Predicting Effects of Land Use and Climate Change on Availability of Suitable Thermal Habitat for Native and Nonnative Salmonids

Climate in the western United States is expected to become warmer and more variable during the next century, and concurrent demands for watershed resources are expected to increase. Accordingly, understanding how changes to broad-scale factors (such as climate) and local-scale factors (such as land use) affect local aquatic ecosystems has become a fundamental concern for fisheries ecologists and managers. In addition, a majority of salmonid populations in the western United States currently persist in fragmented stream networks, and these coldwater fish may be particularly vulnerable to changes in thermal habitat. In this study, we assess the availability of suitable summer stream temperatures for native (Arctic grayling and mountain whitefish) and nonnative salmonids (brown trout, brook trout and rainbow trout) in a mountain-valley stream network. Past climate and stream monitoring data were collected to assess trends in summer air temperature, stream discharge, and stream temperature. To evaluate the present extent of suitable thermal habitat, stream temperatures were surveyed with a combination of remote sensing (thermal infrared-TIR), continuous longitudinal temperature profiling, and fixed-station water temperature loggers. Thermal infrared imaging and continuous longitudinal profiling were cost-effective methods for evaluating explicit spatial patterns in temperature data at multiple spatial scales, and fixed temperature loggers enabled an explicit evaluation of temporal variation in stream temperature. Using this combination of data, we characterized the distribution of thermally suitable habitat during the summer and identified associated habitat attributes (e.g., riparian vegetation, channel morphology, and discharge). Suitable thermal habitat was patchily distributed throughout the study area, and cold-water tributaries likely provide critical cold-water thermal refugia. These spatially and temporally explicit thermal data were incorporated into statistical stream temperature models, and future changes in thermal habitat were assessed based on regional climate change predictions. Under scenarios of increasing and more variable summer air temperatures, we predicted a significant increase in the occurrence of stream temperatures that exceed chronic and acute thermal tolerance thresholds for salmonids. Evaluating spatial patterns in observed and predicted stream temperatures allowed us to identify areas critical to maintaining suitable coldwater habitat in the future.