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E Thhs p a p e r n o t t o c i t e d w i t h o u t p r i o r r e f e r e n c e t o t h e a u t h o r .

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A c o u s t i c Methods i n

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No. 38

I SOME RESULTS OF ABUNDANCE ESTIMATION STUDIES WITH ECHO

INTEGRATORS

L a r s M i d t t u n and Odd Nakken I n s t i t u t e of Marine R e s e a r c h

Bergen, Norway

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

The u s e o f a c o u s t i c methods f o r f i s h abundance e s t i m a t i o n i s now i n a s t a t e o f p r o g r e s s i v e improvement. A l r e a d y a t p r e s e n t s u c h methods h a v e proved t o g i v e v a l u a b l e i n f o r m a t i o n s on

s t o c k s i z e o f b o t h e x p l o i t e d and u n e x p l o i t e d f i s h p o p u l a t i o n s (BLINDHEIM and NAKKEN 1971, DRAGESUND 1970a and b , DRAGESUND e t a i . 1 9 7 3 , M I D T T U N and NAKKEN 1 9 7 2 , THORNE and WOODEY 1 9 7 0 , THORNE e t a l . 1 9 7 1 ) . The s u c c e s s o f a n a c o u s t i c abundance e s t i - m a t i o n depends s t r o n g l y on t h e b e h a v i o u r and t h 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 q u e s t i o n . The b e s t c o n d i t i o n s f o r t h e e c h o i n t e g r a t i o n t e c h n i q u e a p p l i e d - b y t h e I n s t i t u t e o f Marine R e - s e a r c h , Bergen, a r e g i v e n when t h e f i s h s t o c k i n q u e s t i o n i s d i c t r i b u t e d w i t h i n a d e f i n e d a r e a , unmixed w i t h o t h e r s p e c i e s and i n c o n t i n o u s s c a t t e r i n g l a y e r s a t m o d e r a t e d e p t h s i n mid- w a t e r . C o n d i t i o n s a r e more c o m p l i c a t e d when t h e f i s h a r e

forming d e n s e , s c h o o l s o r a r e mixed w i t h o t h e r s p e c i e s , and

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iinfavourable s i t u a t i o n s a r e found wlien the f is11 a r e d i s t r i b u t e d c l a s e t o t h e bottom o r n e a r t h e s e a s u r f a c e .

;n p r a c t i c a l a p p l i c a t i o n i t i s t h s i e f o r e o f t h e g r e a t e s t importance t o t a k e i n t o c o n s i d e r a t i o n f i s h behaviour and t o c a r r y o u t t h e s u r v e y s aiming a t an abundance e s t i m a t i o n when c o n d i t i o n s a r e a s f a v o u r a b l e a s p o s s i b l e . T h i s maY be r e l a t e d

t o s p e c i a l spawning o r f e e d i n g s e a s o n s .

I n t h e f o l l o w i n g we s h a l l g i v e two examples of a c o u s t i c f i s h s t o c k abundance e s t i m a t i o n . The f i r s t one i s t h e e s t i m a t i o n of an e x p b i t e d f i s h s t o c k , namely t h e B a r e n t s Sea C a p e l i n . The second example i s s i z e measurement of t h e u n e x p l o i t e d

b l u e w h i t i n g s t o c k . The s u r v e y s of both s p e c i e s a r e u n d e r t a k e n a t t h e time of t h e y e a r when t h e c o n d i t i o n s seem t o f a v o u r

t h e t e c h n i q u e used.

ME THOD

The method a p p l i e d i s d e s c r i b e d by FORBZS _and NAKKEN ( 1 9 7 2 ) . Wllen a 2 0 l o g R + Z C : R time v a r i e d g a i n i s ilsed, t h e i n t e g r a t e d

( ~ r averaged) echo intensi-;:y is p r o p o r t i c : ~ z L to t h e number c f £i.oh p e r uni t a r e a ,

w h e r e p A i s t h e number of f i s h p e r u n i t a r e a and M i s t h e i n t e g r a t e d echo i n t e n s i t y . I n p r a c t i c a l app!.ication M i s accumulated o v e r each r i a u t i c a l m i l e s a i l e d . The c o n s t a n t b c a n be r e g a r d e d a s a t r e s h o l d d e n s i t y below which no c o n t r i - ^, b u t i o n t o t h e i n t e g r a t e d echo i n t e n s i t y o c ~ - u r s .

When aiming a t a,n e s t i m a t e o f s t o c k s i z e , t h e two main p o i n t s t o be c o n s i d e r e d a s e :

1. Detexmination of m and b i n egilation ( I ) , s o t h a t r e l a t i v e d e n s i t i e s , Y , can be ~ ~ n v e r t e d t o a h z o l u t e d e n s i t i e s

y

A '

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L , C o l l e c t o b s e r v a t i o n s o f M t h r o u g h o u t t h e t o - t a l a r e a o f

~ 3 i s t r i b u t i o n f o r t h e stoclc u n d e r c o n s i d e r a t i o n .

I

I n o r d e r t o d e t e r m i n e m and b i n e q u a t i o n ( I )

,

c o r r e s p o n d i n g v a l u e s o f ? and M a r e needed. THORNE and WOODEY (1970) ob-

A

t a i n e d e s t i m a t e s o f P A by p e l a g i c t r a w l i n g . M I D T T U N and NAKKEN (1971) s u g g e s t e d a d i f f e r e n t way o f a p p r o a c h : When t h e f i s h s p e c i e s i n q u e s t i o n a r e r e c o r d e d a s i n d i v i d u a l s t h e number o f f i s h t r a c e s which a p p e a r on t h e r e c o r d i n g p a p e r a r e c o u n t e d . A s e r i e s o f o b s e r v a t i o n s o f M and N (number o f f i s h w i t h i n t h e d e p t h i n t e r v a l o f i n t e g r a t i o n ) a r e t h e n o b t a i n e d . The number o f f i s h , N, a r e d i v i d e d by t h e s a m p l i n g a r e a o f t h e s o u n d e r , w i t h i n t h e i n t e r v a l o f c o u n t i n g and i n t e g r a t i o n ,

g i v i n g

PA.

The s a m p l i n g a r e a o f t h e s o u n d e r c a n be found i n two ways. When t h e e c h o s o u n d e r u s e d , h a s a c a l i b r a t e d r e c o r d e r g a i n s w i t c h , t h e d i f f e r e n c e between t h e r e c o r d e r g a i n a t normal s e t t i n g and t h e g a i n g i v i n g a j u s t v i s i b l e marking o f t h e p a p e r f o r t h e f i s h i n q u e s t i o n , a r e u s e d t o

f i n d t h e s a m p l i n g a n g l e i n t h e d i r e c t i v i t y d i a g r a m . The

s a m p l i n g a n g l e ( d e t e c t i o n a n g l e ) c a n a l s o be found by c o u n t i n g t h e number of e c h o e s from e a c h f i s h (MIDTTUN and NAKKEN 1 9 7 1 ) . I t s h o u l d be n o t e d t h a t i t i s n o t n e c e s s a r y t o do t h e o b s e r - v a t i o n s o f f A and M w i t h t h e same e c h o s o u n d e r . When u s i n g two s o u n d e r s , t h e one which i s used t o o b t a i n e s t i m a t e s o f N s h o u l d h a v e a 40 l o g R + 2 d R time v a r i e d g a i n 1 , a s t h i s w i l l s i m p l i f y t h e l a t e r c a l c u l a t i o n s .

The t o t a l amount o f f i s h , T , o f a g i v e n s p e c i e s w i t h i n a n a r e a , A , c a n be w r i t t e n

The most c o n v e n i e n t way t o c a l c u l a t e T i s t h e r e f o r e t o do a n a r e a i n t e g r a t i o n o f 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

(x

_A ^MdA)

.

Then a r e l a t i v e measure o f f i s h abundance i s a c h i e v e d . I f e s t i m a k e s of m i s a v a i l a b l e , a b s o l u t e e s t i m a t e s o f f i s h abun- d a n c e , T , c a n be c a l c u l a t e d .

(4)

mien mixed r e c o r d i ~ g s o c c u r , i t i s n e c e s s a r y t o d i s c r i m i n a t e bekween t h e s p e c i e s , and f i n d t h e c o n t r i b u t i o n t o t h e i n t e - g r a t o r r e c o r d i n g s from each s p e c i e s . On board t h e Norwegian r e s e a r c h v e s s e l s i t i s t h e r e f o r e r o u t i n e p r o c e d u r e t o i d e n t i f y t h e echo r e c o r d i n g s by t r a w l c a t c h e s whenever t h e r e c o r d i n g s a p p a r e n t l y change.,

.

^{I t}^{i s}a l s o r o u t i n e t o s c r u t i n i z e t h e a c o u s t i c d a t a each day and t o d e c i d e which s p e c i e s of f i s h have c o n t r i b u t e d t o t h e i n t e g r a t e d echo i n t e n s i t y . T h i s

a n a l y s i s i s done by e x p e r i e n c e d p e o p l e on b a s i s o f examination of t r a w l c a t c h e s and echo r e c o r d i n g s .

During t h e i n v e s t i g a t i o n s r e p o r t e d i n t h i s p a p e r t h e 6 echo i n t e g r a t o r s onboard t h e G.O.Sars were connected t o t h e Simrad s c i e n t i f i c sounder E K 3 8 . The 5 upper c h a n n e l s were normaily a d j u s t e d t o i n t e g r a t e i n 50 m d e p t h s l i c e s down t o 250 m , w h i l e t h e 6 t h c h a n n e l worked between 250 and 449 m . During

t h e b l u e w h i t i n g s u r v e y s a s l i g h t l y d i f f e r e n t s e t t i n g was adopted i n o r d e r t o i n c l u d e a l s o t h e d e e p e r p a r t of t h e f i s h l a y e r s . A l l c h a n n e l s have a bottom s t o p f u n c t i o n which s t o p s t h e i n t e g r a t i o n j u s t above bottom.

The i n t e g r a t e d echo i n t e n s i t i e s were r e a d p e r n a u t i c a l m i l e , averaged f o r each 25 n a u t i c a l m i l e s ( r u n n i n g means) and

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

RESULTS AND DISCUSSION

D e t e r m i n a t i o n of m and b.

Table 1 and F i g s 1-2 show come r e s u l t s of t h e d e t e r m i n a t i o n of t h e c o n s t a n t s m and b f o r b l u e w h i t i n g and c a p e l i n . The

d i f f e r e n c e i n t h e c o n s t a n t b f o r b l u e w h i t i n g a r e caused by t h e d i f f i c u l t y of d e f i n i n g ' t h e " j u s t v i s i b l e mark" on t h e r e c o r d i n g p a p e r . The d i f f e r e n c e s i n t h e s l o p e s , m , from J a n u a r y t o March a r e h a r d e r t o e x p l a i n . I n March a r e l a t i v e narrow range o f f i s h d e n s i t y o b s e r v a t i o n s were used and i t might be t h a t v a l u e s from J a n u a r y a r e t h e most r e l i a b l e . The mean of a P P t h r e e s l o p e s i s 1570 individuals/mm p e r n a u t i c a l m i l e e n a u t i c a 1 m i l e 2 c o r r e s p o n d i n g t o approxirna t e l y 0 . 4 t o n / mm p e r n a u t i c a l m i l e - n a u t i c a l m i l e 2 ^e

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t a r g e t s t r e n g t h a t s e a o f -45 dB. F u r t h e r , t h e r e s u l t s ob- t a i n e d by NAKKEN and OLSEN (1973) i n d i c a t e t h a t t h e b l u e

w h i t i n g h a s a d o r s a l a s p e c t t a r g e t s t r e n g t h which i s s l i g h t l y lower t h a n t h a t o f cod. The d i f f e r e n c e between b l u e w h i t i n g and c a p e l i n which a p p e a r s from o u r o b s e r v a t i o n s i s t h e r e f o r e

- 1

r e a s o n a b l e . S i n c e t h e c a l i b r a t i o n r e s u l t s f o r t h e G.O.Sars show ^- no s i g n i f i c a n t d i f f e r e n c e s in. equipment c h a r a c t e r i s t i c s between 1971 and 1973 t h e r e s u l t s o b t a i n e d w i t h i n t h i s p e r i o d a r e a

comparable,

The b i a s i n t r o d u c e d i n t h e e s t i m a t e s of m, by o t h e r s p e c i e s

l

mixing w i t h t h e b l u e w h i t i n g and c a p e l i n d u r i n g t h e c a l i b r a t i o n

l l

r u n s i s b e l i e v e d t o be s m a l l . P e l a g i c . t r a w l c a t c h e s i n d i c a t e d

l

t h a t b o t h s p e c i e s were i n " p u r e " c o n s e n t r a t i o n s

.

I t i s s e e n from e q u a t i o n (11) t h a t even i f t h e magnitude of t h e ^l c o n s t a n t b exceeds t h a t o f m , b w i l l h a v e a q u i t e i n s i g n i f i c a n t

i n f l u e n c e on t h e t o t a l amount o f f i s h which a p p e a r i n t h e

d i s t r i b u t i o n c h a r t s ( F i g s 3 and 4 ) . I n c a l c u l a t i n g t h e amount of f i s h p r e s e n t 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 e f o l l o w i n g

forms o f e q u a t i o n I1 were t h e r e f o r e used:

Blue whi t i n g : T = 0 , 4

j

MdA ( t o n ) A

C a p e l i n : T = 1 , 4 *

/

^{M ~ A}( t o n )

A

i

T h e magnitude o f t h e v a r i a n c e s a s s o c i a t e d w i t h t h e e s t i m a t e s o b t a i n e d from e q u a t i o n s (111) a r e t o some e x t e n t unlcnown,

and a s t h e v a r i a n c e w i l l v a r y from s p e c i e s t o s p e c i e s and from

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year t o y e a r t h e e s t i m a t e s f o r t h e two s p e c i e s w i l l be d i s c u s s e d ^I s e p e r a t e l y .

C a p e l i n

The r e s u l t i n g abundance e s t i m a t e s from t h e two c a p e l i n s u r v e y s a r e t a b u l a t e d below

1 9 7 1 1972

R e l .

.(l

^{M ~ A )} ^Abs.^{( T )}

mm- (nm) 2 t o n

R e l .

(j

M ~ A ) Abs. ( T ) mm- (nm) ² t o n

l l

The a b s o l u t e v a l u e s i n t h i s t a b l e a r e l o w e r t h a n t h o s e r e p o r t e d 1 by M I D T T U N and NAKKEN (1972)

.

The r e a s o n f o r t h i s d i s c r e p a n c y 1 i s t h e lower v a l u e s o f m. The v a l u e s o f m r e p o r t e d i n t h e ^I^l

11

above mentioned p a p e r were found a t by t a k i n g t h e r a t i o s be-

i

j

tween t h e a r i t h m e t i c means o f

R

^and^M i n t a b l e 1. The magni- t u d e o f t h i s r a t i o i s d e p e n d i n g on t h e magnitude o f b and t h e p r o c e d u r e may l e a d t o s e r i o u s e r r o r s e s p e c i a l l y when many low

11 l

v a l u e s o f M and

gA

a r e u s e d .

l

The c a p e l i n s u r v e y i n 1 9 7 1 ( F i g . 3 ) were c a r r i e d o u t u n d e r

f a v o u r a b l e c o n d i t i o n s . The f i s h were d i s t r i b u t e d o v e r wide \

I

a r e a s i n s c a t t e r i n g l a y e r s and t h i n s c h o o l s w e l l above t h e bottom and p r a c t i c a l l y unmixed w i t h o t h e r s p e c i e s

.

E x c l u d i n g

a l l u n c e r t a i n , M-values ( d i s r e g a r d i n g t h e r e a s o n f o r t h e

I!

u n c e r t a i n t i ) , t h e e s t i m a t e o f T d e c r e a s e d w i t h e i g h t p e r c e n t .

The v a l u e o f m f o r c a p e l i n was o b t a i n e d d u r i n g t h i s s u r v e y and ^'I_li i s b a s e d on mean v a l u e s o f N from f o u r d i f f e r e n t o b s e r v e r s .

'i

When m i s c a l c u l a t e d f o r e a c h o f t h e s e f o u r s e t s o f d a t a t h e ^'i

8 I h i g h e s t and l o w e s t v a l u e s w i l l be 1 , 6 and 1 , 3 ton/mm p e r ^I'l l n a u t i c a l m i l e - s q u a r e n a u t i c a l m i l e r e s p e c i t v e l y . Using t h e

#i

I l

above i n f o r m a t i o n t o c a l c u l a t e a n u p p e r and lower v a l u e s o f T one a r r i v e s a t 7 , 71-106 t o n and 5 , 7 0 1 0 ~ t o n r e s p e c t i v e l y .

The p e r c e n t a g e m a t u r e f i s h i n t h e t r a w l c a t c h e s were found t o ^,

/

be 35-40 p e r c e n t (DRAGESUND and NAKKEN 1972) and a c c o r d i n g l y ^lI I l

(7)

t h e spawning s toclc ainoun t e d t o somewhat be tween 2 . 0

l o 6

t o n s ::~ld 3 , 1 0 1 0 6 t o n s , These f i y u ~ : e s c o i n c i d e v e r y w e l l w i t h

cslililalies kroin egg and l a r v a l s u r v e y s arid t a y g i n g e x p e r i n i c n t s (DRAGESUND, GJ@SErrER arid MONCTAD 1373)

.

The c o n d i t i o n s f o r d o i n g echo i n t e g r a t i o n i n August 1972 ( ~ i g . 3 ) were more d i f f i c u l t t h a n i n September 1971. I n t h e w e s t e r n p a r t o f t h e a r e a t h e c a p e l i n was d i s t r i b u t e d c l o s e t o

t h e bottom o v e r wide a r e a s . F a r t h e r t o t h e e a s t , c a p e l i n and p o l a r cod v e r y o f t e n were e x t e n s i v e l y mixed, and a t t i m e s i t

was i m p o s s i b l e t o d i s c r i m i n a t e between t h e two s p e c i e s on t h e ^l r e c o r d i n g p a p e r . The p o r t i o n m a t u r e f i s h i n t h e t r a w l c a t c h e s

were e s t i m a t e d t o be 75-50 p e r c e n t (GJ~SÆTER e t a l . 1972)

r e s u l t i n g i n e s t i m a t e s o f 6 , 8 0 1 0 ~ tons and 7 , 2 4 1 0 ~ t o n s f o r

l

t h e spawning stoclc. O b s e r v a t i o n s made by MONSTAD and KOVALYOV

l

(1973) i n November-December 1972 i n d i c a t e d a much lower p e r - c e n t a g e o f m a t u r e f i s h (30-40 p e r c e n t ) a m a t t e r which w i l l

r e d u c e t h e e s t i m a t e s o f t h e spawning stoclc c o n s i d e r a b l y .

1

Blue w h i t i n g

S u r v e y s f o r e s t i m a t i n g t h e spawning s t o c k o f t h e b l u e w h i t i n g

l

w e r e c a r r i e d o u t i n 1972 and i n 1973, b o t h y e a r s j u s t b e f o r e o r d u r i n g t h e spawning s e a s o n , March-April. I n t h i s p e r i o d t h e b l u e w h i t i n g i s d i s t r i b u t e d w i t h i n a r a t h e r w e l l d e f i n e d a r e a n o r t h w e s t o f t h e B r i t i s h I s l e s ( F i g . 4 ) . The f i s h a r e m o s t l y c o n s e n t r a t e d i n p e l a g i c s c a t t e r i n g l a y e r s a t d e p t h s between a b o u t 350 and 550 m e t r e s and unmixed w i t h o t h e r s p e c i e s . Both i n 1972 and i n 1973 t h e r e were a c t u a l l y

c a r r i e d o u t two i n d e p e n d a n t s u r v e y s e a c h y e a r and t h e r e s u l t s ^l a r e p r e s 5 n t e d below:

I

R e l . (/M&) Abs. ( T )

mrno (nm) 2 t o n

x) e x c l u d i n g t h e R o c k a l l Rank and t h e P o r c u p i n e Banlc s o u t h o f 5 4 O l a t i t u d e

.

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^{i s}a g r e a t p l e a s u r e t o acknowledge t h e i n v a l u a b l e h e l p o f

I

S.H. J a k u p s s t o v u w i t h whom w e h a v e c o o p e r a t e d d u r i n g t h e work on b l u e w h i t i n g .

REFERENCES

BLINDHEIM,J. and NAKKEN,O. 1971. Abundance e s t i m a t i o n of t h e ^l I spawning L o f o t e n cod 1971. Coun.Meet.int.Coun.

DRAGESUND,O. 1970 a . D i s t r i b u t i ~ n ~ a b u n d a n c e and m o r t a l i t y o f

l

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 h a r e n g u s h inne) i n r e l a t i o n t o

s u b s e q u e n t y e a r - c l a s s s t r e n g t h . ~ i s k . ~ i r . S k r . S e r .

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(ed.) 1970 b. International O-group f ish surveys

' ^I

-.

in the Barents Sea 1965-1968. Coop.res.rep.int.

Coun.Explor.Sea, 1970, Ser.A (18) : 1-81. fiklirneo].

DRAGESUND, O. and NAKKEN, O. 1972. Lodde- og polartorskunder- s$kelser i Barentshavet

i

august-september 1971.

[capelin and Polar cod investigations in the Barents Sea in August-September 19711. Fiskets Gang, 58: 145-148.

I

l DRAGESUM),O., GJ@SÆTER,J. and MONSTADIT. 1973. Estimates of

I stock size and reproduction of the Barents Sea

l

capelin in 1970-1972. Fisk.Dir.Skr.~er.HavUnders., (in press).

l

I FORBES,S.T. and NAKKENJO. (ed.) 1972. Manual of methods for l ! fisheries resource survey and appraisal. Part 2.

I FAO Manual in Fisheries Science, No.5, 1972.

I

I GJ@SÆPERIJ.l MIDTTUNIL., MONSTADIT., NAKKEN,O., SMESTADIO*M*,

I

SÆTRE,R. and ULLTANG,@. 1972. UndersØkelser av fiskeforekomster i Barentshavet og ved Spitsbergen i august-seitmeber 1972. [~nvesti~ations on fish distribution and abundace in the Barents Sea and

I

off Spitsbergen in August-September 19723

.

Fiskets Gang,58: 1010-1021.

~

ⁱ

JAKUPSST0VU.S.H. and MIDTTUNIL. 1972. ~olmuleundersØkelser nordvest for De Britiske @yer i februar-mars 1972.

[~iue whiting surveys northwest of the ~ r i t i s h Isles in February-March 19721

.

Fiskets Gang, 58.

428-433.

M1DTTUN.L. and NAKKEN.0. 1971. On acoustic identification.

sizing and abundance estimation of fish. Fisk.qir.

Skr.Ser.HavUnders., 16: 36-48.

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MIDTTUNIL. and NAKKEN,O. 1972. Application of acoustic stock

I

abundance estimation on capelin and blue whiting.

Coun.Mect .Int .Coun.Explor .Sea 1972 ( B : 16) 1-8,

-

6 Figs. [~imeo]

.

MONSTADIT. and KOVALYOV,S,M. 1973. Loddeunders$kelser i

l

Barentshavet i november-desember 1972. [capelin

I

investigation in the Barents Sea in November- December 19721. Fiskets Gang, 59: 350-353.

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

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

l

integration and its application to ~uvenile

Sockeye salmon in Lake Washington. University of Washington, College of Fisheries, Circular no.7012,

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T a b l e 1. Echo i n t e g r a t o r d e f l e c t i o n s M (mm p e r n a u t i c a l m i l e ) and c o r r e s p o n d i n g d e n s i t i e s o f f i s h

PA

(number o f f i s h p e r s q u a r e m i l e ) o b t a i n e d by;counting o f f i s h t r a c e s ,N. on t h e r e c o r d i n g p a p e r . N and N2 a r e o b t a i n e d from t h e same f i s h t r a c e s by t h e p e r s o n e l on

1

d i f f e r e n t watches. The r e s u l t s o f a l e a s t mean s q u a r e r e g r e s s i o n ,

PA

⁼m ~ + b , a r e shown a t t h e bottom o f t h e t a b l e .

J a n . 1973

Blue whi t i n g

March 1973

C a p e l i n S e p t . 1971

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/ /

LOO-

300 -

200

-

INTEGRATED ECHO INTENSITY, M,mm PER NAUTICAL MILE ^p

F i g . 1. C o r r e s p o n d i n g v a l u e s o f i n t e g r a t e d e c h o i n t e n s i t y , M , and f i s h d e n s i t y ,

P A ,

( c a l c u l a t e d from c o u n t s on t h e r e c o r d i n g p a p e r ) f o r b l u e w h i t i n g . , 1) and 2 ) e s t i m a t e s from two d i f f e r e n t w a t c h e s c o u n t i n g t h e same r e c o r d i n g p a p e r i n J a n u a r y 1 9 7 3 , 3 ) e s t i m a t e s from March 1973.

' /

F i g . 2 . C o r r e s p o n d i n g v a l u e s o f i n t e g r a t e d e c h o i n t e n s i t y , M ,

t and f i s h d e n s i t y ,

P A ,

( c a l c u l a t e d from c o u n t s on t h e

l

r e c o r d i n g p a p e r ) f o r c a p e l i n .

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