The conundrum of cold winter effects in Hudson Bay

This is a follow-up on my last post regarding Beaufort Sea ringed seal declines and polar bear mortalities in cold winters. In that post, I discussed studies showing that when shorefast ice in the eastern Beaufort was thick, some polar bear females were found starving and many cubs died because newborn ringed seals (food!) were not available.

In these cold winters (months of Jan/Feb/March), the breeding-age male seals and pregnant females that would normally set up territories in the shorefast ice in preparation for the birth of pups and mating, appear to have moved offshore into the pack ice before pups were born (pups may or may not have been lost and mating may or may not have occurred).

These papers do not always refer specifically to ‘shorefast ice’ when they talk about thick or heavy ice during especially cold winters. However, we can presume that they mean shorefast ice because Arctic biologists (e.g. Kelly et al. 2010) now consider virtually all ringed seals to use shorefast ice as breeding/pupping grounds, despite compelling evidence from observational and genetic studies that a substantial proportion of the global ringed seal population must breed and give birth in the offshore pack ice (more on this in a later post, but see Davis et al. 2008; Finley et al. 1983; Wiig et al. 1999).

The deadly effects of cold winters in the eastern Beaufort Sea were very well documented for the winter of 1973-74, with seal and polar bear declines lasting until 1976. The evidence for a similar decline in both polar bears and ringed seals from 1984 through 1986 (Stirling and Lunn 1997:174) is somewhat less well documented but still quite pronounced (see graph below, from Stirling 2002).

Incomplete data on seals from other years, derived primarily from native hunters harvesting ringed seals for food, suggests that similar declines in ringed seals (and therefore polar bears) may also have occurred from 1992-1994, following heavy ice conditions the winter of 1992 and in 1964-1966, following a heavy ice year in 1964 (Stirling and Lunn 1997:178).

Figure from Stirling’s 2002 paper. Note that “ovulation rate” and “natality” are measures used to assess reproductive success in ringed seals and polar bears, respectively. In effect, what it shows is that during cold, heavy ice winters (1973-74 and 1984-85), the numbers of probably-pregnant seals declined and as a consequence, overall polar bear reproduction declined. These two cold events are the only ones with reasonably good data on both ringed seals and polar bears. Click to enlarge

So, the years of cold winters and heavy shorefast ice development in the eastern Beaufort appear to have been 1963-64, 1973-74, 1983-84, 1991-92.

[Interestingly, 1962-63 (the year before the Beaufort Sea cold winter) was the coldest year in Britain for more than 200 years.]

See the Beaufort Sea ice chart below for 1972-1977 for a sense of how offshore ice conditions were in 73/74 compared to other years.

Here is what offshore sea ice looked like in the Beaufort Sea in winter and spring from 1972-1977. This graph shows development of ice by week from Jan. 1 of each year (ringed seals give birth beginning early April in this region) through July 16. The especially cold 1973-74 season is highlighted, when heavy ice impeded ringed seal pupping/breeding in the shorefast ice. Graph generated from Canadian Ice Service data (average is 1981-2010).Click to enlarge

Note the sea ice graph above (generated from this CIS page) does not depict the shorefast ice, which is what is impacting ringed seals and polar bears. As far as I know, there is no data on shorefast ice thickness for this area or any other, although a paper published in June 2012 (Galley et al. 2012) describes the extent of this ice mass and breakup/freezeup dates from 1983-2009 for this region (see Quote archive, Featured Quote #1 for details). Breakup dates may not be the best proxy for estimating how thick the shorefast ice was in any one year in the historical record, but for now it seems to be all that is available.

It would appear from the Beaufort Sea data that especially cold winters make nearshore life difficult for ringed seals and deadly for nursing polar bear females and their cubs. But is it always the case?

Perhaps not. Data from Hudson Bay seems to tell another story. There were declines of ringed seals in western Hudson Bay (see map below showing both regions) in the 1990s when it was colder than average, but apparently, there were no negative effects on polar bears or their cubs.

This ringed seal decline was describe in a recent paper by Chambellant and colleagues (2012), see a summary of the paper here.

Map showing the western Hudson Bay region and the eastern Beaufort Sea, Canada. From Stirling and Lunn 1997:168. Click to enlarge

Based on data from subsistence hunters (for 1991, 1992, 1998-2000, and 2003-2006), analyzing results by decade rather than by individual year, Chambellant et al. found that [my bold]:

ringed seal reproductive parameters and survival of ringed seal pups were low in the 1990s and increased in the 2000s, indicating that conditions in western Hudson Bay were less favorable for the species in the 1990s than they were in the 2000s. The beginning of the 1990s was characterized by late dates of breakup (e.g., 21 July in 1992, 25 days later than average) and short open-water periods (126 days in 1991 and 1992), cold temperatures, and a high ice cover in late spring, suggesting heavy ice conditions.

Graph of western Hudson Bay breakup dates used for ringed seal study by Chambellant et al. (2012:274). Note that this graph uses Julian calendar dates, so the latest breakup date shown (1992, above the red line) was July 21, 25 days later than the 1990-2007 average (Julian day 177). Breakup for the year 2000 (marked by red line) was apparently not as late in this region (July 13) as it was in southwestern Hudson Bay (see next graph), but it was still later than 1991. Note that 2009 almost certainly had a late breakup date as well, see text for details. Click to enlarge

However, Chambellant and colleagues point out that western Hudson Bay polar bears appeared to benefit from the cold and heavy ice in the 1990s! Polar bears, they report, were in better condition than previously and produced more cubs, citing (among others) the Stirling and Lunn 1997 paper mentioned above.

…polar bear body condition and natality increased in western Hudson Bay in the early 1990s. This increase was associated with the delayed breakup dates in the early 1990s that created longer spring hunting seasons…

While no data earlier than the 1990s exists for ringed seals, studies on western Hudson Bay polar bears have been going on since the late 1960s. At least one of those studies (Stirling et al. 2004:20) indicate that there were late breakup dates in the early 70s and mid-80s (see graph below), as there was in the Beaufort, and a few late dates in the 90s also. [Note that this Stirling et al. 2004 paper used a different method to calculate breakup dates than others have done, such as Stirling used in his new polar bear book] and in Stirling and Parkinson 2006.

SW Hudson Bay breakup dates for 1971-2001 used in Stirling et al. 2004. “Area 5″ (the filled dots) overlaps with the Chambellant et al. 2012 study area around Churchill (others are further south). Top red line indicates the latest breakup date recorded (July 31), lower line is approximately July 21 (latest breakup date recorded in the Chambellant et al. study). Note 2009 breakup date was probably later than any of these.Click to enlarge

But the 1990s were quite a while ago now. Have there been any cold years with late breakup dates since then? Yes, apparently so. The graph from Stirling et al. 2004 (above) shows the latest breakup date for Area 5 (around Churchill) was in 2000. Breakup in Area 5 was later in 2000 than both the early 70s and mid-80s. Not only that, breakup was almost certainly just as late, or later, in 2009 (depending on the method of calculating the date).

As I mentioned in my review of Ian Stirling’s new polar bear book we know that in 2009, the Port of Churchill, Manitoba (on western Hudson Bay) had the latest opening date since records began because of late breakup of the sea ice. [see this new post at Polar Bear Alley on the recent history of the Port of Churchill.]

So, were polar bears in good condition in 2009?

Apparently, yes. It seems it’s a bit too soon to expect published reports of studies that include 2009 data but there is good anecdotal information that polar bears did quite well in 2009.
For example, according to a press release (PR) issued by Frontiers North Adventures in Churchill on Aug. 18, 2009, “due to colder-than-usual subarctic weather this year, healthier polar bears are being spotted along the west Hudson Bay coast.” An ice specialist interviewed for the story said this about the ice:

“This year [2009] there is more ice on average,” explains Dr. David Barber, a Canada Research Chair in Arctic System Science and the director of the Centre for Earth Observation Science at the University of Manitoba. “Hudson Bay in particular has seen a very cold winter and essentially a late spring.”

The president, Robert Buchanan, of Polar Bears International (an activist conservation group that funds many scientific studies) was quoted as saying “What we’re seeing this year [2009] is an aberration compared to the long-term trend.”

However, it is clear from the data above that that statement is spectacularly misleading.

The cold winter of 2009 may be an ‘aberration’ relative to the trend lines constructed by some researchers (see Scott and Marshall 2010 for an alternative view). But it is certainly not an ‘aberration’ in the sense of late breakup dates being unusual events over the last 40 years. It may indeed be an “aberration” in the sense of being the latest breakup date ever recorded but since no one has yet published an ‘official’ breakup date for 2009 (using either passive microwave data or CIS ice charts), we still don’t know precisely how 2009 compares to other cold years.

We can conclude, however, that polar bears in Hudson Bay, in contrast to those in the eastern Beaufort, appear to have been in especially good condition in colder-than-average years.

What is it about Hudson Bay that might account for this difference? I’ll explore some of the unique aspects of Hudson Bay in a later post.

But for now, consider this statement made by Stirling and Lunn (1997:171) in the portion of their paper that describes the study on reproductive success (natality) in western Hudson Bay polar bears:

In the early to mid-1980s, the natality of female polar bears in western Hudson Bay was the highest recorded anywhere in polar bear range, and nowhere else did females successfully wean cubs at 1.5 years of age instead of at the normal age of 2.5 years. Subsequently, a long-term decline in condition of adult female polar bears and survival of their cubs was documented from the 1970s through the late 1980s (Derocher & Stirling 1992), as reflected by a significant decline in condition indices…

This decline did not constitute a threat to the population because even when natality was at its lowest in the late 1980s, the rates were still higher than the upper range of values for bears elsewhere in the Arctic (e.g. Stirling et al. 1976, 1980).

…the more important (but unanswered) question is probably not why natality declined from the early 1980s but how could natality have been sustained at a level so much higher than other polar bear populations in the first place, what facilitated the successful weaning of yearlings there but nowhere else in their range, and how could females manage these physiological feats in a habitat where pregnant females must also fast for 8 months or more?”

Very interesting questions indeed! Perhaps if some polar bear biologists had been expending less effort over the last few decades looking for evidence of global warming, they might have found some answers to those questions.

References cited
Chambellant, M., Stirling, I., Gough, W.A. and Ferguson, S.H. 2012. Temporal variations in Hudson Bay ringed seal (Phoca hispida) life-history parameters in relation to environment. Journal of Mammalogy 93:267-281.

Davis, C.S., Stirling, I., Strobeck, C., and Coltman, D.W. 2008. Population structure of ice-breeding seals. Molecular Ecology 17:3078-3094.

Finley, K.J., Miller, G.W., Davis, R.A., and Koski, W.R. 1983. A distinctive large breeding population of ringed seal (Phoca hispida) inhabiting the Baffin Bay pack ice. Arctic 36:162-173.

Holst, M., Stirling, I. and Calvert, W. 1999. Age structure and reproductive rates of ringed seals (Phoca hispida) on the northwest coast of Hudson Bay in 1991 and 1992. Marine Mammal Science 15: 1357-1364.

Kelly, B. P., Bengtson, J. L., Boveng, P. L., Cameron, M. F., Dahle, S. P., Jansen, J. K., Logerwell, E. A., Overland, J. E., Sabine, C. L., Waring, G. T. and Wilder, J. M. 2010. Status review of the ringed seal (Phoca hispida). NOAA Technical Memorandum NMFS-AFSC-212.

Scott, J.B.T. and Marshall, G.J. 2010. A step-change in the date of sea-ice breakup in western Hudson Bay. Arctic 63:155-164. Available here (open access)

Stirling, I. 2002. Polar bears and seals in the eastern Beaufort Sea and Amundsen Gulf: a synthesis of population trends and ecological relationships over three decades. Arctic 55 (Suppl. 1):59-76.

Stirling, I. and Lunn, N.J. 1997. Environmental fluctuations in arctic marine ecosystems as reflected by variability in reproduction of polar bears and ringed seals. In Ecology of Arctic Environments, Woodin, S.J. and Marquiss, M. (eds), pg. 167-181. Blackwell Science, UK.

Stirling, I. and Parkinson, C.L. 2006. Possible effects of climate warming on selected populations of polar bears (Ursus maritimus) in the Canadian Arctic. Arctic 59:261-275.

Stirling, I., Lunn, N.J., Iacozza, J., Elliott, C. and Obbard, M. 2004. Polar bear distribution and abundance on the southwestern Hudson Bay coast during open water season, in relation to population trends and annual ice patterns. Arctic 57:15-26.

Wiig, O., Derocher, A.E., and Belikov, S.E. 1999. Ringed seal (Phoca hispida) breeding in the drifting pack ice of the Barents Sea. Marine Mammal Science 15:595-598.

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