A Rapid Rangewide Assessment of Bull Trout Distributions: A Crowd-Sourced, Edna-Based Approach with Application to Many Aquatic Species

Cold-water salmonids in the northwestern U.S. have declined in many locations from an array of factors, including climate change. To address the need for reliable information about habitats that are suitable now and might be in the future, we developed the Climate Shield model that makes accurate (78–85%) and spatially explicit predictions about natal habitat for bull trout and cutthroat trout and identifies likely future habitats. This model identified over 5,000 potential natal habitats for bull trout, but thousands of these have rarely or never been sampled. To address this information gap, we developed an environmental DNA (eDNA) sampling method that provides precise, robust information about the presence of bull trout (and other species) that can be collected quickly and at low cost across a species' range. The method often has better detection efficiency than electrofishing, costs less, and takes less time; a single person can inventory an entire 6^th-code watershed in one day. Initial studies were aimed at precisely delineating the distribution of bull trout within select watersheds, as well as confirming their absence from potential habitats and discovering previously unknown populations. We are now using eDNA surveys to identify occupied bull trout habitat across its U.S. range.