Precocious Male Maturation as a Driver of Domestication Selection in Hatchery Spring Chinook Salmon

Salmon supplementation hatchery programs have been designed with the intention of increasing natural production and harvest opportunities while limiting ecological and genetic risk to native populations.  Reduced size at age and age of maturation in hatchery fish compared to wild fish has been previously documented.  In male spring Chinook salmon the physiological decision to mature as a full sized anadromous adult (age-3-5) or a precociously mature "sneaker" parr or minijack (age 1-2) can be described as a conditional strategy where variation in life-history type is under polygenic control and expression of a phenotype depends on exceeding some threshold condition (i.e. size or energy reserves).  Over the past decade we have monitored the frequency of minijacks in a supplemented stock of spring Chinook salmon in the Yakima River, Washington, USA and documented average rates in excess of 40%. This program also includes a hatchery domestication monitoring effort that compares several traits of a supplemented (Natural) genetic line with those of a "domesticated" (Hatchery) control line founded from first generation hatchery adults in 2002.  Minijack rates were significantly lower in the Hatchery line compared to the Natural line in 4 of 6 years.  These data suggested that while minijack rates were quite high in progeny of first generation Yakima hatchery fish, domestication selection against the minijack phenotype could be occurring relatively rapidly in the Hatchery line.  To follow-up, we conducted a “common garden” experiment exploring the effects of limited years in hatchery culture on minijack maturation in the Natural (0-1 generations) and the Hatchery (2 generations) lines reared under identical laboratory conditions.  Growth rates and size were not significantly different between the lines.  However, minijack rates were significantly lower in the Hatchery compared to the Natural line and logistic regression revealed that the threshold weight needed to achieve maturity was significantly higher (19%) in the Hatchery than the Natural line.  These data provide evidence that the Hatchery line at the Yakima Hatchery is undergoing domestication selection in the size threshold for minijack maturation after only 1-2 generations.  With regard to this important demographic trait (age of maturity), these data would suggest that an integrated hatchery strategy (Supplemented Natural line) might reduce or slow the rate of selection on this and associated traits, but help maintain high minijack rates.  Long-term, segregated hatchery rearing (Hatchery line) likely results in lower minijack rates and a larger threshold size for initiation of early male maturation.