Microsatellite DNA data point to extensive but incomplete admixture in a marble and brown trout hybridisation zone

Marble trout, endemic to the Adriatic drainage basin, is severely threatened by hybridisation with non-native brown trout. In the present study, we analysed 12 microsatellite DNA loci to assess genetic population structure and differentiation between sympatric phenotypic marble and brown trout at nine sampling sites in the upper Etsch/Adige River system. F _ST and AMOVA analyses revealed significant genetic differentiation between marble and brown trout samples. Thus, admixture between brown and marble trout appears to be incomplete. However, factorial correspondence analysis depicted marble trout, Atlantic brown trout and intermediate genotypes. Bayesian-based individual assignment tests identified indigenous marble trout at five sampling sites. In four other samples no ‘pure’ marble trout were detected. Bidirectional, first-generation hybridisation, involving both sexes of both parental species was observed. In locations where ‘pure’ marble trout still exist, post-F1 hybridisation appears to be directed towards brown trout. This has likely slowed the rate of hybridisation between the two trout species and the decline of relic marble trout populations. Based on these results, restoration management actions are proposed, such as the abandonment of brown trout stocking activities, sharper angling policies, establishment of indigenous marble trout breeding strains and the elaboration of a conservation priority list.