genetic diversity, there was no evidence of genetic differentiation among samples; common haplotypes occurred in similar frequencies in all samples. These results are consistent with high levels of gene flow among localities throughout the Pacific Ocean and between Pacific and Atlantic Ocean localities. Ward et al. (1994) examined four polymorphic loci encoding allozymes and mtDNA with two informative restriction enzymes among seven samples from the western, central, and eastern Pacific Ocean, and a fifth polymorphic enzyme in eastern and central Pacific samples. They found no significant frequency differences among localities in mtDNA haplotypes or allozymes at four loci. The frequencies for GPI-F, however, were not significantly different between two eastern Pacific samples (southern California and southern Mexico), but were significantly different between these samples and samples from the central and western Pacific Ocean (Coral Sea, Philippines, Kiribati) and two samples taken near Hawaii. Their results were consistent with an earlier study (Sharp, 1978) in identifying heterogeneity between these areas. Ward et al. (1994) concluded that gene flow between eastern and western Pacific yellowfin tunas was severely restricted, with only a few individuals per generation moving between the two regions. The lack of concordance of the other four polymorphic loci and the mtDNA haplotypes with the PGI-F locus, suggests minimally that additional study of mtDNA in yellowfin tuna is warranted. The study of Ward et al. (1994) does emphasize the need for multiple molecular genetic techniques for examining the stock structure of a given species (i.e., the same conclusion may not have been reached with the use of any one technique).
The combined results of Suzuki et al. (1958, 1959), Suzuki (1962), and Fujino (1970) for the Tg blood group of albacore tuna showed little allele-frequency heterogeneity between albacore tuna sampled from the north and south Pacific Ocean, suggesting that fish in this area consist of a single, genetically homogeneous population. There were, however, allele-frequency shifts between samples from the Atlantic and Indian Oceans, indicating population-level differentiation. In another study of albacore tuna, Keyvanfar (1962) found significant frequency differences in blood group alleles between samples from the Atlantic Ocean and the Mediterranean Sea. He also found qualitative immunodiffusion differences between albacore tuna from the Atlantic Ocean and Mediterranean Sea; Atlantic fish had an antigen that was apparently lacking in Mediterranean fish. The genetic basis of this difference is unknown. Graves and Dizon (1989) analyzed mtDNA between albacore tuna from southern Africa (n = 11) and San Diego (n = 12). They found six fragment length variants in individual fish but virtually no differentiation between samples from the Atlantic and Pacific Oceans. The high proportion of shared haplotypes is strong evidence for recent or ongoing gene flow between oceans. Similar results have recently been obtained