Bull. Jpn. Soc. Fish. Oceanogr. 72(3), Page 189-199, 2008   Long-term fluctuations of the water quality in Tokyo Bay judged from a data set of Chiba Prefecture Mitsuhiro Ishii*, Ken-ichi Hasegawa and Jun Kakino** Tokyo Bay Fisheries Research Laboratory, Chiba Prefectural Fisheries Research Center, 3091 Kokubo, Futtsu, Chiba 293-0042, Japan * Chiba Prefectural Fisheries Research Center, 2492 Chikuracho Hiraiso, Minamiboso, Chiba 295-0024, Japan † e-mail: m.ishi26@ma.pref.chiba.lg.jp Long-term fluctuations in water quality properties such as water temperature, salinity, density, pH, dissolved oxygen, NH4-N, PO4-P and transparency in Tokyo Bay were examined using a data set observed by Chiba Prefectural Fisheries Research Center (CPFRC) for fifty-eight years during 1948-2005. As a result, it was shown that the changes of the properties were related to each other. Water temperature was gradually increasing for a long term especially in autumn and winter. This increase caused to prolong the existence of hypoxic water to late autumn. Also, difference in density between bottom water to surface water as indicator of the stratification strength was related to the horizontal coverage of hypoxic water. When the stratification was weak, the annual occurrence of red tides was small. Eutrophication factors such as hypoxic water, annual occurrence of red tides, NH4-N, PO4-P and transparency were most remarkable in the 1960s. NH4-N in 2004 has decreased to the same level of the first half of the 1960s. PO4-P was higher in bottom water than in surface water because of the dissolution from reductive mud under hypoxia. It has decreased in the late 1990s, especially in March. Transparency increased to the same level as in the 1960s but hypoxic bottom water and annual occurrence of red tides have not always decreased. Eutrophication in Tokyo Bay has caused a large decrease in catch of the bivalves, flatfish and others. In addition, the recent rise of water temperature in autumn and winter has shortened the culture period of laver Porphyra yezoensis. Moreover, fall of PO4-P in March has damaged the laver culture because nowadays no enough PO4-P for the growth of laver remains in the water after diatom blooming. Key words: Tokyo Bay, water temperature, oxygen deficient water, NH4-N, PO4-P, transparency