Diaz-Jaimes and Uribe-Alcocer: Allozyme and RAPD variation in Thunnus albacares 



775 



similar sizes (Collette an(i Nauen, 1983). Because recruit- 

 ment to the original tuna schools has been reported as 

 well (Kimley and Holloway, 1999), random processes could 

 also induce differences in genotypic frequencies that favor 

 aggregation of some genotypes, while segregating some 

 others, causing a kind of Wahlund effect that is reflected by 

 a heterozygous deficit as shown by the homozygous excess 

 for loci and locations having HW deviations, especially as 

 shown in the Gulf of California sample. 



The estimations of population structure based on allo- 

 zymes showed a small but significant value different from 

 zero (6=0.048; P<0,01). The Gulf of California sample con- 

 tributed to the significant subdivision value as shown when 

 that collection was excluded from the regional subdivision 

 analysis, as well as to significant heterogeneity of its allele 

 frequencies when paired comparisons were made. 



The small value of for overall estimations on RAPD 

 data is probably due to the small sample size. The nega- 

 tive values of 6 from overall and regional estimations re- 

 sulted from subtracting the large value of the correction 

 derived from the variation expected of the sample sizes 

 from the small value of variation due to fluctuations in al- 

 lele frequencies. The fact that RAPD data are considered 

 dominant could reduce information about the true allele 

 distributions by subestimation of null allele frequencies 

 notwithstanding the correction applied to recessive geno- 

 types, which is dependent on the sample sizes (Lynch and 

 Milligan, 1994). Other assumptions for RAPD data limit 

 the value of this marker, especially when estimations are 

 derived from a small number of loci and sample sizes. Ad- 

 ditional constraints are related to the limited number of 

 alleles (two) to estimate dominant markers, which tend 

 to subestimate the polymorphism and thus reduce the 

 significance of relatively small discrepancies in allele 

 distributions. 



No differentiation between coastal and offshore samples 

 was found in our study because of the slight, nonsignifi- 

 cant differences in the estimation of the subdivision by 

 regions. Although the overall estimation was not different 

 from zero, the allele homogeneity analysis showed allele- 

 frequency heterogeneity between coastal and offshore 



samples, and nonheterogeneity between coastal and inter- 

 mediate samples. 



These results are consistent with the migration reports 

 through tagging studies; evidence exists for the presence 

 of two main yellowfin tuna groups in the eastern Pacific 

 that mix to some extent (Fink and Bayliff, 1970) and that 

 migrate longshore from around the 20°N to the mouth of 

 the Gulf of California and to the zone between the Revilla- 

 gigedo and the Clipperton islands, and back again (Joseph 

 et al, 1964; Fink and Bayliff, 1970), although longitudi- 

 nal movements are restricted to the limits of yellowfin 

 regulatory area (CYRA). Similarly, important northward 

 movements along the coasts to the mouth of the Gulf of 

 California, and subsequently to the western coasts of Baja 

 California, have been reported. Although the estimation of 

 6 for allozymes showed a significant value, it was notably 

 influenced by the heterogeneity found between the Gulf of 

 California sample and all other samples. 



Discarding the variation displayed by loci La, Lgg, 

 and Pap-F*, originating mainly from Gulf of California 

 sample, the estimation of subdivision was still marginally 

 significant after Bonferroni correction, which should be 

 considered as evidence that the Gulf of California sample 

 may represent a partially isolated population with differ- 

 ent allele frequencies. Oceanographic conditions inside the 

 Gulf are somewhat different from those of the Pacific Ocean 

 where there are warmer waters at the end of the year, es- 

 pecially during yellowfin tuna spawning seasons. There is 

 also high productivity characterized by the presence of sig- 

 nificant biomass abundance of sardine or anchovy schools 

 (Cisneros-Mata et al., 1995), which represents opportuni- 

 ties to establish the feeding and consequently the spawn- 

 ing grounds for eastern Pacific yellowfin tuna. Likewise, 

 there is a trend of migratory movements through the Gulf 

 of California by different groups of yellowfin tunas (Fink 

 and Bayliff, 1970). These movements promote stock mix- 

 ing and help to explain the wide polymorphism displayed 

 in this sample, in contrast to the weak variation found in 

 other samples from the coast and offshore regions. Further 

 genetic research, including sequential temporal sampling 

 of young fishes in order to ensure the presence of individu- 



