589 



Allozyme variability in two samples 

 of swordfish, Xiphias gladius L., 

 in the North Pacific Ocean 



Jose M. Grijalva-Chon 



CICESE, Laboratories de Ecologia Pesquera 



PO. Box 2732, Ensenada, Baja California, Mexico 



Present address Centra de Investigaciones Cientificas y Tecnol6gicas 



Universidad de Sonora 



Rosales y Nirios Heroes s/n, Hermosillo, Sonora. 83000 Mexico 



Jorge De la Rosa-Velez 



Universidad Aut6noma de Baja California, Facultad de Ciencias Marinas 

 PO. Box 453, Ensenada, Baja California, Mexico 



Oscar Sosa-Nishizaki 



CICESE, Laboratono de Ecologia Pesquera 



PO Box 2732, Ensenada, Baja California, Mexico 



The swordfish, Xiphias gladius L., 

 is a cosmopolitan species found in 

 all tropical and temperate oceans 

 (Nakamura, 1983). It is an es- 

 teemed food fish and subject to a 

 multinational fishery throughout 

 its range of distribution (Saka- 

 gawa, 1989). In 1991 the Pacific 

 catch was 29,245 metric tons (t) or 

 about 44% of the total global catch 

 of67,142t(FAO, 1993). 



A variety of population 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-Nishi- 

 zaki and Shimizu, 1991). Grijalva- 

 Chon et al. ( 1994) examined restric- 

 tion-fragment-length polymor- 

 phisms (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 popula- 

 tion structure in fishes, especially 

 in fishery resources, because of 

 their relative simplicity (Utter et 



al., 1974, 1987; Ihssen et al., 1981). 

 Several studies of mtDNA have 

 demonstrated increased resolution 

 for detecting population substruc- 

 ture (Ferris and Berg, 1987), and 

 the methods used in them have 

 complemented each other because 

 they involve different modes of in- 

 heritance: with protein electro- 

 phoresis, phenotypic data from 

 nuclear loci and from alleles trans- 

 mitted by sexual reproduction can 

 be analyzed, whereas with the 

 mtDNA method, fragments of hap- 

 loid DNA that are (usually) mater- 

 nally inherited and evolve rapidly 

 can be analyzed (Allendorf et al., 

 1987). The nuclear allelic variants 

 involve sexual exchange among in- 

 dividuals, and one can infer the 

 population fitness or reproductive 

 success through genetic variation 

 (Nevo, 1978). In the case of a fish- 

 ery resource, fishing mortality al- 

 ters the frequency of adaptative 

 traits (Nelson and Soule, 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 



swordfish in two localities in the 

 North Pacific were genetically ho- 

 mogeneous. 



Materials and methods 



Samples of white muscle, liver, and 

 heart from 44 individuals from the 

 N-NE Hawaiian Islands and from 

 50 individuals from Mexico off Baja 

 California were used to study the 

 allozymic variation (see Grijalva- 

 Chon et al. [ 19941 for sampling and 

 storage details). 



Samples of approximately 5 g of 

 tissue were homogenized in equal 

 volumes of a solution consisting of 

 0.1 M Tris-HCl pH 8, NAD+, NADP, 

 and polyvinylpyrrolidone (100:0.1: 

 0.1:1, v:w:w:w), with an electric 

 homogenizer (Tissumizer Tekmar). 

 In the case of liver, 0.5 mL of toluene 

 was added. Homogenized samples 

 were centrifuged at 20,000 xg for 20 

 minutes at 4°C, and supernatants 

 transferred into capped tubes and 

 stored at -70°C. 



The allozyme analysis comprised 

 15 enzyme systems that encoded 26 

 presumptive loci (Table 1). In gen- 

 eral, the method described by 

 Aebersold et al. (1987) was fol- 

 lowed, with the exception that agar 

 was not used in the staining proce- 

 dure. The support media were 12% 

 starch gels (Sigma S4501). 



The criteria of Grant and Utter 

 ( 1980 ) and Grant et al. ( 1984) were 

 followed for zymogram interpreta- 

 tion. Locus nomenclature followed 

 that of Shaklee et al. ( 1990), which 

 designates the loci in ascending or- 

 der beginning with number 1 for the 

 most cathodal locus. Alleles for poly- 

 morphic loci were designated by us- 

 ing relative electrophoretic mobili- 

 ties: the most common allele at each 

 locus was designated to be 100. 



Data were analyzed with 

 BIOSYS-1 (Swofford and Selander, 

 198 1 ). A locus was considered poly- 



Manuscript accepted 19 April 1996. 

 Fishery Bulletin 94:589-594 ( 1996). 



