Feeding behaviour of reared and wild cod and the effect of learning: two strategies of feeding on the two-spotted goby

This study investigates possible differences in the feeding behaviour of reared and wild cod Gadus morhua and the effect of learning in connection with sea-ranching experiments. In two laboratory experiments, single individuals, reared or wild, were allowed to prey upon 15 two-spotted gobies   trials of 30 min duration. Reared and wild cod seldom attacked stationary gobies and generally attacked moving gobies, and reaction times were similar (c.0·25 s). Reared and wild cod ingested similar numbers of gobies per trial. Reared cod, however, caught more gobies in pursuits, while wild cod took more gobies in lunges. Reared cod had higher costs as measured by a higher index of energy expenditure and a higher opercular beat frequency, and wild cod were thus more efficient (benefit divided by cost). The efficiency of both groups tended to increase as they gained experience. The results indicate that reared cod more often used a pursuit strategy and wild cod more often a lunge strategy. Differences in previous experience may have initiated the strategies, and the success of each strategy in the experimental situation, in addition to the potential cost of changing strategy, may have maintained them. For cod released into a natural habitat, a pursuit strategy may lead to low efficiency vis-à-vis mobile prey and could also involve a high risk of predation, forcing reared cod to change strategy.