Body size dictates physiological and behavioural responses to hypoxia and elevated water temperatures in Murray cod ().

Increasing drought frequency and duration pose a significant threat to fish species in dryland river systems. As ectotherms, fish thermal and hypoxia tolerances directly determine the capacity of species to persist in these environments during low flow periods when water temperatures are high and waterbodies become highly stratified. Chronic thermal stress can compound the impacts of acute hypoxic events on fish resulting in significant fish mortality; however, it is not known if all size classes are equally susceptible, or if the allometric scaling of physiological processes means some size classes are disproportionately affected. We investigated the physiological responses of Murray cod () over a four-fold body size range (0.2-3000 g) to acute changes in water temperature and oxygen concentration following 4 weeks of acclimation to representative spring (20°C) and summer (28°C) water temperatures. We recorded maximum thermal tolerance ( ), oxygen limited thermal tolerance (P ), lowest tolerable oxygen level (as the oxygen level at which lose equilibrium; O) gill ventilation rates and aerial surface respiration threshold, blood oxygen transport capacity and lactate accumulation. Acclimation to elevated water temperatures improved thermal and hypoxia tolerance metrics across all size classes. However, body size significantly affected thermal and hypoxia responses. Small were significantly less hypoxia tolerant than larger individuals, while larger fish were significantly less thermal tolerant than smaller fish. Hypoxia constrained thermal tolerance in , with both small and large fish disproportionally compromised relative to mid-sized fish. Our findings indicate that both very small/young (larvae, fry, fingerlings) and very large/older in dryland rivers are at significant risk from the combined impacts of a warming and drying climate and water extraction. These data will inform policy decisions that serve to balance competing demands on precious freshwater resources.