vigilance levels & health in hawaiian spinner

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VIGILANCE LEVELS & HEALTH IN HAWAIIAN SPINNER DOLPHINS By Megan O’Toole Dr. Dave Johnston, Advisor April 2012 Masters project submitted in partial fulfillment of the requirements for the Master of Environmental Management degree in the Nicholas School of the Environment of Duke University 2012

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VIGILANCE LEVELS & HEALTH IN HAWAIIAN SPINNER DOLPHINS

By

Megan O’Toole

Dr. Dave Johnston, Advisor

April 2012

Masters project submitted in partial fulfillment of the

requirements for the Master of Environmental Management degree in

the Nicholas School of the Environment of Duke University

2012

MeganO’Toole

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Abstract

Overthepastseveraldecades,Hawaiianspinnerdolphinshavebecomea

speciesofinterestforbothscientistsandtouristsalike.Spinnerdolphinscanbe

foundrestinginthebaysofcoastalHawaiialmostdaily,andtheirhabitshavebeen

notedbytheburgeoningecotourismindustryandincreasedhumanpresence.While

noindepthstudieshavebeenperformedonthedolphinpopulations,these

increasedinteractionshaveresearchersconcernedaboutthevulnerabilityofthe

dolphinpopulation.

Dukas&Clark(1995)hypothesizethatakeyunexploredfactorinanimal

healthisvigilancelevel,ortheabilityoftheanimaltoprocesscomplexpatterns

suchasforagingordetectingpredatorsbasedontheamountofresttheyareableto

obtain.Asthelifestyleofspinnerdolphinsputsthematextremeriskforlowered

vigilancelevels,theequationsputforwardinthearticlewereusedtocreateabio‐

energeticsmodelthatwouldillustrateitspotentialeffectsontheirabilitytocapture

preyandevadepredators.

Whileshort‐termvigilancelossdoesnotappeartobesignificantlydamaging

tothedolphin’sabilitytofeedandprotectitself,along‐termlossmaygreatlyaffect

aspinnerdolphin’soverallhealth.Thesefindingsindicatethatthecurrenthuman

presenceinthebaysduringthespinnerdolphins’restinghourshasthepotentialto

detrimentallyaffecttheirhealthandconsequentlytheirpopulationnumbersand

shouldserveasastartingpointforfurthervigilancebasedresearch.

Introduction

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Spinnerdolphinshavebeenstudiedforoverfortyyears,butstillmuchabout

theirbiologyisunknown.Thefederalgovernmentprovidesfortheirwellbeing

undertheMarineMammalProtectionAct(1972)butnostatelawsorprograms

providethesamelegalcoverage.InthebaysnearKona,Hawaii,thepresenceof

spinnerdolphinsisaneverydayoccurrence.Inadditiontothepodsofdolphins,

hundredsofpeople,bothtouristsandlocals,swimmingandboatingalsofillthebays

eachday(Timmeletal.2008).Theincreasingpresenceofhumansinbaysiscausing

morefrequentinteractionsbetweenthehumansandthedolphins,buttheeffectsof

theseinteractionsarestilllargelyunknown.ThewesternsideofHawaii’sbigisland

isoneofthemostnotableplaceswherehumaninteractionwithspinnerdolphins

occursdaily(Delfour2007).

Tobetterunderstandtheuniquenatureofthespinnerdolphinandtheir

susceptibilitytohumanactivities,theirdailyhabitsshouldalsobeaddressed.

Spinnerdolphinsaresmallcetaceanseasilyidentifiedbyathreetone‐greypattern

andgettheircommonnamefromtheaxialspinsandleapstheyperformduring

socialencounters(“SpinnerDolphin”2011).Theyareuniqueinthatunlikemost

othercetaceanstheyforageatnightintheopenoceanfeedingonmesopelagicfish,

shrimp,andsquid(Benoit‐Bird&Au2009)andspendthedayrestingand

socializinginbays(Norris&Dohl1980).Preyofthespinnerdolphinliveatabout

400to700mbeneaththesurfaceofthewaterduringthedaybutbecausespinners

arethoughttoonlybeabletodivetodepthsof400mmaximumtheymustwaituntil

nightfall,whenthemesopelagiclayerrisestodepthseasilyreachedbythedolphins

forabout11hoursperday(Benoit‐Bird2003).Theleadinghypothesisforrestingin

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thebaysisthatbyrestinginthebaysduringtheday,thepredationriskismuch

lowerthanintheopenoceanbecauseoftheenclosedareaaswellastheincreased

abilitytoseetheshadowofapredatoronthesandybottom(Heenehan2010).Just

aswithpeople,restingallowsthedolphinstorecoverfromtheirdaily,ornightly

efforts.Thedolphinsrestwithuni‐hemisphericsleepmeaningonlyonehalfofthe

brainisshutdownatatimewhiletheotherenablestheanimaltoperformfunctions

necessaryforsurvival(Cirelli&Tononi2008).Theabilitytosleepwithonlyonehalf

oftheirbrainatatimeallowsthedolphinstocontinueswimmingandreturntothe

surfacetobreathewhilealsowatchingforpredatorssotheotherhalfoftheircan

brainrest(Cirelli&Tononi2008).Currently,theirbehavioristocomeintothese

baystorestwheretheywillbelesslikelytobeattackedbypredatorsandcansleep

withlessinterruption.However,withthegrowingpresenceofhumans,this

behaviorisbeingdisruptedastheywakeupduetotheunexpectedcommotion.

MoststudiesfocusonthewesternsideofthebigislandofHawaii.Thefour

baysthatarecurrentlybeingstudiedindepthareKeauhako,Kealeakekua,

Honaunau,andMakakobays.Eachofthesebayshistoricallyhavehadfrequent

spinnerdolphinpresenceduringthedayandsohavebecomefavoredtostudy

spinnerdolphinandhumaninteractions(Norrisetal.1994).Thesebaysare

characterizedbyshallowwater,sandybottoms,areprotectedfromwindandstrong

oceancurrents,andarealllessthan1.5kmacross.Byhavingthissetof

characteristics,thebaysareideallocationsforspinnerdolphinrestingareas(Norris

etal.1994).

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Additionally,Konabaysarealsohometoanisolatedsub‐stockofspinner

dolphins.AccordingtotheNationalMarineFisheriesService(NMFS),theentire

chainofHawaiianislandsishosttoasinglestockofspinnerdolphins,butrecent

dataindicatesthatsixindividualstocksaremostlikelypresentalongtheislands

(Andrewsetal.,2010).TheMMPAdefinitionofastockis“agroupofmarine

mammalsofthesamespeciesorsmallertaxainacommonspatialarrangement,that

interbreedwhenmature,”andAndrewsetal.foundthatlittlegeneticmixingof

individualsinbetweenthesub‐stocksoccurs.Consequently,shouldoneofthe

individualpopulationsaroundHawaiibecomeunsustainablysmall,itisunlikelylike

thatthedolphinstockselsewhereinHawaiiwouldbeabletobolsterthedwindling

populationthroughemigrationtothesmallerstock.Additionally,spinnerdolphins

havebeenobservedtohave“long‐termsitefidelity,”meaningthatevenifconditions

ofthebaysdeterioratetothepointwherethedolphinsaredetrimentallyaffected,

theanimalswillnotseekbetterrestingbayswhichcouldexacerbatetheissueof

isolatedstocks(Marten&Psarakos1999).Bynotmovingtomorerestful,less

human‐influencedareasthedolphinsnotonlywillbecontinuingtheirexposureto

poorconditionsbutwillalsoeliminatethepossibilityofmixingwithahealthier

population.

Withtheincreasedinteractionsbetweenhumansanddolphinscomes

increasedconcernforthedolphins’wellbeingandwhetherpeople’sactivitiessuch

askayakingorscubahaveaneffectontheirbehaviorandconsequentlytheirhealth.

Thehumanpresenceinthebaysisunliketoabateanytimeintheforeseeablefuture

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asmillionsofdollarsayeararebroughtintoHawaiifromtouristswhowishtobe

abletoseeandorswimwiththespinnerdolphins

NoofficialpopulationestimateisavailablefortheHawaiianspinnerdolphin

(DepartmentofCommerce2011).However,arecentstudyputsthebestavailable

populationestimateforthebigislandbetween600and700dolphinsinthearea

(Hilletal.tobepublished).Theestimateisconcerningwhencomparedtothe

initialabundanceestimatedonebyNorrisof1000to2000dolphinsaboutthirty

yearsago(Norrisetal.1994).Thedecrease,however,cannotbeattributedtoany

specificfactorforseveralreasons.Littleisknownontrendsofthedolphin

populationchangingovertimeduetoalackofconsistentresearchanddifficulty

findingconclusivecorrelatingfactors.

Formanypeople,swimmingwiththedolphinshasnowbecomeviewedasa

right,whetheritisbecausetheyareontheirdreamvacationorthey’vedoneitall

theirlives.Somefeelaspiritualneedtoswimwiththedolphinswhileothersview

themasaliensfromanotherworld.Boatingtoursthatadvertiseswimmingwith

dolphinsbeganinHawaiiaround1991andhavenowbecomeamajorportionofthe

tourismindustry.Ecotourismasawholehasbeenincreasingannuallyatarateof

about10%(Delfour,2007).Whateverthereason,eachdaydolphinsandhumans

sharethesebaysandwithincreasingfrequencywillinteractwitheachother.This

includestourisminHawaii,muchofwhichoccursonthewaterandhighlightsall

varietiesofmarinemammalsincludingpilotwhales,beakedwhales,andspinner

dolphins.FabienneDelfourconductedastudyinwhichtheeffectsofkayaking,

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boatingandswimminginthebaysonspinnerdolphinbehavior(2007).Thestudy

foundthatasthenumberofhumanactivitiesinthebayincreased,thenumberof

timesachangeindolphinswimmingdirectionoccurredfollowinganencounterwith

humans(Delfour2007).Otherstudieshavefoundalterationsinanimaldistribution

(Lammers2004),speed(Ribeiroetal.2005),swimmingpatterns(Acevedo1991),

andbehavioratthesurface(Hastieetal.2006).Additionally,Samuelsetal(2000)

reportedthatthenumberofdolphinsreturningtothebayswithhighesthuman

presencedecreasedovertime,indicatingpoorconditionsfordolphinresting.

Humanactivityisclearlyaffectingthebehaviorofthedolphins,buttheextentto

whichthismayaffecttheirhealthisstillunknown.

Inmanycases,lawsonconservationorenvironmentalissueshaveboth

federalandstatecounterparts.However,withmarinemammalsthisisnotthecase.

Onthestatelevel,Hawaiihaslawsforbiddinginteractionwithanimalswithin

sanctuaries,butasnoneofthebaysareprotectedthestatedoesnothaveany

enforcementpower.OfthefourbaysstudiedoffoftheBigIslandofHawaii,atleast

oneisconsideredaMarineLifeConservationDistrictbythestateofHawaii,but

theirlawsmakenomentionofdolphinswithinthebay,theyonlyapplytonon‐

marinemammalspecies.In2006,thestateofHawaiiwasauthorizedtoassistinthe

enforcementofdolphinprotectionbutsofarthishasbeenmostlylimitedto

spreadingtheawarenessofMMPAguidelines(“DepartmentofLandandNatural

Resources”2006).

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TheonlylegislationthatcurrentlyappliestospinnerdolphinsistheMarine

MammalProtectionAct(MMPA).ThisisbecausetheMMPAprohibitsthecreationof

anystatepoliciesdealingwiththemanagementofmarinemammals,andsoHawaii

cannotcreatetheirownregulationstodealwithanypotentialissuesarisingfrom

humaninteractionswiththedolphins.Theenforcementofdolphinsunderthe

MMPAfallstoNMFSProtectedResources,andthenisdividedupintosixregions,

withthePacificIslandRegionalOffice(PIRO)dealingwiththeHawaiianSpinner

dolphins.UnderTitleI,whichaddressestheconservationandprotectionofmarine

mammals,theMMPAmakesitillegaltoharassorattempttoharassmarine

mammals.Thisincludesthealteringoftheirbehaviors,whichmayoccurinthese

bayswhenpeoplearecomeintoclosecontactwiththedolphins(MMPA1994).As

theratesofhumanactivitiesintheHawaiianbaysareincreasing,NMFSisreceiving

highernumberofcomplaintsthattheswimmersandboatersareharassingthe

dolphins(Timmeletal.2008).Whilethe1994amendmentstotheMMPAessentially

makesswimmingwithdolphinsillegalunderLevelBharassment,or“havingthe

potentialtodisturbamarinemammalstockinthewildbycausingadisruptionof

behavioralpatterns”(MMPA1994),lackofconclusivefindingsonthepartof

researchersaswellasthedifficultyofprovingsuchoccurrencesincourthavekept

theregulationdifficulttoenforce.ManyofthestudiesperformedintheHawaiian

baysarethereforeattemptingtoprovethisportionoftheMMPAisbeingviolatedin

ordertoproveacauseforstricterpolicyormoreenforcementinthesebays.

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AnotheroneofthegoalsoftheMarineMammalProtectionActistomaintain

thestocksofmarinemammalsabovetheoptimumsustainablepopulation(MMPA

1994).“Optimumsustainablepopulation”isthenumberofanimalsthatwillresult

inthemaximumproductivityofthepopulationkeepinginmindthecarrying

capacityofthehabitatandthehealthoftheecosystemofwhichtheyforma

constituentelement.Spinnerdolphinsarecurrentlyconsideredas“depleted”by

NMFS,whichmeansthatthespecieshavefallenbelowtheiroptimalsustainable

level(“SpinnerDolphin”2011).ThisdegradationoftheirstatuspromptedNMFS

PIROtolookforwaystomitigatehumaninteractionswiththedolphinsonaregular

basis.

Lackofenforcementinthebaysaswellasthedifficultyofprovingwhether

aninteractionhasalteredthebehaviorofthedolphinspromptedNMFStopublish

voluntarySpinnerdolphinviewingguidelines.Theseguidelines,whicharenomore

thanthenormalsuggestedbehaviorwhenencounteringanymarinemammal,are

distributedtotouroperatorsaspartofanoutreachprogramtoincreaseawareness

oftheMMPAandthepossibleeffectsofhumaninteractionsinanattempttodeter

humanpursuitofdolphinswithoutnewregulations.Theguidelinesinstructboatsto

stayover50metersawayfromthedolphinsatalltimes,tolimitobservationtimeto

halfanhour,andtomakesurethedolphinsarenotencircledortrappedbetween

boatsortheshoreatanytime(“SpinnerDolphin”2011).Additionally,outreach

programstotouristsexploringtheareasalertthemtothenocturnalnatureofthe

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spinnerdolphinsaswellastherequirementsoftheMMPA.Inthesummerof2011,

NOAAbeganimplementationoftheDolphinSMARTprograminHawaii,where

touristsareabletofindalistofdolphintouroperatorswhohaveagreedtoabideby

theNOAAdolphinviewingguidelines(“SwimmingwithSpinnerDolphins,”2011).

In2006,NMFSissuedanoticeofintenttocreatenewpoliciestodealwith

humaninteractionswiththespinnerdolphinsinthesebays(Departmentof

Commerce,2006).Inearly2011,NMFSannouncedtheirplanofclosingKealakekua,

Honaunau,MakakoandKauhakobaysduringthemorninghourswhenspinner

dolphinshavemostfrequentlybeenobservedtoberesting(Johnston,2011).These

fourbayshavebeenstudiedintensivelyforthepasttwoyearsinordertocollect

dataforanalysisonthebehaviorofdolphinsinthepresenceofhumans.Afterthe

closurethestudywillcontinueinanattempttodeterminewhethertheabsenceof

humanactivitybringsaboutanychangeinbehavioralpatterns.Alreadybacklash

hasbeenseenagainstthisnewpolicy,andnotwithoutwarrant.A2009reportby

theInternationalFundforAnimalWelfareestimatestheannualrevenuefortourist

relatedboatingactivitiesinHawaiiisover$40million.Anordertoshutdownfour

ofthemajortouristbaysonthebigislandduringprimebusinesshourscouldcause

millionsofdollarsinlossesforthetouroperatorsfrequentingthoseareas(“Whale

Watching”2009).WhileprotectingmarinemammalsisapriorityforNMFS,the

alienationofalargerevenuestreaminHawaiimaynotbethebestlongtermplanif

protectionofspinnerdolphinhabitatistocoexistwiththetourboatindustry.This

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isespeciallycontentiousasNMFSisoftenunabletoprovetheMMPAhasbeen

violated.

Oneconceptwhichhasnotbeenexploredindepthasawaytomeasure

spinnerdolphinhealthandcouldgreatlyaidinthediscussionofpotential

ramificationsofhumanactivitiesistheeffecttheamountofsleep,orlackthereof,

hasonthespinnerdolphin.InDukas&Clark’s1995articleSustainedvigilanceand

animalperformance,theydiscusstheconceptthatalackofrestleadstotheinability

ofthecentralnervoussystemto“sustainahighqualityofinformationprocessing

foranextendedperiodoftime.”A2008studyconcludedthatdolphinssubjectto

sleepdeprivationweremorelikelytohaveslowerresponsetimestostimuli,

indicatingthatacontinuousinterruptionofsleeppatternscouldpotentiallyincrease

themortalityriskofspinnerdolphinsduetoinabilitytosuccessfullyforageor

predation(Cirelli&Tononi2008).However,theseverityorproportionalresponse

tolackofsleephasnotyetbeenmeasured.Dukas&Clarkcreatedamodelwhich

measuresthevigilance,inthisinstancedefinedasa“stateofbehavioralalertnessto

predators…ageneralconditionofenhancedabilitytoprocessinformation”anda

seriesofequationswhichcorrelatemeasuresofhealthsuchasabilitytoforagefor

food,mortalityrisk,andnetenergygain.Usingatechniquesuchasmodelingwould

allowforthestudyofpossibleeffectsofhumanpresenceinthebayswithout

interferingbybecomingapresencethemselves.Fromtheseequations,amodelwas

createdspecificallytargetedtowardsthespinnerdolphininanattempttoassessthe

relationshipbetweenrest,vigilancelevel,andhealth.

MaterialsandMethods

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Inordertodisplayspinnerdolphinbehavioraldatawithinamodeling

framework,theenvironmentalmodelingprogramSTELLAwasused.Equations

createdbyDukasandClark(1995)wereemployedwithsomeslightmodifications

duetotheneedtotakeintoaccountcharacteristicsspecifictothespinnerdolphins

inadditiontothefeasibilityofcreatingaworkingmodelfromequationsthathave

previouslyonlybeenappliedtheoretically.Becausetheequationsinthepaperare

designedforanimalsingeneral,whencreatingaStellamodelvaluessuchas

metaboliccostoflivingandpreydensitywhileforaginghavetobealteredinorder

totailorthemodeltospinnerdolphins.Valuesfortheseparametersweretaken

frommultiplepapersdiscussingtheforaging,socializing,andsleepinghabitsofthe

dolphins(e.g.referencesandseebelow).

TheBasicModel

Thefirstsimulationrunfromthemodeldemonstratedanidealsituationin

whichthevigilanceofadolphinbeginsatthemaximumvigilancelevelbefore

foragingandendsatmaximumvigilancelevelafterarestingbout(seeTable1).

Table 1. Symbols and values used in the initial model

Symbol Description Value

t Time 0 < t < 24 υ Vigilance level 0 < υ < 100 γ Resting multiplier τ Time delay between foraging and resting 8 hours η Time Delay multiplier α Rate of vigilance decrement during foraging activity .07/hr β Rate of vigilance recovery during rest .19/hr θ Foraging multiplier

λ Number prey encountered per hour 215.7 animals

f Long-term average rate of food intake

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µ Predation risk µ Chance of encountering a predator

ρ Percent chance of encountering a predator while resting 0.002

ρ Percent chance of encountering a predator in the open ocean 0.02 M Overall mortality risk

c Metabolic cost per hour foraging 173.75 kcal/hr

Eprey Calories of prey item E Net rate of energy gain F Total expected lifetime energy gain L Length of prey

Thefirststepindemonstratingthevariationsinvigilanceinspinnerdolphins

isdeterminingtherateofchangeofvigilanceoverthecourseoftheiractivities.

ConstantproportionalratesofchangeareassumedintheequationsbyDukas&

Clarkof:

‐αυwhileforaging β(1‐υ)whileresting

Vigilancelevelinthismodelisdemonstratedbyν,where0<υ>100,with0

representingthedolphinatadepletedvigilanceleveland100representing

maximumvigilance(Dukas&Clark1994).Themodelwasrunforasimulation

period24hourswiththestartingpointusingavigilancelevelof100,assuming

maximumvigilanceafteraperiodofrestasthedolphinsswitchtoforaging

behaviors.Thevalueforrateofvigilancedecrement,orα, issetataproportionof

vigilanceof.07perhourandβissetatarateof0.19vigilancerecoveryperhour.

Whilethevaluesofα andβhavenotbeenrecordedinanystudy,theassumptionis

madethattherateofrecoverywouldbemuchhigherthantherateofdecrementin

animalsasamuchlargerproportionoftimeisspentawakethanasleep.For

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simplicitypurposes,vigilancelevelonlyincreasesduringrestingperiodsandonly

decreasesduringforagingbouts.Giventhatthedolphinsforageforan

approximately11hourseachnightwhilethemesopelagiclayerisavailabletothem

(Benoit‐Bird),itwouldmeanthatthedolphin’sdecrementtime,forthepurposeof

thismodel,wouldbelimitedtothose11hours.Additionally,KennethNorris

observedrestingspinnerdolphinstoremaininthebaysforaminimumof5hours

(1994).Therefore,thewindowoftimeinwhichvigilancerecoveryoccursissetto5

hoursper24hourcycle.Tocreatetheidealsimulation,anydecrementthatoccurs

duringtheforagingperiodwouldneedtobefullyrecoveredduringtherestingbout.

Therefore,withaninitialplaceholderrateofdecrementof.07percentageof

vigilancelostperhouroveran11hourperiod,therateofrecoverywouldhavetobe

.19perhourinordertocreatethebaseline.Inadditiontohavingboutsofforaging

andresting,atimedelayisalsotakenintoaccountforthetransitionsbetween

foragingandresting.Asdolphinsdonotforageinthesamelocationtheyrest,travel

istakenintoaccountaswellassocializationtime.Forthepurposesofthismodel,

movingfromtherestinglocationtotheforaginglocationorviceversaandanytime

spentsocializingiscategorizedasatimedelayinwhichthevigilancelevelneither

diminishesnorincreases.Forthismodel,thetimedelayissetat4hoursbetween

eachtransition,totaling8hoursperday.Consequently,themodelwouldrunwith

theforagingperiodforthefirstelevenhours,afourhourtimedelayfortraveland

socializing,arestingperiodforthenextfivehoursandthenswitchingfromrestto

foragingfor21:00‐24:00hrs.

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Theequationusedtomodelthevigilancedynamicsovera24hourperiodis

then

υ=‐αυθ + γβυ+ηυ

where

θ =1 t<110t>11

γ=0 t<16,t>20 1 16>t<20

η =0 t<11,16<t<20111>t<16,t>20

Assumingtheaveragerateoffoodintakeduringaforagingboutis

proportionaltothevigilancelevelatthattimeandthatanypreyencounteredbythe

dolphiniscaughtandconsumed

Averagerateoffoodintake=λυ

wecanthencalculatethetheoreticalamountoffoodadolphinwouldbeableto

consumeduringanygivenforagingperiod

f (υ) = λυ0

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AccordingtoDukas&Clark(1995),increasingtheamountoftimespent

foragingshouldincreasetherateatwhichtheyareabletoconsumepreydueto

increasedtimetoencounterfood.Foodintakeisalsousedtoestimatetheoverall

healthofthedolphinassumingthattheamountofpreycaughtcanindicatefuture

successintermsofgainingsizeandthusbecominglessatriskfrompredationas

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wellasbeinganindicatoroffuturereproductivehealth(Read2011).Therefore,the

higherthefoodintakeoverthedolphin’sdailyforagingperiod,thehigherthe

assessmentofthedolphin’shealth.

MortalityRiskandMetabolicCosts

Upuntilthispoint,predationandmetaboliccostarenotaccountedforinthe

equations,butbyutilizinganotherequationbyDukas&Clark(1995)predationrisk,

orthedolphin’ssusceptibilitytoapredationattemptcanbecalculatedby

=

Thisequationsimplyillustratesthatasforagingtimeprogressesandcauses

vigilancetodecreaseadolphin’sriskofpredationwillrise.Foragingtimeinthis

instanceincludestheactivitiesof“bothfeedingandscanningtheenvironmentfor

predators”(Dukas&Clark,1995).Withoutrestthisactivityreducesthecapacityof

thedolphintobothfeedandlookoutforpredatorsinthefuture.Assumingthereare

noothercausesofmortalityandatmaximumvigilancethedolphinwillbeableto

escapeanypredationattempts,DukasandClarktranslatethispredationriskintoan

overallmortalityrisk(M)shownasanincreasingfunctionofforagingeffortby

Ametaboliccost(c)isthentakenintoaccount.Byincludingthemetaboliccostinto

theequation,overallenergygaincanbecalculatedandanestimatedoverallcaloric

intakecanbeproduced,leadingmostlikelytoamoreaccuratepredictionoffuture

dolphinhealth(Dukas&Clark1995).Netdailyenergygainismodeledbythe

equation

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Themetaboliccostoflivingalsovariesbetweenrestingandforagingstates.

Atalltimes,thedolphinismovingaswellasperforminginvoluntarybodily

processeswhichrequireenergyofmaintenance.Becausenostudieshavebeen

publishedonthemetabolicratesofspinnerdolphins,thecalculatedvaluescame

fromstudiesonbottlenosedolphins.Yazdi,etal(1999)discussthevariousenergy

costsassociatedwithbaselineswimmingaswellasthecostofincreasedspeedsand

diving.Forthesimulation,themodelisdividedintotwometabolicrates,therates

whileforagingandtherateswhilenotforaging(duringthetimedelayandresting

bouts).Therestingmetabolicrateofbottlenosedolphinswasfoundtobeanenergy

consumptionrateof2.15W/kg(Yazdietal.,1999).However,duringperiodsof

efficientswimmingYazdietal.foundthemetabolicratetobeabout10%higher.

Assumingthatthedolphinsareswimmingefficientlyfortheperiodswhilenot

foraging,themetaboliccostmustfactorinthecostoftransportwhilethedolphins

travelandrest(astheyarestillswimmingduringrestingbouts).Themetaboliccost

oftransportduringtheseperiodsisthensettobeat1.16 ,whichconverts

to0.00027 (Yazdietal.,1999).Iftheaveragespinnerdolphinweighs65kg

andisswimmingatthemostefficientrateof2.5m/s(Yazdietal.1999),thenthe

dolphinwillbeusing157.95kilocaloriesperhourwhiletravellingandresting.

Williamsetal.(1999)foundthatthemetabolicrateofdolphinsincreasesupto20%

fromtherestingmetabolicrateduringdivingcausingthemetaboliccostofforaging

tobesetat173.75kilocaloriesperhour.Betweenthesetwometabolicrates,the

averagedailymaintenancerequirementforaspinnerdolphinisabout61kcal/kg.

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UsingtheresultsofBenoit‐Bird(2003)onpreycaloricvaluesandassuminga

61kcal/kgmaintenancerequirement,thedailyenergyneedsarecalculatedtobe

about3,965kilocaloriesperday.Themainpreyitemsofspinnerdolphinsare

knowntobefish,shrimpandsquid,withanaverageequationmeasuringthecaloric

contentofthedolphin’sdietbeing

Amajorityofthepreyfoundinspinnerdolphins’stomachsweremyctophidfish

withanaveragelengthof7.8cm(Benoit‐Bird2003)andsothiswasusedasthe

lengthfortheaveragepreyitemobtained.Whenpluggedintotheequationabove,

thecaloriccontentoftheaveragepreyofaspinnerdolphinis1,975caloriesor

1.975kilocaloriesandsothisvaluewasusedastheparameterforthecaloriesof

eachfooditemeaten.Usingthe11hourperiodasthelimitforhowlongthedolphin

willhavetoobtain3,965kilocalorieswecandeterminethatinorderforthisenergy

requirementtobemet,atleast2,007.59preyitemsmustbeeateneveryday.With

theassumptionstatedfortheaveragerateoffoodintake,thespinnerdolphinwill

eatallpreyitcomesintocontactwithshoulditbeatitshighestvigilancelevel.As

thevigilanceleveldiminishes,thesmallerpercentageofpreythedolphinwillbe

abletocapture.Therefore,thenumberofpreyencounteredperhourusedinearlier

equationswouldneedtobeatleasttheminimumamountoffoodadolphinneeds

perday.Usingthisdata,thenumberofpreyencounteredperhourissetat215.7

animalssothatwithitsvaryingvigilanceunderidealcircumstancesthedolphincan

meetitsminimummaintenanceenergyneeds.Asfoodavailabilityisnotalimiting

factorfordolphinenergymaintenance(Benoit‐Bird2003),thisnumberwould

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actuallybealowestimateofpreyencounters.Additionally,spinnerdolphins

activelyseekhighaggregationsofprey(Benoit‐Bird&Au2008)aswellas

cooperativelyforagesotheirforagingareaswouldhavehigherchancesof

encounteringpreythantheopenoceanorthemesopelagiclayerasawhole.

Byusingaconservativeestimateofhowmuchaspinnerdolphinwouldeatina

singleforagingperiodandaddinginthemetaboliccost,wecanthenseewhetherthe

dolphinisabletofulfillitscaloricneedsoverthecourseofthedayandareableto

haveanothercriteriabywhichtomeasurespinnerdolphinhealth.

Creatingsuboptimalconditions

Thefirstsimulationofthemodelwasrununderoptimalconditionsinwhich

thedolphinwasabletorestuntilitsvigilancewasfullyrestoredinordertocreate

baselinenumbersforoutputssuchasfoodintakeperforagingboutaswellasnet

energygainoverthelifetimeoftheanimal.Subsequentsimulationswhenthe

dolphinisonlyabletoobtain80,60,and40percentoftheminimumamountof

sleepobservedbyNorris(1994).Thesesimulationswilldepictpossible

consequencesofprevalenttouristdisruptionofdolphinsrestinginthebays.

Whilemimickingoptimalconditions,themodelissetuptorestorevigilance

toitsoriginallevelattheendofeachdaywhichprovidesthedolphinwiththe

optimalamountofvigilancetoperformthenextday’sactivities.Whilethese

conditionscontinue,thedolphinwillbeabletorestoreitsvigilanceindefinitely.

Shouldsuboptimalconditionsoccur,vigilancelevelswouldthenbealteredwithin

themodel.Takingintoaccountthepossibilitythatlongtermeffectsofsleep

deprivationmaybedifferentthanshortterm,foragingabilityaswellasmortality

MeganO’Toole

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riskmaybemoreevidentwhenlookedatfromalongtermperspectiveandsothe

simulationwillbeextendedfromitsoriginal24hourperiodtoathirtydayperiodin

ordertoviewpotentiallongtermeffects.

Results

OptimalScenario

Vigilancelevelintheoptimalscenarioisshowntobeginat100%withthe

dolphinbeingfullyabletoprocessitssurroundings,decreaseoverthecourseofthe

foragingboutandnotriseagainuntilthedolphinisabletorestagaininthebays

withthevigilancelevelreturningtomaximumcapacityattheendoftheresting

period(Figure1).

Thevigilancedynamicwasthenusedtomodeltherateoffoodintakeadolphin

wouldbeabletoperform,asfispositivelycorrelatedwithvigilancelevel.Therate

offoodconsumptionishighestatthebeginningoftheforagingboutanddecreases

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14 16 18 20 22 24

VigilanceLevel(Percentage)

Time(Hours)

Figure1.Vigilancedynamicovera24hourperiod

Vigilance

MeganO’Toole

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asvigilancelevelslower.Aftertheforagingboutends,therateoffoodintake

consequentlydropsto0fortherestoftheday(Figure2).

AscanbeseeninTable2,thelongertheforagingboutthemorefoodthedolphinis

abletoobtain.However,therateatwhichthedolphinisabletocaptureitsprey

decreasesovertimeanditbecomesalessefficientpredator.

Table2.Rateofpreyintakeandtotalpreyintakeduringaforagingbout

Time(hours)PreyConsumedperHour

TotalPreyIntake(numberofanimals)

0 215.76 215.761 200.66 431.522 186.61 632.173 173.55 818.784 161.40 992.335 150.10 1153.736 139.59 1303.837 129.82 1443.428 120.73 1573.249 112.28 1693.98

10 104.42 1806.2611 97.11 1910.68

Final 2007.80

0

50

100

150

200

250

0 1 2 3 4 5 6 7 8 9 10 11

RateofPreyConsumption

Vigilancelevel

Time(Hours)Figure2.Rateofpreyconsumptionperhouroveraforagingboutcomparedwithvigilancelevel

NumberofPreyConsumed

Vigilance

MeganO’Toole

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MortalityRiskandNetEnergyGain

Predationriskwasfoundtobenegativelycorrelatedwithvigilancelevels

andsoastheforagingboutprogressed,thedolphin’sriskofpredationrisesasitis

nolongerbeabletoprocessitssurroundingsasthoroughlyasitcouldatthe

beginningoftheforagingbout.Additionally,asthedolphinrestsanditsvigilance

wouldrise,theriskofpredationlowers(Figure3).

Nextanalyzedwastheoverallmortalityrisk,whichdiffersfromthe

predationriskinthatittakesintoaccountthelikelihoodofthedolphin

encounteringapredatorratherthanjustitsriskofbeingpredatedshouldan

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14 16 18 20 22 24

Percentage

Time(Hours)Figure3.Predationriskandvigilancelevelovera24‐hourperiodwherethepredationriskrepresentsthepercentchance,shouldapredatorattackthatthedolphinfailstonoticethepredator.

Vigilancelevel

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interactionoccur.Fromthisequationitwillbenoticedthatthemortalityriskison

therisewhilethedolphinforagesanditsvigilancelevellowers.However,the

mortalityriskdropsdrasticallywhenreachingtherestingperiodbecauseofthe

highlyimprobablepresenceofpredatorsintherestingbays(Figure4).

Thefinalwayspinnerdolphinhealthwasmeasuredisthroughtheanimal’s

abilitytoobtainitsminimumenergyrequirementsinordertomaintainits

metabolicneeds.Themodeledequationdemonstratesthatthedolphinisableto

captureenoughfoodforitselfovertheforagingbouttoprovideitselfwithenough

energytoperformdailyfunctions.Asthemodelissettothedolphineatingonly

enoughtomaintainitsenergyneeds,theoverallenergygainisaround0attheend

oftheday,justintimeforthedolphin’snextforagingbouttobegin(Figure5).

0

5

10

15

20

25

30

35

40

0 2 4 6 8 10 12 14 16 18 20 22 24

Percent

Time(Hours)

Figure4.Adolphin'smortalityriskduetopredationovera24‐hourperiod

MortalityRisk

MeganO’Toole

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However,thisisaconservativeapproach,astheanimalswouldlikelyexceedtheir

dailyminimumrequirements.

Usingthisoptimalsituationinwhichthedolphinisablenotonlytoreplenish

itsvigilancelevelsattheendofeachdayaswellascaptureenoughpreytomeetits

metabolicneedsnowallowsforacomparisontosimulationsinwhichthedolphinis

notabletoobtainasmuchrestasisneeded.

SuboptimalSimulations

Inordertostudythepotentialeffectsofdisruptedsleepingtimesimulations

werethenrunat80%,60%,and40%ofthedolphin’soriginal5hoursofsleep.As

showninFigure6,theshortertherestingperiodmeansthedolphinsleavethebays

earlierinthedayandtoalongerforagingbout.Lessrestandmoretimesearching

forpreymeansthedolphin’svigilanceislowerattheendofeachforagingboutwith

lesstimetorecover,andwhenthedolphin’snextforagingboutbeginsitsvigilance

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

NetEnergyGain(Kilocalories)

Time(Hours)Figure5.Thenetenergygainandlossduetothemetaboliccostofaspinnerdolphin

NetEnergyGain

MeganO’Toole

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isnotrestoredtoitsoptimallevel.Theamountofrestlostisproportionaltothe

amountofvigilancenotrestoredattheendofeachday(seeFigure6).

Withloweredvigilancelevels,thedolphincontinuestocapturefoodduring

itsextrahoursofforagingbutatlowerratesastimewearson(Table3).

Table3.Rateofpreyintakeandtotalpreyintakeduringaforagingbout

Time(hours) PreyConsumedperHour TotalPreyIntake0 215.76 215.761 200.66 431.522 186.61 632.173 173.55 818.784 161.40 992.335 150.10 1153.736 139.59 1303.837 129.82 1443.428 120.73 1573.249 112.28 1693.98

10 104.42 1806.2611 97.11 1910.6812 90.32 13 83.99 14 78.11

Final(optimal) 2007.80Final(80%rest) 2098.11

0102030405060708090100

0 2 4 6 8 10 12 14 16 18 20 22 24

VigilanceLevel(Percent)

Time(Hours)

Figure6.Comparativevigilancelevelsatvaryinglengthsofrest

OptimumRest

80%rest

60%rest

40%rest

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Final(60%rest) 2182.11Final(40%rest) 2260.22

Whileintheshortterm,foragingforlongerperiodsoftimemaybebeneficialtothe

dolphinasincreasedcaloricintakecanbeusedasameasurementofoverallfitness

(Dukas&Clark1994)butifthelongerforagingboutsandshorterrestperiods

continuelongtermitmayaffecttheirhealthindifferentways,apossibilitythatwill

beanalyzedlaterinthepaper.

MortalityRiskandNetEnergyGain

Asvigilancelevelsfailtoincreasebacktotheirmaximumlevelattheendofa

restingperiod,theamountoffoodgainedisnottheonlyaspectofthedolphin’sday

thatisaffected.Mortalityrisk,afunctionofvigilancelevel,alsodemonstrates

markedlydifferentresultswithlesssleepthanfromthatfoundintheoptimal

scenario.Figure2,whichshowsthemortalityriskofthedolphinfallingbacktozero

attheendoftheday,iscomparedwiththedifferingmortalityrisksasaresultofa

lossofsleeptime.Figure7illustratesthedrasticdifferenceonthelevelofmortality

riskbasedontheloweredvigilancelevels.Previously,evenwhenthedolphinsleft

therestingbaysathour20theirriskofmortalityduetoincreasedchanceof

encounteringapredatordidnotrisesignificantlybecausetheirvigilancelevelwas

sohighitkepttheirpredationriskextremelylow.Duetoashorterrestingperiod,

thevigilancelevelisloweredandasaresulttheirmortalityriskjumpsexponentially

astheyexitthebayscausingwhatwaspreviouslyalmostnonexistentmortalityrisk

tobeashighas31%(Figure7).

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Intermsofenergygain,whileusingitasameasureofthedolphin’soverall

healthitwouldseemthatintheshorttermlongerforagingboutsandshorter

restingperiodswouldsuitthedolphin’sneedsofgrowthandreproductive

behaviorsasitproducesagreaternetenergygainattheendoftheday(seeFigure8)

asthemetaboliccostremainsthesameregardlessofvigilancelevel.

0.005.0010.0015.0020.0025.0030.0035.0040.0045.00

0 2 4 6 8 10 12 14 16 18 20 22 24

MortalityRisk(Percent)

Time(Hours)Figure7.Comparisonofvaryingmortalityriskbasedonrestinglengths

Optimalrest

80%rest

60%rest

40%rest

0200400600800100012001400160018002000

0 2 4 6 8 1012141618202224

NetEnergyGain(kilocalories)

Time(Hours)Figure8.Comparisonofnetenergygainwithvaryingrestinglengths

EnergyGainwithOptimalrest

EnergyGainwith80%rest

EnergyGainwith60%rest

EnergyGainwith40%rest

MeganO’Toole

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However,whilethenetenergygainmaybehigherattheendofasingledaythe

overallhealthofthedolphinmaysufferevenwithregardstoenergygaininthelong

term.

ExtendedSimulation

Forthepurposesofthismodel,anextendedsimulationofthedatawasrun

for30dayswiththedolphinbeingabletoobtain80%ofitsidealrestingperiod.

Whenrunforonlyoneday,vigilancelevelwasrestoredto78%ofitsmaximum

level,enoughforthedolphintostillmaintainitsenergyneedsaswellaskeepits

mortalityriskstoareasonablelevel.However,afterbeingrunfor30daysthe

dolphin’svigilancelevelendsat0.74%ofitsmaximumlevelandquitefrequently

hitszeroduringforagingbouts(seeFigure8).Vigilancelevelsdroppingtozerodoes

notmeandeathandinrealityaspinnerdolphinwouldmostlikelyneverreacha

vigilancedecrementthatsevere.Thescalefromzerotoonehundredpercentis

merelyanarbitraryscaleusedtodemonstratethedecrementandrecoveryof

vigilanceratherthanaprecisemeasurement.

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Therateoffoodintakewasalsodrasticallyaffectedoverthelongterm.

Previously,whenrunattheoptimalrestingtime,thedolphinisabletocatch71.68

preyitemsperhour,butattheendof30dayslosingonly20%ofitsrestingtimethe

dolphinonlycatchesanaverageof15.79preyitemsperhourduetothedecreased

rateatwhichitisabletocatchpreybecauseofloweredvigilancelevels.Inthelatter

halfofthemonththedataoftenindicatesthedolphin’sabilitytocaptureonlyoneto

twopreyitemsperhourofforaging.Onthethirtiethdayofthesimulation,thetotal

preyintakeisjustunder19preyitems,afractionofthe2008preyitemscaptured

undertheoptimalscenarioorevenafterasingledayof80%rest(SeeTable4).

Table4.Rateofpreyintakeandtotalpreyintakeduringaforagingboutafter30daysat80%optimalsleep

Time(hours)PreyConsumedperHour TotalPreyIntake

0 2.02 2.011 1.88 3.892 1.74 5.633 1.62 7.26

0102030405060708090100

0 48 96144192240288336384432480528576624672720

VigilanceLevel(Percent)

Time(hours)Figure9.Simulationrunfor30days(or720hours)withthedolphinrestingat80%ofitsideallength

VigilanceLevel

MeganO’Toole

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4 1.51 8.765 1.40 10.176 1.30 11.477 1.21 12.698 1.13 13.829 1.05 14.87

10 0.98 15.8411 0.91 16.7512 1.02 17.77

Final 18.78

MortalityRiskandNetEnergyGain

Asshowntheoptimalscenario(Figure4),thedolphinbeginseachdaywith

itslowestmortalityriskitwillhaveforthenext24‐hoursuntilitisabletorecover

vigilanceduringtherestingperiod.However,whensufficientrestisnotpossiblethe

dolphinfailstoregainmaximumvigilancebytheendofthedayandconsequentlyits

mortalityriskdoesnotfalltoitsideallevelsaroundzero.Overtime,thelowestlevel

ofmortalityriskrisesasthedolphinisnotabletoregainallofitslostvigilanceand

theamountunrecoveredgrowslargerasthemonthwearson.Figure10showsthis

gradualincreaseinoverallmortalityriskovera30dayperiodasthedolphin’s

vigilancelevelscontinuetodecrease.

MeganO’Toole

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Intheshort‐termsimulations,itwouldseemthatthelongertheforaging

bout,thebetteroffthedolphinisintermsoffoodcaptureandenergygained.

However,ifthebehavioroflongerforagingboutsisexpandedtolookatthe

dolphin’snetenergygainovertheperiodofamonththebenefitquicklydisappears

andwhatintheshorttermwasbeneficialbecomesdetrimentaltothedolphin’s

overallhealth(Figure11).

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

1 3 5 7 9 11131517192123252729

MortalityRisk(Percent)

Time(Days)Figure10.Mortalityriskoveramonthperiodwith80%optimalsleepingtime

MortalityRisk

‐5000‐4000‐3000‐2000‐1000

010002000300040005000

1 4 7 10 13 16 19 22 25 28

Kilocalories

Time(Days)

Figure11.Totalkilocaloriesconsumedperdayandnetenergy

KilocaloriesConsumedperday

NetEnergy

MeganO’Toole

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Ofcourse,themodeldoesnottakeintoaccountamultitudeoffactorsincludingthe

knowledgespinnerdolphinsoftenparticipateincooperativeforaging(Benoit‐Bird

etal2009)whichwouldreducetheamountofeffortexpendedperpreyitem

captured.Whilethisisanextremeprojectionandishighlyunlikelytobeaccurateit

showsageneraltrendthatlongtermlossofsleepandconsequentlyvigilancewill

severelyaffecttheabilityofthedolphintoprovidefoodforitself.

Discussion

Asdolphinsperformvariousactivitiesthroughouttheday,theirdecisionsto

participateincertainbehaviorsareduetoavarietyoffactors.Someofthebehavior

maybeattributedtofoodavailabilityorenergyconservationorevenpredationrisk,

butDukas&Clark(1995)arguethatvigilancedecrementisanotheroneofthese

importantdeterminingfactors.Theirequationssimulateascenarioinwhichmuch

ofthelifeofaspinnerdolphinisaffectedbyvigilancelevels.Whilecertainlynotthe

onlydeterminingfactor,Dukas&Clarkmayhavetouchedonanaspectofan

animal’slifethathasnotbeenexploredandispossiblyquitepertinenttothe

spinnerdolphincontroversies.

Vigilancemaybedefinedconciselyas“complexpatternrecognition”(Dukas

2002),butthescopeofitsinfluenceisclearlymuchgreaterthanitinitiallymay

appear.Fromthesesimulationsitwouldseemthatthevigilancelevelofananimal

affectsmorethanjustitsawarenessofitssurroundings;itaffectshowtheanimalis

abletocareforandprotectitselfeveryday.Theoptimalscenarioprovidesunder

somegeneralassumptionsthefactorswhichareacceptedtobeaffectedbyvigilance

levelandhowtheyrespondtoitsriseandfall.Intheidealscenario,thedolphinis

MeganO’Toole

33

abletoregainallofthevigilanceithaslostoverthecourseoftheforagingbout,keep

mortalityrisktoaminimumandconsumeenoughpreyitemstomeetitsmetabolic

needs.

Assoonastheamountofrestisdiminished,thedolphin’svigilancelevelfails

toreturntoitsmaximumlevelattheendofitsdailyrestingperiod.Initially,the

consequenceoffailuretoobtain5hoursofrestdoesnotseemofconsequence,as

evengetting40%ofitsoptimumsleepingperiodthevigilancelevelonlydropsto

aroundfiftypercent.However,itisnotthelevelofvigilancethatindicatesthe

dolphin’soverallhealthbutrathertheotherfactorswhichareafunctionofthe

vigilancelevelthatindicatethetrueeffectsofalackofrest.Therateofpreycapture

ispositivelycorrelatedwithvigilancelevel(Figure2)andsothelongertheforaging

boutthelowerthevigilancelevelmeaningthedolphinisslowertodetectand

captureprey.Onthefirstday,thelongerforagingboutwiththeshorterresting

periodisdemonstratedtobebeneficialforthedolphinintermsofenergygain.

However,themortalityriskindicatesthatevenwithminimallossofsleep,the

dolphinsaresignificantlymoreatrisktopredationthanwithafullrestingbout.As

showninFigure7,withafullrestingperiodof5hours,thevigilancelevelis

restoredtothepointthatevenafterthedolphinleavesthebaysfortheopenocean,

wherethechanceofencounteringapredatorjumpsexponentially,themortalityrisk

isstillveryclosetozero.Withlessrestvigilanceisnotrestoredto100%and

consequentlythepredationriskremainshighersothatthemortalityriskafter

losingonly20%restingtimeleadstoajumpinmortalityriskjustbyleavingthe

restingbays(seeFigure7).

MeganO’Toole

34

Whiletheshort‐termscenariosprovideinsightintohowsensitivethevarious

factorsaretothedynamicsofvigilancelevels,along‐termviewdemonstratesinto

thepotentialeffectsofhumaninterruptionofadolphin’srestingtime.Thethirty

daysimulationwasrunwiththedolphinsleeping4hoursperdayinordertogeta

betterideaoflongtermramificationsofsleepdeprivationwhileusingalow

estimateoftheamountofsleeplost.However,theresultsarestillquitedrastic.As

canbeseeninFigure9,vigilancelevelsdecreaseoverthethirtydayperioduntil

maximumvigilanceachievedattheendofarestingboutisonlyaquarterofa

percentofthemaximumpotential.Additionally,themortalityriskgrowsasthe

monthprogressesthedolphinisunabletohaveenoughrestingtimetorecoverits

vigilancelevelsandisthereforelesslikelytobeabletodetectapredatorintimeto

fendofftheattackorescapeasitsresponsetimewillbeslowed.

Inthemostextremeoutcomeoftheextendedsimulation,themodelpredicts

thataspinnerdolphinhavingsleptforfourhoursadayfor30daysinarowwill

onlybeabletocaptureapproximately19preyitemsoveratwelvehourforaging

boutonaccountofdiminishedvigilance.Whilethisisunrealistictoexpectthisina

realworldscenario,theindicationisclearthatalong‐termlossofsleepand

consequentlyvigilancewillseverelyaffecttheabilityofthedolphintoprovidefood

foritself.Additionally,Figure11showsthenetgainofenergytofallbelowzero

afteronlytheseconddayofgetting80%ofoptimalrestingtime.Resultssuchas

thismayindicatelongtermeffectsonthedolphin’shealthduetopoornutritionor

reproductivehealth.Oncenetenergygainedfallsbelowzero,itindicatesthatthe

dolphinmustutilizeenergyfromitsstoresinitsblubberinordertomeetits

MeganO’Toole

35

metabolicneeds.Dolphinblubberprovidestheanimalwithinsulationandpositive

buoyancy(Struntz,etal2004),importantcharacteristicsforalifelivedunderthe

water.Ifthedolphinisajuvenile,thiswouldalsodiminishgrowthcapacityasall

energywouldbeusedmerelyformaintenancepurposes.

Therearemultipleweaknessesinthismodelandmuchroomfor

improvement.Firstly,aminorprobleminthismodelisitsfailuretoaddressthe

vigilancedecrementwhichsurelyoccursduringthetravelandsocializationtime

whichoccursbetweenforagingandrestingbouts.Additionally,atthecurrentrateof

vigilancedecrement,eventheslightestlossofsleepleadstohugejumpsinthe

mortalityriskofthedolphinsaswellaslongtermenergygain.Theparametersmay

havetobechangedtoreflectthefactthatthedolphinsarenotstarvingtodeath

eventhoughtheyappeartobegettinglessthan4hoursofsleepoveraperiodlonger

thanonemonth.However,failuretoaddressthisinthemodeldoesnotnegatethe

potentialramificationsoflackofrest,asthegeneraltrendsarestillpertinentand

valuableinformationtothosestudyingthespinnerdolphins.Withlesssleepwill

comelessabilitytodetectpreyandoverthelongtermthedolphin’snutritionwill

sufferbutperhapsnottotheextentindicatedbythemodel.

Furthermore,thenetenergygainequationsandparameterscouldbemade

moreaccurate.Byonlycalculatingtheminimumamountoffoodcapturedandeaten

inasingledaybyaspinnerdolphin,themodelismostlikelytoosensitivetoa

loweredvigilancelevelinthatitalmostimmediatelyindicatesthatthedolphinwill

sufferfrommalnutritionfromtwoboutsofpoorsleep.However,thiswould

introduceanotherprobleminthatinsteadofhavingacalculatednumberbasedon

MeganO’Toole

36

studiesandestimatesformaintenanceneedsofthespinnerdolphinamore

arbitrarynumberwouldhavetotakeitsplace,makingthemodellessbasedonfacts

andmorebasedonassumptionsofhowmuchoveritsmaintenanceneedsadolphin

mayeat.

Ofcourse,aswithallmodels,theseresultsmustbetakenasmoreofa

guidelinethanfactthatwithvigilancedecrementcomesanexactproportional

responseinallotheraspectsofadolphin’slife.Eveniftheamountofvigilancelost

leadstoadecreaseinforagingabilitylessseverethanthedatafromthemodel

indicates,thetrendisclear:longtermvigilancedecrement,nomattertheamount,

caninfactcauseasizeabledecreaseintheabilitytodetectbothpreyandpredators

andleadtohigherinstancesofmortalityinthespinnerdolphin.Vigilancelevelis

nottheonlyfactorthataffectsdolphinhealthandmortality,butasoneofmanythis

modelcanprovideaninsightintothepotentialramificationsofnearconstant

humanpresenceintheirrestingbays.Ifotherfactorsweretakenintoaccount,the

resultsfromalackofsleepwouldnotbeasdrastic.However,themodelscreated

giveanideaofhowvigilancemayfactorinandbeapartofthedolphin’sdeclining

healthinthewild.

Thismodelcouldbetakenevenonestepfartherandperhapsattemptsome

sortofoptimalforagingsothatwhenthemortalityriskbecomestoohighforthe

dolphinanditisnotabletocaptureenoughpreyitwouldceaseforagingandstart

anotheractivity.Byaddingthistothemodel,perhapsitwouldcreateamore

realisticscenarioinwhichthedolphinwouldnotforageuntilitsvigilancelevelisso

lowthatitwouldbealmostimpossibletorecovertoitsmaximumcapacity.

MeganO’Toole

37

Conclusion

Vigilanceinthisinstancedoesnotprovideasolutiontotheseissuesnoran

additionalproblem.Itmerelyservesasanothersteppingstonetobetter

understandingpotentialfactorsinthedecreaseinspinnerdolphinhealthinthese

bays.Additionally,thedrasticdecreaseinallofthespinnerdolphinhealthmeasures

(mortalityrisk,totalpreycapture,netenergygain)mayprovideadditionalleverage

onthepartofthefederalgovernmenttoillustratethattheconstanthumanpresence

inthebaysisinfactaviolationoftheMMPA.Thedataproducedbythemodel

supportstheactiononthepartofthefederalgovernmenttohavepartialclosuresin

baysknowntohaverestingspinnerdolphins.

Additionalresearchmustbedoneinordertofullyunderstandthebiology

andlifehistoryofthespinnerdolphinnotonlytohelpmakethismodelmore

accuratebutalsobecausewithoutfurtherknowledgeitispossiblethatallactionsto

protectthedolphinscouldprovefutileandsubsequentregulationoftheareawould

bepremature.Greatercontrols,possiblythroughregulationandenforcementneed

tobeenactedinthebays.Spinnerdolphinhealthisdecliningandthenumbersof

peopleinthebaysareincreasingwithoutadequatemanagementofactivitiesor

thoughttoconsequencesofhumanpresence.Thenumberofcomplaintsrelatedto

spinnerdolphinharassmentincreaseseachyearandyetenforcementhasnot

reactedaccordingly.Formanytouroperatorsafiledcomplaintsimplyleadstoa

slaponthewrist,andthebusinessistoolucrativeforsuchasmallgovernmental

responsetoceasetheirbehavior.Vigilancemodelingprovidesyetanotherinsight

intoacomplexproblemwithagrowingneedforasolution.Dolphinconservationis

MeganO’Toole

38

oneoftheuniqueenvironmentaltopicsthatintermsoftheissue‐attentioncycle

(Downs1972),itisabletomaintainlong‐termpublicenthusiasm.Thisstatus

providestremendousopportunitytopreventthegradualdeclineofaspeciesintoan

endangermentofextinctionandshouldnotbewasted.

MeganO’Toole

39

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