Summer sea ice melt and polar bear maternity dens

The yearly sea ice minimum extent is almost upon us, which has recently been the seasonal signal for excitable biologists and their activist groupies to resume their breathless rants about what sea ice loss could mean for polar bears.

Never mind that the summer minimum extent reached in September, no matter how low it goes, is pretty much irrelevant to polar bear health and survival. As I’ve discussed before, what’s really important is the presence of not-too-thick ice during the spring, so they can catch lots of young seals and put on lots of fat.

But to a lesser degree, the extent at mid-to-late summer is important because this is when pregnant females that prefer to make their maternity dens on shore are looking for good places to spend the winter.

So the topic for today is this: how much does the extent of ice at the height of summer dictate where polar bear females make their winter dens?

Primary denning areas around the Arctic are marked below on a map showing ice extent on 9 August 2014 (Fig. 1). This shows that more than half of all such regions still had some sea ice associated with them on that date, including Western Hudson Bay and the Southern Beaufort.

Figure 1. Ice extent at 9 August 2014 in relation to major known polar bear denning areas around the Arctic (Obbard et al. 2010). Less than 50% of all such den habitat was without associated sea ice at this time (blue circles). Note that most pregnant females in the Southern Beaufort region (Alaska) are known to den on the sea ice rather than on land, as do a fairly number of Barents Sea bears. Major areas in dark magenta/blue, minor areas in pale pink/light blue. Map courtesy MASIE.

The presence of ice at this time is relevant because in some areas, it dictates where and when females go ashore. Note that bears are good swimmers and can paddle to land from off-shore ice but keep in mind that they don’t always have to go ashore. Most bears in the Southern Beaufort (and probably many in Russia as well), for example, simply stay with the ice as it retreats.

Some pregnant females do head to shore in the Southern Beaufort to prepare dens for the winter, but most den out on the ice. Recall from last year (Fig. 2, from last August), that by September, four females captured on land earlier in the year by USGS biologists were on shore but four were still on the ice (see September post here).

That fifty-fifty ratio isn’t necessarily representative however, and it may change year to year. One study (discussed here) found that less than 5% of Southern Beaufort bears spent the ice-free period (September/October) on shore.

Figure 2. Movements of 8 satellite-tagged polar bears for the month of August, 2013 in the Southern Beaufort. Kaktovik is just to the right of Prudhoe Bay. Out of eight females with collars, five were on the ice and only three were on shore. All were on the ice in July. More details here and here.

Barents Sea bears also have this choice: some den on land around the archipelago but some make their dens out on the ice.

If pregnant Barents Sea females can’t get to the denning area they prefer (many tend to den in the same area year after year), they go somewhere else — as documented in a recent study discussed here. That kind of resilience and adaptability is what has made polar bears so successful over the long term in this ever-changing habitat.

The extent of ice still present at 9 August this year means that the yearly summer fast and the search for onshore winter dens began around this time for most polar bears across the Arctic. For others — like those in Baffin Bay (off northwest Greenland) and along the coast of Labrador — it began a few weeks earlier (Fig. 3).

Figure 3. Sea ice extent for Canada on 7 July 2014. Note the dwindling ice in Baffin Bay off northwest Greenland and along the Labrador coast (the approaches to Hudson Bay). Courtesy Canadian Ice Service.

To put this year’s late summer polar bear habitat into perspective, note that yesterday, the National Snow and Ice Data Center (NSIDC) issued it’s prediction for the lowest extent of the year (about a month away), which you might be surprised to find is nothing like the “Arctic Ice Death Spiral“ predicted so loudly a few years ago by the agency’s Mark Serreze.

This year, says the NSIDC:

“As of mid-August, sea ice extent remains on track to end up somewhere between the sixth and the tenth lowest sea ice minimum….Most likely this year’s minimum will be between 5.0 and 5.5 million square kilometers (1.9 and 2.1 million square miles).” [my bold]

[Last year’s prediction here]

To put it another way (in words that are less negative), the extent of sea ice come September is likely to be as high as it was in 2005 (5.31 m sq. km) and nowhere near the record-breaking low we had in 2012 (3.36 m. sq. km). Not that it really matters to polar bears, but that’s what’s happening in their world.

References
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/