Understanding and Estimating Effective Population Size for Practical Application in Marine Species Management

Effective population size (N-e) determines the strength of genetic drift in a population and has long been recognized as an important parameter for evaluating conservation status and threats to genetic health of populations. Specifically, an estimate of N-e is crucial to management because it integrates genetic effects with the life history of the species, allowing for predictions of a population's current and future viability. Nevertheless, compared with ecological and demographic parameters, N-e has had limited influence on species management, beyond its application in very small populations. Recent developments have substantially improved N-e estimation; however, some obstacles remain for the practical application of N-e estimates. For example, the need to define the spatial and temporal scale of measurement makes the concept complex and sometimes difficult to interpret. We reviewed approaches to estimation of N-e over both long-term and contemporary time frames, clarifying their interpretations with respect to local populations and the global metapopulation. We describe multiple experimental factors affecting robustness of contemporary N-e estimates and suggest that different sampling designs can be combined to compare largely independent measures of N-e for improved confidence in the result. Large populations with moderate gene flow pose the greatest challenges to robust estimation of contemporary N-e and require careful consideration of sampling and analysis to minimize estimator bias. We emphasize the practical utility of estimating N-e by highlighting its relevance to the adaptive potential of a population and describing applications in management of marine populations, where the focus is not always on critically endangered populations. Two cases discussed include the mechanisms generating N-e estimates many orders of magnitude lower than census N in harvested marine fishes and the predicted reduction in N-e from hatchery-based population supplementation.