Patterns of Genetic Connectivity in Southern European <scp><i>Salmo trutta</i></scp> L. Populations

ABSTRACT Salmonid fish species are highly threatened by climatic and anthropogenic pressures, since they are very sensitive to thermal stress and habitat degradation. Salmo trutta L. is a salmonid with a distinct lifestyle, such as the holobiotic (brown trout) and the anadromous (sea trout) ecotypes. Near the southern limit of the species distribution, Iberian trout populations are arguably more vulnerable to environmental stressors. To analyse the genetic diversity, structure and migratory patterns of trout populations in the Iberian Atlantic coast, 705 trout representing both ecotypes were sampled. Molecular fingerprinting methodologies were used, applying a set of 14 microsatellite loci developed for salmonids. Results suggest a latitudinal genetic pattern, with higher diversity among the northern populations and genetic differentiation between the northern populations and the southernmost populations. Following a latitudinal gradient of abundance, the anadromous trout emerges as one of the main drivers of gene flow between these populations. Our results also reveal a longitudinal genetic pattern within river systems, with trout populations fragmented not only by large hydropower dams but also by successive smaller barriers, resulting in distinct genetic groups. River barriers were clearly shown to promote significant isolation and a decrease in genetic variability of upstream trout populations. Therefore, restoring longitudinal connectivity is a key action that should be prioritised. This study provides a breakthrough in understanding the genetic structure of southern European trout populations, offering essential insights for the effective management and conservation of this threatened and highly valuable species.