Western Hudson Bay polar bears reportedly still on ice as of 17 July 2017

Today, polar bear biologist Andrew Derocher posted a progress report via twitter on the annual journey ashore of the Western Hudson Bay bears tagged by his University of Alberta research team that shows virtually all of the bears are still out on the sea ice.

Churchill_Polar_Bear_2004-11-15 Wikipedia

After months of gloomy reports on the state of the Hudson Bay sea ice, it’s clear from the map Derocher posted (below) that only one bear out of 12 still transmitting has come ashore so far, although he comments that “some tags haven’t reported lately” (the purple icons are ear tags put on males &/or young bears while the blue icons are collars put on adult females):

Oddly, the same comment was made almost a month ago about these same bears and the suggestion was made that these animals “may be swimming to shore”:

Money quote: Today Derocher remarked that “bears may be shifting behaviour to stay out on less ice” to explain why the tagged bears have still not come ashore as he expects them to do.

Perhaps if he used a different ice chart, it might make more sense (see below). However, the same thing has been happening year after year: WHB polar bears stay on the ice much longer than Derocher predicts but he does not change his expectations or the type of ice chart he uses to track the bears.

As I’ve pointed out before (because this is what field researchers have stated), polar bears have a tough time catching seals after about mid-June or so but they may still prefer to be on the sea ice than on land, even if it’s low concentration ice.

The ice charts Derocher uses appears to show only ice that is more than 50% concentration. Compare the ice on his tweet from today (17 July) — which shows barely any ice at all and might be interpreted as showing polar bears in open water.

But the Canadian Ice Service chart for today shows much more ice:

Sea ice Canada 2017 July 17

Moreover, their “stage of development” charts for northern and southern HB show that virtually all of this ice is still classified as thick first year ice (> 1.2m thick):

Hudson Bay North daily ice stage of development 2017_July 17

Hudson Bay south daily ice stage of development 2017_July 17

The concentration is not uniform however, and as Derocher’s charts also show, there are only a few patches in the northern portion of the bay that are still 90-100% concentration but that clearly has not been driving his marked polar bears to shore in a hurry:

Hudson Bay North daily ice concentration 2017_July 17

Hudson Bay south daily ice concentration 2017_July 17

However, satellites have trouble recognizing some ice at this time of year because of melt water that forms on top of ice (which the satellite may interpret as open water), so there may actually be more ice than is shown even on the CIS charts.

The bears will come ashore eventually, of course — probably within the next week or so, depending on how the melt proceeds. Some are already ashore and others may stay out longer.

Keep in mind, however, that the latest research (Castro de la Guardia 2017 – see abstract below) shows that Western Hudson Bay polar bears in recent years (to 2015) spent only about 3 weeks longer onshore (i.e. 21 days) than they did in the 1980s. Bears left the ice about 2 weeks earlier and left shore about 1 week later — but this hasn’t changed since 2003 (Regehr et al. 2007).

Which means that there has been no change, on average, in the amount of time that most WHB polar bears spent on shore since 2003 and thus, the ice-free season in recent years has not been any worse for the bears than it was 14 years ago.

Castro de la Guardia et al 2017 Abstract highlighted

References

Castro de la Guardia, L., Myers, P.G., Derocher, A.E., Lunn, N.J., Terwisscha van Scheltinga, A.D. 2017. Sea ice cycle in western Hudson Bay, Canada, from a polar bear perspective. Marine Ecology Progress Series 564: 225–233. http://www.int-res.com/abstracts/meps/v564/p225-233/

Regehr, E.V., Lunn, N.J., Amstrup, S.C., and Stirling, I. 2007. Effects of earlier sea ice breakup on survival and population size of polar bears in Western Hudson Bay. Journal of Wildlife Management71: 2673-2683. http://onlinelibrary.wiley.com/doi/10.2193/2006-180/abstract

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