Polar bears of W Hudson Bay utilizing a substantial patch of thick first year ice
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There’s been no word as yet, either from tour operators or polar bear researchers, that Western Hudson Bay polar bears have come ashore for the summer/fall season. Andrew Derocher reported at the end of June that the bears tagged by his team were still on the ice and as I write this, has not yet reported them ashore.
That pattern is consistent with the presence of thick ice along the west coast of the Bay — from well north of Churchill to the south of Wapusk National Park — for the last few weeks. The weekly chart for 3 July 2017 below from the Canadian Ice Service shows that virtually all of the ice remaining is thick first year ice (>1.2m, dark green on this map):
By the 9 July, the extent of this patch of ice was somewhat reduced but still a very prominent feature over the Bay, suggesting that if adult seals are using this ice as a refuge while molting, some bears may still be attempting to hunt even though their success rate may not be very high:
Bottom line: As has been the pattern for more than a decade, 2017 will not go down in history as an especially early year for WHB polar bears coming off the ice for the summer/fall season but instead may be as late as last year, when lots of bears were reported off the ice by mid-July at Seal River (just north of Churchill), all in excellent condition.
It remains to be seen if the condition of bears will be as good this year as they were in 2016, given the late start to the season. But it does mean that the lack of trend in breakup dates since 2001 continues: breakup of the sea ice in WHB since 2001 has been about one week later than it was before 1998 (Castro de la Guardia 2017; Cherry et al. 2013; Lunn et al. 2016).
If some polar bear struggle to survive this year it will be due to the late freeze-up date last fall combined with challenging winter conditions over Hudson Bay, not because of an early breakup of the sea ice.
And while it is certainly true that the overall trend in time spent onshore by WHB polar bears since 1979 has increased by about three weeks, the lack of a continued trend since 2001 is not what was expected or predicted, especially given the marked decline in global sea ice levels that have made headlines since 2007 (Crockford 2017), and the predictions of how devastating such low levels of ice would be to polar bears in areas like Hudson Bay that have to deal with a total disappearance of sea ice in summer and early fall.
Stirling et al. (2004), copied below, illustrated what he and other polar bear specialists consider ‘normal’ sea ice breakup patterns for on Hudson Bay (1971-2000), with ice remaining north of Churchill into mid-July (medium grey) but retreating to the southwest coast by late July (dark grey) — WHB bears generally leave the ice in areas V through III (Cherry et al. 2013), then migrate north along the coast in late fall, if necessary, to meet the newly-forming ice:
As of 7 July this year, according to CIS — the end of the first week of July — the ice strongly resembled Stirling et al.’s ‘normal’ for mid-July (medium grey in above figure), keeping in mind that more ice is generally present than satellites can discern because they have trouble distinguishing between open water and melt-water ponds on top of ice:
As of this morning (10 July 2017, below), that swath of ice has diminished some but not disappeared:
Sea ice conditions in the 2nd week of July this year on Hudson Bay are most similar to those in 2014, below (10 July 2014):
Last year (10 July 2016) there was much more ice over all at this date and bears came ashore in excellent condition:
And in 2015, the extensive ice was distributed further south and east and most polar bears stayed on the ice as long as possible:
References
Castro de la Guardia, L., Myers, P.G., Derocher, A.E., Lunn, N.J., Terwisscha van Scheltinga, A.D. 2017. Sea ice cycle in western Hudson Bay, Canada, from a polar bear perspective. Marine Ecology Progress Series 564: 225–233. http://ift.tt/2lruEp5
Cherry, S.G., Derocher, A.E., Thiemann, G.W., Lunn, N.J. 2013. Migration phenology and seasonal fidelity of an Arctic marine predator in relation to sea ice dynamics. Journal of Animal Ecology 82:912-921. http://ift.tt/ZcUKVG
Crockford, S.J. 2017. Testing the hypothesis that routine sea ice coverage of 3-5 mkm2 results in a greater than 30% decline in population size of polar bears (Ursus maritimus). PeerJ Preprints 19 January 2017. Doi: 10.7287/peerj.preprints.2737v1 Open access. http://ift.tt/2jB2S8i
Lunn, N.J., Servanty, S., Regehr, E.V., Converse, S.J., Richardson, E. and Stirling, I. 2016. Demography of an apex predator at the edge of its range – impacts of changing sea ice on polar bears in Hudson Bay. Ecological Applications, in press. DOI: 10.1890/15-1256
Stirling, I., Lunn, N.J., Iacozza, J., Elliott, C., and Obbard, M. 2004. Polar bear distribution and abundance on the southwestern Hudson Bay coast during open water season, in relation to population trends and annual ice patterns. Arctic 57:15-26. http://ift.tt/2t5LGIJ
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July 10, 2017 at 10:56AM
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