Contrary to initial predictions, the cleanup of Ushigafuchi moat at the Imperial Palace that began Tuesday yielded just two small bluegill, government officials said.

The Environment Ministry started cleaning the moat, which is about 1 meter deep, to remove garbage and nonnative fish species. The moat is one of 13 surrounding the Imperial Palace. On Monday, the ministry drained the water level to about 40 cm.

Clad in waterproof coveralls, five employees of an engineering firm commissioned by the ministry waded into the moat to start the cleanup work.

Items removed so far include bicycles, push carts and electric fans. More than 20 fish, of four species, were also captured, but of these, only two were not native to Japan, officials said.

The ministry will initially focus on removing garbage before turning its attention to capturing the nonnative fish, including bluegill and black bass, starting Thursday.

The Imperial Palace moats have been home to a variety of native fish dating back to the Edo Period (1603-1867), including stone “moroko,” a type of carp, and chaenogobius laevis, a goby, according to the ministry.

In recent years, however, the moats have been dominated by bluegill and black bass. In an Environment Ministry study conducted in 2000, officials netted 3,400 fish in Ushigafuchi and found 99 percent of the catch were nonnative fish, overwhelmingly bluegill.

Bluegill were introduced to Japan in the 1960s, and black bass in the 1920s. Being omnivorous, these two species rapidly expanded in lakes and rivers while feeding on other fish, shrimp, aquatic plants and eggs of other fish.

Environment Ministry officials supervising the Imperial Palace moats first detected the presence of black bass in 1975 and bluegill in 1984.

The ministry has been trying to capture the nonnative fish in the moats since fiscal 2001.

Its work has been hampered, however, as fishing nets often become tangled with garbage dumped there.

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