I n t e r n a t i o n a l Council f o r t h e E x p l o r a t i o n o f t h e Sea
C.M. 1 9 8 5 / ~ : 2 8
F i s h Capture Cornmittee
I N SITU FISH TARGET STRENGTHS DERIVED WITH A SPLIT-BEAM ECHO SOUNDER
Kenneth G. F o o t e , A s g e i r Aglen and Odd Nakken I n s t i t u t e o f Marine Research
5011 Bergen, Nouway
ABSTRACT
i n s i t u t a r g e t s t r e n g t h s o f s i x f i s h e s were determined w i t h a s p l i t - beam echo sounder d a r i n g c r u i s e ~
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a b o u t Lofoten i n March 1984 and t h eS h e t l a n d I s l a n d s i n J u l y 1984. The s p e c i e s and l e n g t h s , mean and s t a n d a r d d e v i a t i o n , a r e t h e f o l l o w i n g : cod (Gadus morhua), 8 2 1 1 1 cm, s a i t h e
( P o l l a c h i u s v i r e n s ) , 5 7 i 6 cm; Norway p o u t ( T r i s o p t e r u s e s m a r k i ) , 1 8 * 2 and 1 5 1 1 cm; h e r r i n g (Clupea h a r e n g u s ) , 281 2 cm; r e d f i s h o r ocean p e r c h
( S e b a s t e s m a r i n u s ) , 20 I 9 cm; and g r e a t e r s i l v e r s m e l t ( A r g e n t i n a s i l u s ) , 371 4 cm. E x t r a c t i o n o f u n b i a s e d mean t a r g e t s t r e n q t h s by a method o f t h r e s h o l d compensation i s d e s c r i b e d .
RESUME:
I n - s i t u d e t e r m i n a t i o n d ' i n d e x d e r e f l e x i o n obtenue avec ~ r n sondeur v e r t i c a la
f a i s c e a u s c i n d 6Determination d ' i n d e x d e r e f l e x i o n i n - s i t u g r a c e 2 un sondeur v e r t i c a l 3 f a i s c e a u s c i n d 6 au c o u r s de campagnes e f f e c t u e e s a u t o u r d e s L o f o t e n en mars 1984 e t d e s i l e s S h e t l a n d e n j u i l l e t 1984. Les e s p e c e s e t u d i e e s avec
l e u r t a i l l e moyenne/ecart t y p e s o n t l e s s u i v a n t e s : morue (Gadus m o r h u a ) , 82 111 cm; l i e u n o i r ( P o l l a c h i u s v i r e n s ) , 5 7 f 6 cm; t a c a u d n o r v e g i e n
( T r i s o p t e r u s e s m a r k i ) 1 8 1 2 e t 1 5 I 1 cm; hareng (Clupea h a r e n g u s ) , 2 8 * 2 cm; s e b a s t e ( S e b a s t e s m a r i n u s ) ,
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LO I 9 cm; e t grande a r g e n t i n e ( ~ r g e n t i n a s i l u s ) , 3 7 k 4 cm.INTRODUCTION
The need f o r knowledge o f f i s h t a r g e t s t r e n q t h s i s w e l l known (Midttun 1 9 8 4 ) . I n s i t u measurements a r e p a r t i c u l a r l y v a l u a b l e f o r
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r e p r e s e n t i n g t h e a c o u s t i c s c a t t e r i n g p r o p e r t i e s o f f i s h under t h e a c t u a l c o n d i t i o n s o f t h e i r s u r v e y i n g . Such d a t a a c q u i r e a g r e a t e r s i g n i f i c a n c e when u s e d t o d e t e r m i n e t h e l e n g t h dependence o f t a r g e t s t r e n g t h , a s t h e r e s u l t i n g r e l a t i o n can t h e n be used on f i s h of d i f f e r e n t l e n g t h s t h a n
o r i g i n a l l y observed and a l s o , under c e r t a i n c i r c u m s t a n c e s , on f i s h o f d i f f e r e n t s p e c i e s .
Development of t h e f i r s t commercial split-beam echo sounder, by SIMRAD, was t h u s welcomed f o r i t s e v i d e n t u s e f u l n e s s i n determining i n s i t u t a r g e t s t r e n g t h s . By p r o v i d i n g a means of d i r e c t measurement, t h e split-beam technique a v o i d s many of t h e problems i n t r i n s i c t o i n d i r e c t methods (Ehrenberg 1 9 8 3 ) . I t i s a d d i t i o n a l l y s u p e r i o r i n p r i n c i p l e , i £ n o t i n p r a c t i c e t o o , t o t h e o n l y o t h e r d i r e c t
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i n s i t u method, t h a t o f d u a l beams (Ehrenberg 1974, 1 9 7 9 ) .The simple purpose of t h i s paper i s t o p r e s e n t some r e s u l t s from t h e f i r s t a p p l i c a t i o n s of t h e new sprit-beam echo sounder. T h i s was, i n f a c t , t h e f i r s t model of t h e ES380 system, which was s p e c i a l l y adapted f o r r e s e a r c h use (Foote, K r i s t e n s e n and S o l l i 1 9 8 4 ) . However, while t h e b a s i c s i n g l e - f i s h echo d a t a were e a s y t o g a t h e r , t h e p r e s e n c e of b o t h weak- and s t r o n g - s i g n a l t h r e s h o l d s , o r c u t o f f s , i n t h e p r s e a s s i n g complicated t h e a n a l y s i s . I t i s hoped t h a t d e s c r i p t i o n h e r e of t h e
method of t h r e s h o l d compensation w i l l a i d o t h e r c u r r e n t o r p o t e n t i a l u s e r s of t h e new split-beam echo sounder
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o r o t h e r thresho1d.-affected systems o r t e c h n i q u e s f o r t h a t m a t t e r-
i f only by u r g i n g c a u t i o n i n t h ei n t e r p r e t a t i o n of o s t e n s i b l y unambiguous d a t a .
MATERIALS
The primary m a t e r i a l s c o n s i s t of t h e a c o u s t i c and b i o l o g i c a l d a t a c o l l e c t e d on a number of s p e c i e s d u r i n g c r u i s e s w i t h R/V G.O. SARS about Lofoten i n March 1984 and t h e S h e t l a n d I s l a n d s i n J u l y 1984. The form of t h e a c o u s t i c d a t a g a t h e r e d with t h e SIMRAD ES380 split-beam echo sounder has a l r e a d y been d e s c r i b e d i n d e t a i l (Foote, K r i s t e n s e n and S o l l i 1 9 8 4 ) . I n b r i e f , each s i n g l e - f i s h echo i s c h a r a c t e r i z e d by t h r e e d a t a : t h e ping number, echo range t o t h e n e a r e s t d e c i m e t e r , and t a r g e t s t r e n g t h . Each of t h e s e numbers i s recorded f o r each r e s o l v e d s i n g l e - f f s h echo. The t a r g e t s t r e n g t h i s expressed a s one of 80 t a r g e t s t r e n g t h c l a s s e s evenly spaced ( over t h e range from -50 t o -20 dB, hence w i t h 0.375 dB r e s o l u t i o n . The split-beam echo sounder, w i t h 38 kHz o p e r a t i n g frequency, was c a l i b r a t e d with t h e 60 mm copper sphere (Foote e t a l . 1981, Foote 1982) a t l e a s t once d u r i n g each c r u i s e .
The a c o u s t i c d a t a a r e v a l u a b l e o n l y when accompanied by good b i o l o g i c a l d a t a on r a t h e r pure f i s h a g g r e g a t i o n s . For p r e s e n t purposes, t h e p u r i t y i s s u f f i c i e n t f o r unambiguous assignment of t h e a c o u s t i c d a t a t o t h e r e s p o n s i b l e f i s h e s i n e i t h e r of two s i t u a t i o n s : a c l e a r l y dominant s i n g l e s p e c i e s , o r two s p e c i e s of d i s t i n c t l e n g t h groups. Thus, of t h e 11 d a t a s e r i e s w i t h split-beam echo sounder measurements of f i s h during t h e March Lofoten c r u i s e , o n l y f i v e a r e u s a b l e f o r determining mean t a r g e t s t r e n g t h s . Of t h e 14 d a t a s e r i e s d u r i n g t h e J u l y S h e t l a n d I s l a n d s c r u i s e , only f o u r a r e u s a b l e , and of t h e seven s e r i e s d u r i n g t h e March and A p r i l c r u i s e t o survey b l u e w h i t i n g (Micromesistius poutassou) west of t h e B r i t i s h I s l e s , none i s c o n s i d e r e d u s a b l e because o f s u s p e c t e d problems w i t h s p e c i e s and l e n g t h s e l e c t i v i t y of t h e t r a w l i n g g e a r .
T a b l e 1. B i o l o g i c a l d a t a accompanying i n $ i t u t a r g e t s t r e n g t h m e a s u r e m e n t s made d u r i n g two 1 9 8 4 c r u i s e s .
Assumed l e n g t h d i s t r i b u t i o n i n s i m u l a t i o n s
T r u n c a t i o n No. s p e c i m e n s F i s h l e n g t h (cm) N(mean,s.d.) l i m i t s Data S u r v e y
s e r i e s d a t e F i s h C a u g h t S i z e d Mean S.D. Min. Max. Mean S.D. Min. Max.
1 12/3 Norway p o u t 223 223 17.6 1 . 6 1 0 2 1 1 7 . 6 1.6 1 4 . 4 2 0 . 8
S a i t h e 52 5 2 59.7 4.9 4 8 6 9 5 7 . 2 6 . 0 45.2 69.2
2 1 3 / 3 C a i t h e 1 8 6 3 7 3 57.2 6 . 0 4 5 9 1 57.2 6 . 0 45.2 6 9 . 2
liedfis11
3 92 9 2 1 9 . 7 8.7 9 43 19.7 8.7 1 1 . 0 37.1
S a i t h e 1 5 1 5 56.3 5 . 1 46 6 5 57.2 6.0 45.2 6 9 . 2
7 1 5 / 3 Cod 1 3 1 3 81.7 1 0 . 6 6 0 9 8 81.6 1 1 . 4 5 8 . 8 1 0 4 . 4
8 1 5 / 3 G r . s. smelt. 1813 1 8 1 3 37.2 4.4 25 50 37.2 4.4 28.4 4 6 . 0
11 1 8 / 3 Cod [ ~ n s p e c . 9 5 3 81.6 1 1 . 4 50 1 0 5 + ] 81.6 1 1 . 4 5 8 . 8 1 0 4 . 4
1 5 25/7 H e r r i n g 1 6 5 1 6 5 2 8 . 8 2.0 24 34 28.5 2.0 24.5 32.5
2 5 29/7 H e r r i n g 22 22 28.0 2.7 25 34 28.5 2.0 24.5 32.5
26 30/7 Norway p o u t 2250 1 0 7 1 4 . 8 1.1 1 2 1 9 1 4 . 8 1.1 1 2 . 6 1 7 . 0
A summary of t h e b i o l o g i c a l d a t a i s p r e s e n t e d i n Table 1. I n every c a s e e x c e p t one t h e l e n g t h d i s t r i b u t i o n i s a p p a r e n t l y normal. I t i s assumed t o be normal i n modeling t h e t h r e s h o l d e f f e c t , w i t h mean and
s t a n d a r d d e v i a t i o n g e n e r a l l y e q u a l t o t h e corresponding sample v a l u e s , and w i t h t r u n c a t i o n a t two s t a n d a r d d e v i a t i o n s from t h e mean. I n t h e
e x c e p t i o n a l c a s e , t h a t of r e d f i s h , t h e d i s t r i b u t i o n was r a t h e r e l o n g a t e d , and i s c h a r a c t e r i z e d by a normal d i s t r i b u t i o n t r u n c a t e d a t one s t a n d a r d d e v i a t i o n below t h e mean and two s t a n d a r d d e v i a t i o n s above t h e mean.
A l l of t h e l e n g t h d a t a i n t h e t a b l e were determined d i r e c t l y from c a t c h e s o b t a i n e d d u r i n g simultaneous measurement w i t h t h e split-beam echo sounder e x c e p t f o r t h o s e of cod on 18 March. Because t h e s e measurements were made i n an a r e a of i n t e n s i v e , commercial cod f i s h i n g , t r a w l i n g was impossible. Recourse was t h e r e f o r e had t o Danish-seine c a t c h d a t a c o l l e c t e d a t Svolvær and Henningsvar over t h e p e r i o d 8-14 March. The s i m i l a r i t y of t h e s e d a t a w i t h t h o s e determined by t r a w l i n g by R/v G.O.
SARS on 1 5 March i s noted.
Because o f t h e s m a l l n e s s of t h e h e r r i n g c a t c h e s shown i n Table 1, t h e parameters o f t h e assumed l e n g t h d i s t r i b u t i o n were determined on t h e b a s i c of f i v e c a t c h e s . These i n c l u d e d t h o s e taken d u r i n g d a t a s e r i e s 15 and 25 and t h e o t h e r t h r e e c a t c h e s from t h e same p e r i o d and a r e a .
When t r a w l i n g d u r i n g t h e March Lofoten c r u i s e , a bottom t r a w l , t h e co-called shrimp t r a w l , was used f o r a l 1 r e p o r t e d c a t c h e s e x c e p t t h a t of cod on 1 5 March, f o r which t h e p e l a g f c t r a w l was used. For t h e r e p o r t e d c a t c h e s from t h e J u l y S h e t l a n d I s l a n d s c r u i s e , t h e p e l a g i c t r a w l was used.
The a c o u s t i c d a t a a r e s w a r i z e d through t h e histograms of i n s i t u t a r g e t s t r e n g t h s i n F i g . 1. A d d i t i o n a l d a t a i n t h e form of ping number
1 7 Norway p o u t and s a i t h e
2Ø
R e d f i s h and s a i t h e
e . S e r . 8
G r e a t e r s i l v e r s m e l t
. ;o;. i i a
/ 1.
S e r . 26Norway p o u t
50 -40 -30 -20
Fig. 1. 'Farget s t r e n g t h hist.ograms of t e n d a t a s e t s d e r i v e d w i t h t h e s p l i t - b e a m echo sounder. See T a b l e s 1 and 2 f o r f u r t h e r d e t a i . 1 ~ .
and d e p t h , which a r e a t t a c h e d t o each r e s p e c t i v e t a r g e t s t r e n g t h datum, a r e n e g l e c t e d h e r e . Thus, according t o F i g . 7 i n Foote, K r i s t e n s e n and S o l l i ' s paper ( 1 9 8 4 ) , some of t h e measurements i n c l u d e d i n t h e histograms d e r i v e from t h e same f i s h , observed r e p e a t e d l y during passage of t h e v e s s e l and echo sounder beam. Such m u l t i p l e o b s e r v a t i o n s , which may i n v o l v e from a b o u t 15% o r l e s s t o more t h a n 50% of t h e t o t a l number of s i n g l e - f i s h d a t a , a r e n o t expected t o b i a s t h e r e s u l t s , although
r e f e r e n c e t o t h e o r i g i n a l d a t a and recomputation could d e c i d e t h e m a t t e r i f necessary.
Some circumstances of t h e a c o u s t i c d a t a c o l l e c t i o n a r e given i n Table 2 . Both t h e depth range and b o a t speed r e f e r t o t h e analyzed d a t a . The depth i s a c t u a l l y t h e sum of t h e d e p t h of hull-mounted t r a n s d u c e r , which i s about 5 m , and t h e t a r g e t range. Howevex, s i n c e t a r g e t s a r e a c c e p t e d by t h e ES380 system o n l y i f l y i n g w i t h i n 4.94 deg o f t h e a c o u s t i c a x i s , t h e depth e s t i m a t e i s o n l y n e g l i g i b l y b i a s e d .
Often fewer d a t a were analyzed t h a n were a v a i l a b l e . Reasons f o r t h i s i n c l u d e d t h e d e s i r e t o m a i n t a i n a homogeneous d a t a s e t , a s , f o r example, by l i m i t i n g t h e v e s s e l speed t o a narrow range, o r by l i m i t i n g t h e f i s h echoes t o a narrower depth range than was a c t u a l l y employed d u r i n g t h e o b s e r v a t i o n s .
Table 2. Conditions of acoustic data collection.
Measuring time
Starting position No. single-fish ~ e p t h range Boat speed
data (m) (knots)
Data Period
series Date Hour (min) Lat. (ON) Long. (O) Total Analyzed in. Max. Meaii S.D.
METHODS
Two b a s i c problems must be addressed i n a n a l y z i n g t h e d a t a p r e s e n t e d i n F i g . 1. I n t h e c a s e of t h o s e d a t a c o n s i s t i n g of mixed s p e c i e s , namely d a t a s e r i e s 1 and 3, t h e t a r g e t s t r e n g t h d a t a i n F i g s . l a and c must be a s s i g n e d t o t h e i n d i v i d u a l f i s h e s . The s o l u t i o n t o t h i s problem i s
r e f e r r e d t o below a s t h e method of " s e p a r a t i o n " . The second problem, t h a t of e x t r a c t i n g t h e mean t a r g e t s t r e n g t h , i s common t o a l l d a t a s e t s . T h i s
would be t r i v i a l indeed i f t h e d a t a were u n a f f e c t e d by t h r e s h o l d i n g , b u t t h i s i s p a t e n t l y n o t t h e c a s e . C o n s i d e r a t i o n of t h e range i n f i s h s i z e s and l i k e l y corresponding t a r g e t s t r e n g t h s (Nakken and Olsen 1977, F00te and Nakken 1978) s u g g e s t s t h a t t h e t a r g e t s t r e n g t h s of t h e l a r g e s t cod have n o t been r e p r e s e n t e d because of t h e upper t h r e s h o l d , o r c u t o f f , of -20 dB. S i m i l a r l y , t h e t a r g e t s t r e n g t h s of f i s h s h o r t e r t h a n 30 cm o f t e n l i e w e l l below t h e lower t h r e s h o l d o f -50 dB. Thus t h e e f f e c t o f
t h r e s h o l d i n g must b e considered i n computing mean t a r g e t s t r e n g t h s from t h e split-beam measurements i f , f o r example, it i s i n t e n d e d t o use t h e s e i n t y p i c a l echo i n t e g r a t i o n work. The reason i s simply t h a t s t a n d a r d echo i n t e g r a t o r s r e g i s t e r f i s h echoes o v e r a much g r e a t e r dynamic range than t h e 30 dB of f i n a l r e g i s t r a t i o n i n t h e SIMRAD split-beam system.
S e p a r a t i o n of composite t a r g e t s t r e n g t h histograms
T h i s a p p l i e s t o t h e d a t a of s e r i e s 1 and 3. It i s a p p a r e n t from Table 1 t h a t t h e s a i t h e l e n g t h d i s t r i b u t i o n s resemble t h a t of s e r i e s 2 . I n f a c t , t h e geographical a r e a s of t h e t h r e e s e r i e s a r e e s s e n t i a l l y t h e same, b e i n g t h e f i s h i n g banks w e s t of Lofoten, where s a i t h e spa-dn i n t h e s p r i n g . Thus t h e r e l a t i v e c o n t r i b n t i o n of t h e s a i t h e t o t h e composite t a r g e t s t r e n g t h histograms o f F i g s . l a and c a r e known. Because t h e second s p e c i e s of the two d a t a s e r i e s , Norway pout and r e d f i s h ,
~ e s p e c t i v e l y , a r e s m a l l e r t h a n t h e s a i t h e , t h e g r e a t e s t t a r g e t s t r e n g t h s of t h e l a r g e s t Norway pout and r e d f i s h , w i l l undoubtedly be s u b s t a n t i a l l y less t h a n t h e q r e a t e s t t a r g e t s t r e n g t h s of t h e l a r g e s t s a i t h e (Nakken and Olsen 1 9 7 7 ) .
The d i f f e r e n c e i n peak t a r g e t s t r e n g t h s o f t h e s e v e r a l f i s h e s can be e s t i m a t e d by r e f e r e n c e t o Nakken and O l s e n ' s t a r g e t s t r e n g t h d a t a . The a p p r o p r i a t e e q u a t i o n s s h a r e t h e common form
where TSmax i s t h e maximum 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 i n u n i t s of d e c i b e l s , R i s t h e f i s h l e n g t h i n c e n t i m e t e r s , and t h e c o e f f i c i e n t s m and b a r e determined by a least-mean-squares r e g r e s s i o n a n a l y s i s . For s a i t h e t h e r e s u l t i s
TS = 23.4 l o g R
-
65.1 maxo r , x e q u i r i n g t h a t m=20,
TS = 20 109 R
-
60.2 maxI f a nominal l e n g t h of 70 cm i s used f o r t h e l a r g e s t s a i t h e i n each o f d a t a s e r i e s 1 and 3 , t h e n t h e maximum 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 i s expected t o be about -23 o r -22 dB. T h i s a g r e e s e x a c t l y w i t h t h e o b s e r v a t i o n s i n F i g s . l a and c .
Norway pout was n o t measured by Nakken and Olsen. I t i s a gadoid,
hence f o r p r e s e n t purposes might be r e p r e s e n t e d a s having a t a r g e t s t r e n g t h roughly comparable t o t h a t o f o t h e r gadoids o f s i m i l a r l e n g t h . For want of a c l o s e r k i n s h i p , t h e maximum t a r g e t s t r e n g t h r e l a t i o n f o r Norway pout i s based on t h e combined cod, s a i t h e and p o l l a c k d a t a of Nakken and Olsen. I t i s
TSmax = 24.5 l o g R
-
67.1 o r , r e q u i r i n g m=20,TS = 20 l o g R
-
60.5 maxThus f o r t h e l a r g e s t observed Norway p o u t , w i t h R=21 cm, t h e maximum t a r g e t s t r e n g t h i s expected t o be about -35 o r -34 dB.
Redfish was a l s o o m i t t e d by Nakken and Olsen i n t h e i r ~neasurements.
Were t h e gadoid d a t a a p p r o p r i a t e , although r e d f i s h i s n o t a gadoid, a maxilnum t a r g e t s t r e n g t h of about - 2 8 o r -27 dB could be expected from t h e l a r g e s t caught specimen of 43 cm. However, comparison of t h e t a r g e t s t r e n g t h h i s t a g r a m s of Figs. l b and c s u g g e s t s a p o s s i b l e g r e a t e s t r e d f i s h t a r g e t s t r e n g t h of -30.5 dB.
S e p a r a t i o n of t h e s a i t h e c o n t r i b u t i o n from t h e composite histograms i n F i g s . l a and c i s accomplished by a t t r i b u t i n g a l l d a t a above t h e l i k e l y g r e a t e s t t a r g e t s t r e n g t h o f t h e second f i s h e s t o s a i t h e . The number of r e p r e s e n t e d s a i t h e d a t a above t h i s c u t o f f r e p r e s e n t s t h e same f r a c t i o n of t h e e n t i r e s a i t h e d i s t r i b u t i o n a s does t h e comparable p a r t of t h e pure- s a i t h e t a r g e t s t r e n g t h histogram i n Fig. l b . The p u r e - s a i t h e histogram can t h u s b e s c a l e d a b s o l u t e l y , and t h e p a r t below t h e c u t o f f s i n F i g s . l a and c can be s u b t r a c t e d d i r e c t l y from t h e composite histogram. The r e s u l t o f applying t h i s procedure t o t h e composite d a t a i n F i g s . l a and c i s shown i n F i g s . 2a and b , r e s p e c t i v e l y .
a ra
I Ia . S e r . 1 remainder
-
Norway poutb . S e r . 3 remainder
-SP1 -40 -30 -20 -50 -40 -30 -20
TS (DB) TS
(DB1
F i g . 2. T a r g e t s t r e n g t h histograms d e r i v e d from F i g s . l a and c by removal of t h e s a i t h e c o n t r i b u t i o n s .
T h r e s h o l d coinpensation
The e f f e c t o f t h e s h o l d i n g by t h e s p l i t - b e a m echo sounder a t -50 and -20 di3 i s e s t i m a t e d by comparing t h e p u r e - s p e c i e s t a r g e t s t r e n g t h h i s t o g r a m s o f F i g s . l b , d - j , and 2 w i t h t a r g e t s t r e n g t h h i s t o y r a m s s i m u l a t e d f o r
comparable s p e c i e s from Nakken and O l s e n ' s d a t a , b u t w i t h o u t i m p o s i t i o n o f t h r e s h o l d s . The e m p i r i c a l d i s t r i b u t i o n i s t h e n extended by appending t h e t a i l s o f t h e s i m u l a t e d h i s t o g r a m : t h o s e p a r t s l y i n g below a b o u t -50 dB and above -20 dB, w h i l e t h e o r i g i n a l e m p i r i c a l h i s t o g r a m i s s c a l e d down i n p r o p o r t i o n t o t h e c e n t r a l p a r t o f t h e s i m u l a t e d h i s t o g r a m , namely t h a t l y i n g between t h e two t h r e s h o l d s .
The t a r g e t s t r e n g t h s i m u l a t i o n d a t a were d e r i v e d d i r e c t l y from Nakken and O l s e n ' s d a t a . To form a uniform b a s i s h a v i n g t h e same l e n g t h d i s t r i b u t i o n a s t h e o b s e r v e d , o n l y t h o s e t a r g e t s t x e n g t h f u n c t i o n s were used w i t h f i s h l e n g t h s l y i n g w i t h i n t h e t r u n c a t i o n l i m i t s shown i n T a b l e 1. The measured
(
t a r g e t s t r e n g t h f u n c t i o n s were t h e n s c a l e d b o t h i n magnitude and a n g l e t o s i m u l a t e a s e r i e s o f f i s h spanning t h e l e n g t h r a n g e . T a r g e t s t r e n g t h v a l u e s l o s t by c o n t r a c t i o n o f t h e o r i g i n a l f u n c t i o n s were r e p l a c e d by v a l u e s
d e r i v e d from t h e v e r y approximate r e l a t i o n TSmi,=30 l o g R
-
1 0 0 , where R i s t h e s i m u l a t e d f i s h l e n g t h i n c e n t i m e t e r s . Computed t a r g e t s t r e n g t hh i s t o g r a m s f o r e a c h o f t h e s e were s u b s e q u e n t l y compounded a c c o r d i n g t o a t r u n c a t e d normal d i s t r i b u t i o n h a v i n g t h e mean and s t a n d a r d d e v i a t i o n g i v e n i n t h e same t a b l e .
R e p r e s e n t a t i o n o f t h e s e v e r a l f i s h e s i n t h e s i m u l a t i o n was one-to-one f o r cod, s a i t h e and h e r r i n g . F o r b o t h Norway p o u t and t h e non-gadoid b u t p h y s o c l i s t o u s r e d f i s h , Nakken and O l s e n ' s combined d a t a f o r cod, s a i t h e and p o l l a c k were u s e d . The non-clupeoid, phyostomous g r e a t e r s i l v e r s m e l t was r e p r e s e n t e d i n i t i a l l y by h e r r i n g , b u t t h e a p p l i c a b l e d a t a were s o few t h a t t h e s i m u l a t i o n was r e p e a t e d on t h e b a s l s o f t h e gadoid d a t a , a n d t h e s e p a r a t e r e s u l t s were a v e r a g e d .
I n a t t a c h i n g t h e lower t a i l of t h e s i m u l a t e d h i s t o g r a m t o t h e s p l i t - beam h i s t o g r a m , e i t h e r -50, -48.5 o r -47 dE was u s e d a s t h e a t t a c h m e n t 1 p o i n t . The e x a c t c h o i c e was made t o o p t i m i z e t h e agreement o f t h e two
h i s t o g r a m s . Three h i g h e r c u t o f f s , -45.5, -44 and -42.5 dB, were a l s o examined b u t were c l e a r l y e x c e s s i v e a s t h e agreement was much p o o r e r .
Another i n g r e d i e n t f o r s i m u l a t i n g t h e t a r g e t s t r e n g t h d i s t r i b u t i o n i s knowledge o f t h e f i s h b e h a v i o u r a s e x p r e s s e d through t h e tilt a n g l e d i s t r i b u t i o n . N o t w i t h s t a n d i n g a f e v e r i s h i n t e r e s t i n t h e s u b j e c t ( F o o t e 1980a, F o o t e and Ona 1 9 8 5 ) , and rumours o f development o f a t r a n s p o n d i n g t i l t - a n g l e - m e a s u r i n g t a g , tilt a n g l e d i s t z i b u t i o n s have been d e t e r m i n e d a t s e a f o r o n l y t h r e e s p e c i e s (Olsen 1971, Carscadden and M i l l e r 1980,
B u e r k l e 19831. Given t h e s e n s i t i v i t y o f t h e tilt a n g l e d i s t r i b u t i o n t o b e h a v i o u r , f o r example, d i r e c t e d h o r i z o n t a l swimming v e r s u s f e e d i n g v e r s u s d i v i n g , t h i s i s c l e a r l y unknown f o r t h e o b s e r v e d fiski.
The s t a t e o f n e a r l y t o t a l i g n o r a n c e o f f i s h b e h a v i o u r was remedied by assuming a r a n g e o f b e h a v i o u r modes, p e r f o r m i n g t h e d e s c r i b e d
computations f o r e a c h , and a v e r a g i n g t h e r e s u l t s o v e r t h e e n t i r e s e t . A s i n g l e assumption was made a b o u t t h e b e h a v i o u r : t h a t it was n o t extreme.
T h i s h y p o t h e s i s was t h e o r e t i c a l l y s u s t a i n e d , i n f a c t , f o r s i m u l a t e d t a r g e t
s t r e n g t h d i s t r i b u t i o n s f o r mean tilt a n g l e s g r e a t e r t h a n 10 deg from t h e h o r i z o n t a l g e n e r a l l y l a c k o r u n d e r r e p r e s e n t t h e l a r g e s t observed t a r g e t s t r e n g t h s . T h e r e f o r e , i f t h e t a r g e t s t r e n g t h measurements of Nakken and Olsen (1977) and t h e i r a p p l i c a b i l i t y (Foote 1983) can be b e l i e v e d , t h e n s t r o n g avoidance r e a c t i o n s with d i v i n g (Olsen 1979, 1981) a r e simply incompatible w i t h t h e o b s e r v a t i o n s .
The non-extreme behaviour modes were c h a r a c t e r i z e d by normal d i s t r i b u t i o n s i n tilt a n g l e w i t h means of -10, -5, 0 , 5 and 10 deg and s t a n d a r d d e v i a t i o n s of 5 , 10 and 1 5 deg. I n averaging t h e t a r g e t s t r e n g t h s w i t h r e s p e c t t o t h e normal d i s t r i b u t i o n s , t h e e f f e c t of p e r s p e c t i v e
(Foote 1980b) was i n c o r p o r a t e d by i n c r e a s i n g t h e f i r s t two s t a n d a r d d e v i a t i o n s t o 5.5 and 10.2 deg, r e s p e c t i v e l y , while l e a v i n g t h e t h i r d unchanged. These v a l u e s were determined f o r a c i r c u l a r beam p a t t e r n whose edge i s 5 deg from t h e a c o u s t i c a x i s assuming an e q u a l l y l i k e l y
p r o b a b i l i t y of occurrence anywhexe i n t h e h o r i z o n t a l p l a n e (Foote 1 9 8 5 ) . Computation of t h e mean t a r g e t s t r e n g t h
TS
f o r a p a r t i c u l a r behaviour mode was accomplished i n t h e i n t e n s i t y domain. Thus-
TS = 10 l o g-
-'
47r I
where t h e average 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
i s determined from t h e s e t of p r o b a b i l i t i e s {hi, i = 1 , 2 , . . . , n ) d e s c r i b i n g t h e extended e m p i r i c a l histogram according t owhere ai i s t h e average 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 o r t h e p a r t i c u l a r t a r g e t s t r e n g t h i n t e r v a l . I f t h i s e x t e n d s from TSi t o TSi+l, t h e n
Computation of t h e mean t a r g e t s t r e n g t h TS* w i t h r e s p e c t t o a l 1 1 5 i n v e s t i g a t e d non-extreme behaviour modes i s a l s o e f f e c t e d i n t h e i n t e n s i t y domain. The sample v a r i a t i o n i n t h i s mean v a l u e was e s t i m a t e d by computing t h e s t a n d a r d d e v i a t i o n s Au* of t h e averaged average 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*,
and then computingo* I Ao*
TS* = 10 l o g
f 4n
RESULTS
Threshold-cornpensated mean i n s i t u t a r g e t s t r e n g t h s d e r i v e d w i t h t h e new split-beam echo sounder a r e shown i n Table 3 . The s t a n d a r d d e v i a t i o n s i n t a r g e t s t r e n g t h r e f l e c t u n c e r t a i n t y o v e r t h e e x a c t behaviour mode, o r tilt a n g l e d i s t r i b u t i o n , assumed i n t h e course o f complementing t h e
T a b l e 3. Threshold-compensated mean i n s i t u t a r g e t s t r e n g t h s d e r i v e d w i t h t h e SIMRAD ES380 s p l i t - beam echo sounder.
Boat s p e e d T a r g e t c t r e n g t h bzo Data
F i s h Length (cm) Depth (m) ( k n o t s ) No. d a t a (dB) (dB) s e r i e s
Cod 8 1 . 6 I 1 1 . 4 7 0 - 1 6 5 2 . 7 5 0 . 2 4400 -28.7 I 0 . 8 -66.9 7
Cod 81.6 I 11.4 85
-
160 11.3 I 0.4 9600 - 2 9 . 0 1 0 . 8 -67.2 I l aCod 8 1 . 6 I 1 1 . 4 8 5 - 1 6 0 3 . 3 1 0 . 3 9000 -28.5 I 0.7 -66.7 l l b
S a i t h e 57.2 1 6 . 0 105
-
130 2.9 I 0.2 3000 -30.3 I 0.0 -65.5 2Norway p o u t 17.6 I 1.6 105
-
240 3.9 I 2.6 9800 - 4 2 . 0 1 0 . 8 -66.9 1N o r w a y p o u t 1 4 . 8 I 1 . 1 8 5 - 1 1 5 4 . 2 i 3 . 7 4201 -44.7 I 1.0 -68.1 26
H e r r i n g 28.5 I 2.0 65
-
95 7 . 1 I 2.9 6545 -43.3 I 0.4 -72.4 1 5H e r r i n g 28.5 I 2.0 1 5
-
45 5.5 I 3.8 2687 -42.5 I 0.4 -71.6 25o r i g i n a l , g e n e r a l l y t r u n c a t e d d a t a s e t s . The s t a n d a r d d e v i a t i o n was computed a s t h e a r i t h m e t i c mean o f TS* and TS!, which i n c u r r e d no e r r o r b e c a u s e o f t h e s i m i l a r s m a l l n e s s o f t h e e x c u r s i o n s from t h e mean TS*.
For comparison p u r p o s e s t h e q u a n t i t y
bZo
= TS*-
20 l o gR ,
( 8 )where
R
i s t h e mean f i s h l e n g t h , i s incl-uded.The mean t a r g e t s t r e n g t h d e r i v e d by e q u a l w e i g h t i n g o f t h e t h r e e cod d a t a i s -28.9
a.
I f t h i s i s used t o g e t h e r w i t h t h e t a b u l a t e d d a t a f o r t h e o t h e r g a d o i d s , t h e n t h e r e s u l t of r e g r e s s i n g t a r g e t s t r e n g t h ont h e l o g a r i t h m o f mean f i s h l e n g t h
R
i si
TSgadoidc = 21.9 l o g
R -
6 9 . 7which o b t a i n s w i t h a s t a n d a r d e r r o r o f 1.0 dB. I f t h e l e n g t h dependence i s c o n s t r a i n e d t o be 20 l o g
R,
t h e n. -
TSgadoids = 20 l o g R
-
66.8w i t h s t a n d a r d e r r o r o f 1.1 dE3. I n c a s e each o f t h e t a b u l a t e d cod d a t a i s weighted e q u a l l y w i t h each o f t h e o t h e r t h r e e gadoid d a t a , t h e n t h e
r e s u l t i n q e q u a t i o n s a r e TS=21.1 l o g
-
68.7 and TS=20 l o gR -
66.9, which o b t a i n w i t h t h e r e s p e c t i v e s t a n d a r d e r r o r s o f 0.9 and 1 . 0 dE3.If t h e m a t t e r o f t h e d e p t h dependence of t h e h e r r i n g d a t a i s ignoxed, and t h e two t a r g e t s t r e n g t h s a r e accorded e q u a l w e i g h t , t h e n t h e a v e r a g e t a r g e t s t r e i i g t h o f 2 8 . 5 cm h e r r i n g i s -42.9 dB. I f t h i s s i n g l e datum i s
' a l l o w e d t o d e t e r m i n e t h e c o e f f i c i e n t b i n t h e e q u a t i o n ==20 l o g
R +
b , t h e nDISCUSSION
The aim o f t h i s p a p e r h a s been a s i m p l e p r e s e n t a t i o n o f
--
i n s i t u t a r g e t s t r e n g t h s , d e r i v e d w i t h t h e new s p l i t - b e a m echo s o u n d e r , f o ra p p l i c a t i o n i n echo i n t e g r a t i o n . However, t h e d i f f e r e n c e i n t h r e s h o l d i n g p r a c t i c e s between t h e s p l i t - b e a m system and t y p i c a l echo i n t e g r a t o r s i s s u b s t a n t i a l : echo i n t e g r a t o r s t y p i c a l l y r e g i s t e r f a r weaker s i g n a l s t h a n t h o s e c o r r e s p o n d i n g t o t h e l o w e r , -50 dB t h r e s h o l d o f t h e s p l i t - b e a m s y s t e m , and s a t u r a t i o n o c c u r s a t t y p i c a l d e t e c t i o n r a n g e s o n l y f o r much s t r o n g e r s i g n a l s t h a n t h o s e c o r r e s p o n d i n g t o t h e u p p e r , -20 dB t h r e s h o l d o r c u t o f f o f t h e s p l i t - b e a m system. Thus a c a r e f u l examination o f t h e t h r e s h o l d i n g h a s been n e c e s s a r y .
Compensation f o r t h e t h r e s h o l d e f f e c t s h a s been a c h i e v e d through a combined comparison and e x t r a p o l a t i o n p r o c e d u r e b a s e d on s i m u l a t e d t a r g e t s t r e n g t h d i s t r i b u t i o n s . These depend on t h e v a l i d i t y o f t h e b a s i s t a r g e t s t r e n g t h d a t a , presumed e s t a b l i s h e d ( ~ o o t e 1 9 8 3 ) , and knowledge o f t h e f i s h b e h a v i o u r a s e x p r e s s e d t h r o u g h t h e t f L t a n g l e d i s t r i b u t i o n . Given n e a r l y cornplete i g n o r a n c e o f t h e p a r t i c u l a r b e h a v i o u r p a t t e r n s , a r a n g e o f non- extreme b e h a v i o u r modes h a s been assumed. Averaging o f t h e r e s p e c t i v e mean t a r g e t s t r e n g t h s h a s r e v e a l e d a r a t h e r low v a r i a n c e , w i t h no s t a n d a r d d e v i a t i o n e x c e e d i n g 1 . 0 dB a c c o r d i n g t o Table 3. T h i s i s f o r t u n a t e , f a r i n d i c a t i n g a b a s i c i n s e n s i t i v i t y o f threshold-compensated i n s i t u t a r g e t
--
s t r e n g t h s t o t h e p a r t i c u l a r b e h a v i o u r mode, which i s b o t h unknown and d i f f i c u l t t o know.
There i s , however, c l e a r s u p p o r t f o r t h e e x c l u s i o n o f extreme
b e h a v i o u r p a t t e r n s from t h e a n a l y s i s o f e a c h d a t a s e r i e s h e r e . I t i s t h e a b s e n c e of r e l a t i v e l y l a r g e t a r g e t s t r e n g t h s i n t h e s i m u l a t e d d i s t r i b u t i o n s . I f t h e mean tilt a n g l e were, f o r i n s t a n c e , t o d e v i a t e from t h e h o r i z o n t a l by more t h a n a b o u t 10 d e g , t h e n it would be d i f f i c u l t , i £ n o t i m p o s s i b l e , t o e x p l a i n t h e l a r g e t a r g e t s t r e n g t h s t h a t were o b s e r v e d . I n a word, t h e p r e s e n t a n a l y s i s i n d i c a t e s t h a t f i s h d e t e c t e d w i t h i n t h e a c o u s t i c beam were n o t s e r i o u s l y a f f e c t e d by t h e p a s s a g e of t h e v e s s e l . I t i s i m p o r t a n t t o n o t e t h a t most of t h e d a t a were c o l l e c t e d a t moderate s p e e d s . I n t h e c a s e of cod, however, d a t a were c o l l e c t e d a t each of s e v e r a l d i s t i n c t s p e e d s , v a r y i n g from l e s s t h a n 3 k n o t s t o more t h a n 11 k n o t s , y e t n e i t h e r s y s t e m a t i c n o r s i g n i f i c a n t d i f f e r e n c e s i n t a r g e t s t r e n g t h were found.
J u s t i f i c a t i o n f o r t h e t h r e s h o l d compensation i s p r o v i d e d by a
comparison o f t h e compensated mean t a r g e t s t r e n g t h s w i t h t h e c o r r e s p o n d i n g mean t a r g e t s t r e n g t h s computed d i r e c t l y from t h e uncompensated s p l i t - b e a m d a t a . Only i n t h e c a s e o f s a i t h e a r e t h e e s t i m a t e s i d e n t i c a l , which i n d i c a t e s t h a t t h e o b s e r v e d t a r g e t s t r e n g t h d i s t r i b u t i o n f o r s a i t h e i s e x p e c t e d t o l i e wholly w i t h i n t h e a c c e p t a n c e r a n g e of t h e echo sounder.
For t h e o t h e r f i s h e s , t h e e f f e c t o f compensation, a s b a s e d s o l e l y on t h e mean v a l u e s , v a r i e s from -1.8 t o 1 . 8 dB.
While p r e p a r i n g t h e s p l i t - b e a m d a t a f o r a v e r a g i n g , two i n s t a n c e s o f mixed-species d a t a were e n c o u n t e r e d . I n each o f t h e s e , t h e d i s t r i b u t i o n
form o f t h e component w i t h t h e l a r g e r t a r g e t s t r e n g t h s , namely s a i t h e , was w e l l known. T h i s a l l o w e d s u b t r a c t i o n o f t h e e n t i r e l a r g e - f i s h c o n t r i b u t i o n , l e a v i n g t h e s m a l l - f i s h d i s t r i b u t i o n a s t h e remainder f o r f u r t h e r a n a l y s i s . A d e g r e e o f j u s t i f i c a t i o n f o r t h i s p r o c e d u r e l i e s i n t h e f i n a l r e s u l t s : t h e t a r g e t s t r e n g t h s o f t h e Norway p o u t o f 17.6 cm mean l e n g t h and t h e r e d f i s h a r e i n l i n e w i t h o t h e r p h y s o c l i s t
--
i n s i t ut a r g e t s t r e n g t h s , b o t h a s determined i n t h i s s t u d y and a s d e t e r m i n e d e l s e w h e r e . Exemplary, i n d e p e n d e n t l y d e r i v e d t a r g e t s t r e n g t h d a t a a r e p r o v i d e d by a s e r i e s o f rneasurements o f w a l l e y e p o l l o c k ( T h e r a g r a
chalcogramma) w i t h t h e dual-beam echo sounder (Traynor and Ehrenberg 1979, Ehrenberg e t a l . 1981, Ehrenberg 1983, Txaynor and Williamson 1983,
Traynor 1 9 8 4 ) .
I t i s t e m p t i n g t o compare t h e p r e s e n t r e s u l t s w i t h o t h e r i n s i t u
--
/d a t a i n s y s t e m a t i c f a s h i o n , b u t t h i s exceeds t h e scope o f t h i s work. i
Two o t h e r comparisons o u g h t , however, t o b e made b e f o r e p a s s i n g . (1) Gadoid t a r g e t s t r e n g t h . The r e l a t i o n d e r i v e d on t h e s o l e
b a s i s of 1 3 p o l l a c k swimbladders and 2 s a i t h e swimbladders, and assumption o f cod b e h a v i o u r a s d e s c r i b e d by Olsen ( 1 9 7 1 ) , i s ( F o o t e 1985)
which i s t o b e compared w i t h Eq. ( 9 b ) .
( 2 ) H e r r i n g t a r g e t s t r e n g t h . The r e l a t i o n recommended by t h e 1983 P l a n n i n g Group on ICES-Coordinated H e r r i n g and S p r a t A c o u s t i c Surveys
(Anon. 1983) i s
which i s t o be compared w i t h Eq. ( 1 0 ) .
Much more remains t o b e done w i t h t h e d a t a a n a l y z e d h e r e . Three examples o f s t u d i e s f o r f u t u r e p r o s e c u t i o n i n c l u d e t h e f o l l o w i n g :
(1) d e t e r m i n a t i o n o f t h e d e p t h dependence o f t h e h e r r i n g t a r g e t s t r e n g t h , (2) compensation f o r t h r e s h o l d i n g on t h e b a s i s o f d a t a s i m u l a t e d from swimbladder morphometries, and ( 3 ) i n v e s t i g a t i o n o f a v o i d a n c e r e a c t i o n s t h r o u g h a s t a t i s t i c a l a n a l y s i s o f echo t r a c e l e n g t h s (Midttun 1984,
Aksland 1 9 8 5 ) . Another s t u d y which c o u l d be p r o f i t a b l y undertaken i n t h e f u t u r e , were b e t t e r b e h a v i o u r a l d a t a forthcoming i n t h e i n t e r i m , i s a r e f i n e m e n t o f t h e p r e s e n t t a r g e t s t r e n g t h v a l u e s b a s e d on more c e r t a i n s p e c i f i c a t i o n o f t h e a p p l i c a b l e k i l t a n g l e d i s t r i b u t i o n s .
I t i s i n t e r e s t i n g r e t r o s p e c t i v e o f t h e i n t r o d u c t i o n o f t h e s p l i t - b e a m echo sounder one y e a r ago ( F o o t e , K r i s t e n s e n and S o l l i 1984) t o n o t e t h a t t h e p o t e n t i a l o f t h e i n s t r u m e n t i s b e i n g r e a l i z e d . However, i t i s e x c e e d i n g l y i m p o r t a n t t o o t o c a 1 1 t h e a t t e n t i o n o f c u r r e n t and f u t u r e u s e r s o f t h e equipment t o t h e h a z a r d s o f i g n o r i n g t h e t h r e s h o l d s , t h e S c y l l a o f -20 dB, t h e Charybdis o f -50 'dB.
ACKNOWLEDGEMENTS
A . Raknes i s thanked f o r c o l l e c t i n g t h e a c o u s t i c d a t a on cod on 1 8 March 1984. N . Diner i s thariked f o r t r a n s l a t i n g t h e a b s t r a c t , which was i n c o m p l e t e when s e n t t o him.
Aksland, M. 1985. B a s i c e c h o - t r a c e - l e n g t h s t a t i s t i c s . ICES c . M . / ~ : 3 2 , 12 pp. [mimeo].
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h e r r i n g and s p r a t a c o u s t i c s u r v e y s . ICES C . M . / H : ~ ~ , 11 pp. [mimeo].
B u e r k l e , U . i 9 8 3 . F i r s t look a t h e r r i n g d i s t r i b u t i o n s w i t h a bottom r e f e r e n c i n g underwater towed i n s t r u m e n t a t i o n v e h i c l e "BRUTIV". FAO F i s h . Rep., ( 3 0 0 ) : 125-130.
Carscadden, J . E . , and M i i l e r , D. S. 1980. E s t i m a t e s o f "Lilt a n g l e o f c a p e l i n u s i n g underwater p h o t o g r a p h s . ICES c . M . / ~ : 5 0 , 4pp. [mimeo].
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Ehrenberg, J. E . 1979. A comparative a n a l y s i s o f
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1981. Improved c a l i b r a t i o n o f h y d r o a c o u s t i c equipment w i t h copper s p h e r e s . ICES C.M./B:20, 1 8 pp. [mimeo].
Foote, K. G. 1982. Optimizing copper s p h e r e s f o r p r e c i s i o n c a l i b r a t i o n of hydroacoustic equipment. J . a c o u s t . Soc. A m . , 71: 742-747.
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Olsen, K. 1979. Observed avoidance behaviour i n h e r r i n g i n r e l a t i o n t o passage of an echo s u r v e y v e s s e l . ICES c . M . ( ~ : 1 8 , 21 pp. [mimeo].
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Traynor, J . J . , and Ehrenberg, J. E . 1979. E v a l u a t i o n of t h e d u a l beam a c o u s t i c f i s h t a r g e t s t r e n g t h measurement method. J . F i s h . Res.
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Traynor, J. J . 1984. Dual beam measurement o f f i s h t a r g e t s t r e n g t h and r e s u l t s of an echo i n t e g r a t i o n survey of t h e e a s t e r n Bering Sea walleye p o l l o c k (Theragra chalcogramma). T h e s i s . U n i v e r s i t y of Washington, 153 pp.
