Hybridization Dynamics in Declining Southernmost Atlantic Salmon Populations:  30-year study

Global change severely alters riverine ecosystems, shifting interspecific dynamics and potentially increasing hybridization rates among closely related species. In Southern Europe, Atlantic salmon (Salmo salar) populations are facing dramatic declines, which hypothetically drives an increase in interspecific crossings with the more stable brown trout (Salmo trutta) due to a shortage of conspecific mates. This study aims to determine recent hybridization rates in Asturian rivers (Spain), evaluate historical trends through the comparison with the results from Castillo et al. (2008) and identify cross directionality. A total of 2,509 juvenile salmonids were sampled in several catchments from six Asturian river basins in 2023, 2024 and 2025. Species and hybrid identification were confirmed using nuclear molecular markers (5S rDNA and H3p) based on amplicon size polymorphism. The maternal origin of detected hybrids was resolved by sequencing the mitochondrial cytochrome c oxidase subunit I (cox1) gene. These results were compared with the ones obtained in the same region, species and methodology by Castillo et al. (2008), who analysed 1,561 samples from 1989-1994 and 1,630 from 2004. An overall hybridization rate of ~1.2 % was detected for the 2023-2025 period, showing no significant variation between years or river basins. While significantly lower than historical rates from the stocking era (4.22% in the 1985–1994 period), hybridization has remained remarkably low and stable since 2004, when the rate in these same rivers was of 1.09%. Mitochondrial cox1 analysis revealed asymmetric inter-specific gene flow, with Atlantic salmon acting as the maternal parent in all successful cases. Hybridization between S. salar and S. trutta remains a rare and stable event at their southernmost range margin despite salmon population declines. The strong asymmetric directionality points toward specific behavioral or evolutionary reproductive barriers. Further research should assess the possibility of introgression between both parental genomes, what may have evolutionary implications.