Nitrogen excretion patterns and postprandial ammonia profiles in Black Sea turbot (Scophthalmus Maeoticus) under controlled conditions

Measurements of the rate of nitrogenous excretion were carried out in two batches of young Black Sea turbot (small 42 g, large 72 g) at 12±1°C under natural light conditions (10 h light:14 h dark and 13 h light:11 h dark, respectively). The ammonia nitrogen excretion rates of fish starved for 48 hours were 0.20±0.05 mg-N for the small fish and 0.18±0.09 mg-N/100g fish/h for the large. Fish were then fed a pellet diet containing 8.3% nitrogen at average rations of 0.67% and 0.59% of the body weight, respectively, for four days. On the fourth day, ammonia nitrogen excretion rates were evaluated. In both batches, the rates were 2-3 times higher immediately after feeding than in the starved fish, reaching a peak 3-6 hours after feeding and declining afterwards. For the small and large fish, respectively, 21% and 20% of the consumed nitrogen was excreted as ammonia nitrogen, 6% and 7% as urea nitrogen, and 8% and 4% as feces nitrogen within 24 hours after feeding.