Aliens cruising in: Explaining alien fouling macro-invertebrate species numbers on recreational yachts.

The extent of biofouling on recreational vessels has been used as a proxy for the presence of alien species and has been linked with vessel characteristics. However, the relationship between these factors and alien species numbers has not been examined, despite the importance of this metric for invasive species management. This study assessed physical characteristics, maintenance regimes and travel patterns of yachts and their relationship with the number of macro-invertebrate alien species present on vessels from four South African marinas. Overall, 88% of yachts were fouled with macro-invertebrate alien species. The only factor that influenced alien species numbers in the context of this study was the primary use of yachts, with cruising yachts supporting significantly more alien species than racing yachts. This is likely linked to differences in cleaning regimes, as racers are subject to rigorous and frequent cleaning. These findings suggest that cruising vessels may play a key role in the intra-regional transfer of alien species and that racing yachts likely pose a lower biosecurity threat. • The use of yachts was the only factor explaining alien species richness. • Cruising vessels supported significantly more alien species than did racers. • Travel among local marinas by these vessels poses an intra-regional transfer risk. • The elevated invasion risk associated with cruisers needs to be managed. [ABSTRACT FROM AUTHOR]