Session C5: Impact of Water Intake for Drinking Water Supply, on Downstream Eel Migration in the Fremur River

Abstract: The Fremur is a small coastal river located in northern Brittany, which discharges into the English Channel near Saint Malo (2°060W, 48°340N). Its catchment area spread over 60 km2 with a 17 km main stem. Three major dams (Pont Avet, Pont es Omnes and Bois Joli) located on the river, were previously impassable for fish, until the construction of fishpasses in 1996 (Feunteun et al., 1998). The highest one, Bois Joli dam, is 14 m high and holds a 3,9 million m3 reservoir used for drinking water supply. Downstream eels (Anguilla anguilla) migration is possible over the crest of the dam when overflowed as well as through the pumping pipe since 2012, following the pumping station construction. One kilometer downstream, at Pont-es-Omnès dam, a wolf trap allows to study eel downstream migration since 1996 (Acou et al., 2008). To study the potential impact of the water intake on eel downstream migration, in March 2012, a net was installed at the exit of a pipe, receiving the washing water, coming from the first filter preventing big materials, such as fish, to go further into the pumping station. The scientific follow-up stated in 2012 has allowed to trap more than 700 individuals belonging to 7 different species (6 of fish, 1 of crayfish). More than 170 eels were caught, including 89 during the 2012-2013 campaign. The study showed that eels use the pumping pipe independently of overflowing periods. Two periods of migration were highlighted, one in summer with a majority of yellow eels and the other one in winter with a majority of silver eels. The direct mortality was estimated between 2 and 100 %. The majority of eels showed symptoms of pathology like cutaneous erosions and bleedings. Since January 2013, the delayed mortality is studied by mark-release-recapture. Eels were marked with blue alcyan ink using a “Dermojet” at capture in the net and there were recaptured at the wolf trap at Pont-es- Omnès dam. First results indicate a high delayed mortality. These results highlight the major impact of water intake on downstream migrating eels. They underline the need to protect or to adapt water intake especially in the current context of eel decline