Physiological and behavioural thermoregulation of juvenile yellowfin tuna Thunnus albacares in subtropical waters

Fish of the genus Thunnus are unusual because they are regional endotherms. In this study, archival tag data were used to demonstrate behavioural and physiological thermoregulation in juvenile yellowfin tuna, Thunnus albacares (35-52 cm fork length). Tags inserted into the peritoneal cavity were recovered from 23 yellowfin tuna caught mainly around Ishigaki Island, Japan, in 2009-2012. Peritoneal and environmental temperatures and depth data revealed that juvenile yellowfin tuna made daytime bounce dives repeatedly with a median dive duration of 16 min, and they remained at the surface during night-time. Body temperature daily averages were slightly higher than those of ambient temperature, and the body temperatures varied according to ambient temperature changes associated with vertical movements. Heat exchange coefficients (.) during dives were estimated by a heat budget model. The thermal inertia component of. was estimated in an experiment with dead juvenile yellowfin tuna. The value of. exceeded the measured thermal inertia and varied between ascents and descents, thus providing evidence that physiological thermoregulation contributes to. in juvenile yellowfin tuna and varies according to environmental temperature. Physiological thermoregulation helps maintain relatively stable body temperatures during dives in which tuna behaviourally escape cool deep waters and enter warm surface waters. Comparison of the obtained physiological parameters of. and heat production (..T m) with those previously recorded in Pacific bluefin tuna indicate that yellowfin tuna has a higher. and lower.T m than Pacific bluefin tuna, which may restrict yellowfin tuna to warm tropical and subtropical surface waters.