Supplementation with Local, Natural-Origin Broodstock May Minimize Negative Fitness Impacts in the Wild

We investigate relative reproductive success (RRS) of an ongoing supplementation program for Chinook salmon in Johnson Creek, Idaho.  Pedigrees of natural and hatchery-origin fish from five broodyears were tracked over two generations with molecular markers.  Results show that hatchery-reared females had no detectable difference in fitness relative to natural-origin females (overall RRS=1.03, p=0.55), but hatchery-reared males had lower fitness than their natural-origin counterparts (overall RRS=0.84, p=0.04; jack male RRS=0.91, p=0.05).  Since individuals that do not produce returning adult offspring have no direct genetic effect on the population, we also compared RS between the hatchery and natural-origin fish that contributed at least one offspring to the next generation and found that RS was not significantly different for either sex (female RRS=1.05, p=0.99; male RRS=0.96, p=0.79; jack male RRS=1.24, p=0.35).  Additionally, RRS of hatchery-reared fish (H) that mated with natural-origin fish (HxN matings) were equivalent (RRS=1.00, p=0.71) to those between two natural fish (NxN), with HxH matings having lower overall RRS of 0.85 that was not statistically significant (p=0.43).  Results suggest that supplementation with 100% local, natural-origin broodstock may be a practice that can successfully boost population size with minimal negative genetic impacts to wild populations.