Why Catch-Per-Unit-Effort is Insufficient as an Index of Abundance for the White Sands Pupfish (Cyprinodon tularosa)

Authors: Damon Peterson, Colleen A. Caldwell

Year: 2016 (xlviii)

Abstract

An important goal amongst researchers and managers is to accurately quantify population parameters to detect shifts in survival and abundance. Some organisms represent a monitoring challenge when they are locally abundant, short-lived, and alter their reproductive strategy as environmental conditions vary. Here, we present evidence that the use of minnow traps combined with a commonly calculated index of abundance for pupfish (i.e., catch-per-unit-effort CPUE) can be affected by basic methodological choices such as trap soak time (i.e., effort) as well as the timing and location of trap placement. We investigated movement into and out of torpedo-shaped (Gee) traps by White Sands pupfish (Cyprinodon tularosa) and the effect movement would have on CPUE. White Sands pupfish exited traps at frequencies that were nearly equivalent to the rate of entering, suggesting total catch and thereby CPUE, are strongly influenced by factors that determine the probability of an animal exiting the trap. In add ition, CPUE was weakly correlated with a more robust estimate of abundance (Lincoln-Peterson estimator) suggesting CPUE is not a good index for abundance especially when trap soak times are variable. We recommend the use of capture-recapture protocols to account for differences in capture probabilities due to factors unrelated to abundance such as habitat and capture effort.