| Summary: | The behaviour of migrating silver eels (Anguilla anguilla) was studied at 6 hydroelectric power-plants on the Gave de Pau river from 2007 to 2009. Nearly 200 eels were radiotracked along 60 km in highly variable hydrological conditions. Only 34% of the eels that were released in the upper part of the study area successfully passed the 6 power-plants. Depending on hydrological conditions, eels alternate motionless periods with periods of active migrations with speeds ranging from 0.05 km/h to more than 10km/h. Migratory activity and especially obstacles passage are largely influenced by hydrological conditions: though downstream movements were observed in river flows ranging from 25 m3/s to 530 m3/s, only 19% of obstacles passages have been observed when the river flow was inferior to the average discharge while half of the passages have occurred when the flow was superior to 2.3 times the average river discharge. A large majority (88%) of eel movements occurred during high river flows, mainly during the rising phase (73%). A model predicting obstacles passage was derived from those observations: the model assumes that obstacles passages are uniformly distributed among 5 river discharge classes: Q75; Q90; Q95; Q97.5; Q99. Regarding the time of the day, most passages occurred at twilight or at the beginning of the night (86% of passages from 5pm to 8 am, and 53% from 6pm to 12am). Fish passage routes were variable among obstacles and depending on hydrological conditions. On the whole, 66% of the observed passages have been carried out through spillways, 31% through the turbines, 2% through bypasses primarily designed for smolts and 1% through upstream fish passes. The passage route was largely influenced by three factors: (i) the ratio of spill flow to total river flow, (ii) the configuration of the water intake (e.g., position relative to the direction of the river flow) and (iii) the trashrack bar spacing. Passages through the turbines increase when the turbine flow increases, while passages by spillway are in a ...
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