Japan hosts at least one free-roaming river otter, but it’s not likely the country’s endemic variety, which was last documented decades ago and has been “officially” extinct since 2012. That’s the assessment of the country’s Environment Ministry following surveys on Tsushima Island in the Korea Strait, where an otter was caught on motion-sensor camera back in February.

A camera trap set in Tsushima's mountains captured footage of the otter early in the morning of February 6, 2017. Image: University of the Ryukyus

That discovery raised hopes that the Japanese river otter – once common across Japan’s main islands but hammered from the nineteenth century onward by the fur market, habitat degradation, and other anthropogenic stressors – might still be around. The last sighting was in 1979 on Shikoku, the smallest of the main islands.

But the recent fieldwork, bolsters an alternative explanation already suspected by some scientists: the mustelid caught on film early this year is probably a Eurasian otter.

“The possibility that it is an offspring of Japanese river otters in Shikoku is low." Professor Hiroshi Sasaki of Chikushi Jogakuen University, who led the surveys, told The Asahi Shimbun. "Therefore, it is not a Japanese river otter in the narrow sense.”

Researchers collected a clutch of otter scat (technical term: spraint) and found a set of likely-looking pawprints. DNA analysis suggested several of the spraints came from a male otter, but it’s not clear whether multiple animals might be out and about on Tsushima (as preliminary analysis of scat gathered on the island in July hinted at). And the taxonomic affiliation appears to be with the Eurasian otter, a remarkably widely distributed water-weasel found from the British Isles and North Africa to Siberia and Southeast Asia.

Eurasian otters inhabit the Korean Peninsula, and it’s entirely possible that they swam from South Korea or drifted over on currents. Image: Mario Madrona/Flickr

Along with Russia’s Sakhalin Island – only about 40 kilometres from northernmost Japan, Hokkaido – Eurasian otters inhabit the Korean Peninsula, basically a stone’s throw or two from Tsushima. Biologists say it’s entirely possible the island’s otter (or otters) swam from South Korea or drifted over on currents. The Japan Times reports that the Environment Ministry also acknowledged the possibility that river otters have persisted on Tsushima since historical times.

The native otters of Tsushima may well have been Eurasian otters to begin with, given the proximity to the East Asian mainland. The exact taxonomy of the Japanese river otter, meanwhile, isn’t known: It may have been a distinct species or simply a Japanese subspecies of the Eurasian type.

The Environment Ministry plans to continue researching the otter presence on Tsushima, and may deploy motion-sensor cameras there. (The camera that documented the otter in February was part of a study on leopard cats.) So hey – watch this space!



Header image: Lex McKee