Original Research Article
Use of an acoustic camera to monitor seaward migrating silver-phase eels (Anguilla anguilla) in a regulated river
Eamonn S. Lenihan *, T. Kieran McCarthy, Colin Lawton
AnimalEcologyandConservationUnit,Zoology,SchoolofNaturalSciences,RyanInstitute,NationalUniversityofIreland,Galway Ireland
1. Introduction
TheworldwidedeclineofAnguillideels,andespecially thebetterresearchedtemperatespecies(e.g.Haroetal., 2000),hasresultedinseriousconcernsaboutthestatusof these stocks. In the case of the European eel, Anguilla
anguilla(L.),recruitmentlevelsandfisheryyieldstatistics showthat sincethe1980s thespecieshasundergonea dramatic decline throughout its range (Moriarty and Dekker, 1997). Anthropogenicobstacles forhydropower generation or other river regulation purposes that ad- verselyaffectdownstreammigratingsilver-phaseeels(e.g.
Winteretal.,2006;Jansenetal.,2007;Piperetal.,2015) are among the factors thought to have reduced eel spawning stocks. Delayed spawning migration, and mortalityathydropowerdams,throughimpingementon ARTICLE INFO
Articlehistory:
Received21February2018 Accepted13July2018 Availableonlinexxx
Keywords:
Silver-phaseeel Populationdynamics DIDSON
Swimmingbehaviour Hydropower Swimmingspeeds
ABSTRACT
Downstreammigrationdynamicsofsilver-phaseeelshavetraditionallybeendocumented using fisheries catch data. However, improved monitoring protocols and more comprehensiveanalyses areneeded because ofwidespread declines in Anguillid eel stocks.Therefore, daily catches of silver-phase Europeaneel (Anguilla anguilla) were recordedataneelfishingweironthelowerRiverErneinnorthwestIreland.Inparallel, eelsdetecteddownstreamwithanacoustic(DIDSON)cameraweresurveyedandthe resultswerecomparedwithcatchdataobtainedattheeelweir.Theprincipalobjective wastoevaluatetheuseoftheacousticcameratomonitormigrationpatternsandeel swimmingbehaviourinaregulatedriver.Ahighlysignificantrelationshipbetweenthe acoustic cameraeel counts and fishing weir catches was observed (p<0.001). This indicatesthatfisheries-independentestimatesofeelspawnerbiomassescapementwillbe possible in the future using acoustic cameras. Acoustic camera observationson eel swimmingbehaviour indicatedsignificantly more eelsswimming downstream when dischargewascontinuousthroughthenight(p<0.05).Onnightswithnodischargemore eelswereobservedswimmingupstream(p<0.05).Theswimmingspeedsofeelsdiffered significantly (p<0.05) between continuousand low discharge conditions. European rivers are increasingly affected by anthropogenic control of discharge to facilitate hydropowergeneration,floodcontrolandnavigation,andthisaffectsnaturalpatternsof eelmigration.Theresultsofthisstudydemonstratethepotentialuseofacousticcameras formonitoringeelmigrationdynamicsandforbehaviouralanalysesofeelmigrationin suchregulatedrivers.
ß2018EuropeanRegionalCentreforEcohydrologyofthePolishAcademyofSciences.
PublishedbyElsevierB.V.Allrightsreserved.
* Correspondingauthor.
E-mailaddress:[email protected](E.Lenihan).
ContentslistsavailableatScienceDirect
Ecohydrology & Hydrobiology
j our na l ho me pa g e : ww w . e l se v i e r . com / l oca t e / e co hy d
https://doi.org/10.1016/j.ecohyd.2018.07.001
1642-3593/ß2018EuropeanRegionalCentreforEcohydrologyofthePolishAcademyofSciences.PublishedbyElsevierB.V.Allrightsreserved.
trash screens or during turbine passage, have been frequentlydocumented (e.g. Winter et al., 2006; Calles etal.,2010;Pedersenetal.,2012;Piperetal.,2013).
Ireland,likeotherEuropeanUnionmemberstates,has implementedconservationmeasuresthatareintendedto contributetotherestorationoftheEuropeaneelstockin linewiththeEUregulationECNo.1100/2007.Closureof eelfisheries and measuresto reduce adverse effects of hydropowerdams,areamongthemajorelementsofthe Irishnationaleelmanagement plan(DCENR2008).Asa short to medium term measure, silver-phase eels are caughtinthreeriverbasins,theRiversShannon,Erneand Lee, and released below hydropower dams (McCarthy etal.,2014;MacNamaraandMcCarthy,2014).Monitoring andevaluationofthesetrapandtransportprogrammesis undertaken on an annual basis. This presents good researchopportunitiesforanalysisoftheseawardmigra- tionsofeelsandforthedevelopmentofnewmonitoring methods. Downstream migration has traditionally been quantifiedbasedoncatchdata(Haro,2003),butreliable catchdataisnotalwaysavailable.
Acousticcameras,forexampledual-frequencyidentifi- cation sonar (DIDSON), are increasingly being used for fisheriesmanagementastheyarefast,quantitativeandnon- invasive (Foote 2009; Martignac et al., 2015). Acoustic camerashavebeenusedinpreviousstudiestoanalysefish migrations(MaxwellandGove,2004;Baumgartneretal., 2006; Crossmanet al., 2011)and may provide a robust means of assessing the downstream migration of eels.
Acousticcamerashavetheabilitytoprovidegoodquality images in the darkand turbid waters which typify the nocturnaldownstream migrations of eels.While species identificationisaknownlimitationofacousticcameras,eels areeasilyidentifiablefromtheirsinusoidalmovementand elongatemorphology(Webb,1982).Aswellasproviding
quantitativedataoneelabundance,acousticcamerascan providequalitativeinformationoneelbehaviour,suchas swimmingdirectionorspeed(Martignacetal.,2015).
Theaimsofthepresentstudywere:(1)Toestablishthe relationshipbetweenthenumberofeelscountedusinga dual-frequency identificationsonar(DIDSONTM)acoustic cameraandnightlycatchesatafishingweirontheRiver Erne and to evaluate the potential of acoustic camera surveysin futuremonitoring ofsilver-phaseeelescape- ment and (2)To usetheacousticcameratoinvestigate behaviouralresponsesofsilver-phaseeelstochangesin theregulateddischargeoftheriver.
2. Studyarea
TheRiverErne(Fig.1)systemis locatedintheNorth WesternInternationalRiverBasinDistrictofIrelandand drainsanareaof4374km2(McCarthyetal.,2014).There aretwohydropowerdams,CliffHPSandCathaleen’sFall HPS,locatedinthelowermostsectionoftheriversystem (Fig. 1), above which there is a total wetted area of 26,197ha(McCarthyetal.,2014).Dischargeontheriver, whichisregulatedforfloodcontrol,navigationandpower generation,variesfrom<10m3s1to382m3s1witha mean annual discharge of 92m3s1 (McCarthy et al., 2014).Dischargeisregulatedatthehydropowerdamsand at Portoraregulationweir(Fig.1).Thetwo hydropower dams are separated by the man-made Assaroe Lake (235ha)(Fig.1).Thefishingweirmonitoredinthisstudy islocatedatRoscorBridge,750mdownstreamfromthe outflow of Lower Lough Erne (Fig. 1) (5482803200N, 880101600W),whiletheacousticcamerawasmountedon apontoon5kmfurtherdownstreamatasiteontheriver (5482804800N,880504900W)adjacenttothetownofBelleek.
Fig.1.MapoftheErneCatchment(withsitesmentionedintextindicated)andofIrelandshowingthegeographicallocationoftheRiverErne.
3. Methods
Downstream migrating silver-phase European eels were caught at the Roscor Bridge fishing weir by an experiencedfishingcrewon84nightsbetween29/09/16 and08/02/17.Thesitewasfishedusingthreenetsattached tothebridge.Netsweresetatduskandliftedatdawn.
The acoustic camera (DIDSON 300m, Soundmetrics Corp.;http://www.soundmetrics.com),wasusedtocount migratingsilver-phaseeelsandtoinvestigatetheresponse oftheseeelstochangesindischarge.Theacousticcamera wasattachedtoafloatingpontoonwhichwassecuredto theriverbedandbankwithanchoredchains.Theacoustic camera unit was held on a custom-made frame which couldbeloweredinandoutofthewateronpivotingbars.
When submerged, theacoustic camera unit wasfacing perpendiculartotheflowoftheriver.Inthis study,the sonarwassettooperateatlowfrequency(1.1MHz)which haslowerresolutionimagesbutagreaterrange.Despitea reductioninresolution,eelswerestilleasilydistinguished based on their swimming behavior and distinctive morphology. The insonified field of view wasa conical frustumandwasprogrammedtostartat6mfromtheunit andextendafurther20minwaterrangingindepthfrom 6.5mto7.6m.Itwaspossiblethateelscouldpassaboveor belowtheconicalbeamundetected.Aprimaryaimofthe study was to investigate the relationship between eel countsrelativetoeelweircatch,andabsoluteeelcounts werenot required. Theobjective ofestablishing a good index of eel numbers did, however, require that the methodology was consistent. The acoustic camera recorded continuously from the 21/11/2016 until 3/2/
2017 (n=74 nights), except for three nights due to a poweroutage.Correspondingcatchdatawasavailablefor 42nightsduringthisperiod.
Nightly discharge data for the sampling period was recorded at Cliff dam. The discharge for nights with corresponding catch and acoustic camera data (n=42) wasassignedtothreedistinctcategories:(1)continuous (n=22), (2) interrupted (n=14) and (3) no discharge (n=6). Continuous discharge can be either high (>150m3s1; McCarthy et al., 2014)or low discharge (<150m3s1)andremainsreasonablyconstantthrough- outthenight.Interrupteddischargewascharacterizedby continuousflowforaportionofthenightfollowedbya rapiddroptonearzerom3s1.Linearregressionanalysis was used tocompare acoustic camera eel counts with catch from the Roscor Bridge fishing weir (n=42).
Followingthis,linearregressionanalyseswereconducted comparingacousticcameraeelcountswithcatchforeach discrete dischargecategory.Using theacousticcamera’s software (DIDSON V5.21, Soundmetrics Corp.) acoustic survey data wasmanuallyanalysed tocounteels. Only nocturnalhourswereanalysedtocoincidewiththetime whennetswereset.
The impact of variable discharge patterns on the swimmingdirectionofsilver-phaseeelswasinvestigated usingtheacousticcameradata.Themeannumberofeels observedmovingupstreamanddownstreamduringeach of the three discharge categories were compared using independent-samples t-tests.The timingof cessation of
downstream migration during nights with interrupted dischargecomparedtonightsofcontinuousdischargewas alsoevaluatedtoestablishtheimpactofinterruptionof discharge on eel swimming behaviour. The acoustic camera was also used to calculate swimming speed of eels moving both downstream and upstream in the presence and absence of discharge. Swimming speed was defined as the distance covered by an eel divided bythetimetaken(ms1),inexcessofthewatervelocity.
Distancewascalculatedasthelengthbetweenthepoints wherethe eelenteredand exitedtheacousticcamera’s insonifiedfieldofview.Swimmingspeedwascalculatedat variousdistancesalongtheinsonifiedcone,rangingfrom 7.7mto20.0m. Swimming path distance rangedfrom 5.5mto10.1m, andtimestakenrangedfrom13.2sto 25.3s, allowing enough time to calculate swimming speeds. Using stratified random sampling 4 groups of 20 eels were selected for analyses of downstream and upstream swimming speeds in continuous and no dis- charge conditions (n=80). Groups were initially com- pared using a Kruskal–Wallace test to investigate if significantdifferencesexistedbetweengroups,andapost hocGames–Howelltestwasthenusedtoconfirmwhere differencesoccurred.
4. Results
Over the entire fishing season 5050kg of eels was caughtat the fishingweir, with4249kgcaught during acousticcameradeployment.Duringthemainsilver-phase eel migration period starting in Autumn 2016, mean dischargewas77.53m3s1.Thedailymaximumrecorded was 337m3s1. Size frequencies produced from weir catchrevealedthat78.1%ofeelswerefemale(eelswith totallength(Lt)440mm,McCarthyetal.,2014).
A total of 587 eels were observed during acoustic cameraobservations withswimmingdirectionrecorded for each. It took approximately 105h to process the 42 nights of data. Of these, 303 (52%) were observed swimming downstream, and 284 (48%) were observed swimming upstream. 280 (92%) of the downstream swimmingeelsand91(32%)oftheupstreamswimming eels were observedduring continuous discharge condi- tions. 20 (7%) of the downstream swimming eels were observedduringnightswithinterrupteddischarge,while 57(20%)oftheupstreamswimmingeelswereobserved.
Theremaining3 (1%)downstreamswimming eelswere observedduringnodischargeconditionswhile136(47%) upstream swimming eels were observed. Direction of swimmingwasshowntodifferwithdischargeconditions (Fisher’sexacttest,p<0.0001).
Acousticcamera counts and weir catch were highly significantly and positively related (R2=0.905, n=42, p<0.001). Nights with continuous discharge (n=22) accountedfor98.5%ofthecatchduringacousticcamera deployment.Analysisofthissubsetofdataindicatedthat acousticcameracountsandweircatchweremorehighly significantlyrelated(R2=0.962,n=22,p<0.001)(Fig.2).
The relationship between acoustic camera counts and catchfor nights withinterrupted discharge (R2=0.247, n=14, p>0.05) and no discharge (R2=0.021, n=6,
p>0.05), were not significant. These catch categories whencombinedonlyaccountedfor1.5%ofthetotalseason catch.
Significantly more eels moved downstream during continuousdischarge than upstream(p<0.05)(Fig. 3).
When discharge was interrupted, being continuous for onlyapartofthenightbeforedroppingrapidlytonearzero m3s1,there wasno significantdifference inthe mean number of eels moving downstream or upstream (p>0.05) (Fig. 3). However, during nights with no discharge, the mean number of eels moving upstream wassignificantlyhighercompared tothemeannumber movingdownstream(p<0.05)(Fig.3).Itwasobserved repeatedlywhencomparingthenumberofdownstream migratingeelsonnightswithinterrupteddischargeand continuousdischargethateelsstoppedmigratingdown- streamassoonasdischargedroppednearlytozero,while eelscontinuedtomovedownstreamthroughoutthenight duringcontinuousflow(Fig.4).
Asummaryofthemeanswimmingspeedsofeelscanbe foundinTable1.Therewasasignificantdifferenceinthe swimmingspeedofeelsbetweengroups(p<0.05).The
swimmingspeedofeelsmovingdownstreamincontinu- ous discharge was significantly different to the other groups (p<0.05) while eels swimming upstream in continuous discharge and those swimming upstream and downstream in no discharge conditions were not significantlydifferentfromeachother(p>0.05).
5. Discussion
Conservationmeasuresaimingtoaidintherecoveryof eel stocks frequently rely on fisheries catch data to quantify production and escapement (Amilhat et al., 2008; MacNamara and McCarthy, 2014; Charrier et al., 2011). There are limitations associated with theuse of catch data (e.g. Bilotta et al., 2011). Biases can be introduced whenthe fishingseason does not coverthe entire migration period of eels, or when catch data otherwise is missing, thus leading to discontinuities in catchdatasetsusedinthecalculationofproductionand escapement(Pooleetal.,2018). Duetosuchlimitations, production and escapement estimates are increasingly basedon models(Aprahamianetal.,2007;Priggeetal., 2013).However, thereremainsa needforaquantitative datasourcetocalibrateandvalidatethesemodels(Bilotta et al., 2011). In this study on the River Erne, we demonstratedthatacousticcameraeelcountscanenable fisheries independent estimates of spawner biomass escapement.
The roleof increasingdischargein stimulatingsilver eelstomigrateiswellestablished(TeschandWhite,2008;
Vøllestadetal.,1986;CullenandMcCarthy,2003).Inthis study, regulated variationin dischargeimpactedon the swimming directionand speedof eels which mayhave implications for timing of eel migration from the river Fig.2.Therelationshipbetweennumbersofmigratingsilver-phaseeels
countedfromDIDSONsonarsurveysandthecorrespondingindexnet catches at Roscor Bridge during continuous discharge (n=22).
Spearman’s=0.962, p<0.001. Dash line represents 95% confidence intervals(CI).
Fig.3.Themeannumberofeels(Standarderror) observedpassing upstreamanddownstreamforeachofthedischargecategoriesrespectively.
*=significantatthep<0.05levelandns=notsignificant.
Fig.4.Twoexamplesofnight-timenumbers(perhour)ofeelsmoving downstreamin(A)interrupteddischargeconditions(15/12/16)and(B) continuousdischargeconditions(26/12/16).
system. Under continuous discharge conditions, the majority ofeels migrateddownstream, withonly a few individualsswimmingupstream.Thisisthemostnatural behaviourassociated withseawardmigrating eels,with largenumbersofindividualsmovingdownstreamduring migration peaks (Bruijs and Durif, 2009). Theupstream movementofeelsduringhighdischargemayreflectthe failureofindividualstosuccessfullypassthedamlocated 750mdownstreamoftheacousticcamera,whichwasalso observedbyCallesetal.(2010),ortheresponseofeelsto altered flow fields around the entrance to dams (Piper et al.,2015). On nightswithinterrupted discharge,eels were observed swimming downstream until discharge ceased(Fig.4).Whendischargedroppedtonearzerom3s-
1,eelsbegantomoveupstreamwithveryfewindividuals observed swimming downstream. Some eels were ob- servedswimmingintotheacousticcamera’sfieldofview, hesitatingandswimmingbackinthedirectiontheycame.
Eelsareknowntoactivelysearchbeforechoosingaroute (Calleset al.,2010), this behaviourhaspreviouslybeen referredtoas‘‘recurrence’’(Jansenetal.,2007).Behrmann- GodelandEckmann(2003)notedthateelsapproachinga damindischargeconditionslessthan200m3s1tended toexhibitcirclingbehaviour,andescapedupstreamasan avoidancebehaviour.Thismightexplaintheoccurrenceof someeelsswimmingupstreamduringtimesofcontinuous discharge as they attempt to avoid the dam 750m downstream.
The mean swimming speed (x=1.10ms1) of eels migratingdownstreamduringcontinuousdischargecon- ditionsinthisstudyfallswithintherangeofswimming speedsobservedinpreviousriverinestudies(Tesch,1994;
Haro,2003;McCarthyetal.,2008;Barryetal.,2016).The mean swimming speed of eels moving upstream (x=0.71ms1)in continuousdischargeconditionswas belowtheexpectedrangereportedbyTesch(1994).Mean swimmingspeedsforeelsmovingupstream(0.53ms1) anddownstream(0.58ms1)duringnodischargecondi- tionswerealsolowerthantherangedescribedbyTesch (1994).VandenThillartetal.(2009)estimatedthateels swim 4–6times moreefficiently than salmonids. Given theirremarkablylowlevelsofenergyuse,itseemslikely that eels swimming in no discharge conditions, while making no progress towardsthe sea, are wasting little energy. Swimming speedscalculated using theacoustic cameraagreedwithexistingcalculationsbasedonvarious othermethodssuchastelemetry(Barryetal.,2016),and otherformsofhydroacoustictechnology(McCarthyetal., 2008),aswellasestimatedfromswimtunnels(Vanden Thillartetal.,2009).ThissuggeststhatDIDSONprovidesa robustmeansofcalculatingfishswimmingspeed.
DischargepatternsontheRiverErneareregulatedfor hydropowergeneration,navigationandfloodcontrol.At RoscorBridgeduringtimeswhendischargeislow,fishing issometimesnotpossibleastheprocessisinefficientor because fishing nets become entangled. Conversely, in previous fishing seasons on the River Shannon, during timesofflooding,fishingbecameahazardandhadtobe abandonedforsafetyreasons(MacNamaraandMcCarthy, 2014). In both scenarios, and particularlyfor thelatter, valuablecatchdataisunavailableforcalculationsofsilver- phaseproduction andescapement. Asthisstudy shows, acousticcameracanprovideestimatesofspawnerbiomass escapementinsuchtimes.LowerLoughErneupstreamof theacousticcameradeploymentsiteactsasasinkinwhich debrisgathers.Aswellasthis,thereisanotherconserva- tionfishingsite locatedattheoutletof this lakewhich effectively filtersoutall debris.Therefore, inthis study, there was little risk of debris damaging the acoustic cameraor reducingthequality of datacollected during floodconditions.
Acousticcameras(DIDSON)havenowbeenusedfora variety of uses in the field of fisheries including:
aquaculture(Han etal., 2009); investigationson fishing gear selectivity and efficiency (Rakowitz et al., 2012);
ecology, such as analysis of size, abundances and behaviour (Boswellet al., 2008); and interactions with both natural and man-made interfaces, like turbines (Viehmanand Zydlewski,2015). Acousticcameras have alsobeensuccessfullyusedinstudiesonthemigrationof several eelspecies (Baumgartneret al., 2006; McCarthy etal.,2014;MacNamaraandMcCarthy,2014;Eggetal., 2017).
However,acousticcameras,likeDIDSON,havelimita- tions(Martignacetal.,2015).HighInitialcapitalinvest- mentmeansthatacousticcameras,suchasDIDSON,are unlikelytobewidelyusedinmonitoringprogrammesat present(Bilottaetal.,2011).However,asthisstudyand others(e.g.Bilottaetal.,2011;Baumgartneretal.,2006) havedemonstrated,thereareadvantagestousingacoustic cameras instead of traditional netting methods. Once acoustic camera counts are validated they provides a meansofquantifyingabundance,validating models,but also provides a means of directly observing silver eel behaviour.Ittookapproximately105htoprocessthedata generated over 42 nights. There is currently no robust means of automatically counting fish using DIDSON (Rakowitz,2009),whichmeansthatmanuallyprocessing the data remains the best option. In the future, the procedure maybecomefully, orsemi-automated which willsignificantly reducethelabour associatedwiththis methodology. Some limitations associated with the Table1
Meanswimmingspeedsinexcessofwatervelocityforeelsswimmingupstreamanddownstreaminvariousdischargeconditions.N=numberofeels monitoredforeachcategory.Thedistance(km)aneelcouldbeexpectedtotravelinanhouriscalculatedbasedonmeanswimmingspeed.
Dischargeconditions Swimmingdirection Samplesizen Meanswimmingspeed(ms1SE) Range(ms1)
Continuous Downstream 20 1.100.14 (0.31–1.40)
Continuous Upstream 20 0.710.13 (0.48–1.74)
None Downstream 20 0.580.09 (0.29–1.10)
None Upstream 20 0.53+0.06 (0.27–0.85)
methodologydescribedinthisstudydoremain.Unlessfish are prevented from returning in the opposite direction thereisnowaytoensurethesamefishisnotcountedmore thanonce,asdescribedbythe‘multi-transit’hypothesis (Brehmeretal.,2006).Themethodologydescribedherefor estimating spawner biomass escapement from acoustic camera surveys should be applicable to an array of migratoryfishspecies,includingotherAnguillidspecies.
However,therelationshipdescribedinthisstudywillnot bedirectlyapplicabletootherriversystems,meaningthat an initial means of quantifying catch will be required, against which the acoustic camera counts can be calibrated.
Large scale studies on eel population dynamics are traditionallyveryreliantondataobtainedfromcommer- cialfisheries.However,theextensiveanalysesofseaward migrating eel populations and estimation of spawner biomass requiredby Eel Management Plans (EMPs) are oftennotpossibleduetotheabsenceoforlimitedextentof silver eel fishing. This hasled to increasedreliance on populationmodelinginmanyEuropeancountries.Acous- ticcameras,likeDIDSON,canprovidea directmeansof assessingmigrationdynamicsofsilvereels.Quantification of spawner biomass and validation of models is also enabledwhen calibrationofDIDSON populationcounts, relative to catch data, is possible. Acoustic camera observationsoneelswimmingbehaviourcanalsoprovide valuableinformationforconservationandmanagementof migratingeelpopulations.
6. Conclusion
Quantificationof the downstream migrationsof eels hastraditionallyreliedonsilvereelcatchdata.However, eelfishingdoesnotalwayscovertheentireeelmigration season, which can lead to biases when calculating productionandescapement.Inthisstudy,weinvestigated thepotentialuse ofan acoustic camerato monitor the downstreammigrationpatternsandswimmingbehaviour ofEuropeaneels(A.anguilla).Linearregressionanalysisof dailyeelcountsfromanacousticcamerasurveyandeel catch from a nearby fishing weir revealed a highly significant relationship. Such a strong relationship can enable fisheries-independentestimates of spawner bio- massescapementintheabsenceofcontinuouseelcatch datasets.
Theacousticcamerasurveyalsoshowedthatregulation ofriverdischargeimpactedontheswimmingbehaviourof eels.Whendischargewascontinuous,themajorityofeels wereobserved swimmingdownstream. However, when discharge ceased, eels were observed swimming back upstream. Delays to migration caused by alteration of dischargecouldhaveconsequencesforthetimingofeel spawning.
Acoustic cameras have limitations, primarily high initialcostand thegenerationof large amountsofdata to process. However, this study shows that acoustic camerashaveadvantagesovertraditionalnettingmethods which,wherepossible,warrantstheir inclusioninsilver eelmonitoringprogrammes.
Conflictofinterest Nonedeclared.
Ethicalstatement
Authorsstatethattheresearchwasconductedaccord- ingtoethicalstandards.
Acknowledgements
We also want to thank D. Nowak for technical assistanceindeploymentoftheDIDSONcameraandother fieldwork. The handling of eels during this study was conductedunderprojectauthorisationAE19125issuedby theHealthProductsRegulatoryAuthority(HPRA),Irelands competent authority responsiblethe implementation of EUDirective2010/63/EU.
Fundingbody
We wish to thank the Electricity Supply Board for financiallysupportingthis researchandfor provisionof hydrometricdata.
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