Estimating Abundances of Age-0 Rainbow Trout by Mark–Recapture in a Medium-Sized River

Abstract

Abstract We developed and evaluated a sampling methodology to obtain mark–recapture data to estimate abundances of age-0 rainbow trout Oncorhynchus mykiss in 70–125-m-wide reaches of the Henrys Fork of the Snake River, Idaho. Sampling by electrofishing was concentrated in sample areas that were 100 m long and extended from bank to bank; these areas were electrofished 3–5 times within periods of 3–17 d. Adjacent 50-m-long areas upstream and downstream were sampled to quantify movements out of the 100-m sample areas. We evaluated assumptions—closed population and equal catchability—using the field data, and we used simulation to identify the most appropriate abundance estimator for sparse data. Both closed and open population abundance estimators were evaluated. Most trout (84%) were recaptured in the area where they were marked, but about 10% had moved downstream and about 6% were recaptured upstream. Multistrata model analyses confirmed that apparent mortality rates, and hence movement rates, were low. Th...