Aquatic nuisance species are carried across the seas not only inside ships but attached to the outside as well. Organisms like barnacles, mussels, sponges, algae and sea squirts attach themselves to the hulls of ships, fouling them. These organisms then colonize the hull and "hitch a ride" from one port or bioregion to the next. Invasions can occur when these fouling organisms come in contact with structures in a new port or release their larvae into its waters. Under the right conditions, these invaders may establish themselves in the new port and spread to nearby areas within that bioregion.
Historically, hull fouling was considered a primary vector for transporting species. However, the modern use of metal hulls and anti-fouling paints as well as decreased port residency times and faster ship speeds contribute to lessened hull fouling. Although these advances help decrease the transfer of invasive species via hull fouling, an increase in shipping may counteract their benefits. 438 million m2 of wetted hull surface area (WSA) arrives into U.S. bioregions per year. 67% of this WSA is coming into the U.S. from bioregions outside the U.S. The other 33% is crossing bioregions within the U.S. And even the best maintained vessels have 5-20% of their WSA fouled. Shipping companies as well as the U.S. military spend millions of dollars each year in increased fuel costs and antifouling coating research and application.