Open Collections

UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Social behavior and feeding ability of two phenotypes of Gasterosteus aculeatus in relation to their… Larson, Gary Lee 1972

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
831-UBC_1972_A1 L37.pdf [ 4.99MB ]
Metadata
JSON: 831-1.0101273.json
JSON-LD: 831-1.0101273-ld.json
RDF/XML (Pretty): 831-1.0101273-rdf.xml
RDF/JSON: 831-1.0101273-rdf.json
Turtle: 831-1.0101273-turtle.txt
N-Triples: 831-1.0101273-rdf-ntriples.txt
Original Record: 831-1.0101273-source.json
Full Text
831-1.0101273-fulltext.txt
Citation
831-1.0101273.ris

Full Text

SOCIAL BEHAVIOR AND FEEDING ABILITY OF TWO PHENOTYPES OF GASTEROSTEUS AOULSATUS IN RELATION TO THEIR SPATIAL AND TROPHIC SEGREGATION IN A TEMPERATE LAKE by GARY LEB LARSON B.Sc, University of Washington, 1966 M.Sc, University of Washington, 1969 A THESIS SUBMITTED IN PARTIAL FULFILMENT OP THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of Zoology We accept t h i s thesis as conforming to the required standard In p r e s e n t i n g 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 o f 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 Co lumbia , 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 re ference and s tudy . I f u r t h e r agree t h a t pe rmiss ion f o r e x t e n s i v e copying o f 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 r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l ga in s h a l l not be a l lowed wi thout my w r i t t e n p e r m i s s i o n . Department o f Z o o l o g y The U n i v e r s i t y o f B r i t i s h Columbia Vancouver 8, Canada Date September 1, 1972. i i ABSTRACT S i g n i f i c a n t differences i n morphology, v e r t i c a l d i s t r i b u t i o n , aggressiveness, and feeding habits were found between limnetic and benthic forms of the threespine s t i c k l e -back (Gasterosteus aculeatus) i n a small coastal B r i t i s h Columbian lake. In deep laboratory aquaria, simulating lake pelagic and benthic regions, the benthic form was aggressive, s o l i t a r y , and a t t r a c t e d to bottom cover, i n which i t remained. The l i m n e t i c form, i n contrast, was gregarious, non-aggressive, attr a c t e d to surface cover, and seldom found near the bottom. In shallow aquaria, representing l i t t o r a l lake regions, limne t i c stickleback were aggressive and non-gregarious u n t i l cover was added, whereafter most i n d i v i d u a l s established residence i n i t and became s i g n i f i c a n t l y l e s s aggressive. Benthic stickleback put i n t o shallow aquaria huddled i n small groups u n t i l cover was added, a f t e r which they spaced them-selves out. These r e s u l t s were consistent with differences i n the behavior of the two forms of stickleback i n the f i e l d . The l i m n e t i c form fed heavily on zooplankton i n the lake, while the benthic form mostly ate macro-benthos, Gammarus being i t s p r i n c i p a l prey. Laboratory studies compared the feeding rates of both phenotypes; that of the limnetic form was highest on plankton, while that of the benthic form i i i was highest on araphipods. The extensive d i s s i m i l a r i t i e s demonstrated i n s o c i a l behavior and feeding a b i l i t y of the two forms of stickleback indicate that t h e i r s p a t i a l and trophic segregation In Paxton Lake are due mainly to innate behavioral differences, rather than i n t e r a c t i o n . TA3LE OP CONTENTS ABSTRACT TABLE OP CONTENTS LIST OF FIGURES LIST OF TABLES LIST OF APPENDICES ACKNOWLEDGMENTS INTRODUCTION DESCRIPTION OF STUDY AREA GENERAL METHODS F i e l d Laboratory FIELD RESULTS Physical and Chemical Limnology Zooplankton Macro-Benthos Stickleback Morphology Body size D i s t r i b u t i o n and substrate preference Summer a c t i v i t y Interactions Feeding habits Salmon V LABORATORY RESULTS 40 So c i a l Behavior 40 V e r t i c a l d i s t r i b u t i o n and aggression 40 Adults 40 Pry 42 Individual and pairs with groups 44 Groups of llmnetics with single benthics 51 Cover e f f e c t s 58 D i s t r i b u t i o n and aggression i n shallow water 64 Feeding A b i l i t y 69 The e f f e c t of starvation on hunger 72 Feeding ' 76 On Tublfex 76 On Artemla s a l i n a n a u p l i i 77 On Daphnla 79 On Hy a l e l l a 81 Maximum prey size 84 Manipulation time 84 Time between successful grasps 87 Search time 87 Search time amongst cover 91 DISCUSSION 91 LITERATURE CITED 107 APPENDIX 111 v i LIST OP FIGURES FIGURE Page 1. Paxton Lake, B r i t i s h Columbia, showing 4 lo c a t i o n of the limn o l o g i c a l stations (^ ), bottom trap area, dock, pump, outlet dam and boundary between the basins ( ). Depth contours i n meters. 2. Water l e v e l f l u c tuations i n Paxton Lake f o r 6 1969 (0), 1970 ( A ) , and 1971 ( A ) compared with the mean and range of 1957 - 1968 ( ' 9 i ). Based upon data provided by Texada Mines Ltd. 3. Isotherms (°C) i n basin 1 of Paxton Lake f o r 11 1969 and 1970. 4. Mean number of zooplankton per m-* f o r 0 - 5 m 12 tows along the bottom trap area i n Paxton Lake during 1969 and 1970. R o t i f e r data are i n units of 102, the other genera are 10^. 5. Mean body lengths of the crustacean zooplankton 14 i n Paxton Lake from May - September 1969. and July - October 1970. Symbols: copepod ( a ) , Bosmlna (a), Polyphemus ( 9 ) , and Daohnla (0). 6. Mean c u r l length (95 per cent confidence l i m i t s ) 17 of Gammarus l a c u s t r i s i n Paxton Lake from June -October, 1970. Numbers r e f e r to sample s i z e s . 7. Relationship between the body length and body 20 weight of benthic ( A ) and limnetic (X ) threespine stickleback i n Paxton Lake. 8. Relationship between the body length and jaw 21 length of benthic (——) and limnetic ( ) threespine stickleback i n Paxton Lake. Data points are not presented but the l i n e s ( f i t t e d by regression analysis) are based upon 95 benthic and 52 limnetic, f i s h . 9. Relationship between the body length and mouth width of benthic (®) and limnetic (0) threespine stickleback i n Paxton Lake. 22 v i i FIGURE Page 10. Relationship between the body length and the r a t i o 23 of the mm of the l e f t eye visable from the v e n t r a l side of the fishes head (VE) to the body length (SBL) of the Paxton Lake benthic ( A ) and limnetic (X) stickleback. 11. Frequency d i s t r i b u t i o n of standard body lengths of 24 Paxton Lake stickleback captured i n 1969 - 1971. 12. D i s t r i b u t i o n of Paxton Lake limnetic (hatched bars) 26 and benthic ( s o l i d bars) stickleback i n 1969 and 1970 taken i n surface and bottom traps. Data are presented as per cent frequency of occurrence. Numbers i n parentheses give the sum of the number of f i s h captured/trap/depth zone. 13. Relationship between the per cent vegetative cover 30 (Chara) and the t o t a l number of limnetic and benthic stickleback/m2 i n the l i t t o r a l observation areas at Paxton Lake i n 1970. 14. Relationship between the t o t a l number of benthic 32 and limnetic stickleback/m^ i n the l i t t o r a l observation areas at Paxton Lake, July 17 - 18, 1970. 15. Summer d i e l a c t i v i t y of Paxton Lake stickleback 33 captured from bottom and suspended trap samples. Data are expressed as the number of f i s h captured per three hour i n t e r v a l s during 24 hour periods except f o r pooled data between 00 and 06 hours. The i n i t i a l time of each i n t e r v a l i s given. 16. Feeding habits of Paxton Lake stickleback, 1969 37 and 1970. Data are expressed as per cent frequency of occurrence and numbers without parentheses indicate the number of stomachs containing each prey type. Numbers i n paren-theses r e f e r to the number of f i s h examined. The abbreviations are defined as: zooplankton (hatched bars) - copepods (Z), Daphnia and Bosmina (D), Polyphemus pedicuius (P); macro-benthos ( s o l i d bars) - Gammarus l a c u s t r i s (A), ostracods (0), chironomid larvae (0), baetid nymphs (B), Sphaeriidae (S), gastropods (G), and miscellan-eous (open bar;M). v i i i FIGURE Page 17. Relationship between the body length of Paxton 38 Lake benthic stickleback and the average number of zooplankton consumed per f i s h size class (bars). Data points are also given. 18. Feeding habits of coho salmon i n Paxton Lake, 41 1969 to 1971. Data are expressed as per cent frequency of occurrence and numbers without parentheses indicate the number of stomachs containing each prey type. Numbers i n parentheses r e f e r to the number of f i s h examined. The abbreviations are defined as: zooplankton (hatched bars) - Polyphemus pedlcuius (P), Daphnia (D); macro-benthos ( s o l i d bars) - Gammarus l a c u s t r i s (A), Gastropods (G), chironomid larvae (0), Chaoborus L); insects ( I ) , and stickleback (S). 19. Total d a i l y number of Paxton Lake benthic and 45 limne t i c stickleback f r y i n the upper halves of 35 1 aquaria. 2 0 . Average., time subordinant benthic and l i m n e t i c 50 stickleback spent at j a r groups without and with dominant fishes present i n aquaria. Symbols: Dom - dominant, Sub - subordinant, B - benthic, L - l i m n e t i c . 21. Average number of aggressive acts (nips) between 52 subordinant and dominant Paxton Lake stickleback i n the paired rover t e s t s . Data compares benthic-vs benthic, limnetic vs l i m n e t i c , and benthic vs limnetic rovers. Symbols: Dom - dominant, Sub -subordinant, L-limnetic, and B - benthic. 22. Relationship between the t o t a l number of nips to 53 f i s h i n the jars when benthic rovers were s o l i t a r y and the number of nips they made to other rovers i n paired t e s t s . 23. Total number of limnetic stickleback i n the 57 upper h a l f of a standard 339 1 aquarium per session without and with i n d i v i d u a l benthics of d i f f e r e n t body lengths In test 1 and t e s t 2. 24. Relationship between the mean number of l i m n e t i c 59 stickleback i n the upper h a l f of a 339 1 aquarium and the mean number of benthic nips to limnetics each day. Symbols: test 1 ( 0 ) , test 2 (0). i x FIGURE Page 25. Aggression between single benthics of d i f f e r e n t 60 lengths and groups of 20 limnetics, Benthic data presented as the mean number of nips to limneti c stickleback each day; limnetic data as the mean nips to benthic stickleback each day (test 1 (®), t e s t 2 (0). 2 6 . T o t a l number of benthic stickleback i n the upper 62 h a l f of the standard 339 1 aquarium each day without and with cover i n the lower and upper halves of the water column. Maximum possible t o t a l count i n the upper h a l f each day would be 75. 27. The e f f e c t s of cover and benthic aggression on 63 the t o t a l number of limnetic stickleback i n the upper h a l f of standard 339 1 aquaria each day. Maximum possible t o t a l count i n the upper h a l f each day would be 120. 2 8 . Relationship between l i g h t i n t e n s i t y (lux) at the 66 water surface and the t o t a l number of limnetics together i n groups of 4 or more each day i n shallow (X) and deep (A) aquaria.'Maximum possible t o t a l number of f i s h together each session would be 98. 29. Relationship between l i g h t Intensity (lux) at the 67 water surface and the t o t a l number of nips by limnetics each day i n the shallow aquarium (test 1 (X), test 2 (-*)). 30. The e f f e c t s of cover and an i n d i v i d u a l benthic on 68 the aggression (nips) and aggregation behaviors of li m n e t i c stickleback i n shallow aquaria. Symbols as t o t a l each day f o r : number together i n groups of 4 or more (9~~—#), number i n tank section where cover was placed (0 0), limnetic to li m n e t i c aggression (A A) $ a n ( i benthic to limneti c aggression ( • — • ) . 31. The e f f e c t s of cover on benthic stickleback 71 aggregation behavior i n the shallow aquarium. Symbols as t o t a l each day f o r : number together i n groups of 4 or more (9 »), and number i n tank section where cover was placed ( 0 — 0 ) . Maximum possible together each session would be 98. X FIGURE Page 32. Relationship between hours of deprivation and 74 •weight (gm) of Tub!fex eaten by three 69 mm benthic stickleback. A l l data are proportioned to the heaviest i n d i v i d u a l (4.5 gm). 33* Relationship between hours of deprivation and 75 the proportion of the maximum food r a t i o n eaten f o r a l l limnetic and benthic stickleback up to 24 hours (p<.0 .05» ), and i n d i v i d u a l benthics (69 mm(®),£59 mm (n)) and limnetics (x) at 48 hours. 34. Relationship between number of grasps per feeding 78 session and index of stomach f u l l n e s s at the end of the feeding session f o r benthic ( 9 ) and limnetic (0) stickleback. 35. Relationship between f i s h length and i n t e r v a l 80 between grasps, manipulation time, and search time f o r benthic ( 9 ) and limnetic (0) stickleback feeding on Artemia s a l i n a n a u p l i i . 36. Relationship between f i s h length and i n t e r v a l 82 between grasps, manipulation time, and search time f o r benthic (9) and limnetic (0) s t i c k l e -back feeding on Daphnia. 37. Relationship between prey density and i n t e r v a l 83 between grasps f o r the 60 mm limnetic (0) and the 51 mm benthic (@) stickleback feeding on Daphnia. Ninety-five per cent confidence l i m i t s are included. 38. Relationship between f i s h length and maximum 85 size of H y a l e l l a consumed by limnetic (A) and benthic (#) stickleback. Projecting l i n e s i n d i c a t e the next a v a i l a b l e prey s i z e which was not eaten. 39. A comparison between H y a l e l l a length and manipulation 86 time f o r benthic and limneti c stickleback of s i m i l a r s i z e . Data f o r a large benthic are also presented. Symbols: benthics - 48 mm (*>), 51 mm ( r ) , 54 mm (v), 59 mm ( 9 ) , and 69 mm (0); limnetics - 54 mm (*•), 58 mm («a), and 60 mm (<i). 40. Relationship between mouth width and slopes of 88 x i FIGURE Page manipulation time vs amphipod size (data from Pig. 39) f o r benthic ( A ) and limnetic (x) stickleback. 41. Relationship bet-ween H y a l e l l a length and time 89 between successful grasps f o r benthic and limnetic stickleback of s i m i l a r and d i f f e r e n t s i z e s , Symbols: benthics - 48 mm (+), 51 mm (x), 59 mm (§), 64 mm (P ), and 69 mm (0); limnetics - 54 mm (^7), 58 mm ( A ) , and 60 mm ( A ). 42. Relationship between f i s h body length and 90 amphipod length consumed at a feeding rate of 10 seconds f o r benthic (®) and limnet i c (0) stickleback. 43. Relationship between search time and Hy a l e l l a 92 length f o r benthic and limnetic stickleback of s i m i l a r and d i f f e r e n t lengths. Symbols: benthics - 48 mm (+), 51 mm (x), 59 mm (®)» 64 mm (•), and 69 mm (0); limnetics - 58 mm ( A ) and 60 mm ( A ) . 44. Mean search time amongst l e a f l i t t e r f o r benthic 93 (#) and limnetic (0) stickleback between 30 and 75 mm long. LIST 0? TABLES TABLE Page 1. Total number of macro-benthic organisms/m 2 15 c o l l e c t e d from samples taken at 1-m depth i n t e r v a l s from 0 - 8 m i n Paxton Lake, 1970. 2 . Depth d i s t r i b u t i o n of Gammarus l a c u s t r l s /m2 16 along the bottom trap area (0 - 8 m ) i n Paxton Lake from June to October, 1970. One sample was taken per month. 3. Some morphological differences between adult 18 benthic and limnetic threespine stickleback i n Paxton Lake. 4. Depth and substrate preference of Paxton Lake 28 stickleback i n summer 1970. Data presented .as average number of f i s h observed per 10 m2. About 1000 m2 of lake bottom were surveyed each month. 5. Number of stickleback observed together In 29 standard shore and cover type areas of Paxton Lake during summer, 1970. 6. Mean size of plankters consumed by Paxton Lake 39 stickleback. 7. Mean number of Paxton Lake stickleback i n upper 43 and lower halves of standard 339 1 aquaria. 8 . S t a t i s t i c a l analysis of the time s o l i t a r y 48 benthic and limnetic stickleback (rovers) spent with f i s h of e i t h e r phenotype In the j a r s . 9. S t a t i s t i c a l analysis of aggressive a c t i v i t y of 49 s o l i t a r y benthic and limnetic stickleback (rovers) toward f i s h of eith e r phenotype i n the j a r s . 10. S t a t i s t i c a l analysis of the aggressiveness of 54 benthic stickleback captured from shallow and deep areas i n Paxton Lake, 1970. x i i i LIST 0? APPENDICES APPENDIX Page 1. Mean rations of Tubifex and per cent body weight 111 of food eaten at 12, 24, 48, and 72 hours of deprivation by benthic and limnetic stickleback. 2. Regression analysis of manipulation times on 112 Hy a l e l l a by Paxton Lake stickleback. 3. Selected examples of v a r i a t i o n around the mean 113 size of Hya l e l l a i n the time between successful grasps experiment. ACKNOWLEDGMENTS The National Research Council provided f i n a n c i a l support. I am pleased to express gratitude to my supervisor, Dr. Thomas Gordon Northcote, for h i s guidance and c r i t i c i s m s throughout the study. Special thanks are given to Dr. J. D. McPhail and Mr. R. Jones f o r t h e i r assistance i n the f i e l d and discussions of t h e i r studies of Paxton Lake stickleback. Thanks are given to the B r i t i s h Columbia Fish and W i l d l i f e Branch f o r permitting use of t h e i r f i e l d equipment. I extend my sincere appreciation to Mr. R. Sjolund f o r his e f f o r t s i n the f i e l d and many hours l o c a t i n g f i e l d equipment. Mrs. A. Bryan i d e n t i f i e d Gammarus l a c u s t r l s . Drs. C. Wehrhan, P. A. Larkin, and J . Bryan helped analyse the data. Drs. C. J . Walters, D. Chitty, N. L i l e y , I. E f f o r d , G. Scudder, and G. Orlans k i n d l y gave t h e i r time to discuss aspects of the r e s u l t s . I appreciated t h e i r help. I thank Dr. N i l s - A r v i d Nilsson f o r h i s suggestions when the study f i r s t began. Drs. C. S. H o l l i n g , D. Chitty, and G. Scudder, read the manuscript and offered many valuable suggestions. To my wife, Ingrld, my sincere gratitude for her assistance and encouragement. 1 INTRODUCTION Many people have studied competition f o r food,space, or "both among fishes In temperate waters (Larkin, 1956). Some authors agree that competition Is r e a l (e.g., Nilsson, 1967); others question whether i n t e r s p e c i f i c competition e x i s t s to any measurable extent, and suggest natural s e l e c t i o n favors species tolerant to the unstable environment (e.g., Larkin, 1956). Part of the disagreement l i k e l y develops from the i n a b i l i t y to define the niche of each species, and also the supply and demand of the resources competed f o r . This information i s necessary f o r an accurate i n t e r p r e t a t i o n of competitive.situations i n the f i e l d . A f t e r an extensive l i t e r a t u r e review, and many years of f i e l d study of a l l o p a t r i c and sympatric populations of a r t i c char (Salvelinus alpinus) and brown trout (Salmo trutta) Nilsson (1955, ' 6 0 , ' 6 3 , '65, and '67) developed the hypothesis that c l o s e l y related sympatric species of f i s h i n temperate waters probably segregate through competitive i n t e r a c t i o n . Using the nomenclature of Brian (1956), Nilsson categorized the segregation as i n t e r a c t i v e , l»e, the r e s u l t of i n t e r a c t i o n (competition) between two or more species f o r the same environ-mental resources. He suggested that overlap i n ec o l o g i c a l requirements emphasized differences between the species, r e s u l t i n g i n a refined p a r t i t i o n i n g of the environmental resources available to each. Nilsson also ref e r s to another 2 form of segregation i n f i s h e s , termed s e l e c t i v e , i . e . , the species s e l e c t t o t a l l y d i f f e r e n t habitats by i n s t i n c t i v e behavior. Selective segregation may be the t h e o r e t i c a l end-r e s u l t of i n t e r a c t i v e segregation, and may best apply to sympatric fishes i n t r o p i c a l waters (Nilsson, 1967). The discussion above only ref e r s to those forms which, because they r e t a i n t h e i r i d e n t i t i e s through reproductive I s o l a t i o n , are c l a s s i f i e d as species. Relating the discussion to sympatric subspecies of the same species i s d i f f i c u l t because they lack the necessary i s o l a t i n g mechanisms to prevent gene mixing (Mayr, 1965). The occurrence of sympatric subspecies i s possible; however, t h e i r segregation i s l i k e l y maintained p r i m a r i l y by i n t e r a c t i o n , i . e . , i n t r a s p e c i f i c competition (Mettler, et a l . 1969). Two major problems often facing an analysis and Interpretation of such competitive s i t u a t i o n s are: ( l ) lack of knowledge of whether the species or subspecies evolved i n a l l o p a t r y or sympatry; (2) i d e n t i f i c a t i o n of s p e c i f i c status between sympatric forms, i . e . , species, subspecies, or races. Nevertheless, i n v e s t i g a t i n g the segregation of sympatric forms of the same species or genus may lead to a better understanding of the role of competition i n evolution, because many of the differences seen from a l l o p a t r i c populations may have evolved through sympatric competition. 3 An opportunity arose to study the s p a t i a l and trophic segregation of two "forms" of stickleback inhabiting a lake. The s p e c i f i c status of these forms, be they species or sub-species, i s uncertain, but i s discussed l a t e r . Quantitative f i e l d observations and c o l l e c t i o n s were made of feeding habits, d i s t r i b u t i o n , and s o c i a l behavior of both forms of stickleback. Abundance and d i s t r i b u t i o n of food, and d i s t r i -bution of vegetative cover and other l i m n o l o g i c a l character-i s t i c s of t h e i r habitats were also measured, laboratory experiments were conducted to quantify the s o c i a l behavior and feeding a b i l i t y of the two forms held separately and together. With these laboratory data I hoped an accurate i n t e r p r e t a t i o n could be made of the competitive s i t u a t i o n between the two stickleback forms In the lake. In addition, contemporaneous studies of the e c o l o g i c a l genetics and habitat s e l e c t i o n of these forms by Dr. J. D. McPhail and Mr. R. Jones , when completed, may contribute to an o v e r a l l d e f i n i t i o n of the segregation mechanisms involved. DESCRIPTION OP STUDY AREA Paxton Lake l i e s at an elevation of 61 m on Texada Island, a small coastal i s l a n d In Georgia S t r a i t , B r i t i s h Columbia (Pig. 1). The lake i s divided i n t o two basins. I t s outlet o r i g i n a l l y had access to the sea, but was dammed late i n 1956, r a i s i n g the water l e v e l of the lake approximately 1.5 m. Since 1957 water has been pumped from the system f o r i n d u s t r i a l purposes r e s u l t i n g i n marked annual f l u c t u a t i o n s of the water 4 Pig. 1. Paxton Lake, B r i t i s h Columbia, showing location of the limnological stations ( • ) , bottom trap area, dock, pump, outlet dam and boundary between the basins ( ). Depth contours i n meters. 5 l e v e l ( F i g . 2). In 1969 the f l u c t u a t i o n was nearly normal except for the draw-down In the f a l l . The water l e v e l was abnormally low i n 1970, but high i n 1971. The surface area of the lake at a l e v e l of 48 cm (Fig. 2) i s 17 ha., 10 and 7 ha. f o r basins 1 and 2, r e s p e c t i v e l y . The maximum depth i s 16 m* Trees and brush along the o r i g i n a l shoreline to the present high water l e v e l are dead as a r e s u l t of damming. Many dead trees have f a l l e n i n t o the lake, providing a d d i t i o n a l l i t t o r a l cover. Physical c h a r a c t e r i s t i c s of the l i t t o r a l areas vary with water l e v e l . At high water t e r r e s t r i a l plants dominate the vegetation, but at lower water l e v e l s the l i t t o r a l regions may be covered by aquatic vegetation (predominately Chara), marl, or ooze. In 1969 Chara began to grow along the shoreline once the water l e v e l s t a b i l i z e d between September and October. In 1970 tlie water l e v e l was more stable and Chara grew to depths of over 3 m. In August, 1971» high water l e v e l prevented Chara growth, however, the previous years stand was s t i l l evident between 3 and 6 m . Cutthrout trout (Salmo c l a r k l ) and stickleback apparently are indigenous to the lake. The former was almost completely exterminated when the lake was drawn-down to 11 m i n 1957. In 1968, five-thousand coho (Oncorhynchus MONTHS Pig. 2. Water l e v e l fluctuations i n Paxton Lake for 1969 (0), 1970 ( A ) , and 1971 ( A ) compared with the mean and range of 1957 - 1968 ( i — • — i ) Based upon data provided by Texada Mines Ltd. 7 klsutch) f i n g e r l i n g s were planted i n the lake. GENERAL METHODS F i e l d Observations of stickleback d i s t r i b u t i o n and i n t e r -action were made with a viewing box from the dock, the shoreline, and a boat. The maximum viewing depth was about 3.5 m. Sticklebacks weire sampled by dipnet and with wire minnow traps (No. 12562, Canada Fishing Tackle and Sports Ltd.) every three hours f o r a 24 hour period at approximately monthly i n t e r v a l s . The traps were placed along the lake bottom and also suspended from f l o a t i n g cover ( i . e . , the dock and trees) to c o l l e c t samples of benthic and limnetic forms of stickleback. Most bottom samples of f i s h were taken within a single sampling area ( F i g . 1). The traps were placed on the lake bottom at 1 - a depth i n t e r v a l s from 0 - 9 m. Suspended traps were placed about 0.5 m below the lake surface. Only f i s h greater than 30 mm i n length were captured.by the traps. Coho salmon and cutthrout trout were c o l l e c t e d by angling and g i l l n e t t i n g . Plankton was sampled with a no. 10 Wisconson type net (25 cm mouth diameter) i n 1969 and with standard Clarke-Bumpus gear i n 1970 (no. 10 net). Most samples were taken over the bottom trap area, but some were taken near the 8 dock and at the lim n o l o g l c a l stations (Fig. 1). Horizontal tows were made at the surface and v e r t i c a l tows from 0 - 5 m i n each area. Usually the samples were enumerated by t r i p l i c a t e subsamples and the r e s u l t s averaged. In some cases the entire sample was counted. Body measurements of crustacean zooplankters were made along the maximum carapace axis of 30 randomly selected i n d i v i d u a l s of each genus (Brooks and Dodson, 1965; Galbraith, 1967). Bottom fauna was sampled with an Ekman dredge (175 cm-). Samples were taken along the bottom trap area at 1 - m depth i n t e r v a l s from 0 - 8 m between June and October, 1970. Most f i n e debris was removed at the lake by washing the samples i n a bucket with a screen bottom (mesh size 1 mm). Each sample was l a t e r sorted three times f o r organisms. The organisms were preserved i n 10 per cent formalin. Growth rate of the amphipod Gammarus l a c u s t r l s was estimated by measuring i t s " c u r l length". This species curled when attacked by stickleback and when preserved after, c o l l e c t i o n . Curl length was measured by holding the cephalothorax p a r a l l e l to the l a s t p osterior abdominal appendage and taking the distance between these structures. Water temperature, oxygen concentration, and conductance were usually recorded at the lim n o l o g i c a l stations ( F i g . 1 ) , although some readings of temperature and oxygen were also 9 taken along the l i t t o r a l areas close to the lake bottom. Water temperatures were recorded with a thermister (model 44TB - Y. S. I. Co. Inc.) or with a F. B. A. oxygen- tem-perature probe. Dissolved oxygen concentrations were determined by the winkler method i n 1969 and with a P. B. A. oxygen-temperature probe i n 1970. Conductance was determined with an I n d u s t r i a l Instrument Co. meter, model RB2 - 3341. Laboratory In d i v i d u a l f i s h were kept i n 35 1 glass aquaria (51 x 25 x 28 cm). Groups of f i s h were kept i n 339 1 wood aquaria (118 x 48 x 60 cm) with glass f r o n t s . The aquaria were cleaned at monthly i n t e r v a l s . To reduce f i s h mortality 2 - 3 oz of rock s a l t and 2 - 3 drops of malachite green were added to each tank a f t e r water was renewed. Each aquarium had an insid e f i l t e r and a i r was continually supplied. Water tem-perature was maintained at 15° 0 (± 2°). A 12 hour l i g h t and dark photoperiod was controlled automatically. Light i n t e n s i t i e s were measured at the surface of the water with a photometric c e l l (G. M. Submarine Photometer, model 15 - m -02/1). Stomach analysis of the f i s h followed the per cent frequency of occurrence method outlined by Hynes (1950). Standard body length was taken f o r each f i s h . FIELD RESULTS Physical and Chemical Limnology Seasonal changes of water temperature i n 1969 and 1970 10 were s i m i l a r ( F i g . 3). In both years the maximal surface temperatures (about 23° C) occurred between June and July. The thermocline was usually between 2 to 5 m during summer. In winter water temperatures dropped to about 5° C. Oxygen was present throughout the water column and along the l i t t o r a l bottom regions during winter and early summer, but was much reduced or absent below 7 m from mid- summer to the f a l l turnover. HgS was detected below 7 m i n July and August, 1969. The surface conductivity of Paxton Lake was-high f o r a coastal lake (260 jxVthos/ cm5 at 25° C on May 23, 1969; see Northcote and Larkin, 1956). The surface pH was s l i g h t l y above n e u t r a l i t y (7.7 on November 16, 1970). Of the major cations (Na, K, Mg, and Ca) calcium was predominant (58.2 mg/l), the others being l e s s than 2.5 mg/l when determined from surface water taken on November 16, 1970. Zooplankton The zooplankton was dominated numerically by clado-cerans, e s p e c i a l l y Bosmina sp., Paphnia sp., and Polyphemus  pedicuius. Oopepods and r o t i f e r s were also present but not i d e n t i f i e d . The population densities of a l l genera were high i n 1969, Bosmina being the most abundant with a maximum density of about 105,000/ m3 i n August ( F i g . 4). However, a l l genera declined i n abundance i n 1970, except Paphnia 11 A M , J J A S D N D J F M A M J J A S D 1969 1970 MONTHS Pig. 3. Isotherms (°0) i n basin 1 of Paxton Lake f o r 1969 and 1970. 12 50-i 25 -UJ CD ID Z 60 5 75 25 ^ copepods rotifers - NBosmina / N / ^ — * — i • • Daphnia \ • A. .Polyphemus : . / - X - - . W \ M J J A S O N D J F M A M J J A S O 1969 1970 Pig. 4. Mean number of zooplankton per m^  f o r 0 - 5 m tows along the bottom trap area i n Paxton Lake during 1969 and 1970. R o t i f e r data are i n units of 10*, the other genera are 103. 13 and Polyphemus. Japhnla was the most abundant form i n 1970 reaching a maximum mean density of about 6 0 , 0 0 0 / m i n J u l y . The mean body size of most zooplanlcters was s i m i l a r i n both years studied (Pig. 5 ) . In 1970 i n d i v i d u a l Japhnia were four times l a r g e r than i n 1969, reaching up to 1.3 mm i n length. Macro-benthos The macro-benthos was dominated i n number and probably biomass by a single species of amphipod, Gammarus l a c u s t r i s (Table 1 ) . Most amphipods were found i n the Ohara, which was di s t r i b u t e d along the l i t t o r a l areas (Table 2). The average size of Gammarus (Pig. 6) approximately doubled between June (2 . 6 mm) and October ( 4 . 9 mm). Stickleback Morphology Many phenotypes of threespine stickleback occur i n Paxton Lake, The predominant forms, "limnetics" and "benthics", occur i n the pelagic and benthic regions of the lake respective-l y . The other forms are not abundant and appear to be i n t e r -mediate between limnetics and benthics ( J . E. McPhail, personal communication). Limnetics and benthics d i f f e r i n the number of body plates, number of g i l l rakers, and the presence or absence of a p e l v i c g i r d l e and pe l v i c spines (Table 3 ) . Benthics 14 1 " 5 + 1 9 6 9 1 9 7 0 MONTHS MONTHS Pig. 5. Mean body lengths of the crustacean zooplankton i n Paxton Lake from May - September 1969, and July - October 1970. Symbols: copepods ( a ) , Bosmina ( • ), Polyphemus ( f ) , and Daphnla (0). 15 Table 1. To t a l number of macro-benthic organisms/m col l e c t e d from samples taken at 1-m depth i n t e r v a l s from 0 - 8 m i n Paxton Lake, 1970. Month Gammarus Chironomid Baetid Gastropods Sphaeriidae l a c u s t r i s larvae nymphs June 3168 88 44 220 178 July 624 88 0 44 44 August 3679 44 0 44 0 September 4011 0 0 220 0 October 1100 44 44 44 44 16 Table 2. Depth d i s t r i b u t i o n 1 of Gammarus l a c u s t r i s / m~ along the bottom trap area (0 - 8m) i n Paxton Lake from June to October, 1970, One sample was taken per month. Depth June July August September October Mean Per cent m Occurrence 0 3124? 360 1012 3327 308 1626 80.4 1 0 0 0 0 528 106 5.2 2 0 44 44 44 44 35 1.7 3 44 264 220 88 88 141 7.0 4 0 0 88 352 132 114 5.7 5 0 0 0 0 0 0 0.0 6 0 0 0 0 0 0 0.0 7 0 0 0 0 - 0 0.0 8 0 0 0 0 0 0.0 1 Depths not adjusted f o r fluctuations of water l e v e l . 2 V e r t i c a l l i n e represents the depth d i s t r i b u t i o n of Chara. 17 x i — z: n a o_ i—i X 0_ 5 . . A . . 3. 2 . 1 . 0 . 3 2 i J N J U 31 4 3 A U S E 1 9 7 0 MONTHS 2 2 •C F i g . 6. Mean c u r l length (95 per cent confidence l i m i t s ) of Gammarus l a c u s t r l s i n Paxton Lake from June -October, 1970. Numbers r e f e r to sample s i z e s . Table 3 . Some morphological differences between adult benthic and l i m n e t i c threespine stickleback i n Paxton Lake. Phenotype Number of Number of Pelvi c P e l v i c Number of dorsal spines body plates g i r d l e spines g i l l rakers Benthic 2 - 3 0 absent absent 1 1 - 2 1 Limnetic 3 8 - 18 present present 18 - 25 1 Data provided by R. L. Jones. 19 had s i g n i f i c a n t l y (p<0.05; analyses of covariance) heavier bodies, longer jaws, and wider mouths than limnetics of the same body length (Pigs. 7, 3, and 9). The perceptual f i e l d s of the two forms were not determined, but positioning of the eyes d i f f e r e d between the phenotypes. Limnetics probably could see w e l l i n f r o n t a l , caudal, dorsal, and ventral directions because of t h e i r narrow bodies and protrusive eyes, as could benthics l e s s than about 55 mm i n length. However, f o r l a r g e r benthic stickleback, v i s i o n was oriented more to dorsal and f r o n t a l d i r e c t i o n s . Their v e n t r a l v i s i o n (as indicated by the mm of eye visable from the v e n t r a l side of the head) was usually l e s s than that f o r limnetics of the same length and was apparently n e g l i g i b l e above lengths of about 85 mm (Pig. 10). Body siz e Benthic stickleback frequently attained body lengths of over 65 mm, while limnetics seldom grew l a r g e r than 60 mm (Pig. 11). In 1970 and 1971, mean body lengths of both phenotypes decreased from those observed i n 1969 (Pig. 11). Young-of-the-year of both forms attained between 30 - 40 mm i n length by October. D i s t r i b u t i o n and substrate preferences Both phenotypes bred near the shoreline between A p r i l and early J u l y . Nests of benthic stickleback were located i n cover, 20 ID 3Z ID M Ld >-• • 7 - 0 . B - 0 . 5 - 0 . 4 - 0 . 3 - 0 . B » 0 l 6:e 0 - 8 0 - 7 0 ' 0 -0 -6 5 4. 0 - 3 . 0 - B . 0 - 1 . AA AA X AA> X 4fe X ^ X 4? X ' x-E O S O 4 0 5 0 B O 7 0 B O BODY L E N G T H (MM) Pig. 7. Relationship between the body length and body weight of benthic (A ) and limnetic (X) threespine stickleback i n Paxton Lake. 21 - B O . . 2 0 3 0 4 0 5 0 BO 7 0 B O BODY L E N G T H (MM) Pig. 8. Relationship between the body length and jaw length of benthic (—) and limnetic ( ) threespine stickleback i n Paxton Lake. Data points are not presented but the l i n e s ( f i t t e d by regression analysis) are based upon 95 benthic and 52 limnetic f i s h . 22 I Q I I-3 o 2 8 7 J 6 5 4 3 2 O • •* o CD O O O o CO o • © 0 o 20 30 4 0 5 0 60 7 0 8 0 BODY LENGTH (MN/fi Flgv 9. Relationship between the body length and mouth width of benthic (•) and limnetic (0) threespine stickleback i n Paxton Lake. 23 •03+ m \ UJ > •OS... •01.. x x X X X X X •00. 30 AO 50 60 70 90 BODY LENGTH (MM) Pig. 10. Relationship between the body length and the r a t i o of the mm of the l e f t eye visable from the ventral side of the fishes head (VE) to the body length (SBL) of the Paxton Lake benthic ( A ) and limnetic (X) stickleback. 24 BENTHIC LIMNETIC 3 0 75 SO IO IO-IO IO IO IO IO >IGTH IT* O O 1/v O O ci | ir> i o i o 1 1 o o o o m m •V 3> in o o JULY 22 1909 AUGUST 5 AUGUST 28 SEPT NOV I960 T O MAY 1970 J U N E TO J U L Y AUGUST SEPT O C T AUGUST 1971 MM 11. Frequency d i s t r i b u t i o n of standard body lengths of Paxton Lake stickleback captured i n 1969 -1971. 25 and limne t i c nests were found i n open regions ( J . JJ. McPhail, personal communication). After spawning, most limnetic stickleback i n 1969 formed large aggregations near the lake surface, usually associated with surface cover such as trees or the dock (Fig. 12) . In 1969, benthic stickleback were usually taken at the lake bottom, i n a l l but the deoxygenated regions below 7 m, where HgS was present ( F i g . 12) . They were observed usually as s o l i t a r y i n d i v i d u a l s amongst the Chara along the l i t t o r a l zone, but small groups occurred near sunken logs. Benthics sometimes were found with aggregations of limnetic stickleback. Such i n d i v i d u a l s were generally l e s s than 60 mm i n length. Both phenotypes were captured during winter, but only i n the bottom traps. Most limnetics were captured i n the deepest traps, and most benthics i n the shallowest ones (Fig. 12) . The summer d i s t r i b u t i o n of both phenotypes was altered i n 1970 from that observed i n 1969 (Fig. 12) . Limnetics were much closer to the shoreline, r a r e l y formed large aggregations i n open water, and moved near the shore when t r a v e l l i n g from one area of cover to another. Benthic stickleback were In shallower water than i n 1969 and most were associated with cover along the lake bottom. Information on stickleback depth and substrate 26 DEPTH SUMMER FALL WINTER SUMMER FALL PER CENT OCCURRENCE . 5.0 . . 5.0 . . 5.0 . 5.0 . 5.0 SURFACE \\\\\\ ^ 6 . 1 ) Pi (32.0^ I (7O.5) % O - 2 5 3 - 4 6 7 + a / -r 1 (283.3) ([59.5; JULY AUG AUG 28 ' NOV SEPT MARCH Pig. 12. D i s t r i b u t i o n of Paxton Lake limnetic (hatched bars) and benthic ( s o l i d bars) stickleback i n 1969 and 1970 taken i n surface and bottom traps. Data are presented as per cent frequency of occurrence. Numbers i n parentheses give the sum of the number of f i s h captured/trap/depth zone. 27 preferences i n summer were made by counting f i s h i n standard portions of the shoreline representing d i f f e r e n t cover types and i n f i v e l i t t o r a l areas near the dock. . Data from the shore surveys demonstrated segregation i n depth and substrate habitat between the phenotypes (Table 4). Limnetics were usually i n shallower water than large benthics and associated with Ohara or branch substrate. Small benthics were s i m i l a r to limneti c stickleback i n depth d i s t r i b u t i o n and substrate preference except they often occupied open mud areas i n greater d e n s i t i e s . Large benthics were deeper than limnetics and near mud with cover areas. The extension of Chara from 0.5 m i n July to about 3 m by September increased the amount of bottom cover, apparently permitting limnetics and small benthics to occupy deeper water (Table 4).In addition, increased cover appeared to aggregate limnetic stickleback and disperse benthic stickleback (Table 5). Fis h i n f i v e l i t t o r a l areas near the dock (data not shown) were s i m i l a r i n substrate preference to those recorded i n the shore surveys (Table 4). However, aggression ifrom large benthics influenced the d i s t r i b u t i o n of limnetic stickleback i n the l i t t o r a l areas. In July large benthics frequently chased l i m n e t i c s , f o r c i n g them to seek refuge above Chara , where large benthics were l e s s numerous ( F i g . 13). 28 Table. 4. Depth and substrate preferences of Paxton Lake stickleback i n summer, 1970. Data presented as average number of f i s h observed per 10 m2. About 1000 m2 of lake bottom were surveyed each month. July August September Depth (m) L 1 B650 2 3>50 L B£50 B>50 L B<50 shore - 1 7.8 4.1 0 . 7 3.0 0.8 0.3 4.0 2.2 0 1 - 2 0.1 12.5 0 .6 2.9 0.3 0.4 19.1 3.2 1.4 2 - 3 0.3 92.5 2 . 3 0 . 5 0.1 0 .6 7.0 1 .3 2.2 Substrate Above Chara 9.2 1.1 0 2 .7 0.3 0 30.8 3.8 0 In Chara 0 1.5 0.1 0 0.1 0 0 0 0 Edge of Chara 0 0.4 1.3 0 0 0 . 3 0 0 1.2 Open mud 0 91.1 0.2 1.0 0 . 6 0 0 0.9 0 Mud with cover 0 14 .3 34.0 0 0.2 0.8 0 1.4 2 . 3 Branches 0 0 0 7.1 3 .6 0.1 0 0 0 1 Limnetic stickleback 2 Benthic stickleback less than or equal to 50 mm i n length 29 Table 5. Number of stickleback observed together i n standard shore and cover type areas of Paxton Lake during* summer, 1970. Number Together Month Stickleback Form 1 2 3 * 5 5 Limnetic 1 1 1 0 0 1 July Benthic 50<mm 20 12' 6 ' 0 2 5 Benthic 50>mm 1 1 2 4 2 1 0 Limnetic • 0 1 3 0 0 7 August Benthic 50<mm 31 2 2 0 0 0 ' Benthic 50>mm 35 0 0 0 0 0 30 500i [ ;300 u < OQ LU _J u ^ i 0 0 6-4-2 O lOOi a UJ CO I I 50-JULY JULY AUGUST O 50 IOO PER CENT CHARA F i g . 13. Relationship between the per cent vegetative cover (Chara) and the t o t a l number of limnetic and benthic sticklebacks /m2 i n the l i t t o r a l observation areas at Paxton Lake i n 1970. 31 Density of limnetic stickleback In the f i v e l i t t o r a l observation areas of the lake was negatively re l a t e d (p< 0.05) to the number of large benthics there (Pig. 14) , In August large benthics were r a r e l y seen In the observation areas. The number of limnetics then decreased as the per cent vegetative cover (Chara) i n an area increased ( F i g . 13) . Thus, these data suggest that large benthics forced lim-n e t i c stickleback above the Chara i n July. In summer of 1971 both stickleback forms were only found i n the vegetative region along the lake bottom from 3 - 6 m. Summer a c t i v i t y The frequency of f i s h capture i n minnow traps was assumed a measure of f i s h a c t i v i t y , though trap capture e f f i c i e n c y was possibly reduced during darkness. In summer of 1969 benthic stickleback were most active In the evening and l a t e morning (Fi g . 15) . Limnetics were most active from l a t e morning to darkness. Both phenotypes were most active during the l a t e afternoon i n 1970. Interactions A s e r i e s of aggressive encounters was recorded among the two stickleback forms In July and August, 1970. Benthic stickleback i n i t i a t e d most c o n f l i c t s , which occurred i n about equal frequency toward eith e r phenotype. The encounters by both phenotypes are outlined below: 32 5 0 0 v CO (J 3 0 0 Lu O I ICO] 1 D z o l o 8 N U M B E R O F B E N T H I C S Pig. 14. Relationship between the t o t a l number of benthic and limnetic stickleback /m2 i n the l i t t o r a l observation areas at Paxton Lake, July 17 - 18, 1970. 33 H O U R S Pig. 15. Summer d i e l a c t i v i t y of Paxton Lake stickleback captured from bottom and suspended trap samples. Data are expressed as the number of f i s h captured per three hour i n t e r v a l s during 24 hour periods except for pooled data between 00 and 06 hours. The i n i t i a l time of each i n t e r v a l i s given. 34 Limnetic encounters (1) Twenty limnetic f r y (plus four adults) were d i s t r i b u t e d i n an area of about 0.75 m2 amongst t a l l vegetation i n water 1 - m deep. The adults were about 20 cm apart and occupied the shadiest regions. The f r y were about 5 to 10 cm from each other. Interactions were not observed between f r y and adults, but several aggressive c o n f l i c t s occurred within each group (average nips per f i v e minutes f o r f r y 1.1, adults 0.3). One adult limnetic stickleback attacked a 60 mm benthic swimming near the lake bottom beneath the vegetation. The l i m n e t i c nipped the benthic behind the head and then returned to the vegetation. The benthic quickly moved out of the area. (2) A group of limnetics (53 f r y and 10 adults) was d i s t r i b u t -ed above Chara and on mud with cover regions i n 1 - m water near the dock. Several aggressive c o n f l i c t s occurred between Individuals on the mud with cover regions; though no encounters were observed between i n d i v i d u a l s d i s t r i b u t e d above the Chara. Benthic encounters (1) A 65 nm benthic nipped a 40 mm benthic when the l a t t e r swam within 10 cm of the former. The c o n f l i c t occurred above the shore vegetation i n 0 .5 m of water. The 65 mm benthic held i t s p o s i t i o n and the smaller f i s h moved into the Chara. 35 (2) Three aggressive encounters occurred between two 40 mm benthics located above Chara i n water 1 - m deep. Both f i s h retreated i n opposite d i r e c t i o n s . (3) An aggregation of 8 benthic stickleback ( a l l greater than 50 mm i n length) was d i s t r i b u t e d by a sunken log i n 2- m of water. One f i s h was about 10 cm from the group and i t remained i n the shadiest region. By nipping the other f i s h i t appeared to keep them out of i t s area. (4) Two adult limnetics were moving close to the lake bottom on open mud i n 1 - m of water. A 60 mm benthic attacked and nipped both limnetics three times; the l a t t e r retreated above the Chara and were not followed by the benthic. (5) A group of 25 limnetic f r y was close to the lake surface i n 1 - m of water near the dock. A 65 mm benthic entered the area and quickly chased the limnetics into deeper water. No physical contact was made. (6) A group of 58 limnetic f r y and 10 adults occupied an area of about 2 m2 i n water 1 m deep near the dock. The f i s h were d i s t r i b u t e d above Chara and on mud with cover regions. A 70 mm benthic entered the area and chased the limne t i c s . No physical contact was made but the limnetics moved Into an area of about 0.5 m2 above the Chara. (7) A group of 50 limnetic stickleback f r y was attacked and nipped 5 times by a 65 mm benthic when the group passed near the l a t t e r ' s hiding place i n 0.5 m of water. 36 The limnetics moved from the area and the benthic returned to i t s hiding place. (8) A group of 30 limnetics was swimming along the edge of "t* 1 6 Shara i n about 2 -m of water. A 55 mm benthic attacked the group when they passed near the benthic's hiding place. No physical contact was made but the limnetics quickly moved from the area. The benthic returned to i t s hiding place. Feeding habits Benthic stickleback fed mainly upon Gammarus, chiron-omid larvae, and ostracods (Fig. 1 6 ) . Some benthics (mostly f i s h < 55 mm i n length) also fed upon plankton i n 1969 and 1970 (Fig. 17) . In summer and f a l l of 1969 limnetic stickleback fed mostly upon plankton (predominantly cladocera). During winter and spring macro-benthos contributed about h a l f of t h e i r d i e t . In summer and f a l l of 1970 limnetics s t i l l fed mostly upon plankton but more macro-benthos was consumed than during the same periods i n 1969. The increased contribution of macro-benthos to limnetic diets i n 1970 l i k e l y resulted from t h e i r greater association with the lake bottom ( F i g . 1 2 ) . Phenotypes of s i m i l a r lengths usually consumed zooplankton of approximately the same size (Table 6 ) . Salmon In f a l l of 1968, 5000 coho salmon f i n g e r l i n g s (Onoorhynchus kisutch) were planted i n Paxton Lake. The 37 1 LIMNETICS P R E Y I960 I97Q 1 LIMNETICS SUMMER FALL WINTER SPRING SUMMER FALL 1 LIMNETICS PER CENT OCCURP 5.0 5.0 5.0 .SO IENCE 5.0 5.0 1 LIMNETICS Z A O C B G S M 1 : (3 6) 1 H (34) 1 3 1 LIMNETICS 3 1 4 1 (.95) 2 \ P s • (114) \ n (40) 1 1 ! (77) 1 3 BENTHICS P I S M FI (454) 3 1 4 •i N 1(133) 4 (.3 8) fi (8 8) 1 8 la II n (116) 1 16 • 38 (.97) ' l .1 JULY AUG 28 NOV MAY JULY SEPT AUG 5 SEPT MARCH J U N E AUG O C T F i g . 16. Feeding habits of Paxton Lake stickleback, 1969 and 1970. Data are expressed as per cent frequency of occurrence and numbers without parentheses indicate the number of stomachs containing each prey type. Numbers i n paren-theses r e f e r to the number of f i s h examined. The abbreviations are defined as: zooplankton (hatched bars) - copepods (Z), Daphnla and Bosmina (D), Polyphemus pedicuius (P); macro-benthos ( s o l i d bars) - Gammarus l a c u s t r l s (A), ostracods (0), chironomld larvae (0), baetid nymphs (B), Sphaeriidae (S), gastropods (G), and miscellan-eous (open bar; M). 38 >350 -cr UJ CD 3 LU < cr UJ 300 X u < 250 D (— 01 \ 200 LD CH UJ t— 150 Z < 1 Q_U_ 100 O 50 .. 0 30 40 50 BO 70 BODY LENGTH (MM) SO Pig. 17. Relationship between the body length of Paxton Lake benthic stickleback and the average number of zooplankton consumed per f i s h size class (bars). Data points are also given. Table 6. Mean size (mm) of plankters consumed by Paxton Lake stickleback Date Phenotype Pish Size (mm) Daphnia Bosmina Copep< July Limnetic 30 - 40 0.40 0.32 1969 41 - 55 0.36 0.32 0.71 Benthic 41 - 55 0.35 0.57 55 - 75 - 0.35 0.64 August Limnetic 41 - 55 0.42 0.36 0.91 1969 55 - 65 0.40 0.34 0.62 Benthic 41 - 55 0.40 0.38 -October Limnetic 41 - 55 1.15 1970 Benthic 41 - 55 1.14 - -t Based upon body measurements of at lea s t 20 i n d i v i d u a l s of each predator type. 40 f i s h ware 12 - 15 cm i n length next summer and attained lengths up to 34 cm "by August 1971. Salmon consumed mostly insects i n July and August, 1969, and May, 1970 (Pig. 18) . In June, 1970, they fed mainly upon insects and Gammarus. Chaoborus, Gammarus, and stickleback were the predominant prey i n July. Daphnla and Gammarus were the predominant prey i n October. In August, 1971, stickleback were the predominant prey. The salmon attacked l i m n e t i c stickleback near the dock during daylight hours i n July, 1970, frequently causing the l a t t e r to move onto the l i t t o r a l region. Observations were made f o r 30 minutes once per 3 hours from 0600 - 2100. The mean rate of attack was 2 per 30 minutes. LABORATORY RESULTS So c i a l Behavior V e r t i c a l d i s t r i b u t i o n and aggression Adults An experiment was conducted to determine i f adult benthic and limneti c stickleback d i f f e r e d i n v e r t i c a l d i s t r i b u t i o n and aggressive behavior i n standard 339 1 aquaria, simulating pelagic and benthic habitats i n the lake. The f i s h were fed Tubifex once each day. A l l f i s h were held under standard laboratory conditions f o r 2 - 3 months before t e s t i n g . Three groups of each phenotype 41 19 69 JULY AUGUST MAY I97Q JUNE JULY SEPT OCT I97I AUGUST PER CENT OCCURRENCE SO 5Q 5.Q 5Q 5Q_ > LU cr a. (44) 22 00 I I 26 L S3 2. 3 11 (35") I (12) 4 I F i g . 18. Feeding habits of coho salmon i n Paxton Lake, 1969 to 1971. Data are expressed as per cent frequency of occurrence and numbers without parentheses indicate the number of stomachs containing each prey type. Number i n paren-theses re f e r to the number of f i s h examined. The abbreviations are defined as: zooplankton (hatched bars) - Polyphemus pedicuius (P), Daphnia (D); macro-benthos ( s o l i d bars) -Gammarus l a c u s t r i s (A), gastropods (G), chironomid larvae (C), Chaoborus (L); insects (I) , and stickleback ( S j l 42 (20 f i s h / group) were tested separately. A h o r i z o n t a l l i n e on the glass front divided the water column int o upper and lower halves. V e r t i c a l d i s t r i b u t i o n s of the f i s h were recorded as the number of i n d i v i d u a l s i n the upper and lower halves of the aquaria. Counts were made from behind a p a r t i t i o n once per hour f o r 6 hours on each of three days. Light Intensity was 10.4 lux at the water surface. Limnetic stickleback were i n the upper h a l f of the aquaria more than benthics were (Table 7). They usually remained i n a group and showed l i t t l e aggressive behavior. Benthic stickleback usually remained i n the lower h a l f of the aquaria i n scattered groups of up to 10 i n d i v i d u a l s . A l l f i s h were aggressive, e s p e c i a l l y s o l i t a r y i n d i v i d u a l s . After termination of some experiments water temperatures i n the aquaria dropped below 8°C. In such cases most Individuals of both phenotypes were observed on the aquaria bottoms where they remained i n a c t i v e . Fry An experiment s i m i l a r to the above was conducted with benthic and limnetic stickleback young-of-the-year ( f r y ) . Benthic f r y were hatched and reared i n the laboratory. No pregnant limnetics were a v a i l a b l e , so lake reared f r y were used. Limnetics were heavier than benthics (mean weights 0.21 and 0.14 gm, r e s p e c t i v e l y ) . Both forms seemed i n good condition. Table 7 Mean number of Paxton Lake stickleback i n the upper and lower halves of the standard 339 1 aquaria. Phenotype Group Number i n Number i n , upper h a l f lower h a l f Limnetic 1 56 64 2 48 72 3 40 80 Benthic 1 5 115 2 10 110 3 9 111 44 Standard 35 1 aquaria were used f o r rearing and t e s t i n g . Pour groups of twenty f i s h of each phenotype were used; two of each phenotype were fed chopped Tubifex, the r e s t were fed Artemia s a l i n a n a u p l i i . V e r t i c a l d i s t r i b u t i o n s were recorded as f o r adults. Counts were made once per hour f o r 5 hours each day. Light i n t e n s i t y at the water surface ranged between 13 - 15 lux. No food was present during observation periods. Limnetics were In the upper h a l f of the water column more than benthic stickleback (p<0.05; Pig. 19). Pood types did not influence t h i s r e l a t i o n s h i p between the phenotypes (Pig. 19). Most limnetics remained together, generally In the c e n t r a l tank region, and r a r e l y hovered close to the tank bottom. Benthics generally hovered i n small groups close to the tank bottom, usually i n corners. Aggression was r a r e l y observed within either phenotype. Individuals and pairs with groups An experiment was conducted to determine i f i n d i v i d u a l adult benthic and limnetic stickleback were equally attracted and aggressive to groups of f i s h and i f equal s o c i a l status was established between paired fishes of the same and d i f f e r e n t phenotype. A standard 339 1 aquarium was divided i n h a l f with a v e r t i c a l wooden p a r t i t i o n . Single clear glass one gallon jars (16 cm diameter) were suspended near the water surface 45 i o c n NAUPLI I TUBIFEX F i g . 19. Total d a i l y number of Paxton Lake benthic and limnetic stickleback f r y i n the upper halves of 35 1 aquaria. 46 i n each h a l f of the tank. A l i n e was drawn around the middle of the jar p a r a l l e l to the tank 'bottom, d i v i d i n g i t into upper and lower sections. Light i n t e n s i t y was 1 0 . 4 lux at the water surface. Five f i s h of the same phenotype were placed i n each j a r , and i n d i v i d u a l f i s h ("rovers") added to each half of the tank. Rover behavior was recorded for four 5 minute periods, each 20 minutes apart. The time rovers spent along the upper and lower sections of the jar (within 15 cm of i t ) and the number of nips to f i s h i n the jars were recorded. An additional rover was then added to the aquarium and the positions of each at the jar as well as aggression between them were recorded continuously for 15 minutes immediately a f t e r introduction and then for three a d d i t i o n a l 5 minute periods, each 2 0 minutes apart. Rover pairs of the same and d i f f e r e n t phenotypes were tested. Body lengths of the f i s h tested ranged between 4 5 - 7 5 mm for benthics and 4 5 - 61 mm f o r limnetics. S o c i a l status in paired rover tests were recorded as dominance when one f i s h apparently controlled, the jar position of the other, and no dominance when both f i s h had free access to jar positions. A two way analysis of variance tested single rover data for time at the jars and aggression toward the f i s h i n the jars. Analysis of jar positions compared the subordinate 4 7 fishes frequency at upper and lower .jar sections while s o l i t a r y and when paired wlth anothar rover. Analysis of aggression i n paired ieeis? compare J (1) aggression by dominant f i s h to subordinate of each phenotype, and ( 2 ) aggression by subordinate •• to dominants of each phenotype, A p i l o t series was conducted with no f i s h i n the j a r s . Limnetic rovers never came to the ja r ; benthic rovers occasionally v i s i t e d the jar f o r short periods. The analysis of s o l i t a r y rover behavior (Tables 8 and 9 ) indicated : ( 1 ) i n d i v i d u a l rover benthics spent more time with the jar groups of either phenotype than did limnetic rovers: ( 2 ) ' benthic rovers spent more time with groups of benthics, while limnetic rovers spent equal time with groups of either phenotype; ( 3 ) for each rover phenotype aggression was equal toward i n d i v i d u a l j ar groups of benthics and limnetics; and ( 4 ) benthic rovers were more aggressive than limnetics to the groups. Dominance was observed i n a l l paired rover t e s t s . In the tests with mixed phenotypes, benthics dominated limnetics s i g n i f i c a n t l y more frequently (sign t e s t , p ~ 0 . 0 1 , N~- 19» X= 4 ) . Subordinate f i s h moved to the upper jar section when dominated by benthics ( p < 0 . 0 5 ) » but not when dominated by limnetics ( p > 0 . 0 5 » F i g . 20 ); however, subordinate were positioned a few centimeters from the dominant limnetics Table 8 . S t a t i s t i c a l analysis of the time s o l i t a r y benthic and limnetic stickleback (rovers) spent with f i s h of either phenotype i n the jars Source of V a r i a t i o n df SS MS F Significance ; ; (p = 0 .05) Treatments 3 12681.9 4227.3 16.3 s L 1 x B 1 vs Rovers 1 10848.0 10848.0 41 .9 s L vs B with B Rovers 1 1826.0 1826.0 7.1 s L vs B with I? Rovers 1 7.9 7 .9 0 . 0 3 NS Within 49 12685.0 258.9 Total 52 1 L = Limnetic stickleback i n the jars B = benthic stickleback i n the jars 2 Benthic rovers 3 Limnetic rovers Table 9. S t a t i s t i c a l analysis of aggressive a c t i v i t y of s o l i t a r y benthic and limnetic stickleback (rovers) toward f i s h of either phenotype i n the j a r s . Source of Va r i a t i o n df SS MS P Significance (P = 0.05) Treatments 3 52375.5 17458.5 8.85 S L 1 and B 1 vs rovers 1 47581.0 -47581.0 24.12 S p L vs B with B rovers 1 4793.0 4793.0 2.43 NS L vs B with I? rovers 1 1.5 1.5 < 0.00 NS Within 54 106504.0 1972.3 -Total 57 1 L = Limnetic stickleback i n jars B = Benthic stickleback i n jars 2 Benthic rover 3 Limnetic rover 50 DOM B DOM B DOM L DOM L S U B B SUB L SU tj SUB L Pig. 2 0 . Average time subordinate benthic and limnetic stickleback spent at j a r groups -without and with dominant fishes present i n aquaria. Symbols: Pom - dominant, Sub - subordinate, B - benthic, L - limnetic 51 at the bottom of the j a r . Dominant benthics were more aggressive toward sub-ordinants than were dominant limnetics ( p< 0 . 0 5 ; Fig« 2 1 ) . However, dominants of either phenotype were most aggressive toward subordinant benthics (p<0.05) . Subordinant f i s h were equally aggressive to dominants of either phenotype ( p < 0 . 0 5 ) . The above data indicated that benthic rovers were more aggressive toward Individuals and groups of e i t h e r phenotype than were limnetic rovers. Most benthics attacked the f i s h . S o l i t a r y limnetic rovers showed l i t t l e aggressive behavior and usually positioned themselves near the j a r groups. Defence of the j a r bottom by dominant limnetics from other rovers i n paired tests was probably s i m i l a r to t h e i r t e r r i t o r i a l behavior i n t a l l vegetation. A s i g n i f i c a n t r e l a t i o n s h i p (p< 0 .05) was observed between the t o t a l number of aggressive acts (attempted nips) to f i s h i n the jars by each benthic rover i n s o l i t a r y t e s t s , and the number of nips they made to other rovers i n paired tests (Pig. 2 2 ) . The number of nips to f i s h i n the jars was used as a test of the aggressiveness of s o l i t a r y shore-benthics and i n d i v i d u a l s from groups deeper i n Paxton lake. Shore benthics were more aggressive (Table 1 0 ) . Groups of limnetics with single benthics An experiment was conducted to determine i f i n d i v i d u a l 52 &GNTH8C VS. BENTHIC D O M T O S U B SUB TO D O M I 00- L I M N E T I C VS L I M N E T I C 5 0-oc uJ to -1 OJ D O M T O S U B 3 \, SU15 TO D O M < BENTHIC VS L IMNET IC \ \ * L; ^ g \ \ i — • 111 V 1 D O M B SUB B D O M L SU B L TO TO T O TO SU B L DOM L SUB 3 D O M B Pig. 2 1 * Average number of aggressive acts (nips) between subordinate'and dominant Paxton Lake stickleback i n the paired rover t e s t s . Data compares benthic vs benthic, limnetic vs limnetic, and benthic vs limnetic rovers. Symbols: Dom - dominant, Sub - subordinate, B - benthic, and L - limnet i c . 53 2 0 0 -cr UJ > BENTHIC NIPS TO JAR Pig. 22. Relationship between the t o t a l number of nips to f i s h i n the jars when benthic rovers were s o l i t a r y and the number of nips they made to other rovers i n paired t e s t s . 54 Table 10. S t a t i s t i c a l analysis of the aggressiveness of benthic stickleback captured from shallow and deep areas i n Paxton Lake, 1970. Depth Number Mean Nips . t-Test Significance Captured of Pish ( i 95$ CL.) unpaired p = 0.05 1 m 5 17,2 ± 7.3 c a l c . = 3.12 p< 0 . 0 5 3m 4 4.4 ± 1.8 df = 34 1 95 % confidence l i m i t s 55 benthic stickleback were aggressive to groups of l i m n e t i c s , and i f they a l t e r e d the l a t t e r ' s v e r t i c a l d i s t r i b u t i o n , A standard 339 1 aquarium was used i n a l l t e s t s . A h o r i z o n t a l l i n e across the tank front divided the water column int o upper and lower sections. Pish positions were recorded photographically every 15 minutes from 1100 to 17*5 hours and scored as the number of f i s h i n each section. S t a t i s t i c a l analysis compared limnet i c v e r t i c a l d i s t r i b u t i o n s before and during contact with each benthic. Aggression was recorded by counting the number of nips between the phenotypes f o r f i v e minutes once per hour. Each benthic was tested separately; the smallest benthics were used f i r s t i n each t e s t , followed by i n c r e a s i n g l y large f i s h . The number of days of recording per f i s h , body size of each f i s h , order of benthics used, and the conditioning period before t e s t i n g are presented i n Table 11. Light i n t e n s i t y was 15 lux at the water surface. The f i s h were fed at 1800 hours each day. A l l benthic stickleback usually remained near the tank bottom. V e r t i c a l d i s t r i b u t i o n of limnetic stickleback s h i f t e d downward a f t e r contact with the very aggressive 53 mm benthic i n t e s t 1, and during contact with the 55 mm benthic i n test 2 (p < 0 . 0 5 ) ; thereafter the limnetics remained down i n the aquaria with or without l a r g e r benthics In each test. (Pig. 23). The 45 mm benthic In test 2 did not Table 11. Number of days per test, body length of the f i s h , order of benthics used, and the conditioning period before each test Test Days Each Benthic Mean Length Benthic Lengths and Conditioning Examined Limnetics Order Used Period 1 5 55.7 53, 65, and 75 mm 5 months 2 2 50.9 45, 55, 65, and 1 month 75 mm 57 9 0 0 F E S T I Pig. 23. Total number of limnetic stickleback i n the upper h a l f of a standard 339 1 aquarium per session without and with i n d i v i d u a l benthics of d i f f e r e n t body lengths i n test 1 and test 2. 58 change the v e r t i c a l d i s t r i b u t i o n of limnetics (p > 0 . 0 5 ) . Benthic aggression generally increased a f t e r 1400 hours, probably i n response to the limnetics increased occupation of the upper water column during that time i n " a n t i c i p a t i o n " of food. The r e l a t i o n s h i p between the mean number of limnetics i n the upper h a l f of the water col'amn each day and the mean number of benthic nips to limnetics each day was s i g n i f i c a n t (p < 0 . 0 5 ) » i n d i c a t i n g that when limnetic stickleback remained i n the lower hal f of the water column benthic aggression was reduced (Fig. 2 4 ) . Intermediate sized benthics were more aggressive than were the smaller or larger ones used (Fig. 2 5 ) . Limnetic stickleback showed l i t t l e aggression. The data indicated that benthic stickleback are aggressive to groups of limnetics and can change the l a t t e r ' s v e r t i c a l d i s t r i b u t i o n . The downward s h i f t of the limnetics distribution, was l i k e l y caused by t h e i r confinement to a small aquarium, since they showed no such behavior i n the f i e l d . Cover effects An experiment was conducted to determine i f the limnetics association with f l o a t i n g cover and the benthics association with bottom cover i n Paxton Lake were normal behavior patterns. A standard 339 1 aquarium was used, f o r a l l t e s t s . V e r t i c a l d i s t r i b u t i o n s were determined by counting the number 59 I 3 5 7 NUMBER OF LIMNETICS IN UPPER HALF 24. Relationship between the mean number of limnetic stickleback i n the upper h a l f of a 339 1 aquarium and the mean number of benthic nips to limnetics each day. Symbols: test 1 (0), test 2 ( t ) . 60 IO 6-2 -BENTHIC o t a ^ 64 LIMNETIC I 45 53 55 65 75 BODY LENGTH (MM) OF BENTHIC Pig. 25. Aggression between single benthics of d i f f e r e n t lengths and groups of 20 limnetics. Benthic data presented as the mean number of nips to limnetic stickleback each day; limnetic data as the mean nips to benthic stickleback each day (test 1 ( § ) , test 2(0)). 61 of f i s h i n the upper and lower halves of the water column at hourly i n t e r v a l s (1100 to 18OO f o r l i m n e t i c s , and 1200 to 1600 f o r benthics). One group (15 f i s h ) of each phenotype was tested without and with cover ( a dead pine branch) i n the tank:. Cover was positioned i n upper and lower halves of the water column, occupying about h a l f the volume of each h a l f . One aggressive 65 mm benthic was used In the second t e s t with limne t i c stickleback. When cover was absent most benthics remained i n the lower portion of the water column ( F i g . 2 6 ) . No d i s t r i b u t i o n a l changes occurred when cover was i n the lower h a l f of the water column (p>0.05); however, 1 - 4 benthics defended portions of the cover from other i n d i v i d u a l s . The number of benthics i n the upper h a l f of the water column increased when f l o a t i n g cover was present (p<.0.05) , but two i n d i v i d u a l s accounted f o r most of the Increase. The number of limnetic stickleback i n the upper portion of the water column Increased when cover was i n e i t h e r portion of the water column (p<0.05; F i g . 27). However, most i n d i v i d u a l s moved Into the upper h a l f when f l o a t i n g cover was present ( F i g . 27). The aggressive benthic did not a l t e r the limnetics v e r t i c a l d i s t r i b u t i o n when f l o a t i n g cover was present (p>0.05; F i g . 27). However, i t increased the number of limnetics In the upper h a l f when cover was i n the lower h a l f 62 NO COVER COVER COVER NO COVER U P P E R LOWER U P P E R COVER F i g . 2 6 . Total number of benthic stickleback i n the upper ha l f of the standard 339 1 aquarium each day without and with cover i n the lower and upper halves of the water column. Maximum possible t o t a l count i n the upper h a l f each day would be 75 . 63 IOO • UJ a 5 0 o t— r MO COVER COVER U P P E R HALF I r COVER LOWER HALF COVER UPPER HALF i r COVER LOWER HAL F "i r NO COVER BENTHIC ABSENT BENTHIC PRESENT Pig. 27. The-effects of cover and benthic aggression on the t o t a l number of limnetic stickleback i n the upper h a l f of standard 339 1 aquaria each day. Maximum possible t o t a l count i n the upper h a l f each day would be 120. 64 of the water column (p < 0.05). When cover was removed the number of limnetics i n the lower h a l f of the water column increased from that observed p r i o r to the addition of cover or the benthic (p<0.05).. The s e l e c t i o n of d i f f e r e n t cover p o s i t i o n by the two stickleback forms i n the lake was also found i n the laboratory experiments. The data strongly suggest the summer d i s t r i b u t i o n of limnetic stickleback near f l o a t i n g cover and benthic stickleback near submerged cover to be normal features of t h e i r behavior and not the r e s u l t of i n t e r -ference between the two forms. D i s t r i b u t i o n and aggression i n shallow water An experiment was conducted to examine aggression and aggregation behavior of each phenotype i n aquaria representing l i t t o r a l regions of Paxton Lake. Two groups of limnetic and one group of benthic stickleback were used. A group contained 14 f i s h and was randomly divided into l o t s and placed into deep or shallow aquaria. Later the l o t s were switched to opposite aquaria and retested. The deep aquarium (a standard 339 1 tank) was divided into three v e r t i c a l sections by l i n e s on the glass f r o n t . The shallow tank measured 116 x 91 x 20 cm, held 211 1 of water, and was marked into 6 sections (each 38.5 x 45.5 cm) by l i n e s on i t s bottom. Dis t r i b u t i o n s were recorded as the sum of Individuals 65 i n groups of four or more per tank section from observations made once per h a l f hour f o r 7 hours each day. In lim n e t i c tests aggression was recorded f o r f i v e minute periods once per hour f o r 7 hours each day. Aggression was not recorded i n benthic t e s t s . Limnetic and benthic stickleback were tested without and with cover (a dead pine branch) i n the shallow tank. The cover f i l l e d one tank section. Light i n t e n s i t y was held at 0.7, 5.0, or 8.6 lux In the limnetic tests when cover was absent, and was 8.6 lux i n others. One aggressive 65 mm benthic was used i n the second tests with limne t i c stickleback. When cover was absent limnetic stickleback were l e s s gregarious i n the shallow tank than i n the deep tank at a l l l i g h t i n t e n s i t i e s (Pig. 28). However, i n the shallow tank the number of f i s h together increased as did aggression with increasing l i g h t i n t e n s i t y (Pigs. 28 and 29). The r e l a t i o n s h i p between the number of f i s h together and the number of aggressive acts each day was s i g n i f i c a n t (p<0.05). Aggression was low i n the deep tank and apparently not influenced by l i g h t i n t e n s i t y . When cover was added to the shallow tank most limnetics moved into i t and the t o t a l number of f i s h together each day increased s i g n i f i c a n t l y (p<0.05; F i g . 30). The additi o n of the benthic further increased the number of limnetics occupying the cover i n run 1 (p<0.05), but not i n run 2 66 98 90 SO g 70 O 50 g 30 3 20 Z 10 0 A. A -40 x X. X 0 3 6 LUX AT SURFACE x K x-X -tt-f-Pig. 28* Relationship between l i g h t i n t e n s i t y (lux) at the water surface and the t o t a l number of limnetics together i n groups of 4 or more each day i n shallow (X) and deep ( a ) aquaria. Maximum possible t o t a l number of f i s h together each session would be 98. 67 120 110 100 30 Q. Z SO 70 U_ GO O 50 cr UJ 40 CQ UM 30 z 20 10 0 3 6 LUX AT SURFACE Pig. 29. Relationship between l i g h t i n t e n s i t y (lux) at the water surface and the t o t a l number of nips by limnetics each day i n the shallow aquarium (test 1 (X), test 2 ( A ) ) . 68 Fi g . 30. The effects of cover and an i n d i v i d u a l benthic on the aggression (nips) and aggregation behaviors of limnetic stickleback i n shallow aquaria. Symbols as t o t a l each day f o r : number together i n groups of 4 or more (•——§), number i n tank section where cover was placed (0 0), limnetic to limnetic aggression (A—A), and benthic to limnetic aggression (•—•). 69 (p>0.05). In run 2 benthic aggression Increased and forced some limnetics from the cover. Limnetic - limnetic aggression decreased when cover was added to the shallow tank (p<0 . 0 5 ; Table 12) and further decreased when the benthic was present (Fig* 30). Most benthic stickleback hovered i n small groups i n the corners of both aquaria when cover was absent. When cover was added to the shallow tank they dispersed (p < 0.05; F i g . 31) and 1 - 3 i n d i v i d u a l s defended portions of the cover from other f i s h e s . The laboratory r e s u l t s on the aggression and aggregation behavior of the two forms of Paxton Lake stickleback on simulated lake regions of open mud and mud with t a l l vegetation were consistent with f i e l d observations, strongly suggesting that these a c t i v i t i e s represent normal features of t h e i r behavior. Feeding A b i l i t y A series of experiments was conducted to compare the feeding a b i l i t y of the two stickleback forms on types of prey they u t i l i z e d i n Paxton Lake. A l l tests were conducted i n standard 35-1 aquaria illuminated with sing l e 40 and 10 watt bulbs (surface l i g h t i n t e n s i t y of 14.9 l u x ) . Observations were made from behind one-way p l a s t i c mirrors. A l l data were recorded on a Hustrak 4 channel tape recorder. Tublfex sp., Artemia s a l i n a n a u p l i i , Daphnia sp., and the amphipod H y a l e l l a azteca were used f o r prey. 70 Table 12. S t a t i s t i c a l analysis of the ef f e c t s of cover on the aggressive behavior between limnetic stickleback In the shallow aquarium. Tank Condition Number of Mean nips t - Test Observations per day 1 No cover 41.8 ca l c . = 6.9 p < 0.05 Cover 6 23.5 1 Level of si g n i f i c a n c e = 0.05 71 40i NO COVER NO COVER COVER C O V E R GROUP I GROUP 2 Pig. 31 . The effects of cover on benthic stickleback aggregation behavior i n the shallow aquarium. Symbols as t o t a l each day f o r : number together i n groups of k or more (• § ) , and number i n tank section where cover was placed (0 0). Maximum possible together each session would be 98. 72 The e f f e c t of starvation on hunger Benthic stickleback weigh more than limnetics of the same body length (Pig. 7)» and therefore may have higher rates of food consumption. Consequently the weight of food consumed by each phenotype a f t e r being starved f o r various periods of time were determined i n order to standardize stomach f u l l n e s s at the s t a r t of each feeding experiment. The amount of Tubifex eaten was determined as the d i f f e r -ence between the r a t i o n provided and that remaining a f t e r 30 minute feeding sessions. Prey was f i l t e r e d on. Watman paper (No. 1) f o r two minutes with constant suction to remove excess moisture before weighing. The basic methodology of H o l l i n g (1966) was followed to determine the amount of food eaten. Pish were starved 12, 24, 48, and i n two cases, 72 hours between feeding. It was assumed that no food would be consumed with zero hours of starvation. Por s t a t i s t i c a l analysis mean rations f o r each f i s h ( f i v e r e p l i c a t e s per f i s h per deprivation l e v e l ) were proportioned to the body weight of the heaviest benthic (4.5 gm; Appendix 1 ) . The amount of food eaten i s a measure of hunger (H) i f i t represents the quanity of food passing from the gut during the times of deprivation. Thus, dH/dTF = AD (HK - H) (1) where TP i s the time of food deprivation, AD i s a constant, 73 the rate of food disappearance, and HK i s the maximum amount of food the gut can hold. This equation Integrates to -AD(TP) H = HK(1 - e ), (2) which i n turn can be transformed into the l i n e a r form of -AD (TP) (HK - H)/HK = e (3) or HK l T 1 ( \ = AD (TP) (4) HK - H The l a s t equation (4) provides a way to test the descr i p t i v e powers of the o r i g i n a l assumption i n equation 1, f o r i f equation 2 does describe the r e l a t i o n s h i p adequately, then a plot of l n ( ) vs TP should y i e l d a straight HK - H l i n e passing through the o r i g i n , with slope AD (Holling, 1966). Benthic and limnetic stickleback consumed equal amounts of food i n proportion to t h e i r body weights. Per cent body weight of food consumed at a l l l e v e l s of deprivation ranged from 2.47 - 7.41 and 2.18 - 8.48 f o r benthics and limnetics, r e s p e c t i v e l y (Appendix 2). HK was 0.31 at 72 hours of deprivation f o r 69 mm benthics (Pig. 32). Hunger did not d i f f e r between the phenotypes up to 24 hours of deprivation (p >0.05> Pig. 33). However, a f t e r 24 hours of deprivation hunger decreased f o r a l l limnetics and those benthics i 59 mm i n length (benthics between 60 and 68 mm were not examined). Such a decrease may r e f l e c t an incapacity to consume as much food 74 HOURS OF DEPRIVATION F i g . 32. Relationship between hours of deprivation and weight (gm) of Tublfex eaten by three 69 mm benthic stickleback. A l l data are proportioned to the heaviest i n d i v i d u a l (4.5 gm). 75 3 . + 2 .. x x i X 1 0 x X * X / x / 1 8 X / X X 0 12 24 36 HOURS OF DEPRIVATION —H 48 Pig. 33. Relationship between hours of deprivation and the proportion of the maximum food ration eaten f o r a l l limnetic and benthic stickleback up to 24 hours (p<0.05; ), and i n d i v i d u a l benthics (69 mm (•), 6 59 mm ( • )) and limnetics (X) at 48 hours. a f t e r 48 hours of starvation, than an actual decrease i n the rate of food passage from the gut. The two 69 mm benthics tested showed no such decrease i n food intake up to 72 hours of deprivation (Pig. 3 3 ) . A l l f i s h were starved 24 hours before subsequent experiments. Feeding On Tubifex An experiment was conducted to examine e f f e c t s of mouth siz e on feeding rates of benthic and limneti c stickleback. The mean d a l l y r a t i o n of Tubifex f o r each f i s h was placed i n t o a 10 x 5 x 1 cm aluminium cup on the tank bottom. Prey animals were allowed to bunch together before each f i s h was tested. The number of grasps at the food by each f i s h tested as well as i t s stomach f u l l n e s s were recorded a f t e r 30 minute feeding periods. The amount of food eaten was determined as outlined i n the previous experiment. The index of stomach f u l l n e s s was determined from the following r e l a t i o n s h i p : Obs H = Max where H i s an index of stomach f u l l n e s s , Obs the amount of food consumed, and Max the mean d a i l y r a t i o n of each f i s h . Limnetic stickleback of 45, 55» and 60 mm and benthics of 5 5 s 60, and 69 mm i n length were used, Benthic stickleback usually consumed more prey per grasp than did limnetics (Fig. 34); they frequently took 9 0 per cent or more of t h e i r t o t a l r a t i o n i n 10 or fewer grasps. Most benthics grasped loose worms.after consuming much of t h e i r r a t i o n and while t h i s a c t i v i t y greatly increased the" number of grasps, a d d i t i o n a l prey added l i t t l e to the f i n a l H values. Oh Ar tern l a s a l l n a n a u p l i l This experiment was conducted to compare feeding a b i l i t y of the two stickleback forms on a small prey, Artemla. sji-.lina nauplii„ Brine shrimp n a u p l i i were hatched and separated from unhatched eggs and empty egg cases following the method-ology of Larson ( 1 9 7 0 ) . Test f i s h were fed n a u p l i l f o r at le a s t two weeks p r i o r to experiments. Large benthics (> 5 5 mm) fed poorly on n a u p l i i during the conditioning period, and t h e i r diets were supplemented p e r i o d i c a l l y with Tubifex. Nauplii usually sank to the tank bottom, and most were captured there by each phenotype. Nauplii used i n the experiment were 0.6 - 0 . 7 mm i n length, presented i n densities of 1750 - 2300/ tank* Data on feeding a b i l i t y were obtained by recording 78 .O CO UJ z CO 3 O. 6 . X u < H 0 . 2 CO LL o X Ixl a IO 3 0 5 0 NUMBER O F GRASPS Pig. 34. Relationship between number of grasps per feeding session and index of stomach ful l n e s s at the end of the feeding session f o r benthic (•) and limnetic (0) stickleback. 79 the i n t e r v a l between grasps at i n d i v i d u a l n a u p l i i , the manipulation (swallowing) time, and the search + f i x a t i o n + approach time ( c o l l e c t i v e l y c a l l e d search time). Search time was equal to the i n t e r v a l between grasps minus manipulation time. Data represent means of at l e a s t 40 consumptions per f i s h . Most f i s h were tested 2 - 3 times. Limnetics ^ 50 mm i n length had the shortest i n t e r v a l between grasps, manipulated the fas t e s t and searched the l e a s t ( F i g . 35). Small benthics ( £ 4 5 mm) performed as well as limnetics over 50 mm i n length. Larger benthics fed at a low rate and those longer than 64 mm ate no n a u p l i i at a l l . Manipulation time contributed most to the i n t e r v a l between grasps for limnetics, while search time contributed most f o r benthics, e s p e c i a l l y the larger I n d i v i d u a l s . On Daphnla To examine feeding a b i l i t y of limnetic and benthic forms of stickleback on prey swimming a c t i v e l y i n the water column a series of experiments was conducted using Daphnla. The prey was s i m i l a r i n size (average 1.09 - 0.14 mm) to l a r g e r Daphnla i n Paxton Lake ( F i g . 5). Prey density was fi x e d at 2.9 per 1 at the s t a r t of one set of experiments ( F i g . 36) and tested over a range of 0.1 to 2.9 per 1 i n another ( F i g . 37). Data were co l l e c t e d from 7 - 2 5 consumptions per f i s h i n experiments with variable 80 I/) < Q HI 5 U UJ UJ 2 Z O -J z < 2 ui 2 x < UJ ( D ° 0 • • I 1 4 0 50 60 70 DODY LENGTH MM Pig. 35. Relationship between f i s h length and i n t e r v a l between grasps, manipulation time, and search time for benthic (t) and limnetic (0) stickleback feeding on Artemla sali n a n a u p l i i . prey densities and from 25 consumptions per f i s h i n the other t e s t s . A l l f i s h were fed Daphnia fo r two weeks "before the experiment started. Only f i s h of each phenotype with the highest feeding rate on Daphnla were used i n the experiment with variable prey d e n s i t i e s . Limnetic stickleback searched l e s s , manipulated the prey f a s t e r , and consumed them at a higher rate than did most benthics (Pig. 36). Large limnetics had the highest feeding and manipulation rates; however, search time was about the same f o r limnetics of a l l s i z e s . Small ( 6 51 mm) benthic stickleback fed at higher rates, manipulated prey f a s t e r , and searched less than large benthics. Search time contributed most to the low feeding rates benthic stickleback, e s p e c i a l l y for large i n d i v i d u a l s ; manipulation time made the l a r g e s t contribution to the high feeding rates of limnetic stickleback. The 60 mm l i m n e t i c fed at a higher rate than the 51 mm benthic at a l l prey densities ( F i g . 37). Feeding rate did not Increase with prey densities above 1.1 per l i t e r f o r e i t h e r phenotype. On H y a l e l l a A f i n a l block of experiments examined various aspects of benthic and l i m n e t i c stickleback feeding upon amphipods, an important food item f o r the benthic form i n Paxton Lake 82 a. < DC O z 111 u UJ UJ CO v / J i I l l 5 z g Q. Z < 2 UJ I-O O o _ o O o 4 1 8.8 < ° • H 3 < HI in • ••• o o o o 4 0 50 60 7 0 DODY LENGTH MM 36. Relationship between f i s h length and i n t e r v a l between grasps, manipulation time, and search time for benthic (t) and limnetic (0) s t i c k l e -back feeding on Daphnla. 83 8 • O 2 . O I o l.'O 2 DAPHNIA PER LITER 3 37. Relationship between prey density and i n t e r v a l between grasps for the 60 mm limnetic (0) and the 51 mm benthic (•) stickleback feeding on Daphnia. Ninety-five per cent confidence l i m i t s are included. 84 Maximum prey :•/.<:-Fish were fed Hj^e_13.a. f o r one vreek before the experiments started,. During tests four or f i v e amphipods of known size were placed into experimental aquaria and exposed to predation by i n d i v i d u a l f i s h f o r 45 minutes.' Consumed prey were recorded. Each f i s h was tested at least four times. Over the size range examined, benthic stickleback consistently consumed larger amphipods than did limnetics (Fig. 3 8 ) . The 60-mm limnetic and 45-mm benthic consumed prey of s i m i l a r s i z e . Benthics l a r g e r than 51—mm were not tested because such i n d i v i d u a l s could consume the largest amphipods i n Paxton Lake. Manipulation time Up to 6 amphipods were fed In d i v i d u a l l y to each f i s h tested and manipulation (swallowing) times recorded. Manipulation time did not equal handling time because prey were'often rejected several times before being consumed* Manipulation times increased l i n e a r l y f o r each predator as prey size increased (Fig. 3 9 ) and a l l slopes (from regression analysis) were s i g n i f i c a n t l y d i f f e r e n t from zero except for the 54-mm limnetic (Appendix 2 ) . Slopes decreased as predator size Increased and were smaller for benthics than limnetics of s i m i l a r body length (Fig. 40)• 85 6 + 30 40 50 60 BODY LENGTH (MM) Pig. 38. Relationship between f i s h length and maximum size of Hyalella consumed by limnetic (A ) and benthic (t) stickleback. Projecting l i n e s indicate the next available prey size which was not eaten. 86 39. A comparison between Hy a l e l l a length and manipulation time for benthic and limnetic stickleback of s i m i l a r s i z e . Data f o r a large , benthic are also presented. Symbols: benthics-48 mm (e>), 51 mm (• ), 54 mm (V ), 59 mm (§), and 69 mm (0); limnetics - 54 mm ( ^ ) , 58 mm ), and 60 mm ( ^ ) . 87 However, f o r limnetics and benthics with about the same mouth width, the former manipulated prey f a s t e r (Fig, 40)• Time between successful grasps Each stickleback was offered a range of prey s i z e s . F i f t e e n ampbipods were used i n each t e s t . Variances around mean prey sizes were small (Appendix 3). The i n t e r v a l s between successful grasps (that grasp before a prey was consumed) of i n d i v i d u a l prey were recorded. For benthic and limnetic stickleback of comparable length, the former had shorter i n t e r v a l s between successful grasps on prey of the same size (Fig„ 41). Grasping rates decreased as prey size increased f o r a l l predators; however, the 69 -mm benthic presumably had an optimal prey size between 3 and 4 mn? before i t s feeding rate decreased. Furthermore, benthics consumed larger amphipods than did limnetics of comparable length feeding at most success-f u l grasping rates, e.g., 10 seconds (Fig. 42; data from Fig, 41). Search time Limnetics (58 and 60 mm) searched le s s than did benthics of comparable and longer body size on prey le s s than 2 mm i n length (Fig. 4 3 )„ With l a r g e r prey, limnetics •searched more than did benthics; limnetics seemed "reluctant" to consume any prey not immediately swallow-able. However, small benthics (48 and 51 mm) searched le s s than 88 10 £ 6 Ld Q_ D A i _J 2 .. 0 MOUTH WIDTH (MM) Pig. 40. Relationship between mouth width and slopes of manipulation time vs amphipod size (data from Pig. 39) f o r benthic ( A ) and limnetic (X) stickleback. 89 2 0 t 3 5 AMPHIPOD • ^ a i * / i I* / / o 3 5 LENGTH (MM) F i g . 41. Relationship between Hyalella length and time between successful grasps f o r benthic and limnetic stickleback of si m i l a r and d i f f e r e n t siz e s . Symbols: benthics - 48 mm ( +)> 51 mm (X), 59 mm (•), 64 mm ( o ) , and 69 mm (0); limnetics - 54 mm (V ), 58 mm ( A ) and 60 mm ( A ) . 90 2 UJ M CO > UJ cr a 4 0 5 0 6 0 7 0 BODY LENGTH (MM) Pig. 42. Relationship between f i s h body length and amphipod length consumed at a feeding rate of 10 seconds f o r benthic (•) and limnetic (0) stickleback. 91 did l imnetics f o r prey of any size (Pig. 43). For each large benthic (59» 64, and 69 mm) search time was minimal within a s p e c i f i c range of prey s i z e s ; however, with smaller or l a r g e r prey the search time increased. The optimal range of prey sizes s h i f t e d to l a r g e r prey as benthics Increased i n s i z e . Search time amongst cover Loose l e a f l i t t e r f i v e cm i n depth was placed on the bottom of each tank. Approximately twenty H y a l e l l a of various lengths were present i n the cover f o r each t e s t . Search time amongst cover was recorded over three separate 10 minute periods f o r each f i s h . Benthic stickleback spent s i g n i f i c a n t l y more time searching the l e a f l i t t e r than did limnetics (unpaired t -t e s t , c a l c . t = 2.479, d.f. = 20; l e v e l of s i g n i f i c a n c e = 0.05). The 51 mm benthic often spent periods of 2 - 3 minutes amongst the cover ( F i g . 44). Limnetics u s u a l l y searched the water column or the surface of the cover. DISCUSSION In the present study I attempted to evaluate the importance of s o c i a l behavior and feeding a b i l i t y as mechanisms maintaining the segregation of two forms of thresspine stickleback (Gasterosteus aculeatus) i n a small coastal lake. 92 43. Relationship between search time and Hyalella length f o r benthic and limnetic stickleback of si m i l a r and d i f f e r e n t lengths. Symbols: benthics - 48 mm ( + ) , 51 mm (X), 59 mm (•), 64 mm ( O ) , and 69 mm (0); limnetics - 58 mm ( A ) and 60 mm ( A ) . 93 UJ cr < UJ CO O o o 3 0 4 0 5 0 6 0 BODY LENGTI-7 0 (MM) 8 0 Pig. 44. Mean search time amongst l e a f l i t t e r f o r benthic (•) and limnetic (0) stickleback between 30 and 75 mm long. 94 A d i s t i n c t s p a t i a l segregation occurred between the two forms of stickleback i n the lake, during summer 1969. The l i m n e t i c form (limnetics) was u s u a l l y found i n large aggregations associated with f l o a t i n g cover, while the benthic form (benthics) was usually s o l i t a r y and associated with bottom cover. Both forms were found along the lake bottom during winter, but limnetics were deeper than benthics. Segregation of l i m n e t i c and benthic stickleback was a l t e r e d In summers of 1970 and 1971, presumably owing to severe predation. from salmonlds. Large benthics ( > 50 mm) along the shoreline of Paxton Lake were s o l i t a r y , and from laboratory tests seemed more aggressive than those deeper i n the lake. Limnetics which moved i n t o shoreline areas i n 1970 apparently contended with t h i s aggresaiveness by occupying regions uninhabited by large benthics, i . e . , t a l l vegetation and open water above Chara. Limnetics were aggressive and held small t e r r i t o r i e s when i n t a l l vegetation, but were gregarious and non-aggressive when above Chara. In the absence of large benthics, limnetics did occupy shore areas with open mud or mud with cover substrate, and became s l i g h t l y t e r r i t o r i a l and aggressive toward each other. Trophic differences between the two stickleback forms were greatest during summer. Limnetics consumed mostly 95 plankton, and benthics fed heavily on macro-benthos. Limnetic stickleback ate more macro-benthos during winter than i n summer. Laboratory studies demonstrated s i g n i f i c a n t differences i n s o c i a l behavior between limnetic and benthic s t i c k l e b a c k , which seemed relevant to t h e i r behavior i n the f i e l d . In deep aquaria (representing pelagic and benthic regions of Paxton Lake) benthics were: (1) near the bottom of the water column; (2) aggressive; (3) t e r r i t o r i a l ; and (4) attracted to bottom cover. In contrast limnetics were:(l) high i n the water column; (2) only s l i g h t l y aggressive; (3) gregarious; and (4) a t t r a c t e d to surface ( f l o a t i n g ) cover. In shallow aquaria (representing l i t t o r a l regions of Paxton Lake) limnetics were aggressive and not gregarious i f cover was absent. When cover was present most i n d i v i d u a l s went int o i t and became l e s s aggressive. Behavior of benthic stickleback i n shallow aquaria d i f f e r e d from that of the l i m n e t i c s . They aggregated i n small groups when cover was absent, and became t e r r i t o r i a l when cover was present, 1 - 3 i n d i v i d u a l s defending the cover from other f i s h . In the laboratory experiments limnetics fed on plankton at a higher rate than did benthics. Their prey manipulation and search a b i l i t i e s probably contributed most to t h i s success. Benthics fed on amphlpods at a higher rate than did l i m n e t i c s . A wide mouth probably contributed much to the benthics feeding advantage on t h i s prey type 0 The phenotypic differences between the two stickleback, forms have a genetic basis, and a r t i f i c i a l " h y b ridisation" does not produce discontinuous phenotypes ( J . D, McPhail, personal communication). However, behavior of "hybrids" may be intermediate between that of the parental phenotypes (R. L. Jones, personal communication). Thus, i t Is u n l i k e l y that polymorphism contributed to the behavior and morphology of the Paxton Lake stickleback. The morphological and s o c i a l behavioral differences between lake-reared adult stickleback were also found between limnetic and benthic ycung-of~the-year that were hatched and reared under standard laboratory conditions (R. L. Jones, personal communication).It appears that non«genetlc differences make, at best, a minor contribution to the behavior and morphology of the two stickleback forms. Benthic and limnetic stickleback appear to d i f f e r g e n e t i c a l l y , but t h e i r s p e c i f i c status i s i n question. Males of benthic and limnetic stickleback usually b u i l d nests i n d i f f e r e n t shore regions of Paxton Lake; the former seeking covered areas, while the l a t t e r nests i n the open* Nothing Is known about mate preferences, but limnetic males were observed to successfully court several benthic females i n 1 9 6 9 . Some benthic females appeared^ however, too large to 97 enter the nests. Perhaps t h i s could he. a .form of pre-mating i s o l a t i o n mechanise preventing, to some extent benthic females from depositing eggs i n limnetic nests. The extent of interbreeding between the two stickleback forms i s unknown, but i t does occur ( J . D. McPhail, personal communication)• In the laboratory 9 offspring from, a r t i f i c i a l crosses between limnetics and benthics appear v i a b l e (F1 and backcross) and possibly occupy intermediate habitats (R. L. Jones, personal communication). In the f i e l d they have intermediate feeding habits. The above information could suggest that the two stickleback forms are species, but u n t i l further data are available we must consider them possible subspecies (ra.ces)« I f the two stickleback forms represent d i f f e r e n t species, t h e i r segregation should be of the i n t e r a c t i v e type-as suggested by the hypothesis of M i s s o n (1967). However, It i s not clear that the mechanisms proposed by M l s s o n adequately cover t h i s as a case of Interactive segregation, as may be i l l u s t r a t e d by considering the proposed mechanisms i n r e l a t i o n to the r e s u l t s of t h i s study: l) E x p l o i t a t i o n - This Is defined as the a b i l i t y of one species to use vacant resources f a s t e r , or get there f i r s t , than another species (Brian, 1956), Pood seems the main concern i n the Paxton Lake case. The two stickleback ate 98 s i m i l a r foods during winter, but the f i s h were l e s s active at the lower temperatures. E x p l o i t a t i o n then seemed u n l i k e l y or unimportant. In summer, 1970, l i t t l e trophic overlap occurred between the two stickleback forms. Limnetics moved near the shoreline i n 1970 and along the lake bottom, between 3 and 6 m i n 1 9 7 1 , probably i n response to Increased salmonid predation. Both stickleback forms ate more of the same foods than i n summer 1 9 6 9 , and food exp l o i t a t i o n may have occurred. 2 ) T e r r i t o r i a l i t y - This could be a possible mechanism f o r maintaining s p a t i a l segregation between the stickleback forms. Benthic stickleback were shown to be t e r r i t o r i a l and aggressive, attacking aggregations of limnetic stickleback moving along l i t t o r a l regions i n 1 9 7 0 . Breakdown of s p a t i a l segregation a f t e r 1969 probably increased the frequency of benthic - limnetic i n t e r a c t i o n . In the laboratory, benthic stickleback were shown to interact with both forms of stickleback, e s p e c i a l l y t h e i r own form, causing changes i n v e r t i c a l d i s t r i b u t i o n of subordinate f i s h e s . Thus, benthle-limnetie i n t e r a c t i o n i n the f i e l d l i k e l y represents a normal feature of the benthics' behavior. V e r t i c a l d i s t r i b u t i o n of i n d i v i d u a l groups of limnetic and benthic stickleback i n aquaria were d i s s i m i l a r , but consistent with f i e l d observations. 99 Futhermore, differences between the stickleback i n morphology, s o c i a l behavior, and feeding a b i l i t y were consistent with that of other f i s h species i n h a b i t i n g pelagic or benthic environments (see page 105 ). There-fore, the i n t e r a c t i o n between the two stickleback forms l i k e l y played only a minor role maintaining t h e i r segregation i n 1969. 3) Food-fighting - This was never observed i n the lake or laboratory. In the laboratory benthics grasped food and moved to an unoccupied tank section to consume i t . Limnetic stickleback usually remained together while feeding. 4) Predation - A few large benthics preyed upon both forms of stickleback i n 1969, but the l e v e l of cannibalism was low. Thus, predation probably was unimportant as a factor maintaining segregation of the two forms. 5) Other interferences - Nilsson (1967) suggested that avoidance experiments of Ivlev (1961) and Breder (1929) indicated that stimuli released by strange shapes, move-ments, sounds, or odours could be important to segregation i n f i s h e s . N i l s s c n (1967) mentions that a l t e r a t i o n of the environment by introduced species may also be important, c i t i n g Cahn (1929), Threlnen and Helm (1954) as examples. The impact of salmon upon decreased s p a t i a l and trophic differences between the stickleback forms i n 1970 and 1971 may be most relevant to interferences of t h i s type. 100 However, l i t t l e i s known of such interferences between stickleback, or between .salnonids and stickleback p r i o r to 1970. The discussion above suggested that i n summer 1969, i n t e r s p e c i f i c competition was of reduced importance i n maintaining the segregation of the two stickleback forms. This i s not unexpected because evolution i s a continuous process and can lead to reduced Interference between sympatric species. Reduction of i n t e r s p e c i f i c interference through character displacement (Kohn and Orlans, 1962; Mayr, 1965) i s dependent on many processes which l i k e l y vary i n importance with s p e c i f i c s i t u a t i o n s , but always an important one i s "environmental s t a b i l i t y . Interference between clo s e l y related species, or unrelated species with s i m i l a r e c o l o g i c a l demands may be greatly reduced I f an environment i s s u f f i c i e n t l y stable to permit adaptation by species to s p e c i f i c portions of the environment. Adaptive r a d i a t i o n may be extensive, but as Fryer' ( 1 9 5 9 ) has shown i t may be r e s t r i c t e d with many related species or genera frequently co-existing i n the same habitats and consuming the same foods, Fryer emphasized that for the c i c h l i d fishes i n Lake Nyasa s p e c i a l i z a t i o n has been enhanced by the lakes large s i z e , long s t a b i l i t y , and spar e number of habitats as well as by predation and mouth breeding c h a r a c t e r i s t i c s of Its f i s h e s . Competition i s 101 probably not absent between the c i c h l i d f i s h e s , but no doubt these mechanisms have greatly reduced interference between the species. Species adaptations are more generalized i n less stable environments. The o v e r a l l s t a b i l i t y of a temperate lake, f o r example, i s low due to seasonal f l u c t u a t i o n s i n temperature, vegetative cover, food type, food density, etc. Therefore s e l e c t i o n favors retention of generalized features of species. However, s t a b i l i t y s t i l l exists but on a more coarse l e v e l i n that species adapt to regions of lakes such as pelagic or benthic rather than the more s p e c i f i c micro-habitats of the type noted by Fryer ( 1959) . Several species might inhabit pelagic or benthic regions of a lake, but each may possess v a r i a t i o n s i n behavior, mouth s i z e , mouth po s i t i o n , or body shape providing an advantage f o r e x p l o i t i n g c e r t a i n environmental resources better than the others. However, these adaptations, while reducing i n t e r -s p e c i f i c i n t e r a c t i o n , are not complete, maintaining within each species a p l a s t i c i t y to environmental changes. Two examples of supporting evidence are provided from the study by Keast and Webb (1966) on 14 cohabiting f i s h species i n Lake Opinicon and an experimental study of feeding behavior and i n t e r a c t i o n of coastal cutthroat trout and d o l l y varden char by Schutz (MS 1969). The extensive adaptations of fishes i n t r o p i c a l lakes (e.g., Lake Nyasa) 102 as compared with those i n temperate waters may inpart bs due to the greater age of the former* I f the two stickleback forms represent a single species, t h e i r extensive adaptations l i k e l y represent an attempt to optimize the p o t e n t i a l resources of the pelagic and benthic regions of Paxton Lake, Their f a i l u r e to perfect i s o l a t i n g mechanisms to a t t a i n s p e c i f i c status may have resulted from i n s u f f i c i e n t time, or a response (possibly introgression) to the less stable conditions since 1957* as a means of maintaining genetic v a r i a t i o n that increases the species chances of s u r v i v a l . Nonetheless, the level, of i n t r a s p e c i f i c competition appeared to be low i n summer, 1 9 6 9 . The evolution of the Paxton Lake stickleback i s not cl e a r . A l l o p a t r i e processes are'possible: (1) each s t i c k l e -back form could have evolved In one of the two lake basins.; (2) double invasion of the freshwater (leiurus) and marine (trachurus) forms of Gasterosteus. Sympatric dimorphic s e l e c t i o n may also have been operative. I n t r a s p e e i f i c competition for food, space, reproduction s i t e s , etc., could have provided a separating force for sel e c t i o n of phenotypic extremes i n the population (I.e., disruptive selection) subjected to s e l e c t i v e pressures i n d i f f e r e n t lake regions ( i . e . , pelagic and benthic), Suoh divergences would require s t a b i l i z e d s e l e c t i o n ( i . e . , favoring average Individuals and eliminating extreme variants) a f t e r 103 the phenotypes were separated (Mayr, 1965). The extensive adaptations and genetic differences between the two stickleback forms may suggest the establishment of s t a b i l i z e d s e l e c t i o n i n t h i s case. Whether a l l o p a t r i c or sympatric processes are at work i s thus i n question, but perhaps the following mechanisms may provide a d d i t i o n a l i n s i g h t i n t o the evolution of the two stickleback forms: 1) P a r a s i t i c influence - Gasterosteus i s frequently infested with Schistocephalls. The parasite Is acquired from eating copepods. Infested Individuals have a higher metabolic rate than do those uninfested (Lester, 1971). Lester suggested t h i s metabolic increase as a reason f o r the segregation of infested from uninfested i n d i v i d u a l s , the former seeking the high oxygenated water near creek mouths or at lake surfaces. Such a s i t u a t i o n might be applicable to the Paxton Lake stickleback. A high percentage of limnetics were infested with the parasite i n 1969. If the t r a i t f o r plankton feeding had a genetic basis and i f infested i n d i v i d u a l s l i v e d long enough to spawn, with time d i r e c t i o n a l s e l e c t i o n might favor o f f s p r i n g with increased f i t n e s s to l i v i n g a pelagic existence. This process could r e s u l t i n the evolution of a limnetic phenotype from a common gene pool, or r e f i n e an i n t e r a c t i o n between two gene pools. \ 104 2) Stickleback predation - A few large benthic stickleback consumed stickleback as prey i n 1969. This occurred at a low l e v e l , but the suggestion that more intense predation occurred i n the past i s obvious. I t i s possible that benthic predation on limnetics and possibly other benthics, generated s u f f i c i e n t d i r e c t i o n a l force to eit h e r r e f i n e or develop the present e c o l o g i c a l r e l a t i o n s h i p s between the two stickleback forms. 3) Salmonld predation - Salmon!d predation may have acted as a s e l e c t i v e force favoring those phenotypes occupying the pelagic and benthic regions of the lake (see Fryer, 1959, f o r a discussion of the role of predation i n f i s h evolution). Laboratory studies suggested that the two forms of s t i c k l e -back became well adapted to d i f f e r e n t resources of Paxton Lake. Limnetic stickleback's " a t t r a c t i o n " to surface cover and shore behavior, and the benthic stickleback's t e r r i t o r i a l behavior and occupation of bottom cover may have been mechanisms reducing salmonid predation before the cut-throat trout population decreased i n 1957. This i n t e r -pretation seems reasonable, since limnetics were usually associated with surface cover i n 1968 (before salmon were planted i n Paxton Lake; J . D, McPhail, personal communication), and In 1969 when salmon predation was low; but they moved near and into l i t t o r a l areas when predation l e v e l s 105 i n c r e a s e d In 1970* High water i n 1971 a p p a r e n t l y p e r m i t t e d salmon to explore shallow shore areas where no s u i t a b l e cover was present f o r the stickleback., Doth forms of s t i c k l e b a c k then sought refuge i n the Chara between 3 and 6 m a l o n g the l a k e bottom. Hoogland e_t aJU (1958) presented e x p e r i m e n t a l evidence t h a t s t i c k l e b a c k p e l v i c spines are important defense mechanisms.- Despined I n d i v i d u a l s were eaten more o f t e n than were spined i n d i v i d u a l s . A p p l i c a t i o n of these r e s u l t s to Paxton lake s t i c k l e b a c k , however, presents many d i f f i c u l t i e s . Since l i m n e t i c s t i c k l e b a c k l i k e l y has a. h i g h e r r i s k of p r e d a t i o n to salmonids than b e n t h i c s t i c k l e b a c k , t h e i r s p i n e s could serve as a defence mechanism. But what about b e n t h i c s t i c k l e b a c k which had n e i t h e r p e l v i c s p i n e s nor body p l a t e s ? Perhaps the l o s s of armorment p e r m i t t e d them to move q u i c k l y i n t o bottom cover as a means of a v o i d i n g p r e d a t i o n . B e n t h i c and l i m n e t i c f i s h e s d i f f e r i n d i s t r i b u t i o n , as w e l l as s o c i a l and f e e d i n g b e h a v i o r (e.g., b e n t h i c specl.es-•IMl^ L S2Xi^H.a» Brawn, 1 9 6 9 ; Hypsypons rublcunda, C l a r k e , 1 9 7 0 ; Cottus asper and C. rhotheus, Northcote, 1 9 5 4 : l i m n e t i c s p e c i e s - Clupjia harengus, B l a x t e r , 1 9 6 5 , and B l a x t e r and H o l l i d a y , 1 9 6 3 ; and E n g r a u l l s mordas, Leong and O'Connel, 1 9 6 8 ) , In g e n e r a l , b e n t h i c s p e c i e s are a g g r e s s i v e and t e r r i t o r i a l ; they remain near bottom cover, possess a l a r g e 106 mouth ( r e l a t i v e to body s i z e ) , and consume mostly benthic foods. Limnetic fishes are gregarious and non-aggressive; they inhabit open waters, possess a small mouth, and consume mostly plankton. These c h a r a c t e r i s t i c s also pertain to benthic and limnetic stickleback In Paxton Lake. Furthermore, experimental demonstration of these c h a r a c t e r i s t i c s suggested that the s p a t i a l and trophic segregation of the stickleback was not the r e s u l t of intense interference i n 1 9 6 9 . In conclusion, t h i s study indicated that the two stickleback forms, whether species or subspecies, had evolved means for co-existence with much reduced interference p r i o r to salmonld introduction. I f they represented two species, I believe the mechanisms of t h e i r segregation do not f u l l y support the hypothesis of Nilsson that c l o s e l y related species, or unrelated species with s i m i l a r e c o l o g i c a l demands, segregate through competitive i n t e r a c t i o n . L i t t l e evidence was gathered to support t h i s hypothesis. In f a c t , I suggest th i s segregation to be, i n N i l s s o n f s terms, closer to the s e l e c t i v e , than the i n t e r a c t i v e type. I f the two stickleback forms are subspecies of Gasterosteus aculeatus. a lack of intense i n t r a s p e c i f i c competition was strongly suggested. 107 LITERATURE CITED Blaxter, J . H. S. 1965. The feeding of herring larvae and t h e i r ecology i n r e l a t i o n to feeding. Cal. Coop. Oceanic Pish. Invest. Reports, 10: 79 - 8 8 . and P. G. T. K o l l i d a y . 1963. The behavior and physiology of the herring and other clupeolds. In: Advances In Marine Biology, 1:261 - 393. Brawn, V. M. 1969. Feeding behavior of cod (Gadus morhua). J. Pish. Res. Bd. Canada, 2 6 : 583 - 596. Brian, M. V. 1956. Segregation of species of the ant genus Myrmica. J. Anim. Ec o l . , 25: 319 - 337. Brooks, John Landon, and Stanley I. Dodson. 1965. Predation, body s i z e , and composition of plankton. Science, 150: 28 - 3 5 . Clarke, Thomas A. 1970. T e r r i t o r i a l behavior and population dynamics of a Pomacentrid f i s h , the g a r i b a l d i , Hypsypops rubicunda. Ecolog. Monogr., 40 (2) : 1 8 9 - 2 1 2 . Fryer, Geoffrey. 1959. Some aspects of evolution i n Lake Nyasa. Evolution, 13: 440 -451. Galbriath, J r . , Merle G. 1967. S i z e - s e l e c t i v e predation on Daohnia by rainbow trout and yellow perch. Trans. Amer. Fish. S o c , 96( 1 ) : 1 - 10. H o l l i n g , C.S. 1966. The functional response of i n v e r t i -brate predators to prey density. Mem. Entomol. Soc. Canada, 48: 1 - 86. 108 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 f i s h e s . J. Anim. E c o l . , 19(1): 36 - 58. Keast, A. and D. Webb. 1966. Mouth and body structure r e l a t i v e to feeding ecology i n f i s h fauna of a small lake. J . Pish. Res. Bd. Canada, 23 M 845 - 1874. Kohn, Alan J . and Gordon H. Orians. 1962. E c o l o g i c a l data i n the c l a s s i f i c a t i o n of c l o s e l y r e l a t e d species. Systematic Zoology, 11: 119 - 127. Larkin, P. A. 1956. I n t e r s p e c i f i c competition and population control i n freshwater f i s h e s . J . Pish. Res. Bd. Canada, 13: 327 - 342. Larson, Gary L. 1970. A simple chamber fo r separating brine shrimp n a u p l i i from unhatched eggs and empty egg cases. Prog. Pish.-Cult., 32(4): 208. Leong, R. J . H. and C. P. O'Connel. 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 . Pish. Res. Bd. Canada, 26: 557 - 582. Lester. R. J . G. 1970. The influence of Schistocephalls plerocercoids on the r e s p i r a t i o n of Gasterosteus and a possible r e s u l t i n g e f f e c t on the behavior of the f i s h . Canadian J . Zool., 4 9 : 361 - 366. Mayr, Ernst. 1965. Animal species and evolution. Cambridge Massachusetts, Harvard University Press, pp. 797. 109 Mettler, L. E. and T. G. Gregg. 1969. Population genetics and evolution. Prentice-Hall, pp.212. Nilsson, N i l s - A r v i d . 1955. Studies on the feeding habits of trout and char i n northern Swedish lakes. Rep. Inst. Freshw. Res. Brottningholm, 36:163 - 225. 1960. Seasonal fluctuations i n the food segregation of trou t , char and whitefish i n 14 north-Swedish lakes. Rep. Inst. Preshw. Res. Drott-ningholm, 41 : 185 - 205. . 1963. Interaction between trout and char i n Scandinavia. Trans. Amer. Pish. S o c , 92: 276 -285. 1965. Pood segregation between salmonid species i n northern Sweden. Rep. Inst. Preshw. Res. Drottningholm, 46: 58 - 78. 1967. Interactive segregation between f i s h species. In: Gerking, S. D. (Ed.), The b i o l o g i c a l basis of freshwater f i s h production. Black-w e l l S c i e n t i f i c Publications, Oxford and Edinburgh, p. 295 - 313. Northcote, Thomas G. 1954. Observations on the comparative ecology of two species of f i s h , Cottus asper and Oottus rhotheus i n B r i t i s h Columbia, Copeia: 25 - 28. 110 Northcote, T. G. and P. A. Larkin. 1955. Indices of productivity i n B r i t i s h Columbia lakes. J . Pish. Res. Bd. Canada, 13: 515 - 54-0. Schutz, David C. 1969. An experimental study of feeding behavior and i n t e r a c t i o n of coastal cutthroat trout (Salmo c l a r k i c l a r k i ) and d o l l y varden (Salvelinus  malma) M. Sc. Thesis, Department of Zoology, University of B r i t i s h Columbia. 81 p. APPENDIX t . Mean rations of Tubifex and per cent body weight of food eaten at 12, 24, 48, and 72 hours of deprivation by benthic and limnetic stickleback. Phenotype Tank Size Weight Weight of food eaten at mm gm deprivation l e v e l s of : (hours) 12 24 48 12 Benthic A 69 4.48 .172 .253 .288 .283 t (6.29) 1 (3.82) (5.62) (6.40) Benthic B .69 4.50 .183 .267 .286 .332 (4.09) (6.00) (6.40) (7.41) Benthic 0 69 4.39 .128 (2.92) .201 (4.58) .277 (6.31) — Limnetic D 60 2.11 .046 (2 . 18) .074 (3 .51) .062 (2.94) — Benthic E 47 1.50 .069 (4.60) .037 (2.47) .075. (5.00) • — Limnetic ? 54 1.62 .083 (5.12) .099 (6.11) .077 (4.75) — Benthic G 58 2.75 .097 (3.42) .140 (5.09) .112 (4.07) — Benthic H 54 2.25 .097 (4.31) .128 (5.69) .104 (4 .62) — Limnetic I 47 1.12 .036 (3.21) .074 (6.61) .050 (4.45) — Limnetic J 54 1.71 .051 (2.98) .099 (5.79) .145 (8.48) — Limnetic K 45 0.94 .046 (4.90) .088 (9.36) .036 (3.83) --1 ( ) = per cent body weight 112 APPENDIX 2. Regression analysis of manipulation times on amphipods by Paxton Lake stickleback Phenotype Size mm Regression 1 n Prob. b = 0 Sign. Benthic 69 y - -.14+ 1.05X 5 .038 s 59 y = -.15 + 1.48X 5 . 0 3 3 s 56 y - -.22 + 2.21X 5 .000 s 51 y = - . 1 9 + 3.39X 7 .000 s 45 y - . 5 8 + 7.19X 7 .000 s Limnetic 60 y = -.37 + 3.92X 5 .000 s 58 y -.60 + 5 . 8 3 X 7 .000 s 54 y -.83 + 9.32X 4 .104 NS 1 Number of amphipods presented to each predator. 113 APPENDIX 3. Selected exaTnples of v a r i a t i o n around mean size °^ H y a l e l l a i n the time between grasps experiment Date Phenotype Fish Mean H v a l e l l a size length - 95% C. L. ram mm May 6 Benthic 6 9 4 . 11 (3.95 - 4.27) Benthic 51 3.30 (3.15 - 3.44) Benthic 59 3.19 (3.06 - 3,33) limnetic 60 3.20 (3.00 - 3,40) May 9 Benthic 69 2.01 (1.84 - 2 . 1 8 ) Benthic 48 2 . 3 1 ( 2 . 1 6 - 2.46) Benthic 59 2.43 (2.34 - 2.53) Limnetic 60 2.43 (2.33 - 2.53) Limnetic 5S 2.37 ( 2 . 2 8 - 2.47) 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.831.1-0101273/manifest

Comment

Related Items