Modeled Population Connectivity across the Hawaiian Archipelago

25 Pages·2016·2.78 MB·English

RESEARCHARTICLE
Modeled Population Connectivity across the
Hawaiian Archipelago
JohannaL.K.Wren1,2*,DonaldR.Kobayashi3,YanliJia4,RobertJ.Toonen2
1 JointInstituteforMarineandAtmosphericResearch,UniversityofHawai‘iatMānoa,Honolulu,Hawai‘i,
UnitedStatesofAmerica,2 Hawai‘iInstituteofMarineBiology,SchoolofOceanandEarthScienceand
Technology,UniversityofHawai‘iatMānoa,Kāne‘ohe,Hawai‘i,UnitedStatesofAmerica,3 Ecosystemsand
OceanographyProgram,PacificIslandsFisheriesScienceCenter,NationalOceanographicandAtmospheric
Administration,Honolulu,Hawai‘i,UnitedStatesofAmerica,4 InternationalPacificResearchCenter,
UniversityofHawai‘iatMānoa,Honolulu,Hawai‘i,UnitedStatesofAmerica
a11111
*[email protected]
Abstract
Wepresentthefirstcomprehensiveestimateofconnectivityofpassivepelagicparticles
releasedfromcoralreefhabitatthroughouttheHawaiianArchipelago.Potentialconnectivity
OPENACCESS
iscalculatedusingaLagrangianparticletransportmodelcoupledofflinewithcurrentsgener-
Citation:WrenJLK,KobayashiDR,JiaY,Toonen
atedbyanoceanographiccirculationmodel,MITgcm.Theconnectivitymatricesshowasur-
RJ(2016)ModeledPopulationConnectivityacross
prisingdegreeofself-recruitmentanddirectionaldispersaltowardsthenorthwest,fromthe
theHawaiianArchipelago.PLoSONE11(12):
e0167626.doi:10.1371/journal.pone.0167626 MainHawaiianIslands(MHI)tothenorthwesternHawaiianIslands(NWHI).Weidentify
threepredictedconnectivitybreaksinthearchipelago,thatis,areasinthemidandnorthern
Editor:GiacomoBernardi,UniversityofCalifornia
SantaCruz,UNITEDSTATES partofthearchipelagothathavelimitedconnectionswithsurroundingislandsandreefs.
Predictedregionsoflimitedconnectivitygenerallymatchobservedpatternsofgeneticstruc-
Received:April22,2016
turereportedforcoralreefspeciesintheuninhabitedNWHI,butmultiplegeneticbreaks
Accepted:November17,2016
observedintheinhabitedMHIarenotexplainedbypassivedispersal.Thebettercongru-
Published:December8,2016
enceinourmodelingresultsbasedonphysicaltransportofpassiveparticlesinthelow-lying
Copyright:Thisisanopenaccessarticle,freeofall atollsoftheuninhabitedNWHI,butnotintheanthropogenicallyimpactedhighislandsofthe
copyright,andmaybefreelyreproduced,
MHIbegsthequestion:whatultimatelycontrolsconnectivityinthissystem?
distributed,transmitted,modified,builtupon,or
otherwiseusedbyanyoneforanylawfulpurpose.
TheworkismadeavailableundertheCreative
CommonsCC0publicdomaindedication.
DataAvailabilityStatement:Dataisavailablehere:
https://www.bco-dmo.org/dataset/665467. Introduction
Funding:ThispaperwasfundedbyNSF(OCE12- Determininglevelsandpatternsofconnectivityisvitalforunderstandingmetapopulation
60169),andinpartbyagrant/cooperative
dynamicsandpersistence,andisessentialforeffectiveresourcemanagementsee[1–5].Over
agreementfromtheNationalOceanicand
ecologicaltimescales,populationpersistencedependsoneithertheabilitytoretainlocally
AtmosphericAdministration,ProjectR/SS-13,
whichissponsoredbytheUniversityofHawaiiSea producedlarvae,i.e.self-recruitment,ortheabilitytoimportlarvaefromnearbyareas,i.e.
GrantCollegeProgram,SOEST,underInstitutional connectivity[6–8].Self-recruitmentisametricdescribinghowopenorclosedapopulationis,
GrantNo.NA14OAR4170071fromNOAAOfficeof whichinturndescribesitsresilience[7,9].Openpopulationsreceiveaninfluxoflarvaefrom
SeaGrant,DepartmentofCommerce(forRJTand
outsidesources,makingthemmoreresilienttolocaldisturbancesbutlimitedinpotentialfor
JLKW).Theviewsexpressedhereinarethoseof
localadaptation[10,11].Closedpopulationsaremoresensitivetolocaldisturbancesandpos-
theauthorsanddonotnecessarilyreflecttheviews
sessagreaterpotentialforlocaladaptationsincetheyaredependentonlocallyproducedoff-
ofNOAAoranyofitssubagenciesThisisSea
GrantcontributionUNIHI-SEAGRANT-JC-13-16, springandhaveamoredirectlinkbetweenlocalproductionandrecruitment.Marine
PLOSONE|DOI:10.1371/journal.pone.0167626 December8,2016 1/25
PopulationConnectivityinHawaii
SOESTcontributionnumber9638andcontribution populationstudieshavehistoricallyworkedundertheassumptionthatmarinefishpopulations
number1661fromtheHawai‘iInstituteofMarine areopen—thatis,theyreceivelarvaefromotherpopulationssomedistanceaway[12]dueto
Biology.
thedispersalabilityandrelativelylonglarvaldurationofmarinefishlarvae.However,studies
CompetingInterests:Theauthorshavedeclared inrecentyearshavechallengedthisnotion,showingthatdespiteastronglarvaldispersalabil-
thatnocompetinginterestsexist. itymanymarinereefpopulationsappearclosed,withlarvaestaying“closetohome”[6,13–16].
Wenolongerassumeallmarinepopulationstobeopen,andthefocusisnowondetermining
theextenttowhichmarinepopulationsexchangelarvae(see[1,2]).Knowingtheconnected-
nessofapopulationisvitalineffectivelymanagingthepopulationanddesigningfunctioning
marinereserves.
Mostcoastalmarinespecieshaveabiphasiclifecycle,inwhichdispersaltakesplacepre-
dominantlyduringthepelagiclarvalstageofthelifecycle[17].Somespecieslaybenthiceggs
thatdevelopintopelagiclarvae,whereasothersspawngametesdirectlyintothewatercolumn,
wheretheydriftaspassiveparticlesuntiltheydevelopswimmingabilitiessimilartobenthic
hatchlings.Larvaecanbefeedingornon-feedinginthewatercolumn,andthepelagiclarval
phasemaylastforminutestomonthsinthepelagosbeforetheyreturntothebenthostosettle.
Eachoftheselife-historydifferenceshavepredictableimpactsonobservedpopulationgenetic
structure[18,19],butthebiologicalandphysicalfactorsdrivingdispersalintheseaarenot
wellunderstoodanddifficulttogeneralize.Factorscontrollingsuccessfuldispersalcanbespe-
ciesspecific[15,20–22],dependontimingofspawningevents[23,24],andvaryamongloca-
tions[25–29].
TheHawaiianArchipelago,locatedinthesubtropicalNorthPacificOcean,isa2,500km
longchainofvolcanicislandsandatolls,stretchingfrom19˚NintheMHIto30˚Ninthe
NWHI.TheHawaiianArchipelagoisoneofthemostisolatedontheplanet,andhometoone
ofthelargestmarinereservesintheworld,PapahānaumokuākeaMarineNationalMonument
(PMNM).ThereisahighlevelofendemismintheHawaiianArchipelago[30,31],anddueto
itsremotelocation,hasuniquemanagementneeds[32,33].WhiletheMHIarepopulatedwith
activefisheriesandheavyanthropogenicloading,theNWHIareuninhabitedandfullypro-
tectedwithlittleanthropogenicinfluence[34].OneofthehopesforestablishingPMNM,
whichwasthelargestMPAontheplanetatthattime,wasaspillovereffectwheretheprotected
fishpopulationsinPMNMwouldreplenishfishstockintheMHI.Unfortunately,thishope
hasbeenlittlesupportedamongstudiestodateofbothinvertebratesandfishes[35–38].The
lackofspilloverfromPMNMtotheMHIhasbeenattributedtotheprevailingsurfacecurrents
movinglarvaeuptheislandchainfromtheMHItowardstheNWHI[35,39].
BecausemanagementneedsvarygreatlybetweentheheavilypopulatedMHIandtheunin-
habitedPMNM,itisvitalthatweunderstandthepopulationdynamicsbetweentheseareasas
wellaswithinthem.Well-connectedpopulationswithnumerousdispersalpathwaysamong
sitesaremoreresilient,thatis,morelikelytorecoverfromdisturbance.Conversely,isolated
populationsthatarehighlydependentonself-recruitmentforpopulationmaintenanceareless
likelytorecoverafteradisturbanceandfaceagreaterriskofextinction[3–5,7].
Extensivepopulationgeneticworkhasbeendonetocharacterizepopulationstructurefor
fishandinvertebratestoinferexchangeamongsitesthroughouttheHawaiianArchipelago
(reviewedby[20,22]),butonlyahandfulofstudieshavefocusedonestimatingdispersaldur-
ingthelarvalstage[35,36,39–45].Todate,allsuchstudiesfocusoneitherasinglespeciesof
interest,asmallregionofthearchipelagooraverylimitedtimeperiod.Here,wepresentthe
firstcomprehensivedatasetdescribingmodeledpotentialconnectivityamongsitesthroughout
theentireHawaiianArchipelagousingabiophysicalmodelcoupledwitheddyresolvingocean
currents.Weuseapurelyphysicalmodelwithpassiveparticlestodeterminelikelypatternsof
potentialconnectionswithinthearchipelagoandJohnstonAtollbecausedetailedinformation
onlarvalbehavior,mortalityratesandpopulationsizesarenotcurrentlyavailableforthevast
PLOSONE|DOI:10.1371/journal.pone.0167626 December8,2016 2/25
PopulationConnectivityinHawaii
majorityofspeciesinHawai‘i.Theresultsfromthisstudywillsetthegroundworkforfuture
studiestousemorerealisticbiophysicalmodelsthatincorporatesuchfactorsaslarvalbehavior
astheybecomeavailable.
Methods
Dispersalmodel
MITgcm. TheMassachusettsInstituteofTechnologygeneralcirculationmodel
(MITgcm)solvestheincompressibleNavier-Stokesequationsonasphereindiscretizedforms
employingafinite-volumetechnique[46].TheregionalMITgcmimplementationforthe
HawaiianArchipelagoextendsfrom175˚Eto150˚Wandfrom15˚Nto35˚Nata0.04˚(~4km
intheregion)resolution.Intheverticaldirection,thewaterdepthisdividedinto50layers
withathicknessrangingfrom5mnearthesurfaceto510mnearthebottom.Itisforcedatthe
surfacebywindsderivedfromtheAdvancedScatterometer(ASCAT)observationswitha
0.25˚resolution,andbyheatandfreshwaterfluxesobtainedfromtheEuropeanCenterfor
Medium-RangeWeatherForecast(ECMWF)InterimReanalysisata1.5˚resolution.The
oceanstate,asestimatedbytheglobalHYCOMpredictionsystemata0.08˚resolution[47],is
usedtodefinetheinitialandopenboundaryconditions.Thesimulationperiodrunsfrom
May2009toMay2014.Weusetheflowfieldsinthe100mmodellayertodisperseparticlesin
ourLagrangiantrackingexperiments(seebelow),asthislayerhasshowntobethebestpredic-
torofsettlementintheregion[42,43].
Habitat. Forthisstudy,weincludedallavailablecoralreefhabitatintheHawaiianarchi-
pelagoandJohnstonAtoll.JohnstonAtollisthenearestreeftotheHawaiianArchipelago,
located1390kmsouthwestoftheBigIslandofHawai‘i.WechosetoincludeJohnstonAtollin
thehabitatdefinitionbecausethereareshownbiogeographictiesbetweenJohnstonAtolland
theHawaiianArchipelago[41,48–51].Togenerateourhabitatmap,weusedhabitatdefinedas
‘coralreef’inIKONOS-deriveddatafortheNorthwestHawaiianIslands[52,53]andthedata
setpresentedin[54]fortheMHI,andcreateda4-km2gridofthathabitat,totaling687habitat
pixels.Thehabitatpixelswereadditionallygroupedinto31differentislands/banks/atolls
(hereafterreferredtoasislands)toallowforislandscalecomparisons(Fig1).
Modelinitialization. Toinvestigatetheexchangeofparticlesamonglocationsinthe
HawaiianArchipelago,weusedaLagrangianbio-physicalparticletrackingmodel[40,42]cou-
pledwiththeflowfieldsfromtheMITgcmsimulationdescribedabove.Eddydiffusivitywas
setto250m2/sec,consistentwithdriftersinHawaiianwaters(following[35]).Wereleased50
particles(virtuallarvae)dailyfromMay2,2009,untilApril10,2014,from687coralreefhabi-
tatpixelstotalingjustover62millionreleasedparticlesforeachmodelrun.Weusedapelagic
larvalduration(PLD)of45days,representativeformostreeffish[55].Previousstudiesshow
thatPLD’slongerthan45daysdonotsignificantlyaltersettlementprobabilitiesintheMHI
[42].Foraparticletobeconsidered“settled”,ithadtobewithina5kmradiusofthecenterof
ahabitatpixelonthelastdayofitsPLD(onday45)(Table1).Preliminarystudiesshowno
changeinconnectivitypatternswhenparticlesareallowedtosettleacrossarangeofPLD,so
wechoseastaticPLDasopposedtorangeofPLD’sorasettlementwindowbecausewewere
interestedinthephysicaldriversofdispersal.Thedispersalmodelwasrunthreetimesandthe
outputaveragedforconsistency.
Totesttherobustnessofthemodelwithrespecttooceancirculationmodelresolution[56]
weranidenticalbiophysicalmodelruns,forcingthemodelwithcurrentvelocitiesfromthe
globalHYCOMata0.08˚resolutionandaregionalimplementationofHYCOMat0.04˚
(availablefortheMHIonly)resolutioninadditiontotheMITgcm(S1File).
PLOSONE|DOI:10.1371/journal.pone.0167626 December8,2016 3/25
PopulationConnectivityinHawaii
Fig1.MapoftheHawaiianArchipelago.ToppanelshowingtheHawaiianArchipelagodomainofthedispersalmodelwiththemajorsurfacecurrents(in
green)identified(afterLumpkin1998).Bathymetrylinesdenote1000and50misobaths.BottompanelsshowcoralreefhabitatpixelsfortheNorthwestern
HawaiianIslandsandMainHawaiianIslandsrespectively,witheachisland’shabitatpixelsshownasaseparatecolor.BathymetrylinesinNWHIdenote
50mdepthisobath.
doi:10.1371/journal.pone.0167626.g001
PLOSONE|DOI:10.1371/journal.pone.0167626 December8,2016 4/25
PopulationConnectivityinHawaii
Table1. Glossaryofoftenusedterms. TermsanddefinitionsusedthroughouttheHawaiianArchipelago
connectivitystudy.
Potential Themodeledestimateofconnectivityofasiteusingphysicaloceanographicattributes
connectivity andlimitedbiologicalfactorsinfluencingdispersalabilityoftheparticles.
Settlement Inthisstudywedefinesettlementasanyparticlewithin5kmofthecenterofahabitat
pixelonday45afterrelease
Self-recruitment Aparticlethatsettlesbackontothesameislandfromwhichitwasreleased
Connectivitybreak Anareaoverwhichfew,ifany,particlesareexchanged
Source-sinkindex Theratioofexportofparticlesandimportofparticlestoanarea,dividedbythetotal
numberofsuccessfullytransportedparticles.
doi:10.1371/journal.pone.0167626.t001
Statisticalanalysis
Wearefocusingonpotentialconnectivityinthisstudy,whichestimatestheconnectivityofa
siteusingphysicaloceanographicattributesandlimitedbiologicalfactorsinfluencingdispersal
ability(inourcasePLDandhabitat)[57,58].Toevaluatepatternsofpotentialconnectivityin
theHawaiianArchipelago,wecreatedaconnectivitymatrixthatmeasuresthelikelihoodof
particleexchangebycurrentsamongsites.Themodelgeneratesa687x687settlementmatrix
(Sij)containingthenumberofparticlesreleasedfromhabitati(sourcesite)thatsuccessfully
reachedhabitatj(receivingsite)forthefullrunofthemodel(fiveyears).Tocreatetherear-
wardprobabilitymatrix,wescaledSijtoislandspecifictotalreleasedparticles.Rearwardprob-
abilitymatricesreportoriginsitesofparticlesarrivingatthereceivingsiteandcanbewritten:
P =S /∑S.
ij(rearward) ij j
Wethenbinnedthe687habitatsitesusedinthedispersalmodelbyisland,resultingina31
x31islandmatrix,toallowforasimplercomparisonofpotentialconnectivity.Theresulting
probabilitymatrix(P )showstheoriginislandofsuccessfullytransportedparticlesateach
ij
island.ThenumberineachcelloftheP matrixistheprobabilityofaparticletransportedto
ij
islandjhavingoriginatedfromislandiforthefiveyearsthemodelwasrun,andeachrowin
thematrixsumsto1.Thediagonaloftheprobabilitymatrixshowstheself-recruitmentfor
eachisland.ForwardprobabilitymatriceswerealsogeneratedandaredescribedinS1File.
BecausethemajorityofcoralreeffishspawnduringMay-June[59,60],wecalculatedallmet-
ricsonbothyear-roundreleasesandreleasesrestrictedtoMay-Juneofeachyear.Allmatrices
wereplottedusingthesoftwareprogramGenericMappingTools(GMT)4.5.11[61].
Subtractionmatricesweregeneratedbysubtractingtheprobabilitymatrixforyearround
releasesfromthematrixforMay-JunereleasesusingthesubroutinegrdmathinGMT4.5.11.
Theresultingsubtractionmatrixshowswherethetwoconnectivitymatricesdiffer.Onlyfor-
wardmatriceswerecomparedwitheachother,andrearwardmatriceswitheachother.We
usedmanteltestsforeachpairofconnectivitymatricesusingfunctionmantelintheVegan
packageversion2.2–1inthestatisticalsoftwareR[62]tocalculatethecorrelationbetweenthe
probabilitymatrices.
Successfultransport,definedasanyparticlewithin5kmofthecenterofahabitatpixelon
day45afterrelease,wascalculatedbytallyingthedailynumberofsuccessfullytransportedpar-
ticlesforallislandsanddividingitbythetotalnumberofdailyparticlesreleasedforthefive-
yearmodelrun,allowingustodetermineannualandseasonalvariability.Additionally,wecal-
culatedislandspecific“settlement”successoverfiveyears.
Dispersaldistance,thegeographicdistancebetweenthereleasesiteandreceivingsitefora
successfullytransportedparticleatthereceivingsite,wasdeterminedbyfirstcalculatingdis-
tancesbetweenall687habitatpixelsusingthedistancematrixfunctiondistmwiththeHaver-
sineformulaintheR-packagegeosphere[63].Thedistmfunctioncalculatesthegreatcircle
PLOSONE|DOI:10.1371/journal.pone.0167626 December8,2016 5/25
PopulationConnectivityinHawaii
distance(Haversineformula)betweentwopointsusingtheirlatitudesandlongitudesin
degreesandcreatesa687x687distancematrix(Dij)withthereleasesites(i)onthex-axis
(rows)andreceivingsites(j)onthey-axis(columns).Wemultipliedthesettlementmatrix
(Sij)generatedbythebiophysicalmodel(seeabove)withthedistancematrix(Dij)togenerate
aproductmatrix(Pij).Becausethereismorethanonespawningandsettlementsite(hence-
forthhabitatsite)perisland(forexample,BigIslandhas129habitatsites,O‘ahuhas62and
KureAtollhas13),weaddedallthedistancesforallthehabitatsitesintheproductmatrix
belongingtoeachisland,generatinga31x31matrixcontainingthesumofallthedistancesof
alltheparticlesforeachislandcalledtheislandproductmatrix(PIij).Thesameprocedurewas
followedtogenerateanislandsettlementmatrix(SIij);asquare31x31matrixcontainingthe
totalnumberofsuccessfulsettlersforeachisland.Wethendividedthecolumnsumsfromthe
islandproductmatrixwiththecolumnsumsoftheislandsettlementmatrixtoobtainthe
meandispersaldistanceforsuccessfullysettledparticlesateachisland.Thesecalculationswere
performedforyear-roundreleases,aswellasforparticlereleasesconfinedtoMayandJuneof
eachyeartoallowustoexploreseasonalpatterns.
Self-recruitment,definedastheproportionofsuccessfullytransportedparticlesateach
islandthatoriginatedfromthatsameisland,isanimportantmetricwhenevaluatingtheper-
sistenceofapopulation[8,64].Wecalculatedself-recruitmentforthedurationofthemodel
runforeachislandbydividingthenumberofreleasedparticlesfromanislandthatweretrans-
portedbacktothatislandbythetotalnumberof“settlers”there.Thisallowsustodetermine
howdependentanislandisonrecruitmentfromoutsidelocationstomaintainthepopulation.
Source-sinkdynamicswereassessedbycalculatingasource-sinkindexfollowingHolstein
etal.[21].Wedefineasourceasanislandthatexports(outgoing)moreparticlesthanit
imports(incoming),andasinkislandimportsmoreparticlesthanitexports[21,64].The
source-sinkindexisaratioofthedifferencebetweensuccessfultransportoutoftheisland
(export)andsuccessfultransportintotheisland(import),dividedbythetotalofallsuccessfully
transportedparticlesinandoutoftheisland[21,64].Becausetheindexlooksatthedifference
inthetotalfluxofparticlesintoandoutofeachisland,itallowsustocompareislandswith
varyingamountofhabitatandislandsthathavetotalnumbersoftransportedparticlesthatdif-
fersbyordersofmagnitude.Theindexspansfrom-1to1,andapositiveindeximpliesasource
siteandanegativeindeximplyasinksite.Thestrongertheindexthemorelikelythesiteisto
beapersistentsourceorsinksite.Azeroindexindicatesthatthefluxofparticlesthataresuc-
cessfullytransportedontotheislandandoutoftheislandsarethesame.Thisindexallowsus
tocompareislandsinthearchipelagoandevaluatessource-sinkdynamicsonaregionalscale,
whereasself-recruitmentallowsustocharacterizeislandsassourcesorsinksonalocalscale.
Results
Potentialconnectivity
Theprobabilitymatrixshowsanisolation-by-distancepatternwithsitesfarawayfromeach
otherhavinglittleornopotentialconnectivityandconsiderableself-recruitmentformost
islandsacrosstheArchipelago(Fig2).RestrictingparticlereleasetothetypicalMay-June
spawningseasonminimallyalterstheoverallpotentialconnectivitypatterns(r=0.932)(Fig2b,
S1Fig).DuringspawningseasonO‘ahuandMaroReefshowstrongerconnectionswithneigh-
boringislandswhileNi‘ihauandKaua‘ibecomelessconnected.Self-recruitmentismore
importantforKureandMidwayAtolls(Fig2)duringspawningseason,whereasRaitaismore
dependentonself-recruitmentyearround(Fig2).
ThereislimitedpotentialconnectivitybetweentheNWHIandtheMHI,andthedirection
ofdispersalispredominantlyfromtheMHItotheNWHI.ParticlesoriginatingintheMHI
PLOSONE|DOI:10.1371/journal.pone.0167626 December8,2016 6/25
PopulationConnectivityinHawaii
Fig2.PotentialConnectivitymatrixfortheHawaiianArchipelago.(A)Thevaluesineachcellare
“settlement”probabilitiesscaledtothereceivingsiteforyear-roundparticlerelease.Arrowsindicatedthe
breaksmentionedinthetext.Eachrowinthematrixaddsupto1.Highvalues(red)indicatehighconnectivity
andlownumbers(blue)indicatelowconnectivity,andwhitecellsdenotenoconnectivity.(B)Differencematrix
showingthedifferenceinconnectivitybetweenyear-roundandMay-Juneparticlerelease.TheMay-June
PLOSONE|DOI:10.1371/journal.pone.0167626 December8,2016 7/25
PopulationConnectivityinHawaii
releasematrixissubtractedfromtheyear-roundreleasematrix(inAabove).Positivevalues(red)denotea
higherconnectivityvalueforyear-roundreleasesandanegativenumber(blue)denoteshigherconnectivity
forMay-Junereleasedparticles.
doi:10.1371/journal.pone.0167626.g002
form37uniqueconnectionswithsitesintheNWHI(markedareaintheupperrightcornerof
Fig2a),whileparticlesoriginatingfromtheNWHIonlyform24uniqueconnectionswith
sitesintheMHI(markedareasinthebottomleftinFig2a).Fourtimesasmanyparticlesare
successfullytransportedfromtheMHItotheNWHIthanviceversa(3.1%fromMHIto
NWHIand0.77%fromNWHItotheMHIofthetotalsuccessfullysettledlarvae).TheMHI
donotexportanyparticlesnorthwestofMokumanamana,andislandslocatedbetweenKaula
andNihoainthecenterofthearchipelagoaretheonlyislandsintheNWHItocontributepar-
ticlestotheMHI.MostparticlesreleasedfromNihoaarelosttothesystem,indicatedbythe
lowself-recruitment(<1%)andlowcontribution(6.067E-4%–0.72%)tothe“settlement”at
nearbyislands(Fig2).
WhiletransportbetweentheMHIandtheNWHIispredominantlynorthwestward,total
transportbetweenallislandsinthearchipelagoisreversed,with37%ofthesuccessfullytrans-
portedparticlesarriveatislandstothesouth(islandsbelowthediagonalinFig2a),while32%
ofparticlesaretransportedtoislandstothenorth(islandsabovethediagonalinFig2a).How-
ever,96.3%ofthesuccessfullytransportedparticlesoriginatingintheMHIendupsettling
withintheMHIand3.73%aresuccessfullytransportedtotheNWHI,while95.1%ofsuccess-
fullytransportedparticlesoriginatingintheNWHIaresuccessfullytransportedtositeswithin
theNWHIand4.87%ofparticlesaresuccessfullytransportedtotheMHI.
Therearethreebreaksintheconnectivitymatrixpresentforbothyear-roundandseasonal
particlerelease.Veryfewparticlessuccessfullycrossthesebreaks.Thesebreaksaremorepro-
nouncedduringspawningseasonreleases(Fig2b),andaremoredistinctintheforwardmatri-
ces(S2andS3Figs).ThesouthernmostbreaklocatedbetweenNihoaandMokumanamanais
themostpronounced.NoparticlescrossthisbreakintooroutoftheMHI,effectivelycutting
theMHIofffromtheNWHI.ThecentralbreakatGardnerPinnaclesandMaroReefistra-
versedonlybyparticlestoandfromRaitaBank.ThenorthernbreakbetweenLisianskiand
PearlandHermesAtolleffectivelyisolatesKureAtollandMidwayIslands,resultinginhigh
self-recruitmentforthenorthernmostislandsinthearchipelago.
Usingflowfieldsfromdifferentoceanographiccirculationmodelsatdifferentspatialreso-
lutionsallowsustotestwhetherthepotentialconnectivitypatternsarerobusttomodelresolu-
tion.Thereisastrongcorrelationbetweenthepotentialconnectivitydescribedaboveandthe
connectivitymatrixgeneratedfromadispersalmodelrunthatusedcurrentvelocitiesfromthe
coarserglobalHYCOM(r=0.9291)(S4Fig).FortheMHI,wecomparedconnectivitymatrices
generatedfromthreedispersalmodelrunsthatusedcurrentvelocitiesfromthe0.04˚MITgcm
(S5aFig),0.04˚regionalHYCOM(S5bFig),and0.08˚globalHYCOM(S5cFig).Potential
connectivityfortheMHIgeneratedfromthemodelrunusingdifferentresolutionsofthe
HYCOMcurrentsshowedthestrongestcorrelation(r=0.974)(S6aFig),followedbyconnec-
tivitymatricesgeneratedfromthemodelrunswiththesamespatialresolutionoftheflow
field,MITgcmand0.04regionalHYCOM(r=0.9533)(S6bFig).Weobservedthelargestdif-
ferencebetweenpotentialconnectivitygeneratedfrommodelrunsusingMITgcmandthe
0.08kmHYCOMflowfields(r=0.9305)(S6cFig).
Total“settlement”
Successfultransportacrossallislandsishighlyvariablewithameanof1.416%(SE7.708e-5)of
allreleasedparticlessuccessfullyarrivingatareceivingsiteoverthefive-yearmodelrun.The
PLOSONE|DOI:10.1371/journal.pone.0167626 December8,2016 8/25
PopulationConnectivityinHawaii
Fig3.TotalpercentsuccessfulsettlementforallsitesintheHawaiianArchipelagoforthefive-yearmodelrun.Thegreenbarsrepresentparticles
spawnedduringpeakspawningseasonMay-Juneeachyear.
doi:10.1371/journal.pone.0167626.g003
lowesttotalsuccessfultransportwasseenonJuly6,2011(0.682%),andthehighesttotalsuc-
cessfultransportonNovember2,2012(2.405%).Thereisnodiscernibleseasonalpatternin
totalarrivalsobservedforthearchipelagoasawhole(Fig3).Thehighestratesofsuccessful
transportin2009(2.27%)and2010(2.22%)coincidedwithparticlesreleasedduringpeak
spawningseason(markedbygreenbarsinFig3);however,thefollowingthreeyearshadsome
ofthelowestratesofsuccessfultransportforparticlesreleasedinMay-June(0.68%,0.95%,and
0.96%).
TheMHIhaveoveralllargerrelativeprobabilityofsuccessfularrivalswhileislandsnearthe
observedbreaksintheconnectivitymatrixhavethelowestrelativeprobabilityofsuccessful
arrivals.JohnstonAtollhasthelowestrelativesuccessfularrivalvalueofallat0.0637%for
year-roundreleaseand0.0337%forpeakspawningseasonreleases(Fig4).AtKaula,therela-
tivearrivalsuccessisalmosttwoordersofmagnitudelargercomparedwithJohnstonAtoll,
with3.574%foryear-roundspawning.Weseethelargestrelativearrivalsforseasonalrelease
atLāna‘iwith2.713%.Hawai‘iIslandistheonlyMHItoshowhigherarrivalsuccessforparti-
clesreleasedduringspawningseason(2.523%)comparedtoyear-roundreleases(2.298%).
Distancetraveled
Thespatiallyaveragedmeandistancetraveledis112.32km(SE=1.705)foryear-roundparti-
clerelease.Particlesreleasedduringpeakspawningseasontravelfurther,withmeandistance
of124.37km(SE=2.372).Mediandistancesareshorter,101.39kmand110.80kmforyear-
roundandMay-Junereleaserespectively,indicatingthatafewparticlesdisperse,traveling
PLOSONE|DOI:10.1371/journal.pone.0167626 December8,2016 9/25
PopulationConnectivityinHawaii
Fig4.Totalpercentsuccessfulsettlementateachislandforthefive-yearmodelrun.GreenbarsshowsettlementforparticlesspawnedduringMay-
June,graybarsshowsettlementforyear-roundspawning.
doi:10.1371/journal.pone.0167626.g004
significantlylongerdistancesanddrivingupthemean.Thisisalsoevidentfromthelongright
tailonthedensitykernel(Fig5).
Particlesarrivingatislandsinthecenterofthearchipelagotravelthelongestmeandis-
tances,whileparticlesatJohnstonAtolltraveltheshortest(100%self-recruitment)(Fig6).Par-
ticlessuccessfullytransportedtothebankjustsouthofNihoadispersedmorethandoublethe
meandistanceforothersitesacrossthearchipelago:onaverage341.4kmduringMay-June
release,and277.2kmduringyear-roundrelease.Consistentwithtotaldispersaldistancesfor
allislands,island-specificdispersaldistancesaregreaterforparticlesreleasedduringspawning
season,for23outof31islands(Fig6).IntheMHI,dispersaldistancesareconsistentthrough-
outtheyearexceptforparticlesreleasedfromKaua‘iwhichhasamuchlongerdispersaldis-
tanceduringMay-Junerelease.Kaua‘idispersaldistancesaremoresimilartoislandslocated
inthecenterofthearchipelago,likelyduetothepredominantlynorthwestdirectionofdis-
persal(Fig2)andthelongerdistancesbetweenhabitatsintheNorthwesternHawaiianIslands.
Particlesreleasedfromislandlocatedtothenorthwestofeachconnectivitybreak(Pearland
HermesAtoll,MaroReefandMokumanamanaIsland)haveshorterdispersaldistancescom-
paredtotheislandjustsoutheastofthebreak(LisianskiIsland,GardnerPinnacles,andNihoa
Island)by45.6%,63.4%,and73.9%respectively.
Self-recruitment
Themeanself-recruitmentforthearchipelagois25.2%(SE=0.0414)butvariesgreatlyfrom
islandtoisland.JohnstonAtollreliessolelyonself-recruitment(100%)forpopulationpersis-
tence,whileatNihoaIslandself-recruitmentaccountsforlessthan1%oftotalsettlement(Fig
7).DuringpeakspawningseasonNihoa,alongwithGardnerPinnacles,importalltheirparti-
cles.Islandspecificself-recruitment(Fig7,anddiagonalintheconnectivitymatrixinFig2)is
strongestatKure(year-round50.5%),PearlandHermesAtoll(year-round80.67%,May-June
PLOSONE|DOI:10.1371/journal.pone.0167626 December8,2016 10/25