Life History Pathways of Wild Spring Chinook Salmon in the Willamette River Basin, Oregon

Spring Chinook salmon from the Willamette River basin follow a diversity of life history pathways between fry emergence and migration to the Columbia River estuary as smolts. We quantified the number of juveniles following different pathways by sampling and tagging juveniles in different rearing habitats throughout the basin. Newly emerged fry were caught in spawning areas and in mainstem habitats as far as 150–200 km from spawning areas. Juvenile Chinook salmon were captured with seines in habitats that ranged from headwater spawning areas to the Willamette River and fish were implanted with passive integrated transponder (PIT) tags to track migration. Scales were sampled from adult carcasses in spawning areas and analyzed to determine if fish entered saltwater before the first annulus was formed (subyearling or age-0 smolt) or after annulus formation (yearling or age-1 smolt). We found that the proportion of returning adult salmon with a subyearling or yearling life history varied among years and subbasin populations, indicating that a variety of life history tactics can contribute to the overall survival and resilience of a population.  Differences among brood years were compared to environmental conditions experienced as juveniles to investigate factors that may influence growth and freshwater migration patterns.