The effects of marine heatwaves on a coral reef snapper: insights into aerobic and anaerobic physiology and recovery

Marine heatwaves (MHW) are intensifying, posing a grave threat to coral reefs. We exposed a coral reef mesopredator, Lutjanus carponotatus, to MWH conditions (+1 degrees C and +2 degrees C) for 4 weeks and found increased oxygen consumption, recovery time, haemoglobin, and lactate. Interestingly, several effects persisted for at least 2 weeks post-MWH. Marine heatwaves (MHWs) are increasing in frequency and intensity. Coral reefs are particularly susceptible to MHWs, which cause mass coral bleaching and mortality. However, little is known about how MHWs affect coral reef fishes. Here, we investigated how MHWs affect the physiology of a coral reef mesopredator, Lutjanus carponotatus. Specifically, we exposed mature adults to two different MHW intensities, +1 degrees C (29.5 degrees C) and + 2 degrees C (30.5 degrees C) and measured physiological performance at 2 and 4 weeks of exposure and at 2 weeks post-exposure. At these time points, we measured oxygen consumption at rest and after a simulated fishing capture event, recovery time, excess post-exercise oxygen consumption (EPOC) and associated biochemical markers in the blood (baseline lactate, post-capture lactate, glucose, haemoglobin levels and haematocrit proportion). We found that 2 weeks of exposure to MHW conditions increased resting oxygen consumption (+1 degrees C = 23%, +2 degrees C = 37%), recovery time (+1 degrees C = 62%, +2 degrees C = 77%), EPOC (+1 degrees C = 50%, +2 degrees C = 68%), baseline lactate (+1 degrees C = 27%, +2 degrees C = 28%), post-capture lactate (+1 degrees C = 62%, +2 degrees C = 109%) and haemoglobin levels (+1 degrees C = 13%, +2 degrees C = 28%). This pattern was maintained at 4 weeks of exposure except for post-capture lactate which was reduced (+1 degrees C = -37%, +2 degrees C = 27%). In combination, these results suggest a greater reliance on anaerobic glycolysis to maintain homeostasis in MHW conditions. At 2 weeks post-exposure, when compared to control fish, we found that capture oxygen consumption was increased (+1 degrees C = 25%, +2 degrees C = 26%), recovery rate was increased (+2 degrees C = 38%) and haemoglobin was still higher (+1 degrees C = 15%, +2 degrees C = 21%). These results show that MHW conditions have direct physiological demands on adult coral reef snapper and ecologically relevant residual effects can last for at least 2 weeks post-MHW; however, individuals appear to recover from the negative effects experienced during the MHW. This provides new insight into the effects of MHWs on the physiological performance of coral reef fishes.