The European Eel: A Conservation Journey

The Mystery of the European Eel: For centuries, the life cycle of the European eel (Anguilla anguilla) remained one of the greatest mysteries in marine biology. This elusive species, which migrates thousands of kilometres from European rivers to the Sargasso Sea to spawn, baffled scientists, and fishermen alike. The question of where and how eels reproduce was so perplexing that even Aristotle speculated that eels emerged spontaneously from the mud. The mystery persisted until the early 20th century when Danish scientist Ernst Johannes Schmidt made groundbreaking discoveries.

Ernst Johannes Schmidt's Discoveries: Born in Jaegerspris, Denmark, Schmidt dedicated his career to solving the "Eel Question." Through extensive research funded by the Carlsberg Foundation, Schmidt established that European eels undertake a long reproductive migration to the Sargasso Sea. His meticulous work from 1904 to 1923, including numerous expeditions and sampling campaigns, culminated in identifying the Sargasso Sea as the eel's primary spawning ground. This pivotal discovery provided the first clear understanding of the eel's life cycle, significantly advancing marine biology and conservation efforts.

The life cycle of the European eel (Anguilla anguilla). 

The Life Cycle of the European Eel: The European eel is known for its complex and highly migratory life cycle, which includes several distinct stages spanning marine and freshwater environments. The eel's life begins in the Sargasso Sea, where it hatches and drifts towards the coasts of Europe and North Africa as larvae (leptocephali). Upon reaching the continental shelf, they metamorphose into glass eels, migrate into freshwater or brackish environments, and grow into yellow eels. After several years, they transform into silver eels and migrate back to the Sargasso Sea to spawn. This species is characterized by its elongated body, adaptability to diverse habitats, and extensive migratory behaviour.

Documenting the Decline: The decline of the European eel was first documented in scientific literature in the late 1970s. During various sessions of the European Inland Fisheries Commission (EIFAC) and ICES symposia, significant reductions in eel populations and recruitment rates were identified. The earliest comprehensive mention of the decline can be traced to the EIFAC's 1968 session in Rome and subsequent meetings, where concerns were raised about decreasing eel fisheries and recruitment failures [“].

Causes of Decline: Several ecological issues have contributed to the decline of the European eel. Illegal trade, and overfishing, particularly of glass eels and silver eels, has severely impacted their populations. Habitat loss due to the construction of barriers such as dams and weirs disrupts their migratory routes, preventing access to essential spawning and growth habitats. Additionally, pollution and changes in water quality have degraded their environments. Climate change has altered oceanic currents and temperature regimes, further affecting eel spawning and larval survival rates. The introduction of parasites like the swimbladder nematode (Anguillicola crassus) has exacerbated the decline by impairing eel health and reducing their survival.

Challenges in Conservation: Conserving the European eel poses numerous challenges due to its intricate life cycle and extensive migratory patterns. Effective conservation requires international collaboration, as eels migrate across multiple jurisdictions from their spawning grounds in the Sargasso Sea to European and North African waters. Establishing and enforcing consistent regulations across these regions is difficult. The eel's long generation time means that population recovery is inherently slow, often taking decades. Limited knowledge of their oceanic life stages further complicates comprehensive management strategies. Current conservation efforts include habitat restoration, construction of eel passes to facilitate migration, and stricter fishing regulations to protect vulnerable life stages.

Eel Conservation Efforts in Denmark: In Denmark, several conservation efforts are underway to address the decline of the species. These initiatives are part of broader European efforts aimed at protecting this critically endangered species. The most important regulations and organizations for eel conservation in Denmark include:

  • EU Eel Regulation (EC 1100/2007): This mandates the protection and sustainable use of the European eel stock. It requires EU member states, including Denmark, to implement Eel Management Plans (EMPs) aimed at reducing human impacts to levels that allow the species to recover.
  • Sustainable Eel Group (SEG): SEG promotes the sustainable management of eel populations through the SEG Standard, which includes responsible fishing practices, traceability, and habitat restoration efforts. SEG also works on raising public awareness and advising policy makers.
  • European Eel Alliance: Coordinated by FishSec, this alliance comprises over 100 conservation staff across Europe. It aims to strengthen management and protection of European eels at all levels, supporting the development of international and regional action plans.
  • Danish Environmental Protection Agency: This national body implements the EU Eel Regulation in Denmark, overseeing the development and execution of the country’s Eel Management Plan.

How Denmark is Fighting the Decline

Copenhagen Harbor Eel Sanctuary

WWF Denmark and By & Havn have developed the world's first eel sanctuary in Copenhagen Harbor, creating vital habitats for eels within the urban environment to support conservation efforts and raise awareness. This project includes 80 "fish kindergartens," or biohuts, designed for juvenile and small fish. The initiative has motivated others, including companies, to add 120 more biohuts, making Copenhagen Harbor the leading urban eel conservation area. The purpose of these biohuts is to provide safe and nurturing habitats, significantly enhancing the local aquatic ecosystem.

Eel Pass Installation Projects

Denmark has been actively involved in installing eel passes on rivers and streams to facilitate the migration of eels past barriers such as dams and weirs. According to the Fourth Danish Progress Report (2021), Denmark has installed numerous eel passes across various rivers, significantly enhancing the connectivity of aquatic habitats. These passes help eels bypass obstacles that would otherwise prevent them from reaching vital feeding and spawning grounds. The installation of eel passes has contributed to an increase in the escapement of silver eels, which are the mature eels ready to migrate back to the Sargasso Sea to spawn. The latest data indicate that the current escapement of silver eels is 122.3 tons. Although still below the target level, this represents a positive trend facilitated by improved migration pathways. By providing safer passage routes, eel passes have reduced mortality rates associated with hydropower plants and other barriers. For example, the implementation of bypass channels at hydropower facilities has mitigated the impact of turbine mortality on migrating eels.

Restocking Programs

As part of Denmark's Eel Management Plan, restocking efforts relocate glass eels from high-density areas to habitats with low eel densities, boosting survival rates and aiding population recovery. Since 2009, the target of 0.8 million eels annually has been surpassed, with 1.2 to 1.6 million eels restocked each year. Lakes and rivers are primary sites, supporting eel growth and migration to the Sargasso Sea. The National Institute of Aquatic Resources monitors these efforts, finding that smaller eels (2-5 grams) have better survival and growth rates, significantly contributing to the production of silver eels ready for migration.

By combining regulatory measures, organizational efforts, and practical conservation initiatives, Denmark continues to play a significant role in the conservation of the European eel, striving to ensure the species' long-term recovery and sustainability. In conclusion, the European eel is a species with a unique and complex life cycle, facing significant ecological challenges that have led to its dramatic decline. Conservation efforts are ongoing but are hindered by the need for international collaboration and the long timescales required for population recovery. Continued research and adaptive management strategies are essential to ensure the survival of this iconic species.


Åström, M., & Dekker, W. (2007). When will the eel recover? A full life-cycle model. ICES Journal of Marine Science, 64(7), 1491–1498.

Cancio, I. (2021). Ernst Johannes Schmidt, the detective that solved the ‘Eel question’ with a little bit of help from good Carlsberg beer | EMBRC. European Marine Biological Resource Centre (EMBRC).

Carder, M. (2022). Status Assessment 2022—European eel. OSPAR Assessment Portal.

Cresci, A., Durif, C., Shema, S., Skiftesvik, A., & Browman, H. (2019). Glass eels (Anguilla anguilla) imprint the magnetic direction of tidal currents from their juvenile estuaries. Communications Biology, 2.

Eel—European Commission. (2023, December 12).

Moriarty, C. (ed), & Dekker, W. (ed). (1997). Management of the European Eel [Technical Report]. Marine Institute.

New eel habitat makes Copenhagen Harbor the world’s first eel sanctuary. (2023, November 27). WWF Baltic.

van Ginneken, V. J. T., & Maes, G. E. (2005). The European eel (Anguilla anguilla, Linnaeus), its Lifecycle, Evolution and Reproduction: A Literature Review. Reviews in Fish Biology and Fisheries, 15(4), 367–398.