A new database harmonises information from France, Spain and Portugal to improve monitoring of the European eel throughout its migration

• An international team of scientists has published the largest harmonised dataset on the European eel and the river ecosystems it inhabits across France, Spain and Portugal.
• The work, coordinated by AZTI within the framework of the SUDOANG project, integrates information from more than 515,000 river sections, 106,400 barriers and nearly half a million eel records collected over more than three decades, with the aim of enabling more accurate assessments of the species and supporting the management and conservation of its habitats.

Sukarrieta, June 2, 2026. Understanding how the European eel is distributed across thousands of kilometres of rivers, and the factors influencing its presence, remains one of the major challenges in conserving this species, which is classified as Critically Endangered by the International Union for Conservation of Nature (IUCN). To contribute to this understanding, an international team of scientists has published a database in the journal Scientific Data that compiles and harmonises information on the species and its river habitats across France, Spain and Portugal.

The work is the result of several years of collaboration between research centres, universities and public bodies from the three countries, coordinated within the framework of the European INTERREG SUDOE SUDOANG project and supported by the Spanish General Secretariat for Fisheries through the DIADROMAS project.

The publication provides the scientific community and public authorities with a shared resource integrating information obtained through monitoring programmes, sampling campaigns and hydrographic databases developed by numerous organisations. Harmonising these datasets enables comparable analyses across territories and provides a more robust scientific basis for assessing eel populations at regional and international level.

Shared data for a species without borders

The database brings together information on 515,011 river sections, 106,400 barriers within river systems — including dams, weirs and fish passes — and 23,080 sampling stations, as well as 494,163 individual eel records collected between 1985 and 2020.

In addition to data on the species’ presence and abundance, the database includes detailed information on hydrographic networks, aquatic habitat surface areas, river connectivity and various human pressures that may affect eels throughout their life cycle. This information makes it possible to relate the species’ distribution to environmental characteristics and to analyse how factors such as river barriers or habitat availability may influence eel populations.

The results can be explored and visualised through VISUANG, an interactive platform that allows users to consult this harmonised information, together with other outputs from the project.

“One of the main challenges in assessing the status of the European eel is that the available data come from a wide variety of sources and are generated using different methodologies. This work has made it possible to integrate and harmonise all this information within a single common structure, facilitating large-scale analyses that were previously extremely complex,” explains María Mateo, researcher at AZTI and lead author of the study.

“Having a shared reference framework will help us better understand how the species is distributed, which factors determine its presence and how its populations evolve across different European regions,” she adds.

The authors emphasise that establishing a common database is an important step towards improving scientific assessments of the species and advancing more standardised monitoring methods across countries. The resource may also prove valuable for studies on other migratory fish species and for research related to the management and conservation of river ecosystems.

The challenge of tracking a highly migratory species

The European eel undergoes one of the most fascinating life cycles in the animal kingdom. It is born in the Sargasso Sea, in the western Atlantic, and its larvae travel thousands of kilometres driven by ocean currents before reaching European and North African coasts. From there, the species colonises estuaries, rivers, lagoons and wetlands across both the Atlantic and Mediterranean regions, where it spends much of its life before beginning the return journey to the ocean to reproduce.

This extraordinary migration means that what happens in each river stretch can have consequences for a population sharing a common origin and destination. For this reason, having harmonised information on the species’ distribution, habitat quality and the presence of barriers throughout its range is essential for understanding how different environmental and human pressures affect it in each territory.

The authors highlight that this shared database represents a benchmark tool for improving international monitoring of the species, advancing towards more comparable assessment methodologies between countries, and supporting future research on eels and other migratory fish. At a time when populations continue to face multiple threats, access to harmonised and openly shared information is essential for improving our understanding of the species’ status and underpinning management decisions with a solid scientific basis.

Scientific article:

Mateo, M., Briand, C., Korta, M. et al. A database of eels and their freshwater habitats in southwestern Europe. Sci Data (2026). https://doi.org/10.1038/s41597-026-07213-3