LAUSR

This study sought to compare the survival of juvenile fall-run Chinook Salmon Oncorhynchus tshawytscha released from the U.S. Fish and Wildlife Service’s Coleman National Fish Hatchery using standard release practices to experimental releases occurring at earlier dates and smaller fish sizes, with the goal of providing managers with information to assess the trade-offs between experimental and standard release strategies. Coded-wire-tagged hatchery fall-run Chinook Salmon smolts were released at different times of the year, with releases occurring in April (standard timing) and March (early timing) during 2016, 2017, 2019, and 2020. A Bayesian modeling framework was used to investigate if fall-run Chinook Salmon smolt release timing was associated with variations in smolt-to-adult returns (SARs) while accounting for biotic and abiotic factors that were experienced by fish following release. Results indicated that release timing (day) was related to variation in SARs, with smolts released earlier in the year significantly more likely to return to the hatchery than those released at standard times in 3 out of 4 years with experimental releases. Results from the best-fitting model indicated that, on average, the odds of SARs decreased an estimated 30.6% (95% credible interval = 29.2–42.7%) for every 10 d later in the year smolts were released from the hatchery. Larger-sized smolts were more likely to return as adults to the hatchery, with the odds of SARs increasing by an estimated 5.9% (4.2–7.7%) for each 1-mm increase in mean fork length. Several environmental indices were also associated with SARs, including river flows, air temperature, the Pacific Decadal Oscillation, and ocean upwelling. Results suggest that releasing fish earlier in the year is a viable management tool to increase SARs of fall-run Chinook Salmon to Coleman National Fish Hatchery despite their smaller size at release. Benefits are especially great in years when environmental conditions in the Sacramento River are expected to be poor.