Analysis of existing data on sand flathead larval and juvenile recruitment in Port Phillip Bay

Abstract

Sand flathead, Platycephalus bassensis, is important for recreational and commercial fishing in bays, inlets and coastal waters of southern mainland Australia and Tasmania. In Victoria, sand flathead are most important in Port Phillip Bay. Historically, the ability to reliably catch sand flathead in Port Phillip Bay has been an expectation of recreational fishers. The importance of sand flathead to commercial fishers has declined over recent time for a variety of reasons including changes in fisher behaviour and targeting preferences.

Fishery independent monitoring data collected by the annual Fisheries Victoria baywide trawl program since 1990 has clearly indicated that the abundance of sand flathead in Port Phillip Bay has declined and that the rate of decline has increased since the late 1990s. The long-term decline in abundance of sand flathead in Port Phillip Bay has recently been recognised by stakeholders and has become an important issue for Fisheries Victoria. The reasons for the decline remain unclear, as is the likelihood of recovery in the near future.

The annual baywide trawl program provides data on adult sand flathead, but poorly samples small juveniles. One reason for a decline in fishery production is prolonged failure of recruitment of young. The roles of juvenile recruitment failure and factors influencing juvenile recruitment success as drivers of the declining adult sand flathead population in Port Phillip Bay are unclear.

This project integrated data from two sampling programs to provide new information on sand flathead early life-history and juvenile recruitment dynamics. The two programs are:
• The summer baywide ichthyoplankton monitoring program being conducted as part of the Channel Deepening Baywide Monitoring Program (CDBMP) (2004/05, - 2010/11)
• The ongoing small beam trawl survey which monitors annual recruitment of 0+ age snapper in Port Phillip Bay each March (2000- ongoing).

An important aim of this work was to provide recruitment time series for juvenile (pre-recruit) sand flathead of age 0+ (i.e. young-of-the-year) and 1+ years for comparisons with the data on adult (age 2+) sand flathead from the baywide trawl program. This provides the basis for ongoing collection of sand flathead pre-recruit data from the small beam trawl monitoring program which currently only reports on 0+ age snapper.
Information extracted from the baywide ichthyoplankton monitoring program included distribution and abundance of both sand flathead (P. bassensis) and yank flathead (P. Speculator) larvae. Identification of the larval stages of these two species was also confirmed by DNA analyses. Age, length and growth rates were determined, and spawning dates were estimated for sand flathead larvae collected during the November-January sampling period. Key results were that both sand and yank flathead larvae are widely distributed throughout Port Phillip Bay, including the Port Phillip Heads region. Both species can be found as newly-hatched stages in the north and east of Port Phillip Bay indicating they were derived from local spawning within the bay, although we cannot discount the possibility that some larval stages are advected into the bay from spawning outside the bay. Sand flathead larvae were most abundant at eastern sites in the bay (Mordialloc, Carrum, Frankston) and the entrance to the Geelong Arm (Point Wilson). Sand flathead larvae were found in all depths from 5–24 m, but were more abundant in the 10–15 m depth range. Yank flathead larvae were most abundant in outer Hobsons Bay, the east of the bay (Mordialloc and Frankston) and at Point Wilson, and were found in all depths from 5–24 m, with highest catches generally in the 8–20 m depth range. Finally, otolith ageing of sand flathead larvae indicated they have a larval duration of 20–30 days and that spawning can continue until early January in Port Phillip Bay.

Information extracted from the small beam trawl program included 11 years of data on the overall abundance and size distribution of sand flathead at 7 sites in Port Phillip Bay. Further, by confirming representative size cut-offs based on otolith ageing, this data set was broken down to provide abundance indices for pre-recruit sand flathead of 0+ and 1+ years age, and adult sand flathead (i.e. >2 years age) across the 11 years.

The 11 years of beam-trawl sampling indicated that recruitment of 0+ age sand flathead was highly variable in space and time. Overall, the densities of 0+ age sand flathead over the last 11 years appeared to be low. Highest recruitment of 0+ age sand flathead occurred in 2004 (i.e. 2003/04 spawning season) and there was an indication of increased recruitment over the recent period from 2008-2010. The peak in abundance of 0+ age sand flathead observed in 2004 was clearly evident in the size distribution data and was detected the following year as a peak in abundance of 1+ age fish, and again the following year as a peak in abundance of age 2+ fish by the larger baywide otter trawl program. This provides some evidence that dynamics of recruitment to adult population are linked to juvenile recruitment dynamics, and therefore to the dynamics of egg production and or early life-history survival rates.

Comparison of the recent 11 years of 0+ and 1+ age recruitment data from the small beam-trawl program with the 20 years of age 2+ recruitment data from the baywide trawl program confirmed that recruitment of juvenile sand flathead has been low over the past 11 years. The strongest cohort detected for the past 11 years (i.e. 2003-04 spawning), is poor compared to recruitment events in the 1990s, and did not appear to have a sustained impact on the overall abundance of adult sand flathead. The poor juvenile recruitment over the last 11 years would be an important driver of the continued decline in the abundance of sand flathead within Port Phillip Bay over the last 10 years.
Other information extracted from the small beam- trawl program included depth distributions of sand flathead of different life-stages in Port Phillip Bay. Preliminary daily otolith ageing data also indicated the broad potential spring-summer spawning period of sand flathead within Port Phillip Bay. Juvenile and adult sand flathead were found across the same depths and habitats from 5–24 m, although it appeared that 0+ age fish were more abundant in shallower depths from 5–15 m, 1+ age fish were most abundant in intermediate depths between 10-20 m and adults in depths from 15–24 m. Catch rates of 0+ and 1+ age sand flathead were highest at Hobsons Bay and the eastern bay sites (Mordialloc, Carrum, Frankston), compared to the centre of the bay and Point Wilson.
The small beam-trawl program aimed at 0+ age snapper can provide ongoing annual abundance indices for sand flathead in Port Phillip Bay, with the important inclusion of the pre-recruit 0+ and 1+ year age classes. This additional data can be provided at little extra cost or modification to the existing sampling program and can provide a valuable indicator of spawning success and future population trends of sand flathead.

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Larvalandjuvenilesandflatheadrecruitment
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Analysisofexistingdataonsandflathead
larvalandjuvenilerecruitmentinPort
PhillipBay
PaulHamer,JodieKemp,JuliaKent
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FisheriesVictoria
ResearchReportSeriesNo.50
December2010

Larvalandjuvenilesandflatheadrecruitment
ii
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Hamer,P.,Kemp,J.,andKent,J.(2010)Analysis
ofexistingdataonsandflatheadlarvaland
juvenilerecruitmentinPortPhillipBay.Fisheries
VictoriaResearchReportSeriesNo.50.
ISSN 1448‐7373
ISBN978‐1‐74264‐525‐4
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PaulHamer
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Larvalandjuvenilesandflatheadrecruitment
iii
ExecutiveSummary
Sandflathead,Platycephalusbassensis,is
importantforrecreationalandcommercial
fishinginbays,inletsandcoastalwatersof
southernmainlandAustraliaandTasmania.In
Victoria,sandflatheadaremostimportantin
PortPhillipBay.Historically,theabilityto
reliablycatchsandflatheadinPortPhillipBay
hasbeenanexpectationofrecreationalfishers.
Theimportanceofsandflatheadtocommercial
fishershasdeclinedoverrecenttimeforavariety
ofreasonsincludingchangesinfisherbehaviour
andtargetingpreferences.
Fisheryindependentmonitoringdatacollected
bytheannualFisheriesVictoriabaywidetrawl
programsince1990hasclearlyindicatedthatthe
abundanceofsandflatheadinPortPhillipBay
hasdeclinedandthattherateofdeclinehas
increasedsincethelate1990s.Thelong‐term
declineinabundanceofsandflatheadinPort
PhillipBayhasrecentlybeenrecognisedby
stakeholdersandhasbecomeanimportantissue
forFisheriesVictoria.Thereasonsforthedecline
remainunclear,asisthelikelihoodofrecoveryin
thenearfuture.
Theannualbaywidetrawlprogramprovides
dataonadultsandflathead,butpoorlysamples
smalljuveniles.Onereasonforadeclinein
fisheryproductionisprolongedfailureof
recruitmentofyoung.Therolesofjuvenile
recruitmentfailureandfactorsinfluencing
juvenilerecruitmentsuccessasdriversofthe
decliningadultsandflatheadpopulationinPort
PhillipBayareunclear.
Thisprojectintegrateddatafromtwosampling
programstoprovidenewinformationonsand
flatheadearlylife‐historyandjuvenile
recruitmentdynamics.Thetwoprogramsare:
• Thesummerbaywideichthyoplankton
monitoringprogrambeingconductedaspart
oftheChannelDeepeningBaywide
MonitoringProgram(CDBMP)(2004/05,‐
2010/11)
• Theongoingsmallbeamtrawlsurveywhich
monitorsannualrecruitmentof0+age
snapperinPortPhillipBayeachMarch
(2000‐ongoing).
Animportantaimofthisworkwastoprovide
recruitmenttimeseriesforjuvenile(pre‐recruit)
sandflatheadofage0+(i.e.young‐of‐the‐year)
and1+yearsforcomparisonswiththedataon
adult(age2+)sandflatheadfromthebaywide
trawlprogram.Thisprovidesthebasisfor
ongoingcollectionofsandflatheadpre‐recruit
datafromthesmallbeamtrawlmonitoring
programwhichcurrentlyonlyreportson0+age
snapper.
Informationextractedfromthebaywide
ichthyoplanktonmonitoringprogramincluded
distributionandabundanceofbothsandflathead
(P.bassensis)andyankflathead(P.Speculator)
larvae.Identificationofthelarvalstagesofthese
twospecieswasalsoconfirmedbyDNA
analyses.Age,lengthandgrowthrateswere
determined,andspawningdateswereestimated
forsandflatheadlarvaecollectedduringthe
November‐Januarysamplingperiod.Keyresults
werethatbothsandandyankflatheadlarvaeare
widelydistributedthroughoutPortPhillipBay,
includingthePortPhillipHeadsregion.Both
speciescanbefoundasnewly‐hatchedstagesin
thenorthandeastofPortPhillipBayindicating
theywerederivedfromlocalspawningwithin
thebay,althoughwecannotdiscountthe
possibilitythatsomelarvalstagesareadvected
intothebayfromspawningoutsidethebay.
Sandflatheadlarvaeweremostabundantat
easternsitesinthebay(Mordialloc,Carrum,
Frankston)andtheentrancetotheGeelongArm
(PointWilson).Sandflatheadlarvaewerefound
inalldepthsfrom5–24m,butweremore
abundantinthe10–15mdepthrange.Yank
flatheadlarvaeweremostabundantinouter
HobsonsBay,theeastofthebay(Mordiallocand
Frankston)andatPointWilson,andwerefound
inalldepthsfrom5–24m,withhighestcatches
generallyinthe8–20mdepthrange.Finally,
otolithageingofsandflatheadlarvaeindicated
theyhavealarvaldurationof20–30daysand
thatspawningcancontinueuntilearlyJanuaryin
PortPhillipBay.
Informationextractedfromthesmallbeamtrawl
programincluded11yearsofdataontheoverall
abundanceandsizedistributionofsandflathead
at7sitesinPortPhillipBay.Further,by
confirmingrepresentativesizecut‐offsbasedon
otolithageing,thisdatasetwasbrokendownto
provideabundanceindicesforpre‐recruitsand
flatheadof0+and1+yearsage,andadultsand
flathead(i.e.>2yearsage)acrossthe11years.

Larvalandjuvenilesandflatheadrecruitment
iv
The11yearsofbeam‐trawlsamplingindicated
thatrecruitmentof0+agesandflatheadwas
highlyvariableinspaceandtime.Overall,the
densitiesof0+agesandflatheadoverthelast11
yearsappearedtobelow.Highestrecruitmentof
0+agesandflatheadoccurredin2004(i.e.
2003/04spawningseason)andtherewasan
indicationofincreasedrecruitmentoverthe
recentperiodfrom2008‐2010.Thepeakin
abundanceof0+agesandflatheadobservedin
2004wasclearlyevidentinthesizedistribution
dataandwasdetectedthefollowingyearasa
peakinabundanceof1+agefish,andagainthe
followingyearasapeakinabundanceofage2+
fishbythelargerbaywideottertrawlprogram.
Thisprovidessomeevidencethatdynamicsof
recruitmenttoadultpopulationarelinkedto
juvenilerecruitmentdynamics,andthereforeto
thedynamicsofeggproductionandorearlylife‐
historysurvivalrates.
Comparisonoftherecent11yearsof0+and1+
agerecruitmentdatafromthesmallbeam‐trawl
programwiththe20yearsofage2+recruitment
datafromthebaywidetrawlprogramconfirmed
thatrecruitmentofjuvenilesandflatheadhas
beenlowoverthepast11years.Thestrongest
cohortdetectedforthepast11years(i.e.2003‐04
spawning)ispoorcomparedtorecruitment
eventsinthe1990s,anddidnotappeartohavea
sustainedimpactontheoverallabundanceof
adultsandflathead.Thepoorjuvenile
recruitmentoverthelast11yearswouldbean
importantdriverofthecontinueddeclineinthe
abundanceofsandflatheadwithinPortPhillip
Bayoverthelast10years.
Otherinformationextractedfromthesmall
beam‐trawlprogramincludeddepth
distributionsofsandflatheadofdifferentlife
stagesinPortPhillipBay.Preliminarydaily
otolithageingdataalsoindicatedthebroad
potentialspring‐summerspawningperiodof
sandflatheadwithinPortPhillipBay.Juvenile
andadultsandflatheadwerefoundacrossthe
samedepthsandhabitatsfrom5–24m,although
itappearedthat0+agefishweremoreabundant
inshallowerdepthsfrom5–15m,1+agefish
weremostabundantinintermediatedepths
between10‐20m,andadultsindepthsfrom15–
24m.Catchratesof0+and1+agesandflathead
werehighestatHobsonsBayandtheeasternbay
sites(Mordialloc,Carrum,Frankston),compared
tothecentreofthebayandPointWilson.
Thesmallbeam‐trawlprogramaimedat0+age
snappercanprovideongoingannualabundance
indicesforsandflatheadinPortPhillipBay,with
theimportantinclusionofthepre‐recruit0+and
1+yearageclasses.Thisadditionaldatacanbe
providedatlittleextracostormodificationtothe
existingsamplingprogramandcanprovidea
valuableindicatorofspawningsuccessand
futurepopulationtrendsofsandflathead.





Larvalandjuvenilesandflatheadrecruitment
v
TableofContents
ExecutiveSummary............................................................................................. iii
Introduction............................................................................................................ 1
Background.................................................................................................................................................................1
Reviewofsandflatheadspawningandearlylife‐history .................................................................................2
Spawning .................................................................................................................................................................2
Larvalstage..............................................................................................................................................................2
0+age........................................................................................................................................................................2
ProjectDesignandMethods ............................................................................... 4
Baywideeggandlarvalmonitoringprogram.......................................................................................................4
Dataanalysesandlaboratorymethods................................................................................................................5
Beam‐trawlmonitoringprogram ...........................................................................................................................5
Dataanalysesandlaboratorymethods................................................................................................................6
Results...................................................................................................................... 8
Baywideeggandlarvalmonitoringprogram.......................................................................................................8
Larvalidentificationsuccess .................................................................................................................................8
Abundanceanddistribution .................................................................................................................................8
Ageandgrowthoflarvae......................................................................................................................................9
SpawningdatesofPlatycephalusbassensislarvaesampledfromNovember‐January ...................................9
Beam‐trawlmonitoringprogram .........................................................................................................................14
Confirmationofsizecut‐offforthe0+agegroupinMarchbeam‐trawlsamples.......................................14
Spatialanddepthdistributions...........................................................................................................................14
Interannualabundancevariationby..................................................................................................................17
samplingarea ........................................................................................................................................................17
Interannualabundancevariation–pooledacrosssamplingareas................................................................17
Comparisonofbeam‐trawlandbaywideottertrawlsurveydata.................................................................22
Sizedistributions ..................................................................................................................................................24
Estimatedspawningdatesfrom0+ageotoliths...............................................................................................27
Discussion............................................................................................................. 28
Newinsightsintoearlylifehistory......................................................................................................................28
Recruitmentvariation .............................................................................................................................................29
Anewpre‐recruitindexforsandflathead ..........................................................................................................30

Larvalandjuvenilesandflatheadrecruitment
vi
Acknowledgements ............................................................................................ 31
References ............................................................................................................. 32

ListofFigures
Figure1AdultPlatycephalusbassensis...................................................................................................................... 3
Figure2Platycephalusbassensislarva,approx.9mmSL ....................................................................................... 3
Figure3Platycephalusspeculatorlarva,approx.10mmSL ................................................................................... 3
Figure40+agePlatycephalusbassensiscollectedbybeam‐trawlinPortPhillipBayinMarch2010,
demonstratingtheconsiderablesizerangepresentofthe0+agegroup.................................................... 3
Figure5MapofPortPhillipBayshowingichthyoplanktonsamplingarea...................................................... 4
Figure6Sagittalotolithfroma10.3mmSLP.bassensislarva,x400magnification. ........................................ 5
Figure7Diagramoftheplumbstaffbeamtrawlusedtosample0‐agesnapper.a)mainbodyofnet,4m
long,12mmstretch,4mm2apertureknotlessraschelmesh.b)codendbag,1mlong,8mmstretch,3
mm2apertureknotlessraschelmesh.c)4.7mheadrope.d)2.2mbreastlines.e)1.7mlowerbridle,
firstmetreoflowerbridleis2cmx3cmx0.5cmchain.f)1.8mupperbridle.g)4.8mticklerchain,
chaintypeasprevious.h)5.6mfootrope.i)5.8kgcurveddetachableleadweights(divinghip
weights).j)9.5cmdiameterfoamfloats.k)3msteelbeam,3cmdiameterpipe.l)4.1mbeambridle
rope.m)stainlessswivel.n)emergencyretrievalline. ................................................................................. 7
Figure8MapofPortPhillipBayshowingbeam‐trawlsamplingareas(sitescentral1andcentral2
sampledsince2004,allothersitessampledsince2000). .............................................................................. 7
Figure9Imagesofpreparedtransversesectionsof;a)0+ageotolithfordailyageingwithprimordium
indicatedbyarrow‐aandthemicrostructuretransitionzone(settlementmark)indicatedbyarrow‐
b,b)otolithfromage6+yearsadultwitharrowdenoting1stannualincrement,c)otolithfromage1+
juvenileshowinglocationof1stannualincrement. ....................................................................................... 8
Figure10MapofPortPhillipBayshowingdistributionofPlatycephalusbassensislarvaesampledfromlate
NovembertoearlyJanuarypooledacross6years(2004/05–09/10). ........................................................... 9
Figure11ScatterplotofcatchratesofPlaycephalusbassensislarvaeagainstdepthofthesamplinglocations.
............................................................................................................................................................................ 10
Figure12MapofPortPhillipBayshowingdistributionofPlatycephalusspeculatorlarvaesampledfrom
lateNovembertoearlyJanuarypooledacross6years(2004/05–09/10). .................................................. 11
Figure13ScatterplotofcatchratesofPlaycephalusspeculatorlarvaeagainstdepthofthesampling
locations............................................................................................................................................................. 12
Figure14Scatterplotsandlinearregressionofstandardlengthversusagefor;a)Platycephalusbassensis,
andb)PlatycephalusspeculatorlarvaesampledinPortPhillipBayduringsummer. .............................. 13
Figure15FrequencydistributionofhatchdatesofPlatycephalusbassensislarvaesampledbetweenlate
NovemberandmidJanuaryinPortPhillipBaypooledacrosssixyears(2004/05–09/10)..................... 13
Figure16Distributionofsizesof0+and1+agePlatycephalusbassensissampledinlateMarchfromPort
PhillipBay. ........................................................................................................................................................ 14
Figure17Comparisonofmeanabundances(pooledacross11years)ofPlatycephalusbassensislife‐stages
amongbeam‐trawlsamplingareasinPortPhillipBay.Errorbars±SE. ................................................. 15
Figure18Scatterplotsofbeam‐trawlcatchratesofPlaycephalusbassensislife‐stagesagainstdepthofthe
samplinglocations. .......................................................................................................................................... 16

Larvalandjuvenilesandflatheadrecruitment
vii
Figure19Interannualvariationinabundanceof0+agePlatycephalusbassensisrecruitsfrombeam‐trawl
samplinginMarchcomparedacrosssamplingareasinPortPhillipBay.Errorbars±SE.................... 18
Figure20Interannualvariationinabundanceof1+agePlatycephalusbassensisrecruitsfrombeam‐trawl
samplinginMarchcomparedacrosssamplingareasinPortPhillipBay.Errorbars±SE.................... 19
Figure21InterannualvariationinabundanceofadultPlatycephalusbassensisfrombeam‐trawlsampling
inMarchcomparedacrosssamplingareasinPortPhillipBay.Errorbars±SE..................................... 20
Figure22Interannualvariation(pooledacrosssamplingareas)inabundanceofdifferentlife‐stagesof
Platycephalusbassensisacross11yearsofbeam‐trawlsamplinginMarchinPortPhillipBay.Error
bars±SE. ........................................................................................................................................................... 21
Figure23Interannualvariationinabundanceofage2+yearPlatycephalusbassensissampledbythe
baywideottertrawlprogramsince1990(NB:datarestrictedtoMornington,HobsonsBayand
Beaumaristransectsandsites>12m)............................................................................................................ 23
Figure24SizefrequencydistributionsofallPlatycephalusbassensissampledbythesmallbeam‐trawlin
PortPhillipBayinMarchcomparedover11years.Datapooledacrosssamplingareas...................... 24
Figure25FrequencydistributionofhatchdatesofPlatycephalusbassensisatthemonthlyscaleestimated
fromotolithsof0+agefishsampledinMarch1997and2010fromPortPhillipBay............................. 27


Larvalandjuvenilesandflatheadrecruitment
1
Introduction
Background
Sandflathead,Platycephalusbassesnsis(Fig.1),isa
demersalfishspeciesthatisimportantfor
recreationalandcommercialfishinginbaysinlets
andcoastalwatersofsouthernmainland
AustraliaandTasmania(Kailolaetal.1993).In
Victoria,sandflatheadaremostimportantin
PortPhillipBaywheretheyareconsideredthe
‘breadandbutter’fishery.Historically,theability
toreliablycatchsandflatheadinPortPhillipBay
hasbeenanexpectationofrecreationalfishers.
Thevalueofsandflatheadtocommercialfishers
hasdeclinedovertimeforavarietyofreasons
includingchangesinfisherbehaviourand
targetingpreferences(Jenkins2010).
Fisheryindependentmonitoringdatacollected
bytheannualFisheriesVictoriabaywidetrawl
programsince1990hasclearlyindicatedthatthe
abundanceofsandflatheadinPortPhillipBay
hasdeclinedoverthelast20yearsandthatthe
rateofdeclinehasincreasedsincethelate1990s
(Parryetal.2009;Jenkins2010).Thedata
indicatesadeclineofapproximately80%inthe
abundanceandbiomassofsandflatheadinthe
deepandintermediatedepthzones(>12m)of
PortPhillipBaysincetheearly1990s,andaless
pronounced,butstillsignificant,declineinthe
shallowerdepths(<12m)andthewesternregion
ofthebay(Parryetal.2009;Jenkins2010).The
long‐termdeclineinabundanceofsandflathead
inPortPhillipBayhasrecentlybeenrecognised
bystakeholdersandhasbecomeanimportant
issueforFisheriesVictoria.Thereasonsforthe
declineremainunclear,asisthelikelihoodof
recoveryinthenearfuture.
Understandingwhythelocalabundanceoffish
changesovertimeiscomplex.Attheveryleasta
knowledgeofrecruitmentdynamicsofyoung
andchangesinmortalityratesofolderfishis
requiredtodeterminewhetheradeclineisdueto
prolongedjuvenilerecruitmentfailureand/or
reducedsurvivalratesofolderfish.Establishing
whichofthesepopulationprocessesisdrivinga
fisherydeclineischallengingwithoutlong‐term
historicalmonitoringofrecruitmentand
mortalityrates.
Whilechangesindistributionandormigration
habitsmayalsoinfluencechangesinlocal
abundanceoffish,sustainedlong‐termdeclining
orincreasingtrendswouldbemorelikelyto
relatetoprocessessuchasrecruitmentand
survivalwhichmaybedrivenbychangesin
environmentalsuitability,habitatcarrying
capacityandorpredation/exploitationrates.
Ifthepopulationprocess(es)drivingafishery
declinecanbedetermined,thenextchallengeis
todeterminewhatenvironmental,ecologicalor
humanrelated(includingexploitation)factors
maybecontributingtotheprocessofdecline.
Onlywiththisinformationcanresource
managersactivelymanagethefisherywiththe
aimofpreventingfurtherdeclineandpromoting
populationrecovery.
Muchofourcurrentunderstandingoftrendsin
sandflatheadabundanceinPortPhillipBayhas
beenderivedfromthepreviouslymentioned
annualbaywidetrawlprogram,whichcollects
dataonavarietyofdemersalspeciesusinga
commercialsizeottertrawl.Duetothelarge
meshsizeofthistrawlitprimarilysamplessand
flatheadof2yearsofageandolderanddoesnot
providereliabledataonrecruitmentvariationof
0+agefish(i.e.young‐of‐the‐year),orontheir
spatialdistribution.Poorunderstandingofthe
earlylife‐historyandtheprocessofjuvenile
recruitment(i.e.spawningthroughfirstyearof
life)remainsacriticallimitingfactorinour
abilitytoexplainpopulationchangesofsand
flatheadinPortPhillipBay.
Thisprojectaimedtofillsomeoftheinformation
gapsaroundsandflatheadearlylife‐historyand
recruitmentdynamicsinPortPhillipBayby
drawingtogetherdatafromtwomonitoring
programsthathavebeenundertakeninPort
PhillipBayoverthelast11years,concurrentwith
thepreviouslymentionedbaywideottertrawl
program.Theseprogramsare:
• Thesummerbaywideichthyoplankton
motoringprogrambeingconductedaspart
oftheChannelDeepeningBaywide
MonitoringProgram(CDBMP)(2004/05,‐
2010/11)
• Thesmallbeam‐trawlsurveywhich
monitorsrecruitmentof0+agesnapperin
PortPhillipBay(2000‐ongoing).
Inthisproject,datafromthesetwoprogramsare
extractedtoprovidenewinformationonsand
flatheadearlylife‐historyincludingrecruitment

Larvalandjuvenilesandflatheadrecruitment
2
dynamicsofthe0+agelifestage,larvaland0+
distributionsandabundances,andspawning
dates.Animportantoutcomeofthisworkalso
involvedthecollationofallthesandflathead
datacollectedbythesmall‐beamtrawlsurvey
overthepast11years.Althoughthisprogram
primarilytargets0+agesnapper(<10cmlength),
dataonallsandflatheadcollectedhavebeen
recordedonfieldsheets.Thisprojectinvolved
formalcompilationandanalysesofthesedata
bothtoidentifypopulationlevelrecruitment
variation/trendsandtodevelopa0+age
recruitmentindexforsandflatheadinPort
PhillipBaythatcanbecollectedinconjunction
withtheongoingsnapperrecruitment
monitoring.
Reviewofsandflathead
spawningandearlylife‐history
Spawning
Sandflathead(Fig.1)reachsexualmaturity
betweenthelengthsof19‐21cmTL(totallength)
inPortPhillipBay,withmostindividuals>22cm
TLand>2yearsofagebeingmature(Brown
1977).Similarly,inTasmanianwaters,50%ofthe
populationisobservedtobematureat21.0and
23.5cmTLformalesandfemales,respectively
(Jordan1998).Spawningofsandflatheadoccurs
inestuaries,coastalembaymentsandshelf
waters(Neiraetal.2000;Jordan2001).Spawning
inTasmanianwatershasbeenreportedtooccur
fromOctobertoMarch,butcanvaryspatially
andinterannually(Jordan2001;Baniand
Moltschaniwskyj2008;Banietal.2009).
SpawninginPortPhillipBayhasbeenreported
tooccurfromAugusttoOctober;however,
individualswithrunningripegonadshavebeen
observedaslateasDecember(Brown1977).
Thesestudiessupporttheideaofanextended
spring/summerspawningperiodforsand
flathead.However,itremainsunclearwhether
successfulrecruitmentisderivedfromparticular
timeperiodswithinthispotentialbroad
spawningwindow.
Larvalstage
MonthlyichthyoplanktonsurveysofPortPhillip
Bayconductedinthemid1980s(Jenkins1986)
identifiedtwotypesofplatycephalidlarvae.
Larvaltype1wascollectedfromJunetoJulyin
1983andfromOctobertoAprilin1984,and
larvaltype2wascollectedonlyinNovemberand
December.BasedontheinformationinBrown
(1977)andJordan(2001),itisthoughtthatlarval
type1wasP.bassensis(Fig.2),whereaslarval
type2wasyankflathead,P.speculator(Fig.3).
Overall,mostplatycephalidlarvaewere
observedbyJenkins(1986)duringNovemberoff
QueenscliffandStLeonards;however,
platycephalidlarvaewereobservedthroughout
PortPhillipBay.
Sandflatheadlarvaehavealsobeencollected
duringsummer(December)inVictoriancoastal
waters,mostlybeingfoundinthesurfaceto50m
depthzone(Neiraetal.2000).InTasmanian
waters,sandflatheadlarvaehavegenerallybeen
foundconcentratedinmiddepthwaters,which
arethoughttoretainlarvaeinshorebecausecross
shelfsub‐surfacecurrentsarepredominantly
onshore(Jordan2001).
Sandflatheadlarvaesettletothebenthichabitat
atsizesof<21mmTL(Jordan1998),although
theexactsizeatsettlementisunclear.Recently‐
settledsandflatheadarefoundonunvegetated
habitats(EdgarandShaw1995),althoughlittleis
knownaboutthespecifichabitatpreferencesor
trophicdependenciesoftheseearlylife‐stages.
Aspectsoflarvalecologysuchaslarvalduration
andsizeatsettlementareunknownforVictorian
waters.
0+age
Morphologically,0+agesandflathead(Fig.4)
showallthecharacteristicsofadults,andare
relativelyeasytodistinguishinthefieldusing
thepre‐opercularspines(lowerspinetwicethe
lengthofupperspine)andcaudalfin
pigmentation(Gomonetal.2008).Thereisno
publishedinformationonthehabitats,depth
preferencesandtrophicdependenciesof0+age
sandflatheadinPortPhillipBay.InTasmania
juvenilesandflatheadaresuggestedtoshowa
preferenceforunvegetatedhabitatsinnearshore
waters,asopposedtoseagrasshabitats,anditis
thoughtthattherearenomajorhabitatshifts
withage(Jordan2001).
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Larvalandjuvenilesandflatheadrecruitment
3
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Figure1AdultPlatycephalusbassensis
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Figure2Platycephalusbassensislarva,approx.
9mmSL
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Figure3Platycephalusspeculatorlarva,approx.10
mmSL
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Figure40+agePlatycephalusbassensiscollected
bybeam‐trawlinPortPhillipBayinMarch2010,
demonstratingtheconsiderablesizerange
presentofthe0+agegroup.
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7cm
14
cm

Larvalandjuvenilesandflatheadrecruitment
4
ProjectDesignandMethods
Baywideeggandlarval
monitoringprogram

Thebaywideeggandlarvalsamplingprogramis
anichthyoplanktonsamplingprogramthathas
occurredinPortPhillipBay,includingPort
PhillipHeads,fromlateNovemberthroughto
midJanuaryeachyearsince2004/05andwill
finishin2010/11.Thissamplingprogramisan
integralpartoftheCDBMPandmonitors
interannualvariationinabundanceoffishlarvae
inPortPhillipBay,withafocusonanchovyand
snapper(Acevedoetal.2008).However,
platycephalidspecieshavealsobeencollected
duringthesamplingprogramandthesesamples
areutilisedinthecurrentproject.
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Ichthyoplanktonsamplingwasalsoconductedas
partofanotherprojectattwoareasincoastal
watersimmediatelyadjacenttothePortPhillip
Headsinthesummersof2004/05,2005/06and
2006/07(Hameretal.2010b).Althoughdatafrom
thesecoastalsamplesarenotpresentedinthis
reportduetotheincompletetaxonomic
identificationofallsamples,Platycephalus
bassensislarvaehavebeenpositivelyidentified(J.
Kent,pers.comm.).Thisfurtherindicatesthat
thisspeciesspawnsincoastalwatersadjacentto
PortPhillipBay.Thesamplesareavailablefor
futureinvestigationofcoastalspawning.
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Figure5MapofPortPhillipBayshowingichthyoplanktonsamplingarea.

Larvalandjuvenilesandflatheadrecruitment
5
Dataanalysesandlaboratorymethods
Thedetailedmethodforthecollectionof
ichthyoplanktonsampleshasbeendescribed
previouslybyAcevedoetal.2008andHameret
al.2010b.Samplingsitesareindicatedinfigure5.
Allplatycephalidlarvaewereextractedfromthe
available6yearsofethanolpreserved
ichthyoplanktonsamples.Duetothesimilarityin
morphologyofplatycephalidlarvae,separation
ofthecommonflatheadspeciesinPortPhillip
Bay,Platycephalusbassensis(Fig.2)and
Platycephalusspeculator(Fig.3),ischallenging,
particularlyforsmallerspecimens.Toconfirm
theaccurateidentificationoflarvaeofthesetwo
speciesbasedonstandardtaxonomickeys,
morphologyandpigmentation,weselecteda
sub‐sampleof20Platycephalidaelarvae;
including11specimensidentifiedbasedon
morphologyasP.speculator,7specimens
identifiedbasedonmorphologyasP.bassensis
and2dummysamplesofthefamily
Scorpaenidae.Thesesamplesrangedinsizefrom
2.3—11.7mmSL(mean:5.7mmSL)andwere
senttotheUniversityofTasmaniafor
confirmationofspeciesidentitybyDNA
analysis.
DNAwasextractedfromthesamplesandCO1
(cytochromeoxidase1)amplificationwas
achievedbyPCR(polymerasechainreaction).
TheresultantDNAwassequencedandthe
sequencescomparedwiththoseavailablefor
Platycephalidaeinthe‘BarcodesofLife’
database.Thisdatabasehassignificantcoverage
ofthePlatycephalidaeandincludessequences
forbothP.bassensisandP.speculator.Ofthe20
samplessentforanalyses,PCRamplification
failedfor4samplesincludingthe2dummy
samples.
AlthoughP.bassensiswasthefocusofthis
project,P.speculatordatawerealsoincludedfor
comparison.Dataonthedensity(CPUE)ofsand
flatheadandyankflatheadlarvaeweremapped
tocomparedistributionsoflarvaeacrossthe
samplingareaswithinPortPhillipBayand
acrossPortPhillipHeads.Thelarvaldensitydata
werealsographedinrelationtowaterdepth.
BecauseofthegenerallylownumbersofP.
bassensislarvaecollectedinanyparticularyear,
thedatawerepooledacrossallyearsforboth
species.
Allplatycephalidlarvaeextractedfrom
ichthyoplanktonsamplesweremeasuredfor
standardlength(tipofsnouttotipofcaudle
peduncle,nearest0.1mm)underadissecting
microscopefittedwithanocularmicrometer.
Sagittalotolithswereextractedfrom62xP.
bassensisand47xP.speculatorlarvae.Theotoliths
wereimmersedinimmersionoilandtheirages
wereestimatedbyviewingunderacompound
microscopeatX400magnificationassuming
dailyincrementformation(Fig.6).Ageestimates
wereplottedagainstlengthandwereusedto
back‐calculatehatchdates(indicativeof
spawningtimes)forP.bassensis.Hatchdates
werenotdeterminedforP.speculatorsinceall
larvaeanalysedinthecurrentstudywere
sampledfromone24hrsamplingperiodaspart
ofavalidationofdailyincrementformationin
platycephalidlarvae(J.Kent,unpublisheddata).
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Figure6Sagittalotolithfroma10.3mmSLP.
bassensislarva,x400magnification.

Beam‐trawlmonitoringprogram

Theannualbeam‐trawlsurveyisanight‐time
samplingprogramaimedatmonitoring
interannualvariationinrecruitmentof0+age
(young‐of‐year)snapperinPortPhillipBay
(HamerandJenkins2004).Theprogramis
conductedinlateMarcheachyearandhasbeen
ongoingsince2000.Thebeam‐trawlusedissmall
(~3mmouthwidth)andisconstructedofafine
meshallowingittosamplesmallnewly‐recruited
snapperandotherspecies(Fig.7).
Thebeam‐trawlsurveyhasbeenconductedat7
sitesfortheentire11yearsoftheprogram,and9
sitessince2004(Fig.8).Detailedmethodsforthe
trawlprocedurecanbefoundinHamerand
Jenkins(2004).Whiletheprogramisfocussedon
0+agesnapper,dataonsandflatheadhasalso
beenroutinelycollectedtopotentiallyaddvalue
100
μ
m

Larvalandjuvenilesandflatheadrecruitment
6
totheprogram.However,thesedatahavenot
previouslybeenenteredtocomputerfromfield
sheetsoranalysedindetailandarecurrentlynot
routinelyincludedinserviceagreementreports
tofisheriesmanagers.
Duetothebehaviouralcharacteristicsofsand
flatheadcoupledwiththenight‐timesampling,
thesmallbeam‐trawlsamplesincludesand
flatheadacrosstheirentiresizerange,unlike
snapper,whereefficiencyislowforfisholder
than1yearofage.Selectivityofthesampling
gearwasassumednottohavevariedacross
years.Thedatasetthereforehas11yearsofdata
onsandflatheadrecruitmentacrossvarioussize
andagegroups,includingtheimportant0+age
group.
Dataanalysesandlaboratorymethods
Allsandflatheadcollectedbythebeam‐trawl
monitoringprogramwerecountedand
measuredinthefieldtothenearest0.5cmtotal
length(tipofsnouttotipofcaudalfin,TL).A
sub‐sampleofindividuals≤20cmTLwere
retainedinsomeyearsforpotentialfutureusein
dailyageingstudies,andparticularlytoconfirm
thesizerangeacrosswhichindividualscanbe
confidentlyidentifiedasbeingofthe0+age
group(below).
Toextractatimeseriesof0+agerecruitment
fromthe11yeardataset,wefirsthadto
determinethesizerangeof0+agesandflathead
inMarchsamples,andassesshowreliably0+age
fishcouldbediscriminatedfrom1+agefish
basedonsize.Todeterminethesizecut‐offfor0+
agesandflatheadcollectedinMarch,we
examinedthesizedistributiondataacrossthe11
yearstoprovideaninitialsizerangelikelyto
representthe0+and1+agegroups.Toconfirm
thesizecut‐offfor0+agefish,theotolithswere
extractedfromasampleof60sandflathead
acrossthesizerangeindicatedfromthesize
distributiondata(i.e.40—200mmTL,results
below).Theotolithsweresectionedandpolished
inthetransverseplaneandthepreparedotolith
sectionswereviewedunderacompound
microscopeatX200magnificationtoconfirmthe
presenceorabsenceofthe1stannulus(Figs.9
b,c).Allotolithswereexaminedwithoutprior
knowledgeoffishsize.
Forselectedpreparationsofconfirmed0+age
fish,dailyageestimationwasattemptedbased
onassumeddailyincrementformation(Fig.9a).
Dailyincrementcountswereestimatedbytwo
readersofsimilarexperience.Thedailyage
estimateswereusedtoestimatehatchdates.
Dataoncatchrates(i.e.CPUE,number1000m‐2,
seeHamerandJenkins2004)wereanalysedfor
thesizecategories;≤15cm,15‐20cm,and>20cm
TL.Basedonthesizedistributiondataandthe
otolithanalysesdiscussedabove,thesesize
groupsrepresentthe0+age,1+ageand≥2year
age(i.e.adults)groupsrespectively.Size
distributionswerequalitativelycomparedacross
the11yearsofsampling.Dataforeachofthe
threesize/agecategorieswerealsocompared
acrosscapturedepthsandsites.
Todeterminehowwellthe0+agerecruitment
datapredictsrecruitmentofthe1+agegroupthe
followingyear,thedataforthe0+and1+age
groupwerecomparedstatisticallyusing
correlation(Pearson’s).Finally,thestatistical
significanceofinterannualvariationin
abundanceofthedifferentsize/agecategories
wasanalysedusingthenon‐parametricKruskal‐
WallistestduetotheinabilitytotransformCPUE
datatomeettheassumptionsofstandard
parametricANOVA(QuinnandKeough2002).
Tofurtherreducetheinfluenceofzerodatafor
individualtrawlshots,thedatawereaveraged
foreachsitetoprovidethereplicationwithin
eachyear.Theorderofranksumsofthe
individualyearsisusedtoindicatethestrongest
toweakestyear‐classes.
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Larvalandjuvenilesandflatheadrecruitment
7
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Figure7Diagramoftheplumbstaffbeamtrawlusedtosample0‐agesnapper.a)mainbodyofnet,4m
long,12mmstretch,4mm2apertureknotlessraschelmesh.b)codendbag,1mlong,8mmstretch,3mm2
apertureknotlessraschelmesh.c)4.7mheadrope.d)2.2mbreastlines.e)1.7mlowerbridle,firstmetre
oflowerbridleis2cmx3cmx0.5cmchain.f)1.8mupperbridle.g)4.8mticklerchain,chaintypeas
previous.h)5.6mfootrope.i)5.8kgcurveddetachableleadweights(divinghipweights).j)9.5cm
diameterfoamfloats.k)3msteelbeam,3cmdiameterpipe.l)4.1mbeambridlerope.m)stainless
swivel.n)emergencyretrievalline.
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Figure8MapofPortPhillipBayshowingbeam‐trawlsamplingareas(sitescentral1andcentral2
sampledsince2004,allothersitessampledsince2000).
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n
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j
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Pt. Wilson
Hobsons Bay Outer
Hobsons Bay Inner
Central 1
Central 2
Mordialloc
Carrum
Frankston 1
Frankston 2
Pt. Wilson
Hobsons Bay Outer
Hobsons Bay Inner
Central 1
Central 2
Mordialloc
Carrum
Frankston 1
Frankston 2

Larvalandjuvenilesandflatheadrecruitment
8
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Figure9Imagesofpreparedtransversesectionsof;a)0+ageotolithfordailyageingwithprimordium
indicatedbyarrow‐aandthemicrostructuretransitionzone(settlementmark)indicatedbyarrow‐b,b)
otolithfromage6+yearsadultwitharrowdenoting1stannualincrement,c)otolithfromage1+juvenile
showinglocationof1stannualincrement.

Results
Baywideeggandlarval
monitoringprogram
Larvalidentificationsuccess
DNAanalysesconfirmedthatidentificationof
PlatycephalusbassensisandP.speculatorlarvae
basedonmorphologyandpigmentationwas
highlyaccurate.Allsandflatheadlarvae
identifiedbymorphologyandpigmentation
werepositivelyconfirmedbyDNAanalysis.
Only1ofthe11yankflatheadlarvaeidentified
bymorphologyandpigmentationwasnot
positivelyconfirmedbyDNAanalyses.This
specimenwasidentifiedasatoothyflathead,P.
aurimaculatus.Theresultsdemonstratedthat
bothsandandyankflatheadlarvaecouldbe
reliablyidentifiedbylaboratorystaffbasedon
standardtaxonomickeysusingmorphologyand
pigmentation.
Abundanceanddistribution
Overthe6yearsofichthyoplanktonsampling,
483platycephalidlarvaewereextractedfrom
samples.Ofthese,only64wereidentifiedas
sandflathead,whereas354wereidentifiedas
yankflathead,and32specimenswereidentified
as`other’or`unidentified’Platycephalidae.
Platycephalusbassensis
Sandflatheadlarvaewerefoundatall8
samplingareaswithinPortPhillipBay(Figs.10,
11).Highestabundancesweregenerallyinthe
10‐15mdepthzone(Fig.11),particularlyoff
Carrum,Mordialloc,Frankston,PtWilson,and
HobsonsBay(Fig.10).Fewlarvaewerecollected
atthedeepercentralbaysite(Fig.10).P.
bassensislarvaealsooccurredatthesampling
sitesacrossPortPhillipHeads(Fig.10).
Platycephalusspeculator
Yankflatheadlarvaewerecommonacrossall
samplingareasandfoundinalldepths(Figs.12,
a)
b) c)
a
b
a)
b) c)
a)
b) c)
a)
b) c)
a
b

Larvalandjuvenilesandflatheadrecruitment
9
13).Highestabundanceswereinthe10‐15m
depthzone(Fig.13),particularlyoffFrankston,
HobsonsBayandPointWilson(Fig.12).P.
speculatorlarvaealsooccurredatthesampling
sitesacrossPortPhillipHeads(Fig.12).
Ageandgrowthoflarvae
Platycephalusbassensis
Dailyageestimatesforsandflatheadwere
obtainedfrom50larvaeranginginsizefrom
3.1–11.8mmSL(Fig.14a).Agesestimates
rangedfrom3–25days(Fig.14a).Theregression
ofageonlengthindicatedanaveragegrowth
rateof0.36mmday‐1(Fig.14a).
Platycephalusspeculator
DailyageestimatesforYankflatheadwere
obtainedfor33larvaesampledovera24hour
period(14‐15/1/2010).Theselarvaerangedin
sizefrom3.1–8.1mmSL(Fig.14b).Age
estimatesrangedfrom3–10days(Fig.14b).The
regressionofageonlengthindicatedanaverage
growthrateof0.57mmday‐1(Fig.14b).
SpawningdatesofPlatycephalus
bassensislarvaesampledfrom
November‐January
Thedistributionofhatchdatesforthe50larvae
sampledfromNovember‐January,pooledacross
the6years,indicatedthatspawningofsand
flatheadinPortPhillipBaycouldoccurthrough
toatleastmid‐January(Fig.15).However,most
ofthelarvaewerederivedfromspawningprior
tomidDecember(Fig.15).
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Figure10MapofPortPhillipBayshowingdistributionofPlatycephalusbassensislarvaesampledfromlate
NovembertoearlyJanuarypooledacross6years(2004/05–09/10).
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Platycephalus bassensis
Density (number 1000 m
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Depth (m)
Platycephalus bassensisPlatycephalus bassensis
Density (number 1000 m
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Depth (m)

Larvalandjuvenilesandflatheadrecruitment
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Figure11ScatterplotofcatchratesofPlaycephalusbassensislarvaeagainstdepthofthesamplinglocations.
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Platycephalus bassensis
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Larvalandjuvenilesandflatheadrecruitment
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Figure12MapofPortPhillipBayshowingdistributionofPlatycephalusspeculatorlarvaesampledfrom
lateNovembertoearlyJanuarypooledacross6years(2004/05–09/10).
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Platycephalus speculator
Density (number 1000m
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Platycephalus speculatorPlatycephalus speculator
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Depth (m)

Larvalandjuvenilesandflatheadrecruitment
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Figure13ScatterplotofcatchratesofPlaycephalusspeculatorlarvaeagainstdepthofthesampling
locations.
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Platycephalus speculator
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Figure14Scatterplotsandlinearregressionofstandardlengthversusagefor;a)Platycephalusbassensis,
andb)PlatycephalusspeculatorlarvaesampledinPortPhillipBayduringsummer.












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
Figure15FrequencydistributionofhatchdatesofPlatycephalusbassensislarvaesampledbetweenlate
NovemberandmidJanuaryinPortPhillipBaypooledacrosssixyears(2004/05–09/10).

Platycephalus bassensis
0102030
Age (days)
0
5
10
15
Standard length (mm)
a)
Platycephalus speculator
024681012
Age (days)
1
2
3
4
5
6
7
8
9
Standard length (mm)
b)
SL (mm) = 0.36 x age + 2.56 (r
2
= 0.86) SL (mm) = 0.57 x age + 1.41 (r
2
= 0.84)
Platycephalus bassensis
0102030
Age (days)
0
5
10
15
Standard length (mm)
a)
Platycephalus speculator
024681012
Age (days)
1
2
3
4
5
6
7
8
9
Standard length (mm)
b)
Platycephalus bassensis
0102030
Age (days)
0
5
10
15
Standard length (mm)
a)
Platycephalus bassensis
0102030
Age (days)
0
5
10
15
Standard length (mm)
a)
Platycephalus speculator
024681012
Age (days)
1
2
3
4
5
6
7
8
9
Standard length (mm)
b)
Platycephalus speculator
024681012
Age (days)
1
2
3
4
5
6
7
8
9
Standard length (mm)
b)
SL (mm) = 0.36 x age + 2.56 (r
2
= 0.86) SL (mm) = 0.57 x age + 1.41 (r
2
= 0.84)
0
1
2
3
4
1-
Nov 6-
Nov 11-
Nov 16-
Nov 21-
Nov 26-
Nov 1-
Dec 6-
Dec 11-
Dec 16-
Dec 21-
Dec 26-
Dec 31-
Dec 5-
Jan 10-
Jan
Frequency
Hatch date
n=50
Platycephalus bassensis
0
1
2
3
4
1-
Nov 6-
Nov 11-
Nov 16-
Nov 21-
Nov 26-
Nov 1-
Dec 6-
Dec 11-
Dec 16-
Dec 21-
Dec 26-
Dec 31-
Dec 5-
Jan 10-
Jan
Frequency
Hatch date
n=50
Platycephalus bassensis

Larvalandjuvenilesandflatheadrecruitment
14
Beam‐trawlmonitoringprogram
Confirmationofsizecut‐offforthe0+
agegroupinMarchbeam‐trawl
samples
Otolithswereassessedfrom56juvenilesand
flatheadcapturedinlateMarchranginginsize
from44to195mmTL(Fig.16).Thelargest
sampleidentifiedasbeingof0+agewas150mm
TLandthesmallestsamplesidentifiedasbeing
of1+agewere145mm(Fig.16).Thesedata
justifyasizecut‐offof150mmTLforthe0+age
groupinbeam‐trawlsamplescollectedinlate
March.Allsamplesbetween151and200mm
wereassessedasbeingof1+age(Fig.16);
however,itisunclearwhetherornotsome1+
agefishcanbelargerthan200mmwhen
sampledinlateMarch.Basedonthesesize‐age
datathesamplescollectedfromthebeam‐trawl
programwereallocatedtothreecategories:0+
age:≤15cmTL,1+age:15‐20cmTL,adults:>20
cmTL.











Figure16Distributionofsizesof0+and1+agePlatycephalusbassensissampledinlateMarchfromPort
PhillipBay.


Spatialanddepthdistributions
Theavailabledatawereinitiallypooledacross
yearstoprovideageneralpictureofthe
variationindensitiesofthethreeage/size
categoriesamongthesamplingareasanddepths
inPortPhillipBay.
0+agesandflatheadwerefoundatallsampling
areas,withhighestabundancesoverallatthe
northernandeasternsites(HobsonsBayInner,
HobsonsBayOuter,Carrum,Mordialloc,
Frankston1andFrankston2)(Fig.17).Asimilar
patternwasobservedforthe1+agegroup(Fig.
17).Adultsweresimilarlyabundantatthe
northernandeasternsites;however,in
comparisonwiththe0+and1+agegroups,
abundanceofadultswaslowatHobsonsBay
InnerandhigheratthetwoCentralbaysites
(Fig.17).
0+agesandflatheadwerefoundinallsampling
depthsfrom5–23m;highestcatchrateswere
generallyinthedepthrange5–15m(Fig.18).
The1+agegroupwasmostabundantinthe10–
20mdepthrange(Fig.18).Adultswerefoundin
allsamplingdepthsfrom5–24mwithhighest
catchratesinthe15–20mdepthrange(Fig.18).
1+
0+
Age group
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
Total length (mm)
1+
0+
Age group
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
Total length (mm)

Larvalandjuvenilesandflatheadrecruitment
15


























Figure17Comparisonofmeanabundances(pooledacross11years)ofPlatycephalusbassensislife‐stages
amongbeam‐trawlsamplingareasinPortPhillipBay.Errorbars±SE.












Carrum
Central 1
Central 2
Frankston 1
Frankston 2
Hobsons Bay
Inner
Hobsons Bay
Outer
Mordialloc
Pt Wilson
Site
0
10
0+ age
1+ age
adults
5
Density (number 1000 m-2)
Carrum
Central 1
Central 2
Frankston 1
Frankston 2
Hobsons Bay
Inner
Hobsons Bay
Outer
Mordialloc
Pt Wilson
Site
0
10
0+ age
1+ age
adults
5
Density (number 1000 m-2)

Larvalandjuvenilesandflatheadrecruitment
16
































Figure18Scatterplotsofbeam‐trawlcatchratesofPlaycephalusbassensislife‐stagesagainstdepthofthe
samplinglocations.

Depth (m)
0
5
10
15
0+ age
0
5
10
15
0102030
0
10
20
30
Density (number 1000 m
-2
)
1+ age
Adults
Depth (m)
0
5
10
15
0+ age
0
5
10
15
0
5
10
15
0102030
0
10
20
30
Density (number 1000 m
-2
)
1+ age
Adults

Larvalandjuvenilesandflatheadrecruitment
17
Interannualabundancevariationby
samplingarea
0+agegroup
Abundanceof0+agerecruitsvariedacrossyears
atallsamplingareas.Whilethepatternsof
interannualvariationdiffereddependingonthe
samplingarea,mostsamplingareasexperienced
arecruitmentpeakin2004(Fig.19).
1+agegroup
Abundanceof1+agerecruitsvariedacrossyears
attheHobsonsBayOuter,Mordialloc,Carrum,
Frankston1andFrankston2samplingareas;
however,abundanceswereconsistentlylowand
variedlittleacrossyearsatthePtWilson,
Central1,Central2andHobsonsBayInner
samplingareas(Fig.20).Consistentwiththe
peakin0+ageabundancein2004(Fig.19),a
peakintheabundanceof1+agefishwas
observedin2005attheHobsonsBayOuter,
Mordialloc,Carrum,Frankston1andFrankston
2samplingareas(Fig.20).
Adults
Abundanceofadultsandflatheadvariedacross
yearsatmostsamplingareas,althoughvariation
wasmostnotableattheCentral2,HobsonsBay
Outer,Carrum,Frankston1andFrankston2
samplingareas(Fig.21).Therewasaclear
declineinabundanceofadultsfrom2005to2006
attheCarrum,Frankston1,Frankston2and
Central2samplingareas(Fig.21).
Interannualabundancevariation–
pooledacrosssamplingareas
Comparisonsacrossyearsofthedatapooled
acrosssamplingareas(i.e.samplingareaas
replicates,NB:excludingCentral1andCentral2
duetotheseareasnotbeingsampledinall
years)showsaclearpeakinrecruitmentofthe
0+agegroupin2004andthe1+agegroupin
2005(Fig.22).Correlationbetweenthetime
seriesof0+ageand1+age(laggedbackby1
year)abundancedatawashighlysignificant(r=
0.973,n=10,P<0.001).Variationinrecruitment
ofthe0+agegroupexplained95%ofthe
variationinrecruitmentofthe1+agegroupthe
followingyear.
Interannualvariationwassignificantforthe0+
agegroup(Kruskal‐Wallis,P<0.001).Therank
sumorderofyearsforthe0+agegroupwas:
2004>2009>2008>2001>2002>2010>2005>
2000>2003=2007>2006.
Interannualvariationwasnot‐significantforthe
1+agegroupwithallsamplingareasincluded
(Kruskal‐Wallis,P>0.05);however,variation
wassignificantwhentheconsistentlylow
abundanceareasofPtWilsonandHobsonsBay
Innerareexcludedfromtheanalysis(Kruskal‐
Wallis,P<0.01).Theranksumorderofyears
forthe1+agegroupwas:2005>2010>2002>
2006>2004>2009>2003>2000>2001>2007>
2008.
Interannualvariationwasnot‐significantforthe
adultgroupwithallsamplingareasincluded
(Kruskal‐Wallis,P>0.05);however,variation
wassignificantwhentheconsistentlylow
abundanceareasofPtWilsonandHobsonsBay
Innerareexcludedfromtheanalysis(Kruskal‐
Wallis,P<0.01).Theranksumorderofyearsfor
theadultgroupwas:2004>2005>2003>2001>
2002>2007>2000>2009>2010>2008>2006.
Withtheexceptionof2000,meantotal
abundanceofsandflatheadwaslowerfrom
2006onwardsthanthesixyearsprior(Fig.22).



















Larvalandjuvenilesandflatheadrecruitment
18




























Figure19Interannualvariationinabundanceof0+agePlatycephalusbassensisrecruitsfrombeam‐trawl
samplinginMarchcomparedacrosssamplingareasinPortPhillipBay.Errorbars±SE.





2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
0
5
10
15 Carrum
Central 1 Central 2
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
Year
Frankston 1
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
Frankston 2
0
5
10
15 Hobsons Bay Inner Hobsons Bay Outer Mordialloc
0
5
10
15 Pt Wilson
Density (number 1000 m
-2
)
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
0
5
10
15 Carrum
Central 1 Central 2
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
Year
Frankston 1
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
Frankston 2
0
5
10
15 Hobsons Bay Inner Hobsons Bay Outer Mordialloc
0
5
10
15 Pt Wilson
Density (number 1000 m
-2
)

Larvalandjuvenilesandflatheadrecruitment