The inclusion of passive interrogation antenna (PIA) detection data has promise to increase precision of population abundance estimates ([Formula: see text]). However, encounter probabilities are often higher for PIAs than for physical capture. If the difference is not accounted for, [Formula: see text] may be biased. Using simulations, we estimated the magnitude of bias resulting from mixed capture and detection probabilities and evaluated potential solutions for removing the bias for closed capture models. Mixing physical capture and PIA detections (pdet) resulted in negative biases in [Formula: see text]. However, using an individual covariate to model differences removed bias and improved precision. From a case study of fish making spawning migrations across a stream-wide PIA (pdet ≤ 0.9), the coefficient of variation (CV) of [Formula: see text] declined 39%–82% when PIA data were included, and there was a dramatic reduction in time to detect a significant change in [Formula: see text]. For a second case study, with modest pdet (≤0.2) using smaller PIAs, CV ([Formula: see text]) declined 4%–18%. Our method is applicable for estimating abundance for any situation where data are collected with methods having different capture–detection probabilities.
Evaluation of Fish Movements, Migration Patterns, and Population Abundance with Streamwidth PIT Tag Interrogation Systems, Final Report 2002.