Monitoring and Managing Fishes that Are Invisible and Keep Moving Around: Influences of an Invasive Species and Environmental Factors on Capture Probability

Understanding fish population status and trends are fundamental to effective research and management. Challenges in understanding population status include recognizing and accounting for sources of variation in capture probability (p^) that can obscure patterns in count data and bias inferences about the population. In systems where management actions such as invasive species removals are implemented based on population triggers, errors in abundance estimation can propagate to missed management opportunities simply due to variation in p^, rather than actual population status. We assessed the relative importance of individual heterogeneity (species and fish size), environmental (stream discharge, temperature) and spatial variation, and interspecific interactions (density) on electrofishing p^ for native (Speckled Dace Rhinichthys osculus, Bluehead Sucker Catostomus discobolus, Flannelmouth Sucker Catostomus latipinnis) and invasive (Brown Trout Salmo trutta, Rainbow Trout Oncorhynchus mykiss) fishes. We fit closed‐population depletion models with combinations of covariates using 6 years of data collected during invasive salmonid suppression efforts. We found that the relative importance of spatially and temporally varying environmental influences on p^ differed between native and introduced species. Temperature was important for explaining variability in p^ for two of three native species, and discharge had a significant influence on p^ for both Brown Trout and Rainbow Trout. We also found that p^ declined with increasing Brown Trout density, which would limit precision and potentially bias abundance, for two of the three native species. These results illustrate the potential importance of interspecific interactions in influencing p^ and understanding implications of management actions, such as invasive species suppression. Our results demonstrate the need for cautious interpretation of uncalibrated catch‐per‐unit‐effort data for abundance‐triggered management actions, including those designed to promote conservation of endangered or economically valuable species.