Barents Sea polar bears adapt to changing sea ice conditions

Sea ice development in eastern Svalbard this fall is lagging well behind the rest of the Arctic. Several polar bear researchers (e.g., Andrew Derocher here and here) have recently been raising the alarm that this might be devastating for Barents Sea females looking for suitable denning sites.

Barents Sea ice 2015 Dec 2_NIS_sm

The implication is that pregnant polar bear females are inflexible – so fixated on one location to give birth that they are unable or unwilling to choose an alternative if conditions preclude using their preferred location this year. However, the available facts do not support such a pessimistic attitude.

A recent interview with Norwegian polar bear biologist Jon Aars (7 December 2015 : Lack of ice causing major problems for female polar bears) suggests that few, if any, pregnant polar bear females will reach their preferred maternity den locations on the islands off the east coast this year.

The Google-translated article begins: “The polar bears are sleeping, but where? That is a mystery. A near-record absence of sea ice is destroying access to the traditionally important denning areas in Svalbard.

Around Svalbard, the date that sea ice surrounds traditional denning areas (like Kong Karls Land) determines whether or not females can reach them. Last year, there was lots of ice early in the fall and there were many females with cubs seen the following spring, and many bears were in excellent condition this summer.

Andersen et al 2012 w Derocher_Svalbard dens_fig 1

Polar bear denning areas found around Svalbard from 1972-2010, from Andersen et al. 2012.

That was not the case in 2013. That fall, the ice was about as late as this year and few new cubs were spotted in the traditional Svalbard denning areas (especially Kongsoya and Svenskoya in Kong Karls Land, see map above) the following spring, generating a number of doomsday-like comments from Dr. Aars (here and here).

Fast-forward to this year: the early December 2015 Svalbard Posten  interview with Aars continues (my bold):

“Ice charts for Svalbard show small narrows with open sea in all directions. Such conditions stretch across the archipelago to Kong Karls Land and Hopen, the most important denning areas for polar bears in the archipelago.

The deadline for pregnant females to hibernate is here. In about one month they will begin to give birth.

“When ice is as it is now, we are certain that there will be few bears that hibernate in these places,” said Jon Aars, a polar bear researcher at the Norwegian Polar Institute.

“At Hopen there are virtually no lairs because of the ice conditions now. By comparison, there can be more than 30 in good years.

“There will be almost none that come to Hopen if the ice is not settled by Dec. 1,” he said.”

“Polar bears are good at swimming, but it doesn’t seem like they’re doing it mainly to reach denning areas,” Aars said.

Where does a bear go if she can’t get to her planned den site?

That’s one of many unanswered questions surrounding the mysterious life of the bears, Aars said.

“We don’t know,” he said. “There are two possibilities. Either she goes and hibernates somewhere else, or she skips giving birth for one year. How much energy will she use looking for a worse [sic: better?] alternative?”

An opportunity exists for some to follow the ice edge eastward to Franz Josef Land and hibernate there, instead of pointing their snouts south toward solid ground in Svalbard. But data for such movement is too thin to conclude anything.

“Another question is how much we understand about the probability that there is good ice in and around those areas,” Aars said. “Why if a female bear, standing up on ice edge and realizing that there is little ice in the fjords of Svalbard, would she therefore choose to go to Franz Josef Land instead?”

Read the whole thing here.

See the IUCN Polar Bear Specialist Group map below showing Svalbard (which belongs to Norway) and Franz Josef Land (which belongs to Russia): Franz Josef Land almost always has ice before Svalbard in the fall (as is the case this year, see ice map above).

Barents Sea with Franz Josef Land

Hopen Island (so small it is not shown on the PBSG map) is the furthest south area in this region that is known to be used for denning and it is not unexpected that it is only used when sea ice conditions are ideal.

However, Derocher and colleagues (2011: 277) noted in a study of denning on Hopen that only two dens were recorded there between 1900 and 1950 even though sea ice conditions were presumably more extensive and reliable. The authors suggest this may have been because the polar bear population was so low prior to 1950 (“related to the difference in the number of adult females in the Barents Sea polar bear population“).

If that’s the case, it means that extensive denning on Hopen is not only a function of sea ice conditions but also of the size and local density of the pregnant female portion of the polar bear population. In other words, Hopen Island is not used unless other denning areas in eastern Svalbard are densely occupied by other females and the sea ice reaches that far south early enough in the fall for pregnant females to reach it. That suggests Hopen Island is not an optimal denning location by any definition of the term and that has probably always been true.

In contrast, Franz Josef Land is an excellent alternative denning area.  There are a lot of islands scattered across an area of shallow continental shelf that is no doubt attractive to ringed and bearded seals. Previous research (Mauritzen et al. 2002) showed that bears that den in eastern Svalbard also use the area around Franz Josef Land during other parts of the year.

In fact, Mauritzen and colleagues concluded (pg. 87) that

…our results suggest no sharp population boundaries between Svalbard and the Barents [southern Svalbard islands plus western Franz Josef Land islands] and Kara Seas [eastern Franz Josef Land islands and islands to the east], indicating one continuous population within this area.” [my bold]

In an earlier paper by some of the same authors, Mauritzen and colleagues (2001) distinguished between ‘nearshore’ females and ‘pelagic’ females. They implied that pelagic females (who were captured on the ice in spring and spent the entire year on the ice) made their dens on the sea ice, although they did not say so explicitly.

In addition, a more recent study of Svalbard area polar bear dens (Andersen et al. 2012; see map above) could not rule out the possibility that some females made their dens on the sea ice. The authors also stated (pg. 503):

Telemetry also revealed that some polar bears caught in Svalbard or in the central Barents Sea den in the Franz Josef Land archipelago in Russia.

As I reported last year (April 2014), Geoff York (now with Polar Bears International), who led a team of WWF personnel that worked with Norwegian Polar Institute researchers on Svalbard just as females should have been emerging from their maternity dens (after the late fall ice advance mentioned above), had this to say (my bold):

There is some evidence that the Svalbard population is moving away from traditional denning sites on the Norwegian islands. The bears need to be close to sea ice to hunt when they emerge from their dens. One possibility is that they are moving further east [to Franz Josef Land] where the ice survives longer.”

So, despite the assertion that Svalbard area females always use the same den locations year after year, it is clear that this is not always what happens – it is in fact well known that Svalbard area females are willing and able to shift to Franz Josef Land to make their dens, especially if sea ice conditions change. The species would never have survived the dramatic sea ice changes over the last 30,000 years if all polar bear females were as inflexible in their denning preferences as comments by Aars and Derocher imply.

Is it a catastrophe if females den in Franz Josef Land rather than eastern Svalbard? It does not seem so to me – instead, it seems like a comforting confirmation that polar bears  are flexible enough to survive these relatively small changes in distribution and timing of Arctic sea ice formation.

However, it is also clear we won’t know precisely how many bears move back and forth between Svalbard and Franz Josef Land in relation to recent sea ice changes without full cooperation and input from Russian authorities and Russian biologists. This summer (2015) that lack of cooperation meant Norwegian biologists couldn’t survey bears in Franz Josef Land to get a total count for the entire Barents Sea region (see reports here and here).

)      Ei binne er immobilisert og får påført et satellitthalsbånd. En 7 måneder gammel unge ser på

Magnus Andersen and Jon Aars [summer 2015] observe a female polar bear and her cub waiting for the tranquilliser to have its full effect so that they can start work taking samples from the bear safely.

References
Andersen, M., Derocher, A.E., Wiig, Ø., and Aars, J. 2012. Polar bear (Ursus maritimus) maternity den distribution in Svalbard, Norway. Polar Biology 35:499-508.

Derocher, A.E., Andersen, M., Wiig, Ø., Aars, J., Hansen, E. and  Biuw, M. 2011. Sea ice and polar bear den ecology at Hopen Island, Svalbard. Marine Ecology Progress Series 441:273-279. Open access.

Mauritzen, M., Derocher, A.E. and Wiig, Ø. 2001. Space-use strategies of female polar bears in a dynamic sea ice habitat. Canadian Journal of Zoology 79:1704-1713. http://www.nrcresearchpress.com/doi/abs/10.1139/z01-126#.U0spVlda9lo

Mauritzen, M., Derocher, A.E., Wiig, Ø., Belikov, S.E., Boltunov, A.N., Hansen, E. and Garner, G.W. 2002. Using satellite telemetry to define spatial population structure in polar bears in the Norwegian and western Russian Arctic. Journal of Applied Ecology 39:79-90.

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