FISHERIES SCIENCE 2006; 72: 1166–1176 Blackwell Publishing AsiaMelbourne, AustraliaFISFisheries Science0919-92682006 Blackwell Science Asia Pty LtdDecember 200672611661176Original Article Temperature effect on bluefin tuna habitat T Kitagawa et al. *Corresponding author: Tel. 81-3-5351-6686; Fax: 81-3-5351-6686. E-mail: takashik@ori.u-tokyo.ac.jp Received 31 March 2006. Accepted 14 June 2006. The effect of water temperature on habitat use of young Pacific bluefin tuna Thunnus orientalis in the East China Sea Takashi KITAGAWA, 1 * Aida SARTIMBUL, 2 Hideaki NAKATA, 3 Shingo KIMURA, 1 Harumi YAMADA 4 AND Akira NITTA 5 1 Ocean Research Institute, University of Tokyo, Nakano, Tokyo 164-8639, 2 Graduate School of Science and Technology, Nagasaki University, Nagasaki 852-8521, 3 Faculty of Fisheries, Nagasaki University, Nagasaki 852-8521, 4 National Research Institute of Far Seas Fisheries, Fisheries Agency of Japan, Shimizu, Shizuoka 424-8633, and 5 Japan NUS, Minato, Tokyo 108-0022, Japan ABSTRACT: 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 hab- itat 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 fur- thest north in 1998. The differences were probably caused by changes in SST associated with La Niña (1996) and El Niño (1998) events. Another densely populated area was identified in offshore waters of latitude 28–30 °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°C into the region immediately to the north-east. KEY WORDS: archival tag, Bluefin tuna, East China Sea, El Niño, habitat use, La Niña. INTRODUCTION The main spawning ground of Pacific bluefin tuna Thunnus orientalis is located to the south of Japan. After hatching, the fish are transported by ocean currents to coastal areas of Japan, on both the western (Sea of Japan) and eastern (Pacific) sides. They migrate northward in summer and then return to the south in autumn. 1 The East China Sea (ECS) (Fig. 1) is the winter habitat for bluefin tuna aged 0–3 years, where they are targeted by trolling and purse seining fisheries. However, the catch of young bluefin varies interannually from 150 to 2000 tonnes, 2 and the fluctuation mechanism has yet to be definitively established. It is therefore of importance to identify the factors that control the interannual differences in fish habitat use in the ECS. The schools of bluefin that congregate around Tsushima Island in the ECS during October– November later migrate to the south, and occur west of the Goto Islands in January. 3 These fish then migrate further south in February to the mid-ECS for overwintering. In March, when surface temper- atures warm, a northerly migration begins. In June, the major proportion appears to the west of the Goto Islands, while a smaller number migrate along Tsushima Island. They enter the Sea of Japan in June and July. 3 This description is based on anal- ysis of a single year of fishery catch data. 3 Recent progress (since the 1990s) in data logging techniques such as ‘archival tags’, has provided considerable information about the migratory and swimming ecology of free-ranging bluefin tuna. 4–14 Several papers have described the migration of young bluefin tuna in the western North Pacific. Kitagawa et al. 6 found that in winter, in the ECS, the bluefin remain within the surface mixed layer, but move closer to the surface during the night, that is, swimming depth displayed diel periodicity. Fish displaying more marked diel periodicity tended to migrate further offshore, where greater