Searching for the critically endangered European eel in oceanic islands: A pioneer study in the freshwater systems of Madeira, Macaronesia

The globally threatened European eel (Anguilla anguilla) is the only freshwater fish native to Macaronesia. Despite being a keystone species to freshwater habitats, little is known about its population, environmental associations and conservation requirements in oceanic islands. The density and geographical-related factors influencing the distribution of A. anguilla in the subtropical Archipelago of Madeira (Portugal) were examined. Data on the species occurrence was initially collected using an online survey addressed to citizens. Eels were then sampled through electrofishing in 31 sampling sites along 11 streams on Madeira Island, and eel-habitat associations were investigated using LMMs and GLMMs. One stream was also sampled in Porto Santo Island. The presence of A. anguilla was confirmed in the two islands of the archipelago with streams (Madeira and Porto Santo). Eels are distributed throughout Madeira Island, being more abundant - particularly small size individual- at lower altitudes (mostly below 150 m). The number of weirs was an important predictor of eel density, suggesting that these flood-control structures limit their upstream migration. This study represents a pioneer sampling effort of the European eel in Madeira and results provide much-needed baseline information on the species distribution and related environmental factors in oceanic islands. While most eel research has historically focused on mainland habitats where dams pose a significant threat to eel habitats, our study underscores the critical importance of addressing smaller, often overlooked barriers that may also cause habitat fragmentation. The European eel is a panmictic species, thus, the conservation of the Macaronesian subpopulations can have substantial benefits for its global recovery. A comprehensive investigation of the impact of man-made obstacles on eel habitat is still needed. Similarly, other common threats such as interactions with non-indigenous species and infection by Anguillicola crassus should be a research priority in future studies.