiners (Lindsey, 1950a and b, and others) i n various lakes, reveal an enormous range o f d i e t a r y tolerance. The a b i l i t y of both species 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 tends to reduce the i n t e n s i t y of competition. While amphipods have been severely depleted i n Paul Lake, both t r o u t and shiners have replaced them i n t h e i r d i e t s . That t h i s new d i e t i s not as s a t i s f a c t o r y as the o l d one f o r young trou t may be i n f e r r e d from t h e i r slower growth rate - but large t r o u t now grow f a s t e r than beofre as a r e s u l t of feeding on shiners (Crossman, 1957). Here the d i f f e -rence i n a b i l i t i e s of large and small f i s h to change t h e i r d i e t s r e s u l t s i n completely opposite e f f e c t s of competition w i t h i n d i f f e r e n t s i z e groups o f the same species. In conclusion, the present study i n d i c a t e s t h a t competition i n nature may 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 emphasis. The p h y s i c a l and b i o l o g i c a l environment and the d i s t r i b u t i o n and behaviour of competitors may be i n states of continual f l u x i n which case the "niches" of the competitors cannot be considered constant as i s a basic 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 competitive r e l a t i o n s h i p s can be considerably more complicated and v a r i a b l e than s i t u a t i o n s described by the most e l a s t i c of e x i s t i n g models. 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 in the predator-prey relationship of rainbow trout (Salmo gairdneri Richardson) and redside shiners (Richardsonius balteatus Richardson) in 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 in Paul Lake, British Columbia. Jour. Amer. Fish. Soc. for 1959. In press. Dobzhansky, T. 1950. Heredity, environment and evolution. Science, HI: 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 in a community of freshwater fishes. J. Anim. Ecol., 17(1): 1-14. Hasler, A. D. 1954* Odour perception and orientation in fishes. J. Fish. Res. Bd. Canada, 11(2): 107-129. Lagler, K. F. 1944. Problems of competition and predation. Trans. N. A". Wildlife Conf., 9: 212-219. Larkin, P. A. 1956. Interspecific competition and population control in 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 in Paul Lake, British Columbia. Trans. Am. Fish. Soc, 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) in 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 in British Columbia. Sci. Rept., B. C. Game Commission. 8 pp. Lindsey, C.C. 1953* Variation in anal fin ray count of the redside shiner Richardsonius balteatus (Richardson). Canadian J. Zool. 31: 211 - 255 44 Lindstrom, T. 1955* On the r e l a t i o n f i s h s i z e - food s i z e , Rept. from Inst. Freshw. Res. Drottningholm. 33: 70-165. MacFadyen, A. 1957. Animal Ecology: Aims and Methods. Putnam and Sons. London. McLeod, C. 1957. The growth rate of rainbow t r o u t i n Pinantan Lake, B. C. MS. I n s t i t u t e of F i s h e r i e s , Univ. of B r i t i s h Columbia. Mottley, C. McC. 1932. The propagation of trout i n the Kamloops d i s t r i c t . Trans. Am. F i s h . Soc. f o r 1932. 62: 144-151. Mottley, C. McC. and J . C. Mottley. 1933. The food of Kamloops t r o u t . Annual Rept. B i o l . Bd. Canada. 1933: 91-92. Nicholson, A. J . 1954* An o u t l i n e of the dynamics o f animal populations. Austr. J . Zool., 2(1): 9-65. Nilsson, N. 1955. Studies on the feeding habits o f t r o u t and char i n north Swedish l a k e s . Rept. from I n s t . Freshw. Res. Drottningholm. 29: 108-111. Rawson, D. S. 1934. P r o d u c t i v i t y studies i n lakes of the Kamloops region, 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 at some large Alpine lakes i n western Canada. Ecology, 23. Ricker, W. E. 1932. The u t i l i t y of nets i n fresh-water plankton i n v e s t i g a -t i o n s . TCrans. Am. F i s h . Soc. f o r 1932. 62. Ricker, ¥. E. 1936. The food and the food supply of sockeye salmon (Oncor-hynchus nerka Walbaum) i n Cultus Lake, B r i t i s h Columbia. J . B i o l . Bd. Canada, 3(5): 450-4-68. Ricker, W. E. and J . Gottschalk. 1941. An experiment i n removing coarse f i s h from a l a k e . Trans. Am. F i s h . Soc. for 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 plankton production and f i s h production i n ponds. Trans. Am. F i s h . Soc. f o r 1938. 68: 309-315. Solomon, M. E. 1949. The natural c o n t r o l of animal populations. J . Animal E c o l . 18: 1-35. Southern, R. 1933. The food and growth o f brown t r o u t . Salmon & trout Mag. June, 1932. C i t e d i n Nilsson, (1955). S t a r r e t , W. C. 1950. Food r e l a t i o n s h i p s of the minnows of the Des Moines River, Iowa. Ecology, 31(2): 216-233. Stringer, G. E. and ¥. S. Hoar, 1955. Aggressive behaviour of underyearling Kamloops t r o u t . Canadian Jour. Zool. 33: 48-160. 45 Swingle, H. S. and E. V. Smith. 1941* The management of ponds f o r the production o f game and pan f i s h . In a Symposium on Hydrobiology. Tihiv. Wisconsin press, Madison, pp. 218-226. Udvardy, M. D, F. 1951* The s i g n i f i c a n c e of i n t e r s p e c i f i c competition i n b i r d l i f e . Qikos, 3(1): 98-123. } 

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