Efficiency of the future fish lift of the hydroelectric power plant of Baigts on the Pau river | Etude de l'efficacité du futur ascenseur à poissons de l'aménagement hydroélectrique EDF de Baigts de Béarn sur le gave de Pau

français. L'objet de l'étude était l'évaluation de l'efficacité du futur ascenseur à poissons de Baigts sur le gave de Pau dont la construction de la partie basse s'est achevée au printemps 2000. Cette étude a consisté d'une part à piéger l'ensemble des poissons s'engageant à l'intérieur du dispositif afin de vérifier le bon fonctionnement général de la passe et d'autre part à suivre les déplacements de saumons radiomarqués au niveau de l'aménagement afin de déterminer précisément l'impact de l'ouvrage en terme de % de franchissement et de durée de blocage. Le piégage en 2000 a permis, pour la première fois, d'estimer l'abondance du saumon sur le gave de Pau qui est au minimum de 300 poissons. Une bonne connaissance du comportement du saumon au niveau de l'aménagement ainsi que la possibilité de comparer le comportement des poissons à Baigts et Castetarbe, laissent à penser que seule une augmentation des incursions et des temps de stationnement au niveau du dispositif de franchissement peut permettre d'améliorer de façon notable la franchissabilité de l'ouvrage.

anglais. The efficiency of the ongoing fish lift installed at Baigts hydroelectric power plant on the Pau River was assessed shortly after building of the lower part of the pass in spring 2000. The present study consisted in (i) trapping all fish entering the fishpass to test its comprehensive functioning and (ii) monitoring small-scale movements of radio-tagged salmon nearby the obstacle to specify its impact both in terms of proportions of upstream passages and migration delays. From 14/06 to 03/12/2000, 5,305 fish from 26 different fish species including 6 diadromous migratory species were trapped, among which 19 allis shad, 1 twaite shad, 59 lamprey, 26 eel, 55 sea trout and 257 Atlantic salmon. Sampling sizes especially for shad, lamprey and eel did not account for their abundance in the Pau River in 2000 as field sampling starting nearly to the end of the migration period for both shad and lamprey and because of a selective bar-spacing effect of the trapping device on eel. For salmon, its abundance in the Pau River was found to attain a minimum of 300 specimens in 2000. Radio-tracking of 28 returning adult salmon gave evidence of a significantly improved upstream passing of the obstacle. The rates of upstream passage averaging 34% in the period 1995-1997 increased up to 50% to 80% in 2000 which may be partly attributed to increased fishpass attractivity. Nevertheless, considering the migration delays still too long (median blockade: 53 days; min.: 0.4 day; max.: 137 days) and also the position of the obstacle on the river, located further downstream of major spawning grounds, the efficiency of such a fish pass has yet to be improved so as to make the hydroelectric power plant of Baigts compatible with strategies for restoring Atlantic salmon stocks on the Pau River. A deep under-standing of salmon behaviour at the two different fish passes of Baigts and Castetarbe plants let then suppose that only sustained incursions and presence duration of fish near the fish pass entrance might most likely significantly enhance successful upstream passages. In the present plant configuration with the fish pass located on the left bank, i.e. on the opposite side from the tailrace, it was found that increasing fish presence in the immediate vicinity of the fish pass indeed required increasing spillage from the dam. From in situ experiments it was shown that for maximum turbine discharges of less than 40 m3/s, from 40 m3/s to 60 m3/s and greater than 60 m3/s, dam flow discharges of respectively 7.5 m3/s, 9.5 m3/s and 11.5 m3/s would probably highly increase the attractivity of the left bank.