Helgolfinder Meeresunters. 33, 687-699 (1980)
Fisheries m a n a g e m e n t and c o n s e r v a t i o n in the Irish Sea
K.
B r a n d e r
Ministry of Agriculture, Fisheries and Food, Fisheries Laboratory; Lowestoft, Suffolk, NR 33 OHT, UK
ABSTRACT: The Irish Sea ia a relatively small, enclosed sea area which is subject to a wide range of human uses including navigation, oil terminals, dumping of sewage and industrial sludge, cooling for nuclear power stations, gravel extraction, gas and oil prospecting and fishing. Commer- cial fishing is affected by the other uses and at the same time it provides a means of monitoring their effects on a part of the ecosystem. Regular samples taken from fish markets provide a long series of age-composition data of the main commercial species - cod, whiting, plaice and sole - from which population changes can be assessed. More recently groundfish trawl surveys have been carried out to provide more detailed information on the distribution of all demersal fish species seasonally and in relation to area, depth and sediment type. Advice on the management of commercial fish species is prepared by a working group of the International Council for the Exploration of the Sea (ICES}, and is based mainly on analytical single-species models. There are obvious shortcomings of such models in an area of mixed fishery and high diversity such as the Irish Sea. The objectives adopted in these models and in fisheries management generally are examined critically in relation to the possible aims of conservation.
I N T R O D U C T I O N
T h e Irish S e a (ICES D i v i s i o n VIIa, s e e Fig, 1) has a n a r e a of a p p r o x i m a t e l y 45 000 k m 2 a n d is a r e l a t i v e l y e n c l o s e d s e a w h i c h is s u b j e c t to a w i d e r a n g e of h u m a n uses. Its m a x i m u m depth, in t h e N o r t h C h a n n e l , is o v e r 200 m b u t m o s t of t h e a r e a is less t h a n 50 m d e e p . In t e r m s of a n n u a l fish y i e l d it is o n e of the l e a s t p r o d u c t i v e a r e a s a r o u n d t h e British Isles (Brander, 1977) a n d this m a y b e l i n k e d to t h e a p p a r e n t l y short a n d l a t e p r o d u c t i o n c y c l e (Colebrook, 1979) w h i c h m a y in t u r n b e d u e to strong t i d a l m i x i n g a n d l o w solar r a d i a t i o n d u r i n g t h e s p r i n g m o n t h s (Dickson & Reid, in preparation).
In s e v e r a l r e s p e c t s t h e Irish S e a is t h e most p o l l u t e d sea a r e a a r o u n d t h e British Isles. T h e l e v e l s of zinc, copper, r e a c t i v e m e r c u r y a n d m a n g a n e s e a r e all h i g h e r t h a n t h o s e in t h e N o r t h S e a or E n g l i s h C h a n n e l (Sheets 50 a n d 51 of L e e & Ramster, 1979). This is d u e p a r t l y to h i g h n a t u r a l l e v e l s a n d runoff from old m i n e w o r k i n g s , b u t m a i n l y to i n d u s t r i a l inputs a n d s l u d g e d u m p i n g , t h e s c a l e of w h i c h is d o c u m e n t e d b y t h e D e p a r t m e n t of the E n v i r o n m e n t (1978) a n d b y M u r r a y & N o r t o n (1979). T h e m a j o r d i s c h a r g e of r a d i o a c t i v e w a s t e i n t h e UK t a k e s p l a c e at W i n d s c a l e in C u m b r i a a n d l e v e l s of c a e s i u m - 1 3 7 in the Irish S e a are up to t h r e e orders of m a g n i t u d e h i g h e r t h a n t h o s e in the N o r t h S e a (Mitchell, 1977).
O t h e r a c t i v i t i e s a f f e c t i n g life i n the s e a are" n a v i g a t i o n b y oil tankers, c a r g o v e s s e l s a n d ferries - the s c a l e of w h i c h c a n b e j u d g e d from s h e e t 7 of Lee & R a m s t e r (1979); oil t e r m i n a l s - t h e r e are m a j o r t e r m i n a l s at M i l f o r d H a v e n a n d A n g l e s e y ; g r a v e l e x t r a c t i o n
688 K. B r a n d e r
b y d r e d g e r s - m a i n l y in L i v e r p o o l Bay (sheet 23 of Lee & Ramster, 1979); oil a n d g a s p r o s p e c t i n g - a g a i n m a i n l y in L i v e r p o o l B a y at p r e s e n t .
A l l of t h e s e h a v e c a u s e d c o n c e r n a b o u t t h e state of t h e m a r i n e e n v i r o n m e n t b o t h b e c a u s e of t h e d a n g e r to life in t h e s e a and, in t h e c a s e of p o l l u t a n t s , b e c a u s e of t h e d a n g e r of r e t u r n p a t h w a y s to m a n . M o n i t o r i n g p r o g r a m m e s h a v e b e e n e s t a b l i s h e d , p a r t i c u l a r l y w i t h the l a t t e r in m i n d (Mitchell, 1977; M u r r a y , 1979; P o r t m a n n , 1979). T h e d i r e c t effect of p o l l u t a n t s on life in t h e s e a is v e r y difficult to e s t a b l i s h , p a r t i c u l a r l y o u t s i d e t h e littoral, s i n c e so l i t t l e is k n o w n a b o u t n a t u r a l p o p u l a t i o n l e v e l s a n d t h e i r fluctuations. E v e n w h e r e l e t h a l effects a r e not o b s e r v e d t h e r e m a y b e less e a s i l y d e t e c t a b l e s u b l e t h a l ones, a l t h o u g h the d i s t i n c t i o n is not a p a r t i c u l a r l y i m p o r t a n t one in t e r m s of p o p u l a t i o n d y n a m i c s , s i n c e in e i t h e r c a s e a c h a n g e is b r o u g h t a b o u t in one or m o r e of t h e p o p u l a t i o n p a r a m e t e r s (growth, m o r t a l i t y , n e t r e p r o d u c t i o n , m i g r a t i o n ) .
It h a s b e e n s u g g e s t e d t h a t toxic w a s t e s , p a r t i c u l a r l y PCBs, m a y h a v e b e e n a c o n t r i b u t o r y factor in t h e i n c r e a s e d i n c i d e n c e of e p i d e r m a l l e s i o n s in t h e flatfish p o p u l a t i o n s of t h e N o r t h East Irish S e a in 1971 (Perkins et al., 1972). S h e l t o n & W i l s o n (1973) c o l l e c t e d further d a t a on e p i d e r m a l l e s i o n s a n d c o n c l u d e d t h a t " . . . of a l l the e n v i r o n m e n t a l v a r i a b l e s w h i c h c a n b e i n v o k e d to e x p l a i n t h e s e results, p o l l u t i o n is the l e a s t l i k e l y . " S i n d e r r n a n n (1977) q u o t e s o n l y t h e e a r l i e r of t h e s e s t u d i e s a n d a d d s , e r r o n e o u s l y , t h a t " F i n d a m a g e , u n k n o w n b e f o r e 1970 (in t h e Irish Sea), w a s o b s e r v e d in p l a i c e a n d d a b . "
T h e r o l e of p o l l u t i o n s t u d i e s is w e l l k n o w n , b u t in a d d r e s s i n g t h e p r o b l e m of p r o t e c t i n g life in the s e a it s e e m s s t r a n g e t h a t t h e n e e d for s t u d i e s of f i s h e r i e s is s o m e t i m e s q u e s t i o n e d . T h e r e are at l e a s t two r e a s o n s for this state of affairs w h i c h h a v e r e c e n t l y c o m e to the a u t h o r ' s notice. O n e is t h a t f i s h e r i e s b i o l o g i s t s a r e r e g a r d e d as h a v i n g t h e i n t e r e s t s of t h e f i s h i n g i n d u s t r y r a t h e r t h a n t h o s e of the m a r i n e e c o s y s t e m at h e a r t a n d t h e o t h e r is t h e i d e a t h a t f i s h e r i e s statistics a r e s u b j e c t to too m a n y u n c e r t a i n - ties to b e of a n y u s e i n m o n i t o r i n g p o p u l a t i o n c h a n g e s d u e to pollution.
This p a p e r s e e k s to e x a m i n e t h e v a l i d i t y of t h e s e criticisms a n d to s h o w t h a t f i s h i n g h a s h a d a v e r y g r e a t effect on Irish S e a fish p o p u l a t i o n s . S t u d i e s of t h e f i s h e r i e s a r e t h e r e f o r e e s s e n t i a l w h e n c o n s i d e r i n g the a i m s of m a r i n e c o n s e r v a t i o n in t h e a r e a a n d a r e also n e c e s s a r y w h e n d r a w i n g c o n c l u s i o n s a b o u t t h e p o s s i b l e effects of p o l l u t a n t s .
STUDIES O F C O M M E R C I A L FISHERIES
L O N G - T E R M C A T C H TRENDS A N D T O T A L D E M E R S A L SURPLUS P R O D U C T I O N
Statistics of a n n u a l l a n d i n g s of fish a n d s h e l l f i s h b y c o u n t r y h a v e b e e n r e c o r d e d in t h e B u l l e t i n S t a t i s t i q u e of t h e I n t e r n a t i o n a l C o u n c i l for t h e E x p l o r a t i o n of t h e S e a (ICES) s i n c e 1909. T h r o u g h o u t m o s t of this p e r i o d l a n d i n g s w e r e r e p o r t e d for t h e Irish S e a a n d Bristol C h a n n e l c o m b i n e d (ICES D i v i s i o n s VIIa a n d VIIf, s e e Fig. 1) a n d it h a s t h e r e f o r e b e e n n e c e s s a r y to k e e p t h e m c o m b i n e d i n e x a m i n i n g the l o n g - t e r m c a t c h t r e n d s a n d i n c a l c u l a t i n g t o t a l d e m e r s a l p r o d u c t i o n . T h e 1977 B u l l e t i n r e c o r d s l a n d i n g s of 30 d e m e r s a l fish s p e c i e s , 5 p e l a g i c fish s p e c i e s a n d 15 s h e l l f i s h s p e c i e s from t h e Irish Sea. T h e c o u n t r i e s t a k i n g p a r t i n t h e f i s h e r y a r e UK, I r e l a n d , F r a n c e , B e l g i u m a n d t h e N e t h e r - l a n d s . T h e p r i n c i p a l d e m e r s a l s p e c i e s a r e cod, w h i t i n g , p l a i c e , sole a n d t h e r a y s p e c i e s . H e r r i n g m a k e u p t h e b u l k of t h e p e l a g i c c a t c h a n d t h e m a i n s h e l l f i s h s p e c i e s a r e N o r w a y lobster, q u e e n scallops, s c a l l o p s a n d m u s s e l s . In 1977 i n t e r n a t i o n a l l a n d i n g s of d e m e r s a l fish in ICES D i v i s i o n VIIa t o t a l l e d a b o u t 34 000 t o n n e s , p e l a g i c fish a b o u t 22 000 t o n n e s a n d s h e l l f i s h a b o u t 21 000 t o n n e s . T h e h i g h p r o p o r t i o n of shellfish, o v e r 27 % of t h e t o t a l
8 ~ 7 ~ 6 ~ 5 ~ 4 o 3 ~ 2 ~
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w e i g h t l a n d e d , is a n o t e w o r t h y f e a t u r e of t h e Irish S e a fisheries, p a r t i c u l a r l y s i n c e s e v e r a l of t h e s p e c i e s are of h i g h v a l u e .
F i g u r e 2 shows t h e t r e n d s i n total d e m e r s a l , t o t a l p e l a g i c a n d s h e l l f i s h l a n d i n g s from D i v i s i o n s VIIa p l u s VIIf s i n c e 1922. S h e l l f i s h l a n d i n g s p r i o r to 1950 w e r e s m a l l a n d h a v e not b e e n i n c l u d e d b e c a u s e t h e units a r e i n c o m p a t i b l e . A l l t h r e e c a t e g o r i e s h a v e s h o w n g r e a t i n c r e a s e s o v e r t h e p e r i o d , b u t t h e r e a s o n s for t h e i n c r e a s e are v a r i e d . P e l a g i c l a n d i n g s r o s e s h a r p l y from 1970 o n w a r d s d u e p a r t l y to g o o d h e r r i n g r e c r u i t m e n t i n t h e Irish S e a a n d i n c r e a s e d e x p l o i t a t i o n of t h e C e l t i c S e a stock, b u t m a i n l y b e c a u s e c a t c h e s of m a c k e r e l from t h e W e s t e r l y s t o c k a r o u n d C o r n w a l l a r e s o m e t i m e s t a k e n i n D i v i s i o n VIIf. T h e r e is no i n d i c a t i o n t h a t p e l a g i c c a t c h e s in D i v i s i o n VIIa c o u l d b e m a i n t a i n e d a b o v e 20 000 t o u n e s on a s u s t a i n e d basis. F i s h i n g on the M o u r n e h e r r i n g stock h a s b e e n b a n n e d a n d t h e M a n x stock c o u l d e a s i l y follow t h e s a m e d o w n w a r d p a t h if t i g h t controls are not e n f o r c e d .
T h e i n c r e a s e d s h e l l f i s h l a n d i n g s a r e p r o b a b l y d u e to i n c r e a s e d f i s h i n g effort on t h e m as t h e m a r k e t for s h e l l f i s h h a s i m p r o v e d . F i s h i n g effort on
Nephrops
is too h i g h {Brander, 1977), b u t t h e m e s h size h a s r e c e n t l y b e e n i n c r e a s e d a n d t h e stocks s e e m to s t a n d u p to t h e f i s h i n g p r e s s u r e r e m a r k a b l y well.T h e d e m e r s a l l a n d i n g s h a v e r i s e n c o n s i d e r a b l y too o v e r the p a s t t h i r t y y e a r s a n d r e a c h e d b y far t h e h i g h e s t l e v e l of t h e c e n t u r y in 1973, s i n c e w h e n t h e r e h a s b e e n a s l i g h t d e c l i n e . O n t h e i r o w n t h e s e statistics c a n n o t b e t a k e n as a n i n d i c a t i o n of p o p u l a t i o n size s i n c e t h e f l u c t u a t i o n s i n c a t c h m a y b e a r e s u l t of v a r y i n g f i s h i n g i n t e n s i t y r a t h e r t h a n p o p u l a t i o n c h a n g e s . Statistics of f i s h i n g effort (fishing i n t e n s i t y = f i s h i n g effort/area) a r e less c o m p l e t e t h a n t h o s e of i n t e r n a t i o n a l catch a n d t h e r e are
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difficulties in d e f i n i n g t h e u n i t s of effort a n d in s t a n d a r d i s i n g t h e m b e t w e e n d i f f e r e n t c o u n t r i e s a n d m e t h o d s of fishing. N e v e r t h e l e s s , a n i n d e x of t h e f i s h i n g effort on d e m e r s a l fish h a s b e e n p r o d u c e d for t h e p e r i o d s i n c e 1954, w h i c h is d e r i v e d from English, W e l s h a n d B e l g i a n t r a w l data, c o r r e c t e d for i n c r e a s e s in h o r s e p o w e r (Brander, 1977). T h e r i s i n g t r e n d i n t h e t o t a l i n t e r n a t i o n a l f i s h i n g effort t h u s d e r i v e d is s h o w n in F i g u r e 3 w h i c h a l s o s h o w s t h e r e s u l t i n g d e c l i n e in total d e m e r s a l c a t c h p e r effort. C a t c h p e r effort is t a k e n as a n i n d i c a t o r of a v a i l a b l e stock b i o m a s s a n d from t h e s e f i g u r e s it t h e r e f o r e a p p e a r s t h a t t h e t o t a l d e m e r s a l fish b i o m a s s i n t h e Irish S e a a n d Bristol C h a n n e l h a s f a l l e n to little m o r e t h a n a t h i r d of its 1954 l e v e l as a r e s u l t of fishing. Before g o i n g on to l o o k f u r t h e r into t h e s e f i g u r e s it is p e r h a p s w o r t h g i v i n g m o r e d e t a i l a b o u t f i s h i n g i n t e n s i t y a n d its d i s t r i b u t i o n . A r o u g h c a l c u l a t i o n i n d i c a t e s t h a t t h e total a r e a s w e p t b y t r a w l s in 1978 w a s a b o u t 147 000 k m 2 w h i c h is a b o u t 2'/2 t i m e s t h e t o t a l a r e a of D i v i s i o n s VIIa a n d VIIf. S i n c e f i s h i n g effort is n o t e v e n l y d i s t r i b u t e d this m e a n s t h a t p o p u l a r g r o u n d s a n d tows m a y h a v e b e e n t r a w l e d o v e r m a n y m o r e t i m e s t h a n this, w h e r e a s l a r g e a r e a s a r e not f i s h e d at all.
T h e d i s t r i b u t i o n of i n t e r n a t i o n a l f i s h i n g effort w i t h i n t h e Irish S e a c a n o n l y b e g i v e n a p p r o x i m a t e l y b e c a u s e not a l l c o u n t r i e s c o l l e c t t h e i n f o r m a t i o n , b u t F i g u r e 4 s h o w s t h e English, W e l s h a n d B e l g i a n c o n t r i b u t i o n s in 1976, w i t h m o s t f i s h i n g t a k i n g p l a c e in t h e E a s t e r n Irish Sea. T h e r e m a i n i n g countries, Eire, N o r t h e r n I r e l a n d a n d F r a n c e fish m a i n l y in t h e c e n t r a l a n d w e s t e r n parts, p a r t i c u l a r l y in t h e a r e a s w e s t a n d s o u t h w e s t of t h e Isle of M a n .
A y i e l d curve for t h e d e m e r s a l s p e c i e s c a n b e d e r i v e d from F i g u r e 3 u s i n g t h e m e t h o d s d e s c r i b e d b y B r a n d e r (1977} a n d A n o n y m o u s (1978, 1979}, a n d this is s h o w n in F i g u r e 5. B e a r i n g in m i n d t h e l i m i t a t i o n s of t h e m e t h o d , it p r o v i d e s s o m e u s e f u l first a p p r o x i m a t i o n s . T h e m a x i m u m s u s t a i n a b l e y i e l d (MSY) from t h e d e m e r s a l f i s h e r y of VIIa a n d VIIf is j u s t u n d e r 50 000 t o n n e s ( n o m i n a l weight} a n d t h e b i o m a s s of d e m e r s a l fish (of c a t c h a b l e size) at MSY is a b o u t 100 000 t o n n e s , s i n c e f i s h i n g is r e m o v i n g a b o u t h a l f of t h e b i o m a s s e a c h year. In t h e a b s e n c e of f i s h i n g t h e b i o m a s s w o u l d b e a r o u n d
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Fig. 3. Relationship between total demersal catch rate and fishing effort for ICES Divisions VIIa +
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200 000 t o n n e s . T h e c u r r e n t l e v e l of f i s h i n g is g r e a t e r t h a n t h a t n e e d e d to t a k e t h e M S Y a n d d e m e r s a l s t o c k b i o m a s s , as i n d i c a t e d b y c a t c h p e r effort, is j u s t o v e r a t h i r d of t h e v i r g i n s t o c k b i o m a s s . T h e m o d e l is little m o r e t h a n a s u m m a r y of t h e c h a n g e s w h i c h h a v e t a k e n p l a c e in t h e d e m e r s a l stocks o v e r t h e l a s t t h i r t y y e a r s , b u t it d o e s fit t h e d a t a w e l l a n d is c o n s i s t e n t w i t h t h e m o r e d e t a i l e d s i n g l e s p e c i e s a n a l y t i c a l m o d e l s . As such it forms p a r t of t h e m a n a g e m e n t c a s e for r e s t r a i n i n g a n d r e d u c i n g t h e i n t e r n a t i o n a l f i s h i n g effort.
S I N G L E SPECIES A N A L Y T I C A L M O D E L S O F F I S H P O P U L A T I O N S
W h e r e a s t h e t o t a l d e m e r s a l m o d e l a l l o w e d o n l y r a t h e r b r o a d g e n e r a l i s a t i o n s a b o u t t h e c u r r e n t s t a t e of Irish S e a fish stocks, t h e s i n g l e s p e c i e s m o d e l s a r e b a s e d on q u a r t e r l y or a n n u a l a g e c o m p o s i t i o n s of t h e l a n d i n g s o b t a i n e d from r o u t i n e s a m p l i n g on fish m a r k e t s a n d on l a r g e n u m b e r s of a g e d e t e r m i n a t i o n s from otoliths. For cod, p l a i c e , sole a n d w h i t i n g w e k n o w t h e a g e s t r u c t u r e of t h e l a n d i n g s for a n u m b e r of y e a r s a n d a r e a b l e to r e c o n s t r u c t t h e a g e structure of t h e p o p u l a t i o n s u s i n g v i r t u a l p o p u l a t i o n a n a l y s i s {Pope, 1972).
T h u s w e h a v e i n f o r m a t i o n on the r a t e s of growth, m o r t a l i t y a n d r e c r u i t m e n t for t h e s e s p e c i e s e x t e n d i n g b a c k o v e r 10 to 15 y e a r s a n d u p d a t e d a n n u a l l y b y a w o r k i n g g r o u p of ICES ( A n o n y m o u s , 1979). W i t h t h e e x c e p t i o n of f i s h i n g m o r t a l i t y , t h e c h a n g e s w h i c h h a v e t a k e n p l a c e in t h e s e r a t e s a r e for t h e m o s t p a r t s m a l l a n d w i t h o u t trend. F o r e x a m p l e , F i g u r e 6 s h o w s t h e d e v i a t i o n in p l a i c e g r o w t h a b o u t t h e l o n g - t e r m m e a n for t h e p e r i o d 1964-78. R e c r u i t m e n t too h a s a r a n d o m s c a t t e r (Fig. 7) in s p i t e of a m a r k e d d e c l i n e in t h e s t o c k b i o m a s s of cod, p l a i c e a n d sole. O n t h e o t h e r h a n d f i s h i n g m o r t a l i t y h a s i n c r e a s e d s t e a d i l y on p l a i c e a n d cod, as one w o u l d e x p e c t from t h e t r e n d in t o t a l i n t e r n a t i o n a l effort {Fig. 3). Sole, w h i c h is t a k e n m a i n l y i n a d i r e c t e d s e a s o n a l fishery, h a s not a p p a r e n t l y b e e n s u b j e c t e d to i n c r e a s e d f i s h i n g m o r t a l i t y a n d t h e w h i t i n g a s s e s s m e n t is c o m p l i c a t e d b y h i g h m o r t a l i t y in s m a l l m e s h f i s h e r i e s for
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Fig. 7. Deviations in annual recruitment of plaice, sole, whiting and cod from the geometric long- term mean for ICES Division VIIa (data from Anonymous, 1979)
l a n d i n g s from t h e North C h a n n e l . Its d i s a p p e a r a n c e s e e m s to h a v e b e e n u n n o t i c e d or at least u n r e c o r d e d , a n d it is d o u b t f u l w h e t h e r a m a n a g e m e n t p o l i c y to protect it from f i s h i n g could h a v e b e e n d e v i s e d i n a n y case. T h e s a m e is p r o b a b l y true for the other ray species a n d this is a clear e x a m p l e of the l i m i t a t i o n s of the p r o t e c t i o n afforded b y fisheries m a n a g e m e n t .
A s h o r t c o m i n g of the a n a l y t i c a l m o d e l s b a s e d o n c o m m e r c i a l m a r k e t s a m p l i n g is that for most p u r p o s e s it is i m p o s s i b l e to work o n a f i n e r scale t h a n the w h o l e area (in this case D i v i s i o n VIIa) b y quarters of the year. In t h e case of p l a i c e a n d cod it is k n o w n that t h e r e are different s p a w n i n g areas a n d c o n s i s t e n t growth rate differences b e t w e e n the e a s t e r n a n d w e s t e r n Irish Sea, b u t b e c a u s e it is i m p o s s i b l e to d i v i d e the total catch b e t w e e n these areas the a s s e s s m e n t c a n n o t b e split. W h e n more d e t a i l e d i n f o r m a t i o n o n a p a r t i c u l a r s m a l l area is n e e d e d , it is u s u a l l y n e c e s s a r y to u s e s p e c i a l r e s e a r c h surveys a n d the k i n d of i n f o r m a t i o n w h i c h c a n b e o b t a i n e d with these is d e s c r i b e d i n the n e x t section.
RESEARCH SURVEYS OF FISH POPULATIONS
A series of g r o u n d f i s h trawl s u r v e y s i n the North East Irish Sea has b e e n c a r r i e d out over the last 5 years. T h e s u r v e y s take p l a c e e v e r y 4 m o n t h s u s i n g a s m a l l (16.7 m l e n g t h ) c h a r t e r e d f i s h i n g vessel. Details of the m e t h o d a n d some of the results are g i v e n b y B r a n d e r & W a l l a c e (1976). T h e a d v a n t a g e s of this k i n d of s u r v e y are that f i s h i n g positions c a n b e s e l e c t e d a n d m e t h o d s k e p t s t a n d a r d ; s m a l l m e s h covers are u s e d a n d all fish w h e t h e r c o m m e r c i a l or n o t are recorded; s u p p l e m e n t a r y b i o l o g i c a l i n f o r m a t i o n o n f e e d i n g , condition, etc. c a n b e collected.
A m a j o r p r o b l e m w h i c h has a l w a y s c o n f r o n t e d studies of fish p o p u l a t i o n s is h o w to s a m p l e t h e m a n d i n p a r t i c u l a r w h e t h e r the i n h e r e n t v a r i a b i l i t y i n catches t a k e n b y g e a r such as trawls is so l a r g e as to o b s c u r e a n y fine differences i n distribution. O u r surveys h a v e a t t e m p t e d to d e m o n s t r a t e c o n s i s t e n t differences i n fish d i s t r i b u t i o n d u e to depth, d i s t a n c e from the coast, s e a s o n a n d s e d i m e n t type a n d i n this t h e y h a v e m e t w i t h some
S u c c e s s .
As a n e x a m p l e , the d i s t r i b u t i o n of t w e l v e species i n r e l a t i o n to s e d i m e n t a n d m o n t h is g i v e n i n T a b l e i, w h i c h shows the g e o m e t r i c m e a n of t h e n u m b e r p e r haul, the overall coefficient of variation, the p r o p o r t i o n of the total s u m of s q u a r e s d u e to h a u l - t o - h a u l (within stratum) v a r i a b i l i t y a n d the s i g n i f i c a n c e levels for the effects of s e d i m e n t type a n d month. T h e w h o l e a n a l y s i s w a s carried out o n log t r a n s f o r m e d n u m b e r s p e r h a u l (log e (I -bN)) b e c a u s e the f r e q u e n c y d i s t r i b u t i o n s are h i g h l y skewed. E v e n so, m a n y of the coefficients of v a r i a t i o n are a b o v e 100 % and, b e c a u s e this m a y l e a d to excessive n u m b e r s of s i g n i f i c a n t F ratios i n the a n a l y s i s of v a r i a n c e , the m o n t h x s e d i m e n t i n t e r a c t i o n t e r m has b e e n i n c l u d e d i n the "error" s u m of s q u a r e s a n d o n l y the m a i n effects h a v e b e e n tested.
696 K. B r a n d e r
o v e r a l l v a r i a b i l i t y a m o n g t h e c a t c h e s of s p r a t in t h e 30 h a u l s a n a l y s e d is v e r y h i g h , as i n d i c a t e d b y t h e c o e f f i c i e n t Of v a r i a f i o n of 1~8 o/0 for t h e t r a n s f o r m e d d i s t r i b u t i o n , b u t t h e h a u l - t o - h a u l v a r i a b i l i t y w i t h i n s t r a t a is low, a c c o u n t i n g for o n l y 17 % of t h e t o t a l s u m of s q u a r e s . As c a n b e s e e n from t h e m e a n v a l u e s , s p r a t a r e far m o r e a b u n d a n t o v e r m u d d y s a n d t h a n o v e r t h e o t h e r s e d i m e n t s , a r a t h e r s u r p r i s i n g r e s u l t w h i c h is h o w e v e r confir- m e d b y o t h e r s s t u d y i n g t h e s p e c i e s e l s e w h e r e (Johnson, p e r s o n a l c o m m u n i c a t i o n ) .
Of t h e o t h e r s p e c i e s o n l y p o o r cod a n d p o u t w h i t i n g c a n b e s h o w n to b e i n f l u e n c e d b y s e d i m e n t t y p e a n d o n l y cod a n d w h i t i n g s h o w s t a t i s t i c a l l y s i g n i f i c a n t s e a s o n a l f l u c t u a t i o n s .
U s i n g i n f o r m a t i o n of this k i n d one c a n p r e d i c t t h e s p e c i e s c o m p o s i t i o n a n d a b u n - d a n c e w h i c h m i g h t b e e x p e c t e d in p a r t i c u l a r a r e a s a n d s e a s o n s a n d h e n c e d e t e c t s i g n i f i c a n t d e v i a t i o n s from e x p e c t a t i o n w h i c h m i g h t i n d i c a t e s o m e o t h e r factor at work. H o w e v e r , t h e y a r e o n l y t h e first s t e p i n u n d e r s t a n d i n g t h e structure of t h e fish c o m m u - n i t y a n d t h e i r m a i n u s e at p r e s e n t s e e m s to lie in i n s t i l l i n g c a u t i o n w h e n a s c r i b i n g a c a u s e to a p a r t i c u l a r d e c l i n e or a b s e n c e . For e x a m p l e , C o r l e t t & O ' S u l l i v a n (1972) t e n t a t i v e l y c o n c l u d e d on t h e b a s i s of two t r a w l h a u l s t h a t t h e s l i g h t l y r e d u c e d n u m b e r of s p e c i e s s o u t h e a s t of t h e L i v e r p o o l B a y s l u d g e d u m p i n g a r e a m i g h t b e a n i n d i c a t i o n of d e t r i m e n t a l c o n d i t i o n s , b u t our m o r e d e t a i l e d s a m p l i n g i n c o m p a r a b l e d e p t h s a n d t h e s a m e b o t t o m t y p e on a r e a s 15 m i l e s w e s t of t h e d u m p site s h o w e d s i m i l a r n u m b e r s of s p e c i e s to t h o s e n e a r t h e d u m p i n g g r o u n d a n d c o m p a r a b l e s t r a t a off the Isle of M a n s h o w e d c o n s i s t e n t l y l o w e r n u m b e r s . In t h e l a t t e r c a s e t h e r e d u c e d n u m b e r of s p e c i e s m a y b e d u e to g r e a t e r t i d a l currents, b u t this factor is e x t r e m e l y difficult to i n c l u d e in t h e s a m p l i n g d e s i g n .
T h e p r o b l e m of s a m p l i n g fish p o p u l a t i o n s in o r d e r to s t u d y the factors i n f l u e n c i n g t h e i r d i s t r i b u t i o n a n d a b u n d a n c e is n o t a n i n s u p e r a b l e one, b u t the n u m b e r of factors i n v o l v e d is l a r g e a n d t h e r e f o r e t h e u n e x p l a i n e d r e s i d u a l v a r i a n c e is also l i k e l y to r e m a i n f a i r l y b i g for m o s t s p e c i e s . T h e a n a l y s i s of v a r i a n c e d e s c r i b e d h e r e m a y a l l o w one to s e l e c t s p e c i e s w h o s e d i s t r i b u t i o n a p p e a r s to b e w e l l e x p l a i n e d b y t h e factors i n c l u d e d a n d s m a l l p l a i c e a r e a g o o d e x a m p l e . A f u r t h e r a n a l y s i s h a s l o o k e d at t h e i r n u m b e r s i n r e l a t i o n to d e p t h , d i s t a n c e from the coast, s e a s o n a n d y e a r , b u t k e e p i n g s e d i m e n t t y p e constant. Not s u r p r i s i n g l y t h e i r d i s t r i b u t i o n c a n b e s h o w n to d e p e n d s i g n i f i c a n t l y on a l l of t h e s e a n d one is left w i t h a f a i r l y c l e a r p i c t u r e of h o w t h e i r n u m b e r s w o u l d b e e x p e c t e d to vary.
D I S C U S S I O N
Table 1. Analysis of variance of mean number of fish caught per one hour trawl haul Species Sediment (6 hauls) Month (10 hauls) Muddy Sand Mud, Shelly Lag Signifi- March June Novem- sand sand & sand gravel cance ber gravel level Dab Limanda limanda 134.5 67.1 60.4 4.4 99.7 Plaice Pleuronectes platessa (~ 25 cm) 39.9 8.4 2.8 3.6 28.7 Sprat Sprattus sprattus 33.2 4.3 3.2 0.6 0.4 Flounder Platichthys flesus 4.4 2.0 1.0 1.4 0.6 Poor cod Trisopterus minutus 4.4 379.3 68.6 52.5 328.6 Plaice Pleuronectes platessa (> 25 cm) 13.4 29.7 18.1 3.9 25.9 Dragonet Callionymus lyra 0.3 5.1 11.7 1.4 1.9 Pout whiting Trisopterus luscus 3.2 38.6 5.5 54.8 13.7 Cod Gadus morhua 0.9 3.9 7.4 8.6 6.8 Lesser spotted dogfish Scyliorhinus caniculus 1.2 0.9 1.3 6.3 2.4 Whiting Merlangius merlangus 57.6 39.6 84.7 33.1 96.8 Thornback ray Raia davata 3.2 2.4 3.1 1.3 9.5 * Significant {p < 0.05) * * Highly significant (p < 0.01) * 101.0 68.8 16.5 * * 24.0 12.5 3.5 * 4.5 13.1 0.1 ** 5.8 1.4 0.1 ** 75.2 33.1 167.2 13.3 16.0 16.4 3.9 3.7 1.2 * 10.4 25.8 15.6 1.3 5.4 11.5 0.7 2.8 4.9 31.4 95.4 62.8 3.0 3.7 3.2
Signifi- cance level Coeffi- cient
of
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148 114 56 50 104 69 75 101 25 68
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698 K. B r a n d e r
the Irish Sea a n d e l s e w h e r e , w h i c h c u r r e n t excessive f i s h i n g p r e s s u r e represents, w i l l b e c u r b e d w i t h i n the n e x t few years. If this d e s i r a b l e state is a c h i e v e d it will b e a n o t a b l e success for fisheries m a n a g e m e n t , b u t will b e r e g a r d e d b y most Of those c o n c e r n e d a b o u t m a r i n e life as o n l y t h e first step o n a l o n g road.
T h e o b j e c t i v e s of fisheries m a n a g e m e n t are l i m i t e d to m a i n t a i n i n g y i e l d s of com- m e r c i a l species a n d practical c o n s i d e r a t i o n s m e a n that only a very few of these are p r o p e r l y s t u d i e d a n d protected. T h e p a r a l l e l w i t h agriculture, w h i c h h u s b a n d s a l i m i t e d n u m b e r of species to the d e t r i m e n t of m a n y others is i n e s c a p a b l e . O n e of the m a i n aims of c o n s e r v a t i o n is to m a i n t a i n the diversity of species, b u t o n l a n d the c o n s e r v a t i o n m o v e m e n t owes m u c h of its force to a e s t h e t i c a n d a m e n i t y considerations, w h i c h m a y b e h a r d to a p p l y b e y o n d littoral areas. As o n l a n d , there m a y b e some conflict b e t w e e n the o b j e c t i v e s of m a n a g e m e n t to o b t a i n a y i e l d a n d those of g e n e r a l conservation, b u t s t u d i e s of the fisheries w i l l p l a y a m a j o r part i n i d e n t i f y i n g a n d r e s o l v i n g t h e s e a n d are i n a n y case e s s e n t i a l to a n u n d e r s t a n d i n g of the m a r i n e ecosystem a n d of the d a n g e r s to it.
T u r n i n g to the q u e s t i o n w h e t h e r fisheries d a t a such as those u s e d i n the a n a l y t i c a l m o d e l could b e u s e d to detect c h a n g e s d u e to pollution, the first point, w h i c h is obvious b u t n e e d s to b e m a d e a g a i n , is that o n e c e r t a i n l y c a n n o t i n t e r p r e t a n y c h a n g e s i n the m a r i n e ecosystem w i t h o u t t a k i n g a c c o u n t of the m a s s i v e effects of f i s h i n g o n fish p o p u l a t i o n s and, directly or indirectly, o n other life i n the sea. For e x a m p l e , the i n c r e a s e d i n c i d e n c e of l e s i o n s i n Irish Sea flatfish r e p o r t e d b y P e r k i n s et al. (1972) m i g h t b e e x p l a i n e d b y fishery i n d u c e d stress, p a r t i c u l a r l y since the i n c i d e n c e was h i g h e r a m o n g a d u l t fish l i v i n g i n less p o l l u t e d offshore a r e a s t h a n a m o n g y o u n g fish o n i n s h o r e areas w h i c h are more polluted, b u t less h e a v i l y fished. T h e r e is no e v i d e n c e that f i s h i n g c h a n g e d at that time i n a w a y w h i c h m i g h t h a v e b r o u g h t this about, b u t n e i t h e r is there a n y direct e v i d e n c e that p o l l u t a n t s d i d either. T h e effects of f i s h i n g are so great a n d w i d e s p r e a d that it is i n d e e d d o u b t f u l w h e t h e r p o l l u t i o n i n d u c e d c h a n g e s i n growth, n a t u r a l mortality, r e c r u i t m e n t or m i g r a t i o n c o u l d b e d e t e c t e d a g a i n s t them, u n l e s s the c h a n g e s are p a r t i c u l a r l y disastrous or persist for a l o n g e n o u g h period.
C O N C L U S I O N S
(1) T h e Irish Sea is h e a v i l y p o l l u t e d i n s e v e r a l respects a n d is also s u b j e c t to m a n y other h u m a n activities i n c l u d i n g n a v i g a t i o n , recreation, gravel, oil a n d gas p r o s p e c t i n g a n d e x t r a c t i o n a n d fishing. To d a t e o n l y f i s h i n g c a n b e s h o w n to h a v e a w i d e s p r e a d effect o n the m a r i n e ecosystem; it r e m o v e s over half of the a d u l t p o p u l a t i o n s of several c o m m e r c i a l species e a c h year.
(2) In g e n e r a l , the d e m e r s a l fisheries are p r o d u c i n g y i e l d s w h i c h are as h i g h as or h i g h e r t h a n e v e r before, b u t there is little d o u b t t h a t most species are overfished. F i s h i n g has p r o b a b l y l e d to the d i s a p p e a r a n c e of Raia batis from the Irish Sea a n d other species of R a i i d a e are also i n d a n g e r .
(4) S t u d i e s of t h e f i s h e r i e s c a n g i v e i n f o r m a t i o n o n l o n g - t e r m c h a n g e s i n p o p u l a t i o n
l e v e l s , o n t h e g r o w t h , m o r t a l i t y a n d r e c r u i t m e n t of f i s h p o p u l a t i o n s a n d o n t h e i r d e t a i l e d
d i s t r i b u t i o n i n r e l a t i o n to e a c h o t h e r a n d t o e n v i r o n m e n t a l f a c t o r s . S u c h s t u d i e s a r e
e s s e n t i a l i n d e t e r m i n i n g t h e p o s s i b l e c a u s e s of c h a n g e s i n t h e f i s h p o p u l a t i o n s a n d f o r m
a n i m p o r t a n t p a r t of o u r u n d e r s t a n d i n g of t h e m a r i n e e c o s y s t e m .
L I T E R A T U R E C I T E D
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Fish. Res. Lowestoft 36, 1-40.
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North East Irish Sea. - C. M. - ICES I#'.11, 1-5.
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c o p e p o d s in t h e North Atlantic O c e a n a n d t h e North Sea. - Mar. Biol. 51, 23-32.
Corlett, J. & O ' S u l l i v a n , A. J., 1972. F i s h e r i e s in Liverpool Bay. In: O u t of sight, out of m i n d . H.M.S.O., L o n d o n 2, 441-461.
Dept. of t h e E n v i r o n m e n t , 1978. D i g e s t of e n v i r o n m e n t a l p o l l u t i o n statistics. H.M.S.O., London, 94 pp.
H e r d m a n , W. A. & D a w s o n , R. A., 1902. Fish a n d f i s h e r i e s of t h e Irish Sea. - M e m . Lancs. Sea Fish.
C o m m . 11, 1-98.
Lee, A. J. & Ramster, J. W., 1979. Atlas of t h e s e a s a r o u n d t h e British isles. - Fish. Res. Tech. Rep.,
M A F F Direct. Fish. Res., Lowestoff 20, 1-4; chart 1-51.
Mitchell, N. T., 1977. Radioactivity i n surface a n d coastal w a t e r s of t h e British Isles, 1976. Part 1:
T h e Irish Sea a n d its environs. - Tech. Rep. Fish. radiobiol. Lab., Lowestoft 13, 1-15.
Murray, A. J., 1979. Metals, o r g a n o c h l o r i n e p e s t i c i d e s a n d PCB r e s i d u e levels i n fish a n d s h e l l f i s h l a n d e d i n E n g l a n d a n d W a l e s d u r i n g 1974. - A q u a t . environ. Monit. Rep., MAFF Direct. Fish. Res., Lowestoft 2, 1-11.
Murray, A. J., IAndsay, A. & Norton, M. G., 1979. T h e c o m p o s i t i o n of d r e d g e d spoils d u m p e d at s e a
from E n g l a n d a n d Wales. - Fish. Res. Tech. Rep., MAFF Direct. Fish. Res., Lowestoft 52, 1-10.
Perkins, E. J., Gilchrist, J. R. S. & Abbott, D. J., 1972. I n c i d e n c e of e p i d e r m a l l e s i o n s in fish of t h e
N o r t h East Irish Sea. - Nature, Lond. 238, 101-103.
Pope, J. G., 1972. A n i n v e s t i g a t i o n of t h e a c c u r a c y of virtual p o p u l a t i o n a n a l y s i s u s i n g cohort analysis. - Res. Bull. int. C o m m n N.W. Atl. Fish. 9, 65-75.
P o r t m a n n , J. E., 1979. C h e m i c a l m o n i t o r i n g of r e s i d u e levels in fish a n d s h e l l f i s h l a n d e d in E n g l a n d a n d W a l e s d u r i n g 1970-73. - A q u a t . environ. Monit. Rep., M A F F Direct. Fish. Res., Lowestoft 1, 1-70.
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140-141.
S i n d e r m a n n , C. J., 1977. Recent d a t a on p o s s i b l e a s s o c i a t i o n of c o a s t a l / e s t u a r i n e p o l l u t i o n w i t h fish
