Utilising Magnetism as a means to reduce Elasmobranch bycatch
1. Thomas Overy OSUC07
Utilising Magnetism as a means to reduce Elasmobranch bycatch
Abstract
Elasmobranchsare well knowntobe veryimportanttothe world’soceanecosystems. Many
are apex or keystone predatorsandofferpopulationcontrol overtheirpreyspecies. Sharks,rays
and chimaeras(chondrichthyans) are caughtasbycatch by manyfisheriesgloballyandthisiscausing
dramaticdeclinesinthe global populationof manyvital species. Effective meansof reducingthe
levelsof bycatchneedtobe researchedanddiscoveredtoavoidextinctionof endangered
elasmobranchspeciesandtopreventtrophiccascades. The mostpromisingmethodisusing
magnetismtopreventelasmobranchfromtakingbaitonlonglinesandtodeterthemfromgetting
caught innets. Many testshave beenperformedandthe resultsshow thatmagnetismisan
effectivedeterrentbutthe sharkstestedquicklybecame accustomedtoitandthe effectiveness
declinedovertime. More researchisneededasmagnetismisahighlypromisingareaof
elasmobranchdeterrentsandcouldhelpreduce orevenstopbycatch.
Sharksare well knowntobe importanttothe ecologyof the world’soceans butmany
speciesare becomingendangeredthroughoverfishingandbothcommercial andrecreational
bycatch. It’snot onlysharksthat are caughtbut alsomany speciesof raysandchimaeras
(chondrichthyans)are takenasbycatch as well,meaningall elasmobranchspeciesare indangerof
populationdecline. A methodtoreduce bycatchneedstobe foundsothat the delicate ecosystem
of the oceanscan be protected. One suchmethodisthoughtto be usingthe elasmobranchsunique
electrosense tohelpthemavoidcapture.
Sharks are oftenseenasat the apex of the trophicsystemthat theyare in (Kitchell etal
2002) andif theyare not at the top, theyare near to itand theyare oftenkeystone predators
(Hinman,1998); therefore reductionsinnumbersof sharkswill have asubstantial effectonthe
whole foodweb.
A shark-targetedsurvey bythe University of NorthCarolina(UNC) hasbeenconducted
continuouslyfrom1972 to presentoff the coast of NorthCarolinawhichhasshownsignificant
decreasesinpopulationof large sharks(speciesabove 2minlength). The percentage decrease
rangesfrom87% for sandbarsharksup to 99% or greaterfor bull,duskyandsmoothhammerhead
sharks(Myers et al, 2007). Thisis sucha dramaticallyhighlevel of decliningpopulationsthat action
mustbe taken. It isthoughtto be due both to the exploitationof sharksforsharkfinsoupand other
such luxuryproductsandalsothroughbycatch fromthe many fisheriesoperatinginthe shark’s
territory.
750000 tonnes(Bonfil,1994; Stevens etal, 2000) of Elasmobranchspeciesare caught
accidentallyinthe world’sfisheries everyyear. Thisisasubstantiallyhighnumberwhencompared
to the average annual global catch fromelasmobranchfisheries,whichis824364 tonnes(FAO
FisheriesDepartment,2010). Most of the animalscaughtas bycatch are deadwhentheyare
retrievedandmanyothersare seriouslyinjured. Netsthattrapthe animalsstopthemfrommoving
and therefore suffocate themaswatercanno longerflow overtheirgills, stoppingthe oxygen
intake. Thisisshownbybeach netsinparticularas theycan seriouslydepletewhole populationsin
certainareas(DudleyandCliff,1993, Stevens etal, 2000).
Reducingbycatch isan importantmissionforthe world’sfisheriestoundertake assharks
needtobe protected. The elasmobranchsare vital tothe ecologyof oceansacrossthe globe and
2. Thomas Overy OSUC07
unlessactionistaken to stemthe rapidlyincreasingpopulationdecline,trophic cascadescouldstart
to occur and have verydamagingeffectsonfisheriesandecosystems.
Ascan be seeninFig.1 there are several sharkspeciesintertwinedwithmanyprey
organisms. Blue sharkscan be seentopreyon squid,flyingsquidandmesopelagicfish;brown
sharkspreyon squid,small scombridsandepipelagicfishplusskipjacktuna. The large sharkspecies
preyon all of the above. Whatis alsonoticedisthat nothingfeedsonthe sharks,thustheyare at
the top of this particularfoodchain(asare the blue marlinandthe biggertuna species). Thismeans
that theyare important predatorsandpossiblythe mostinfluential of theirtrophicguild. Thistrend
holdstrue,notonlyinthe NorthPacificbutall across the planetwhere sharksare found. Theyare
almostalwaysthe topof the foodchain,since theyhave few natural predators(Orcas,Humansand
otherSharks),andthismeansthat if theirpopulationstartstodecline ordwindlethe resultingtipin
the foodwebsbalance couldcause majorproblemsforthe ecologyof the area andevenforfisheries
that operate there through trophiccascade (Myerset al, 2007).
A trophiccascade,as definedbythe EncyclopaediaBritannica,isaphenomenoncausedby
the removal (oraddition) of atop predator(or toppredators) inan ecosystemwhichcauses
correspondingchangesinthe populationsof prey(orpredators) throughthe foodchaincausing
dramaticchangesin ecosystemstructure. Thiscanhappeninany ecosystem, marineorterrestrial,
and can have longtermeffectsonthe diversityof the affectedarea. A trophiccascade worksas
follows:atoppredatoris removed;thiscausesanincrease inthe populationof the nexttrophic
level;thisthendecreasesthe populationof the nextlowesttrophiclevel whichisoftenthe producer
and thisdecrease inthe lowestlevel’spopulationcauses otherspeciesthatfeedonittodecline in
numbersasresourcesare now more limited. The decrease inpopulationthenoccursupthe food
chainas there are fewerandfewerpreyorganisms. Thisdetrimental effectcanbe catastrophic
unlessdealtwithquicklyandefficiently. Thisisthe mainconsequence of the overfishingof sharks if
it isnot stoppedor,at the veryleast,slowed.
As observedbyMyers et al 2007, the abundance of 11 shark speciesthatfedonother
elasmobranchsdecreased over35years (1970 to 2005) whichcausedanincrease intheirprey’s
population,inparticularthe cownose ray whichfeedsonbayscallops. The increasednumbersof
Figure 1: Food
web of the
higher trophic
levels in the
Central North
Pacific (Kitchell
J.F. et al)
3. Thomas Overy OSUC07
these rayscausedthe collapse of a 100 yearoldscallopfisherydue tothe drasticrise inpredationof
the scallops,withalmostthe entire stockbeingdecimatedtothe pointthatthere were toofew
organismstocontinue a successful breedingsystem. Thisexampleshowshow muchof an impact
the depletionof sharkpopulationscanbe toecosystemsandtohumans.
The dramaticeffectthatsharksand elasmobranchshave onthe world’soceansis the
reasonwhymany scientistsandorganisationsare startingto seek effectivewaysof reducing
elasmobranchbycatchwithoutreducingeffectivenessand profitabilityof the fisheriesinvolved.
Several methodshave been researched,all usingthe shark’ssensoryorganstomake themavoid
capture themselves. The mostpromisingseemstobe utilisingmagnetismtoprovoke anavoidance
response inthe elasmobranch’suniqueampullae of Lorenzini,whichisthe topicreviewedhere.
Otherfishare not knownto have electroreceptors thatare as highlyadaptedandevolvedat
elasmobranchsandthis maybe a meanstodetersharksand rays etc. from fishingtrawlsand
longlineswithoutdeterringanyof the targetspecies. Exploitingthe knownfactsaboutthe ampullae
of Lorenzini couldhelpreduce the worldwide bycatch quickly.
The ampullae of Lorenzini are special electroreceptorsfoundonthe snoutof elasmobranchs;
exclusivelyaroundthe rostrum(BoordandCampbell,1977). Theyare a seriesof jellyfilledpores
withinthe epidermisanddermisthatare openatthe surface endandclosedat the other. At the
closedendof the canal there are ampullae onthe wallsof the alveoli (Riggetal,2009). Here they
gatherinformationfromthermal,mechanical andelectrical stimuli whichtheysend,viathe lateral
line nerves,tospecial receptorterminalswithinthe central nervoussystem(BroodandCampbell,
1977). Theycan alsodetectdirectcurrentfieldsandlow frequencyalternatingcurrentfieldsfor
bothlivingandnon-livingsources(BroodandCampbell,1977) and theyare sensitiveenoughto
detectthe Earth’s magneticfield(Rigg etal, 2009) and elasmobranchscanorientate themselvesto
thisfor navigation. Theyare knowntodetectmagneticfieldsthroughamechanismknownas
‘indirect-basedmagnetoreceptionviaelectromagneticinduction’(Kalmijn,1984).
Since itis knownthatelasmobranchscandetectmagneticfields,studieshave been
conductedtosee if there isany significantdifferenceindetectionof differenttypesof magnets.
There are fourmaintypesof magnetsthat have been(orshouldbe) investigated. Theyare ferrite
magnets,rare-earthmagnets, electromagnets andelectropositivemetals. Ferrite,rare-earthand
electropositive metalsare all typesof permanent magnetwhereaselectromagnetsare non-
permanent. Permanentmagnetshave beentested muchmore thannon-permanentdue tothe fact
that theyare simple andsafe touse underwater. Electromagnets,however,are more complex and
couldbe hazardousto work withinsalt-water.
Ferrite magnetsare magneticcompoundsthatcanbe formedintomanyshapesastheyare
ceramics. Alnicoisan alloymade of three ferromagneticmetals(Iron,cobaltandnickel)(Hunt,
1998). The biggestadvantage of ferritesisthattheydonot conductelectricity. Rare-earthmagnets
are the strongestpermanentmagnets. The twotypesare neodymiummagnetsandsamarium
cobaltmagnets;howeverneodymiumisthe strongestandmore available soitisusedmore oftenin
studies. The biggestproblemwithrare-earthmagnetsisthattheyare brittle andeasilycorroded
(StonerandKaimmer,2008) whichis a majorissue whenitcomesto workinginsalt-water.
Electropositive metalsare metals thatproduce avoltage whenimmersedinseawater. Theyare a
mixture of the lanthanide elements(cerium,neodymium, lanthanumandpraseodymium) (Brill etal.
2009). Theyare currentlybeingusedasshark repellentsbysome fisheriesbutmore researchis
beingconductedtotesttheireffectiveness.
4. Thomas Overy OSUC07
Electromagnetsare verydifferenttopermanentmagnetsastheyuse anelectrical currentto
produce a magneticfield. The simplestformof an electromagnetiswhencopperwire iswrapped
tightlyaroundan ironringor tube (see figure 2). Electricityisthenpassedthroughthe wire andthis
producesa magneticfield. The biggestadvantage of electromagnetsisyouhave the abilitytoturn
the magnetismonand off forwhenitis needed(givingoff pulsesissomethingthatneedstobe
studiedespeciallywell foruse assharkdeterrents). The majorproblemwithelectromagnetsisthat
theyrelyonelectricitytoworkandthis couldbe a problemwhenworkingwithseawater.
Findingthe magnettype whichismosteffective atdeterringelasmobranchsfromnetsor
hooksisvital to stoppingthe vastnumbersof animalsbeingcaughtandkilledeachyear. However,
lookingatonlythe most effective isnotenough. There hastobe a balance betweeneffectiveness
and longevity,notonlyinthe sense of the magnetsunderwaterlifespanbutalsohow longbefore
elasmobranchsbecome usedtothe magneticfieldandtherefore become lesssensitive toit
(O’Connell etal 2011). Thisis where electromagnetsshow promise astheycanpulse fieldsrather
than be permanent removingthe possibilityof elasmobranchshabitualizingtothe constant
magneticfieldof permanentmagnets. Thisisan area thatneedsto be furtherresearched asitcould
holdthe keyto reducingbycatchinthe longterm.
There have been several studies investigatingthe effectivenessof magnetismisdeterring
sharksfrom fishingnets,longline hooksandbeachnetsaswell. However, there needstobe more
researchdone before any conclusionscanbe drawn. Currently,magnetsare showntohave a
relativelygoodeffectonelasmobranchsbutoftenwithmagnetsthatare expensive ordetrimental to
the environment(Stoneretal 2008). Hopefully,furthertestingwillprovideananswertowhich
magnetscan be usedsafely,cheaplyand efficiently.
An excellentstudythatprovidesastartingpointforusingpermanentmagnetsasa meanstoprevent
unnecessaryelasmobranchdeathswasconductedin2011 by Craig O’Connellandothers. They
testedthe responsesof Negaprion brevirostris(Lemonsharks) whentheywerefacedwitha
simulatedbeachnetwithmagneticbarriersinplace. The aimof thisstudywasto see if permanent
magnetscouldreduce sharkdeathsinbeachnetsby beingusedasrepellents. Theytestedthe
effectiveness of the magnetsintwoways;the firstwasusingtonicimmobilityandthe secondwas
usinga constructedbarrierto see howthe sharksrespondedtomagneticexposurewhile swimming
freely.
Tonicimmobilityisanunlearned,coma-likestasisfoundinmanycreatures,bothterrestrial
and aquatic(Brooks et al 2011). Many elasmobranchsexhibittonicimmobilityanditisthoughtto
be a response derivedfromevadingapredatorbyreducingthe chance of detection(Marx etal
Figure 2: An example of a
simple electromagnet. When
the switchison the current
fromthe powersource
createsa magneticfield.
(How Magnets Work,2012)
5. Thomas Overy OSUC07
2008, Ratner,1967). The characteristicsshownbysharkswhile inatonicimmobilitystate isthatof a
limpmuscle tone (Whitman etal, 1986) and alsoof a rhythmicpumpingof seawateroverthe gills
(insharksthat have thisability)(WatskyandGruber,1990). It is causedbya quickdorsoventral
inversion(beingturnedover) andcanhappennaturallyorforcibly(WatskyandGruber,1990).
The test performed onthe sharkswhile intonicimmobilitywassimple. The sharkswere
placedintotonicimmobility,hadtheireyescoveredtoprevent visual stimuli affectingthe results
and thena researchermovedhissubmergedhandtowardsthe blinderof the sharkat approximately
5cm/s from a distance of 100cm away. Thiswas the control. Afterthe response of the control was
recordedthe same procedure wascarriedout witha submergedgrade C8barium-ferritemagnet.
The response wasthenrecordedona scale of 0 to 3. 0 beingnoresponse and3 beinga violentbend
away fromthe stimulusresultinginthe thrashingof the shark. The shark wasthen given60 seconds
to recoverbefore the testwasrepeatedforatotal of 6 times. There were atotal of 6 sharksinthis
trial,4 male and2 female (all releasedbacktothe wilduponcompletion). Table 1has the resultsof
thisexperiment.
Response 1 Response 2 Response 3 Response 4 Response 5 Response 6
Control 0 ± 0 0 ± 0 0 ± 0 0 ± 0 0 ± 0 0 ± 0
Magnet 2.67 ± 0.52 2.5 ± 0.84 1.83 ± 0.752 0.667 ± 0.52 0.5 ± 0.55 0 ± 0
The resultsintable 1 showthat overtime the sharks became more accustomedtobeing
nearthe magnets. Thisisseenbythe decrease inlevelsof response duringtonicimmobility,starting
as a violentresponseanddecliningsteadilywitheachexposure until therewasnoresponse atall.
Thiscouldbe showingthe sharkshabitualizingtothe magneticexposuremeaningthatmagnetsof
thistype may notbe a permanentsolution.
The secondhalf of the experimentwasthe free swimmingportion. Forthistrial a 6m
diameterpenwasbuiltandhada meshbarrierbisectingitthroughthe centre. Inthismeshthere
were two0.4m x 0.4m openings. Onone openingthere were fourclaybricksattachedtothe edge
of the opening,onthe otherwere foursimilarlysizedgrade C8barium-ferritemagnets (seefigure3).
Both the bricksand the magnetswere wrappedsoasto remove visual biases. The sharkwas placed
ina containmentpen(whichswitchedside foreachtesttoremove side preference behaviour) anda
baitbag was submergedfor30 secondsopposite itsothat a scenttrail wouldextendthrougheach
opening. Once the sharkswere released,three behaviourswere observed:visits,entrancesand/or
avoidances.
The resultsof thisexperimentshowedthatthere wasasignificantdecreaseinthe response
to magneticstimuli afterprolongedexposure,justasthe tonicimmobilitytestshowed. Thiscould
meanthat permanent magnetsare noteffectiveatrepellingsharksinthe longterm;howeverthese
were juvenile sharksthatwere retestedseveral timesinquicksuccession. Adultandyoungsharksin
the wildwouldnotbe aroundmagnetsforthis periodof time andtherefore itcouldbe safe to
assume thatthe initial responsesshowthatmagnetscanbe usedonfishinglinesandnetseffectively
as youonlyneedto deterthe sharkonce for it to avoidbeingcaptured. O’Connell etal (2011)
suggestusinga rotational beamnear beachnetsto avoidthe local sharksbecomingusedtothe
Table 1. ResultstakendirectlyfromO’Connell etal’s 2011 papershowinga lineardecrease in
responsivenesstomagneticexposure withrepeatedexposure intonicimmobility.
6. Thomas Overy OSUC07
magneticfieldspresentasthe rotationwouldchange the magneticfluxinthe water. Thisiswhere
electromagnetscouldbe usedmosteffectively,if theycanbe usedat all.
Stonerand Kaimmer(2008) conducteda laboratoryexperimenteddesignedtotestthe use
of rare-earthmetalsinreducingelasmobranchbycatchonlonglineswithoutreducingthe normal
catch of targetfishspecies. Theytestedthe effectsof neodymium-iron-boridemagnetsandcerium
mischmetal onspinydogfish(Squalusacanthius) andPacifichalibut(Hippoglossusstenolepis) against
the controlsof stainlesssteel nutsandanaluminiumplate. The magnetsandthe controlswere
suspendedinthe waterwithbaitattachedtoa 6cm lengthof twine underneaththem. Baitwas
attachedusingcable tiesbecause hookshave beenshowntoinfluence willingnesstotake baitin
repeatedtests(StonerandOttmar,2004). There wasbetween16and 20 independenttrialsoneach
speciesincludingdifferinglevelsof fooddeprivation.
The halibutshowednoflinching;irritationorrepulsionbyanyof the materials(magnetsor
metals) andthe resultssuggestthatusingeithermagnetinthe wildwill notcause anydownturnin
the numbersof Pacifichalibutcaught. Dogfish,however,showedstrongflinchingwhenapproaching
the magnets. Theyalsoseemedtobecome disorientedwhenclosetothe mischmetal (lessthan
10cm away). None of thisoccurred whentheywere nearthe controlsandso the authors assumed
that the magnetswere definitelycausingalarge amountof irritationtothe dogfishandcreating
momentsof hesitation thatresultedinfishcirclingthe bait,whereasinthe control trialsthe baitwas
approacheddirectlyandtakenwithouthesitation. This showspromise butonce the dogfishwere
deprivedof foodforlongerperiods(2daysinsteadof 1 and 4 days insteadof 2) theystill tookthe
baitdespite beingirritatedbythe magnetbutittook a significantlylongertime forthe sharkto
attack than the controls.
Thistrial also testedtonicimmobilityandfoundthe exactsame findingsasthe previously
described studybutthere waslessof a decline inresponse tothe mischmetal suggestingthatit
couldbe much more effective thanothermagnets;theyfoundthatrare-earthmagnetshadvery
little effecton spinydogfishbutithasbeenshowntobe effectiveonotherspecies. There isa
problemthatthe researchersdiscoveredwhenusingmischmetal andthatwasthat it hasrapid
Figure 3.Diagram
showingthe set
up forthe free
swimmingtest.
(O’Connel etal,
2011)
7. Thomas Overy OSUC07
Figure 4. Michael Hermann’ssimple fish
hookdesigntoreduce elasmobranch
bycatch.(Smartgear,2012)
hydrolysisinsaltwater. Thismeantthatthinwaferscouldnotbe usedmore thanonce whichwould
increase the monetaryoutputtouse mischmetal asa deterrentinfisheriesglobally. Stonerand
Kaimmer(2008) suggestselectivelyimprovingfishinggearasa more ecologicallyandeconomically
friendlywaytoreduce bycatchas differentfisheriescoulduse differenttypesof magneticstimuli to
reduce the accidental capture of the local speciesfoundinthe areastheyfish.
There have beenotherstudiesintothisarea(CraigO’Connell of Sharkdefensehasdone
manytestson many speciesinrecentyears) andtheyare all showingsimilarlypromisingresults.
Sharksare repelledbymagnets,to varyingdegrees,andthisshouldbe usedtodramaticallyreduce
the numbersof sharksbeingcaught as bycatch. There
are people whoare devisingcosteffectivesolutionsto
the bycatch issue. Michael Herrmannwonthe 2006
SmartgearGrand Prize forhis ideaof puttingrare-earth
magnetsonfishhooksto detersharks(Figure 4).
However,itseemstobe a longway fromreachingthe
worldwide fisheriesas,despitewidespread
acknowledgementof bycatchas an issue,notmany
mitigationschemesare inplace. The oceansof the
worldare currentlyonlypolicedbythe UnitedNations
Conventiononthe Lawof the Sea(UNCLOS) andit does
not have specificruleson the total allowable catchof
elasmobranchspecies andnordoesithave legislation
regardingbycatch. There is noway to stopbycatch
throughlawsand rulessothe onlyway to stopor at
leastreduce itisto get the equipmentusedtoactivelystopelasmobranchsfrombeingcaughtinthe
firstplace.
As the studiesdiscussedhave shown,the use of magnets isa viable optiontoreduce the
level of bycatchof elasmobranchspecies. Sharksshow adefinitive aversiontohighlevelsof
magnetismandcan be forcedto hesitate whenfeeding. Alsotheyhave beenseentoavoidmagnets
whenswimmingsothere isdefinitepromiseforthe use of magnetisminbycatchreduction.
However,itmaynot be a longterm solutionif certainspeciescanbecome accustomedtothe type of
magnetused. Thiswouldbe a problemwithlocal populationsespeciallywherelonglinesare usedas
the animalswouldbe aroundthe highlevelsof magnetismforregular,extendedperiodsof time.
More researchintothisarea isneededanditisnecessarytofindotherpossible alternativestousing
magnetismincase itis onlya shortterm solution. Thisisanarea that needsextensive andquick
researchas manyspeciesof elasmobranchare endangered andneedprotectionbefore theygo
extinct.
8. Thomas Overy OSUC07
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