Distribution and growth of blue whiting.pdf (1.804Mb)

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DISTRIBUTION AND GROWTH OF BLUE WHITING I N THE NORTH-EAST ATLANTIC 1980

- 1988

T e r j e Monstad

I n s t i t u t e of Marine Research P.O. Box 1870 Nordnes N-5024 Bergen, Norway

ABSTRACT

The d i s t r i b u t i o n p a t t e r n of t h e b l u e w h i t i n g spawning s t o c k have g r a d u a l l y changed i n r e c e n t y e a r s , and c o n c e n t r a t i o n s a r e recorded more o f f s h o r e and more t o t h e s o u t h than b e f o r e . The a c t i v i t y of t h e f l e e t w i t h i t s s h i f t i n g of f i s h i n g l o c a l i t i e s , g i v e s knowledge of t h e b l u e w h i t i n g ' s m i g r a t i o n r o u t e d u r i n g t h e spawning s e a s o n .

Also d u r i n g t h e summer season t h e d i s t r i b u t i o n p a t t e r n h a s changed.

The r e c o r d i n g s o f t h e h i g h e s t c o n c e n t r a t i o n s have s h i f t e d from n o r t h and northwest of t h e warmer s i d e of t h e Norwegian Sea t o more s o u t h e r n p a r t s . The a c o u s t i c e s t i m a t e s , which a r e c o n s i d e r e d a s i n d i c e s o n l y , have decreased n o t i c e a b l y s i n c e t h e beginning o f t h e 1 9 8 0 ' s . I n some y e a r s they have been lower than t h e spawning s t o c k assessments, and t h u s demonstrated t h a t t h e t o t a l s t o c k h a s n o t been p r o p e r l y covered d u r i n g t h e f e e d i n g season.

For t h e l a s t 5-6 y e a r s t h e s t r u c t u r e of t h e b l u e w h i t i n g s t o c k has been dominated by t h e s t r o n g of 1982- and 1 9 8 3 - y e a r c l a s s e s .

The growth of b l u e w h i t i n g , based on observed l e n g t h s a s w e l l a s back- c a l c u l a t e d l e n g t h s , i s d e s c r i b e d f o r v a r i o u s y e a r s and y e a r c l a s s e s a s o c c u r i n g i n v a r i o u s a r e a s . E s p e c i a l l y t h e 1982 and 1983 y e a r c l a s s e s when, a s j u v e n i l e s , b e i n g b a s i s f o r t h e mixed i n d u s t r i a l f i s h e r y i n

t h e North Sea.

The growth p a t t e r n v a r i e d s i g n i f i c a n t l y among t h e v a r i o u s y e a r c l a s s e s , and t h e r a t e i n c r e a s e d with i n c r e a s i n g y e a r s up t o t h e 1978-yearclass.

T h e r e a f t e r t h i s e f f e c t was r e v e r s e d up t o t h e 1983-yearclass.

A f t e r t h e age of

5

y e a r s t h e b l u e w h i t i n g growth p a t t e r n may d i v e r s e s i g n i f i c a n t l y between some a r e a s , and o f f t h e Norwegian Coast b l u e w h i t i n g grow t o a l a r g e r s i z e t h a n more o f f s h o r e i n t h e Norwegian Sea and west of t h e B r i t i s h Isles. However, while t h e growth i n t h e Hebrides a r e a was q u i t e s i m i l i a r t o t h a t i n t h e Norwegian S e a , i t d i f f e r e d s l i g h t l y from t h e growth i n t h e Porcupine bank a r e a . T h i s i n d i c a t e s i n f l u e n c e of b l u e w h i t i n g from o t h e r a r e a s which d o n ' t t a k e p a r t i n t h e m i g r a t i o n back t o t h e Norwegian Sea. Analysis of annua1 zone measurements a l c o i n d i c a t e s d i f f e r e n c e s i n t h e s t o c k .

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LIFE HISTORY

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b l u e w h i t i n g D i s t r i b u t i o n and growth of b l u e w h i t i n g

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INTRODUCTION

Blue w h i t i n g (Micromesistius poutassou) i n t h e n o r t h e a s t A t l a n t i c system probably c o n s i s t s of s e v e r a l s t o c k s , which most l i k e l y o v e r l a p each o t h e r . From t h e f e e d i n g a r e a i n t h e Norwegian Sea, t h e m a j o r i t y of b l u e w h i t i n g each s p r i n g undertakes a spawning migration t o t h e main spawning ground i n t h e a r e a west of t h e B r i t i s h Isles. The ICES' Blue Whiting Assessment Working Group i n 1980 decided t o t r e a t t h e e n t i r e a r e a a s a s i n g l e assessment u n i t with southern boundary a t t h e Porcupine bank, i. e. along l a t i t u d e

5Z0

30' N (Anon. 1980)

.

The Porcupine bank a r e a is considered an a r e a where b l u e w h i t i n g from t h e s o u t h , from t h e n o r t h and from l o c a l p o p u l a t i o n s mix. Some y e a r s , however, c a t c h e s taken on t h e southern s l o p e of t h e bank have been thought belonging t o t h e n o r t h e r n s t o c k and consequently confined t o t h e n o r t h e r n a r e a . The Working Group t h e r e f o r e found i t n a t u r a l a l s o t o have t h e boundary between t h e s o u t h e r n and t h e n o r t h e r n s t o c k s t o be f u r t h e r s o u t h , and i n 1988, f o r p r a c t i c a l reason decided t o have i t l o c a t e d a t l a t i t u d e

480

N (Anon. 1989).

I n t h e 1 9 7 0 ' s t h e i n t e r e s t i n commercial e x p l o i t a t i o n o f b l u e w h i t i n g had a r a p i d i n c r e a s e . I n t e r n a t i o n a l f i s h e r y took p l a c e i n t h e f e e d i n g a r e a a s w e l l a s i n t h e spawning a r e a , and i n a d d i t i o n b l u e w h i t i n g was caught a s by-catch i n t h e mixed i n d u s t r i a l f i s h e r y i n t h e North Sea. A number of approximately

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n a t i o n s have used t o e x p l o i t t h e b l u e w h i t i n g s t o c k .

Since t h e 1960's t h e b l u e w h i t i n g s t o c k has been surveyed i n t h e f e e d i n g a r e a d u r i n g summer o r autumn by r e s e a r c h v e s s e l s from v a r i o u s c o u n t r i e s , f o r exempel USSR (Ushakov and Mazhirina 1978, Shevchenko and I s a e v 1 9 8 3 ) , Norway (Blindheim e t . a l . 1971, Blindheim and Jakupsstovu 1976, Monstad and Blindheim 1978, 1986) and Germany (Sarhage and Sshgne 1980). I n t h e p e r i o d 1982

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86 t h e summer surveys have been coordinated by ICES and r e s e a r c h v e s s e l s from The Faroes, I c e l a n d , GDR, Norway, USSR and Denmark have p a r t i c i p a t e d (Anon. 1982, 1983b- 1986b).

The assessments from t h e s e s u r v e y s , considered a s r e l a t i v e i n d i c e s because o f , most probably incomplete coverage of t h e t o t a l n o r t h e r n s t o c k , i n d i c a t e a s t e a d y d e c r e a s e of t h e s t o c k from 1980 onwards (Monstad and Blindheim 1986). I n t h e Working Group's VPA-results an i n c r e a s e of t h e s t o c k s i z e due t o t h e s t r o n g 1982 and 1983 y e a r c l a s s e s i s r e f l e c t e d . From 1986 a s l i g h t d e c r e a s e set i n again (Anon. 1989).

Every y e a r s i n c e t h e beginning of t h e 1 9 7 0 ' s t h e spawning s t o c k o f t h e n o r t h e r n b l u e w h i t i n g has been monitored a c o u s t i c a l l y i n t h e s p r i n g season by v e s s e l s from one o r more c o u n t r i e s . However, due t o incongruence of time and a r e a covered, t h e r e s u l t i n g e s t i m a t e s show a g r e a t discrepancy (Anon. 1983a, 1984a, 1986 a , 1989).

The assessments of t h e s t o c k s i z e i n t h e spawning a r e a i n g e n e r a l have been, however, h i g h e r than t h e assessments i n t h e f e e d i n g a r e a . T h i s i n d i c a t e s a b i a s i n methodology, and i n 1985 an ICES Workshop d e a l i n g with t h i s m a t t e r was set ^up. It concluded t h a t t o o b t a i n proper r e c o r d i n g s of t h e s t o c k i n t h e Norwegian Sea t h e s i z e of i t h a s t o be above a c e r t a i n l e v e l o r d i s t r i b u t e d i n a s m a l l e r a r e a i n l a r g e r c o n c e n t r a t i o n s than was t h e c a s e some of t h e y e a r s , e s p e c i a l l y i n

1983

and

1984

(Anon

1985

a ) .

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b l u e w h i t i n g

D i s t r i b u t i o n and growth of b l u e w h i t i n g

...

The change o f t h e b l u e w h i t i n g d i s t r i b u t i o n p a t t e r n i n t h e Norwegian Sea i n t h e 1 9 8 0 ' s may n o t o n l y be due t o a s t o c k d e c r e a s e . It can a l s o have been caused by changes i n t h e enviromental c o n d i t i o n s d e s c r i b e d by Shevchenko and I s a e v (1983, 1 9 8 5 ) , (Monstad and Blindheim 1986).

The growth o f b l u e w h i t i n g has been d i s c u s s e d by B a i l e y ( 1 9 8 2 ) . H e compares parameters o b t a i n e d by s e v e r a l a u t h o r s and show a g r e a t v a r i e t y i n t h e growth. T h i s may i n d i c a t e t h e e x i s t e n c e of s e p a r a t e s t o c k s . The growth r a t e i s h i g h e s t i n t h e female which a l s o grow t o a l a r g e r s i z e than t h e male. Within i t s f i r s t y e a r of l i f e t h e b l u e w h i t i n g may r e a c h a s i z e of 20-25 cm (Gjøsæter, Beck and Monstad

1979) .

MATERIAL AND METHODS

Blue w h i t i n g was recorded d u r i n g surveys by u s e of echo sounders. For ^I measurements of f i s h d e n s i t y t h e echo sounder was connected t o an echo

i n t e g r a t o r . Ssme of t h e survey r e s u l t s l i k e t h e c r u i s e t r a c k with s t a t i o n s , t h e d i s t r i b u t i o n , s t r u c t u r e and e s t i m a t e o f t h e s t o c k have p r e v i o u s l y been published i n survey r e p o r t s and o t h e r s .

The d e n s i t y symbols of t h e d i s t r i b u t i o n p a t t e r n are based on r e l a t i v e v a l u e s a d j u s t e d t o t h e abundance e s t i m a t e f o r comparison o f t h e v a r i - ous y e a r s i n t h e a r r a y . The p a t t e r n s d u r i n g t h e s p r i n g season a r e a l l based on Norwegian surveys (Monstad and Midttun 1980,1981; Dahl 1982;

Midttun 1983; Monstad

1984, 1986,

1987a,

1988a),

e x c e p t f o r

1985

when Norway d i d n o t survey t h e spawning a r e a . For t h a t y e a r t h e r e s u l t from a Faroese survey i s used (Jakupsstovu and Thomsen

1985).

The p a t t e r n s d u r i n g t h e summer season are based on Norwegian surveys a l o n e f o r t h e y e a r s 1980,

i981

and

1988

(Hamre e t a l . 1980; Blindheim and Monstad 1981; Monstad 1988b). For t h e y e a r s 1982-1986 t h e d i s t r i - b u t i o n p a t t e r n s a r e based on t h e ICES-coodinated s u r v e y s (Anon. 1982, 1983b-1986b) and f o r

1987

on combined r e s u l t s of Norwegian and GDR s u r v e y s (Danke 1987; Monstad 1987b).

During t h e j o i n t s u r v e y s , d a t a l i k e echo i n t e g r a t o r r e a d i n g s , t r a w l c a t c h i n f o r m a t i o n s , b l u e w h i t i n g l e n g t h measurements and temperature o b s e r v a t i o n s were exchanged between t h e v e s s e l s from p a r t i c i p a t i n g c o u n t r i e s . For a l l t h e y e a r s concerned, however, t h e age compositions f o r b o t h t h e s p r i n g and t h e summer s i t u a t i o n s , a r e based on o t o l i t h r e a d i n g s of Norwegian samples only.

For t h e Norwegian Sea age-length keys were used on e i t h e r s e p a r a t e o r combined l e n g t h measurements, weighted by t h e echo abundance o b t a i n e d w i t h i n t h e v a r i o u s a r e a s surveyed. For t h e spawning a r e a t h e l e n g t h measurements were a l l from Norwegian r e s e a r c h v e s s e l s , e x c e p t i n 1985 when samples from commercial c a t c h e s were used. The d e s c r i p t i o n of t h e t o t a l age compositions i s based on t h e r e s u l t s from t h e Working Group's VPA-run, and hence i n c l u d e s t h e t o t a l c a t c h of b l u e whiting p r e s e n t e d a s number of f i s h p e r age group (Anon. 1989).

The d e s c r i p t i o n of t h e f i s h e r y p r o g r e s s d u r i n g s p r i n g season i s based on c a t c h s t a t i s t i c s from t h e D i r e c t o r a t e of F i s h e r i e s .

The b i o l o g i c a l samples used i n t h e length-growth a n a l y s i s were i n g e n e r a l c o l l e c t e d w i t h r e s e a r c h v e s s e l s , supported by complementing samples from commercial c a t c h e s . The o t o l i t h s were k e p t whole and soaked i n water while b e i n g aged and measured under microscope. The annual zones were measured along t h e l a r g e s t d i a m e t e r u s i n g t h e i n n e r

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...

edge of t h e h y a l i n e a r e a .

Von B e r t a l a n f f y ' s e q u a t i o n was used i n t h e a n a l y s i s , and t h e v a l u e s f i t t e d t o length-growth curves a s descibed by Ricker

(1975).

The c a l - c u l a t i o n s were run by a computer programme. A p l o t of f i s h l e n g t h a g a i n s t o t o l i t h s i z e was used f o r " b a c k - c a l c u l a t i o n " , and mean v a l u e s o f t h e zones were e n t e r e d t h e graph t o g e t t h e corresponding f i s h l e n g t h s (Tesch 1 9 6 8 ) .

I l l u s t r a t i o n of t h e growth of j u v e n i l e b l u e whiting i n t h e North Sea, was made p o s s i b l e by u s e of age-length keys on t h e m a t e r i a l of l e n g t h measurements o b t a i n e d from commercial c a t c h e s a t t h e f i s h p l a n t s by t h e D i r e c t o r a t e of F i s h e r i e s .

For d e s c r i p t i o n of t h e temperature t r e n d s a t d i f f e r e n t d e p t h t s d u r i n g t h e s p r i n g and t h e summer season were used o b s e r v a t i o n s made with CTD- sonde from Norwegian r e s e a r c h v e s s e l s only. The 2O-isoline a t 200m depth d u r i n g summer i s o b t a i n e d from t h e Norwegian sea s u r v e y s mentioned above.

RESULTS AND DISCUSSION D i s t r i b u t i o n ;

1) I n t h e spawning a r e a .

D i s t r i b u t i o n and d e n s i t y of b l u e whiting i n t h e spawning season, i . e . March-May, of 1980-1988 a r e shown i n F i g . l a - c . O u t l i n e s of t h e a c t u a l a r e a s surveyed a r e a l s 0 marked on t h e f i g u r e . Due t o more o r less c o n s t a n t movement of t h e s t o c k w h i l e m i g r a t i n g t o and from t h e spawning grounds, t h e p i c t u r e of d i s t r i b u t i o n , a s w e l l a s t h e a c o u s t i c abundance e s t i m a t e s , v a r i e s from y e a r t o y e a r depending on t h e t i m e p e r i o d and t h e a c t u a l a r e a surveyed.

The t e x t t a b l e below shows t h e biomass ( i n m i l l . tonnes) o f t h e b l u e w h i t i n g spawning s t o c k e s t i m a t e d from t h e Norwegian a c o u s t i c s u r v e y s , and t h e corresponding VPA-results o b t a i n e d by t h e Working Group (Anon.

1989)

Survey:

5.4

6.0

4.4

2 . 1

-

^{2 . 1}

4 . 1

^{6 . 8}

VPA :

4 . 4 3.5

^3.0 ^2.6 ^2.6

3.5 4.5

^{4 . 2}

4.3

The s t o c k s i z e was found t o vary through t h i s p e r i o d and t h e lowest v a l u e s was observed i n

1984

and 1986. During t h e n e x t two y e a r s a l a r g e r s t o c k was measured and i n 1988 i t had reached t h e l e v e 1 of t h e s t o c k s i z e observed i n t h e beginning of 1 9 8 0 ' i e s . The VPA-results, however, r e f l e c t a more even p i c t u r e , b u t w i t h t h e h i g h e s t v a l u e i n 1986 when t h e a c o u s t i c one was l o w e s t .

I n g e n e r a l t h e h i g h e s t d e n s i t i e s of b l u e w h i t i n g were found n e a r t h e c o n t i n e n t a l s l o p e , b u t some y e a r s a l s o more w e s t e r l y o v e r deep w a t e r . During t h e a r r a y of y e a r s concerned i t i s , however, p o s s i b l e t o d i s t - i n g u i s h a s l i g h t change from a mainly n o r t h e r n t o a more s o u t h e r n p a t t e r n .

I n t h e y e a r s 1980-1984 t h e b e s t r e c o r d i n g s were made, and hence t h e g r e a t e r p a r t of t h e spawning s t o c k observed mainly i n t h e a r e a s o f f t h e Hebrides. Blue w h i t i n g was, however, found a l l t h e way from t h e

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blue whiting

Distribution and growth of blue whiting

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blue whiting Distribution and growth of blue whiting

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blue whiting

Distribution and growth of bPue whiting

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b l u e whiting D i s t r i b u t i o n and growth of b l u e whiting.

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Porcupine bank t o The Faroes-Shetland a r e a , except f o r 1982 when only a s m a l l e r a r e a was surveyed. I n

1981

blue whiting was observed around The Faroes and i n

1984

on e i t h e r s i d e of t h e Rockall bank.

I n 1985 t h e b e s t recordings of blue whiting were made i n an a r e a more t o t h e south than i n t h e previous y e a r s , i . e . on t h e s h e l f between t h e Hebrides and I r e l a n d .

I n t h e s p r i n g of 1986 b l u e whiting was found s c a t t e r e d d i s t r i b u t e d over an a r e a from t h e Porcupine bank t o n o r t h of The Faroes. An extended survey demonstrated t h e continuation of t h e d i s t r i b u t i o n f u r t h e r northwards along t h e s h e l f edge west of Norway up t o t h e Lofoton Isles (Monstad 1986). However, mostly weak recordings were o b t a i n e d , and hence probably only a p o r t i o n of t h e spawning s t o c k was recorded t h a t year.

8 1 8 2 83 8 4 85 86 8 7 88 80 8 1 8 2 8 3 8 4 8 5 8 6 8 7 8 8 Year

Fig. 2. Temperature t r e n d s a t

5 ,

50, 100, 200, and 400 m depth f o r t h e period 1980-1988. A ) I n t h e spawning a r e a west of t h e Hebrides i n March/April near p o s i t i o n : 5 8 O ~ , 10°W.

B) I n t h e Norwegian Sea i n August a t t h e o u t e r s t a t i o n of t h e Svin~y-NW s e c t i o n , p o s i t i o n : 64040' N , 00° 00' E/W.

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...

I n 1987 t h e d e n s i t y was found t o be s i g n i f i c a n t l y b e t t e r t h a n i n 1986.

Recordings of b l u e w h i t i n g were made from s o u t h o f I r e l a n d t o o v e r t h e Porcupine bank and n o r t h of i t , and f u r t h e r a l o n g t h e s h e l f edge t o t h e a r e a n o r t h of S h e t l a n d and The Faroes. The h i g h e s t concen- t r a t i o n s and t h e m a j o r i t y of t h e s t o c k were found t o be l o c a t e d more t o t h e s o u t h t h a n p r e v i o u s l y , i . e . t h e western p a r t o f t h e Porcupine bank and o v e r deep water n o r t h of i t .

I n s p r i n g 1988 an even l a r g e r p a r t of t h e spawning s t o c k was recorded t h a n i n 1987, d i s t r i b u t e d from s o u t h of I r e l a n d t o t h e Faroes-Shetland a r e a . Very h i g h c o n c e n t r a t i o n s of b l u e w h i t i n g were found b o t h t o t h e s o u t h and t o t h e n o r t h , i . e . n e a r t h e s l o p e w e s t and n o r t h of t h e Porcupine bank and west and southwest of t h e Hebrides. I n a d d i t i o n b l u e w h i t i n g appeared i n n o t a b l e c o n c e n t r a t i o n s f a r t h e r o f f t h e c o n t i n e n t a l s h e l f and o v e r deeper water t h a n u s u a l l y observed.

The observed g r a d u a l change i n t h e d i s t r i b u t i o n p a t t e r n could i n d i c a t e t h a t d u r i n g t h e l a s t 2-3 y e a r s t h e Porcupine bank a r e a h a s become more important a s t h e main spawning ground t h a n t h e a r e a o f f t h e Hebrides.

The temperature c o n d i t i o n s on t h e spawning ground were found t o be v e r y homogeneous i n t h e water coloumn, w i t h only minor y e a r t o y e a r changes, v a r y i n g s l i g h t l y around

g0

C ( F i g . 2 a ) .

1 0 0 0 T o n n e s

\ N o r w e g i a n S e a

\

M i x e d I n d u s t r i a l e4--9---.-,-.m-=-.,C-\\ 5" d @ - - - -3&.-o-o,

-0-

.-.-.

---.-.-.-.C~~ ' - ~ - - m - - - - -e==--

7

Fig.

3.

Annua1 l a n d i n g s of b l u e w h i t i n g , from t h e main f i s h e r i e s and t o t a l , of t h e n o r t h e r n s t o c k , 1978-1987. Source: Working Group Report of 1988.

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LIFE HISTORY

-

b l u e whiting D i s t r i b u t i o n and growth o f b l u e w h i t i n g

...

2 ) During t h e spawning f i s h e r y season.

From a modest f i s h e r y i n t h e f i r s t h a l f of t h e 1 9 7 0 ' s , t h e commercial e x p l o i t a t i o n of t h e n o r t h e r n b l u e whiting s t o c k i n c r e a s e d t o maxima i n 1979 and 1980 with annua1 c a t c h e s of more than 1 m i l l . tonnes ( F i g . 3 ) . A few y e a r s l a t e r t h e c a t c h r a t e was c u t down t o approximately t h e h a l f , and then again i n c r e a s e d t o a new maximum i n 1986.

I n t h e y e a r s 1978

- 1981

t h e bulk of t h e c a t c h e s w a s taken i n t h e Norwegian Sea w i t h t h e USSR a s l e a d i n g n a t i o n ( F i g . 4 ) . S i n c e 1982, however, t h e l a r g e s t q u a n t i t i e s of b l u e whiting have been taken i n t h e spawning a r e a w e s t of t h e B r i t i s h Isles. I n t h i s f i s h e r y Norway use t o t a k e s i g n i f i c a n t l y more than t h e o t h e r n a t i o n s and i s r e s p o n s i b l e f o r approximately h a l f of t h e l a n d i n g s .

I n t h e mixed i n d u s t r i a l f i s h e r y i n t h e North Sea, where b l u e w h i t i n g i s taken a s by-catch, Denmark and Norway are t h e main a c t o r s . The

1 0 0 0 tonnes

N o r w e g i a n S e a

D e n m a r k O t h e r

3 0 0 S p a w n i n g A r e a

200 1 0 0

I

M i x e d I n d u s t r i a l 1 O0

Fig.

4.

Annua1 l a n d i n g s of b l u e w h i t i n g by n a t i o n s from t h e main f i s h e r i e s of t h e n o r t h e r n s t o c k , 1978-1987. Source: Working Group Report of

1988.

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LIFE HISTORY

-

b l u e w h i t i n g 237 D i s t r i b u t i o n and growth of b l u e w h i t i n g

...

annua1 l a n d i n g s , however, exceeded 100 000 tonnes o n l y i n t h e y e a r s 1982

-

1984 ( F i g .

3) .

During s p r i n g t h e migration of b l u e whiting c o n c e n t r a t i o n s w i t h i n and from t h e spawning a r e a , can be i l l u s t r a t e d by t h e a c t i v i t y of t h e f i s h i n g f l e e t . P r e f e r a b l y , t h e f l e e t w i l l always o p e r a t e on t h e d e n s e s t c o n c e n t r a t i o n s , and t h u s i t s t r a n s f e r r o u t e through t h e s e a s o n , t o a g r e a t e x t e n t c a n be a p a r a l l e l t o t h e m i g r a t i o n r o u t e . F i g . 5a-c, which i s p u r e l y based on Norwegian c a t c h s t a t i s t i c s a l l o c a t e d on s t a t i s t i c a l r e c t a n g l e s , shows t h e f i s h e r y p r o g r e s s i n t h e s p r i n g season of 1980

-

^1988.

I n g e n e r a l t h e f i s h e r y s t a r t s a t t h e Porcupine bank i n March o r l a t e February, c o n t i n u e s i n t h e a r e a o f f t h e Hebrides i n A p r i l and t e r m i n a t e s i n Faroes-Shetland waters by t h e end o f May o r beginning of June. The bulk of t h e c a t c h e s i s u s u a l l y landed i n A p r i l from t h e a r e a between t h e Hebrides and Faroe I s l a n d s .

There has been a tendency f o r t h e f i s h e r y t o s t a r t g r a d u a l l y e a r l i e r each y e a r . While i t began i n t h e middle of March i n 1980 and

1981,

i t was i n i t i a t e d i n l a t e February i n 1982 and

-83,

o f f I r e l a n d and t h e Hebrides r e s p e c t i v e l y . S i n c e then t h e f i s h e r y h a s taken p l a c e i n February every y e a r i n t h e a r e a o f f t h e Hebrides.

From

1985

onwards i t a l r e a d y s t a r t e d i n e a r l y January i n Faroes waters and continued southwards t o t h e a r e a w e s t of t h e Hebrides i n February.

A f t e r t h a t t h e f i s h e r y f e l l back t o i t s t r a d i t i o n a l p a t t e r n , with a c t i v i t y on t h e Porcupine bank a r e a i n March, o f f t h e Hebrides a g a i n i n A p r i l and i n Faroes and Shetland waters i n May and e a r l y June.

Some y e a r s t h e f i s h e r y could t a k e p l a c e o v e r e x t e n s i v e a r e a s with c a t c h i n g b o t h a t t h e edge of t h e Porcupine bank and o f f t h e Hebrides, o r o f f t h e Hebrides and i n t h e Faroes-Shetland a r e a a t same t i m e . I n

1985

and 1986 t h e c a t c h i n g of b l u e w h i t i n g i n t h e Faroes a r e a i n e a r l y January, was a c o n t i n u a t i o n of c a t c h i n g which a c t u a l l y had s t a r t e d up on t h o s e f i s h i n g grounds i n December t h e p r e v i o u s y e a r . I n

1987

and

1988

t h e Norwegian f i s h e r y f o r b l u e w h i t i n g i n t h e a r e a west of I r e l a n d , i n a d d i t i o n t o an extreme western l o c a t i o n took p l a c e more t o t h e s o u t h t h a n e v e r b e f o r e .

I n g e n e r a l t h e bulk of t h e c a t c h e s is taken n e a r t o t h e c o n t i n e n t a l s l o p e , i . e . t h e p e l a g i c t r a w l i s used very c l o s e t o , and some times a c t u a l l y a t t h e bottom. I n

1988,

however, t h e f l e e t had t o work more o f f t h e s l o p e a r e a and over deeper water than b e f o r e t o g e t t h e b e s t c o n c e n t r a t i o n s of b l u e whiting. The c a t c h e s taken n e a r e r t o bottom a t t h e r s l o p e c o n t a i n e d a s i g n i f i c a n t p a r t of g r e a t e r s i l v e r smelt

(Argentina s i l u s ) a s by-catch. T h i s was e s p e c i a l l y t h e c a s e i n t h e o f f t h e Hebrides i n May.

Viewing t h e f i s h i n g f l e e t s o p e r a t i o n l o c a l i t i e s through t h e p e r i o d 1980

- 1988

a s shown on F i g .

5,

i t i s obvious t h a t g r e a t e r p a r t s of t h e b l u e w h i t i n g s t o c k have annua1 o s c i l l a t i o n between t h e Norwegian Sea and t h e s l o p e a r e a west of t h e B r i t i s h Isles. However, t o what e x t e n t a l s 0 p a r t s of t h e spawning s t o c k m i g r a t e t o o t h e r a r e a s i s s t i l l a v a l i d q u e s t i o n .

Due t o a f i s h e r y ban i n t h e a r e a s o u t h of 5b030'N and e a s t of l L O W , t h e r e i s a gap i n t h e f i s h e r y a c t i v i t y between t h e Porcupine Bank a r e a

(12)

LIFE HISTORY

-

...

Fig.

S A .

Progress of the Norwegian blue whiting fishery in the spawning area, 1980-1982.

(13)

LIFE HISTORY

-

blue whiting

...

Fig.

5B.

Progress of the Norwegian blue whiting fishery in the spawning area,

1983-1985.

(14)

LIFE HISTORY

-

...

J a n u a r y / F s b r u a r y 1ii;rcli A p r i l I ' i a y / J a r i i

<

500 t o n n e c 500-5000 t o n n e s 5000-15000 t o n n e s

1

> l 5 0 0 0 t o n n e s

Fig. 5C. Progress of the Norwegian blue whiting fishery in the spawning area,

1986-1988.

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LIFE HISTORY - b l u e w h i t i n g

...

and o f f t h e Hebrides. P a r t of t h e s t o c k a t t h e Porcupine Bank may n o t m i g r a t e northwards, b u t r a t h e r southwards t o Biscaya and t h e Azores a r e a , o r even westward and north-westward t o East-Greenland and s o u t h of I c e l a n d .

3)

I n t h e f e e d i n g a r e a .

The d i s t r i b u t i o n and d e n s i t y of b l u e w h i t i n g i n t h e Norwegian Sea d u r i n g t h e summer season of 1980

- 1988

a r e shown i n F i g . 6a-c. The a r e a surveyed i s a l s 0 marked i n t h e f i g u r e t o g e t h e r with t h e t e r m o l i n e of 2O C a t 200 m depth. The f i s h were recorded from t h e s u r f a c e down t o mainly 300 m d e p t h , b u t f r e q u e n t l y even d e e p e r , i . e . t o 500 m. The o v e r a l l m a j o r i t y of t h e r e c o r d i n g s , however, was o b t a i n e d a t 200 m d e p t h , and hence t h e t e r m o l i n e i n t h a t depth i s chosen.

I n t h e t e x t t a b l e below is given t h e t o t a l estimates ( i n m i l l . tonnes) of t h e b l u e w h i t i n g biomass from t h e Norwegian Sea surveys i n 1980-88 of which t h e r e s u l t s i n 1982-86 a r e from t h e i n t e r n a t i o n a l s u r v e y s , and t h e corresponding VPA-results of t h e t o t a l s t o c k o b t a i n e d by t h e Working Group.

Survey: 9.1* 4.9" 4 . 6 2.8 3.8 4.9 3.0 2.5** 2.6"

VPA :

5.1

^{4 . 3}

4 . 1 4.8 5.4 5.8

6.4 6.2

5.8

* Norwegian surveys o n l y ,

* *

Combined r e s u l t of Norwegian and GDR s u r v e y s

The h i g h e s t a c o u s t i c e s t i m a t e i n t h e Norwegian s e a was made i n 1980 a s a l s 0 t h e spawning s t o c k was observed a t a comparative high l e v e 1 west o f The B r i t i s h Isles. I n

1981

t h e r e was a s i g n i f i c a n t drop i n t h e

" t o t a l " s t o c k e s t i m a t e , a s was t h e c a s e a l s 0 from 1982 t o

1983.

T h e r e a f t e r t h e s t o c k had an i n c r e a s e due t o t h e numerous 1982- and 1 9 8 3 - y e a r c l a s s e s . However, t h e new d e a c r e a s e observed i n 1986 and t h e low e s t i m a t e s o b t a i n e d i n 1987 and 1988 a r e i n no correspondance w i t h t h e abundance e s t i m a t e s i n t h e spawning a r e a o v e r t h e same p e r i o d of t i m e .

Though t h e a r e a surveyed d i d n o t cover t h e e n t i r e s t o c k i n t h e Norwegian S e a , i t was most probable a main and i m p o r t a n t p a r t . A s mentioned e a r l i e r , t h e r e c o r d i n g s of b l u e w h i t i n g d u r i n g t h e summer season i n t h e Norwegian s e a when t h e s t o c k i s d i s p e r s e d o v e r v a s t e a r e a s and a t g r e a t d e p t h s , do n o t r e f l e c t t h e t r u e p i c t u r e . The e s t i - mates have t o be t r e a t e d a s i n d i c e s o n l y , w i t h t h e o b s e r v a t i o n s t o a c e r t a i n degree d e s c r i b i n g t h e d i s t r i b u t i o n p a t t e r n .

The VPA-results, l i k e t h e a c o u s t i c e s t i m a t e s show a drop from 1980 t o 1981 and -82, b u t t h e r e a f t e r an s t e a d y i n c r e a s e i n t h e s t o c k s i z e up t o 1986-87.

For a l l t h e y e a r s concerned t h e b l u e w h i t i n g was observed s c a t t e r e d o v e r t h e g r e a t e r p a r t of t h e a r e a surveyed, i . e . on t h e e a s t e r n and warmer s i d e of t h e Norwegian Sea. Most y e a r s , however, d e n s e r concen-

t r a t i o n s of b l u e w h i t i n g were most of t h e y e a r s found i n minor and l i m i t e d a r e a s only.

The h i g h e s t abundance was measured i n 1980, w i t h t h e b e s t concen- t r a t i o n s t o a g r e a t e x t e n t confined t o t h e n o r t h i n an a r e a between J a n Mayen and Bear I s l a n d ( F i g . 6 a ) . I n t h e succeeding y e a r s t h e

(16)

Fig.

6A.

Distribution and densities of blue whiting with 2OC isoline at 200 m depth, during summer 1980-1982. Single hatch: area with scattered recordings, double hatch: dense recordings, black: very dense recordings. Stippled line indicates the area surveyed.

(17)

LIFE HISTORY

-

blue whiting

...

(18)

LIFE HISTORY

-

...

u .d C, C, 'd b;)

7 .d

5 f:CC"

k .d C

C, 4 .d

V) O -d V) m n .d 'd U3 U

.d b;)

h

(19)

LIFE HISTORY

-

...

abundance decreased, and t h e b e s t c o n c e n t r a t i o n s were found more t o t h e s o u t h i n a " b e l t " from I c e l a n d t o western Norway.

I n t h e y e a r s 1982-1985 t h e d e n s e s t c o n c e n t r a t i o n s were s o l e l y recorded i n t h e s o u t h . During t h i s p e r i o d t h e c o n t r i b u t i o n s of t h e s u c c e s s f u l 1982- and 1983-yearclasses were s i g n i f i c a n t , and t h e t o t a l s t o c k abundance i n c r e a s e d . The j u v e n i l e p a r t of t h e s t o c k , however, s t i l l appeared mostly i n t h e s o u t h e r n p a r t of t h e Norwegian Sea. I n

1985

high d e n s i t i e s were observed on t h e s h e l f around t h e Faroe I s l a n d s ( ~ i g . 6 b ) .

From 1986 t o 1988 t h e abundance of b l u e w h i t i n g decreased once more and mostly very s c a t t e r e d recordings were made i n t h e f e e d i n g a r e a . Dense c o n c e n t r a t i o n s t h e s e y e a r s were only o b t a i n e d w i t h i n s e p a r a t e minor a r e a s i n t h e s o u t h and i n t h e n o r t h ( F i g . 6 ~ ) .

The western l i m i t of t h e d i s t r i b u t i o n could i n some y e a r s be r e l a t e d t o t h e 2O - i s o l i n e a t 200 m d e p t h , e s p e c i a l l y i n t h e northwestern p a r t i n

1981 -

1986. I n 1987 and 1988 water of temperature around 2O C a t 200 m was o n l y observed w i t h i n a narrow a r a n o r t h of t h e Faroe I s l a n d s .

The temperature c o n d i t i o n s i n t h e Norwegian Sea d i f f e r s i g n i f i c a n t l y from t h e c o n d i t i o n s observed f u r t h e r s o u t h on t h e spawning grounds. I n F i g 2b a r e shown t h e o b s e r v a t i o n s made a t t h e o u t e r s t a t i o n o f t h e s t a n d a r d hydrographic s e c t i o n Svin~y-NW, i . e a t p o s i t i o n 64O40' N 00°00' E/W, i n August 1980-1988. This s t a t i o n may r e p r e s e n t t h e south- e a s t e r n a r e a o f t h e Norwegian Sea, where c o l d e r water from n o r t h p e n e t r a t e s i n t h e deep and t h u s mixes i n t h e water coloumn w i t h warmer water from s o u t h .

The August temperature i n t h e s u r f a c e l a y e r v a r i e d between

lo0

^{and 12O}

d u r i n g t h e p e r i o d , while a t 400 m i t v a r i e d from O.SO i n 1980 t o 2O C i n 1988. The g r e a t e s t v a r i a t i o n was observed i n t h e medium depth l a y e r of 100

-

²⁰⁰^m where a g r e a t e r p a r t of t h e b l u e w h i t i n g s t o c k u s u a l l y i s recorded.

A t 200 m depth t h e August temperature i n c r e a s e d from

1.9O

i n 1983 t o

5.8O

i n 1986, dropped t o 3.S0 t h e n e x t y e a r and t h e n i n c r e a s e d r a d i - c a l l y t o 6.2O C i n

1988

when t h e i n f l u x of c o l d e r water from t h e n o r t h was reduced once more. The same p a t t e r n of v a r i a t i o n could a l s 0 be observed a t 100 m d e p t h , o n l y a t a O. 5O

-

2O h i g h e r temperature l e v e l . The d i s t r i b u t i o n p a t t e r n i n t h e f e e d i n g a r e a t o

a

g r e a t e x t e n t i s i n f l u e n c e d by t h e a v a i a b i l i t y o f food, b u t a l s 0 by t h e hydrographic c o n d i t i o n s b o t h d i r e c t l y and i n d i r e c t l y . Plekhanova and Soboleva (1982) found t h a t i n y e a r s with high plankton p r o d u c t i o n t h e b l u e w h i t i n g s t o c k is widely d i s p e r s e d , and i n y e a r s w i t h low production t h e c o n c e n t r a t i o n s i s mainly found i n t h e a r e a s where t h e production i s b e s t .

Due t o i n c r e a s e d e x t e n t of A r t i c water with temperature below Jo C t h e A r t i c f r o n t a t 200 m depth s h i f t e d from 1980 t o

1984

about 300 km eastward. This may have a c t e d a s a b a r r i e r which h a s f o r c e d t h e s t o c k t o chose a more e a s t e r n m i g r a t i n g r o u t e than e a r l i e r (Monstad and Blindheim 1 9 8 6 ) .

The migration r o u t e t o t h e Norwegian Sea t h u s followed t h e s o u t h e r n s l o p e of t h e Faroe-Shetland Channel a s d e s c r i b e d by Shevchenko and I s a e v (1983, 1985). This i s a l s 0 e v i d e n t from t h e i l l u s t r a t i o n s of t h e

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LIFE HISTORY

-

...

Mest o f B r i t i s h I s l e s ( s u r v e y l R o r w e e i a n S e a ( s u r v e y l T o t a l [ V P A I s p r i n g

20 i 1 9 8 0

1 0 -

F i g .

7.

Age composition (N%) of b l u e w h i t i n g i n t h e p e r i o d 1980-1988 i n t h e spawning s t o c k west of t h e B r i t i s h Isles d u r i n g s p r i n g , i n t h e s t o c k recorded i n t h e Norwegian Sea d u r i n g summer and i n t h e t o t a l s t o c k as c a l c u l a t e d by VPA.

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LIFE HISTORY

-

...

o p e r a t i o n p o s i t i o n s of t h e Norwegian f i s h i n g f l e e t ( F i g .

5

a - c ) . While t h e f i s h i n g season was t e r m i n a t e d a t t h e western s i d e of t h e Faroes i n 1980 i t was a t t h e e a s t e r n s i d e from

1981

t o 1987.

T h i s a l o n e does n o t g i v e t h e e x p l a n a t i o n t o t h e reduction of congrea- g a t i o n s i n t h e f e e d i n g a r e a s more n o r t h . The s h i f t of r o u t e , however, may somehow have l e d t h e s t o c k i n t o a r e a s where i t could d i s p e r s e i n t o deeper water and t h u s been more d i f f i c u l t t o r e c o r d .

Age composition.

Age compositions of b l u e w h i t i n g i n t h e y e a r s 1980

- 1988

are shown i n F i g .

7

f o r t h e spawning s t o c k , t h e s t o c k observed i n t h e Norwegian Sea d u r i n g summer and f o r t h e t o t a l s t o c k a s o b t a i n e d from VPA-results. I n t h e t h r e e sets of i l l u s t r a t i o n s t h e s t r o n g 1982- and 1983-yearclasses dominate t h e p i c t u r e .

I n 1980 f i s h o f

5 -7

y e a r s o f age were t h e most numerous ones i n t h e spawning s t o c k . Then, i n

1981

t h e 1978-yearclass was t h e most abundant w i t h more t h a n 20% of t h e s t o c k , and up t o

1983

became t h e dominating y e a r c l a s s .

I n t h e Norwegian Sea t h e 1978-yearclass was a l s o found t o b e t h e most s i g n i f i c a n t one i n t h i s p e r i o d , and i s r e f l e c t e d i n t h e VPA- r e s u l t s a s a r a t h e r abundant y e a r c l a s s . I n

1985

i t c o n t r i b u t e d w i t h

13%

t o t h e spawning s t o c k , b u t t h e r e a f t e r o n l y t r a c e s of i t were found.

Already a s O-group t h e 1982-yearclass was observed t o be a numerous y e a r c l a s s (Anon.1982). A minor p a r t of i t appeared i n t h e spawning s t o c k a l r e a d y i n 1983. I n 1984 a g r e a t e r p a r t o f i t had become mature and dominated i n t h e spawning s t o c k with a c o n t r i b u t i o n of

53% .

Likewise t h e 1983-yearclass was observed t o b e a r i c h y e a r c l a s s a s O-group (Anon. 1983b). Minor p a r t s o f i t appeared f o r t h e f i r s t t i m e i n t h e spawning s t o c k a s one and two y e a r o l d , and i n 1986, a s t h r e e y e a r o l d , i t c o n t r i b u t e d t o t h e spawning s t o c k i n t h e same degree a s t h e 1 9 8 2 - y e a r c l a s s , i . e . with approximately 40%.

I n

1987

and

1988

t h e 1982-yearclass was found o n l y i n modest measure i n t h e samples, and t h e c o n t r i b u t i o n t o t h e spawning s t o c k i n t h e two y e a r s hence was observed t o have dropped t o j u s t above 10%

.

^The

1 9 8 3 - y e a r c l a s s , however, made up more t h a n h a l f o f t h e spawning s t o c k i n

1987

and

34%

of i t i n

1988.

I n t h e Norwegian Sea t h e 1982- and 1 9 8 3 - y e a r c l a s s e s were found a t e q u a l c o n t r i b u t i o n l e v e l a s one y e a r o l d s , namely more t h a n 60% i n

1983

and 1984 r e s p e c t i v e l y . The sudden drop i n

1987

of t h e 1982- y e a r c l a s s abundance was recorded a l s 0 i n t h e Norwegian Sea. The abundance of t h e 1983-yearclass a l s o decreased from

1987

onwards, b u t n o t a s s i g n i f i c a n t l y a s t h e 1982-yearclass. The VPA-results r e f l e c t a more modest r e d u c t i o n of t h e two y e a r c l a s s e s ' dominance ( F i g . 7 )

Growth ;

1) By observed l e n g t h s

The ~ r o w t h of b l u e w h i t i n g i n t h e n o r t h - e a s t A t l a n t i c - i s d e s c r i b e d f o r

6

v a r i o u s a r e a s i n d i c a t e d i n Fig.

8

and a p p r o p r i a t e l y named a s : Vestf j o r d , Norwegian Coast, North S e a ,

Norwegian S e a , Hebrides

,

Porcupine.

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LIFE HISTORY

-

b l u e whiting D i s t r i b u t i o n and growth of blue whiting

...

Fig.

8.

Borders of t h e s i x d i f f e r e n t a r e a s mentioned i n t h e t e x t .

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LIFE HISTORY

-

D i s t r i b u t i o n and growth of b l u e whiting

...

²⁴⁹

The samples used were c o l l e c t e d i n t h e s p r i n g season, i . e . February- A p r i l f o r t h e Hebrides and Porcupine a r e a s , and i n t h e summer season, i . e . May-September with t h e bulk i n August f o r t h e o t h e r a r e a s .

Using von B e r t a l a n f f y ' s equation

t h e v a l u e s f o r Lin

,

t h e " r a t e " K and t h e h y p o t h e t i c a l age when t h e f i s h would have i e e n z e r o l e n g t h , to ( R i c k e r 1975) were obtained f o r t h e s i x d i f f e r e n t a r e a s i n t h e p e r i o d 1980

-

1988, f o r males, females and both s e x e s combined. These a r e shown i n Table 1.

The d a t a were f i t t e d t o von B e r t a l a n f f y ' s growth curves and a r e shown f o r t h e s e x e s combined i n Fig.

9

^{f o r} a l l t h e a r e a s , s c a l e d t o r e s p e c t i v e season f o r each y e a r .

Except f o r V e s t f j o r d e n t h e growth up t o t h e age o f 5 y e a r s was found tobe very s i m i l a r i n t h e v a r i o u s a r e a s . A f t e r

5

y e a r s t h e growth d i v e r s e d s i g n i f i c a n t l y . However, t h e p a t t e r n i n t h e Hebrides a r e a was a l l t h e way up t o

14

y e a r s of age, very much l i k e t h e p a t t e r n i n t h e Norwegian Sea a r e a , i n d i c a t i n g t h e same kind of f i s h . T h i s may v e r i f y t h e m i g r a t i o n from t h e spawning a r e a o f f t h e B r i t i s h y I s l e s t o t h e f e e d i n g a r e a i n t h e Norwegian Sea. The s l i g h t d i f f e r e n c e of t h e growth p a t t e r n i n t h e Porcupine and t h e Hebrides a r e a s , i n d i c a t e s t h e i n f l u e n c e of "other" b l u e w h i t i n g i n t h a t "mixing a r e a " , i . e . i t could be i n d i v i d u a l s from t h e s o u t h o r from l o c a l s t o c k s .

Table 1. Growth parameters of von B e r t a l a n f f y ' s e q u a t i o n f o r both s e x e s and combined of b l u e whiting from s i x a r e a s i n 1980-88.

(24)

LIFE HISTORY

-

...

...---•

N o r t h S e a - . . : s - - ~ e s t f j o r d

3 6

.- .: .,

/L* ,. ^-

M-0-*-* ^-__-.e -e- Norw. C o a s t 35

-

P o r c u p i n e

34

-

Norw. S e a

H e b r i d e s 33

-

32

-

3 1

-

30

-

^to

~ e s t f j o r d : 36.8 0 . 2 8 -2.63 29

-

28

-

27

-

26

-

L .,LLU'f-.-- . -

24 2 3 22

-

20

-

14

.

â ⁱ ^t Î Î ^l Î Î Î Î Î Î Î Î Î

Norwegian Coast: 35.9 . ^{O. 26} ^-2.67

t:

'.

.

North Sea : 37.9 0 . 2 1

L'

: -3.30

b. ,

~ o r w e g i a n Sea : 33.9 0.35 -2.20

,y" [t-

H e b r i d e s : 3 3 . 8 0 . 3 6 - 1 . 5 2

!

~ n r r i i n i n e 1 34.2 0 . 3 5 - 1 - 4 8

b a n k

.

- I -

1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 YEARS

Fig.

9.

von Bertalanffy's growth curves and parameters of blue whiting, combined sexes, from six different areas in the North-east Atlantic.

(25)

LIFE HISTORY

-

...

cn

36 - ^{A .} ^{T h e}H e b r i d e s

35 -

2

34 -

33

-

32

-

- - C - - -

ar

31

-

30 -

29 -

28 - ^{t o}

27

-

⁹ 3 5 . 7 ^{O . 3 2} - 1 . 7

26 - ^d ^32.4 ^{0 . 3 8} ^{- 1 . 5}

2 5 -

24 -

23

-

22

-

21

-

1

20 -

39

1 2 3 4 5 6 7 B 9 1 0 1 1 12 13 14 Yearc

F i g . 10. von B e r t a l a n f f y ' s growth curves and parameters o f b l u e w h i t i n g , male and female from A ) t h e Hebrides and B) t h e Norwegian Coast.

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LIFE HISTORY

-

...

The growth p a t t e r n f o r t h e Hebrides a r e a d i f f e r a b i t from t h e o b s e r v a t i o n s B a i l e y (1982) d i d f o r t h e a r e a . For both s e x e s he found a s m a l l e r Lin

,

b u t a h i g h e r growth r a t e K. I n t h e Porcupine a r e a t h e gowth p a e t e r n a l s o d i f f e r from what Ushakov and Mazhirina (1978) c a l c u l a t e d from d a t a c o l l e c t e d i n 1970-1976, t h e Linf f o r b o t h s e x e s combined b e i n g 34.2 cm and K 0.35, a g a i n s t t h e i r s 37.2

cm

and 0.16.

B a i l e y ( 1 9 8 2 ) , however p r e s e n t s a wide v a r i e t y of growth parameters fom v a r i o u s a u t h o r s , and s u g g e s t s t h a t t h i s v a r i a t i o n may be t h e r e s u l t of r e a l v a r i a t i o n i n growth r a t e , b u t could a l s o be due t o b i a s e s i n sampling. The d i f f e r e n t i a l m i g r a t i o n s could be r e l a t e d t o s i z e r a t h e r than t o age.1n t h e Hebrides a r e a where presumedly a d u l t f i s h dominate, n o t a l l of t h e y e a r c l a s s e s may be f u l l y r e c r u i t e d and hence a d i f f e r e n t r a t e K could be observed h e r e t h a n i n f e e d i n g a r e a . Blue w h i t i n g o f f t h e c o a s t of Norway grow t o a l a r g e r s i z e t h a n t h e b l u e w h i t i n g i n more o f f s h o r e a r e a s o f t h e Norwegian Sea and west of t h e B r i t i s h Isles. T h i s may a l s o s u g g e s t t h e appearance of l o c a l s t o c k s n o t t a k i n g p a r t i n t h e major spawning migration.

The d i f f e r e n c e i n growth p a t t e r n of male and female i s i l l u s t r a t e d i n Fig.lOa-b, by von B e r t a l a n f f y ' s growth curves from t h e Hebrides and t h e Norwegian Coast. A t an age of

5

y e a r s t h e d i f f e r e n c e i s approximately

1.5

cm, and a t 1 0 y e a r s i t h a s i n c r e a s e d t o almost

3

cm.

Females i n t h e V e s t f j o r d and t h e North Sea a r e a s grew t o t h e l a r g e s t s i z e , while males i n t h e Porcupine a r e a had t h e s m a l l e s t growth, t h e c a l c u l a t e d Linf being

5

cm s h o r t e r .

The d i f f e r e n t growth p a t t e r n s of t h e v a r i o u s y e a r c l a s s e s a r e demonstrated i n Fig. I l a - c by t h e 1971

-

1983 y e a r c l a s s e s when they appeared i n t h e Hebrides a r e a a t 1 -6 y e a r s of age. For t h e 1971-1974 y e a r c l a s s e s t h e growth was q u i t e s i m i l a r up t o t h e age of

4

y e a r s . A f t e r t h a t i t d i v e r s e d , with t h e 1974 y e a r c l a s s c l e a r l y showing t h e

f a s t e s t growth t h e f o l l o w i n g two y e a r s .

The 1976

- 1978

y e a r c l a s s e s had a "congruence-point" between t h e age of

4

^and

5

y e a r s , and a t a 1 . 5 cm h i g h e r l e v e 1 t h a n t h e p r e v i o u s group of y e a r c l a s s e s . For t h e n e x t group,

1979 - 1983,

no such common p o i n t appeared from t h e a n a l y s i s . Except f o r t h e 1979 y e a r c l a s s , which grew a b i t jslower than t h e 1980 y e a r c l a s s , t h e y e a r c l a s s e s had p a r a l l e l growth p a t t e r n s a t d i f f e r e n t r a t e s from t h e age of 1 y e a r . The two numerous y e a r - c l a s s e s of 1982 and

1983,

however, had v e r y s i m i l a r growth p a t t e r n s up t o t h e age of

5

y e a r s , b e i n g r e l a t i v e l y low and a t t h e same l e v e l a s t h e 1971 and 1992 y e a r c l a s s e s . The o v e r a l l p i c t u r e i n d i c a t e s an i n c r e a s i n g growth r a t e of t h e y e a r c l a s s e s w i t h i n c r e a s i n g y e a r s up t o

1978

y e a r c l a s s , when t h i s e f f e c t i s r e v e r s e d f u r t h e r up t o

t h e

1983

y e a r c l a s s ( F i g . l l a - c ) .

The growth p a t t e r n s of t h e y e a r c l a s s e s of 1979-1983 when t h e y appeared i n t h e v a r i o u s a r e a s a t t h e age from 1 up t o

8

y e a r s , a r e i l l u s t r a t e d i n Fig. 12 f o r male and female s e p a r a t e . For some of t h e y e a r c l a s s e s t h e d a t a b a s i s was n o t s u f f i c i e n t t o produce a r e l i a b l e f i t t n e s t o t h e von B e r t y a l a n f f y ' s curves when s p l i t on s e x . The d e c r e a s i n g tendency i n growth by i n c r e a s i n g y e a r s of t h e y e a r c l a s s e s was obvious f o r a l l t h e

5

a r e a s f o r t h e a r r a y s of 1979

- 1983.

(27)

LIFE HISTORY

-

blue whiting

253 ...

g. m - N

h hhh

.d ri

- k i

b o a m

c

ai-

*d a m

L) ari

.d (d

3 1112

_cd_o

a, 7 ( U n

d k r n P .d

d

-

bi a-

O h t -

o ^o\

m 4

a l r i , # 3 k r i ri,

k ( d t - a ,

7 a,cnri, o hri ri,

C @ O (d d L) ri,C o

3 k @ O .d k h k - @ h O x < a , m .ri k h

-

m o r r i h ha

h a , cn h C ( d r i

k cn

d k (dr-

( d o cn

@ k m r i k a, a, - 5 a,

m r n . i - r ~

U k *

c

X P o a, a,- 3 m x u

(28)

LIFE HISTORY

-

blue whiting D i s t r i b u t i o n and growth of blue whiting

...

Y e a r c l a s s e s

c m 1979 19 80 1981 ¹⁹⁸² ¹⁹⁸³

36'1 Norwegian C o a s t

3 2- 28-

2c-

_ / - -

/ ,-

/ l'

/ /

I l ~ l ~ ' l ' l ' l ~ 1 l ' ' 1~ ~~ 1- ~' ~ ~1 ' ~1 ~ 1 1

36-

321

²⁸

2L- M-

Age in years Norwegian S e a

c

^I

^// ^{

, , . , .

2 c b 6 2^l ^~~ ^l 6^' ^l8 ^' 2^l ^'6 ^l 8^' ^l2 ^~ L ^l 6^~Î2 ^'^lÎL ^'^~6Î ^~ ^s ^~ ^~ ^~

36

28- 2L

-

2 0

-

Fig. 12. von B e r t a l a n f f y ' s growth curves of t h e

1979-1983

year c l a s s e s of blue whiting, males and females, a s appearing f i v e d i f f e r e n t a r e a s during t h e l a s t decade.

North S e a

H e b r i d e s

327 r-(- ^[ ^/

36

28 2L-

l . I - l . 1 ~~ ~ ' l ' l ' l ~ l ~ l ~ l1 ' ~1 ' ~I ~ l K l n l ~

327

^/ _l _. _l _~ _l

^[

_- _~ _l _~ _l _' _l _' _l _~

^/

_l _l ^/'_'_~^{y C C}_l_l _~_'^/^/^/_l^/'_l_m _'_~ _~ _~ _~ _~ _~ _~

(29)

LIFE HISTORY

-

blue whiting

255 ...

Fig. 13A. Length distribution (N%) by month of blue whiting in

samples from the mixed industrial fishery in the North Sea, 1982 and 1983.

(30)

LIFE HISTORY

-

...

2260

21 2

1 0 - JUN

1 2 14 1 6 1 8 2 0 2 2 24 2 6 2 8 3 0 3 2 3 4 3 6 CM

Fig. 13B. Length distribution (N%) by month of blue whiting in

samples from the mixed industrial fishery in the North Sea, 1984 and 1985.