Tracking Western Alaskan polar bears in the Beaufort in Sept – all 3 chose Canada

All three polar bear females tagged in the Southern Beaufort Sea far west of Kaktovik (near Barrow) spent all or most of September onshore in the Northern Beaufort area of Canada.

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Original caption: “Movements of 3 satellite-tagged polar bears for the month of September, 2016. Polar bears were tagged in 2016 on the spring-time sea ice of the southern Beaufort Sea. All 3 of these bears have satellite collar transmitters. Polar bear satellite telemetry data are shown with AMSR2 remotely-sensed ice coverage from 29 September, 2016. ” [Note that the circles with the polar bear icons are the end points (final positions for the month), while the tail ends of the strings are the positions at the first of the month]. Original image here.

We won’t know for sure until November whether these females are just taking refuge onshore in the Northern Beaufort (two on northwestern Banks Island and one on Prince Patrick Island) or if they are pregnant and making ready to prepare their maternity dens for the winter.

However, it is possible that some (or all) of these three will stay in the north for the winter, which would put them in a totally different subpopulation than they were in when captured near Barrow. It shows how ‘leaky’ subpopulation boundaries can be.

Such movement may explain (in part), why numbers in the Northern Beaufort in the past appeared to fluctuate. In other words, there may be more movement between the western Southern Beaufort and the northern portion of the Northern Beaufort (depending on ice conditions and availability of Arctic seal prey) than has been documented previously. For perspective on how this might affect population counts, see Jim Steele’s guest post How science count bears.

Previous research has shown (summarized here, with references) that Chukchi Sea and SB bears in the west (around Barrow) mix rather freely, and similarly that SB bears around Tuktoyaktuk in the eastern SB mix with Northern Beaufort bears. In 2014, the SB/NB boundary was moved west by Environment Canada to account for this movement.

However, the USGS tracking data this summer are showing bears from around Barrow in western Alaska ending up in the northern portion of the Northern Beaufort – which appears to be new phenomenon, since bears captured near Barrow are more apt to move into the Chukchi Sea (Amstrup et al. 2001, 2005). ec_polarbearstatusmapcanada_oct-26-2014_direct

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Movement of Alaskan Southern Beaufort bears into the Northern Beaufort this summer cannot be plausibly blamed on reduced summer ice, since ice conditions have been remarkably similar in this region since 2007. Tracking information of collared Southern Beaufort bears has been made available starting in 2010, yet since then only one bear has previously chosen to summer on land in the Northern Beaufort (in 2013).

September 2010

August/September 2011

September 2012 and October 2012

September 2013  (one bear on Banks Island)

September 2014

September 2015

August 2016

Perhaps this is not significant in the larger scheme of things – it may just be a one-off event. But it could be another indication that more bears exist in this region than have previously been assumed, generating some competition for space. Time will tell.

References

Amstrup, S. C., McDonald, T. L. and Stirling, I. 2001. Polar bears in the Beaufort Sea: A 30-year mark-recapture case history. Journal of Agricultural, Biological, and Environmental Statistics 6:221-234. http://link.springer.com/article/10.1198/108571101750524562

Amstrup, S.C., Durner, G. M., Stirling, I. and McDonald, T. L. 2005. Allocating harvests among polar bear stocks in the Beaufort Sea. Arctic 58:247-259. http://arctic.journalhosting.ucalgary.ca/arctic/index.php/arctic/article/view/426
Pdf here.

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