Escapement of a silver-phase eel population, Anguilla anguilla, determined from fishery in a Mediterranean lagoon (Or, France)

Escapement of silver eels from a Mediterranean lagoon was estimated by a capture–tagging–recapture and automated tag-reading study. The population of silver-phase eels in the lagoon was estimated to be 13.2 kg ha21, with an escapement rate from the commercial fishery of 76.8%

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

When "safe" dams kill: analyzing combination of impacts of overflow dams on the migration of silver eels

International audienceThe drastic decline in European eel Anguilla anguilla stock is now widely recognized. However, while various causes for this decline have been identified, the relative importance of each cause remains unclear.2.During the catadromous migration of silver eels, the negative impact of dams is frequently highlighted, but mainly for powered dams (with turbines) or where connectivity is completely ruptured. Mechanical impact due to turbine blades is often considered the most important cause of mortality of silver eels during downstream migration. Consequently, non-powered dams equipped with spillways are often considered safe for the passage of migrating silver eels.3.We hypothesized that, to understand the negative impacts of dams, a much wider context must be considered than turbine mortality alone. Using an acoustic telemetry survey of silver eels, we demonstrated the negative effects of non-powered dams on downstream migration.4.Five main impacts on eel populations were highlighted: (i) the attenuation or loss of triggering factors, leading to an absence of or delay in migration; (ii) extra delays and extra distances travelled when crossing the dam; (iii) extra energetic costs of the additional distance travelled as result of exploring the dam and the reservoir to find other escape passages; (iv) the selection of a more risky behavioral phenotype, i.e., bold eels; and (v) direct blocking once migration has started. Mortality was evaluated as a supplementary impact. Some of these effects (attenuation of triggers, extra delays to cross the dam) might be more important than the same effect from powered dams, probably due to the constant high water discharge required for turbines that facilitate the passage of eels.5.As these “safe” dams are very widespread, they must be considered a potential threat to effective eel conservation

Disrupted downstream migration behaviour of European silver eels (Anguilla anguilla, L.) in an obstructed river

International audienceIn the European eel (Anguilla anguilla, L.), the steep decline of reproductive silver eels is partly due to disorientation and mortality during their downstream migration, when facing turbines, but also reservoirs and dams. In the Frémur, an obstructed river in Brittany, which is representative of the western coastal hydrosystem of France, five hydrophones were used to study the downstream migration patterns of twenty acoustically-tagged silver eels. Using this acoustic telemetry design, we showed that, despite exceptionally favourable environmental conditions, silver eels experienced important issues to move downstream the river. Indeed, 75 % of eels were delayed and up to 65 % were definitively stopped in their downstream migration. The 14 m high Bois-Joli dam, located at 5 km from the estuary, and its reservoir were the major obstacles to downstream movements. Eels that managed to move downstream only passed over the dam crest, during the night, and under highly favourable environmental conditions: river flow >1.2 m3.s−1 and water level at the dam >28.26 mNGF (Niveau Général de la France; baseline mean sea level for France). Three different downstream migration behaviours were observed: “successful migrants”, “uncertain migrants” and “unsuccessful migrants”. None of them were related to biological traits, suggesting a behavioural plasticity of silver eels. This study provides useful information to manage eel populations in such water basins that are very likely to be applied to all water reservoirs and dams, which are widespread through the distribution range of European eels

Le programme français de repeuplement en civelles - Bilan des trois premières années de transferts

La présente expertise, réalisée dans le cadre du GIS GRISAM, a mobilisé 10 experts dans le but de produire un état des lieux sur l'efficacité des "repeuplements" mis en ½uvre dans le cadre du Plan national anguille ; ainsi que des recommandations sur les protocoles de déversement et de suivi, dans la perspective d'un triple objectif. Informer les gestionnaires de l'anguille impliqués dans ces opérations - Servir de base au rapportage de la Commission européenne en 2015 - Permettre l'optimisation de la mise en ½uvre de la seconde phase de transferts, si celle-ci devait continuer à figurer dans le PGA français

Modelling past migrations to determine efficient management rules favouring silver eel escapement from a large regulated Floodplain Lake

International audienceAs human activities caused a dramatic decline of European eel population since the 1970 s, the European Union has set targets to ensure a 40% escapement to the sea of the silver eel biomass by considerably reducing anthropogenic impact. Thus, human obstacles to fish migration like dams and hydropower plants should enable efficient management measures ensuring safe passage for eels during the migration. In order to provide a quick and efficient assessment of eel conservation measure applied to the sluice gates of a large floodplain lake, we implemented a novel evaluation method using predictions on past migration seasons when no management measure were applied. For this purpose, we collected acoustic telemetry monitoring data over three migration seasons and fitted a predictive model based on Boosted Regression Trees (BRTs) to describe the influence of environmental parameters on migration. The water level difference over two days proved to be decisive, along with early migration occasions in the season, as an increase of at least 10 cm water level was associated with a sharp increase of migration probability. We then used the BRT model to predict migration occasions at the dam over 8 past seasons and forecast the impact on escapement of management measures if they had been applied. Thanks to this original prospective assessment of dam management measures, we identified an enhanced opening decision rule, capable of increasing the eel escapement for each year of the study. The management measure is particularly efficient during the years with poor hydrological conditions (i.e. droughts), that is the periods with the lowest initial escapement rates. Finally, efficient management measures to increase silver eel escapement were based on increasing the number of gate opening days by only a few days per year (c.a. 15%). However, the management measure only focused on silver eel migration, so that further investigations should consider the impacts of other threats occurring during the life cycle. This study also provides a highly operational approach for fast evaluation of conservation measures, avoiding lengthy and expensive monitoring campaigns of classical ex-post assessments