How did bluegill fishes gifted to Japan’s future emperor in 1960 cause an ecological crisis?

Fifteen bluegill fish gifted by Chicago’s mayor to then Crown Prince Akihito of Japan in 1960 turned into an invasive species crisis, with their population reaching 25 million by 2007. Bluegill fish could tolerate murky shorelines and grew rapidly as native species suffered. Emperor Akihito later said it hurt him to think that the fish had caused such ecological havoc.