696 



Fishery Bulletin 91(4), 1993 



tropical oceans, suggested by circumtropical occurrence 

 oflarvae (Nishikawa et al., 1985), would permit unob- 

 structed gene flow throughout the species' distribu- 

 tion. 



Both genetic and morphological analyses revealed 

 considerable variation in yellowfin tuna; however, ge- 

 netic analyses indicated that differentiation does not 

 occur. Evidence that morphological characters are en- 

 vironmentally influenced was provided by comparison 

 of our data to the morphological differentiation found 

 in Schaefer (1991, 1992). In our analysis, 20 fish from 

 four locations (Ecuador, Mexico, Hawaii, and Austra- 

 lia) were the same as those used in Schaefer s studies. 

 Although no genetic differences were found among 

 these locations, morphometric characters and gill-raker 

 counts differed significantly, which we conclude is the 

 result of phenotypic plasticity. The finding of greater 

 morphological variability among Pacific samples than 

 occurred between the Atlantic and Pacific (Schaefer 

 and Walford, 1950) also supports the hypothesis that 

 phenotypic plasticity is the cause of morphological dif- 

 ferentiation in the Pacific. 



The null hypothesis that yellowfin tuna in the Pa- 

 cific Ocean share a common gene pool could not be 

 rejected in this analysis. Our results were consistent 

 with the alternate hypothesis that yellowfin tuna main- 

 tain sufficient gene flow among areas to prevent the 

 accumulation of significant genetic differentiation. As 

 theoretical models indicate that very low levels of mi- 

 gration (only a few individuals per generation) are 

 needed to prevent genetic differentiation among large 

 populations (Allendorf and Phelps, 1981; Hartl and 

 Clark, 1989), the present data indicate only that some 

 minimal amount of exchange is occurring. Other ap- 

 proaches are necessary to quantify the amounts of mix- 

 ing among regions and to judge more accurately 

 whether or not separate stocks should be distinguished 

 for management purposes. 



Acknowledgments 



Kurt Schaefer either directly collected or arranged col- 

 lection of all specimens from the Pacific. Antony Lewis 

 and Ziro Suzuki kindly made regional literature avail- 

 able for our use. William Bayliff, Mark Chittenden, 

 Bruce Collette, Edward J. Heist, and Kurt Schaefer 

 critically reviewed the manuscript. 



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