Demographic and Genetic Consequences of Permitting Captively Bred Chinook Salmon to Colonize Following Modification of An Impassable Dam

Captively reared animals can provide an immediate demographic boost in programs where a species is reintroduced for conservation purposes but they may reduce the genetic diversity or fitness of colonizing populations.  This trade-off was explored in a population of Chinook salmon Oncorhynchus tshawytscha recently granted access to 33 km of spawning habitat via modification of Landsburg Diversion Dam on the Cedar River, WA, USA.  We thoroughly sampled adult salmon at the onset of colonization (2003 – 2009) and used microsatellite DNA markers to evaluate the demographic and genetic consequences of permitting hatchery origin salmon to spawn if they volitionally entered the fish ladder.  A severely male biased sex ratio indicated that the reproductive capacity of the population was limited by females.  Permitting hatchery females to spawn more than doubled (2.7x) the total number of second generation recruits, and there was no consistent difference in the reproductive success (RS: number of returning adult offspring) between hatchery and natural origin females across three cohorts.  In contrast, hatchery males were less productive than natural origin males in all three cohorts (range in relative RS: 0.70 – 0.90).  Although this difference was not statistically significant (p > 0.05), it demonstrated the potential for a genetic fitness cost with little demographic benefit because it is unlikely that any females would have failed to spawn had the hatchery males been excluded.  Hatchery and natural origin salmon had similar patterns of genetic diversity and effective population size, so there was no evidence that inclusion of the hatchery fish reduced either parameter.  We conclude that in the first generation, the demographic benefits of the hatchery females certainly outweighed the genetic consequences, but not for the males.