Boat harbour design can exacerbate hull fouling

Hull fouling is a major cost for owners of small vessels and an important pathway for the spread of non-indigenous aquatic species. The extent of fouling depends on a hull's susceptibility to recruitment by aquatic organisms and the local availability of competent planktonic propagules ('propagule pressure'). Management strategies have typically been concerned with increasing resistance of the hull to recruitment through the use of toxic paints. Here we tested the hypothesis that fouling is influenced by the design of the harbour in which the boat is moored. We compared recruitment of sessile invertebrates to available surfaces in two types of recreational boat harbours: marinas that were partially enclosed by a permanent breakwall, and marinas that lacked breakwalls. Recruitment in the marinas was compared to coastal reference sites that were not used for mooring. At each location, recruitment tiles were deployed for 4 weeks on four separate occasions over a period of 2 years. Measurements of current velocities and spatial patterns of water flow at each location showed that permanent breakwalls created complex patterns of circulation that retained water within the marina basin for up to 12 h d(-1). Despite large regional and temporal variability in fouling over time, most organisms recruited in greatest numbers to surfaces in partially enclosed marinas, and were often several orders of magnitude more abundant in the enclosed marinas than in unenclosed marinas or coastal reference locations. Harbour design has an important influence on the rate at which fouling organisms recruit to available surfaces within marinas. Entrainment of water in enclosed marinas may limit the dispersal of planktonic propagules by advective currents but effectively increases propagule pressure to available surfaces, including resident boat hulls. This is likely to accelerate the development of hull-fouling assemblages and increase the chances of transport of non-indigenous species that establish populations in the harbour basin.