Do hydrodynamic changes affect abundances of the striped Venus clam? A preliminary analysis in the Southern Adriatic Sea.

The striped venus clam (Chamelea gallina Linnaeus, 1758) is one of the most important commercially valuable bivalves distributed in the Italian sandy soft bottoms of the Adriatic Sea (Central Mediterranean Sea). The impacts of hydraulic dredge used in the harvesting of this bivalve have been widely investigated in this area, while poor data are available on the relationships between juveniles’ recruitment and water circulation. This preliminary study explores the dynamics of C. gallina populations in the Northern Gargano Area during the period 2018-2019, in which a strong decrease in abundance of juveniles and adults was detected. The analysis on abundance data (N/100 m2) and the distribution along the depth gradient were integrated with outputs on water currents (speed m/s, and direction) obtained from the 3D hydrodynamic numerical model MIKE 3 FM HD. A significant decrease in abundance was observed from 2018 to 2019, while a slight regression of the depth range for the juveniles distribution was observed in 2019. Relevant changes in the water circulation were observed between the two years, with a higher current speed and larger cyclonic gyres in 2018 than in 2019. These preliminary results suggest the occurrence of sudden hydrodynamic changes in the investigated area, which can affect the dispersal of larvae towards unsuitable muddy bottoms for the larval settlement. Further analysis should be carried out with an integration of other oceanographic variables and an improvement in the modelling of the temporal scale resolution.