Hudson Bay ecosystem

Named for the Dutch explorer Henry Hudson, Hudson Bay is the second-largest bay in the world after the Bay of Bengal. It is a marine bay and marginal sea in northern Canada that drains most of the central area of the country as well as parts of the upper Midwest of the United States, about 1.5 million square miles (3.9 million square kilometers) in all. Hudson Bay connects with the North Atlantic Ocean via the Hudson Strait, and to the Arctic Ocean via the Northwest Passage.

Hydrology and Geography

Many streams and rivers contribute to Hudson Bay’s waters, significantly reducing its salinity compared to the open ocean. Hudson Bay is surrounded by vast expanses of wetlands, taiga or boreal forest, and tundra ecosystems.

Hudson Bay is a relatively shallow saltwater body, with an average depth of 330 feet (100 meters). It is about 900 miles (1,450 kilometers) long north to south—including its southernmost lobe, James Bay—and about 650 miles (1,050 kilometers) across at its widest. Total surface area is approximately 475,000 square miles (1.23 million square kilometers). Hudson Bay is ice-covered for half the year; ice begins to form in early November, and the whole bay has very low year-round average temperatures. The ice cover begins to melt in mid-June, usually clearing from the eastern shores first. Water temperature rises to 46–48 degrees F (8–9 degrees C) on the western side of the bay by late summer.

While it is a saltwater or brackish body, Hudson Bay receives freshwater inflow at an annual volume of roughly 216 cubic miles (900 cubic kilometers); even more comes from precipitation and ice melt. Consequently, and also due to its limited hydraulic exchange with the Atlantic Ocean, the bay is less salty than the ocean.

Rich Base of Life

Because of its vast dimensions, the Hudson Bay marine ecosystem has many aquatic and coastal ecozones with varied habitats that are used year-round by Arctic and sub-Arctic species, as well as seasonally by migratory fish, marine mammals, and birds. The sea ice, for instance, supports the seals upon which polar bears depend. Millions of geese and shorebirds feed and breed in vast coastal salt marshes. Productive submarine eelgrass beds provide food for waterfowl migrating to and from breeding habitat in the Arctic Islands, and shelter for crustacean and fish populations. Numerous estuaries provide habitat corridors for anadromous fish like salmon and Arctic char, and support beluga whales seasonally.

The Hudson Bay supports great numbers of large mammals, both aquatic and terrestrial. Migratory marine mammal species such as belugas frequent the region as permitted by ice conditions across the northern channels. Other cetaceans, such as orca and minke whales, are rarer visitors. Large concentrations of more than 50,000 beluga whales are found in the estuaries of the Nelson and Churchill Rivers in July and August. Ringed seals, bearded seals, and harbor seals, as well as walruses, reside in the bay year-round, while harp and hooded seals visit seasonally. Subsistence harvesting of these marine mammals is important to the aboriginal peoples and the economy of the bay. Arctic foxes and polar bears reside on coasts in the summer, and on sea ice during the rest of the year; they, too, are hunters of seals.

At least 130 species of birds migrate to the Hudson’s waters each summer and depart in late fall. Waders, waterfowl, and raptors share pelagic, intertidal, and wetland habitats here. Most such species—at least a hundred—find breeding zones on shore. Cranes, plovers, herons, bitterns, loons, pelicans, ducks, geese, swans, osprey, falcons, eagles, and owls share the Hudson Bay.

Less is known about the prevalence and range of marine fish species here, due to the near-absence of commercial fisheries; even traditional subsistence fishing has been somewhat limited. Some sixty or so fish species are known in these waters, however, and many of these are adapted to the relatively shallow, brackish environment here. Of the recognized species, many spend their whole lives in the marine environment. Half that many species are marine but frequent the estuaries as seasonal nursery grounds; another nine feed in the brackish coastal realm, but spawn in freshwater zones. As many as sixteen freshwater species have evolved salinity tolerances sufficient to spend some time here in brackish coastal reaches or estuaries. One species—the Atlantic salmon—is anadromous, spending much of its life in between saltwater  and freshwater, spawning migrations.

Conservation and Climate Change

The governance of the Hudson Bay area is largely a series of compromise decisions and shared responsibilities between federal, provincial, local, and First Nations bodies. As global warming proceeds—and faster here in its effects than in many if not most other parts of the world—such organizations will have to work more closely in concert to help determine the growing number of issues. These will range from plans for hydroelectric dams and other water diversion schemes; new deepwater port facilities and related marine transport strategies; mining, agriculture, grazing, and aquaculture rights; tourism pursuits such as hunting; and real estate development. Encompassing all of these areas is the issue of environmental conservation.

There has already been considerable focus on the impact of climate change and transboundary contaminants on Hudson Bay. Historic commercial whaling, particularly for bowheads and belugas, depleted these populations, though some bowhead populations have substantially recovered. The steady increase in regional temperatures over the last 100 years has caused a lengthening of the ice-free period and continues to jeopardize the sustainability of ice-dependent species such as polar bears and seals. Research has directly linked the decline of the Western Hudson Bay polar bear population to shrinking sea ice, with studies indicating a decline of roughly 50 percent since 1979 due to longer ice-free periods that reduce hunting opportunities and cause energetic stress. These ice-dependent marine mammals are vulnerable to both airborne contaminants and global warming. Because these carnivorous animals are high in the food chain, they accumulate contaminants, such as heavy metals and persistent organic pollutants in their tissues. In December 2025, researchers documented a rare case of a female polar bear adopting a cub in the Western Hudson Bay population, highlighting behavioral adaptability even under environmental stress.

Diminished sea-ice cover and increased precipitation are some of the long-term trends already detected in the Hudson Bay. Each of these vectors is projected to intensify, along with other effects such as sea-level rise, which will have as yet undetermined but certainly serious impacts on the habitats here.

Bibliography

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Ferguson, Steve H., et al., editors. A Little Less Arctic: Top Predators in the World’s Largest Northern Inland Sea, Hudson Bay. Springer, 2010.

Frey, David. “Hudson Bay Polar Bears May Not Survive Climate Change.” The Wildlife Society, 2 July 2024, wildlife.org/hudson-bay-polar-bears-may-not-survive-climate-change/. Accessed 14 Feb. 2026.

Goldsmit, Jesica, et al. “Screening for High Risk Marine Invaders in the Hudson Bay Region, Canadian Arctic.” Frontiers in Ecology and Evolution, 11 Feb. 2021, doi:10.3389/fevo.2021.627497. Accessed 14 Feb. 2026.

“Hudson Bay.” Encyclopædia Britannica,, 14 Feb. 2026, www.britannica.com/place/Hudson-Bay. Accessed 14 Feb. 2026.Marsh, James H. “Hudson Bay.” The Canadian Encyclopedia, Historica Canada, 16 Apr. 2025, www.thecanadianencyclopedia.ca/en/article/hudson-bay/. Accessed 14 Feb. 2026.

Polan, Jason, and Jeff Wells. “A Proposed Marine Conservation Area along Hudson and James Bays Makes Significant Progress.” National Audubon Society, 29 Feb. 2024, www.audubon.org/news/proposed-marine-conservation-area-along-hudson-and-james-bays-makes-significant-progress. Accessed 14 Feb. 2026.

Ritchie, J. C. Postglacial Vegetation of Canada. Cambridge University Press, 1987.

Vallerand, Joan, et al. “Climate Variations in Eastern Hudson Bay over the Past 3000 Years.” Quaternary Science Reviews, vol. 339, 2024, doi:10.1016/j.quascirev.2024.108862. Accessed 14 Feb. 2026.

Venkatraman, Sakshi. “Polar Bear Mother Adopts Cub in Rarely Documented Case.” BBC, 17 Dec. 2025, www.bbc.com/news/articles/cvg803v3erko. Accessed 14 Feb. 2026.

Peters, Michelle. “Why Beluga Whales Migrate to Churchill, Canada, Each Summer.” Nathab.com, 16 Nov. 2025, www.nathab.com/blog/why-beluga-whales-migrate-to-churchill-canada-each-summer. Accessed 14 Feb. 2026.