533-562.pdf (880.0Kb)

~ l b d e v i g e n r a p p o r t s e r . , 1, 1984. ISSN 0333-2594 The P r o p a g a t i o n of Cod Gab ma4hua L.

INFERENCES REGARDING PREDATION ON THE EARLY LIFE STAGES OF COD

AND OTHER FISHES

J . R . Hunter

N a t i o n a l Oceanic and ~ t m o s p h e r i c A d m i n i s t r a t i o n N a t i o n a l Marine F i s h e r i e s S e r v i c e

Southwest ~ i s h e r i e s C e n t e r 8604 La J o l l a S h o r e s Drive La J o l l a Ca. 92038 USA

ABSTRACT

J . R . H u n t e r , 1 9 8 4 . I n f e r e n c e s r e g a r d i n g p r e d a t o r s on t h e e a r l y l i f e s t a g e s of cod and o t h e r f i s h e s . I n : E. Dahl, D.S. D a n i e l s s e n , E . Moksness and P. Solemdal ( ~ d i t o r s ) , The P r o p a g a t i o n of Cod Gadu6 monhua L. ~ l b d e v i g e n r a p p o r t s e r . , 1, 1984: 5 3 3 - 5 6 2 .

L i t t l e i n f o r m a t i o n e x i s t s on t h e p r e d a t i o n on t h e e a r l y l i f e s t a g e s of m a r i n e f i s h i n g e n e r a l and even l e s s on cod.

Thus t h e approach used i n t h i s review was t o i n f e r t h e p r e d a t o r y p r o c e s s e s t h a t may b e s i g n i f i c a n t i n t h e e a r l y l i f e h i s t o r y o f cod u s i n g t h e g e n e r a l l i t e r a t u r e on t h e p r e d a t o r s of m a r i n e f i s h eggs and l a r v a e . The m o r t a l i t y of cod d u r i n g t h e embryonic p e r i o d i s h i g h i n d i c a t i n g a h i g h r a t e of p r e d a t i o n and p r e d a t i o n may c o n t i n u e t o b e t h e dominant s o u r c e of m o r t a l i t y i n l a t e r l a r v a l s t a g e s . The known p r e d a t o r s of eggs and l a r v a e a r e d i s c u s s e d a s a r e i n t r i n s i c and e x t r i n s i c f a c t o r s t h a t a f f e c t v u l n e r a b i l i t y t o p r e d a t i o n . Recommendations a r e made f o r f u t u r e r e s e a r c h .

The o b j e c t i v e of t h i s review was t o c h a r a c t e r i z e t h e p r o c e s s of p r e d a t i o n on cod eggs and l a r v a e i n t h e s e a . L i t t l e i n f o r m a t i o n e x i s t s on t h e p r e d a t i o n on t h e e a r l y l i f e s t a g e s of marine f i s h i n g e n e r a l and even l e s s on cod.

Thuq. t h e approach I have used i n t h i s review was t o i n f e r t h e p r e d a t o r y p r o c e s s e s t h a t may b e s i g n i f i c a n t i n t h e e a r l y l i f e h i s t o r y o f cod u s i n g t h e g e n e r a l l i t e r a t u r e on t h e p r e d a t o r s of marine f i s h eggs and l a r v a e .

P r e d a t i o n , s t a r v a t i o n , and c e r t a i n a b i o t i c e f f e c t s (wave a c t i o n , UV r a d i a t i o n , t e m p e r a t u r e and s a l i n i t y c h a n g e s ) have been proposed a s p o s s i b l e s o u r c e s of t h e g r e a t m o r t a l i t y e x p e r i e n c e d by p e l a g i c marine f i s h e g g s and l a r v a e i n t h e s e a . During t h e embryonic p e r i o d ( e g g and y o l k - s a c s t a g e s ) o n l y a b i o t i c e f f e c t s and p r e d a t i o n a r e s o u r c e s of m o r t a l i t y b e c a u s e t h e embryo s u b s i s t s on y o l k . A b i o t i c e f f e c t s

l

o c c a s i o n a l l y may b e i m p o r t a n t i n t h e embryonic s t a g e s s i n c e I t h e s e s t a g e s a r e t h e most s u s c e p t i b l e t o p h y s i c a l damage.

l

Moribund eggs and l a r v a e have been observed i n p l a n k t o n c a t c h e s b u t t h e s e r e p o r t s have been d i s c o u n t e d i n r e c e n t y e a r s and a t t r i b u t e d t o f a i l u r e t o wash p l a n k t o n n e t s t h o r o u g h l y between tows (May, 1 9 7 4 ) . S o l e i m ' s (1942) o b s e r v a t i o n of a l a r g e number of dead o r dying yolk-sac h e r r i n g l a r v a e , presumably k i l l e d by changes i n t e m p e r a t u r e and s a l i n i t y , a p p e a r s t o be a n a u t h e n t i c c a s e a s t h e p r o p e r p r e c a u t i o n s were t a k e n t o a v o i d c o n t a m i n a t i o n from p r e v i o u s n e t h a u l s (Wiborg, 1 9 7 6 ) . N e v e r t h e l e s s , changes i n t e m p e r a t u r e and s a l i n i t y s u f f i c i e n t t o k i l l l a r g e numbers of eggs o r l a r v a e i n t h e open s e a a r e p r o b a b l y r a r e , and e x p e r i m e n t a l e v i d e n c e i n d i c a t e s t h a t o t h e r a b i o t i c c a u s e s of m o r t a l i t y a r e u n l i k e l y . For example, p e l a g i c eggs appear t o be h i g h l y r e s i s t a n t t o damage from wave a c t i o n (Pommeranz, 1974) and s i g n i f i c a n t damage from UV r a d i a t i o n seems t o r e q u i r e s u s t a i n e d exposure f o r days i n t h e upper 5 m of t h e w a t e r column, which seems u n l i k e l y i n w e l l mixed s u r f a c e

I

w a t e r s ( ~ u n t e r e t a l . , 1 9 8 2 ) . Thus t h e m o r t a l i t y d u r i n g t h e

embryonic p e r i o d i s good e v i d e n c e f o r t h e e x i s t e n c e of p r e d a t i o n a s a c a u s e of n a t u r a l m o r t a l i t y .

The m o r t a l i t y r a t e s of embryonic marine f i s h e s a r e o f t e n v e r y h i g h i n d i c a t i n g h i g h r a t e s o f p r e d a t i o n . M o r t a l i t y r a t e s ( 2 ) o f cod eggs from t h e North S e a range from 0.03 t o 0 . 4 1 o v e r 11 y e a r s and a v e r a g e d a b o u t 0.25, i m p l y i n g a l o s s r a t e of a b o u t 22% p e r day (Daan, 1979, 1 9 8 1 ) . C o n s i d e r i n g t h e l o n g i n c u b a t i o n p e r i o d of cod e g g s (15-18 d a y s a t 5-7'C) t h e s e r a t e s seem s u r p r i s i n g l y h i g h , b e i n g comparable t o t h o s e o f t e m p e r a t e n o r t h e r n anchovy which h a s o n l y a 3 day i n c u b a t i o n p e r i o d ( T a b l e 1 )

.

Thus i n t h e North Sea t y p i c a l l y 98% of t h e cod eggs a r e p r o b a b l y consumed by p r e d a t o r s b e f o r e t h e y h a t c h .

TABLE 1

M o r t a l i t y r a t e s of p e l a g i c eggs and l a r v a e i n p e r c e n t p e r d a y .

F i s h Year ( S ) S t a g e Mean Range R e f e r e n c e

Cod 1968-1977 E99

P l a i c e 1947-1977 E99

P l a i c e 1947-1971 Yolk-sac

l a r v a e

Anchovy 1980-1983 E99

Anchoveta 1981 E99

S o l e 1969-1970 E99

22 3-33 Daan, 1976

7 2-13 Harding e t

a l . , 1978

1 0 3-22 Harding e t

a l . , 1978 21 13-36 Unpub. d a t a

SWFC'

-

67 S a n t a n d e r e t a l . , 1983

-

^42-65 R i l e y , 1974 'southwest F i s h e r i e s C e n t e r , La J o l l a , C a l i f o r n i a 92038, USA.

The r a n g e of m o r t a l i t y r a t e s f o r cod eggs and f o r t h o s e of o t h e r s p e c i e s i n d i c a t e s t h a t p r e d a t i o n - r a t e s v a r y g r e a t l y

from y e a r t o y e a r , and such v a r i a t i o n might a c c o u n t f o r t h e observed v a r i a t i o n s i n r e c r u i t m e n t , p a r t i c u l a r l y i f t h e s e p r e d a t i o n r a t e s p e r s i s t e d o v e r s u b s e q u e n t l i f e s t a g e s . Some e v i d e n c e e x i s t s t h a t t h e h i g h p r e d a t i o n r a t e encountered i n t h e egg s t a g e p e r s i s t s through t h e embryonic p e r i o d , i . e . , u n t i l t h e development of a f u n c t i o n a l eye and jaw and t h e o n s e t of f e e d i n g . I n p l a i c e and n o r t h e r n anchovy t h e m o r t a l i t y r a t e s of eggs do n o t d i f f e r much from t h a t of y o l k - s a c l a r v a e i n d i c a t i n g no s h a r p t r a n s i t i o n i n r a t e s d u r i n g t h e embryonic p e r i o d ( ~ a r d i n g e t a l . , 1978; Hunter, 1 9 8 2 ) .

A f t e r t h e o n s e t of f e e d i n g t h e argument f o r p r e d a t i o n a s t h e p r i n c i p l e s o u r c e of m o r t a l i t y i s l e s s compelling because s t a r v a t i o n may a l s o p l a y a n i m p o r t a n t r o l e . A l l l a b o r a t o r y s t u d i e s on p e l a g i c f i s h l a r v a e such a s cod, h e r r i n g , anchovy, and p l a i c e ( H u n t e r , 1981) i n d i c a t e t h a t t h e y a r e h i g h l y v u l n e r a b l e t o s t a r v a t i o n a t t h e o n s e t of f e e d i n g owing t o t h e i r s m a l l s i z e , s h o r t s i g h t i n g d i s t a n c e f o r p r e y , slow c r u i s i n g s p e e d s d u r i n g t h e s e a r c h f o r f o o d , low c a p t u r e s u c c e s s , and h i g h food d e n s i t y r e q u i r e m e n t s . On t h e o t h e r hand, new s t u d i e s i n l a r g e s e m i - n a t u r a l e n c l o s u r e s seem t o

i n d i c a t e t h a t p r e d a t i o n i s t h e major s o u r c e of m o r t a l i t y i n t h e s e a . I n such e n c l o s u r e s , 3-108 of cod l a r v a e s u r v i v e d t h r o u g h metamorphosis a t food d e n s i t i e s e q u i v a l e n t t o t h o s e encountered i n t h e s e a ( E l l e r t s e n e t a l . , 1 9 8 1 ) . S i n c e t h e s e s u r v i v a l r a t e s a r e much g r e a t e r t h a n t h o s e of l a r v a e i n t h e s e a , t h e absence of p r e d a t o r s i n t h e e n c l o s u r e a p p e a r s t o be t h e r e a s o n f o r t h e remarkably h i g h s u r v i v a l .

New t o o l s now e x i s t which c a n be used t o d e t e r m i n e i f t h e a p p a r e n t v u l n e r a b i l i t y t o s t a r v a t i o n i n f e r r e d from l a b o r a t o r y work t r a n s l a t e s i n t o a h i g h r a t e of n a t u r a l m o r t a l i t y . I n f a c t , i d e n t i f i c a t i o n of s t a r v a t i o n a s a c a u s e o f m o r t a l i t y i n t h e s e a is now e a s i e r t o a s s e s s

q u a n t i t a t i v e l y t h a n t h e e f f e c t s of p r e d a t i o n , b e c a u s e s t a r v i n g l a r v a e c a n now b e a c c u r a t e l y i d e n t i f i e d u s i n g h i s t o l o g i c a l c r i t e r i a ( T h e i l a c k e r , 1978; O ' C o n n e l l , 1 9 8 0 ) , and slow growth of l a r v a e c a n b e d e t e c t e d u s i n g d a i l y i n c r e m e n t s on l a r v a l o t o l i t h s (Methot and Kramer, 1 9 7 9 ) . Such measurements h o l d t h e k e y t o d e t e r m i n i n g t h e r e l a t i v e i m p o r t a n c e o f s t a r v a t i o n and p r e d a t i o n ( b y t h e d i f f e r e n c e between s t a r v a t i o n m o r t a l i t y and t o t a l m o r t a l i t y ) d u r i n g t h e post-embryonic p h a s e of l a r v a l l i f e . Sea s t u d i e s u s i n g t h e s e new t e c h n i q u e s a r e j u s t b e g i n n i n g ( n o d a t a e x i s t on c o d ) b u t p r e s e n t e v i d e n c e i n d i c a t e s p r e d a t i o n p l a y s t h e dominant r o l e i n l a r v a l m o r t a l i t y ( O ' C o n n e l l , 1980; Methot and Kramer 1 9 7 9 ) .

These l i n e s o f e v i d e n c e seems t o i n d i c a t e t h a t p r e d a t i o n d u r i n g t h e embryonic p e r i o d i s h i g h and v a r i a b l e , and it p r o b a b l y remains t h e p r i n c i p l e s o u r c e of m o r t a l i t y t h r o u g h o u t t h e r e s t of t h e e a r l y l i f e h i s t o r y o f cod. The l a t t e r c o n c l u s i o n i s of c o u r s e s p e c u l a t i o n , a l t h o u g h t h e r e s u l t s of t h e e n c l o s u r e s t u d i e s seem q u i t e c o n v i n c i n g i n t h i s r e g a r d . Proof w i l l a w a i t a s s e s s m e n t of t h e r a t e s of s t a r v a t i o n i n t h e s e a .

K I N D S OF PREDATORS

C r u s t a c e a and c h a e t o g n a t h s ,

Consumption of m a r i n e f i s h l a r v a e by copepods, e u p h a u s i i d s h r i m p , h y p e r i i d amphipods and c h a e t o g n a t h s h a s been s t u d i e d i n s m a l l (0.2-3.5 l i t r e ) c o n t a i n e r s i n t h e l a b o r a t o r y . For example, i n t h e C a l i f o r n i a C u r r e n t r e g i o n , 11 s p e c i e s of c a l a n o i d copepods, and e u p h a u s i i d shrimp were c a p a b l e of c a p t u r i n g o r f a t a l l y i n j u r i n g y o l k - s a c anchovy l a r v a e

( L i l l e l u n d and L a s k e r , 1971; T h e i l a c k e r and L a s k e r , 1 9 7 4 ) , and t o t h e n o r t h i n Dabob Bay, Washington, t h e c a l a n o i d Euchaeta e L o n g a t a and s e v e r a l s p e c i e s of c r a b l a r v a e have

b e e n i d e n t i f i e d a s p r e d a t o r s of y o l k - s a c hake l a r v a e ( B a i l e y and Yen, 1 9 8 3 ) . A d d i t i o n a l s p e c i e s of c a l a n o i d s observed p r e y i n g on l a r v a e a r e mentioned by Lebour ( 1 9 2 5 ) . L a b o r a t o r y s t u d i e s seem t o show t h a t s m a l l c a r n i v o r b u s a r t h r o p o d s ( c a l a n o i d copepods, amphipods and e u p h a u s i i d s ) l c a n a t t a c k and k i l l s i g n i f i c a n t numbers of m a r i n e y o l k - s a c l

I f i s h l a r v a e i f t h e c o n c e n t r a t i o n s of l a r v a e a r e s u i t a b l y h i g h ( T a b l e 2 ) . Consumption of f i s h l a r v a e by c h a e t o g n a t h s h a s been f r e q u e n t l y i n f e r r e d from e x a m i n a t i o n of p r e s e r v e d

,

_specimens t a k e n i n p l a n k t o n tows (Lebour 1922, 1923;

Alvarifio 1 9 8 0 ) . On t h e o t h e r hand, l a b o r a t o r y e x p e r i m e n t s o n two s p e c i e s ( S a g i t t a eLegann and S . netona) seem t o

I

l i n d i c a t e t h a t t h e y a r e n o t i m p o r t a n t p r e d a t o r s . S a g i t t a

1

r e q u i r e d a 24h s t a r v a t i o n p e r i o d b e f o r e consuming any l a r v a l I f i s h , t h e y s t r o n g l y p r e f e r r e d copepods t o l a r v a e , and t h e y

l

a t e o n l y f i r s t f e e d i n g l a r v a e , a s younger l a r v a e ( y o l k - s a c ) and o l d e r o n e s were n o t e a t e n (Kuhlmann, 1 9 7 7 ) . C l e a r l y , more i n f o r m a t i o n i s needed on l a r v a l p r e d a t i o n by c h a e t o g n a t h s

.

Few o f t h e s m a l l i n v e r t e b r a t e p r e d a t o r s s t u d i e d t o d a t e seem t o b e e f f e c t i v e p r e d a t o r s of p e l a g i c f i s h e g g s . L a b o r a t o r y t e s t s i n d i c a t e t h a t n e i t h e r Euphaunia ( ~ i l l e l u n d and L a s k e r , 1971; B a i l e y and Yen, 19831, S a g i t t a (Kuhlmann, 1977) n o r c r a b l a r v a e ( P a n a t h e m i n t o and o t h e r s ) and shrimp l a r v a e , ( B a i l e y and Yen, 1983) a r e s i g n i f i c a n t p r e d a t o r s of p e l a g i c f i s h e g g s . P r e s e n t l y , t h e o n l y p e l a g i c c r u s t a c e a n i d e n t i f i e d a s a p r e d a t o r of p e l a g i c eggs i s t h e amphipod CatLiopiun ( B a i l e y and Yen, 1 9 8 3 ) , b u t v e r y few o f t h e v a s t number of p e l a g i c c r u s t a c e a have been c o n s i d e r e d .

Only t h e y o l k - s a c s t a g e of m a r i n e f i s h l a r v a e a p p e a r s t o b e v u l n e r a b l e t o t h e s e s m a l l c a r n i v o r o u s a r t h r o p o d s . These p r e d a t o r s a p p e a r n o t t o be a b l e t o consume s i g n i f i c a n t numbers of p e l a g i c eggs and t h e number of f i s h l a r v a e a t t a c k e d d r o p s r a p i d l y a t t h e end of t h e y o l k - s a c p e r i o d . For example, t h e number of y o l k - s a c anchovy l a r v a e k i l l e d by Labidocena f e m a l e s d e c l i n e d from 16 newly h a t c h e d anchovy

% " ^m

.8

-

k m r l f i 4:

S $ $

8 . 0

2:8$;

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I ^{4 . 4r.a} 8%:

8-,533

i 2 5 sZP ,p

3 g*-

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5 2

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l a r v a e p e r day t o a b o u t 7 p e r day f o r 168-h-old l a r v a e . The p r e s e n c e of an a l t e r n a t i v e prey ( c o p e p o d s ) d e p r e s s e s t h e r a t e of l a r v a l p r e d a t i o n i n t h e c a r n i v o r o u s copepods

( L i l l e l u n d and L a s k e r , 1971; B a i l e y and Yen, 1983) b u t a p p a r e n t l y n o t i n e u p h a u s i i d s h r i m p s . For example, a t e q u a l d e n s i t i e s of Pneudocatanun and hake l a r v a e t h e p r e s e n c e of P n e u d o c a t a n u ~ d e p r e s s e d t h e p r e d a t i o n r a t e of Euchaeta on hake l a r v a e by 60% ( B a i l e y and Yen, 1983 )

.

S i n c e copepods, r a t h e r t h a n l a r v a l f i s h dominate most p l a n k t o n assemblages, t h i s i s an i m p o r t a n t e f f e c t . The p r e d a t i o n r a t e s observed i n t h e l a b o r a t o r y probably a r e seldom r e a l i z e d i n t h e s e a b e c a u s e t h e l a r v a l f i s h d e n s i t i e s a r e much lower i n t h e s e a and more numerous a l t e r n a t i v e prey e x i s t f o r t h e p r e d a t o r s . Yolk-sac h e r r i n g a r e a p o s s i b l e e x c e p t i o n a s t h e y a p p e a r t o

a c h i e v e remarkably h i g h d e n s i t i e s i n t h e s e a ( 2 0 l a r v a e / l i t r e a t t h e s u r f a c e ; Westernhagen and R o s e n t h a l , 1 9 7 6 ) .

F i e l d e v i d e n c e a l s o i n d i c a t e s t h a t , h y p e r i i d amphipods and c a r n i v o r o u s c a l a n o i d copepods consume s i g n i f i c a n t numbers of f i s h l a r v a e a l t h o u g h s p e c i f i c i d e n t i f i c a t i o n of remains of l a r v a e i n t h e stomachs i s u n c e r t a i n . H y p e r i i d amphipods CO-occur w i t h yolk-sac h e r r i n g l a r v a e f o r 40 days i n D e p a r t u r e Bay, B r i t i s h Columbia, and remains of f i s h l a r v a e a r e t h e most abundant i t e m i n t h e i r g u t s (Westernhagen and R o s e n t h a l , 1976; Westernhagen, 1 9 7 6 ) . F i s h l a r v a e , e s p e c i a l l y h e r r i n g and Ammodqten, make up 23%

of t h e food of t h e h y p e r i i d amphipod. P a t a t h e m i d t o g a u d i c h a u d i d u r i n g A p r i l and June i n t h e North Sea ( S h e a d e r and Evans, 1 9 7 5 ) . F i s h l a r v a e (presumably yolk-sac s t a g e s o f h a k e ) o c c u r r e d i n 30 t o 43% of t h e Euchaeta stomachs examined by B a i l e y and Yen ( 1 9 8 3 ) .

F i e l d e v i d e n c e from f r e s h w a t e r l a k e s i n d i c a t e s t h a t marine c y c l o p o i d copepods might be a s i m p o r t a n t p r e d a t o r s a s t h e p r e d a c i o u s c a l a n o i d s . Adult female f r e s h w a t e r l a k e c y c l o p o i d copepods ( V i a c q c t o p b t h o m a o i and A c a n t h o c q c t o p ~ v e t n a t i o ) a r e r e p o r t e d t o f e e d on a l e w i f e IALoAa

I pneudohahengun) l a r v a e : 1-4% of a l e w i f e l a r v a e ( t y p i c a l l y 3-

1

l 5 mm) t a k e n i n p l a n k t o n tows had a t t a c h e d c y c l o p o i d copepods; and a l l were t a k e n a t n i g h t , i n d i c a t i n g t h a t t h e l a r v a e may be more v u l n e r a b l e when l e s s a c t i v e ( ~ a r t i g e t a l . , 1 9 8 2 ) . S i m i l a r l y , Brewer e t a l . , (1983) found t h e c y c l o p o i d copepod ~C0hy~eUA angficc~n) a t t a c h e d t o 5% o f w h i t e c r o a k e r l a r v a e (Genqonemun fieatun) t a k e n i n n e t tows from n e a r s h o r e w a t e r s of S o u t h e r n C a l i f o r n i a . Only s m a l l l a r v a e were s u s c e p t i b l e t o c a p t u r e a s 93% of t h e l a r v a e w i t h a t t a c h e d copepods were l e s s t h a n 4 mm long. I t i s p o s s i b l e t h a t t h e p r e d a t i o n observed i n t h e s e r e p o r t s o c c u r r e d i n t h e p l a n k t o n n e t . N e v e r t h e l e s s , i t seems i m p o r t a n t t o e v a l u a t e t h e p r e d a t o r y p o t e n t i a l of marine c y c l o p o i d s b e c a u s e of t h e i r g r e a t abundance. For example, c y c l o p o i d copepods c o n s t i t u t e 20-30% of t h e t o t a l copepods o c c u r r i n g i n Norwegian c o a s t a l p l a n k t o n (Wiborg, 1 9 5 4 ) .

P r e d a t i o n by J e l l y f i s h and Ctenophores

L a b o r a t o r y and f i e l d o b s e r v a t i o n s over t h e y e a r s have i d e n t i f i e d a t l e a s t 34 s p e c i e s of medusae and a b o u t 5 s p e c i e s of c t e n o p h o r e s a s p r e d a t o r s o f t h e eggs and l a r v a e of marine f i s h e s . T h i s p r o b a b l y r e p r e s e n t s a s m a l l f r a c t i o n of p o t e n t i a l p r e d a t o r s g i v e n t h e hundreds of s p e c i e s and t h e i r c a r n i v o r o u s f e e d i n g h a b i t s . These r e c o r d s a r e based l a r g e l y on c a s u a l o b s e r v a t i o n i n t h e l a b o r a t o r y o r food h a b i t s t u d i e s of s e a c a u g h t i n d i v i d u a l s where f i s h eggs and l a r v a e a r e t y p i c a l l y a n i n c i d e n t a l p a r t of a zooplankton d i e t . Thus, i t seems p o i n t l e s s t o l i s t a l l s p e c i e s and r e f e r e n c e s h e r e , b u t t h e m a j o r i t y a r e mentioned i n v a r i o u s reviews o r s u r v e y s ( ~ e b o u r , 1922, 1923, 1925; F r a s e r , 1969;

B i e r i , 1961, 1970; Larson, 1976; M o l l e r , 1980; A r i and Hay, 1982) and a d d i t i o n a l r e f e r e n c e s a r e mentioned i n t h e c o n t e x t of t h i s review.

Consumption r a t e s of f i s h l a r v a e by l a r g e medusae and c t e n o p h o r e s a t t i m e s can be v e r y h i g h . F r a s e r ( 1 9 6 9 ) , i n

h i s r e v i e w , s p e c u l a t e s t h a t a l i f e t i m e consumption of 50-250 l a r v a l f i s h by each hydromedusa, a b o u t 450-500 by each Auhetia, and a b o u t 15,000 by e a c h Cyanea i s p r o b a b l e . S t e v e n s o n ( 1 9 6 2 ) commented t h a t a s many a s 45% of t h e P a c i f i c h e r r i n g l a r v a e t a k e n i n a sample had been o r were b e i n g devoured by c t e n o p h o r e s (PLeuhobhachia s p p . ) on one o c c a s i o n i n 1947, b u t h e c o n t i n u e s t h a t p r e d a t i o n by c t e n o p h o r e s i s g e n e r a l l y n o t a s e r i o u s s o u r c e of m o r t a l i t y ; i n a l m o s t 4,000 p l a n k t o n samples c o n t a i n i n g l a r v a l P a c i f i c h e r r i n g , e v i d e n c e of such a m o r t a l i t y o c c u r r e d i n fewer t h a n 100 samples. I n a s h a l l o w cove i n t h e Gulf of C a l i f o r n i a , t h e siphonophore Rh.izophy6a ey~enhahdZ.i was e s t i m a t e d t o consume a b o u t 8 . 8 f i s h l a r v a e p e r siphonophore p e r day which may have r e p r e s e n t e d a b o u t 26% of t h e v a r i o u s s p e c i e s of f i s h l a r v a e p r e s e n t i n t h e cove ( ~ u r c e l l , 1 9 8 1 ) . Major blooms of t h i s s p e c i e s i n t h e Gulf a r e p r o b a b l y s p o r a d i c , however.

The b e s t documented example o f p r e d a t i o n by j e l l y f i s h on f i s h l a r v a e i s f o r t h e scyphomedusa AuheLia a u h i f a which a p p e a r s t o be an i m p o r t a n t p r e d a t o r of yolk-sac h e r r i n g l a r v a e . I n K i e l F j o r d ( w e s t e r n B a l t i c ) consumption r a t e s of AuheLia averaged a b o u t f o u r l a r v a e p e r medusa p e r day d u r i n g May, i n d i c a t i n g t h a t a t l e a s t 2-59; of t h e t o t a l s t o c k o f y o l k - s a c h e r r i n g l a r v a e a r e e a t e n p e r day ( ~ g l l e r , 1 9 8 0 ) . No c o r r e l a t i o n e x i s t e d between t h e s i z e of t h e medusa and t h e p e r c e n t a g e of medusae c o n t a i n i n g l a r v a e b u t t h e number of l a r v a e p e r medusa i n c r e a s e d w i t h i n c r e a s i n g medusa d i a m e t e r from up t o 10 l a r v a e i n 12 mm medusae t o 68 l a r v a e i n 42 mm medusae. L a b o r a t o r y work i n d i c a t e s t h a t Auhetia (12-14 mm) have t h e c a p a c i t y t o k i l l h e r r i n g l a r v a e a t r a t e s up t o 7/h w i t h s a t i a t i o n o c c u r r i n g a f t e r consumption of 5-6 l a r v a e ( B a i l e y and B a t t y , 1 9 8 3 ) . These a u t h o r s a l s o p o i n t o u t t h a t b o t h e f f i c i e n c y o f c a p t u r e and e n c o u n t e r r a t e s i n c r e a s e g r e a t l y w i t h medusa s i z e , c o r r o b o r a t i n g M o l l e r ' s f i e l d o b s e r v a t i o n s . The e v i d e n c e f o r t h i s s p e c i e s and p e r h a p s o t h e r l a r g e j e l l y f i s h being a major p r e d a t o r of cod

l a r v a e i s n o t s t r o n g . AuheLia, and o t h e r l a r g e j e l l y f i s h a r e most abundant d u r i n g t h e summer, whereas peak months of spawning of cod a r e F e b r u a r y and March ( a t l e a s t i n t h e B a l t i c ; M o l l e r , 1 9 7 9 ) . On t h e o t h e r hand, t h e e p h y r a e s t a g e might b e i m p o r t a n t b e c a u s e t h e y become abundant i n March ( ~ 6 l l e r 1979) and e p h y r a e o f AuheLia o n l y a few d a y s o l d w i l l e a t l a r v a l f i s h e s (Lebour, 1 9 2 2 ) .

The s i z e - s p e c i f i c v u l n e r a b i l i t y o f cod and o t h e r f i s h l a r v a e t o j e l l y f i s h i s e s s e n t i a l f o r e s t i m a t i n g o v e r a l l impact on t h e p o p u l a t i o n and i n t e r p r e t i n g m o r t a l i t y r a t e s , y e t l i t t l e d a t a e x i s t o t h e r t h a n f o r t h e y o l k - s a c s t a g e s . The v u l n e r a b i l i t y o f cod must d e c l i n e g r e a t l y o v e r t h e l a r v a l s t a g e s s i n c e l a t e l a r v a l and p e l a g i c j u v e n i l e cod a r e known t o accompany and s e e k s h e l t e r b e n e a t h l a r g e j e l l y f i s h (Cqanea c a p i L L a t a , AuneLia a u h i t a , Rhizobtoma pulmo) a s d o many o t h e r gadoid f i s h e s (Ward, 1912; Mansueti, 1 9 6 3 ) . Young w h i t i n g , Gadub mehLangub, which show t h e same b e h a v i o u r a p p e a r t o have developed a c e r t a i n d e g r e e o f immunity from t h e d i s c h a r g e of nematocysts of Cyanea and presumably cod a r e s i m i l a r l y immune ( D a h l , 1 9 6 1 ) . Thus t h e s e l a r g e j e l l y f i s h seem u n l i k e l y p r e d a t o r s of o l d e r cod l a r v a e and j u v e n i l e s .

Consumption o f p e l a g i c f i s h eggs by j e l l y f i s h and c t e n o p h o r e s seems t o b e mentioned much l e s s f r e q u e n t l y i n t h e l i t e r a t u r e t h a n p r e d a t i o n on l a r v a e . About f i v e s p e c i e s o f medusae (Lebour, 1922, 1923; P h i l l i p s e t a l . , 1969;

B a i l e y and Yen, 1 9 8 3 ) , two chondrophora ( B i e r i , 1961, 19701, and two c t e n o p h o r e s , PLeuhobhachia (Lebour, 1922, 1923;

B a i l e y and Yen, 1983) and Mnemqopbib t e i d q i ( B u r r e l l and Engel, 19761, have been r e p o r t e d t o consume p e l a g i c f i s h eggs a s w e l l a s l a r v a e . PLeufiobhachia might b e a s i g n i f i c a n t p r e d a t o r of cod eggs and e a r l y l a r v a e a s it r e a c h e s peak abundance i n January-February i n t h e B a l t i c , which c o r r e s p o n d s t o peak p r o d u c t i o n of cod l a r v a e ( ~ h l e r , 1 9 7 9 ) . A t f i l t e r i n g r a t e s t h a t r a n g e from 5 ml/day f o r a n i m a l s l e s s t h a n 0 . 8 mm t o 1 , 1 5 0 ml/day f o r 7.2-8.3 mm

a n i m a l s (Reeve and W a l t e r , 1 9 7 6 ) , d e n s e swarms of PLeuhobhachia c e r t a i n l y would have t h e c a p a c i t y f o r removing s i g n i f i c a n t q u a n t i t i e s of eggs and y o l k - s a c l a r v a e . Eggs and l a r v a e t h a t c o n c e n t r a t e a t t h e s e a s u r f a c e a r e probably h i g h l y v u l n e r a b l e t o t h e n e u s t o n i c j e l l y f i s h VeLeeLa and Pofipifa (Chondrophorae) ( B i e r i , 1961, 1 9 7 0 ) . Off t h e c o a s t of C a l i f o r n i a i n May, f i s h eggs (Thachuhub b ymmeihicub ) were t h e most i m p o r t a n t and common food i t e m (48% of p r e y ) o f VeLeLLa ( B i e r i , 1 9 6 1 ) .

The o c c u r r e n c e of swarms of c t e n o p h o r e s o r j e l l y f i s h a p p e a r s t o be v e r y e p i s o d i c . Ctenophore p o p u l a t i o n s , f o r example, a p p e a r t o be c a p a b l e of v e r y r a p i d growth r e s u l t i n g i n t h e sudden appearance of dense swarms. Reeve and Walter (1976) conclude from s t u d i e s of growth of PLeuhobnachia i n l a r g e bags ( 6 8 m3) t h a t p o p u l a t i o n s a r e c a p a b l e of i n c r e a s i n g by more t h a n 50% p e r day. Although p a t c h e s o r swarms of c t e n o p h o r e s and j e l l y f i s h a r e known t o o c c u r i n t h e open s e a , t h e most f r e q u e n t r e p o r t s a r e from i n s h o r e and o f t e n p a r t i a l l y e n c l o s e d a r e a s such a s b a y s , f j o r d s and e s t u a r i e s . B u r r e l l and Engel (1976) mention t h a t c o n c e n t r a t i o n s of c t e n o p h o r e s seem t o be s h o r t l i v e d i n n e r i t i c o r o c e a n i c w a t e r b u t they may p e r s i s t f o r months i n c o n f i n e d a r e a s such a s e s t u a r i e s . Thus, s i g n i f i c a n t p r e d a t i o n by j e l l y f i s h and c t e n o p h o r e s may o n l y o c c u r on t h e o c c a s i o n s when spawning and o u t b u r s t s of j e l l y f i s h occur t o g e t h e r ; t h e p r o b a b i l i t y o f such e v e n t s seems t o be g r e a t e r i n p a r t i a l l y e n c l o s e d a r e a s t h a n i n t h e open s e a . An i n v e r s e r e l a t i o n s h i p between AuheLia and l a r v a l h e r r i n g abundances i n K i e l F j o r d c l e a r l y i l l u s t r a t e s t h i s p o i n t , a s t h e h i g h e s t l a r v a l abundances o c c u r r e d i n y e a r s i n which s i g n i f i c a n t numbers of h e r r i n g had h a t c h e d b e f o r e t h e a n n u a l o u t b u r s t o f AuneLia ( ~ G l l e r , 1 9 8 2 ) . These i n v e r s e c o r r e l a t i o n s between j e l l y f i s h o r c t e n o p h o r e abundance and l a r v a l abundance a r e r e l a t i v e l y common ( A l v a r i i i o , 1 9 8 0 ) , b u t o n l y i n h e r r i n g i s it good e v i d e n c e f o r consumption by t h e medusae, s i n c e h e r r i n g have g e o g r a p h i c a l l y f i x e d spawning

grounds and t h e p a r e n t s c a n n o t move e l s e w h e r e t o a v o i d c o n c e n t r a t i o n s of medusae. On t h e o t h e r h a n d , low abundance of eggs and l a r v a e of p e l a g i c spawners c o u l d a l s o b e a t t r i b u t e d t o a v o i d a n c e of such r e g i o n s by t h e p a r e n t s . T h i s seems l i k e l y f o r p l a n k t i v o r o u s f i s h e s s i n c e t h e z o o p l a n k t o n f o o d s of t h e s e f i s h e s a r e consumed by medusae and c t e n o p h o r e s , and spawning s c h o o l s may n o t remain long i n a r e a s where f o r a g e i s low.

O t h e r i n v e r t e b r a t e s

Marine p o l y c h a e t e s and s q u i d undoubtably consume m a r i n e f i s h l a r v a e b u t l i t t l e i s known r e g a r d i n g t h e i r i m p o r t a n c e i n l a r v a l m o r t a l i t y . Lebour ( 1 9 2 3 ) mentions t h a t t h e p o l y c h a e t e TomopXenin h e t g o t a n d i c a consumes h e r r i n g , and o t h e r f i s h l a r v a e and s q u i d a r e known p i s c i v o r e s . Owing t o t h e i r s i z e , a g i l i t y , and p i s c i v o r o u s h a b i t s , t h e y a r e c a p a b l e of f e e d i n g on many l i f e s t a g e s of f i s h e s , n o t j u s t t h e y o u n g e s t l a r v a l s t a g e s , a s i s t h e c a s e f o r most i n v e r t e b r a t e p r e d a t o r s . Newly h a t c h e d s q u i d ( L a t i g o o p a t e n c e n n ) p r e f e r f i s h l a r v a e t o b r i n e shrimp a d u l t s , and t h e d a i l y r a t i o n of a newly h a t c h e d i n d i v i d u a l i s e q u i v a l e n t t o 1 4 y o l k - s a c anchovy p e r d a y , and a t 8 mm t h e i r d a i l y r a t i o n i s 135 anchovy l a r v a e ( H u r l e y , 1 9 7 6 ) .

F i s h e s

Ample e v i d e n c e e x i s t s t h a t p e l a g i c p l a n k t i v o r o u s f i s h e s consume p e l a g i c eggs and l a r v a e . I n t h e s o u t h e r n North Sea cod eggs a r e consumed by h e r r i n g , s p r a t , and s t i c k l e b a c k . The h i g h e s t mean r a t e s were 5-6 cod eggs p e r s p r a t o r h e r r i n g w i t h t h e eggs of p l a i c e and o t h e r s p e c i e s b e i n g consumed i n c o n s i d e r a b l e q u a n t i t i e s a s w e l l (Daan, 1 9 7 6 ) . Cod eggs were more abundant i n t h e p l a n k t o n t h a n p l a i c e b u t h e r r i n g and s p r a t seemed t o consume more p l a i c e e g g s . T h i s might b e , r e l a t e d t o t h e f a c t t h a t p l a i c e have l a r g e r eggs o r

p o s s i b l y t h a t t h e eggs of p l a i c e were more c o n c e n t r a t e d t h a n t h o s e of cod. D a a n ' s s t u d y a l s o i d e n t i f i e d h e r r i n g a s a m a j o r consumer of f i s h l a r v a e , a s h e r r i n g l a r v a e comprised 10-60% of t h e w e i g h t of t h e stomach c o n t e n t s of h e r r i n g i n some o f t h e c o l l e c t i o n s .

T h a t h e r r i n g and o t h e r p l a n k t i v o r o u s f i s h e s a r e m a j o r consumers of f i s h eggs and l a r v a e i s a l s o i n d i c a t e d by a number of o t h e r s t u d i e s . For example, 54% of t h e stomach c o n t e n t s of h e r r i n g and 45% of s p r a t were composed of f i s h eggs and l a r v a e i n t h e s o u t h e r n North Sea (Pommeranz, 19811, and i n s o u t h e r n New England w a t e r s and Georges Bank, e g g s and l a r v a e of f i s h e s r e p r e s e n t e d 4.5% o f t h e A t l a n t i c m a c k e r e l d i e t by w e i g h t and 0.4% o f t h a t of h e r r i n g (Maurer, 1 9 7 6 ) . S i m i l a r l y , 83% of t h e p r e y consumed by A t l a n t i c m a c k e r e l l a r v a e (13-19 mm) were o t h e r mackerel l a r v a e . Cannibalism p r o b a b l y o c c u r r e d a t n i g h t d u r i n g a n upward m i g r a t i o n of t h e l a r g e r l a r v a e a s t h e y p a s s e d t h r o u g h l a y e r s where s m a l l e r i n d i v i d u a l s were abundant (Grave, 1 9 8 1 ) . P a c i f i c mackerel l a r v a e , Scombeh j a p o n i c u b , a r e a l s o l a r v a l f i s h p r e d a t o r s ( H u n t e r and K i m b r e l l , 1 9 8 0 a ) . Along t h e P e r u v i a n c o a s t , f i s h l a r v a e and a p p e n d i c u l a r i a n s were t h e p r i n c i p a l f o o d s of P a c i f i c mackerel l a r v a e , w i t h s a r d i n e and P a c i f i c mackerel l a r v a e o c c u r r i n g i n l a r v a l mackerel stomachs; l a r v a e a s s m a l l a s 3.7 mm were p i s c i v o r o u s ( L i p s k a y a , 1 9 8 2 ) . Young p e l a g i c s t a g e s of cod CO-occur w i t h A t l a n t i c mackerel i n t h e North Sea ( J o n e s , 1 9 8 3 ) ; c o n s e q u e n t l y n e a r l y a l l l i f e s t a g e s of m a c k e r e l c a n be c o n s i d e r e d a s p o t e n t i a l p r e d a t o r s of cod l a r v a e . Harding e t a l . (1978) examined t h e stomach c o n t e n t s of f i s h p r e d a t o r s t a k e n i n egg p a t c h e s i n t h e s o u t h e r n North S e a . S i x t y - t w o p e r c e n t of t h e p r e y i t e m s i n t h e stomachs of h e r r i n g , and 96% o f t h o s e i n t h e stomachs of anchovy ( E n g h a u t i b e n c f i a b i c h o l u b ) were f i s h eggs o r l a r v a e , whereas o t h e r p o t e n t i a l p r e d a t o r s , Mehtangiub mehtangun, Metanoghammub a e g l e d i n u h

,

Limanda l i m a n d a , E u t n i g l a guhnahduh

,

and Sphatkub b p h a t t ~ A c o n t a i n e d o n l y a few f i s h eggs o r l a r v a e .

C a p e l i n a r e a l s o i n d i c a t e d a s p o s s i b l e consumers of cod eggs and l a r v a e s i n c e cod e g g s , a s w e l l a s o t h e r f i s h e g g s and l a r v a e have been i d e n t i f i e d i n t h e i r stomachs (Pozdnyakov, 1 9 6 1 ) .

Cod l a r v a e and p e l a g i c j u v e n i l e s a r e l i k e l y p r e d a t o r s of l a r v a l cod. Laurence e t a l . (1981) i n f e r r e d t h a t c a n n i b a l i s m o c c u r r e d i n cod r e a r i n g t a n k s b e c a u s e v a r i a b i l i t y i n l e n g t h d e c l i n e d and s u r v i v a l d e c r e a s e d i n g r o u p s o f cod l a r v a e m a i n t a i n e d a t low food densities.

V a r i a b i l i t y i n l e n g t h i n c r e a s e d u n t i l an age of one month a f t e r which it d e c r e a s e d i n d i r e c t p r o p o r t i o n t o t h e d e n s i t y o f p r e y i n t h e c o n t a i n e r , i n d i c a t i n g t h e e x i s t e n c e of c a n n i b a l i s m . E l l e r t s e n e t a l . (1981) a l s o s u g g e s t s t h a t c a n n i b a l i s m o c c u r r e d i n t h e i r e n c l o s u r e s t u d y b e c a u s e t h e s u r v i v a l of a g r o u p o f cod l a r v a e r e a r e d w i t h o l d e r , n e a r metamorphic i n d i v i d u a l s , was much lower t h a n e x p e c t e d .

T h i s d i s c u s s i o n i n d i c a t e s t h a t p l a n k t i v o r o u s f i s h e s such a s h e r r i n g , s p r a t , p i l c h a r d , mackerel and o t h e r s consume eggs and l a r v a e of cod and o t h e r marine f i s h e s . T h i s c o u l d b e e x p e c t e d s i n c e p e l a g i c f i s h eggs and l a r v a e d i f f e r l i t t l e i n s i z e and a v o i d a n c e c a p a b i l i t i e s from t h e t y p i c a l f o o d s consumed by s u c h f i s h e s , and i f e n c o u n t e r e d t h e y would b e consumed a l o n g w i t h t h e i r t y p i c a l foods (copepods and e u p h a u s i i d s ) .

P e l a g i c p l a n k t i v o r o u s f i s h e s c a n b e abundant i n cod spawning h a b i t a t s and t h e r e f o r e c o u l d be r e s p o n s i b l e f o r a s i g n i f i c a n t p r o p o r t i o n of t h e m o r t a l i t y o f cod e g g s and l a r v a e a s w e l l a s t h a t of o t h e r s p e c i e s . For example, Edwards and Bowman ( 1 9 7 9 ) l i s t 16 i m p o r t a n t s p e c i e s of p e l a g i c p l a n k t i v o r o u s f i s h e s from a cod h a b i t a t i n t h e w e s t e r n A t l a n t i c ( c o n t i n e n t a l s h e l f o f f New J e r s e y and Nova S c o t i a ) w i t h o u t even c o n s i d e r i n g t h e p e l a g i c p l a n k t i v o r o u s l i f e s t a g e s o f t h e d e m e r s a l f i s h e s i n t h a t a r e a . They e s t i m a t e d t h e z o o p l a n k t o n consumed by t h e two most abundant p l a n k t i v o r o u s s p e c i e s ( A t l a n t i c h e r r i n g and A t l a n t i c m a c k e r e l ) v a r i e d from 2 ' 106 t o 26 ' 106 t o n s a n n u a l l y f o r

h e r r i n g and from 4 ' 106 t o 28 ' 106 t o n s f o r m a c k e r e l depending on t h e i r p o p u l a t i o n s i z e s . C l e a r l y , f l u c t u a t i o n s i n t h e s i z e of t h e s e two major p l a n k t i v o r o u s f i s h s t o c k s m i g h t i n d u c e major v a r i a t i o n i n t h e m o r t a l i t y o f cod eggs and l a r v a e even i f cod e a r l y l i f e s t a g e s were an i n c i d e n t a l p a r t of t h e i r d i e t . T h i s s u g g e s t s t h a t a d i r e c t c a u s a l r e l a t i o n s h i p might be r e s p o n s i b l e f o r t h e o b s e r v e d a n t a g o n i s t i c b e h a v i o u r of p e l a g i c s t o c k s ( h e r r i n g and s p r a t ) and demersal s t o c k s ( c o d and p l a i c e ) i n t h e North S e a (Daan, 1 9 7 6 ) .

The p r o p o r t i o n of egg and l a r v a l m o r t a l i t y t h a t c a n be a t t r i b u t e d t o p r e d a t i o n by p l a n k t i v o r o u s f i s h e s h a s r a r e l y been determined f o r any s p e c i e s . Daan (MS p r e s e n t e d a t t h e ICES c o u n c i l meeting i n 1 9 8 3 ) e s t i m a t e s t h a t a t l e a s t 1% of t h e eggs produced by cod and a t l e a s t 7% of t h o s e produced by p l a i c e a r e consumed by h e r r i n g . T h i s may b e a n u n d e r e s t i m a t e b e c a u s e h e assumed t h e d i g e s t i o n t i m e f o r a n egg was one day and b e c a u s e t h e h e r r i n g s t o c k i n t h e North Sea i s s t i l l a t a r a t h e r low l e v e l . The o n l y p u b l i s h e d s t u d y where c a n n i b a l i s m o r p r e d a t i o n h a s been r e l a t e d t o n a t u r a l m o r t a l i t y i s t h a t of Hunter and Kimbrell ( 1 9 8 0 b ) . They e s t i m a t e d t h a t n o r t h e r n anchovy stomachs c o n t a i n e d a mean of 5 . 1 anchovy eggs/stomach and u s i n g an i n s t a n t a n e o u s c o e f f i c i e n t of g a s t r i c e v a c u a t i o n of 0.71/h t h e y e s t i m a t e d t h e d a i l y consumption of eggs t o be 85.8 e g g s / f i s h o r a b o u t 17% of t h e d a i l y egg p r o d u c t i o n . MacCall ( 1 9 8 0 ) , u s i n g a new e s t i m a t e of n a t u r a l m o r t a l i t y of eggs ( Z = 0 . 3 9 ) and a few o t h e r m o d i f i c a t i o n s , e s t i m a t e d from t h e i r d a t a t h a t c a n n i b a l i s m on eggs c o u l d a c c o u n t f o r 28% of t o t a l egg m o r t a l i t y .

Hunter and K i m b r e l l ( 1 9 8 0 b ) n o t e d t h a t t h e eggs were e a t e n by f i l t e r i n g , b u t t h i s p r o c e s s a p p e a r s t o b e non- random, s i n c e consumption of eggs i n t h e s e a i n c r e a s e d a s t h e 1 . 6 power of egg abundance a l t h o u g h t h e form o f t h e r e l a t i o n s h i p i s u n c l e a r . A t t i m e s anchovy s c h o o l s p r o b a b l y a g g r e g a t e on p a t c h e s of eggs r a t h e r t h a n t a k i n g them

i n c i d e n t a l l y when f e e d i n g on o t h e r z o o p l a n k t o n , b e c a u s e egg d e n s i t i e s i n t h e s e a a r e a t t i m e s h i g h enough t o s t i m u l a t e s e l e c t i v e f i l t r a t i o n ( ~ u n t e r and Dorr, 1 9 8 2 ) .

VARIATION I N VULNERABILITY

The v u l n e r a b i l i t y o f a l a r v a t o i t s p r e d a t o r s i s c o n t r o l l e d by t h e i n t r i n s i c c h a r a c t e r i s t i c s of s i z e and m a t u r a t i o n a l s t a t e , and by v a r i o u s e x t r i n s i c v a r i a b l e s i n c l u d i n g l a r v a l f o r a g e , t e m p e r a t u r e , a l t e r n a t i v e p r e y f o r t h e p r e d a t o r , and v i s i b i l i t y .

~ n t r i n s i c f a c t o r s

The most i m p o r t a n t f a c t o r a f f e c t i n g v u l n e r a b i l i t y of l a r v a e t o p r e d a t i o n i s w i t h o u t a d o u b t t h e m a t u r a t i o n a l s t a t e . The l e a s t m a t u r e s t a g e s ( e g g and y o l k - s a c ) a r e a l s o t h e most d e f e n c e l e s s , s u f f e r t h e h i g h e s t l o s s e s , and a r e v u l n e r a b l e t o t h e g r e a t e s t v a r i e t y o f p r e d a t o r s r a n g i n g from s m a l l i n v e r t e b r a t e s t o f i s h e s . I know of o n l y two s t u d i e s i n which t h e d e c l i n e i n v u l n e r a b i l i t y t o p r e d a t i o n w i t h l a r v a l m a t u r a t i o n h a s been d i r e c t l y s t u d i e d , a l t h o u g h t h e marked d e c l i n e i n t h e c a p t u r e s u c c e s s of p l a n k t o n n e t s w i t h l a r v a l s i z e i s a n i m p o r t a n t s o u r c e of i n d i r e c t e v i d e n c e ( s m i t h and Richardson, 1977; H u n t e r , 1 9 8 1 ) . I n h e r t h e s i s work, L. Dowd ( u n i v e r s i t y o f Miami, p e r s o n a l communication, 1 9 8 3 ) f i n d s t h a t t h e consumption r a t e s of j u v e n i l e bay anchovy (Anchoa mi,tchil.Li) d e c l i n e e x p o n e n t i a l l y from a b o u t 80 s e a bream l a r v a e p e r h o u r (Ahchobangub hhomboida.Lib) f o r 2.5 mm l a r v a e t o a b o u t 1 0 / h f o r 4.5 mm l a r v a e . She found t h a t t h e y o l k - s a c s t a g e s and eggs were l e s s v u l n e r a b l e t h a n 2.5 mm l a r v a e b e c a u s e lower a c t i v i t y l e v e l s made t h e yolk- s a c l a r v a e l e s s v i s i b l e . Webb ( 1 9 8 1 ) p r o v i d e d a s t u d y on changes i n v u l n e r a b i l i t y o f n o r t h e r n anchovy t o c a p t u r e by t h e aquarium f i s h Amphiphion p e h c u l a . The s u c c e s s of anchovy i n a v o i d i n g c a p t u r e i n c r e a s e d l i n e a r l y w i t h s i z e ,

w i t h t h e p e r c e n t a g e of l a r v a e t h a t responded t o a t t a c k i n c r e a s i n g from 9 % i n yolk-sac l a r v a t o 85% i n 12 mm l a r v a e . The improvement i n t h e avoidance of p r e d a t o r s w i t h m a t u r a t i o n may b e due t o d e c r e a s e s i n t h e l a t e n c y o f t h e e s c a p e r e s p o n s e , i n c r e a s e s i n t h e speed and s t a m i n a o f avoidance movements o r some combination of t h e s e f a c t o r s . Webb ( 1 9 8 1 ) found t h a t t h e c r i t i c a l element i n t h e enhanced a b i l i t y o f l o n g e r l a r v a e t o e s c a p e t h e p r e d a t o r

was t h e l a t e n c y o f t h e s t a r t l e r e s p o n s e , a s o n l y l a r v a e t h a t responded t o o l a t e o r n o t a t a l l were c a u g h t . Thus t h e m a t u r a t i o n of s e n s o r y systems seems t o be more i m p o r t a n t t h a n a n i n c r e a s e i n b u r s t speed f o r t h i s p a r t i c u l a r p r e d a t o r .

The mode of a t t a c k o f a p r e d a t o r d e t e r m i n e s which f a c t o r s a r e o f g r e a t e s t importance i n a v o i d i n g c a p t u r e . For example, t h e s t r i k i n g b e h a v i o u r of f i s h e s range o v e r a continuum from s u s t a i n e d p u r s u i t t o sudden l u n g e s (Webb and Skadsen, 1 9 8 0 ) . Lungers u s u a l l y s t a r t an a t t a c k w i t h t h e body p o s i t i o n i n an "S" s h a p e , s t r i k e a t t h e p r e y a t h i g h speed from s h o r t range and seldom pursue t h e p r e y i f t h e a t t a c k f a i l s , whereas p u r s u e r s s t a r t w i t h t h e body p o s i t i o n e d i n a " C " s h a p e , s t r i k e a t slower speed from g r e a t e r d i s t a n c e s , and p e r s i s t e n t l y c h a s e t h e p r e y i f t h e f i r s t a t t e m p t f a i l s (Webb and Skadsen, 1980: Webb, 1983;

Webb, Univ. o f Mich., p e r s o n a l communication, 1 9 8 3 ) . C l e a r l y m a t u r a t i o n of s e n s o r y systems w i l l be o f utmost importance i n a v o i d i n g t h e a t t a c k of l u n g i n g p r e d a t o r s , whereas b o t h locomotor and s e n s o r y systems a r e i m p o r t a n t i n a v o i d i n g p u r s u i n g p r e d a t o r s .

E x t r i n s i c f a c t o r s

Two of t h e most i m p o r t a n t e x t r i n s i c f a c t o r s t h a t a f f e c t v u l n e r a b i l i t y t o p r e d a t i o n a r e t e m p e r a t u r e and t h e abundance of l a r v a l f o r a g e . Low t e m p e r a t u r e o r l a c k o f food slows growth and t h e r e b y i n c r e a s e s t h e d u r a t i o n t h a t l a r v a e remain

i n t h e s m a l l e r and most v u l n e r a b l e l i f e s t a g e s . Poor y e a r - c l a s s e s of P a c i f i c s a r d i n e were a s s o c i a t e d w i t h c o l d w a t e r t e m p e r a t u r e s and h i g h food abundance, and it may have been t h e p r o l o n g a t i o n of t h e l a r v a l p h a s e by low t e m p e r a t u r e r e s u l t i n g i n h i g h e r p r e d a t i o n t h a t l e d t o weak y e a r - c l a s s e s ( ~ u r ~ h y , 1961 )

.

^{I n} s u p p o r t of t h i s argument Murphy c a l c u l a t e d t h a t a 3' d e c l i n e i n w a t e r t e m p e r a t u r e (17-14' C ) c o u l d d e c r e a s e by 10 f o l d t h e number of s a r d i n e l a r v a e t h a t a t t a i n 21 mm. P r o l o n g a t i o n of t h e e a r l y l i f e s t a g e s of cod by low t e m p e r a t u r e might a l s o i n c r e a s e m o r t a l i t y r a t e s a s a 3' d e c l i n e i n t e m p e r a t u r e (10-7" C ) i n c r e a s e s t h e t i m e from f e r t i l i z a t i o n t o metamorphosis by a b o u t 20% ( L a u r e n c e and Roger, 1976; Laurence, 1 9 7 8 ) .

An i n c r e a s e i n v u l n e r a b i l i t y t o p r e d a t o r s a s a r e s u l t of slow growth can b e caused by low food abundance a s w e l l a s by t e m p e r a t u r e . Dowd ( U n i v e r s i t y of Miami, p e r s o n a l

communication, 1983) found t h a t s e a bream l a r v a e r e a r e d on low food r a t i o n s were much more v u l n e r a b l e t o p r e d a t i o n by j u v e n i l e bay anchovy t h a n w e r e t h o s e r e a r e d on a h i g h e r r a t i o n . She d e m o n s t r a t e s t h a t t h e i n c r e a s e i n v u l n e r a b i l i t y c a n b e e x p l a i n e d e n t i r e l y by t h e s m a l l e r s i z e of l a r v a e f e d t h e lower r a t i o n . Lack o f food may a l s o i n c r e a s e v u l n e r a b i l i t y b e c a u s e of a d e t e r i o r a t i o n of e s c a p e r e s p o n s e of s t a r v e d l a r v a e . Twice a s many hake l a r v a e were a t t a c k e d by t h e c a r n i v o r o u s copepod E u c h a e t a when t h e y were s t a r v e d f o r 3-4 days t h a n when n o t s t a r v e d ( B a i l e y and Yen, 1 9 8 3 ) . A s t h e l a r v a e d i f f e r e d l i t t l e i n s i z e t h e i n c r e a s e d v u l n e r a b i l i t y may b e a t t r i b u t e d t o a d e g e n e r a t e d e s c a p e r e s p o n s e .

O t h e r e x t r i n s i c f a c t o r s such a s p r e s e n c e of a l t e r n a t i v e p r e y , abundance of p r e d a t o r s , and p a t c h i n e s s of eggs and l a r v a e were mentioned p r e v i o u s l y . I w i l l n o t d w e l l on t h e s e f a c t o r s h e r e o t h e r t h a n t o p o i n t o u t t h a t t h e p r e s e n c e of a l t e r n a t i v e p r e y c o n s i s t e n t l y r e d u c e s p r e d a t i o n r a t e s i n most l a b o r a t o r y s t u d i e s t o d a t e . O t h e r f a c t o r s which have y e t t o be s t u d i e d i n l a r v a l f i s h e s a r e t h e e f f e c t of t h e

o n s e t of s c h o o l i n g , s h e l t e r - s e e k i n g b e n e a t h j e l l y f i s h , e f f e c t s of v e r t i c a l movements o r c u r r e n t s which may c o n c e n t r a t e eggs o r l a r v a e , v i s i b i l i t y of p r e d a t o r s and p r e y a t d i f f e r e n t t i m e s of d a y and i n d i f f e r e n t w a t e r t y p e s , and t e m p e r a t u r e - s p e c i f i c a c t i v i t y l e v e l s of l a r v a e , any o f which may a l t e r t h e a t t r a c t i v e n e s s of l a r v a e f o r c e r t a i n p r e d a t o r s .

DISCUSSION

A l l t h e w e l l documented a c c o u n t s f o r consumption of eggs and l a r v a e by i n v e r t e b r a t e p r e d a t o r s and most of t h o s e f o r f i s h , a r e f o r s p e c i e s o t h e r t h a n cod; t h e c l u p e o i d s , a n c h o v i e s and h e r r i n g comprise t h e m a j o r i t y o f such r e c o r d s . Owing t o t h e s i m i l a r i t y i n s i z e of p e l a g i c eggs and l a r v a e , i t seems l i k e l y t h a t cod l a r v a e a t t h e same l i f e s t a g e would be v u l n e r a b l e t o t h e p r e d a t o r s d i s c u s s e d i n t h e p r e v i o u s s e c t i o n s i f cod l a r v a e CO-occurred w i t h them. Two undocumented a c c o u n t s seem t o s u p p o r t t h i s view; Lee (1966) mentions t h a t t h e amphipods P a x a t h e m i ~ t o LibcLLuta and P~eudoEibxotun s p . consume cod l a r v a e , and i t i s noted i n F r a s e r (1962) t h a t PLeunobhachia a l s o feed on cod l a r v a e .

The w e l l documented h i g h m o r t a l i t y o f cod eggs i s c e r t a i n l y t h e b e s t e v i d e n c e f o r a h i g h r a t e of p r e d a t i o n d u r i n g cod e a r l y l i f e h i s t o r y , y e t , t h e i d e n t i t y o f t h e p r e d a t o r s p r i m a r i l y r e s p o n s i b l e f o r t h e s e l o s s e s s t i l l remains i n d o u b t . The p l a n k t i v o r o u s f i s h e s , s p r a t , h e r r i n g and o t h e r s consume cod eggs b u t on t h e b a s i s o f p r e s e n t e v i d e n c e it seems u n l i k e l y t h a t t h e y can a c c o u n t f o r a l l t h e l o s s e s . C e r t a i n l y , medusae and c t e n o p h o r e s ( p a r t i c u l a r l y PEeuxobhachia) may consume s i g n i f i c a n t q u a n t i t i e s o f cod eggs. The i d e n t i f i c a t i o n of t h e major p r e d a t o r s o f cod eggs i s n o t a t r i v i a l q u e s t i o n s i n c e cod e x i s t a s eggs f o r about 2 0 % of t h e p e r i o d between f e r t i l i z a t i o n and metamorphosis.

I n a d d i t i o n , consumers of eggs ( t h e p l a n k t i v o r o u s f i s h e s and

c t e n o p h o r e s ) a r e a l s o l i k e l y t o b e i m p o r t a n t consumers of y o l k - s a c s t a g e s a s w e l l , b u t yolk-sac s t a g e s a r e l e s s commonly d e t e c t e d because of f a s t d i g e s t i o n r a t e s ( s e e n e x t s e c t i o n ) . I n f a c t , m o r t a l i t y r a t e s of anchovy ( H u n t e r , 1982) and p l a i c e (Harding e t a l . , 1 9 7 8 ) o f t e n do n o t d i f f e r much between egg and y o l k - s a c s t a g e s , i n d i c a t i n g t h a t eggs and yolk-sac s t a g e s may b e v u l n e r a b l e t o t h e same p r e d a t o r s .

The t a x a of p r e d a t o r s t h a t may be t h e most i m p o r t a n t i n t h e e a r l y l i f e h i s t o r y o f cod a t any l i f e s t a g e a r e unknown.

The l i t e r a t u r e I have d i s c u s s e d f o r o t h e r s p e c i e s of f i s h e s i n d i c a t e s t h a t t h e s m a l l c r u s t a c e a n s (copepods, e u p h a u s i d s and amphipods) a r e i m p o r t a n t consumers of o n l y t h e y o l k - s a c s t a g e s , whereas p l a n k t i v o r o u s f i s h e s consume eggs, y o l k - s a c s t a g e and o l d e r l a r v a e a s w e l l . The l a r g e r i n v e r t e b r a t e s ( c t e n o p h o r e s , s q u i d , and some of t h e medusae) a r e a l s o c e r t a i n l y a b l e t o consume l a r g e r l a r v a e a s w e l l . The consumption r a t e s of f i s h e s i n c r e a s e much more w i t h an i n c r e a s e i n egg o r l a r v a l d e n s i t y t h a n do consumption r a t e s o f t h e s m a l l e r i n v e r t e b r a t e s . H e r r i n g o r s p r a t would be c a p a b l e of consuming t h o u s a n d s of cod eggs o r y o l k - s a c l a r v a e p e r day i f t h e d e n s i t y were s u f f i c i e n t l y h i g h , whereas t h e s m a l l e r i n v e r t e b r a t e s s a t i a t e r a p i d l y and maximum a t t a c k r a t e s a r e o n l y 2-6 l a r v a e p e r day. T h i s i s an i m p o r t a n t d i s t i n c t i o n b e c a u s e it i n d i c a t e s t h a t p r e d a t i o n by f i s h e s i s more l i k e l y t o have a s t r o n g e r dependence on p r e y d e n s i t y t h a n p r e d a t i o n by s m a l l i n v e r t e b r a t e s . I t

seems a p p r o p r i a t e , owing t o t h e l a c k o f i n f o r m a t i o n on p r e d a t i o n and t h e i n h e r e n t d i f f i c u l t y of such s t u d i e s , t o conclude t h i s review w i t h a d i s c u s s i o n of t h e problems i n h e r e n t i n i n v e s t i g a t i o n s of t h e p r e d a t o r s of marine f i s h eggs and l a r v a e .

Problems i n e s t i m a t i n g e f f e c t s of p r e d a t o r s

A c c u r a t e e s t i m a t i o n of t h e e f f e c t of p e l a g i c p r e d a t o r s on t h e s u c c e s s of y e a r - c l a s s e s of f i s h e s i s perhaps t h e most

d i f f i c u l t o f a l l r e s e a r c h on . r e c r u i t m e n t p r o c e s s e s . Some of t h e major d i f f i c u l t i e s i n a q u a n t i t a t i v e a s s e s s m e n t of p r e d a t i o n on eggs and l a r v a e a r e : t h e i n c i d e n t a l n a t u r e of eggs and l a r v a e i n t h e d i e t s of t h e i r p r e d a t o r s , which r e q u i r e s a n a s s e s s m e n t of t h e r o l e of a l t e r n a t i v e p r e y ; t h e p a t c h i n e s s of t h e y o u n g e s t s t a g e s ( e g g and y o l k - s a c s t a g e s ) and t h e r a r i t y o f t h e o l d e r s t a g e s i n t h e p l a n k t o n ; t h e r a p i d d i g e s t i o n of e a r l y eggs and e a r l y l a r v a e i n t h e stomach of p r e d a t o r s ; b i a s e s r e s u l t i n g from p r e d a t o r s f e e d i n g w i t h i n n e t c a t c h e s , and from undersampling l a r g e j e l l y f i s h and c t e n o p h o r e s ; a n d , of c o u r s e , t h e g r e a t d i v e r s i t y o f p o t e n t i a l p r e d a t o r s . I d i s c u s s some of t h e s e i s s u e s below and s u g g e s t some a p p r o a c h e s .

I t h a s l o n g been r e c o g n i z e d t h a t abnormal f e e d i n g may o c c u r among r a p t o r i a l copepods

,

amphipods, and c h a e t o n a t h s i n t h e crowded c o n d i t i o n s of p l a n k t o n samples. Many o f t h e food h a b i t s t u d i e s may y i e l d u n n a t u r a l l y h i g h p r e d a t i o n r a t e s on l a r v a e b e c a u s e chance e n c o u n t e r s a r e g r e a t l y i n c r e a s e d . Records of f e e d i n g on l a r v a e by c h a e t o n a t h s might f a l l i n t h i s c a t e g o r y b e c a u s e l a b o r a t o r y work d o e s n o t seem t o i d e n t i f y them a s i m p o r t a n t l a r v a l p r e d a t o r s ( ~ u h l m a n n , 1 9 7 7 ) . ~ n s e r t i o n of dyed l a r v a e w i t h i n t h e n e t p r i o r t o a tow m i g h t b e a method of c o r r e c t i n g f o r t h i s p o s i b l e b i a s a s t h i s t e c h n i q u e h a s been s u c c e s s f u l l y used i n stomach a n a l y s e s of t r a w l c a u g h t f i s h e s ( L a n c r a f t and Robinson, 1979; Robinson and B a i l e y , 1 9 8 2 ) .

The r a p i d d i g e s t i o n of y o l k - s a c l a r v a e by j e l l y f i s h and f i s h e s , and t h e i r t o t a l d e s t r u c t i o n by c r u s t a c e a n p r e d a t o r s g r e a t l y r e d u c e s t h e chance of d e t e c t i o n of p r e d a t i o n . W i t h i n 5 h o u r s , y o l k - s a c h e r r i n g l a r v a e a r e d i g e s t e d beyond r e c o g n i t i o n by Aufieeia a t 10-12'C ( ~ S l l e r , 1 9 8 0 ) , and d i g e s t i o n of t h e s m a l l e r y o l k - s a c l a r v a e of p e l a g i c spawners p r o c e e d s much more r a p i d l y . L a r v a e of American f l o u n d e r and Gadun naLda a r e t o t a l l y d i g e s t e d e x c e p t t h e e y e s i n 1 . 5 h o u r s i n AuheLia stomachs and t a k e o n l y 2 h o u r s t o d i s a p p e a r c o m p l e t e l y ( F r a s e r , 1 9 6 9 ) . S i m i l a r l y , t h e y o l k - s a c t o f i r s t

f e e d i n g s t a g e s of n o r t h e r n anchovy a r e d i g e s t e d beyond r e c o g n i t i o n i n t h e stomachs of a d u l t anchovy i n 0.5 h o u r s a t 1 6 " ~ ( ~ u n t e r and ~ i m b r e l l , 1 9 8 0 b ) . Eggs i n anchovy stomachs d i s a p p e a r a t a r a t e of a b o u t 50%/h; eggs a r e d e t e c t a b l e somewhat l o n g e r t h a n yolk-sac anchovy l a r v a e , presumably b e c a u s e t h e c h o r i o n i s more r e s i s t a n t t o d i g e s t i o n t h a n i s t h e l a r v a . The r a p i d d i g e s t i o n of eggs and e a r l y l a r v a e h a s two i m p o r t a n t i m p l i c a t i o n s . Low a v e r a g e numbers i n t h e stomachs of p r e d a t o r s may i n d i c a t e a much g r e a t e r impact t h a n a n t i c i p a t e d b e c a u s e t h e d a i l y r a t i o n i n c r e a s e s markedly when a d j u s t e d by a r a p i d g a s t r i c e v a c u a t i o n r a t e ( H u n t e r and K i m b r e l l , 1 9 8 0 b ) . Secondly, n e t tows o r t r a w l s s h o u l d be of s h o r t d u r a t i o n s i n c e t y p i c a l d u r a t i o n of t r a w l h a u l s may exceed t h e d u r a t i o n of d e t e c t a b i l i t y o f yolk-sac s t a g e s . To my knowledge no one h a s examined t h e change i n t h e r a t e of d i g e s t i o n of l a r v a e i n p r e d a t o r stomachs a s a f u n c t i o n of l a r v a l s i z e , a l t h o u g h it i s c l e a r t h a t o l d e r l a r v a e may remain i n t h e stomach f o r much l o n g e r p e r i o d s t h a n y o l k - s a c s t a g e s . I r o n i c a l l y , t h e most abundant s t a g e s which have t h e h i g h e s t p r o b a b i l i t y o f o c c u r r i n g i n p r e d a t o r stomachs, d i s a p p e a r from t h o s e stomachs a t t h e h i g h e s t r a t e , whereas l a r g e r l a r v a e which a r e presumably more r e s i s t a n t t o d i g e s t i o n may b e s o r a r e t h a t q u a n t i t i v e assessment through stomach examination may b e i m p r a c t i c a l .

The m a c e r a t i o n of l a r v a e when i n g e s t e d by c r u s t a c e a make s p e c i f i c i d e n t i f i c a t i o n of l a r v a e and e s t i m a t e s of consumption r a t e s d i f f i c u l t . There i s some r e a s o n f o r optimism, however, s i n c e c r u s t a c e a n p r e d a t o r s seem t o have low d a i l y consumption r a t e s , and immunological t e c h n i q u e s which g i v e o n l y i n c i d e n c e might b e u s e f u l f o r a s s e s s m e n t of consumption i n c r u s t a c e a n p r e d a t o r s . These t e c h n i q u e s have been used t o t r a c e food webs (Boreham and Ohiagu, 1978;

F e l l e r , 1 9 8 2 ) . L a b o r a t o r y s t u d i e s a r e r e q u i r e d t o check f o r c r o s s r e a c t i o n s and t o d e t e r m i n e t h e d u r a t i o n t h a t l a r v a e can be d e t e c t e d i n p r e d a t o r stomachs u s i n g immunological t e c h n i q u e s .

A c c u r a t e e s t i m a t e s of t h e impact of p r e d a t o r s a l s o r e q u i r e t h a t t h e consumption by p r e d a t o r s be measured a t t h e h e i g h t of t h e spawning s e a s o n , and s i n c e consumption by p r e d a t o r s may i n c r e a s e n o n - l i n e a r l y w i t h t h e abundance of eggs and l a r v a e , measurements of b o t h consumption r a t e s and l a r v a l abundance must be done a t t h e same p l a c e and t i m e . Eggs and e a r l y l a r v a e a r e s u f f i c i e n t l y c o n c e n t r a t e d a t t i m e s f o r p r e d a t o r s t o feed s e l e c t i v e l y r a t h e r t h a n consuming them i n c i d e n t l y w i t h o t h e r foods. T h i s seems t o be t h e c a s e f o r s c h o o l s of anchovy f e e d i n g on t h e i r own eggs ( H u n t e r and K i m b r e l l , 1980a) and may b e i m p o r t a n t f o r t h e p l a n k t i v o r o u s p r e d a t o r s of cod a s w e l l , b u t such e f f e c t s can o n l y be d e t e c t e d by s i m u l t a n e o u s p l a n k t o n and p r e d a t o r c a t c h e s .

The g r e a t v a r i e t y o f p o t e n t i a l p r e d a t o r s of cod eggs and l a r v a e makes it i m p r a c t i c a l t o s t u d y s y s t e m a t i c a l l y e v e r y one. Thus t h e most r e a s o n a b l e approach i s t o s e l e c t s e v e r a l t h a t seem t o be t h e most i n f l u e n t i a l . Such p r e d a t o r s must have a h i g h p o p u l a t i o n biomass, a h i g h p o p u l a t i o n consumption r a t e , and a p r e f e r e n c e f o r t h e s i z e o r t a x o n i n q u e s t i o n , b u t s t i l l be a b l e t o m a i n t a i n a h i g h abundance on a l t e r n a t i v e prey and have r e c r u i t m e n t t h a t i s r e l a t i v e l y i n d e p e n d e n t of t h e prey organism abundance ( R o t h s c h i l d and Rooth, 1 9 8 2 ) . The p l a n k t i v o r o u s f i s h e s ( h e r r i n g and s p r a t ) and some of t h e i n v e r t e b r a t e s may f i t t h i s r o l e f o r cod eggs and l a r v a e .

The p r e d a t i o n problem seems s o complex t h a t it may t a k e a long time b e f o r e m o r t a l i t y r a t e s of cod can be p a r t i t i o n e d among t h e d i f f e r e n t g r o u p s of p r e d a t o r s o r p r e d a t o r - f e e d i n g models t e s t e d a g a i n s t a c c u r a t e f i e l d e s t i m a t e s of consumption. Thus, a s a f i r s t s t e p , i t may be p r e f e r a b l e t o a s s e s s t h e i n c i d e n c e of s t a r v a t i o n i n t h e s e a , and t h e r e b y d e t e r m i n e by d i f f e r e n c e t h e p r o p o r t i o n of t o t a l m o r t a l i t y caused by p r e d a t i o n . A c c u r a t e f i e l d work can be done now on s t a r v a t i o n u s i n g h i s t o l o g i c a l c r i t e r a and growth i n c r e m e n t s on o t o l i t h s , whereas f i e l d work on p r e d a t i o n h a s many more u n c e r t a i n t i e s . I n a d d i t i o n , v a r i o u s l a b o r a t o r y s t u d i e s on