395-440.pdf (1.435Mb)

F l 4 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 o f Cod G a d u s m o r h u a L.

LARVAL FISH TROPHODYNAMIC STUDIES ON GEORGES BANK: SAMPLING

STRATEGY AND INITIAL RESULTS

R. G . Lough

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 N o r t h e a s t F i s h e r i e s C e n t e r

Woods Hole L a b o r a t o r y

Woods H o l e , M a s s a c h u s e t t s 02543 USA

ABSTRACT

Lough, R. G . , 1984. L a r v a l f i s h trophodynamic s t u d i e s on Georges Bank: Sampling s t r a t e g y and i n i t i a l r e s u l t s . I n : E . Dahl, D.S. D a n i e l s s e n , E. Moksness and P. Solem- d a l ( E d i t o r s )

,

The P r o p a g a t i o n o f Cod G a d u s m o r h u a L . F l 6 d e v i g e n r a p p o r t s e r . , 1, 1984: 395-434.

A s a m p l i n g s t r a t e g y i s o u t l i n e d t o s e r v e a s a framework f o r d e t e r m i n i n g t h e f i n e - t o m i c r o - s c a l e v e r t i c a l d i s t r i - b u t i o n o r f i s h l a r v a e and t h e i r p r e y on Georges Bank i n a s i n g l e v e s s e l , i n t e r d i s c i p l i n a r y mode o f o p e r a t i o n . A major o b j e c t i v e o f t h i s s a m p l i n g program i s t o c h a r a c t e r i z e t h e development and t e m p o r a l - s p a t i a l v a r i a b i l i t y o f t h e s e d i s - t r i b u t i o n s t o e v a l u a t e growth and s u r v i v a l o f l a r v a l popu- l a t i o n ~ . The o p e r a t i o n a l p l a n , s a m p l i n g g e a r and i n s t r u - m e n t a t i o n , a s w e l l a s s p e c i a l t e c h n i q u e s employed a r e d i s - c u s s e d i n t e r m s o f t h e u s e f u l n e s s o f t h e p a r a m e t e r s measured.

I n i t i a l r e s u l t s a r e p r e s e n t e d from a t w o - p a r t s t u d y c o n d u c t e d i n April-May 19 81, f o c u s e d on haddock ( ~ e l a n o y r a m m u s a e g l e - f i n u s L . ) and Cod adus us m o r h u a L . ) l a r v a e .

I n A p r i l , a g a d i d egg p a t c h w i t h r e c e n t l y - h a t c h e d l a r v a e

( C . 91% haddock) was l o c a t e d on t h e s o u t h e a s t e r n p a r t o f Georges Bank, between t h e t i d a l l y - w e l l - m i x e d f r o n t ( c . 60-m i s o b a t h ) and t h e s h e l f / s l o p e - w a t e r f r o n t ( c . 100 m ) . The w a t e r column a l o n g t h e s o u t h e r n f l a n k was s t i l l well-mixed i n A p r i l and t h e l a r v a e were b r o a d l y d i s t r i b u t e d w i t h a w e i g h t e d mean d e p t h between 30 and 40 m. D e n s i t y o f t h e i r dominant copepod p r e y was r e l a t i v e l y low n e a r t h e s u r f a c e

( c 3 p r e y / l ) b u t i n c r e a s e d w i t h d e p t h (5-10 p r e y / l ) .

When t h e same l a r v a l p o p u l a t i o n was s u r v e y e d a g a i n i n May i t had moved t o t h e s o u t h w e s t a t a r a t e c o n s i s t e n t w i t h t h e r e s i d u a l c u r r e n t s . By May t h e w a t e r column was s t r a t i f i e d a l o n g t h e s o u t h e r n f l a n k . A s e a s o n a l t h e r m o c l i n e was ob- s e r v e d between 10 and 20 m and f i s h l a r v a e and t h e i r p r e y

(50 p r e y / l ) were c o n c e n t r a t e d i n t h i s zone. A s t o r m swept t h e r e g i o n and d i s p e r s e d t h e l a r v a e and p r e y (5-10 p r e y / l ) t h r o u g h o u t t h e w a t e r column. On t h e c r e s t of t h e bank i n t h e well-mixed w a t e r s ( < 6 0 m ) , l a r v a e and t h e i r p r e y (10-25 p r e y / l ) were b r o a d l y d i s t r i b u t e d v e r t i c a l l y , b u t t h e mean d e p t h o f t h e l a r v a e c o i n c i d e d w i t h t h e h i g h e s t d e n s i t y o f p r e y a t middepth. The i m p l i c a t i o n of t h e s e o b s e r v a t i o n s t o haddock and cod s u r v i v a l a r e d i s c u s s e d .

INTRODUCTION

O t h e r t h a n c a t a s t r o p h i c l o s s e s , t r o p h i c ( f e e d i n g ) i n t e r - r e l a t i o n s h i p s i n v o l v i n g b o t h growth and p r e d a t i o n a r e con- s i d e r e d t o be t h e b a s i c f a c t o r s c o n t r o l l i n g l a r v a l m o r t a l i t y . The m o r t a l i t y p r o c e s s a t t h e i n d i v i d u a l l e v e l i s t h o u g h t t o be a f u n c t i o n o f chance e n c o u n t e r s by l a r v a e w i t h t h e i r pred- a t o r s and zooplankton p r e y which ( l i k e t h e l a r v a e t h e m s e l v e s ) a r e d i s t r i b u t e d c o n t a g i o u s l y o r i n p a t c h e s ( L a s k e r , 1975;

Vlymen, 1977; Beyer, 1 9 8 0 ) . I t i s b e l i e v e d t h a t t h e d e g r e e t o which l a r v a e a r e a b l e t o grow r a p i d l y t h r o u g h a s u c c e s s i o n o f d e c r e a s i n g p r e d a t o r y f i e l d s , t h e r e b y r e d u c i n g m o r t a l i t y , d e t e r m i n e s t h e i r p o t e n t i a l p o p u l a t i o n s i z e . However, t h i s p r o c e s s i s a complex f u n c t i o n of t h e d e n s i t y d i s t r i b u t i o n

( p a t c h i n e s s ) of t h e l a r v a e , t h e i r p r e y and p r e d a t o r s , and p o s s i b l e c o m p e t i t o r s o r o t h e r forms which may b e a l t e r n a t i v e p r e y o f l a r v a l p r e d a t o r s . S i n c e p r e y abundance below some l e v e l w i l l b e a c r i t i c a l f a c t o r i n f l u e n c i n g l a r v a l s u r v i v a l , i t i s n e c e s s a r y t o know how f e e d i n g of l a r v a e i n t h e f i e l d i s a f f e c t e d by t h e f i n e - s c a l e ( p a t c h y ) d i s t r i b u t i o n o f plank- t o n communities and t o u n d e r s t a n d t h e b i o l o g i c a l and p h y s i c a l p r o c e s s e s which l e a d t o t h e f o r m a t i o n and d i s s i p a t i o n o f such p a t c h e s .

A t t h e N o r t h e a s t F i s h e r i e s C e n t e r (NEFC), t h e Marine Ecosystems D i v i s i o n i s c o n d u c t i n g a broad-based r e s e a r c h program (MARMAP) on t h e C o n t i n e n t a l S h e l f , i n v o l v i n g b o t h m o n i t o r i n g and p r o c e s s - o r i e n t e d s t u d i e s , d i r e c t e d towards a b e t t e r u n d e r s t a n d i n g of t h e r e c r u i t m e n t p r o c e s s (Gross- l e i n e t a l . , 1979; Sherman, 1980)

.

I n t h e l a s t d e c a d e , p r o c e s s - o r i e n t e d s t u d i e s have been c a r r i e d o u t by t h e NEFC i n t h e Georges Bank a r e a a d d r e s s i n g t h e r e c r u i t m e n t problem.

The f i r s t major s t u d y i s r e p r e s e n t e d by t h e autumn 1978 L a r v a l H e r r i n g P a t c h S t u d y which was conducted a s an i n t e r - n a t i o n a l , m u l t i - s h i p , m u l t i - d i s c i p l i n a r y e x p e r i m e n t (Lough, 1 9 7 9 ) . The p r i m a r y o b j e c t i v e was t o d e f i n e and f o l l o w a p a t c h (homologous c o h o r t ) o f h e r r i n g l a r v a e a s a d i s s i p a t i v e f e a t u r e t o g a i n a b e t t e r u n d e r s t a n d i n g of t h e p h y s i c a l pro- c e s s e s a f f e c t i n g i t s d i s p e r s a l . The sampling s t r a t e g y was d e s i g n e d t o p r o v i d e s h o r t - t e r m e s t i m a t e s of l a r v a l growth and m o r t a l i t y i n r e l a t i o n t o t h e p r e y - p r e d a t o r f i e l d a s t h e p a t c h a d v e c t e d . More r e c e n t s t u d i e s have been conducted on haddock and cod l a r v a e s i n c e s p r i n g 1980 i n a s i n g l e v e s s e l , i n t e r - d i s c i p l i n a r y mode o f o p e r a t i o n . Most of t h e sampling e f f o r t i n t h i s mode i s t o d e t e r m i n e t h e f i n e - t o m i c r o - s c a l e v e r t i c a l d i s t r i b u t i o n o f l a r v a e and t h e i r p r e y (copepods) i n well-mixed and s t r a t i f i e d w a t e r s . A major o b j e c t i v e i n t h i s c a s e i s t o c h a r a c t e r i z e t h e development and t e m p o r a l v a r i - a b i l i t y of t h e s e d i s t r i b u t i o n s f o r u s e i n s i m u l a t i o n models.

These s t u d i e s r e q u i r e d i f f e r e n t s a m p l i n g s t r a t e g i e s w i t h i n t h e c o n s t r a i n t s o f a v a i l a b l e r e s o u r c e s t o meet t h e d e s i r e d o b j e c t i v e s .

Each s a m p l i n g s t r a t e g y must be u n i q u e l y d e s i g n e d f o r t h e s p e c i f i c o b j e c t i v e s and h y p o t h e s e s i n v e s t i g a t e d , t a k i n g i n t o a c c o u n t t h e p e c u l i a r i t i e s o f t h e t a r g e t s p e c i e s and i t s b i o - l o g i c a l and p h y s i c a l environment. However, a s an i n v e s t i - g a t i o n of l a r v a l f i s h growth and m o r t a l i t y i s i n h e r e n t l y complex, i n v o l v i n g t h e i n t i m a t e i n t e r a c t i o n of t h r e e t r o p h i c l e v e l s s i m u l t a n e o u s l y (Shepherd and Cushing, 1980; L a u r e n c e , 1 9 8 1 ) , a m u l t i - f a c e t e d sampling s t r a t e g y i s r e q u i r e d t o

r e s o l v e p a t t e r n s and i n t e r a c t i o n s o c c u r r i n g on t h e over- l a p p i n g time-space s c a l e s (Haury e t a l . , 1 9 7 8 ) . I n t h i s p a p e r o u r s a m p l i n g s t r a t e g y i s p r e s e n t e d on t h e haddock- cod s t u d y which h a s e v o l v e d i n p a r t from t h e r e s u l t s o f t h e L a r v a l H e r r i n g P a t c h Study. The e x p e r i m e n t a l o b j e c t i v e s , sampling g e a r and i n s t r u m e n t a t i o n employed a r e d i s c u s s e d i n terms o f t h e u s e f u l n e s s of t h e p a r a m e t e r s measured and h i g h l i g h t e d w i t h d a t a a n a l y z e d t o - d a t e .

T a r g e t S p e c i e s

Haddock ( ~ e l a n o g r a m m u s a e g l e f i n u s L.) was chosen a s t h e main t a r g e t s p e c i e s , f o l l o w e d by cod (Gadus morhua L . ) , be- c a u s e of i t s commercial and e c o l o g i c a l importance and t h e b e s t o v e r a l l b a s e o f l i f e h i s t o r y d a t a . T h i s d a t a b a s e i n c l u d e s e x t e n s i v e l a b o r a t o r y e x p e r i m e n t a l d a t a , an i n d e x o f y e a r - c l a s s s t r e n g t h a t t h e '0-group' s t a g e , and f e c u n d i t y and spawning p o p u l a t i o n biomass d a t a . The n o r t h e a s t e r n p a r t o f Georges Bank i s a p r i n c i p a l spawning ground f o r haddock and cod and t h e i r e a r l y l i f e h i s t o r i e s a r e s i m i l a r i n many r e s p e c t s . T h e i r spawning s e a s o n s o v e r l a p , b u t f o r cod i t i s c o n s i d e r a b l y l o n g e r and a l s o i t s spawning d i s t r i b u t i o n a p p e a r s t o e x t e n d f u r t h e r s o u t h t h a n t h e h a d d o c k ' s ( C o l t o n e t a l . , 1 9 7 9 ) . Cod spawn from l a t e autumn i n t o April-May, whereas haddock spawn from F e b r u a r y t o J u n e . Peak spawning

f o r b o t h cod and haddock o c c u r s i n t h e s p r i n g w i t h cod spawning a b o u t a month e a r l i e r t h a n haddock. The o n s e t and d u r a t i o n o f haddock spawning a p p e a r s t o b e a s s o c i a t e d w i t h i n c r e a s i n g w a t e r t e m p e r a t u r e (Marak a n d L i v i n g s t o n e , 1970)

.

F e r t i l i z e d cod and haddock e g g s h a t c h i n a b o u t 2 - 3 weeks a t a v e r a g e s p r i n g t e m p e r a t u r e s (Marak and C o l t o n , 1961;

Laurence and Rogers, 19 76)

,

and t h e l a r v a e a r e p l a n k t o n i c f o r s e v e r a l months t h e r e a f t e r . The l a r v a e h a t c h a t c . 4 mm SL ( C o l t o n and Marak, 1969) and y o l k s a c r e s o r p t i o n i s

completed 6-7 d a y s p o s t - h a t c h a t ~ O C ( L a u r e n c e , 1 9 7 4 ) .

~ a b - r e a r e d l a r v a e were c o n s i d e r e d metamorphosed ( c . 10 mm, 1000 pg d r y w t ) i n 30 days a t 90C and 40-50 d a y s a t 70C. F i g . 1 d e p i c t s t h e p r i n c i p a l haddock spawning time and a r e a on Georges Bank, t h e g e n e r a l i z e d egg and l a r v a l d r i f t , and a r e a s where d e m e r s a l 0-group f i s h a r e most a b u n d a n t 6-8 months l a t e r ( G r o s s l e i n and Hennemuth, 1 9 7 3 ) . The d i s t r i b u t i o n of l a t e s t a g e e g g s and r e c e n t l y - h a t c h e d l a r v a e i n d i c a t e t h a t d i s p e r s i o n from t h e spawning c e n t e r on n o r t h e a s t Georges f o l l o w s t h e g e n e r a l p a t t e r n o f d r i f t , p r e d o m i n a n t l y t o t h e s o u t h w e s t a t 1-4 m i l e s / d

(2-7 km/d) (Walford, 1938; Marak and C o l t o n , 1961; C o l t o n , 1965 ; Smith e t a l .

,

^{19 79)}

.

During April-May, h i g h concen- t r a t i o n s o f l a r v a e ( > O . l / r n 3 ) can be found a l o n g t h e s o u t h e r n

f l a n k o f Georges between t h e 60 and 100 m i s o b a t h s . Some

F i g . 1. P r i n c i p a l haddock spawning a r e a on Georges Bank and g e n e r a l i z e d l a r v a l d r i f t ( i n d i c a t e d by a r r o w s ) and a r e a s where d e m e r s a l 0-group haddock a r e most abundant 6-8 months l a t e r .

p o r t i o n o f t h e l a r v a e a p p a r e n t l y a r e t r a n s p o r t e d n o r t h on t h e w e s t e r n s i d e o f Georges Bank, b u t l i t t l e i s known a b o u t p o s s i b l e l o s s e s o f l a r v a e o f f t h e bank. The 0-group f i s h t e n d t o be c o n c e n t r a t e d on t h e n o r t h e r n p a r t o f t h e bank i n d i c a t i n g a f a v o r a b l e e n v i r o n m e n t f o r t h e i r s u r v i v a l .

Hydrography o f Georges Bank

The r e s i d u a l d r i f t of Georges Bank i s d e s c r i b e d a s a semi- e n c l o s e d c l o c k w i s e c i r c u l a t i o n w i t h a mean s p e e d of a p p r o x i - m a t e l y 10 cm/s o r 5 km/d ( F i g . 2 ) . A c o u n t e r - c l o c k w i s e c i r - c u l a t i o n d e v e l o p s i n t h e Gulf o f Maine and b o t h g y r e s i n t e n - s i f y i n t h e summer (Bumpus and L a u z i e r , 1 9 6 5 ) . I n w i n t e r t h e

F i g . 2 . S c h e m a t i c r e p r e s e n t a t i o n o f t h e well-mixed and s t r a t i f i e d w a t e r s on Georges Bank and mean c i r c u l a t i o n flow

( a r r o w s ) d u r i n g s p r i n g and summer.

n e a r s u r f a c e f l o w i s g e n e r a l l y d r i v e n by t h e w i n d s ; t h e mean t r a n s p o r t i s o f f s h o r e . Recent s t u d i e s summarized by Butman e t a l . (1982) c o n c l u d e d t h a t t h e o b s e r v e d mean f l o w a t 10 m h a s a permanent c l o c k w i s e c i r c u l a t i o n around Georges Bank w i t h a mean c i r c u i t t i m e of c . 2 months f o r a p a r c e l moving a l o n g t h e 60 m i s o b a t h . D e s p i t e t h e c o n s i d e r a b l e v a r i a b i l i t y t h a t c o u l d o c c u r i n t h e t r a j e c t o r y o f s u c h a p a r c e l , t h e y i n f e r r e d t h a t t h e c l o c k w i s e c i r c u l a t i o n a r o u n d t h e c r e s t of t h e bank may p r o v i d e a mechanism f o r p a r t i a l r e t e n t i o n o f p l a n k t o n .

The w a t e r on Georges Bank s h o a l e r t h a n 60 m i s v e r t i c a l l y well-mixed t h r o u g h o u t t h e y e a r by t h e s e m i - d i u r n a l , r o t a r y

t i d a l c u r r e n t s t h a t have s p e e d s up t o >2 k n o t s (103 cm/s) (Bumpus, 1 9 7 6 ) . P r o g r e s s i v e v e c t o r diagrams of t h e t i d a l e l i p s e s a r e o r i e n t e d NW-SE on t h e c r e s t w i t h t h e i r l o n g a x e s r a n g i n g 4-8 m i l e s (7-15 km) i n l e n g t h . Summing t h e h o u r l y s p e e d s o v e r a 1 2 h p e r i o d , an a p p r o x i m a t i o n of t h e d i s t a n c e t r a v e l l e d by a p a r c e l of w a t e r ranged 10-20 m i l e s

(19-37 km) o v e r t h e s h o a l s and 5-6 m i l e s (9-11 km) o v e r t h e d e e p e r p a r t s .

B e s i d e s t h e dominant t i d a l e n e r g y on t h e s h e l f , s t o r m s a t 4-5 d i n t e r v a l s have an i m p o r t a n t r o l e i n s h e l f w a t e r dynam- i c s ( B e a r d s l e y e t a l . , 1976)

.

I n w i n t e r t h e well-mixed w a t e r i s s e p a r a t e d from a d j a c e n t w a t e r masses by two f r o n t s . On t h e s o u t h e r n f l a n k , t h e s h e l f / s l o p e - w a t e r f r o n t i n t e r s e c t s t h e bottom a t a b o u t 80 m and s e p a r a t e s t h e c o o l e r , f r e s h e r s h e l f w a t e r from t h e warmer, more s a l i n e s l o p e w a t e r . On t h e n o r t h e r n s i d e , a s u b s u r f a c e f r o n t s e p a r a t e s t h e Georges Bank w a t e r from t h e Gulf o f Maine w a t e r . I n l a t e spring-summer a s e a s o n a l t h e r m o c l i n e (20-30 m) d e v e l o p s i n w a t e r s g r e a t e r t h a n 60 m. A s u b s u r f a c e band o f c o o l w i n t e r w a t e r i s found a l o n g t h e s o u t h e r n f l a n k between t h e 60 and 100 m i s o b a t h s ,

Gulf Stream warm-core e d d i e s moving n e a r t h e s o u t h e r n edge o f t h e bank may p l a y an i m p o r t a n t r o l e i n t h e movement of s h e l f / s l o p e - w a t e r , b o t h on and o f f t h e s h e l f , and t h e e n t r a i n m e n t o f o r g a n i s m s r e s i d i n g t h e r e (Lough, 1 9 8 2 ; Joyce and Wiebe, 1983)

.

O b j e c t i v e s and Sampling S t r a t e g y

The main f o c u s o f t h e haddock-cod s t u d y t o - d a t e i s t o d e s c r i b e t h e s p a t i a l - t e m p o r a l v a r i a b i l i t y o f l a r v a e and t h e i r p r e y (copepods) d u r i n g t h e i r f i r s t month of l i f e on Georges Bank. O b s e r v a t i o n s a l s o a r e made t o b e t t e r under- s t a n d f a c t o r s g o v e r n i n g t h e i r p r o d u c t i o n and t o s u r v e y p o s t - l a r v a e and p o t e n t i a l p r e d a t o r s o f l a r v a l f i s h by sampling t h e macro-plankton and micro-nekton components on t h e same c r u i s e . Our s a m p l i n g program i s p r e s e n t l y d e s i g n e d t o i n v e s t i g a t e t h e f o l l o w i n g h y p o t h e s e s which we f e e l a r e i m p o r t a n t i n o r d e r t o u n d e r s t a n d t h e f e e d i n g dynamics and s u r v i v a l of l a r v a e r e t a i n e d on Georges Bank:

1. Growth of l a r v a e i s r e l a t e d t o t h e d e n s i t y o f micro- zooplankton p r e y .

2 . Micro-zooplankton a r e c o n c e n t r a t e d i n a r e a s of r e - l a t i v e l y h i g h p h y t o p l a n k t o n biomass.

3. Micro-zooplankton a r e c o n t a g i o u s l y d i s t r i b u t e d (clumped)

.

4. S t r a t i f i c a t i o n o f t h e w a t e r column a l o n g t h e s o u t h e r n f l a n k o f Georges Bank i n l a t e s p r i n g s e r v e s t o c o n c e n t r a t e zooplankton and f i s h l a r v a e v e r t i c a l l y .

5 . Feeding s u c c e s s i s a s t o c h a s t i c p r o c e s s of random e n c o u n t e r s w i t h ' p a t c h y ' p r e y .

S u p p o r t i v e e v i d e n c e f o r t h e f i r s t f o u r hypotheses can be made by f i e l d o b s e r v a t i o n s ; t h e f i f t h h y p o t h e s i s must be i n v e s t i - g a t e d through p r o b a b i l i s t i c food e n c o u n t e r models o r q u a s i - r e a l i s t i c l a b o r a t o r y experiments. The t h e r m o c l i n e i s poten- t i a l l y i m p o r t a n t because b i o l o g i c a l p r o d u c t i v i t y a p p e a r s con- c e n t r a t e d n e a r t h i s l a y e r and l a r v a l and j u v e n i l e haddock a p p e a r t o be u n i q u e l y a s s o c i a t e d w i t h it ( M i l l e r e t a l . , 1963; C o l t o n , 1965, 1972; Houghton and Marra, 1983). During s p r i n g when r e c e n t l y - h a t c h e d l a r v a e a r e p r e s e n t , t h e s e a s o n a l t h e r m o c l i n e i s beginning t o form, v e r t i c a l l y s t r a t i f y i n g t h e w a t e r column ( > 6 0 m bottom d e p t h ) . The p r e s e n c e of a d i s - c o n t i n u i t y l a y e r r e s u l t i n g i n a g r e a t e r d e g r e e of s t r u c t u r e and p a t c h i n e s s o f t h e p l a n k t o n may be c r i t i c a l t o t h e s u r - v i v a l o f l a r v a e i n t h i s r e g i o n . There i s a need t o measure prey a v a i l a b i l i t y p r i o r t o , d u r i n g , and a f t e r t h e r m o c l i n e formation i n o r d e r t o e v a l u a t e t h e importance o f t h i s phe- nomenon.

A f i e l d program a d d r e s s i n g t h e s e hypotheses r e q u i r e s sampling on s p a t i a l s c a l e s r a n g i n g from c e n t i m e t e r s t o k i l o - m e t e r s and temporal s c a l e s from minutes t o weeks. Consider- a b l e emphasis i s g i v e n t o t h e s m a l l e r s c a l e s of p a t t e r n a s i n d i v i d u a l l a r v a e e n c o u n t e r t h e i r p r e y on t h e m i c r o - s c a l e l e v e l (1 cm t o 1 m ) ; however, a l a r v a ' s swimming c a p a b i l i t i e s soon develop t o where i t can m i g r a t e v e r t i c a l l y 1 0 ' s of m e t e r s i n a m a t t e r of h o u r s . Sampling l a r v a e a t t h e popu- l a t i o n l e v e l r e q u i r e s d i s c r e t e samples a t t h e f i n e - s c a l e l e v e l (1 m t o 1 km), f o r example, t o r e s o l v e v e r t i c a l migra- t i o n p a t t e r n s . To d e f i n e a coherent p a t c h of l a r v a e , o r t o sample p o s t - l a r v a e o r l a r g e r p r e d a t o r s , r e q u i r e s sampling on a c o a r s e s c a l e (1 t o 100 km). S y n o p t i c , t h r e e - d i m e n s i o n a l sampling of t h e v a r i a b l e f i e l d s i s needed, b u t o u r p r e s e n t technology and sampling t e c h n i q u e s u s u a l l y o n l y p e r m i t q u a s i - s y n o p t i c sampling of t h e p a r a m e t e r s o r organisms of i n t e r e s t

(Kelley, 1 9 7 6 ) . The sampling g e a r used s h o u l d be d i r e c t e d towards c o l l e c t i n g d i s c r e t e samples of t h e t a r g e t organism

a s s y n o p t i c a l l y a s p o s s i b l e a t t h e p o p u l a t i o n l e v e l . However, s i n c e p o p u l a t i o n s of l a r v a e , t h e i r p r e y and p r e d a t o r s u s u a l l y o c c u r a t d i f f e r e n t s c a l e s , an a r r a y o f sampling g e a r i s r e - q u i r e d which t e n d s t o n e g a t e s i m u l t a n e o u s s a m p l i n g , u n l e s s more t h a n one r e s e a r c h v e s s e l i s used. N e v e r t h e l e s s , we can approach n e a r s y n o p t i c l t y f o r some e l e m e n t s of t h e s a m p l i n g program u t i l i z i n g j u s t one v e s s e l .

The r o t a r y t i d e s (12.4 h p e r i o d ) a r e t h e dominant f o r c i n g f u n c t i o n on t h e bank s o t h a t e x p e r i m e n t s s h o u l d be n e s t e d w i t h i n i t s s p a c e - t i m e domain. According t o t h e N y q u i s t theorem, which s t a t e s t h a t a f u n c t i o n c a n be d e t e c t e d i f i t s p e r i o d i s a t l e a s t t w i c e t h e sampling f r e q u e n c y , s t a t i o n s a m p l i n g on a g r i d would have t o be t a k e n a t l e a s t once e v e r y 6 h a t a s a m p l i n g d i s t a n c e between 5 and 20 m i l e s ( 9 and 3 7 km) depending on bottom d e p t h . And i n o r d e r t o encom- p a s s a b e f o r e and a f t e r s t o r m e v e n t , o b s e r v a t i o n s s h o u l d be r e p e a t e d e v e r y 2 d o v e r a t l e a s t an 8-10 d p e r i o d . Sameoto (1975, 1978) found t h a t zooplankton v a r i a b i l i t y was s i m i l a r o v e r a b r o a d a r e a of t h e S c o t i a n S h e l f s o t h a t an a c c u r a t e and e f f i c i e n t e s t i m a t e of p o p u l a t i o n means c o u l d be made by t a k i n g 2 n e t samples 6 h a p a r t a t a f i x e d s t a t i o n .

Our b a s i c f i e l d s t r a t e g y i s t o l o c a t e and c h a r a c t e r i z e a p o p u l a t i o n of l a r v a e and t h e i r p r e y , and t h e n t o compare and c o n t r a s t t h e i r f i n e - t o m i c r o - s c a l e d i s t r i b u t i o n w i t h i n s t r a - t i f i e d and well-mixed w a t e r s on Georges Bank. P r e v i o u s ex- p e r i e n c e from t h e 1978 L a r v a l H e r r i n g P a t c h Study i n d i c a t e d t h a t r e l a t i v e l y c o h e r e n t and s t a b l e p a t c h e s o f l a r v a e and

z o o p l a n k t o n c o u l d be d e f i n e d w i t h c o n v e n t i o n a l s a m p l i n g t e c h - n i q u e s (bongo-net samples) and f o l l o w e d f o r a number o f d a y s t o weeks a t a s p a t i a l s c a l e somewhat g r e a t e r t h a n t h e t i d a l e x c u r s i o n ( > 5 m i l e s o r > l 0 km). I t was assumed f o r s a m p l i n g p u r p o s e s t h a t v a r i a b i l i t y w i t h i n t h e t i d a l regime was s i m i l a r a s mixing p r o c e s s e s dominate on t h i s s c a l e . A l s o , by f o l - lowing a drogue f o r s t a t i o n t i m e - s e r i e s o b s e r v a t i o n s , one assumed t h e same p a r c e l of w a t e r was b e i n g sampled w i t h t h e

same l a r v a e - p r e y p o p u l a t i o n . Thus, by r e d u c i n g h o r i z o n t a l v a r i a b i l i t y , a l i a s i n g o f o b s e r v a t i o n s v e r t i c a l l y would be r e d u c e d i n o r d e r t o c o n d u c t t i m e - s e r i e s o b s e r v a t i o n s o v e r a minimum o f two t i d a l c y c l e s . The l i m i t a t i o n s o f time- s e r i e s a n a l y s e s i n m a r i n e e c o s y s t e m s a r e d i s c u s s e d by Denman and P l a t t ( 1 9 7 8 ) .

The deployment o f moored c u r r e n t m e t e r a r r a y s c a n pro- v i d e a t r u l y s y n o p t i c t h r e e - d i m e n s i o n a l p i c t u r e of t h e h o r i z o n t a l c u r r e n t f i e l d w i t h i n t h e s t u d y a r e a . C o a r s e t o meso-scale MARMAP p l a n k t o n - h y d r o g r a p h y s u r v e y s con- d u c t e d on Georges Bank and c o n t i g u o u s w a t e r s d u r i n g t h e same t i m e p r o v i d e a b r o a d e r background i n which t o com- p a r e o u r more i n t e n s i v e f i n e - s c a l e s t u d i e s . Remote s e n - s i n g o f f e r s t h e p o t e n t i a l o f r e g i o n a l s y n o p t i c i t y f o r a number of n e a r - s u r f a c e p a r a m e t e r s s u c h a s ocean tempera- t u r e and c o l o r ( C h a m b e r l i n , 1982; Gower, 1982)

.

METHODS

Gear, I n s t r u m e n t a t i o n , and S p e c i a l Techniques

B o n g o - n e t s a m p l e r

S t a n d a r d MARMAP bongo-type s a m p l e r s a r e used t o make i n t e - g r a t e d water-column h a u l s from 5 m above t h e bottom t o t h e s u r - f a c e t o c o l l e c t z o o p l a n k t o n (Posgay and Marak, 1 9 8 0 ) . A 61-cm bongo s a m p l e r (505 and 333 vm mesh n e t s ) and 20 cm bongo s a m p l e r ( 2 5 3 and 165 Mm n e t s ) a r r a y a r e towed o b l i - q u e l y a t 1 1/2 k n o t s ( 7 8 cm/s) and lowered a t a w i r e s p e e d o f 5 0 m/min and r e t r i e v e d a t 20 m/min. Water f i l t e r e d t h r o u g h e a c h n e t i s measured by a flowmeter and t h e tow d e p t h p r o f i l e i s measured w i t h a time-depth r e c o r d e r .

M O C N E S S

A M u l t i p l e O p e n i n g / C l o s i n g Net and E n v i r o n m e n t a l S e n s i n g System (MOCNESS; Wiebe e t a l . , 1976; 1982) w i t h t h r e e s e p a r a t e u n d e r w a t e r s a m p l i n g u n i t s (1/4 m , 1 m, 10 m) p r o v i d e s u s w i t h wide s p e c t r u m c a p a b i l i t i e s o f s a m p l i n g d i s c r e t e v e r t i c a l s t r a - t a encompassing t h r e e t r o p h i c l e v e l s from m i c r o - p l a n k t o n , f i s h l a r v a e - z o o p l a n k t o n , t o m i c r o - n e k t o n i c o r g a n i s m s . MOCNESS i s a r e c t a n g u l a r s a m p l e r whose n i n e s e r i a l l y l i n k e d n e t s can be opened and c l o s e d s e q u e n t i a l l y by commands t h r o u g h a con- d u c t i n g c a b l e from t h e s u r f a c e v e s s e l , t h u s p e r m i t t i n g sam- p l i n g o f up t o n i n e d i s c r e t e d e p t h l e v e l s o r h o r i z o n t a l s e r i e s i n a s i n g l e h a u l . The t h r e e u n d e r w a t e r s a m p l e r s a r e d e s i g n e d t o be h a u l e d a t 1 1/2 k n o t s ( 7 8 cm/s), 45O n e t a n g l e , f o r an e f f e c t i v e mouth a r e a of 1/4 m2, 1 m 2 , and 10 m2. S t a n d a r d n e t mesh s i z e f o r t h e u n d e r w a t e r u n i t s a r e 64 pm, 3 3 3 um, and 3 mm, r e s p e c t i v e l y . On-deck, r e a l - t i m e m o n i t o r i n g i n - c l u d e s d e p t h ( p r e s s u r e )

,

n e t a n g l e , number of t h e n e t p r e - s e n t l y f i l t e r i n g w a t e r , volume o f w a t e r f i l t e r e d , t e m p e r a t u r e and c h l o r o p h y l l f l u o r e s c e n c e ( A i k e n , 1 9 8 1 ) . P a r a m e t e r d a t a a r e s t o r e d on a n HP-85 computer s y s t e m f o r r e a l - t i m e X-Y p l o t s o f t e m p e r a t u r e and f l u o r e s c e n c e v s . d e p t h , which a r e u s e f u l i n s e l e c t i n g s a m p l i n g d e p t h s ( s e e F i g . 3 ) . A North- s t a r Loran C u n i t w i t h p l o t t e r a l s o i s i n t e g r a t e d w i t h t h e MOCNESS f o r r e c o r d i n g t h e p o s i t i o n a t e a c h n e t r e l e a s e . O t h e r s e n s o r s s u c h a s s a l i n i t y , l i g h t , and oxygen w i l l be i n t e g r a t e d w i t h MOCNESS.

P l a n k t o n p u m p

I n 1 9 8 1 a l-hp s u b m e r s i b l e w e l l pump was u s e d t o sample micro-zooplankton a t d e p t h . The pump i s t y p i c a l l y d e p l o y e d a t t a c h e d t o 1/4" ( 6 . 4 mm) w i r e w i t h a 45 kg l e a d b a l l . De- l i v e r y o f w a t e r from d e p t h t o a deck m a n i f o l d f i t t e d w i t h fine-mesh n e t s (20 and 5 3 pm mesh) i s by a 7.5 cm d i a m e t e r PVC d i s c h a r g e hose. Water i s t y p i c a l l y pumped from f i v e

MOCNESS 191

START FINISH

21 MAY l Q B l 22,541 47 21 MAY l Q B l 23,568 18 40 55.10N 67 16. Q51 4 0 54.85N 67 15.531 BOTTOM DEPTH 7BM BOTTOM DEPTH B0M

F i g . 3. Real-time t e m p e r a t u r e - d e p t h p l o t o f l m MOCNESS h a u l 191. A s o l i d t e m p e r a t u r e l i n e i s drawn a s n e t i s s e t t o maximum d e p t h and d o t t e d a f t e r f i r s t n e t i s opened and s a m p l i n g s e q u e n c e b e g i n s .

TEMPERATURE ('Cl

d e p t h l e v e l s i n t h e u p p e r 5 0 m of w a t e r f o r 10 min e a c h d e p t h t o f i l t e r 1 m3 o f w a t e r . S i n c e t h e 1982 s e a s o n , a l a r g e r s u b m e r s i b l e pump h a s been u s e d t o f i l t e r 1 m3 of

8

18

28

38

g N

-

I 58 l- a W 0 m

78 88 88

i88

w a t e r i n 1 min.

5 18 15 20 25

1

JET 7

-' 3E

⁶

1 ^-

-

-

1;

-

C T D - f l u o r o m e t e r

A N e i l Brown CTD m i c r o - p r o f i l i n g s y s t e m w i t h a G e n e r a l O c e a n i c s N i s k i n b o t t l e r o s e t t e i s u s e d f o r r a p i d c o n t i n u o u s p r o f i l i n g o f t e m p e r a t u r e and s a l i n i t y w i t h d e p t h . The w a t e r

b o t t l e c o l l e c t i o n s a l s o a r e u s e d t o make d i s c r e t e o b s e r v a - t i o n s o f m i c r o - z o o p l a n k t o n , n u t r i e n t s , and p h y t o p l a n k t o n biomass m e a s u r e s by c o n v e n t i o n a l methods. Continuous i n - s i t u f l u o r e s c e n c e i s measured a t t h e same t i m e by d e p l o y i n g an ENDECO s u b m e r s i b l e f l u o r o m e t e r ( T u r n e r Designs Model) w i t h on-deck r e c o r d i n g o f d e p t h , f l u o r e s c e n c e , and tempera- t u r e v i a c o n d u c t i n g c a b l e . A r e c e n t l y a c q u i r e d V a r i o s e n s i n - s i t u f l u o r o m e t e r w i l l be i n t e r f a c e d w i t h t h e CTD.

R e a l - t i m e z o o p l a n k t o n p r o c e s s i n g

I n p r o c e s s - o r i e n t e d s t u d i e s t h e r e i s need f o r r e a l - t i m e r e s u l t s s o t h a t d e c i s i o n s c a n be made t o o p t i m i z e t h e ex- p e r i m e n t a l o p e r a t i o n s . A method we employ a t s e a t o make r o u t i n e , q u a n t i t a t i v e a n a l y s e s o f p l a n k t o n - n e t s a m p l e s u s i n g s i l h o u e t t e p h o t o g r a p h y t e c h n i q u e s c o u p l e d w i t h a m i c r o f i c h e r e a d e r , an e l e c t r o n i c d i g i t i z e r , and a s m a l l p e r s o n a l computer i s d e s c r i b e d by Lough and P o t t e r ( 1 9 8 3 ) . More t h a n 90% o f t h e o r g a n i s m s can be i d e n t i f i e d t o s p e c i e s

l e v e l and l i f e s t a g e , a n d a subsample enumerated w i t h i n 20 min a f t e r c o l l e c t i n g by t h i s method.

A H I A C C r i t e r i o n PC320 12-channel p a r t i c l e c o u n t i n g and s i z i n g s y s t e m (Pugh, 1978; Tungate and Reynolds, 1980) h a s been a c q u i r e d f o r development a s a r e a l - t i m e t o o l f o r t h e q u a n t i f i c a t i o n o f m a r i n e p l a n k t o n . Three s e n s o r s (CMH-150, CMH-600, E-2500) a r e u s e d t o c o u n t p a r t i c l e s i n t h e r a n g e o f 5-2500 um. However, a t p r e s e n t we p r o c e s s N i s k i n b o t t l e w a t e r s a m p l e s o n l y i n a b a t c h mode. The H I A C u n i t h a s been i n t e r f a c e d w i t h a C a n b e r r a Multi-Channel A n a l y z e r and an HP-85 computer s y s t e m t o c o n t r o l a l l s e t t i n g s and f u n c t i o n s . The i n s t r u m e n t i s b e i n g m o d i f i e d f o r i n - s i t u p a r t i c l e p r o - f i l i n g a l o n g t h e l i n e s r e p o r t e d by T i l s e t h and E l l e r t s e n

(1984)

.

L a r v a l c o n d i t i o n and g r o w t h i n d i c e s

S p e c i a l c o l l e c t i o n s of l a r v a e , p r e s e r v e d t h r o u g h o u t t h e c r u i s e , a r e a n a l y z e d i n t h e l a b o r a t o r y f o r b i o c h e m i c a l con- t e n t , h i s t o l o g i c a l and m o r p h o l o g i c a l a s s e s s m e n t , and o t o l i t h i n c r e m e n t d e p o s i t i o n . L a b o r a t o r y s t u d i e s by Buckley (1979, 1981) have d e m o n s t r a t e d r e l a t i o n s between food a v a i l a b i l i t y and l a r v a l RNA/DNA r a t i o s and growth r a t e . A r e g r e s s i o n model h a s been d e v e l o p e d r e c e n t l y (Buckley, 1982) between t e m p e r a t u r e , RNA-DNA r a t i o , and mean d a i l y p r o t e i n growth r a t e which a c c o u n t s f o r s h o r t - t e r m growth o v e r t h e p r e v i o u s 2 - 4 d a y s . T h i s s e n s i t i v e t e c h n i q u e i s now b e i n g u s e d t o s t u d y t h e r e l a t i o n s between e n v i r o n m e n t a l c o n d i t i o n s and l a r v a l growth and s u r v i v a l i n t h e f i e l d . From t h e same samples l a r v a e a r e b e i n g a n a l y z e d h i s t o l o g i c a l l y ( O ' C o n n e l l , 19 7 6 ) and m o r p h o m e t r i c a l l y ( T h e i l a c k e r

,

19 81) t o e v a l u a t e t h e i r c o n d i t i o n and d e v e l o p c r i t e r i a f o r d e t e c t i n g s t a r v e d and weakened l a r v a e . P o p u l a t i o n mean age and long-term a v e r a g e growth o f l a r v a e can be e s t i m a t e d by r e l a t i n g o t o l i t h growth i n c r e m e n t s t o l a r v a l s i z e ( B o l z and Lough, 1 9 8 3 ) . An i n d i v i d u a l l a r v a ' s p a s t e n v i r o n m e n t a l growth h i s t o r y a l s o may be r e v e a l e d w i t h p r o p e r l a b o r a t o r y v e r i f i c a t i o n of t h e i r o t o l i t h s (Radtke, 19 84)

.

P r e y s e l e c t i o n

Larvae from s e l e c t e d MOCNESS h a u l s a r e p r o c e s s e d f o r g u t c o n t e n t s by t h e methods d e s c r i b e d i n Cohen and Lough (1983) and Kane ( i n p r e s s ) .

F i e l d O p e r a t i o n a l P l a n

A c o n c e n t r a t i o n of l a r v a e ( o r e g g s ) on Georges Bank i s l o c a t e d from a p r e v i o u s MARMAP b r o a d - s c a l e s u r v e y , o r a t t h e t i m e o f t h e c r u i s e by e x p l o r a t o r y t r a n s e c t s u s i n g s t a n d a r d bongo-net g e a r i n l i k e l y a r e a s . Then a g r i d of

40-50 s t a t i o n s , 5 m i l e s a p a r t , i s o c c u p i e d w i t h i n a 2 d p e r i o d t o c h a r a c t e r i z e t h e l a r v a l f i s h , p l a n k t o n , and t e m p e r a t u r e - s a l i n i t y f i e l d i n a n a r e a s u f f i c i e n t l y l a r g e

( C . 30 X 50 m i l e s [56 X 9 3 km]) t o encompass t h e a n t i c i -

p a t e d d i s p e r s a l o f p l a n k t o n h a v i n g a r e s i d u a l d r i f t o f 4 m i l e s / d ( 7 km/d) i n which t h e f i n e - s c a l e s t a t i o n s t u d i e s w i l l b e c a r r i e d o u t o v e r 4-6 d. The s u r v e y g r i d u s u a l l y i s s i t u a t e d s o t h a t s t a t i o n s o v e r l a p t h e s h o a l f r o n t o f t h e well-mixed w a t e r s (c60 m ) and t h e s o u t h e r n s h e l f / s l o p e - w a t e r f r o n t ( c . 100 m) bounding t h e s t r a t i f i e d w a t e r s on t h e bank.

A bongo h a u l and XBT d r o p a r e made on e a c h g r i d s t a t i o n , and s u r f a c e t e m p e r a t u r e , s a l i n i t y and f l u o r e s c e n c e a r e m o n i t o r e d c o n t i n u o u s l y .

Based upon r e a l - t i m e sample a n a l y s e s made d u r i n g t h e g r i d s u r v e y , a s t a t i o n i s s e l e c t e d f o r t h e f i n e - s c a l e t i m e - s e r i e s o b s e r v a t i o n s and a drogue i s d e p l o y e d a t t h e d e p t h c o r r e s - p o n d i n g , i d e a l l y , t o t h e w e i g h t e d c e n t e r o f g r a v i t y o f t h e l a r v a l p o p u l a t i o n . On one o c c a s i o n , a drogue was d e p l o y e d w i t h an a r r a y o f v e c t o r - a v e r a g i n g c u r r e n t m e t e r s (VACM) p o s i t i o n e d t o measure c u r r e n t v e l o c i t y and t e m p e r a t u r e a t s e l e c t e d d e p t h s t o d e t e r m i n e s h e a r i n t h e w a t e r column.

On s t a t i o n , t h e s a m p l i n g scheme used i s a c o m b i n a t i o n o f f i n e - t o m i c r o - s c a l e o b s e r v a t i o n s i n o r d e r t o sample f i s h l a r v a e and t h e i r p r e y , and o t h e r e n v i r o n m e n t a l p a r a m e t e r s . T h i s scheme a l l o w s 2-4 o b s e r v a t i o n s o f e a c h k i n d d u r i n g a t i d a l p e r i o d (12.4 h ) . On e a c h d r o g u e - f o l l o w e r s t a t i o n , t i m e - s e r i e s o b s e r v a t i o n s a r e made f o r a minimum o f 30 h and sometimes a s l o n g a s 50 h encompassing 2-4 t i d a l p e r i o d s . A c o m p l e t e s e r i e s o f o b s e r v a t i o n s i s made e v e r y 6 h i n t h e f o l l o w i n g s e q u e n c e : CTD-fluorometer c a s t , MOCNESS l m h a u l , p l a n k t o n pump c a s t , CTD-fluorometer c a s t , and MOCNESS 1/4 m h a u l .

C T D - f l u o r o r n e t e r c a s t

The o b j e c t i v e of t h i s o p e r a t i o n i s t o o b t a i n a v e r t i c a l p r o f i l e (and v a r i a b i l i t y ) of t e m p e r a t u r e , s a l i n i t y , and c h l o r o p h y l l a f l u o r e s c e n c e on a m i c r o - s c a l e l e v e l . C a s t s may be r e p e a t e d f o r s h o r t - t e r m v a r i a b i l i t y . N i s k i n w a t e r b o t t l e samples a r e c o l l e c t e d a t s e l e c t e d d e p t h s f o r c a l i - b r a t i o n p u r p o s e s and p a r t i c l e s i z e a n a l y s i s u s i n g t h e H I A C PC320 system. A n c i l l a r y o b s e r v a t i o n s i n c l u d e a l i g h t - m e t e r c a s t t o d e f i n e t h e l i g h t e x t i n c t i o n c u r v e , and a b o t t o m - t r i p N i s k i n b o t t l e c a s t t o c o l l e c t a p h y t o p l a n k t o n sample w i t h i n a m e t e r of bottom.

M O C N E S S 1 m h a u l

The o b j e c t i v e of t h i s h a u l i s t o d e t e r m i n e t h e v e r t i c a l d i s t r i b u t i o n and abundance o f f i s h l a r v a e and l a r g e r zoo- p l a n k t o n from n e a r bottom ( < 5 m) t o s u r f a c e w i t h 10 3r 5 m r e s o l u t i o n . An a d e q u a t e sample o f l a r v a e (30-100 i n d i v i d u a l s ) i s u s u a l l y o b t a i n e d by f i l t e r i n g 250 m3 of w a t e r which t a k e s a b o u t 5 min f o r e a c h n e t . During t h i s 5 min t h e n e t t r a v e l s a h o r i z o n t a l d i s t a n c e o f c. 235 m.

l

P l a n k t o n p u m p c a s t

Micro-zooplankton samples a r e c o l l e c t e d a t 4-6 d i s c r e t e d e p t h l e v e l s b a s e d upon t h e v e r t i c a l d i s t r i b u t i o n of t h e f i s h l a r v a e and e n v i r o n m e n t a l c o n d i t i o n s . A t e a c h d e p t h l e v e l , 1 m3 o f w a t e r i s pumped on deck and f i l t e r e d t h r o u g h 20 and 53 um mesh n e t s . Sampling r e s o l u t i o n i s 1-2 m v e r t i c a l l y and 1 0 ' s o f m e t e r s h o r i z o n t a l l y , depending on t h e r a t e o f pumping and s h i p ' s d r i f t .

M O C N E S S 1 / 4 m h a u l

The o b j e c t i v e o f t h i s h a u l i s t o d e t e r m i n e t h e v e r t i c a l d i s t r i b u t i o n and abundance of micro-zooplankton r e t a i n e d by 64-um mesh n e t s o v e r t h e v e r t i c a l d i s t r i b u t i o n r a n g e of f i s h l a r v a e . About 20-36 m 3 o f w a t e r i s f i l t e r e d by e a c h n e t

(1-3 min) w i t h i n an i n t e g r a t e d s t r a t a of 1 0 , 5 , o r 2-m r e s o l u t i o n (94-170 m h o r i z o n t a l d i s t a n c e t r a v e l e d ) .

Following t h e f i n e - s c a l e s t a t i o n o b s e r v a t i o n s , t h e g r i d of s t a t i o n s may be r e s u r v e y e d and new t r a n s e c t s added i n t h e d i r e c t i o n of t h e r e s i d u a l c u r r e n t , o r MOCNESS 10-m h a u l s may be made on a t r a n s e c t of s t a t i o n s i n t h e s t u d y a r e a . The

10 m MOCNESS i s used t o d e t e r m i n e t h e v e r t i c a l d i s t r i b u t i o n and abundance of p o t e n t i a l m i c r o - n e k t o n i c p r e d a t o r s and p o s t - l a r v a e w i t h 15 o r 25 m r e s o l u t i o n , e a c h n e t f i l t e r i n g 7000-14000 m 3 o f w a t e r i n 15-30 min (705-1410 h o r i z o n t a l d i s t a n c e t r a v e l e d ) . A l m MOCNESS h a u l u s u a l l y i s made immediately b e f o r e o r a f t e r t o c o l l e c t l a r v a l f i s h o r o t h e r food p r e y .

RESULTS AND DISCUSSION

Some o f t h e i n i t i a l r e s u l t s a r e p r e s e n t e d h e r e from a two- p a r t s t u d y conducted a b o a r d R/V ALBATROSS I V , 15-30 A p r i l 1981 and 18-30 May 1981. On t h e A p r i l c r u i s e a w e l l - d e f i n e d c o n c e n t r a t i o n of g a d i d e g g s was l o c a t e d on t h e s o u t h e a s t p a r t of Georges Bank between t h e 60 and 100 m i s o b a t h s by t h e bongo sampling g r i d of s t a t i o n s ( F i g s . 4 - 8 ) . R e c e n t l y - h a t c h e d haddock and cod l a r v a e (3-5 mm SL) were found most a b u n d a n t l y towards t h e s o u t h e a s t e r n p a r t of t h e g r i d and a r a t i o of t h e i r abundance i n d i c a t e d t h a t a b o u t 91% of t h e g a d i d e g g s were had- dock, t h e o t h e r 9% cod. The m a j o r i t y of e g g s were a t a l a t e s t a g e of development ( C o l t o n and Marak, 1962) and were e s t i - mated t o have been spawned 8-10 d p r e v i o u s l y i n t h e ~ O C w a t e r . E a r l y s t a g e e g g s were more abundant t o t h e n o r t h e a s t n e a r t h e

F i g . 4 . Haddock l a r v a l d i s t r i b u t i o n s from A p r i l and May 1 9 8 1 g r i d s u r v e y s . D e n s i t i e s c o n t o u r e d by f a c t o r l e v e l o f 4 .

16-18 APRIL 1981 610333

Fig. 5. Cod larval distributions from April and May 1981 grid surveys. Densities contoured by factor level of 4.

, ' - . I

,'-- 19-21 MAY LARVAE

..,

, .%

,' c<-.:,

SPRING 1981

F i g . 6 . Haddock and cod e g g and l a r v a l d i s t r i b u t i o n s g e n e r a l i z e d from t h e A p r i l and May 1 9 8 1 g r i d s u r v e y s .

MELANOGRAMMUS AEGLEFIMUS

60

r

^{16-18 APRIL 1981}

19-21 MAY 1981

-.

₈ 30

L

20 L

2

$

^to

2

0

LENGTH ( m m )

F i g . 7. L e n g t h - f r e q u e n c y d i s t r i b u t i o n s o f haddock l a r v a e c o l l e c t e d on t h e A p r i l and May 1 9 8 1 g r i d s u r v e y s .

h i s t o r i c a l spawning grounds. Cod l a r v a e were more w i d e s p r e a d t h a n haddock and t h e i r g r e a t e r s i z e r a n g e was i n d i c a t i v e of t h e i r e a r l i e r spawning i n February-March.

By May, a c o n c e n t r a t i o n o f l a r v a l haddock and cod was l o c a t e d a l o n g t h e s o u t h e r n f l a n k o f Georges t o t h e s o u t h w e s t o f t h e A p r i l d i s t r i b u t i o n , s i t u a t e d between t h e s h o a l t i d a l f r o n t and t h e d e e p e r s h e l f / s l o p e - w a t e r f r o n t . The mean l e n g t h of b o t h l a r v a l p o p u l a t i o n s sampled on t h e g r i d was 6 mm and i s c o n s i s t e n t w i t h l a b o r a t o r y growth r a t e s o v e r t h e p e r i o d of t i m e between h a t c h i n g i n A p r i l and t h e May s u r v e y

( L a u r e n c e , 19 78; Bolz and Lough, 1983)

.

Also, an e s t i m a t e d t r a n s p o r t o f 1-2 m i l e s / d , which i s c o n s i s t e n t w i t h t h e long- t e r m r e s i d u a l c u r r e n t s r e p o r t e d f o r t h i s a r e a , would a c c o u n t

GADUS MORHUA

16-18 APRIL 1981

19-21 MAY 1981

\

LENGTH ( m m )

F i g . 8. Length-frequency d i s t r i b u t i o n s o f cod l a r v a e c o l - l e c t e d on t h e A p r i l and May 1981 g r i d s u r v e y s .

f o r t h e d i s p l a c e m e n t between t h e h i g h e s t c o n c e n t r a t i o n o f e g g s i n A p r i l and l a r v a e i n May. Coupled w i t h t h e f a c t t h a t no o t h e r egg o r l a r v a l c o n c e n t r a t i o n s were found i n t h e a r e a , t h e s e o b s e r v a t i o n s s u p p o r t t h e view t h a t t h e egg and l a r v a l c o n c e n t r a t i o n s d e f i n e d belonged t o t h e same spawning popula- t i o n .

An i m p o r t a n t f e a t u r e o f t h e s e egg and l a r v a l c o n c e n t r a t i o n s i s t h e i r c o h e r e n c e and s t a b i l i t y which p r o v i d e c o n t i n u i t y i n t h e sampling program. The g r i d s t a t i o n d e n s i t i e s have been c o n t o u r e d by a f a c t o r of 4 a s t h e c o e f f i c i e n t of v a r i a t i o n of a s i n g l e p l a n k t o n h a u l t y p i c a l l y i s i n t h e r a n g e of 22-44%

( C a s s i e , 1 9 6 3 ) . Note t h e i n t e r n a l c o n s i s t e n c y of t h e s t a t i o n v a l u e s w i t h i n t h e c o n t o u r e d a r e a s . Resampling a g r i d t r a n - s e c t once on t h e A p r i l s u r v e y and a g a i n i n May 4-7 d l a t e r

produced egg and l a r v a l c o n c e n t r a t i o n s n e a r l y i d e n t i c a l t o t h e p r e v i o u s s t a t i o n v a l u e s ( w i t h i n a f a c t o r of 4 ) . Using a l l a v a i l a b l e i n f o r m a t i o n , t h e haddock and cod egg and l a r v a l c o n c e n t r a t i o n s have been g e n e r a l i z e d i n F i g . 6 t o show t h e i r s i z e , s h a p e , and d i s p e r s a l between s u r v e y s . The h i g h e s t con- c e n t r a t i o n s of e g g s and l a r v a e c o n t o u r e d were e l l i p t i c a l i n s h a p e w i t h major and minor a x e s o f a b o u t 30 X 1 5 m i l e s

(56 X 28 km). The s m a l l e s t p a t c h r e s o l v e d i s a b o u t 10 X 5 m i l e s (19 X 9 km), which i s on t h e s c a l e o f t h e t i d a l e x c u r - s i o n s and t h e sampled g r i d of s t a t i o n s . The l o w e s t concen- t r a t i o n of l a r v a e d e f i n e d and c o n t o u r e d a s a p a t c h was a b o u t 60 m i l e s ( I l l km) l o n g between t h e s h e l f / s l o p e - w a t e r f r o n t and t h e t i d a l f r o n t . I f one assumes t h a t t h e p a t c h dimen- s i o n s a r e r e a s o n a b l y a c c u r a t e , an e s t i m a t e of m o r t a l i t y can be made between t h e e g g s i n A p r i l and t h e l a r v a e i n May.

Using methods s i m i l a r t o t h o s e d e s c r i b e d i n Lough e t a l . ( 1 9 8 0 ) , m o r t a l i t y of haddock and cod from t h e i r h a t c h i n g m i d p o i n t t h r o u g h t h e 6-mm s i z e c l a s s (18-24 d p o s t - h a t c h ) was e s t i m a t e d t o be 6-8%/d. These l o s s r a t e s a r e c o n s i s - t e n t w i t h t h e range of r a t e s (5-15%/d) r e p o r t e d by S a v i l l e

(1956) f o r F a r o e haddock l a r v a e .

I t a l s o i s o f i n t e r e s t t o n o t e t h a t t h e l a r g e s t and p r e - sumably o l d e s t l a r v a e c o l l e c t e d on t h e g r i d s u r v e y were found t o t h e e x t r e m e s o u t h w e s t and on t h e s h o a l s (c60 m ) . T h i s p a s t May 1983, u s i n g t h e 10 m MOCNESS, r e l a t i v e l y h i g h den- s i t i e s (70-450/10 000 m3) o f cod p o s t - l a r v a e (15-50 m m ) and s a n d e e l , A m m o d y t e s s p . (45-80 m m ) , were c o l l e c t e d through- o u t t h e s h o a l e r p a r t s o f w e s t e r n Georges Bank, b o t h o f which have been o b s e r v e d t o p r e y upon young f i s h l a r v a e .

I n A p r i l , w i n t e r c o n d i t i o n s s t i l l p r e v a i l e d ; t h e w a t e r column was well-mixed t h r o u g h o u t t h e s t u d y a r e a , i s o t h e r m a l

( ~ O C ) from s u r f a c e t o bottom. Only d u r i n g t h e f i n a l days of t h e c r u i s e was a s l i g h t warming of s u r f a c e w a t e r s o b s e r v e d , i n d i c a t i n g t h e o n s e t bf s p r i n g t h e r m a l s t r a t i f i c a t i o n on t h e f l a n k o f t h e bank. Net-phytoplankton (220 um) biomass i n - c r e a s e d w i t h d e p t h from 1-2 mg c h l a/m3 n e a r t h e s u r f a c e t o 5-

10 mg c h l ./m3 n e a r t h e bottom, a p p a r e n t l y due t o s i n k i n g o f l a r g e r d i a t o m s and d i n o f l a g e l l a t e s (Busch and Mountain, 19 82)

.

Nanno-phytoplankton ( c 2 0 um) biomass was e v e n l y d i s - t r i b u t e d t h r o u g h o u t t h e w a t e r column a t 1-2 mg c h l a/m 3

.

The v e r t i c a l d i s t r i b u t i o n o f g a d i d e g g s was low a t t h e s u r - f a c e and a l s o g e n e r a l l y i n c r e a s e d i n d e n s i t y w i t h d e p t h t o a maximum a t t h e b o t t o m ( F i g . 9 ) . The cod l a r v a e were s e p a - r a t e d i n t o two s i z e g r o u p s f o r a n a l y s i s (3-8 mm and > 8 mm)

25-29 APRIL 1981 MOC 168-186

3-8mm LARVAE > 8mm LARVAE

NO/IOOM NO / I O O M ~

0 40 20 0 20 40 60 60 40 20 0 20 40 60

4 0 40

50 50

60 60

GBDlD EGGS MOC 174 NO. / 1 0 0 f l

400

GADUS MORHUA GEORGES BANK MOCNESS - IM

I; WEIGHTED MEAN DEPTH

F i g . 9. V e r t i c a l d i s t r i b u t i o n o f cod l a r v a e and g a d i d e g g s c o l l e c t e d by 1 m MOCNESS ( 3 3 3 um mesh) on t h e s o u t h e a s t p a r t o f Georges Bank (41020'N 66053'W), 25-29 A p r i l 1981.

b e c a u s e o f r e p o r t e d d i f f e r e n c e s i n b e h a v i o r o f t h e l a r g e r l a r v a e (Wiborg, 1960; M i l l e r e t a l . , 1 9 6 3 ) . T h e i r mean day and n i g h t abundances w i t h i n 10 m s a m p l i n g s t r a t a o v e r a 54 h p e r i o d a r e shown i n F i g . 9. The s i z e r a n g e o f l a r v a e c o l l e c t e d by t h e 1 m MOCNESS a r e e s s e n t i a l l y t h e same a s t h a t c o l l e c t e d by t h e 6 1 cm bongo n e t shown i n F i g s . 7 and 8. Both s i z e g r o u p s o f cod l a r v a e a r e b r o a d l y d i s t r i b u t e d t h r o u g h o u t t h e w a t e r column w i t h w e i g h t e d mean p o p u l a t i o n d e p t h s between 30 and 40 m i n w a t e r 66-70 m b o t t o m d e p t h . More cod l a r v a e a r e u s u a l l y c a u g h t by n i g h t t h a n d a y , e s - p e c i a l l y i n t h e u p p e r 20 m. A s i g n i f i c a n t v e r t i c a l d i s p l a c e - ment between day and n i g h t i s shown by t h e l a r g e r s i z e group.

N i g h t mean abundance o f t h e s e l a r v a e i n t h e u p p e r 20 m o f t h e w a t e r column (mean l e n g t h o f 11 mm) was g r e a t e r by a f a c t o r of 14-26 t h a n t h a t o f t h e mean day abundance.

By mid-May, t h e w a t e r column was w e l l - s t r a t i f i e d a t b o t t o m d e p t h s g r e a t e r t h a n 60 m. A t t h e f i r s t t i m e - s e r i e s s t a t i o n (80 m ) , 2 1 May, t h e s u r f a c e t e m p e r a t u r e approached 100C, a s t r o n g t h e r m a l g r a d i e n t (0.750C/m) was e v i d e n t be- tween 15 and 20 m, and below t h e t h e r m o c l i n e t h e w a t e r was 5.g°C t o b o t t o m ( r e f e r F i g . 3 ) . Both n e t - and nanno-phyto- p l a n k t o n biomass were r e d u c e d t o < l mg c h l a/m 3

,

b u t showed

a s l i g h t i n c r e a s e i n t h e nanno-phytoplankton biomass above 20 m. Both haddock and cod l a r v a e were a l m o s t e x c l u s i v e l y c o n f i n e d t o t h e u p p e r 20 m o f t h e w a t e r column w i t h maximum abundance w i t h i n t h e t h e r m o c l i n e ( F i g s . 10 and 1 1 A , MOC 191)

.

An i n t e n s e s t o r m s w e p t t h e a r e a w i t h h i g h n o r t h e a s t e r l y w i n d s , 35-40 k n o t s (18-21 m / s ) , and upon resuming o p e r a t i o n s a t t h e same s i t e s e v e r a l days l a t e r on 24 May, i t was e v i d e n t t h a t t h e w a t e r column was well-mixed, c . 7OC i s o t h e r m a l . Phyto- p l a n k t o n biomass was u n i f o r m l y d i s p e r s e d from t o p t o bottom.

Haddock and cod l a r v a e now were b r o a d l y d i s t r i b u t e d through- o u t t h e w a t e r column w i t h a w e i g h t e d mean d e p t h between 30 and 42 m , a l t h o u g h t h e r e was a s u g g e s t i o n o f a n u p p e r s h i f t i n t h e v e r t i c a l d i s t r i b u t i o n o f l a r v a e d u r i n g t h e n i g h t ( F i g s . 10 and 1 1 A , MOC 193-207)

.

On 28 May, a s i n g l e MOCNESS h a u l

A ) STRATIFIED STATION

21 MAY 1981 MOC 191 24-26 MAY 1981 MOC 193-207 28 MAY 1981 MOC 220

NO / / 0 0 h t 3 N O / / O O M ~ NO /f00ht3

B ) W E L L - M I X E D STATION 27-29 MAY 1981 MOC 209-224

NO / t 0 0 h t 3

140 80 0 80

MELANOGRAMMUS A EGL EFINUS GEORGES BANK MOCNESS - IM

0 SINGLE OBS f WEIGHTED MEAN DEPTH

F i g . 1 0 . V e r t i c a l d i s t r i b u t i o n o f haddock l a r v a e on (A) s t r a t i f i e d s t a t i o n (40°55'N 6 7 O 1 6 ' ~ ) b e f o r e and a f t e r s t o r m , 22-24 May 1 9 8 1 , and on (B) s h o a l , well-mixed s t a t i o n

( 4 1 ° 0 7 ' ~ 67O35'W), 27-29 May 1981.

( 2 2 0 ) showed t h a t a s h a l l o w t h e r m o c l i n e had formed and t h e l a r v a e were r e a g g r e g a t i n g i n t h e u p p e r 20 m a s s o c i a t e d w i t h t h e r e s t r a t i f i c a t i o n . By p l o t t i n g w a t e r column d e n s i t y ( s i g m a - t ) v a l u e s d u r i n g t h i s p e r i o d i n F i g . 1 2 , one c a n s e e t h e p r o c e s s of r e s t r a t i f i c a t i o n between t h e t i m e t h e s t o r m a b a t e d s u f f i c i e n t l y t o resume s a m p l i n g on 24 May (MOC 1 9 3 ) a n d t h e l a s t h a u l on 28 May (MOC 2 2 0 ) . At t h i s r a t e i t would t a k e a t o t a l o f a b o u t 7-10 d f o r t h e w a t e r column and

f i s h l a r v a e t o r e s t r u c t u r e t o t h e same d e g r e e o b s e r v e d p r i o r t o t h e s t o r m . M i l l e r e t a l . ( 1 9 6 3 ) , i n a mid-May 1958 v e r - t i c a l d i s t r i b u t i o n s t u d y o f l a r v a l haddock a r o u n d t h e f l a n k

A ) STRATIFIED STATION

21 MAY 1981 MOC 191 24-26 MAY 1981 MOC 193-207 28 MAY 1981 MOC 220

NO. / 1 0 0 h f 3 NO. /100hf3 NO. /100hf3

B)

WELL-MIXED STATION GACV.5 MoRh'uA

27-29 MAY 1981 MOC 209-224 GEORGES BANK

NO. /1003 MOCNESS

-

I M

0 6 4 2 0 2 4 6 8 1 0

-.

^{' 0}

20 0 SINGLE OBS

30 WEIGHTED MEAN DEPTH

Q 40

F i g . 11. V e r t i c a l d i s t r i b u t i o n o f c o d l a r v a e on ( A ) s t r a t i - f i e d s t a t i o n ( 4 0 ~ 5 5 ' ~ 67O16'W) b e f o r e and a f t e r s t o r m , 22- 24 May 1 9 8 1 , a n d on ( B ) s h o a l , w e l l - m i x e d s t a t i o n ( 4 1 ° 0 7 ' ~ 67O35'W), 27-29 May 1981.

of G e o r g e s Bank, f o u n d t h a t 84% o f t h e l a r v a l p o p u l a t i o n o c c u r r e d w i t h i n t h e d i s c o n t i n u i t y l a y e r , t h e c o n f i n e s o f a t h e r m o c l i n e , w h i c h o c c u p i e d a b o u t 25% o f t h e w a t e r column.

A s h o a l - w a t e r s t a t i o n (50 m b o t t o m d e p t h ) was o c c u p i e d f o r 25 h , 27-29 May, w h e r e t h e w a t e r column was w e l l - m i x e d , 8 - ~ O C . Haddock and c o d l a r v a e w e r e b r o a d l y d i s t r i b u t e d t h r o u g h t h e w a t e r column w i t h w e i g h t e d mean d e p t h s b e t w e e n 20 a n d 30 m ( F i g s . 10 and 1 1 B ) . T h e r e was n o s i g n i f i c a n t d i f f e r e n c e b e t w e e n t h e i r d a y a n d n i g h t v e r t i c a l d i s t r i b u t i o n .

F i g . 1 2 . Water-column d e n s i t y ( s i g m a - t ) p r o f i l e s on s t r a t i - f i e d s t a t i o n ( 4 0 ° 5 5 ' ~ 67016'W) b e f o r e a n d a f t e r s t o r m , 22- 24 May 1981. C o r r e s p o n d i n g MOCNESS h a u l numbers shown.

P h y t o p l a n k t o n b i o m a s s was u n i f o r m l y low t h r o u g h o u t t h e w a t e r column w i t h a n o t i c e a b l e i n c r e a s e i n t h e b o t t o m few m e t e r s , b u t s l i g h t l y h i g h e r (1-2 mg c h l a/m 3 ) t h a n t h e d e e p e r s t a t i o n

(80 m ) .

The d o m i n a n t c o p e p o d s o n G e o r g e s Bank i n l a t e - w i n t e r a n d s p r i n g a r e P s e u d o c a l a n u s s p . , C a l a n u s f i n m a r c h i c u s , and O i t h o n a s i m i l i s . P s e u d o c a l a n u s t e n d s t o b e more a b u n d a n t o n t h e s h o a l a r e a o f G e o r g e s w h i l e C a l a n u s d e v e l o p s h i g h abun- d a n c e i n t h e n e a r - s u r f a c e w a t e r s o f t h e s t r a t i f i e d zone a l o n g t h e s o u t h e r n f l a n k . O i t h o n a , a s m a l l c o p e p o d , i s wide- s p r e a d i n i t s d i s t r i b u t i o n . P r e y s e l e c t i o n s t u d i e s o f l a r v a l

haddock and cod show t h a t t h e n a u p l i a r and c o p e p o d i t e

s t a g e s o f P s e u d o c a l a n u s and C a l a n u s a r e t h e i r most i m p o r t a n t p r e y (Sherman e t a l . , 1981; Kane, i n p r e s s )

.

Eggs of t h e s e two s p e c i e s can sometimes comprise a s i g n i f i c a n t number o f p r e y i t e m s f o r t h e s m a l l e s t l a r v a e ( < 6 m m ) , e s p e c i a l l y f o r t h e more p a s s i v e l y f e e d i n g haddock l a r v a e . The p r e f e r r e d p r e y s i z e o f f o u r l e n g t h g r o u p s o f l a r v a e i s d e p i c t e d i n F i g . 13. Note t h a t cod f e e d upon l a r g e r p r e y a t a s m a l l e r

s i z e t h a n haddock. Both s p e c i e s o f l a r v a e ( < l 0 mm) s e l e c t 50-80% o f t h e i r p r e y i n t h e 0.10-0.19 mm w i d t h c l a s s . Re- c e n t l y - h a t c h e d l a r v a e , 3.5-5.9 mm, a r e p a r t i c u l a r l y depen- d e n t on t h i s s i z e c l a s s of p r e y which encompasses t h e nau- p l i u s 111 t h r o u g h c o p e p o d i t e I1 s t a g e s of P s e u d o c a l a n u s and t h e n a u p l i u s 1 1 - V s t a g e s o f c a l a n u s .

H A D D O C K C O D

L E N G T H GROUP l M M I

F i g . 13. P r e f e r r e d p r e y s i z e o f l a r v a l haddock and cod

l e n g t h g r o u p s from May 1980 Georges Bank s t u d y (Kane, i n p r e s s ) .