Integrative taxonomy reveals non-native Squalius hybrids in the Iberian Peninsula

The introduction of non-native species poses a critical threat to biodiversity in Mediterranean freshwater ecosystems. This study documents the first record of non-native Squalius hybrids in the Ave River (Iberian Peninsula), combining morphological and molecular data to confirm their presence and assess their potential origin. The specimens exhibit hybrid characteristics, with most morphological and meristic traits overlapping those of both S. cephalus and S. squalus . Almost all individuals analysed showed an admixed genomic background for both species, suggesting a hybrid origin likely within the species’ native range before their introduction into Portugal through human-mediated activities, likely related to recreational fishing. Furthermore, molecular data also supports hybridization with the native S. carolitertii , raising conservation concerns given the negative impacts associated with hybridization with non-native species. By combining data from social media and citizen science platforms, we were able to map the rapid expansion of this invasive chub in the Ave River from 2018 until now. Our findings emphasize the urgent need for population control measures, while enhancing monitoring, genetic studies, and increased public awareness to mitigate the impacts of invasive species on Iberian freshwater biodiversity.