McMurtrie,P.;KA.Edge,D.Crouchley,D.Gleeson,M.J.Willans,andA.J.Veale.Eradicationofstoats( Mustela erminea )fromSecretaryIsland,NewZealand Eradication of stoats ( Mustela erminea ) from Secretary Island, New Zealand P. McMurtrie 1, K-A. Edge 1, D. Crouchley 1, D. Gleeson 2, M. J. Willans 3, and A. J. Veale 4 1Department of Conservation, Te Anau Area Offi ce, PO Box 29, Lakefront Drive, Te Anau 0640, New Zealand. <[email protected]>. 2Landcare Research, PB 92170, Auckland, NZ. 3The Wilderness, RD Te Anau-Mossburn Highway, Te Anau, NZ. 4School of Biological Sciences, The University of Auckland, Private Bag 92019, Auckland Mail Centre, Auckland 1142, NZ. Abstract Stoats( Mustelia erminea )areknowntobegoodswimmers. FollowingtheirliberationintoNewZealand, stoatsreachedmanyoftheremotecoastalislandsofFiordlandaftersixyears. StoatsprobablyreachedSecretaryIsland (8140ha)inthelate1800s. Reddeer( Cervus elaphus )aretheonlyothermammalianpestpresentonSecretaryIsland; surprisingly,rodentshaveneverestablished.Thesignifi cantecologicalvaluesofSecretaryIslandhavemadeitanideal targetforrestoration. TheeradicationofstoatsfromSecretaryIslandcommencedin2005. Ninehundredandfortyfi ve stoattrap tunnels,eachcontainingtwokilltraps,werelaidoutalongtracksatadensityof1tunnelper8.6ha. Traps werealsoputinplaceontheadjacentmainlandandsteppingstoneislandstoreducetheprobabilityofrecolonisation. Prebaitingwasundertakentwice,fi rstinJuneandtheninearlyJuly2005. InlateJuly,thetrapswerebaited,setand clearedtwiceover10days. Ninetyfi vestoatswerecapturedinthisperiod.Subsequenttrap checkshavetakenplacethree timeseachyear:inNovember,FebruaryandbetweenMayJuly. FortyfourstoatswerecaughtinFebruary2006,with successivecapturesdecreasingtobetween0and9eachtrappingperiod,withmostcaughtinautumn.Geneticanalysisof stoatscapturedtoJune2008indicatesthatthesestoatswereamixtureofresidentsandafewimmigrants. A signifi cant stoatplagueeventduringsummer20062007mayhaveincreasedthelikelihoodofnewstoatssubsequentlyarrivingon SecretaryIsland. Whileeradicationhasnotyetbeenachieved,manyofourconservationobjectivesarebeingmet.The experimentalnatureofthisprogrammehasopenedthedoorfortestingnewgroundinthefi eldofislanderadicationsand challengingsomeofthepreviouslyheldviewsofwhatshouldandshouldnotbeattempted. Keywords: Eradication,immigration,trapping,monitoring,geneticanalyses,restoration INTRODUCTION Stoats ( Mustela erminea ) were fi rst introduced into Thescaleandexperimentalnatureofthisprogramme mainland New Zealand in the late 1880s in response to required that, in addition to determining whether the feral rabbit ( Oryctolagus cuniculus ) plagues that were outcome of these objectives is met, opportunities for destroyingpasturelandsandposingaseriousthreattothe learning must also be undertaken. Stoats captured after NewZealandeconomy. Stoatshavehaddramaticeffects theinitialeradicationcampaigncouldbeusedtodetermine on New Zealand’s naïve native animal species, many of the level of subsequent reinvasion to Secretary Island. whichevolvedwithoutterrestrialpredators(King1984). Herewepresentalltrappingdatafrom2005toDecember Stoatsareverymobileandarecapableswimmers(Taylor 2009andapreliminaryanalysisofstoatreinvasionusing and Tilley 1984) and were observed by Richard Henry, moleculargenetictechniquesdescribedbyGleeson et al. curatoronResolutionIslandinFiordland,by1900(Hill (2010). A demographicstudyoftheSecretaryIslandstoat andHill1987).Thestoatsprobablyinvadedotherremote population prior to trapping and after the initial knock islandsinFiordland,includingSecretaryIsland,ataround downisunderway(A.Vealeunpubl.data). Dietanalysis thesametime. ontheoriginaltrappedpopulationhasbeenundertakenbut SecretaryIslandisadministeredbytheNewZealand isnotincludedinthispaper(E.Murphypers.comm.). Department of Conservation. In 2004, it became the focusofa10yearprogrammetoeradicatestoatsandred METHODS deer ( Cervus elaphus ) (Edge et al. 2011). Techniques for eradicating stoats from islands had been piloted Study area successfullyonseveralsmallerislandsinFiordland(Elliott Secretary Island (8140 ha; 1196 m), at the entrance et al .2010). Stoatshadalsobeentrappedon19islands toDoubtfulSoundonthewesterncoastlineofFiordland ranginginsizefrom1- 67haandwithinvaryingdistances National Park (Fig. 1), is the second largest island on from the mainland over a fouryear period, in order to the Fiordland coast and the third highest island in New produceapredictivemodelofstoatreinvasion;of46stoats Zealand. Theislandisseparatedfromthemainlandportion captured, only one was caught on an island further than of Fiordland byThompson Sound to the east (minimum 304moffshore. Basedontheseresults,Elliott et al .(2010) distancebetweenthetwoisC.950m),andbyDoubtful concludedthatlargeislandslikeSecretaryandResolution Sound to the south (minimum distance to closest stoat wouldbesuitableforstoateradicationattempts. Ourpaper populationistwopassagesC.170mand600mviaBauza describes an approach to the eradication of stoats from Islandwhichislargelystoatfree). SecretaryIslandbasedonanoperationalplanwithtwokey In 1963, Secretary Island was designated a ‘Special objectives(Golding et al. 2005): Area’ withinFiordlandNationalParkbytheNewZealand 1. To eradicate stoats from Secretary Island. The Government due to the island’s unmodifi ed vegetation plandefi ned‘eradication’ asthecompleteremovalofthe andthereal(orapparent)absenceofintroducedbrowsing residentstoatpopulationandtheestablishmentofalong or grazing animals including the brushtail possum term control and monitoring programme to manage re (Trichosurus vulpecula andreddeer. Introducedrodents invasion. were also absent, making Secretary Island the largest inshoreislandinNewZealandfreeofsuchpests. Stoats 2. To achieve and maintain a zerodensity stoat weretheonlymammalianpestsknowntobepresent.Red populationonSecretaryIslandsothatindigenousspecies deerprobablyarrivedinthelate1950s(MarkandBaylis currentlyexistingontheislandorintroducedtotheisland 1975),butittooksometimeforapopulationtoestablish canthrive. (Crouchley et al .2011). Pages 455-460 In: Veitch, C. R.; Clout, M. N. and Towns, D. R. (eds.). 2011. Island invasives: eradication and management. IUCN, Gland, Switzerland. 455 Island invasives: eradication and management Stoat trapping on Secretary Island Full details of methods for the trapping programme onSecretaryIslandareprovidedintheOperationalPlan (Golding et al .2005). Inbrief,theseinvolvedthefollowing techniques. A 108 km network of trap lines was established on SecretaryIslandfromOctober2004toApril2005along mainridgelinesandspurs,habitatboundaries,waterways and traversable terrain (Fig. 2). Based on previous successfuleradicationsofstoatsfromislandsinFiordland, weneededaminimumaveragedensityofonetrap tunnel per9ha(Elliott et al. 2010.).Wealsoneededtoensurethat everystoatontheislandwouldencounteratrap (Parkes 1990;Parkes et al .2002). Homerangeestimatesforstoats varyaccordingtogender,season,andfoodavailability,soit wasimportanttodeterminethesmallestlikelyhomerange in order to decide the maximum spacing between traps. Homerangesaresmallestwhenprey,especiallyrodents,is easilyavailable.Forexample,averagehomerangeswere 93(SE±7)haforfourmalestoatsand69(SE±8)haforfi ve femalestoatsinaFiordlandbeech( Nothofagus sp.)forest whenrodentswereabundant(MurphyandDowding1995). Larger stoat home ranges were reported in areas where rodentsarescarce,withestimatesof204haformalesand 124haforfemales(MurphyandDowding1995),223ha formalesand94hafemales(Alterio1998),and210ha formalesand89hafemales(Miller et al .2001). Without similar home range information for stoats on Secretary Island, ourtrap networkwasbasedonthesmallerhome rangesizesofMurphyandDowding(1995),whichmeant Fig. 1 Location of Secretary Island within Fiordland thatwhereverpossibletrapsshouldbenomorethan700 National Park. mapart. However,duetotheextremelyruggedterrainon theislandthereweresevenlocationswherethisdistance exceeded700m(seeFig.2). A totalof945tunnelseachcontaining2MarkIV Fenn killtraps(DBSpringsLtd.Worcestershire,England)were placedat135mintervalsalongthetrap linesandat150m intervalsalongtheeasterncoastline(accessiblebyboat), yieldinganaveragetunneldensityof1tunnelper8.6ha (Fig.2). Twotunneltypeswereusedtohousetraps:300wire meshtunnelswithwoodenbasesand645woodentunnels withwiremeshends. Woodenandwiremeshtunnelswere placedinarepeatedsequencealongtrap linescomprising onewiretunnelfollowedbytwoofwood. Thevariation intunneltypeswasusedtoovercomeanypossibilitythat a few stoats were unwilling to enter either one type of tunnel. Previous stoat eradications in Fiordland used pre baitingwherebystoatswerefreetoentertrapssetwiththe safetycatchonandwithbaitwasleftinsideandoutsidethe trap entrance. Itisnotpossibletodeterminehowcrucial prebaitinghasbeentothesuccessoftheseprogrammes. Prebaitingisrelativelyinexpensiveandtheamountofbait takeobservedduringtheprebaitingphasesuggestsitmay reducethetimetakentoachievetheinitialknockdown. TrapswereprebaitedtwiceonSecretaryIsland:20June- 26Juneand5July- 11July2005.Duringprebaiting,each tunnelsitewasbaitedwithonefreshhen’seggandapiece ofmeat(ca3cmcubeofbeef,rabbitorvenison)onthe baitblockbetweentraps.Anadditionalhen’seggwasalso placedoutsidethetrap tunnelonthegroundandanother approximately1moffthegroundonatree. Fig. 2 Secretary Island trap lines and mainland traps Stoat trapping began on Secretary Island from 20 buffered to 700m. - 30July2005usingthe prebaitingregime.Trapswere 456 McMurtrie et al .: Eradication of stoats, Secretary Island checkedtwiceduringthisinitialtrappingperiodandwere MainlandfromJuly2005- January2008. Subsampling