Potential Effects of Climate Change on Native Cutthroat Trout throughout the Upper Colorado River Basin: Analyzing Thermal Habitat for Fragmented Populations

Analyzing how climate change may influence fish populations hinges on accurate projections for future climate patterns, models to relate these to fish habitat, and data on the current ecological context for target populations.  Within the Rocky Mountain West, warmer summer temperatures have already been recorded, and are predicted to increase by 2.7 °C in some areas by 2050.  Colorado River cutthroat trout (Oncorhynchus clarkii pleuriticus; CRCT) are native to the upper Colorado River basin, and are now restricted to 14% of their historic range.  Populations of CRCT have been highly fragmented by non-native fishes and various land uses, and most populations are isolated in high-elevation streams.  To investigate the synergistic effects of population fragmentation and potential changes in thermal habitat from future climate change, we used a detailed range-wide database of CRCT distributions and developed a predictive stream temperature model based on air temperature, empirical stream temperature data hydrology, and geomorphic characteristics.  Specifically, our stream temperature model was created from a database of stream thermographs from high elevation lotic systems (>1700m), NHD⁺ stream segment geomorphic attributes, daily stream flow records from USGS gage sites, and daily air temperature records from SNOTEL sites.  We focused our analysis on stream segments containing CRCT conservation populations (i.e., genetically pure, isolated from non-natives, or displaying unique life history characteristics). We also developed a set of eco-physiological criteria from existing lab and field results relating stream temperatures to potential growth, reproduction, and survival of CRCT.  Using these tools we suggest limited negative consequences of projected warming on CRCT thermal habitat.  However, some of the most fragmented populations restricted to short stream reaches may be more sensitive to even subtle changes in thermal conditions.  These results emphasize the importance of considering multiple sources of potential climate change influences on fish populations, an approach that should be paramount to current and future aquatic conservation efforts.