Assessment of Bull Trout Passage during Operation of the Imnaha River Weir 2020 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 assess 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 91.9%, higher than the yearly benchmark (76.5%) for conversion. Through the 2020 migration year, the 4-year running average for conversion was 91.4%, slightly less than the benchmark (92.9%). Overall, the median time it took Bull Trout to move from approximately 50 m below to approximately 100 m above the weir was 1.0 days. The benchmarks for median delay in June, July, and August (6, 4, and 2 days, respectively) were met. Maximum delay in June (43 days) and July (13 days) exceeded the benchmark for maximum delay (8 days), whereas maximum delay in August (1 day) met the benchmark. In addition, Bull Trout detected moving upstream to within approximately 50 m of the weir were more likely to exhibit milling behavior than those detected at sites approximately 100 m upstream and approximately 43 km downstream of the weir. Thus, in 2020, the operation of the weir may have slowed the rate at which some Bull Trout moved past the weir. Overall, Bull Trout passage at the weir was similar to that in 2018 and 2019 and improved relative to that in 2017. These improvements were 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 in 2018.