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Afin d’atteindre son objectif d’absence de perte nette de biodiversité, la séquence « Éviter, Réduire, Compenser » (ERC) doit permettre d’apporter des gains au moins équivalents aux pertes liées aux projets d’aménagement. Cela peut être réalisé grâce à des actions de restauration écologique qui visent à accélérer le rétablissement des écosystèmes. Le dimensionnement de la compensation passe par une estimation des gains en amont de la réalisation des projets, étape cruciale mais qui reste très incertaine, en raison notamment du manque de retour d’expérience du succès des actions de restauration écologique. Dans cet article, nous proposons des pistes d’amélioration de cette estimation. Pour cela, nous comparons des estimations de gains apportés par des mesures de restauration sur l’île de Kembs (Alsace), en utilisant une méthode d’évaluation de l’équivalence (ECOVAL) et des données de suivis récoltées juste après la mise en place de ces mesures. Plusieurs analyses sont réalisées et concernent (i) la justesse des estimations et le lien avec le degré d’incertitude attribué à l’estimation des gains (faible, moyen ou fort) et (ii) les sources des écarts observés entre les valeurs estimées et issues des suivis. Globalement, la moitié des estimations sont justes à court terme, mais une majorité est soumise à un délai trop court pour observer une évolution des indicateurs. Les bonnes estimations se retrouvent pour tous les degrés d’incertitude et quasi tous les taxons et milieux évalués. Les erreurs d’estimations proviennent principalement d’aléas pendant les travaux et d’une évolution naturelle des milieux non prévue. De plus, plus l’incertitude est importante et plus les écarts entre valeur estimée et issue des suivis est grande. Nos résultats soulignent l’importance de la mise en place de suivis réguliers sur plusieurs années (et notamment à long terme) pour vérifier que la biodiversité évolue bien comme estimé et pour rectifier les mesures compensatoires au besoin avec de la gestion adaptative. Le principal levier d’amélioration des estimations réside dans la capitalisation de retours d’expérience en portant une attention particulière à l’homogénéisation entre les données de suivis et celles de l’état initial.

International audience; In response to biodiversity erosion caused by human activities, biodiversity offsets are widely used to compensate for negative impacts with an objective of "No Net Loss" (NNL). One major challenge is to measure the biodiversity that will be impacted and compensated for. In most European countries and in France particularly, there are no recommended and available standardized indicators to measure biodiversity for offsets. This leads to heterogeneous practices with no guarantee of achieving NNL. In response to this fact we suggest a set of organized indicators to measure terrestrial biodiversity, which can be used in European and French offset programs to conciliate scientific knowledge, offset policies requirements and operational constraints that stakeholders have. To do this, we proceeded in four steps: (1) agreeing on a scientific definition of biodiversity; (2) Identifying the components of biodiversity targeted by the offset policy (3) developing a framework in which the relevant indicators will be organized and (4) prospecting for and selecting a set of indicators based on data collected on eight sites in France. We argue that the set of indicators address the legal requirements as well as the good practices recommended by the scientific sphere, while remaining operational for the stakeholders using it for their projects. It can also easily be adapted to other contexts where offset policy and ecosystems are different. Finally, we identify perspectives for losses and gains calculation, which is the next step to evaluate ecological equivalence between impacts and offsets.

International audience; Multiple abiotic and biotic processes structure trout population dynamics at different space and time scales [1], [2]. The key role of hydrological variability in structuring brown trout populations is well-established [3-5]; [6]; [7]. All age-stages were considered as potentially influenced by high spring floods because spates have a major impact on 0+ fish [8] [9] [10] and may be strong enough to influence the survival and dispersal of older stages [11]. Nevertheless, authors often failed to identify constraining abiotic conditions for juveniles and adults (e.g.,[4]) except after exceptional events (e.g., a 50-year flood in [11]). For instance, discharge thresholds (e.g.,maximum mean daily flood) have often been used, although they correspond to very different hydraulic constraints depending on rivers. Describing high flow based on standardized quantitative variables for the hydraulic habitat of brown trout (e.g., depth, velocity) might reduce these inconsistencies. In this communication, we pointed out the results quantifying the effect of floods on trout densities obtained through three different approaches taking into account abiotic and biotic parameters [12, 13]. We used an extensive data set collected in 45 river reaches, including 22 reaches located downstream a hydropower facility. Electrofishing surveys and detailed physical habitat characteristics (e.g. hydraulics, water temperature, and cover) were available at all reaches.

International audience; La gestion et la protection efficaces des populations d’animaux sauvages reposent sur une bonne compréhension de leurs cycles biologiques. Parce que la dynamique des populations dépend des interactions complexes de processus biologiques et écologiques à différentes échelles, de nouvelles approches sont nécessaires pour expliquer la variabilité des processus démographiques et des paramètres associés dans une hiérarchie d’échelles spatiales. Un modèle bayésien hiérarchique du cycle biologique des truites fario (Salmo trutta) résidentes a été élaboré pour évaluer l’influence relative de déterminants locaux et généraux de la mortalité. Le modèle a été calé sur un vaste ensemble de données provenant de 40 tronçons de rivière, qui combine des données d’abondance et de conditions du milieu (hydraulique, température de l’eau). La mortalité des juvéniles dépendante de la densité augmente à faibles températures et diminue quand la disponibilité d’abris augmente. Des températures de l’eau élevées accroissent la mortalité densité-dépendante chez les adultes. Le modèle pourrait aider à prédire la mortalité mensuelle des juvéniles et des adultes dans des scénarios de réchauffement climatique et de modifications de la disponibilité d’abris découlant de la dégradation ou de la restauration d’habit

[Departement_IRSTEA]Eaux [TR1_IRSTEA]QUASARE [ADD1_IRSTEA]Systèmes aquatiques soumis à des pressions multiples; International audience; Assessments of fish population in large environment are highly challenging and extremely expensive. Indeed, fish populations are estimated using repeated counts (electrofishing, net catches) characterized by a strong over dispersion that provide uncertain estimates of actual densities. Previous studies showed that fish-population time series allow detecting significant trends and shifts linked to continuous and sudden environmental changes (caused by global warming, anthropogenic alterations and restoration actions). However, those ecological time series exhibited also a high interannual variability still poorly understood. In this study, we proposed to use symbolic dynamics approach combined with techniques from Information Theory to identify the processes that drive the strong interannual variability in fish-population time series. We analysed four extensive data sets (fish populations in 40 streams, the Rhône River, the Gironde estuary, and in the Bay of Biscay) characterized by relatively long-term ecological time series (~ 30 years) and including a detailed description of the environmental conditions. We transformed continuous environmental and ecological time series into symbolic sequences based on the time series trajectory at each time-point and calculated the mutual information between symbolic sequences to evaluate the degree of synchrony between time series. We expected synchronisms between 1) fish populations of the closest sites, 2) juveniles and adults, 3) environmental drivers and fish densities. A lack of significant results for these three evident hypotheses might suggest a too important sampling noise to be able to examine the variability in fish populations at the interannual time scale and a reconsideration of these costly extensive fish samplings.

International audience; The structure and abundance of brown trout populations varies greatly among years and stream reaches. Among the numerous environmental key factors explaining this variability, the effect of floods is predominant but rarely quantified. Here, we summarise the results of a PhD thesis that aimed to identify the demographic processes and the influence of biotic and abiotic drivers structuring the age-stages of trout populations in time and space. We used an extensive data set collected in 45 river reaches, including 22 located downstream a hydropower facility. Electrofishing surveys and detailed physical habitat characteristics (e.g. hydraulics, water temperature, cover) were available at all reaches. Firstly, an analysis of the determinants of synchrony in densities at a large scale, pointed out that the recruitment of close populations (up to 75 km apart) could be synchronized by large floods during emergence of fry and/or by the remobilization of spawning substratum. Secondly, the application of a deterministic trout population dynamic model, independently calibrated on nine reaches, revealed the influence of various local environmental drivers on population dynamics. Finally, a hierarchical model involving all reaches showed that density-dependent mortality among juveniles and adults was a key biotic process. The strength of the competition was greater in absence of shelters and varied with water temperature. The results of this work pointed out that trout populations share common processes that can be modelled as a function of quantitative environmental parameters. They provide a scientific basis to hydropower facility managers to reduce their influence on trout populations and respond to regulatory demands.

International audience; The key role of hydrological variability in structuring brown trout populations is well established. However, the influence of additional drivers is more difficult to identify. The implementation of long-term monitoring and the development of reliable tools can help to reveal fine local drivers structuring fish populations in contrasted flow regimes. This study used data series for nine reaches monitored for nine to nineteen years in four French salmonid streams. Study reaches were within five bypassed sections influenced by instream flow. A deterministic trout population dynamics model was applied on each reach, with calibration and validation procedures. Results revealed that biological drivers structured all reaches similarly. In addition, seven other drivers were identified. Among these additional drivers, hydrology mainly explained temporal fluctuations in trout density, regardless of reach. Three drivers independent of hydrology were also revealed: poor water quality, limited spawning area and the effect of power plant operations (overtopping, flushing or plant shutdown). All drivers influenced the whole bypassed section and were never limited to the scale of the reach (sampling area). Further analyses of each driver are now needed, to regionalise and quantify their respective impact precisely. Thus, assessment of trout population status would be simplified , enabling implementation of efficient management rules.

[Departement_IRSTEA]Eaux [TR1_IRSTEA]QUASARE; International audience; Les facteurs environnementaux peuvent causer des fluctuations synchrones de densités entre populations. Une meilleure compréhension des processus expliquant la synchronie est fondamentale pour prédire des pertes de résilience des métapopulations sujettes à des changements environnementaux. Nous étudions la synchronie des chroniques de densités de trois classes d’âge de la truite brune (Salmo trutta) (0+, 1+ et adultes) entre 36 tronçons de cours d’eau. Nous utilisons des tests de Mantel pour discriminer les effets relatifs de la proximité géographique, de la synchronie de variables environnementales clefs (débit, température de l’eau, conditions hydrauliques et mobilité du substrat) et de la dispersion densité-dépendante. La synchronie environnementale expliquait fortement la synchronie de la truite jusqu’à des distances de 75 km. Cet effet était dû en partie à l’influence négative sur les 0+ des hauts débits pendant l’émergence et une influence de la mobilité du substrat pendant la période de ponte. La dispersion entre tronçon influençait faiblement nos résultats. Les densités de juvéniles et d’adultes étaient fortement structurées par des processus de survie, mais n’étaient pas influencées par la synchronie des conditions environnementales. Les résultats suggèrent que l’environnement peut avoir des effets généraux sur la dynamique de population qui peuvent influencer la résilience des métapopulations

International audience; Managing brown trout (Salmo trutta fario) populations is of high concerns for many stakeholders (e.g. angling associations or hydropower companies). However, building trout life cycle models remains challenging. Here, we describe a general hierarchical Bayesian model of brown trout population dynamics that combines global (e.g. an average value of age-class annual survival) and local parameters (e.g. shelter availability influencing local carrying capacity). To fit the model, we used annual fish samplings (i.e., two-pass electrofishing followed by density estimation) conducted at 40 reaches. Reaches were located in natural (n=21) or bypassed (i.e. downstream dam; n=19) stream sections across France, thus offering different habitat characteristics. Habitat characteristics were measured at each reach, including daily flow, daily hydraulics conditions, daily temperature and shelter availability. This study (1) allowed us to understand between-reach differences in survival processes, (2) was consistent with previous studies and explained spatial variations in resident brown trout population dynamics and (3) offers a promising framework for future developments.

Le retour au premier plan de la notion de santé des écosystèmes met en évidence la préoccupation de la société à mettre en oeuvre des mesures pour la protection des milieux anthropisés. Les critères d’évaluation de la santé des peuplements piscicoles mis en oeuvre sont rarement débattus au sein de la communauté scientifique. L’exercice proposé ici devait permettre d’avoir ce débat sur la truite commune (Salmo trutta), espèce phare des cours d’eau situés en tête de bassin, qui représentent la majorité du réseau hydrographique français. La démarche visait à recueillir la conception qu’avait un nombre restreint d’experts, sur le fonctionnement de ces populations et sur les critères d’évaluation de leur fonctionnement. Les paramètres clé régulant chaque stade de développement ont été identifiés et hiérarchisés. Un consensus s’est dégagé sur le fait que, dans l’état actuel des connaissances, le diagnostic peut être établi sur la base de l’analyse des paramètres abiotiques essentiels pour la biologie et, plus difficilement, sur l’analyse des paramètres biotiques. Pour tous les stades de développement, les paramètres identifiés sont liés à l’habitat (qualité du substrat, conditions hydrauliques, température et qualité de l’eau), l’hydrologie et la connectivité. Des connaissances demandent d’être acquises pour définir des critères biologiques. Cela implique de renforcer la surveillance biologique à long terme et les actions de recherche pour caractériser la variabilité des paramètres biologiques et identifier les échelles spatio-temporelles pertinentes et les processus fonctionnels., The renewed emphasis on the concept of ecosystem health highlights society’s interest in taking measures to protect environments affected by human activity. The criteria used for evaluating fish population health are rarely discussed within the scientific community. The present study attempts to discuss these for brown trout (Salmo trutta), a flagship freshwater species typical of headwater streams which represent most of the French hydrographic network. This initiative aimed at gathering ideas from a limited number of experts on the criteria for evaluating trout population health. The main key parameters were identified and organized into a hierarchical framework for each development stage. A consensus emerged on the fact that at the current state of knowledge, a diagnosis can be established based on the analysis of essential abiotic parameters but with more difficulty on the analysis of biotic parameters. For all development stages, the identified parameters are linked to habitat (substrate, hydraulic parameters, temperature and water quality), hydrology and connectivity. Further knowledge is required to identify relevant biological criteria. This implies long-term biological monitoring and research programs to better understand and identify biological parameters and their variability as well as relevant spatiotemporal scales and functional processes.

Brown trout (Salmo trutta) ecology was largely studied. However, the process structuring the spatiotemporal patterns of population dynamics remains unclear. The objective of this thesis was to identify the demographic processes (e.g. survival or displacements) and the influence of biotic (between-individuals interactions) and abiotic (environmental conditions) processes structuring the age-stages (fry, juveniles and adults) of these populations in time and space.I studied (1) the role of density-dependence on survival and (2) the influence of environmental conditions experienced by trouts (hydraulics and water temperature). To assess the results’ transferability, I studied (3) the spatial scale (global or local) of influence of the processes and (4) if those processes varied among populations (hierarchical approach).I considered the dynamics of 45 trout populations, 22 being located downstream a hydropower facility. At a large scale, the recruitment of distant populations (up to 75 km apart) may be synchronized by large floods during emergence of fry or by spawning substratum displacements. We summarized the results of a determinist population dynamics models, locally calibrated on nine reaches showing well-described environmental conditions. This summary revealed the influence of various local drivers on population dynamics. Finally, a hierarchical model showed that density-dependent mortality among juveniles and adults was a key biotic process. The strength of the competition was greater in absence of shelter (