Individual aerobic performance and anaerobic compensation in a temperate fish during a simulated marine heatwave

Marine heatwaves (MHWs) are becoming more frequent and intense due to climate change and have strong negative effects on ecosystem. Few studies have reproduced the complex nature of temperature changes of a MHW, while it is suggested that ectotherms may be more vulnerable to rapid changes such as during MHWs. Effects of an experimental MHW were investigated in the golden grey mullet Chelon auratus. Juveniles acclimated to 20 °C were exposed to a rapid 5 °C increase in temperature, followed by a five-day period at 25 °C, before quickly returning to 20°C. Metabolic variables (SMR-standard, MMR-maximum rate, AS-aerobic scope, EPOC-excess post‑oxygen consumption) and critical swimming speed (U_crit) were measured at different phases of this MHW and after a thermally stable recovery phase. Although the pattern was only significant for the SMR, the aerobic three variables describing aerobic metabolism (SMR, MMR and AS) immediately increased in fish exposed to the acute elevation of temperature, and remained elevated when fish stayed at 25 °C for five days. A similar increase of these metabolic variables was observed for fish that were progressively acclimated to 25 °C. This suggests that temperature increases contribute to increases in metabolism; however, the acute nature of the MHW had no influence. At the end of the MHW, the SMR remained elevated, suggesting an additional cost of obligatory activities due to the extreme event. In parallel, U_crit did not vary regardless of the thermal conditions. Concerning EPOC, it significantly increased only when fish were acutely exposed to 25 °C. This strongly suggests that fish may buffer the effects of acute changes in temperature by shifting to anaerobic metabolism. Globally, this species appears able to cope with this MHW, but that's without taking into consideration future projections describing an increase in both intensity and frequency of such events, as well as other stressors like pollution or hypoxia.