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Hudson Bay Ringed Seal Populations Declined Significantly Due to Heavy Ice in the 1990s

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.
The ringed seal (Phoca hispida) is a small Arctic species, the only seal that maintains breathing holes in the sea ice with its teeth and claws. This unique behavior enables the ringed seal to occupy the vast areas of largely unbroken sea ice that form close to shore over the winter (called 'shorefast ice'), a habitat that other Arctic seals cannot use. Ringed seals give birth on the ice in early spring within a snow cave (called a 'subvivean lair') excavated by the mother over one or more of her breathing holes. This snow cave offers the white-coated pup some protection from predators and the harsh Arctic weather. While warm temperatures and rain in the spring can melt the cave prematurely, which leaves the pups exposed (Ferguson et al. 2005), very cold winters can make the shorefast ice too thick to maintain breathing holes, forcing breeding seals further offshore (Vibe, 1965; 1967). So, either extreme -- cold winters and warm springs -- can negatively affect pup production in the shorefast ice and cause ringed seal population numbers to decline (see Stirling 2002, Stirling and Lunn 1997).

In the present study, Chambellant et al. (2012) collected data from seals of all ages and sexes that were harvested in the fall by Inuit subsistence hunters between 1991 and 2006 (specifically: 1991, 1992, 1998-2000 and 2003-2006) in western Hudson Bay (672 seals altogether, 304 in the 1990s and 368 in the 2000s). They recorded the size, age, reproductive status and body condition of the seals and collected data on environmental conditions for Hudson Bay (including maximum snow depth between February and May, spring temperatures, cumulative rainfall from April-June, chlorophyll-alpha concentrations, ice cover in May-June, length of the open water season, and sea ice breakup dates). Sea ice thickness was not measured. 'Pups' were animals born in the spring of the year prior to the fall harvest and juveniles were seals 1-5 years of age.

Chambellant and colleagues found that "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."

The difference in population numbers between the two decades was marked in this study: there were three times as many ringed seal pups in the 2000s than there were in the early 1990s. The author suggest that some of the increase in the 2000s was due to immigration of young seals (including young adult females) from other regions, rather than improved pup production locally. Statistical analyses suggested an optimum range of sea ice breakup dates (mid-June to mid-July) and snow depth (25-52 cm) might be most favorable for ringed seal pup production in this area.

Although sea ice thickness was not measured in this study, the authors note that "heavy ice" [presumably this means 'thick ice' but it is not actually defined] in the western Arctic in the mid-1970s and 1980s had been proposed "as the major cause of ringed seal reproductive failures and abundance decline." (e.g. Stirling 2002; Stirling and Lunn 1997). Chambellant et al. further suggest that "the poor ringed seal reproductive and survival performances in the 1990s could have been triggered by the cold and heavy ice conditions that prevailed in western Hudson Bay at the beginning of the decade." In contrast to ringed seals, however, the authors point out that the polar bear (Ursus maritimus) seemed to have benefitted from the cold conditions in western Hudson Bay in the early 1990s -- polar bears were in better condition and produced more cubs.

While much has been made of the vulnerability of Hudson Bay ringed seals to earlier seasonal retreat of sea ice in Hudson Bay predicted for the future (e.g. Derocher et al., 2004; Ferguson et al., 2005), the devastating toll that cold winters and thick shorefast ice have had on this small seal elsewhere in the Arctic is not often discussed. And because polar bears depend on ringed seals for food, they are not everywhere immune to the effects of heavy ice despite the apparent benefit they enjoyed in western Hudson Bay in the early 1990s. For example, in the Beaufort Sea heavy ice conditions occurred in the early 1990s, the mid-1970s, and mid-1980s; all three times, both polar bear and ringed seal numbers declined significantly (Stirling 2002, Stirling and Lunn, 1997). In 1974 and 1975 in particular, ringed seal pup production fell by 80% or more in nearshore habitats and as a result, survival of polar bear cubs "plummeted" (numbers not given) and many polar bear females were found in extremely poor condition (Stirling and Lunn, 1997; Stirling, 2002).

Little was known about ringed seal populations in western Hudson Bay before the early 1990s (Holst et al., 1999), so it is not known if pup production declined during the 1970s and 1980s as they did during the 1990s. However, the polar bears of western Hudson Bay have been under study, in a manner similar to the Beaufort Sea population, since the late 1970s, and since late breakup dates in the 1970s and 1980s have been noted in those studies (e.g. Stirling et al., 2004), it suggests that heavy sea ice conditions were not confined to the Beaufort Sea those years.

In summary, heavy shorefast ice has been shown to be associated with devastating declines in pup production for nearshore-breeding ringed seals in two regions of the Arctic (western Hudson Bay and the Beaufort Sea). These heavy ice conditions are known to have occurred during each decade between the 1970s and the 1990s. Because female polar bears that den close to shore are dependent on nearshore-breeding ringed seals to successfully rear their cubs, polar bear numbers have also crashed dramatically during heavy ice years, although so far this has only been documented in the Beaufort Sea.

Additional References
Derocher, A.E., Lunn, N.J. and Stirling, I. 2004. Polar bears in a warming climate. Integrative and Comparative Biology 44: 163-176.

Ferguson, S.H., Stirling, I. and McLoughlin, P. 2005. Climate change and ringed seal (Phoca hispida) recruitment in western Hudson Bay. Marine Mammal Science 21: 121-135.

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.

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., 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.

Vibe, C. 1965. The polar bear in Greenland. In, Proceedings of the First International Scientific Meeting on the Polar Bear. Fairbanks, Alaska 6-10 September 1965, IUCN Polar Bear Specialist Group. University of Alaska International Conference Proceedings Series, No. 1. pg. 17-25. Washington, DC.

Vibe, C. 1967. Arctic animals in relation to climatic fluctuations. Meddelelser om Grønland. 170(5). C. A. Reitzels Forlag, Copenhagen.

Archived 15 May 2012