Data exists that show polar bears’ body condition improves during sea ice summers – Rise for that?

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

“Anyone who says sea ice at this time of year doesn’t affect polar bears is ignoring research that shows their body condition continues to increase throughout the summer if they stay out of the ice.. ” [Andrew Derocher 28 June 2022]

I really wanted to know in which newspaper this data appeared but of course, he did not provide that information. Instead, 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 object to my literature review of ‘before and after’ measurements of the same polar bear in spring (mid-May) and fall after a summer on ice that have not been reported. I fight the polar bear experts assumption that all polar bears spend the summer (i.e., July to September) on sea ice either gain or lose much less weight than if they were on land eating nothing but they don’t actually know this is true. If I’m wrong, show me the data and I’ll correct my statements on the subject.

The only published article that I know of like It provides this information is a report on a study from 1983-1994 in the South Beaufort Sea.

“Gears of similar length are usually heavier in the fall than in the spring.” [Durner and Amstrup 1996:483]

Note that the researchers didn’t capture the same bears in spring and fall: they captured a different group of bears each season. Therefore, the data do not prove that individual bears gain weight during the summer, as the researchers might assume.

Furthermore, since the aim of the study was to create a formula for predicting bear mass based on girth and length measurements (since it is difficult to weigh bears in the field), the authors do not give We know how much the bears weigh. Autumn, the only difference is statistically important to their model, no biologically significant for the bears. Is it 2-3kg or 20-30kg? They didn’t say.

Because we cannot know from this study how much more bears weigh more in the fall than in the spring, it is not possible to assess whether the vaguely described ‘additional weight’ for bears in general is should be interpreted as evidence of a net survival benefit for bears. Certainly, the study authors did not claim a benefit.

Likewise, Derocher has repeatedly said that Hudson Bay West bears staying longer on broken ice from July to early August continue to hunt seals and thus increase their overall survival benefit: today he said the same.

But the video below shows why polar bears rarely succeed in 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]

Presenter

Durner, GM and Amstrup, SC 1996. Mass and body size relationships of polar bears in northern Alaska. Wildlife Association Newsletter 24 (3): 480-484. https://pubs.er.usgs.gov/publication/70185398