Evaluating suitable thermal habitat for native and nonnative salmonids across multiple scales

The upper Big Hole River watershed in southwestern Montana supports populations of both native and nonnative cold-water salmonids. In this stream network, low summer discharge and warm stream water temperatures threaten the persistence of native Arctic grayling Thymallus arcticus, and harsh thermal conditions may exasperate negative interactions between grayling and introduced trout species. To evaluate the extent of suitable thermal habitat, spatial and temporal patterns of stream temperature were assessed 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 late 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 provide a baseline for evaluating potential effects of climate change, habitat restoration, and shifts in non-native salmonid distributions.