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

We examined precision of size, age, growth, and mortality parameters for fo ur reef fishes at sample sizes ranging from 25 to 1000 using bootstrapped p opulation samples. The results are illustrative rather than prescriptive in that we do not determine "optimum" sample sizes, but rather describe impro vements in precision with increasing sample size. Furthermore, we do not ad dress 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 Ber talanffy growth parameters were up to an order of magnitude more precise wh en constraints were placed upon the fitting process. Asymptotic lengths, L- infinity, were up to eight times as precise as Brody growth coefficients, K . Catch curves were generally less precise than two other mortality estimat ors, but we cannot advocate any estimator until accuracy is addressed. We p ropose a general rule of collecting an average of 7-10 fish per age-class t o estimate a variety of parameters. However, we more strongly suggest apply ing similar analyses for focal species and, where possible, with considerat ion of the application of parameters (e.g., sensitivity analyses).