The behaviour of returning adult Atlantic salmon (Salmo salar L.) in the vicinity of Baigts hydroelectric power plant on the Pau River as determined by radiotelemetry.

The behaviour of 19 radiotagged adult Atlantic salmon (Salmo salar L.) was studied during the summer and autumn of three successive years (1995 to 199 7) in the vicinity of the Baigts hydroelectric plant. Most of the time, the fish stayed away from the dam. Visits to the site and the time which they remained downstream were generally short lived (less t han 1 hour), with the fish generally being within 300 m downstream from the plant. When fish were absent for a longer time, i.e. more than 6 hours, th ey were often observed in a pool located between 500 m and 1.2 kilometer do wnstream from the obstacle. Behavioural rhythmicity of fish was pointed out :they approached the plant mainly at dawn and were more present and active during the day. Mean daily temperature and turbine discharge influenced salmon behaviour in the vicinity of the plant. Fish were more often found approaching the plan t when the temperature was higher than 16 degrees C but stayed less time in the vicinity of the obstacle. Presence of fish in the tailrace and at the dam was influenced by temperature and by operation of the hydroelectric pow er plant with fish being less present in the tailrace when temperature exce eded 20 degrees C and more present at the dam when the turbine discharge wa s lower than 50 m(3)/s. Studies revealed that the plant impeded the upstream migration of salmon wi th fish being delayed up to 137 days and only a small percentage (34%) bein g able to pass over the obstacle. The location of the fish pass, at the opp osite side of the tailrace, was not responsible for its poor efficiency sin ce all fish were monitored near the entrance. The conditions encountered by fish at the entrance of the fish pass were not optimal with most of the vi sits lasting less than 5 minutes. Moreover, only 0.86% of the visits result ed in fish moving into the pass.