Fouling and ships' hulls: how changing circumstances and spawning events may result in the spread of exotic species.

Organisms fouling ships' hulls are continually in transit worldwide. Although effective antifouling paints incorporating organotins have considerably reduced fouling biomass these paints have a limited period of effectiveness, which may be less than the ships' inter-docking period, depending on sea temperature and abrasion. Vessels immersed over several years can allow fouling communities to develop and spread beyond their native distribution. This process of establishment is not fully understood. This review proposes that short rapid turn-around of vessels with mature attached biota can result in synchronized spawnings and production of sufficient zygotes to form a founder population. Spawning may be induced by changes in temperature or salinity on entry into a port, according to season. The diversity of taxa in transit on ships' hulls includes commercial molluscs, which have the potential to transmit their diseases or pests to port regions. Several factors may act in the further enhancement of exotic species establishment including changes of in-port berthing regions to more marine conditions. Ships today are generally larger, and faster, and have a high frequency of port visits thereby increasing the number of spawning opportunities, perhaps with a larger inoculum size. With trade expansion, new trading routes, political events and changes in climate, new pathways for invasion will emerge. Greater controls on industrial discharges, improved treatments of urban wastes and better management of waste runoff into rivers as well as a phasing out of organotin antifoulants will mean a reduced toxicity in port regions. This may enable a smaller inoculum to colonize by creating opportunities for establishment not present in the previous 25 years. Some invaders will have unwanted consequences for the environment, economies and human health.