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Iriomote Ishigaki NP, Okinawa
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Urauchi River has yielded 407 recorded fish species—about twice the number reported from the much longer Shimanto. A 2016 review applying Japan’s 2013 Red List found 43 threatened brackish- and freshwater species here.
Urauchi’s lower reaches are a bull-shark nursery: surveys from 2014–2019, plus records dating to 1979, repeatedly found newborns and young-of-the-year. Sightings are commonly reported in May and June.
In just 1.5 km of the upper freshwater reach, researchers recorded 13 fish species that grow beyond 30 cm. Seven are marine-origin fish that spend most of their lives in the river—a remarkable sea-to-stream twist.
The Urauchi estuary is home to 5 mangrove species, including kinds that grow near the northern edge of their global range. That makes this river mouth an especially important northern outpost for these tropical trees.
At Urauchi’s river mouth, mangrove roots give young fish shelter from predators, while nutrient-rich phytoplankton feeds them. The tangled forest is more than a shoreline backdrop: it helps stock the river’s remarkable fish community.
The name Mariyudu comes from water gathering in its broad, round plunge pool. Farther upstream, Kanbire means “where the gods sit”; local legend says the gods gathered there to plan the making of the islands.
Around 1960, about 15 households lived in Inaba, upriver from the mouth. Rice farms averaged about 2 hectares, and a merchant brought household goods from Ishigaki by boat—showing how the river linked this remote community to the wider world.
Children from Inaba, a former village on the river, walked about 6 km each way to school over mountain paths—12 km a day. The village had no electricity and was abandoned around the early 1970s.
After Inaba’s rice paddies became dry grassland, small ponds were dug there as a restoration experiment. Aquatic creatures missing from the initial survey—including threatened water beetles—were found again soon afterward.
On the Utara tributary, a giant ohirugi mangrove was measured at 8.4 m tall with a 312 cm trunk circumference; its age was estimated at 400 years. It is one of the government’s “Forest Giants 100.”
The nearby Utara coal mine grew to employ almost 1,000 people in the 1930s. Its official history records long hours, coupon wages instead of cash, and harsh punishment for escape attempts; the mine closed in 1943.
Island divers began monitoring 6 endangered fish species in the upper river after concerns arose about a water intake installed in 2015. Their study stretch runs from Gunkani Rock toward Mariyudu Falls.
