Criteria for managing fishery stocks under the Endangered Species Act of 1973 (ESA) have been discussed by Waples (1991) and under the Magnuson Fishery Conservation and Management Act of 1976 (MFCMA) in a recent National Research Council report (NRC, 1994). The MFCMA (Public Law 94-265, 16 U.S.C. 1801 et seq.) specifies that "an individual stock of fish shall be managed as a unit throughout its range," but does not provide criteria for defining a stock. The original ESA also did not provide criteria for defining a stock or management unit, but the ESA amendments of 1978 (Public Law 95-632 , 92 Stat. 3751) defined a "species" as "any subspecies of fish or wildlife or plants, and any distinct population segment of any species of vertebrate fish or wildlife which interbreeds when mature." In the past few decades, biochemical and molecular genetic methods have been applied to fishery management issues, leading to an expansion of the stock concept to include interpopulation genetic variability. This expanded stock concept is intended to facilitate the conservation of biologically meaningful management units (Utter, 1981; Waples, 1991) that may be uniquely adapted to a particular area. Waples (1991) proposed that a distinct population segment should be defined as an "evolutionarily significant unit" (ESU) that is "substantially reproductively isolated from other conspecific populations units'' and that "represents an important component in the evolutionary legacy of the species." Under this concept, populations are defined as groups of individuals that share a common space, interbreed, and are totally or partially isolated from other such groups. The degree of isolation, brought about by reduced gene flow among breeding areas and the amount of time the populations have been isolated from one another, influences the degree of genetic differentiation among groups.
Dizon et al. (1992) proposed a scheme of population classification based on genetic and geographic criteria. Category I populations are geographically separated groups of individuals that are more closely related genetically to each other than they are to individuals in other groups. Such populations have the highest probability of being evolutionarily significant units. Category II populations are also differentiated genetically but are only marginally separated geographically. Category III populations show little genetic differentiation from one another but are geographically separated and therefore likely to be isolated reproductively. Genetic differences among geographically isolated populations are expected eventually to increase. Category IV populations show little genetic differentiation because of extensive gene flow and are unlikely to be evolutionarily significant units. Geographic distribution, parasite markers, microconstituent analysis, tag-recapture data, population parameters, morphological variability, and genetic information can be used to assign populations to these categories. A variety of evidence is available for assessing the population structure of Atlantic bluefin tuna, but much of the data are equivocal. The sections below present these data and their interpretations.