Tag Archives: ice formation

Polar bear specialist calls Hudson Bay freeze-up ‘late’ yet bears were moving offshore 2 weeks ago

Predictably, polar bear specialist Andrew Derocher has finally posted a tracking map of the Western Hudson Bay bears his team has fitted with collars and eartags–two weeks after bears were released from Churchill’s polar bear jail, which is the local signal that there is enough sea ice for bears to leave shore. As I reported two weeks ago, release of jailed bears happened this year on 10 November. And as I predicted in that post, by waiting so long after that event to post his map, Derocher can make it seem to his naive followers on Twitter that the bears are just starting to leave now (the last map he posted was on 11 November, when none of his bears had moved). He reinforces this by calling WH freeze-up “late”, when by all objective measures (including local informants reporting bears on the ice) it was as early as it had been in the 1980s (Castro de la Guardia et al. 2017).

Recall that fall is the second-most important feeding season for polar bears, due to the fact that seals are strongly attracted to newly-forming sea ice. It’s their chance to regain some or all of the weight lost over the summer, before the long winter fast begins (while bears indeed hunt when they can, they are not often successful during the depths of the Arctic winter: most bears are at their lowest weight by March).

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New polar bear hunting habitat forming already along the coast of the Laptev Sea: A new trend?

Not even three weeks after the yearly minimum of sea ice extent was reached this year, new shorefast ice is already forming off the coast of Siberia, which is critical fall hunting habitat for polar bears.

Polar bears on a seal kill in new ice, 31 October 2020 in W. Hudson Bay via webcam.

So, not only was this year’s sea ice extent for September at the very lowest extreme of predicted levels for late summer, given ever-increasing CO2 concentrations in the atmosphere, new ice seems to be forming earlier in the fall as well, which bodes well for winter ice formation. It’s looking to me like the decade-long increasing trend of September ice extent since 2012 (see below) may indicate a change more biologically relevant to ice-dependent Arctic animals than the zero trend since 2007.

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Sea ice growth after the summer minimum begins in the Canadian Arctic Archipelago

The Arctic sea ice minimum was declared to have been reached on 16 September this year (4.72 mkm2), breaking no records.

Ice extent can only go up from this point forward but at this time of year, it happens slowly and isn’t noticeable in the Arctic Basin as much as it is in the Canadian Arctic Archipelago. As I wrote about last year, new ice development in the fall next to shore creates upwelling conditions that attract fish and seals, and therefore provides feeding opportunities for polar bears.

The same process likely happens when new ice forms next to old ice. These few weeks of developing sea ice, wherever they occur, are the last chance polar bears have to replace weight lost over the summer before the cold and darkness of winter reduces hunting opportunities to virtually nil.

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Shorefast ice formation and the fall feeding season for polar bears

What may seem like a silly question is actually fundamental to polar bear survival: in the fall, why do Western Hudson Bay bears correctly expect to find seals in the new ice that forms offshore? Why are seals attracted to that new ice – called ‘shorefast ice’ or ‘fast ice’ – when they would clearly be safer out in the open water where there is no ice and no bears?

As the picture below attests, polar bears can and do kill ringed seals in the new ice that forms off the coast of Western Hudson Bay even when it is but a narrow strip of thin ice – and so close to shore their successes can be caught on camera.

Three adult male polar bears share a seal kill on the newly-formed ice off Wapusk National Park, Western Hudson Bay. 5 November 2020. Buggy cam, Explore.org

A different bear was also filmed killing another seal on 31 October. And these are only the kills we know about along a very short stretch of coast – the killing is almost certainly going on up and down the entire coast, into James Bay (see below), where there is just as much ice but no cameras.

As far as I am aware, this seal killing by polar bears goes on in newly-formed shorefast ice everywhere across the Arctic in early fall, not just in Hudson Bay. Although the timing varies, virtually everywhere in the peripheral seas of the Arctic Ocean (Barents, Kara, Laptev, Chukchi, Beaufort, as well as Baffin Bay and Davis Strait), shorefast ice forms before the mobile ice pack expands to meet the ice developing from shore.

This shorefast ice formation in fall provides a predictable but short-lived source of prey for polar bears as they strive to regain some of the weight lost over the summer.

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