ISSN: 2239-5172

National Research Council of Italy

Department of Earth and Environment

Marine Research at CNR

Editorial Board: Enrico Brugnoli, Giuseppe Cavarretta, SalvatoreMazzola, Fabio Trincardi, Mariangela Ravaioli, Rosalia Santoleri

Editorial Office: Daniela Beatrici, Paolo Braico, Margherita Cappelletto,Elisabetta Gallo, Luigi Mazari Villanova, Pier Francesco Moretti

Roma, November 2011

Volume DTA/06-2011

Consiglio Nazionale delle RicercheDipartimento Terra e AmbienteP.le Aldo Moro, 700185 Romatel. 06 4993 3886 fax 06 4993 3887email: direttore.dta@cnr.itweb: www.dta.cnr.it

Population Dynamics of the Clam, Chamelea gal-lina, in the Adriatic Sea (Italy)

E. Betulla Morello1, M. Martinelli1, B. Antolini1, M.E. Gramitto1, E.Arneri2, C. Froglia1

1, Institute of Marine Sciences, CNR, Ancona, Italy2, Food and Agriculture Organization, FAO AdriaMed Project, Roma, Italyb.morello@ismar.cnr.it

Abstract

In the western Adriatic Sea the fishery for the clam, Chamelea gallina, by meansof hydraulic dredgers is by far the most important bivalve fishery, having reachedlandings of over 100000 t in the early 1980s. Owing to this intense fishery, theclam population has undergone profound modifications and substantial reductions inbiomass throughout the years. Thus, between 1984 and 2001 the scientific commu-nity was given the task of annually assessing the state of this resource. This paperis aimed at presenting the results obtained from the surveys carried out in the An-cona (AN) and S. Benedetto (SB) Maritime Districts between 1984 and 2001. Theresults are indicative of a resource and a fleet heavily dependent on stochastic sub-stantial recruitment events. A stressed resource was revealed which was unable tocope with the intense exploitation. Large recruitment events followed by significantnatural mortality episodes and the paucity of older individuals may suggest a shiftin the allocation of energy, from growth to reproduction. Following the collapse ofthe resource in 2001, in SB especially, part of the vessels were redistributed in thenearby AN district. This caused a considerable increase in fishing effort and a furtherdepletion of the resource as highlighted by the long closure periods. Despite this nofurther surveys were funded. Its is our hope that this contribution may, somehow, actas a catalyst for the resumption of the assessment of a crucial coastal resource.

1 Introduction

Chamelea gallina is an infaunal clam ofthe Veneridae family (Bivalvia: Lamelli-branchiata: Veneridae), locally known as‘vongola’ or ‘lupino’. Venerid clams aremostly found in temperate and tropical re-gions where they inhabit the particulatesands of the infralittoral and circalittoralzones [1]. Chamelea gallina is a gonocho-ristic species with a long spawning period(April to August), during which spawningtakes place at intervals, and seems to be

followed by a resting stage [2]. Follow-ing spawning a trochophore larva is pro-duced which, once the egg membrane isshed, becomes free-swimming and pelagic[3]. The larval shell is then secreted bring-ing the larva into the veliger stage. Thelength of such pelagic life is not known,but probably does not extend beyond 20to 30 days [1], following which the lar-vae settle in the substratum, becoming ben-thic. In the central Adriatic the growthrate of C. gallina is high in the first threeyears of life [1] and then progressively de-

Fishery and Sea Resources

creases, reaching a maximum length of 49-50 mm (≈ 6 years of age [4]). Neverthe-less, growth is slow and it takes one yearto reach a size of 18 mm and two years toreach the minimum landing size (MLS) of25 mm [1]. Chamelea gallina appears toexhibit density-dependent growth rate andmortality: years of exceptional recruitmentmay lead, the following year, to very poorstocks of large individuals due to juvenilemortality, or to very large stocks of smallindividuals due to growth inhibition and re-source competition. It has been reportedthat a proportion of the population reachesmaturity by the end of the first year of age(18 - 19 mm) and all individuals are readyto reproduce within the second year [1].In the western Adriatic Sea the fishery forC. gallina is by far the most important bi-valve fishery, having reached landings inexcess of 100,000 t in the early 1980s [1].This species is also exploited in Albania,France, Spain and Turkey [5, 1, 6]. Ow-ing to the introduction of the modern hy-draulic dredge into the Adriatic, the C. gal-lina population has been subjected, for thepast 40 years, to constant and consistent ex-ploitation. The pioneer hydraulic dredgewas introduced in the late 1960’s and bythe 1980s it had been replaced by the mod-ern version used today. With the advent ofthe modern hydraulic dredge, the C. gal-lina fishery in the western Adriatic Sea es-calated progressively, reaching a status ofextreme importance for the economy of thefishing community: by 1994 the vesselsfishing hydraulic dredges had increased by115% (808). Whilst the stock was stillhealthy in 1984, the management proce-dures adopted were inadequate to protect itfrom overexploitation: landings increasedby approximately 20% between 1974 (an-nual catch = 80,000 t) and 1984; fishingeffort saw a 50% increase [1]. It is esti-

mated that by 1984 the fleet had expandedso much as to be able to cover the wholearea of the fishing grounds in one yearwhilst it takes two years for a clam to reachthe MLS of 25 mm. The precarious situ-ation of the C. gallina resource in the en-tire Adriatic Sea was acknowledged by thestakeholders, and from 1984 the scientificcommunity was given the task of assess-ing the state of the resource in each Districtwith the aim of giving management guide-lines for the year following. This contribu-tion summarises the results obtained fromthe analysis of the long time series of sur-vey data collected from the Ancona (AN)and S. Benedetto (SB) Maritime Districtsbetween 1984 and 2001. These two ar-eas have been in conflict since the begin-ning of the fishery; conflict which escalatedin 2001 when the resource collapsed andthe fishery in the SB District was closedprompting a redistribution of the SB ves-sels into the adjacent AN District and thecreation of a new “Fishing District” in thearea of overlap (Civitanova Marche).

2 Methods

The annual surveys aimed at analysing andquantifying the clam population in the ANand SB Districts were carried out from1984 to 2001 by ISMAR-CNR Ancona,with an interruption in 1988-1990. For thispurpose, the entire AN District was dividedinto 21, 2 mile-wide transects perpendicu-lar to the coast, between the mouth of theriver Cesano and the mouth of the riverChienti (Figure 1). Similarly, the SB Dis-trict was divided into a further 13 transects,between the mouth of the river Chienti andthe mouth of the river Tronto (Figure 1).Within each transect, a sample was taken atevery metre depth between 3 and 12 m. For

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Figure 1: Map of sampled stations (black dots) within the Ancona (AN) and S. Benedetto(SB) Maritime Districts during the annual C. gallina stock assessment surveys showingDistrict boundaries (solid line) and sub-area (SA) boundaries (dashed lines).

statistical analysis purposes, and for theirintrinsic differences, the entire area com-prising the two Districts was divided into 5sub-areas (SA), three of which in the ANDistrict and two in the SB District (Figure1):

• SA 1: ‘North of Ancona’: between Seni-gallia and Ancona harbour;

• SA 2: ‘Conero’: between Ancona har-bour and Sirolo;

• SA 3: ‘South of Conero’: between Siroloand Civitanova;

• SA 4: ‘North Aso’: between Civitanovaand Altidona;

• SA 5: ‘South Aso’: between Altidona

and S. Benedetto del Tronto.

The sampling was carried out using char-tered commercial vessels mounted with ex-perimental hydraulic dredges. The exper-imental gear was divided into 3 sectionseach 80 cm wide, the lateral sections hav-ing a grid-spacing of 12 mm (as requiredby law) and the central one of 6 mm (thislatter grid size allows the dredge to retainall individuals above 14 mm and a propor-tion of smaller ones). The vessel steamedto the chosen transect and depth and de-ployed the dredge which was towed, par-allel to the coast, using the anchor tech-nique, for a distance of 50 +/- 5 m (for de-

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tails see [7]). The catch retained by thecentral portion of the dredge representedthe benthic community present on 40 m2,the total area covered by the dredge in onetow being 120 m2. On each tow, a bag ofcatch collected in the central section wasweighed and retained and the remainderwas weighed and returned to sea. The re-tained material was frozen at -18°C andsubsequently examined in the laboratorywhere each sample was sorted. All organ-isms were identified, counted and weighed.Chamelea gallina individuals were mea-sured to the lowest millimetre along themajor axis (termed length hereafter). Thedata were treated using the formulae pro-posed for the application of stratified ran-dom sampling by Cochran (1963, in Rus-sell [8]). The area of each stratum (stratawere defined by depth intervals of 2-4 m,5-6 m, 7-8 m, 9-10 m and 11-12 m for SAs1, 3, 4 and 5, and depth intervals of 3-9m and 10-12 m for SA2) was calculated,and the sampling units and the individualtows were estimated in areal terms, allow-ing to express the area of a stratum in termsof the total possible number of samplingunits contained within it [8]. This enabledthe calculation of mean catch per stratum(both in terms of biomass and abundance),its variance and ultimately total biomass,abundance and their variances (Cochran,1963, in [8]). Both biomass and abundancewere calculated using these equations. Toestimate biomass a mean weight was as-signed to each size class based on the fol-lowing length-weight relationship (Froglia,unpublished data):

Log10Wt = 2.7554 ∗ Log10L− 3.1787,

where: Wt = weight (g); L = length (mm).When necessary the identification of statis-tically significant trends in time was car-ried out using Kendall’s coefficient of rank

correlation test applied to trend lines calcu-lated using Kendall’s robust line-fit methodfor nonparametric regression [9]. The sig-nificance criterion for all tests was set at al-pha = 0.05.

3 Results

Overall, both Maritime Districts have un-dergone considerable year-to-year fluctua-tions in total biomass (Figure 2a). Mini-mum total biomass estimates were reachedin 1991 (1063 t) in AN and in 1994(468 t) in SB, whilst maximum valueswere recorded in 1999 (97270 t) and 1998(96160 t) in the two Districts, respectively.No significant decreasing trend in biomassthrough time was discernible for either Dis-trict, but each District experienced col-lapses of the resource that resulted in tem-porary closures of the fishery (AN in 1991,SB in 1991, 1994-1995, 2001). Similarlyto total biomass, commercial biomass (Fig-ure 2b) fluctuated widely with estimatesfor AN ranging between 490 t (1991) and12351 t (2000) and for SB between 309 t(1994) and 8955 t (1985). Low estimates ofcommercial biomass appear to have beenthe rule for the SB with the exception of1984 and 1985. In 1986 the commercialstock fell in both Districts, reaching ex-tremely low values in the early 1990s whenthey were comparable. 1994 saw a re-covery of the AN commercial stock thatwas not paralleled by a similar recoveryin SB, where the relatively high estimatesrecorded in 1985 were never to be reachedagain. Since 1994 commercial biomass es-timates have been considerably higher inAN consistently across the years and thiswas especially evident in 1999 and 2000when commercial clam estimates were ap-proximately five times greater in AN. A

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marked decline of the exploitable fractionof the resource occurred in 2001 in bothDistricts, when estimates fell to 3127 t inAN and 671 t in SB. Inter-annual fluctu-ations were even more marked upon con-sideration of the younger portion of the ex-amined population (≤ 18 mm) with yearsof extremely low biomass being followedby very high biomass estimates in the suc-cessive year (e.g. SB 1997 cf 1998), orwith several subsequent years in which re-cruitment was virtually absent (e.g. AN1984-1987) (Figure 2c). Figure 2c appearsto indicate recruitment events as sporadicevents. Successful recruitment in one yearwill result in high estimates of individualssmaller than 19 mm in the successive year.Upon comparison of the two Districts, SBappears to have had the most successful re-cruitment events since 1984 and this wasespecially so in 1998 when the juvenilefraction reached 78597 t. The results showthat years of intense recruitment (e.g. 1996and 1998 in SB) were not coupled withproportional increases in biomass of thecommercial fraction in subsequent years.On the other hand, a clear relationshipemerged between the juvenile (≤ 18 mm)and ‘medium’-sized (≥19 ≤ 25 mm) frac-tions of the population. The attempt madeat evaluating whether a relationship existedbetween stock and recruitment in termsof abundance highlighted the fact that re-cruitment is largely independent of stocksize having consistently low values acrossthe range of stock abundance considered,with few exceptions (Morello, unpublisheddata). In order to gain information on theprofitability of the resource from an eco-nomic point of view, the surface area cor-responding to increasing commercial clamdensities was calculated for each year andDistrict (Figure 3) and considerable differ-ences were revealed between Districts in

this respect. The percentage area support-ing no clams was consistently higher inSB (Figure 3b) compared with AN (Figure3a), whilst the converse was true for thepercentage surface area supporting ≥12.5kg·1000 ·m−2. The higher percentage ofhighly productive grounds in AN was fur-ther confirmed upon comparison of surfaceareas supporting densities greater than 75kg·1000m−2, which, in AN have been vari-able but with peaks around 45 km2 in 1999and 2000, whilst in SB they have been veryclose to zero since 1986 (Figure 3b). Thelower overall exploitable area available inSB is likely to play only a partial role inthis difference. The results illustrated inFigure 4 show that there has been a pro-gressive decrease in the mean length of thecommercial (≥ 25 mm) fraction of the C.gallina population examined from 1984 to2001 in both Maritime Districts. Kendall’scoefficient of rank correlation test revealedthat this ‘progressive decrease’ could be at-tributed to statistically significant decreas-ing trends with time in both Districts. Fig-ure 5 illustrates the length-frequency dis-tributions (LFD) of C. gallina in the ANMaritime District between 1984 and 2001.Overall the time series of LFDs indicatesa drastic change in the size compositionof the population across the years whichis likely to be strictly connected with thefishery. In the early years (1984 and 1985;Figure 5a, 5b) the population was healthy,skewed towards the larger sizes (maximumsize ≈ 45 mm) with bi- or tri-modal LFDshaving cohorts at modal sizes around 22mm and 32 mm. From 1987 onwards thereappears to have been a progressive deple-tion of the larger individuals, a decreasein the modal sizes (e.g. 15 mm in 1994,Fig 5h; 19 mm in 1999, Figure 5m) andleft-skewed LFDs. Any recruitment eventrecorded (e.g. 1991, Figure 5e) appears to

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have grown slowly disappearing promptlyas soon as it reached 22-25 mm. TheLFDs are not shown here for lack of space,but the situation of the SB District clampopulation is very similar though high-lighting an even more precarious situationthan that described for AN. The paucity ofadult commercial-sized individuals, whichbecame particularly evident in AN from1994, appears to be an intrinsic character-istic of the SB C. gallina population andwas evident from the first year scientificsampling took place. The LFDs for thisarea are thus characterised by an alarmingscarcity of large clams, left-skewness andsmall modal sizes of the main mode (≈19mm), most likely due to a fishery that hasnot allowed the population to grow muchlarger than 23 mm. A study carried out onthe discards of the hydraulic dredge fisheryin the two Districts, in fact, strongly em-phasized the differences in gear selectivitybetween vessels from the AN fishing fleetand those from SB. In AN, the commer-cial portion of the catch was never smallerthan 21 mm and the percentage represen-tation of the 21-22 mm size category wasalways very low. In contrast, in SB, the ap-pearance of individuals at 19 mm in com-mercial catches was very frequent and thepercentage representation of the 21-22 mmsize class was conspicuous (Morello, un-published).

4 Discussion

Overall, the results obtained from the an-nual surveys to assess the Chamelea gal-lina stock in the Maritime Districts ofAncona (AN) and S. Benedetto (SB) re-vealed considerable year-to-year fluctua-tions in the total population, as well asin the commercial (≥ 25 mm) and juve-

nile (≤ 18 mm) fractions of the popula-tion, and no significant decreasing trendswith time were evident. The high commer-cial biomass estimates obtained for bothDistricts in 1984 and 1985 drastically de-creased in 1986. The decrease can mostprobably be attributed to concurring fac-tors, amongst which the fishery but also ad-verse environmental conditions (e.g. sed-iment resuspension consequent of stormsand increased freshwater inputs in the au-tumn of 1985 were the likely cause of amass mortality event in SA5 [10]). Noscientific surveys took place in the pe-riod 1988-1990 and when they resumed in1991 an extremely depleted commercial re-source was revealed, which in both Dis-tricts reached the historic minima. Verylow biomass estimates were reported forthe same year in virtually all Maritime Dis-tricts of the Adriatic Sea [11]. The sum-mers of 1988 and 1989 coincided with theoccurrence of abundant floating gelatinousaggregates in the coastal areas of the west-ern Adriatic Sea [12] which likely reducednewly settled C. gallina through suffoca-tion, having direct consequences on thecommercial stock (≥ 25 mm) two yearslater, in 1991 [11]. The high biomass ofjuvenile clams in 1991 in the SB Districtmay have been the result of a recruitmentevent which occurred in 1990 as a reactionto the stress induced by the presence of ma-rine snow in the previous year. Mass mor-tality of older cohorts has been reported tobe one of the preconditions triggering un-usually successful settlement in sedentarycontagiously distributed populations [13].The years following 1994 saw a recoveryof the C. gallina stock in the AN Dis-trict, which increased to attain very highcommercial biomass estimates in 1999 and2000. The same was not true for SB wherethe commercial fraction of the population

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never equalled the high production levelsrecorded in the mid-1980s. The overall sur-face area covered by the clam grounds isvery different in the two Districts (304 km2

in AN; 216 km2 in SB), but not enough toaccount for the differences in commercialbiomass estimates. In a similar manner,the percentage surface area devoid of C.gallina was consistently greater in SB, andthat supporting over 5 kg·1000 ·m−2 washigher in AN across the years, especiallyfrom 1996. A density of 5 kg·1000 ·m−2

is considered to be the lowest economi-cally viable density of the resource in de-pleted conditions [11]. These results fur-ther confirm the precarious situation of theclam stock in SB where most of the sur-face area supported clam densities belowthis lower-limit value. A drastic reductionin biomass was observed between 2000 and2001 in both Maritime Districts. In SB,the biomass was reduced to such an extentas to prompt a call for the complete cessa-tion in commercial fishing activity (start-ing September 2001). A reason, addi-tional to high exploitation, can be given forthe crash of both populations in 2001 andthis is the very high freshwater input andparticle suspension which was recorded inNovember 2000, when the Po river floodedas a result of very heavy rainfall attainingdischarge values around 12100 m3 · s−1

(Pecora & Frollo, www.arpa.emr.it) com-pared with a mean discharge of 1560m3 · s−1 (www.ilfiumepo.net). The inter-annual variability in juvenile biomass esti-mates was even greater than that recordedfor commercial biomass and this was truefor both Districts. The results point to thefact that high recruitment events are a spo-radic event. Chamelea gallina is a ratherlong-lived species, reported to reach eightyears of age [4]. Consequently, under idealconditions (i.e. no fishing) the species has

no need to produce excessive numbers ofrecruits every year, especially in light ofthe fact that growth, and natural mortal-ity, are reported to be density-dependent[1, 11]. Overall, the results obtained al-low no general considerations to be madeon the relationship between stock size andrecruitment; recruitment being largely in-dependent of stock size. This is a situ-ation widely reported for many infaunalbivalve stocks [13]. It may well be thatthis absence of relationship has saved theC. gallina resource in the area from com-mercial extinction under this regime of in-tensive exploitation. The results indicatedstatistically significant decreasing trends inthe mean size of commercial estimates be-tween 1984 and 2001 in both Districts.It is well known that fishing, by target-ing large economically profitable individu-als, has the overall effect of removing theolder cohorts from the harvested popula-tion and the direct consequence of this isa decrease in the mean size of the popu-lation, particularly of the commercial frac-tion [14, 15, 16]. For this reason, the mean,or median, size of a harvested population isconsidered as a biological reference point,i.e. a yardstick against which the status of apopulation can be gauged [17, 16]. Furthe-more, Trenkel and Rochet [16] in an anal-ysis of the performance of several biolog-ical reference points, highlighted the factthat mean length of catch was one of themost precisely estimated and most power-ful indicators of fishing impact. The sig-nificant decreasing trend in the mean sizeof the commercial C. gallina fraction of thepopulation is, thus, a good indication of thenegative effects of the hydraulic dredgingfleets have had across the years on the clamresource in the study area. The length-frequency distributions obtained from bothDistricts revealed that the medium-sized

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fraction of the resource has been heavilyfished throughout the years. The fisheryappears to have been responsible for main-taining the population in a stable state cen-tred around medium-sized individuals. In-deed, Marrs et al. [18] reported that thefleet seeks areas supporting high clam den-sities and not areas where the size of indi-viduals is large; once the densities drop thefleet changes area (rotation of areas). Thisinevitably results in the medium-sized (un-dersized) fraction of the population beingharvested to yield high quantities, ratherthan seeking large individuals that wouldyield proportionately lower catches perunit time. The size frequency distribu-tions, the biomass estimates and the lackof a relationship between stock size and re-cruitment, as well as the removal strategyadopted by the fleet, highlight the fact thatthe resource and the entire fleet, are heavilydependent on the stochastic event of sub-stantial clam recruitment events; and onlysuch events will yield a cohort so conspic-uous as to withstand and survive the fish-ing pressure to which it is subjected. Inthe past several years, large recruitmentshave occurred and significant natural mor-tality events, mostly of adult clams (≥19mm in size), have been observed towardsthe end of the spawning season. The ori-gin of these mortality events is still unclearas these phenomena were recorded un-der normoxic conditions both in areas thatwere heavily exploited and in areas closedto dredging for several months. Earlyand protracted spawning, significant andprotracted recruitment events followed bymass mortalities, and the scarcity of clamsover 30 mm, may all be indications ofa shift in energy-allocation strategy fromone favouring growth to one favouring re-production. The adoption of a more r-oriented life strategy inevitably generates a

series of micro-cohorts which will furtherincrease variability and further impede theability to follow a specific cohort throughtime. The repercussions on the life cy-cle may be multiple; a progressive reduc-tion, through harvesting, in the older frac-tion of Atlantic cod, Gadus morhua, for ex-ample, has been reported to have led to areduction in mean age at first maturity andto a modification of the temporal succes-sion of spawning events within one season[15]. Little is known about annual fluctu-ations in recruitment success in Chameleagallina, as for many other inshore bivalves(e.g. Spisula substruncata, [19]). Adopt-ing a precautionary approach, the fishingeffort (e.g. in terms of number of licences)allotted to an area (be it an entire Mar-itime District or a single sub-area) shouldbe based on resource estimates obtainedfor years of low recruitment in the case ofspecies with a more r-orientated strategysuch as Chamelea gallina. Buchanan [20]has stressed that biomass is no guide to pro-ductivity in infaunal echinoderms and thisis equally true of bivalves. Just becausea resource is plentiful now does not meanthat it can sustain heavy and prolonged ex-ploitation. Chamelea gallina in the Adri-atic has been widely reported to reproducein its first year of benthic life, around 18mm in length [1], well below the MLS of25 mm set by law. The high intensity offishing to which the stock is subjected inthe study area is such as to ensure the cap-ture of the majority of an annual cohortas soon as it reaches at least 22 mm shelllength on average. This may create seriousproblems for the resource in years of lowrecruitment. Despite this, in light of the bi-ology of the species in the area, an MLSof 25 mm can be considered a precaution-ary size and this may be one other reasonwhy the resource has not become commer-

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cially extinct. In light of these results, aneffective control of fishing effort and ac-tive enforcement of existing rules regard-ing the characteristics of C. gallina dredgesand sieving equipment are deemed funda-mental to maintain this ‘high-tech’ fishery;but over and above this it is especially im-portant that the stock be assessed, via sur-veys, at least yearly. The rotational nature

of the fishery and the occurrence of massmortalities, make it very difficult to followa cohort through time, especially on an an-nual basis. This could be remedied by car-rying out more surveys in one year, Marrset al. [18] suggest two, so as to gain a morerealistic quantification of the two sourcesof both fishing and natural mortality.

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Figure 2: Estimates of (a) total, (b) commercial (≥25mm) and (c) juvenile (≤18mm)Chamelea gallina biomass (+ 95% C.I.) in the Ancona (AN) and S. Benedetto (SBT)Districts for 1984-2001.

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Figure 3: Surface area (%) corresponding to increasing commercial Chamelea gallinadensities (0 = 0 kg·1000m−2. * = 0-5 kg·1000m−2; ** = 5-12.5 kg·1000m−2; *** = >12.5 kg·1000 ·m−2) in the (a) Ancona (AN) and (b) S. Benedetto (SB) Districts between1984 and 2001.

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Figure 4: Variation in mean length of commercial Chamelea gallina (≥ 25 mm) be-tween 1984 and 2001 in the (a) Ancona (AN) and (b) S. Benedetto (SB) Districts withthe trend line and non-parametric regression equation calculated according to Kendall’srobust line-fit method [9], indicating the results of the Kendall’s coefficient of rank cor-relation (tau) used to establish the significance of such trends (* = significant at alpha =0.05).

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Figure 5: Length-frequency distributions of the sampled Chamelea gallina populationin the Ancona Maritime District (AN) from 1984 to 2001. Dotted bars represent theunderestimated fraction of the population.

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Invertebrate Fisheries: their Assessment and Management. pages 507–524, 1989.

[2] G. Valli and G. Zecchini-Pinesich. Alcuni aspetti della riproduzione e della biome-tria in Chamelea gallina (L.) (Mollusca: Bivalvia) del Golfo di Trieste. Atti delConvegno delle Unita Operative Afferenti Sottoprogetti Risorse Biologiche ed In-quinamento Marino, pages 343–351, 1982.

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[12] M. Giani, A.M. Cicero, F. Savelli, M. Bruno, G. Donati, A. Farina, E. Veschetti,and L. Volterra. Marine snow in the Adriatic Sea: a multifactorial study. Scienceof the Total Environment, Suppl:539–550, 1992.

[13] J.F. Caddy. Recent developments in research and management for wild stocks ofbivalves and gastropods. In Marine invertebrate fisheries: their assessment and man-agement. pages 665–700, 1989.

[14] R.C. Babcock, S. Kelly, N.T. Shears, J.W. Walker, and T.J. Willis. Changes in com-munity structure in temperate marine reserves. Marine Ecology Progress Series,189:125–134, 1999.

[15] S.A. Murawski, P.J. Rago, and E.A. Trippel. Impacts of demographic variation inspawning characteristics on reference points for fishery management. ICES Journalof Marine Science, 58:1002–1014, 2001.

[16] V.M. Trenkel and M.J. Rochet. Performance of indicators derived from abundanceestimates for detecting the impact of fishing on a fish community. Canadian Journalof Fisheries and Aquatic Sciences, 60:67–85, 2003.

[17] J.F. Caddy and R. Mahon. Reference points for fisheries management. FAO Fish-eries Technical Paper, 341:1–83, 1995.

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