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Polar Bear Population Status in the Northern Beaufort Sea, Canada, 1971–2006

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Polar Bear Population Status in the Northern Beaufort Sea, Canada, 1971–2006 Ecological Applications, 21(3), 2011, pp. 859–876 Ó 2011 by the Ecological Society of America Polar bear population status in the northern Beaufort Sea, Canada, 1971–2006 1,2,5 3 1,2 4,6 4,7 IAN STIRLING, TRENT L. MCDONALD, E. S. RICHARDSON, ERIC V. REGEHR, AND STEVEN C. AMSTRUP 1Wildlife Research Division, Science and Technology Branch, Environment Canada, 5320-122nd Street, Edmonton, Alberta T6H 3S5 Canada 2Department of Biological Sciences, University of Alberta, Edmonton, Alberta T6H 3S5 Canada 3Western EcoSystems Technology, Inc., 2003 Central Ave., Cheyenne, Wyoming 82070 USA 4USGS Alaska Science Center, Biological Science Office, 4210 University Drive, Anchorage, Alaska 99508 USA Abstract. Polar bears (Ursus maritimus) of the northern Beaufort Sea (NB) population occur on the perimeter of the polar basin adjacent to the northwestern islands of the Canadian Arctic Archipelago. Sea ice converges on the islands through most of the year. We used open- population capture–recapture models to estimate population size and vital rates of polar bears between 1971 and 2006 to: (1) assess relationships between survival, sex and age, and time period; (2) evaluate the long-term importance of sea ice quality and availability in relation to climate warming; and (3) note future management and conservation concerns. The highest- ranking models suggested that survival of polar bears varied by age class and with changes in the sea ice habitat. Model-averaged estimates of survival (which include harvest mortality) for senescent adults ranged from 0.37 to 0.62, from 0.22 to 0.68 for cubs of the year (COY) and yearlings, and from 0.77 to 0.92 for 2–4 year-olds and adults. Horvtiz-Thompson (HT) estimates of population size were not significantly different among the decades of our study. The population size estimated for the 2000s was 980 6 155 (mean and 95% CI). These estimates apply primarily to that segment of the NB population residing west and south of Banks Island. The NB polar bear population appears to have been stable or possibly increasing slightly during the period of our study. This suggests that ice conditions have remained suitable and similar for feeding in summer and fall during most years and that the traditional and legal Inuvialuit harvest has not exceeded sustainable levels. However, the amount of ice remaining in the study area at the end of summer, and the proportion that continues to lie over the biologically productive continental shelf (,300 m water depth) has declined over the 35-year period of this study. If the climate continues to warm as predicted, we predict that the polar bear population in the northern Beaufort Sea will eventually decline. Management and conservation practices for polar bears in relation to both aboriginal harvesting and offshore industrial activity will need to adapt. Key words: aboriginal hunting; Arctic; Beaufort Sea; climate warming; open-population capture– recapture models; polar bears; population estimation; sea ice; survival rates; Ursus maritimus. INTRODUCTION Although polar bears may occasionally capture a seal in open water (e.g., Furnell and Oolooyuk 1980), they are Polar bears are distributed throughout the ice-covered fundamentally dependent upon sea ice as a platform waters of the circumpolar Arctic in 19 relatively discrete from which to hunt seals in both winter and summer populations (Aars et al. 2006). Their preferred habitat is (Stirling 1974, Stirling and Latour 1978, Smith 1980). the annual ice over the relatively shallow waters (,300 Thus, changes in the distribution, total amount, and m) of the continental shelf and interisland channels of types of sea ice, and the patterns of freeze-up and various archipelagos. These areas are more biologically breakup, have the potential to significantly influence the productive and seals are more abundant than in the deep survival and reproductive success of polar bears (e.g., polar basin (Stirling et al. 1982, 1993, Kingsley et al. Regehr et al. 2006, 2010, Stirling and Parkinson 2006). 1985, Stirling and Øritsland 1995, Durner et al. 2009). In this study, we used capture–recapture data to estimate age class-specific annual survival rates and population trend for the northern Beaufort Sea (NB) Manuscript received 28 April 2010; revised 19 July 2010; accepted 9 August 2010. Corresponding Editor: P. K. Dayton. population (Fig. 1) from the mid-1970s to 2006, to assess 5 E-mail: [email protected] factors that might influence survival, particularly those 6 Present address: U.S. Fish and Wildlife Service, 1011 E. related to habitat (i.e., sea ice) loss. We were particularly Tudor Rd., MS341, Anchorage, Alaska 99503 USA. interested in how sea ice habitat might be correlated 7 Present address: Polar Bears International, 810 N. Wallace, Suite E, P.O. Box 3008, Bozeman, Montana 59772 with NB demographic parameters because NB is USA. adjacent to the southern Beaufort Sea (SB) population, 859 860 IAN STIRLING ET AL. Ecological Applications Vol. 21, No. 3 FIG. 1. Northern Beaufort Sea (NB) population boundary and study area in relation to the southern Beaufort Sea (SB) population boundary. which appears to be declining as a result of reduced and northwest to remain on largely multiyear ice along access to suitable sea ice habitat, especially that over the the southern edge of the polar pack, where they can continental shelf, during the open-water season in continue to hunt seals until the ice refreezes again in the summer and fall (Hunter et al. 2010, Regehr et al. 2010). fall (e.g., Amstrup et al. 2000). Around the edge of the polar basin, in areas such as Since 1979, when it first became possible to monitor that occupied by NB, much of the annual ice along the patterns of breakup and freeze-up of sea ice over the coast melts in early summer. The bears then move north entire Arctic Ocean using satellite images, the total April 2011 NORTHERN BEAUFORT SEA POLAR BEARS 861 amount of ice remaining at the annual minimum in late the sustainable annual harvest may be closer to 50 summer has declined at a rate of 9.8% per decade (Lunn et al. 2006). Regardless, the annual harvest has (Comiso 2006). In recent years, there have been several been less than 40 bears for over 15 years (Lunn et al. record sea ice minima in the Arctic (Comiso 2006, 1998, 2002, 2006), largely because of difficult travel Serreze et al. 2007, Stroeve et al. 2007). One conse- conditions for hunters and, to some degree, a reduced quence has been a shift in the position of the southern hunting effort in parts of the area. Even though the edge of the perennial (or multiyear pack) ice over the annual harvest has remained well below the allowable Chukchi and southern Beaufort seas. The southern edge limit, subsequent evaluations of change in the maximum of the pack ice, which used to persist over the sustainable yield, along with recognition that the polar continental shelf through the summer, now retreats far bears’ sea ice habitat is changing, emphasize the to the north over the deep polar basin, where biological importance of a new estimate of population size and productivity is much lower (Pomeroy 1997). In SB, demographic values for the NB population. correlated with the trend toward a longer open-water STUDY AREA season and sea ice being farther offshore (in particular beyond the edge of the continental shelf), there have The NB population is distributed over the sea ice of been several indications that the polar bear population is eastern and northern Amundsen Gulf, the south and west being nutritionally stressed (e.g., Amstrup et al. 2006, coast of Banks Island, and the western end of M’Clure Stirling et al. 2008, Rode et al. 2010). The southern Strait up to the southwestern coast of Prince Patrick Beaufort Sea (SB) population now appears to be in Island (Fig. 1). A defining feature of the marine decline due to decreased recruitment and survival ecosystem in NB is that it borders the Arctic Ocean, (Regehr et al. 2006, 2010, Hunter et al. 2010). from which it receives a steady inflow of cold and In contrast to SB, during the open-water period in relatively unproductive polar water (Pomeroy 1997) via a NB, at least some sea ice remains in most years over the continuous clockwise current, the Beaufort Gyre (Wilson continental shelf along the west coast of Banks Island 1974). This current flows south from the polar basin and Prince Patrick Island and M’Clure Strait. along the west coast of Banks Island through the Cape Occasionally, some ice remains in the western Bathurst Polynya, where it mixes with westerly currents Amundsen Gulf, south of Banks Island. Thus, in recent from Amundsen Gulf, passes westward along the Alaska years, the polar bears in NB have still had access to ice coast, and then flows back north toward the pole. In over the continental shelf during winter and, most almost all months, there is at least some open water in the importantly, through the critical feeding period in spring shore lead and polynya system that parallels the coast and early summer when seals are more abundant there from Prince Patrick Island south through the Cape than they are over the deep polar basin (Stirling et al. Bathurst Polynya and west along the mainland coast 1982). Later in summer, as the ice breaks up, most bears (Smith and Rigby 1981, Stirling 1997). The distributions move back north and northwest toward whatever ice of ringed (Phoca hispida) and bearded (Erignathus remains over the continental shelf to the west of Banks barbatus) seals, and consequently also those of the polar Island and farther offshore until freeze-up later in the bears that hunt them, are influenced strongly by the autumn.
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