Evaluating active and passive sampling methods to quantify crayfish density in a freshwater wetland

We evaluated the sampling efficacy of 1-m(2) throw traps (active sampler) and baited minnow traps (passive sampler) across an experimental density gradient (1, 2, 4, 6, 8, 10, and 15/m(2)) of the slough crayfish (Procambarus fallax) in 2 trials with different crayfish populations. In both trials, throw-trap density estimates were highly correlated with actual crayfish density (r(2) = 0.96). The form of the relationships between density estimates and stocked densities was consistent between trials, and indicated that throw traps captured a similar proportion of the stocked crayfish regardless of the stocked density. When we adjusted the relationships to account for clearing efficiency (proportion of captured animals actually recovered from the trap), the slopes of the regressions were not significantly different from 1 in either trial. Size distributions and sex ratios of crayfish collected by the throw traps accurately reflected those of the stocked populations. Baited minnow traps performed inconsistently between the 2 trials. Catch-per-unit-effort (CPUE) and density were significantly correlated only in Trial 2 (r(2) = 0.82). The slope of the regression in Trial 2 (0.621) was significantly < 1, and the intercept was positive and nearly significant (p = 0.074), indicating that minnow traps captured increasingly smaller proportions of the stocked crayfish as the stocked density increased (i.e., differences between CPUE values underestimated actual differences between stocked densities along the gradient). Minnow traps were biased toward capturing large male crayfish, but the form of the relationships between CPUE and density did not improve when large-male CPUE was used in the regressions. Our results suggest that 1-m(2) throw traps provide better estimates than baited minnow traps of crayfish densities in shallow vegetated habitats.