Risk to Genetic Effective Population Size should be an Important Consideration in Fish Stock-Enhancement Programs

Conservation of single-locus and quantitative genetic variation in managed populations requires maintenance of sufficiently large (genetic) effective population sizes, but the reductive effect exerted by large-scale fish-stocking programs on the effective sizes of natural populations has been largely ignored in conventional stocking guidelines. Using a model described by Ryman and Laikre (1991), we evaluate the effects of stocking on the effective sizes of fish populations and the accompanying genetic risks. We examine the parameters of the model as they relate specifically to the supplementation of fish populations. Then employing the model we present case studies of two species, red drum (Sciaenops ocellatus) and Gulf sturgeon (Acipenser oxyrinchus desotoi), which differ greatly in biology and demography and are both currently subjects of proposed or ongoing aquaculture programs designed for supplementation of wild stocks. The red drum is a pelagic, marine species comprising few, extremely large, randomly mating populations. Under typical stocking conditions, the red drum population we examined is unlikely to experience significant loss of single-locus or quantitative (polygenic) variability. The diadromous Gulf sturgeon (a “threatened species”) is divided into several discrete populations, each of very limited abundance. The stocking program proposed for Gulf sturgeon could reduce their effective population sizes well below suggested threshold values; this reduction could result in the rapid loss of rare alleles and in other drift-associated alterations in frequencies of selectively neutral alleles, as well as in greatly reduced polygenic variability. These case studies illustrate the need to weigh the long-term risks to genetic variation, both single-locus and quantitative, in natural fish populations against the immediate benefits of artificially increasing their abundance.