Bull. Jpn. Soc. Fish. Oceanogr. 77(3), Page 155-163, 2013   Vertical swimming behavior and habitat of age-0 skipjack tuna, Katsuwonus pelamis, based on archival tag data in the subtropical North Pacific during winter Suguru Okamoto1†, Hidetada Kiyofuji1, Mitsuhiro Takei2, Hiromu Fukuda1, Yoichi Ishikawa3, Hiromichi Igarashi3, Shuhei Masuda4, Nozomi Sugiura4 1 National Research Institute of Far Seas Fisheries, Fisheries Research Agency, 5-7-1 Orido, Shimizu, Shizuoka, Shizuoka 424-8633, Japan 2 Miyazaki Prefectural Fisheries Experimental Station, 6-16-3 Aoshima, Miyazaki, Miyazaki 889-2162, Japan 3 Data Research Center for Marine-Earth Sciences, Japan Agency for Marine-Earth Science and Technology, 3173-25 Showa-machi, Kanazawa, Yokohama, Kanagawa 236-0001, Japan 4 Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-Cho, Yokosuka, Kanagawa 237-0061, Japan † E-mail: suoka@affrc.go.jp Two age-0 skipjack tuna Katsuwonus pelamis (42.5 and 44.0 cm fork length) were recaptured by pole and line one week after released with implanted archival tags in the subtropical North Pacific in February 2011. Data of pressure, peritoneal cavity and ambient temperature data recorded every 20 min for one week were downloaded from two archival tags. The vertical habitat-use during daytime was distributed mainly in the layer of 30-70 m depth, which was deeper than that during nighttime (20-50 m). The deepest swimming depth was 127 m (ambient temperature＝22.4°C). Recaptured skipjack tunas spent more than 95% of their time above the thermocline (depth＝110-125 m), where the ambient temperature was more than 23.8°C. We defined the area where water mass warmer than 23.8°C was distributed as the potential main habitat (PMH) for age-0 skipjack tuna in February, and investigated the interannual variation of the PMH distribution using the ocean reanalysis product by 4-dimensional variational data assimilation. The PMH in 2003 was wider and deeper than that in 1998. Every year from 1990 to 2006, the PMH in the east of 140°E spread to further north (1-3°) than that in the west of 140°E. Compared with the region off Sanriku-Joban in a previous study, warm water mass and the vertical habitat-use of skipjack tuna during daytime were distributed deeply in the subtropical North Pacific. These differences could be attributed to the differences in the availability and catchability of skipjack tuna for fisheries such as pole and line. Key words: age-0 skipjack tuna, archival tag, vertical behavior, vertical habitat-use, subtropical North Pacific