Abstract: Biological invasion occurs when species establish self-sustaining populations beyond their historical geographic ranges. Marine invasions have received relatively little attention compared to terrestrial and freshwater communities. Nonindigenous species become numerically dominant in invaded marine communities, just as those elsewhere. They have significant impacts on population, community and ecosystem-level processes. Oil tankers arriving in Prince William Sound deliver approximately 17 million cubic meters of non-oily ballast water annually. Tankers that arrive in Prince William Sound from politically foreign ports are required to undergo mid-ocean ballast exchange. Most ballast water delivered to Prince William Sound, however, comes from domestic ports, including San Francisco Bay, Long Beach, and Puget Sound. Vessels from ports such as these are requested to voluntarily conduct open-ocean exchange of ballast water before reaching their destination port. Ships practice two basic types of ballast water exchange that replaces coastal water with ocean water: Flow through Exchange, and Empty Refill Exchange. Experiments were done with both types of exchange aboard commercial tankers arriving to Port Valdez to quantify the efficacy of ballast water exchange in reducing transfer of coastal organisms. The study demonstrated that ballast water exchange on oil tankers arriving in Prince William Sound was highly effective, with the empty-refill method being more effective than flow-through method. The efficacy of flow-through is variable dependent upon the configuration of the ballast water tank how that allows the water to enter and exit.
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