Population structure of Atlantic salmon (Salmo salar L.): a range-wide perspective from microsatellite DNA variation

Atlantic salmon (n = 1682) from 27 anadromous river populations and two non anadromous strains ranging from south-central Maine, USA to northern Spain were genotyped at 12 microsatellite DNA loci. This suite of moderate to hig hly polymorphic loci revealed 266 alleles (5-37/locus) range-wide. Statisti cally significant allelic and genotypic heterogeneity was observed across l oci between all but one pairwise comparison. Significant isolation by dista nce was found within and between North American and European populations, i ndicating reduced gene flow at all geographical scales examined. North Amer ican Atlantic salmon populations had fewer alleles, fewer unique alleles (t hough at a higher frequency) and a shallower phylogenetic structure than Eu ropean Atlantic salmon populations. We believe these characteristics result from the differing glacial histories of the two continents, as the North A merican range of Atlantic salmon was glaciated more recently and more unifo rmly than the European range. Genotypic assignment tests based on maximum-l ikelihood provided 100% correct classification to continent of origin and a veraged nearly 83% correct classification to province of origin across cont inents. This multilocus method, which may be enhanced with. additional poly morphic loci, provides fishery managers the highest degree of correct assig nment to management unit of any technique currently available.