Labrador polar bears face a longer ice-free season than Hudson Bay bears, but do well

Pretty typical ice levels in both regions for this time of year – Davis Strait polar bears (especially those in Labrador) are still onshore while Hudson Bay bears (even those in the south) have their sea ice hunting platform back.

Davis Strait Hudson Bay freeze-up at Nov 25 2014_PolarBearScience

Funny thing is, the Davis Strait subpopulation may still be increasing despite a longer ice-free season than Western Hudson Bay. And bears in the south of that region – who spend the summer onshore in Labrador – have the longest ice-free season of all1 yet according to the latest survey they were even doing better than bears in northern Davis Strait.

That apparent paradox has an easy explanation – sea ice extent in late summer/early fall (length of the ice-free season) has much less of an impact on polar bear health and abundance than the state of the food supply in the spring. More seals in spring, polar bears do well; few seals in spring, polar bears starve.

Harp seal female with nursing pup, DFO Canada.

Harp seal female with nursing pup, DFO Canada.

It turns out that having more seal pups to eat in spring (in this case, harp seal pups) has kept Labrador polar bears in good condition and reproducing well despite a longer ice-free season – exactly the same reason that Chukchi Sea bears are doing well (in that case, with lots of ringed seals).

The harp seal population in the northwest Atlantic is the highest it’s been since the mid-19th century (DFO 2012), with a sharp increase since 2007. The main pupping region for harp seals is off southern Labrador (see DFO map below), which puts newborn and newly-weaned pups in easy reach of southern Davis Strait polar bears.

Harp seals Canada DFO 2012 Fig1

DFO 2012 map showing pupping locations of harp seals off Labrador.

 

Said Peacock and colleagues (2013) in the most recent Davis Strait polar bear study:

Survival and reproduction of bears in southern Davis Strait was greater than in the north and tied to a concurrent dramatic increase in breeding harp seals (Pagophilus groenlandicus) in Labrador.”

Litter size differed significantly among all sub-regions in Davis Strait. In southern Davis Strait, litter size of COY (1.71) was greater, but not statistically, than in the other sub-regions (1.33 in northern Davis Strait), possibly because of the greater availability of harp seals in the south.” [my bold]

And have a look (below) at what the ice conditions off Labrador looked like last spring, when polar bears were out hunting seal pups: the ice was at least 10% higher than the 30 year average last spring in Davis Strait.

Sea ice extent Canada 2014 April 1_CIS

Footnote 1. As Crane 1978 pointed out in his study of sea ice in Davis Strait between 1964 and 1974:

On average, sea ice is present in this area for up to 40 weeks of the year in the Davis Strait and for 30 weeks off the northern Labrador coast.”

In all years the Labrador coast is generally the first area to be clear of ice. In the south there is open water by 25 June but along the northern Labrador coast there can be a three week delay in clearance in late ice years.

Early ice years, on the other hand, tend to be characterised by ice spreading south in Davis Strait and east along Hudson Strait from Foxe Basin. In both cases and Labrador coast is generally the last to freeze.” [my bold]

References
Crane, R.G. 1978. Seasonal variations of sea ice extent in the Davis Strait-Labrador Sea area and relationships with synoptic-scale atmospheric circulation. Arctic 31:434-447. http://arctic.journalhosting.ucalgary.ca/arctic/index.php/arctic/article/view/2671 Pdf here.

DFO. 2012. Current Status of Northwest Atlantic Harp Seals, (Pagophilus groenlandicus). Department of Fisheries and Oceans Canada Science Advisory Secretariat Science Advisory Report. 2011/070.

Peacock, E., Taylor, M.K., Laake, J., and Stirling, I. 2013. Population ecology of polar bears in Davis Strait, Canada and Greenland. Journal of Wildlife Management 77:463–476. http://onlinelibrary.wiley.com/doi/10.1002/jwmg.489/abstract?deniedAccessCustomisedMessage=&userIsAuthenticated=false

Comments are closed.