Precocious Maturation of Hatchery‐Raised Spring Chinook Salmon as Age‐2 Minijacks Is Not Detectably Affected by Sire Age

Juvenile males produced in spring Chinook Salmon Oncorhynchus tshawytscha hatchery programs can exhibit high rates of maturation in freshwater as 2‐year‐old “minijacks.” This phenomenon is associated with high feeding rates and increased size and/or growth that juveniles experience in the hatchery environment, though studies also support a genetic component affecting age of maturation among salmonids, including precocious maturation in freshwater. This prompted a study to test whether the age of natural‐origin spring Chinook Salmon broodstock affects the rate at which their hatchery‐raised male progeny mature as age‐2 minijacks. In three consecutive brood years, we factorially mated age‐4 adult females with age‐3, age‐4, and age‐5 adult (jacks) male broodstock. In the latter two brood years, we also incorporated age‐1 precocious parr (microjacks) as sires. After communal rearing to the smolt stage (age 1), male juveniles were characterized as immature or as maturing minijacks based on plasma 11‐ketotestosterone concentration, and each was identified to its respective full‐sib progeny group via genetic parentage analysis. A generalized linear mixed model, performed for each brood year separately, was used to characterize expected precocious maturation rates by sire age, while controlling for potential effects of smolt body weight and individual parent identities. Multiple comparisons across sire ages within brood years were used to evaluate relative rates of precocious maturation. Estimates of the probability of minijack maturation among families within sire ages and brood years varied from as much as 0% to 100%, and no consistent effect of sire age on precocious maturation rate was observed. Exploratory analyses investigating additional effects of egg size, dam length, and spawn date also failed to identify consistent predictors of precocious maturation. Instead, variability was largely attributed to both dam‐ and sire‐specific effects, indicating a heritable component to precocious maturation, though not detectably associated with other measured attributes.