Bull. Jpn. Soc. Fish. Oceanogr. 84(4), Page 271-284, 2020   Relationship between sea temperature variation and fishing ground formations of chub mackerel in the Pacific Ocean off Tohoku Takeshi Okunishi1† ,Katsumi Yokouchi1,Daisuke Hasegawa1,Takahiro Tanaka1,Takashi Setou2, Ryuji Yukami2,Hiroshi Kuroda2,3,Kazuyoshi Watanabe4 and Akinori Takasuka2a 1Tohoku National Fisheries Research Institute, Japan Fisheries Research and Education Agency, 3–27–5 Shinhama-cho, Shiogama, Miyagi 985–0001, Japan 2National Research Institute of Fisheries Science, Japan Fisheries Research and Education Agency aPresent: Graduate School of Agricultural and Life Sciences, The University of Tokyo 3Hokkaido National Fisheries Research Institute, Japan Fisheries Research and Education Agency 4Japan Fisheries Information Service Center †E-mail: okunishi@affrc.go.jp Chub mackerel Scomber japonicus is one of the most commercially exploited fish species in the northwestern Pacific. In the present study, the relationships between commercial purse seine fisheries records of chub mackerel catches and oceanographic conditions in the region off Tohoku, northeastern Japan, were examined for years 1995 to 2015. The surface layer temperature and the fishing ground distributions showed vast interannual fluctuations. Sea temperature variations greatly impacted the fishing ground formations of chub mackerel. This was especially evident when the offshore Oyashio front shifted south (north) in spring, resulting in relatively low sea temperatures in the southern region off Sanriku before the fishing season began in spring, which seemingly shifted fishing grounds southward (northward) off Sanriku in summer and autumn. Overall, results from this work highlight that regional oceanographic conditions leading up to the start of the fishing season can provide a good indication of the main chub mackerel fishing grounds. Key words: purse seine fisheries, chub mackerel, fish ground variation, Pacific Ocean off Tohoku