Genetic risks of marine hatchery enhancement: The good, the bad, and the unknown

As global capture fisheries level off or decline, hatchery enhancement of marine species is being undertaken on a large, though poorly documented, worldwide scale. Hatchery enhancement can potentially reduce the genetic diversity of natural stocks because the enormous fecundity of many marine fish and shellfish species enables the swamping of populations with the progeny of a very few adults. Much of the impact of hatchery supplementation on genetic diversity can be predicted from its effects on the effective size of the natural population. A simple mixing model (Ryman and Laikre, 1991) allows calculation of the effective population size of an enhanced population, if the effective sizes of the hatchery and prestocked natural populations and the fraction that each contributes to the mix are known. This model has been applied to a hatchery supplementation program for the endangered Sacramento River winter chinook salmon (Oncorhynchus tshawytscha). This example shows that a reduction in effective population size is not inevitable and that supplementation can increase the effective sizes of small populations. For highly fecund marine species, on the other hand, a severe reduction in effective population size appears to be a more likely result because hatchery populations are generally small and contributions to populations are large. Unfortunately, a paucity of data prevents application of the model to any of the major marine supplementation programs. The precautionary approach to fisheries management suggests slowing the development and implementation of marine enhancement programs until and unless sufficient data are available to evaluate their potential impact on effective population size and genetic diversity.