A number of minks native to North America have been found living in the wild in Fukushima Prefecture, northern Japan. Experts say the animal can damage the eco-system in the area.
52 male and female minks were captured around a river in Koriyama City from late August to October 1st.
The minks are believed to be descendants of American minks that were brought to northern parts of Japan after World War 2 to be farmed for their fur.
Sadao Ihara of Ohu University says 6 to 7 minks were found to be living per kilometer along the river, and that this density is higher than that of other mink habitats around the world.
Ihara warns that if the minks prey on too many frogs, the population of frogs that eat crop pests will decrease, causing damage to local farming.
He is calling for measures to get rid of the harmful non-native animal.
The breeding and sale of American minks has been prohibited in Japan since 2006, when they were designated as an invasive alien species that could harm the indigenous ecosystem.