Data from: Effective size of a wild salmonid population is greatly reduced by hatchery supplementation

Many declining and commercially important populations are supplemented
with captive-born individuals that are intentionally released into the
wild. These supplementation programs often create large numbers of
offspring from relatively few breeding adults, which can have substantial
population-level effects. We examined the genetic effects of
supplementation on a wild population of steelhead trout (Oncorhynchus
mykiss) from the Hood River, Oregon, by matching 12 run-years of hatchery
steelhead back to their broodstock parents. We show that the effective
number of breeders producing the hatchery fish (broodstock parents; Nb)
was quite small (harmonic mean Nb=25 fish per brood-year vs. 373 for wild
fish), and was exacerbated by a high variance in broodstock reproductive
success among individuals within years. The low Nb caused hatchery fish to
have decreased allelic richness, increased average relatedness, more loci
in linkage disequilibrium, and substantial leve ls of genetic drift in
comparison to their wild-born counterparts. We also documented a
substantial Ryman-Laikre effect whereby the additional hatchery fish
doubled the total number of adult fish on the spawning grounds each year,
but cut the effective population size of the total population (wild and
hatchery fish combined) by two thirds. We demonstrate that, in this
system, the Ryman-Laikre effect is most severe when (1) more than 10% of
fish allowed onto spawning grounds are from hatcheries and (2) hatchery
fish have high reproductive success in the wild. These results emphasize
the tradeoffs that arise when supplementation programs attempt to balance
disparate goals (increasing production while maintaining genetic diversity
and fitness).