The uses of ethology in fish-farming

The uses of ethology in fish-farming

100 THE USES OF ETHOLOGY I N F I S H - F A R M I N G N.B. M E T C A L F E Department of Zoology, Glasgow University, Glasgow G12 8QQ (Gt. Britain) ...

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100

THE USES OF ETHOLOGY I N F I S H - F A R M I N G

N.B. M E T C A L F E

Department of Zoology, Glasgow University, Glasgow G12 8QQ (Gt. Britain)

ABSTRACT

In this review, I m a i n l y c o n s i d e r t h e rearing of s a l m o n i d fish ( s a l m o n , r a i n b o w a n d b r o w n t r o u t ) , since these are t h e m a j o r species f a r m e d in Britain. A n a d r o m o u s (migrat o r y ) species o f s a l m o n i d , such as t h e A t l a n t i c s a l m o n , s p e n d 1--4 years as juveniles (parr) in f r e s h w a t e r b e f o r e m i g r a t i n g t o t h e sea. A f u r t h e r 1--2 years are s p e n t at sea b e f o r e the fish r e t u r n to t h e i r n a t a l river to spawn. Strategies o f rearing s a l m o n range f r o m m e r e l y raising t h e eggs for s u b s e q u e n t release as parr, t h r o u g h to captive-rearing for the entire life-cycle using sea " c a g e s " for t h e m a r i n e phase. C o m m e r c i a l developm e n t o f s a l m o n f a r m i n g has o n l y o c c u r r e d in t h e U.K. w i t h i n t h e last 20 years. As a c o n s e q u e n c e , m a n y o f t h e b e h a v i o u r a l p r o b l e m s associated w i t h captive rearing o f these fish have y e t to be fully e x p l o r e d , especially since it is very difficult for fish-farmers t o observe the b e h a v i o u r of t h e i r fish. T h e r e is t h e r e f o r e c o n s i d e r a b l e scope for applied ethological studies of the foraging a n d social b e h a v i o u r of f a r m e d fish species, tackling j u s t t h e same t y p e s o f p r o d u c t i v i t y p r o b l e m s as are e n c o u n t e r e d w i t h t h e c o n v e n t i o n a l f a r m i n g o f livestock. C o n s i d e r feeding b e h a v i o u r . J u v e n i l e s a l m o n i d s are " s i t - a n d - w a i t " p r e d a t o r s , resting o n t h e b e d o f s t r e a m s a n d d a r t i n g o u t to i n t e r c e p t f o o d carried past in t h e c u r r e n t . T h e y are u n a b l e to bite pieces off the prey, a n d so are r e s t r i c t e d in t h e i r diet to i t e m s smaller t h a n the m o u t h b r e a d t h . However, t h e smaller t h e items, the s h o r t e r t h e d i s t a n c e over w h i c h t h e y can be d e t e c t e d , a n d h e n c e the lower t h e p r o p o r t i o n c a u g h t b y t h e fish. O p t i m a l foraging a p p r o a c h e s to this t r a d e - o f f b e t w e e n visual c o n s t r a i n t s , prey energetic c o n t e n t a n d h a n d l i n g t i m e have b e e n used t o p r e d i c t t h e m o s t e n e r g e t i c a l l y - p r o f i t a b l e size of f o o d pellet to feed to t h e fish, a n d t h e successful testing o f these p r e d i c t i o n s has led to increases in f a r m i n g efficiency. T h e spatial b e h a v i o u r a n d social o r g a n i s a t i o n of juvenile s a l m o n i d s has also b e e n a m a j o r ( t h o u g h o f t e n u n r e c o g n i s e d ) a q u a c u l t u r a l p r o b l e m . Parr are n a t u r a l l y despotic, restricting p o p u l a t i o n densities b y territorial aggression. F o r c i n g t h e m to live at high densities m a y result in d o m i n a n t fish m o n o p o l i s i n g food sources; s u b o r d i n a t e s m a y t h e n e i t h e r incur u n e c o n o m i c a l l y high energetic costs if t h e y a t t e m p t to c o m p e t e w i t h d o m i n a n t s , or o p t o u t of the c o m p e t i t i o n a l t o g e t h e r a n d subsist o n a m a i n t e n a n c e r a t i o n , w i t h a c o n s e q u e n t d e p r e s s i o n o f g r o w t h rate. Since t h e age at w h i c h juvenile s a l m o n migrate to sea (and h e n c e t h e t i m e to h a r v e s t i n g ) is s i z e - d e p e n d e n t , t h e r e are considerable e c o n o m i c advantages in m i n i m i s i n g such d o m i n a n c e effects. A n ethological app r o a c h has b e e n to e l i m i n a t e t h e spatial variability in the f o o d s u p p l y w i t h i n t h e t a n k , a n d m a k e use of the n a t u r a l h a b i t a t p r e f e r e n c e s of parr to m a n i p u l a t e t h e i r spatial dist r i b u t i o n ; t h e o u t c o m e is a t a n k design w h i c h allows all fish t h e same o p p o r t u n i t y to o b t a i n f o o d , so r e d u c i n g t h e size discrepancies w i t h i n t h e p o p u l a t i o n . Studies such as these, w h i c h i n c o r p o r a t e b e h a v i o u r a l c o m p o n e n t s as m a j o r criteria for t h e design of rearing systems, reveal t h e t r e m e n d o u s scope for e t h o l o g i s t s in aquaculture, w h i c h p r o m i s e s to be an increasingly i m p o r t a n t aspect of a n i m a l p r o d u c t i o n in Britain.