Fish and Fisheries. 2017;1–17. wileyonlinelibrary.com/journal/faf  | 1© 2017 John Wiley & Sons Ltd

Received:25November2016  |  Accepted:21June2017DOI: 10.1111/faf.12238

O R I G I N A L A R T I C L E

Distribution patterns and population structure of the blue shark (Prionace glauca) in the Atlantic and Indian Oceans

Rui Coelho1  | Jaime Mejuto2 | Andrés Domingo3 | Kotaro Yokawa4 |  Kwang-Ming Liu5 | Enric Cortés6  | Evgeny V. Romanov7 | Charlene da Silva8 |  Fábio Hazin9 | Freddy Arocha10 | Aldrin Masawbi Mwilima11 | Pascal Bach12 |  Victoria Ortiz de Zárate13 | William Roche14 | Pedro G. Lino1 |  Blanca García-Cortés2 | Ana M. Ramos-Cartelle2 | Rodrigo Forselledo3 |  Federico Mas3 | Seiji Ohshimo4 | Dean Courtney6 | Philippe S. Sabarros12 |  Bernardo Perez13 | Ciara Wogerbauer14 | Wen-Pei Tsai15 | Felipe Carvalho9 |  Miguel N. Santos1

1PortugueseInstitutefortheOceanandAtmosphere(IPMA),Olhão,Portugal2InstitutoEspañoldeOceanografía(IEO),C.O.ACoruña,ACoruña,Spain3DirecciónNacionaldeRecursosAcuáticos(DINARA),LaboratoriodeRecursosPelágicos,Montevideo,Uruguay4NationalResearchInstituteofFarSeasFisheries(NRIFSF),Shizuoka-City,Shizuoka,Japan5InstituteofMarineAffairsandResourceManagement,NationalTaiwanOceanUniversity,Keelung,Taiwan6NationalOceanographicandAtmosphericAdministration,NationalMarineFisheriesService(NOAA-NMFS),SoutheastFisheriesScienceCenter,PanamaCityLaboratory,PanamaCity,FL,USA7CentreTechniqued’AppuiàlaPêcheRéunionnaise(CAPRUN–HydrôRéunion),LePort,ÎledelaRéunion8DepartmentofAgriculture,ForestryandFisheries;Branch:FisheriesResearchandDevelopment,InshoreResearch,CapeTown,SouthAfrica9DepartamentodePescaeAquicultura,UniversidadeFederalRuraldePernambuco(UFRPE),Recife,Brasil10InstitutoOceanográficodeVenezuela,UniversidaddeOriente,EstadoSucre,Venezuela11MinistryofFisheriesandMarineResources,NationalMarineInformationResourceCenter,Windhoek,Namibia12InstitutdeRecherchepourleDéveloppement(IRD),UMR248MARBEC,SèteCedex,France13InstitutoEspañoldeOceanografía(IEO),C.O.Santander,Santander,Spain14InlandFisheriesIreland,Dublin24,Ireland15DepartmentofFisheriesProductionandManagement,NationalKaohsiungMarineUniversity,Kaohsiung,Taiwan

CorrespondenceRuiCoelho,PortugueseInstitutefortheOceanandAtmosphere(IPMA),Olhão,Portugal.Email:rpcoelho@ipma.pt

Present addressesFelipeCarvalho,NOAAPacificIslandsFisheriesScienceCenter,Honolulu,HI,USA

MiguelN.Santos,InternationalCommissionfortheConservationofAtlanticTunas(ICCAT),Madrid,Spain

Funding informationSeeacknowledgmentssection

AbstractTheblueshark(Prionace glauca)isthemostfrequentlycapturedsharkinpelagicoce-anicfisheries,especiallypelagiclonglinestargetingswordfishand/ortunas.Aspartofcooperativescientificeffortsforfisheriesandbiologicaldatacollection, informationfromfisheryobservers,scientificprojectsandsurveys,andfromrecreationalfisheriesfromseveralnations in theAtlanticand IndianOceanswascompiled.Datasets in-cludedinformationonlocation,sizeandsex,inatotalof478,220bluesharkrecordscollected between 1966 and 2014. Sizes ranged from 36 to 394cm fork length.Considerablevariabilitywasobservedinthesizedistributionbyregionandseasoninbothoceans.Largerbluesharkstendtooccurinequatorialandtropicalregions,and

2  |     COELHO Et aL.

1  | INTRODUCTION

Theblueshark(Prionace glauca,Carcharhinidae)isoneofthewidestranging of all pelagic shark species, found throughout tropical andtemperateseasfromlatitudesofabout60°Nto50°S(Last&Stevens,2009).Itisapelagicspeciesmainlydistributedfromtheseasurfacetodepthsofabout350m,eventhoughdeeperdivesdownto1,000mhavebeenrecorded(Campanaetal.,2011).Thebluesharkisanoce-anic species capable of long-range migrations (e.g. Campana etal.,2011; da Silva, Kerwath,Wilke,Meÿer,& Lamberth, 2010;Queirozetal.,2005),butcanalsooccasionallyoccurclosertoshore,especiallyinareaswherethecontinentalshelfisnarrow(Last&Stevens,2009).Thesporadicpresenceofbluesharkrecruitshasbeendescribedveryclosetoshoreinsomeareas(e.g.north-eastAtlantic,Mejuto,García-Cortés,Ramos-Cartelle,&Abuin,2014).

Blue sharks are capturedby avarietyof fishinggears, butmostcatches that have been reported take place as by-catch in pelagiclonglines targeting tunas (Thunnus spp.) and/or swordfish (Xiphias gladius), where it is the most prevalent shark captured (Castro,Serna,Macías,&Mejuto,2000;Coelho,Fernandez-Carvalho,Lino,&Santos,2012;Hazin,Broadhurst,Amorin,Arfelli,&Domingo,2008;Mejuto,1985;Mejuto&García-Cortés,2005;Mejuto,García-Cortés,Ramos-Cartelle,&Serna,2009;Romanov,Bach,&Romanova,2008).Dependingonthefisheries,areasandseasons,bluesharkcatchescanbeverysignificantintheoverallcatchandinsomespecificcasescanaccountformorethan50%ofthetotalfishcatchandaround85–90%ofthetotalelasmobranchcatch(Coelhoetal.,2012).

IntheAtlantic,theaveragebluesharklandingsreportedtoICCAT(International Commission for the Conservation of Atlantic Tunas)over the last few years (2010–2014) were approximately 64,000t, ofwhichapproximately58%were from theNorthand42% fromthe South Atlantic. Overall, this represents approximately 8.5% ofthetotalpelagic fish landings inweight for theAtlantic,considering

that the average annual landings (all species combined) reported toICCATduringthesameperiodwereapproximately756,000t(Anon.,2014).IntheIndianOcean,theaverageannualbluesharklandingsre-portedtoIOTC(IndianOceanTunaCommission)overthe2010–2014periodwereapproximately28,000t(Anon.,2015a),whichrepresentsapproximately1.6%of the totalpelagic fish landingsconsideringanaverageannuallanding(allspeciescombined)reportedtoIOTCofap-proximately1,700,000tforthesameperiod(Anon.,2015a).However,comparedtotheAtlanticOcean,thecatchandlandingsofbluesharkinthe IndianOceanare likelyconsiderablyhigherthanthereportedvaluesduetounder-reportingandlackofspecies-specificidentifica-tionformanysharkspeciesinsomefisheries.Overthesameperiod(2010–2014), the reported landings of “sharks nei—not elsewhereincluded” for the IndianOceanwereapproximately47,000 t (Anon.2015a),whichconsideringtheprevalenceofbluesharkinpelagicgearcatchesislikelycomposedofalargeproportionofbluesharks.

Understandingthespatio-temporaldynamicsofmarinespeciesisextremelyimportantforfisheriesmanagementandconservation,asitallowsabetterunderstandingofthespeciesdistributionandpotentialimpactsbyfisheries.Somepreviousstudieshavefocusedonthedis-tributionofcatchratesofbluesharkinspecificareasoftheAtlantic,including the works of Hazin, Boeckmann, Leal, Lessa etal. (1994),MejutoandGarcía-Cortés(2005),Domingo,Mora,andCornes(2002),Montealegre-Quijano and Vooren (2010) and Carvalho etal. (2011)in the south-west Atlantic; Cortés, Brown, and Beerkircher (2007)andTavares,Ortiz,andArocha (2012) in thewesternNorthAtlantic;Megalofonou, Damalas, andDeMetrio (2009) in theMediterranean;andVandeperre,Aires-da-Silva,Santosetal.(2014),Vandeperre,Aires-da-Silva, Fontes etal. (2014) in the Central NorthAtlantic. Previousstudieshavealsoinvestigatedsizedistributionsofbluesharksinbroadareas of the North and SouthAtlantic, such asMejuto and García-Cortés(2005),andinmorespecificareasoftheAtlantic,suchasTavaresetal.(2012)offVenezuelaintheCaribbeanSeaandadjacentwaters,

smaller specimens inhigher latitudes in temperatewaters.Differences in sex ratioswere also detected spatially and seasonally. Nursery areas in the Atlantic seem tooccurinthetemperatesouth-eastoffSouthAfricaandNamibia,inthesouth-westoffsouthernBrazilandUruguay,andinthenorth-eastofftheIberianPeninsulaandtheAzores.Parturitionmayoccurinthetropicalnorth-eastoffWestAfrica.IntheIndianOcean,nurseryareasalsoseemtooccurintemperatewaters,especiallyinthesouth-westIndianOceanoffSouthAfrica,andinthesouth-eastoffsouth-westernAustralia.Thedistributionalpatternspresentedinthisstudyprovideabetterunderstandingofhowbluesharkssegregatebysizeandsex,spatiallyandtemporally,andimprovethescientificadvicetohelpadoptmoreinformedandefficientmanagementandconserva-tionmeasuresforthiscosmopolitanspecies.

K E Y W O R D S

AtlanticOcean,fisheryobserverprogrammes,IndianOcean,pelagicfisheries,sizedistribution,spatialdistribution

     |  3COELHO Et aL.

Carvalho etal. (2010) in the south-westAtlantic, and da Silva etal.(2010)offtheAtlantic–Indianconfluencezone.FortheIndianOcean,thecurrentlyavailableinformationonbluesharkisstillveryscarceandincludesmainlyobservationsonbiologicalaspectsanddistribution(e.g.Gubanov&Grigor’yev,1975;Sellesetal.,2014),andsize,sex,catchratesandreproductiveparameters(Mejuto&García-Cortés,2005).

Ecological risk assessment (ERA) methods have been used bysomet-RFMOs(tunaRegionalFisheriesManagementOrganizations)toprovide indicatorsof thevulnerabilityofpelagic shark species tofishinggears.In2012,asemi-quantitativeERAforpelagicsharkswasdevelopedintheIndianOcean,wherethebluesharkreceivedame-diumvulnerabilityrankingastheywerecharacterizedtobethemostproductivesharkspeciesbutalsohighlysusceptibletopelagiclonglinegear(Muruaetal.,2012).IntheAtlantic,ERAsforpelagicsharkswereconducted in2008and2012, andalso showed that theblue sharkhadanintermediatevulnerabilitylevel,alsocharacterizedbyhighpro-ductivitywithin thepelagic sharksandhighsusceptibility topelagiclonglinefishinggear(Cortésetal.,2010,2015).

The lateststockassessmentsofbluesharkfortheAtlanticwerecarriedoutbyICCATin2015.FortheNorthAtlanticstock,allscenar-ios indicatedthatthestockwasnotoverfishedandthatoverfishingwasnotoccurring,butduetothehighlevelsofuncertainty,thepossi-bilityofthestockbeingoverfishedandoverfishingoccurringwasnotcompletelyruledout(Anon.,2015b).FortheSouthAtlantic,thesce-nariosandmodelsvariedfrompredictingthatthestockwasnotover-fishedandthatoverfishingwasnotoccurring,tolessoptimisticcaseswherethestockcouldbeoverfishedandoverfishingcouldbeoccur-ring.Thehighuncertaintyincatchestimatesanddeficiencyofsomeimportant biological parameters, particularly for the SouthAtlantic,were identifiedasobstaclesforobtainingmorereliableestimatesofthecurrentstockstatus(Anon.,2015b).ThelateststockassessmentconductedfortheIndianOceanbyIOTCalsotookplacein2015,andfromthevariousmodel runs, therewasa suggestion that thestockcouldbesubjecttooverfishingbutnotyetoverfished;however,therewashighuncertaintyintheresults,andassuch,thestockstatusre-maineduncertain(Anon.,2015c).Asinmostpelagicspecies,thereisstillconsiderableuncertaintyinthestockstatusadviceforbluesharkcurrentlyprovidedbothfortheAtlanticandIndianOceans.

Todate, anoceanic-wideand fleet-combinedstudyon the size-structureanddistributionpatternsofbluesharkislacking.However,this type of information is needed to provide better managementadviceforthepopulationsatanoceanic-levelscale.ResearcheffortshavebeencarriedoutinrecentyearsbyscientistsbothintheAtlanticand IndianOceans, incollaborationwith themajor fishing fleets, toprovide and analyse such scientific data in support ofmanagementadvice.Thisincludestheprovisionofsize-baseddataforlength-based,age-structured integrated stock assessmentmodels that have beenusedmorerecentlybythet-RFMOs.

Themaingoalofthisstudyisthereforetoprovideareviewofthedetailed size distribution data available for the blue shark from themajoroceanicfleetsthattargettunasand/orswordfishintheAtlanticand Indian Oceans, especially pelagic longline fisheries that canhave relativelyhighcatch ratesofbluesharks.Additionaldata from

recreational fisheries and scientific projects and surveys were alsoused.Thespecificobjectivesofthisreviewareto:(i)analysethesizedistributionandseasonalpatternsofthebluesharkintheAtlanticandIndianOceans;(ii)providetime-seriestrendsofthesizedistributionineachregion;(iii)analysethedistributionofsexratiosatoceanic-widescales;(iv)characterizethemainareasofconcentrationofparticularlifestagesincludingjuveniles/immatureandadults/maturespecimens;and(v)modeltheexpectedsizedistributionoveroceanic-widescalesintheAtlanticandIndianOceans.

2  | MATERIALS AND METHODS

2.1 | Data collection

Bluesharkrecordsanddatawerecollectedmainlybynationalscien-tificobserverson-boardcommercialvessels.Additionaldatawereob-tainedfromdetailedlogbooksandportsamplersworkingonnationaldatacollectionprogrammes,andfromscientificprojectsfromseveralfishingnationsintheAtlanticandIndianOceans,mainlysurveyingpe-lagic longlinefisheries.Mostofthedatacamefromthecommercialdriftingpelagiclonglines,includingshallownightsettinglonglinestar-getingswordfishinbothtemperateandtropicalregions,deeperdaysettinglonglinestargetingtropicaltunasinmoretropicalregions,anddeeper setting longlines in high latitudes of theNorthAtlantic tar-getingbluefintuna(Thunnus thynnus;ICCAT,2006–2016).AdditionaldatausedcamefromartisanalpelagiclonglinesintheBayofBiscay,fromscientificpelagic longlinesurveyscarriedoutbysomenationsbetweenthe1960sand1980s (JapanandUSSR),and fromtaggingundertakenbyanglingchartervesselsoffIreland(Greenetal.,2009).A summaryof thedata collected, compiled andused for this studyisprovided inTable1.A limitationofthisstudy is thatthemajorityof the data collected came from fishery-dependent sources, whichaffected the length compositions anddetectionof blue sharks (seeDiscussionformoredetails).

Datawerecollectedacrossawidegeographicalrangeinthetwooceans. In theAtlantic, the twohemisphereswere separatedat the5°Nparallel,asrecommendedintheICCATManualforsharkspecies(ICCAT,2006–2016;Figure1).Furthermore,eachhemispherewasdi-videdintofourareas(NW,NE,SW,SE)takingintoconsiderationtheICCATsamplingareas forsharks (ICCAT,2006–2016)aswellas thedistributionpatternsof thefleetsandthecharacteristicsof thedis-tributionsofsizesofbluesharksinthesample.FortheIndianOcean,onlyonebluesharkstockwasconsideredasusedbytheIOTC,dividedintofourareas(NW,NE,SW,SE)basedmainlyonthecharacteristicsofthedistributionsofsizesofbluesharksinthesampleanddistribu-tionofthefleets(Figure1).

For captured specimens, data on size, sex, capture location anddatewererecorded.Thesizemeasurementmostoftentakenwastheforklength(FL),butthereweresomeexceptionsassomeofthena-tionalprogrammesrecordothermeasurements(e.g.TL—totallength;PCL—pre-caudal length; LW—live or round weight; DW—dressedweight). In those cases, all sizes andweightswere converted to FLusingequationsavailableatthenationalresearchinstitutes(Table2).

4  |     COELHO Et aL.

2.2 | Data analysis

Size-frequencydistributionsbyareaandtrendsinmeansizedistribu-tionswereanalysedandplottedbyyear,area,sexandquarteroftheyear.SizedataweretestedfornormalitywithKolmogorov–SmirnovnormalitytestswiththeLillieforscorrection(Lilliefors,1967),andforhomogeneityofvarianceswithLevenetests(Levene,1960).Specimensizeswerecomparedamongregions,sexesandquartersoftheyearusingnonparametrick-samplepermutationtests(Manly,2007).

Sex ratios were calculated and mapped over a 5°×5° (lati-tude×longitude) grid for both theAtlantic and IndianOceans.Thecomparison among areas was carried out with contingency tablesandPearson’schi-squared tests.Thesex ratioswerealsocomparedamongseasonsoftheyearandsize-classes(categorizedbythe20thpercentilesofthedata),takingintoaccountthevariousregions,usingCochran–Mantel–Haenszel(CMH)chi-squaredtests.Thisallowedthedetectionofseasonalityandsize-relatedeffectsinthesexratioscon-ditionaltoeachoftheregionsanalysed.

The proportions of immature versus mature specimens in eachregionandseasonwerecalculated. In theAtlantic, themedian sizesatmaturity(FL)usedtodefineimmatureandmaturespecimenswerebased on the ICCAT SharkWorking Group report (Anon., 2014) asfollows:NorthAtlantic:females=182.1cmFL,males=197.0cmFL;SouthAtlantic: females=173.8cmFL,males=175.5cmFL.For theIndianOcean,themediansizesatmaturity(FL)weredefinedaccordingtotheIOTCExecutiveSummaryforbluesharkproducedbytheIOTCScientificCommittee(Anon.,2015d)asfollows:females=194cmFL;

males=201cmFL.Thekerneldensitiesofthedistributionofyoungjuvenile (age<=1), immature (juveniles of all age classes) and adultsharksintheAtlanticandIndianOceanswerecalculatedona5°×5°grid.Kerneldensitieswereestimatedonthisgridusingbivariatenormaldistributions (Wand,1994).Forplottingthedensitiesofyoung juve-niles(ages0and1),thesize-at-agedefinitionsofSkomalandNatanson(2003)wereused,specificallyage0females:60.9cmFL;age0males:66.1cmFL;age1females:97.0cmFL;andage1males:97.4cmFL.

Ageneralizedadditivemodel(GAM)withaGaussianerrorstruc-tureandidentitylinkfunctionwasusedtopredicttheexpectedbluesharksizedistributionsasa functionof location (latitudeand longi-tude) and quarter of theyear in each ocean.The predictors in thismodelweregivenbythesmoothfunctionsoflatitudeandlongitudeplusaparametriccomponentforthequarters.Thesmoothtermsforthe location covariateswereestimatedbymaximum likelihoodwiththin plate regression splines (Wood, 2003). The significance of themodel parameterswas testedwith likelihood ratio tests comparingnestedmodels,includingthesignificanceoftheinteractionsbetweenlatitude, longitude andquarterof theyear.Goodnessof fitwas as-sessedwithAkaikeinformationcriterion(AIC;Akaike,1973)andwiththe final devianceexplained.A residual analysiswas carriedout formodelvalidation.Theexpectedmean sizesweremapped along thestudyareaineachoceanandforeachquarteroftheyear.

The analysis for this studywas carried out using the R languageforstatisticalcomputingversion3.2.0. (RCoreTeam,2015).Additionalpackagesused included the following libraries: “car” (Fox&Weisberg,

TABLE  1 Summaryofthedatacompiledandanalysedforthisstudybyfleetandgeartype,withinformationonthesamplesizeinnumberofspecimens(N),thesizerangeofthespecimens(FL—forklength,cm)andtherangeofyearsineachdataset

Ocean Country/fleet Gear Activity Sample (N) Size range (FL, cm) Years range

Atlantic Brazil Pelagiclongline Commercial 6,242 43–320 2004–2008

EU.Spain Pelagiclongline Commercial 99,053 41–310 1993–2013

EU.Spain Artisanallongline Commercial 26,889 69–310 1998–2001

EU.Ireland Rod and reel Recreational 3,520 40–240 1970–2013

EU.Portugal Pelagiclongline Commercial 87,490 45–370 1997–2013

Japan Pelagiclongline Commercial 33,206 42–328 1997–2014

Namibia Pelagiclongline Commercial 11,578 38–352 2004–2013

Taiwan Pelagiclongline Commercial 59,107 40–394 2004–2013

Uruguay Pelagiclongline Commercial 69,157 36–305 1998–2012

USA Pelagiclongline Commercial 2,685 41–335 1992–2014

Venezuela Pelagiclongline Commercial 1,376 50–355 1994–2013

SouthAfrica Pelagiclongline Commercial 521 107–265 2012–2014

Indian EU.France Pelagiclongline Commercial 305 89–300 2007–2014

EU.France Pelagiclongline Research 53 100–270 2003–2011

EU.Portugal Pelagiclongline Commercial 15,276 80–299 2011–2014

Japan Pelagiclongline Commercial 39,978 41–369 1992–2014

Japan Pelagiclongline Research 4,163 62–307 1967–2002

Taiwan Pelagiclongline Commercial 10,275 51–350 2004–2013

USSR Pelagiclongline Research 2,975 57–311 1966–1989

SouthAfrica Pelagiclongline Commercial 4,371 70–322 2012–2014

     |  5COELHO Et aL.

2011), “classInt” (Bivand,2013), “ggplot2” (Wickham,2009), “gmodels”(Warnes,Bolker,Lumley,&Johnson,2013),“KernSmooth”(Wand,2015),“lme4”(Bates,Maechler,Bolker,&Walker,2013),“maps”(Becker,Wilks,Brownrigg, & Minka, 2013), “mapplots” (Gerritsen, 2013), “maptools”(Bivand&Lewin-Koh,2013),“mgcv”(Wood,2006,2011),“perm”(Fay&Shaw,2010),“plyr”(Wickham,2011),“rgdal”(Bivand,Keitt,&Rowlingson,2013),“scales”(Wickham,2012)and“shapefiles”(Stabler,2013).

3  | RESULTS

3.1 | Spatial distribution

Atotalof478,220bluesharkswererecordedandusedforthiswork,with400,824 from theAtlantic and77,396 from the IndianOcean.Specimensranged insizefrom36to394cmFL intheAtlantic,andfrom41to369cmFLintheIndianOcean,coveringmostoftheknownsizerangeofthespecies.Asummaryofthesamplesize(N)andspeci-men size ranges by ocean and fleet is provided in Table1, and thedistributionmapofthesampleinbothoceansisshowninFigure2.

Size datawere not normally distributed (Lilliefors test:D=0.036,p<.001),andthevarianceswereheterogeneousamongregions(Levenetest: F=2005.2, df=11, p<.001), quarters (Levene test: F=250.8,df=11,p<.001)andsexes (Levenetest:F=12.584,df =1,p<.001).Usingunivariatenonparametricstatisticaltestsrevealedthatsizessig-nificantlydifferamongregions(permutationtest:chi-squared=138440,df =12, p<.001), quarters (permutation test: chi-squared=5484.8,df=3,p<.001)andsexes(permutationtest:chi-squared=1358,df=1,p<.001).

Considerable variability was observed in the size distributionsof both male and female blue sharks among areas (Figures1–3).However,with the areas structured as described above, blue sharksizedistributionswithineachareaweremostlyunimodalexcept forslightevidenceofbimodaldistributions insomeareas (NAT-NEandNAT-SW;Figure3). In theAtlantic, smaller specimens tended to becaptured in more temperate waters (NAT-NE, SAT-SW; Figure3),while larger specimens tended to be captured more frequently intropicalwaters, especially betweenWestAfrica and the CaribbeanSea(NAT-SE,NAT-SWandSAT-SE; Figure3). Similarly, in the Indian

F IGURE  1 LocationofthestudyareaintheAtlanticandIndianOceanswiththelimitsofthesizesampledistributionsshadedinthelightgreyarea.Thestockandregionnomenclatureusedandthespatialdistributionofthesamplesarealsoindicated.Specifically,theICCATandIOTCstockmanagementunitsforsharksareidentifiedassolidblacklines(NorthAtlantic,SouthAtlanticandIndianOcean),andthefourareas(quadrants)withineachstockasdefinedforthisstudyareidentifiedwithdashedlines

TABLE  2 Morphometricrelations(length–length,length–weightandweight–weight)fromunpublisheddataavailableatnationalinstitutes,usedtoconvertandstandardizethemeasurementsusedinthisstudy.Themeasurementsareforklength(FL),totallength(TL),pre-caudallength(PCL),liveorroundweight(LW)anddressedweight(DW).Allsizedataareincmandallweightdataareinkg.DatacomefromIPMA(PortugueseInstitutefortheOceanandAtmosphere),NRIFSF(NationalResearchInstituteofFarSeasFisheries)andYugNIRO(SouthernScientificResearchInstituteofMarineFisheriesandOceanography;E.Romanov,unpublisheddata)

Ocean Relation Equation Source

Atlantic Livetodressedweight DW=0.0068+LW*0.4167 IPMA

Forklengthtoliveweight LW=0.0000015*FL^3.2907 IPMA

Totaltoforklength FL=−1.122+TL*0.829 NRIFSF

Totaltopre-caudallength PCL=−2.505+TL*0.762 NRIFSF

Indian Pre-caudaltoforklength FL=0.9095+PCL*1.0934 YugNIRO

Totaltoforklength FL=3.6291+TL*0.8215 YugNIRO

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F IGURE  2 Locationandsizedistributionofsamples(FL,cm)ofblueshark(Prionace glauca)recordedforthisstudyintheAtlanticandIndianOceans.Thecategorizationofsize-classeswascarriedoutusingthe0.2quantilesofthedata(valuesinthelegendrepresentthelowerandupperlimitsofeachsize-class).TheICCATandIOTCstockmanagementunitsforsharksareidentifiedassolidblacklines(NorthAtlantic,SouthAtlanticandIndianOcean).Thefourareas(quadrants)withineachstockasdefinedforthisstudyareidentifiedwithdashedlines.[Colourfigurecanbeviewedatwileyonlinelibrary.com]

F IGURE  3 Size-frequencydistributionsofmaleandfemaleblueshark(Prionace glauca)caughtinthedifferentregionsoftheAtlanticandIndianOceans.NAT=NorthAtlantic,SAT=SouthAtlanticandIO=IndianOcean.Withineachmajorarea,therearefourquadrantsasdefinedforthisstudy(NW,NE,SWandSE,seeFigure1).Theverticallinesrepresentmediansizeatmaturityineachregion(solidlines=males,dashedlines=females).[Colourfigurecanbeviewedatwileyonlinelibrary.com]

     |  7COELHO Et aL.

Ocean, smaller specimensalso tended tobecaptured inmore tem-peratewaters (IO-SE and IO-SW, Figure3), while larger specimenswerecapturedmorefrequentlyintropicalwaters(IO-NEandIO-NW;Figure3).Thesegeneraltrendstendedtobecommonforbothmalesand females.However, in someareas, thereweremoremarkeddif-ferencesinthesize-frequencydistributionofeachsexwiththemalesbeingnoticeablysmallerthanthefemales(IO-SE;Figure3).

3.2 | Annual and seasonal variability

Thereweredifferencesintimeseriesofthemeansizesamongregions,withsomeregionsshowingrelativelymorestabletrendsthanothers.The timeserieswere relatively stable in theNAT-NEandSAT-NW(Figure4). In contrast, higher variabilitywas found in theNAT-NWandNAT-SE(Figure4).

Nomajor trends in the timeserieswerenoticeable formost re-gions.However, in some cases, such as the IO-SW, therewere rel-ativelypronounced trendswith largerblue shark sizes in the1970s(researchcruisedata),followedbyaperiodwithsmallersizesbetween1992 and 2006, and then another periodwith larger sizes inmorerecentyears(Figure4).

Seasonalityandsexalsoinfluencethesizeofbluesharkscaught.In some areas, similar trendswere observed formales and femalesthroughouttheyear.Forexample,intheSAT-NE,IO-SEandIO-SW,bothmaleandfemalesizestendedtodecreasethroughouttheyear(Figure5). In contrast, in the IO-NW, both male and female sizestendedtoincreasealongthequartersoftheyear(Figure5).

3.3 | Sex ratios

Ofall blue sharkswith sex recorded (417,552 specimens), 352,797were from the Atlantic and 64,755 from the Indian Ocean. In theAtlantic, 165,229 specimens (46.8%) were females and 187,568(53.2%)weremales, representinganoverall sex ratioof1.14malesforeachfemale.IntheIndianOcean,32,819specimens(50.7%)werefemalesand31,936(49.4%)weremalesrepresentinganoverallsexratioverycloseto1:1,specifically1.03femalesforeachmale.

IntheAtlantic,bothspatialandseasonalvariabilityinsexratioswasevidentwhencalculatedandmappedovera5°×5°gridforeachquar-teroftheyear(Figure6).Inthetemperatenorth-eastAtlantic,thereweremorefemalesinthehigherlatitudes(northof45°N),especiallyevidentinquarters3and4.Incontrast,inlowerlatitudesoftemperate

F IGURE  4 Timeseriesofthemeansizeofblueshark(Prionace glauca)bysexcaughtinthedifferentregionsoftheAtlanticandIndianOceans.NAT=NorthAtlantic,SAT=SouthAtlanticandIO=IndianOcean.Withineachmajorarea,therearefourquadrantsasdefinedforthiswork(NW,NE,SWandSE,seeFigure1).Theerrorbarsare95%confidenceintervals.[Colourfigurecanbeviewedatwileyonlinelibrary.com]

8  |     COELHO Et aL.

north-easternwaters, between20 and40°N, therewere in generalmoremales,especiallyinquarters2and3.IntemperatewatersoftheNAT-NW,therewashighvariabilityinthesexratios,whileintropicalwaters inthecentralAtlantic,therewasa largeconcentrationoffe-males,particularlyinquarter3.IntheSouthAtlantic,between0and20°S,thesexratioswerehighlyvariable,whileinwaterssouthof20°S,therewereingeneralmoremales,bothinthesouth-westandsouth-eastAtlanticandespeciallyinquarters1,2and3.IntheareaoftheGulfofGuinea(north-eastquadrantoftheSouthAtlantic),therewasatendencyforthepresenceofmoremalesinquarters3and4.ThedifferencesobservedintheAtlanticsexratiosweresignificantwhencompared among the geographic areas as defined in Figure1 (pro-portiontest:chi-squared:3,501.5,df=7,p<.001)andseasonscon-ditionallywithineacharea (CMHtest: chi-squared=1,808.1,df=3,p<.001).Therewerealsosignificantdifferencesdetectedinthesexratioscomparingsizestestedconditionallywithintheeacharea(CMHtest:chi-squared=1,518.5,df =4,p<.001).

Similarly,intheIndianOcean,therewasalsoevidenceofvariabil-ity inthesexratioswhencalculatedandmappedovera5°×5°gridforeachquarteroftheyear(Figure7).Ingeneral,thereweremorefe-males recorded in southern latitudes both in the south-eastern andthesouth-westernIndianOcean,especiallysouthof40°S.Incontrast,therewasatendencyforthepresenceofmoremalesimmediatelytothenorthofthisparallel, inwatersbetweenca.40°Sand30°S,alsobothintheSEandSWIndianOcean.Thesexratiosinsoutherntropical

watersweremorevariable,withmorefemalesinquarters1and2,andmoremalesinquarter3,especiallyintheeasternareas.InthetropicalNorthIndianOcean(northoftheequator),therewereingeneralmoremales throughout theyear inmostareas.Thedifferences in thesexratiosobservedintheIndianOceanweresignificantwhencomparedamong the geographic areas asdefined inFigure1 (proportion test:chi-squared: 3,755.9,df=3,p<.001) and seasonswithin each area(CMHtest:chi-squared=956.5,df=3,p<.001).Therewerealsosig-nificantdifferencesdetectedinthesexratioscomparingsizestestedconditionallywithineacharea(CMHtest:chi-squared=696.3,df=4,p <.001).

3.4 | Distribution of life stages

Considerablevariabilitywasobservedinthedistributionofyoungju-venileandadultspecimensinbothoceanswhenconsideringregionsand quarters. In theAtlantic,more immature blue sharks, includingyoung-of-the-year(age0)andverysmalljuveniles(age1),werecap-tured in thenorth-east (GulfofBiscay),centraleast (Azores IslandsandwaterswestoftheAzores)andsouth-west (offsouthernBraziland Uruguay) regions (Figure8), while adults were more abundantintheequatorialandtropicalEasternAtlantic, intheGulfofGuineaand closer to the Cabo Verde Archipelago (Figure9). In the IndianOcean, thedensitiesof juvenileswerehigher in the south-westoffSouthAfrica,andsouth-eastoffAustralia(Figure8),whileadultswere

F IGURE  5 Meansizeofmaleandfemaleblueshark(Prionace glauca)bysexandquarteroftheyearcaughtinseveralregionsoftheAtlanticandIndianOceans.NAT=NorthAtlantic,SAT=SouthAtlanticandIO=IndianOcean.Withineachmajorarea,therearefourquadrantsasdefinedforthiswork(NW,NE,SWandSE,seeFigure1).Theerrorbarsare95%confidenceintervals.Thehorizontallinesrepresentmediansizeatmaturityineachregion(solidlines=males,dashedlines=females).[Colourfigurecanbeviewedatwileyonlinelibrary.com]

     |  9COELHO Et aL.

distributed along wider areas, including the eastern Indian Ocean,closertoIndonesia(Figure9).

3.5 | Modelling size distribution

Therewasalsoconsiderablevariabilityintheexpectedsizedistribu-tionsofbluesharkbothintheAtlanticandIndianOceanswhentakingintoconsiderationthecatchlocationandquarteroftheyear.IntheAtlantic,thelargerbluesharkswerepredictedtooccurmainlyalongtheequatorialandtropicalregions,particularlyintheCentralEasternAtlantic,alongEquatorialwatersandintheGulfofMexico.Bycon-trast,thesmallerspecimenswerepredictedtooccurmainlyinhigherlatitudesboth in thenorthernandsouthernhemispheres,especially

inthenorth-eastandsouth-westregionsoftheAtlantic(Figure10).Similarly,intheIndianOcean,thelargermeanbluesharksizeswerealsopredictedmainlyalongtheequatorialandtropicalregions,whilethesmallerspecimenswerepredictedtooccurinhigherlatitudesandmore temperate waters of the Southern Indian Ocean (Figure11).In the IndianOcean, therewasalsosomevariabilitywith longitude,withthelargerspecimenspredictedtooccurmainlyinthenorth-westandmediumsizesinthenorth-eastregions(Figure11).ForboththeAtlanticandIndianOceans,thefinalestimatedGAMsconsideredthenonparametricsmoothtermsforlocation(latitudeandlongitude,withinteractions)andtheparametrictermofquarterusedasafixedfac-tor.Thetotaldevianceexplainedbythefinalmodelswas43.2%fortheAtlantic and 46.5% for the IndianOcean. The residual analysis

F IGURE  6 Blueshark(Prionace glauca)sexratiosrecordedin5°×5°(latitude×longitude)squaresduringthisstudyineachquarteroftheyear(a=quarter1,b=quarter2,c=quarter3,d=quarter4)fortheAtlanticOcean.Circlesizesarefixedandnotproportionaltosamplesizewithineach5°×5°square.[Colourfigurecanbeviewedatwileyonlinelibrary.com]

(a) (b)

(c) (d)

10  |     COELHO Et aL.

revealednomajor trendsorpatterns in the residuals that couldbeconsideredproblematic.

4  | DISCUSSION

This work provides the most comprehensive study on blue sharkpopulationstructureandsizedistributionpatternsevercarriedoutintheAtlanticandIndianOceans,includingdatafromscientificfisheryobserver programmes, fishery-independent sampling programmesand surveys, projects and research cruises. The results provide animportantcontributiontothestudyofthespatialandseasonaldy-namicsofthemostwidelydistributedandcapturedpelagicsharkin

oceanicwaters. Intermsofgeographicalcoverageanddistribution,recordsofbluesharksrangingfrom62°Nto54°SintheAtlanticandfrom25°N to48°S in the IndianOceanwereprovided.Theprevi-ouslyreportedglobalareaofdistributionofbluesharkrangedfromabout60°Nto50°S(Last&Stevens,2009).Assuch,thisgeneralwidelatitudinalrangeofdistributionisconfirmed,andwealsoexpandthepreviously reported values in both hemispheres, especially for theAtlantic.

Significantdifferenceswerefoundinthelength-frequencydistri-butions,sexratiosandproportionsofimmatureandmaturespecimensacrosssubregionsoftheAtlanticandIndianOceans.Ofparticularim-portanceistonotetheclearlatitudinalstratificationofbluesharksinbothoceans,withthelargermaturespecimenstendingtooccuralong

F IGURE  7 Blueshark(Prionace glauca)sexratiosrecordedin5°×5°(latitude×longitude)squaresduringthisstudyineachquarteroftheyear(a=quarter1,b=quarter2,c=quarter3,d=quarter4),fortheIndianOcean.Circlesizesarefixedandnotproportionaltosamplesizewithineach5°×5°square.[Colourfigurecanbeviewedatwileyonlinelibrary.com]

(a) (b)

(c) (d)

     |  11COELHO Et aL.

theequatorialand tropical regionsofbothoceans,and thesmaller-sized immature specimens occurringmainly in temperatewaters inhigher latitudes. In theAtlantic, immature sharks occur both in thetemperatenorthandtemperatesouth,especiallyinthenorth-eastandinthesouth-westAtlantic,whileintheIndianOceanimmaturesharksoccur in temperate southernwaters, as theNorthern IndianOceandoesnothaveatemperatewatersystem.ThisgeneralsizesegregationcorroboratesthepatternspreviouslydescribedbyMejutoandGarcía-Cortés (2005) forblueshark in theseoceans.However, thisgenerallatitudinalgradientisoppositetothepatternsfoundinsomeotherpe-lagicsharkspecies.Oneexampleisthebigeyethresher(Alopias super-ciliosus)intheAtlanticOcean,wherethesmallerandyoungersharkstendtoconcentratepredominantly inthetropical regions,whilethelargerspecimensseemtoprefertemperateareasofthenorthernandsouthernAtlantic(Fernandez-Carvalhoetal.,2015).

There are also longitudinal gradients in size distribution alongbothoceans. IntheAtlantic,the largerspecimenswerepredictedtooccurmainlyinthenorth-westandsouth-eastequatorialandtropical

regions,especiallyintheGulfofGuineaandinthecentralandwesterntropicalAtlantic,whileimmaturesharksoccurredmainlyinthenorth-east and south-west.Again, these results corroborate the previousfindingsfromMejutoandGarcía-Cortés(2005).Similarly,intheIndianOcean, the larger specimenswerealsopredicted tooccurmainly inthetropicalnorth-westernarea. Inthesouth-westernIndianOcean,trophicecologystudieshaveshownanontogenicshiftinthedietofblueshark,withthelargerspecimensdisplayingmoreoffshoretropicalforaginghabitats(Rabehagasoaetal.,2012).

Ingeneral, themovementofsharkscanbe influencedbymigra-tionofprey(e.g.Carey,Scharold,&Kalmijn,1990),watertemperature(e.g.Nakano,1994),reproductivestate,sexandsizesegregation(e.g.Kohler,Turner,Hoey,Natanson,&Briggs,2002;Montealegre-Quijano&Vooren,2010;Nakano&Seki,2003;Pratt,1979;Strasburg,1958).The reasons for the specific differences detected in the blue sharkdistributionpatternsseemtobemainlyrelatedtomigratoryandhab-itatsegregationpatterns,whichareinturnrelatedtospatio-temporalchangesingrowthandreproductivestages.SpecificallyfortheSouth

F IGURE  8 Kerneldensitydistributionsforyoung-of-the-yearandsmalljuveniles(ageclasses0and1,seetextinMethodsfordefinitions)intheAtlantic(a)andIndianOceans(c);andjuvenilesofallageclassesofblueshark(Prionace glauca)intheAtlantic(b)andIndianOceans(d).[Colourfigurecanbeviewedatwileyonlinelibrary.com]

(a)

(c)

(b)

(d)

12  |     COELHO Et aL.

Atlantic, Hazin, Pinheiro, and Broadhurst (2000) hypothesized thatadultbluesharkscopulateoffsouth-easternBrazilfromDecembertoFebruary,andovulationandfertilizationtakeplaceoffnorth-easternBrazilthreetofourmonthslater(Hazin,Boeckmann,Leal,Otsuka,&Kihara,1994).PregnantfemaleswouldthenmoveacrosstheAtlanticto theGulfofGuineawhereearlypregnancystagesare foundfromJune toAugust (Castro &Mejuto, 1995). Finally, parturitionwouldlikelytakeplaceinmoretemperatewatersoffSouthAfrica(daSilvaetal.,2010;Hazinetal.,2000),asconfirmedbythepresenceofne-onate sharkswith umbilical scars and femaleswith post-parturitionscars.Thepatternsinthesizedistributionreportedinourstudylendsomesupporttothishypothesis,asthelargerspecimensarefoundintropicalandequatorialareas,especially in theGulfofGuinea,whilesmaller specimens, including young age 0 and 1 juveniles, occur inmore temperatewaters offNamibia and SouthAfrica in the south-eastAtlantic.However,ahighdensityofsmaller-sizedspecimens intemperate south-westwaters off southern Brazil andUruguaywasalsofound,whichisnotfullyconcordantwiththeprevioushypothesis.Still, ingeneral, thepresenceofsmall juvenilebluesharkshasbeen

associatedwithcolderandmoreproductivewaters(Mejuto&García-Cortés,2005),whichwould justifythisprevalenceofsmall juvenilesinthetemperateandmorecoastalwatersofthesouth-westAtlantic.Basedonourstudy, themainnurserygrounds forblueshark in theSouthAtlanticwouldthereforebeintemperatewatersofthesouth-eastAtlanticoffSouthAfricaandNamibia,andalsointhesouth-westAtlanticoffsouthernBrazilandUruguay.

FortheNorthAtlantic,Pratt (1979)suggestedthatmatingtakesplaceoffsouthernNewEnglandinlateMayandearlyJune,andthattheembryos take9–12months todevelopandareborn fromAprilto July. Based mainly on tagging data, Stevens (1990) added thatadultsharksinthenorth-westAtlanticcouldmoveoffshoreintotheGulfStreamorsouthalong themarginsof theGulfStream into theCaribbean.Nurseryareas for thespecies in theNorthAtlantichavebeenproposedintheMediterraneanSeaandofftheIberianPeninsula,andintheCentralNorthAtlanticofftheAzoresIslands(Aires-da-Silva,Ferreira, & Pereira, 2008; Vandeperre, Aires-da-Silva, Santos etal.,2014,Vandeperre,Aires-da-Silva,Fontesetal.,2014).Thesizedistri-butionpatternsreportedinourstudycorroborateandexpandthese

F IGURE  9 Kerneldensitydistributionsforadultmale(a,c)andfemale(b,d)blueshark(Prionace glauca)intheAtlantic(a,b)andIndianOceans(c,d).[Colourfigurecanbeviewedatwileyonlinelibrary.com]

(a) (b)

(c) (d)

     |  13COELHO Et aL.

previoushypotheses,as intheNorthAtlanticthemainareasforag-gregationoflargematureadultspecimensappeartobeinthetropicalNortheast,whilelargeaggregationsofsmallerimmaturesharksweredetectedparticularly in the temperateNortheastandCentralNorthAtlantic. Areas of particular abundance for young-of-the-year andsmalljuvenilesaremainlyofftheIberianPeninsulaandintheBayofBiscayinthenorth-eastAtlantic,andofftheAzoresIslandsandwestoftheAzoresintheCentralNorthAtlantic,whichconfirmsthattheseareasmaybethemainnurserygroundsforthebluesharkintheNorth

Atlantic. Our study also pinpointed a large concentration of adultspecimens,especially large females, in thetropicalNortheast regionaroundtheCaboVerde IslandsandoffWestAfrica, inaregionthathad been previously reported byNakano and Stevens (2008) as animportantareaofconcentrationforpregnantfemales.Litvinov(2006)suggestedafiner-scaleheterogeneityofthesex-specificdistributionofbluesharks,describingdenseaggregationsofadultmalesincertainslopeandseamountareas,wherethemales’prevalencecouldreach80%–90%.Litvinov (2006)hypothesized the functional roleof such

F IGURE  10 Predictionofthesizedistributionofblueshark(Prionace glauca)caughtintheAtlanticOceanbyquarteroftheyear(a=quarter1,b=quarter2,c=quarter3,d=quarter4),fromaGeneralizedAdditiveModel(GAM).Thesizerangeconsideredwas36–394cmFL,andthesexesaremodelledtogether.[Colourfigurecanbeviewedatwileyonlinelibrary.com]

(a) (b)

(c) (d)

14  |     COELHO Et aL.

maleaggregationswith the increasingprobabilities tocopulatewithmaturefemalespassingontheirmigratoryroutes.

Limitedworkhasledtofewhypothesesonthelarge-scaledistri-butionofbluesharkintheIndianOceantodate,withtheexceptionofsomeanalysesrestrictedmainlytotheIndian/Atlanticconfluencezone(daSilvaetal.,2010).Infact,theremaybesomeconnectivitybetweenthesouth-eastAtlanticandsouth-westIndianOceans,ashasbeende-scribedforotherpelagicsharks(e.g.daSilva-Ferretteetal.,2015;forthecrocodileshark(Pseudocarcharias kamoharai,Pseudocarchariidae).Ourresultssuggestthatimmaturesharks,includingyoung-oftheyear,juvenileandpre-adultsharks,concentratemainlyintemperatewatersof the south-west IndianOceanoff SouthAfrica, and in the south-east Indian Ocean off south-westernAustralia, implying that thesemaybe the twomainnurserygrounds for the species in the IndianOcean.LargermatureandadultbluesharksaremorewidelyspreadalongtheIndianOcean,includinginmoretropicalandequatorialwa-ters,butthereisalsoalargeconcentrationofadultsinthesouth-westtemperateregion,whichcombinedwiththepresenceofyoungspec-imens,may representaparturitionground for theblue shark in theIndianOcean.Apredominanceof females inearlypregnancystageshasalsobeendescribedforthenorth-westIndianOcean(Gubanov&Grigor’yev,1975),especiallyduringthefirsthalfoftheyear.

ForthePacificOcean,andparticularlyintheNorthPacific,Nakano(1994)suggestedthatmatingtakesplaceinearlysummerat20–30°N,andthatpregnantfemalesthenmovenorthtoparturitiongroundsinmoretemperatewatersat35–45°N.Thepuppingandnurseryareas

arelocatedinthesecolderwaterregions,wherethereisalargerpreybiomassforthejuveniles,whichcanremaintherefor5–6yearspriortomaturity (Nakano &Nagasawa, 1996). By contrast, adults occurmainlyfromequatorialwaterstoareassouthofthenurserygrounds(Nakano&Stevens,2008).TheseresultsforthePacificaresimilartowhatisnowdescribedinthisworkespeciallyfortheAtlantic,withtheadultsoccurringmainlyalongequatorialandtropicalwatersandthesmalljuvenilesincoldertemperatewatersofbothhemispheres.

Alimitationofourstudywasthatthedatausedweremostlyfish-ery dependent, obtained frommultiple fishing fleets,with differentfishingmétiersthattargetdifferentspecies.Asaresult,thesizerangesandabundancereportedbyeachfleetforeachregionmayalsobeaf-fectedbyareacoverageandgearselectivity(e.g.hookshapeandsize,baittype,useofwireleaders,targeting,day/nightfishinganddepthofhooks).Intermsofthesetdepthofthehooks,ithasbeenshownthattheverticalcatchratepatternsofbluesharkdonotseemtoclusteronparticulardepth ranges, as ismorecommonlyobserved in tunasand billfishes (Nakano, Okazaki, & Okamoto, 1997; Yokawa, Saito,Kanaiwa,&Takeuchi,2006).However,the influenceofdepth inthecatch-at-size is stillnotcompletelyunderstood.Theothervariables,suchashookandbait type,useofwire leadersand targeting,havebeenshowntoaffectsharkcatchrates.

Itisalsoimportanttonotethatmostofthedatausedinthisworkcomefromoceanicpelagiclonglines,setinoceanicwatersandtarget-ingmainlyswordfishortunas,withtheexceptionofthedatafromtheartisanallonglinesintheBayofBiscay,whichoperateinamuchmore

F IGURE  11 Predictionofthesizedistributionofblueshark(Prionace glauca)caughtintheIndianOceanbyquarteroftheyear(a=quarter1,b=quarter2,c=quarter3,d=quarter4),fromaGeneralizedAdditiveModel(GAM).Thesizerangeconsideredwas41–369cmFL,andthesexesaremodelledtogether.[Colourfigurecanbeviewedatwileyonlinelibrary.com]

(a)

(c)

(b)

(d)

     |  15COELHO Et aL.

coastalregion.Assuch,theresultsobtainedprovidemainlyavisionofthefractionofthebluesharkpopulationthatispresentinoceanicwa-tersandavailableto,andselectedby,thesefishinggears.Oneimport-antresultfromthisstudyisthatthecaptureofverysmallspecimens(youngjuveniles)wasingenerallowinoceanicwaters.Thiscanbedueeithertotheverysmallsharksoccurringmainlyinmorecoastalwaters,thatisnotbeingpresentinhighnumbersinoceanicwaters,orpossiblyduetofishinggearselectivity,thatis,smalljuvenilesalsooccurringinoceanicwatersbutnotcapturedbytheseoceanicpelagiclonglines.Inthissense,NakanoandStevens(2008)pointedoutthatjuvenilebluesharksremaininthenurseryareasanddonottakepartinextensivemigrationsuntilreachingasizeofabout130cm.Mejutoetal.(2014)notedthepresenceofsmallrecruitsinverycoastalareasofthenorth-eastAtlantic(offnorth-westSpain),suggestingthattheseverysmalljuvenilesmay,infact,prefermorecoastalandproductivewatersofthetemperateregions.Therefore,small juvenilebluesharksmaynotbepresentinhighabundancesinoceanicwaters,makingthatcomponentofthepopulationlesssusceptibletooceanicfisheries.

Evenwiththelimitationsinherenttothefisheries-dependentna-tureof thedata, our studyprovides an important improvementontheunderstandingof thespatio-temporaldynamicsandpopulationstructureofbluesharkpopulationsintheAtlanticandIndianOceans.Whileourstudyprovidesageneraloverviewofthedistributionpat-ternsatoceanic-widescales,alimitationisthefactthattheanalysesandmodelsusedfocusonmajorlarge-scale,spatio-temporaleffectsover entireocean-basin areas.There are likelyother finer-scale ef-fectsandlocalvariabilitypatternsaffectingdistributionthatarenotcaptured in our large-scale models and analyses. Therefore, whilethisstudy is importantasageneraloverviewproviding thegeneralandmajortrendsintheAtlanticandIndianOceans,itisimportanttoemphasizetheneedtocontinueconductingmoredetailedandlocalanalysesforspecificregionsoftheseoceans.BluesharksarerevealedtooccurfromtemperatetotropicalregionsoftheAtlanticandIndianOceans,andthisisalsothecaseinthePacificOcean(Nakano&Seki,2003),indicatingthatthebluesharkislikelyoneofthemostthrivingandwidelydistributedfishamongthehighlymigratoryspecies.

In conclusion, the distribution patterns presented in this studyprovideabetterunderstandingofdifferentaspectsofthebluesharkdistributionanddynamicsintheAtlanticandIndianOceans.There-sultshavebeenprovidedtotheICCATSharkSpeciesGroupandtheIOTCWorkingPartyonEcosystemsandBycatchandhavebeenincor-porated, tosomeextent, in the latestbluesharkstockassessmentscarriedoutbytheset-RFMOs.Weexpectthatthisandfurthersim-ilaranalyseswillcontinuetobeusedinfuturestockassessmentsofthisandothersharkspecies,astheyallowtheuseofmoreadequatestockassessmentmodels,withinclusionofbothbiologicalandspatial-seasonaldynamicsofthespecies,andultimatelyhelpmanagersadoptmoreinformedandefficientmanagementandconservationmeasures.

ACKNOWLEDGEMENTS

ThisworkwascarriedoutaspartofcooperativestudiesconductedbytheICCATSharkSpeciesGroupandtheIOTCWorkingPartyon

EcosystemsandBycatch.Theauthorsaregrateful toall the fisheryobserversandfishingvesselskippersandcrewsfromallthenationsinvolved that have contributed data for the analysis. PortuguesedatawerecollectedbytheNationalDataCollectionProgram(PNAB)withintheEUDataCollectionFramework(DCF).Frenchdata(ReunionIsland,IndianOcean)werecollectedbytheNationalDataCollectionProgramwithin the EUData Collection Framework, and from pro-jectsEUFP7MADEandGEFSWIOFP.SamplingfromUruguaywasconductedbyobserversfromPNOFA.SamplingfromVenezuelawasconductedbyobserversoftheICCATsponsoredEnhancedProgramforBillfishResearch(EPBR).TagginginIrelandwasundertakenbyrec-reational angling charter vessel skippersunder theNationalMarineSportfishTaggingProgramme,managedby InlandFisheries Ireland.USSR data were collected during the Soviet Indian Ocean TunaResearch Longline Program (SIOTLLRP) funded by the Ministry ofFisheriesofformerUSSRandcarriedoutbytheSouthernScientificResearchInstituteofMarineFisheriesandOceanography(YugNIRO),Kerch,Crimea.SamplingfromtheUSAwasconductedbyobserversfromtheNOAA-NMFS-SEFSCPelagicObserverProgram.RuiCoelhoissupportedbyan Investigador-FCTcontract (Ref: IF/00253/2014)from thePortugueseFoundation for Science andTechnology (FCT,Fundação para a Ciência e Tecnologia)supportedbytheEU European Social FundandthePrograma Operacional Potencial Humano.

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How to cite this article:CoelhoR,MejutoJ,DomingoA,etal.Distributionpatternsandpopulationstructureoftheblueshark(Prionace glauca)intheAtlanticandIndianOceans.Fish Fish.2017;00:1–17.https://doi.org/10.1111/faf.12238