Closed Blue CraB shedding systems: understanding Water Quality

Marc Turano

glossaryDO DissolvedoxygenNH3

- Un-ionizedammoniaNH4

+ IonizedammoniaNO2

- NitriteNO3

- NitrateTAN Totalammonianitrogen

about the authorMarcTuranoisNorthCarolinaSeaGrant’smaricultureandbluecrabspecialist.HeisalsothecoordinatorfortheN.C.BlueCrabResearchProgram,wherehehelpspotentialapplicantswithgrantproposalsandprojectdesign,aswellasmonitorsongoingbluecrabresearch.

TuranoreceivedhisPh.D.inzoologyfromNorthCarolinaStateUniversity.Healsoholdsabachelor’sinmarinebiologyfromtheUniversityofNorthCarolinaatWilmingtonandamaster’sinmariculturefromTexasA&MUniversity-CorpusChristi.

about the n.C. Blue Crab research ProgramTheN.C.BlueCrabResearchProgram(BCRP)isfundedbytheN.C.GeneralAssemblyandadministeredbyNorthCarolinaSeaGrant.TheBCRPencouragesinnovativeproposalsthatwillimprove,protectandrestoreNorthCarolina’sbluecrabresources.Whereappropriate,BCRPseekstopartnercommercialandrecreationalfishers,andpersonsintheseafoodindustrywithacademicresearcherstoensureuseful,well-documentedresults.

AlistofpotentialresearchtopicsandalistofSeaGrantandN.C.DivisionofMarineFisheriescontacts,aswellasongoingandpreviousBCRPprojects,areavailableatwww.ncseagrant.org.Under“FundingOpportunities,”selecttheBlueCrablink.InformationisalsoavailablefromNorthCarolinaSeaGrantofficesinRaleigh,Manteo,MoreheadCityandWilmington.

PhotosKenBlevinsandMarcTurano

Crab artDuaneRaver

Closed Blue CraB shedding systems: understanding Water Quality

ThesoftcrabindustryinNorthCarolinaisasignificantpartofthestate’sbluecrabfishery.From2001to2005,theaverageannualland-ingsforpeelercrabsandsoftcrabsinthestatewere1.6millionpoundswithatotalvalueof$4.8million,accordingtotheN.C.DivisionofMarineFisheries.In2007,therearemorethan300licensedcrabsheddersinNorthCarolinaoperatingmorethan5,500sheddingtanks.Theseoperatorseitherpurchasepeelercrabsforshedding,orharvestandshedtheirowncrabs.In2002,NorthCarolinaSeaGrantresearchersDavidEgglestonandJuanChavezreportedmortalitiesin11softsheddingsystemsthatrangedfrom10to30percent.Becausesoftcrabsareworthuptoseventimesthevalueofhardcrabs,suchlossescansignificantlyreduceprofitability.Further,bluecrabsinNorthCarolinaarelistedasaspeciesofconcernandexcessivemortalitiesexperiencedinthesoftcrabfisheryrepresentaninefficientcomponentofthefishery.

ThreecrabsheddingtechniquesareusedinNorthCarolina:floatingtrays,onshoreflow-throughsystemsandclosedrecirculatingsystems.Mostoperationsinthestateutilizetheonshoreflow-throughsystemdesign,whichrequirestheleastmaintenance.However,thisdesignreliesontheavailabil-ityofaconsistentsupplyofcleanwater,whichcanfluctuatewithchangingenvironmentalconditions.Closedsheddingsystemsminimizefluctuationsinenvironmentalconditionsandallowcrabsheddingoperationstomovetolessexpensiveinlandareas.Thesesystemsrecyclewaterthroughaseriesoffilterstominimizetheneedfornewwater.However,closedsheddingoperationshavenotincreasedinNorthCarolina.Thisislikelyduetoanincompleteunderstandingofthetechniquesrequiredtomanagethesesystems.

Closedsheddingsystemsrequireproperdesignandmaintenance.Operatorsofthesesystemsmustmaintainsufficientwaterqualityforcrabstosurvive.Parameters,suchastotalammonianitrogen(TAN),nitrite(NO2

-),nitrate(NO3

-),salinityand—mostimportantly—dissolvedoxygen(DO),shouldbetestedregularly,andmorefrequentlyduringpeaksheddingseasons.Torunclosedsheddingsystemssuccessfully,anoperatormusthaveathoroughunderstandingofhowthesewaterqualityvariablesinteract.

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the good and the BadTheclosedsystem,likeothersheddingsystems,hasbothadvantagesanddisadvantages.

advantagesTheclosedsystemallowsforincreasedcontroloverthesheddingenvironment—thewater.Inaclosedsystem,salinityandtemperaturecanberegulated,minimizingpotentialproblemswithexcessiverainfallordrought.Addition-ally,inareaswheretidalmovementscausedbywindpatternscancompletelyremoveavailablewater,closedsystemscancontinuetooperate.Siltationproblems,suchasmuddywater,areminimizedbecauseaconstantsupplyofcleanwaterisnotnecessary.Anotherimportantadvantageistheflexibilityinlocatingthesystem.Withouttheneedforawatersourcenearby,closedsystemscanbeconstructedawayfromexpensivewaterfrontproperty.

disadvantagesClosedsystemsaremorecomplexthanflow-throughsystems.Watermustbefilteredsothatrecycledwaterdoesnotdegradetheenvironmentinthetanksandharmthesheddingcrabs.

Dependingonthelevelofsophistication,closedsystemscanbecostly,particularlyduringthestart-upphase.Filtersaddtotheexpendi-turesfortanks,pumpsandplumbingsuppliesusedinflow-throughsystems.Closedsystemsalsorequiremoremaintenance.Regularfiltermaintenanceandwaterqualitymonitoringarerequiredtoensureoptimalholdingconditionsthatminimizestressonthecrabs.Closedsystemsdonotofferthedilutionfactorofflow-throughsystems.Ifaclosedsystemexperiencesasuddenboutofpoorwaterquality(forexample,aspikeinammonialevels),theonlymethodofrapidremovaliswithwaterexchange.

Anotherdisadvantageofclosedsystemsistheinabilitytofillthetankswithwaterandimmediatelystockthemwithcrabs.Closedsystemsneedtobestart-edearlierintheseasonbecausethesystemneedstobe“cycled”toestablishapopulationofhealthybacteria.Furthermore,ifasheddingoperationcontinuesthroughoutthehardcrabbingseason,thebacteriaestablishedatstart-upmustbefedtosustainsufficientlevelsforperiodicstockingofpeelers.

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When is a Closed system appropriate?Crabsheddingoperatorsmustconsiderseveralfactorswhendecidingifaclosedsystemisthemostefficientmethodofsheddingcrabs.Factorstoconsiderinclude:

•Location •Levelofproduction/risk •Cost

Locationisoneofthemostimportantfactorswhenconsideringaclosedsystem.Often,anoperatordoesnothaveachoiceinthelocationforacommercialoperation.Considerthesourceofwateratthechosenlocation.Somesheddingoperationsusewellwaterandaddcom-merciallyavailableseasalt,whileothershaulorpumpwaterfromnearbyrivers,creeksorestuar-ies.Thewatersourceshouldbechosenbasedonquantity,qualityandcost.Althoughlargevolumesofwaterarenotnecessaryforaclosedsystem,itisbeneficialtohaveaccesstoagoodwatersourcefor

emergencies,aswellasforfillingand/orflushingthesystem.

ThewaterforaclosedsheddingsystemshouldhavecertaincharacteristicsincludingbalancedpHandalkalinity.Italsoshouldbefreeoftoxiccompoundssuchasheavymetalsandpesticides.Althoughthesewaterqualityvariablescanbemanipulatedwithchemicals,itisbesttouse“good”wateratthebeginningtominimizecostlychemicaladditions.

Anotherconsiderationisthedesired level of production/risk.Operatorsmustmanageallsystemstoensurelowcrabmortality.Removalofdeadandnewlymoltedcrabsarerequiredforthesystemtoremainoperableandprofitable.However,closedsystemsrequireagreaterdegreeofmanagement.Ideally,DOlevelsshouldbecheckeddaily,andallotherparameters—TAN,NO2

-,NO3-,

salinityandpH—weekly.

Althoughthereisnoexactschedule,waterqualitymonitoringshouldincreaseduringthepeakseason.Duringtimesoflowerstockingdensities,monitor-ingcanbereduced.Responsetimeisakeycomponentwhenrespondingtoawaterqualityproblem.Thecrabscannotmoveastheywouldtoescapepoorconditionsinthewild.Routinewaterqualitysamplingandrecordingofthesemeasurementscansignificantlyreduceunexpectedchangesinwaterquality.

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Foroperators,experienceisthebestteacherformanagingclosedsystems.Fornewcomers,anappropriatedesigniskey,alongwithfollowingmanagementrecommendations(forexample,maintainingwaterquality).However,becausesheddingcrabsremainssomewhatofanart,themoreexperiencedanoperatorbecomes,theeasierthesystemistomanage.Closedsystemsrequireadditionaltimeforwaterqualitymaintenance,whichshouldbeconsideredwhendecidingwhichtypeofsystemtoconstruct.

Inmanycases,costisthelimitingfactorwhenoperatorsconsiderwhattypeofsheddingsystemtoconstruct.Unfortunately,budgetconstraintsoftenresultinpoorlybuiltandineffectivesheddingsystems.

Whendesigningasystem,operatorsshouldidentifythemaximumnumberofsheddingtankstoinstall—immediatelyandinthefuture.Thisistypi-callyaccomplishedthroughthebusinessplan,whichaccountsforthenumberofcrabsthatneedtobeproduced,alongwithmarketdemand/pricingandoverhead.Purchasepumpsandfiltersthatarecapableofhandlingthefuturecapacity.Oversizedpumpsandfilterswillnotharmasheddingsystemandwillminimizefuturecostsofupgrades.Conversely,underestimatingthesizeofthesecomponentscanunderminethesuccessofasheddingoperation.

Ifaclosedsystemisappropriateforaspecificlocation,properdesignisessen-tial.Closedsheddingsystemsutilizethesamesheddingtrays,pumpsandpipingasflow-throughsystems.Thedifferenceisinthefilters.Beforedesigningappropriatefiltersforaclosedsheddingsystem,opera-torsmustunderstandinteractionsinwaterqualitythatoccurinthesesystems—and,inturn,thepurposeforthevariousfilter-ingcomponents.Theremainderofthisdocumentwillfocusonthespecificwaterqualityparametersthatmustbemonitoredinclosedsheddingsystemsforpropersystemmaintenance.Theseinclude:DO,temperature,nitrogenouswastes(TAN,NO2

-,NO3-),pHandalkalinity.

Water Quality interactionsdissolved oxygen (do)Themostimportantwaterqualityparameterinclosedsheddingsystemsisdissolvedoxygen(DO).Properoxygenlevelsmustbemaintainedinashedding

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systemtoprovideanoptimalenvironmentforcrabmoltingandsurvival,aswellasbiologicalfiltration.

DOlevelsshouldbemaintainedatorabove5partspermillion(ppm)or5milligramsperliter(mg/l)inthesheddingtrays.Oxygenlevelscanbeaffectedbysalinity,temperatureanddensityofcrabsstocked.Astemperature,salinityandcrabdensityincreaseinasheddingsystem,theabilityofthatwatertoholdoxygendecreases.DOlevelswillbemostproblematicinfull-strengthseawatersystemsduringthewarmersummermonths.MeasuringDOissimpleusingvarioustestkits.However,comparedwithaDOmeter,testkitscanbelessaccurate,difficulttouseandcumbersomewhenmultiplemeasurementsareneeded.Operatorsareencouragedtopur-chaseaDOmeter.Theyrangeinprice,dependingonqualityandadditionalmeasurements,suchastemperature,pHandsalinity.Selectameterthatcanwithstandfrequentuseandexposuretoaharshenvironment.

Oxygenlevelsmustbecheckedoftenandinmultipleplaces.Checkthelevelsdailytoensurethesystemisoperatingproperly.Duringperiodsofveryhighstockingdensities(severalhundredcrabspertray),checkDOlevelsmultipletimesthroughouttheday.Oxygenlevelsshouldbemonitoredinthetraysaswellasatthesumporbiofilter.Becausethebiofilteris“alive,”thebacterianeedsufficientoxygentosurvive.Dissolvedoxygenlevelsshouldbeatleast5ppminthewaterleavingthetanksandenteringthebiofilter.DOlevelsabove5ppmareoptimalforsheddingandcrabsurvival.

temperatureBecausepeelercrabsdonotbegintoappearincrabharvestsuntilthewaterwarmstoabout65F,sheddingsystemsbenefitfrombeingmaintainedabovethistemperature.Althoughan“optimal”temperatureforcrabsheddinghasnotbeendetermined,mostclosedsheddingoperationsmaintainwatertemperaturebetween70and80F.Further,temperatureplaysanimportantroleinTANtoxicity(see the TAN section).

Considerwhethertohousethesysteminaclosedbuilding.Temperaturecanbekeptstablefortheentiresheddingseasoninindoorsystems.Theabilitytowarmwaterinthespringallowstheoperatortostartearlierstockingofpeelercrabsandprovidesamoreconsistenttemperaturethroughouttheseason.Whenthesystemisfullyenclosed,heatingorcoolingtheairintheroomwilladjustthewatertemperature.Outdoorsheddingsystemsaretypicallynotmaintainedfortemperatureduetothecostsassociatedwithheatingorcoolingthewater.

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nitrogenous Wastes and the nitrification ProcessNitrogenouswastes—wasteproductswithnitrogen—originatefromanumberofsources.Theseincludecrabexcretionandosmoregulation(howcrabsmaintaintheirbloodchemistry),anddebrismaterialsfromharvest.Theprocessofnitrificationoccursinthebiofilter,wherenitrogenouswastesareconvertedintonontoxicforms.Ammoniaisproducedmainlyfromcrabwasteandisconvertedintoarelativelynontoxicformofnitrogen,nitrate(NO3

-).Fig. 1showstwochemicalequationsforthenitrificationprocess.

Fig. 1: Chemical reactions in nitrification

Becauseoxygenisessentialtothisprocess,ensuringthatDOlevelsinthesheddingtraysaremaintainedat5ppmorgreaterwillresultinsufficientDOtothebiofilter.

Total Ammonia Nitrogen (TAN)Ammoniaisaformofnitrogen.TANisthecombinationoftwoformsofammonia—un-ionized(NH3

-)andionized(NH4+).NH3

-,theun-ionizedform,istoxictocrabsatlowlevels,whileNH4

+isrelativelynontoxic.Bothformsofammoniaarepresentinclosedsheddingsystems.However,thoseamountsaredependentontemperatureandpH.BecausemosttestkitsprovidemeasurementsasTAN,theoperatormustknowthetemperatureandpHtocalculatetheleveloftoxicammonia.

Ammoniareleasedbycrabsaccumulatesinasheddingsystem.Othersourcesincludethebreakdownofcrabwaste(feces)andothernitrogen-richmateri-als,suchassoilanddebrisfromharvest.Bluecrabsdonotridthemselvesofammoniabyurinating.Rather,theyreleaseammoniathroughtheirgillstomaintainpropersaltandwaterbalance.Asammoniabuildsinthesheddingsystem,crabscannotcontinuetoreleaseammonia,therebyaccumulatingitwithintheirbodies.Toomuchammoniainthecrabscanleadtotissuetoxicityanddeath.

Ammoniaisremovedfromclosedsheddingsystemsbythenitrificationprocessinthebiofilter,orbyflushing.Biologicalfilters,knownas“biofilters,”are

Equation 1: 2NH4+ + 3O2 2NO2

- + 2H2O

Equation 2: 2NO2- + O2 2NO3

-

(Ammonium) (Oxygen) (Nitrite) (Water)

(Nitrite) (Oxygen) (Nitrate)

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chambersthathousesubstrate(plasticbeads,oystershell,sand,etc.)onwhichbacteriacangrow.Thesebacteriawill“nitrify,”orconvertthetoxicammoniaintoitseventualform,NO3

-(refer to the equations in Fig. 1).Forthebiofiltertofunctionproperly:

•Thefiltermustbesizedappropriately(withsufficientsubstratematerial). •Sufficientwaterflowfromthetanksmustbeprovidedtothefilter. •Waterflowingtothefiltermustbeoftheappropriatewaterquality(see System Maintenance section).

Ammoniaistypicallymeasuredwithatestkit.Besuretheappropriatetestkitisused,becausesaltwaterandfreshwaterkitsusedifferentreagents.Mostclosedsystemsoperateatlowsalinity—5to10partsperthousand(ppt)—wherefreshwatertestkitscanbeused.Closedsystemsoperatingongreaterthan10pptwillrequireasaltwatertestkit.Also,eachtestkitmaybediffer-entintheresultingtestvalue,withmostreportingTANratherthansolelythetoxicformofammonia,NH3

-.ThisdiscrepancycanleadtosomeconfusionbecausecrabscanwithstandhighlevelsofTANbutnothighlevelsofNH3

-.

Table 1: Un-ionized ammonia factors

ChangesinpHand/ortemperaturewillresultinchangesbetweenthepercentofbothtoxicandnontoxicformsofammonia.Table 1displaystherelation-shipbetweenpH,temperatureandtheamountofun-ionizedammonia.AstemperatureandpHincrease,theamountofun-ionized(toxic)ammoniaincreases.Fig. 2isanexampleofcalculatingNH3

-fromTAN.TocalculateNH3

-fromTAN,findthepHandtemperatureinthetableofassociatedvaluesusuallyfoundwiththetestkittogetanun-ionizedammoniafactor(see Table 1).ThenmultiplythefactorbytheppmofTAN.

temperature ph oC oF 6.5 7.0 7.5 8.0 8.5 9.0

10.0 50.0 0.0006 0.0019 0.0058 0.1082 0.0556 0.1567

15.0 59.0 0.0009 0.0027 0.0086 0.0268 0.0801 0.2159

20.0 68.0 0.0001 0.0040 0.0124 0.0383 0.1118 0.2847

25.0 77.0 0.0002 0.0059 0.0175 0.0532 0.1510 0.3598

30.0 86.0 0.0026 0.0081 0.0251 0.0751 0.2045 0.4482

Standard methods for the examination of water and wastewater.AmericanPublicHealth Association(APHA),AmericanWaterWorksAssociationandWaterPollutionControl Federation.15thed.APHA,NewYork.1980.

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Fig. 2: Example of calculating un-ionized ammonia (NH3-)

in Total Ammonia Nitrogen (TAN)

Nitrite (NO2-)

Aby-productintheconversionofammonia,NO2-istoxictocrabsatlow

levelsandshouldbekeptbelow0.5to1.0ppm.Excessivenitritecansuffocatecrabsbecauseitdecreasestheabilityofcrabbloodtotransportoxygen.Likeammonia,NO2

-isremovedthroughbiofiltrationand/orflushing.Sufficientoxygenlevelsarerequiredinthebiofilteraswellasinthesheddingtanks.TheequationsinFig. 1showthatoxygenisrequiredtoconvertammoniatoNO3

-.SomesheddingsystemshaveproblematicNO2-levels,acondition

likelyduetoinsufficientoxygentoconvertammoniatoNO3-.

Insystemsusinglowersalinitywaters,increasingthesalinitycanreducesomeofthenegativeeffectsofNO2

-toxicity.Incrabblood,NO2-attachesto

hemocyanin,theoxygen-carryingmoleculeinblood.NO2-takesupsiteson

thehemocyaninmolecule,leavinglessroomforoxygentoattach.Addingsalt(NaCl)increasestheamountofchlorideions(Cl-)inthewater.TheadditionalCl-outcompetestheNO2

-onthegillsofthecrabandpreventsNO2-from

bindingtohemocyanin.Maintainingsalinitylevelsat10pptandabovemayprovidecrabssomeprotectionfromNO2

-toxicity.

NO2-ismanagedsimilarlytoammonia—weeklymeasurementswithmore

frequentmeasurementsduringpeakloading.SimpletestkitsareavailableandnocalculationsarenecessarytoobtainthelevelofNO2

-.

Nitrate (NO3-)

Thefinalproductofnitrificationisnitrate,orNO3-.Thisproductisgener-

allynottoxictocrabs,unlessitexceeds500ppm.Asammoniaisbrokendown,NO3

-levelswillgraduallyincrease.Generally,anNO3-“spike”will

notbeobserved.

Example: • pH = 8.0 • Temperature = 77 F • TAN = 1.5 ppm

Using Table 1, locate the factor that intersects at a pH of 8.0 and temperature of 77 F:0.0532

Next, multiply 0.0532 x 1.5 ppm = 0.0798 ppm NH3-.

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OnceNO3-levelsincreaseabove500ppm,operatorsmustchangethewater

toremoveNO3-fromthesystem.NO3

-canbeconvertedtonitrogengas,butthisonlyoccursinareaswithoutoxygen,whichisnotbeneficialtoshed-dingsystems.Typically,monthlyorbi-monthlywaterexchanges,dependingonloading,willbesufficienttokeepNO3

-levelsincheck.Duringawaterexchange,aportionofthewaterisdrainedfromthesystemandreplacedwithnewwater.

NO3-levelscanbetestedusingtestkitssimilartothatforNO2

-.Atthestart,testingshouldoccurtwicepermonth.Afterseveralmonthsoftesting,shed-dingsystemoperatorsshouldbeabletodetermineifthewaterexchangesaresufficienttoremoveNO3

-andminimizefuturetesting.

ph“Perhydroniumion,”orpH,measurestheacidityoralkalinityinthewater.Themeasurementisreportedonascaleof1(acidic)to14(basicoralkaline).WaterwithapHof7.0isconsideredneutral.ThepHvaluecanvarywiththesourceofwater.Full-strengthseawater—32ppt—usuallyhasapHof8.2.WatersamplednearcreeksorditchescouldhaveapHbelow7.0becauseoftheincreaseinacidsoriginatingfromthedecompositionofgrass,leavesanddeadorganisms.

ThemeasureofpHinasheddingsystemisimportantforseveralreasons.First,crabsmusttrytomatchtheirinternalpHwiththatoftheenvironment.Second,pHisoneofthedeterminingfactorsinhowmuchammoniabecomestoxic.Table 1showsthatincreasesinpHandtemperatureresultinahigherpercentageofammoniainthetoxicform.Lastly,aproperpHisnecessaryforbacteriatothriveinthebiologicalfilter.

ThepHinasheddingsystemshouldbemaintainedbetween6.8and8.2.Typically,pHdecreasesinasystemovertime.Thisdecreaseistheresultofnitrificationandcrabrespiration,bothofwhichincreasecarbondioxide(CO2).CO2isaweakacid.Therefore,anincreaseinCO2leadstomoreacidinthewaterandadecreaseinpH.

Inmanyclosedsheddingsystems,addingbufferingmaterialsuchasoystershellcanpreventdecreasesinpH.Oystershelliscomposedofcalciumcarbon-ate,acompoundwithabasicpH(>7.0).AsthepHofthewaterdecreases,oystershellsdissolve,andcalciumcarbonateisreleased.Thecalciumcar-bonateactstobufferthelowpH,increasingthepHofthesystem.Periodicflushingalsohelpstoremovesomeofthewasteinthesystem,preventingadecreaseinpH.

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MonitoringpHissimplewithatestkitorpHmeter.Itshouldbemonitoredweekly,andmoreoftenduringpeakloadingperiods.

alkalinityAlkalinity,alsoreferredtoasbufferingcapacity,istheabilityofwatertoresistanincreaseordecreaseinpH.ProperalkalinityhelpspreventlargechangesinpH.ThenitrificationprocessproducesacidthatdecreasespH.Thepresenceofbasicmaterials—thosewithahigherpH,suchasoystershell—inthesystemcanprovideabuffertomaintainahigherpHlevel.Commercialsheddingoperationsshouldincludeoystershellorsimilarmaterialinthebiofilterorotherpartsofthesystemtohelpincreasealkalinity.Alkalinityshouldbemeasuredatthestartofasheddingoperationandperiodicallythroughouttheseason.Alkalinitylevelsof100ppmorgreaterarepreferred.

Water Quality management in Closed shedding systemssystem start-up

Fig. 3: Changes in the Concentration of Ammonia, Nitrite and Nitrate Over Time

Oneofthedisadvantagesofusingaclosedsheddingsystemisthatitcannotbefilledandstockedimmediately,aswithaflow-throughsystem.Becausebacterianeedtoestablishthemselvesinthebiofilter,newsheddingsystemsshouldbecycledforfourtoeightweeks,dependingontemperature,beforeaddingcrabs.

Fig. 3 isatypicalexampleofthenitrificationprocessduringstart-up.Ammoniaaccumulatesinthesystemfromwasteby-productsoftheinitialstockingorganisms.Simultaneously,Nitrosomonas sp.,thebacteriaresponsibleforconvertingammoniatoNO2

-,beginstomultiply.OnceasufficientcolonyofNitrosomonas sp.isestablished,ammoniadecreases,andNO2

-builds.Along

Conc

entr

atio

n

Time

Ammonia

NitriteNitrate

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withthedecreaseinammonia,Nitrobacter sp.,thebacteriaresponsibleforcon-vertingNO2

-toNO3-,alsobeginstomultiply,causingadecreaseinNO2

-.TheresultingproductisNO3

-,whichgraduallybuildsinthesystem,andisremovedthroughperiodicalflushing.

Tostartanewsystem,placehardshelljimmycrabsand/orotherspeciesthataretolerantofvaryingwaterqualityintothetraysandfeedthemdaily.Bacteriapresentinthefillwaterandintroducedbytheanimalswill“seed”thebiofilterbedwithnitrifyingbacteria.Wastefromfeedingthecrabsand/orfishprovidetherawmaterialforbacteriatobeginconvertingammoniaintoNO2

-andthenNO3-.OncetheNO2

-spikedeclines,itissafetobeginstock-ingpeelercrabs.Theoverallprocessvarieswithtemperatureandsalinity,butgenerallyrangesfromfourtoeightweeks.Addpeelersgraduallytoavoidshockingthesystem,asthebacteriabedwillcontinuetogrowwithincreasesincrabdensity.

system maintenance Onceclosedsystemsarecycledwithbacteriaestablishedinthebiofilter,theymustbemaintainedataconsistentstockingrate.Followingapeeler-crabrun,asheddingsystemtypicallyhasfewcrabsstocked.Topreparethesystemforanotherpeelerrun,stockandfeedhardcrabsorotherspeciesduringthedownperiod.Thebiofilterinaclosedsystemmustbefedcontinuouslytobeproper-lymaintainedandreadyforstockingpeelers.“Shockloading,”oraddinglarge

numbersofpeelers(withoutmaintain-ingsomeloadonthesystem)inashortperiodoftime,shouldbeavoided.

Conductwaterexchangesmonthlyoronceeveryotherweek.Ideally,operatorsshouldreplace25percentofthesystemwaterwithnewwaterduringanex-change.Promptlyremovingsettledsol-idsinthesystemalsowillhelpmaintaingoodwaterquality.Finally,operatorsshouldensurealgaedoesnotaccumulatein,oron,thebiofiltermedia.Algaeinthebiofilterwillreducethefilter’scapac-

itytoconvertammoniaintoNO3-becausethealgaeout-competethebacteria

forspaceonthemedia.Table 2showsoptimalparametersforcrabsheddinginaclosedsheddingsystem.

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Table 2: Optimal Parameters for Crab Shedding in a Closed System

Theparameterslistedhereare“optimal”butmaybedifficulttomaintainatpeakloadingdensities.However,maintaininglevelsclosetothesewillhelpensurecrabsurvival.Individualmeasurementsshouldbetakenonaregularbasisandrecordedinalog.Temperatureanddissolvedoxygenlevelsshouldbemonitoreddaily,whileTAN,NO2

-,NO3-andpHshouldbemonitored

weekly.Salinityandalkalinitytestingshouldbeperformedatinitialsetupandaftereachwaterexchange.Performadditionaltestingifthereisanin-creaseinobservedmortalities,abnormalcrabbehavior,orifthecolorand/orsmellofthesystemwaterchanges.

summaryClosedsheddingsystemshaveaplacewithinNorthCarolina’sbluecrabfish-eryiffishersworktounderstandhowtomaintainwaterquality.Theadvan-tageofmovingawayfromwaterfrontpropertywithoutsacrificingsheddingsuccessmakesthissystemdesignattractive.Keypointstorememberifyoudecidetoconstructasystemare:•Designthesystemforthemaximumnumberoftankstobeused.Consider notonlycurrent,butalsofuturegrowthplans.Tankscanalwaysbe added,butpurchasinglargerpumpsand/orfilterslaterwillsubstantially reduceprofits.•Retainadditionalwaterinthesystem.Moreisbetterwhenitcomesto water.Oversizethesumptoincreaseoverallwaterquality.•Waterchemistrymustbeunderstoodtooperateanyclosedcrabshedding system,howeverdesigned.Monitordissolvedoxygen,nitriteandammonia levelsmoreaggressivelyduringtheactiveseason.Keepalogforwater qualitymaintenancerecordstohelpidentifyproblemsbeforeandafter theyoccur.•Stockthesystemslowly.Newsheddingsystemsrequirealeadtimeoffour tosixweeksbeforeaddingcrabs.“Shockloading”ofpeelercrabsalmost certainlycausesunnecessarycrabdeaths.

Parameter measurement

DO 5 ppm in trays

pH 6.8 to 8.2

Toxic Ammonia 0.5 to 1.0 ppm

Nitrite 0.5 to 1.0 ppm

Salinity Maintain within 5 ppt of capture water

Alkalinity >80 ppm

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Formoreinformationonsoftshellcrabsorsheddingsystems,refertothefollowingpublications. •Reducing Peeler and Soft Crab Mortality: From Harvesting to Delivery.Wayne Wescott.NorthCarolinaSeaGrantPublication#:UNC-SG-02-02 •Closed Crab Shedding System: Protein Skimmers.MarcTurano.NorthCarolina SeaGrantPublication#:UNC-SG-03-06 •Closed Crab Shedding System: Quick Reference Guide.MarcTurano.North CarolinaSeaGrantPublication#:UNC-SG-02-01 •Design of Recirculating Blue Crab Shedding Systems.RonaldMaloneand DanielBurden.LouisianaSeaGrantPublication#:LSU-T-88-003-C2 •Blue Crab Shedding Systems: Water Quality Concerns.HarrietPerry.Maryland SeaGrantPublication#:MASGC-G-85-003-C2 •Manual for Handling and Shedding Blue Crabs (Callinectes sapidus).Michael Oesterling.VirginiaSeaGrantPublication#:VSGCP-H-88-003

ContactsMarcTurano919/513-0122ormarc_turano@ncsu.eduBobHines252/222-6312orrjhines@ncsu.edu

additional references

north Carolina sea grantNorth Carolina State University

Campus Box 8605Raleigh, NC 27965-8605

919/515-2454www.ncseagrant.org

UNC-SG-07-04

A publication of North Carolina Sea Grant. Funding was provided by the North Carolina Blue Crab Research Program.

1,000 copies of this document were printed at $1.11 per copy.

Printed on recycled paper.