Search

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]

The availability of suitable habitat is often viewed as one of most important limiting factors for animal populations. In this study, we examined the composition and spatial distribution of stream habitat based on summer water temperature using airborne thermal imagery, floating temperature surveys, and fixed temperature data loggers in a regulated and unregulated segment of the Bear River in Idaho and Wyoming. We also used temperature-sensitive radio telemetry tags to measure water temperature in habitats used by Bonneville cutthroat troutOncorhynchus clarkii utah. We found that when available water temperatures increased in the Bear River, cutthroat trout in the regulated segment continuously selected cooler habitats that followed a similar rate of increase as available water temperatures. In the unregulated segment, cutthroat trout did not select significantly cooler available water temperatures when measured over the entire study period. However, cutthroat trout in the unregulated segment did select cooler water temperatures during the peak period of high water temperatures in July. In the regulated river segment, patches of habitat with hospitable water temperatures were small, infrequent, and widely distributed. Whereas, in the unregulated river segment, habitat patches with hospitable water temperatures were larger, more frequent, and in closer proximity to one another. Our data indicate that during the peak of summer temperatures the availability of habitat below the thermal maxima of cutthroat trout becomes very limited, but they are able to locate and use these small patches of habitat during the warmest part of the summer. Received December 5, 2011; accepted July 27, 2012 [ABSTRACT FROM PUBLISHER]