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Akan-Mashū NP, Hokkaido
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Lake Akan’s mud is a marimo time capsule: researchers used buried DNA and water-flea remains to reconstruct 200 years of change, estimating early-1900s marimo biomass at 10–100 times today’s.
A marimo ball isn’t one alga: it’s a woven colony of branching filaments. Lake Akan’s largest recorded spheres reach 30 cm, and a 6-cm ball can take 150–200 years to grow.
Akan’s “lake lobster” is an invasive North American crayfish, introduced to Japan as a food resource in the 1920s. It can threaten marimo, yet the local fishery now sells it as soup.
Marimo algae grow in other lakes, but Lake Akan is the world’s only known place where large spherical colonies thrive together. The marimo became a national natural monument in 1921 and a special natural monument in 1952.
Lake Akan’s himemasu are kokanee—sockeye salmon whose ancestors were landlocked by volcanic changes. Each July, the local Ainu community honors this native fish with prayers and a dugout-canoe fishing reenactment.
In 1950, local residents and the Akan Ainu Association began the Marimo Festival as a campaign to protect and return marimo to the lake. It continues as an annual ceremony with kamuy-nomi prayers.
Marimo aren’t shaped by hand: in shallow bays, waves roll the unattached algae balls over the lakebed, brushing off sediment and exposing their surfaces to sunlight. Lake winds help set this slow tumble in motion.
At Lake Akan’s shore, the Bokke bubbles gray mud and volcanic gas. Its name comes from the Ainu word pofuke, “boiling up,” and geothermal heat keeps nearby patches snow-free even in winter.
In 1897, botanist Takiya Kawakami named the round algae 毬藻 (marimo, “ball algae”) after collecting it at Lake Akan. He published the name and his findings the following year.
Akan’s wakasagi smelt were transplanted from Lake Nishi-Abashiri in 1929, according to town records. Today they’re part of the lake’s winter fishery and are also made into local tsukudani, a savory preserve.
The familiar tale that two Ainu lovers became marimo isn’t documented as traditional Ainu folklore. A local marimo researcher traced the marimo version to a 1922 Japanese story adapted from an account the author said he heard from Ainu people.
