European Ocean Biodiversity Information System

Methodology of data compilation: Initially, an ICAS catalogue was created, including species classified as of high impact in the European Alien Species Information Network (EASIN), or as invasive or of high impact in recent review papers. A thorough search was conducted to look for information on the presence of these species in the Greek Seas. Georeferenced data, along with information on the reported date / year of sighting, depth range, abundance and habitat type were compiled using all available types of data sources, including published scientific literature, grey literature (e.g., theses, technical reports, and newspaper reports), offline databases (including records from past projects of universities, research institutes, the ELNAIS database, and citizen science initiatives), online scientific or citizen science databases (Algaebase, GBIF, OBIS, iNaturalist - only research-grade data), social networks (e.g., Instagram and Facebook), as well as through field work involving scientific diving and interviews with marine scientists, fishers and divers that were carried out during the ALAS project. In situ underwater sampling during the ALAS project was carried out in 2020 at 156 sites, and all sampling was conducted using non-destructive visual sampling techniques via SCUBA or free diving, down to a maximum of 20 m depth. Moreover, an unstructured interview approach was applied during the respective interviews with marine scientists, fishers and divers, while a photographic guide of the targeted ICAS was produced and provided during the interviews to enhance the species verification process. Information from social networks was specifically retrieved from the websites of diving centers, which were thoroughly checked for images of ICAS, and once relevant images were found the corresponding information was further validated through direct communication. All data were organized within an Excel spreadsheet and all duplicate records were deleted.