Bull. Jpn. Soc. Fish. Oceanogr. 80(1), Page 1-19, 2016   Quantification of the demersal marine ecosystem structure in the northern district of the Tohoku sea area, northeastern Japan based on Ecopath approach Shiroh Yonezaki1†, Masashi Kiyota1, 2, Yoji Narimatsu3, Tsutomu Hattori3 and Masaki Ito4 1 National Research Institute of Far Seas Fisheries, Fisheries Research Agency, 2-12-4 Fukuura, Yokohama, Kanagawa 236-8648, Japan 2 Tokyo University of Marine Science and Technology, 4-5-7 Konan, Minato-ku, Tokyo 108-8477, Japan 3 Tohoku National Fisheries Research Institute, Fisheries Research Agency, 25-259 Same-machi, Hachinohe, Aomori 031-0841, Japan 4 Hokkaido National Fisheries Research Institute, Fisheries Research Agency, 116 Katsurakoi, Kushiro, Hokkaido 085-0802, Japan † E-mail: yonez@affrc.go.jp An Ecopath model for the demersal marine ecosystem (46 functional groups) was constructed based on fisheries resources survey data and commercial fisheries catch statistics in the northern district of northeastern Japan. This is the first Ecopath model for demersal marine ecosystems in the sea area around Japan. We used outputs of the Ecopath model and network analysis tools to quantitatively describe the food web structure and examine the fisheries impacts on the ecosystem. Macrobenthos, mesopelagic fishes, and Pacific krill Euphausia pacifica were identified as important prey species, but prey selectivity of these species differed by predators. Keystone functional groups were demersal sharks, Pacific cod Gadus macrocephalus (2+years), and giant grenadier Coryphaenoides pectoralis. In primary production required (PPR%), Danish seine-trawl fishery was lower than pair-trawl fishery, whereas in relative loss in production (L-index), Danish seine-trawl fishery was higher than pair-trawl fishery. Key words: Ecopath, demersal fishes, marine ecosystem, Trawl fishery, Fishing impact