PNNL

Abstract

Hatchery-reared steelhead Oncorhynchus mykiss that do not emigrate as smolts shortly after release may negatively impact wild fish communities through ecological interactions. We used systematic, stratified snorkeling surveys to document the relative abundance of wild rainbow trout O. mykiss, bull trout Salvelinus confluentus, and westslope cutthroat trout O. clarki lewisi as well as the upstream limit of residual hatchery steelhead (hatchery-reared steelhead that had failed to emigrate before June 1). Our objective was to determine whether residual hatchery steelhead had migrated upstream from their release point into an area containing a threatened population of bull trout and cutthroat trout. Hatchery steelhead made up a larger portion of the salmonid community in the sites near their release location (mean = 52.5%, range = 29-79%), and constituted a lower proportion (mean = 4.8%, range = 0-14%) of the salmonid community as distance upstream of the release location increased. However, residual hatchery steelhead had migrated over 12 km upstream into an area containing a threatened stock of bull trout and westslope cutthroat trout O. clarki lewisi.