Efficiency of a quadrat sampling technique for estimating riffle-dwelling crayfish density

Quantitative sampling of crayfish can be challenging as a result of clustered distributions produced by a variety of factors. We tested the efficiency of a 1-m² quadrat sampler for estimating the density of riffle-dwelling crayfish. Sampling efficiency was evaluated in an Ozark Plateau stream through a mark-recapture study. We stocked three marked crayfish per square meter into 12 enclosed riffle sections (range, 46-53 m²) and then randomly sampled five 1-m² quadrats in each of the 12 sections. Sampling efficiency was determined by comparing the estimates of crayfish density to the known density of marked crayfish. We found crayfish sampling efficiency (mean, 69%) to be comparable to that in studies evaluating the quadrat sampler for stream fish, although efficiency across our study riffles ranged widely, from large underestimates (27%) to positively biased overestimates (140%). Coefficients of variation revealed that replicating habitat units within a stream or stream reach may provide more precise crayfish density estimates than replicating quadrat samples within a single habitat unit. None of the physical habitat or water chemistry variables that we measured were correlated with sampling efficiency. A power analysis of confidence interval (CI) precision demonstrated that moderate precision (i.e., CI half-width =20% of mean) was achievable at a reasonable sampling effort (40 quadrats). The 1-m² quadrat sampler can be effective for quantitatively sampling lotic crayfish. This information should benefit researchers and managers interested in estimates of crayfish density in streams.