MITOCHONDRIAL-DNA DIFFERENTIATION AND POPULATION-STRUCTURE IN RED DRUM (SCIAENOPS-OCELLATUS) FROM THE GULF-OF-MEXICO AND ATLANTIC-OCEAN

Variation in mitochondrial (mt)DNA was examined among 473 red drum (Sc iaenops ocellatus) sampled in 1988 and 1989 from nearshore localities in the northern Gulf of Mexico (Gulf) and the Atlantic coast of the so utheastern United States (Atlantic). Data were combined with those fro m a previous study to generate a total of 871 individuals sampled from 11 localities in the Gulf and 5 localities in the Atlantic. Individua ls assayed were from the 1986 and 1987 year-classes. A total of 118 co mposite mtDNA genotypes (haplotypes) was found. The percentage nucleot ide sequence divergence among the 118 haplotypes ranged from 0.184 to 1.913, with a mean (+/- SE) of 0.878 +/- 0.004. MtDNA nucleon diversit ies and intrapopulational nucleotide-sequence divergence values were s imilar over all Gulf and Atlantic localities, and were high relative t o most fish species surveyed to date. These data indicate that the per ceived decline in red drum abundance appears not to have affected the genetic variability base of the species. Significant heterogeneity in the frequencies of at least four haplotypes was detected between poole d samples from the Gulf vs pooled samples from the Atlantic. No hetero geneity was found among localities from the Gulf or localities from th e Atlantic. High levels of gene flow among all localities were inferre d from F(ST) values (a measure of the variance in mtDNA haplotype freq uencies) and from Slatkin's qualitative and quantitative analyses. Par simony and phenetic analyses revealed no strong evidence for phylogeog raphic cohesion of localities, although there was weak support for coh esion of four of five localities from the Atlantic. These data indicat e that the red drum population is subdivided, with weakly differentiat ed subpopulations (stocks) occurring in the northern Gulf and along th e Atlantic coast of southeastern USA. Spatial autocorrelation analysis and heterogeneity tests of haplotype frequencies among regions within the Gulf supported the hypothesis of increased gene flow among neighb oring localities; i.e., migration of individuals within the Gulf may b e inversely related to geographic distance from an estuary or bay of n atal origin. Estimates of evolutionary effective female-population siz e indicate that the red drum subpopulations may be large.