Southern Beaufort Sea (SB)

Status table outtake

Table comment: Population estimate is from an analysis of mark-recapture data from 2001 - 2006. Estimated risk of future decline is based on vital rates estimated from the 2001-2006 data used in matrix-based demographic models that incorporate sea ice forecasts. The SB-NB boundary is being re-considered, which may affect estimates of the size and status of both subpopulations.

Research on polar bears in the SB has been ongoing since 1967 (Amstrup et al. 1986, Stirling 2002). Radio-telemetry and mark-recapture studies through the 1980s indicated that polar bears in the region comprised a single subpopulation, with an eastern boundary between Paulatuk and Baillie Island, Northwest Territories, Canada, and a western boundary near Icy Cape, Alaska (Amstrup et al. 1986, Amstrup and DeMaster 1988, Stirling et al. 1988). Analyses of more recent satellite relocations using probabilistic models indicate that, rather than exhibiting distinct boundaries, there are areas of overlap between the SB and adjacent subpopulations (Amstrup et al. 2004b; Amstrup et al. 2005). At Barrow, Alaska, USA in the west, 50% of polar bears are from the SB subpopulation and 50% are from the Chukchi Sea (CS) subpopulation. At Tuktoyaktuk, Northwest Territories, Canada, in the east, 50% of polar bears are from the SB subpopulation and 50% are from the northern Beaufort Sea (NB) subpopulation. Based on this analysis, polar bears in the vicinity of the current eastern boundary near Pearce Point, Northwest Territories, are rarely members of the SB subpopulation. To address this issue, user groups, scientists and resource managers are discussing a western shift of the SB-NB boundary. One proposal has been to move it to 133° W longitude but a line further east is also under consideration. A decision on the potential boundary shift is expected in 2010, at which time the abundance and status of the SB and NB subpopulations will be re-evaluated. A similar boundary shift, or a change in the way harvest is allocated among subpopulations, may be required on the western side of the SB subpopulation where it borders the CS subpopulation (Amstrup et al. 2005). Sound management requires that current scientific information be used to define biologically relevant polar bear subpopulations. This presents an increasing challenge, as sea ice loss and increased variability in sea ice extent have the potential to affect polar bear movements and distribution, including the breakdown of historic subpopulation boundaries (Derocher et al. 2004).

The size of the SB subpopulation was first estimated to be approximately 1,800 animals in 1986 (Amstrup et al. 1986). Survival rates of adult females and dependent young were estimated from radio-telemetry data collected from the early 1980s to the mid-1990s (Amstrup and Durner 1995). Through the 1980s and early 1990s, observations suggested that the SB subpopulation was increasing. Amstrup et al. (2001) found that the SB subpopulation may have reached as many as 2,500 polar bears in the late 1990s. However, that estimate was not considered reliable due to methodological difficulties, and management decisions continued to be based on a population size of 1,800. Results from an intensive mark-recapture study conducted from 2001-2006 in both the USA and Canada indicated that the SB subpopulation included 1,526 (95% CI = 1,211 – 1,841) polar bears in 2006 (Regehr et al. 2006). This suggests that the size of the SB subpopulation declined between the late 1990s and 2006, although low precision in the previous estimate of 1,800 precluded a statistical determination. Subsequent analyses of the 2001-2006 data using multistate and demographic models indicated that the survival and breeding of polar bears during this period were affected by sea ice conditions, and that population growth rate was strongly negative in years with long ice-free seasons, such as 2005 when Arctic sea ice extent reached a record low (Hunter et al. 2007, Regehr et al. 2010). Thus, the SB subpopulation is currently considered to be declining due to sea ice loss. If the region continues to lose polar bear habitat as forecasted by global climate models (Durner et al. 2009), it is likely that the SB subpopulation will face extirpation in the next 100 years (Hunter et al. 2007).