Patterns and drivers of marine bioinvasions in eight Western Cape harbours, South Africa

In South Africa, fouling is the dominant vector of marine invasions, being responsible for 48% of the 86 alien introductions that are known. This study aimed to document alien species in fouling assemblages in eight Western Cape harbours and to assess patterns and potential drivers of these invasions. In each harbour, 10 visual (1 m x 1 m) and 10 scrape (15 cm x 15 cm) subtidal samples were randomly collected at depths between 1 and 5 m. In total, 22 marine alien species were detected and, although no statistical differences were evident in the number of alien species recorded per harbour, the number of alien species per m(2) differed significantly among harbours, with Hout Bay Harbour having the highest. The brachiopod Discinisca tenuis, documented for the first time outside of aquaculture facilities, was found to have spread from Saldanha Bay to St Helena Bay. A previously unknown alien amphipod, Ericthonius difformis, was detected in Simon's Town Harbour. CART (classification and regression tree) analyses revealed that relatively small harbours with yachts supported more alien fouling species than other harbours. This finding could be useful for managers wishing to prioritise Western Cape harbours for monitoring.