Western Hudson Bay (WH)

Abundance estimate of 842 (95% CI: 562-1121) from 2016, few observations of females with cubs. Declining survival, birth rates and body condition have been linked to earlier ice-break up.

Status table outtake

Subpopulation size Subpopulation trend Sea ice metrics 1979-2018 Human-caused removals 2013/2014–2017/2018
Estimate and uncertainity Method and type of evidence Year and citation Long term (approx 3 generations) Short term (approx 1 generation) Change in date of spring ice retreat / fall ice advance (days per decade) Change in summer sea ice area (percent change per decade) 5-year mean
Quota (bears per year) Actual (% of total population)
842
562-1121
Mark-recapture distance sampling2016Very likely decreased (1995 to 2016)Likely decreased (2011 to 2016)-5.9/3.2-21.831.629.6 (3.5%)
See also the complete table (all subpopulations)

Comments, vulnerabilities and concerns

Concerns include harvest, increased time onshore due to changing dates of breakup and freeze-up, declines in body condition, and lower productivity. Earlier declines in size of subpopulation linked to reduced survival due to timing of sea ice breakup. 2016 abundance estimate was 18.3% lower than 2011 estimate; similar rate of change in abundance over same time period in adjacent Southern Hudson Bay subpopulation.

Status and delineation

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

Hudson Bay is a relatively shallow inland sea that is ice covered in winter and ice free in summer (Hochheim et al. 2010).  Although three subpopulations of polar bears (Foxe Basin, Southern Hudson Bay, and Western Hudson Bay) occur on the sea ice in winter and spring, they appear to be largely segregated during the open-water season (Derocher and Stirling 1990; Peacock et al. 2010; Viengkone et al. 2016). During the ice-free period, Western Hudson Bay polar bears exhibit strong fidelity to terrestrial summering areas in northeastern Manitoba (Stirling et al. 1977; Derocher and Stirling 1990; Cherry et al. 2013; Stapleton et al. 2014; Lunn et al. 2016). The current Western Hudson Bay subpopulation boundary is based largely 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).

Although the size of the Western Hudson Bay subpopulation was unknown until the 1990s (Derocher and Stirling 1995a), Inuit have observed a larger number of bears in recent decades relative to the historic levels of the early 1900s to 1970s (McDonald et al. 1997; Tyrrell 2006, 2009; Nirlungayuk and Lee 2009; Henri et al. 2010; Kotierk 2012). A significant factor likely contributing to this observed increase was a population-level response to decreased hunting pressure that occurred in the 1950s and 1960s resulting from 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 (SE = 51). However, this estimate was considered conservative because the study had not covered the southern portion of the range east of the Nelson River (Calvert et al. 1995; PBSG 1995) and, therefore, for management purposes the population size was adjusted to 1,200 (Calvert et al. 1998). In 1994 and 1995, Lunn et al. (1997) expanded the capture program to sample animals to the Western Hudson Bay/Southern Hudson Bay management boundary and estimated abundance to be 1,233 (SE = 209) in 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 and also documented that the survival rates of cubs, sub-adults, and old bears (>20 years) were negatively correlated with the date of sea ice breakup.

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 Western Hudson Bay/Southern Hudson Bay 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. 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, which is consistent with Western Hudson Bay 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 37 years, which has likely contributed to declining reproductive success (Derocher and Stirling 1995b; Stirling et al. 1999; Sciullo et al. 2016; Lunn and McGeachy 2018).

Lunn et al. (2016) evaluated the demography and status of the Western Hudson Bay subpopulation 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 as well as 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 the abundance estimates from the aerial survey and capture-recapture model are broadly similar with overlapping confidence intervals, it is difficult to make direct comparisons because the studies differed with respect to spatial and temporal perspectives and with the assumptions of each method (Lunn et al. 2016). The aerial survey provides a snapshot estimate of the total number of polar bears in the Western Hudson Bay management area at the time of the survey whereas the point estimate from the capture-recapture model is based on the number of bears that moved through the smaller, capture-recapture sampling area over multiple years.

The most recent estimate of abundance comes from a mark-recapture distance sampling study in 2016 to update subpopulation status (Dyck et al. 2017). Pre-survey consultations with Nunavut Hunters’ and Trappers’ Organizations, Kivalliq communities, and with the Manitoba Department of Sustainable Development were conducted in order to include local and traditional knowledge in the study design. Dyck et al. (2017) estimated there to be 842 bears (95% CI: 562–1121) that, although not statistically significant from the previous aerial survey estimate, represented an 18% decline in abundance between 2011 and 2016. Over the same period of time and using similar methods, Obbard et al. (2018) documented a 17% decline in abundance for the neighbouring Southern Hudson Bay subpopulation. Similar to observations from the 2011 survey, cubs-of-the-year and yearling cubs comprised a small proportion of the sample size (Dyck et al. 2017) and suggested that low reproductive performance of the Western Hudson Bay subpopulation has continued.

From the 1930s through the 1960s, Inuit report that encounters with polar bears in the interior of the Kivalliq mainland and along the Kivalliq coast of Hudson Bay were rare (Nirlungayuk and Lee 2009; Tyrrell 2009). Within the last few decades more bear-human encounters have occurred resulting in increased concerns for human safety and property damage (Tyrrell 2006, 2009; Henri et al. 2010).
 

References

Reference list