Fish and Fisheries. 2017;1–32. wileyonlinelibrary.com/journal/faf  | 1© 2017 John Wiley & Sons Ltd

Received:9January2017  |  Accepted:20July2017DOI: 10.1111/faf.12242

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

Alien freshwater fish species in the Balkans—Vectors and pathways of introduction

Marina Piria1  | Predrag Simonović2,3 | Eleni Kalogianni4 | Leonidas Vardakas4 |  Nicholas Koutsikos4,5 | Davor Zanella6 | Milica Ristovska7 | Apostolos Apostolou8 |  Avdul Adrović9 | Danilo Mrdak10 | Ali Serhan Tarkan11 | Dragana Milošević10 |  Linda N Zanella6 | Rigers Bakiu12 | F Güler Ekmekçi13 | Metka Povž14 |  Kastriot Korro15 | Vera Nikolić2 | Rifat Škrijelj16 | Vasil Kostov17 |  Andrej Gregori14 | Michael K Joy18

1DepartmentofFisheries,Beekeeping,GameManagementandSpecialZoology,FacultyofAgriculture,UniversityofZagreb,Zagreb,Croatia2FacultyofBiology,UniversityofBelgrade,Belgrade,Serbia3InstituteforBiologicalResearch“SinišaStanković”,UniversityofBelgrade,Belgrade,Serbia4InstituteofMarineBiologicalResourcesandInlandWaters,HellenicCentreforMarineResearch,Anavyssos,Greece5DepartmentofEnvironment,UniversityoftheAegean,Mytilene,Greece6FacultyofScience,UniversityofZagreb,Zagreb,Croatia7FacultyofSciencesandMathematics,St.CirilandMethodiusUniversity,Skopje,Macedonia8InstituteofBiodiversityandEcosystemResearch,BulgarianAcademyofSciences,Sofia,Bulgaria9DepartmentofBiology,FacultyofSciences,UniversityofTuzla,Tuzla,BosniaandHerzegovina10FacultyofSciencesandMathematics,UniversityofMontenegro,Podgorica,Montenegro11FacultyofFisheries,MuğlaSıtkıKoçmanUniversity,Muğla,Turkey12DepartmentofAquacultureandFisheries,AgriculturalUniversityofTiranaKodërKamëz,Tirana,Albania13BiologyDepartment,FacultyofScience,HacettepeUniversity,Ankara,Turkey14InstituteofNaturalSciences,Umbra,Ljubljana,Slovenia15FacultyofVeterinaryMedicine,AgriculturalUniversityofTirana,Tirana,Albania16FacultyofScience,UniversityofSarajevo,Sarajevo,BosniaandHerzegovina17InstituteofAnimalScience,St.CyrilandMethodiusUniversity,Skopje,Macedonia18InstituteofAgriculture&Environment,MasseyUniversity,PalmerstonNorth,NewZealand

CorrespondenceMarinaPiria,FacultyofAgriculture,DepartmentofFisheries,Beekeeping,GameManagementandSpecialZoology,UniversityofZagreb,Zagreb,Croatia.Email:mpiria@agr.hrandDavorZanella,FacultyofScience,UniversityofZagreb,Zagreb,Croatia.Email:davor.zanella@biol.pmf.hr

Funding informationUniversityofZagreb,Croatia;MinistryofEducation,ScienceandTechnologicalDevelopmentofSerbia,Grant/AwardNumber:173025

AbstractFishintroductions,particularlyinareasofhighbiologicaldiversityandendemism,rep-resentamajorthreatforbiodiversity. IntheBalkanPeninsula,60fishspecieshavebeenintroducedtodate,ofwhich36havebecomenaturalizedininlandwaters.SincetheBalkansareoneoftheworld’s35biodiversityhotspots, this largepresenceofalienfishspeciesposesaseriousthreatforthestabilityoffreshwaterecosystemsandthesurvivalofthenativeichthyofaunaandofaquaticbiodiversityingeneral.Themo-tivation for the introductions, and the historical timeline, varies among the Balkanstates.Despiterecentattemptstoimplementandalignlegislationaimedatpreventingthe introductionofpotentially invasivespecies,andthe implementationofrigorouscontrols of introductions and increasedprotection of openwaters, themajority of

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1  | INTRODUCTION

Human-inducedbiological introductions represent amajor threat toglobalbiodiversity,byreducingspeciesdiversityandleadingtobiotichomogenizationatlargespatialscales(McKinney&Lockwood,1999;Toussaint, Beauchard, Oberdorff, Brosse, &Villéger, 2016; Villéger,Blanchet, Beauchard, Oberdorff, & Brosse, 2015). In the previousdecades,thenumberofintroducedspeciesworldwidehasincreasedexponentially(Hulme,2009;Vilàetal.,2010)andalienspecies’inva-sionsarenowwidelyconsideredtobeoneofthemainthreatstobio-diversityandthesecondleadingcauseofanimalextinctions(seeMEA,2005;Clavero&García-Berthou,2005).Freshwaterfishintroductionshaveincreasedsubstantiallyduringrecentdecades,mainlyduetothenegativesocio-economic,evolutionaryandecologicalimpactsofglo-balizationonnative freshwaterbiodiversity (Cucherousset&Olden,2011; Gozlan, Britton, Cowx, & Copp, 2010; Hulme, 2009; Vitule,Freire,&Simberloff,2009).Morespecifically,theintroductionofalienspeciestogetherwithanthropogenichabitatlossanddegradation,hy-drologicalalterationandpollutionoftenactsynergisticallytowardsthereductionorextinctionofnativefreshwaterfishspecies(Arthington,Milton,&McKay, 1983;Dudgeon etal., 2006; Joy, 2014; Kennard,Arthington, Pusey, & Harch, 2005; Olden etal., 2010). Invasivespecies are often superior competitors in relation to the evolution-ary isolated native species populations (Mills, Rader, & Belk, 2004;Townsend, 1996), and they have broader environmental tolerances,being thus able to thrive in degradedhabitats (Courtenay&Meffe,1989;Kennardetal.,2005).Theirnegativeimpactsonnativespeciesincludepredation,trophiccompetition,behavioural interference,hy-bridization,spreadofnovelparasitesanddiseases,alterationoffoodwebs and modification of biochemical cycles (Arthington & Lloyd,1989;Millsetal.,2004;Keller&Brown,2008;Kalogianni,Giakoumi,Andriopoulou,&Chatzinikolaou,2014;forreviews,seeLeunda,2010;Cucherousset&Olden,2011).However,theinvasionprocesscontainsaseriesofstages,fromintroduction,survivalandreproductiontodis-persal,with species having to overcome barriers to reach the nextstep,withdifferinginvasionsuccess(Blackburnetal.,2011).Severalattemptshavebeenmade to classify alien species according to themagnitudeoftheirenvironmentalimpacts,basedonthemechanismofimpactthroughwhichtheyexerttheireffect,thatispredation,compe-tition,hybridization,diseasetransmission(seeBlackburnetal.,2014).Theobviousaimremainsastandardizedmethodofimpactassessment

thatwouldbeapplicableatawiderangeofspatialscales,fromglobaltoregionalandnational.Theeffectsofalienspeciescanrangefromminimal tomassive, and the various stages of the invasion processrequire different management interventions, with special emphasisonthehaltingoftheirsecondaryspread,followingtheestablishmentofisolatedpopulations(seeBlackburnetal.,2011;VanderZanden&Olden,2008).However,undercurrentclimatechangescenarios,inva-sivealienspeciesareexpectedtoaccentuateclimaticstresseffectsbyreducingthenumberofnativespeciesand/ortheir functional typeswithin the ecosystem and by increasing ecosystem susceptibility toclimaticperturbation(Masters&Norgrove,2010).

In Europe, lists of alien species have been compiled for severalcountries [e.g.Spain (Elvira&Almodóvar,2001),Germany (Gollasch&Nehring, 2006), Italy (Gherardi etal., 2008)], and an inventory offreshwateralienspeciespresentinEuropehasbeencompiledbytheEuropeanAlienSpeciesInformationNetwork(EuropeanAlienSpeciesInformationNetwork,EASIN;Katsanevakisetal.,2012);basedonthisinventory,arecentstudyprovidedthefirstEurope-wideassessmentofthemajorpathwaysandgatewaysoffirst introductionsforfresh-wateralienspeciesinEurope(excludingtheBalkans,Nunes,Tricarico,Panov,Cardoso,&Katsanevakis,2015).Detailedanalyses,however,oftheinvasionhistoryofasingletaxonomicgroupsuchasfreshwaterfishesinaspecificregionofEurope,suchastheBalkans,mayprovideuseful insights intopatternsanddriversofbiological invasions,con-tributingtothedevelopmentoflocalstrategiesforthemanagementof alien species (Hulme, 2009).Hence, such analyses could provideinformationonkeyrecipientareasofintroductionforthepurposesofsurveillanceandprevention,inlinewiththerequirementsofthenewEuropeanRegulationonInvasiveAlienSpecies(EU2014),thatobligesEUMemberStatestoconductacomprehensiveanalysisofthepath-waysofintroductionofIASintheirterritory,identifyingthepathwaysthatrequirepriorityaction.

This study focuses on the alien fish species of the BalkanPeninsula, one of the world’s biodiversity hot spots (Mittermeier,Turner,Larsen,Brooks,&Gascon,2011)thatpossessthehighestpro-portionofrangerestrictedendemicfishspeciesinEurope(Bănărescu,2004;Barbierietal.,2015;Kottelat&Freyhof,2007;Simonovićetal.,2013).RecentsurveysfromseveralBalkancountries,however,haverevealed that 15%–23% of their fish fauna is alien (Barbieri etal.,2015;Piria,Tomljanovićetal.,2016;Piria,Povžetal.,2016;Simonovićetal.,2013),withcertaincatchments,suchastheDanubeRiverand

currentintroductionsremainintentional,primarilyviaaquaculture.Thisreviewarticleprovidesahistoricaloverviewoffreshwaterfishintroductions,themotivationbehindthemandthecurrentdistributionofalienfreshwaterfishesintheBalkans.Theeco-logicalimplicationsandfutureperspectivesconcerningalienfishspeciesintheregionarealsodiscussed.

K E Y W O R D S

BalkanPeninsula,distribution,inlandwaters,introductionshistory,invasives,non-nativefish

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Pamvotis Lake (Greece), having an ichthyofauna comprised ofmorethan50%and80%ofalienfishes,respectively (Leonardos,Kagalou,Tsoumani,&Economidis,2008;Simonovićetal.,2013).Furthermore,owing to the high level of endemism and the great conservationalvalue of theBalkan freshwater fish species, introductions can havesevere negative and irreversible impacts on the ichthyodiversity ofthePeninsula (Barbierietal.,2015;Ćaletaetal.,2015;Karapetkova& Zhivkov, 1995; Mrdak, Nikolić, Tošić, & Simonović, 2012; Povž,Gregori,&Gregori,2015;Snoj,Razpet,Tomljanović,Treer,&Sušnik,2007; Sušnik etal., 2007). Although there is national legislation inplacethatregulatesfishandotheralienspecies introductions intheBalkans(Piriaetal.,2017),andelsewhereinEurope(Coppetal.,2005),this legislationisrarelyenforcedbynationalenvironmentalagenciesandarepracticallyineffective.Furthermore,inmostBalkanscountries,measurestoeffectivelycontroltheintroductionsortranslocationsofalien fish species are still lacking. In addition,within the frameworkoftherecentEuropeanIASregulation,severalissuesrelatedtoinva-sive species controlwhich are bydefinition cross-border cannot beaddressed without a collaboration between the various countries,especially since EUMember States along the EU’s borders are po-tentiallyatgreaterriskofnewbioinvasionsfromtheirnon-EUneigh-bours(EC2008),sincenon-EUborderingcountriesarenotobligedtoenforceEUlawsconcerningIAS.Thus,acommonframeworkguidingtheimplementationofIASlegislationbetweenEUandneighbouringnon-EU countries is urgently required (Piria etal., 2017), aswell asanalignmentof the legal frameworksamongtheBalkanstates.ThelargenumberoftransboundaryriversandlakesintheBalkans,actingascorridorsofdispersalbeyondstateboundaries,makesthiscommonframeworkanevenmoreurgentpriority.Transboundaryagreements,suchastherecentonebetweenGreeceandBulgariafortheNestosRiver,ortheagreementbetweenFYRMacedonia,GreeceandAlbaniafor thePrespaLakesoncoordinatedmanagementandplanning,arepositiveinitiativestowardsthatgoal.

Given the high rate of alien fish species introductions in theBalkansandthethreattheyrepresenttothelocalendemicfishfauna,the identificationof thevectorsandpathwaysof their introduction,their currentdistributionand theecological implicationsof their in-troduction,especiallyinviewofclimatechangeprojections,becomesofparamountimportancefordesigningandimplementingappropriatecontrolandmitigatingactions.However,therearenostudiesthathavefollowedtheinvasionofanyofthealiensintheBalkanareafromtheirverybeginning,andconsequently,theinitialpathwayandsubsequentdispersionofmanyintroducedspeciesremainunknown.Similarstud-iesontheintroduction(establishment/dispersal)ofalienspecieshavebeenconducted inthe IberianPeninsula.However, the introductionphase remains the least researchedand themostunderrepresentedinthescientificliterature,incontrasttootherphasesofbiologicalin-vasionsthathavereceivedwiderattention,suchastheestablishmentsuccess(Clavero&García-Berthou,2005;García-Berthou,2007).Thisstudyattempts to fill thisgap in theknowledgeaboutalienspeciesin theBalkans by providing the first comprehensive reviewof alienspeciesintroductions,aimingtoadvancetheknowledgeofthehistoryofintroductions,currentdistribution,andpresentstatusofalienfish

speciesandtheirdocumented impactonnativespecies,basedonathoroughreviewofthescientificandgreyliterature.FutureprospectsconcerningalienfishdispersalintheBalkans,especiallyundercurrentclimatechangescenarios,arealsodiscussed.

2  | STUDY AREA

Thisreviewcompiledthelistofintroducedspeciesbycollectingalltheavailableinformation,includingpeer-reviewedarticles,conferencear-ticles,greyliterature,thesesandtechnicalreports.Unpublishedsur-veydata from field surveyswerealso applied in somecases,whentheavailablematerialwascollectedbytheauthorspersonallyinfieldsamplingcampaigns.Attentionwasgiventoascertainthevalidityofeachparticularrecord.Hence,the inclusionofaspeciesrequiredatleastonereliablesource,orafterreferencecross-checkingortore-ceiveconfirmationoftheauthorsduringfieldsurveys.Translocatedspecies(seeCoppetal.,2005fordefinition)wereexcludedfromthislist. Introduced species were grouped into three categories basedontheircurrentstatus:acclimatized,naturalizedorunknown,basedon the available information and following the definitions providedbyCoppetal.(2005).Additionally,theywerecategorizedusingtheir“statusofinvasion”accordingtoBlackburnetal.(2011).Fishsystem-atictaxonomywasbasedonKottelat&Freyhof,2007andFroese&Pauly,2015.Theyearordecadeofthefirstdiscoveryofanintroducedspecieswasbasedeitheronthedateoffirstoccurrenceorthedateoffirstpublication.Inthecurrentstudy,speciesdistributedinallmajorbasins andwater bodies of a country (rivers, lakes) are consideredwidespread,specieswithamoderatedistributionarethosefoundinasinglebasinofonecountry,whilespecieswitharestricteddistributionarepresentinonlyclosedwaterbodies(alpineorartificiallakes,res-ervoirsandisolatedsmallriverbasins).Unknowndistributionincludesspeciesfound/caughtonlyonce,sporadically,orthoseintroducedinthepast,buttheircurrentstatusremainsunknown.

2.1 | The geography and climate of the Balkans

Thestudyareaencompassedthecountriesofsouth-easternEuropethat have historically been identified as Balkan countries, within apoliticalandculturalframework.Itisroughlydelineatedtothenorthby theDanube–Sava–Sočaborder, excluding a small partof south-easternRomania. TheBalkanPeninsula is bordered by theAdriaticandIonianSeastothewest,theMediterraneanSeatothesouthandtheAegean,MarmaraandBlackSeastotheeast.

Thehighfishbiodiversityisaresultoftheregion’sgeologicalandpalaeoclimatichistoryandthegeophysicalvarietyof its inlandwaterbodies (Griffiths, Krystufek, & Reed, 2004; Skoulikidis, Economou,Gritzalis, & Zogaris, 2009). Themajor biogeographical barrier in theBalkansistheDinarides–Hellenidesmountainchainthatseparatesthewestern andeastern faunas,with the early isolationof theWesternBalkansintheMioceneleadingtoarichendemicfauna(Bianco,1986;Ćaletaetal.,2015;Gascetal.,1997;Skoulikidisetal.,2009). Incon-trast, the EasternBalkans have a lower degree of endemicity, but a

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higherrichnessofaquaticbiota,andareinfluencedbyadjacentbiogeo-graphicalregions,suchastheBlackSeaandWesternAnatolianregions.

TheclimaticdifferencesbetweenthevariouspartsoftheBalkansfurther contribute to these biogeographical differences. Climate isgenerallycharacterizedbyadistinctbimodalseasonalityandastrongnorth–south(N–S)gradient,withincreasingtemperatureanddecreas-ing precipitation towards the south–south-east (S–SE) (Skoulikidisetal.,2009).Furthermore, theEasternBalkansarecharacterizedbymuch lower precipitation than the Western Balkans. According toPeel,Finlayson,andMcMahon(2007),withsubsequentcorrectionsbyKoutsoyiannisetal.(2008),themodifiedKöppen-GeigerclimatetypemapoftheBalkansshowstherearefourmainclimatetypesinthere-gion(Figure1a).Thedominantclimatetypebylandareaistemperate(C,61.3%), followedby cold (D,31.1%), polar (E,5.6%) andarid (B,2.1%). In contrast, Europe as awhole is dominated by the cold cli-mate type (D,44.4%), followedbyarid (B,36.3%,Peeletal.,2007).Overall, thereare13subtypeswithintheBalkanPeninsula, thema-jorityofwhichbelongtothetemperate/mesothermalclimates(groupC,Figure1a)with the twodominant temperate types,CsaandCfa,encompassing more that 50% of the Balkans. More specifically, inthesouthernBalkanPeninsula(whichencompassesmostofAlbania,GreeceandtheEuropeanpartofTurkey(Turkey-Thrace),andpartofthe Former Yugoslav Republic of Macedonia (FYR Macedonia), thedominantclimaticsubtypeisthehot-summerMediterraneanclimate(Csa, 27.7% of the total). The next subtype, mostly in the centralBalkans(partsofBulgaria,MontenegroandSerbia,tonorth-westernCroatiaanda smallpartofBosnia–Herzegovina), is thehumid tem-peratehot-summerclimatetype(Cfa,25.2%).Conversely,majorpartsof the northern and central eastern Balkans (Bosnia–Herzegovina,easternCroatia,Slovenia,andareaswithinBulgaria,FYRMacedonia,northern Greece and Serbia) are covered by continental/microther-malclimates,withthecontinentalwarm-summerclimateDfb(20.9%)beingdominant.SimilartomostofEurope,someclimatetypesintheBalkansextendfurthersouththanexpected(Peeletal.,2007);forex-ample, in the areaofMt.Triglav (46.38N,13.88E) in Slovenia andMusala(42.18N,23.58E)inBulgaria,thereareregionsofthetundraETclimatetype,accountingforatotalof5.4%.

2.2 | The river network and the lentic ecosystems of the Balkans

2.2.1 | The inland waters of the Balkan Peninsula

TheBalkanPeninsulaischaracterizedbyextremehydrographicfrag-mentation, with hundreds of autonomous river basins, numerousnaturallakesandartificiallargeandsmallreservoirs,fallingwithinfourmajorcatchmentbasins:AdriaticSea,IonianSea,AegeanSeaandBlackSea (Figure1b). Themajor divide separating the Aegean Sea basinfromtheremainingthreemaindrainageareasisthepeakDrmanska

Glava (elevation926m) inKosovo.Abelletal. (2008)proposed thedivisionoftheBalkansintosevenecoregionsbasedontheirfreshwa-terfishfauna:GulfofVenice,Dalmatia,south-eastAdriatic(AdriaticSeabasin),Ioniandrainages(IonianSeabasin),Thrace,Vardar,Aegeandrainages(AegeanSeabasin)andUpperDanubeandDniester–LowerDanube (BlackSea).TheThraceecoregionshowssomeoverlapbe-tweentheAegeanSeaBasinandtheBlackSeaBasin(Table1).

TheAdriaticSeabasinisseparatedfromtheBlackSeabasintotheeastbytheDinaricMountainRange.AccordingtoAbelletal.(2008),itincludesthreeecoregions:GulfofVenice,Dalmatiaandsouth-eastAdriatic ecoregions.TheGulf ofVenice ecoregion, extends into theBalkanPeninsulafromthewest,isboundedbytheJulianAlpsintheeastandbytheDinaricAlpstothesouth. IntheBalkanpartofthisecoregion, themost important river is the Isonzo/SočaRiver (Italy/Slovenia), which is characterized by a pluvial regime. The Dalmatiaecoregionischaracterizedbyshortandisolatedrivercatchmentsanda karst landscape,with springing and sinking riversofvariable flowregimeswithatendencytodryout,andthecommonoccurrenceofconnected aboveground and underground flows. The largest riveris the Neretva River (BiH/Croatia),which forms awide delta at itsmouth.OtherimportantriversincludetheZrmanja,KrkaandCetinarivers(Croatia)andTrebižatandTrebišnjica(BiH).Duetotheperme-ablekarst landscape, lakesarenotcommon.ImportantlakesincludetwolakesbothcalledVranskoLake(CresIslandandBiograd),BaćinaLakes(Croatia)andBuškoLake,BlidinjeLakeandBoračkoLake(BiH).Bosnia–Herzegovina hasmore than 20 natural lakes, including highmountain lakes, that is Idovačko Lake (1,830m). The south-eastAdriaticecoregionisboundedbytheNorthAlbanianAlps,theEasternHighlandsandPindusMountains.ItincludesallofAlbaniaandmostofMontenegroandasmallportionoftheterritoryofSerbia,Kosovo,FYRMacedoniaandnorthernGreece.Theecoregionismountainous,drop-pingtoalluvialandcoastalterrainas itapproachestheAdriaticSea.ThemostimportantcatchmentistheDrinRiver(Albania)anditstribu-tarynetwork:WhiteDrimandPlavska(Serbia/Kosovo)andBlackDrin(Albania)(Popovska&Bonacci,2007).OtherimportantriversystemsaretheMoračaRiveranditstributaryZeta(Montenegro)inthenorthand theAoos/VjosëRiver (Greece/Albania) in the south.The trans-boundaryBojana/BünaRiver (Montenegro/Albania)connectsSkadarLaketotheAdriaticSea,whereitformsawidedelta.Theecoregioncontains anumberof transboundary tectonic lakes: Skadar/Shkodra(Montenegro/Albania), Ohrid (Albania/FYR Macedonia) and PrespaLakes(Albania/FYRMacedonia/Greece).

The Ionian Sea Basin is considered a single ecoregion byAbelletal. (2008), the Ioniandrainageecoregion,boundedtotheeastbythePindusRange.ImportantriversaretheKalamas,Acheron,Louros,ArachthosandAcheloosinnorthernGreecethatflowintotheIonianSeaand lakesPamvotisandTrichonis, the latterbeingthe largest inGreece. On the Peloponnese Peninsula, important rivers are theAlfeiósandEvrotas.

F IGURE  1 Studyarea,(a)KoppenGeigerclimateclassificationsysteminBalkans;(b)thefreshwaterriversandlakesnetworksystemintheBalkanPeninsulasensuAbelletal.(2008).withboundariesofthemaindrainagebasins;codesfrom1to113arepresentedinTable1.[Colourfigurecanbeviewedatwileyonlinelibrary.com]

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TABLE  1 FreshwaterecoregionsoftheBalkanPeninsulaaccordingtoAbelletal.(2008)(R.=River;R.s.=Rivers;Res.=Reservoir;Res.s=Reservoirs;L.=Lake;L.s=Lakes)

Code Water Bodies Ecoregion

1 AcheloosR. IonianDrainages

2 AcheronR. IonianDrainages

3 AlfiosR. IonianDrainages

4 AliakmonR. Vardar

5 Aoos/VjosëR. South-eastAdriaticDrainages

6 ArachthosR. IonianDrainages

7 ArdaR. Thrace

8 Aro-RižanaR.s. Dalmatia

9 Axios/VardarR. Vardar

10 BlackDrinR. South-eastAdriaticDrainages

11 Bled-BohinjL.s Dniester–LowerDanube

12 Blidinje-BuškoL.s Dalmatia

13 Bojana/BünaR. South-eastAdriaticDrainages

14 BoračkoL. Dalmatia

15 BosnaR. Dniester–LowerDanube

16 ĆehotinaR. Dniester–LowerDanube

17 CerknicaL. Dniester–LowerDanube

18 CetinaR. Dalmatia

19 CrnaR. Vardar

20 Danube Dniester–LowerDanube

21 Doirani/DojranL. Vardar

22 DospatL. Thrace

23 DragovišticaR.-LisinaRes.

Thrace

24 Drava R. UpperDanube

25 Drin R. South-eastAdriaticDrainages

26 Drina R. Dniester–LowerDanube

27 ErgeneR. Thrace

28 Evros/Maritsa/MeriçR. Thrace

29 FierzaRes. South-eastAdriaticDrainages

30 GalaL. Thrace

31 GorgopotamosR. AegeanDrainages

32 GrahovskoL. Dalmatia

33 IronGate/DjerdapI&IIRes.s

Dniester–LowerDanube

34 IskarR. Dniester–LowerDanube

35 IskarRes. Dniester–LowerDanube

36 JasenikR. Dniester–LowerDanube

37 KalamasR. IonianDrainages

38 KamchiyaR. Dniester–LowerDanube

(Continues)

Code Water Bodies Ecoregion

39 KardzhaliRes. Thrace

40 KerkiniRes. Thrace

41 KlinjeRes. Dalmatia

42 KokinBrodRes. Dniester–LowerDanube

43 Kolpa/KupaR. Dniester–LowerDanube

44 KremastaRes. IonianDrainages

45 KrkaR.-KrkL. Dalmatia

46 Krupac-SlanoRes.s.-LiverovićiL.

South-eastAdriaticDrainages

47 LadonasR. IonianDrainages

48 LepenacR. Dniester–LowerDanube

49 LessiniR. IonianDrainages

50 LikaR. Dalmatia

51 LimR. Dniester–LowerDanube

52 LišticaR. Dalmatia

53 LourosR. IonianDrainages

54 LugomirR. Dniester–LowerDanube

55 MedvednicaR. Dniester–LowerDanube

56 Mesta/NestosR. Thrace

57 MladostRes. Vardar

58 MoračaR. South-eastAdriaticDrainages

59 MuraR. UpperDanube

60 NeraR. Dniester–LowerDanube

61 NeretvaR. Dalmatia

62 OhridL. South-eastAdriaticDrainages

63 OtilovićiRes. Dniester–LowerDanube

64 OvcharitzaRes. Thrace

65 Palić-LudašL. Dniester–LowerDanube

66 PamvotisL. IonianDrainages

67 PancharevoRes. Dniester–LowerDanube

68 PčinjaR. Vardar

69 PerućaL. Dalmatia

70 Pinios(centralGreece)R. Vardar

71 PivaR.-PivskoRes. Dniester–LowerDanube

72 PlavL. Dniester–LowerDanube

73 PlavskaR. South-eastAdriaticDrainages

74 PlitviceL.s Dniester–LowerDanube

75 PlivaL.s Dniester–LowerDanube

76 PolifitosRes. Vardar

77 PračaR. Dniester–LowerDanube

78 PrespaL.s South-eastAdriaticDrainages

79 PtujL. UpperDanube

TABLE  1  (Continued)

(Continues)

     |  7PIRIA et Al.

TheAegeanSeaBasinconsistsof threeecoregionsaccording toAbelletal. (2008):Vardar,AegeandrainagesandThrace.TheVardarecoregionincludespartofSerbia,FYRMacedoniaandGreeceandisboundedbytheDinaricAlpsandPindusRange in thewest, theŠarMountainsinthenorth-west,theOsogovskaandMaleševskemoun-tainsintheeastandMt.Olympusinthesouth.Themostimportantrivers are the Vardar/Axios River (FYR Macedonia/Greece) and itstributaries: Lepenac and Pčinja rivers (Serbia), Crna and Brejalnica(FYRMacedonia).OthermajorriversincludetheAliakmonandPinios

(Greece). The Vardar ecoregion includes also the transboundaryDoiraniLake(FYRMacedonia/Greece).TheAegeandrainagesecore-gionliestothesouthoftheVardarecoregionandisboundedbythePindusrangetothewestandMt.Othrystothenorth,encompassingsouth-easternGreece.Theecoregionischaracterizedbysmall inter-mittedwatercoursespronetodryout insummer,withmanyfedbykarst springs.Themost important river is theSpercheios,withover60tributariesandcreatinga largedeltaat itsmouth.Other import-antwatercourses are theAssopos andBoeoticKifissos.TheThraceecoregion lies to theeastof theVardarecoregionandsouthof theDnieper–LowerDanubeecoregionandspansbothEuropeandAsia.TheriversofthisecoregionarenotlimitedtotheAegeanSeaBasin,assomefallwithintheBlackSeaBasin.ThemostimportantriversinthewesternpartoftheecoregionareStruma/Strymon(Bulgaria/Greece),with its tributariesDragovištica (Serbia/Bulgaria) andStrumica (FYRMacedonia/Bulgaria),andtheMesta/NestosRiver(Bulgaria/Greece).In the Eastern Thrace ecoregion, important rivers are theMaritsa/Evros/Meriç (Bulgaria/Greece/Turkey-Thrace)with itsmain tributar-iesTundzha(Bulgaria/Turkey-Thrace)andArda(Bulgaria/Greece).TheRezovo River (Bulgaria/Turkey-Thrace) springs inTurkey-Thrace andflowsthroughBulgariabeforedrainingintotheBlackSea.Therearemorethan40naturallakesinthisecoregion,includingLakeVolviandLakeVegoritisinthewesternpartoftheecoregion,andmorethan10naturallakes,includingLakeGala(Balık,1985)inEasternThrace.

TheBlackSeaBasin includes twoecoregionsaccording toAbelletal. (2008): Dniester–Lower Danube and Upper Danube. TheDniester–LowerDanubeecoregionisavastarea,encompassingmanycountries,andisprimarilyafloodplainregion.IntheBalkanPeninsula,delineatedbytheSoča-Sava-Danubebordertothenorth,thisecore-gion includes Slovenia, Croatia, Bosnia–Herzegovina, Montenegro,Serbia,BulgariaandTurkey-Thrace.Thedominantriverintheecore-gion,andintheBlackSeaBasin,istheDanubeRiver(intheBalkans:Croatia/Serbia/Bulgaria),which formsavastdeltaat itsmouth.TheDanube is considered to be one of Europe’smost important riversand its catchment encompassesvast areas of the inlandwater net-workwithintheBalkanPeninsula.Forexample,thiscatchmentdrains81%ofSlovenia,65%ofCroatia,76%ofBosnia–Herzegovina,51%ofMontenegro92%ofSerbiaand43%ofBulgaria (ICPDR2009).ThelongesttributaryistheTisa/TiszaRiver(Romania/Serbia),whichformsthelargestsub-basin.ThetransboundarySavaRiver(Slovenia/Croatia/BiH/Serbia/Montenegro)isthesecondlargestsub-basinandcontrib-utesthemostwatertotheDanube,anditsmainsubtributariesaretheKrkaandKupa(Slovenia/Croatia),Una(Croatia/Bosnia–Herzegovina),VrbasandBosna(BiH),Drina(BiH/Serbia,withmaintributariesTara,PivaandLim(Montenegro).TheIskarRiver(Bulgaria)isthelargesttrib-utaryoftheDanubeinBulgaria,followedbytheVitRiver(Bulgaria).Other independentandshorterriversdraineasternBulgariadirectlyintotheBlackSea.Thereareanumberofnaturallakesinthisecore-gion.Sloveniahaslakesofglacialorigin,suchastheTriglav,BledandBohinj,andofkarsticorigin,suchasCerknicaLakeorthenumeroussmallerpolje (karstfield) lakes.Montenegrohassmallhighmountainglacial lakes, themost importantofwhich isLakePlav (Popovska&Bonacci,2007),whileinBosnia–Herzegovina,thereistheGreatPliva

Code Water Bodies Ecoregion

80 Rezovo Dniester–LowerDanube

81 SavaR. Dniester–LowerDanube

82 SkadarL. South-eastAdriaticDrainages

83 Soča/IsonzoR. GulfofVeniceDrainages

84 SpercheiosR. AegeanDrainages

85 Struma/StrymonR. Thrace

86 TamišR. Dniester–LowerDanube

87 TaraR. Dniester–LowerDanube

88 TavroposRes. IonianDrainages

89 Thermopylaehotsprings AegeanDrainages

90 ThisavrosRes. Thrace

91 TikveshRes. Vardar

92 TimokR. Dniester–LowerDanube

93 TisaR. Dniester–LowerDanube

94 ToplaVodaR. Dniester–LowerDanube

95 TrbojeL. Dniester–LowerDanube

96 TrebišnjicaR.-BilećaL. Dalmatia

97 TrichonisL. IonianDrainages

98 Triglav/TriglavskajezeraL.s

Dniester–LowerDanube

99 TundzhaR. Thrace

100 VaraždinRes. UpperDanube

101 VegoritisR. Vardar

102 VelikaMoravaR. Dniester–LowerDanube

103 VitR. Dniester–LowerDanube

104 VlasinaL. Dniester–LowerDanube

105 VogrščekRes. GulfofVeniceDrainages

106 VolviL. Thrace

107 VouliagmeniL. AegeanDrainages

108 VranskoL.s Dalmatia

109 VranskoL.(CresIsland) Dalmatia

110 VrbasR. Dniester–LowerDanube

111 WhiteDrinR. South-eastAdriaticDrainages

112 Yliki-ParalimniL.s AegeanDrainages

113 ZetaR. South-eastAdriaticDrainages

TABLE  1  (Continued)

8  |     PIRIA et Al.

Lake.InSerbia,therearethePalićandLudašLakes.Bulgariahasthelargestnumberofnaturallakes,about400(highmountainglaciallakes,several landslidelakes,BlackSeacoastalandDanuberiparianlakes/marshes).TheUpperDanubeecoregionintheBalkansencompassesonlyasmallpartofSloveniaandCroatiaandincludestheDravaRiver(Slovenia/Croatia)anditstributaryMura(Slovenia/Croatia).TheDravaRiverformsthefourthlargesttributaryoftheDanube.

2.2.2 | Artificial reservoirs

InSlovenia,dammingcreatedtheartificialLakePtuj(DravaRiver),thelargest inSlovenia,LakeMostnaSoči (SočaRiver)andLakeTrboje(SavaRiver). InCroatia, thereare three largeartificial reservoirsontheDravaRiverandoneontheCetinaRiver.InBosnia–Herzegovina,therearedamsontheNeretvaRiver,althoughthelargestreservoirisBilećaLake(TrebišnjicaRiver),neartheborderwithMontenegro. InMontenegro,thereareseveralrelativelylargeartificiallakes,inrivercanyonsorrivervalleys,mostlyforelectricpowerproduction(PivskoandOtilovićiinthenorthandSlano,Krupac,LiverovićiandGrahovskoincentralMontenegro).InAlbania,mostlowlandreservoirsareusedforaquacultureorirrigation.InFYRMacedonia,therearemorethan120reservoirs(21large)createdforwatersupply,hydropowerpro-duction,irrigationandfishfarming.InGreece,thereareover160largereservoirs(ICOLD2015),mostlyinthenorththatprimarilyserveforirrigationanddrinkingwatersupply,whileseverallargereservoirsareforhydropowerproduction, that isLakePolyfytos (AliakmonRiver),LakeKremasta(AcheloosRiver)andLakeThisavros(NestosRiver).InEasternThrace,thereare≈70reservoirs(mostlysmall)usedforirriga-tion andwater supply. In Serbia, several reservoirs havebeen con-structedforhydropower,watersupplyorerosioncontrol.ThelargestistheDerdap(IronGate)reservoirontheDanubeRiver.InBulgaria(totalsurfaceareaof395km2),largereservoirs(>2km2)areusedforirrigation,hydropowergeneration,watersourcescontrol,fishfarming,drinkingwatersupply,floodcontrolandrecreation.The3,000smallreservoirsareusedmainlyforirrigationandfishfarming;themajor-ityaremountainreservoirs(elevationfrom600to1,800m;Stefanov,2007).

Finally,abriefmentionshouldbemadeofponds,thatissmallandshallow lentic water bodies characterized by plant growth, namelyrootedplants,asopposed to lakes thataremuchdeeperand largerwithnoplantgrowth.Pondsareoftentherecipientsofalienspecies,similarlytootherlenticsystems.

3  | VECTORS OF INTRODUCTIONS

Thisstudyshowedthatthetwomainmodesofalienspeciesintroduc-tionintheBalkancountrieswereasfollows:(i)intentional,thatisforrecreationalandsportfishing,aquaculture,ornamentaltradeandbio-logicalcontrol)and(ii)unintentional,thatistheaccidentalspreadofaspeciesasaconsequenceofintroductionforaquaculturepurposes,orthenaturalspreadofaspeciesoutsideitsnaturalhabitat(Figure2a).Datasincethelate19thcenturyindicatethatintroductionscontinued

atahighraterightuptothe1980s. Inthesubsequentdecades, in-troductionsofboth typeswere reduced substantially (Figure2b–k).By country, the highest number of unintentionally introduced spe-ciesoverthelast15yearshasbeenrecordedinCroatiaandGreece(Figure2e,f).Themainvectorforintroductions,withthehighestnum-ber of fish species inmost Balkan countries,was intentional intro-ductionforaquaculture,except inSerbiaandTurkey-Thrace,wheremostspecieswereintroducedunintentionally.Recreationalandsportfishing(angling)isanothermajorvectorofalienspeciesintroductionanddispersal (Figure2a). InSlovenia,thishasbeenacommonprac-ticeformorethan100years(Figure2j),whileinotherBalkancoun-tries,recreationalfisheriesandstockingactivitiesbeganafterWorldWarII.Anotherimportantintroductionvectorwasbiologicalcontrol.ThispracticebeganinAlbania(Figure2b)andspreadthroughouttheBalkansafter the1960s. It is interestingthatno introductionshaveeverbeenreportedforbiologicalcontrolinBosnia–HerzegovinaandSerbia(Figure2d,i)orforrecreationalandsportfishinginAlbania,FYRMacedoniaandTurkey-Thrace(Figure2b,h,k).Finally,althoughitap-pearsthattheornamentaltrade(Figure2a–k)isnotahighlyimportantvectorof introduction, somehavewarned that thiswill becomeanincreasingprobleminEuropeandtheBalkansinthefuture(Chucholl,2013).

Freshwateraquaculture,whichrepresentsamajorvectorfortheinterwatershed dispersion of many alien speciesworldwide (Nunesetal., 2015), has a long tradition in theBalkans, datingback to the16thand17thcenturies inthecaseofSlovenia,whenfishbreedingwasperformedby thechurch,monasteriesandsquires. InSlovenia,thefirstfishfarmforartificialcarpbreedingwasestablishedin1870,with salmonid farming beginning a decade later (Govedič, 2005).Similarly,inCroatiaandBulgaria,thefirsttroutfarmswereestablishedinthelate19thcentury,followedbythefirstcarpfarmsatthesametimeinCroatia(Piria,Tomljanovićetal.,2016),andinBulgariainthe1940s(Uzunova&Zlatanova,2007).InGreece,incontrast,thefresh-wateraquaculturesectorwasverylimiteduntilthe1980s(Perdikaris,Gouva,&Paschos,2010).

Freshwater aquaculture in the Balkans is focused on the pro-duction of both warm-water (cyprinid or carp-like) and cold-water(salmonid or trout-like) species,with common carp (Cyprinus carpio, Cyprinidae)and rainbowtrout (Oncorhynchus mykiss, Salmonidae)asthepredominantspecies,respectively.

Indeed,statisticsshowthatcommoncarpdominateswarm-wateraquacultureproduction incountrieswitha largeaquaculturesector,accountingfor85%–90%ofthetotalwarm-waterspeciesproductionof8,000–10,000tonnesinSerbiaandover50%oftheproductioninCroatia(2,100tonnesofatotalof2,884tonnes)andBulgaria(1,736ofa totalof3,248tonnes–2012data,FAO2016).Thesame isalsoevidentincountrieswithpredominantlycold-wateraquaculture,suchasGreece.

Morespecifically,warmfreshwateraquacultureincountrieswithan extensive sector such asCroatia (eight large and several smallerunits), Bulgaria (451 fish farms) and Serbia (36 large and severalsmallerfarms)mainlyinvolvesthefarmingofcommoncarp,eitherinmonocultureorinpoly-culturewithotherspecies,suchasgrasscarp

     |  9PIRIA et Al.

F IGURE  2 Vectorsoffishintroductionsintothe(a)BalkanPeninsula(Kosovoexcluded),n=numberofspecies(AL,Albania;BG,Bulgaria;BH,Bosnia–Herzegovina;CRO,Croatia;GR,Greece;KS,Kosovo;ME,Montenegro;MK,FYRMacedonia;RS,Serbia;SLO,Slovenia;TR,Turkey-Thrace);(b–k)eachcountryseparately,sincethebeginningofthe20thcentury.[Colourfigurecanbeviewedatwileyonlinelibrary.com]

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AL BG BH CRO GR ME MK RS SLO TR

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Aquaculture

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(f) (g)

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AL

10  |     PIRIA et Al.

(Ctenopharyngodon idella,Cyprinidae),silvercarp(Hypophthalmichthys molitrix, Cyprinidae) and bighead carp (Hypophthalmichthys nobilis,Cyprinidae),(FAO2016).InBulgaria,channelcatfish(Ictalurus puncta-tus,Ictaluridae)israisedinnetcagesinthecoolinglakeoftheMaritza-Iztok 2 Thermal Electrical Plant, while sturgeons, such as Danubesturgeon (Acipenser gueldenstaedti, Acipenseridae), stellate sturgeon(Acipenser stellatus,Acipenseridae),beluga(Huso huso,Acipenseridae)and starlet (Acipenser ruthenus,Acipenseridae), are reared for stock-ing,togetherwithhybrids,alsoforhumanconsumptionandstocking.Finally,inbothGreeceandEasternThrace,commoncarpisthedom-inantspeciesofwarm-wateraquaculture,togetherwithDanubestur-geonandNiletilapia(Oreochromis niloticus,Cichlidae),whichwasalsostockedinGreeceinthepastforcommercialpurposes.

Thegeographical,climaticandwaterconditionsincertainBalkancountries, suchasSloveniaorMontenegro,aremore favourable forcold-water fish husbandry, with small-scale fish farms on montanestreamsandriversdominatingaquaculture in thesecountries.Cold-water aquaculture in Bulgaria concentrates alsomainly on rainbowtrout,whichaccountedfor98%ofthetotalcold-wateroutputofal-most 2,800 tonnes in 2012, followed by brown trout (Salmo trutta,Salmonidae) and brook trout (Salvelinus fontinalis, Salmonidae),and the whitefish species, European whitefish (Coregonus lavare-tus, Salmonidae) and peled (Coregonus peled, Salmonidae). Similarly,rainbow trout dominates cold-water fish farm production in Serbia,withtotalproductionvaryingfrom1,600to2,500tonnes,inBosnia–Herzegovinaat97farmsfrom3,000to4,000tonnes,wherebrowntroutandbrooktroutarealsorearedin55fishfarms,inAlbaniaat57troutfishfarms,inFYRMacedoniaat64farmsandinEasternThraceatsevenlargefarmsthatproduce1,000tonnesofrainbowtrout.Thedominance of rainbow trout rearing in the Balkans ismore evidentinCroatia,where 345 of 351 tons of all salmonid fish produced in2013at27troutfishfarmswererainbowtrout (FAO2016).Finally,inGreece,mostly in itsnorth-westernpart, themostcommoncold-wateraquaculturespeciesisrainbowtrout,followedbycohosalmon(Oncorhynchus kisutch,Salmonidae),rearedin≈80aquacultureunits.

Warm-water aquaculture represents amajor source of invasive-ness in theBalkans; there is evidenceof established self-sustainingpopulationsoftheAsiancarpsbasedonevidenceofsexualmaturationandoftheincreasingnumberofthesespecies(Janković,1998).Thereisevidenceofthepresenceofsilvercarpandgrasscarpjuveniles(0+age)intheBulgariansectionoftheDanube(A.Apostolou,personalob-servation).Warm-wateraquacultureisalsoamajorvehicleforthein-terwatersheddispersionofseveralotherhighlyinvasivespecies,suchasthegibelcarp(Carassius gibelio,Cyprinidae),pumpkinseed(Lepomis gibbosus, Centrarchidae) and topmouth gudgeon (Pseudorasbora parva,Cyprinidae)(Nunesetal.,2015;Savinietal.,2010;Uzunova&Zlatanova,2007).Furthermore,aliensturgeonspeciesbreedingneartheDanube inBulgarianwaterscouldalso impactnatural stocksbyhybridization, while the possible detrimental effects of paddlefish(Polyodon spathula,Polyodontidae)escapeesfromRomanianfishfarmsarecurrentlyunknown.

Unlikewarm-wateraquaculture,cold-wateraquaculturedoesnotrepresent a major vector of invasiveness, as alien salmonids do not

reproducenaturallyinmostoftheBalkanwaters.However,therearedocumentedcasesof rainbow trout reproductiondatingback to theearly20thcenturyinSlovenia(Franke,1913;Mršić,1935)andtotheearly1970sinCroatia(MacCrimmon,1971);formorerecentreports,seethereviewbyStanković,Crivelli,andSnoj(2015).Surprisingly,therearealsoindicationsofareproducingrainbowtroutpopulationinariverinsouthernCroatia(D.Zanella,personalobservation)andinsouthernGreece (Koutsikos etal., 2012). In addition, the impacts of escapeesfromcold-waterfishfarmsonlocalbiodiversitycertainlywarrantfur-therstudies,assuggestedelsewhere(Tarkan,Marr,&Ekmekçi,2015).

Sport and recreational fishing (angling) is also amajorvector ofthe spread of alien freshwater fishes in the Balkans, through theirtranslocationbetweenwatersheds,anactivitythatisverydifficulttomonitor.Inthepast,anglersoftenengagedinthispractice,byinten-tionallyintroducingspeciesoffisheriesinterestinwatersconsideredto have “impoverished” fauna, as is the case of the introduction ofnorthernpike (Esox lucius,Esocidae)andofanalienstrainofbrowntrout toAdriaticwater systems in Croatia. Similarly, the transfer ofalien species, such as the gibel carp, betweenwater bodies in FYRMacedoniaorEasternThraceiscurrentlyalsoextensive(Aydınetal.,2011),whereasforothercountries,suchasBosnia–Herzegovina,in-formationontheseactivitiesislacking.Anglershavealsolikelybeenresponsibleforthespreadofmanysmallalienspecies,usingthemaslivebaitandthustransferringthemfromcatchmenttocatchment.InCroatiaandSlovenia,forexample,small-bodiedfishesfromthelowerDanubeareroutinelytransportedaslivebaitforanglinginAdriaticriv-ers(Dalmatiaandsouth-eastAdriaticecoregions),despitetheprohibi-tionofthispracticebylaw(Pofuk,Zanella,&Piria,2017).InGreece,incontrast,thescaleofanglingandcommercialfisheriesinlargelakesandreservoirsisscarce,andthus,thespreadofalienspeciesthroughthesepracticesisratherlimited,incomparisonwiththeotherBalkancountries. Stocking with alien species has been banned by legisla-tioninmanyBalkancountries(in2000inSerbia;in2001inCroatia),although dispersal via the Danube River as a prominent southernInvasiveCorridor(BijdeVaate,Jazdzewski,Ketelaars,Gollasch,&VanderVelde,2002)continues.However,unintentional introductionsofalienfishspeciesandalientroutstrains (Simonovićetal.,2015)stilloccurincertaincountries,suchasSerbia.

Theornamental tradehasalsobeen recognizedasan importantvectorfortheintroductionofinvasivespecies,linkedtoover150spe-cies’ invasionsinnaturalecosystemsaroundtheworld(Fuller,2003;Padilla&Williams,2004;Siguan,2003).Thevastmajorityofthestud-iesontheornamentaltradeasavectorhavebeenconductedinNorthAmerica,with few such studies conducted in Europe (Copp& Fox,2007;Duggan, Rixon,&MacIsaac, 2006;Maceda-Veiga, Escribano-Alacid,DeSostoa,&García-Berthou,2013;Padilla&Williams,2004).FortheBalkanregion,datacollectionandmonitoringoftheornamen-talfishtradeisvirtuallynon-existent,asnoofficialdatasetsareavail-able.Recently,Papavlasopoulou,Vardakas,Perdikaris,Kommatas,andPaschos(2014)concludedthattheaquariumfishsectorisundervir-tuallynocontrolinGreece,giventheexistenceofthreatenedspecies,speciespotentiallyharmfultohumansandspeciescapableofestab-lishingnon-indigenouspopulations,ifreleasedintothewild.

     |  11PIRIA et Al.

Todate,theornamentaltradeaccountsforalimitedbutsteadilygrowingproportionoffishintroductionsintheBalkanregion(Zenetosetal.,2009).Sofar,onlynineornamentalfishspecieshavebeenre-corded inthefreshwatersoftheBalkans (TableS1).Onlythegold-fish(Carassius auratus,Cyprinidae)andthepumpkinseed, introducedin thenorthernBalkans in the19thcentury,occurwidely in there-gion, while the rest are restricted to individual basins (Economouetal., 2007). For example, the guppy (Poecilia reticulata, Poeciliidae)hasonlybeenintroducedtoAlbania,whereitscurrentdistributionisunknown, to Bulgaria in the artificial Lake Pancharevo on the IskarRiver,atributaryoftheDanubeandtoSerbia,intheToplaVodather-malStream,whichsupportsaself-sustainingpopulationofthespecies(Milenković, Žikić, Stanković, &Marić, 2014). Similarly, theAmazonsailfin catfish (Pterygoplichthys pardalis, Loricariidae) has only beenrecordedintheSerbian(middle)sectionoftheDanubeRiver (afirstrecordintheinlandwatersofEurope,seeSimonović,Nikolić,&Grujić,2010).However,despitethe lownumberofornamentalfishspeciesreleasedintheBalkans,thepossibilityofmoreornamentalfishbeingintroduced, either intentionally or unintentionally, and subsequentlybeingestablishedintothewild,withunknownimpactsonthenativefauna,stillremains.Furthermore,thereisnoknowledgeofhowthesespeciesaffect thenative fish fauna,asno relevant researchhasyetbeenconductedintheregion.

Finally, only three species have been introduced in the Balkansfor biological control: the grass carp, the silver carp and the highlyinvasive eastern mosquitofish (Gambusia holbrooki, Poeciliidae). Thegrass carpwas introduced to control the growthof aquaticvegeta-tion(Economidis,1991)andthesilvercarptocontrolplanktonicas-semblages (Vuković & Kosorić, 1978), although these introductionshad negative implications for inlandwater communities (Domaizon& Dévaux, 1999; Leonardos, Kagalou etal., 2008). The easternmosquitofish was introduced in the Balkans and elsewhere in theMediterraneaninthe1920sformosquitocontrol,withquestionableresults (Stephanides, 1964). Today, the eastern mosquitofish is thesecondmostwidespreadalienfishspecies in theMediterraneanre-gionandthemostwidespreadinGreece,occurringin49.5%ofitsriverbasins(Economouetal.,2007).

4  | PATHWAYS OF FISH INTRODUCTIONS IN THE BALKANS

4.1 | Biogeographical origin of introduced species and current state

The majority of introduced species in the Balkans are of NorthAmerican origin (25.0%), followed by Asian (23.3%) and EasternEuropean species (23.3%), which coincides with the chronology ofthefirstintroductions(Figure3).Untiltheearly1950s,introductionswere primarily of North American and Asian species (Copp etal.,2005; Fijan, Petrinec, & Đorđević, 1989; Holčik, 1991; Livadas &Sphangos,1941;Nežić,1938;Povž&Šumer,2005),whiletheinterestforspeciesfromNorthernandWesternEuropearoselater(TableS1).Other introduced species originated from Northern Europe (8.3%),

SouthAmerica(5.0%),Asia/Europe(6.7%),Africa(5.0%)andWesternEurope(3.3%,Figure3).Itshouldbenotedthatsincetheichthyofau-nal research inKosovo is sparse (Gashi etal., 2016), only thenew-est data are presented in Table S1. The historical data for KosovoareincludedinthepresenteddataforFYRMacedonia,Montenegro,AlbaniaandSerbiaandarenotcalculatedasaseparatecountryduetothelackofdata.

In total, 60 fish species have been introduced in the BalkanPeninsula, intentionally, accidentally or by natural dispersal (TableS1).Thefirst introductionsweredocumentedinthe19thcenturyinBulgaria (two species),Croatia (one species) andSlovenia (four spe-cies). Known introductionswere performed, from1901 to 1920, inBosnia–Herzegovina and Croatia, from 1921 to 1940 in Croatia,Serbia,Bosnia–HerzegovinaandBulgariaandafter1940 inAlbania,Greece,MontenegroandSerbia.Inmostcountries,thehighestnumberofintroductionstookplacebetween1960and1980(Figure4).Since2000,Turkey-Thracehasnotdocumentedanynewalienfishspeciesintroductions, as opposed toCroatia,which has recorded the high-estnumberofnewalienspeciessincethen(Ćaleta,Jelićetal.,2011;Ćaleta,Tutmanetal.,2011;Jelkić&Opačak,2013;Piria,Treeretal.,2011;Piria,Špremetal.,2011;Safneretal.,2013;Šandaetal.,2013),althoughthismaybetheresultofintensiveichthyologicalresearchintheperiodafter2000ratherthannewintroductions(Figure4).

Themajority (38.46%)ofspecies intheBalkanPeninsulahavealimiteddistribution,includingalpinelakes(e.g.arcticcharr(Salvelinus alpinus, Salmonidae) in Slovenia, reservoirs (e.g. pikeperch (Sander lucioperca,Percidae)inFYRMacedonia,artificiallakesandreservoirs(e.g. Aristotle’s catfish (Silurus aristotelis, Siluridae) in Bulgaria andisolatedandsmallriverbasins(e.g.Europeangrayling(Thymallus thy-mallus, Salmonidae) inMontenegro. In thecurrent study,33.17%ofall introduced fisheswere found tobewidespread.Themostwide-spreadspeciesaregibelcarp,pumpkinseed,brownbullhead(Ameiurus nebulosus,Ictaluridae),Easternmosquitofishandtopmouthgudgeon,duetotheirhighinvasivepotential(Perdikarisetal.,2016;Piriaetal.,2016b;Simonovićetal.,2013).Specieswithamoderatedistribution(15.38%) include the monkey goby (Neogobius fluviatilis, Gobiidae)and bighead goby (Ponticola kessleri, Gobiidae) in Croatia, the gold-fish inAlbania andGreece and the silver andbighead carp inmostcountries.For12.98%species, theirBalkandistribution isunknown,includingspeciesfound/caughtonlyonce(e.g.short-snoutedpipefishintheDanubeRiver),sporadically(e.g.MississippipaddlefishinSerbia,CroatiaandBulgaria)orintroduced,buttheirstatusremainsunknown(e.g. brook trout in Skadar Lake), (Figure5a).The prevalence of thealien species in theBalkan countries (Figure5b) suggests thatmosthavebecomewidespread inSerbia,Slovenia,Croatia,Turkey-ThraceandAlbania. Themajority of these species have a restricted distri-bution in Greece, Bulgaria, FYRMacedonia andMontenegro.Mostintroduced fish species in the Balkans have naturalized (Figure5c),with the exception of Turkey-Thrace, where most introduced fishhave acclimatized (Figure5d).A categorization of alien species sta-tusofinvasionsensuBlackburnetal.(2011)showedthatthehighestpercentageofspecies (29%)belongstotheC3category (individualssurviving in the wild at locations where introduced, reproduction

12  |     PIRIA et Al.

occurringandpopulationself-sustaining),23%belongstotheB3cat-egory(individualstransportedbeyondthelimitsoftheirnativerange,anddirectlyreleasedintonovelenvironment)and24%totheEcate-gory(fullyinvasivespecies,withindividualsdispersing,survivingand

reproducing atmultiple sites across a greater or lesser spectrumofhabitatsandextentofoccurrence).Otherspecies(21%)belongtotheC0,C1andC2categorieswhichrefertopopulationsthatarenotself-sustaining(Figure5e).OnlyAlbania,BulgariaandCroatiareportedthe

F IGURE  4 Temporalpatternofintroductionof alienfishspeciesintoBalkanareasincethebeginningofthe20thcentury.Periodfrom2001–2015represents14years.[Colourfigurecanbeviewedatwileyonlinelibrary.com]

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F IGURE  3 OriginofintroducedalienfishspeciesintheBalkanPeninsula(AFR=Africa;AS=Asia;EE=EasternEurope;NE=NorthernEurope;NA=NorthAmerica;SA=SouthAmerica;WE=WesternEurope).Numbersinarrowsdenotethenumberofnon-nativespeciesofvaryingoriginsintroducedtotheBalkans(insetgraphshowsthepercentagecontributionoforigins).ThestandardJenksNaturalbreaksclassificationmethod(ArcGIS,version10.1)wasusedtodefinetheclassrangesofspeciesper1,000km2groups.[Colourfigurecanbeviewedatwileyonlinelibrary.com]

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     |  13PIRIA et Al.

C0category[individualsreleasedintothewild(i.e.outsideofcaptivityorcultivation)toalocation,butwereincapableofsurvivingforasig-nificantperiod].TheC2category(individualssurvivinginthewildatthe site of introduction, reproduction occurring, but population notself-sustaining) was reported Bulgaria, Bosnia Hercegovina, CroatiaandFYRMacedonia.TheC1category[individualssurvivinginthewild(i.e.outsideofcaptivityorcultivation)atthelocationofintroduction,butwithout reproduction]was reported inall countriesand in totalrepresents21%(Figure5e,f).

4.2 | North American introductions

RainbowtroutwasthefirstNorthAmericanspeciesintroducedtotheBalkanregion.ThisspecieswaslikelyintroducedfirstfromGermanytotheSamokovfishfarminBulgariain1878(Uzunova&Zlatanova,

2007;Welcomme, 1988), followedby its introduction fromAustriain1883 to theČabar fish farm inCroatia (Bojčić,1997),andnot in1894 to theMt. Medvednica fish pond near Zagreb as previouslyreported (Stanković etal., 2015). In 1891, eggswere shipped fromotherAustrianfarmstoSlovenia(Franke,1913;Povž&Ocvirk,1990).The first introductions into open waters were likely conducted inSlovenia in1893,and in1895, fishweretranslocatedtothehatch-eryonMedvednicaStreamnearZagreb inCroatia (Stankovićetal.,2015).By1908, rainbow troutwere introduced intomany suitablewatersthroughoutcentralSlovenia(Franke,1913;Stanković&Snoj,2013)andby1935tootherareasinthecentralandAdriaticpartsofSloveniaandCroatia(Stankovićetal.,2015).Attheturnofthe20thcentury, rainbow troutwere introduced fromCalifornia to Bosnia–Herzegovina in a fish farm at the springs of the Bosna River nearSarajevo (Mršić, 1935). Fry originating from these shipments were

F IGURE  5 Alienspecies(a)totalprevalence;(b)prevalencebycountry;(c)statusofacclimatization;(d)statusofacclimatizationofintroducedspeciesbycountry,expressedasnumberofspecies;(e)invasioncategoryaccordingBlackburnetal.,2011(forexplanationofcodesB3,C0,C1,C2,C3,EseeTableS1);and(f)invasioncategorybycountry;(W,widespreadspecies;M,moderatedistribution;R,restricted;U,unknowndistribution;A,acclimatized;A*,acclimatizedinrestrictedareas;N,naturalized;UN,unknownstatus).[Colourfigurecanbeviewedatwileyonlinelibrary.com]

(a) (b)

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05

1015202530354045

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14  |     PIRIA et Al.

used for seed introductions into theRiverBosna, LakePlivsko andBoračkoLakeand intoseveralsinkingkarststreamsnearLivnoandin Trebišnjica River. All these seed introductions were reported assuccessful(seeStankovićetal.,2015).Thesecondwaveofintroduc-tions began in the 1930swith the first report for Serbia (Lenhardtetal.,2011)andregularimportsintoBulgaria(Uzunova&Zlatanova,2007). The third introductionwave occurred in 1951with the im-port of rainbow trout eggs from Switzerland to two state hatcher-ies innorthandnorth-westernGreece(Perdikarisetal.,2010).Thatsameyear,rainbowtroutstockingbegan inMontenegroforanglingpurposes (Drecun, 1951) and later (in 1965) in FYRMacedonia forfishfarming(Naumovski,1995).Themostrecentintroductionswerereported fromAlbania (Shumka, Paparisto,&Grazhdani, 2008) andfromTurkey-Thrace(Çelikkale,2002)inthe1970s.AlthoughthefirstintroductionsinGreeceandAlbaniawerenotedinthe1950sandlate1970s,respectively,thefirstattemptsofintroductionmusthaveoc-curredmuchearlier,namelyinthe1880s,asdocumentedbyHolčik(1991).

Brook trout is native to north-eastern America, fromNewfoundlandtoHudsonBayinCanadatoGeorgiainthesouthernUSA,andhasbeenintroducedworldwide.AccordingtoHolčik(1991),brooktroutwasintroducedtothewesternBalkansin1869,whichcoincideswiththeSlovenianrecordin1884(Povž&Ocvirk,1990)whenitwasintroducedtotheLjubljanaareaforsportfishingandasafoodfish(Povžetal.,2015).Attheturnofthe20thcentury,brooktroutwas introduced to the Sarajevo region (Željeznica, Zujevina,BosnaandPračarivers)andin1913tothekarstriversSturbaandŠuica (Vuković&Kosorić,1978). In the1930s, itwas imported toBulgariaalongside rainbowtrout.These twospecies remaindomi-nantinBulgariancold-waterfarming(Uzunova&Zlatanova,2007).IntheperiodbetweenthetwoWorldWars,brooktroutwasintro-duced to Serbia (Vlasina and Lisina Lakes) and successfully rearedin theVrla fishpondatSurdulica (Lenhardtetal.,2011;Simonović,2006).Much later,brooktroutwas introducedtoFYRMacedonia,between1965and1980(Naumovski,1995)andtoMontenegroin1985(Marić,1991).InMontenegro,thisspecieswasintroducedtotheMarezafishfarmnearTitogradandlatertranslocatedtoSkadarLake for cage farming (Marić, 1991), fromwhichmany specimensescaped.Thereisnoknowledgeonanyfurtherrestockingorfarmingactivities inMontenegro orAlbania, although this species is pres-ent in Skadar Lake (Talevski etal., 2009). It is not clearwhen theintroductionofbrook troutoccurred inCroatia,but this species isfarmed (Vardić etal., 2007) in the inlandwaters of theDniester–LowerDanube ecoregion (Jelić, Jelić, Žutinić,&Ćaleta, 2012) andalso in the south-eastAdriatic ecoregion (Muhamedagić,Gjoen,&Vegra,2010).ThelatestintroductionoccurredinGreecewheneggsofbrook troutwere introduced into theEdessa statehatchery forincubation, and fingerlings were subsequently released into LakeVegoritisandtheTavroposReservoir(Economidis,Dimitriou,Pagoni,Michaloudi,&Natsis, 2000) during the1980s, although its estab-lishmentintheTavroposReservoirisdoubtful(Barbierietal.,2015).

Coho salmon is a non-indigenous species from the NorthernPacific.IntheBalkans,itwasfirstintroducedforcommercialpurposes

in Serbia in the1950s (Janković&Raspopović, 1960a,b), inGreecein the 1980s (Crivelli, Catsadorakis, Malakou, & Rosecchi, 1997;Economidis,Dimitriouetal.,2000;Tsekosetal.,1992)andinSloveniaandCroatia(Teskeredžić,Teskeredžić,Tomec,&Hacmanjek,1989).InGreece,eggsfromCanadawereimportedtotheEdessastatehatcheryinNorthernGreece,whilelivespecimenswerereleasedatnearbyLakeVegoritisandTavroposReservoir(AcheloosRiver)inCentralGreeceinthe1980sand in fish farms in theGorgopotamosRiver (Spercheiosdrainage,Economidis,Dimitriouetal.,2000).AlthoughnotestablishedinGreece,escapeesorreleasedindividualshavebeenreportedintheTavropos Reservoir, LakeVegoritis and Gorgopotamos River and intheupperLadonasRiver(Alfiosdrainage)inthePeloponnese(Barbierietal.,2015). Insomecases,suchasatTavroposReservoir,repeatedstockinghasoccurredinthepast(Economouetal.,2001).InCroatia,SerbiaandSlovenia,thisspecieshassincedisappeared.

Brown bullhead and black bullhead (Ameiurus melas, Ictaluridae)wereintroducedatanearlydateintotheBalkans.Thefirstintroduc-tionsoccurred inCroatia in1905 (Fijan etal., 1989) and fromhereto other European countries (Movchan, Talabishka, &Velikopolskiy,2014).Itwasoriginallybelievedthatonlybrownbullheadwasintro-ducedtoEurope (Fijanetal.,1989;Movchanetal.,2014),althoughfollowingthediscoveryofthepresenceofblackbullhead,itbecameapparentthatbothbullheadspecieswereintroducedatthesametime(Holčik,1991)andweresubsequentlymisidentified (Movchanetal.,2014).The firstoccurrenceofbrownbullhead in inlandwaterswasreportedin1926fortheDanubeRiver(Balón,1964),followedbyre-cordsfromSerbiaandBosnia–Herzegovinasince1930(Lenhardtetal.,2011;Simonović,2006;Sofradžija,2009;Tutman,Glamuzina,Dulčić,&Zovko,2012;Vuković& Ivanović,1971)andSlovenia since1935(Povž&Ocvirk,1990).Morethan40yearslater(in1978),thisspecieswasobservedinMontenegro(Knežević,Vuković,&Ražnatović,1978),FYR Macedonia (between 1970 and 1995, Naumovski, 1995) andBulgariain1975(Uzunova&Zlatanova,2007).ThelackofreportsofbrownbullheadinAlbaniaisquestionableasthisspecieswasreportedforSkadarLakeinMontenegro(Kneževićetal.,1978),alakesharedbybothcountries.AccordingtoRakajandFlloko(1995),blackbullheadwasindeedintroducedintoAlbania,althoughtherearenodataonitscurrentpresence. InGreece,a recordofbrownbullhead from2012remainsunconfirmed(Barbierietal.,2015).

Thedistributionofthesetwospeciesoutsidetheirnativedispersalarea is not known (Fijanetal., 1989), a fact confirmedbydata col-lectedforthisarticle(TableS1).Onlyfourcountrieshavereportedthepresence of black bullhead (Bosnia–Herzegovina, Croatia, SloveniaandSerbia),withtheperiodbetweenfindingsinCroatia(Fijanetal.,1989)andSerbia (Cvijanović,Lenhardt,&Hegediš,2005)extendingover100years. InBosnia–HerzegovinaandSlovenia,theexacttimeofappearanceisnotknown(Adrović,Skenderović,Salihović,&Stjepić,2012;Povž&Šumer,2005).RecentresearchinCroatiahasconfirmedthedominantpresenceofblackbullheadinbothbasins(AdriaticandBlackSeas),whilethebrownbullheadisrestrictedonlytotheNeretvaRiver and its tributaries (Novosel, 2010). In countries borderingwithCroatia (Slovenia andBosnia–Herzegovina),most reports refertobrownbullhead (Adrovićetal., 2012;Povž&Šumer,2005), thus

     |  15PIRIA et Al.

indicating ambiguities and possible misidentifications of these twospecies.

Channel catfish was introduced more recently in the Balkans,namely to the formerYugoslavia in 1968 (Fijan etal., 1989;Holčik,1991).ThisspeciesisonlystillpresentinaquacultureinFYRMacedonia,whereithasbeenrecordedintheinlandwaters(Nastova-Gjorgjioska&Kostov,2000).TheintroductionofchannelcatfishtoBulgariaoccurredin1975foraquaculturepurposes(Uzunova&Zlatanova,2007),withincreasing trendsof production (Hadjinikolova,Hubenova,&Zaikov,2010).Duetothesporadicrecordsofchannelcatfishininlandwaters(AApostolski,unpublisheddata),itisconsiderednaturalized(Uzunova&Zlatanova,2007),althoughitsimpactisunknown.

Three more species were introduced to the Ovcharitza andKardzhalireservoirsofBulgariainthelate1970s(Holčik,1991):small-mouth buffalo (Ictiobus bubalus, Catostomidae), bigmouth buffalo(Ictiobus cyprinellus, Catostomidae) and black buffalo (Ictiobus niger,Catostomidae).Theirreproductionisassumedtobenatural,althoughtheircurrentstatusisunknown,astherearenoknownrecordssincethen(Uzunova&Zlatanova,2007).

Pumpkinseed is one the most successful introduced fishes inEurope,withwidespread introductions inthe late19thcentury (Fox&Copp,2014).AccordingtoHolčik(1991),theintroductionandes-tablishmentofpumpkinseed inseveralEuropeancountriesbegan in1885.ThisyearcouldalsobethetimeofintroductionintotheBalkans,specificallySlovenia(Povž&Šumer,2005)andBulgaria(Holčik,1991).The pumpkinseed has been reported inGreece since 1885 (Holčik,1991),althoughthefirstestablishedpopulationwascited100yearslater by Economidis, Kattoulas, and Stephanidis (1981; AliakmonRiver). In Serbia, pumpkinseed was first recorded in the 1930s intheTisa River (Lenhardt etal., 2011) and inBosnia–Herzegovina atthe same time (Sofradžija, 2009). The species was reported in theCroatian part of the Danube River basin in the 1920s and 1930s(Plančić,1946). InCroatia, itwas translocated from theCrnaMlakafishfarmintothekarstLikaRiverbylocalfishermenasearlyas1937(Plančić, 1946; Vuković & Ivanović, 1971). Pumpkinseed was firstreported in the Turkey-Thrace in the Ipsala Canal-Edirne (Erk’akan,1983) andGala Lake in the1980s (Özcan, 2007) and in theVardarRiverinFYRMacedoniaatthesameperiod(Kostov,Rebok,Slavevska-Stamenković,&Ristovska,2010).IntheGreatPrespaLakesharedbyAlbania,GreeceandFYRMacedonia,pumpkinseedwasobserved in1994(Shumkaetal.,2008),whilethemostrecentoccurrenceofthisspeciesintheBalkanswasintheKrupacReservoirinMontenegroin2003(D.Milošević,persobs.).Pumpkinseeddispersal,aidedlargelybylocalfishermenusingthemasbait,hasbeenenhancedwithcommoncarp stocking practices (Ağdamar etal., 2015). Pumpkinseed is cur-rentlydistributedinvirtuallyallthewaterbodiesoftheBalkans.

Largemouth (black) bass (Micropterus salmoides, Centrarchidae)was introduced repeatedly to theBalkanPeninsula (Povž&Ocvirk,1990). Itwasfirst introducedtoEuropeandtotheBalkans in1877(Holčik,1991) foraquaculturepurposes. InCroatia, itwas rearedatseveral fish farms until 1935 (Fijan, 1966).At that time, several at-tempts of adaptation to inlandwatersweremade, all unsuccessful(Fijan,1966;Povž&Ocvirk,1990).The first reports in theDanube

Riverbasindatebackto1957(Lenhardtetal.,2011),althoughintheSerbianpartoftheDanube,itwasnotreporteduntil1984(Maletin,1988).InCroatia,largemouthbasswereintroducedasecondtimeinthe late 1980s for aquaculture (M. Piria, personal observation) andwerefound in1990 inVaraždinhydropowerreservoir,obviouslyre-leased for angling (Mrakovčić, 1992). For the same purpose, large-mouthbasswasreleasedinSloveniain1993intheVogrščekreservoirand likely translocated in 2004 into stillwater (Prekmurje) (Povž &Šumer,2005).InBosnia–Herzegovina,itwasfoundintheVrbasRiverinthe2000s(Radević,2001),althoughthetimeandpathofintroduc-tionisunknown.Althoughpresent intheDanubebasin,andseveralclosedfreshwatersystemsintheBalkans,ithasnotachievedsuccessinnaturalreproductionandexpansion(D.Zanella,personalobserva-tion).Nevertheless, largemouthbasswasobservedin2003neartheestuariesoftheLessinistreaminGreece,asalikelyescapeeandwasstatedascauseofconcern(Corsini-Foka&Economidis,2007).

The eastern mosquitofish and the common mosquitofish(Gambusia affinis,Poeciliidae)wereintroducedtoEuropeinthe1920s(Vidal,García-Berthou,Tedesco,&García-Marín,2010).Theirinvasionwassuccessful,andnegativeimpactsonnativecommunitieshaveledthemtobeconsideredamongthe100worstinvasivespeciesworld-wide(Cote,Fogarty,Weinersmith,Brodin,&Sih,2010andreferenceswithin).Severalhundredsofspecimensofeasternmosquitofish,andnotonly12ascitedinVidaletal.(2010),weresuccessfullyintroducedintoSpain(Cáceresprovince)in1921tocontrolmosquitolarvaeandpupae.In1922,theyweretransferredtoLagodiPorto,Ostia,PaludiPontine andVetralla in Italy (Radošević, 2013) and then to almosteveryEuropeancountry,aswellastoAsiaandtheMiddleEast(Coppetal.,2005;Radošević,2013)andhavesincespreadsuccessfully toover40countries(Welcomme,1988).ThesameeasternmosquitofishpopulationfromSpain(namedGambusia patruelis,Poeciliidae)arrivedatCroatiainspring1924,whereintroductionsbeganintheTrogirareaof Dalmatia. In June 1924, eastern mosquitofish was translocatedtothePorečmarshandtheenvironsofVrsar,RovinjandPulaintheIstrian Peninsula and not in Italy as stated byVidal etal. (2010). InAugust 1924, easternmosquitofishwere translocated toKrk Island(Radošević,2013).Twelvespecimensofcommonmosquitofisharrivedin1926toRovinj,Croatiadirectly fromtheUSA,as theyweresup-posedtobemoreresistanttocoldweatherthantheeasternmosqui-tofish(Radošević,2013;Vidaletal.,2010).InSlovenia,mosquitofish(speciesunknown)arrived in theKoper salternarea in1927,wheretheymanagedtosurvive(Radošević,2013).

Recently, a single male common mosquitofishwas identified inSlovenia,inapondbetweentheRižanaandArorivers.However,thegonopodiummorphologyofthisspecimen,andthevariabledorsalfinraycountofotherspecimensfromthesamelocality,indicatesthatthecommonmosquitofishwasalsointroducedandsubsequentlyhybrid-izedwiththeeasternmosquitofish(Veenvliet,2007).InNorthAmerica,thesetwospecieshybridizeandeasternmosquitofishgenotypestendtooutcompeteandevenreplacethoseofthecommonmosquitofishwhere they coexist naturally or by introductions (Vidal etal., 2010;Walters&Freeman,2000).ArecentgeneticstudysuggestedthatthemosquitofishinEuropeshouldbereferredtoaseasternmosquitofish;

16  |     PIRIA et Al.

unlessthegonopodiummorphology,finraycountsorgeneticidenti-fication clearly demonstrates that it is commonmosquitofish (Vidaletal.,2010andreferenceswithin).

AtaroundthesametimeasintroductionsbeganinCroatia(1924),theeasternmosquitofishalsoarrivedatBosnia–Herzegovinaandsoondispersed throughout the whole territory of the former Yugoslavia(Vuković & Ivanović, 1971),with the exception of Serbia (Lenhardtetal.,2011;Simonovićetal.,2013).InFYRMacedonia,easternmos-quitofishwerefirstintentionallyintroducedintoDojranLakein1924(referredtoascommonmosquitofish,Apostolski,Petrovski,Popovska,&Sidorovski,1956).ThespeciesspreadrapidlytoinhabitthelargestriversofFYRMacedonia (VardarRiver,Bregalnica,etc.). InBulgaria,itwasintroducedin1924(Uzunova&Zlatanova,2007)andthefirstsuccessful breeding attempts were made near the Tundzha River(Mikov,2005).Thesefishweremassivelybredandintroducedasabi-ologicalagenttoBulgarianmalaria-richareas,suchasswamps,lakes,coastallakesandwetlandsneartheBlackSeashoreandtheDanubeRiver(Zarev,2012).InMontenegro,easternmosquitofishwereintro-ducedintothemarshesaroundthecityofBar inthe1930sforbio-controlpurposes (Tischler,1950)andnot in1957when itwas firstreported(Drecun,1957),asmalariawasvirtuallyeradicatedinEuropein the1950s (Gachelin&Opinel,2011).According toShumkaetal.(2008),easternmosquitofishwerefirstintroducedtoAlbaniain1979,although the first introduction likely occurred earlier, in the 1950s,when active eradication campaigns againstmalariawere conducted(Berger, 2015). The first documented introduction of mosquitofishtoGreecewasG. patruelisfromItalyandFrance,between1927and1937(Livadas&Sphangos,1941),appearingalsoinEasternThraceatthesametime,alsointroducedforbiologicalcontrol(Geldiay&Balık,1988).

Paddlefish is native to the Mississippi River basin in the USA(McClanes,1974).BurtsevandGershanovich(1976)andReshetnikovetal.(1997)reportedtheywerefirstintroducedintoRussiaforfarmingand ornamental purposes in 1970 and 1974, respectively.They en-terednaturalwaters,althoughnaturalreproductionwasnotrecorded(Reshetnikov,2003a).PaddlefishwererecordedintheupperDanubeRiver following introductions for aquaculture in Hungary (1986),Germany (1987) andAustria (1990) (Zauner, 1997), althoughHolčik(2006)claimsthisspecieswasreleasedintonaturalwatersbyaquarists.InBulgaria,paddlefishwereintroducedfrom2002to2004(Uzunova&Zlatanova,2007).IntheSerbianstretchoftheDanubeRiver,paddle-fishwerefirstrecordedintheLowerDanubeRiveratrkm426in2000(Simonović,Marić,&Nikolić,2006)anddownstreamoftheIronGate(Đerdap) IIdam (rkm863) in2006 (Lenhardtetal.,2006;Simonovićetal.,2006).PaddlefisharecurrentlyrearedatfishfarmsinGermany,Austria,CzechRepublic,Bulgaria,RomaniaandHungary(Bogutskaya&Naseka,2006;Hubenova,Zaikov,&Vasileva,2005;Prokeš,Baruš,&Peňáz, 2000;Vedrasco, Lobchenko,&Billard, 2001) andwere in-troducedtotheGospičfishfarminSloveniain2012,alsoforfarming(Povž,2012).VassilevandPehlivanov(2005)reportedthepresenceofpaddlefishfryinthelowerDanube,implyingtheirsuccessfulacclima-tizationandpossiblenaturalization.DespiteElvira’s(2000)assumptionthatpaddlefishhavelikelybecomenaturalizedinRussianrivers,Holčik

(2006)consideredthatnaturalizationofpaddlefishintheDanubeRiverishighlyunlikelyduetothescarcityoffloodplainzones,oxbowlakes,lagoonsand innerdeltas in theUpperDanube, andDniester–LowerDanube ecoregions as favourable habitats for the larger planktoniccrustaceansthatformthedietofthepaddlefish.Despitethis,themostrecentadultpaddlefishspecimenwascaughtintheCroatiansectionofthelowerDanubeRiveratrkm1418,neartheKopačkiRitNatureParkin2011,withoutmentionofwhetherthisspecimenwasintroducedinanearlylifestageordevelopedinnature(Jelkić&Opačak,2013).InGreece,paddlefishwasintroducedfromHungary(1995)andtheUSA(1997)tofish-farmpensnearLakePamvotis(Ioannina);however,ac-cordingtoLeonardos,Kagalouetal.(2008)wasunabletoacclimatizeandwassoonextirpatedfromthelake.AccordingtoPaschos(2002)andBarbierietal. (2015), there isnoevidenceofany releaseof thespeciesinthewildanywhereinGreece.

Striped/whitebass(Morone saxatilis × M. chrysops) isasignificanthybridofastripedbass(Morone saxatilis,Moronidae)femaleandwhitebass(Morone chrysops,Moronidae)male,originallyculturedintheUSA.ItwasintroducedintoEuropeandNorthernAfricaasanaquaculturefish(Nelson,1994).In2010,amalespecimenofthewiper/sunshinebasshybridwascaughtbycommercialfishingnetintheCroatiansec-tionoftheDanubeRiverandwasassumedtohaveescapedfromanaquaculture facility (Safneretal.,2013).However,according toTateandHelfrich(1998),thesehybridsarefertilebutcanproduceonlyafewoffspring,althoughHodson (1989) stated that femalesproducesufficient number of eggs for successful reproduction once a year.Althoughnotcurrentlybelievedtobeaseriousthreattothefishcom-munity(Safneretal.,2013),cautionshouldremain.

Sailfin molly (Poecilia latipinna, Poeciliidae) is a North Americanspecies thatwas introduced to several countries around theworldthroughtheornamental fish trade. In theBalkans, there isonlyoneknown population in Greece; it is a female-dominated feral mollypopulation that inhabits the brackish, geothermally heated LakeVouliagmeninearAthens(Koutsikosetal.,2012).Thispopulationwaspositively identified as the sailfinmolly based onmeristic traits (N.Koutsikos,unpublisheddata).

4.3 | Asian introductions

Thefirstfishintroductions,ingeneral,beganwithcommoncarp(Balón,1995), although their origins remain unknown (Balón, 2006; Coppetal.,2005).Recordsfrom7000to6000BC indicatethat thisspe-cieswasnativetotheriversdrainingintotheEuropeanpartofPonto-Caspianregion(Tsepkin,1995),originallydomesticatedinEuropebytheRomans(Balón,2006),whileitwaslatertranslocatedbetweenandwithinEuropeandAsiaasafoodsource(Coppetal.,2005).InmostBalkancountries, commoncarp is considerednativeand isonly re-portedasintroducedinSloveniaandBosnia–Herzegovina(TableS1),whilethedebateonitsorigincontinues(Govedič,2005).

The history of goldfish introductions in Europe dates back to1611 (Copp etal., 2005), although the exact time of introductionto theBalkans isunknown. Itwas likely first introduced toSloveniaandCroatia in the late19thcentury forornamentalpurposes (Povž

     |  17PIRIA et Al.

&Ocvirk, 1990;Povž&Šumer, 2005).Currently, fiveBalkan coun-triesreportthisspeciesaspresent.Insome,goldfisharewidespread,whileitsdistributioninothersismoderatetorestricted(TableS1).Thissuggests threepossibilities: (i) theydonotspreadeasily; (ii) there isa lackofdatafortheBalkanarea;or (iii) theyaremistakenforgibelcarp or crucian carp (Carassius carassius, Cyprinidae) as reported byKalous,Bohlen,Rylková,andPetrtýl(2012).However,forAlbaniaandMontenegro,it isstillpossiblethatgoldfishisdispersedthroughtheOhrid-Drin-Skadarsystemandtransportedtoneighbouringwatersys-tems,suchasPrespaLake(Liaskoetal.,2010).

Introductions of gibel carp to Europe are linked with releasesof common carp imported to Europe from Chinese aquaculture inaround1611–1691 (seeCoppetal.,2005),althoughthehistoryofits introductions, taxonomical status and native distribution is stillnotclear.Someauthorshavesuggested thatgibel carp is commoninDanubianmarshes(Kovachev,1922),althoughgibelcarpandcru-ciancarpwerementionedasvarietiesandnotasseparatespecies.ThiswasalsostatedforBulgaria(Drensky,1948),withnotesofthepresenceofbothsexesintheDanubeandthesuggestionthattwo-year-oldfemalesbecamemale(Busnita,1938;citedbyBerg,1964asanincorrecthypothesis). Is itpossiblethatthephysicalsimilarityofthebrownvarietiesofgoldfish,gibelcarpandcruciancarpresultedinthemisidentificationoftheseintroducedspeciesforthenativecru-ciancarp,assuggestedbyCoppetal. (2005)?AlthoughithasbeendebatedwhethergibelcarpisalieninallofEurope(Lusková,Halačka,Vetešník,&Lusk,2004),newresearchbasedonmolecularanalysissuggeststhatgibelcarppopulations inCentralEuropeandpartsofEasternEuropearenative, resulting fromnaturalpost-glacial rangeexpansion, while the rest of Europe was colonized anthropogeni-cally (Bohlen, Lamatsch,&Petrtýl, 2013).Basedon this, gibel carpinBulgaria isconsideredtobeanativespecies,translocatedtotheNorthernAegean basin, especially Strymon andNestos rivers.Thefirstfindingsinthisarearevealedonlythepresenceofgynogeneticfemales (Economidis, 1974),while a sex ratioof3-4: 1of females:malesinStrymonwasreported(Apostolou,2002),indicatingthatthespeciesisarecentinvaderintheseareas.ThearrivalofgibelcarpintheDniester–LowerDanubeecoregionisthoughttohaveoccurredin1912asaconsequenceofitsnaturalspreaddowntheDanube(Coppetal., 2005; Perdikaris etal., 2012). More than five decades later,inthelate1950s,gibelcarpwasdocumentedintheSerbianpartoflowerDanubeattheborderwithRomania(Plančić,1967),whenrapiddispersalintotheBalkansbegan.Bythe1980s,ithadalreadyspreadthroughout most of former Yugoslavia, even reaching the Adriaticdrainage,andwasbecomingcommerciallyrelevant (Vuković,1982).Thereareseveralsuggestionsastohowthisspecieswasintroducedto the freshwaters of Greece (Economidis, Dimitriou etal., 2000;Tsoumani,Liasko,Moutsaki,Kagalou,&Leonardos,2006).Morere-cent studies suggest that the specieswas either translocated fromLakeKerkini(StrymonRiver)toLakePamvotisorwasintroducedfromItalytothesamelakeduringthe1950sorlater(Leonardos,Kagalouetal.,2008;Perdikarisetal.,2012).

Themost recent findof anewCarrasius complex species in theBalkans is the gin-buna carp (Carassius langsdorfii, Cyprinidae), first

detectedinGreecein2009(Tsipas,Tsiamis,Vidalis,&Bourtzis,2009).ThisspecieswasfirstreportedinEuropein2000(Kalous,Šlechtová,Bohlen, Petrtýl, & Švátora, 2007) and seems to have only recentlybeen introduced. The pathways and vectors of its introduction toEurope are not known, but itwasmost likely an unintended intro-ductionaccompanyingothercyprinidfishes(Rylková,Kalous,Bohlen,Lamatsch,&Petrtýl,2013).Geneticdatahaverevealedtheexistenceofatleasttwolineagesamongthegin-bunacarpthathavebeenin-troducedtoEurope (Kalousetal.,2007;Tsipasetal.,2009),bothofwhicharefoundinGreece,andoneisfoundinBosnia–Herzegovina(Rylkováetal.,2013).

TheinvasionofgibelcarpcoincidedwiththeintroductionoftheChinesecarps,that isgrasscarp,silvercarp,bigheadcarpandade-clineofthenativecruciancarppopulations,attributedtoashiftfromcleartoturbidwater-preferringspeciesintheDanubeRiver(seeCoppetal.,2005).ThefirstknownintroductionfortheBalkanareawasre-cordedinAlbania(1959),whenfingerlingsofgrasscarpandsilvercarpwereimportedfromChina,withreintroductionoccurringin1968withtheimportsofbigheadcarpandlikelyWuchangbream(Megalobrama amblycephala, Cyprinidae), (Shumka etal., 2008). The same authorsmentioned that all these introduced species, including crucian carp,successfullyreproducedin1972.ForthemajorityoftheBalkanscoun-tries,cruciancarparenativefishalthoughtheyareconsideredalienforAlbaniaandTurkey-Thrace(Shumkaetal.,2008;Tarkanetal.,2015).According to the latest information, crucian carpwere unintention-ally introducedtoLakeTaşkısı inTurkeyinthe1950s(S.Tarkan,un-publisheddata),andnotin1990,aspreviouslyreported(Tarkanetal.,2015),asitwasobviouslymisidentifiedduetoitssimilaritywithotherCarassius species. Reports of crucian carp introductions by Shumkaetal. (2008) inAlbaniacouldbeerroneousduetoa lackofdataongibelcarppresence.IsitpossiblethatthisfishalsoarrivedfromChinatogetherwithotherChinesecarpsandwasmistakenforgibelcarp?ThiswassuggestedbyrecentresearchofSkadarLakeinMontenegro,whereanabundantgibelcarppopulationwasfound,thoughnocru-ciancarp(Mrdak,2009).

Dueto the roleofAsiaticherbivorous fish in increasingproduc-tion in fish ponds and their commercial significance, grass carp fin-gerlingswereimportedtoSerbiaattheendof1963fromHungarianponds(Mihajlović&Ćirić,1969).Thereafter,probablyin1964,silvercarpwerealsoimportedfromHungarytothesamefishfarm(Vuković&Kosorić,1978).It isnotdocumentedwhenbigheadcarpwereim-ported intotheBalkans,although itseemsthat this ratheroccurredtogetherwithsilvercarpinSerbia(Fijan&Vojta,1969).Inthesameperiod, Chinese carp species were introduced to Bulgarian ponds(Uzunova&Zlatanova, 2007), and severalyears later, in 1966, alsointoCroatianfishponds,alsofromHungary(Fijan&Vojta,1969).InSlovenia, the first introductionswere performed fromCroatian fishfarms(Povž&Ocvirk,1990;Povž&Šumer,2005)butafter1966(Fijan&Vojta,1969),andnotin1963aspreviouslyreported(Povž&Šumer,2005;Welcomme,1988).FishesweretranslocatedamongtheformerYugoslavnationswithoutexactknowndates(Povž&Ocvirk,1990).InFYRMacedonia,thefirstattemptstorearChinesecarpspecieswerein theperiod1965–1975at several fish farms locatedon theCrna

18  |     PIRIA et Al.

andVardar rivers,without a known source of introduction (Kostov,Ristovska, Prelić, & Slavevska-Stamenković, 2011). Silver carp andgrasscarpinGreecewereintroducedbetweenthe1970sand1980sforthebiologicalcontrolofplantsandplanktonicorganismsinaqua-cultureponds(Zenetosetal.,2009).

TopmouthgudgeonwereintroducedintoEurope(Romania)inthe1960sfromtheYangtzeRiver (WuhanProvince)concomitantlywithChinesecarpspecies’ importsand thenspread throughtheDanubeRiver basin (Gozlan etal., 2010). It has also been documented thattopmouthgudgeonwereintroducedintoLakeSkadarinAlbaniaasafood item for predatory fishes in hatcheries (Wildekamp,VanNeer,Küçük,&Ünlüsayın,1997),possiblypriorto1970(Gozlanetal.,2010;Kneževićetal.,1978;Wildekampetal.,1997),whichcouldrepresentasecondsourceofdispersal.BothsourceswerementionedaspotentialintroductionroutesforSmallPrespaLake,GreatPrespaLakeandtheAliakmonRiverinGreeceinthe1970s(Bianco,1988),forŠaskoLakeinMontenegroin1977(Kneževićetal.,1978),fortheLugomirRiverinSerbiain1978(Cakić,Lenhardt,Kolarević,Micković,&Hegediš,2004)andfortheMericRiver-İpsalainTurkey-Thraceinthe1980s(Ekmekçi&Kırankaya,2006).AthirdsourceofdispersalcouldbetheUkraine,whereChinesecarpspecieswereimportedforaquaculture,asthefirstBulgarianrecord in1976isthoughttobetheresultofthis invasion(Marinov,1979).ThesouthernBulgarianpopulationcouldbethere-sultof invasion fromsouth-eastEurope (Albania) (Wildekampetal.,1997).InAlbania,topmouthgudgeonoccurredintheperiodbetween1960and1970(Rosecchi,Crivelli,&Catsadorakis,1993;Wildekampetal.,1997)andnotsince1998asclaimedbyShumkaetal. (2008).Later, likely via dispersal within the Upper Danube and Dniester–LowerDanubeecoregions,thisspecieswasrecordedintheSavaRiverinCroatia in1985 (Habeković&Popović,1991)and in theJasenekstreamnearCelje inSlovenia in1986 (Povž&Ocvirk,1990).Muchlater, the presence of the topmouth gudgeon in FYR Macedonianopenwaterswas first recorded in 1996 (Georgiev, 2000), althoughtheywereobviouslypresentsincethe1970sintheGreatPrespaLake,sharedbyAlbania,GreeceandFYRMacedonia(Rosecchietal.,1993).

In addition to gibel carp, topmouth gudgeon and the Chinesecarps,white amur bream (Parabramis pekinensis, Cyprinidae)was al-readyintroducedtotheGreatPrespaLakebythelate1970s(Rosecchietal., 1993). Thus, they could have also occurred then in GreeceandFYRMacedonia, despite a lackof reports.However, theywereintroduced into Small Prespa Lake much earlier (in 1959) by theAlbanians, either intentionally (Crivelli, 1995; Economidis, Dimitriouetal.,2000;Rosecchietal.,1993),oraccidentallyasescapees fromfishfarms(Perdikarisetal.,2010andreferenceswithin).Nevertheless,itappearsthatwhiteamurbreamhasnotbecomeestablished,sincenone have been recently caught for several years (Perdikaris etal.,2010)andthestatusofthisspeciesremainsunknown(Crivelli,1995;Froese&Pauly, 2015).TwomoreAsian species, blackAmur bream(Megalobrama terminalis,Cyprinidae)andblackcarp(Mylopharyngodon piceus,Cyprinidae),wereintroducedintoSkadarLakesharedbetweenAlbaniaandMontenegro,likelybytheAlbaniansin1963(Holčik,1991)andwererecordedinMontenegroin1973(Vuković&Knežević,1978)and1983(Knežević&Marić,1986).Inthesameperiod,amurbream

wasintroducedintoBulgaria(in1964),whereitwasusedforthebio-logicalcontrolofthezebramussel(Dreisena polymorpha,Dreissenidae)population in theOvcharitza reservoir (Uzunova&Zlatanova,2007)andrecentlyinSlovenia(2004)foraquacultureandanglingpurposes(Povž,2009a).Boththesespecieshaveacclimatizedtotheirnewen-vironments,althoughnaturalreproductionhasnotbeenconfirmed.

Chinese sleeper (Perccottus glenii, Odontobutidae) is one of themost invasive fish species in Eastern and Central Europe in recentdecades (Coppetal., 2005).Chinese sleeperpossessesanopportu-nisticandaggressivebehaviourmakingittheperfectinvader(Ćaleta,Jelić etal., 2011). Its accelerated expansion has been documentedonseveraloccasionsinassociationwithAsiancyprinidstockings(seeGrabowskaetal.,2011)andthisspecieswasintroducedonmultipleoccasionsintovariouspartsofAsiaandeasternEurope(Bogutskaya&Naseka,2002;Uzunova&Zlatanova,2007).SinceitsfirstappearanceinthecatchmentoftheTisaRiver(Hegedišetal.,2007;Koščo,Lusk,Halačka,&Luskova,2003;Simonovićetal.,2006), its spread in theDanubeRiverwasonlyamatteroftime(Hegedišetal.,2007;Jurajda,Vassilev,Polačik,&Trichkova,2006).ItsfirstoccurrenceintheBalkanPeninsulawas in 2003 in the Serbian section of the lowerDanubeRiver(Gergely&Tucakov,2003).ItwasrecordedintheBulgarianpartof theDanubeRiverand itsmarshesand lakes in2006 (Uzunova&Zlatanova,2007),andthelatestrecordwasfrom2008fromachannelneartheSavaRiverinCroatia(Ćaleta,Jelićetal.,2011). Intheearlystagesofitsinvasion,highdensitiesofChinesesleeperwererecordedatmany locations inEurope(Bogutskaya&Naseka,2002),althoughits prevalence in the Balkans remains moderate, as is the case forSerbia (Simonović etal., 2006) and restricted in Bulgaria (Uzunova&Zlatanova,2007),while inCroatia, following the first findings, itspresencewasnotlaterconfirmed(Ćaleta,Jelićetal.,2011).Althoughsomehavepredictedthatthisspecieswillbeabletoestablishasta-blepopulationinthenearfuture(Uzunova&Zlatanova,2007),havinganadverse impactonthenative ichthyofauna (Reshetnikov,2003b),andthusstrongrestrictivemeasureshavebeenadvised(Ćaleta,Jelićetal.,2011), itappearsthatithasfailedtoinvadetheupperpartoftheDanubebasin.AccordingtoKoščoetal. (2003),thisspecies isapoorswimmeranditmaybeexpectedthatitsdispersalwithinariversystemwouldmainlybedownstream.However,thepossibilitythatitisstillgoingthroughtheprocessofacclimatizationshouldnotberuledout(Simonovićetal.,2006).

4.4 | European introductions

4.4.1 | Eastern Europe

FiveofthePonto-Caspian(P-C)gobies(Pisces,Gobiidae)havebeenreportedintheBalkanstodate:bigheadgoby,monkeygoby,roundgoby (Neogobius melanostomus, Gobiidae), racer goby (Babka gym-notrachelus,Gobiidae)andwesterntubenosegoby(Proterorhinus sem-ilunaris, Gobiidae).Native distributions of (P-C) gobiesweremostlyconfined to the lower reachesof theDanubeRiver and the littoralzoneoftheBlackSea;thus,theyarenativespeciesforBulgariaandTurkey (Vassilev, Apostolou, Velkov, Dobrev, & Zarev, 2012). The

     |  19PIRIA et Al.

Djerdap Gorge represented the uppermost range boundary of big-headgoby,monkeygoby,racergobyandroundgoby(Miller,2003).Historically,thewesterntubenosegobyhadthefarthestrangeuptheDanubeofallthePonto-CaspianGobies,beingfoundasfarupstreamasViennaandthemouthoftheMoravaRiverontheCzech/Slovakborder (Roche,Janač,&Jurajda,2013).Thus,thewesterntubenosegobyisconsideredasnativetoCroatia(Jakovlićetal.,2015;Polačiketal., 2008), but not to Serbia or Bosnia–Herzegovina (Simonović,2006). Thedistribution, invasion and rangeexpansionof theseP-Cgobieshasbeenwelldocumented(Rocheetal.,2013)andtheycon-tinue to expand their distribution across the European continent(Jakovlićetal.,2015).ThemostrecentrecordofrangeexpansionwasforthebigheadgobyintheSlovenianpartoftheSavaRiverin2015(Simonovićetal.,2017)andforthemonkeygobyintheEvrosRiverin Greece in 2011 (Zogaris & Apostolou, 2011). However, cautionisneeded inacceptinganalienstatus for this record inGreeceduethelackofpreviousichthyologicalstudiesoftheEvrosRiverandthecloseproximityofthisrivertothespecies’nativedistributionrange;althoughitisunlikelythataconspicuousmedium-sizedfishwouldgounnoticed(Zogaris&Apostolou,2011),whichmayindicatenewrangeexpansion inGreece inlandwaters.P-Cgobiesarenot reported forAlbaniaandMontenegro.

Aristotle’scatfishisaspeciesendemictothelowerAcheloosRiversystem(westernGreece,Economidis,1991)listedintheIUCNRedListofThreatenedSpecies asdatadeficient (Crivelli, 2006).PopulationsoccurintheTrichonis,Lysimachia,OzerosandAmvrakiaLakes(LowerAcheloosbasin),aswellasthePamvotis,VolviandYlikiLakesviatrans-location (Barbieri etal., 2015; Leonardos,Tsikliras, Batzakas,Ntakis,&Liousia,2009).Recently,twospecimensofAristotle’scatfishwereidentified froma small artificial lake in theMontanadistrict (north-west Bulgaria), which is the first find outside Greece. In all likeli-hood,thespecieswas introducedtoBulgariaasaseedingfishfromMacedonia(northernGreece,Pehlivanov&Atanasov,2007).

Pikeperch was introduced in the Mladost reservoir in FYRMacedoniain1967alongwithcarpoffspringfromCroatia(Georgiev&Naumovski,1982),where they successfully reproduced. Itsdistri-butionisrestrictedonlytothisreservoir.Inthe1980s,pikeperchwasalso introduced toFierzaLake inAlbania for aquaculture, also fromtheformerYugoslavia,althoughitscurrentstatusisunknown(Shumkaetal.,2008).

Eurasianperch(Perca fluviatilis,Percidae)wasintroducedtoFierzaLakeinAlbaniaataboutthesametimeandforthesamepurposeaspikeperch(Shumkaetal.,2008),withpopulationsbecomingnatural-izedintheinlandwatersofAlbania.

Short-snoutedpipefish (Syngnathus abaster, Syngnathidae) is na-tive to theMediterranean andBlackSeas and to theAtlantic coastfromGibraltartothesouthernBayofBiscay.Thefirstrecordoftheshort-snoutedpipefishinSerbiawasin1998(Sekulić,Cakić,Lenhardt,Vucić,&Budakov,1999).ThisintroductionwaslikelyviaballastwateroriginatingfromtheBlackSea(Cakić,Lenhardt,Mićković,Sekulić,&Budakov,2002).AstudyonthefishcommunitiesofthelowerDanubeRiver (Djerdap reservoir) revealed an increase in the abundance ofshort-snoutedpipefishingrassylittoralhabitats(Simić&Simić,2004),

althoughtheirdispersalremainslimitedandtheirimpactinsignificant(P.Simonović,personalobservation).

Commonbream(Abramis brama,Cyprinidae) isnativetomostofEurope(includingtheBalkanPeninsula)andwesternAsiabutnottoFYRMacedonia(Kostovetal.,2015andreferenceswithin)where22specimenswere first captured in theTikvesh Reservoir, Crna River(Vardar River basin), in 2014. Common bream was possibly intro-ducedfromBulgariaorSerbia,accidentallyimportedwithcagesfromBulgariabetween2006and2008orwithfryfromSerbiaforstockingsportandrecreationalfisheriesin2008(Kostovetal.,2015).Theim-pactofcommonbreamonnativefishpopulationsandecosystemsinFYRMacedoniaisstillunknown.

European grayling is native to the Black Sea drainage area andto theDalmatiaecoregion, includingSloveniaandCroatia (Froese&Pauly,2015;Mrakovčićetal.,2006),butnottothesouth-eastAdriaticecoregion,thatisAlbaniaandMontenegro.Europeangraylingwasin-troducedtotheSkadarLakedrainageareainthe1960s(Drecun,1962;Knežević, 1981) and is present in the inlandwatersofMontenegro(MoračaRiver)andmayalsobepresentinSkadarLake(Talevskietal.,2009).InBulgaria,Europeangraylingwasalsointroducedinthe1970sinasmallhighlandreservoir(Uzunova&Zlatanova,2007)butitisas-sumedtohavesincedisappeared.

Tench (Tinca tinca, Cyprinidae) is native tomost of Europe, butnottoAlbania.Thisspecieswasintroducedtothiscountryinthelate1980sforaquaculturepurposes(Rakaj&Flloko,1995;Shumkaetal.,2008)andhassincebecomeestablishedinthetransboundaryPrespaLake(Shumka,Aleksi,Sandlund,Cake,&Mali,2013).

Letnicatrout(Salmo letnica,Salmonidae)wasintroducedtoSerbiaandGreecefromLakeOhrid.InGreece,itwasintroducedtothetrans-boundary Prespa Lakes (Crivelli etal., 1997) in the 1950s,where itmayhavehybridizedwith the localendemic troutSalmo peristericus Karaman,1938.ItscurrentstatuswithinGreekterritoryisunknown(Barbierietal.,2015);however,asingle individualwascaughtagainin2011inGreatPrespaLake(Koutseri,2012).IntroductionstoSerbiabegan in 1953 and 1954, when fry (106 each year) were stocked(Janković&Raspopović,1960a).Afterwards,fertilizedroewastrans-ferredtothenewhatcherysetontheLakeVlasinareservoirfortheestablishment of a commercial trout fishery there. Since the late1960s,roewasreintroducedalmosteveryyear,sinceitseemsthatthisspeciesdidnotnaturalizeinthereservoir,despiteitsfastgrowthandgreatyield. It isnotknownwhetherthisspecieshybridizeswiththenativebrowntrout,butfromtheregularroeimportsoveralong-termperiod, itappears that letnica troutwere feral thereanddiminishedafterthecessationofreservoirstocking.

4.4.2 | Northern Europe

Five fish species from Northern Europe have been introduced tothe Balkans, that is Europeanwhitefish, peled, vendace (Coregonus albula, Salmonidae), Arctic charr and Atlantic salmon (Salmo salar,Salmonidae).

ThefirstknownintroductionoftheCoregonusgenusdatesbackto1937,whenlarvaeofCoregonussp.wasimportedfromAustriatothe

20  |     PIRIA et Al.

PlitviceLakesinCroatia(Habeković,1978).Thisintroductionwasnotsuccessful,butoneyearlater,anewshipmentoflarvaewasimportedand released (Plančić,1938). It isnotknownwhether this introduc-tionsucceeded(Taler,1949),althoughCoregonussp.formspartoftheichthyofaunaofthePlitviceLakes(D.Zanella,personalobservation).ThesecondintroductioninCroatiawascarriedoutin1977and1978,whenlarvaeofEuropeanwhitefishandpeledwereimportedfromtheformerCzechoslovakiaandintroducedtotheDraganićifishfarmandPeručareservoirontheCetinaRiver(Habeković,1978;Jevtić,1991).Between these two introductions inCroatia, therewasa successfulintroductiontotheTavroposreservoir inGreeceinthe1950s, likelyforanglingpurposes(Petridis&Sinis,1993).Forthesamereason,ven-dacein1964,peledin1970andEuropeanwhitefishin1978werein-troducedtoBulgariaandsuccessfullyadaptedtotheconditionsofthelargeranddeeperBulgarianuplandreservoirs.Twoofthesespecies,peledandEuropeanwhitefish,naturalizedinseveralverylargemoun-tain lakes, such as Iskar and Dospat (Uzunova & Zlatanova, 2007).Much later,peledwas introducedtoSerbia intheDrinareservoir in1991foraquacultureandlaterforanglingpurposes(Simonović,2006)butthecurrentstatusofthisspeciesinSerbiaisunknown.Thesameyear,EuropeanwhitefishwasrecordedintheSlovenianpartofDravaRiver,followingitsdownstreammovementfromtheAustrianpartofthe river, and it has since been found continuously (Povž& Šumer,2005).Themost recent introduction of peled, in 1999, occurred inBosnia–HerzegovinaduringtherestockingofBoračkoLake.This in-troduction was successful, as confirmed by recent angler catches(Hamzić,Muhamedagić,&Lelo,2011).

The first known introductionof charr in theBalkans likelydatesbackto1928,whenSalvelinussp.fromItalywasintroducedtoKrnskoLakeinSlovenia.ThesecondintroductiontoSloveniafromAustriaoc-curredin1943inBohinjLakeforsportfishingpurposes(Povž&Ocvirk,1990).However,forbothSlovenianrecords,itisunclearwhetherthespecies introducedwereArctic charr or lake charr (Salvelinus umbla, Salmonidae)asbothscientificnameshavebeenusedintheliterature(Piriaetal.,2016b;Povž&Ocvirk,1990;Povž&Šumer,2005;Povžetal.,2015).ThenativerangeoflakecharraretheAlpineandsubalpinelakesinItaly(Trentino,AltoAdige),France(LakeBourget),Switzerland,GermanyandAustria,whiletheEuropeanrangeofArcticcharrextendsfromthenorthernAtlanticsouthwardtosouthernNorway,IcelandandsouthernGreenland,includingisolatedpopulationsintheNorthernUK,Scandinavia,Finlandand theAlps (Froese&Pauly,2015).AccordingtoEnglbrecht,Schliewen,andTautz(2002),thealpinepopulationsofArcticcharrwereusuallyincludedintoS. alpinus,butwerelaterrevisedtoS. umbla(Kottelat,1997).CouldthesourceoftheSlovenianpopu-lationsbeS. umblafromalpinepopulationsorS. alpinusintroducedtoItalyorAustria?InCroatia,theintroductionofarcticcharrisreferredtoasS. alpinus,whichwasintroducedfromBohinjLaketoKozjakLake(oneofthePlitviceLakes)in1963(Pažur,1970).Later,anglersreleasedarcticcharrinseveralinlandwatersorspecimensescapedfromfarms,althoughthedistributionofthisspecieshasnotyetbeenrevised(D.Jelić,personalcommunication).Recently,arcticcharrwasrecordedinthe Peruča reservoir andNeretva River (J. Budunski, personal com-munication). Itmayoccur intheNeretvaRiverduetotherestocking

ofseveralriversintheNeretvacatchment,e.g.LišticaRiver.ThiswasconfirmedbyŠkrijelj(2002),whostatedthattheNeretvahydropowerreservoirswererepeatedlyrestockedwiththisspecies.Furthermore,arcticcharrwasfirstintroducedtothewatersofBosnia–Herzegovinain1943,asS. alpinus toPlivaLakenearJajce, and this species soonpopulatedseveralreservoirsinthecatchmentsoftheDrina,NeretvaandTrebišnjicarivers,andKlinjeLake(Sofradžija,2009).TheSerbianliterature also refers to S. alpinus as an introduced species, first inKokinBrod reservoir in1943, then later inVlasinaLake (Simonović,2006),whileMontenegrindatasuggestthatS. alpinuswasintroducedin1955(Drecun,1960;Marić,1991). Incontrast,Welcomme(1988)suggested1928astheyearofS. alpinusintroductionfromSwitzerlandandAustriatoSerbiaandMontenegro.

The first introduction of Atlantic salmon in Croatia occurred inthe1980sforexperimentalhatcherypurposes intheestuaryoftheKrkaRiver,althoughthiswasnotsuccessful(Skaramuca,Teskeredžić,&Teskeredžić,1997).ThenextrecordinCroatiadatesfromthe2000sintheDravaRiver,whenanglerscaughtseveralspecimensofAtlanticsalmon(N.Šprem,personalcommunication),theiroriginremainingun-known.AtlanticsalmonwasintroducedtoGreecein1985forrearingintheEdessahatcheryinnorthernGreeceandtohatcheriesincentralGreece,PeloponneseandCrete(Economidis,Dimitriouetal.,2000).

4.4.3 | Western Europe

Thethree-spinedstickleback(Gasterosteus aculeatus,Gasterosteidae)isa small teleost fishconsidered tobeoneof themostwidelydis-tributedfreshwaterfishesintheworld(Froese&Pauly,2015).IntheBalkans, it isnativeto theriversof theAdriatic, IonianandAegeanbasinsofSlovenia,Croatia,Montenegro,AlbaniaandGreece(Barbierietal.,2015;Zanellaetal.,2015)andtotheDanubeRivermouthandBlackSeacoastofBulgaria(Biserkovetal.,2015).Thefirstrecordofthethree-spinedsticklebackinSerbiawasintheDanubeRiveratrkm92 in 1995 (Cakić, Lenhardt, & Petrović, 2000), although there areunverifiedearlierrecords(Lenhardtetal.,2011).Thespecies ispre-sentintheMuraRiver(UpperDanube)inSlovenia(Povžetal.,2015)andwas recently recorded in theCroatianpartof theMuradue todownstream spreading following a flood event (Lisjak etal., 2015).ThepopulationsoftheMuraandDravariversinSloveniaandCroatiaare of central European origin, as confirmed by genetic analysis (L.Zanella,unpublisheddata).Ahnelt,Pohl,Hilgers,andSplechtna(1998)reported that aquarists were responsible for the introduction andspreadofthethree-spinedsticklebackintheDanuberiversystemattheendof19thcentury,althoughthepossibilitythatthisspecieshadalreadyexistedintheDanubecannotbeexcluded.Thespecieshasnoeconomicvalue,anditsimpactonnativefishspecieshasnotyetbeeninvestigated(Lenhardtetal.,2011).

Brown trout is one of themost attractive recreational salmonidspecies.ItwasfirstintroducedtotheSočaRiver(GulfofVeniceecore-gion)in1906(Ocvirk,1989;Povž&Ocvirk,1990),whereithybridizedsuccessfullywithendemicsalmonids(Crivelli,1995).Inthelate1980s,onlyeightgeneticallypure(i.e.notintrogressed)marbletrout(Salmo marmoratus, Salmonidae) populations were identified in the upper

     |  21PIRIA et Al.

Sočabasin(Fumagallietal.,2002),whereasallotherpopulationswerefoundtobeintrogressedwithalientroutofDanubianandAtlanticor-igin(Berrebi,Povž,Jesenšek,Cattaneo-Berrebi,&Crivelli,2000;Snojetal.,2000).StockingofbrowntroutofAtlanticlineagehasalsotakenplace intheJadroRiver inCroatia (Dalmatiaecoregion), likely inthe1990s, in the habitat of the highly endangered endemic populationof softmouth trout (Salmo obtusirostris, Salmonidae), thought to beofancientorigin(Snojetal.,2007).Population-specificmicrosatellitealleleprofilesindicatehybridizationbetweenbrowntroutofAtlanticlineageandJadrosoftmouthtrout(Sušniketal.,2007).Thishasraisedconcerns that introductions of brown trout ofAtlantic andDanubelinages intotheAdriaticbasincatchmentmay leadtotheextinctionofendemictroutspecies (Mrdaketal.,2012;Snoj,Bogut,&Sušnik,2008).SupportiveevidenceforthiscanbefoundintheDanubeRiverbasin, that is introgressionof alienAt1haplotypeofAtlanticbrowntrout(Marić,Sušnik,Simonović,&Snoj,2006),whichcausedthelossof intraspecificvariability, following the introductionof alien strainsandachange ingeneticcompositionofnativebrowntroutstockofDanubelineage(Simonovićetal.,2014).Consideringtheimpactofin-troducedforeign-sourcedbrowntroutonnativebrowntroutstocksinSerbia,browntroutoftheAtlanticlineagewasfoundtobethemostinvasive alien brown trout strain in Serbia (Simonović etal., 2015).Introductions of brown trout ofAtlantic origin in the Balkanswerereported in the2000sformostcountries (TableS1),although intro-ductionslikelybeganwithearlyangleractivitiesandrestockingwithforeignlineages,suchasinCroatiainthe1970s(Jadan,Čož-Rakovac,TopićPopović,&Strunjak-Perović,2007).Despiteheavystockingwithalienstrainsof trout (Snojetal.,2009),nohaplotypesofAtlanticorDanubianbrowntroutphylogeneticlineageswerefoundinAlbania.

4.5 | African introductions

African species have only recently been recorded in the BalkanPeninsula.ThefirstintroductionwithNiletilapiaoccurredinGreeceinthe1980s,intofishfarmsnearthetownsofEpidavros(Peloponnese),Arta and Nafpaktos (western Greece, Katsaros & Fousekis, 1997;Perdikarisetal.,2010)andtheKremastaReservoir(westernGreece,Perdikarisetal.,2010;Barbierietal.,2015).NiletilapiainGreeceisapotentialnewcandidatespeciesforthediversificationofconventionalwarm-water aquaculture production, as local environmental condi-tions ingeothermalwatersgiveexcellentpotential for itsacclimati-zation(Perdikarisetal.,2010).ItsoverwinteringandsurvivalindeepditcheshavealreadyresultedinestablishedpopulationsinthelowerpartofArachthosRiver(nearArta,westernGreece,Perdikarisetal.,2010)andintheThermopylaehotspringscatchment(centralGreece,Barbieri etal., 2015). A similar situationwas reported for Slovenia,whereNiletilapiawasintroducedintothegeothermalsystemoftheToplastrugachannelnearBrežicein2008whereithasbecomeestab-lished(Povž,2009b).Theimpactofthisspeciescouldbeadverseonself-recruitingnativespeciesduetothestrongcompetitionofitshigh-density populations and dominant behaviour at spawning grounds(Perdikaris etal., 2010). Slovenian populations could be a potentialsource fordispersalofNile tilapiavia theSavaRiver system to the

LonjskoPolje floodplain area inCroatia,where theywere recordedin 2008, although thismight be the result of unintentional releasefromhome aquaria (D. Zanella, personal observation).Mozambiquetilapia (Oreochromis mossambicus,Cichlidae)was introduced intotheOvchariza reservoir inBulgaria in the1990s (Uzunova&Zlatanova,2007).Recently,onlyisolatedspecimensofMozambiquetilapiacouldbefoundinBulgaria(Uzunova&Zlatanova,2007),althoughtheactualpopulationsizeisunknown(A.Apostolou,personalobservation).

NorthAfricancatfish (Clarias gariepinus,Clariidae)haveoccurredinSloveniasince1997ingravelpitsneartheHungarianborder,likelyasaconsequenceofsportfishingactivities(Povž,2007),andin2007,theywererecordedintheOvcharitzaReservoirinBulgaria(Uzunova&Zlatanova,2007),althoughthesourceandstatusofbothintroductionsareunknown.Themostrecent introductionofNorthAfricancatfishoccurredin2010attheDraganićifishfarminCroatiaforaquaculturepurposes(MislavŠarić,personalcommunication).

4.6 | South American introductions

Thepirapitinga(Piaractus brachypomus,Serrasalmidae)isatropicalfishcharacteristic of theAmazon andOrinoco basins,where it inhabitslarge flooded rivers and lakes (Jégu, 2003).Outside its natural dis-tribution range, thepirapitinga ismostcommon in theopenwatersoftheUSA,duetoaquariumrelatedreleasesandfishfarmescapes(Nico&Fuller,2010).PirapitingahavealsobeenrecordedinEurope:in Spain (Leunda, 2010), Slovakia (Hensel, 2004), Poland (Nowak,Szczerbik,Tatoj,&Popek,2008),Croatia(Ćaleta,Tutmanetal.,2011),Montenegro(D.Mrdak,personalcommunication)andGreece(2012),where itwas possiblymisidentified as red piranhaPygocentrus nat-tereriKner,1858(Perdikarisetal.,2016),whichstillrequiresclarifica-tion (L.Vardakas, pers comm).All these individuals are assumed tohavebeenreleasedbyaquarists,andtheirestablishmentandimpactarecompletelyunknown(Froese&Pauly,2015).

Theguppyisoneofthemostpopularaquariumfishes.Itsviablepopulations have become established inmany hot springs orwarmeffluents of power plants inmany European countries (Hanel etal.,2011). In the Balkans, it is present in Pancharevo Lake near Sofia,Bulgaria(Uzunova&Zlatanova,2007),withanunknowndateofintro-duction.SeveralauthorsmentionitspresenceinAlbania(Haneletal.,2011;Shumkaetal., 2008),butwithoutanyother information.Thecurrentstatus,distributionorpotential impactof theguppy inbothcountries is unknown.However, recent research in Serbia suggeststhattheguppy(foundin1970)hasbecomenaturalizedinathermalstream, together with other ornamental organisms, allochthonousplantspecies(Vallisneria spiralis, Elodea canadensis and Lundwigia rep-ens)andthealiensnailMelanoides tuberculate(Milenkovićetal.,2014).

TheAmazonsailfin catfish,native to theAmazonRiverBasinofBrazilandPeru,isanewnon-indigenousfishspeciesrecordedintheSerbiansectionoftheDanubeRiveratriverkm1026,makingthisthefirst report for the inlandwatersofEurope (Simonovićetal.,2010).ThestatusofthisspeciesintheDanubeisunknown,althoughitmaynegativelyimpacttheaquaticfoodbaseandnativeinvertebrateandvertebratespecies(Nico&Martin,2001).

22  |     PIRIA et Al.

4.7 | Multiple origin introductions

Eurasian ruffe (Gymnocephalus cernua, Percidae), which is native totheCaspian,Aral,Black,BalticandNorthSeabasins,Siberia(Petriki,Naziridis,Apostolou,Koutrakis,&Bobori,2014),occursintheBalkancountriessituated intheDanubeRiverBasin.Thisspecieshasbeenrecorded in several aquatic ecosystems outside of its native range,both in Europe and in North America (Brown, Selgeby, & Collins,1998; Hume, Adams, Bean, &Maitland, 2013). In FYRMacedonia,EurasianruffewasfirstfoundintheTikveshreservoirconnectedwiththeCrna River in 1997. Itmay have been an unintentional releasefromfishpondstockingwithcommoncarpnearthemiddleandupperreachesoftheCrnaRiverinPelagonia(Georgiev&Kostov,1998).ThemostrecentrecordwasfortheStrymonRiverBasin,sharedamongBulgaria, Greece, FYR Macedonia and Serbia, with two specimensfoundinKerkiniLake in2012. Itwashypothesizedthatthespecieshasnotyetestablishedlargepopulationsinthisriversystem(Petrikietal.,2014).However,itisconsideredanefficientinvaderduetoitsadaptability and tolerance to awide rangeof environmental condi-tions,aswellasitshighfecundityandstrongreproductivepotential(Lorenzoni,Pace,Pedicillo,Viali,&Carosi,2009).

Sterletwas introducedtotheTikveshReservoir,FYRMacedoniapriorto2006 (Kostov,Georgiev,Nastova-Gjorgjioska,&Naumovski,1998).Therearetwopossiblereasonsforthisintroduction,thatisitwasintroducedalongwithcarpstockingfromBulgaria,oritwasim-ported fromSerbia in 1997 for ornamental fish breeding, and thenreleasedintothewatersoftheCrnaRiver(Kostovetal.,2011).

Danube sturgeon and Siberian sturgeon (Acipenser baerii,Acipenseridae) were imported to Greece from Russia in 1997 aseggs to the Lake Pamvotis hatchery for rearing in closed systems(Economidis, Dimitriou etal., 2000). Danube sturgeonwas also im-portedtoFYRMacedoniainthe2000sfromBulgarianhatcheriesforaquaculturepurposes(Kostov,2008).AccordingtoLeonardos,Tsiklirasetal. (2008), there isarecordofthereleaseofA. baerii inPamvotisLake(north-westernGreece),whileaccordingtoPaschos(2002)andBarbierietal.(2015),thereisnoevidencethatSiberiansturgeonwereever released (intentionally or accidentally) to the wild in Greece.Recently,inSeptember2016,SiberiansturgeonwasreleasedintotheSavaandMurariversinSlovenia,duetomisidentificationofthespe-cieswithsterlet(M.Povž,personalobservation).

5  | ECOLOGICAL IMPLICATIONS

Theintroductionofnewspeciescanhaveunexpectednegativecon-sequences(Gozlan,St-Hilaire,Feis,Martin,&Kent,2005),itmaynothaveanyreportedecologicalimpactonthenativeecosystem(Gozlan,2008), or the species may disappear without causing any ecologi-cal impacts (Lehtonen,2002). In theBalkans, thereare few reportson the ecological impacts of introduced fishes. Most publicationsarestudiesoutlining thegeneral risksassociatedwith the introduc-tionof alien fish species (Vilizzi, 2012). It hasbeenpostulated thatmanyintroducedalienspeciescauseadverseeffects(Britton,Gozlan,

&Copp, 2011;Copp etal., 2005; Economidis, Koutrakis, &Bobori,2000;Economidis,Dimitriouetal.,2000;Gozlanetal.,2005,2010),althoughthereisanongoingscientificdebateaboutwhether,insteadof causing significantdisturbances in theecosystems, the introduc-tionofalienspeciescanresultmerely inan increaseofbiodiversity(Gozlan,2008),orcanevenhavepositive impactsonnativespecies(Rodriguez, 2006; Schlaepfer, Sax,&Olden, 2011). In somenaturallakes(andreservoirs)withanimpoverishedfishfaunaornofishfauna,theintroductionofalienspeciesthroughconcertedstate-sponsoredstocking programmes and private initiatives could be deemed ben-eficial, by increasing fisheries production or by contributing to theecological balance of eutrophic lentic ecosystems (for Greece, seeEconomidis, Dimitriou etal., 2000). In many other lentic systems,however,introductionsofalienspecieshavedramaticallyalteredtheirecological communities. Extensive stocking, mainly for commercialpurposesandsportfishing,coupledwithunintentionalintroductions,haveradicallyaffectedthefishfaunaofthoseaquaticsystems,lead-ingeventotheextinctionofnativespecies.AnexampleistheTikveshReservoir,thelargestartificiallakeinFYRMacedonia,whereofatotalof20fishspecies,sevenarenowalien,andthenon-indigenousbrownbullheadhasbecomedominantinthelake(Kostovetal.,1998,2011).Thesameisalsotruefornaturallakes,suchasSkadarLake,whichwasextensively stocked by Albanian authorities to support commercialfishing.AnotherexampleisLakePamvotis (Greece),anacuteexam-pleofthecombinedeffectsofrapiddegradationofecologicalqualityand alteration of the trophic complexes on a lentic ecosystem duetostocking; thenumerousand largelyuncontrolled introductions inthis lake forextensivestocking,alongwithseveralaccidental intro-ductions, resulted in theseveredeclineornearextinctionofnativespecies (Leonardos, Kagalou etal., 2008; Zenetos etal., 2009). Thefish faunaof the lakecurrentlyconsistsof20 introduced (includingtranslocated)andonlyfournativespecies,ofwhichatleastone,thelocalendemicEpirusminnow(Pelasgus epiroticus,Cyprinidae),isnearextinctormostlikelyextinct,whilethepopulationofAlbanianbarbel(Luciobarbus albanicus, Cyprinidae) in the lakehas already collapsed(Leonardos,Paschos,&Prassa,2007;Leonardos,Kagalouetal.,2008).In thepast,mountainstreamsand riverswerealsostocked,usuallywith trout species forangling,as is thecase inMontenegro,wherevirtuallyallriverswerestockedwithrainbowtroutandbrooktrout(D.Mrdak,personalobservation).

Inadditiontoothernegativeimpactsofalienspeciesonfreshwa-terecosystems,theintroductionofalieninfectiousagentsrepresentsoneof thegreatest risks associatedwith this globalmovement andintroductionofspecies(Brittonetal.,2011).Assuch,topmouthgud-geoninEuropeisacarrierofseveralpathogens(Gozlanetal.,2005),posingarisknotonlytofreshwateraquaticspecies,asthisthreathasalsoexpanded tomarine fishes, forexampleaswith the transferoftheS. destruenspathogentoEuropeanseabass (Dicentrarchus labrax,Moronidae),(Ercanetal.,2015).Fromtheperspectiveoftheentryofpathogens into Balkan freshwaters, few studies have given reports(Nikolić & Simonović, 2002; Nikolić, Simonović, & Karan Znidaršić,2007), and none has focused on the consequences and/or impactmechanisms on native populations. Little is still known about the

     |  23PIRIA et Al.

consequencesoftheintroductionoffreshwaterfishesandofvariouspathogens,indicatingtheneedforfuturemonitoringandresearchef-fortsconcerning this issue (Brittonetal.,2011).Ontheotherhand,duetoclimaticreasons,severalalienspecieshaveraretononaturalreproductionintheBalkansandasaresult,dispersionisoftenlimited.For example,Asian carps breed naturally only in theDanubeRiver,andonlyinthoseyearswhenhigherwaterlevelsconcurwithhighertemperatures(Janković,1998).However,tolerantspeciessuchastheeasternmosquitofish,pumpkinseedandtopmouthgudgeonoccurinmostrivercourses,plainreservoirsandlakesoftheBalkans.Similarly,theMediterraneanandsub-MediterraneanclimateintheAdriaticSeabasincanprovideappropriateenvironmentalconditionsforintroducedspecies,suchastheNorthAfricancatfish,firstreportedinSloveniain1997andcurrentlyfoundinfishfarmsinthatcountry(Povž,2007).Althoughthespeciesiswidelytolerantofextremeenvironmentalcon-ditions(Froese&Pauly,2015),itisunabletosurvivewinterconditionsinthecontinentalpartoftheseareas,whenwatertemperaturefallsbelow10°C.Nevertheless,apotentialtranslocationofthisspeciestothermalspringhabitatsconstitutesathreat,asseenbytheadaptationofNiletilapiainSlovenia(Povž,2009b).

Undercurrentclimatechangescenarios,impactsonBalkanfresh-water biodiversity are expected to increase as a general rule, sincethenumberofalienspeciesinvolvedisgrowingandthevulnerabilityof ecosystems to invasions is increasing. Risingwater temperatureswillparticularlyaffectfreshwaterfishes(Cochrane,DeYoung,Soto,&Bahri,2009)andtheproblemisexpectedtobesevere,forexample,inGreece,where39%ofallnativefishspeciesarealreadyclassifiedasthreatened(Barbierietal.,2015).Moreover,changesinthesealevelwiththeconsequentflooding,erosionandsalinizationofcoastallow-landsandestuaries (Handisyde,Ross,Badjeck,&Alisson,2007)cancause furtheralterations in thebalanceofaquatic systemsandcre-ateopportunitiesfortropicalandsubtropicalalienspeciestoinvadefragileBalkancoastalwetlands, lagoonsandspring-fedpools,whichhoststrictlyendemicspecieswithhighlylocalizeddistributions.Onesuch example isGreece,with the endemic species almiri toothcarp(Aphanius almiriensis,Cyprinodontidae),Corfu toothcarp (Valencia le-tourneuxi,Valenciidae)andothers.Ontheotherhand,thehighdegreeofintermittencyinthefreshwaterresourcesofthesouthernBalkansmaypossiblybearestrictivefactorforthespreadofinvasivespecies.Forinstance,theAdriaticSeabasinischaracterizedbysmallisolatedriverbasins,manyofwhicharesubjecttoextremehydrographicfrag-mentation,which could reduce the rangeexpansionofnewly intro-ducedfreshwaterfishspecies.Nevertheless,climatechange-inducedalterations of ecosystem conditions, that is in the large number oflarge-scaletransboundaryriverbasinsintheBalkans,mayenablethespread of invasive species and favour the creation of habitats andconditionssuitablefornewlyintroducedinvasivespecies(EPA,2008).Indicativeofthat,isthefindingthatthemajorityoffreshwateralienspeciesintheinlandwatersofGreecehavebeenrecordedinNorthernGreece,wheremostlenticandloticsystemsaretransboundary.Thus,climatechangemayeithercreateadditionalopportunitiesforinvasionor, alternatively, conditions unsuitable for certain invasive species,thus the relative importance of climate change impacts on invasive

freshwaterfishspeciesintheBalkansdependsprimarilyontheichthy-oregionconsideredandthetraitsofeachfishspecies.

6  | DISCUSSION AND CONCLUSIONS

The findingspresentedhere indicate that theBalkanPeninsulahadcommonroutesofintroductionofmanyspeciesinthepast,occurringinseveralwaves.Thefirstintroductionstookplacebetween1883and1891, not only in theBalkans, but also throughout Europe (Holčik,1991). Thesewere primarily aimed at introducing various salmonidspeciesfromNorthAmerica(Povžetal.,2015;Stankovićetal.,2015),whichdidnotsucceedinsecuringself-sustainingpopulationsatalllo-calities(Povž&Ocvirk,1990).Inadditiontothesalmonids,thebrownbullheadwasalsoconsideredaveryattractivespeciesforbreeding,whichlaterprovedincorrectduetoitspoorergrowthinEurope(Fijanetal.,1989;Ivančić,1946)anditshighinvasivepotential(Novomeská&Kováč,2009).FollowingWorldWarI,thesecondwaveofintroduc-tionstookplace,intheperiodfrom1924to1937,whichcontinuedwiththeintroductionofNorthAmericanspecies,primarilysalmonids.Thosespeciesthatfailedtoformsustainablepopulationsinthefirstwaveexperiencedrepeatedintroductions(Povž&Ocvirk,1990).

AfterWorldWar II, therewasatrueboomof introductionsofawiderangeoffishspeciesintheBalkanPeninsula,fromvariouspartsoftheworld.Thiswavecontinueduntilthe1980s.Oneofthemostimportantreasonsforintroductionwasashortageoffood,andaqua-culture played a significant role in resolving this issue. During thisperiod, significant advances were made in aquaculture (Stickney &Treece,2012),with increasedproductionofnativeandalienspecies(Turk, 1997). However, problems arose with the escape of farmedfish into openwaters and the subsequent invasions of certain spe-cies(Vuković&Kosorić,1978).Thus,brownbullheadbecameoneofthemain invaders inopenwaters (Novomeská&Kováč,2009), andthegibelcarp,withitsgynogeneticreproduction,becameathreatnotonlytowildcyprinids,butalsotofreshwateraquacultureproductioninmostBalkancountries(Perdikarisetal.,2012;Povž&Šumer,2005;Savinietal.,2010;Simonovićetal.,2013).

As a consequence, primarily of aquaculture activities but alsoothermotivesforintroduction,suchasangling,biologicalcontrolandtoalesserdegreetheornamentaltrade,therewasrecentlyanincreaseinthenumberofthetotalfishspeciesineachcountryoftheBalkanPeninsula suggesting an ongoing process of biotic homogenization.Studies of alien fish introductions inMediterranean-climate regionshaveemphasizedthelossoffaunaluniqueness,albeitwithaconcur-rentincreaseinthetotalnumberoffishspecies.A7%averageincreasein taxonomic and functional similarity of freshwater fish fauna hasbeendocumentedasaresultofhumanactivities (Marretal.,2013).AllMediterraneanregionsaroundtheworldaredisplayingtaxonomicandfunctionalhomogenizationinmorethan50%oftheircatchments,withtheexceptionofthesouth-westernCape,CentralAnatoliaandthe Aegean Sea drainages (Marr etal., 2013). Overall, catchmentsexhibitingtaxonomichomogenizationarealsohomogenizedintermsof their functional trait composition, which may have important

24  |     PIRIA et Al.

consequences for the functioningof theseecosystems (Olden, Lise,&Giam,2016).Thesestudiesonbiotichomogenization,however,alsoincludedtranslocatedspecies,whichmaypossiblyplayastrongerrolethanexotic species (Villéger,Grenouillet,&Brosse,2014),butwerenot considered in the current study.Thus, the temporal changes ingenetic,taxonomicandfunctionaldissimilaritiesforfreshwaterfishesintheBalkanscouldbeevengreaterandfurtherresearchonthistopiciscertainlyrequired.

For theCsa climate type,which is the predominant type in theMediterranean regions of most of the Balkan countries, as well asSpainandItaly(Peeletal.,2007),concernshavebeenexpressedoverimpactson freshwaterbiota (Vilizzi,2012).Furthermore, theBalkanPeninsula isthe localitywiththe largestnumberofendemicspecies(Oikonomou,Leprieur,&Leonardos,2014)thataredirectlythreatenedby the introduction of alien species (Ćaleta etal., 2015; Piria etal.,2016b; Ribeiro& Leunda, 2012; Snoj etal., 2007).TheCfa climatetypeisalsofoundinAsiaandNorthAmerica,wheretheDfbclimatetypeisalsosignificantlyrepresented(Peeletal.,2007).ItisfromtheCfaandDfbclimateareasofNorthAmerica that thepumpkinseed,brown bullhead, black bullhead and Eastern mosquitofish originate(Froese&Pauly,2015),andtherefore, theclimaticconditions intheBalkansaresuitablefortheiracclimationandnaturalization(Fox,Vila-Gispert,&Copp,2007;Movchanetal.,2014;Vidaletal.,2010).MostofNorthernandWesternEuropearecharacterizedbytheDfaandCfbclimatetypes(Peeletal.,2007),whichalsocorrespondstopartoftheBalkan Peninsula, and therefore, the introduced species from theseareasarealsocapableofnaturalization,asseeninthecaseofEuropeanwhitefishandpeled(Hamzićetal.,2011;Uzunova&Zlatanova,2007).In contrast, the African and South American climate types do notcorrespond to theclimaticconditions in theBalkans.Therefore, thesurvivalofspeciesofthatoriginislimitedtothermalwaters,suchastheNiletilapiainSlovenia(Povž,2009b),althoughthereisarealisticthreatofnaturalization inCsbareas, forexample, theNila tilapia inGreece(Barbierietal.,2015;Perdikarisetal.,2010).Pursuanttothis,itwouldbedesirabletoprioritizetheBalkanPeninsulaforthepurposeofactionstoremediateand/orcontrolintroduced/translocatedfishes,which has been lacking, following the example of countrieswhereprogresshasbeenmade in themanagementof invasives, forexam-pleofcommoncarpinAustraliaortopmouthgudgeoninEnglandandWales.Nonetheless,theseattemptsdemonstratethateradicationmaynotbepossible,althoughmanagementeffortscanaimtocontroltheirdistributionanddispersalintheenvironmentandreducetheirimpactsonnativespeciesandecosystems(Brittonetal.,2011).

Thecurrentstudyhasshownthatalargeproportionofalienspe-ciesintheBalkansarewidespreadandarefullyinvasivespeciesbythecriteriaofBlackburnetal.(2011).Majorcorridorsfortheirspreadarethe transboundary rivers and lakes, suchasOhrid-Drin-Skadar riversystemdraininganumberofcountriesintheBalkans,ortheDanubeRiverdraininga large sectionofCentral andEasternEurope. In theBalkans,mostmajorriversandlakesaretransboundary,creatingcon-flictsofinterestsincewaterresourcesareunevenlydistributedamongthecountries.Thereisastrongneedforcoordinationinrivermanage-mentissues,suchaspollutioncontrol,hydroelectricenergyproduction

andthecontrolofalienfishes.Thecurrentstudyhasalsorevealedthetrueextentofthegapsintheknowledgeonthecurrentstate,distri-butionandimpactsofmanyalienspeciesintheBalkans.Thesegapsshouldbetargetedthroughcoordinatedresearchthatcouldassistef-fectiveconservationefforts.

IntheBalkanPeninsula,thereremainsanimmeasurablewealthoffreshwaterfishdiversity,despitethefactthatthereareveryfewfresh-waterecosystemsleftintactbyintroduction,translocationorstockingwithaliengeneticmaterial.Aboveall,thereisagreatobligationandresponsibilitytopreservetheseuniqueecosystemsforfuturegener-ations,whichcanbesavedonlythroughrationalandjointlong-termconservationactions.

ACKNOWLEDGEMENTS

M.PiriawassupportedbytheUniversityofZagreb,Croatia,andP.Simonovićbygrant#173025oftheMinistryofEducation,ScienceandTechnologicalDevelopmentof Serbia.Τhe research concerning thealienspeciesofGreeceispartofthePhDdissertationbyN.KoutsikosattheDepartmentofEnvironment,UniversityoftheAegean,Greece.

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How to cite this article:PiriaM,SimonovićP,KalogianniE,etal.AlienfreshwaterfishspeciesintheBalkans—Vectorsandpathwaysofintroduction.Fish Fish. 2017;00:1–32. https://doi.org/10.1111/faf.12242