1971_B_15.pdf (488.2Kb)

I n t e r n a t i o n a l Council f o r t h e E $ p l o r a t i o n o f t h e Sea

d- ' Li

-3

,.,+,k+ ^s G e c o l m i t t e e

Ref, :Denersal F i s h ( N ) C o m i t t e e

l ^{. r - + i ,}

1 _.d & & ! \ J i : Ø b ; 7 ' ~ ~ " ..(A ,- 4 t , < , -,.,r, -

p ,

4 , ^{b , - ,- r .,'o,(? ,}

?C"C",+. w / ' d Y 4 - ( t '

ABzUNDAPTCE ECTIT4ATION O F THE SpAWbII2TG LOFOTEN COD 1971

J, Blindheim and 0 , Nakken I n s t i t u t e o f Marine Research

Bergen, Norway

S i n c e 1935 t h e L o f o t e n area h s s r e g u l a r l y been echo surveyed durinc:

t h e spawning s e a s o n o f t h e A r c t i c cod (Sund 1 9 3 5 ) , The r e s u l t s a f t e r t h e s e s u r v e y s h a v e , s o f a r , been g i v e n a s e s t i m a t e s o f r e l a t i v e es li-^

dance o f cod (Monstad e t a1,1969), No a b s o l u t e v a l u e s h a v e been

p u b l i s h e d s i n c e the e s t i m a t e s e v i d e n t l y would h a v e been t o o low, The.

r e s s o n f o r t h i s w a s p r e s u n e d t o be t h n t a n e r r o n e o u s sampling volwne o f t h e echo sounder was u s e d i n t h e c a l c u l a t i o n s and t h a t t h e s u r v e y g r i d w a s i n s u f f i c i e n t l y dense,

I n Marck 1971 t h e L o f o t e n s u r v e y was c a r r i e d o u t w i t h t h e more e f f i - c i e n t equipment onboard the new r e s r a r c h v e s s e l "G.O,SarsH, T h e s i m o5 t h e i n v e s t i g a t i o n was f i r s t l y t o e s t i m a t e t h e number o f cod w i t h i n

the main spawning a r e a ( F i g , 1) and s e c o n d l y t o f i n d o u t t h e d e n s i t y s u r v e y g r i d t o be a p p l i e d i n t h e a r e a , I n t h i s p a p e r sone p r e l i m i n a r g r e s u l t s o f t h e 1971 s u r v e y i s p r e s e n t e d ,

KATERI AL A l D METHODS

The a r e 3 was surveyed 6 t i m e s d u r i n g t h e c r u i s e (Table l ) , and a s demonstrated i n Fig, l the s u r v e y t r a c k s were l a i d v e r y c l o s e , onLy 1 n a u t i c a l m i l e a p a r t , The p o s i t i o n s were d e t e r m i n e d v e r y accurltzl:7 by b e a r i n g s ( r a d a r o r v i s u a l l y ) and f u r t h e r s u p p o r t e d by e x c e l l e n t d e c c a coverage.

The m a t e r i a l w a s c o l l e c t e d w i t h 3 echo i n t e g r i t t o r s which i n t e g r a t e d the squared v o l t a g e s from a 38 m z echo sounder, ^A 120 k4Sz echo

sounder was als0 used t o o b t s i n b e t t e r r e s o l u t i o n , With t h e s e t t i n g s used t h e main c h a r a c t e r i s t i c s o f t h e two sounders were:

S o u r c e l e v e 1 136 124 db//l u B a r r e f 2 : *

V o l t a g e r e s p o n s e

+

^7,8

-

^0.6 db//l v o l t p e r u ^.'I-:

P u l s e l e n g t h 0.6 0,3 m i l l i s e c o n d s

Beamwidth 5,0° and 5,5O 4,5O c i r c u l l r between 3 db poir,"cs W G 20 l o g R+2 x R 40 l o g R+2 x R

The g a i n s e t t i n g s were choosen s o t h a t t h e minimum reeoordable s i g n a l corresponded t o a t a r g e t s t r e n ~ t h o f -52 db.

The echo sounders and i n t e g r a t o r s were connected o n - l i n e t o a com-

p u t e r onboard t h e s h i p , T h i s p e m i t t e d a c c u a r a t e and r e l i a b l e rcad!-r~:~

o f t h e i n t e g r a t o r s , and s m p l i n g of echo sounder d a t a p r o v i d e d p o c s i - b i l i t y f o r d e t e r m i n i n g t h e sampling volume o r a r e a o f t h e echo sounder::

(Midttun cind Nakken 1 9 7 1 ) , The 3 echo i n t e g r a t o r s s m p l e d v o l t a g e s from 6 d e p t h s l i c e s , T h e 6 ckiannels were r e a d b y the computer whieh gave a n o u t p r i n t each n a u t i c a l m i l e as shown i n t a b l e 2, ^{A s} t h e v o l - t s g e i s squared b e f o r e i n t e g r a t i o n t h e v a l u e s i n t i t b l e 2 are propor- t i o n a l t o echo i n t e n s i t y , and a WG o f 20 l o g R a 2 x R make t h e o u t p u t o f t h e i n t e g r a t o r p r o p o r t i o n a l t o nuinber of f i s k per u n i t a r e a (Midttun and Hakken 1971, Bodholt 1 9 6 9 ) ,

The i n t e g r a t o r s have a bottom s t o p system, However, when p s s s i n g

3 s t e e p l y s l o p i n g bottom, t h e bottom echo i s weak and w i l l b e i n t e g r a t e d ,

- A s t h e bottom topography i n t h e a r e a i s i r r e g u l s r , t h i s occured r a t h e r .

o f t e n , T h i s e r r o e w a s c o r r e c t e d d u r i n g t h e d a i l y s c r u t i n y of t h e echo r e c o r d s and t h e correspbnding integrams, During this s c r u t i n y i t w a s a l s o decided what k i n d o f f i s h t h a t 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 , and c o n t r i b u t i o n s from f a l s e echoes ( i n t e r - f e r e n c e s , n o i c e ) w e r e a e l e t e d ,

The v a l u e s o f i n t e g r a t e d echo i n t e n s i t y , 34, were c o n v e r t e d t o number of f i s h p e r u n i t a r e a ,

7 .

u s i n g the f o m u l s (Midttun and Nakken 1971):

M r i t i n g C =

4

^{= N}

M M ~ A ( 11 )

where N i s number o f f i s h and A i s t h e s u r f s c e a r e a w i t h i n which t h e f i s h e s are d i s t r i b u t e d , C can b e c i l c u l a t e d when a s s o c i a t e d v a l u e s o f M, M and A a r e observed,

l

2.J was Sound b y c o u n t i n g f i s h t r a c e s on t h e r e c o r d i n g p a p e r when o n l y s i n g l e f i s h e s w e r e recorded, The corresponding s a p l i n g a r e a , A, wqs c a l c u l a t e d from t h e formula:

where R i s t h e mean d e p t h o f t h e d e p t h s l i c e and @m x i s the athwardships width o f t h e beam. Frequency d i s t r i b u t i 8 n s of the d e t e c t i o n s e c t o r a n g l e ,

q,

w e r e o b t a i n e d ( X i d t t u n and Nakken 1971)

(Pmax Sound by t h e f o r n u l a :

^CC

8

^{max =}

_CIL

^{( IV}

The f u r t h e r management o f *e n c o u s t i c d a t a was t o p l o t t h e s p s i n t a b l e 2 i n t o c h a r t s ( Fig, 1 ) , Two methods have been a p p l i e d f o r o b t a i n i n g t h e t o t a l number o f f i s h w i t h i n t h e a r e a , F i r s t l y iso- l i n e s w e r e drawn through p o i n t s of e p a l i n t e g r a t o r d e f l e c t i o n and t h e s e v a l u e s m u l t i p l i e d by t h e a p p r o p r i a t e a r e a s , Secondly t h e area covered was d i v i d e d i n t o s u b a r e a s e o f 4 s q u a r e n a u t i c a l m i l e s , A l l o b s e s v a t i o n s wikhin each s u b a r e a were averaged and t h e a v e r a g e s w e r e accumulated t o

1

^- g i v e t h e t o t a l . A n s t t e m p t t o d e t e r m i n e t h e n e c c e s s a r y c l o s e n e s s of . -

c o u r s e l i n e s was made by u s i n g o b s e r v a t i o n s from e v e r y second and

l

1 - e v e r y t h i r d t r a c k as b a s i s f o r t h e e s t i m a t e s of t h e t o t a l s ,

Fig, 1 shows r e l a t i v e d e n s i t y d i s t r i b u t i o n s o f cod w i t h i n t h e a r e a d u r i n g survey no, I1 and no,V. A comparison o f t h e two s u r v e y s

i l l u s t r a t e s c l e a r l y t h a t t h e most dense c o n s e n t r a t i o n s a r e found a t d i f f e r e n t l o c a l i t i e s , However, F i g , 1 i l l u s t r a t e s t h e two s u r v e y s w i t h t h e most d i f f e r e n t p i c t u r e s w i t h r e g a r d t o l o c s t i o n s of maximum

f i s h c o n s e n t r a t i o n s , The v a l u e s o f total echo abundance a c c o r d i n g t o survey a r e g i v e n i n T a b l e l and Fig, 2 f o r t h e 3 d i f f e r e n t survey g r i d s and t h e two methods o f c a l c u l a t i o n ,

A frequency d i s t r i b u t i o n o f , observed w i t h t h e 120 )iHz &cho- sounder i s g i v e n i n t a b l e 3 , f n d t h e v a l u e of p m a x w a s 5 , s when e q u a t i o n I V w a s a p p l i e d , Equation 111 g i v e s t h e n a s m p f i n g a r e a , A, _0,73 p e r nau i c a l _{" 10"}

5

s q u a r e n a u t i c a l m i l e stecuned i n t h e d e p t h m i l e . s l i c e 55-100 m of A=

The r e s u l t s o f a series o f p a p e r c o u n t s of i n d i v i d u a l f i s h on t h e 120 IcHz echosounder and 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 o r d e f l e c - t i o n s i n t h e d e p t h s l i c e 55-100 m are g i v e n i n T a b l e 4, The mean

N

^{= 0.24} f i s h / u n i t d e f l e c t i o n , a p p l i e d i n e q u a t i o n II g i v e s

. - M

..

N

-

c

= -

- -

0,24 ^*

^{l o L}

M-A 0.73 3 3 f i s h / u n i t d e f l e c t i o n ( n a u t i c a l m i l e )

The t o t a l number of f i s h w i t h i n the a r e a i s now found from T a b l e 1 when m u l t i p l y i n g t h e f i g u r e s of t o t a l echo abun- 6nnce by 3 3 , g i v i n g 6.8-lo6 f i s h and 13.5 10 f i s h a s 6 t h e lower and u p p e r l i m i t s r e s p e c t i v e l y , These numers are b a s e d on t h e " i s o l i n e n method and 1 n u t i c a l m i l e between t h e s u r v e y t r a c k s .

DISCUSSION

C a l c u l a t i o n o f t h e sampling a r e a , A , from e q u a t i o n (111)

r e q u i r e s f u l l o v e r l a p by t h e beam from p i n g t o p i n g . With t h e a c t u a l speed, beamangle and p u l s e r a t e u s e d , however, t h e e r r o r i n t r o d u c e d i s i n s i g r i i f i c a n t .

Even t h e h i g h e s t o f the p r e s e n t e s t i m a t e s i s e v i d e n t l y o n l y a f r a c t i o n o f the t o t a l spawning s t o c k (Anon 1970, Hylen and Dragesund 1 9 7 0 ) . I n s p i t e o f t h i s t h e r e s u l t s a r e l i k e l y t o b e r e a s o n a b l e , I t must be c o n s i d e r e d t h a t even t h o u g i t h e main spawning grounds were surveyed, t h i s i s o n l y a p a r t o f t h e t o t a l a r e a where spawning o c c u r r e d .

I t i s w e l l known t h a t c o n s i d e r a b l e c o n c e n t r a t i o n s o f cod m i g r a t e d t o t h e n o r t h - e a s t e r n p a r t o f Lofoten. T h i s a r e a was n o t surveyed b e c a u s e many s k e r r i e s and s h a l l o w s make

t h e w a t e r s t o o d i f f i c u l t t o s u r v e y w i t h t h e r e l a t i v e l y

b i g r e s e a c h v e s s e l . I t i s a l c o o f importance t h a t a spawning cod do n o t s t a y i n t h e a r e a d u r i n g t h e whole spawning season.

C o n c e g i e n t l y i t i s v e r y u n l i k e l y t h a t t h e whole spawning s t o c k was p r e s e n t i n L o f o t e n a t t h e t i m e o f t h e s u r v e y s .

A s a l r e a d y mentioned i n t e g r a t i o q o f bottom echo o c c u r r e d r a t h e r o f t e n , Although t h e u t m o s t c a r e w a s t a k e n t o c o r r e c t f o r t h i s i t i n f l u e n c e d t h e r e s u l t s . However, e x a c t l y the same s u r v e y t r a c k s w e r e f o l l o w e d on e a c h s u r v e y . a n d t h e

c o r r e c t i o n s were u n d e r t a k e n i n t h e same way e v e r y time. T h i s e r r o r s h o u l d t h e r e f o r e e f f e c t t h e r e s u l t s o f a l l t h e s u r v e y s i n t h e same manner. The v a r i a t i o n E r & s u r v e y t o s u r v e y i s , t h e r e f o r e c a u s e d by o t h e r f a c t o r s , such as f i s h movements t o and from t h e a r e a . A s s e e n i n f i g , 1 t h e r e w a s con-

s i d e r a b l e f i s h movement a l s o w i t h i n t h e a r e a between s u r v e y s , a m a t t e r which was s u p p o r t e d b y t h e fishermens o p i n i o n ,

A comparison of t h e two methods ( " I s o l i n e t t a n d M ~ q u a r e t ' ) f o r o b t a i n i n g t h e t o t a l number o f f i s h d o e s n o t i n d i c a t e any c l e a r d i f f e r e n c e . A s a mean f o r t h e 6 s u r v e y s t h e numbers d e r i v e d by t h e v s q u a r e t h e t h o d a r e 3% below t h e

" i s o l i n e " method.

A f i r s t glance---&--Fig. 2 g i v e s t h e impression t h a t t h e r e i s no c ~ n s i d e r a b l e d i f f e r e n c e between -._results o f t h e t h r e e g r i d d e n s i t i e s . From Table 1, however, it can be s e e n t h a t d e v i a t i o n s r e a c h i n g 38% a r e observed when a g r i d w i t h 2 n a u t i c a l m i l e s between t h e s u r v e y t r a c k s i s a p p l i e d . The r e s u l t s o b t a i n e d by t h e i s o l i n e method and the most dense g r i d i s t h e n t a k e n a s 10W. The same comparison when o n l y e v e r y t h i r d t r a c k i s used, g i v e s d e v i a t i o n s up t o 54 %,

A conseguence o f t h i s must b e t h a t a v e r y dence s u r v e y g r i d i s needed i f a r e l i a b l e number s h a l l b e achieved.

SUMMARY

T h i s paper p r e s e n t s come p r e l i m i n a r y r e s u l t s of a c o u s t i c s u r v e y s i n Lofoten d u r i n g t h e spawning season of t h e A r c t i c cod i n 1971, The aim o f t h e s u r v e y s was t o e s t i m a t t h e

d number o£ cod w i t h i n t h e main spawning a r e a ,

T h e a r e a was surveyed 6 t i m e s and a v e r y dence survey g r i d were a p p l i e d , t h e s u r v e y t r a c k s b e i n g o n l y 1 n a u t i c a l m i l e a p a r t ,

The s u r v e y s r e s u l t e d i n numbers v a r y i n g between 6.8 and

13.5 m i l l i o n s of cod w i t h i n t h e survey a r e a . The s e a s o n f o r t h e g r e a t f l u c t u a t i o n of t h e numbers i s l i k e l y t o b e f i s h move-

ments i n and o u t of t h e a r e a . Bottom echoes i n f l u e n c e d on t h e r e s u l t s t o some d e g r e e , b u t t h i s was n o t d i f f e r e n t from survey t o survey.

E s t i m a t e s were a l s o worked o u t zpplying o n l y e v e r y second and e v e r y t h i r d s u r v e y t r a c k . T h i s r e s u l t e d i n g r e a t e r f l u c t u a t i o n s and a c c o r d i n g l y i n d i c a t e d t h a t a dense survey g r i d i s n e s s e c a r y ,

REFEIIENCES

Anon, 1970. Report of t h e n o r t h - e a s t a r c t i c f i s h e r i e s working

Coop, Res. R e p . S e r , A , i n t . Coun, Explor, Sea, 16:

1-60.

Bodholt, H, 1969, Q u a n t i t a t i v a measurements of s c a t t e r i n g l a y e r s , Simrad B u l l . , 3: 1-11.

l i

Hylen, A. and Dragesund, 0. 1970. Recruitment oE young arcto- norwegian cod and haddock in relation im parent stock size. Stock and ~ecruitment. Int. Coun.

Explor. Sea, S y m p . , Copenh.

,

^{1970 (19) :} 1-14. Mimeo.

Midttun, L. and Nakken, 0. 1971. On acoustic identification, sizing and abundance estimation of fish.

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

Monstad, T., Nakken, 0. and Nævdal, G. 1969. Skreiinnsiget 1969. (The migration of arctic cod into Lofoten 1969)

.

Fiskets Gang, 55: 571-573. Fisken og Havet 1969 (2) : 3-5,

Sund, 0, 1935. Echo sounding in fishery research.

Nature, 135: 953.

Tablr 1 . Total echo abundance x i n t h e investigated area d u r i n g the 6 s u s v e y s , Vaiues a r e worked out by _{t w s} d i f f e r e n t rnethods and with three grid densities,

»T ST.

BETMEEN

!

S U R V E Y K U M B E R A N D D A T E

COURSE

LINES I

1

_I ^I1

20 M,hRCIi 121 MARCtl

LUZTN-

^BRUN-

1

SQVARE I 1 190 9 3

l

l

T a b l e 2, P r i n t out of echo abundance in 6 echo integrator channeas,

total far all channels (right) and log readings ( l e f k ) , D e p t h i n t e r v a % for each charanel PS indicated.

l DEPTH INTERVALS, METREC ,

l

T a b l e 3. Frequency d i s t r i b u t i o n o f t h e d e t e c t i o n s e c t o r a n g l e ,

, observed w i t h t h e 120 kHz echosounder.

Degrees 1 2 3 4 5 6 7 8 9

N u m b e r 1 6 53 3 1 17 1 7 20 25 9 3

T a b l e 4, R a t i o s between numbers of f i s h on t h e p a p e r record,N, o f t h e 120 kHz echosounder and u n i t s of d e f l e c t i o n , M, o f t h e

i n t e g t a t o r f o r 7 d i f f e r e n t n a u t i c a l m i l e s i n t h e d e p t h s l i e s 55-100 m,

-

^{N : 0,23 0 , 2 0 0,19 0,22 0,23 0.31 0,30} Mean 0,24

M