Abstract. -The genetic basis of 

 the population structure of yellow- 

 fin tuna, Thunnus albacares, in the 

 Pacific Ocean was investigated with 

 restriction fragment length polymor- 

 phism (RFLP) analysis of mitochon- 

 drial DNA (mtDNA). Samples of 20 

 yellowfin tuna were examined from 

 each of five Pacific locations and one 

 Atlantic location. MtDNA analysis 

 with 12 informative restriction en- 

 donucleases demonstrated consider- 

 able genetic variation, as evidenced 

 by an overall nucleon diversity of 

 0.84 and a mean nucleotide sequence 

 diversity of 0.91%. Estimates of 

 within-sample variation were re- 

 markably consistent across all six lo- 

 cations. Despite high levels of varia- 

 tion, there was no evidence of genetic 

 differentiation among samples. Com- 

 mon genotypes occurred with simi- 

 lar frequencies in all samples, and, 

 with one exception, all genotypes 

 that were represented by more than 

 one individual occurred at more than 

 one location. We could not reject the 

 null hypothesis that all yellowfin 

 tuna share a common gene pool. Our 

 results are consistent with the al- 

 ternate hypothesis that there is suf- 

 ficient gene flow within the Pacific, 

 as well as between the Atlantic and 

 Pacific oceans, to prevent the ac- 

 cumulation of significant genetic 

 differentiation. 



Genetic analysis of the population 

 structure of yellowfin tuna, 

 Thunnus albacares, from the 

 Pacific Ocean* 



Daniel R. Scoles 



Virginia Institute of Marine Science 



School of Marine Science, College of William and Mary 



Gloucester Point. VA 23062 



John E. Graves** 



Virginia Institute of Marine Science 



School of Marine Science, College of William and Mary 



Gloucester Point. VA 23062 



Inter-American Tropical Tuna Commission 

 8604 La Jolla Shores Drive, La Jolla, CA 92037 



Yellowfin tuna (Thunnus albacares) 

 occur in the tropical and subtropical 

 oceans and support major commer- 

 cial fisheries throughout their range 

 (Collette and Nauen, 1983). The eco- 

 nomic importance of this species is 

 indicated by high annual catches that 

 have increased from 596,764 metric 

 tons (t) in 1981 to 986,529 1 in 1990, 

 of which 66 to 69% were from the 

 Pacific Ocean (FAO, 1992). Recently, 

 purse-seine and longline fisheries in 

 the western Pacific (120°E to about 

 180°) provided a major share of 

 yellowfin tuna landings, with a catch 

 of 342,921 1 in 1990 (Lawson, 1991). 

 In the eastern Pacific (east of 130 C W) 

 record landings near 270,000 1 oc- 

 curred in each of the past 3 years 1 . 



A thorough understanding of yel- 

 lowfin tuna population structure is 

 necessary for the effective man- 

 agement of this economically impor- 

 tant, marine resource. A variety of 

 studies, including tagging, morpho- 

 metric, fishery statistic and genetic 

 analyses, have been used to infer 

 population structure. However, the pro- 

 posed population structures differed. 



Tagging studies have indicated 

 that movements of yellowfin tuna 

 in the Pacific Ocean tend to be geo- 

 graphically restricted. Fink and Bay- 

 liff (1970) analyzed tag return data 

 in the eastern Pacific and proposed a 

 northern and southern group of fish 

 with some exchange between groups. 

 There was very limited westward 

 movement of tagged fish reported in 

 the study; however, as the eastern 

 Pacific fishery expanded westward in 

 subsequent years, several returns 

 were obtained from farther offshore 

 indicating possible mixing between 

 eastern and central Pacific fish (Bay- 

 liff, 1984). In the western Pacific tag- 

 ging studies showed that most indi- 

 viduals remained within the western 

 Pacific region and did not make ex- 

 tensive movements (Itano and Will- 

 iams, 1992; Lewis, 1992). Although 

 the majority of recaptured yellowfin 

 tuna in all studies showed limited 

 movement, some returns were ob- 

 tained which demonstrated the po- 

 tential for fish to move large dis- 



Manuscript accepted 12 August 1993. 

 Fishery Bulletin 91:690-698 (1993). 

 690 



'Anonymous. 1992. Inter-American Tropical 

 Tuna commission. Annual Report, 1991. 



"Contribution No. 1811 of the Virginia Insti- 

 tute of Marine Science, 



**To whom reprint requests and correspon- 

 dence should be addressed. 



