Thermoregulation of ocean sunfish in a warmer sea suggests their ability to prevent heat loss in deep, cold foraging grounds

During behavioural thermoregulation, it has been demonstrated that fish alter their heat exchange rates between warming and cooling. In the case of the ocean sunfish (Mola mola), there is a several-fold difference in the rate of heat exchange between foraging in the deep, cooler water and recovering body temperature at the warmer surface. Increased heat gain during warming has been suggested, but there is also the possibility of preventing heat loss during cooling. To examine this, we measured the vertical movement and muscle temperature of three captive sunfish that were released into the field under warmer conditions and compared them with previously reported sunfish that foraged in cooler deep water. The released sunfish vertically moved from near the surface, where the water temperature was 21–25 °C, to depths of 150–200 m, where the water temperature was 15–17 °C. All sunfish occasionally appeared on the sea surface and sometimes remained at the surface for <41 min. However, they reduced their muscle temperature by diving before their muscle temperature exceeded 21–22 °C, suggesting behavioural thermoregulation using the cooler water temperature at depth. In addition, the feeding events detected by video were limited to depths shallower than 60 m. This is in contrast from previously observed sunfish behaviour, in which fish were reported as recovering their body temperature using warmer temperatures at the surface after foraging in deep, cooler water. The heat-budget models indicated that the whole-body heat-transfer coefficients during warming in this study were similar to those of previously reported sunfish that had recovered their body temperature at the sea surface after foraging in deep, cooler water. In contrast, the heat-transfer coefficients at cooling in this study were higher than those of sunfish foraging in deep, cooler water. This suggests that ocean sunfish may have the ability to prevent heat loss in the physiological regulation of heat exchange rate during thermoregulation, while also indicating that sunfish have the ability to respond to changes in the temperature environment by altering their behaviour and heat exchange rate.