Hypoxia tolerance and metabolic coping strategies in Oreochromis niloticus

The Nile tilapia (Oreochromis niloticus) is widely farmed in tropical and subtropical pond culture. O. niloticus is recognized as a species that is tolerant of hypoxic conditions, a trait that may largely be responsible for the success of this species in aquaculture. Until now, neither coping mechanisms nor a comparison of various indices of hypoxia tolerance to characterize the response to hypoxia, have been described. In the present study, Nile tilapia were subjected to hypoxia of increasing severity and duration to examine effects on metabolic rate (MO₂) and post hypoxic oxygen debt. MO₂ was measured during periods of severe hypoxia at 2.1 kPa O₂ (10% oxygen saturation) lasting between 2 and 24 h at 27 °C. Hypoxia tolerance was assessed by determining the critical oxygen tension (Pcrit) and the pO₂ at which loss of equilibrium (LOE) occurred. We show that the tolerance of Nile tilapia to severe hypoxia is largely achieved through a capacity for metabolic depression. Despite prolonged exposure to dissolved oxygen levels below Pcrit, the fish showed little excess post-hypoxic oxygen consumption (EPHOC) upon return to normoxic conditions. LOE did not occur until conditions became near-anoxic. Blood pH was not affected by severe hypoxia (2.1 kPa O₂), but a significant acidosis occurred during LOE, accompanied by a significant elevation in lactate and glucose levels. The results from the present study indicate that Nile tilapia do not switch to anaerobic metabolism during hypoxia until pO₂ falls below 2.1 kPa.