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Size-selective predation by the threespine stickleback Burko, Thomas 1975

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SIZE-SELECTIVE PREDATION BY THE THREESPINE STICKLEBACK by THOMAS BURKO B.Sc, UNIVERSITY OF TORONTO, 1973 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE In the Department of Zoology We accept t h i s thesis as conforming to the required standard The University of B r i t i s h Columbia February, 1975 In presenting t h i s t h e s i s in p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission for extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s representatives. It i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission. Department of *Z- OOt-O The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8, Canada Date :-6 i i ABSTRACT T h i s t h e s i s examines some f a c t o r s t h a t l i m i t and d e f i n e s i z e - s e l e c t i v e p r e d a t i o n by the t h r e e s p i n e s t i c k l e b a c k , G a s t e r o s t e u s a c u l e a t u s . Behavioural? p h y s i c a l and e n e r g e t i c f a c t o r s were c o n s i d e r e d . E n c l o s u r e s t u d i e s i n a shallow o l i g o t r o p h i c l a k e and i n a small creek were used to determine the a c t u a l s i z e l i m i t s f o r d i f f e r e n t prey taxa over a range o f s t i c k l e b a c k s i z e s . The prey organisms were the amphipods H y a l l e l a a z t e c a and Crangonyx richmondensis, the iso p o d A s e l l u s o c c i d e n t a l i s m chironomid l a r v a e and the c l a d o c e r a n Sida c r v s t a l l i n a . The major q u e s t i o n o f the t h e s i s i s what determines the l a r g e s t prey organism a s t i c k l e b a c k can handle? The h y p o t h e s i s t h a t p h y s i c a l f a c t o r s , p a r t i c u l a r l y mouth s i z e , are l i m i t i n g was t e s t e d . The observed l i n e a r r e l a t i o n s h i p between maximum prey s i z e ( H y a l l e l a and A s e l l u s ) and f i s h s i z e support t h i s h y p o t h e s i s . F u r t h e r evidence f o r mouth s i z e as the l i m i t i n g f a c t o r , i s pro v i d e d by a l a b o r a t o r y experiment u s i n g A s e l l u s as prey. A r t e m i a s a l i n a n a u p l i i were fed to l a r g e and small s t i c k l e b a c k s i n an attempt to determine the importance o f r e l a t i v e prey and pr e d a t o r s i z e on f e e d i n g performance. Large f i s h d i d r e l a t i v e l y p o o r l y , but the e f f e c t s o f prey s i z e changes were i n c o n c l u s i v e due to confounding f a c t o r s . i i i The r o l e o f e n e r g e t i c s i n s i z e - s e l e c t i v e p r e d a t i o n was examined v i a a model which gave minimum capture r a t e s necessary to s u s t a i n f i s h at maintenance l e v e l s f o r a g i v e n prey s i z e , predator s i z e and temperature. The model can be used to e x p l a i n v a r i o u s o b s e r v a t i o n s i n the f e e d i n g ecology l i t e r a t u r e . The r o l e of v i s i o n i n s i z e - s e l e c t i v e p r e d a t i o n was b r i e f l y c o n s i d e r e d . Evidence from the l i t e r a t u r e i n d i c a t e d t h a t n o n - l i n e a r r e l a t i o n s h i p s e x i s t between: 1) r e a c t i v e d i s t a n c e and prey s i z e 5 and 2) prey r i s k and predator' d i s t a n c e . These n o n - l i n e a r i t i e s p o i n t to the importance o f prey s i z e i n p r e dator-prey i n t e r a c t i o n s . i v TABLE OF CONTENTS Page ABSTRACT i i TABLE OF CONTENTS i v LIST OF TABLES v i LIST OF FIGURES v i i ACKNOWLEDGEMENTS i x INTRODUCTION 1 GENERAL METHODS 9 FIELD -r 9 Marion Lake 9 U. B. C. Endowment Lands 10 Animal Measurements 11 LABORATORY 11 RESULTS 13 FIELD ENCLOSURE STUDIES 13 Marion Lake 13 Endowment Lands 23 Conclusions 31 LABORATORY EXPERIMENTS 32 ASELLUS FEEDING EXPERIMENT: THE UPPER LIMIT TO SIZE-SELECTIVE PREDATION 32 Introduction 32 Methods 33 Results 34 Conclusions 48 ARTEMIA SALINA NAUPLII FEEDING EXPERIMENT: FACTORS AFFECTING THE LOWER LIMIT TO SIZE-SELECTIVE PREDATION 49 V Page I n t r o d u c t i o n 49 Methods 50 R e s u l t s and i n t e r p r e t a t i o n 52 Conclusions 60 ENERGETICS AND SIZE-SELECTIVE PREDATION 61 I n t r o d u c t i o n 61 Minimum d a i l y energy requirements ( e x c l u s i v e o f growth) 61 Prey l e n g t h and weight 63 The f i n a l model 64 A p p l i c a t i o n s of the model 69 Conclusions 73 VISION AND SIZE-SELECTIVE PREDATION 75 I n t r o d u c t i o n 75 Evidence 75 Conclusions 79 GENERAL DISCUSSION 81 LITERATURE CITED 85 APPENDIX: BIAS IN ELECTIVITY INDICES 89 LITERATURE CITED IN APPENDIX 94 v i LIST OF TABLES Table Page I. Prey consumed i n Marion Lake enclosures 15 II. Comparison of H v a l l e l a size d i s t r i b u t i o n s i n f i s h stomachs and benthos 20 I I I . Comparison of H v a l l e l a size d i s t r i b u t i o n i n stomachs of three size classes of stickleback 20 IV. Comparison of Sida size d i s t r i b u t i o n i n stomachs of three size classes of stickleback 20 V. Prey consumed i n Endowment Land enclosure 23 VI. Comparison of prey size d i s t r i b u t i o n s i n f i s h stomachs and i n benthos for Asellus and chironomid larvae 27 VII. Comparison of prey size d i s t r i b u t i o n s between f i s h size classes for Asellus and chironomid larvae — 27 VIII. Number of fast and slow giving up responses by large and small sticklebacks feeding on Asellus - 41 IX. Number of give up responses for small and large sticklebacks feeding on Asellus .9-1.0 times maximum length • 41 X. Performance of sticklebacks on Asellus cut i n h a l f and 1.1 times as wide as the experimental maximum 43 XI. One t a i l e d t - t e s t s for brine shrimp experiment — 56 XII. Relative consumption of large and small s t i c k l e -backs feeding on two sizes of Artemia sal i n a n a u p l i i 58 XIII. Data from the l i t e r a t u r e concerning length-weight relationships f o r various entomostracan taxa 65 XIV. Capture rates necessary for maintenance of the ninespine stickleback at 15°Cj for various prey sizes 67 v i i LIST OF FIGURES Figure Page 1. Size-selective predation within the framework of animal trophic ecology 3 2. Sizes of organisms consumed i n Marion Lake enclosures versus stickleback standard length 17^7 3. E f f i c i e n c y of sorting various size classes of H. azteca from benthic samples 18 7' 4. Size d i s t r i b u t i o n s of H. azteca i n stomachs of Marion Lake enclosure f i s h and i n benthos 19} 5. Mouth width of Chemainus Lake sticklebacks and Lake Errock sticklebacks plotted against standard f i s h length 22 6. Mean length of Daphnia magna eaten by d i f f e r e n t sizes of stickleback during a 15 minute period — 24( 7. Size d i s t r i b u t i o n s of Asellus occidentalis and chironomid larvae i n stomachs of d i f f e r e n t size classes of sticklebacks and i n benthic sample from Endowment Lands enclosure 26 8. Sizes of chironomid larvae and Asellus occidentalis consumed by sticklebacks of d i f f e r e n t lengths i n Endowment Lands enclosure 28 9. Chironomid head width versus t o t a l body length — 29 10. E f f i c i e n c y of benthic sort from Endowment Lands - 30 11. Asellus o c c i d e n t a l i s i n normal standing and curled positions 37 12. Maximum size of Asellus eaten i n laboratory versus f i s h standard length for two populations of s t i c k l e -backs having d i f f e r e n t mouth widths 38 13. Handling time and giving up time versus f r a c t i o n maximum prey (Asellus) length (Chemainus f i s h ) — 40 14. Handling time versus f r a c t i o n maximum Asellus length taken by Errock f i s h i n laboratory 45 v i i i Figure Pa; 15. (a) Maximum width of Asellus taken by Chemainus f i s h i n laboratory and i n creek i n Endowment Lands versus mouth width. (b) Maximum width of H v a l l e l a and Crangonyx taken i n Marion Lake enclosures versus mouth width 46 16. Proportion of times Chemainus sticklebacks gave up on Asellus prey r e l a t i v e to t o t a l number of opportunities (expressed as a p r o b a b i l i t y Pr) versus size class of prey ( f r a c t i o n of maximum prey length) 47 17. Mean attack rates of small sticklebacks feeding on large and small n a u p l i i (Artemia salina) and large sticklebacks feeding on large and small n a u p l i i at three d i f f e r e n t d ensities 53 18. Attack rate during a ten minute t r i a l (high n a u p l i i density) for two small sticklebacks 54 19. Mean attack rates for seven large sticklebacks feeding on large brine shrimp n a u p l i i (900 /um over a 10 minute period (high n a u p l i i density) — 55 20. Lines representing combinations of prey length (P) and f i s h length (F) for which maintenance requirements are just obtained (ninespine s t i c k l e -21, Brown trout (Salmo t r u t t a ) growth rates, mean Asellus aquaticus weight, Asellus biomass and numbers of Asellus plotted against month of the year for a Swedish Lake (adapted from Berglund, back) 68 1968) 72 22. Relationships between reactive distance, prey r i s k and prey size 78 i x ACKNOWLEDGEMENT S I wish to thank my s u p e r v i s o r , Dr. J . D. Mc P h a i l f o r a l l o w i n g me the o p p o r t u n i t y t o undertake t h i s study and f o r h i s support a t a l l stages o f the work. I thank Dr. W. N e i l l and Dr. J . Smith who c r i t i c a l l y read the manuscript and o f f e r e d v a l u a b l e suggestions f o r i t s improvement. I am g r a t e f u l t o Kim Hyatt f o r h i s constant moral support and even more constant b i t i n g c r i t i c i s m . F i n a l l y , I wish to thank R i s a whose p a t i e n c e and encouragement made a l l the d i f f e r e n c e . 1 INTRODUCTION Size-selective predation may be defined as a predator's d i f f e r e n t i a l e x p l o i t a t i o n of various prey size classes. The importance of t h i s phenomenon i n trophic ecology i s best expressed by Ivlev . (1961): "...the problem of selective feeding has become one of the most important questions i n contemporary trophic ecology... together with the degree of concentration of the food, the size of the food organism i s the basic determining factor of selective feeding. M(pg. 6-7) Considering the d i r e c t r e l a t i o n s h i p between quantity and quality of food and reproductive output? the implications of s i z e - s e l e c t i v e predation for natural selection and evolution become clear. A s i m p l i f i e d view of the p o s i t i o n of s i z e -selective predation i n the study of animal trophic ecology i s i l l u s t r a t e d i n Fig. 1. Aspects of s i z e - s e l e c t i v e predation have been studied i n i n t e r t i d a l s t a r f i s h (Menge, 1972)? i n congeneric species of birds (Schoener, 1965; Gibb and Betts, 1963)? and i n the predaceous copepod, Mesocyclops edax (Confer, 1971). These studies deal primarily with i n d i r e c t food r e l a t i o n s , that i s e i t h e r competitive interactions between predators (Menge; Schoener; Gibb and Betts) or interactions between prey species (Confer). Various b i r d studies have looked at the importance of prey size as i t concerns i n d i v i d u a l predator species (direct food r e l a t i o n s : the redshank feeding on the 2 amphipod Corophium, Goss-Custard, 1970; the great t i t feeding on lepidopteran larvae? Royama, 1970; the house martin feeding on a variety of insect taxa, Bryant, 1973). Early studies of size selection by f i s h are la r g e l y d e scriptive and tend to place emphasis on t h i s phenomenon solely as i t re l a t e s to the species studied (Allen, 1935; Hynes, 1950). However, a number of common observations emerge from such studies. The most general i s that as f i s h grow the d i e t changes. Young f r y start on small zooplankton ( r o t i f e r s , ostracods, Bosmina) and progressively switch to larger zooplankton (Daphnia, Sida), chironomid larvae, large benthic invertebrates (amphipods, caddisfly larvae) and ultimately, i f they grow large enough, to other vertebrates ( f i s h and salamanders). Not a l l species progress through t h i s entire sequence, but c e r t a i n l y some ontogenetic change i n d i e t seems general i n fishes. Recent work focuses on the ef f e c t s of s i z e - s e l e c t i v e predation on zooplankton community structure (Brooks and Dodson, 1965; Galbraith, 1967; Brooks, 1968; Brooks, 1969; Dodson, 1970; Cramer and Marzolf, 1970; Wells, 1970; Stenson, 1972; Nilsson, 1973; Dodson, 1974; Northcote and Clarotto, 1974). Here, f i s h predation generally favours the persistence of small-sized cladoceran species (less than 1 mm at maturity) apparently because large species such as Daphnia pulex are consumed i n greater numbers. When the reverse pattern (large sized species 3 F i g . - l . S i z e - s e l e c t i v e p r e d a t i o n w i t h i n the framework of animal t r o p h i c ecology. ANIMAL TROPHIC ECOLOGX / DIRECT FOOD RELATIONS INTENSITY SELECTIVE OF FEEDING FEEDING INDIRECT FOOD RELATIONS I COMPETITIVE INTERACTIONS 1) prey concentrat ion 1) hunger of predator 2) patchiness of prey 3) p o p u l a t i o n and community s t ruc ture o f feeding animals 2) d e n s i t y of predator 3) experience o f predator U) m o b i l i t y of prey 5) prey p r o t e c t i v e devices - c a s e s - s p i n e s - p a l a t a b l l l t y -concealment behaviour 6) p rey concentrat ion 7) prey patchiness 8) s i z e o f predator and prey DETERMINES 1) quant i ty aad q u a l i t y o f food to predator ( f i t n e s s ) 2) s p e c i f i c prey m o r t a l i t i e s EVOLUTION.OF PREDATOR AND PREY 4 favoured) i s encountered invertebrate s i z e - s e l e c t i v e predation i s (invoked; as a causative agent. Certainly invertebrate predators such as the midge, Chaoborus, tend to choose the smaller entomostracan species such as Bosmina l o n g i r o s t r i s (Dodson, 1970). The l i t e r a t u r e c i t e d so f a r i s concerned with the e f f e c t s of s i z e - s e l e c t i v e predation, and as Fig. 1 i l l u s t r a t e s the ultimate e f f e c t i s predator and prey evolution. However, factors that l i m i t and s p e c i f i c a l l y define s i z e - s e l e c t i v i t y are of great importance. Ultimately i t i s these factors upon which natural s e l e c t i o n must work. Behavioural, physical and energetic factors may be recognized. Research on the behavioural factors underlying s i z e -selective predation i s minimal. Ivlev (1961) correlated a behavioural t r a i t (rapacity) with skewness of the prey size d i s t r i b u t i o n taken by four species of f i s h . Highly rapacious piscivores? such as pike, c h a r a c t e r i s t i c a l l y show a prey size d i s t r i b u t i o n skewed toward t h e i r upper l i m i t s . In contrast, "peaceful" planktivores, such as bleak, show a d i s t r i b u t i o n skewed i n the other d i r e c t i o n . Hyatt (1974) indicates that behavioural factors underlie the differences i n size of food p a r t i c l e s taken by rainbow trout, Salmo gairdneri, and kokanee, Oncorhyncus nerka, i n Marion Lake, B.C. Trout prefer large benthic organisms such as large amphipods (Crangonyx  richmondensis) and odonate nymph's,?,; while kokanee prefer small 5 benthic invertebrates such as chironomid larvae, small amphipods (Hvallela azteca) and zooplankters (Sida  c r y s t a l l i n a ) . These preferred size differences are r e f l e c t e d i n r e l a t i v e foraging heights. Foraging trout cruise approxi-mately 20 cm above the lake bottom. Kokanee tend to move just above the mud-water interface. I f we assume equal v i s u a l c a p a b i l i t i e s , t h i s difference i n foraging heights enables the f i s h to d i f f e r e n t i a l l y encounter t h e i r respective prefer-red prey items. Physical factors, such as mouth width, i n two morphs of threespine stickleback (Larson, 1972) and v i s u a l acuity i n sticklebacks (Beukema, 1968) and i n rainbow trout (Ware, 1971), have also been investigated. Larson found that the benthic morph of stickleback feeds on large benthic invertebrates, and has a wider mouth than the limnetic morph that feeds l a r g e l y on zooplankton. Beukema examined the p r o b a b i l i t y of discovery of the sludgeworm, Tubifex, versus the distance from f i s h to prey and found the r e l a t i o n s h i p i s inverse but approximately l i n e a r over a wide range. Ware examined a similar r e l a t i o n s h i p , reactive distance versus prey size and found an inverse square r e l a t i o n s h i p . The importance of physical l i m i t a t i o n s (mouth size and v i s u a l acuity) are a major concern of t h i s thesis. 6 A f i n a l set of factors influencing food size selection are those of an energetic nature. As an organism grows? i t s energy requirements increase? and either feeding rates must increase or larger prey must be procured. In an aquatic medium t h i s i s e s p e c i a l l y true for particulate feeders as opposed to f i l t e r feeders? since the l a t t e r can f i l t e r larger volumes of water per unit time as they grow? but the former must always feed on one prey organism at a time. The common ontogenetic change i n prey size for most f i s h i s probably a necessary consequence of t h i s s i t u a t i o n. A l l e n (1935) noted the importance of an increase i n prey size. He reasoned that to sustain i t s e l f on copepods of a similar size, a four inch perch would have to feed at eight times the rate of a two inch perch. This necessitates an unreasonable capture rate of four per second. Later studies extended t h i s approach by considering the e f f e c t s of food size on: growth rates (Lindstrom? 1954? for a r c t i c char); growth e f f i c i e n c y (Paloheimo and Dickie? 1966? for brook tro u t ) ; and maintenance f e a s i b i l i t y (Leong and O'Connell? 1969? for the northern anchovy). Leong and O'Connell showed that the northern anchovy? Engraulis mordax? could not meet energy maintenance requirements by indiscriminate f i l t e r feeding? and concluded that p a r t i c u l a t e feeding on large prey was probably of great importance. 7 Using energy maximization as a goal? optimal foraging models may be constructed to determine precisely which prey sizes form the optimal set when such factors as handling time and search time are considered. Werner and H a l l (1974) and Werner (1974) devised such a model for the sunfish, Lepomis. In the present study I have also developed a model to demonstrate the importance of predator-prey size relationships-, i n determining the minimum size prey that can maintain a given predator. The model i s used to investigate c e r t a i n findings i n the current l i t e r a t u r e on the feeding ecology of f i s h . My size selection experiments use the threespine stickleback, Gasterosteus aculeatus» as the predator. This animal i s easy to handle i n the laboratory and f i e l d , and re a d i l y acclimates to experimental conditions. Because the f i s h used i n t h i s study were taken from a population recently introduced into Marion Lake, B.C., i t should be possible to make some int e r e s t i n g predictions (based on mouth size consi-derations) of the d i r e c t e f f e c t s of the introduction on the lake's benthic community and the i n d i r e c t e f f e c t s on the p o t e n t i a l l y competing species rainbow trout and kokanee. The basic objective of t h i s thesis i s to contribute to our understanding of the prey size selection process i n fishes. However, t h i s rather broad objective was approached through the following more s p e c i f i c thesis objectives: 8 To demonstrate the existence of, and describe the nature of, s i z e - s e l e c t i v e predation by the threespine stickleback. To examine the upper food size l i m i t i n an attempt to test the hypothesis that a trophic dimension, mouth size, l i m i t s upper food size. To examine the lower food size l i m i t i n an attempt to demonstrate that energetic and v i s u a l considerations are of major importance. 9 GENERAL METHODS FIELD Marion Lake. To q u a l i t a t i v e l y and quantitatively describe the nature of s i z e - s e l e c t i v e predation i n sticklebacks? an enclosure study was carried out i n Marion Lake. This shallow oligotrophic lake i s located 50 km east of Vancouver? B.C. The sticklebacks were from Chemainus Lake on Vancouver Island? and were a part of an introduction into Marion Lake. Enclosures were constructed of metal screening and contained c i r c u l a r areas of 0.1 square meters. Ten enclosures? one f i s h i n each? were placed i n the lake on four d i f f e r e n t occasions from early July to early August, 1974. The f i s h ranged i n size from 20-55 mm. Enclosures were placed at 40-60 cm depths over open mud substrate and spaced 5-10 meters apart. A l l size classes (Table 1) of f i s h were represented i n each set of ten. The f i s h were not fed for at lea s t 36 hours before being put into the lake. Lake l e v e l s did not vary by more than +20 cm between experimental periods. Surface temperatures varied from 16°C - 22°C during the period of the study. The f i s h were removed 10 to 20 hours a f t e r placement? and benthic samples (one per enclosure) were taken with a 225 cm^ - Hargrave sampler. The samples were then sorted for the amphipod species H v a l l e l a azteca and Crangonvx richmondensis. 10 Together with chironomid larvae, these formed the bulk of benthic prey items. For various p r a c t i c a l reasons, chironomid a v a i l a b i l i t y * was not examined, although i t was considered i n a separate enclosure study. Soon a f t e r completion of the enclosure study, f i v e control benthic samples were taken near the enclosures and each was sorted three times to obtain an estimate of s i z e -selective sorting bias. U.B.C. Endowment Lands. To provide comparative f i e l d data for the As e l l u s laboratory experiment (discussed i n the following section) and supporting data for the Marion Lake study, an additional enclosure study was carried out i n a creek i n the U.B.C. Endowment Lands. Thirteen Chemainus Lake sticklebacks, starved for 48 hours, were placed i n a pool 1 x 2 m i n surface dimensions and ten cm i n depth. The substrate was mud and l e a f l i t t e r and the water temperature was 14°C. The pool was sectioned o f f from the rest of the creek by wire fencing. Eleven of the thirteen f i s h placed i n the pool were s t i l l i n the same .33 m^  subsection 24 hours l a t e r . A l l the f i s h were removed and the top two cm of t h i s subsection was scooped up (with a trowel) into a large p l a s t i c bag. This material served as a benthic sample. Two three hour sorts were made to estimate *The term a v a i l a b i l i t y , as used i n t h i s study, refers to the r e l a t i v e number of in d i v i d u a l s present i n a category. 11 size class sorting e f f i c i e n c y . Animal Measurements. A l l animals (stomach contents and benthos) were preserved i n 70% alcohol. For whole amphipods, c u r l length (Larson, 1972) and depth (greatest dorsal extremity to end of telson) were measured to the nearest .1 mm with c a l i p e r s . Isopod length was measured from the base of the f i r s t p a i r of antennae to the posterior margin of the telson. Crustaceans with severed heads were not included i n the analysis. Chironomid head capsule width and zooplankton length (anterior margin of head to posterior margin of the body exclusive of spines, t a i l s or other projections) were measured to the nearest .04 mm with an ocular micrometer under a compound microscope. Standard lengths and mouth widths (corner to corner of apparent o r a l aperture) were taken for each f i s h . LABORATORY To examine the upper food size l i m i t f o r s t i c k l e -backs and the importance of mouth size i n determining t h i s l i m i t , a series of feeding experiments were performed using the isopod Asellus o c c i d e n t a l i s (determined from a key provided by Williams, 1970) as prey. The experiments were carried out i n 22 l i t r e wooden aquaria (25 x 25 x 35 cm) with glass fronts. Neither gravel nor f i l t e r s were used and the bottom was covered with white p l a s t i c to f a c i l i t a t e recognition of small prey 12 (by both the f i s h and myself). P l a s t i c a i r hoses were used to keep the water oxygenated for the duration of the experiment. To examine the lower food size l i m i t , a feeding ex-periment was carried out using Artemia s a l i n a n a u p l i i as prey. The experiment was conducted i n 16 l i t r e glass aquaria (40 x 20 x 20 cm). Temperatures i n the laboratory were maintained at 18°C +2°C and the photoperiod was 13 hours. 13 RESULTS FIELD ENCLOSURE STUDIES Marion Lake. Of the 40 enclosure f i s h j 36 were successfully retrieved. Table I shows the r e s u l t s of the stomach analysis for three size classes of stickleback. The size l i m i t s were chosen so that equal numbers of f i s h occurred i n each class. Three standard importance parameters are used: percent occur-rence} percent number and percent by volume (Hynes? 1950). The use of volume alone does not adequately take into account time as an important resource so that the mean of the three indices i s given as an o v e r a l l estimate of importance. The large numbers of Sida c r y s t a l l i n a eaten* r e l a t i v e to other prey» i s probably an a r t i f a c t of the experiment. Sida tend to c o l l e c t on the sides of enclosures. This observation i s confirmed by data from sticklebacks caught i n minnow traps i n Marion Lake during the same period (Kim Hyatt> unpublished data). The column headed "other Entomostracans" i s probably a better i n d i c a t o r of the importance of zooplankton i n the d i e t . The data indicate that the smallest size class f i s h (19-26 mm) eat primarily zooplankton and small dipteran larvae (chironomids). Fish smaller than 19 mm were not retained by the enclosures} however ten f i s h 6-12 mm i n length were dipnetted from Marion Lake. The largest of these (9-12 mm) contained 14a ' Table I. Prey consumed i n Marion Lake enclosures. %0 = 7o occurrence, %N = % number, yoV = %volume. Other entomostracans include ostracods, the copepod Cyclops  bicuspidatus and the cladocerans Polyphemus pediculus and Bosmina l o n g i r o s t r i s . The volume weightings are approxima-tions based on mean dimensions of prey eaten. I i s an o v e r a l l estimate of importance: I = (%0 + %N + %V)/3 14b FISH SIZE -(mm.) NO. OTHER ENTOMO-STRACANS SIDA DIPTERAN LARVAE AMPH'IPODS DIPTERAN PUPAE TRICHOPTERAN LARVAE 10 %N %V I to tn % v i to w ty i to tn tM i to tn %v i _ 19-26 • -12' 58 6b 12 4b 67 19-16 34 67 12 60 43 17 3 6 9 17 1 16 11 30-41 •• 12 42 3 0 15 99 84 31 72 67 5 9 27 92 6 46 '48 , 33 2 14.15 42-55. 12 17 1 0 6 92 82 9 61 42 2 1 15 99 11 83 65 67 4 9 27 VOLUME WEIGHT (mm3) ING .02 .1 .5 0.25 s* 2.00 m 7.00 1 2 * s= small f i s h , m=medium f i s h , l=large fish o s t r a c o d s j Bosmina l o n g i r o s t r i s P o l y p h e m u s p e d l c u l u s and small Sida c r y s t a l l i n a , and the s m a l l e s t ones (6-9 mm) contained o n l y the remains o f r o t i f e r s . The l a r g e r s i z e c l a s s e s o f f i s h f e d p r o p o r t i o n a t e l y l e s s on zooplankton ( d i s c o u n t i n g Sida) and d i p t e r a n l a r v a e and fed more h e a v i l y on l a r g e r b e n t h i c i n v e r -tebrates} e s p e c i a l l y amphipods. I n general? s i n c e l a r g e prey items tend to be b e n t h i c r a t h e r than l i m n e t i c ? the d i f f e r e n c e s i n f o r a g i n g n i c h e s between l a r g e and small s t i c k l e b a c k s can be viewed from the p o i n t o f s i z e s e l e c t i o n . Another mode o f s e l e c t i v i t y occurred w i t h prey s p e c i e s such as H y a l l e l a a z t e c a . * The maximum H y a l l e l a l e n g t h f i s h l e n g t h c o r r e l a t i o n ( F i g . 2d) i s based only on f i s h l e s s than 45 mm i n ]Jeng^ 'h>. T h i s i s because prey l a r g e r than 4.5 mm are very r a r e , and t h e r e f o r e the l a r g e s t f i s h have no o p p o r t u n i t y to eat l a r g e r prey. As a r e s u l t the prey s i z e curve appears to l e v e l o f f . The l a r g e r amphipod Crangonyx richmondensis appears to show the same p a t t e r n as H. az t e c a , however, due to h i g h v a r i a n c e and small sample s i z e the c o r r e l a t i o n i s 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 ( F i g . 2c). Data from the H. a z t e c a a v a i l a b i l i t y a n a l y s i s ( F i g . 4) i n d i c a t e s more p r e c i s e l y the nature o f the s i z e s e l e c t i v i t y . Large f i s h consume p r o p o r t i o n a t e l y more l a r g e H y a l l e l a than *The H y a l l e l a e f f i c i e n c y s o r t ( F i g . 3) d i d not i n d i c a t e any s o r t i n g b i a s and t h e r e f o r e the da t a was analyzed d i r e c t l y . E f f i c i e n c y was c a l c u l a t e d as: NO. ANIMALS IN 1ST SORT/NO. FROM ALL SORTS. 16 were available, while the reverse i s true for the smaller size classes of f i s h . The d i s t r i b u t i o n s are analysed i n Tables II and III by the Kolmogorov-Smirnoff test of goodness of f i t . In addition, a percentage overlap index i s used to measure the area that the d i s t r i b u t i o n s share i n common. This index i s equal to 1 minus one h a l f the sum of the absolute differences between the proportions eaten and the proportions present for each size c l a s s . The complement of t h i s index with 1 i s an o v e r a l l measure of the degree of size s e l e c t i v i t y occurring or " t o t a l e l e c t i v i t y " (see Appendix). A l l pairs of d i s t r i b u t i o n s are s i g n i f i c a n t l y d i f f e r e n t except for medium size f i s h versus a v a i l a b i l i t y . Overlap indices p a r a l l e l these r e s u l t s . Crangonyx could not be analysed i n a similar way since r e l a t i v e l y few were eaten. The two smaller size classes of f i s h ate four immature Crangonyx (2-4 mm) while the largest f i s h ate a t o t a l of eight immature and two mature (6 mm) prey. The benthic samples contained 79% immature and 21% mature individ u a l s (N = 120). Apparently size s e l e c t i v i t y i s les s intense f o r the larger f i s h , perhaps because of the higher minimum size of Crangonyx (2 mm versus 1 mm for H y a l l e l a ) . I f mouth size i s l i m i t i n g , a l i n e a r r e l a t i o n s h i p between maximum prey size and f i s h size i s expected (assuming that mouth width increases i n d i r e c t proportion to body length). Eor t h i s population of G. aculeatus t h i s seems to be the case 17a Fig . 2. Sizes of organisms consumed i n Marion Lake enclosures versus stickleback standard length. (a) chironomid larvae (head width) versus f i s h length. r ( a l l data) =-.141, p>.05 (b) Sida c r y s t a l l i n a length versus f i s h length. r(max. values only) = .355, p>.05 (c) Crangonvx richmondensis length versus f i s h length. r ( a l l data) = .339, p >.05 r(max. values only) = .657, p>.05 (d) H v a l l e l a azteca length versus f i s h length. r ( a l l data) = .562, p <.01 r(max. values only) = .720, p<.01 Points i n (a) and (c) represent i n d i v i d u a l prey. Points i n (b) and (d) represent means and v e r t i c a l l i n e s are ranges. Maximum prey size regression based on points to 45 mm f i s h length i s shown i n (d). 17b I i i-Q ft" O , a-i o I o I o 30 4 0 FISH LENGTH (mm) 50 (a) OS 0.4 25 35 t$-FISH LENGTH (mm) (b) L 1 I L _ 0 40 FISH LENGTH (mmj E - 3 /4 1 i 1 FISH LENGTH (mm.) (d) 18a F i g . 3. E f f i c i e n c y o f s o r t i n g v a r i o u s s i z e c l a s s e s of H. a z t e c a from b e n t h i c samples. 18b 19a Fi g . 4. Size d i s t r i b u t i o n s of H. azteca i n stomachs of Marion Lake enclosure f i s h and i n benthos. Size classes of the amphipod are based on length (mm): 1 = 1.5; 2 = 1.5-1.9; 3 = 2-2.4; 4 = 2.5-2.9; 5 = 3-3.4; 6 = 3.5-3.9; 7 = 4-4.4; 8 = 4.5-4.9; 9 = 5-5.4 (a) Large f i s h 42-55 mm (12). Number of prey = 62 (b) Medium size f i s h 30-41 mm (12). Number of prey = 33 (c) Small f i s h 19-26 mm (12). Number of prey = 5 N Benthos = 391 19b 2 5 -i (a) 50H 2 5 H >-u z LU 3 1 0 0 " o u Dd u. 75-f I L. L _ _ J ,(b) D I E T IN B E N T H O S 50H 2 5 -(c) 3 4 5 6 7 8 9 H Y A L L E L A SIZE C L A S S 20 Table I I . Comparison of H y a l l e l a size d i s t r i b u t i o n i n f i s h stomachs and benthos. % Overlap Significance (Kolmogorov-Smirnoff) Large f i s h versus a v a i l a b i l i t y 53 p< .01* Medium f i s h versus a v a i l a b i l i t y 67 P>.20 Small f i s h versus a v a i l a b i l i t y 33 p < . 01* Table I I I . Comparison of H y a l l e l a size d i s t r i b u t i o n i n stomachs of three size classes of stickleback. % Overlap Significance (Kolmogorov-Smirnoff * Large versus medium f i s h 47 p <.01* Medium f i s h versus small 30 p < .05* Large versus small f i s h 8 p <.01* Table IV. Comparison of of three size classes of Sida size d i s t r i b u t i o n i n stomachs stickleback. 7a Overlap Significance (Kolmogorov-Smirnoff) Large versus medium f i s h 82 P > .20 yiedium versus small f i s h 82 p < . 05* Large versus small f i s h 74 p < . 05* 21 (Fig. 5). A test of curvature (second degree polynominal f i t ) was not s i g n i f i c a n t at the .05 l e v e l , and the same test indicated no s i g n i f i c a n t curvature (p> .05) for f i s h length versus maximum Hy a l l e l a length (for f i s h up to 45 mm i n length). This organism grows s u f f i c i e n t l y large to be unavailable to smaller size classes of sticklebacks, as attempts to feed adult H y a l l e l a to small sticklebacks (20 mm) i n the laboratory confirmed. Chironomid larvae and Sida, on the other hand, did not pose problems for the size range of f i s h tested. This i s indicated by the lack of positive c o r r e l a t i o n between maximum prey size and f i s h size (Fig. 2a and 2b). Feeding of another cladoceran Daphnia magna to sticklebacks i n the laboratory however, indicated that f i s h l e s s than 25 mm had d i f f i c u l t y i n consuming Daphnia greater than 2 mm i n length. This was expressed as a lower mean size of Daphnia eaten (Fig. 6). The p o s s i b i l i t y that large cladocerans are les s l i k e l y to be taken by small f i s h i s confirmed by a Komolgorov-Smirnoff test on the Sida prey size d i s t r i b u t i o n s for the three f i s h size classes. Small f i s h (19-26 mm) ate s i g n i f i c a n t l y smaller Sida than the other size classes. The o v e r a l l c o r r e l a t i o n test was apparently too coarse to resolve these differences. However, although Sida were s e l e c t i v e l y preyed upon, t h i s s e l e c t i v i t y was of a fa r lower degree than for H y a l l e l a (higher percent overlaps for Sida) (Table IV). 22a Fig. 5. Mouth width of Chemainus Lake sticklebacks (#) and Lake Errock sticklebacks (0) plotted against standard f i s h length. Chemainus r = .956 p< .01; Errock r = .936 p <.01 M O U T H W I D T H ( m m ) IV) 4^ 0) _ j — ! [- 1 O 00 m z o —I CO O 3 ^ o l o o o o o O Oo « o o • • o • o • ° tt • : • Vt to to cr. 2.3a Fig. 6. Mean length of Daphnia magna eaten by d i f f e r e n t sizes of stickleback during a 15 minute period. 23b E E 1.9 z LU _ l 1.7 < z Q_ < 1.5| 2 5 4 5 6 5 F I S H L E N G T H (mm.) :j 24 Endowment Lands. Chironomid larvae and A s e l l u s o c c i d e n t a l i s formed the greater part of the d i e t of f i s h i n the enclosure (at l e a s t 90% by volume). Small f i s h ( < 30 mm) tended to concentrate on chironomids compared to larger f i s h . These depended on Asellus to a greater extent (Table V). Table V. Prey consumed i n "Endowment Land enclosure. %N=% number, %V=% volume, s=small f i s h , m=medium f i s h , l=large f i s h Fish size # Copepods Asellus Chironomid Other (mm) Larvae Invertebrates %N %V %N %V %N %V %N 16-27 5 43 3 12 14 41 83 3 31-44 4 10 0 70 97 20 3 0 48-58 4 1 0 56 86 38 14 5 Volume weighting .02 3 4 .5 s .0 m .0 1 1.0 not included Zooplankton (cyclopoid copepods) were of importance only to the smallest f i s h . Figure 7a shows the Asellus size preferences for three size classes of stickleback (class l i m i t s were chosen so that numbers i n each class were about the same). The extent of size s e l e c t i v i t y i s analysed r e l a t i v e to a v a i l a b i l i t y , 25 and among size classes of stickleback, i n Tables VI and VII. Overlap indices are given and chironomid larvae are included for comparison (Fig. 7b). The re s u l t s indicate that Asellus undergoes greater size s e l e c t i o n by sticklebacks than do chironomid larvae. I f mouth width i s l i m i t i n g , one would expect a l i n e a r relationship between maximum prey size and f i s h size. Figure 8b shows that such a relat i o n s h i p exists f ° r Asellus. However chironomid larvae display no such pattern (Fig. 8d). A l l sizes of chironomid larvae were rea d i l y consumed by a l l sizes of f i s h tested.* Sorting e f f i c i e n c y was high for larger Asellus (75%) and r e l a t i v e l y low for small i n d i v i d u a l s (50%) (Fig. 10). A sim i l a r s i t u a t i o n existed f o r chironomid larvae. The e f f e c t of these biases would be to skew a v a i l a b i l i t y d i s t r i b u t i o n s to the ri g h t , however the appropriate corrections' factors have been applied to the data**. * In the chironomid analysis, sizes are given as head capsule width which correlates well with t o t a l body length up to 10 mm, even though many species are involved (Fig. 9). ** For Asellus the mean sorting e f f i c i e n c y for the f i r s t two and l a s t s i x s i z e ^ classes were applied to the corresponding data. The f i r s t size class e f f i c i e n c y and the mean of the l a s t four were used for the chironomids. 26a Fig . 7. Size d i s t r i b u t i o n s of Asellus o c c i d e n t a l i s and chironomid larvae i n stomachs of d i f f e r e n t size classes of sticklebacks and i n benthic sample from Endowment Lands enclosure. (a) Asellus. Size classes: 1 =.£1.9; 2 = 2-2.4; 3 =2.5-2.9; 4 = 3-3.4; 5.= 3.5-3.9; 6 = 4-4.4; 7 = 4.5-4.9; 8 = ^ 5 (length i n mm) ( i ) 4 large f i s h 48-58 mm, N prey = 46; ( i i ) 4 medium f i s h 31-44 mm, N prey = 21; ( i i i ) 5 small f i s h 16-27 mm, N prey =10 N benthos = 120 (b) Chironomid larvae. Size classes: 1 = .08-.16; 2 = .2-.28; 3. = .32-.4; 4 = .44-.52; 5 =>.52 (head capsule width i n mm) ( i ) 4 large f i s h 48-58 mm, N prey = 31; ( i i ) 5 small f i s h 16-27 mm, N prey.= 34 N Benthos = 152 FREQUENCY Q) cr o o -> o D ro to co m n > to to o _L_ F R E Q U E N C Y ^ to 5 2. ( % ) o o J L_ q9Z 27 Table VI. Comparison of prey size d i s t r i b u t i o n s i n f i s h stomachs and i n benthos (Kolmogorov-Smirnoff t e s t ) . % Overlap Significance Large f i s h 71 p<.05* Asell u s Medium f i s h 72 • P> -20 Small f i s h 55 p < .05* Chironomid Large f i s h 82 p > .20 larvae Small f i s h 87 P >-20 Table VII. Comparison of prey size d i s t r i b u t i o n s between f i s h size classes (Kolmogorov-Smirnoff t e s t ) . % Overlap Significance Large versus medium f i s h 74 P > .20 Asell u s Medium versus small f i s h 50 p >.05 Large versus small f i s h 29 p < . 01* Chironomid Large versus small f i s h 78 P >.20 larvae ^Medium size f i s h not included due to small prey sample size. 28a Fig. 8. Sizes of chironomid larvae and Asellus occidentalis consumed by sticklebacks of d i f f e r e n t lengths i n Endowment Lands enclosure. (a) Chironomid larvae. r ( a l l data) = .111, p>.05 A l l data points are shown. (b) Asellus. r ( a l l data) = .491, p<.01 r(max. values) = .889, p<.01 Points represent means. V e r t i c a l l i n e s represent ranges. Maximum prey size regression i s shown i n (b). 9 O x £ £ x < LU X Q O o: x u • i r »> I • t J 1 • • i 10 30 FISH L E N G T H (mm) 50 ( a ) £ E 2 U l _ l LO 3 LU LO < / • A t 10 30 50 FISH LENGTH (mm.) ( b ) oo cr 29a Fig. 9. Chironomid head width versus t o t a l body length. Regression l i n e i s based on points up to 10 mm body length, r = .849, p <.01 30a Fig . 10. E f f i c i e n c y of benthic sort from Endowment Lands. (a) Asellus occidentalis (b) Chironomid larvae Size classes as i n Fig. 8. 30b 1 2 3 4 5 chironomid size class 31 CONCLUSIONS FROM ENCLOSURE STUDIES: 1) Different size sticklebacks tend to concentrate t h e i r feeding e f f o r t s on d i f f e r e n t taxonomic groups of prey and these i n turn represent d i f f e r e n t foraging habitats (planktonic versus benthic). These prey groups have d i f -ferent size d i s t r i b u t i o n s . 2) D i f f e r e n t size sticklebacks exploiting the same species of prey tend to do so i n a si z e — s e l e c t i v e manner (r e l a t i v e to one another and r e l a t i v e to what i s available} 3) The degree of size s e l e c t i v i t y varies depending on the prey type. Prey species with dimensions that create handling d i f f i c u l t i e s for smaller f i s h (Asellus? Hvallela? Crangpnvx)? are used more s e l e c t i v e l y . Most indivi d u a l s of small species (Sida? chironomid larvae) can be eaten by a l l sizes of f i s h tested? so that maximum and mean prey sizes taken tend to be more equal. In t h i s case, s e l e c t i v i t y probably functions through factors a f f e c t i n g the lower size l i m i t , when that l i m i t e x i s t s . For the sizes of f i s h tested, t h i s l i m i t was apparently not reached. 32 LABORATORY "EXPERIMENTS ASELLUS FEEDING EXPERIMENT: THE UPPER LIMIT TO S I Z E -SELECTIVE PREDATION INTRODUCTION To examine the r o l e mouth s i z e p l a y s i n d e t e r m i n i n g the upper l i m i t to s i z e - s e l e c t i v e p r e d a t i o n , a s e r i e s of l a b o r a t o r y experiments were c a r r i e d out u s i n g A s e l l u s  o c c i d e n t a l i s as prey. The hypotheses t e s t e d are: 1) A. A r e l a t i o n s h i p e x i s t s between p r e d a t o r s i z e and maximum prey s i z e , and i f p h y s i c a l c o n s i d e r a t i o n s are o f major importance ( i . e . mouth dimensions) t h i s r e l a t i o n s h i p w i l l be l i n e a r . B. A p a r t i c u l a r prey dimension and a p a r t i c u l a r p r e d ator mouth dimension (width) are important. 2) A. I f h a n d l i n g time (HT = time from f i r s t a t t a c k to swallowing of prey) i s p l o t t e d a g a i n s t prey s i z e , there should be a sharp r i s e i n t h i s curve as maximum prey s i z e i s approached. Such an abrupt r i s e i n HT would suggest t h a t under f i e l d c o n d i t i o n s prey near a f i s h ' s maximum would not be taken. E n e r g e t i c reasons may be i n v o l v e d . ( i . e . l a r g e prey are o f t e n dropped and l o s t ; t h i s wastes time and energy). B. I f g i v i n g up time (GUT = time from f i r s t a t t a c k to c e s s a t i o n o f a t t a c k i n g f o r at l e a s t 15 seconds) i s p l o t t e d 33 against prey size an abrupt decline i n the curve i s expected for prey sizes just above maximum. Again, i t i s maladaptive for a predator to waste time on prey that cannot be consumed ( i . e . predators should be able to recognize prey that are too large). 3) Small f i s h , feeding on prey that are proportionally as large, should give up more e a s i l y than large f i s h because of differences i n foraging experience. Because of t h e i r plank-tivorous foraging habits, smaller f i s h (20-30*mm) rarely encounter prey large enough to require long handling times. Lacking such experience, small f i s h should give up more e a s i l y on r e l a t i v e l y large prey. METHODS Twenty-three Chemainus Lake sticklebacks, 15-48 mm i n length and ten Lake Errock sticklebacks, 19-43 mm i n length, were fed Asellus occidentalis over a period of ten days. A l l f i s h were fed Asellus under experimental conditions f o r three days before the experiment started. Half of the Chemainus f i s h were fed prey k i l l e d by immersion i n hot water (4:5°C) while the other h a l f were fed l i v e prey. This was to determine whether there i s a size related protective e f f e c t of the cu r l i n g response of Asellus.. This experiment f a i l e d because other e f f e c t s (such as generalized prey struggling) confounded the 34 e f f e c t s of cu r l i n g . The maximum prey size regressions for l i v e and dead prey were not s i g n i f i c a n t l y d i f f e r e n t ( t - t e s t , p >.05) so the results were combined. F i g . 11 shows Asellus i n the normal standing and curled positions. The Lake Errock f i s h were used to check the importance of mouth width. This population tends to have narrower mouths than Chemainus f i s h of similar length (Fig. 5). The isopod's length (end of telson to base of f i r s t pair of antennae) and maximum width ( f i f t h thoracic segment) were recorded to the nearest 0.1 mm with c a l i p e r s . In each t r i a l prey were deposited i n the tank i n the v i c i n i t y of the f i s h . Based on data from Larson (1972) I assumed hunger l e v e l s to be maximum for a l l f i s h a f t e r 24 hours (the i n t e r v a l between t r i a l s ) . I only used data f o r the f i r s t prey offered each day since i t was clear that an i n i t i a l f a i l u r e affected the subsequent performance of the f i s h . RESULTS Attack Behaviour. Normally, the prey were attacked while f a l l i n g through the water column. However, once on the bottom, moving prey e l i c i t e d quicker responses than stationary prey. For larger prey, only end-on attacks were successful. Broadside attacks were successful only on r e l a t i v e l y small prey. The observed attack sequence was as follows: F i g . 11. A s e l l u s o c c i d e n t a l i s i n normal standing (a) and c u r l e d (b) p o s i t i o n . 36; 1) Prey recognition (manifested as a d i r e c t o r i e n t a t i o n toward the prey). 2) Approach and grasp i n one swift motion. The seizures were from a variety of di r e c t i o n s (end-on to broadside). I t was d i f f i c u l t to get r e l i a b l e measures since the attacks varied greatly i n duration and i n t e n s i t y , i n addition to angle. 3) Swallow and spit out (from 0 to 5 times). The a b i l i t y to swallow (remove prey from my sight) normally indicated success. However, occasionally a f t e r being spit out, the prey was not reswallowed, and sometimes a prey was regurgitated a f t e r a f u l l minute. Predator Size versus Prey Size. A plot of maximum prey length (mean of largest size eaten and next largest size offered but not eaten) against f i s h length (Chemainus) y i e l d s a good l i n e a r c o r r e l a t i o n (Fig. 12a). A curvature test showed no s i g n i f i c a n t departure from l i n e a r i t y (p>.05). A plot of mouth width against prey width also y i e l d s a good c o r r e l a t i o n ; however, i n t h i s case the slope i s near 1 so that mouth width equals prey width (Errock slope = .94 + .27 (1 S.E.);:Chemainus slope = 1.08 + .12 (1 S.E.) Fig. 12b)*. This r e l a t i o n s h i p implicates mouth width and prey width as the c r i t i c a l l y l i m i t i n g dimensions, and the l i n e a r i t y supports the hypothesis that physical l i m i t a t i o n s are '*Five large Chemainus f i s h were not included because they had had inadequate opportunity to demonstrate t h e i r max. prey size due to lack of large prey. F i g . 12. Maximum s i z e o f A s e l l u s eaten i n l a b o r a t o r y versus f i s h standard l e n g t h f o r two p o p u l a t i o n s o f s t i c k l e b a c k s w i t h d i f f e r e n t mouth widths. Chemainus f i s h (•) Errock f i s h (0) _ _ , _ _ _ „ ^ _ (a) Maximum prey l e n g t h . R e g r e s s i o n s l o p e s are d i f f e r e n t ( t - t e s t , p<.02). H o r i z o n t a l bars r e p r e s e n t next s i z e o f f e r e d but not eaten. (b) Maximum prey width. R e g r e s s i o n s l o p e s not d i f f e r e n t (p > .2). AX PREY LENGTH (mm; M A X P R E Y WIDTH (mm) 38. of major importance i n determining maximum prey size. The regression l i n e s using prey length are s i g n i f i -cantly d i f f e r e n t for Errock and Chemainus f i s h (t = 2.7, p <.02, Fig . 12a). This i l l u s t r a t e s the eff e c t s of difference i n mouth width between the two populations. When the l i n e s are based on width dimensions there i s no s i g n i f i c a n t difference (t = .77, p>.5, Fig. 12b), i l l u s t r a t i n g the importance of t h i s dimension. Handling Time and Giving up Time. Handling time (HT) and giving up time (GUT) data are based on ten Chemainus f i s h fed l i v e prey and seven f i s h fed dead prey. Of the remaining six f i s h , two were i l l toward the end of the t r i a l s , therefore only maximum prey size data were used (based on e a r l i e r t r i a l s ) . The four remaining f i s h required very large prey items, supplies of which were exhausted soon a f t e r the t r i a l s started. The HT and GUT curves have the predicted shapes. The HT curve (Fig. 13b) shows a steep r i s e occurring at approxi-mately 80% of the maximum prey size, while the GUT curve (Eig. 13a) shows a steep i n i t i a l drop that l e v e l s o f f near 1.2 times the maximum prey size. Werner (1974) found similar HT curves for two species of sunfish (Lepomis) feeding on a r t i f i c i a l (beef kidney) and natural prey (Daphnia magna and sunfish f r y ) . The significance of the abrupt r i s e i n HT w i l l be discussed shortly. F i g . 13. (a) GUT ( g i v e up time) and (b) HT ( h a n d l i n g time) versus f r a c t i o n o f maximum A s e l l u s (prey) l e n g t h consumed by Chemainus s t i c k l e b a c k s i n the l a b o r a t o r y . L i v e prey ( # ) Dead prey (o) 18 14 S 10r E • * « 1.1 .9 1 FRACTION MAX PREY SIZE 13 —r —i 14 15 ( a ) 8 0 0 a •5 «6 .7 .8 .9 1 FRACTION. MAX PREY LENGTH ( b ) VO cr •40 • The early decline i n the GUT curve i s consistent with the hypothesis that sticklebacks can quickly perceive prey that are too large to be handled (larger than 1.2 times the maximum i n t h i s case). The high v a r i a b i l i t y i n both curves can be partly attributed to prey measurement error. Large versus Small Fish. Small f i s h (less than 28 mm) gave up sooner (p <. 005) and more often (p <.025) than large f i s h when feeding on large prey .9-1.0 times the r e l a t i v e maximum (Tables VIII and IX). This supports the behavioural differences i n i t i a l l y proposed, that i s , small f i s h by virt u e of t h e i r l a r g e l y planktivorous experience, are predisposed to deal with prey requiring r e l a -t i v e l y short handling times. Prey Width and Predator Mouth Width. Since the data displayed i n Fig. 12b implicated prey width and stickleback mouth width as p a r t i c u l a r l y relevant dimensions, i t seemed worthwhile pursuing t h i s topic. In the l a s t three t r i a l s of the Asellus feeding experiment, six Lake Errock and six Chemainus Lake f i s h were offered prey cut i n h a l f between the fourth and f i f t h thoracic segments. Only the posterior h a l f representing approximately 55% of the t o t a l body length, was used. The prey were chosen so that the width was approximately 10% greater than the mean 41 Table VIII. Number of fast and slow giving up responses by large and small sticklebacks feeding on Asellus. Small Fish « 2 8 mm) Large (>28 Fish mm) Obs. Obs. Exp. Number of giving up times les s than 2 min 30 4 9.5 X2=11.9 (p<.005) Number of giving up times greater than 2 min 11 9 3.5 Table IX. Number of give up responses for small and large sticklebacks feeding on Asellus .9-1.0 times maximum length. Small Fish (< 28 mm) Large (> 28 Fish mm) Obs. Obs. Exp. Number of give up responses . 15 2 6.4 Xz=5.6 (p<.025) Number handled 18 12 7.6 42 of the experimental maximum and the next largest size offered but not eaten. Each f i s h was tested from one to three times for a t o t a l of 24 t r i a l s . The res u l t s are given i n Table X. Based on length alone, a l l 24 prey should have been e a s i l y consumed, however only six were actually eaten and the handling times for these six were abnormally high. However, t h i s i s not r e a l l y a f a i r comparison, since a prey 107o broader should also be 10% thicker. The r e l a t i v e volume of the prey i s therefore 1.1 x 1.1 x .55 = .666 of the predator's maximum. This volume can be redistributed to create a whole hypothetical prey having dimensions, r e l a t i v e to the maximum of: .666 = .87 length x .87 width x .87 thickness Therefore an entire prey of volume equal to those offered, would have a length of .87 of the r e l a t i v e maximum. No predator ever gave up on t h i s length prey (Fig. 13a) so that the expected number of successes i s 12 even i f volume (or biomass) instead of length were the c r i t e r i o n . The shape of Asellus i s therefore of great importance i n determining i t s v u l n e r a b i l i t y to s t i c k l e -backs. 43 Table X. Performance of sticklebacks on Asellus cut i n ha l f and 1.1 times as wide as the experimental maximum. # T r i a l s # Successes HT for Successes Obs. Exp. (min) Chemainus f i s h 12 3 12 .3, 6.4, 15 Errock f i s h 12 3 12 .3, 3.7, 6.3 Handling Time, Prey Size and Predator Preference. I f the maximum width of Asellus from the enclosure study i s plotted against predator mouth width, a l l points f a l l well below those obtained i n the laboratory (Fig. 15a). Actually most of the points f a l l below a l i n e of slope =.8. If one observes the HT data for Chemainus f i s h (Fig. 13a) and Errock f i s h (Fig. 14), i t i s apparent that handling times generally do not exceed 30 seconds u n t i l a value of .8 times the maximum prey size i s reached. With few exceptions "high" handling times do not occur u n t i l beyond t h i s point. From the previous considerations i t i s l i k e l y that maximum prey size obtained i n the f i e l d i s often well below the physical c a p a b i l i t i e s of the predator as determined i n the laboratory. High handling times are a l i k e l y reason. In the laboratory, i f a f i s h had handling problems the prey was often dropped, however, since there was nowhere 4 a F i g . 14. HT versus f r a c t i o n maximum A s e l l u s l e n g t h taken Errock f i s h i n l a b o r a t o r y . 44b A A i 1 A i A, A* ^ 4 * 4 * 1 A A J ^*AAA I t •5 .6 .7 .8 .9 FRACTION M A X PREY LENGTH 45a F i g . 15. (a) Maximum width o f A s e l l u s taken by Chemainus f i s h i n l a b o r a t o r y (») and i n creek i n Endowment Lands(o) versus mouth width. (b) Maximum width o f H v a l l e l a (•) and Crangonvx (o) taken i n M a r i o n Lake e n c l o s u r e s versus mouth width. cr .46' to f l e e i n the tank the prey could not escape? even though i t was s t i l l quite capable of escape manoeuvres. Fi g . 16 indicates that i n the laboratory experiment there was a good chance (>50%) of success for f i s h feeding on prey .95-.99 of the maximum. I f t h i s figure i s v a l i d i n the f i e l d , then the low maximum prey sizes cannot be explained on energetic grounds, since i t would be worthwhile for t h e i f i s h to spend a few extra minutes handling a large prey item. However, i n nature i f a prey i s dropped on a heterogeneous l e a f l i t t e r substrate (the habitat of A s e l l u s ) , i t i s l i k e l y to escape and i f the escape rates of large prey are higher i n the f i e l d than i n the laboratory, then the f i e l d observation can be accounted for. The Asellus f i e l d data was partly biased because the larger f i s h apparently did not havef?many large prey available. To strengthen the evidence, I analysed the data for H y a l l e l a azteca and Crangonyx richmondensis from the Marion Lake enclosure study i n the same manner as I did for Asellus (Fig. 15b). Although I had no data on handling times for these two species, I believe that a plot of the data r e l a t i v e to a l i n e of slope .8 and passing through the o r i g i n , would s t i l l be a reasonable q u a l i t a t i v e test. The re s u l t s support the hypothesis that the maximum size of animals taken i n the f i e l d tends to be substantially less than the physical l i m i t . 47a F i g . 16. P r o p o r t i o n of times Chemainus s t i c k l e b a c k s gave up on A s e l l u s prey r e l a t i v e to t o t a l number o f o p p o r t u n i t i e s (expressed as a p r o b a b i l i t y ( P r ) ) versus s i z e c l a s s o f prey ( f r a c t i o n o f maximum prey length). 47b .8-34 .85-39 .9-94 .9c>.99 >1 A S E L L U S S I Z E C L A S S 48 CONCLUSIONS - ASELLUS FEEDING EXPERIMENT: 1) Mouth width i s important i n determining the maximum size Asellus a stickleback can eat. Asellus width i s a p a r t i c u l a r l y relevant prey dimension. 2) Handling times r i s e abruptly at prey sizes near 80% of the maximum prey size for given f i s h . Beyond t h i s point there i s a d i s t i n c t p r o b a b i l i t y of giving up. In the f i e l d s t i c k l e -backs ra r e l y take prey near t h i s physical upper l i m i t . This may be due to high handling times which i n turn r e s u l t i n a high p r o b a b i l i t y of the prey escaping. Giving up time decreases rapidly as prey exceed the maximum prey size. Such behaviour should be advantageous as time i s not wasted on prey too large to be consumed. 3) Small f i s h feeding on proportionally large prey give up sooner, and more often, than large f i s h . This behaviour may r e f l e c t differences i n experience. 4 9 ARTEMIA SALINA NAUPLII FEEDING EXPERIMENT: FACTORS AFFECTING THE LOWER LIMIT TO SIZE-SELECTIVE PREDATION. INTRODUCTION In the p r e v i o u s chapter i t was demonstrated t h a t although the maximum food s i z e s t i c k l e b a c k s take i n the l a b o r a t o r y may be determined by mouth s i z e , under f i e l d c o n d i t i o n s prey l a r g e r than 80% o f the maximum s i z e are r a r e l y taken. T h i s d i f f e r e n c e i s p o s s i b l y due to h a n d l i n g d i f f i c u l t i e s and the p r o b a b i l i t y o f escape i n the f i e l d . To understand how the breadth o f the food s i z e niche dimension i s determined, i t i s a l s o necessary to look at f a c t o r s e f f e c t i n g the lower l i m i t to s i z e s e l e c t i v i t y . U s i n g b r i n e shrimp n a u p l i i (Artemia s a l i n a ) as prey, an experiment was conducted to determine the e f f e c t s of p r e d a t o r s i z e , prey s i z e and prey d e n s i t y on the f e e d i n g performance o f s t i c k l e b a c k s ( a t t a c k r a t e ) . These parameters appeared to be b a s i c i n the s i z e s e l e c t i o n process. A prey s i z e c h o i c e experiment was a l s o performed to check on the r e s u l t s o f the f i r s t experiment. The s p e c i f i c hypotheses t e s t e d are: 1) L a r g e r f i s h should d i s p l a y g r e a t e r h e s i t a n c y (a lower and more d i s c o n t i n u o u s a t t a c k r a t e r e l a t i v e t o small f i s h when f e e d i n g on small prey at low d e n s i t i e s . T h i s may be the r e s u l t of e n e r g e t i c l i m i t a t i o n s . I f the prey are s u f f i c i e n t l y small maximum f e e d i n g r a t e s would not s a t i s f y maintenance 50 requirements. I t would be imprudent for a predator to exert a great deal of e f f o r t under such circumstances and face c e r t a i n e x t i n c t i o n when other alternatives ( i n nature) normally e x i s t . 2) Large f i s h begin feeding on larger prey at lower densities. This may be the r e s u l t of energetic and/or v i s u a l considera-tions. 3) A l l f i s h prefer larger prey when given a choice. METHODS Five small sticklebacks, 20-27 mm i n length, and fi v e large ones, 43-49 mm i n length, were tested i n d i v i d u a l l y for six consecutive days on two sizes of Artemia s a l i n a n a u p l i i presented at three densities. The six combinations occurred i n random order. The experiment was then replicated to y i e l d twelve t r i a l s f or each f i s h . The n a u p l i i lengths were 912 + 86 jam and 480 + 13 yum (1 S.E.). Maximum body diameters were 184 + 23 /Jtn and 180 + 13 pm f o r large and small n a u p l i i respectively. Nauplii were grown i n aerated saltwater at 25°C + 2°C for 24 hours (small n a u p l i i ) and 72 hours (large n a u p l i i ) . The bulk of the eggs and egg cases were removed by allowing the mixtures to s e t t l e f o r f i v e minutes and removing n a u p l i i v i a suction through a p l a s t i c tube placed i n the growing chamber (1500 cc j a r ) . A l l f i s h were fed from the same mixture each 51 day. I t was therefore assumed that the remaining eggs did not bias the r e s u l t s . The three densities used were 4-6 nauplii / 1 , 14-95 nauplii/1, and 210-640 nauplii/1. The n a u p l i i were separated from the s a l t water by means of a 73 yum mesh net. They were then deposited i n the centre of the tank. Attack rates were recorded each minute for the ten minutes following the f i r s t attack. Each f i s h was allowed ten minutes to begin feeding otherwise i t was assigned an attack rate of zero. Following the t r i a l s each day, the f i s h were fed to s a t i a t i o n on Tubifex and the remaining n a u p l i i were removed from the tank. Larson (1972) had d i f f i c u l t y main-tain i n g adult sticklebacks on brine shrimp n a u p l i i . For t h i s reason Tubifex were used to supplement the d i e t when the f i s h were being conditioned to Artemia during the ten day period p r i o r to the experiment. For unknown reasons, several f i s h died during the course of the experiment. These f i s h were immediately replaced by f i s h of equivalent size and experience. As an additional test of prey size preference and as a possible way of i s o l a t i n g energetic and v i s u a l e f f e c t s , both prey sizes were offered simultaneously, i n a known proportion, to ten large and four small f i s h . 52 RESULTS AND INTERPRETATION The E f f e c t s of Prey Size, Predator Size and Prey Density on Attack Rate. The data are presented i n terms of t o t a l number of attacks i n a ten minute period and maximum number of attacks/ min over the same ten minute period (Fig. 17). Total attack rate f o r small f i s h i s not an accurate measure for comparative use with large f i s h because some of the smaller f i s h tended to become satiated toward the end of the ten minute period (Fig. 18) compared to large f i s h which did not (Pig. 19). Differences i n t o t a l number of attacks and maximum attack rate f o r the two prey sizes, fed on by small f i s h , were not s i g n i f i c a n t at any density ( t - t e s t , p> .2), therefore the r e s u l t s were combined (Table XI). Attack rates tend to increase with density, however, small f i s h begin feeding rapidly at a lower density than large f i s h . Small f i s h have higher maximum attack rates than large f i s h at a l l d e n s i t i e s . The e f f e c t of prey size i s s i g n i f i c a n t for large f i s h at the intermediate density only. Large f i s h begin feeding on large prey at a lower prey density than they do with small prey. The lower feeding rates of large f i s h are not necessarily due to hesitance or preference. They could be due to i n a b i l i t y to feed well on t h i s prey type ( i n a b i l i t y to search, capture e t c . ) . The actions of the f i s h while feeding negate t h i s p o s s i b i l i t y . Often large f i s h would swim across 53a Fig. 17. Mean attack rates of f i v e small sticklebacks feeding on large and small n a u p l i i (Artemia salina) («) and f i v e large sticklebacks feeding on large (o) and small (A) nauplii> at three d i f f e r e n t d e n s i t i e s . V e r t i c a l l i n e s indicate 1 S.E. (a) Maximum number of attacks i n a one minute i n t e r v a l versus n a u p l i i density. (b) Total number of attacks over a ten minute t r i a l versus n a u p l i i density. 54a Fig. 18. Attack rate during a ten minute t r i a l (high n a u p l i i density) f o r two small sticklebacks (a) and (b). T r i a l 1 (o) T r i a l 2 (t) 54b (a) 2 on LU 2 n r 3 4 i 5 6 o (b) 7 8 9 1b 2 o - T -3 4 5 6 7 TIME (MIN) 9 10 55a F i g . 19. Mean attack rates for seven large sticklebacks, feeding on large brine shrimp n a u p l i i (900 yum) over a ten minute period (high n a u p l i i density). 55b 161 14 cr LU u < £ ioi < ~i 1 1 r 1 T 1 r ~ 3 4 5 6 7 8 9 10 T I M E (MIN) 56 the tank (40 cm) to attack a prey when there were hundreds immediately next to i t . In addition, large f i s h often f a i l e d to attack during a t r i a l . At the lowest density? eleven of seventeen t r i a l s resulted i n no feeding within ten minutes. The corresponding figure for small f i s h i s one i n f i f t e e n (X^ < .005). Large f i s h feeding on small Daphnia magna (less than 1 mm) have a similar behavioural response. When large D. magna (greater than 1 mm) are substituted, a l l sizes of f i s h appear to feed equally well. I t may be concluded therefore, that large f i s h display an element of size related hesitancy with brine shrimp n a u p l i i i n general and the smaller size class i n pa r t i c u l a r . This i s further investigated i n the following section. Table XI. One t a i l e d t - t e s ts for brine shrimp experiment. 5 Nauplii/1. 55 Nauplii/1. 435 Naup/1. Large f i s h versus small f i s h feeding on large n a u p l i i (max. attack rate) p<f .01* p< .01* p <.05* Large f i s h feeding on large versus small n a u p l i i (max. attack rate) P> .2 p <.005* P>.2 Large f i s h feeding on large versus small n a u p l i i ( t o t a l attack rate) p > .1 p< .05* •p=. 1 57 Choice of Prey Size. To determine prey size preference, four large (38-45 mm) and four small (22-28 mm) sticklebacks were allowed to feed on both sizes of brine shrimp n a u p l i i , presented at the same time, at a t o t a l density of 500/1 i n the proportion large: small = 2.42 + .16 :1. The proportion was determined from counts of four one ml„ aliquots. The feeding period was ten minutes. Only two of the four large f i s h fed during the experimental period, so the test was replicated for an addi-t i o n a l eight large f i s h (38-5,1 mm). The proportion of large to small prey was 2.82 + .52 :1. The r e s u l t s are given i n Table XII. Of the eight large f i s h that fed, three ate s i g n i -f i c a n t l y more small n a u p l i i , four ate s i g n i f i c a n t l y more large n a u p l i i , and one showed no preference at a l l . Three of four small f i s h ate s i g n i f i c a n t l y more large n a u p l i i , while one showed no preference. I f v i s i o n i s t'ae major factor a f f e c t i n g choice of prey, then based on prey surface area (refer to section on v i s i o n and s i z e - s e l e c t i v e predation), twice as many large prey should be taken r e l a t i v e to t h e i r proportion. This should be true for both sizes of f i s h , assuming equal v i s u a l acuity. I f energetic considerations are r.'ae. major factor determining choice, then large f i s h v w i l l consume r e l a t i v e l y more large prey than small f i s h do. 58 Table XII* R e l a t i v e consumption o f l a r g e and small s t i c k l e backs f e e d i n g on two s i z e s of Artemia s a l i n a n a u p l i i . F i s h Length (mm) # Small (S) N a u p l i i # Large (L) N a u p l i i L/S Exp. L/S Obs. 2 2 . 2 .16 1 3 6 2 . 4 2 8 . 5 * 2 8 . 8 17 306 2 . 4 2 1 8 . 0 * 2 7 . 0 62 261 2 . 4 2 4 . 2 * 2 8 . 3 61 111 2 . 4 2 1 . 8 4 1 . 3 26 10 2 . 4 2 0 . 4 * 3 8 . 0 70 86 2 . 4 2 1 . 2 * 4 2 . 2 34 22 2 . 8 2 0 . 6 5 * 4 4 . 1 8 74 2 . 8 2 9 . 3 * 5 1 . 3 7 111 2 . 8 2 1 5 . 9 * 3 8 . 2 50 166 2 . 8 2 3 . 3 4 2 . 8 23 181 2 . 8 2 7 . 9 * 4 1 . 3 8 48 2 . 8 2 6 . 0 * • - —' * I n d i c a t e s s i g n i f i c a n c e at . 0 5 l e v e l ( x 2 ) 59 The r e s u l t s indicate a high degree of v a r i a b i l i t y among large f i s h , therefore no conclusive statement can be made concerning the e f f e c t of n a u p l i i size differences. Several other prey c h a r a c t e r i s t i c s probably helped to confound the e f f e c t of size differences. The larger n a u p l i i are l i g h t e r i n pigmentation and have translucent peripheral and marginal extremities. The e f f e c t i v e size i s therefore l e s s than previously supposed. In addition, larger n a u p l i i have d i f f e r e n t swimming patterns than the smaller i n d i v i d u a l s . The former move by long, moderately fast strides while the l a t t e r move by short very fast s t r i d e s . These factors could explain why no uniform preference was observed for large f i s h feeding on the two d i f f e r e n t prey. The i n t e r p r e t a t i o n for small f i s h i s more d i f f i c u l t , because three of the four preferred large prey. Unfortunately, the sample size could not be increased because the supply of f i s h was l i m i t e d . To determine whether or not " l a r g e r " n a u p l i i represent a larger food packet, two r e p l i c a t e weight determinations were performed on an electrobalance for groups of approximately 100 n a u p l i i . The "small" n a u p l i i r e p l i c a t e s y i e l d 2.20 and 2.41 micrograms dry weight per i n d i v i d u a l while the values for "large" n a u p l i i are 1.86 and 1.78 micrograms. Based on length and diameter measurements, the larger n a u p l i i should weigh twice as much as they do. The larger apparent size was l i k e l y the 60 r e s u l t o f water a b s o r p t i o n . T h i s would account f o r the r e l a t i v e t r a n s l u c e n c y o f the lon g e r n a u p l i i . CONCLUSIONS: Large f i s h feed p o o r l y compared t o small f i s h when the prey are small (Artemia s a l i n a n a u p l i i ) . The reason i s probably not p h y s i c a l f e e d i n g a b i l i t y but r a t h e r a lower m o t i v a t i o n a l s t a t e i n l a r g e r f i s h which may be the r e s u l t o f e n e r g e t i c l i m i t a t i o n s . 61 ENERGETICS AND SIZE-SELECTIVE PREDATION INTRODUCTION In the previous experiment i t was demonstrated that large sticklebacks feed poorly on small prey. An energetic l i m i t a t i o n was considered as a possible explanation. To investigate t h i s hypothesis and to determine the nature of the l i m i t a t i o n s that energetic considerations must impose i n general? a model was constructed r e l a t i n g prey size, predator size and minimum rate of capture necessary to sustain f i s h at maintenance l e v e l s (exclusive of growth). The model i s also used to explain some observations made i n the current l i t e r a t u r e on the feeding of fishes. MINIMUM DAILY ENERGY REQUIREMENTS (EXCLUSIVE OF GROWTH) The model i s based on metabolic data determined by Cameron, Kostoris and Penhale (1973) for the ninespine s t i c k l e -back (Pungitius pungitius). Differences i n r e l a t i v e metabolic requirements between the two species of stickleback w i l l a f f e c t the model i n a l i n e a r fashion. The model i t s e l f , however, operates geometrically (length-weight relationship) and i s therefore f a i r l y i n s e n s i t i v e to metabolic differences between species, assuming equivalent temperature responses ( i . e . tempe-rature changes are assumed to have the same af f e c t on metabolic rates). A temperature response parameter i s included i n the 6 2 f i n a l e q u a t i o n to make the model a p p l i c a b l e to a v a r i e t y o f f i s h s p e c i e s . The model i s based on a 3 0 mm s t i c k l e b a c k ( 2 9 9 mg) and i s d e r i v e d r e l a t i v e t o a temperature o f 1 5 ° C . At t h i s temperature a 3 0 mm f i s h r e q u i r e s 3 8 . 8 c a l . / d a y a t normal a c t i v i t y l e v e l s minus the growth requirement. T h i s i s equ i v a -l e n t to approximately 7 . 7 6 mg o f dry food ( 5 c a l = 1 mg). The requirement f o r metabolism i s a c t u a l l y 207o l e s s ? however a s s i m i l a t i o n l o s s e s must be taken i n t o account. The weight o f a s t i c k l e b a c k i n c r e a s e s as the 2 . 7 4 5 power o f i t s l e n g t h . The requirement i n /ag/min (R) f o r a f i s h o f l e n g t h F i s : p- 7 » 7 6 0 / F . 2 . 7 4 5 x . 8 2 / - , ^ R " 7 2 0 ( 3 0 ; ( U where the m e t a b o l i c requirement i n c r e a s e s as the . 8 2 power o f the r e l a t i v e f i s h weight. Twelve hours o f d a y l i g h t ( 7 2 0 min) and a v i s u a l predator are a l s o assumed. The e q u a t i o n thus f a r i s v a l i d f o r 1 5 ° C o n l y . Tempe-r a t u r e i s i n c l u d e d as a v a r i a b l e as f o l l o w s : R _ 7 . 7 6 0 / F _ . 2 . 7 4 5 x . 8 2 i n r ( T - 1 5 ) m 7 2 0 ( 3 0 } 1 0 { 2 ) where T i s the temperature i n ° C and r i s the temperature response parameter equal t o . 0 9 4 f o r P . p u n g i t i u s . S i m p l i f y i n g the e q u a t i o n we have: R= . 0 0 5 1 F 2 ' 2 5 1 0 r ( T " 1 5 ) ( 3 ) 63 PREY LENGTH AND WEIGHT When considering zooplankton species as the prey, the length-weight r e l a t i o n s h i p i s the most sensitive aspect of the model? primarily because: 1) d i f f e r e n t species appear to follow d i f f e r e n t length-weight relationships; 2) available data shows a great deal of v a r i a t i o n within genera or species. In Table XIII I have calculated the wet weight of individual zooplankters 1 mm i n length, using equations co l l e c t e d from the l i t e r a t u r e by Edmonson and Winberg (1971). Four of these equations? i n the form Weight = a Length x, are given to i l l u s t r a t e the v a r i a b i l i t y between taxa. Data from LeBrasseur and Kennedy (1972) are also given, dry weights being calculated as 14-26% of the wet weight. Because t h e i r techniques for measuring wet weight are d i f f e r e n t than those commonly employed, a proportion of 7.57, i s used to convert the wet weights (cited by Edmonson and Winberg) to dry weights (Brasseur and Kennedy note that the range normally considered i s 5-107o). The present model assumes a cubic rel a t i o n s h i p between prey length and weight and a dry weight value of 3.5 jug for a 1 mm i n d i v i d u a l . I t would therefore be most d i r e c t l y applicable to a daphnid species. The general formula for prey weight i s : W = — h (4) where P i s prey length, X i s the appropriate degree of the 64 Table XIII. Data from the l i t e r a t u r e concerning length-weight relationships for various entomostracan taxa. Taxon Relationship Length (mm) Wet Weight (pg) Conversion Factor Dry Weight (pg) * Copepods 1 29.8 .075 2.2 **Diaptomus 1.1 11 .19 2.1 * Copepods W=.055L2-73 1 55 .075 4.1 **Cvclops .96 6 .26 1.6 * BosnvLna lon,girostri£ 1 166 .075 12.4 * Bo,smi,na W=.124L 2 , 1 8 1 1 124 .075 9.3 * Daphnia hvalina 1 13.8 .075 1.0 * Daphnia hvalina 1 54.8 .075 4.1 * Daphnia longispina 1 54.8 .075 4.1 * Daphnia W=.052L 3 , 0 1 2 1 52 .075 3.9 **Daphnia ' .9 10 .19 1.9 * Diaphanosoma W=.092L 2' 4 4 9 1 92 .075 6.9 **Holopedium .9 17 .14 2.4 W = wet weight i n mg L = length i n mm *Edmonson and Winberg (l97l) **LeBrasseur and Kennedy(l972) 65 re l a t i o n s h i p (normally between 2 and 3) and y and h i s a cor-responding pair of prey length and dry weight respectively. Total dry weights do not represent the portion of the prey that can be digested by the f i s h . Ash free dry weights are a better estimate? however, for Daphnia they are normally only 4-13% less than the t o t a l dry weight (Wlssing and Hasler 1968). C h i t i n content may impose a greater error (up to 407o dry weight; B i l l N e i l l , personal communication) because c h i t i n i s l a r g e l y ashable but i n d i g e s t i b l e . THE FINAL MODEL The minimum energy requirements of a f i s h can now be expressed i n terms of a capture rate (C): C = R / W = , Q 0 5 i F 2 ; 2 5 i o r ( T - 1 5 ) p x h k 5 ) y which i n the present case reduces to: C = . 0 0 1 4 6 F 2 - 2 5 1 0 - 0 9 4 ( T - 1 5 ) ( 6 )  p3 Two questions may now be asked: 1) What average capture rate i s necessary f o r a f i s h to maintain i t s e l f at a given temperature and for a given size of prey? 2) Is t h i s capture rate feasible? Table XIV provides the answer to the f i r s t question. Even when lacking s p e c i f i c data, the second question can often be answered, because the required capture 66 Table XIV. Capture rates necessary for maintenance of the ninespine stickleback at 15°C (based on eq. (6)) for various prey sizes. F i s h lengths are extrapolated to y i e l d approxi-mations for other species. Capture rates are i n numbers/min. Fish Length (cm) .15 .3 .6 .9 1.2 1.5 1.8 .7 35 4.3 .6 .2 .1 •k 1 77. 9.7 1.2 .4 .2 .1 •k 2 368 46 5.8 1.7 .7 .4 .2 3 916 115 14 4.2 1.8 .9 .5 4 •k-k 219 27 8.1 3.4 1.8 1.0 5 •k-k 362 45 13 5.6 2.9 1.7 6 •k-k 545 68 20 8.5 4.4 2.5 10 •k-k 215 63 26 13 8 20 •k-k •k-k •kk 303 128 65 38 30 •k-k •k-k •k-k 755 318 163 94! 50 •k-k •k-k •k-k •k-k 514 298 For 5°C divide rates by 8 For 10°C divide by 3 For 20°C multiply by 3.4 * implies l e s s than .1 **implies greater than 1000 67 r a t e s may be ord e r s o f magnitude apart when c o n s i d e r i n g prey and p r e d a t o r s o f d i f f e r e n t s i z e s . F i g . 20 provi d e s the same i n f o r m a t i o n i n a d i f f e r e n t manner. For any g i v e n maximum capture r a t e , i t i s p o s s i b l e to q u i c k l y a s c e r t a i n a l l the preda t o r - p r e y s i z e combinations t h a t are f e a s i b l e . The e f f e c t o f a change i n temperature or a change i n maximum capture r a t e (when c o n s i d e r i n g d i f f e r e n t s p e c i e s o f pre d a t o r and prey) i s to change the i s o c l i n e c o n s i d e r a b l y . 68a Fig . 20. Lines represent combinations of prey length (P) and f i s h length (F) for which maintenance requirements are just obtained when feeding at rates (a) = 10 prey/min; (b) = 30 prey/min; (c) = 90 prey/min. These values apply for a water temperature of 15°C. For a feeding rate of 10 prey/min (b) also represents values f o r 10°C and (c) represents 5°C. (based on equation 6) 68b (cm) 69 APPLICATIONS OF THE MODEL A) Artemia salina n a u p l i i . A major suggestion of the brine shrimp experiment i s that large sticklebacks feed poorly on both sizes of n a u p l i i , compared to small sticklebacks. I t was hypothesized that a lack of motivation was responsible. I t would therefore be of interes t to see i f energy l i m i t a t i o n s could be a cause of t h i s . Using equation (5) and a mean n a u p l i i weight of 2 pg> at 18°C the rate of capture required f o r a 45 mm stickleback i s 25.7/min and for a 20 mm f i s h the rate i s 4.1/min. Although the required capture rate could be achieved by a large stickleback at high prey d e n s i t i e s , the s i t u a t i o n i s c l e a r l y marginal. B) Rotifers. Of ten very small f i s h dipnetted from Marion Lake i n August of 1974, those l e s s than 10 mm i n length contain a s i g n i f i c a n t proportion of r o t i f e r s (>10% by volume) 0.15 mm long. Table XIV indicates that a f i s h 7 mm i n length would have to consume r o t i f e r s at the rate of about one every two seconds at 15°C i n order to maintain i t s e l f . A 10 mm f i s h would have to feed at more than twice that rate; a highly unattractive prospect, e s p e c i a l l y i f larger food p a r t i c l e s are available. 70 C) Daphnia. Galbraith (1967) finds that yellow perch (7-25 cm) and rainbow trout (20-43 cm) r a r e l y eat Daphnia less than 1.3 mm i n length. Numerous studies carried out i n the past few years show t h i s to be the case for many species of f i s h and cladocerans of equivalent size (refer to studies c i t e d i n the Introduction to the t h e s i s ) . Table XIV shows that although the smallest f i s h Galbraith uses would have to feed at rates of 10-30/min on Daphnia 1.2 mm i n length, the larger f i s h would have to feed at rates approaching 500/min. These f i s h would be unable to survive i f Daphnia of t h i s size were the only source of food available. Galbraith also notes that Daphnia form a smaller proportion of the trout d i e t during the summer than during the winter. This may be a consequence of the enormous influence of temperature on the metabolic require-ments of the f i s h , as well as the presence of a larger proportion of smaller Daphnia due to cyclomorphotic changes ( B i l l N e i l l , personal communication). Assuming temperatures of 5°C and 20°C for winter and summer respectively, the required capture rates would be i n the r a t i o 1:25. D) Asellus aquaticus. The general model (equation 5) can be applied to any prey species. Berglund (1968) presents data for brown trout, Salmo t r u t t a , feeding on Asellus aquaticus i n a Swedish lake. 71 The study i s p a r t i c u l a r l y useful i n terms of applying the energy model because Asellus constitutes 90-100% of the die t of the trout (numerically) throughout the year. Growth rate data f o r four large trout (800-1000 g) and data on numbers, biomass and mean size (weight) of Asel l u s were extracted from t h i s study and c o r r e l a t i o n c o e f f i c i e n t s calculated between growth and each of the three Asellus parameters. F i g . 21 indicates how these values changes with respect to one another? over a 17 month period. The c o r r e l a t i o n c o e f f i c i e n t with respect to number/m i s .044 (N.S. at p = .05). Biomass/m^ correlates better with growth rate (r = .418) but i s s t i l l not s i g n i f i c a n t at the .05 l e v e l . Only mean Asellus size has a s i g n i f i c a n t c o r r e l a t i o n with growth rate (r = .780? p<.01). Trout smaller than 400 grams tend to have uniform growth rates. I t appears that the larger f i s h are p a r t i c u l a r l y sensitive to changes i n the nature of t h e i r food supply, p a r t i c u l a r l y mean size. During the winter, when large trout growth i s greatest, mean Asel l u s size i s approximately 15 mg. The average -require-ment per minute f o r a six hour day at 5°C i s 1.7 mg. This i s equivalent to approximately seven average size prey per hour. Although nearly three times as much daylight i s available i n the summer, metabolic requirements are at least eight times as great and mean prey size i s ha l f the winter value. This would 72a F i g . 21. Brown t r o u t (Salmo t r u t t a ) growth r a t e s ( s o l i d ) , mean A s e l l u s aquaticus weight (long dashes), A s e l l u s biomass (short dashes) and numbers of A s e l l u s (dots) p l o t t e d against month of the year f o r a Swedish l a k e (adapted from Berglund, 1968). B = biomass i n grams per m^  ( A s e l l u s ) 9 N = number of A s e l l u s per m^  W = mean weight of an i n d i v i d u a l A s e l l u s i n mg G = growth r a t e of t r o u t i n grams per gram of f i s h (mean of the four l a r g e s t f i s h f o r which data was provided) B N 50 25 40 20H 30 15H 20 10^ 10 i15 00 -f lo -01 H5 -02 cr 73 necessitate a capture rate of nearly 40 prey/hour, a rather high figure for a benthic predator (Kim Hyatt, personal commu-nic a t i o n ) . In addition, the prey size d i s t r i b u t i o n i s sharply bimodal with few very large indi v i d u a l s and numerous small indivi d u a l s being present i n the population (approx. 1/500 of the biomass of the large i n d i v i d u a l s ) . Under these circumstances the advantages of being small and having a lower energy require-ment are apparent. These considerations could account for the observed growth patterns. CONCLUSIONS: Although the energy model i s based on predation on one prey type at a time, whereas f i s h may feed on the d i f f e r e n t prey types (species and size classes) as they occur, the model nevertheless indicates which prey classes cannot be of great importance from an energetic point of view. Only a minimum amount of time and e f f o r t should be expended i n the search and capture of these prey i f energy i s the only consideration. I t i s possible, however, that c e r t a i n small prey represent an important vitamin-nutrient source for the predator. I f t h i s i s the case, the model can be used to determine which prey classes w i l l f a l l into t h i s category. In summary we may conclude: 1) Predator size, prey size and temperature l e v e l s determine the minimum capture rate that a stickleback must maintain i n 74 o r d e r t o s a t i s f y m e t a b o l i c r e q u i r e m e n t s . 2 ) T h e r e s u l t s f r o m a n e n e r g y m o d e l b a s e d o n t h e a b o v e v a r i a b l e s c a n h e l p t o e x p l a i n t h e p r e y s i z e c h o i c e s a n d s p e c i e s c h o i c e s ( i f t h e r e a r e s i g n i f i c a n t s i z e d i f f e r e n c e s b e t w e e n s p e c i e s ) t h a t a f i s h m a k e s . 3) I n c a s e s w h e r e t h e l a r g e i n d i v i d u a l s o f a p r e y s p e c i e s f l u c t u a t e g r e a t l y i n a b u n d a n c e d u r i n g t h e y e a r , s u b s e q u e n t e f f e c t s o n g r o w t h r a t e s , e s p e c i a l l y o f t h e l a r g e r f i s h , may be e x p e c t e d . 75 VISION AND SIZE-SELECTIVE PREDATION INTRODUCTION The enclosure r e s u l t s and the Asellus feeding experi-ment demonstrate the importance of a mechanistic factor(mouth size) i n determining the upper l i m i t to size s e l e c t i v i t y by sticklebacks. The feeding model was used to show the importance of energy considerations i n determining the lower l i m i t to size s e l e c t i v i t y . A mechanistic factor based on v i s u a l considerations can also be shown to be a major element i n determining a predator's choice of prey size. The magnitude of the l i g h t stimulus reaching the eyes of a v i s u a l predator should be of prime importance i n determining the l i k e l i h o o d of a response to that stimulus. I f t h i s i s the case, reactive distance w i l l vary as the square root of the prey surface area or stimulus ( l i g h t ) emitting region. This follows because l i g h t i s reduced i n proportion to the inverse square of the distance from the source. This argument i s v a l i d only when contrast levels are uniform, as differences i n contrast w i l l greatly a f f e c t reactive distance (Ware, 1971). EVIDENCE Ware (1973) provides data on reactive distance f o r the rainbow trout, Salmo gairdneri ? versus prey length (pieces of chicken l i v e r ) . Length of h i s prey correspond to surface area and therefore v i s u a l stimulus, because length i s the only 76 dimension t h a t changes duri n g the experiment. When prey s i z e i s p l o t t e d against the square of the r e a c t i v e d i s t a n c e , a s t r a i g h t l i n e seems to be the r e s u l t ( r = .99, p < . O l ) . A t e s t of curvature (second degree polynominal f i t ) however, was s i g n i f i c a n t at the .05 l e v e l , although j u s t b a r e l y so. The l i m i t e d amount of data may have been the cause of t h i s ( F i g . 22a). Ware's model f o r determining r e a c t i v e d i s t a n c e compares w e l l w i t h h i s experimental r e s u l t s . I t would be i n s t r u c t i v e at t h i s p o i n t to examine h i s f i n a l equation: D = ln(C/Ki) + bB + ((ln(C/K)+ b B ) 2 + 4*ap)^ ( 7) loc where D i s r e a c t i v e d i s t a n c e , o< i s the c o e f f i c i e n t of l i g h t a t t e n t u a t i o n i n water, B i s the background i l l u m i n a t i o n , p i s prey l e n g t h and a l l other l e t t e r s r e f e r to constants that i n v o l v e c o n t r a s t c o n s i d e r a t i o n s . Given t h a t the background i s o f constant i l l u m i n a t i o n (the mud-bottom of a l a k e , that i s i n view to a f o r a g i n g s t i c k l e b a c k at any given i n s t a n t ) and the water of uniform c l a r i t y , equation (7) reduces t o : D = f + (g + hp)^ (8) where f,g , and h are constants. I n t h i s form i t can be seen that r e a c t i v e d i s t a n c e v a r i e s as the square root of the prey le n g t h . The c o r r e c t form of the equation may have been a r r i v e d at f o r the wrong reasons. The i m p l i c i t assumption i n Ware's 77a Fi g . 22. Relationships between reactive distance? prey r i s k and prey size. (a) The reactive distance (RD) squared for rainbow trout attacking d i f f e r e n t lengths of chicken l i v e r . Data from Ware (1971). (b) Square root of prey r i s k ( probability of discovery) (R) versus inverse of distance between predator (European thrush) and prey (stationary a r t i f i c i a l prey). Data from Smith (1974). (c) P r o b a b i l i t y of discovery (prey r i s k ) versus the inverse square of the distance between predator (threespine s t i c k l e -back) and prey (Tubifex). Data from Beukema (1968). 77b •32r .16h cc ( a ) J L. J L ,003 - 007 PREY LENGTH (rrO .011 .015 •8r •6 ( b ) .02 1/D trrj ~& L" ui -6, O A (D;ST/\NCE) * 103 (cm)** ( O 78 model i s that only one dimension i s of importance to the predator. There i s no physiological or physical p r i n c i p l e to support such an assumption. A more appropriate quantity i n l i e u of p would be zp 2 to represent prey surface area, where z i s an appropriate dimensional constant. The res u l t s of (7) can therefore be considered to be fortuitous since the prey width i s held constant. An experiment from Beukema (1968) ( i n which threespine sticklebacks fed on Tubifex) can be used to examine the import-ance of the v i s u a l stimulus i n determining prey r i s k . Beukema's r i s k values are p r o b a b i l i t i e s of discovery of the prey, calculated as a proportion equal to the number of discoveries from a given distance over the number of opportunities at that distance. The proportion of captures (prey r i s k ) i s expected to be d i r e c t l y proportional to the prey's apparent size (strength of v i s u a l stimulus). Size i s constant and therefore the pr o b a b i l i t y of discovery versus 1 / D 2 should be l i n e a r . Fig. 22c shows Beukema's data replotted on the appropriate scale. A test of curvature on the points representing distances of 20 cm and greater i s not significance at the .05 l e v e l , however the l i n e a r regression i s s i g n i f i c a n t (p< .01). For distances l e s s than 20 cm, r i s k changes very slowly (there i s always some chance that the prey i s not detected even when near the predator). Smith (1974) provides additional data using the European blackbird as predator and stationary a r t i f i c i a l prey. 79 The re l a t i o n s h i p ./Risk = -D + constant, i s proposed to describe the data points, however an inverse f i t of ./Risk" against 1/D (equivalent to Risk against 1/D2 as i n Beukema's study) showed no s i g n i f i c a n t curvature at the .05 l e v e l up to a threshold distance of 50 cm (Fig. 22b). The two studies described above demonstrate the non-l i n e a r e f f e c t of distance on prey r i s k . I f i n addition a predator's choice could be shown to vary non-linearly with prey length, then a strong mechanistic basis for size s e l e c t i v i t y would be available to explain the size preference evident i n many predaceous f i s h (Galbraith, 1967; Brooks,1968). Brooks (1968) provides data that indicate the importance of prey size i n determining prey r i s k (alewives feeding on Diaptomus minutus), however a detailed analysis of h i s data would not be j u s t i f i e d because of fa u l t y experimental design. The Artemia n a u p l i i experiment of the present study was also designed to examine the nature of the prey size-prey r i s k r e l a t i o n s h i p . The r e s u l t s were not consistent, i n d i c a t i n g that other factors such as degree of pigmentation, prey swimming patterns and translucency differences between the two prey sizes, were confounding the ef f e c t of length differences. CONCLUSIONS: With respect to the relationship between prey size and prey r i s k , the most important consideration i s not the 80 p r e c i s e m a t h e m a t i c a l n a t u r e o f t h e r e l a t i o n s h i p b e c a u s e t h i s w i l l u n d o u b t e d l y v a r y d e p e n d i n g o n t h e s p e c i f i c s i t u a t i o n . E m p i r i c a l r e l a t i o n s h i p s w i l l n o r m a l l y s u f f i c e f o r m o d e l l i n g p u r p o s e s . T h e s i g n i f i c a n t o b s e r v a t i o n s f r o m a q u a l i t a t i v e v i e w a r e : 1) P r e y r i s k d e c l i n e s r a p i d l y a s p r e y s i z e i s r e d u c e d . 2 ) I n t e r m s o f r e a c t i v e d i s t a n c e ? l a r g e r p r e y w i l l be n o t i c e d f r o m a g r e a t e r d i s t a n c e t h a n s m a l l e r p r e y . 3) B o t h o f t h e a b o v e r e l a t i o n s h i p s t e n d t o be n o n - l i n e a r so t h a t s m a l l c h a n g e s i n s i z e ( o n e d i m e n s i o n a l ) r e s u l t i n v e r y l a r g e c h a n g e s i n r i s k o r r e a c t i v e d i s t a n c e . 4 ) A c h a n g e i n t h e d i s t a n c e b e t w e e n p r e d a t o r a n d p r e y h a s a n o n - l i n e a r e f f e c t o n p r e y r i s k t h a t i s s i m i l a r t o t h e e f f e c t o f a c h a n g e i n p r e y s i z e . 81 GENERAL DISCUSSION The purpose of t h i s thesis has been to investigate the nature of s i z e - s e l e c t i v e predation by the threespine stickleback, and i n so doing, to determine some of the factors that define and l i m i t t h i s phenomenon. In recent years a great deal of research has been devoted to studying the e f f e c t s of size s e l e c t i v i t y by f i s h on zooplankton communities i n lakes (Brooks and Dodson, 1965; Brooks, 1968; Stenson, 1972). This work deals primarily with prey organisms that approachec^ the lower size l i m i t s of the fishes concerned. The e f f e c t s of upper l i m i t s are l a r g e l y neglected. Dodson (1970) demonstrates the importance of a lower size l i m i t for salamanders (Ambystoma tigrinum) feeding on cladocerans i n a lake. He suggests that t h i s lower l i m i t allowed a l a r v a l Chaoborus population to coexist with the salamanders. Salamander predation favours the existence of smaller cladocerans i n the range .7-1 mm, the preferred range f ° r Chaoborus larvae. The salamanders eat cladocerans which are normally greater than 1 mm i n length. Dodson (1974) also demonstrates the importance of an upper food size l i m i t for species interactions. A predaceous copepod Diaptomus shoshone was found to prevent small Daphnia minnehaha from coexisting with the larger Daphnia mi ddendorffiana because of selective predation on the former. 82 Similar sorts of analyses can also be done with benthic communities. The r e s u l t s of the present study indicate that the upper food size l i m i t for Gasterosteus aculeatus i s l a r g e l y determined by mouth width. For the prey species examined, Asellus o c c i d e n t a l i s and H y a l l e l a azteca, a p a r t i c u l a r prey dimensions (width) was found to be c r i t i c a l . Using these r e s u l t s i t should be possible to predict some of the e f f e c t s of a stickleback introduction on the structure of a benthic community. With reference to the introduction of sticklebacks i n Marion Lake, B.C., one might predict that the amphipod H. azteca w i l l decline i n numbers r e l a t i v e to the larger Crangonyx  r i chmond en s i s . Meagre data presented for Crangonyx (Fig. 3) suggest that the i n d i v i d u a l s of reproductive size w i l l be too large for most sticklebacks to consume. H y a l l e l a of reproductive size could be taken by a larger proportion of the stickleback population. These changes i n the benthic community structure could i n turn a f f e c t predator interactions because kokanee, Oncorhvncus nerka, are also present and take s i g n i f i c a n t quantities of H y a l l e l a at c e r t a i n times of the year (Hyatt, 1974). With -reference to the lower prey size l i m i t , I found that large sticklebacks fed poorly on very small prey (brine shrimp n a u p l i i ) compared to smaller f i s h . Although behavioural, energetic and v i s u a l reasons can be considered, the l a t t e r factor i s probably unimportant with respect to the p a r t i c u l a r experiment 83 performed because small f i s h fed well and have i n a l l pr o b a b i l i t y a v i s u a l acuity no better than large f i s h (Protasov, 1961). The visual factor may act uniformly for d i f f e r e n t sizes of f i s h . I t should nevertheless r e s u l t i n proportionally fewer small prey being taken when a l l sizes of predators are considered. The observed performance of the large f i s h may be the re s u l t of behavioural pre-adaptations for foraging on benthos, i n i t i a l l y selected for on the basis of energetic factors (the energy model showed that maximum possible energy intake from n a u p l i i was marginal at best). Energetic factors may then be assumed to be partly responsible for the differences i n feeding niches (benthic versus planktonic) between large and small f i s h . In addition to mouth dimensions determing upper food size l i m i t s , energetic considerations as manifested i n the model presented, can be employed as an aid i n determining s p e c i f i c e f f e c t s of f i s h introductions on prey communities by establishing lower size l i m i t s f or the predators concerned. The model demonstrates that the lower prey size l i m i t of 1 to 1.3 mm found i n the l i t e r a t u r e (Galbraith, 1967; Brooks and Dodson, 1965) can be explained on energetic grounds. Concerning the importance of size s e l e c t i v i t y for the f i s h , Ivlev (1961) finds that an increase i n aggregation(clumping) of prey r e s u l t s i n higher feeding rates for his f i s h , although t o t a l density of prey remains constant. Paloheimo and Dickie (1966) postulate that such clumping i s i n fact equivalent to 84 creating "larger." prey and c i t e evidence i n the form of increased growth e f f i c i e n c e s f o r brown trout, Salmo t r u t t a , fed progressively larger prey items. My analysis of Berglund's data (Berglund, 1968) tends to support the importance of large food p a r t i c l e s (large Asellus) for the growth of large f i s h . In conclusion, the findings of t h i s study would support the hypothesis that s i z e - s e l e c t i v e predation i n the threespine stickleback i s l i m i t e d by physical, behavioural and energetic factors. The precise manifestation of the f i r s t two of these i s i n a l l l i k e l i h o o d an evolutionary compromise with the t h i r d . 85 LITERATURE CITED All e n , K. R. 1935. The food and migration of the perch (Perca f l u v i a t i l i s ) i n Windermere. J. Animal E c o l . 4: 264-273. Berglund, T. 1968. The influence of predation by brown trout on Asellus i n a pond. Inst, of Fresh Water Resources Drottningholm 48: 77-101. Beukema, J . J . 1968. Predation by the threespine stickleback. Behaviour 31: 1-126. Brooks, J . L. and Dodson, S. I. 1965. Predation, body size and composition of plankton. Science 150: 28-35. Brooks, J. L. 1968. The e f f e c t s of prey size selection by lake planktivores. Systematic Zoology 17: 273-291. Brooks, J. L. 1969. Eutrophication and changes i n the composition of the zooplankton. Proc. Int. Symp. on Eutrophication. Bryant, D. M. 1973. The factors influencing the selection of food by the house martin (Delichon urbica (L.)) . J . Animal Ecol. 42: 539-564. Cameron, J. N., Kostoris, J . and Penhale, P. A. 1973. Preliminary energy budget of the ninespine stickleback (Pungitius pungitius) i n an a r t i c lake. J . Fish, Res. Board Can. 30: 1179-1189. Confer, J . L. 1971. Intrazooplankton predation by Mesocvclops  edax at natural prey densities. Limn, and Ocean. 16(4): 663-666. Cramer, J. D. and Marzolf, G. R. 1970. Size-selective predation by gizzard shad. Trans. Am. Fish. Soc. 99(2): 320-332. Dodson, S. I. 1970. Complementary feeding niches sustained by si z e - s e l e c t i v e predation. Limn, and Ocean. 15(1): 131-137. Dodson, S. I. 1974. Zooplankton competition and predation: an experimental test of the s i z e - e f f i c i e n c y hypothesis. Ecology 55: 605-613. 86 Edmonson, W. T. and Winberg, G.G. 1971. A manual on methods fo r the assessment of secondary productivity i n fresh water. IBP handbook No. 17. Blackwell S c i e n t i f i c Publications, Oxford and Edinburgh. Galbraith, M. G. 1967. Size-selective predation on Daphnia by rainbow trout and yellow perch. Trans. Am. Fish Soc. 96(1): 1-10. Gibb, J . A. and Betts, M. M. 1963. Food and food supply of nestling t i t s (Paridae) i n breckland pine. J . Animal Ecol. 32: 489-533. Goss-Gustard, J . D. 1970. Responses of redshank to density of t h e i r prey. J. Animal "Ecol. 39: 91-113. Hyatt, K. D. 1974. Feeding "behaviour of rainbow trout and kokanee i n Marion Lake, B.C. Phd thesis at the Univ. of B r i t i s h Columbia, i n preparation. Hynes, H. B. N. 1950. The food of fresh water sticklebacks (Gasterosteus aculeatus and Pygosteus pungitius) with a review of methods used i n studies of the food of fis h e s . J . Animal Ecol. 19: 36-58. Ivlev, V. S. 1961. Experimental ecology of the feeding of fis h e s . Yale University press, Hew Haven. 302 pages. Larson, G. L. 1972. Social behaviour and feeding a b i l i t y of two phenotypes of Gasterosteus aculeatus i n r e l a t i o n to t h e i r s p a t i a l and trophic segregation insa temperate lake. Phd. thesis, Univ. of B r i t i s h Columbia. LeBrasseur, R. J. and Kennedy, D. D. 1972. The f e r t i l i z a t i o n of Great Central-Lake I I . Zooplankton standing stock. Fishery B u l l e t i n 70(1): 25 T36. Leong, R. J . H. and O'Connell, C. P. 1969. A laboratory study of p a r t i c u l a t e and f i l t e r feeding of the northern anchovy (Engraulis mordax). J . Fish. Res. Bd. Can. 26: 557-582. Lindstrom, T. 1954. On the r e l a t i o n f i s h size-food size. Inst, of Fresh Water Research, Drottningholm 36: 133-147. Menge, B. A. 1972. Competition for food between two i n t e r t i d a l s t a r f i s h species and i t s e f f e c t on body size and feeding. Ecology 53(4): 633-644. 87 Nilsson, N. and P e j l e r , B. 1973. On the r e l a t i o n between f i s h fauna and zooplankton composition i n Northern Swedish lakes. Inst. Fresh Water Research, Drottningholm 53: 51-77. Paloheimo, J . E. and Dickie, L. M. 1966. Food and Growth of fishes I I I . Relations among food, body size and growth e f f i c i e n c y . J . F i s h * Res. Bd. Can. 23(8): 1209-1248. Protasov, V. R. 1970. V i s i o n and near o r i e n t a t i o n of f i s h . I s r a e l programme for s c i e n t i f i c t r anslations. 175 pg. Royama, T. 1970. Factors governing the hunting behaviour and selection of food by the Great T i t (Parus major (L.)) J. Animal E c o l . 39: 619-668. Schoener, T. W. 1965. The evolution of b i l l size differences among sympatric congeneric species of birds. Evol. 19: 189-213. Smith, J . N. M. 1974. The food searching behaviour of two European thrushes. I I . The adaptiveness of the search patterns. Behaviour 49: l£61. Stenson, J . A. E. 1972. Fish predation e f f e c t s on the species composition of the zooplankton community i n eight small forest lakes. Inst. Fresh Water Research, Drottningholm 52: 132-148. Ware, D. M. 1971. The predatory behaviour of rainbow trout. Phd thesis, Univ. of B r i t i s h Columbia. Ware, D. M. 1973. Risk of epibenthic prey to predation by rainbow trout (Salmo gair d n e r i ) . J . Fish. Res. Bd. Can. 30: 787-797. Wells, La Rue 1970. E f f e c t s of alewife predation on zooplankton populations i n Lake Michigan. Limn, and Ocean. 15(4): 556-565. Werner, E. E. 1974. The f i s h size, prey size, handling time r e l a t i o n i n several sunfishes and some implications. J . Fish Res. Bd. Can. 31: 1531-1536. Werner, E. E. and H a l l , D. J . 1974. Optimal foraging and the size selection of prey by the b l u e g i l l sunfish (Lepomis  macrochirus). Ecology 55: 1042-1052. 88 Wllliamsj W. D. 1970. A r e v i s i o n of the N. A. epigean species of Asellus (Crustacea: Isopoda). U. S. Smithsonian Inst. Contributions to Zoology '#49, 80 pages. Wissing, T. E. and Hasler, A. D. 1968. C a l o r i f i c values of some invertebrates i n Lake Mendota, Wisconsin. J . Fish. Res. Bd. 25(11): 2515-2518. 89 APPENDIX: BIAS IN ELECTIVITY INDICES The forage r a t i o R/P and I v l e v 1 s e l e c t i v i t y index (R-P)/(R+P), where R i s the proportion of a food type i n the d i e t and P i s i t s proportion r e l a t i v e to a l l food types present, are frequently used to quantify food selection. The l a t t e r index e s p e c i a l l y i s often described as being non-bias and l i n e a r and therefore the appropriate one to use (Ivlev, 1961; Cramer and Marzolf, 1971; Burbidge, 1974). Dodson (1970) suggests using R/P because d i f f e r e n t predators feeding on the same food type i n the same area could then be compared (P i s constant). The problem of unequal range of avoidance and preference could be overcome by using log (R/P). Jacobs (1974) notes that with the common indices, a change i n P causes a disproportionate change i n e l e c t i v i t y (E), depending on the exact value of P. The index that he proposes, however, i s just as dependant on P (E = (R-P)/(R+P-2RP)). The problem with using any index at a l l i s that R i s forced to relate to P i n a precise manner determined by the d e f i n i t i o n of the index. The data i s therefore transformed making any i n t e r p r e t a t i o n t o t a l l y dependant on the transformation i t s e l f . Two d i f f e r e n t indices can therefore r e s u l t i n two d i f f e r e n t sets of conclusions. As an example, I have presented a series of curves (Fig. l a ) taken from Ivlev (1961). The curves i l l u s t r a t e the change i n e l e c t i v i t y for carp feeding on four 90;a F i g . 1. Changes i n e l e c t i v i t y ('£) o f carp w i t h changes i n the r e l a t i v e d e n s i t y o f chironomid l a r v a e . T o t a l prey d e n s i t y remains constant. chironomid l a r v a e o; amphipods • freshwater isopods o; m o l l u s c s © (a) E = (R-P)/fc+P) (b) E = (R-P) Data from I v l e v (1961). R i s % i n r a t i o n ; P i s 7» a v a i l a b l e . 90b g r o u p s o f p r e y a s one o f t h e p r e y ( c h i r o n o m i d l a r v a e ) c h a n g e s i n r e l a t i v e d e n s i t y . I v l e v c o n c l u d e s t h a t when a p r e f e r r e d p r e y ( c h i r o n o m i d l a r v a e ) i n c r e a s e s i n r e l a t i v e d e n s i t y , i t i s s e l e c t e d t o a l e s s e r e x t e n t a n d t h o s e p r e y t h a t a r e l e s s p r e f e r r e d a r e s e l e c t e d t o a g r e a t e r e x t e n t w i t h t h e m a j o r c h a n g e s o c c u r r i n g a t l o w p r e y d e n s i t i e s . T h e s e r e s u l t s a n d c o n c l u s i o n s a r e l e s s a r e s u l t o f t h e d a t a a n d m o r e a r e s u l t o f t h e n a t u r e o f t h e i n d e x u s e d t o d e s c r i b e t h e d a t a . F i g . 2 d e s c r i b e s t h e b i a s i n h e r e n t i n t h e i n d e x . F o r s m a l l v a l u e s o f P ( o r R ) I v l e v ' s i n d e x i s h i g h l y s e n s i t i v e t o s m a l l s h i f t s i n P . When P i s l a r g e r c h a n g e s i n E a r e m u c h s m a l l e r . T h i s a c c o u n t s , tco a a g r e a t e x t e n t , f o r t h e s h a p e s o f t h e c u r v e s i n F i g . l a . A l i n e a r i n d e x E = R - P g i v e s d i f f e r e n t r e s u l t s . F i g . l b i n d i c a t e s t h a t t h e l e s s f a v o u r e d p r e y i n c r e a s e m o r e s l o w l y i n t e r m s o f e l e c t i v i t y a s t h e i r a v a i l a b i l i t y i n c r e a s e s , T h e two i n d i c e s p r e d i c t d i r e c t l y o p p o s i t e r e s u l t s f o r t h e c h i r o n o m i d l a r v a e . T h e l i n e a r i n d e x R - P i s t h e e x c e s s o r d e f i c i t o f t h e r a t i o n o v e r t h e s u p p l y o f a n y g i v e n f o o d t y p e . I t i s b i a s e d a s w e l l b e c a u s e i t m a k e s t h e a s s u m p t i o n t h a t t h e r e l a t i o n s h i p b e t w e e n R a n d P i s l i n e a r . T h e b e s t c o u r s e t o f o l l o w t h e r e f o r e , i s t o p l o t R a g a i n s t P w h e n p o s s i b l e o r p e r f o r m a m u l t i p l e r e g r e s s i o n s h o u l d a t h i r d v a r i a b l e ( e . g . t e m p e r a t u r e ) a l s o be o f i n t e r e s t . •92a Fig. 2. Bias i n e l e c t i v i t y indices. The manner i n which two e l e c t i v i t y indices are affected by changes i n r a t i o n (R) at high and low l e v e l s of a v a i l a b i l i t y (P) i s shown. The curved l i n e s represent Ivlev's index E = (R-P)/(R+P) and the straight l i n e s represent a l i n e a r index E = (R-P). 93 The index R-P may be e a s i l y converted to a measure of s i m i l a r i t y or overlap (0), as i t i s employed i n t h i s thesis. 0 = 1 - %£/R, - P./ i = l 1 1 I t has been used i n t h i s form as a measure of niche overlap (Emlen, 1973). LITERATURE CITED Burbidge, R. G. 1974. D i s t r i b u t i o n , growth, selective feeding and energy transformations of the young-of-the-year blueback herring, Alosa a e s t i v a l i s (Mitchell) i n the James R., V i r g i n i a . Trans. Am. Fish. Soc. 103(2): 297-311. Cramer, J. D. and Marzolf, G. Rv 1970. Size selective predation by gizzard shad. Trans. Am. Fish. Soc. 99(2): 320-332. Dodson, S. I. 1970. Complementary feeding niches sustained by size selective predation. Limn, and Ocean. 15(1): 131-137. Emlen, J. M. 1973. Ecology: an evolutionarv approach, Addison-Wesley, P h i l i p p i n e s . 493 pages. Ivlev, B. S. 1961. Experimental ecology of the feeding; of fi s h e s . Yale University press, New Haven. 302 pages. Jacobs, J. 1974. Quantitative measurement of food selection. Oecologia 14: 413-417. 

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