Breakup of sea ice on track in Canada as critical feeding period for polar bears ends
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Relative to recent years and potential impacts on polar bear health and survival in Canada, there is nothing out of the ordinary in the pattern or speed of sea ice breakup for 2017, either over Hudson Bay, the southern Beaufort, or the eastern high Arctic.
Last year at this time (see map below), there was more open water in Hudson Bay and in the Southern Beaufort yet the polar bears came ashore in fine shape that summer and there was no hue-and-cry of dying bears anywhere. Breakup this year is on track to be about 3 weeks earlier than it was in the 1980s, as it has been since at least 2001, a conclusion reached by polar bear specialists (Castro de la Guardia et al. 2017; Lunn et al. 2016), who examined sea ice breakup to 2015.
Here are critical words to remember (more details here) from biologist Martin Obbard and colleagues (2016:29) on the relationship between body condition and sea ice for Southern Hudson Bay (SH) polar bears, which apply equally well to bears in other regions:
“Date of freeze-up had a stronger influence on subsequent body condition than date of break-up in our study. Though models with date of freeze-up were supported over models with other ice covariates, we acknowledge that lower variability in freeze-up dates than in ice duration or break-up dates could have influenced the model selection process. Nevertheless, we suggest that a stronger effect of date of freeze-up may be because even though break-up has advanced by up to 3-4 weeks in portions of Hudson Bay it still occurs no earlier than late June or early July so does not yet interfere with opportunities to feed on neonate ringed seal pups that are born in March-April in eastern Hudson Bay (Chambellant 2010). Therefore, losing days or weeks of hunting opportunities during June and July deprives polar bears of the opportunity to feed on adult seals, but does not deprive them of the critical spring period (Watts and Hansen 1987) when they are truly hyperphagic. No doubt, the loss of hunting opportunities to kill adult seals has a negative effect on body condition, but it appears that for bears in SH a forced extension of the fast in late fall has a greater negative effect on subsequent body condition.” [my bold]
In other words, by mid-June at least (maybe earlier), polar bears have largely finished their intensive feeding that’s so critical to their survival over the rest of the year.
That’s why the latest count of SH polar bears (Obbard et al. 2015) showed a stable population (and see this recent post on WHB polar population estimates). But freeze-up was late last year and that’s what will make the difference to polar bears over the coming year.
Here’s the CIS graph for ice cover over the NW section of Hudson Bay (the area most relevant to Western Hudson Bay breakup dates), which shows levels only slightly below average in the first week of June in 2017, with more than 70% of the region ice-covered (more than 2016 at the same time and much more than 2015).
All that said, freeze-up over Hudson Bay was quite late last year (2016) — not the latest it’s ever been (the El Niño year of 1998, at December 16) but still late enough to be worrisome for the fate of polar bears in Western and Southern Hudson Bay who had been onshore since mid-to-late July. Which means that if ice and snow conditions over the bay were not optimal for polar bear hunting in early April especially, more bears than usual may have died over the winter and more bears than usual may not come ashore fat enough to get through the 2017 summer fast.
We’ll have to wait and see.
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
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
Obbard, M.E., Stapleton, S., Middel, K.R., Thibault, I., Brodeur, V. and Jutras, C. 2015. Estimating the abundance of the Southern Hudson Bay polar bear subpopulation with aerial surveys. Polar Biology 38:1713-1725.
Obbard, M.E., Cattet, M.R.I., Howe, E.J., Middel, K.R., Newton, E.J., Kolenosky, G.B., Abraham, K.F. and Greenwood, C.J. 2016. Trends in body condition in polar bears (Ursus maritimus) from the Southern Hudson Bay subpopulation in relation to changes in sea ice. Arctic Science, in press. 10.1139/AS-2015-0027
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June 8, 2017 at 04:44AM
