Bull. Jpn. Soc. Fish. Oceanogr. 75(4), Page 211-220, 2011   Water mass structure in the spawning area of walleye pollock (Theragra chalcogramma) on the Pacific coast of southern Hokkaido, Japan Itsuka Yabe1, Akira Kusaka2, Tomonori Hamatsu3†, Tomonori Azumaya3 and Akira Nishimura4 1 Japan NUS Co. Ltd., Loop-X Bldg., 7F 3-9-15 Kaigan, Minato-ku, Tokyo 108-0022, Japan 2 National Research Institute of Fisheries Science, Fisheries Research Agency, 2-12-4 Fukuura Kanazawa, Yokohama, Kanagawa 236-8648, Japan 3 Hokkaido National Fisheries Research Institute, 116 Katsurakoi, Kushiro, Hokkaido 085-0802, Japan 4 Seikai National Fisheries Research Institute, Fisheries Research Agency, 1551-8 Taira-machi, Nagasaki, Nagasaki 851-2213, Japan † e-mail: azumaya@fra.affrc.go.jp Relationships between physical environmental factors (temperature, salinity and depth) and the formation of the spawning area of walleye pollock (Theragra chalcogramma) off the North Pacific coast of Hokkaido, Japan were examined. Water temperature inside and outside of Funka Bay decreased from January to March and especially in Hidaka Bay because of the Coastal Oyashio Water (COW) intrusion into the surface layer. From quotient analysis, mean spawning environmental conditions of walleye pollock were 4.43 (SD=2.04)°C, and a salinity of 33.28 (SD=0.42), both are typical of the water mass of Oyashio Water (OW). The spawning area moved anti-clockwise from Tomakomai to the mouth of the Tsugaru Strait during the spawning period. Water mass which eggs mainly experienced in the sea surface was OW in January and February, and COW in March. The duration to hatch was from 8 to 20 days in Tsugaru Warm Water (TW), from 15 to 25 days in OW and 25 days or more in COW. We conclude that the relative distribution of water masses affects the location of the spawning area and subsequently the rate of development of the eggs. Finally, we propose a practical method to estimate the spawning area of walleye pollock using the physical environment. Key words: walleye pollock, eggs, spawning environment, Coastal Oyashio, Oyashio, Tsugaru Warm Current