Fur Seal Foraging Behavior Reflects Prey Stock Structure ...and fur seal foraging to help inform...
Transcript of Fur Seal Foraging Behavior Reflects Prey Stock Structure ...and fur seal foraging to help inform...
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Jeremy T. Sterling1, Mike E. Goebel2 , Sara J. Iverson3, Stan Kotwicki4 , Noel A. Pelland1, Rolf R. Ream1, Alan M. Springer5
1 NMML, AFSC, NMFS, NOAA, 7600 Sand Point Way N.E., Seattle, Washington 98115 [email protected] SWFSC,3DalhousieUniversity,4RACEAFSC,5UAF
AbstractClimatemodelspredictincreasingBeringSeatemperaturesanddeclinesinrecruitmentofwalleyepollock.Predictingtheimpactonthenorthernfurseal,aspeciesheavilyreliantonpollockforfood,requirestheneedformultiyearobservationaldatalinkingclimate,pollock,andfursealforagingtohelpinformnumericalecosystemmodels,whichisthegoalofthisstudy.TwonorthernfursealstudiesconductedonthePribilofIslandsduringthebreedingseasonsof1995and1996and2005and2006setouttobetterunderstandhowthebiophysicalenvironmentaffectsfursealforagingsuccess.Thesetwostudiesfocusedon136adultfemalesprovisioningdependentpupsandrecorded83,136hoursand595,236divesduring502foragingtripstosea.Growthratesof107pupsofthesefemalesweredeterminedattheendoftheseason.Oceanographically,1995and2006weresimilarwithrespecttobelowaverageBeringSeashelfbottomwatertemperaturesandextensivecoldpools.Incontrast,1996and2005wereoceanographicallywarmwithmuchsmallercoldpoolsresidingfarthertotheNEandoutoffursealforagingrange.Inallyears,availablepreyforfursealsdiffereddemographicallyandspatially,resultinginfursealbehavioralresponsesthattrackedtheknownage-relatedbehavior,distribution,andabundanceofwalleyepollockandthevariablecoldpooloccupancyofPacificherring.Approximately40%offursealdivesoccurredintheBeringSeabasin,aregiondominatedbymesoscalevariability.Basineddyandedgedeterminationhelpedexplainfursealmovementanddivebehavioraswellasinterannualvariabilityinbasinoccupancy.Eddyedges,type,andalignmentwithinfursealforaginghabitatwereallimportantfactors(seeanimation).In2005and2006,stormsalteredfursealforagingbehavior,causingsealsinthebasintoseekforaging opportunitiesontheshelfordeeperintothebasin, whileshelfforaginghotspotswereabandoned,resultinginlonger
Background and Methods• Harveststrategiesexplained thePribilof Is.fursealdecline andrecovery inthelate19th andearly20th centuries and70%ofthefirstoftwodeclinesbeginning inthelate1950s(Fig.1).
• Pupproduction nowis~25%ofitspeakandthecurrentdecline remainsunexplained.
• Theinteraction betweensummerandwinterforagingconditions couldberesponsible forthemostrecentdeclines (Fig.1top).
• Weexaminedaspectsofthefeedingenvironment, effectsonadultfemalebehavior,andpupgrowthwiththeaimtoinformnumericalecosystemmodelsandexplainPribilof fursealdemography.
• SealArgosanddivedataweremodeledtoproduce hourlylocationsanddivelocations. Divedepthswereclassifiedasbottom,midwater,andsurface.
• Gentry1984andSinclairetal.1994showedthatdivinganddiettrackedpollockyearclassstrength.Alexander1894,Serobaba 1970,andWyllie-Echeverria1998showedthatsealandpollockdistributionsshiftinresponsetothelocationoftheBeringSeacoldpool.Wepredictedthatdivedepthsandlocationswouldtrackpollockspatialandverticaldistributions.
• AneddydetectionandedgedeterminationalgorithmwasappliedtoAVISOSSHdatatoidentify,track,andexplainsealbehaviorrelativetoeddies.
• NCEPwindspeedanddirectionwereexaminedwithrespecttosealbehavior.
• SealbehaviorsandpupgrowthrateswerecomparedtoBogoslof I.,BeringSea,AK,afursealcolonyexperiencingsignificantpopulationgrowthsince1980(Fig.1top).
• Resultswerecomparedtopupweights,numbersofpollock,St.Paulwindspeeds andtemperaturedata,andotheryearswithfemaletripdurationsandpupweights.
Fur seals and pollock
“…whileinstreaksindiscoloredwater…ThematereportedseeinganabundanceofAlaskapollockjumpingandmanytravelingsealsin
pursuitofthem”
A.B.Alexander1894
• Dietstudiesdatebacktothe1890s
• Pollockandsquidoccurmostfrequently• Speciesconsumptionvariesspatially–morepollockconsumedontheshelfandmoresquidinthebasin
• Contentsofsealstomachsamplesmatcheddominantpollockyearclasses
• Dietcollectionsofstomachs,scats,andspewsindicatesealsconsumebothjuvenileandadultpollock
• Durationofsealtripswereshorterinyearswithlargepollockyearclasses
• ClimatechangepredictionsindicatesignificantdeclinesinpollockrecruitmentintheBeringSea
• Somestudiesconcludejuvenilepollockare,“likelydetrimentaltotheir[furseals]populationhealthandreproductivefitness”
Results
• Timeadultfemalesforagedandnursedpupsaffectedpupgrowth,thusfactorsaffectingtripbehaviorarekeytounderstandingpupgrowth(Fig.2).
• Coldpoolsin1995&2006causedsealstoforagemoretothewestandsouth.Inwarmyears1996&2005sealsforagedmoretothenortheast(Fig.3).
• Inallyears(only2006showninFig.4)foragingeffortwasconcentratedinsimilarthermalzonesaswaspollockbiomassconcentration.
• Divesweredeepestin1995(40.1± 2.2mSE),followedby2005(32.4± 2.5mSE)and2006(27.8± 2.6mSE).Shallowdivesoccurredin1996(26.6± 2.9mSE;Fig.5).
• Eddiesin1996movedalongtheshelfbreakandledtoshortertripsandmoretimespentineddies.Moretimeineddieswasrelatedtogreaterpupgrowth(Fig.6).
• Stormsin2005&2006alteredtrips.Sealsrespondedbyreturningtothecolonyorextendingtheirtrips(Figs.3&7).
• Divingdepthvariedseasonallyandchangedduringstormsbyshiftingawayfromthebottomtowardthesurface(only‘06shownFig.8).
• 1996eddyvs.shelftripdurationsweresimilar(Fig.7),therewerefewstorms,agoodeddyfield,shallowdiving,morepollock,andaverageairtemperatures.
• Tripdurationstothebasininotheryearsweregenerallylongerthanshelftrips,with2006showingthegreatestdifferenceduetoapooreddyfield(Figs.6&7).
• Pupgrowthincreasedwithincreasingnumbersofpollock(Fig.9).Althoughthisstudycoveredonly4years,otherstudiessupportedourresults(Peterson1965&Gentry1984).Linearmodelsalsoshowedsex,temperature,andnumbersofpollockexplained76%oftheSt.PaulI.pupweightvariability.
• Residualpupgrowthin2005reflectedwarmsummerairtemperatures(Figs.1&9).
TherecommendationsandgeneralcontentpresentedinthisposterdonotnecessarilyrepresenttheviewsorofficialpositionoftheDepartmentofCommerce,theNationalOceanicandAtmosphericAdministration,ortheNationalMarineFisheriesService.
AcknowledgmentsWewouldliketothankRodTowell forpupweightanalysesandNickBondforhelpfuldiscussionsonatmosphericeffectsontheupperocean.WealsothankalltheresearchersintheAlaskaEcosystemsProgramthatcontributedtothefieldresearch,includingJimThomason,Kate Call,BrianFadely,andAlisonBanks.Thisresearchwasconductedunderpermitnos.14328&782-1708.
Fursealforagingbehaviorreflectspreystockstructure,abundance,andresponsestovariableoceanographyandstorms
tripstosea.Theresultsofthisstudyshowtheinterrelationshipsbetweenthefursealforagingbehavior,variablepreyfields,andclimatestatesandhighlightshowthesefactorsaffectpupgrowth.FutureworkseekstoaidongoingBeringSeaecosystemmodelingeffortsandprovideindicestohelpexplainpastandcurrentdemography.
Figure1.St.PaulI.(blackdots)andBogoslof I.(reddots)pupproduction,andanomaliesinnumbersofpollock(pinkbars),St.Paulmalepupweight(blackbars),temperature(redbars)andwindspeed (bluebars)anomalies.Verticaldashedlinesaretheyearsofthisstudy.
Figure4.Sealverticaldistancetraveled(diveeffort)overlaidonKotwicki’s combinedbottomtrawl,mid-waterpollockbiomassassessment,andbottomtemperatureisotherms(top).Normalizedthermalzonesofpollockbiomassandfursealeffort(bottom).
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Figure8.Seasonalproportionofsealverticaldistancetraveled(diveeffort)groupedbydiveclassifications(middle)andalignedwithfrictionvelocity(windspeed)cubed(top).
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Figure3.Spatialdistributionplotsoffursealdiveclassificationsfor1995and1996(top)andbeforeandafterSeptemberstormsin2005(middle)and2006(bottom).
Conclusions“Intermsofpreyspeciescomposition,thesummerdietoffemales
andjuvenilemalefursealsdoesnotappeartohavechangedsincetheturnofthecentury.” Sinclairetal.1994
“…over500otoliths fromSt.Paulwerenearlyallfromsmallgadids ranginginlengthfrom
4-5inchestoafoot.”KenyonandWilke 1957
Figure9.Pupgrowthinthisstudy,andin1963and1984,andatBogoslof,andregressionmodelresultspredictingpupweights.
Figure2.Pupgrowthvs.momforagingbehavioratthePribilofs andBogoslof.
Figure6.Timespentineddiesandpupgrowthofbasinforagers.
Figure7.Tripdurations,fittedGAMs,andwindspeed forallsealsinallyears.
• Fursealforagingbehaviorreflectedboththephysicalenvironmentanditseffectonpollock,theirprimaryprey.
• Coldpools,eddyfields,storms,andpollockstockstructureaffectedsealdiving,foraginglocation,andtripdurations.
• Tripandshoretimespentwithpups,andairtemperaturesatthecoloniesinfluencepupgrowth.
• Higherpupweightsandpupgrowthrateswereseeninyearswithgreaternumbersofpollockandwithalong-shelfbreakeddyfields.
• Modelsindicatepupgrowthincreasesfrom0.3– 1.1gramsperdayforeachadditionalbillionpollock.
• Thisadds1.2- 4.6kgtopupweaningmasswhengoingfromanaveragepollockyeartoayearofmaxestimateof87billionfish.
• Putintocontextofthemostrecentdeclinesoffurseals,1984hadthesecondlargestpupweightanomaly.Pupproductionhadstabilizedandbeguntoincreasefollowingthe1978,1982,and1984largepollockyearclasses,suggestinggoodrecruitmentfromthoseyears;andwhereSt.PaulI.airtempsandwindswerenormal.Divinganddietsreflectedlargepollockyearclasses.
• Conversely,30%ofthedeclinefrom1956-1979isunexplained.Airtemperatures,pupweights,andpollocknumberswerealllow,whilewindspeeds andtripdurationswerehigh.Webelieve thislikely explainspartofthe30%declinegivenourresults.
• Bogoslof pupgrowthratesprovideinsightsintooptimalconditionsleadingtogoodrecruitment.
Figure5.Fractionofverticaldistancetraveledgroupedbydivedepthclassificationin(A)1995and(B)1996.Moreeffortwasclassifiedonthebottomin1995,whilemoreeffortoccurredabovethethermoclineandnearsurfacein1996.Thelarge1992(age-3)yearclasswasdistributedintheoutershelfhabitatandnearbottomwheredivesoccurred.Incontrast,1996hadanaboveaverageage-0yearclassandage-0pollocktypicallyoccupywatersabovethethermoclineandgraduallymigratetomid-waterastheygrow.
”…sealsalwaystravelwithafair
wind.”C.LHooper1892