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HAVFORSKNINGSINSTITU'ITETS EGG- OG LARVEPROGRAM (HELP)

ABUNDANCE INDICES FOR THE ARCTO-NORWEGIAN COD IN 1979-1986 BASED ON LARVAE INVESTIGATIONS.

BY

H. Bjørke and S. Sundby I n s t i t u t e of Marine Research P.O. BOX 1870, N-5024 BERGEN

Norway

ABSTRACT

P o s t l a r v a e surveys have been c a r r i e d o u t i n t h e western p a r t of t h e B a r e n t s Sea i n J u n e / J u l y s i n c e

1977.

The aim of t h e i n v e s t i g a t i o n s h a s been t o s t u d y t h e d i s t r i b u t i o n p a t t e r n of f i s h l a r v a e and t h e i r r e l a t i o n t o t h e d i s t r i b u t i o n of water masses; and t o e s t a b l i s h abundance i n d i c e s . It h a s been shown t h a t mesoscale f e a t u r e s of t h e l a r v a l d i s t r i b u t i o n a r e very pronounced, and t h a t they a r e connected t o t h e c i r c u l a t i o n f e a t u r e s . The survey s t r a t e g i e s were changed i n

1983,

and emphasis was l a i d on t h e i n v e s t i g a t i o n of cod l a r v a e . Each y e a r , except f o r 1986, p o s t l a r v a l cod have been e s p e c i a l l y abundant a t t h e Tromsøflaket bank. T h i s i s p a r t l y due t o reduced d i f f u s i o n by t h e e x i s t e n c e of an a n t i - c y c l o n i c v o r t e x above t h e bank. A f t e r t h r e e s t r o n g y e a r c l a s s e s i n 1983,

1984

and 1985, t h e r e s u l t s f o r 1986 i n d i c a t e a very poor y e a r c l a s s , and p r a c t i c a l l y no l a r v a e were found a t Tromsøflaket. The anomalous d i s t r i b u t i o n i n

1986

i s d i s c u s s e d . P o s t l a r v a e and O-group i n d i c e s a r e d i s c u s s e d and compared, and a c o r r e l a t i o n c o e f f i c i e n t of 0.82 was found f o r t h e p e r i o d 1979-85.

INTRODUCTION

The Norwegian s h e l f c o n s t i t u t e important spawning grounds f o r f i s h of c o n s i d e r a b l e economical i n t e r e s t such a s Northeast A r c t i c cod, s a i t h e

,

c a p e l i n and Norwegian s p r i n g spawning h e r r i n g (ANON, 1 9 7 9 ) . Eggs and l a r v a e of t h e s e s p e c i e s a r e t r a n s p o r t e d northwards and eastwards by t h e r e s i d u a l c u r r e n t s .

The I n s t i t u t e of Marine Research has s i n c e 1948 been sampling f i s h eggs and l a r v a e a t d i f f e r e n t l o c a l i t i e s along t h e Norwegian c o a s t ( e . g . WIBORG, 1960; HOGNESTAD, 1969; DRAGESUND, 1970; GJØSÆTER and SÆTRE

, 1974

; ELLERTSEN

,

SOLEMDAL, STRØMME

,

SUNDBY, TILSETH

, WESTGARD

and ØIESTAD, 1981; BJØRKE, 1981, 1984; SUNDBY and SOLEMDAL 1984;

SUNDBY and BRATLAND 1986). The sampling has taken p l a c e d u r i n g t h e s p r i n g and summer seasons and has p a r t l y been aimed a t t h e s t u d y of s i n g l e s p e c i e s , such a s h e r r i n g and cod and c a p e l i n .

Sampling of o l d e r l a r v a e ( p o s t l a r v a e ) o f f s h o r e s t a r t e d i n 1977. The aim was t o s t u d y t h e d i s t r i b u t i o n . of l a r v a e i n J u n e / J u l y and i f p o s s i b l e c a l c u l a t e an abundance index of t h e d i f f e r e n t s p e c i e s . R e s u l t s from t h e p e r i o d

1977 -

1983 and a d i s c u s s i o n of t h e sampling s t r a t e g y a r e p r e s e n t e d by BJØRKE and SUNDBY

(1984).

RANDA (1981, 1984) found a c o r r e l a t i o n c o e f f i c i e n t o f 0.92 between t h e e s t i m a t e d O-group i n d i c e s produced i n August/September and t h e VPA e s t i m a t e s f o r t h e same y e a r c l a s s a t age

3

i n t h e p e r i o d 1970-1976.

The r e l i a b i l i t y of an abundance e s t i m a t e i n c r e a s e s i f i t i s supported by o t h e r independent e s t i m a t e s . The aim of t h i s paper i s t o p r e s e n t t h e r e s u l t s of t h e p o s t l a r v a e i n v e s t i g a t i o n s with r e s p e c t t o cod and t o compare t h e r e s u l t s with t h e O-group i n d i c e s f o r cod.

The USSR h a s s i n c e

1959

been conducting egg and l a r v a e s u r v e y s p a r t l y o v e r l a p p i n g t h e a r e a p r e s e n t e d i n t h i s work. The sampling p e r i o d s have been April/May and J u n e / J u l y , e . g . BARANENKOVA and KOKHLINA 1964;

BARANENKOVA, SOROKINA and KOKHLINA 1973, M I N A 1980 and MUKHINA and D V I N I N A

1983.

MATERIALS AND METHODS

The geographical d i s t r i b u t i o n of l a r v a e was found by f i s h i n g with a p e l a g i c t r a w l . A t r a w l with an e f f e c t i v e opening of

4

x 10 m wac used i n

1977

and from

1978

u n t i l

1985

a t r a w l w i t h an opening o f

18

x

18

m was used. I n

1985

a t r a w l with an opening of 29 x 29 m was i n t r o d u c e d . The d i f f e r e n t types of t r a w l s had d i m i n i s h i n g mesh s i z e s towards t h e end and a

4

m l o n g n e t w i t h mesh s i z e

4

mm ( s t r e t c h e d ) a t t h e i n n e r p a r t of t h e cod end (wrongly d e s c r i b e d a s

15

mm mesh s i z e i n BJØRKE and SUNDBY,

1984).

Attempts made i n

1985

t o compare t h e f i s h i n g e f f i c i e n c y between t h e l a t t e r two k i n d s of t r a w l s f a i l e d u n f o r t u n a t e l y , and t h e only way t o compare t h e

1985

d a t a with t h o s e of p r e v i o u s y e a r s was t o r e g a r d t h e opening of t h e two t r a w l s . Regarding t h e reduced avoidance f o r l a r g e r g e a r s we used t h e r a t i o 2 . 8 : l f o r f i s h i n g e f f i c i e n c y between t h e l a r g e s t and smallest t r a w l .

During t h e p e r i o d 1978-1981 two h a u l s were made a t each s t a t i o n , one w i t h t h e h e a d l i n e i n 40 and 20 m depth and a towing t i m e of

15

minutes i n each d e p t h i n t e r v a l . The o t h e r hau1 was made a t t h e s u r f a c e with f i v e b i g f l o a t s on t h e h e a d l i n e . Towing time was 30 minutes f o r t h e l a t t e r .

Only one kind of hau1 was made a t each s t a t i o n from

1982

onwards, From 1982 u n t i l 1985 t h e depths and towing t i m e were t h e same a s t h e p r e v i o u s y e a r s and f i v e b i g f l o a t s were used on t h e h e a d l i n e d u r i n g t h e h a u l . The same procedure was followed i n

1985

and 1986, b u t t h e towing time a t each depth i n t e r v a l was halved.

Towing speed d u r i n g a l l t h e y e a r s was 2-3 k n o t s . The c a t c h i n g e f f i c i e n c y of t h e t r a w l i s n o t known. T h e r e f o r e t h e computed index i s based on t h e number of l a r v a e caught p e r t r a w l hour. Oceanographic o b s e r v a t i o n s were made with a Nei1 Brown CTD-microprofiler down t o t h e bottom o r t o 500 m depth.

A l l t h e f i s h l a r v a e were i d e n t i f i e d , and a subsample of 50 l a r v a e was taken t o measure t h e i r l e n g t h .

The r e s u l t s from t h e y e a r s 1979, 1980,

1981,

1983,

1984, 1985

and 1986 a r e p l o t t e d on h o r i z o n t a l maps and t h e i n d i c e s were e s t i m a t e d by u s i n g a p l a n i m e t e r t r a c k i n g t h e i s o l i n e s . I n

1977

a s m a l l e r t r a w l was used and i n 1978 and 1982 t h e s t a t i o n n e t was n o t dense enough t o r e v e a l t h e mesoscale f e a t u r e s , and t h e r e f o r e t h e abundance i n d i c e s f o r t h e s e y e a r s a r e n o t c a l c u l a t e d . From 1983 t h e sampling program was s t r a t i f i e d t o map t h e mesoscale f e a t u r e s more c o r r e c t l y .

The abundance index may be formulated

where A i s t h e abundance index

N i s t h e c o n c e n t r a t i o n of l a r v a e expressed a s number caught p e r t r a w l hour

x and y a r e t h e geographical c o o r d i n a t e s .

The abundance i n d i c y s , a p e a r i n g i n Table 1 , has t h e u n i t no. l a r v a e p e r t r a w l hour x km x10

k? .

RESULTS AND DISCUSSION

F i g u r e 1 shows t h e l o c a l names used i n t h e t e x t . F i g u r e s 2

- 8

show t h e h o r i z o n t a l d i s t r i b u t i o n of p o s t l a r v a l cod f o r t h e y e a r s 1979

-

1981

and 1983

-

1986. Except f o r 1986 t h e r e i s one pronounced f e a t u r e commen f o r a l l t h e y e a r s : The p o s t l a r v a l cod were p a r t i c u l a r l y abundant a t t h e T r o m s ~ f l a k e t bank a t t h e e n t r a n c e of t h e B a r e n t s Sea. Between 45% and 90% of t h e cod were found w i t h i n t h i s l i m i t e d a r e a .

Higher c o n c e n t r a t i o n s of p o s t l a s v a l cod were a l s o found a t t h e North Cape Bank (Nordkappbanken), These f e a t u r e s correspond w e l l with t h e c i r c u l a t i o n f e a t u r e s i n t h e a r e a of i n v e s t i g a t i o n . There i s a clock- wise c i r c u l a t i o n above both banks which reduces t h e d i f f u s i o n of l a r v a e

.

A f t e r t h r e e s t r o n g y e a r c l a s s e s i n

1983 -

1985 t h e r e s u l t s from 1986 i n d i c a t e a very poor y e a r c l a s s . Only i n 1980 was t h e p o s t l a r v a e index lower. The h o r i z o n t a l d i s t r i b u t i o n was a l s o e x c e p t i o n a l i n 1986 inasmuch a s no l a r v a e were found a t T r o m s ~ f l a k e t ( F i g u r e 8 ) . That y e a r t h e l a r v a e was d i s t r i b u t e d i n t h r e e main p a t c h e s . One s e p a r a t e d p a t c h e x t e n d s w i t h i n t h e S p i t s b e r g e n c u r r e n t branch of t h e Norwegian c u r r e n t between Norway and Bear I s l a n d ( B j ~ r n ~ y a ) . These l a r v a e c o n s t i t u t e d 9% of t h e p o s t l a r v a e index. Another main p a t c h was found a t t h e North Cape Bank (Nordkappbanken) e x t e n d i n g eastwards t o t h e

Thor I v e r s e n Bank, and t h e l a r v a e i n t h i s p a t c h c o n s t i t u t e d

43%

of t h e p o s t l a r v a e index. The t h i r d main p a t c h (shaded i n F i g u r e

8 )

extended along t h e c o a s t of East-Finnmark and p a r t l y i n t o t h e E a s t Finnmark f j o r d s , and t h e l a r v a e i n t h i s p a t c h c o n s t i t u t e d

45%

of t h e p o s t l a r v a e index. Most probably t h e s e l a r v a e o r i g i n a t e from t h e spawning of c o a s t a l cod i n t h e Finnmark f j o r d s . The c o a s t a l cod i n Finnmark has been d e s c r i b e d by JAKOBSEN (1986). It m i g r a t e s from t h e f e e d i n g a r e a s a t t h e c o a s t of Finnmark t o spawn i n t h e E a s t Finnmark f j o r d s . Eggs and l a r v a e a r e s u b j e c t e d t o t h e t r a n s p o r t of e s t u a r i n e c i r c u l a t i o n i n t h e f j o r d s and then t o t h e eastwards t r a n s p o r t by t h e Norwegian c o a s t a l c u r r e n t . The h o r i z o n t a l d i s t r i b u t i o n of t h e l a r v a e w i t h i n t h e shaded a r e a i n F i g u r e

8

corresponds w e l l w i t h such a c i r c u l a t i o n

(SÆTRE and LJØEN, 1 9 7 1 ) .

The extremely e a s t e r l y d i s t r i b u t i o n i n 1986 may have s e v e r a l r e a s o n s . F i r s t l y t h e spawning of cod i n Lofoten was v e r y low, due p a r t l y t o a s m a l l spawning s t o c k (RAKNES and SUNNANÅ. 1986). The low spawning a c t i v i t y i n Lofoten could be due t o t h e f a c t t h a t a l a r g e r f r a c t i o n of t h e s t o c k may have been spawning a t t h e n o r t h e r n spawning a r e a between Andøya and Sørøya, o r t h a t t h e s u r v i v a l of l a r v a e i n Lofoten was lower than a t t h e n o r t h e r n spawning a r e a s . Eggs and l a r v a e from t h e s e spawning a r e a s w i l l tend t o have a more e a s t e r l y d i s t r i b u t i o n a t t h e p o s t l a r v a l s t a g e than t h e eggs and l a r v a e from Lofoten. The l a r g e f r a c t i o n of t h e p o s t l a r v a e t h a t o r i g i n a t e s from t h e Finnmark f j o r d s c o n t r i b u t e s a l s o t o an e a s t e r l y d i s t r i b u t i o n . F i n a l l y t h e e a s t e r l y d i s t r i b u t i o n may be due t o an extreme anomaly of t h e atmospheric f o r c i n g o c c u r r i n g on t h e c u r r e n t system p r e v i o u s t o t h i s i n v e s t i g a t i o n . A t p r e s e n t t h i s has n o t been i n v e s t i g a t e d .

I n s p i t e of t h e anomalous d i s t r i b u t i o n of t h e p o s t l a r v a l cod t h e d i s t r i b u t i o n of watermasses does n o t show anomalous f e a t u r e s . F i g u r e 9 shows t h e d i s t r i b u t i o n of s a l i n i t y a t 20 m depth and i n d i c a t e s a normal developed v o r t e x which have been observed p r e v i o u s y e a r s above t h e Tromsøflaket bank (BJØRKE and SUNDBY

1984).

Table 1 shows t h e abundance i n d i c e s f o r cod from t h e p o s t l a r v a e surveys i n J u n e / J u l y and from t h e O-group s u r v e y s i n August/September (ANON, 1985). The l a t t e r i n d i c e s a r e l o g a r i t h m i c and c a l c u l a t e d according t o RANDA (1984). Figure 10 shows t h e two s e t s of i n d i c e s and t h e r e g r e s s i o n l i n e .

Table 1. Abundance i n d i c e s f o r cod from t h e p o s t l a r v a e surveys i n J u n e / J u l y and from t h e O-group surveys i n August/September.

Year

1979

1980

1981 1983 1984 1985

1986

Post-

l a r v a e 7 . 2

0.4

15.4 74.7 23.5 56.5

1.9

o -

group 0.40 0.13 0.10 1.69 1.55 2.46

-

( l o g a r i t h m i c )

Assuming t h a t l o g a r i t h m i c t r a n s f o r m a t i o n normalize t h e O-group i n d i c e s , a Pearson product moment c o r r e l a t i o n c o e f f i c i e n t can be c a l c u l a t e d from t h e d a t a i n Table 1. The computed c o e f f i c i e n t e q u a l s 0.82, s i g n i f i c a n t a t t h e

5

p e r c e n t l e v e l .

Both i n

1981

and 1983 a very l a r g e f r a c t i o n of t h e p o s t l a r v a e index

r e s u l t e d from t h e abundance of l a r v a e a t T r o m s ~ f l a k e t . Both y e a r s t h e O-group i n d i c e s were low compared t o t h e p o s t l a r v a e i n d i c e s . The abundance based on t h e O-group e s t i m a t e s of b u t h t h e s e y e a r s have been a d j u s t e d upwards d u r i n g t h e l a s t years(AN0N 1982,

1984

and 1986)and r e c e n t e s t i m a t e s from a c o u s t i c surveys i n d i c a t e t h a t t h e 1981 year c l a s s i s 1 / 5 of t h e 1983 y e a r c l a s s . (HYLEN, JAKOBSEN, NAKKEN and SUNNANÅ,

1985) .

T h i s may imply t h a t t h e O-group s t a t i o n g r i d may f a i l t o r e c o r d very patchy d i s t r i b u t i o n s such a s t h o s e recorded on T r o m s ~ f l a k e t by t h e p o s t l a r v a e surveys i n 1981 and

1983.

The p o s t l a r v a e index f o r t h e 1984 y e a r c l a s s i s obviously an underestimate s i n c e t h e northernmost a r e a of d i s t r i b u t i o n was n o t examined. The d i r e c t i o n of t h e s e adjustments of t h e O-group e s t i m a t e s i n

1981

and

1983

a r e i n d i c a t e d i n Fig-ure 10. I n view o f t h e s e f a c t o r s i t i s l i k e l y t h a t t h e c o r r e l a t i o n between t h e r e a l abundance of cod l a r v a e a t t h e s e two s t a g e s i s b e t t e r t h a n i n d i c a t e d from t h e two s e t s of d a t a i n F i g u r e 10. Bearing i n mind t h e improved coverage of t h e cod l a r v a e d i s t r i b u t i o n s i n c e

1983

i t seems r e a s o n a b l e t o b e l i e v e t h a t t h e p o s t l a r v a e i n d i c e s produced i n J u l y g i v e a f a i r l y good i n d i c a t i o n f o r t h e y e a r c l a s s s t r e n g t h of cod a s O-group.

CONCLUSIONS

A f t e r t h r e e s t r o n g y e a r c l a s s e s of cod produced i n

1983-85

t h e p o s t l a r v a e index f o r 1986 i n d i c a t e s a very poor y e a r c l a s s .

Except f o r t h e y e a r 1986 p o s t l a r v a l cod were e s p e c i a l l y abundant a t t h e T r o m s ~ f l a k e t bank i n t h e southwestern p a r t of t h e B a r e n t s Sea.

P r e v i o u s l y have

45 -

90% of t h e p o s t l a r v a e have been found over t h i s bank. T h i s was caused by t h e presence of t h e a n t i - c y c l o n i c v o r t e x above t h e bank which g r e a t l y reduces t h e d i f f u s i o n of p o s t l a r v a l cod.

The absence of p o s t l a r v a e a t T r o m s ~ f l a k e t and t h e extreme e a s t e r l y d i s t r i b u t i o n i n 1986 may have been caused by a more n o r t h e r n l y spawning of cod, a lower s u r v i v a l of cod l a r v a e a t t h e s o u t h e r n spawning a r e a s a n d l o r an extreme anomaly of t h e c i r c u l a t i o n system.

The p o s t l a r v a e i n d i c e s produced i n J u l y seem t o g i v e a r e a s o n a b l e good i n d i c a t i o n f o r t h e y e a r c l a s s s t r e n g t h a s O-group cod.

REFERENCES

ANON. 1979. The b i o l o g y , d i s t r i b u t i o n and s t a t e of e x p l o i t a t i o n of f i s h s t o c k s i n t h e ICES a r e a . P a r t 11. I n t . Coun. Explor. Sea Coop. R e s . Rep., 1977(86): 1-202.

ANON. 1982. Report of t h e A r t i c F i s h e r i e s Working Group. Copenhagen, 16-23 September

1981.

Coun. Meet. i n t . Coun. Explor. Sea,

1982

(G:2):1-53. (mimeo.)

ANON. 1984. P r e l i m i n a r y r e p o r t of t h e i n t e r n a t i o n a l O-group f i s h survey i n t h e Barents Sea and a d j a c e n t waters i n August-September 1984. Coun. Meet. i n t . Coun. Explor. Sea,

1984

(H:36) : 1-28.(mimeo.)

ANON.

1985.

P r e l i m i n a r y r e p o r t of t h e i n t e r n a t i o n a l O-group f i s h survey i n t h e Barents Sea and a d j a c e n t waters i n August-September

1985.

Coun. Meet. i n t . Coun. Explor. Sea, i985 (G:75) : 1-27.(mimeo.)

ANON. 1986. Report of t h e A r c t i c F i s h e r i e s Working Group.

Copenhagen, 25 September

-

2 October 1985. Coun. Meet. i n t . Coun. Explor. Sea, 1986 (Assess :

4 )

:

1-53.

(mimeo

.

⁾

BARANENKOVA, A.S. and KHOKHLINA, N.S. 1964. On t h e c o n d i t i o n of formation of t h e Arcto-Norwegian cod s t o c k i n t h e 1956, 1960 and 1961 y e a r c l a s s e s d u r i n g t h e f i r s t y e a r of t h e i r l i f e . Trudy PINRO, Vyp. 16 : 195-214.

BARANENKOVA, A.S., SOROKINA, G . B . , KHOKHLINA, N.S.

1973.

D i s t r i b u t i o n and abundance of eggs and l a r v a e of main commercial f i s h e s of t h e B a r e n t s Sea on d r i f t r o u t e s from spawning a r e a s i n A p r i l - J u l y 1969. Trudy PINRO, Vyp.

33

:

34-81.

( I n Russian, with E g l i s h a b s t r a c t ) .

BJØRKE, H.

1981.

D i s t r i b u t i o n of f i s h eggs and l a r v a e from Stad t o Lofoten d u r i n g A p r i l 1976-80. Pp 583-603

3

Roald Sætre and Martin Mork (Eds

.

⁾ The Norwegian Coas t a l

u urr rent.

Universi t y of Bergen

1981:

895 p.

BJØRKE, H.

1984.

D i s t r i b u t i o n of eggs and l a r v a e of gadoid f i s h e s from S t a d t o Lofoten d u r i n g A p r i l 1 9 7 6 - 1 9 8 3 . I n : ~ . Dahl, D.S.

Danielsen, F. Moksness and P. Solemdal ( e d s . ) Flgdevigens Rapportser: l .

1984

: 365-394.

BJØRKE, H. and SUNDBY, S.

1984.

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Fig. 1.

Local names used in the text.

Fig.

2.

Distribution

of

postlarval cod

20

~ u n e - l 0 July

1979.

Number per

trawi hour.

7L0

Fig.3. Distribution of postlarval cod 24 June-l0 July 1980.

per trawl hour.

Number

Fig. 4. Distribution of postlarval cod 29 June-23 July 1981. Number per

trawl hour.

Number

per

Fig. 6. Distribution of

postlarval cod 24

June-20 July 1984. Number

per

trawl hour.

Fig. 7. Distribution of postlarval cod

21

June-15 ~ u l y

1985.

Number

per

trawl hour.

Fig.

9.

Salinity at 20

m

depth in 1986.

70

-

60-

x so-

W a

i

W

g c o -

* 4

+

"l O

a 30-

20-

10-

PS 1.0 1.5 2.0

1

2.5 3.0

- O-GROUP IYOEX

Fig. 10. The regression line between the cod postlarvae and the

O-group indices . Arrows indicate direction of ad justments .

Denne rapport serien har begrenset. dåstribusj on, Opplysninger om programmet og rapportene kan rettes til

Programledelsen for BELP

Fiskeridirektoratets Havforskningsinstitutt Postboks 1870

5024 Bergen

1987

Nr.

1.

!?.Solemdal og P-Bratland: KlekkeforlØp for

lodde

i

Varangerfjorden

1 9 8 6 .

Nr. 2. T.Haug og SeSundby: Kveitelarver og miljØ.

UndersØkelser på gytefeltene ved SØrØya.

Nr. 3. H.BjØrke, K.Hansen og S.Sundby: Postlarveunder- sØkelser i

1 9 8 6 .

Nr,

4 ,

H.BjØrke and S.Sundby: Abundance indices for

the Arcto-Norwegian cod in

1979-1986

based on

larvae investigat'ions,