Citizen warnings and post checkout molecular confirmations using eDNA as a combined strategy for updating invasive species distributions

Citizen warnings alerting about the presence of invasive species has been claimed as fundamental in the strategies addressed to manage aquatic invasions. A great volume of online information coming from fishermen and ordinary citizens, about detection of new species in their communities is currently available. Unfortunately, it is not always implemented as a useful tool by agencies, and within stakeholders protocols, to prevent and manage biological invasions. In this work, we have used as a case study the wels catfish Silurus glanis Linnaeus, 1758. Online blogs, webpages, videos, magazines, and newspapers, were searched for news about the presence and spread of S. glanis in the Iberian Peninsula until June 2016. This information was compiled with official reports (scientific papers and regional and national governmental reports) to create a map showing the putative current pattern of S. glanis invasion through Iberian freshwater ecosystems. The current situation pointed to Silurus's presence in six of the seven main river basins of the Iberian territory. Since non-official reports need post-alerts confirmations, we have also set up a molecular pilot study designing genus specific primers to detect this species in environmental DNA (eDNA) from the reported locations. In the pilot study, primers were tested on eDNA samples extracted from experimental aquariums and on real environmental samples taken from different basins in Spain (Ebro, Douro and Tagus). In all these basins S. glanis was detected. Official Silurus reports were confirmed with two molecular markers in five out of the six cases (83%) assayed in this work, and in two out of three non-official reports (66%) coming from fishermen websites and newspaper reports. The proposed combined strategy (citizen alerts and the eDNA detection method) can be a helpful tool in early detection of invasive species allowing fast and effective management actions by stakeholders.