Environmental drivers of fish spatial distribution and activity in a reservoir with water level fluctuations

The aquatic ecosystem structuration in human influenced environment, is closely dependent of the associated uses, which are generally fluctuant. We conducted an extended field monitoring on a reservoir under water level fluctuations (WLF), in order to study the responses of fish fauna to changes in environmental conditions. The study design was based on a monitoring of fish behaviour by telemetry in a reservoir with a particular attention to the littoral zone because of its front line position during WLF. The results of this study, which was conducted on the Bariousses reservoir, located on the Vezere river (Correze, France), are summarized in this article. The study revealed that WLF induced a temporal variability in the littoral zone surface. In addition, we observed a gradual decline in structural complexity of littoral habitats with a tendency towards homogenisation (dominance of fine substrates and absence of vegetation) in relation with the drop in water level. Behavioural individual responses of pikeperch, perch and pike were highly variable in relation to environmental fluctuations. Temperature and photoperiod were the two main parameters controlling fish activity and spatial distribution. Water level affected part of fish assemblage: some individuals were more mobile and left the littoral zone when inshore habitats were less complex (low water level).