Marimo Population in Hokkaido’s Lake Akan Dramatically Declined in Past 120 Years; 10-100 Times More Abundant Before

The Yomiuri Shimbun

6:00 JST, April 21, 2025

SAPPORO — There were 10-100 times more marimo moss balls in Hokkaido’s Lake Akan 120 years ago, a research team has found.

The team, consisting of researchers from Tohoku University, Kushiro International Wetland Centre and other institutions, made the estimation based on DNA preserved in sediments on the bottom of the lake, which is in Kushiro in the prefecture.

The researchers said that the decrease was caused by an influx of muddy water in the lake due to deforestation in the early 20th century. They believe that another factor is fluctuations in the lake’s water level caused by the construction of a hydroelectric power plant along a river flowing into the lake.

The results of the study were published in an international journal in March.

It had been believed that the lake’s marimo, a national protected species, started to decline in the first half of the 20th century, but until now there had been no scientific evidence to support that claim, the team said.

In July 2021, the researchers collected sediment samples, which included marimo’s DNA. By comparing the amounts of DNA from the marimo and other creatures and using among other methods, the researchers were able to estimate the population of the marimo in the lake over 200 years.

As a result, the team concluded that the population of marimo was abundant until about 1900 but had declined sharply by around 1950. The timeline coincides with a period of sediment influx and major water level fluctuations.

Since 1950, forest and water level management has been tightened, and measures have been taken to purify the lake’s water. However, the marimo population has not recovered.

“Our research clearly shows that there used to be at least 10 times as many marimo as today,” said Jotaro Urabe, a professor emeritus of Tohoku University and an expert in marine plant ecology who is a member of the team. “To get the environment in Lake Akan back to how it was, we need a ten-fold increase in the population of marimo.”

It has been pointed out that the number of large marimo balls in Lake Akan may have decreased drastically in recent years due to an overgrowth of waterweeds.

“We have obtained valuable results that will help us restore Lake Akan to the way it should be,” said Isamu Wakana, a member of the team and researcher at the Kushiro International Wetland Centre. “Reconstruction of the conservation system is urgently needed,”