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NextImg:Winter sea ice habitat for polar bears still abundant enough to sustain a thriving species

From Polar Bear Science

Susan Crockford

In contrast to summer sea ice, winter ice in the Arctic was again abundant this year. The slight decline since 1979 has so far been no cause for concern to polar bears, who are thriving.

According to the US National Snow and Ice Data Center report (5 April 2023), the average ice extent for March was 14.44 mkm2, considered the “winter”value (as compared to “summer” which is the average for September). The extent for this year was certainly below the long-term average but nowhere near being gone for good and nowhere near the low extent for winter at the height of the Eemain interglacial, when there was no ice at all in the Bering Sea (Polyak et al. 2010).

Numbers don’t tell the whole story. Critically for polar bears, there was abundant ice in every subpopulation region where the species depends on newborn seals for food at this time of year, from the Bering Sea to the Barents Sea. There is even a fair amount off southeast Greenland, home to the newest subpopulation of bears.

While some researchers and the media focus on the fact that some Arctic ice is getting thinner, this is not an issue for polar bears who require ice less than 2 m thick at this time of year (Derocher et al. 2004).

That means abundant first year ice, especially near shore and over Hudson Bay (shown below), is excellent news for polar bears.

In their 2004 paper, Derocher and colleagues state this unequivocally in the first sentence of their abstract:

Polar bears (Ursus maritimus) live throughout the ice-covered waters of the circumpolar Arctic, particularly in near shore annual ice over the continental shelf where biological productivity is highest.

That means abundant first year ice, especially near shore and over Hudson Bay, is excellent news for these species.

Moreover, a study by George Durner and colleagues (2019:8625) on the use of sea ice in summer and winter by Southern Beaufort Sea polar bears over two recent decades (1996-2006 and 2007-2016) compared to a 1985-1995 decade baseline concluded:

During the ice maximum season (i.e., winter), polar bears used the best habitat available, which changed relatively little across the three decades of study.

Ian Stirling and colleagues (1999:294) could not have said it more clearly in their paper about Western Hudson Bay polar bears:

Polar bears reach their lightest weights of the year in late March, just before the birth of the next cohort of ringed seal pups. This fact suggests it is the success of their hunting in spring and early summer that enables them to maximize the body reserves necessary for survival, reproduction, and nursing of cubs through the rest of the year.

References

Derocher, A.E., Lunn, N.J., and Stirling, I. 2004. Polar bears in a warming climate. Integrative and Comparative Biology 44(2):163-176. https://doi.org/10.1093/icb/44.2.163

Durner, G.M., Douglas, D.C., and Atwood, T.C. 2019. Are polar bear habitat resource selection functions developed from 1985-1995 data still useful? Ecology and Evolution 9(15):8625-8638. https://doi.org/10.1002/ece3.5401

Polyak, L., Alley, R.B., Andrews, J.T., et al. 2010. History of sea ice in the Arctic. Quaternary Science Reviews 29:1757-1778. https://doi.org/10.1016/j.quascirev.2010.02.010

Stirling, I., Lunn, N.J., and Iacozza, J. 1999. Long-term trends in the population ecology of polar bears in Western Hudson Bay in relation to climate change. Arctic 52(3):294-306. https://doi.org/10.14430/arctic935