Claim: data exists showing polar bear body condition improves over summer on sea ice

Do polar bears increase their body condition if they stay on the sea ice over the summer? Do they continue to hunt successfully from broken ice in July and early August in areas like Hudson Bay where ice eventually melts out completely? There seems to be an assumption that they do but one polar bear specialists repeatedly claims there is data showing this is the case.

A polar bear breaks through thin Actic Ocean ice Aug. 23, 2009.

Anyone saying sea ice at this time of year doesn’t affect polar bears is ignoring research showing their body condition continues to increase through summer if they’re out on the ice.” [Andrew Derocher 28 June 2022]

I would seriously like to know which paper or papers this data appears in but of course, he doesn’t provide that information. Instead, it’s ‘trust me, I’m the expert’.

[See a similar claim from 2019, in which he claims there is “new research” showing that bears continue to accumulate fat stores in July (“more July sea ice=fatter polar bears”), but he doesn’t say where we can find those research results.]

I contend from my review of the literature that explicit ‘before and after’ measurements of the same polar bears in spring (at mid-May) and fall after a summer on the ice have not been reported. I contend polar bear specialists assume that all polar bears which spend the summer (i.e. July-September) on sea ice either gain weight or lose much less weight than if they’d been on land eating nothing but they do not actually know this is true. If I’m wrong, show me the data and I’ll revise my statements on the topic.

The only published paper of which I am aware that looks like it provides this information is a report on a study from 1983-1994 in the Southern Beaufort Sea.

“Bears of similar length were consistently heavier in autumn than in spring.” [Durner and Amstrup 1996:483]

Note that the researchers did not capture the same bears in spring and fall: they captured a different set of bears in each season. As a consequence, the data does not prove that individual bears gained weight over the summer, as researchers may assume.

Moreover, because the purpose of the study was to generate a formula for predicting the mass of bears based on girth and length measurements (because weighing bears in the field is difficult), the authors don’t tell us how much heavier the bears were in autumn, only that the difference was statistically significant for their model, not biologically significant for the bears. Was it 2-3kg or 20-30kg? They didn’t say.

Because we cannot know from this study how much more individual bears weighed in the autumn vs. the spring, it is not possible to assess whether the vaguely described ‘additional weight’ measured for bears in general should be interpreted as proof of a net survival benefit to individual bears. Certainly, the study authors didn’t claim a benefit.

In a similar vein, Derocher repeatedly states that Western Hudson Bay bears that stay out longer on broken sea ice through July and early August continue to hunt seals and thus add to their overall survival benefit: he said so again today.

But the video below shows why polar bears are rarely successful at hunting adult seals during the summer on broken ice, whether in the Barents Sea or on Hudson Bay. [Hungry Polar Bear Ambushes Seal | The Hunt | BBC Earth, 30 June 2017]

References

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. https://pubs.er.usgs.gov/publication/70185398

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