Note on the distribution of the wels catfish, Silurus glanis Linnaeus, 1758 in the Serchio River (Central Italy): eDNA analysis and specimen occurrence data

The spread of invasive alien species is a significant ecological issue due to the competition for resources, disease transmission, and ecosystem damage it can cause. Management strategies focus on prevention, eradication or containment, approaches that require huge efforts and continuous monitoring. The wels catfish (Silurus glanis) is considered a generalist freshwater predator, raising major concerns given its voracity and remarkable adaptability. Introduced in Italy in the early 20th century, it is now widespread across northern and central regions. This study used online surveys, anglers’ interviews, and quantitative real-time PCR (qPCR) protocols with species-specific probes and primers on environmental DNA (eDNA) to assess the distribution of S. glanis upstream and downstream of an artificial ecological barrier, the Ripafratta (PI) weir, in the Serchio River basin (Central Italy). Online and stakeholder surveys as well as qPCR analyses did not detect the species upstream of the weir in 2022. As with any eDNA-based approach, this result reflects nondetection rather than confirmed absence. However, more recent reports from anglers indicate that the species may now occur upstream of the barrier. However, the mechanism behind this occurrence remains uncertain. Future efforts should focus on evaluating the influence of small weirs on the dispersal of large freshwater fishes such as the wels catfish to improve their correct management, especially when planning fish ladders to increase river connectivity.