Interreplicate variance and statistical power of electrofishing data from low-gradient streams in the southeastern united states

I assessed the relationship between interreplicate variance and the mean for electrofishing catch per unit effort (defined as number of fish per 100 m² of stream surface area) from 56 sites in 22 first- through fourth-order streams on the coastal plain of South Carolina. Variance and mean were strongly and positively related. This relationship was unaffected by type and quantity of instream structure and was similar among schooling and nonschooling species. Interreplicate variance was unaffected by replicate reach lengths ranging from 60 to 130 m. Replicate lengths near the lower end of this range required less total effort (number of replicates times replicate length) than longer replicates for comparable precision. Statistical power computations indicated that large numbers of samples (>25) were required to detect small (<20%) differences among means, and that the required number of samples was greater when catch per unit effort was low, indicating that power can be increased if multiple passes are used to increase the catch. However, it required less total time to use a relatively large number of single-pass replicates than a smaller number of multiple-pass replicates to achieve a given level of precision. Investigation of several types of data transformations indicated that the log transformation was most efficient at reducing the correlation between the variance and the mean, as required for parametric statistical analysis.