Bull. Jpn. Soc. Fish. Oceanogr. 81(3), Page 230-244, 2017   Spatio-temporal dynamics and mortality of mega-benthos in relation to the development of hypoxia in an inner bay: A model study in Mikawa Bay, Japan Ryota Sone1, 4 †, Mitsuyasu Waku1, 5, Satoshi Yamada1, 6, Teruaki Suzuki2 and Teruhiro Takabe3 1 Aichi Fisheries Research Institute, 97 Wakamiya, Miya, Gamagori, Aichi 443-0021, Japan 2 Graduate School of Environmental and Human Science, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya, Aichi 468-8502, Japan 3 Research Institute of Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya, Aichi 468-8502, Japan 4 Present: Fisheries Administration Division, Department of Agriculture, Forestry, and Fisheries, Aichi Prefectural Governmental Office, 3-1-2 Sannomaru, Naka-ku, Nagoya, Aichi 460-8501, Japan 5 Present: Higashi Mikawa Agriculture, Forestry, and Fisheries Office, Aichi Prefectural Government, 5-4 Hatchodo-ri, Toyohashi, Aichi 440-0806, Japan 6 Present: Nishi Mikawa Agriculture, Forestry, and Fisheries Office, Aichi Prefectural Government, 1-4 Myodaijihoncho, Okazaki, Aichi 444-0860, Japan † E-mail: riyouta_sone@pref.aichi.lg.jp To investigate spatio-temporal responses of mega-benthos species to hypoxia, we conducted a bottom trawl survey with environmental observation in Mikawa Bay monthly from June to October. The number of deaths for each main mega-benthos species was estimated from standing stock in relation to fishing, recruitment, and growth. Further, natural mortality coefficient (Mt) and death ratio (Dnratio) under hypoxia expansion (i.e., June to September) were calculated. Distribution areas and standing stocks of each mega-benthos species decreased as hypoxia developed. Although Mt (month-1) changed with the calculation period, the mean from June to September was estimated at 0.41-0.85, 0.42-0.46, 0.46-0.76 and 1.16-1.26 in flounder, swimming crab, mantis shrimp, and cockle, respectively. These values were higher than well-known natural mortality coefficients, which were calculated from biological parameters or observations conducted in normoxic regions. Calculated Dnratio was more than 50% in all analyzed species. The results of this study indicate that developed hypoxia causes mass mortality of main mega-benthos species. Moreover, expansion of hypoxic areas incidental to coastal development have potentially deteriorated the ecological corridor for the stone flounder and kuruma prawn populations, which use the inner area of the bay as a nursery. Key words: dissolved oxygen, environmental quality standards, hypoxia, mega-benthos, mortality, spatio-temporal distribution, ecological corridor