Sea ice, beluga whales, and polar bear densities in the Gulf of Boothia

As I discussed in my last post, the Gulf of Boothia subpopulation in the central Canadian Arctic has the highest density of polar bears anywhere in the world. The question is, why?

For example, is the sea ice in the Gulf of Boothia region so markedly different from its nearest subpopulation-neighbor, M’Clintock Channel (Fig. 1), that it accounts for the wide disparity in polar bear densities between the two? The differences, remember, are dramatic: Gulf of Boothia, 18.3 bears per 1000 km² vs. M’Clintock Channel, 1.9. And while M’Clintock Channel may be low in part due to recent over-harvests (see footnote 1), even the density before over-harvests occurred in M’Clintock Channel were only 4.7, compared to 10.4 bears per 1000 km² in Gulf of Boothia (see Table 1 in previous post).

Today, I’ll take a look at sea ice and ringed seal habitat in the Gulf of Boothia and M’Clintock Channel, as well as information from a study on polar bear diets, which together shine some light on why the Gulf of Boothia is such a great place for polar bears.

Figure 1. Map showing the side-by-side relationship of M’Clintock Channel and the Gulf of Boothia. From Barber and Iacozza (2004: Fig. 1).

Barber and Iacozza (2004) provide 21 years of data on sea ice and ringed seal habitat in the Gulf of Boothia and M’Clintock Channel (1980-2000). Their study primarily tracked two types of sea ice over time: “first-year ice” comprised all first year ice types (including thin first-year ice, medium first-year ice, and thick first-year ice), while “thick ice” included old ice, second-year ice and multiyear ice.

They had this to say about Gulf of Boothia (GB) sea ice:

“From 1987 to 1993, there was a regular reversal in the proportion of ice types between the first-year ice class and the thick ice class. After 1993, the first-year ice class dominated, covering more than 65% of the GB area each year.”

Figure 2. Proportion of ice types present within (a) M’Clintock Channel (left) and (b) Gulf of Boothia (right) from 1980 to 2000, derived from Canadian Sea Ice Service weekly ice charts, last week of March. Both are considered to be in the same sea ice ecoregion. Both had a mix of first year (filled circle) and multiyear ice (open square), although some years, the Gulf of Boothia had much more multiyear ice than M’Clintock Channel (e.g. 1980, 1988, 1993). After 1993, the mix of multiyear and first year ice was very similar, yet the two regions support very different densities of polar bears. This graph is Barber and Iacozza’s Fig. 2.

The “reversals” in proportion of ice types in the Gulf of Boothia referred to by Barber and Iacozza, which make the Gulf of Boothia ice history distinctive (see Fig. 2), occurred over a relatively short period – only 7 years (1987-1993). But the first population assessment was done prior to that period, in 1986, and the density calculated based on that estimate was the highest density of all regions examined at that time.

So, density was already high before the reversal of ice types occurred and despite them, the bear population in the Gulf of Boothia increased rapidly.

OK, ice thickness and variability differences don’t really help explain polar bear densities in Gulf of Boothia. What was happening with ringed seals, the primary prey of polar bears, while all this was going on?

Barber and Iacozza tracked what they called “sea ice characteristics indicative of ringed seal habitat” – not ringed seals themselves. As Taylor et al. (2009:792) put it:

“Barber and Iacozza (2004) found no trends in Gulf of Boothia sea ice conditions or ringed seal habitat suitability indices in the interval 1980-2000.”

OK then, it looks like the relative amounts of ice types preferred by ringed seals doesn’t help explain the high density of polar bears in Gulf of Boothia either. It might help if we had some data on ringed seal density differences between the Gulf of Boothia and M’Clintock Channel to help us decide, but it doesn’t look like that’s available.

However, there is some information on polar bear dietary diversity in Gulf of Boothia – how much of each kind of prey they consume. A study published by Gregory Thiemann and colleagues in 2008 examined the fat of polar bears in 10 Canadian subpopulations to see what they had been eating (deduced from the species-specific ‘fatty acids’ left in the fat). They found that Gulf of Boothia bears consumed mostly ringed seal (70%) but also bearded seal (20%) and beluga whale (10%), with small amounts of walrus and narwhal.

But that’s not the end of the story.

Compared to the nine other Canadian subpopulations these authors sampled, bears in Gulf of Boothia consumed the most walrus (although still not very much, compared to other species). In addition, Gulf of Boothia was also the only region (except Lancaster Sound, which lies just to the north), where there were no differences in prey consumed according to age or sex. That means all bears in the population were eating the same thing year round, which could be significant.

Ringed seal would be available year round (even over the winter), which almost certainly explains the high percentage of ringed seal in polar bear diets. The availability of other seal species would likely change with the seasons, however.

Brown and Fast (2012:23), for example, reported that bearded seals as well as ringed seals are known by local Inuit to use Gulf of Boothia as pupping grounds, making the fat pups of both species available to polar bears in the spring (March through June, see previous post here).

In addition, Thiemann et al. (2008:608) found that:

“Bears in the Gulf of Boothia shifted their diets from ringed seals in fall-winter to beluga whale in spring-summer (Fig. 10c). These bears also showed a slight seasonal increase in their consumption of narwhal, which summer in the area (Laidre and Heide-Jørgensen 2005).” [my bold]

So, after the critical seal pupping season is over, polar bears also have access to the beluga whales and narwhal that migrate into the gulf for the summer. Remember from Fig. 2 that there is still some ice left in the Gulf of Boothia most years.

Rapidly changing ice conditions in summer might trap groups of beluga and narwhal within small areas of open water, making it relatively easy for polar bears to kill them (see previous post here and a pertinent 2008 news story here, back up pdf here).

This might be it then: having easy access to beluga whales in summer might make the difference. Polar bears in the Gulf of Boothia have access to a particularly diverse mix of prey that includes fat-rich belugas in the summer, when the number of easily-hunted seal pups declines.

The fact that polar bears in the Gulf of Boothia have access to beluga whales and narwhals during the summer, along with a bit of ice to hunt them from, could be the answer to the conundrum of why bears in this region are doing so much better than others.  And while empirical evidence from a study designed to address this issue would be much more convincing, research seems to be focused elsewhere.

Footnote 1.: The M’Clintock Channel subpopulation declined from an estimated 700 bears in the mid-1970s to 284 ± 59.3 bears in the spring of 2000 (Obbard et al. 2010:50). Surprisingly, the numbers of bears hunted in the Gulf of Boothia and in M’Clintock Channel were very similar – about 40 bears/year each during the 1990s (Taylor et al. 2006). It seems odd that this number was too many for M’Clintock Channel but not for Gulf of Boothia. Perhaps there were other factors at work?

References
Barber, D.G. and Iacozza, J. 2004. Historical analysis of sea ice conditions in M’Clintock channel and the Gulf of Boothia, Nunavut: implications for ringed seal and polar bear habitat. Arctic 57:1-14. http://arctic.synergiesprairies.ca/arctic/index.php/arctic/article/view/478/508

Brown, L. and Fast, H. 2012. An overview of important ecological and biological marine features in Nunavut based on local knowledge. Canadian Manuscript Report of Fisheries and Aquatic Sciences 2976. Fisheries and Oceans Canada, Winnipeg. publications.gc.ca/collections/collection_2012/…/Fs97-4-2976-eng.pdf‎

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/

Taylor, M.K., Laake, J., McLoughlin, P.D., Cluff, H.D., and Messier, F. 2006. Demography parameters and harvest-explicit population viability analysis for polar bears in M’Clintock Channel, Nunavut, Canada. Journal of Wildlife Management 70:1667-1673. http://onlinelibrary.wiley.com/doi/10.2193/0022-541X%282006%2970%5B1667:DPAHPV%5D2.0.CO;2/abstract

Taylor, M.K., Laake, J., McLoughlin, P.D., Cluff, H.D., and Messier, F. 2009. Demography and population viability of polar bears in the Gulf of Boothia, Nunavut. Marine Mammal Science 25:778-796. http://onlinelibrary.wiley.com/doi/10.1111/j.1748-7692.2009.00302.x/abstract

Taylor, M., and Lee, J. 1995. Distribution and abundance of Canadian polar bear populations: a management perspective. Arctic 48:147-154. http://arctic.synergiesprairies.ca/arctic/index.php/arctic/article/view/1236/1261

Thiemann, G.W., Iverson, S.J., and Stirling, I. 2008. Polar bear diets and Arctic marine food webs: insights from fatty acid analysis. Ecological Monographs 78:591-613. http://www.esajournals.org/doi/abs/10.1890/07-1050.1