Natural flow regimes in dryland river systems are becoming increasingly augmented due to climate change and water extraction pressure. During unnaturally protracted drought periods in Australia’s Murray-Darling Basin, native fish persist in refuge waterholes which routinely become hypoxic and undergo significant warming. These conditions compromise fish physiological function by simultaneously elevating metabolic demand (warming) and reducing the availability of oxygen (hypoxia) required to fuel aerobic processes. This study investigated whether juvenile golden perch (Macquaria ambigua), could mitigate the deleterious effects of hypoxia and warming through mechanisms of physiological plasticity and behavioural changes.
Juvenile golden perch were acclimated to either normoxic (100% air saturation) or hypoxic (50% air saturation) waters warmed to 30°C for six weeks. Following acclimation, metabolic physiology, blood oxygen carrying capacity, hypoxia tolerance, upper thermal tolerance and behavioural responses to hypoxia were assessed for both acclimation groups under normoxic and hypoxic conditions.
Prolonged exposure to moderate hypoxia improved the overall hypoxia tolerance and additionally produced cross-tolerance to elevated temperatures in golden perch. Physiological plasticity of the heart and spleen also resulted following hypoxia acclimation, with both increasing in size to support an elevated haematocrit. This was inferred to have improved blood-oxygen carrying capacity, facilitating the maintenance of aerobic scope in hypoxia-acclimated fish despite conditions of low environmental O2. Fish behaviour was also modulated by hypoxia acclimation, observed as a reduced reliance on normoxic refuges when faced with severe, acute hypoxia.
We found strong evidence that golden perch undergo significant physiological remodelling and behaviour shifts to offset moderate chronic hypoxia stress. This capacity to ‘adapt’ to environmental change may be crucial to the long-term population resilience of fishes against future drought-induced stress. These results importantly inform management of waterhole refugia through improving the accuracy of habitat suitability models which can be used to predict and prevent fish kills.