The microsatellite DNA data are the most extensive and therefore our inferences are based most strongly on them. The other data are consistent with the microsatellite DNA data, however, which increases our confidence. For allozyme and microsatellite markers, there is broad regional divergence among European (national) populations and somewhat less among North American (provincial or state) populations, the latter having less spatial separation. The RAPD results parallel those for the allozymes and microsatellites to the extent that the sampling frame was comparable. For mtDNA markers, the patterns within Europe and within North America are reversed with respect to the allozyme and microsatellite results, more variation occurring among North American regions than among European regions. The mtDNA markers show about the same watershed-to-watershed divergence as the nuclear markers, but they show less variation within watersheds. Within either continent, genetic similarities are slightly higher in populations from different tributaries within major watersheds than in populations from different major watersheds.
Collectively, the results show the same pattern of hierarchical genetic divergence as that shown by the previous extensive allozyme data (May et al. 1994 and references therein) and that shown by the less-extensive protein and DNA analyses of Atlantic salmon from North America (e.g., Verspoor 1986, Ståhl 1987, Bermingham et al. 1991, King et al. 1993, Schill and Walker 1994, Kornfield 1994, Taggart et al. 1995, McConnell et al. 1997, King et al. 2000). The results show large divergence between continental (North American and European) populations; broad regional divergence on both sides of the Atlantic, European national populations being about twice as divergent as North American provincial and state populations; and comparable interwatershed divergence within Maine and within the Canadian provinces. In addition, intertributary divergence in a major watershed (e.g., the Penobscot River) is sometimes substantial, with predictable temporal variation within a given sampling locality. This is the typical pattern seen in salmon and their relatives (Ryman 1983, Ståhl 1987, Allendorf and Waples 1996).