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Aso-Kuju NP, Oita
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On 27 December 2008, researchers recorded pale chub (oikawa) spawning in a stream flowing into Kinrin-ko—months outside its usual June–August season. A hot-spring-fed stretch there stays about 30°C.
Older residents recalled that Kinrin-ko and the lakeside Shitan-yu bath once shared a water level, so fish sometimes swam straight into the tub. Children swam in the lake, then warmed up there—a summer pastime.
Kinrin-ko inspired Kinrin-mochi, a local sweet shaped like a golden fish scale. Its soft Oita-grown rice cake wraps sweet potato with kabosu citrus, sake, and fragrant Yufuin-style yuzu paste.
Kinrin-ko receives hot-spring water at 32–35°C and cooler spring water at 15–20°C. When cold air meets its warmer surface, the lake’s famous winter steam makes perfect sense.
Hot-spring channels around Kinrin-ko shelter the Onsen-gomatsubo, a freshwater snail just 4 mm long. Japan’s Environment Ministry lists these channels as its sole habitat; Oita ranks the snail Critically Endangered.
Yufuin oral tradition tells of a goddess ordering Gongen to break the rim of a vast basin lake. As the water drained, an old dragon crawled to the mountain foothills and begged for refuge; Kinrin-ko was left as its small home.
Before it was called Kinrin-ko, local people knew it as Onyu-no-ike or Takemoto-no-ike. The name “Golden Scales Lake” is linked to scholar Mori Kūsō’s 1884 visit, when sunset lit fish scales on the water.
Physicist Ukichiro Nakaya remembered Kinrin-ko as crowded with wild carp over 50 cm long, their scales shining gold. In a 1960 essay, he said easier road access later brought people who caught them.
Physicist Ukichiro Nakaya wrote in 1960 that a breeder’s carp turned from silver to gold after a year in Kinrin-ko water—and reportedly gained 50% more weight over winter than carp in ordinary ponds.
Kinrin-ko’s tiny life is remarkably varied: researchers recorded 147 diatom taxa from 46 genera. The species dominating hot-spring-influenced water differed from those found in cooler parts of the lake.
Before the lakeside became a busy visitor spot, villagers came to Tenso Shrine morning and evening to fill containers with clear spring water and carry it home on their backs. The spring still flows beside the shrine.
Kinrin-ko has an unwelcome aquatic resident: introduced tilapia have become established in the lake. Yufu City reports that removal efforts have not fully eradicated them, so protecting the lake’s ecology remains ongoing work.
