Trends in the abundance of Bonneville cutthroat trout and nonnative trout in the Bear River basin of Idaho.

Bonneville cutthroat trout (Oncorhynchus clarkii utah) have experienced substantial declines in their historical distribution and abundance, and recent status assessments have noted a particular lack of information on abundance trends for the species. From 1993 to 2020, a total of 186 backpack electrofishing surveys were conducted across 34 index reaches to monitor abundance of Bonneville cutthroat trout and nonnative salmonids in southeastern Idaho streams. Trout abundance (all species combined) for fish ≥100 mm (total length) averaged 7.6 fish/100 m2 of stream (3.1 fish/100 m). Bonneville cutthroat trout population growth rate (λ) was generally stable through time (mean λ = 1.04 across all reaches; 95% confidence interval, 0.98-1.10), whereas for nonnative trout considered collectively, estimates of λ in general were declining over the entire study period (mean λ = 0.93; 0.89--0.97). Although mean density of Bonneville cutthroat trout was higher at reaches where nonnative trout were absent (x = 7.7 fish/100 m²) than where they were present (x = 4.2 fish/100 m²), estimates of λ for cutthroat trout were not related to the abundance of nonnative trout. Bonneville cutthroat trout λ was also unrelated to all the reach-scale environmental conditions we measured except for conductivity, which was positively associated with λ. Conductivity is normally associated with the productivity of a water body, but it is also correlated to other important cations and anions (e.g., alkalinity and water hardness) that can influence fish populations in a number of ways; thus, we cannot ascertain whether the relationship we observed was causative or correlative. Bonneville cutthroat trout abundance was higher in years when both winter and summer discharge were higher in the previous year, which concurs with a large body of literature demonstrating that reduced baseflow during winter or summer can adversely affect salmonid recruitment, food resources, predatory avoidance, survival, and stream habitat conditions. [ABSTRACT FROM AUTHOR]