Repeated rapid assessment surveys reveal contrasting trends in occupancy of marinas by non-indigenous species on opposite sides of the western English Channel

Rapid assessment surveys of non-indigenous species (NIS) of sessile invertebrates were made at seven marinas in NW France and 10 marinas in SW England in 2010, and repeated in 2013. Fourteen NIS were recorded, 12 of which were seen on both coasts. Site occupancy differed between the opposite sides of the western English Channel. In Brittany, most species occurred at most sites in both 2010 and 2013. In 2010, site occupancy in Devon & Cornwall was distinctly lower; by 2013, the difference compared to Brittany had narrowed considerably, largely because of rapid colonisation of additional sites by species that were infrequent in 2010. Three more of the recent NIS are present in Devon & Cornwall but have still not become widespread. It is concluded that the recently introduced fouling animals studied here are longer established in NW France than in SW England, and have probably spread northwards across the Channel.