Abstract Objective Age of maturation in Chinook Salmon Oncorhynchus tshawytscha is phenotypically plastic, influenced by both genotype and environmental factors. Salmon hatchery programs often rear fish under accelerated growth regimes using high-lipid diets that can result in younger age at maturity, including increased prevalence of age-2 males (minijacks). In the first phase of this investigation (previously published), we demonstrated that minijack prevalence could be decreased by up to 65% through a combination of reduced feeding frequency and dietary lipid. The goal of the second phase of this investigation was to report on the dietary treatment effects on (1) juvenile travel time and apparent river survival, (2) adult recovery rates and demography, and (3) the relationship between the prevalence of minijacks and adult recovery. Methods Juvenile Umatilla River hatchery fall Chinook Salmon were reared at Bonneville Hatchery, Oregon, under four dietary treatments across four replicate rearing cohorts. The dietary treatments included two feeding frequencies (standard fed 7 d/week and reduced fed 4 d/week) and two dietary lipid levels (standard 18% and reduced 12%) in a 2 × 2 factorial design. The fish were differentially implanted with coded wire tags according to treatment, and some of the fish were tagged with passive integrated transponder tags. The downstream passive integrated transponder tag detections at John Day Dam were used to determine juvenile travel time and river survival. The coded wire tags were used to determine adult recoveries in the marine and freshwater environment. The recovery data were analyzed within the context of significantly variable ocean conditions over the course of the four ocean entry years. Results The downstream travel time of the out-migrating juveniles was significantly affected by the dietary treatments, with the smallest treatment fish traveling slower (mean travel time = 68.2 d) than the largest treatment fish (mean travel time = 45.2 d). However, juvenile downstream survival to John Day Dam was not statistically different between treatments. Dietary treatment had a significant effect on total adult recovery. More adult fish (ages 4–6) were recovered from the low feeding frequency–low dietary lipid treatment (0.51%) than from the high feeding frequency–high dietary lipid treatment (0.42%) over the four ocean entry years. In addition, despite being smaller at release, this treatment produced older and larger adults (mean age at recovery = 4.46 years) than were produced by the largest smolts at release (mean age at recovery = 3.72 years). However, interannual variation in the ocean conditions had a more substantial effect on adult recovery than did any hatchery-rearing regime. Conclusions The results of this investigation provide insights for customizing feeding regimes to reduce precocious male maturation while increasing overall adult size, recovery rates, and age at recovery in salmon hatchery programs within the context of variable ocean conditions.
Harstad et al. (Thu,) studied this question.