Bull. Jpn. Soc. Fish. Oceanogr. 81(3), Page 211-221, 2017   Factors affecting distribution of hypoxia off the Banzu tidal flat in Tokyo Bay Satoshi Ohata1a†, Makoto Kajiyama1 and Hiroshi Yagi2b 1 Tokyo Bay Fisheries Laboratory, Chiba Prefectural Fisheries Research Center, 3091 Kokubo, Futtsu, Chiba 293-0042, Japan a Chiba Prefectural Fisheries Research Center, 2492 Hiraiso, Chikura-cho, Minamiboso, Chiba 295-0024, Japan 2 National Research Institute of Fisheries Engineering, 7620-7 Hasaki, Kamisu, Ibaraki 314-0408, Japan b National Defense Academy, 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan † E-mail: s.ooht@pref.chiba.lg.jp The phenomenon of hypoxia distribution off the Banzu tidal flat of Tokyo Bay was classified into two patterns from 2007-2008 observation data. One pattern occurred when cold and heavy seawater entered the bottom layer from outside the bay when a south wind persisted. At this time, hypoxia that was distributed in the bottom layer of the bay center moved to the middle layer in the same sea area and also moved off the Banzu tidal flat. In the offshore area of the Banzu tidal flat, hypoxia was distributed very shallowly, so the risk of this spreading to the Banzu tidal flat increased. The other pattern was that when a north wind continued blowing for more than a few days, hypoxia distributed in the bottom layer of the bay center moved off the Banzu tidal flat. At this time, hypoxia was distributed only in the bottom layer off the Banzu tidal flat, so it was unlikely that hypoxia affected shellfish in the tidal flat. However, fishery resources such as Japanese cockles are located in this area, and it seemed that these fishing ground formations could be influenced by hypoxia. Key words: hypoxia, Tokyo Bay, Banzu tidal flat, oceanic water