Cooperative Fish and Wildlife Research Units Program: Colorado
Education, Research and Technical Assistance for Managing Our Natural Resources

Trout species often segregate along elevational gradients, yet the mechanisms driving this pattern are not fully understood. On the Logan River, Utah, USA, exotic brown trout (Salmo trutta) dominate at low elevations but are near-absent from high elevations with native Bonneville cutthroat trout (Onchorhynchus clarkii utah). We used a Bayesian, spatial modeling approach to evaluate abiotic conditions (describing mechanisms related to temperature and physical habitat) potentially influencing the distribution of brown trout in this system. We also considered the influence of propagule pressure, including the intensity of past stocking pressure and distance from a concentrated stocking area. The distribution of brown trout was best explained by models describing: 1) average summer temperature interacting with unit stream power, our measure of gravel availability for spawning (R2=0.54), and 2) the prevalence of anchor ice between each reach and a high-intensity stocking area, also interacting with our measure of gravel availability (R2=0.52). Based on these findings, effects of cold summer temperatures on age-0 survival and/or the lack of movement upstream due to anchor ice are likely mechanisms contributing to low abundance of brown trout at high elevations. The potential role of anchor ice in affecting longitudinal distributions of brown trout has not been well-explored in other research. These findings have implications for future brown trout expansion upstream given some evidence suggesting anchor ice prevalence on the Logan River has decreased significantly over the last several decades, likely in response to climatic changes.