Age Structure and Impacts of Release Timing on Adult Returns of Coded-Wire Tagged Spring Chinook Salmon in the Upper Willamette River Basin

Twelve-million coded wired tagged spring Chinook salmon were released from upper Willamette River (UWR) hatcheries for brood years 2000 through 2008. Using the Regional Mark Information System and statistical software package R we looked for impacts of fall/spring release timing on smolt to adult returns (SARs), age structure, and stray.

Our analysis demonstrated that release timing had a subbasin-specific impact on SARs, and a basin-wide impact on age structure. South Santiam subbasin fall releases returned at a higher rate than spring releases. McKenzie subbasin spring releases returned at a higher rate than fall releases. Basin-wide, fall releases returned significantly younger than spring releases.

Generally, stray rates for UWR reared spring Chinook salmon were under 5%. South Santiam releases strayed to the spawning grounds and were caught in fisheries at a higher rate than other releases. Other releases with high stray rates have since been modified or eliminated.

The results from this project suggest that SARs for UWR hatchery spring Chinook salmon are subbasin specific and are informative for ensuring hatchery releases reflect mitigation goals. Further research could look at the impact of age on stray rates, and evaluate the impact of in-river conditions at time of release on SARs.