Genetic variation within and among domesticated Atlantic salmon broodstocks in British Columbia, Canada.

Atlantic salmon have been reared in the British Columbia, Canada aquaculture industry since the early 1980s. No breeding programmes spanned the entire production period and pedigree records were not kept for broodstocks prior to or since importation. Of the three recognized industry strains, two are of European ancestry ('Mowi' from Norway and 'McConnell' from Scotland) and one is of North American heritage ('Cascade' from Gaspe, Quebec). We evaluated the amount and distribution of genetic variation within industry broodstocks by surveying microsatellite variation at 11 loci in 20 broodstock groups sampled from major production facilities. Allelic richness averaged 10.9 (range 5.8-13.8), compared with a value of 20.3 obtained for a North American wild population. Pairwise genetic distances (D(S)) between samples within strains were generally less than those between strains, with samples attributed to the same strain clustering together in a neighbour-joining dendrogram. Nevertheless, average distances between samples within the European strains were high (0.41 for Mowi; 0.71 for McConnell) but lower (0.06) for the Cascade strain. The reduced intra-sample and increased intra-strain genetic variation observed for the BC domesticated samples compared with wild populations was similar to observations for European domesticated Atlantic salmon. Evidence of introgression of the Cascade strain into European broodstocks was provided by the presence of large Ssa202 alleles (confined to North America in wild populations) in some Mowi and McConnell samples. Introgression likely also contributed to the decreased intercontinental genetic distance for the domesticated samples of this study compared with that observed for wild populations.