1999; Stirling 2002).
annually migrate to feed in the coastal shoal habitats of the
Bearded seals ( Erignathus barbatus (Erxleben, 1777)) are
Alaskan Arctic from July through October (Moore et al.
also available to polar bears year-round in the southern Beau-
2000). Although there is no official subsistence hunt for
fort Sea. Adult bearded seals are significantly larger than
gray whales in Alaska, a limited number of beach-cast gray
ringed seals, thereby providing a greater biomass (per ani-
whales do occur along the coast in the summer and autumn
mal) than ringed seals. In Svalbard, Derocher et al. (2002)
months (J. George, North Slope Borough – Department of
compiled data from 17 years of tracking bears by helicopter
Wildlife Management, unpublished data (2004)). These
and reported the numerical composition of kills. Polar bear
whales present another potential low trophic level dietary
kills were dominated by ringed seal (75%), followed by
source for polar bears along Alaska’s Beaufort Sea coast;
bearded seal (16%), and harp seal ( Pagophilus groenlandicus
however, the numbers available in winter are likely low.
(Erxleben, 1777); 9%). Examination of stomach contents
Most dietary studies of polar bears have been observa-
from hunter-killed bears in the same area reported similar
tions of killed prey (Stirling and Archibald 1977; Hammill
proportions of ringed and bearded seals in the diet of Sval-
and Smith 1991; Derocher et al. 2002). The relative impor-
bard polar bears (Lønø 1970). Of polar bear kills observed
tance of each of the potential prey species in the diet of po-
while tracking in the Alaskan portion of the southern Beau-
lar bears is poorly understood and likely varies with sex,
fort Sea, ringed seals predominate, although bearded seal
age, location, time of year, and fluctuations in annual avail-
carcasses are frequently observed (S.C. Amstrup, personal
ability. Stable isotope analyses of carbon (13C/12C, d13C) and
communication (2005)).
nitrogen (15N/14N, d15N) have been used as a tool for evalu-
In addition to ringed and bearded seals, polar bears along
ating trophic relationships in the Arctic marine food web
Alaska’s Beaufort Sea coast, and elsewhere in the Arctic,
(Hobson and Welch 1992; Atwell et al. 1998; Pauly et al.
have been observed consuming beluga whales ( Delphinap-
1998; Hoekstra et al. 2002; Dehn et al. 2005, 2006). In par-
terus leucas (Pallas, 1776)) and walruses ( Odobenus rosma-ticular, d15N composition of an organism has been used as
rus (L., 1758)) (Lowry et al. 1987; Calvert and Stirling
an indicator of its relative trophic position. There is gener-
1990; Smith and Sjare 1990; Rugh and Shelden 1993).
ally a 3%–5% increase in d15N values with each trophic
Both walrus and beluga are migratory species that overwin-
transfer (Hobson and Welch 1992; Kelly 2000), which pro-
ter in the Bering Sea (Sease and Chapman 1988; Hazard
vides an integrated measure of the trophic position of an or-
1988), but may be accessible to Beaufort Sea polar bears
ganism over time (Atwell et al. 1998; Pond and Gilmour
on a limited basis. Beach-cast walrus carcasses have been
1997). In addition to measuring between-species trophic re-
observed along the Beaufort Sea coast in June and July, but
lationships in the Arctic marine food web, d15N and d13C
fewer than five carcasses were recorded during aerial sur-
values have been used to trace dietary history and nutritional
veys conducted in 1995 and 1997 (Kalxdorff 1998). Winter
ecology within populations, as well as movements between
availability of beluga whales and walruses to Beaufort Sea
geographically distinct ecosystems (Abend and Smith 1995;
polar bears is likely limited. However, the overall contribu-
Hilderbrand et al. 1996; Pond and Gilmour 1997; Hobson
tion of these species to polar bear diets remains unclear.
and Schell 1998; Bowen et al. 2005). By using stable iso-
When available, polar bears will scavenge on the remains
topes of two elements and mass balance equations, it is pos-
of bowhead whales ( Balaena mysticetus L., 1758). Along
sible to estimate the proportions of three isotopically distinct
Alaska’s Beaufort Sea coast, bowhead whale carcasses are
prey items that may make up the diet of the predator (Hil-
utilized by scavenging polar bears following the traditional
derbrand et al. 1996; Hilderbrand et al. 1999; Phillips and
hunt of the Inupiat people (Miller et al. 2004). These com-
Gregg 2001; Ben-David et al. 2004).
munities have harvested bowhead whales for generations
In this study, we used d13C and d15N analyses to describe
(Reeves 2002), and currently, the Alaska Eskimo Whaling
the feeding ecology of Beaufort Sea polar bears, focusing on
Commission, with permission of the International Whaling
three known fall and winter foods (i.e., ringed seals, bearded
Commission, allows an annual quota of whales to be har-
seals, and remains of bowhead whales). Ringed and bearded
vested. Among the three whaling communities along Alas-
seals are both high trophic level predators with mixed diets
ka’s portion of the Beaufort Sea, 26 whales were landed in
consisting of varying proportions of fish and marine inverte-
fall 2002 and 13 whales were landed in fall 2003 (Suydam
brates (Lowry et al. 1980; Dehn et al. 2005). Although both
et al. 2002; Suydam et al. 2003). Following these successful
ringed and bearded seals occupy a similar trophic level as
hunts, polar bear aggregations of up to 60 individuals were
measured by d15N, their mean d13C signatures differ by ap-
reported around bowhead harvest sites (Miller et al. 2004).