The Relative Weight Index in Fisheries Management: Status and Needs

Abstract

The relative weight (Wr) index allows easy interpretation of condition for fish of various species and lengths. Rapid proliferation of index use, with a piecemeal approach to research on the index, has led to a confusing array of equations and methods for developing these equations. We recommend that the regression-line-percentile (RLP) technique be adopted as a standardized method for developing standard weight (Ws) equations, as good statistical consistency has been demonstrated for this approach. Equations should be modeled on 75th-percentile-weight data for consistent interpretation, and should be developed from data sets that represent the entire geographical range of a species. Standard weight should be considered a benchmark for comparison rather than a management target. Although the concept of a generalized “optimal” target range for Wr is attractive, Wr targets should be established for specific management objectives. Standard weight equations should be evaluated for length-related biases, and minimum applicable lengths for Wr use should be defined. Standard weight equations proposed should be published in peer-reviewed literature to facilitate effective application and evaluation by the management community. Application of rigorous criteria to development of Ws equations would standardize and clarify the theoretical basis of Wr applications. Further research is needed to clarify the relation of condition indices to other measures of population structure and dynamics, and to develop valid Ws equations for juvenile fish.