ORIGINAL PAPER
Charles Monnett Æ Jeffrey S. Gleason
Observations of mortality associated with extended open-water swimming
by polar bears in the Alaskan Beaufort Sea
Received: 25 April 2005 / Revised: 20 December 2005 / Accepted: 20 December 2005 / Published online: 12 January 2006

Springer-Verlag 2006
Abstract During aerial surveys in September 1987–2003,
a total of 315 live polar bears were observed with 12
(3.8%) animals in open water, defined for purposes of
this analysis as marine waters >2 km north of the
Alaska Beaufort Sea coastline or associated barrier is-
lands. No polar bear carcasses were observed. During
aerial surveys in early September, 2004, 55 polar bears
(Ursus maritimus) were seen, 51 were alive and of those
10 (19.9%) were in open water. In addition, four polar
bear carcasses were seen floating in open water and had,
presumably, drowned. Average distance from land and
pack ice edge for live polar bears swimming in open
water in 2004 (n=10) were 8.3±3.0 and 177.4±5.1 km,
respectively. We speculate that mortalities due to off-
shore swimming during late-ice (or mild ice) years may
be an important and unaccounted source of natural
mortality given energetic demands placed on individual
bears engaged in long-distance swimming. We further
suggest that drowning-related deaths of polar bears may
increase in the future if the observed trend of regression
of pack ice and/or longer open water periods continues.
Introduction
Polar bears (Ursus maritimus) depend on the vast ex-
panses of Arctic sea ice for hunting their primary prey
species, ringed (Phoca hispida) and bearded (Erignathus
barbatus) seals (Stirling and Derocher 1993, Stirling
et al. 1993) and locating potential mates. In some loca-
tions, they rely heavily on fat stores to meet energetic
demands incurred during the open water fasting period
(Ramsay and Stirling 1988; but see Derocher et al.
1993).
There is evidence that total extent of Arctic sea ice
has declined at an annual rate of 3–5% over the past
several decades, although these declines are not consis-
tent across the Arctic (Comiso 2003, 2005; Stroeve et al.
2004). Warming trends in the Arctic (Comiso 2003) also
appear to be affecting thickness of multiyear ice in the
polar basin (Rothrock et al. 1999) and perennial sea ice
coverage (declines 9% per decade) (Comiso 2002a, b).
Concerns have been raised over apparent declines in the
minimum extent of summer ice pack and temporal in-
creases in the open water period, and the potential for
impacts on polar bears and their populations (Stirling
and Derocher 1993; Stirling 2002; Derocher et al. 2004).
Evidence of potential negative effects from reductions
in Arctic sea ice comes from long-term monitoring of
polar bears in Canada. Mean monthly (April–June)
surface air temperatures in western Hudson Bay in-
creased at a rate of 0.2–0.3C per decade during 1950–
1990 (Skinner et al. 1998) and annual ice breaks up 2.5
weeks earlier than 30 years ago (Stirling et al. 1999;
Derocher et al. 2004; Gough et al. 2004). Female polar
bears in this region are now coming ashore roughly 2
weeks earlier and in poorer condition (Derocher and
Stirling 1992; Stirling and Lunn 1997; Stirling et al.
1999). Long-term declines in birth rates and reduced cub
weights in conjunction with delayed fall dispersal onto
the ice by females and associated young are indicative of
negative population effects, at least at a regional scale
(Stirling and Derocher 1993; Stirling et al. 1999; Der-
ocher et al. 2004). Though these sublethal effects are
now recognized, no mention has been made of potential
for lethal effects of reduced sea ice on individual polar
bears. The net effect of global climate change on polar
bear populations remains largely unknown (but see
Stirling et al. 1999; Derocher et al. 2004), but its po-
tential for negative impacts may pose one of the greatest
conservation challenges to the management of polar
bears (Norris et al. 2002; Hassol 2004).
The primary purpose of the Minerals Management
Service (MMS) Bowhead Whale Aerial Survey Project
(BWASP) is to monitor the fall migration of bowhead
C. Monnett (&) Æ J. S. Gleason
Minerals Management Service, Environmental Studies Section,
3801 Centerpoint Drive, Suite 500, Anchorage, AK, 99503 USA
E-mail: charles.monnett@mms.gov
Polar Biol (2006) 29: 681–687
DOI 10.1007/s00300-005-0105-2

whales in the Alaskan Beaufort Sea. However, systematic
observations have been made of all marine mammals
encountered, including polar bears. Herein, we discuss
our observations of polar bears swimming offshore in the
Alaskan Beaufort Sea and the circumstances surrounding
subsequent observations of dead, and presumably
drowned, polar bears.
Study area
The BWASP is based on a set of random field transects
within established geographic blocks overlapping or near
Chukchi and Beaufort Sea lease sale areas offshore of the
north Alaskan coast (140–157W and south of 72N,
Fig. 1). The study area roughly overlaps a ‘‘core area’’ of
polar bear activity (see Amstrup 2000, Amstrup et al.
2000, 2001 for further details). In the Beaufort Sea,
landfast ice forms during the fall and may eventually
extend up to 50 km offshore by the end of winter (Norton
and Weller 1984). The pack ice, which includes multiyear
ice averaging 4 m in thickness with pressure ridges up to
50 m thick (Norton and Weller 1984), becomes contig-
uous with new and fast ice in late fall. From early-
November to mid-May, the Beaufort Sea remains almost
totally ice covered. In mid-May, a recurring flaw lead can
form just seaward of the stable fast ice followed by
decreasing ice concentrations and large areas of open
water in summer (LaBelle et al. 1983). A detailed
description of local weather patterns off the coast of
northern Alaska is provided by Brower et al. (1988).
Methods
The BWASP survey was conducted from 1987 to 2004
from a de Havilland Twin Otter Series 300 aircraft
equipped with two medium-sized bubble windows be-
hind the cabin bulkhead and one on the aft starboard
side that afforded complete trackline viewing. Data
Fig. 1 Distribution of polar bears (Ursus maritimus) observed
during fall annual bowhead whale (Balaena mysticetus) aerial
surveys in the Beaufort Sea, 1987–2004. Open circles represent live
bears that were observed swimming. Closed circles represent dead
bears observed in 2004. Numbers within open circles indicate year
in which the observation occurred. Circles without numbers
represent observations in 2004
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