Bull. Jpn. Soc. Fish. Oceanogr. 79(1), Page 19-30, 2015   Water quality environment and variation in hypoxic water in Mikawa Bay Yoshito Honda1†, Yuzumi Toda1, Keisuke Ninokata1, Yasuo Nakashima1 and Teruaki Suzuki2 1 Aichi Fisheries Research Institute, 97 Wakamiya, Miya, Gamagori, Aichi 443-0021, Japan 2 Meijo University Graduate School of Environmental and Sciences, 1-501 Siogamaguchi, Tenpaku, Nagoya, Aichi 468-8502, Japan † E-mail: yoshito_honda@pref.aichi.lg.jp The water qualities observed in Mikawa Bay, such as water temperature, salinity, dissolved oxygen (DO), chemical oxygen demand (COD), chlorophyll a, particulate organic nitrogen (PON), total nitrogen (TN), total phosphorus (TP), and dissolved inorganic nitrogen (DIN), and dissolved inorganic phosphorus (DIP) were analyzed using the monitoring data since the 1970s. As a result, it was found that the water-quality properties of the horizontal distribution differ locally, and that there is a long-term trend in the variations of water properties. The transparency was low in the water column in the first half of the 1990s when TN, and TP concentrations were higher and red tide had occurred frequently in the same time. After that, TN and TP concentration decreased due to the reduction of nitrogen and phosphorus from the land. However, the hypoxic water area tended to expand in contrast with the decrease in TN, TP, total number of red tide days, and the increase in transparency. The results of multiple regression analysis, from 1980 to 1997, fishery catches of short-necked clam (Rudittapes philippinarum) had greatly contributed most to the suppression of hypoxic water. On the other hand, the hypoxic water area tended to expand regardless of fishery catches of short-necked clam that had been increasing since the second half of the 1990s. The water temperature rapidly increased between 1997-1998, and the cause of the expansion of the hypoxic water area was thought to be environmental change of Mikawa Bay. However, since 2011, the hypoxic water area has been shrinking. This was caused by the weakening of the stratification by a decrease in the water temperature, an increase in short-necked clam resources, and construction of artificial tidal flats. Key words: Mikawa Bay, hypoxic water, seawater quality, eutrophication, Rudittapes philippinarum