TOKYO -- Standing beside the Hibiya Moat bordering the Imperial Palace, Tetsuo Uchida holds in his hands the attackers who threaten one of Japan's most hallowed sanctuaries. "This is a bluegill, and this is a largemouth bass," he says, a fish in each palm. The bluegill, about 4 inches long, flops up once, gives a few last gasps, and dies. Score one for the home team. But the battle is only beginning.

Foreign brokerage firms and car makers are gobbling up Japanese rivals, but this time the intruders have gone too far: The bluegill and largemouth bass, both native to North America, have invaded eight of the 13 moats surrounding the palace at the heart of Tokyo where Emperor Akihito lives with his family. In the Hibiya Moat, the two fish account for more than 99% of the total fish population, thanks to what officials describe as their "extremely strong breeding power."

The bluegill, to quote an Environment Agency statement on the matter, "eat everything" - including the eggs of the haze, or goby, a stubby, mud-loving fish native to Japan that long predominated in the moats. The largemouth bass, meanwhile, munch on goby minnows. Adding insult to imperial injury, the goby is a suborder of fish that Emperor Akihito, a trained ichthyologist, has spent decades studying.

"That's why this is such a problem," frets Nobuo Ichihara, deputy superintendent of the Environment Agency office that oversees 12 of the 13 moats. (The Imperial Household Agency controls the 13th.) "This place is like a symbol of Japan. It's scary to think what may happen if we do a survey in five years - the native species may be all gone," says Mr. Ichihara.

And, so, the government has mobilized Mr. Uchida, the fish expert, and other specialists to save the moat. So seriously do the Japanese take their royal mission that one pundit here was inspired to call the American fish "black ships" - a reference to the U.S. warships that forced open Japan to foreign trade in 1854 - and to raise the specter of a "joi campaign." "Joi," which means "expel the barbarians," was the cry of pro-emperor nationalists in the 1860s as Western influence grew in Japan.

All of this raises a big question: Who let the barbarians in the moats? Officials think some Japanese prankster was probably responsible, with the first bluegill making its way into the moat sometime between 1975 and 1984. But that leads to the even touchier question of how bluegill entered Japan in the first place. Here, some people - very discreetly - are pointing fingers at none other than His Majesty.

In 1960, when he was crown prince, Akihito visited Chicago and received a gift of bluegill from Mayor Richard J. Daley. The emperor brought the fish back to Japan, thinking they would make good fishing for city children. But they soon found their way into Lake Biwa near Kyoto, and this worried Akihito, according to a 1977 account by a fish expert, Eizo Kimura, who met with the crown prince.

Shigeru Yamashita, chairman of the Japan Bass Pro Tournament Association, thinks the emperor's connection to the bluegill explains why many people, including the Environment Agency, were reluctant until now to launch an all-out attack on the foreign fish. But Kazuo Kidokoro, superintendent of the palace outer grounds, denies this. "There's no connection between the bluegill here and the emperor's bluegill," he insists, standing beside the Hibiya Moat. A palace spokesman said the emperor hasn't commented on the invasion of his moat and couldn't be contacted for his views on bluegill.

The emperor, who celebrates his 10th anniversary on the throne this year, has enough to worry about. His realm's economy is feeble, and his daughter-in-law, Harvard-educated Crown Princess Masako, who married Crown Prince Naruhito in 1993, has yet to give birth to an heir. Technically, Akihito's feelings aren't the Environment Agency's concern, but fortunately, there are perfectly legitimate scientific grounds for wanting the foreign fish out of his moat. Around the world, introduced species are threatening native species, and governments from Hawaii to Madagascar are making efforts to preserve biological diversity.

So, expel the barbarians it is. But how? That's where Mr. Uchida, the fish expert, comes in. He's part of a team of fish experts that the Environment Agency has summoned to tackle the problem. On a sunny autumn morning recently, the team sets out - six people on a 12-foot metal boat, steered by an old man with a long bamboo pole. They cast the wide, flat nets known as toami that Japanese fishermen traditionally have used.

After an hour, they return to shore with thousands of tiny bluegill and bass, and a few fairly large ones - up to about 7 1/2 inches. But they find only a handful of gobies. Officially, this operation is just an "investigation," meaning there will be no blood on the court's hands. But the gobies are carefully placed in a plastic bag full of water, so they can be thrown back once they're measured. The bluegill don't get such royal treatment: They're stuffed into dry bags. After they're measured, the agency will bury them - or "return them to earth," as Mr. Uchida delicately puts it.

The "investigation" will reduce the foreigners' numbers temporarily, but everyone admits it isn't a permanent solution, because the bluegill and bass breed so fast. "It's true that the toami net has its limits, but you have to ask what other choices we have," says Mr. Kidokoro, the superintendent. "There are no really good solutions." Mr. Kidokoro's family name, incidentally, means "castle place," although he says that's not why he got his job. Because castles and moats were a prominent feature of early modern Japanese cities, many Japanese have the family name Hori ("moat") or Horinouchi ("inside the moat"), which, coincidentally, is the name of Mr. Kidokoro's boss.

Foreign fish are a problem in lakes around Japan, and some towns have turned to anglers for help. Lake Biwa, the nation's largest lake and the one inundated with foreign fish after Akihito brought back his souvenirs from Chicago, started an anti-bluegill campaign this year, encouraging people to fish for the predators and either eat them or throw them in the garbage.

But Mr. Ichihara recoils at the suggestion that anglers be drafted to solve the imperial moat's problem. "This isn't a suitable place for the general public to fish," he says. "It's not a place of recreation." Even Mr. Yamashita of the bass association deems angling an unseemly option. "We couldn't bear it if you had all these fishermen lined up at the symbol of Japan," he says. Worse, some people might go after the wrong fish. As Emperor Akihito notes in his contribution to "The Fishes of the Japanese Archipelago," the Japanese have long dined on some types of the goby, his research subject.