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Mapping the marine migration of an IUCN endangered species

The spawning grounds of the European eel (Anguilla anguilla L.) have been a mystery for centuries and with the declining population, making it a critically endangered species on the IUCN Red List, it is important to unravel this mythical place and the migration routes leading to it. Understanding where eels are coming from and the obstacles they face during their migration are essential to take appropriate management actions to restore the population. Yet, it is not trivial to do. Eels migrate over 6000 km through the marine realm to reach their spawning grounds, presumably in the Sargasso Sea. This area has been identified as the spawning site since 100 years ago, when the Danish researcher Johannes Schmidt found the smallest and youngest eel larvae in those waters. However, nowadays technology allows to track eels through the deep depths of the Atlantic Ocean on their spawning migration.

Keywords: European eel, Anguilla anguilla, migration, data storage tags, English Channel, North Sea

“Technological advances allow the unprecedented mapping of fish migration routes, such as the enigmatic spawning migration of the European eel.”

Pieterjan Verhelst

Pieterjan Verhelst is a fish biologist at the Research Institute for Nature and Forest, where he focuses on fish migration research. He has been involved in topics such as testing fish passages and studying fish migration behaviour for more than 10 years. The European eel has been playing a key role in his research from the beginning since it is considered a symbol species for fish migration research and management.


We aimed to identify the marine spawning migration routes of eels swimming into the Atlantic Ocean. It is evident that eels leaving Belgium took a shorter route through the English Channel. However, our results indicated that some eels migrate north and reach the Atlantic Ocean over the UK. It is uncertain if both eels consume an equal amount of energy for migration, let alone spawning, and therefore contribute to the next generation equally. It does show that the animal adopts a flexible spawning migration strategy.


Eels were externally fitted with pop-off data storage tags. These devices log the temperature and depth records that the animal encounters at that moment. This information is used to model their migration trajectories. One disadvantage of this method is that the tags need to be retrieved to download the data. This is not a trivial thing for a device attached to an animal that is swimming away from continental Europe. However, the tags we used have a pop-off mechanism that ensures the tag detaches from the fish after six months, floats to the surface and then starts drifting with currents and wind and hopefully washes ashore so it can be found by beachcombers. A €50 reward stimulates the retrieval rate and indeed, about 35% of our 238 deployed tags have been recovered. By comparing the temperature at depth registered by the tag with those of marine maps we can reconstruct the trajectory the eel swam. In some occasions the data provided additional information of the tags fate. As an example, it occurred that the temperature suddenly rose to 36°C and showed substantial up and down movement in the water column, spanning almost 800 m. This behaviour is typical for a pilot whale (Globicephala sp), indicating our tagged eel got predated! The longest track we retrieved from the data was about 1200 km. However, most tags pop much earlier and before the preprogrammed pop-off time. We are not sure if this is attributed to the tag-attachment method or, for instance, underestimated predation. Future and more research will hopefully tell!


Used components of the LifeWatch Infrastructure

This research was supported through LifeWatch who funded part of the data logging devices, called archival tags. The European Tracking Network (ETN) database is used as data archive and for the (meta)data management and greatly helped to keep track of all the retrieved and downloaded datasets.



Van Wichelen, J., Verhelst, P., Perneel, M., Van Driessche, C., Buysse, D., Belpaire, C., Coeck, J. & De Troch, M. (2022). Glass eel (Anguilla anguilla L. 1758) feeding behaviour during upstream migration in an artificial waterway. Journal of Fish Biology.

Goossens, J., Buyse, J., Bruneel, S., Verhelst, P., Goethals, P., Torreele, E., Moens, T. & Reubens, J. (2022). Taking the time for range testing: an approach to account for temporal resolution in acoustic telemetry detection range assessments. Animal Biotelemetry 10: 1-13.

Verhelst, P., Aarestrup, K., Hellström, G., Jepsen, N., Koed, A., Reubens, J., Sjöberg, N., Svendsen, J.C. & Kristensen, M. L. (2022). The effect of externally attached archival data loggers on the short-term dispersal behaviour and migration speed of European eel (Anguilla anguilla L.). Animal Biotelemetry 10: 1-8.

Verhelst, P., Reubens, J., Coeck, J., Moens, T., Simon, J., Van Wichelen, J., Westerberg, H., Wysujack, K. & Righton, D. (2022). Mapping silver eel migration routes in the North Sea. Scientific reports 12: 1-10.

Verhelst, P., Reubens, J., Buysse, D., Goethals, P., Van Wichelen, J., Moens, T. (2021). Towards a roadmap for diadromous fish conservation: the Big Five considerations. Frontiers in Ecology and the Environment, online published.

Van Wichelen, J., Verhelst, P., Buysse, D., Belpaire, C., Vlietinck, K., Coeck, J. (2020). Glass eel (Anguilla anguilla L.) behaviour after artificial intake by adjusted tidal barrage management. Estuarine, Coastal and Shelf Science, 107127.

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Bruneel, S., Verhelst, P., Reubens, J., Luca, S., Coeck, J., Moens, T., Goethals, P. (2020). Combining disciplines: Dealing with observed and cryptic animal residencies in passive telemetry data by applying econometric decision-making models. Ecological Modelling 438: 109340.

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Reubens, J., Verhelst, P., van der Knaap, I., Wydooghe, B., Milotic, T., Deneudt, K., Hernandez, F., Pauwels, I. (2019). The need for aquatic tracking networks: the Permanent Belgian Acoustic Receiver Network. Animal Biotelemetry 7, 1 – 6.

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Verhelst, P., Reubens, J., Aelterman, B., Van Hoey, S., Goethals, P., Moens, T., Coeck, J., Mouton, A. (2018). Movement behaviour of large female yellow European eel (Anguilla anguilla L.) in a freshwater polder area. Ecology of Freshwater Fish 27: 471-480

Verhelst, P.; De Meyer, J.; Reubens, J.; Coeck, J.; Goethals, P.; Moens, T.; Mouton, A.M. (2018). Unimodal head-width distribution of the European eel (Anguilla anguilla L.) from the Zeeschelde does not support disruptive selection. PeerJ 6: e5773.

Verhelst, P.; Bruneel, S.; Reubens, J.; Coeck, J.; Goethals, P.; Oldoni, D.; Moens, T.; Mouton, A. (2018). Selective tidal stream transport in silver European eel (Anguilla anguilla L.) – Migration behaviour in a dynamic estuary. Est., Coast. and Shelf Sci. 213: 260-268.

Verhelst P.; Baeyens R.; Reubens J.; Benitez J.; Coeck J.; Goethals P.; Ovidio M.; Vergeynst J.; Moens T.; Mouton A.. (2018). European silver eel (Anguilla anguilla L.) migration behaviour in a highly regulated shipping canal. Fish. Res. 206: 176 - 184.

Verhelst, P.; Reubens, J.; Pauwels, I.; Buysse, D.; Aelterman, B.; Van Hoey, S.; Goethals, P.; Moens, T.; Coeck, J.; Mouton, A. (2018). Movement behaviour of large female yellow European eel (Anguilla anguilla L.) in a freshwater polder area. Ecol. Freshw. Fish. 27(1): 471-480.

Verhelst, P.; Buysse, D.; Reubens, J.; Pauwels, I.; Aelterman, B.; Van Hoey, S.; Goethals, P.; Moens, T. (2018). Downstream migration of European eel (Anguilla anguilla L.) in an anthropogenically regulated freshwater system: Implications for management. Fish. Res. 199: 252-262.

Verhelst, P.; Boets, P.; Van Thuyne, G.; Verreycken, H.; Goethals, P.L.M.; Mouton, A. (2016). The distribution of an invasive fish species is highly affected by the presence of native fish species: evidence based on species distribution modelling. Biological Invasions 18(2): 427-444.

Reubens, J.; Verhelst, P.; van der Knaap, I; Deneudt, K.; Moens, T.; Hernandez, F. (2018). Environmental factors influence the detection probability in acoustic telemetry in a marine environment: results from a new setup. Hydrobiologia Online: 1–14.

Bruneel, S.; Gobeyn, S.; Verhelst, P.; Reubens, J.; Moens, T.; Goethals, P. (2018). Implications of movement for species distribution models - rethinking environmental data tools. Sci. Total Environ. 628-629: 893-905.

Breine, J.; Pauwels, I.; Verhelst, P.; Vandamme, L.; Baeyens, R.; Reubens, J.; Coeck, J. (2017). Successful external acoustic tagging of twaite shad Alosa fallax (Lacépède 1803). Fish. Res. 191: 36-40.

Huisman, J.; Verhelst, P.; Deneudt, K.; Goethals, P.L.M.; Moens, T.; Nagelkerke, L.A.J.; Nolting, C.; Reubens, J.; Schollema, P.P.; Winter, H.V.; Mouton, A. (2016). Heading south or north: novel insights on European silver eel Anguilla anguilla migration in the North Sea. Mar. Ecol. Prog. Ser. 554: 257-262.

De Smet, B., D'Hondt, A.-S., Verhelst, P., Fournier, J., Godet, L., Desroy, N., Rabaut, M., Vincx, M., Vanaverbeke, J. (2015). Biogenic reefs affect multiple components of intertidal soft-bottom benthic assemblages: the Lanice conchilega case study. Estuarine, Coastal and Shelf Science 152: 44–55.



As the laureate of the 2016 VLIZ Communication Award, Pieterjan could benefit from a personal communication coaching by the VLIZ Communication division. This collaboration resulted in an educational video on eel migrations and acoustic telemetry.


News and outreach


Radio interview with Pieterjan



Useful links

Fish acoustic receiver network: Learn about the DST tags and fish telemetry network of the Belgian LifeWatch Observatory.
ETN: Access, store and share fish tracking data on the European Tracking Network data platform.
Data explorer: Access and explore the acoustic fish detection data with the LifeWatch data explorer.