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Habitat shifts and behavioural interactions between sympatric and experimentally allopatric cutthroat.. Andrew, Joyce H. 1985

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HABITAT SHIFTS AND BEHAVIOURAL INTERACTIONS BETWEEN SYMPATRIC AND EXPERIMENTALLY ALLOPATRIC CUTTHROAT TROUT AND DOLLY VARDEN CHAR by JOYCE H. ANDREW B.Sc, Simon Fraser University, 1975 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF. MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA December 1985 © Joyce H. Andrew, 1985 ln p resen t ing this thesis in partial fu l f i lment of t h e requ i rements for an a d v a n c e d d e g r e e at the Univers i ty o f Brit ish C o l u m b i a , I agree that t h e Library shall m a k e it freely avai lable for re fe rence a n d s tudy . I further agree that p e r m i s s i o n fo r ex tens ive c o p y i n g o f this thesis for scho la r l y p u r p o s e s may be g ranted by the h e a d o f m y d e p a r t m e n t o r by his o r her representat ives . It is u n d e r s t o o d that c o p y i n g o r pub l i ca t i on of this thesis fo r f inancia l gain shall not b e a l l o w e d w i t h o u t m y wr i t ten pe rm iss ion . D e p a r t m e n t o f T h e Un ivers i ty o f Brit ish C o l u m b i a 1956 M a i n M a l l V a n c o u v e r , C a n a d a V 6 T 1Y3 DE-6(3 /81) i i ABSTRACT The role of competition in structuring a lacustrine community of two salmonid species, cutthroat trout (Salmo c l a r k i Richardson) and Dolly Varden char (Salvelinus malma Walbaum), was investigated in three coastal B.C. lakes. Habitat u t i l i z a t i o n of both species alone ( a l l o p a t r i c ) and in coexistence with each other (sympatric) was determined by g i l l netting at depth contours from lake surfaces to bottoms such that l i t t o r a l , epipelagic, pelagic, and epibenthic habitats were sampled. From June to October, trout u t i l i z e d mainly surface habitats ( l i t t o r a l and epipelagic) in sympatry and a l l o p a t r y . Char u t i l i z e d a l l habitats in allopatry, and exhibited generalist feeding behaviour by o p p o r t u n i s t i c a l l y u t i l i z i n g d i f f e r e n t habitats as prey abundance varied between sampling periods. However, in sympatry, char s h i f t e d to deeper habitats not occupied by trout. In sympatry, trout and char were s p a t i a l l y segregated with depth. However, temporal segregation was not pronounced. The habitat s h i f t by char supports an hypothesis of competition between sympatric trout and char for habitat resources, where competition acts more strongly on char. However, food abundance partly explained patterns in f i s h d i s t r i b u t i o n . The hypothesis that habitat segregation between sympatric trout and char is based on behavioural interactions was investigated in laboratory experiments. There were changes in the type and intensity of interaction between trout and char with irradiance l e v e l that were consistent with their d i s t r i b u t i o n and depth of habitat. At high irradiance l e v e l s such as occur in surface habitats, trout were more aggressive to char than at low irradiance l e v e l s . In sympatry with trout, char may seek refuge from aggression by trout in deeper habitats with lower irradiance l e v e l s . The feeding performance of char in interspecies pairs dominated by trout increased with decreasing intensity of behavioural interactions. However, the feeding performance of these char did not improve at low irradiance l e v e l s , presumably because char continued to display subordinate behaviours while confined in an aquarium with dominant trout. Whether the s h i f t to deeper habitats by lake- dwelling sympatric char i s a result of interference mechanisms is not c l e a r . However, an hypothesis involving an interactive mechanism of segregation and interference competition along irradiance l e v e l gradients cannot be rejected by th i s study. TABLE OF CONTENTS ABSTRACT i i LIST OF TABLES iv LIST OF FIGURES v ACKNOWLEDGEMENTS v i 1 .0 INTRODUCTION 1 2.0 MATERIALS AND METHODS 7 2.1 Study Area 7 2.2 Spatial and Temporal D i s t r i b u t i o n 10 G i l l Netting 10 Limnological Sampling 13 Data Analyses 14 2.3 Laboratory Experiments 16 2.3.1 Irradiance Levels 19 2.3.2 Behavioural Interactions 20 2.3.3 Data Analyses 22 3.0 RESULTS 23 3.1 Study Lake Environmental Conditions 23 3.1.1 Morphometric Comparisons 23 3.1.2 Temperature, Oxygen, and Irradiance Levels .... 23 3.1.3 Fish Prey Di s t r i b u t i o n s 27 3.2 Length, Weight, and Age of Trout and Char 29 3.3 Ef f e c t s of Coexistence on Spatial and Temporal Di s t r i b u t i o n 35 3.3.1 Trout 37 Sympatric Trout in Loon Lake 37 A l l o p a t r i c Trout in Eunice Lake 40 Sympatric versus A l l o p a t r i c Trout 40 3.3.2 Char 45 Sympatric Char in Loon Lake 45 A l l o p a t r i c Char in Katherine Lake 48 Sympatric versus A l l o p a t r i c Char 51 3.3.3 Trout versus Char 54 3.4 Ef f e c t s of Irradiance Level on Behavioural and Feeding Interactions 59 3.4.1 General Behaviour 59 Establishment of Dominance 59 Swimming Behaviour 61 Feeding Behaviour 63 3.4.2 Behavioural Interactions 64 3.4.3 Feeding Performance 69 4.0 DISCUSSION 76 4.1 Spa t i a l and Temporal D i s t r i b u t i o n 76 4.2 Behavioural Interactions and Irradiance Level 93 4.3 Concluding Statement 102 5.0 REFERENCES 105 V LIST OF TABLES Table 1. Fish sizes and order of irradiance l e v e l treatments 18 Table 2. Agonistic, swimming, and feeding behaviours recoreded in the experiment 21 Table 3. Physical and chemical c h a r a c t e r i s t i c s of Loon, Eunice, and Katherine lakes, University of B r i t i s h Columbia Research Forest 24 Table 4. Zooplankton species in Loon, Eunice, and Katherine lakes 28 Table 5. Comparison of fork length of trout and char captured in Loon, Eunice, and Katherine lakes in 1976 and 1982 30 Table 6. Kruskal-Wallis analyses of variance of g i l l net catch per unit e f f o r t for Loon Lake sympatric trout by time of day, habitat, and month 39 Table 7. Kruskal-Wallis analyses of variance of g i l l net catch per unit e f f o r t for Eunice Lake a l l o p a t r i c trout by time of day, habitat, and month 42 Table 8. Kruskal-Wallis analyses of variance of g i l l net catch per unit e f f o r t for Loon Lake sympatric trout versus Eunice Lake a l l o p a t r i c trout by lake, time of day, habitat, and month 43 Table 9. Kruskal-Wallis analyses of variance of g i l l net catch per unit e f f o r t for Loon Lake sympatric char by time of day, habitat, and month 47 Table 10. Kruskal-Wallis analyses of variance of g i l l net catch per unit e f f o r t for Katherine Lake a l l o p a t r i c char by time of day, habitat, and month 50 Table 11. Kruskal-Wallis analyses of variance of g i l l net catch per unit e f f o r t for Loon Lake sympatric char versus Katherine Lake a l l o p a t r i c char by lake, time of day, habitat, and month 52 Table 12. Kruskal-Wallis analyses of variance of g i l l net catch per unit e f f o r t for Loon Lake sympatric trout and char by species, time of day, habitat, and month 55 Table 13. Kruskal-Wallis analyses of variance of g i l l net catch per unit e f f o r t for Eunice Lake a l l o p a t r i c trout versus Katherine Lake a l l o p a t r i c char by species, time, habitat, and month 57 Table 14. Swimming a c t i v i t y of trout and char in feeding t r i a l s and t r i a l s without prey present 65 LIST OF FIGURES Figure 1. Map of g i l l net sampling stations and lake habitats used in the analysis of s p a t i a l d i s t r i b u t i o n of trout and char in Loon, Eunice, and Katherine lakes, U.B.C. Research Forest 8 Figure 2. Temperature, dissolved oxygen concentration, Secchi depth, and midday irradiance p r o f i l e s of Loon, Eunice, and Katherine lakes 25 Figure 3. Length frequency d i s t r i b u t i o n of trout and char captured from Loon, Eunice, and Katherine lakes 31 Figure 4. Length-weight relationship and functional regression l i n e s of trout and char from Loon, Eunice, and Katherine lakes 33 Figure 5. Age composition of trout and char from Loon, Eunice, and Katherine lakes 34 Figure 6. Age-length relationship of trout and char from Loon, Eunice, and Katherine lakes 36 Figure 7. Spatial and temporal d i s t r i b u t i o n of sympatric trout in Loon Lake 38 Figure 8. Spatial and temporal d i s t r i b u t i o n of a l l o p a t r i c trout in Eunice Lake 41 Figure 9. Spatial and temporal d i s t r i b u t i o n of sympatric char in Loon Lake 46 Figure 10. Spatial and temporal d i s t r i b u t i o n of a l l o p a t r i c char in Katherine Lake 49 Figure 11. Behaviours associated with the establishment of dominance of trout over char. . J 60 Figure 12. Horizontal position in aquarium during the establishment of dominance of trout over char 62 Figure 13. Type and intensity of behavioural interactions between dominant trout and subordinate char at four irradiance le v e l s 66 Figure 14. Swimming a c t i v i t y of dominant trout and subordinate char at four irradiance le v e l s 68 Figure 15. Effe c t of behavioural interactions between dominant trout and subordinate char on the feeding performance of char 70 Figure 16. Feeding performance of dominant trout and subordinate char at four irradiance le v e l s 71 Figure 17. Type and intensity of behavioural interactions between dominant trout and subordinate char during feeding at four irradiance le v e l s 72 Figure 18. Swimming a c t i v i t y of dominant trout and subordinate char during feeding at four irradiance lev e l s 74 Figure 19. V e r t i c a l position in the water column of subordinate char at four irradiance le v e l s 75 Figure 20. Schematic diagram of habitat overlap of sympatric Loon Lake trout and char and experimentally a l l o p a t r i c Eunice Lake trout and Katherine Lake char. .. 83 v i i ACKNOWLEDGEMENTS I am grateful to my supervisor, Dr. Tom Northcote, for the opportunity to conduct th i s study, for research funding and summer support, and valuable c r i t i c i s m on my work. Constructive comments on my research proposals and thesis manuscript were also made by my other research committee members, Dr. B i l l N e i l l , Dr. Kim Hyatt, and Dr. J. Don McPhail. I thank the many individuals at the Insti t u t e of Animal Resource Ecology who helped me develop my ideas while providing encouragement and support, especially Dave Bernard, Sandie O'Neill, A l i s t a i r Blachford, Linda Berg, Dr. Lee Gass, and Dr. Tony S i n c l a i r . Accomodation in the U.B.C. Research Forest was kindly provided by the research forest s t a f f . My collaborators in the f i e l d study, Dr. Bror Jons'son, K j e t i l Hindar, and Nina Jonsson, provided valuable expertise in g i l l netting techniques and many weeks of f i s h dissections and age determination. F i e l d assistance was provided by Dr. Tom Johnston, Al Stockwell, and Brian Emerson. Video equipment for the laboratory study was provided by Dr. Kees Groot, Dr. Carl Walters, and Dr. Lee Gass. Most of a l l , I am grateful to those individuals who encouraged me to pursue an M.Sc. in f i s h e r i e s biology, namely my father Fred Andrew, B i l l P h i l l i p s , and Dr. Glen Geen, and to my family for their continued support throughout my studies. 1 1.0 INTRODUCTION The continuing debate during the past twenty years over the role of i n t e r s p e c i f i c competition in structuring animal communities has remained largely unresolved (Connell 1983, Roughgarden 1983, Simberioff 1983). A great deal of research has focused on testing the 'competitive exclusion p r i n c i p l e ' , which states that two non-interbreeding populations occupying the same niche cannot coexist i n d e f i n i t e l y (Harden 1960). Either one species w i l l become extinct l o c a l l y (Hutchinson 1957), or ecological differences between the species w i l l be magnified and the two species w i l l segregate into d i f f e r e n t niches. I n t e r s p e c i f i c competition determines the number of species that can coexist at stable population lev e l s in an area, and l i m i t s the s i m i l a r i t y of competing species in re l a t i o n to the abundance and d i v e r s i t y of c r i t i c a l resources (Werner 1977). Therefore, i n t e r s p e c i f i c competition i s p o t e n t i a l l y strongest between cl o s e l y related species because their preferred niches are often similar or overlapping. Niche s h i f t s in food or habitat u t i l i z a t i o n between species coexisting (sympatric) in one location and separately ( a l l o p a t r i c ) in d i f f e r e n t locations are generally considered to produce the strongest evidence supporting the premise that competition influences the structure of p a r t i c u l a r communities (Zaret and Rand 1971, Schoener 1974a, 1975, Werner and Hall 1976, Connell 1983). The addition or removal of competitors a l t e r s niches or abundances by varying l e v e l s of competition (Connell 1975, Colwell and Fuentes 1975). Competition has been implicated as an important selective 2 force in f i s h communities (Yoshiyama 1980, Schoener 1983). A number of studies have demonstrated the occurrence of competition between fishes as well as food and habitat p a r t i t i o n i n g , and temporal segregation (reviews in Schoener 1983, Connell 1983). Fish species may exclude potential competitors from pa r t i c u l a r habitats by either depleting food resources ( i . e . e x p l o i t a t i v e competition) or through aggressive interactions ( i . e . interference competition). Nilsson (1967) i d e n t i f i e d two types of segregation - interactive and selective - in f i s h communities. Interactive segregation implies that ecological differences such as food or habitat selection are magnified through d i r e c t behavioural interaction, while selective segregation occurs between species which have evolved differences s u f f i c i e n t l y great to be e c o l o g i c a l l y isolated in their use of one or more c r i t i c a l resources (see also Brian 1956). Many f i s h species possess a broad repertoire of behaviours (Hoar 1951), and several studies have shown that s p a t i a l segregation of species results from i n t e r s p e c i f i c aggression and t e r r i t o r i a l behaviour of salmonids in streams (Kalleberg 1958, Hartman 1965, Hartman and G i l l 1968, Everest and Chapman 1972, Cunjak and Green 1984) as well as other fishes on coral reefs (Low 1971, Myrberg and Thresher 1974). In a review of 164 studies on competition among animals, Schoener (1983) found that t e r r i t o r i a l competition prevailed among fishes. Natural sympatric populations of two salmonid species, 3 Dolly Varden char (Salvelinus malma Walbaum) and cutthroat trout (Salmo c l a r k i Richardson), coexist in Loon Lake, B.C. Individuals from these populations were experimentally segregated in 1974 - 1976 (Hume 1978), thereby creating a l l o p a t r i c populations of cutthroat trout (herein referred to as trout) and Dolly Varden char (herein referred to as char) in Eunice and Katherine lakes, respectively. These a l l o p a t r i c populations are self - s u s t a i n i n g , and several naturally produced generations are now present in these lakes. Spatial segregation between trout and char in Loon Lake and other nearby lakes was c l e a r l y demonstrated by Andrusak and Northcote (1971), Armitage (1973) and Hume (1978). Trout and char segregate with depth, trout inhabiting surface and midwater zones and char occupying deeper water. Evolved differences between these'populations have also been demonstrated. S o l i t a r y and paired trout and char from natural sympatric populations d i f f e r in their orientation in the water column in laboratory studies (Schutz and Northcote 1972) and each i s more e f f i c i e n t than the other at feeding on prey items at i t s own preferred height in the water column (Schutz and Northcote 1972, Hume 1978). There i s a major dietary overlap in limnetic zooplankton (Armitage 1973) although trout have higher capture e f f i c i e n c i e s on zooplankton species (Hume 1978). Henderson and Northcote (1985) found that irradiance l e v e l i s an important factor which determines the success of prey acqui s i t i o n by trout and char in Loon Lake. Trout are superior at prey detection and foraging e f f i c i e n c y in r e l a t i v e l y high 4 i r r a d i a n c e (e.g. l i t t o r a l and shallow l i m n e t i c h a b i t a t s ) and char are s u p e r i o r at v i s u a l d e t e c t i o n and chemoreception of benthic and p e l a g i c prey i n r e l a t i v e l y low i r r a d i a n c e (e.g. d i u r n a l deep water h a b i t a t s and n o c t u r n a l shallow water f e e d i n g ) . Despite evidence that t r o u t were dominant over char in i n t e r s p e c i f i c p a i r s used i n f e e d i n g t r i a l s (Schutz and Northcote 1972) and i n stream aquaria (Rosenau 1978), i t has been concluded i n p r e v i o u s s t u d i e s that s e g r e g a t i o n of lake p o p u l a t i o n s i s l a r g e l y s e l e c t i v e r a t h e r than i n t e r a c t i v e . However, as i n d i c a t e d by Henderson (1982), s e g r e g a t i o n between sympatric t r o u t and char need not always be i n t e r a c t i v e or s e l e c t i v e . I n t e r a c t i v e s e g r e g a t i o n may occur i n the e a r l y stage of c o e x i s t e n c e when t r o u t dominates and outcompetes char f o r food, but then s e l e c t i v e p r e s s u r e s may cause g e n e t i c changes in v a r i o u s morphological and b e h a v i o u r a l c h a r a c t e r s , r e s u l t i n g i n s e l e c t i v e s e g r e g a t i o n . There are two major o b j e c t i v e s i n t h i s study. The f i r s t i s to determine whether h a b i t a t s e g r e g a t i o n between t r o u t and char i n Loon Lake i s due to c o m p e t i t i v e i n t e r a c t i o n , and the second o b j e c t i v e i s to determine whether the b e h a v i o u r a l i n t e r a c t i o n s between t r o u t and char d i f f e r e d with i r r a d i a n c e l e v e l i n a way that would support a hypothesis of s e g r e g a t i o n based on i n t e r s p e c i f i c a g g r e s s i v e i n t e r a c t i o n s . The hypotheses to be t e s t e d are as f o l l o w s : 5 1. Trout and/or char have undergone a spatio-temporal habitat s h i f t from sympatry to allo p a t r y such that experimentally segregated trout and char occupy more similar habitats than when in sympatry. This hypothesis w i l l be examined by comparing s p a t i a l and temporal habitat uses by sympatric populations in Loon Lake with those of experimental a l l o p a t r i c populations of trout and char (Loon Lake stock) in Eunice and Katherine lakes, respectively. If differences are found, t h i s would suggest that habitat segregation may be due to competition between the two sympatric populations. A habitat s h i f t by only one species would indicate that the species which does not s h i f t i s the superior competitor. Based on the results of the f i e l d study reported herein, the following hypothesis w i l l be tested to investigate the possible r e l a t i o n s h i p between irradiance l e v e l and the effectiveness of aggressive behaviour by trout in excluding char from habitats: 2. There are changes in the intensity and type of agonistic behaviour between trout and char with changes in irradiance l e v e l that are consistent with their d i s t r i b u t i o n with depth in Loon Lake. This hypothesis w i l l be examined in a laboratory experiment using i n t e r s p e c i f i c pairs of trout and char. If there i s a reduction in aggressive behaviours with decreasing irradiance l e v e l , low irradiance habitats may provide refugia for char (the i n f e r i o r competitor, according to the f i e l d results reported 6 h e r e i n ) i f i t i s a b l e to a c q u i r e food resources i n these h a b i t a t s . T h e r e f o r e , the f o l l o w i n g hypothesis w i l l be t e s t e d : 3. The r a t e of p l a n k t i v o r o u s f e e d i n g of char ( i n the presence of t r o u t ) i s reduced by a g g r e s s i v e behaviour of t r o u t . 7 2.0 MATERIALS AND METHODS 2.J_ Study Area The present study was conducted in three small, oligotrophic lakes in the University of B r i t i s h Columbia Research Forest (49° 19'N, 122° 34'W), near Haney, B.C. (Figure 1). The lakes are situated in coastal mountain uplands at elevations between 340 m (Loon Lake) and 505 m (Katherine Lake). The surrounding topography i s characterized by steep slopes covered by western hemlock forest with stands of alder, birch, and planted Douglas f i r ( F e l l e r 1975). There are g r a n i t i c outcrops of quartz io d i t e in the northern portion of the Research Forest and gradual slopes of forest-covered g l a c i a l t i l l in the south (Roddick and Armstrong 1956). The climate i s wet and mild (Efford 1967). Eunice and Katherine lakes freeze over in winter but Loon Lake i s ice-covered only in occasional winters. Each of the three lakes i s almost e n t i r e l y surrounded by forest to the water's edge. Loon Lake contains patches of water l i l y pads (Nuphar polysepalum) along approximately one-fourth of the shoreline, and beds of h o r s e t a i l (Equisetum f l u v i a t i l e ) and pondweed (Potamogeton spp.) in the l i t t o r a l zone near the south end of the lake. In Eunice Lake, there are f l o a t i n g mats of bog vegetation along the south shore of the main part of the lake. S i m i l a r l y to Loon Lake, Eunice Lake contains patches of water l i l y pads, but also has sparse patches of skunk cabbage (Lysichiton americanum), ferns, and shrubs near the shoreline. 2 & 1 0 2 0 4 0 S A M P L I N G D E P T H C O N T O U R ( m ) Figure 1. Map of g i l l net sampling stations and lake habitats used in the analysis of s p a t i a l d i s t r i b u t i o n trout and char in Loon, Eunice, and Katherine lakes, University of B r i t i s h Columbia Research Forest. 9 Katherine Lake has more abundant growth of aquatic macrophytes in the l i t t o r a l zone than the other two lakes, e s p e c i a l l y near i t s north and south ends. The l i t t o r a l zone contains patches of grasses and reeds (Graminae), water l i l y pads, and other submerged vegetation. Loon Lake contains coexisting native populations of cutthroat trout and Dolly Varden char, but no other f i s h species. Abundance of adults of these species was estimated by the Schnabel method during the period 1974 - 1976 to be 7300 and 3100 f i s h , respectively (Hume 1978). Eunice and Katherine lakes were both f i s h l e s s u n t i l 1974. Between October 1974 and June 1976, a t o t a l of 1571 cutthroat trout and 881 Dolly Varden char were transplanted from Loon to Eunice and Katherine lakes, respectively (Hume 1978). The numbers transferred were s u f f i c i e n t to assume genetic homogeneity between donor and transplanted stocks (Ryman and Stahl 1980). The transplanted populations reproduced successfully in each of the new lake systems so that by 1982 there could have been up to eight successive year classes recruited to them and at least two generations that completed th e i r entire l i f e cycle within the recipient lakes. 10 2.2 Spat i a l and Temporal D i s t r i b u t i o n Spatial d i s t r i b u t i o n s of f i s h populations were assessed during three sampling periods in 1982: (1) 22 June to 5 July (hereafter referred to as the June sampling period), (2) 16 to 25 August, and (3) 30 September to 10 October (hereafter referred to as the October sampling period). G i l l Netting Fish were captured in nylon monofilament g i l l nets. Each g i l l net gang was composed of seven 5 m long panels of increasing mesh sizes (20, 25, 31, 38, 44, 51, and 60 mm stretched diagonal mesh). Nets were either 2, 5, or 10 m deep and were marked at 1 m intervals to f a c i l i t a t e determination of capture depth of f i s h . Sampling stations at Loon and Eunice lakes were located along the 2, 5, 10, 20, and 40 m depth contours, and in Katherine Lake at the 2, 5, 10, and 20 m depth contours (Figure 1). Stations were marked with buoys and retained throughout the study. G i l l nets were set during day (8.0 ± 4.5 h) and night (13.5 ± 4.0 h) periods. During each sampling period a l l stations were sampled from surface to bottom at least twice. At the 10, 20, and 40 m stations g i l l nets were set successively so that a l l depths at every station were sampled. For example, at the 40 m station, a 10 m deep g i l l net was set at 0-10 m, 10-20 m, 20-30 m, and 30-40 m on four sequential days to complete one "day sample". In addition to experimental sampling, an extra set was performed in Katherine 11 Lake to supplement low August catches. The net was set overnight on the bottom 2 m, the net extending from the shore to deep benthic habitats of the lake. One potential problem with t h i s sampling design i s that catch per unit e f f o r t may decrease with repeated sampling at each station. Since each depth was sampled four times (two day and two night sets), day or night catches may be higher i f the f i r s t sampling was respectively a day or a night set. Day and night catches per unit e f f o r t were s i g n i f i c a n t l y d i f f e r e n t for each population in June (x2=3.84, p<.05, df=l), and in three out of four f i s h populations the greater catch was obtained during the time consistent with f i r s t net exposure. However, differences in day and night catches should not af f e c t t h i s part of my study whose purpose was to compare d i s t r i b u t i o n s of f i s h populations and not t o t a l catch between lakes. F i r s t net exposure may bring about a non-random depletion of the population in the v i c i n i t y of each station so that by f i s h i n g f i r s t at the shallower layers of the water column, the catches at depths of 10 m to 40 m may be reduced, which would bias d i s t r i b u t i o n s and make them appear shallower. However, every population was fished in the same manner and d i s t r i b u t i o n s therefore would be biased in the same way i f such depletions occur. There are some d i f f i c u l t i e s in determining f i s h density and d i s t r i b u t i o n by inference from results of g i l l net sampling (Andreev 1955), although researchers frequently have used g i l l nets for t h i s purpose (e.g. Horak and Tanner 1964). G i l l nets 12 are passive sampling devices in that the capture of f i s h is due to their swimming into the net and becoming g i l l e d and entangled. If f i s h are more active at certain times of the day or in certain habitats (presumably due mainly to foraging or spawning a c t i v i t y ) , a higher catch per unit e f f o r t w i l l result in that sample. In addition, f i s h may be able to v i s u a l l y perceive and avoid capture by g i l l nets better during the day than night. Because the e f f i c i e n c y o.f g i l l nets varies with irradiance l e v e l and a c t i v i t y of f i s h , the accuracy of the interpretation of catch per unit e f f o r t as " f i s h density" varies between samples. However, conditions of irradiance with depth and the d i e l illumination cycle were similar among lakes and habitats, and sampling dates of f i s h populations. Furthermore, since foraging or reproductive a c t i v i t i e s are obviously related to the "importance" of habitats to f i s h , a higher catch per unit e f f o r t biased by these a c t i v i t i e s i s i n d i c a t i v e of a habitat which i s "useful" to f i s h . In any case, biases in the measurement of catch per unit e f f o r t probably were similar for a l l populations, lakes, and habitats so that comparisons between them would be biased in much the same way. Fish removed from g i l l nets were sampled at a f i e l d laboratory at Loon Lake. Depth of capture (within 1 m depth intervals) was recorded as f i s h were removed from g i l l nets. At the laboratory, species, date and location (lake and station) of capture, fork length (± 1.0 mm), weight (± 0.1 g), sex and state of sexual maturity were recorded. Females with eggs up to pinhead size and males with testes enlarged up to half the body 13 cavity length were recorded as immature (juvenile) f i s h . If gonad development was more advanced the state of sexual maturity was recorded as mature (adult), following Dahl (1917). Ages were determined later using o t o l i t h s (details in Jonsson et a l . 1984). Limnoloqical Sampling To test the hypothesis that niche u t i l i z a t i o n of a l l o p a t r i c and sympatric populations is the same, lake environments should be i d e n t i c a l with respect to limnological features, as well as prey types and sizes. Such ideal conditions are rarely i f ever met in whole lake experiments. However, to determine whether there were important differences between lakes and seasonal differences within lakes, limnological measurements and sampling of prey types were conducted during each of the three g i l l netting periods. Temperature, dissolved oxygen concentration, and l i g h t penetration p r o f i l e s were determined at the deepest point of each lake (Figure 1). Temperature and oxygen concentration were measured using a YSI Model 57 meter with a 15 m cable and probe. Measurements of temperature and dissolved oxygen at depths greater than 15 m were obtained using the same apparatus, but water samples were brought to the surface in a 3 1 Van Dorn bottle . Light penetration was measured by a standard Secchi disc and/or Licor Model LI-185A l i g h t meter. Invertebrate prey types, densities and d i s t r i b u t i o n s were determined by Hindar et a l . (in prep.) concurrently with my study and are summarized in Section 3.1.3. Zoobenthos were 1 4 sampled with a 9 x 9" Ekman dredge at the g i l l netting stations in a l l sampling periods. Five p a r a l l e l samples were taken at each st a t i o n . Zooplankton were sampled by diagonal hauls with Clarke-Bumpus gear (0.08 mm net) from the depths 0-5, 5-10, 10- 20, and 20-40 m. Surface arthropods were sampled with a net (frame size 30 x 30 cm, 0.2 mm mesh) at distances of 50-150 m towed from the bow of a boat along the shore l i n e and in mid- water (6 samples per lake per month). Data Analyses Numerical catch data from the three lakes were used to determine s p a t i a l and temporal habitat use. of trout and char. The d i s t r i b u t i o n of each species in sympatry and allopatry was compared to determine whether one or both species had undergone a habitat s h i f t . A second test was performed to determine the r e l a t i v e s i m i l a r i t y of habitat use by the two species by comparing habitat use by trout and char in sympatry with habitat use in al l o p a t r y . An hypothesis of competition predicts that the two species prefer more similar habitats than they occupy in sympatry. Spatial and temporal d i s t r i b u t i o n s of populations were compared using the Kruskal-Wallis extention to two and three factor nonparametric analyses of variance on ranked values (Zar 1984, p.219-222, 249). In the model for analysis of variance (ANOVA), depth i s nested in sta t i o n , because va r i a t i o n in catch with depth i s confounded by the sloping lake bottom. For example, the catch at depth equal to .2 m at the 2 m contour has 1 5 a benthic influence whereas this component i s absent at the same depth at the other stations. To avoid t h i s confoundment, results from g i l l netting stations and depths were assigned to four habitat zones.- The l i t t o r a l zone included the 2 m and 5 m stations, the epipelagic habitat included the upper 5m of the 10 m, 20 m, and 40 m stations, and the pelagic habitat included the 5 to 15m depth zone at the 20 m station and 5 to 35 m depth zone at the 40 m station. Katherine Lake had reduced sampling e f f o r t in the l a t t e r three habitat zones because i t lacked a 40 m contour. A 5 m deep g i l l net set was considered to be one sampling unit. A l l samples were corrected to catch per unit e f f o r t (individuals captured/100 m2 net area/12 h set). To determine the s p a t i a l and temporal d i s t r i b u t i o n of f i s h populations, catch per unit e f f o r t ANOVAs were determined with respect to habitat, time of day, and sampling month. Di e l and seasonal movements of f i s h between habitats were inferred when the r e l a t i v e catch per unit e f f o r t in lake habitats changed with time of day or month, respectively. Whether habitat s h i f t s occurred between sympatric and. a l l o p a t r i c populations was determined by catch per unit e f f o r t ANOVAs of either trout or char with respect to lake (Loon Lake versus Eunice or Katherine Lake), sampling month, time of day, and habitat. Tukey's multiple comparison of means (p=.05) was used to determine homogeneous sets of means from ANOVAs (Zar 1984, p.199). Genlin software was used throughout the analyses. 16 2.3 Laboratory Experiments Trout and char were c o l l e c t e d in May 1984 from experimental a l l o p a t r i c populations (Hume and Northcote 1985) from Eunice and Katherine lakes, respectively. Collections were made with monofilament g i l l nets of stretched mesh sizes 20 to 60 mm. Despite a r e l a t i v e l y high i n i t i a l mortality in the laboratory, adequate numbers were maintained there for four months before experiments started. The two species were held separately at temperatures seasonally ranging from 6.0 to 12.5 °C in large oval fibreglass aquaria (137 x 78 x 70 cm deep) with flow- through providing water replacement every 2.3 hours. Fish were fed d a i l y rations of chopped chicken l i v e r ; some char would not eat l i v e r and were fed Neomysis mercedis, a mysid shrimp. Fish used in the experiments were segregated from the rest of the stock and held i n d i v i d u a l l y or in mixed-species pairs for 3-6 days in similar but smaller aquaria with water replacement rates of approximately one hour (112 x 50 x 36 cm deep). These f i s h were fed ad libitum d a i l y rations of Neomysis mercedis. Agonistic behaviour of six mixed-species pairs of trout and char was recorded at four irradiance l e v e l s . Each pair of f i s h was considered to be one r e p l i c a t e . Each pair was held for 2-5 days at the highest irradiance l e v e l u n t i l one f i s h became "dominant" and the other "subordinate". Based on the results of my f i e l d study, trout are superior competitors to char in Loon Lake, therefore only pairs in which the trout was dominant were used in the experiment. Although trout were dominant in eight out of the ten size-matched interspecies pairs used in the 17 laboratory, there was no s i g n i f i c a n t difference at p=.05 in frequency of dominance by trout and char with t h i s small sample size (binomial test, one-tailed, p=.055). Since decreased l i g h t l e v e l was the experimental treatment, the highest irradiance l e v e l was the control, and was used to establish baseline levels of aggression. The three levels were then presented in the next three consecutive days. Rather than using d i f f e r e n t f i s h pairs at each treatment l e v e l , behaviours were recorded for each f i s h pair at each treatment l e v e l . Although t h i s procedure was used to avoid l o g i s t i c a l problems, i t viol a t e d the assumption of i n f e r e n t i a l s t a t i s t i c s of independence of data at treatment l e v e l s . To p a r t i a l l y circumvent t h i s problem, treatments were presented to f i s h pairs in random order so that prior experience at other irradiance levels was randomized (Table 1). Fish were allowed at least one day of acclimation to each l i g h t l e v e l before data on behaviour were recorded. Two t r i a l s of 30 minutes duration were performed each day, one before and another after feeding. T r i a l s were conducted between 10:00 and 17:00 PST. Each replicate pair was held in the experimental aquarium u n t i l data on behaviour at each l i g h t l e v e l had been recorded. Following each r e p l i c a t e , f i s h were anaesthetized in 2- phenoxyethanol and were measured and weighed to v e r i f y s i z e - matching of pairs (Table 1). Pairs were held together in the experimental aquarium prior to treatments u n t i l the f i s h were acclimated to the aquarium and aggression between the trout and char had s t a b i l i z e d (see Sect ion 3.4.1). 18 Table 1. Fish sizes and order of irradiance l e v e l treatments. Trout Char Order of Length Weight Length Weight Replicate P a i r 1 Treatments 2 (cm) (gm) (cm) (gm) 1 T1--C1 I-111 -IV -II 24 1 1 25 0 23 0 1 1 2 5 2 T2--C2 I-I I -III -IV 22 4 1 1 5 0 22 3 98 0 3 T3--C3 I-IV- III -II 25 5 1 57 5 25 8 1 56 0 4 T3--C4 I - I I -IV- III 25 5 1 57 5 26 5 1 75 5 5 T4--C5 I-111 -II -IV 24 0 1 22 5 23 2 108 .0 6 T4--C6 I-111 -IV -II 24 0 1 22 5 24 .5 1 1 4 .0 1T=trout; C=char 2Irradiance l e v e l treatments: I = 3.0 X 1 0 1 8 photons/m2/s II = 1.5 X 1 0 1 6 photons/m2/s III = .5.0 X 1 0 1 5 photons/m2/s IV = 3.0 X 1 0 1 5 photons/m2/s 19 During the establishment of dominance and the period of data recording, f i s h in the experimental aquarium were fed da i l y rations of 25 l i v e Neomysis mercedis. Neomysis were coll e c t e d from the Main Arm of the Fraser River, east of the George Massey tunnel. Neomysis were not present in Eunice and Katherine lakes, therefore both species were equally inexperienced with this prey prior to the experiment. Neomysis i s a r e l a t i v e l y large planktonic or epibenthic invertebrate, and i t s swimming movements make i t a highly v i s i b l e prey to both trout and char. The mean length of Neomysis used was 11.23 ± 2.76 mm (mean ± standard deviation) t o t a l length (anterior end of carapace to t i p of telson), and was not s i g n i f i c a n t l y d i f f e r e n t between samples (F-test, p>.05, F=2.46, df=4,115). 2.3.]_ I r radiance Levels Experiments were conducted in a glass-fronted brown wooden aquarium (118 x 56 x 30 cm deep) with a sand substrate and flow- through providing water replacement every 1.8 h. Water temperature varied seasonally from 9.0 to 13.0 °C. The tank was illuminated by two V i t a - l i t e fluorescent tubes, mounted in a light-proof housing and suspended 50 cm above the water surface. The spectral d i s t r i b u t i o n of V i t a - l i t e s approximates that of the sun (Henderson 1982, his Figure 2). The three lower irradiance lev e l s were obtained by s l i d i n g a board with a lengthwise 0.64 cm s l i t under the housing, and placing layers of black c l o t h over the s l i t . The highest irradiance l e v e l used (3.0 x 10 1 8 photons/m2/s) 20 was greater than the saturation irradiance threshold (SIT) of char and near that of trout. The SIT i s the minimum quantity of irradiance that maximizes reaction distance to prey (Henderson and Northcote 1985). According to Henderson and Northcote, both species use vi s u a l prey detection above the SIT and trout always use v i s u a l prey detection. The lowest irradiance l e v e l used (3.0 x 10 1 5 photons/m 2/s) was at the visual irradiance threshold (VIT) of trout but greater than that of char. The VIT is the maximum quantity of irradiance r e s u l t i n g in zero reaction to prey, below which prey targets are not detected v i s u a l l y (3.0 x 10 1 5 and 7.0 x 10 1" photons/m 2/s for trout and char, respectively; Henderson and Northcote 1985). 2.3.2 Behavioural Interactions Several categories of agonistic behaviour, which included both aggression and submission, were recorded (Table 2). Despite improved v i s i b i l i t y to the observer using the special video camera, observations at the lower irradiance levels were lim i t e d . Therefore, only r e l a t i v e l y obvious behavioural acts were recorded to ensure reg u l a r i t y and r e l i a b i l i t y in recording. More subtle behaviours such as threat postures (e.g. f i n raising) were not recorded. Similar procedures were used on in t e r - and i n t r a s p e c i f i c pairs as well as s o l i t a r y f i s h of both spec i e s . Observations of f i s h were made from outside the light-proof room which housed the experimental aquarium. For purposes of observation, the aquarium was illuminated with infrared l i g h t of 21 Table 2. Agonistic, swimming, and feeding behaviours recorded in the experiment. Behavioural Act Description A. Aggressive Behaviour Charge Aggressor rapidly darts at body of submissive f i s h , but aggressor does not chase submissive f i s h i f i t attempts to escape. Chase Aggressor chases submissive f i s h down length of aquarium, usually at burst swimming speed. Nip Aggressor bites or nips t a i l or other body parts of submissive f i s h . B. Submissive Behaviour Avoidance Submissive f i s h avoids an aggressive interaction by fast swimming (usually < burst speed) down length of aquarium when the aggressor approaches. C. Swimming Behaviour Swimming a c t i v i t y Movement in horizontal position in aquarium to a d i f f e r e n t q u a r t i l e = one unit of a c t i v i t y . Recorded only during l u l l s in aggression. Bottom rest Occurred in char only; resting on substrate on pectoral and caudal f i n s . Diagonal hover Submissive f i s h hovers in water column, usually near surface, in a non-horizontal position (approximately 30° angle) with i t s head up. Fish may be stationary or move forward slowly, but most movements are balancing movements, mainly of the pectoral f i n s . D. Feeding Behaviour Feeding s t r i k e Rapid forward movement at prey, not necessarily r e s u l t i n g in capture; occurred in feeding t r i a l s only. 22 two incandescent lamps shielded by 12.7 cm Kodak Wratten Series 88A f i l t e r s . The lamps were placed at an angle on top of the aquarium near the front so as not to block l i g h t from the V i t a - l i t e s . Henderson (1982) found that the mean reaction distances of trout and char were not s i g n i f i c a n t l y d i f f e r e n t in the presence or absence of infrared l i g h t when V i t a - l i t e levels were 4.2 x 10 1 7 and 3.0 x 10 1 5 photons/m2/s for trout and char, respectively. However, other f i s h species can perceive far-red l i g h t up to 740 nm (Beauchamp et a l . 1979), and there is no evidence that salmonids do not have similar high red s e n s i t i v i t y in s u f f i c i e n t l i g h t intensity (R.D. Beauchamp, pers. comm.). Fish were observed on a video monitor through a Sanyo S i l i c o n Diode video camera (VCS 3000) f i t t e d with a Fujinon T.V. EE 1:1.4 25 mm photomultiplier lens and a V i t i c o n tube which i s sensitive to infrared l i g h t . Behaviours were recorded on an electronic hand-held event recorder (Observational Systems OS- 3) . 2.3.3 Data Analyses The s t a t i s t i c a l test used to analyze the experimental results was one-way ANOVA for single-factor experiments with repeated measures (Winer 1971, Rodgers 1977). In a l l ANOVAs, the number of behavioural interactions was transformed using log,o(behavioural interactions + 1.0) to normalize the Poisson d i s t r i b u t i o n of the data. Minitab software was used throughout the analyses. 23 3.0 RESULTS 3.j_ Study Lake Environmental Conditions 3_.j_.j_ Morphometric Comparisons Loon Lake i s larger and deeper than Eunice and Katherine lakes based on surface area, depth, and volume comparisons (Table 3). Loon Lake has over twice the surface area and volume of either of the other two lakes. The three lakes have similar shoreline development ( D t ) , which ranges from 1.5 (Eunice Lake) to 2.2 (Loon Lake). However, Katherine Lake has a larger percentage of i t s surface area formed by l i t t o r a l zone than does Eunice or Loon Lake (Table 3). Almost one-fourth of the surface area of Katherine Lake i s less than 2 m deep, while Loon and Eunice have much smaller l i t t o r a l zones (7.1% and 10.4% of their surface areas, re s p e c t i v e l y ) . 3̂ J_.2 Temperature, Oxygen, and I rradiance Levels During each of the three sampling periods, a l l lakes were thermally s t r a t i f i e d with well-developed thermoclines (Figure 2). The ep i l i m n i a l depths were similar for a l l lakes (Table 3). Maximum ep i l i m n i a l temperatures- were similar in a l l lakes (approximately 20 °C) during summer but decreased (12-15 °C) and deepened (6.5-8.5 m) during autumn. In October, erosion of epilimnia had begun but f a l l turnover had not yet occurred. During each of the three sampling periods, the dissolved 24 T a b l e 3. P h y s i c a l and c h e m i c a l c h a r a c t e r i s t i c s of L o o n , E u n i c e , and K a t h e r i n e l a k e s , U n i v e r s i t y of B r i t i s h Columbia R e s e a r c h F o r e s t . Loon E u n i c e K a t h e r i n e E l e v a t i o n (m) 340 480 505 S u r f a c e a rea (ha) . 48. 6 18.2 20.7 Maximum depth (m) 62 42 29 Mean d e p t h (m) 27. 5 15.8 7.5 Volume (m3 x 1 0 4 ) 1 1 336 288 175 S h o r e l i n e development ( D t ) 1 2. 2 1 .5 1.9 Shal low l i t t o r a l area (0-2 m) (percent of l a k e area) 7. 1 10.4 24.5 E p i l i m n i o n d e p t h i n 1982 (m) June 5. 5 3.5 4.5 August 6. 5 6.0 6.0 October 8. 5 6.5 7.5 S e c c h i d i s c t r a n s p a r e n c y i n 1982 (m) June 9. 3 8.5 9.1 August 8. 1 4.0 7.3 October 7. 5 - - I r r a d i a n c e e x t i n c t i o n c o e f f i c i e n t (TJ) 1 . 1 1 .7 1 .4 p H 2 6 . 4 - 6 . 7 6.4 6.6 C o l o r (Pt u n i t s ) 2 <5 1 5 1 0-15 T o t a l d i s s o l v e d s o l i d s ( m g / L ) 2 32 1 6 1 5 'Hume 1978 2 N o r t h c o t e and C l a r o t t o 1975 2 5 LOON EUNICE KATHERINE TEMPERATURE (C) AND DISSOLVED OXYGEN (mg/L) 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 o o CO o TEMPERATURE c z m o. UJ a ' y l S\ / 1 ' / 1 1 ( 1 1 / / / 1 / / / / 10 .15 1017 1019 10'" 10" 1017 1019 1021 10" 10 IRRADIANCE LEVEL (photons/m /s) „15 > c c cn o n -* o ro m 33 19 21 10 10 F i g u r e 2. T e m p e r a t u r e , d i s s o l v e d o x y g e n c o n c e n t r a t i o n , a n d m i d d a y i r r a d i a n c e p r o f i l e s o f L o o n , E u n i c e , a n d K a t h e r i n e l a k e s . ( S u r f a c e c o n d i t i o n s i n O c t o b e r : 7.53 x 1 0 1 S p h o t o n s / m 2 / s ; f l a t c a l m ; n o c l o u d ) 26 oxygen p r o f i l e s of the lakes usually exhibited positive heterograde curves. The only exceptions were Katherine and Eunice lakes in October, where dissolved oxygen concentration decreased with depth. The dissolved oxygen concentration maximum at the thermocline may be a result of decreased s o l u b i l i t y of oxygen in the epilimnion due to high summer temperatures, oxygen consumption in the hypolimnion ( t y p i c a l of clinograde reduction with depth), and production of oxygen by phytoplankton at the thermocline (Wetzel 1983). The l a t t e r process was probably not important, as dissolved oxygen maxima seldom exceeded 110%. Eunice Lake was anoxic near the sediment in deep parts of the lake during August. Secchi disc transparencies of the lakes were similar during June (Table 3). In a l l lakes, water transparency decreased between June and October, perhaps due to accumulated phytoplankton biomass over the growth season. Irradiance p r o f i l e s in October indicate that l i g h t extinction with depth i s more rapid in Eunice and Katherine lakes than in Loon Lake (Table 3). However, the pattern of l i g h t extinction in the three lakes i s t y p i c a l of coastal oligotrophic lakes. The study lakes tend to be s l i g h t l y a c i d i c (pH 6.4-6.7), and Pt values range from <5-15 units (Table 3). Although Loon Lake has approximately twice the t o t a l dissolved s o l i d s content of Eunice or Katherine lakes, a l l three lakes are within the low range t y p i c a l of coastal B r i t i s h Columbia lakes (Northcote and Larkin 1956). 27 3.j_.2 Fish Prey Distributions The study lakes share similar prey types (Table 4), densities, and d i s t r i b u t i o n s (Hindar e_t a l . in prep.). The densities of surface arthropods (mainly winged insects) were not s i g n i f i c a n t l y d i f f e r e n t between the lakes in 1982, and were less than 2 individuals/m 2 during a l l sampling periods except Eunice in August and Katherine in October, when both exceeded 4 individuals/m 2. The proportion of large (̂ 4 mm) surface arthropods was highest in July and in Katherine in October. Limnetic zooplankton densities were highest (9000 - 23000 individuals/m 3) at 0-5 m, and decreased with depth in a l l samples except Katherine in October, when the density was highest at 5-10 m (18846 individuals/m 3 at 5-10 m versus 8734 individuals/m 3 at 0-5 m). From spring to autumn, densities at depths greater than 10 m (10-20 and 20-40 m sampling intervals) were always higher in Loon (379-4264 individuals/m 3) than the other lakes (287-1023 individuals/m 3 and 489-1887 individuals/m 3 in Eunice and Katherine lakes, respectively). There was a r e l a t i v e l y low proportion of large size classes of zooplankton in a l l lakes in July and in Eunice Lake in August. The densities of l i t t o r a l zoobenthos (mainly Hyalella azteca, Pisidiurn sp., and chironomid larvae) were highest at the 2 m depth contour in October in a l l lakes (1500 - 2200 individuals/m 2). There was a s i g n i f i c a n t increase with depth in the proportion of large (£8 mm) zoobenthos in a l l lakes except Katherine in July. Profundal zoobenthos was almost exclusively chironomid larvae, and showed density maxima at 40 m in Loon, Table 4. Zooplankton species in Loon, Eunice, and Katherine lakes. 1 Loon Eunice Katherine , Cladocera Daphnia rosea Bosmina l o n g i r o s t r i s Holopedium gibberum Diaphanosoma brachyurum Polyphemus pediculus Leptodora k i n d t i i Ceriodaphnia pulchella , Copepoda Diaptomus Diaptomus Diaptomus Diaptomus Cyclopoda kenai leptopus oregonensi s t y r r e l l i *** 2 * * * ** ** ** ** *** ** ** * * * *** * * * * * * * * * * * * ** ** * ** ** ** ** ** * * * * * * ** ** ** ** ** ** 1Data from Hindar et a_l. in prep., Hume and Northcote 1985, and Northcote and Clarotto 1975. 2*** v e r y common; ** common; * uncommon; - rare 29 usually at 20 m in Eunice, and at 10 m in Katherine Lake. 3.2 Length, Weight, and Age of Trout and Char Size differences between f i s h in sympatric and a l l o p a t r i c populations were not as pronounced in 1982 as they were in 1976 (Table 5). Trout and char transferred in 1974 - 1976 to Eunice and Katherine lakes, respectively, grew quickly following the experimental transfers to previously f i s h l e s s lakes with abundant food resources (Hume and Northcote 1985). This conclusion was based on comparisons of length d i s t r i b u t i o n s , length-weight relationships, and increases in the growth rates of individual f i s h from sympatric and a l l o p a t r i c populations. In 1976, the mean lengths of a l l o p a t r i c trout and char (209.9 and 237.2 mm, respectively) were s i g n i f i c a n t l y greater than mean lengths of f i s h in their sympatric donor populations (180.0 and 172.0 mm, respectively; t - t e s t s , two-tailed, p<.00l; Hume 1978). Six years l a t e r , the mean length of a l l o p a t r i c char (197.2 mm) was s t i l l s i g n i f i c a n t l y greater than sympatric char (182.2 mm; t- t e s t , two-tailed, p<.00l) although the difference was not so great as in 1976, but the mean length of a l l o p a t r i c trout (165.9 mm) was s i g n i f i c a n t l y less than sympatric trout (178.4 mm; t- test, two-tailed, p<.00l; Table 5, Figure 3). Differences in f i s h sizes between years may be attributed to the wider range of g i l l net mesh sizes used in 1982 (20-60 mm) than in 1976 sampling (25-51 mm stretched diagonal mesh). Since trout and char are native in Loon Lake and have coexisted for centuries, these populations may be assumed to be 30 Table 5. Comparison of fork length of trout and char captured in Loon, Eunice, and Katherine lakes in 1976 and 1982. Fork Length (mm) t-Test t-Test Between Between Range Standard Lakes Years N min,max Mean Deviat ion (df) 1 (df ) 2 A. 1976 (Hume 1978) Trout Loon 218 1 1 1 ,233 180.0 18.4 7 . 272***3 Eunice 214 116,310 209.9 41.8 (430) Char Loon 25 112,217 1 72.0 24.8 6.957*** Kather ine 1 25 134,337 237.2 45.4 (148) B. 1982 Trout Loon 1066 77,332 178.4 30. 1 .000ns 21 .776*** (1282) (1981) Eunice 917 82,270 1 65.9 26.5 19.311*** (1129) Char Loon 288 96,220 182.2 14.7 3.105** 5.633*** (31 1 ) (678) Katherine 392 100,323 1 97.2 43.4 8.861*** (515) 1H 0: Mean length in experimental lakes = mean length in Loon Lake 2H 0: Mean length in 1976 = mean length in 1982. 3*** p<.00l; ** p<.0l; * p<.05; ns=not s i g n i f i c a n t p>.05 31 TROUT o in o o in o > 2 u z 3 o a ui cr ««- o }— m z 111 O g - UJ a. o o CV) o o J (1066) t = 4 - (922) o in o o CO o OJ CHAR 60.0 o J o in (291) in -< z "0 > o o o CM O (462) r o •a > —< 33 n <99 120 160 200 240 »260 <99 120 160 200 240 £260 FORK LENGTH (mm) F i g u r e 3 . L e n g t h f r e q u e n c y d i s t r i b u t i o n o f t r o u t a n d c h a r c a p t u r e d f r o m L o o n , E u n i c e , a n d K a t h e r i n e l a k e s . ( S a m p l e s i z e s a r e shown i n p a r e n t h e s e s . ) 32 stable, and the Malthusian parameter (r) equal to zero (Jonsson et a l . 1984). The mean length of trout in Loon Lake did not d i f f e r s i g n i f i c a n t l y between 1976 and 1982 ( t - t e s t , two-tailed, p>.05), but that of char was s i g n i f i c a n t l y longer in 1982 (t- test, two-tailed, p<.0l). The result for char, however, is questionable due to the r e l a t i v e l y low sample size in 1976 (n=25). The mean lengths of a l l o p a t r i c trout and char in 1982 were s i g n i f i c a n t l y shorter than those in 1976 ( t - t e s t s , two- t a i l e d , p<.00l). Sympatric trout and char captured in 1982 had the same patterns of length versus weight as those captured in 1976. Functional regression of the logs of weight and length (Ricker 1973) of 1982 Loon trout resulted in a slope which overlapped the 95% confidence l i m i t s of the length-weight rel a t i o n s h i p for 1976 (slope=2.73 ± 0.105, cf Figure 4). Loon char captured in 1982 also had a length-weight rel a t i o n s h i p that overlapped the 95% confidence l i m i t s of those captured in 1976 (slope=2.82 ± 0.305, cf Figure 4). This provided further evidence that the native f i s h populations in Loon Lake were stable with respect to their length-weight relationship. Trout and char captured in 1982 ranged in age between 0+ and 12+ years (Figure 5). Fish captured in 1975 - 1976 were determined to be a maximum of age 4+ years using pr o b a b i l i t y paper analysis (Hume 1978), which was corroborated with scale analysis (Armitage 1973). The maximum age of f i s h between 1976 and 1982 probably did not d i f f e r by eight years, but rather the age difference i s an a r t i f a c t of the difference in technique of 3 3 I 19 UJ o o in o o o in in o o in A. TROUT Sympatric Log Wt=2.81(Log Len)-4.57 Slope+95% C.L.=2.81+.079 N=624 r=.98 Allopatric Log Wt=2.51(Log Len>-3.91 Slope+95% C.L. = 2.51+.076 N=663 r=.93 SYMPATRIC ALLOPATRIC o o o in in B. CHAR / Sympatric Log Wt=2.62(Log Len)-4.16 / / Slope+95% C.L. = 2.62+145 / / N=185 r=.82 / Allopatric Log Wt= 3.01 (Log Len)-5.02 / / Slope+95% C L =3.01+. 116 / N=286 r=.99 yy // // // / / / / / / / / / / / / / / 50 100 200 400 LENGTH (mm) Figure 4 . Length-weight relationship and functional regression l i n e s of trout and char from Loon, Eunice, and Katherine lakes. (Data points are omitted for c l a r i t y . ) 3 4 TROUT CHAR u z UJ o U J cr u. z UJ u tr UJ a. o in o O m o 01 o o o ru (291) i i i II il cn -< z u > (462) E L > r r Q "0 > n 10 12 AGE IN YEARS Figure 5. Age composition of trout and char from Loon, Eunice, and Katherine lakes. (Ages were determined using o t o l i t h s . Sample sizes are shown in parentheses.) i 35 age determination. A l l o p a t r i c trout and sympatric char had r e l a t i v e l y strong age 3+ and 4+ year classes, respectively (Figure 5). This i s also r e f l e c t e d in length d i s t r i b u t i o n s (Figure 3) and age-length relationships (Figure 6). A l l o p a t r i c trout are shorter at a given age than sympatric trout, but a l l o p a t r i c char are longer (Figure 6). There was no s i g n i f i c a n t difference in growth rate between a l l o p a t r i c and sympatric trout, but a l l o p a t r i c char had a faster growth rate than sympatric char (Jonsson et a_l. 1984, their Table 2). In 1976, the larger length classes of a l l o p a t r i c trout and char of a given length tended to weigh more than the same species in Loon Lake (Hume 1978). In 1982, t h i s was s t i l l true of char, but a l l o p a t r i c trout were not able to maintain the same growth rates in weight throughout the lengths sampled (Figure 4). 3.3 E f f e c t s of Coexistence on Spatial and Temporal D i s t r i b u t i o n In Tables 6-7 and 9-10, the ANOVA terms that indicate differences in s p a t i a l use of habitat and d i e l differences in habitat use of individual populations are "habitat" and "time*habitat", respectively. Seasonal differences in habitat use and d i e l movement are indicated by the interaction terms "month*habitat" and "month*time*habitat", respectively. In Tables 8 and 11, the ANOVA terms that provide information on differences in s p a t i a l and d i e l use of habitats between f i s h populations are the interaction terms "lake*habitat", and "lake*time*habitat", respectively. In Tables 12-13, the ANOVA 36 TROUT CHAR 156 O O • cn o o • CM 6 ,6 O r— 13 Z UJ cr o u. o o' cn o o o o • 220 274 219 81 / (1066) 75 2 2 10 6 -H-H- (291) 237 46 _368 1 9 6 54 1 2 2 / 0 2 4 6 8 10 12 (922) 125 7 1 29 , (0 < x "0 • STANDARD H DEVIATION n RANGE • i i i i i ' i i i i i 2 1 (462) 107 65 76 70 •A 104 0 2 4 6 8 10 12 AGE IN YEARS r r o "D > H 31 M n Figure 6. Age-length relationship of trout and char from Loon, Eunice, and Katherine lakes. (Sample sizes are shown on bars. Total sample sizes are shown in parentheses. ) 37 terms that indicate differences in s p a t i a l and d i e l use of habitats between trout and char populations are the interaction terms "species*habitat" and "species*time*habitat", respectively. 3 . 3 . j_ Trout Sympatric Trout in Loon Lake Sympatric trout mainly occupied depths between 0-10 m from June to October (Figure 7), and were most dense (in the sense that CPUE was highest; see Section 2.2) in l i t t o r a l habitat, and secondarily in epipelagic habitat, while epibenthic and es p e c i a l l y pelagic habitats were l i t t l e u t i l i z e d , according to r e l a t i v e CPUEs between habitats (Kruskal-Wallis H-test (herein referred to as H-test), p<.00l; Table 6). There were no s i g n i f i c a n t d i e l or seasonal movements between habitats from June to October (H-test, p>.05; Table 6). (Movement between habitats i s inferred when the r e l a t i v e CPUEs change between day and night (diel) or from month to month (seasonal movement); see Section 2.2). However, there was some evidence of a shoreward movement at night (Figure 7). Trout u t i l i z e d pelagic habitat to a greater extent in October than during June and August although th i s trend was not s t a t i s t i c a l l y s i g n i f i c a n t (H-test, p>.05; Table 6D). DAY NIGHT zzzr (96) VAVAA o CM (236) (93) 2 x r— O. Ill Q o . o . CO 22 ( 1 7 0 ) I n d . / 1 0 0 m / 1 2h >15 S . 1 - 1 5 2 . 1 - 5 0 . 1 - 2 • 0 ( 1 1 4 ) y//Y/AV/// ML 1 0 (357) yA/VAAYAA/ yA/V/AA/Ay 22 c 2 m o o H O CD m 73 1 0 2 0 4 0 SAMPLING DEPTH CONTOUR (m) Figure 7 . Spatial and temporal d i s t r i b u t i o n of sympatric trout in Loon Lake. (Mean catch per unit e f f o r t in 5 deep g i l l net sets in units of individuals/100 m2 net area/12 h set. Numbers of individuals captured are shown in parentheses.) 39 Table 6. Kruskal-Wallis analyses of variance of g i l l net catch per unit e f f o r t for Loon Lake sympatric trout by time of day (D=day, N=night), habitat ( L = l i t t o r a l , EP=epipelagic, P=pelagic, EB=epibenthic), and month (j=June, A=August, O=0ctober). Tukey Multiple H - s t a t i s t i c , Probability Comparison of ANOVA Factor Deg. Freedom of H Means (p=.05)1 A. June (N=332) time of day 2 .97, 1 ns 2 ns habitat 50 .52,3 ** * L EP EB P time*habitat .93,3 ns ns B. August (N=263) time of day .05, 1 ns ns habitat 42 .03,3 * * * L EP EB P t ime*habitat .30,3 ns ns C. October (N=471) time of day .91,1 ns ns habitat 39 .68,3 *** L EP P EB time*habitat .54,3 ns ns D. Pooled (N=1066) sampling month 4 .15,2 ns ns time of day 2 .93, 1 ns ns habitat 1 17 .33,3 *** L EP EB P month*time 1 .22,2 ns ns month*habitat 3 .43,6 ns ns time*habitat .80,3 ns ns month*time*habitat .87,6 ns ns 'Factor levels are l i s t e d in descending order of means; homogeneous subsets are underlined. 2 * * * P<.001; ** p<.0l; * p<.05; ns=not s i g n i f i c a n t p>.05 40 A l l o p a t r i c Trout in Eunice Lake From June to October, a l l o p a t r i c trout mainly occupied depths from 0-5 m (Figure 8), and were most dense in epipelagic habitat and secondarily in l i t t o r a l habitat, while pelagic and epibenthic habitats were l i t t l e u t i l i z e d (Table 7). In June, the mean CPUE of trout in epibenthic habitat was higher than in pelagic habitat (Table 7D), but t h i s pattern was not maintained in August or October (Table 7B and C). There were no s i g n i f i c a n t d i e l or seasonal movements between habitats from June to October (H-tests, p>.05; Table 7). In August and October, there seemed to be a shoreward movement to the l i t t o r a l zone at night (Figure 8), but t h i s trend was not s t a t i s t i c a l l y s i g n i f i c a n t (H-tests, p>.05; Table 7B and C). Sympatric versus A l l o p a t r i c Trout In August and in a l l sampling months pooled, sympatric and a l l o p a t r i c trout were d i s t r i b u t e d d i f f e r e n t l y between habitats (H-tests, p<.05; Table 8B and D). Sympatric trout u t i l i z e d (in order of decreasing CPUE) l i t t o r a l , epipelagic, epibenthic, then pelagic habitats, whereas a l l o p a t r i c trout u t i l i z e d epipelagic, l i t t o r a l , pelagic, then epibenthic habitats. However, both populations were most abundant in l i t t o r a l and epipelagic habitats, while fewer trout were found in epibenthic and pelagic habitats. In general, the v e r t i c a l d i s t r i b u t i o n of a l l o p a t r i c trout was more r e s t r i c t e d to shallow habitats than that of sympatric trout. There was no difference in d i e l patterns of 41 DAY NIGHT £ x i -o. HI Q 10 20 40 SAMPLING DEPTH CONTOUR (m) Figure 8. Spatial and temporal d i s t r i b u t i o n of a l l o p a t r i c trout in Eunice Lake. (Mean catch per unit e f f o r t in 5 m deep g i l l net sets in units of individuals/100 m2 net area/12 h set. Numbers of individuals captured are shown in parentheses.) 42 Table 7. Kruskal-Wallis analyses of variance of g i l l net catch per unit e f f o r t for Eunice Lake a l l o p a t r i c trout by time of day (D=day, N=night), habitat (L= l i t t o r a l , EP=epipelagic, P= pelagic, EB=epibenthic), and month (J=June, A=August, O=0ctober) Tukey Multiple H - s t a t i s t i c , Probability Comparison of ANOVA Factor Deg. Freedom of H Means (p=.05)1 A. June (N=411) time of day 1 .32, 1 ns 2 ns habitat 48 .66,3 * * * EP L EB P time*habitat 8 .51,3 ns ns B. August (N=176) time of day .07, 1 ns ns habitat 40 .09,3 * * * EP L P EB time*habitat .46,3 ns ns C. October (N=330) time of day .02, 1 ns ns habitat 42 .87,3 *** EP L P EB time*habitat 1 .32,3 ns ns D. Pooled (N=917) sampling month 5 .05,2 ns ns time of day .24, 1 ns ns habitat 1 30 .70,3 *** EP L P EB month*time 1 .50,2 ns ns month*habitat 3 .74,6 ns ns time*habitat 1 .12,3 ns ns month*time*habitat 1 .74,6 ns ns 1Factor l e v e l s are l i s t e d in descending order of means; homogeneous subsets are underlined. 2*** p<.00l; ** p<.0l; * p<.05; ns=not s i g n i f i c a n t p>.05 43 Table 8. Kruskal-Wallis analyses of variance of g i l l net catch per unit e f f o r t for Loon Lake sympatric trout versus Eunice Lake a l l o p a t r i c trout by lake (L= Loon, E=Eunice), time of day (D= day, N=night), habitat ( L = l i t t o r a l , EP= epipelagic, P=pelagic, EB=epibenthic), and month (j=June, A=August, O=0ctober). Tukey Multiple H - s t a t i s t i c , P r o b a b i l i t y Comparison of ANOVA Factor Deg. Freedom of H Means (p=.05)1 A. June (Loon N=332, Eunice N=411) lake .27,1 ns 2 ns time of day .06, 1 ns ns habitat 90.31 ,3 * * * EP L EB P lake*time 4.10,1 * E-D L-N E-N L-D lake*habi tat 1.13,3 ns ns t ime*habitat .41,3 ns ns lake*time*habitat 1.75,3 ns ns B. August (Loon N= 263; Eunice N= 1 76) lake 1.71,1 ns ns time of day .13,1 ns ns habi tat 72.35,3 *** L EP EB P lake*t ime .00, 1 ns ns lake*habi tat 9.60,3 * L-L E-EP L-EP E-L L-EB L-P E-P E-EB t ime*habi tat .62,3 ns ns lake*time*habitat .17,3 ns ns C. October (Loon N =471; Eunice N = 330) lake 1.87,1 ns ns time of day .61,1 ns ns habitat 78.35,3 *** L EP P EB lake*time .26, 1 ns ns lake*habitat 3.35,3 ns ns time*habitat 1.78,3 ns ns lake*time*habitat .33,3 ns ns ..Continued 44 Table 8. Continued 1 . 7, D. Pooled (Loon N=1066; Eunice N=917) lake month time of day habitat lake*month lake*time lake*habitat 85 08, .68, 236.02 68 month*habitat time*habitat lake*month*habitat lake*time*habitat 1 2 1 3 2. ,2 2.33,1 10.45,3 3.67,6 1.62,3 3.55,6 .33,3 ns * ns * * * ns ns * ns ns ns ns ns 0 J A ns L EP P EB ns ns L-L E-EP L-EP E-L L-EB L-P E-P E-EB ns ns ns ns 1Factor lev e l s are l i s t e d in descending order of means; homogeneous subsets are underlined. 2*** p^.001; ** p<.0l; * p<.05; ns=not s i g n i f i c a n t p>.05 45 h a b i t a t use ( H - t e s t , p>.05; T a b l e 8 ) , nor s e a s o n a l d i f f e r e n c e s i n h a b i t a t use ( H - t e s t , p>.05; T a b l e 8D) between s y m p a t r i c and a l l o p a t r i c t r o u t . 3 . 3 . 2 Char , S y m p a t r i c Char i n Loon Lake S y m p a t r i c char were c a p t u r e d w i t h the h i g h e s t CPUE i n e p i b e n t h i c h a b i t a t , s e c o n d a r i l y i n p e l a g i c h a b i t a t , t h i r d l y i n e p i p e l a g i c h a b i t a t , and most sparse i n l i t t o r a l h a b i t a t ( F i g u r e 9; H - t e s t , a l t h o u g h not s i g n i f i c a n t at the u s u a l l e v e l of p=.05, was s i g n i f i c a n t at p=.068; T a b l e 9D) . T h i s p a t t e r n of d i s t r i b u t i o n was found i n August ( H - t e s t , p < . 0 l ; T a b l e 9B) and October ( H - t e s t , a l t h o u g h not s i g n i f i c a n t at the u s u a l l e v e l of p=.05, was s i g n i f i c a n t at p=.058; T a b l e 9C) w i t h the e x c e p t i o n t h a t char were more dense i n l i t t o r a l than e p i p e l a g i c h a b i t a t i n O c t o b e r . Char were not d i s t r i b u t e d d i f f e r e n t l y between h a b i t a t s i n June ( H - t e s t , p>.05; T a b l e 9 A ) , a l t h o u g h at n i g h t , most char were found between depths 0-10 m at the 5-20 m c o n t o u r s . There were s e a s o n a l d i f f e r e n c e s i n the d i s t r i b u t i o n of char between h a b i t a t s ( H - t e s t , p < . 0 l ; T a b l e 9D) , which i n v o l v e d a movement from r e l a t i v e l y s h a l l o w h a b i t a t s i n June to h a b i t a t s g r e a t e r than 5 m deep i n August and October ( F i g u r e 9 ) . There were no d i e l movements between h a b i t a t s ( H - t e s t , p>.05; T a b l e 9 ) , a l t h o u g h t h e r e was some e v i d e n c e t h a t char u t i l i z e d s h a l l o w e r h a b i t a t s a t n i g h t i n August and October ( F i g u r e 9 ) . I t s h o u l d be noted t h a t char spawn d u r i n g autumn. A l t h o u g h many char i n D A Y N I G H T (8) (109) Y//X//A I c 2 m I (31) m '////, m. 5 10 2 0 4 0 2 6 1 0 2 0 4 0 S A M P L I N G D E P T H C O N T O U R ( m ) F i g u r e 9. S p a t i a l and temporal d i s t r i b u t i o n of sympatric char i n Loon Lake. (Mean cat c h per u n i t e f f o r t i n 5 deep g i l l net sets i n , u n i t s of i n d i v i d u a l s / 1 0 0 m2 net area/12 h s e t . Numbers of i n d i v i d u a l s captured are shown i n parentheses.) 47 Table 9. Kruskal-Wallis analyses of variance of g i l l net catch per unit e f f o r t for Loon Lake sympatric char by time of day (D= day, N=night), habitat (L= l i t t o r a l , EP=epipelagic, P=pelagic, EB=epibenthic), and month (J=June, A=August, O=0ctober). Tukey Multiple H - s t a t i s t i c , Probability Comparison of ANOVA Factor Deg . Freedom of H Means (p=.05)1 A. June (N=117) time of day habitat time*habitat 8.79,1 5.07,3 .69,3 ** 2 ns ns N D ns ns B. August (N=54) time of day habitat time*habitat .78, 1 14.52,3 1 .09,3 ns ** ns ns EB P EP L ns C. October (N=117) time of day habitat time*habitat 2.05,1 7.49,3 3.47,3 ns ns 3 ns ns ns (EB P L EP) ns D. Pooled (N=288) sampling month time of day habitat month*time month*habitat 3.59,2 9.34,1 7.14,3 2.29,2 18.28,6 A-P ns ** ns" ns ** O-L J-L ns N D ns (EB P EP L) ns J-EP O-EB A-EB 0-PA J-EB J-P O-EP A-EP A-L4 time*habitat month*time*habitat 1.10,3 5.35,6 ns ns ns ns 1Factor l e v e l s are l i s t e d in descending order of means; homogeneous subsets are underlined. 2*** p < . o o i ; ** p<.0l; * p<.05; ns=not s i g n i f i c a n t p>.05 3p=.058 flp=.068 48 spawning coloration were captured during October, no spawning aggregations were observed. A l l o p a t r i c Char in Katherine Lake A l l o p a t r i c char occupied depths from the surface to the bottom of Katherine Lake (Figure 10), but were d i s t r i b u t e d d i f f e r e n t l y between habitats in each sampling month (H-test, P<.01; Table 10). In June and August, CPUEs were highest in epibenthic habitat, secondarily in l i t t o r a l habitat, and t h i r d l y in either pelagic or epipelagic habitat (H-tests, p<.05; Table 10A and B). The extra g i l l net set overnight in Katherine Lake in August captured 11.71 char/100 m 2/l2 h in l i t t o r a l habitat, and 15.24 char/100 m 2/l2 h in epibenthic habitat. These supplemental catches corroborated the results from experimental sampling. In October, char were most dense in epipelagic habitat and l i t t o r a l habitat, and least dense in pelagic and epibenthic habitat (H-test, p<.05; Table 10C). During October, char were observed to r i s e for surface prey over the whole lake. Due to the small sample sizes of a l l o p a t r i c char, especially during the day in June (N=16) and August (N=2), the d i s t r i b u t i o n s were "driven" by nocturnal catches. The large number of char captured at night in October (N=209) came mainly from the l i t t o r a l zone and were c h i e f l y large adult males and females in spawning co l o r a t i o n . Although there were no s i g n i f i c a n t d i e l movements between habitats (H-test, p>.05; Table 10), in June and October there was a marked increase in the density of char in l i t t o r a l habitat DAY NIGHT SAMPLING DEPTH CONTOUR (m) Figure 10. Spatial and temporal d i s t r i b u t i o n of a l l o p a t r i c char in Katherine Lake. (Mean catch per unit e f f o r t in 5 m deep g i l l net sets in units of individuals/100 m2 net area/12 h set. Numbers of individuals captured are shown in parentheses. Note change in depth scale from Figures 9-11.) 50 Table 10. Kruskal-Wallis analyses of variance of g i l l net catch per unit e f f o r t for Katherine Lake a l l o p a t r i c char by time of day (D=day, N=night), habitat ( L = l i t t o r a l , EP=epipelagic, P= pelagic, EB=epibenthic), and month (J=June, A=August, O=0ctober) Tukey Multiple H - s t a t i s t i c , Probability Comparison of ANOVA Factor Deg. Freedom of H Means (p=.05)1 A. June (N=112) time of day habitat time*habitat B. August (N=27) time of day habitat time*habitat 1 1 .92, 1 9.05,3 3.35,3 15.91,1 15.51,3 5.97,3 * * * 2 * ns ** * ns N D EB L P EP ns N D EB L EP P ns C. October (N=253) time of day 5.92,1 ns ns habitat 5.58,3 * EP L P EB time*habitat 5.53,3 ns ns D. Pooled (N=392) sampling month 25.60,2 *** 0 J A time of day 14.86,1 *** N D habitat 3.48,3 ns ns month*time 1.59,2 ns ns month*habitat 18.44,6 ** Q-EP Q-L J-EB J-L O-P A-EB O-EB J-P A-L A~EP J-EP A-P ,time*habitat 5.51,3 ns ns month*time*habitat 5.33,6 ns ns 1Factor l e v e l s are l i s t e d in descending order of means; homogeneous subsets are underlined. 2*** P<.001; ** p^.01; * p<.05; ns=not s i g n i f i c a n t p>.05 51 at night (Figure 10). Sympatric versus A l l o p a t r i c Char In June and October, sympatric and a l l o p a t r i c char were dist r i b u t e d d i f f e r e n t l y between habitats (H-tests, p<.05 and P<.01, respectively; Table 11A and C), but in August and in a l l months pooled, there was no s i g n i f i c a n t difference in their d i s t r i b u t i o n between habitats (H-tests, p>.05; Table 11B and D). The small number of char captured in August created a r e l a t i v e l y large variance in catch per unit e f f o r t which may have obscured differences in d i s t r i b u t i o n between habitats. Although both populations were most dense in epibenthic habitat in August, a l l o p a t r i c char were found in greater r e l a t i v e abundance in l i t t o r a l habitat than sympatric char. Sympatric and a l l o p a t r i c char exhibited s i g n i f i c a n t l y d i f f e r e n t seasonal movements between habitats (H-test, p<.00l; Table 11D). Seasonal differences in habitat use were due to opposite trends of v e r t i c a l movement between the two populations. Sympatric char u t i l i z e d shallow habitats in June and mainly epibenthic habitat in August and October, but a l l o p a t r i c char u t i l i z e d epibenthic habitat in June and August and shallower habitats in October. There was no s t a t i s t i c a l difference in d i e l use of habitats between sympatric and a l l o p a t r i c char (H-tests, p>.05; Table 11). However, in October, there was some evidence that sympatric char used shallower parts of the water column at night while a l l o p a t r i c char (mainly spawners) used l i t t o r a l habitat to a greater extent at night than during the day. 52 Table 11. Kruskal-Wallis analyses of variance of g i l l net catch per unit e f f o r t for Loon Lake sympatric char versus Katherine Lake a l l o p a t r i c char by lake (L=Loon, K=Katherine), time of day (D=day, N=night), habitat ( L = l i t t o r a l , EP= epipelagic, P= pelagic, EB=epibenthic), and month (J=June, A=August, O=0ctober) Tukey Multiple H - s t a t i s t i c , P robability Comparison of ANOVA Factor Deg . Freedom of H Means (p=.05)1 A. June (Loon N=117; Katherine N=112) lake 7.11,1 * * 2 K L time of day 20.12, 1 *** N D habitat 1.66,3 ns ns lake*tirne .58, 1 ns ns lake*habitat 1 1 . 17,3 0 * K-L K -EB K-P L-EP L-L L -EB L-P K-EPJ time*habitat 2.53,3 ns ns lake*time*habitat 1 .20,3 ns ns B. August (Loon N = 54; Katherine N=27) lake .02, 1 ns ns time of day 4.86,1 * N D habitat 22.05,3 *** EB P L E-P lake*time 1.87,1 ns ns lake*habitat 3.66,3 ns ns time*habitat 3.60,3 ns ns lake*time*habitat .66,3 ns ns C. October (Loon : N=117; Katherine N=253) lake 19.09,1 *** K L time of day 3.29,1 ns ns habitat .42,3 ns ns lake*t ime .00,1 ns ns lake*habitat 12.20,3 ** K-EP K-L K-P K-•EB L-EB L-P L-L L-•EP time*habitat 6.75,3 ns ns lake*time*habitat .60,3 ns ns ..Continued 53 Table 11. Continued D. Pooled (Loon N= 288; Katherine N=392) lake 18.31,1 * * * K L month 19.21,2 *** 0 J A time of day 22.79,1 *** N D habitat 6.12,3 ns ns lake*month 10.70,2 ** K-0 K-J L-0 L-J j L-A K-A lake*time 1.29,1 ns ns lake*habitat 2.93,3 ns ns month*habitat 10.26,6 ns ns time*habitat 5.06,3 ns ns lake*month*habitat 23.86,6 * * * K-O-EP K-O-L K-J L-A-L lake*time*habitat 1.99,3 ns ns f a c t o r l e v e l s are l i s t e d in descending order of means; homogeneous subsets are underlined, z*** P<.001; ** p<.0l; * p<.05; ns=not s i g n i f i c a n t p>.05 54 3>.3.3 Trout versus Char There were s i g n i f i c a n t differences in habitat use between sympatric trout and char during each sampling month and a l l sampling months pooled (H-tests, p<.00l; Table 12). In June, segregation between sympatric trout and char was based on differences in u t i l i z a t i o n of l i t t o r a l and limnetic zones, where trout were most dense in l i t t o r a l habitat and char were most dense in epipelagic habitat. However, during August and October, differences in habitat u t i l i z a t i o n were based on segregation with depth, where trout u t i l i z e d mainly l i t t o r a l and epipelagic habitats and char u t i l i z e d mainly epibenthic and pelagic habitats. There were s i g n i f i c a n t differences in habitat use between a l l o p a t r i c trout and char during June, August, and a l l sampling months pooled (H-tests, p<.00l; Table 13A, B, and D). Although differences in habitat u t i l i z a t i o n between a l l o p a t r i c trout and char were mainly based on depth of habitat, l i t t o r a l habitat was among the two more heavily used habitats for both trout and char. During October, differences in habitat use were not so s i g n i f i c a n t as'during other months (H-test, p>.05 (cf p<.00l); Table 13C). In October, both a l l o p a t r i c populations were most dense in epipelagic, l i t t o r a l , pelagic, then epibenthic habitats. However, trout were much less dense in pelagic and epibenthic habitats r e l a t i v e to the two heavily used (epipelagic and l i t t o r a l ) habitats (Table 13C). There was no s i g n i f i c a n t difference in d i e l movements between habitats between sympatric trout and char or between a l l o p a t r i c trout and char in any month or pooled months (H- 55 Table 12. Kruskal-Wallis analyses of variance of g i l l net catch per unit e f f o r t for Loon Lake sympatric trout and char by species (T=trout, C=char), time of day (D=day, N=night), habitat ( L = l i t t o r a l , EP=epipelagic, P=pelagic, EB=epibenthic), and month (J=June, A=August, O=0ctober). ANOVA Factor H - s t a t i s t i c , Deg. Freedom Probability of H Tukey Multiple Comparison of Means (p=.05)1 A. June (Trout N=332; Char N=117) spec ies 12.27,1 time of day 8.21,1 habitat 34.38,3 species*time .31,1 species*habitat 16.89,3 time*habitat .54,3 species*time*habitat .72,3 * * * 2 ** * * * ns *** ns ns T C N D. EP L EB P ns T-L T-EP C-EP T-EB)  C-L C-EB T-P C-P ns ns • B. August (Trout N=263; Char N=54) spec ies 12.06,1 time of day ' .39,1 habitat 11.23,3 species*time .11,1 species*habitat 45.62,3 time*habitat .70,3 species*time*habitat .13,3 * * * ns * ns *** ns ns T C ns L EP EB P ns T-L T-EP C-EB T-EB. C-P T-P C-EP C-L ns ns C. October (Trout N=471; Katherine N=117) spec ies 17.93,1 t ime of day 1.83,1 habitat 11.98,3 species*time .00,1 species*habitat 34.40,3 time*habitat 1.02,3 species*time*habitat 1.10,3 * * * ns ** ns *** ns ns T C_ ns L EP P EB ns T-L T-EP C-EB T-P, C-P T-EB C-L C-EP ns ns .Continued 56 Table 12. Continued D. Pooled (Trout N=1066; Katherine N=288) species month time of day habitat species*month species*time spec ies*habitat 40.41,1 6.66,2 7.88,1 47.88,3 .31,2 .32, 1 89.14,3 month*habitat 8.96,6 time*habitat .87,3 species*month*habitat 5.39,6 species*time*habitat .50,3 *** * ** *** ns ns * * * ns ns ns ns T C 0 J A D N L EP EB P ns ns T-L T-EP C-EB T-EB. C-P T-P C-EP C-L ns ns ns ns 1Factor le v e l s are l i s t e d in descending order of means; homogeneous subsets are underlined. 2*** P<.001; ** p<.0l; * p<.05; ns=not s i g n i f i c a n t p>.05 57 Table 13. Kruskal-Wallis analyses of variance of g i l l net catch per unit e f f o r t for Eunice Lake a l l o p a t r i c trout versus Katherine Lake a l l o p a t r i c char by species (T=trout, C=char), time of day (D=day, N=night), habitat ( L = l i t t o r a l , EP= epipelagic, P=pelagic, EB=epibenthic), and month (J=June, A= August, O=0ctober). Tukey Multiple H - s t a t i s t i c , Probability Comparison of ANOVA Factor Deg. Freedom of H Means (p=.05)1 A. June (Trout N=411 ; Char N=112) spec ies 5.78,1 * 2 T C time of day 1 .29, 1 ns ns habitat 32.62,3 * * * L EP EB P spec ies*time 8.79,1 ** T-D C-N T-N C-D spec ies*habitat 31 .07,3 * * * T-EP T-L C-L C-EB, C-P T-EB T-P C-EP time*habitat 1.69,3 species*time*habitat 1.83,3 B. August (Trout N=176; Char N=27) spec ies 12.83, 1 time of day 2.13,1 habitat 24.30,3 species*time 1.05,1 species*habitat 26.22,3 time*habitat 1 . 04 , 3 species*time*habitat 1.23,3 ns ns *** ns * * * ns *** ns ns ns ns T C_ ns EP L EB P ns T-EP T-L C-EB T-P C-L C-EP C-P T-EB ns ns C. October (Trout N=330; Char N=253) spec ies .13,1 ns time of day .98,1 ns habitat 49.40,3 *** species*time .40,1 ns species*habitat 5.39,3 ns time*habitat 5.68,3 ns species*time*habitat 1.24,3 ns ns ns EP L EB P ns ns ns ns .Continued 58 Table 13. Continued D. Pooled (Trout N=917; Char N=392) spec ies 9 . 0 8 , 1 ** T C month 1 9 . 1 6 , 2 *** 0 J A time of day 3 . 8 9 , 1 * N D habitat 9 5 . 7 7 , 3 * * * EP L EB P spec ies*month 8 . 1 2 , 2 * C-0 T-J T-0 T-AT C-J C-A spec ie s * t ime 6 . 4 8 , 1 * T-D C-N T-N C-D species*habitat 5 3 . 6 1 , 3 * * * T-EP T-L C-EB C-L . C-EP C-P T-P T-EB month*habitat 1 1 . 0 7 , 6 ns ns time*habitat 4 . 7 5 , 3 ns ns spec ies*month*habitat 7 . 5 0 , 6 ns ns spec ie s * t ime*habitat 1 . 6 2 , 3 ns ns 1Factor lev e l s are l i s t e d in descending order of means; homogeneous subsets are underlined. 2 * * * p < . o o i ; ** p < . 0 l ; * p < . 0 5 ; ns=not s i g n i f i c a n t p > . 0 5 59 tests, p>.05; Tables 12 and 13). However, during June there was some evidence of a difference in the pattern of d i e l movement of a l l o p a t r i c trout and char between habitats (Figures 8 and 10). During June, although a l l o p a t r i c trout were most dense in epipelagic and l i t t o r a l habitats during day and night, they were more dense in epibenthic habitat during the night than the day, and although a l l o p a t r i c char were most dense in epibenthic habitat o v e r a l l , they were more dense in l i t t o r a l habitat during the night than the day. These d i e l habitat s h i f t s indicate that both a l l o p a t r i c trout and char make nocturnal use of habitats which are t y p i c a l of the other species. 3_.£ Ef f e c t s of Irradiance Level on Behavioural and Feeding Interactions _3.4.1 General Behaviour Establishment of Dominance The establishment of dominance in f i s h pairs prior to experimental treatments followed a regular pattern of behaviour. To i l l u s t r a t e t h i s pattern, agonistic interactions of one pair of f i s h are shown in Figure 11. I n i t i a l l y , the f i s h explored the aquarium and although swimming a c t i v i t y of both f i s h was r e l a t i v e l y high, there were few behavioural interactions. This i n i t i a l response was followed by a phase of r e l a t i v e l y high interaction and reduced swimming a c t i v i t y , when dominance by trout was established. Once established, t h e i r dominance was 60 F i g u r e 11. Behaviours a s s o c i a t e d with the establishment of dominance of t r o u t over char. 61 maintained for the duration of the experiment in regular bouts of aggression, but the number of aggressive interactions and time spent in bouts markedly decreased (Figure 11). During t h i s phase, the swimming a c t i v i t y of the char was very low and i t s use of the aquarium was r e s t r i c t e d to one end (Figure 12). If the char strayed from t h i s position, the trout immediately "responded" with aggressive behaviour. Once dominance was established, a l l aggressive behaviours were performed by the trout and a l l submissive behaviours by the char. Although data on establishment of dominance were not recorded for a l l pairs of f i s h , t h i s sequence of behaviours was easy to recognize, and in each r e p l i c a t e , irradiance l e v e l treatments were not commenced u n t i l agonistic behaviours had s t a b i l i z e d . Swimming Behaviour The swimming a c t i v i t y of trout was s i g n i f i c a n t l y greater than that of char (F-test, p<.00l, F=37.55, df=1,23) in i n t e r s p e c i f i c pairs (e.g. days 4-6, Figure 11B). Char in every replicate spent the majority of their time at either end of the aquarium, whereas trout either swam back and forth in the tank or hovered at the opposite end to the char (e.g. Figure 12). Char had a d i f f e r e n t position in the water column than trout. Char often "rested" on the substrate, whereas trout always swam or hovered in the water column. Char in i n t e r s p e c i f i c pairs rested on the substrate more often at low irradiance le v e l s and always at the end of the aquarium. S o l i t a r y char behaved in the same way, but rested more 62 A . Q U A R T I L E I ( L E F T - H A N D E N D ) C H A R T R O U T ~n E3n rm H 1 B . Q U A R T I L E I I o in o • P7 V C . Q U A R T I L E I I I _C1 & ] EL D . Q U A R T I L E I V ( R I G H T - H A N D E N D ) 1 3 D A Y F i g u r e 12. H o r i z o n t a l p o s i t i o n i n a q u a r i u m d u r i n g t h e e s t a b l i s h m e n t o f d o m i n a n c e o f t r o u t o v e r c h a r . 63 frequently near the centre of the aquarium. Although char seemed to "prefer" resting on the substrate, char in i n t e r s p e c i f i c pairs may have assumed th i s resting posture in an attempt to escape from aggression from trout. Char often swam to the bottom and became very s t i l l during an aggressive bout, leaving t h i s position only to avoid the trout i f i t approached again to continue the bout. However, in some pairs, trout i n i t i a t e d aggressive bouts when char assumed a resting posture. Another swimming behaviour performed by char but not trout in i n t e r s p e c i f i c pairs was hovering in a non-horizontal position. This behaviour was performed by subordinate f i s h , and never by s o l i t a r y individuals or dominant trout or char. Although the relevance of t h i s behaviour i s not known, i t may be used by subordinate f i s h as a submissive display to reduce their apparent size from a v e r t i c a l viewpoint. Feeding Behaviour During feeding t r i a l s , there were several behavioural changes in trout and char. Following the introduction of Neomysis, behavioural interactions were reduced i n i t i a l l y for approximately 10 min while the f i s h exploited the prey, then interactions between trout and char became very frequent, but decreased within 30 minutes. However, there was no s i g n i f i c a n t difference in the intensity of behavioural interactions between feeding and non-feeding t r i a l s (F-test, p>.05, F=3.25, df=1,23). The increase in swimming a c t i v i t y of both trout (F-test, p<.00l, F=20.67, df=1,23) and char (F-test, p<.0l, F=9.31., df = 1,23) was 64 highly s i g n i f i c a n t , mainly due to increased searching a c t i v i t y (Table 14). However, the swimming a c t i v i t y of trout was s i g n i f i c a n t l y greater than that of char whether prey were present (F-test, p<.00l, F=14.73, df=1,23) or absent (F-test, P<.001, F = 37.55, df=1,23) . The v e r t i c a l position of char in the water column ( i . e . "resting" on bottom, swimming in the water column, or diagonal hover) was not changed from non-feeding t r i a l s . 2L4.2! Behavioural Interactions The frequency of behavioural interactions between trout and char was reduced with decreasing irradiance l e v e l (F-test in two-way ANOVA (irradiance l e v e l (repeated measures) and order), p<.05, F=4.52, df=3,l0; Figure 13). There were v i r t u a l l y no interactions at the lowest irradiance l e v e l . The order in which irradiance levels were, presented to each pair did not s i g n i f i c a n t l y a f f e c t the frequency of behavioural interactions (F-test in two-way ANOVA (irradiance l e v e l (repeated measures) and order), p>.05, F=0.23, df=2,l0). At a l l irradiance l e v e l s , behavioural interactions were predominantly submissive acts of avoidance by the char. At the three lowest irradiance l e v e l s , there was an even greater predominance of submissive acts by the char, and aggression by the trout was v i r t u a l l y n i l (Figure 13), although the dominance relati o n s h i p was maintained at a l l irradiance l e v e l s . At the lower irradiance l e v e l s , v i s u a l perception of char by trout may be limited due to reduced swimming a c t i v i t y of the 65 Table 14. Swimming a c t i v i t y of trout and char in feeding t r i a l s (F) and t r i a l s without prey present (NF). (Data from a l l irradiance l e v e l s pooled; N=24.) NF Trout F Char NF F Swimming a c t i v i t y per 30 minutes (mean ± standard deviat ion) 43.9±21 .2 107 .6±83.5 9.8±17.2 31.9±48.6 z n T Q ro " s cn r-u < _J < cr o > < X UI ca o o rvj o i n o o o i n a J a i n A. IRRADIANCE LEVEL 3.0 x 10 1 8 prtotons/m 2/s 93.55 ± 31.02 a c t s LH 2 zzzz Avoid Change Chase Nip o o a i n o J B. IRRADIANCE LEVEL 1.5 X 1016 p h o t o n s / m 2 / s 19.39 ± 10.79 a c t s o i n o J C. IRRADIANCE LEVEL 5.0 x 10 1 5 photons/m 2/s 9.19 ± 4.53 acts D. IRRADIANCE LEVEL 3.0 x 10 1 5 photons/m 2/s 0.50 ± 0.34 acts o J 1 2 3 4 5 6 REPLICATE gure 13. Type and intensity of behavioural interactions between dominant trout and subordinate char at four irradiance l e v e l s . (Mean behavioural acts per 30 min standard error are shown.) 67 trout. With decreasing irradiance, trout must a c t i v e l y seek out char in order to perform aggressive acts. There was no s i g n i f i c a n t difference in swimming a c t i v i t y of trout in i n t e r s p e c i f i c pairs with char with decreasing irradiance l e v e l (F-test, p>.05, F=1.32, df=3,15; Figure 14). However, s o l i t a r y trout and i n t r a s p e c i f i c trout pairs exhibit reduced swimming a c t i v i t y with decreased irradiance l e v e l . The same i s true of s o l i t a r y and i n t r a s p e c i f i c pairs of char, but char in i n t e r s p e c i f i c pairs with trout showed a highly s i g n i f i c a n t reduction in swimming a c t i v i t y with decreasing irradiance l e v e l (F-test, p<.0l, F=5.44, df=3,15; Figure 14). Therefore, the swimming a c t i v i t y of trout appears to be influenced by the presence of char. Swimming a c t i v i t y of trout (p<.05, r=.43, n=23) but not char (p>.05, r=.37, n=24) was correlated with intensity of behavioural interactions, although the r value for trout was not much greater than that of char. It appears that under low irradiance, char continue to exhibit submissive behaviours ( i . e . "avoidance"; Table 2) used at higher irradiance levels to avoid aggression by trout, and that subordinate char are less active than dominant or s o l i t a r y f i s h . However, in several t r i a l s at the lower irradiance l e v e l s , char did not avoid trout, and some char appeared to be torpid in low irradiance (3.0 x 10 1 5 photons/m 2/s) because they did not respond to nudging by trout. 6 8 z x o ro < Z n X X on o m o A. IRRADIANCE LEVEL 3.0 x 10 1 8 photons/m 2/s TROUT 34.96 + 8.15: CHAR 30.44 ± 10.27 uni t s TROUT CHAR in tL I o m o B. IRRADIANCE LEVEL 1.5 x 1016 photons/m 2/s TROUT 51.47 ± 4.50; CHAR 2.76 + 1.52 u n i t s 1 L I . C. o . 00 o IRRADIANCE LEVEL 5.0 x 10 1 5 photons/m 2/s TROUT 52.90 + 9.97: CHAR 2.26 ± 0.91 u n i t s 1 o m IRRADIANCE LEVEL 3.0 x 10 1 5 photons/m 2/s TROUT 35.81 + 10.01: CHAR 3.46 ± 1.58 u n i t s o 1 l I jzm. I L 3 4 REPLICATE Figure 14. Swimming a c t i v i t y of dominant trout and subordinate char at four irradiance l e v e l s . (One swimming a c t i v i t y unit i s entry to a new horizontal q u a r t i l e of aquarium. Mean a c t i v i t y per 30 min ± standard error are shown.) 69 3._4.3 Feeding Performance The feeding performance of char in i n t e r s p e c i f i c pairs dominated by trout increased with decreasing intensity of behavioural interactions ( t - t e s t , two-tailed, p>.05, t=5.41, df=4; Figure 15). Data used in th i s test were r e s t r i c t e d to feeding t r i a l s at irradiance levels that maximized the reaction distance for both species (3.0 x 10 1 8 photons/m 2/s). The difference in feeding performance of trout and char in i n t e r s p e c i f i c pairs dominated by trout was highly s i g n i f i c a n t (F-test, p<.00l, F=24.29, df=1,23; Figure 16). Trout made more feeding s t r i k e s than char in a l l r e p l i c a t e s at a l l irradiance le v e l s except for one t r i a l (Replicate 2; Figure 16C). Neither trout (F-test, p>.05, F=2.39, df=3,l5) nor char (F-test, p>.05, F=1.92, df=3,15) fed less frequently under lower irradiance l e v e l . The feeding performance of trout declined more rapidly with irradiance l e v e l than did that of char although t h i s result was not s t a t i s t i c a l l y s i g n i f i c a n t (Figure 16). During feeding t r i a l s , char maintained similar behaviour to non-feeding t r i a l s , including subordinate behaviours, swimming a c t i v i t y , and orientation in the water column (Figures 17-19). As in t r i a l s without prey present, there was a highly variable intensity of interaction at the three highest irradiance l e v e l s , but there was a r e l a t i v e l y low intensity of interaction and v i r t u a l l y no aggression at the lowest l e v e l in feeding t r i a l s (Figure 17, cf Figure 13). However, trout were dominant even at the lowest irradiance l e v e l , and the feeding performance of char was affected at a l l irradiance l e v e l s . There was a highly 70 Figure 15. Effect of behavioural interactions between dominant trout and subordinate char on the feeding performance of char. (Irradiance l e v e l = 3.0 x 10 1 8 photons/m 2/s. Bars indicate ± standard error. Sample sizes are shown in parentheses.) 7 1 A. IRRADIANCE LEVEL 3 TROUT 27.92 + 5.83; .0 x 1018 photons/m2/s CHAR 8.76 ± 3.86 strikes o O 1/ TROUT CHAR 1 1 o B. IRRADIANCE LEVEL 1 TROUT 19.10 + 4.68 5 x 1016 photons/m2/s CHAR 2.08 + 1.24 strikes o m in UJ (ft LO Z n Q UJ UI U. O CM 1771 O O CM C. IRRADIANCE LEVEL 5 TROUT 13.70 ± 3.33 0 x 1015 photons/m2/s CHAR 4.57 t 2-50 strikes 1 1 <2 L o D. IRRADIANCE LEVEL 3.0 x 1015 photons/m2/s TROUT 11.90 + 4.43; CHAR 1.28 ± 0.92 strikes o CM I 1 i a 3 4 REPLICATE Figure 16. Feeding performance of dominant trout and subordinate char at four irradiance l e v e l s . (Mean feeding s t r i k e s per 30 min ± standard error are shown.) 7 2 a in a o o in A. IRRADIANCE LEVEL 3.0 X 10 1 8 photons/m 2/s 54.72 + 26.84 acts mi i — i o ro \ ui I -u < _J < tr O i-t > < I UJ CD O in o o o in o in B. IRRADIANCE LEVEL 1.5 x 1016 photons/m 2/s 41.94 ± 22.62 acts I | Avoid ttmH Change Y///A Chase Nip m a in o o - Q m o J C. IRRADIANCE LEVEL 5.0 X 10 1 5 photons/m 2/s 49.71 t 25.28 acts m L 1 0. IRRADIANCE LEVEL 3.0 X 10 1 5 photons/m 2/s 8. 13 + 2.01 acts J _ C I 1 3 A REPLICATE F i g u r e 17. Type and i n t e n s i t y of b e h a v i o u r a l i n t e r a c t i o n s between dominant t r o u t and s u b o r d i n a t e c h a r d u r i n g f e e d i n g a t f o u r i r r a d i a n c e l e v e l s . (Mean b e h a v i o u r a l a c t s p e r 30 min ± s t a n d a r d e r r o r a r e shown.) 7 3 s i g n i f i c a n t decrease in behavioural interactions in feeding t r i a l s with decreasing irradiance l e v e l (F-test, p<.00l, F=15.13, df=3,40). The swimming a c t i v i t y of char in feeding t r i a l s was s i g n i f i c a n t l y greater than non-feeding t r i a l s (F- test, p<.0l, F=9.31, df=1,40) but decreased with irradiance l e v e l s i m i l a r l y to non-feeding t r i a l s (F-test, p<.001, F=9.84, df=1,15; Figure 18, cf Figure 14). The orientation of char in the water column was remarkably similar between feeding and non- feeding t r i a l s (Figure 19). However, v e r t i c a l orientation varied between individual char. For example, the char in replicate 4 hovered in a diagonal position much more frequently than any other char. At the three lowest irradiance l e v e l s , the fiv e other char never used the diagonal hover behaviour (with the exception of replicate 2 at 1.5 x 10 1 6 photons/m 2/s), but used bottom resting behaviour more frequently than at the highest irradiance l e v e l . 74 o ro v. >- u < L9 z n z x n Z g „ A. IRRADIANCE L E V E L 3 . 0 X 10 1 8 photons/m2/s TROUT 2 0 2 . 2 6 ± 4 2 . 1 5 ; CHAR 9 3 . 0 7 ± 2 4 . 1 7 units o o I TROUT CHAR I I 1 o o • CU IRRADIANCE L E V E L 1.5 x 10 1 6 photons/m2/s TROUT 9 0 . 4 6 ± 2 8 . 5 8 : CHAR 9 . 2 1 + 3 . 9 7 units 1 I I VTA I o o • CU C. IRRADIANCE L E V E L 5 . 0 x 1 0 1 5 photons/m2/s TROUT 7 6 . 7 0 ± 1 2 . 3 6 : CHAR 1 9 . 6 1 ± 1 0 . 6 6 units I 1 o o H cu 0 . IRRADIANCE L E V E L 3 . 0 x 10 1 5 photons/m2/s TROUT 6 0 . 8 1 ± 1 0 . 7 6 ; CHAR 3 . 9 3 ± 1 .55 units m 3 4 R E P L I C A T E Figure 18. Swimming a c t i v i t y of dominant trout and subordinate char during feeding at four irradiance l e v e l s . (One swimming a c t i v i t y unit i s entry to a new horizontal q u a r t i l e of aquarium. Mean a c t i v i t y per 30 min ± standard error are shown.) 75 c E L U 3E O o. OJ A. IRRADIANCE LEVEL • Off Bottom 3.0 x 1018 photons/mVs ^3 Diagonal Bottom o. O . OJ B. IRRADIANCE LEVEL 1.5 x 10 1 6 photons/mVs C. IRRADIANCE LEVEL 5.0 x 10 1 S photons/mVs D. IRRADIANCE LEVEL 3.0 x 1015 photons/m /s NF F NF F NF F NF F NF F NF F 1 2 3 4 5 6 REPLICATE Figure 19. V e r t i c a l position in the water column of subordinate char at four irradiance l e v e l s . (NF=no prey present; F= Neomysis mereedis present. See text for explanation of terms.) 76 4.0 DISCUSSION 4 .J_ Spatial and Temporal D i s t r i b u t i o n Habitat p a r t i t i o n i n g i s common in f i s h communities (Nilsson 1967, Keast 1970, 1978, Zaret and Rand 1971, Moyle 1973, Werner et a l . 1977, Gorman and Karr 1978). A "habitat" i s a place with a p a r t i c u l a r kind of environment inhabited by organisms (e.g. l i t t o r a l zone), where "environment" i s a c o l l e c t i v e term for the conditions in which an organism l i v e s (e.g. temperature, irradiance l e v e l , other organisms). The preferred habitat of a species i s commonly thought to be based mainly on the abundance and type of food resources present. The habitats used in my study were defined by their proximity to either the lake surface or bottom, or a combination of the two boundaries: epipelagic, proximity to the surface; epibenthic, proximity to the bottom; l i t t o r a l , proximity to both; and pelagic, proximity to neither boundary. Each habitat had similar environmental conditions of temperature, oxygen, and irradiance l e v e l in the study lakes. In addition, prey types were d i s t r i b u t e d in similar r e l a t i v e abundance among habitats in the lakes (with some exceptions, as discussed below). If resource use patterns are affected by i n t e r s p e c i f i c competitive interactions, then addition or removal of competitors should cause species to " s h i f t " their niche in response, either away from or towards the resources used by the competitor, respectively (Eadie 1982). A niche i s a p a r t i c u l a r role or set of relationships of an organism in an ecosystem, 77 which may be f i l l e d by d i f f e r e n t species in di f f e r e n t geographical areas. A niche i s composed of many "dimensions", the most important of which are trophic relationships, habitat or s p a t i a l dimension, and temporal dimension or time of a c t i v i t y (Pianka 1975). Species may undergo s h i f t s in p a r t i c u l a r dimensions of their niche. A habitat s h i f t i s the divergence of sympatric species from each other so that each then occupies a d i f f e r e n t part of the s i t e , thereby allowing coexistence under the pressure of competition (Schoener 1974a, Connell 1980). Although species may compete for space per se (e.g. Fausch and White 1981), competition between f i s h for space is usually c l o s e l y associated with competition for food resources contained within p a r t i c u l a r habitats (Magnuson 1962, Gustafson e_t a l . 1969, D i l l e_t a_l. 1981). Segregation by habitat i s one of the most important means by which e c o l o g i c a l l y similar species p a r t i t i o n food resources (Schoener 1974a). Niche or habitat s h i f t s are often c i t e d as evidence of competition in f i s h communities (Nilsson 1960, 1963, Schoener 1974b, 1974c, 1975, Werner and Hall 1976, 1977, 1979, Werner 1977, Nilsson and Northcote 1981, Magnan and F i t z g e r a l d 1982, Larson and Moore 1985). An hypothesis of competition between trout and char in Loon Lake would be supported by a habitat s h i f t i n one or both species between sympatry and a l l o p a t r y . There were s i g n i f i c a n t differences between habitat use of the sympatric and experimental a l l o p a t r i c trout populations in August and a l l sampling months pooled. A l l o p a t r i c trout were captured in greatest r e l a t i v e abundance in epipelagic habitat, 78 sympatric trout were captured in greatest r e l a t i v e abundance in l i t t o r a l habitat, and epibenthic and pelagic habitat were less u t i l i z e d by both populations. Differences in prey d i s t r i b u t i o n s between Loon and Eunice lakes could in part explain this apparent habitat s h i f t . Zoobenthos, zooplankton, and surface prey were di s t r i b u t e d in a similar way within each lake in June and August, except that surface arthropods were much more abundant in Eunice (4.06 individuals/m 2) than in Loon (0.23 individuals/m 2) during August (Hindar e_t a_l. in prep.). A l l o p a t r i c Eunice trout may have u t i l i z e d epipelagic habitat in August to consume surface arthropods. The d i s t r i b u t i o n of trout in Eunice mainly in the upper 5 m of the water column rather than the upper 10 m as in Loon i s consistent with the hypothesis that trout in epipelagic habitat in Eunice were consuming the abundant surface arthropods in August. However, Hindar e_t a l . (in prep.) found that sympatric and a l l o p a t r i c trout consumed the same prey types, and in similar r e l a t i v e proportions during each month. In any case, despite the apparent preference of a l l o p a t r i c trout for epipelagic habitat, sympatric trout probably preferred l i t t o r a l over epipelagic habitat, or at least were not outcompeted or excluded by char from epipelagic habitat, because char were not abundant in epipelagic habitat in Loon Lake. There was no s i g n i f i c a n t difference between d i e l or seasonal use of habitats by sympatric and a l l o p a t r i c trout. I conclude that these data provide no clear evidence of a habitat s h i f t in trout between sympatry and allopatry in habitat use, d i e l differences in use of habitats, 79 or seasonal differences in use of habitats. There were s i g n i f i c a n t differences in habitat use between sympatric and a l l o p a t r i c char in June and October, but no difference in August or in a l l months pooled. In June, a l l o p a t r i c char were most dense in epibenthic and l i t t o r a l habitats but sympatric char u t i l i z e d a l l habitats equally, including epipelagic and l i t t o r a l habitat that was u t i l i z e d by trout. This s h i f t in habitat use in June does not support an hypothesis of competition between trout and char in Loon Lake because sympatric char shifted to habitats which were more similar to t y p i c a l trout habitat than the epibenthic and l i t t o r a l habitats most heavily u t i l i z e d by a l l o p a t r i c char. However, in June, the main food item of a l l o p a t r i c char was Zygoptera larvae, which were abundant in the l i t t o r a l zone (Hindar e_t a_l. in prep.). The high abundance of t h i s prey in Katherine Lake during June probably had a strong influence on the a t t r a c t i o n of a l l o p a t r i c char to l i t t o r a l , and possibly epibenthic, habitats. In October, the results of habitat s h i f t s contradict those of June, as epipelagic and l i t t o r a l habitats that were used most heavily by a l l o p a t r i c char were least u t i l i z e d by the sympatric population. These results support an hypothesis of competition between trout and char in Loon Lake, because sympatric char shifted to habitats that were less similar to trout habitat than habitats used most heavily by a l l o p a t r i c char. During October, a l l o p a t r i c char were probably attracted to the epipelagic zone to prey upon surface arthropods, as t h i s prey type was over twice as dense during 80 t h i s sampling period (4.5 individuals/m 2) than in any other lake or month (Hindar et a l . in prep.). During October 1982, char were observed to r i s e to the lake surface to consume fl o a t i n g prey items, and surface arthropods were a more important dietary item to char than in any other lake or month. During October, a l l o p a t r i c char were probably not attracted to epipelagic habitat (0-5 m) to consume zooplankton prey, as t h i s prey type was more dense at 5-10 m in pelagic habitat. Since the density of surface arthropods in Loon Lake during October was not comparable to that in Katherine Lake, habitat s h i f t s should be interpreted with caution, because prey abundance may partly explain the s h i f t . The d i s t r i b u t i o n of a l l o p a t r i c char from the surface to the bottom of the lake and of sympatric char from approximately 5 m deep to the lake bottom also support the hypothesis that trout and char compete in Loon Lake. There was a s i g n i f i c a n t difference in seasonal use of habitats between sympatric and a l l o p a t r i c char. The seasonal movement of sympatric char from mainly surface habitats in June to primarily epibenthic and pelagic habitats in August and October (H-test, p<.00l; Table 12D) resulted in a greater segregation between sympatric trout and char from June to August and October. These data indicate that sympatric char undergo a habitat s h i f t in October to habitats that are not u t i l i z e d by trout. Sympatric char may also undergo a habitat s h i f t in August, but the evidence for th i s s h i f t i s not conclusive. There were no s t a t i s t i c a l differences between the two 81 populations in d i e l use of habitats, although in August and October there was some evidence that sympatric char used shallower habitats at night, while a l l o p a t r i c char used l i t t o r a l habitat more at night than during the day. Prior to sampling in the lakes, I predicted that sympatric, but not a l l o p a t r i c , char would move at night to shallow habitats occupied by trout as irradiance le v e l s decreased. In shallow habitats at night, char could exploit abundant zooplankton or even surface arthropod prey, since char are more e f f i c i e n t than trout in the detection of prey at low irradiance levels (Henderson and Northcote 1985). Even though sympatric char do not c l e a r l y show thi s pattern of d i e l movement, the d i e l v e r t i c a l movement of a l l o p a t r i c char i s puzzling. Why are a l l o p a t r i c char not d i s t r i b u t e d in shallow habitats during the day? One possible reason i s that risk of avian predation is greater at the surface than in deeper habitats. Risk of predation i s known to affe c t the choice of foraging habitat by f i s h (Mittlebach 1981, 1984, Werner et a l . 1983). Belted kingfisher (Meqaceryle alcyon), great blue heron (Ardea herodias), and common loon (Gavia immer) have been observed in the U.B.C. Research Forest (J. Werring, pers. comm.) and, although these bird species are not common, may act as predators at the lake surface during the day but not at night at a l l three study lakes. Another reason may be that char prefer the deepest water and lowest irradiance l e v e l that s t i l l allows maximum reaction distance to prey. Without hindering v i s u a l perception of food items, u t i l i z i n g low irradiance l e v e l habitats may provide concealment from predators. The reaction 82 distance of char i s maximized at a r e l a t i v e l y low irradiance l e v e l (>3.0 x 10 1 6 photons/m2/s; Henderson and Northcote 1985) and during the day t h i s irradiance l e v e l i s r e l a t i v e l y deep (>40 m on a clear July day; Henderson and Northcote 1985, their Figure 4). However, at night, when irradiance levels at a l l depths decrease, char must migrate to shallower water to maintain maximum reaction distance (Henderson and Northcote 1985, their Figure 5). These data do not provide evidence that char in sympatry with trout undergo a temporal niche s h i f t , as both sympatric and a l l o p a t r i c char used shallower limnetic or l i t t o r a l habitats at night. An hypothesis of competition between trout and char in Loon Lake predicts, in addition to habitat s h i f t s of one or both species, that habitat u t i l i z a t i o n of the two species is more similar when both species are in allopatry than when they coexist in Loon Lake. Habitat u t i l i z a t i o n of sympatric trout versus char, and of a l l o p a t r i c trout versus char were s i g n i f i c a n t l y d i f f e r e n t during June and August (H-tests, p<.00l; Tables 12 and 13). However, in October, there was no s i g n i f i c a n t difference in habitat u t i l i z a t i o n between a l l o p a t r i c populations (H-test, p>.05; Table 13D), while that of the sympatric populations remained s i g n i f i c a n t l y d i f f e r e n t (H-test, P<.001; Table 12C). The result in October i s consistent with the pattern of habitat overlap shown in Figure 20. There were no differences in d i e l use of habitats between sympatric populations or between a l l o p a t r i c populations from June to October. However, at night in June, there was some evidence 8 3 A . S Y M P A T R I C C H A R >-u z UJ Z> C3 UJ tr D E P T H O F H A B I T A T Figure 20. Schematic diagram of habitat overlap of sympatric Loon Lake trout and char and experimentally a l l o p a t r i c Eunice Lake trout and Katherine Lake char. 84 that a l l o p a t r i c trout moved to t y p i c a l char habitat (epibenthic) and a l l o p a t r i c char moved to t y p i c a l trout habitat ( l i t t o r a l ) . F ish d i s t r i b u t i o n s in October support an hypothesis of competition between trout and char in Loon Lake based on a comparison of habitat separation between the two sympatric populations with that of the two a l l o p a t r i c populations, but there i s i n s u f f i c i e n t evidence in June and August to support the hypothesis. The lack of evidence in June and August i s due to the fact that the habitat u t i l i z a t i o n of trout and char was s i g n i f i c a n t l y d i f f e r e n t at a l l times, thereby preventing "quantification" of niche separation. Although trout did not undergo a habitat s h i f t or expand their v e r t i c a l d i s t r i b u t i o n in all o p a t r y , the habitat s h i f t of char and expansion of their habitat u t i l i z a t i o n to the surface layers of the lake in allo p a t r y i s consistent with the hypothesis that i n t e r s p e c i f i c competition occurs between trout and char in Loon Lake. In addition, the habitat s h i f t by char but not trout i s consistent with the hypothesis that competition acts more strongly on char. Although niche s h i f t s are generally accepted as evidence of competition within communities, there are s p e c i f i c c r i t e r i a that must be met for competition to occur. Competition occurs when two or more organismic units use the same resources that are in short supply, and t h i s reduces the fit n e s s and/or equilibrium population size of each (Pianka 1983, p.184). Trout and char populations in Loon Lake consume at least some of the same prey species (Schutz and Northcote 1972, Hume and Northcote 1985, Hindar e_t a_l. in prep.). In allopatry, 85 similar genetic stocks of these species consume v i r t u a l l y a l l of the same prey categories as each other, although not in the same r e l a t i v e proportions (Hindar et a_l. in prep.). The rapid growth of Loon Lake trout and char stocks transferred to previously f i s h l e s s lakes with abundant food resources (Hume and Northcote 1985) indicates that these species are food-limited in Loon Lake. The limited growth of trout and char in Loon Lake most l i k e l y results in reduced reproductive potential ( i . e . fecundity) and potential population size, and therefore reduces their f i t n e s s . Although Pianka's c r i t e r i a have not been addressed d i r e c t l y in thi s study, they could provide a focus for further studies on these f i s h populations. Part of the d i f f i c u l t y in drawing a strong conclusion from t h i s f i e l d experiment is the lack of a r i g i d control for the experimental transfer of f i s h between whole lake environments. Although the study lakes had similar limnological features and f i s h prey d i s t r i b u t i o n s , even subtle differences in lake environments may influence how f i s h u t i l i z e habitats. Differences in lake morphometry, especially l i t t o r a l development, and invertebrate d i s t r i b u t i o n s are probably among the most important of these factors in thi s experiment. In addition, i n t r a s p e c i f i c competition pressure influences how populations respond to i n t e r s p e c i f i c competition, and this has not been addressed in this experiment. The r e l a t i v e importance of these factors on habitat u t i l i z a t i o n by trout and char i s d i f f i c u l t to quantify. The conclusion that trout i s a superior competitor to char 86 i s corroborated by evidence from diets and physiological performance based on growth rates and si z e . Char, but not trout, showed a niche s h i f t in diet r e l a t i v e to the sympatric donor stock (Hindar et a l . in prep.). Sympatric and a l l o p a t r i c trout had a marked food resource overlap from June to October, and fed mainly on l i t t o r a l zoobenthos, surface insects, and cladocerans. Sympatric and a l l o p a t r i c char overlapped in food resources in October only, although food resource a v a i l a b i l i t y may partly explain t h i s r e s u l t . Sympatric char consumed mainly l i t t o r a l zoobenthos, chironomids, and zooplankton, whereas a l l o p a t r i c char consumed mainly l i t t o r a l zoobenthos in June and August, and surface arthropods and zooplankton in October (Hindar et a l . in prep.). A l l o p a t r i c char improved r e l a t i v e to the sympatric population with respect to the l i f e history variables growth rate (higher, p<.00l) and size (fork length longer, p < . 0 0 O , while growth rate of a l l o p a t r i c trout was the same as that of the sympatric stock (p>.05) and mean fork length was shorter (p<.00l; Jonsson et a_l. 1984, their Table 2). However, differences in growth rates and size between sympatric and a l l o p a t r i c stocks as indicators of competitive pressure in Loon Lake must be interpreted with caution, as a l l o p a t r i c populations are not necessarily at carrying capacity, while the populations in Loon Lake are stable (Section 3.2). The habitat segregation of trout and char in Loon Lake i s mainly with depth of habitat. Seasonal and da i l y differences are not common. The morphological and ecological s p e c i a l i z a t i o n s , or selective differences, of these c l o s e l y 87 related species are in accordance with their pattern of habitat use. Char feed more successfully on benthic prey, and trout on surface prey in laboratory experiments involving s o l i t a r y individuals and interspecies pairs exposed to food in benthic, surface, and both locations (Schutz and Northcote 1972, see also Hume 1978). Differences in feeding performance on prey types may be related to differences in mouth morphology of trout and char (Hespenheide 1973). The mouth of char i s subterminal and "directed" downwards at benthic prey whereas the mouth of trout i s terminal, which may allow trout to feed more e f f e c t i v e l y on zooplankton or surface prey. Deep-dwelling char have a SIT and VIT that are one and two orders of magnitude lower, respectively, than those of the more surface-dwelling trout (Henderson and Northcote 1985). These differences in vi s u a l a b i l i t y of trout and char are related to differences in the eye morphology and the r a t i o of rods to cones in the retina (Henderson 1982), and enable each species to detect prey in i t s habitat. In addition to v i s u a l perception of prey, char are capable of chemoreception of prey below their VIT, and the greater maximum reaction distance and foraging v e l o c i t y of trout enable trout to v i s u a l l y search a volume of water seven times greater than char for a zooplankter such as Diaptomus kenai on a summer day (Henderson and Northcote 1985). The s p e c i a l i z a t i o n s of trout make i t superior to char in the exploitation of food resources in shallow, well-illuminated habitats, and vice versa for char in deeper, less well-illuminated habitats. However, these s p e c i a l i z a t i o n s do not explain the absence of char from 88 shallow habitats in sympatry with trout in Loon Lake. In addition to selection of habitat based on food resources, trout and char may also use other environmental factors such as temperature and irradiance l e v e l to provide a cue for habitat p a r t i t i o n i n g with depth. Apart from differences in prey type and abundance with depth, the limnetic zones of lakes are homogeneous in many respects. Temperature is an important determinant of physiological and biochemical rates (Fry 1971). The behavioural thermoregulation of f i s h i s well- documented (Ferguson 1958, Brett 1971, N e i l l and Magnuson 1974, Coutant 1977), and. the success of a f i s h in achieving i t s fundamental thermal niche can contribute to i t s f i t n e s s in terms of growth (Brett 1970, Magnuson et a l . 1979). Recent studies have documented thermal habitat p a r t i t i o n i n g by f i s h in lake bottom habitat (Brandt et a l . 1980), seasonal habitat s h i f t s along temperature gradients (Matthews et a l . 1985), thermal habitat s h i f t s resulting from competitive interaction (Beitinger and Magnuson 1975, Crowder and Magnuson 1982), and complementarity in the use of food and thermal habitats in a lake (Crowder et a l . 1981). Magnuson et a l . (1979) stressed that temperature i s an ecological resource and i s one axis of an animal's multidimensional niche. The fundamental temperature niche of trout and char i s probably similar to that of juvenile rainbow trout, which McCauley and Pond (1971) found to be 17- 20 °C. It i s i n t u i t i v e that the fundamental irradiance niche of both trout and char i s one which maximizes the reaction distance to prey targets. Therefore, habitat preference of both trout 89 and char based on temperature and irradiance l e v e l preferences is in warm epil i m n i a l waters of l i t t o r a l or epipelagic habitat. Since the reaction distance of char is maximized at a lower irradiance level than trout (Henderson and Northcote 1985), the fundamental habitat of char extends deeper in the water column than that of trout. In allopatry and sympatry, trout occupy their fundamental niche based on temperature and irradiance preferences. Although sympatric char occupy habitats deeper in the water column than trout, in allopatry they are found in shallower habitats that are more similar to their preferred thermal and irradiance niche. Therefore, an hypothesis of habitat selection based on temperature and irradiance l e v e l preferences i s supported by the d i s t r i b u t i o n of a l l o p a t r i c trout and, to a lesser extent char. The habitat s h i f t of char in sympatry to colder habitats with lower irradiance levels i s in accordance with the hypothesis that competition between trout and char in Loon Lake acts more strongly on char. When competition concerns d i r e c t u t i l i z a t i o n of l i m i t i n g resources and deprives other individuals of the benefits to be gained from those resources, the mechanism of competition i s said to be exp l o i t a t i v e • If competitive a b i l i t y i s based on interference phenomena and individuals harm each other by aggressive encounters, producing toxins, and so on, which prevent a competitor from gaining access to resources, the mechanism of competition i s said to be interference (Crombie 1947, Elton and M i l l e r 1954, Brian 1956, M i l l e r 1967, Schoener 1983, Pianka 1983). Local extinction occurs only where species 90 niches overlap, thereby allowing populations to coexist in contiguous allopatry ( M i l l e r 1964). Based on his work with gopher species, M i l l e r (1964) stated that, as a general p r i n c i p l e , whenever competitive exclusion occurs and the fundamental niche of one species i s included within the fundamental niche of another species, the f i r s t species with the spec i a l i z e d niche must be the superior competitor in order to survive. M i l l e r ' s results are p a r a l l e l e d by the results of the present study, where the preferred habitat ( i . e . fundamental niche) of trout i s included within that of char, and trout i s the superior competitor. Connell (1961), Werner and Ha l l (1977), and Nilsson (1960, 1963) also obtained similar r e s u l t s . Connell found that the barnacle Cthamalus s t e l l a t u s survived at a l l water lev e l s in the i n t e r t i d a l zone, but persisted in competition with the superior competitor Balanus balanoides only by occupying a part of the environment where B. balanoides did not survive. Werner and H a l l found that b l u e g i l l sunfish (Lepomis macrochirus) were more f l e x i b l e in habitat use than green sunfish (L. cyanellus), which were limited to l i t t o r a l habitats. In sympatry with aggressive green sunfish, b l u e g i l l s s h i f t e d to smaller, less preferred food items in the open water column, while green sunfish remained in the l i t t o r a l zone and exploited larger food items. The open water column provided a competitive refuge for the b l u e g i l l , which handled small foods more e f f i c i e n t l y than did green sunfish. Nilsson found that brown trout (Salmo trutta) and a r c t i c char (Salvelinus alpinus) in Sweden preferred similar 91 prey, but in sympatry, char shifted to offshore prey, primarily zooplankton, whereas trout continued to feed on preferred prey types in the l i t t o r a l zone. The trout were more ef f e c t i v e than char in exploiting preferred prey items and were much more t e r r i t o r i a l and aggressive in i n t e r s p e c i f i c encounters. In each case c i t e d above, the competitor that was a s p e c i a l i s t in habitat or food selection (e.g. trout in the present study) was a superior competitor to the generalist competitor (e.g. char), and interference competition was the mechanism of exclusion of the generalist from i t s usual habitat. Habitat selection behaviour of species evolves because organisms in some habitats leave more descendants than organisms in other habitats; such behaviour can be very exact and sp e c i a l i z e d in predictable environments (Krebs 1978). However, generalists often occur where they have few competitors (Morse 1980), perhaps due to their competitive exclusion by superior s p e c i a l i s t competitors. A l l o p a t r i c populations of trout and char used in the present study both demonstrated p l a s t i c i t y in prey selection in the 18 mo following segregation in 1974 - 1976 (Hume and Northcote 1985). Both populations switched to abundant Chaoborus larvae in pelagic habitats. However, th i s switch in prey type represented a greater change in habitat selection by char than trout. Individual char were previously highly benthofagous in sympatry with trout, whereas trout occupied shallow habitats in Loon Lake. Therefore, char can be concluded to be more generalist, opportunistic predators than trout. In 92 the present study, a l l o p a t r i c char were more general in habitat u t i l i z a t i o n than trout. However, fluctuations in abundances of d i f f e r e n t prey types may partly explain the apparent diet expansion of a l l o p a t r i c char because Katherine Lake may have had more e r r a t i c fluctuations in food types and abundances than Loon or Eunice lakes (Hindar et a_l. in prep.). The habitat s h i f t of char to less preferred habitats in the presence of trout may be based on interference by trout. Schutz and Northcote (1972) found that trout were aggressively dominant to char in most, interspecies pairs in laboratory aquaria, and Rosenau (1978) found that trout were more aggressive than char in stream aquaria. In summary, the present study showed evidence consistent with the hypothesis that trout and char were in competition in Loon Lake. Based on d i s t r i b u t i o n between habitats in allopatry, char are generalists in habitat selection and trout are r e l a t i v e s p e c i a l i s t s . In sympatry, trout remain in shallow, habitats s i m i l a r l y to a l l o p a t r i c trout, and may competitively exclude char from t h i s zone. D i e l and seasonal temporal differences are not important to habitat u t i l i z a t i o n of trout. The mechanism of competition of trout and char in Loon Lake i s at least p a r t i a l l y e x p l o i t a t i v e , based on sel e c t i v e differences between the two species. However, a mechanism of e x p l o i t a t i v e competition does not explain why char are not present in t y p i c a l trout habitat. Other studies suggest that interference competition is usually the mechanism of competition when a generalist competitor i s excluded from the preferred habitat of a s p e c i a l i s t competitor. 93 In addition, the superior aggressiveness of trout i s consistent with an hypothesis of segregation with depth based on interference competition. It i s suggested that trout and char are segregated with depth in Loon Lake based on interference competition by trout, and that t h i s mechanism i s moderated by irradiance l e v e l , an environmental cue which provides structure in the pelagic environment. 4.2 Behavioural Interactions and Irradiance Level If interactive segregation occurs between populations, the following c r i t e r i a must be met: 1. The populations must be segregated s p a t i a l l y and/or temporally, at least during c r i t i c a l periods of resource a c q u i s i t i o n . 2. The populations must be competitors, or potential competitors, for an es s e n t i a l resource such as food or space. 3. The populations must have a communication system of recognizable signals, which may take the form of aggressive or agonistic behaviours that signal dominance or t e r r i t o r i a l i t y to individuals or groups of the other population. 4. To avoid l o c a l extinction, both populations must be able to maintain growth and reproduction. In accordance with C r i t e r i o n 1, trout and char in Loon Lake are segregated s p a t i a l l y with depth (Armitage 1973, Hume 1978, 9 4 my f i e l d study). Furthermore, in accordance with C r i t e r i o n 2 , I presented evidence in my f i e l d study that trout and char are in competition. In sympatry, there i s a habitat s h i f t by char, but trout occupy their preferred habitat. Trout in sympatry with char in Loon Lake have similar s p a t i a l and temporal d i s t r i b u t i o n s as trout in allopatry in Eunice Lake; they are most dense in l i t t o r a l and epipelagic habitats. However, char in Loon Lake have a r e s t r i c t e d s p a t i a l and temporal d i s t r i b u t i o n in r e l a t i o n to char in allopatry in Katherine Lake. In allopatry, char occupy the entire water column, but in sympatry with trout, char are found in deep limnetic water. It was concluded from the f i e l d study that trout and char in Loon Lake are in competition and that trout i s the superior competitor. However, at t h i s point i t has not been shown whether competition i s of the exploitative or interference type. My laboratory experiments address C r i t e r i a 3 and 4 , and investigate behavioural interaction (interference competition) as a possible mechanism of interactive segregation between trout and char. In conjunction with C r i t e r i a 3 , one requisite of the establishment of a dominance relat i o n s h i p i s a communication system of recognizable signals between dominant and subordinate ind i v i d u a l s . The repertoire of behaviours encompassing aggressive behaviours performed by dominant individuals and submissive behaviours performed by subordinate individuals are termed "agonistic behaviour". The aggressor i s i d e n t i f i e d by overt defense of i t s t e r r i t o r y as attacking, chasing, or threatening, or displays which may be overt or r i t u a l i z e d (Morse 95 1980). I n t e r s p e c i f i c patterns of aggressive behaviour in animal groups studied to date resemble i n t r a s p e c i f i c patterns (Morse 1980, p.267), and in clos e l y genetically related species such as trout and char, communication signals may be more similar than behavioural signals of species which are not cl o s e l y related. Aggressive behaviour and dominance-mediated i n t e r s p e c i f i c relationships have been reported for many animal taxa, including mammals, birds, f i s h , l i z a r d s , salamanders, s t a r f i s h , insects, crustaceans, spiders, and limpets (reviewed in Morse 1980, p.267). These relationships occur over a wide range of s o c i a l situations, including both s t r i c t t e r r i t o r i a l situations and ones in which no stationary area i s defended (Morse 1980). If the dominance relationship involves t e r r i t o r i a l i t y , aggressive behaviour i s associated with a clear reference point in space (concept developed by Schjelderup-Ebbe 1922, c i t e d in Morse 1980). The function of aggressive behaviour, then, i s to gain access to resources (e.g. food, space, or mates) while r e s t r i c t i n g the a v a i l a b i l i t y of resources to another individual or group. In accordance with C r i t e r i o n 3, there i s evidence that trout and char possess a common system of communication. Aggressive behaviours of trout such as nip, charge, and chase, and the submissive behaviour of char such as avoiding or fle e i n g from i t s aggressor combined to produce a dominance relat i o n s h i p in interspecies pairs in the present study. The dominance rel a t i o n s h i p in the laboratory aquarium may be associated with t e r r i t o r i a l i t y , as evidenced by the r e s t r i c t i o n of char to one 96 end, and the more general use of the aquarium by dominant trout (Figure 12). Fish may use the aquarium walls or substrate to v i s u a l l y locate t e r r i t o r i e s . However, there would seem to be an absence of such s p a t i a l markers to locate t e r r i t o r i e s in lake pelagic environments. Unless f i s h are cl o s e l y associated with the lake surface or bottom, there are few v i s u a l cues for a f i s h as to i t s location. Therefore, how is segregation between Loon Lake populations of trout and char maintained? One environmental cue which may provide a v e r t i c a l " s p a t i a l " marker for trout and char in Loon Lake i s irradiance l e v e l . Irradiance l e v e l i s an important factor in selective differences between the two species in the procurement of food resources (Henderson 1982). Since trout are less able to perceive prey in low irradiance, the preferred habitats of trout have r e l a t i v e l y high irradiance l e v e l s . If trout are dominant to char in Loon Lake, char w i l l experience strong aggression in high irradiance habitats occupied by trout and char may fl e e to low irradiance habitats to seek refuge. The laboratory experiments reported herein provide evidence for reduced intensity of behavioural interactions between trout and char as irradiance decreases to the vis u a l threshold of trout. Assuming that aggression i s based on v i s u a l cues, one explanation for reduced aggression by trout with decreasing irradiance l e v e l i s that the a b i l i t y of trout to v i s u a l l y perceive char declines over the range of experimental irradiance tested. The v i s u a l a b i l i t y with respect to reaction distance to prey of trout and char c e r t a i n l y declines over t h i s range 97 (Henderson 1982). In addition to the reduced c a p a b i l i t y of trout to see char with decreasing irradiance l e v e l , the swimming a c t i v i t y of trout i s another factor which would a f f e c t the frequency of v i s u a l contact of char by trout. Although the swimming a c t i v i t y of trout in i n t e r s p e c i f i c pairs with char did not decrease s i g n i f i c a n t l y with irradiance l e v e l (F-test, p>.05), s o l i t a r y trout were less active in low irradiance. Therefore, the presence of subordinate char may stimulate the swimming a c t i v i t y of trout to reinforce the dominance rela t i o n s h i p . The confinement of trout and char in r e l a t i v e l y small laboratory aquaria at almost f i f t y times the natural f i s h density in Loon Lake "forced" more intense interactions between the f i s h than they would l i k e l y experience in their natural environment. Since trout in Loon Lake are not confined in close proximity with char as they are in experimental aquaria, their swimming a c t i v i t y would probably be more similar to that observed in s o l i t a r y trout. Therefore, in low irradiance habitats in Loon Lake, trout would have very l i t t l e v i s u a l contact with char due to reduced v i s u a l a b i l i t y and swimming a c t i v i t y . Although aggression by trout decreased to a very low int e n s i t y at their v i s u a l irradiance threshold, the dominance rel a t i o n s h i p with char persisted. Although most behavioural interactions at a l l irradiance l e v e l s were submissive acts by char ( i . e . "avoidance"), the proportion of submissive acts increased with decreasing irradiance l e v e l . In Loon Lake, a dominance relationship between individual interacting pairs of 98 trout and char would possibly not be established nor would i t p e r s i s t as observed in the experiments, because the char would be able to f l e e from i t s aggressor. In low irradiance habitats, char would have a r e l a t i v e l y low encounter rate with trout, and the low intensity of aggression might then not be strong enough to es t a b l i s h a dominance rela t i o n s h i p . In Loon Lake, trout and char segregate s p a t i a l l y on an irradiance gradient, where trout use r e l a t i v e l y high irradiance l e v e l habitats and char use deeper habitats with lower irradiance l e v e l s . Competitively i n f e r i o r char experience differences in intensity of aggression by trout along t h i s gradient, and the d i s t r i b u t i o n of the two species i s consistent with these differences. If char can learn to associate r e l a t i v e l y high irradiance l e v e l with a greater intensity of aggression from trout, the hypothesis that habitat segregation is maintained by aggressive i n t e r s p e c i f i c interactions by competitively superior trout cannot be rejected. The a b i l i t y of char to associate irradiance l e v e l with aggression by trout was not tested in t h i s study. Further investigations, p a r t i c u l a r l y d i r e c t f i e l d observations, are required to test the application of this laboratory study to interactions of trout and char at irradiance le v e l s in their natural lake environment. A more rigorous laboratory test of the hypothesis that segregation between trout and char i s maintained by a mechanism of interactive segregation based on irradiance l e v e l s would be to conduct similar experiments to mine but in larger aquaria. Use of 10 x 10m 99 aquaria would scale the experimental f i s h density to approximately the natural density in Loon Lake. In aquaria of th i s size, behavioural interactions between interspecies pairs would probably be much less intense than I observed, but would probably r e f l e c t more accurately the natural rate of interaction. However, the results of my laboratory experiments may be indicative of interactions that take place in the natural environment in a more subtle form. The fourth c r i t e r i o n for interactive segregation i s that populations must be able to obtain adequate food resources to maintain growth and reproduction, thereby avoiding l o c a l e x t i n c t i o n . It is not known whether the trout and char populations in Loon Lake are at stable levels but the fact that char have persisted in Loon Lake for many decades i f not centuries indicates that they have been able to obtain adequate food resources for growth and longterm reproduction (see also Jonsson et §_1. 1 984). However, in coexisting populations of trout and char, the exclusion of char from i t s preferred habitat or d i s t r i b u t i o n probably means that char are r e s t r i c t e d to less than optimal foraging patches, since habitat selection i s ultimately based in i t s contribution to f i t n e s s of the individual (Alcock 1975, Werner et a l . 1981). A reduction in energy intake per foraging e f f o r t by char due to less dense prey, smaller prey, and increased search and/or handling time in such patches obviously results in decreased f i t n e s s . A strong reduction in f i t n e s s would result in the eventual extinction of the population. One mechanism the char may use to obtain 100 adequate food resources i s to occupy low irradiance habitats to forage unhindered by aggressive trout. The feeding performance of char may improve when aggression by trout i s reduced. The feeding experiments in t h i s study were performed to test whether the feeding performance of trout and char was consistent with an hypothesized mechanism of interactive segregation on irradiance l e v e l gradients in Loon Lake. My experiments provide evidence that in an irradiance l e v e l that maximized reaction distance of char and very nearly maximized that of trout (3.0 x 10 1 8 photons/m 2/s), the feeding rate of char on Neomysis mercedis was adversely affected by aggressive behaviour of trout. Char which had more behavioural interactions with trout made fewer feeding s t r i k e s (Figure 15). However, in Loon Lake habitats below the v i s u a l irradiance threshold of trout (3.0 x 10 1 5 photons/m 2/s), trout would not detect prey items v i s u a l l y (Henderson and Northcote 1985) and would presumably not see char, thereby removing the stimulus for aggressive behaviour of trout. Moreover, below th i s threshold, trout would not r e s t r i c t the feeding performance of char through aggressive behaviour, and char could forage as i f trout were not present, detecting prey items v i s u a l l y to their v i s u a l irradiance threshold (7.0 x 101 * photons/m 2/s), and below th i s l e v e l detecting prey using chemoreception (Henderson 1982). The feeding rate of char was i n f e r i o r to that of aggressively dominant trout at a l l irradiance l e v e l s above the v i s u a l irradiance threshold of trout. As described in the foraging model of Henderson and Northcote (1985), the greater 101 reaction distance and swimming a c t i v i t y of trout (Figures 14 and 18) allowed trout to search a larger volume of water than char thereby increasing prey encounter rate. In my experiment, char were subordinate at a l l irradiance levels and were r e s t r i c t e d to one end of the aquarium. They behaved l i k e " s i t and wait" predators, and were only able to search for prey in a hemispherical volume of water, with reaction distance as the radius. The feeding performance of char in i n t e r s p e c i f i c pairs dominated by trout (8.76 ± 9.47 (mean ± standard deviation) st r i k e s per 30 minutes, n=6) was s i g n i f i c a n t l y less than that of s o l i t a r y char (48.14 ± 6.86 s t r i k e s per 30 minutes, n=2), but was not s i g n i f i c a n t l y d i f f e r e n t from that of char dominating i n t e r s p e c i f i c or i n t r a s p e c i f i c pairs (27.4 ± 20.1, n=4 and 7.72 ± 7.81, n=2 strikes per 30 minutes, respectively). The feeding performance of both trout and char was reduced with decreasing irradiance l e v e l . However, feeding strikes by trout declined more rapidly than those of char (Figure 16). It should be noted that feeding s t r i k e s included both successful and unsuccessful attacks on Neomysis prey, and that the proportion of unsuccessful s t r i k e s probably increased with decreasing irradiance l e v e l , e specially for trout. Prior to performing the experiment, i t was expected that the feeding performance of char would improve with decreasing irradiance l e v e l while aggression by trout was less frequent. However, at a l l irradiance levels above the v i s u a l irradiance threshold of trout, the feeding performance of trout was superior to that of char (Figure 16). This i s probably because the dominance relationship between the 1 02 f i s h persisted to the visua l irradiance threshold of trout. For reasons already given, in Loon Lake, the dominance relat i o n s h i p would probably break down in low irradiance habitats, and therefore the feeding performance of char might then improve. In any case, char are c e r t a i n l y more capable than trout of procurement of food resources in low irradiance habitats (Henderson and Northcote 1985). Henderson showed that, although the maximum reaction distance for visu a l prey detection by char at their saturation irradiance l e v e l of 3.0 x 10 1 6 photons/m 2/s, v char use v i s u a l prey detection down to their v i s u a l irradiance threshold of 7.0 x 10 1 4 photons/m 2/s, below which they use chemoreception of prey. Trout are only able to use vi s u a l prey detection down to an irradiance l e v e l of 3.0 x 10 1 5 photons/m 2/s, which corresponds to a depth of below 40 metres in Loon Lake on a sunny summer day. For approximately 5.5 h per night, not even surface waters are illuminated s u f f i c i e n t l y for prey detection by trout (Henderson and Northcote 1985, the i r Figure 5). Therefore char are able to capture prey in these darker spatio-temporal habitats in the absence of trout. 4.3 Concluding Statement Segregation of trout and char in Loon Lake i s c e r t a i n l y s e l e c t i v e due to behavioural (Schutz and Northcote 1972) and physiological (Henderson 1982) differences that af f e c t prey a c q u i s i t i o n . However, competition plays a role in habitat u t i l i z a t i o n of sympatric trout and char. Trout occupy surface habitats whether a l l o p a t r i c or in sympatry with char. On the 1 03 other hand, char undergo a habitat s h i f t between sympatry and a l l o p a t r y . In allopatry, char occupy d i f f e r e n t habitats seasonally, in accordance with food abundance, but in sympatry, char s h i f t to deeper, less well-illuminated habitats. Temporal segregation between trout and char was not pronounced. It i s concluded that trout are competitively superior to char, based on the lack of habitat s h i f t by trout and the accompanying habitat s h i f t by char, although t h i s result i s interpreted with caution since differences in prey d i s t r i b u t i o n s partly explain habitat u t i l i z a t i o n . D i f f i c u l t i e s in drawing strong conclusions from the f i e l d r e sults arise due to the lack of r i g i d controls in such whole lake experiments. In p a r t i c u l a r , the influence of differences in l i t t o r a l development and prey d i s t r i b u t i o n s on habitat u t i l i z a t i o n by f i s h are d i f f i c u l t to quantify. Other studies have shown that trout are very aggressive towards char in lake (Schutz and Northcote 1972) and stream (Rosenau 1978) laboratory aquarium studies. It i s concluded from my laboratory experiments that behavioural interactions between dominant trout and subordinate char decrease with irradiance l e v e l . If t h i s holds true in lake environments, char may seek refuge from aggression by trout by s h i f t i n g to low irradiance habitats. Char do switch to such habitats in sympatry with trout, but whether their habitat s h i f t in Loon Lake i s a result of interference mechanisms i s not c l e a r . This r e l a t i o n s h i p might be confirmed in an appropriate study, based on f i e l d observations. Although the feeding performance of char improves with decreasing intensity of aggression by trout, my 104 laboratory experiment did not show that decreasing irradiance l e v e l per se produced the same e f f e c t . This i s probably because in my experiment, the dominance relationship between trout and char persisted in conditions of low irradiance due to the continued confinement of f i s h pairs in aquaria. This study corroborates the scenario proposed by Henderson (1982) that when trout and char invade a f i s h l e s s lake, trout through their aggressive highly competitive behaviour, are able to occupy their "optimal" habitat based on food preferences and r e s t r i c t char to other portions. However, the habitat occupied by char in sympatry with trout may be "optimal" for char, since food, competition, and predators are important variables determining the habitat for each species. Segregation of populations need not be exclusively selective or interactive, and although the segregation of trout and char in Loon Lake is c e r t a i n l y s e l e c t i v e , an hypothesis involving an interactive mechanism of segregation and interference competition along irradiance l e v e l gradients cannot be rejected. 105 5.0 REFERENCES Alcock, J. 1975. Animal behavior: an evolutionary approach. Sinauer Associates, Inc., Sunderland, Mass. 547p. Andreev, N.N. 1955. 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( i n preparation) The e f f e c t of i r r a d i a n c e l e v e l on behavioural i n t e r a c t i o n s between c u t t h r o a t t r o u t and Do l l y Varden char. 8. Andrew, J.H., N. Jonsson, T.G. Northcote, B. Jonsson and K. Hindar. (i n preparation) S p a t i a l and temporal d i s t r i b u t i o n of sympatric and experimentally a l l o p a t r i c c u t t h r o a t trout and D o l l y Varden char. 9. Hindar, K., J.H. Andrew, B. Jonsson and T.G. Northcote. ( i n preparation) Feeding s t r a t e g i e s of sympatric and experimentally a l l o p a t r i c c utthroat t r o u t and Do l l y Varden char. RESEARCH PRESENTATIONS 1. P a c i f i c Ecology Conference, Bamfield, B.C., 21-24 February 1985. 2. SCWIST Society f o r Canadian Women i n Science and Technology Symposium, Cowichan Bay, B.C., 26-28 A p r i l 1985. 3. I n s t i t u t e of Animal Resource Ecology, UBC Research seminar, 9 September 1985

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