Oxygen uptake of rainbow trout during acute warming: effects of oxygen supersaturation and aerobic exercise

Naturally occurring oxygen supersaturation (hyperoxia) could provide a refuge for fish from heat waves if thermal tolerance is limited by the ability to take up sufficient oxygen to meet cellular oxygen demands during acute heat stress, as previously proposed. While several studies have assessed warming tolerance (CTmax) in water breathers under hyperoxia, none have assessed whether warming tolerance during aerobic exercise (CTSmax) is affected by hyperoxia. Here, we assessed whether hyperoxia could increase the thermal tolerance and metabolic rate of rainbow trout (Oncorhynchus mykiss) both at high swimming speeds (CTSmax; 85% of maximum swimming speed) and in traditional CTmax trials. To do so, n=16 rainbow trout were subjected to thermal tolerance trials under two treatments: hyperoxia [150% dissolved oxygen (DO)] and normoxia (100% DO). For these trials, fish were warmed at a rate of 2°C h-1, with estimates of oxygen consumption (ṀO2) generated for each 30 min (1°C) increment. Hyperoxia caused an apparent increase of ca. 0.6°C in CTSmax but the effect disappeared after controlling for random effects and differences in variance among trial types. Likewise, hyperoxia had no effect on CTmax. Surprisingly, we found no effect of hyperoxia on ṀO2 during the CTSmax trials. Interestingly, although all fish increased their ṀO2 during the acute warming trials, many did not reach ṀO2,max based on earlier critical swimming speed (Ucrit) trials at a benign temperature. Together, our results provide insight into the effects of ecologically relevant levels of oxygen saturation on thermal tolerance in fish.