Bowed down in a sea of troubles: the role of recreational boats in the spread of alien species in the Mediterranean Sea

A large-scale study on the biofouling communities on recreational vessels and marinas was undertaken in the Mediterranean Sea. Approximately 600 boat owners in total from 25 marinas (in France, Italy, Malta, Greece, Turkey and Cyprus) were surveyed on their boat specifics, antifouling practices and travel history. Additionally, samples were collected from these same boat-hulls to first identify which NIS they were carrying, and then statistical analyses were tested to determine which factors drive the higher species richness on boat- hulls. About 500 of the vessels contained fouling, the remainder having been recently cleaned. Of all sampled vessels, 70% hosted at least one NIS, up to a maximum of 11. NIS richness in marinas is found here to strongly correlate to NIS richness on their boats. Of ten factors tested to determine influence of higher NIS richness on boat-hulls, the most significant were: time since last cleaning (which includes the last antifouling coating) and the visible hull fouling estimate of niche areas (observed from the boat hulls). Boat hull material, average cruising speed, and increased marina visits did not contribute here to higher NIS richness on boat- hulls. We warn here that boats moored in the Eastern Mediterranean region especially pose a much higher risk of spreading NIS due to a higher divergence in species occurrences between marinas and the boats within them.