Bluefin tuna are members of the family Scombridae, an assemblage that contains 15 genera and about 48 species of epipelagic fish. The scombrid genus Thunnus contains seven species, including the bluefin tuna. Results of molecular genetic studies of Thunnus, as well as species that are related to bluefin tunas (mackerel, bonitos, and billfishes), have provided information on the amount of genetic differentiation that might be expected among the global populations of bluefin tuna. Several recent studies have used mtDNA sequence variation to examine patterns of molecular genetic divergence and infer evolutionary relationships among tunas and other scombroid fish. Bartlett and Davidson (1991) examined a 290-base-pair (bp) sequence of the cytochrome b (cyt b) gene in four species of tuna from the northeast Atlantic Ocean: bluefin (n = 33), yellowfin (n = 33), bigeye (n = 32), and albacore (n = 12). The main result of this study was the demonstration that one could use these 290 bp to distinguish each of the four species and that only a small amount of tissue (100 mg) is required for analysis. This was significant given that carcasses (which do not have identifying morphological characters), often end up at auctions and could be sampled and used in the genetic assessment of population structure. Also, identification of larval and juvenile specimens, which are often difficult to sort to species, is now possible with genetic techniques.

In broader studies to understand the relationships among tunas, mackerels, and billfishes, Block et al. (1993) and Finnerty and Block (1994) examined 600 nucleotides of the cyt b gene among 30 species. These studies included nine species of tunas and provided direct comparison of sequence variability between northern and southern bluefin tunas. The inferred phylogeny provided strong support for the monophyly of tuna genera (Thunnus, Katsuwonus, Euthynnus, and Auxis) and indicated that species of the genus Thunnus are closely related to one another. Only three nucleotide sites differed between southern (n = 2) and northern (n = 2) bluefin tuna. Sequence differences between these populations are small (0.5%) compared to the maximum intraspecific sequence difference detected among other members of the suborder (i.e., 1.8% for blue marlin, Makaira nigricans, over a similar region of cyt b [Finnerty and Block, 1992]). These data call into question the validity of separating northern (T. thynnus) and southern bluefin tuna (T. maccoyi) into separate species or indicate the presence of Northern bluefin tuna in southern oceans. Additional molecular data, particularly from nuclear genes, are needed to determine whether the inferences made in these studies can be corroborated.

In a third study of mtDNA variability in tunas, Chow and Inoue (1993)