Hokkaido Plans Waterweed Removal from Lake Akan Marimo Moss Balls Colony

The Yomiuri Shimbun

14:50 JST, December 24, 2025

As marimo moss balls, which are designated as special natural monument, have been in decline at Lake Akan in Kushiro, Hokkaido, the city plans to remove waterweeds from a marimo colony in the northern part of Churui Bay of the lake next summer.

The plan is part of the measures to help preserve the unique spherical moss balls, which are endemic to the lake.

Regarding the sharp decline of marimo in Lake Akan, it has been pointed out that the proliferation of waterweeds has weakened water flow, leading to insufficient rotation and hindering the marimo’s growth. Starting in late June, the Board of Education of the city conducted a comparative survey of changes in density at four locations — a test area where waterweeds were extensively removed south of the colony, and a nearby area where waterweeds were not removed.

According to the report, density increased at three of the four locations, however 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 the outside of it, between June and July next year.