closed blue crab shedding systems: understanding water quality
TRANSCRIPT
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Closed Blue CraB shedding systems: understanding Water Quality
Marc Turano
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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
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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/[email protected]/[email protected]
additional references
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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.
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