Widespread human alteration of natural barriers threatens Pacific salmon biodiversity

Amy Fingerle , Matthew R. Sloat, William I. Atlas, Thomas W. Buehrens, Caleb N. Jetter, Michael R. Miller, Jonathan W. Moore, Maya Scanlon, Ted Grantham, Stephanie Carlson

Anthropogenic barriers such as dams are a pervasive threat to freshwater diversity, yet human-mediated increases in aquatic connectivity can also have unintended consequences. Natural, flow-dependent barriers such as waterfalls structure the upstream limits of Pacific salmon and steelhead by completely or partially obstructing passage. Across the Pacific Northwest, natural barriers were modified to improve passage for adult salmonids, yet this history is poorly recognized. Here we document natural barrier modifications from California to British Columbia. We present case studies that highlight unintended impacts of natural barrier modification to early-migrating runs, whose persistence is contingent on exclusive habitat access facilitated by natural barriers, and we discuss strategies for mitigating biodiversity impacts in modified systems. This study suggests that management and policy can more effectively conserve biodiversity by recognizing natural barrier modifications and the linkages between landscape features and salmon biodiversity.