Spatial distribution, abundance and habitat use of the protected fan mussel Pinna nobilis in Souda Bay, Crete

The spatial distribution and abundance of the endangered fan mussel Pinna nobilis was investigated in Souda Bay, Crete, Greece. A density surface modelling approach using survey data from line transects, integrated with a geographic information system, was applied to estimate the population density and abundance of the fan mussel in the study area. Marked zonation of P. nobilis distribution was revealed with a density peak at a depth of similar to 15 m and practically zero densities in shallow areas (< 4 m depth) and at depths > 30 m. A hotspot of high density was observed in the southeastern part of the bay. The highest densities occurred in Caulerpa racemosa and Cymodocea nodosa beds, and the lowest occurred on rocky or unvegetated sandy/muddy bottoms and in Caulerpa prolifera beds. The high densities of juvenile fan mussels (almost exclusively of the first age class) observed in dense beds of the invasive alien alga C. racemosa were an indication of either preferential recruitment or reduced juvenile mortality in this habitat type. In C. nodosa beds, mostly large individuals were observed. The total abundance of the species was estimated as 130 900 individuals with a 95% confidence interval of 100 600 to 170 400 individuals. Despite poaching, incidental killing and marine pollution, the fan mussel population of Souda Bay remains of high importance as one of the largest documented in Greece, and protection measures are urgently needed to secure its sustainability. We provide a baseline for the Souda Bay fan mussel population.