A quick stop at an aquarium turned into one of the most studied invasive species stories in modern science. In 2006, Kouichi Kawamura and colleagues published a genetic study in the journal Molecular Ecology that found that almost all the bluegill fish swimming in Japan’s lakes and rivers today are descendants of just 15 fish. This is not an estimate. The study analyzed DNA from throughout Japan, Korea, and the United States and traced the lineage back to a section of the Mississippi River near Guttenberg, Iowa.

Crown Prince Akihito of Japan was in Chicago in October 1960 as part of a goodwill tour to mark 100 years of diplomatic relations between the US and Japan. He had about 21 hours in the city, and his one personal request was to see the Shedd Aquarium. He was 26, and a serious ichthyologist already. Richard J Daley, Chicago's mayor and an avid angler, gave him a gift: 18 bluegill, a fish common to Illinois's lakes and ponds. In some versions, Daley scoops them out of a tank.

The breeding project got off to a rocky start. Bluegills, it turned out, were not very good fish to raise at scale for food, and the idea was pretty much dropped by the late 1960s. But some fish were released into lakes and ponds, including Japan's largest and oldest, Lake Biwa.

By the 2000s, bluegill had spread to nearly every prefecture. Officials estimated the population of the species nationwide to be about 25 million fish in 2007. Native species, including the rosy bitterling, a small fish native to Japan, were badly affected. In 2005, Japan's central government banned breeding or releasing the species.

Shiga Prefecture, home to Lake Biwa, invests heavily in its removal projects each year and has encouraged the “catch and eat campaign,” urging fishermen to cook bluegill rather than release it back into the lake. There are also recipes that now range from fried bluegill all the way to bluegill burgers. One local manufacturer reportedly even tried fermenting bluegill following the same technique used for funazushi, a famous salted crucian carp from Shiga.