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  • VIEWPOINT OF A PARTNER

    -Anne Pénalba \ France Hydro Electricité
    "The R&D programme turned out to be very fruitful. Each project produced new knowledge and a number of points were cleared up..."

Understanding and reducing the impact of river structures to save the eel

Over a period of three years, the Eels & Installations R&D programme, a partnership between Onema, Ademe and hydroelectric companies, set up 18 research projects. The projects produced very useful data for evaluating the impact of river structures on eels and a set of tools and technical solutions for restocking the species.

After three decades of decline, a national management plan to save the European eel was set up in 2007 to work on the factors causing eel mortality, namely pollution and habitat degradation, overfishing, hydroelectric turbines and obstacles to migration. It was precisely to address these points that the Eels & Installations R&D programme was launched from 2009 to 2011 by Onema, Ademe, Compagnie nationale du Rhône, EDF, France Hydro Electricité, GDF Suez and Société hydroélectrique du Midi. An initial line of work dealt with upstream migration, a decisive factor for colonisation of freshwater environments. Starting even in river estuaries, young eels are confronted with a large number of installations (tidal doors, floodgates, locks, etc.). One project (Onema/ Irstea/Unima) confirmed in situ the effectiveness of partial opening of tidal doors and allowing entry of a limited volume of salt water. A capture-mark-recapture method (Onema/ Irstea/Gironde and Charente-Maritime fishing federations) was used on different sites located just upstream of the tidal reaches of rivers. Glass eels, marked with an elastomer tag, were released downstream of obstacles and then recaptured upstream. This project produced information on how they overcome obstacles and on the effectiveness of the brush passes installed for their upstream migration. Farther upstream, large dams can also block migration. At the EDF Golfech dam, an innovative brush pass, including a buffer basin to prevent travel back downstream, was successfully tested (Onema/ EDF/Migado). On the same site, teams developed an automatic resistive device that provides precise biological information on elvers reaching the top of the pass.

Impacts on downstream migration

Toward the end of their life, adult females set off to the ocean and, during the downstream migration, confront hydroelectric turbines which cause high mortalities. A capture-mark-recapture project (EDF/CNR), filled out with a review of the literature on existing results (Onema), quantified mortalities for the turbines installed in France. Mortality rates ranged from 5% to 10% for large, lowhead turbines and exceeded 80% for some small turbines. To estimate the mortalities caused by a given installation, it was necessary to know the percentages of eels passing through each channel (turbines, locks, dams, bypasses). This aspect was studied (Onema/ EDF) on six structures in the Gave de Pau river, using radio-monitoring techniques. The results showed that a majority of the fish escaped via the bypasses (68% on average). This data will now make it possible to evaluate the cumulative damages along an entire river. An integrated model, the first of its kind, was developed by Onema. When applied to a river comprising 26 installations, the model indicated an overall percentage of escaping eels between 33% and 66%, depending on the hydrological conditions of the given year.

Local modifications and turbine management

A number of modifications on a given installation can reduce mortalities. Tests on the VLH (very low head) turbine on the Moselle river (MJ2-Ecogea) confirmed a mortality rate close to zero. For high heads, another project (EDF) contributed to the development of the Alden “fish-friendly” turbine. A further study (Institut P’) in an experimental channel looked into sizing water-intake screens with a relatively small distance between bars and evaluated the production losses. An infrasonic repulsion device was also tested (Onema/ EDF/CNR) on the Gave de Pau river as an alternative means to hinder eels from entering the turbines. The test was unfortunately not successful, no changes in fish behaviour were noted.

Another solution to reduce mortalities is to halt generation during peak migration periods. A number of approaches were tested to determine the periods in advance. A “biomonitor” on captive eels (Galway university) succeeded in signalling peak periods, but not in advance. Experimental fishing campaigns (EDF) proved more successful, but were costly and applicable only to certain installation configurations. The most promising technique lies in the development of predictive models, similar to that used on the Loire river (MNHN/EDF). The model links 20 years of capture data with other parameters (flow rates, water turbidity, weather, etc.) and the current version can predict 80% of the observed peaks.

Concerted implementation

The time has come for operational deployment. It is through concerted implementation by the various participants and on the different management levels that the technical solutions and tools produced by the R&D programme will contribute to the eel stock-recovery goals.

Contact : philippe.baran@onema.fr