Invaders are frequently introduced across oceans and along coastlines through the ballast water in ship hulls, water that often includes plankton and larval stages of marine and estuarine species. Large vessels need this water for balance as they load and unload cargo. However, by dumping ballast water in their ports of entry, they accidentally bring in new species that can alter or damage the local ecosystem. In 2004 policymakers thought they had found a solution: have cargo vessels exchange their ballast water in the open ocean, at least 200 nautical miles from land. This method, called "open-ocean exchange," flushes out or kills potential invaders by exchanging coastal water for water from the deep ocean.
But some ships do not use the practice and many more cannot without veering drastically off course. In perhaps the most comprehensive study to date, Whitman Miller and a team of scientists from SERC looked at all international ships entering the contiguous U.S. over three years. Published in the journal BioScience, the study analyzed approximately 105,000 vessel reports from January 2005 to December 2007. While most ships opted not to discharge their ballast water at all, a substantial number continued to dump unexchanged or improperly exchanged water into their ports of entry.
Not all coasts are affected equally. The Gulf of Mexico and the East Coast received much larger fractions of unexchanged ballast water than the West Coast. Roughly 5 percent of the ballast water discharged on the West Coast had not undergone open-ocean exchange. By contrast 21 percent of the discharged water in the Gulf and 23 percent on the East Coast went unexchanged....
US Coast Guard image of ship pumping ballast water