European eels travel thousands of kilometers. Why are our rivers a trap for them?

European eels undertake an extraordinary journey once in their lifetime: from the muddy bottom of a river or lake to the crystal-clear waters of the Sargasso Sea. Their spawning migrations are becoming increasingly difficult. This is due to hydrotechnical barriers, but also to water pollution and parasites.

The mysterious life cycle of the European eel

The European eel (Anguilla anguilla) is a rare example of a species that lives in muddy freshwater habitats but changes to a marine environment once in its lifetime. To spawn, it always travels to the same place: the crystal-clear waters of the Sargasso Sea in the North Atlantic. Some individuals must travel as far as 6,000 km.

The details of the European eel’s life are not fully understood. It is not known why they choose the Sargasso Sea as their spawning ground, and the act of egg-laying has never been observed.

What we do know is that they are exceptionally long-lived fish. In the wild they can live up to 20 years, and in captivity even up to 80 years. The first, larval stage of life lasts from several months to three years, after which young eels begin their migration from the Sargasso Sea toward freshwater habitats on the European continent.

Why is the European eel population declining?

In its latest report, ICES (the International Council for the Exploration of the Sea) states that the European eel population is in a critical condition. It has declined by 90 percent over the past decades.

There are several reasons for the decline of the European eel population. The European Commission points, among other things, to excessive fishing in both marine and freshwater habitats. It also notes that migrations to spawning grounds are obstructed by dams, weirs, and hydropower plants. Hydropower facilities are particularly dangerous because of their turbines. Studies indicate that eel mortality caused solely by hydrotechnical barriers may reach 60 percent in the Vistula River basin and 40 percent in the Oder River basin.

The decline of the European eel population is also driven by water pollution, which accumulates in the fish’s body, and by parasites, especially the invasive swim bladder nematode. Climate change may be an indirect factor, affecting lower water levels and changes in the temperature of ocean currents that enable eel migration.

How can the decline of the European eel population be reversed?

Scientists and conservation organizations have been calling for stronger protection of the European eel for more than 20 years. Their efforts have led to restrictions on eel fishing. However, these measures have not produced the expected results, and the population has not recovered. Current recommendations therefore call for a complete ban on commercial and recreational fishing, as well as on the capture of juvenile eels for stocking purposes.

Strengthening species protection should go hand in hand with measures in the field of water management. When eels encounter migration barriers in rivers and lakes such as dams, hydropower plants, and weirs, they cannot freely reach their spawning grounds. To some extent, this problem can be addressed by fish passes, structures designed to help fish move past barriers. However, this is not a perfect solution: eels do not always find the passage, and mechanical components such as turbines remain a major threat.

Neither species protection nor well-designed infrastructure will work on its own. To stop the decline of the European eel population, integrated action is needed. ICES points to the need to halt fishing, improve water quality, and above all restore European rivers to a more natural state.

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