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Energetics of vertical migration in Chaoborus trivittatus larvae Swift, Michael Crane 1974

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1. / -ENERGETICS OF VERTICAL MIGRATION IN CHAOBOBOS TRIVITTATOS LARVAE by MICHAEL CRANE SWIFT B. Sc., U n i v e r s i t y of C a l i f o r n i a , Davis, 1966 M. A., U n i v e r s i t y of C a l i f o r n i a , Davis, 1968 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE EEGREE OF DOCTOR OF PHILOSOPHY i n the Department of Zoology He accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLOMBIA FEBRUARY, 1974 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h Columbia, I agree t h a t 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 r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y purposes may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f The U n i v e r s i t y o f B r i t i s h Columbia Vancouver 8, Canada D a t e S4/<^;/??Y i ABSTRACT One o f t h e r e c e n t t h e o r i e s f o r t h e a d a p t i v e v a l u e o f v e r t i c a l m i g r a t i o n ( M c L a r e n 1963) s t a t e s t h a t m i g r a n t s g a i n an e n e r g e t i c a d v a n t a g e o v e r n o n - m i g r a n t s b e c a u s e by a l t e r n a t i n g b e t w een a r e a s o f h i g h and low t e m p e r a t u r e s t h e y a r e a b l e t o p a r t i t i o n e n e r g y i n t o g r o w t h more e f f i c i e n t l y t h a n n o n - m i g r a n t s . The e n e r g e t i c s o f t h e v e r t i c a l m i g r a t i o n o f f o u r t h - i n s t a r C h a o b o r u s t r i v i t t a t u s l a r v a e i n E u n i c e L a k e , B r i t i s h C o l u m b i a were s t u d i e d t o i d e n t i f y and g u a n t i f y t h i s h y p o t h e s i z e d e n e r g e t i c a d v a n t a g e . F o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e u n d e r g o a r e g u l a r , s y n c h r o n o u s , d i e l v e r t i c a l m i g r a t i o n w h i c h e x p o s e s them t c a wide r a n g e o f t e m p e r a t u r e a nd p r e y d e n s i t y . F e e d i n g o c c u r s p r i m a r i l y a t n i g h t , and n e a r t h e s u r f a c e . A l t h o u g h a l l 20oplankton i n E u n i c e L a k e a r e p o t e n t i a l p r e y , Diajgtomus k e n a i c o n s t i t u t e s t h e m a j o r i t y o f f h e b i o m a s s i n t h e d i e t o f f o u r t h - i n s t a r l a r v a e . I n E u n i c e L a k e C. t r i v i t t a t u s l a r v a e grew more s l o w l y t h a n t h e i r p o t e n t i a l g r o w t h r a t e b e c a u s e o f low f o o d a v a i l a b i l i t y . S e v e r a l e n e r g e t i c s p a r a m e t e r s i n c l u d i n g c a r b o n a s s i m i l a t i o n e f f i c i e n c y , t h e e f f e c t o f t e m p e r a t u r e cn r e s p i r a t i o n r a t e , and t h e e f f e c t s o f r a t i o n s i z e and t e m p e r a t u r e on l a r v a l g r o w t h were measured i n t h e l a b o r a t c r y . C a r b o n a s s i m i l a t i o n e f f i c i e n c y o f b o t h c o p e p o d s and c l a d o c e r a n s by C. t r i v i t t a t u s i s a b o u t 6 8 % . R e s p i r a t i o n r a t e i n c r e a s e s l i n e a r l y w i t h t e m p e r a t u r e o v e r t h e r a n g e 5-25°, a l t h o u g h t h e r e i s a s u g g e s t i o n of a p l a t e a u i n c x y g e n c o n s u m p t i o n e v e r t h e t e m p e r a t u r e r a n g e t h e l a r v a e a r e e x p o s e d t o d u r i n g t h e i r m i g r a t i o n . T e m p e r a t u r e and r a t i o n s i z e i n t e r a c t t c d e t e r m i n e l a r v a l g r o w t h r a t e ; f l u c t u a t i n g t e m p e r a t u r e s l i m i t e d g r o w t h r e g a r d l e s s c f p r e y d e n s i t y w h i l e a t 2 0 ° p r e y d e n s i t y l i m i t e d t h o g r o w t h r a t e . E m p i r i c a l d a t a f r c m t h e f i e l d and l a b o r a t o r y were i n c o r p o r a t e d i n t o a g e n e r a l i z e d c o m p u t e r s i m u l a t i o n model c f t h e e n e r g e t i c s o f a v e r t i c a l l y m i g r a t i n g l a r v a . The mcdel was used t o e x a m i n e t h e e f f e c t s o f v a r i o u s m i g r a t i o n p a t t e r n s , p h y s i c a l p a r a m e t e r s , and b i o l o g i c a l p a r a m e t e r s on l a r v a l g r o w t h . A n a l y s i s o f s e v e r a l p o s s i b l e m i g r a t i o n s t r a t e g i e s shewed t h a t , on an e n e r g e t i c s b a s i s a l o n e , g r o w t h w i l l be m a x i m i z e d by e i t h e r s t a y i n g n e a r t h e s u r f a c e where t h e r e i s f e e d , o r by v e r t i c a l l y i r i g r a t i n g w i t h a p h y s i o l o g i c a l l y d e t e r m i n e d p e r i o d i c i t y b a s e d on i n d i v i d u a l f e e d i n g h i s t o r y . The r e s u l t s c f l a b o r a t o r y g r o w t h e x p e r i m e n t s and c o m p u t e r s i m u l a t i o n s a g r e e d w i t h t h e s e two a l t e r n a t i v e s t r a t e g i e s . However, i l i l i t t a t u s l a r v a e i n E u n i c e L a k e do n o t f o l l o w e i t h e r o f t h e s e p a t t e r n s . No a l t e r n a t i v e h y p o t h e s i s t c e x p l a i n t h e i r m i g r a t i o n p a t t e r n i s a t t r a c t i v e ? , and I c o n c l u d e t h a t i t i s a r e l i c t o f p r e v i o u s s e l e c t i o n f o r t h i s p a t t e r n i n l a k e s c o n t a i n i n g d i u r n a l v e r t e b r a t e p r e d a t o r s . i i i TABLE OF CONTENTS Page A b s t r a c t i Table of Contents i i i L i s t of Fi g u r e s v i L i s t of Tables v i i i Acknowledgements i x I. General I n t r o d u c t i o n 1 I I . General Methods 5 Study Area, Sampling, Holding F a c i l i t i e s 5 C o n t r o l l e d Temperature F a c i l i t i e s 6 Carbon-14 Assays 8 L a r v a l Dry Weights and C a l o r i m e t r y 9 Nomenclature 10 I I I . F i e l d S t u d i e s 12 I n t r o d u c t i o n 12 Methods 13 Resul t s 15 Temperature 15 V e r t i c a l M i g r a t i o n 16 Zooplankton D i s t r i b u t i o n 18 Feeding 20 L a r v a l Growth i n the F i e l d 25 D i s c u s s i o n 27 Summary v. •• 33 i v IV. E n e r g e t i c s S t u d i e s .............................. 34 I n t r o d u c t i o n 34 Methods 37 S t r i k e E f f i c i e n c y , Contact E f f i c i e n c y , Handling Time 37 F r i c t i o n C o e f f i c i e n t 38 A s s i m i l a t i o n 39 R e s p i r a t i o n Rates 41 Growth Experiment I 46 Growth Experiment I I 47 R e s u l t s 4 8 S t r i k e E f f i c i e n c y , Contact E f f i c i e n c y , Handling Time 48 Energy Cost of V e r t i c a l M i g r a t i o n 51 A s s i m i l a t i o n E f f i c i e n c y 53 R e s p i r a t i o n 55 Growth Experiment I 59 Growth Experiment II 61 D i s c u s s i o n 65 Summary 77 V. S i m u l a t i o n s t u d i e s 79 I n t r o d u c t i o n 79 T h e o r i e s on the Adaptive Value of V e r t i c a l M i g r a t i o n 79 The S i m u l a t i o n Model 85 Methods 86 R e s u l t s . 91 S e n s i t i v i t y 91 The E f f e c t of Food . 96 The E f f e c t of Temperature , 101 D i s c u s s i o n 104 Summary 119 L i t e r a t u r e C i t e d 121 Appendix I 128 Appendix II 135 v i L I S T OF FIGURES F i g u r e Page 1 D i e l f l u c t u a t i n g t e m p e r a t u r e r e g i m e f o r g r o w t h e x p e r i m e n t s I (A) and I I ( B ) , ........ 7 2 S c h e m a t i c d i a g r a m o f t h e a p p a r a t u s u s e d t o p r o d u c e f l u c t u a t i n g t e m p e r a t u r e s i n a w a t e r b a t h . 8 3 I s o t h e r m s i n E u n i c e L a k e i n 1970-1971 and 1971-1972 16 4 D i e l v e r t i c a l d i s t r i b u t i o n o f o l d f o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e i n E u n i c e L a k e . 17 5 T o t a l z o o p l a n k t o n i n E u n i c e L a k e i n 1972. ...... 19 6 Time o f f e e d i n g o f o l d f o u r t h - i n s t a r £• t r i v i t t a t u s l a r v a e on f i v e d a t e s i n 1 9 7 T - 1 9 7 2 . . 7 2 1 7 D e p t h o f f e e d i n g o f o l d f o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e on 20-21 S e p t e m b e r , 1971. 23 8 The s p e c i e s c o m p o s i t i o n o f t h e d i e t o f f o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e and o f z o o p l a n k t o n i n E u n i c e L a k e i n 1972. 25 9 G r o w t h o f C h a o b o r u s l a r v a e i n E u n i c e L a k e — 1971-1973. . . 7 26 10 C o n s t a n t p r e s s u r e r e s p i r o m e t e r d e s i g n . 42 11 I n c u b a t i o n a p p a r a t u s f c r m i c r o - W i n k l e r o x y g e n c o n s u m p t i o n m e a s u r e m e n t s . 44 12 S t r i k e and c o n t a c t s u c c e s s o f f o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e a s a f u n c t i o n o f p r e y s i z e . 49 13 R e s p i r a t i o n r a t e s o f o l d f o u r t h - i n s t a r l a r v a e (1972 y e a r - c l a s s ) a t d i f f e r e n t t e m p e r a t u r e s 56 14 R e s p i r a t i o n r a t e s o f young f o u r t h - i n s t a r l a r v a e (1972 y e a r - c l a s s ) a t d i f f e r e n t t e m p e r a t u r e s 57 v i i 15 L a r v a l growth during Growth Experiment I. ...... 60 16 L a r v a l growth during Growth Experiment I I . ..... 62 17 G e n e r a l i z e d flow diagram of the s i m u l a t i o n model 88 18 The e f f e c t s of changes i n the parameter values of s e l e c t e d parameters on l a r v a l growth 92 19 The e f f e c t of migration p a t t e r n on l a r v a l growth — n a t u r a l prey d e n s i t i e s 97 20 The e f f e c t of migrat i o n p a t t e r n and s u r f a c e prey d e n s i t y on l a r v a l growth. .............. 98 21 The e f f e c t of migrat i o n p a t t e r n and prey d e n s i t y p r o f i l e cn l a r v a l growth 100 22 The e f f e c t of migrat i o n p a t t e r n and temperature p r o f i l e on l a r v a l growth. ....... 102 v i i i LIST OF TABLES Table Page 1 Comparison cf f e e d i n g by f o u r t h - i n s t a r l a r v a e during the day and n i g h t . 22 2 Mean and standard e r r o r of the time r e q u i r e d f o r f o u r t h - i n s t a r l a r v a e to i n g e s t d i f f e r e n t prey s i z e s and s p e c i e s , and the a n a l y s i s of varia n c e t a b l e f o r the d i f f e r e n c e s between s i z e s and s p e c i e s . ........ 50 3 Experimental measurements of l a r v a l s i n k i n g v e l o c i t y and c a l c u l a t e d f r i c t i o n c o e f f i c i e n t s . 53 I 4 Carbon a s s i m i l a t i o n e f f i c i e n c i e s of f o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e fed on twc prey types. The values are means + 95?? confidence l i m i t s . 54 5 Terms of the r e g r e s s i o n e g u a t i c n s : In (B) =ln (a)+b (In (H)) d e s c r i b i n g the r e l a t i o n s h i p between oxygen consumption (R) and dry weight (H) 59 6 Re s u l t s of a two-way a n a l y s i s c f varia n c e with r a t i o n and temperature as main e f f e c t s i n Growth Experiment I I . 64 i x ACKNOWLEDGEMENTS F i n a n c i a l s u p p o r t f o r t h i s s t u d y was p r o v i d e d by N a t i o n a l R e s e a r c h C o u n c i l R e s e a r c h G r a n t NRC 67-3454 t o Dr. T. G. N o r t h c o t e . S e v e r a l t e a c h i n g a s s i s t a n t s h i p s i n t h e D e p a r t m e n t o f Z o o l o g y a r e g r a t e f u l l y a c k n o w l e d g e d . The a d m i n i s t r a t i o n o f t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a R e s e a r c h F o r e s t p r o v i d e d l o g i s t i c s u p p o r t . W i t h o u t t h e h e l p o f many p e o p l e t h i s s t u d y c o u l d n o t have been c o m p l e t e d . R e g i n a C l a r o t t o d e s e r v e s s p e c i a l r e c o g n i t i o n and t h a n k s f o r c o u n t i n g z o o p l a n k t o n and h e l p i n g w i t h r o u t i n e l a b o r a t o r y e x p e r i m e n t s . Dr. T e r e s a S t a c h u r s k a p r o v i d e d h e l p f u l d i s c u s s i o n s on b i o e n e r g e t i c s and o b t a i n e d one o f t h e r e s p i r o m e t e r s u s e d i n t h e s t u d y f r o m P o l a n d . D o l o r e s L a u r i e n t e and t h e s t a f f o f t h e b i o l o g y d a t a c e n t e r c h e e r f u l l y d o n a t e d t i m e and a d v i c e on c o m p u t e r p r o g r a m m i n g . C a r l W a l t e r s and P i e r r e K l e i b e r h e l p e d i n t h e f o r m u l a t i o n and c o n s t r u c t i o n o f t h e s i m u l a t i o n m o d e l . P i e r r e was an i n v a l u a b l e s o u r c e o f i d e a s , a p p a r a t u s , and e n t h u s i a s m t h r o u g h o u t t h e c o u r s e o f my r e s e a r c h . T u r i n g many h o u r s o f d i s c u s s i o n , Kim H y a t t p r o v i d e d a c l e a r l o g i c a g a i n s t which t o t r y my e n t h u s i a s t i c b u t o f t e n p o o r l y d e v e l o p e d i d e a s . D r s . C. J . W a l t e r s , G. G. E. S c u d d e r , T. H. C a r e f o o t , and I . E. E f f o r d a d v i s e d me d u r i n g t h e e a r l y s t a g e s o f t h e r e s e a r c h , and D r s . W a l t e r s , S c u d d e r , and C a r e f o o t r e v i e w e d t h e X t h e s i s . D r . T. G. N o r t h c c t e p r o v i d e d a j u d i c i o u s m i x t u r e o f p a t i e n c e , a d v i c e , and c r i t i c i s m d u r i n g h i s t e n u r e as my r e s e a r c h s u p e r v i s o r . F o r t h i s I am e x t r e m e l y g r a t e f u l . He p r o v i d e d d i r e c t i o n w i t h o u t d i c t a t i n g , c r i t i c i s m t h a t was c o n s t r u c t i v e and f r i e n d l y , and p a t i e n c e enough t o a l l o w me t o f i n d my own way. My s p e c i a l t h a n k s gc t o A l i c e F e d o r e n k o who f o r n e a r l y t h r e e y e a r s s h a r e d i n e v e r y p h a s e o f t h i s s t u d y . Her u n s t i n t i n g e f f o r t s i n t h e f i e l d and l a b o r a t o r y , w h i l e p u r s u i n g h e r own r e s e a r c h , and h e r c o n s t a n t i n t e r e s t i n C h a o b o r u s a r e r e s p o n s i b l e , t o a g r e a t e x t e n t , f o r t h e s u c c e s s o f t h i s s t u d y . I w o u l d e s p e c i a l l y l i k e t o t h a n k Nancy K l e i b e r , K a t h y K r e b s , and my p a r e n t s f o r p r o v i d i n g m o r a l s u p p o r t w h i l e I was i n v o l v e d i n t h i s s t u d y . 1 I . GENERAL INTRODUCTION The v e r t i c a l m i g r a t i o n o f p l a n k t c n i c a n i m a l s i s cne o f t h e most s t u d i e d and l e a s t u n d e r s t o o d phenomena i n a q u a t i c b i o l o g y . B i o l o g i s t s have been i n t e r e s t e d i n v e r t i c a l m i g r a t i o n i n b o t h m a r i n e and f r e s h w a t e r s f o r a t l e a s t 250 y e a r s . E a r l y u n d e r s t a n d i n g o f t h e m i g r a t i o n p r o c e s s came f r o m work cn m a r i n e f o r m s , b u t f r e s h w a t e r s t u d i e s a r e w e l l r e p r e s e n t e d i n t h e v e r t i c a l m i g r a t i o n l i t e r a t u r e ( H u t c h i n s o n 1 9 6 7 ) . As t e c h n i q u e s and u n d e r s t a n d i n g a d v a n c e d , t h e r e has been a p r o g r e s s i o n f r o m d e s c r i p t i v e s t u d i e s on v e r t i c a l d i s t r i b u t i o n , t o e x p e r i m e n t a l work on t h e s t i m u l i e l i c i t i n g and mechanisms c o n t r o l l i n g t h e m i g r a t i o n , and t o e x p e r i m e n t a l and t h e o r e t i c a l s t u d i e s a i m e d a t e x p l a i n i n g t h e a d a p t i v e v a l u e o f t h i s phenomenon ( R u s s e l l 1927, H a u c h l i n e and F i s h e r 1969) . V e r t i c a l m i g r a t i o n s o c c u r i n a l m o s t a l l g r o u p s o f p l a n k t o n and i n some f i s h ( C u s h i n g 1 9 5 1 ) . L i g h t became g e n e r a l l y a c c e p t e d a s t h e s t i m u l u s i n i t i a t i n g t h e m i g r a t i o n , and l i g h t , i n c o n j u n c t i o n w i t h p h y s i o l o g i c a l e f f e c t s and o t h e r e n v i r o n m e n t a l f a c t o r s , h a s been i m p l i c a t e d i n t h e c o n t r o l o f t h e m i g r a t i o n . R u s s e l l (1927) r e v i e w s e a r l y work on v e r t i c a l d i s t r i b u t i o n and c o n t r o l m e chanisms. C u s h i n g (1951) p r o p o s e d a g e n e r a l scheme f o r t h e m i g r a t i o n p a t t e r n o f a l l v e r t i c a l l y m i g r a t i n g z c o p l a n k t c n b a s e d on l i g h t a s t h e i n i t i a t i n g and c o n t r o l l i n g s t i m u l u s . H i s p a t t e r n h a s f o u r p h a s e s : 1) a s c e n t f r o m t h e day d e p t h , 2) 2 midnight s i n k i n g , 3) dawn r i s e , 4) sharp descent to the day depth. T h i s p a t t e r n , though developed with r e s p e c t to marine forms, i s v a l i d f o r freshwater forms as well (Hutchinson 1967). S e v e r a l t h e o r i e s have been proposed to account f o r the adaptive value of v e r t i c a l m i g r a t i o n : escape from predators ( M a n t e i f e l 1959a, b, Pearre 1973), h o r i z o n t a l t r a n s p o r t (Hardy and Gunther 1935, Hardy 1956, David 1961), s o c i a l c c n t r o l of p o p u l a t i o n s i z e (Wynne-Edwards 1962), energy gain (McLaren 1963, K e r f c c t 1970), a combination of the above (Hutchinson 1967, Mauchline and F i s h e r 1969), and demographic e f f e c t s (McLaren 1974). The merits of these t h e o r i e s are considered i n S e c t i o n V. McLaren's (1963) hypothesis f o r the adaptive value cf v e r t i c a l m igration i s the only proposal to date that c o n s i d e r s the energy budget of the m i g r a t i n g animal. He concludes that an "energy boost" accrues to migrants from the more e f f i c i e n t p a r t i t i o n i n g of energy to growth at low temperatures and that t h i s "energy boost" can be used f o r growth, f e c u n d i t y , e t c . The o b j e c t of t h i s study was to examine the e n e r g e t i c s of a v e r t i c a l l y m igrating zooplankter, to see i f McLaren's hypothesized "energy boost" could be demonstrated, and to determine the p h y s i c a l and b i o l o g i c a l f a c t o r s which a f f e c t i t s magnitude. Chaoborus l a r v a e were chosen as the experimental animal because they are well known v e r t i c a l m i g r a t o r s , they 3 are r e a d i l y c o l l e c t e d and e a s i l y maintained i n the l a b o r a t o r y , and, s i n c e they are predators, t h e i r food consumption i s e a s i l y monitored. The study c o n s i s t e d of three phases. 1) F i e l d s t u d i e s were done to examine the v e r t i c a l m i g r a t i o n , f e e d i n g , and growth of c. t r i v i t t a t u s l a r v a e . 2) l a b o r a t o r y experiments were c a r r i e d out to measure the elements of the energy budget eguations C=P+R+F+U and A=P + R (C i s consumption, P i s growth, R i s r e s p i r a t i o n , F i s f e c e s , U i s excreted m a t e r i a l and A i s a s s i m i l a t i o n ) . 3) A g e n e r a l i z e d computer s i m u l a t i o n model of v e r t i c a l m i g r a t i o n was used i n c o n j u n c t i o n with the r e s u l t s of f i e l d and l a b o r a t o r y s t u d i e s to examine the e f f e c t s of v a r i o u s v e r t i c a l m igration s t r a t e g i e s , p h y s i c a l parameters, and b i o l o g i c a l parameters on l a r v a l growth. Larvae of the "phantom midge", Chaoborus, are well known i n the fauna of l e n t i c f r e s h w a t e r . They t y p i c a l l y spend the d a y l i g h t hours b u r i e d i n the mud, emerge at dusk and migrate i n t o the water column to f e e d , and burrow i n t o the mud again at dawn. Although t h e i r migration has f a s c i n a t e d b i o l o g i s t s s i n c e i t was f i r s t d i s c o v e r e d , most of the l i t e r a t u r e on t h i s aspect of t h e i r b i o l o g y has been, and i s s t i l l , d e s c r i p t i v e (Juday 1921, Worthington and Ricardo 1936, Wood 1956, Roth 1968, Sikorowa 1973). Some q u a n t i t a t i v e experimental work has been done on the nature and c o n t r o l of the m i g r a t i o n (Eerg 1937, Teraguchi and Northcote 1966, LaRow 1968, 1969, 1970, 4 C h a s t o n 1 9 6 9 ) . S t u d i e s on t h e e c o l o g y o f C h a o b o r u s have been p a r t i c u l a r l y c o n c e r n e d w i t h t h e i r m i g r a t i o n , b u t some i n f o r m a t i o n on t h e i r f e e d i n g i s a v a i l a b l e (Dodscn 1970, Parma 1971, F e d o r e n k o 1 9 7 3 ) . S t u d i e s on C h a o b o r u s p h y s i o l o g y have been w i d e i n s c o p e and i n c l u d e t h e f o l l o w i n g a r e a s : l o c o m o t i o n ( N a c h t i g a l l 1 9 6 5 ) , d i g e s t i o n ( G e r s c h 1952, M o n t s h a d s k y 1 9 4 5 ) , r e s p i r a t i o n ( J o n a s s o n 1972, W e l c h 1 9 6 8 ) , p u p a t i o n and e m e r g e n c e ( D e o n i e r 1943, Parma 1971, B r a d s h a w 1 9 7 3 ) , a n a e r o b i o s i s ( L indeman 1942, S i k o r o w a 1968, and P r o k e s o v a 1 9 5 9 ) . A C h a o b o r u s b i b l i o g r a p h y has been c o m p i l e d by R o t h and Parma (1970) w h i c h i n c l u d e s most o f t h e i n f o r m a t i o n on t h i s f a s c i n a t i n g a n i m a l p u b l i s h e d b e f o r e 1970. / 5 I I . GENERAL METHODS S t u d y A r e a , S a m p l i n g , H o l d i n g F a c i l i t i e s E x p e r i m e n t a l work i n t h i s s t u d y was c a r r i e d o u t u s i n g f o u r t h - i n s t a r l a r v a e o f C h a o b o r u s (Shadanojahasma) t r i v i t t a t u s (Loew) c o l l e c t e d f r o m E u n i c e L a k e , B r i t i s h C o l u m b i a . E u n i c e L a k e i s a s m a l l o l i g o t r o p h i a l a k e i n t h e U n i v e r s i t y o f E r i t i s h C o l u m b i a R e s e a r c h F o r e s t n e a r Haney, B. C. The l a k e l i e s a t an e l e v a t i o n c f 480m, has a mean d e p t h o f 15.8m, a maximum d e p t h c f 42m, and a s u r f a c e a r e a o f 18.2 ha. I t i s u s u a l l y i c e c o v e r e d f r o m mid-December t o A p r i l o r May. D e t a i l s o f r o u t i n e s a m p l i n g m e t h o d o l o g y and t h e g e n e r a l e c o l o g y o f C. t r i v i t t a t u s i n t h e l a k e a r e g i v e n e l s e w h e r e ( F e d o r e n k c and S w i f t 1 9 7 2 ) . S p e c i a l s a m p l i n g t e c h n i q u e s u s e d f o r s p e c i f i c e x p e r i m e n t s a r e d e s c r i b e d i n t h e p e r t i n e n t s e c t i o n s . L a r v a e f o r e x p e r i m e n t a l use were h e l d i n t h e d a r k a t a c o n s t a n t t e m p e r a t u r e o f 6° i n t h e l a b o r a t o r y . A l l e x p e r i m e n t s were r u n w i t h i n a few d a y s o f when t h e l a r v a e were c a p t u r e d . L a r v a e were h e l d w i t h o r w i t h o u t f o o d d e p e n d i n g on t h e e x p e r i m e n t f o r w h i c h t h e y were t o be u s e d ; l a r v a e were f e d m i x e d z o o p l a n k t o n f r o m E u n i c e L a k e when n e c e s s a r y . 6 C o n t r o l l e d T e m p e r a t u r e F a c i l i t i e s C o n s t a n t t e m p e r a t u r e s o f 5° and 20° ± 1° were m a i n t a i n e d i n P e r c i v a l i n c u b a t o r s ( P e r c i v a l , Boone, I o w a ) . F l u c t u a t i n g t e m p e r a t u r e s (7 - 15° and 5 - 20°, F i g . 1) were p r o d u c e d i n a w a t e r b a t h by a l t e r n a t e l y c y c l i n g c o l d and h o t w a t e r . The r a t e o f c h a n g e was c o n t r o l l e d by t h e d e g r e e o f m i x i n g o f t h e two w a t e r s u p p l i e s . The w a t e r s u p p l y s y s t e m c o n s i s t e d o f two 900 l i t e r t a n k s e a c h c o n t a i n i n g a s u b m e r s i b l e pump ( L i t t l e G i a n t , model 1-42A) w h i c h s u p p l i e d t h e w a t e r b a t h . W a t e r r e t u r n t o t h e t a n k s was c o n t r o l l e d by s o l e n o i d v a l v e s ( A s c o Nc. 759 1S, A s c o l e c t r i c B r a n t f o r d L t d . , B r a n t f o r d , O n t a r i o ) i n e a c h d r a i n l i n e ( F i g . 2 ) . The s u b m e r s i b l e pump and d r a i n v a l v e o f e a c h t a n k were w i r e d t o t i m e s w i t c h e s s u c h t h a t t h e d r a i n v a l v e s were open when t h e pumps were r u n n i n g . Water t e m p e r a t u r e s were m a i n t a i n e d u s i n g r e f r i g e r a t i o n u n i t s b u c k i n g a g a i n s t room t e m p e r a t u r e (5° t a n k ) and a g a i n s t a h e a t i n g c o i l s e t above 20° (20° t a n k ) . O p e r a t i o n o f t h e two s y s t e m s was u n d e r t h e c o n t r o l o f two t i m e s w i t c h e s ( I n t e r m a t i c , M o d e l T 1 0 1 ) . C a r b o n - m A s s a y s A l l c a r b o n - 1 4 a s s a y s were done by l i q u i d s c i n t i l l a t i o n c o u n t i n g ( N u c l e a r C h i c a g o , I s o c a p / 3 0 0 o r Mark I) w i t h e x t e r n a l s t a n d a r d g u ench c o r r e c t i o n . S o l u b l e a q u e o u s s a m p l e s (up t o 1 ml) were c o u n t e d i n 10 ml o f B r a y ' s s c i n t i l l a t i o n s o l u t i o n (4g PPO, 2g POEOP, 60g n a p t h a l e n e , 100 ml m e t h a n o l , 20 ml e t h y l e n e 7 FIGDRE 1 D i e l f l u c t u a t i n g t e m p e r a t u r e r e g i m e f o r g r o w t h e x p e r i m e n t s I (A) and II ( B ) . T e m p e r a t u r e s a r e shown f o r one week i n b o t h c a s e s . 7a 8 FIGURE 2 S c h e m a t i c d i a g r a m o f t h e a p p a r a t u s u s e d t o p r c d u c e f l u c t u a t i n g t e m p e r a t u r e s i n a w a t e r b a t h . L e g e n d : 1. I n l e t h o s e f r o m 20° t a n k , 2. I n l e t hose f r o m 5° t a n k , 3. O u t l e t t o 20° t a n k , 4. O u t l e t t o 5° t a n k , 5. H e a t i n g c o i l , 6. S u b m e r s i b l e pumps, 7, 8, 9. E x p e r i m e n t a l a g u a r i a . 9 g l y c o l , made up t o 1 l i t e r w i t h 1, U - C i o x a n e ) . P a r t i c u l a t e m a t e r i a l on f i l t e r s was c o m b u s t e d o r c o u n t e d i n B r a y ' s s o l u t i o n w i t h C a f c - 0 - S i l ( f i n e l y d i v i d e d s i l i c a ) a d ded t o f o r m a g e l t o keep p a r t i c u l a t e m a t e r i a l i n s u s p e n s i o n . P a r t i c u l a t e m a t e r i a l was d r i e d and c o m b u s t e d i n a t u b e f u r n a c e ( L i n d b e r g H e v i - D u t y ) a t 500-600° u n d e r a s t r e a m o f o x y g e n . C a r b o n d i o x i d e i n t h e o u t f l o w s t r e a m was c o l l e c t e d i n 8 ml o f e t h a n o l a m i n e t r a p p i n g s o l u t i o n ( J e f f a y and A l v a r e z 1961) i n a V i g r e u x c o l u m n . F o l l o w i n g c o m b u s t i o n and 10 m i n u t e s o f f l u s h i n g , t h e t r a p p i n g s o l u t i o n was r i n s e d i n t o a s c i n t i l l a t i o n v i a l w i t h 10 ml o f a t o l u e n e s c i n t i l l a t i o n s o l u t i o n (5g PPO, 0.3g POPOP, made up t o 1 l i t e r w i t h t o l u e n e ) and t h e n c o u n t e d . The e f f i c i e n c y o f t h i s c o m b u s t i o n p r o c e d u r e has been c h e c k e d w i t h v a r i o u s a m o u n t s o f p a r t i c u l a t e m a t e r i a l and known amounts o f 1 * C - g l u c c s e , and t h e r e c o v e r y was f c u n d t o be v i r t u a l l y 100%. Raw c o u n t s were c o n v e r t e d t o d i s i n t e g r a t i o n s p e r m i n u t e (dpm) and c o r r e c t e d f o r b a c k g r o u n d b e f o r e a n a l y s i s . l a r v a l Dry W e i g h t s and C a l o r i m e t r y L a r v a e were c o l l e c t e d u s i n g d i a g o n a l h a u l s f r o m 20-0m e v e r y two weeks i n 1971 and e v e r y week i n 1972 w i t h a 30cm o r 60cm s q u a r e n e t . S e v e r a l s a m p l e s were t a k e n d u r i n g t h e w i n t e r o f 1971-1972. S a m p l e s were s o r t e d and d r i e d a t 100° i n 1971 and 60° i n 1972. The l a r v a e were s e p a r a t e d f r o m p o t e n t i a l f o o d j u s t a f t e r c a p t u r e and no l a r v a e w i t h f r e s h l y c a u g h t p r e y i n 10 t h e i r c r o p s were u s e d . G e n e r a l l y 5 o r more r e p l i c a t e w e i g h t s were m e a s u r e d p e r i n s t a r w i t h t h e number o f l a r v a e p e r r e p l i c a t e v a r y i n g f r o m 50-100 f o r t h e f i r s t and s e c o n d i n s t a r s t o 10 f o r t h e f o u r t h i n s t a r . A l l s a m p l e s were w e i g h e d cn a Cahn Gram E l e c t r o b a l a n c e . C a l o r i c c o n t e n t o f young and o l d f o u r t h - i n s t a r l a r v a e was measured i n e a r l y s p r i n g u s i n g a P h i l l i p s c n o x y g e n m i c r o t o m b c a l o r i m e t e r ( P h i l l i p s o n 1964) m a n u f a c t u r e d by G e n t r y - W e i g e r t I n s t r u m e n t s I n c . , A i k e n , S o u t h C a r o l i n a . S a m p l e s were d r i e d a t 60° f o r t h r e e d a y s and s t o r e d i n a d e s i c c a t o r u n t i l u s e d . F i v e r e p l i c a t e s o f e a c h s a m p l e were c o m b u s t e d . C a l o r i c c o n t e n t was c a l c u l a t e d a s c a l o r i e s p e r gram d r y w e i g h t . N o m e n c l a t u r e B e c a u s e C. t r i v i t t a t u s h a s a two y e a r l i f e c y c l e ( F e d o r e n k o and S w i f t 1972) t h e r e a r e two y e a r - c l a s s e s o f f o u r t h - i n s t a r l a r v a e p r e s e n t i n t h e l a k e d u r i n g p a r t o f t h e summer. I have used t h e t e r m s " o l d " and " y o u n g " t o d i s c r i m i n a t e b e t w e e n t h e y e a r c l a s s e s . O l d f o u r t h - i n s t a r l a r v a e a r e t h o s e l a r v a e t h a t a r e i n t h e i r s e c o n d summer o r w i n t e r , and y oung f o u r t h - i n s t a r l a r v a e a r e t h o s e i n t h e i r f i r s t summer o r w i n t e r . A l l t e m p e r a t u r e s r e f e r r e d t o a r e d e g r e e s C e n t i g r a d e . I have u s e d g e n e r i c names t o r e f e r t o t h e v a r i o u s a n i m a l s d i s c u s s e d a f t e r t h e y a r e f i r s t m e n t i o n e d e x c e p t f o r t h o s e g e n e r a w h i c h a r e r e p r e s e n t e d by more t h a n one 11 s p e c i e s ; these are r e f e r r e d to by t h e i r generic and s p e c i f i c names. 12 I I I . FIELD STUDIES I n t r o d u c t i o n In order to assess the r o l e of e n e r g e t i c s i n the adaptive value of v e r t i c a l m i g r a t i o n , i t i s necessary to know the timing and magnitude of the migration and the temporal c h a r a c t e r i s t i c s of those b i o t i c and a b i o t i c parameters which a f f e c t the e n e r g e t i c budget of the migrator. P r e l i m i n a r y a n a l y s i s of Chaoborus v e r t i c a l migration suggested that temperature, through i t s e f f e c t on v a r i o u s temperature dependent r a t e s , and prey d e n s i t y and d i s t r i b u t i o n , through t h e i r e f f e c t on l a r v a l f e e d i n g , are the two most important f i e l d parameters a f f e c t i n g the e n e r g e t i c s of Chaoborus l a r v a e . The f i e l d p o r t i o n of t h i s study was designed to answer fo u r questions. 1) What i s the v e r t i c a l migration p a t t e r n of £• t r i v i t t a t u s i n Eunice Lake ? 2) what are the p h y s i c a l and b i o l o g i c a l c h a r a c t e r i s t i c s of the environment the l a r v a e l i v e i n ? 3) What are the c h a r a c t e r i s t i c s of l a r v a l feeding? 4) What are the c h a r a c t e r i s t i c s of the growth of the l a r v a e i n the lake? The r e s u l t s of these f i e l d s t u d i e s provide a r e f e r e n c e data base f o r use i n the c o n s t r u c t i c n of a g e n e r a l i z e d model cf a v e r t i c a l l y migrating Chaoborus l a r v a . 13 M e t h o d s T e m p e r a t u r e , p l a n k t o n v e r t i c a l d i s t r i b u t i o n , a n d l a r v a l g r o w t h w e r e m o n i t o r e d u s i n g t h e m e t h o d s o f F e d o r e n k o a n d S w i f t ( 1 9 7 2 ) . T e m p e r a t u r e a n d l a r v a l g r o w t h w e r e m e a s u r e d w e e k l y f r o m M a y t c N o v e m b e r . V e r t i c a l d i s t r i b u t i o n o f l a r v a e a n d z o o p l a n k t o n w a s m o n i t o r e d u s i n g a C l a r k e - B u m p u s ( C - B ) p l a n k t o n n e t w i t h a N o . 2 0 ( 0 . 0 8 m m ) n y l o n n e t . S a m p l e s w e r e t a k e n w e e k l y a t n e o n a n d m i d n i g h t u s i n g d i a g o n a l h a u l s s p a c e d e v e r y 2m f r o m 0 - 2 0 m . T h e d i a g o n a l h a u l s w e r e m a d e b y s l o w l y r a i s i n g t h e C - B s a m p l e r 2m w h i l e t h e b o a t w a s m o v i n g . F o o d o f t h e l a r v a e w a s d e t e r m i n e d d i r e c t l y b y d i s s e c t i n g t h e c r o p a n d e n u m e r a t i n g i t s c o n t e n t s f o l l o w i n g t h e m e t h o d o f S w i f t a n d F e d o r e n k o ( 1 9 7 3 ) . C r o p s w e r e c h a r a c t e r i z e d b y t h e p r e s e n c e o r a b s e n c e o f f o o d , a n d t h e d e g r e e o f d i g e s t i o n o f t h e f o o d . P r e y i t e m s f r o m t h e c r o p s w e r e d e s i g n a t e d a s " f r e s h " i f t h e y w e r e i d e n t i f i a b l e a s e n t i r e , i n d i v i d u a l z o o p l a n k t e r s . T h i s d e f i n i t i o n e x c l u d e d a n i m a l s w h o s e e x o s k e l e t o n w a s m a c e r a t e d f r o m t h e " f r e s h " c a t e g o r y . T h e r a t e o f d i g e s t i o n v a r i e d w i t h t e m p e r a t u r e a n d p r e y t y p e ; B o s m i n a r e m a i n e d " f r e s h " f o r a b o u t o n e h o u r , a n d D . k e n a i r e m a i n e d " f r e s h " f o r a b o u t t h r e e h o u r s a t s u m m e r s u r f a c e t e m p e r a t u r e s . T h e s e p a r t i a l d i g e s t i o n t i m e s w e r e l o n g e r a t l o w e r t e m p e r a t u r e s . R e s u l t s o f t h e s e a n a l y s e s a r e e x p r e s s e d a s t h e p e r c e n t a g e o f c r o p s t h a t c o n t a i n e d a n y a m o u n t o f f o o d a n d t h e p e r c e n t a g e o f c r o p s t h a t c o n t a i n e d f r e s h f o o d . 14 T h e t i m e o f d a y t h a t t h e l a r v a e f e d w a s d e t e r m i n e d b y s a m p l i n g e v e r y t h r e e h o u r s o v e r a 2 4 h o u r p e r i o d . T h i s w a s d o n e o n 2 9 S e p t e m b e r , 1 9 7 1 , a n d o n 17 J u l y , 8 A u g u s t , 6 S e p t e m b e r , a n d 7 O c t o b e r , 1 9 7 2 . I n 1 9 7 1 h o r i z o n t a l h a u l s w e r e m a d e a t 2m i n t e r v a l s ( 1 - 2 1 ) w i t h a 3 0 c m s q u a r e n e t w i t h t h e f r a m e f i x e d t o a w e i g h t e d l i n e . T h e r e w a s p r o b a b l y l i t t l e c o n t a m i n a t i o n f r o m d e p t h s n o t b e i n g s a m p l e d b e c a u s e t h e n e t o p e n i n g w a s h e l d v e r t i c a l l y w h e n t h e n e t w a s b e i n g r a i s e d o r l o w e r e d ; t h e l a r v a l d i s t r i b u t i o n b a s e d o n t h i s m e t h o d a g r e e d w e l l w i t h t h a t b a s e d o n C - B s a m p l e s f r o m t h e s a m e d e p t h s . I n 1 9 7 2 s a m p l e s f o r d e t e r m i n i n g t i m e o f f e e d i n g w e r e c o l l e c t e d u s i n g v e r t i c a l h a u l s w i t h a 1m n e t . T h e 24 h o u r s a m p l e s f r o m 1 9 7 1 w e r e u s e d i n c c n j u n c t i c n w i t h r a t e s o f d i g e s t i o n t o d i r e c t l y d e t e r m i n e t h e d e p t h a t w h i c h f e e d i n g t a k e s p l a c e . 15 R e s u l t s i T e m p e r a t u r e T h e t e m p e r a t u r e r e g i m e i n E u n i c e L a k e w a s s i m i l a r d u r i n g t h e t w o y e a r s o f t h i s s t u d y ( F i g . 3 ) . I n b o t h y e a r s a s t a b l e t h e r m o c l i n e d e v e l o p e d a t a b o u t 4m d u r i n g t h e s u m m e r m c n t h s . T h e s h a r p d e c r e a s e i n t e m p e r a t u r e i n e a r l y J u l y 1 9 7 2 w a s d u e t o e x t r e m e l y h i g h r a i n f a l l o v e r a p e r i o d c f t h r e e d a y s . V e r t i c a l M i g r a t i o n T h e m i g r a t i o n p a t t e r n o f C . t r i v i t t a t u s l a r v a e i n E u n i c e L a k e g e n e r a l l y f o l l o w s t h e p a t t e r n d e s c r i b e d b y C u s h i n g ( 1 9 5 1 ) . T h e p a r t i c u l a r f o r m o f t h e d e p t h d i s t r i b u t i o n a n d m i g r a t i o n i s i n s t a r s p e c i f i c a n d h a s b e e n d e s c r i b e d b y F e d o r e n k o a n d S w i f t ( 1 9 7 2 , F i g . 9 ) . T h e l a r v a e c o n s i d e r e d i n t h i s s t u d y a r e t h e o l d f o u r t h - i n s t a r l a r v a e . D u r i n g t h e s u m m e r t h e y s p e n d t h e d a y a t a b o u t 1 2 m , b e g i n t o m o v e u p w a r d a t 1 8 0 0 h o u r s , r e a c h 3m b y 2 1 0 0 h o u r s , a n d t h e n s i n k s l o w l y t c t h e d a y d e p t h b y 0 9 0 0 h o u r s ( F i g . 4 ) . T h e r e i s l i t t l e s u g g e s t i o n c f a d a w n r i s e . O n l y a b o u t o n e h o u r i s s p e n t a t 3 m . T h i s p a t t e r n o f d i e l m i g r a t i o n i s f o u n d f r o m M a y t o N o v e m b e r . I n t h e w i n t e r t h e l a r v a e a r e d i s t r i b u t e d t h r o u g h o u t t h e w a t e r c o l u m n ( F e d o r e n k o a n d S w i f t 1 9 7 2 ) . 16 FIGURE 3 Isotherms i n Eunice Lake i n 1970-1971 and 1971-1972. Temperatures are i n degrees Centigrade. i FIGURE 4 D i e l v e r t i c a l d i s t r i b u t i o n of o l d f o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e i n Eunice Lake. The width of the k i t e diagrams r e p r e s e n t s numbers per 100 l i t e r s ( a f t e r Fedorenko and S w i f t 1972, F i g . 9). T I M E - H O U R S 1200 1800 2100 2400 0300 0600 0900 18 Z o o p l a n k t o n D i s t r i b u t i o n F i v e s p e c i e s , e x c l u s i v e o f C h a o h c r u s , d o m i n a t e t h e z o o p l a n k t o n i n E u n i c e L a k e o n b o t h a n u m b e r a n d b i o m a s s b a s i s . T h e s e a r e t h e c o p e p o d s D i a g t o m u s k e n a i a n d D . t _ ^ r e l l i a n d t h e c l a d o c e r a n s D a p h n i a r o s e a , ] j o l o £ s d i u m _ g i b b e r u m , a n d ^ i l £ i l 3 I 1 2 S o m a t i a c h ^ u r u m ( F e d o r e n k o a n d S w i f t 1 9 7 2 ) . T h e z o o p l a n k t o n a r e d i s t r i b u t e d t h r o u g h o u t t h e w a t e r c o l u m n ( F i g . 5 , d a t a f r o m A . F e d o r e n k o ) b u t t h e y a r e m o s t n u m e r o u s a b o v e 4 - 6 m e t e r s . T h e p r e y d e n s i t y i n t h e e p i l i m n i o n i s a s h i g h a s 6 0 0 - 1 0 0 0 p e r 100 l i t e r s f o r a s h o r t t i m e i n J u n e a n d J u l y b u t i s g e n e r a l l y a b o u t 4 0 0 p e r 1 0 0 l i t e r s . T h e 2 0 0 a n i m a l s p e r 100 l i t e r s i s o p l e t h i s d e e p e r t h a n 6m o n l y d u r i n g l a t e S e p t e m b e r . T h r o u g h o u t t h e y e a r t h e d e n s i t y i n t h e h y p o l i m n i o n i s l o w — 0 - 1 0 0 a n i m a l s p e r 1 0 0 l i t e r s . T h e r e i s s o m e d i e l c h a n g e i n t h e v e r t i c a l d i s t r i b u t i o n o f s o m e s p e c i e s , b u t o n l y D . k e n a i a n d D . r o s e a a r e f o u n d b e l o w 6 m ; a t n i g h t D . k e n a i s p r e a d s o v e r t h e e n t i r e w a t e r c o l u m n a s d e e p a s 20m f r o m i t s d a y d e p t h o f 3m a n d D . r o s e a m i g r a t e s f r o m i t s d a y d e p t h o f a b o u t 9m u p t o 2m ( A . F e d o r e n k o , p e r s . c o m m . ) . FIGURE 5 T o t a l z o o p l a n k t o n i n E u n i c e L a k e i n 1 9 7 2 . T h e l i n e s a r e z o o p l a n k t o n d e n s i t y i s o p l e t h s ( n u m b e r p e r 1 0 0 l i t e r s ) . S a m p l i n g w a s d o n e a t n o o n o n t h e d a t e s i n d i c a t e d b y a r r o w s . N a u p l i i , r o t i f e r s , a n d C h a o b o r u s l a r v a e a r e n o t i n c l u d e d . >o 03 20 F e e d i n g O n l y a s m a l l p r o p o r t i o n o f t h e l a r v a l p o p u l a t i o n f e e d s o n a g i v e n d a y ( F i g . 6 ) . A t m o s t o n l y 40% o f t h e l a r v a e e x a m i n e d h a d f u l l c r o p s a n d l e s s t h a n 20% h a d f r e s h p r e y i n t h e i r c r o p s . T h e r e w a s n o s i g n i f i c a n t d i f f e r e n c e i n t h e n u m b e r o f l a r v a e w i t h f u l l c r o p s d u r i n g t h e d a y ( f i r s t 4 s a m p l i n g t i m e s i n F i g . 6) a n d n i g h t ( s e c o n d 4 s a m p l i n g t i m e s ) o n a n y d a t e s a m p l e d i n 1 9 7 1 o r 1 9 7 2 ( T a b l e 1 ) . I n J u l y a n d O c t o b e r 1 9 7 2 s i g n i f i c a n t l y m o r e l a r v a e h a d f r e s h p r e y i n t h e i r c r o p s a t n i g h t t h a n d u r i n g t h e d a y ; t h e r e w a s n o s i g n i f i c a n t d i f f e r e n c e o n t h e o t h e r d a t e s s a m p l e d ( T a b l e 1 ) . L a r v a e w i t h n o f o o d i n t h e c r o p a r e c a p a b l e o f f e e d i n g a t a n y t i m e o f t h e d a y o r n i g h t i f p r e y a r e a v a i l a b l e . T h e l a r v a e i n f r e s h l y c a u g h t , c o n c e n t r a t e d z o o p l a n k t o n s a m p l e s a l l c a t c h p r e y w i t h i n a n h o u r o f t h e t i m e t h e y a r e c o l l e c t e d . D a t a f r o m h o r i z o n t a l t o w s t a k e n o v e r a 2 4 h o u r p e r i o d i n 1 9 7 1 i n d i c a t e t h a t o l d f o u r t h - i n s t a r l a r v a e b e g i n f e e d i n g a b o u t 1 8 0 0 h o u r s w h e n t h e y a r e a t a d e p t h o f 10m a n d c o n t i n u e t o f e e d d u r i n g t h e n i g h t ( F i g . 7 ) . T h e r e i s s o m e e v i d e n c e t h a t f e e d i n g o c c u r s d u r i n g t h e d a y , b u t i t i s p r o b a b l y a n a r t i f a c t o f s l o w d i g e s t i o n o f p r e v i o u s l y c a p t u r e d p r e y r e s u l t i n g f r o m l o w d a y - d e p t h t e m p e r a t u r e s . F o u r t h - i n s t a r C . t r i v i t t a t u s l a r v a e a r e a b l e t o f e e d o n t h e e n t i r e s i z e r a n g e o f p r e y i n t h e l a k e f r o m r o t i f e r s ( 0 . 1 m m ) t o D . k e n a i ( 2 . 3 m m ) . T h e r e a r e s e a s o n a l c h a n g e s i n FIGURE 6 T i m e o f f e e d i n g o f e l d f o u r t h - i n s t a r C . t r i v i t t a t u s l a r v a e o n f i v e d a t e s i n 1 9 7 1 - 1 9 7 2 . P e r c e n t a g e f u l l c r o p s ( s o l i d c i r c l e s a n d s g u a r e s ) a n d p e r c e n t a g e c r o p s c o n t a i n i n g f r e s h p r e y ( o p e n c i r c l e s ) a r e p l o t t e d a g a i n s t s a m p l i n g t i m e . T h e s g u a r e s a r e 1 9 7 1 d a t a . BL3 2 2 T a b l e 1 . C o m p a r i s o n o f f e e d i n g b y f o u r t h - i n s t a r l a r v a e d u r i n g t h e d a y a n d n i g h t . F u l l C r o p s C r o p s c o n t a i n i n g f r e s h f c o d D a t e d . f . X 2 P X 2 F 21 S e p t . 1 9 7 1 1 2 . 3 7 . 1 - . 2 5 • 17 J u l y 1 9 7 2 1 0 . 3 0 8 . 5 - . 7 5 8 . 26 < . 0 0 5 8 A u g . 1 9 7 2 1 0 . 1 5 3 . 5 - . 7 5 0 . 3 5 . 5 - . 7 5 6 S e p t . 1 9 7 2 1 2.140 . 1 - . 2 5 3 . 0 5 . 0 5 - . 1 7 O c t . 1 9 7 2 1 0 . 6 5 . 2 5 - . 5 1 2 . 0 4 < . 0 0 5 F I G U R E 7 D e p t h o f f e e d i n g o f o l d f c u r t h - i n s t a r C . t r i v i t t a t u s l a r v a e o n 2 0 - 2 1 S e p t e m b e r , 1 9 7 1 . P e r c e n t a g e f u l l c r o p s i s p l o t t e d a g a i n s t d e p t h a t e a c h s a m p l i n g t i m e . A t e a c h d e p t h p l o t t e d , n > 2 0 . 24 p r e y s p e c i e s d e n s i t i e s , b u t a l m o s t t h e e n t i r e s p e c t r u m o f p r e y t y p e s i s f o u n d i n t h e d i e t t h r o u g h o u t t h e s u m m e r ( F i g . 8 a ) . T h r e e s p e c i e s - - E . k e n a i , D . t _ y r e l l i , a n d H c l o j j e d i u n i m a k e u p m o s t o f t h e z o o p l a n k t o n b i o m a s s i n t h e l a k e ( F i g . 8 d ) . O f t h e s e t h r e e , o n l y p . k e n a i a n d p . t j j r e l l i a r e e a t e n t o a n y g r e a t e x t e n t . H o l o j a e d i u m i s s e l d o m f o u n d i n t h e d i e t b e c a u s e i t i s t o o l a r g e t o b e e a s i l y c a p t u r e d . M o s t c f t h e b i o m a s s i n t h e d i e t o f o l d f o u r t h - i n s t a r l a r v a e c o m e s f r o m e a t i n g D . k e n a i . T h i s c c p e p o d c o n s t i t u t e d 3 0 $ o r m o r e o f t h e b i o m a s s o f t h e d i e t i n e a c h m o n t h s a m p l e d , a n d a s m u c h a s 70?? ( F i g . 8 b ) . O l d f c u r t h - i n s t a r l a r v a e a p p e a r t o f e e d s e l e c t i v e l y c n D . k e n a i ; t h i s s p e c i e s i s f o u n d i n t h e d i e t i n m u c h h i g h e r p r o p o r t i o n s t h a n i n t h e l a k e . B o s m i n a i s n u m e r i c a l l y m u c h m o r e a b u n d a n t i n t h e l a k e t h a n i n t h e d i e t , a n d p . t j r e l l i i s e a t e n r o u g h l y p r o p o r t i o n a l l y t c i t s d e n s i t y . L a r v a l G r o w t h i n t h e F i e l d T h e p a t t e r n o f g r o w t h o f C . t r i v i t t a t u s l a r v a e i n t h e f i e l d w a s e s s e n t i a l l y t h e s a m e i n 1 9 7 1 a n d 1 9 7 2 . T h e e g g s h a t c h i n raid-June a n d t h e l a r v a e g r o w f o r t w o s u m m e r s b e f o r e e m e r g i n g ( F i g . 9 , f r o m F e d o r e n k o 1 9 7 3 ) . fit a n y t i m e d u r i n g t h e y e a r , e x c e p t f o r a s h o r t t i m e b e f o r e t h e e g g s h a t c h i n t h e s p r i n g , t h e r e a r e t w o y e a r c l a s s e s p r e s e n t i n t h e l a k e . F r o m O c t o b e r t o J u n e b o t h y e a r c l a s s e s a r e i n t h e f o u r t h i n s t a r . I n t h e i r s e c o n d y e a r t h e l a r v a e g r o w a t a f a s t e r r a t e t h a n t h e y d o i n t h e i r f i r s t y e a r ( F i g . 9) ; t h e s e t w o g r o w t h p a t t e r n s FIGURE 8 The s p e c i e s composition of the d i e t of f o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e and of zooplankton i n Eunice Lake i n 1972. A. Percentage frequency of occurrence of prey i n l a r v a l crops. B. Percentage composition by weight of prey i n l a r v a l crops. C. Percentage freguency of occurrence of some zooplankton i n Eunice Lake. D. Percentage composition by weight of some zooplankton i n Eunice Lake. R o t i f e r s and Chaoborus are not i n c l u d e d i n C and D. The s p e c i e s cede' i s as f o l l o w s : B — r o t i f e r s , B — Bosmina, P — HO-iY-P-hemus , D — Dap__hn i a , H — Hclo^e^l"iS» c Di^Jlhanc^onia a n <3 m i s c e l l a n e o u s c l a d o c e r a n s , F — D. Kenai, T — D. T v r e l l i , 0 — Chaoborus l a r v a e , X — u n i d e n t i f i e d m a t e r i a l . Percent Composit ion of Prey ( o j & c n c o r o ^ c n c o F O f e O l C O r o j ^ m a ) o o o o o o o o o o o o o o o o rO 0) FIGURE 9 Growth of Chaoborus l a r v a e i n Eunice L a k e — 1971-1973. The p o i n t s are mean dry weights ± 95$ co n f i d e n c e l i m i t s . The numbers i n d i c a t e the year c l a s s of the l a r v a e . 2.0T 1971 1972 1973 27 were s i m i l a r i n both years of t h i s study. Although the weight at the end of the f i r s t and second summer appears r e l a t i v e l y constant over the three years s t u d i e d , the l a r v a e have the p o t e n t i a l to grow at a much f a s t e r r a t e . In a 28 day i n s i t u experiment t e s t i n g growth at high fcod d e n s i t i e s , e l d l a r v a e reached a higher weight than t h e i r c o u n t e r p a r t s i n the lake c o n t r o l (1.58mg as compared to 1.28mg); young l a r v a e grew to the same weight as the o l d l a r v a e and four times the weight of t h e i r c o u n t e r p a r t s i n the l a k e during the course of the experiment (1.78mg as compared to 0.38mg) (Fedorenko 1973). D i s c u s s i o n The most prominent f e a t u r e of the b i o l o g y of Chaoborus l a r v a e i s t h e i r r e g u l a r , d i e l v e r t i c a l m i g r a t i o n . The migration p a t t e r n i n Eunice Lake i s s i m i l a r to that found i n most chaoborid p o p u l a t i o n s except that these l a r v a e spend the d a y l i g h t hours i n the deep c o l d waters of the hypolimnion i n s t e a d of being buried i n the sediment (Fedorenko and S w i f t 1972). Teraguchi and Northcote (1966) found a s i m i l a r m i g r a t i o n p a t t e r n i n C. f l a v i c a n s i n Corbett Lake, B r i t i s h Columbia. The d e s c r i p t i v e c h a r a c t e r i s t i c s of the m i g r a t i o n , although i n t e r e s t i n g , are not as important i n t h i s study as the p h y s i o l o g i c a l consequences r e s u l t i n g from migration through the p h y s i c a l and b i o l o g i c a l g r a d i e n t s present i n the lake (temperature, oxygen c o n c e n t r a t i o n , l i g h t , and prey density) . 28 T h e s t a b l e t h e r m o c l i n e i n E u n i c e L a k e i s t y p i c a l o f s m a l l , d e e p , t e m p e r a t e l a k e s i n t h e s u m m e r . T h e p r e s e n c e o f t h e t h e r m o c l i n e t h r o u g h o u t t h e s u m m e r h a s a c o n s i d e r a b l e e f f e c t o n t h e d a i l y t h e r m a l r e g i m e c f t h e v e r t i c a l l y m i g r a t i n g l a r v a e . T y p i c a l l y , o l d f o u r t h - i n s t a r l a r v a e e n c o u n t e r a w i d e r a n g e o f t e m p e r a t u r e ( 5 ° - 2 0 ° ) d u r i n g t h e i r m i g r a t i o n i n J u l y a n d A u g u s t , a n d a n a r r o w e r r a n g e i n J u n e , S e p t e m b e r , a n d O c t o b e r . T h e e n t i r e w a t e r c o l u m n i n E u n i c e L a k e i s w e l l o x y g e n a t e d t h r o u g h o u t t h e y e a r ( F e d o r e n k o a n d S w i f t 1 9 7 2 ) . A l t h o u g h l e w o x y g e n c o n c e n t r a t i o n h a s b e e n s u g g e s t e d a s a r e g u l a t o r y m e c h a n i s m f o r v e r t i c a l m i g r a t i o n i n C . £ u j j c t i p e n n i s ( L a R o w 1 9 7 0 ) , i t s e e m s u n l i k e l y t h a t o x y g e n c o n c e n t r a t i o n h a s a n y e f f e c t o n v e r t i c a l m i g r a t i o n i n E u n i c e L a k e . B e c a u s e o f l i g h t a t t e n u a t i o n w i t h i n c r e a s i n g d e p t h a n d t h e l i g h t - d a r k c y c l e , l i g h t h a s l o n g b e e n i m p l i c a t e d a s a c o n t r o l l i n g f a c t o r i n t h e v e r t i c a l m i g r a t i o n o f z o o p l a n k t o n . T e r a g u c h i a n d N o r t h c o t e ( 1 9 6 6 ) s u g g e s t e d t h a t l i g h t i n t e n s i t y w a s a c o n t r o l l i n g f a c t o r i n C h a o b o r u s m i g r a t i o n s a n d L a R o w ( 1 9 6 9 ) s h o w e d t h a t a c r i t i c a l l o w l i g h t i n t e n s i t y w a s r e q u i r e d t o i n i t i a t e m i g r a t i o n . T h e v e r t i c a l m i g r a t i o n o f £ * i £ i l i i £ a t u s i n E u n i c e L a k e i s p r o b a b l y u n d e r p h o t o p e r i o d i c c o n t r o l . I n l a k e s l i k e E u n i c e L a k e w h i c h h a v e a l o w t r a n s p a r e n c y 2 9 (Secchi depth 5-6m), most primary production takes place near the s u r f a c e i n the r e l a t i v e l y w e l l mixed e p i l i m n i c n . T h i s l a y e r a l s o supports the h i g h e s t d e n s i t y of herbivorous zooplankton — the food of Chaoborus. The d i s t r i b u t i o n of t o t a l zooplankton i n Eunice Lake c l e a r l y demonstrates the sharp g r a d i e n t i n p r o d u c t i v i t y with depth. One of the e x p l a n a t i o n s commonly given f o r v e r t i c a l m i gration i s that zooplankton, whether herbivorous or c a r n i v o r o u s , migrate i n t o the f o c d - r i c h e u p h c t i c zone i n order to feed (Hardy 1956, McLaren 1963). The depth at which f o u r t h - i n s t a r l a r v a e fed was determined by examining the time and depth where feeding occurred, and by comparing known predator and prey d i s t r i b u t i o n s with the a c t u a l d i e t . Although the two ways of examining t h i s q uestion gave somewhat d i f f e r e n t r e s u l t s , i t was c l e a r that most f e e d i n g occurred i n the e p i l i m n i o n a t night. One of the most s u r p r i s i n g f i n d i n g s was that a l a r g e p r o p o r t i o n of the l a r v a l p o p u l a t i o n had no food i n t h e i r crops at any time. I t appears that many l a r v a e do not r e a d i l y feed e i t h e r at the r e l a t i v e l y high prey d e n s i t i e s fcund i n the e p i l i m n i o n , or at the r e l a t i v e l y low prey d e n s i t i e s i n the hypolimnion. Estimates of f e e d i n g time based on f u l l crop data were found to be l e s s i n d i c a t i v e of the a c t u a l f e e d i n g time than estimates based on the occurrence of f r e s h prey because of the d i f f e r e n t d i g e s t i o n times of the v a r i o u s prey types. Thus, no 30 d i e l f e e d i n g p e r i o d i c i t y was i n d i c a t e d by the f u l l crop data, but a f e e d i n g peak at night was i n d i c a t e d i n J u l y and October by the occurrence of f r e s h prey i n the crops. A n a l y s i s of d i e l f e e d i n g p e r i o d i c i t y on some prey s p e c i e s taken i n d i v i d u a l l y shows a c l e a r f e e d i n g peak at night (Fedorenko 1973). Data on the depth where feeding occurs support the f r e s h prey occurrence r e s u l t s . Feeding a c t i v i t y , shown by the percentage of f u l l crops at d i f f e r e n t depths, occurs at n i g h t when the l a r v a e are i n the e p i l i m n i o n and are feed i n g on prey that are r e s t r i c t e d to the e p i l i m n i o n . The apparent f e e d i n g during the day (0900-1500) i s probably the r e s u l t of slow d i g e s t i o n of prey captured p r i o r to the sampling time. Prey capture at the day depth i s u n l i k e l y because of low prey d e n s i t y . The i n c r e a s e i n f e e d i n g at 1800 hours takes place at the depth and time that the upward migrating l a r v a e o v e r l a p with the downward migrating D. ken a i . Knowing the feeding f u n c t i o n a l response curves of o l d f o u r t h - i n s t a r l a r v a e and the zooplankton depth d i s t r i b u t i o n , one can p r e d i c t the time that f e e d i n g i s most l i k e l y to occur. For o l d f o u r t h - i n s t a r l a r v a e the feeding f u n c t i o n a l response curves f o r p. kenai and p. t ^ r r e l l i show fe e d i n g s a t u r a t i o n at prey d e n s i t i e s of about 10 and 100 prey per l i t e r r e s p e c t i v e l y (Fedorenko 1973). Since these prey d e n s i t i e s do not occur at the day depth of old f o u r t h - i n s t a r l a r v a e , one can i n f e r t h a t feeding must occur at n i g h t . A n a l y s i s of the l a r v a l d i e t shows 31 t h i s t o b e t h e c a s e . T h e o l d f o u r t h - i n s t a r l a r v a e f e e d m o r e o r l e s s i n p r o p o r t i o n t o p r e y a b u n d a n c e . On a n u m e r i c a l b a s i s B o s m i n a a n d p . t v r e l l i , t h e t w o m o s t a b u n d a n t p r e y , a r e e a t e n m o s t f r e q u e n t l y t h r o u g h o u t t h e s u m m e r . T h e y a r e b o t h f o u n d a b o v e 5m a t a l l t i m e s . O n a b i o m a s s b a s i s , f e e d i n g d o e s n o t a p p e a r t o t a k e p l a c e e n t i r e l y a t t h e s u r f a c e . p i a £ t o m u s k e n a i , t h e p r i n c i p a l p r e y t h r o u g h o u t t h e s u m m e r , i s d i s t r i b u t e d b e l o w 5m d u r i n g t h e n i g h t . S i n c e t h e l a r v a e o v e r l a p s p a t i a l l y w i t h p . k e n a i t h r o u g h o u t t h e m i g r a t i o n p e r i o d ( d u s k - d a w n ) , p . k e n a i may f o r m t h e p r i n c i p a l p r e y t a k e n b e l o w t h e t h e r m o c l i n e . I f t h i s i s t h e c a s e , o l d f o u r t h - i n s t a r l a r v a e c o u l d b e f e e d i n g p r i m a r i l y w h i l e b e l o w t h e t h e r m o c l i n e . T h e p h y s i o l o g i c a l e f f e c t s o f e x p o s u r e t o t h e p h y s i c a l a n d b i o l o g i c a l g r a d i e n t s d i s c u s s e d a b o v e d e t e r m i n e l a r v a l g r o w t h . F e d o r e n k o a n d S w i f t ( 1 9 7 2 ) d i s c u s s t h e g r o w t h o f C- £ £ ± £ i t t a t u s l a r v a e a n d s u g g e s t t h a t t h e t w o y e a r l i f e c y c l e s e e n i n t h i s s p e c i e s i s d u e , i n p a r t , t o l o w f o o d a v a i l a b i l i t y . A f i e l d t e s t o f t h i s h y p o t h e s i s h a s s h o w n t h a t t h e l a r v a e h a v e t h e p o t e n t i a l t o g r o w v e r y r a p i d l y i f p r o v i d e d w i t h l a r g e a m o u n t s o f f o o d ( F e d o r e n k o 1 9 7 3 ) . B o t h o n e y e a r o l d l a r v a e a n d y o u n g o f t h e y e a r l a r v a e g r e w t c t h e i r n o r m a l p u p a t i o n w e i g h t a f t e r o n l y 2 8 d a y s o f h i g h f o o d ( F e d o r e n k o 1 9 7 3 ) . T h e e x p e r i m e n t w a s t e r m i n a t e d i n l a t e f a l l a n d i t w a s n ' t p o s s i b l e t o d e t e r m i n e w h e t h e r t h e s e " h e a v y " o n e y e a r o l d l a r v a e w o u l d p u p a t e i f g i v e n t h e a p p r o p r i a t e l i g h t c o n d i t i o n s . W h i l e l a r g e a m o u n t s o f f o o d a l l o w t h e l a r v a e t o 32 grow much f a s t e r than i n the f i e l d , i t i s i n t e r e s t i n g to lcok at the d i f f e r e n c e s i n i n s i t u growth among y e a r - c l a s s e s . The d i f f e r e n c e s i n f i n a l weight of the three y e a r - c l a s s e s s t u d i e d (1969, 1970, 1971) were most probably due to v a r i a t i o n s i n o v e r a l l food l e v e l s . Prey d e n s i t i e s were higher i n the summer of 1971 than they were i n 1972, although the s p e c i e s composition d i d not change. 33 Summary 1. F o u r t h - i n s t a r l a r v a e i n Eunice Lake undergo a r e g u l a r d i e l v e r t i c a l m i g r a t i o n throughout the summer. 2 . The migration exposes the l a r v a e to a wide range of temperatures and prey d e n s i t i e s . 3 . Because of the zooplankton d i s t r i b u t i o n i n Eunice Lake, most feeding by o l d f o u r t h - i n s t a r l a r v a e takes place at night near the s u r f a c e . However, p. kenai may be eaten below the the r m o c l i n e . 4. The e n t i r e spectrum of prey s p e c i e s i n Eunice Lake i s v u l n e r a b l e to pr e d a t i o n by o l d f o u r t h - i n s t a r l a r v a e . The larv a e appear to feed on the v a r i o u s prey s p e c i e s i n pr o p o r t i o n to t h e i r d e n s i t y ; p. kenai i s the main source of biomass i n the d i e t . 5. L a r v a l growth i n Eunice Lake appears to be lower than i t s p o t e n t i a l maximum because of low fcod a v a i l a b i l i t y . V a r i a t i o n s i n zooplankton d e n s i t y probably account f o r the d i f f e r e n c e s i n growth among y e a r - c l a s s e s . 34 IV. ENERGETICS STUDIES I n t r o d u c t i o n The energy budget of an animal i s s t a t e d (Ricker 1971) by the equations C=P+R+F+U and A=P + R where C i s consumption — the t o t a l i n take of food d u r i n g a s p e c i f i e d time i n t e r v a l , P i s p r o duction (growth) — i n c r e a s e i n biomass, R i s r e s p i r a t i o n — that p a r t of a s s i m i l a t i o n which i s converted to heat or mechanical energy and i s used up i n l i f e processes, F i s egesta — that p a r t of the t o t a l food i n t a k e that i s not absorbed, U i s excreta — that part of the m a t e r i a l absorbed that i s passed from the body as u r i n e or through the g i l l s or s k i n , and A i s a s s i m i l a t i o n ( p h y s i o l o g i c a l l y u s e f u l energy) — the food absorbed l e s s the e x c r e t a . A l l the elements of these equations except C and U were measured i n d i v i d u a l l y f c r old f o u r t h - i n s t a r l a r v a e i n t h i s study. Consumption (C) data were a v a i l a b l e (Fedorenko 1973). Excreta (U) was not estimated because i t could not be separated from l i q u i d egesta. Temperature, pressure, and l i g h t are the most v a r i a b l e environmental f a c t o r s encountered by migrating l a r v a e . The pressure change experienced by these l a r v a e i s r e l a t i v e l y s m a l l (1-4 atmospheres), and l i g h t has not been shown to have much e f f e c t cn p h y s i o l o g i c a l r e a c t i o n r a t e s i n i n v e r t e b r a t e s . The e f f e c t s of these two f a c t o r s were not examined. Temperature, however, i s known to a f f e c t p h y s i o l o g i c a l 35 r e a c t i o n r a t e s , and the l a r v a e were exposed to a wide temperature range (8-20°) during t h e i r m i g r a t i o n . The e f f e c t of temperature on r e s p i r a t i o n r a t e was measured d i r e c t l y , s i n c e r e s p i r a t i o n probably r e p r e s e n t s the major energy l o s s of the l a r v a e . Consumption and a s s i m i l a t i o n have been shown t c be u n a f f e c t e d by temperature changes (Fedorenko 1973 f o r C, Lawton 1970 f o r A). The i n t e r a c t i o n of temperature and r a t i o n s i z e was measured to provide i n f o r m a t i o n about consumption (C) and growth (P). The e f f e c t of temperature on d i g e s t i o n r a t e was measured by Fedorenko (1973). Any c o n s i d e r a t i o n of the o v e r a l l e n e r g e t i c s of v e r t i c a l m igration r e q u i r e s an assessment of the metabolic c o s t of the swimming i n v o l v e d . Most previous s t u d i e s have assumed, i m p l i c i t l y or e x p l i c i t l y , that t h i s cost i s high. Recent s t u d i e s (Hutchinson 1967, Vlyroen 1970) have suggested t h a t t h i s i s not the case. T h e r e f o r e , an attempt was made to assess the swimming co s t of o l d f o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e . Consumption i s the only component of the prey capture process which appears i n the energy budget equation. Feeding e f f i c i e n c y , the p r o p o r t i o n of attempted captures that are s u c c e s s f u l , i s an important part of any study of f e e d i n g . Experiments to g u a n t i f y t h i s parameter are i n c l u d e d i n t h i s s e c t i o n . The i n t e g r a t i o n of a l l the f a c t o r s a f f e c t i n g energy gain and l o s s to an animal i s expressed as a change i n the weight 36 of the animal. In an attempt to g u a n t i f y the e f f e c t of prey density and temperature on energy gain and l o s s , I conducted two growth experiments. The r e s u l t s of these experiments were used to t e s t the g e n e r a l i z e d model cf v e r t i c a l m i gration. The r e s u l t s of the f o l l o w i n g s t u d i e s on the parameters of the energy budget equation, on the swimming c o s t of mi g r a t i o n , and on the f e e d i n g e f f i c i e n c y were used, i n c o n j u n c t i o n with f i e l d r e s u l t s , as input data f o r the general model of Chaoborus v e r t i c a l m i g r a t i o n d i s c u s s e d i n a l a t e r s e c t i o n . 37 Methods S t r i k e E f f i c i e n c y , Contact E f f i c i e n c y , Handling Time The s t r i k e e f f i c i e n c y , (captures) / ( s t r i k e s + c o n t a c t s + captures) * 1005?, and contact e f f i c i e n c y , (captures) / (contacts + captures) * 100$, were measured by watching l a r v a e s t r i k e at d i f f e r e n t prey s i z e s . The prey were copepods (D. kenai) and cl a d o c e r a n s (D. rosea) v a r y i n g from 0.6mm to >3.0mm i n l e n g t h . Experiments were c a r r i e d cut at va r i o u s times of the day with 20-25 l a r v a e i n 200-250ml of water at about 10° and incandescent or f l u o r e s c e n t room l i g h t i n g . A l l l a r v a e were starved f o r between one and two days before being used i n the experiments. The l a r v a e were exposed to twc prey at a time and the number of s t r i k e s , c c n t a c t s , and captu r e s was recorded over a p e r i o d of 20 minutes to one hour. For the sm a l l e s t prey the l a r v a e were placed i n d i v i d u a l l y i n t o 200ml of water with about 200 prey. A s t r i k e was d e f i n e d as a d e f i n i t e s t r i k i n g movement at a prey i n c l o s e proximity to the l a r v a ; t h i s d e f i n i t i o n excluded s t r i k e - l i k e movements which occurred when no prey were near. A cont a c t was de f i n e d as a s t r i k e which succeeded i n h i t t i n g or holding the prey item but was not fo l l o w e d by s u c c e s s f u l i n g e s t i o n . A capture was defin e d as the s u c c e s s f u l i n g e s t i o n of a prey item. Captures were scored as c o n t a c t s a l s o . Confidence l i m i t s f o r the pr o p o r t i o n captured f o r n >30 were c a l c u l a t e d from the normal approximation to the binomial d i s t r i b u t i o n (Snedecor and 38 Cochran 1967) and f o r n <30 from t a b u l a t e d values of confidence l i m i t s of p r o p o r t i o n s (Crow 1956), Handling time was measured, using a stopwatch, from the time c o n t a c t was made to the time the prey had f u l l y passed the p o s t e r i o r margin of the head c a p s u l e . In most cases i t was p o s s i b l e tc see whether the prey had been in g e s t e d head or t a i l f i r s t . $ F r i c t i o n C o e f f i c i e n t Neither the 14-carbon nor the oxygen method was s u i t a b l e f o r d i r e c t l y measuring the metabolic c o s t of v e r t i c a l m i g r a t i o n . It was necessary, t h e r e f o r e , t c approximate t h i s c ost with some measureable q u a n t i t y . I chose the c o s t of overcoming f r i c t i o n over the d i s t a n c e migrated as an estimate of the c o s t of v e r t i c a l m i g r a t i o n . A body s i n k i n g through the water has an upward f o r c e of VDwg+fv and a downward f o r c e of VDag where V i s the volume, Dw i s the d e n s i t y of water, Da i s the d e n s i t y of the body, g i s the a c c e l e r a t i o n due tc g r a v i t y , v i s the v e l o c i t y of the body, and f i s the f r i c t i o n c o e f f i c i e n t . At constant v e l o c i t y , VDwg+fv=VEag and f=g(VDa-VDv)/v. The f r i c t i o n c o e f f i c i e n t was measured e x p e r i m e n t a l l y from the s i n k i n g r a t e of f o u r t h - i n s t a r t r j . v i t ^ a t u s l a r v a e which were a r t i f i c i a l l y weighted by i n s e r t i n g p i e c e s of i n s e c t pin the length of the body c a v i t y . The time i t took the l a r v a plus pin to sink through 100cm of 39 water was measured. The p i n was e n t i r e l y within the body c a v i t y and the l a r v a was allowed to sink 20cm before r e a c h i n g the measured 100cm. Only t r i a l s i n which the body of the l a r v a c o n t a i n i n g the p i n was i n i t s n a t u r a l p o s i t i o n ( h o r i z o n t a l and d o r s a l s i d e uppermost) were used i n the c a l c u l a t i o n of v. S i x d i f f e r e n t l a r v a e were used to measure v and 6-7 measurements cf v were made with each l a r v a before i t l o s t i t s n a t u r a l shape and was d i s c a r d e d . The parameters VDa and VDw were measured by weighing the l a r v a c o n t a i n i n g the pin (W2), a known volume of water (W1), and the water + l a r v a and pin -displacement (W3). Thus VDa=W2 and VDw= (W1+W2)-W3. The a c c e l e r a t i o n due to g r a v i t y i s a constant. A s s i m i l a t i o n A s s i m i l a t i o n e f f i c i e n c y of o l d f o u r t h - i n s t a r £• t r i v i t t a t u s l a r v a e was measured f o r both a c l a d c c e r a n (D. rosea) and a copepod (D. k e n a i ) . The method e s s e n t i a l l y followed that of Sorokin (1968). L a b e l l e d algae were prepared by growing c u l t u r e s of Scenedesmus or Chlam_ydomonas i n 1 l i t e r of B r i s t o l ' s medium with approximately 42.5 uC of 1 *C-bicarbonate added to i t . The l a b e l l e d algae were c e n t r i f u g e d and resuspended i n un c h l o r i n a t e d water. Ba^hnia were grown on the l a b e l l e d algae by s t a r t i n g with females c a r r y i n g embryos that were almost ready to hatch. The newly hatched Da£hnia were allowed to grow 40 on the l a b e l l e d algae f o r about one week. Diaj:tjomus were put i n t o the l a b e l l e d medium as copepodites and allowed to feed f o r about a week. These procedures produced prey having about 15,000 cpm per i n d i v i d u a l . Chaoborus l a r v a e were fed a s i n g l e l a b e l l e d prey item and immediately placed i n t o u n c h l o r i n a t e d water with a pH of about 9. Ten d i s h e s , each c o n t a i n i n g f i v e l a r v a e i n 200ml of water, were incubated at 15° f o r 3-5 days. Each day the l a r v a e were t r a n s f e r r e d to f r e s h water c o n t a i n i n g u n l a b e l l e d food, and the water they had been i n was assayed f o r r a d i o a c t i v e carbon i n the p a r t i c u l a t e , , and d i s s o l v e d o r g a n i c f r a c t i o n s . At the end of the experiment the l a r v a e were assayed f o r 14-carbon. The t o t a l amount of l a b e l l e d carbon ingested was taken as the sum of a l l the 14-carbon recovered p a r t i c u l a t e , C0 2, d i s s o l v e d o r g a n i c , and l a r v a l . The a s s i m i l a t i o n e f f i c i e n c y was taken as: (C02-dpm + DOM-dpm + larval-dpm) / total-dpm. A s s i m i l a t i o n e f f i c i e n c y was a l s o measured by comparing the weights of consumed food and undigested m a t e r i a l (A=C-F-U). For t h i s experiment I assumed t h a t , on a weight b a s i s , l i q u i d egesta (a p o r t i o n of F) had a n e g l i g i b l e weight r e l a t i v e to s o l i d egesta (the remainder of F) and may be ignored. The formula f o r a s s i m i l a t i o n e f f i c i e n c y reduces to A=C-F/C *100?o. S i n g l e p. kenai from a p o p u l a t i o n with a known average dry weight were fed to f o u r t h - i n s t a r l a r v a e which were 41 allowed to egest any undigested m a t e r i a l . F i v e d i s h e s , each c o n t a i n i n g 10 fed l a r v a e i n 200ml of water were held at 15° f o r 24 hours. The l a r v a e were removed and the water f i l t e r e d through pre-weighed Ha M i l l i p o r e f i l t e r s (pore s i z e 0.45u). The f i l t e r s were d r i e d at 50° and weighed. The amount a s s i m i l a t e d was taken as (weight of egested material) / (weight of one ccpepod) * 1003. R e s p i r a t i o n Rates R e s p i r a t i o n r a t e s of young and old f o u r t h - i n s t a r l a r v a e were measured i n the f a l l of 1972 using constant pressure r e s p i r o m e t e r s . Two r e s p i r o m e t e r s were used: one with a measuring c a p a c i t y of 5.5ul c o n s t r u c t e d from a design of Klekowski (1968), and one with a 50ul c a p a c i t y from Dr. Klekowski's l a b o r a t o r y ( F i g . 10). The r e s p i r o m e t e r chambers were 5ml round bottom f l a s k s connected to the r e s p i r c m e t e r with ground g l a s s j o i n t s . The chambers contained 2-3ml of lake water and one old or two young f o u r t h - i n s t a r l a r v a e ; carbon d i o x i d e r e s p i r e d by the l a r v a e was absorbed i n about 0.1ml of 20% KOH on f i l t e r paper. R e s p i r a t i o n r a t e s were measured at 5, 10, 15, 20, and 25°; temperatures were maintained within 1° using a water bath. The l a r v a e were held with food at 6° a f t e r being c o l l e c t e d . A l l r e s p i r a t i o n measurements were done within one week a f t e r capture. There was no food i n the crops of the FIGURE 10 Constant pressure r e s p i r c m e t e r design. A. D e t a i l of the s c a l e tube and adjustment rod. B. D e t a i l of the sidearm and r e a c t i o n f l a s k . C. View of the e n t i r e apparatus. Legend: 1. Mercury l e v e l adjustment rod, 2. Mercury, 3. Scale tube, 4. Index mark, 5. Opening from f l a s k s tc the atmosphere, 6. Reaction f l a s k , 7. F i l t e r paper f o r c a r r y i n g KOH, 8. C o n t r o l f l a s k , 9. Experimental f l a s k , 10. P l e x i g l a s stand, 11. Water l e v e l during o p e r a t i o n , ( a f t e r Klekowski 1968). 42a 43 l a r v a e used i n the experiments. A f t e r the l a r v a e were placed i n the f l a s k s a cne hour e g u i l i b r a t i c n p e r i o d was allowed before measurements were begun. Incubation time v a r i e d according to the temperature but was between three and 30 hours. Measurements were made every 15 minutes during s h o r t i n c u b a t i o n s and every few hours during long ones. A f t e r the measurements were completed the l a r v a e were k i l l e d i n hot water, d r i e d at 50°, and weighed. Oxygen consumption at STP was c a l c u l a t e d per i n d i v i d u a l and per m i l l i g r a m dry weight. Mean r e s p i r a t i o n r a t e s at each temperature were f i t t e d tc the equation R e s p i r a t i o n Rate = aWb where a i s a c o n s t a n t , W i s the dry weight of the l a r v a e , and b i s the s l o p e of the r e g r e s s i o n l i n e of In r e s p i r a t i o n r a t e against In dry weight. R e s p i r a t i o n r a t e s are given i n u n i t s of u l mg - 1 h r - 1 and u l i n d i v i d u a l - 1 h r - 1 . R e s p i r a t i o n r a t e s of young f o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e were measured at 5, 10, 15, and 20° using a micro-Winkler method f o r determining oxygen c o n c e n t r a t i o n . Incubations were c a r r i e d out i n 50ml f l a s k s with 10ml s y r i n g e r e s e r v o i r s connected to them by needles i n s e r t e d through rubber stoppers and with S i l a s t i c - f i l l e d sampling p o r t s i n the s i d e s ( F i g . 11). The water used i n the i n c u b a t i o n s was s a t u r a t e d with oxygen at 20° and c c o l e d to 6 ° . The f o l l o w i n g procedure was used f o r each experiment. A f l a s k was f i l l e d with water and 8-10 l a r v a e were added. A rubber stepper was FIGURE 11 Incubation apparatus f o r micro-Winkler oxygen consumption measurements. Legend: 1. 5 ml s y r i n g e , 2. S i l a s t i c - f i l l e d rubber stopper, 3. 50 ml f l a s k , U. S i l a s t i c - f i l l e d sampling p o r t . 44a 45 i n s e r t e d which d i s p l a c e d water i n t o the s y r i n g e . The s y r i n g e b a r r e l was f i l l e d with water, a needle was i n s e r t e d through the sampling port and the s y r i n g e plunger was pushed i n t o the b a r r e l so that a i r was excluded and about 3ml of water were d i s p l a c e d cut the sampling p o r t . Water removed from the f l a s k through the sampling port during sampling was replaced by water from the s y r i n g e r e s e r v o i r . The f l a s k s were placed at the experimental temperature f o r two hours before measurements began. I n i t i a l and f i n a l oxygen c o n c e n t r a t i o n s (three r e p l i c a t e s each) were measured with a 6-24 hour i n c u b a t i o n period depending on the experimental temperature. / The d i s s o l v e d oxygen determinations were made i n the f o l l o w i n g way. A f t e r r i n s i n g the s y r i n g e with 0.2ml cf water from the f l a s k , a 1.2ml sample was drawn i n t o the s y r i n g e and 0.05ml of manganous sulphate and a l k a l i n e - i o d i d e - a z i d e s o l u t i o n s were added deep i n the sample through the s y r i n g e t i p . The needle was replaced and 0.2ml of sample was d i s p l a c e d . The needle was stoppered, and the s y r i n g e was shaken and stood on end so that the p r e c i p i t a t e would s e t t l e . One ml of 5% s u l p h u r i c a c i d was p u l l e d i n t o the syringe to l i b e r a t e the i o d i n e and t h i s was t i t r a t e d to the s t a r c h endpoint with a 100mg per l i t e r s o l u t i o n of sodium t h i o s u l p h a t e s t a n d a r d i z e d each day a g a i n s t 0.0005M b i i o d a t e . A f t e r i n c u b a t i o n the l a r v a e were d r i e d at 100° and weighed. Oxygen consumption at STP was c a l c u l a t e d as the 46 d i f f e r e n c e i n oxygen c o n c e n t r a t i o n between i n i t i a l and f i n a l measurements and converted to u l mg - 1 h o u r - 1 . Growth Experiment I F o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e were fed excess food from August 17 to October 12, 1971 under three temperature regimes to examine the e f f e c t of temperature on growth. Two hundred l a r v a e were held i n a q u a r i a at constant 5° and 20°, and f l u c t u a t i n g 8-16° temperature regimes. The f l u c t u a t i n g temperature regime was 8° f o r 16 hours (0400-2000), 16° f o r four hours (2200-0200), and was changing f o r f c u r hours ( F i g . 1). T h i s was approximately the temperature change experienced by the l a r v a e migrating i n the lake during summer. The l a r v a e at a l l three temperature regimes were fed mixed Eunice Lake zooplankton ( p r i m a r i l y p. kenai, p. t _ y r e l l i , and Da_ph nia) every two days f o r s i x weeks; excess food was present at high d e n s i t i e s at a l l times. The 5° and 20° tanks were exposed to a 16:8 l i g h t : d a r k regime i n i n c u b a t o r s . The f l u c t u a t i n g temperature aquaria were exposed to an approximately 12:12 l i g h t : d a r k regime. Hore accurate l i g h t c o n t r o l was impossible in the f a c i l i t i e s a v a i l a b l e . Larvae were removed f o r dry weight measurements p e r i o d i c a l l y during the course of the experiment. Growth was taken as the d i f f e r e n c e between the i n i t i a l and subsequent dry weights. 47 G r o w t h E x p e r i m e n t I I G r o w t h e x p e r i m e n t s w e r e r e p e a t e d u s i n g t h r e e t e m p e r a t u r e r e g i m e s a n d t h r e e f o o d l e v e l s . T h e e x p e r i m e n t s r a n f r o m N o v e m b e r 3 t o N o v e m b e r 2 4 , 1 9 7 1 . T w o h u n d r e d l a r v a e w e r e h e l d i n s e v e n l i t e r a q u a r i a a t 5 ° , 2 0 ° , a n d f l u c t u a t i n g 5 - 2 0 ° t e m p e r a t u r e r e g i m e s . T h e f l u c t u a t i n g t e m p e r a t u r e r e g i m e w a s 5 ° f o r 16 h o u r s ( 0 3 0 0 - 1 9 0 0 ) , i n c r e a s i n g t c 2 0 ° f o r t h r e e h o u r s ( 1 9 0 0 - 2 2 0 0 ) , 2 0 ° f o r t h r e e h o u r s ( 2 2 0 0 - 0 1 0 0 ) , a n d d e c r e a s i n g t o 5 ° f o r t w o h o u r s ( 0 1 0 0 - 0 3 0 0 ) ( F i g . 1 ) . L i g h t w a s c o n t r o l l e d a t 1 6 : 8 l i g h t : d a r k i n t h e 5 ° a n d 2 0 ° i n c u b a t o r s . T h e l i g h t r e g i m e f o r t h e f l u c t u a t i n g t e m p e r a t u r e t a n k s w a s u n c o n t r o l l e d b u t w a s u s u a l l y 1 2 : 1 2 l i g h t : d a r k . F o o d c o n s i s t e d o f m i x e d p l a n k t o n f r o m E u n i c e L a k e . T h e t h r e e f o o d l e v e l s w e r e n o p r e y , 100 p r e y , a n d 6 0 0 p r e y a q u a r i u m - 1 d a y - 1 a n d c o r r e s p o n d e d t o 0 p r e y l a r v a - 1 d a y - 1 , 0 . 5 p r e y l a r v a - 1 d a y - 1 , a n d 3 p r e y l a r v a - 1 d a y - 1 r e s p e c t i v e l y , T h e m i d d l e p r e y d e n s i t y a p p r o x i m a t e d t h e l a k e p r e y d e n s i t y . P r e y l e v e l s w e r e m a i n t a i n e d b y s a m p l i n g t h e r e m a i n i n g p r e y e a c h d a y a n d a d d i n g e n o u g h p l a n k t o n t o b r i n g p r e y d e n s i t i e s u p t o t h e r e q u i r e d l e v e l . G r o w t h w a s m e a s u r e d a s b e f o r e . 48 R e s u l t s S t r i k e E f f i c i e n c y , Contact E f f i c i e n c y , Handling Time S t r i k e e f f i c i e n c y and c o n t a c t e f f i c i e n c y decreased as prey s i z e i n c r e a s e d ( F i g . 12). The upper s i z e l i m i t f o r Dajjhnia was between 2.2 and 2.6mm. The l a r v a e were more l i k e l y to avoid or push weakly away from these l a r g e Danhnia r a t h e r than s t r i k e at them. The upper s i z e l i m i t f c r Cia£tomus was probably about the same, but no copepods i n t h i s s i z e range were a v a i l a b l e f o r t e s t i n g . The s i z e at which these two prey types became toe s m a l l to be captured was l e s s than 0.6irm and was below the s i z e s a v a i l a b l e f o r t e s t i n g . S t r i k e e f f i c i e n c y was v i r t u a l l y the same on a l l s i z e s of both prey types. Contact e f f i c i e n c y was c o n s i d e r a b l y higher cn the copepods p. kenai and p. t _ y r e l l i than on Da£hnia but only when prey s i z e was g r e a t e r than 1mm. The time r e q u i r e d to i n g e s t a captured prey item i n c r e a s e d as prey s i z e i n c r e a s e d f o r both prey s p e c i e s (Table 2) . Larvae i n g e s t e d Diajotomus f a s t e r than they i n g e s t e d DajDhnia. The s m a l l e s t prey t e s t e d were i n g e s t e d f a s t e r than could be measured manually — g e n e r a l l y i n about two seconds. A two way a n a l y s i s of v a r i a n c e was used tc t e s t the n u l l hypothesis that there was no d i f f e r e n c e i n i n g e s t i o n time between the two prey s p e c i e s and between the prey s i z e s w i t h i n each s p e c i e s . A l l the data were transformed using the In FIGURE 12 S t r i k e and contact success cf f o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e as a f u n c t i o n of prey s i z e . Data are ireans ± 955? confidence l i m i t s f o r s t r i k e success (Dap_hnia— s o l i d c i r c l e s and D i a ^ t c m u s — open c i r c l e s ) and contact success (Da_ghnia— s c l i d squares and piap_tomus— open sguares). PERCENT o ro o o o oo o o o Tl cn N m -©--B-50 Table 2. Mean and standard e r r o r of the time r e q u i r e d f o r f o u r t h - i n s t a r l a r v a e to i n g e s t d i f f e r e n t prey s i z e s and s p e c i e s , and the a n a l y s i s of va r i a n c e t a b l e f o r the d i f f e r e n c e s between s i z e s and s p e c i e s . Prey Type n Prey S i z e (mm) In g e s t i o n Time (sec) mean mean SE pap_hnia 28 1.0 24. 3 5.6 24 1.4 74. 2 10. 8 21 1.8 210.5 44. 1 Diaptomus 16 1.0 5.0 0. 2 22 1.4 11.4 1. 1 35 1.8 103.64 28.0 A n a l y s i s of Variance Table Source d.f. SSQ MS F p Species 1 2. 98 2. 980 60, .92 <0. ,001 S i z e 2 4. 62 2. 310 47. . 16 <0. .001 Species X Siz e 2 0. 1 1 0. 055 1. .13 0. , 326 Er r o r 130 127. 50 0. 049 51 t r a n s f o r m a t i o n to c o r r e c t f o r unequal v a r i a n c e s . A s i g n i f i c a n t d i f f e r e n c e was found i n i n g e s t i o n times between prey s p e c i e s and between s i z e s w i t h i n each s p e c i e s (Table 2). There was no s i g n i f i c a n t i n t e r a c t i o n between s i z e and s p e c i e s on i n g e s t i o n time. Energy Ccst cf V e r t i c a l M i g r a t i o n £ll§2£P.Ei!s: l a r v a e are able to r e g u l a t e t h e i r d e n s i t y over a c o n s i d e r a b l e range by means of two p a i r s of a i r bladders (Damant 1924). When a l a r v a i s n e u t r a l l y buoyant at a p a r t i c u l a r depth i t i s at an unstable e q u i l i b r i u m p o i n t — an i n c r e a s e i n d e n s i t y w i l l cause i t to s i n k , and a decrease i n d e n s i t y w i l l cause i t to r i s e . In a d d i t i o n to the p o s s i b l e d e n s i t y mechanism f o r changing depth, mentioned above, the l a r v a e can migrate by swimming, i . e . by f l e x i n g and extending t h e i r bodies r a p i d l y and then g l i d i n g . The g l i d e may be i n any d i r e c t i o n . Whatever mechanism i s used to migrate, a l a r v a must at l e a s t overcome the f o r c e of f r i c t i o n along i t s migration path. The energy r e q u i r e d f o r the m i g r a t i o n was c a l c u l a t e d from the equation E= ( f v s ) / ( e f f ) where E i s the energy r e q u i r e d (ergs), f i s the f r i c t i o n c o e f f i c i e n t f o r the l a r v a moving p e r p e n d i c u l a r l y to the long a x i s of i t s body (gm/sec), v i s the v e l o c i t y of movement (cm/sec), s i s the d i s t a n c e t r a v e l l e d (cm), and e f f i s the metabolic e f f i c i e n c y (the e f f i c i e n c y of 52 c o n v e r t i n g the p o t e n t i a l energy i n food tc k i n e t i c energy). Experimental values f o r v were s i m i l a r between t r i a l s with the same l a r v a and f o r d i f f e r e n t l a r v a e (Table 3). The f r i c t i o n c o e f f i c i e n t was between 0.0.and 1.0 gm/sec i n a l l t r i a l s . High values of v (0.33 cm/sec, Teraguchi and Northcote 1966), f (1.0 gm/sec), and s (20m), and a low value of e f f (195) were used i n the c a l c u l a t i o n i n order to produce a high estimate of the e n e r g e t i c c o s t of v e r t i c a l m i g r a t i o n (E). T h i s c a l c u l a t i o n y i e l d s the r e s u l t : E = 6.6 x 10* ergs. Using an o x y - c a l o r i f i c c o n v e r s i o n f a c t o r of 4.89 x 1 0 - 3 c a l per u l cxygen, E = 0.32 u l oxygen f o r one h a l f cf the d a i l y m i g r a t i o n . Using a r e s t i n g metabolic r a t e of about 24 u l oxygen per day (1.0 u l oxygen i n d i v i d u a l - 1 h o u r - 1 at 10°, F i g 13) the c a l c u l a t e d c o s t of a complete migration (0.64 u l oxygen) i s l e s s than 3% of the d a i l y metabolic r a t e . A s s i m i l a t i o n E f f i c i e n c y The e f f i c i e n c y of carbon a s s i m i l a t i o n by f o u r t h - i n s t a r t r i v i t t a t u s l a r v a e i s shown f o r a c l a d o c e r a n and a copepod (Table 4). A s s i m i l a t i o n e f f i c i e n c y was e s s e n t i a l l y the same f o r both of these prey types. The low mean a s s i m i l a t i c n e f f i c i e n c y f o r the one day experiment i s the r e s u l t of one very low value. There was good agreement between a s s i m i l a t i c n e f f i c i e n c i e s f o r copepods as measured by the 14-carbon and the 53 Table 3. Experimental measurements of l a r v a l s i n k i n g v e l o c i t y and c a l c u l a t e d f r i c t i o n c o e f f i c i e n t s . T r i a l S i n k i n g V e l o c i t y F r i c t i o n C o e f f i c i e n t niean + 1 S. D. 1. 8.76 + 0.29 0.39 2. 9.35 ± 0.46 0.42 3. 9.08 i 0.37 0.68 4. 8.14 ± 0.08 0.47 5. 8.41 ± 0.39 0.53 6. 9.13 + 0.52 0.84 54 Table 4. Carbon a s s i m i l a t i o n e f f i c i e n c i e s of f o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e fed on two prey types. The values are means ± 95% confidence l i m i t s . Prey Type Method Dap_hnia majgna Cia£tomus kenai Radiocarbon 1 day 56.4 ± 14.8 3 day 67.4 ± 7.2 76.4 ± 4.0 Weight 1 day 66.9 55 p e r c e n t d i g e s t e d m e t h o d s . R e s p i r a t i o n W e i g h t - s p e c i f i c o x y g e n c o n s u m p t i o n ( u l m g - 1 h r - 1 ) a n d i n d i v i d u a l o x y g e n c o n s u m p t i o n ( u l i n d i v i d u a l - 1 h r - 1 ) o f o l d f o u r t h - i n s t a r l a r v a e i n c r e a s e d l i n e a r l y w i t h t e m p e r a t u r e ( F i g . 1 3 ) . T h e r e i s a s u g g e s t i o n o f a p l a t e a u i n t h e i n d i v i d u a l r a t e o v e r t h e t e m p e r a t u r e r a n g e 1 0 - 1 5 ° a n d i n t h e w e i g h t - s p e c i f i c r a t e o v e r t h e 1 5 - 2 0 ° r a n g e . T h e s e p l a t e a u s o c c u r o v e r t h e t e m p e r a t u r e r a n g e t h e l a r v a e a r e e x p o s e d t o d u r i n g t h e i r d i e l v e r t i c a l m i g r a t i o n . W e i g h t - s p e c i f i c a n d i n d i v i d u a l o x y g e n c o n s u m p t i o n r a t e s o f y o u n g f o u r t h - i n s t a r l a r v a e a l s o i n c r e a s e d w i t h t e m p e r a t u r e ( F i g . 14) b u t t h e m i c r o - W i n k l e r m e a s u r e m e n t s a t 5 a n d 1 0 ° w e r e s i g n i f i c a n t l y l o w e r t h a n t h e m i c r o - r e s p i r o m e t e r m e a s u r e m e n t s ( t - t e s t : 5 ° , p = . 0 3 2 7 ; 1 0 ° r p = . 0 2 1 8 ; 1 5 ° , p = . 3 6 5 8 ; 2 0 ° , p = . 3 2 1 4 ) . B o t h m e t h o d s s h o w e d a s h a r p i n c r e a s e i n w e i g h t - s p e c i f i c r e s p i r a t i o n r a t e b e t w e e n 10 a n d 1 5 ° . I n d i v i d u a l o x y g e n c o n s u m p t i o n o f t h e y o u n g l a r v a e w a s l o w e r t h a n t h a t o f t h e o l d l a r v a e ( F i g s . 13 a n d 1 4 ) . W e i g h t - s p e c i f i c r e s p i r a t i o n o f t h e y o u n g l a r v a e w a s h i g h e r t h a n t h a t o f t h e o l d l a r v a e a t 1 5 , 2 0 , a n d 2 5 ° b u t w a s n o t s i g n i f i c a n t l y d i f f e r e n t b e t w e e n t h e t w o t y p e s o f l a r v a e a t 5 a n d 1 0 ° ( t - t e s t : 5 ° , p = . 1 8 3 9 ; 1 0 ° , p = . 8 3 0 7 ; 1 5 ° , p = . 0 2 6 ; 2 0 ° , p = . 0 3 4 3 ; 2 5 ° , p = . 0 4 5 8 ) . T h e c x y g e n c o n s u m p t i o n o f b o t h o l d a n d y o u n g 56 F I G U R E 13 R e s p i r a t i o n r a t e s o f o l d f o u r t h - i n s t a r l a r v a e ( 1 9 7 2 y e a r - c l a s s ) a t d i f f e r e n t t e m p e r a t u r e s . T h e d a t a p o i n t s a r e m e a n s ± 9 5 $ c o n f i d e n c e l i m i t s . » FIGURE 14 R e s p i r a t i o n r a t e s of young f o u r t h - i n s t a r l a r v a e (1972 y e a r - c l a s s ) at d i f f e r e n t temperatures. The data p o i n t s are means ± 95$ con f i d e n c e l i m i t s f o r measurements with micro-respirometers ( s o l i d and open c i r c l e s ) and with the micro-winkler method (Xs) . 57a 58 f o u r t h - i n s t a r l a r v a e was used to determine the e f f e c t s of s i z e and temperature on r e s p i r a t i o n r a t e . The equation R = aWb (R i s oxygen consumption, W i s body weight, a and b are constants) d e f i n e s the r e l a t i o n s h i p between body weight and metabolic r a t e . A l o g a r i t h m i c p l o t of R versus W can be f i t t e d with a s t r a i g h t l i n e , the slope of which i s the c o e f f i c i e n t of r e s p i r a t i o n (b). A l i n e a r r e g r e s s i o n was c a l c u l a t e d f o r In oxygen consumption on In dry weight f o r 5, 10, 15, 20, and 25°. The terms of these equations are given i n Table 5. The s l o p e s of the r e g r e s s i o n l i n e s r e l a t i n g w e i g h t - s p e c i f i c r e s p i r a t i o n and dry weight are more v a r i a b l e than those r e l a t i n g i n d i v i d u a l r e s p i r a t i o n and dry weight. The l a t t e r equations f o r each temperature are used i n the s i m u l a t i o n model. Growth Experiment I The i n i t i a l experiment was run with three temperature regimes -- constant 5° and 20°, and f l u c t u a t i n g 7-15° — with an excess feed r a t i o n . Growth was r a p i d at 20°, slower at f l u c t u a t i n g temperatures, and slowest at 5° ( F i g . 15). At the end of 20 days l a r v a e l i v i n g at 20° were more than double the weight of l a r v a e of the same y e a r - c l a s s l i v i n g i n the l a k e , l a r v a e l i v i n g under f l u c t u a t i n g temperatures had grown somewhat more than those i n the lake ( F i g . 9, C. t r i v i t t a t u s '71, Sept. '71), and l a r v a e at 5° weighed l e s s than the lake 59 Table 5. Terms of the r e g r e s s i o n equations: In (R)=ln (a)+b (In (W)) d e s c r i b i n g the r e l a t i o n s h i p between oxygen consumption (R) and dry weight (W) . R= u l mg - 1 h o u r - 1 and W= mg °C n In (a) b p of slope being 0.0 5 15 -1.053 -0.3700 0.204 10 16 -0.6577 -0.3127 0.999 15 10 -0.0859 -0.3552 0.014 20 12 0.0128 0.3407 0.046 25 12 0.3919 0.4020 0.019 R= u l i n d i v i d u a l - 1 h o u r - 1 and W= mg °C n In(a) b p of slope being 0.0 5 15 -1.0530 0.6300 0.040 10 16 -0.6639 0.8851 0.003 15 10 -0.0859 0.6447 <0.001 20 12 0.0128 0.6593 0.001 25 12 0.3919 0.5979 0.002 60 F I G U R E 15 L a r v a l g r o w t h d u r i n g G r o w t h E x p e r i m e n t I . T h e d a t a p o i n t s a r e m e a n l a r v a l d r y w e i g h t s ± 95% c o n f i d e n c e l i m i t s a f t e r 21 a n d 4 5 d a y s a t t h e e x p e r i m e n t a l t e m p e r a t u r e s . 61 p o p u l a t i o n . By the end of the experiment (46 days) the l a r v a e l i v i n g at 20° weighed more than the two year o l d lake l a r v a e and many had pupated. Under f l u c t u a t i n g temperatures growth was twice that of the one year o l d l a r v a e i n the l a k e , and at 5° growth was equal to that of the one year o l d l a r v a e i n the l a k e . There was c o n s i d e r a b l e m o r t a l i t y at 20°, and too few larv a e s u r v i v e d to allow c o n f i d e n c e l i m i t s to be placed cn the f i n a l weight shown. Growth Experiment I I No Food: At 5° there was a slow but continuous l o s s i n weight (F i g . 16). At the f l u c t u a t i n g and 20° temperatures there was no l o s s i n weight and perhaps a s l i g h t gain i n weight over the 21 day experimental p e r i o d . High Food: As i n the f i r s t growth experiment growth was g r e a t e r under a l l three temperature regimes than i n the f i e l d (Figs. 16 and 9), The g r e a t e s t growth occurred at 20° where the la r v a e t r i p l e d t h e i r i n i t i a l weight; l a r v a e under f l u c t u a t i n g c o n d i t i o n s doubled t h e i r weight, and l a r v a e at 5° gained weight s l i g h t l y . Larvae i n the lake did not measureably grow during the experimental p e r i o d . 62 F I G U R E 16 L a r v a l g r o w t h d u r i n g G r o w t h E x p e r i m e n t I I . T h e d a t a p o i n t s a r e m e a n l a r v a l d r y w e i g h t s ± 95% c o n f i d e n c e l i m i t s a f t e r 13 a n d 21 d a y s a t t h e e x p e r i m e n t a l c o n d i t i o n s . 6 3 L o w F o o d : T h e r e w a s s o m e g r o w t h u n d e r l o w f o o d r a t i o n s b u t l e s s t h a n u n d e r h i g h r a t i o n s ( F i g . 1 6 ) . L a r v a e a t t h e 2 0 ° a n d f l u c t u a t i n g t e m p e r a t u r e r e g i m e s g r e w t h e s a m e a m o u n t , d o u b l i n g t h e i r i n i t i a l w e i g h t d u r i n g t h e e x p e r i m e n t . A t 5 ° g r o w t h w a s s l i g h t . T h e a m o u n t o f g r o w t h a t 5 ° a n d a t f l u c t u a t i n g t e m p e r a t u r e s w a s t h e s a m e w h e t h e r t h e r a t i o n w a s l o w o r h i g h . A t 2 0 ° t h e a m o u n t o f g r o w t h w a s v e r y d i f f e r e n t b e t w e e n t h e s e t w o r a t i o n s . A t w o w a y a n a l y s i s o f v a r i a n c e w a s u s e d t o t e s t t h e n u l l h y p o t h e s i s t h a t t h e r e i s n o i n t e r a c t i o n b e t w e e n t e m p e r a t u r e a n d r a t i o n i n t h e i r e f f e c t o n l a r v a l g r o w t h . T h e no f o o d r a t i o n w a s n o t c o n s i d e r e d i n t h e a n a l y s i s . T h e r e s u l t s ( T a b l e 6) s h o w t h a t t h e r e i s a h i g h l y s i g n i f i c a n t i n t e r a c t i o n e f f e c t b e t w e e n r a t i o n s i z e a n d t e m p e r a t u r e o n g r o w t h . 64 Table 6. Results cf a two-way a n a l y s i s of v a r i a n c e with r a t i o n and temperature as main e f f e c t s i n Growth Experiment I I . Source cf d.f., Sum of Mean Variance Squares Square Ration 1 .155 .155 19.03 .C002 Temperature 2 1.36 .682 83.47 <.0001 R X T 2 .175 .087 10.70 .C004 Er r o r 27 .221 .008 T o t a l 32 1.91 65 D i s c u s s i o n The s t r i k e and contact e f f i c i e n c i e s were, as expected, i n v e r s e l y p r o p o r t i o n a l to prey s i z e over the s i z e range t e s t e d ; these two e f f i c i e n c i e s undoubtedly i n c r e a s e to a peak and then drop r a p i d l y as prey s i z e decreases. The d i s c r e t e prey s i z e range which can be s u c c e s s f u l l y handled by each i n s t a r i s determined by the s i z e of the l a r v a l head capsule and mouth-parts. F o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e are l a r g e r than most Chaoborus l a r v a e and are a b l e t c i n g e s t zooplankton t h a t are g u i t e l a r g e (2.2-2.6mm f o r Daohnia) . Eodson (1970) reported a maximum l e n g t h of 1.7mm f o r zooplankton eaten by C. n j b l a e i and C. f l a v i c a n s . The f a c t that l a r v a e respond to very l a r g e Daohnia (>2.6mm) i n a manner very d i f f e r e n t from t h e i r response to more v u l n e r a b l e prey suggests t h a t the l a r v a e have a sensory system that can assess the s i z e of p o t e n t i a l prey r a t h e r a c c u r a t e l y . The s i m i l a r s t r i k e e f f i c i e n c y on copepods and cladocerans and higher c o n t a c t e f f i c i e n c y on copepods suggest t h a t the l a r v a e are not able to d i s c r i m i n a t e between these prey types before s t r i k i n g , but t h a t once the prey i s contacted, copepods are much more e a s i l y i n g e s t e d than c l a d o c e r a n s . The more e f f i c i e n t capture of copepods has been a t t r i b u t e d to t h e i r swimming motion (Roth 1971). The s t r i k e e f f i c i e n c y curve should show the same d i f f e r e n c e between prey types as does the contact e f f i c i e n c y curve i f l a r v a e are able t c d i f f e r e n t i a t e 66 between the two prey types on the b a s i s of t h e i r swimming motion. T h i s was not the case. I t i s more l i k e l y that s i z e , shape, and prey behavior a f t e r c o n t a c t are the determinants cf s u c c e s s f u l capture. The d i f f e r e n t c o n t a c t success curves f o r copepods and c l a d o c e r a n s support Roth's suggestion t h a t s t r e a m l i n e d , c y l i n d r i c a l copepods are more e a s i l y i n g e s t e d than i r r e g u l a r l y shaped c l a d o c e r a n s . The d i f f e r e n c e i n c o n t a c t success decreases with decreasing prey s i z e and a l l prey <1.0mm i n length are i n g e s t e d with apparently egual ease. The r e l a t i v e ease of i n g e s t i o n of these two prey types i s r e f l e c t e d i n t h e i r r e s p e c t i v e i n g e s t i o n times. Smaller prey were i n g e s t e d more e a s i l y than l a r g e prey and copepcds were ingested f a s t e r than cladocerans of the same s i z e . Such d i f f e r e n c e s have been observed by others (Roth 1971, S p r u l e s 1970). Swuste et a l . (1972) r e p o r t e d a s e l e c t i o n mechanism i n £• £I§vicans t h a t operates to accept or r e j e c t prey a f t e r contact i s made. T h i s may be a means of r e j e c t i n g u n p a l a t a b l e prey of any s i z e . Every attempt was made i n the c a l c u l a t i o n of the energy cost of v e r t i c a l migration to produce a maximum estimate cf t h i s c o s t . I f t h i s approximation could be shown to be n e g l i g i b l e with r e s p e c t to the d a i l y metabolic c o s t , and t h e r e f o r e cf l i t t l e consequence to the l a r v a , the c o s t of migration would not have to be measured d i r e c t l y . The c a l c u l a t e d r e s u l t (<3% of the d a i l y metabolic rate) was low r e l a t i v e to the d a i l y metabolic r a t e and the a c t u a l energy 6 7 c o s t w a s p r o b a b l y m u c h l o w e r t h a n t h e c a l c u l a t e d v a l u e . T h e m e t a b o l i c e f f i c i e n c y ( e f f ) w a s u n d o u b t e d l y h i g h e r t h a n t h e 1$ v a l u e u s e d i n t h e c a l c u l a t i o n ; t h i s w o u l d l o w e r t h e c a l c u l a t e d e n e r g y c o s t c o n s i d e r a b l y . I f t h e s e l a r v a e s w i m u p w a r d a n d d o w n w a r d s a c t i v e l y , t h e m e t a b o l i c c o s t o f m i g r a t i o n m i g h t b e h i g h e r t h a n t h a t c a l c u l a t e d b e c a u s e o f e n e r g y r e g u i r e d f o r a c t i v e s w i m m i n g . H o w e v e r , t h i s i n c r e a s e m i g h t b e o f f s e t s o m e w h a t b y a l o w e r f r i c t i o n c o s t d u e t c t h e l a r v a e m o v i n g p a r a l l e l t c t h e l o n g a x i s o f t h e b o d y r a t h e r t h a n p e r p e n d i c u l a r l y t o i t . I f t h e m i g r a t i o n i s a c c o m p l i s h e d s c l e l y b y p a s s i v e l y r i s i n g a n d s i n k i n g , u s i n g " t h e a i r b l a d d e r s t o c h a n g e l a r v a l d e n s i t y , t h e e n e r g y c o s t m i g h t b e v e r y l o w . T h e a c t u a l m e c h a n i s m i s p r o b a b l y a c o m b i n a t i o n o f a c t i v e s w i m m i n g a n d d e n s i t y c h a n g e , a n d t h e a c t u a l m e t a b c l i c c o s t o f t h e m i g r a t i o n i s p r o b a b l y b e l o w t h e h i g h e s t i m a t e c a l c u l a t e d . T h e e s t i m a t e d m e t a b o l i c c o s t o f o v e r c o m i n g f r i c t i o n a g r e e s w e l l w i t h t h e r e s u l t s r e p o r t e d b y V l y m e n ( 1 9 7 0 ) f o r t h e e n e r g y c o s t o f v e r t i c a l m i g r a t i o n i n c o p e p o d s . He f o u n d t h e c o s t t o b e 0 . 3 $ o f t h e b a s a l m e t a b o l i c r a t e a s s u m i n g 1 0 0 $ e f f i c i e n c y ; b e c a u s e m e t a b o l i c e f f i c i e n c y i s l e s s t h a n 1 0 0 $ , h i s e s t i m a t e i s p r o b a b l y s o m e w h a t l o w . H u t c h i n s o n ( 1 9 6 7 ) c o n c l u d e d o n a t h e o r e t i c a l b a s i s t h a t l e s s t h a n 0 . 5 $ o f t h e o r g a n i c m a t t e r i n t h e b o d y w o u l d b e o x i d i z e d d u r i n g a 50m m i g r a t i o n a s s u m i n g 1% e f f i c i e n c y . He p o i n t e d o u t t h a t t h i s i s o f l i t t l e c o n s e q u e n c e t o a f i l t e r f e e d i n g o r g a n i s m e a t i n g i t s b o d y w e i g h t i n f o o d e v e r y d a y . E v e n t h o u g h a p r e d a t o r l i k e 68 Chaoborus eats r e l a t i v e l y i n f r e q u e n t l y , (Fedcrenko, 1973), a migrat i o n cost of 3% of the d a i l y metabolic cost i s probably n e g l i g i b l e . Very c l o s e agreement was found between the a s s i m i l a t i o n e f f i c i e n c i e s c a l c u l a t e d using two independent methods -- the 14-carbon method and the formula A=C-F/C (symbols as d e f i n e d on page 34). T h i s agreement i n d i c a t e s that the c a l c u l a t e d values are probably good es t i m a t e s of the a c t u a l a s s i m i l a t i o n e f f i c i e n c i e s . The carbon a s s i m i l a t i o n e f f i c i e n c i e s r e p o r t e d here agree well with those r e p o r t e d f o r other i n v e r t e b r a t e predators (Lawton 1970, Monakov 1972); over a wide range of i n v e r t e b r a t e groups the a s s i m i l a t i o n e f f i c i e n c y was g e n e r a l l y greater than 60% and o f t e n as high as 95%. S c h i n d l e r (1968) found an i n c r e a s e i n a s s i m i l a t i o n e f f i c i e n c y with temperature in B. magna. Lawton (1970) found no e f f e c t of temperature, fee d i n g r a t e , or diapause on a s s i m i l a t i o n e f f i c i e n c y i n h i s study of the predaceous d a m s e l f l y nymph Pjjrrhosoma. He d i d f i n d a s i g n i f i c a n t d i f f e r e n c e i n the e f f i c i e n c y with which d i f f e r e n t feed types were a s s i m i l a t e d . I t has been suggested that a s s i m i l a t i o n e f f i c i e n c y i n c r e a s e s as c a l o r i c content of the food i n c r e a s e s ( S c h i n d l e r 1968), but Lawton (1970) found no c o r r e l a t i o n between a s s i m i l a t i c n e f f i c i e n c y and e i t h e r ash content or c a l o r i f i c value i n Pirrhosoma. In p o i k i l o t h e r m s oxygen consumption g e n e r a l l y i n c r e a s e s d i r e c t l y with temperature (Prosser and Brown 1961). Jonasscn 69 (.1972) reported a l i n e a r i n c r e a s e i n i n d i v i d u a l cxygen consumption f o r C. f l a v i c a n s i n response to i n c r e a s i n g temperature. R e s p i r a t i o n r a t e s of the l a r v a e i n h i s study were higher (0.7-7.8 u l i n d i v i d u a l - * hour-*, at 7-24°) than those of C. t r i v i t t a t u s i n the present study (0.4-2.0 u l i n d i v i d u a l - 1 h o u r - 1 , at 5-25° f o r old f o u r t h - i n s t a r l a r v a e and 0.3-0.8 u l i n d i v i d u a l - 1 h o u r - 1 , at 5-25° f o r young f o u r t h - i n s t a r l a r v a e ) . His l a r v a e were about the same s i z e as young f o u r t h - i n s t a r l a r v a e and smal l e r than o l d f o u r t h - i n s t a r l a r v a e . Both young and o l d f o u r t h - i n s t a r l a r v a e i n c r e a s e d t h e i r oxygen consumption as temperature i n c r e a s e d over the temperatures t e s t e d . However, there i s a suggestion that the o l d f o u r t h - i n s t a r l a r v a e r e g u l a t e t h e i r cxygen consumption over the 10-15° temperature range. There was no apparent reason f o r the plateau i n i n d i v i d u a l oxygen consumption tc be s h i f t e d by 5° from that i n w e i g h t - s p e c i f i c oxygen consumption. The plateau extends over the middle and upper temperatures that the l a r v a e are exposed to during t h e i r d i e l v e r t i c a l m i g r a t i o n . R e g u l a t i o n of oxygen consumption has been found i n ether p o i k i l c t h e r m s (Moshiri 1969 i n the predaceous cladoceran i§£i°dora. T e a l 1959 i n a crab (Oca), A. W. Knight, pers. comm., i n the shrimp Neomysis). Fur t h e r study i s necessary to c o n c l u s i v e l y demonstrate a r e g u l a t o r y p l a t e a u i n the cxygen consumption—temperature curve of o l d f o u r t h - i n s t a r C, t r i v i t t a t u s l a r v a e . Temperature i s the environmental f a c t o r which most 70 a f f e c t s oxygen consumption, and v e r t i c a l l y migrating animals are exposed to wide d i e l temperature changes. Since up to 90$ of a s s i m i l a t e d energy may be l o s t t c an animal f c r maintenance metabolism ( P h i l l i p s o n 1966), any r e d u c t i o n i n the magnitude of t h i s energy c o s t would be advantageous to the animal by making a d d i t i o n a l energy a v a i l a b l e f o r growth or r e p r o d u c t i o n . Regulators (animals which can maintain an e s s e n t i a l l y constant rate of oxygen consumption over a wide temperature range) n e u t r a l i z e the e f f e c t of r i s i n g temperature cn oxygen consumption so that t h e i r maintenance metabolic c o s t i s r e l a t i v e l y constant. It seems strange that t h i s energy saving mechanism i s not more widespread c o n s i d e r i n g the number of animal groups which migrate v e r t i c a l l y and the time these organisms have had to evolve such a d a p t a t i o n s to d i e l temperature f l u c t u a t i o n s . Perhaps the emphasis cn a c c l i m a t i z a t i o n i n many r e s p i r a t i o n s t u d i e s has prevented wider exposure of mechanisms of t h i s s o r t . T e a l and Carey (1967) found that temperature but not pressure a f f e c t e d the r e s p i r a t i o n r a t e of v e r t i c a l l y m igrating, e p i p e l a g i c copepods t h a t are l a r g e l y h e r b i v o r o u s . These copepods r e s p i r e d l e s s at the low temperature of t h e i r day depth than they d i d at n i g h t . T e a l (1971) found, however, that r e s p i r a t i o n of v e r t i c a l l y m i g r a t i n g , predaceous decapods was constant at a l l times. The d e p r e s s i n g e f f e c t of d e c r e a s i n g temperature on oxygen consumption was o f f s e t by an i n c r e a s e i n metabolic r a t e as pressure i n c r e a s e d . T e a l p o s t u l a t e d that a 71 constant metabolic r a t e i s r e q u i r e d f o r them t c remain e f f e c t i v e p r e d a t o r s throughout t h e i r depth range, by day and by n i g h t . Chaoborus l a r v a e are a l s o v e r t i c a l l y migrating predators whose metabolic r a t e decreases as temperature decreases. The e f f e c t of pressure on oxygen consumption was not s t u d i e d e i t h e r by i t s e l f or i n c o n j u n c t i o n with temperature. The presence of a pressure e f f e c t that i n c r e a s e s metabolic r a t e as pressure i n c r e a s e s would probably not a f f e c t feeding i n these l a r v a e s i n c e they feed r e a d i l y at temperatures as low as 4° i f prey are a v a i l a b l e . The r e l a t i o n s h i p between oxygen consumption and dry weight f i t s the general p a t t e r n of Brody (1945). I n d i v i d u a l oxygen consumption was g r e a t e r f o r the l a r g e r old f o u r t h - i n s t a r l a r v a e than f o r the smaller young f o u r t h - i n s t a r l a r v a e , and w e i g h t - s p e c i f i c oxygen consumption was g r e a t e r f o r the young f o u r t h - i n s t a r l a r v a e than f o r the e l d f o u r t h - i n s t a r l a r v a e . The lack of an i n c r e a s e i n w e i g h t - s p e c i f i c r e s p i r a t i o n of young f o u r t h - i n s t a r l a r v a e between 5 and 10° may be the r e s u l t of a temperature t h r e s h o l d i n oxygen consumption. Young f o u r t h - i n s t a r l a r v a e are found above 10° most of the time and t h e i r r e s p i r a t i o n r a t e may be at a minimum at t h i s temperature. Thus they wouldn't show a response to a change i n temperature from 5 to 1 0 ° . The c o e f f i c i e n t of r e s p i r a t i o n (b) i s v a r i a b l e i n p o i k i l o t h e r m s over p h y l o g e n e t i c and e c o l o g i c a l groups 72 (Hemmingsen 1960, Huebner 1973). The values of b r e p o r t e d here l i e between .59 and .89 f o r i n d i v i d u a l oxygen consumption and are w i t h i n the range c f values g e n e r a l l y r e p o r t e d . The high value at 10° (.89) was the r e s u l t of a low i n d i v i d u a l r e s p i r a t i o n r a t e f o r young l a r v a e at that temperature i n c o n j u n c t i o n with a sharp i n c r e a s e i n oxygen ccnsumpticn between 5 and 10° f o r o l d l a r v a e . A c t i v i t y i n f i s h has been shown to respond n o n - l i n e a r l y to an i n c r e a s e i n temperature ( F i s h e r and S u l l i v a n 1958, c i t e d i n Fry 1971). T h i s n o n - l i n e a r i t y may produce an apparent plateau i n the oxygen consumption—temperature curve i f a c t i v i t y i n c r e a s e s and then decreases as temperature i s i n c r e a s e d (Schmein-Engberding 1953, c i t e d i n Fry 1971). The r e l a t i o n s h i p between temperature and a c t i v i t y i n Chaoborus remains to be s t u d i e d . However, in r e s p i r a t i o n experiments over a wide range of temperatures, the l a r v a e were never observed to move about more than they did i n holding tanks. They t y p i c a l l y hang motionless i n the water column except f o r o c c a s i o n a l sharp s t r i k i n g or swimming movements. Although some e f f e c t of a c t i v i t y may be i n c l u d e d i n the r e s p i r a t i o n r a t e s r e p o r t e d here, i t i s not at a l l c l e a r whether i t would b i a s cxygen consumption upward or downward. Any a n a l y s i s of the i n d i v i d u a l e f f e c t s of environmental parameters such as temperature and prey d e n s i t y on the elements of the energy budget equation i g n o r e s the complex 73 i n t e r a c t i o n of such parameters i n determining the net energy change i n the i n d i v i d u a l over a given time p e r i o d . Attempts to examine the e f f e c t s of these i n t e r a c t i o n s are disc u s s e d below. The f i r s t growth experiment showed c o n c l u s i v e l y that young f o u r t h - i n s t a r l a r v a e have the p o t e n t i a l to e x p l o i t high prey d e n s i t i e s and grow at a much f a s t e r r a t e than i n the lake. For t e c h n i c a l reasons the f l u c t u a t i n g temperature regime in t h i s experiment was not the same as that which the l a r v a e experienced i n the lake. The second growth experiment was designed to measure the i n t e r a c t i o n cf r a t i o n s i z e and temperature on the e n e r g e t i c budget of f o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e as represented by the energy budget eguation C=P+R+F+U (see p 34). Egesta and ex c r e t a (F and U) were not measured during the experiment. Metabolic energy c o s t (R) was v a r i e d by using three temperature regimes, and consumption (C) was v a r i e d by changing the prey d e n s i t y . Growth (P) was chosen as the index cf the e f f e c t of these two parameters because i t r e f l e c t s a l l the p h y s i c a l and p h y s i o l o g i c a l i n t e r a c t i o n s which a f f e c t the energy budget. The maintenance energy c o s t was tc be measured d i r e c t l y i n the zero r a t i o n treatment. Since no food was provided, C and F were zero and P*R+U=0. The weight l o s s of the l a r v a e during the experimental p e r i o d would be a minimum estimate of the maintenance metabolic cost f o r each temperature regime 74 (-P=B+U). The use of the zero r a t i o n treatment as a measure of R assumed that there was no consumption i n t h i s treatment. T h i s assumption was not j u s t i f i e d s i n c e a s l i g h t gain i n weight was recorded at 20° and at f l u c t u a t i n g temperatures. Cannibalism occurred a t a l l three temperature regimes, and i t probably i n c r e a s e d with temperature. Seme m o r t a l i t y did occur i n each treatment, but i t c o u l d not be p a r t i t i o n e d i n t o " n a t u r a l m o r t a l i t y " and c a n n i b a l i s m . However, there i s evidence that c a n n i b a l i s m could only have been very low. At a l l t h ree temperatures the l a r v a e i n the no food treatment grew much l e s s than those i n the low food treatment. I f c a n n i b a l i s m had been a s i g n i f i c a n t source of f c o d , the l a r v a e at 20°, at l e a s t , should have grown more than they d i d . The presence of an unknown but probably low amount of c a n n i b a l i s m i n t h i s treatment was unfortunate. With no measure of the absolute value of R + U, i t was impossible to c a l c u l a t e consumption, one of the v a r i a b l e s t h a t the experiment was designed to examine. N e v e r t h e l e s s , the c o n c l u s i o n s from these r e s u l t s are not a f f e c t e d by the inadequacy of the no fcod treatment. The energy budget equation C=P+R+F+U can be used to analyze the c o n d i t i o n s necessary f o r growth. Assuming a s s i m i l a t i c n e f f i c i e n c y (A) i s constant over the temperature range i n these experiments, n c n - a s s i m i l a t e d m a t e r i a l (F) i s constant and can be n e g l e c t e d , and C=P+R+U. In order f o r growth to occur (P p o s i t i v e ) , C must be g r e a t e r than R+U. I f 75 m a i n t e n a n c e c o s t (R+U) i s assumed t c be c o n s t a n t f o r a l l p r e y d e n s i t i e s a t a g i v e n t e m p e r a t u r e r e g i m e , t h e n g r c w t h |P) must be d i r e c t l y p r o p o r t i o n a l t o c o n s u m p t i o n ( C ) . On a p h y s i o l o g i c a l b a s i s , t e m p e r a t u r e - d e p e n d e n t p r o c e s s e s s u c h as p r e d a t o r - p r e y i n t e r a c t i o n r a t e s , d i g e s t i o n r a t e s , b l c o d s u g a r u t i l i z a t i o n r a t e s , e t c . , i n c r e a s e w i t h t e m p e r a t u r e . Thus, a t h i g h t e m p e r a t u r e s , f o o d t u r n o v e r i s f a s t e r and c o n s u m p t i o n may p o t e n t i a l l y be g r e a t e r , at 20°, growth was g r e a t e r a t a h i g h r a t i o n t h a n a t a low r a t i o n . T h i s was not t h e c a s e under t h e 5° and f l u c t u a t i n g t e m p e r a t u r e r e g i m e s where growth was t h e same a t low and h i g h r a t i o n s . T h e r e a p p e a r t o be two i n t e r r e l a t e d c o n t r o l mechanisms d e t e r m i n i n g growth i n t h e s e t r e a t m e n t s - - t e m p e r a t u r e and r a t i o n s i z e . Under t h e 5° and f l u c t u a t i n g t e m p e r a t u r e r e g i m e s t e m p e r a t u r e was the g r c w t h l i m i t i n g f a c t o r . These l a r v a e were f e e d i n g a t t h e i r maximum p h y s i o l o g i c a l r a t e a t t h e low r a t i o n ; t h e y c o u l d n o t r e s p o n d to an i n c r e a s e i n f o o d by i n c r e a s i n g t h e i r c o n s u m p t i o n . At 20° the t u r n o v e r r a t e o f f o o d and t h e m e t a b o l i c c o s t were h i g h e r . At the low r a t i o n the l a r v a e f e d below t h e i r maximum p h y s i o l o g i c a l r a t e , and t h e i r growth was l e s s t h a n t h a t a t t h e h i g h r a t i o n . A t t h e h i g h r a t i o n t h e l a r v a e were a b l e t o i n c r e a s e c o n s u m p t i o n and t h i s r e s u l t e d i n a g r e a t e r weight g a i n . T h e r e i s t h u s a c l e a r i n t e r a c t i o n between t h e e f f e c t of r a t i o n s i z e and t e m p e r a t u r e on growth. Maximum r a t i o n s i z e (a f u n c t i o n o f t e m p e r a t u r e ) a p p a r e n t l y i n c r e a s e s more r a p i d l y w i t h t e m p e r a t u r e t h a n does m e t a b o l i c r a t e , so t h a t p o t e n t i a l 76 growth r a t e i n c r e a s e s with temperature. The maintenance l e v e l cf metabolism i s reached when consumption eguals maintenance c o s t s . When C<R+Uf weight l o s s o c c u r s ; when OR+U, the amount of growth i s determined by the i n t e r a c t i o n cf temperature and prey d e n s i t y on consumption. The amount of weight gained i s determined by the balance between the energy gains from higher food turnover r a t e s at the higher temperatures, and energy l o s s e s due to the higher metabolic r a t e s a s s o c i a t e d with these temperatures. The i n t e r a c t i o n of food and temperature has important i m p l i c a t i o n s f o r the e n e r g e t i c s of v e r t i c a l m i g r a t i o n . Because of d i g e s t i o n r a t e c o n s t r a i n t s imposed by the temperature regime they are exposed to, migrating l a r v a e are unable to u t i l i z e high prey d e n s i t i e s even when exposed to them. Larvae l i v i n g c o n t i n u o u s l y a t high temperatures are abl e to e x p l o i t the d i f f e r e n c e s between in c r e a s e d d i g e s t i o n rate and maintenance cost and can u t i l i z e high prey d e n s i t i e s to maximize growth. Because of the d i s t r i b u t i o n cf prey, non-migrants are u s u a l l y exposed to higher prey d e n s i t i e s than migrants, and t h i s r e s u l t s i n f a s t e r growth r a t e s . These experiments demonstrate that a s t r a t e g y of no migratio n i s best f o r maximizing growth. 77 Summary 1. Old f o u r t h - i n s t a r l a r v a e were r e l a t i v e l y i n e f f i c i e n t at c a t c h i n g both copepods and c l a d o c e r a n s . T h e i r e f f i c i e n c y of i n g e s t i n g a prey item a f t e r c o n t a c t i n g i t i n c r e a s e d r a p i d l y as prey s i z e decreased to the minimum s i z e t e s t e d (0.6mm), The maximum s i z e i n g e s t e d was 2.6mm f o r cladocerans and probably about the same f o r copepods. Copepods of a l l s i z e s t e s t e d were ing e s t e d f a s t e r than cladocerans of the same s i z e . 2. The metabolic c o s t of moving through the d i s t a n c e covered by the d i e l v e r t i c a l migration was estimated to be small r e l a t i v e tc the d a i l y metabolic c o s t . 3. The carbon a s s i m i l a t i o n e f f i c i e n c y f o r both copepods and c l a d o c e r a n s was about 68$. 4. R e s p i r a t i o n r a t e s f o r both young and o l d f o u r t h - i n s t a r l a r v a e i n c r e a s e d l i n e a r l y with temperature. The suggestion of a pl a t e a u over the 10-20° range may i n d i c a t e some a b i l i t y to r e g u l a t e oxygen consumption over part of the temperature range that the l a r v a e are exposed to during t h e i r d i e l v e r t i c a l m i g ration. The r e l a t i o n s h i p between body s i z e and oxygen consumption was t y p i c a l ; w e i g h t - s p e c i f i c oxygen consumption decreased as s i z e i n c r e a s e d , and i n d i v i d u a l oxygen consumption i n c r e a s e d as s i z e i n c r e a s e d . 5. L a r v a l growth was a f u n c t i o n of temperature and prey d e n s i t y . Temperature a f f e c t s growth through i t s e f f e c t cn 78 s e v e r a l p h y s i o l o g i c a l r a t e s ; prey d e n s i t y a f f e c t s growth through i t s e f f e c t on consumption and thus on the a b i l i t y of the l a r v a e to feed at t h e i r maximum p o t e n t i a l . I t was shown that growth was g r e a t e r i n l a r v a e l i v i n g at constant high temperature than i n those l i v i n g under a f l u c t u a t i n g temperature regime. 79 V. SIMULATION STUDIES I n t r o d u c t i o n T h e o r i e s on the Adaptive Value of V e r t i c a l M igration The v a r i o u s t h e o r i e s advanced to e x p l a i n the a d a p t i v e value of v e r t i c a l m i g r a t i o n have been reviewed by McLaren (1963) and Mauchline and F i s h e r (1969). They f a l l i n t o the s i x c a t e g o r i e s p r e v i o u s l y mentioned: escape from predators, h o r i z o n t a l t r a n s p o r t , s o c i a l c o n t r o l of r e p r o d u c t i v e r a t e , e n e r g e t i c s , a combination of these, and demographic e f f e c t s . P o t e n t i a l predators can undoubtedly see t h e i r prey at depths w e l l below those to which many zoop l a n k t e r s migrate. Nevertheless, escape from predators i s probably a component of the adaptive value of v e r t i c a l m i g r a t i o n and the primary reason f o r descent by day r a t h e r than by n i g h t . V e r t i c a l m i g r a t i o n may a i d escape from p r e d a t i c n by reducing prey a v a i l a b i l i t y to v i s u a l p r e d a t o r s , and by p r o v i d i n g r e f u g e s . The r e a c t i v e f i e l d of a f i s h (the volume within which i t can detect a prey item) i s determined i n part by l i g h t i n t e n s i t y and prey behavior. As l i g h t i n t e n s i t y decreases the r e a c t i v e f i e l d s h r i n k s . A decrease i n prey a c t i v i t y a s s o c i a t e d with low day-depth temperatures w i l l f u r t h e r decrease the r e a c t i v e f i e l d of v i s u a l predators. V e r t i c a l migration may a l s o provide a refuge from p r e d a t o r s which cannot f c l l c w the migration 80 because of t h e r m o c l i n e s , low oxygen zones, or other p h y s i c a l and chemical g r a d i e n t s . However, migrati o n might j u s t as e a s i l y expose migrants to a d i f f e r e n t set of predators at t h e i r day depth. M a n t e i f e l (1959a,b) , G i r s a (1959) , and Pearre (1973) have suggested that predator avoidance i s the main reason f o r v e r t i c a l m i gration. S e v e r a l t h e o r i e s have c o n s i d e r e d the value of v e r t i c a l m i gration i n u t i l i z i n g h o r i z o n t a l water t r a n s p o r t to e f f e c t changes i n the h o r i z o n t a l d i s t r i b u t i o n of the migrants. The adaptive value o r i g i n a l l y proposed f o r h o r i z o n t a l t r a n s p o r t was the avoidance of t o x i c e f f e c t s from areas of high phytoplankton d e n s i t y (Hardy and Gunther 1935). Though these t o x i c e f f e c t s have been shown to e x i s t , there i s no evidence t h a t zooplankton avoid dense phytoplankton patches. Since t h i s theory was advanced, s e v e r a l authors, i n c l u d i n g Hardy (1956), have suggested that h o r i z o n t a l t r a n s p o r t a i d s i n the " c o l o n i z a t i o n " of p r e v i o u s l y u n e x p l o i t e d patches at the s u r f a c e . In order to a r r i v e at patches which occur randomly i n space and time and to remain i n them, migrants would have to be able to modify the extent and timing of t h e i r m i g r a t i o n . The r e g u l a r nature of most migration p a t t e r n s suggests that zooplankton are not responding to d i s c r e t e patches. However, throughout the year s u r f a c e waters are uniformly more productive than deep water; s e l e c t i o n f o r i n c r e a s e d food i n t a k e could q u i t e c o n c e i v a b l y have produced a migration to the s u r f a c e . T h i s advantage does not, by. i t s e l f , e x p l a i n the 81 development of the downward movement i n a d i e l v e r t i c a l m i g r a t i o n . David (1961) suggested that h o r i z o n t a l t r a n s p o r t • r e s u l t i n g from v e r t i c a l m igration prevents the formation cf h i g h l y s p e c i a l i z e d s p e c i e s with l i t t l e c a p a c i t y to adapt to changes i n the environment. T h i s occurs because the h o r i z o n t a l "mixing" of zooplankton p r o v i d e s a d d i t i o n a l chances f o r genetic recombination. Because of t h e i r almost complete lack of s e x u a l i t y , i t seems u n l i k e l y that h o r i z o n t a l t r a n s p o r t would a f f e c t recombination i n those c l a d o c e r a n s which v e r t i c a l l y migrate; t h i s theory c e r t a i n l y doesn't apply tc the v e r t i c a l migration of Chaoborus l a r v a e because they emerge before breeding. There i s l i t t l e evidence that non-migratory marine copepods (Oithona s i m i l i s group) are more prcne to s p e c i a t i o n than migratory ones (Calanaus f i n m a r c h i c u s group) (McLaren 1963). David's theory assumes that the time s c a l e of mixing necessary f o r adequate g e n e t i c recombination t c prevent extreme s p e c i a t i o n i s s h o r t e r than the time s c a l e of p h y s i c a l mixing o b t a i n a b l e without v e r t i c a l m i g r a t i o n . There i s no evidence t c support t h i s assumption. Wynne-Edwards (1962) suggested that v e r t i c a l migration has evolved to produce aggregations of animals near the s u r f a c e or at other depths i n order to c a r r y out e p i d e i c t i c d i s p l a y s . These d i s p l a y s allow the p o p u l a t i o n to asses i t s numbers and r e g u l a t e i t s r e p r o d u c t i o n r a t e i n order to hold 82 i t s d e n s i t y a t , or r e s t o r e i t t c , the optimum. There i s no evidence that 20oplankters are able to c o n t r o l t h e i r r e p r o d u c t i o n r a t e ; McLaren (1963) c i t e d c o n s i d e r a b l e evidence that growth and r e p r o d u c t i o n r a t e s are p r i m a r i l y dependent on food and temperature. Kerfoot (1970) di s c u s s e d the adaptive value of o r i e n t i n g to l i g h t r a t h e r than to pressure i n terms of the b i o e n e r g e t i c b e n e f i t s r e s u l t i n g from these two o r i e n t a t i o n s t r a t e g i e s . He concluded that o r i e n t a t i o n to l i g h t and a n o c t u r n a l f e e d i n g p a t t e r n , r a t h e r than o r i e n t a t i o n to p r e s s u r e , maximizes exposure to food over the course of a year i n temperate l a t i t u d e s . T h i s i s p a r t i a l l y a r e s u l t of night length (feeding time) being l o n g e s t at the time of year t h a t primary production i s lowest. Animals o r i e n t i n g tc pressure would be l e s s able to respond to changes i n the depth at which peak p r o d u c t i v i t y o c c u r s . M i l l e r et a l . (1972) c r i t i c i s e d K e r f o c t ' s theory on the grounds that an animal should stay i n the most pr o d u c t i v e r e g i o n at a l l times i n s t e a d of spending the d a y l i g h t hours i n regions of low primary p r o d u c t i v i t y . N e i t h e r Kerfoot nor h i s d e t r a c t o r s considered the p o t e n t i a l r o l e of temperature i n the adaptive value of v e r t i c a l m i g r a t i o n . McLaren (1963) has proposed another theory f o r the adaptive value of v e r t i c a l m i g r a t i o n based on e n e r g e t i c s . He c o n s i d e r s growth and r e p r o d u c t i o n as f u n c t i o n s of temperature alone; f e e d i n g i s assumed to occur when the animal i s at the 83 s u r f a c e . H i s theory i s concerned with the p o t e n t i a l e n e r g e t i c b e n e f i t s from an a l t e r n a t i o n of high and low temperatures as a r e s u l t of v e r t i c a l m i g r a t i o n . Any e n e r g e t i c b e n e f i t r e s u l t i n g from the a l t e r n a t i o n of high and low temperatures depends on the p a r t i t i o n i n g of energy to r e s p i r a t i o n and growth. The p r o p o r t i o n of a v a i l a b l e energy that goes to growth can c n l y i n c r e a s e i f r e s p i r a t i o n r a t e decreases f a s t e r with decreasing temperature than d i g e s t i o n and a s s i m i l a t i o n r a t e s do. McLaren p r e d i c t e d that the energy gain from a l t e r n a t i n g temperatures i n c r e a s e s as the d i f f e r e n c e between the high and low temperature i n c r e a s e s . Thus, as the temperature d i f f e r e n t i a l between the s u r f a c e and day-depth i n c r e a s e s , i t becomes more and more advantageous t c migrate. C r i t i c i s m s of McLaren's theory c e n t e r on h i s assumptions about f e e d i n g and the r e l a t i o n s h i p between temperature and r e s p i r a t i o n and d i g e s t i c n r a t e s ; the theory w i l l be c r i t i c a l l y examined below (p 107). Mauchline and F i s h e r (1969) accepted McLaren's assessment cf p r e - e x i s t i n g t h e o r i e s but c r i t i c i s e d h i s f e e d i n g assumptions. Since migrants u l t i m a t e l y depend on s u r f a c e p r o d u c t i v i t y , these authors concluded that there are a number cf p o t e n t i a l b e n e f i t s d e r i v e d from v e r t i c a l m i g r a t i o n . The f o l l o w i n g b e n e f i t s were i n c l u d e d i n t h e i r a n a l y s i s : the general p a t t e r n of day descent may decrease p r e d a t i o n ; f o r t u i t o u s h o r i z o n t a l t r a n s p o r t may enable parts cf a po p u l a t i o n to e x p l o i t new phytoplankton patches; d i f f e r e n t l i g h t i n t e n s i t y p r e f e r e n c e s among s p e c i e s may allow the 84 p a r t i t i o n i n g of r e s o u r c e s . In t h e i r view, v e r t i c a l migration produces the g r e a t e s t p o s s i b l e u t i l i z a t i o n of r e s o u r c e s . McLaren (1974) proposed a model of the e f f e c t of v e r t i c a l m i g r a t i o n i n t h e r m a l l y s t r a t i f i e d waters on the r a t e of i n c r e a s e cf a p o p u l a t i o n of marine copepods. The mcdel does not i n c l u d e the metabolic c o n s i d e r a t i o n s upon which h i s e a r l i e r (McLaren 1963) theory depends. The metabolic model was r e q u i r e d to produce a more r a p i d grcwth r a t e , and thus a g r e a t e r number of eggs, i n migrants than i n non-migrants i n a given time p e r i o d . McLaren's new theory does not r e q u i r e that migrants grow f a s t e r than non-migrants; by i n c l u d i n g the slower grcwth rate of migrants i n h i s model, i t i s not necessary f o r him to i n c o r p o r a t e the metabolic assumptions he needed b e f o r e . His new model i s based on l i f e h i s t o r y s t r a t e g y theory and r e q u i r e s the f o l l o w i n g f i v e assumptions: 1) f i n a l s i z e i s a negative f u n c t i o n of temperature; 2) f e c u n d i t y i n c r e a s e s with s i z e ; 3) the r e q u i r e d r e p r o d u c t i v e p e r i o d i c i t y can be maintained d e s p i t e prolonged development; 4) age s p e c i f i c m o r t a l i t y i s greater on young stages than on o l d stages; 5) the p o p u l a t i o n i s near e q u i l i b r i u m . When these assumptions are s a t i s f i e d , there w i l l be a p a r t i c u l a r set of c o n d i t i o n s ( f e c u n d i t i e s , m o r t a l i t y r a t e s , etc.) i n which the i n c r e a s e d m o r t a l i t y due to prolonged development time i s o f f s e t by i n c r e a s e d f e c u n d i t y due to l a r g e r s i z e , and migrants w i l l produce more eggs than non-migrants. E m p i r i c a l data on the marine copepod Pseudocalanus minutus support h i s theory. 85 McLaren's demographic model may e x p l a i n the adaptive value of v e r t i c a l m i g r a t i o n f o r some organisms. However, because i t i s dependent on a d e l i c a t e balance among so many parameters, i t i s not l i k e l y t c be very widely a p p l i c a b l e . I t w i l l c e r t a i n l y s t i m u l a t e i n t e r e s t i n the a p p l i c a t i o n of l i f e - h i s t o r y s t r a t e g y theory to the phenomenon of v e r t i c a l m i g r a t i o n . The S i m u l a t i o n Model The f i e l d and l a b o r a t o r y experiments with C. t r i v i t t a t u s l a r v a e d i s c u s s e d p r e v i o u s l y (Sections I I I and IV) were designed t c e l u c i d a t e the e f f e c t s of temperature on the major e n e r g e t i c processes a f f e c t i n g growth. These r e l a t i o n s h i p s were i n c o r p o r a t e d i n t o a computer s i m u l a t i o n model of v e r t i c a l m i g r a t i o n . The s i m u l a t i o n model was used to examine the e f f e c t s of v a r i o u s m i g r a t i o n s t r a t e g i e s , p h y s i c a l parameters, and b i o l o g i c a l parameters on l a r v a l growth. The s i m u l a t i o n experiments were suggested by the r e s u l t s of f i e l d and l a b o r a t o r y experiments, and by a c o n s i d e r a t i o n of McLaren's pred i c t i o n s . The f o l l o w i n g g e n e r a l questions were examined: 1) i s there a demonstrable "energy boost" as a r e s u l t of v e r t i c a l m i g r a t i o n with n a t u r a l prey d i s t r i b u t i o n s ? 2) what i s the nature of the i n t e r a c t i o n between prey d e n s i t y and d i s t r i b u t i o n and v e r t i c a l m i g r a t i o n s t r a t e g y ? 3) what migration strategy, i s "best" f o r l a r v a l growth on an 86 e n e r g e t i c s b a s i s ? Methods The s i m u l a t i o n model was designed to f e l l o w the d a i l y energy balance of a v e r t i c a l l y migrating l a r v a . The energy gains from f e e d i n g and the energy l o s s e s due to metabolism were c a l c u l a t e d over 20 minute i n t e r v a l s f o r 30 days. Energy p a r t i t i o n i n g i n the model f o l l o w e d the energy budget equation C=P+R+F+U (Ricker 1971) as modified by Warren and Davis (1967) except where e m p i r i c a l data were mi s s i n g . Change i n weight was the index used to express the net energy change because of the a v a i l a b i l i t y of data i n mass u n i t s . Appendix I c o n t a i n s a d e t a i l e d d e s c r i p t i o n of the model. Graphs of t y p i c a l i n p u t data, flow diagrams of the g e n e r a l model and of the f e e d i n g and d i g e s t i o n s u b r o u t i n e s , a l i s t of parameter values and t h e i r sources, and a l i s t of the FORTRAN v a r i a b l e s used i n the model and t h e i r d e f i n i t i o n s are i n c l u d e d i n t h i s appendix. Appendix I I c o n t a i n s a l i s t i n g of the FORTRAN programs used i n running the model. A l l s i m u l a t i o n s were run cn an IBM 1130 computer. Each s i m u l a t i o n begins by s e t t i n g the p o s i t i o n of the l a r v a a t 20m and the time equal to zero. The water temperature and prey d e n s i t i e s at the depth occupied by the l a r v a are i n t e r p o l a t e d from temperature and prey d e n s i t y p r o f i l e s based on e m p i r i c a l data from mid-summer (except when temperature cr 87 prey d e n s i t y are manipulated). The migrati o n p a t t e r n i s pre-set i n g r a p h i c a l form and the p o s i t i o n of the l a r v a i n subsequent time i n t e r v a l s i s determined by i n t e r p o l a t i o n . The general form of the model i s shown i n F i g u r e 17. A f t e r the a p p r o p r i a t e depth, temperature, and prey d e n s i t i e s have been determined, the energy gain to the l a r v a during the c u r r e n t time i n t e r v a l i s c a l c u l a t e d . The model e n t e r s a feed i n g phase or a d i g e s t i o n phase depending on the degree of f u l l n e s s of the crop. I f the crop i s f u l l the f e e d i n g phase i s bypassed and only d i g e s t i o n o ccurs. I f the crop i s net f u l l , the model enters the f e e d i n g phase and then the d i g e s t i o n phase. The model i s designed so that once the crop i s f u l l the f e e d i n g phase i s bypassed u n t i l a set p r o p o r t i o n of the food i n the crop has been d i g e s t e d , and the crop has been emptied. T h i s f o r m u l a t i o n i s designed to mimic the d i g e s t i v e process i n c J i i l 2 l 2 2 r . 3 1 i * l a r v a e i n which there i s a d i g e s t i v e pause a f t e r the crop i s f u l l f ollowed by the e g e s t i o n of undigested m a t e r i a l from the crop. The model next c a l c u l a t e s the energy l o s s e s due to r e s p i r a t i o n and movement. The net energy change i s c a l c u l a t e d from the energy gained from d i g e s t i o n and energy l o s t to r e s p i r a t i o n and movement. The l a r v a l weight i s then incremented, and the next time i n t e r v a l i s begun by the c a l c u l a t i o n of the new p o s i t i o n of the l a r v a . The f e e d i n g phase of the model c a l c u l a t e s the number and weight of l a r g e and small prey eaten per time i n t e r v a l . The number eaten i s s t o c h a s t i c a l l y determined from a Poisson 88 FIGURE 17 G e n e r a l i z e d flow diagram of the s i m u l a t i o n model. The diagram r e p r e s e n t s the sequence of o p e r a t i o n s which occurs during one time i n t e r v a l . 8 8a T I M E _— f D E P T H C A L C U L A T E T E M P E R A T U R E P R E Y D E N S I T Y C A L C U L A T E A M O U N T E A T E N Y E S V C A L C U L A T E A M O U N T D I G E S T E D 'C A L C U L A T E R E S P I R A T I O N C O S T C A L C U L A T E S W I M M I N G C O S T N O 1* C A L C U L A T E N E T E N E R G Y C H A N G E I N C R E M E N T L A R V A L W E I G H T I N C R E M E N T T I M E 89 d i s t r i b u t i o n . The mean of the d i s t r i b u t i o n v a r i e s with prey d e n s i t y a c c o r d i n g to the H o l l i n g d i s k equation f o r two prey types: U= (ALPH 1 *BFUD)/ (1+ALPH1*BFUD*H1) + (ALPH2*SFUD*H2) f o r l a r g e prey and U= (ALPH2*SFUD)/ (1 + ALPH1*BFUD*H1)+ (ALPH2*SFUD*H2) f o r s m a l l prey, where U i s the mean of a Poisson d i s t r i b u t i o n , ALPH 1 i s the " c a t c h a b i l i t y " of l a r g e prey, ALPH2 i s the " c a t c h a b i l i t y " of smal l prey, BFUD i s the l a r g e prey d e n s i t y , SFUD i s the smal l prey d e n s i t y , H1 i s the handling rime f o r la r g e prey, and H2 i s the ha n d l i n g time f o r smal l prey. The s t o c h a s t i c element i s introduced to make the feeding phase of the model more c l o s e l y i m i t a t e the n a t u r a l f e e d i n g p a t t e r n cf £hl2^2£H§ l a r v a e . Feeding depends on random encounters with prey items, and the expected number of encounters i n cne time i n t e r v a l i s s m a l l . My use of the disk equation i s not s t r i c t l y c o r r e c t s i n c e the random element i s the encounter r a t e (ALPH1*BFUD and ALPH2*SFUD) not the mean capture r a t e (U). This f o r m u l a t i o n would permit a l a r v a to eat f a s t e r than i s p h y s i c a l l y p o s s i b l e , based on handling time, i f a l a r g e Poisson number i s encountered. However, handling time i s shor t r e l a t i v e to the 20 minute time i n t e r v a l s , and the crop volume has a maximum val u e . These c o n s t r a i n t s , and the low p r o b a b i l i t y of g e t t i n g a high Poisson number, mean th a t the p o t e n t i a l e r r o r from my use of the random element i n c a l c u l a t i n g prey capture i s very s m a l l . The t o t a l amount cf 90 food i n the crop i s updated during the fe e d i n g phase. I f the t o t a l food eaten (large and small) i n a time i n t e r v a l would f i l l the crop to more than i t s maximum c a p a c i t y , the amount of food i n the crop i s set equal to the maximum c a p a c i t y of the crop and the model bypasses the feeding phase i n subseguent time i n t e r v a l s u n t i l the crop i s emptied. T h i s occurs when the d i g e s t i b l e p o r t i o n of a c r o p - f u l l of food has been d i g e s t e d . T h i s seguence cf crop f i l l i n g and emptying i s based on the ob s e r v a t i o n that l a r v a e do net s e l e c t i v e l y egest a p o r t i o n of the crop contents and then feed again. The amount of d i g e s t i b l e fcod i n the crop i s c a r r i e d as a separate v a r i a b l e which i s in c r e a s e d by a p r o p o r t i o n of the t o t a l food eaten i n each time i n t e r v a l . T h i s p r o p o r t i o n (68%) i s equal t c the a s s i m i l a t i o n e f f i c i e n c y measured i n S e c t i o n IV. The d i g e s t i v e phase i s entered during every time i n t e r v a l whether or not the model entered the f e e d i n g phase i n t h a t i n t e r v a l . The amount di g e s t e d during a time i n t e r v a l i s c a l c u l a t e d by i n t e r p o l a t i o n from e m p i r i c a l t e m p e r a t u r e - - d i g e s t i o n r a t e data. The food t h a t i s dige s t e d i s added to a stored food pool that i s used i n l a t e r c a l c u l a t i o n s . The amount of food d i g e s t e d i s su b t r a c t e d frcm the t o t a l food i n the crop and from the d i g e s t i b l e food. I f the amount t o be di g e s t e d i s g r e a t e r than the amount a v a i l a b l e f o r d i g e s t i c n , the amount a v a i l a b l e i s dige s t e d and both food v a r i a b l e s are s e t to zero. The model w i l l begin to enter the feeding phase again when t h i s occurs. 91 R e s p i r a t i o n and movement are the sources of energy l o s s c onsidered i n the model. R e s p i r a t o r y energy l o s s i s c a l c u l a t e d from the equation: R=aWb The values of a and b are from S e c t i o n IV. For intermediate temperatures R i s i n t e r p o l a t e d . The energy c o s t of a migration v was estimated i n S e c t i o n IV, and t h i s estimate i s added to the r e s p i r a t o r y energy l o s s when the l a r v a i s m i g r a t i n g . The t o t a l energy l o s s i s used i n l a t e r c a l c u l a t i o n s . The net energy change i s c a l c u l a t e d as the d i f f e r e n c e between the s t o r e d energy and the t o t a l energy l o s s . Stcred energy i s the d i g e s t e d food minus a constant percentage l o s s to s p e c i f i c dynamic a c t i o n . The net energy change i s converted to mass u n i t s and used to increment the l a r v a l weight. R e s u l t s S e n s i t i v i t y A number of s i m u l a t i o n s were dene t c assess the s e n s i t i v i t y of the model to changes i n i t s parameter v a l u e s . The r e s u l t s ( F i g . 18) show t h a t the e f f e c t of parameter changes on l a r v a l growth was q u i t e v a r i a b l e . Small changes i n handling time (HI, H2) had l i t t l e e f f e c t cn l a r v a l grewth because the f o r m u l a t i o n of the d i s k equation i s such that they have l i t t l e e f f e c t on the number of prey eaten. As FIGURE 18 The e f f e c t s of changes i n the parameter values of s e l e c t e d parameters on l a r v a l growth. The l i n e s on each graph are the r e s u l t s of s i m u l a t i o n s using the values a s s o c i a t e d with the l i n e s as input parameter values f o r the model. 92 a 93 " c a t c h a b i l i t y " of s m a l l prey (ALPH2) was v a r i e d the number cf small prey eaten v a r i e d , but, because of t h e i r small s i z e , there was l i t t l e e f f e c t on l a r v a l growth. Within the range of reasonable values f o r the weight of l a r g e prey (WEF) and s p e c i f i c dynamic a c t i o n (SDA), these parameters had a s m a l l e f f e c t on l a r v a l growth. SDA a c t s only on the amount of food d i g e s t e d , which i s low i n any time i n t e r v a l . Because there i s a maximum weight of food which the crop can held, any e f f e c t cf WBF i s c o n s t r a i n e d , and i t s e f f e c t on l a r v a l growth i s l e s s than i t would be i f the crop s i z e was u n l i m i t e d . Changing the p r o p o r t i o n of fcod eaten that could be a s s i m i l a t e d |AE) had a r e l a t i v e l y l a r g e e f f e c t on l a r v a l growth because t h i s p r o p o r t i o n d i r e c t l y a f f e c t s the energy gain to the l a r v a . The " c a t c h a b i l i t y " of l a r g e prey (ALPH1) a l s o has a l a r g e e f f e c t cn growth. I t a f f e c t s the number of l a r g e prey captured i n the f e e d i n g phase of the model. Because of the s i z e of l a r g e prey, even small changes i n the number eaten have a l a r g e e f f e c t cn growth. The random number i n i t i a l i z e r (RNI) a f f e c t s the number cf prey eaten by changing the s e r i e s of random numbers used i n the f e e d i n g phase of the model. The e f f e c t of t h i s s t o c h a s t i c element i s to produce a v a r i a b l e p a t t e r n of growth and f i n a l weight ( F i g . 18). The v a r i a b i l i t y i n t r o d u c e d by the s t o c h a s t i c element i n the model i s r e l a t i v e l y low compared to the e f f e c t s of temperature, prey d e n s i t y , and some of the other parameters. For t h i s reason I b e l i e v e t h a t the q u a l i t a t i v e r e s u l t s presented below represent e f f e c t s of the manipulated 9 4 v a r i a b l e s r a t h e r than v a r i a b i l i t y b u i l t i n t o the model. P r e d i c t i o n s from the model depend cn s e v e r a l assumptions and f u n c t i o n a l r e l a t i o n s h i p s . L a r v a l m i g r a t i o n i s assumed to occur every day. There i s l i t t l e evidence that t h i s does not occur i n Eunice Lake, but i f migration extended over s e v e r a l days i t would have a l a r g e e f f e c t on the e n e r g e t i c s of the l a r v a e . Prey v u l n e r a b i l i t y i s assumed to be constant through the day and n i g h t . I f t h i s were net t r u e , i t might have important consequences f o r the e n e r g e t i c s of animals s t a y i n g at the s u r f a c e . However, Chaoborus are ambush predators which use v i b r a t i o n r e c e p t o r s to l o c a t e t h e i r prey so that day-night d i f f e r e n c e s would not be expected on the part of the predator. Zooplankton may be able to see and avoid Chaoborus l a r v a e during the day, but there are no r e s u l t s to t e s t t h i s p o s s i b i l i t y . In the model I have assumed that once the crop i s f i l l e d , no more feed i n g i s p o s s i b l e u n t i l that amount of food i s d i g e s t e d , and the remains egested. I f continuous f e e d i n g were p o s s i b l e the energy gain to the l a r v a at high d e n s i t i e s might be higher than p r e d i c t e d . There i s evidence that Chaoborus e x h i b i t a d i g e s t i v e pause, but e f f o r t s to q u a n t i f y i t f a i l e d . For ease i n computation I have assumed that a l l digested m a t e r i a l i s a s s i m i l a t e d and that the p r o p o r t i o n of a prey item that i s u l t i m a t e l y d i g e s t e d eguals the a s s i m i l a t i o n e f f i c i e n c y measured f o r f o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e ; t h i s seems to be a reasonable r e p r e s e n t a t i o n of d i g e s t i o n i n Chaoborus l a r v a e . The estimated energy c o s t c f migration i s 95. assumed to be constant with r e s p e c t to temperature. T h i s i s probably a n e g l i g i b l e source of e r r o r because c f the s m a l l amount of energy i n v o l v e d . The o x y - c a l o r i f i c c o n v e r s i o n f a c t o r used assumes a constant r e s p i r a t o r y , q u o t i e n t . The RQ i s probably not constant, but the v a r i a b i l i t y i n v o l v e d i s probably low. Some of the parameter values were measured and some are reasonable estimates. Of those d i s c u s s e d above to which the model was s e n s i t i v e (ALPH1, WBF, and AE), only ALPH1 was not measured d i r e c t l y ; i t was estimated from l a b o r a t o r y s t r i k e experiments. Experiments were done to c h a r a c t e r i z e and q u a n t i f y the e f f e c t s of temperature on metabolism and grcwth (Section IV). Less emphasis was placed on the r e l a t i o n s h i p between f e e d i n g r a t e and prey d e n s i t y . The feeding part of the model i s t h e r e f o r e l e s s r i g o r o u s than the metabolism p a r t . Because of the parameter estimates and the form of the feeding part of the model, i t i s not p o s s i b l e to analyze f i n e d e t a i l s cf e n e r g e t i c cost and b e n e f i t . However, using the model, f c o d , temperature, and migration p a t t e r n are shown t o N h a v e a l a r g e e f f e c t on l a r v a l growth. I f e e l that the model i s adequate to demonstrate q u a l i t a t i v e d i f f e r e n c e s i n the e f f e c t of v a r i o u s combinations of food, temperature, and migration p a t t e r n on growth. An i n c r e a s e i n d e t a i l i n the feeding part and more accurate parameter estimates would allow a more exact q u a n t i f i c a t i o n cf growth and of the d i f f e r e n c e s between migration s t r a t e g i e s , but the r e s u l t s and p r e d i c t i o n s would 96 probably be the same. The E f f e c t o f Food The i n t e r a c t i o n of food and migration s t r a t e g y on grcwth was examined i n three ways -- with n a t u r a l prey d e n s i t i e s , with v a r i o u s " s u r f a c e " prey d e n s i t i e s , and with v a r i o u s prey d e n s i t y p r o f i l e s . In the f i r s t s et of s i m u l a t i o n experiments the prey d e n s i t y was set at n a t u r a l summer values f o r Eunice l a k e , and the migr a t i o n p a t t e r n was v a r i e d . Growth under these food c o n d i t i o n s was g r e a t e s t i f the l a r v a stayed at the s u r f a c e ( F i g . 19) cr followed a p h y s i o l o g i c a l migration p a t t e r n i n which downward migration i s t r i g g e r e d by a f u l l crop, and upward migration i s t r i g g e r e d by an empty crop. Larvae f o l l o w i n g the n a t u r a l Eunice Lake migrat i o n p a t t e r n of 4 hcurs at the s u r f a c e and 16 hours at the bottom grew l e s s than l a r v a e which stayed at the top but more than l a r v a e which stayed at the bottom. The second set of s i m u l a t i o n experiments compared s t a y i n g at the s u r f a c e and f o l l o w i n g a p h y s i o l o g i c a l p a t t e r n tc the n a t u r a l m i g r a t i o n p a t t e r n , as s u r f a c e prey d e n s i t i e s v a r i e d from zero to summer f i e l d d e n s i t i e s . Prey d e n s i t y was v a r i e d from 0 t c 300 each of l a r g e and smal l prey per 100 l i t e r s at the s u r f a c e . No food was provided below 7m — a c o n d i t i o n s i m i l a r to that found i n the l a k e . The r e s u l t s ( F i g . 20) again FIGURE 19 The e f f e c t of migration p a t t e r n on l a r v a l grcwth n a t u r a l prey d e n s i t i e s . The l i n e s are r e s u l t s from s i m u l a t i o n s with the migration p a t t e r n set as l a b e l l e d . A - P h y s i o l o g i c a l B-Su r f a ce C - N a t u r a l D-Bo t tom 98 FIGURE 20 The e f f e c t of migration p a t t e r n and s u r f a c e prey d e n s i t y on l a r v a l growth. The data p o i n t s are the f i n a l l a r v a l weights of s i n g l e s i m u l a t i o n s . 9 8 a 1.2 -I O P h y s i o l o g i c a I O CT _ N a t u r a J= x S u rf a c e l.o-l -+—' _C D) CD £ .8-03 > o x Sl ctf .6-c .— .4 o 1 -1 1 1 1 1 — 0 50 100 150 200 250 300 P r e y D e n s i t y at S u r f a c e 99 show that larvae f o l l o w i n g the migration p a t t e r n found i n Eunice Lake grow l e s s than l a r v a e f o l l o w i n g a p h y s i o l o g i c a l m i g r a t i o n p a t t e r n or s t a y i n g at the s u r f a c e . There was l i t t l e d i f f e r e n c e between the " s t a y - a t - s u r f a c e " and p h y s i o l o g i c a l p a t t e r n s because at the prey d e n s i t i e s examined the crop seldom f i l l s , and the l a r v a e f o l l o w i n g the p h y s i o l o g i c a l pattern experience the same c o n d i t i o n s as those which stay at the s u r f a c e . In the t h i r d s et of s i m u l a t i o n s with v a r i a b l e f c o d , the food p r o f i l e was v a r i e d to i n c r e a s e the amount of food a v a i l a b l e at the day depth. The fou r food p r o f i l e s had 300 of each prey s i z e at the s u r f a c e and 50, 150, 200, and 250 of each prey s i z e at the bottom. The f o u r mig r a t i o n p a t t e r n s used i n the previous experiments were t e s t e d with each prey p r o f i l e . The r e s u l t s are q u a l i t a t i v e , but they show a gradual s h i f t i n the mig r a t i o n p a t t e r n which maximizes growth from s t a y i n g at the s u r f a c e when food i s low at the day depth {Fig. 21 A,B) to s t a y i n g at the bottom when food i s abundant at the day depth ( F i g . 21 D). There may be some c r i t i c a l prey d e n s i t y p r o f i l e which prov i d e s maximum growth under the n a t u r a l Eunice Lake migration p a t t e r n , but i t i s not demonstrated i n t h i s s et of s i m u l a t i o n s . With a prey d e n s i t y of 200 at the bottom, the n a t u r a l migration p a t t e r n i s best i n terms of growth, but the d i f f e r e n c e s i n growth between the fou r migration p a t t e r n s are within the s t o c h a s t i c v a r i a t i o n i n the model. As i n the previous s e t of s i m u l a t i o n s there was l i t t l e d i f f e r e n c e 100 F I G U R E 21 T h e e f f e c t o f m i g r a t i o n p a t t e r n a n d p r e y d e n s i t y p r o f i l e o n l a r v a l g r o w t h . T h e l i n e s a r e r e s u l t s f r o m s i m u l a t i o n s w i t h t h e m i g r a t i o n p a t t e r n s e t a s l a b e l e d ( a , b , c , d ) . T h e p r e y d i s t r i b u t i o n i s a s f o l l o w s : 3 0 0 o f e a c h p r e y s i z e a t t h e s u r f a c e , a n d 50 ( A ) , 150 (B) , 2 0 0 (C) , a n d 2 5 0 (D) o f e a c h p r e y s i z e a t t h e b o t t o m . 1 0 0 a ( 6 U J ) J l l B l S A A | B A J B ~] 101 between the " s t a y - a t - s u r f a c e " and p h y s i o l o g i c a l p a t t e r n s . The E f f e c t of Temperature The e f f e c t of temperature on growth was examined by running s i m u l a t i o n experiments with n a t u r a l prey d e n s i t i e s , the n a t u r a l migration p a t t e r n , and v a r i o u s temperature p r o f i l e s . The f o l l o w i n g temperature p r o f i l e s were t e s t e d : constant 5, 10, 15, and 20°, and p r o f i l e s with a 1, 2, and 3° d i f f e r e n c e between s u r f a c e and bcttom temperatures at s u r f a c e temperatures of 10, 15, and 20° (for example: 10, 9; 10, 9, 8; 10, 9, 8, 7; 15, 14; e t c . ) . The constant 5, 10, 15, and 20° p r o f i l e s were a l s o t e s t e d with the " s t a y - a t - s u r f a c e " migration p a t t e r n . Ten s i m u l a t i o n s were run with randomly chosen i n i t i a l i z e r s f o r each temperature p r o f i l e ; the mean growth was p l o t t e d a g a i n s t s u r f a c e temperature. At a l l the temperature p r o f i l e s t e s t e d , growth was low when the la r v a e followed the n a t u r a l migration p a t t e r n ( F i g . 22). There was l i t t l e d i f f e r e n c e between constant temperatures and p r o f i l e s with 1, 2, and 3° d i f f e r e n c e s between the s u r f a c e and bottom so these r e s u l t s were pooled. The 5° point i s f o r constant 5° c n l y (the model wasn't set up to handle temperatures below 5 ° ) . At a l l surf a c e temperatures growth was much gr e a t e r when the l a r v a e stayed at the s u r f a c e than when they migrated. As the s u r f a c e temperature i n c r e a s e d , growth decreased under both migration p a t t e r n s . FIGURE 22 The e f f e c t of m i g r a t i o n p a t t e r n and temperature p r o f i l e cn l a r v a l growth. The data p o i n t s f o r the s u r f a c e migration p a t t e r n ( s o l i d c i r c l e s ) and the n a t u r a l m i g r a t i o n p a t t e r n at 5° (open c i r c l e ) are the mean f i n a l weights of 10 s i m u l a t i o n s . The data p o i n t s f o r the n a t u r a l m i g r a t i o n p a t t e r n a t 10, 15, and 20° ( s o l i d c i r c l e s ) are pooled r e s u l t s of the four temperature p r o f i l e s d e s c r i b e d i n the t e x t . 102 a 2.2-2.0-1.8-1.6-1.4" o>1.2-• O Na tu ra l S u r f a c e 751.0H LL .8-.21 1 0 5 10 15 S u r f a c e T e m p e r a t u r e 20 103 The r e s u l t s of the s i m u l a t i o n experiments agree well with l a b o r a t o r y growth s t u d i e s . In the l a b o r a t o r y grcwth experiments the l a r v a e were exposed to constant prey d e n s i t i e s and the temperature was v a r i e d to match the temperature p r o f i l e encountered during a mi g r a t i o n . In the s i m u l a t i o n experiments, however, the l a r v a e were exposed to v a r i a b l e temperature and v a r i a b l e prey d e n s i t y . For t h i s reason the l a b o r a t o r y s i t u a t i o n i s analagous to only the " s u r f a c e " s i m u l a t i o n s and those i n which the prey d e n s i t y p r o f i l e was v a r i e d . In both growth experiment II ( F i g . 16) and the s i m u l a t i o n experiments with v a r i a b l e s u r f a c e food ( F i g . 20), growth was a f u n c t i o n of prey d e n s i t y when the l a r v a e were at surf a c e temperatures (no m i g r a t i o n ) . When the prey d e n s i t y p r o f i l e was v a r i e d so that i t approached constant prey d e n s i t y throughout the water column (Fig 21, C,D) l a r v a l growth was the same or grea t e r f o r migrating l a r v a e than f o r non-migrants. T h i s matched the l a b o r a t o r y r e s u l t s f o r the low food treatment ( F i g . 16). The s u r f a c e prey d e n s i t i e s i n t h i s set of s i m u l a t i o n s were too low f o r the l a r v a e to grow enough to match the l a b o r a t o r y r e s u l t s f o r the high feed treatment; l a r v a l grcwth i n the s i m u l a t i o n experiments was prey d e n s i t y l i m i t e d at the s u r f a c e prey d e n s i t i e s . ( 104 D i s c u s s i o n A c o n s i d e r a t i o n of the p o t e n t i a l s t r a t e g i e s of v e r t i c a l m igration suggests three b a s i c migration p a t t e r n s . They are, i n order of i n c r e a s i n g complexity: 1) stay i n one place, 2) a d i e l m i g r a t i o n p a t t e r n of some s o r t that i s synchronous ever the whole p o p u l a t i o n , and 3) a p a t t e r n t h a t i s under the c o n t r o l of the i n d i v i d u a l animal's p h y s i o l o g i c a l s t a t e so that the p o p u l a t i o n i s mig r a t i n g asynchronously. The e n e r g e t i c i m p l i c a t i o n s of f o l l o w i n g these pa t t e r n s are analyzed i n terms of t h e i r a b i l i t y to maximize net energy gain to the organism. In c o n s i d e r i n g the energy c o s t s and b e n e f i t s of these p a t t e r n s I have assumed that r e s p i r a t i o n r a t e v a r i e s with temperature, that i t decreases f a s t e r than d i g e s t i o n and a s s i m i l a t i o n r a t e s with d e c r e a s i n g temperature, and that the prey v u l n e r a b i l i t y i s constant over time. The s i m p l e s t p a t t e r n i s t h a t of no m i g r a t i o n . Many zooplankton groups have r e p r e s e n t a t i v e s which do not migrate (Hutchinson 1967). On an e n e r g e t i c s b a s i s there are two cases of non-migration that must be considered — s t a y i n g i n c o l d , food-poor, deep water and s t a y i n g i n warm, f c o d - r i c h , n e a r - s u r f a c e water. In e i t h e r case the maintenance metabclic c o s t w i l l be r e l a t i v e l y c onstant. Staying i n deep water where primary and secondary production are low w i l l r e s u l t i n death i f consumption i s below the l e v e l r e g u i r e d f o r maintenance of the organism. At best, growth w i l l be slow because of low food I 105 consumption. The d i g e s t i o n and a s s i m i l a t i o n r a t e s would have to be g r e a t e r than the r e s p i r a t i o n r a t e f o r any growth to occur. S t a y i n g at the s u r f a c e presents an e n t i r e l y d i f f e r e n t problem. R e s p i r a t i o n and d i g e s t i o n r a t e s and prey d e n s i t y are almost u n i v e r s a l l y higher than i n deep water. Above the maintenance l e v e l of food consumption growth i s a f u n c t i o n cf prey d e n s i t y ( F i g s . 16 and 20). D i e l v e r t i c a l m i g r a t i o n s are found widely i n nature (Cushing 1951, Teraguchi and Northcote 1966, Mauchline and F i s h e r 1969). C h a r a c t e r i s t i c a l l y the m i g r a t i o n p a t t e r n has a r e g u l a r p e r i o d i c i t y , and most of the p o p u l a t i o n migrates synchronously; the p e r i o d i c i t y i s u s u a l l y one that can be modified by exogenous s t i m u l i . An animal migrating with a r e g u l a r d i e l m igration p a t t e r n cannot maximize energy gain i n an environment c o n t a i n i n g sharp b i o l o g i c a l and p h y s i c a l g r a d i e n t s . Nc matter what the form of the p a t t e r n i s , an animal f o l l o w i n g a d i e l m igration p a t t e r n would p e r i o d i c a l l y migrate down out of the f o o d - r i c h zone with i t s gut empty, thus d e c r e a s i n g i t s p r o b a b i l i t y of f e e d i n g to n e a r l y zero u n t i l i t re-ascends on the next m i g r a t i o n c y c l e . T h i s p a t t e r n would be p a r t i c u l a r l y bad f o r predators s i n c e they feed more i n f r e g u e n t l y than h e r b i v o r e s . An a d j u s t a b l e d i e l migration p a t t e r n would be e s p e c i a l l y advantageous i f p a r t i c u l a r combinations of time and depth are b e t t e r i n some way than o t h e r s . These "more advantageous" combinations of depth and time are u s u a l l y assumed to be the r e s u l t of p r e d a t i o n , but* 106 McLaren (1963) suggests they are e n e r g e t i c i n nature. The roost complex migrati o n p a t t e r n envisaged i s one that i s c o n t r o l l e d by the p h y s i o l o g i c a l s t a t e of the animal. Using a c o n t r o l mechanism of t h i s s o r t , an animal could move up i n t o warm, f o o d - r i c h water and feed u n t i l i t s gut was f u l l and then move down i n t o c o o l , water u n t i l i t s gut was empty, E n e r g e t i c a l l y , t h i s p a t t e r n i s the most a t t r a c t i v e because the animal makes the most e f f i c i e n t use of a l t e r n a t i o n between warm and c o l d water. An animal using t h i s mechanism to t r i g g e r migration i s n e i t h e r l e f t at the s u r f a c e with a f u l l gut nor l e f t i n deep water when i t s gut i s empty. There i s no gccd evidence that t h i s mechanism i s very widespread, t u t i t has been suggested r e c e n t l y f o r chaetcgnaths (Pearre 1973). A p o s s i b l e reason that t h i s mechanism has seldom been r e p o r t e d i s t h a t the po p u l a t i o n would be migrating asynchronously. Each i n d i v i d u a l wculd migrate with i t s own p e r i o d i c i t y and an e a s i l y d e t e c t a b l e mass migratio n would seldom occur. The a n a l y s i s of the mig r a t i o n p a t t e r n s presented above i s based s o l e l y on e n e r g e t i c s . The c o n c l u s i o n that a s u r f a c e pattern or p h y s i o l o g i c a l l y based p a t t e r n i s most advantageous i s at odds with previous theory (McLaren 1963) on the en e r g e t i c advantages r e s u l t i n g from v e r t i c a l m i g r a t i o n . I t i s necessary, t h e r e f o r e , to c r i t i c a l l y examine McLaren's theory. The g e n e r a l c o n c l u s i o n s of McLaren's (1963) theory on the adaptive value of v e r t i c a l m i g r a t i o n are presented above (page 107 83). He concluded t h a t the migratio n p a t t e r n most commonly seen i n zooplankton (an endogenous r h y t h m i c i t y capable c f being modified by exogenous s t i m u l i ) i s e n e r g e t i c a l l y the most advantageous. T h i s c o n c l u s i o n i s at odds with the a n a l y s i s of the e n e r g e t i c s of v a r i o u s migration p a t t e r n s presented' abcve. The d i f f e r e n c e between the two analyses of e n e r g e t i c advantage l i e s i n McLaren's assumptions. The general c r i t i c i s m of McLaren's theory i s that i t ignores the complex i n t e r a c t i o n between food consumption and temperature dependent d i g e s t i o n and r e s p i r a t i o n r a t e s . He makes s e v e r a l assumptions, e x p l i c i t l y or i m p l i c i t l y , which are c r i t i c a l t c the proper f u n c t i o n i n g of h i s theory on the e n e r g e t i c advantage of a l t e r n a t i n g between high and low temperatures. These assumptions are the f o l l o w i n g : 1) " a l l necessary f e e d i n g " can occur at the s u r f a c e ; 2) r e s p i r a t i o n rate i s a monotonically i n c r e a s i n g f u n c t i o n of temperature; 3) d i g e s t i o n r a t e i s constant or r e s p i r a t i o n r a t e must decrease f a s t e r with r e s p e c t to temperature than d i g e s t i o n and a s s i m i l a t i o n r a t e s do. McLaren's f i r s t assumption i s i n v a l i d f o r two reasons. I t has been pointed out t h a t not a l l organisms that migrate feed e n t i r e l y at the s u r f a c e . Mauchline and F i s h e r (1969) found that many euphausiids feed throughout t h e i r m i g r a t i o n ; these authors a l s c guestion whether g r a z e r s are able to f i l t e r enough food during the s h o r t dark period i n the summer t c meet 108 t h e i r d a i l y requirements. S e v e r a l predators have been shown to feed i r r e g u l a r l y and to feed both a t the s u r f a c e and i n deep water (Teal 1971, Fedorenko 1973, Pearre 1973). The concept of " a l l necessary f e e d i n g " i n t r o d u c e d by McLaren i s impossible to d e f i n e . I t can r e f e r e q u a l l y well to a maintenance r a t i o n or to a maximum r a t i o n . McLaren's i m p l i c i t assumption that r e s p i r a t i o n r a t e i s a monotonically i n c r e a s i n g f u n c t i o n of temperature has important consequences f o r the e n e r g e t i c c o s t a s s o c i a t e d with moving upward i n t o warm water or downward i n t o c c l d water. The presence cf metabolic a d a p t a t i o n s to decrease the e f f e c t of high temperature on r e s p i r a t i o n r a t e would tend to make i t l e s s c o s t l y t c move up i n t o warm water. Although not widespread, the presence of t h i s type of metabolic " r e g u l a t i o n " has been suggested f o r C. t r i v i t t a t u s (Section IV) and f o r Neomysis ,(A. H. Knight, pers. comm.) and r e p o r t e d f o r the v e r t i c a l l y m igrating cladoceran, Le_ptodora (Moshiri 1969). I f r e s p i r a t i o n r a t e did not decrease as temperature decreased, the p o t e n t i a l energy gain from a reduced metabclic r a t e at low temperature would not be r e a l i z e d . T e a l (197 1) showed that the r e s p i r a t i o n r a t e of predaceous, mesopelagic decapod Crustacea d i d not decrease at the c o l d temperature cf t h e i r day depth. T h i s phenomenon might well be widespread i n v e r t i c a l l y m i g rating p r e d a t o r s , and i t could represent a s i g n i f i c a n t energy l o s s . 109 Any e n e r g e t i c b e n e f i t r e s u l t i n g from the a l t e r n a t i o n of high and low temperatures depends on the p a r t i t i o n i n g of energy to r e s p i r a t i o n and growth. If d i g e s t i o n and a s s i m i l a t i o n r a t e s are constant with r e s p e c t to temperature, a decrease i n r e s p i r a t i o n r a t e at low temperatures w i l l i n c r e a s e the p r o p o r t i o n of a v a i l a b l e energy that goes to growth. Dnder these c o n d i t i o n s the e n e r g e t i c "boost" p r e d i c t e d by McLaren could e x i s t . However, d i g e s t i o n r a t e v a r i e s with temperature (Brett and 'Higgs 1970, Fedorenko 1973). Under these c o n d i t i o n s , f o r a change i n the p a r t i t i o n i n g of a v a i l a b l e energy to occur such that more energy i s d i r e c t e d i n t o grcwth at low temperatures, the r e s p i r a t i o n r a t e must decrease f a s t e r than the d i g e s t i o n r a t e as temperature decreases. I f one accepts t h i s reasoning, the most e f f i c i e n t m i g r a t i o n p a t t e r n i s the p h y s i o l o g i c a l one, not the one p r e d i c t e d by McLaren. From the a n a l y s i s of the e n e r g e t i c s of v e r t i c a l migration i t i s c l e a r that the m i g r a t i o n s t r a t e g y maximizing net energy gain i s that cf s t a y i n g at the s u r f a c e or f o l l o w i n g a p h y s i o l o g i c a l rhythm based on the f i l l i n g and emptying of the gut. F o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e i n Eunice Lake f e l l o w n e i t h e r of these p a t t e r n s . I t i s necessary t h e r e f o r e to c o n s i d e r other, non-energetic e x p l a n a t i o n s f o r t h e i r migration p a t t e r n . The d i e l d i s t r i b u t i o n of these l a r v a e might simply be a r e f l e c t i o n of the d i e l d i s t r i b u t i o n of t h e i r prey. T h i s 110 e x p l a n a t i o n i s u n l i k e l y s i n c e almost a l l the z c c p l a n k t c n s p e c i e s eaten by C. t r i v i t t a t u s are found above 5m. Diajgtonius kenai, the prey s p e c i e s which makes up most of the biomass of the d i e t , makes a d i e l v e r t i c a l m igration which i s opposite to that made by f o u r t h - i n s t a r l a r v a e . There i s no experimental evidence to support or d i s p u t e the hypothesis that d i s e a s e or p a r a s i t e s might be more abundant i f these l a r v a e stayed i n warm water. In three years cf study no moribund or o b v i o u s l y p a r a s i t i z e d l a r v a e were encountered. I t i s c o n c e i v a b l e that prey v u l n e r a b i l i t y i s not constant. A decrease i n prey v u l n e r a b i l i t y during the day might be expected i f zooplankton are b e t t e r able to d e t e c t the presence of a Chaoborus l a r v a during d a y l i g h t . Since Chacbcrus l a r v a e a p p a r e n t l y use v i b r a t i o n r e c e p t o r s r a t h e r than v i s i c n to detect t h e i r prey (Horridge and Boulton 1967), and are able to capture prey i n complete darkness (Duhr 1955), t h e i r a b i l i t y t c d e t e c t prey should be constant throughout the day and n i g h t . C. t r i v i t t a t u s l a r v a e are able to feed on both copepods and c l a d o c e r a n s i n the l i g h t with apparent ease, and < there i s no apparent avoidance response by t h e i r prey. A decrease i n v u l n e r a b i l i t y might make i t e n e r g e t i c a l l y l e s s a t t r a c t i v e to stay a t the s u r f a c e during the day i f f e e d i n g hadn't yet occurred by dawn. The e f f e c t of changes i n prey v u l n e r a b i l i t y was examined by a d j u s t i n g the prey d e n s i t y by a 111 v u l n e r a b i l i t y c o e f f i c i e n t (BVUL, SVUL) i n a few s i m u l a t i o n s . The r e s u l t s of these s i m u l a t i o n s showed that prey v u l n e r a b i l i t y must be 100 times lower during the day than at night to s h i f t the migration s t r a t e g y maximizing growth from the " s t a y - a t - s u r f a c e " p a t t e r n to the n a t u r a l p a t t e r n . A change i n prey v u l n e r a b i l i t y of t h i s magnitude, however, seems u n l i k e l y i n t h i s predator-prey system. Competition between C. t r i v i t t a t u s and C. americanus i s u n l i k e l y to have produced the migration p a t t e r n seen i n t h i r d and f o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e . F o u r t h - i n s t a r C. §!f=ricanus l a r v a e s t a y near the s u r f a c e at a l l times, and, based on morphological and d i e t data, they appear to be r e s t r i c t e d to a narrower range of p o t e n t i a l prey than are f o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e . T h e i r prey capture apparatus i s s m a l l e r (head l e n g t h , mouth gape, antenna l e n g t h ) ; they are l e s s e f f i c i e n t at c a p t u r i n g l a r g e prey items; they feed on a more r e s t r i c t e d d i e t than do f o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e (Fedorenko 1973). C. americanus l a r v a e are a l s o n u m e r i c a l l y l e s s abundant than C. t r i v i t t a t u s l a r v a e . The f o l l o w i n g two t h e o r i e s have been proposed to account f o r the upward movement of Chaoborus. Neither of them e x p l a i n s why migrants should move down out of t h e i r food source. Goldspink and S c o t t (1971) have suggested that seme aspects of the v e r t i c a l migration of C. f l a v i c a n s i n Lochan Dubh agree 112 with the e p i d e i c t i c concept c f Wynne-Edwards (1962). T h i r d and f o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e i n Eunice Lake v e r t i c a l l y migrate about 18 months before they pupate and emerge. I t seems u n l i k e l y t h at t h e i r v e r t i c a l migration can have any s i g n a l l i n g f u n c t i o n that would a f f e c t f e c u n d i t y that f a r i n advance. Goldspink and S c o t t do not c o n s i d e r causes of the downward m i g r a t i o n . Hunt (1958) and LaRcw (1970) have suggested that low oxygen c o n c e n t r a t i o n i n the hypolimnion or sediments s t i m u l a t e s the l a r v a e t c migrate. Oxygen c o n c e n t r a t i o n i s high at a l l depths i n Eunice Lake (Fedorenko and Sw i f t 1972) so i t seems u n l i k e l y that t h i s e x p l a n a t i o n can account f o r the upward v e r t i c a l m i g r a t i o n of C. t r i v i t t a t u s l a r v a e . The downward migration i s not co n s i d e r e d i n these s t u d i e s . Avoidance of d i u r n a l predators i s an a t t r a c t i v e hypothesis to e x p l a i n the occurrence of n o c t u r n a l a c t i v i t y p a t t e r n s . Welch (1968) showed t h a t C. £uncti_pennis l a r v a e which were p l a n k t o n i c at night and benthic during the day were preyed upon very l i t t l e by f i s h i n a shallow pond. MacDonald (1956) found t h a t chaoborid l a r v a e i n Lake V i c t o r i a avoided p r e d a t i o n by Borm_yrus kannume by being p l a n k t o n i c when the f i s h were f e e d i n g on the benthos. Northccte (pers. comm.) suggested t h a t C. f l a v i c a n s may avoid heavy f i s h p r e d a t i o n i n Corbett Lake by spending the day i n the anoxic hypolimnion and migrating up to feed at n i g h t . Predator avoidance i s not so simple as the above d i s c u s s i o n suggests. Pope et a l . (1973) 113 found that some Chaoborus s p e c i e s were c n l y found i n lakes without f i s h , some were only found i n lakes with f i s h , and some ( i n c l u d i n g C. t r i v i t t a t u s ) occurred i n lakes whether f i s h were present or not. Cr e p u s c u l a r predators are most a c t i v e at dusk and dawn; these are the times of day that most Chaobcrus are m i g r a t i n g (LaRow 1969, S e c t i o n I I I ) . Avoidance of f i s h p redation cannot be co n s i d e r e d as a proximal cause of the v e r t i c a l m i g r a t i o n p a t t e r n of f o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e i n Eunice Lake because there are no f i s h i n the l a k e . Notonectids are v i s u a l predators that are known to be able to capture Chaoborus l a r v a e and are abundant i n the lake. They are unable to forage deeper t h a t abcut 2m and are unable to reach C. t r i v i t t a t u s during the day. The downward migration of C. t r i v i t t a t u s might allow them to escape d i u r n a l p r e d a t i o n by no t o n e c t i d s . However, n o t o n e c t i d s probably would exert l i t t l e i f any p r e d a t i o n pressure on C. t r i v i t t a t u s even i f the l a r v a e did not migrate; these l a r v a e never come c l o s e r to the s u r f a c e than about 3-4m, even during the n i g h t . The m i g r a t i o n p a t t e r n seen i n Eunice Lake may be simply the p e r s i s t a n c e of a m i g r a t i o n p a t t e r n adapted to some previous s e l e c t i v e pressure which i s now re l a x e d or absent. A behavior p a t t e r n as complex as a d i e l migration p a t t e r n , whether i t be endogenous or exogenous i n c r i g i n , could be c o n s e r v a t i v e i n i t s e v o l u t i o n i n the face of r e l a x e d s e l e c t i o n pressure. However, i f such a behavior p a t t e r n i s n e i t h e r n e u t r a l nor advantageous, s e l e c t i o n should act to change i t . 1 14 I t appears from l a b o r a t o r y and s i m u l a t i o n s t u d i e s , that i t i s e n e r g e t i c a l l y advantageous f o r l a r v a e i n Eunice Lake to remain at the s u r f a c e or migrate on a p h y s i o l o g i c a l b a s i s ; i f t h i s i s the case, the migration p a t t e r n observed i n Eunice Lake would not be expected to p e r s i s t . I t i s p o s s i b l e that there i s an unknown s e l e c t i v e f o r c e t h a t i s a c t i n g on C. t r i v i t t a t u s l a r v a e i n Eunice Lake t h a t maintains the observed migration p a t t e r n , or i t may be that there i s f a s t enough exchange between p o p u l a t i o n s i n nearby l a k e s that any p o p u l a t i o n i n a lake with no s e l e c t i o n pressure f a v o r i n g d i e l v e r t i c a l m i gration i s only r e c e n t l y derived from a p a r e n t a l p o p u l a t i o n in a lake with a high s e l e c t i o n pressure f o r t h i s behavior p a t t e r n . S t a h l (1966) suggests t h a t one of the reasons f o r the high frequency of co-occurrence of Chaoborus s p e c i e s i n l a k e s i s the chance d i s p e r s a l cf a d u l t s tc lakes ether than the ones they emerged from. T h i s mechanism could provide a continuous source of h i g h l y adapted l a r v a e so t h a t the gradual degradation of the h i g h l y adapted behavior p a t t e r n a s s o c i a t e d with the removal of s e l e c t i o n pressure i s not n o t i c e a b l e . I t i s i m p o s s i b l e t c study t h i s type of mechanism i n a s h o r t study. S i g n i f i c a n t l y , Pope et a l . (1973) found C, t r i v i t t a t u s i n l a k e s which contained f i s h and i n those which di d not; they did not record the m i g r a t i o n p a t t e r n s of these p o p u l a t i o n s . Within a few miles of Eunice Lake i n the U. B. C. Research F o r e s t there are s e v e r a l l a k e s which c o n t a i n Chaoborus l a r v a e ; the s p e c i e s represented i n c l u d e C. f l a y i c a n s and 1 15 C. t r i v i t t a t u s i n l a k e s c o n t a i n i n g f i s h and C. t r i v i t t a t u s i n lakes without f i s h . I t i s c l e a r from f i e l d data (Section III) t h a t f o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e i n Eunice lake are migrating d i e l l y with a migration p a t t e r n which s h i f t s with changing day length and i s cued by l i g h t . Except f o r the l a c k cf a b e n t h i c phase, the migration p a t t e r n of C. t r i v i t t a t u s i n Eunice Lake i s e s s e n t i a l l y the same as that g e n e r a l l y r e p o r t e d f o r Chaoborus l a r v a e . Since t h i s p a t t e r n does not appear t c be the most advantageous on an e n e r g e t i c s b a s i s , i t i s necessary to look elsewhere f o r an e x p l a n a t i o n of the a d a p t i v e value of v e r t i c a l m i g r a t i o n i n Chaoborus l a r v a e . Near-surface f e e d i n g i s s u f f i c i e n t to e x p l a i n the upward movement. The c r i t i c a l part of the migration which must be e x p l a i n e d i s the downward movement out of the f o o d - r i c h s u r f a c e waters. For Chacbcrus l a r v a e i n g e n e r a l , the most probable e x p l a n a t i o n f o r the downward movement i s the avoidance of p r e d a t i o n — e s p e c i a l l y f i s h p r e d a t i o n -- i n the s u r f a c e waters by day. None of the e x i s t i n g t h e o r i e s , however, c o n v i n c i n g l y e x p l a i n s the v e r t i c a l migration of c. t r i v i t t a t u s i n Eunice Lake. The e x p l a n a t i o n which seems most l i k e l y f o r the p a r t i c u l a r form of the migration of C. t r i v i t t a t u s l a r v a e i n Eunice Lake i s that i t s p e r s i s t e n c e i n the f a c e of a p p a r e n t l y r e l a x e d s e l e c t i o n pressure i s due to p e r i o d i c immigration from p o p u l a t i o n s exposed to s t r o n g s e l e c t i o n f o r t h i s behavior p a t t e r n . 116 The young stages of many v e r t i c a l l y migrating animals do not migrate, but r a t h e r spend the e n t i r e day near the s u r f a c e (McLaren 1963, Teraguchi and Northcote 1966). F i r s t and second i n s t a r C. t r i v i t t a t u s are no exception (Fedorenko and S w i f t 1972). These small l a r v a e l i v e about 3m deep from the time they hatch u n t i l they reach t h i r d i n s t a r and begin to migrate. What are the advantages a s s o c i a t e d with s t a y i n g at the s u r f a c e when small? I t may be advantageous f o r young stages to grow as r a p i d l y as p o s s i b l e i n order to gain a s i z e refuge from some predators. T h i s would he f a c i l i t a t e d by s t a y i n g i n the warm, su r f a c e waters where many of the smaller z c c p l a n k t o n s p e c i e s are found. I t may be that young animals are l e s s t o l e r a n t of food shortages and by s t a y i n g at the s u r f a c e where food i s , more abundant they minimize t h i s r i s k . Whatever the advantage, we must r e s o l v e why they g r a d u a l l y s h i f t to a v e r t i c a l l y migrating h a b i t a t some point d u r i n g t h e i r development. I t may be t h a t commencement of v e r t i c a l migration i s r e l a t e d to s i z e i n those s p e c i e s that migrate. As s i z e i n c r e a s e s the s t r a t e g y of migrants may s h i f t from one of maximizing grcwth with^ an attendant high metabolic c o s t when they are young, to one i n v o l v i n g slower growth but more e f f i c i e n t p a r t i t i o n i n g cf energy tc growth as they get o l d e r . The p h y s i o l o g i c a l migration p a t t e r n would s t i l l be best i f t h i s were the case. Growth r a t e s of young C. t r i v i t t a t u s l a r v a e are higher than those of t h i r d and f o u r t h - i n s t a r l a r v a e and the l a t t e r l a r v a e appear to spend l e s s time at the s u r f a c e as they get c i d e r . 117 V e r t i c a l l y migrating c a r n i v o r e s , whether zooplankton or f i s h , have been l a r g e l y ignored i n c o n s i d e r a t i o n s of the adaptive value of v e r t i c a l m i g r a t i o n . However, they present some problems when these t h e o r i e s are extended to i n c l u d e them. One such problem, that of the maintenance of a constant metabolic r a t e throughout the m i g r a t i o n (Teal 1971), has been di s c u s s e d p r e v i o u s l y (Section I V). Various t h e o r i e s concerning the a d a p t i v e value of v e r t i c a l m igration d i s c u s s e d above have been considered f o r f i s h (Brett 1971). He concludes that j u v e n i l e sockeye salmon i n Babine l a k e , B.C., behavioura1ly thermoregulate to maximize energy gain. U n l i k e McLaren (1963) B r e t t i n c l u d e s an a n a l y s i s of the r o l e of food; however, he c o n s i d e r s only the case of maximum r a t i o n . His c o n c l u s i o n s are su b j e c t to the same c r i t i c i s m s as McLaren's. A l t e r n a t i o n of high and low temperatures can a l s o occur i n the h o r i z o n t a l plane. Hyatt (pers. comm.) i s stu d y i n g f c r a g i n g movements of sockeye salmon which feed i n warm shallow water at the edge of Marion Lake and refuge i n a c o l d water s p r i n g i n the centre cf the l a k e . The temperature regime experienced by these f i s h during a h o r i z o n t a l m i g r a t i o n i s s i m i l a r to that experienced by f i s h i n Babine Lake or by zooplankton during t h e i r v e r t i c a l m i g r a t i o n s . The b i o e n e r g e t i c conseguences of a l t e r n a t i o n of high and low temperatures are unc l e a r , but they may play an important part i n maintaining these d i v e r s e migration p a t t e r n s . 118 The a n a l y s i s of Chaoborus v e r t i c a l m igration i n Eunice Lake can be viewed as a model f o r the wider c o n s i d e r a t i o n cf the adaptive value of v e r t i c a l m i g r a t i o n . McLaren's (1963) prov o c a t i v e theory does not p r o p e r l y e s t a b l i s h the r e l a t i o n s h i p between food and temperature and the e f f e c t of t h i s i n t e r a c t i o n on the e n e r g e t i c s of v e r t i c a l l y migrating animals. I n c l u d i n g food i n an a n a l y s i s of v e r t i c a l m igration based s o l e l y on e n e r g e t i c s l e a d s to the two m i g r a t i o n s t r a t e g i e s d i s c u s s e d above as the most adaptive — s t a y i n g at the s u r f a c e or f o l l o w i n g a p h y s i o l o g i c a l l y mediated migration p a t t e r n . Some animals do not migrate at a l l , and few, i f any, have been shown to f o l l o w a p h y s i o l o g i c a l m i g r a t i o n p a t t e r n . Most a c t i v e l y m i grating zooplankton f o l l c w a s e a s o n a l l y a d j u s t a b l e , l i g h t mediated d i e l m i g r a t i o n p a t t e r n t h a t places them near the s u r f a c e during the night and deeper during the day. As Mauchline and F i s h e r (1969) p o i n t out, i t i s u n l i k e l y that any s i n g l e theory can account f o r the adaptive value cf v e r t i c a l m i gration. Rather, there are a v a r i e t y of b e n e f i t s which vary i n importance from group to group. The idea of an e n e r g e t i c advantage d e r i v e d from the a l t e r n a t i o n of high and low temperatures i s an a t t r a c t i v e one, but nc adeguate theory i s p r e s e n t l y a v a i l a b l e that w i l l demonstrate such an advantage. Such a theory would have tc take i n t o account temperature dependent r e s p i r a t i o n and d i g e s t i o n r a t e s , prey d i s t r i b u t i o n , and the p o s s i b i l i t y of i r r e g u l a r feeding. 119 Summary 1. A review of e a r l y t h e o r i e s on the adaptive value of v e r t i c a l m i g r a t i o n confirms the l a c k of any s i n g l e theory t h a t can account f o r v e r t i c a l m i g r a t i o n . A number of b e n e f i t s have been suggested f o r v e r t i c a l m i g r a t i o n i n a v a r i e t y of s p e c i e s . 2. McLaren's (1963) theory cn the e n e r g e t i c advantage of a l t e r n a t i o n of temperature i s shown to depend on assumptions which are not j u s t i f i e d . C o n s i d e r i n g e n e r g e t i c s only, animals should stay at the s u r f a c e or f o l l o w a p h y s i o l o g i c a l migration p a t t e r n i n order to maximize growth. A s i m u l a t i o n model of the e n e r g e t i c s of v e r t i c a l m i g r a t i o n i n Chaoborus l a r v a e confirmed t h a t , on an e n e r g e t i c s b a s i s , the two migration p a t t e r n s c i t e d above are b e t t e r f o r l a r v a l growth than the m i g r a t i o n p a t t e r n a s s o c i a t e d with McLaren's theory. 3. The d i f f e r e n c e s between the e x i s t i n g migration p a t t e r n of f o u r t h - i n s t a r C. t r i v i t t a t u s l a r v a e and the p a t t e r n s suggested as the most advantageous on an e n e r g e t i c b a s i s are d i s c u s s e d . With no obvious s e l e c t i v e f o r c e a c t i n g i n Eunice Lake to maintain the v e r t i c a l migration p a t t e r n observed, i t i s concluded t h a t t h i s p a t t e r n i s probably maintained by slow l o s s of the m i g r a t i o n p a t t e r n by the Eunice Lake p o p u l a t i o n and by immigration from populations i n other l a k e s which are s u b j e c t to s e l e c t i o n pressure f a v o r i n g t h i s migration p a t t e r n . 4. With r e s p e c t to v e r t i c a l m i g r a t i o n i n general, i t i s 120 concluded that the adaptive value of the m i g r a t i o n v a r i e s frcm group to group. One of the p o t e n t i a l b e n e f i t s from migrating i s an energy gain due to a l t e r n a t i n g high and low temperatures. To date there has been no cohesive theory on the e n e r g e t i c s of v e r t i c a l m igration that i n c l u d e s a d i s c u s s i o n of food a v a i l a b i l i t y . 121 LITERATURE CITED Berg, K. 1937. 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Chaoborus f l a y i c a n s (Meigen) (Diptera: Chaoboridae): an a u t e c o l o g i c a l study. PhD. T h e s i s . Groningen. 128 p. Pearre, S. J r . 1973. V e r t i c a l migration and f e e d i n g i n S a ^ i t t a elecjans V e r r i l l . Ecology 54: 300-314. 125 P h i l l i p s o n , J. 1964. A miniature bomb c a l o r i m e t e r f o r s m a l l b i o l o g i c a l samples. Oikos 15: 130-139. P h i l l i p s o n , J . 1966. E c o l o g i c a l e n e r g e t i c s . S t . M a r t i n ' s P r e s s , New York. 57 p. Pope, G. F., J . C. H. C a r t e r , and G. Power. 1973. The i n f l u e n c e of f i s h on the d i s t r i b u t i o n of Chaoborus spp. (Diptera) and d e n s i t y of l a r v a e i n the Matamek R i v e r system, Quebec. Trans. Amer. F i s h . Soc, 102: 707-714. Prokesova, V. 1959. H y d r o b i o l o g i c a l r e s e a r c h of two n a t u r a l l y p o l l u t e d pools i n the wcody in u n d a t i o n area of the E l b e . Vest. C s l . S p o l . Z o o l . [= Acta Sec. Z o o l . Bohemoslov]. 23: 34-69. Prosser, C. L. , and F. A. Brown. J r . 1961. Comparative animal p h y s i c l o g y . 2nd. ed. Saunders. 688 p. B i c k e r , W. E. 1971. I n t r o d u c t i o n , p. 1 to 6. In Methods f o r the assessment of f i s h p r o duction i n f r e s h waters. I. B. P. Handbook No. 3. E l a c k w e l l , Oxford. 348 p. Roth, J . C. 1968. Benthic and l i m n e t i c d i s t r i b u t i o n of three Chaoborus s p e c i e s i n a southern Michigan lake ( D i p t e r a , Chaoboridae). Limnol. Oceanogr. 13: 242-249. Roth, J . C. 1971. The fcod of Chaoborus, a plankton predator, i n a southern Michigan Lake. PhD. T h e s i s . U n i v e r s i t y of Michigan. Roth, J . C , and S. Parma. 1970. A Chaoborus b i b l i o g r a p h y . B u l l . Entomol. Soc. Amer. 16: 100-110. R u s s e l l , F. S. 1927. The v e r t i c a l d i s t r i b u t i o n of plankton i n the sea. B i o l . Rev. Cambridge P h i l . Soc. 2: 213-262. S c h i n d l e r , E. W. 1968. Feeding, a s s i m i l a t i o n and r e s p i r a t i o n r a t e s cf Daphnia ma<jna under v a r i o u s environmental c o n d i t i o n s and t h e i r r e l a t i o n to production e s t i m a t e s . J . Anim. E c o l . 37: 369-385. Schmein-Engberding, F. 1953. Die Vorzugstemperaturen e i n i g e r Knochenfische und i h r e p h y s i o l o g i s c h e Bedeutung. Z. F i s c h . 2: 125-155. Sikorowa, A. 1968. The behavior of Chaoborus L i c h t l a r v a e under unfavorable oxygen c o n d i t i o n s . E k o l . p o l . Ser. A 16: 1-8. Sikorowa, A. 1973. Morphology, b i o l o g y , and ecology of s p e c i e s belonging to the genus Chaoborus L i c h t e n s t e i n ( D i p t e r a , 126 Chaobcridae) o c c u r r i n g i n Poland. [ i n P o l i s h , E n g l i s h summary]. Zesz. nauk. ART O l s z t . (105). Ochrona Wed I Rybacto Srodladowe 1:1-121. Snedecor, G. W., and W. G. Cochran. 1967. S t a t i s t i c a l methods. 6th ed. Iowa State U n i v e r s i t y Press. 593p. 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R e s p i r a t i o n of crabs i n Georgia s a l t marshes and i t s r e l a t i o n to t h e i r ecology. P h y s i o l . Z c o l . 32: 1-14. T e a l , J. M. 1971. Pressure e f f e c t s on the r e s p i r a t i o n of v e r t i c a l l y migrating decapod C r u s t a c e a . Am. Z c o l . 11: 571-576. T e a l , J . M., and F. G. Carey. 1967. E f f e c t s c f pressure and temperature on the r e s p i r a t i o n of eu p h a u s i i d s . Deep-Sea Res.. 14: 725-733. Teraguchi, M., and T. G. Northcote. 1966. V e r t i c a l d i s t r i b u t i o n and migrati o n of Chaoborus f l a v i c a n s l a r v a e i n C c r b e t t Lake, B r i t i s h Columbia. Limnol. Oceanogr. 11: 164-176. Vlymen, W. J . 1970. Energy expenditure of swimming copepods. Limnol. Oceanogr. 15: 348-356. Warren, C. E., and G. E. Davis. 1967. Laboratory s t u d i e s cn the f e e d i n g , b i o e n e r g e t i c s , and growth of f i s h . P. 175 to 214. In S. C. Gerking [ e d . ] The b i o l o g i c a l b a s i s cf freshwater f i s h p r o d u c t i o n . B l a c k w e l l , Oxford. 495p. 127 Welch, H. E. 1968. Energy flow through the major macroscopic components of an a q u a t i c ecosystem. PhD. T h e s i s . U n i v e r s i t y of Georgia. 97p. Wood, K. G. 1956. Ecology of Chaoborus (Diptera: C u l i c i d a e ) i n an O n t a r i o l a k e . Ecology 37: 639-643. Worthington, E. B., and C. K. Ricardo. 1936. S c i e n t i f i c r e s u l t s of the Cambridge e x p e d i t i o n to East A f r i c a n Lakes 1930-31. No. 17. The v e r t i c a l d i s t r i b u t i o n and movements of the plankton i n l a k e s Rudolf, Naivasha, Edward, and Bunyoni. J . L i n n . Soc. London Z o o l . 40: 33-69. Wynne-Edwards, V. C. 1962. Animal d i s p e r s a l i n r e l a t i o n to s o c i a l behavior. O l i v e r and Boyd, Edinburgh. 653 p. 128 APPENDIX I Figure Page 1. Flow diagrams of s u b r o u t i n e s EATIT and DIGST. 129 2. Flow diagram of su b r o u t i n e DOIT. ............... 130 3. Graphs of the input v a r i a b l e s f o r the model. ... 131 Table Page 1. The meaning of the FORTEAN v a r i a b l e s used i n the s i m u l a t i o n model 132 2. The meaning, source, and value of the parameters i n the s i m u l a t i o n model 134 129 FIGURE 1 Flow diagrams of s u b r o u t i n e s EATIT and DIGST. SUBROUT INE EAT1T B F C T R = Y N T ( T I M E , T M , B V U D S F C T R = Y N T ( T I M E , T M , B V U L ) B F U D = B F O O D * B F C T R S F U D = S F O O D x S F C T R U = C A L P H I X B F U D ; / ( I + ( A L P H I > « B F U D X H I 3 + C A L P H 2«S F U D * H 2; B F E = P O I S ( U ) U = ( A L P H 2 * S F U D ) / 1+ F A L P H 1 * B F U D * H 1 ) + < A L P H 2 * S F U D * H 2 ) S F E = P O I S ( U ) W B F E = W B F ^ B F E W S F E = W S F * S F E T F E = W B F E + W S F E d F A = F A + ( 0 . 6 8 * T F E ) S T F E = S T F E + T F E SUBROUTINE DIGST A D = Y N T ( T E M P , T M T U R , D P ) Y E S F P I G = A D V C = V C - A D F A = F A - A D A D = F A v ^ R E T U R N J 03 130 FIGURE 2 Flow diagram of subroutine DOIT. 1 3 0 a II LG= I W - WO I PIG = 5FPIG II A3S = 0. FA = O. VC = 0. moiro. STFE=0 TIME=0. DEPTH= YNT (TIME. TIM. Z) TEMP = YNT (DEPTH; OT. TEM) BFOOD= Y NT (DEPTH, DF, BFUDE ) SFOOD = YNT (DEPTH. DF, SFUDE) R F E = O. SFE = 0. YES CALL EAT IT IF (VC GE. VCMAX) IFLG=2 K= 1.30 j J = 1. 72 R R R L " EXP(A(L- l ) J «W» 'B (L - l ) RHRH = G>:i'{A|l.|]*W<*B(L) RRR = RRRL + (RRRH-RRRL)*<T EMP-TMP(L-1)))/( XM P(L)-TMP(L- U) RRR =EXP (A(l.))yWXXB(L) | T = = -RC = RRR 3. SC = 0. IF (T !M E .GE. 24. .AND. TIME .LT. 30. ) SC = .0-15 IF (TI M E .GE.42. .AND. TIME .LT. 48. ) SC = .0 45 RESP=(RC+ SCIKVVF ESTRD = FPIG - (GDA*FPIG) WG = ESTRD-HES P V/ = V/ + WG TIME e TIME +1 *  OUTPUT ROUTINES 131 FIGUBE 3 Graphs of the input v a r i a b l e s f o r the model. 131 a 132 Table 1. The meaning of the FORTRAN v a r i a b l e s used i n the s i m u l a t i o n model. V a r i a b l e Meaning A C o e f f i c i e n t i n the r e s p i r a t i o n r a t e c a l c u l a t i o n AD Weight of food d i g e s t e d per time i n t e r v a l BFCTR Large prey a v a i l a b i l i t y c o e f f i c i e n t BFE Number of l a r g e prey eaten per time i n t e r v a l BFOOD Large prey d e n s i t y BFUD Adjusted l a r g e prey d e n s i t y BFUDE Input d e n s i t y of l a r g e prey BVUL V u l n e r a b i l i t y c o e f f i c i e n t f o r l a r g e prey DEPTH Depth DP D i g e s t i o n r a t e DT Depth counter f o r i n t e r p o l a t i o n EASS Weight of food a s s i m i l a t e d per time i n t e r v a l ESTRD Weight of food s t o r e d per time i n t e r v a l FA Food a v a i l a b l e f o r a s s i m i l a t i o n FPIG Food pool i n the gut IFLG F l a g f o r switch from f e e d i n g c y c l e tc d i g e s t i v e pause RESP T o t a l weight c o s t per time i n t e r v a l RC R e s p i r a t i o n c o s t RRR R e s p i r a t i o n r a t e RRRH Upper i n t e r p o l a t i o n point i n the c a l c u l a t i o n of ESR RRRL Lower i n t e r p o l a t i o n p o i n t i n the c a l c u l a t i o n of RRR SC Swimming co s t SFCTR Small prey a v a i l a b i l i t y c o e f f i c i e n t SFE Number of s m a l l prey eaten per time i n t e r v a l SFOOD Small prey d e n s i t y SFUD Adjusted s m a l l prey d e n s i t y SFUDE Input d e n s i t y f o r smal l prey STFE Cumulative weight of food eaten SVUL V u l n e r a b i l i t y c o e f f i c i e n t f o r smal l prey TEH Temperature TEMP Temperature TFE Weight of a l l prey eaten per time i n t e r v a l TIM Time counter f o r depth i n t e r p o l a t i o n TIME Time i n t e r v a l i n the s i m u l a t i o n TM Time counter f o r the prey v u l n e r a b i l i t y i n t e r p o l a t i o n TMTUR Temperature U Mean of the Poisson d i s t r i b u t i o n i n EATIT 133 V a r i a b l e Meaning VC Crop volume W L a r v a l weight WBFE Weight of l a r g e prey eaten per time i n t e r v a l WG Gain i n weight per time i n t e r v a l WSFE Weight of small prey eaten per time i n t e r v a l Z Depth 134 Table 2. The meaning, source, and value of the parameters in the s i m u l a t i o n model. Parameter Value U n i t s Meaning Source ALPH1 0.000128 ALPH2 0.000064 H1 H2 SDA SEST SFPG VCMAX 0. 1 0. 30 0.30 0.0015 0.0 0. 16 WBF 0.08 WF 0.00485 WSF 0.006 Number per 100 l i t e r s per hour Number per 100 l i t e r s per hour hours hours WO 0. 15 mg mg mg mg mg per minute mg mg Capture r a t e per u n i t l a r g e prey d e n s i t y during s e a r c h i n g time Capture r a t e per u n i t small prey d e n s i t y during s e a r c h i n g time Handling time f o r l a r g e prey Handling time f o r small prey S p e c i f i c dynamic a c t i o n I n i t i a l weight s t o r e d I n i t i a l food pool i n the gut Maximum crop c a p a c i t y Weight of l a r g e prey Conversion f a c t o r : oxygen consumed to mg Weight of small prey I n i t i a l weight Measured Measured S e c t i o n IV S e c t i o n IV H y p o t h e t i c a l H y p o t h e t i c a l H y p o t h e t i c a l Measured Measured Hargrave (1971) Measured H y p o t h e t i c a l APPENDIX I I FUNCTION YNT (TX,X,Y) DIMENSION X (20) , Y (20) 1=2 IF (X(I)-TX) 20,30,40 1=1+1 GO TC 19 YNT=Y (I) GO TO 50 YNT=Y (1-1) + ( (Y (I) -Y (1-1) ) * (TX-X (I-1) ) ) / (X (I) -X (I- 1) ) RETURN END FUNCTION POIS (U) R=RANDV (0) XN=0. XNF=1. P = 0. E=EXP (-U) UN=1 P=P+UN*E/XNF IF(P.GF.R) GO TO 10 XN=XN+ 1. XNF=XNF*XN UN=UN*U GO TO 5 POIS=XN RETURN END THIS SUEROUTINE DRAWS A GRID FOR OUTPUT OF THE SIMULATION RESULTS. SUBROUTINE MOUT COMMON D1,SDA,WO,XMAX,YMAX,YMIN,TF,WF,SFPG,SEST,ALPH1 1,ALPH2,H1,H2,WBF,WSF,VCMAX,STFE,VC,FPIG,BFOOD,SFOOD,E 1 FE,SFE, FA, TEMP, TIME, Z (6) , TIM (6) , TEM (6) ,BFUDE(11) , SFUD 1E (11) ,RR (6) ,DP (6) , TMTUR (6) ,A (5) ,B (5) ,TMP (5) ,TM (6) , EVU 1L (6) ,SVUL (6) ,BFUD,SFUD, BFCTR,SFCTR,NPAR CALL CATSW (0,JQ) GO TC (3,4) ,JQ XSC = 6./XMAX YSC = 6./YMAX CALL SCALF (XSC,YSC,0.,0.) CALL FGRID (0,0.,0.,XMAX/30.,30) CALL FGRID (1,0.,0.,YMAX , 0) x=o. Y=WO CALL FPLOT (-2,X,Y) IXMAX=XMAX DO UO I=1,IXMAX BEAD ( I ' l ) Y IF (Y-YMAX) 30,30,31 Y = YMAX IF (Y-YMIN) 33,34,34 Y = YMIN X=I CALL FPLOT (0,X,Y) CONTINUE CALL PENUP BETUBN END THIS SUBBOUTINE CONTAINS THE NECESSABY INSTRUCTIONS FOB MODIFYING THE INPUT PABAMETEBS. SUBROUTINE MMOD DIMENSION PAR (40) COMMON D1,SEA,WO,XMAX,YMAX,YMIN,TF,WF,SFPG,SEST,ALPH1 1,ALPH2 ,H1,H2,WBF,WSF,VCMAX,STFE,VC,FPIG,BFOOD,SFOOD,B 1FE,SFE,FA,TEMP,TIME,Z (6) , TIM (6) , TEM (6 ) , BFUDE (11) , SFUE 1E (11) ,RB (6) ,DP (6) , TMTUB (6) , A (5) , B (5) ,TMP (5) ,TM (6) , BVU 1L(6) ,SVUL(6) ,BFUD,SFUD,BFCTB,SFCTR,NPAB EQUIVALENCE (PAR(1),D1) M=6 TO CHANGE A PABAMETEB VALUE CALL E ATSW (1 , JQ) GO TO (1 1 ,20) , JQ WBITE (1,12) FORMAT (» ID/NEW VALUE') BEAD (M)IB,X PAR (ID)=X GO TC 10 TO CHANGE 'TIM' VALUES CALL EATSW(2,JQ) GO TO (21 ,30) , JQ WE IT E (1,22) FOBMAT ('NEW TIM VALUES') BEAD (M) (TIM (I) , 1=1,6) GO TO 20 TO CHANGE «Z' VALUES CALL D ATSW (3, JQ) GO TO (31,40) ,JQ WRITE(1,32) FOBMAT ('NEW Z VALUES') READ (M) (Z (I) ,1=1 ,6) GO TO 30 TO CHANGE 'TEM' VALUES CALL BATSW(4,JQ) 137 GO TO (41,50) ,JQ 41 WRITE(1,42) 42 FORM AT ('NEW TEM VALUES') READ (M) (TEM (I) ,1=1,20) GO TO 40 C... TO CHANGE 'EFUDE• VALUES 50 CALL DATSW(5,JQ) GO TO (51 ,60) , JQ 51 WRITE(1,52) 52 FORMAT ('NEW LARGE FUDE VALUES') READ (M) (EFUDE(I) ,1=1,11) GO TO 50 C... TO CHANGE * SFUDE* VALUES 60 CALL D ATSW (6, JQ) GO TC (61,70) ,JQ 61 WRITE (1,62) 62 FORMAT ('NEW SMALL FUDE VALUES') READ (M) (SFUDE (I) ,1=1, 11) GO TO 60 C... TO CHANGE »A« VALUES 70 CALL DATSW(7,JQ) GO TO (71, 100) , JQ 71 WRITF(1,72) 72 FORMAT ('NEW A VALUES') READ (M) (A (I) ,1=1 ,5) GO TO 70 C... TO CHANGE »B' VALUES 100 CALL DATS W (9,JQ) GO TO (101 , 80) , JQ 101 WRITE (1,102) 102 FORMAT ('NEW B VALUES') READ (M) (B (I) ,1=1,5) GO TO 100 80 CALL DATSW (10,JQ) GO TO (81,90) ,JQ 81 WRITE(1,82) 82 FORMAT (' NEW DP VALUES') READ (M) (DP (I) ,1=1 ,6) GO TO 90 C... TO CHANGE *TM' VALUES 90 CALL DATSW(11,JQ) GO TO (91 , 110) , JQ 91 WRITE(1,92) 92 FORMAT ('NEW TM VALUES') READ (M) (TM (I) , 1=1,6) GO TO 110 C... TO CHANGE 'BVUL' VALUES 110 CALL DATSW (12,JQ) GO TO (1 1 1 , 120) , JQ 111 WRITE(1,112) 112 FORMAT ('NEW BVUL VALUES') READ (M) (BVUL (I) , 1=1,6) 138 GO TC 120 C... TO CHANGE 'SVUL' VALUES 120 CALL DATSW(13,JQ) GO TC (121,130) , JQ 121 WRITE(1,122) 122 FORMAT ('NEW SVUL VALUES') READ (M) (SVUL (I) , 1=1,6) GO TO 130 130 RETURN END C... THIS SUBROUTINE CALCULATES THE NUMBER OF LARGE AND C... SMALL PREY EATEN PER TIME INTERVAL AND CALCULATES THE C. . .VOLUME OF THE CROP AFTER EATING. U IS THE CAPTURE RATE C... OF LARGE OR SMALL PREY CALCULATED FROM HCILING'S C... DISK EQUATION FOR TWO PREY TYPES. ALPH1 AND ALPH2 ARE C... CAPTURE RATES PER UNIT PREY DENSITY DURING SEARCH C... TIME. LFOOD AND SFOOD ARE THE PREY DENSITIES OF LARGE C... AND SMALL PREY. H1 AND H2 ARE HANDLING TIMES FOR LARGE C... AND SMALL PREY. WBF AND WSF ARE THE WEIGHTS OF LARGE C... AND SMALL PREY. POISS IS A FUNCTION WHICH GIVES THE C... NUMBERS OF LARGE AND SMALL PREY EATEN USING U AS THE C... MEAN OF A POISSON DISTRIBUTION AND A RANDOM NUMEER C... GENERATOR. SUBROUTINE EATIT COMMON D1,SDA,WO,XMAX,YMAX,YMIN,TF,WF,S FPG,S EST,ALPH1 1,ALPH2,H1,H2,WBF,WSF,VCMAX,STFE,VC,FPIG,EFCOD,SFOOE,E 1FE, SEE, FA, TEMP,TIME, Z (6) , TIM (6) , TEM (6) , BFUDE (1 1) , SFUD 1E(11) ,RR (6) ,DP (6) , TMTUR (6) , A (5) , B (5) ,TMP (5) ,TM (6) ,EVU 1L (6) ,SVUL (6) ,BFUD,SFUD, BFCTR,'SFCTR, NPAR BFCTR=YNT(TIME, TM, BVUL) BFUD=BFOOD*BFCTR SFCTR=YNT(TIME, TM, SVUL) SFUD=SFOOD*SFCTR U= (ALPH1*BFUD) /(1 + (ALPH1*BFUD*H1) + (ALPH2*SFUD*H2) ) BFE=POIS (U) U=(ALPH2*SFUD)/(1+ (ALPH1*BFUD*H1) + (ALPH2*SFUD*H2)) SFE=POIS (U) WBFE=WBF*BFE WSFE=WSF*SFE TFE=WBFE+WSFE IF (TFE.GT. (VCMAX-VC) ) TFE= (VCMAX-VC) VC=VC+TFE FA=FA+ (0.68*TFE) STFE=STFE+TFE RETURN END C... THIS SUBROUTINE CALCULATES THE AMOUNT DIGESTED FROM C... THE CROP DURING A TIME INTERVAL. SUBROUTINE DIGST COMMON D1,SDA,WO,XMAX,YMAX , YMIN,TF,WF,SFPG,SEST,ALPH1 1 39 1 , A L P H 2 , H 1 , H 2 , W B F , W S F , V C M A X , S T F E , V C , F P I G , B F C O D , S F O O D , E 1 F E , S F E , F A , T E M P , T I M E , Z ( 6 ) , T I M ( 6 ) , T E M ( 6 ) , B F U D E ( 1 1) , S F U D 1 E ( 1 1) , BB (6) ,DP ( 6 ) , T M T U R ( 6 ) , A ( 5 ) R B ( 5 ) , T M P ( 5 ) , TM |6) , B V U 1 L ( 6 ) , S V U L ( 6 ) , B F U D , S F U D , B F C T R , S F C T R , N P A B A D = Y N T ( T E M P , T M T U R , D P ) I F ( A D . G T . F A ) A D = F A F P I G = A E V C = V C - A D F A = F A - A D R E T U R N E N D C... T H I S S U B R O U T I N E D O E S T H E A C T U A L C A L C U L A T I O N S O F E N E R G Y C,.. G A I N S A N D L O S S E S D U R I N G E A C H T I M E I N T E R V A L . S U B R O U T I N E D O I T ( D F , D T , R T O T , H , I D ) COMMON D 1 , S D A , W O , X M A X , Y M A X , Y M I N , T F , W F , S F P G , S E S T , A L P H 1 1 , A L P H 2 , H 1 , H 2 , W B F , W S F , V C M A X , S T F E , V C , F P I G , B F O O D , S F O O D , B 1 F E , S F E , F A , T E M P , T I M E , Z { 6 ) , T I M ( 6 ) , T E M ( 6 ) , B F U D E ( 1 1 ) , S F U E 1 E ( 1 1 ) ,RR ( 6 ) ,DP ( 6 ) , T M T U R ( 6 ) , A ( 5 ) , B ( 5 ) , T M P ( 5 ) , TM ( 6 ) , B V U 1 L ( 6 ) , S V U L ( 6 ) , B F U D , S F U D , B F C T R , S F C T R , N P A R C I N I T I A L I Z E I F L G = 1 N T E S T - 3 W=WO F P I G = S F P G E S T R E = S E S T E A S S = 0 . F A = 0 . V C = 0 . R T O T = 0 . S T F E = 0 . C... L O O P A R O U N D T I M E I N T E R V A L S — 3 0 * 2 8 8 F I V E M I N U T E I N T E R V A L S I T E S T = 0 DO 2 0 0 K = 1 , 3 0 T I M E = 0 . DO 1 0 5 J = 1 , 7 2 D E P T H = Y N T ( T I M E , T I M , Z ) T E M P = Y N T ( D E P T H , D I , T E M ) B F O O D = Y N T ( D E P T H , D F , B F U D E ) S F O O C = Y N T ( D E P T H , D F , S F U D E ) C... I F L G T R I G G E R S T H E P R O G R A M I N T O T H E F E E D I N G C Y C L E OR C... D I G E S T I V E P A U S E ( D I G E S T I V E C Y C L E O N L Y ) . I T I S C... D E P E N D E N T ON T H E C R O P V O L U M E . E F E = 0 . S F E = 0 . GO TO ( 4 0 0 , 5 1 0 ) , I F L G 4 0 0 C A L L E A T I T I F ( V C . G E . V C M A X ) I F L G = 2 5 1 0 C A L L D I G S T >• I F ( F A . G T . O . ) GO TO 5 5 0 I F L G = 1 V C = 0 . 140 FA=0. C... ENERGY CALCULATIONS: ASSIMILATION, MAINTENANCE, C . . . STORAGE, GROWTH, AND RESPIRATION (RRR), 550 L=2 4 IF (TMP (L) -TEMP) 3,2,1 3 L=L+1 GO TO 4 2 RRR=EXP (A (L) ) *W**B (L) GO TC 5 1 RRRL=EXP (A (L-1))*W**B (L-1) RRRH=EXP (A (L) ) *W**B (L) RRR=RRRL+ ( (FRRH-RRRL) * (TEMP-TMP (L-1) ) ) / (TMP (L) -TMP (L- 1) ) GO TC 5 5 RC=RRR/3. SC=0.0 IF (TIME. GE. 24. .AND. TIME. LT. 30.) SC=.045 IF (TIME.GE. 42. .AND. TIME.LT. 48.) SC-.045 RESP= (RC+SC)*WF ESTR E= FPIG-(SDA*FPIG) WG= (ESTRD-RESP) W=W+WG TIME=TIME+1. CALL DATSW(15,JQ) GO TC (600,601) , JQ 600 WRITE(3,602) T1MB,DEPTH,TEMP,BFOOD,SFOOD,BFE,SFE,FPIG,FA, 1RRR, RESP, ESTRD, WG, W 60 2 FORMAT (1X,F11.0,9(1X,E10.'3)) WRITE (3,7 00) L,RRRL,BRRH,RRR, A (L) , B (L) ,TMP (L) ,TEMP 700 FORMAT (1X,11,7(5X,F1 0.4)) WRITE (3, 1000) VC,IFLG 1000 FORMAT (2X,»***', (F10.6) ,»***»,12,'***') 601 CONTINUE RTOT=RTOT + RESP ITEST=ITEST+1 IF (MOE (ITEST,NTEST) .EQ.O) WRITE (TID)W 105 CONTINUE 200 CONTINUE RETURN END C... THIS IS THE MAINLINE PROGRAM WHERE ALL DATA INPUTS AND C... OUTPUTS ARE HANDLED. DIMENSION DT (20) , DF (11) ,PNAME (2) , PAR (40) COMMON D1,SDA,WO,XMAX,YMAX,YMIN,TF,WF,SFPG,SEST,ALPH1 1,ALPH2,H1,H2,WBF,WSF,VCMAX,STFE,VC,FPIG,BFOOD,SFOOD,B 1FE,SFE,FA,TEMP,TIME,Z (6) , TIM (6) , TE M (6 ) , BFUDE (1 1) ,SFUE 1E (11) ,RR (6) ,DP (6) , TMTUR (6) , A (5) , B (5) ,TMP (5) , TM (6) , BVU 11(6),SVUL(6),BFUD,SFUD,BFCTR,SFCTR,NPAR EQUIVALENCE (PAR(1),D1) DEFINE FILE 1(730,2,U,ID) CALL P1130 M=6 c.. 104 112 120 READ IN PARAMETERS AND GRAPHS. READ (2,2) NPAR FORMAT (12) DO 120 1=1 , NPAR READ (2,104) K, PNANE,PAR(K) FORMAT (I3,7X,2A4,2X,F10.0) WRITE(3,112) K, PNAME, PAR (K) FORMAT (3X,I2,1X,2A4,2X,E14.7) CONTINUE READ (2,100) READ (2,100) READ (2,100) READ (2,100) READ (2,100) READ (2,100) READ (2,100) READ (2,100) READ (2,100) READ (2, 100) READ (2 ,100) READ (2,100) READ (2 ,100) READ (2,100) READ (2 ,100) (Z(I) ,1=1,6) (TIM (I) ,1=1,6) (TEM (I) ,1=1,20) (DT(I) ,1=1,20) (BFUDE (I) ,1= 1,11) (SFUDE (I) ,1=1 , 11) (DF(I),1=1,11) (TMTUR (I) ,1=1 ,6) (DP(I) ,1=1,6) (A(I),I=1,5) (B (I) ,1=1,5) (TMP (I) ,1=1,5) (TM (I) ,1=1,6) (BVUL (I) ,1=1 ,6) (SVUL (I) ,1=1,6) 100 FORMAT (8F10.0) WRITE(3,602) Z WRITE(3,602) TIM WRITE(3,602) TEM WRITE(3,602) DT WRITE (3,602) BFUDE WRITE (3,602) SFUDE WRITE (3,602) DF WRITE (3,602) TMTUR WRITE (3,602) DP WRITE(3, 602) A WRITE (3,602) B WRITE (3,602) TMP WRITE(3,602) TM WRITE (3,602) BVUL WRITE (3,602) SVUL 800 PAUSE ID=1 READ (6, 333) C 333 FORMAT (F3.0) CALL RANDI (C) C...TO MODIFY INPUT CALL DATSW (8,JQ) GO TO (81 ,90) , JQ 81 CALL M MOD 90 CALL DOIT(DF,DT, WRITE (3,2000) W 2000 FORMAT (1X,'FINAL RTOT,W,ID) WEIGHT=',E14.7) 142 P=W-WO RATIO= P/(P+RTOT) WRITE (3, 2001) RTOT ,P,RATIO 2001 FORMAT (30X,»RTOT =•,E14.7,5X,«P = ',E11.4,5X,'RATIO = », 1E14. 7) WRITE (3,3000) STFE 3000 FORMAT ( 5X,E12.5) CALL MOUT C,..TO MAKE ANOTHER RUN CALL DATSW (14,JQ) GO TO (800,801) , JQ 801 CALL EXIT 602 FORMAT (1X,10 (1X,F 10.4)) END EXECUTION TERMINATED T=5.40 DR=8 $SIG 

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