Western & Southern Hudson Bay polar bears experience earliest freeze-up in decades

Reposted from Polar Bear Science

Posted on November 3, 2020

This is shaping up to be one of the shortest ice-free seasons in at least 20 years for both Western and Southern Hudson Bay polar bears.

Hudson Bay sea ice at 2 November 2020. NSIDC Masie chart.

Last week, sea ice started forming along the shore of Hudson Bay, from the north end all the way south into James Bay. So far, the shorefast ice that’s forming is only a narrow strip along the coast but is thickening and becoming broader each day, which means that unless something changes dramatically, the bears should all be on the ice at the end of the week, an exodus from shore that hasn’t happened this early in WH since 1993 (the earliest since 1979).

The last WH tagged polar bear didn’t leave the ice this year until 21 August, which means if it’s on the ice by the end of this week it will have spent only 11 weeks onshore – less than 3 months. Even the first bears that came ashore in mid-July will have only spent about 16 weeks on land – at least a month less than they did a decade ago (Stirling and Derocher 2012). Four months spent ashore was the historical average for Western Hudson Bay bears in the 1970s and 1980s (Stirling et al. 1977, 1999). This year, most polar bears will have spent only about 13-14 weeks on land because they did not come ashore until early August.

Freeze-up dates since 1979

I am using a definition of “freeze-up” that describes the behaviour of polar bears to newly formed ice, not the date when fall ice coverage on the bay reaches 50% (e.g. Lunn et al. 2016). According to a recalculation of WH data that goes up to 2015 and back to 1979 (Castro de la Guardia 2017, see graph below), in the 1980s bears left for the ice at freeze-up (10% sea ice coverage) about 16 November ± 5 days. The earliest the bears left the ice was in 1991 and 1993, on 6 November (Julian day 310).

The first week of November is very early for bears leaving for the ice.

castro-de-la-guardia-et-al-derocher-2017-fig-3-no-captioncastro-de-la-guardia-et-al-derocher-2017-fig-3-no-captioncastro-de-la-guardia-et-al-derocher-2017-fig-3-no-caption
Figure 3 from Castro de la Guardia (2017) showing freeze-up and breakup dates and ice-free days 1979-2015 for Western Hudson Bay, showing that the earliest freeze-up dates since 1979 (top panel) came on 6 November, Day 310 (in 1991 and 1993).

Therefore, freeze-up dates of 10-12 November or so (Day 314-316) for 2017, 2018, and 2019 are some of the earliest freeze-up dates recorded since 1979 (the earliest being 6 November, Day 310, in 1991 and 1993), even earlier than the average for the 1980s. And 2020 is earlier still.

Virtually all Western Hudson Bay bears leave the shore within about 2 days of sea ice concentration reaching 10% (Castro de la Guardia 2017), although Southern Hudson Bay bears leave when it reaches about 5%: in other words, the bears go as soon as they possibly can. As I discussed in 2016 regarding newly-published studies (Obbard et al. 2015, 2016) on the status of Southern Hudson Bay (SH) bears:

“…SH polar bears left the ice (or returned to it) when the average ice cover near the coast was about 5%. This finding is yet more evidence that the meteorological definition of “breakup” (date of 50% ice cover) used by many researchers (see discussion here) is not appropriate for describing the seasonal movements of polar bears on and off shore.”

The earliest freeze-up date for Southern Hudson Bay appears to be about 11 November (Julian day 315), based on data in a paper by Obbard and colleagues in 2016. So freeze-up is early for these bears as well.

Despite this being the best of six very good years for the polar bears of Western Hudson Bay, activist polar bear scientists continue to sell the public their false message of doom based on data from years ago. As I’ve mentioned previously, polar bear data from Western Hudson Bay prior to these good years (i.e. up to 2009 only) was used for the latest model (Molnar et al. 2020) predicting future conditions for polar bears elsewhere in the Arctic. These good years for sea ice and bears have simply been ignored in long-term projections.

Hiding the good news

Polar bear biologist Derocher recently said the timing of sea ice formation this year is ‘normal’:

Yesterday’s satellite image has little ice forming near Rankin Inlet. It’s -11 C (12 F) so ice should form soon. Timing is about normal. NW Hudson Bay is where the 1st ice forms. Ice moves south on winds & piles up along shore east of Churchill where many polar bears are waiting. https://t.co/EQH390707H pic.twitter.com/vbWnbT0p0c— Andrew Derocher (@AEDerocher) October 28, 2020

However, charts from the Canadian Ice Service indicate otherwise.

In their ‘Departure from normal’ chart for the week of 2 November 2020, sea ice formation along the entire western coast of Hudson Bay is ‘greater than normal’ and ‘much greater than normal’ (blue and dark blue), below. Only the very northern portions are slightly less than normal (pink):

Daily charts for Hudson Bay north and south are below for 3 November: dark purple areas (‘grey ice’) were light purple the day before (‘new’ ice):

WH polar bears and sea ice photos

Probably the same triplet litter as spotted in September was seen again on 31 October getting ready to leave for the ice. How many more of these triplet litters are out there? They are rarely seen now although they used to be common. Last one before this was photographed in 2017 north of Churchill. But while these large litters may now be more rare than they used to be, they have not disappeared. In other regions, triplet litters are an indicator of a healthy population.

Mother with triplet cubs, 31 October 2020. Dave Allcorn photo.

All of the bears spotted hanging around waiting for the sea ice have been fat and healthy (e.g. below).

Three fat bears, 31 October 2020. Wakusp National Park.

On Saturday, a polar bear mother with two cubs went out on the ice and caught a seal (below).

Polar bears on a seal kill, 31 October 2020. Wakusp National Park.

Mother with two cubs, sea ice in the background. Wakusp National Park, 3 November 2020.

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/

Lunn, N.J., Servanty, S., Regehr, E.V., Converse, S.J., Richardson, E. and Stirling, I. 2016. Demography of an apex predator at the edge of its range – impacts of changing sea ice on polar bears in Hudson Bay. Ecological Applications 26(5): 1302-1320. DOI: 10.1890/15-1256

Molnár, P.K., Bitz, C.M., Holland, M.M., Kay, J.E., Penk, S.R. and Amstrup, S.C. 2020. Fasting season length sets temporal limits for global polar bear persistence. Nature Climate Change. https://doi.org/10.1038/s41558-020-0818-9

Obbard, M.E., Stapleton, S., Middel, K.R., Thibault, I., Brodeur, V. and Jutras, C. 2015. Estimating the abundance of the Southern Hudson Bay polar bear subpopulation with aerial surveys. Polar Biology 38:1713-1725.

Obbard, M.E., Cattet, M.R.I., Howe, E.J., Middel, K.R., Newton, E.J., Kolenosky, G.B., Abraham, K.F. and Greenwood, C.J. 2016. Trends in body condition in polar bears (Ursus maritimus) from the Southern Hudson Bay subpopulation in relation to changes in sea ice. Arctic Science 2: 15-32. DOI: 10.1139/AS-2015-0027

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. doi:10.1111/j.1365-2486.2012.02753.x

Stirling I, Jonkel C, Smith P, Robertson R, Cross D. 1977. The ecology of the polar bear (Ursus maritimus) along the western coast of Hudson Bay. Canadian Wildlife Service Occasional Paper No. 33. pdf here.

Stirling, I., Lunn, N.J. and Iacozza, J. 1999. Long-term trends in the population ecology of polar bears in Western Hudson Bay in relation to climate change. Arctic 52:294-306. http://arctic.synergiesprairies.ca/arctic/index.php/arctic/article/view/935/960

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November 5, 2020 at 12:39PM

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