| Summary: | 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 ...
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