Untitled - CNR

Marine research at CNRmarked 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 fluctuationswere even more marked upon considerationof the younger portion of the examinedpopulation (≤ 18 mm) with yearsof extremely low biomass being followedby very high biomass estimates in the successiveyear (e.g. SB 1997 cf 1998), orwith several subsequent years in which recruitmentwas 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 recruitmentevents 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) fractionsof the population. The attempt madeat evaluating whether a relationship existedbetween stock and recruitment in termsof abundance highlighted the fact that recruitmentis 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 economicpoint of view, the surface area correspondingto increasing commercial clamdensities was calculated for each year andDistrict (Figure 3) and considerable differenceswere revealed between Districts inthis respect. The percentage area supportingno 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 furtherconfirmed upon comparison of surfaceareas supporting densities greater than 75kg·1000m −2 , which, in AN have been variablebut with peaks around 45 km 2 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 progressivedecrease 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 attributedto statistically significant decreasingtrends with time in both Districts. Figure5 illustrates the length-frequency distributions(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 depletionof 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 to1911