Posted on November 11, 2019 |
This is the third year in a row that freeze-up of Western Hudson Bay (WH) ice has come earlier than the average of 16 November documented in the 1980s. Reports by folks on the ground near Churchill confirm polar bears are starting to move onto the sea ice that’s developing along the shore after almost 5 months on land. After five good sea ice seasons in a row for WH polar bears, this repeat of an early freeze-up means a sixth good ice season is now possible for 2019-2020.
Polar bear family on the ice off Churchill Manitoba (taken from a helicopter), courtesy Explore.org
When mothers with cubs are out on the ice (see photo above), it’s pretty certain the mass movement from land to sea ice is well underway because these family units are usually the last to leave.
Current ice conditions
Weather in Churchill was very cold today, -36C with the wind chill. The slight moderation in temperature in the forecast for the rest of this week is still very conducive to ice formation:
The Canadian Ice Service charts for 10 November 2018 below (the overall picture and the details for ice development in northern Hudson Bay) show the ice conditions last year at the time that bears left for the ice:
Below is what the ice looks like this year (11 November 2019): while the band of ice is not quite as thick as last year at this time, recent cold weather has led to solid ice formation along the west coast of Hudson Bay and into James Bay (home to Southern Hudson Bay bears). This ice is guaranteed to widen and thicken over the next few days, putting this year only a day or two behind last year and 2017.
Just to round out the comparison, below is the detailed ice development chart for 11 November 2017:
Freeze-up dates since 1979
Like Andrew Derocher’s student Laura Castro de la Guardia, 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 while in recent years (2004-2008) they left about 24 November ± 8 days, a difference of 8 days. In other words, the relative change in the dates that WH bears left the shore between the 1980s and recent years is only about 1 week (with lots of variation).
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.
Virtually all Western Hudson Bay bears leave the shore within about 2 days of sea ice concentration reaching 10% (Castro de la Guardia 2017; Cherry et al. 2013), 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.”
Here is the week 19 report from the 2018 Churchill Polar Bear Alert Program (November 4-11 — almost 5 months ashore), confirming that bears were moving onto the rapidly forming ice by the first week of November last year:
For 2019, the town of Churchill is behind in their posting of problem bear reports (the last one listed is 28 October) but I’ll insert the relevant status sheets for the season’s end here as soon as they are available.
My 2017 Southern Hudson Bay post (with its list of references) is worth another look for its discussion of the following points: the definition of freeze-up; the relationship of official freeze-up and breakup dates to the dates that bears depart; the overall health and survival of Western and Southern Hudson Bay polar bears.
A final note: if PBI spokesperson Amstrup had been right about his predictions of Arctic sea ice and polar bear survival back in 2007 when he was the head of the US Geological Survey’s polar bear research team, there would be no polar bears at all in Hudson Bay right now (Crockford 2017, 2019), not a thriving population of fat, healthy bears moving offshore as early as bears did in the 1980s.
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/
Cherry, S.G., Derocher, A.E., Thiemann, G.W., Lunn, N.J. 2013. Migration phenology and seasonal fidelity of an Arctic marine predator in relation to sea ice dynamics. Journal of Animal Ecology 82: 912-921. http://onlinelibrary.wiley.com/doi/10.1111/1365-2656.12050/abstract
Crockford, S.J. 2017. Testing the hypothesis that routine sea ice coverage of 3-5 mkm2 results in a greater than 30% decline in population size of polar bears (Ursus maritimus). PeerJ Preprints 2 March 2017. Doi: 10.7287/peerj.preprints.2737v3 Open access. https://peerj.com/preprints/2737/
Crockford, S.J. 2019. The Polar Bear Catastrophe That Never Happened. Global Warming Policy Foundation, London. Available from Amazon in paperback and ebook formats.
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
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