Abstract

Immature Pacific bluefin tuna, Thunnus thynnus orientalis, marked with archival tags, were released off Tsushima Island in the East China Sea. Timed data on swimming depth, ambient temperature and peritoneal-cavity temperature were recorded every 128 s to evaluate seasonal and spatial changes and the effects of ambient temperature on vertical distribution. The tuna swam within the surface mixed layer in the East China Sea during both night and day in winter, whereas they spent most of their time at the surface in summer when a thermocline developed. Bluefin tuna migrating into the Sea of Japan and the western North Pacific usually stayed at the surface and did not show large vertical migrations. This was presumably because the vertical distribution of their prey in these regions is markedly different from that in the East China Sea. This suggests that seasonal and spatial differences in the vertical distribution of water temperature and prey determine the pattern of vertical distribution and movements of bluefin tuna.