Salmonids, a group of coldwater adapted fishes of enormous ecological and socio-economic value, historically inhabited a variety of freshwater habitats throughout the Pacific Northwest (PNW). Over the past century, however, populations have dramatically declined due to habitat loss, overharvest, and invasive species. Consequently, many populations are listed as threatened or endangered under the U.S. Endangered Species Act. Complicating these stressors is global warming and associated climate change. Overall, aquatic ecosystems across the PNW are predicted to experience increasingly earlier snowmelt in the spring, reduced late spring and summer flows, increased winter flooding, warmer and drier summers, increased water temperatures, and expansion of invasive species. Understanding how effects of climate change might influence habitat for native salmonid populations is critical for effective management and recovery of these species. Scientists at the USGS and University of Montana used novel techniques and empirical data to study how climate change may drive landscape scale impacts that affect freshwater habitats and populations of key salmonid species (bull trout, cutthroat trout, and steelhead) throughout the PNW. Results showed strong linkages between climatic drivers (temperature and flow regimes) and the distribution, abundance, and genetic diversity of native salmonids across the PNW. Specifically, warming temperatures and shifting flow regimes are expected to fragment stream systems and cause salmonids to retreat upstream to headwater areas, thereby decreasing fish population abundance and genetic diversity, both of which are critical for persistence in a changing landscape. Climate-change‐induced periods of decreasing spring snowmelt and increases in stream temperatures are likely to decrease native biodiversity by fostering cross-breeding between invasive and native trout species. The study also developed a new framework for assessing the vulnerability of freshwater species to climate change and other stressors in complex stream networks, which will aid managers in proactively implementing conservation programs to increase resiliency and adaptive capacity of aquatic species.
