Japan's Emperor Akihito has said he "feels pained" that he introduced an aggressive non-native species of fish to Japan nearly 50 years ago.
The bluegill wiped out the Japanese royal bitterling
Several bluegill fish were given to the emperor during a visit to the US.
He hoped the fish could be bred for food but some escaped from research centres and have now spread through Japan, threatening native species.
The emperor urged an audience of marine experts to take measures to protect native species.
Speaking at an annual convention on fishing and conservation, Emperor Akihito said he was distressed by the results of the bluegill experiment.
The fish were given to him by the mayor of Chicago during a visit to the Chicago Aquarium nearly 50 years ago, when he was still crown prince.
Emperor Akihito urged experts to prevent further extinctions
They were passed to a research facility next to Lake Biwa, Japan's largest lake, to investigate whether they could provide an abundant food source during a time of protein shortages in Japan.
It soon became clear that the fish had spread into the wild.
They have now spread throughout Japan's waterways, wiping out the Japanese royal bitterling and drastically reducing the number of some other native fish.
The emperor said he was distressed that the attempt to provide food had resulted in a threat to native species.
He urged marine experts to ensure that no further extinctions would occur as a result of the bluegill's success.