Genetic markers for the discrimination of native and non-native Atlantic salmon in Europe.
Different types of genetic markers, developed to study the interactions of native and non-native Atlantic salmon Salmo s&r in Europe, were assessed for their discriminatory capacity. These included a Cbanding polymorphism on chromosome 8, 13 variable enzyme loci, mtDNA haplotype variation
identified by restriction endonuclease analysis, and five VNTR loci detected by probes isolated from Atlantic salmon and brown trout partial genomic libraries. Fifty fish from each of three wild stocks, the rivers Shin (Scotland), Blackwater (Ireland), and Esva ( Spain), and one farm strain of Norwegian origin, were screened.
No absolute discriminators were identified but stocks differed with respect to variant types present and showed significant frequency divergence with single locus D values greater than 0.9. This level of divergence is adequate to allow most individuals in two stock mixtures to be assigned to stock using stock-specific genotypes and differences in the expected occurrences of non-stock specific
genotype combinations. In this regard, two quasi-real scenarios are evaluated - stocking of the Rio Esva with River Shin fish and the presence of farm fish in the River Blackwater. In both scenarios, the two most powerful discriminators are VNTR loci followed by one (Esva/Shin) or two (farm/Blackwater) protein loci and then mtDNA. The proportion of individuals, within each stock, identifiable to stock origin based on the possession of a stockspecific genotype in the Esva/Shin comparison was O/O, 1.8/29.4,0/~6, and 74.8154.1 for the cytological, protein, mtDNA and VNTR markers respectively. In comparison, the figures were O/O, O/3.9,0/12.0, and 13.3/85.5 when the farm and Blackwater are compared. In the former case, further assignment using non-stock specific variation boosts classification success to > 95% with an error rate of less than 1% for most
proportional mixtures of the two stocks using the most discriminating of all four marker types. Our work suggests absolute stock markers are likely to be rare among European salmon. In spite of this, good discrimination of native and non-native salmon would still appear feasible in many situations for simple stock mixtures using differences in variant frequencies; the exact extent to which
this can be achieved and the usefulness of the different markers are likely to be situation specific. Discrimination can be enhanced in controlled situations by the genetic manipulation of stock groups prior to release. Practical considerations such as the selective neutrality of markers, desirability of non-destructive sampling, and the extent of population divergence would appear to make VNTR loci and mtDNA the markers of choice though against this is the high cost and technical complexity of the laboratory analysis.