A Power Analysis on the Monitoring of Bull Trout Stocks Using Redd Counts

Abstract

The bull trout Salvelinus confluentus is listed as a federally threatened species in the Columbia and Klamath river drainages. A priori establishment of levels of decline or increase in bull trout redd numbers that will be considered biologically significant and levels of statistical significance that will be used to identify changes in redd numbers is essential to the success of future recovery plans. A prospective statistical power analysis indicates that with standard significance levels and two-tailed testing procedures the yearly variation in redd numbers typifying many stocks of bull trout in Montana limits the power of detecting less than 50% of changes in population size per generation to less than 0.8 during the first 15 years of a monitoring program. The limitations of monitoring bull trout stocks with redd counts, coupled with the critical nature of identifying future population changes, justifies the need to (1) identify and reduce the level of measurement error involved in redd counts, (2) use levels of statistical significance that adequately balance the risks of committing type I and type II errors, (3) use one-tailed testing procedures for identifying population declines during the initial and other critical years of a monitoring program, and (4) explore the use of other methods of monitoring.