Sustainable management of Bass Strait scallops

8 Pages·1994·1.4 MB·

by  Richard J Mcloughlin

SUSTAINABLEMANAGEMENTOF BASS STRAITSCALLOPS
RICHARD MCLOUGHMN
J.
McLaughlin, R.J. 199408 10: SustainablemanaeementofBassStraitscallops.Memoirsof
theQueenslandMuseum36(2): 307-314. Brisbane. ISSN0079-8835.
ThehistoryofscallopfishinginBassStrait,itsmanagementandresearchisbrieflyrevicwecL
Theextent ofthe known biology and ecology of the species is discussedinrelation to the
current management ofthe fishery, and an assessment of the current policy is presented.
Growthand recruitmentoverfishing as it relates to this fishery is discussed in light ofthe
'twospawnings'criterionunderpinningthecurrentmanagementplan.
Richard McLaughlin. CS1HO Division ofFisheries, CSIRO MarineLaboratories, G.P.O.
Box )538, Hohun. Tasmania 7(H)};present address: DepartmentofPrimary Industryand
Fisheries, C.P.O. Box6l9F. Hobart, Tasmania 700/; 9March, 1994.
Largeconcentrationsofthe 'commercial' scal- regenerate or provide additional year classes of
lop, Pecten fumatus* were located off Lakes scallops in the lime frame of the current fishery
EntranceinBassStraitin 1970 Researchsurveys c. 20years.
sponsored by the Tasmanian Government be-
tween 1971 and 1973 located promising scallop MANAGEMENTAND RESEARCH
beds along northern Tasmania. These were first
exploitedin 1973,a yearthatsawaresurgenceof
fishing activity in Port PhillipBay. Discovery of Zachaiin (1990)andGwyther(1990)described
majornew bedsoffthe Furneaux IslandGroupin themanagementofBassStraitscallopsfromboth
easternBassStrait in the late 1970ssparked offa Tasmanian and Victorian perspectives. Present
periodofrapidexpansion in thescallop industry. managementresultedfromconcernsbyStateand
Fishing activity in the region increased dramati- CommonwealthGovernments(andindustry)fol-
cally,and total landings reachedarecord high in lowing expansion during 1979-1983. The Bass
19S2-83 when the total catch (live weight) ap- Strait InterimManagementRegimeofNovember
proached 12,000 tonnes and the number of ves- 1983saw 97Victorian and 134Tasmanianbased
sels participating tripled in two years to 231 vessels gaining access to the whole Bass Strait
vessels (Young& Martin.1989; Zacharin.1990). fishery (i.e. their respective state waters and the
Commonwealth controlled central zone greater
By 1985, the main beds in Bass Strait were than 20 nm from the coastline ofthe two states
depleted and the decline in landings was just as
(Fig.l)).
dramatic as the rise, Banks Strait, the last major The Commonwealth Government then estab-
scallop bed in Bass Strait, was fished out during lished the Bass Strait Scallop Task Force
1986. The Tasmanian zone of southern Bass
(BSSTF),consistingofgovernmentand industry
Straitwasclosed toscallop fishing following the representatives, whose brief was to develop a
t1h9e8r7esheaassonb,eeanndlsiuttrlveeyssubhsaevqeuesnintcersechrouwintmtehnatt rmeasnoaurgceem;e2n.tWpalsanatchcaetp:t1a,bElfefteoctailvleplayrtuiteisl;isaenddth3e.
(Zacharin,1987(1989; McLoughlin ct al.,1988: Waslegallyenforceable.TheTaskForeewasnot
Martinetal.,1989, Martin,1990). able to develop a management plan agreeableto
Overthis 20yearperiod, few. ifany, commer- all parties, and the final recommendation
cially fished scallop beds have supported ex- presented to the 1985 meeting of the Australian
ploitation for more than 2 consecutive seasons. Fisheries Council was to effect a high degree of
Few ofthesebedsconsistedofscallopscompris- separation between theTasmanianand Victorian
ing morethan I or2yearclasses. Theconclusion based fleets. Access to the Commonwealth con-
is thateffortand capacity in this fishery h3S built trolled central /.onewas restricted toscallopves-
up to the point that single recruitment events, sels that qualified cither for a Tasmanian or
resulting in discrete scallop beds ofsingle year Victorian state license and that had an endorse-
classes, arequickly fished out as the majority of ment of their Commonwealth Fishing Boat
the bed reaches a size at which they become Licence. Theseparation was finalisedunderOff
commercially viable to land (McLoughlin et ul shore Constitutional Settlement agreements be-
1991). These beds do not, in general, appearto, tween thecommonwealth andstategovernments
308 MEMOIRSOFTHEQUEENSLAND MUSEUM
high priority should be to regular monitoring of
thedistributionandabundanceofrecruitsandthe
sizeandcondition ofscallopson thefew remain-
ing beds. Howeverthis monitoring work was not
carried out as it was not funded. The last survey
ofscallopstocksintheregionwasthatcarriedout
by CSIRO in May-June 1988. The decision by
theCommonwealthtoclosethezonein 1990was
made in response to what appeared to be (he
imminent resumption ofunregulated fishing fol-
lowing the collapse 3 years earlier, and concern
as to the impact this might haveon the recovery
ofscallop stocks. A bed ofapparently immature
scallops found near Deal Island in 1990 was
FIG.I, MapofmanagementzonesforBass Straitscal- surveyed in June of the same year and, despite
lops."Hieboundariesforthe(wo<tatevlie20nautical somediscussionbasedoninterpretationofmodes
mmialneisaosftfalsehowraet,erwsi.th the islands belonging to Tas- binytCheSIleRnOgthanfdretqhueenBcuyrdeaatua,otfheRyurwaelreReassosuerscseesd
inJune 1986(Zacharin.1990). Thismanagement y(eBaRrRc)laasssbsecianlglopprsedwoimtihnaatmeliynocrom(p6%o)se2d+ofye1a+r
plan had no biological or objective fishery
managementprinciples axits basis, sclcaaslslocposmtphoantewnats(Matarrtiisnk o19f9a0)r.eIstuwmapstitohinsobfeduno-f
Apartfromlimitationsonentry intothecentral regulated fishing, and which prompted mini-
zone fishery, no other effort or catch control
sterial action.
regulationswere imposed untilJune 1991], when
the then Commonwealth Minister for Primary In December 1990, the Commonwealth, Tas-
rnanianandVictorianPrimaryIndustry Ministers
Industries and Energy, Hon. John Kerin MP, an-
jointlyannouncedanew managementplanforthe
nouncedclosureoftheBassStraitcentral zone to
allscallop fishing, in his media release the Min- dceanmternaltazloncehsacnaglleopinfismhaernya;gitermeepnrtesepnhtieldoasofpuhny-
ister staled that he hud no option hut to close the (Anon. 1991). Two aspects of the new arrange-
arreecaovuentrieldthteoraewleavselclwehaircehviwdoeunlcedtshuaptpsotrotckasshuasd- mentswere: 1,thattheCommonwealthwishedto
tained and substantial commercial fishery The work towards handing over management to the
Commonwealth decision was (apparently) twostates,with an agreedjurisdiction lineacross
rpercoompmtmeedndbaytitownoicnoenasrildyer1at9i8o9nst:hat1.nTohfeisChiSnIgRbOe zmtihoneniesSttwrroaatuiitlodnf;obreandpdeupr2ep,notdsheeasnttooupfpeonsitnanttgeheoff"ispthrheeersiecenescnteardao-lf
allowedonanyofthefew remainingbedsinBass
commercial bedsthathavehadtheopportunityto
Strait until stocks recovered (Fishing Industry
spawn twice*.
Researchand DevelopmentCorporationgrantno
1985/83 final report); and 2, Reports of limited OVERFISHING AND EXPLOITATION
fishing on beds ofapparently immature scallops
STRATEGIES
in the central zone by Victorian fishermen
Only one major study ofthe biology, ecology
and fishery for scallops in Bass Strait has been By the end of the 1987 season the fishery had
carried out (FIRDC 1985/83). It was the final collapsed. The total catch by Tasmanian vessels
report of this study to the funding body that In 1987 was less than 500 tonnes, representing a
included a recommendation for a *tW0 95% drop in annual landings in six years
spawnings* criterion. Regional surveys during (Zacharin.1990) with Victorian vessels landing
theCSIROstudyindicatedseverestockdepletion 220tonnes, a90% drop incatchesoverthe same
and a lack of recruitment (McLoughlin el a!., period- CPUE for the same period dropped to
1988; MartinelaL,1989.) The research indicated 13% of 1982/83 levels; industry and managers
thatspawning age stockshad fallentosucha low accepted that overfishing wasoccurring
level that failure to protect existing beds could Althoughgenerallyapplicabletoallfishstocks,
precluderecover) ofscallopslocks in Bass Strait Sinclair el al. (1985) distinguished two types of
forsomeyears. overfishingofscallops; 'recruitmentoverfishing*
The CSIRO study also recommended thai ;i and 'growthoverfishing'.Thefirstconceptinvol-
W
MANAGEMENTOFBASS STRAITSCALLOPFISHERY
Pectenfumalus- BassSiraitCalendar
Yearclass IX I u j 3+
AgeImuMhyl 7-10 12 19-22 M 31-M 36
Moiiih Oci Nov June-Sept—•»-Nov -^•Juno-Sept Nov Juno-Sept Nov Junt-Scpt
il
Kveni
Gonadsmaturefor Gonadsmaturefor Gonadsuiauirefor Gonadsmaturefor
Reproductive fusttunc.Number secondtime.Eggs thethirdtime.Eggs thefourthikne,Hggs
slatin somfaelglgasnpdrtohdeusceumdaiys orcicpuernsa,nTdhsbpaiwsntihneg orcicpeunrsa.nTdhisspaiwsniihneg oricpceurr)sa.tiTJhsispaIwsn\ihneg
notbespawned 1stmajorspawning. 2ndmajorspawning. 3rdmajorspawning.
Harvestingaithis Harvestingatthl* ManestmgatlhK
[jointwillremove nounwillrerno* pointwillremove
Effect(iI"fishing mostorallofyear mostorallofyea- mostorallofyear
seasonstarting classbeforethe? classbeforethey Classbeforethey
inAprileachyear cancompletetheir cancompletetheir cancompletetheir
1stmajorspawning. 2ndmajorspawning. 3rdmajorspawning.
Theindirecteffects Theindirecteffects Theindirecteffects
offishingmaykill offishingmaykill ofFuhinpmaykill
thosescallopsnot thosescallopsnot thosescallopsnot
taughtbydredging, caughtbydredging. caughtbydredging.
F1G.2. Biological calendarofscallopreproduction and fishingpractice in BassStraitscallops
ves the self-reproducing capacity ofthe popula- formation, as they do not generally consider
tionanddescribesaleveloffishingthatbeginsto eitherenvironmental orfishery relatedvariations
limit the ability of the mature spawning popula- in annual recruitment.
tion to effectively provide adequate future For example, the yield-per-recruit approach
recruitment - in this case, adequate in terms of will generate the same advice on optimal fishing
providingacommercial fishery. Thesecondcon- strategies whetherornotrecruitmentoverfishing
ceptisrelativelybetterunderstood,anddescribes is occurring, and yet it would be critical for a
fishing at a size or age at which the maximum fishery manager to modify the strategies for
yield in weight (or dollars ifeconomic informa- management ifsuch recruitmentoverfishingwas.
tion is available) is not realized; that is, fishing occurring. Martin el al. (1990) examined the
the resource at a 'small' size when a larger size problems of simple yield-per-recruit manage-
would provide betterreturns. mentstrategies fortheBassStraitscallopfishery,
Havingrecognised theneedforactivemanage- andconcluded that they wouldgenerallyprovide
ment in Bass Strait, a number of strategies .\rc poor results. These problems were further dis-
worthy of consideration, including maximising cussed by Young & Martin (1989).
yield from individual beds or populations. One It is still not certain whether recruitment of
approachto maximising yield from a population scallops in Bass Strait is dependent upon supply
whereannual recruitment tothesame population of larvae from nearby beds. However, regular
can be ignored is to calculate the annual balance spatmonitoringovertwoyearsatsixsitesinBass
betweengrowthofthescallopsandremoval from Strait, and advection modelling of larval trajec-
the population by death (Mohn,1986). This ap- torieswithreal windandtidedatausingaverified
proach is relatively easy to evaluate given infor- circulation model (Fandry,1983), showed that
mation about growth rates, mortality rates atage larvae are conserved within Bass Strait in all but
and the effective gear selectivity rates for each the windiest years (Young et al.,1992). A posi-
age. The problem lies in the assumptions of the tive relationshipbetweencommercialcatchrates
'yield-per-recruif models used to derive this in- and spatfall in the same 1° square in Bass Strait
Jin MEMOIRSOFTHHQUFGNSLANDMUSEUM
also provides some evidence ora stock recruit-
mentrelationship(Youngelal.,1990).Thusscal- Vi (1)
lop stocks are probably self-sustaining in Bass
Smtarianittaiannedd.a Avlitahbloeugshpaawnmiinngimstuomckstsohcokulldevbeel where Y is the annual yield. Ft is the instan-
cparenvneonttbreecdreufiitnmeedn,t oavecrofnissehrivnagtisvheoualpdpbreoaicnhcort-o atabnuenoduasncfeisohfin<ghemcoorthaolrittaytattimteimt,eant.dNWttiiss tthhee
porated into the management plan. Several meat weight in grams of the individual scallops
authors have identified thepossibility ofrecruit- aptlatciemoent.thTehseammoedceolhoarstsouvmeersatnhautmbfiesrhionfgyteaakress,
ment overfishing in Bass Strait (McLoughlin ct and the management aim in this case is lo target
aYmlao„nu1an9gg8e8&m;enMMtaarrpttliiannn. i1ent9i8l9iaa)lt;.e.dt1h9ien89B1,a9s9Zs1acaShtiramarsiitntos.c1aa9vl9ol0io;dp fiasipspmhriaonxgaicomhnisttehode aegxSeianmcclliaansiesr (etothreatli.emfe(f)1e9cwt8h5e)onfutsvheaedryyitiehnligds
thisproblem management strategies in the Canadian fishery
for the sea scallop Placopecten magellanicus,
AGEAND JOCUNDITY where up to 13 year classes may be present and
fishing does not usually target on scallops until
A they are 5-6 years old. Their analysis showed
biological 'calendar' ofscallopreproduction
and fishing (Fig, 2) shows that to achieve two clearoverall yieldbenefitsintargetingolderyear
classes
majorspawnings, scallops must be in their third
year ofgrowth. Fecundity ofscallops ofvarious Thesamesituationofmultipleyearclassesand
ages and from various areas of Bass Strait was annual recruitment tobeds does notoccur in the
first determined by CSIRO during their 3 year Bass Strait fishery. Here, scallop beds are
research program (Martin et al.,1990). While generally composed of a single dominant year
fecundity was found to be variable, there ap- class, are fished to 'extinction* in the same year
peared a relationship of size (age) and egg as fishing commences, and very little survival
production, with 3+yearclass scallops shedding occursintothenextyear Themodeldoesnolthen
3-5timesas manyeggsas 1+scallops.These 3+ have to formally take into account fishing mor-
scallopswere75-85mmshell height. Althougha tality in each year, since typically it is always in
linearage/fecundityscheduleforBassStraitscal- excess ofthe capacity ofthe bed to survive into
lops isdrawn for simplicity (Fig.2) it isprobable the next year (McLoughlin et ah,J991). The
thai the relationship is non-linear, reaching an model reduces to calculating the yield in each
asymptote at some value approximate to maxi- yearfromagiven age/size structure,exploitation
mum size ofthis species at around 140mm shell rate(E) and stock size (N), traded offagaiast an
height. annual natural mortality (M). Anequationtocal-
culate yield at time I thus reduces to:
STOCK MANAGEMENT
»££, N, W, (2).
. .
Despitetheinherentproblemswiththeassump-
while Ihc numberofscallops available forcap-
tuisoenfsulutnodceornlsyiidnegraynicelxd-apmeprl-reeocrfugirtowmotdheolvse,rfiitshi-s ture in each yearis
ing modelled fora hypothetical Bass Strait seal
lopbed.Typically,themodelusedisananalytic;)I Nl+1=Nt(1-M) (3).
yield modeldeveloped forexploited fishpopula-
tions(e.g., Bcvcrton & Holt,1957). Sinclairetal. This has been modelled for three management
(1985) forexample, used such a model modified strategies, using data from average meat weights
foruse on scallops toestimateyieldasa function at age for Bass Strait scallops and a 'hypo-
of fishing effort and gear selectivity-at-age, thetical' scalloppopulation, where:
wheretheannualcatchfromthepopulationunder Nt: Although used only as an example,
agiven fishing strategy equals the catch thatcan Zacharin(pers.comm. 1990)calculatedthestock
be taken from a single cohort throughout its life sizeavailableforcaptureatabednearDeal Island
under the same strategy.The yield is maximised asc.26millionscallops,assuminganoverall30%
by maximising the following relationship for a dredgeefficiency duringthesurvey ofJune 990
1
singlecohort, tMartin.1990; McLoughlin et al..l991). Assum-
MANAGEMENTOFBASSSTRAITSCALLOPFISHERY 311
TABLE I, Analytical (meat) yield model results for older, and this trend is continued ifthey are noc
three management stiategics, where (a) is scallops fished until the dominant cohort is 3 years old.
s3offcinr+aswolhomalenrodawpdgasfserrdwo2sim+,thoaanganwedasre1(ld+des)c,otiins(vwcisa)tcryiadslfslsa,occpat(slohlr)foiopissfsh5fesid0csah%flerldoofopmfrsraotfghmieesahg11e++ed ecHBlaaoocswshseevlySyeterraair,intt.itryepSiliscacawtaloillorlonytphsrtaocinonagnceysteiuiadanrelsr1iifznicegsohfhtirohnirogstmsrpe5risaOnucl-Jttui7mlc0eyomroimenf
shell height, but it is unlikely that many would
AgefYrs) Populaiion Av.Wifg) Catch either be landed or processed that were smaller
than about60mm shellheight (ignoringthe legal
(000,000) (tonnes)
minimum size limit of 70mm), thus explaining
(a) ] 23.5 7.7 180.9 the necessity for calculation of some effective
2 1.5 11.2 16.8 exploitation rate, E. Assuming anormal size dis-
3 1.0 14.0 14 tribution ofscallops in the cohort, only one half
TOTAL ,i i ? ofthe cohort would thenbeconvened to 'yield*.
(b) 2 12.2 11.2 136.8 This has been calculated in Table 1 (d), where it
3 0.7 14.0 10.9 is clear that yield from this bed is substantially
4 0.5 17.0 8.5 reduceditthisstrategyisused,ratherthanleaving
TOTAL 156.2 the bed until the major cohort was 3 years old.
This size selectivity can normally be accounted
<o 3 6.3 14 88.9 for, and adjusted at will in yield models, but is
4 0.4 17.0 6 8 keptseparate here forsimplicity.
5 0.3 18.0 4.6 Regardlessoftheyieldimplications, theimpor-
TOTAL 100.3 tant result for a management strategy utilizing
(d) i 12.2 7.7 93.9 scenario(b)and(c)isinpotentialeggproduction
as a measure of recruitment overfishing. Usine
2 1.5 11-2 16.8
(Fig. 3):
3 1.0 14 14 Fecundity (millionsofeggs'i= 1.086 (years
TOTAL 124.7 old) +0.148 (5),
(d) modified (50%) selectivity foryear 1 cohort. it is a simple matterto calculate the difference
in age-based egg production from the three
ingarealisticcohortageonthisbed(from length managementstrategies(Table 1).Thedifferences
sftrreuqcuteunrceyddpaotap)u,lathtiisohnasofbe2e3n.c5onmvielrlitoendt1o+anyaegaer TABLE 2. Egg production for three management
scallops (90.3%), 1.5 million 2+ year scallops satnrdataeggiee/sf,ecbuansdeidtyonscahnaeldyutliecaolfyFiiegldurmeo3d;elwhoefrTeab(ai)ci1s
(5.7%) and 1 million 3+yearscallops (4%). scallops fished fromage 1+onwards, (IV) isscallops
Wt: While average meat weights vary sig- fishedfromage2+onwards,and(c)isscallopsfished
nificantlyin BassStrait,particularly forthe King fromage 3+onwards
Island beds, yields are given for average meat
weights for age/size classes encountered in \ge Pop.size f-ee Cumulative
spawningcondition (000.000) pnxtiii'imti eggproduction
tan
M: a fixed annual natural mortalityof0.52 has
been applied to individual cohorts, While this is III* 1 23.5 29
the only &published figure for this species 2 1.5 3.5
(Gwyther McShane,1988). it is likely to be 3 1.0 3.4
highly variable, This figure is used as an average TOTAL 359 35.9
valueonly.
Thescenariosmodelled undertheassumptions (b) 2 12.2 28.3
above (Table 1) have the same population (a) 3 0.8 2.S
fished afterinitial discovery as acombination of 4 0.5 2.2
I-,2-and3*yearoldcohorts,(b)leftforoneyear TOTAL 33/2 fi& 1
and fished at 2-, 3- and 4+ year old cohorts, and (c) 3 0.3 ttf
(c) left for twoyears and fished as 3-, 4- and 5+ 4 0.4 1.8
yearoldcohorts. Inthis simpleexamplesubstan- M
5 0.3
sticaallldoepcsreuanstielsainmayjioerlidtyreasrueltagferdom2+leyaevairnsgatnhde TOTAL 248 93 9
m
MEMOIRSOFTHEQUEENSLAND MUSEUM
6 problemsofsocialandeconomicpressuresontrie
scallop fishing industry and how these relate to
5iy=0.14662-1- 1.0862* RA2=0.528 stock management(Caddy,1989)
A structural problem in the Bass Strait fishery
4 isthatthese samesocial andeconomic pressures,
and a lack of management foresight in the late
197G's and early 1980's, have resulted in a fleet
capableofannualoverfishingofavailablestocks.
The managemenl plan introduced in 1991 con-
tains a mechanism for avoidance of the most
serious long term problem, recruitmentoverfish-
ing. Thetwospawningscriteriongoesalongway
tosolvingthischronicoverfishingproblemwhile
alsoattempting to maximisetheyieldfromexist-
ing recruits, but the economics of fishing will
Age (years) remain marginal while stocks remain low. This
linking of *yicld-per-recruit' with 'eggs-per-
FIsGc.al3l.opAgbeeds(.years)versusfecundityforsixBassStrail ryeieclrduimfodisela avanlduawbalseuesxetdenbsyioMnoohfntheet aaln.al(y1t9i8c4al)
to develop management strategics for Georges
in egg production from these strategics are evi- Bank scallops (Gabriel et al.,1989). The
dentfrom Tabic2, where itisobvious thai while mechanism for achieving the two spawnings
mosteggsarereleased whenpopulation size isat policy, that is, a trashing rate tied to asize limit,
itshighest, removing the population at this point docsnotreduceitsmediumandlongtermvalidity
will result in only a fraction of the total egg and is also a valuable management tool for con-
production that would have been realised if the servation ofjuvenile stocks. The trashing rate
population had been Left until the major cohort concept will also become increasingly important
was 3years old, that is, the population spawns in as (hopefully) stocks rebuild and new juvenile
each yearofthe threeyears untileaughi. beds arediscovered.
The difference in moving from management Similar policies forshiftingfishingeffortfrom
strategy (d) to strategy (c) will result in only a youngeragegroupstoolderandlargeragegroups
20% drop in total yield, but a 260% increase in have resulted in substantial gains in long term
egg production. Overthe medium to long term it yields and egg production in Canadian scallop
is probable that ihis will be reflected in the fisheries (Caddy,1989). Targeting of older age
recruitment strength ofscallop stocks, as well as group scallops had a number of beneficial im-
inhigheroverallpricesfromlargersizedscallops. pacts on fishing strategics not characteristic of
simple minimumsizelimits,whichhasbeensug-
DISCUSSION gested as an alternative policy forthe Bass Strait
fishery. A minimum size limit without a viable
Given the high per-unit value and the relative system of protecting dense patches of new
easeofcatchingscallopswhenthey areabundant, recruits by local area closures (particularly in
it is hardly surprising that continued consumer multiple yearclassbeds)wouldreturnthe fishery
demand has had a significant impact on invest- to the destructive practice of fishing areas of
ment andeffort in the Bass Strait fishery. Other predominately small shell in ordertocull out the
complicatingfactorsinclude 'diversified* fishing few large scallops, whileinflictinghigh levelsof
license policies in Tasmania, which maintains a incidental mortality on unlandedjuveniles (Me-
largepool ofpotential effort, andconversely, the I .oughlin ctal.,I988; Caddy,1989). The fact that
lack of diversified license policies in Victoria thispracticedoesoccuratlowstocklevelsinBass
whichforceseffort intothefisherybecauseofthe Strait is evident from examination of the 1987
inabilityofthe licence holderstospread effort to season (Martin et al.,1989).
other fish stocks. There is no easy management The principleofthe 'twospawnings* criteria is
solution here, but is becoming evident that there nothing more or less than one strategy for stock
isaneed forbioeconomicmodelingtodetermine rebuilding.However,astockrebuildingstrategy.
not only effectsofecological and environmental with the specific objective of increasing spawn-
variables on scallop stocks, but also the critical ing stock abundance, is just one example of a
MANAGEMENTOFBASS STRAITSCALLOPFISHERY 313
fishery managementstrategy (Sainsbury,1992a). linking catch and stock, such asproportional es-
Themanagementstrategy may include the use of capement, constant escapement or proportional
biological or fishery reference points and a harvestingratecanbe,orarc,beingapplied.What
specifiedwayinwhichthereferencepointsareto is being applied is a constant quota which is
beused inthe managementofthe fishery(e.g.no generally recognisedasa highriskstrategy since
fishing on stocks until they have completed two itignores interannualrecruitmentvariability.For
spawnings). However, as Sainsbury (1992a)ex- example, one possible scenario is thateven with
plained, the essential point is that the manage- a twospawnings strategy, three adverse years of
mentstrategy tobeevaluatedisbutoneaspectof environmentalconditionsforrecruitment will see
aprocessthaiincludesslockdynamics,economic all stocks (beds) available for fishing, and with
dynamics,obscrvalions (offleet and fisheryper- the existingexcess fishing capacity and with an-
formance), estimation procedures, management nual natural mortality, stock collapse would be
decisions and management implementation, all onceagainareal possibility. Further, theexisting
operating under a management policy with management plan theoretically allows ap-
specific goals orobjectives proximately 18,000 tonnes of scallops (live
The exact causes and mechanisms of recruit- weight) to be landed in any year (ie.. No. of
ment collapse are poorly known, although often vessels in fleet x monthly quotax no. ofmonths
a combination ofhigh fishing mortality and en- availableforfishing),despitetheknowledgethat
vironmental variability isindicated,andecologi- in the fustory of the fisherynomore than 12,000
cal interactionsarcsuspected.Thecomplexity of tonnes has been landed in any one year.
theseinteractionsissuchthatthereislittleexpec- What then might be a course of action for
tation that the population size at which recruit- considering these problems? Initially,anassess-
ment collapse will happen can be accurately ment oftheexisting policy with regards tostock
predicted (Sainsbury,1992b). However, recruit- recovery will be necessary Assuming that the
ment collapse has occurred in numerous marine stockdocsrecovertosome level consistent with
resources, and it is strongly suspected in Bass amoreconstantlevelofannual recruitment, then
Straitscallops from 1986-1990. inthemediumtermalinkingofannualcatchwith
Ultimately, the reliability and success of any stock size will become necessary for some level
managementstrategy(forBassStraitscallops) is ofsustainabilily (thatis,anestimateofminimum
seen to be dependent on the ability to forecast spawning stock biomass-per-recruit). Ofcourse,
accurately (Mohn.1986), although this may re- thisisinextricablylinkedwithprofitabilityforthe
quirealongtimeseriesofcatchdatatobereliable fleet, with theeconomicsoffishingfortheexist-
(Orensanz et al ,1991). However, with large in- ing fleet only being viableatrelatively highstock
terannualfluctuationsin recruitment,growth and sizes-financialviabilityatlowerstocksizeswill
mortality typically occurring in Bass Strait scal- rely on the reduced catch being shared among
lops such a predictive capability is not ye? pos- fewer operators. A policy objective in the
sible. Management must therefore rely on medium term may well be an appropriate reduc-
maximisingprobability forboth maximal annual tion in fleet size to a level at which economic
yield and recruitment success in subsequent . [ability ismaintainedataverageannual catches.
years, and these strategies must be maintained
overa suitable time period to determine it they
arc successful.
ACKNOWLEDGEMENTS
In respect of Bass Strait scallops, the current
management strategy is a stock rebuilding
strategybasedonoutputcontrols: twospawnings
busedon an averagesizeatage,andcatch restric- This paper has grown out ofdiscussions with
tions,unrelatedtostocksizebutimplementedfor manypeople,includingpeopleatall levels in the
orderly marketing and processing. However, fishery - fishermen, managersandscientists. Par-
there remains the risk that the underlying struc- ticular thanks to AFMA, industry and state
tural problems remain unattended. These are, government members of the Bass Strait scallop
critically, {1) overall fleet size, and (2) a lack of Consultative Committee, Mr Will Zacharin and
knowledge ofstock size, resilience and produc- Drs. PeterYoung.TonySmithandTony Koslow.
tivity The tact; of a link between annual loial Mr Richard Martin produced the biological
catch and stock size isparticularly worrisome as calendarforscallops,and1thankhimforpermis-
none of the usual stock management strategics sion touse it in this paper.
1
MP.MOIRSOFTHEQUEENSLAND MUSEUM
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