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