Hokkaido Plans Waterweed Removal from Lake Akan Marimo Moss Balls Colony

The Yomiuri Shimbun

14:50 JST, December 24, 2025

KUSHIRO CITY, Hokkaido — The city of Kushiro, Hokkaido, plans to remove waterweeds from part of Lake Akan next summer, in an attempt to counter a decline in marimo moss balls in the lake.

The plan is part of measures to help preserve the unique spherical moss balls, which are endemic to the lake and have been designated a special natural monument.

It has been pointed out that the proliferation of waterweeds has weakened water flow, leading to insufficient rotation and hindering the marimos’ growth. Starting in late June, the Board of Education of the city conducted a comparative survey of changes in density at four locations — two test areas where waterweeds were extensively removed south of a colony in the lake’s Churui Bay and two areas where waterweeds were not removed.

According to the report, density increased at three of the four locations, but no clear effect from the removal was found at one of the locations.

Isamu Wakana, a marimo researcher at the city’s World Natural Heritage Promotion Office, said, “[The lack of effect at one test site] is likely because waterweeds were covering the colony.”

As an emergency measure, the city’s Board of Education is considering removing waterweeds within a 4,200-square-meter area inside the colony, instead of outside, between June and July next year.