Physiological plasticity v. inter-population variability: understanding drivers of hypoxia tolerance in a tropical estuarine fish

Physiological plasticity and inter-population variability (e.g. local adaptation) are two key drivers in determining the capacity for species to cope with environmental change, yet the relative contribution of each parameter has received little attention. Here, we investigate the acclimation potential of two geographically distinct populations of the barramundi (Lates calcarifer) to diel hypoxia. Fish were exposed to a daily hypoxia challenge of 6 h below 62% saturation, down to a minimum of 10 ± 5% saturation, followed by a return to normoxia. Respiratory and haematological variables were assessed after 8 and 16 days of daily hypoxia exposure. Hypoxia tolerance (measured as the critical oxygen tension; [O₂]_crit) was not different between populations and not different from control fish after 8 days ([O₂]_crit = 20.7 ± 2.8% saturation), but improved similarly in both populations after 16 days ([O₂]_crit = 16.5 ± 3.1% saturation). This improvement corresponded with increases in haematocrit and haemoglobin, but not an increase in the mean cell haemoglobin concentration. Given the similarity of the response between these two geographically distinct populations, we conclude that hypoxia tolerance for barramundi may be more dependent on physiological plasticity than inherent variability between populations.