Tracking polar bears in the Beaufort Sea in April 2016 and early polynya formation

putting_collar_on_polar_bear_slider_USGS

Here’s the update on the polar bears fitted with satellite collars or ear tags in the Beaufort by USGS biologists over the last two years. Five new bears were added last month, which means there are now thirteen bears being tracked. Ice conditions are somewhat different than they have been in the past but concluding that such a situation means trouble is premature, I think (see here).

Beaufort tracking USGS bear-movements-April 2016 sm

Caption from USGS: Movements of 13 satellite-tagged polar bears for the month of April, 2016. Polar bears were tagged in 2015 and 2016 on the spring-time sea ice of the southern Beaufort Sea. Eight of these bears have satellite collar transmitters and 5 of these bears have glue-on satellite transmitters. Polar bear satellite telemetry data are shown with MODIS imagery from 22 April, 2016. See original high resolution image here and close-up below. More up-to-date sea ice images are also below.

There is no indication this year of the thick heavy sea ice that has dogged Beaufort Sea bears in the recent past and that’s a good thing, because those think ice years could be spectacularly deadly (see previous post Biggest threat to polar bears reconsidered).

For example, in the mid-1970s and mid-2000s, polar bear numbers declined by close to 50% because there were simply not enough newborn ringed and bearded seal pups for polar bears to eat – the thick spring ice conditions drove the seals offshore or out of the region altogether.

This year the spring ice in the Beaufort is rather fractured, with the usual polynyas opening a bit earlier than usual and extending a bit further west (see previous posts here and here). My sense is that this is a good thing because open water is what seals seek at this time of year and areas where many seals congregate are good hunting grounds for polar bears (Figs. 1 and 3).

Beaufort tracking USGS bear-movements-April 2016 lg closeup

Figure 1. Close-up of Beaufort Sea ice at 22 April 2016 and USGS tagged polar bear locations. Interesting that yellow bear (upper left) is heading west into the Chukchi Sea after denning on the sea ice over the winter. See last month’s map here.

From last year (Fig. 2), comparison of polynya development recorded in 1975 vs. 2015 at mid-May (which expand as spring progresses).

Bathurst and W Beaufort polynyas_1975 vs 2015_PolarBearScience

Figure 2. West Barrow polynya at 17 May 1975 (Smith and Rigby 1981: Fig. 15c) and the polynya this year at 14 May (Naval Research Laboratory). Click to enlarge.

As you can see (Fig. 3, below), the open water in the eastern Beaufort this year was quite a bit more extensive by the first week of May than it usually is a week or so later (Fig. 2):

Sea ice extent Canada 2016 May 8 CIS

However, there is no evidence that a bit more open water than usual at this time of year is a bad thing for polar bears – there is still lots of hunting habitat available. The notion that more open water means more “dangerous swims” for polar bears at this time of year is really rather ludicrous. It turns out the only firm data biologists have about swimming bears in summer comes from one bear (that happened to undertake the longest swim) and she lost less weight during a combined swim/walk over ice than she would have done if she’d stayed onshore (see previous post here). The hype on this topic is over-shadowing the facts.

As I noted last year about open water in spring:

“…here is what marine mammal biologists Ian Stirling and colleagues had to say about polar bears and the Cape Bathurst polynya in spring (Stirling et al. 1981:49):

Polar bears prey mainly upon ringed seals and, to a lesser degree, on bearded seals. Polar bears appear to be more abundant in polynya areas and along shoreleads, probably because the densities of seals are greater and they are more assessable. For example, between March and June in the Beaufort Sea from 1971 through 1975, 87% of the sightings of polar bears were made adjacent to floe edges or in unstable areas of 9/10 or 10/10 ice cover with intermittent patches of young ice.” [my bold]

Later, they discussed why these areas of open water can be so important in the Southern Beaufort area (Stirling et al. 1981:54):

“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.”[my bold]

In other words, Stirling and colleagues suggested in 1981 that the marked declines in ringed seal, bearded seal and polar bear numbers in the mid-1970s (discussed many times on this blog) was due primarily to the fact that the Cape Bathurst polynya did not develop as usual because thick ice conditions prevented it.

Polynya References
Dunbar, M.J. 1981. Physical causes and biological significance of polynyas and other open water in sea ice. In: Polynyas in the Canadian Arctic, Stirling, I. and Cleator, H. (eds), pg. 29-43. Canadian Wildlife Service, Occasional Paper No. 45. Ottawa.

Stirling, I. and Cleator, H. (eds). 1981. Polynyas in the Canadian Arctic. Canadian Wildlife Service, Occasional Paper No. 45. Ottawa.

Smith, M. and Rigby, B. 1981. Distribution of polynyas in the Canadian Arctic. In: Polynyas in the Canadian Arctic, Stirling, I. and Cleator, H. (eds), pg. 7-28. Canadian Wildlife Service, Occasional Paper No. 45. Ottawa.

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 pertinent excerpts of above papers here.

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