Sea of Japan

• 1. Introduction

  ›

  The Sea of Japan sits in near-total isolation from the Pacific Ocean, enclosed by the Japanese archipelago, the Korean Peninsula, Sakhalin, and the Russian Far East. That enclosure shapes almost everything about it: the salinity is lower than in the open ocean, the tidal range is modest, and the biological community has developed along its own lines. The sea covers a surface area of about 1,050,000 square kilometres, reaches a maximum depth of 4,568 metres, and stretches more than 2,255 kilometres from north to south. What drives a body of water so large to behave almost like a lake? And why does a dispute over what to call it still echo in international institutions today? Those questions pull in directions the sea itself makes visible: its geology, its currents, its fish, and the peoples and nations that have argued over it for generations.

• 2. A Name Contested

  ›

  Japan calls it Nihon-kai. South Korea calls it Donghae, the East Sea. North Korea uses the name East Sea of Korea. Russia calls it Yaponskoye more. China has recorded it as both Riben hai and, originally, Jinghai, the Sea of Whales. Each name carries a political weight that the word "naming" barely captures.

  Japan argues that the name Sea of Japan has been the internationally accepted English term since the early 19th century. South Korea counters that the term East Sea was in use before Japanese colonial rule and asks that both names appear alongside each other on maps and in publications. The International Hydrographic Organization's reference document, Limits of Oceans and Seas, known as S-23, used the name Japan Sea from its first publication in 1928. That document has not been updated since 1953. In 2020, the IHO decided to replace S-23 with a new digital standard, S-130, which will assign unique numerical identifiers to maritime areas rather than names. The IHO Secretary-General confirmed that the 1953 version would remain publicly available as a record of the transition from analogue to digital charting. The naming question, in other words, was not resolved; it was sidestepped by technology.

• 3. Built by Deep Time

  ›

  Before there was a sea here, there was a land bridge. The Sea of Japan was landlocked when East Asia's land bridge existed, and the basin only began opening in the Early Miocene when the Japan Arc started to form. During that long Miocene interval, the northern and southern parts of the Japanese archipelago separated from each other, and the sea expanded as the arc extended.

  The straits that now connect the sea to its neighbours are geologically young and shallow, and that shallowness has consequences. The oldest of the five straits are the Tsugaru and Tsushima straits, and their formation interrupted elephant migration into the Japanese islands at the end of the Neogene Period, roughly 2.6 million years ago. La Perouse Strait is the most recent, forming somewhere between 60,000 and 11,000 years ago; mammoths had moved to northern Hokkaido along the route it now closes. All five straits have a minimal depth of around 100 metres or less, which throttles water exchange and keeps the sea's biology distinct from the Pacific.

  The eastern margin of the sea may host an incipient subduction zone. Large earthquakes struck there in 1940, 1964, 1983, and 1993. During the ice ages, as sea levels fell across the world, the exit straits dried one by one. There is genuine scientific controversy about whether the sea level ever fell far enough to close even the deepest exit, the western channel of Korea Strait, turning this basin into a vast cold inland lake with a fresh-water surface layer that froze over in winter.

• 4. How the Water Moves

  ›

  Currents in the Sea of Japan circle counterclockwise. The Kuroshio Current, the Tsushima Current, and the East Korea Warm Current carry warmer and more saline water northward, where they merge into the Tsugaru Current and exit through Tsugaru Strait into the Pacific. Part of that flow feeds the Soya Current, which exits through La Perouse Strait into the Sea of Okhotsk. The return branch, composed of the Liman, North Korea, and Central Japan Sea currents, brings cold fresh water southward along the Asian coast.

  The sea's average salinity is 34.09 parts per thousand, slightly below the Pacific's because enclosure reduces the mixing that keeps ocean salinity high. In summer, salinity ranges from 31.5 parts per thousand in the north to 34.5 parts per thousand in the south. The Korea Strait admits salty water and helps keep the south comparatively saline even as ice melt in spring freshens the north.

  Dissolved oxygen is notably high throughout the water column, reaching 95 percent of saturation near the surface and dropping to about 70 percent at 3,000 metres. Cold winter air drives convection in the northern part of the sea, pushing oxygenated surface water downward. That oxygen-rich cold layer, formed in the north in winter and carried south by currents, sits at roughly 1,000 metres and persists through the entire year. The seawater's transparency is about 10 metres, and its colour runs from blue to green-blue.

• 5. The Life Within

  ›

  More than 800 species of aquatic plants and more than 3,500 animal species inhabit the Sea of Japan. Among the animals are more than 900 species of crustaceans, about 1,000 species of fish, and 26 species of mammals. The coastal zones hold several kilograms of biomass per square metre.

  Pelagic fish species include saury, mackerel, sardines, anchovies, herring, sea bream, squid, and various species of salmon and trout. The sea bottom supports cod, pollock, and Atka mackerel. Seals and whales represent the mammals; the ancient Chinese name for the basin, Jinghai, translates as Sea of Whales, which suggests how visible the whales once were. Shrimps and crabs account for a large share of the crustacean life. Because the straits are shallow, there is no characteristic oceanic deep-water fauna; what lives here evolved in something closer to an oversized lake than a genuine oceanic margin. The flora and fauna unique to this region carry the scientific designation Japan Sea elements.

  American, Canadian, and French whaleships worked the sea between 1847 and 1892. During the peak years of 1848 and 1849, more than 170 vessels cruised the basin, more than 60 in 1848 and more than 110 in 1849. They primarily hunted right whales from March to September, with peak catches in May and June. As right whale numbers declined, crews shifted to humpbacks. They also attempted blue and fin whales, but both species invariably sank after being killed, making them commercially useless at the time. Most fleets entered through Korea Strait and left through La Perouse Strait.

• 6. Exploration and Science

  ›

  For centuries the sea shielded Japan from land invasion, most notably against Mongol attempts. European engagement with it came later. Russian expeditions of 1733-1743 mapped Sakhalin and the Japanese islands. In the 1780s, Jean-Francois de Galaup, comte de Laperouse, sailed northward through the strait that now bears his name. William Robert Broughton, a British naval officer, explored the Strait of Tartary and the eastern coasts of the Russian Far East and Korean Peninsula in 1796.

  Adam Johann von Krusenstern, sailing around the globe aboard the Nadezhda between 1803 and 1806, examined the sea and the eastern shores of the Japanese islands in passing. Gennady Nevelskoy discovered the strait between the Asian continent and Sakhalin in 1849 and mapped the northern Strait of Tartary. Russian expeditions of 1853-1854 and 1886-1889 measured surface temperatures, recorded tidal patterns, and documented the counterclockwise cyclonal character of the sea's currents. The American North Pacific Exploring and Surveying Expedition ran from 1853 to 1856; the British Challenger expedition followed from 1872 to 1876. V. K. Brazhnikov described the aquatic life between 1899 and 1902, and P. Yu. Schmidt continued that work from 1903 to 1904. Japan's own systematic scientific study of the sea did not begin until the 1920s, a late start relative to European and American efforts.

• 7. Fishing, Ports and Politics

  ›

  Fishery has long been the dominant economic activity on and around the Sea of Japan. Herring, sardines, and bluefin tuna are the main commercial targets on and near the continental shelves, though all three species have been depleted since World War II. Squid is caught primarily near the centre of the sea, salmon near the northern and south-western shores, and seaweed production is well developed along multiple coasts.

  The importance of fishing to the region is visible in territorial disputes: Japan and South Korea contest Liancourt Rocks, and Japan and Russia contest the Kuril Islands, with fishing rights central to both conflicts. Vladivostok serves as the base for Russia's whaling fleet and as the Pacific terminus of the Trans-Siberian Railway, handling substantial inland cargo. Russian international freight moves more through Nakhodka and Vostochny, which maintain active exchange with Japan and South Korea. Cold political relations between the bordering states have historically kept shipping across the sea at a moderate level; Japan's largest ports developed on its Pacific coast instead. The main Japanese ports on the Sea of Japan side are Niigata, Tsuruta, and Maizuru. South Korea's major ports, Busan, Ulsan, and Pohang, sit on the south-eastern coast of the Korean Peninsula but primarily serve trade partners beyond the sea's immediate neighbours. Shipment volumes are growing as East Asian economies expand, and the sea also holds magnetite sands and natural gas and petroleum fields near northern Japan and Sakhalin Island.