The relative influence of temperature versus geomorphic factors on invasion potential of brown trout (Salmo trutta) in a mountain stream

Expansion of brown trout (Salmo trutta) into higher elevations of mountain rivers due to climate change or habitat alterations could negatively impact native fish species.  The Logan River, Utah is a coarse-bed, mountain stream in which brown trout are nearly absent from higher elevations dominated by native Bonneville cutthroat trout (Oncorhynchus clarki utah) despite the absence of known physical barriers or physiological limitations. We hypothesize that relationships between geomorphic-hydrologic factors that change along an elevational gradient, rather than temperature-related factors, most affect the distribution of brown trout.  At 20 representative reaches, we estimated annual, winter, and summer water temperatures.  Using measurements of bankfull discharge, pebble counts, and longitudinal profile surveys, we also estimated geomorphic-hydrologic variables including reach stream power, reach D50, and % spawning gravels. While both temperature and hydrologic-geomorphic factors showed significant relationships to brown trout spawning densities along at least a portion of the river, no one factor alone explained the entire distribution.  Particularly at higher elevations, the presence of tributaries, side channels, and backwaters may augment habitat for brown trout, allowing the species to thrive in patches with less suitable habitat conditions.