Seto Inland Sea: islands, routes and environmental management

Basins and channels

Islands and coastlines divide the sea into broad basins and narrower passages. Tidal currents in straits affect navigation, helping explain ports associated with waiting for tides or favourable winds. This is not a sealed pool: channels connect it to other seas, and conditions vary across the region.

Navigation at Kurushima Strait

Kurushima Strait shows how currents shape a route. Vessels travelling with the tide use the middle channel, while those travelling against it use the western channel. When the current reverses, the channel used for a given direction of travel changes. The local vessel-traffic centre traces the practice to the Meiji and Taisho periods, with its first formal codification in 1929.

Maritime exchange connected shores from ancient times, and medieval ports supported trade. Early-modern coastal routes, including the western route and Kitamaebune shipping, linked ports, goods and merchants. Steamships, railways and modern harbours later changed transport. Fishing, aquaculture, salt-making, farming, shipbuilding and industry belong to this history alongside tourism.

Scenery protection and water quality

Setonaikai National Park was among Japan’s first national parks in 1934, but does not encompass the entire inland sea. Postwar urban and industrial growth increased wastewater loads. Red tides and other problems helped drive 1970s regulation. Sewage treatment and discharge controls improved water quality along Hyogo’s coast from the 1980s, while red-tide incidents declined. Scenic protection and industrial-pollution control remained distinct tasks.

Cooperation among coastal governments

Cooperation crossed prefectural borders. In 1971, governors from eleven prefectures and mayors of three designated cities formed a conservation conference. Interim legislation followed in 1973 and became the Special Measures Law in 1978. A sea used by many communities required coordinated measures beyond a single local authority.

Changes in fish catches and nori production involve nutrients, food webs and production conditions, among other factors. In Hyogo’s coastal waters, catches began declining in the late 1990s and nori discolouration became frequent. Insufficient nitrogen and phosphorus have been identified as one contributor. Legal changes in 2015 and 2021 supported more differentiated management. Hyogo’s October 2022 plan sought to increase nutrient supplies in designated waters while maintaining water-quality safeguards.