Bull. Jpn. Soc. Fish. Oceanogr. 83(3), Page 151-163, 2019   Development of water-mass cluster analysis software to analyze the water-mass variations off the coast of Tohoku Japan: A case study for future application to analyze fisheries catch variation Takahiro Tanaka1†, Yutaka Imamura2, Takuya Kodama3, Toshiyuki Oikawa3, Asagi Yagura4, Mitsuhiro Saeki4, Kouhei Makabe5, Yuuya Suzuki6,11, Satoshi Ohata7, Hitoshi Kaneko1,10, Yuji Okazaki1, Hiroshi Kuroda8, Taku Wagawa9, Daisuke Hasegawa1, Shigeho Kakehi1 and Takeshi Okunishi1 1 Tohoku National Fisheries Research Institute, Japan Fisheries Research and Education Agency, 3–27–5 Shinhama-cho, Shiogama, Miyagi 985–0001, Japan 2 Fisheries Research Institute, Aomori Prefectural Industrial Technology Research Center, 10 Tsukidomari, Moura, Hiranai-machi, Higashi Tsugaru, Aomori 039–3381, Japan 3 Iwate Fisheries Technology Center, 3–75–3 Hirata Kamaishi, Iwate 026–0001, Japan 4 Miyagi Prefecture Fisheries Technology Institute, 97–6 Sodenohama, Watanoha, Ishinomaki, Miyagi 986–2135, Japan 5 Fukushima Prefectural Fisheries and Marine Science Research Centre, 35 Yoko-machi, Onahama, Iwaki, Fukushima 971–8101, Japan 6 Ibaraki Prefectural Fisheries Research Institute, 3551–8 Mitsuduka, Hiraiso-cho, Hitachinaka, Ibaraki 311–1203, Japan 7 Chiba Prefectural Fisheries Research Center, 2492 Hiraiso, Chikura-cho, Minamiboso, Chiba 295–0024, Japan 8 Hokkaido National Fisheries Research Institute, Japan Fisheries Research and Education Agency, 116 Katsurakoi, Kushiro, Hokkaido 085–0802, Japan 9 Japan Sea National Fisheries Research Institute, Japan Fisheries Research and Education Agency, 1–5939–22 Suido-cho, Chuou-ku, Niigata, Niigata 951–8121, Japan 10 (Current affiliation) Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba, Japan, 5–1–5 Kashiwanoha, Kashiwa, Chiba 277–8564, Japan 11 (Current affiliation) Freshwater Branch Office, Ibaraki Prefectural Fisheries Research Institute, 1560 Tamatsukuriko, Namekata Ibaraki 311–3512, Japan † E-mail: takahirot@affrc.go.jp Hydrographic data off the Pacific coast of Tohoku and off the Boso Peninsula were analyzed using newly developed software that conducts a statistical cluster analysis to reveal the spatial distribution and temporal variation of occurrence frequency of water-masses in surface and subsurface layers. The interannual variation of water-mass occurrence frequency off the coast of Tohoku in summer was most associated with the interannual meridional shift of the Oyashio first branch, and the cold, fresh (warm, saline) clusters were more frequently observed when the Oyashio first branch shifted southward (northward). This tendency was also detected at a 10-m depth, where the previous water-mass classification method was difficult to apply. At a 100-m depth, multiple clusters were classified as Tsugaru Warm Current water, but those that could not be formed by isopycnal mixing between Kuroshio and Oyashio waters were limitedly distributed along the coast of Tohoku. It was suggested these clusters originated from the Tsugaru Strait. Off the coast of the Boso Peninsula in spring, the occurrence frequency of cold water-mass tended to increase when the Kuroshio axis departed from the coast. To relate the water-mass variations to fisheries catch variations, the number of clusters may need to be determined by considering the physiological characteristics of each fish species, such as optimum temperature range. By expanding the analysis to different regions and fish species and accumulating knowledge, the software is expected to provide a useful tool for extracting characteristic water-mass variations and clarifying the relationship between water-mass and fisheries catch variations. Key words: cluster analysis, Tohoku region, water-mass classification, fisheries catch variation