Hybridization With a Non‐Native Congener Threatens the Catalan Chub ( <i>Squalius laietanus</i> ): Optimization of a DNA‐Based Early Warning Tool

Hybridization with non-native congeners represents an increasingly recognized threat to the long-term viability and genetic integrity of endemic freshwater fishes. The Catalan chub (Squalius laietanus), a leuciscid endemic to northeastern Iberian river basins and currently listed as Vulnerable on the International Union for the Conservation of Nature Red List, has recently been exposed to the expansion of other Squalius species within its range, particularly the European chub (Squalius cephalus), raising concerns regarding interspecific hybridization and introgression. Here, we develop and apply a rapid and cost-effective nuclear DNA assay that reliably discriminates native, non-native, and hybrid genotypes. Using this approach, hybridized individuals were consistently detected in known contact zones, as well as in river sections where only S. laietanus had previously been identified based on mitochondrial DNA barcoding. These results provide molecular evidence of spatially widespread hybridization between S. laietanus and non-native S. cephalus across the core distribution of the endemic species. The observed pattern of hybridization adds another threat to S. laietanus, and it is therefore directly relevant for its future Red List assessment.