Background: Recent improvements in fixed acoustic monitoring receivers allow the tracking of individual aquatic animals over long periods of time with regular fine-scale positions. The VEMCO Positioning System (VPS) is now widely used, but various methodological issues remain to be clarified. The aim of this study was to analyze the spatial distribution of the probability of location and the positioning error over the entire surface of a hydropower reservoir, prior to analyzing fish behavior. Findings: Filtering the data set by the horizontal position error (HPE) significantly reduced the positioning error. Retaining only the positions with an HPE less than 15 retained 79% of VPS positions and decreased the positioning error by 33% (mean = 3.3 m, SD = 3.3 m). A higher probability of location was observed inside than outside the receiver array (44% and 36%, respectively). Moreover, the positioning error significantly differed inside (n = 243, mean = 2.4 m, SD = 2.1 m) and outside (n = 253, mean = 4.2 m, SD = 4.0 m) the receiver array (P < 0.001). Finally, the lowest positioning errors were detected in the area with the highest receiver density. Conclusions: The VPS measures fish positioning in a reservoir, under suitable conditions, with satisfactory accuracy. We showed that the probability of location and the positioning error differed spatially in accordance with previous results in other conditions. Consequently, these analyses are recommended as a prerequisite to further spatial analyses using VPS-derived data.

Water-level fluctuations are a major function of reservoirs that influence the littoral zone of the lake, and prove to be of high importance for the whole biological communities of lakes. Working on a French hydropower reservoir, we studied the influence of water-level fluctuations on intra-annual variations of littoral habitat availability and their consequences for the structure of fish assemblages inhabiting the littoral zone. As the water level decreased significantly, habitat conditions tended to be much more homogeneous. The proportion of sites with a thin substrate and low slope increased, while submerged vegetation and riparian shade disappeared. The relationship between habitat complexity and fish assemblage changed along the water-level gradient. The habitat effect on assemblage structure was strongest when the water-level conditions were high and very high, and weaker for low and very low water-level conditions. A homogenization of fish assemblages was observed when the water-level condition reached a threshold. These results suggest an effect of water-level management in structuring fish assemblages of the littoral zone of a reservoir due to a decrease of habitat complexity.

Perch Perca fluviatilis is a widespread predator in European reservoirs, frequent in open waters but also known to spend a lot of time in the littoral zones. To get insight into how adult perch used and selected their habitat in an environment subject to water level fluctuations, 21 perch were continuously tracked using acoustic telemetry over 2 years in the Bariousses reservoir (France). The different available habitats were characterized by depth classes and substrate types, presence of emerging trees, and presence of tree stumps in the littoral zone. We showed that perch habitat preferences were strongly dependent on the season, except for substrate type, and in line with their habitat use. Surprisingly we did not find any influence of the water level which however reduced the structural complexity of the littoral zone when lowering. In spring and summer, whatever the water level, we observed a strong preference for the littoral zone and complex habitats. In autumn and winter, perch migrated into deeper waters. However, the individual variability of the habitat preferences was quite high. This type of research helps to understand the spatial ecology of fish and provides useful guidance to hydromorphological restoration for fish populations.

The diversity of predatory species plays a key role in ecosystem functioning but our understanding of the mechanisms underlying their coexistence is limited, particularly in freshwater ecosystems. Northern pike Esox lucius, European perch Perca fluviatilis and pikeperch Sander lucioperca are three widespread predatory species in European lakes, where they often coexist. As potential competitors, we hypothesised that partitioning habitat is a determinant of species coexistence. This was tested by quantifying the variability of their habitat use in tracking adult individuals in the Bariousses reservoir (France, 86.6 ha, mean depth 7.1 m). Specifically, we investigated their distribution along the littoral–pelagic and depth axes along the daily cycle and across seasons. From littoral to pelagic waters were first found pike, then perch and finally pikeperch. Pike was the closest to the surface while pikeperch was the deepest. This general pattern was, however, variable across seasons with the three species located in the upper layer in summer during reservoir stratification. Individuals were more evenly distributed along the littoral–pelagic axis and closer to the bottom when water was mixing (autumn, winter). In summer, perch used more intensively in the pelagic zone during the daytime. Other species did not show any diel change of habitats. Our results highlighted that species coexistence is associated with habitat partitioning among these three predators, with perch showing a more variable behaviour regarding habitat characteristics. Now more than ever, in the context of global change which modifies habitats, it is of crucial importance to understand the coexistence mechanisms of species that shape ecosystems.

-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 Vézère river (Corrèze, 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).Keywords -reservoir; littoral habitat; fish; water level fluctuations; acoustic telemetry.Résumé -Le fonctionnement des milieux, aquatiques soumis à des pressions anthropiques, est étroitement dépendant des usages générant des fluctuations de l'environnement des communautés biologiques. Nous avons mené une étude sur une retenue soumise à des fluctuations du niveau de l'eau (WLF), afin d'étudier les réponses de l'ichtyofaune aux changements des conditions environnementales. Un suivi du comportement des poissons par télémétrie acoustique a été mené avec une attention particulière portée à la zone littorale car elle est fortement soumise aux WLF. Les résultats de cette étude sur la retenue des Bariousses, localisé sur la Vézère (Corrèze, France) sont synthétisés dans cet article. Nous avons mis en évidence que les fluctuations du niveau de l'eau induisent une variabilité Hydroécol. Appl. © EDF, 2018 https://doi.org/10.1051/hydro/2018001 https://www.hydroecologie.org temporelle de la surface occupée par la zone littorale. De plus, une diminution progressive de la complexité structurelle des habitats littoraux avec une tendance à l'homogénéisation (dominance des substrats fins et de l'absence de végétation) est observée suite à un abaissement du niveau de l'eau. Les réponses comportementales du sandre, de la perche et du brochet étaient fortement variables en fonction des conditions environnementales. La température et la photopériode représentent deux paramètres structurant majeurs de l'activité et du choix des habitats. Le niveau d'eau affecte une partie du peuplement ; certains individus sont plus mobiles et ont tendance à fréquenter de façon moindr...