Polar Bears Came Ashore in Western Hudson Bay as Late in 2009 as They Did in 1992
Cherry, S.G., Derocher, A.E., Thiemann, G.W. and 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.
How much difference do advances in methods make, especially to studies on polar bears?
Cherry et al. (2013) recently re-examined the phenomenon of sea ice breakup in western Hudson Bay and put satellite collars on adult females between 1991 and 1997, and again between 2004-2009, to determine when they came ashore in the summer and when they went onto the ice in the fall. The bears were captured between Churchill, Manitoba and the Ontario border, in August or September.
They determined that the date when ice reached a 30% concentration level (based on passive microwave data) was the most useful for predicting when polar bears came ashore, rather than the 50% date, as had been used in previous studies (e.g., Stirling et al. 1999). Similarly, rather than freeze-up being defined as the point when 50% ice coverage is reached, they found that 10% was the value most relevant for polar bears.
Cherry et al. used these new 30%/10% values to recalculate past breakup and freeze-up dates for 1991 to 2009 (cf. Stirling and Parkinson 2006, which calculated breakup dates from 1980 to 2004) and see how the revised dates correlated with the behavior of radio-collared bears. They found that "throughout the study, bears arrived ashore a mean of 28.3 day (S.E. = 1.8) after 30% ice cover (Fig. 2)… Collared polar bears departed from shore an average of 2.5 days, S.E. = 0.7, after 10% freeze-up (Fig. 2)".
Their figure 2 shows that between 1991 and 2009, there was considerable variability in breakup dates: bears left the ice later in 2009 than they did in 1992, negating the suggestion that there has been a relentless recession of breakup dates since 1991 in Western Hudson Bay. Perhaps most importantly for polar bears, only nine out of 19 breakup dates in this period fell in June (with none falling in the first half of the month), suggesting there was little to no impact on the critical November-June feeding period. The graph also shows a discernible trend towards later freeze-up dates in the fall and the subsequent departure of polar bears from shore. However, while Cherry et al. conclude that "throughout our study, collared polar bears showed trends towards arriving onshore earlier during break-up and departing from shore later during freeze-up," neither trend is marked on the graph.
Surprisingly, they also found that "polar bears displayed high degrees of fidelity [to areas where they came ashore] even when sea ice disappeared relatively early, suggesting that coming ashore in a familiar location may be more important that remaining longer on the ice." But they also observed that such fidelity was not always high: in some years (particularly 2007), bears did come ashore in unfamiliar territory. They emphasize that "polar bears tended to show less fidelity when there was relatively more ice present in the southern Hudson Bay zone [along the Ontario coastline] compared to the western Hudson Bay zone [in Manitoba], and this pattern was exacerbated when rates of ice disappearance were relatively high."
Knowing that polar bears usually leave the ice each year about 28 days after sea ice coverage declines to 30% and then depart the shore only a few days (2.5) after there is 10% ice coverage in the fall makes it much easier to predict the seasonal movements of Western Hudson Bay bears each year. In contrast to the old method, which Stirling and Derocher (2012) used to show a significantly advancing trend in breakup dates between 1979 and 2007, there does not appear to have been more than a slight advance in breakup dates between 1991 and 2009 using this new method. However, contrary to Gagnon and Gough (2005), who found no trend in freeze-up dates for Western Hudson Bay between 1971 and 2003 using the 50% concentration method, there appears to be a clear trend toward later freeze-up over time using this new method. Finally, while some bears apparently choose a favorite location to come ashore even if there is enough ice to spend more time hunting, many bears are flexible enough in their choice of where and when they leave the ice in the summer to adapt to yearly variability in ice conditions.
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