Reproductive success of hatchery- and natural-spawning sockeye salmon, Auke Creek, Alaska

Evaluating salmon hatchery supplementation programs requires assessing not
only program objectives but identifying potential risks to wild
populations as well. Such evaluations can be hampered by difficulty in
distinguishing between hatchery- and wild-born returning adults. Here, we
conducted three years (2011–2013) of experimental hatchery supplementation
of sockeye salmon in Auke Lake, Juneau, Alaska where a permanent weir
allows sampling and genotyping of every returning adult (2008–2019). We
identified both hatchery- and wild-born returning adults with parentage
assignment, quantified the productivity (adult offspring/spawner) of
hatchery spawners relative to that of wild spawners, and compared run
timing, age, and size at age between hatchery- and wild-born adults.
Hatchery-spawning females produced approximately six to 50 times more
returning adults than did naturally spawning females. Supplementation had
no discernable effect on run timing and limited consequences for size at
age, but we observed a distinct shift to younger age at maturity in the
hatchery-born individuals in all three brood years. The shift appeared to
be driven by hatchery-born fish being more likely to emigrate after one,
rather than two, years in the lake but the cause is unknown. In cases when
spawning or incubation habitat is limiting sockeye salmon production,
hatchery supplementation can be effective for enhancing the number of
returning adult fish but not without the risk of phenotypic change in the
recipient population, which can be an undesired outcome of hatchery
supplementation. This study adds to a growing body of evidence suggesting
that phenotypic change within a single generation of captive spawning
might be widespread in salmon hatchery programs.