| Summary: | An in situ experiment on an electrical screen installed in the first part of the tailrace of an hydroelectric power plant was performed to test its efficiency both in terms of deterrent effect and impact on fish behaviour, in particular its efficiency to guide fish toward the by-passed section of the river. The study site was the hydroelectric power plant of Peyrouse on the Pau River on which basic knowledge was gained both on upstream fish passage and migratory behaviour when displacements of about 20 adult salmon had been monitored from 1995 to 1997 so before the implementation of the screen in 1998. From late July to early December 2000, 23 tagged fish were released at a few kilometres downstream from Peyrouse plant and their on-site trajectories monitored by radiotracking. 21 fish were recorded in the vicinity of the electrical screen, the other two entering directly the by-passed branch when arriving on site. The electrical barrier exerted a marked repulsive effect on fish as 85% (117 out of 135) of attempts failed in passing over the screen. Nevertheless, 10 of the 21 fish (48%) succeeded in passing over it, and 7 among them in their first attempt. In contrast the screen was shown much less permeable to downstream fish passage: only 14 of the 360 attempts (3.9%) resulted in downstream fish passage. Straight after passing over the barrier, fish remained "trapped" in the tailrace area which resulted in much higher migration delays. The screen capability of guiding fish toward the by-passed section of the river was low as less than 25% (27 out of 115) of approaches to the barrier were followed by such displacements toward the by-passed branch, fish in fact moving much often downwards. In turn, only 8 of these 27 incursions in the by-passed branch were followed by upstream movements to the dam. When comparing results of 1995-97 and 2000, the screen as currently functioning proved to be of only minor relevance: 10 of the 13 fish (77%) which had never passed over the electrical screen succeeded in passing over the plant while only 2 of the 10 fish (20%) which had passed over the electrical screen once at least did pass over it. Considering all fish, at present only 12 out of 23 (52%) succeeded in passing over the obstacle while rates of upstream passage reached 76% (16/21 tagged fish) in 1995-97 experiments. Consequently even so totally impermeable, a screen by itself appeared insufficient to really improve upstream passing of obstacles such as Peyrouse. Hydraulic conditions (flow discharges and velocities) in the by-passed section of the river were thought as much important. For plants with tailrace of about 100-200 m, implementation of such a screen appeared unjustified especially when hydraulic conditions (flow discharges and velocities) at the confluence by-passed branch/tailrace as well as further upstream in the by-passed branch were found adequately attractive for fish. But for plants with longer tailrace (of about 1 km and more), a screen when totally impermeable might deeply decrease migration delays. As formerly, attention had to be paid on hydraulic conditions both at the confluence with the tailrace and in the by-passed branch. In intermediary situations (tailrace of a few hundred metres long), it seemed difficult to foresee migration delays for fish entering the tailrace of a given plant and then the opportunity of an electrical screen. / L'objet de l'étude était l'évaluation in situ d'un écran électrique installé en début de canal de fuite d'un ouvrage hydroélectrique, à la fois en terme d'efficacité à repousser les saumons et d'influence sur leur comportement, notamment sa capacité de guidage vers le tronçon court-circuité. Le site choisi a été l'aménagement de Peyrouse sur le gave de Pau, sur lequel on possédait un "état zéro" sur la franchissabilité de l'obstacle et le comportement migratoire du poisson, les déplacements d'une vingtaine de saumons avaient été étudiés de 1995 à 1997 préalablement à l'installation de l'écran en 1998. La comparaison des résultats obtenus de 1995 à 1997 et en 2000 ne permet pas de mettre en évidence un réel intérêt de l'écran dans ses conditions de fonctionnement actuelles.
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