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
