Physiological and behavioural plasticity improves resilience to chronic hypoxia and warming in a dryland river fish

Freshwater organisms are increasingly imperiled by episodes of chronic environmental hypoxia and warming which have become more frequent and widespread as climate change impacts intensify globally. Dryland rivers sustain populations of fish in arid and semi-arid places on Earth. However, rising water temperatures and increasing levels of aquatic hypoxia are compounding the challenges already faced by fish in these often-extreme environments. These interacting stressors compromise fish health by simultaneously elevating metabolic demand for oxygen while reducing its availability to fuel aerobic processes. However, the capacity for fish to offset some of these negative effects through behavioural and/or physiological plasticity is recognised but poorly understood. This study investigated whether juvenile golden perch (Macquaria ambigua) could mitigate the deleterious effects of hypoxia and warming through physiological phenotypic plasticity and behavioural changes. Juvenile golden perch were acclimated for six weeks to either normoxic (100 % air saturation) or moderately hypoxic waters (50 % air saturation) maintained at 30 °C. Following acclimation, metabolic physiology, blood oxygen transport capacity, tolerance of hypoxia and thermal stress, and behavioural responses to hypoxia and predation were assessed under conditions of both normoxia and hypoxia. Prolonged exposure to moderate hypoxia improved tolerance of individuals to both hypoxia and elevated temperatures. Physiological plasticity of the heart also resulted following hypoxia acclimation indicated by increased ventricular mass to support an elevated haematocrit. This improved blood‑oxygen transport capacity facilitated the maintenance of aerobic scope in hypoxia-acclimated fish despite conditions of low environmental oxygen. Fish acclimated to hypoxic conditions had a reduced preference for normoxic environments but did not display changed antipredator behaviour, with the risk of predation outweighing the risk of severe hypoxia for both acclimated and non-acclimated fish. Acclimated golden perch adjusted their morphology, physiology and behaviour to offset moderate chronic hypoxia stress, with these changes also conferring cross-tolerance to thermal stress. The ability to induce resilience against two significant environmental stressors of fish has important implications for water resource management in dryland rivers which needs to balance the demands of water users and the environment during resource-limited dry periods.