Dinamica di popolazione di Ruditapes Decussatus (L) e insediamento di Rudutapes Philippinarum (Adams&Reeve) in Sardegna (Italia)

Ruditapes decussatus is a traditional resource of fishing activity in Sardinia Island. This species is not cultured, but fishermen picked them up from the bottom of brackish areas. The distribution and the population structure of Ruditapes decussatus were investigated in six coastal lagoons: Tortolì (OT) S. Giovanni Santa Gilla (CA), Marceddì, Corru S’ittiri (OR), Calich (SS) and in the Olbia gulf. In this last site were also studied the growth and the mortality. In the same area, during the study the settlement of the allochthonous species Ruditapes philippinarum was found, so the population structure of this bivalve mollusc was also studied. The samplings were seasonally, started in autumn 2008 and finished in summer 2009. Ruditapes were collected using a shellfish rake in different stations for each site (five repetitions for each of 29 sampling stations). In spring was also collected the sediment in all stations. With regard to Ruditapes decussatus, the analysis of density demonstrates a highly heterogeneous situation in the different sites. The maximum of density was measured at San Giovanni (CA), where it has reached 1,315 *m-2 specimens. In the same site, the maximum of density (considering the average between the repetition in each station) was reached equal to 507 * m-2 specimens in summer. This value is close to that observed in the Olbia Gulf and Corru s' Ittiri lagoon in spring. In other sites densities were lower, but the maximum always corresponded to spring or summer, except Marceddì (OR) where the maximum value was detected in the winter sampling. In this case, the excessive rainfall caused very low salinity and the death of almost all Ruditapes in the lagoon. Also the population structure was very heterogeneous in the sites. The length frequency distribution (oro-aboral length) was statistically different for at least two seasons for all comparisons (χ2 test, P<0,01). The main reason of this difference is the recruitment, very different among the sites. In all lagoons, except Calich (SS) and Marceddì (OR), the recruitment was evident in spring and often has continued even in the summer. San Giovanni (CA) was the only case where specimens of shell length below 15 mm were recorded during all year. Several nursery areas were found in different sites. A station of the Olbia Gulf presented the highest densities of juveniles found during the study (487±368 * m-2 specimens). The cause of this variability, in addiction to the different fishing effort, was the recruitment, which impacted heavily on the distribution and density of juveniles. The growth of Ruditapes decussatus in the Olbia Gulf has been studied for each station because the analysis of the entire sample did not make possible the identification of different age groups. The estimates for the Von Bertalanffy model show that growth rates were marginally different in the stations. The function obtained with the average of parameters in the different station are: K=0.68 ±0.07, L∞ =47.02±4.32 mm, to= -0.17±0.11. The shell length- high (h= 0.6805*l 0.9853) and length-thickness (s = 0.3876*l 1.1176) relationship has shown a growth not perfectly linear. The mortality function for the Ruditapes decussatus in the Olbia gulf is N(t+1)=204.27* e (-1.3509 t). The sediment in all sites was mainly sandy. The granulometry of sites was statistically different for all comparisons (χ2 test, P<0,01). A univocal correlation between density of Ruditapes decussatus and granulometry was not found while exist, in the Olbia Gulf, a strong significant negative correlation between the fraction “fine silt” and the growth constant K. Ruditapes philippinarum was found in S. Gilla lagoon and in the Olbia gulf. In this site it has reached 19% of the total number of two species. In the gulf Ruditapes philippinarum is distributed everywhere but not in all seasons; the density and the shell length are statically different in the sampling stations except in winter (Kruskall-Wallis test P<0,01). The maximum of density for station was 106±83 specimens in spring. The jouveniles below 15 mm were present mainly in spring and summer, especially in a river mouth. The research has picked out the good situation of Ruditapes decussatus in Sardinia island in respect to other countries were Ruditapes philippinarum has almost replaced Ruditapes decussatus. The settlement of Ruditapes philippinarum in some Sardinian sites need some precaution because the allochthonous species are not diffused to avoid a dangerous competition against the autochthonous species.