A 1 lozyme variability in two samples of swordfish , Xiphias gladius L . , in the North Pacific Ocean

The swordfish, Xiphias gladius L., is a cosmopolitan species found in all tropical and temperate oceans (Nakamura, 1983). It is an esteemed food fish and subject to a multinational fishery throughout its range of distribution (Sakagawa, 1989). In 1991 the Pacific catch was 29,245 metric tons (t) or about 44% of the total global catch of 67,142 t (FAO, 1993). A variety ofpopulation structures have been proposed for Pacific swordfish. From one to four stocks have been proposed on the basis of fishery data from all of the Pacific Ocean (Bartoo and Coan, 1989; Skillman, 1989; and Sosa-Nishizaki and Shimizu, 1991). GrijalvaChon et a1. (1994) examined restriction-fragment-length polymorphisms (RFLP) of mitochondrial DNA (mtDNA), and their results supported the hypothesis of one stock in the North Pacific. Allozyme analyses have been used extensively to study population structure in fishes, especially in fishery resources, because of their relative simplicity (Utter et aI., 1974, 1987; Ihssen et aI., 1981). Several studies of mtDNA have demonstrated increased resolution for detecting population substructure (Ferris and Berg, 1987), and the methods used in them have complemented each other because they involve different modes of inheritance: with protein electrophoresis, phenotypic data from nuclear loci and from alleles transmitted by sexual reproduction can be analyzed, whereas with the mtDNA method, fragments ofhaploid DNA that are (usually) maternally inherited and evolve rapidly can be analyzed (Allendorf et aI., 1987>. The nuclear allelic variants involve sexual exchange among individuals, and one can infer the population fitness or reproductive success through genetic variation (Nevo, 1978). In the case of a fishery resource, fishing mortality alters the frequency of adaptative traits (Nelson and SouIe, 1987). The intent of this study was to examine the genetic variation at protein-coding loci in swordfish in a preliminary way to test whether Materials and methods