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Xerox University Microfilms 300 North Zoeb Road Ann Arbor, Michloan 48106 1 I 74-6111 RASMUSSEN, Glen Allan, 1947A STUDY OF THE FEEDING HABITS OF FOUR SPECIES OF FISH, Alosa pseudoharenqus. Coreqonus hoyl, Perea flavescens, AND Osmerus mordax. STTHree sITeS o n LAKE MICHIGAN, AS"COMPAftCb TO THE ZOOPLANKTON, PHYTOPLANKTON AND WATER CHEMISTRY OF THOSE SITES. Michigan State University, Ph.D., 1973 Ecology University Microfilms, A XEROXC om pany, Ann Arbor, Michigan A STUDY OF THE FEEDING HABITS OF FOUR SPECIES OF FISH, A lo sa p s e u d o h a re n g u s . Coregonus h o y i . P e r e a f l a v e s c e n s . AND Osmerus m ordax. AT THREE SITES ON LAKE MICHIGAN, AS COMPARED TO THE ZOOPLANKTON, PHYTOPLANKTQN AND WATER CHEMISTRY OF THOSE SITES. By G len A l l a n Rasmussen A THESIS S u b m itte d t o M iohlgan S t a t e U n iv e r s ity i n p a r t i a l f u l f i l l m e n t o f th e r e q u ir e m e n ts f o r t h e d e g re e o f DOCTOR OF PHILOSOPHY D e p a rtm e n t o f Zoology 1973 ABSTRACT A STUDY OF IRE FEEDING HABITS OF FOUR SPECIES OF FISH, A lo s a p se u d o h a re n g u s, C o reg o n u s h o y i. P e r o a f l a v e s c e n s . AND O sm erus raordax. AT THREE SlEES ON IAKE MICHIGAN, AS COMPARED TO THE ZOOPLANKTON, PHYTOPLANKTON AND WATER CHEMISTRY OF THOSE SITES. BY Glen A l l a n Pasmussen F o u r s p e c ie s o f f i s h , A losa p s e u d o h a re n g u s. Coregonus h o y i . Peroa f l a v e s c e n s a n d Osmerus m ordax. w ere s tu d ie d a t t h r e e s i t e s on L ake Miohig&n i n term s o f a g e and grow th c h a r a c t e r i s t i c s , depth s e l e c t i o n and food p r e f e r e n c e s . A l l f o u r s p e c ie s showed r e p e a t a b l e d e p th s e l e o t i o n p a t t e r n s on a y e a r ly b a s i s and a l l show ed o v e r la p i n t h o ir f o o d s e l e o t i o n . A ll f o u r s p e c i e s a r e s e l e c t i v e z o o p la n k to n p r e d a t o r s i n th e s e n s e o f Brooks a n d Dodson (1 9 6 5 ) h u t th e a l e w i f e , A losa i s th e most e f f i o i e n t o f th e f o u r i n term s o f a b i l i t y to a c q u i r e t h i s f o o d s o u r c e . L a rg e sm e lt and p e r c h o v e r s ix i n c h e s i n l e n g t h t e n d to p i s o i v o r y . Z o o p la n k to n and p h y to p la n k to n num bers w ere a l s o fo llo w e d a t t h e t h r e e stu d y s i t e s a s w e ll a s b a s i c c h e m ic a l p a ra m e te rs* over t i m e . A ll showed f l u c t u a t i o n s I t i s h y p o th e s iz e d t h a t s e l e c t i v e p r e d a t i o n i n f l u e n c e s z o o p la n k to n numbers w h ic h i n tu r n i n f l u e n c e s p h y to p la n k to n a b u n d an o e , ACKNOWLEDGMENTS I would l i k e v e ry much t o th a n k th e r o l l owing p e o p le w ith o u t whom t h i s t h e s i s w o u ld h av e been im p o s s ib le ; D r . Jam es B ra d d o c k , who p e r s e v e r e d w i t h me fo r f o u r y e a r s and who gave i n v a l u a b l e h e lp , D r . Eugene R o o lo f s , who w as ray E n g li s h c o n s u lta n t* Dr* C la re n c e McNabb, who g a v e me b o th p h i l o s o p h i c a l a n d s o h o la r ly h e l p , and D r. P e t e r Tack, who gave me a o o e s s t o d a t a and p r o v id e d o r i t i o a l e v a lu a tio n c o f t h e m a n u s o r ip t. I w ould a l s o l i k e t o thank D r . C h a rle s L i s t o n and orew a n d B ie C onsum ers Power Company f o r t h e d a ta c o l l e c t e d a t t h e L udlngton s i t e d u rin g 1 9 7 2 . And s p e o i a l t h a n k s goes to n y W ife Susan who h e lp e d i n a ll* ways t o g e t th e p r o j e o t d o n e . ii TABLE OF CONTENTS Pago LIST OF FIGURES ..................................................................................................................... v LIST OF TABIES ........................................................................................................................ vi INTRODUCTION....................................... 1 B ack g round M a t e r i a l .................................................................... 2 METHODS AND MATERIALS ....................................................................................................... 16 F is h C o l l e c t i o n s .......................................................................................................... 16 Z o o p la n k to n ..................................................................................................................... 16 P h y s i c a l a n d C hem ical S t u d i e s ............... 17. F h y t o p l a n k t o n .................................................................................................................. 17 C o n t r o ll e d F e e d in g E x p e r i m e n t ............................................................................ 18 Stom ach A n a ly s e s .......................................................................................................... 19 RESULTS........................................................................................................................................ 20 D epth D i s t r i b u t i o n o f F i s h ................................................................................... 20 O v e r a ll Food H a b i t s ............................................................................ *.................... 22: Food H a b its o f t h e B l o a t e r .................................................................................. 25 Food H a b its o f t h e A l e w i f e .................................................................................. 30 Food o f A le w iv e s < 6 . 0 In c h e s L o n g .................................................................. 30 Food o f A le w iv e s 30 6 .0 In o h e s an d G r e a t e r .................................................... Food H a b its o f t h e S m e l t .......................... 31 Food H a b its o f t h e P e ro h . .................................................................................. 35 L a b o r a to r y F e e d in g E x p e r i m e n t ............................................................................ 39 lii TABLE OF CONTENTS (continued) Pag® Z o o p la n k to n ...................................................... W H i s t o r y ...................................................................................... 46 R e s u l t s an d D i s o u s s i o n ................................................................................... 46 P h y to p la n k to n ............................... 48 P h y s i c a l a n d C hem ioal S t u d i e s .............................................................................. 54 SUMMARY........................................................................................................................................ 84 BIBLIOGRAPHY............................................................................................................................ 86 I iv LIST OF FIGURES r Page F ig u r e 1 Lake M ichigan show ing d i v i d i n g l i n e b e tw e e n t h e N o rth and South B asin b a s e d on c u r r e n t p a t t e r n s a n d subm arine r i d g e s ........................................................................................................................ 5 2 N o rth B a sin B ig S a b le P o i n t .......................................... *............. 7 3 S o u th B a s i n ................................................................................................................ 9 4 L u d in g to n Power P r o j e c t s i t e o f Consum ers Power C om pany 11 5 P o s s ib le f a o t o r s w h ich may e x p l a i n th e c u r r e n t s t a t u s o f th e f i s h e r i e s i n Lake M i c h i g a n ....................................................................... 13. Y e a rly d e p th s e l e o t i o n b y a le w i v e s , s m e l t , p e r oh and b l o a t e r s in Lake M ic h ig a n (1 9 7 0 -1 9 7 2 ) fro m L u d in g to n , B ig S a b le P o in t an d S a u g a tu o k ................................................................... 23 L en g th d i s t r i b u t i o n o f f e m a le D anhnia r e t r o o u r v a from sam ples i n 195 4, 1 9 6 6 , 1 9 7 0 , 1 9 7 1 , and 1 9 7 2 . ............................ **9 8A-H F h y to p la n k to n t r e n d s i n L a k e M ichigan 1 9 7 0 -1 9 7 2 ............................. 57 6 7 v LIST OF TABLES T able 1 a b 2a 2b 3a 3b 4a 4b 5a 5b 6a 6b 6o Page O v e r a l l F ood H a b its o f Four F is h S p e c ie s i n t h e L ake a s M easu red b y Stom ach A n a l y s i s ........................................................ P e r c e n t F re q u e n c y o f O ocurrenoe o f V a rio u s Food O rganism s • . . . P e r c e n t Volume o f V a rio u s Food O rganism s ................................................. P e r c e n t F re q u e n c y o f O c cu rre n ce o f V a rio u s Food Ite m s i n th e Stom ach o f t t h e B l o a t e r , C oregonus h o y i ............................................... P e r c e n t Volume o f V a rio u s Food Item s i n t h e Stom aoh o f th e B l o a t e r , C oregonus h o y i ................................................................... ............. P e r c e n t F re q u e n c y o f O ccu rre n ce o f V a rio u s Food I te m s i n t h e Stom ach o f t h e A le w ife , A lo s A Tpseudoharengua ................................. P e r c e n t Volume o f V a rio u s Food Ite m s i n t h e Stom aoh o f th e A le w if e , A lo sa p s e u d o h a r e n g u s .............................. .......................... . . . . P e r c e n t F re q u e n c y o f O c cu rre n ce o f V a rio u s Food Ite m s i n th e Stom aoh o f t h e S m e lt, Osmerus m ordax . . . » ............................................. P e r c e n t Volume o f V a rio u s Food Ite m s i n t h e Stom aoh o f t h e S m e lt, Osmerus m o r d a x ..................................................................................... P e r c e n t F re q u e n c y o f O oourrenoe o f V a rio u s Food Ite m s i n t h e Stom ach o f t h e P e rc h , P e re a f la v e s c e n s ................................................ P e r c e n t Volume o f V a rio u s Food Ite m s i n t h e S tom ach o f th e P e r o h , P e ro a f l a v e s c e n s ................................ ................................................ P e r c e n t F re q u e n c y o f O oourrenoe o f V a rio u s Food Ite m s i n th e S to m achs o f F o u r F is h S p e c ie s ................................................................... P e r c e n t Volume o f V a rio u s Food Item s i n t h e S tom aohs o f F o u r F is h S p e o ie s .......................................................................................................... P e r c e n ta g e o f T o ta l Volume o f a l l Stom achs (4 8 0 ) Sam pled C o n ta in in g a P a r t i c u l a r Food S o u r c e ....................... 26 26 26 28 29 33 34 36 37 40 41 43 43 43 7 Food s o u r c e s i n L ake M ich ig an - Z o o p lan k to n a n d L im n e tic C r u s ta c e a ........................................................................................................ 45 8 Z o o p la n k to n an d L im n e tic C r u s ta c e a from Lake M ic h ig a n I . . 1 9 7 0 -1 9 7 2 ................................................................................................................. 47 P h y to p la n k to n S p e o ie s ..................................... 55 10 W a te r c h e m is tr y an d t r a n s p a r e n c y .................................................................. 73 11 F l u c t u a t i o n i n Z o o p la n k to n Numbers Over Time ....................................... 79 9 vi INTRODUCTION On t h e b a s i s o f c u r r e n t p a t t e r n s a n d su b m arin e r i d g e s , Lake M ichigan can be d i v id e d i n t o two m a jo r b a s i n s , h e r e i n d e s i g n a t e d a s n o r t h an d s o u th . I n th e s e s t u d i e s , t h e s o u th e r n b a s i n was c o n s id e r e d t o be t h a t a r e a s o u th o f a l i n e o o n n e o tin g M uskegon, M iohigan an d M ilw aukee, W is c o n s in , w h ile t h e n o r t h e r n b a s i n i n c lu d e d e v e r y th in g n o r th o f t h i s l i n e e x c lu d in g Green Bay an d t h e M ackinao S t r a i t s a r e a (F ig u r e 1 ) . Eaoh o f t h e s e b a s in s showB c o n s i d e r a b l e I n t e r n a l m ix in g a s w e l l a s m ix in g b etw een b a s i n s (A yers e t a l « , 1 9 5 3 ). T h ree s tu d y s i t e s w ere c h o se n , one i n t h e s o u th e rn b a s i n - t h r e e t o f i v e m ile s n o r t h o f t h e Kalamazoo R iv e r , A lle g a n C o unty, M ichigan (1 9 7 0 -1 9 7 2 ), a n d two i n t h e n o r th e r n b a s i n - one o f f B ig S a b le P o i n t , Mason C o m ity , M iohigan (1 9 7 0 -1 9 7 1 ) an d a n o th e r f o u r m ile s s o u th o f L u d in g to n , Mason C o u n ty , M ichigan (1 9 7 2 ). l o n g i t u d e s f o r th e s e s i t e s a r e g iv e n i n F ig u re s 2-4-. L a titu d e s and These s i t e s w ere ch o sen on t h e a ssu m p tio n t h a t t h e i n t e r n a l m ix in g c h a r a c t e r i s t i c s o f th e b a s i n s w ould o r e a te d i f f e r e n t p h y s i o a l and c h e m io a l e f f e c t s a t th e d i f f e r e n t s i t e s , th u s a f f e o t i n g p h y to p la n k to n , z o o p la n k to n , an d f i s h c h a ra c te ris tic s . (1 ) The p u rp o s e o f t h e s e s t u d i e s was t h u s s e v e r a l f o l d : To d e te rm in e i f t h e i n t e r n a l m ix in g was g r e a t enough a n d t h e i n t e r - b a s i n m ixing s m a ll enough t o a f f e o t z o o p la n k to n f o o d 1s to o k s i n suoh a m anner a s t o change t h e e a t i n g h a b i t s o f th e f i s h a t t h e S au g atu o k s i t e v e r s u s t h e L u d in g to n -B ig S a b le P o i n t s i t e s . 1 2 (2 ) To d e te r m in e IT t h e s e l e c t i v e fe e d in g h y p o th e s is o f B rooks and Dodson (1 9 6 5 ) c o u ld b e s u b s t a n t i a t e d b y stom ach a n a ly s e s o f f i s h from Lake M ic h ig a n . ( 3 ) To d e te r m in e i f t h i s s e l e c t i v e fe e d in g o o u ld a l t e r z o o p la n k to n num bers. (Jf) To s e e i f f l u c t u a t i o n s i n z o o p la n k to n num bers an d s p e o i e s w ould c a u se f l u c t u a t i o n s i n t h e num bers and s p e o ie s o f p h y to p la n k to n * (5 ) To d e te r m in e t h e e f f e c t o f te m p e r a tu re , s e x , a n d ag e on th e d i s t r i b u t i o n a n d f e e d i n g h a b i t s o f c e r t a i n f i s h s p e o ie s i n L ake M io h ig an . Of p a r t i c u l a r I n t e r e s t w ere t h e a l e w i f e , A lo sa p s e u d o h a re n g u s . th e b l o a t e r , C o regonus h o y i , t h e y e llo w p e r c h , P ero a f l a v e s c e n s , an d th e s m e lt, Osmerus m o rd ax . la k e i n term B o f : An e f f o r t was made t o exam ine t h e s e f o u r s p e o i e s i n th e fo o d p r e f e r e n c e s , d e p th d i s t r i b u t i o n th r o u g h o u t t h e y e a r , and i n t e r - a n d i n t r a - s p e o i f i o c o m p e titio n f o r fo o d an d f o r spaw ning s ite s . (6 ) To d e te r m in e fo o d a c q u i s i t i o n a b i l i t y and s e l e c t i v i t y o f f e e d in g under c o n tr o lle d c o n d itio n s . (7 ) To d e te r m in e , b y a n a l y s i s o f a le w iv e s w ashed u p on t h e b e a o h , w h e th er t h e s e f i s h w ere s t a t i s t i c a l l y i d e n t i c a l i n te rm s o f s i z e , s e x , a g e , and w h e th e r r e p r o d u c tio n h a d been co m p leted a s oom pared t o th e g e n e ra l p o p u la tio n in th e la k e . B ackground M a te r ia l The a l e w i f e , A lo sa p se u d o h a re n g u s (W ils o n ), was i n tr o d u c e d i n t o Lake M ichigan v i a t h e E r i e a n d W e lla n d C a n a ls , I t h a s a p p a r e n t l y b e a n a llo w e d to s u c c e e d b e c a u s e o f t h e la o k o f p r e d a t io n b y su c h l a r g e p i s c i v o r o u s s p e o ie s a s t h e l a k e t r o u t , S a l v e l i n u s nam aycush (W albaum ); t h e w a lle y e , 3 S t i z o s t e d l o n v itr e u m v ltre u m ( M i t c h e l l ) ; a n d t h e b u r b o t , L o ta l o t a (L in n a a u s ) , s p e c ie s whose num bers w ere d e p l e t e d b y t h e s e a la m p re y , Petrom vzon m arln u s ( L in n a e u s ) , a n d a l s o b y f i s h i n g p r e s s u r e . o f t h i 3 c an b e fo u n d i n S m ith ( 1968a an d 1 9 6 8 b ). An a c c o u n t S in c e th e a l e w i f e 's f i r s t s ig h tin g i n Lake M ich ig an i n 1 9 4 9 , i t i n c r e a s e d i n num bers u n t i l 1 9 6 7 , Vhen 4-2,000,000 pounds w ere ta k e n i n o o m n e ro ia l t r a w l s . S in c e t h a t i tim e t h e s p e o ie s h a s d e c r e a s e d i n num bers an d a p p e a rs t o b e c o n tin u in g t o d e c lin e . I The s m e lt, Osmerus m ordax ( M i t c h i l l ) , was i n tr o d u c e d i n t o C r y s t a l Lake i n B e n zie C o u n ty , M ichigan i n 1906 (Van O o s te n , 1937)* I t re a c h e d Lake M ichigan th ro u g h a o h a n n e l c o n n e c tin g t h e tw o l a k e s and p r o l i f e r a t e d , becom ing a f a c t o r i n t h e f i s h e r i e s a ro u n d 1930* 4 . 8 m i l l i o n pounds w ere c a p tu r e d i n 1 9 4 0 , b u t d i s e a s e s t r u o k t h e p o p u l a t i o n a n d re d u o e d i t s ;l num bers d r a s t i c a l l y (Van O o s te n , 1 9 4 7 ). The c a t c h a g a in olim b ed t o 9 .1 m i l l i o n pounds i n 1 9 5 8 , b u t t h e o u r r e n t l e v e l i s a p p ro x im a te ly l / 8 o f t h a t maximum. C o m p e titio n w ith t h e a le w i f e i s a p r o b a b le r e a s o n f o r t h i s d e c lin e . The b l o a t e r , C oregonus h o y i ( G i l l ) , ( L e u o io th y s h o y i b y S m ith , 1 9 6 8 a ), i s one o f se v e n s p e o ie s o f "C hubs” o r *C iscos” o f th e f a m ily S a lm o n id ae s u b fa m ily C o re g o n in a e , t h a t i n h a b i t s L ake M iohiganJ C qregonus p i g r i n i n n i s ( G i l l ) - b l a c k f i n c i s c o CJ jo h a n n a e CWagner) - d e e p w a te r o i s c o Cj a lp a n a e ( K o e ltz ) - lo n g ja w o is c o c l k i v l ( K o e ltz ) - k i y i Cl r e i g h a r d l (K o e ltz ) - s h o r tn o s e o is c o CJ z e n ith if lu s (Jo rd o n a n d Everm ann) - s h o r t ja w o is c o (3. h o y i ( G a n ) - b l o a t e r Of th e s e se v en s p e o i e s , t h e b l o a t e r now c o n s t i t u t e s a b o u t 98$ o f a l l t h e chub s to a k (S m ith , 1968a ) a n d a b o u t 9 7 $ o f a l l ch u b s ta k e n d u r in g th e s e s tu d ie s . T h is i s down s l i g h t l y fro m 1 9 6 4 , when t h i s s p e o ie s c o n s t i t u t e d 99•2$ o f a l l ch u b s ta k e n ( W e lls , 1 9 6 6 ). A c o o rd in g t o W ells ( 1966) la m p re y p r e d a t i o n , f i s h i n g p r e s s u r e , an d a le w i f e c o m p e titio n a p p e a rs t o hav e b e e n th e m a jo r f a c t o r s i n t h e d e m ise o f t h e s e se v en s p e c ie s . F ig u re 5 g r a p h i c a l l y shows some o f t h e s e r e l a t i o n s h i p s . I t is i n t e r e s t i n g t o n o te t h a t t h e b l o a t e r , i n t h e a b s e n c e o f t h e o t h e r s i x chub s p e c i e s , i n c r e a s e d b o th i n s i e e and num bers b e f o r e c o m p e titio n from th e a le w i f e and a c e r t a i n am ount o f f i s h i n g p r e s s u r e c a u se d i t s num bers to l e s s e n (S m ith , 1 9 6 8 ). The y e llo w p e r c h , P e ro a f l a v e s c e n s ( M i t c h e l l ) , i s l a r g e r th a n th e a l e w i f e , s m e lt, o r c h u b , a t t a i n i n g l e n g t h s o f 15 - 18 in c h e s i n t h e G re a t L ak es. As s u c h , p e ro h r e p r e s e n t a v a lu a b le f i s h e r y , 5*8 m i l l i o n pounds b e in g h a r v e s t e d i n 196**. A lth o u g h t h e s p e o i e s : 'i s p la n k tiv o r o u s when young, o l d e r I n d i v id u a ls te n d t o p i s o i v o r y a n d , i n t h e a b se n c e o f th e la k e t r o u t , t h e p e ro h was th e m a jo r p r e d a t o r i n t h e l a k e . The 196** o a to h l e v e l o f 5*8 m i l l i o n pounds r e p r e s e n t s an i n c r e a s e o v e r p r e v io u s f i g u r e s . This: i s p resu m ab ly a s s o o i a t e d w ith two f a c t o r s - a d e c r e a s e i n p r e d a tio n on young p e rc h b e c a u se o f th e d e o re a s e i n t h e l a k e t r o u t p o p u la tio n and th e e x p l o i t a t i o n o f a new fo o d s o u ro e - t h e b u rg e o n in g a le w i f e p o p u la tio n . However, a s th e a le w if e was r e a c h in g i t s p e a k i n 1967 th e p e ro h p o p u la tio n was show ing a s i g n i f i c a n t d e c l i n e (6 0 0 ,0 0 0 pounds c a u g h t i n 1968) . I t is my a ssu m p tio n and t h a t o f S m ith ( 1968a ) t h a t t h e c o n c e n tr a ti o n o f a le w iv e s in s h o r e d u r in g th e spaw ning a n d h a to h in g p e r i o d o f th e p e ro h c a u se d a d e c r e a s e i n th e r e c r u i tm e n t o f t h e p e r c h s p e o i e s . From c u r r e n t d a t a , th e p e rc h a r e m aking a s i g n i f i c a n t com eback an d s h o u ld a g a in make u p a s ig n if ic a n t p a r t o f th e f i s h e r i e s . 5 F ig u r e 1 - Lake M ichigan show ing d i v i d i n g l i n e b etw een t h e N o rth an d S o u th B a sin b a s e d on c u r r e n t p a t t e r n s and su b m a rin e r i d g e s r The num bers 1 - 6 , 7-11* and 1 2 -1 7 r e p r e s e n t l o c a t i o n s o f s tu d y s i t e s s e t up i n 1 9 7 0 -7 2 . F ig u r e s 2 - ^ show f u r t h e r d e t a i l o f th e s tu d y s i t e s . L a titu d e a n d L o n g itu d e g iv e n h o r i z o n t a l a n d v e r t i c a l ax es. “ 1 - 6, N o rth e rn B a sin L u d in g to n 1 2 -1 7 A pproxim ate D iv id in g L in e -N o rth & S o u th B a s in s 7 -1 1 S o u th e rn B a sin S a u g a tu pk 7 F ig u r e 2 - N o rth B a sin B ig S a b le P o i n t . L o c a tio n o f s t u d y s i t e s 1 - 6 shown i n c l u d i n g maximum d e p th a t t h o s e s i t e s . L a t i t u d e a n d L o n g itu d e o f s i t e s g iv e n a t r i g h t o f m ap. (A d a p te d fro m t £ S . G e o lo g ic a l S u rv e y Map* Mason C o u n ty , M ic h ig a n .) 8 '3 0 ' 25' M a n iste e Co« Mason C ounty 10 M 20 M. 60 M< Lake M ichigan 50 M, 30 M, 5 - 86°2 6 »00"W 6 - 86°2V 00"W W#-°10'00"N Lake 'o l n t S o a le o f M iles 9 F ig u r e 3 - S o u th B a s in . L o o a tio n o f S tu d y S i t e s 7 -1 1 i n o lu d i n g ■ maximum d e p th s a t e ao h s i t e . S tu d y c o n d u c te d 1 9 7 0 -1 9 7 2 . L a t i t u d e and L o g itu d e g iv e n a t r i g h t . A d a p te d from U .S . G e o lo g ic a l S u rv e y H ap, A lle g a n C o u n ty , M ic h ig a n . 10 8 6 ?1 7 ,3 0 M 86 - 5 1 10 11 8 .5 M 2 7 .2 M 9 50 M 42H 0" ALLEGAN COUNTY 1 8 .If M LAKE MICHIGAN R iv e r 86 l V 0 0 " t f 42 42*00"N 42 4 2 '3 0"N 8 6 ; i 7 , 00"W 42~ 43, 30"N 8 6 ° l4 'l6 " W 4 2 °4 4 '1 9 "N 86”13*30"W 4 2 ''4 4 , 3 0 " n S c a le o f M ile s 11 F ig u r e 4- - L u d in g to n Pow er P r o j e c t s i t e o f Consum ers Pow er Company* S tu d y S i t e s 1 2 -1 7 show n, i n c l u d i n g maximum d e p th s a t e a c h s i t e . L a t i t u d e s a n d L o n g itu d e s g iv e n a t r i g h t . (A d ap ted from L is to n an d T ack, 1972) 12 Per© M a rq u e tte Lake S c a le o f M iles - 86° 26 * 50**W t# ° 5 2 'l* 5 nN - 86®27*20"W ^3 53*05W N 16 ,x 21m Pumped S to ra g e Basin - 86■ '27'10"W & 54*50"N - 86°27'35"U to 51*920**N - 8 6 °2 9 ' 00"W t o 53*30"N - 86®27*20»W 4 3 ° 5 1 '0 0 WN LAKE MICHIGAN ! Figure 5 P o s sib le f a c to r s which nay e x p la in th e c u rre n t s ta tu s o f th e f i s h e r i e s i n Lake M ichigan. Data in c lu d e s th e f a c to r s o f p re d a tio n , com petition and f i s h e r i e s p re s s u re . E x p lan atio n f o r th e symbols i s given below: C - Com petition P = P red atio n R = R ecruitm ent t* • * *• *• LTA = Lake T rout A dults LTT = Lake T rout Young LT ~ Lake T rout (A dults and Young) LCA = Large Chubs - A d u lts - In clu d es Coregonus k i y i C. n ig r ip in n is C. z e n ith ic u s C. alpenae C. r e ig h a r d i C. johannae LCY = Large Chubs - Young CS - Sm all Chubs (B lo a te r - Coregonus h o y i) PA = Perch A dults FT =. Perch Young S = Smelt HC - Medium Chubs - th e rem nants o f th e p o p u la tio n s o f la r g e chubs from b e fo re 1950 - in clu d e s th e fo llo w in g sp e c ie s C. z e n ith ic u s C. alpenae C. r e ig h a rd i C. k i y i Adapted from W ells (1966) and Smith (1968a), G reat Lakes F is h e rie s Catch S t a t i s t i c s . % 14 LCA LTT FT 1910-1940 F is h e r y e ry 1 9 4 0 -1 9 5 0 FT l * 15 I960 'ey m i F is h e r y PA l o a te l 1222 F is h e ry 2MO— C oho/ & V Chinook ew ifi PA METHODS AMD MATERIALS F i s h C o lle o tio n s U sin g t h e sy stem o f H orak a n d T anner ( i 960) , f i s h w ere o a u g h t i n 1 - i n c h , 2 - i n c h , a n d 2£ - i n c h m esh g i l l n e t s , e a c h n e t e i t h e r 12 f e e t b y 50 f e e t o r 6 f e e t by 50 f e e t . The n e t s w ere s e t a t e a c h s t a t i o n f o r a 2 4 -h o u r p e r io d a t e a c h o f t h r e e d e p th s ( s u r f a o e , 4 - 5 m e t e r s , and n e a r t h e b o tto m )* N e ts w ere s e t i n b lo c k s o f t h r e e so t h a t a p a n e l o f eao h m esh s i z e was p r e s e n t a t e a c h s t a t i o n and a t e a c h d e p th . A lth o u g h i t was n o t a lw a y s p o s s i b l e t o g e t o u t on t h e l a k e on t h e same d a t e s e ao h y e a r d u e t o in c le m e n t w e a th e r , l i f t s d i d ta k e p l a c e e a c h y e a r d u r i n g s p e c i f i e d p e rio d s o f d ay s. A f t e r t h e n e t s w ere s e t f o r 24 h o u r s , th e y w ere b r o u g h t on b o a rd an d t h e f i s h w ere im m e d ia te ly m e a su re d , w eig h ed and se x e d (w here p o s s i b l e ) . S c a le s w ere ta k e n t o d e te rm in e a g e a n d , w here d e te r m i n a ti o n o f se x w as a p ro b le m , t h e f i s h w ere p la c e d on i c e an d ta k e n a s h o r e f o r e x a m in a tio n . A lth o u g h t o t a l number and w e ig h t o f e a c h o f th e s p e o ie s o f i n t e r e s t was o b t a i n e d , n o t a l l f i s h w ere exam ined i n d e t a i l b e c a u s e o f t h e l a r g e num bers in v o lv e d . F is h w i t h in e ao h s p e o ie s w ere ta k e n a t random f o r m ore c o m p le te e x a m in a tio n . In a l l , 2598 p e r c h , 1232 s m e l t , 2520 a le w i v e s , a n d 1870 b l o a t e r s w ere exam ined i n d e t a i l . Z o o p lan k to n Z o o p la n k to n w ere ta k e n a t t h e same tim e and s i t e s a s t h e f i s h , p h y to p la n k to n an d c h e m ic a l s t u d i e s . A m e te r p la n k to n n e t (180 m esh) was tow ed a t a p p ro x im a te ly 2 -3 m il e s / h o u r f o r 100 m e te r s a t d e p th s o f 0 -1 m e te r , 4 - 5 m e t e r s , an d n e a r t h e b o tto m . Sam ples w ere f i x e d i n c o n c e n tr a te d f o r m a lin ( a 3856 aq u eo u s s o l u t i o n o f fo rm a ld e h y d e ) f o r one 16 week an d th e n tw o a l i q u o t p o r t i o n s w e re ta k e n and c o u n te d . S p e o le s w ere en u m e rated and e x p re s s e d a s num ber/m ^. P h y s i c a l a n d C h em ical S tu d ie s The f o llo w in g p a ra m e te rs w e re s t u d i e d - te m p e r a tu r e p r o f i l e , t r a n s p a r e n c y , pH, a l k a l i n i t y , d i s s o l v e d osygen and o a rb o n d i o x i d e , an d d is s o lv e d s o l i d s . T u r b i d i t y was m ea su re d a t t h e L u d in g to n s t a t i o n s b u t a t no o t h e r s . W ater te m p e r a tu r e s w ere m ea su re d a t each s t a t i o n u s in g a t h e r m is t o r a s g i l l n e t s w ere b e in g lo w e re d . in te rv a ls a t a l l s ta tio n s . tim e u s i n g a S e c c h i d i s k . T e m p era tu re s w ere ta k e n a t tw o -m e te r W ater tr a n s p a r e n c y was m ea su re d a t th e same S ta n d a rd c h e m ic a l a n a l y t i c a l m ethods w ere u s e d t o m easu re D . O . , CO,,, a l k a l i n i t y , and d i s s o l v e d s o l i d s . A pH m e te r (made b y B eckm an), c a l i b r a t e d b e f o r e e a c h o u t in g a g a i n s t a O.OO8695M KH^PO^ + 0.0304M Na2 HP0^ s o l u t i o n w ith a pH o f 7-4-13 ( a t 2 5 ° C ), was u s e d i n a l l pH c a l c u l a t i o n s , a l l v a lu e s b e in g te m p e r a tu r e c o r r e c t e d u s i n g t h e sy ste m o f B a te s ( 1 9 6 2 ) . A H t e s t s w ere r u n on sa m p le s ta k e n on a b i- w e e k ly b a s i s u s in g a 1 - l i t e r m o d ifie d Kemmerer s a m p le r from d e p th s o f 0 -1 m e te r , 4 -5 m e te r s , a n d 1 m e te r from t h e b o tto m . I f th e s i t e was g r e a t e r , th a n 40 m e te r s i n d e p th , a f o u r t h sam ple was o f t e n ta k e n a t 2 0 m e te r s . P h y to p la n k to n P h y to p la n k to n w ere e r r u m e r a te d u s in g th e te c h n iq u e o f MoNabb ( i 9 6 0 ) ; 4-00-ml sam p les w ere t a k e n , i n d u p l i c a t e , from w a te r ta k e n fro m th e s u r f a c e , 4—5 m e t e r s , 1 m e te r fro m th e b o tto m , and fro m 20 m e te r s w here a v a ila b le . T hese 4 00-m l sa m p le s w ere f i x e d w ith 1 6 m l o f c o n o e n tr a te d f o r m a lin a n d 50 m l o f t h e sam ple was f i l t e r e d th r o u g h a .4 5 u m i l l i p o r e ;18 filte r. o il. The f i l t e r was d r i e d f o r 2k h o u rs an d th e n c l e a r e d w ith e m e rsio n A c o v e r g l a s s was p la c e d on t h e s l i d e a n d 3Q f i e l d s w ere sc a n n e d a t 450 X. C o n c e n tr a tio n was e x p r e s s e d a s num bers p e r m i l l i l i t e r o f l a k e w a te r . C o n tr o lle d F e e d in g E x p e rim e n t A c o l l a t e r a l e x p e rim e n t t o d e te rm in e f e e d in g p r e f e r e n c e s was a tte m p te d u s in g t h e f o llo w in g sy s te m : f i s h o f a l l f o u r s p e o ie s w ere c a u g h t u s in g a 1 - in c h m esh t r a w l n e t d u r in g t h e m onths o f A p r i l , J u n e , a n d A u g u st. T hese f i s h w e re p la c e d i n h o ld in g ta n k s ( i o f an u n u se d 5 0 0 - g a l l o n o i l - f u e l ta n k ) f o r 2k h o u r s t o i n s u r e t h a t t h e y h a d n o t e a t e n f o r a t l e a s t t h a t p e r i o d o f tim e . They w e r e 'th e n i n tr o d u c e d i n t o a p o o l (15 f e e t i n d ia m e te r b y k f e e t i n d e p th ) i n t o w hioh Lake M ichigan w a te r h a d b e e n p la c e d . The f i s h w ere p l a c e d i n t h e p o o l i n s i n g l e s p e o ie s g ro u p s o f 5* o r 1 0 , o r m ixed s p e o ie s g ro u p s o f t e n I n d i v i d u a l s ( f o r exam ple - 5 p e r o h , 5 s m e lt , o r 2 p e r c h , 2 s m e l t , 3 a le w iv e s , 3 b l o a t e r s ) . The f i s h w ere a llo w e d t o rem a in i n t h e p o o l f o r 3 0 m in u te s t o f e e d on known c o n c e n t r a t i o n s o f z o o p la n k to n . U n d is tu rb e d f i s h w ould b e g in f e e d i n g w i t h in 15 m in u te s o f i n t r o d u c t i o n . O nly f i s h l e s s th a n o r e q u a l t o 6 .0 in o h e s i n l e n g t h (1 5 cm) w ere u s e d . A f t e r e a c h t e s t , t h e p o o l was sam p led t o d e te rm in e re m a in in g z o o p la n k to n c o n c e n tr a ti o n a n d t h e f i s h w e re o u t open t o d e te rm in e stom ach c o n t e n t s . I n a l l , 120 f i s h o f e a c h s p e o ie s w ere sa m p le d . As n i l f e e d i n g e x p e rim e n ts w ere o o n d u o te d i n a c lo s e d s h e d w ith a r e d l i g h t s o u r c e , t h e z o o p la n k to n a n d l a r g e o r u s ta a e a n s w ere d i s t r i b u t e d f a i r l y u n if o r m ly th ro u g h t h e w a te r m ass. The f i s h w ere a o o lim a te d f o r th e 2k h o u r p e r i o d i n t h i s l i g h t t o e l i m i n a t e a s many d e p th c u e s a s p o s s ib le . Ample oxygen was pumped th ro u g h t h e sy s te m an d c o n s t a n t num bers X9 a n d c o n c e n tr a ti o n s o r fo o d o rg a n ism s v e r e u s e d th r o u g h o u t A p r i l , Ju n e an d A u g u st. Stom ach A n a ly se s On t h e 8220 f i s h whose sto m ach c o n te n ts w ere exam ined i n d e t a i l , t h e f o llo w in g sy s te m was u s e d : (1 ) 'Dio f i s h t o b e sam p led w ere b r o u g h t b a c k t o t h e l a b o r a t o r y on i c e t o slo w d e c o m p o s itio n o f a n y m a t e r i a l i n t h e sto m ach . ( 2 ) The stom ach was q u i c k l y rem oved a lo n g w ith a one in c h s e c t i o n ( 2 . 5 cm) o f g u t on e i t h e r en d o f th e sto m ach . ( 3 ) The o o n te n ts o f t h e e n t i r e segm ent w as p l a c e d i n a s m a ll v i a l c o n ta in i n g 72$ e th a n o l an d s e t a s i d e f o r e x a m in a tio n . ( 4 ) The m a t e r i a l was exam ined a t 10X, w ith h i g h e r m a g n i f ic a ti o n b e in g u s e d f o r somei c l o s e r i d e n t i f i c a t i o n o f t h e s m a lle r a o o p la n k to n fo rm s . F o llo w in g t h e sy ste m o f W e lls an d B o sto n (1 9 6 3 ) , s m a ll num bers o f p l a n k t e r s w ere c o u n te d w h ile l a r g e r num bers w ere e s t i m a t e d . was made t o k e y d a m s an d o s tr a o o d s t o s p e o i e s . No a tt e m p t Z o o p la n k to n w ere d i v id e d o n ly i n t o C la d o c e ra n s a n d Copepods w ith num bers e x p r e s s e d b o th i n te rm s o f p e r c e n ta g e f r e q u e n c y o f o c c u re n c e an d p e r c e n t vo lu m e. RESULTS D epth D i s t r i b u t i o n o f F is h D epth d i s t r i b u t i o n s v a r y from s p e b i e 3 t o s p e o ie s w ith tim e o f t h e y e ar. W ith in some s p e o i e s , d e p th d i s t r i b u t i o n v a r i e s w ith s e x and a g e , b u t a v e ra g e d e p th i s g iv e n i n F ig u r e 6 . B l o a t e r s ( C oregonus h o y i ) w e re seldom c a u g h t i n d e p th s l e s s th a n 20 m e te rs an d even more seldom c a u g h t i n n e t s s e t more t h a n 10 m e te rs o f f th e b o tto m . Young b l o a t e r s (u p t o 2 y e a r s ) w ere c a u g h t i n m id w a te r ra n g e s (2 0 -4 0 m e te rs i n d e p th ) b u t a f t e r 2 y e a r s j o in e d t h e a d u l t s i n a y e a r l y m ig r a to r y t r e n d from d e e p e r w a te r ( g r e a t e r th a n 60 m e te r s ) i n t h e w i n t e r t o s h a llo w e r w a te r ( 20+ m e te r s ) b y m id-sum m er. A f t e r t h i s tim e th e p o p u la tio n b eg an a p r o g r e s s i v e movement b a c k to w a rd d e e p e r w a t e r , r e g a i n in g 60 m e te rs b y l a t e A u g u st o r e a r l y S e p te m b e r. B o th s e x e s w erb c a u g h t t o g e t h e r and t h e r e seem: t o h a v e b e en n o d e p th - s e x i n t e r a c t i o n s . A lew iv es (A lo sa o s e u d o h a re n g u s ) show movement fro m d e p th s g r e a t e r th a n 60 m e te rs i n e a r l y Kay t o d e p th s o f a b o u t 3 m e te r s b y l a t e Hay. They rem ain b e tw ee n 3 and 10 m e te rs u n t i l l a t e A u g u s t, a n d th e n move to w a rd th e d e p th s . Young a l e w iv es (1 - 2 y e a r s ) i n h a b i t t h e same m id w a te r p e l a g i c zone o c o u p ie d b y t h e b l o a t e r an d u t i l i z e t h e same fo o d s o u r c e s . D a ta s i g n i f i c a n t a t t h e p < .0 5 l e v e l i n d i c a t e t h e f o llo w in g : (1 ) L arg e a le w iv e s o f t h e 1 -2 y e a r o l a s s i n h a b i t t h e 0 -2 0 m e te r zone i n t h e s p r i n g , b u t seem t o b e d i s p l a c e d o f f - s h o r e a s t h e l a r g e r , o l d e r i n d i v i d u a l s oome u p from g r e a t e r d e p th s . ( 2 ) The l a r g e s t members o f t h e o l d e r age c l a s s e s (3 y e a r s and o l d e r ) seem t o move i n t o s h a llo w w a te r b e f o r e s m a lle r i n d i v i d u a l s o f th o s e same age c l a s s e s . As th e te m p e r a tu re r i s e s i n th e summer, t h e l a r g e r 20 ' 21 i n d i v i d u a l s a r e a g a in t h e f i r s t t o m ove, t o 20 m e te rs i n e a r l y A ugust an d t o g r e a t e r th a n 5 0 m e te r s b y e a r l y S e p tem b er. S m e lt (Q sm erus m ordax) o c cu p y a zone a b o u t 4 0 -5 0 m e te rs i n d e p th d u r in g t h e w i n t e r and th e n move in s h o r e v e r y e a r l y i n t h e s p r i n g t o c o m p lete b r e e d in g . They re m a in i n s h a llo w w a te r ( l e s s th a n 10 m e te r s ) u n t i l e a r l y Ju n e and th e n g r a d u a l l y move d e e p e r , r e a c h in g 40 m e te rs b y l a t e O c to b e r. A v a ila b le d a t a i n d i c a t e t h a t a l l b r e e d in g s m e lt move i n s h o r e en m a sse , w ith none o f t h e s i z e d i f f e r e n c e s shown b y a le w iv e s . H ie re d o e s n o t a p p e a r t o b e a d e p th d i f f e r e n c e r e l a t e d t o s i z e o r s e x . P e rc h ( P e r e a f l a v e s c e n s ) i n h a b i t a w ide zone o f 0 -4 0 m e te rs i n d e p th a l l y e a r ro u n d , c o n c e n t r a t i n g t h e i r a c t i v i t i e s i n w a te r s l e s s th a n 25 ; '' m e te rs d e e p . H iey move i n t o s h a llo w w a te r ( l e s s th a n 10 m e te r s ) i n m id - J u n e , r e t u r n i n g t o t h e i r w i n t e r i n g d e p th b y t h e end o f S e p te m b e r. A t th e p < . 0 5 l e v e l , t h e d a t a s u g g e s t t h e f o llo w in g : ( 1 ) H ie l a r g e s t I n d i v i d u a l s o f ag e g ro u p s I an d I I a r e fo u n d i n s h a llo w w a te r i n th e summer a n d i n d e e p e r w a te r i n t h e f a l l , a p a t t e r n a l s o e x h i b i t e d b y t h e p o p u l a t i o n i n g e n e r a l , a lth o u g h t h e s e y o u n g e r f i s h a r e n o t in v o lv e d i n b r e e d i n g . H ie i n s h o r e movement i s p ro b a b ly key ed t o fo o d a c q u i s i t i o n . ( 2 ) Male p e rc h move i n t o t h e s h a llo w w a te r b r e e d in g and f e e d in g a r e a s so o n e r th a n do t h e f e m a le s , an d rem a in t h e r e l o n g e r . ( 3 ) More fe m a le s w ere o a u g h t i n m id -w a te r n e t s t h a n w ere m a le s , w h ile more m a le s w ere c a u g h t i n n e t s s e t on th e b o tto m th a n w e re f e m a le s , s t r o n g l y s u g g e s tin g t h a t p e r c h show d i f f e r e n c e s i n f e e d i n g h a b i t s and l o c a l e s b a s e d on s e x . T h is i s b o rn e o u t b y stom ach a n a ly s e s ( p . 4 9 )* ( 4 ) Male p e rc h p r e f e r s h a llo w e r w a te r (5 -1 5 m e te r s ) on a y e a r ro u n d b a s i s w h ile fe m a le s a r e d i s t r i b u t e d u n ifo r m ly o v e r a l l d e p th s a t a l l s e a s o n s ( 0 -4 0 m e t e r s ) . T h is i s p r o b a b ly key ed t o fo o d a c q u i s i t i o n , m ales b e in g p r i m a r i l y b o tto m f e e d e r s , fe m a le s b e in g p r i m a r i l y p e l a g i c f e e d e r s . I b i s i n t u r n a f f e o t s t h e i r fo o d h a b i t s ( s e e p . ffO -Jfl). T h ere w ere no s i g n i f i c a n t d i f f e r e n c e s i n te rm s o f l e n g t h , w e ig h t o r se x c o m p o s itio n o f p o p u l a t i o n s o f a le w iv e s , s m e lt , p e r c h , o r chubs c a p tu r e d a t S a u g a tu c k , L u d in g to n , o r B ig S a b le P o i n t . I t was e x p e o te d t h a t t h e r e m ig h t b e d i f f e r e n c e s b a s e d on d i f f e r e n c e s i n z o o p la n k to n and p h y to p la n k to n c o n c e n tr a ti o n s a n d on w a te r c h e m is tr y v a r i a t i o n s betw een th e t h r e e s i t e s , b u t no d i f f e r e n c e s s i g n i f i c a n t a t t h e p < .0 5 l e v e l w ere d e te c te d . O v e r a ll Food H a b its A lth o u g h q u a n t i t i e s o f v a r i o u s fo o d o rg a n ism s oonsumed ch an g es th r o u g h o u t th e; y e a r on t h e b a s i s o f a g e , d e p th , s e a s o n , l o o a t i o n , and se x i n some s p e o i e s , i t w ould s t i l l seem v a l i d , g iv e n t h e l a r g e number o f i n d i v i d u a l s sa m p le d , t o oompare o v e r a l l p e r c e n ta g e s o f fo o d . T h is h a s b een done f o r f o u r s p e o i e s , b o t h a s a p e r c e n ta g e o f t h e f i s h t h a t u t i l i z e t h a t fo o d s o u r c e , and a s t h e p e r c e n ta g e o f d r y w e ig h t t h a t t h i s fo o d s o u rc e m akes u p on a y e a r l y b a s i s (T a b le s l a - 5 a and l b - 5 b ) . D e f i n i t e c o m p e titio n f o r fo o d s o u ro e s a p p e a rs t o e x i s t . A lew iv es and b l o a t e r s u s e t h e same m a jo r fo o d s o u r o e s , l a r g e z o o p la n k to n an d o r u s ta c e a n s t o c k s , b u t s m e lt f e e d on o s tr a o o d s and i n s e o t l a r v a e t o a f a r g r e a t e r d e g re e th a n a n y o f t h e o t h e r t h r e e s p e o i e s , a n d p e rc h consume f i s h a s a m a jo r p e r c e n ta g e (31$ o v e r a l l , b u t 4 8 .8 $ i n t h e l a r g e s t f i s h ) o f t h e i r d i e t , th o u g h n o t t o t h e e x c l u s i o n o f o t h e r fo o d o rg a n is m s . Fem ale p e rc h to o k a g r e a t e r p e r c e n ta g e o f f i s h th a n d i d m ale p e r o h , s u g g e s tin g d i f f e r e n t i a l f e e d in g h a b i t s r e l a t e d t o l o o a t i o n . In d e e d a g r e a t e r F ig u re 6 - Y early depth s e le c tio n by a le w iv e s, sm e lt, p e rc h and b lo a te r s i n Lake Michigan (1970-1972) from L udington, Big Sable P o in t and Saugatnck. Depth . (M eters) EARLY 10 20 LATE EARLY LA1E EARLY UTE ■ ' BLOATER 0 D ^PERCH A A 25 p e r c e n ta g e o f fe m a le s th a n m a le s w e re c a p tu r e d b o th I n s h o r e and I n p e l a g i c w a t e r s , w h ile a g r e a t e r p e r c e n ta g e o f m ales w e re c a p tu r e d I n n e t s s e t on t h e b o tto m I n d e e p e r w a t e r s . A le w iv e s , b l o a t e r s , a n d s m e lt seem n o t to show t h i s d i f f e r e n t i a l f e e d in g r e s p o n s e r e l a t e d t o s e x . B l o a t e r s showed a se x r a t i o : v e r y much i n f a v o r o f fe m a le s among th o s e g r e a t e r th a n 7 in c h e s lo n g (a lm o s t 3 0 :1 i n t h e l a r g e s t s p e c im e n s ). The young b l o a t e r s showed a g r e a t e r p e r c e n ta g e o f fe m a le s th a n m a le s b u t n o t t o a s g r e a t an e x te n t, T h is s e x r a t i o h a d no e f f e c t on t h e ty p e s o f fo o d s u t i l i z e d . F e e d in g re s p o n s e chan g ed f o r a l l f i s h s p e o ie s fro m m onth t o m onth, p a r t i c u l a r l y among t h e z o o p la n k to n f e e d e r s a s t h e num bers o f p a r t i c u l a r s p e o ie s o f z o o p la n k to n ( e x . C y c lo p s « D a p h n ia ) r o s e and f e l l . S in o e t h e s e ch an g es c o u ld b e fo llo w e d f o r t h r e e y e a r s and s in o e t h e y c o u ld b e lum ped i n b l o c k s , t h e m onths M arch, A p r i l , an d May w ere com bined a s S p rin g d a t a ; J u n e , J u l y , and A ugust a s Summer d a t a ; an d S e p te m b e r, O c to b e r, an d November a s F a l l d a t a . T ren d s w ere s i m i l a r enough d u r in g e a c h o f th e s e 3 -m onth b lo c k s t h a t no s t a t i s t i c a l s i g n i f i c a n c e was l o s t . In d e e d , s i g n i f i c a n c e was g a in e d b e o a u s e o f t h e g r e a t e r num bers o f i n d i v i d u a l s i n eac h b lo c k . F o o d -H a b its o f t h e B l o a t e r B l o a t e r s w ere c o l l e c t e d a t a l l d e p th s (0 -6 0 M) d u r in g s p r i n g , summer an d f a l l . Stom aohs o f 2265 f i s h w ere exam ined o f w h ich 1870 c o n ta in e d i d e n t i f i a b l e fo o d , 203 w ere e m p ty , an d 192 c o n ta in e d fo o d w hich was u n id e n tifia b le . R e fe re n c e s h e r e a r e made o n ly t o th o s e 1870 i n d i v i d u a l s w ith i d e n t i f i a b l e fo o d . Of t h e 133 b l o a t e r s l e s s t h a n 6 in c h e s i n l e n g t h w hich o o n ta in e d f o o d , 98 came from t h e zone b e tw e e n 10 an d 4-0 m e te rs i n d e p th . The O v erall Food H abits o f Four F ish S pecies i n th e Lake a s Measured by Stomach A nalysis Table l a . S pecies B lo a te r A lew ife Sm elt Perch Average * of F ish 1870 2520 1232 2598 Pontoporeia 44.1 6 3 .4 27.3 5 7 .0 '4 7 .9 5 P e rc e n t Frequency o f Occurrence of V arious Food Organisms Ifrs is 4 2 .2 3 4 .6 2 3 .2 4 5 .0 I3 o .2 5 Copepods 33-9 7 3 .4 7 2 .1 4 0 .5 54.9 C ladocerans 1 7 .6 6 3.0 6 8 .1 _36_.8 . 1 6 .3 8 Clams 22.6 1 .1 0.6 7.3 .7 - 9 F ish Eggs and Fry 2 .8 1 2 .5 1 1 .8 3 7 .4 ... 16.12 T Other Midges I n s e c ts 7 .4 1 .5 1 .2 1 .8 8 .8 2 1 .9 4 4 .4 4 4 .6 “F .T f yt------------------O stracods 5 .2 8 .9 64.0 1 8 .1 2 4 .1 i Pontoporeia 3 5 .6 3 4 .4 8 .5 1 3 .5_ ttv sis 3 0 .0 2 4 .6 7 .6 1 2 .8 18.75 Copepods 1 8.5 21.2 28.5 1 5 ,0 • 2 0 .8 Cladocerans 1 7 .1 23-9 13.7 OO # of S pecies F ish B lo a te r 1870 A lew ife .3520 Sm elt 1232 2598 Perch Average P e rc e n t Volume o f V arious Food Organisms K Table l b . m mis 1 .6 0 .2 0 .2 1 .6 0 .9 F ish Eggs and Fry 1 .0 1 .5 3 .4 31.0 Midges 3 .5 0.3 2 .7 St2 Other In s e c ts 0 .6 0.3 5 .5 4 .6 72.5J5 O stracods 1 .1 0 .6 2 0 .1 i*° 27 re m a in in g 35 f i s h w ere c a u g h t a t d e p th s g r e a t e r th a n 4 0 m eters* Z o o p la n k to n made u p t h e e n t i r e d i e t o f t h e f i s h c a u g h t i n t h e m id -d e p th r a n g e s , an d 72$ (volum e b a s i s ) o f th e d i e t o f b o tto m f e e d e r s a t g r e a t e r th a n 4 0 m e te r s . Z o o p la n k to n s p e o ie s o f g r e a t e s t im p o rta n c e w ere C yclo p s b i o u s p i d a t u s « D iaptom us s p . , an d D aphnla g a l e a t a . The p e rc e n ta g e s o f z o o p la n k to n i d e n t i f i e d i n stom ach a n a l y s i s changed o v e r t h e t h r e e y e a r p e r i o d i n c lo s e c o r r e l a t i o n w ith th e ch an g es m ea su re d i n t h e z o o p la n k to n s to c k s . ( C o e ffic ie n t ~ .8 7 ) . Midge l a r v a e a l s o c o n s t i t u t e d a r e l a t i v e l y l a r g e p r o p o r t io n o f t h e fo o d o f b o tto m f e e d in g i n d i v i d u a l s ( 1 2 . 1$ ) . F is h 6 .0 t o 6 .9 in c h e s lo n g f e l l i n t o ag e c l a s s e s I an d I I and te n d e d t o show a m id -d e p th p r e f e r e n c e . Z o o p la n k to n was t h e m o st im p o r ta n t fo o d f o r b o th m id -d e p th an d b o tto m f e e d e r s , a lth o u g h P o n to p o re ia a n d M vsis w ere oonsumed t o a f a i r l y h ig h d e g re e b y t h e b o tto m f e e d e r s . Mo o t h e r o rg a n ism s c o n s t i t u t e d a s i g n i f i c a n t p e r c e n ta g e o f t h e fo o d a t t h i s age. A te n d e n c y fo u n d i n t h e 1 -2 age o l a s s b u t r e v e r s e d i n o l d e r f i s h i s t h a t M vsis i s a more im p o r ta n t fo o d o rg an ism th a n P o n to p o r e ia . As i n t h e s tu d y b y W e lls a n d B eeto n (1 9 6 3 ), P o n to p o re ia was t h e m o st im p o r ta n t fo o d o f b l o a t e r s g r e a t e r th a n 7 in c h e s i n l e n g t h , fo llo w e d b y M vsis an d l a r g e z o o p la n k to n s p e o i e s . Some stom achs c o n ta in e d o v e r 200 P o n to p o r e ia . p a r t i c u l a r l y i n t h e l a r g e b l o a t e r s f e e d in g on t h e b o tto m i n s h a llo w w a te r (1 0 -4 0 m e t e r s ) . Of th e z o o p la n k to n e a t e n , C y o lo p s. D iap to m u s. and D aphnia w ere fo u n d t o p re d o m in a te i n te rm s o f num bers consum ed, number o f f i s h consum ing t h e s e o rg a n is m s , a n d p e r c e n ta g e d r y w e ig h t, I b i s c o r r e l a t e s w e l l w ith a s e l e c t i v e f e e d in g h y p o th e s is b a s e d on s i z e an d num ber o f o rg a n ism s a v a i l a b l e . Table 2a. Item Length <6.0 i n . 6 .0 - 6 .9 7 .0 - 7 .9 8 .0 - 9 .9 1 0 .0 -1 1 .9 Percent Frequency of Occurrence of Various Food Items in the Stomach of the Bloater, Coregonus hoyi. # of Fish Pontoporeia Hvsis Copepods Cladocerans dams 133 666 454 316 301 4 .4 27.0 4 3 .2 65*6 80.3 5 7 .0 5 1 .0 5 1 .4 4 6 .0 87.0 4 1 .6 21.3 16.5 3 .0 49*6. 1 7 .5 10.6 5 .4 3 .0 0 .0 2 .5 21.6 3 8 .5 Weighted Average Season Spring 851 Summer 556 F all 463 Weighted A1jrerage DepthCm) 4 7 .2 4 9 .2 2 9 .4 13.9 5 4 .1 3 8 .1 4 0 .1 4 6 .8 28.3 4 7 .2 3 0 .8 4 6 .5 3 4 .5 36.3 1 0 .0 5 1 .2 71.2 56.3 1 3 .0 41.3 72.3 51.4 3 .3 67.4 3 1 .0 0-20 m 20-50 n ^ 50 m 313 1047 610 Weighted Average 5.4 ?L.l 39-5 49.7 . Fish Eggs Other and Fry Midges Insects Ostracods 0 .0 0 .5 1.3 4 .2 _____ 8.3 1 8 .6 2 .7 2 .6 6 .5 6J? 1 .0 2 .3 2 .7 0.9 0 .8 2 .0 3 .1 4 .5 6 .7 30,9 2 .5 5.2 1 .8 4 .7 5 .3 17.2 30.3 1 5 .0 26.3 16.2 2 5 .4 23.1 5 .1 1 .2 2 .1 3 .2 7 .1 7 .1 _ 8 .0 1 .3 1 .5 3 .3 5 .2 1.7 . . . 7.1 7.3 1 .5 4 .8 1 4 .4 2 4 .4 14.1 I ? .? 2 4.9 2 7 .4 I 'M 22.7 1 .1 2 .0 5.3 3 .2 9 .6 7*5 5 .1 2 .6 1 .4 0 .5 6.7 1.1 2 .3 4 .7 8 .6 6 .0 f t t 1? . 0-8 _ Table 2a* # of F ish P e rce n t Volume o f V arious Food Item s i n th e Stomach of th e B lo a te r , Coregonus h o y i. PontoDoreia Item Length <6 .0 i n . 5 .0 133 2**. 0 6 . 0- 6 .9 666 3 0 .0 7 . 0- 7 .9 454 1*7.0 8 . 0- 9 .9 316 72.0 101 1 0 .0 -1 1 .9 W eighted Average 3 5 .7 Season 1*8 .2 S pring 851 Summer 556 32.3 F a ll te l 26.3 3 8 .1 W eighted Average Denth(m) 0—20 m 113 15-7 20-50 m 10*1-7 5 8 .5 610 <50 o 3 2 .7 W eighted Average # * .0 Mysis Conenods C ladocerans Clams F ish Eggs and Fry Midges Other in s e c ts O stracods 7 .5 35-5 1*9.0 3 8 .0 2 0 .0 3i*.7 1*9.0 26.3 1 1 .0 5 .0 1 .0 1 6 .5 2 3 .0 8 .8 6 .0 4 .0 0.3 6 .6 0 .0 1 . 1* 1 .5 1 .7 3-2 1 .6 0 .0 0.3 1 .2 1 .5 2 .1 1 .0 1 2 .1 2 .1 1 .0 1 .1 1 .3 2 .2 1 .0 0 .8 0.3 0 .5 0.3 0 .6 2 .3 2 .0 0 ,1 0 .8 0.3 1 .0 2 3 .6 3 4 .3 1*2 .1 31.3 1 6 .1 2 2 .8 1 6 .6 18.2 i* .l 8 .0 1 3 .1 7 .5 1 .0 1 .1 2 .7 1-5 2 .1 0.9 0 .0 1 .2 3 .8 4 .2 2 .5 3 .6 0 .6 •0.5 0 .7 0 .6 1 .1 1 .0 1 .2 1 .1 15-7 3 1 .3 53. I 3 5 .4 21*. 1* 1 5 .7 1511* 1 5 .3 . 8 .3 10.7 6 .2 ....8-5 1 .2 2 .6 1 .0 1 .8 0 .0 1 .0 2 .0 1 .2 3 .5 6 .1 0 .9 3-9 0 .6 0 .6 0 .5 0 .5 1 .3 1 .3 0 .7 1 .0 ...... 3b Food H a b its o f t h e A le w ife Stom achs w ere exam ined from 3110 f i s h , o f w hich 2520 o o n ta in e d i d e n t i f i a b l e fo o d o rg a n is m s , 3 9 6 w ere em pty, a n d t h e re m a in d e r (2 1 4 ) h a d u n i d e n t i f i a b l e sto m ach o o n t e n t s . F re q u e n c ie s o f o o c u rre n o e and p e r c e n t d r y w e ig h t w ere c a l c u l a t e d , T a b le s 3& And 3 b . Food o f A le w iv es •<’ 6 .0 I n c h e s Long F i s h fro m 1 .7 t o 5 -9 in o h e s w ere sa m p le d . N e a rly a n (9 5 $ ) h a d z o o p la n k to n i n t h e i r stom aohs a n d z o o p la n k to n c o n s t i t u t e d 76*5 p e r c e n t o f t h e d r y w e ig h t o f t h e c o n t e n t s . No p r e f e r e n c e c o u ld b e shown f o r p a r t i c u l a r z o o p la n k to n i n sp eo im en s l e s s th a n 4 in o h e s l o n g . From 4 .0 t o 5*9 in c h e s t h e a le w i f e s t a r t e d d e m o n s tr a tin g s p e c i f i c i t y to w a rd s t h e • l a r g e r fo rm s o f z o o p la n k to n ( C yolops b l c u s p i d a t u s « M esooyolops e d a x . D aphnla g a l e a t a rae n d o ta . D. r e t r o o u r v a . E p ls o h u ra l a o u s t r i s . and D iaptom us s lo H ls ) . A c e r t a i n num ber o f t h e l a r g e s t fo rm s (L e p to d o ra k i n d l i . Lim nocalam us m a o ru ru s , P o n to p o r e ia a f f i n i s , an d H y sis r e l l o t a ) w ere a l s o fo u n d , b u t t h e s e a r e n o t a s i m p o r ta n t a s i n l a r g e r f i s h , p ro b a b ly f o r two re a so n s. F is h o f t h i s s i z e a r e u s u a l l y p e la g io a n d t h e l a r g e s t c r u s ta c e a n s a r e fo u n d v e r y n e a r t h e b o tto m , an d t h e s m a ll f i s h may n o t b e a b le t o h a n d le t h e l a r g e r fo rm s o f o r u s ta o e a n s . I h i s h y p o th e s is i s g iv e n s u p p o r t b y t h e f a c t t h a t t h e young a le w iv e s seld o m consume f r y o f t h e i r own o r o t h e r s p e c i e s , p re s u m a b ly b e c a u s e o f an I n a b i l i t y t o h a n d le o rg a n ism s o f t h i s s i z e . Food o f A le w iv e s 6 .0 In o h e s a n d G r e a te r As a le w iv e s a p p ro a c h l e n g t h s p e la g io m id -d e p th zone w h ich t h e y h a v e i n h a b i t e d u p t o t h a t tim e an d b e g in 31 t o show v e r t i c a l m ig r a tio n w i t h t h e s e a s o n s . I t i s a l s o a b o u t t h i s tim e , when t h e a le w iv e s a r e a b o u t 3 y e a r s o ld * t h a t t h e y f i r s t b e g in t o show p ro n o u n ce d I n s h o r e movements d u r in g t h e b r e e d in g se aso n * Ju n e th ro u g h A u g u st. e a t. T h is h a s a m arked i n f l u e n c e on t h e am ount an d ty p e s o f fo o d th e y A lth o u g h z o o p la n k to n w e re s t i l l fo u n d i n a h ig h p e r c e n ta g e o f sto m ach s i n 6 - in c h f i s h (a v e ra g e 79$ v s 95$ i n l e s s th a n 6 in c h e s ) * th e p e r c e n t d r y w e ig h t b e g in s t o show a d e o re a s e a t t h i s tim e (fro m 77*5$ f o r f i s h l e s s th a n 6 in o h e s t o 5 9 * 0 $ i n 6 .0 -6 * 9 in c h f i s h ) . T h is t r e n d i n c r e a s e s i n o l d e r f i s h * 'where z o o p la n k to n fo rm s a n i n c r e a s i n g l y s m a lle r p o r t i o n o f th e d i e t . On th e o t h e r hand* K v sls an d P o n to p o r e ia become i n c r e a s i n g l y m ore im p o r ta n t w i t h i n o r e a s e i n s i z e . P o n to p o r e ia makes up g r e a t e r th a n b tim e s th o p e r c e n t d r y w e ig h t o f t h e stom ach c o n te n ts o f a 7 .0 + in c h f i s h a s i t d o e s i n a n a le w i f e l e s s th a n 6 in o h e s i n l e n g t h . T h ere a l s o seem s t o b e a s e a s o n a l an d d e p th v a r i a b i l i t y i n fo o d a c q u i s i t i o n . F o r exam ple* a le w iv e s i n th e s p r i n g a n d f a l l w ere fo u n d t o hav e h ig h e r c o n c e n tr a ti o n s o f M ysis th a n i n t h e summer. From F ig u r e 6* a le w iv e s a t t h e s e two s e a s o n s w ere a t g r e a t e r d e p th s * w here M ysls a p p e a rs i n g r e a t e r c o n c e n tr a ti o n (B eeton* I 9 6 0 ) . P o n to p o r e ia . on t h e o t h e r hand* show a te n d e n c y t o h av e t h e i r h i g h e s t num bers i n w a te r l e s s th a n 50 m e te rs i n d e p th (W e lls a n d B eeton* 1963)* a t r e n d r e f l e o t e d i n t h e p e r c e n t d r y w e ig h t o f t h i s s p e o ie s i n th e sto m ach s o f alew ives* c a u g h t i n s h a llo w e r w a te r. The z o o p la n k to n show a s i m i l a r d i s t r i b u t i o n p a t t e r n t o P o n to p o re ia and th u s w ere fo u n d i n g r e a t e r c o n c e n tr a ti o n i n a le w i f e stom achs from l e s s th a n 50 m e te r s ir v 'd o p th . F is h eg g s a n d f r y * a s e x p e c te d * show t h e i r g r e a t e s t o c c u rre n c e i n a le w if e sto m achs d u r in g summer a n d e a r l y f a l l : i n t h e s h a llo w e r d e p th s * w ith t h e l a r g e r a le w iv e s consum ing t h e g r e a t e s t p e r c e n t a g e s c o f t h i s fo o d . 32 Other food sources show no d e f in it e s t a t i s t i c a l trends but are included to show comparison* I t i s in te r e s tin g to n ote th a t fin g e r n a il clams, midges, and other in s e c ts form a fa r greater p a rt o f the d ie t o f b lo a ters than o f the alew ives (about 4 | times as g r e a t). Since a fa r greater percentage o f b lo a te r s e a t clams (2 2 .6£ vs 1 .1 # fo r the a le w iv e s), t h is could be a fa cto r in the su rv iv a l o f the b lo a te r in com petition w ith the a le w ife . Another fa c to r which may be o f importance i s the amount o f time the two s p e c ie s , a le w ife and b lo a te r , spend in th e same depth zone o f the lak e (2 months), thus minimizing d ir e o t com petition to t h is 2 month period. Food Habits o f the Smelt S m e lt i n Lake M ichigan a p p a r e n t l y a ro s e fro m an in tr o d u c e d s to c k from C r y s t a l L a k e , M ichigan ( C r e a s e r , 1 9 2 6 , Van O o s te n , 19 5 3 )* w hich i n t u r n came fro m G reen L ak e, M aine ( K e n d a ll, 1927)* S e x r a t i o s i n Lake M ich ig an seem t o b e a p p ro x im a te ly 60# fe m a le ; 40# m a le , d a t a c o n s i s t e n t w ith Beckman (196*0 who fo u n d a 6 l# :3 9 # r a t i o i n C r y s t a l L a k e , and B a r le y (1 9 6 4 ) who fo u n d a 68$ :3 2 # r a t i o . T h is r a t i o o f fe m a le s t o m ales te n d s t o i n o r e a s e w ith a g e , a f a o t s u b s t a n t i a t e d b y Beckman, B a i l e y , and McKenzie (1 9 6 4 ). However t h i s d i f f e r e n t i a l s e x r a t i o d o e s n o t a p p e a r t o h av e a s i g n i f i c a n t e f f e c t on t h e ty p e o f fo o d u t i l i z e d , so s e x e s a r e oom bined i n T ab les 4 a a n d 4 b . I n G u ll L ak e , M ic h ig a n , B u rb id g e (19&9) fo u n d a te n d e n c y to w a rd e a t i n g z o o p la n k to n i n t h e s p r i n g an d f a l l and C haoborus and t e n d lp e d id s d u r in g t h e summer, s u g g e s tin g a change i n fo o d a v a i l a b i l i t y h a b i t s th r o u g h th e y e a r . No suoh ohange was fo u n d i n Lake M ich ig an s m e l t . P e rh a p s t h e d e c r e a s e i n v a r io u s z o o p la n k to n form s w hich o c c u r i n L ake M ichigan o o o u rs Table 3a. Percent Frequency of Occurrence of Various Food Items in the Stomach of the Alewife, Alosa pseudoharengus♦ * of Item F ish Length < 6 .0 i n . 31^ 6 . 0- 6 .9 956 7 .0 - 7 .9 907 8 . 0- 9 .0 308 > 9 .0 i n . *3 W eighted Average Season S pring 307 1602 Summer F a ll 611 W eighted Average Depth 0-20 m 762 20-50 m 1239 >50 m 519 W eighted Average Pontoporeia Mvsis Copepods C ladocerans damct F ish Eggs and Fry Midges Other In s e c ts 4 1 .0 5 1.0 78.0 71.0 76.0 62.29 1 2 .0 1 8 .0 4 5 .0 4 1 .0 4 8 .0 30.24 91.0 85.0 82.0 64.0 4 5 .0 81.43 9 3 .0 7 2.0 6 8 .0 4 8 .0 3 4 .0 49.62 <1 <1 1 .0 1 .6 2 .0 1 .1 3 .0 1 1 .5 1 5 .0 1 7 .5 1 9 .0 6 .0 <1 <1 1 .0 1 .7 .2 .5 1 .1 <1 1 .3 1 .0 2 .3 4 .0 1 .3 6 .0 7 .0 8 .5 9 .8 _i3 .r 7 .8 6 64.0 5 8 .0 76.0 9 - 4 1 .0 2 5 .0 3 8 .0 30.D- 61.0 83.0 76.0 78.3 73.0 68.0 4 8 .0 63.5 1 .3 <1 1 .6 1 .3 2 .0 2 4 .0 1 5 .0 1 9 .1 1 .3 <1 _ 1 .5 1 .3 3 .0 1 .0 1 .4 1 .4 9 .8 1 1 .0 6 .9 9 .8 3 4 .0 85.0 69.0 66.6 2 4 .0 3 3 .0 4 7 .0 3 3 .6 68.0 8 3 .0 38.0 68.5 1 .0 1 .8 0 .5 1 .1 :3 .3 31:0 4 .2 1 7 .0 3 .4 2 .0 0 .0 2 .0 1 1 .1 13.2 2 .4 1 0 .4 81.0 88.0 51.0 -7 1 -9 5 1 .8 1 .3 0 .4 1. 2: O stracods Table 3b. Percent Volume of Various Food Items in the Stomach of the Alewife, Alosa pseudoharengus. # of F ish Item Length <(5.0 i n . 316 6 . 0- 6 .9 956 7 .0 - 7 .9 907 308 8 . 0- 8 .9 > 9 .0 i n . 43 W eighted Average Season S p rin g 307 Summer 1602 F a ll 611 W eighted Average Depth 0—20 m 762 20-50 m 1239 >50 m 519 W eighted Average Pontoporeia Ifrs is Copepods C ladocerans Clams F ish Eggs and Fry 1 2 .1 19.6 41.0 48.0 51.2 30.3 1 0 .0 1 8 .5 3 1 .0 3 1 .5 3 2 .6 2 0 .8 4 1 .8 3 1 .6 15-3 9 .0 8 .2 2 0 .0 3 5 .7 2 7 .4 9 -8 8 .0 19 A 0 .0 0 .1 0 .1 0 .2 0 .4 0 .1 ‘ 0 .1 1 .3 1 .2 2 .1 3 .0 1-3 3 4 .4 3 5 .8 ?3 ?4•? .8 2 6 .6 17.2 2 1 .7 22.5 2 6 .1 • 21.9 2 1.6 23.27 2 0 .7 1 5 .6 1 5 .2 1 6 .6 0.3 0.3 0 .1 0.3 3 7 .6 4 2 .2 2 3 .4 3 6 .8 1 5 .5 2 6 .7 3 1 .6 2 4 .2 2 6 .4 28.2 7 .0 23.2 2 0 .6 2 2 .7 8 .0 1 8.9 0 .2 0 .2 0 .1 0 .2 Other In s e c ts O stracods 0.3 0 .4 0 .5 0 .1 0 .0 0 .4 0.3 0 .5 0 .5 0.3 0 .0 0 .4 0 .7 0 .6 0 .6 0 .8 0 .1 0 .6 0 .0 2 .4 2 .0 1 .8 0-.3 0 .4 0 .4 0 .4 0 .4 0 .6 0 .2 0 .5 0 .4 0 .7 0 .6 0 .6 1 .9 2 .3 0 .2 1 .9 0 .4 0.3 0 .1 0 .3 - 0 .6 0.3 0 .0 0 .4 0 .3 0 .8 0 .6 0 .5 Midges 35 t o a g r e a t e r e x t e n t i n G u ll L a k e , n e c e s s i t a t i n g a c h an g e t o l a r v a l fo o d so u rc es. L ack ey ( 1 9 6 9 ) , w o rk in g on a M iln e L a k e , d i d n o t f i n d t h a t z o o p la n k to n t o t a l l y d i s a p p e a r e d fro m t h e d i e t , a s d i d B u r b id g e , b u t z o o p la n k to n num bers d i d d e o r e a s e a n d i n s e c t s a n d I s o p o d s made u p a g r e a t e r p e r c e n ta g e o f t h e fo o d . Xn L ak e M ic h ig a n , t h e j u v e n i l e s m e lt (u p t o a b o u t 6 in c h e s i n l e n g t h ) f e d m a in ly on c o p e p o d s , o l a d o c e r a n s , an d o s t r a o o d s . I h e s e d a t a a g re e s u b s t a n t i a l l y w ith th o s e o f K e n d a ll ( 1 9 2 7 ) , G reene ( 1 9 3 0 ) , an d Gordon ( 1 9 6 1 ) . A d u lt s m e lt w e re fo u n d t o consum e M r s is . P o n t o p o r e ia . i n s e o t s , a n d f i s h ( p r i m a r i l y y o u n g - o f te n o f t h e i r own s p e o i e s ) i n a d d i t i o n t o t h e c o p e p o d s, c l a d o c e r a n s , a n d o s t r a o o d s . T h is a g r e e s w i t h d a t a o f S c h n e b e rg e r (1 9 3 7 )* B ald w in ( 1 9 5 0 ) , a n d F e rg u so n (1 9 6 5 )* The d i e t o f t h e s m e lt d o e s v a r y s e a s o n a l l y i n t h e G r e a t L a k e s , a n d t h i s v a r i a t i o n f o l lo w s t h e v a r i a t i o n i n fo o d s t o c k p o p u l a t i o n s , s u g g e s t i n g t h a t , s m e lt p io k t h e m ost a b u n d a n t fa u n a a v a i l a b l e . S m e lt a b u n d an c e a l s o seem s t o b e k e y e d t o d e p th b u t m a in ly b e c a u s e s m e lt w e re o n ly r a r e l y o a u g h t b e lo w 5 0 m e t e r s , and th e n o n ly v e r y e a r l y i n t h e s p r i n g . O f t h e 1712 s m e lt c a u g h t , 1232 c o n ta in e d fo o d i n a i d e n t i f i a b l e s t a t e , 266 w e re e m p ty , a n d 3 1 4 c o n ta in e d u n id e n tif ia b le fo o d . I h e d a t a a r e p r e s e n t e d i n T a b le s 4 a an d 4 b . Food H a b its o f t h e P e rc h L ake M ich ig an p e r c h a t t a i n g r e a t e r l e n g t h s an d w e ig h ts th a n a n y o f th e o t h e r 3 s p e o i e s o f f i s h s t u d i e d . I n d i v i d u a l s sa m p le d d u r i n g t h e 3 y e a r s r a n g e d fro m 2*5 t o 1 6 .1 in o h e s i n l e n g t h . Of t h e 3462 p e ro h s a m p le d , 2598 o f t h e s e c o n ta in e d i d e n t i f i a b l e f o o d , 488 c o n ta in e d u n i d e n t i f i a b l e fo o d an d 3 7 6 c o n ta in e d no f o o d a t a l l . As w i t h t h e o t h e r 3 s p e o i e s , a v e ra g e c o n te n ts v a r i e d o v e r b o t h s t a t i o n a n d d e p t h , b u t am ple num bers a llo w e d c l o s e r a p p ro x im a tio n s th a n i s a v a i l a b l e i n m o st s t u d i e s , T h a rra tt Table 4a. Percent Frequency of Occurrence of Various Food Items in the Stomach of the Smelt, Osmerus mordax. # of F ish Item Pontoporeia Leneth 6 .0 i n . 106 1 8 .0 6 . 0- 6 .9 26.0 407 7*0- 7 .9 385 25*5 242 8 . 0- 8 .9 33*0 9*0 3 4 .0 ?2 1W eighted Average 27.2 Season S p rin g 26.5 665 Summer 346 28.3 F a ll 221 2 7 .1 M felghted Average 1 2 7 .1 Denth 0-20 m 438 31.3 20-50 m 718 3 6 .4 50 m 13.3 76 | W eighted Average 1 33*3 F ish Eggs and Frv Midges Other In s e c ts O stracods 0 .0 6 .4 1 2 .6 1 3 .8 2 4.0 1 0 .6 1 .0 8 .2 1 0 .4 1 0 .7 1 3 .6 9 .2 1 6 .0 1 8 .5 1 8 .0 27.6 2 9 .5 2 0 .8 9 2 .0 7 3 .2 61.8 4 5 .6 4 7 .4 6 4 .1 0 .7 1 .1 0 .0 0 .7 1 4 .7 1 4 .6 1 0 .1 1 1 .5 1 0 .6 4 .3 9 .9 1 6 .1 2 5 .6 2 4 .0 2 0 .2 74.3 4 7 .6 7 0 .1 5 5.2" 0 .6 1 .0 0 .0 0 .8 1 5 .6 1 7 .2 2 .6 1 5 .8 14.3 1 0 .8 1 .3 ^*5„. 2 2 .6 2 7 .6 15 .5 2 5 .1 7 5 .4 6 8 .6 4 8 .0 69.8 K vsis Copepods C ladocerans d am s 9 .0 2 7 .0 3 1 .5 3 7 .6 4 1 .0 3 0 .1 9 7 .0 83.5 7 4 .4 60.8 44.9 74.6 9 5 .0 77.8 66.5 61.0 4 0 .2 6 9 .8 0 .0 0.3 0 .6 0 .8 1 .1 O.J 27.3 17.9 2 5 .4 2 5 .4 97.0 5 8 .2 61.2 79.8 7 9 .6 5 1 .6 73.1 70.7 1 2 .1 3 1 .7 2 5 .8 2 4 .4 9 6 .1 84.3 3 6 .0 85.6 8 5 .6 8 0.2 4 8 .5 8 0 .2 Table 4b. Percent Volume o f Various Food Items in the Stomach of the Smelt, Osmerus mordax. f o£. Fish' P ontoporeia Item Length < 6 .0 i n . 106 407 6 . 0- 6 .9 381 7 .0 - 7 .9 242 8 . 0- 8 .9 >9.0 93 W eighted Average Season - . - . . S pring 665 Summer 343 F a ll 221 W eighted Average Depth 0-20 m 428 20-50 m 727 >50 m 7* W eighted Average 1 .1 3 -5 7.3 1 1 .8 1 8 .6 7 .1 (Hawra F ish Eggs a n d 'F ry Midges Other I n s e c ts O stracods 24.6 0 .0 0 .1 0 .2 0.3 0 .4 0 .2 0 .0 0 .6 3 .4 4 .6 0 .5 2 .7 0 .2 1 .6 3 .7 3 .8 4 .3 2 .7 3 .6 3 .5 4 .8 6 .7 6 .9 4 .8 24.3 2 4 .6 21.3 1 6 .5 1 3 .9 2 1 .1 Mvsis Copepods 1 .7 2 .6 1 1 .4 1 0 .1 1 2 .4 7 .4 3 7 .0 3 4 .0 2 6 .2 2 4 .7 2 0 .5 28.9 3 2 .1 3 0 .5 2 1.7 1 9 .5 *. ................. C ladocerans . 1 2 .5 4 .2 8 .8 9 .5 8 .4 ‘6 .2 8 .2 7 .7 3 0 .2 19.9 3 5 .4 28.2 28.2 19-3 23.3 24.8 0 .1 0.3 0 .2 0 .2 4 .8 5 .1 1 .3 4 .0 2 .4 3 .6 0.3 2 .3 3 .8 9 .4 3 .3 5 .3 1 1 .0 3 2 .0 17.3 1 8 .0 9 .1 8 .1 8 .0 8 .4 4 .8 5 .1 1 2 .9 3 0 .6 3 2 .7 2 2 .2 3 1 .3 23-3 26.1 1 8 .5 2 4.6 0 .2 0.3 0 .1 0 .2 6 .4 3 .5 0.3 4 .3 3 .3 3 .2 1 .6 3 -1 5 .2 6 .3 5 .0 5 .8 1 5 .0 1 7 .6 3 0 .7 17-5 (1 9 5 9 ) s t u d i e d p e r c h fro m S a g in aw B ay, a s d i d E l- Z a r k a (1 9 5 9 ) "but l a r g e v a r i a n c e a n d s m a ll sa m p le s i z e s i n t h e i r s t u d i e s made m e a n in g fu l a n a l y s i s d iffic u lt. E schm eyer ( 1 9 3 8 ) , W e lle r ( 1 9 3 8 ) , J o b e s (19 5 2 )# a n d o t h e r s h a v e s u g g e s te d t h a t y e llo w p e r c h s c h o o l b y se x e s* A lth o u g h t h e d a t a a s p r e s e n t e d i n T a b le 5 a a n d 5 8 do n o t show t h i s , t h e s u g g e s t i o n a p p e a r s t o have some v a l i d i t y i n p e r c h g r e a t e r th a n 6 in c h e s i n l e n g t h . T hese o l d e r p e rc h (2 + y e a r s ) - a p p e a r e d t o c h o o se d i f f e r e n t fo o d o r g a n is m s , fe m a le s t a k i n g a g r e a t e r p e r c e n t a g e o f f i s h a n d m a le s t a k i n g a h i g h e r p e r c e n ta g e o f m id g es a n d l a r g e c r u s ta c e a n s * I n p e r c h l e s s t h a n 6 .0 i n c h e s l o n g , z o o p la n k to n a n d i n s e c t l a r v a e w ere o f t h e g r e a t e s t im p o rta n c e * T h a r r a t t (1 9 5 9 ) fo u n d i n s e c t l a r v a e t o compose t h e g r e a t e s t f r a c t i o n i n h i s s t u d i e s , b u t a l l h i s s t a t i o n s w ere i n s h a llo w w a te r ( 6 - 2 1 m e te r s i n d e p th ) an d a l l sa m p lin g w as d o n e w i t h b o tto m t r a w l s . I n t h e p r e s e n t s t u d i e s , z o o p la n k to n , b o t h co p ep o d s a n d c la d o c e r a n s , w ere fo u n d t o b e o f t h e g r e a t e s t im p o r ta n c e , p a r t i c u l a r l y i n f i s h l e s s th a n 4 .0 in c h e s i n l e n g t h . I n f i s h 4 .0 t o - 6 .0 i n o h e s : i n l e n g t h , o t h e r o rg a n is m s (M v s is . P o n to p o re ia * Gammarus* c la m s , s n a i l s , o s t r a c o d s , a n d s m a ll f i s h ) b e g a n t o b e fo u n d i n i n c r e a s i n g n u m b ers, a lth o u g h o la d o o e r a n s a n d c o p e p o d s s t i l l form ed a l a r g e p o r t i o n o f t h e d ie t. I n p e r c h g r e a t e r t h a n 6 . 0 in c h e s i n l e n g t h , z o o p la n k to n beoome a l e s s im p o r t a n t f a c t o r i n t h e d i e t , w ith t h e e x c e p tio n o f some L lm noo alan u s and L e p to d o r a . M v sis a n d P o n to p o r e ia become m ore i m p o r t a n t , e s p e c i a l l y t o t h e m a le s ; a n d f i s t } p a r t i c u l a r l y m innow s, young a le w i v e s , young s m e l t , and young p e r c h , i n o r e a s e i n im p o r ta n c e i n t h e fe m a le f i s h . The minnows r e f e r r e d t o h e r e i n c l u d e t h e e m e ra ld s h i n e r , N o tr o p is a t h e r n o r d l s . i n 132 p e rc h sto m a c h s ; s p o t t a i l s h i n B r , N o tr o p is h u d s o n iu s « i n 168 p e r oh stom aohs 39 n in e s p i n e s t i c k l e b a c k , P r in g ltiu s r r u n g i t i u s « i n 334- p e r c h sto m a c h s; a n d some u n i d e n t i f i a b l e f i s h i n 85 p e r c h sto m a c h s. M ost i d e n t i f i a b l e f i s h re m a in s h ow ever w ere e i t h e r y o u n g s m e lt ( p r i m a r i l y i n l a t e summer, i n 4 5 6 p e rc h s to m a c h s ) , young a le w iv e s ( m a in ly i n l a t e sum m er, i n 353 p e r c h s to m a c h s ) , an d young p e rc h ( c h i e f l y i n summer a n d f a l l , i n 412 s to m a c h s ) . Of sto m a c h s c o n ta in i n g re m a in s w h ic h w ere i d e n t i f i a b l e f i s h i n o r i g i n ( 9 7 1 ) , 6 5 4 w ere fro m fe m a le s a n d 317 w e re fro m m a le s ,, a b o u t 2 : 1 , e v e n th o u g h m a le s o u tn u m b e red fe m a le s a p p ro x im a te ly 1 .5 * 1 * C o n v e r s e ly , m id g e s , P o n to p o r e ia . Gammarus, a n d M vsis w e re p r e s e n t i n g r e a t e r p e r c e n ta g e i n m a le s th a n i n fe m a le s (5 7 $ o f a l l m a le s v s . 3 1 $ o f a l l f e m a l e s ) . a g r e e s i n p r i n c i p l e , w i t h th e f i n d i n g s o f T h a r r a t t (19 5 9 )* T h is The e v id e n c e s u g g e s t s t h a t o l d e r fe m a le p e r c h f e e d i n b o th open w a te r a n d on t h e b o tto m w h ile m ale p e r c h te n d t o f e e d m a in ly on t h e b o tto m . H its d i f f e r e n c e i n f e e d i n g h a b i t s c o u ld a l s o e x p l a i n d i f f e r e n c e s i n num bers fo u n d a t i n s h o r e s t a t i o n s , w here f e m a le s o f t e n p re d o m in a te d , i f t h i s movement was k e y ed t o movement o f a fo o d s o u r c e (minnows o r s m e lt f o r e x a m p le ). L a b o r a to r y F e e d in g E x p e rim e n t F o u r h u n d re d an d e i g h t y f i s h , 1 2 0 o f e a c h o f t h e f o u r s p e o ie s o f i n t e r e s t , w ere g iv e n fo o d s t o c k s ta k e n from L ake M ic h ig a n on w h ic h t o f e e d t o d e te r m in e i f f e e d i n g i n t h e l a k e was b a s e d on s e l e o t i o n o f a p a r t i c u l a r fo o d s o u r c e o r m e r e ly a v a i l a b i l i t y . R e s u l ts a g r e e f a i r l y c l o s e l y w i t h th o s e o f f i s h sa m p le d i n t h e l a k e , w ith t h e e x c e p tio n t h a t P o n to p o r e ia an d M vsis make u p a s l i g h t l y l a r g e r p e r c e n ta g e u n d e r t h e t e s t c o n d i t i o n s th a n i n t h e l a k e f o r a l l s p e o i e s t e s t e d , im p ly in g a p o s s i b l e l a c k o f a v a i l a b i l i t y o f t h i s fo o d s o u r c e , a t l e a s t f o r f i s h o f t h i s s i z e , in th e la k e . S e c o n d ly , o s t r a c o d s make u p a Table 5a. Percent Frequency of Occurrence of Various Food Items in the Stomachs of the Perch, Perea flavescens. t of Item F ish •- l e n g t h <6 .0 i n . 316 6. 0- 6 .9 665 7 . 0- 7 .9 753 8 . 0- 9 .9 642 > 1 0 .0 in 222 Weighted Average Season S pring 473 Summer 1561 F a ll 564 W eighted Average Depth 0-20 m 1795 20-50 m 635 > 50m 168 Weighted Average Pontoporeia . Mysis Copepods 23.0 61.0 68.0 63.0 6 9 .0 5 9 .6 1 8 .0 5 2 .0 5 3 .0 5 3 .5 4 8 .2 4 8 .2 9 5 .0 4 8 .0 3 2 .0 1 5 .0 1 2 .0 J 7 .8 68.5 46.3 56.2 J 2 .5 61.6 1 7 .4 55.0 3 1 .8 48.3 60.2 62.5 52.1 18.8 4 8 .6 6 6 .6 29.2 C ladocerans Clams F ish Eggs and Fry Midges Other In s e c ts O stracods 9 1 .0 4 3 .0 2 6 .0 1 2 .0 8 .0 3 4 .5 2 ;o 5 .5 8 .6 9 .0 1 0 .4 lS .9 1 4 .6 3 4 .5 3 7 .6 4 6 .8 53.0 3 0 .2 65 .0 4 9 .0 4 3 .0 3 4 .0 3 2 .0 4 4 .0 61 .0 5 3 .0 4 4 .0 3 6 .0 2 8 .0 4 5 .0 1 5 .0 2 2 .6 19 .5 1 8 .5 17.6 2 4 .5 4 3 .7 3 4 .8 4 3 .0 38.2 4 6 .1 29.2 3 5 .1 3 3 .5 6 .0 1 1 .4 4 .5 8*9 3 1 .4 48.3 32.5 3 7 .6 3 1 .5 64.3 3 7 .0 5 2 .4 48.8 4 7 .9 3 6 .5 4 5 .6 2 1 .4 1 6 .5 1 7 .9 1 9 .0 5 5 .2 5 1 .6 19.7 51.7 4 9 .2 4 1 .8 1 6 .4 4 6 .0 U '.8 ~ 7 .6 2 .5 10.7 3 6 .3 tt.7 4 4 .2 21-JL 6 0 .5 5 1 .2 2211 55-7 51.8 50.3 2 1 .1 5 6 .4 23.8 1 8 .9 1 3 .1 2 1 .9 . Table 5b. Percent Volume of Various Food Items in the Stomach of the Perch, Perea flavescens. # of F ish Pontoporeia Item Length 4 .0 < 6 .0 i n . 3 l6 1 0 .4 6 .0 - 6 .9 665 14.8 7 .0 - 7 .9 753 642 8 . 0- 9 .9 1 7 .9 > 1 0 .0 in 222 20.3 W eighted Average 13.6 Season 13.6 Spring 473 1561 Summer 1 3 .4 564 F a ll 13.5 W eighted Average 1 3 .5 Depth 0-20 m 1 0 .1 1795 20-50 m 15.2 635 >50 m 168 1 5 .2 W eighted Average 1 1.7 Other I n s e c ts 4 .8 27.6 3 3 .7 40.2 4 8 .8 3 1 .5 _ 6 .0 6 .6 5 .8 5 -4 5 .2 5 .9 _ 5 .7 5 .0 4 .6 4 .0 4 .0 4 .6 3 .0 4 .5 4 .5 4 .3 4 ,3 4 .3 _ 2 .3 1 .1 1 .4 1 .4 2 6 .2 3 6 .8 3 0 .0 3 3 .4 5 .7 5 .5 5 ,5 5 .5 4 .0 4 .6 5 .2 4 .6 3 .8 4 .2 4.D 4 .1 2 .2 1 .3 1-3 2 8 .7 28.2 3 6 .1 2 9 .1 6 .8 5 .0 4 .0 6 .2 6.3 4 .0 4 .0 2 .9 5 .1 Copepods 3 .0 9 .6 13.9 16.9 1 8 .7 1 2 .6 3 8 .0 1 8 .6 10.6 5 .0 3 .0 13.9 3 5 .0 1 6 .4 9 .5 4 .4 3 .2 1 2 .6 0 .5 1 .3 1 .7 1 .8 2 .6 1 .6 1 3 .4 1 1 .6 1 2 .2 1 2 .1 16.5 13.2 14.3 14.3 12.7 1 4 .1 13.3 1 7 .5 18.6 8 .9 17.2 1 7 .5 1 5 .1 8 .5 16.3 7 .9 1 1 .8 1 7 .5 9-5 Cladocerans _ F ish Eggs and Fry Midges Jfrsis Clams i-9 3*5 5 .6 O stracods 5-8 , k2 sm aller volume in sm elt stomachs under experim ental co n d itio n s than in the la k e. This may or may not be s ig n if ic a n t . The peroent volume in perch stomachs d iffe r e d in lab and lak e data but t h is could be due to the lack o f sm all f is h fr y as an a lte r n a tiv e food souroe. In the absence o f f i s h eggs and f r y in t h is experim ent, some organisms may have been preyed on more h e a v ily than oth erw ise, but t h is food source does n ot appear to be very important in f i s h l e s s than 6 .0 inches in le n g th , as determined in n atural p o p u la tio n s, and should not have created too g reat a discrepancy. The r e s u lts o f th e experiment are li s t e d in Tables 6a and 6b. In terms o f e f f ic ie n c y o f acquiring th ese fo o d s, the a lo w ife i s more e f f ic ie n t than the other three sp e c ie s as shown by a comparison o f the amount o f food consumed per u n it tim e. These data are presented in Table 6c. Given the con d itions o f the experim ent, alew ives were more e f f i c i e n t predators on zooplankton and the la rg e orustaoeans Mvsis and Pontoporeia than were the other s p e c ie s . The a le w ife a ls o took greater proportions o f the major copepods and oladocerans. B loaters took the g r e a te st volume o f midge larvae in the stud y, sm elt the g r e a te st p ortion o f o stra co d s, and perch, given th ese c o n d itio n s, were g e n e r a lis ts in th e ir food gath erin g. Given a d iffe r e n t age group and d iffe r e n t i n i t i a l food sou rce, the percentages are certa in to be d if f e r e n t , but t h is experiment g iv e s some idea o f the food preferences fo r t h is age and s iz e group and shows where competition' could e x i s t . S e le c tiv e predation on zooplankton by the a le w ife has been proposed by Brooks and Dodson (1 9 6 5 ) and Smith (1 9 7 0 ) • These in v e s tig a to r s in ferred s e le c t iv e predation on the b a sis o f numbers o f various zooplankton Table 6a. Percent Frequency of Occurrence of Various Food Items in the Stomachs of Four Fish Species. # of F ish 120 120 120 120 S p ecies B lo a te r A lew ife Sm elt Perch Table 6b . Pontonoreia 8 .0 4 6 .0 23.0 26.0 Mys i s 9-3 1 4 .7 1 2 .0 2 3 .0 Copepods 93.0 97.0 97.0 9 5 .0 Cladocerans 8 6 .0 9 2 .0 95.3 8 9 .5 Clams 1 .0 1 .0 0 .0 3 .4 Midges 21.2 4 .2 4 .6 6 1 .0 Other I n s e c ts 4 .6 4 .0 3 .5 5 8.3 O stracods 3 .8 6 .0 9 2 .0 3 7 .0 P e rce n t Volume o f V arious Food Item s in th e Stomachs o f Four F is h S p e c ie s. S pecies B lo a te r A lew ife Sm elt Perch # of F ish 120 120 120 120 Table 6c . Percentage o f T o ta l Volume o f a l l Stomachs (480) Sampled C ontaining a P a r tic u la r Food Source. Species B lo ater A lew ife Sm elt Perch t of F ish 120 120 120 120 Pontoporeia 7 .5 12.9 5-9 7 .8 P ontoporeia 1 .6 4 .0 1 .0 1 .2 Mysis 8 .2 1 0 .6 4 .6 6 .1 Mysis 2 .0 3 .4 1 .1 1 -5 Copepods k6. 0 3 9.5 3 4 .4 34.0 Copepods 1 0 .1 12.3 7 .4 7 .4 Cladocerans 22.8 3 4 .1 3 2 .0 3 1 .6 C ladocerans 5 .6 8 .7 7 ,0 7 .1 Clams <1 <1 0 .0 1 .1 Clams <0 .1 <0 .1 0 .0 0.25 Midges 1 2 .2 0 .6 3 .6 6 .8 Midges 3-3 1 .0 1 .0 1 ,3 _ Other I n s e c ts 1 .0 0 .5 3 .2 6 .4 O ther I n s e c ts 0.25 0 .1 1 .0 1 .4 O stracods 2 .4 1 .5 16.3 8 .2 O stracods 0 .6 1 .2 4 .0 2 .7 T o ta l 2 3 .6 3 0 .8 2 3 .5 2 2 .9 fo rm s b e f o r e i n t r o d u c t i o n o f t h e a l e w i f e . The c u r r e n t s e t o f s t u d i e s t e n d t o i n d i c a t e , on t h e b a s i s o f sto m ac h a n a ly s e s a s com pared t o z o o p la n k to n c o u n ts i n t h e l a k e , t h a t n o t o n l y a le w i v e s , b u t a l s o b l o a t e r s , p e r c h a n d s m e lt a r e s e l e c t i v e z o o p la n k to n p r e d a t o r s , b o th i n t h e l a b o r a t o r y .i h d i n th e la k e . Tho J u s t i f i c a t i o n f o r t h i s s t a te m e n t l i e s i n t h e f o l l o w i n g : A v e rag e num bers/m ^ o f 1 5 z o o p la n k to n a n d l i m n e t i c o r u s t a o e a s p e o i e s w e re t a b u l a t e d fro m T a b le 1 1 an d t h e a v e ra g e volum e / i n d i v i d u a l ( i n m ap) w as fo u n d b y two m e th o d s, i d e a l i z e d c a l c u l a t i o n s b a s e d on i n t e g r a t i o n o f b o d y l e n g t h a n d d ia m e te r a n d b y w a te r d is p la c e m e n t. The a v e ra g e volum e i s g i v e n i n colum n A o f T a b le 7 . I n t h e s e c o n d oolum n o f T a b le 7 i s a f i g u r e r e p r e s e n t i n g t h e a v e r a g e num ber o f i n d i v i d u a l s o f e a c h s p e o i e s p e r on a y e a r l y b a s i s . T h is f i g u r e f l u c t u a t e s t o a g r e a t d e g re e on a m o n th ly b a s i s b u t oolum n B g i v e s t h e a v e r a g e /n P /y r * Column C o f T a b le 7 i s t h e p r o d u o t o f oolum ns A tim e s colum n B a n d r e p r e s e n t s t h e a v e r a g e d e n s i t y i n m m ^/m ^/yr t h a t a n y one s p e o i e s o o o u p ie s in th e la k e . On t h e b a s i s o f oolumn C, z o o p la n k to n on a y e a r l y b a s i s r e p r e s e n t s a p p r o x im a te ly 24-32 m nrtyra^/yr, w h e re a s H v s is an d P o n to n o r e ia r e p r e s e n t o n ly 80.4- m m ^ /n ^ /y r, a r a t i o o f a p p r o x im a te ly 3 0 : 1 . H ow ever, i n a o t u a l sto m ach a n a l y s e s , Msrsis a n d P o n to p o r e ia oompose 2.4- tim e s t h e volum e o f z o o p la n k to n i n t h e b l o a t e r , 1*5 tim e s a s much i n t h e a l e w i f e , 92$ a s much i n t h e s m e lt , an d 3 0 $ a s much i n t h e p e r c h . E ven i n t h e p e r c h , t h e l a r g e l i m n e t i c o r u s ta o e a compose 10 t im e s a s much volum e a s w o u ld b e e x p e o te d b y random c h a n c e a l o n e . On t h i s b a s i s t h e n , s e l e c t i v e p r e d a t i o n on p a r t i c u l a r fo o d ite m s c an b e i n f e r r e d a n d t h e h y p o t h e s i s o f B ro o k s an d Dodson ( 1 9 6 5 ) , . an d S m ith ( 1 9 7 0 ) , seem s v a l i d . 45 T a b le 7* Food s o u r c e s i n L ake M ic h ig a n - Z o o p la n k to n a n d L im n e tic C r u s ta c e a : A v e ra g e Volume p e r i n d i v i d u a l , A v e rag e Number o f I n d i v i d u a l s P e r C u b ic M e te r o f L ake W a te r, A v e ra g e Volume o f E ach S p e o ie s P e r C ubic M e ter o f W a te r On A Y e a r l y B a s i s , Ave. Volume (mm?) Daphnia g a le a ta D. retrocurua D. lon girem is Episohura la c u s t r is Diaptomus s i c i l i s D. ash lan d i Polyphemus p ed icu lu s Cyclops b icu sp id a tu s Mosooyclops edax Diaphenosoma brachyurum Leptodora k in d t ii Limnocalanus maorurus M /sis r e lio t a Pontoporeia a f f i n i s Bosmina lo n e ir o s t r is 1 .8 1 .8 1 .3 0 .5 0 .8 2 0 .2 5 0 .1 5 1 .1 1 .4 0 .8 2 3 -9 2 2 .0 3 4 .5 3 4 .5 0 .0 5 Ave. # / n r On A Y early B asis Density-Volume U nits Ave. V o l. / n r On A Y earlv B asis 1 2 5 .0 1 4 1 .0 1 .5 2 1 .0 4 4 3 .0 1 0 1 .0 9 .2 6 0 7 .0 7 2 .5 1 .8 6*5 3 7 5 .0 2 3 .3 2 3 .3 2 2 5 .0 2 5 3 .8 2 .0 1 0 .5 3 6 3 .3 2 5 .3 1 .3 8 6 6 7 .7 1 0 1 .5 1 .5 2 5 .5 7 5 0 .0 8 0 .4 8 0 .4 4 .6 9 3 .0 T o ta l Z o o p la n k to n =* 2432 V o l . / n ? / y r . T o ta l l'fy sis & P o n to p o r e ia ~ 8 0 .3 8 5 V o l . / n ? / y r . k6 Z o o p la n k to n H is to ry V ery fe w p a p e r s h a v e b e e n p u b l i s h e d on l i m n e t i c o r u s t a o e a i n l a k e M ic h ig a n . E a r l y w orks ( B ir g e 1 8 8 2 , 1 8 9 4 ; F o rb e s 1 8 8 2 ; Maroh 1 8 9 5 ) m a in ly c o n s i s t o f b r i e f a c o o u n ts o f c e r t a i n s p e o i e s . N ot u n t i l t h e p a p e r s o f Eddy (1 9 2 7 ) was a n y th o ro u g h a n a l y s i s a tte m p te d . I b i s w ork s to o d a lm o s t a lo n e u n t i l W e l l 's s t u d i e s w e re p u b l is h e d i n i 9 60 a n d 1 9 6 8 , s t u d i e s w h ich p r o v id e much o f t h e b a c k g ro u n d f o r t h i s p a p e r . R e s u l ts a n d D is c u s s io n T a b le 1 1 g i v e s z o o p la n k to n c o n c e n t r a t i o n i n n u m b e rs /m e te r^ f o r 15 s p e c i e s o f z o o p la n k to n i n L ake M lo h ig an (n u m b e rs/1 0 m e t e r s ^ f o r P o n to p o r e ia a f f i n i s a n d M ysis r e l i o t a ) • A lth o u g h i n d i f f e r e n t f a m i l i e s , P o n to p o r e ia an d M ysis show s i m i l a r d i s t r i b u t i o n p a t t e r n s ( B e e to n , 1 9 5 7 ) en d a r e lum ped i n T a b le 1 1 . T h ese num bers a r e a v e r a g e s o v e r a l l d e p th s a n d s t a t i o n s a n d do n o t r e f l e o t t h e t r u e d i s t r i b u t i o n a t tim e o f c a p t u r e . S in o e W e l l 's i 960 p a p e r g o e s i n t o d e t a i l on d e p th d i s t r i b u t i o n , 1 o m itte d t h e s e d a t a h e re . Of t h e 3 5 s p e o i e s i d e n t i f i e d (T a b le 8 ) , 19 show ed v e r t i c a l m ig r a tio n on a d i e l b a s i s , f u r t h e r c o m p lic a tin g d i s t r i b u t i o n p a t t e r n s . The 15 s p e c i e s shown i n T a b le 1 1 show o h an g es c o n s i s t e n t w i t h t h o s e fo u n d b y W e lls i n te rm s o f n u m b e rs /m e te r^ d u r i n g t h e v a r i o u s m onths o f t h e y e a r . They a l s o show c h a n g e s i n f r e q u e n o y on a y e a r t o y e a r b a s i s ( T a b le U ) . Forms t h a t d e o l i n e d s h a r p l y fro m 1954 t o 1966 (W e lls X 9 7 0 ), an d a r e now i n c r e a s i n g i n n u m b e rs, a r e t h e l a r g e s t o la d o o e r a n s ( L e p to d o ra k i n d t i i . D ap h n ia g a l e a t a . a n d D ap h n ia r e t r o o u r v a ) . t h e l a r g e s t o a la n o id co p ep o d s (L im nonflla n u s m a o ru ru s . E p is o h u r a l a o u s t r i s . a n d D iap to m u s s i o i l i s ) , t h e l a r g e s t c y o lo p o id oopepod (M sso o v o lo p s e d a x ) , t h e m y sid M y sis r e l i o t a . an d 47 T a b le 8 . Z o o p la n k to n a n d L im n e tic C r u s ta c e a fro m L ake M ic h ig a n 1 9 7 0 -1 9 7 2 (S a u g a tu c k , L u d in g to n , a n d B ig S a b le P 6 i n t ) Mean L e n g th (mm) Range o f L e n g th s (mm) 0 .9 1 0 .6 0 - 1 .2 0 Cyclopoid Copepods Cyclops b icu sp id atu s Cyclops v e r n a lis Mesocyclbps edax Tropolycyclops parasim is Eucyclops a g i l i s D.9X 0.66- 1.13 1 .1 5 * * 0 .8 7 - 1 .3 0 C a la n o id Copepods D iaptom us a s h l a n d i i D iaptom us o i n u t u s D iap to m u s o r e g o n e n s is D iaptom us s i o i l i s L im nocalrrus m ao ru ru s E u ry te m e ra a f f i n i s E p is c h u r a l a c u s t r i s S e n e c e lla c a la n o id e s 0 .8 5 0 .8 3 1 .0 8 1 .2 5 2.4-5 1 .0 7 1 .7 0 2 .8 9 0 . 680 .6 5 0 .9 5 1 .0 7 2 . 00. 0 . 920 .8 7 2 . 00- O.9 6 0 .9 2 1 .1 7 1 .4 0 2 .6 5 1 .1 8 1 .9 8 3 .2 6 0 .4 5 0 .4 8 1 13 1 .1 3 0 .8 5 0185 0 .8 4 0 .5 5 0 .6 0 * 0 .7 0 0 .5 6 0 .2 7 0 .2 7 0 .6 5 0 . 600. 500 .3 8 0 .4 2 0 .3 7 0 .4 0 ♦ 0. 450. 452 .36* O.5 6 0 .6 9 1 .8 7 1 .8 5 1 .6 1 1 .0 6 1 .0 0 0 .7 0 0 .7 5 C la d o c e ra B osm ina l o n g i r o s t r i s Bosm ina c o re g o n ia D ap h n ia g a l e a t a m en d o tae D ap h n ia r e t r o a u r v a D ap h n ia l o n g ir e m i s D iaphanosom a b rac h y u ru m D iaphanosom a le u o h te n b e rg ia n u m C e r io d a p h n ia q u a d ra n g u la C e rio d a p h & ia l a c u s t r i s E u r y c e rc u s l a m e l l a t u s H olopedium g ib b e r u a Polyphem us p e d i c u iu s L e p to d o ra k i n d t i i C hydorus s p h a e r i c u s S id a o r y s t a l l i n a . 5-00 * * 1 .0 0 0 .9 6 5 .8 0 Amphipoda P o n to p o r e ia a f f i n i s ^ •1 1 .7 0 7 . 00- 1 3 .0 0 Ify s id a c e a Ify s is r e l i o t a = 0 2 .4 0 8 .0 0 - 1 4 .0 0 V ery r a r e s p e c i e s D ap h n ia s o h o d l e r i D ap h n ia am bigua S c a p h o le b r is a u r i t a S im n o c e p h a lis s p . M oina m io r u r a * * ♦ * * * * * * * * Too fe w c a p t u r e d T or a d e q u a te e s t i m a t i o n o f mean l e n g t h o r ra n g e , 48 the araphipod Pontoporeia a f f i n i s . Of th e se Daphnia g a le a ta and Meso cryc l ops edax became extrem ely rare by 19 66, but are now p resen t in numbers equal to th e 1954 popu lation s i z e . Some o f th e medium and sm all s p e o ie s, which in oreased in numbers from 1 9 5 4 to 1 9 6 6 , and which are now on th e d e c lin e » are th e oladocerans Daphnia lo n g ir e m is. Polyphemus p ed icu lu s, and Bosmina lo n g ir o s t r is « and th e oopepods Cyolops b iouspid atus and Diaptomus a sh la n d i. Some o f the m iddle s iz e d forms show the same pattern as th e la r g e r form s, but on a sm aller so a le ( i . e . Diaphanosoma brachyurum) . Hie zooplankton com position o f 1972 has s h if t e d baok towards th a t o f 1 9 5 4 . F ig u r e ? show s a n o th e r f a o e t o f t h i s c h a n g e . The a v e r a g e s i z e , an d s i z e a t o n s e t o f m a t u r i t y , o f D a p h n ia r e t r o c u r v a h a s a l s o ch an g e d th r o u g h th e; y e a r s . I h e 1972 p o p u l a t i o n o f t h i s s p e o i e s h a s s h i f t e d fro m t h a t o f 1966 t o r e s e m b le m ore c l o s e l y t h a t o f 1 9 5 4 . P h y to p la n k to n Of a l l s p e c i e s o f p h y to p la n k to n i d e n t i f i e d fro m L ake M ic h ig a n , (T a b le 9 ) o v e r 9 9 $ w e re d ia to m s . D u rin g t h e t h r e e - y e a r c o u n tin g p e r i o d , 24-71 d ia to m s l i d e s w eref made a n d e n u m e ra te d . B e ca u se o f t h e l a r g e num bers o f s l i d e s i n v o l v e d , a d i r e c t c o u n t o f f r u s t u l e s / m l w as i m p o s s i b l e . C o n s e q u e n tly t h e m ethod o f MoNabb ( i 9 6 0 ) w as u s e d , w i t h t h e f o llo w in g c h a n g e ; s i n c e t h i s m eth o d o a n n o t e s t i m a t e t h e num ber o f f r u s t u l e s f o r c h a in - f o r m in g d ia to m s s u c h a s F r a g i l a r i a a n d T a b e l l a r i a o r t h e num bers i n a " c o m p o s ite * d ia to m s u c h a s A s t e r i o n e l l a . I m u l t i p l i e d t h e num ber o f d ia to m s d e te r m in e d b y MoNabb*s m ethod b y t h e a v e r a g e num ber o f c o m p o s ite p a r t s , o r a v e r a g e c h a in l e n g t h t o o b t a i n a n a v e r a g e f o r num ber o f f r u s t u l e s . Q his i s i n c l o s e a g re e m e n t ( . 0 5 l e v e l o f s i g n i f i c a n c e ) w i t h a o t u a l L en g th d i s t r i b u t i o n o r fe m a le D aphnia r e t r o o u r v a from sa m p le s i n 1 9 5 ^ t 1 9 6 6 , 1 9 7 0 , 1 9 7 1 , an d 1 9 7 2 . (D a rk ly sh a d e d p o r t i o n s r e p r e s e n t m a tu re i n d i v i d u a l s . ) $> Frequency Vn 0 $ Frequency # Frequency H o Vn i> Frequency H O Vn —T VJl □ o> El H © i> Frequency Sji S’ O' OS 1 § P- uJa r //:>// 2 § K Vj\ '/•//// f/ *r/ .7* "Ji r,j £ -J o o UJ I -%3 O g 8n 51 f r u s t u l a c o u n ts . A v erag e num bers a r e g iv e n I n T a b le 9. D ie s e a s o n a l a b u n d an c e o r 12 s p e c i e s b y s e a s o n a n d d e p th i s g r a p h i c a l l y shown i n F i g u r e s 8A^H. C y o lo te lla .b o d a n lc a a n d C y o l o t e l l a oom ta a r e b o t h l i s t e d on one g ra p h a s a sum o f t h e tw o s p e o i e s on t h e b a s i s o f s i z e , s h a p e , an d s i m i l a r i t y o f tre n d s . B o th s p e o i e s a r e fo u n d i n h i g h l y o l i g o t r o p h i o c o n d i t i o n s ( H o lla n d , 1 9 6 8 ) , a s t h e s m a l l e r C y o l o t e l l a s p e o i e s , jf r r tz in g ta n a a n d m lc h lg a n ia n a . b o th o f w h ic h a r e a l s o l i s t e d on one g r a p h . S k v o r tz o v (1 9 3 7 ) r e p o r t e d t h a t Qvni n-fcal-Ta b o d a n ic a w as common among d ia to m s fro m t h e C h icag o c i t y m a in s i n J a n u a r y 1936* a lo n g w i t h C . o o m ta . C. mi nM prar>iana. an d £ • o o e l l a t a . T hese f o r m s , th o u g h s t i l l p r e s e n t i n t h e s o u t h e r n b a s i n , h a v e g iv e n way t o o t h e r fo rm s (Stoe^rm er a n d Y ang, 1 9 7 0 ) . C. m io h ig a n ia n a , a fo rm en d em ic t o l a k e M ic h ig a n , i s a p p ro x im a te ly t w i c e a s common i n t h e n o r t h e r n b a s i n a s i n t h e s o u t h e r n ( H o lla n d , 1969)* As t h e g ra p h s sh o w , a l l o f t h e s e s p e o i e s r e a c h g r e a t e r num bers i n t h e n o r t h e r n b a s i n t h a n i n th e s o u th e rn , in d ic a tin g o le a n e r w a te rs t o th e n o r th . The g ra p h s a l s o i n d i c a t e summer maxima s l i g h t l y l a t e r i n t h e n o r t h e r n b a s i n . A n o th e r t r e n d , n o t shown h e r e i s a te n d e n c y f o r t h e num bers o f o l i g o t r o p h i o fo rm s t o i n c r e a s e i n num bers a s one moved f h r t h e r o f f s h o r e o r f u r t h e r fro m a " p b l l u t i n g ,,. s o u r c e ( i . e . - t h e K alam azoo R i v e r ) . I n c o n t r a s t t o t h e s e h i g h l y o l i g o t r o p h i o f o r m s , one a l s o f i n d s many e u r y t o p i o s p e o i e s , some e u t r o p h i c , a n d some w h ic h a r e fo u n d i n h y p e r e u tr o p h io h a b i t a t s . Among t h e e u r y t o p i o s p e o i e s a r e M e lo s ir a : i s l a n d i o a . F r a g i l a r i a o ro to n e n s is K itto n . v a r . o r o to n e n s ia . n fo rm o sa H a s s . v a r . fo rm o s a . T a b e l l a r i a f e n e s t r a t a ( J y n g b .) K u tz . v a r . fe n e s tra ta . ‘ T a b e l l a r i a f l o o o u l o s a (R o th ) K u tz . v a r . f l o o c u l o s a . S y n e d ra a o u s K u tz . v a r . a o u s . a n d S v n e d ra n a n a M e i s t . • b e d is c u s s e d . E ach o f t h e s e w i l l 52 M e lo s ir a i s l a n d i o a i s a e u r y to p io d o m in a n t fo rm a n d h a s b e e n so f o r many s p e o ie s w h ic h i s t h e u s u a l s p r i n g y e a r s (A h ls tr o m , 1 9 3 6 ) . T h is s p e o ie s i s a b u n d a n t i n o l i g o t r o p h i o l a k e s i n t h e m o u n ta in a n d n o r t h e r n r e g i o n s o f E u ro p e (S to e rm e r a n d Yang* 1 9 7 0 ) , a n d a p p e a r s t o b e f a v o r e d b y low l e v e l s o f e n ric h m e n t, a s i t i s re d u o e d a ro u n d p o l l u t e d h a r b o r s i n L ake M ich ig an a n d i n G reen Bay (S to e rm e r an d Y ang, 1 9 7 0 ) . M. i s l a n d i o a p e a k e d i n l a t e A p r i l - e a r l y May a t S a u g a tu o k a n d i n m id-M ay a t L u d in g to n , d e c re a s in g t o l e s s th a n 1 0 org an ism s/m L b y l a t e J u n e i n b o t h a r e a s . M e lo s ir a i s l a n d i o a i s re p la c e d as t h e d o m in a n t fo rm i n t h e e a r l y summer b y F r a g i l a r i a o r o t o n e n s is a n d A s t e r i o n e l l a f o r m o s a . F . o ro to n e n s is a l s o p e a k s, a t t h e b e g in n in g o f May, b u t d o e s n o t d w in d le t o l e s s th a n 1 0 0 f r u s t u l e s / m l u n t i l a b o u t t h e en d o f S e p te m b e r. I t i s th e m o st c o n s is te n t m a jo r d o m in a n t i n t h e la k e a n d i s re d u c e d o n ly a d j a c e n t t o p o l l u t e d h a r b o r s (S to e rm e r and Y ang, 1 9 7 0 ) . A s t e r i o n e l l a fo rm o s a . t h e o t h e r summer d o m in a n t, show s an odd d is trib u tio n p a tte rn . I n t h e s o u th e r n b a s i n o f L ak e M ic h ig a n , i t i s one o f t h e d o m in a n t fo rm s i n May and J u n e , b u t o n ly a t s h a l lo w d e p th s ( l e s s th a n 5 m e t e r s ) . S e p te m b e r. The s p e o i e s th e n v i r t u a l l y d i s a p p e a r s u n t i l l a t e By c o n t r a s t , d a t a fro m B ig S a b le P o i n t a n d t h e ' L u d in g to n P r o j e o t show, a u n ifo r m d e p th d i s t r i b u t i o n w ith t h e h i g h e s t l e v e l s o c o u r r in g i n l a t e Ju n e t h r o u g h S e p te m b e r, d a t a w h ic h a g re e : w ith ' th o s e o f S to e rm e r a n d Yang ( 1 9 7 0 ) , b u t d i s a g r e e s w ith t h o s e o f G holnbky (1 9 6 8 ) w ho, i n s m a l l e r l a k e s , fo u n d p a t t e r n s m o re . c o n s i s t e n t w i t h t h o s e f o u n d i n t h e s o u th e r n b a s i n . T h is w o u ld a p p e a r t o r e l a t e t o t h e r m a l s t r a t i f i c a t i o n an d a v a ila b ility o f n u trie n ts . T f lb a lla y ia f e n e s t r a t a a n d T a b e l l a r i a f l o o u l o s a a r e g ro u p e d t o g e t h e r i n F ig u r e 8 b e c a u s e o f t h e i r o lo s e a f f i n i t y . In d ee d , b ecau se o f t h e i r 53 s i m i l a r d i s t r i b u t i o n p a t t e r n , t h e tw o s p e o i e s a r e o f t e n c o n s i d e r e d tw o e x p r e s s io n s o r fo rm s o f t h e same g e n o ty p e (K nudson, 1 9 5 2 )* T h e ir p p a k s i n i L ake M ic h ig a n 's tw o m a jo r b a s i n s c o in c id e q u i t e c l o s e l y b u t a p p r o x im a te ly l £ tim e s a s many f r u s t u l e s w e re c o u n te d a t t h e p e a k n e a r S a u g a tu o k , a g a in s t r e s s i n g t h e m ore a d v a n c e d e u t r o p h i c a t i o n o f t h a t a r e a . S y n e d ra a c u s a n d S y n e d ra n a n a w ere g ro u p e d f o r c o n v e n ie n c e . T h e ir num bers p e a k e d d u r i n g t h e summer (J u n e -A u g u s t) and se ld o m d u r i n g t h e c o u n tin g p e r i o d d ro p p e d b e lo w 100 f r u s t u l e s / m l , e x o e p t a t t h e c l o s e r i n s h o r e L u d in g to n s t a t i o n s . Numbers a v e ra g e d 6 0 0 /m l a t t h e S a u g a tu o k s t a t i o n s i n m idsum m er, JfOO/ral i n t h e L u d in g to n a r e a . One fo rm , M e lo s ir a g r a n u l a t a . shows s i g n i f i c a n t d i f f e r e n c e s b e tw e e n t h e tw o b a s i n s . T h is fo rm , th o u g h n o t d o m in a n t i n L ak e M ic h ig a n , i s o f t e n d o m in a n t i n e u t r o p h i o , a l k a l i n e l a k e s i n n o r t h e r n E u ro p e a n d N o rth A m erica ( C le v o - E u le r , 1951* H u b e r - P e s t a l o z z i , 19*t2; S to e r m e r a n d Y ang, 1 9 7 0 ). I t r e a c h e s p e a k s o f g r e a t e r th a n tw io e a s m any i n d i v i d u a l s a t S a u g a tu o k a n d r e a c h e s t h e s e p e a k s a m onth t o a m onth a n d a h a l f e a r l i e r th a n t h e s m a ll e r p e a k a t L u d in g to n . As i t i s n o t y e t a d o m in a n t, i t seem s t o im p ly t h a t L ake M ich ig an i s , a s y e t , r e l a t i v e l y o l i g o t r o p h i o o r m e s o tr o p h io . T h is i m p l i c a t i o n i s f u r t h e r s tr e n g th e n e d b y t h e l o w - l e v e l p r e s e n o e o f S te p h a n o d is c u b t e n u i s , a fo rm a s s o c i a t e d w i t h d i s t u r b e d h a b i t a t s (S to e rm e r and Y ang, 1 9 7 0 ). T h is s p e o i e s h a s b e e n r e p o r t e d i n s a m p lin g fro m Lake O n ta rio (N a le w a jk o , I 9 6 0 ) , Lake E r i e (H ohn, 1 9 6 9 ) a n d G re e n Bay a n d o t h e r p a r t s o f L ake M ic h ig a n ( H o lla n d , 1 9 6 9 ) . I t h a s l o n g b e e n p r e s e n t i n L ake M ich ig an b u t r e a c h e d l e v e l s o f 1 $ o f a l l p h y to p la n k to n o n l y r e c e n t l y (S to e rm e r an d Y ang, 1 9 7 0 ) . A o o o rd in g t o S to e rm e r a n d Y a n g , i t s ab u n d an o e i s r e s t r i c t e d a s y e t to n e a r sh o re a re a s an d h a r b o r s . T h is i s b o rn e o u t 5** in t h i s s e t o f s t u d i e s , th e in s h o re zones c o n tr ib u tin g th e g r e a t e s t num bers o f i n d i v i d u a l s , w i t h t h e «on e j u s t n o r t h o f t h s K g l a w o o R iv e r c o n t r i b u t i n g t h e h i g h e s t num bers o f a l l i n t h e s e s t u d i e s , (5 2 5 f r u s t u l e s / m l i n e a r l y Ju n e i n 1971)* S to e rm e r a n d Yang (1 9 7 0 ) p r o j e c t a n i n o r e a s e i n t h e num bers o f t h i s s p e o i e s a s p o l l u t i o n i n c r e a s e s . The a v e ra g e num bers o f some o f t h e l a r g e fo rm s ( g r e a t e r th a n 1 5 u i n s i z e ) o f p h y to p la n k to n s u c h a s A s t e r i o n e l l a fo rm o sa* S y n e d ra u l n a . M e lo s ir a i s l a n d i o a a n d C y o l o t e l l a b o d a n lo a h a v e shown d e c r e a s e s o v e r t h e t h r e e y e a r p e rio d * T h is d e o r e a s e w as aooom panied b y a n i n o r e a s e i n t h e num bers o f some o f t h e s m a l l e r C y o l o t e l l a fo rm s a n d t h e sm a ll S te n h a n o d is c u s fo rm s . T h ere h a s b e e n no o onoom m itant d e c r e a s e o r i n o r e a s e i n t h e s i z e o f any o f th e s p e o ie s . C o lle a g u e s a s s o o i a t e t h i s d e o r e a s e i n l a r g e fo rm s w i t h d e c r e a s i n g l e v e l s o f p h o s p h a te , n i t r a t e , a n d s i l i c a t e i n t h e l a k e , a n a s s o c i a t i o n w i t h o u t a d e q u a te s t a t i s t i c a l b a c k in g . I b e lie v e th e d e o re a se t o b e a s s o c i a t e d w i t h i n o r e a s i n g num bers o f l a r g e zoopQ Lankters s i n c e 1 9 6 7 , when t h e a l e w i f e , a s e l e c t i v e p r e d a t o r on l a r g e z o o p la n k to n fo rm s (B ro o k s a n d D odson, 1965» W e l ls , 1 9 7 0 ) , r e a c h e d i t * s p e a k a n d s t a r t e d t o d e o l i n e , a llo w in g th e l a r g e z o o p la n k to n t o i n c r e a s e i n n u m b e rs. T h is r i s e i n num bers c o u ld h a v e e f f e c t e d t h e d e o r e a s e i n num bers o f l a r g e p h y to p la n k to n i n tw o w ays ( l ) l a r g e z o o p la n k to n consum e a g r e a t e r q u a n t i t y o f p h y to p la n k to n /b o d y volum e a n d (2 ) l a r g e z o o p la n k to n a r e t h e o n ly o n e s t h a t c a n h a n d le l a r g e r ( g r e a t e r th a n 1 5 u ) p h y to p la n k to n * P h y s i c a l an d C h e m ica l S t u d i e s T e m p e ra tu re s r a n g e d fro m *2°C t o 2 2 .3 °C o v e r t h e t h r e e y e a r p e r i o d . On m o st d a t e s , e x a e p t : ih a t t h e b o tto m d e p th s * s h a l lo w w a t e r , t e m p e r a t u r e s w e re s l i g h t l y lo w e r T h erm al s t r a t i f i c a t i o n was o b s e r v e d d a r i n g t h e ' 55 T a b le 9 P h y to p la n k to n S p e o ie s S p e o ie s A c h n a n th e s c l e v e l A m p h ip le u ra p e l l u o i d a A m p h ip o ro ra o r n a t a Amphora o v a l i s Amphora p e r p u s i U a C e r a to n e ls a r c u s ♦ Q y c l o t e l l a b o d a n ic a ♦ C y o lo t e l l a oom ta ♦ C y o lo t e l l a g lo m e r a ta ♦ C y o lo t e l l a k u t z i n g i a n a C y o l o t e l l a m e n e g h in ia n a ♦ C y o lo te lla m ic h ig a n ia n a C y o lo te lla o o e ll a t a ♦ C y o lo te lla s t e l l l g e r a C y m ato p leu ra s o l e a Q ym bella t u r g l d a ♦D iatom a te n u e v . e lo n g a tu m D iatom a v u l g a r e E u c o c c o n e is f l e x e l l a F r a g i l a r i a o a p u o ln e a F r a g i l a r i a o o n s tr u e n s ♦ F ra g ila ria o ro te n e n s ls F r a g i l a r i a i n te r m e d i a ♦ F ra g ila ria p in n a ta F r a g ila r ia v ire s c e n s Gomphonema o liv a c e u m ♦ M e lo s ira am bigua M e lo s ir a b in d e r a n a M e lo s ir a d i s t a n s ♦ M e lo s ira g r a n u l a t a ♦ M e lo s ira i s l a n d i o a M e lo s ir a v a r i a n s M e rid io n c i r c u l a r e N a v ic u la a n g l i o a N a v ic u la r a d i o s a N a v ic u la r e i h h a r d t i i N a v ic u la s o u t e l l o i d e s N itz s c h ia a o io u la r is O pephora m a r t y i R h o ic o s p h e n ia o u r v a ta H h iz o s o le n ia e r i e n s i s ♦ S te p h a n o d is o u s a s t r a e a ♦ S te p h a n o d is c u s n i a g a r a e ♦ S te p h a n o d is o u s t e n u i s S u r i r e l l a a n g u s ta S u r ir e lla te n e ra ♦ S y n ed ra ao u s L o c a tio n N o r t h i o r S o u th B a s in # F ru s tu le s / C h a in o r Complex b o th 1 .0 1 .0 rt n tt vv tr : i .o M a in ly N o rth n h n M a in ly N o rth M a in ly S o u th M a in ly N o rth b o th n n it tt ti M a in ly N o rth b o th tt it it tt tt tt tt M a in ly S o u th b o th it it tt it it tt n it tt tt it tt tt tt M a in ly S o u th b o th tt n 1 .0 1 .0 1 .0 1 .0 1 .1 4 .0 3 .0 1 .5 2 .3 2 .1 2 .0 1 .0 1 .0 1 .7 1 .5 1 .1 6 .0 6 .3 7 .2 4 .2 4 .5 4 .0 1 .0 5 .0 6 .8 5 .0 5 .4 3 -9 3 .8 5 .6 1 .0 1 .0 1 .0 1 .0 1 .0 1 .0 1 .0 1 .0 1 .6 1 .1 4 .0 1 .0 1 .4 1 .4 56 T a b le 9 ( c o n tin u e d ) S p e o ie s ♦ S y n ed ra n a n a ♦ S y n ed ra u l n a S y n e d ra v a u c h e r i a e ♦ T a b e lla ria f e n e s t r a t a ♦ T a b e U a r ia f l o o o u l o s a C o c c o n e is i n t e r r u p t u s ♦ A s t e r i o n e l l a fo rm o sa L o c a tio n N o rth o r S o u th B a s in b o th " ** ” " n " ♦ P r in c ip a l s p e c ie s # F ru s tu le s / C h ain o r Complex 1*0 1*0 1*0 1 2 .4 1 0 .1 1*7 5*8 F ig u r e 8A-8B. F h y to p la n k to n t r e n d s i n L ake M ic h ig a n 1 9 7 0 -1 9 7 2 . 58 Figure 81A FKUSTUI£S/ML. times 10 600 A l l D iato m a (S o u th B a s in ) 300 200 f/*f • 100 70 60 50 4o 30 20 10 0 -2 M 4 -5 M 600 10 M A l l D ia to m s (N o rth B a s in ) FRUSTULES/ML. tines 10 300 200 ioo m 70 60 * • .* 50 40 30 20 10 4 59 F ig u re 8-B 100 flYwint-ftTla bodanioa-comta ($cruth) **• **71 **i 20 «/ •I •t * »/ * 10 o: m . 4 -5 M , 10 M 100 C y o lo te lla bodanica-oomta (North) 20 10 M A H J J A S 0 N 60 Figure 8-B (oontlnued) 100 C V olotella m iohiganiana-kutzingiana (South) 0 M 4 -5 M 10 M 100 C y o lo tella m iohiganiana k u tzin g ia n a (North) 20 10 M A M J J A S 0 N 61 F igu re 8-C F r a g ila r ia o ro ten en sis (South} 600 frustules/ ml. 300 200 •• • • • • 100 20 10 M A M J J A 600 F r a g ila r ia oroton en sis (North) frdstdles/ ml, 300 200 100 20 10 S 0 N 62 F igure 8-D 100 M e lo s ir a I s l a n d i o a (S o u th ) 20 10 S u rfa c e k -5 M 10 M 100 M a lo s lr a i s l a n d i o a (N o rth ) 20 10 63 Figure 8-D (oontinued) 100 M e lo s ir a /• *1-0 30 20 10 ML S u rfa o e k -5 M , . . . . 10 M 100 M elosira pranulata (North) ‘.40 20 10 J &+ F ig u re 8-E 100 Synedra u lna (South) 20 4> m 10 A M J J A S 0 N ■ ■■■■ Surfaoe —— M 10 M 100 Synedra ulna M A M J J (North) A S 0 N 65 F igure 8-E (con tin u ed ) 100 formosa ( S o u th ) 50 40 30 20 1 10 A M J J A S 0 N Surface Jfc-5 M 10 M 100 20 10 A ste r lo n a lln formosa (North) 66 Figure 8-F 600 m• 300 T a b e lla ria fe n e s tra ta f l o o o u lo s a (S o u th ) FHOSTULES/ML. 200 100 20 10 h. 600 fe n e s tra ta -flo o o u lo s a (N o rth ) 300 200 /# • Surface 4 -5 M 10 M FRUSTULES/ML. 100 70 60 50 40 30 20 • ## : 10 67 F igure 8-G 600 Sraedra acus-nana ^m (South) m 200 100 20 10 600 S v n e d ra a c u a -n a n a S u rfa c e 4 -5 M (N o rth ) 10 M 300 200 100 70 ■ 56 00 1+0 30 1 20 ■ • ' 10 * M V A 1 M * J 1 J A 1. S O :.T-------»- N 68 600 300 frdstules/ ml. 200 Stephanodlsous te n u is (South) 100 70 60 50 if 0 30 20 jt-t. 10 M A M J J A s 0 N Surface 600 *K5 M 10 M frustules/ ml. 300 200 100 20 10 Stephanodisous te n u is (North) 69 summer a t B ig S a b le P o i n t S t a t i o n s 2 , 3 , a n d 4-; S a u g a tu o k s t a t i o n 9 cuid L u d in g to n s t a t i o n 1 6 . i s t o b e e x p e c te d h e r e . T h ese a r e t h e d e e p - w a te r s t a t i o n s a n d s t r a t i f i c a t i o n D u rin g s t r a t i f i c a t i o n , t e m p e r a t u r e d i f f e r e n c e s o f u p t o 1 3 °C w e re o b s e r v e d b e tw e e n t h e s u r f a c e a n d b o tto m v a l u e s . A lth o u g h t u r n o v e r a n d i n i t i a l w arm ing o c c u r r e d a t d i f f e r e n t t im e s o v e r a t h r e e - y e a r p e r i o d , a g r a d u a l w arm ing t r e n d i s a p p a r e n t w h ic h a f f e o t s t h e o l o s e r i n s h o r e w a t e r s s o o n e r th a n t h e w a t e r s f u r t h e r o f f s h o r e . T h is w arm ing t r e n d show s w id e f l u c t u a t i o n s d u e t o w in d s a n d u p w e llln g c u rre n ts . N o r t h e r l y w in d s , a s e x p e c te d , te n d t o lo w e r t h e t e m p e r a t u r e s w h ile s o u t h e r l y w in d s t e n d t o i n o r e a s e te m p e r a tu r e s i n s h o r t (2 4 h o u r ) p e r i o d s o f t im e . T hese c h a n g e s a r e p a r t i c u l a r l y e v i d e n t d u r i n g J u n e , J u l y , an d A u g u st when s u r f a c e w a t e r n o r m a lly a t t e m p e r a t u r e s a ro u n d 15°C c an b e lo w e re d u p t o 8 .5 ° C i n on e 2 4 - h o u r p e r i o d . I n t h e s h a l lo w e r d e p t h s , b o tto m te m p e r a tu r e s t e n d t o f o l l o w t h e same o u rv e s a s t h e s u r f a c e w a t e r . N e v e r t h e l e s s ,^ t e m p e r a t u r e s d i d show a c o n s t a n t w arm ing t r e n d a n d r e a o h e d maximum te m p e r a tu r e s o f 1 7 .8 ° C a t B ig S a b le P o i n t ( s t a t i o n 6 ) , 1 8 .2 ° C a t L u d in g to n ( s t a t i o n 1 6 ) , a n d 2 2 .3 ° C a t S a u g a tu o k ( s t a t i o n 7 ) d u r i n g t h i s s tu d y . The w arm ing t r e n d b e g in s e a r l i e r i n t h e s o u t h e r n b a s i n t h a n i n t h e n o r t h e r n , a n d h i g h e r maximum t e m p e r a t u r e s a r e re a o h e d i n t h e s o u th e r n b a s in . Minimum te m p e r a tu r e s f o r t h i s s tu d y w e re 2 .2 ± 2°C f o r a l l t h r e e lo c a tio n s o v er a l l th r e e y e a r s . T hese lo w s w e re r e c o r d e d i n M arch a n d A p r i l an d a g a i n i n l a t e O c to b e r a n d November f o r a l l d e p t h s . I n Mar oh a n d A p r i l t h e r e i s o f t e n i c e on t h e l a k e , b u t a l l sam p le s i t e s w e re o u t b ey o n d t h i s d r i f t i o e , t h u s y i e l d i n g t h e s l i g h t l y w arm er t e m p e r a t u r e s t h a n m ig h t be e x p e c te d o t h e r w i s e . W ater t r a n s p a r e n c y a s d e te r m in e d b y S e o o h i d i s k i s shown i n T a b le 1 0 . The g r e a t e s t t r a n s p a r e n c y w as r e o o r d e d i n A p r i l a t t h e B ig S a b le P o i n t 70 s t a t i o n s 39 4 , a n d 5* O i l s t r a n s p a r e n c y l e v e l o r 1 1 m e te r s I s I n s u b s t a n t i a l a g re e m e n t w i t h t h e s tu d y o f A y e rs e t a l . w a te r o f t h i s c l a r i t y o f f F r a n k f o r t d u r i n g J u n e . (1 9 5 8 ) who fo u n d Low S e o o h l d i s k r e a d i n g s a t B ig S a b le P o i n t a n d a t l u d i n g t o n w e re g e n e r a l l y p r e c e d e d b y h i g h w in d an d wave c o n d i t i o n s w h ic h s t i r r e d u p s i l t a n d o l a y fro m t h e b o tto m . Low t r a n s p a r e n c y r e a d in g s f o r t h e S a u g a tu o k s t a t i o n s 9 w e re m o st o f t e n a s s o c i a t e d w ith a w in d fro m t h e s o u th w h ic h c a u s e d t h e plum e o f t h e Kalam azoo R iv e r t o t u r n n o r th w a r d . The K alam azoo c a r r i e s a h i g h l o a d o f su sp e n d e d s o l i d s ( s i l t a n d o r g a n io s m a in ly ) w h ic h d r a m a t i c a l l y lo w e r " th e tr a n s p a r e n c y , o f t h e w a t e r . The s t a t i o n s f u r t h e r o f f s h o r e ( s t a t i o n 9 ) an d f u r t h e r n o r t h (num bers 1 0 an d 1 1 ) w ere l e s s a f f e c t e d b y t h i s w e a th e r c o n d itio n . Bio s o u th e r n b a s i n t e n d s t o h a v e a g r e a t e r l o a d o f s u s p e n d e d s o l i d s an d s u p p o r t s a g r e a t e r p h y to p la n k to n g ro w th ( s e e p h y to p la n k to n ) a n d th u s show ed lo w e r o v e r a l l t r a n s p a r e n c y l e v e l s t h a n t h e n o r t h e r n b a s i n o v e r th e c o u rs e o f t h e t h r e e y e a r s t u d y p e r i o d . O th e r d a t a fro m c h e m ic a l d e t e r m i n a t i o n s a r e shown i n T a b le . A ll t h e d a t a a r e i n t h e fo rm o f r a n g e s an d m eans f o r d e p th s o f 0 -1 m e t e r s , 4 - 5 m e t e r s , a n d a t t h e b o tto m . A lth o u g h d a t a a r e a v a i l a b l e f o r some d a t e s an d s t a t i o n s a t 20 m e t e r s , th o s e d a t a a r e s t a t i s t i c a l l y i d e n t i o a l ( .0 5 l e v e l ) w i t h t h e b o tto m d a t a a n d a r e e x o lu d e d f o r t h e s a k e o f o l a r i t y . pH i n o r e a s e d fro m a b o u t 7*0 (S a u g a tu o k # 7 ) o r 7 . 8 (L u d in g to n an d G rand S a b le P o i n t ) t o a ro u n d 8 . 5 t o 8 .6 d u r i n g t h e m o n th o f J u n e a t a l l s t a t i o n s , w i t h t h e e x c e p ti o n s o f G rand S a b le P o i n t s t a t i o n s 4 a n d $ 9 w h ic h show ed a s lo w e r r a t e o f c h an g e a t d e p th s o f 5 0 -6 0 m e t e r s . The ohange i s a t t r i b u t e d t o p h y to p la n k to n g ro w th and p h o t o s y n t h e s i s , which* c h a n g e s t h e f r e e C0g b a l a n c e , a n d h e n c e pH ( R u t t n e r , 1 9 6 9 ) . I t i s o f n o te t h a t th e g r e a t e r i n i t i a l r e s e r v e s o f CO^ i n th e s o u t h e r n b a s i n w e re u s e d u p a t 71 a p p r o x im a te ly t h e same tim e a s t h o s e I n t h e n o r t h e r n b a s i n , im p ly in g t h a t p h o t o s y n t h e s i s b e g a n s o o n e r , o r a t a g r e a t e r r a t e i n t h e s o u t h e r n b a s in * T h is i s c o n s i s t e n t w i t h e a r l i e r w a n n in g o f t h e s u r f a c e l a y e r s i n t h e s o u th e r n e n d o f t h e la k e * I t i s a l s o o f n o t e t h a t b o t h t h e n o r t h e r n an d s o u th e r n b a s i n s r e a c h t h e sam e pH e n d p o i n t , s t r e s s i n g t h e f a c t t h a t t h e w a te r s o f t h e tw o a r e a s show c o n s i d e r a b l e m ix in g a n d p r o b a b l y h a v e a l a r g e b ic a rb o n a te b u f f e r in g o a p a o ity . COg l e v e l s , i n c o n n e c tio n w i t h pH c h a n g e s m e n tio n e d a b o v e , c h an g e d fro m 3 * 6 - ^ * 0 ppm t o 0 .0 ppm b y t h e f i r s t w eeks i n J u l y a t a l l s t a t i o n s * As a b o v e , t h e l e v e l s b e g a n d r o p p in g s o o n e r i n t h e s o u t h e r n b a s i n t h a n i n t h e n o r t h e r n , w i t h i n s h o r e l e v e l s b e in g t h e f i r s t t o d r o p . F r e e CO,, was a g a in f o u n d a t G rand S a b le P o i n t s t a t i o n s 2 , 3 , a n d 4 , a t L u d in g to n s t a t i o n 1 6 , a n d a t S a u g a tu o k s t a t i o n 9 a t t h e b e g in n i n g o f A u g u s t. C o n c e n tr a tio n v a r i e d f o r t h e t h r e e s t a t i o n s o v e r t h e c o u r s e o f t h e t h r e e y e a r s , b u t f r e e COg d i d a p p e a r i n q u a n t i t y d u r i n g A u g u s t. A fte r t h i s t i m e , q u a n t i t i e s o f f r e e CO,, f l u c t u a t e d i n t h e s u r f a o e a n d d e e p e r w a t e r s a s t h e e f f e o t s o f w in d m ix in g , t e m p e r a t u r e , r a t e o f p h o t o s y n t h e t i o a c t i v i t y , a n d u p w e llin g c u r r e n t s w e re f e l t , b u t a t n o tim e d i d t h e l e v e l d r o p t o 0 . 0 ppm a g a i n . D is s o lv e d oxygen r a n g e d fro m 1 0 - 1 5 ppm a t G rand S a b le P o i n t , fro m 10-14- ppm a t L u d in g to n , an d 6-14- ppm a t S a u g a tu o k . H ie h i g h l e v e l s w e re fo u n d e a r l y a n d l a t e i n t h e y e a r , w hen 0^ l e v e l s w e re o o n s t a n t th r o u g h o u t t h e w a t e r c o lu m n . L e v e ls d r o p p e d a f t e r t h i s t i m e , p a r t i c u l a r l y a t S a u g a tu o k w h ere! l e v e l s d ro p p ed - t o 6 ppm a t s t a t i o n 7 d u r i n g J u l y , p r o b a b ly d u e t o a l a r g e i n f l u x o f d eco m p o sin g o r g a n io m a t t e r fro m t h e K alam asoo R iv e r. T h is i s s u b s t a n t i a t e d b y h i g h d i s s o l v e d s o l i d l e v e l s a t t h i s t im e . Deep w a t e r 0g l e v e l s d e c r e a s e d u n d e r t h e summer th e r m o c l i n e on calm d a y s . 72 M easu rem en ts ta k e n a f t e r s to rm s w i t h f o r c e 4 w in d s ( a b o u t 8 f t . w a v es) show ed d i s s o l v e d oxygen l e v e l s t o b e e q u a l th r o u g h o u t t h e w a t e r colum n im p ly in g w in d - g e n e r a te d t u r n o v e r o f t h e w a te r co lu m n . None o f t h e s e r e a d i n g s a r e lo w enough t o p r e v e n t u t i l i z a t i o n o f t h e w a t e r colum n b y a n y * o f t h e f i s h s p e c i e s i n h a b i t i n g t h e l a k e , b u t some o f t h e t r o u t a n d salm on s p e c i e s t e n d t o a v o id t h e lo w e r l e v e l s o f Og e x h i b i t e d a ro u n d S a u g a tu c k i f o t h e r , h i g h e r l e v e l s w ere a v a i l a b l e ( F r e y , I 9 6 0 ) . - A l k a l i n it y l e v e l s w e re lo w e r a t t h e s o u t h e r n b a s i n s i t e s th a n a t t h e n o rth e rn s i t e s ( .0 1 l e v e l o f s ig n i f ic a n c e ) . an d t o pH l e v e l s a n d was t o b e e x p e c te d . T h is i s r e l a t e d b o th t o CO,, A d i s c u s s i o n o f COg, pH, a n d a l k a l i n i t y i s fo u n d i n R u t tn e r ( 1 9 6 9 ) a n d R e id ( 1 9 6 1 ) . D is s o lv e d s o l i d s w e re s i g n i f i c a n t l y h i g h e r i n t h e s o u t h e r n b a s i n th a n in th e n o rth e rn . T h is c a n a p p a r e n t l y b e a t t r i b u t e d t o a com plex o f fa c to rs : (1 ) The K alam azoo R i v e r , w h io h c a r r i e s a h i g h l o a d o f s e d im e n t an d o r g a n i o s , e n t e r s l a k e M ic h ig a n n e a r t h e s tu d y s i t e s . I n d e e d when t h e w in d was fro m t h e s o u th t h e am ount o f d i s s o l v e d s o l i d s w e n t u p . (2 ) The S a u g a tu c k s i t e r e c e i v e s a g r e a t e r n u t r i e n t l o a d , a s m ea su re d b y c o n c e n t r a t i o n o f d i s s o l v e d s o l i d s ( T a b le 1 0 ) t h a n d o e i t h e r o f t h e n o rth e rn b a s in s i t e s . I n t e r n a l c u r r e n t s y s te m s a s m e a su re d b y A yer e t a l (1 9 5 8 ) m a i n t a i n t h e s e m a t e r i a l s w i t h i n t h e two b a s i n s , a lth o u g h some m ix in g d o e s o c o u r. The h i g h e r n u t r i e n t l e v e l s a t t h e S a u g a tu o k s i t e s h o u ld s u p p o r t a g r e a t e r z o o p la n k to n a n d b e n th o s o ro p t h a n a t L u d in g to n o r B ig S a b le P o i n t . Table 10 Water chemistry and transparency: ranges and means from surface, middle and bottom waters, 1970 and 1971* Big Sable Point (#1-3) S ta tio n 1 S ta tio n 2 S ta tio n 3 5 40 1 5 50 10-15 13 10-15 « 10-15 13 10-15 13 10-15 33 9-15 1 2 .5 0. 0-3.8 0. 0-3.8 0. 0-3.8 0. 0- 3 .9 0. 0- 3 .9 0. 0- 3 .9 0 .0 -4 .0 O.O-3 .5 0 .0 -3 .5 range mean 7 .8 -8 .5 8 .3 7 .8 -8 .5 8 .3 7 .8 -8 .5 8 .2 7 .8 -8 .5 8 .3 7 .8 -8 .5 8 .3 7 -8 -8 .5 8 .3 7 .8 - 8 .5 8 .3 7 .S -8 .5 8 .3 7 .8 -8 .5 8 .2 A lk a lin ity (ppm) range mean 126-140 132 126-140 132 126-140 132 126-140 132 126-140 132 126-140 132 128-144 334 128-144 134 128-144 134 D isso lv ed s o lid s (ppm) range mean 170-181 169-181 166-178 175 • 175 172 169-177 174 168-177 174 164-177 166 169-176 173 166-175 171 164-170 166 Secchi Disk Reading (M) range mean 4 .0 - 8 .0 5 .6 Depth (m eters) D isso lv ed oxygen (ppm) range mean co2 ..(ppw) range 1 5 10-15 13 10-15 13 30 10-15 13 .1 pH 4 .4 - 8 .0 5 .8 4 .7 -1 1 .0 6.1 Table 10 (continued) Water chemistry and transparency: ranges and means from surface, middle and bottom waters, 1970 and 1971» Big Sable Point (#4-6) S ta tio n 4 S ta tio n 5 S ta tio n 6 1 5 60 1 5 20 1 5 10 10-15 13 10-15 13 8-15 12.3 10-15 13 10-15 13 10-15 13 10-15 13 10^15 13 9-15 13 range mean 0.0 - 4 .0 0. 0- 3 .8 0 .0 -3 .8 0.0 - 4 .0 0 . 0- 3 .8 0 .0 -3 .6 0 .0 -3 .1 0 .0 - 3 .4 0 .0 -3 .6 range mean 7 .9 - 8 .6 8 .3 7 .8 -8 .5 8.3 7 .9 - 8 .6 8 .2 8 .3 8 .3 8 .3 8 .3 8 .3 8 .3 A lk a lin ity (ppm) range mean 128-144 135 128-144 135 128-144 135 128-144 134 128-144 134 128-144 135 126-140 133 126-140 134 126-140 133 164-172 168 160-165 162 168-177 174 166-177 173 160-171 164 172-190 180 172-189 180 170-188 178 Depth (m eters) D isso lv ed oxygen (ppn) range mean C02 D isso lv ed s o lid s (ppm) range mean S ecchi D isk Reading (M) range mean t\3 M pH 4 .8 -1 1 .0 7 .2 4 .8 -1 1 .0 7 .2 4 .2 - 8 .7 5 .9 Table 10 (continued) Water chemistry and transparency: ranges and means from surface, middle and bottom waters, 1970, 1971, and 1972 (#7-9) S ta tio n 7 S ta tio n 8 S ta tio n 9 20 1 1 5 7 .5 1 5 6-13 12 6-13 12 7-13 11 8-13 12 8-13 12 8-12 11 10-14 13 9-14 13 8-12 12 range (ppm) 0. 0- 3 .6 0. 0-3 .6 0. 0- 3 .6 0 .0 -3 .6 0. 0- 3 .6 0. 0- 3 .6 0 . 0- 3 .6 0. 0- 3 .6 0. 0- 3 .6 range mean 7.0 - 8 .4 7 .5 7 .0 -8 .4 7 .5 7 .0 -8 .4 7.7 7 .1 -8 .3 7 .7 7 .2 -8 .3 7 .7 7 .2 -8 ,4 7 .8 7. 2- 8 .4 7 .7 7 .2 -8 .4 7 .8 7 .3 -8 .4 7 .8 A lk a lin ity (ppm) .range mean 17-94 61 17-94 6l 17-94 61 26-72 56 28-72 56 28-94 60 28-94 56 28-94 54 37-94 54 155-260 195 160-230 202 155-200 180 125-135 130 116-170 130 114-130 118 Depth (m eters) D issolved oxygen Xppn) range ‘mean C0? 5 46 D issolved s o lid s (ppm) range mean Secchi Disk Beading (M) range mean 155-285 . 155-280 200 198 0 .9 -4 .0 2 .2 1 . 2- 5 .0 3 .2 CD O pH : 3 . 0- 6 .0 4 .5 Table 10 (continued) Water c h e m istry and transparency: ranges and means from surface, middle and bottom waters, 1970, 1971 and 1972, Saugatuck (#10-11) S ta tio n 10 1 5 8-13 13 8-13 12 o . 0- 3 .6 ; range mean A lk a lin ity (ppm) range mean D isso lv ed s o lid s (ppm) range mean Secchi Disk Heading (M) range mean 8 .5 8-12 12 7-13 12 7-13 12 7-12 11 0. 0- 3 .6 0. 0- 3 .6 0. 0- 3 .6 0. 0- 3 .6 O.O-3 .6 7 -3 -8 .3 7 .7 7 .3 -8 .3 7 .7 7*3-8.4 7 .7 7 .2 -8 .3 7 .6 ? .2 -8 .3 7 .7 7 .2 -8 .3 7 .7 37-72 56 37-72 56 37-94 58 28- 72' 60 28-72 61 28-72 61 155-200 158 135-160 140 118-138 125 155-230 160 150-230 152 2 .5 -5 .0 3 -6 o pH 5 $ C02 (ppm) range . 1 6 D isso lv ed oxygen (ppm) range mean 20 ON 1 Depth (m eters) S ta tio n 11 2 .5 -4 .0 3 .1 Table 10 (continued) Water chemistry and transparency; ranges and means from surface and bottom depths, 1972 Ludington (#12-14) S ta tio n 12 « •* • S ta tio n 13 S ta tio n 14 * , 1 8 1 14 1 12 10-13 12 10-13 12 10-13 12 10-13 12 10-13 12 10-13 12 0. 0- 2 .8 0. 0- 3 .5 0 .0 -3 .5 O.O-3 .5 0 . 0- 3 .5 0 . 0- 3 .5 7 .9 -8 ,5 8.3 7 .8 -8 .6 8 .3 7 .8 -8 .6 8 .3 7 .9 - 8 .6 8 .3 7 .8 -8 .5 8 .3 7 .8 -S .6 8 .3 A lk a lin ity (ppm) range mean 116-120 118 114-120 117 116-118 117 114-118 117 116-120 117 116-118 117 D isso lv ed s o lid s (ppm) range mean 172-188 179 172-190 180 172-185 179 172-184 179 174-181 178 175-188 179 Depth; (m eters) D isso lv ed oxygen (ppm) range mean C02 (ppm) range pH range mean Table 10 (continued) Water chemistry and transparency: ranges and; means from surface, and bottom depths, 1972 Ludington (#12-14) S ta tio n 15 Depth (m eters) S ta tio n 16 1 6 1 11-12 12 10-14 12 11-13 13 0 .0 -3 ,7 0 .0 -3 .5 range mean 7 .8 -8 .5 8 .3 A lk a lin ity (ppm) range mean D isso lv ed s o lid s (ppm) range mean D isso lv ed oxygen (ppm) range mean C02 (ppm) range S ta tio n 17 24 1 12 9-13 12 10-13 12 10-13 11 0. 0- 3 .5 0 .0 -3 .5 0 .0 -3 .3 0 .0 -3 .5 7 .8 -8 .5 8 .3 7 .8 -8 .5 8 .3 7 .8 -8 .5 8 .1 7 .8 -8 .5 8 .3 7 .8 -8 .5 8 .3 114-120 118 316-122 118 114-118 117 116-120 118 108-124 116 114-120 116 175-184 180 175-181 179 170-181 175 169-179 174 172-180 176 173-180 177 pH 79 Table 11 Fluctuation in Zooplankton Numbers Over Time. 1 Month M arch A p ril E a r l y May L a te May E a r l y Ju n e L a te Ju n e E a rly J u ly L a te J u l y A u g u st S e p tem b er O c to b e r November M arch A p ril E a r l y May L a te May E a r l y Ju n e L a te Ju n e E a rly J u ly L a te J u l y A iig u st S e p tem b er O c to b e r November M arch A p ril E a r l y May L a te May E a r l y Ju n e L a te Ju n e E a rly J u ly L a te J u l y A u g u st S e p tem b er O c to b e r November 1970 j 1971 S a u g a tu c k ! L u d in g to n I S a u g a tu o k 1 L u d in g to n D ab h n ia a a l e a t a m en d o ta - 1 .1 5 mm 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 10 17 20 37 3 2 120 32 125 100 288 120 38 108 117 765 35 4 80 188 1 52 15 708 108 8 297 165 14 8 169 D abhnia r e t r o c u r v a - 1 .1 5 mm 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 .5 15 7 210 4 : i 5 25 1400 47 30 5 .5 1410 85 88 47 1 08 1 35 785 30 220 82 0 137 1 00 0 0 52 D ab h n ia l o n s i r e m i s - C.8 5 mm 0 0 0 0 0 0 0 0 : 1 1 0 2 1 0 1 .2 3 0 4 .2 6 1 -5 1 .1 4 3 .2 6 .5 1 4 6 3 -7 0 2 2 3 .5 0 10 8 .4 1 .5 0 2 8 5 .5 0 0 0 1 0 0 0 0 1 1 . 1 . 1 . 1972 S a u g a tu c k 1 L u d in g to n 0 0 0 1 2 .5 1 50 2 10 720 1350 2 20 H5 26 0 0 0 0 0 A 1 1 .8 31 70 125 210 502 810 185 •• 0 0 0 A 1 0 0 140 310 1500 1500 880 75 0 0 0 0 1 0 1 15 68 68 154 20 81 0 0 0 0 A A 1 0 0 1 1 1 1 5 0 0 0 0 1 .5 If 1 A 0 1 0 ■« 80 Table 11 (continued) Fluctuation in Zooplankton Numbers Over Time M onth March A p ril E a r l y May L a te May E a r l y Ju n e L a te Ju n e E a rly J u ly L a te J u l y A u g u st S ep tem b er O c to b er November ... . March A p ril E a r l y May L a te May E a r l y Ju n e L a te Ju n e E a rly J u ly L a te J u l y A u g u st S ep tem b er O c to b e r November March A p ril E a r l y May L a te May E a r l y Ju n e L a te Ju n e E a rly J u ly L a te J u l y A u g u st S ep tem b er O cto b er November 1 1972 1971 1 1970 S a u g a tu o k 1 L u d in g to n S a u g a tu o k 1 L u d in g to n 1 S a u g a tu c k L u d in g to n E o is c h u r a iL a o u s tris - 1 .6 6 mm 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 . 1 1 0 1 0 1 .1 3 1 1 3 .3 1 11 18 8 11 8 11 68 47 18 42 37 17 1 06 9° 85 88 95 65 66 68 3 6 55 32 43 3 0 4 5 3 2 4 7 18 33 18 23 16 28 25 18 10 10 17 _ 6 3 4 8 mm D iantoraus s i o l l i s - 1..2 1300 1176 808 892 977 785 1500 1000 1008 880 780 895 1100 • 892 1000 816 800 755 550 542 514 4-30 525 475 208 190 157 88 109 95 88 80 78 77 85 lZ 85 66 ■75 75 45 _62* 14 12 21 25 52 15 10 5 13 14 9 33 14 1 11 5 16 9 188 200 197 186 115 175 2000 2100 1350 _ 1100 950 888 D iaotom us a s h la r ) d i - 0 .8 5 mm 12 16 16 15 13 25 12 14 11 16 28 17 _ ■. 1 4 24 17 37 37 1■4^ 26 17 32 28 38 46 26 3 8 3 2 43 49 150 49 88 87 92 127 200 2 84 199 302 251 287 209 208 296 284 300 275 118 147 186 152 202 215 42 45 88 65 55 67 : 12 26 17 20 17 18 217 2D8 212 244 190 208 81 T able U (co n tirru ed ) F lu c tu a tio n s i n Z ooplankton Numbers Over Time 1 Month M arch A p ril Early* MayL a te May E a r ly Ju n e L a te Ju n e E a rly J u ly L a te J u l y A u g u st S e p tem b er O c to b e r November M arch A p ril E a r l y May L a te May E a r l y Ju n e L a te J u n e E a rly J u ly L a te J u l y A u g u st S ep tem b er O c to b e r November Maroh A p ril E a r l y May L a te May E a r l y Ju n e L a te J u n e E a rly J u ly L a te J u l y A u g u st S e p tem b er O o to b er 1971 1970 S a u g a tu c k 1 l u d in g t o n Saliva tu c k 1 L u d in g to n B osm ina l o n g i r o s t r i s - 0 .4 5 ran 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2 30 19 29 31 210 210 210 245 210 222 288 267 188 304 395 613 288 308 168 139 150 102 201 160 10? 88 120 142 :1 5 32 15 17. D iabhanosom a b ra c h v u ru m - 0 .8 5 ran 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 . 0 0 0 0 0 0 0 0 0 0 1 2 . 1 0 0 2 2 0 1 .1 1 3 .9 2 3 .5 3 -8 1 .8 13 2 .4 0 0 0 0 L e D to d o ra k i n d t i i - 2 5 mm 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 4 .6 0 1 .2 3 .8 1 4 .6 1 6 .8 6 .7 7 .8 1 5 .8 22 6 .4 10 8 2 8 6 1 6 .5 6 6 .8 1 .4 6 6 4 1 2 1 1 .8 0 1 1972 __ S a u g a tu c k 1 L u d in g to n 0 0 0 0 3 245 45 41 28 34 62 24 0 0 0 0 0— * 3 .6 7 .4 2 .8 2 .6 10 18 8 .7 _ 0 0A 0 •a 0* 16 190 195 320 85 3*5— 65 12 0 0 0 0 0 0 1 2 .1 0 1 1I, 24 12 • 0 0 0 0 1 .2 193 2 8 .9 29 3 6 .4 1 0 .2 1 .1 1 0 0 a. 0 1 1*5 15*5 26 18 1 0 .2 1«■11, .7a. 4 .7 82 Table 11 (contlrued) F lu c tu a tio n s i n Zooplankton Numbers Over Time. 1 Month M arch A p ril E a r l y May L a te May E a r l y Ju n e L a te Ju n e E a rly J u ly L a te J u l y A u g u st S e p te m b e r O c to b e r November M arch A p ril E a r l y May L a te May E a r l y Ju n e L a te J u n e E a rly J u ly L a te J u l y A u g u st S e p tem b er O o to b e r November M arch A p ril E a r l y May L a te May E a r l y Ju n e L a te Ju n e E a rly J u ly L a te J u l y A u g u st S e p tem b er O o to b er 1972 1971 I 1970 S a u g a tu c k 1 L u d in g to n S a u g a tu c k 1 L u d in g to n S a u g a tu c k 1 L u d in g to n 1 Polvohem us o e d i c u i u s - 0 . 56 mm 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 .6 4 4 .5 4 4 1 2 .2 3 .8 6 .2 16 5 .8 15 18 84 142 168 60 25 10 18 15 12 15 15 0 4 0 0 0 5 0 0 0 0 0 0 0 0 0 0 0 0 C v c I o d s b i c u s n i d a t u s - 0 .9 1 mm 66 52 64 88 64 67 188 123 122 167 128 155 388 600 605 295 500 490 710 802 895 762 750 867 UOO 880 1030 800 901 900 620 1200 1400 1410 902 950 1100 900 625 1310 1100 1500 700 986 833 1008 865 987 3 02 808 677 870 800 633 180 342 546 400 800 780 205 4 36 165 298 405 622 200 200 2 4 4 256 308 302 15 mm M esoovolons e d a x - 1 . 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 26 55 65 11 33 15 208 205 88 108 119 151 174 485 255 161 206 149 226 214 182 222 180 158 200 106 187 188 47 117 144 105 26 14 • 49 48 12 0 0 0 0 5 83 T a b le 11 ( c o n tin u e d ) F l u c t u a t i o n s i n Z o o p la n k to n Numbers Over Tim e. 1972 1970 1971 1L u d in g to n S a u g a tu c k S a u g a tu c k 1 L u d in g to n S a u g a tu c k 1 L u d in g to n Month T.lmrtoeAlarms m a o ru ru s - 2 .5 0 mm 46 48 38 0 12 March 31 500 24 7 276 225 202 241 A p ril 1700 1100 801 980 701 502 E a r l y May 1350 1800 866 1012 641 488 L a te May 600 500 412 118 223 109 E a rly Ju n e 201 470 226 325 108 L a te J u n e 33 68 130 96 43 1 0 .8 E a rly J u ly 15 32 90 16 92 11 6 .8 L a te J u l y 280 A u g u st 331 88 157 89 34 107 198 90 285 S e p tem b er 54 65 720 562 402 607 O c to b er 231 332 1000 _ 1100 890 98 1 46 6 39 1 a f T i n i s e a . 12 mm # /1 0 ou m e te r s M vsis r e l i c t a a n d P o n to p o r e ia 0 0 2 0 0 0 March 10 12 2 3 .2 0 0 A p ril 52 3 0 2 2 4 3 4 20 2 0 E a r l y May 106 3 0 220 84 24 176 L a te May 32 106 135 2 2 .6 84 20 E a r l y Ju n e 2 3 .2 22 27 9 .6 18 88 L a te Ju n e 2 6 .4 6 1 7 .2 8 2 .4 1 .2 E a rly J u ly 8 1 0 .6 2 13 8 .4 0 .8 L a te J u l y 11 7 .6 3 .2 9 .4 7 .6 0 .8 A u g u st 20 1 3 .6 1 5 .8 1 2 .2 S ep tem b er 2 .2 3 .‘6 20 8 33 1 3 .6 O o to b er 25 5 135 18 1 3 .6 5 . 6 8 November 32 1 . SUMMARY A s t u d y w as u n d e r ta k e n a t t h r e e s i t e s on L ake M ic h ig a n t o s tu d y t h e f e e d in g h a b i t s o f f o u r s p e c i e s o f f i s h , A lo s a p s e u d o h a r e n g u s . C oregonus h o y i« P e r e a f l a v e s c e n s . a n d Osmerus m o rd ax . a n d t o d e te rm in e i f t h e s e s p e c i e s w ere s e l e c t i v e f e e d e r s i n t h e s e n s e o f B ro o k s a n d Dodson (1 9 ^ 5 ) • To a c c o m p l i s h * t h is , sto m ach a n a l y s e s w ere p e rfo rm e d on 2520 a le w i v e s , 1870 b l o a t e r s * 2598 y e llo w p e ro h a n d 1232 a m e ric a n sm e lt* F i s h w e re c o l l e c t e d t h r e e o f t h e f o u r s e a s o n s o f t h e y e a r a t d e p th s fro m 0 t o 60 m e te rs* S m a ll i n d i v i d u a l s o f a l l f o u r s p e c i e s p r e y e d p r i m a r i l y on s m a ll C le d o c e ra n s a n d Copepods a s w e l l a s t h e l a r g e r l i m n e t i c c r u s t a c e a P o n to p o r e ia a f f i n i s a n d M v sis r e l i c t a . L a rg e r i n d i v i d u a l s a l s o consum ed t h e s e o rg a n is m s b u t a d d e d o t h e r f o o d i t e m s . A d u lt p e rc h f e d h e a v i l y on f i s h e g g s a n d y o u n g an d t h e s m e lt consum ed l a r g e q u a n t i t i e s o f o s t r a o o d s . A p p ro x im a te ly t h r e e tim e s a s m any fe m a le p e r c h a s m u le s w ere f o u n d t o c o n ta in f i s h * s u g g e s t i n g s c h o o l in g b y s e x i n t h i s s p e c i e s a s r e l a t e d t o fo o d a c q u i s i t i o n * T h is d i f f e r e n t i a l s c h o o lin g i s c o n firm e d b y s t a t i s t i c a l e v a l u a t i o n o f c a to h l o c a t i o n * N e i th e r a le w iv e s * b l o a t e r s n o r s m e lt show ed s i m i l a r s c h o o l in g o r f e e d i n g d i f f e r e n c e s b a s e d on s e x . F e e d in g d i f f e r e n c e s w e re f o u n d f o r a l l f o u r s p e c i e s b a s e d on d e p th * tim e o f t h e y e a r * and a v a i l a b i l i t y o f v a r i o u s fo o d s o u r c e s . A l a b o r a t o r y e x p e r im e n t u s i n g 120 f i s h o f e a c h o f t h e f o u r s p e c i e s was a l s o p e rfo rm e d t o d e te r m in e i f p a r t i c u l a r s p e c i e s o f z o o p la n k to n w e re consum ed i n g r e a t e r num bers a n d w o u ld b e e x p e c te d i f t h e f i s h w ere e a t i n g a t random* A l l f o u r s p e o i e s consum ed g r e a t e r p r o p o r t i o n s o f t h e l a r g e l i m n e t l o c r u s t a o e a P o n to p o r e ia a n d M ysis th a n w o u ld b e e x p e c te d on a random fe e d in g b a s i s . la k e i t s e l f . T h is s e l e c t i v e f e e d i n g i s a l s o fo u n d i n f i s h ta k e n fro m t h e Z o o p la n k to n c o u n ts w e re made o v e r a t h r e e y e a r p e r i o d a n d Ql* 85 compared to data from stomach a n a ly se s. The la r g er lim n e tic orustacea and zooplankton were found to co n trib u te a g rea ter percentage In stomach a n alyses than t h e ir volume in th e la k e would su g g e st. The h ypothesis o f Brooks and Dodson (1 9 6 5 ) thus seems t o be upheld f o r th e se four s p e c ie s .' F h y to p la n k to n s t u d i e s w e re a l s o u n d e rta k e n a s w e l l a s c h e m ic a l a n d p h y s i c a l s t u d i e s t o d e te r m in e i f f i s h from t h e t h r e e s i t e s w ere a f f e c t e d ' b y t h e s e p a r a m e te r s . A lth o u g h d iffe re n c e s w e re d e t e c t e d i n p a r a m e t e r s , no m a jo r d i f f e r e n c e s i n s i z e , a g e c o m p o s itio n o r s e x c o m p o s itio n c o u ld fo u n d i n t h e f o u r f i s h s p e o ie s s to c k s t h a t c o u ld b e c o r r e l a t e d w ith c h a n g in g p h y s i c a l an d c h e m ic a l p a ra m e te r s o r c h a n g in g p h y to p la n k to n be s to o k s . BIBLIOGRAPHY A h ls tro m , E . H. 1936* H ie d e e p - w a te r p la n k to n o f L ake M ic h ig a n , e x c l u s i v e o f t h e C r u s t a c e a . T r a n s . A n. M ic ro sc o p . S o c . , 5 5 1 286 - 290 . AFHA, AWWA, WPCA. 1 9 7 1 . S ta n d a r d m eth o d s f o r t h e e x a m in a tio n o f w a te r an d w a s te w a te r . Amer. P u b l. H e a lth A s s n ., W a sh ., D. C. XXXV + 874 pp. A p p le g a te , V ernon C ., a n d H a rry D. Van M s te r. 1 9 7 0 . A b r i e f h i s t o r y o f c o m m e rc ia l f i s h i n g i n Lake E r i e . U. S . F i s h a n d W il d l . S e r v . , F i s h . L e a f l . 6 3 0 , 28 p p . A y e r s , J . C ., D. C. C h a n d le r, G. H. L a u f f , C. F . Pow ers an d E . B. H enson. 1958* C u r r e n ts a n d w a te r m a sse s o f Lake M ic h ig a n . U n iv . M ic h ig a n , G r e a t L ak es R e s. I n s t . P ub. 3* 169 p . B a i l r y , M e r r y ll M. 1 9 6 4 . 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