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Fuji-Hakone-Izu NP, Yamanashi
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On 2 December 2025, researchers filmed wild kunimasu spawning in Lake Sai for the first time anywhere. A lake-bed camera caught fish choosing gravel and spawning—a behavior never before filmed in the wild.
When acidic Tamagawa water entered native Lake Tazawa in 1940, kunimasu was thought extinct—but 100,000 eggs sent to Sai in 1935 had survived. A dark fish in local nets led to its rediscovery in 2010.
Sai’s traditional masu-miso starts with grilled himemasu mixed with scallions, ginger, bonito flakes and, depending on the household, egg yolk. The local fishery chief describes it as a distinctive home-style dish.
An 864 Fuji lava flow split ancient Ko-Senoumi into today’s Sai, Shoji and Motosu lakes. Their surfaces still sit at the same elevation, and they are thought to share water underground.
Researchers found 8 kunimasu spawning beds 27–31 m below Sai’s surface; the smallest is just 1×2 m. Groundwater seeps through every surveyed patch, making these tiny gravel spots important habitat.
On Sai’s south shore, people once prayed for rain to the dragon said to live in Ryugu Cave. This 60 m lava tunnel became a Fuji-pilgrimage shrine, and its Dragon Festival is still held every 2 August.
Until roads reached the area in the 1960s, Sai’s lakeside villages relied on boats to reach one another. Researchers think pilgrims on the old Fuji pilgrimage crossed the lake by boat, too.
Sai was part of the Fuji faith’s “Inner Eight Lakes” circuit. After climbing Fuji, pilgrims made a 1–2-day water-purification journey around eight sacred lakes—including all five Fuji Five Lakes.
A debris flow hit lakeside Nenba before dawn on 25 September 1966, damaging 37 of its 41 thatched houses. Forty years later, 20 recreated thatched homes brought the hamlet’s former landscape back.
At Nenba, one of the few houses to survive the 1966 debris flow is a late-Edo sericulture home. Its airy upper floor still shows traces of silkworm raising beneath a distinctive kabuto-style thatched roof.
Nenba residents reroofed houses together—and also pushed for paved roads, piped water, improved cookstoves and telephones. The village was even honored by a newspaper as a “new-life model village.”
