Abstract:
The distribution of Lahontan cutthroat trout Oncorhynchus clarki henshawi was assessed
in a high-desert stream in southeastern Oregon where beaver Castor canadensis are
abundant. Longitudinal patterns of beaver ponds, habitat, temperature, and Lahontan
cutthroat trout age group distribution were identified throughout Willow Creek. Three
distinct stream segments were classified based on geomorphological characteristics. Four
beaver-pond and four free-flowing sample sections were randomly located in each of the
three stream segments. Beavers substantially altered the physical habitat of Willow Creek
increasing the depth and width of available habitat. In contrast, there was no measurable
effect on water temperature. The total number of Lahontan cutthroat trout per meter was
significantly higher in beaver ponds than free-flowing sections. Although density (fish/m²) showed no statistically significant (P<0.05) increase, values in beaver ponds were
two-fold those of free-flowing sections. Age-1 and young-of-the-year trout were absent or
in very low numbers in lower Willow Creek because of elevated temperatures, but high
numbers of age-2 and 3 (adults) Lahontan cutthroat trout were found in beaver ponds
where water temperatures reached lethal levels (>24°C). Apparently survival is greater in
beaver ponds than free-flowing sections as temperatures approach lethal limits.
