Geostatistical approach to investigate spatial patterns of the endangered fan mussel Pinna nobilis (Linnaeus, 1758)

The manta tow survey technique combined with the geostatistical approach was used to identify the reliability of these tools to identify the position of the banks of P. nobilis and evaluate the spatial distribution and abundance of its population. A large spatial scale study was carried out through a geostatistical approach to identify the spatial distribution and abundance of the endangered fan mussel Pinna nobilis populations in the Sant’Antioco Island, Sardinia, Tyrrhenian Sea. A smaller spatial scale approach, which was carried out by using the standard method of quadrates by scuba diving, was used to study the size distribution and to supply correction factors for the estimates obtained by geostatistics. Kriging mapping revealed the presence of a very dense population with values of abundance as high as up to 70 individuals 100 m−2, corresponding to a total number upscaling to 3.9 million individuals over the whole area. The small spatial scale study showed that the fan mussel population had a spatially heterogeneous size structure, with each location characterized by a different size/age structure mirroring the observed patterns of spatial autocorrelation. The comparison between the density of specimens directly observed by scuba diving and those estimated by the geostatistical technique showed that estimated value was overestimated when the density was low and underestimated when the density was high. We therefore conclude that the technique proposed in this study is a less time-consuming technique for prospection studies for the assessment of the presence/absence of P. nobilis population and their abundances in shallow waters, which are the first step for the development of conservation and management strategies of this endangered species.