Estimation of size at sexual maturity: an evaluation of analytical and resampling procedures

Size at 50% maturity (l50%) is commonly evaluated for wild populations as a point of biological reference (see Table 1). To estimate l50%, a sample of organisms known to have just reached sexual maturity could be available and their arithmetic mean size can be used as an estimator. However, the sample needed to obtain such a designbased estimator (Smith, 1990) for wild populations might be too expensive and would involve time-consuming histological procedures. Fisheries biologists prefer to conceive size at first maturity as the average size at which 50% of the individuals are mature. With this conception, the estimator is not based on a sampling design but on a model of the relation between body size and the number of individuals that are mature from a total number at each of many size intervals. The variance of a designbased estimator is determined by sampling design (Thompson, 1992). The variance of a model-based estimator is not as easily obtained. A sample of published works in the fisheries literature provides a measure of the frequency with which Estimation of size at sexual maturity: an evaluation of analytical and resampling procedures