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The removal of the two dams on the Sélune River since 2019 has led to profound changes in the aquatic ecosystem. Lentic habitats bordered by forest had shifted to new conditions (running water and sparsely vegetated riverbanks) therefore shaping organisms’ assemblies. We studied how the reestablishment of aquatic lotic habitats in interaction with riparian vegetation could mediate the restoration of important ecological functions in the new river. Six stations located along the river continuum were surveyed for 3 years after dam removal: two control stations upstream the former reservoir, three restored stations within the former reservoir, and one control station downstream. We monitored physico-chemical characteristics, phytobenthos biomass and the river’s benthic metabolism, and assessed the functional composition of macroinvertebrate communities. We compared the recorded variables among upstream, downstream and restored sampling stations. We observed a rapid recolonization by invertebrates, but a still low phytobenthic primary production in restored stations. Such a low primary productivity was also reflected in the functional composition of invertebrate communities. Three years after dam removal, there was still a significant time lag between communities recovery and expected ecosystem functioning restoration. We observed a quick colonization by aquatic running-water invertebrate communities of new lotic reaches, but a slower recovery of important ecological functions rates such as those observed in control stations.

Introduction: Diadromous fish populations have strongly declined over decades, and many species are protected through national and international regulations. They account for less than 1% of fish biodiversity worldwide, but they are among the most perceptible linkages between freshwater and marine ecosystems. During their migration back and forth, diadromous fish species are subjected to many anthropogenic threats, among which river damming can severely limit access to vital freshwater habitats and jeopardize population sustainability. Here, we developed a method based on a double-observer modeling approach for estimating the abundance of diadromous fish during their migration in rivers. Methods: The method relies on two independent and synchronous records of fish counts that were analyzed jointly thanks to a hierarchical Bayesian model to estimate detection efficiencies and daily fish passage. We used simulated data to test model robustness and identify conditions under which the developed approach can be used. The approach was then applied to empirical data to estimate the annual silver eel run in the Touques River, France. Results: The analysis of simulated datasets and the study case gives evidence that the model can provide robust,accurate, and precise estimates of detection probabilities and total fish abundance in a set of conditions dependent on the information provided in the data (annual distribution of fish passage, annual number of observation, pairing period, etc.). Discussion: Then, the method can be applied to various species and counting systems, including nomad acoustic camera devices. We discuss its relevance for programs on river continuity restoration, notably to quantify population restoration associated with dam removals.

The contribution of two basal energy sources – detrital organic matter and primary producers – as part of aquatic food webs varies typically along river continua. A host of barriers to river flow increase the water residence time and sediment and nutrient retention in reservoirs worldwide, and potentially alter the balance between detritus-based and algae-based energy pathways in the downstream food webs. We explored this issue on the Sélune River (Normandy, France), a small coastal stream that drains an agricultural catchment with high sediment runoff. Seasonal measurements of the following parameters were compared upstream and downstream of the reservoirs of two large dams (16 m and 36 m high): sediment fluxes, nutrient and chlorophyll a concentrations, algal communities in the epilithic biofilm (taxonomic composition, biomass and growth), and benthic invertebrate communities (abundance and trophic guild structure). As anticipated, annual sediment fluxes were much lower downstream of the reservoirs, where significant decreases in water turbidity, phosphate and silicate concentrations were recorded. A higher chlorophyll a concentration in water and a higher contribution of pelagic algae taxa to the photosynthetic biofilm suggested drifting and deposition of reservoir-borne phytoplankton downriver. Photosynthetic biofilm growth was higher downstream of the reservoirs in spring and fall, and so was the abundance of herbivores in the invertebrate community, notably scrapers and algae eaters. Energy pathways within riverine food webs were traced using stable isotope analyses of carbon (C) and nitrogen in the tissues of aquatic consumers (invertebrates and fish). Mixing models revealed a discontinuity in the origin of the C entering the food webs along the river continuum, confirming a greater contribution of algal C to aquatic consumers downstream of the reservoirs. These results illustrate mechanisms whereby large reservoirs can modulate C flow in food webs along a sm

Acoustic cameras are increasingly used in monitoring studies of diadromous fish populations, even though analyzing them is time-consuming. In complex in situ contexts, anguilliform fish may be especially difficult to identify automatically using acoustic camera data because the undulation of their body frequently results in fragmented targets. Our study aimed to develop a method based on a succession of computer vision techniques, in order to automatically detect, identify and count anguilliform fish using data from multiple models of acoustic cameras. Indeed, several models of cameras, owning specific technical characteristics, are used to monitor fish populations, causing major differences in the recorded data shapes and resolutions. The method was applied to two large datasets recorded at two distinct monitoring sites with populations of European eels with different length distributions. The method yielded promising results for large eels, with more than 75% of eels automatically identified successfully using datasets from ARIS and BlueView cameras. However, only 42% of eels shorter than 60 cm were detected, with the best model performances observed for detection ranges of 4–9 m. Although improvements are required to compensate for fish-length limitations, our cross-camera method is promising for automatically detecting and counting large eels in long-term monitoring studies in complex environments.

La pêche à l’électricité est une méthode classiquement utilisée pour les inventaires de biodiversité des poissons et crustacés dans le cadre de la surveillance réglementaire de l’état écologique des cours d’eau. Elle pose un certain nombre de difficultés en territoires ultramarins tropicaux, de par certaines spécificités physiques (conductivité de l’eau basse à très basse) et écologiques (communautés mixtes de poissons et de crustacés, biodiversité forte à très forte) qui les caractérisent. Issues du partenariat OFB-INRAE, les recherches menées en collaboration avec l’entreprise HYDRECO-Guyane ont permis de définir des conditions spécifiques d’application de la pêche à l’électricité pour les petits cours d’eau (< 6 mètres de large) tropicaux. Grâce à des essais au laboratoire et in situ, en Guyane, aux Antilles et à La Réunion, nous avons identifié les réglages électriques les plus efficaces pour la capture, l’estimation de la richesse spécifique et de l’abondance des espèces et le respect de la santé animale. Ces recherches se placent dans l’objectif plus large d’améliorer la qualité des données bancarisées pour le suivi environnemental des masses d’eau. Elles donnent lieu à des recommandations pour la maitrise d’œuvre et la maitrise d’ouvrage de la pêche à l’électricité en petits cours d’eau tropicaux.

Flight is a key feature of the reproduction and dispersal of emerging aquatic insects. However, morphological measurements of insect flight are mostly available for terrestrial taxa and dragonflies, while aquatic insects have been poorly investigated. We analyzed seven flight-related morphological parameters of 32 taxa belonging to five orders of emerging aquatic insects (Ephemeroptera, Trichoptera, Plecoptera, Diptera, and Megaloptera) with different life history traits related to flight (dispersal strategy, voltinism, adult lifespan, and swarming behavior). After correcting for allometry, we used an a priori-free approach to cluster the individuals according to their flight-related morphology. Then, we explored the levels of agreement between these clusters, taxonomy, and several life history traits of the taxa. All orders were scattered among several clusters, suggesting a large range of flight capacities, particularly for Diptera. We found swarming taxa in each cluster, showing that morphological adaptations to swarming are not identical in all aquatic insects. The clusters did not match the expected dispersal capacity of the taxa as derived from the literature or databases. Heavy wide-winged insects notably gathered taxa traditionally described as good or weak dispersers. Flight capacities based on morphology partly matched with the taxonomy and life-history traits of aquatic insect imagoes. Other parameters such as flight propensity, energy stores, and wing kinematics should help refine their flying and dispersal capacity.

Le Pôle sur la gestion des Migrateurs Amphihalins dans leur Environnement (MIAME) regroupe quatre organismes (OFB, INRAE, L’Institut Agro et Université de Pau et des Pays de l’Adour), soit 39 agents titulaires participant au niveau national en 2022. Les travaux ont une vocation opérationnelle pour la gestion de cette biodiversité remarquable de poissons et macrocrustacés, ainsi que les écosystèmes continentaux, estuariens et marins qui l’abritent, en métropole et outre-mer. Sur cette thématique, les missions concernent la recherche-développement, l’expertise, le pilotage des observatoires nationaux, la centralisation des connaissances, le développement de sites pilotes, l’innovation-transfert et la formation. Le Pôle fait l’objet d’une convention de coopération public-public entre les organismes constitutifs, qui couvre la période 2019-2024. Ce rapport présente l’organisation du Pôle et les activités développées au cours de l’année 2022.

Competition arises when species share a limited resource, but this can be avoided through niche partitioning. Despite the large body of literature on diadromous fishes, very few studies have focused on niche partitioning when competing for resources. Diadromous fishes are suffering a global decline throughout their range in part due to their peculiar life history traits as they migrate from the sea to freshwater to spawn or the reverse. They are particularly sensitive to river fragmentation induced by barriers. Dams for instance are expected to alter the spatial distribution and resource exploitation of diadromous fishes as well as other organisms. Here, we studied the ecological niche of six taxa of diadromous fishes, temporally co-occurring in the same land–sea continuum obstructed by two river dams. We used Bayesian mixing models run on C and N isotopes to infer the various habitats used and the origin of the trophic carbon in muscle and scale tissues of diadromous fishes. Results showed that the sub-adults of Anguilla anguilla, juveniles of Petromyzon marinus, and Salmo trutta exploit mainly the freshwater, or marine part for adults of Lampetra fluviatilis, whereas juveniles of Platichthys flesus and Mugilidae use all the habitats of the land–sea continuum. In the freshwater habitat, the microphytobenthos and biofilm are the main sources of carbon used, while in the marine habitat, the basal carbon sources are more diverse. The analyses of niche overlaps between diadromous fishes demonstrate that the widest isotopic niches, observed for A. anguilla and P. flesus, also have more impact within the community. Results are discussed in terms of intra- and interspecific competition between these diadromous fishes and their generalist/specialist strategies in an obstructed catchment.