Adfluvial Life History in Spring Chinook Salmon from Quartzville Creek, Oregon

Through spawning ground and snorkel surveys, we confirmed the presence of adfluvial spring Chinook salmon Oncorhynchus tshawytscha behind Green Peter Dam, a high-head dam in the upper Willamette River, Oregon. During our first survey year (2012), we recovered six carcasses of adfluvial Chinook salmon, identified nine live adults, and counted nine redds.  Scale analyses from carcasses revealed that adult fish were age 5 or 6, with no evidence of accelerated growth as associated with ocean entry for anadromous salmon. Otolith microchemistry results from an unmarked adult female Chinook did not indicate ocean residence, and no hatchery thermal marks were observed. In 2013, we observed one live, unclipped adult and several juvenile Chinook salmon.  We conclude that a small adfluvial population of spring Chinook salmon exists in Green Peter Reservoir. Although this is the first documentation of an adfluvial Chinook population in Oregon, a growing body of evidence suggests that similar populations may exist above other Willamette River reservoirs and needs to be considered in the context of ongoing research, monitoring, reintroduction, and recovery actions pertaining to threatened Willamette spring Chinook salmon. We recommend that future research and monitoring efforts be designed to identify and describe additional adfluvial populations.