Bull. Jpn. Soc. Fish. Oceanogr. 73(1), Page 1-7, 2009   Implication of effects of fluctuations in oceanographic condition and maturation process on migration of common mackerel (Scomber japonicus) in the fishing grounds around Izu Islands Kyu Okabe1†, Shizuo Iwata2 and Seiichi Watanabe3 1 Kanagawa Prefecture Fisheries Technology Center, Jougashima, Misaki, Miura, Kanagawa 238-0237, Japan 2 Japan Fisheries Information Service Center, Toyomi-shinko Bldg 6F, 4-5 Toyomi-cho, Chuo-ku, Tokyo 104-0055, Japan 3 Faculty of Marine Science, Tokyo University of Marine Science and Technology, 4-5-7 Konan, Minato-ku, Tokyo 108-8477, Japan † e-mail: okabe.4wwh@pref.kanagawa.jp Effects of oceanographic conditions and maturity on schooling and migration of common mackerel, Scomber japonicus, were analyzed based on fishing log of dip net fishing, oceanographic conditions, tagging experiments and maturation process around the fishing ground in Izu Islands. Spawning schools of the mackerel were caught at the shoal less than 200 m of water depth where was covered with warm water from Kuroshio. Sea surface temperature (SST) of the fishing ground was approximately 16°C in the sexually maturing period of the mackerel until mid-February, and ranged 18-20°C during spawning period until mid-June, indicating SST suitable for schooling coincides that for spawning. When the preparation of spawning was complete, the mature mackerel moved southward toward warm shoals where SST was optimum for spawning conditions by the influence of shifts of Kuroshio axis. Due to the rise of the temperature by the influence of warm water, the mackerel moved northward for cooler SST. Intrusion of warm water over 22°C induced the mackerel to cease spawning and migrate northward for feeding in the late fishing season in mid-June. Thus, the intrusion of warm water from Kuroshio is deeply involved in schooling, spawning and feeding migration of the mackerel around the fishing ground in Izu Islands. Key words: Scomber japonicus, common mackerel, migration, oceanographic condition, maturation process