Bull. Jpn. Soc. Fish. Oceanogr. 79(4), Page 266-276, 2015   Changes of dominant megabenthos caught by small-bottom trawl in relation to bottom dissolved oxygen concentration in Ise Bay, central Japan Manabu Hibino1*†, Takashi Aoyama2, Tadashi Matsuzawa1 and Koutarou Tani1 1 Marine Resources Research Center, Aichi Fisheries Research Institute, 2-1 Toyohama, Minami-chita, Chita-gun, Aichi 470-3412, Japan 2 Department of Stock Enhancement, Aichi Fisheries Promotion Fund, 1-3 Ichizenmatsu, Konakayama, Tahara, Aichi 441-3618, Japan * Fisheries Administration Division, Department of Agriculture, Forestry, and Fisheries, Aichi Prefectural Governmental Office, 3-1-2 Sannomaru, Naka-ku, Nagoya, Aichi 460-8501, Japan † E-mail: manabu_hibino@pref.aichi.lg.jp To estimate the changes in dominant megabenthos catch in relation to dissolved oxygen (DO) concentration in bottom water in Ise Bay, Japan, we conducted a field study with small-bottom trawl. The catch per unit effort (CPUE) of non-fishery targets increased at 22-36% DO saturation, around the edge of the hypoxic area (approximately less than 10% DO saturation) regardless of the surveys being conducted during day or night. CPUE of fisheries targets was also high at the same hypoxic edge; however, this was indicated only in the nighttime survey for the large catch of the conger eel Conger myriaster. These results indicated that hypoxia of the semi-closed bay induced an avoidance behavior in megabenthos due to the mass non-selective catch of the bottom trawl, but it simultaneously increased the catch. The number of species and the modified Shannon-Wiener diversity index (H*) were positively correlated with DO (% saturation), but not with CPUE. It was suggested that the increase of CPUE could be an index of the avoidance behavior in megabenthos in facing to hypoxic water. Both CPUE and economic profits decreased under DO30% saturation and only few megabenthos were caught under DO20%. Different avoidance behavior were taken by the dominant 15 species of non-fishery targets as observed from the different relationships between CPUE and DO saturations. The species that showed relatively slow avoidance against the hypoxia evaluated from CPUE-weighted average DO were goneplacid crab, Carcinoplax vestiva （DO26%; 1.92 mg/l）; Japanese mantis shrimp, Oratosquilla oratoria (DO30%; 2.24 mg/l); and small dragonets, Repomucenus valenciennei (DO30%; 2.21 mg/l). The abnormal distribution and behavior of megabenthos, including that of fishery targets, occurred at higher DO saturation levels than those in experimental LC50 and affected bottom-trawl fishery. Key words: megabenthos, distribution, hypoxic edge, small bottom trawl, dissolved oxygen (DO) concentration, Ise Bay