Life History Observations of Adfluvial Chinook Salmon prior to Reintroduction of Anadromous Salmonids

Abstract

Pacific salmon Oncorhynchus spp. vary in life history and degree of anadromy, but information on populations inhabiting only freshwater throughout their life cycle is limited. We confirmed the presence of a self-sustaining population of adfluvial Chinook Salmon O. tshawytscha upstream of century-old hydroelectric dams in the Skokomish River system, Washington. Snorkel, redd, and juvenile-trapping surveys revealed their life history attributes prior to planned reintroductions of anadromous salmonids above the dams. Adult Chinook Salmon in Lake Cushman (a reservoir) were large-bodied fish (mean length = 610 mm; mean weight = 5.4 kg), up to 4 years of age, and migrated into the inlet river to spawn in October and November. Annual peak counts of adult spawners were chronically low (35 or less) based on interannual snorkel surveys since 1994. Chinook Salmon parr (n = 780; mean length = 105 mm) of a single cohort were captured (June–August) at the upper dam and had faded parr marks and bright, silvery coloration typical of smolts. Our study confirmed that Chinook Salmon reared and reproduced exclusively in freshwater and suggested a life history pattern most closely resembling anadromous fall-run Chinook Salmon. We reviewed annual hatchery release records from 1926 to 2016 and found no evidence of Chinook Salmon being planted above the dams, in contrast to landlocked Chinook Salmon occurring in Oregon and California reservoirs. These adfluvial Chinook Salmon may be descendants of the native Skokomish River anadromous population, including fish from below the dam, and represent adaptations to the reservoir after dam construction. Lake Cushman Chinook Salmon are federally threatened, the only reported landlocked population in Puget Sound and coastal Washington, and may represent a unique evolutionary legacy worthy of protection.

Received March 7, 2017; accepted July 5, 2017 Published online October 20, 2017

ACKNOWLEDGMENTS

We thank many biologists for conducting snorkel and spawner surveys throughout the last 22 years. We also thank Matt Bleich, David Cogswell, Elisabeth Cordner, and Florian Leischner of Tacoma Power (juvenile data); Rick Endicott of Long Live the Kings (size and age data of adults); Joe Anderson and Catie Mains of Washington Department of Fish and Wildlife (hatchery release data); and Jim Myers of National Oceanic and Atmospheric Administration Fisheries (background information). Thanks to Pat Crain, Jeff Duda, Tom Quinn, and two anonymous reviewers for their helpful comments on an earlier version of this manuscript. Funding for this work was provided by Olympic National Park and Tacoma Power. The use of any trade names was for descriptive purposes only and does not constitute endorsement by the National Park Service. The opinions reflected herein are those of the authors and do not necessarily represent those of the Washington Department of Fish and Wildlife.