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T h i s paper n o t t o be c i t e d w i t h o u t p r i o r reference t o t h e a u t h o r s , 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 of t h e sea

C.M. 1 9 7 9 / G : 5 1

D e r m e r s a l f i s h ( N ) C o m r n i t t e e R e f . F i s h i n g T e c h n o l o g y

C o m m i t t e e .

ACOUSTIC METHOD FOR E S T I M A T I N G ABSOLUTE ABUNDANCE O F YOUNG COD AND HADDOCK I N THE BARENTS S E A

I

John D a l e n and O d d M. S m e d s t a d I n s t i t u t e of M a r i n e R e s e a r c h , B e r g e n

ACOUSTIC METHOD FOR ESTIMATING ABSOLUTE ABUNDANCE OF YOUNG COD AND HADDOCK IN THE BARENTS SEA

John Dalen and Odd M. Smedstad Institute of Marine Research, Bergen

where printed should read

p.l., 1.10 -method to assess absolute- -method to estimate absolute- from below

11 1. 5 4 r/e where 4 r/3 where p 5, 1. 3 C = [ I v 2

-

Cre ,o,l-l

O P

2

2 cre

<,,]-l

C = [ I v

- O P

2

Il Il

1. 4 in the density is the density

p.7, 1. 8 For the [jthl category For the -jth category ^l

p.9, 1. 7 235 dB 11.1 yPa ref. lm, 235 dB//lvPa ref. lm,

Il Il 1.10 bandwidth: band width:

Il li

1.13 having siz depth having six depth References

p.4, 1.13 for censuser and

any

mixing of Yes rations?

for censuses and

any age obser-

vations?

Yes

II on the process of echo inteqra- of the acoustic method tion on assessing of estimating absolute Fig.2. of the instruments and sampling of the echo integration

equipment. process.

ACOUSTIC METHOD F O R ESTIE./IATING ABSOLUTE ABUNDANCE OF YOUNG COD AND HADDOCK I N THE BARENTS SEA.

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

A c o u s t i c equipment h a s been u s e d f o r o b s e r v i n g and m o n i t o r i n g f i s h s t o c k s f o r s e v e r a l d e c a d e s . When t r y i n g t o estimate

abundances o f f i s h s t o c k s l e e h o c o u n t i n g and e c h o i n t e g r a t i n g have been t h e most employed methods (BLINDHEIM & NAKKEN 1971, BUZETA & NAKKEN 1975, C R A I G & FORBES 1969, CUSHING 1968,

EHRENBERG 1974, JOHANNESON & LOSSE 1977, MIDTTUN & NAKKEN 1977,

NAKKEN & DOMMASNES 1975, THORNE 1977, THORNE, REEVES AND

MILLIKAN ,1971, THOTWE & WOOBEY 1 9 7 0 ) . Out of t h e s e twu methods echo i n t e g r a t i o n t e c h n i q u e s have shown up t o b e t h e most a p p l i -

i

c a b l e one.

The p r o j e c t on a c o u s t i c abundance o b s e r v a t i o n of d e m e r s a l f i s h i n t h e B a r e n t s Sea s t a r t e d i n 1970. I n i t i a l l y we c o n c e n t r a t e d on f i n d i n g t h e most s u i t a b l e t i m e o f t h e y e a r f o r abundance e s t i m a t i o n b o t h from b i o l o g i c a l and s u r v e y i n g p r o s p e c t s . From 1974 we g r a d u a l l y p u t i n c r e a s i n g e f f o r t i n t o a s s e s s i n g a b s o l u t e abundances o f r e c r u i t s of n o r t h - e a s t A t l a n t i c eod and haddock.

S i n e e 1976 t h e s u r v e y s have f o l l o w e d a d e t a i l e d and w e l 1 -

d e f i n e d s c h e d u l e and working p l a n which w e c o n s i d e r y i e l d s and a c c e p t a b l y h i g h q u a l . i t y of c o l l e c t e d i n f o r m a t i o n .

2 . Methods.

2 . 1 . Fundamentals o f t h e method. -".---m--- .

,

@ m

&;

The a c o u s t i e ntethod t o -s=s a b s o l u t e abundances o f f å c h w e d e f i n e a s c o n t a i n i n g t h e f o l l o w i n g p r o c e s s e s :

I

-

o b s e r v i n g and r e c o r d i n g i n t e g r a t e d e c h o i n t e n s i t i e s by means o f c a l i b r a t e d e c h o s o u n d e r s and e c h o i n t e - g r a t o r s ^l

2

-

i d e n t i f y i n g Your r e c o r d i n g s on s p e c i e s by means o f

echograms, i n t e g r a . t o r r e a d i n g s , and b i o l o g i c a l s a m p l e s , 3

-

d e t e r m i n i n g t h e s i z e d i s t r i b u t i o n o f r e c o r d e d f i s h by

means o f b i o l o g i c a l s a m p l e s , and t o same e x t e n t by echo s t r e n g t h c l a s s i f i . c a . t i o n ,

4

-

a p p l y i n g m a t h e m a t i c a l models t o e s t i m a t e f i s h d e n s i t y , amounts o % f i s h and d i s t r i b u t i o n s o f l e n g t h , w e i g h t and a g e .

t" """c*-

N ~ t i c e ~ t h a t we d e f i n e t h e a c o u s t i c methods a s c o n s i s t i n g o f more a c t i v i t i e s o f e q u a l s i g n i f i c a n c e t h a n j u s t u s i n g a c o u s t i c

i n s t r w n e n t s o r making a c o u s t i c o b s e r v a t i o n s . The q u a l i t y o f e a c h o f t h e p r o c e s s e s and t h e i r e f f e c t s on t h e abundance estimates have been d i s c u s s e d by s e v e r a l i n v e s t i g a t o r s (BODHOLT 1977, CUSHING 1968, DOUBLEDAY 1976, EHRENBERG & LYTLE 1977, FOOTE 1979, NAKKEN 1975, SHOTTON & DOWD 1 9 7 5 ) . T h i s t o g e t h e r w i t h e x p e r i e n c e from own i n v e s t i g a t i o n s i s surnmarized i n DALEN 1979.

Echo i n t e g r a t i o n .

The i n t e n s i t y o f a n u n d e r w a t e r a c o u s t i c s i g n a l s c a t t e r e d back from a c o l l e c t i o n o f s c a t t e r e r s i s p r o p o r t i 6 n a l t o t h e d e n s i t y o f t h e s c a t t e r e r s . T h i s h o l d s f o r a wide d e n s i t y r a n g e (FOOTE 1978, RØTTINGEN 1976) and i s t h e b a s i c p h i l o s o p h y f o r a p p l y i n g echo i n t e g r a t i o n when a s s e s s i n g abundances of f i s h s t o c k s . The p r o c e s s i s i l l u s t r a t e d by a b l o c k d i a g r a m i n F i g . l.

W e now c o n s i d e r a n e n s o n i f i e d volume,

vs from which w e may r e c e i v e e c h o e s s i r n u l t a n e o u s l y . T h i s volume may c o n t a i n : 1

-

randomly, homogenlously d i s t r i - b u t e d f i s h e s , i . e . t h e

l o c a t i o n o f e a c h f i s h i s i n d e p e n d e n t of t h e l o c a t i o n o f t h e o t h e r s , w i t h a d e n s i t y p v p e r c u b i c m e t e r , and

2

-

p l a n k t o n i c o r g a n i s m s , b u b b l e s and o t h e r s m a l l p a r t i c l e s . Assuming t h a t t h e p h a s e s o f t h e e c h o e s of t h e i n d i v i d u a l f i s h a r e randomly d i s t r i b u t e d we c a n e x p r e s s t h e back s c a t t e r e d i n t e n s i t y a s :

where

=o

-

s o u r c e l e v e l [dB//

h

l vPa a t l

m l ,

v

-

t h e p r e s s u r e t o v o l t a g e c o n v e r s i o n f a c t o r o f t h e t r a n s d u e e r r

and f i x e d s y s t e m g a i n ,

T

-

e f f e c t i v e p u l s e l e n g t h , which i s found t o b e a p p r o x i r n a t e l y e

4 T / $ where r i s t r a n s m i t t e d p u l s e l e n g t h (MITSON 1 9 7 6 ) ,

3

f3

-

a t t e n u a t i o n c o e f f i c i e n t [dB/ m],

r

-

d i s t a n c e from t r a n s d u c e r t o c o n s i d e r e d volume, G ( r ) - t i m e v a r y i n g g a i n f u n c t i o n ,

s

-

volume back s c a t t e r i n g c o e f f i c i e n t ,

v

b ^{(€l,@) d} beam p a t t e r n d i r e c t i v i t y f a c t o r f o r f i s h a t a n g u l a r

c o o r d i n a t e s 8 and @,

52

-

s o l i d a n g l e d e t e r m i n i n g t h e a n g u l a r c o o r d i n a t e s o f t h e volume.

W e have p u t t h e d i r e c t i v i t y beam p a t t e r n f b n c t i o n s d u r i n g t r a n s m i t t i n g and r e c e i v i n g a s b e i n g e q u a l . T h i s i s t r u e o n l y when h a v i n g s t a b i l i z e d t r a n s d u c e r s which w e used h e r e . W e d e f i n e t h e volume back s c a t t e r i n g c o e f f i c i e n t a s :

where o i s t h e b a c k s c a t t e r i n g c r o s s s e c t i o n o f a f i s h .

The f i r s t t e r m o f e q , 2 i s t h e f r a c t i o n o f t h e back s c a t t e r i n g i n r e s p o n s t o t h e t a r g e t s o f i n t e r e s t and t h e s e c o n d t e r m i s i n r e s p o n s e t o what we may d e n o t e a s " n o i s e " ( i . . e . c a u s e d by

p l a n k t o n , bubbles, o t h e r p a r t i c l e s and a m b i e n t n o i s e ) .

'O

P r

I f w e a p p l y a tirrle v a r y i n g g a i n f u n c t i o n G ( r ) = re

,

eq. 1 now becomes s

The two i n t e g r a l s of eq, ₃ c a n b e e s t i m a t e d when t h e f o l l o w i n g terms a r e known:

1

- the directivity function of the transducer,

2

- the backscattering cross section as a function of species, size and tilt angle, and

3

- the behaviour pattern (distribution of the tilt angle) of the species.

We define the effective backscattering cross section as:

where 5 is the orientation distribution-averaged value of

o / 4 n [FOOTE

19791. and the effective "noise" volume back-scat- tering coefficient as:

This yields for eq.

3.

The number of fish within a depth interval rl to r2 can be expressed as:

We define the echo abundance,

M,

as the sum of intensities from each transmission over the depth interval rl to r2 per unit

distance. 1.

' 'a

where

p =

Pl/s is the number of transmissions over 1,

P

is the

rate of transmission (pulse repetition frequency), and s is the

vessel speed.

is the area fish density per unit distance.

P

3

in the density coefficient.

can be regarded as a threshold density.

This yields:

For the echo abundance, M , to be a consistent, general term we have to compensate for the rate of transmissions, P, and the vessel speed,

s.

In most integrators this is done automatically by presetting the value of P and sensing a signal proportional

$0 the speed, s.

P 0 determines mathematically the lower limit of the echo abun- danee, M, f o r which eq. 12 can be applied. On the other hand

the

magnitude and hence the effect of .p can be controlled by

a

threshold generator within the integrator.

Active use of the threshold control can to some extent be useful

En

suprescing noise siynals and signals from smaller organisms.

A

p a r t i c ~ ~ a r effect o% this threshold should be stressed because it makes the integrator system generally discriminating against small targets as discussed by EHRENBERG & WEIMER 1974 and WEIMER

& EHRENBERG 1975.

-

The density coefficient, C, is besides being a function of the instrument characteristics such as source level, receiving

s e n s i t i v i t y , s y s t e m g a i n and e f f e c t i v e p u l s e l e n g t h , a f u n c t i o n o f t h e e f f e c t i v e back s c a t t e r i n g c r o s s s e c t i o n of t h e f i s h under o b s e r v a t i o n . T h i s l a t t e r p a r a m e t e r i s a m u l t i v a r i a b l e f u n c t i o n which t a k e s c a r e o f s p e c i e s , l e n g t h o r w e i g h t d e p e n d e n c e s , modes o f b e h a v i o u r , s p a t i a 1 d i s t r i b u t i o n o f t h e f i s h , and beam p a t t e r n f u n c t i o n o f t h e t r a n s d u c e r . J u s t r e c e n t l y a s a t i s f a c t o r y a n a l y - t i c a l t r e a t m e n t h a s been done on t h e e f f e c t i v e b a c k s c a t t e r i n g c r o s s s e c t i o n

-

f i r s t t o some e x t e n t by LOVE 1977 and t h e r e a f t e r t o a h i g h e r l e v e l by FOOTE 1979.

2.3. E s t i m a t i o n ^i---iii--model f o r c a l c u l a t i n q a b s o l u t e abundances ^{iii--iiiii------------}

I

T h i s model i s t o c o n v e r t e c h o i n t e n s i t y i n t o f i s h d e n s i t y on t h e b a s i s o f t h e i n f o r m a t i o n from t h e b i o l o g i c a l s a m p l i n g . The e c h o abundance produced by a p o p u l a t i o n of f i s h i s g e n e r a l l y a f u n c - t i o n o f t h e t o t a l abundance o f t h e f i s h , t h e d i s t r i b u t i o n o f s p e c i e s and s i z e s o f t h e d i f f e r e n t s p e c i e s . We d e f i n e t h e t e r m

" c a t e g o r y " a s c o n s i s t i n g o f a c e r t a i n l e n g t h g r o u p from a c e r t a i n s p e c i e s

.

Echo i n t e n s i t i e s , Mi, from " n n c a t e g o r i e s c o n t r i b u t e s i m u l t a n e - o u s l y t o t h e t o t a l v a l u e o f M (FORBES & NAKKEN 1 9 7 2 ) , and t h e s e y i e l d s :

The f i s h d e n s i t y and t h e i n t e g r a t e d e c h o i n t e n s i t y form a l i n e a r r e l a t i o n f o r e a c h c a t e g o r y " i " , c o n f . e q . 1 2 .

Applying a p r o p e r t h r e s h o l d s e t t i n g t o g e t h e r w i t h b e i n g aware o f any e f f e c t from n o i s e s o u r c e s , t h e i n f l u e n c e o f p. i s removed t h r o u g h t h e p r e p r o c e s s i n g o f t h e a c o u s t i c d a t a .

T h i s y i e l d f o r e q . 1 4 :

Having a s s i g n e d a p a r t i c u l a r e c h o abundance M s t o a s p e c i e s " s "

o r a g r o u p o f s p e c i e s t h r o u g h t h e s c r u t i n i z i n g p r o c e s s e s , w e have t o d i s t r i b u t e Msto t h e s p e c i f i e d c a t e g o r i e s . The n e c e s s a r y i n f o r m a t i o n f o r t h i s i s drawn from t h e f r e q u e n t t r a w l s a m p l i n g

. by assuming t h a t t h e s a m p l e s y i e l d t r u e d e n s i t y r a t i o s by number between t h e d i f f e r e n t c a t e g o r i e s o f t h e a c t u a l s p e c i e s which a r e o b s e r v e d . For t h e -&j t h & c a t e g o r y w e have:

k j ⁼ (number o f f i s h o f c a t e g o r y " j ^" i n t h e c a t c h ) / ( t o t a l number of f , i s h i n t h e c a t c h which have c o n t r i b u t e d t o t h e a s s i g n e d echo abundance,

Ms )

From t h e a s s u m p t i o n a b o u t t r u e s a m p l i n g which i m p l i e s t h a t t h e nurnber o f f i s h o f c a t e g o r y " j " i n t h e c a t c h i s p r o p o r t i o n a l t o t h e a r e a l d e n s i t y o f t h e f i s h o f t h e same c a t e g o r y , w e may w r i t e :

-

^'aj

k j

-

n E 'ai

F o r a l l v a l u e s of " i " we may w r i t e :

l

l

Eq. 18 together with eq. 14 yields:

l

By inserting eq. 19 into eq. 13, M i can be expressed by i, Ci and Ms.

We obtain our desired functions by eq. 20 and eq. 14 which express the absolute fish density,

pai, from ki, C i and Ms.

2 . 4 . ---..m--- I n s t r u m e n t s and s a m ~ l i n q - e q u i ~ m e n t ' .

--- ----

The v e s s e l w e have u s e d , R.V "G.O. S a r s " i s e q u i p p e d w i t h SIMIPAD s c i e n t i f i c e c h o s o u n d e r s

-

b o t h w i t h hull-mounted t r a n s d u c e r s and w i t h towed o n e s . The p a r t i c u l a r s o u n d e r u s e d f o r abundanee e s t i m a t i o n h a s t h e f o l l o w i n g main c h a r a c t e r i s t i c s :

j

S o u r c e l e v e l :

t r a n s m i t t i n g f r e q u e n c y :

/ / 4 C

235 d ~ &+l @ a r e f .

1 m,

38 kHz,

p u l s e l e n g t h _-- : 0 , 6 m s ,

b a n s i d t h ; 3 kHz and

beam w i d t h , h a l f power l e v e l : 5 . 5 ^O a p p r o x . c i r c u l a r .

The s o u n d e r i s c o n n e c t e d t o a n a n a l o g u e e c h o i n t e g r a t o r system h a v i n g si.% )3 d e p t h c h a n n e l s . The s c h e m a t i c s o f t h e s a m p l i n g and p r o c e s s i n g o f d a t a a r e shown by a b l o c k d i a g r a m i n f i g . 2 ,

When o b c e r v i n g d e m e r s a l s p e c i e s s p e c i a l a t t e n t i o n h a s t o b e p a i d t o t h e d e p t h column c l o s e t o t h e bottom. T h e r e a r e l i m i t a t i o n s of n e a r bottom fis11 d e t e c t i o n which p r a c t i c a l l y a r e d e t e r m i n e d by o

1 t h e beam a n g l e o f t h e t r a n s d u c e r , 2 e f f e e t i v e p u l s e l e n g t h ,

3 s i z e o f t h e f i s h t a r g e t s , 4

-

r o u g h n e s s o f t h e bottom, and 5

-

s l o p e o f t h e bottom.

T h e s e problems a r e d i s c u s s e d i n d e t a i l by MITSON _{1976 and} DALEN 1979. The c o n c l u s i o n i s t h a t a n e c h o s o u n d e r s h o u l d f u l f i l

r e q u i r e n i e n t s of h a v i n g a narrow beam t r a n s d u c e r and working w i t h s h o r t p u l a e s when e s t i m a t i n g n e a r bottom f i s h p o p u l a t i o n s .

I t i s n e e e s s a r y t o have a n i n t e g r a t o r w i t h a p r o p e r bottom s t o p funetioar and a l s o a bot.tom l o c k f u n c t i o n . To e x t r a c t a s much

i n f o r m a t i o n a s p o s s i b l e a b o u t f i s h s t a n d i n g c l o s e t o t h e bottom, t h e c h a n n e l s h a v i n g t h e two bottom mode f u n c t i o n s a r e r u n p a r a l - l e l . The g a i n e d i n f o r m a t i o n comes from comparison o f t h e i n t e - g r a t o r r e a d i n g s from t h e two c h a n n e l s o r t h e d i f f e r e n c e between them. These v a l u e s a r e a l s o e s p e c i a l l y u s e f u l i n p r o c e s s i n g e c h o i n t e n s i t i e s when h a v i n g a rough o r uneven bottom.

A s i m p o r t a n t a s a c o u s t i c a l d a t a a r e t h e b i o l o g i c a l d a t a when a s s e s s i n g a b s o l u t e abundance o f t h e a c t u a l s p e c i e s . R e q u i s i t e b i o l o g i c a l d a t a from ground f i s h w e c o l l e c t b o t h from bottom t r a w l s and p e a l g i c t r a w l s w i t h fine-meshed n e t i n t h e cod end.

An o v e r a l l o b j e c t i v e i s t o t r y t o m i n i m i z e t h e e f f e c t of t h e known s o u r c e s o f e r r o r i n a s s e s s m e n t methods and t o o p t i m i z e w i t h r e s p e c t t o a l l i m p o r t a n t p a r a m e t e r s which i n f l u e n c e t h e a c c u r a c y o f a n abundance e s t i m a t e . Some s i g n i f i c a n t p o i n t s w i l l b e d i s c u s s e d .

2.5.1. B i o l o g i c a l b a s i s .

While d e v e l o p i n g t h e s u r v e y d e s i g n - t h r o u g h t h e y e a r s from 1970

i ,)

w e have b e e n l o o k i n g f o r any f a v o u r a b l e b i o l o y i c a l s i t u a t i o n s from t h e a c o u s t i c p o i n t of view f o r e s t i m a t i n g young cod and haddock. I n t h i s c o n t e x t w e r e q u i r e :

1

-

t o s u r v e y t h e s p e c i e s when i t h a s t h e most a d v a n t a g e o u s s p a t i a l l o c a t i o n o r s p r e a d w i t h i n i t s a r e a o f r e s i d e n c e , 2

-

t h a t t h e g r e a t e r number o f t h e f i s h have t o s t a y o f f t h e

2.

bottom t o b e o b s e r v e d a c o u s t i c a l l y ,

3

-

a minimum o f m i g r a t i n g movements o f t h e f i s h w i t h i n t h e s u r v e y e d a r e a t o minimize c e r t a i n b i a s e s i n t h e e s t i m a t e s , and

4

-

n o t t o o s t r o n g i n t e r f e r e n c e o f o t h e r s p e c i e s among t h o s e b e i n g a s s e s c e d .

For youny cod and haddock i n t h e B a r e n t s Sea we have found t h a t t h e p e r i o d J a n u a r y t o t h e m i d d l e of March i s f a v o u r a b l e f o r a p p l y i n g a c o u s t i c methods.

2 . 5 . 2 . S u r v e y n e t and d i s t r i b u t i o n o f t r a w l s t a t i o n s .

We t r y t o d e s i g n t h e s u r v e y p a t t e r n and t o d i s t r i b u t e t h e t r a w l sampling t o o b t a i n t h e most p r e s i c e abundance e s t i m a t e f o r a g i v e n s u r v e y p e r i o d . T h i s i s on t h e b a s i s o f :

l

-

t h e e x p e c t e d e x t e n s i o n a r e a o f cod and haddock,

2

-

t h e p r o b a b l e d i s t r i b u t i o n o f t h e s p e c i e s i n t h e a r e a , 3

-

t h e p r o b a b l e s i z e d i s t r i b u t i o n o f t h e s p e c i e s , and 4

-

t h e 1 a . r g e - s c a l e b e h a v i o u r p a t t e r n s of t h e s p e c i e s .

W e a l s o r e q u i r e s u f f i c i e n t t o t a l s a m p l i n g e f f o r t b o t h a c o u s t i - c a l l y and b i o l o g i c a l l y t o o b t a i n r e p r e s e n t a t i v e c o v e r a g e o f t h e d i f f e r e n t s t r a t a when t h e whole s u r v e y e d a r e a i s d i v i d e d inta smaller u n i t s ,

C o n s i d e r i n g t h e l a r g e - s c a l e b i o l o g i c a l s a m p l i n g , t h i s a l w a y s t a k e s p l a c e whenever t h e p a t t e r n o f t h e echo r e c o r d i n g s o f d e m e r s a l s p e e å e s c h a n g e s o r when b i o l o g i c a l d a t a i s needed i n r e c p o n s e t o d b s t r i b u t i o n and c o v e r a g e . S t a n d a r d towing d i s t a n c e i s t h r e e n a u t i s a 2 r n i 1 . e ~ t o g i v e a n a p p r o x i m a t e l y e q u a l l y e f f e c - t i v e s w e p t volurne b o t b f o r d e m e r s a l and p e l a g i c t r a w l s t a t i o n s , The s u r v e y p a t t e r n may g e n e r a l l y b e d e s s r i b e d a s a z i g z a g path,, c r o s s i n g z i g z a g p a t h , p a r a l l e l l i n e s o r u n s y s t e m a t i c l i n e s t o

r e a c h a p o i n t / s r n a l l a r e a a t randorn. The s t a t i s t i c a l e f f e c t s o f ^i*

t h e v a r i a n c e of a11 e s t i m a t e when c h o o s i n g d i f f e r e n t p a t t e r n s , have been exarnined by s e v e r a l i n v e s t i g a t o r s (HANSEN, HURWITZ MADOW 1 9 5 3 , NICMERSON ^& DOWD 1.977, SHOTTON & DOWD 1975, YATES

1 9 6 5 ) * For a r e l a t e d d i s c u s s i o n s e e DALEN 1979.

Our a p p r o a e h h a s d e v e l o p e d i n t o a p a r a l l e l l i n e p a t t e r n . I t g i v e s w e l l d e f i n e d t r a n s e c t s t h r o u g h t h e s u b a r e a s , and a t

a v e r a g e t h e L i n e s a r e e q u i d i s t a n t , b u t s l i y h t l y d e n s e r i n a r e a s

w i t h h i g h e r f i s h c o n c e n t r a t i o n s . The t o t a l s u r v e y a r e a i s d i v i d e d i n t o s u b a r e a s which w e d e n o t e a s s t a t i s t i c a l s q u a r e s . The d i m e n s i o n s of t h e s g u a r e s a r e 30 n a u t i c a l m i l e s i n t h e

n o r t h - s o u t h d i r e c t i o n and 60 ^e c o s a n a u t i c a l m i l e s

-

^{where a i s}

d e g r e e l a t i t u d e , i n t h e e a s t - w e s t d i r e c t i o n . T h i s p a r t i c u l a r s i z e i s d e t e r m i n e d from t h e d i s t r i b u t i o n o f :

1

-

t h e s p e c i e s c o m p o s i t i o n ,

2

-

t h e s i z e ( l e n g t h / a g e ) o f t h e specimen, and

3

-

t h e d e n s i t y o f t h e f i s h , i . e . t h e v a l u e s o f t h e e c h o abundance.

W i t h i n e a c h s q u a r e t h e s e f u n c t i o n s a r e found t o c o n t a i n minor and a c c e p t a b l e v a r i a t i o n s (DALEN, HYLEN & SMEDSTAD 1977 a,b,

DALENF~SMEDSTAD 1979)

.

W e t r a n s e c t between 7 0 and 1 0 0 % of t h e s u b a r e a s d e p e n d i n g on t h e e x t e n s i o n and d e n s i t y of t h e f i s h d i s t r i b u t i o n . Come o f t h e

b a s i c i d e a s b e h i n d t h i s p r e - p l a n n e d , w e l l d e f i n e d s u r v e y p a t t e r n , a r e t o e s t a b l i s h a r e p e a t a b l e program from y e a r t o y e a r . T h i s y i e l d s c l e a r l y comparable i n f o r m a t i o n a b o u t y e a r l y c h a n g e s of f i s h d e n s i t y , d i s t r i b u t i o n and s i z e c o m p o s i t i o n .

26. S c r u t i n i z i n q and ~ r e ~ r o c e s s i n q

--

o f d a t a .

The main o b j e c t i v e o f t h e p r e p r o c e s s i n g i s t o a t t a c h t h e r i g h t echo abu.ndance t o a l l o b s e r v e d s p e c i e s o r g r o u p s o f s p e c i e s and t o remove unwanted e f f e c t s on t h e e c h o abundances from non- b i o l o g i c a l s o u r c e s .

A d a i l y s c i e n t i f i c s t a f f m e e t i n g i s h e l d t o u t i l i z e t h e r u n n i n g i n f o r m a t i o n from:

1

-

t h e e c h o abundance o b s e r v a t i o n

-

by e c h o s o u n d e r and i n t e g r a t s r ,

2

-

t h e b i o l o g i c a l s a m p l i n g

-

by t r a w l s ,

3

-

t h e o c e a n o g r a p h i c s a m p l i n g

-

by CTB-sonde and thermograph, and

4

- any influence from the weather on the acoustic data ac- q u i s i t i ~ n ~ i.e. wind and wave induced air bubbles in the surface layer which wil.1 cause additional absorbtion 05 the transceived echo energy (LØVIK

&

DALEN 1979) .

The acoustic data are scrutinized and preprocessed both £or mapping purposes and for the computer aided estimation of all relevant biological quantities.

3.

Some resu=.

I

The period of investigation for the different years are:

1976: 1 February to 20 February 1977:

9 0

January to

4

February 1978: 30 January to

E5

March 1979:

2

February to 20 March.

In 1976 we had relatively short time for the investigations,

buk the weather was good. In 1977 the eastern part of the Barents Sea was badly investigated because of bad weather. In 1978 the weather was excellent and the whole area was well investigated.

In 1979 there was again trouble with the weather so that the eastern part of the area was not investågated. However, this did not matter this year because of the extreme cooling of the

water masses in the eastern area.

The geographical distribution of the different age groups were very similar from year to year. The results £rom 1978 can be used as exarnpbes (Pigs 3-9). For cod it seems that 2-3 years

**

old fish are most numerous in the eastern part of the Barents Sea (Figs

5

and 6), while the older cod have a more westerly distribution. However, in L979 the picture was quite different.

This winter the tounge of warm water which usually is situated from the Skolpen Bank to the Goose Bank and Novaya Zemlja

disappeared. East of about

3 6 O ~

all temperature observations

were below

OOC

from the surface to the bottom. This cooling

forced the fish consentrations to move westwards. Most of the young cod were in February 1979 found between 30'~ and 3 5 O ~ . The haddock prefer warmer water than the cod and are therefore

found more westerly than the cod (Figs

8

and 9).

Table

1

gives the estimated abundances of the year classes of cod in the different years. It shows that the I-group always are greatly underestimated and to some extent also the 11-group.

Fish older than

6

years are also underestimated because they are mostly distributed west of the investigated area. It is thus the abundances of 3 to 5 years old cod that are best estimated by this method in the Barents Sea. However, in 1977 the estimates of the three to four youngest year classes was not good because of the bad investigation of the eastern area.

The abundances of the different year classes found by the acous- tic method are in good agreement with the O-group indexes (Anon 1979). Both 1976 and 1974 are found to be weak year classes, while 1975 is strong. The 1973 year class is, however, found to be somewhat weaker than indicated by the O-group index,

Comparing the estimates from the acoustic method with the esti- mates from the VPA made by the Arctic Fisheries Working Graup

(ANON

1979), we find that the acoustic estimates are between 35%

and 70% of the VPA estimates except for the 1975 year class, For this year class the acoustic estimates are much higher than the VPA. Normally the acoustic method will underestimate the year class strength. This indicates that there are same errors in the input data of the VPA.

Table

2

gives the estimates of the abundances of the haddock year classes. Haddock older than

5

years are mainly distributed west of the investigated area and therefore almost lacking in our material.

Comparing the acoustic estimates with the VPA we find greater

variances than for cod and also higher percentages. However,

also for haddock the 1975 year class seems to be estimated to0

low in the VPA.

REFERENCES

ANON 1979. R e p o r t on t h e A r c t i c F i s h e r å e s WorIcIng Group#

C h a r l o t t e n l u n d , 2 6 May

-

1 J u n e . ^-Coun. ^P Meet. int.

-

Coun. E x p l o r . ^.-- Sea. 1 9 7 9 (G: 1 : ^1-35 j!$imeo>*

BLINDHEIM, J . and NAKKEN, O. 1971. Abundanee e s t i m a t i o n of t h e spawning L o f o t e n cod 194P, I n t . CounO E x p l o r . Sea, Coun. Meet. 1971 ^--p(B15) ^-: 1-9 & i m e o ] ~

BODHOLP, H. 1977, V a r i a n c e e r r o r i n e c h o i n t e g r a t o r s u t p u % .

-

I n Hydro-Acoustics i n F i s h e r i e s ^-PR e s e a r c h , ^- e d , & . W , M a r g e t t s

(Rapp* P.-V. R6un. Cons. i n t . E x p l o r . Mer 1'70, 1977),

CRAIG, R.E. and FOWBES, S. 1969. A s o n a r f o r Sish c o u n t i n g . F i s k D i r , S k r . S e r . HavUnders. ^p 1 5 : 2 1 0 - 2 1 9 .

CUSHING, D , H . 1 9 6 8 . U i r e c t e s t i m a t i o n o f a f f s h p o p u l a t i o n Bd. Canada, 2 5 ( 1 1 ) 2349- a c o u s t i c a 1 1 y . J . F i s h , R e s *

2364.

DALEN, J. 1979. A e o u s ~ c meth.ods f o r a s s e s s i n g iabundancec of dexncrsall f i s h stoclcs w i t h s p e c i a 1 a t t e n t i o n t o cod and haddock i n t h e l3aren.k.s Sea, F i s k l l i r . l__.-.-__--- C k r , S e r .

-

HavUnders ^-W

.

^{[1n -prep7}

DALEN,

J . ,

HYLEM, A . og SMEDSTAD, 0 , M . 1 9 7 7 a , A k u s t i s k mengde- m å l i n g a v t o r s k og h y s e i B a r e n t s h a v e t i F e b r u a r 1 9 7 6 : ~ Acsust%c abundance e s t i r n a t i o n a f eod and haddock i n

t h e B a r e n t s Sea i n F e b r u a r y 1 9 7 6 .

.

^m-----.P i s k e n Hav, 1 9 7 7 ( 2 ) :

--

3-15,

1 9 7 7 b , I n v e s t i g a t i a n s on derneraal f i s k i n t h e B a r e n t s Sea i n w i n t e r 1 9 7 7 , Coun. M e e - k , i a l t . C:oun,- .--^I---

Sea, 1977 ( F : 2 3 ) 1 - 1 9 [~limeo].

------p-

DALEN, J. og SMEDSTAD, O.M. 1979. Akustisk mengdemåling av torsk og hyse i Barentshavet vinteren 1978. Acoustic

abundance estimation of cod and haddock in the Barents Sea in winter 1978.

.

Fisken Hav., 1979 G n pint].

DOUBLEDAY, W.G. 1976. A pilot study of a survey design for

a

combined acoustic and other-trawl groundfish survey.

Coun. Meet. int. Coun. Explor. Sea, 1976(B:30):

1-15,

EHRENBERG, J.E. 1974. Two Applications for a Dual-Beam Transduc~

in Hydroacoustic Fish Assessment Systems. OCEAN174.

Proceeding of OCEANV74 IEEE International Conference on Engineering in the Ocean Environment, 1974.

EHRENBERG, J.E. and LYTLE, D.W. 1977. Some signal processing techniques for reducing the variance in acoustic stock abundance estimates. In - Hydro-Acoustics in Fisheries Research, ed. A.R. Margetts (Rapp. P.-v. Weun. Cons.

int. Explor. 170, 1977), 205-213.

EHRENBERG, J.E. and WEIMER, R.T. 1974. Effects of thresholds on th.e estimated fish seattering cross section obtaåned with

a

dual-beam transducer system, Appl. Phys. Lab., Univ. of Wash. Report No. 7421.

FOOTE, K.G. 1970. AnaLysis of empirical observations on the scatt.ering of sound by encaged aggregations of fish.

FickDir. Skr. Ser. HavUnders., 16: 423-456.

-- 1979a. Averaging of fish target strength functions.

1979b, On representing the Length Dependence o f Acoustic Target Strengths of Fish. ^PJ. Fåsh. Res. Bd. ^- Canada, @n press].

---

HANSEN, M . H . , H U R W I T Z , W.N, and MADOW, W.G. 1953, Sample s u r v e y methods and tl~eoacy, V o l . l , Wl.ley P1 Sons, I n c . New York 1 9 5 3 , 6 3 8 p.

JOHANNESON, K.A. and LOSSE, (3°F. 1977, Methodology of a c o u s t i c e s t i m a t i a n s o f f i s h abundance i n come UNDP/FAO re-

s o u r c e s u r v e y p r o j e c t s . I n Hydro-Acsustlcs _-- -m----- i n Fisheries R e s e a r c h ,

-

e d . A , R . M a r g e t t a (Tapp. P.-V. Rcun. Gans, i n t , E x p l o r . 1 7 0 , 1 9 7 7 ) , 296-318.

LOVE, ?.H. 1977, T a r g e t s t r e n g t h o f a n i n d i v i d u a l f i s h a t any a s p e c t . J . A c o u s t . Soc. Am. 6 2 : 1394-1403,

LØVIK, A. and DALEN, J . 1979. On t h e i n f l u e n c e of wind i n d u c e d b u b b l e s on e c h o i n t e g r a t i o n a u r v e y s

.

[In p r e p

.l .

MIDTTUN L. and NAKKEN, 0 . 1977. Come r e s u l t s of abundance esti- m a t i o n , S t u d l e s w i t h e c h o i n t e g r a t o r s . I n Hydro-

A c o u s t i c i n F i s h e r i e s R e s e a r c h , e d . _--w-- A . R , M a r g e t t s

---p

(Rapp. P.-W. R6un. c o n s . i n t o E x p l o r . 1 7 0 . 1 9 7 7 ) . 2 5 3 - 258.

NAKKEN, 0 . and DOmSNESS, A. 1975.

The

a p p l i c a t i o n of

an

e c h o i n t e g r a t i o n s y s t e m i n L n v e s t i g a t i o n s on

the

s k a c k

s t r e n g t h o f t h e B a r e n t s Sea cape1.i.n ( M a e l o t u s v i l l o s u s , M i i l l e r ) 1971-1974. I n t . Coun, ^W--.---- Fxpbor, ---.---P" Sea, Coun.

Meet. 1975 ( ~ ~ 2 5 ) : 1-9, @irneo.]

.

----p.---

NICHERCON, T , B . and DOWD, R . G . 197'7. Design and s p e r a t i o n of

'S

s u r v e y p a t t e r n s f o r d e m e r s a l f i s h e s u s i n g t h e compu- t e r i z e d e e b o c o u n t i n g systenn, g p p . P.-v. -- ----.- R6un0 Coxass a n t , E x p l o r . M e r , , 170: 2 3 2 - 2 3 6 ,

-P

RØTTINGEN, I . 1 9 9 6 , On t h e r e l a t i o n b e t w e e ~ i e c h o intensity and f i s h d e n s i l y , P i s k D i r , S k r . S e r . _- ----_._-.--~.-.--- HavUnders., 16: 301- 314,

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THORNE, R.E. 1977. A New Digital Hydro-acoustic Data Processor and Some Observations in Herring in Alaska.

-- J,

FLsh.

Res. Bd. Canada 34: 2288-2294,

THQRNE, R.E., REEVES, J.E. and MILLIKAN, A.E. 1971. Estimatlon of the hake population in Port Susan, Washington, using an echo integrator. J, Fish. Res. Bd, Canada.

28 (9) : 1275-1284.

THORNE, R.E. and WOODEY, J.C. 1970. Stock assessment

by

echo integration and its application to juvenile sockeye ^t

1

salmon in Lake Washington. Univ. of Wash, Coll. FPsh.

Circular no, 70-2, 1970.

WEIMER, R.T. and EHRENBERG, J.E. 1975. Analysis of Threshold- Induse Bias Inherent in Acoustic Scattering Cross- Section Estimates of individual Fish. J. Fish, Res,

Bd.

Canada. 32 (12) : 2547-2551.

cp

YATES,

F.

1965. Sampling methods for censuser and surveys.

Table 1. Estimates on year class abundances of cod (thousands), and the relative

l

abundance compared with the VPA (ANON 1979).

Year class

-

Table 2. Estimates on year class abundances of haddock (thousands), and the relative

l

abundance compared with the VPA (ANON 1979).

Y e s

D i s t r i b u t e M o n

1

s p e c i e s ; M i ,

_I

C a l c u l a t e lengtti E s t a b l i s h a g e -

d i s t r i b u t i o n s f o r l e n g t h k e y s f o r e a c h s p e c i e s e a c h s p e c i e s

n s f o r e a c h

C a l c u l a t e n u r n b e r

I

of f i s h e s w i t h i n of f i s h e s w i t h i n age(

l e n g t h g r o u p s of a n d l e n g t h g r o u p s

e a c h s p e c i e s f o r of e a c h s p e c i e s f o r

e a c h siatistica1 a r e a

I

i

- -.- --

I

i

C a l c u l a t e t o t a l

E s t a b l i s h w e i g h t - n u m o e r of f i s h e

o f s p e c i f i r d c a t e - l e n g t h r e l a t i o n -

. s u r v e v e d a r e a

C a l c u l a t e t o t a l w e i g h t of f i s h - of s p e c i f i e d c a t e g o r i e s f o r t o t a l s u r v e y e d

r--I

Fig.

x.

Block diagram on the prosesc of echo integration on assessing abundances of fish stocks.

Acoiistic signals

T r a n s d u c e r T r a n s m i t t e

Time varied

I

^t

Fig.

t .

Block diagram of the instruments and sampling equipment.

Fig. 3. Survey routes, hydrographical stations, and trawl stations in February

-

March 1978. 1) Ice boarder, 2) Bathytermograph, 3 ) Nansen bottles, 4) CTD-sonde, 5) pelagic trawl, 6) bottom trawl. 7) Juday-net.

Fig. 4. Echo abundance of cod and haddock in 1978.

Fig. 5. Distribution of 2 years old cod in 1000 per (nautical mile) in 1978. 2

Fig. 6. Distribution of 3 years old cod in 1000 per (nautical mi1e12 in 1978.

Fig. 7. Distribution of 4 years and older cod in 1000 per (nautical mile) in 1978.

F i g . 8. D i s t r i b u t i o n o f 2 y e a r s o l d haddock i n 1000 p e r ( n a u t i c a l m i l e ) i n 1978.

F i g . 9. D i s t r i b u t i o n o f 3 y e a r s and o l d e r haddock i n 1000 p e r ( n a u t i c a l m i l e ) i n 1978.