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  • Management plan to save the eel. Optimising the design and management of installations

    The Eels & Installations R&D programme
    is the result of a partnership between Onema, Ademe and five hydroelectric companies. Over the past three years, the programme launched 18 research projects to optimise the design and management of installations to protect migrating eels.


    Onema ran a study on the Charente estuary to remove obstacles to the movement of juvenile eels. Efforts are required for eels and other species so they can access the rivers and wetlands where they can grow.

    Access of eels to continental waters and littoral wetlands is important for restoration efforts in favour of the species. Unfortunately, during the winter upstream migration, glass eels are blocked by tide gates in estuaries that stop seawater from reaching freshwater upstream areas (marshes, canals). In the framework of the national management plan for eels, one part of the R&D programme examined how to allow the free movement of glass eels through tide gates. From 2009 to 2013, experiments were carried out on the Charente estuary by several partners in the project, including Onema, Irstea, Unima1*, the departmental council, the Charente public river-basin territorial agency and the fishing federations.

    Achieve continuous input of seawater

    Unima and the departmental council proposed two sites in the Charente estuary with double tide gates, namely the Charras canal and the Biard lock. On the first, a 10-centimetre chock held the gates slightly ajar while on the second, a notch was cut out on one of the gates to allow a regular, but limited flow of seawater upstream. "The downstream flow of freshwater was too fast for the glass eels to overcome, so they gathered in front of the tide gates during the 90 minutes to two hours prior to each high tide. As soon as the current changed direction, the juveniles were carried by the tide through the gates, which explains why it is important to allow for a continuous flow of seawater", explains Philippe Baran (Onema). However, a number of constraints must be taken into account, notably the risk of flooding, water salinity levels and the transport of suspended matter.

    47 measurement campaigns, night and day

    To determine the effectiveness of the various methods, Onema and Irstea spent over a year defining capture techniques for glass eels. "We had to find trapping systems, generally based on the techniques used by fishermen and notably by poachers", notes Alain Alric (Onema). Once the study protocol had been validated, 47 measurement campaigns were organised (32 on the Charras canal and 15 at the Biard lock). "The campaigns, under Onema management, were fairly difficult to organise because they were carried out both night and day, and mobilised teams of 7 to 8 people, from Onema (from the Ecohydraulic centre and the regional and local offices), the departmental council, Unima, the fishing federations and the Charente public river-basin territorial agency", adds Alain Alric. The study produced information on the kinetics of glass-eel passages and on their behaviour (passages through the gates generally at night and along the bottom).

    Photo: tide gates in the Charente estuary.

    Easier access to neighbouring wetlands

    The results showed that approximately 10% of the glass eels passed through the gates, representing several dozen kilos each season. Other species, such as shrimp, lampreys, sticklebacks and comb jellies also take advantage of the openings to access the neighbouring upstream wetlands.

    The results of the research project with those of other, similar experiments carried out recently in France will be published in a technical guide. Drafted by Grisam2* and Onema, the guide will "present the diverse techniques employed as well as the parameters and constraints weighing on each site, the goal being to assist water managers in selecting the best solution for their site", notes Christian Rigaud (Irstea).

    • 1*. Association for marshes in the Charente-Maritime department.
    • 2*. Scientific group for diadromous fish.

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