| Summary: | National audience Radio tracking of 114 returning adult Atlantic salmon on the Pau River during a 3-year study (1995-1997) was used to determine the impact of 31 of the 37 obstacles built on the river. Five obstacles located downstream of spawning areas have a major effect on upstream migration with respect to the proportion of fish being able to pass over and the migration delays. An improvement in the facilities allowing fish to pass over these obstructions would enable more than 80% of them (as opposed to 13% currently) to reach good spawning grounds. Migration delays can prevent a large proportion of the population from reaching the spawning grounds in time, and this is especially true for fish which enter the river later in autumn or fish which stop their migration for a "quiescent period" in the lower part of the river after being delayed by several obstacles. The lowest weirs (2.5 m) have variable effects on migration depending on the type of fishpass. Pool passes and natural bypass channels are the most efficient. Denil fishpass with floor baffles are not very efficient and this is mainly due to limited operational flow and higher sensitivity to upstream flow level variations. Effects of hydroelectric power plants without diversion depend on the location of fish pass entrances and the flow in the fishway : minimum effects on migration have been observed at plants fitted with fish passes with significant flow (2-4 m3/s) located in the tailrace. The effect of plants with diversion canal, all fitted with fish passes at dam, are related to the restricted flow in the by-passed section of the river. Dam flow discharge with values greater than 500 l/s/m of width at the confluence with the tailrace attract fish. Most upstream passing of obstacles (85.6%) occurs during the day. Only natural bypass channels enable a significant proportion of fish to pass over the obstacles at night. Le suivi par radiotélémétrie de la migration anadrome de 114 saumons atlantiques sur le Gave de Pau, réalisé de 1995 à 1997, a ...
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