Recreational boats routinely transfer organisms and promote marine bioinvasions

Recreational boats and their marinas are important components of the hub-and-spoke model of invasion in coastal marine systems. Like most vectors, however, species transfers by boats are under-sampled and the extent of invasion among coastal bays is patchily sampled, with a bias towards larger urbanized bays with commercial port systems. We assessed both the hubs (bays with marinas) and the spokes (recreational boat movements and their vector biota) that form the basis of a non-shipping hub-and-spoke system for the coast of California. Non-native invertebrates and algae have been reported from most bays in California (26 of 27), with only five of these having commercial shipping ports. Recreational boats offer frequent opportunities for transfers of species among bays; 292,000 recreational boats were registered in coastal California, and more than half (54%) of surveyed recreational boat owners made voyages outside their home bays. Destinations for recreational boaters often included bays without commercial ports and areas of conservation value inside National Marine Sanctuaries. We recorded 158 taxa from the hulls and niche areas of 49 transient recreational boats upon arrival at Californian marinas. These included several non-native species that are already well-established at arrival locations and four that are not known from the state or sampling location. Biofouling abundance on vessels ranged from zero to remarkably high levels (estimated at 10⁶ organisms) for vessels that traverse open coastal systems. Management opportunities and models exist for the recreational boat vector but, absent operational incentives, these are challenging to implement broadly. An initial management focus on areas recognised for their distinct marine habitats and biodiversity may be an effective implementation approach.