Prospectsforseascaperepair:Three casestudiesfromeasternAustralia
ByColinCreighton,VishnuN.Prahalad, IanMcLeod, MarcusSheaves,MatthewD. TaylorandTerryWalshe
Threecasestudiesspanning tropical,subtropicaland temperateenvironments highlighttheminimumpotential benefitsofinvestinginrepairof coastalseascapes.Fisheries,a marketbenefitindicatorreadily understoodbyarangeof stakeholdersfrompolicymakers tocommunityadvocates,were usedasasurrogatefor ecosystemservicesgenerated throughseascapehabitat restoration.Foreachcasestudy, whilerecognisingthatbiological informationwillalwaysremain imperfect,theprospects forseascaperepairare compelling.
Keywords: coastalwetlands,ecological restoration,ecosystemservices,fisheries, saltmarshes.
ColinCreighton, IanMcLeod and Marcus Sheaves arefromTropWATER,JamesCookUniversity,Townsville,Queensland,Australia. MarcusSheaves isHeadofMarineBiology andAquacultureattheCollegeofScienceand Engineering,JamesCookUniversity,Queensland,Australia,(JamesCookUniversityTownsville4811,Queensland,Australia.Emails: colinmwnrm@bigpond.com,ian.mcleod1@jcu. edu.au&marcus.sheaves@jcu.edu.au); Vishnu Prahalad isLecturerofPhysicalGeographyat theDisciplineofGeographyandSpatialSciences, UniversityofTasmania,Hobart,Australia(PO Box78,Hobart7001,Tasmania,Australia. Email:vishnu.prahalad@utas.edu.au); Matthew D.Taylor isAssociateDirectorFisheries
Figure1. SeascapehabitatsfromeasterncoastalAustraliausedascandidateexamplesto illustratethebenefitsfromrestoration:(a)tropicalcasestudy:cast-nettinginactionalongamangrove-linedcreekinAustralia’swet–drytropics;(b)subtropicalcasestudy:aremnantsubtidal marshchannelontheLakeWooloweyahdelta,northernNewSouthWales(thechannelextends inabout20mandthenhitsthedyke);(c)temperatecasestudy:pop-netsinactionathightideon asaltmarshinTasmania’snorth-westcoast,and(d)thefocalspeciesYellow-eyeMullet(Aldrichettaforsteri).
ResearchatthePortStephensFisheriesInstitute,NewSouthWalesDepartmentofPrimary Industries,TaylorsBeachRd,TaylorsBeach, NewSouthWales,Australia&ConjointProfessorattheSchoolofEnvironmentalandLifeSciences,UniversityofNewcastle,NewSouth Wales,Australia(Callaghan,NewSouthWales 2308,Australia.Email:matt.taylor@dpi.nsw. gov.au); TerryWalshe isfromtheCentrefor EnvironmentalandEconomicResearch,UniversityofMelbourne(Victoria3010,Australia. Email:twalshe@unimelb.edu.au).
Introduction
Coastalseascapesareamosaicoftidallyinfluencedhabitatsthatinclude channels,gutters,mudflats,mangrove clumps,mangrove-linedchannelsand variouscommunitiesofseagrass,saltmarshesandtidalfreshwaterwetlands.Theirgenerallyflatprofileand proximitytothecoastandhumansettlementsmakethemamenableto beingdrained,filledandconverted
tofarmland,sportsfields,housesand canalorindustrialestates(Lee etal. 2006;Sheaves etal. 2014;Rogers etal.2016).Saltmarsheshaveoften bornethebruntofanthropogenic impactsduetotheir‘frontline’position,beingmostexposedtohuman settlementsandactivities.Alongthe Australiancoast,seascapesandespeciallytheirsaltmarshcomponents havebeencleared,drained,filled andleveesconstructedtoexclude tidalinundation(Sinclair&Boon 2012;Prahalad2014).Moregenerally, modificationtoseascapes especially barrierstowaterflowandconnectivity,suchasbundwalls,orroads occursalongalmosteveryriverand estuaryinthemorepopulatedparts ofAustralia(NLWRA2002;Creighton etal. 2015).
Functionally,theseascapecontinuumdrivescoastalecologicalproductivityandprovidesarangeof ecosystemservices(e.g.Laegdsgaard 2006;Mount etal. 2010;Boon etal. 2011;Creighton etal. 2015).Anumberoftheimportantregulating,supportingandprovisioningservices suchascarbonsequestration(Lawrence etal. 2012)andcommercial andrecreationalfisheries(Creighton etal. 2015;Taylor etal. 2017a,2017b)aredependenton hydrologicalconnectivitybeingmaintained,sothatfreshandtidalwaters haveadequateopportunitiestomeet. Reinstatingtidalconnectivityto ensurebiological,chemicaland hydrologicalfluxesiskeytorestoring ecosystemfunctionandecosystem services(e.g.Raposa&Talley2012). Indeed,theAustralianGovernment’s conservationadvicefortherecovery ofcoastalsaltmarshlistedasathreatenedecologicalcommunityunder the EnvironmentProtectionandBiodiversityConservationAct1999 (EPBCAct)clearlyidentifiestheneed for‘maintenanceofecologicalfunctionandincreasedresilience’through ‘permanentorintermittentconnectionwiththesea;functioningtrophic pathways;[and]structuralhabitat ’
(TSSC,2013,p.23).Thereisestimatedtobe164,000–245,000haof saltmarshcoveredunderthislisting, withdataabouttheirdeclineinextent andconditionhighlyvariableacross regions(forexamplesofhigh-resolutiondata,seeSinclair&Boon2012 andPrahalad2014).
Recognisingthevalueofcoastal seascapehabitats,theongoingthreats totheirecosystemservicesandthe needforecologicalmanagementand restoration,thetwocentralquestions weseektoaddressare:(i)whatare thepotentialbenefitsthatcanbe derivedfromseascaperepair?and (ii)howdothesebenefitsoutweigh thecostsforrepairunderdifferent riskscenarios?Weenvisagethisinformationwouldprovidequantifiable potentialbenefitsaspartofbusiness casesthatmightthenattractpublic orprivateinvestmentinrepair.This approachaccordswiththeextended attentionbeingpaidtoenvironmental assets(NaturalInfrastructure)in nationalaccounts(BureauofMeteorology2013),andtheincreasingnumberofrobustvaluationsofecosystem services(e.g.throughtheUnited NationsSystemofEnvironmental-EconomicAccountsframework:United Nations2014).
Althoughthetwoquestionswe addressarepertinenttoallofAustralia’sseascapehabitats,wefocus onsaltmarshinparticular,duetoits vulnerablestatusundertheEPBC Actandtheneedtoaddressrepairas partoftheproposedRecoveryPlan (TSSC2013;Rogers etal. 2016).The focusonsaltmarshisfurtherjustified giventheaddedeffectsofclimate changeandsealevelrisethatrequire coastalwetlandstoretreatinland,furtherincreasingland-useconflictsand opportunitycostsforrepair(Abel etal. 2011;Prahalad etal. 2019a).
Toaddressourquestions,weused threecasestudies(Taylor&Creighton 2018;Prahalad etal. 2019b;Abrantes etal.2019)developedaspartofa researchprogramsupportedby AustralianGovernment’sNational
EnvironmentalScienceProgram.The casestudiesspanarangeofbiophysicalandpolicysettingsacrosstropical,subtropicalandtemperate Australia(Figs1,2,Table1).Across thesecasestudies,wesoughtindicators(cf.UnitedNations2014)that (i)aresupportedbycalculationsthat areclear,simpleandreadilyunderstoodbypolicymakertocommunity advocate;(ii)reflectvaluationsthat arewellfoundedandbasedonAustralia’sexistingcommoditymarkets; and(iii)areconservativeandgenerallylowerboundplausibleestimates ofvalue,withonlyselected,usually singlebenefitstreamsusedinthevaluationprocess.Here,weemploykey prawnandfishspeciesaseasilypubliclyunderstoodexemplarindicators forestimatingthepotentialbenefits ofseascaperepair.Benefitstreams areaccompaniedbylistsofecologicallysustainableassumptionsthat clearlydemonstratethatthevalues areconservative.Wealsolistadditionallikelybenefitstherebyalso demonstratingtheconservativenatureoftheresults.
Theterm‘value’usedhererefersto marketclearingpricesoftradable commodities.Thesedollar(AUD)valuesreflecttheeconomiccostsand potentialbenefitsifthereisinvestmentinrepair(orbenefitsforgoneif thereisnorepair).Byusingcommerciallyrecognisedspeciesandtheir dollarvalueinthemarketplace,we seektotranslatewhatcanbean obscuresetofecosystemservicesinto commonlyandreadilyunderstood metrics.Indoingso,weprovide groundworkfordevelopingmore detailed,contextuallynuancedand locallyspecificbusinesscasesforseascapeconservationandrepair.We acknowledgethoughthattheinterpretationsofvalueencompassawide rangeofattributesbeyondthescope ofthepresentpaper(e.g.nonmarket andnonusevalues),andnotallof theseattributesareamenableoreven suitableforeconomicvaluation(see Boon&Prahalad2017).
Figure2. LocationofthethreecasestudiesfromeasterncoastalAustraliausedtosignifythepotentialfisheriesbenefitsthatcanbederivedfrom repairoftropical,subtropicalandtemperateseascapeenvironments.TheslightangulartiltinthemapisduetheTransverseMercatorprojectionused.
CaseStudies
Thefollowingthreecasestudiessignifythepotentialbenefitsthatcan bederivedfromrepairofcoastal saltmarshspanningtropical,subtropicalandtemperateseascapeenvironments.Theeastcoasttropicaland subtropicalstudiesselectedprawn speciesasindicatorsforestimating benefits(i.e.potentialincreasesin prawnbiomass)fromseascape repair.Thisisbecauseprawnsare iconicseafoodproductsinthetropicalandsubtropicalregions,generally inhighdemand,andarewellunderstoodasanindicatorofpotential marketbenefitbyarangeofstakeholders.Prawnsarealsoannual, highlyfecundspeciesthatwill rapidlyexpandinpopulationsize
byexploitingrepairedhabitat.In comparison,thereislimitedunderstandingofseafoodderivedfrom saltmarshesintemperateregions (Wegscheidl etal. 2017).Theeast coasttemperatestudytherefore examinedthefishassemblagein generalandidentifiedthemostdominantseafood/fishspeciesofcommercialandrecreationalinterestto illustratebothcurrentandpotential fisheryvalue.
Casestudy1:Eastcoast tropicalsaltmarsh restoration(Bowling GreenBay,north Queensland)
TheBananaPrawn(Fenneropenaeus merguiensis)fisherywaschosenas themarketbenefitindicator.This
speciesusestropicalestuariesasnurserygrounds(Vance etal. 1990; Sheaves etal. 2012),wheretheyrely onsaltmarshvegetationforpartof theirnutritionalsupport(Abrantes& Sheaves2009).TheBananaPrawnis acommerciallyimportantfoodspeciesandimportanttargetofrecreationalfishersthroughoutnorth Queenslandestuaries.Thespeciesis alsovitalpreyofotherhighprofile commercial/recreationalspeciessuch asBarramundi(Latescalcarifer). BananaPrawnishighlyfecundand willrecruitrapidlytorepairedenvironments.Finally,BananaPrawnis anidealtargetspeciesbecausethey canbesampledusingcastnets,agear typethatisparticularlysuitablefor smallmangrove-linedestuaries (Fig.1a),andprovideaccurate
Table1. Casestudyinrelationtolocalpolicycontext(cf.Rogers etal.2016),proposedlikelypolicychanges,andtargetedecosystemservice subsidiesresultingfromseascaperepair(selectedonthebasisthattheyarereadilyunderstoodbypolicymakersanddecision-makers) Climate zone Casestudy area Policycontext(forboth conservationandrestoration,if applicable)
Prospectsforseascape conservationandrepairusing fisheriesasapolicysurrogate
Changesintermsofincreased fisheriesproductionoutputs resultingfromseascaperepair TropicalBowling GreenBay, north Queensland
Saltmarshes,mangrovesandtidal channelsdesignatedasfishhabitat areasprotectedunderQueensland FisheriesAct1994 (Rogers etal. 2016).Protectiondoesnotextendto nondesignatedwetlandareas(e.g.on pastureland).Majorinvestmentin ecosystemrepairproposedunderthe AustralianGovernment Reef2050 Long-termSustainabilityPlan
Conserveexistingsaltmarshaskey fishhabitatsthroughcooperation withStateFisheriesagencies(e.g. asmarineprotectedareasunderthe Queensland FisheriesAct1994). Investintherepairofdegraded saltmarshbyremovaloftidal barrierstoreinstatetidalflows.
Increaseincommerciallyand recreationallyimportantspecies populations,suchasBananaPrawns (Fenneropenaeusmerguiensis)and theirkeypredatorsBarramundi (Latescalcarifer).Indirectadditional increasesincommercialand recreationalpiscivorousfishspecies abundanceandbiomassthrough enhancedfoodchainsresultingin increasedbiomassofpreytaxasuch asHerrings(Clupeidae)andMullet (Mugilidae).
SubtropicalClarence River estuary,New SouthWales (NSW)
Coastalsaltmarshhabitatand associatedecologicalcommunityare listedasan‘endangeredecological community’underNSW Threatened SpeciesConservationAct1995 (Rogers etal. 2016).The NSWMarine EstateManagementStrategy2018–2028 seeksto‘reducethecumulative impactsofexistingagricultural infrastructureonfreshwaterflowsand estuarinehydrology’(e.g. reinstatementoftidalflowsto saltmarsh).
Investintherepairofdegraded saltmarshbyremovaloftidal barrierstoreinstatetidalflows(e.g. throughthe NSWMarineEstate ManagementStrategy2018–2028).
Increaseintherecruitmentand trophicproductivityofSchoolPrawn (Metapenaeusmacleayi),a commerciallyandrecreationally importantspecies.Additionalgains infisheriesproductivitythrough exportofbiomass(through outwelling)fromsaltmarshtoother seascapehabitats.
TemperateCircular Headregion, north-west Tasmania
Norecognitionofsaltmarshesand theirvalueswithinStatelegislation (exceptforafewlistedspeciesand thoseareaswithinexistingreserves). Someprotectionaffordedunderthe statewideplanningregime,subjectto enforcement(seePrahalad etal. 2019a).
estimateswithahighnumberofreplicatescollected(Johnston&Sheaves 2007).
Theeastcoasttropicalstudy (Abrantes etal.2019)foundthatestimatesofproductivityofindividual componentsoftheestuarywere highlyvariableanddependedona numberofassumptions,whichare difficulttovalidate(Ronnback etal. 1999;Minello etal. 2008;Rozas& Minello2011).Incomparison,estimatesatthewhole-of-estuarylevel, theseascapeslevel,inlinewithcurrentunderstandingofestuarinespeciesrelianceonamosaicofhabitats (Nagelkerken etal. 2015;Sheaves 2017),requiredarelativelylow
Conserveexistingsaltmarshaskey fishhabitatsthroughliaisonwith StateFisheriesagencies(e.g.as marineresourcesprotectedareas underthe LivingMarineResources ManagementAct1995).Investin therepairofdegradedsaltmarshby removalofleveestoreinstatetidal flows(seeFigure4).
numberofassumptionsandproduced estimateswithrelativelylowvariability.Abrantes etal. (2019)foundasa conservativeestimate,amaximum juvenileprawnbiomassof6.5g/m2 forthe2mwidebandsalongthe estuaryedgewhereprawnsarefound. Fortheestuarystudied,withanedge areaof5.6ha,theconservativetotal biomassofjuvenilepawnswas0.36 tonnes.
Theactualestuaryproductivity wouldlikelybemuchhigherbecause thisestimateonlyrelatestothemaximumjuvenilestockforasampling occasionanddoesnottakeinto accountcontinualmovementsof prawnstooffshoreadulthabitatonce
Increaseinthreecommerciallyand recreationallyimportantspecies populations,especiallyofYellow-eye Mullet(Aldrichettaforsteri). Additionalfoodsubsidiesto piscivorousfishthataretargetedby bothcommercialandrecreational fishersfromSilversides (Atherinidae)andGobies(Gobiidae).
theyreachasufficientsize.Tomore preciselycalculateestuaryproductivity,informationwouldbeneeded onpatternsofrecruitment,growth rates,mortality,predationandemigration.Sufficeittosayanestimate ofBananaPrawnproductivityof 0.36tonnesisprobablyordersof magnitudebelowtotalestuaryproductivity(Abrantes etal. 2019). Whilethisprovidesabaselineestimatethatcanbeusedtodemonstrate thepotentialbenefitsofseascape repair,muchmoreextensivestudies wouldberequiredtolinkproduction ofBananaPrawntoparticularareas ofsaltmarshhabitat(Sheaves&Johnston2010;Sheaves etal. 2012).
Casestudy2:Eastcoast subtropicalsaltmarsh restoration(Clarence Riverestuary,northern NewSouthWales)
TheSchoolPrawn(Metapenaeus macleayi)fisherywaschosenasa marketbenefitindicator.School Prawnishighlyreliantonestuarine nurseryhabitatandprimaryproductivityderivedfromestuarinehabitats forrapidgrowththroughtheirearly life-historystages(Hart etal. 2018; Raoult etal. 2018).Thespeciesis importanttobothcommercialand recreationalfisheriesinNewSouth Wales(Taylor etal. 2017a).School Prawnisfastgrowingandhighly fecund,andgivenreasonablefreshwaterinflowtoestuaries,itisunlikelyto experiencestock-relatedlimitations torecruitment.Thespeciesismostly commerciallyharvested;thiscommercialharvestprovidesasought-after productforhumanconsumptionand isthemostwidelyusedbaitforrecreationalfisheriesinsouth-easternAustralia.Giventhelife-history characteristicsoftheSchoolPrawn, benefitsfromhabitatrestorationare likelytobeevidentinthisspecies overatmosttwotothreeyears.
Basedonassumptionsdetailedby theeastcoastsubtropicalstudy(Taylor&Creighton2018),estimatesindicatethatreinstatementof connectivityof27.6haofshallow subtidalcreeksandsubsequentutilisationbySchoolPrawns(assuming goodjuvenilerecruitment)could yield ~2500kgofproduct,equating toagrossvalueof AUD24,000and associatedtotaloutputof ~AUD140,000peryear.Whenconvertedbacktoaper-hectareestimate, thesevaluesequateto ~AUD900and AUD5000perhaperyear,respectively,forseascapehabitat.
Thebenefitsofhabitatrepairare notlimitedtothevaluesestimated fromdirectusageofthehabitatfor SchoolPrawn.Seascapehabitatscontainimportantprimaryproducersthat
contributetotheoverallproductivity oftheestuary,andconsequently,they makesubstantialcontributionstothe exploitedbiomassharvestedfrom estuarinesystems(Taylor etal. 2017a,2017b).Potentialgainsinprimaryproductivitywhenthesehabitatsarereconnectedtothebroader estuarywillbeoutwelledtoother areasacrosstheestuarinesystem.This canoccurthroughmechanisms includingthetransportofparticulate organiccarbon(POC),transportof dissolvedorganiccarbon(DOC),or consumptionofmarshplantsbysmall nektononthemarshsurface(when inundated),andsubsequentmovementthroughouttheestuary.These additionalbenefitsarenotcaptured inthisanalysis,butcouldcontribute toafishery-derivedvalueofupto AUD20,000perhaperyearofareal saltmarshthatisreconnectedtothe estuaryintheClarenceRiversystem (Taylor etal. 2017a).
Anyreconnectedsubtidalchannels arisingfromrepair(Fig.1b),aswellas outwelledproductivity,willalsoprovidehabitattodirectlysupportother targetspeciessuchasMudCrab (Scyllaserrata),DuskyFlathead (Platycephalusfuscus),Yellowfin Bream(Acanthpagrusaustralis),Luderick(Girellatricuspidata)andSea Mullet(Mugilcephalus)(Morton etal. 1987;Mazumder2009;Webley etal. 2009).Directsupportofadults and/orjuvenilesoftheseexploited specieswillproducefisherybenefits thatcontributeadditionalvaluefrom habitatrepair.Boththesefactorswill seeflow-onbenefitsforrecreational andcommercialfisheriesalike.
Casestudy3:Eastcoast temperatesaltmarsh restoration(CircularHead region,north-west Tasmania)
Theeastcoasttemperatestudy(Prahalad etal. 2019b)wasthefirstdocumentationoffishusageofTasmanian saltmarshes.Thefocusonfishand theselectionofnorth-westCircular
Headregionstudyareastemmedfrom anumberofreasons.TheCircular Headregionishometoaboutafourth ofallcoastalsaltmarshesinTasmania andformspartofarichseascape matrixwithexpansivetidalflats,seagrassbedsandbuffering Melaleuca ericifolia swampforests(Mount etal. 2010).Theregionisveryimportantforcommercialandrecreational fisheriesinTasmania.TheCircular Headregionsaltmarsheshavebeen subjecttomostextensiveclearing andagriculturaldrainageworks,with thelargestpotential(~629haor55% ofcurrentextent)forhabitatrepair throughtidalrestoration(Prahalad 2014).
Prahalad etal. (2019b)found11 fishspeciesusingCircularHeadsaltmarsheswithahighmeandensityof >72fishper100m2 (sampledata fromApriltoMay2017;Fig.1c). ThefamilyAtherinidae(Silversides) contributedthreespeciesand74%of thetotalcatchnumbers.Commercial andrecreationalspeciesthatutilise thesesaltmarshesinnorth-westTasmanianseascapesincludethefollowing:Yellow-eyeMullet(Aldrichetta forsteri),AustralianSalmon(Arripis truttaceus)andGreenbackFlounder (Rhombosoleatapirina).Thesethree speciescontributedcloseto20%of thetotalcatchnumbers.Ofthese,Yellow-eyeMullet(Fig.1d)wasmost abundantandcommon,presentin 24(65%)ofthe37netsthatcaught fishandmadeup19%ofthetotal catch.Extendedsamplingthroughout theyearmayrevealfurtherspecies usingsaltmarshes.
Yellow-eyeMullet,AustralianSalmonandGreenbackFlounderare amongthesevenkeyspeciestargeted byrecreationalfishersinTasmania (Lyle etal. 2014).Notably,YelloweyeMulletandAustralianSalmonhelp underpinrecreationalfisheriesinthe north-westregionofTasmania,with byfarthegreatestproportionofMullet andSalmon(74%and23%ofstatewide recreationalcatchin2012–13)being caughtfromthisregion(Lyle etal.
2014).ThecommercialcatchofYellow-eyeMulletpeakedin1999/2000 andhasdecreasedsince,with2tonnes reportedtobecaughtin2015/16 (Emery etal. 2017).AlthoughtheTasmanianstockofYellow-eyeMulletis classifiedas‘sustainable’,anyrepair andexpansionoftheirnurseryhabitat arelikelytosupportandenhanceits carryingcapacity,andhenceitssustainabilitystatus.Forexample,given thatanaverageof13.6individualsof Yellow-eyeMulletwerefoundina 100m2 areaofsaltmarsh(Prahalad etal. 2019b),restoringtidalflowsto anominal100haofsaltmarshcould translatetoanincreaseinthespecies populationby136,000individuals (seeFig.4).Therewasalsoevidence forrapidrecruitmentpotential.Samplestakenfromrehabilitatingsaltmarshesbehindpreviouslybreached leveessupportedsimilarfishassemblagestonearbyunalteredmarshes withoutlevees.Thisindicatesthat removingtidalbarrierstoreconnect marshescurrentlybehindleveesis likelytoreturnimmediatebenefitsfor fishusethroughexpandedhabitat andfoodresources(cf.Roman etal. 2002;Raposa&Talley2012).
WhileSilversides(Atherinidae)are notdirectlytargetedbyfishersinTasmania,theyprovideanabundantfood sourceforotherpiscivorousfishthat aretargetedbybothcommercialand recreationalfishers(cf.Mazumder etal. 2011).Mostimportantly,these arepartofthesuiteofspeciesthatcontributetooverallmarinebiodiversity andproductivityofthesetemperate systems.Theseseascapescontribute morebroadlytothemarinefoodweb viaexportofplantandanimalmatter tocoastalwaters(Melville&Connolly 2003;Svensson etal. 2007).
ASimpleFrameworkfor BuildingaBusinessCase forInvestmentinSeascape Repair
Whileacknowledgingasuiteof ecosystemservicesassociatedwith
repair(e.g.Jenkins etal. 2010),this researchhasemphasisedbenefits stemmingfromincreasedharvestfor recreationandhumanconsumption ofasubsetofspecies readilyvalued benefits.Ifthesebenefitsareestimatedtobegreaterthanthecostsof implementation,thenaprospective repairprojecthasabenefit–costratio of atleast 1(andusuallymuchhigher: seedeGroot etal. 2013).
Ourbiologicalunderstandingofthe magnitudeofstockincreasesassociatedwithanyspecificrepairactions remainsrudimentary.Predictingwith certaintythepay-offofinvestmentin repairprojectsisclearlydifficult. Insufficientinformationshouldalways providetheimpetusforcarefulconsiderationofpotentialrisksandacautionaryapproach.However,riskand uncertaintyareubiquitousfeatures ofmanykindsofinvestment.Delaying decision-makingwhileuncertaintyis furtherreducedorentirelyresolved carriesthecostofforegonebenefits, bothgross(e.g.increasedyields)and net(e.g.avoidedrisks).Repaircosts areverylikelytoincreaseinthefuture duetodecliningresourcecondition relativetodemand,andhighercapital andlabourcosts(Blignaut&Aronson 2008).Italsoignoresthebenefitsof learningviaimplementationthrough adaptivemanagement(Walters1986; Burley etal. 2012).Here,weusethe eastcoastsubtropicalcoastalwetland restoration(ClarenceRiverestuary, northernNewSouthWales)case studytolaygroundworkbyoffering abasicdecisionsupportframework forconsideringinvestmentinseascaperepairunderuncertainty.
Aprimarysourceofuncertaintyis thesizeoftheincreaseinyieldor quotaarepairprojectmightbring. Forexample,forSchoolPrawn,one ofthekeyvariablesforwhichthere waslargeuncertaintywastherecruitmentsubsidyassociatedwithrepairof adiscreteareaofhabitatanditsimplicationsforbiomassandharvest(Taylor&Creighton2018).Assumethat weareconsideringrepairforthree
hypotheticalcandidatesites,A,B andC,withintheClarenceRiverestuary,allofwhicharemotivatedprimarilybyanincreaseinSchoolPrawn abundanceandavailability.Although wemaynotknowthetruemagnitude oftherecruitmentsubsidy,wecan useexpertjudgementtoestimate theprobabilityofadiscretesetofpossibilitiesandestimateassociated improvementsinquotas.TheillustrativejudgementsshowninTable2 forthreehypotheticalsitesarethe authors’own(cf.Taylor&Creighton 2018),butinothersettingsanalysts canformallyelicitjudgementsusing accessibleandprovenmethods(Hemming etal. 2018).
ConsideringsiteAfirst,theriskneutralapproachistocalculatethe expectedbenefitusingtheprobability-weighteddifferencebetweenestimateswithandwithoutrepair.That is,ourrisk-neutralbestestimateof thepay-offforrepairatsiteAisan additionalharvestof375kg/year,on average(Table2).IftheclearingmarketpriceforSchoolPrawnis AUD10kg 1 (Taylor&Creighton 2018),wecannowestimatethepresentvalue, PV,ofthebenefit: PV ¼ A r 1 1 1þr ðÞh ,where A isthe annualbenefit, r isthediscountrate (orinterestrate),and h isthetime horizon(inyears)overwhichthe repairprojectistobeassessed.For A = $3,750, r = 4%or0.04and h = 30years, PV = $64,845.Ifthe (discounted)costsofimplementing theprojectarelessthan$64,845, thentherisk-neutraldecision-maker willproceedwithimplementation, knowingthattheexpectedratioof benefittocostexceeds1.Ifcosts areintheinterval(AUD$25,938–$95,106;seeTable3:siteA),thedecision-makerneedstoconsidertheir attitudetorisk,andperhapsotherservicesthatmaybecomevaluablein future(e.g.carbonandnitrogenstorage,recreation:Jenkins etal. 2010). Inaddition,theprospectsfortransferringlearningoutcomes(knowledge spillover)tootherspeculative
projectsandinvestmentsmaybe worthconsidering.
Afterapplyingthecalculationsand dataforSchoolPrawnshownabove tositesBandC,wereportbestestimatesandplausibleboundsforthe presentvalueofthebenefitofrepair ateachofthethreesitesinTable3. Theestimatedcostsofrepairforour hypotheticalsitesareshownin Table4.Up-frontcostsincludecapital worksandcompensatorypaymentsto landholdersforinundationofotherwiseproductiveland,amongother possibleimpacts.Ongoingcostsare tobeincurredformaintenance.Using thesameformulaaboveforcalculatingthepresentvalueofmaintenance costs(againwitha30-yeartimehorizonanda4%discountrate),we obtaintotalcostsforeachcandidate project.Outcomesaresummarised
as(uncertain)benefit–costratiosin Figure3.
Therisk-neutraldecision-maker focusesonbestestimates.Risk-averse decision-makersfocusonlower bounds,andriskseekersonupper bounds.Thepriorityorderofthe threeprojectsdependsonriskattitudewhereBis(weakly)preferred toA,andCisnonviableforthe risk-neutraldecision-maker;Ais (weakly)preferredtoB,andBispreferredtoCforthosethatarerisk seeking,andnoneoftheprojects mayappealtoarisk-aversedecisionmaker.
Figure3. Benefit–costratiosofthethree hypotheticalcandidaterepairprojects,with plausiblebounds.Asdiscussedintext,these areconservativeestimatesusingonlyour indicatorspeciesandtheactualbenefit-tocostratioisgenerallymuchhigher(seede Groot etal. 2013).
Table2. Estimatedannualharvestrates(kgperyear)forthreehypotheticalcandidaterepair siteswithintheClarenceRiverestuary WithrepairWithoutrepair
Site A† 250700950100300400
Site B 4009001200200550700
Site C 200600800150400500
†ForsiteA,asanexample,theprobabilityweighteddifferencebetweenestimateswithand withoutrepair:0.25 9 (250–100) + 0.50 9 (700–300) + 0.25 9 (950–400) = 375kg/year.
The4%discountratewiththe30yeartimehorizonhasbeenusedby similarassessmentsfocusedonwetlandrestoration(e.g.Jenkins etal. 2010).Althoughsocialinvestments whichaccruebenefitsforthefuture havebeensubjecttoalower‘social discountrate’(andusuallylowerthan private/individualdiscountrates), basedonbothmarketandethical principles(Harrison2010;United Nations2014).Areviewof2160economistsbyWeitzman(2001)indicated apreferencetousediscountratesof lessthan4%anddecreasingtoless than1%forthedistantfuture(i.e.a timehorizonof >76years)forclimate changemitigation.Landmanagers themselvesmaychooserepairunder lowdiscountratesforbothmarket andnonmarketreasonsduetovarying riskperceptions,andatrialauction processcouldhelprevealcosts(e.g. Stoneham etal. 2003).
Table3. Bestestimatesandplausibleboundsforthepresentvalueofbenefitsforeachofthree hypotheticalcandidaterepairprojects
PresentvalueofbenefitSiteASiteBSiteC Lowerbound$25,938$34,584$8646
Bestestimate$64,845$60,522$32,423 Upperbound$95,106$86,460$51,876
Table4. Costsforeachofthreehypotheticalcandidaterepairprojects SiteASiteBSiteC
Costsofcapitalworks$8000$7000$10,000
Costsoflandholdercompensation$10,000$25,000$20,000 Annualcostofongoingmaintenance$1500$500$1000 Presentvalueoftotalcosts$43,938$40,646$47,292
Thepurposeofthesimpleframeworkwehaveoutlinedhereisto demonstratehoweffectiveseascape repairdecisionscanbemadedespite uncertainty.Itcanbereadilyadapted todifferentdiscountratesandtime horizonsandextendedtoinclude continuousprobabilisticjudgements andadditionalsourcesofuncertainty (e.g.costtofishers).Wenote,importantly,thatexpertjudgementneed notbeacriticalbottleneckinadaptingthisframeworktodevelopmore detailed,contextuallynuancedand locallyspecificbusinesscases.There
Figure4. Locationoftheeastcoasttemperatestudywiththepotentialforsaltmarshrepairmappedinhighresolution(basedonPrahalad2014) andthelikelymarginalincreaseinthefocalspeciesYellow-eyeMullet(Aldrichettaforsteri;mi-yem,classifiedusingJenksnaturalbreaks).Theinset obliqueimageprovidesacloserviewofthepotentialforsaltmarshrepairthroughrestitutionoftidalflows,throughengagementwithprivatelandmanagers.BasedatafromtheLIST(www.thelist.tas.gov.au,StateofTasmania).TheslightangulartiltintheinsetmapisduetheTransverseMercator projectionused.
aresimpleandaccessibleprotocols availableforelicitingthekindsof judgementsusedinourhypothetical examplehere(Burgman etal. 2011; Hemming etal. 2018).Theframeworkexplicitlyarguesagainstuseof uncertaintyasanexcuseforinaction (alsoseedeGroot etal. 2013).Even whereuncertaintymakesthestandalonemeritofacandidaterepairprojectunclear,thebenefitstobegained fromlearningthroughimplementationandsubsequentmonitoringmay makeimplementationworthwhile (Burley etal. 2012).Alsoofimportance,particularlyinthecontextof seascapehabitatsandtheircapacity
forcarbonstorage,isthe‘socialwelfarevalue’ofrepairthatwould includeavoideddamagesduetomitigationofclimaterisks(Jenkins etal. 2010).Therearemanyotherconsiderationsforleverage,suchasbenefits derivedfromjobcreationandtraining,aswellassustainingculturalvalues(Blignaut&Aronson2008),such asconnectiontoplace(e.g.Aboriginal‘SeaCountry’).
ConcludingComments
Thethreediversecasestudieshave demonstratedthesubstantialindicativebenefitsthatcanaccruefrom
seascaperepairandmayassistinthe formulationoftheproposedRecovery Planforcostalsaltmarshlistedunder theEPBCAct.Whileonlymarketbenefitindicatorspeciesthatarereadily understoodbythecommunitywere usedforillustration,thetotalbenefits (aspositiveexternalities)ofrepairare multiple.Equallyimportantly,even withjustthevalueofthemarketbenefitindicatorspeciesused,theargumentforinvestmentinrepairis compelling(Blignaut&Aronson 2008;Turner&Daily2008).Thechallengeremainsthatwhilerepairdeliversmultiplepublicandprivate benefits,currentlythesedrained
seascapeareasaregenerallyinprivate ownershipandarerestrictedfrom functioningasfisherieshabitats(e.g. Fig.4).Theopportunitycostsfor restoringthesefisherieshabitatsneed tobebroughtintosharperfocusfor policymakerstocommunityadvocatesbyincreasingtherecognition oftherelativecostsandbenefitsof competinglanduses.
Astothespecificcostsofrepair works,activitiesareinmostcasesrelativelysimple generallyinvolving minorearthworksinremovingsmall bundsandanyinfilltoreinstatetidal connectivityandre-establishtidal channels(e.g.Prahalad2014;Prahalad etal. 2019b).Thesearelikelyto berelativelyinexpensiveandcould berapidlyundertakenbyequipment suchasatractor-mountedbackhoe. Thesecostscanbeintegratedapart ofabusinesscasedevelopedfrom thegroundworkwehaveprovided, focusingonareadilyunderstood potentialmarketbenefitindicatoras asurrogateforecosystemservicebenefitsaccruingfromseascaperepair. Anybusinesscaseforrepairwillalso needtoaddresstheneedsforgreater clarity,rigouranddemonstrablemerit inidentificationofsuitablerepairsites andtargets.Indeed,thisprovidesa keychallengeforscientists,determiningamongmanyprospectiverepair sitesandmarketbenefitindicators, allofthemindividuallyworthyto varyingdegreesforseascapefunction, whichofthesesitesandindicators willincreasetheprospectsformuchneededinvestmentinsaltmarshand seascaperepair.
Acknowledgements
Thisworkwasundertakenforthe MarineBiodiversityHub,acollaborativepartnershipsupportedthrough fundingfromtheAustralianGovernment’sNationalEnvironmental ResearchProgram(NESP).RachelTribouthelpedprepareFigure2.NicBax (CSIRO)andanonymousreviewers providedvaluablecomments.