Population structure of the protected fan mussel Pinna nobilis in the south Aegean Sea (eastern Mediterranean)

Abstract

Pinna nobilis is an endemic Mediterranean species of high economic and cultural importance. Nowadays, it is under strict protection as a dramatic decline in its populations has been recognized, assigned to the combined effect of fisheries and habitat degradation. This study attempts to evaluate, by non-destructive sampling, the current status of P. nobilis populations in the south Aegean Sea (Dodecanese island complex), where they have been affected for centuries by practically uncontrolled harvesting. Population density and total length of individuals was estimated by diving along strip transects at six islands. Two stations were randomly selected per island, and two depth zones were surveyed per station. Additionally, temporal sampling was carried out at Astypalaia Island from March 2004 to June 2006, to assess relevant trends in density of individuals and size structure of the population. In total 1372 P. nobilis individuals were measured in situ to estimate maximum shell length. The studied populations showed increased density in the shallower depth zone and sheltered sites; small-scale environmental regimes are proposed as shaping factors of P. nobilis distribution. Increased density was also detected during the cold period of the year, probably related to recruitment success. In the lower depth zone the P. nobilis population consisted of larger individuals. A significant decrease in size was detected temporally as mean length was smaller in December 2004; in the same month the presence of small-sized juveniles was depicted in a left-shift in size-frequency distribution. These observations are probably related to the reproductive biology of the species.