Abstract: The alteration of structural connectivity in fuvial networks is important for the genetic dynamics of aquatic species. Exploring the efects of network fragmentation through genetic analysis is crucial to assess the conservation status of riverine species. In this study, we investigated the genetic consequences of the altered connectivity of brown trout in the Deva Cares catchment (northern Spain). We investigated (1) genetic diversity, (2) genetic diferentiation and genetic structure, (3) migration rates and efective population size and (4) genetic diferentiation and riverscape characteristics. Analysis of the genetic variation among 197 individuals from the 13 study sites revealed a high degree of genetic diferentiation (FST=0.181). Belowbarrier study sites had higher genetic diversity and lower FST values, while headwater and above-barrier study sites had lower genetic diversity and higher FST values. Most of the genetic groups identifed were separated by one or more impermeable barriers. We reported an abrupt decrease in genetic diversity and efective population size in upper course tributaries and isolated reaches. Likewise, a downstream-biased gene fow was found, and it was most likely related to the fragmentation caused by barriers, since the results from migration indicated that gene fow between groups without impermeable barriers was higher bidirectionally. Isolation by impermeable barriers played a more important role than hydrological distance in determining the genetic structure. Most of the genetic groups showed small efective population sizes. Genetic analysis at the river network scale provides evidence for the role of barriers in determining genetic diversity patterns, highlighting the importance of maintaining and restoring river longitudinal connectivity.

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