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AcoucLic Methods i n F i s h e r i e s R e s e a r c h

No. 2 6

ECHO ABUNDANCE INDICES OF O-GROUP FISH I N THE BARENTS SEA 1965-1972

Anders Haug and Odd Nakken I n s t i t u t e o f Marine R e s e a r c h

Bergen, Norway

I N T R O D U C T I O N

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D u r i n g l a t e summer and e a r l y autumn f i s h f r y of many s p e c i e s o c c u r p e l a g i c a l l y i n t h e B a r e n t s Sea. S i n c e 1965 j o i n t i n t e r n a t i o n a l s u r v e y s have been c a r r i e d o u t y e a r l y i n t h i s a r e a d u r i n g 12-15 d a y s i n l a t e August and e a r l y September ( F i g . 1 ) . The p r i m a r y aim of t h e s e s u r v e y s i s t o map t h e g e o g r a p h i c a l d i s t r i b u t i o n and t o o b t a i n e s t i m a t e s o f y e a r c l a s s s t r e n g t h of O-group f i s h . Y e a r l y p r e l i m i n a r y r e s u l t s from t h e s u r v e y s have been r e p o r t e d t o t h e ICES annua1 m e e t i n g (ANON. 1965-72 and DRAGESUND 1 9 7 0 ~ ) . DRAGESUND

(1970 a and b ) and HYLEN and DRAGESUND ( 1 9 7 0 ) have made f u r t h e r a n a l y s e s o f some of t h e r e s u l t s . I n t h e annua1 r e p o r t s t h e y e a r - c l a s s s t r e n g t h of t h e d i f f e r e n t s p e c i e s h a s been s t a t e d t o b e p o o r , a v e r a g e o r s t r o n g , and t h i s g r a d i n g h a s s o f a r been based m a i n l y on t h e e x p e r i e n c e o f t h e p a r t i c i p a t i n g s c i e n t i s t s . F o r some of t h e i n v o l v e d s p e c i e s , i n d i c e s o f echo abundance have been u s e d t o c l a s s i f y t h e y e a r c l a s s s t r e n g t h (DRAGESUND and NAKKEN 1 9 7 0 ) .

DRAGESUND (1971) h a s compared t h e e c h o abundance i n d i c e s a t t h e O-group s t a g e w i t h abundance i n d i c e s o b t a i n e d by o t h e r methods a t

>

a g e - 3 y e a r s f o r come s p e c i e s , and he c o n c l u d e s : "A v e r y c l o s e c o r r e l a t i o n i s f o u n d b e t w e e n t h e two i n d e p e n d e n t e s t i m a t e s o f

y e a r c l a s s s t r e n g t h and i t i s c o n c l u d e d t h e r e f o r e , t h a t t h e ^l a b u n d a n c e i n d i c e s o f O-group f i s h o b t a i n e d from t h e a c o u s t i c

s u r v e y s g i v e a f a i r l y good e s t i m a t e o f y e a r c l a s s s t r e n g t h " .

Echo a b u n d a n c e i n d i c e s f o r t h e m o s t numerous s p e c i e s o f O-group f i s h sampled d u r i n g t h e i n t e r n a t i o n a l s u r v e y s a r e p r e s e n t e d i n t h i s p a p e r . The method o f c a l c u l a t i o n i s d e s c r i b e d , and t h e r e s u l t s a r e d i s c u s s e d .

MATERIAL AND METHODS

The d i s t r i b u t i o n a n d d e n s i t y o f t h e p e l a g i c s c a t t e r i n g l a y e r s h a v e b e e n e s t i m a t e d from t h e e c h o s o u n d e r p a p e r r e c o r d s , a n d t h e o r g a n i s m s f o r m i n g t h e s c a t t e r i n g l a y e r s w e r e i d e n t i f i e d f r o m s a m p l i n g w i t h s m a l l meshed p e l a g i c t r a w l s (DRAGESUND, M I D T T U N and OLSEN 1 9 7 0 ) . I n t h e r e p o r t s t o t h e ICES a n n u a 1 m e e t i n g , d i s t r i b u t i o n c h a r t s f o r e a c h s p e c i e s h a v e b e e n g i v e n . F i g . 2

shows t h e d i s t r i b u t i o n o f r e d f i s h i n 1 9 7 2 .

The d e n s i t y o f t h e s c a t t e r e r s h a s b e e n e s t i m a t e d by v i s u a l

g r a d i n g o f t h e p a p e r r e c o r d i n g s and c l a s s i f i e d a c c o r d i n g t o t h e f o l l o w i n g c o d e :

C l a s s i - No Very S c a t t e r e d Dense Very

f i c a t i o n r e c o r d i n g s c a t t e r e d d e n s e

D u r i n g t h e c r u i s e t h e c l a s s i f i c a t i o n h a s b e e n c a r r i e d o u t on- b o a r d t h e v e s s e l s f o r e a c h n a u t i c a l m i l e s a i l e d . The d i s t r i b u L t i o n c h a r t s ( F i g . 2 ) o f O-group f i s h h a v e , a s a r u l e , b e e n

g i v e n two d e n s i t y g r a d e s , s c a t t e r e d and d e n s e . T h i s g r a d i n g h a s p a r t l y b e e n b a s e d on t h e v i s u a l p a p e r g r a d i n g a n d p a r t l y on t h e amount o f f i s h i n t h e t r a w l c a t c h e s . By c a r e f u l e x a m i n a t i o n o f a l l t h e d i s t r i b u t i o n c h a r t s i t a p p e a r s t h a t t h e c r i t e r i a u s e d t o d i s c r i m i n a t e between s c a t t e r e d and d e n s e may h a v e v a r i e d

somewhat from y e a r t o y e a r . The f o l l o w i n g method f o r c a l c u l a t i n g

t h e a b u n d a n c e i n d i c e s was t h e r e f o r e a d o p t e d .

4

A ) A c e r t a i n number o f f i s h p e r h a u 1 was u s e d t o d i s c r i m i n a t e b e t w e e n s c a t t e r e d a n d d e n s e i n t h e d i s t r i b u t i o n c h a r t s . T h i s number was f o u n d by e x a m i n i n g t h e t r a w l c a t c h e s , which c o n t a i n e d o n l y one s p e c i e , and compared t h e number o f f i s h p e r h a u 1 w i t h t h e c o r r e s p o n d i n g v i s u a l d e n s i t y g r a d i n g ( T a b l e 1 ) . Assuming a l i n e a r r e l a t i o n s h i ~ b e t w e e n t h e l o g a r i t h m o f t h e c a t c h a n d t h e v i s u a l l y e s t i m a t e d d e n s i t i e s , o n e a r r i v e d a t t h e f o l l o w i n g

v a l u e s f o r d i s c r i m i n a t i o n b e t w e e n s c a t t e r e d a n d d e n s e :

S p e c i e s Cod C a p e l i n R e d f i s h P o l a r c o d No. o f f i s h

p e r h a u 1

B ) N e w d i s t r i b u t i o n c h a r t s were drawn up f o r e a c h s p e c i - e s where t h e s e v a l u e s w e r e u s e d f o r t h e a b o v e m e n t i o n e d d i s c r ' i m i n a t i o n .

C ) The a b u n d a n c e i n d i c e s , T , w e r e c a l c u l a t e d from t h e f o r m u l a

Where As a n d A d e n o t e t h e a r e a s w i t h i n which t h e d e n s i t i e s d

w e r e f o u n d t o b e s c a t t e r e d ( A s ) a n d d e n s e ( A d ) The c o e f f i c i e n t k , i s t h e r a t i o b e t w e e n f i s h d e n s i t i e s , c l a s s i f i e d a s d e n s e

( 3 a n d 4 ) and s c a t t e r e d '1 a n d 2 ) . T h i s c o e f f i c i e n t was d e t e r - mined by DRAGESUND ( 1 9 7 0 b ) t o h a v e a n d a p p r o x i m a t e v a l u e o f 1 0 . T a b l e 2 shows t h a t t h e e c h o i n t e g r a t o r r e a d i n g s , o b t a i n e d

d u r i n g t h e 1 9 7 1 c r u i s e , i n d i c a t e a v a l u e o f k b e t w e e n 6 a n d 8 . The c a l c u l a t e d a b u n d a n c e i n d i c e s f o r t h e p e r i o d 1965-1972, u s i n g k = 1 0 , a r e l i s t e d i n T a b l e 3.

F o r t h e p e r i o d 1965-1972 two a v e r a g e i n d i c e s h a v e b e e n c a l c u l a t e d f o r e a c h s p e c i e s ( T a b l e 4 ) . A v e r a g e 1 i s t h e a r i t h m e t i c mean o f a l l e i g h t v a l u e s and A v e r a g e 2 i s t h e a r i t h m e t i c mean o f

s i x o f t h e v a l u e s , t h e h i g h e s t a n d l o w e s t a r e e x c l u d e d . F i n a l l y a n i n t e r v a l was s e l e c t e d ( T a b l e 3 ) w i t h i n which t h e i n d i c e s

c o r r e s p o n d s t o a n a v e r a g e y e a r c l a s s s t r e n g t h . The r a t i o b e t w e e n

t h e u p p e r a n d l o w e r l i m i t o f t h e s e i n t e r v a l s was c h o s e n t o b e a p p r o x i m a t e l y 2 , a n d new e s t i m a t e s o f y e a r c l a s s s t r e n g t h were o b t a i n e d by c o m p a r i n g t h e i n d i c e s f o r e a c h y e a r w i t h t h e

a v e r a g e s .

RESULTS AND DISCUSSION

The r e s u l t s o f t h e c a l c u l a t i o n , a n d t h e c o n t r i b u t i o n t o t h e i n d i c e s from s c a t t e r e d and d e n s e c o n s e n t r a t i o n s a r e shown i n T a b l e 3 . T a b l e 4 shows t h e c l a s s i f i c a t i o n o f y e a r c l a s s

s t r e n g t h g i v e n i n t h e a n n u a 1 r e p r t s (E1) and f o u n d f r o m t h e c a l c u l a t e d a v e r a g e i n d i c e s ( E 2 ) .

F o r most o f t h e s p e c i e s t h e c o i n c i d e n c e b e t w e e n t h e two c l a s s i - f i c a t i o n s i s g o o d , b u t f o r c a p e l i n some d i s c r e p a n c i e s o c c u r .

I t seems t h a t t h e a n n u a 1 e s t i m a t e s made j u s t a f t e r t h e c r u i s e s a r e more o p t i m i s t i c t h a n t h o s e b a s e d on t h e c a l c u l a t i o n s .

L a t e r o b s e r v a t i o n s on t h e s p a w n i n g s t o c k o f c a p e l i n h a v e shown t h a t t h e y e a r c l a s s e s 1967 and 1969 w e r e a p ~ r o x i m a t e l y e q u a l i n

.

s t r e n g t h w h i l e t h e 1968 y e a r c l a s s was weaker (DRAGESUND e t a l . 1 9 7 3 ) . T h i s may i n d i c a t e t h a t t h e i n d i c e s i n T a b l e 3 f o r c a p e l i n a r e s u b j e c t e d t o l a r g e e r r o r s .

When c o m p a r i n g t h e i n d i c e s i n T a b l e 3 t h e v a r i o u s s o u r c e s o f e r r o r , a n d t h e i r i n f l u e n c e on t h e c a l c u l a t e d numbers, s h o u l d b e k e p t

i n mind. The m o s t i m p o r t a n t s o u r c e s o f e r r o r a r e :

1. The r a t h e r c r u d e method o f c l a s s i f i c a t i o n o f d e n s i t y from t h e r e c o r d i n g p a p e r . A l t h o u g h c a r e h a s b e e n t a k e n t o a v o i d b i a s e s c a u s e d by d i f f e r e n c e s i n power a n d g a i n s e t t i n g s o f t h e e c h o - s o u n d e r s by i n t r o d u c i n g c a l i b r a t i o n

-

and c o n t r o l c o u r s e s , t h e a c c u r a c y o f t h i s c l a s s i f i c a t i o n w i l l a l w a y s d e p e n d on t h e i n t e r - p r e t a t i o n o f t h e d i f f e r e n t s c i e n t i s t s .

2 . The e r r o r s i n v o l v e d i n t h e c h o i s e o f c a t c h p e r h a u 1 f o r d i s c r i - m i n a t i o n b e t w e e n s c a t t e r e d and d e n s e . A g a i n t h e i n t e r p r e t a t i o n made by t h e p e r s o n e l l i s a n i m p o r t a n t b u t unknown f a c t o r , and t h e nurnber o f f i s h p e r h a u 1 shows l a r q e v a r i a n c e s ( T a b l e 1 ) .

The l a r g e d i f f e r e n c e s i n r e l a t i v e u n i t s between c a t c h e s and i n t e g r a t o r d e f l e c t i o n s ( T a b l e 2 a n d 3 ) a r e p r o b a b l y c a u s e d by a s y s t e m a t i c u n d e r e s t i m a t i o n o f low f i s h d e n s i t i e s when u s i n g

t r a w l s

.

3 . The d i f f e r e n c e i n mean l e n g t h o f t h e s p e c i e s e a c h y e a r , w h i c h a f f e c t s t h e a v e r a g e t a r g e t s t r e n g t h o f t h e i n d i v i d u a l s and t h u s a l s o t h e c o n t r i b u t i o n t o t h e i n d e x . T h i s m i g h t b e t h e r e a s o n f o r t h e low i n d e x f i g u r e f o r t h e 1967 y e a r c l a s s o f c a p e l i n a s compared t o t h e 1968 y e a r c l a s s . I n 1967 t h e O-group c a p e l i n

was s i g n i f i c a n t l y s m a l l e r t h a n i n 1968 (DRAGESUND 1 9 7 0 c F i g . 3 1 ) . F o r o t h e r s p e c i e s w h i c h show l e s s d i f f e r e n c e s i n mean l e n g t h

f r o m y e a r t o y e a r , t h i s m a t t e r i s b e l i e v e d t o b e o f l e s s i m p o r t - a n c e .

4 . The s e l e c t i o n o f k . The e f f e c t o f u s i n q a k o f 6 o r 8 i n s t e a d o f 1 0 c a n e a s i l y b e s t u d i e d i n T a b l e 3 . T h i s w i l l r e d u c e t h e h i g h e s t e s t i m a t e s , b u t n o t c h a n g e t h e r a n k o f t h e i n d i c e s .

I n f o r m a t i o n on t h e c o n t r i b u t i o n s t o t h e t o t a l b i o m a s s f r o m t h e d i f f e r e n t s p e c i e s c a n , o n l y t o a l i t t l e e x t e n t , b e e x t r a c t e d from T a b l e s 3 a n d 4 . I n o r d e r t o make s u c h a c o m p a r i s o n , know- l e d g e o f t h e sound r e f l e c t i n g p r o g e r t i e s and much more a c c u r a t e i n f o r m a t i o n o f r e l a t i v e d e n s i t i e s a r e n e e d e d . F o r r e l a t e d

s p e c i e s l i k e cod and h a d d o c k , which c a n b e assumed t o h a v e e q u a l r e f l e c t i o n p r o p e r t i e s , a c o m p a r i s o n m i g h t h o l d .

However, e v e n t h o u g h c o n s i d e r a b l e i n a c c u r a c i e s a r e p r e s e n t i n t h e f i g u r e s t h e r e i s no d o u b t t h a t t h e i n d i c e s f o r e a c h s p e c i e s r e f l e c t r e l a t i v e s t r e n g t h s o f y e a r c l a s s e s o f t h e O-group f i s h e s i n t h e B a r e n t s S e a .

REFERENCES

ANON. 1965. Preliminary Report of the joint Soviet-Norwegian

investiagations in the Barents Sea and adjacent waters September 1965. Coun.Meet.int.Coun.Explor.Sea, 1965

(161) : 1-5, 10 figs. [~limeo.]

ANON. 1966. Preliminary Report of the joint international O-group fish survey in the Barents Sea and adjacent waters August/September 1966. Coun.Meet.int.Coun.Explor.Sea,

1966 (H23): 1-9, 13 figs. [~imeoJ

ANON. 1967. Preliminary Report of the international O-group fish

survey in the Barents Sea and adjacent waters August/ ⁱ September 1967. Coun.Meet.int.Coun.Explor.Sea, 1967

(H31) : 1-8, 14 figs. r ~ i m e o a

ANON. 1968. Preliminary Report of the O-group fish survey in the Barents Sea and adjacent waters in August-September 1968. Coun.Meet.int.Coun.Explor.Sea, 1968 (F33): 1-4, 13 figs. [~imeoa

ANON. 1969. -Preliminary Report of the O-group fish survey in the

I

Barents Sea and adjacent waters in August-September 1969. Coun.Meet.int.Coun.Explor.Sea, 1969 (F34): 1-5, 15 f igs.[Mimeo]

ANON. 1970. Preliminary Report of joint Soviet-Norwegian O-qroup fish survey in the Barents Sea and adjacent waters in August-September 1970. Coun.Meet.int.Coun.Explor.Sea,

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1970 (H34) : 1-5, 16 figs. [~imeo,]

ANON. 1971. Preliminary Report of the international O-group fish survey in the Barents Sea and adjacent waters in

August-September 1971. Coun.Meet.int.Coun.Ex~10r.Sea~

1971 (H32) : 1-5, 17 figs.CMimeoa

ANON. 1972. Preliminary Report of the international O-group fish survey in the Barents Sea and adjacent waters in August-September 1972. In prep.

DRAGESUND, 0. 1970 a. Factors influencing year-class strength of Norwegian spring spawning herring (Clupea harengus - Linne). FiskDir.Skr.Se~HavUnders., 15(4): 381-450.

DRAGESUND, 0. 1970 b. Distribution, abundance and mortality of young and adolescent Norwegian Spring spawning herrin?

(Clupea harengus Linne) in relation to subsequent year-class strength. Ibid, 15(4): ^y 451-556.

DRAGESUND, O . (ed.). 1 9 7 0 ~ . International O-cjroup fish surveys in

l

the Barents Sea 1965-1968. Coop.res.rep.int.Coun.Explor.

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Sea, 1970, Ser. A(181: 1-81. b i m e d

DRAGESUND, 0 . 1971. Comwarative analysis of year-class strength among fish stoclcs in the North Atlantic. Ibid, 16 (2) :

49-64.

DRAGESUND, O,, G J f l S K T E K , J. and MONSTAD, T. 1973. Estimates of stock size and reproduction of the Barents Sea Capelin in 1973-1972, FiskDir.SIcr,Ser,HavUnderc.,

(in press).

DF.AGESUND, O., MIDTTUN, L. and OLSEN, S D 1970. Methods for esti- mating distribution and abundance. of O-group fish.

In: Dragesund, O. (ed.): International O-qroup fish surveys in the Barents.Sea 1965-1968. Coop.res.re9.

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int.Coun.Explor.Sea, -.-...-.-- 1970, Ser.A(l8): 1-81. L~irneod

DRAGESUND, 0. and NAKKEN, 0. 1970. Relationship of parent stock size and year-class strength in Norwegian spring spawning herring. Int.Coun.Explor.Sea, Symp. 1970

r -T

HYLEN, A. and DRAGESUND, 0. 1970. Recruitment of young Arcto- Morweoian Cod and Haddock in relation to parent stock size. Coun.Meet.int.Coun.Explor.Sea, 1970 (19): 1-11, 22 figs. kimeo] -

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