Something fishy is lurking in the near-sacred moat ringing the Imperial Palace — an invasion of American species that Japan says must be stopped.

Fearful that the moat’s indigenous fish are being rapidly wiped out by foreign species like bluegill and black bass, the Environment Ministry said it will begin draining sections to kill off the North American invaders and keep the palace pure.

“We just cannot stand there watching our indigenous fish such as gobies being eaten up by such strong foreign species,” ministry spokeswoman Sae Sato said.

The decline of native species, including the southern top-mouthed minnow, deep crucian carp and the flat bitterling, has been a growing concern since voracious foreign fish were introduced decades ago by sports anglers.

Particularly worrisome are robust black bass and ravenous bluegill, which tend to feed on local fish. Bluegill have been found in Japan since 1960, when some were presented to Emperor Akihito, then the Crown Prince, by the mayor of Chicago.

Today, the Environment Ministry says the two species inhabit at least eight of the 13 segments of the stone-lined Imperial moat around the palace in central Tokyo. The moat is a popular tourist attraction, but fishing there is banned.

There are 17 species of native fish living in the moat. To halt their adversaries’ advance, officials will begin on Feb. 25 a 2.5 million yen, two-week task of draining a 400-meter section. The water level will be lowered so the foreign fish can be scooped up and buried. Other sections will be drained in turn, with natives being reintroduced once each is refilled.

“To date, we have tried all sorts of ways, such as casting nets, to catch black bass and bluegills, but to no avail,” Sato said.

It will be first time the moat will be drained since 1973, when water was removed for cleaning, she said.

Environmentalists say the rapid spread of foreign fish in Japan endangers the diversity of local populations. Weeding out nonnative species from the moat — an isolated body of water where a reinvasion is likely to be gradual — could help create a “survival zone” for Japanese species.

Large mouth black bass can grow to 80 cm in length and weigh 10 kg. The fish are believed to have been brought to Japan from California by a businessman in 1925. Bluegill grow up to 23 cm and weigh about 250 grams.

Efforts to cull nonnative fish have been undertaken elsewhere in Japan as well.

In 1998, Shiga Prefecture set a goal of catching 300 tons of bluegill and black bass a year in Lake Biwa, Japan’s largest lake, to halve their population over 10 years. But the effort has fallen well short of that goal, in part because of opposition from anglers.

There are an estimated 5 million bass anglers alone in Japan.

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