Following up on my previous post, it appears sea ice conditions on Hudson Bay this year might be headed for a late breakup due to the dominance of thick first year ice. That would mean a relatively longer on-ice season for polar bears in Western and Southern Hudson Bay.
As of the 1st week in May 2018, most of Hudson Bay, Foxe Basin and Hudson Strait were covered with thick 1st year ice (dark green, >1.2 m thick):
Compare that to the 1st week of May 2016, which had much less thick first year ice than 2018 and more medium first year ice (70-120cm, bright green):
To update the situation, at the end of May this year (week of 28 May), thick first year ice covered even more of the bay with a large patch of open water in the NW corner:
Thick first year ice does not melt as quickly as medium or thin first year ice (lime green) under most conditions, so the amount of thick first year ice present in May strongly affects the rate of breakup of the ice over the summer (temperature and wind also contribute). Here are some charts of ice melt sequences from the past (2016 and a couple others) that give a hint at what might be in store for Hudson Bay polar bears this year.
A recent example: 2016
By late June 2016 in the melt season, there was still lots of ice over the central bay area:
By early July 2016, much of the thick first year ice remained:
And by mid-July, there was still enough ice for some bears to remain offshore:
By early August, there was a little thick, first year ice close to shore but most bears were ashore and said to be in great shape:
A late breakup year: 2004
Before considering ice melt patterns for 2004, look again at the coverage this year at the middle of May (15 May):
Fourteen years ago, in 2004, virtually all of Hudson Bay, Foxe Basin and Hudson Strait were covered in thick first year ice at mid-May:
The melt sequence that followed is below (21 June, 7 July, 2 August):
2004 was indeed a relatively late breakup year for Western Hudson Bay, according to polar bear researchers (Lunn et al. 2016). The graph below from Lunn and colleagues shows breakup for 2004 at about 1 July, and bears would have left the ice 25-30 days later:
But as a more recent analysis (to 2015), using a different method and data source, shows not a steady decline of breakup dates but a marked step-change at 1995 for both breakup and freeze-up (Castro de la Guardia 2017), with bears coming ashore about two weeks later in the 2005-2015 period than they did in 1980-1989, about 20 days after the sea ice breakup date:
For previous perspectives on dates ashore at breakup in WH, see Cherry et al. (2013); Regehr et al. (2007); and Stirling and Parkinson (2006).
An early breakup year: 2011
Finally, compare the above to a truly early breakup year for Western Hudson Bay like 2011, when the 50% ice threshold was reached about a month earlier than 2004 (4 June), which means bears came ashore in late June.
That year, there was much less thick first year ice in Hudson Bay and Hudson Strait than in 2016:
By the third week in June, there was quite a bit of open water and the thick ice had moved around as thin ice broke up and melted:
By early July, virtually all that was left was thick first year ice in the middle of the bay:
By late July, the ice was essentially gone except for a patch of thick first year ice well offshore in Western Hudson Bay and bears had come ashore by XXXX:
Will this year’s breakup of Western Hudson Bay ice be similar to 2004? Or will it be more like 2016? Either way, the state of the ice at mid-May suggests it is unlikely to be an early year for polar bears ashore in western and southern Hudson Bay.
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://www.int-res.com/abstracts/meps/v564/p225-233/
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://onlinelibrary.wiley.com/doi/10.1111/1365-2656.12050/abstract
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
Regehr, E.V., Lunn, N.J., Amstrup, S.C., and Stirling, I. 2007. Effects of earlier sea ice breakup on survival and population size of polar bears in Western Hudson Bay. Journal of Wildlife Management71: 2673-2683. http://onlinelibrary.wiley.com/doi/10.2193/2006-180/abstract
Stirling, I. and Parkinson, C.L. 2006. Possible effects of climate warming on selected populations of polar bears (Ursus maritimus) in the Canadian Arctic. Arctic 59:261-275. http://arctic.synergiesprairies.ca/arctic/index.php/arctic/issue/view/16.
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