Abundance estimate of 1030 (95% CI: 754-1406) from 2011, few observations of females with cubs. Declines in survival and birth rates and body condition have been linked to earlier ice-break up.
Status table outtake
Status and delineation
The current population boundaries of the Western Hudson Bay subpopulation are based on capture, recapture, and harvest of tagged animals (Stirling et al. 1977, Derocher and Stirling 1990, 1995a, Taylor and Lee 1995, Lunn et al. 1997). This subpopulation appears to be geographically segregated from the Southern Hudson Bay subpopulation to the southeast and the Foxe Basin subpopulation to the north during the open-water season, although all three subpopulations mix on the Hudson Bay sea ice during the winter and spring (Stirling et al. 1977, Derocher and Stirling 1990, Stirling and Derocher 1993, Taylor and Lee 1995).
During the 1960s and 70s, abundance likely increased with the closure of the fur trading post at York Factory, withdrawal of military personnel from Churchill, and the closure of hunting in Manitoba (Stirling et al. 1977, Derocher and Stirling 1995a). Derocher and Stirling (1995a) estimated the mean population size for 1978-1992 to be 1,000 ± 51 (SE). This estimate was considered conservative because the study had not covered the southern portion of the range east of the Nelson River; for management purposes, population size was adjusted to 1,200 (Calvert et al. 1995, Wiig et al. 1995). In 1994 and 1995, Lunn et al. (1997) expanded the capture program to sample animals to the boundary between WH and SH. Using these additional data, abundance was estimated to be 1,233 ± 209 in autumn 1995. Regehr et al. (2007) reported a decline in abundance from 1,194 (95% CI = 1,020-1,368) in 1987 to 935 (95% CI = 794-1,076) in 2004. Further, the survival rates of cubs, sub-adults, and old bears (>20 years) were negatively correlated with the date of sea ice breakup (Regehr et al. 2007). A mark-recapture distance sampling study resulted in an abundance estimate of 1,030 (95% CI = 754-1,406) in 2011 (Stapleton et al. 2014). During this survey, 711 bears were observed and more bears, particularly adult males, were observed in the coastal areas east of the Nelson River towards the WH/SH boundary than were documented during the late 1990s (Stirling et al. 2004). Stapleton et al. (2014) suggested that a distributional shift may have negatively biased abundance estimates derived from capture samples. The mean litter size (cubs-of-the-year, 1.43 ± 0.08; yearlings, 1.22 ± 0.10) and number of cubs observed as a proportion of total observations (cubs-of-the-year, 0.07; yearlings, 0.03) were lower than those recorded for the neighboring subpopulations of Foxe Basin and Southern Hudson Bay and consistent with WH having low reproductive productivity (Regehr et al. 2007, Peacock et al. 2010, Stapleton et al. 2014). The body mass of solitary adult female polar bears has declined over the past 30 years, which has likely contributed to declining reproductive success (Derocher and Stirling 1995b, Stirling et al. 1999, Sciullo et al. 2016, Lunn unpubl. data).
Lunn et al. (2016) evaluated the demography and population status of WH for the period 1984-2011, using a Bayesian implementation of multistate capture-recapture models, coupled with a matrix-based demographic projection model, to integrate several types of data and to incorporate sampling uncertainty, and demographic and environmental stochasticity across the polar bear life cycle. Their analysis resulted in an estimate of 806 (95% CI = 653,984) for polar bears in the core area of study north of the Nelson River in 2011. Although both the aerial survey and capture-recapture estimates are broadly similar with overlapping confidence intervals, it is difficult to make direct comparisons because the geographical area covered differed. The aerial survey likely provides an accurate “snapshot” estimate of the total number and distribution of polar bears in the WH management area at the time of the survey. The point estimate of abundance from the capture-recapture model, represents the number of bears that move through the smaller, capture-recapture sampling area.
In addition to an estimate of abundance, Lunn et al. (2016) documented a significant relationship between sea ice conditions and survival of female polar bears of all age classes. For the recent decade 2001-2011, the growth rate of the female segment of the population was 1.02 (95% CI = 0.98, 1.06), which may be due in large part to short-term stability of ice conditions in western Hudson Bay.
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