Estimating the Abundance of Adfluvial Bull Trout Spawning in Cougar Creek

In response to a general decline in abundance across their native range, Bull Trout (Salvelinus confluentus) were listed as threatened under the Endangered Species Act in 1999. Gaining a better understanding of the reproductive component of a population is important for Bull Trout recovery and persistence. Accurately monitoring the trend in abundance of spawners is essential to inform future management actions that may affect populations in the North Fork Lewis River subbasin. To estimate the abundance of the adfluvial Bull Trout spawning population, a resistance board weir and underwater video system were operated on Cougar Creek, a tributary to Yale Reservoir on the North Fork Lewis River, from July 29, 2020 through November 12, 2020. A total of 36 observations of adults moving upstream through the weir were recorded primarily from mid-August through late September, with the peak occurring in mid-September. The Bull Trout observed were all relatively large, migratory fish and length estimates derived from laser scaling ranged from 507 – 790 mm FL (mean, 695 mm FL). Since an individual could pass the weir multiple times during the spawning season, the number of recorded upstream observations may have been an overestimate of the true population size. To address this concern, we used PIT tag detections in addition to a photo-identification technique to allow recognition of individuals based on natural marks, such as colors, spots, scars, and fin shapes and to estimate the number of individuals that passed upstream of the weir. The estimated total number of spawning Bull Trout in Cougar Creek during 2020 was 34 unique individuals. The estimated number of males and females in the spawning population was 14 and 20, respectively. These data, combined with a redd count of 27 during 2020, suggest a spawner/redd ratio of 1.3. In future years, additional PIT-tagging efforts and improved PIT tag detection capability should further improve population estimates.