No evidence that long-distance swimming contributed to Beaufort Sea polar bear population crash of 2004-2006

The air is thick with desperation on the polar bear front:

“[Andrew] Derocher said the polar bear population in the Beaufort Sea has fallen more than 50 per cent in the past 10 years.

“So it is a concern that this is probably one of the factors associated with the population decline,” he said.

Derocher_CBC news 19 April 2016

As the CBC report in which this quote appears states immediately afterwards, there is no evidence for such a thing in the paper under discussion:

“The study found no direct evidence of that – all polar bears appeared to survive the swims recorded in the study.”

There is no truth to Derocher’s first statement either. Desperation – you don’t have to be a scientist to sense it. And the media wonder why people don’t trust them…

The population crash in the Southern Beaufort was caused by thick spring ice that drove pregnant ringed seals away – this phenomenon was not only well-documented, but had occurred about every 10 years previously, back to at least the 1960s (discussed in detail in Crockford 2015, fully referenced, with quotes).

By all measures and accounts, the mid-2000s event was as severe as the event of 1974-1976 and every bit as deadly but it wasn’t caused by global warming; young bears didn’t die because they had to swim too much in the summer but because they starved to death due to lack of ringed seal pups to eat in the spring.

The decline in 2004-2006 was not “more than 50 percent” – in fact, let me quote the original paper regarding the amount of the Southern Beaufort decline (Bromaghin et al. 2015:647):

“Conservatively, the decline seems unlikely to have been less than 25% but may have approached 50%. Improved survival and stability in abundance at the end of the investigation are cause for cautious optimism.”

All of the recent news “reports” [Washington Post & National Post, 21 April; CBC News, 19 April) about the new study on polar bear swimming in the Beaufort Sea and Hudson Bay (Pilfold et al. 2016) highlight the hype and leave out the important facts, which I quoted in my original post on this paper two days ago:

“….91% (91/100) of the swims in the BS occurred before the annual September minimum sea ice extent had been reached.  In the BS, 81% (29/36) of swims started and ended in pack ice…

I also pointed out:

“However, no Beaufort Sea bears died as a result of swims undertaken between 2007 and 2012, even in 2012 when summer sea ice was the lowest it has been since 1979 or in 2007, when the summer ice was second-lowest. No new evidence is presented in the Pilfold el al. paper of any harm or ill-effect of swimming the distances required.”

The only evidence of “harm” came from a previous study (Pagano et al. 2012; Durner et al. 2011) that found one bear [ONE BEAR], in 2008, lost weight (49 kg or 22% of her starting body weight) after a 9-day swim and 53 day walk over ice (that’s more than 2 months)  – at a time she would have been losing weight regardless (because bears eat little during the summer even if they are on the ice). She was not, however, dangerously thin (she was  177 kg after her “ordeal”), and while it is true she lost her cub at some point, it is also true that the researchers did not know what happened to it. It could have drowned (as they assumed – and tell every reporter they talk to) or it could have been eaten by another bear or suffered some other kind of accidental death sometime after the swim was over. No one knows how the cub died – they just know it wasn’t with its mother when she was re-captured.

An antidote to this sort of hype can be found in an Earth Day essay by Pat Michaels (21 April 2016: “Save the Polar Bears? They’re Fine, Actually”). I have only one quibble: polar bears are a separate and distinct species in every sense of the word. Occasional hybridization with grizzly bears does not negate the species status of polar bears.

Bromaghin, J.F., McDonald, T.L., Stirling, I., Derocher, A.E., Richardson, E.S., Rehehr, E.V., Douglas, D.C., Durner, G.M., Atwood, T. and Amstrup, S.C. 2015. Polar bear population dynamics in the southern Beaufort Sea during a period of sea ice decline. Ecological Applications 25(3):634–651.

Crockford, S.J. 2015. “The Arctic Fallacy: sea ice stability and the polar bear.” GWPF Briefing 16. The Global Warming Policy Foundation, London. Pdf here.

Durner, G.M., Whiteman, J.P., Harlow, H.J., Amstrup, S.C., Regehr, E.V. and Ben-David, M. 2011. Consequences of long-distance swimming and travel over deep-water pack ice for a female polar bear during a year of extreme sea ice retreat.  Polar Biology 34: 975-984.

Pagano, A.M., Durner, G.M., Amstrup, S.C., Simac, K.S. and York, G.S. 2012. Long-distance swimming by polar bears (Ursus maritimus) of the southern Beaufort Sea during years of extensive open water. Canadian Journal of Zoology 90: 663-676.

Pilfold, N.W., McCall, A., Derocher, A.E., Lunn, N.J., and Richardson, E. 2016. Migratory response of polar bears to sea ice loss: to swim or not to swim. Ecography in press.

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