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Hydroelectric works, which constitute an obstacle to upward migration, are also likely to disturb going down. Particularly when this involves transit through turbines, the migrating fish can be physically affected. In situ studies aiming at estimating the potential mortality risks have been conducted by EDF in collaboration with CEMAGREF and they involve the three following dams : Pout?s in the Allier, Mauzac and Tuili?re in the Dordogne. The results strictly depend on the characteristics of the turbine. In Mauzac and Tuili?re, mortalities are restricted. In Poutes where there is a high percentage of potential risks, a device has been made available to avoid risks in the turbines.Les ouvrages hydro?lectriques qui constituent un obstacle ? la migration de mont?e sont aussi susceptibles de perturber la d?valaison. En particulier, en cas de transit par les turbines, les migrateurs peuvent subir des dommages. Des ?tudes in situ visant ? estimer les risques potentiels de mortalit? ont ?t? conduites par EDF en collaboration avec le CEMAGREF sur trois barrages de caract?ristiques diff?rentes : barrages de Pout?s sur l'Allier, Mauzac et Tuili?re sur la Dordogne. Les r?sultats sont ?troitement d?pendants des caract?ristiques de la turbine. A Mauzac et Tuili?re, les mortalit?s sont limit?es. A Pout?s, o? les risques potentiels sont ?lev?s, il a ?t? proc?d? ? la mise au point d'un dispositif permettant d'?viter l'entra?nement dans les turbines.

The writer synthesizes knowledge basically used for constructing devices which enable migrating fish to pass through basin catchments. He describes the dimensioning criteria and utilization conditions for various types of ladders (fish ladders in successive basins, slow-motion passes, lifts and sluices). The emphasis is put on the significance of the situation and attractiveness of these undertakings which must concern the level of improvement and rivers.L'auteur fait la synth?se des connaissances servant de base ? la conception des dispositifs permettant le franchissement des barrages par les poissons migrateurs. Les principes de fonctionnement, les crit?res de dimensionnement et les conditions d'utilisation des diff?rents types de passes (passes ? bassins successifs, passes ? ralentisseurs, ascenseurs et ?cluses) sont ?voqu?s. L'accent est mis sur l'importance de la situation et de l'attractivit? de ces ouvrages qui doivent ?tre ? l'?chelle de l'am?nagement et du cours d'eau.

Natural and artificial physical and chemical pressures affecting rivers thus restrict the growth of a large number of species, especially amongst migrating amphibiotic specifies. The various species affected in the aquatic environment resulting in the rarefactlon of those species which are currently protected under the Law for Preserving Natural Environment are illustrated by some examples. The course of the vital cycle of the various species of fish includes several phases : reproduction and embryonic evolution, juvenile development, adult development and several functions (reproduction, feeding, sleep) generally situated in two different locations. The completion of a complete cycle depends on a perfectly suitable environment responding to the needs of the species throughout these various phases (habitat and quality of water) and possibilities to circulate between the corresponding locations.Les pressions physiques et chimiques d'origine naturelle ou artificielles s'exer?ant sur les cours d'eau limitent le d?veloppement d'un nombre important d'esp?ces particuli?rement parmi les migrateurs amphibiotiques. Les diff?rents types d'atteinte au milieu aquatique entra?nant la rar?faction de ces esp?ces maintenant prot?g?es au titre de la loi sur la protection de la nature sont illustr?es par quelques exemples. Le d?roulement du cycle vital des diverses esp?ces de poissons comprend plusieurs phases : reproduction et d?veloppement embryonnaire, d?veloppement juv?nile, d?veloppement adulte et plusieurs fonctions (reproduction, alimentation, repos) g?n?ralement situ?es en des sites diff?renci?s. L'accomplissement du cycle complet est tributaire de l'ad?quation de l'environnement aux besoins des esp?ces au cours de ces diff?rentes phases (habitat et qualit? des eaux) et des possibilit?s de circulation entre les sites correspondantes.

In the first part of this paper, the present state of design of fishways is reviewed, focusing on the interest and the limits of each type of fish facility. The second part deals with downstream migration problems at hydroelectric powerplants: fish damage in spillways and hydraulic turbines and design of fish screening and other diversionary techniques used to prevent entry of downstream migrant into intakes.

After a brief reminder of the reasons why migrating fish need to move freely up and down rivers, this article describes current techniques in the area of upstream fish passage facilities, emphasising the value and limitations of each type: poll-type fishways, fish locks and lifts, by pass channels and preliminary weirs. The authors then define what is meant by efficiency in this context, before going on to describe fishway monitoring methods. The second part of the article reviews current knowledge on dowstream fish migration through hydropower stations, focusing on damage caused by fish passing through turbines or by spillways, and on the design of facilities (physical and behavioural barriers) used to prevent fish from entering water intakes. The study highlights surface bypasses which have been used successfully to guide migrating fish safely past existing water intakes. The authors conclude by emphasising that whatever the facilities used, small-scale hydropower stations virtually always have some impact on migratory fish populations, especially when a series of installations along their migratory route causes the impact to become cumulative.