The Yellow Sea is a marginal sea of the Western Pacific Ocean located between mainland China and the Korean Peninsula, and can be considered the northwestern part of the East China Sea. It is one of four seas named after common colour terms (the others being the Black Sea, the Red Sea and the White Sea), and its name is descriptive of the golden-yellow colour of the silt-laden water discharged from major rivers.
The innermost bay of northwestern Yellow Sea is called the Bohai Sea (previously Pechihli Bay or Chihli Bay), into which flow some of the most important rivers of northern China, such as the Yellow River (through Shandong province and its capital Jinan), the Hai River (through Beijing and Tianjin) and the Liao River (through Liaoning province). The northern extension of the Yellow Sea is called the Korea Bay, into which flow the Yalu River, the Chongchon River and the Taedong River.
Since 1 November 2018, the Yellow Sea has also served as the location of "peace zones" between North and South Korea.
The sea is rich in seaweed (predominantly kelp, Laminaria japonica), cephalopods, crustaceans, shellfish, clams, and especially in blue-green algae which bloom in summer and contribute to the water color (see image above). For example, the seaweed production in the area was as high as 1.5 million tonnes in 1979 for China alone. The abundance of all these plant and animal species increases toward the south and indicates a high sea productivity, accounting for the diversity of fish species and high fish yield from the sea. Several species of goby new to science have been discovered recently in the Yellow Sea.
The southern part of the Yellow Sea, including the entire west coast of Korea, contains a 10 km-wide (6.2 mi) belt of intertidal mudflats, which has the total area of 2,850 km2 (1,100 sq mi) and is maintained by 4–10 m (13–33 ft). Those flats consist of highly productive sediments with a rich benthic fauna and are of great importance for migratory waders and shorebirds. Surveys show that the area is the single most important site for migratory birds on northward migration in the entire East Asian – Australasian Flyway, with more than 35 species occurring in internationally significant numbers. Two million birds, at minimum, pass through at the time, and about half that number use it on southward migration. About 300,000 migrating birds were transiting annually only through the Saemangeum tidal flat area. This estuary was however dammed by South Korea in 1991–2006 that resulted in drying off the land. Land reclamation also took 65% of the intertidal area in China between the 1950s and 2002, and as of 2005 there were plans to reclaim a further 45%.
Populations of oceanic megafauna, such as marine mammals, sea turtles, and larger fish, have decreased in modern times, not only due to pollution but also due to hunting. Japanese industrial whaling and illegal mass operations by the Soviet Union with support from Japan have been major drivers of population decline. Species that reside in the area today include spotted seals, and cetaceans such as minke whales, killer whales, false killer whales, and finless porpoises, but nonetheless all the remnants of species listed could be in very small numbers. Historically, large whales were very abundant either for summering and wintering in the Yellow and Bohai Seas. For example, a unique population of resident fin whales and gray whales were historically presented, or possibly hosted some North Pacific right whales and Humpback whales (3 whales including a cow calf pair was observed at Changhai County in 2015) year-round other than migrating individuals, and many other migratory species such as Baird's beaked whales.Even blue whales, Japanese sea lions, dugongs (in southern regions only), and leatherback turtles used to breed or migrate into Yellow and Bohai seas.
Spotted seals are the only resident species of seal in the Yellow Sea. A sanctuary for these seals is situated at Baengnyeongdo, which is also known for its finless porpoises. Great white sharks have also been known to prey on seals in the area.