Chronic ammonia toxicity in juvenile turbot (Scophthalmus maximus)

Chronic effects of ammonia were studied in three batches of turbot (Scophthalmus mrrrimus) juveniles (14, 23 and 104 g) exposed for 4-6 weeks to constant ammonium chloride solutions. Under the environmental conditions used (16.5-17.5 degrees C, pH 7.92-8.03, salinity 34.5 ppt, over 80% oxygen saturation), no mortalities occurred up to 0.4 mg unionised ammonia (UIA-N) l(-1) i.e. 10 mg total ammonia nitrogen (TAN) l(-1). Estimated 28-day LC(50)s averaged 0.95 mg UIA-N l(-1) and growth was stopped from 0.8 mg UIA-N l(-1). In 14 and 23 g turbot, LOEC (lowest-observable-effect concentration) values for growth were 0.41 and 0.21 mg UIA-N l(-1), respectively, compared with 0.10 mg UIA-N l(-1) in 104 g turbot. In 14 g fish, the most tolerant group in terms of growth, NOEC (no-observable-effect concentration) and MATC (maximum-acceptable-toxic-concentration) values were 0.18 and 0.30 mg UIA-N l(-1), respectively. Reduced growth rate was due to a decrease in food intake, not to poorer food utilisation. The food conversion ratio was 0.9 and PER (protein efficiency ratio) and PUC (protein utilisation coefficient) averaged 2.5 and 45%, respectively. No significant changes in body composition were observed up to 0.4-0.5 mg UIA-N In adapted small turbot, no major physiological disturbances were observed up to 0.4-0.5 mg UIA-N l(-1), while large turbot were more sensitive to ammonia. Major changes were observed in blood plasma TAN contents which were positively correlated with ambient ammonia concentrations. Chronic lethal levels and chronic adaptation levels of TAN were respectively about 20 and 13-15 mg l(-1) plasma. Significant increases in plasma urea-N contents and urea-N daily excretion rates were only observed for the highest ammonia concentrations tested. (C) 1997 Elsevier Science B.V.