Growth,food utilization and effect of social interaction in the European bass Dicentrarchus labrax

Fingerling and juvenile bass, Dicentrarchus labrax (L.) fed on filleted saithe, Gadus virens, exhibited much higher K ₁ energy and nitrogen utilization efficiencies (growth as % energy or nitrogen intake) than those fed on trout pellets or shore crabs, Carcinus maenas, although the efficiency declined during prolonged feeding. Mean organic absorption efficiency for saithe was 99.5% and was independent of food intake but the efficiency for pellets and crab declined with increasing food intake from 99 to 92%. For each diet the regression of relative growth rate on food intake was linear up to the regression of relative growth rate on food intake was linear up to the highest feeding level and independent of body weight, enabling construction of a simple model for food transformation. The regression coefficient (equivalent to K ₃, the ratio of growth to food consumed in excess of maintenance) for saithe was about 3 times that for pellets in terms of energy and twice that for pellets in terms of nitrogen. The more efficient utilization of saithe for growth and of pellets for maintenance were related to differences in their nutritional value. With each diet, K ₃ and maximum K ₁ for energy were about twice those for nitrogen. Social interaction had a significant effect on food utilization in juvenile bass. Isolated fish showed poor growth and mean K ₁ efficiency in comparison with bass in pairs or groups. K ₃ efficiency and mainternance requirement were similar in individuals and groups, but were significantly greater in pairs. These results are related to behavioural observeations and environmental stress, and the limitations of extrapolating the results of laboratory feeding experiments on fish to other situations is discussed.