Experimental hyperoxia (O₂ supersaturation) reveals a gill diffusion limitation of maximum aerobic performance in...

Several studies have demonstrated that hyperoxia increases the maximal O₂ consumption rate (ṀO_2max) in fish, but exactly how this occurs remains to be explained. Here, we tested the hypothesis that hyperoxia improves arterial oxygenation in rainbow trout during exhaustive exercise. We demonstrate a 35% higher ṀO_2max in hyperoxia (200% air saturation) relative to normoxia, which was achieved through a combined 15% increase in cardiac output due to elevated peak heart rate, and a 19% increase of the arterial-venous (A-V) O₂ content difference. While arterial O₂ partial pressure (PaO₂) and O₂ saturation of haemoglobin declined post-exhaustive exercise in normoxia, this did not occur in hyperoxia. This protective effect of hyperoxia on arterial oxygenation led to a 22% higher arterial O₂ content post-exhaustive exercise, thereby allowing a higher A-V O₂ content difference. These findings indicate that ṀO_2max is gill diffusion limited in exhaustively exercised rainbow trout. Moreover, as previous studies in salmonids have demonstrated collapsing PaO₂ in normoxia at maximal swimming speed and at acutely high temperatures, a diffusion limitation may constrain ṀO₂ in other situations eliciting peak metabolic demand. These findings, along with the fact that hyperoxia increases ṀO_2max in several other fishes, suggest that gill diffusion limitations of ṀO_2max may be widespread in fishes.