Photo/Illutration The area surrounding the mouth of the Tamagawa river with the Tamagawa Sky Bridge where “shirauo” icefish have been found (Provided by a joint venture of Penta-Ocean Construction Co., Hitachi Zosen Corp., Fudo Tetra Corp., Yokogawa Bridge Corp., Honma Corp. and Takadakiko Co.)

“Shirauo” icefish that were thought to have died out in the capital more than 50 years ago are in fact thriving in the mouth of the Tamagawa river that flows into Tokyo Bay, experts said.

Project team members from the Public-Private Cooperation Forum for Tokyo Bay Restoration collected adult specimens and eggs of the fish after a series of surveys in the area.

However, they conceded that the origin of the fish remains unknown as they vowed to press on with their research.

Shirauo is a tiny fish with a translucent body that measures less than 10 centimeters and lives in brackish water.

Once representing “Edomae” (fish caught in Tokyo Bay), shirauo was a specialty of the Tsukudajima district along the Sumidagawa river. It was apparently cherished by the warlord Tokugawa Ieyasu (1543-1616), founder of the Tokugawa Shogunate.

Shirauo were harvested in the Tamagawa river until the mid-Showa Era (1926-1989), but then numbers dwindled rapidly due to land reclamation projects and a deterioration in water quality.

The Tokyo metropolitan government’s red data book on endangered species says the species went extinct in Tokyo in or around 1970.

In 2017, the Kawasaki municipal government ordered a series of environmental impact surveys in the area surrounding the Tamagawa Sky Bridge connecting the city’s waterfront and Haneda Airport to coincide with construction of the span.

City officials caught five shirauo in winter 2021, one in winter 2022, 77 in autumn the same year and one in winter 2023.

But they concluded it was too early to publicize their findings with a positive spin.

Researchers from the project team who also serve on an environmental advisory panel for the bridge construction carried out additional surveys. After they collected 223 eggs in 2022 and 98 in 2023 in the same spot, they decided to announce their findings on Sept. 11 based on solid scientific evidence.

There seems to be no doubt that shirauo have established a foothold in the Tamagawa river because researchers from the land ministry and the Tokyo metropolitan government each found one in the river in 2021, 2022 and 2024, they said.

It is possible that shirauo were able to maintain a presence following efforts to clean up the river by pumping less raw sewage into the water and a sandy mudflat suitable for egg laying that formed because of a 2019 typhoon.

That hypothesis was offered by Kana Takeyama, who leads the project, and Toshio Furota, a professor emeritus at Toho University.

However, a DNA analysis failed to determine where the shirauo originated.

The team said there is a high possibility that the fish migrated from the Inbanuma marsh in Chiba Prefecture, which boasts a natural distribution of shirauo.

However, others assert that a civic group released shirauo from Lake Ogawarako in Aomori Prefecture in northern Japan into the Sumidagawa river on several occasions in or around 2019.

In that case, the increased population is due more to human efforts than natural causes.

“We hope they won’t release any more (shirauo) and let the population recover naturally,” Furota said. “It is our wish to see catches increase so that these fish can be served on ‘yakatabune’ leisure boats or enjoyed on other occasions.”

The Kawasaki city government completed its surveys after the bridge’s completion in 2022, but the project team said it intends to work with locals to carry out more surveys.

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