A genetic cline shapes population structure of the round sardinella along its West African distribution

Small pelagic fish play an important role in marine trophic webs and human food security. Among those, Sardinella aurita represents an extremely valuable resource in West Africa, where it is assumed to conform a transboundary stock. During the last decade, quantitative stock assessment has not been possible in NW African waters due to the limitation of the indices used; however, a critical biomass decline has been observed, and the stock is considered overexploited. In order to delineate the population structure of the species in West Africa, twelve microsatellites were developed ad hoc and genotyped on some 1300 individuals collected from Morocco to Angola. The significant differentiation detected between the northernmost samples (Morocco to Guinea) and the southernmost ones (Liberia to Angola) was due to one candidate locus to positive selection. The signal of this locus resided uniquely in the frequency of allele SauMS002_131, which displayed a remarkable latitudinal cline. Allele frequency, starting around 47%-63% in Moroccan-Guinean waters and dropping to 0% off Gabon and southwards, accounts for the North-South declining trend and can be used as a meaningful tool to assist population structure outline for this species in West Africa for management purposes in order to ensure sustainable regionally shared fisheries.