Use of Redd Counts to Detect Trends in Bull Trout (Salvelinus conf luentus) Populations

Abstract

Bull trout (Salvelinus confluentus) is a species of growing interest in the Pacific Northwest (J.S.). It was recently proposed for listing under the Endangered Species Act (U.S. Fish and Wildlife Service 1997); however, uncertainty and debate regarding the status of remaining populations continues. Biologists throughout the region have expressed concern regarding the loss of historical populations and the risks facing those that remain (Howell & Buchanan 1992; Rieman & McIntyre 1993), but published data demonstrating such problems are few. A variety of monitoring data are available from management agencies but, as with other species (e.g., Pechmann et al. 1991; Reed & Blaustein 1995), time series are often short or highly variable, potentially limiting the power of simple tests for trends in the data. As a result, in a recent public policy conference regarding the future management of bull trout and the lands influencing their habitats, some argued that there is little statistically supported evidence that populations are declining and therefore no clear risk of extinction. The most consistent and extensive monitoring information we could locate for bull trout is represented by redd (nest) counts maintained by the Idaho Department of Fish and Game and the Montana Department of Fish, Wildlife and Parks. We used the data provided by these agencies to determine whether the patterns within and among populations suggest any trends and to evaluate the difficulty of detecting potentially important trends. Bull trout associated with three large lake systems in the Clark Fork River basin of northern Idaho (Pend Oreille Lake) and northwestern Montana (Flathead and Swan lakes) exhibit a migratory life history (Rieman & McIntyre 1996) analogous to that of anadromous salmonids. Adults spawn and juveniles rear in tributary streams. Juveniles