The two-stock hypothesis currently used by the International Commission for the Conservation of Atlantic Tunas (ICCAT) is based in part on the assumption that mixing of western Atlantic bluefin tuna and eastern Atlantic/Mediterranean bluefin tuna is limited (ICCAT, 1992, 1993). The two management units of Atlantic bluefin tuna include a western management unit (west of 45°W) and an eastern management unit (east of 45°W and in the Mediterranean Sea; see Figure 2-1). Data indicate that while spawning is limited to two discrete areas, the Gulf of Mexico and the Mediterranean Sea, there is movement of individuals between western and eastern management units. A key issue is the extent of movement.

The first studies of population structure in Atlantic bluefin tuna date to the early part of this century. The earliest written work is attributed to several reports by M. Sella (1926, 1927, 1929; cited in Brunenmeister, 1980), who, in the mid-to late 1920s, inferred origins and movement patterns from fishing tackle characteristics of different eastern Atlantic Ocean and Mediterranean Sea fisheries. Sella hypothesized that bluefin tuna moved from the eastern Atlantic Ocean into the Mediterranean Sea, that tuna moved from the south of Spain to Norway after spawning, and that small and medium-sized tuna could swim long distances. Although criticized because of concerns that tackle types were not reliable indicators of hooking localities, Sella's hypotheses agree with movement patterns inferred from tagging experiments carried out since the early 1900s.

An early review of research on population structure for Atlantic bluefin tuna is an unpublished report in the early 1970s by F.J. Mather and A.C. Jones (cited in Murphy, 1990), which suggested that there were three populations: one in the western Atlantic Ocean, one in the eastern Atlantic Ocean, and one in the Mediterranean Sea. They also suggested that a separate population might exist in the south Atlantic Ocean. Mather et al. (1974) suggested two alternative hypotheses: a single Atlantic population and one or more Mediterranean populations; or two Atlantic populations, one spawning in the western Atlantic Ocean and the other in the eastern Atlantic Ocean or the Mediterranean Sea, or both, and one or more Mediterranean populations. They believed there was evidence of a two-stock hypothesis but noted that the "evidence is insufficient to permit clear-cut conclusions." Brunenmeister (1980) also reviewed evidence for population structure but was unable to support any hypothesis. Finally, Murphy (1990) argued that the presently accepted two-population hypothesis was not adequately flexible to fit available data. He proposed that bluefin tuna in the northern Atlantic Ocean represented one population and that the interchange between the Atlantic and Mediterranean populations is sufficiently small such that Mediterranean bluefin tuna may represent a second population. His hypothesis appears to be based on three assumptions: (1) rates of movement between western and eastern Atlantic bluefin tuna are significantly higher than those between eastern Atlantic and