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Description:This report summarizes the current state of knowledge about ballast water exchange (BWE) as a management strategy by ships to reduce the risk of invasions, with emphasis on two major U.S. ecosystems, the Great Lakes and Chesapeake Bay. Today, most global trade occurs by shipping among ports, creating unintended opportunities for transfer of aquatic species that result in biological invasions. Ships transfer organisms in their ballast tanks and on their hulls. To reduce the risk of invasions from ballast water discharge, estimated annually to exceed 70,000,000 metric tons in the U.S., ships arriving from foreign ports are required to conduct ballast water exchange (BWE) or alternative treatment before discharging ballast. This management strategy became mandatory in the United States for the Great Lakes and upper Hudson River in 1993, and it has been required for ships arriving to the Chesapeake Bay and all other ports since September 2004. BWE consists of flushing coastal water from ballast tanks by replacing it with oceanic waters and is intended to reduce the concentration of coastal organisms that may become established in subsequent ports upon ballast discharge. Most oceanic organisms are considered unlikely to colonize coastal habitats, just as many coastal organisms cannot persist in the open-ocean. In addition to removal of organisms, BWE will also expose any residual coastal organisms in the ballast tanks to full-strength seawater, which may cause high mortality for organisms from freshwater ecosystems that are likely to survive in the Great Lakes and upper Hudson River. Such “salinity stress” may thus increase the efficacy of BWE.
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Sea Grant Document Number:MICHU-T-07-001
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Rights Information:Public Domain
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