We estimated size-specific capture probabilities of age-0 rainbow trout Oncorhynchus mykiss in the Lee's Ferry Reach of the Colorado River, Arizona, by backpack and boat electrofishing at discrete shoreline sites using both depletion and mark-recapture experiments. Our objectives were to evaluate the feasibility of estimating capture probability for juvenile fish in larger rivers; to determine how it is influenced by fish size, habitat, flow, density, and recovery period; and to test population closure assumptions. There was no mortality among the 351 rainbow trout that were captured by electrofishing, marked, and held for 24 h. Of a total of 2,966 fish that were marked and released, only 0.61% were captured outside of mark-recapture sites, and total emigration from mark-recapture sites was 2.2-2.6%. These data strongly suggest that populations within discrete sites can be treated as effectively closed for the 24-h period between marking and recapture. Eighty percent of capture probability estimates from 66 depletion experiments and 42 mark-recapture experiments ranged from 0.28 to 0.75 and from 0.17 to 0.45, respectively, and the average coefficient of variation of estimates was 0.26. There was strong support for a fish size-capture probability relationship that accounted for the differences in vulnerability across habitat types. Smaller fish were less vulnerable in high-angle shorelines that were sampled by boat electrofishing. There was little support for capture probability models that accounted for within-day and across-month variation in flow. The effects of fish density on capture probability were challenging to discern, variable among habitat types and estimation methodologies, and confounded with the effect of fish size. As capture probability estimates were generally precise and the closure assumption was met, our results demonstrate that electrofishing-based mark-recapture experiments at discrete sites can be used to estimate the abundance of juvenile fish in large rivers.
