Table S1. Genetic variability at the microsatellite loci studied in Salmo trutta for the rivers Narcea (Na), Sella (Se), Cares (Ca) and Nive (Ni). Acronyms: allelic richness (AR); mean heterozygosity expected (Hs) and observed (Ho), with standard deviation (SD); HWE: proportion (in percent) of loci under Hardy–Weinberg equilibrium, after Bonferroni correction (Rice, 1989); sample size (n).

Table S2. Genetic units estimated with STRUCTURE (STR; Pritchard et al., 2000) and BAPS (Corander et al., 2003) software for the studied areas (rivers Narcea, Sella, Cares and Nive). K: number of genetic units. STR: STRUCTURE.

Table S3. Analyses of the Molecular Variance (AMOVA) in each zone considering the genetic units estimated for brown trout samples by STRUCTURE (Pritchard et al., 2000) and BAPS (Corander et al., 2003) software. Significance level: **P < 0.01; NS: not significant. AMOVA was not possible to do in the river Sella with BAPS groups because in this case there were not populations within groups.

Table S4. Pairwise FST between samples from the studied rivers (above diagonal) and their corresponding P-values (below diagonal). Significance level: *< 0.05; **< 0.01. (A) River Narcea; (B) River Sella; (C) River Cares; (D) River Nive.

Table S5. Estimates of gene flow (Nm) among genetic units of the studied rivers; Nm: number of migrants per generation; M = m/μ; U: migration to upstream areas (maximum likelihood); D: migration to downstream areas (maximum likelihood). Private alleles estimation: GENEPOP software (Raymond & Rousset, 1995); Coalescent estimation: MIGRATE software (Beerli, 2004). (A) STRUCTURE genetic units; (B) BAPS genetic units.

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