We hear endlessly about the polar bears ‘forced’ to go without food for months because of receding summer sea ice — what about all the bears that stay out on the ice over the summer? Presumably, those bears keep hunting for seals – but how many do they actually catch?
[Update 9 February 2015: Just to be clear, this post is based on the facts available in the peer reviewed literature — if you think I have missed something, let me know via the “Contact us” page above]
As it turns out, not very many – and for some unlucky bears in late summer, probably none. While they probably eat a bit better in late fall, if they’re lucky and persistent, by the time winter comes, biologists assume most bears again eat very little. This explains why all polar bears are at their lowest weight in late winter (March), just before Arctic seal pups are born.
[Winter – January to March; Spring – April to June; Summer – July to September; Fall – October to December (e.g. Pilfold et al. 2015, in press)]
To put it another way, the reason that polar bears in some areas easily survive an onshore fast of 4 months or more over the late summer/early fall is that they would get very little to eat (if anything) even if they stayed out on the ice. It’s the fat put on in spring/early summer (from gorging on baby seals) that carries them over the summer, no matter where they spend it.
So why do some polar bear biologists and others (e.g. Stirling and Derocher 2012) keep insisting that if late summer sea ice retreats even more in the future than it has in recent years, polar bear survival will be seriously compromised because they won’t be able to eat? For example, Morrison and Kay (2014) made the following astonishing claim last September on the Polar Bears International website, apparently approved by PBI’s ‘chief scientist’ Dr. Steve Amstrup:
“Summer is the limiting season because polar bears depend on sea ice as a platform to hunt seals. With summer and early fall sea ice losses, polar bears have less time on the ice, eat less, and become skinnier.”
Really? Take a look at the facts.
The importance of spring seals
Hammill and Smith (1991:132), studying polar bear predation on ringed seals in the Lancaster Sound region of the central Canadian Arctic (Barrow Strait, see Fig. 1 below), had this to say:
“Prior to late April, we found little evidence of predation in Barrow Strait. In late spring, polar bears enter a period of intense feeding (Stirling and McEwan 1975, Ramsay and Stirling 1988), which begins with the onset of the ringed seal pupping season. …Feeding on young seals continues throughout the spring and early summer as bears replenish depleted fat reserves. After ice breakup, bears move ashore and begin another period of little feeding (Stirling and McEwan 1975, Ramsay and Stirling 1988).” [my bold]
OK, but don’t polar bears keep hunting during the late summer and early fall in areas where sea ice is available, like the Beaufort, Chukchi and Barents Seas? Indeed they do but it appears that polar bears spend less time hunting, and are often far less successful, than they are in spring. 1
Evidence for rates of successful summer hunting
Ian Stirling (1974) reported his summer observations of polar bears from a high observation tower available on southern Devon Island (near Barrow Strait) in the central Canadian Arctic between 24 July and 8 August (Fig. 1). He noted that only five seals were caught during 602.7 hours of bear observations – and that was in 1973, before summer sea ice declines were an issue.
Stirling attributed this “low success rate” to the fact that there are a large number of natural holes in the ice during the summer, which offers seals more escape routes. Stirling noted that the bears he observed spent about 30% of their time sleeping, 12% just lying around, 25% walking and 4% swimming (usually without any apparent purpose) – compared to spending just 2% of their time feeding.
Ironically, that’s slightly less time feeding than Southern Hudson Bay polar bears spent during their time onshore during the summer! A study of James Bay bears in 1969 and 1970 found they spent 3.2% of their time feeding on geese and berries (Knudsen 1978).
Stirling and a colleague later added more feeding frequency data to Stirling’s 1973 observations, gathered from the same area in 1974 and 1978. Stirling and Øritsland (1995) used that data to create a generalized, month-by-month estimate of the number of ringed seals killed per day of polar bear hunting effort (Fig. 2, below). In this analysis, bears out on the pack ice were assumed to kill only one seal per month over the late fall/winter months of December through February and two per month in the late fall (October/November). They explained:
“A monthly kill of zero was set for August and September because open water prevails in most areas, although we recognize that bears on drifting ice still kill some seals. There are no data on the rates at which polar bears kill seals in the late fall after freeze-up when young annual ice is widespread and seals become accessible again. However, from observations in the Beaufort Sea and on the western coast of Hudson Bay, where polar bears summer on the multiyear pack ice and land, respectively, the bears move onto the young annual ice to hunt as soon as possible after freeze-up and the remains of seals killed there by bears have been observed (Stirling 1974b; Latour 1981; Derocher and Stirling 1990; 1. Stirling, unpublished data). Although this suggests that fall feeding during freeze-up could be important, in the absence of data we conservatively set the kill rate at 15 days of hunting per seal killed per bear. There are no data for the winter months, so an arbitrary rate of one seal kill per bear per month was assigned.” [my bold]
Another study provided no information on rates of polar bear feeding episodes but did report month-by-month evidence of seal kills found on the ice between 1984 and 2001 in the Svalbard/Barents Sea area (March to October). Derocher and colleagues (2002:449) recorded only 24 seal kills (various species, including ringed, bearded and harp seals) in August (the only summer month reported), less than half of the 58 kills recorded in April (highest kills for a spring month); only one kill was recorded in October. So, while it is clear that some bears were eating over the summer in the Barents Sea, we don’t have any idea how many bears were responsible for the seal kills found on the ice.
Finally, confirmation that this pattern of limited hunting success from summer through winter is considered typical of all polar bear populations comes from a just-released paper on polar bear feeding by Nicholas Pilfold and colleagues Andrew Derocher, Ian Stirling and Evan Richardson (Pilfold et al. 2015 in press).
These authors suggest that the feeding budget presented by Stirling and Øritsland in 1995 (Fig. 2) is still true now, even for the eastern Beaufort Sea bears they studied:
“Polar bears are hyperphagic in spring and may acquire as much as two-thirds of the energy requirement for the year, providing fat reserves for survival through periods of low prey access during the summer melt season (Stirling and Øritsland 1995).” [my bold]
Two-thirds of a year’s food supply consumed over three months (April – June) is the same as eight twelfths (2/3 = 8/12). That means 4 months worth of food is generally consumed over the remaining 9 months of the year. That’s not very much per month, spread over summer, fall and winter — especially since most of it is consumed during late fall2.
Some polar bear biologists, and others, continue to claim that future declines in summer sea ice coverage are a serious threat to polar bear survival because it reduces their access to their prey (e.g., Stirling and Derocher 2012). However, observations made well before summer ice declines became dramatic3, polar bears were rarely successful at seal hunting in summer because it was so much easier for seals to escape.
This means that even under the best of conditions, bears spending summers on the sea ice do not eat very often and some probably don’t eat at all. Even for bears that achieve a limited degree of hunting success, this reduced summer feeding probably approaches a “semi-fasting” situation, while unsuccessful hunters may fast as completely as bears spending summers onshore.
This underscores the importance of the spring and early summer feeding period for all bears and shows how misleading it is to focus on recent declines — and potential future declines — of sea ice coverage at the end of summer (September).
Footnote 1: Keep in mind that young bears that haven’t mastered the tricks of successful hunting will always be at risk of starvation, as they have always been – if they can’t put on enough weight in spring, they won’t last through the following summer, fall and winter of low feeding opportunities. Steven Amstrup (2003) pointed out that starvation is probably the leading cause of death for young and old bears alike.
“Starvation of independent young as well as very old animals must account for much of the natural mortality among polar bears… Also, age structure data show that subadults aged 2-5years survive at lower rates than adults (Amstrup 1995), probably because they are still learning hunting and survival skills.”
“I once observed a 3-year-old subadult that weighed only 70 kg in November. This was near the end of the autumn period in which Beaufort Sea bears reach their peak weights (Durner and Amstrup 1996), and his cohorts at that time weighed in excess of 200 kg. This young animal apparently had not learned the skills needed to survive and was starving to death.” [my bold]
Footnote 2: This statement by Pilfold and colleagues suggests that the conclusion drawn by Durner and Amstrup in (1996), regarding weights of Southern Beaufort Sea bears in Alaska (1983-1994), was misleading and/or mistaken. Durner and Amstrup stated that “bears of similar length were consistently heavier in autumn than in spring” (although they didn’t actually say by how much – it could have been only a kg or two). Their result was derived from a model designed to estimate body weights from girth measurements and body length, not a conclusion based on comparing weights over time of individual bears. Durner and Amstrup interpreted this result to mean that individual Beaufort Sea bears were lighter in spring than they were in autumn (when captured in October or November) – in contrast to all other populations studied (see also Amstrup 2003, quoted in footnote 1). While that could be interpreted to mean that Beaufort Sea bears continue to feed reasonably often over the summer, it is perhaps more plausible to suggest that they are more successful than average at hunting in the early fall — and thus regain lost summer weight more quickly.
Footnote 3: Sea ice changes since 1979 shown below, from NOAA’s “2014 Arctic Report Card,” in the sea ice chapter. Summer sea ice (September minimum, red) has declined dramatically since 1979 but spring ice (March maximum, black) has declined only slightly.
Amstrup, S.C. 2003. Polar bear (Ursus maritimus). In Wild Mammals of North America, G.A. Feldhamer, B.C. Thompson and J.A. Chapman (eds), pg. 587-610. Johns Hopkins University Press, Baltimore.
Derocher, A.E., Wiig, Ø., and Andersen, M. 2002. Diet composition of polar bears in Svalbard and the western Barents Sea. Polar Biology 25 (6): 448-452. http://link.springer.com/article/10.1007/s00300-002-0364-0
Durner, G.M. and Amstrup, S.C. 1996. Mass and body-dimension relationships of polar bears in northern Alaska. Wildlife Society Bulletin 24(3):480-484.
Hammill, M.O. and Smith T.G. 1991. The role of predation in the ecology of the ringed seal in Barrow Strait, Northwest Territories, Canada. Marine Mammal Science 7:123–135.
Knudsen, B. 1978. Time budgets of polar bears (Ursus maritimus) on North Twin Island, James Bay, during summer. Canadian Journal of Zoology 56(7): 1627- 1628. http://www.nrcresearchpress.com/doi/abs/10.1139/z78-224#.VNZWHy5v_gU
During the summers of 1969 and 1970 polar bears (Ursus maritimus) were observed on North Twin Island (53°20′ N; 80°00′ W), James Bay. Almost all of their time (86.8%) was spent resting. They spent only 3.2% of their time feeding, primarily on geese (Branta canadensis) and crowberries (Empetrum nigrum). There was no difference between the time budgets of lone bears and females accompanied by cubs.
Morrison, A. and Kay, J. 2014. “Short-term Sea Ice Gains Don’t Eliminate Long-term Threats.” Polar Bears International, “Scientists & Explorers Blog” posted 22 September 2014. http://www.polarbearsinternational.org/news-room/scientists-and-explorers-blog/short-term-sea-ice-gains-dont-eliminate-long-term-threats
Pilfold, N. W., Derocher, A. E., Stirling, I. and Richardson, E. 2015 in press. Multi-temporal factors influence predation for polar bears in a changing climate. Oikos. http://onlinelibrary.wiley.com/doi/10.1111/oik.02000/abstract
Stirling, I. 1974. Midsummer observations on the behavior of wild polar bears (Ursus maritimus). Canadian Journal of Zoology 52: 1191-1198. http://www.nrcresearchpress.com/doi/abs/10.1139/z74-157#.VR2zaOFmwS4
Stirling, I. and Derocher, A.E. 2012. Effects of climate warming on polar bears: a review of the evidence. Global Change Biology 18:2694-2706 http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2486.2012.02753.x/abstract
Stirling, I. and Øritsland, N. A. 1995. Relationships between estimates of ringed seal (Phoca hispida) and polar bear (Ursus maritimus) populations in the Canadian Arctic. Canadian Journal of Fisheries and Aquatic Sciences 52: 2594 – 2612. http://www.nrcresearchpress.com/doi/abs/10.1139/f95-849#.VNep0y5v_gU