Temporal trends in the body condition of Southern Hudson Bay polar bears

Abstract

Ecological change in the Arctic as a result of climatic warming has been suggested as a significant threat to the conservation of polar bears (Ursus maritimus) (Lunn et al. 2002). The impacts of climatic warming on Canadian polar bear populations may occur first near the southern edge of the range in James Bay and Hudson Bay (Stirling and Derocher 1993, Arctic Climate Impact Assessment 2004, Derocher et al. 2004). In particular, the break-up of sea ice earlier in the year is believed to reduce opportunities for polar bears to feed and store fat needed for prolonged fasting during the ice-free season (Stirling et al. 1999). If polar bears have access to their primary prey, ringed seals (Pusa hispida) (Stirling and Archibald 1977), for a shorter period then it is likely that they will have difficulty gaining sufficient body mass during the ice-covered period, which may have effects at both the individual and population level. For example, Stirling et al. (1999) documented a long-term decline in body condition and evidence of reduced reproductive success in polar bears from the Western Hudson Bay population, which they attributed to a trend towards earlier melting of the sea ice in summer in western Hudson Bay. More recently, Gough et al. (2004) and Gagnon and Gough (2005) demonstrated trends towards earlier break-up in James Bay, along the southern shore of Hudson Bay, and in western Hudson Bay during the period 1971-2003. Trends towards later freeze-up were found for northern and northeastern Hudson Bay (Gagnon and Gough 2005); trends in other areas of Hudson Bay were not statistically significant but were in the direction of later freeze-up. Over the past 3 decades, break-up dates are occurring earlier by about 9.5 days per decade in northern James Bay and by between 5 and 8 days per decade along the southern Hudson Bay coast of Ontario.