diet composition Svalbard.pdf

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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