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science, particularly our ability to determine appropriate target catches and to estimate actual fishing mortality or stock size as a basis for recommending effort or catch controls to meet these targets. Even when science is adequate, the effectiveness of management in achieving the desired control (i.e., control the exploitation rate) may be uncertain (Walters and Parma, 1996; Walters and Pearse, 1996; Walters, 1998). Experience and simulation analyses have shown that stock assessment methods sometimes are prone to errors exceeding 50%, even when costly monitoring programs are in place (NRC, 1998a). Worse, errors tend to be correlated from year to year, compounding their effects over time. Retrospective analysis often reveal biases, with stock size initially overestimated or underestimated for several consecutive years (Sinclaire et al., 1991; Parma, 1993). When scientists and managers depend on catch data from the fishery itself (i.e., fishery-dependent data), levels of bycatch and discards at sea often are unknown, and these sources of fishing mortality may not be included properly in assessments. Fundamental parameters, such as the rate of natural mortality, can be specified only in a rather broad range, based on life-history correlates. Indices of abundance derived from research surveys are valuable, but they too can be imprecise or, in many fisheries, simply unavailable.

It has been argued (Walters and Pearse, 1996; Lauck et al., 1998; Walters, 1998) that uncertainty in stock assessments is simply too large to manage fisheries sustainably using conventional tools. Three main approaches have been proposed to address this uncertainty: (1) choose substantially lower catch rates as fishing targets than in the past (Mace, 1994; Restrepo and Powers, 1999); (2) implement management tools that are less dependent on stock assessments, such as reserves (Roberts, 1997a; Lauck et al., 1998; Walters, 1998; Murray et al., 1999) and size limits (Myers and Mertz, 1998), and (3) generate institutional incentives that encourage responsible behavior on the part the fishers, such as different forms of user rights (NRC, 1999b; Hilborn et al., in press). These three approaches are not exclusive, and all may have to be considered for fishery management to be successful. Marine reserves, as an alternative to conventional management, also have uncertainties associated with their performance. Sources of costs and benefits of some of these approaches are discussed in more detail in Chapter 4.

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