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.