Influence of boat anchoring on Pinna nobilis : a field experiment using mimic units

Abstract

Anchoring by recreational boats causes several different effects on marine habitats and species. The influence of boat anchoring on the Mediterranean endemic bivalve Pinna nobilis was assessed experimentally in the field by deploying non-biological mimic units of the bivalve around the islands of Mallorca and Malta. In each of these two localities, two study areas having different levels of boat anchoring activities, namely, 'control' (no anchoring allowed) and 'affected' (anchoring allowed), were used for the field experiment. A significant number of P. nobilis mimic bivalves were affected by boat anchoring; the influence was three times higher at anchoring sites than in control areas. Boat anchoring also had an adverse effect on Posidonia oceanica meadows, with seagrass cover, shoot density and global density being lower in the anchoring areas. Moreover, lower values of natural P. nobilis density and size classes were recorded from the anchoring areas.