Size at Release, Size‐Selective Mortality, and Age of Maturity of Willamette River Hatchery Yearling Chinook Salmon

Abstract

We analyzed scales from returning Willamette River yearling Chinook salmon Oncorhynchus tshawytscha to explore the effects of size at release on subsequent adult returns. We tested the hypothesis that survival to adulthood is independent of size at ocean entry for yearling Chinook salmon. Significant size‐selective mortality, indicated by a larger size at ocean entry among surviving adults than among all released juveniles, was observed for Chinook salmon released in 2002–2004 but not for those released in 2005. Juvenile Chinook salmon released in 2002–2004 that entered the ocean at less than 150 mm in fork length were underrepresented in the returning adult populations. We also investigated the relationships between age at maturity and size at ocean entry, timing of release, circulus spacing, and size at the end of the first ocean year. We observed significant differences in fork length at the end of the first ocean winter among returning age‐4, ‐5, and ‐6 Chinook salmon; the younger returning fish were larger after their first year of ocean growth. Fork length at ocean entry and average first‐ocean‐year circulus spacing were significantly greater for age‐4 than for age‐5 and age‐6 Chinook salmon but were similar for age‐5 and age‐6 fish. Our analyses indicate that yearling Chinook salmon may experience size‐selective mortality, although the relationship between the intensity of size‐selective mortality and smolt‐to‐adult survival remains unclear.