Past and Present Processes Influencing Genetic Diversity and Effective Population Size in a Natural Population of Atlantic Sturgeon

Abstract

Threats such as habitat loss, invasive species, and overexploitation cause species extinctions; however, stochastic processes can accelerate extinction rates as census sizes decline. Using molecular and ecological data, we explored the influence of these processes on the demography of a candidate species under the U.S. Endangered Species Act—the Atlantic sturgeon Acipenser oxyrinchus oxyrinchus. We used molecular microsatellite markers to estimate the effective population size (N_e ) and effective number of breeders (N_b ) and we used mark–recapture data to estimate the number of spawners (N_a ) for Atlantic sturgeon of the Altamaha River, Georgia. We found that estimates of N_b were 7–45% less than the estimated N_a over four consecutive cohorts and that skewed sex ratios could explain the relative decrease of N_b to N_a . Our estimate of contemporary N_e was 125 (95% confidence interval = 75–348) and was at least an order of magnitude less than our estimate of historical N_e . To explain the large discrepancy between these estimates, we tested several alternative evolutionary scenarios that might explain the observed pattern of genetic diversity. Our results indicated that the observed genetic data were indeed best explained (i.e., 0.998 posterior probability of the data given the hypothesis) by overexploitation during the last half of the 20th century.

Received October 4, 2010; accepted June 28, 2011

ACKNOWLEDGMENTS

We thank P. Schueller and M. Bednarski for sample collection. We thank S. Clemmensen and A. Williams for providing laboratory assistance and B. Sloss and V. Mudrak for insightful comments, editorial assistance, and helpful discussion. M. Bartron provided initial PCR multiplex conditions. Computer programs used for life table analyses were kindly made available by P. Jorde and B. Spearman. Use of trade names does not constitute endorsement by the U.S. Government.