Assessment of Bull Trout Passage during Operation of the Imnaha River Weir 2018 Annual Progress Report

Bull Trout are listed across their entire range in the U.S. (coterminously) under the Endangered Species Act as a threatened species. A potential threat to Bull Trout that has recently received considerable attention is the operation of weirs and the resulting influence on Bull Trout migrations. Some Bull Trout in the Imnaha River Core Area migrate from the Snake or lower Imnaha rivers, past a weir operating to collect Chinook Salmon, to spawning areas in the upper Imnaha River. A team of biologists investigated whether the operation of the Imnaha River weir impacted (based on previously derived benchmarks) Bull Trout during their spawning migration. Marking of bull trout with individual unique Passive Integrated Transponder (PIT) tags and monitoring their detections at PIT tag arrays were used to monitor the migratory behavior of Bull Trout when the weir was in operation. The percentage of Bull Trout that passed the weir (conversion) was estimated to be 93.8%, higher than the yearly benchmark (76.5%) and the 4-year running average benchmark (92.9%) for conversion. Overall, the median time it took Bull Trout to move from just below to just above the weir was one day. The benchmarks for median delay in June and July (6 and 4 days, respectively) were met, whereas the benchmark for median delay in August (2 days) was not. However, only a small portion (3%) of the individuals included in the analysis passed through the study area in August. Maximum delay in June (16 days), July (12 days), and August (25 days) exceeded the benchmark for maximum delay (8 days). Thus, in 2018, the operation of the weir may have slowed the rate at which some Bull Trout moved past the weir during their spawning migration. Overall, Bull Trout passage at the weir improved relative to that in 2017, perhaps due at least partly to improved passage conditions in openings at the base of the weir designed to provide passage for resident fish, and to the modification of the design of the trap entrance.