A bloody underwater battle rages in the lakes of Japan. On one side swims the aggressive American bluegill – a feisty swarm of five-pound fish that has invaded Japan. On the other side is, well, any native species that gets in its way.

Japan’s Emperor Akihito expressed “heartfelt distress” this week over the bluegill bullies that have pushed some Japanese fish to extinction. The Emperor’s pain is especially strong since this onslaught is, to put it bluntly, his fault.

While touring the US in 1960, the then-Prince Akihito accepted some live bluegills as a gift from the Mayor of Chicago. The Japanese government hoped to turn this present into a welfare program, with officials breeding the bluegills – in captivity – as an emergency food reserve in case of a nationwide protein shortage.

(Illustration: Monster-Watch staff.)

But, like the Hawaiian Cane Toads in Australia, the American fish grew out of control. Several bluegills escaped from the Japanese research facility and began fighting for turf in Lake Biwa, the country’s largest lake. By 2002, the fugitive fish had fathered 50 million offspring and the quarrelsome kin overran many Japanese waterways. The brutal Americans wiped out Japan’s royal bitterling and have seriously endangered many other aquatic species.

"We had high expectations of raising them for food, and I'm deeply troubled by how it turned out," the Japanese Emperor said in a speech on Sunday (as translated by the AP).

The Japanese government recently launched a $1.8 million extermination campaign, which has cut the bluegill’s numbers in half. But Japan must fend off other invasions as well. The country’s new push for “ecological nationalism” – marked by the passing of the Invasive Alien Species Act last year – has done little to hold off the encroachment of foreign snakes, lizards, beetles, and the much-feared European dandelion.

I’m sure Steven Colbert would laud the bluegill as a symbol for American freedom, strength, and world domination.