DARR 2.0: Updated Software for Estimating Abundance from Stratified Mark-Recapture Data

摘要

Estimating abundance of downstream migrants in anadromous salmonid populations typically relies on trapping a portion of the population as it passes a fixed point on a stream and expanding this count using capture probabilities estimated with mark-recapture techniques. Numerous factors cause the probability that an individual passing the trap will be captured to vary over time, and such variation, if not accommodated in the analysis, can seriously bias the resulting estimate of abundance. Likewise, substantial time intervals between the release of marked fish and their subsequent susceptibility to recapture can bias abundance estimates if such intervals are not measurable and accounted for in the analysis. Stratified mark-recapture experiments, in which marks applied to individuals are changed over time so that recaptured individuals can be assigned to the period in which they were released and recaptured, allow the use of statistical estimators that explicitly accommodate variation in capture probability and the distribution of marked individuals among the sampling periods or strata. However, issues related to small sample sizes can hinder the application of such estimators in small populations, particularly in cases where time intervals between release and recapture can be substantial$ such conditions are commonly observed in populations of coho salmon (Oncorhynchus kisutch) and steelhead (O. mykiss) in small coastal streams. DARR 2.0 updates a software application for estimating abundance from stratified mark-recapture data sets. DARR 2.0 applies a series of algorithms to a stratified mark-recapture data set to aggregate strata as necessary to yield an admissible estimate of abundance (using the Darroch (1961) stratified-Petersen estimator) while preserving as much structure as possible in the data. This software is provided to encourage and support the use of statistically rigorous abundance estimators in monitoring programs that focus on estimating the abundance of downstream migrants in populations of anadromous salmonids, but may also be used to analyze other temporally or spatially stratified mark-recapture data sets in which the number of tagging and recapture periods are equal.