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Polar Biol (2002) 25: 448–452
DOI 10.1007/s00300-002-0364-0
ORIGINAL PAPER
Andrew E. Derocher Æ Øystein Wiig Æ Magnus Andersen
Diet composition of polar bears in Svalbard
and the western Barents Sea
Accepted: 21 January2002 / Published online: 20March 2002
Springer-Verlag 2002
Abstract Weestimatedboththenumericalandbiomass
compositionofthepreyofpolarbears(Ursus maritimus)
from135opportunisticobservationsofkillsinSvalbard
and the western Barents Sea collected from March to
October 1984–2001. By number, the prey composition
was dominated by ringed seals (Phoca hispida) (63%),
followed by bearded seals (Erignathus barbatus) (13%),
harp seals (P. groenlandica) (8%) and unknown species
(16%). However, when known prey were converted to
biomass, the composition was dominated by bearded
seals (55%), followed by ringed seals (30%) and harp
seals(15%).Resultsindicatedthatbeardedsealsarean
important dietary item for polar bears in the western
Barents Sea. We believe that different patterns of space
usebydifferentbearsmayresultingeographicvariation
of diet within the same population.
relative energetic contribution of prey species and the
seasonal composition of prey.An earlier study ofpolar
bear diet in Svalbard also suggested that ringed and
bearded seals were the main prey (Lønø 1970). Uncer-
taintyindietcompositionarisesbecausepolarbearsalso
consume a variety of other species, including walrus
(Odobenus rosmarus) (Calvert and Stirling 1990), white
whales (Delphinapterus leucas) and narwhal (Monodon
monoceros) (Lowry et al. 1987; Smith and Sjare 1990),
harp seals (P. groenlandica) (Lønø 1970), seabirds
(Stempniewicz 1993) and carrion (Christiansen 1981).
However, most studies concur that seals are the main
prey.
In this paper, we document polar bear predation on
ringed, bearded and harp seals in Svalbard and the
westernBarentsSeafrom observations ofseal killsites.
We also discuss the relative importance of these species
in the diet of polar bears.
Introduction
Materials and methods
ThemostcarnivorousoftheUrsidae,polarbears(Ursus
maritimus) are thought to prey largely on ringed seals
(Phoca hispida)and,toalesser extent,onbeardedseals
(Erignathus barbatus) (Stirling and Archibald 1977;
Smith 1980; Gjertz and Lydersen 1986; Stirling and
Øritsland1995).However,thedietofpolarbearsisstill
poorly understood, with limited information about the
Samplingoccurredfrom1984to2001,fromMarchtoOctober,on
an opportunistic basis. The total sampling area is approximately
contained by the outermost locations of seal kills (Fig. 1). Polar
bearsinthesamplingarearepresentedallage,sexandreproductive
classes in the population, and we believe the seal kills were repre-
sentative of those taken by the population.
Sealkillsontheseaiceareeasytoidentifyduetothepresence
ofblood,scavengers(glaucousgulls, Larus hyperboreus,ivorygulls,
Pagophila eburnea,arcticfox, Alopex lagopus),orbearsatthekill.
Wewereabletoexcludearcticfoxpredationbasedonpresenceor
absence of tracks. Most (n=113) seal kill sites were located while
tracking polar bears by helicopter, during research on polar bear
ecology. Observations collected during August (1999) were ob-
tained during an aerial survey to estimate polar bear abundance.
Some information on kills was collected from other researchers
working in the area. We included 22 samples noted in earlier
studies of seal predation to increase geographic coverage. We
included 6 ringed seal kills from Gjertz and Lydersen (1986), 1
ringedsealkillfromLydersenandGjertz(1986)and15ringedseal
kills from Wiig et al. (1999).
Limited information from kills was available from most ob-
servations. Identification of species and age class of the kill was
based on assessment from the air, ship or by examination of
) Æ M. Andersen
Norwegian Polar Institute,
9296, Tromsø,Norway
E-mail: derocher@npolar.no
Tel.: +47-77-750524
Fax: +47-77-750501
Ø. Wiig
Zoological Museum,
University of Oslo,
P.O. Box 1172 Blindern, 0318,
Oslo, Norway
&
A.E. Derocher (
449
Results
Fig. 1. Distribution of ringed seal, bearded seal, harp seal and
unknown seal species killed by polar bears in Svalbard and the
westernBarentsSeain1984–2001.Thestudyareaisapproximately
delineated by the outermost kill sites
Information was obtained on 135 seal kills (Table 1)
with120(89%)collectedin1995–2001.Numerically,the
majorityofthekillswereringedseals(63%),followedby
bearded seals (13%), harp seals (8%) and 16% of un-
known species. The numerical kill composition of
known species (n=114) was 75% ringed seal, 16%
bearded seal and 9% harp seal. Most (14/21) of the
unknownpreyspecieswerefromAugust,whensampling
wasconductedduringanaerialsurveyandcheckingkills
onthegroundwasnotpossible.Oftheringedsealkills,
pups composed 72% (54/75) of those classified to age
group. For bearded seals, 31% (5/16) were pups. The
prey composition by biomass (n=98) was estimated to
be composed of 50% adult bearded seal, 20% adult
ringedseal,10%ringedsealpup,15%harpsealand5%
bearded seal pup.
TheseasonaldistributionofpredationwasdiKcultto
assess due to non-representative sampling through the
months. However, ringed seal predation appeared to
dominate in spring during the pupping season, and
predationonbeardedsealswasmoreevenlydistributed
through the sampling period. Observation of harp seal
predationwasrestrictedtoJune.Twenty-fourkillswere
observed during August, with eight in multiyear ice.
While manyofthese killswere notidentified tospecies,
beardedsealswereabundantintheareaandtwooffour
adultbeardedsealkillsinAugustwereinmultiyearice.
Insu
K
cient data were available to determine the spatial
patternsofthekillsbutitappearedthatpredationevents
occurred throughout the study area (Fig. 1).
remains.Itwassometimesnotpossibletoidentifythepreyspecies
or age as only bone fragments or blood remained. During the
spring pupping season, kills at digs in rough pressure-ridged ice
with little blood in the area were classified as ringed seal pups.
Adultbeardedandharpsealswereobviousfromtheirsize.Because
teeth and claws were available from only a few samples (less than
ten animals), ages from these werenot determined.
We estimated prey biomass in the diet of polar bears using a
mean mass as an approximation of the size for each seal species.
Weexcludedallpreyofunknownspeciesandageclassforbiomass
estimation. The mass of adult seals was obtained from growth
curvesusingthemeanasymptoticsizesoffemalesandmales.Adult
mass of ringed seals was set at 57kg (Lydersen and Gjertz 1987),
273 kg for bearded seals (Andersen et al. 1999) and 131 kg for
harp seals (Innes et al. 1981). We pooled juvenile and adult seals
and used adult size in calculations to treat the three species in a
similarmanner, butrecognisedthatthismaybea sourceof error.
We averaged the birth and weaning masses to estimate pup body
mass, from published studies. For ringed seals, we used 11kg,
based on a birth mass of 4.55 kg and a weaning mass of 18 kg
(Lydersen et al. 1992). Similarly, we used 62 kg for bearded seal
pups,whichwasbasedonabirthmassof33kg(Burns1981)anda
weaning mass of 92kg (growth of 3.3 kg/day over 18 days)
(Lydersen et al. 1994).
Discussion
Estimating the diet of any free-ranging animal is a dif-
ficult undertaking and prone to inaccuracies. Given the
inaccessibility of polar bear habitat, it is extremely
problematic to obtain an overview of their diet, partic-
ularly in remote areas, during summer when the ice is
melting, and during the winter dark period. The meth-
ods used in our study were opportunistic and classifi-
cation into age classes was approximate. Using a mean
pup mass and asymptotic mass for adults may result in
biasesofestimatingintakeif,forexample,beardedseals
Table 1. Number of seal kills
found opportunistically in
Svalbard and the western
Barents Sea from1984 to 2001
and attributed to polar bear
predation, in relation to month
Month Ringed seals
Bearded seals
Harp seals
Unknown
Ad/Juv Pup Unk Ad/Juv Pup Unk Ad/Juv Unk
March 1 2 – – – – – – –
April 9 35 6 3 1 – – – 4
May 7 17 1 2 4 1 – – 2
June – – – 2 – – 7 4 –
August 3 – 3 4 – – – – 14
October 1 – – – – 1 – – 1
Total 21 54 10 11 5 2 7 4 21
450
killed were well below the asymptote. However, we feel
that the methods used are a reasonable representation
and any method would result in some level of bias. We
could not assess possible sampling bias of the different
prey. For example, it is possible that a fresh kill of a
large bearded seal may be more easily detected than a
smaller ringed seal killed some days earlier. Therefore,
interpretation of our data must proceed with caution.
Marine-mammalresourcesavailabletopolarbearsin
the study area are poorly understood and quantitative
estimates are unavailable for most potential prey.
Bearded seals are widely distributed throughout
Svalbard and the western Barents Sea (Benjaminsen
1973),andtheirdistribution overlaps substantially with
that of polar bears. Bearded seals are largely benthic
feedersandcandivetodepthsupto400 m(Burns1981;
Gjertz et al. 2000), so that most of the Barents Sea,
which is less than 300 m deep, may provide feeding
habitat.Theabundanceofbeardedsealsisuncertainbut
maynumberinthe300,000rangeintheNorthAtlantic
(Burns 1981).
TheringedsealpopulationsizeintheSvalbardareais
unknown but the global population likely numbers in
themillions(Reeves1998).InSvalbardandthewestern
Barents Sea, there is ringed seal reproduction in both
land-fast ice (Smith and Lydersen 1991) and drifting
pack-ice(Wiigetal.1999).Duringstudiesofringedseal
breedinghabitatinSvalbard,adiscrepancywasnotedin
theproductionofringedsealsandthenumberofringed
seals required to support the polar bear population in
the area (Smith and Lydersen 1991). Smith and Lyder-
sen (1991) suggested that pack-ice production of ringed
seals may be an important contribution to the popula-
tion. Our results further confirm the findings of Wiig
et al. (1999) that ringed seals breed in the drifting pack
ice of the Barents Sea, particularly northwestof Hopen
Island. Ringed seals are available to all bears in the
study population.
The Barents Sea harp seal population is approxi-
mately 2.2 million animals (Nilssen et al. 2000) and
represents apotentially abundant foodsourcefor polar
bears. However, harp seals do not reach polar bear
habitatuntilApril/Mayandthenincreaseinabundance
alongthedrift-ice edgeuntilOctober,when theyreturn
south(Haugetal.1994;Nordøyetal.1998).Harpseals
areavailabletopolarbearsparticularlyinearlysummer,
whenbothspeciesselectopenice(10–60%cover)(Haug
et al. 1994). However, some harp seals are pelagic and
this portion of the population is unavailable to polar
bears(Nordøyetal.1998).Furtherresearchisneededto
quantifytheimportanceofharpsealstothispolarbear
population.
The only previous study of polar bear diet in the
study area comes from bears harvested throughout the
yearnearSvalbard;52ringedseals,10beardedsealsand
6harpsealswerefoundinstomachs(Lønø1970).Harp
seals were only found during June/August and most
beardedseals(9/10)werefoundinthesameperiod.The
prey composition from this study was 76% ringed seal,
15% bearded seal and 9% harp seal, and was very
similartothecompositionofthe114samplesofknown
species in our study. Numerically, similar to earlier
studies, ringed seals are the dominant prey of polar
bears. However, on a biomass basis, the results from
Lønø(1970),togetherwithours,suggestthatthedietof
polarbearsinSvalbardandthewesternBarentsSeahas
asignificantcontribution from beardedseals. However,
in the eastern Barents Sea, a Russian study reported
68% ringed seal, 22% walrus and miscellaneous other
items for the diet of polar bears (Parovshchikov 1964)
andmayreflectfurthergeographicvariationinthesame
population. Studies of fatty-acid profiles of polar bears
suggest that geographic variation in diet may be large
(Iverson et al. 1999).
Polar bears in Svalbard and thewestern BarentsSea
area are part of a common population that extends as
far east as Franz Josef Land (Mauritzen et al. 2002).
Polar bears living in the study area have two different
space use patterns: one group lives near shore and has
smallannualrangeswhereastheotherlivesoffshoreand
haslargerranges(Mauritzenetal.2001).Annualrange
size of adult females ranged from 185 to 373,539 km
2
and dietary differences were postulated to explain the
different space use patterns (Mauritzen et al. 2001). In
particular, Mauritzen et al. (2001) suggested that near-
shore bears relied more on the land-fast ice and preyed
largelyonringedsealsduringspringwhilepelagicbears
preyed more on bearded and harp seals over a longer
period. Our results support this hypothesis given that
most of the kills observed in June and August were in
multiyear pack-ice where the pelagic bears tend to
summer.
Sampling of polar bear kills is not easily accom-
plishedandmostotherstudieshavebeenconductedover
a relatively brief period during the spring when ringed
seals pup (Stirling and Archibald 1977; Smith 1980).
Further, these studies have been restricted to the Ca-
nadianArchipelagowherestableicecreatesgoodringed
seal breeding habitat (Hammill and Smith 1989, 1991;
Furgal et al. 1996). The results from our study suggest
thatpolarbearsintheBarentsSea,whichsummerinthe
multiyear ice, may feed on seals all year.
Polar bears are opportunistic and other prey species
are available to them in the study area. Hooded seals
(Cystophora cristata)rangenorthwardtotheiceedgein
summerandoverlapwithpolarbears(Gjertz1991).We
found no instances of walrus predation but, as the
walrus population in Svalbard recovers from overhar-
vest (Gjertz and Wiig 1995), it is possible that walrus
predation may increase. In addition, harbour seals
(Phoca vitulina) are found in the Svalbard Archipelago
(Gjertz et al. 2001) and, during the ice-free period in
August 2001, we observed polar bears attempting
aquatic stalks of hauled-out harbour seals in Van
KeulenFjordonthewestcoastofSvalbard.Wedidnot
observe successful predation but did observe several
polar bear tracks in the bottom sediments near shore
and saw two bears swimming toward hauled-out seals.
451
Polar bears are also opportunistic scavengers. In
summer2001,polarbearswereobservedfeedingonboth
a white whale carcass and a sperm whale (Physeter
macrocephalus) carcass in northern Svalbard (J.O.
Scheie, personal communication). In these 2 observa-
tions,upto14and17bears,respectively,wereobserved
onthecarcasses,suggestingthatscavengingisimportant
formanyindividuals.Further,observationsofpredation
and scavenging of reindeer (Rangifer tarandus
platyrhynchus) (Derocher et al. 2000) attest to the
diversity of diet.
Prey composition is an important element for un-
derstanding the ecotoxicology of polar bears. In Sval-
bard, there was speculation that harp seals were
responsible for the high levels of polychlorinated bi-
phenylsin polar bears (Kleivaneet al.2000;Gabrielsen
andHenriksen2001).However,ourresultsmakeitclear
thatmoredetailedstudyofpolarbeardietisrequiredto
understand trophic transfer ofpollutants.In particular,
mother-offspring transfer of pollutants can result in
nursing young having higher pollution loads than the
mother (Tanabe and Tatsukawa 1991; Polischuk et al.
1995; Beckmen et al. 1999). Given the large number of
ringedandbeardedsealpupsconsumedbypolarbears,
it is important that the pollution load of seal pups be
studied. However, careful quantification of the season-,
sex- and age-specific diet of polar bears is required
before trophic-level transfer of pollution can be under-
stood.Similarly,ifclimatechangealtersthedistribution
and abundance of prey (Stirling and Derocher 1993),
better documentation of current predation patterns is
essentialforunderstandingtheeffectsofclimatechange
on polar bears.
In summary, similar to other areas, the diet of polar
bears in Svalbard and the western Barents Sea is domi-
natedbyringedsealsonanumericalbasis,butbearded
seals make a significant contribution to the diet when
biomass is considered. Harp seals likely play an impor-
tant, but lesser, role in the diet of bears living in more
pelagic habitats, but only during the summer months.
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Acknowledgements This research was funded by the Norwegian
Polar Institute and the Norwegian Research Council. We are
grateful for reports of seal kills from others working in the Sval-
bard area, particularly Georg Bangjord, Wayne Lynch and Chris-
tianLydersen.JonOveScheie,EnvironmentalO
K
cer,Governorof
Svalbard, kindly provided the reports of polar bear feeding on
white and sperm whales.
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