Addressing depletion failure and estimating gear efficiency using back-calculation of capture probabilities

• Program MARK was used to back-calculate capture probabilities (p) of fish.
• We used a novel approach to address depletion failure and removal gear selection.
• Useful for identifying and avoiding issues associated with depletion failure.
• Optimal removal gear can be selected based on back-calculated estimates of p.
• Two pilot studies with fish in small systems were used to illustrate the approach.

Removal abundance (N) estimation methods are commonly used in fisheries but if capture probability (p) is not accurately estimated accurate estimates of N cannot be obtained. Further, if adequate depletions are not achieved during removal, this must be addressed to improve data collection and estimation procedures. Here, two disparate research projects are used as case studies to illustrate: (1) a method in Program MARK for back-calculating p estimated by the removal method following depletion failure; and (2) a Program MARK modeling approach used to estimate efficiency of gear deployed in removal studies. In the first case study, a depletion failure occurred in white sucker Catostomus commersonii populations sampled concurrently with rainbow trout Oncorhynchus mykiss populations using a weighted seine. Back-calculation of closed capture–recapture estimates of p was used to account for depletion failure and obtain per-pass estimates of p for both species. The second case study describes a pilot experiment to determine the efficacy of two removal gears deployed to estimate N of fathead minnows Pimephales promelas in cattle troughs. Back-calculation of p was used to select the appropriate gear to complete removal estimates in future studies. Results indicate that using Program MARK to back-calculate estimates of p allows researchers to detect problems associated with capture that may have resulted in depletion failure. Further, the method allows for comparisons of gear efficiency prior to full-scale experimentation.