150
A. Kudo and R.R. Turner
particulate Hg in the water column measured in November 1975 and an assumed
water exchange rate of 0.4 to 1.2 million m 3 /day to estimate an export of 20 to
60 kg/year. This estimated range is much lower than that calculated by Kudo
(1992) based on the increase of sediment mercury burden (average 3.7 tons/year)
in the Yatsushiro Sea. Even using the ten fold higher water exchange rate given
by Kudo (1992) for the bay does not bring these estimates of mercury export
from the bay any closer. Regardless of the reasons for the discrepancy, sediment
transport out of the bay has been an important process.
As discussed in several of the publications by Kudo and his coworkers, natural
forces have played the dual roles of both dispersing and burying mercurycontaminated sediments from Minamata Bay. A record rainfall event (200-year
return interval) occurred in 1982 producing millions of tons of storm water borne
soil eroded from the hilly lands surrounding the bay and sea which blanketed the
bed sediments of the Yatsushiro Sea. The result was burial of more contaminated
surface sediments and lower levels of mercury in surface sediments in the years
following the event. This natural event occurred within the period when artificial
decontamination (dredging) of bed sediments in Minamata Bay was underway
and thus its effects on the rate of recovery of Minamata Bay itself cannot be
determined.
3
Mercury in Fish and Shellfish
The mercury content of fish and shellfish from Minamata Bay during the 1960s
and early 1970S has been summarized in many publications (e.g., Fujiki and
Tajima 1973, 1992; Nishimura and Kumagai 1983; Environment Agency of Japan
1992). Figures 3 and 4 display the general temporal trends observed through the
early 1970S. No more recent data on mercury in shellfish from Minamata Bay
could be found in the English language literature. Fujiki and Tajima (1992)
updated the situation for fish through 1989 by stating that only 16 species of fish,
out of a total of 87, continued to contain over 0.4 mg/kg total mercury (the
Provisional Regulatory Standard). The intensive fish monitoring program
prescribed by the Kumamoto prefectural government (see section below) for
the period of dredging and thereafter generated considerable data, little of which
has been seen thus far in English language publications . A glimpse of some of
these data was provided by Nakanishi (1992). The graphs of the temporal trend in
the content of mercury in two important commercial fish species through 1990
given in her report have been updated here through early 1996 (Fig. 5) using data
provided to the authors by the Kumamoto Prefecture.
Overall, the available information on biological contamination in Minamata
Bay shows remarkably rapid decreases in mercury content of both fish and
shellfish following cessation of direct discharges of mercury to the bay.
Subsequently, the responses appear to have been slower or absent until
restoration work in the bay was commenced in the early 1980s. As shown in
A. Kudo and R.R. Turner
particulate Hg in the water column measured in November 1975 and an assumed
water exchange rate of 0.4 to 1.2 million m 3 /day to estimate an export of 20 to
60 kg/year. This estimated range is much lower than that calculated by Kudo
(1992) based on the increase of sediment mercury burden (average 3.7 tons/year)
in the Yatsushiro Sea. Even using the ten fold higher water exchange rate given
by Kudo (1992) for the bay does not bring these estimates of mercury export
from the bay any closer. Regardless of the reasons for the discrepancy, sediment
transport out of the bay has been an important process.
As discussed in several of the publications by Kudo and his coworkers, natural
forces have played the dual roles of both dispersing and burying mercurycontaminated sediments from Minamata Bay. A record rainfall event (200-year
return interval) occurred in 1982 producing millions of tons of storm water borne
soil eroded from the hilly lands surrounding the bay and sea which blanketed the
bed sediments of the Yatsushiro Sea. The result was burial of more contaminated
surface sediments and lower levels of mercury in surface sediments in the years
following the event. This natural event occurred within the period when artificial
decontamination (dredging) of bed sediments in Minamata Bay was underway
and thus its effects on the rate of recovery of Minamata Bay itself cannot be
determined.
3
Mercury in Fish and Shellfish
The mercury content of fish and shellfish from Minamata Bay during the 1960s
and early 1970S has been summarized in many publications (e.g., Fujiki and
Tajima 1973, 1992; Nishimura and Kumagai 1983; Environment Agency of Japan
1992). Figures 3 and 4 display the general temporal trends observed through the
early 1970S. No more recent data on mercury in shellfish from Minamata Bay
could be found in the English language literature. Fujiki and Tajima (1992)
updated the situation for fish through 1989 by stating that only 16 species of fish,
out of a total of 87, continued to contain over 0.4 mg/kg total mercury (the
Provisional Regulatory Standard). The intensive fish monitoring program
prescribed by the Kumamoto prefectural government (see section below) for
the period of dredging and thereafter generated considerable data, little of which
has been seen thus far in English language publications . A glimpse of some of
these data was provided by Nakanishi (1992). The graphs of the temporal trend in
the content of mercury in two important commercial fish species through 1990
given in her report have been updated here through early 1996 (Fig. 5) using data
provided to the authors by the Kumamoto Prefecture.
Overall, the available information on biological contamination in Minamata
Bay shows remarkably rapid decreases in mercury content of both fish and
shellfish following cessation of direct discharges of mercury to the bay.
Subsequently, the responses appear to have been slower or absent until
restoration work in the bay was commenced in the early 1980s. As shown in