Late fall polar bear habitat 2020 compared to some previous years

Reposted from Polar Bear Science

Posted on December 22, 2020 | 

It’s time to look at sea ice habitat at 15 December (Julian Day 350), now that virtually all bears except pregnant females throughout the Arctic are either out on the sea ice attempting to hunt for seals or hunkered down against the darkness.

As is usual at this time of year, the Canadian Archipelago, the Beaufort, East Siberian and Laptev Seas are well covered in ice (see regions on map below). As for the rest, despite what one polar bear specialist has implied there is no evidence that a slower-than-usual fall freeze-up in the other peripheral seas of the Arctic negatively affects polar bear health or survival.

In fact, because of the attractiveness of the ice edge for seals in the fall, as I discussed last month, it’s possible that the longer the ice edge persists in fall, the more successful polar bears will be in hunting seals – except those above the Arctic Circle where lack of daylight from early November may cause polar bears to hunker down and rest rather than try to hunt through the darkness. But we’ll never know for sure, because bears have never been studied at this time of year – experts simply make assumptions about what happens (e.g. Stirling and Oritsland 1995).

Sea ice thickness also varies year to year throughout the season but does not matter much to polar bears, who hunt most successfully in first year ice less than 2m in thickness, which comprises all of the regions currently purple in the ice thickness chart below.

HUDSON BAY

This year at mid-December, there was more ice than usual in central and southern Hudson Bay (below) and somewhat less than usual in the eastern portion.

However, the ice is forming so fast now that by 18 December there was hardly any open water remaing over Hudson Bay and the ice to the north was solidifying (below). Recall that a similar freeze-up pattern left a pod of a dozen or so killer whales stranded in mid-January 2013 and killed four others in 2016. Such ice-entrapment suggests that despite a ‘warming’ Arctic, freeze-up patterns would have to change very dramatically for Hudson Bay to be an attractive place for killer whales. A recent DFO report concluded:

Killer whale ice entrapments are almost always fatal and can wipe out entire family groups, with long-lasting demographic impacts. Ice entrapments could therefore slow Arctic killer whale range expansions, particularly in areas where killer whales that are unfamiliar with sea-ice patterns fail to exit prior to ice formation in winter.

Compare Hudson Bay weekly stage of development charts (below) for this year back to 2014, from the Canadian Ice Service archives. You’ll see that this year appears to have more extensive 1st year ice (light green, ca. 30-70 cm) than any other year (although last year had almost as much) and that 2016 was notable as being a very late freeze-up year:

Western Hudson Bay polar bears with collars or tags deployed by Andrew Derocher and his University of Alberta crew (below) are spread out over the ice of the bay (two on land are denning females), a number are on the thickest ice in the north but others are on thinner ice to the south and east:

BAFFIN BAY, FOXE BASIN, AND DAVIS STRAIT

Ice covereage in the Canadian Eastern Arctic at mid-December is about average this year, according to CIS charts – only a bit of red and pink indicating ‘below normal’ in the east (off of Greenland):

Pack ice has moved down from the north through Baffin Bay into Davis Strait (below) and will soon be off the coast of Labrador, which has somewhat less ice than usual this year at mid-December:

The ice off Labrador at this time is shorefast ice developing and thickening in place (below). As far as we know, few polar bears summer on the northern Labrador coast, so this late ice development is unlikely to affect local bears. However, pack ice will move down during January and February until it engulfs the area north of Newfoundland, bringing some polar bears with it.

GREENLAND AND BARENTS SEA

Freeze-up in the Greenland Sea is progressing a bit faster than usual for the last five years (below), but not remarkably so:

Ice cover in the Barents Sea (below) has been slow so far but has been progressing faster over the last few weeks. There is now ice off the east coast of Novaya Zemlya, shorefast ice that should allow any bears summering there to hunt for seals just as Western Hudson Bay bears do during early freeze-up stages. Within the next few weeks, the Arctic pack ice will move south into the Kara Sea, allowing bears to move more freely.Ice off Svalbard has been much below normal (below), as it has been for years now, which is why virtually all Barents Sea pregnant females currently make maternity dens in Franz Josef Land or on the sea ice to the north. These alternative areas for safely giving birth is the primary reason that the much reduced sea ice around Svalbard in recent years has not impacted Barents Sea polar bear health or survival.

KARA SEA

Ice cover in the Kara Sea at 15 December (below) is lower compared to the last five years but it’s unclear how much effect this will have on local polar bears.

Animals that have opted to spend the ice-free season on Novaya Zemlya or on the Russian mainland will have had a long wait for ice, but those that spent the summer on the Severnya Zemlya archipelago to the east had access to sea ice before the end of November. There has been no word from Belushaya Guba on whether the polar bear problems they had because of poorly maintained garbage dumps in December 2018 that went on until February 2019 have recurred this year.

Despite what Andrew Derocher claims (below), there is no evidence that slightly less sea ice in the fall is detrimental to polar bear health or survival in the Kara Sea or elsewhere. It is possible that it might but no one has studied it, so to suggest that low sea ice cover is ‘trouble’ for polar bears at this time of year is very misleading.

CHUKCHI/BERING SEA

Sea ice cover over the Chukchi Sea is a bit lower than it has been over the last few years, about as low as it was in 2017 (below).

Chukchi Sea polar bears at 14 December (below) had abundant sea ice habitat.

In 2016, when Chukchi polar bears were counted for the first time, there was a similar amount of ice at this time of year (below):

REFERENCES

Stirling, I. and Øritsland, N. A. 1995. Relationships between estimates of ringed seal (Phoca hispida) and polar bear (Ursus maritimus) populations in the Canadian Arctic. Canadian Journal of Fisheries and Aquatic Sciences 52: 2594 – 2612. http://www.nrcresearchpress.com/doi/abs/10.1139/f95-849#.VNep0y5v_gU