Resolving Mixed Stocks in Highly Migratory Species Using Bayesian Clustering of SNP Data Characterized with High-Resolution Melting

Atlantic swordfish (Xiphias gladius) is a highly migratory epipelagic species subject to heavy commercial exploitation.  Previous population genetic studies provide concordant evidence of significant differentiation between Mediterranean, Northwest Atlantic, and South Atlantic swordfish. However, the extent of population admixture is unknown making Atlantic swordfish an ideal model for inferring population admixture with Bayesian clustering.  Nine nuclear genetic markers were developed and genotyped with high-resolution melting analysis (HRMA).  Analysis of 291 adult swordfish from the Northwest Atlantic (n=96), Mediterranean (n=91), South Atlantic (n=104) revealed significant differentiation between Mediterranean, Northwest Atlantic, and South Atlantic stocks.  Analysis of individuals in boundary areas indicate an expansion of South Atlantic swordfish around the northern coast of Africa.  Areas of admixture of North and South stocks were identified near the Azores whereas admixture of Mediterranean and Atlantic swordfish was identified to be minimal.  We found HRMA for SNP discovery and genotyping to be an inexpensive, high-throughput, closed-tube approach ideal for genetic population studies.  Bayesian individual assignment using the newly developed nuclear marker set can provide managers the tools to resolve population admixture in Atlantic swordfish and serve as model for other highly migratory species.