“The Arctic” is a bit hard to define. While the Arctic Circle works as a good boundary for some purposes and the 100C isotherm for July for others, neither work for polar bears because several subpopulations live well south of these limits (Fig. 1).
In the east, Western Hudson Bay, Southern Hudson Bay and Davis Strait are all located well south of the Arctic Circle and the first two (and half of Davis Strait) are beyond the 100C July isotherm as well. In the western Arctic, the Chukchi Sea subpopulation is within the 100C July isotherm but at least half of its bears reside south of the Arctic Circle (Fig. 1) in the Bering Sea (see previous post here).
Unique amongst all of these is Southern Hudson Bay – all of its polar bears make maternity dens and/or spend the summer south of 600N.
Southern Hudson Bay (SH) bears live in the Canadian provinces of Ontario, while Western Hudson Bay (WH) bears reside in Manitoba and Nunavut. The two groups mix over the winter but appear to spend the summer/fall in their respective regions (Stirling et al. 2004). [See previous posts on Western Hudson Bay bears here, here, and here]
“Further south” in the Arctic usually means warmer, with open water present more weeks every summer, sea ice for fewer weeks over the winter. So, shouldn’t the bears of Southern Hudson Bay be already suffering more harm from global warming than virtually all other subpopulations, including those in Western Hudson Bay?
After all, Western Hudson Bay bears appear to have experienced a statistically significant decline in numbers, among other effects (Regehr et al. 2007; Stirling and Derocher 2012) — surely Southern Hudson Bay bears are doing worse?
You’d think so, but they aren’t.
UPDATED October 28, 2014: Reference added, Obbard et al. 2013 (aerial survey results).
Southern Hudson Bay ice breakup and freeze-up
The primary reason that Southern Hudson Bay polar bears are not exposed to a longer ice-free period than Western Hudson Bay bears lies in the characteristics of ice formation (freeze-up) and melt (breakup) in Hudson Bay. These processes do not occur on a strictly north-south gradient as you might expect.
In 2003, sea ice breakup in SH was considered to be about 28.5 days (~4 weeks) earlier in the James Bay portion of the region than it was in 1971, and about 15-24 days (~2-3.5 weeks) earlier along the southwest shore of the subpopulation that borders the WH subpopulation (Fig. 2).
Obbard et al. (2006), summarizing polar research there, put it this way:
“…over the past 3 decades, break-up dates are occurring earlier by about 9.5 days per decade in northern James Bay and by between 5 and 8 days per decade along the southern Hudson Bay coast of Ontario.”
In their recent summary paper, Ian Stirling and Andrew Derocher (2012, see previous post here), say that breakup dates in Western Hudson Bay declined by about 3 weeks between 1979 and 2007 (data to 2011 was not supplied and data from 2005-2007 is unpublished).
In other words, there has been no appreciable difference in breakup dates between Western Hudson Bay and the southwest shore of the Southern Hudson Bay region over the previous 30 years (ending 2003), and breakup in James Bay occurred only about one week earlier than in Western Hudson Bay.
Have a look at Figs. 4 and 5 to see how breakup and freeze-up progressed in the spring of 2013 and the fall of 2012. These ice maps illustrate how the southwest shore, which lies within Southern Hudson Bay territory, can have a slightly later date of breakup than Western Hudson Bay to the north (when you’d think it would be earlier). The ice formation processes are largely east-west, not north-south.
Freeze-up in 2012 below
All of this means that, contrary to what you might expect, Southern Hudson Bay polar bears do not have a markedly longer on-shore period than bears in Western Hudson Bay.
OK, you might think, since the Western Hudson Bay subpopulation appears to be declining and displaying other negative effects of global warming, Southern Hudson Bay bears should be similarly affected. Nope, wrong again.
Status of Southern Hudson Bay polar bears
The Southern Hudson Bay subpopulation is one of the few regions considered to be ‘stable’ by the Polar Bear Specialist Group in their latest assessment (Obbard et al. 2010) – in other words, neither declining nor increasing.
[Update October 28, 2014: an aerial study conducted 2011 and 2012 (Obbard et al. 2013, added to references below, pdf provided) confirmed this conclusion]
The current estimate is 900-1000 bears, based on 2005 data, with about 70-110 of these denning and/or summering on the islands in southern-most James Bay (Obbard and Walton 2004 [ref. added July26, 2013]).
A comprehensive study of Southern Hudson Bay bears, completed in 2005 by PBSG member Martyn Obbard and colleagues (2007), reported the results this way:
“Abundance in the Southern Hudson Bay population was unchanged between two intensive capture-recapture periods, which were separated by almost 20 years (1984-86 vs. 2003-05). This was so despite the evidence for a decline of 22% in abundance for the neighbouring Western Hudson Bay population over roughly the same period (i.e., 1987-2004; Regehr et al. 2007).”
“It appears that changes in environmental factors such as sea ice distribution and duration…have not yet resulted in unambiguous changes in survival or to a consequent reduction in population size in the SH population to this date.” [my bold]
And in an earlier paper (published in 2006), that focused on body condition (relative fatness) specifically, Obbard and colleagues had this to say about their results:
“[body condition was] significantly poorer for bears captured from 2000 to 2005 than for bears captured from 1984 to 1986,”
…the significance of a trend toward poorer body condition over time is unknown.” [my bold]
So, no decline of the polar bear population in Southern Hudson Bay, even though the sea ice declined at least as much as it did in Western Hudson Bay. And a slight decline in body condition for SH bears but no decline in numbers, even though reported declines in body condition in WH were accompanied by a drop in population of 22%.
Polar bear biologists and activists hardly ever talk about this successful population living right next door to the one they claim is suffering so badly from the effects of global warming, even though ice conditions in both have changed in virtually identical fashion over the last 30 years. Weird, eh?
It also appears that the status of Southern Hudson Bay polar bears (decline in body condition, advance in breakup dates, no decline in population numbers) are much like those I described recently (here and here) for Davis Strait polar bears, which rarely get mentioned either. It would appear that the Southern Hudson Bay subpopulation is yet another example of how moderately declining (or highly variable) sea ice has not necessarily caused harm to polar bears.
Despite living under virtually identical conditions as the Western Hudson Bay subpopulation, touted by polar bear biologists and activists alike as the most severely affected by global warming, Southern Hudson Bay bears appear to be doing just fine.
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.
Obbard, M.E., Cattet, M.R.L., Moody, T., Walton, L.R., Potter, D., Inglis, J. and Chenier, C. 2006. Temporal trends in the body condition of southern Hudson Bay polar bears. Ontario Ministry of Natural Resources, Applied Research and Development Branch, Sault Ste, Marie, Canada. Climate Change Research Information Note 3. Available from http://sit.mnr.gov.on.ca
Obbard, M.E., McDonald, T.L., Howe, E.J., Regehr, E.V. and Richardson, E.S. 2007. Polar bear population status in southern Hudson Bay, Canada. Administrative Report, U.S. Department of the Interior- U.S. Geological Survey, Reston, VA.
Obbard, M.E., Middel, K.R., Stapleton, S., Thibault, I., Brodeur, V. and Jutras, C. 2013. Estimating abundance of the Southern Hudson Bay polar bears subpopulation using aerial surveys, 2011 and 2012. Ontario Ministry of Natural Resources, Wildlife Research and Monitoring Section, Science and Research Branch, Wildlife Research Series 2013-01. Peterborough, Ontario. Pdf here.
Obbard, M.E., Theimann, G.W., Peacock, E. and DeBryn, T.D. (eds.) 2010. Polar Bears: Proceedings of the 15th meeting of the Polar Bear Specialists Group IUCN/SSC, 29 June-3 July, 2009, Copenhagen, Denmark. Gland, Switzerland and Cambridge UK, IUCN. http://pbsg.npolar.no/en/meetings/
Obbard, M. E. and Walton, L.R. 2004. The importance of polar bear provincial park to the southern Hudson Bay polar bear population in the context of future climate change. Proceedings of the Parks Research Forum of Ontario (PRFO):105-116. [added July 26, 2013] pdf here.
Regehr, E.V., Lunn, N.J., Amstrup, S.C., and Stirling, I. 2007. Survival and population size of polar bears in western Hudson Bay in relation to earlier sea ice breakup. Journal of Wildlife Management 71: 2673-2683.
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://arctic.synergiesprairies.ca/arctic/index.php/arctic/article/view/479/509
Vongraven, D. and Peacock E. 2011. Development of a pan-Arctic monitoring plan for polar bears: background paper. Circumpolar Biodiversity Monitoring Programme, CAFF Monitoring Series Report No. 1, CAFF International Secretariat, Akureyri, Iceland. Available at http://www.caff.is/publications/