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I n t e r n a t i o n a l C o u n c i l f o r t h e E x p l o r a t i o n o f t h e Sea

C.M. 1986/H:51

P e a l e g i c F i s h C o m m i t t e e

THE EFFECTS OF A MINIMUM LAMDING S I Z E REGULATION I N THE FISHERY OF NORWEGIAN SPRING-SPAWNING HERRING

J o h a n n e s H a m r e and R e i d a r T o r e s e n I n s t i t u t e o f M a r i n e R e s e a r c h P.O. Box 1 0 7 0 , N-5011 B e r g e n , Norway

A b s t r a c t

E f f e c t s o f m i n i m u m l a n d i n g s i z e ( L C ) i n t h e f i s h e r y o f N o r w e g i a n s p r i n g - s p a w n i n g h e r r i n g h a v e been i n v e s t i g a t e d . T h e h e r r i n g i s a s s u m e d t o be a unit s t o c k , in w h i c h t h e g r o w t h o f i m m a t u r e s d i f f e r s a c c o r d i n g t o t h e d i s t r i b u t i o n area. T w o i m m a t u r e s t o c k c o m p o n e n t s a r e d i s t i n g u i s h e d . O n e d i s t r i b u t e d in N o r w e g i a n c o a s t a l w a t e r s s o u t h o f 7 0 d e g r e e s n o r t h and o n e o f f N o r t h e r n N o r w a y and i n t h e B a r e n t s Sea.

S t o c k and yield per r e c r u i t f o r t w o c o r r e s p o n d i n g s t o c k c o m p o n e n t s w e r e c a l c u l a t e d ( I C E S s t a n d a r d p r o g r a m m e s ) using i n p u t p a r a m e t e r s a s o b s e r v e d i n t h e r e s p e c t i v e areas. T h e w e i g h t in c a t c h by a g e o f i m m a t u r e s i s s e t e q u a l t o t h a t o b s e r v e d i n a u t u m n c a t c h e s ( e x p e c t e d t o be t h e m a i n f i s h i n g s e a s o n o f i m m a t u r e s ) w h e r e a s t h e w e i g h t in c a t c h o f a d u l t s i s t h e a v e r a g e o v e r t h e year. T h e w e i g h t by a g e i n t h e s t o c k e q u a l s o b s e r v e d w e i g h t o f p r e s p a w n e r s . T h e m a t u r i t y o g i v e by c o m p o n e n t i s c a l c u l a t e d by u s i n g t h e l e n g t h d e p e n d e n t m a t u r i n g s c a l e d e s c r i b e d by T o r e s e n ( 1 9 8 6 ) .

T h e a s s e s s m e n t s h o w s t h a t i f t h i s s t o c k i s m a n a g e d at F o a l , t h e r e i s l i t t l e t o g a i n in yield and s t o c k per r e c r u i t by i n t r o d u c i n g a m i f i m u m l a n d i n g s i z e a b o v e 2 0 c m in N o r w e g i a n c o a s t a l w a t e r s ( s o u t h o f 7 0 N ) , w h e r e a s s o m e i n c r e a s e in yield m a y b e o b t a i n e d i n t h e B a r e n t s Sea.

I n t r o d u c t i o n

T h e A t l a n t o - S c a n d i a n H e r r i n g and C a p e l i n W o r k i n g G r o u p c o n s i d e r e d t h e e f f e c t s o f a m i n i m u m l a n d i n g s i z e o f h e r r i n g (1,) at i t s 1 9 8 5 m e e t i n g ( A n o n 1 9 8 6 1 . Based o n an a s s u m e d a v e r a g e g r o w t h p a t t e r n s i m i l a r t o t h a t o b s e r v e d i n t h e s t r o n g y e a r c l a s s e s 1959 and 1 9 6 0 and a n a t u r a l m o r t a l i t y r a t e o f 0.13 for a l l a g e g r o u p s , t h e W o r k i n g G r o u p c a l c u l a t e d s t o c k and yield per r e c r u i t f o r m i n i m u m l a n d i n g s i z e s o f 2 0 c m , 2 5 c m , 27 c m and 3 0 cm. T h e r e s u l t s s h o v e d that s o m e g a i n c o u l d be o b t a i n e d i n t h e p o t e n t i a l yield by i n c r e a s i n g t h e m i n i m u m l a n d i n g s i z e f r o m 2 0 c m t o 2 5 c m a t a n e x p l o i t a t i o n l e v e l o f F O . , , but p r a c t i c a l l y n o g a i n in yield c a n be e x p e c t e d by i n c r e a s i n g 1, a b o v e 25 cm. T h e g r o w t h p a t t e r n and n a t u r a l m o r t a l i t y r a t e o f i m m a t u r e h e r r i n g m a y d i f f e r a c c o r d i n g t o t h e d i s t r i b u t i o n a r e a o f t h e immatures. T h e j u v e n i l e h e r r i n g o f f n o r t h e r n N o r w a y and i n t h e B a r e n t s S e a h a s a s l o w e r g r o w t h r a t e t h a n t h e j u v e n i l e h e r r i n g d i s t r i b u t e d o n t h e N o r w e g i a n c o a s t f u r t h e r t o t h e s o u t h . It i s a l s o r e a s o n a b l e t o a s s u m e t h a t n a t u r a l m o r t a l i t y o f h e r r i n g i s h i g h e r i n t h e B a r e n t s Sea t h a n i n t h e c o a s t a l area d u e t o p r e a d a t i o n by cod. T h e a i m o f t h e p r e s e n t p a p e r i s t o i n v e s t i g a t e t h e e f f e c t s o f m i n i m u m l a n d i n g s i z e s o f h e r r i n g f o r t h e t w o h e r r i n g c o m p o n e n t s s e p a r a t e l y , r e g a r d i n g t h e t w o g r o u p s o f i m m a t u r e s t o b e r e c r u i t s t o o n e u n i t a d u l t stock.

M a t e r i a l and m e t h o d s

S u s t a i n e d yield ( Y ) and c o r r e s p o n d i n g s p a w n i n g s t o c k ( S ) per r e c r u i t h a v e b e e n c a l c u l a t e d by u s i n g t h e I C E S s t a n d a r d p r o g r a m m e s . T h e input p a r a m e t e r s o f w e i g h t by a g e , f i s h i n g p a t t e r n , m a t u r a t i o n o g i v e and n a t u r a l m o r t a l i t y a r e d e t e r m i n e d a s f o l l o w s :

W e i g h t by a g e

T h e w e i g h t s by a g e i n c a t c h and s t o c k by c o m p o n e n t a r e g i v e n i n T a b l e 1 . T h e w e i g h t s i n c a t c h a r e c a l c u l a t e d m e a n w e i g h t in t h e c a t c h e s i n A u g u s t - O c t o b e r f o r h e r r i n g y o u n g e r t h a n 6 y e a r s and m e a n w e i g h t i n t h e c a t c h e s t h r o u g h o u t t h e y e a r f o r f i s h 6 y e a r s o r older. T h e w e i g h t s by a g e i n s t o c k a r e c a l c u l a t e d m e a n w e i g h t by a g e o f s t o c k s a m p l e s f r o m t h e t h e f i r s t t h r e e m o n t h s o f t h e year.

F i s h i n g p a t t e r n

T h e p r o g r a m m e d e t e r m i n e s f i s h i n g p a t t e r n a s r e l a t i v e F by a g e g r o u p s . T h e m i n i m u m l a n d i n g s i z e m a y d e t e r m i n e s t h e f r a c t i o n o f a n a g e g r o u p o f i m m a t u r e s b e i n g r e c r u i t e d t o t h e c a t c h a b l e s t o c k and t h e r e l a t i v e F i s e s t i m a t e d a s t h e proportion o f herring w i t h i n t h e a g e g r o u p s beeing a b o v e t h e 1,

.

M a t u r i t y o g i v e

T h e p r o g r a m m e c a l c u l a t e s m a t u r e s t o c k a s t h e s u m o f t h e p r o p o r t i o n s o f m a t u r e h e r r i n g in a l l a g e g r o u p s . T o r e s e n ( 1 9 8 6 ) h a s s h o w n t h a t t h e m a t u r i t y o g i v e o f h e r r i n g i s l e n g t h d e p e n d e n t , and t h e r e f o r e d e p e n d s

on t h e g r o w t h r a t e . The p r o p o r t i o n o f mature h e r r i n g by age groups have been e s t i m a t e d as i t ' s p r o p o r t i o n i n t h e spawning s t o c k w h i l e m a t u r i n g ( a t 3 , 4 and 5 y e a r s ) r e l a t i v e t o i t ' s p r o p o r t i o n when f u l l y r e c r u i t e d (mean p r o p . a t 6, 7 and 8 y e a r s ) . These p r o p o r t i o n s v a r i e s a c c o r d i n g t o t h e g r o w t h o f t h e immatures. The i n p u t f i g u r e s o f m a t u r e h e r r i n g o f t h e c o a s t a l component a r e e s t i m a t e d t o 102, 2 5 % , 60% and 90Z f o r t h e age groups 3 t o 6 r e s p e c t i v e l y and 100% above 6 y e a r s . For t h e B a r e n t s Sea component t h e m a t u r i n g i s c a l c u l a t e d t o be d e l a y e d by one y e a r . The f i g u r e s a r e based on v a l u e s f r o m t h e r e s p e c t i v e components o f t h e y e a r c l a s s e s 1959 and 1960.

N a t u r a l m o r t a l i t y

I n accordance w i t h t h e same b a s i c c o n s i d e r a t i o n as d e s c i b e d i n t h e Working Group r e p o r t (Anon. 19861, t h e e f f e c t s o f minimum l a n d i n g s i z e s a r e c a l c u l a t e d f o r t h e f o l l o w i n g v a l u e s o f M:

( 1 ) M = 0.15 f o r a l l ages i n b o t h components;

( 2 ) M1 = M2 = 0.35 and M>2 = 0.15 i n b o t h components and ( 3 ) M1 = M 2 = 0 . 5 0 , M3 = 0.35 and M>g = 0.15 i n The B a r e n t s Sea. The M-values o f 0.35 on j u v e n i l e s i n t h e c o a s t a l component a r e d e r i v e d f r o m a c o u s t i c s t o c k abundance e s t i m a t e s , whereas t h e M-values o f j u v e n i l e h e r r i n g i n t h e B a r e n t s Sea a r e judged a c c o r d i n g t o expected p r e d a t i o n p r e s s u r e

f r o m cod.

S u s t a i n a b l e y i e l d ( Y ) and c o r r e s p o n d i n g spawning s t o c k ( S ) p e r r e c r u i t as one y e a r o l d s ( R 1 ) a r e c a l c u l a t e d as f u n c t i o n o f F f o r 1,-values o f 20 cm, 25 cm, 27 cm (recommended by ACFM) and 30 cm. The r e s u l t s a r e shown i n F i g u r e s 1 t o 5. For t h e coastal. component ( F i g u r e 1 and 2 ) t h e r e i s L i t t l e t o g a i n i n t h e Y / R 1 f o r F - v a l u e s below 0.5 even i f M on immatures s h o u l d be as l o w as 0 . 1 5 . A r e s t r i c t i o n on t h e s m a l l h e r r i n g f i s h e r y i s on t h e o t h e r hand o f much more i m p o r t a n c e f o r t h e r e c r u i t m e n t t o t h e spawning s t o c k . For F = 0.5 an i n c r e a s e i n 1, f r o m 20 t o 27 cm may d o u b l e t h e spawning s t o c k i f M on immature h e r r i n g i s l e s s t h a n 0.35.

For The B a r e n t s Sea component a c o n s i d e r a b l e g a i n i n y i e l d can be e x p e c t e d b y a minimum l a n d i n g s i z e r e g u l a t i o n when t h e e x p l o i t a t i o n i s h i g h . For F = 0.5 t h e g a i n i n Y/R1 i s c a l c u l a t e d t o some 30% if t h e e f f e c t i v e 1, i s i n c r e a s e d f r o m 20 t o 27 cm i n t h e case o f l o w M

( F i g u r e 31, and even i n t h e case o f t h e h i g h e s t M-value ( F i g u r e 5 ) t h e g a i n i n y i e l d i s above 1 5 X . I n a d d i t i o n , t h i s r e g u l a t i o n i s e x p e c t e d t o i n c r e a s e t h e spawning s t o c k by some 300%.

ACFM has as a g u i d L i n e o f e x p l o i t a t i o n r a t e recomended t h a t h e r r i n g s t o c k s s h o u l d p r e f e r a b l y be e x p l o i t e d a t F O , l

.

The assessment shows t h a t i f t h i s s t o c k i s f i s h e d a t F O . l , t h e r e i s p r a c t i c a l l y n o t h i n g t o g a i n i n y i e l d and s t o c k p e r r e c r u i t by i n t r o d u c i n g a mintmum l a n d i n g s i z e above 20 cm i n Norwegian c o a s t a l w a t e r s ( s o u t h o f 70 N ) , whereas some i n c r e a s e i n y i e l d may be o b t a i n e d i n t h e B a r e n t s Sea p r o v i d e d t h a t t h e M-value on t h e immatures i s b e l o w 0.35.

Norway has introduced a 25 cm minimum landing size of herring fished in her fishery none north o f 6 2 north. Based on the present study it 0

seems fair t o conclude that this is a sufficient restriction on the

small herring fishery in case the overall fishing mortality i s regulated t o a reasonably l o w level. This fishery is regulated by catch quotas and the F is cept below F o a l

.

Anon. 1986. Atlanto-Scandian Herring and Cagelin Working Group Report, 29 October-1 November 1985. ICES C . M . 1986/Assess:7.

Toresen, R . 1986. Length and age at maturity of Norwegian

Spring-spawning herring for the year-classes 1959-61 and 1973-78. ICES C . M . 1986/H:42.

Table 1 . Input parameters of weight ( g r a m ) in catch and stock by component.

T a b e l 2. Input p a r a m e t e r s o f f i s h i n g p a t t e r n by c o m p o n e n t and m i n i m u m l a n d i n g sizes. T h e f i g u r e s d e n o t e r e l a t i v e Fs by age.

C = c o a s t ' c o m p o n e n t 8 = B a r e n t s S e a c o m p o n e n t

T a b l e 3. Y i e l d and s p a w n i n g s t o c k per r e c r u i t e s t i m a t e s ( g r a m ) at F 0 . 1

C o a s t a l comp. B a r e n t s S e a comp.

FISHING HOTALllY

F i g u r e

1.

(SSB) f o r 20 c n , 2 5 cr1,27 c m and 30 cm minimum l a n d i n g s i z e . M=0.15

.I200

1100

. I N 0

,900

. _BOO

t 3

- 600 s - 500

B

VI VI

- ^LOO - ³⁰⁰

FISHING MOTALITY - - -- - --

F i g u r e 2 . C o a s t a l component o f Norwegian s p r i n g spawning h e r r i n g . Y i e l d and spawning s t o c k p e r r e c r u i t

(SSB) f o r 2 0 cm,25 cm,27 cm and 30 cm minimum l a n d i n g s i z e . M1=M2=0.35. M3+=0.15

1100

1030

900

800

700

600

500

LOO

300

200

100

--- .---- -.. .

. . _{91 Q2 9 3 0.4 0.5 0,6 0-7 0) 0.9 I D 1,l 12 t3 1.4 IS}

FISHING HOTALITY

Figure 3. Barents Sea component of Norwegian spring spawning herring. Yield and spawning stock per recruit

(SSB) for 20 cm,25 cm,27 cm and

cm minimum landing size. M=0.15

FISHING EDTALITY

Figure

Barents Sea component of Norwegian spring spawning herring, Yield and spawning stock per recruit

(SSB) for 20 cm,25 cm,27 cm and 30 cn minimum

landing size. M1=M2=0.35. M3+=0.15

FISHING MOTALlTY

Figure 5. Barents Sea component of Norwegian spring spawning herring. Yield and spawning stock per recruit

(SSB) for 20 cm,25 cr,z,27 cm and 30 cm minimum

landing size. M1=M2=0.5. M

-0.35. M4+=0.15