DietofgreatcormorantsPhalacrocoraxcarbo sinensiswinteringinMalause(South-WestFrance)
FrédéricSantoul1,Jean-Baptiste Hougas1,AndyJ.Green2and
SylvainMastrorillo1
With1figureand2tables
Abstract:ThedietofcormorantsroostingatMalauseontheGaronneRiver(South- WestFrance)wasstudiedbyanalyzingpelletscollectedduringthewinter2001–2002. Thisroost held ameanof550cormorantsbetweenOctober2001andMarch2002, makingitoneofthemostimportantwinterroostsinFrance.Thedietcontained14fish species,including10cyprinids.Cyprinidswerethemostabundantprey,representing
90%ofindividual fish.BreamAbramisbramabetween100and150mminlength werethemostabundantfooditems,whilelargebream(300mmtotallength)were alsoconsumed. Theproportionofpelletscontainingbreamincreasedfrom26%in October–Novemberto69%inFebruary–March.Therewasnosignificantchangein thesizeofthebreamovertime. ThenextmostabundantspecieswaspikeperchSander lucioperca presentin15.4%ofthepelletsfromDecember–Januarybut only2.6%in February–March.
Keywords:cormorants,wintering,dietcomposition,pellets.
Introduction
Althoughtheforagingbehaviour anddiet ofcormorantshasbeenstudiedpre- viouslyinbreeding andnon-breeding areas,therehasbeennopublishedwork oninteractions betweencormorantsandthefishcommunitiesinFrenchrivers. Althoughtheimpactofcormorantsonsomefishpondsmaybesignificant(Im
Hafner1984,DobrowolskiDejtrowski1997),littleisknownabout theirfeedinginfreshwaterrivers(FelthamDavies1997,Noordhuisetal.
1997,Suter1997).Asaconsequence oftheincreaseofthecormorant asa
1 Authors’addresses:Laboratoire d’EcologiedesHydrosystèmes, UMRCNRS/UPS
5177,Bâtiment4R3b2,UniversitéPaulSabatier,118RoutedeNarbonne,31062Tou- louseCedex4,France;E-mail:(correspondingauthor).
2 EstacionBiologicadeDoñana, AvenidadeMariaLuisas/n, PabellondelPeru,41013
Sevilla,España.
breedingbirdinNW-Europesincetheearly1980s(Marion1997),thenum- bersofwinteringindividualsinthe Midi-PyrénéesRegionofsouth-west Francehaveincreasedsubstantially(Bousquet1992).Thisincreaseinthe populationofcormorantshascausedmanycomplaints fromanglers,whohave calledformeasurestoreducecormorantsnumber.Inthisstudywecensused thecormorantpopulationandexamineditsdietatthemostimportantsitein thisregion.
Methods
MalauseissituatednearMoissacattheGaronneandTarnRivers confluence inthe centralpartoftheGaronnefloodplain.Weekly censusesofcormorantswerecarried outfromSeptember2001toApril2002atanightroostlocatedatMalauseonanisland intheGaronneRiver.Countsweremadeusingatelescope(20´60)atduskwhenall thebirdswerepresent.
Dietofwinteringcormorants wasstudiedusingpelletanalyses.Pelletswerecol- lectedunderthenightroostonceaweekfromOctober2001toMarch2002.Onlyfresh andcomplete onesweretaken.Pharyngalboneswereseparatedandcyprinidspecies wereidentified.Dentalandotherhardpartsallowedtheidentificationofotherfish suchaspredators (formethods seeMathieu1997andCarssMarquiss, 1997).No attemptatcorrectionofwearwasmade(onlynon-erodedhardpartswereused toesti- matetotalfishlength).Thepercentoccurrenceoffish itemsinthepellets(i.e.theper- centageofindividualpelletsinwhicheachfishspecieswasrecorded)wascalculated andcomparedamongthreedifferentperiods(Oct–Nov,Dec–JanandFeb–Mar)for
eachspecies,usingYatescorrectedc2tests.
Results
DuringourstudyperiodamigrantpeakwasnotedinOctober(850birds),then thenumberofcormorantsremainedrelativelystablefromNovembertoJanu- ary(about700birds).Thenumberofwinteringbirds startedtodecreasein February,thelastbirdsleavingthesiteattheendofApril.
Identifiablefishremainswerefoundin68%ofthepellets(n=425),22%
onlycontainedunidentifiableremains,while10%werecompletelyempty.A totalof545individualpreyitemswasfound(for343ofthemmeasureswere taken),belongingto14differentspeciesoffish(Table 1).Thesizerangeofthe fishtakenwasconsiderable; fishfrom58to480mmwerefound.Cyprinids were dominantwith10species,representing90%ofallindividualfish.Within this family,bream(Abramisbrama)wasthemostcommonpreyspecies(Ta- ble2).Silverbream(Bliccabjoerkna),rudd(Scardiniuserythrophthalmus) androach(Rutilusrutilus)werealsocommonpreyspecies.Thepercidpike- perch(Sander lucioperca)wasalsoabundantinthefoodofthecormorants.
Table1.Fishspeciespreyeduponbycormorantsinthewinterof2001/02atMalause, South-WestFrance.
ScientificnameNameFishlength,cmN
Mean / Min / MaxAbramisbrama(L.,1758) / Bream / 16.4 / 6.2 / 40.2 / 162
Albumusalbumus(L.,1758) / Bleak / 8.7 / 1
BarbusBarbus(L.,1758) / Barbel / 17.4 / 13.3 / 23.8 / 6
Bliccabioerkna(L.,1758)
Carassiuscarassius(L.,1758)
Cyprinuscarpio(L.,1758) / Silverbream
Cruciancarp
Carp / 20.8
19.5 / 8.9
17.0 / 37.5
27.2 / 36
5
Leuciscuscephalus(L.,1758) / Chub / 21.6 / 5.8 / 37.6 / 19
Rutilusrutilus(L.,1758) / Roach / 20.7 / 5.8 / 43.0 / 34
Scardiniuserythrophthalmus(L.,1758) / Rudd / 12.2 / 6.3 / 24.1 / 35
Tincatirca(L.,1758) / Tench / 3
Esoxlucius(L.,1758) / Pike / 43.0 / 38.1 / 48.0 / 2
Ictalurusmelas(Rafinesque,1820) / Blackbullhead / 2
Percafluviatilis(L.,1758) / Perch / 1
Sanderlucioperca(L.,1758) / Pikeperch / 30.9 / 19.0 / 43.0 / 43
Table2.PercentoccurrenceoffishitemsinthepelletsofcormorantswinteringinMa- lause.
Name / Oct–Nov(N=82) / Dec–Jan(N=130) / Feb–Mar(N=78)Bream / 25.6 / 46.2 / 69.2
Bleak / 1.2 / 0.0 / 0.0
Barbel / 11.0 / 0.8 / 1.3
Silverbream / 11.0 / 12.3 / 6.4
Cruciancarp / 1.2 / 1.5 / 0.0
Carp / 13.4 / 4.6 / 1.3
Chub / 8.5 / 5.4 / 3.8
Roach / 8.5 / 10.0 / 9.0
Rudd / 9.8 / 12.3 / 9.0
Tench / 1.2 / 2.3 / 0.0
Pike / 1.2 / 1.5 / 2.6
Blackbullhead / 0.0 / 2.3 / 0.0
Perch / 0.0 / 1.5 / 0.0
Pikeperch / 11.0 / 15.4 / 2.6
Differencesinpercentoccurrencebetweenthethreeperiods(Oct–Nov,Dec– Jan,andFeb–Mar)weretested forthetwomostabundantpreyspecies,bream andpikeperch. Forbream,asignificantincreasewasnotedfromOct–Nov(n
=26)toDec–Jan(n=46)andfromDec–JantoFeb–Mar(n=69)(Fisher’s
exacttest:P= 0.018andP= 0.012).Nosignificantdifferenceswerefoundfor thepikeperchbetweenOct–NovandDec–Jan,butasignificantdecreasewas
Fig.1.Sizefrequencyofbream(Abramisbrama)caughtbycormorantsfromSeptem- ber2001toApril2002.
detectedfromDec–Jan(n=15)toFeb–Mar(n=3)(Fisher’sexacttest:P
0.001).Nosignificantchangeinthesizeofthebreamwasfoundbetweenthe different periods (Mann-Whitney test).Most breamwere between 100–
150mmintotallength.However,largebreamupto300mmwerealsooften consumed(Fig.1).
Discussion
Pelletanalysisisacommonlyusedmethodtoassessthe dietoffish-eating birds,although therearesomepotentialbiases.Theprincipaloneisthatsmall fishcanbeunderrepresentedinpelletsowingtodigestionofidentifiablere- mains(Duffy Laurenson1983,SuterMorel1996,ZijlstaVan eerden1995).However,wedonot expectthatweoverlooked anypreyspe- cies.Thepelletanalysisisconsidered arather“weak” methodbysomeau- thorswhileothersconsideritgoodenoughandalsoveryimportant(Carsset al.1997,McKay etal.2003).
This isthefirststudyoncormorantpredationonafish communityina
Frenchriver.Theincreasingnumbersofwinteringcormorantsinsouth-west Francearelikelytohaveanincreasing impactonthefishpopulationsinthis region.WelacksufficientdataonfishdensitiesintheGaronneRiverandon distancesmovedbyfeedingbirdstomakeagoodestimateofwhatproportions offishstockswereconsumedduringourstudy.However,giventhateachcor- morant’sdailyfoodrationisabout500g(FelthamDavies1997,Keller
Visser1999),weestimatethatthecormorantpopulationwasconsuming about450kgoffishperdaywhenthenumbersofbirdspeakedinOctober.
Hence,forthewhole winteringperiodtheestimationofbird predationwas about65tonsoffish. Somecormorants usingourroostsitefedinadjacent wetlandsoftheGaronneRiver(e.g.gravelpits)andtheproportionofbirds doingthisisdifficulttoestimate.Thus,moreworkisrequiredtoestimatethe impactofcormorantsonfishstocksineachwetland.
Numerousstudieshavefoundthatthemainpreyspeciesofcormorantsare
cyprinids,representingbetween50%and morethan90%ofthespeciesfound incormorants’diet(Keller1995,Veldkamp1995,BoudewijnDirksen
1997,Van eerden Zijlstra 1997,Privileggi 2003,Trauttmansdorff
2003).However,numerous publications(Voslamber1988,Boudewijn Dirksen1991,Platteeuwetal.1992)statethatbreamarerarelyeatenby cormorantsbecauseoftheirhigh,laterallyflattenedbody.Inourstudy,bream wasthemostimportant speciescaughtbycormorants,andalthoughthebulks ofthesepreywerebetween100–150mm,ourresultsshowedthatcormorants canalsoforageonlargebream (seealsoVeldkamp1995).Largebreamare relativelyabundantintheGaronneRiverandisolatedfishingcormorantsoften eattheselargefishaswewitnessedduringourfieldwork.
Thedietofcormorantsmayvary considerably betweenlocationsandsea- sons(Kirbyetal.1996).IntheMalauseroost,breamdominated thefood compositionandtheproportion ofthisspeciesinthedietofthecormorantsin- creasedduringthewinteringperiod.Noordhuisetal.(1997)suggestedthat theseasonalchangesinthespeciescomposition ofthepelletsareprobablyre- latedtochangesinwatertemperatureandindistributionandbehaviourofthe fish.Theoftenhighlyturbidwatersinriverscanforcethebirdstoturntomass flockfishingonshoalsofsmallfish(Noordhuisetal.1997).Solitaryfeeding birdsgenerallytakelargerfish(Suter1997).IntheGaronneRiver,bothfish- ingtechniquesareusedregularly (Santoul,unpubl.data).
Unlikeinourstudy,pikeperchwerealmost absentfrommanyprevious studies ofcormorantdiet(e.g.Keller1995,1998).Mostofthepikeperch presentinourpelletswerejuvenilefish(mean30.9cm).Theyshoalinwinter andmaybecomemoreavailableformassflockfishing.Thismayexplainthe higherpredationwedetectedfromOctobertoJanuary.Thepercentageofoc- currenceofpikeperchinpelletsdecreasedinFebruaryandMarch,thebegin- ningofthespawningperiodforpikeperchwhenfishleaveshoalsandbecome less availableforcormorants(Craig 1987).Thedynamicsofpredatoryfish canfluctuateconsiderably(Craig 1987),thusagoodreproductive success of pikeperchin2000mayalsoexplaintheabundanceofpikeperchinthedietof cormorantsinourstudy.
ThisinitialresearchoncormorantsinaFrench riverestablishesthecom-
positionofdietthroughoutthewinter.Furtherstudiesofboththispredatorand itspreyspecies arerequiredtoquantifytheimpactofcormorantsonfishcom- munities.
Acknowledgements
ThisworkwaspartiallyfundedbytheAgencedel’EauAdour-Garonne andthe Groupe- mentd’IntérêtScientifiquesurl’ECOlogieetl’ECOnomieduBassinAdour-Garonne (GISECOBAG).WethanktheNaturalScienceSocietyofTarnetGaronne.Wealso thanktworefereesfortheirusefulcomments.
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Submitted:11September2003;accepted:17December2003.