Worldwide, obstructions on watercourses have interfered with migratory pathways of fish species, reducing life-cycle success and often eliminating diadromous fish species altogether from river basins. Over the last century, efforts to mitigate these effects were initially directed at developing fishways for upstream, high-value migrant adult salmon. In more recent years, efforts have turned to developing fishways for other species. Results of past research suggest that the development of effective fishways requires biological knowledge of fish behaviour when encountering variable flows, velocity and turbulence, combined with hydraulic and civil engineering knowledge and expertise to develop facilities that provide appropriate hydraulic conditions that fish will exploit. Further, it often requires substantial financial resources for biological and hydraulic testing as well as engineering design, particularly where prior knowledge of the behaviour of target fish species does not exist. Where biological or engineering knowledge (or both) is absent, development of effective passage facilities must take on a trial and error approach that will almost certainly require years to attain success. Evaluations of existing adult and juvenile fish passage facilities, where they have been carried out, suggest that migrant fish reject areas with hydraulic conditions they determine unsuitable. Even well designed fish ladders or nature-like bypass channels for upstream migrants, even those with good attraction flows, will fail if incorrectly sited. Although progress has been made, developing successful installations for downstream migrants remains much more difficult, probably because downstream fish move with the flow and have less time to assess cues at entrances to any bypasses that they encounter.

OVERVIEWDownstream fish passage technology is much less advanced than it is for upstream fish passage facilities. This is simply due to the fact that efforts to re-establish free movement for migrating fish began with the construction of upstream fish passage facilities, and that downstream migration problems have only been recognised and addressed more recently. It is also because it is much more difficult and complex to develop effective facilities for downstream migration. This situation is not restricted to France, since as yet, no country has found satisfactory solutions for downstream migration problems, especially where large installations are involved (EPRI, 1994). SPECIES AND STAGES INVOLVED IN DOWNSTREAM MIGRATIONDownstream migration, i.e. migration descending a river towards the sea or a lake, concerns fish at different stages of development, depending on the species. The diadromous and potadromous species which are legally recognised and listed as protected migratory species in France, and the particular development stages involved in downstream migration, are as follows: -Atlantic salmon (Salmo salar) and sea trout (Salmo trutta trutta): juveniles (smolts) migrating to the sea to reach their feeding grounds, and adults (kelts) after spawning. With respect to kelts it should be noted that the provision of downstream passage facilities for adult sea trout is much more important than for salmon because multiple spawning is much more frequent in trout than in salmon.-Allis shad (Alosa alosa): mainly juveniles (since most adults die after spawning).-Twaite shad (Alosa fallax): both juveniles and adults.-Eels (Anguilla anguilla): adults only, during migration to their marine spawning grounds.-Sea lamprey (Petromyzon marinus) and River lamprey (Lampetra fluviatilis): juveniles of both, and also adults in the case of river lamprey.-Brown trout (Salmo trutta fario) and Grayling (Thymallus thymallus): juveniles migrating to downstream feeding grounds, and adults after spawning.-Pike (Esox lucius): both adults and juveniles.

Efficiencies of two types of bypass, a surface and a bottom sluice, were tested for the natural downstream migration of silver eels Anguilla anguilla at a small hydroelectric power plant at Halsou, on the River Nive in France. Naturally migrating eels were caught after their passage through either bypass. A total of 637 eels were trapped during the three-year study. Total efficiency for both bypasses, evaluated on the basis of downstream movement of radiotagged eels, ranged from 56% to 64%. Given a bias due to hydrological conditions at the time of the runs, the precise efficiency of each separate bypass was not calculated. However, preferred passage through the bottom bypass for both tagged and untagged eels was confirmed by telemetry, as three to four times as many eels transited through the bottom bypass compared to the surface one. The behaviour of 74 individuals released in the forebay was observed by radiotelemetry. Close to half of the radiotagged eels returned up the headrace after their release, and most eventually migrated downstream over the dam with appropriate environmental conditions. Upon arrival at the power plant, eels displayed foraging behaviour in the forebay with frequent displacement interrupted by long resting periods in zones with low current. The repulsive effect of the trashrack located in front of the turbine intake increased with increasing turbined discharge. The study indicated that a trashrack with a smaller bar-spacing (around 20 mm), associated with an appropriate bypass, could deflect a large proportion of the female eels from the turbines. However, this solution needs to be tested on site to quantify the risk of mortality due to impingement on the trashrack.

Key-words:silver eel, downstream migration, behaviour, hydroelectric plant, trashracks, radio-tracking Between 2004 and 2007, 116 downstream migrant silver eels (Anguilla anguilla) were monitored at a hydropower plant on the Gave de Pau river in South-West France using radio and PIT telemetry. The objectives of the study were: (i) to determine the environmental conditions when eels arrived and passed the facility; (ii) to determine the rate of eel escapement (passage other than via the turbines); (iii) to describe the behaviour of eels faced with the intake structure and the permeability of the intake trashracks for the different sizes of eel; and (iv) to determine whether surface bypasses originally designed for salmon could be effective for eels. Five types of behaviour of silver eels in the forebay and at the plant intakes were identified. The study showed the key factor influencing both eel behaviour and the route taken through the plant was variation in river discharge. Escapement rate was related to eel length and the spill flow to river flow ratio, which could be described by a logistic regression model. The surface bypasses originally designed for salmon were found to aid downstream eel migration significantly. At velocities < 0.40 m·s −1 , no eels, even the largest, for which the racks are a physical barrier, were found impinged on the trashracks.

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