Recreational boats as potential vectors of marine organisms at an invasion hotspot

Abstract

With more than 200 aquatic nonindigenous species (NIS), San Francisco Bay (California, USA) is among the world's most invaded harbors. Hard-substratum benthic (biofouling) organisms, which dominate NIS richness, have arrived primarily as a result of shipping and aquaculture activity over past centuries. To date there has been no assessment of the leisure craft vector in the Bay. We aimed to characterize (1) biofouling on boats' submerged surfaces and (2) boater behavior likely to affect the risk of NIS transfers. We used an underwater pole-cam, specimen collections, and a boater questionnaire to quantify the extent and composition of biofouling on recreational boats and to eval- uate boater behavior at a subset of the Bay's marinas. Several NIS, already established within the Bay, were recorded from vessel hulls, including the bryozoans Bugula neritina, Membranipora chesa- peakensis and Watersipora sp., the ascidians Botrylloides violaceus, Styela clava and Ciona intesti- nalis, the polychaete Ficopomatus enigmaticus, and the sponge Clathria prolifera. Only 16% of ques- tionnaire respondents had traveled to sites outside the Bay in the previous 12 mo. Frequency of hull painting and cleaning varied substantially, but we did not find strong patterns of biofouling extent associated with hull husbandry or boat usage. The potential for within-Bay and coastwise regional spread of NIS is high, and recreational boats probably interact in close proximity to other vectors (e.g. commercial ships), causing a ratchet effect of vector events; however, there remains a gap in under- standing the levels and condition of biofouling on transient boats. Transient vessels from San Fran- cisco Bay and other West Coast sites should be the focus of future studies to evaluate the extent to which organisms are being transferred among bays and how vector management could be applied to prevent NIS transfers and impacts.