Adult Returns of Subyearling and Yearling Fall Chinook Salmon Released from a Snake River Hatchery or Transported Downstream

Abstract We compared the release-to-adult returns of coded-wire-tagged groups of fall chinook salmon Oncorhynchus tshawytscha in a 2 × 2 factorial experimental design: subyearlings and yearlings released directly from a hatchery versus those barged below two main-stem hydroelectric dams on the Snake River, Washington. Releases comprised six brood years over a 6-year period. In every release year, chinook salmon released as yearlings performed better than subyearlings; this was true both for returns to the Snake River and for their contribution to Pacific Ocean and lower Columbia River fisheries. We found no significantly consistent differences in rates of return between transported and on-station releases for either age-class. Return rates of both subyearlings and yearlings varied significantly with year of release. Median migration speeds of branded chinook salmon released on-station varied directly with Snake River flows. However, there was no general relation between flow (and attendant spill) in the S...