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 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.

The institute raised the fish in Lake Biwa on a trial basis, spreading double nets around its fish farm. But by the 1960s it was confirmed that the fish had started reproducing by themselves in the lake. As a result of the fish’s rapid multiplication, the catches of native fish such as Nigorobuna carp have dropped dramatically.