2023-12-10 12:00:05
Dismantling of the Vezins dam (Manche), on the Sélune river, in August 2019. LOU BENOIST/AFP
A large bucket filled with an anesthetic liquid, an all-terrain camping table, an electronic scale… On this autumn morning, Anthony Acou, research engineer at the French Biodiversity Office (OFB), deployed his equipment on the bank of the Yvrande, a modest tributary of the Sélune, a coastal river upstream of the bay of Mont Saint-Michel. Around him, around ten agents from the OFB and the National Institute of Agronomic Research (INRAE) are busy.
Almost mechanically, he extracts the sculpins, chubs, trout and eels taken from the stream using an electric fishing system from the basin, in order to weigh and measure them. “Height and weight are fundamental data for analyzing population dynamics”, explains the engineer. On his laptop screen, the Excel spreadsheet data tells the story of a rebirth. After more than a century of absence, migratory fish, notably Atlantic salmon and European eel, two protected species, have returned to the Sélune and its tributaries.
“Migration cycles restored”
Until recently, two hydroelectric dams (Vezins, 36 meters high) and La-Roche-qui-Boit (16 meters high), built in 1914 and 1932, prevented any upwelling of fish for nearly ninety-one kilometers. The dismantling of these installations, recorded during the Grenelle de l’environnement in 2009, began in June 2019, after ten years of intense debates and numerous oppositions. The work, completed at the end of 2022, constituted the largest leveling of dams ever carried out in Europe.
In a few months, nature took back its rights. “It was a very quick transition”, rejoices Laura Soissons, Inrae research engineer and coordinator of the Sélune scientific program. Jean-Marc Roussel, researcher at INRAE, explains: “Thanks to the removal of the dams, migratory cycles between fresh water and salt water could be re-established. » Invertebrates have also benefited from the improvement in water quality and vegetation has reappeared on the banks, which were once submerged. Generally speaking, the ecological state of the river has improved. “These are very promising first results”added Laura Soissons. According to the National Biodiversity Observatory, in 2018, only 43.4% of French watercourses were considered to be “in good ecological condition”.
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Another observation: downstream, the dams and the reservoir heated the water by more than two degrees in summer. An anomaly erased by dismantling and which could encourage the arrival of new species. “It’s a real victory, especially in the context of global warming”estimates Laura Soissons.
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