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and subsequent discharge of water from ballast tanks. It has been estimated that more than 3,000 species of animals and plants are transported daily around the world in ballast water, which is required for safe operation under a range of conditions.1 For the purposes of this study, the term ballast includes sediment, which is the debris suspended in ballast water as it is loaded that subsequently accumulates on horizontal surfaces in ballast tanks. Ship owners since time immemorial have endeavored to avoid using ballast, preferring to carry revenue-earning cargo. Nevertheless, ballast is always necessary for the successful and safe operation of ships.
Any approach to managing ballast water and controlling introductions of nonindigenous aquatic species must take into account that there may be several source regions and release sites of ballast water on any sea voyage. The biota in ballast water are correspondingly diverse, and predicting the presence of a particular unwanted species in a particular vessel or certifying a particular vessel as free of or safe from all unwanted species is extremely difficult. The diversity of potential introductions and the numerous environmental factors (e.g., water temperature, nutrient levels, and the extent and nature of pollution) determining the fate of organisms discharged with ballast water make it impossible to predict what the next introduction will be or when and where it will occur. Nevertheless, it can be stated with confidence that further introductions will take place and that ballast water is an important vector contributing to the dispersal of nonindigenous aquatic organisms.
POTENTIAL CONTROL STRATEGIES
Changing ballast at sea is currently the favored technique for reducing the risk of introducing nonindigenous aquatic organisms into the marine environment through discharged ballast water. Ballast water loaded in port or taken on board while transiting inshore waters is charged with ocean water during passage between ports of call. This method is usually effective because most freshwater estuarine, and inshore coastal organisms cannot survive when discharged into the ocean environment. Similarly, freshwater, estuarine, or inshore coastal waters are inhospitable to oceanic organisms. One of the main functions of ballast is to ensure the stability and manageability to ships at sea. Therefore, altering the ballast condition while under way may jeopardize vessel safety. In addition, the design of most ballast systems does not permit the removal of all ballast and associated biota. Thus, while changing ballast may be an acceptable and effective control method under certain circumstances, it is neither universally applicable nor totally effective, and alternative strategies are needed.
Ballast water is generally used in preference to solid ballast because of reduced loading times and increased vessel stability during a voyage.