Long term changes.pdf (601.4Kb)

LONG TERM CHANGES OF GROWTH AND MATURATION I N THE NORWEGIAN SPRING SPAWNING HERRING

Reidar Toresen

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

ABSTRACT

Two p e r i o d s with g r e a t d i f f e r e n c e s i n y e a r c l a s s abundance and r e c r u i t m e n t where chosen f o r growth and m a t u r a t i o n s t u d i e s , 1950

-

1960 and 1973

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1983. I n t h e f i r s t p e r i o d , t h e mean y e a r c l a s s s t r e n g t h a t

3

y e a r s was 6 . 8 x 109 i n d i v i d u a l s while i n t h e second p e r i o d i t was 0.4 x 109 i n d . . The growth i n each p e r i o d a s observed by t h e l e n g t h a t age, i s compared t o t h e y e a r c l a s s s t r e n g t h and t h e abundance of t h e s t o c k a t t h e t i m e . V a r i a t i o n s i n growth a r e observed and b e f o r e maturation s t r o n g y e a r c l a s s e s h a s remarkably less growth t h a n weak y e a r c l a s s e s . The l a t t e r p e r i o d i s t h e r e f o r e c h a r a c t e r i z e d by f a r b e t t e r growth, e s p e c i a l l y i n t h e e a r l y y e a r s o f l i f e , than t h e former. According t o growth r a t e s of t h e v a r i o u s y e a r c l a s s e s , t h e age a t m a t u r i t y is s u b s t a n t i a l l y h i g h e r i n t h e f i r s t p e r i o d than i n t h e l a t t e r . However, t h e l e n g t h a t m a t u r i t y i s found t o be f a i r l y c o n s t a n t showing t h e maturation t o be s i z e dependent r a t h e r t h a n dependent on t h e age.

Long term changes of growth and m a t u r a t i o n . . . .

INTRODUCTION

A f t e r t h e g r e a t r e d u c t i o n i n t h e abundance of t h e Norwegian s p r i n g spawning h e r r i n g i n t h e l a t e s i x t i e s , t h e growth a s observed by t h e l e n g t h a t age and weight a t age i n c r e a s e d s i g n i f i c a n t l y , (Jargensen 1979). A t t h e same time, t h e a d u l t s t o c k t o t a l l y changed i t s migration p a t t e r n a s i t ceased m i g r a t i n g i n t o t h e Norwegian Sea a f t e r spawning, (Dragesund e t a l . 1980). S i n c e 1970 t h e d i s t r i b u t i o n h a s been l i m i t e d t o t h e f j o r d s of t h e western and n o r t h e r n Norway ( e x c e p t f o r t h e 1983 y e a r - c l a s s ) . The observed change i n growth and maturation i s explained by t h e s e v e r e r e d u c t i o n o f t h e abundance of t h e s t o c k a s observed i n o t h e r h e r r i n g s t o c k s , (Jakobsson and Halldorsson

1984).

However, t h i s change i n growth i s n o t n e c e s s a r i l y d e n s i t y dependent.

Fig. 1. The a r e a s i n which growth i s s t u d i e d .

It should be noted t h a t growth i n t h e Norwegian s p r i n g spawners i s extremely v a r i a b l e , depending s t r o n g l y on t h e a r e a i n which they spend t h e i r a d o l e s c e n t phase. Herring l o c a t e d o f f Finnmark and i n t h e Barents Sea d u r i n g t h e a d o l e s c e n t phase have a markedly slower growth

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h e r r i n g

Long t e r m changes of growth and maturation

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t h a n t h o s e found o f f t h e c o a s t between Møre and Lofoten. A s t h e maturing p r o c e s s i s dependent on t h e l e n g t h r a t h e r t h a n t h e age i n t h e h e r r i n g (Toresen 1 9 8 6 ) , t h e p a r t s of t h e s t o c k spending t h e i r f i r s t y e a r s of l i f e s o u t h of Lofoten mature e a r l i e r than t h o s e found f u r t h e r t o t h e n o r t h o r i n t h e Barents Sea. Accordingly, immature h e r r i n g o l d e r t h a n t h r e e y e a r s a r e s c a r c e s o u t h of Lofoten, whereas o f f Finnmark h e r r i n g i n t h e adolescent s t a g e a r e common up t o f o u r y e a r s of age. T h i s p a t t e r n i s probably independent of t h e s i z e of t h e year- c l a s s e s . What i s most c e r t a i n l y dependent on t h e y e a r c l a s s s t r e n g t h i s t h e r e l a t i v e abundance o f t h e s t o c k i n t h e v a r i o u s a r e a s and t h e growth w i t h i n t h e s e a r e a s .

S i n c e t h e r e c r u i t m e n t t o t h i s s t o c k i s s o v a r i a b l e , i t is of g r e a t importance i n t h e assessment t h a t t h e p r e d i c t i o n of growth i s c o r r e c t , and i t i s t h e r e f o r e v a l u a b l e t o know t h e mechanisms t h a t govern t h e growth

.

I n t h i s paper v a r i o u s a s p e c t s of growth i n t h e Norwegian s p r i n g spawners a r e e n l i g h t e n e d e s p e c i a l l y with r e s p e c t t o t h e q u e s t i o n of growth and y e a r c l a s s abundance. Two p e r i o d s with q u i t e s i g n i f i c a n t d i f f e r e n c e s i n abundance, r e c r u i t m e n t and growth a r e chosen f o r t h i s purpose.

MATERIAL AND METHODS

The immature p a r t of t h e s t o c k i s t r e a t e d s e p a r a t e l y from t h e a d u l t s because t h e b i o l o g i c a l samples from t h e two p a r t s of t h e s t o c k are n o t taken i n t h e same p e r i o d of t h e y e a r . The samples from t h e spawning s t o c k a r e taken d u r i n g t h e spawning i n s p r i n g while t h e samples from t h e immatures a r e taken d u r i n g autumn.

The immatures

Growth, a s expressed by t h e mean l e n g t h a t age i n t h e autumn (Sept- Nov) i s s t u d i e d w i t h i n

4

r e g i o n s along t h e Norwegian c o a s t ( F i g . 1 ) . Average v a l u e s of l e n g t h a t age f o r t h e f u l l p e r i o d s a r e compared between t h e r e g i o n s . The

1983

y e a r c l a s s i s t r e a t e d s e p a r a t e l y because i t is a very s t r o n g one showing a tendency t o a l e v e 1 of growth which i s s i m i l a r t o t h a t of t h e f i f t i e s .

The m a t e r i a l from t h e f i f t i e s i s very poor, a s no s y s t e m a t i c sampling of t h e j u v e n i l e p a r t of t h e s t o c k was c a r r i e d o u t a t t h a t t i m e . No weight a t age d a t a a r e a v a i l a b l e .

The a d u l t s

The g e o g r a p h i c a l r e g i o n s a r e h e r e c u t down t o one, t h e main spawning grounds i n a r e a 07 ( F i g 1 ) . Sampling i s done d u r i n g spawning i n s p r i n g (Feb-Apr). Mean v a l u e s of l e n g t h a t age each y e a r i n t h e two p e r i o d s a r e h e l d up a g a i n s t spawning s t o c k abundance. I n a d d i t i o n , t h e mean l e n g t h a t age f o r each y e a r c l a s s i s compared t o t h e y e a r c l a s s s t r e n g t h a t t h r e e y e a r s . F u r t h e r on, growth a s expressed by l e n g t h increments i n t h e age i n t e r v a l l from 5-10 y e a r s i s compared between t h e p e r i o d s and t h e mean l e n g t h a t age f o r a l l y e a r c l a s s e s i n each p e r i o d a r e compared i n a p l o t .

A t l a s t , t h e growth a s expressed by t h e weight a t age r e l a t i o n s h i p i s d i s c u s s e d b r i e f l y .

Long t e r m changes of growth and m a t u r a t i o n . . . .

l

^Age a t m a t u r i t y . The h e r r i n g s t o c k i n t h e s e v e n t i e s i s d i v i d e d i n two ^{l I} components, one n o r t h e r n type and one s o u t h e r n type. These components I

were c a r a c t e r i z e d by d i f f e r e n c e s i n t h e growth p a t t e r n , a s t h e s o u t h e r n one was more f a s t growing. Maturity ogives are c a l c u l a t e d f o r

both components s e p a r a t e l y . ^l

l

When a y e a r c l a s s i s f u l l y r e c r u i t e d t o t h e spawning s t o c k t h e r a t i o between t h e number of i n d i v i d u a l s i n t h i s y e a r c l a s s and t h e number i n t h e o l d e r y e a r c l a s s e s i s c o n s t a n t i f t o t a l m o r t a l i t y ( Z ) by age group i s c o n s t a n t .

1

^{I n} t h e spawning s t o c k , t h e y e a r c l a s s e s mix v e r y w e l l and i n t h e samples taken from t h e spawning s t o c k t h e r a t i o between t h e v a r i o u s y e a r c l a s s e s i s q u i t e c o n s t a n t although i t may vary somewhat from one y e a r t o a n o t h e r . However, no t r e n d i n t h i s v a r i a n c e was d e t e c t e d and t h e p r o p o r t i o n s of each y e a r c l a s s i n t h e spawning s t o c k was c a l c u l a t e d a s f o l l o w s :

where p i is t h e p r o p o r t i o n o f y e a r - c l a s s i i n t h e spawning s t o c k , Ni i s t h e number of f i s h i n t h e samples from t h i s y e a r - c l a s s and k i s t h e number of y e a r - c l a s s e s i n t h e spawning s t o c k t h a t i s f u l l y r e c r u i t e d . The r e l a t i v e abundance of t h e f u l l y r e c r u i t e d y e a r - c l a s s e s were then c a l c u l a t e d a s t h e mean of t h e age groups

6, 7

and

8

y e a r s ( p i ) . The m a t u r i t y o g i v e by y e a r - c l a s s i s then c a l c u l a t e d a s t h e p r o p o r t i o n s of t h e number of f i s h i n t h e younger age groups t o p i i n t h e samples of t h e spawning s t o c k .

RESULTS AND DISCUSSIONS The immatures

The observed d a t a of l e n g t h a t age f o r t h e two p e r i o d s i n f o u r a r e a s along t h e Norwegian c o a s t a r e shown i n Table 1 a-d. The d a t a g i v e n a r e mean v a l u e s of a l l samples of young h e r r i n g from September til1 November each y e a r . The mean v a l u e s of l e n g t h a t age might have been a f I e c t e d by t h e t i m e t h e samples a r e taken i n t h e autumn. However, t h e growth d u r i n g t h e s e months i s marginal and i t i s assumed t h a t t h e growth f o r t h e season i s f i n i s h e d . A s can be s e e n from t h e t a b l e s , t h e d a t a from t h e f i f t i e s and e a r l y s i x t i e s is very poor. However, t r e a t e d t o g e t h e r , t h e few samples p r e s e n t g i v e an i d e a of t h e growth l e v e l i n t h e f u l l p e r i o d .

Comparing t h e l e n g t h a t age w i t h i n t h e two p e r i o d s b u t between t h e d i f f e r e n t r e g i o n s show a c l e a r t r e n d of i n c r e a s i n g growth when moving from n o r t h t o s o u t h along t h e c o a s t (Table 1 a - d ) . T h i s phenomenon i s e a r l i e r d e s c r i b e d by Dragesund (1970), and i s probably r e l a t e d t o environmental c o n d i t i o n s . The l e n g t h d i s t r i b u t i o n of t h e O-group h e r r i n g may b e s i m i l a r a l l along t h e c o a s t , b u t a d i f f e r e n c e i n l e n g t h

a t age i s i n i t i a t e d v e r y soon and t h i s d i f f e r e n c e i n c r e a s e s through t h e y e a r s . T h i s f e a t u r e r e s u l t s i n earlier m a t u r a t i o n o f t h e components o f t h e s t o c k spending t h e adolescence f u r t h e r t o t h e s o u t h

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Long term changes of growth and maturation.

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A R E A 0 5 T I M E P E R I O D S

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T I M E P E R I O D S

I d) T I M E P E R I O D S

A G E ( Y E A R ) A G E ( Y E A R )

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3 2 -

2 8 -

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^{2 4 -}

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l- w

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1 6 -

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Fig. 2a-d. P l o t s of length a t age i n t h e four regions i n t h e two periods.

A R E A 07 ,

/

1 I I I I

o 1 2 3 4

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h e r r i n g Long term changes of growth and maturation....

than those found i n t h e north o r i n t h e Barents Sea, a s t h e maturation process i s r e l a t e d t o growth r a t h e r than t o age. The recruitment t o

t h e spawning s t o c k i s t h e r e f o r e l a r g e l y dependent on t h e geographical d i s t r i b u t i o n of t h e year c l a s s e s i n t h e e a r l y s t a g e s of l i f e .

The d i f f e r e n c e i n growth between t h e two periods a r e s t u d i e d by comparing t h e average l e n g t h a t age values f o r each period and region.

Figure 2 . a-d shows t h e p l o t s of l e n g t h a t age i n t h e f o u r regions i n each period. The

1983

year-class i s p l o t t e d s e p a r a t e l y . I n a l l t h e r e g i o n s , growth i s f a r b e t t e r i n t h e s e v e n t i e s than i n t h e f i f t i e s . A s t h e p l o t s c l e a r l y show, t h e growth r a t e i s a l s o higher i n t h i s l a t t e r period l e a d i n g t o g r e a t e r d i f f e r e n c e s i n l e n g t h a t age through t h e y e a r s . This higher growth a l s o l e a d s t o maturation a t a much younger age f o r t h e year c l a s s e s i n t h e s e v e n t i e s , which was observed down t o

3

^years.

Is t h i s l a r g e d i f f e r e n c e i n growth between t h e s e two periods d e n s i t y dependent? The annual recruitment was a t a much higher l e v e l i n t h e f i f t i e s than

it

t h e s e v e n t i e s . The mean year c l a s s s t r e n g t h a t

3

y e a r s was

6

790 x 10 i n d i v i d u a l s i n t h e f i f t i e s , while i n t h e s e v e n t i e s t h i s number was reduced t o 436 x 10

.

^{However, i t i s} a f a c t t h a t b i g year c l a s s e s of Norwegian spring-spawning h e r r i n g have q u i t e another geographical d i s t r i b u t i o n than small ones. Weak y e a r c l a s s e s spend more o r l e s s e n t i r e l y t h e adolescence i n t h e f j o r d s and along t h e c o a s t . The r e l a t i v e d i s t r i b u t i o n of each year c l a s s i n t h e d i f f e r e n t regions i s determined a t t h e O-group s t a g e . Abundant year c l a s s e s a l s o d i s t r i b u t e i n t h e f j o r d s , but a t a c e r t a i n l e v e l of abundance, t h e O-group h e r r i n g a l s o g e t d i s t r i b u t e d i n t h e open watermasses of t h e Barents Sea. The mechanism governing t h i s d i s t r i b u t i o n p a t t e r n i s n o t y e t known. However, t h e conditions f o r high growth r a t e s a r e l e s s favourable i n t h e open watermasses of t h e oceans than i n t h e f j o r d s and along t h e c o a s t , where t h e temperature i s higher and t h e d e n s i t y of zooplankton i s g r e a t e r . The d e n s i t y of h e r r i n g i n t h e f j o r d s f o r t h e s e b i g year c l a s s e s i s s t i l l much higher than f o r small year c l a s s e s , probably leading t o reduced growth due t o higher d e n s i t y . I n t h i s way, we have two main reasons f o r reduced growth i n b i g year c l a s s e s of Norwegian spring-spawners; d e n s i t y dependent growth i n t h e f j o r d s and reduced growth due t o d i s t r i b u t i o n i n t h e open watermasses of t h e Barents Sea. The l a t t e r f a c t o r i s a secondary e f f e c t of high d e n s i t y but t h e reduced growth i s n o t d e n s i t y dependent i t s e l f .

Maturation

The r e s u l t s a r e shown i n Table 2. It i s q u i t e c l e a r t h a t t h e recruitment t o the spawning stock s e t i n e a r l i e r i n t h e s e v e n t i e s than i n t h e f i f t i e s and s i x t i e s . E s p e c i a l l y t h e year c l a s s e s ,

1973

and

1974

mature t o spawn a t a very e a r l y age and they a r e almost f u l l y r e c r u i t e d a t four. For t h e year c l a s s e s 1959-61, they p a r t l y mature a t t h r e e , but only t o a very small e x t e n t . F u l l recruitment d o e s n ' t occur u n t i l they a r e s i x o r seven.

These maturity ogives a r e s t r o n g l y r e l a t e d t o t h e growth. F a s t growing h e r r i n g mature t o spawn e a r l y , but more o r l e s s a t t h e same l e n g t h a s t h e slow growing ones.

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Long term changes of growth and m a t u r a t i o n . . . .

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F i g . 3a & b. Mean l e n g t h a t age p e r y e a r and spawning s tock abundance

.

The a d u l t s

Long term changes of growth and m a t u r a t i o n . . . .

Length a t age. The observed d a t a of mean l e n g t h a t age f o r t h e two p e r i o d s a t t h e spawning grounds i n Area 07 i s shown i n Table

3.

^{A l l}

samples taken i n t h e a c t u a l time, February

-

A p r i l , a r e p r e s e n t i n t h e m a t e r i a l .

Mean l e n g t h a t age p e r y e a r i n t h e two p e r i o d s and spawning s t o c k abundance i s shown i n F i g . 3 , a and b. The spawning s t o c k d e c r e a s e s from a l e v e l of about 10 m i l l tonnes i n

1957

til1 about

3

m i l l tonnes i n

1965

and f u r t h e r t o n e a r l y z e r o i n 1969. Through t h e s e y e a r s , t h e growth, a s expressed by t h e mean l e n g t h a t age p e r y e a r , show an i n c r e a s i n g t r e n d throughout t h e f i f t i e s . I n t h e beginning of t h e s i x t i e s however, t h e growth suddenly s t a r t t o f a l l shown by t h e low v a l u e s o f mean l e n g t h a t age f o r t h e y e a r c l a s s e s 1958-59 and -60. The c o r r e l a t i o n between t h i s p a t t e r n of growth and t h e s t o c k abundance i s n o t very c l e a r because t h e change of growth p a t t e r n d i f f e r s from s n e age group t o a n o t h e r . I n f a c t , i t i s obvious t h a t t h e growth p a t t e r n changes i n correspondance with t h e y e a r c l a s s s t r e n g t h r a t h e r than t h e s t o c k abundance.

Y E A R C L A S S

Fig.

4.

Mean l e n g t h a t age p e r y e a r c l a s s s t r e n g t h a t t h r e e y e a r s i n t h e two p e r i o d s .

Figure

4

shows t h e mean l e n g t h a t age p e r y e a r c l a s s and t h e corresponding y e a r c l a s s s t r e n g t h i n t h e two p e r i o d s . I t i s c l e a r , from t h e p l o t t h a t t h e r e i s a p o s i t i v e c o r r e l a t i o n between t h e l e n g t h a t age f o r t h e d i f f e r e n t y e a r c l a s s e s and t h e y e a r c l a s s abundance.

The o v e r a l l growth p a t t e r n shows an i n c r e a s i n g t r e n d u n t i l

1957.

^The

1958

y e a r c l a s s h a s f a r less growth than t h e two o r t h r e e o l d e r y e a r c l a s s e s and t h i s d e c r e a s i n g tendency t o growth c o n t i n u e s f o r t h e

1959

year c l a s s f l a t t e n i n g o u t f o r t h e 1960 y e a r c l a s s , T h i s p a t t e r n i s set b e f o r e t h e h e r r i n g mature. A s shown i n Fig.

4,

t h e p a t t e r n i s c l e a r a t

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Long term changes of growth and maturation.

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y e a r s , smoothening a l i t t l e u n t i l t h e age of t e n . The observed drop i i n growth f o r t h e weak 1958 y e a r c l a s s i s most c e r t a i n l y caused by i n t e r a c t i o n s between t h i s year c l a s s and t h e very s t r o n g 1959 y e a r

i I c l a s s .

i i

1

I n t h e second p e r i o d , only weak y e a r c l a s s e s o c c u r , e x c e p t f o r t h e l 1983 y e a r c l a s s which i s comparable t o s t r o n g y e a r c l a s s e s i n t h e

1

f i f t i e s . It i s d i f f i c u l t t o draw any conclusions from t h e appearing / I growth p a t t e r n . However t h e r e i s a tendency t o d e c r e a s i n g v a l u e s o f

l l e n g t h a t age throughout t h e y e a r s ( F i g .

3

b ) . This d e c r e a s e might

I

I have been a f f e c t e d by t h e i n c r e a s e i n t h e abundance of t h e t o t a l 1

i s t o c k . However, a c l e a r drop i n growth i s observed f o r t h e s t r o n g

1983

i y e a r c l a s s .

l

I Comparing t h e curves of mean l e n g t h a t age by y e a r c l a s s between t h e i two p e r i o d s i n Figure

4

g i v e s an impression of b e t t e r growth i n t h e

1

i s e v e n t i e s t h a n i n t h e f i f t i e s . However, t h e b e t t e r growth i s

I r e s t r i c t e d t o t h e immature p a r t of t h e s t o c k o n l y , a s t h e mean l e n g t h

1

increments f o r t h e age i n t e r v a l l 5-10 y e a r s i s h i g h e r i n t h e former

I

l p e r i o d than i n t h e l a t t e r ,

5.87

and 5.08 cm r e s p e c t i v e l y . The l e n g t h

I increment i n t h e age i n t e r v a l l 5-10 y e a r s f o r t h e y e a r c l a s s e s with

i

lowest v a l u e s of l e n g t h a t

4

y e a r s i s much h i g h e r than f o r year-

l

c l a s s e s with high v a l u e s of mean l e n g t h a t

4.

T h i s phenomenon is e a r l i e r d e s c r i b e d by Watkin ( 1 9 2 7 ) , and Østvedt (1958) and Jakobsson &

j Halldorsson

(1984)

has d e s c r i b e d t h i s f o r Atlanto-scandian h e r r i n g . It

1

l e a d s t o less v a r i a t i o n i n l e n g t h w i t h i n t h e same y e a r c l a s s a s t h e

j f i s h grow o l d e r .

F i g u r e 5 shows t h e p l o t s of mean l e n g t h a t age f o r a l l y e a r y e a r c l a s s e s i n t h e two p e r i o d s . The p l o t i l l u s t r a t e s t h e h i g h e r v a l u e s o f mean l e n g t h a t age i n t h e s e v e n t i e s than i n t h e f i f t i e s and t h e d i f f e r e n c e i s c o n s t a n t i n t h e whole age i n t e r v a l 5-10 y e a r s . The F i g u r e s 2 and

5

compared show very w e l l t h a t t h e d i f f e r e n c e s i n growth i n l e n g t h i s i n i t i a t e d e a r l y i n l i f e , b e f o r e m a t u r a t i o n .

I

^{T I M E} P E R I O D S

l i

A G E ( Y E A R )

l

F i g .

5.

Mean l e n g t h a t age f o r a l l y e a r c l a s s e s i n t h e two p e r i o d s .

Long term changes of growth and m a t u r a t i o n . . . .

Weight a t age. Table 2 shows t h e mean weight a t age each y e a r f o r t h e two p e r i o d s . These d a t a show e x a c t l y t h e same p a t t e r n of growth a s t h e growth of l e n g t h d a t a d i d ( F i g .

4

and

5 ) .

There i s a more o r less c o n s t a n t d i f f e r e n c e of 35-60 g between t h e mean weight a t age v a l u e s f o r t h e age groups i n t h e two p e r i o d s , These d i f f e r e n c e s follows t h e l e n g t h a t age d i f f e r e n c e s and v a r i e s randomly d u r i n g t h e age i n t e r v a l l 5-10 y e a r s .

MANAGEMENT CONSIDERATIONS

Without any doubt, t h e r e i s a r e l a t i o n s h i p between y e a r c l a s s abundance and growth. The reduced growth due t o h i g h abundance might b e d e n s i t y dependent i n c e r t a i n a r e a s , b u t i n most c a s e s , it i s most c e r t a i n l y due t o d i s p e r s i o n of young h e r r i n g i n a r e a s w i t h lower c o n c e n t r a t i o n s of food p a r t i c l e s and lower temperatures.

The reduced growth l e a d s t o l a t e r maturation because t h e m a t u r a t i o n i s s i z e dependent r a t h e r t h a n age dependent. The r e c r u i t m e n t of l a r g e y e a r c l a s s e s t o t h e spawning s t o c k t h e r e f o r e tend t o be delayed compared t o t h e r e c r u i t m e n t of s m a l l y e a r c l a s s e s . T h i s d e l a y may be

2-3

y e a r s o r even more. The r e c r u i t m e n t t o t h e f i s h e r y w i l l a l s 0 be l a t e r f o r l a r g e r y e a r c l a s s e s t h a n f o r less abundant y e a r c l a s s e s s i n c e t h e f i s h e r y i s r e g u l a t e d by a minimum l a n d i n g s i z e r e g u l a t i o n . I f t h e p r e d i c t i o n o f growth i s n o t thoroughly e v a l u a t e d i n t h e assessment work, t h e d e l a y i n r e c r u i t m e n t may l e a d t o an o v e r - e x p l o i t a t i o n of t h e o l d e r y e a r c l a s s e s i n t h e s t o c k . S i n c e t h e TAC i s given i n weight r a t h e r t h a n number o f i n d i v i d u a l s , a reduced growth a l s o may l e a d t o a g e n e r a l o v e r - e x p l o i t a t i o n of t h e s t o c k .

REFERENCES

Dragesund, 0 . 1970. D i s t r i b u t i o n , abundance and m o r t a l i t y of young and a d o l e s c e n t Norwegian s p r i n g spawning h e r r i n g (Clupea harengus L . ) i n r e l a t i o n t o subsequent y e a r - c l a s s s t r e n g t h . F i s k . D i r . S k r . S e r . Havunders

. ^{, 15} ( 4 )

:

451 -

^556.

Dragesund, O., Hamre, J . and U l l t a n g , Ø. 1979. Biology and p o p u l a t i o n dynamics of t h e Norwegian s p r i n g spawning h e r r i n g . ICES Symposium on t h e b i o l o g i c a l b a s i s of p e l a g i c f i s h s t o c k management,

3:

80 pp. (mimeo).

Jakobsson, J. and H a l l d o r s s o n , 0.

1984.

Changes i n b i o l o g i c a l para- meters i n t h e I c e l a n d i c summer spawning h e r r i n g . ICES CM 1984/H:43, 12 pp. (mimeo).

Jørgensen, K . 0 . 1979. Norsk vårgytende s i l d (Clupea h a r e n m s L . ) Endringer i b i o l o g i s k e parametre i perioden 1950

- 1978.

T h e s i s , U n i v e r s i t y of Bergen,

75

pp. ( I n Norwegian) (mimeo)

.

LIFE HISTORY

-

h e r r i n g

Eong term changes of growth and m a t u r a t i o n . . . .

Toresen, R . 1986. Length and age a t m a t u r i t y of Norwegian s p r i n g - spawning h e r r i n g f o r t h e y e a r - c l a s s e s 1959-61 and 1973-78.

ICES CM 1 9 8 6 / ~ : 4 2 ,

4

pp. (mimeo)

.

Watkin, E.E. 1927. I n v e s t i g a t i o n s on Cardigan Bay Herring. P a r t V.

Rep. Mar. and Fresh Water Inv. Dept. Zool. Univ. C o l l . Wales, 2 .

Ø s t v e d t , O . J . 1958. Some c o n s i d e r a t i o n s concerning t h e homogeneity of t h e Atlanto-scandian h e r r i n g . Rapp. P.-v. Reun. Cons. i n t . Explor. Mer,

143: 53-57.

Long term changes of growth and maturation....

Table 1 a . Mean l e n g t h a t age.

AREA

04

Age Year

1950 51 52 53 54 5 5 5 6 57 58 59 60

Mean

Mean

O 1 2

3 4

19.4

21.1

20.8 19.8 23 3

17.6 24.0

23.4 22.3

20.7 16.5 16.8 10.8 15.6

12.1

11.45 16.40 19.44 21.73 22.55

9.93 17.35 24.12 28.68 31.50

LIFE HISTORY

-

h e r r i n g

Long term ckanges of growth and maturation....

Table 1 b. Mean l e n g t h a t age.

AREA

05

Age Year

1950 51

O

1 2

3 4

22.2 19.0

57 58 59

60 Mean

14.5

10.9 18.4

10.9 17.55 20.34 23.20

1973

7 4 7 5 7 6

77

78

7 9 80

8

1 82

83

Mean

19.2 25.2 29.7 33.2 13.5 22.6 29.2 32.4

34.1

21.2 29.0 12.6

16.8

28.2

20.2

9.8 19.4

23.3 30.4

10.3 18.2 28.5 32.4

12.2 24.7 29.5 34.2

28.2 31

3

32

7 9.7 18.5

24.4 29

3

31.5 9.6

19.4

21

.g

24.3 29

3

11.35 19.51 26.53 30.12 32.64

LIFE HISTORY

-

h e r r i n g Long t e r m changes of growth and maturation....

Table 1 c . Mean l e n g t h a t age.

AREA O6

Age

Mean

1

^11.50

^17.78

^{22.00 23.00}

Year O 1 2

3 4

LIFE

HISTORY -

h e r r i n g

Long term changes of growth and m a t u r a t i o n . .

..

Table 1 d. Mean l e n g t h a t age.

LIFE BISTORY

-

h e r r i n g Long term changes of growth and maturation..

..

Table

4.

Weight a t age i n t h e spawning s t o c k .

Age Year

Mean

4 5 6 7 8 9

10

200.2

244.6 269.3 290.4 309.5 322.8 342.0

Mean ; Diff;

239.5 281.1 320.6 346.0 368.4 377.2 382.8

39.3 36.5 51.3 55.6 58 9 54.4 40.8