economically feasible or otherwise practical, but implementing a system of ballast water management and controls can reduce the probability that unwanted or harmful introductions will occur. Selecting the appropriate level of risk of introducing unwanted organisms has important implications for worldwide requirements for ballast water management and control.

Current guidelines for controlling introductions of nonindigenous aquatic nuisance species do not identify an acceptable level of risk. For example, avoiding ballasting in known "hot spots," in areas where the sediment content of the water column is high, at night, or at certain times of the year are crude but effective measures for managing risk by reducing the number of organisms loaded with ballast water. Although these measures may be effective, they are required or recommended on the basis of very limited knowledge about the risk of introductions. Therefore, the ecological effectiveness and economic efficiency of control measures cannot be quantified.

The fundamental question that remains to be addressed is what level of ballast water control is effective. Biological science cannot currently answer this question. The answer depends in part upon the receiving country's specific concerns about which potential introductions pose economic, ecological, human health, and other risks. In addition, it is not known how many individuals of a given species are needed to establish a viable, self-reproducing population at a new site. Even if this number were known, it would not be consistent from one receiving environment to another.

Issues such as these can be addressed by risk analysis. Risk analysis is used in many settings as a strategic tool for setting priorities and optimizing management and control measures. In the United States, for example, risk assessment is used to establish safe levels of exposure to carcinogens and other toxic chemicals. The Aquatic Nuisance Species Task Force is developing a generic risk analysis for nonindigenous aquatic organisms (Risk Assessment and Management Committee, 1995). In Australia, the coastal water guidelines working group is developing a voyage risk assessment and management process. The proposed risk-based approaches to managing ballast water use quantitative risk assessment methodology to combine a determination of the likelihood of introduction of a specified unwanted organism with an assessment of the economic and social impact resulting from the introduction (see Box 3-3). Lessons learned from successful efforts that use risk analysis as a strategic decision aid underscore the importance of standardizing methods and assuring that assumptions and methods are clear to all (NRC, 1994).

Even though detailed scientific information, such as species-specific data on probabilities of establishment, is generally lacking, risk analysis methodology can help decision makers in two ways. First, it can assist regulators in establishing appropriate levels of protection by helping them understand and balance technical, economic, and social factors. Second, when a specific organism is of concern, risk analysis methods can help evaluate alternatives and optimize the selected control strategy. Thus, a probabilistic risk analysis based on limited available

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