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Immature Pacific bluefin tuna Thunnus thynnus orientalis, marked with archival tags, were released near Tsushima Island in the eastern East China Sea (58 fish on 7-14 Dec 1995; 47 fish on 29 Nov 1996), to investigate thermoconservation mechanisms of immature bluefin under low ambient temperature. A total of 15 fish were recovered and time-series data for ambient water and peritoneal cavity temperatures, recorded every 128 s, were analyzed. In winter, the difference between ambient and peritoneal cavity temperatures increased only slightly as ambient temperature decreased. In sum- mer the difference became appreciably large, as ambient temperature decreased due to repeated dives to depths below the thermocline for short periods (~640 s), 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 dur- ing dives. 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. The mean temperature difference for a half- day period was larger in summer than in winter and in addition, the difference increased with body size. These results imply that the ability to maintain peritoneal cavity temperature develops with growth from winter to summer. The heat budget model further suggests that an ability to maintain peritoneal cavity temperature could result from decreasing both the whole-body heat-transfer coeffi- cient (k) and internal heat production ( . Tm), and that the significant increase in temperature difference from winter to summer could be attributed to a lower decreasing rate of . Tm compared with that of k during this period. The significantly larger temperature difference in the daytime, compared to that in the nighttime, suggested higher internal heat production resulting from higher internal activity in the daytime. However, it was also revealed that vertical diving activity could be related to physical condi- tions such as light intensity. The bluefin made few dives on days when solar radiation was compara- tively low, implying that low visibility may prevent dives to depths below the thermocline.

Thermoconservation mechanisms inferred from peritoneal cavity temperature in free-swimming Pacific bluefin tuna Thunnus thynnus orientalis