Small boats provide connectivity for nonindigenous marine species between a highly invaded international port and nearby coastal harbors.

While considerable variation exists in ecological and economic impacts among nonindigenous species (NIS), the potential magnitude of cumulative impacts for each species increases with increasing area occupied. In the marine environment, large commercial ships have often transferred NIS across ocean basins to new continents. However, following such initial invasions, small craft (recreational and fishing boats) likely play an important role in the secondary, coastwise spread of NIS, thus increasing the geographic range and potential magnitude of impact. In this study, we assess the connectivity among bays in terms of small vessel movement and associated biofouling organisms in central California (USA), examining flux between a heavily invaded international port, San Francisco Bay (SFB), and three small marinas on the adjacent coast. We estimated vessel flux among locations, using data from 405 boater questionnaires and 4,000 transient boat records, and found a strong bi-directional connection between SFB and the nearby coastal marinas. Video surveys of 36 boats that had recently traveled revealed macrofauna on the underwater surfaces on 80% of boats, and at least 27 taxa (including 7 NIS) were present on boats sampled by SCUBA. Importantly, while we provide evidence for strong connectivity, our data most certainly underestimate the flux of vessels and cumulative transfers of organisms among these locations during this short-term study, as vessel movement data are incomplete and surveys are limited even in this small region. This study underscores the opportunities for biofouling organisms, many with limited selfdispersal capacity, to spread via small boats and suggests that effective management strategies to minimize NIS spread and impacts must address the small-boat vector.