The genetic structure and phylogeography of Atlantic salmon (Salmo salar) across the Baltic Sea basin and neighbouring areas (eastern Atlantic Ocean, North Sea, Barents Sea, White Sea, and two Russian lakes, Onega and Ladoga) were studied to resolve the partly contradictory hypotheses of the species' postglacial colonization history. Thirty-eight populations (total of 2180 individuals) were studied for nine DNA microsatellite loci. Within the Baltic Sea, the anadromous populations formed three clear groups, corresponding to the northern (Gulf of Bothnia), eastern (Gulf of Finland and eastern Baltic Main Basin), and southern regions (western Baltic Main Basin). The genetic differences among these three groups were clearly greater (G GB 5.6%; G GB being the proportion of diversity components between regions within basins) than were those among population groups in the eastern Atlantic Ocean (G GB 2.2%) from Ireland to the White Sea. The isolation-by-distance model explained part of the differentiation within, but not between, the regions. The results strongly indicate colonization of the Baltic Sea by at least three glacial lineages. Potential refugium areas for each lineage are proposed.Résumé : Nous avons étudié la structure génétique et la phylogéographie du saumon atlantique (Salmo salar) de part en part du bassin de la Baltique et dans les régions avoisinantes (est de l'Atlantique, mer de Barents, mer Blanche et deux lacs russes, les lacs Onega et Ladoga) pour résoudre les hypothèses en partie contradictoires sur l'histoire de la colonisation postglaciaire de l'espèce. Nous avons analysé neuf locus ADN microsatellites chez 2180 individus appartenant à 38 populations. Au sein de la Baltique, les populations anadromes forment trois groupes distincts qui correspondent aux régions du nord (golfe de Bothnie), de l'est (golfe de Finlande et bassin principal de l'est de la Baltique) et du sud (bassin principal de l'ouest de la Baltique). Les différences génétiques entre ces trois groupes sont nettement plus importantes (G GB 5,6 %; G GB représente la proportion d'éléments de diversité entre les régions à l'intérieur des bassins) que celles qui existent entre les groupes de populations de l'est de l'Atlantique (G GB 2,2 %), de l'Irlande à la mer Blanche. Le modèle de l'isolement en fonction de la distance explique une partie de la différentiation au sein des régions, mais non entre elles. Nos résultats indiquent fortement une colonisation de la Baltique par au moins trois lignées glaciaires. Nous proposons des zones possibles de refuge pour chaque lignée.[Traduit par la Rédaction] Säisä et al. 1904

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Microsatellite DNA based analysis of the pattern of genetic diversity among three coastal and five freshwater populations of pikeperch Sander lucioperca in the northern part of the Baltic Sea drainage basin indicated marked genetic differentiation between the coastal and lake populations. The F(st) between these population groups was as high as 0.25 and R(st) =0.32. In general, the lake populations showed higher genetic diversity than the coastal ones. In terms of genetic distance, the three coastal populations (Vanhankaupunginlahti, Västanfjärd and Taivassalo) grouped tightly together. The freshwater samples formed a looser group, in which the northern Lake Kemijärvi showed greater distance from the southern lakes than these did from each other. The two lake populations originally established through stockings (Lakes Painio and Averia) grouped near to their source population of Lake Lohjanjärvi and their diversity level was nearly the same. Safeguarding the unique Baltic coastal populations of S. lucioperca against gene flow from increasing hatchery releases using freshwater S. lucioperca should be a high management priority.

Patterns of genetic diversity and differentiation among five wild and four hatchery populations of Atlantic salmon in the Baltic Sea were assessed based on eight assumedly neutral microsatellite loci and six geneassociated markers, including four expressed sequence tag (EST) linked and two major histocompatibility complex (MHC) linked tandem repeat markers (micro-and mini-satellites). The coalescent simulations based on the method of Beaumont and Nichols (1996, Proc. R. Soc. Lond. Ser. B -Biol. Sci., 263, 1619-1626) indicated that two loci (MHCIIa and Ssa171, with the lowest and highest overall F ST estimates, respectively) exhibited significant departures (P<0.05) from the neutral expectations. Another coalescentbased test for selective neutrality (Vitalis et al. 2001, Genetics, 158, 1811-1823 further supported the outlier status of the Ssa171 microsatellite locus but not of the MHCIIa linked minisatellite. In addition, actin related protein linked microsatellite locus was identified with this test as an outlier in six pairwise population comparisons. All genetic diversity estimates revealed more genetic variation in hatchery stocks than in the small wild salmon populations from the Gulf of Finland. However, the wild populations possessed alleles at gene-associated markers (e.g. MHCI and IGF) not found in the hatchery stocks, which together with moderate genetic differentiation and distinctive environmental conditions justifies the special conservation measures for the last remaining native salmon populations in the Gulf of Finland.

The genetic consequences and gene flow of pikeperch (Sander lucioperca) stocking were assessed in three boreal lakes based on admixture model analysis and comparison of the pre- and post-release patterns of genetic variability at 9 DNA microsatellite loci in the recipient populations. In two out of the three cases, the releases of fish from foreign populations caused significant changes in the genetic structure of the recipient population. The largest changes were observed in Lake Oulujärvi, where the post-release sample was almost identical to the released Lake Vanajanselkä population, and about 90% of the catch was composed of the released population. The genetic composition of Lake Lohjanjärvi pikeperch also shifted markedly towards that of the released Lake Vanajanselkä population, and about half of the later catch was of released Vanajanselkä origin. In Lake Vanajanselkä, in contrast, releases of pikeperch from lakes Painio and Averia had only a small impact on the genetic structure of the pikeperch population. These results indicate that the current stocking practices create an effective artificial gene flow that may strongly shape and reduce the genetic differentiation among the remaining native pikeperch populations. A common feature of all three cases was the lack of prior appraisal of the potential genetic and ecological risks in relation to the expected benefits of the release programmes.