Understanding the Significance of Redd Counts: A Comparison between Two Methods for Estimating the Abundance of and Monitoring Bull Trout Populations

Abstract

Abstract For salmonids that exhibit multiple life history forms within a single population, it may be necessary to evaluate the inconsistencies associated with population monitoring techniques. We compared mark– resight population estimates with those based on annual redd counts for bull trout Salvelinus confluentus in eastern Oregon. Our data suggest that across years, the trends in population estimates based on expanded redd count data and those based on the mark–resight method may be similar within basins. Across basins, however, there appear to be inconsistencies between mark–resight population estimates for different size-classes of bull trout and the expanded redd count data. In some systems, only the larger, potentially migratory fish are represented in redd counts, whereas in others some combination of small resident and large, potentially migratory fish is represented. The disparity between redd counts and population estimates for the reproductive population suggests that caution be invoked when choosing the monitoring techniques used to set recovery or monitoring goals for bull trout populations.