An Evaluation of Redd Counts as a Measure of Bull Trout Population Size and Trend

Abstract

AbstractThe use of redd counts to monitor abundance and trend of bull trout Salvelinus confluentus has been clouded by uncertainties concerning measurement error, life history variation, and correspondence of redd counts to adult population size. We compared census redd counts with population estimates of mature females for a migratory fluvial population of bull trout (primarily ≥ 300 mm fork length) and for a population of small (<200 mm), likely resident, bull trout. We also compared the measurement error of the experienced surveyors who conducted the redd counts to that of a group of inexperienced surveyors. Although the regression of redd counts on adult females for the migratory population was statistically significant, a large proportion of the variation in the relationship was unexplained (r 2 = 0.47). Despite that variation, redd counts accurately reflected a greater than 50% decline in the population over 10 years; however, 5‐year trends in redd counts could be misleading. Power analysis parameterized by using the variation in the number of females per redd and measurement error of experienced surveyors indicated that minimum declines of 44–56% or increases of 78–118% over 10–15 years would be necessary for detection using traditional statistical criteria. Geometric mean abundance of migratory adults derived from redd counts and adults‐per‐redd values from the present study and published averages were similar to measured adult numbers in most cases. For both migratory and resident populations, redd counts by experienced surveyors were substantially more accurate and precise than those by inexperienced surveyors. Counts of migratory bull trout redds were more accurate and precise than counts of resident bull trout redds, which were significantly smaller and consistently underestimated. Thus, bull trout redd counts can be used to estimate abundance levels and to detect substantial longer‐term changes in abundance, particularly for migratory populations. However, the reliability of the counts depends on the skill of the surveyors.Received April 4, 2011; accepted August 8, 2011