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Unzen-Amakusa NP, Kumamoto / Nagasaki
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The strait’s whirlpools have a volcanic backstory: its underwater terrain ranges from about 20 m shallows to 150 m depths. Different depths speed the tide unevenly, helping it curl into eddies.
Hayasaki Seto Arakabu is a registered brand for scorpionfish caught here. The local fisheries cooperative says the fish live at 80–120 m and that the fast current helps give their flesh a firm texture.
Tide-driven currents in Hayasakiseto can exceed 11 km/h—about the pace of a brisk runner. The flow reverses with the tides, helping make this narrow passage a strikingly dynamic stretch of water.
The resident dolphins have intricate social lives: photo-identification research around Tsūji Island found male pairs formed non-random alliances over several years, sometimes associating with females.
Local residents launched dolphin-watching trips in August 1993; 3,219 visitors came that first year, rising to 38,997 by 1997. In 1996, “Dolphins of the Itsuwa Sea” was selected among Japan’s 100 Soundscapes.
Around Futae, traditional fishing centered on free-diving for shellfish and seaweed. A local museum account suggests this way of fishing helped people and dolphins share the waters, without targeting the small fish dolphins eat.
Near Tsūji Island and Oniike, dolphins often swim just 1–1.5 km offshore. Their wider range can extend to Yushima, Tomioka Peninsula and, at times, as far as Shimabara Port.
A local research team identifies dolphins by features such as their dorsal fins, snouts and body markings. By the end of fiscal year 2023, it had individually identified about 130 dolphins off Tsūji Island.
Researchers are exploring the strait’s current as a renewable-energy source. A 2023–24 sea trial tested a floating turbine off Miyazaki-bana, with a local fisheries cooperative, university and high school among the participants.
