Reproductive success of captively bred and naturally spawned Chinook salmon colonizing newly accessible habitat

Captively reared animals can provide an immediate demographic boost in reintroduction programs but may also reduce the fitness of colonizing populations. Construction of a fish passage facility at Landsburg Diversion Dam on the Cedar River, WA, USA, provided a unique opportunity to explore this trade-off. We thoroughly sampled adult Chinook salmon (Oncorhynchus tshawytscha) at the onset of colonization (2003 – 2009), constructed a pedigree from genotypes at 10 microsatellite loci, and calculated reproductive success (RS) as the total number of returning adult offspring. Hatchery males were consistently but not significantly less productive than naturally spawned males (range in relative RS: 0.70 – 0.90), but the pattern for females varied between years. The sex ratio was heavily biased towards males, so inclusion of the hatchery males increased the risk of a genetic fitness cost with little demographic benefit. Measurements of natural selection indicated that larger salmon had higher RS...