Western Hudson Bay (WH)

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

Size Sea ice metrics Human-caused removals 2010–2014
Estimate /
95% CI
Year Method 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 sampling-5.2/3.6-16.32524.82828
See also the complete table (all subpopulations)

Comments, vulnerabilities and concerns

Concerns include harvest, declines in body condition, and lower productivity compared to adjacent FB and SH subpopulations. Decline in size of subpopulation 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.

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 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.
 

References

Calvert, W., M. Taylor, I. Stirling, G. B. Kolenosky, S. Kearney, M. Crête, and S. Luttich. 1995. Polar bear management in Canada 1988-92. Pp. 61-79 in Ø. Wiig, E. W. Born, and G. W. Garner, eds. Polar Bears: Proceedings of the Eleventh Working Meeting of the IUCN/SSC Polar Bear Specialist Group. IUCN, Gland, Switzerland and Cambridge, UK.

Derocher, A. E., and I. Stirling. 1990. Distribution of polar bears (Ursus maritimus) during the ice-free period in western Hudson Bay. Canadian Journal of Zoology 68:1395-1403.

Derocher, A. E., and I. Stirling. 1995a. Estimation of polar bear population size and survival in western Hudson Bay. Journal of Wildlife Management 59:215-221.

Derocher, A. E., and I. Stirling. 1995b. Temporal variation in reproduction and body mass of polar bears in western Hudson Bay. Canadian Journal of Zoology 73:1657-1665.

Derocher, A. E., N. J. Lunn, and I. Stirling. 2004. Polar bears in a warming climate. Integrative and Comparative Biology 44:163-176.Dyck, M. G., and R. K. Baydack. 2004. Vigilance behaviour of polar bears (Ursus maritimus) in the context of wildlife-viewing activities at Churchill, Manitoba, Canada. Biological Conservation 116:343-350.

Lunn, N. J., S. Servanty, S., E. V. Regehr, S. J. Converse, E. Richardson, and I. Stirling. 2016. Demography of an apex predator at the edge of its range – impacts of changing sea ice on polar bears in Hudson Bay. Ecological Applications 26:1302-1320.

Lunn, N. J., I. Stirling, D. Andriashek, and G. B. Kolenosky. 1997. Re-estimating the size of the polar bear population in Western Hudson Bay. Arctic 50:234-240.

Molnár, P. K., A. E. Derocher, T. Klanjscek, and M. A. Lewis. 2011. Predicting climate change impacts on polar bear litter size. Nature Communications 2:186, DOI: 10.1038/ncomms1183.

Molnár, P. K., A. E. Derocher, G. W. Thiemann, and M. A. Lewis. 2010. Predicting survival, reproduction and abundance of polar bears under climate change. Biological Conservation 143:1612-1622.

Peacock, E., A. E. Derocher, N. J. Lunn, and M. E. Obbard. 2010. Polar bear ecology and management in Hudson Bay in the face of climate change. Pp. 93-115 in S. H. Ferguson, L. L. Loseto, and M. L. Mallory, eds. A Little Less Arctic: Top Predators in the World’s Largest Northern Inland Sea. Springer, New York.

Regehr, E. V., N. J. Lunn, S. C. Amstrup, and I. Stirling. 2007. Effects of earlier sea ice breakup on survival and populaiton size of polar bears in western Hudson Bay. Journal of Wildlife Management 71:2673-2683.

Sciullo, L., G. W. Thiemann, and N. J. Lunn. 2016. Comparative assessment of metrics for monitoring the body condition of polar bears in Western Hudson Bay. Journal of Zoology 300:45-58.

Stapleton S, Atkinson S, Hedman D, and Garshelis D. 2014. Revisiting Western Hudson Bay: using aerial surveys to update polar bear abundance in a sentinel population. Biological Conservation 170:38-47.

Stirling, I., and A. E. Derocher. 1993. Possible impacts of climatic warming on polar bears. Arctic 46:240-245.

Stirling, I., A. E. Derocher, W. A. Gough, and K. Rode. 2008. Response to Dyck et al. (2007) on polar bears and climate change in western Hudson Bay. Ecological Complexity 5:193-201.

Stirling, I., C. Jonkel, P. Smith, R. Robertson, and D. Cross. 1977. The ecology of the polar bear (Ursus maritimus) along the western coast of Hudson Bay. Canadian Wildlife Service Occasional Paper 33, 64 pages.

Stirling, I., N. J. Lunn, and J. Iacozza. 1999. Long-term trends in the population ecology of polar bears in western Hudson Bay in relation to climatic change. Arctic 52:294-306.

Stirling, I., N. J. Lunn, J. Iacozza, C. Elliott, and M. Obbard. 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.

Taylor, M., and J. Lee. 1995. Distribution and abundance of Canadian polar bear populations - a management perspective. Arctic 48:147-154.

Wiig, Ø., E. W. Born, and G. W. Garner, eds. 1995. Polar Bears: Proceedings of the Eleventh Working Meeting of the IUCN/SSC Polar Bear Specialist Group. IUCN, Gland, Switzerland and Cambridge, UK.