Assisted migration of fluvial Bonneville Cutthroat Trout on the Cub River, Idaho

Coleman, T., DeRito, J., & Kennedy, P. (2024). Utah Assisted migration of fluvial Bonneville

Cutthroat Trout on the Cub River, Idaho. Utah Chapter of the American Fisheries

Society, St. George, Utah. DOI: 10.13140/RG.2.2.35945.67683

Abstract: The loss of available habitat and fragmentation has led to the decline of Cutthroat Trout (Oncorhynchus clarkii) populations and migratory life histories across the Western United States. On the Cub River (Idaho), a fish ladder and manual trap were installed in 2007 at an impassable dam to promote fluvial Bonneville Cutthroat Trout (Oncorhynchus clarkii utah) spawning migration and continue preventing upstream encroachment of non-native Brown Trout (Salmo trutta). The number of Cutthroat Trout trapped has declined from an average of 115 (range 31 to 275) for years 2007 to 2015, to 29 (range 9 to 41) for years 2020 to 2022. The goal of this study was to assess if a daily operation and maintenance schedule of the ladder versus previous years which were a three-times-per-week schedule, would increase the number of Cutthroat Trout trapped in 2023. The fish ladder was operated for ~10.5 weeks from May 5 until July 17 with a total of 38 fish caught, 25 (66%) Cutthroat Trout and 13 (34%) Brown Trout. Despite the increased personnel effort in 2023, the number of Cutthroat Trout trapped was less than the average of recent past years. The relatively low numbers of cutthroat may be explained in part by the record snowpack which led to an unusually high spring run-off event making fish access to the ladder at the start of the season unlikely. However, if low catch persists, either the ladder/trap or access to, are not functioning as intended, or the fluvial life-histories may be approaching extirpation.