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Volume 1 (Issue #3), 1993 STOCK DENSITY INDICES ig diaries had a size structure similar to electrofishing samples. Anderson listed objective ranges for PSD of largemouth bass and bluegill caught by g for various pond management strategies that could be used by Missouri owners. SELECTION OF SAMPLING SITES Biologists often choose subjective sampling sites based on the likelihood of captur- ing target species. One justification for such a technique is that greater sample size generally can be obtained with less effort. However, Hubbard and Miranda (1986) found that PSD and RSD of largemouth bass samples were overestimated when electrofishing took place in subjectively selected sites compared to randomly selected sites. Therefore, biologists should balance the need for sufficient sample site at lowest necessary effort with the potential for resulting size structure bias. The biases we have discussed do not preclude the proper use of stock density indices. Many other assessment tools used by fisheries biologists also can be affected b~ biased samples. A good example would be mortality rates. Length-related biases due to sampling gear or seasonal influences would result in overestimation or urhderestimation of mortality rates. Biologists should be aware of these biases, deal w}th as many as possible by selection of appropriate sampling gear and date, and carefully interpret their data with consideration of the biases. Similarly, biased length-frequency data can be useful as trend data over time, as long as a biologist properly interprets these data. D~ SAMPLE SIZE I Sa;tnple size is an important consideration when using stock density indices to quantify length-frequency data and assess fish populations. In our opinion, people often calculate stock density indices from insufficient sample sizes. For example, few biologists would think of showing alength-frequency histogram from a sample of 13 fish -especially in a refereed publication. However, biologists are often willing to calculate, report, and use a PSD from that sample of 13 fish. Two methods are available to easily determine whether sufficient sample size has been obtained to reliably calculate stock density indices. Weithman et al. (1979) reported a method by which sequential samples of a fish population could be assessed for reliability using confidence intervals. Once the sample was deemed reliable, sampling could stop. Management biologists may not have the time available to sequentially assess their samples and determine whether more effort is needed. Often, a certain effort is allotted to a body of water, and the resulting data must be assessed. Thus, we suggest that biologists under such constraints use the tables provided by Gustafson (1988) to obtain confidence intervals for stock density indices. Weithman et al. (1979) described the calculation of confidence intervals, and Gustafson (1988) provided tables for simple estimation of 80 or 95% confidence intervals that can be used for PSD or any other RSD. Biologists using these confidence intervals would at 'least know how much confidence to place in their data and what type of change would have to occur to statistically indicate a change in the population size structure. r 211