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Understanding the effect of human induced landscape fragmentation on gene flow and evolutionary potential of wild populations has become a major concern. Here, we investigated the effect of riverscape fragmentation on patterns of genetic diversity in the freshwater resident brook lamprey (Lampetra planeri) that has a low ability to pass obstacles to migration. We also tested the hypotheses of i) asymmetric gene flow following water current and ii) admixture with the closely related anadromousL. fluviatilisecotype having a positive effect onL. planerigenetic diversity. We genotyped 2472 individuals, including 225L. fluviatilis, sampled in 81 sites upstream and downstream from barriers to migration, in 29 West-European rivers. Linear modelling revealed a strong positive relationship between the distance to the source and genetic diversity, consistent with expected patterns of decreased gene flow into upstream populations. However, the presence of anthropogenic barriers had a moderate effect on spatial genetic structure. Accordingly, we found evidence for downstream-directed gene flow, supporting the hypothesis that barriers do not limit dispersal following water flow. DownstreamL. planeripopulations in sympatry withL. fluviatilisdisplayed consistently higher genetic diversity. We conclude that genetic drift and slight downstream gene flow mainly drive the genetic make up of upstreamL. planeripopulations whereas admixture between ecotypes maintains higher levels of genetic diversity inL. planeripopulations sympatric withL. fluviatilis. We discuss the implications of these results for the design of conservation strategies of lamprey, and other freshwater organisms with several ecotypes, in fragmented dendritic river networks.

Salmonids are characterized by a large diversity of life histories, but their study is often limited by the imperfect observation of the true state of an individual in the wild. Challenged by the need to reduce uncertainty of empirical data, recent development in medical imaging techniques offered new opportunities to assess precocious maturation in Atlantic salmon parr. Traditional phenotypic (external) examination and ultrasound (internal) examination were compared and recommendations on fish handling and ultrasound image interpretation are provided. By allowing to see the unseen, portable ultrasound imaging offers great opportunities for ecological studies in the wild, such as the assessment of individual sexual maturation.

This study was funded by the Agence Francaise pour la Biodiversite (research program supervised by J.M.P.) and the Ministere de l'Education Nationale, de l'Enseignement Superieur et de la Recherche (Ph.D. grant to N.B.). This work was carried out under the licenses 08/2015 and 07/2016 delivered by the Prefecture de la Loire-Atlantique. We thank the Parc Natural Regional de Briere, notably J.P. Damien, for supporting our research activities on amphibian ecology. The authors declare that they have no conflict of interest. We are also grateful to two anonymous reviewers for their constructive comments on an earlier version of the manuscript.; International audience; Stable isotope analyses (SIA) are now widely used in ecology research to quantify the trophic implications of a large range of ecological processes. Non-destructive sampling methods have been successfully developed for most vertebrates, but the validation of such methods is missing in amphibians. Filling this methodological gap is critical to reduce the significant lack of knowledge on the trophic ecology of this imperilled group. The relevance of tail fin as a surrogate for muscle, a tissue routinely used for SIA, was assessed in four amphibian species, including tadpoles and adult newts, by (1) testing the link between the isotope ratios of carbon and nitrogen in fin and muscle, and addressing the additional practical issues of (2) the effects of a topical anaesthetic on isotope ratios, and (3) the minimum quantities of fin needed for SIA to limit the severity of sampling. The isotope ratios were highly correlated between the two tissues in all four species. Equations were provided to predict muscle estimates from fin values for each species separately and grouped by their taxonomic order (anurans or urodeles), which represent a valuable solution for any other species. Topical anaesthetics had little influence on isotope ratios, and the surface of tail fin needed for SIA was small. We conclude that the tail fin of amphibians can be used as a non-lethal surrogate for muscle in SIA, and this offers promising prospects for research on the trophic ecology of amphibians, and potentially on their conservation.; Les analyses d'isotopes stables (AIS) sont maintenant largement utilisées en recherche en écologie pour quantifier les implications trophiques d'une large gamme de processus écologiques. Des méthodes d'échantillonnage non-létales ont été développées chez la plupart des vertébrés, mais une telle validation est absente chez les amphibiens. Il est essentiel de combler cette lacune de façon à réduire le manque de connaissances sur l'écologie trophique de ce groupe menacé. La pertinence de la nageoire caudale comme substitut au muscle, un tissu régulièrement utilisé pour les AIS, a été évaluée chez quatre espèces d'amphibiens, incluant des têtards et des tritons, (1) en testant le lien entre les ratios isotopiques du carbone et de l'azote du muscle et de la nageoire, et en répondant à des questions pratiques comme (2) l'effet d'un anesthésique local sur les ratios isotopiques, et (3) les quantités minimales de nageoire nécessaires aux analyses isotopiques, dans le but de limiter la sévérité de l'échantillonnage. Les ratios isotopiques étaient hautement corrélés entre les deux tissus chez les quatre espèces. Des équations ont été fournies pour prédire les valeurs du muscle à partir de celles de la nageoire en considérant chaque espèce séparément et en les regroupant selon leur ordre taxonomique (anoures ou urodèles) ce qui représente une solution intéressante pour d'autres espèces. L'anesthésique topique a eu une faible influence sur les ratios isotopiques, et la surface de nageoire nécessaire pour les AIS était faible. Nous concluons que la nageoire caudale des amphibiens peut être utilisée en tant que substitut au muscle pour conduire des AIS, ce qui offre des perspectives prometteuses pour la recherche en écologie trophique des amphibiens, et potentiellement pour leur conservation.

Stable isotope analysis of organic matter in sediment records has long been used to track historical changes in productivity and carbon cycling in marine and lacustrine ecosystems. While flow dynamics preclude stratigraphic measurements of riverine sediments, such retrospective analysis is important for understanding biogeochemical cycling in running waters. Unique collections of riverine fish scales were used to analyse N-15 and C-13 variations in the food web of two European rivers that experience different degrees of anthropogenic pressure. Over the past four decades, dissolved inorganic N loading remained low and constant in the Teno River (70 degrees N, Finland); in contrast, N loading increased fourfold in the Scorff River (47 degrees N, France) over the same period. Archived scales of Atlantic salmon parr, a riverine life-stage that feeds on aquatic invertebrates, revealed high N-15 values in the Scorff River reflecting anthropogenic N inputs to that riverine environment. A strong correlation between dissolved inorganic N loads and C-13 values in fish scales was observed in the Scorff River, whereas no trend was found in the Teno River. This result suggests that anthropogenic N-nutrients enhanced atmospheric C uptake by primary producers and its transfer to fish. Our results illustrate for the first time that, as for lakes and marine ecosystems, historical changes in anthropogenic N loading can affect C cycling in riverine food webs, and confirm the long-term interactions between N and C biogeochemical cycles in running waters.

Estuaries are essential fish habitats because they provide nursery grounds for a number of marine species. Previous studies in the Bay of Vilaine (part of the Bay of Biscay, France) have underlined the estuarine dependence of juvenile common sole (Solea solea, L) and shown that the extent of sole nursery grounds was positively influenced by the variability of the river flow. In the present study, stable carbon and nitrogen isotopes were used to describe the trophic network until the young-of-the-year sole and to compare interannual variations in the dominant trophic pathways in the sole nursery areas in this bay. Particulate organic matter (POM), sediment organic matter (SOM), microphytobenthos, benthic invertebrate sole prey and young-of-the-year common sole were collected during the summer over 4 years characterised by contrasting river discharges. POM isotopic signatures were used to identify the origins of nutrient and organic matter assimilated into the estuarine food web through benthic organisms to juvenile common sole. Interannual spatial variations were found in the POM carbon stable isotope signatures, with the importance of these variations depending on the interannual fluctuations of the river flow. Moreover, the spatio-temporal variability of this POM isotopic signature was propagated along the food webs up to juvenile sole, confirming the central role of river discharge and terrigeneous subsidy input in the estuarine benthic food web in determining the size of the sole nursery habitat. (C) 2009 Elsevier B.V. All rights reserved.