How Will Climate Change Affect Cutthroat Trout?

Bonneville Cutthroat Trout

Cutthroat trout are the only native trout species across much of the Rocky Mountains and the Great Basin. Although historically abundant, their populations declined drastically during the 20th century due to habitat degradation and aggressive stocking of non-native species. Populations in many areas are now heavily fragmented and constrained to small areas of habitat. Climate-related changes across the western US are occurring especially rapidly and present a new set of management challenges for cutthroat trout.

To help address these challenges, Trout Unlimited (TU) initiated a large-scale study of how projected climate change will impact the distribution of inland cutthroat trout over the next 70+ years across much of the species' historic range. The study was funded by the National Fish and Wildlife Foundation, the US Geological Survey and the Forest Service and conducted in cooperation with the US Forest Service's Rocky Mountain Research Station (RMRS) and the University of Washington's Climate Impacts Group (CIG). Work was coordinated by Seth Wenger of TU, with other TU scientists including Helen Neville, Dan Dauwalter, and Jack Williams. Other team members included Dan Isaak, Charlie Luce, Bruce Rieman, Mike Young, Dave Nagel, Dana Horan and Gwynne Chandler of RMRS; Aland Hamlet and Marketa McGuire Elsner of CIG; Jason Dunham of the US Geological Survey; and Kurt Fausch of Colorado State University.

In the first phase of the study, researchers at CIG and TU used a macroscale hydrologic model to produce predictions of stream flows for historical conditions and future, climate change conditions. The result was the first broad-scale database of flow metrics for small and mid-sized streams across the western US. This is valuable because flows are important in determining suitable habitat for fish, and are predicted to change under climate change. A validation of the dataset was published in Water Resources Research [1]. The dataset itself can be downloaded here [2].

In the second phase of the study, the research team studied the role of climate in determining distributions of cutthroat trout, brook trout, and bull trout in the Interior Columbia Basin. They found that bull trout and brook trout showed high climate sensitivity, but cutthroat trout appeared to be less sensitive. However, cutthroat trout displayed a strong negative response to brook trout presence. This study was published in the Canadian Journal of Fisheries and Aquatic Sciences [4]

In the third phase of the study, the team evaluated the role of climate and other variables in determining cutthroat distributions of cutthroat trout, brook trout, and bull trout in the Interior Columbia, Upper Missouri, Upper Colorado, and Great Basin. They then projected distributions of the species under future climate scenarios. In contrast to the previous analysis, they found that cutthroat trout were likely to lose substantial habitat under climate change - an estimated 58%. Brook trout were predicted to lose 77% of their suitable habitat, brown trout 48%, and rainbow trout 35%. The total decline in habitat suitable for any of the four trout species was 47%. The results were published in the journal Proceedings of the National Academy of Sciences and can be downloaded here [5]. The article received substantial attention in the popular media, with articles in more than 50 newspapers, magazines, and web outlets, including the New York Times science blog, the Denver Post, Field and Stream, Science, Nature, the major national Canadian newspapers and even some outlets in Europe.

This plot shows the length of predicted suitable habitat for four species of trout under current conditions and under climate-change scenarios for the 2040s and 2080s.