Coping strategy and stress response of European sea bass Dicentrarchus labrax to acute and chronic environmental hypercapnia under hyperoxic conditions

Abstract Four groups of sea bass Dicentrarchus labrax (202 ± 42 g, mean weight ± SD) kept in a recycling system were exposed for 45 days to four different levels of water carbon dioxide partial pressure (PCO 2 ): 2 mm Hg (2.8 mg/l; control), 13.9 mm Hg (16.9 mg/l; low), 24.7 mm Hg (32.8 mg/l; medium) and 37.8 mm Hg (51.2 mg/l; high). Mean water oxygen saturation was 136–137%. Blood gas status, acid–base balance and stress response were measured during acute (0, 3, 6, 24, 48 and 96 h) and chronic (8, 22, and 45 days) exposure. Blood PCO 2 , [tCO 2 ] and pH significantly increased in hypercapnic groups according to water CO 2 concentrations and were significantly higher compared to control groups throughout the trial. Blood pH increase was due to blood HCO 3 − accumulation followed by Cl − loss. Blood PO 2 significantly decreased in all hypercapnic groups compared to controls during both acute and chronic exposure. Htc and tHb increased during acute exposure in response to environmental hypercapnia and significantly decreased at the end of the trial in high CO 2 groups. Serum cortisol increased significantly during acute exposure as a function of time and CO 2 , resulting in higher values in medium and high CO 2 groups compared to low and control groups. Cortisol levels did not change during the chronic phase in relation to CO 2 and no differences were observed among groups after 45 days. Serum glucose increased significantly in high hypercapnic groups after 8 days. Serum osmolality, [Na + ] and [K + ] did not differ among hypercapnic and control groups during the trial. A significant positive correlation was found between reactive oxygen metabolites (ROMs) and total antioxidant status (AOP) but neither parameter changed versus CO 2 concentration. No significant differences in growth performance (body weight, standard length, condition factor and specific growth rate) were observed between controls and hypercapnic groups. Mortality was found to be negligible ( Sea bass displayed fast and efficient acid–base compensatory mechanisms in response to environmental hypercapnia up to 38 mm Hg PCO 2 . However, acute and chronic exposure above 25 mm Hg PCO 2 (> 30 mg/l CO 2 ) was found to be stressful, potentially impairing the welfare of sea bass.