At the Department of Aquatic Resources (SLU Aqua), we conduct research and monitoring of the European eel (Anguilla anguilla).
We lead and carry out Sweden's data collection program for eel, which is part of the EU's Data Collection Framework (DCF). We also conduct work related to Sweden's Eel Management Plan, including the triennial stock assessment. In addition to monitoring and data collection, we are involved in several research projects aimed at improving our understanding of the European eel and helping to improve the status of its population.
About the European eel
European eels can live to a very old age. We do not know exactly how old they can become, but at least 80 years, and probably much older. The oldest individuals are females; males generally live only a few years.
Eels pass through several life stages. They hatch from eggs as flat, leaf-shaped larvae (leptocephali), then transform into glass eels, followed by pigmented elvers (juvenile eels). During their growth phase they are known as yellow eels and have a yellow-brown colouration. They later transform into silver eels, developing a silvery-grey colour. At this stage they stop feeding, their eyes and pectoral fins enlarge, and they migrate back to the Sargasso Sea, where they were born, to spawn.
Eels migrate extraordinary distances. Their eggs hatch in the Sargasso Sea, and the larvae are carried by ocean currents to the coasts of Europe and North Africa—a journey of up to 8,000 km. They may then migrate long distances into rivers and lakes before returning all the way to the Sargasso Sea. During this return migration they alternate between swimming at approximately 400 meters depth at night and about 800 meters during the day.
We know surprisingly little about eels. For example, no one has successfully tracked a single adult eel all the way from the coasts of Europe or North Africa back to the Sargasso Sea. No one has ever observed an adult eel in the Sargasso Sea. We know almost nothing about how they reproduce. We also do not know why they choose to begin their migration at a particular age. In Sweden, some females begin their return migration at around 15 years of age, while others wait until they are 40 years old or even older.
The eel is a panmictic species, meaning that all European eels throughout their entire range belong to one single breeding population. This differs from species such as salmon and trout, which consist of hundreds of distinct populations. European eels are panmictic because all individuals, regardless of where they spent their juvenile lives in Europe or North Africa, reproduce together in the Sargasso Sea, where all genes are mixed.
View the latest assessment of eel populations in Sweden's marine, coastal, and freshwater environments through the Fiskbarometern (currently only available in Swedish).
A more detailed assessment is available in Sweden's triennial stock assessment, most recently completed in 2024.
An international stock assessment covering the entire distribution of the European eel is conducted annually by the WGEEL (Working Group on Eels). The latest report is available online.
In this two-year research project we will investigates habitat selection by yellow eels in Swedish waters during three different periods:
The present day
Before hydropower expansion in Sweden (before 1920)
The ICES management reference period (1960–1979), before the dramatic decline of eel populations across Europe
The goal is to understand how habitat use varies among yellow eels and whether it has changed over time.
To do this, we analyse the concentrations of different chemical elements in eel otoliths (ear stones) from birth until capture. These chemical signatures reflect environmental conditions such as water salinity, allowing us to reconstruct where an eel has lived throughout its lifetime.
The project investigates the effects of glass eel stocking on the critically endangered European eel by comparing locally recruited and imported, quarantined eels in laboratory and field studies, analysing 40 years of monitoring data, and compiling a systematic evidence review – providing a stronger knowledge base for future eel management.
The project investigates how hydropower plants affect the survival and migration of the critically endangered European eel by comparing mortality and migration routes before and after fish passage facilities were constructed at three hydropower plants – providing improved knowledge to support future hydropower licensing reviews and eel management.
This project aims to improve the management of salmon and eel across Europe by strengthening the scientific knowledge base throughout their entire life cycles and geographic distribution.
A large amount of eel data is available through the national and international stock assessments (see the links under "How us the european eel doing?").
Additional datasets are available through the Eel Data Portal, including information on stocking, recruitment, recaptures, production, fisheries, survival, and more.
Electrofishing data for eels and other fish species are also available through a separate application.
To ensure the quality of the eel data collected under the national DCF (Data Collection Framework) program, several quality assurance measures are applied. More information about these procedures is available in the documentation on data quality assurance.
The Last Eel (in Swedish, Uppdrag Granskning, Swedish Television).
Interested in learning more about eels through accessible, science-based communication? Watch and listen to this informative music video about eels on YouTube.
Members of SLU Aqua's Eel Biology Unit (from left to right): Eirik Ryvoll Åsheim, Birgitta Jacobson, Rob Van Gemert, Philip Jacobson, Elin Myrenås, Lisa Spotowitz, and Josefin Sundin. Photo: Ida Ahlbeck Bergendahl.
Contact
Person
Josefin Sundin, Researcher and Deputy Head of Division
