TOXICITY OF AMMONIA TO TURBOT JUVENILES - I - EFFECTS OIL SURVIVAL, GROWTH AND FOOD UTILIZATION

Long-term effects of constant exogenous ammonia concentrations were in vestigated in two different batches of turbot juveniles (53 and 73 g) under controlled environmental and feeding conditions. Over the 84-day experiments, ambient un-ionized ammonia (UIA-N, NH3) concentrations w ere steady (coefficient of variation, 12-16 %) and water pH range was 7.88-7.99. Survival was maximum up to 0.33 mg.l(-1) UIA-N and at 0.73, 50 % mortalities were observed on day 52 (73 g) or day 77 (53 g). No- growth concentration was 0.73 mg.l(-1) UIA-N in the two groups and the 84-day lowest-observable-effect concentration range 0.14-0.33 mg.l(-1 ). In the most tolerant group (53 g), the 84-day no-observable-effect concentration was 0.14 mg.l(-1) UIA-N. The estimated 28-day efficient concentration that gave 50 % of the specific growth rate of controls r anged from 0.60 to 0.75 mg.l(-1) UIA-N. Specific growth rate to ambien t ammonia concentration patterns were different in the two groups for the lowest concentrations (0.14-0.34) and similar to food intake to am monia pattern. Food efficiency was negatively correlated to ambient am monia concentration, with major changes in food conversion ratio, and protein utilisation observed above 0.33-0.70 mg.l(-1) UIA-N. The main change in body composition was an increase in water content in all amm onia-exposed fish. A significant increase in urea-N excretion rate was also observed over 0.33 mg.l(-1) UIA-N. Thresholds for growth were af fected by exposure duration. Turbot juveniles growth capacity may be d epressed by usual ambient ammonia concentrations under intensive farmi ng conditions.