Discarded trash and other debris is being carried by rivers running through Aichi, Gifu and Mie prefectures and washing ashore on Toshi Island in Ise Bay, an Environment Ministry study found.
The island, located just off Toba, a key fishing and tourism port, is the largest in Mie Prefecture.
The ministry said Mie and the other two prefectures need to work together to rectify the problem.