Physiological responses of European sea bass Dicentrarchus labrax to different stocking densities and acute stress challenge

Stocking density is considered a priority topic in aquaculture research due to its bearing on the welfare of farmed fish and the need for future recommendations governing stocking density management on fish farms. In this study, two consecutive experiments were carried out in order to investigate the physiological response of sea bass Dicentrarchus labrax to different stocking densities and to an acute stress challenge. Three duplicate groups of sea bass (mean BW= 139.8 g; TL = 18.6 cm) were reared in six recirculating tanks (5 m(3)) at three stocking densities (15, 30 and 45 kg/m(3)) for 6 weeks, and then crowded at 100 kg/m(3) for 15 min. Sampling was performed after 2 weeks and at the end of the stocking density experiment (6 weeks) and at 1, 6, 24, 48, 72 h and 10 days after crowding. Blood samples were analyzed for cortisol, glucose, non-esterified fatty acids (NEFA), total proteins, triglycerides and cholesterol concentrations. Univariate and multivariate analyses were applied to the data sets. No differences in growth and survival were observed in sea bass kept at 15, 30 and 45 kg/m(3) for 6 weeks. Serum cortisol levels measured at 2 and 6 weeks were not significantly different among density groups. A significant increase in NEFA concentration was observed in sea bass kept at 45 kg/m(3) after 6 weeks, suggesting enhanced lipid mobilization. In response to an additional acute crowding stressor, the levels of cortisol and NEFA were significantly higher and of glucose significantly lower in sea bass reared at 45 kg/m(3) compared to fish kept at 15 and 30 kg/m(3). However, pre-stress levels were recovered after 24-48 h in all groups. Multivariate analysis did not reveal any discrimmant variables among groups, indicating similar physiological conditions in the sea bass experiencing different stocking densities and similar recovery patterns after crowding. Results indicate that stocking density at 45 kg /m(3) for 6 weeks did affect the energetic status of sea bass and their sensitivity to a subsequent crowding stressor. Taking into account that aquaculture practices usually involve multiple stressors, a proper recovery time from stress is recommended to enable sea bass to preserve their physiological status. (c) 2007 Elsevier B.V. All rights reserved.