Interactive effects of oxygen saturation and ammonia on growth and blood physiology in juvenile turbot

The effects of dissolved oxygen concentration combined with sublethal doses of un-ionised ammonia nitrogen (UIA-N) on growth and physiology were investigated in juvenile turbot. Fish with a mean ± SD initial weight of 19.1 ± 4.1 g, were reared in replicate at normoxic (80¿85% O2 saturation) and hyperoxic (110¿115% O2 saturation) conditions, combined with either control or two sublethal concentrations of un-ionised ammonia (0, 0.17 and 0.31 mg UIA-N l¿ 1) added for 64 days at 18 °C and pH 8.04 in a flow-through system. The subsequent effect of fixed O2 and UIA concentrations on growth and blood physiology was measured every 22 days. The results show that UIA-N concentrations only had a negative effect on growth under normoxic conditions, suggesting that hyperoxic conditions may increase tolerance to un-ionised ammonia in juvenile turbot. Only minor effects on the acid-base balance were found. A generally higher blood CO2 and PCO2, along with an accumulation of HCO3¿ resulting in an increased blood pH was found in the hyperoxic groups towards the end of the experiment. We conclude that, under normoxic conditions, a chronic exposure of ammonia above 0.17 mg UIA-N l¿ 1 will have a negative effect on growth and that hyper-oxygenation can be a possible solution to counteract this harmful effect