Evaluating the Ryman-Laikre effect for marine stock enhancement and aquaculture

The Ryman-Laikre (R-L) effect is an increase in inbreeding and a reduction in total effective population size (N-eT) in a combined captive-wild system, which arises when a few captive parents produce large numbers of offspring. To facilitate evaluation of the R-L effect for scenarios that are relevant to marine stock enhancement and aquaculture, we extended the original R-L formula to explicitly account for several key factors that determine N-eT, including the numbers of captive and wild adults, the ratio of captive to wild N-e/N (beta), productivity of captive and wild breeders, and removal of individuals from the wild for captive breeding. We show how to provide quantitative answers to questions such as: What scenarios lead to no loss of effective size? What is the maximum effective size that can be achieved? and What scenarios insure that N-eT will be no smaller than a specified value? Important results include the following: (1) For large marine populations, the value of beta becomes increasingly important as the captive contribution increases. Captive propagation will sharply reduce NeT unless the captive contribution is very small or beta is very large (similar to 10(3) or higher). (2) Very large values of beta are only possible if wild N-e/N is tiny. Therefore, large wild populations undergoing captive enhancement at even modest levels will suffer major reductions in effective size unless wild Ne is a tiny fraction of the census size (about 10(-4) or lower).