GENETIC ANALYSIS OF THE STOCK STRUCTURE OF THE YELLOWFIN TUNA, Thunnus albacares, IN THE SOUTHWEST EQUATORIAL ATLANTIC OCEAN AS SUGGESTED BY MITOCHONDRIAL DNA SEQUENCES

Tuna fisheries is a growing industry in northeastern Brazil, where the yellowfin tuna (Thunnus albacares, Bonnaterre 1788) is the most abundant species. The distribution of the populational stock of T. albacares in the equatorial Atlantic Ocean has been considered continuous, however the International Commission for Conservation of the Atlantic Tuna (ICCAT) has worked with a discontinuous population. For practical reasons a “two stock” hypothesis has been used in most assessments and the two stocks are separated at 30° W. In the present paper, a DNA sequencing technique was used to investigate the genetic variation in the T. albacares stock from the southwest equatorial Atlantic Ocean off northeastern Brazil, and the phylogenetic position of this species in the genus Thunnus. Within the 401-bp amplified segment of the cytochrome b gene, a single variable nucleotide site was identified among the 35 sampled individuals of T. albacares. This level of nucleotide sequence divergence is very low and it is not enough to determine the presence of two stocks in the studied area. The monophyly of the genus Thunnus was supported by the phylogenetics analyses. Yellowfin tuna from Canada and the U.S.A. were more closely related to each other than to the yellowfin tuna from Brazil. The species more closely related to T. albacares were T. atlanticus and T. tonggol. Based on the results obtained in this study, we suggest that the fisheries authoritiesfrom the ICCAT and Brazilian Institute of Environment (IBAMA) should consider a unique stock of T. albacares in the southwest equatorial Atlantic Ocean for fisheries management purposes.