Effect of stocking density and feeding level on energy expenditure and stress responsiveness in European sea bass Dicentrarchus labrax

European sea bass (initial weight 72 ± 4 g) were stocked in 200-L tanks at two densities: a low density (LD) ∼ 5.5 kg m − 3 and a high density (HD) ∼ 36 kg m − 3 . The tanks were part of a recirculation system and were equipped to carry out frequent oxygen measurements. At each density the fish were fed at increasing levels from around maintenance requirement up to apparent satiation. The experiment was carried out for 10 weeks at 22 °C, after which the density in the LD treatment had increased to ∼ 10 kg m − 3 and to ∼ 60 kg m − 3 in the HD treatment. At the end of the trial blood samples were taken from several fish to determine the basal levels of cortisol and glucose. Furthermore, to assess the responsiveness to an acute stressor, additional fish were subjected to individual confinement in submerged nets, blood was sampled and cortisol and glucose analysed. At the end of the trial there was no significant difference in growth performance and voluntary feed intake between the groups raised at different densities. The partitioning of energy demand for maintenance and growth highlighted a slightly higher energy maintenance requirement in the LD fish (50.9 kJ (kg) 0.80 day − 1 ) compared to the HD groups (43.15 kJ (kg) 0.80 day − 1 ). In accordance with this, an increased oxygen demand for sea bass kept at the low density was detected through weekly measurements. Analyses of the blood parameters showed, that higher stocking density resulted in higher cortisol levels in both control and stressed groups (after netting), but the effect of stocking density on the acute stress response was less pronounced at the higher feeding level.