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FREON

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IGEG Fisheries Acou5t.i’~Gcience G Technology E FAST:) Working Group Meeting, Ostende, Belgium, 1 9 - 2 2 April, 1 9 8 6 ,

Not to be cited without permission of the authors

Ne pas citer sans autorisation des auteurs

A m e t h o d o l o g y f o r i n s i t u measurement o f mean t a r g e t s t r e n g t h i n s m a l l s c h o o l s .

Pierre Freon G François Gerlotto Pôle de Recherche Océanologique et halieutique Caraïbe ORTOM, BP 8 1 , 97256 Fort-de-France Cédex, Martinique (French W.I.1

.I. INTRODUCTION Different methods of T S measurement have been already performed on single fish ( c a g e d , tethered or wild in situ) or o n number of live fish in a cage [Johannesson and Mitson, 1983; Foote, 1987): Each method presents i t s own advantage and limitations. The three main problems to solve are: ( I ) to perform the measure on fish behaving as close a s pos-

sible from their natural: behaviour and physiological condition, (2) pattern,

to

take into account the effect o f the transducer beam c

( 3 ) to take account o f the bias introduced by high fish density (acoustic shadowing or re-radiation) when school echoes are integrated.

During the last decade the scientific effort was oriented toward the resolution o f mostly one o f these three problems at the same time, by measuring in situ individual wild fish when distributed in low density (dual beam or split beam) or by measuring fish in a small cage. Olsen ( 1 9 8 6 ) intended to estimate the sound attenuation under a large herring school, but as f a r as we‘ know, no attempt o f T S measurement a s been done on wild school, although this seems possible when some particular conditions are satisfied. In this case, the above mentionned three problems are 1

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II. EQUIPMENT A n EYM Simrad'sounder with a wide beam transducer (22O) has been used, connected to an Agenor [ I F R E M E R / O R S T O M ) digital echointegrator and to a tape recorder (during t h e future experiment, a analogic-digital converter will be used in order to reduce the possible biases introduced by analogic tape recording).

A wide angle camere was connected to a video tape recorder equipped with a precise revolution counter and allowing a performant slow motion and frame by frame play-back. A microphone was also connected for comments and for checking the synchronisation between tape and video recording. A 6 meter tube ended b y a one meter graduated bar w a s used f o r calibrating the size of the video pictures according to the depth and to the monitor screen size (fig. 1 ) . All this equipment w a s installed on a n instrumented raft, providing a support for the transducer and the camera more stable and shallower than a boat in the coastal area where the experiment was carried on. Some observations have been done on completely wild schools and others o n small schools enclose2 in a large net (purse-seine) used a s a mesocosm and providing an epparent natural behaviour of the school (see fig. 2 and Fréon & Gerlotto, this meeting).

III. METHODOLOGY Using Johannesson E Mitson"s (1983) notation, the mean volume back-scattering strength, we get:

where

Sv

is

where T S is the mean target-strength and F~ the mean density expressed in number of fish per cubic meter. If some conditions are satisfied, the mean density p v o f a thin fish layer can be estimated using a sounder and a camera. This can be done considering the volume V o f the truncated cone delimited on t h e one hand by the camera field o f view and on the other hand by the upper end lower limits o f the layer Cd, and d e l , ' obtained f r o m the sounder (fig. 1 3 . So we get: I

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the

layer

density is likely homogeneous and presents a

f a i r l b donstant thickness ( a s on photo 1 and 2) , and i f the mean" d-epth o f this layer i s rather constant during a f e w seconds, then

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some sampled w i e w r ; a a n b e u s e d for e s t i m a t i n g t h e m e a n d e n s i t - . y i n s i d e t h e volume V. F o r i n s t a n c e o n u s t a b l e 30 s e c o n d s e q u e n c e the sampling frequency c o u l d b e of o n e f r e m e e a c h s e c o n d . The f r a m e by, f r a m e s y s t e m o f t h e v i d e o r e c o r d e r c a n b e u s e d , o r a d i g i t a l i z e d p i c t u r e c a n be a n a l y s e d o n a c o m p u t e r . The s p e c i e s c o m p o s i t i o n and t h e mean f i s h l e n g t h c a n b e est i m a t e d e i t h e r b y f i s h i n g o r b y u s i n g t h e wid-eo f o r m e a s u r i n g t h e fish on the m o n i t o r a n d c a l c u l a t i n g t h e ' r i s i n g f a c t o r from t h e c a l i b r a t i o n r e s u l t s a n d a c c o r d i n g t o t h e mean d e p t h g i v e n b y the s o u n d e r . If t h e l a y e r t h i c k n e s s i s t o o h i g h a n d i n t r o d u c e s a l a r ge variability of t h e a p p a r e n t l e n g t h s measured on t h e s c r e e n , t h e n o n l y t h e l a r g e s t f i s h can be m e a s u r e d , c o n s i d e r i n g t h a t t h e y a r e l o c a t e d i n t h e u p p e r p a r t of t h e l a y e r ( s u c h a method suppos e s a n a r r o w d i s t r i b u t i o n of t h e b o d y l e n g t h s a n d t i l t a n g l e s i n side the schools). O t h e r a p p r o a c h e s c a n be d e v e l o p p e d u s i n g s t e r e o camera o r a s e c o n d v i d e o c a m e r a ( o r p h o t o c a m e r a ) w i t h a l a r g e f o c a l l e n s p r o v i d i n g a n a r r o w d e p t h of f i e l d .

I n t h i s l a s t c a s e , a n a r r o w i n t e r v a l of d e p t h c a n b e s a m p l e d i n s i d e t h e l a y e r by m e a s u r i n g o n l y t h e f i s h p r e s e n t i n g a good resolution. T h e c a l i b r a t i o n o f t h i s s e c o n d camera m u s t b e a c h i e v e d under i d e n t i c a l c o n d i t i o n s t o t h o s e t a k i n g p l a c e d u r i n g t h e exper i m e n t on t h e s c h o o l ( t u r b i d i t y , l i g h t i n t e n s i t y and d i r e c t i o n ) , u s i n g t h e same g r a d u a t e d t u b e o r b e t t e r a d i e d fish. This w i l l p r o v i d e b o t h t h e p r e c i s e mean d e p t h of s a m p l i n g and i t s r a n g e . If the t r a n s d u c e r a n d t h e camera l e n s u r e p r o p e r l y c h o o s e n i n o r d e r t o h a v e t h e angle 8 o f t h e l e n s w i d e l y g r e a t e r t h a n the m e a n a n g l e o f t h e t r a n s d u c e r b e a m a t -5 d B f o r i n s t a n c e , t h e r e f o re t h e S v v a l u r s c a n be a s s u m e d t o b e r e p r e s e n t a t i v e o f t h e m e a n a c o u s t i c r e s p o n s e of t h e t r a n s d u c e r f o r a g i v e n d e p t h , when " t h e layer i s observed on t h e whole s c r e e n s u r f a c e .

Some e x p e r i m e n t s w e r e c o n d u c t e d a f t e r f i x i n g t h e c a m e r a a n d t h e transducer on t h e raft, but others were realized with these d e v i c e s f i x e d o n t h e battom a n d o r i e n t e d t o w a r d t h e s u r f a c e . Acc o r d i n g t o t h e d e p t h of t h e l a y e r a n d t o t h e water transparency, o n e o r t h e o t h e r m e t h o d i s s u i t a b l e . If t h e f i s h d i r e c t i v i t y d i a gram i 5 suppose'd t o p r e s e n t a v e r t i c a l a x i s o f s y m e t r y -as u s u a l ly admittedt h e n t h e r e s u l t s must b e c o n s i s t e n t . Observations from t h e bottom p r e s e n t t h r e e a d v a n t a g e s : first the camera and the transducer are a b s o l u t e l y stable and provide less v a r i a b l e data, second t h e pictures are perfectly contrasted ("shadow show") and t h i r d t h e r e i s a b s o l u t e l y no i n f l u e n c e of t h e equipment o n t h e f i s , h b e h a v i o u r . a n d pv, TS c a n b e e a s i l y c a l c u l a t e d .

Knowing