Table Of ContentMODELLING MANAGEMENTMEASURES INTHEQUEENSLAND SCALLOP
FISHERY
M.C.L DREDGE
Dredge, M.C.L. 1994 OX 10: Modelling management in the Queensland scallop fishery.
MemoirsoftheQueenslandMuseum36(2): 277-282. Brisbane. ISSN0079-8835.
ThesaucerscallopAmusiumjaponicumhailoriisthebasisofatrawlfisherywithan-average
annual meal production ofabout 1,000tonnes in Queensland. A variable size lima (90mm
insummerandautumn,95mminwinterandspring)andabanonfishingduringdaylightarc
significantcomponentsofthemanagement package imposedon the fishery.
Effects ol alternative management regimes on yield per recruit, value per recruit and
spawners per recruit have been evaluated using a modelling procedure. The effect of
variation ingrowthparametershasbeeninterpreted in the model.
Results indicatethat increasing the size limit to95mm throughout theyear would increase
spawners perrecruit minimally while decreasing value per recruit 15-20%. A 95mm size
limil for most ofthe yearwith 24 houra day fishing, would decrease spawners per recruit
by 15-35% in the ranges ofFexamined, while increasingrelative value perrecruit only a)
lowerexploitationlevels.
M.C.L. Dredge, QueenslandDepartmentofPrimaryIndustries,Southern FisheriesCentre,
P.O. Box 76, Deception Bay4508, Queensland; 15April, 1994.
Queensland's saucer scallop {Amusiumjapon- the U.S. and south cast Asia became profitable
icumballoti)fisheryisacomponentofthestate's (DredgeJ985). Effort directed towards scallop
multi-species trawl fishery Some 900 trawlers stocks increased rapidly (Fig.l), total catches in-
(10-20m long), are licensed to fish fora number creased then levelled off, while catch rales
of species of penaeid prawns, slipper lobsters decreasedbyanorderofmagnitudeduring 1978-
(Scyllaridae),crabsandscallops.Catchandeffort 1985.
data were monitored voluntarily in particular The scallop fishery has been managed through
fisheries, but have been compulsory and com- bothinputandoutputcontrols,largelysince 1985
prehensivesince 1988.Thecurrentreturn system (Table 1). Management was initially directed
calls fordaily records ofeffort and catch within towards maximising yield perrecruitthroughthe
30'x30' spatial grids,withsomedatabeingavail- use of a size limit, initially set at 80mm shell
able at a finer spatial resolution (6,x61). height (SH), but later increased (Table I). As
The trawl fishery annually takes c.10?000ton- effort directed at the scallop resource increased
nesofproduct, with variation in total production and catch rates fell, managersexpressedconcern
and species proportions between years. Scallops about the state of the resource and directed
contribute an average of c.1000 tonnes of meat management measures towards maintenance of
annually (Neil Trainor, pers. comra.). spawning stock levels. These included introduc-
All of the state's licensed trawlers are legally tionofa variablesizelimit(90mmSH insummer
entitled to fish for scallops. Not all vessels do and autumn, 95mm in winier and spring)
catch them, however. During 1988-1992, 270- designed to reduce fishing effort during the
360 vessels reported catches of scallops (Neil species' winter spawningseason, introductionof
Trainor,pers.comm.},Thereappearstobeexcess daylighttrawl closures in orderto reduce fishing
fishing capacity in the states trawl fleet in the effort, and the short-lived trialing ofareasclosed
contextoftakingthestate'sannual scallopcatch. to fishing as spawning stockprotection sites.
Queensland's saucer scallop stock was first The industry hasrepeatedlyexpressed concern
fished in the mid 1950's (Rucllo.1975), when about the management package which has
pmwn trawlers working out ofHervey Bay took evolved over the past 10 years. Some fishermen
appreciable quantities. The fishery remained an regard the daylight trawl ban as discriminatory.
irregularOffseason* sourceofincome forprawn Others would prefer to have a year-round size
trawl operators until the mid 1970*s. when limit of °1>or95mm SH. Analternative nioposal
serious attempts to exportsaucerscallopmeat to involves 24 hour a day fishing white naving a
278 MEMOIRSOFTHEQUEENSLANDMUSEUM
1,800 * Catch 20 of computer based
simulations. The results
Effort
1,600 of this evaluation form
^CPUE (kg/boai/dayl
the basis ofthispaper.
1,400 ~
15 MATERIALS AND
V)
1,200 TC3 METHODS
WED
3 Theoretical yield per
1,000 D
= recruitoutcomes from a
10 range of management
w
H 800 >- scenarios were model-
CO
X3 led using QuickBasic
H
600 g programmes based on
O those described in
i- -Q
400. Dredge (1992). The
programmeswerestruc-
turedtocreateaseriesof
200
overlaying two dimen-
¥--*--#-•*-*
ft sional matrices.
A N ##<^##<£ &
A*A A*^%°fl> <$><$><£<£ TheModel
FIG.1.TotallandingsandcatchratesfromtheQueenslandsaucerscallopfishery In the initial matrix,
one axis defined num-
95mm SH size limit for May-January, and a bers in a series of
90mmSHlimitforFebruary-April, inclusive. recruitment cohorts and the other defined time.
Thesealternativeshavebeenevaluatedinterms Theresultantmatrixdevelopedaseriesofcohorts
of meat yield per recruit, value per recruit, and linked to recruitment events over time. All
spawners perrecruit, using outputs from a series simulationswerebasedontwoidentical,normal-
ly distributed recruitment pulses being fed into
TABLE 1. Summaryofmanagementproceduresin thescallopfishery (P. Pond,pers.comm.)
Date Shellsize Gearsize Max. Trawlclosures Designated Preservation zones
mesh shuckingareas
size
11/84 80mm Combined 82mm
headropeand
foolrope <109m
7/84 85mm 75mm
10/87 90mm Daylighttrawl
ban 1/10-31/1
eachyear
12/87 Daylighttrawl
banlifted
11/88 Urangan,Gladstone
& Rosslvn Bay
2/89 Daylighttrawl Three 10-minuteby
ban 10-minuteareas
closedtofishing
3/89 95mm4/89-10/89
90mm 11/90-3/91
5/90 95mm5/90-10/90 Closuresdeleted
90mm 11/90-4/91
MODELLINGMANAGEMENTOFQLDSCALLOPFISHERY 279
the model overa 16weektime period,inorderto
simulate a four month, bi-modal spawning
process commencing in early winter .-_.-,
(Dredge,1981). The model was stepped through
the ltime* axis, both to feed in recruits and to
diminish the numbers ofscallops in each cohort
throughaprocessequivalenttonaturalmortality,
M
i.e. though the process Nt+i = Ni.e~ , where Ni
represents numbMersattimet,K+i arenumbers at
time t+1, and the coefficient of natural mor-
M
tality. was ascribed a value of 0.02 week '
(Dredge,1985).
The second matrix was used toestimate size at
age (shell height) in each cohort at each age.
Growth rates ofscallops are known to vary with
tliodcaaltiroeng,iampepa(rWeinltlliyamass &a fDurnecdtgieon.1o9f81d;epDtrhedagned INTERMEDIATE 2.00C XcJ
& Robins-Troeger,unpubl. data).Threedifferent
growth scenarios were used in this model. Sub-
setsofthismatrixwereusedtoascribesizeatage
forscallops from areas where growth was rapid
00S5n>
(U=105(l-e-< ), intermediWat*e (Li=100(l-e-
fr-051*"), and slow (Lt=97(l-e- 9**), with t in
weeks. Von Bertalanffygrowthparameterswere
derived from Williams & Dredge (1981) and
Dredge &Robins (unpubl. data). The model was
basedupon55%ofscallopbeingtakenfrom 'fast
growth' areas, 35% from 'intermediate growth'
rate areas, and 10% from 'slow growth' areas.
These figures are based on the average spatial
distribution of catch for 1989-1991 inclusive
(Trainorpers. comm.).
A third matrix wasused toconvertshellheight
to adductor weight for scallops in each cohort at
eachage.Monthlyshellheighttoadductorweight
conversions, based on those in Williams &
Dredge (1981), were used for this procedure. A Fishingmor. i
dollar value was ascribed to scallops in each
FIG.2. Valueperrecruitandspawnersperrecruitfrom
cohort at each age by multiplying numbers of scallopswithdifferinggrowthparameters.
survivorsbymeatweightbyunitvalueofmeatin
a fourthmatrix.This required a correction factor
based on the individual meat weights, as there is her ofone and two year old scallops which sur-
an appreciabledifference in scallop prices based vived at the beginning and end of the winter
on individual meat sizes (Hart, this memoir). spawning period. This index has beenused asan
index ofspawners perrecruit.
Fishingwassimulatedthroughaprocesswhich
involvedidentifyingthosecohorts in which scal-
lops were larger than a given ('legal') size and
ManagbmrntScenarios
increasing the mortality rate to include a com-
ponentforfishingmortality(F),rangingbetween Yield per recruit in meat weight and dollar
light fF=0.005 week 1) to very heavy (F=0.040 value,andspawnersperrecruitwereestimatedin
week'*). The resultant 'catch' of both meat the followingmanagementscenarios:
weight and value was accumulated as the model 1) A 90mm SH size limit in summer and
was stepped through time. autumn and 95mm SH size limit in winter and
An index of the numberofspawning scallops spring, with no daylight fishing (the existing
wasdevelopedby averaging thesum ofthe num- managementsituation).
.
280 MEMOIRS OFTHEQUEENSLANDMUSEUM
IB' spawnersperrecruitwereestimatedasafunction
Sizelimit*.90mmNov-Apr.JSmWMbi of fishing mortality (P) between 0.005-0.040
week"1
250 u .
* — * ». *
jt-: RESULTS
it) "ICO
"": a,-' Output from model runs is most readily inter-
preted in graphic form.
'..iiE 50
'" sfiawnarl
d* V ^ .c^ * J* EffectofVariationon Growth Parameters
Spawnersperrecruitandcatchvalueperrecruit
fromequivalentrecruitmentprocesseswerecom-
Si*?limit:9bmmMflrrtMjnd 2W „ pared for populations with 3 sets of growth
u parameters. Variation in growth parameters had
1o 200 U1J an appreciableeffect in 'perrecruit* output (Fig.
01 150 2). Value per recruit of slow growing scallops
Li
a was 1/3-1/2ofthatin fastgrowingscallops,with
n 10C the differential increasing as exploitation rates
increased. Conversely,spawnersperrecruitfrom
""
slow growing scallops remained at near steady
levels as the exploitation rate increased, indicat-
^ o* „« ing how few attained legal sizebefore theycom-
menced spawning. Spawner per recruit levels
Nodav"-.ii'f i'' j assumeeffectiveriskingeffortdou•ble? 9fr0o%mtfoas6t0g%roowfitnhgossecasleleonpsindesclloiwnegdrofwrionmgasbcoault-
lops as the rateofexploitation increased.
in: jqi:ht.|mi, hpi OhmmMay-feb
EffectofVariationon ManagementScenarios
~~ —- -f.«. au Variations in 'per recruit* output as a conse-
quence of varying management scenarios are
_3 depicted in Fig. 3. If the output derived by
' ||:!,-- to modelling the existing management situation is
^ ^ / ^ usedasareferencepoint,alteringsizelimitsfrom
90mm SH (summer and autumn), 95mm SH
Fishing mortality (winter and spring) (management scenario 1) to
95mm SH all year round (management scenario
FIG.3, Variation invalue perrecruitandspawnersper 2) would result in ageneral decrease of20-35%
recruit as a consequence of varying management value perrecruit (rising with increasing F) and a
regimes. commensurate increase of 5-12 % in terms of
spawners perrecruit. Thisis dependant upon the
2) A year round 95mm SH size limit, with no irenlgatsicvaellionppsuttooftfhaestf,isihnetreyr.mediateandslowgrow-
daylight fishing. Resultsfromthemodelsuggestthatconsequen-
3)A 95mm SH size limitin May-January, and cesofchanging the management regime from 90
a 90mm SH limit in February-April (inclusive), mmSH(summerandautumn),95mmSH(winter
with an increase in fishing mortality commen- and spring) (management scenario 1) to one in
surate with 24 houra day fishing throughout the which effective fishing mortality was doubled
year- This was achieved by assuming that the (nodaylightclosure) andsize limits wereheld at
fishing mortality rate (F) doubled as a conse- 90mmSHinFebruary toMay,and 95mm SHfor
quence ofallowing 24 hour aday fishing. the remainderofthe year (scenario 2), value per
Model runs were carried out over a 104 week recruitwouldincreasesubstantially(40%)atlow
timespan,whichapproximatestheeffectivemax- levels ofexploitation, but change little at higher
imum life span of the species (Heald & Caputi, exploitation levels. Spawners per recruit would
1981; Dredge,1985). Yield per recruit and be reduced bv about 10% at lower levels ofex-
MODELLINGMANAGEMENTOFQLDSCALLOPFISHERY 281
ploitation,andupto25% atihehighestexploita- an arbitrary doubling of fishing mortality was
tionlevel examined usedtosimulatetheeffectsofallowing24howa
day trawling. Verification of such an arbitrary
DISCUSSION procedureis notpossible.
Given the current limitations in our under-
The model demonstrates that yield per recruit standing of spawning stock and subsequent
and spawners per recruit will be markedly in- recruitment levels, the model output indicates
fluencedby growthparameters ofscallops taken that the existing management package offers a
reasonablecompromisebetween obtainingmax-
pinartahmeetfeirssheroyf.sTcahlelroepsismaevyidveanrcyectohnastidgcrioawbtlhy imumcatch value fromtheresource while main-
over relatively small distances (Williams & taining broodstock levels.
Dredge.1981; Ansell et al..l991; Ciocco,1991). Bycomparison,theWesternAustralianagency
Such variation in growth parameters can have a whichmanagesafishery forthesamespecieshas
marked effect on optimum age or size at first amanagementphilosophybasedonlimitedentry,
captureforyieldperrecruitmaximisation.Varia- minimising capture costs, and minimising con-
tion in growth parameters have been recognised flict between alternative fisheries in the main
and incorporated into themodeldescribed in this fishing ground (Shark Bay). Maintenance of a
paper.There is, however, noreasonwhypropor- substantia] breeding population is considered
tionsoflandingsfromslow,intermediateandfast critical to management. This is achieved by
growing areas should remain constant, and con- having a summer closure, and a predominantly
sequentlythemodel'soutputshouldbetreatedas winter fishery,Ihusstltowingthebulkofanimals
indicative In years when a high proportion of to spawn early in the (winter) spawning season.
landings come from areas where scallops grow <WJAolU987J9R9). Size limits are not used in the
quickly, yield per recruit may be maximised by fishery, as scallop shucking in the fishery
having a largersize limit, and conversely, when takes place a! sea
scallopsettlementoccurspredominantly in \slow Queensland fisheries managers seek to main-
growth' areas, yield would be increased with a tain biological sustainahility and long term
smaller size limit. Given the lead-in lime and economic viability ofthe (integrated east coast
informationrequirements fora managementsys- trawl) fishery while recognising social values in
tem using flexible size limits, implementation defining management actions (Glaister et al.,
seems unlikely in the shortterm. 1993) The differences in management phil-
The output derived by modelling the fishery osophy have resulted in fisheries which have
under alternative management scenarios indi- markedly different seasonality, input costs and
numbers ofparticipants.
catedthatvariationinexploitationlevelseffected
berth trends And absolute values of yield per
ACKNOWLEDGEMENTS
recruit.
Dredge (1992) suggested that a size limit of
90mmSHmaintainedthroughouttheyearwould Neil Trainor provided many of the catch and
have litlle effect on value per recruit by com effortstatisticsfromtheQueenslandcommercial
parison with the existing 90mm SH (winter and fisheries data base, SUNFISH. Julie Robins-
.spring), 95mm SH (summer ami autumn) suv Troeger prepared the graphics, and the editorial
limits. Output from the model used in this study committeeatSouthernFisheriesCentrereviewed
suggestedthat increasingsizelimitsto95mmSH the manuscript. My thanks toall ofthese people
ona year round basis would induce 9 substantial fortheirefforts.
losstothefisherywitharelativelyminorincrease
in terms ofspawners perrecruit. LITERATURECITED
The management option involving size limits
being set at 90mm SH in February to May. jnd ANSELL A.D.. DAO,J.C.& MASON,J. 1991.Three
95mm SH for the remainder of the yeai, and scallop fisheries: Prctcn mtiximux, Chla/iiyx(Ae
increasingexploitationbyallowing24houraday quipveten)oprrcutarisandC.(Chtamys)varia.In
S.B, Shumway fed.), 'Scallops: biology, ecology
trawling was examined. Results indicated thai andaquaeuhure*. (Elsevier; Amsterdam).
spawners per recruit would be reduced by 10- ClOCCO, N.F. 1991. Difference in individual growth
25%. and value per recruit would be increased rate amongscallop(Chtamys tehuelcha (d'Orb))
only at low levels ofexploitation. This scenario populations from San Jose Gulf (Argentina).
involvedafairdegreeofuncertainty,however, as Fisheries Re<e*th 12; 31^*2.
.
282 MEMOIRS OFTHEQUEENSLANDMUSEUM
DREDGE, M.C.L. 1981. Reproductive biology ofthe growth, recruitment and reproduction in the
saucer scallop Amusiumjaponicum balloti (Ber- southernsaucerscallop,Amusiumballoti(Bernar-
nard!) in central Queensland waters. Australian di,1861) in Shark Bay, Western Australia.
Journal of Marine and Freshwater Research 32: Fisheries Research Bulletin ofWesternAustralia
775-787. 25: 1-33.
DREDGE,M.C.L. 1985.Estimatesofnatural mortality JOLL, L.M. 1987. The Shark Bay scallop fishery.
andyieldperrecruitforAmusiumjaponicumbal- Fisheries Department, Western Australia,
lotiBernardi(Pectinidae)basedontagrecoveries. FisheriesmanagementPaper 11
Journal ofShellfish Research5(2): 103-109. JOLL,L.M. 1989.History,biologyandmanagementof
DREDGE, M.C.L. 1992. Usingsizelimitstomaintain Western Australian stocks of the saucer scallop
scallop stocks in Queensland. In D.A. Hancock Amusium balloti. In M.C.L. Dredge, W.F.
(ed.), 'Legal sizes and their use in fisheries Zacharin & L.M. Joll (eds), 'Proceedings ofthe
management'. Australian Society for Fish Biol- Australasian scallop workshop'. (Tasmanian
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of Rural Resources Proceedings No. 13. RUELLO, N.V. 1975. An annotated bibliography of
(AustralianGovernmentPublishingService,Can- prawnsandtheprawningindustryinAustralia.In
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GLAISTER,J.G.,POND, PC &STOREY,J.G. 1993. seminar'. (Australian Government Publishing
'Framework for management for the East Coast Service: Canberra).
trawl fishery'. (Queensland Fish Management WILLIAMS,M.J. & DREDGE,M.C.L. 1981.Growth
Authority: Brisbane). ofthesaucerscallopAmusiumjaponicumballoti
HART, B. this memoir. Dilemma of the boutique Bernardi in central eastern Queensland.
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