Western Hudson Bay (WH)

Abundance estimate of 1000 (95% CI: 715-1398) 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

Size Trend Sea ice metrics Human-caused removals 2010–2014
Estimate /
95% CI
Year Method Relative to historic level
(approx. 25-yr past)
Current (approx. 12-yr
period centered on present)
Change in spring ice retreat / Change in fall ice advance (days per decade) Change in summer sea ice area (percent change per decade) 5-yr mean Last year
Potential Actual Potential Actual
1030
754-1406
2011Distance samplingReducedStable-5.2/3.6-16.322.623.22832
See also the complete table (all subpopulations)

Comments, vulnerabilities and concerns

Concerns include harvest, projected habitat decline, declines in body condition, and lower productivity compared to adjacent Foxe Basin and Southern Hudson Bay subpopulations. Decline in size of population from late 1980s through late 1990s / early 2000s was linked to reduced survival due to timing of sea ice breakup. Recent analysis indicated stability in subpopulation size from 2001–2010; a period during which there was no significant trend in timing of sea ice breakup or freezeup. This analysis confirmed continued linkage between female survival and sea ice conditions. TEK suggests that subpopulation is increasing.

Status and delineation

Western Hudson Bay subpopulation mapThe Western Hudson Bay area. See also the complete map (all subpopulations).

The current population boundaries of the Western Hudson Bay (WH) subpopulation are based on extensive records of capture, recapture, and harvest of tagged animals (Stirling et al. 1977, Derocher and Stirling 1990, 1995, 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 Derocher1993, Taylor and Lee 1995).

During the 1960s and 70s, subpopulation 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 1995). Derocher and Stirling (1995) estimated the mean population size for 1978-1992 to be 1000 ± 51. 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 1200 (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, they estimated the abundance to be 1233 ± 209 in 1995. Regehr et al. (2007) reported a decline in abundance from 1194 (95% CI = 1020, 1368) in 1987 to 935 (95% CI = 794, 1076) 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 comprehensive aerial survey was conducted in 2011 as part of a program by the Government of Nunavut to continue development of non-invasive techniques to estimate abundance and to address inconsistencies between local knowledge and perceptions that suggest WH is increasing and the research that suggest WH is decreasing (Stapleton et al. 2014). Both distance sampling and sight-resight protocols were used on the survey; a total of 711 bears were sighted. Analysis of the survey data resulted in an abundance estimate of 1,030 (95% CI = 754, 1406). 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. However, in the two years following that of the aerial survey, far fewer bears were seen in the area east of the Nelson River at the same time of year (unpublished data). As well, 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.

Lunn et al. (2014) 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 different methods were used and 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 capture-recapture sampling area.

In addition to an estimate of abundance, Lunn et al. (2014) 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 nonlinearity (i.e., short-term stability) in the long-term observed and forecasted trend toward earlier sea ice break-up in western Hudson Bay.
 

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