1970_B_07.pdf (674.0Kb)

: 3kp.f.aration ~f the Ses

"d

1. INTRODUCTION.

When f i s h t a r g e t s a r e r e c o r d e d w i t h an echo sounder, t h r e e main q u e s t i o n s a r i s e :

1, What k i n d o f f i s h i s i t ?

2 .

mat

i s t h e s i z e o f t h e f i s h ?

3 . What i s t h e f i s h d e n s i t y , i . e . number of f i s h p e r u n i t volume o r p e r u n i t area ?

One o f t h e main problems i n a c o u s t i c f i s h r e c o r d i n g s i s r e l a t e d tro t h e fgct q u e s t i o n . So f a r i d e n t i f i c a t i o n has been done b y c a p t u r e o r underwater photographying ( P a r r i s h and C r a i g 1969) and a l s o t o sone e x t e n t by r e c o g n i t i o n of traces on the r e c o r d i n g p a p e r . While t h e two f i r s t methods are d i f f i c u l t and o f t e n time consuming, t h e t h i r d depends on t h e e x p e r i e n c e and s k i l l o f the o b s e r v e r , and t h e r e a r e no g e n e r a l r u l e s which have been a p p l i e d f o r an a c o u s t i c i d e n t i f i c a t i o n .

I n f o r m a t i o n on t h e s i z e of t h e r e c o r d e d f i s h c a n be o b t a i n e d f r o m knowledge o f t a r g e t s t r e n g t h which may b e found by a n a n a l y z i s of t h e r e c e i v e d echo s i g n a l s (Cushing 1968, ~ r a i g and Forbes 1969).

The t h i r d problem c o n c e r n i n g t h e f i s h d e n s i t y c a n b e r e g a r d e d a s c o n s i s t i n g o f two p a r t s . F i r s t i y , t h e r e i s t h e q u e s t i o n o f c o u n t i n g o r measuring t h e numbers o f f i s h d e t e c t e d , and secondly, t h a t of

f i n d i n g t h e sampling volume.

The p r e s e n t p a p e r a i m s a t a d i r e c t a c o u s t i c i d e n t i f i c a t i o n and sizing of the r e c o r d e d f i s h , ~t f u r t h e r d e s c r i b e s a method for abundance e s t i m a t i o n by t h e a p p l i c a t i o n o f an echo i n t e g r a t o r .

x) Institute of Ilarine Research, Eordnespazken 2,

Bergen, Norway.

2 , IBENTIFICATION AND SIZING.

The d e t e c t i o n s e c t o r angle,

Wnen the sounding s h i p passes over an i n d i v i d u a l t a r g e t , t h e sector angle,

,

within which the t a r g e t i s d e t e c t a b l e , can be determined by counting t h e number of echoes received from i t during sucessive

transmissions (Fig. l),

= 2 a r c t g ^v ( n i l ) 2 D . p

v i s the speed of the s h i p i n cm/sec.

n i s t h e number of echoes received from t h e t a r g e t . D i s t h e depth of the t a r g e t i n cm,

p i s t h e r e p e t i t i o n r a t e of t h e sounder i n pings/sec.

If t h e t a r g e t i s a sphere with a s p h e r i c a l r e f l e c t i v i t y p a t t e r n and passes through a c i r c u l a r beam a number of times a t d i f f e r e n t d i s -

tances from the a c o u s t i c a x i s , t h e frequency d i s t r i b u t i o n of w i l l be a s shown i n Fig. 3 D. The maximum value of

9

⁽

9

^max)

occurs when the t a r g e t passes through the beam c e n t e r . The value

of

<p

max depends on the d i r e c t i v i t y of t h e transducer and t h e

t a r g e t s t r e n g t h ,

Fish t a r g e t s however, do n o t r e f l e c t cound a s does a sphere, The t a r g e t s t r e n g t h of f i s h v a r i e s with t h e i r o r i e n t a t i o n r e l a t i v e t o t h e a c o u s t i c a x i s (Midttun and Hoff 1962, H a s l e t t 1962 and 1965, Love 1969), The d o r s a l - l a t e r a l a s p e c t t a r g e t s t r e n g t h may be a s much as 2 0 db h i g h e r than t h e h e a d - t a i l a s p e c t t a r g e t s t r e n g t h . Schematically t h e t a r g e t s t r e n g t h of an " i d e a l " f i s h can be re-

-. presented a s a three-dimensional p o l a r diagram a s shown i n Fig. 2 . We s h a l l now t r y t o f i n d t h e frequency d i s t r i b u t i o n of t h e d e t e c t i o n

s e c t o r angle

9

when t h e " i d e a l t 1 f i s h passes through our c i r c u l a r beam with d i f f e r e n t h o r i z o n t a l o r i e n t a t i o n s and a t d i f f e r e n t d i s -

tances from t h e a c o u s t i c a x i s . W e assume t h e maximum t a r g e t s t r e n g t h of our f i s h t o be equal t o t h a t of t h e above mentioned sphere.

The maximum angle, y m a x , w i l l occur when the f i s h passes through the c e n t e r of the beam and i s o r i e n t a t e d with i t s long a x i s a t a r i g h t angle t o t h e course l i n e . Then ?max i s t h e same a s f o r the sphere above,

The a n g l e o f a f i s h p a s s i n g through t h e beam c e n t e r w i t h i t s long a x i s p a r a l l e l t o t h e c o u r s e l i n e w i i l be s m a l l e r due t o t h e v a r i a t i o n i n t a r g e t s t r e n g t h (Fig. 2 ) . T h i s v a l u e o f

(P

i s c a l l e d

t h e f i s h a n g l e ,

6 ^.

Thus, t h e a r e a w i t h i n which t h e v e r t i c a l ^l1 looking" c i r c u l a r t r a n s - d u c e r c a n " s e e n t h e " i d e a l t 1 f i s h i s formed a p p r o x i m a t e l y a s an e l l i p s e ( F i g . 4 ) , which a x i s a r e g i v e n by

a = 2 D t g

g

^max

2 ^I b = 2 D t g

*

^(2,

The d e t e c t i o n s e c t o r a n g l e

<P

^{i s} g i v e n by

where i s t h e l e n g t h o f an a r b i t r a r i l y chosen c h o r d o f the

e l l i p s e . I n o r d e r t o e l i m i n a t e t h e d e p t h D,

4

^{i s} e x p r e s s e d i n p a r t s o f t h e l o n g a x i s , a .

For p r a c t i c a l a p p l i c a t i o n s ( 2 ) and ( 4 ) c a n be w r i t t e n

- =

k 4

^b

a

F

^and

- _,

_a

_{, p}

₌

_- f f

The f r e q u e n c y d i s t r i b u t i o n o f

4

^{c a n be} e x p r e s s e d i n : terms o f

y / ?

^{max o r}

d/a. 8

^{i s} a f u n c t i o n o f ^W and

X ,

where ⁰⁽

i s t h e a n g l e between t h e long a x i s o f t h e f i s h and t h e c o u r s e l i n e , and i s t h e h o r i z o n t a l component o f t h e d i s t a n c e from t h e c o u r s e l i n e t o t h e f i s h ( F i g , 4 ) . I£ t h e t r a n s d u c e r i s con- s i d e r e d as o r i g o and t h e c o u r s e l i n e as the y

-

a x i s , we w i l l ^get t h e f o l l o w i n g e q u a t i a n f o r t h e e l l i p s e

2 ⁷

[(x-

X

c o s W

-

y s i n d l

-l-

ry

^{c o s e}

⁺

^(x-

^X

) s i n % = l

9 C )

and

8

^{= yl}

-

^{y 2 f o r x = O} ( 6 1

This qj-ves I ^d I

2 b sin

'<

a cos

d + a

b

Table 1 shows &/a as a function of 0< and

-

X ^{for three}

v a l u e s of b/a. Frequency distributions of &a! is obtained from these tables and shown in Fig, 3, The distribution have marked peaks when -t? equais b or equals

i/$ .

Consequently the fish

angle,

a

^{can be} found when

50

max is known. In Table 2 are listed frequency distributions of

p/j-@

max for different values of

f i / F

^max.

When all the fish recorded have the same fish angle,

SPf ^,

^and

are distributed at random in horizontal orientation and distance from the acoustic axis, the distribution of

p/f

max will be one of the horizontal distributions of Table 2. I£ however, there

is a variation in fish angle, then the distribution of

p/ ⁴

^max

can be considered as be a sum of distributions in Table 2. Let nl be the nurnber of observed

$d?!

^{values, n2} the number of ohserved

VI

values and so on, and let further

g

be the number of fish w i t h

$ 8 X, the number of fish with

f l

⁼

fi

and so on, then, the following set of equations is deduced

The coefficients a to aIol0

12. are taken from Table 2, and the frequency distribution of

Yf

^{is found.}

Results of observations,

Observations of q f o r tod and coalfish are shown in Fig. 5 A.

The corresponding distributions of

f i

as calcuiated from equation

( 8 ) are presented below (Fig. 5 B)

.

The target strength and length distributions from the same obser- vations are presented in Fig, 6. The technique of observation is

d e s c r i b e d by Midttun (1966). T h e t a r g e t s t r e n g t h i s c a l c u l a t e d by a method s i m i l a r t o t h a t d e s c r i b e d by C r a i g and Forbes ( 1 9 6 9 ) . However, we have o n l y used t h e maximurn s i g n a l strengy%h from eaeh

f i s h , and i t i s assumed t h a t t h i s rnaxirnum occured when t h e f i s h p a s s e d t h e t r a n s v e r s e a x i s o f t h e beam. During a l l t h e o b s e r v a t i o n s

the z e r o s i g n a l s t r e n g t h corresponded t o a t a r g e t s t r e n g t h of -40 db.

I n F i g . 7 t h e r e s u l t s o f t h e a n a l y s i s a r e shown i n a

gf -

^TS

diagram The two p o i n t s a r e t h e mean v a l u e s , and t h e r e c t a n g u l a r a r e a s a r e l i m i t e d b y t h e s t a n d a r d d e v i a t i o n s ,

A s s e e n from F i g s . 6 and 7, no s i g n i f i c a n t d i f f e r e n c e was observed,

£ o r t h e two s p e c i e s w i t h r e g a r d t o the t a r g e t s t r e n g t h . T h i s i s n o t s u r p r i s i n g a s t h e l e n g t h s w e r e p r a c t i c a l l y t h e same. The values o f TS appeared t o be r a t h e r low.

Regarding

fl,

^however, a c o n s i d e r a b l e d i f f e r e n c e between the spe- e i e s was observed, and t h i s might i n f u t u r e b e used f o r i d e n t i f i - c a t i o n purposes.

D i s c u s s i o n .

The o b s e r v e d v a l u e s o f

45

( F i g . 5) w e r e lower t h a n t h o s e found from t h e measurements o f Midttun and Hoff (1962). The mean l e n g t h s o f t h e f i s h were, however, l a r g e r i n t h e p r e s e n t e x p e r i m e n t s and t h e r e f o r e s r n a l l e r f i s h a n g l e s may be e x p e c t e d ,

~ l s o t h e observed mean v a l u e s o f t a r g e t s t r e n g t h s w e r e low as cornpared t o t h e v a l u e s r e p o r t e d by Midttun and Hoff, even though t h e f i s h were l a r g e r . T h i s d i f f e r e n c e i s p r o b a b l y c a u s e d b y t h e f i s h h a v i n g an i n c l i n a t i o n from t h e h o r i z o n t a l . Most underwater p i c t u r e s show t h a t f i s h e s a r e u s u a l l y mare o r less i n c l i n e d r e l a -

t i v e t o each o t h e r , and c o n s e q u e n t l y t h e y a r e a l s o i n c l i n e d r e l a t i v e t o t h e h o r i z o n t a l p l a n e , From t h i s f o l l o w s t h a t f i e i d measurements of t a r g e t ' s t r e n g t h s w i l l always b e low compared t o t h e maximum v a l u e s measured i n l a b o r a t o r i e s .

I£ i n F i g , 4 of Midttun and Hoff (1962) we l e t t h e f i s h have a mean i n c l i n a t i o n o f 5' t o t h e h o r i z o n t a l p l a n e , t h e n t h e t a r g e t s t r e n g t h o f cod w i 1 E b e reduced w i t h a mean v a l u e o f 5.5 db, O r i f we t a k e t h e rnaxirnum 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 of an 85 c m cod t o be

-

^{2 0} db, t h e n t h e a v e r a g e i n c l i n a t i o n o f t h e cod i n o u r

f i e l d o b s e r v a t i o n s i s a p p r o x i m a t e l y 7-BO0,

"

The d e t e c t i o n s e c t o r a n g l e and c o n s e q u e n t l y t h e f i s h a n g l e s as d e £ in e d b y u s w i l l b e i n £ luenced by t h e s e t t i n g s o f t h e soundesr, The d i f f e r e n c e o b t a i n e d between cod and c o a l f i s h i n t h i s work i s however, n o t i n f i u e n c e d by t'his, s i n c e a l l the o b s e r v a t i o n s were made w i t h t h e same sounder a t the same s e t t i n g s . Another f a c t o r W i c h w i l l a l t e r t h e d e t e c t i o n a n g l e , i s t h e r o l l and p i t c h o f

t h e v e s s e l (Suornala 1970 F i g . 4 ) . A s no measurements o f p i t h / r o l l a n g l e s were c a r r i e d o u t , w e were n o t a b l e t o a n a l y s e i t s i n f l u e n c e on t h e r e s u l t s ,

We assume t h e f i s h t o be. o r i e n t a t e d a t random b u t w i t h t h e gong a x i s i n t h e h o r i z o n t a l p l a n e . The f i r s t assumption were p r o b a b l y p a r t l y f u l f i l l e d by t h e p a t t e r n o f d i f f e r e n t c o u r s e s used d u r i n g the o b s e r v a t i o n s , The second w a s , as a l r e a d y mentioned, n o t f u l - f i l l e d . c on sider ing t h e t a r g e t s t r e n g t h measurements i t is, how- e v e r , n o t p r o b a b l y t h a t t h e d i f f e r e n c e i n f i s h a n g l e s between cod and c o a l f i s h should be c a u s e d by a s y s t e m a t i c d i f f e r e n c e i n i n c l i n a t i o n between t h e two s p e c i e s d u r i n g t h e o b s e r v a t i o n s ,

The r e a s o n f o r t h i s d i f f e r e n c e i n f i s h a n g l e s i s more l i k e l y t o be found i n t h e s i z e and form o f t h e swimbladders, as p o i n t e d o u t by Midttun and Hoff (1962)

.

I n t h e a u t h o r s o p i n i o n more e x p e r i m e n t a l work should b e c a r r i e d out on a number of s p e c i e s and f o r d i f f e r e n t f i s h s i z e s i n o r d e r t o

f i n d o u t more c o n c l u s i v e l y w h e t h e r t h e fish a n g l e c a n b e o f genera2 v a l u e a s a t o 0 1 i n d i s t i n g u i s h i n g between f i s h s p e c i e s a s i t c o u l d a p p e a r from o u r r e s u l t s on cod and c o a l f i s h ,

O b s e r v a t i o n s should b e c a r r i e d o u t w i t h s t a b i l i z e d t r a n s d u c e r s i n o r d e r t o e l i r n i n a t e e r r o r s c a u s e d by t h e r o l l i n g o f t h e s h i p ,

3 . ABUNDANCE ESTIMATION

Method.

Methods of abundance e s t i m a t i o n a r e d e s c r i b e d i n t h e FAO F i s h e r i e s T e c h n i c a l Paper No. 8 3 q n d FAO F i s h e r i e s R e p o r t No. 78 ( P a r r i s h 1969). I n t h e f o l l o w i n g we s h a l l e x p l a i n the a p p l i c a t i o n o f a n echo i n t e g r a t o r f o r t h e p u r p o s e o f rneasuring f i s h d e n s i t y .

The i n t e g r a t o r we u s e was i n t r o d u c e d by Dragesund and Olsen (1965) and has r e c e n t l y been m o d i f i e d b y Simonsen og Mustad A/S (Bodholt 1969)

.

The s i g n a l v o l t a g e i s now squared before i n t e g r a t i o n and the o u t p u t of t h e i n t e g r a t o r i s t h e r e f o r e p r o p o r t i o n a l t o number of f i s h b o t h when m u l t i p l e and i n d i v i d u a l f i s h t a r g e t s a r e r e c o r d e d , Following Midttun and Nakken (1968) we w r i t e

where M i s t h e r e a d i n g o£ t h e i n t e g r a t o r , N i s t h e number o f f ish g i v i n g t h i s r e a d i n g , and C, i s t h e mean c o n t r i b u t i o n t o M from one f i s h .

A t a c o n s t a n t f i s h d e n s i t y , P ( n u m b e r p e r u n i t volum) applying a TVG p r o p o r t i o n a l t o the f o u r t h power of t h e d e p t h , t h e number of r e c o r d e d f i s h w i l l i n c r e a s e p r o p o r t i o n a l t o the s q u a r e o f t h e d e p t h , D. For a g i v e n i n t e g r a t i o n i n t e r v a l e q u a t i o n (9) c a n t h e n be w r i t t e n

where M D 4 i s t h e i n t e g r a t o r r e a d i n g when t h e TVG i s s e t pro- p o r t i o n a l t o t h e f o u r t h power 06 t h e d e p t h (40 l o g D ) , and D is

t h e mean d e p t h of t h e observed d e p t h i n t e r v a l . From ( 1 0 ) w e g e t

The e x p r e s s i o n on the l e f t s i d e i s p r o p o r t i o n a l t o t h e in- t e g r a t o r r e a d i n g when t h e TVG is p r o p o r t i o n a l t o t h e second power o f t h e d e p t h ( 2 0 l o g D)

.

Consequently, when a TVG p r o p o r t i o n a l t o t h e second power o f t h e d e p t h i s used, t h e i n t e g r a t o r r e a d i n g w i l l be p r o p o r t i n a l t o f i s h d e n s i t y .

The c o n s t a n t C 3 i s now i n d e p e n d e n t of d e p t h , b u t d e p e n d e n t o f TS and

SIf

and t h e c h a r a c t e r i s t i c s of the sounder. If TS and

pIt

o f t h e r e c o r d e d f i s h i s known Cg c a n be found. The most convenierit way t o f i n d Cg, however, i s t o c o u n t s i n g l e f i s h t r a c e s , s a y 30, on the p a p e r record, c a l c u l a t e

p ,

and d i v i d e i t w i t h t h e corre- sponding MD2

.

The o b t a i n e d v a l u e o f Cj c a n be used i n equation

(12) as l o n g as t h e f i s h s p e c i e and s i z e remains unchanged, D i s c u s s i o n ,

Is e q u a t i o n (12) a l s o v a l i d f o r schooLs of f i s h ? I n o t h e r words w i 1 1 one f i s h when member o f a s c h o o l c o n t r i b u t e t o the i n t e g r a t o r v o l t a g e w i t h t h e same v a l u e a s it does when r e c o r d e d as an i n d i v i d u a l ?

Tkie sampling volume w i l l i n c r e a s e w l t h i n c r e a s i n g s c h o o l density, which means t h a t C g should be l a r g e r f o r f i s h as s c h o o l members compared t o s i n g l e f i s h . The increment i n C however, w i l l b e

3'

s m a l l , and t h e p r e s e n t a u t h o r s c o n s i d e r it n e g l i g i b l e .

I n o r d e r t o d e t e r m i n e C _?i8 t h e s a m p l i n g ~ v o l u m e must be known. T h i s max mum

c a n be found from the d i s t r i b u t i o n o f d a r g e t s t r e n g t h o f t h e fish and from t h e d i r e c t i v i t y p a t t e r n o f t h e t r a n s d u c e r , Due t o t h e d i r e c t i v i t y of f i s h , t h i s p r o c e d u r e w i l l g i v e t o o low e s t i m a t e o f f i s h d e n s i t y , a s s e e n from table 2. A t r a n s d u c e r a t t h e s u r f a c e c a n n o t d e t e c t f i s h w i t h h i g h v a l u e s of ^W and X w i t h i n t h e e s t i m a t e d a n g l e max. The d e t e c t a b i l i t y d e c r e a s e s w i t h d e c r e a s i n g

4 .

P£ w e a l l o w t h e f i s h t o be i n c l i n e d r e l a t i v e t o t h e h o r i z o n t a l , t h e n t h e d e t e c t a b i l i t y i n table 2 w i l l be f u r t h e r reduced, T h e r e f o r e , f o r wide beam t r a n s d u c e r s , t h e sampling volume should be c a l c u l a t e d from t h e observed v a l u e s o f

9

i n s t e a d of from t h e d i r e c t i v i t y

diagram o f t h e t r a n s d u c e r .

~ q u a t i o n ( 1 2 ) i s n o t v a l i d for l a r g e fish densities. From echograms we know that below d e n s e f i s h s c h o o l s t h e s t r e n g t h of t h e bottorn echo i s c o n s i d e r a b l y xeduced due t o a t t e n u a t i o n o f mund w i t h i n t h e

s c h o o l , I n such c a s e s v a l u e s of Q c a l c u l a t e d from e q u a t i o n (12) w i l l b e t o o low, However, a t t h e f r o n t o f t h e r e f l e c t e d s i g n a l from a s c h o o l t h e a t t e n u a t i o n might be n e g l e c t e d , and d u r i n g t h e raise time o f t h e echo t h e squared v o l t a g e should be p r o p o r t i o n a l t o t h e number of r e f l e c t o r s w i t h i n one h a l f pulsevolume. _{T h i s} t h e n makes i t p o c s i b l e t o f i n d t h e f i s k d e n s i t y i n t h e uppermost p a r t of t h e s c h o o l .

The r e s p o n s e o f f i s h t o t h e s h i p n o i s e m i g h t c a u s e a lower f i s h

d e n s i t y w i t h i n t h e f i e l d s a n p l e d w i t h an echosounder. Olsen (1969) showed t h a t a t y p i c a l r e s p o n s e o f h e r r i n g t o a n a c o u s t i c s t i m u l u s , was t o t u r n away from t h e sound s o u r c e and s w i m towards t h e area of l e s s sound i n t e n s i t y . I t i s n o t known, however, whether t h e f i s h w i l l r e a c t i n this way t o t h e n o i s e o f a s h i p .

References.

Anon 1969. Technical report of the ICES/FAO acoustic training caurse FAO Fisheries Report No. 83.

Bodholt, H. 1969. ~uantitative measurements of scattering layers.

Simrad Bulletin No. 3.1969.

-SX--..--~.~UI.~~CI~Y~--.:"..-W%. i r,- ,...I.- ^--.i-

Cushing, D. H. 1968. Direct estimation of a fish population acous- tically. ~,Fish.Res, Bd.Canada, 25 (11) : 2349-2364.

craig, R. E. and Forbes, S. 1969. A sonar for fish counting. pisk.

Dir, Skr. Ser. HavUnders., 15: 210-219.

Dragesund, O. and Olsen, S. 1965. On the possibility of estimatinc year-class strength by measuring echo-abundance of

O-group fish. Fisk.Dir.Skr.Ser.HavUnder8. ,l3 (8) :47-75.

Haslett, R. W. G. 1962. Determination of the acouetic backscatterincj patterns and cross sections of f i s h . Srit.~.~ppl.~hys.l3:

349-357.

Haslett, R. W. G. 1965. Acoustic backscattering cross sections of fish at three frequencies and their representation on a universal graph. 3rit.J.Appl.Phys. 16: 1143-1150.

Love, R. H. 1969. Maximum side-aspect target strength of an indi- vidual fish. J.acoust.Soc .Am. 46 (3) : 747-753.

Midttun, L. 1966. Note on measurement of target strength of fish at sea, Coun.Meet.int.Coun.Explor,Sea, F9:l-3,

Midttun, L. and Hoff, I. 1962. Measurements of the reflection of sound by fish. FiskDir.Skr.Ser.HavUnde.rs.,I3(3).

Midttun, L. and Nakken, 0 . 1968. counting of fish with an echo integrator. Coun.Meet.int.Coun.Explor.Sea, B 17: 1-8.

Parrish, Manual methods for fish stock assessment.

Fisheries Technical Paper No, 78,

Suomala, Jr.,J.B. 1970. The application of a digital computer s i m i i - lation to aid in the evaluation o f echo-soundnr design and performance. Technical conference on fish findinq, purse seining and aimed trawling. FAO, Reykjavik 1970

&/a as a f u n c t i o n of +and

o(

for A: b/a = P/8, B: b/a = l / 4 ana C: b/a = 1/2.

C. 3 0.497 0 . 5 0 3 0,521 0 . 5 5 1 O , 598 0 . 6 5 2 9.747 3,%48 c?. 940 U. 980

Frequency distribution in per cents of

p / f

max. for different relations of

/

max. The fish is distributed and orientated at random with its long axis in the horizontal plane.

De tec tabi.- lity in %

Schematic p i c t u r e of a t r a n s d u c e r passage of a t a r g e t .

Schematic p i c t u r e of the reflectivity pattern of an glidealu fish target.

Fig. 3 .

?/q ^MAX

Distribution in per cent of

v / y

^{max for}

6 /q

^max

equal to A: 1/8, B: 1/4, C: 1/2 and D: 1.

Fig. 4. Schematic p r e s e n t a t i o n of the d e t e c t i o n area of a n i d e a l f i s h ,

MAX

Fig* 5 . A: Dirtribution in per cent of observed valuen of

F/$pmax

from coalfish and cod. B

8: Distribution in per cent of the corresponding fish angle values,

gf/ p

^max.

COALFISH

- - -

,o i ^COALFISH

COD

TS = -28,3

s = 3,4

Big. 6. Distribution in per cent of target strength, T S # for observed coalfish and cod, with corresponding length distribution below.

COD

O

Fig. 7.

-

TS diagram showing mean values (points) and standard deviations (straight lines) of observed cod and coalfish.