Population genetic structure and demographic history of blue marlin, Makaira nigricans, in the Pacific and eastern Indian oceans

Blue marlin (Makaira nigricans) is a species that inhabits tropical, subtropical, and temperate waters worldwide. While blue marlin is economically important for fisheries, there are regions in this species’ distribution where little is known about its population structure. This is especially true in the Pacific Ocean, which yields the largest catch of blue marlin, and in the Indian Ocean, where catch has increased consistently over the years and the area is commonly overfished. Sampled Taiwan local market blue marlin shows two morphological characteristics of thick or thin scales. Phylogenetic analysis for taxonomy by mitochondrial DNA (mtDNA) cytochrome b (cyt b) (1140 bp) and control region (CR) (905 bp), these two morphological characteristics of the blue marlin belong to the same species. A total of 183 blue marlin was collected from three Pacific localities, eastern Pacific (EP) (n= 54), western North Pacific (WNP) (n= 56), and South China Sea (SCS) (n= 40) and one Indian Ocean locality (EI) (n= 33), analysis of mtDNA CR revealed that these are both of the ubiquitous clade. Currently blue marlin in the Pacific and Indian Oceans are managed as two distinct management units (the Western and Central Pacific Fisheries Commission [WCPFC] and the Indian Ocean Tuna Commission [IOTC]). This study investigated the genetic structure of blue marlin population inhabiting the Pacific and eastern Indian Oceans based on mtDNA cyt b and CR sequence variation. The results of pairwise ΦST, hierarchical analysis of molecular variance (AMOVA), and spatial analysis of molecular variance (SAMOVA) all support that there is no population differentiation among eastern Indian and Pacific blue marlin, thereby the population are comprised of a single stock. All eastern Indian and Pacific blue marlin possess high haplotype diversity (h) and low nucleotide diversity (π). Neutrality tests, pairwise mismatch distribution, and Bayesian skyline analysis all indicate that eastern Indian and Pacific blue marlin have undergone a rapid population expansion on the order of 0.30 to 0.74 million years ago. This study demonstrates that blue marlin in the Pacific and eastern Indian Oceans constitute a single genetic stock, which provides very important information for stock assessment purpose and for further conservation and sustainable utilization of this resource.