• Kyodo

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Thousands of small fish that have helped the city of Tokushima keep mosquitoes at bay for nearly four decades are now effectively banned as a foreign species.

Since 1968, the municipal government has been releasing thousands of mosquitofish, or Gambusia affinis, into streams and wetland areas as part of efforts to rid the city of mosquitoes.

However, the Environment Ministry designated the mosquitofish in February as a special introduced species, effectively banning their release into domestic waterways. Now the city must receive permission to “raise” the roughly 4,000 fish swimming in its waters by August or get rid of them.

Originally from North America, the freshwater fish measure some 3 to 5 cm in length and mostly eat mosquito larvae. The city, which has many swampy areas, introduced them into irrigation ditches and other waterways as a way to reduce mosquitoes and the diseases they carry, including encephalopathy.

Residents also took to the fish, and the city has been releasing them every summer.

For its part, the Environment Ministry takes issue with the mosquitofish because they threaten the domestic “medaka.”

The municipal government has received offers from residents saying they would be willing to raise the fish, but the new designation also prevents the city from giving the fish away as pets. If it wants to continue “raising” them, the city will have to set up facilities to ensure the fish don’t go beyond their current habitats, such as in the event of typhoons and other situations.

The ministry said that if the city receives permission to keep the fish for the purpose of display, it can continue to have them. However, one city official said, “Where and why should we keep such a large number of fish for display? It would also be difficult to secure the necessary upkeep costs.”

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