Immature Pacific bluefin tuna, Thunnus orientalis, marked with archival tags, were released off Tsushima Island in the East China Sea. Time-series data on swimming depth, and ambient and peritoneal cavity temperatures were recorded every 128 s to investigate their thermoconservation mechanisms under low ambient temperature. In summer the difference between peritoneal and ambient temperatures became appreciably large, as ambient temperature decreased due to repeated dives to depths below the thermocline for short periods, perhaps for feeding. This suggests that peritoneal cavity temperature is maintained during dives. A heat budget model revealed that thermal inertia, or internal heat production in the daytime, was important for thermoconservation during dives to depths below the thermocline. As bluefin could only maintain body temperature for a short period, they had to avoid rapid temperature change at the thermocline through behavioral thermoregulation. This is quite different from the situation for bigeye tuna, as reported elsewhere. In addition, it was also revealed that vertical diving activity could be related to physical conditions such as light intensity. The bluefin made few dives on days when solar radiation was comparatively low, implying that low visibility may prevent dives to depths below the thermocline.
