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

Despite the Mediterranean being both a hotspot for recreational boating and for non- indigenous species (NIS), no data currently exists on the recreational boating sector’s contribution to the spread of NIS in the Mediterranean. To provide recommendations to reduce further introductions and spreading, a wide-scale sampling study on the biofouling communities of recreational vessels and recreational marinas was undertaken surveying over 600 boat owners and sampling their boat hulls for NIS in 25 marinas across the Mediterranean, from France to Turkey, to determine which factors (i.e., boat characteristics, travelling behaviour, home marina) drive higher NIS richness on boat-hulls. The data revealed that Mediterranean recreational vessels travel considerably, averaging 67 travel days and 7.5 visited marinas per annum, resulting in a high potential for spreading NIS, especially as 71% of sampled vessels host at least one (and up to even 11) NIS. Boats with high NIS richness strongly correlated to marinas with high NIS richness. The presence of biofouling in niche areas of the hull (i.e., in the cavities and metallic parts) was the strongest predictor for NIS richness on the hulls along with longer elapsed times since their last cleaning and antifouling applications. Interestingly, colonization of NIS was shown to rapidly occur on boats which recently had their hulls professionally cleaned. We hereby warn that boats moored particularly in the Eastern Mediterranean region pose a higher risk of spreading NIS due to hosting more NIS in general and more cases of new NIS either on boats or in the marinas. To inhibit further introductions and spreading of NIS within the basin, routine marina monitoring for new NIS and pontoon cleaning is recommended for marinas, and a preliminary screening is recommended for incoming vessels. In addition, a visible fouling estimate of niche areas either by camera or snorkelling would help to determine if they require either a power washing/quarantine solution to ensure they are free of NIS.