Untitled - CNR

Marine research at CNRnever equalled the high production levelsrecorded in the mid-1980s. The overall surfacearea covered by the clam grounds isvery different in the two Districts (304 km 2in AN; 216 km 2 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 −2is considered to be the lowest economicallyviable density of the resource in depletedconditions [11]. These results furtherconfirm the precarious situation of theclam stock in SB where most of the surfacearea 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 cessationin commercial fishing activity (startingSeptember 2001). A reason, additionalto 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 m 3 · s −1(Pecora & Frollo, www.arpa.emr.it) comparedwith a mean discharge of 1560m 3 · s −1 (www.ilfiumepo.net). The interannualvariability in juvenile biomass estimateswas even greater than that recordedfor commercial biomass and this was truefor both Districts. The results point to thefact that high recruitment events are a sporadicevent. Chamelea gallina is a ratherlong-lived species, reported to reach eightyears of age [4]. Consequently, under idealconditions (i.e. no fishing) the species hasno need to produce excessive numbers ofrecruits every year, especially in light ofthe fact that growth, and natural mortality,are reported to be density-dependent[1, 11]. Overall, the results obtained allowno general considerations to be madeon the relationship between stock size andrecruitment; recruitment being largely independentof stock size. This is a situationwidely reported for many infaunalbivalve stocks [13]. It may well be thatthis absence of relationship has saved theC. gallina resource in the area from commercialextinction under this regime of intensiveexploitation. The results indicatedstatistically significant decreasing trends inthe mean size of commercial estimates between1984 and 2001 in both Districts.It is well known that fishing, by targetinglarge economically profitable individuals,has the overall effect of removing theolder cohorts from the harvested populationand the direct consequence of this isa decrease in the mean size of the population,particularly of the commercial fraction[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]. Furthemore,Trenkel and Rochet [16] in an analysisof the performance of several biologicalreference points, highlighted the factthat mean length of catch was one of themost precisely estimated and most powerfulindicators of fishing impact. The significantdecreasing 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 lengthfrequencydistributions obtained from bothDistricts revealed that the medium-sized1913