Historical Demography and Genetic Population Structure of the Blackfin Tuna (Thunnus atlanticus) from the Northwest Atlantic Ocean and the Gulf of Mexico

Little is known about the population structure and genetic variability of blackfin\ntuna despite catch increases over the past 25 years. In this thesis, levels of genetic\nvariation contained in 323bp of the mitochondrial DNA (mtDNA) control region-I (CR-I)\nand in six microsatellite loci were characterized for two regions: the Gulf of Mexico\n(GoM) and the Northwest Atlantic. Large amounts of mtDNA diversity (h>0.99; =0.047)\nwere observed in both regions. Mismatch distribution analysis of the CR-I sequence\ndata, using a mutation rate of 1.6% Ma-1for scombroid fishes, indicate blackfin tuna\nunderwent population expansion about 1.4 Ma, a timeline concordant with the expansion\nof other tunas and billfishes. Estimates of female effective population size were very\nlarge at 7.8 million and 12.8 million individuals for the NW Atlantic and the GoM,\nrespectively.\nBoth mtDNA and six microsatellite loci were used to determine blackfin tuna\npopulation structure. Microsatellite and mtDNA AMOVAs revealed significant\ndifferentiation (msat st=0.01, p=0.006 and mtDNA st=0.01, p=0.049) between the GoM\nand the NW Atlantic samples. Migration estimates using mtDNA data indicate that twice as many females enter the NW Atlantic from the GoM (346\nindividuals/generation) than the opposite direction (150 individuals/generation).\nMigration estimates using microsatellite data were substantially smaller, with the Gulf\nreceiving 7 individuals/generation and the NW Atlantic 4 individuals/generation.\nFinally, low levels of genetic differentiation using microsatellite data have been\nreported in other highly abundant marine fishes, which have been attributed to\nhomoplasy in allele size. To test this hypothesis, the allele frequency distributions of\nblackfin and yellowfin tuna at six microsatellite loci were compared. The distances\nbetween species were surprisingly small (Da=4.0%, (delta mu)squared=1.08), with a large degree of\nsimilarity in frequency distributions at four loci. The comparison of bigeye tuna at two\nmicrosatellite loci revealed additional inter-specific similarities. A mutation rate for\nthese loci was estimated by modifying an equation used to estimate time since\ndivergence. Microsatellites in tunas appear to evolve at a rate (4.3x10-7 Ma-1) that is two\norders of magnitude slower than other fishes (1x10-5 Ma-1). Accordingly, microsatellite\nallele size similarities are plesiomorphic and not due to homoplasy.