Spring feeding season almost over for polar bears & sea ice becomes less important

Reposted from Dr. Susan Crockford’s Polar Bear Science

Posted on June 4, 2020 |

Here are ice conditions at the end of May, which signals the near-end of the critical spring feeding period for polar bears. This is because young-of-the-year seals take to the water to feed themselves, leaving only predator-savvy adults and subadults on the ice from some time in June onward (depending on the region).

Spring is the critical feeding period for polar bears (Crockford 2019, 2020; Lippold et al. 2020; Obbard et al. 2016):

“Unexpectedly, body condition of female polar bears from the Barents Sea has increased after 2005, although sea ice has retreated by ∼50% since the late 1990s in the area, and the length of the ice-free season has increased by over 20 weeks between 1979 and 2013. These changes are also accompanied by winter sea ice retreat that is especially pronounced in the Barents Sea compared to other Arctic areas. Despite the declining sea ice in the Barents Sea, polar bears are likely not lacking food as long as sea ice is present during their peak feeding period. Polar bears feed extensively from April to June when ringed seals have pups and are particularly vulnerable to predation, whereas the predation rate during the rest of the year is likely low.“ [Lippold et al. 2019:988]

NISDC comparative graphs shows 2020 ice extent at 31 May 2020 was higher than 2016 on the same date (and about the same as 2015).

Below is the map for Arctic Canada at 31 May 2020, which has a noteable lack of open water in the Beaufort Sea:

Compare to 31 May 2016:

Last year (31 May 2019), however, was quite different again, especially in Hudson Bay and Hudson Strait:

Stirling et al. (1981:54) discussed why polynyas can be so important in the Southern Beaufort and Hudson Bay (my bold):

“One useful approach is to ask what would happen if the polynya was not there? Obviously this is impossible to evaluate on an experimental basis, but by examining the consequences or natural seasonal variation, some useful insights can be gained. For example, the influence of rapidly changing ice conditions on the availability of open water, and consequently on populations of seals and polar bears, has been observed in the western Arctic. Apparently in response to severe ice conditions in the Beaufort Sea during winter 1973-74, and to a lesser degree in winter 1974-75, numbers of ringed and bearded seals dropped by about 50% and productivity by about 90%. Concomitantly, numbers and productivity of polar bears declined markedly because of the reduction in the abundance of their prey species. …If the shoreleads of the western Arctic or Hudson Bay ceased opening during winter and spring, the effect on marine mammals would be devastating.”

Ice cover over Hudson Bay for the last week of May:

Ice cover over the western Canadian Arctic (Eastern Beaufort Sea) for the last week of May:

The Alaskan portion of the Beaufort was still predominantly multiyear ice at the end of May and thick first year ice in the Chukchi Sea (note this map does not show areas of open water):

Areas of open water in the western Beaufort/Chukchi/Bering Seas at end May 2020:

Ice thickness at this time (less relevant for bears and seal, who prefer first year ice):

See previous posts on spring feeding here and here, with references, and on the Beaufort Sea polynya (recurrent open water) here and here.

References

Crockford, S.J. 2019. The Polar Bear Catastrophe That Never Happened. Global Warming Policy Foundation, London. Available in paperback and ebook formats.

Crockford, S.J. 2020. State of the Polar Bear Report 2019. Global Warming Policy Foundation Report 39, London. PDF here.

Lippold, A., Bourgeon, S., Aars, J., Andersen, M., Polder, A., Lyche, J.L., Bytingsvik, J., Jenssen, B.M., Derocher, A.E., Welker, J.M. and Routti, H. 2019. Temporal trends of persistent organic pollutants in Barents Sea polar bears (Ursus maritimus) in relation to changes in feeding habits and body condition. Environmental Science and Technology 53(2):984-995.

Obbard, M.E., Cattet, M.R.I., Howe, E.J., Middel, K.R., Newton, E.J., Kolenosky, G.B., Abraham, K.F. and Greenwood, C.J. 2016. Trends in body condition in polar bears (Ursus maritimus) from the Southern Hudson Bay subpopulation in relation to changes in sea ice. Arctic Science 2:15-32 Doi 10.1139/AS-2015-0027 http://www.nrcresearchpress.com/doi/abs/10.1139/AS-2015-0027#.VvFtlXpUq50

Stirling, I, Cleator, H. and Smith, T.G. 1981. Marine mammals. In: Polynyas in the Canadian Arctic, Stirling, I. and Cleator, H. (eds), pg. 45-58. Canadian Wildlife Service, Occasional Paper No. 45. Ottawa. Pdf of excerpts of above paper here.