Thermal Stability in Hypoxia Tolerance Irrespective of Innate Tolerance Capacity: Associations with Oxygen Demand and Supply Balance in Four Freshwater Fishes

Global warming imposes dual stressors—habitat warming and aquatic hypoxia—that disrupt oxygen demand-supply balance in fishes, threatening their survival and distribution. Thus, the primary aims of the present study were: (1) to investigate the response patterns of hypoxia tolerance to increased acclimation temperatures in two normoxia-adapted and two hypoxia-tolerant freshwater fish species; and (2) to explore the associated physiological mechanisms involved in oxygen demand and supply. We found that, as key fitness-related physiological indicators, all hypoxia tolerance metrics [the critical oxygen partial pressure (PO₂) for maintenance metabolism (Pcrit-SMR) and loss of body equilibrium (PLOE)] were highly stabilized across temperatures in four freshwater fish species. This stability was mainly due to the concurrent increase in oxygen demand and supply with rising temperature. However, under severe hypoxic conditions, hypoxia-tolerant species relied primarily on increased recruitment of anaerobic metabolism and metabolic depression under warm temperatures, rather than increased oxygen supply in normoxia-adapted species. Unexpectedly, one normoxia-adapted species showed a tendency toward enhanced hypoxia tolerance with increasing temperature, which may be attributed to the specific thermal and oxygen regimes in natural habitats. Within each tolerance category, one species lacked thermal stability in predicted Pcrit-MMR (critical PO₂ for maximum metabolic rate), which may be attributable to the possibility that α is not constant across all freshwater fishes. Nevertheless, we demonstrate conserved hypoxia tolerance across thermal gradients in the four freshwater fishes. This underscores the need to elucidate the mechanistic basis of oxygen regulation strategies and hypoxia tolerance, which will enable accurate projection of distribution patterns for aquatic ectotherms in warming environments.