Characterizing fish populations: Effects of sample size and population structure on the precision of demographic parameter estimates

Abstract

We examined precision of size, age, growth, and mortality parameters for four reef fishes at sample sizes ranging from 25 to 1000 using bootstrapped population samples. The results are illustrative rather than prescriptive in that we do not determine "optimum" sample sizes, but rather describe improvements in precision with increasing sam-ple size. Furthermore, we do not address the related issue of accuracy. In general, a sample size needed to be tripled to halve precision at that sample size. Mean lengths and ages were most precise, reaching 10% by a sample size of 75 for all species. von Bertalanffy growth parameters were up to an order of magnitude more precise when constraints were placed upon the fitting process. Asymptotic lengths, L∞ , were up to eight times as precise as Brody growth coeffi-cients, K. Catch curves were generally less precise than two other mortality estimators, but we cannot advocate any es-timator until accuracy is addressed. We propose a general rule of collecting an average of 7-10 fish per age-class to estimate a variety of parameters. However, we more strongly suggest applying similar analyses for focal species and, where possible, with consideration of the application of parameters (e.g., sensitivity analyses).