過去26年間に亘る水俣湾生息魚の総水銀濃度に関する変化

抄録

The Government of Kumamoto Prefecture declared, on July 29, 1997, that the fish and shellfish of Minamata Bay, which had been polluted by mercury discharged from the Chisso plant, had returned to a safe edible state. Simultaneously, the net which had been installed to prevent expansion of mercury pollution was removed from Minamata Bay. In the present paper, the results of the total mercury concentrations in fish caught in Minamata Bay between 1978 and the present are reported. In particular-ly, given the abundance of 3 fish species (smelt-whiting, rock fish, wrasses) captured during sampling, these species were used as indicators to evaluate the state of mercury pollution in Minamata Bay. Moreover, in order to understand the characteristics, the analytical data obtained on total mercury concentrations were distinguished into 3 periods using key events in the history of mercury pollution in Minamata Bay. The first period is the time until the completion of dredgeing in the bay. The second period is from the completion of dredging (March, 1990) until the removal of the net in the bay. The third period is from the removal of the net in Minamata Bay to the present. As a result, in the present report, the total mercury concentration in fish is thought to have gradually decreased. However, fish with total mercury concentration exceeding the safety threshold ware found after the completion of dredging of Minamata Bay. The total mercury concentrations in wrasses, rock fish and smelt whiting respectively ware as follows: the first period (dredging and landfill term: 1978-1989): 0.46. 0.71 and 0.79μg·g^-1 (avg); the second period (net-closure term: 1990-1997): 0.55, 0.59 and 0.95μg·g^-1 (avg); the third period (after net removal: 1998-2004): 0.17, 0.37 and 0.42μg·g^-1 (avg). This trend is predicted to continue in the future.