| Abstract: | The Orlu hydroelectric power plant on the River Oriège is managed by ‘hydropeaking’ and generates significant and frequent daily fluctuations in flow downstream. Observations at this site aimed to determine the impact of these variations on the aquatic environment. Surveys were carried out in June and October, upstream and about 2 km downstream of the plant, with a view to studying fish feeding habits and nutritive resources. In June, when the natural river flow was high, invertebrate populations differed very little between the two sectors. In October, after the low-water period, density and biomass were significantly lower in the downstream sector than they were upstream. Invertebrate drift was studied by taking samples every hour over a 24-hour cycle. Upstream, this drift showed a clear daily rhythm, dominated by nocturnal drift. Downstream in June, invertebrate drift was more abundant than upstream; a similar nychthemeral rhythm was apparent, but there were significant peaks during hydropeaking. In October, this daily rhythm disappeared, and drift was extremely low when river flow was low; on the other hand, a marked increase was observed during hydropeaking. Fish feeding habits were studied parallel to drift by capturing about 20 fish every four hours and analysing their stomach contents. No clear rhythm in the daily food uptake was observed. Stomach content was more abundant downstream. Lastly, the number of prey captured by fish at a given period of the day did not appear to be linked with the effective quantity of invertebrates simultaneously available in the river. |
