Postrelease performance of hatchery yearling and subyearling fall Chinook salmon released into the Snake river

Two rearing treatments are used at Lyons Ferry Hatchery to produce yearling (age-1) and subyearling (age-0) fall Chinook salmon Oncorhynchus tshawytscha for supplementing production of wild fish in the Snake River. We compared four indicators of yearling and subyearling postrelease performance, namely, seaward movement, condition factor, growth rate, and survival. A standard rearing treatment was used to grow yearlings slowly for 14 months to sizes of 152-162 mm (mean fork length). A second standard rearing treatment was used to grow medium subyearlings at a moderate rate for 5 months to 84-89 nm. Two modified rearing treatments were used to produce large subyearlings that were grown rapidly to 90-103 mm and small subyearlings that were grown slowly to 70-76 mm. We released yearlings in April and subyearlings in June on the typical supplementation schedule. Seaward movement, condition factor, growth, and survival varied among rearing treatments. Yearlings moved seaward fastest for reasons related to their large size. Yearlings had the lowest postrelease condition factors and growth rates because they were released when temperatures were cool and they moved seaward quickly, spending little time to feed. Yearlings had the highest survival because they were released when the water was cool, they moved seaward quickly, and their large size reduced susceptibility to predation. Small subyearlings moved seaward the slowest because of their small size and slow growth before release. Small subyearlings had the highest postrelease condition factors and highest growth rates because they lingered and were exposed to relatively high temperatures that were favorable for growth. Small subyearlings had the lowest survival because they lingered and were exposed to low flow and warm water for long durations and their small size increased susceptibility to predation. We conclude that fall Chinook salmon performance after release from Lyons Ferry Hatchery is influenced by release date and by rearing treatment effects on size, prerelease growth rate, and postrelease behavior.