<p>Water temperature strongly influences fish physiology, and body temperature in fishes often changes asymmetrically during cooling and warming. In pelagic fishes, body temperature commonly increases more rapidly during warming than it decreases during cooling, suggesting physiological regulation of heat transfer. However, the contribution of cardiac activity to this asymmetry remains unclear. Ambient water temperature was experimentally manipulated in captive amberjacks (<i>Seriola </i>spp<i>.</i>) and heart rate and abdominal cavity temperature were measured to examine the role of cardiac activity in heat transfer. Heart rate exhibited clear temperature hysteresis, forming a circular relationship with body temperature: heart rate was lower during cooling and higher during warming at the same body temperature. The heat transfer coefficient was consistently higher during warming than during cooling and increased with increasing heart rate. Complementary experiments using a dead fish were conducted to separate physical and physiological components of heat transfer. The heat transfer coefficient of the dead fish was lower than that of live fish, representing the physical component. After subtracting this component, the remaining physiological component also differed between cooling and warming phases. These results demonstrate that heart rate exhibits temperature hysteresis in fishes and contributes to asymmetric heat transfer during cooling and warming.</p>