Phylogeography of the Japanese scad, Decapterus maruadsi (Teleostei; Carangidae) across the Central Indo-West Pacific: evidence of strong regional structure and cryptic diversity.

The Japanese scad Decapterus maruadsi (Carangidae) is an economically important marine species in Asia but its exploitation shows signs of overfishing. To document its stock structure, a population genetic and phylogeographic study of several populations of this species from the central part of the Indo-West Pacific region was conducted using the mitochondrial cytochrome b gene. Genetic homogeneity within the Sundaland region's population, including Rosario (the Philippines) and Ranong (Andaman Sea) populations was revealed with low nucleotide diversity (π = 0.001-0.003) but high haplotype diversity (h = 0.503-0.822). In contrast, a clear genetic structure was observed between this group and the northern Vietnam populations as revealed by F_ST, AMOVA and SAMOVA, while the central Vietnam population of Khanh Hoa is an admixed group between the two differentiated regional populations. The neutrality and mismatch distribution analyses supported a demographic expansion of D. maruadsi in between last Pleistocene to early Holocene period which influenced present day distribution pattern. Contemporary factors such as oceanic currents and different life history traits are also believed to play significant roles in the observed population structure and biogeographical pattern. Based on these results, recommendations on how stocks of the Japanese scad should be managed are offered.