Population Structure and Evolutionary History of Southern Flounder in the Gulf of Mexico and Western Atlantic Ocean

Abstract

The southern flounder Paralichthys lethostigma is a key species in recreational flatfish fisheries in the Gulf of Mexico and the northwestern Atlantic Ocean. Effective management of this species relies on knowing how many populations occur within the range of the species, as knowledge of the underlying genetic structure may be used in stock assessments or in the establishment of management units. Here, the evolutionary history of the southern flounder was elicited with genetic data, and the geographic extent and long-term female effective population size (N_ef ) of extant populations were estimated. Mitochondrial DNA sequence data (415 base pairs, control region) were analyzed with a combination of traditional hypothesis testing (analysis of molecular variance) and the coalescent-based maximum likelihood approach (analysis with the program Migrate). Three main findings resulted from this work. First, two distinct genetic populations of southern flounder exist in the Gulf of Mexico and western Atlantic Ocean, and there is a measured genetic divergence (i.e., F _ST) of 0.374 between them. Within ocean basins, there was little significant genetic structure among samples. Second, coalescent modeling suggests that the two populations of southern flounder have expanded rapidly in numbers in the past 10,000–22,000 years, timing that approximately coincides with the end of the last North American glacial retreat. Third, historical population growth is coupled with high estimates of long-term N_ef , both of which contrast with present-day declines in abundance measured by stock assessments. Present-day southern flounder populations persist in two independent evolutionarily significant units that have divergent evolutionary backgrounds.

Received November 29, 2010; accepted June 27, 2011

ACKNOWLEDGMENTS

This work was funded by federal funds from the Federal Aid in Sportfish Restoration Act (Grant F-179-R) and by matching funds from the Texas Parks and Wildlife Department. We thank the routine monitoring staff of the Texas Parks and Wildlife Department's Coastal Fisheries Division. I. Blandon provided samples from Louisiana and North Carolina. S. Fisk, N. Gargasz, K. Gregalis, and D. Marotta provided Florida samples, which were collected with cooperation from the Florida Fish and Wildlife Conservation Commission. M. Fisher and several anonymous reviewers provided helpful comments on the manuscript draft. Reference to trade names does not imply endorsement by the state of Texas.