Hybridization and invasive species in a threatened freshwater fish community under environmental pressures: Morphometric and molecular evidence

Mediterranean freshwater systems are under threat owing to increased drought driven by climate change, intensive human land uses and non‐native species. This is causing increased fish hybridization in isolated watercourses. The genetic and morphological characteristics of hybrids of sympatric native and non‐native fish species were studied in four streams of the Mediterranean Guadalquivir basin (south‐west Spain). Fish morphology was analysed using geometric morphometrics, and molecular determination of parenthood was inferred through one mitochondrial gene (cytb) and one nuclear gene (Beta‐actin) for all hybrids and a subset of pure parental specimens. Molecular analyses confirmed hybrids between the native Squalius alburnoides and non‐native Alburnus alburnus in a stream with continuous flow. Haplotype analyses suggested that they originated from backcrossing of hybrid offspring. Intergeneric crosses between native species S. alburnoides and Pseudochondrostoma willkommii, and S. alburnoides and Iberochondrostoma lemmingii were detected in streams under reduced connectivity scenarios. Morphometrics revealed that hybrid phenotypes were similar to S. alburnoides. In some cases, molecular markers uncovered hybridization events that were neither detected in the field nor by morphometric analyses, potentially supporting a backcrossing/introgression scenario. Hybridization is likely to be increasing in Mediterranean rivers where S. alburnoides are present owing to increased fragmentation caused by summer drought exacerbated by climate change and human land uses and pressures. This can become a problem for these endemic vulnerable species if genetic diversity is lost, morphological homogenization occurs and hybrids cannot be easily detected in the field. The potential risks could be addressed by monitoring and eradication of non‐native species and segregation from natives. To avoid native–native crosses, habitat quality and desiccation risk could be tackled by improved water quality and riparian reforestation to provide shade and reduce evapotranspiration. This would need increased coordination and intervention between the institutions that share conservation responsibilities in the area.