Bull trout and Pacific lamprey occupancy in the Imnaha River Subbasin, 2016 Final Report

Abstract ─ The goal of the U.S. Fish and Wildlife Service’s studies in the Imnaha River subbasin is to provide information that can be used to inform Bull Trout Salvelinus confluentus recovery and Pacific Lamprey Entosphenus tridentatus conservation efforts.  In 2016, we collected environmental DNA (eDNA) samples in the Imnaha River subbasin to determine if Bull Trout eDNA would be detected in a stream known to support a local Bull Trout population (Big Sheep Creek) and two streams where Bull Trout occupancy was unknown (Horse Creek and the South Fork Big Sheep Creek upstream from a culvert), or if Bull Trout eDNA would be detected in a stream where Bull Trout were known to be absent (Skookum Creek).  We also collected eDNA samples at the mouths of the Imnaha River and Horse, Lightning, and Cow creeks, which are the largest tributaries in the lower Imnaha River, to test for the presence of Pacific Lamprey eDNA.  All of the sites sampled in Big Sheep Creek, and none of the sites sampled in Skookum Creek tested positive for Bull Trout eDNA, providing limited evidence eDNA techniques may yield accurate results.  No Bull Trout eDNA was detected at the sampling sites in Horse Creek, suggesting it may not support a local Bull Trout population.  Bull Trout eDNA was detected just upstream from a culvert near the mouth of the South Fork Big Sheep Creek.  Efforts should be taken in the future to determine if a local Bull Trout population actually exists upstream from that culvert.  No Pacific Lamprey eDNA was detected at any of the sampling sites analyzed for Pacific Lamprey eDNA.  Thus, while Pacific Lamprey are occasionally captured in a rotary screw trap in the Imnaha River near its mouth, it appears they do not inhabit the Imnaha River subbasin continuously or extensively.