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Squid Interspecific Competition InternationalCouncilfor CM2000/Z:01 TheExplorationoftheSea ThemeSessionZ SQUIDINTERSPECIFICCOMPETITION:POSSIBLEIMPACTOFILLEX ARGENTINUSONLOLIGOGAHIRECRUITMENTINTHESOUTHWEST ATLANTIC by AlexanderI.ArkhipkinandDavidA.J.Middleton Abstract FisherystatisticsfortwoabundantSouthwestAtlanticsquid,Illexargentinus (Ommastrephidae)andLoligogahi(Loliginidae),inFalklandwatersbetween1987 and1999wereanalysed.Despitefisheriesregulationproducingreasonablyconsistent fishingeffort,thetotalcatchandCPUEofbothsquidvariedconsiderablyfromyear toyear.Theareasofconcentrationofthetwospeciesareusuallyseparated,withI. argentinusmostabundanttothenorth-westoftheIslandsinFebruary-MayandL. gahitothesouth-eastinFebruary-May(firstseason)andAugust-October(second season).However,insomeyears,I.argentinusdointrudeingreatnumbersinto nurseryorfeedingareasofL.gahiinApril-Maypossiblyaffecting,eitherdirectly(via predation)orindirectly(bycompetitionforfood)theabundanceandrecruitmentof thesecondcohortofL.gahi.CatchesandCPUEofI.argentinusinthefirsthalfofthe season(February-March)didnotcorrelatewiththoseofL.gahiinFebruary-May.In contrast,catchesandCPUEofI.argentinusinthesecondhalfoftheseason(April- May)arenegativelycorrelatedwiththoseofL.gahiinApril-MayandAugust- Octoberofthesameyear.Possiblereasonsforsuchnegativecorrelationsin abundanceofthetwosquidspecies,andtheirimplicationsforfisheriesmanagement, arediscussed. Keywords:squid,fishery,Illexargentinus,Loligogahi,SouthwestAtlantic. AlexanderI.ArkhipkinandDavidA.J.Middleton:FisheriesDepartment,Falkland IslandsGovernment,P.O.Box598,Stanley,FalklandIslands 1 Introduction TheFalklandIslandsInterimConservationandManagementZoneandOuter ConservationZone(FICZandFOCZ)supporttwomainsquidfisheriesinthe SouthwestAtlantictargetingtheommastrephidsquidIllexargentinusandloliginid squidLoligogahi.ThetotalannualcatchwithintheFICZ/FOCZexceeded300,000 tonnesin1999(FIG,2000).Thecatchofeachspecieshasvariedfromyeartoyear, rangingfrom64,000to266,000tonnes(mean130,000tonnes)forI.argentinusand from26,000to98,000tonnes(mean61,000tonnes)forL.gahiin1990-1999(FIG, 2000).Thereasonsforthesevariationsincatch(and,presumably,inabundanceof squid)arelargelyunknown. IllexargentinusisthemostimportantsquidfisheryresourceoftheSouthwest Atlantic.DuringthefirsthalfoftheyearitisfishedintheFICZ/FOCZandArgentine ExclusiveEconomicZone,aswellasininternationalwatersof45-47°S.Thesouth PatagonianandFalklandshelvesareusedasfeedinggroundsbythetwomost abundantwinter-spawninggroupsofI.argentinus:theboanerensisnorthPatagonian stock(BNPS)andthesouthPatagonianstock(SPS)(Brunetti,1988).Squidofboth groupsmigratetothisareafromtheirnurserygrounds(thecontinentalslopeof northernArgentinaandUruguay)inFebruary,feedduringtheaustralsummerand autumnandemigratetotheirspawninggroundsinMay-June(Hatanaka,1988; Haimovicietal.,1998).InboththeFICZ/FOCZandArgentineEEZ,I.argentinusis fishedmainlybyAsianjiggingvessels(Csirke,1987).Twomainwavesofabundance areusuallyobservedintheI.argentinusfisheryonthesouthernPatagonianshelf (Arkhipkin,2000).ThefirstwaveappearsinFebruaryinthenorth-westofthe FICZ/FOCZandfeedsmainlyinthatregion.BythestartofAprilsquidofthiswave areconcentratedontheshelfbreakinthenorth-eastofthezonesbeforetheir northwardmigration,causingasignificantpeakincatches.Thesecondwavestartsto movefromtheArgentineEEZtothewesternpartoftheFICZinthesecondhalfof April.Squidofthiswavemigrateacrossthezonefromthesouth-westtonorth-east, andagainconcentrateontheshelfbreaknorthof50°30’S,resultinginthesecond peakofcatchesinthebeginningofMay.AggregationsofI.argentinusremaininthe zonesuntilthemiddleofJune(FIG,2000). LoligogahiisanotherimportantsquidfisheryresourcewithintheFICZ (Patterson,1988).JuvenileL.gahimovefromtheinnershelftotheoutershelfand 2 shelfbreakoftheFalklandIslands,feedandgrowthereasimmatureandmaturing adultsand,uponmaturation,returntoshallowwaterstospawn(Hatfieldand Rodhouse,1994).ItisassumedthatthereareatleasttwocohortsofL.gahiwith differentspawningperiods(autumnandspring)andgrowthrates(Agnewetal., 1998a).Thissquidistargetedbyatrawlfleetprimarilyduringitsfeedingperiod withinthe‘Loligobox’,theregionlocatedtothesouth-eastoftheFalklandIslands. Thewholefishingseasonissplitintotwoparts(February-MayandAugust-October). Duringthefirstseasonbothcohorts(withapredominanceoftheautumnspawners) arefished,whereasduringthesecondseasononlythesecondcohortisexploited (HatfieldanddesClers,1998). Untilnowlittlehasbeenknownabouttheinteractionsbetweenthetwosquid species.Bothareopportunisticpredatorseatingallpossiblepelagicprey,primarily abundantamphipodandeuphausiidcrustaceans(Guerraetal.,1991;Brunettietal., 1998).Withitsslimmerbodyandsmallersize(meanmantlelength,ML,ofadultsis 140-160mm)L.gahiisamorelikelypotentialpreyforthelargerandmorerobust (320-390mmadultML)I.argentinus,thanthereverse.OnthePatagonianshelfL. gahihasbeenreportedinthedietofI.argentinus(10-15%frequencyofoccurrence) duringtheaustralsummerandautumn(IvanovicandBrunetti,1994). IntheFalklandIslandsFisheriesDepartment,fisheriescatchandeffortdata hasbeengatheredonadailybasissincethebeginningofthelicensedfisheryin1986 (FIG,2000).Thisseriesoffersthepotentialtomakerealprogressinunderstanding fisherydynamicsandinteractions.Inthispaperweexaminetherelationshipbetween abundanceofthemajorsquidspeciesI.argentinusandL.gahi.Ouraimistobetter understandthefactorsthataffectrecruitmenttotheFalklandsquidfisheriesandso improvestockassessmentandpredictioncapabilitiesandguidemanagementfor continuedconservationofthestocks. Materials&Methods TheoveralldistributionofIllexargentinusandLoligogahiintheFalkland Island’sfisheryzoneswascomparedbyconsideringthetotalcatchreportedinthe period1987to1999inclusive(Figure1).AlllicensedvesselsintheFalkland’szones submitdailycatchreportsidentifyingmiddayandmidnightpositionona0.5° longitudeby0.25°latitudegridsystem.ThemajorityoftheI.argentinuscatchis takenbyjiggingvesselssomidnightpositionwasusedtoassigntheirdailycatchtoa 3 particulargridsquare,whereasL.gahiisfishedbytrawlersandmiddaypositionwas usedinthiscase. Fortheseparationofcatchbywaveofabundance,fishingseason,orarea,and forthecalculationofcatchperuniteffort(CPUE),weconsideredonlythosevessels licensedtotargetI.argentinusorL.gahi.Dataforthelicensedfisheryfrom1989to 1999wasincluded.AllcatchesofI.argentinusduringFebruaryandMarchwere consideredtobefromthefirstwaveofabundance(wave1=BNPS;Brunetti,1988) andallcatchesfromMayandJunewereassignedtothesecondwave(wave2=SPS; Brunetti,1988;Arkhipkin,1993,2000).InAprilcatchessouthof49.5°Sandwestof 60.25°W(Figure2)wereassignedtowave2whilecatchesnorthandeastofthisarea wereassignedtowave1.Thisdivisionissupportedbylength-frequencyandmaturity data.InthecaseofL.gahithefisheryoperatesintwoseasons.Thefirstseasonruns from1Februaryto31May,andthesecondseasonfrom1Augustto31October.L. gahilicensesallowfishingtothesouthandeastoftheFalklandIslands.Thisarea wassplitintotwosub-regions:northandsouthof52°S(Figure3).ThefisheryforI. argentinustakesplaceinasingleseason,currentlyrunningfrom15Februaryto15 June.Thefisheryseasonforeitherspeciesmaybeendedearlierthanthenormal seasonclosingdateifwithin-seasonstockassessmentsindicatethestockhasreached aminimumescapementthreshold. CalculationofCPUEforI.argentinuswasrestrictedtothelicensedjigging fleet,whichtakesthemajorityoftheannualcatch(95.4%in1999;FIG,2000).Effort wascalculatedasjiglinehours(vesselsreportthenumberoflinesusedandtime spentjiggingdaily),andCPUEexpressedaskgline-hour-1.Thelicensedfisheryfor L.gahiisrestrictedtotrawlers(whichreportdailytrawlingtime)andCPUEwas calculatedasmetrictonnes(MT)hr-1.ThemedianCPUE(whereCPUEwas calculateddailyforeachlicensedvesselfishing)inagivenperiodwasusedasa generalmeasureofabundance(usuallyreferredtosimplyasCPUEintheremainder ofthispaper).Thiswaschoseninpreferencetothestandardstockestimates (calculatedbydepletionmethods)asthesecurrentlyconsideronlytotalstockforI. argentinus(Basson,etal.1996)andtotalfirst/secondseasonstockforL.gahi (Agnew,etal.1998a)andsodonotallowaneasyseparationoftheestimatedstock sizebyarea(forL.gahi)orbywaveofabundance(forI.argentinus). Tofurtherillustratetheannualpatternsinabundanceforthevariousgroups theannualdeviationinCPUEfromthelongtermmeanCPUEofthegroupoverthe 4 period1989-1999wascalculated.Toallowcomparisonbetweenstocksthisdeviation wasexpressedasaproportionofthelongtermmean. TherelationshipbetweenCPUEforthetwowavesofI.argentinusandCPUE forL.gahiineachregionandseasonwasinvestigatedbycalculatingSpearman’srank correlationbetweenthedatasets.Allstatisticalcalculationswerecarriedoutusingthe Rstatisticalpackage,v1.1.0(IhakaandGentleman,1996).MedianCPUEwasalso calculatedonamonthlybasisforeachstockandcomparisonsbetweenmonthsalso madeusingSpearman’srankcorrelation. Results Distribution GenerallythedistributionsofI.argentinusandL.gahiwithintheFICZ/FOCZ arequitesimilar.BothsquidareencounteredalmosteverywherearoundtheIslands withtheexceptionofthesouthernpartoftheZones(mainlyBurdwoodBank).Inthe east,however,I.argentinustendtooccurfurtheroffshorethanL.gahi.UnlikeL. gahi,I.argentinusdonotappearintheshallownearshorewaters(<50mdepths)of theFalklands.Howeverthedispersalofbothsquidisverydifferent.Thedensest aggregations(and,correspondingly,catches)ofI.argentinusarenotedinthenorth- westernandnorth-easternpartsoftheZones,whereasL.gahiismostabundantinthe southernandeasternpartsoftheFICZ/FOCZ(the‘Loligobox’)(Figure1).Thusthe bulkofthepopulationsofI.argentinusandL.gahiareseparatedspatiallyonthe Falklandshelf. ThedispersalofI.argentinusandL.gahivariesinyearsofhighandlow abundance,toagreaterextentinI.argentinusthanL.gahi.Forexample,inayearof lowI.argentinusabundance(1994)thefirstwavewasobservedbasicallyalongthe
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