Fish and hyperoxia-From cardiorespiratory and biochemical adjustments to aquaculture and ecophysiology implications

Hyperoxia occurs when water oxygen (O-2) levels exceed normal atmospheric pressure (i.e., >100% air saturation). Fish can experience hyperoxia in shallow environments due to photosynthesis or in aquaculture through O-2 supplementation. This review provides a comprehensive synthesis of the effects of hyperoxia on fish, spanning influences on cardiorespiratory function, acid-base balance, oxidative stress and whole animal performance (e.g., thermal tolerance and growth). Fish hypoventilate in hyperoxia, but arterial and venous blood oxygenation increases in spite of reduced convection. Persistently high levels of blood oxygenation in hyperoxia do not commonly result in reduced blood O-2 carrying capacity, but assessments in undisturbed fish are required to clarify this. Hypoventilation also causes the retention of carbon dioxide, hence respiratory acidosis. Another consequence of hyperoxia is increased levels of oxidative stress and concomitant changes to antioxidant defence systems. Despite these changes, however, the bulk of evidence shows no effect of hyperoxia on growth. Hyperoxia does impact the aerobic metabolic rate of fish with either no effect or elevated resting metabolic rate and substantial increases in maximum metabolic rate. There is also evidence that hyperoxia increases aerobic capacity improves cardiac performance and mitigates anaerobic stress during acute warming. Along with improved upper thermal tolerance in some species, these findings collectively suggest that hyperoxia might provide fish a metabolic refuge during acute warming. Since hyperoxia occurs in shallow aquatic habitats, further research establishing the ecophysiological implications of concomitant heat stress and hyperoxia is pertinent, particularly with a changing climate.

Automated summary

This review comprehensively synthesizes the effects of hyperoxia on fish, covering influences on cardiorespiratory function, acid-base balance, and oxidative stress. While hyperoxia can cause hypoventilation and increase aerobic metabolic rate in fish, current evidence suggests it does not significantly impact growth.