Introgression and genetic structure in northern Spanish Atlantic salmon (Salmo salar L.) populations according to mtDNA data

We have used PCR-RFLP analysis to screen ancestral and modern genetic variation of the mitochondrial ND1 region at eight Atlantic salmon rivers in northern Spain. Using old scales, we show that genetic variation in NW Spanish salmon populations has not changed significantly during the past 50 years. Comparing our data with that from the literature we can conclude that, in general, modern salmon populations from NW Spain are distinct from putative northern European donor populations. Taken together, these observations suggest that, despite the non-native introductions carried out during 1970-1993, introgression of northern European mitochondrial genomes in NW Spanish salmon populations has been generally low. Different results were obtained for populations in NE Spain and southern France, where rivers Nansa, Ason, Bidasoa and Nive presented a high frequency of haplotypes characteristic of northern European rivers that reflect introgression. We show that the levels of introgression are correlated with the foreign stocking intensity. Thus, the rivers that were more intensely stocked are more similar to northern European salmon rivers. We also observed an East-West cline in the levels of foreign introductions and introgression. In addition, we provide the first description of the overall genetic structure of Atlantic salmon populations in southern Europe. We have observed evidences of genetic differentiation among populations and isolation by distance as a consequence of salmon homing behaviour. The results obtained do not call for changes in introduction policies in Spain, currently based on the use of native stocks.