Sea of Japan

Category: Marine and Oceanic Biomes.

Geographic Location: Pacific Ocean.

Summary: The sea of Japan’s complex but isolated ecosystem supports a rich variety of marine and avian life.

A marginal sea in the Pacific Ocean between the archipelago of Japan, the Russian and Korean mainland, and the Russian island of Sakhalin, the Sea of Japan has several unique characteristics due to its near-complete enclosure by land and resulting isolation from the rest of the Pacific. Just as islands are home to species found nowhere else, the Sea of Japan has its own unique ecosystem. It is significantly less saline than the surrounding ocean, and, like the Mediterranean Sea, its tidal activity is very low. The Sea of Japan lacks any major bays, capes, or large islands.

Hydrology and Climate

The contribution of river discharge to the water exchange is low—about 1 percent. Most of the rivers flowing into the sea are from the Japanese archipelago. Japan’s four largest rivers—the Shinano, Ishikari, Agano, and Mogami—all empty into the Sea of Japan. The largest river from mainland Asia flowing into the sea is the Tumen River, which begins in Mount Baekdu and forms part of the boundary separating China, North Korea, and Russia.

The water balance of the Sea of Japan is maintained by the inflow and outflow through its five straits: the Strait of Tartary, between the Asian mainland and Sakhalin; La Perouse Strait, between Sakhalin and Hokkaido; the Tsugaru Strait, between Hokkaido and Honshu; the Kanmon Straits (also known as the Straits of Shimonoseki), between Honshu and Kyushu; and the Korea Strait, between South Korea and Kyushu.

Once a landlocked sea, the Sea of Japan became joined to the Pacific Ocean over the Miocene period, as the land fragmented sufficiently for Japan to detach from mainland Asia. The Tsugaru and Tsushima straits formed first, about 2.6 million years ago; La Perouse is the youngest, forming as recently as 11,000 years ago and disrupting the migration of mammoths to and from Hokkaido. Even today, all the straits are shallow, limiting the exchange of water with the Pacific and the migration of marine life to and from the sea.

The isolation of the sea leads to clearly separated layers in the water, except during winter in the northern reaches. Winter temperatures in the central and southern sea hover close to 32 degrees F (0 degrees C) at the bottom of the sea; 34–35 degrees F (1.1–1.6 degrees C) at depths of 1,310–1,640 feet (400–500 meters); 36–40 degrees F (2.2–4.4 degrees C) at 656–1,310 feet (200–400 meters); and as warm as 50 degrees F (10 degrees C) 328 feet (100 meters) below the surface. Because of the counterclockwise circulation of the sea currents, the eastern waters are consistently warmer than the western waters. The difference is greatest—about 10 degrees—in the winter.

The waters of the Sea of Japan thus are predominantly warm, and when the northwestern monsoon wind brings cold, dry air over the sea from the Asian mainland (from October to March), powerful storms and typhoons can result. The western coasts of Japan are regularly pummeled with waves in excess of 25 feet (8 meters). The monsoon also carries seawater evaporation south to the Japanese mountains, where it precipitates as snow and adds to the convection of surface waters.

In the northern area of the sea, particularly in the Strait of Tartary, where the water is especially shallow, the surface freezes for months, beginning as early as October and lasting as late as June. The domination of evaporation over precipitation also leads to higher levels of salinity in the southern sea in the winter; this more-saline water is gradually carried to the north, balancing out the introduction of less-saline water as winter ice thaws in the spring. Summer winds blow in the opposite direction, as warm, humid air from the Pacific is carried over the Sea of Japan to the Asian mainland.

Though tidal activity is limited, it is also complex. The tidal flow of the Pacific pushes through the Korea Strait and the northern portion of the Strait of Tartary twice a day, and pushes through the Tsugaru Strait once a day, causing a daily tide on the Koreas’ eastern shores, the Far East of Russia, and Honshu and Hokkaido in the Japanese archipelago. The amplitude of these tides is low but varies considerably across the Sea of Japan. Near the Korea Strait, for example, it is 10 feet (3 meters); at the southern tip of the Korean peninsula, it is only half that.

Biodiversity

Along with the nutrient richness that is aided by its unique combination of temperature and salinity gradients and tidal cycles, the sea’s waters are extremely rich in dissolved oxygen—as high as 95 percent of the saturation point just below the surface and still as high as 70 percent at depths of 9,840 feet (3,000 meters). This leads to a thriving environment for some 3,500 species of fauna and 800 of marine flora, including nearly 1,000 species of crustaceans.

While lacking the deepwater fauna richness of the greater Pacific Ocean, the Sea of Japan biome does support cod, Atka mackerel, sardines in great abundance, and the anadromous fish such as salmon and trout. Sea lions and cetaceans are here in great populations. Squid spawn in the East China Sea before migrating north into the Sea of Japan via the Korea Strait.

Coastal areas of the northern reaches of the Sea of Japan provide a wintering haven for Steller’s sea-eagle (Haliaeetus pelagicus), and vital nesting areas for the endangered spotted greenshank (Tringa guttifer). Cranes and shorebirds use Furugelm Island, off the Russian coast, as a stopover in migrations.

Along the southern waters of the Sea of Japan, the wetlands and rice paddies of the Lower Maruyama River (in Japan’s Hyogo prefecture) were designated a Ramsar Wetland of International Importance in 2012. Authorities say several thousand species live in or depend on the 1,380-acre (560-hectare) area. It is vital, for instance, to such threatened fish species as chum salmon (Oncorhynchus keta), three-spined stickleback (Gsterosteus aculeatus), and Japanese rice fish (Oryzias latipes latipes). It is here that the previously extirpated oriental white stork (Ciconia boyciana) was successfully reintroduced in the 1950s.

Common fish include saury, mackerel, sardines, anchovies, herrings, sea bream, salmon, trout, squid, cod, pollock, and Atka mackerel. The sardine is the most plentiful pelagic fish, constituting more than 70 percent of the total catch. Squid spawn in the East China Sea before migrating to the Sea of Japan. Seals and whales are common mammals. Because the straits are so shallow, there is no characteristic oceanic deep-water fauna.

Threats and Controversy

A significant ecological problem in the Sea of Japan is oil pollution, particularly along major shipping routes, where the number of accidents of seagoing vessels has increased in the 21st century. There are inconclusive reports of radioactive pollution from Soviet-era reactor equipment and materials disposal at sea. Mercury contamination is a recurring problem, particularly in the Agano River, which feeds into the Sea of Japan near Niigata, site of a mass poisoning event in 1965.

Climate change is projected to warm the average seawater temperatures here, a process that seems to be already underway. Major outbreaks of giant jellyfish (Nemopilema nomurai) have become far more common than they were historically in the Sea of Japan, to the point where commercial fisheries are registering their repeated swarms as a serious threat.

Coral reefs in the Sea of Japan are extending their range further to the north, but this buildup is counterbalanced by the fact that reef-dwelling fish are migrating here from more tropical waters, likely at the expense of at least some of the cold-water species native to the Sea of Japan. Additionally, even the new reefs are threatened by the crown-of-thorns starfish (Acanthaster planci), the classic foe of coral reef-building organisms. This starfish only thrives in warm-water conditions.

While the name Sea of Japan is the most commonly used, other nations have pushed for different nomenclature. South Korea prefers the name East Sea, while North Korea refers to the sea as East Sea of Korea. The South Korean government specifically argues that the name Sea of Japan serves as a reminder of the period when Korea was occupied by imperial Japan. Although Sea of Japan has been the most common name in international use for more than two centuries (in large part because of Japan’s prominence in international trade), the term East Sea was found on Western maps as early as the 1700s.

Sea of Korea was actually the most common Western use in that century, likely because of Japan’s strict isolationist policies from 1633 to 1853, which made Western interaction with the country almost negligible. Oriental Sea was about as common in Western usage as Sea of Japan until the opening of Japan to international trade in the mid-19th century.

Bibliography

Fukuoka, N. “On the Distribution Patterns of the So-Called Japan Sea Elements Confined to the Sea of Japan Region.” Journal of Geobotany, 15, no. 1 (1966).

International Hydrographic Organization. Limits of Oceans and Seas, Special Publication No. 28. Monte Carlo, Monoco: International Hydrographic Organization, 1953.

Tang, Q. and K. Sherman, eds. The Large Marine Ecosystems of the Pacific Rim: Assessment, Sustainability, and Management. London: Blackwell Science, 1999.