Condition-Dependent En Route Migration Mortality of Adult Chinook Salmon in the Willamette River Main Stem

Abstract

Episodically high adult mortality during migration and near spawning sites has hindered the recovery of threatened spring-run Chinook Salmon Oncorhynchus tshawytscha in Oregon’s Willamette River basin. In 2011–2014, we assessed migration mortality for 762 radio-tagged adults along a ~260-km reach of the main stem of the Willamette River. Annual survival of salmon to spawning tributaries ranged from 0.791 (95% CI = 0.741–0.833) to 0.896 (0.856–0.926), confirming concerns about mortality in the migration corridor. In a series of general linear models, descaling, marine mammal injuries, and head injuries to adult Chinook Salmon were linked to reduced survival during migration to tributaries. Many injuries were minor (i.e., epidermal abrasions), which we hypothesize were unlikely to have caused direct mortality but may have increased salmon vulnerability to pathogens or other disease processes. Mortality in the main stem was not significantly associated with salmon body size, energetic status, sex, origin (hatchery, wild), river discharge, or water temperature metrics. The ~10–21% estimates of en route mortality in this study provide an important benchmark for the main stem of the Willamette River. The estimates complement ongoing efforts to quantify mortality of adult Chinook Salmon in Willamette River tributaries and after collection and transport to spawning sites above high-head hydroelectric dams.

Received September 15, 2016; accepted December 1, 2016 Published online March 3, 2017

ACKNOWLEDGMENTS

Many people assisted with the field work, data compilation, and administrative requirements for this study. University of Idaho (UI) personnel included Travis Dick, Charles Erdman, Dan Joosten, Karen Johnson, Matt Knoff, Steve Lee, Mark Morasch, Christopher Noyes, and Theresa Tillson. We thank the UI’s Institutional Animal Care and Use Committee for reviewing the animal procedures used in this study. Oregon Department of Fish and Wildlife (ODFW) personnel provided valuable support and included Todd Alsbury, Lisa Borgerson, Kanani Bowden, Brett Boyd, Ben Clemens, Tom Friesen, Stephanie Gunckel, Tom Murtagh, Dan Peck, Kelly Reis, Kirk Schroeder, Cameron Sharpe, Wayne Vandernaald, Jeff Ziller, and Tim Wright. We thank Stephanie Burchfield and Kim Hatfield (NOAA Fisheries) and Michele Weaver and Shivonne Nisbit (ODFW) for their assistance securing study permits and Bonnie Johnson (U.S. Fish and Wildlife Service) for coordinating the use of AQUI-S 20E. We also appreciate the contributions of Tim Shibahara (Portland General Electric), Doug Drake (Oregon Department of Environmental Quality), and the Eugene Water and Electric Board. Two anonymous reviewers provided constructive comments that substantively improved the paper. This study was conducted under Cooperative Ecosystems Study Unit (CESU) agreement W912HZ-12-2-0004 funded by the U.S. Army Corps of Engineers, Portland District, with assistance by Deberay Carmichael, David Griffith, Fenton Kahn, Rich Piaskowski, Glen Rhett, and Robert Wertheimer.