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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]

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 (x̅ = 7.7 fish/100 m²) than where they were present (x̅ = 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]

Cutthroat trout Oncorhynchus clarkii × rainbow trout O. mykiss hybrids (hereafter hybrids) are difficult to visually distinguish from parental taxa, yet identifying phenotypic traits to separate hybrids from cutthroat trout is needed for conservation and management purposes. We compared phenotypic characteristics against genotype (using 34 species-diagnostic single nucleotide polymorphism loci) for 316 Bonneville cutthroat trout O. clarkii utah, rainbow trout and hybrids in Bear River tributaries. Our phenotypic classifications of fish were 91% accurate for Bonneville cutthroat trout but only 68% accurate for rainbow trout and hybrids combined. Classification errors based on phenotype were observed between parental taxa and hybrids but not between cutthroat trout and rainbow trout. The most useful phenotypic traits for distinguishing Bonneville cutthroat trout from hybrids were the absence of a white leading edge on the pelvic fin, the presence of fewer than 7 spots on the top of the head, and the presence of a prominent throat slash. The degree of hybridization in individual hybrids was associated with the number of spots on the top of their head. However, 16% of >F1 hybrids with a higher proportion of cutthroat trout ancestry and 6% of F1 hybrids exhibited all the phenotypic characteristics of cutthroat trout. The ability to visually detect admixture in hybrids was not related to fish length but was related to admixture level, with logistic regression model results predicting that, for individual hybrids when the proportion of alleles assigned to rainbow trout was >18% (95% CI, 11% to 26%), biologists were more than 50% likely to visually detect O. mykiss traits. While we encourage the use of genetic-based assessments for Bonneville cutthroat trout populations when feasible, our results suggest that phenotypic traits can assist in identifying hybridized populations and hybrid individuals, which will benefit the management and conservation of this species. However, our study included Bonneville cutthroat trout from only the Bear River basin, and further work is needed from the southern portion of the subspecies’ range to support or refute our findings. [ABSTRACT FROM AUTHOR]