Abstract
Comparison of genetic data of serum esterase and transferrin systems of the skipjack tuna, Katsuwonus pelamis Linnaeus, collected from the Atlantic, Indian, and Pacific Oceans together with the results reported earlier, (1) disclosed that fish from the Indian Ocean are distinguishable from those collected both in the Atlantic Ocean and the western Pacific Ocean and (2) suggested that the skipjack tuna, now inhabiting in the worlds major oceans, first appeared in the worlds major oceans, first appeared in the Indian Ocean and then some of the members of the population emigrated to the other oceans, resulting in genetic diversification into the geographical populations, called the Atlantic Subpopulation, Indian Subpopulation, Western Pacific Subpopulation, and Central-eastern Pacific Subpopulation (s), after reproductive isolation.
Regarding the east-west difference of genetic compositions within the Pacific Ocean, analyses of our data on some 500 samples from more than 24, 000 fish indicated the “step structuure” with a gap between 160°E and 175°E more disinctly in the southern hemisphere. Apparent differences in allele frequencies of genetic systems, found recently in 6-phosphogluconate dehydrogenase and phosphoglucose isomerase, between the waters of Japan and Papua New Guinea suggested the necessity of further accumulation of the data before any conclusive statement on population structure in the regions.
