Unique monitoring program for European eels launched on the Rhone

Anguille européenne © Carmie H/Irstea
Anguille européenne © Carmie H/Irstea

As part of a research partnership project, the Compagnie Nationale du Rhône (CNR) and scientists from Irstea Lyon have set up a wide-scale experiment to study the passes in hydroelectric structures used by adult eels starting their seaward migration. The aim is to provide reliable data to meet species conservation regulations, as well as to gain a better understanding of the biological processes that govern the life cycle of this endangered species.

Like polar bears, the European eel is a critically endangered species which has been on the International Union for Conservation of Nature (IUCN) red list since 2008. In line with European regulations, France has implemented a national conservation plan to reduce eel deaths caused by human activities, including hydroelectric power generation. As eels spend most of their lives in fresh water before reproducing in the Sargasso Sea in the Caribbean, hydroelectric facilities can represent an obstacle, or even a risk, to their migration. They can block the eels during their run, when the small eels (glass eels, elvers) born at sea travel upstream, or during their downstream migration, when the adult, sexually mature eels (silver eels) travel back to the sea to reproduce.

Defining eel passes

To meet the regulatory targets of the national conservation plan and its implementation in the Rhone-Mediterranean river basin, the concession operator for navigation, irrigation and electricity production on the river, Compagnie Nationale du Rhône (CNR), launched the DAARAC project in partnership with Irstea's Ecohydrological Dynamics and Modeling Laboratory. The aim of the project is to gain a better understanding of how eels pass through facilities designed and managed by the company during their seaward migration, with a particular focus on the Caderousse site 1 (Vaucluse).

Typically, the hydroelectric facilities of the Rhone river include a dam that blocks the original river bed, redirecting the water towards a diversion channel containing a lock and an electricity plant. While eels may be able to pass through the dam and the lock without damage, passing through the plant's turbines can involve greater risk. "In 2010, we carried out a study that found the survival rate for eels passing through turbines was 92 to 93%. We now have to determine the proportion of eels that pass through the turbines during their seaward migration. The main aim of the DAARAC project is therefore to quantify the number of eels using each of the three possible passes," explains Franck Pressiat, manager of the Environment division at CNR. In order to understand the actual impact of these passes, the project's partners also plan to evaluate the cumulative effect of the three successive facilities the eels must pass through (Caderousse, Avignon, Vallabrègues) before reaching the Camargue delta and the Mediterranean Sea. The study will also provide data on silver eel distribution across both branches of the delta: the Petit Rhône and the Grand Rhône.

A unique monitoring program for the Rhone

To complete the study, CNR joined forces with the Dynam team from Irstea Lyon, who defined the methodology used to monitor the eels and thereafter participated in each stage of the project. The methodology uses acoustic telemetry to record sounds transported through water. During the three years of monitoring, the project will involve capturing around 300 eels reaching sexual maturity and preparing their downstream migration, to mark them (surgically implanting a transmitter) while also placing hydrophones to detect their passage (or not) through the various passageways crossing the Caderousse facility (lock, downstream from the plant, old Rhone bed, etc.).

Installing hydrophones in the Rhone
Installing hydrophones in the Rhone © CNR/Irstea

At the end of the project's first year, the 90km study area has already been fully equipped: from the various points in the Caderousse facility all the way to Arles, gateway to the Camargue delta. In addition to these fixed points, mobile hydrophones fitted to boats will be used to refine the monitoring. The first capture session, carried out in early September, resulted in 72 individuals being fitted with transmitters and released.

"Working with the Irstea team means that not only do we benefit from their technological expertise, which they were able to adapt to the specific nature of the Rhone (river size, interference by various background noises caused by navigation, wind, currents, etc.), but also that we have a partner capable of collecting and interpreting results, locating them within the biological processes of the species," specifies Pressiat.

Scientific and industrial results

By 2021, this project should provide CNR with an improved understanding of the actual and overall impact of its activities on the eel population migrating downstream. The results will be delivered to relevant regional, national and European bodies, and will be used alongside other studies to reflect on the direction taken by the species management plan. In parallel, scientists from the Dynam team who specialize in analyzing the behavior of fish dealing with physical changes to their environment, hope to find answers to several mysteries of eel behavior, such as the triggers for their seaward migration or factors that influence their choice of passageway through the structures they must cross. "As with our other projects, our overall aim is to study the behavior of various fish species and to develop tools to evaluate the impact of hydraulic facilities on their populations. Our ultimate aim is to support facility managers, such as CNR, in making the best design and management choices to maintain their activities while minimizing as far as possible their impact on the environment and the species these contain," concludes Hervé Capra, Ecohydrology research and Irstea's lead scientist for the project.

Project profile
  • Name: DAARAC - Downstream migration of silver eels in the Rhone through CNR facilities
  • Dates: 2017 - 2021
  • Budget (total cost): 1.2 million euros (funding by CNR, Rhone-Mediterranean-Corsica Water Agency, Irstea)
  • Operational partners: Irstea Lyon, Compagnie Nationale du Rhône, French Biodiversity Agency, Rhone-Mediterranean Migratory Fish Association
    For further information

    1- Caderousse is the furthest upstream facility to be equipped with passes for upstream eel migration; the eels use these passes in the opposite direction when migrating downstream.