Around the world, ships moving from port to port and dumping their ballast water have often spread invasive species, including zebra mussels, toxic algae, parasites, and even cholera.
In an effort to crack down on the growing nuisance of exotic species in waterways like the Chesapeake Bay, the U.S. Coast guard in 2004 imposed a new rule for most ships entering American ports. The ships were required to dump their ballast water hundreds of miles away from shore and instead fill up their tanks with water from the open ocean. This deep water typically contains fewer coastal life forms and more salt, which can kill fresh-water creatures like invasive zebra mussels.
But when scientists at the Smithsonian Environmental Research Center in Edgewater, Maryland, studied the impact of the new regulations, they discovered something unexpected: The number of marine hitchhikers multiplied instead of decreasing.