Contemporary population structure and historical demography of sailfish (Istiophorus platypterus) in the Atlantic Ocean
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The Atlantic sailfish (Istiophorus platypterus) is considered over-fished in U.S. waters. Furthermore, preliminary analyses of abundance indicate that a decline in biomass has occurred. While seminal studies have provided useful baseline data about intra- and inter-oceanic variation within sailfish, such studies may have underestimated the amount of genetic variability as a result of small sample sizes and the poor resolution of restriction fragment length polymorphism data. We used 263 base pairs from the hypervariable mitochondrial control region and fragment polymorphisms from five microsatellite loci to assess the contemporary population structure between eastern (n = 30) and western (n = 192) Atlantic sailfish stocks. We failed to reject the hypothesis of panmixia in Atlantic sailfish; however, higher levels of genetic variation were observed within the eastern Atlantic sailfish, and the nature of this difference needs to be investigated further. Having found no evidence of transatlantic differentiation, we pooled the Atlantic samples (n = 222) and compared them to a sample from the eastern Pacific (n = 22) and rejected the null hypothesis, concluding that sailfish from separate ocean basins do not share a common gene pool. We also found evidence of a recent sudden expansion of Clade I (the ubiquitous clade found in both Atlantic and Pacific Oceans) sailfish into the Atlantic that appears to have occurred between 164000 and 351000 years ago, coinciding with interglacial periods during the Pleistocene. This study also presents the first evidence of a recent sudden expansion of sailfish into the eastern Pacific, roughly 85400 to 173000 years ago, following a period when cooling in the eastern Pacific would have restricted the sailfish range to the warmer waters of the western Indo-Pacific. We emphasize that sailfish from this region of the Pacific are phenotypically distinct from those in the rest of the species' range and encourage further studies in order to determine if the eastern Pacific sailfish population should be managed as a separate stock.