In a marine teleost, the significance of oxygen supply for acute thermal tolerance depends upon the context and the endpoint used.

Eight juvenile European seabass were exposed to two thermal ramping protocols with different levels of aerobic activity and tolerance endpoint: the critical thermal maximum for swimming (CTSmax) while exercising aerobically until fatigue and the critical thermal maximum (CTmax) under static conditions until loss of equilibrium (LOE). In the CTSmax protocol, warming caused a profound increase in the rate of oxygen uptake (ṀO2), culminating in a gait transition from steady aerobic towards unsteady anaerobic swimming, then fatigue at 30.3±0.4°C (mean±s.e.m.). Gait transition and fatigue presumably indicate an oxygen limitation, which reflects the inability to meet the combined demands of swimming plus warming. The CTmax protocol also elicited an increase in ṀO2, culminating in LOE at 34.0±0.4°C, which is significantly warmer than fatigue at CTSmax. The maximum ṀO2 achieved in the CTmax protocol was, however, less than 30% of that achieved in the CTSmax protocol. Therefore, the static CTmax did not exploit full cardiorespiratory capacity for oxygen supply, indicating that LOE was not caused by systemic oxygen limitation. Consequently, systemic oxygen supply can be significant for tolerance of acute warming in seabass but this depends upon the physiological context and the endpoint used.