Population structure of gag Mycteroperca microleps (Goode and Bean) in the southeastern United States

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Cushman, Elizabeth Lynn
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The gag (Mycteroperca microlepis) is a large, protogynous grouper important in both the recreational and commercial fisheries. The gag fishery in the United States is currently managed as two separate stock units: the Gulf of Mexico and the Atlantic. A previous study found genetic differentiation between sampling locations for gag from the southeastern United States and the Gulf of Mexico, but questions remained as to whether this patchy structure was temporally or spatially mediated. Utilizing a mitochondrial marker and 11 microsatellite loci, gag from North Carolina, South Carolina, the eastern coast of Florida, and two sites in the eastern Gulf of Mexico were compared to assess the spatial component of genetic variation. On a temporal scale, genotypes of multiple adult cohorts of known ages were compared with young-of-year juveniles from 2005 and archived samples of gag postlarvae from 1985. There was no spatial genetic differentiation among sampling locations in the Atlantic or between the Atlantic and the eastern Gulf of Mexico, and very little evidence of temporal differentiation among cohorts of varying ages. Lack of differentiation suggests that gag represent a single genetically panmictic population from North Carolina to the Florida panhandle. However, coalescent-based genetic simulations indicated that low F ST values may be the result of either broad dispersal (100's-1000's of individuals per generation) at equilibrium between genetic drift and gene flow, or dispersal rates as low as 25 per generation when populations are not at equilibrium. Thus, genetic data can neither support nor refute current management schemes that independently regulate gag in the Gulf and Atlantic.
Fishery management; Gag (Fish); Population genetics