The effect of water temperature on habitat use of young Pacific bluefin tuna Thunnus orientalis in the East China Sea

Immature Pacific bluefin tuna Thunnus orientalis, tagged with archival tags, were released near Tsushima Island in the East China Sea (ECS) during the winters of 1995, 1996 and 1997. Geolocations were estimated using the archival tags from recovered fish. These data, together with sea surface temperature (SST) data from satellite remote sensing, are used to describe the habitat used by these bluefin in the ECS from January to June for 3 years (1996, 1997, 1998), and to asses the effect of water temperature on fish distribution and movement. The results indicate that their geolocations ranged from the area north-east of Tsushima Island to the offshore area in the south-west. However, the area of highest density differed among years, being furthest south in 1996 and furthest north in 1998. The differences were probably caused by changes in SST associated with La Nina (1996) and El Nino (1998) events. Another densely populated area was identified in offshore waters of latitude 28-30 degrees N in 1996 (only), on the cold side of the Kuroshio front. These fish may have been prevented from moving northwards by an intrusion of Kuroshio water of approximately 25 degrees C into the region immediately to the north-east.