Effects of an acute stress challenge on physiological stress response of the sea bass Dicentrarchus labrax (L.) at different stocking densities.

Farmed fish are exposed to different stressing factors and their adaptive response depends both on the intensity and the duration of the stress. Understanding the nature of the physiological stress response and of its measurable consequences has a great importance in improving fish welfare and farm production. High stocking density is considered as an important source of stress in intensive aquaculture as it affects metabolic levels and modulates immune functions. Three duplicated groups of sea bass (mean initial body weight 151.433.5 g) were reared under closed circulated water system conditions at different stocking densities (7, 14 and 21 kg/m3) for 45 days. An acute stress challenge was performed on day 35 in order to investigate the physiological response of sea bass to an additional acute stressor. Fish were crowded for 15 minutes by maintaining them in shallow water inside the tank. Blood samples were drawn from the caudal vein (n=10 for each group) at different sampling times (before crowding, 30 minutes and 6, 24, 48, 72 hours after stress). Final sampling was performed at day 10. Serum cortisol and metabolite levels (glucose, total proteins, cholesterol, triglycerides and total non-esterified fatty acids) were measured. High stocking densities did not influence the physiological status of fish at the endocrine and metabolic levels. No significant variations in serum cortisol and metabolite levels were observed before the stress challenge. Acute stress challenge elicited a significant increase in cortisol, glucose, and non-esterified fatty acids levels in all groups, regardless of stocking density. No change in either serum triglycerides and cholesterol levels was detected. However, a significant quantitative difference in stress response was found among the different groups: fish kept at higher stocking densities showed a greater primary stress response, although recovery occurred within 24-48 hours post stress as in fish at medium and low stocking densities. This study demonstrated that sea bass kept at a high stocking density were more sensitive to an additional acute stressor. Exposure of fish to repetitive and cumulative stressing factors can affect their physiological status and their stress adaptive capacity. Therefore fish welfare should be improved by means of the control of stocking density and husbandry practices.