The Emperor of Japan expressed “heartfelt distress” yesterday for the release of an aggressive American bream into the country’s largest lake, amid growing national concern at the fate of native species.

Emperor Akihito appeared to take responsibility for the introduction to Japan of the American bluegill, whose fierce appetites have caused the extinction of the prized Japanese rosy bitterling.

Emperor Akihito, who was presented with the fish in 1960 as a gift from the Mayor of Chicago, told an audience of marine life experts and environmentalists to “keep a close eye on the creatures of Lake Biwa” so that native breeds would never become extinct again.

But according to the Japanese Environmental Agency, it may already be too late because of the bluegill’s extraordinary breeding power.

National emotion on the subject of alien fish runs high since most of Japan’s lakes and rivers are now overrun with the bluegill. By 2000 the fish had spread throughout the country.

But the bluegill presents a very personal challenge to the Emperor. Not only is Akihito a keen student of marine biology – he is a member of the Linnean Society of London – but the moats of his own palace are also infested with bluegills.

A 1999 report on the state of the 13 moats around the Imperial palace found that eight were plagued by alien species, including the bluegill.

The problem began nearly 50 years ago with what appeared to be a straightforward visit to the United States. Then still the Crown Prince, the young Akihito was on his first foreign excursion since his marriage. He was shown all the usual hospitalities – dinner at the White House, baseball and a trip to Disneyland – but also, given his interest in marine biology, managed to include a detour to the Chicago Aquarium.

It was here that the young prince was given the gift that would transform Japan’s lakes and waterways.

The early 1960s were an era of protein shortage in Japan and the bluegill, the Emperor discovered, was delicious. He gave the fish to the Institute of the Fisheries Agencies expressing the wish that it be considered as a source of food. The fish was for some years farmed under experimental conditions, but somehow it reached the outside world eventually.

Nobody is sure how or when the fish escaped but in a 1977 edition of the Japanese magazine Freshwater Fish, a journalist noted that the bluegill had made its appearance in Lake Biwa. By the 1990s the population exploded.

“It was strongly expected that the fish could be a source of food . . . but I now feel heartfelt distress that it has led to such results,” said the Emperor.

The Emperor’s comments come at a time of heightened “ecological nationalism”. Over the years Japan has allowed a number of foreign species of plants and animals on to its shores and many have proved to be destructive. Many make their way in as pets: exotic snakes and lizards are regular offenders, but the Japanese craze for beetle-collecting has also taken its toll.

Restrictions on beetle imports were relaxed about seven years ago, and in 2005 more than a million beetles were imported to Japan - about 150 times more than in 1999.

In 2006 Japan passed the Invasive Alien Species Act but it has utterly failed to prevent, for example, the march of the European dandelion.

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