THE SPONTANEOUS SWIMMING ACTIVITY OF JUVENILE WHITING (MERLANGIUS-MERLANGUS L) AND COD (GADUS-MORHUA L) UNDER TANK CONDITIONS, WITH REGARD TO FEEDING LEVELS

Groups of juvenile whiting and cod were kept swimming at low sustained swimming speeds in a constant current in a circular tank. The fish we re fed chopped smelt at rations between 2% body weight (bw) d(-1) and adlibitum. The satiation level for this type of food was found to be 5 .9% bw d(-1) for juvenile whiting and 9.1% bw d(-1) for juvenile cod. The whiting showed little spontaneous swimming activity throughout day and night, other than swimming constantly against a slow current. The fish only became active and increased their spontaneous activity duri ng and shortly after feeding or after the light was reduced to dim (ni ght) conditions. This pattern of activity changed little with varying food rations. In general, spontaneous swimming activity in whiting dec lined with declining food consumption. Unlike whiting, juvenile cod in creased their spontaneous swimming activity conspicuously with decreas ing ration. At the lowest ration cod started to increase their swimmin g activity as much as 6 h before feeding, which is interpreted as appe titive or search behaviour. After feeding on the lowest rations, high activity levels continued for another 6 h. Activity declined only grad ually after the meal, and it is thought that the fish were continuing to search for food after ingestion of a dissatisfying meal. It is hypo thesised that with decreasing food availability, the survival strategy of cod is to increase locomotor activity in order to search for more food; the respective strategy of whiting is to reduce energy expenditu re when food availability is low.