Abstract.- Samples of yellow- 

 fin tuna Thunnus albacares from five 

 different areas of the Pacific Ocean, 

 Mexico, Ecuador, Australia, Japan, 

 and Hawaii, collected during Janu- 

 ary to May of 1988, were examined 

 for geographic variation in morpho- 

 metry characters and gill-raker 

 counts. The Kruskal-Wallis test in- 

 dicated a significant difference in the 

 total gill-raker counts among areas. 

 The morphometric data were ad- 

 justed by allometric formulae to re- 

 move size effects. The overall per- 

 cent-correct classification rate for 

 the five groups from the stepwise 

 discriminant analysis, based on 12 

 adjusted morphometric characters, 

 was 77.6%. This is 72.0% (Cohen's 

 kappa statistic) better than would 

 have occurred by chance. These 

 results indicate significant meristic 

 and morphological differences of 

 yellowfin tuna from these areas, 

 which suggests that fish from these 

 areas represent separate groups. 



Geographic Variation in 

 Morphometric Characters and 

 Gill-Raker Counts of Yellowfin Tuna 

 Thunnus albacares from 

 the Pacific Ocean 



Kurt M. Schaefer 



Inter-American Tropical Tuna Commission, Scnpps Institution of Oceanography 

 8604 La Jolla Shores Drive, La Jolla, California 92037 



Manuscript accepted 5 November 1990. 

 Fishery Bulletin, U.S. 89:289-297 (1991). 



Yellowfin tuna Thunnus albacares is 

 an epipelagic species found world- 

 wide in tropical and subtropical 

 oceanic regions, with a nearly con- 

 tinuous distribution in the Pacific 

 Ocean from roughly 40 °N to 40° S 

 (Collette and Nauen 1983). The large- 

 scale industrial fisheries for tuna in 

 the Pacific Ocean landed an estimated 

 471 thousand metric tons of yellow- 

 fin tuna in 1985 (Joseph 1987). Fun- 

 damental to the proper management 

 of yellowfin tuna is the elucidation 

 of population structure. The inter- 

 actions among existing, expanding, 

 or developing fisheries on this re- 

 source cannot be assessed without 

 this knowledge. 



Morphometric studies have pro- 

 vided results useful for identifying 

 marine fish stocks and describing 

 their spatial distributions (Ihssen et 

 al. 1981, Winans 1987). Morphomet- 

 ric characters, used extensively in the 

 analysis of population structure of 

 yellowfin tuna, indicate that there 

 are at least three groups in the 

 Pacific Ocean. Godsil (1948) and God- 

 sil and Greenhood (1951) identified 

 four stocks of yellowfin tuna in the 

 Pacific (Japan, Hawaii, Peru, and the 

 northeastern Pacific) from morpho- 

 metric characters. Morphometric 

 data indicate that yellowfin tuna 

 from southeastern Polynesia, Hawaii, 

 and Central America are different 

 stocks (Schaefer 1955). Kurogane 

 and Hiyama (1957) concluded from 



morphometric data that there are 

 three stocks in the Pacific: western, 

 central, and eastern. Royce (1964), 

 however, concluded that there is an 

 apparent cline in morphometric char- 

 acters along the equator from off 

 Costa Rica to the Caroline Islands. 

 Suzuki et al. (1978) reviewed fisheries 

 and biological data, including mor- 

 phometric data, and concluded that 

 there are at least three relatively in- 

 dependent stocks: western, central, 

 and eastern Pacific. More recently, 

 Schaefer (1989) showed morphomet- 

 ric differences between yellowfin 

 tuna from north and south of 15 °N- 

 20 °N in the eastern Pacific Ocean. 

 With the exception of Schaefer's 

 (1989) study, previous investigations 

 primarily utilized univariate anal- 

 yses of morphometric characters. 

 Although geographic variation in 

 yellowfin tuna morphology can be 

 demonstrated in this manner, uni- 

 variate analyses of single characters 

 do not permit the classification of in- 

 dividual fish into discrete groups or 

 stocks. 



The objectives of the present study 

 were to (1) assess and describe geo- 

 graphic variation in morphological 

 characters and gill-raker counts of 

 yellowfin tuna from five widely- 

 scattered locations of the Pacific 

 basin, (2) test the hypothesis of mor- 

 phometrically distinguishable north- 

 ern and southern groups in the 

 eastern Pacific, and (3) identify the 



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