The Sea of Okhotsk, located in the Russian Far East, is a unique marine environment. Known for its extensive winter sea ice, this marginal sea of the Pacific Ocean experiences extreme conditions. Despite this, the Okhotsk Sea supports rich and diverse marine life, making it a highly productive aquatic region. Its distinct features and climate create a dynamic ecosystem.
Location and Defining Features
The Sea of Okhotsk, in the northwestern Pacific Ocean, borders the Kamchatka Peninsula to its east, the Kuril Islands to the southeast, Japan’s Hokkaido island to the south, and Sakhalin Island along its west. The eastern Siberian coast, including the Shantar Islands, forms its western and northern boundaries. It is almost entirely enclosed by Russian territory, except for a small area touching Hokkaido.
Covering approximately 1,583,000 square kilometers (611,000 square miles), it has a mean depth of about 859 meters (2,818 feet) and a maximum depth of 3,372 meters (11,063 feet) in the Kuril Basin. This basin, west of the Kuril Islands, is the deepest part of the sea. The seabed slopes from north to south, with a continental shelf extending to about 200 meters (650 feet) along the northern and western margins. The remaining 70 percent of the area deepens to the south and east.
The Sea of Okhotsk connects to the Sea of Japan through the Sakhalin Gulf and the Gulf of Tartary to the west, and via the La Pérouse Strait to the south. Continental sediment flows into the sea, predominantly from the Amur River. This freshwater inflow from the Amur River contributes to lower salinity in the upper layers, influencing winter ice formation.
Extreme Climate and Sea Ice
The Sea of Okhotsk has a subarctic climate, making it one of East Asia’s coldest seas. Cold air from Siberia contributes to its extreme cold, particularly in the northern and western parts during winter. This continental influence, combined with a high-pressure system over Siberia and a low-pressure system over the Sea of Okhotsk in winter, creates conditions conducive to chilling the seawater.
Sea ice forms in November in the northwestern basin, expanding southward due to northerly winds and the Okhotsk Sea’s southward flow. This process expels salt from the forming ice, releasing cold, saline, dense water into deeper ocean layers. The sea ice reaches its maximum extent in mid to late March, covering approximately 80 percent of the sea, with a maximum thickness of about 1 meter.
Unique hydrological conditions, including a double-layered structure of different salinities, also play a role. Lower surface layer salinity, influenced by river runoff, prevents deep mixing with denser, saltier intermediate cold water below 50 meters. This barrier allows surface water to cool rapidly to freezing temperatures, facilitating extensive sea ice. It is the southernmost region in the Northern Hemisphere influenced by seasonal sea ice, with ice forming at latitudes as low as 44° N along the Shiretoko Peninsula.
Abundant Marine Life
Despite its extreme conditions, the Sea of Okhotsk is one of the most biologically productive seas globally. Its cold, nutrient-rich waters support diverse marine life. High productivity is attributed to factors like river drainage, intense water mixing by straits and wind, and the upwelling of deep, nutrient-laden ocean waters. Phytoplankton forms the food web’s base, flourishing in spring as sunlight penetrates the water, which supports zooplankton.
This plankton sustains commercially important fish species such as Pacific herring, pollock, salmon, flounder, cod, and capelin. Pollock accounts for a large portion of the total fish catch. Crabs, shrimp, sea mussels, and sea urchins are also found.
It is also home to marine mammals, including seals, sea lions, and whale species. Residents include the northern fur seal, Steller sea lion, and the endangered Kuril harbor seal and gray whale. Many islands, such as the Shantar Islands, serve as breeding grounds for these marine mammals and large seabird colonies like northern fulmars, tufted puffins, murres, and kittiwakes.
Ecological Importance and Human Interaction
The Sea of Okhotsk plays an ecological role, contributing to global fisheries. It accounts for more than half of the Russian Far East’s total fish catches and nearly a quarter of all Russian fish catches. Its rich biological resources support fishing industries, with walleye pollock, herring, and salmon being primary targets.
Human interaction extends beyond fishing to other resource extraction activities. Oil and natural gas deposits have been discovered on the sea’s northern shelf, leading to exploration. These industrial pressures, along with the impacts of climate change, raise concerns about environmental degradation.
Climate change has led to a decline in sea ice, with a reduction equivalent to 3.4% of the sea’s total area per decade observed between 1970/1971 and 2024/2025. This warming also affects migration routes of fish and marine mammals that rely on sea ice for their movements. The Sea of Okhotsk’s sensitivity to these changes underscores the need for ongoing research and collaborative efforts to maintain its ecological balance and ensure the sustainability of its resources.