Genetic variation in Atlantic yellowfin tuna (Thunnus albacares) to assess stock structure and reproductive variance

The population genetic structure of Atlantic yellowfin tuna (Thunnus albacares) has received little attention despite the substantial fishing mortality of juveniles caused by purse seining around fish aggregating devices in the Gulf of Guinea targeting multi-species schools that also include similarly sized skipjack tuna (Katsuwonus pelamis) and bigeye tuna (T. obesus). We used sequence data from 355 bp of the mitochondrial control region I as well as six microsatellite loci to examine: (1) population structure, and (2) to look for evidence of reproductive variance. We analyzed two samples of adults from the Gulf of Mexico (GOM) and one sample of early juveniles (20-50 mm) from the Gulf of Guinea (GOG). We found no evidence of geographic or temporal differentiation among the samples. Accordingly, the null hypothesis of panmixia for yellowfin tuna in the Atlantic Ocean could not be rejected. A sudden expansion analysis based on mtDNA control region I sequence data of yellowfin tuna was highly significant. Time estimates for expansion were between 40,000 and 80,000 years before present. The associated high levels of homoplasy could be masking any existing population structure. Additional sampling from additional locations and across several years will be needed to test the hypothesis of panmixia. We also provide preliminary evidence of the Allendorf-Phelps effect, which may contribute to reproductive variance. This is the first evidence of this effect in any other tuna or pelagic species. Data indicates that early juveniles sharing the same mtDNA control region I haplotype were caught in the same tow and had a significant probability of halfsibship status as calculated from their haplotype and genotype at one microsatellite locus through kinship analysis. Sampling throughout the spawning season and across several years, as well as analysis with additional microsatellite loci that have a more even distribution of alleles, will be needed to more fully identify the sibling status of larvae and early juveniles caught in the same tow as well as the extent of this reproductive variance.