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Aquacultural Engineering

j ou rn a l h o m epa ge :w w w . e l s e v i e r . c o m / l o c a t e / a q u a - o n l i n e

Copepod production in a saltwater pond system: A reliable method for achievement of natural prey in start-feeding of marine ﬁsh larvae

Terje van der Meeren

^a,∗

, Ørjan Karlsen

^a

, Penny Lee Liebig

^b

, Anders Mangor-Jensen

^c

aInstituteofMarineResearch,AustevollResearchStation,HjortCentreforMarineEcosystemDynamics,NO-5392Storebø,Norway

bInstituteofMarineResearch,POBox1870,Nordnes,NO-5817Bergen,Norway

cInstituteofMarineResearch,AustevollResearchStation,NO-5392Storebø,Norway

a r t i c l e i n f o

Articlehistory:

Received28March2014 Accepted8July2014 Availableonline23July2014

Keywords:

Mesocosms Copepodculture Diapauseeggs Pondculture Larvalﬁshnutrition Livefeed

a b s t r a c t

Ahigh-latitudeseawaterpondsystemwasrestartedafter10yearsabsenceofcontrolledbiologicalpro- duction.Watersupplysystemsforemptyingandrefillingthe25,000m³pondwereinstalledalongwith awheelfilterplanktoncollectionunitwhichenabledfractionationandconcentrationoflivezooplank- ton,consistingmainlyofvariousstagesofcopepods.Araftwascentrallylocatedinthepond,servingasa platformforhydrographicalandbiologicalsampling,watermixing,anddeliveryofinorganicnutrientsto boostprimaryproduction.Nocopepodrestingeggsseemedtohavesurvivedthe10-yearrestingperiod, andcopepodeggsandnaupliiwerereintroducedwiththerefillingofseawatertothepond.Abundances ofcopepodnaupliiincreasedabout5monthsafterrefilling,withsubsequentgenerationsofcopepodids andadultcopepods.TheplanktonwasdominatedbythecalanoidcopepodsAcartialongiremisandCen- tropagushamatus.Thepondwasmanagedaccordingtoadistinctyearcycle,withthenaturalproduction seasonlimitedfromMarchtoOctoberfollowedbycooling,quiescence,andcompletedrainingbefore refillinginFebruaryandJulytopreventestablishmentofotherplanktonicorganismsthancopepods.

About4.6and45.4billioncopepodrestingeggswereestimatedtobereadytohatchfromthepondsed- imentatrefillinginFebruarythesecondandthirdyearofoperation,respectively.Thus,theoperational proceduresenabledsynchronoushatchingofcopepodnaupliiduringspringseasonforuseinlargestart- feedingexperimentswithmarinefishlarvae.Further,2.7and1.6billioncopepodsofvariousstageswere harvestedduring40and66daysperiodsin2012and2013,respectively.Thepondhasprovenitselfa reliablesupplierofcopepodswhichsustainedcompletefeeddeliverythroughthewholelarvalperiodin large-scalestart-feedingtrialswithmarinefishlarvae.

1. Introduction

Copepodsarethemostimportantfeedforplanktivorousﬁsh, includingthelarvalstagesofmanyﬁshspecies(Last,1978,1980;

Möllmann et al.,2004).Use of copepods as feedin production of marine ﬁshhas reduced frequencies of skeleton deformities andimprovedpigmentation,survival,andgrowthratesduringlar- val and earlyjuvenile stages (Øiestad et al.,1985; Næss et al., 1995;Shieldsetal.,1999;PayneandRippingale,2000;Støttrup, 2000;Finnetal.,2002;Imslandetal.,2006;Koedijketal.,2010;

LiuandXu,2009;Buschetal.,2010;Barrosoetal.,2013).Com- paredtocommonly usedlivefeedslikerotifers(Brachionussp.) and Artemia, copepods are superior with respect to essential nutrients(Watanabeetal.,1983;Wittetal.,1984;Evjemoetal.,

∗Correspondingauthor.Tel.:+4746956792.

E-mailaddress:[email protected](T.vanderMeeren).

2003;vanderMeerenetal.,2008;Hamreetal.,2013).Forthis reason,therehasrecentlybeenanincreasinginterestinapplying copepodsinlarvalfishrearing.Thisinterestalsoincludestheneed toavoidnutritionallyinducedeffectsinbio-assaystudieswithfish larvaeorotherorganismsthatneedliveplanktonicpreyasfeed (Drilletetal.,2011a),e.g.intoxicitytestingofcompoundsofanthro- pogenicorigin,physiologicalstudies,orecologicalassessmentslike impactsofoceanacidificationorglobalwarming.Copepodsarekey organismsintheaquaticfoodwebs,andwithregardtotheirusein experimentalstudies,attentiontosuchpreyorganismsistherefore expectedtoincrease.

Obtainingenoughcopepodsofdesiredstagesataspecifictime has been one of the barriers for extensive use in aquaculture andexperimentalworkwithfishlarvaeorothercopepod-feeding organisms.Therefore,establishmentofreliableproductionmeth- odsforcopepodsthatcanmeetthequantitativerequirementsof larvalfishesisessential.Harvestingcopepodsfromtheseaisnot anoptionasvariabilityinabundancemakesitdifficulttoobtaina http://dx.doi.org/10.1016/j.aquaeng.2014.07.003

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stablesupply.Inaddition,wildcopepodsmaycarryparasitesthat canimposeathreattolarvalﬁshes(Drilletetal.,2011a).Cope- podcultivationisthereforepreferred,andincludesbothintensive productionundercontrolledconditionsand extensiverearingin tanksor ponds(Støttrup, 2003).Intensiveproduction however, stillstruggles withobtainingenoughcopepods due tothelong generationtimeandrelativelylowcopepoddensitiesinthecul- tures compared to rotifers and Artemia. However,methods for inducingquiescenceinsubitaneouscopepodeggsandsubsequent cold-storagehavebeendeveloped(Drilletetal.,2006;Holmstrup etal.,2006;Højgaardetal.,2008).Thisallowssynchronizedhatch- ingofcopepodeggsinintensivecultures,whichmayincreasethe availabilityofcopepodnauplii.Nevertheless,thenaupliistillhave toberearedthroughthenaupliarandcopepodidstagesiftobeused asafoodsourceforlargerorolderﬁshlarvae.

Ontheotherhand,massproductionofcopepodsinlargevol- umesofwaterlikemesocosms,enclosures,orponds,hasprovento providesufficientamountsofcopepodsfrominitiationofexoge- nousfeedinguntilcompletionofmetamorphosisinlarvalfish(van derMeerenandNaas,1997).Suchsystemshavebeenmanagedas small-scaleecosystemsinwhichthefishlarvaeeitherhavebeen reareddirectlyinthepondwateralongwiththenaturallyoccurring planktonorganisms(Rognerud,1887;KvensethandØiestad,1984;

Pedersenetal.,1989;Blometal.,1991;Engell-Sørensenetal.,2004) ormoreinterestingly,thepond canbemanipulatedtoenhance copepodproductionwherethedesiredcopepodstagescanbecol- lectedandconcentratedbyﬁltersfortheuseasfeedforlarvalﬁshes (Naasetal.,1991;vanderMeerenetal.,1994;Berg,1997;Sørensen etal.,2007;Suetal.,2006).

Onesuch pond system,the “Svartatjern” pond in Austevoll, westernNorway,wasincontinuousoperationfrom1984to2001.

Duringthisperiod,amethodaimingatminimizingcopepodpreda- tors,depletingparasiteinfestation,andmaximizingoverwintering of copepod resting eggsin the sediments was developed. This protocol securedhatching of predictable quantitiesof copepod naupliiinthespringandsubsequentcopepodidgenerations for useinlarvalﬁshrearingexperiments.However,since2001this pondhadbeenaccumulatingrainwater,leavingalayerofanoxic seawateratthebottom.Thepresentstudydescribestherestart ofthispondsystemwithsubsequentestablishmentofamarine copepodcommunity anda seasonal management protocol.The restartmaybeequivalenttostartingupamarinecopepodpond fromscratch,andisthereforevalidforanyothernewestablish- mentofsimilarenclosuresystemstobeusedinaquacultureorin research.

2. Materialsandmethods 2.1. The“Svartatjern”pondfacility

Thepond“Svartatjern”,locatedatN60^◦547,E5^◦1505inthe vicinityoftheInstituteofMarineResearch–AustevollResearch Station(IMR),wasoriginallyasmallfreshwaterlakebeforeconver- sionintoamarinepondthatcouldbedrainedandrefilledbypump systems.Svartatjernasaseawaterrearingfacilitywasestablished in1984andusedforproductionofjuvenilecodbytheextensive methodthatsameyear(Naasetal.,1991;vanderMeerenandNaas, 1997).From1985,thepondwasusedasacopepodproductionunit designedforharvestofvariousstagesofcopepodstobeusedin ecological,genetical,and nutritionalstudiesoflarvalfish(Næss etal.,1995;vanderMeerenandJørstad,2001;vanderMeeren andMoksness,2003;vanderMeerenetal.,2008).TheSvartat- jernfacilitywasterminatedin2001,andalltechnicalequipment usedforpondoperationwasremoved.Duetorenewedinterestin marinecopepods,in2010thedecisionwasmadetorefitthepond

forcopepodproductionagainandfulloperationre-establishedin 2011.

Afterreinstallation,Svartatjernhadadepthof4.5m,anareaof 10,600m²,anapproximatevolumeof25,000m³,wasbowl-shaped withasurfacetovolumeratioof0.48,andhadalayeroforganic sedimentatthebottomthatwasseveralmetrethickinthecentre ofthepond.Seawards,thepondwasclosedbya1mhighconcrete damwithstandpipesforwaterlevelcontrolanddrainageoffresh- waterfromrainandlandrunoff.Thisdrainagetookplaceatthe pondsurface,preventingthesaltwatertounintentionallyleavethe pond.Togainandpreserveheatinthepondwater,athinfresh- waterlayerof10–20cmwasallowedtobuildupfromrainrunoff duringtheproductionseason.Thislayeroffreshorslightlybrack- ishwatercreatesastrongpycnoclinethatwillactastheglassin agreenhouse,leadingtoaccumulationofheatfromincomingsun radiationbyinsulatingthepondwater.Thiswillresultinshorter copepodgenerationtimerelativetowhatisexpectedfromseasonal atmosphericandseawatertemperatures.

2.2. Watersupply,mixing,anddrainagesystems

Apump stationfor seawatersupply, locatedoffshore inthe nearbybay(Fig.1a),consistedofawoodencabinontopofapump basinboltedtotwolargeconcreteblocksrestingonthesandysea bedintheshallowbay.Thewatersupplypipelinewasasubmerged PEHpipe(Ø=355mm)extending270mbeyondthebay,toadepth of35mintothefjord.Thisprovidedseawaterinthesalinityrange 31–34psuandtemperaturesbetween8and12^◦C.Asubmersible Flygtchannelimpellerpump(typeCP3127MT433,4.7kW,230V, 1445rpm)incastironwithupto2m³/mincapacity(XylemWater SolutionsAS,Oslo,Norway:www.flygt.com)wasinstalledinthe pumpstation.Topreventpotentialnegativebiologicaleffectsinthe enclosedpondwater,nozincorothermetalanodeswereusedon thepump(JelmertandvanLeeuwen,2000).Thepumpwasstored ondeckinsidethecabinandloweredbydoubleguidebarswhen connectiontoanotherpipelineusedforpumpingthewatertothe pondwasneeded.Thiswasa170mlongPEHpipe(Ø=200mm) coupledtotwoUNIK-900wheelfilters(UnikFiltersystemAS,Os, Norway:www.unikwater.com),enablingfiltrationdownto80␮m ofallseawaterpumpedtothepond(Fig.1b).Thisfiltrationexcluded smallplanktivorousfishandotherpotentialcopepodpredators.

Outlets of theUNIKfilters trapped considerablevolumesof air anddistributedfineairbubblesintotheeffluents.Therefore,water passingthewheelfilterswascollectedina1.5m³ circularfibre- glasstankforstrippingoftrappedair beforeentering thepond througha95mlongPEHpipe(Ø=305mm)endingnearthebot- tomatthecentreofSvartatjern.Removalofthisairwasnecessary topreventflotationofthepipeextendingintoSvartatjern,andpos- siblyalsoaerationleadingtomixingoftheseawaterandthesurface freshwaterlayerinthepond.

Another submersible Flygt channel impeller pump (type CT3085MT434,1.4kW,230V,1405rpmwithavariable-frequency drive)wasplaced ona raft locatedinthecentre of Svartatjern (Fig.1c).Thispumpwaseitherusedfordrainingthepondcom- pletelyorsupplyingtheplanktonﬁlterswithpondwaterduring ﬁltrationofcopepods.Ithadacapacityof1.5m³/minandwasalso withoutzincorothermetalanodes.Topreventoverload,theraft pumpcouldnotbeoperatedatmorethan45Hz(90%offullspeed).

Thepumpwasattachedtoa2.5mhighmetalarcontheraftdeck byastainlesssteelwire,pulley,andahand-operatedwinch,and couldbeloweredtoanydepthpositioninthepond.Usually,0.3 and2mdepthswereusedforponddrainingandcopepodﬁltration, respectively.Pondwaterwaspumpedthrougha95mlongPEH pipe(Ø=200mm),eithertotheUNIK-900ﬁlterswhereitreturned toSvartatjernthroughtheair-strippingtankandpipesystem,or directlytoanother160mlongPEHpipe(Ø=305mm)fortransfer

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Fig.1.OverviewoftheSvartatjernpondwithoperationalfacilities.Thepumpbasin(A)isconnectedtoaninletpipefromthefjord,andcontainsapumpthatisdirectly coupledtoasupplypipefortheplanktonfiltersystem(B)sothatfilteredseawatercanenterthepondthroughapipeendingnearbyaraftplatformpositionedinthecentre ofthepond(C).Anothersubmersiblepumpislocatedattheraftplatform,usedforsupplyingthefiltersystemduringplanktoncollection,orforemptyingthepondthrough anadditionalpipeendingintheseaoutsidethepumpbasin.Toaccessthepond,thefacilityisequippedwithafloatingdockconnectedtoagangway(shownindarkgrey colour).

to thesea when the pond wasemptied. Flow directions were controlledby hand-operatedvalves placed ontheﬁlter system platform(Fig.1b).

Topreventstratiﬁcationoftheseawaterinthepondastain- lesssteelFlygtcompactmixerwithajetring(type4620,0.75kW, 230V,controlledbyavariable-frequencydrive)wasﬁxedinahor- izontalpositionat2.5mdepthbya5mlongverticalstainlesssteel pipe(Ø=63mm)hingedontheraftdeck.Thisarrangementallowed adjustingtheangleofpropulsionbytiltingthesteelpipetothe desiredposition.

2.3. Theplanktonﬁltersystem

ThetwoUNIK-900wheelfilterswereplacedonawoodenframe about1.1mabove theplatformdeck ontopof thedam atthe entranceofSvartatjern(Fig.1b).Eachfilterwasequippedwithtwo exchangeablefilterwheels(Fig.2)madeofaplanktonnetmounted onafibreglassring(Ø=900mm),separatingtheu-formedfilter tankintothreecompartments.Meshsizesfrom80to350␮mwere usedinfiltrationofcopepods.Thewheelfiltersystemcouldbe operatedbothmanuallyorautomaticallybyatimer.

Whenthefilterswereengaged,incomingunfilteredseawater reachedthefirstcompartmentwherecoarse-filtrationthroughthe firstwheeltookplaceasthewaterenteredthesecondcompart- ment.Next,fine-filtrationoccurredasthewaterpassedthroughthe nextfilterwheelof80␮mmeshsizeandenteredthethirdcham- ber.HereaneffluentpipeofPVCplastic (90^◦bend,Ø=200mm) determinedthewaterlevelinsidethefilters.Thewheelswerecon- tinuouslyrotating,drivenbybeltsfromanelectricenginemounted onanaluminiumframeonthetopofthefilters(Fig.2).Copepods andotherplanktonicparticlesweretrappedonthemeshscreenand broughtoutofwaterontherotatingwheels,beforebeingflushed

offthescreenintoacollectionboxofstainlesssteelwithaplastic lipscrapingtheplanktonnet.Flushingwascarriedoutbyspraying 80-␮mﬁlteredseawaterfromthethirdcompartmentthrough6 nozzlesmountedonaPVCpipe(Ø=25mm)thatcoveredtheradius ofthebackofthewheelvis-à-visthecollectionbox.Astainlesssteel GrundfosCRpump(GrundfosHoldingAS,Bjerringbro,Denmark:

www.grundfos.com)wasinstalledtopowertheflushing.Fromthe collectionboxes,thefiltrateconsistingofcopepodsandparticulate matterweredrainedbygravity,eithertowasteduringrefillingof thepondorcollectedinasetofsix250-Lfibreglasstankswithcon- icalbottomsforaccumulationoflivecopepodsandsedimentation ofnon-livingmaterial(Fig.2).Thetwosizefractionsofthefiltrate couldbedirectedthroughPEHpipes(Ø=32mm)andPVCvalvesto theinletinthebottomofanyofthecollectiontanks,whichwould takeabout50mineachtofill.

Once the collection tanks were filled, the copepod filtrates comingfromthefilterswereautomaticallyswitchedtoanover- flow tube backtothepond through theair-stripping tankand pipesystem.APVCstandpipe(Ø=40mm)wasfittedtothebot- tom,inside theconein eachcollectiontank.Thispipehad four holes(Ø=10mm)inaringabout15cmabovethebottom,which allowedparticlessettlingonthewallsandintheconetostayin thecollectiontankwhenitwasemptied.Sedimentationofnon- livingmaterialstartedautomaticallywhenthetankswerefilled andtheflowstopped,andwasnecessarytoachieveacleancon- centrate of copepods. Due to high densities of copepods with subsequentdeclineinoxygenconcentrationand possiblecanni- balism(Boersmaetal.,2014),itwasessentialthatsedimentation wascarriedoutforashortperiod,andthereforedrainingofthe collectiontanksstartedwithinhalfanhourafterbeingfilled.The designpermittedthecopepodfiltratestoleavethecollectiontanks throughthesametubesastheyenteredthetanks,andforfurther

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Fig.2.Theprinciplesoftheplanktonfiltrationsystem(slightlymodifiedfromMangor-JensenandHolm,2004).Fromthetwoplanktonnetwheelsofthefilter,thecollected copepodswereflushedviacollectionboxestosedimentationtanks,whereanotherconcentrationofthecopepodsthroughaplanktonnettookplacebeforetransporttothe larvalfishrearingfacility.Arrowsinsidetheschematicdrawingindicatedirectionofwatermovement,arrowsoutsidethisdrawingdenotesmovementofcollectedplankton.

Seetextforadetaileddescriptionofthesystemanditsoperation.

volumereductionthefiltratesweresievedthroughaplanktonnet of80␮mmeshsize(Fig.2)thatwassubmergedinanothertank belowtheplatformdeck(Fig.1b).Inthisway,upto1.4m³offiltrate couldbeconcentratedto10–15Lwhicheasilywastransportedin anaeratedcontainertothelarvalrearingfacility.Copepodsurvival duringthisfinalconcentrationandtransporthaspreviouslybeen regardedas100%(vanderMeerenetal.,2008).

2.4. Theseasonalmanagementprotocol

Svartatjernissituatedathighlatitude,withlargeseasonalvari- ationsinlightandtemperature:rangingfrom0to24^◦Cinwater,

−12 to30^◦C in air,and 6–18hsunlight/day.The protocolused formanagingthebiologicalproductioninthepondincluded1–2 completedrainagesperyeartopreventorreduceestablishmentof nuisanceorganismsinthesystemsuchasbivalves,gastropods,bar- nacles,polychaets,cnidarians,orsmallplanktivorousfishes.Such organismsmayeitherfeedonvariousstagesofcopepodsorhave planktonicstagesthatinterferewiththesizesofcopepodscollected inthefiltersforthelarvalfish.About10–12dayswasneededto drainthepond,anditwasleftemptyforanotherweekbeforerefill- ing.Thiswouldtakeanadditional7–8days.Thedurationofthe mandatorydrainageinFebruarywasadjustedtoweathercondi- tions,withtheaimtocompletetherefillingwithinthefirstweek ofMarch.Iceandsnowcovercouldbetolerated,butexposureof sedimentstoairduringextensiveperiodsoffrostshouldbeavoided tominimizecopepodrestingeggmortality(Næss,1991b).

Aproxyforrestingeggabundanceinthesedimentwasesti- matedbyhatchingcopepodnaupliifromsamplesofsediment.Six 6.6cm²and0.5cmthicksurfacesedimentsampleswerecollected withaplastictubefromthe500m²mudﬂatnexttotheconcrete damwiththeplanktonﬁlterplatform(Fig.1b),whenthisareahad beenexposedtoairfor2.5–3weeksduringdraining.Thisareacor- respondstobetween1and1.5mwaterdepthwhenthepondis

ﬁlled.Forremovaloflargeparticles,thesampleswerestirredin 0.5Lof35psuseawaterandwashedthroughplanktonnetsieves of1000and250␮m,respectively.Thenthesampleswererinsed ina40␮msieve,andtheremainsonthissievewereincubatedin glassjarswith0.5Lof35psuseawaterat23^◦Cwithboth24hroof lighting(ﬂuorescenttubes)andoutdoorlightingthroughawin- dow.Hatchedcopepodnaupliiwereremovedandenumeratedon days2and4afterincubation.

After reﬁlling Svartatjern, inorganic granulated agricul- tural fertilizer (22-3-10 NPK, Yara ASA, Porsgrunn, Norway:

www.yara.com) was added regularly to increase and maintain phytoplanktonproduction.Thisfertilizerwasaddedtoaconical 120L tanksituated nexttothemixerontheraft.The fertilizer wasslowlydissolved by a steady water ﬂow supplied through thetankby an impeller pumpset towork for a period of 5h.

Efﬂuent water wasdirected downto themixerat 2.5m depth andspreadinthepond.Theamountoffertilizerwasassessedby Secchidiscreadings,tryingtokeepavisibilityof1.0–1.5mdepth whichwouldsecureenoughlightfornetprimaryproductionin thewholewatercolumnofthepond.Fertilizationwasreducedor stoppedwhenvisibilitydecreasedinthedesiredrange,andvice versa. Alsoperiods of brightsunlight or heavy cloudy weather wereconsideredregardingtheamountoffertilizeraddedaspe- ciﬁcday,tryingtoavoidanoverloadofnutrients.Fertilizerwas added 2–3timesa week.Totals of 99,230, and 90kgfertilizer were added in 2011, 2012, and 2013, respectively. Calculated frommanufacturer’sproductinformation,1kgfertilizerwillgive a 0.34␮M increase in ammonium concentration of the pond water,andsimilarlya0.31␮Mincreaseinnitrate.Theamountof fertilizer added,calculatedas averagenumber ofkg/dayduring the120-day longgrowthperiodsbeforeandaftermid-summer, was0.27 and 0.58 (2011),1.84 and 0.14 (2012), and 0.69 and 0.08 (2013), respectively. On the actual fertilization days, total nitrogennutrientsinthepondwaterwerecalculatedtoincreaseon

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averagebetween1.7and2.5␮M,withmaximumsbetween3.2and 6.4␮M.

AlthoughsilicatepreviouslyhasbeenprovidedtoSvartatjern andothermesocosmsforenhancementofdiatomgrowth(Egge andAksnes,1992),ﬁeldobservationsandlaboratoryexperiments encompassinganumberofcopepodspecieshavefoundthatcer- tainkindsofdiatomsmaydeteriorateeggproductionandhatching causedbyspeciﬁcchemicalconstituentsofthesealgae,e.g.polyun- saturatedaldehydes(PUA)andoxylipins(Pouletetal.,1994;Ban etal.,1997;Chaudronetal.,1996;Uye,1996;Miraltoetal.,1999;

Ianoraetal.,2003,2004,2009,IanoraandMiralto,2010).Despite someuncertaintyaboutthepotentialimpairmentofdiatomson copepodreproductivesuccess(JonesandFlynn,2005;Jónasdóttir etal.,2011;Aminetal.,2011),silicatewasnotusedasaspeciﬁc fertilizerinSvartatjern.

Themixerspeedwastuneddowntotheminimumwhereade- quateoxygenlevelswerekeptatbottomdepthandnostratification occurredin the water column below thethin freshwater layer atthesurface. Toreduce thethicknessof thefreshwater layer, increasesalinity,orchangeotherhydrographicalcharacteristics, seawaterwaspumpedinfromthefjord.Before2001,Svartatjern wasemptiedandrefilledyearlyinJuly.Originally,themainrea- sonwastoberidofanunidentifiedstalkedprotozoanectoparasite thatoccurredinlargenumbersontheexoskeletonof thecope- podsin lateJune and July.Summerdrainagewassuspended in 2011and2012,andconsequentlythisparasitewasseenagainin 2012.However,summerdrainagewascarriedoutin2013.These proceduresweresimilarasinFebruary,andafter1–2weeksair exposureandrefillinginlateJuly,newcopepodgenerationsquickly re-establishedfromhatchingofrestingeggs.Astemperaturefell duringtheautumn,keepinghightemperatureinthepondwater wasfacilitatedbythethinfreshwaterlayer.However,toprovide thebest survival of copepod restingeggs during winterquies- cenceinJanuary(Holmstrupetal.,2006),coolingofthepondwater wasinitiatedinearlyDecemberbytiltingthemixerforbreaking downthesurfacefreshwaterlayer.Exacttimingwasdeducedfrom evaluationofweatherforecastsandactualconditions,particularly regardingrainfallandairtemperature.Duringwinter,theaimfor thehydrographicalconditionsofthestagnantpondwaterwasto keepsalinityabove23psu,andtemperaturebetween0and4^◦C.To bepreparedfordrainageinFebruary,themixerwastakenoutof waterandthepumpontheraftloweredto0.5mdepth.Thiswould ensurethatdrainagecouldstartinFebruary,evenwithicecover onthepond.Lackofmixingwouldallowoxygendepletionofthe waterduringthewinterquiescenceandpossiblyanoxiatobuildup inthebottomwaterandsediment.

2.5. Hydrographicalandbiologicalsampling

Twiceaweekthroughtheproductionseason,hydrographical measurements,Secchidepth, andbiological samplingwerecar- riedout.Hydrographicaldata(temperature,salinityand oxygen saturation)werecollectedbetween09:00and13:00ontheraft (Fig.1c)at0,0.5,1,2,3,3.5,and4mdepthsbyahandheldWTW 310imulti-parameterinstrumentwithConOx-6combinationelec- trodefor oxygen, temperature, and conductivity(WTW GmbH, Weilheim,Germany:www.wtw.de).Secchidepthwasmeasured withawhitedisc(Ø=300mm)inthesameposition,aswascol- lectionofzooplanktonforabundanceestimation.Zooplanktonwas firstcollectedbya12.2-LSchindler-Patalastrap(Schindler,1969) until30thSeptember2011,whenitwasreplacedbya11.7-Ltube sampler.Thetubesamplerwasa2.32mlongPEHpipe(Ø=80mm inside)withavalve ontop.Thetubewasquicklyloweredinto thepondwaterinverticalpositionandthevalveclosedwhenthe tubewasfilled.Theenclosedbodyofwaterwasfilteredthrough a60␮mplanktonnetsieveandfixedina1:50Lugol’ssolution.A

comparisonbetweenthetwoplanktonsamplingdevicesusedgave anestimateof3.8%lessplanktonnumberswiththetubecompared totheSchindlertrap.Thetubeseemedtocollectcopepodnauplii moreefﬁciently(17.9%),whiletheSchindler-Patalastrapwasbet- terforthecopepodidsandadultcopepods(18.9%).Amongthree replicatetubesamples,coefﬁcientofvariationwas8and36%for naupliiandcopepodids,respectively.Sincethetargetoftheplank- ton collectionwasnauplii,and thetubesampledseveraldepth strata,furthersamplingwascarriedoutwiththetube.

The sampleswerestored darkat 4^◦C and enumerated after ﬂushing with fresh water in a 40␮m sieve. To obtain a rea- sonable counting density (minimum 100 individuals of the mostcommon plankton category),a plankton splitter (Motoda, 1959) was used to a maximum of 5 rounds when necessary.

Counting and identiﬁcationwascarried out witha LeicaMZ75 stereomicroscope(LeicaMicrosystemsGmbH,Wetzlar,Germany:

www.leica-microsystems.com),usuallyat25×magniﬁcation.

3. Results

3.1. Hydrographicaldata

Afterfilling Svartatjernin March,salinity wasbetween30.5 and32.5psuwhichreflectedthevariabilityat35mdepthinthe nearbyfjord at thattime of theyear. Asteady drop insalinity occurredthroughouttheproductionseason,endingupintherange of24–25psuinlateNovemberin2011and2012(Fig.3a).In2013, salinitywas33psuatrefillinginJuly,decliningto27psuinlate November.Salinity wasincreased severaltimesby pumpingin seawaterduringtheproductionseasons(Fig.3a).

Atthestart in March,temperatureincreasedfroman initial range3–7^◦Ctoaplateauof15–20^◦CattheendofMay(Fig.3b), increasingfurtherto20–24^◦CinJuneandJuly.Forthetwoyears with no summer drainage (2011 and 2012), the temperature decreasedsteadilyfromlateAugust,toabout8^◦CinlateNovember.

In2013whenSvartatjernwasdrainedinJuly,anewinitialtemper- atureof17^◦Cwasmeasuredatthetimeofcompletedreﬁlling.The temperaturethenincreasedto22^◦CbytheendofAugustandsub- sequentlydeclinedtolateNovemberinaccordancewiththetwo previousyears.

Oxygen saturation increased rapidly after pond reﬁlling in March(Fig.3c),peakingat200–260%inAprilandMay.FromJune toAugust,oxygensaturationvariedbetween90and220%,declin- ingslightlybeforeitfellquicklyinmidOctobertolevelsbelow 60%.Secchidiscreadingsshowedthatvisibilitydeclinedallthree yearsfrom4.5m(bottom)atﬁllinginMarchtobetween2.6and 1.4minMay(Fig.3d).In2011and2012,visibilitywasbetween0.6 and2.1mfromJuneuntillateOctober,beforeitincreasedquickly tomorethan4minNovember.In2013,atechnicalincidentwith thefertilizationpumpmadeitimpossibletoaddfertilizerbetween late Augustand midOctober, resultingin visibilityinthepond remainingabove3.5mduringthatautumn.

3.2. Copepodspeciesandabundances

Theabundanceofcopepodrestingeggsinthesedimentswasnot determinedinFebruary2011.However,veryfewcopepodnauplii orcopepodidswereobservedinthepondwaterduringMarchand Aprilthatyear.Aslightincreaseincopepodidsseemedapparentin lateMay,buttheplanktonwasnotcountedbeforeasamplingpro- grammewasestablishedinmid-June.Lowabundancesofcopepod naupliipersisteduntilincreasingrapidlyinlateJuly.Totaldensities ofcopepodnaupliipeakedbetween40and50/Lseveraltimesfrom AugusttoOctober,andincreasedfurthertoamaximumof90/L inNovember(Fig.4a).Peaksindensitiesofcopepodidswereseen

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Fig.3.HydrographicaldataandvisibilityinthecentreofSvartatjernfromthreeyearsofoperationafterthereconstructionofthepond.

inmid-August,earlyOctober,andNovember,withmaximumsof 110and98/LforstagesCI-CVandCVI,respectively.Cyclopoidand harpacticoidcopepodsoccurredinlownumbers(<12/L)through- out2011,andmiscellaneouszooplanktonlikecladocerans,rotifers, gastropods,bivalves, and polychaets were observed, but rarely exceeded10/L.Ofthecalanoidcopepods,Centropageshamatus(Lill- jeborg)andAcartialongiremis (Lilljeborg)wereidentiﬁedasthe mostabundantspecies(Fig.4b),butalsoAcartiadiscaudata(Gies- brecht)wasobservedinsomequantities.

InFebruary 2012,incubation ofsediments conﬁrmedoccur- rence of resting eggs. An average density of 43.8 viable eggs/cm²±23.9(SD)wasfoundfromcountsofhatchednauplii.

Thiscorrespondedtoatotalof4.6±2.5billionrestingeggsinthe pond,assuminganevendistributionoftheeggsonthesediment area.ShortlyafterreﬁllinginearlyMarch,copepodnaupliipeaked at239/L,followedbysuccessiveabundanceelevationsofcopepo- didsCI-CV(149/L)andcopepodidCVI(63/L)(Fig.4c). Frommid ApriltoendofMaycopepodnaupliivariedbetween87and252/L, andthendroppedbelow83/LuntilendofAugust.Anincreasein copepodnaupliiabundanceoccurredduringautumn,withamaxi- mumof177/Linmid-September,butdeclinedto16/LinNovember.

However,copepodiddensities remainedlow during this period anddidnotexceed33and11/Lfor stagesCI-CVand stageCVI, respectively.In 2012,A. longiremis wasthe dominating species amongthecalanoidcopepods(Fig.4d),andcyclopoidandharpacti- coidcopepodsneverexceeded9/L.Nonewcopepodspecieswere detectedin2012.Miscellaneouszooplanktonwasbelow8/Luntil lateSeptemberbutincreasedduringOctobertoamaximumof66/L duetoabloomofpolychaetlarvae.

InFebruary2013,therestingeggabundancewasdeterminedto 432/cm²±164,givinganestimateof45.4±17.3billionrestingeggs inthepond.Hatchingoftherestingeggsimmediatelyafterreﬁlling ledtocopepodnaupliidensitiesabove161/L,withapeakof584/L atonsetofApril(Fig.5).FromApril,copepodnaupliiabundance

declinedto42/LatendofJunewhensummerdrainagewasiniti- ated.Copepodidsandadultcopepodsfollowedthesamepatternas thenauplii,butwithadelayresultinginapeakof529/Linlasthalfof April.AfterreﬁllinginAugust,copepodnaupliiandoldercopepod stagesneverexceeded72and85/L,respectively(Fig.5).Copepods werenotidentiﬁedtospeciesin2013,butboth Centropagessp., Acartiasp.,andharpacticoidswereobserved.Inparticularharpacti- coidswereseenduringtheautumnof2013.Bivalvelarvae,most likelythecommonEuropeancockle(CerastodermaeduleL.)andthe bluemussel(MytilusedulisL.)thatbothwereobservedinthepond, bloomedduringApril2013,butwerenotenumerated.

3.3. Copepodharvest

Copepodharvestwascarriedoutinboth2012and2013(Fig.6), andaddeduptoatotalof2.6and1.6billionindividualsover40 daysin2012and61daysin2013,respectively.Planktonfiltration wasdoneinperiodsof3.0–16.0h/day(2012)and2.5–13.3h/day (2013).Thefiltrationefficiency,givenasnumberofcopepodsin anystagecollectedperhourfortransporttothelarvalfishhatch- ery (Fig. 6), was on average7.0×10⁶±5.9×10⁶ for 2012 and 4.8×10⁶±3.2×10⁶for2013.

Damage from wheel ﬁlter ﬁltration to the copepods was assessedtobelessthan2%oncopepodidswhencomparingphys- icaldamageoncopepodsfoundinthesedimentedmaterialinthe collectiontankswiththetotalamountsofcopepodscollectedin thethesetanks.

4. Discussion

Copepod production techniques have diversiﬁed by a num- ber of methods ranging from outdoor extensive production in pondstohigh-densityindoorintensivesystemsundercontrolled environments(vanderMeerenand Naas,1997; Støttrup,2003;

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Fig.4.CopepodabundancewithspeciesandstagedistributioninSvartatjernduringthetwoﬁrstyearsofoperationaftertherestartin2011.

Engell-Sørensenetal.,2004;Ogleetal.,2005;Drilletetal.,2011a).

TheseawaterpondSvartatjernbelongstotheextensiveapproach and hasbeen successfully usedin 20 of the last 30 years as a providerofhigh-qualitycopepods(vanderMeerenetal.,2008)for avarietyofscientiﬁcstudieswithlarvalmarineﬁsh(e.g.Kjørsvik

Fig.5.AbundanceofcopepodnaupliiandcopepodidsinSvartatjernin2013.

et al.,1991; vanderMeeren,1991; vanderMeerenand Næss, 1993;Næssetal.,1995;Conceic¸aoetal.,1997;Suthersetal.,1999;

Finnetal.,2002).Marineorbrackishwaterpondsormesocosms producingcopepodsbytheextensivemethodaremostcommonly exploredorusedintropicaltotemperateregions(Leeetal.,2004).

Incontrast,Svartatjernisahigh-latitudemesocosmsystemthat candelivermainlycalanoidcopepodsupto8monthsayear.The advantageofalarge-sizedpondisitsabilitytosupplysubstantial amountsofcopepodsofanystageoveraprolongedperiodwhen high-qualitypreyforlarvalmarinefishisneeded.Thiscaneven bethecasewhencopepoddensitiesarelow,becauseinsuchlarge systemscopepodharvestisamatteroffilteringcapacityandeffi- ciency.Forexample,aftertherestartin2012,Svartatjernsupplied successfullyalltheliveprey,bothcopepodnaupliiandcopepo- dids,inanutritionalstudyofinitially300,000Atlanticcodlarvae (GadusmorhuaL.)overa40-dayperiodfrominitiationofexogenous feedinguntilthelarvaewereweanedonaformulateddiet.

ThecopepodproductioninSvartatjerniscarriedoutinacom- pleteandnaturalecosystemwheremanipulationofthepondkeeps thecopepodsastopgrazersorpredatorsofthefoodweb.Here, thecopepods mayfeedona naturalassemblage ofautotrophic algae,heterotrophicﬂagellates,andprotozoanslikevarioustypes ofciliates.Theassemblagesofsingle-celledorganismshaveshown highdiversitywithregularlyhighdensitiesofmicroalgaeinthe sizerange3–5␮mequivalentsphericaldiameterbeforethepond was closed in 2001 (Fig.7).Ciliates were occurring frequently (Naasetal.,1991),withaveragesbetween40and60ciliates/mL overa yearofweeklysampling(Fig.7).Thelower-trophic-level

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Fig.6.Amountsofcopepodsharvestedandﬁltrationrateoftheplanktonﬁlter systemduringspringseason2012and2013.

planktoncommunities in Svartatjernshouldtherefore ensure a highlydiverseandsufﬁcientfoodsourceforthecopepodsinthe pondwhichinturnmayenhancecopepodgrowthandeggproduc- tionratesbyselectivefeedingandoptimalnutrition(KleinBreteler, 1980;StoeckerandEgloff,1987;WiadnyanaandRassoulzadegan, 1989;TurnerandGranéli,1992;OhmanandRunge,1994;Kleppel andBurkart,1995;MilioneandZeng,2007;Camusetal.,2009;

Dhankeretal.,2013).Optimalnutritionalstatusofthecopepodsin Svartatjernhasindeedbeenveriﬁed(vanderMeerenetal.,2008).

However,despiteapparentlyhighalgal productionas indicated

Fig.7.Averageabundanceofmainphytoplanktongroupsandciliatesamongweekly samplesfrom1mdepthinSvartatjernfromMarchtoOctober(seasonofnetpri- maryproduction)duringtheyears1998,2000,and2001(datafromsamplesfixed inpseudo-Lugol,Verityetal.,2007).Notethelogarithmicordinateaxis.Between 91and98%oftheflagellatesandmonadswereunidentifiedandinthesizerange 2–5␮mequivalentsphericaldiameter.

from previousSvartatjern data (Fig. 7), oscillations in copepod densities showthat there are traits of copepod feeding,repro- duction,andlifehistorythatarenotyetfullyunderstoodinthis system.Forexample,anumberofcopepodspecieshavebeendoc- umentedtobeomnivorousfeedersthatalsocanpreyoncopepod eggsandnauplii(Landry,1978;Conley andTurner,1985;Daan etal.,1988;HadaandUye,1991;LazzarettoandSalvato,1992;Uye andLiang,1998;Boersmaetal.,2014),and theregulatoryforce ofsuchpredationshouldbeincludedinfuturestudiesofcopepod dynamicsinSvartatjernorsimilarsystems.

In2012and2013,thecopepodproductioninSvartatjerndur- ingthespringseasonwasclearlylinkedtooccurrenceofresting eggsinthesediments,asthecopepodnaupliiabundancespeaked shortlyafterrefillinginearlyMarch.Thenaupliiabundancesin Marchwerealsothreetimeshigherin2013comparedto2012, whenamountsofnaupliihatchedfromrestingeggsinthesedi- mentsampleswereten-foldhigherthanin2012.Nosuchearly naupliarpeakwasobservedin2011whenthepondwasrefilled after10yearsofrest.Alsothesmellofsulphidewasprominent whenthe10-year-oldseawaterwasremovedfromtheponddur- ingFebruary2011.Thisindicatesthatathickfreshwaterlayeron top ofa long-lastinganoxicsulphide-richseawater layerprob- ably killedall remnant resting eggsin thesediments fromthe production periodsbefore 2002.Of thethree previously domi- natingcopepodspecies,Eurytemoraaffinis(Poppe)andParacartia grani(Sars)didnotreoccurafterthe10-yearsrestperiod,whileC.

hamatusagainwasobservedin2011alongwithnewAcartiasp., predominantlyA. longiremis.Onlyaone-yearproduction season wasnecessarytoprimethepondwithenoughrestingeggsforuse asapredictablesourceofcopepodharvesting.Underfavourable feedingand environmentalconditions, copepods produce subi- taneous eggsthat will hatch withindays. In addition, calanoid copepods species may produce restingeggs when approaching unfavourableconditions, includingA. longiremisand C. hamatus (Alheitetal.,2005;MarcusandLutz,1998).Restingeggscomprise threetypesofdormancy,fromquiescence(retardeddevelopment), oligopause(delayedhatching),todiapause(arresteddevelopment) (GriceandMarcus,1981;Dahms,1995;Marcus,1996;Chenand Marcus,1997;Marcus,2006;Drilletetal.,2011b).Diapauseeggs mayhaveextremelongevityandhavebeenfoundviableafterabout 70 years in sediments of good conditions (Dahmset al., 2006;

Sichlauetal.,2011).Restingeggsareacommonlifehistorychar- acteristicofmanyneriticcalanoidcopepods,andsucheggsoccur frequentlyathighdensitiesinthesedimentsofNorwegianmarine enclosures,lagoons,andponds(Næss,1991a,1996).Theseeggs willsinktothebottomandaretoleranttoadverseenvironmental conditionslikelargetemperatureﬂuctuations,desiccation,freez- ing,anoxia,andsulphideexposure,aswellasexposurestovarious chemicalslikedisinfectionagents(Næss,1991a,1991b;Næssand Bergh,1994;MarcusandLutz,1998).Alsosubitaneouseggsseem tohavemechanisms toresistdetrimentaleffects ofanoxia and sulphideforsomeperiod(Nielsenetal.,2006).Copepodspecies thatproducerestingeggsareidealtohigh-latitudeenclosureor pondrearing.Theseeggswillsurvivedrainingduringbothwinter andsummerwhichmaybecarriedouttopreventestablishments ofnuisanceorganisms.Itisnotclearwhethertherestingeggsin Svartatjernarequiescence,oligopause,ordiapauseeggs,butcope- podeggswithspinysurfacestructuresresemblingthosedescribed byCastro-Longoria(2001)forAcartiatonsa(Dana)diapauseeggs havebeenobservedinthepond.However,surfacestructuresmay notbeusedasanindicatorofeggtypesinceHansenetal.(2010) described4–5varietiesofsimilarspinysurfacestructuresonAcar- tiaspp.andC.hamatussubitaneouseggs.Næss(1991a)suggested thattheoverwinteringrestingeggsinSvartatjernmostlikelywere diapauseeggs becauseoftheir longevityand tolerance.On the otherhand,thesynchronoushatchingmayindicatethattheresting

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eggsinSvartatjernarequiescenceeggs.Effortstodevelopsecure methodsfordeterminationof eggtypesin copepodsareclearly needed.

Basedon20yearsexperienceinSvartatjern,aseasonalman- agementprotocolhasbeendevelopedwherecoolingandstopin mixingareinitiatedinDecemberforenhancementofrestingegg wintersurvival,followedbydrainagesinFebruaryandJulyforcon- trolofnuisanceorganisms.Theseconddrainingduringsummer wasforvariousreasonsnotcarriedoutin2011and2012,withthe resultofbloomsofpolychaetandbivalvelarvaeduringfall2012 andspring2013,respectively.InDecember,controlledcoolingto 1–4^◦Ciscarriedoutbyadjustingthemixer’sangleandspeed,fol- lowedbyacompletestopinmixingduringJanuarywithoxygen depletionandsulphideformationinthesedimentsandinthewater layersoverthebottominthedeeppartsofthepond.Suchcondi- tionsmaybebeneﬁcialfortherestingeggs,andtheanoxiclayer mayalsoreducethelifeconditionsfor nuisanceorganisms.The combinationoflowtemperature(4–5^◦C)andplausiblylowoxy- genoranoxicconditionshavebeenshowntoprolongthesurvival ofcopepodeggsstoredinsedimentswhencomparedtostorageat highertemperatures(Uye,1980;BanandMinoda,1992).Diapause eggsofC.hamatushavebeenfoundtosurviveaslongas437days atambientﬁeld temperaturesin anoxiacompared tonormoxia (MarcusandLutz,1998).Furthermore,anoxicconditionshavepro- motedgreateraccumulationofviableA.tonsaeggsatthesediment surfacethaninnormoxictreatments(ScheefandMarcus,2011).

Storageconditionswithtemperaturesbelow5^◦C,mediumsalin- ities,andanoxicconditionshavebeenfoundmostoptimalforA.

tonsaquiescenceeggs(Holmstrupetal.,2006).Followingthepro- ceduresofthemanagementprotocolledtohighdensitiesofviable restingeggsintheSvartatjernsedimentinFebruary2013,compa- rabletothehighestrestingeggabundanceobservedinNorwegian pondsandenclosures(Næss,1996).Temperaturesbelow1^◦Cdur- ingthechillingperiod(Fig.1b)didnotseemtoharmrestingegg viabilityinthepond.DrainingofthepondinFebruarywillexpose thesedimentsandtheeggstocombinationsofrain,snow,frost, orsunforaperiodofsomedays(inthecentre)to3–4weeks(in theshallowareas).Reﬁllingimpliesa3–5^◦Cincreaseintempera- tureandelevatedoxygensaturationtonear100%.Thesechanges happenafewweeksbeforespringequinoxwhenthedailychange inphotoperiodisapproachingitsmaximumandbecomesconsis- tentwitha12L:12Dcycle.Althoughthemechanismsforhatching ofrestingeggsinSvartatjernarenotclear,hatchingofdiapause eggshasbeensynchronizedinrelationtoﬂuctuationsintemper- atureandphotoperiod(Marcus,1996;BoyerandBonnet,2013).

Othercueslikeoxygenconcentrationhavealsobeensuggestedas aregulatorymechanism(UyeandFleminger,1976;Dahms,1995).

EggscollectedfromtheSvartatjernsedimentinlateFebruarywere easilyhatchedwithin2–4daysinthelaboratoryunderevenmore extremechangesoftheenvironmentwhenbroughtfromambient outdoorconditionstoindoorwith23^◦C,100%oxygensaturation, and24hrooflightwithanadditional11L:13Dcyclesignalbynat- urallightthroughawindow.

Use of semi-naturalcopepod production in largeenclosures mayintroduce a potentialrisk for transferof diseases or para- sites(Suetal.,2006;Drilletetal.,2011b).Useofzooplanktonfrom largeenclosedlagoonsinNorwegianmarineaquaculturehasgiven infectionsofhelminthparasitesinAtlantichalibut(Hippoglossus hippoglossusL.)larvae(Berghetal.,2001).Similarly,thedigenic trematodeCryptocotylelingua(Creplin)whichusesthegastropod Littorinalittorea(L.)asintermediatehost,havebeenobservedin copepod-rearedturbot(ScophthalmusmaximusL.)larvae(unpub- lisheddata).However,duringthe20yearsthatSvartatjernhasbeen usedasacopepodpondforlarvalﬁshexperiments,infectionsof ﬁshparasitesorpathogenshasnotbeentracedbacktothepond, thoughmeasuresshouldbetakentomonitorthisinthefuture.In

contrasttothelargeenclosedlagoons,Svartatjerncanbedrained completely.Further,thecopepodproductionisbasedonresting eggswithinthesystemandnotcollectionofwildplankton.Inthese ways,intermediatehostswillnotbeallowedtoestablishinthe pond,whichmayexplainthegoodrecordofSvartatjernsofar.

Extensive production in ponds orlarge enclosuresgenerally entailslargenumbersofcopepodsatmoderatetolowdensities (Ogleetal.,2005).Thus,becauseofthelargevolumeofsuchsys- tems,theﬁltercapacitywilllimittheamountthatcanbeharvested.

InSvartatjern,theharvestrateestimatedfromthepumpcapacity wascalculatedto0.24%ofthepondvolumeperhourofﬁltration, whichcorrespondedtoanactualdailyﬁltrationrateintherangeof 0.7–3.8%ofthetotalpondvolumein2012and1.2–2.4%in2013.

Given anexponentialdecay model,it willtake19 and99 days atthehighestandlowestofthesefiltrationrates,respectively,to remove50%oftheplanktoninthepond.Itshouldbenotedthat thesefiltrationratesonlyrepresenttheneedsforlivefeedduring specificstart-feedingexperimentswithmarinefishlarvae.During earlystart-feeding,onlynaupliiareneeded,andcopepodidsare continuouslytransferredbacktothepondwithoutbeingregistered intheharvest.Further,improvementsinfiltrationtechnologycan clearlyimprovethedailyyieldfromSvartatjern.Thequestionis howmuchcanbefilteredperdaywithouthaltingthecopepodpro- duction?Copepodgenerationtimewillvarywithtemperatureand feedconditions,andharvestingrateshouldbelowerthanreproduc- tionrate.MaximumyieldofAcartiatsuensis(ItoTak)from24m³ outdoortanksat24–28^◦Candchlorophyll-alevelsof10␮g/Lwas achievedwhen15to30%oftankvolumewasharvesteddaily(Ohno etal.,1990).Betterreproductiveabilitieswereachievedatlower populationdensities,andhigherpopulationdensitiesresultedin anextensionofdevelopmenttimeforthelastnaupliarstageand allthefivecopepodidstagesthatwerenotcausedbyfoodlimi- tation.Further,a10%dailyharvestrateofEurytemoraaffinisfrom 25m³outdoortanksattemperaturesintherange15–20^◦Cgave stablelevelsofnaupliiabundancewhentheremovedwaterwas replacedbynewseawateroralgalcultures(Nellenetal.,1981).

However,whenthecopepodswereharvestedbynetsandnowater wasreplaced,thenaupliiabundancedecreased.Consideringthese resultsandthechemicalmediatedswitchtodiapauseeggproduc- tionin Eurytemoraaffinisat highpopulationdensities (Ban and Minoda,1994),factorslikeperiodicwaterrenewal,increasedfil- tercapacity,andharvestratemightbebeneficialforthestabilityof thecopepodpopulationsinSvartatjern.Diapauseeggproductionis primarilyinitiatedbychangesinphotoperiodandtemperatureas cuesforsignallingcommenceofenvironmentaladversityforapar- ticularcopepodspecies(Dahms,1995;HairstonandKearns,1995), butalsocrowdingandinsufficientfeedconditionshavebeenshown toinducediapauseeggproductionincalanoidcopepods(Banand Minoda,1994;Drilletetal.,2011b).Detailsofthesemechanisms wouldenablecontrolledrestingeggproductionpriortodrainages ofthepond.Tooptimizecopepodcultureandharvestinalarge enclosurelikeSvartatjern,moreknowledgeisneededaboutcope- podpopulationdynamicsandlifestrategiesatspecieslevelunder variouscultureandharvestconditions.

In conclusion, anenclosure like Svartatjernmay provide all stages of copepodsin adequateamounts forlargestart-feeding experiments withlarvalmarine fish,and where thenutritional requirementsareof mostimportance.The copepods fromsuch enclosurescouldalsobeusedbycommercialhatcheries,atleast forsupplementofnutritionallyhigh-qualitylivepreyduringcrit- icalperiodsoflarvaldevelopment(Næssetal.,1995).Duetothe largeenclosuresize,potentiallimitationsinharvestcausedbyfluc- tuationsofthecopepodabundanceinSvartatjerncanbeovercome byadjustingthedailyfiltrationrate.Further,theseasonalchanges inlightandtemperaturemakethecopeodproductioninSvartat- jernhighlydependentonrestingeggs.Thishasenabledaseasonal