Accelerating the recovery of the European Eel

New evidence published showing hydropower mortality and importance of translocation in Sweden

  • Hydropower turbines are largest cause of eel mortality in Sweden
  • Most freshwater eels in Swedish waterways have been transported in from UK and France

Critical new evidence that re-evaluates the global priorities for eel recovery has been published in Sweden. The report from Sweden’s leading eel scientists, Willem Dekker and Håkan Wickström of the Department of Aquatic Resources at the Swedish University of Agricultural Sciences, reveals that:

  • In inland waters hydroelectric power turbines are now the largest cause of eel mortality in Sweden
  • the vast majority of freshwater eels in Swedish waterways have been relocated from rivers in the UK and France

The groundbreaking new evidence is disclosed by the Sustainable Eel Group, following the scientists’ analysis of 75 years of data from power companies, marine authorities and other research institutes in Sweden.

This showed that while the source of silver eels in earlier times, e.g. 1986, was divided equally between restocking (translocated), assisted immigration and natural recruitment, by 2014 translocation is responsible for the large majority of Swedish silver eel production.

Dekker 1986 restock graph Dekker 2014 restock graph

‘Every time I see this plot I am surprised how important restocking has become in Sweden and how much the composition has changed.’ Willem Dekker, 8 June 2015

The report also shows the fate of silver eels in respective years. In 1986 the fresh water eel fishery and hydropower were each responsible for about 25% of silver eel mortality, and 50% were able to continue their migration. But in 2014, although overall numbers were up, only 26% were able to escape for migration, with 32% mortality due to fishery and 42% due to hydropower (some 147 tonnes of silver eels).

Dekker 1986 mortality graph Dekker 2014 mortality graph

Willem Dekker concludes there are no easy answers, as the principal causes of eel mortality also play a critical role in its sustainable future. “The fishery has a double role, and hydropower is the main mortality,” he said. “But at the bottom line, hydropower is providing the main money for any action on eel, and the fishery is critical for trap and transport initiatives. Without the restocking, there would hardly be an inland stock; without the fishery there would be no trapping for transporting; and without the hydropower there would be no financial basis – a real catch-22 situation.”

“When and where the fishery exceeds sustainable limits, restrictions should be applied – even if the gain would still be annihilated by other impacts. But equally well: where and when hydropower mortality exceeds sustainable limits, restrictions should be applied – even if the full potential will only be achieved after the fishery has been restricted too. And best: When and where the total impact exceeds sustainable limits, parties should cooperate to jointly achieve those limits.”

An explanation of this increase in hydropower mortality is due to an increase in restocking during the 1980s and 1990s, and more recently restocking efforts have been concentrated in westward flowing rivers and on the open west coast of Sweden (Dekker and Wickström). The power companies have also restocked in coastal water to compensate for the hydropower mortalities in rivers and lakes and increase escapement. However this is unlikely to show results yet, given the time to maturity as silver eels of at least 15 years. Trap and Transport has been practiced since 2011 and has resulted in an immediate effect on escapement.

Andrew Kerr, Chairman of Sustainable Eel Group said “This new information is critical to our understanding of how to help the eel. It is critical that we learn as much as possible about the eel and mankind’s impact on it. If we are to help this incredible species to recover, we need an open debate about the challenges it faces, and work collectively across industry, science and conservation to find the best answers.”

Sweden operates a quarantine system to try and eliminate the risk of importing any viruses. The minimum length of the quarantine is 65 days to allow time for all virus tests to be completed. Survival is very high and the total mortality from capture to restocking using glass eels that meet the Sustainable Eel Standard (UK Glass Eels and Scandinavian Silver Eel) is on average less than 10%. The figure of 10% is much lower than any figure reported for the natural mortality in the Severn glass eel fishery. At the restocking release moment the quarantined eels have at least doubled their weight and during the next 15 years they grow an average of 4.4 cm/year (Dekker and Wickström). The authors calculated mortality to be approximately 10%/year and after 15 years there are 15% silver eels remaining.

The research into the navigation patterns, completed by Håkan Westerberg and Niklas Sjöberg in the EELIAD project, prove that the restocked eels leaving Sweden use the same routes into the Atlantic as those silver eels with a completely natural recruitment background. They are therefore likely have the same chances of completing the return migration to the Sargasso Sea.

The 2014 Evaluation Project ( was followed by a steering committee with representatives from Vattenfall hydroelectric AB, the governmental Marine and Water Authority, E.ON Hydro Sweden AB, Fortum Generation, Holmen Energi, Statkraft Sweden AB, Technical Office in Linköping AB and Elforsk / Energy Research Institute; and the report includes an evaluation of the targets set for the program measuring Krafttag Al. The objectives of the measures would be evaluated at the end of the program was decided already when the program started, in 2011. The aim was to get an analysis and documentation of goal attainment. See for the full report.